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DOPED NANOTITANIUM DIOXIDE FOR PHOTOCATALYTIC APPLICATIONS
by
LANGELIHLE NSIKAYEZWE DLAMINI
Thesis in fulfilment of the requirement for the degree
DOCTOR OF PHILOSOPHY
in
CHEMISTRY
in the
FACULTY OF SCIENCE
of the
UNIVERSITY OF JOHANNESBURG
Supervisor : PROF. RWM KRAUSE Co-supervisors : DR. SH DURBACH : PROF. GU KULKARNI
i
DECLARATION
I hereby declare that this dissertation, which I herewith submit for the research
qualification
DOCTOR’S DEGREE IN CHEMISTRY
to the University of Johannesburg, Department of Applied Chemistry, is, apart from
the recognised assistance of my supervisors, my own work and has not previously
been submitted by me to another institution to obtain a research diploma or
degree.
_______________________________ on this ____ day of _______________
(Candidate)
_______________________________ on this ____ day of _______________
(Supervisor)
_______________________________ on this ____ day of _______________
(Co-supervisor)
_______________________________ on this ____ day of _______________
(Co-supervisor)
ii
DEDICATION
I dedicate this work to the following:
My mother, for she has been there for me unconditionally in all situations. A
blessed son I am to have you as my mother. Ndwadwe.
My Princess, HRH Princess Lomakhetfo Dlamini, your influence to me is without
measure. Forever you shall be in my heart. Nkhosi
Mr Richard, my high school chemistry teacher, for he planted the love and
enthusiasm I have for chemistry. Thank you for paving the way.
For I know that I know that I should know your WORD shall come to pass:
לראייה היא עדיין לזמן שנקבע, אך בסופו
של דבר ידבר ולא לשקר. למרות שזה
מתעכב, לחכות שזה; כי זה בוודאי יבוא
' For the vision is yet for an appointed time, but at the end it shall speak and not lie. Though it
linger, wait for it; because it will certainly come and will not delay.'
Habakkuk 2: 3
iii
ACKNOWLEDGEMENTS
I am indebted to many people and institutions that have helped me complete this
thesis.
My advisors, Prof. RWM Krause, Dr. SH Durbach, and Prof. GU Kulkarni, for their
guidance, motivation and genuine interest in my work and to see me grow in
fullness. I am thankful to Jawaharlal Nehru Centre for Advanced Scientific
Research (JNCASR), especially the unit of Chemistry and Physics Materials, for
warmly welcoming me and shaping my research skills and for hosting me during a
research visit.
I acknowledge the University of Johannesburg, Department of Applied Chemistry
for providing me with the platform to further my studies. I extend special
appreciation to Dr K Pillay who was my mentor when I had classes to teach and
Ms H Oosthuizen for believing in me. Thank you to UJ New Generation
Scholarship (NGS) for the consistent financial support and the UJ Center for
Nanomaterials Science and DST/NRF Centre of Excellence in Strong Materials for
project support. I am grateful to the University of Witwatersrand (microscopy unit)
and Council for Scientific and Industrial Research (CSIR) – nanoscale unit for
allowing me to use their facilities. I am also thankful to Dr Paul Franklyn for his
insight and input on preparation of binary metallic oxides.
In the course of my study, I have met incredible people, who have pulled me up
when the going was non-existent, and showered me with love when I felt totally
pathetic, as well as gave me an ear (to the good/bad moments) when I was either
overwhelmed or overjoyed. I am truly grateful to all, even if I don‟t specifically
mention you. You were a friend when I needed one. However, I am particularly
thankful to Lungile Lukhele, Thabile Ndlovu and Soraya Malinga and I hope and
pray I have been a blessing to you just as you have been to me. Machawe “CFO”
Motsa your sarcasm and rebellious nature was a needed dose. My brother from
another mother, Mnqbobi Dlamini, you are awesome. Nomcebo “Tjotjo” Ngwenya,
your humor and stubbornness is for the history books.
iv
St John‟s College (SJC), an institution that completed my training. I am forever
changed by the experience I had in my stay with the college. I am particularly
thankful to the Department of Science which warmly welcomed me into their
space. Dr. C. Henning an exemplary leader you are, Mr. WR Taylor and Mr. R.
MacDonald for allowing me into their classrooms. Mrs. E. Bosman, the fellowship
we shared was always inspirational. Mr. R. Klement, Mrs. K. van Schoor and
Mr. B. Bradley, I appreciated the warmth you showered upon me. Finally, my
mentor, Mrs. N. Stocks what an incredible person you are. I have learnt so much
from you, be it directly or indirectly. Thank you.
SJC isn‟t SJC without its students. Single-handedly you have changed my
perception about students from private schools. I am truly glad to have met you,
from remove to upper six. I am particularly pleased with lower five (A2-set) as their
inquisitiveness and intelligence made me love chemistry all the more. Lastly, I
would like to thank Mr. C Bossert (Nash housemaster) and Mr. C. Milligan (Deputy
housemaster) for allowing to be a stooge at Nash. I appreciate the friendliness and
brotherhood from the fellow stooges (Jarryd Kuiper, Nick Fouche and Pierre le
Roux). These young (for now) Johannains made my stay worthwhile, Bradley
Tshepo Chauke (kind hearted), Andrew Ian Williamson (true academic), Gary
Francis Jackman (my wingman…) S‟bonakaliso MG Nene (outstanding musician),
Molemo Otsile Ponoane (Sotho patriot) and Matthew Henderson (outstanding
actor).
My colorful family (Friday “Boywizzy” Sihlongonyane, Nelisiwe Mamba, Mangaliso
„Mzomba‟ Ndzinisa, Temangilozi Dlamini, Lindelwa Malidzisa, Tfolele Ndwadwe
and Tengetile Malidzisa) whom I love dearly. Thank you for putting up with me. My
father HRH Prince Mandla Dlamini, you are still the greatest.
Finally, the Lord God Almighty, your limitless grace that overlooks my
shortcomings, hence I can never claim any success as mine but yours always. In
you I have found favor and I am forever in awe. I am glad and grateful that you
honor faithfully our covenant. Amen.
v
ABSTRACT
Titanium dioxide (TiO2) – also known as titania – exists in three common
polymorphic crystal phases, i.e. anatase, brookite, and rutile. The most common
phase is rutile while anatase is regarded as the most photoactive. Titania is used
commercially as a white pigment for cosmetics, plastics, paints and sunscreens.
However, a significantly higher interest in TiO2 lies in its application in the
photocatalytic purification of water and air, the production of hydrogen by water
splitting, and its ability to behave as a disinfectant against biological
microorganisms. Despite the potential applications of titania, its use as a pure
material (pristine) is rather limited by a number of physical properties.
The experiments presented in this thesis show that doped titania composites
containing either anionic or binary metal oxide dopands (and composited with
carbon nanotubes (CNTs)) are photocatalytically superior to pristine TiO2.
The synthetic routes of these nanocomposites were based on facile techniques
such as solventless thermal decomposition followed by the “sol-gel” process for
the preparation of binary metal oxides and nanocomposites. For the anionic doped
nanocomposites a single-step (one-pot) sol-gel process was also used. These
doped TiO2 materials were characterized using microscopic and spectroscopic
techniques, to determine the particle size, polymorphic nature, morphology, band
gap and elemental composition. These characterization tools confirmed the
successful synthesis of doped TiO2 and CNTs nanocomposites.
The catalytic properties of these nanocomposites were evaluated by irradiating
samples with ultra-violet and visible light in the presence of model organic
pollutants such as: bromophenol blue and phenol red. From the experimental data,
a general trend could be observed for the anionic doped nanocomposite, when
exposed to different radiation. The work of this thesis was the first to report the
photocatalytic trend of anionic doped (halide) TiO2.
vi
The nanocomposites containing CNTs degraded the model dyes by over 90%
when both UV and visible light were used. The doped nanocomposites were both
more effective and faster in degrading the dyes. In addition, CNT nanocomposites
containing metal oxides were also effective in degrading over 95% of the model
dyes under visible and UV light. The degradation rates of the pollutants varied
with catalyst and dye, but were at least 80% faster than the commercially available
and widely used Degusa P25. The reaction kinetics for the degradation of the
model dyes for all nanocomposites appeared to follow pseudo-first-order rate laws.
Ultra-violet spectroscopy and ion-chromatography were employed to measure the
decrease in the concentration of the dyes and the resultant by-products
respectively.
The synthesis of TiO2, anionic-doped photocatalysts and binary metal oxide
photocatalysts and composites, all based on TiO2, are presented in this thesis
together with their characterisation and performance against model pollutant dyes.
vii
TABLE OF CONTENTS
Section Page
Declaration .............................................................................................................. i
Dedication ............................................................................................................... ii
Acknowledgements ................................................................................................ iii
Abstract .................................................................................................................. iv
Table of Contents ................................................................................................... v
List of Figures ......................................................................................................... x
List of Tables ........................................................................................................ xiii
List of Abbreviations ............................................................................................. xiv
Presentations and Publications ............................................................................ xvi
CHAPTER 1 : INTRODUCTION .......................................................................... 1
1.1 Problem statement ....................................................................................... 1
1.2 Justification .................................................................................................. 2
1.3 Objectives of the Study ................................................................................ 3
1.4 Outline ......................................................................................................... 4
REFERENCES ....................................................................................................... 5
CHAPTER 2 : LITERATURE REVIEW ................................................................ 6
2.1 Introduction .................................................................................................. 6
2.2 Titanium Dioxide .......................................................................................... 8
2.2.1 Surface Perturbation ................................................................... 13
2.2.1.1 Co-deposition of Metals ................................................ 13
2.2.1.2 Anionic Doping .............................................................. 15
2.2.1.3 Composite Semiconductors .......................................... 18
2.2.1.4 Dye Sensitization .......................................................... 20
2.3 Synthesis of Titanium Dioxide Nanoparticles ............................................. 22
2.3.1 Sol-Gel Method ........................................................................... 23
viii
2.3.2 Hydrothermal Method.................................................................. 28
2.3.3 Solvothermal Method .................................................................. 29
2.3.4 Chemical and Physical Vapour Deposition ................................. 30
2.4 Pollutants ................................................................................................... 32
2.5 Dyes........................................................................................................... 36
2.6 Carbon Nanotubes ..................................................................................... 39
REFERENCES: .................................................................................................... 43
CHAPTER 3 : EXPERIMENTAL METHODOLOGY .......................................... 62
3.1 Reagents used ........................................................................................... 62
3.2 Synthesis of Photocatalysts ....................................................................... 63
3.2.1 Anionic-doped Titanium Photocatalyst ........................................ 63
3.2.2 Preparation of Composite Semiconductors ................................. 64
3.2.2.1 CNTs-CoO and CNTs-CoO-TiO2 .................................. 64
3.2.2.2 TiWxOy and CNTs-TiWxOy ............................................. 64
3.3 Characterization ......................................................................................... 65
3.3.1 Transmission Electron Microscopy (TEM) ................................... 65
3.3.2 Scanning Electron Microscopy (SEM) ......................................... 67
3.3.3 Raman Spectroscopy .................................................................. 67
3.3.4 Powder X-ray Diffraction (PXRD) ................................................ 68
3.3.5 Ultraviolet-Vis (UV-Vis) Spectroscopy ......................................... 69
3.3.6 Surface Area Measurements ...................................................... 70
3.3.7 Photocatalytic Measurements ..................................................... 71
REFERENCES ..................................................................................................... 74
CHAPTER 4 Photodegradation of triphenylmethane dyes by anionic doped
titania nanocomposites* .................................................................................... 76
4.1 Introduction ................................................................................................ 76
4.2 Results ....................................................................................................... 77
4.2.1 BET Analysis ............................................................................... 77
4.2.2 Raman Analysis .......................................................................... 80
ix
4.2.3 Optical Studies ............................................................................ 81
4.2.4 Powder X-ray Diffraction (PXRD) ................................................ 83
4.2.5 TEM and SEM ............................................................................. 85
4.3 Photocatalytic Activity of the doped TiO2 and CNTs-TiO2 nanocomposites 87
4.3.1 Photodegradation of Bromophenol Blue ..................................... 88
4.3.2 Photodegradation of Phenol Red (PR) ........................................ 92
4.4 Discussion ................................................................................................. 95
REFERENCES ................................................................................................... 102
CHAPTER 5 Titania-based binary metal oxides and nanocomposites for the
photodegradation of organic pollutants in water* ......................................... 104
5.1 Introduction .............................................................................................. 104
5.2 Experimental ............................................................................................ 105
5.3 Characterization ....................................................................................... 106
5.4 Photocatalytic Activities of pristine TiO2 and various BMO Nanocomposites
115
5.4.1 Photodegradation of Bromophenol Blue (BPB) ......................... 115
5.4.2 Photodegradation of Phenol Red (PR) ...................................... 118
5.5 Discussion ............................................................................................... 120
REFERENCES ................................................................................................... 125
CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS ............................. 127
6.1 Conclusions ............................................................................................. 127
6.2 Future outlook .......................................................................................... 129
APPENDIX ......................................................................................................... 130
x
LIST OF FIGURES
Figure Description Page
Figure 2.1: Representation of the photoexcitation in a semiconductor .................. 7
Figure 2.2: Structures of (a) anatase and (b) rutile.......................................... 9
Figure 2.3: Structure of brookite .................................................................... 10
Figure 2.4: Plausible pathways followed by holes and electrons. ................. 12
Figure 2.5: Absorption of solar spectrum against band gap of titania. .................. 13
Figure 2.6: A representation of the Schottky barrier ............................................. 14
Figure 2.7: Proposed schemes of band gap narrowing of TiO2 by doping with non-
metals. ........................................................................................ 16
Figure 2.8: Plausible mechanism for the fluoride induced enhancement in the
production of free hydroxyl radicals in bulk solution from irradiated
TiO2. ............................................................................................ 17
Figure 2.9: Electron and hole separation in a coupled semiconductor system ..... 19
Figure 2.10 (a) Structure of porphyrin, (b) Structure of metalloporphyrins ........... 21
Figure 2.11: Organic dye (cis – [Ru (dcbH2)LL‟ attached on TiO2. ....................... 22
Figure 2.12: TEM images showing sol-gel prepared titania particles (a) and (b)
uniform distribution of titanium nanoparticles,(c) and (d) reveal
agglomerated nanoparticles. ....................................................... 28
Figure 2.13: SEM images of TiO2 nanoparticles prepared by the hydrothermal
method. ....................................................................................... 29
Figure 2.14: TEM images showing nanocrystals prepared by the solvothermal
method. ....................................................................................... 30
Figure 2.15 (a). Scheme showing the set-up for the synthesis of titanium
nanoparticles by CVD ................................................................ 31
Figure 2.15 (b). Scheme showing the set-up for the synthesis of titanium
nanowires by PVC....................................................................... 32
xi
Figure 2.16: Flow-chat illustrating the different methods used in the removal of dye
from wastewater. ......................................................................... 34
Figure 2.17 Classes and structures of synthetic dye ............................................ 37
Figure 2.17 Classes and structures of synthetic dye ............................................ 38
Figure 2.18: Different allotropes of carbon ........................................................... 39
Figure 2.18: Roll-up of a graphene sheet to form a nanotube. ............................. 40
Figure 2.19: A bottom-up approach of CNTs via CVD route................................. 41
Figure 3.1: Relationship between adsorbed gases compared to pressure ........... 70
Figure 3.2: (a) UV-radiation experimental set-up for the photodegradation of
model pollutants .......................................................................... 71
Figure 3.2: (b) Visible radiation experimental set-up for the photodegradation of
model pollutants .......................................................................... 72
Figure 4.1: Nitrogen sorption isotherm of selected nanocomposites .................... 78
Figure 4.2: Raman spectra of pristineTiO2, dopedTiO2 and doped CNTs-TiO2
nanocomposites .......................................................................... 81
Figure 4.3 Comparative UV absorbance measurements of (a) CNTs-TiO2
nanocomposites (b) doped TiO2.................................................. 82
Figure 4.4: Band gap measurement of pristine TiO2 and iodine doped TiO2 ........ 83
Figure 4.5: PXRD patterns of (A) doped TiO2 , (B) CNT loaded anionic doped
nanocomposites .......................................................................... 84
Figure 4.6: (a, b) TEM images of partly dispersed TiO2 on CNTs (c)
agglomerated TiO2 nanoparticles without CNTs (d) SEM images of
TiO2 nanoparticles without CNTs ................................................ 86
Figure 4.6: ((e, f) EDS spectrum of fluorine and iodine doped CNTs-TiO2
nanocomposites. ......................................................................... 87
Figure 4.7 (a,b) Photodegradation of BPB by anionic doped TiO2 using UV and
visible light .................................................................................. 88
Figure 4.7 (c) reaction kinetics of BPB initiated by visible irradiation .................... 89
xii
Figure 4.8: (a, b) Degradation of BPB by anionic undoped CNTs-TiO2
nanocomposites and CNTs-TiO2-X nanocomposites under UV and
Visible irradiations, ...................................................................... 91
Figure 4.8: (c) Reaction kinetics of CNTs-TiO2-X nanocomposites with visible
light ............................................................................................. 92
Figure 4.9 Photodegradation of PR by doped TiO2 under (a) visible and (b) UV
light (c) reaction kinetics doped TiO2 under UV light. .................. 93
Figure 4.10 (a,b) Photodegradation of PR by CNTs-TiO2-X nanocomposites (c)
reaction kinetics of PR by CNTs-TiO2-X nanocomposites under
visible light. ................................................................................. 94
Figure 4.11: Proposed mechanism of the role played by CNTs upon
photodegradation of dye in water ................................................ 99
Figure 4.12 Generated sulphate ions from the degradation of PR using (a) ultra-
violet and (b) visible light ........................................................... 100
Figure 4.13a Liberated sulphate and bromide ions from the degradation of BPB (a)
CNTs-TiO2, (b) CNTs-TiO2-I, under visible irradiation ............... 101
Figure 4.13b Liberated sulphate and bromide ions from the degradation of (C)
TiO2 (D) TiO2-I under visible irradiation ..................................... 101
Figure 5.1: BET analysis of (a) cobalt oxide titania nanocomposite and (b)
tungsten oxide titania BMOs and nanocomposite ..................... 106
Figure 5.2: Raman spectra of (a) CNTs-CoO-TiO2 (b) TiO2, (c) TiWxOy and (d)
CNTs-TiWxOy ............................................................................ 108
Figure 5.3: (a-b) SEM images of CoO-TiO2 and Ti-WxOy, (c-d) EDS spectra of the
binary oxides ............................................................................. 110
Figure 5.3: (e-h) TEM images of CNTs-CoO-TiO2 and CNTs-Ti-WxOy
nanocomposites, (i) EDS spectra of CNTs-Ti-WxOy nanocomposite
.................................................................................................. 111
Figure 5.4: PXRD pattern of cobalt-titania nanocomposites ............................... 112
Figure 5.5: PXRD patterns of tungsten-titania nanocomposites ......................... 113
xiii
Figure 5.6: Optical studies of the various binary metal oxides, and their
nanocomposites ........................................................................ 114
Figure 5.7: Evaluation of photocatalytic activity of BPB by various TiO2 and BMO
nanocomposites (a, b) under UV irradiation .............................. 115
Figure 5.7: Evaluation of photocatalytic activity of BPB by various TiO2 and BMO
nanocomposites (c,d) under visible irradiation .......................... 116
Figure 5.8: Reaction kinetics of the various (a) TiO2 and (b) BMO nanocomposites
as photocatalysts of BPB using different illumination sources .. 117
Figure 5.9: The photodegradation of PR by TiO2 and BMO nanocomposites (A, B)
under UV irradiation .................................................................. 118
Figure 5.9: The photodegradation of PR by TiO2 and BMO nanocomposites (C,D)
under visible irradiation ............................................................. 119
Figure 5.10: Reaction kinetics of (a) PR photodegradation by various TiO2 and (b)
BMO nanocomposites under UV and visible illumination .......... 119
Figure 5.11: Proposed mechanism for the binary system loaded with CNTs ..... 121
Figure 5.12 Generated sulphate ions from the degradation of PR using (a,c) ultra-
violet and (b,d) visible light ........................................................ 122
Figure 5.12: Generation of bromide and sulphate ions from BPB by TiWxOy and
CNTs- TiWxOy nanocomposites, tested as photocatalysts under
visible and UV light ................................................................... 123
Figure 5.12: Generation of bromide and sulphate ions from BPB by TiWxOy and
CNTs- TiWxOy nanocomposites, tested as photocatalysts under
visible and UV light ................................................................... 124
xiv
LIST OF TABLES
Table Description Page
Table 2.1: Band gaps of semiconductors ....................................................... 7
Table 2.2: Applications of TiO2 ....................................................................... 8
Table 2.3: Crystalline parameters of TiO2 polymorphs ................................. 11
Table 2.4: Physical properties of TiO2 polymorphs ...................................... 11
Table 2.5: Common Alkoxides .............................................................................. 23
Table 3.1 Reagents used ..................................................................................... 62
Table 4.2: Kinetic measurements of the CNTs-TiO2 nanocomposites for each dye
.................................................................................................... 89
Table 5.1 Characterization of photocatalysts ..................................................... 107
Table 5.2 Rate constants of the various TiO2 and BMO nanocomposites as
photocatalysts of BPB using UV and visible light ...................... 117
xv
LIST OF ABBREVIATIONS
AC Activated carbon
AOPs Advanced oxidation processes
BET Brunauer Emmett Teller
BMNPs Bimetallic nanoparticles
BMO Binary metal oxide
BPB Bromophenol blue
CB Conduction band
CCVD Catalytic chemical vapour deposition
CNTs Carbon nanotubes
e- Electron
EDXS Energy dispersive X-ray spectroscopy
Eg Band gap
eV Electron volt
h+ Hole
IC Ion chromatography
JCPDS Joint committee on powder diffraction standards
k Constant
kV Kilovolts
MGDs Millenium development goals
MOs Metal oxides
xvi
nm Nanometer
OECD
Organization for economic cooperation and
development
ppm Parts per million
PR Phenol red
PVCD Physical vapour deposition
PXRDS Powder X-ray diffraction spectroscopy
S(BET) Specific area
sccm Standard cubic centimetre
TEM Transmission electron spectroscopy
UNICEF
United nations international children's
emergency funds
UV Ultra-violet
UV/Vis Ultra-violet spectroscopy
VB Valance band
Vis Visible
Vp Pore volume
xvii
PRESENTATIONS AND PUBLICATIONS
The work presented in this thesis has been submitted to peer-reviewed journals
and presented in conferences as shown below:
Publications
LN Dlamini RW Krause, GU Gulkarni, SH Durbach, Synthesis and characterization
of titania based binary metal oxide nanocomposite as potential environmental
photocatalyst, Materials Chemistry and Physics (2011) 129: 406 – 410
LN Dlamini RW Krause, GU Gulkarni, SH Durbach, Photodegradation of
Bromophenol blue with fluorinated TiO2 composite, Applied Water Science (2011)
1: 19 – 24
LN Dlamini RWM Krause, GU Gulkarni, SH Durbach Photodegradation of
triphenylmethane dyes by anionic doped titania nanocomposites, submitted to the
Journal of Catalysis, under second review.
LN Dlamini RWM Krause, GU Gulkarni, SH Durbach, HI Ezuruike, G Msane,
Improving the efficiencies doped titania nanocomposites, to be submitted to
Materials Chemistry and Physics
Presentations
LN Dlamini, RW Krause, GU Kulkarni, SH Durbach, Fluorinated Titanium dioxide
embedded on carbon nanotubes-co-cyclodextrin polyurethanes for the removal of
organic pollutants in water, Oral Presentation, 1st Young‟s Professional Water
Institute of South Africa (YPWISA), CSIR Pretoria, 19 – 21 January 2010
xviii
LN Dlamini, RW Krause, GU Kulkarni, SH Durbach, Photo-catalytic degradation of
organic pollutants in water with F-TiO2 nanoparticle composites, Poster
presentation, WaterNet symposium, Durban, 12 – 16 April 2010
LN Dlamini, RW Krause, GU Kulkarni, SH Durbach A facile method of preparing
titania based binary metal oxide nanocomposites: CNTs-CoO-TiO2; synthesis and
characterization, Oral presentation, 3rd International Conference for Young
Chemist (ICYC), Penang Malaysia, 23 – 25 June 2010
LN Dlamini, RW Krause, GU Kulkarni, SH Durbach A facile method of preparing
titania based binary metal oxide nanocomposites: CNTs-CoO-TiO2; synthesis and
characterization, poster presentation, 15th international conference on TiO2
photocatalysis: fundamentals and applications, San Diego USA, 15 – 18
November 2010
LN Dlamini RW Krause, GU Gulkarni, SH Durbach, Photodegradation of
bromophenol blue with fluorinated TiO2 composite, poster presentation, 2nd
European symposium on photocatalysis, Bordeaux, Cite Mondiale France, 29 – 30
September 2011
LN Dlamini RW Krause, GU Gulkarni, SH Durbach, Photodegradation of
bromophenol blue with fluorinated TiO2 composite, oral, 2nd TCS International
chemistry conference, Da es Salaam, Tanzania, 5 – 7 October 2011
1
CHAPTER 1:
INTRODUCTION
1.1 Problem statement
Water is not a renewable resource. The demand for water continues to increase
exponentially, doubling every 20 years.1 Farming makes up to 70% of the global
demand of water. In turn, fresh water is lost either through evaporation or leaks,
never returning to the basin. The demand for water has prompted many
economists to refer to water as the “petroleum for the next century.”1 The United
States alone needs up to US$ 1,000 billion in new pipes and wastewater plants by
2020.1 However, some have also seen the increasing demand for water as a
single, powerful catalyst to bring about world war three, as disputes between
countries of cross-border water basins have increased globally.1
The Organization for Economic Cooperation and Developments (OECD) has
warned that competition for shores of water would increase up to 55% by 2050.2
As nations struggle to provide water for their citizens, ironically, pollution of this
rare natural resource is on the rise. According to the 2009 United Nations
International Children‟s Emergency Funds (UNICEF) report, nearly 4000 children
under the age of five die daily from diseases related to contaminated food and
drinking water around the world.3 World vision, estimates eight million children
annually die from drinking unsafe water, exposure to poor sanitation and hygiene-
related diseases.
The Millennium Development Goals (MGDs) established in 2000 by United
Nations (UN), target 7C, aimed at reducing by halve (by 2015) the portion of global
population without sustainable access to safe drinking water.4 However, in the
past decade, increased agricultural and industrial activities have not only
Chapter 1: Introduction
2
increased the demand for water, but have also contributed to the pollution of
surface and ground water. Hence, there is a need for improved functional, cost-
effective and easy to use technologies that would remove and degrade pollutants
in water. Solutions that include the use of freely available solar radiation are
attractive from an energy perspective.
1.2 Justification
Several water treatment techniques such as activated carbon, reverse osmosis,
coagulation and recently nanoporous polymers have been developed to address
water pollution issues.5,6 In our laboratories we have successfully prepared and
tested novel nanoporous polymers capable of removing and degrading organic
pollutants in water.7 The incorporation of bimetallic nanoparticles (BMNPs) onto
these polymers showed a significant degradation of organic pollutants in water.8
As promising and encouraging as the results were, the toxicity of these BMNPs is
of concern and the technology relies on adsorption- which requires regeneration or
disposal at some point. Furthermore, the degradation of pollutants is largely driven
by the ever presence of the catalytic metal in the BMNPs.
Further improvement of green methods is required that will treat water without the
need of adding extra pollutants. At present, photocatalysis is considered to be a
viable and effective method for degrading organic pollutants in water. In this
regard,TiO2 is considered to be an excellent photocatalyst due to its relative non-
toxicity, low cost, thermodynamic stability and excellent chemical and physical
properties.
Based upon the combined characteristics of TiO2 with CNTs, as reported in the
literature, two significant issues were believed to require further study:
(1) The improvement of the interface between TiO2 and CNTs so as to
separate the photogenerated electron-hole pairs efficiently,
(2) The improvement of the photocatalytic activity of TiO2 to enable the use of
visible light.
Chapter 1: Introduction
3
Many authors in the literature have reported on anionic doped TiO2 and CNTs ,
however, at the time of writing this thesis no report was available (as far as could
be ascertained), that specifically investigated the general pattern of Group 7
elements, when doped with TiO2 or as nanocomposites when CNTs are
incorporated.
1.3 Objectives of the Study
Hence, based upon the above-stated shortcomings the objectives of the thesis
were tailored, for the purpose of tackling issues (1) and (2) mentioned previously,
as follows:
(i) To design, synthesise and characterise novel ((b) & (d)) titania
nanocomposites. These include:
a) †Y-TiO2
b) †Y-TiO2-CNTs
c) ‡MOx-TiO2
d) ‡MOx-TiO2-CNTs
†Y: anionic dopants (F,Cl,Br, I); ‡MOx: metal oxide (CoO, WO3)
(ii) To determine the photocatalytic activity of each of these nanocomposites
on organic pollutants in water, using stable and recalcitrant
triphenylmethane dyes as test cases.
(iii) To deduce whether a relationship for the photocatalytic efficiency of various
doped titania nanocomposites exists.
Chapter 1: Introduction
4
1.4 Outline
The research that is reported in this thesis is broken down in the following
sections:
CHAPTER 2: A literature review of the synthesis of titania as well as the
classification and effects of pollutants in water
CHAPTER 3: The experimental procedures followed in this thesis
CHAPTER 4: Characterization and photodegradation of pollutants by
anionic doped titania nanocomposites by UV and visible light.
CHAPTER 5: Characterization and photodegradation of pollutants by titania
based binary metal oxide nanocomposites under UV and
visible light.
CHAPTER 6: Recommendations and general conclusions which can be
drawn from these investigations.
Chapter 1: Introduction
5
REFERENCES
1 . http://www.telegraph.co.uk/finance/newsbysector/utilities/2791116/Water-
crisis-to-be-biggest-world-risk.html accessed 24 April 2012
2 . http://economictimes.indiatimes.com/news/news-by-industry/et
cetera/water-shortage-a-global- threat-without-urgent-reform-
oecd/articleshow/12178269.cms accessed 24 April 2012
3 . http://www.christianpost.com/news/world-vision-shares-8-million-reasons-
to-keep-working-beyond-world-water-day-71856/ accessed 24 April 2012
4 . http://www.un.org/millenniumgoals/environ.shtml accessed 24 April 2012
5 . Forgas E, Cserhati T, Oros G, Removal of synthetic dyes from
wastewaters; a review, Environmental International, (2004) 30, 953 - 971
6 . Gupta GS, Shukla SP, Prasad G, Singh VN, China clay as adsorbent for
dye house waters, Environmental Technology, (1992) 13, 926 – 936
7 . Mamba BB, Krause RW, Malefetse TJ, Gericke G, Sithole SP, Cyclodextrin
nanosponge in the removal of organic matter for ultrapure water in power
generation, Journal of Water Supply: Research and Technology-Aqua,
(2009) 58, 299 - 304
8 . Krause RW, Mamba BB, Dlamini LN, Durbach SH, Fe-Ni nanoparticles
supported on carbon nanotubes – co-cyclodextrin polyurethanes for the
removal of TCE in water, Journal of Nanoparticle Research, (2010) 12 449 -459
6
CHAPTER 2:
LITERATURE REVIEW
2.1 Introduction
Interest in semiconductor photocatalysts rapidly increased since Fujishima (1972)
reported on the photo-electrochemical splitting of water in Nature.1 Since, various
semiconductors such as metal oxides, sulphides, nitrides and selenides have been
investigated.2,3,4 However, metal oxide photocatalysts have been used more
extensively because they were easier to synthesize, relatively stable and
inexpensive. 5,6 Metal chalcogenides were not stable enough and suffered from
photo corrosion. For instance the metal sulphides or selenides (denoted by Y in
the equation below) were easily oxidized by holes generated in the photocatalytic
process (denoted by h+ in the equation below), which resulted in the degeneration
of the photocatalysts (equation. 1).7
MY + 2h+ Y + M2+ [1]
M (metal), Y (chalcogen)
A semiconductor, unlike a conductor, has a non overlapping valance band (VB)
and conduction band (CB). The energy difference between the two levels is known
as the band gap (Eg). Photocatalytic reactions therefore proceed on a
semiconducting material as illustrated in Figure 2.1. Upon illumination by incident
energy equal to or greater than the band gap of the semiconductor, electrons (e-)
receive enough energy and are excited from the VB to the CB, leaving holes (h+)
in the VB. Without the irradiation, holes and electrons are retained in the VB. Each
semiconductor has a minimum theoretical band gap (Table 2.1), corresponding to
the energy required to promote excitation, as represented by equation 2.
Chapter 2: Literature Review
7
Eg
Figure 2.1: Representation of the photoexcitation in a semiconductor
The photogenerated electrons and holes have strong reducing and oxidizing
capabilities respectively, which can be harnessed into various applications
including the production of hydrogen and the decontamination of water.
Table 2.1: Band gaps of semiconductors
Semiconductor Band Gap (eV) Semiconductor Band Gap (eV)
ZrO2 5.0 SiC 3.0
ZnO 3.2 GaP 2.3
TiO2(rutile/anatase) 3.0/3.2 CdS 2.4
WO3 2.8 MoS2 1.8
In2O3 2.7 ZnS 3.6
Fe2O3 2.2 CdSe 1.7
Si 1.1
Band gap = 1240/λnm [2]
CB
VB
Potential (V) hν ≥Eg
Chapter 2: Literature Review
8
2.2 Titanium Dioxide
Titanium dioxide (TiO2) – also known as titania – exists in at least three crystal
modifications: anatase, brookite and rutile.8 The most commonly occurring
polymorph is rutile. Commercially, titania is used as a white pigment for cosmetics,
plastics, paint and sunscreen.8-9 Much of the interest in titania is on its application
in the photocatalytic purification of water and air, its ability to produce hydrogen by
water splitting and its behavior as a disinfectant of biological
microorganisms.10,11,12 Table 2.2, lists some potential applications of TiO2, where
each of these is influenced by one of its properties.13
Table 2.2: Applications of TiO213
Property Category Application
Self-cleaning Buildings and roads Aluminium panels, tiles and windows
Antifogging Optical instruments and vehicles
Tunnel lights, tunnel walls, optical lenses, exterior surfaces windows and headlights
Air cleaning Indoor and outdoor air cleanliness
Room air-cleaner, air conditioner, roadways & concrete for highways
Water purifying Drinking water River water, ground water, lakes and fish feeding tanks
Self-sterilizing Hospitals Hospital garments, ICU walls
Anatase and rutile are the mostly used polymorphs of titania especially in water
and air purification, while brookite is not fully explored.14 Anatase and rutile
structures (Figure 2.2) are described in terms of TiO6 octahedra chains. Each
Chapter 2: Literature Review
9
titanium atom is surrounded by six oxygen atoms and three titanium atoms about
each oxygen atom. The octahedron in rutile is not regular and shows a slight
orthorhombic distortion (Figure 2.2 (b)), while a significant distortion is observed in
anatase (Figure 2.2 (a)). 6,15,16
Figure 2.2: Structures of (a) anatase and (b) rutile15
The Ti-Ti bond distances are 3.79 and 3.04 Å in anatase, as compared to 3.57 and
2.96 Å in rutile. On the other hand, the Ti-O bond distances are far shorter for
(a)
(b)
Chapter 2: Literature Review
10
anatase (1.934 – 1.980 Å) than rutile (1.949 – 1.980 Å).6,15 In the rutile structure,
each octahedron is in contact with ten neighbor octahedra (two sharing edge
oxygen pairs and eight sharing corner oxygen atoms) while in the anatase
structure each octahedron is in contact with eight neighbors (four sharing an edge
and four sharing a corner).15,16 Brookite (Figure 2.3) the other form of TiO2 is
orthorhombic, which has axial ratios of 0.8416:1:0.9444, when compared to the
tetragonal forms of anatase and rutile.17
Figure 2.3: Structure of brookite17
In the brookite structure, the titanium ion is at the center while the oxygen ions are
at the corners of these octahedra. These octahedra share edges and corners with
other crystals stoichiometrically.15,17 The distance between the Ti-O bond is
constant throughout the crystal (1.95 -1.96 Å), while the edges are shortened to
2.50 Å.15,17 A summary of the polymorphs of TiO2 crystalline and physical
properties are presented in Table 2.3 and 2.4.
Chapter 2: Literature Review
11
Table 2.3: Crystalline parameters of TiO2 polymorphs
Parameter Anatase18 Brookite19 Rutile20
Crystal system Tetragonal Orthorhombic Tetragonal
Point group 4/mmm mmm 4/mmm
Space group I41/amd Pbca P42/mmm
Z 4 8 2
Unit cell:
a/Å 3.7845 9.1841 4.5937
b/Å 3.7845 5.4471 4.5937
c/Å 9.5143 5.1450 2.9587
Volume of unit cell/Å3 136.3 257.4 62.4
Table 2.4: Physical properties of TiO2 polymorphs
Physical property @ 298.15 K
Anatase21 Brookite21 Rutile21
∆H°f/kJ.mol-1 -224.6 -941.8 -944.7
∆S°f/kJ.mol-1 49.92 51.56 50.33
∆G°f/kJ.mol-1 -884.5 -886.5 -889.5
C°/J.K-1.mol-1 55.48 53.98 55.02
Hardness/Moh 5.0 – 6.0 5.0 – 6.2 5.0 – 6.5
Density/g.cm-3 3.84 4.26 4.26
Band gap/eV 3.2 2.1-3.54 3.0
Refractive index 2.554 – 2.493 2.583,2.586, 2.791 2.616, 2.903
Mp/K 1820 1825 1830-1850
Bp/K 2500 – 3330 2500 – 3325 2500 – 3000
Solubility H2SO4, alkaline solution
H2SO4, alkaline solution
H2SO4, alkaline solution
Chapter 2: Literature Review
12
Anatase and rutile are the only two polymorphic forms of titania known to be
photoactive. Despite the numerous applications of TiO2 as indicated in Table 2.1,
its use in its intrinsic state is rather limited. The limitations of titania are mainly due
to the following reasons:
a) The fast recombination of the generated photoholes and photoelectrons
from the metal oxide. Every recombination results in the release of energy
in the form of unproductive heat and the subsequent loss of the holes and
electrons. The different fate of the produced holes and electrons upon
excitation by sufficient energy is depicted by Figure 2.4. Routes (a) and (b) show the recombination of the photogenerated species on the metal oxide.
Route (c) shows an electron acceptor specie adsorbed on the surface of the
metal oxide, thereby becoming reduced by the migrating electron. Route (d) depicts the oxidation of an adsorbed specie on the surface of the metal
oxide particle by a photogenerated hole.
Figure 2.4: Plausible pathways followed by holes and electrons.6,22
b) The large band gap of TiO2 (3.0/3.2 eV) only allows the ultra violet portion
(< 380 nm) of the solar spectrum to initiate photoexcitation. The solar
Chapter 2: Literature Review
13
spectrum is mainly composed of about 95% visible light, which doesn‟t have
enough energy to excite titania to be catalytically active (Figure 2.5).11,23
Figure 2.5: Absorption of solar spectrum against band gap of titania.6
Many research groups have worked on improving the surface of titania, so as to
minimize the above mentioned limitations. Through surface and bulk doping, the
phase transformation of titania (i.e. from anatase to rutile) has been stabilized, the
optical band bap has been altered and the ionic/electric conductivity of titania has
been improved.24,25 Primarily there are four ways currently used to overcome the
limitations of TiO2. These include doping with metal ions,26,27 coupling with another
semiconductor,28,29 sensitizing by dyes,30 and doping with non-metals. 31,32,33
2.2.1 Surface Perturbation
2.2.1.1 Co-deposition of Metals
The surface of TiO2 can be modified by metal deposits such as Au, Ag or
Pt.34,35,36,37 Contact between the semiconductor and metal, involves a redistribution
of electric charges and the formation of a double layer, resulting in a space
charge.6 The barrier formed at the metal and semiconductor interface is called the
Schottky barrier. The metal deposited on the semiconductor acts a sink for
Chapter 2: Literature Review
14
photogenerated electrons, mediating the electrons away from the TiO2 surface and
thus minimizing electron-hole recombination.6,11
The deposited metal has a higher work function (Φm) than the semiconductor (Φs),
with which it is in contact, as illustrated by Figure 2.6. Loadings of Pt and Au have
been reported to be more effective than Pd, because of their suitable work function
and electron affinity.38
Figure 2.6: A representation of the Schottky barrier
Care needs to be taken that the metal loading on the surface of titania has to be at
an optimum level, otherwise if excess metal is loaded it in turn acts as a centre for
electron/hole recombination.11 The high costs of noble metals hinders the
practicality of noble-metal doped TiO2, hence low-cost yet effective metals are
being investigated. These include transition metals ions (Fe3+, Cr3+, V3+, Mo5+) and
rare earth metal ions (Y3+, La3+ Os3+).39 Metal ions minimize recombination by
being efficient electron and hole trappers as depicted by the process below:39,40
m
s
Ef
CB
semiconductor - n typemetal
Ef
Ef
metal (eg Pt)Schottkybarrier
hv
TiO2
Chapter 2: Literature Review
15
Mn+ + e-cb → M(n-1)+ (electron trap)
Mn+ + h+vb → M(n+1)+ (hole trap)
The Mn+/M(n-1)+ couple lies below the conduction band edge, thereby acting as an
electron sink; while the Mn+/M(n+1)+ couple is above the valance band edge trapping
the holes formed.41 The introduction of such impurities on the metal oxide, also
improves the optical responsiveness to the visible region through a charge transfer
between a dopant and the CB (or VB) or through a d-d transition in the crystal
field.42
2.2.1.2 Anionic Doping
Non-metal doped TiO2 nanomaterials have been regarded as third generation
photocatalysts after pure TiO2 nanomaterials and metal-doped TiO2 nanomaterials
respectively.6,43 A minimum level of TiO2 doping with non-metals such as B,43 C,44
N,45 F,46 ,S,47 Cl,48 Br48 and I49 is required. The advantage of this is that anion
doping results in the absorption edge of TiO2 being red-shifted to wavelengths
longer than 400 nm. Unlike metal dopants, non-metals do not act as charge
carriers thus do not act as recombination centres.
In 2002 Khan et al for the first time reported water splitting of carbon doped TiO2.50
The addition of carbon reduced the band gap from 3.2 eV to 2.32 eV. The authors
used different carbon sources, glycine, hexamethylene tetramine and
oxalydihydrazine and all dopants showed improved visible activity. The visible
activity was due to the carbide ion substituting oxygen, which forms an isolated
electronic state above the O 2p level. In their study the degradation of methylene
blue (MB) was higher for carbon doped TiO2, using glycine as a carbon source
compared to the other sources, under solar irradiation. The increased degradation
of MB was attributed to the dense surface hydroxyl groups, high surface area,
greater acidic sites and high crystallinity.51 In similar work Kisch and co-workers,
used a variety of alcohols as carbon precursors, where they showed that a highly
condensed coke-like carbonaceous species consequently embedded in the TiO2
which was most likely responsible for visible light sensitization. They reported that
Chapter 2: Literature Review
16
maximum photocatalytic activity was feasible at a calcination temperature of less
than 250°C, as compared to 400°C where the catalyst was inactive against 4-
chloro phenol (CP).52
Asahi and co-workers reported in Science, that nitrogen doped titania showed
significant catalytic activity under visible light irradiation.53 The authors claimed
that the red-shift could be attributed to the narrowing of the band gap, by the
mixing of the N 2p and the O 2p states. Ihara et al, proposed that the visible light
activity was due to the formation of oxygen vacancies which it is believed nitrogen
atoms stabilized.54,55 Serpone argued that the red-shift in titania, when doped with
non-metals, resulted from the formation of colour centres (Ti3+, F, F+, F++) that
absorbed visible light radiation.56 The doping of TiO2 with non-metals and their
effects on the band gap and electronic structure is depicted by Figure 2.7. In
Figure 2.7, (a) represents pristine TiO2 with a band gap of 3.2 eV, while (b) represents TiO2 doped with localized dopant levels near the VB and CB. On the
other hand (c) represents the band gap narrowing due to the broadening of the
VB, as (d) represents the localized dopant levels and electronic transitions to the
CB. Finally (e) represents the electronic transitions from localised levels near the
VB to their corresponding excited states for Ti3+ and F+ centres.53
Figure 2.7: Proposed schemes of band gap narrowing of TiO2 by doping with non-metals.53
Surface fluorination on TiO2 (F-TiO2) largely depends on the pH being between
two and three to be more successful.57,58 Minero et al, were the first to report
enhanced photoactivity of F-TiO2 for the oxidation of phenol, as compared to
unfluorinated TiO2.57 This was believed to have occurred due to an increased
production of hydroxyl radicals and was then confirmed by using DMPO – spin
trap ESR technique. Furthermore, Yu et al used a photoluminescence technique to
confirm the increased production of hydroxyl radicals through the hydroxylation of
terapthalic acid.59
Chapter 2: Literature Review
17
Xu and co-workers have proposed a different mechanism for the excess of free
hydroxyl radicals in F-TiO2.60 They have suggested that the fluoride ions, present
in the Helmholtz layer, promoted the desorption of surface-bound hydroxyl radicals
when TiO2 was irradiated in solution, as depicted in Figure 2.8. Accordingly they
proposed that an increased concentration of fluoride ions in the Helmholtz layer
led to an increase in the desorption of hydroxyl radicals. Likewise their mechanism
proposed that the solution should be maintained at an acidic pH in order for the
desorption of hydroxyl radicals to be highly favoured.60
Figure 2.8: Plausible mechanism for the fluoride induced enhancement in the production of free hydroxyl radicals in bulk solution from irradiated TiO2.60
Tang and co-workers showed that the hydrophilicity of F-TiO2 was improved when
compared to undoped TiO2.61 Here it was suggested that the electronegative
fluorine atom withdrew the electron cloud away from the Ti-O bond and thus
weakened it. This they proposed resulted in the excess availability of oxygen
vacancies under illumination, that in-turn enhanced the hydrophilicity of F-TiO2.
On the other hand, phosphorous doped TiO2 (P-TiO2), using H3PO4 as the source
of dopand, showed enhanced catalytic oxidation of n-pentane in air.62 Here it was
shown that this increased activity was due to the existence of Ti ions that were
tetrahedrally coordinated. These tetrahedrally coordinated Ti ions were thought to
Chapter 2: Literature Review
18
enhance greater adsorption of oxygen and water molecules from the air, thereby
having produced more hydroxyl radicals. This tetrahedral configuration was most
likely responsible for having hindered the recombination of electrons and holes via
stabilization, as they are known to act as hole trappers.62
Similarly iodine doped TiO2 or I-TiO2, due the presence of abundant surface
states, has shown enhanced catalytic activity in the degradation of organic
pollutants, as has been confirmed by PL spectral studies.63 However, work carried
by Liu et al has shown that higher concentrations of water molecules and hydroxyl
groups on such catalysts have decreased their activity.63 64 This result was
contrary to that seen in P-TiO2 and has indicated that the presence of surface
hydroxyl groups does not always result in holes having been trapped and hydroxyl
radicals having been generated. When I-TiO2 was prepared by anodization at
20 V, using KI as an electrolyte, it showed greater methylene orange degradation
under visible light due to the presence of iodine in its multivalency state (I7+, I5+
and I-).65 Theoretical calculations indicated that four I 5p bands appeared within
the band gap states for I (cation) –TiO2 while only two I 5p states (deeply trapped)
were present close to the CB for I (anion)–TiO2.
2.2.1.3 Composite Semiconductors
One major limitation of intrinsic TiO2 is the fast recombination of the
photogenerated charges. Another approach of achieving better charge separation,
involves coupling of two semiconductor particles with different energy levels.
Examples include: CdS-TiO2,66 TiO2-SnO2,67 TiO2-SiO2 68, TiO2-ZrO2 and
CNTs-TiO269 For instance, semiconductor particles having a large bandgap and an
energetically low-lying conduction band (e.g. TiO2 or ZnO) can be combined with
particles having a small bandgap and an energetically high lying conduction band
e.g. CdS or WO3.70 When the two types of particles (as mentioned above) are
combined, light absorption at long (>366 nm ) wavelengths results in efficient
electron transfer to the large bandgap particle, while the holes migrate in the
opposite direction from the holes to the small bandgap particle as illustrated by
Figure 2.9.
Chapter 2: Literature Review
19
Figure 2.9: Electron and hole separation in a coupled semiconductor system
Photocatalytic activity in coupled systems have been reported to have been
increased when compared to the individual performance of the semiconductor.
This has been attributed to efficient charge separation in these systems.55,57
Yu et al were the first to report CNTs-TiO2 composites in photocatalysis.71 These
CNTs-TiO2 composites were prepared by an ultrasound technique and were
monitored for their physicochemical properties and photocatalytic activity for the
oxidation of acetone in air. Here they reported a significant photodegradation of
acetone by the CNTs-TiO2 composite as compared to P25 and AC-TiO2 (activated
carbon). Furthermore these authors attributed the enhanced photocatalytic activity
of TiO2 by CNTs to an increased migration rate of electrons through CNTs.72,73 PL
spectra of these composites confirmed that an increased electron mobility
suppressed the recombination of the e-/h+ pairs. Similarly they also proposed that
this enhanced photocatalytic activity was due the increased adsorption of hydroxyl
groups (hole trappers) on the surface of CNTs-TiO2, leading to an increased
production of hydroxyl radicals. The production of these hydroxyl radicals was
confirmed by EPR spectra of the DMPO-•OH spin adducts.71,74 However, they also
found that higher loadings of CNTs in such composites hindered their
photocatalytic activity. They suggested that the higher loadings of CNTs hindered
VB
CB CB
VB
hν
CdS TiO2
Chapter 2: Literature Review
20
the absorption of UV light, which in turn led to the decrease in the production of
electrons and holes.
Improving on previous approaches, Gao and co-workers developed a novel
surfactant wrapping technique that produced uniformly dispersed TiO2 on the
surface of CNTs via a modified sol-gel method.69 Here the TiO2 film, when
exposed to UV irradiation, significantly decomposed MB under ambient conditions.
The efficiency of this composite was attributed to an improved separation of photo-
generated-electron/hole pairs at the CNTs-TiO2 interface.
2.2.1.4 Dye Sensitization
When a photocurrent is generated with photons less than that of the
semiconductors band gap (Eg ≤ hν), the process is known as sensitization and the
light absorbing dyes are called sensitizers.75 An efficient photosensitizer should
have a) an intense absorption in the visible region, b) be strongly physisorped or
chemisorped on the surface of the semiconductor and c) must efficiently inject
electrons into the conduction band of the metal oxide.6,11,76
Common organic dyes which have been used as sensitizers include, porphyrins
(Figure 2.10a) carboxylated derivatives of anthracene, and transition metal
complexes such as metalloporphyrins (Figure 2.10b) and polypyridine
complexes.30,59,77,78,79,80 The metal centres for the dyes have included: Ru2+, Zn2+,
Mg2+, Fe3+ and Al3+, while the ligands were nitrogen heterocyclics with delocalized
π or aromatic ring systems.60
Chapter 2: Literature Review
21
Figure 2.10 (a) Structure of porphyrin, (b) Structure of metalloporphyrins
Organic dyes have been linked to the semiconductor by various interactions which
have included:
electrostatic interactions, via ion exchange, ion pairing or donor-acceptor
interactions
covalent attachment by directly linking groups of interest or via linking
agents
intermolecular forces, such as hydrogen bonding and van der Waals forces.
Figure 2.11, depicts a metal complex that was attached on a TiO2 nanoparticle by
electrostatic carboxyl interaction (-O-(C=O)-).58,59 The complex was attached to the
metal oxide through an anchoring ligand. The auxiliary ligands were those that
were not directly attached to the surface of the metal oxide and could be used to
tune the spectroscopic response of the dyes.59
Chapter 2: Literature Review
22
N OH
O
NRu
L'L
N
N
O
-O
-O
O
TiO2
L, L'
N
N
CN
N
hv
e-e-
e-
=
Figure 2.11: Organic dye (cis – [Ru (dcbH2)LL’ attached on TiO2. 59
Organic dyes when attached to TiO2 particles have caused a red-shift in the
adsorption edge of the metal oxide. However, most dyes are toxic and unstable in
aqueous solutions, thus making them unsuitable for application in photocatalytic
reactions.49 Furthermore; the photodegradation of the dye may lead to the
formation of stable intermediates which are more harmful than the parent dye. 45,81
2.3 Synthesis of Titanium Dioxide Nanoparticles
Properties influencing the photocatalytic behaviour of titania nanoparticles are
surface area, crystallite size, morphology and crystal structure (phase).82 All these
physical properties can be manipulated by different synthetic methods of TiO2.
Titania can be prepared by the sol-gel method,65,83,84,85,86,87,88,89,90,91 the
hydrothermal method,92,93,94,95,96 the solvothermal method,97,98,99,100,101 the direct
Chapter 2: Literature Review
23
oxidation method,102,103,104,105 by chemical and physical vapour
deposition,106,107,108,109 and by microwave.110,111,112
2.3.1 Sol-Gel Method
A typical sol-gel process involves the hydrolysis and polymerization of a metal
alkoxide (organic) or metal salt (inorganic) resulting in a colloidal suspension.
Metal alkoxide (metalorganic compounds) are often used as precursors in the sol-
gel process because they easily react with water (hydrolysis). Table 2.5 lists some
commonly used alkoxides in the sol-gel method. When the attractive dispersion
forces which are present in the sol during nucleation result in a giant network of
microscopic dimensions, then a gel is formed. Gels are amorphous after drying,
but crystallize when heated.
Table 2.5: Common Alkoxides
Alkoxy Molecular formula
Methoxy OCH3
Ethoxy OCH2CH3
n-propoxy O(CH2)2CH3
iso-propoxy CH3(O)CHCH3
n-butoxy O(CH2)3CH3
sec –butoxy H3C(O)CHCH2CH3
iso-butoxy OCH2CH(CH3)2
tert-butoxy OC(CH3)3
Livage et al, described the sol-gel synthesis using the partial – charge model as
hydrolysis and condensation reactions which can either be oxolation or olation.67
Chapter 2: Literature Review
24
Hydrolysis
During hydrolysis the hydroxyl ion becomes attached to the metal atom (M) as
illustrated by equation (3). Depending on the amount of water and catalyst present
the hydrolysis reaction can be partial (3) or go to completion (4).66-67
M(OR)n + nH2O HO M OR (3)n + ROH
M(OR)n + nH2O (4)+ ROHM(OH)n
(OR) : alkoxide
n
Hydrolysis is influenced by the charge (z), the coordination number (N), the
electronegativity (x°m) of the metal and the pH of the solution.
Condensation
Condensation is a reaction in which molecules are joined together through the
intermolecular elimination of water or an alcohol. Condensation can take place
either by oxolation or olation (see below for further description of these two
processes). Furthermore, condensation can be influenced by two nucleophilic
mechanisms: substitution (SN) and addition (AN), both of which are determined by
the coordination number of the metal. The substituent with the largest partial
negative charge (δ-) is the nucleophile, while the substituent with largest partial
positive charge (δ+) is the leaving group. Nucleophilic substitution (SN) is initiated
when the preferred coordination is satisfied (5). However, when the preferred
coordination is not satisfied, condensation occurs by nucleophilic addition (AN) (6).
Chapter 2: Literature Review
25
M OH XO M M O M
H
X = R, H
(5)
M OH XO M
M O M
H
OX (6)
OX
Oxolation
Oxolation is a condensation reaction in which an oxo bridge (–O–) is formed
between two metals centres as depicted by (7).68 Oxolation proceeds via
nucleophilic addition (AN) in unsaturated metal coordination.66
H2O M OH OH M OH2 M
O
O
M 2H2O
+2H+ (7)
However, for a coordinatively saturated metal, a SN reaction is possible via a two-
step mechanism as shown in (8) and (9).66
Chapter 2: Literature Review
26
M
OH
+ M OH M O M OH
H
(8)
M O M OH
H
M O M + H2O (9)
Olation
Olation condensation is preferred to oxolation when the precursors contain good
water leaving groups. Olation can either be base or acid catalysed.68 In base
catalysis, the base acts as a nucleophile and attacks the partially charged
hydrogen to form water and a metal alkoxide (a stronger nucleophile) (10). This
alkoxide then attacks the partially positively charged end of a second metal
alkoxide, prompting its hydroxide to leave the molecule and depart with its bonding
electrons (11).
M OH + -OH M O- + H2O (10)
M O- + M OH M O M + -OH (11)
In an acid catalyzed reaction, the acid protonates the hydroxide end of the metal
alkoxide the forming an unstable positively charged metal centre (12). Due to the
instability of the complex a water molecule is released, making the hydrogen
highly acidic and prone to nucleophilic attack (by water), forming the hydronium
ion (13), leaving a positive charge behind the metal to propagate the reaction.
Chapter 2: Literature Review
27
M O M OH
H
M O M + H2OH3O+
OH2
(12)
H
M O M
OH2H
1. -H2OM O M + H3O+ (13)
2. H3O+
The advantages of the sol-gel method over other techniques is the control it offers
over the particle size (Figure 2.12a –b) and the shape of nanoparticles. It is a
relatively cheap method compared to other techniques and works at low
temperatures. However, because ultra-fine particles are produced, (sol)
agglomeration (Figure 2.12 b- c) is a major limitation. To minimize agglomeration
reverse micelle and micro-emulsion techniques have been used.70
Micro-emulsion is a thermodynamically stable dispersion of two immiscible fluids.
The system is stabilized by adding one or more surfactants. Different kinds of
micro-emulsions are known and these include (a) water-in-oil (w/o) (b) oil-in-water
(o/w) and (c) water-in-super critical (sc)-CO2 (w/sc-CO2).113 Reverse micelles
result from the self assembly of surfactants in apolar solvents. Metal oxide
nanocomposites are prepared by mixing an oil-soluble metal alkoxide M(OR)x, or
an anhydrous reverse micelle solution containing the alkoxide in the oil phase,
which results in the hydrolysis and condensation of the alkoxide nanoparticles of
metal hydroxide and oxides.114 The advantage of the reverse micelle method is
that it acts as an ideal nanoreactor, providing a suitable environment for controlled
nucleation and growth. Steric stabilization provided by the surfactant layer
prevents the nanoparticles from aggregating.
Chapter 2: Literature Review
28
Figure 2.12: TEM images showing sol-gel prepared titania particles (a) and (b) uniform distribution of titanium nanoparticles,70 (c) and (d) reveal agglomerated nanoparticles. 71
2.3.2 Hydrothermal Method
Hydrothermal synthesis is another example of a wet-chemical method in which
titania nanoparticles can be prepared in a heated aqueous medium or a heated
non-aqueous medium in which water is added as a reactant.60 This method
represents an alternative route to the sol-gel technique as it requires mild
calcination temperatures to promote crystallization. Indeed, it is during heat-
Chapter 2: Literature Review
29
treatment in the sol-gel technique that agglomeration occurs (as previously
mentioned). At low calcination temperatures, through the hydrothermal synthesis,
nanoparticles can be prepared crystalline, uniform in size and with controlled
morphologies.77
Figure 2.13: SEM images of TiO2 nanoparticles prepared by the hydrothermal method.79
2.3.3 Solvothermal Method
The solvothermal method is very similar to the hydrothermal technique of making
nanoparticles. However, one primary difference is that a non-aqueous solvent is
used in this technique.60 Consequently the operating temperatures for such
syntheses can be higher than the hydrothermal method (depending on the organic
solvents used).80 Metal alkoxides readily react with carboxylic acids to form mixed
alkoxy carboxylates (14).81 An oxo bridge could be formed during a condensation
reaction between the two functional groups bonded to the two metal centres, in the
process eliminating an ester (15).81
M OR +H O
O
R' M O C
O
R'
4-x x
xROH+
(14)
Chapter 2: Literature Review
30
M OR+M O C
O
R' M O M + ROCR'
O
(15)
The advantages of the solvothermal method are the high crystallinity and near
mono sized/shaped nanoparticles that form, which can easily be dispersed in
organic-solvents (as depicted by Figure 2.14).
Figure 2.14: TEM images showing nanocrystals prepared by the solvothermal method.80
2.3.4 Chemical and Physical Vapour Deposition
Vapour deposition processes takes place in a vacuum chamber. When there is no
chemical reaction, the process is known to be physical vapour deposition (PVD).60
Chapter 2: Literature Review
31
An horizontal stainless tube reactor is placed at the central part of the furnace and
is heated by a resistive heater up to 1000°C. In chemical vapour deposition
processes (CVD), the precursor is usually a solution of a metal alkoxide which is
bubbled into a fine vapour using a frequency of between 40 kHz and 300 kHz. The
formed vapour is carried over to the reaction chamber by argon or nitrogen,
controlled by a mass flow controller set to between 20 – 70 standard cubic
centimetres (sccm). Oxygen is also mixed in with the carrier gas to oxidize the as-
formed titanium to its oxide. The scheme for this is shown in Figure 2.15a.
When preparing titanium dioxide nanoparticles by PVC, a pure solid metal is
placed on a quartz boat, in a tube furnace, a few millimetres away from the
substrate. Similar to the set-up in CVD, here the temperature is increased to about
1000°C and the carrier gas (N2 or Ar) and oxygen is infused to the mixture to
oxidize the titanium nanoparticles as depicted by Figure 2.15b.
Figure 2.15 (a). Scheme showing the set-up for the synthesis of titanium nanoparticles by CVD 89
(a)
Chapter 2: Literature Review
32
Figure 2.15 (b). Scheme showing the set-up for the synthesis of titanium nanowires by PVC92.
2.4 Pollutants
Water systems are remarkably efficient, for they naturally can purify and renew
themselves, either by sedimentation, or by diluting pollutants to less harmful
concentrations.115 The natural process, however, is lengthy and very difficult to
perform efficiently when larger quantities are continually added to the system.
Pollution sources can be classified as either point sources or non-point sources.96
Point sources are easily identifiable locations of pollution. Examples are factories,
wastewater treatment facilities and sewage leaks. Non-point systems, however,
are not easily identifiable as to their origin. These include run-off sediments,
fertilizers, chemicals and animal wastes from farms.116
Pollutants can be broadly divided into three major categories. The first of these
categories are the inorganic pollutants, such as acids, salts, toxic metals, nutrients
and radioactive materials. The second are organic pollutants, which are pesticides,
plastics, detergents, animal manure and dyes. Lastly, there are microbial
pollutants such as bacteria, parasites and viruses.
(b)
(b)
Chapter 2: Literature Review
33
Organic pollutants, particularly dyes, are a concern because of their persistence
and stability in water. Colour is one obvious visible indicator of wastewater
pollution.117 The presence of synthetic dyes in waters even as low as 1 mg.L-1 can
cause visible colouration of water. If synthetic dyes are present in sufficient
quantities in a water system, they shield light penetration, hinder photosynthetic
activity, slow down the growth of biota and have an affinity to chelate with metals
resulting in micro-toxicity of fish and other organisms. 101
It was estimated in 2008 that the worldwide production of dyestuff was over
7.0 x 105 tons, with a market value over US$10 billion.118 There are over 1000
companies and organizations related to textiles and clothing in South Africa
alone.119 Dyes are used in many fields involving numerous branches in the textile
industry,120,121 leather tanning industry,122,123 paper production,124 food
technology,125,126 agricultural research,127,128 light-harvesting arrays,129
photoelectrochemical cells130 and in hair colourings.131 Due to the high demand
and extensive application of synthetic dyes, proper disposal and treatment of dye
infested effluent is necessary. Synthetic dyes are toxic, non-biodegradable and
form recalcitrant compounds.132 However, due to thousands of different organic
dye compounds, research findings on their carcinogenic, mutagenic and
bactericidal properties are not conclusive. Conventional water treatment methods
have limitations dealing with organic dyes and need to be improved in order to
cope with the growing problem that dye effluents pose. Hence, much research
interest is being focused on developing advanced technologies that would
decolourize and degrade dye effluent. A summary of classical and recent
technologies used to remove pollutants in water is illustrated by Figure 2.16.
Traditional physico-chemical treatments include inorganic (alumina, carbon-based
materials – coal, fly ash, activated carbon and wollastonite) and organic (bagasse)
supports.133,134 One drawback of adsorption methods is their lack of sensitivity.
This is in part due to competition amongst the adsorption sites with dye and other
components in the effluent. Another limitation of adsorption methods is that they
only remove and concentrate the dye on the surface, while the structure of the dye
Chapter 2: Literature Review
34
remains unchanged. Other physico-chemical methods are coagulation, filtration,
Figure 2.16: Flow-chat illustrating the different methods used in the removal of dye from wastewater.99
ion exchange and adsorptive bubble separation techniques (ion floatation, solvent
sublation and adsorption flotation).135,136,137
Chapter 2: Literature Review
35
Chemical methods are more powerful than physical ones and have improved
oxidative and degradative procedures for organic compounds, using ozonation
and oxidation with the hypochlorite ion.138 Combinations of catalytic and
photochemical methods are known as advanced oxidation processes (AOPs) (see
Figure 2.16). AOPs are combinations of: TiO2/H2O2, H2O2/UV, O3/UV, O3/H2O2/UV
and the Fenton reagent (Fe2+/H2O2).98,119,139,140,141 AOPs, primarily rely on the
production of organic radicals and hydroxyl radicals upon photolysis. The
generated radicals are trapped by dissolved oxygen, leading to the production of
very powerful radicals and peroxides.119,120 These radicals improve and hasten the
decolourization and degradation, ultimately mineralizing the dye.122 However,
these methods are generally expensive and might also lead to an incomplete
degradation of the pollutant, producing even more toxic and stable
intermediates.120
The application of microorganisms for the biodegradation and decolourization of
dyes offers significant advantages. The process is economically favourable
compared to the previous methods and the end products are less toxic.113
However, a majority of dye compounds are stable and resistant to microbiological
attack. Microbial biodegeneration and decolourization methods use: mixed
bacterial cultures (either of anaerobic or aerobic kinds),142 pure cultures using
white-rot fungus or bacteria143 or activated sludge which contain both bacteria and
protozoans.144
Lastly, electrochemical (EC) methods have been used to degrade pollutants
(organic and inorganic). EC methods are electrocoagulation, electrochemical
reduction, electrochemical oxidation, indirect electrooxidation with strong oxidants
and photoassisted electrochemical methods. The advantages of EC methods
are99:
rapid separation of organic matter
small amounts of chemicals needed
pH control not necessary except for extreme values
Chapter 2: Literature Review
36
reduction in sludge production
relatively inexpensive techniques.
The disadvantages of EC methods are99:
anode passivation, caused by sludge deposition on these electrodes, which
inhibits the electrolytic process
high concentrations of iron and aluminium ions in the effluent need to be
removed.
2.5 Dyes
Dyes are organic molecules that have a chromophore, (an aromatic structure that
absorbs visible light) and an auxochrome (a group that anchors the dye) which
intensifies the dye colour. Wastewater containing dyes may contain other
auxiliaries like salts, heavy metals, dispersing as well as soothing agents and/or
surfactants. This presents an even greater challenge in the treatment of
wastewater.
Dyes can be broadly classified into three categories. The first is anionic dyes.
These are negatively charged, acidic and reactive. Reactive dyes are brightly
coloured and water soluble, while acidic dyes are more problematic as they pass
through conventional treatment techniques unaffected. The second category is
cationic dyes, which are basic and positively charged. Lastly, is the category of
non-ionic dyes, which are also known as disperse dyes. These dyes don‟t ionize in
an aqueous medium and are deemed as potential carcinogens.
The common chromophores in dyes are azo groups, triphenylmethane dyes,
xanthenes, pthalocyanine, anthraquinone and indigoide. Figure 2.17 illustrates
several classes of dyes and their structure.
Chapter 2: Literature Review
37
Antraquinone Dyes
Figure 2.17 Classes and structures of synthetic dye
BrBr
Br
O
BrBr
SO3N
CH3H3C
NCH3
CH3
NCH3CH3
S O
O
O
OH
OHPhenol red
Crystal violet
Bromophenol blue
O
O
SO3- Na+
OH
OH
O
O
NH2
NH
SO3H
NH
N
N Cl
N
ClMordant red 3
Reactive blue 4
Chapter 2: Literature Review
38
Azo Dyes
Figure 2.17 Classes and structures of synthetic dye
NN
NN
H2N SO-3
NH2
Na+
C
N
N
N(CH3)3
OOH S
N
N
N(CH3)3
OO
OH
p-Methyl red
Congo red
Methyl orange
Chapter 2: Literature Review
39
2.6 Carbon Nanotubes
The science of carbon is an ever evolving field, even in this 21st century, new and
fascinating discoveries are made about carbon. Hence, it is still appropriate that
some researchers refer to carbon as an “old but new material”.145 Carbon has
been known to exist in two naturally occurring crystalline forms or allotropes which
are three and two dimensional respectively i.e. diamond and graphite.146 However,
in the mid 80‟s a „third‟ form of closed caged carbon molecules was discovered by
O‟Brien and his co-workers and named fullerenes.147 Figure 2.18 summarises the
allotropic nature of carbon.
Figure 2.18: Different allotropes of carbon127
Chapter 2: Literature Review
40
Carbon atoms exist in three hybridization states, sp3, sp2 and sp, corresponding to
3D carbon (i.e. diamond), 2D planar carbon (i.e. planar graphite) and 1D chain-like
carbon (i.e. carbyne) respectively.127 Fullerenes and their derivatives are 0D,
connected by directional bonds only by van der Waals-type interactions. It became
apparent that the discovery of fullerenes paved the way for the discovery and
synthesis of other forms of carbon, specifically tubular nanostructures.
Since their rediscovery in 1991 by Iijima, carbon nanotubes (CNTs) have become
the subject of intense research activity worldwide.148 Increased interest in CNTs
arose because of their unique and superior electronic, chemical, physical and
mechanical properties.149 Carbon nanotubes are nanostructures that arise from
folding 2D graphene sheet into a cylindrical shape with hollow internal cavities
(Figure 2.18).150 Depending on the number of rolled graphene sheets, CNTs can
be classified as single walled (one graphene) nanotubes or multiwalled (two or
more) nanotubes.
Figure 2.18: Roll-up of a graphene sheet to form a nanotube.151
Nanostructured materials can be fabricated by two strategies, either by the top-
down approach which is employed primarily by the nanophysics discipline or by
the bottom-up method which is used by the nanochemistry discipline.152 Synthetic
routes of CNTs can be broadly grouped into the following: arc discharge,153,154
chemical vapour deposition (CVD),155,156 and laser ablation.157. The CVD and arc
discharge techniques are most commonly used in CNT synthesis. Both arc
discharge and laser ablation make use of solid-state carbon precursors, which are
vaporised at very high temperatures, to provide the carbon source that is needed
Chapter 2: Literature Review
41
for nanotube growth. The CVD technique represents an example of preparing
CNTs by the bottom-up approach as depicted by Figure 2.19.138 In the CVD
technique, hydrocarbon gases or vaporised organic liquids are used as sources for
carbon atoms, while metal catalysts act as “seeds” for nanotubes growth. The
temperatures employed in this technique are much lower compared to that of the
top-down approach used by arc discharge and laser ablation.158
Figure 2.19: A bottom-up approach of CNTs via CVD route.138
Applications of CNTs arise from their exceptional physical properties. For instance
the electrical properties of CNTs have had a significant impact in field emission in
various electronic devices,132 while their geometrical shape has offered precise
and controlled drug delivery systems.138 Because CNTs have extraordinary
mechanical properties such as high tensile strength and stiffness they have been
used in composites as reinforcements and as cantilever tips in lithographic
techniques.159 Due to the high surface areas of CNTs they have also been used in
hydrogen storage devices,160 in water purification161 as an adsorbent and as
supports for catalysts.162
Chapter 2: Literature Review
42
In the work presented in this thesis, CNTs were used to harness their exceptional
electronic properties when composited with TiO2. Here it was found that CNTs
acted as efficient transporters of the species that were generated on titania upon
photo-illumination and hence they increased the photoactivity of the catalyst by
minimizing electron/hole recombination. Likewise the high surface area of CNTs
assisted in the removal of the model organic pollutants chosen. Equally the
structural integrity of the nanocomposites were also improved due to the excellent
mechanical properties of CNTs. However, the results of these studies will be fully
discussed in the chapters which follow.
Chapter 2: Literature Review
43
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62
CHAPTER 3:
EXPERIMENTAL METHODOLOGY
3.1 Reagents used
Table 3.1 Reagents used
Reagent Supplier Grade of Reagent
Titanium tert-butoxide Sigma-aldrich Used as received
1-Butanol Merk Used as received
Carbon nanotubes Sigma-aldrich Used as received
Ammonia Merk Used as received
Tungstic acid Sigma-aldrich Used as received
Cobalt acetate Sigma-aldrich Used as received
Sodium bromide Riedel-deHaen Used as received
Argon Afrox scientific UHP
Acids
HF, HCl, HBr,HI
Riedel-deHaen Used as received
Chapter 3: Experimental methodology
63
3.2 Synthesis of Photocatalysts
3.2.1 Anionic-doped Titanium Photocatalyst
Pristine titanium dioxide, anionic doped TiO2 and TiO2-carbon nanotubes with and
without anionic dopants (F, Cl, Br and I) were prepared by the sol-gel method. A
slightly modified method as specified by Zhang et al was followed (so as to suit our
desired product).1 Titanium tert-butoxide (10 mL) was added to 1-butanol (52 mL)
under constant stirring for 30 minutes. In the solution, the respective acid (HF,
HCl, HBr and HI) 10 mL of 20 mM was added, each prepared and made to the
mark with 1-butanol. Formic acid (11 mL) was then added to the solution
dropwisely resulting in the formation of a precipitate. The pH of the solution was
maintained at pH 2 to allow the exchange of the hydroxyl group in the gel and
anionic dopants.
The as-formed precipitate was stirred for 2 hours and aged for another 2 hours. In
order to drive out most of the solvent, the precipitate was then oven dried at 80°C
for 3 hours. The dried precipitate was then ground to fine powder and calcined in
air, in a furnace, for 3 hours at a heating ramp of 5°C/min at 400°C.
CNTs were first functionalised before the preparation of nanocomposites with or
without anionic dopants. This was done because the functionalization of pristine
CNTs (f-CNTs) enhanced their solubility and dispersion in organic and aqueous
solvents. At the same time it also improved the linkage between CNTs and TiO2.
This functionalization was achieved by refluxing CNTs in concentrated nitric acid
for 12 hours. The oxidizing power of nitric acid ensured that the tubes were mostly
functionalized with carboxylic groups; yet it minimized the number of defects
introduced on the CNTs. Carboxylic groups have stronger binding power when
compared to alcohols.2,3
CNTs-TiO2 nanocomposites without anionic dopants were initially prepared by
sonicating 0.2% m/m of f-CNTs in 10 mL of 1-butanol for 15 minutes. This mixture
was then added to tert-butoxide (10 mL). Conversely, for nanocomposites
prepared with anionic dopants the f-CNTs were added to solutions of tert-butoxide
Chapter 3: Experimental methodology
64
(10 mL) which had 10 mL of 20 mM of the corresponding acid (HF, HCl, HBr or
HI). Formic acid was added thereafter so that precipitation could occur in a single-
step process. The drying processes were the same as for pristine titania, however,
the calcination process was performed in an inert atmosphere using argon gas at
a flow rate of 10 mLmin-1.
3.2.2 Preparation of Composite Semiconductors
3.2.2.1 CNTs-CoO and CNTs-CoO-TiO2
Functionalized CNTs (1.0 g) were mechanically mixed together with cobalt (II)
acetate (0.4 g) using a pestle and mortar for almost 30 minutes. The mixture was
then placed in a quartz boat in a furnace at 350°C with a ramp rate of 10°Cmin-1
for 3 hours. The mixture was heated in a flowing stream of argon at a flow rate of
60 mLmin-1. The thermal decomposition of the metal acetate coated the wall of
CNTs with the metal oxide (CNTs-CoO).4
The syntheses of CNTs-CoO-TiO2 nanocomposites were achieved by initially
sonicating (0.2% m/m) of CNTs-CoO for 15 minutes in 1- butanol (10 mL) to
disperse the mixture. The mixture was then added to a solution of
tert-butoxide (10 mL) previously “dissolved” in 42 mL of 1-butanol. This mixture
was stirred for 30 minutes at ambient temperature. Thereafter formic acid (11 mL)
was added to the mixture(s) leading to the formation of a grey precipitate. Drying
and calcination processes were the same as described in section 3.2.1.
3.2.2.2 TiWxOy and CNTs-TiWxOy
Tungstic acid (8.00 x 10-4 mol) was dissolved in a solution of 1–butanol (20 mL)
and ammonia (50:50). Titanium tert-butoxide (2.38 x 10-3 mol) was added in a
dropwise fashion to the solution and the as-formed precipitate was stirred for
2 hours and then aged for another 2 hours.5 This precipitate was dried in an oven
at 80°C to drive most off the moisture, then it was ground into fine powder and
calcined in air for 3 hours at 400°C ramped at 5°Cmin-1. Functionalized CNTs
(0.2% m/m) were sonicated for 15 minutes in 10 mL of an ammonia-butanol
Chapter 3: Experimental methodology
65
solution. The dispersed CNTs were then added to a solution of dissolved tungstic
acid (8.00 x 10-4 mol). Thereafter, under constant stirring, titanium tert-butoxide
(2.38 x 10-3 mol) was added dropwise to the mixture. The mixture was then
vigorously stirred for 2 hours. Finally the precipitate was dried and calcined as
described in section 3.2.1.
3.3 Characterization
All samples were characterized by using various microscopic, spectroscopic and
surface area techniques, to elucidate structure, morphology, size distribution and
confirm the nature of the catalyst.
3.3.1 Transmission Electron Microscopy (TEM)
Imaging can be determined by the wavelength of the radiation, while the best
resolution (r) is determined by Rayleigh criterion equation (Equation 1). In
equation 1, λ is the wavelength of the radiation, n is the refractive index of the
viewing medium and α is the semi-angle of collection. In air, n sinα is ~ 1, making r
approximately equal to 0.61 times the wavelength of radiation, as represented by
equation (Equation 2).6,7
r = 0.61λ/nsinα (1)
r = 0.61λ (2)
For an optical microscope, using light with a wavelength with 510 nm, r would be
311 nm, meaning at this wavelength it would not be possible to observe any
isolated nanoparticles. Thus, an alternative radiation with a smaller wavelength
would have to be used to improve the resolvable spacing.
Using the de Broglie equation and the knowledge that particles of matter can
display wave-like behaviour, it is possible to relate the wavelength of these
particles to their momentum by using equation (Equation 3). Here λ is the
wavelength, h is Planck‟s constant and p is the momentum of these particles.
Chapter 3: Experimental methodology
66
Hence this shows that electrons and other particles of matter can be focussed to
form an image. This can be achieved by accelerating electrons, with a large
voltage, where they attain large momenta and consequently extremely small
wavelengths, thus making it possible to view nanoparticles or even atoms,
according to equation (2).5,6
λ = h/p (3)
Therefore by using electron microscopes it is possible to study nanoparticles.
In the characterisation of the photocatalysts synthesised in these studies, small
quantities of these were placed in a glass vial to which about 5 mL of ethanol was
added. Each sample, produced by the methods described earlier, was sonicated
for 15 minutes in an ultrasonic bath. Thereafter, small volumes of the sonicated
samples were pipetted onto a copper grid coated with a polymer and then with
lacy carbon. Transmission electron microscopy (TEM) images were captured by
TECNAI G2 SPIRIT microscope (FEI Company) and JEOL – 3010 microscope
both at an accelerated voltage of 200 kV. Both of the above-mentioned TEMs
were fitted with energy dispersive X-ray analysis (EDX) facilities which were
operated at 15 kV.
Energy dispersive X-ray spectroscopy (which has abbreviations of EDS, EDX and
EDXS), is predominantly used to determine which elements are present in a given
sample. This can be achieved when electrons, from the focused beam in the
microscope, have sufficient energy to collide with different elements in the sample
to dislodge some of their core-shell electrons. When electrons in the outer-shells
of these elements cascade down to replace the core-shell electrons, they release
their energy in the form of X-rays.5 Since the electronic structure of each element
in a sample is unique to that particular element, the energy that its electrons need
to lose to cascade down to their core-shells is also unique to that element. Hence,
by comparing the X-rays that are emitted from the sample to known values, it is
possible to identify each of the elements present in the sample.5,6
Chapter 3: Experimental methodology
67
3.3.2 Scanning Electron Microscopy (SEM)
Alternatively characterisation of samples can be made by SEM, which like TEM is
also an electron microscope technique. However, in SEM the focused beams of
electrons are accelerated at lower voltages than in TEM and samples are required
to be conductive solids. Like as in TEM, SEM can also be coupled with an EDS.
This non-destructive technique reveals various pieces of information about the
sample including: external morphology (texture), chemical structure and the
orientation of materials.
In the characterisation of the various composites (later tested as photocatalysts)
that were synthesised in these studies, small amounts were first sputtered with a
thin layer of carbon to enhance their conductivity. The samples were then mounted
on double sided tape and placed on a SEM stub. SEM images were taken at an
accelerated voltage of 2 kV using Nova NanoSEM 600 (FEI Co., The Netherlands)
and JEOL JSM – 7500F, both equipped with and EDX which was operated at
15 kV.
3.3.3 Raman Spectroscopy
Raman spectroscopy, named after Prof. Sir CV Raman, was the first to
demonstrate the inelastic scattering of a monochromatic excitation source. Raman
spectroscopy is used to identify phases and crystalline polymorphs.7 It is also used
to measure the stress and symmetries of materials that are molecular or
crystalline. Samples can be analysed in all three states of matter at ambient
temperature.6,7
Raman spectra of the photocatalysts synthesised in these studies were collected
with Horiba Jobin-Yvon spectrometer at room temperature. An argon ion (Ar+)
laser was used as the excitation source of 514.5 nm, with a spectral resolution of
2 cm-1. The laser power at the sample was about 3.0 mW, with a collection time of
120 s. Each spectrum was collected in a backscattering configuration which had a
liquid nitrogen cooled charge-coupled device (CCD) detector. The laser beam was
Chapter 3: Experimental methodology
68
focused onto the sample by 100 X magnification objective lens of an Olympus
microscope.
3.3.4 Powder X-ray Diffraction (PXRD)
X-ray diffraction uses X-rays produced in a cathode ray tube by heating a filament
to produce electrons. The electrons produced in this manner are accelerated at a
high voltage (10 – 100 kV) and bombarded into the target sample (which is Cu,
Mo, Cr or Fe), dislodging their inner-shell electrons which produce X-rays. The
X-rays produced this way are then collimated and focused onto the sample and
detector, where the intensity and reflected rays are recorded.7
This non-destructive technique is primarily used to identify unknown crystalline
materials in the fields of: geology, material sciences, chemistry, biology and the
environmental sciences. It is also used to measure the sample purity, determine
the unit cell dimension and average particle diameter.
In the PXRD characterisation of the photocatalysts synthesised in these studies
X-ray scattering patterns were obtained on X‟Pert PANanalytica and Siemens
Siefert 3000TT diffractometers, using the Cu Kα (0.1540 nm) monochromatic
beam. The power settings were 40 kV and 40 mA. Diffraction patterns were
collected with a scan rate of 5°min-1 over the range 20° ≤ 2θ ≤ 60°. The average
crystal sizes, d, were calculated from Scherrer equation (Equation 5) for a given
phase θ, at full width half maximum (FWHM, β) and wavelength (λ) of the X-ray
source.
d = 0.89λ/β cosθ (5)
The X-ray patterns captured were compared to the international centre for Joint
Committee on Powder Diffraction Standards (JCPDS) database, so as to identify
the polymorphs and crystallinity of the materials so formed.
Chapter 3: Experimental methodology
69
3.3.5 Ultraviolet-Vis (UV-Vis) Spectroscopy
UV-Vis spectroscopy is the measurement of the absorption of ultra-violet and
visible light by a sample. The absorbed light causes electronic transitions in the
sample, prompting light to be absorbed or re-emitted by the sample and hence
elucidating the structure of the sample.
Semiconductor crystallites in the diameter ranges of a few nanometers show a
three dimensional size effect in their electronic structure. These quantum size
effects on the band gap absorption energy can be measured by UV-Vis absorption
spectroscopy.7 Thus the band gap of a semiconductor can be calculated by UV-
Vis absorption spectroscopy. The band gaps of the photocatalysts synthesised in
these studies were calculated by using Tauc‟s equation (Equation(s) 6 and 7),
where α is the absorption coefficient and (hν) is the incident photon energy, and Eg
is the band gap.8,9 The constant n accounts for the type of optical transition. In
Tauc‟s equation the values of n could be: ½, 2, 3/2 and 3, which respectively
corresponds to: the allowed direct, allowed indirect, forbidden direct and forbidden
indirect transitions.
αhν ∞ (hν –Eg)n (6)
(αhν)1/n ∞ (hν – Eg) (7)
TiO2 is an example of an indirect semiconductor, hence the band gap was
measured as a plot of (αhν)2 against hν. The exact value of the band gap was
determined by drawing a line that best fit the slope of the absorption edge as well
as drawing another line that extrapolated the background line before the
absorption edge. The intersection of both lines that corresponded to the x-axis was
taken as the band gap. Band gap measurements were carried out on Shimadzu
UV- 2450.
Chapter 3: Experimental methodology
70
3.3.6 Surface Area Measurements
Brunauer, Emmett, Teller (BET) is a surface area analysis named after the
scientists that studied the relationship between the adsorbent and adsorbate.10
Physical adsorption or physisorption of an adsorbent on an adsorbate is
determined by several factors including the temperature, the pressure of the gas,
the interaction between the surface and the gas as well as the surface area of the
adsorbent. The six different interactions between the adsorbate and adsorbent
have been classified by IUPAC as indicated by Figure 3.1
Figure 3.1: Relationship between adsorbed gases compared to pressure12
Type II, IV and VI can be measured by the BET method as the adsorptive –
adsorbent is greater than the adsorptive-adsorbate.10,1112 Typical gases used are
nitrogen, oxygen, carbon dioxide, helium and methane. BET analyses are routinely
performed on catalysts, pharmaceuticals and cements. Typical analysis would
include specific surface area, pore size distribution (pore diameter, pore volume
and pore surface area) and percentage porosity.
The surface areas and pore volume of the nanocomposites synthesised in these
studies were measured by using a Micrometrics TriStar nitrogen adsorption
Chapter 3: Experimental methodology
71
apparatus. Prior to the analysis, the samples were degassed in vacuum at 150°C
for an hour to reach a final pressure of 1 mmHg.
3.3.7 Photocatalytic Measurements
The photocatalytic efficiency of the nanocomposites synthesised in these studies
were measured against model organic pollutants, bromophenol blue (BPB) and
phenol red (PR). The catalysts (0.1 g) were suspended in a solution of each dye
(100 mL, 10 mgL-1). UV and visible light were used to initiate the photocatalytic
activity of the nanocomposites. Prior to the irradiation, the reaction vessel was
magnetically stirred for 30 min in the dark to ensure an adsorption/desorption
equilibrium. The UV and visible radiation experimental setups are schematically
shown in Figure 3.2 (a and b).
UV - lamps@ 366 nm
5 cm
0.1 g catalyst& 100 mL/10mgL-1
Figure 3.2: (a) UV-radiation experimental set-up for the photodegradation of model pollutants
(a)
Chapter 3: Experimental methodology
72
Xenon lamp
Controller
0.1 g catalyst&100 mL/10mgL-1
Figure 3.2: (b) Visible radiation experimental set-up for the photodegradation of model pollutants
The photodegradation of each model organic pollutant was initiated by the UV
lamp (55Watts) was at 366 nm (2 .0 mWcm-2) and the cabin temperature was kept
constant at 45±5°C for all samples (Figure 3.2 (a)). A solar light simulator
equipped with a xenon lamp (300 nm ≤ λ ≤ 800 nm), with a power output of
2.0 mWcm-2 was used in the case of model organic pollutants irradiated by visible
light (Figure 3.2 (b)). The distance between this lamp and the reactor was 10 cm.
A cut-off filter was also installed so as to allow only visible light to the samples,
The cabin material was constructed from a 2 mm thick rectangular (40 cm x 30
cm) cardboard box. This box had a 5 cm circular hole which allowed the visible
light to pass through. An open and close rectangular shuttle was cut out of the
front side of the box in order to allow regular (5 mL) aliquots to be drawn from the
reaction vessel. These aliquots were taken at intervals of 20 min and filtered
through 0.45 µm teflon membranes.
The degradations of BPB and PR were monitored by measuring the absorbances
at λ = 435 nm and λ = 430 nm respectively, as a function of irradiation time, with a
UV-Vis spectrophotometer (Shimadzu UV- 2450). The resulting [Br-] and [SO42-]
(b)
Chapter 3: Experimental methodology
73
ions were measured with an ion chromatograph (IC –Dionex – IC 2000) equipped
with a Dionex ion pac AS18 (2 X 250 mm) column and conductivity detector. The
eluent solution was 30 mM KOH. The photocatalytic degradation was calculated
with the following formulae:
Amount degraded: Ct/C0
where C0 and Ct are the initial and final concentrations of the dyes at t = 0 min and
t = t min.
Chapter 3: Experimental methodology
74
REFERENCES
1 . Zhu J, Zhang J, Chen F, Ino K, Anpo M, High activity TiO2 photocatalyst
prepared by a modified sol-gel method: Characterization and their
photocatalytic activity for the degradation of XRG and X-GL, Catalysis,
(2005) 35, 261 - 268
2 . Krause RW, Mamba BB, Dlamini LN, Durbach SH, Fe-Ni nanoparticles
supported on carbon nanotubes – co-cyclodextrin polyurethanes for the
removal of TCE in water, Journal. Nanoparticle Research, (2010) 12, 449 -
459
3 . Chen YK, Chu A, Cook J, Green MLH, Harris PJF, Heeson R, Humphries
M, Sloan J, Tsang SC, Turner JFC, Synthesis of carbon nanotubes
containing metal oxides and metals of d-block and f-block transition metals
and related studies, Materials Chemistry, (1997) 7, 545 - 549
4 . Lin Y, Watso KA, Fallach MJ, Ghase S, Smith JG, Delozier DM, Cao W,
Crooks RE, Connel JW, Rapid solventless, bulk preparation of metal
nanoparticles decorated carbon nanotubes, Nano, (2009) 3, 871 - 884
5 . Franklyn PJ, Preparation and characterization of novel titanium-tungsten
oxides, Certificate postgraduate studies in chemistry dissertation, University
of Cambridge, (2005)
6 . Christian GD, O‟Reilly JE, Instrumental analysis, Allyn and Bacon, (1986)
2nd ed., Boston, 412
7 . Skoog DA, Holler JF, Nieman TA, Principles of instrumental analysis,
Harcout Brace College Publishers, (1998) 5th ed, Chicago, 272
8 . Slav A, Optical characterization of TiO2-Ge nanocomposite films obtained
by reactive magnetron sputtering, Digest journal of Nanomaterials and
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Biostructures, (2011) 6, 915 - 920
9 . Pankove JI, Optical processes in semiconductors, Dower Publications
(1971) 1st New York
10 . Brunauer S, Emmett PH, Teller E, Adsorption of gases in multilayers,
Journal of the. American Chemical Society, (1938) 60, 309 - 319
11 . Peigney A, Laurent Ch, Flahaut E, Basca RR, Rousset A, Specific surface
area of carbon nanotubes and bundles of carbon nanotubes, Carbon,
(2001) 39, 507 -514
12 Sing KSW, Everett DH, Haul RAW, Mascou L, Pierotti RA, Rouquerol KT,
Siemieniewska T, Reporting physisorption data for gas/solid systems with
special reference to the determination of surface area and porosity, Pure
and Applied Chemistry, (1985) 5, 603 - 619
76
CHAPTER 4
Photodegradation of triphenylmethane dyes by anionic doped
titania nanocomposites*
* Part of the work presented in this chapter has been reported and published
in Applied Water Science (2011)1, 19 - 24
4.1 Introduction
The prevalence and repercussions of dyes in water is fully described in detail in
Chapter 2. In this chapter various photocatalysts (pristine TiO2 and anionic doped
TiO2) as well as undoped/anionic doped TiO2-nanocomposites were monitored for
their efficiency of decomposing bromophenol blue (BPB) and phenol red (PR)
when exposed to either UV or visible radiation. Likewise, the results of the
characterizations and the subsequent performances of these various
photocatalysts are also discussed in this chapter. All photocatalysts reported
herein were characterized by the use of various microscopic, spectroscopic and
surface area techniques, which were performed to elucidate their structures,
morphologies, size distributions as well as to establish their nature. Likewise the
photodegradation by-products of these reactions were monitored with ion
chromatography, where the liberated sulphates and bromides were quantitatively
measured.
Chapter 4: Results and Discussion
77
4.2 Results
4.2.1 BET Analysis
The surface areas of the various photocatalysts (pristine TiO2, anionic doped TiO2
and undoped/anionic doped TiO2-nanocomposites) were investigated by nitrogen
adsorption measurements. All the TiO2-nanocomposites were found to be of the
type IV kind according to the classification in the IUPAC system, which has a
typical hysteresis loop (H2) that is associated with capillary condensation with
mesopores as depicted in Figure 4.1.1 The interconnected hysteresis loops for all
photocatalyst samples tested spanned a broad range of relative pressures i.e.
from about 0.4 to 1.0. This is in accordance with a diverse mesopore size
distribution associated with intra and inter-aggregation of nanocrystals with non-
uniform size.2 The estimated values of the BET specific areas (SBET) and pore
volumes were tabulated in Table 4.1. Here it was observed that there was little to
no difference in the surface areas of pure TiO2 and anionic doped TiO2, where
both had an average surface area of 175 m2g-1. Undoped or anionic doped TiO2-
nanocomposites appeared to have a maximum surface area of 188 m2g-1, which
was larger than that of undoped or anionic doped TiO2, even though the surface
area of CNTs in this study was found to be 260 m2g-1. The relatively low loadings
of CNTs (0.2% m/m) in the relevant nanocomposites was used to account for
these observations.
Chapter 4: Results and Discussion
78
0.0 0.2 0.4 0.6 0.8 1.0
0
20
40
60
80
100
120
140
160
180
TiO2-F
TiO2
CNTs-TiO2
CNTs-TiO2-F
Ads
orbe
d ga
ses(
cm3 /g
)
Relative pressure (P/P0)
Figure 4.1: Nitrogen sorption isotherm of selected nanocomposites
The following table summarises the properties of the nanocomposites, and these
results are further explored in the paragraphs following the table.
Chapter 4: Results and Discussion
79
Table 4.1 Physical properties of nanocomposites
Sample S(BET)(m2/g) Vp (cm3/g) Band gap (eV)
Crystallite size (nm)
Raman IG/ID
TiO2 175 0.27 3.30 9.62
TiO2-F 174 0.38 3.27 9.63
TiO2-Cl 175 0.23 3.26 9.62
TiO2-Br 175 0.30 3.27 9.62
TiO2-I 176 0.24 2.97 9.65
CNTs 260 1.09 13.27 0.92
CNTs-TiO2 180 0.17 3.28 15.40 0.88
CNTs-TiO2-F 182 0.14 3.27 15.26 0.68
CNTs-TiO2-Cl 185 0.15 3.27 15.40 0.88
CNTs-TiO2-Br 183 0.14 3.26 15.68 0.70
CNTs-TiO2-I 188 0.15 2.98 15.86 0.83
Chapter 4: Results and Discussion
80
4.2.2 Raman Analysis
When the samples were analysed by Raman spectroscopy, (Figure 4.2) four
bands which were characteristic of anatase, 638, 515, 196 and 143 cm-1 were
observed.42 The (A1 + B1)g2 and Eg(3) bands were associated with the stretching of
bent Ti-O-Ti modes, while the Eg(1), Eg2 and B1g(1) bands corresponded to the
deformation of Ti-O-Ti modes.3 The presence of the CNTs and the anionic dopants
did not appear to affect the structure of the TiO2 as all the Raman bands were
conclusive to the presence of anatase. The nanocomposites with CNTs (insert)
showed two further significant bands, the D and G bands at 1356 and 1600 cm-1
respectively. The G peak corresponded to the tangential stretching (E2g) mode of
the highly orientated pyrolytic graphite (HOPG) which was indicative of crystalline
graphitic carbon in CNTs. The D (A1g) peak indicated disorder induced features.42
The IG/ID ratio (Table 4.1), which measures the defects in/on CNT structures and
determines their level of crystallinity was calculated to be between 0.70 - 0.83,.4
An IG/ID ratio of less than 0.5 indicates severe structural defects on the walls of the
CNTs.4 Crystalline CNTs are crucial, so as they can act as a “high-way” for
electrons transferred by TiO2 upon photoactivity, thus minimising recombination
and improving photocatalytic activity.
Chapter 4: Results and Discussion
81
0 500 1000 1500 20000
10000
20000
1500 20007000
8000
9000
10000
CNTs-TiO2-Br
CNTs-TiO2-I
G-bandD-band
Inte
nsity
Raman shift/cm-1
TiO2-I
TiO2-BrTiO2
CNTs-TiO2-Br
CNTs-TiO2-I
E g(3)
A 1g+ B
1g(2)
B1g(1)
Eg(1)
Inte
nsity
Raman shift/cm-1
Figure 4.2: Raman spectra of pristineTiO2, dopedTiO2 and doped CNTs-TiO2 nanocomposites
4.2.3 Optical Studies
The optical responsiveness of the doped TiO2 and CNTs-TiO2-X-nanocomposites
(where X = F,Cl, Br, I) were studied with UV-Vis spectroscopy. Most of the doped
CNTs-TiO2 nanocomposite samples (Figure 4.3 A) had strong intense absorptions
in the UV region. This could be accounted for by the promotion of the electron
from the valence band of TiO2 to the conduction band (O2p →Ti3d).5 The optical
response to the visible range may have been hindered when the CNTs-TiO2-
nanocomposite was doped with anions partly because the CNTs acted as electron
quenchers and thus hindered the red-shift. The red-shift was appreciably noted
with doped TiO2, and in particular for iodine doped TiO2. (Figure 4.3B)
Chapter 4: Results and Discussion
82
Figure 4.3 Comparative UV absorbance measurements of (a) CNTs-TiO2
nanocomposites (b) doped TiO2
It was noted that the CNTs-TiO2 nanocomposites had almost the same absorption
edges as pristine TiO2 (de-counting the incorporation of carbon in the lattice of
TiO2). However, some authors have reported a red-shift when CNTs were loaded
(95% m/m).164 However, in these studies the loading of CNTs was kept to a
minimum (0.2% m/m), hence the above-mentioned visible response was not
observed. In fact a minimal loading of CNTs was deliberately used in the
nanocomposites that were synthesised, so as to reduce the possibility of dye
removal due to adsorption.
As previously mentioned (Section 4.3.) the band gaps of the doped TiO2 and
CNTs-TiO2 nanocomposites were measured by plots of (αhν)2 against hν. Iodine
doped TiO2 and corresponding CNTs-TiO2 nanocomposites (Figure 4.4.) showed
a narrower band gap than pristine TiO2 and all the other anionic dopants (Table 4.1). The narrower band gap by iodine-doped TiO2 could be attributed to the larger
atomic size of the iodine molecule which could easily be polarized by long
wavelength light (Vis) as compared to the smaller sized dopants which could be
200 300 400 500 6000.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60TiO2-ITiO2-BrTiO2-ClTiO2-FTiO2
(A)
Abs
orba
nce
(a.u
.)
Wavelength (nm)200 300 400 500 600-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0(A)
TiO2CNTs-TiO2CNTs-TiO2-ICNTs-TiO2-BrCNTs-TiO2-ClCNTs-TiO2-F
Abs
orba
nce
(a.u
.)
Wavelength (nm)
(B) (a))
(b))
Chapter 4: Results and Discussion
83
polarized by shorter wavelengths (UV). Once polarized, it could more easily
transfer electrons to the conduction band of TiO2.
Figure 4.4: Band gap measurement of pristine TiO2 and iodine doped TiO2
4.2.4 Powder X-ray Diffraction (PXRD)
PXRD patterns of doped TiO2 and CNTs-TiO2 nanocomposites are depicted in
Figure 4.5 (A and B respectively) Intense peaks at 2θ, 25.5, 38.2, 48.6, 54.0 and
55.3° corresponding to crystal planes of TiO2 (101), (004), (200), (105) and (211)
respectively, were confirmed by using the JCPDS card (04047). All of the patterns
and planes were exclusive to the anatase polymorphic form of titania. The CNTs
peak at 26.0° plane (002) was overshadowed by the (101) plane of anatase, which
appeared at the same 2θ. However, the presence of CNTs was confirmed by
Raman analysis as previously discussed in Section 4.3.
TiO2
TiO2-I
EgTiO2: 3.30 eV EgTiO2-I: 2.97eV
Chapter 4: Results and Discussion
84
Figure 4.5: PXRD patterns of (A) doped TiO2 , (B) CNT loaded anionic doped nanocomposites
Crystallite sizes were estimated using the Scherrer equation as shown in Table 4.1 (and mentioned previously in section 3.3.4). To avoid the interference of
CNTs overlapping with (101) reflection, the (200) reflection plane (2θ = 48.6°) was
used for the TiO2 loaded with CNTs, but the (101) plane was used for those
20 30 40 50 60
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
12000
13000
(A)
211
105200
112
004
103
002
101
CNTs-TiO2-F
TiO2-commercial
CNTs-TiO2
Cou
nts
(a.u
.)
220 40 60
0
1000
2000
3000
4000
5000
112
103
(A)TiO2-ITiO2-BrTiO2-ClTiO2-FTiO2 (commercial)
211
10820
0
004
101
Cou
nts
(a.u
.)
2
(B)
20 40 60
0
1000
2000
3000
CNTs-TiO2-I
(B)
211
105200
11200
410
3002
101
CNTs-TiO2-Cl
CNTs-TiO2-BrCou
nts
(a.u
.)
2
(a))
(b))
(c))
Chapter 4: Results and Discussion
85
without CNTs. PXRD patterns showed that the presence of CNTs did not affect the
crystallinity of TiO2 but a slight increase in the crystallite size, caused by the
presence of CNTs in the TiO2-nanocomposites, was observed.
4.2.5 TEM and SEM
The textural morphologies of the doped TiO2 and CNTs-TiO2 nanocomposites
were studied by TEM and SEM. TEM images (Figure 4.6 a-b) showed that close
contact between CNTs and titania was made, which could probably be attributed
to van der Waals forces.6 It was suggested that this close contact allowed proper
electron transfer from the conduction band of titania to CNTs, hence minimizing
electron-hole recombination. Of note was the comparison between the TiO2
nanoparticles with and without CNTs. Here a pronounced agglomeration was
noted with the particles without CNTs (Figure 4.6c) suggesting that CNTs acted
as dispersing supports. Spherical particles were also confirmed by SEM images
(Figure 4.6 d) The crystallographic lattice fringes of the nanoparticles and CNTs
with d-spacings of 0.31 nm and 0.36 nm were assigned to the TiO2 (101) and the
CNTs (002) planes respectively.
The sizes of TiO2 particles were found to be between 5 nm and 20 nm, while the
CNTs had an average inner diameter of 15 nm. Elemental analysis which was
obtained through EDS coupled to a TEM (as previously discussed in section 4.3)
to identify the make-up of the doped TiO2 and the CNTs-TiO2 nanocomposites. An
EDS spectrum of fluorinated and iodated TiO2 is shown in Figures 4.6e,f.
Chapter 4: Results and Discussion
86
Figure 4.6: (a, b) TEM images of partly dispersed TiO2 on CNTs (c) agglomerated TiO2 nanoparticles without CNTs (d) SEM images of TiO2 nanoparticles without CNTs
(a) (b)
(d) (c)
Chapter 4: Results and Discussion
87
Figure 4.6: ((e, f) EDS spectrum of fluorine and iodine doped CNTs-TiO2 nanocomposites.
4.3 Photocatalytic Activity of the doped TiO2 and CNTs-TiO2 nanocomposites
The photocatalytic activities of the doped TiO2 and CNTs-TiO2-X nanocomposites
were evaluated for their ability to photodegrade bromophenol blue and phenol red,
both under ultra-visible and visible irradiation. The experimental setups and
conditions were as described in chapter 3 (section.3.3.7) and data were collected
as a set of three replicates.
0 2 4 6 8
0
500
1000
1500
2000
2500
3000
3500
4000
Ti
Ti
I
IO
TiC
Cou
nts(
a.u.
)
k(eV)0 2 4 6 80
1000
2000
3000
4000
5000
6000
7000Ti
Ti
F
O
Ti
C
Cou
nts
(a.u
.)
keV
(f) (e)
Chapter 4: Results and Discussion
88
4.3.1 Photodegradation of Bromophenol Blue
The photodegradation of a concentration of 10 mgL-1 BPB (Figure 4.7a) was
examined under ultra-visible light by doped TiO2. It was observed that TiO2-F
degraded up to 94% of the 10 mgL-1 of BPB in 140 minutes. TiO2 and TiO2-I were
able to degrade 84% and 80% respectively over the same period of time. Under
visible irradiation, TiO2-I degraded 80% of BPB in 140 minutes. Fluorine doped
titania appeared to be the least effective of these, as it was able to only degrade
77 % of the BPB.
Figure 4.7 (a,b) Photodegradation of BPB by anionic doped TiO2 using UV and visible light
0 20 40 60 80 100 120 140 160 180-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
(A)
C/C
0
Time (mins)
TiO2 TiO2-F TiO2-Cl TiO2-Br TiO2-I
0 20 40 60 80 100 120 140 160 180
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9(B)
C/C
0
Time (mins)
TiO2 TiO2-F TiO2-Cl TiO2-Br TiO2-I
Chapter 4: Results and Discussion
89
Figure 4.7 (c) reaction kinetics of BPB initiated by visible irradiation
The photodegradation of BPB both under visible and UV irradiation appeared to
have followed pseudo-first-order reactions (Figure 4.7 b) and their kinetics were
able to be expressed as:
-lnC/C0 = kt
where k was the apparent reaction constant, C0 and Ct were the initial and final
concentrations of BPB. Amongst the doped TiO2 photocatalysts, those that were
doped with fluorine had the highest reaction rate (0.025 min-1) when illuminated
with UV light, while under visible light iodine doped TiO2 had the highest rate
(0.014 min-1). Table 4.2 shows the reaction kinetics of all the other doped TiO2 and
CNTs-TiO2 nanocomposites.
Table 4.2: Kinetic measurements of the CNTs-TiO2 nanocomposites for each dye
(C) (c))
Chapter 4: Results and Discussion
90
Catalysts Bromophenol blue Phenol red
k(UV)min-1 k(Vis)min-1 k(UV)min-1 k(Vis)min-1
TiO2 0.011 0.010 0.005 0.003
TiO2-F 0.025 0.011 0.012 0.003
TiO2-Cl 0.013 0.010 0.006 0.005
TiO2-Br 0.013 0.010 0.006 0.008
TiO2-I 0.012 0.014 0.005 0.009
CNTs-TiO2 0.015 0.012 0.008 0.005
CNTs-TiO2-F 0.067 0.011 0.016 0.005
CNTs-TiO2-Cl 0.021 0.013 0.009 0.012
CNTs-TiO2-Br 0.023 0.018 0.004 0.013
CNTs-TiO2-I 0.017 0.021 0.003 0.015
Chapter 4: Results and Discussion
91
0 20 40 60 80 100 120 140 160 180
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9(B)
C/C 0
Time (mins)
CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-Cl CNTs-TiO2-Br CNTs-TiO2-I
0 20 40 60 80 100 120 140 160 180
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
(A)
C/C 0
Time (mins)
CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-Cl CNTs-TiO2-Br CNTs-TiO2-I
When CNTs were incorporated into the doped TiO2, to form CNTs-TiO2-X
nanocomposites, it was observed that the photoactivity was improved under UV
and visible illumination. For instance, about 98% of 10 mgL-1 BPB was degraded
by CNTs-TiO2-F nanocomposites in 20 minutes while undoped CNTs-TiO2
nanocomposites and CNTs-TiO2-I nanocomposites managed to degrade 85% and
80% respectively in 140 minutes (Figure 4.8a). Under visible irradiation CNTs-
TiO2-I nanocomposites degraded 91% of BPB compared to 82% in 140 minutes.
Kinetic studies revealed that the reactions followed a pseudo-first-order rate law as
observed in Figure 4.8b. CNTs-TiO2-F nanocomposites had the highest reaction
rate (0.067 min-1) when UV light was used, while CNTs-TiO2-I nanocomposites
(0.021 min-1) had the highest reaction rate when visible light was used.
Figure 4.8: (a, b) Degradation of BPB by anionic undoped CNTs-TiO2 nanocomposites and CNTs-TiO2-X nanocomposites under UV and Visible irradiations,
Chapter 4: Results and Discussion
92
Figure 4.8: (c) Reaction kinetics of CNTs-TiO2-X nanocomposites with visible light
4.3.2 Photodegradation of Phenol Red (PR)
The photodegradation of 10 mgL-1 PR by doped TiO2 was evaluated under visible
and ultra-visible light. TiO2-F photodegraded PR to 84% while TiO2 only removed
up to 54% in 140 minutes as depicted in Figure 4.9a. Under visible light, TiO2-I
managed to degrade 78% of PR, while pristine TiO2 achieved 45% in 140 minutes.
The reaction kinetics of all the CNTs-TiO2 nanocomposites (doped or undoped)
once again followed pseudo-first-order reaction kinetics, as illustrated by Figure 4.9b. Table 4.2 shows the different reaction kinetics of all the CNTs-TiO2
nanocomposites under the different conditions. CNTs-TiO2-F nanocomposites had
the fastest reaction rate (0.012 min-1) when UV irradiation was used, while CNTs-
TiO2-I nanocomposites had the quickest reaction rate (0.009 min-1) of all the
CNTs-TiO2 nanocomposites under visible light.
(C) (C)
Chapter 4: Results and Discussion
93
Figure 4.9 Photodegradation of PR by doped TiO2 under (a) visible and (b) UV light (c) reaction kinetics doped TiO2 under UV light.
The efficiency of the CNTs-TiO2 nanocomposites was also monitored with PR. The
introduction of CNTs into the matrix to make a nanocomposite improved the
photodegradation of PR, both under UV and visible irradiations. CNTs-TiO2-F
nanocomposites and CNTs-TiO2–I nanocomposites both degraded 90% of PR in
140 minutes under UV and visible irradiation respectively (Figure 4.10a). On the
other hand the undoped CNTs-TiO2 nanocomposites could only degrade 53%
(UV) and 66% ((Vis). The kinetics of the reactions all followed a pseudo-first-order
rate (Figure 4.10b).
0 20 40 60 80 100 120 140 160 1800.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9(A)
C/C
0
Time (mins)
TiO2 TiO2-F TiO2-Cl TiO2-Br TiO2-I
0 20 40 60 80 100 120 140 160 1800.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9 (B)
C/C
0
Time (mins)
TiO2 TiO2-F TiO2-Cl TiO2-Br TiO2-I
(C)
Chapter 4: Results and Discussion
94
Figure 4.10 (a,b) Photodegradation of PR by CNTs-TiO2-X nanocomposites (c) reaction kinetics of PR by CNTs-TiO2-X nanocomposites under visible light.
(C)
0 20 40 60 80 100 120 140 160 180-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
(A)
C/C 0
Time (mins)
CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-Cl CNTs-TiO2-Br CNTs-TiO2-I
0 20 40 60 80 100 120 140 160 180-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9 (B)
C/C 0
Time (mins)
CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-Cl CNTs-TiO2-Br CNTs-TiO2-I
(B) (A)
(C)
Chapter 4: Results and Discussion
95
4.4 Discussion
Bromophenol blue and phenol red are anionic dyes, making them negatively
charged.7 Undoped TiO2 in a basic medium has a surface which is negatively
charged (equation 1).8 At a higher pH the point of zero charge of TiO2 is negative
and at a lower pH (3) its surface becomes positive according to:
>TiOH + HO- ↔ TiO- (1)
>TiOH + H+ ↔ TiOH2+ (2)
Where: >TiOH = titanol surface group
Based upon this it is suggested that undoped TiO2 was the least photoactive of all
the CNTs-TiO2 nanocomposites due to an increase in the repulsive forces
between the surface of TiO2 and the dye. Photodegradation was only possible
because of the action of a limited amount of hydroxyl radicals which were
produced by the holes from the valence band that reacted with water to neutralize
the OH-, as illustrated by equation (3) and (4) 9
TiO2 hν→ e-CB + h+
VB (3)
{H2O + H+ + OH-} + h+ → OH• + H+ (4)
Another hindrance to the photoactivity of undoped TiO2 was the formation of
surface Ti-O• radicals which were formed by the direct action of holes on the
titanol surface (5) and which acted as hole trappers.
>TiOH + h+ → >Ti-O• (5)
Introducing anionic dopants on titania improved the photocatalytic activity both
under visible and ultra-violet light. Fluorine doped TiO2 performed better than all of
the other halogens when UV light was used. Fluorine being the most
electronegative halogen, partially made the surface of TiO2 positive. It is
Chapter 4: Results and Discussion
96
suggested that this positive surface attracted and bound a negatively charged dye
more strongly than a neutral or negatively charged one. The pH of the solution
was kept at pH 2 during the synthesis of anionic doped nanocomposites, to allow
the exchange of hydroxyl groups with fluorine.
Once the dye (6) was attached on the surface of titania, it was prone to
electrophilic attack by the holes, having produced a positively charged dye surface
R•+(7). The positively charged dye was then attacked by hydroxyl radicals as seen
in (8). This nucleophilic attack mineralized the dye. Morretzi and Selli have also
shown that the number of hydroxyl radicals produced increased when TiO2 was
fluorinated.10 This was suggested to be due to the high potential of fluoride ions
(3.6 V) shielding the surface of TiO2 from being directly attacked by h+, but also
having been able to directly oxidizing water molecules to produce more hydroxyl
radicals (9). Preliminary experiments conducted in our laboratories with
2,2-diphenyl-1-picryhydrazyl (DPPH) have also shown that more hydroxyl radicals
were liberated with fluorine doped TiO2 as compared to pristine TiO2. In that case
it appeared as if an increase in hydroxyl radicals increased the photoactivity of the
nanocomposites. However, in this study it was observed that the photoactivity of
the CNTs-TiO2-X nanocomposites decreased as the electronegativity of the
halogen decreased. Consequently it is proposed that as the polarizing power of
each these halogens decreased, the attraction of the dye to the surface of TiO2
became weaker. With the interfacial distance between the dye and TiO2 having
increased it appears that fewer hydroxyl radicals reached the dye and thus the
photoactivity decreased.
Chapter 4: Results and Discussion
97
R + TiO2 F
R TiO2 F
(6)
TiTi
F F
Ti
F
R R R
+ h+ R + TiO2-F (7)
HO R+ Degradation by-products (8)
TiO2-F + h+ H2O HO + H+ TiO2-F (9)
R
= dye
The data provided from this study suggest that the introduction of TiO2
nanoparticles on CNTs significantly improved the nanocomposites‟s photoactivity,
because these CNTs acted as electron transporters. Since nanoparticles were
perfectly attached on the walls of CNTs (Figure 4. 6a, b) the CNTs acted as
excellent mediators, minimizing electron-hole pair recombination. The trapped
electrons on the CNTs reacted with adsorbed oxygen to form reactive superoxide
radicals (O•2) (10) These superoxide ion radicals were also reactive enough to
degrade the organic dye pollutant (11). However, radicals could also have been
Chapter 4: Results and Discussion
98
neutralized by a proton to form an HO•2 (12) which could have reacted with another
HO•2 to form intermediate hydrogen peroxide H2O2 molecules (13). Hydrogen
peroxide (H2O2) could then have undergone a decomposition (by the reduction of
an electron) to produce more HO• radicals (14) which could have reacted with the
dye molecules to completely mineralize them (15). The increased quantity of OH•
radicals produced by the trapped electrons in CNTs as well as those produced by
the action of h+ from TiO2 (when the holes reacted with the water molecules) most
likely accounted for the significant degradation of the dye. Hence any adsorbed
pollutant on the walls of the CNTs could eventually have been decomposed as
illustrated by Figure 4.11.
TiO2 → e-CB + hVB
+ (3)
O2 + e-/CNTs → O-•2 (10)
O-•2 + dye→ by-products (11)
O-•2 + H+ → HO•
2 (12)
2HO•2 → H2O2 + O2 (13)
H2O2 + e- HO• + HO- (14)
HO• + dye → by-products (15)
Chapter 4: Results and Discussion
99
Figure 4.11: Proposed mechanism of the role played by CNTs upon photodegradation of dye in water
Iodine doped CNTs-TiO2 nanocomposites effectively decomposed the dyes when
visible irradiation was used. Maximum visible light was harnessed by iodine doped
nanocomposites because the band gap of titania was effectively narrowed to
2.97 eV as compared to 3.30 eV in undoped CNTs-TiO2 nanocomposites. The
other anionic dopants were not effective enough in reducing the band gap to make
use of the entire visible spectrum. The formation of Ti3+ surface states have been
reported as colour centres, which assist in the metal oxide to absorb more of the
visible light, have been found to be pronounced in iodine doped TiO2.11 Ti3+ was
able to minimize the re-combination of electron-hole-pair by acting as effective
electron trappers.12 The increased surface area of TiO2-I (176 m2/g) and CNTs-
TiO2-I nanocomposites (188 m2/g) was believed to have enhanced the
photocatalytic activity of the CNTs-TiO2 nanocomposites.
The degradation of BPB and PR was also monitored by the liberation of [SO42-]
and [Br-] using ion chromatography. Only sulphates could be monitored for PR
while both ions were monitored for BPB. The sulphite ion was oxidized to sulphate,
VBh+
e-CB
e-
H+/H2O
HOdye by-products
O2 O-2
dye
H+
by-products
HO2 H2O2 HOe-
CNTs
h dye
Chapter 4: Results and Discussion
100
a more stable (+6) oxidation state of sulphur and hence it was more probable to
measure than the sulphite ion. Fluorine doped TiO2 and corresponding (Figure 4.12a) CNTs-TiO2-F nanocomposites produced the highest amount of the ions
under UV irradiation, while iodine doped (Figure 4.12 b) TiO2 and the
corresponding CNTs-TiO2-I nanocomposites produced the most ions under visible
illumination. Since the ratio of the sulphate to the bromide is 1:4 in the molecule of
BPB, a higher concentration of bromide ions was liberated in BPB (Figure 4.13).
Sulphates have been reported to be irreversibly adsorbed on the surfaces of
titania, which may have also accounted for the low amounts of sulphate ion.8
However, irreversible adsorption of sulphate on the titania doesn‟t affect the
photoactivity of the metal oxide. Devi and co-authors reported enhanced
photocatalytic activity for methyl orange with ammonium persulphate. They
claimed that the enhanced activity that they observed was due to greater
thermostability and the faster kinetic rate of sulphate radicals as compared to
hydroxyl radials.13
Figure 4.12 Generated sulphate ions from the degradation of PR using (a) ultra-violet and (b) visible light
0 20 40 60 80 100 120 140 160 1800.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
[SO
4]2- p
pm
Time (mins)
TiO2 TiO2-F TiO2-I CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-I
0 20 40 60 80 100 120 140 160 1800.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8(B)
[SO
4]2- p
pm
Time (mins)
TiO2 TiO2-F TiO2-I CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-I
(A)
Chapter 4: Results and Discussion
101
Figure 4.13a Liberated sulphate and bromide ions from the degradation of BPB (a) CNTs-TiO2, (b) CNTs-TiO2-I, under visible irradiation
Figure 4.13b Liberated sulphate and bromide ions from the degradation of (C) TiO2 (D) TiO2-I under visible irradiation
0 20 40 60 80 100 120 140 160 180
0
2
4
(A)
Br-
[SO4]2-
Time (mins)
[Br- ] p
pm
0.0
0.2
0.4
0.6
0.8
1.0
1.2
[SO4 ] 2- ppm
0 20 40 60 80 100 120 140 160 180
0
2
4(B)
Br-
[SO4]2-
Time (mins)
[Br- ] p
pm
0
2
[SO4 ] 2- ppm
0 20 40 60 80 100 120 140 160 180
0
2
4
(C)
[Br-]
[SO4]2-
Time (mins)
[Br-
] p
pm
0.0
0.2
0.4
0.6
0.8
1.0
1.2
[SO
4 ]2
- pp
m
0 20 40 60 80 100 120 140 160 180
0
2
4
(D)
[Br-]
[SO4]2-
Time (mins)
[Br-
] p
pm
0
2
[SO
4 ]2
- pp
m
Chapter 4: Results and Discussion
102
REFERENCES
1 . Barrett EP, Joyner LG, Halenda PP, The determination of pore volume and
area distributions in porous substances. I. Computations from nitrogen
isotherm, Journal of American Chemical Society, (1957) 73, 373-389
2 . Sing KSW, Everett DH, Haul RAW, Mascou L, Pierotti RA, Rouquerol KT,
Siemieniewska T, Reporting physisorption data for gas/solid systems with
special reference to the determination of surface area and porosity, Pure
Applied Chemistry, (1985) 57, 603 - 619
3 . Hu G, Meng X, Feng X, Ding Y, Zhang S, Yang M, Anatase TiO2
nanoparticles./carbon nanotubes nanofibers: preparation, characterization
and photocatalytic properties, Journal of Materials Science, (2007) 42, 7162
- 7170
4 . Dresselhaus MS, Ekhund PC, Photons in carbon nanotubes, Advances in
carbon nanotubes, Advances in Physics Letters, (2000) 49, 705- 814
5 . Xu YJ, Zhuang Y, Fu X, New insight for enhanced photocatalytic activity of
TiO2 by doping carbon nanotubes: a case study on degradation of benzene
and methyl orange, Journal of Physical Chemistry C, (2010) 114, 2669 -
2676
6 . Eder D, Carbon nanotubes-inorganic hybrids, Chemical Reviews, (2010)
110, 1348 - 1385
7 . Forgas E, Cserhati T, Oros G, Removal of synthetic dyes from
wastewaters: a review, Environmental International, (2004) 30, 953 - 971
8 . Linsebigler AL, Lu G, Yates JT Jr, Photocatalysis on TiO2 surfaces:
Principles, mechanisms and selected results, Chemical Reviews, (1995) 95,
735 – 758
Chapter 4: Results and Discussion
103
9 . Houas A, Lachheb H, Ksibi M, Elaloui E, Guillard C, Hermann J-M,
Photocatalytic degradation pathway of methylene blue in water, Applied
Catalysis B: Environmental, (2001) 31, 145 - 157
10 . Mrowetz M, Selli E, Enhanced photocatalytic formation of hydroxyl radicals
on fluorinated TiO2, Physical Chemistry Chemical Physics, (2005) 7, 1100 -
1102
11 . Serpone N, Is the band gap of pristine TiO2 narrowed by anion – and
cation-doping of TiO2 in second –generation photocatalysts?, Journal of
Physical Chemistry B, (2006) 110, 24287 - 24293
12 . Fox MA, Dulay MT, Heterogeneous photocatalysis, Chemical Reviews
(1993) 93,341 - 357
13 . Devi LG, Kumar SG,Reddy KM, Munikrishnappa C, Photodegradation of
methyl orange an azo dye by advanced Fenton process using zero valent
metallic iron: influence of various reaction parameters and its degradation
mechanism, Journal of Hazard Materials, (2009) 164, 459 - 467
104
CHAPTER 5
Titania-based binary metal oxides and nanocomposites for the
photodegradation of organic pollutants in water*
* Part of the work presented in this chapter has been reported and published
in Materials Chemistry and Physics (2011)129, 406 – 410, as well as
submitted together with futher work on Ag+ and Ce+ doping of titania.
5.1 Introduction
The use of binary metal oxide (BMOs) systems (i.e. a combination of two metals in
their oxide forms) as photocatalysts has generated much interest because of their
ability to be efficient charge-separators. One major limitation of intrinsic TiO2 is the
fast recombination of the photogenerated charges. One approach of achieving
better charge separation, involves the coupling of two semiconductor particles with
different energy levels. Examples include: CdS-TiO2,1 TiO2-SnO2,2 TiO2-SiO2 3 and
TiO2-ZrO2. For instance, semiconductor particles having a large bandgap and an
energetically low-lying conduction band e.g. TiO2 or ZnO can be combined with
particles having a small bandgap and an energetically high lying conduction band
e.g. CdS or WO3.4 When the two types of particles (as mentioned above) are
combined, light absorption at longer wavelengths (i.e. > 366 nm) result in efficient
electron transfer to the large bandgap particle (i.e. TiO2), while the holes migrate in
the opposite direction from the holes to the small bandgap particle/s (i.e. CdS or
WO3). The minimization of charge recombination increases the photoactivity of
the photocatalysts. Not only do binary systems minimize recombination but they
also improve the photoresponse of the photocatalyst in the visible region as
compared to pristine TiO2.
This chapter of the thesis reports on the efficiency of CoO and WO3 (20% mol)
coupled with TiO2 in the degradation of bromophenol blue and phenol red, under
visible and ultra-visible illumination. Both CoO and WO3 have smaller band gaps
Chapter 5: Results and Discussion
105
i.e. between 2.6 eV and 2.8 eV as compared to TiO2 (3.2 eV), which means that
they would be photoexcited by longer wavelengths while at the same time TiO2
would be comparably poorly excited; yet TiO2 is still a better photocatalyst than
these two metal oxides. To enhance the catalytic activity of the binary system,
CNTs (0.2% m/m) were incorporated into each of the binary metal oxide systems
(i.e. CoO/TiO2 and WO3/TiO2) to make nanocomposites. In addition they served as
efficient electron „bridges‟ between the metal oxides.
5.2 Experimental
Tungsten–titania binary metal oxides were prepared by dissolving tungstic acid
(8.00 x 10-4 mol) in a solution of 1–butanol (20 mL) and ammonia (50:50). Titanium
tert-butoxide (2.38 x 10-3 mol) was added in a dropwise fashion to the solution and
the as-formed precipitate was stirred for 2 hours and then aged for another 2
hours.5 This precipitate was dried in an oven at 80°C to drive most off the
moisture, then it was ground into fine powder and calcined in air for 3 hours at
400°C at a ramp rate of 5°Cmin-1. The tungsten-titania composite was synthesized
by sonicating functionalized CNTs (0.2% m/m) for 15 minutes in 10 mL of an
ammonia-butanol solution. The dispersed CNTs were then added to a solution of
dissolved tungstic acid (8.00 x 10-4 mol). Thereafter, under constant stirring,
titanium tert-butoxide (2.38 x 10-3 mol) was added dropwise to the mixture. The
mixture was then vigorously stirred for 2 hours and the precipitate was dried and
calcined as described in section 3.2.1
Cobalt-titania nanocomposites were also prepared by the sol-gel method, firstly by
thermally decomposing cobalt acetate on CNTs. These decorated CNTs with CoO
were then mixed in a one-step method with titanium tert-butoxide as described in
section 3.2.2.1
Chapter 5: Results and Discussion
106
5.3 Characterization
Cobalt titania nanocomposites and tungsten titania binary oxides and
nanocomposites were studied with nitrogen adsorption BET analysis
(Figure 5.1 a, b). All isotherms represented the type IV classification, which
suggested that the pores followed the “ink bottle” hypothesis.6 The typical
hysteresis loop (H2) arises from the capillary condensation of gases. The
estimated specific areas (SBET) and pore volumes (Vp) were tabulated in Table 5.1.
Cobalt-titania nanocomposites had higher surface areas compared to tungsten
doped nanocomposites.
Figure 5.1: BET analysis of (a) cobalt oxide titania nanocomposite and (b) tungsten oxide titania BMOs and nanocomposite
(a) (b)
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
200
Ads
orbe
d ga
ses(
cm3 /g
)
Relative Pressure(P/P0)
CNTs-CoO TiO2 CNTs-CoO-TiO2
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
200
Ads
orbe
d ga
ses(
cm3 /g
)
Relative pressure (P/P0)
TiWxOy CNTs-TiWxOy TiO2
Chapter 5: Results and Discussion
107
Table 5.1 Characterization of photocatalysts
Photocatalysts S(BET)m2/g Vp(cm3) Crystallite size (nm)
TiO2 175 0.27 9.62
WO3 1.3 0.34 15.56
CNTs-TiO2 180 1.09 13.27
CoO-TiO2 120 0.54 15.34/12.34
Ti-WxOy 5.7 0.35 12.23/12.93
CNTs-CoO-TiO2 145 0.87 11.34/10.78
CNTs-TiWyOy 6.4 0.56 12.11/12.45
Samples prepared (as discussed in section 5.2.) were analysed by laser Raman
spectroscopy using the equipment described in section 3.3.3. The Raman spectra
of cobalt and tungsten BMOs and their nanocomposites (Figure 5.2 a-d)
confirmed the presence of the anatase phase of titania. All samples displayed
strong bands at 143, 395, 516 and 639 cm-1, which are attributed to the Raman
active modes of TiO2 (with symmetries of Eg, B1g, A1g and Eg respectively).7 The
modes A1 + B1(2g) and Eg3 bands were associated with the stretching of bent
Ti-O-Ti modes, while the Eg(1), Eg(2) and B1g corresponded to the deformation of the
Ti-O-Ti modes.2
Chapter 5: Results and Discussion
108
Figure 5.2: Raman spectra of (a) CNTs-CoO-TiO2 (b) TiO2, (c) TiWxOy and (d) CNTs-TiWxOy
The tungsten BMOs displayed bands at 272, 663 and 809 cm-1 which resulted
from the bending of the O-W-O mode due to the bridging oxygen, the stretching of
O-W-O mode due to the bridging of oxygen and the stretching of W=O bands
respectively.8,9 On the other hand in the nanocomposites (i.e. those that had
CNTs), peaks at around 1345 cm-1 were observed which are known as the D-band
and are associated with disordered sp2 carbon.10 Similarly these nanocomposites
displayed related peaks which are known as the G-band, at around 1600 cm-1, that
arose due to well-ordered graphite. One indication of the extent of the crystallinity
(c (d
CNTs-TiWxOy
TiWxOy
0 500 1000 1500 2000
0
200
400
600
800
1000
1200
G-bandA1gB1g
Eg
Eg CNTs-CoO-TiO2
Inte
nsity
Raman shift/cm-1
0 500 1000
0
200
400
600
800
1000
1200
A1gB1g
Eg
Eg TiO2
Inte
nsity
Raman shift/cm-1
(b)
(a)
0 500 10000
5000
10000
0-W
-O
B(1g)
A1g+ B1g(2)
Eg(3)
O-W-O
W=O
Inte
nsity
Raman shift/cm-1
0 500 1000 1500 20009000
10000
11000
12000
13000
14000
15000
16000
17000
Eg(1)
O-W-O
Eg(3)
W=O
G-b
and
D-b
and
Inte
nsity
Raman shift/cm-1
Chapter 5: Results and Discussion
109
of CNTs is usually a measurement of the intensity ratio or area of the G-band
against the D-band i.e. the IG/ID ratio. A larger ratio (i.e. closer to 1) indicates more
crystalline CNTs, while a smaller ratio (closer to 0) infers highly disordered CNTs.
In this research, the IG/ID ratio of the pristine commercial CNTs that were used was
found to be 0.87. Similarly the IG/ID ratios for these CNTs in the cobalt titania
nanocomposites and in the tungsten nanocomposites were measured to be 0.81
and 0.76 respectively. The slight decrease in these ratios was attributed to the
functionalization process as well as the defects introduced by the attachment of
the BMOs on the surface of these CNTs. However, because this decrease was
relatively small the data suggested that the crystallinity of the CNTs had not been
seriously compromised in either nanocomposite.
The morphological attributes of the nanocomposites were examined by both TEM
and SEM techniques. Figure 5.3 a-b show SEM images of CNTs-CoO and the
TiWxOy nanocomposite and BMOs respectively. In Figure 5.3 b it was observed
that the thermally decomposed cobalt acetate was well decorated on the walls of
the CNTs, while titania nanocomposites were more agglomerated. The elemental
composition of the nanocomposites and BMOs was verified by EDS as seen in
Figure 5.3 c,d, indicating the elemental presence of carbon (from CNTs), oxygen
(oxides and functionalized CNTs) and cobalt and tungsten. Figure 5.3 e-h shows
TEM images of the various nanocomposites. The CoO nanoparticles in CNTs-CoO
showed that close contact between the CNTs and CoO nanoparticles had been
made, which was attributed to van-der Waals forces. The interaction between
titania and tungsten was verified by the d-spacings of 0.31 and 0.38 nm which
were assigned to the TiO2 (101) and the WO3 monoclinic (002) planes
respectively. Elemental composition of the TiWxOy nanocomposite was identified
and confirmed by EDS (Figure 5.3 i).
Chapter 5: Results and Discussion
110
Figure 5.3: (a-b) SEM images of CoO-TiO2 and Ti-WxOy, (c-d) EDS spectra of the binary oxides
(a) (b)
0 5 100
500
1000
1500
2000
2500
3000
3500
CoCo
O
C
Cou
nts
keV0 5 10
0
500
1000
1500
2000
TiTi
Ti
W
O
Inte
nsity
(a.u
.)
keV
(c) (d)
Chapter 5: Results and Discussion
111
Figure 5.3: (e-h) TEM images of CNTs-CoO-TiO2 and CNTs-Ti-WxOy nanocomposites, (i) EDS spectra of CNTs-Ti-WxOy nanocomposite
0.31 nm (101)
0.38 nm (001) WO33
TO2
(e) (f)
(g) (h)
0 5 100
500
1000
1500
2000 Ti
W
Ti
C
O
Inte
nsity
(a.u
.)
keV
(i)
Chapter 5: Results and Discussion
112
PXRD patterns for CoO-TiO2 depicted by Figure 5.4 show intense peaks of
cobalt(II) oxide and anatase. All 2θ peaks at 26.22, 31.70, 34.25, 36.11, 42.11 and
54.10° corresponded to (101), (220), (111), (311), (200) and (411) crystal planes
of CoO (JCPDS 43100).11 While anatase had crystal plane reflections of (101),
(004), (200), (105) and (211) respectively corresponding to 2θ 25.3, 37.8, 48.0,
53.9 and 55.1° (JCPDS 04047).2,12 The composite CNTs-CoO-TiO2 showed a
small decrease in 2θ values for CoO crystal planes, indicating the interaction of
TiO2 and CoO in the nanocomposite. The peak for CNTs (001) was overshadowed
by anatase (101) which appeared around the same 2θ. However, the presence of
CNTs in the nanocomposite was fully confirmed by TEM and Raman analysis.
20 30 40 50 60
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
12000
13000
CoO
CoOCoOCoOC
oO
AAA
A
A
TiO2
CoO-TiO2
CNTs-CoO-TiO2
Cou
nts
2
Figure 5.4: PXRD pattern of cobalt-titania nanocomposites
Figure 5.5 shows the PXRD patterns of tungsten-titania nanocomposites. TiWxOy
and CNTs-TiWxOy both showed reflections due to monoclinic WO3 of (002), (020),
(022) and (202) at 2θ 23.12, 23.58, 24.38, 33.21 and 34.23° respectively (JCPDS-
43-1035).3 Monoclinic WO3 is prominent at temperatures between 320 - 500°C,
while orthorhombic WO3 exists above 500°C but less than 700°C. Above 700°C,
the phase of WO3 changed to hexagonal.3,13 Since the nanocomposites were
calcined at 400°C, the presence of monoclinic WO3 was understandable. Using
Chapter 5: Results and Discussion
113
the Scherer equation, the diameter of the WO3 (002) was found to be 10 nm, while
it was found to be 15 nm for TiO2 (101).
20 30 40 50 600
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
6000
WO3
CNTs-TiWxOyTiWxOyTiO2WO3
WO3
WO3WO3
A
A
WO3
A
Cou
nts
2
Figure 5.5: PXRD patterns of tungsten-titania nanocomposites
Optical studies of the nanocomposites were made by UV-Visible spectroscopy.
Figure 5.6 showed that the absorption of all the photocatalysts, and the tungstate-
titania nanocomposites had absorption bands in the visible region i.e. 440 nm or
2.8 eV. On the other hand the CNTs-TiWxOy nanocomposites showed absorptions
that were blue shifted to 410 nm or 3.0 eV. This indicated that there was an
interfacial interaction between TiO2, WO3 and the CNTs, rather than a mixture
between them.
Chapter 5: Results and Discussion
114
200 300 400 500 600 700-0.08-0.06-0.04-0.020.000.020.040.060.080.100.120.140.160.180.200.220.240.260.280.30
Abs
orba
nce
(a.u
.)
Wavelength (nm)
CNTs-CoO-TiO2 CNTs-CoO TiO2 TiWxOy CNTs-TiWxOy
Figure 5.6: Optical studies of the various binary metal oxides, and their nanocomposites
A blue shift of 380 nm (from the expected 440 nm or 2.8 eV) was observed for
CoO nanoparticles. This can be explained by the effect of size quantization of the
nanoparticles.5 CNTs-CoO-TiO2 were observed to absorb at 390 nm because of
the transfer of electrons from the conduction band of TiO2 to CNTs. A strong
absorption in the UV region 360 nm was due to the interband transition (valence
band to conduction band) which could be attributed to the O2- → Ti4+ charge
transfer. The band at ~410 nm in the CNTs-CoO-TiO2 could be attributed to the
Co2+ → Ti4+ intervalence transition.14 Infra-red and near infra-red bands were not
observed, which confirmed that Co2+ was not oxidized to Co3+ during the
calcination step. Literature indicated that at higher temperatures (greater than
650°C), Co2+ and Ti4+ (anatase) would be transformed into rutile and CoTiO3,
which would have led to an increase in IR absorption bands (780 nm).9,15 Such
transformations were not observed in our samples as they were calcined at lower
temperatures (400°C)
Chapter 5: Results and Discussion
115
5.4 Photocatalytic Activities of pristine TiO2 and various BMO Nanocomposites
The photocatalytic efficiencies of the pristine TiO2 and various BMO
nanocomposites were monitored with model organic pollutants, bromophenol blue
and phenol red, under visible and ultra-violet illumination.
5.4.1 Photodegradation of Bromophenol Blue (BPB)
Figure 5.7 (a,c) shows that the CNTs-CoO-TiO2 and the CNTs-TiWxOy had much
higher activity than pure TiO2, CoO-TiO2, TiWxOy and CNTs-TiO2. Here it was
observed that CNTs-CoO-TiO2 had degraded up to 98.7% and 99% of the dye
using UV and visible light respectively in 180 minutes, while CNTs-TiWxOy
managed to degrade 98 and 93% of the dye under the same conditions.
Figure 5.7: Evaluation of photocatalytic activity of BPB by various TiO2 and BMO nanocomposites (a, b) under UV irradiation
0 20 40 60 80 100 120 140 160 180
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 (A)
C/C
0
Time (mins)
TiO2 CNTs-TiO2 TiWxOy CNTs-TiWxOy
0 20 40 60 80 100 120 140 160 180
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 (B)
C/C
0
Time (mins)
TiO2 CNTs-TiO2 TiWxOy CNTs-TiWxOy
(b)) (a)
)
Chapter 5: Results and Discussion
116
Figure 5.7: Evaluation of photocatalytic activity of BPB by various TiO2 and BMO nanocomposites (c,d) under visible irradiation
The photodegradation of BPB by various TiO2 and MBO nanocomposites under
UV and visible light followed pseudo-first-order kinetics. Figure 5.8 (a,b) shows a
plot of –lnC/Co vs t for BPB degradation with these photocatalysts. The values of
the apparent reaction rate constant were obtained from the slopes of the linear
curves in the plot and are shown in Table 5.1
0 20 40 60 80 100 120 140 160 180
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 (C)
C/C
0
Time (mins)
TiO2 CNTs-TiO2 CoO-TiO2 CNTs-CoO-TiO2
0 20 40 60 80 100 120 140 160 180
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 (D)
C/C
0
Time (mins)
TiO2 CNTs-TiO2 CoO-TiO2 CNTs-CoO-TiO2
(c))
(d))
Chapter 5: Results and Discussion
117
Figure 5.8: Reaction kinetics of the various (a) TiO2 and (b) BMO nanocomposites as photocatalysts of BPB using different illumination sources
Table 5.2 Rate constants of the various TiO2 and BMO nanocomposites as photocatalysts of BPB using UV and visible light
Photocatalyst kapp (min-1) UV kapp(min-1) Vis
TiO2 0.011 0.010
CNTs-TiO2 0.015 0.012
TiO2-CoO 0.014 0.010
CNTs-CoO-TiO2 0.021 0.018
TiWxOy 0.009 0.013
CNTs-TiWxOy 0.019 0.013
(a) (b)
Chapter 5: Results and Discussion
118
5.4.2 Photodegradation of Phenol Red (PR)
Figure 5. 9 (a, - d) depicts kinetic curves for the photodegradation of phenol red
over the vvarious TiO2 and BMO nanocomposites as photocatalysts under both
UV and visible light. Increasing the illumination time resulted in a decrease in the
concentration of PR for all the TiO2 and BMO nanocomposites tested as
photocatalysts. The photocatalysts with CNTs showed the highest photocatalytic
activity when compared to pure titania. The concentration of PR decreased from
10 mg/L to 0.5 and 1 mg/L, respectively, corresponding to 95% and 90%
degradation of PR in 180 minutes using UV and visible light over CNTs-TiWxOy.
CNTs-CoO-TiO2 nanocomposites degraded PR by 98% and 96% in 180 minutes
with UV and visible light respectively. The photodegradation of PR in the presence
of UV and visible light also followed pseudo-first-order reaction kinetics as
depicted by Figure 5.10 (a,b).
Figure 5.9: The photodegradation of PR by TiO2 and BMO nanocomposites (a, b) under UV irradiation
(B)
0 20 40 60 80 100 120 140 160 180
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 (A)
C/C
0
Time (mins)
TiO2 CNTs-TiO2 TiWxOy CNTs-TiWxOy
0 20 40 60 80 100 120 140 160 180
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 (C)
C/C
0
Time (mins)
TiO2 CNTs-TiO2 CoO-TiO2 CNTs-CoO-TiO2
Chapter 5: Results and Discussion
119
Figure 5.9: The photodegradation of PR by TiO2 and BMO nanocomposites (c, d) under visible irradiation
Figure 5.10: Reaction kinetics of (a) PR photodegradation by various TiO2 and (b) BMO nanocomposites under UV and visible illumination
(a) (b)
(C)
0 20 40 60 80 100 120 140 160 180
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 (D)
C/C
0
Time (mins)
TiO2 CNTs-TiO2 CoO-TiO2 CNTs-CoO-TiO2
0 20 40 60 80 100 120 140 160 180
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 (B)
C/C
0
Time (mins)
TiO2 CNTs-TiO2 TiWxOy CNTs-TiWxOy
Chapter 5: Results and Discussion
120
5.5 Discussion
The degradation of the dyes (BPB and PR) by the BMOs nanocomposites metal
oxides under the visible and ultra-violet light was over 90% in 180 minutes. All the
nanocomposites with CNTs performed much better than pristine and pure binary
metal oxides. The CoO-TiO2 and TiWxOy BMOs were able to efficiently degrade
the dyes using only visible light because of their smaller band gaps (2.8 eV for -
WO3 and 2.6-2.8 eV for CoO), which meant they could be easily excited by visible
light (for reasons discussed below) and hence transfer this excitation to the TiO2.
Binary metal oxides proved to be efficient charge separators as depicted by
Figure 5.11. Charge separation was achieved as a result of the difference in the
diffusion gradient of the two metal oxides. The conduction band of metal oxide with
the large band gap acted as an electron sink, while the holes produced in its
valence band diffused to the valence band of the metal oxide with the smaller
band gap. The incorporation of CNTs in the composites significantly enhanced
photocatalytic activity because they acted as a “bridge” between the two metal
oxides, and provided a „short-cut‟ as well as an efficient transport of electrons from
(WO3 and CoO) to TiO2.
The holes that were produced were then able to react with water to produce
reactive hydroxyl radicals. These radicals were excellent oxidizing agents and
reacted with the dyes to form their innocuous by-products. The two
photogenerated species were separated efficiently by the system, which lead to
increased photocatalytic activity. On the conduction band, the electrons reacted
with adsorbed oxygen forming superoxide radicals, which were also very reactive
radicals and were able to completely mineralize the dye.
Chapter 5: Results and Discussion
121
Figure 5.11: Proposed mechanism for the binary system loaded with CNTs
The resultant [Br-] and [SO42-] ions were quantified by ion-chromatography to
ascertain the degree of degradation of the dyes. Phenol red degradation (Figure 5.12 a-d), showed a maximum sulphate ion concentration of 2.0 ppm for the
CNTs-CoO-TiO2 nanocomposite under UV light and TiWxOy under visible light in
180 minutes. This signified the complete fragmentation of the sulphate moiety in
the molecule. For bromophenol blue, (Figure 5.12 e- l) the maximum bromide ion
concentration was 4.5 ppm by CNTs-CoO-TiO2 nanocomposites under visible light
while the CNTs-Ti-WxOy nanocomposite liberated 4.1 ppm of bromide ions under
ultra-visible light in 180 minutes. The high concentration of bromide ions compared
to sulphate ions was proportional to the higher atomic ratio of bromide to sulphate
in bromophenol blue.
e- e-
h+ h+
VB VB
CB
hν
OH• dye mineralization
H2O mineralization dye O-•2
O2
Chapter 5: Results and Discussion
122
Figure 5.12 Generated sulphate ions from the degradation of PR using (a,c) ultra-violet and (b,d) visible light
0 20 40 60 80 100 120 140 160 1800.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
(A)
[SO
4]2- p
pm
Time (mins)
TiO2 CNTs-TiO2 CoO-TiO2 CNTs-CoO-TiO2
0 20 40 60 80 100 120 140 160 1800.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
(B)
[SO
4]2- p
pm
Time (mins)
TiO2 CNTs-TiO2 CoO-TiO2 CNTs-CoO-TiO2
0 20 40 60 80 100 120 140 160 1800.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
(C)
[SO
4]2- p
pm
Time (mins)
TiO2 CNTs-TiO2 TiWxOy CNTsTiWxOy
0 20 40 60 80 100 120 140 160 1800.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
(D)
[SO
4]2- p
pm
Time (mins)
TiO2 CNTs-TiO2 TiWxOy CNTsTiWxOy
Chapter 5: Results and Discussion
123
Figure 5.12: Generation of bromide and sulphate ions from BPB by TiWxOy and CNTs- TiWxOy nanocomposites, tested as photocatalysts under visible and UV light
0 20 40 60 80 100 120 140 160 1800
2
4
(E)TiWxOy (UV)
Br-
[SO4]2-
Time (mins)
[Br-
] p
pm
0
2
[SO
4 ] 2-p
pm
0 20 40 60 80 100 120 140 160 1800
2
4
(F)TiWxOy (Vis)
Br-
[SO4]2-
Time (mins)
[Br-
] p
pm
0
2
[SO
4 ] 2-p
pm
0 20 40 60 80 100 120 140 160 1800
2
4
(G)CNTsTiWxOy (UV)
Br-
[SO4]2-
Time (mins)
[Br-
] ppm
0
2
[SO
4 ] 2-ppm
0 20 40 60 80 100 120 140 160 1800
2
4
(H)CNTsTiWxOy (Vis)
Br-
[SO4]2-
Time (mins)
[Br-
] ppm
0
2
[SO
4 ] 2-ppm
Chapter 5: Results and Discussion
124
Figure 5.12: Generation of bromide and sulphate ions from BPB by TiWxOy and CNTs- TiWxOy nanocomposites, tested as photocatalysts under visible and UV light
(b) (a)
0 20 40 60 80 100 120 140 160 1800
2
4
(I)CoO-TiO2 (UV)
Br-
[SO4]2-
Time (mins)
[Br-]
ppm
0
2
[SO4 ] 2-ppm
0 20 40 60 80 100 120 140 160 1800
2
4
(J)CoO-TiO2 (Vis)
Br-
[SO4]2-
Time (mins)[B
r-] p
pm
0
2
[SO4 ] 2-ppm
0 20 40 60 80 100 120 140 160 1800
2
4
(K)CNTs-CoO-TiO2 (UV)
Br-
[SO4]2-
Time (mins)
[Br-
] ppm
0
2
[SO
4 ] 2-ppm
0 20 40 60 80 100 120 140 160 1800
2
4
(L)CNTs-CoO-TiO2 (Vis)
Br-
[SO4]2-
Time (mins)
[Br-
] ppm
0
2
[SO
4 ] 2-ppm
Chapter 5: Results and Discussion
125
REFERENCES
1 . Gopidas KR, Bohorquez M, Kamat PV, Photophysical and photochemical
aspects of coupled semiconductors. charge-transfer processes in colloidal
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2 . Vinodgopal K, Kamat PV, Enhanced rates of photocatalytic degradation of
an azo dyes using SnO2/TiO2 coupled semiconductor thin films,
Environmental Science and Technology, (1995) 29, 841 -845
3 . Fu X, Clark LA, Yang Q, Anderson MA, Enhanced photocatalytic
performance of titania – based binary metal oxides: TiO2-SiO2 and TiO2-
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4 . Fitzmaurice DF, Frei H, Rabani J, Time-resolved optical study on the
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5 . Franklyn PJ, Preparation and characterization of novel titanium-tungsten
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6 . Sing KWS, Adsorption methods for the characterization of porous materials,
Advances in Colloid Interface Science, (1998) 76-77, 3- 11
7 . Hu G, Meng X, Feng X, Ding Y, Zhang S, Yang M, Anatase TiO2
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8 . Ramis G, Busca G, Cristiani C, Lietti L, Forzatti P, Bregani F,
Characterization of tungsta-titania catalysts, Langmuir, (1992) 8, 1744 –
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9 . Chai SY, Kim YJ, Lee WI, Photocatalytic WO3/TiO2 nanoparticles working
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Morawsky AP, Maradyan VE, Obraztsova ED, Sloan J, Terekhov SV,
Zakharov DN, Double-walled carbon nanotubes fabricated by a hydrogen
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19
127
CHAPTER 6
CONCLUSIONS AND RECOMMENDATIONS
6.1 Conclusions
In this study it has been demonstrated that the photocatalytic activity of doped
titania could be improved when both visible and ultra-violet irradiation were
employed.
Synthetic methods discussed in Chapter 3, showed facile methods of preparing
anionic doped TiO2, binary metal oxides and their corresponding nanocomposites.
All samples were characterized by using various microscopic, spectroscopic and
surface area techniques to elucidate structure, morphology, size distribution and
confirm the nature of the catalyst.
The use of anionic dopants as discussed in Chapter 4, showed that fluorine and
iodine doped titania enhanced the photocatalytic activity in the UV and visible light
region respectively. The continuous generation of hydroxyl radicals in fluorine
doped titania enhanced the high degradation rate of the bromophenol blue (BPB)
and phenol red (PR) when UV light was used. This is due to the high potential of
fluoride ions (3.6 V) to shield the surface of TiO2 from being directly attacked by
photogenerated holes (h+), but at the same time to allow direct oxidation of water
molecules to produce more hydroxyl radicals.
On the other hand the photoactivity of iodine-doped titania increased when
exposed to visible light. The reason for this is that iodine-doped TiO2 was able to
absorb visible light due to its narrower band gap. The narrower band gap of iodine-
doped TiO2 could be attributed to the larger atomic size of the iodine molecule,
which could more easily be polarized by long wavelengths of light (i.e. visible light)
Conclusions and Recommendations
128
as compared to the smaller sized dopants, which could be polarized by shorter
wavelengths (UV). Once polarized, it could more easily transfer electrons to the
conduction band of TiO2.
The photoactivity of the nanocomposites of the doped TiO2 were significantly
improved when CNTs were introduced (over 90% of dyes were degraded by all the
photocatalysts). CNTs acted as efficient electron transporters. Since the doped
TiO2 nanoparticles were perfectly attached on the walls of the CNTs, the CNTs
acted as excellent mediators thereby minimizing electron-hole pair recombination.
In addition, the degradation of the dyes, for all photocatalysts tested, followed
pseudo-first-order reaction kinetics.
In Chapter 5, metal oxides were prepared in conjunction with titania (e.g. TiWxOy
and CoO), so as to minimize the recombination of photogenerated species in the
intrinsic TiO2. Here it was found that combinations of metal oxides and TiO2 also
improved their photoresponse as compared to pristine TiO2. This improved
photoresponse can be attributed to the energetically low-lying conduction band of
TiO2, which was combined with metal oxides that have small bandgaps (CoO and
WxOy) and energetically high lying conduction bands. The difference in the band
gaps of the semiconductors meant the binary metal oxides (BMOs) and their
corresponding nanocomposites were able to be excited by longer wavelengths as
compared to pristine TiO2 which was photoexcited by shorter wavelengths.
The incorporation of CNTs to the BMOs improved the photoactivity (over 95 %
degradation of BPB and PR, both under UV and visible illumination because they
acted as “bridges” between the two metal oxides and provided a „short-cut‟ or a
more efficient way to transport of electrons from (WxOy and CoO) to TiO2. This
minimized the recombination between the photogenerated holes and electrons.
Conclusions and Recommendations
129
6.2 Future outlook
Following the facile synthetic route and successful application of the
photocatalysts (doped TiO2, corresponding nanocomposites, binary metal oxides
and nanocomposites of the BMOs) in the degradation of organic dyes in the
presence of UV and visible irradiation, the following potential studies can be
generated. These include:
Performing extensive degradative pathways of the dyes by using
hyphenated-techniques such as gas chromatography – mass spectrometry
or liquid chromatography – mass spectrometry. These analytical techniques
would yield more information about the degradative fragments of the dyes.
Applying extensive iodine doped TiO2 and their nanocomposites in the
textile industries to efficiently degrade these pollutants under visible light.
This could prove very economical to the sector. Further exploration of other
metal oxides which could be coupled with TiO2 could also be undertaken.
Using these photocatalyst to degrade other selected contaminants (organic
and/or inorganic)
Examining the potential of these photocatalysts for water splitting and the
production of fuel (hydrogen).
Conclusions and Recommendations
130
APPENDIX
Photodegradation of BPB under UV light Time
(min)
CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-
Cl
CNTs-TiO2-
Br
CNTs-TiO2-I
C/C0 Error C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01
20 0.5 0.03 0.02 0.02 0.53 0.07 0.56 0.03 0.58 0.02
40 0.44 0.05 0.01 0.03 0.42 0.06 0.43 0.06 0.53 0.03
60 0.36 0.02 0.01 0.04 0.23 0.05 0.31 0.04 0.36 0.04
80 0.19 0.03 0.01 0.05 0.17 0.04 0.17 0.05 0.2 0.05
100 0.16 0.04 0.01 0.06 0.12 0.03 0.13 0.06 0.17 0.08
120 0.09 0.05 0.01 0.07 0.06 0.02 0.05 0.08 0.1 0.07
140 0.08 0.07 0.01 0.05 0.07 0.01 0.06 0.05 0.08 0.06
160 0.07 0.03 0.01 0.04 0.05 0.02 0.03 0.7 0.05 0.05
180 0.06 0.04 0.01 0.03 0.01 0.03 0.01 0.02 0.01 0.04
Conclusions and Recommendations
131
Photodegradation of BPB under visible light Time
(min)
CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-
Cl
CNTs-TiO2-
Br
CNTs-TiO2-I
C/C0 Error C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01
20 0.64 0.03 0.66 0.04 0.65 0.04 0.63 0.04 0.62 0.04
40 0.52 0.02 0.53 0.05 0.52 0.03 0.54 0.03 0.5 0.03
60 0.36 0.04 0.48 0.03 0.46 0.02 0.43 0.02 0.38 0.02
80 0.34 0.05 0.45 0.02 0.42 0.04 0.31 0.04 0.3 0.06
100 0.26 0.08 0.37 0.06 0.33 0.04 0.28 0.04 0.2 0.07
120 0.16 0.05 0.28 0.07 0.23 0.03 0.14 0.03 0.12 0.08
140 0.12 0.03 0.18 0.05 0.16 0.03 0.12 0.03 0.09 0.06
160 0.11 0.02 0.13 0.07 0.12 0.03 0.06 0.03 0.05 0.08
180 0.1 0.04 0.09 0.05 0.06 0.03 0.02 0.03 0.01 0.07
Conclusions and Recommendations
132
Photodegradation of BPB under UV light Time
(min)
TiO2 TiO2-F TiO2-Cl TiO2-Br TiO2-I
C/C0 Error C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01
20 0.72 0.01 0.5 0.04 0.64 0.02 0.64 0.03 0.64 0.04
40 0.49 0.03 0.37 0.03 0.32 0.05 0.31 0.02 0.48 0.03
60 0.38 0.02 0.25 0.02 0.26 0.06 0.28 0.04 0.3 0.02
80 0.34 0.01 0.2 0.04 0.21 0.04 0.2 0.05 0.23 0.04
100 0.27 0.02 0.12 0.04 0.19 0.09 0.18 0.08 0.2 0.04
120 0.19 0.03 0.09 0.03 0.15 0.08 0.14 0.05 0.17 0.03
140 0.16 0.03 0.01 0.03 0.13 0.05 0.11 0.03 0.15 0.03
160 0.16 0.03 0.01 0.03 0.1 0.04 0.1 0.02 0.12 0.03
180 0.16 0.13 0.01 0.03 0.05 0.01 0.05 0.04 0.06 0.03
Conclusions and Recommendations
133
Photodegradation of BPB under visible light Time
(min)
TiO2 TiO2-F TiO2-Cl TiO2-Br TiO2-I
C/C0 Error C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01
20 0.74 0.04 0.7 0.03 0.72 0.04 0.72 0.04 0.72 0.04
40 0.68 0.05 0.68 0.02 0.65 0.03 0.66 0.03 0.6 0.03
60 0.56 0.03 0.58 0.04 0.59 0.02 0.55 0.02 0.42 0.02
80 0.44 0.02 0.42 0.05 0.41 0.04 0.38 0.06 0.34 0.04
100 0.36 0.06 0.33 0.08 0.32 0.05 0.31 0.07 0.28 0.04
120 0.25 0.07 0.26 0.05 0.27 0.04 0.26 0.08 0.25 0.03
140 0.20 0.05 0.23 0.03 0.22 0.06 0.21 0.06 0.2 0.03
160 0.18 0.07 0.16 0.02 0.17 0.05 0.17 0.08 0.15 0.03
180 0.16 0.05 0.16 0.04 0.16 0.07 0.15 0.07 0.12 0.03
Conclusions and Recommendations
134
Photodegradation of PR under UV light Time
(min)
TiO2 TiO2-F TiO2-Cl TiO2-Br TiO2-I
C/C0 Error C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01
20 0.8 0.01 0.67 0.04 0.73 0.02 0.72 0.03 0.8 0.04
40 0.78 0.03 0.53 0.03 0.69 0.05 0.7 0.02 0.74 0.03
60 0.76 0.02 0.44 0.02 0.55 0.06 0.55 0.04 0.69 0.02
80 0.62 0.01 0.31 0.04 0.48 0.04 0.47 0.05 0.61 0.04
100 0.58 0.02 0.28 0.04 0.43 0.09 0.4 0.08 0.54 0.04
120 0.51 0.03 0.25 0.03 0.41 0.08 0.38 0.05 0.5 0.03
140 0.46 0.03 0.17 0.03 0.35 0.05 0.36 0.03 0.44 0.03
160 0.38 0.03 0.11 0.03 0.31 0.04 0.3 0.02 0.39 0.03
180 0.3 0.13 0.09 0.03 0.28 0.01 0.27 0.04 0.26 0.03
Conclusions and Recommendations
135
Photodegradation of PR under visible light Time
(min)
TiO2 TiO2-F TiO2-Cl TiO2-Br TiO2-I
C/C0 Error C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01
20 0.81 0.04 0.76 0.03 0.76 0.04 0.75 0.04 0.74 0.04
40 0.8 0.05 0.74 0.02 0.73 0.03 0.7 0.03 0.72 0.03
60 0.76 0.03 0.69 0.04 0.68 0.02 0.65 0.02 0.6 0.02
80 0.69 0.02 0.61 0.05 0.6 0.04 0.58 0.06 0.52 0.04
100 0.6 0.06 0.59 0.08 0.55 0.05 0.47 0.07 0.44 0.04
120 0.58 0.07 0.56 0.05 0.5 0.04 0.39 0.08 0.32 0.03
140 0.55 0.05 0.5 0.03 0.45 0.06 0.31 0.06 0.28 0.03
160 0.5 0.07 0.48 0.02 0.39 0.05 0.27 0.08 0.19 0.03
180 0.41 0.05 0.46 0.04 0.26 0.07 0.18 0.07 0.16 0.03
Conclusions and Recommendations
136
Photodegradation of PR under visible light Time
(min)
CNTS-TiO2 CNTs-TiO2-F CNTs-TiO2-
Cl
CNTs-TiO2-
Br
CNTs-TiO2-I
C/C0 Error C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01
20 0.78 0.03 0.76 0.04 0.75 0.04 0.75 0.04 0.72 0.04
40 0.76 0.02 0.7 0.05 0.66 0.03 0.65 0.03 0.62 0.03
60 0.75 0.04 0.69 0.03 0.57 0.02 0.53 0.02 0.43 0.02
80 0.6 0.05 0.65 0.02 0.46 0.04 0.44 0.04 0.33 0.06
100 0.51 0.08 0.58 0.06 0.33 0.04 0.28 0.04 0.21 0.07
120 0.47 0.05 0.51 0.07 0.22 0.03 0.2 0.03 0.18 0.08
140 0.42 0.03 0.46 0.05 0.18 0.03 0.17 0.03 0.15 0.06
160 0.34 0.02 0.38 0.07 0.15 0.03 0.11 0.03 0.09 0.08
180 0.3 0.04 0.31 0.05 0.1 0.03 0.07 0.03 0.04 0.07
Conclusions and Recommendations
137
Photodegradation of PR under UV light Time
(min)
CNTS-TiO2 CNTs-TiO2-F CNTs-TiO2-
Cl
CNTs-TiO2-
Br
CNTs-TiO2-I
C/C0 Error C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01
20 0.76 0.03 0.56 0.02 0.78 0.07 0.79 0.03 0.8 0.02
40 0.68 0.05 0.4 0.03 0.75 0.06 0.78 0.06 0.79 0.03
60 0.65 0.02 0.32 0.04 0.62 0.05 0.7 0.04 0.78 0.04
80 0.6 0.03 0.25 0.05 0.59 0.04 0.67 0.05 0.75 0.05
100 0.55 0.04 0.2 0.06 0.53 0.03 0.65 0.06 0.7 0.08
120 0.52 0.05 0.15 0.07 0.48 0.02 0.56 0.08 0.68 0.07
140 0.47 0.07 0.1 0.05 0.31 0.01 0.51 0.05 0.6 0.06
160 0.2 0.03 0.07 0.04 0.19 0.02 0.45 0.7 0.54 0.05
180 0.15 0.04 0.03 0.03 0.13 0.03 0.36 0.02 0.43 0.04
Conclusions and Recommendations
138
Sulphate production during the photodegradation of PR under UV light Tim
e
(mi
n)
TiO2 TiO2-F TiO2-I CNTs-TiO2 CNTs-
TiO2-F
CNTs-
TiO2-I
[SO4
]2-
Err
or
[SO4
]2-
Err
or
[SO4
]2-
Err
or
[SO4
]2-
Err
or
[SO4
]2-
Err
or
[SO4
]2-
Err
or
0 0 0 0 0 0 0 0 0 0 0 0 0
20
0.2
0.0
1 0.3
0.0
4 0.1
0.0
4 0.1
0.0
3 0.2
0.0
2 0.1
0.0
2
40
0.4
0.0
3 0.45
0.0
3 0.2
0.0
3 0.3
0.0
5 0.3
0.0
3 0.2
0.0
3
60
0.5
0.0
2 0.55
0.0
2 0.25
0.0
2 0.4
0.0
2 0.35
0.0
4 0.3
0.0
4
80
0.6
0.0
1 0.6
0.0
4 0.3
0.0
4 0.41
0.0
3 0.38
0.0
5 0.33
0.0
5
100
0.7
0.0
2 0.8
0.0
4 0.33
0.0
4 0.42
0.0
4 0.4
0.0
6 0.42
0.0
8
120
0.8
0.0
3 0.93
0.0
3 0.4
0.0
3 0.45
0.0
5 0.42
0.0
7 0.45
0.0
7
140
0.9
0.0
3 1
0.0
3 0.45
0.0
3 0.5
0.0
7 0.45
0.0
5 0.48
0.0
6
160
0.95
0.0
3 1.3
0.0
3 0.5
0.0
3 0.7
0.0
3 0.5
0.0
4 0.53
0.0
5
Conclusions and Recommendations
139
180
1
0.1
3 1.5
0.0
3 0.6
0.0
3 1
0.0
4 1
0.0
3 0.55
0.0
4
Sulphate production during the photodegradation of PR under visible light Tim
e
(mi
n)
TiO2 TiO2-F TiO2-I CNTs-TiO2 CNTs-
TiO2-F
CNTs-
TiO2-I
[SO4
]2-
Err
or
[SO4
]2-
Err
or
[SO4
]2-
Err
or
[SO4
]2-
Err
or
[SO4
]2-
Err
or
[SO4
]2-
Err
or
0 0 0 0 0 0 0 0 0 0 0 0 0
20
0.1
0.0
4 0.1
0.0
3 0.2
0.0
4 0.1
0.0
3 0.1
0.0
4 0.2
0.0
4
40
0.12
0.0
5 0.13
0.0
2 0.4
0.0
3 0.13
0.0
2 0.15
0.0
5 0.3
0.0
3
60
0.13
0.0
3 0.15
0.0
4 0.6
0.0
2 0.14
0.0
4 0.2
0.0
3 0.5
0.0
2
80
0.14
0.0
2 0.19
0.0
5 0.8
0.0
4 0.15
0.0
5 0.23
0.0
2 0.7
0.0
6
100
0.15
0.0
6 0.21
0.0
8 0.95
0.0
4 0.17
0.0
8 0.3
0.0
6 0.8
0.0
7
120
0.17
0.0
7 0.23
0.0
5 1.25
0.0
3 0.18
0.0
5 0.33
0.0
7 1
0.0
8
140 0.2
0.0
0.25
0.0
1.45
0.0
0.2
0.0
0.36
0.0
1.2
0.0
Conclusions and Recommendations
140
5 3 3 3 5 6
160
0.21
0.0
7 0.27
0.0
2 1.55
0.0
3 0.25
0.0
2 0.38
0.0
7 1.3
0.0
8
180
0.23
0.0
5 0.3
0.0
4 1.6
0.0
3 0.27
0.0
4 0.4
0.0
5 1.6
0.0
7
Conclusions and Recommendations
141
Sulphate and bromide production during the photodegradation of BPB under UV light Ti
m
e
(m
in)
TiO2
TiO2-I CNTs-TiO2
[SO
4]2-
Er
ror
[B
r]
Er
ror
[SO
4]2-
Er
ror
[
B
r]
Er
ror
[SO
4]2-
Er
ror
[B
r]-
Er
ror
[SO
4]2-
E
r
[B
r]-
E
r
0 0 0 0 0 0 0 0 0 0
0.0
1 0
0.0
1 0 0 0 0
20 1.3
0.0
4
0.
1
0.0
3 1.4
0.0
3
0.
3
0.0
4 1.3
0.0
3
0.
2
0.0
7 1.7
0.
04
0.
5
0.
04
40 1.4
0.0
3
0.
2
0.0
2 1.7
0.0
6
0.
5
0.0
3 1.5
0.0
2
0.
25
0.0
6 2.1
0.
03
0.
8
0.
03
60 1.5
0.0
2
0.
3
0.0
4 2
0.0
4
0.
9
0.0
2 1.65
0.0
4
0.
3
0.0
5 2.3
0.
02
1.
2
0.
02
80 1.6
0.0
4
0.
4
0.0
5 2.5
0.0
5
1.
2
0.0
4 1.7
0.0
5
0.
36
0.0
4 2.6
0.
06
1.
3
0.
04
10
0 1.7
0.0
4
0.
5
0.0
8 2.7
0.0
6
1.
4
0.0
5 1.83
0.0
8
0.
42
0.0
3 2.9
0.
07
1.
5
0.
04
12
0 1.9
0.0
3
0.
6
0.0
5 2.9
0.0
8
1.
5
0.0
4 1.95
0.0
5
0.
53
0.0
2 3.2
0.
08
1.
7
0.
03
14
0 2
0.0
3
0.
7
0.0
3 3
0.0
5
1.
7
0.0
6 2.3
0.0
3
0.
67
0.0
1 3.6
0.
06
1.
9
0.
03
16 2.5
0.0 0. 0.0
3.2
0.0 1. 0.0
2.9
0.0 0. 0.0
3.8
0. 2. 0.
Conclusions and Recommendations
142
0 3 95 2 7 8 5 2 8 2 08 1 03
18
0 3
0.0
3 1
0.0
4 3.1
0.0
2 2
0.0
7 3.5
0.0
4 1
0.0
3 3.9
0.
07
2.
3
0.
03
Photodegradation of BPB under UV light by BMOs Time
(min)
TiO2 CNTs-TiO2 TiWXOY CNTs-
TiWxOy
C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.06 0.82 0.08 0.82 0.07 0.82 0.06
20 0.62 0.05 0.5 0.04 0.6 0.08 0.64 0.05
40 0.51 0.06 0.44 0.09 0.55 0.06 0.49 0.07
60 0.5 0.09 0.32 0.06 0.5 0.08 0.38 0.05
80 0.44 0.08 0.17 0.07 0.37 0.04 0.24 0.08
100 0.41 0.06 0.15 0.05 0.35 0.06 0.16 0.06
120 0.39 0.05 0.12 0.04 0.21 0.07 0.1 0.08
140 0.33 0.07 0.09 0.08 0.19 0.06 0.07 0.05
160 0.25 0.02 0.08 0.03 0.17 0.07 0.05 0.07
180 0.2 0.04 0.07 0.06 0.15 0.09 0.02 0.06
Conclusions and Recommendations
143
Photodegradation of BPB under visible light by BMOs Time
(min)
TiO2 CNTs-TiO2 TiWXOY CNTs-
TiWxOy
C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.04 0.82 0.08 0.82 0.05 0.82 0.05
20 0.74 0.05 0.64 0.07 0.62 0.07 0.57 0.04
40 0.64 0.03 0.52 0.09 0.49 0.08 0.43 0.03
60 0.56 0.04 0.36 0.05 0.41 0.04 0.37 0.06
80 0.44 0.03 0.34 0.08 0.33 0.06 0.32 0.07
100 0.36 0.05 0.26 0.03 0.24 0.05 0.22 0.03
120 0.25 0.03 0.16 0.05 0.17 0.08 0.15 0.04
140 0.2 0.06 0.15 0.07 0.14 0.06 0.12 0.06
160 0.18 0.04 0.11 0.06 0.1 0.06 0.08 0.03
180 0.16 0.02 0.09 0.07 0.08 0.05 0.07 0.02
Conclusions and Recommendations
144
Photodegradation of BPB under UV light by BMOs Time
(min)
TiO2 CNTs-TiO2 CoO-TiO2 CNTs-CoO-
TiO2
C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.04 0.82 0.08 0.82 0.05 0.82 0.05
20 0.62 0.05 0.5 0.07 0.73 0.07 0.63 0.04
40 0.51 0.03 0.44 0.09 0.62 0.08 0.43 0.03
60 0.5 0.04 0.32 0.05 0.43 0.04 0.25 0.06
80 0.44 0.03 0.17 0.08 0.33 0.06 0.13 0.07
100 0.41 0.05 0.15 0.03 0.32 0.05 1 0.03
120 0.39 0.03 0.12 0.05 0.25 0.08 0.09 0.04
140 0.33 0.06 0.09 0.07 0.17 0.06 0.06 0.06
160 0.25 0.04 0.08 0.06 0.08 0.06 0.03 0.03
180 0.2 0.02 0.07 0.07 0.06 0.05 0.1 0.02
Conclusions and Recommendations
145
Photodegradation of BPB under visible light by BMOs
Time
(min)
TiO2 CNTs-TiO2 CoO-TiO2 CNTs-CoO-
TiO2
C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.06 0.82 0.08 0.82 0.07 0.82 0.06
20 0.74 0.05 0.64 0.04 0.66 0.08 0.55 0.05
40 0.64 0.06 0.52 0.09 0.5 0.06 0.43 0.07
60 0.56 0.09 0.36 0.06 0.34 0.08 0.33 0.05
80 0.44 0.08 0.34 0.07 0.3 0.04 0.27 0.08
100 0.36 0.06 0.26 0.05 0.23 0.06 0.21 0.06
120 0.25 0.05 0.16 0.04 0.17 0.07 0.18 0.08
140 0.2 0.07 0.15 0.08 0.11 0.06 0.1 0.05
160 0.18 0.02 0.11 0.03 0.09 0.07 0.08 0.07
180 0.16 0.04 0.09 0.06 0.07 0.09 0.01 0.06
Conclusions and Recommendations
146
Photodegradation of PR under UV light by BMOs
Time
(min)
TiO2 CNTs-TiO2 CoO-TiO2 CNTs-CoO-
TiO2
C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.04 0.82 0.08 0.82 0.05 0.82 0.05
20 0.8 0.05 0.72 0.07 0.72 0.07 0.63 0.04
40 0.78 0.03 0.6 0.09 0.66 0.08 0.54 0.03
60 0.76 0.04 0.42 0.05 0.55 0.04 0.43 0.06
80 0.62 0.03 0.34 0.08 0.38 0.06 0.31 0.07
100 0.58 0.05 0.28 0.03 0.31 0.05 0.28 0.03
120 0.51 0.03 0.25 0.05 0.26 0.08 0.14 0.04
140 0.46 0.06 0.2 0.07 0.21 0.06 0.12 0.06
160 0.38 0.04 0.15 0.06 0.17 0.06 0.06 0.03
180 0.36 0.02 0.12 0.07 0.09 0.05 0.02 0.02
Conclusions and Recommendations
147
Photodegradation of PR under visible light by BMOs
Time
(min)
TiO2 CNTs-TiO2 CoO-TiO2 CNTs-CoO-
TiO2
C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.06 0.82 0.08 0.82 0.07 0.82 0.06
20 0.81 0.05 0.71 0.04 0.68 0.08 0.69 0.05
40 0.76 0.06 0.68 0.09 0.65 0.06 0.63 0.07
60 0.69 0.09 0.61 0.06 0.6 0.08 0.58 0.05
80 0.6 0.08 0.53 0.07 0.55 0.04 0.52 0.08
100 0.56 0.06 0.42 0.05 0.41 0.06 0.39 0.06
120 0.55 0.05 0.38 0.04 0.32 0.07 0.28 0.08
140 0.5 0.07 0.28 0.08 0.25 0.06 0.2 0.05
160 0.46 0.02 0.2 0.03 0.18 0.07 0.16 0.07
180 0.41 0.04 0.17 0.06 0.12 0.09 0.14 0.06
Conclusions and Recommendations
148
Photodegradation of PR under UV light by BMOs Time
(min)
TiO2 CNTs-TiO2 TiWXOY CNTs-
TiWxOy
C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.04 0.82 0.08 0.82 0.05 0.82 0.05
20 0.8 0.05 0.72 0.07 0.66 0.07 0.64 0.04
40 0.78 0.03 0.6 0.09 0.53 0.08 0.31 0.03
60 0.76 0.04 0.42 0.05 0.48 0.04 0.28 0.06
80 0.62 0.03 0.34 0.08 0.45 0.06 0.2 0.07
100 0.58 0.05 0.28 0.03 0.37 0.05 0.18 0.03
120 0.51 0.03 0.25 0.05 0.28 0.08 0.14 0.04
140 0.46 0.06 0.2 0.07 0.18 0.06 0.11 0.06
160 0.38 0.04 0.15 0.06 0.13 0.06 0.1 0.03
180 0.3 0.02 0.12 0.07 0.09 0.05 0.05 0.02
Conclusions and Recommendations
149
Photodegradation of PR under visible light by BMOs Time
(min)
TiO2 CNTs-TiO2 TiWXOY CNTs-
TiWxOy
C/C0 Error C/C0 Error C/C0 Error C/C0 Error
0 0.82 0.06 0.82 0.08 0.82 0.07 0.82 0.06
20 0.81 0.05 0.71 0.04 0.72 0.08 0.72 0.05
40 0.76 0.06 0.68 0.09 0.66 0.06 0.66 0.07
60 0.69 0.09 0.61 0.06 0.65 0.08 0.55 0.05
80 0.6 0.08 0.53 0.07 0.38 0.04 0.38 0.08
100 0.58 0.06 0.42 0.05 0.31 0.06 0.28 0.06
120 0.55 0.05 0.31 0.04 0.26 0.07 0.25 0.08
140 0.5 0.07 0.28 0.08 0.21 0.06 0.21 0.05
160 0.48 0.02 0.2 0.03 0.17 0.07 0.16 0.07
180 0.46 0.04 0.17 0.06 0.15 0.09 0.1 0.06