metal oxide nanostructures for water purification · 2019-06-21 · photocatalysis processis one of...
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School of Industrial and Information Engineering
Master degree in Materials Engineering and Nanotechnologies
Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”
METAL OXIDE NANOSTRUCTURES FOR WATER
PURIFICATION Supervisor: Prof. Maria Pia Pedeferri
Co-supervisors: Prof. Maria Vittoria Diamanti Ing. Umberto Bellè
Master degree thesis of:
Hemanth Kumar Seemakurthi
Matr. 858961
Academic Year 2017/2018
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INDEX 1 INTRODUCTION………………………………………………………………………………….1
1.1 WASTEWATER TREATMENT ……………………………………………………………………………….1
1.2 NANOSTRUCTURES FOR WATER PURIFICATION ……………………………………………….. 2
1.3 PHOTOCATALYSIS ………………………………………………………………………………………………3
1.3.1 PRINCIPLES OF SEMICONDUCTOR PHOTOCATALYSIS………………………………………. 3
1.3.2 HETEROGENEOUS PHOTOCATALYSIS………………………………………………………………. 5
1.3.3 MECHANISM…………………………………………………………………………………………………… 6
1.3.4 EFFECT OF PH ………………………………………………………………………………………………… 8
1.4 TITANIUM AND TITANIUM OXIDES …………………………………………………………………… 8
1.4.1 STRUCTURAL PROPERTIES …………………………………………………………………….9
1.4.2 PHOTOCATALYTIC PROPERTIES……………………………………………………………. 10
1.5 IRON AND IRON OXIDES ………………………………………………………………………………….. 12
1.5.1 STRUCTURAL PROPERTIES ………………………………………………………………….. 13
1.5.2 PHOTOCATALYTIC PROPERTIES ………………………………………………………….. 14
1.6 TUNGSTEN AND TUNGSTEN OXIDES ……………………………………………………………….. 16
1.6.1 STRUCTURAL PROPERTIES ………………………………………………………………….. 16
1.6.2 PHOTOCATALYTIC PROPERTIES……………………………………………………………. 17
1.7 METAL OXIDE NANOSTRUCTURES……………………………………………………………………..19
1.7.1 ELECTROCHEMICAL ANODIZATION AND SELF-ORGANIZATION …………….20
2 METHODOLOGY ……………………………………………………………………………….23
2.1 SURFACE TREATMENTS …………………………………………………………………………………… 23
2.2 SAMPLES PRODUCTION ……………………………………………………………………………………24
2.2.1 TITANIUM ……………………………………………………………………………………………………….24
2.2.2 IRON …………………………………………………………………………………………………………….. 24
2.2.3 TUNGSTEN …………………………………………………………………………………………………….. 25
2.2.4 ANNEALING …………………………………………………………………………………………………… 26
2.3 NANOSTRUCTURES CHARACTERIZATION TECHNIQUES …………………………………….27
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2.3.1 SCANNING ELECTRON MICROSCOPY (SEM) …………………………………………………… 27
2.3.2 X-RAY DIFFRACTION (XRD) …………………………………………………………………………….. 28
2.4 PHOTOCATALYTIC ACTIVITY …………………………………………………………………………….. 29
3 RESULTS …………………………………………………………………………………………..33
3.1 MICROSTRUCTURAL ANALYSIS OF TITANIUM OXIDE NANOTUBES……………………33
3.2 MICROSTRUCTURAL ANALYSIS OF IRON OXIDE NANOTUBES …………………………… 35
3.3 MICROSTRUCTURAL ANALYSIS OF TUNGSTEN OXIDE NANOTUBES………………….. 40
3.4 ADSORPTION TESTS AND PHOTOLYSIS TESTS ………………………………………………….. 42
3.5 PHOTOCATALYSIS TESTS ON TITANIUM OXIDE NANOTUBES …………………………… 44
3.5.1 COMPARISON OF REACTION RATES OF TITANIUM OXIDE SAMPLES……………… 47
3.6 PHOTOCATALYSIS TESTS ON IRON OXIDE NANOTUBES …………………………………… 48
3.6.1 COMPARISON OF REACTION RATES OF IRON OXIDE SAMPLES …………… 51
3.7 PHOTOCATALYSIS TESTS ON TUNGSTEN OXIDE NANOTUBES …………………………. 52
3.8 COMPARISON OF PHOTOCATALYTIC TEST PERFORMED ON DIFFERENT METAL
OXIDE SAMPLES ……………………………………………………………………………………………………. 55
4 CONCLUSION ………………………………………………………………………………….. 58
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FIGURE LIST
Figure 1.1
The basic mechanism of Semiconductor Photocatalysis ………………………………….4
Figure 1.2 The potential application of heterogenous photocatalysis …………………………….. 5
Figure 1.3 Representation of steps involved in heterogenous photocatalysis ……..…………. 7
Figure 1.4 crystalline structures of anatase (a), rutile (b) and brookite (c)………………………. 9
Figure 1.5 Band positions of semiconductors with selected redox
potentials….……………………………………………………………………………………………………11
Figure 1.6 Relative positions of various red-ox couples and work functions of various metals
relative to the band edges of TiO2……………………………………………………………………………………………………11
Figure 1.7 Overview of Iron oxide nanomaterial applications …….................................... 12
Figure 1.8 Crystalline structures of hematite (a), magnetite (b) and maghemite (c)……… 13
Figure 1.9 Energy level diagram indicating the CB and VB potentials of Titanium dioxide
and Iron oxides…………………………………………………………………………………………….. 14
Figure
1.10
Stable monoclinic WO3…………………………………………………………………………………. 16
Figure 1.11 Representation of band positions of various semiconductors at NHE…………… 17
Figure 1.12 Overview of different synthesis techniques for metal oxide nanotubes……….. 19
Figure 1.13 Schematic setup for anodization with Ti as an anode…………………………………… 20
Figure 2.1 Samples pictures of a) Titanium; b) Iron; c) Tungsten after sonication…………..23
Figure 2.2 Schematic illustration of SEM apparatus ……………………………………………………… 28
Figure 2.3 Schematic illustration of the plane diffraction in a crystal used for
Bragg’s law……………………………………………………………………………………………………. 29
Figure 2.4 Rhodamine B ………………………………………………………………………………………………...29
Figure 2.5 Calibration curve of RhB in Water solution………………………………………………….… 31
Figure 2.6 Photocatalytic degradation pathway of RhB in presence of water…………….……32
Figure 3.1 SEM image of the annealed TiO2 sample (1)…………………………………………….…….33
Figure 3.2 SEM image of the not annealed TiO2 sample…………………………………………….……34
II
Figure 3.3 XRD results of the annealed TiO2 sample ……………………………………………………..34
Figure 3.4 SEM image of the annealed iron oxide sample (400°C for 1 hour)…………………35
Figure 3.5 SEM image of the not annealed iron oxide sample ……………………………………….35
Figure 3.6 SEM image of the iron oxide sample (400°C for 45 minutes and cooled in
oven)……………………………………………………………………………………………………………..36
Figure 3.7 XRD results of the annealed iron oxide sample (400°C for 1 hour)…………………36
Figure 3.8 XRD results of the iron oxide sample (400°C for 45 minutes and cooled in
oven)……………………………………………………………………………………………………………..37
Figure 3.9 SEM image of the annealed iron oxide sample (400°C for 1 hour)………………… 37
Figure 3.10 SEM image of the not annealed iron oxide sample………………………………………..38
Figure 3.11 SEM image of the iron oxide sample (400°C for 45 minutes and cooled in
oven)……………………………………………………………………………………………………………..38
Figure 3.12 XRD results of the annealed iron oxide sample (400°C for 1 hour)…………………39
Figure 3.13 XRD results of the iron oxide sample (400°C for 45 min and cooled in the
oven)................................................................................................................ 39
Figure 3.14 SEM image of the annealed WO3 sample ……………………………………………………..40
Figure 3.15 SEM image of the not annealed WO3 sample ……………………………………………….40
Figure 3.16 XRD results of the annealed WO3 sample ……………………………………………….......41
Figure 3.17 Typical trend of absorbance during the photodegradation test ……………………43
Figure 3.18 Degradation curves of Titanium 1 sample in RhB at pH 6 and pH 11……………..45
Figure 3.19 Influence of pH on apparent rate constant …………………………………………………..45
Figure 3.20 Degradation curves of ‘Titanium 2’ samples with and without annealing ……..46
Figure 3.21 Influence of annealing on apparent rate constant ………………………………………..46
Figure 3.22 Degradation curves of ‘Titanium 1’ & ‘Titanium 2’ samples (with and without
annealing) …………………………………………………………………………………………………….47
Figure 3.23 Influence of annealing and anodization parameters on apparent rate
Constant ……………………………………………………………………………………………………… 48
Figure 3.24 Degradation curves of ‘Iron 3’ sample with oven cooling ……………………………..49
III
Figure 3.25 Degradation curves of ‘Iron 3’ sample with oven cooling and without oven
cooling(quenched)………………………………………………………………………………………...50
Figure 3.26 Influence of cooling rate on the apparent rate constant ………………………………..50
Figure 3.27 Degradation curves of ‘Iron 3’and ‘Iron 4’ samples ………………………………………..51
Figure 3.28 Influence of annealing and anodization parameters on apparent rate
Constant…………………………………………………………………………………………………………51
Figure 3.29 Degradation curves of ‘Tungsten 2’ samples with and without annealing……….53
Figure 3.30 Influence of annealing on apparent rate constant ………………………………………… 53
Figure 3.31 Degradation curves of ‘Tungsten 2’ on the same and different samples…………54
Figure 3.32 Influence of reuse/repeatability on apparent rate constant…………………………..55
Figure 3.33 Apparent rate constants of different samples in the degradation of RhB in
pH 6………………………………………………………………………………………………………………56
Figure 3.34 Apparent rate constants of different samples in the degradation of RhB in pH
11…………………………………………………………………………………………………………………..56
IV
TABLE LIST Table 1.1 Merits and Demerits of current methods for wastewater treatment …..……………… 1 Table 1.2 Some physical properties of TiO2 polymorphs …..…………………………………………………10 Table 1.3 Some physical properties of Iron oxides……………………………………………………………….13 Table 1.4 Iron oxides crystalline states at different temperatures……………………………………….13
Table 1.5 Historic development of oxide nanotubes by anodization…………………………………….21 Table 2.1 Anodizing and annealing conditions of different metal oxide nanotubular
samples………………………………………………………………………………………………………………..26 Table 3.1 Names of different samples used for photocatalysis…………………………………………….41 Table 3.2 Absorbance values of different solutions for adsorption tests in the dark…………….42 Table 3.3 Absorbance values of different solutions for photolysis test…………………………………42
ABSTRACT
The report made by UN-WATER states that global water use has increased by a factor of six
over the past 100 years and continues to grow steadily at a rate of about 1 % per year. Every
day more than 2 million tons of sewage, industrial and agricultural waste are discharged into
worlds water. But no more than 30% of the water is properly treated. More than 80% of the
industrial wastewater is released into rivers without proper purification in developing
countries. There is a need to develop sustainable water purification techniques to satisfy the
growing water needs of people by eliminating the dangerous and toxic by-products with
minimal energy consumption.
Photocatalysis process is one of such techniques showing promising results. Photocatalysis
consists of the oxidation of harmful compounds present in water or air into carbon dioxide
and water, through photo-driven activation of a semiconducting material. Titanium dioxide is
one of the most used semiconductors because of its availability and intrinsic properties. Due
to the advancement in the nanotechnology, it is possible to increase the surface area of these
semiconductors by creating nanotubular structures.
The use of titanium dioxide is not completely successful due to its inability to work in the
visible spectrum, so we can only exploit most of its photocatalytic properties in the UV
spectrum, which is a very small portion in the solar light we are receiving. Tungsten oxide and
iron oxide seems to be a successful alternative because of their lower bandgap compared to
the titanium dioxide one.
This thesis work is primarily focused on the synthesis of iron oxide and tungsten oxide
nanotubes and comparing the photocatalytic reaction rates of these metal oxides with
titanium dioxide ones.
Chapter 1 introduces the topics on different wastewater treatment techniques with their
advantages and limitations, introduction to the photocatalysis and synthesis of metal oxide
nanotubes by electrochemical anodization.
Chapter 2 deals with the structural and photocatalytic properties of titanium oxide, iron oxide
and tungsten oxide along with their anodic and annealing parameters for the synthesis of
nanotubes.
In chapter 3, the experimental methodology is described for the preparation of samples and
photocatalytic tests performed by dye discolouration through absorbance variations. The
absorbance data is then converted to dye concentration by using the absorbance-
concentration linear correlation provided by beer Lambert law. Tests are performed to study
the effect of pH (= 6, 11) on the dye degradation rate.
All the results of experimental investigations like structural characterization techniques (SEM
& XRD) and numerical data of photocatalytic tests are collected in chapter 4, along with the
comparison of reaction rate during photocatalysis of these metal oxide nanotubes and with
some suggestions for future analyses.
ABSTRACT
Il rapporto di UN-WATER afferma che l'uso globale di acqua è aumentato di un fattore sei
negli ultimi 100 anni e continua a crescere costantemente ad un tasso di circa l'1 % all'anno.
Ogni giorno più di 2 milioni di tonnellate di acque reflue, rifiuti industriali e agricoli vengono
scaricati nell'acqua del mondo. Ma non più del 30 % dell'acqua viene trattata adeguatamente.
Oltre l'80 % delle acque reflue industriali viene rilasciato nei fiumi senza un'adeguata
purificazione nei paesi in via di sviluppo. È necessario sviluppare tecniche di purificazione
dell'acqua sostenibili per soddisfare il crescente fabbisogno idrico delle persone, eliminando
i sottoprodotti pericolosi e tossici con un consumo energetico minimo.
Il processo di fotocatalisi è una di queste tecniche che mostra risultati promettenti. La
fotocatalisi consiste nell'ossidazione di composti nocivi presenti nell'acqua o nell'aria in
anidride carbonica e acqua, attraverso l'attivazione foto-guidata di un materiale
semiconduttore. Il biossido di titanio è uno dei semiconduttori più utilizzati a causa della sua
disponibilità e delle sue proprietà intrinseche. A causa del progresso nella nanotecnologia, è
possibile aumentare la superficie di questi semiconduttori creando strutture nanotubulari.
L'uso del biossido di titanio non ha completamente successo a causa della sua incapacità di
lavorare nello spettro visibile, quindi possiamo sfruttare solo la maggior parte delle sue
proprietà fotocatalitiche nello spettro UV, che è una porzione molto piccola della luce solare
che stiamo ricevendo. L'ossido di tungsteno e l'ossido di ferro sembrano essere un'alternativa
di successo a causa del loro bandgap inferiore rispetto a quello del biossido di titanio.
Questo lavoro di tesi si concentra principalmente sulla sintesi di nanotubi di ossido di ferro e
ossido di tungsteno e confronta le velocità di reazione fotocatalitiche di questi ossidi di
metallo con quelle di biossido di titanio.
Il capitolo 1 introduce gli argomenti relativi alle diverse tecniche di trattamento delle acque
reflue con i loro vantaggi e limiti, l'introduzione alla fotocatalisi e alla sintesi di nanotubi di
ossido di metallo mediante anodizzazione elettrochimica.
Il capitolo 2 riguarda le proprietà strutturali e fotocatalitiche dell'ossido di titanio, dell'ossido
di ferro e dell'ossido di tungsteno insieme ai loro parametri anodici e di ricottura per la sintesi
dei nanotubi.
Nel capitolo 3, la metodologia sperimentale è descritta per la preparazione di campioni e test
fotocatalitici eseguiti mediante colorazione del colorante attraverso variazioni di assorbanza.
I dati di assorbanza vengono quindi convertiti in concentrazione di colorante utilizzando la
correlazione lineare di concentrazione di assorbanza fornita dalla legge Lambert della birra. I
test vengono eseguiti per studiare l'effetto del pH (= 6, 11) sulla velocità di degradazione del
colorante.
Tutti i risultati di indagini sperimentali come le tecniche di caratterizzazione strutturale (SEM
e XRD) e i dati numerici dei test fotocatalitici sono raccolti nel capitolo 4, insieme al confronto
della velocità di reazione durante la fotocatalisi di questi nanotubi di ossido di metallo e con
alcuni suggerimenti per analisi future.
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1. INTRODUCTION
1.1 Wastewater Treatment
One of the major challenges the world is facing today is the availability of fresh and
clean water which is a primary requirement for drinking and industries like food,
pharmaceuticals, electronics. According to the report of the United Nations’ World Water
Development 2018, the clean water demand is expected to increase by almost one third by
2050. It was estimated that by 2025 around 50% (WHO,2014) of the population will be living
in water-stressed areas. Therefore, it is necessary to develop new methods to meet the
demands for the purification of polluted water as the current infrastructure shows difficulties
to keep pace with the growing demand for clean water. A low cost and high-efficiency water
treatment technology is an immediate requirement.
Polluted water comprises suspended solids, coliforms, soluble refractory organic
compounds, toxic materials and carcinogenic substances which are expensive to treat.
Common methods available for wastewater treatment are listed in Table 1.1.
Table 1.1 Merits and Demerits of current methods for wastewater treatment [1]
2
Therefore, one can say that the methods that are currently available for water treatment
possess major disadvantages. A more economical and effective wastewater treatment
technology is needed, one such technique is Advanced Oxidation Technology (AOT) which
uses increased oxidation power towards organic pollutants [2].
Hydroxyl radical (OH•) which have strong oxidation potential (compared to other oxidants
that are normally used for oxidation of water) can be produced by Advanced Oxidation
Processes (AOP) which can help decompose organic pollutants to CO2 and H2O. Ozonation,
Fenton technology, photochemical techniques are some of the techniques that are used in
AOP [3,4,5,6,7,8].
1.2 Nanostructures for Water Purification
Ongoing advances in the control of nanomaterials have encouraged the utilization of
nanotechnology in water and wastewater treatment. In the previous decades, water
nanotechnology has gotten adequate consideration as a potential enhancement to the
customary treatment techniques. Nanomaterials are commonly defined as materials in which
at least one dimension is smaller than 100nanometers [9]. At such a small scale, materials
frequently show exceptional physical or chemical properties over their massive counterparts.
For instance, nanomaterials have a huge number of active sites per unit mass because of their
bigger specific area. Furthermore, nanomaterials have higher surface reactivity due to their
more prominent surface free energy.
As of recently, various examinations have demonstrated that nanomaterials have the
immense capacity and potential in water and wastewater treatment, specifically in the zones
of adsorption [10], membrane process [11], disinfection, catalytic oxidation and sensing [12].
It is a pity that the vast majority of the nanomaterials are still in the phase of laboratory
evaluation or only a proof of the idea. One of the accessible nanotechnologies is the utilization
of zero-valent iron nanoparticles by infusion [13, 10]. Be that as it may, there are as yet
characteristic disservices for direct utilization of free nanoparticles in water and wastewater
treatment process. Firstly, nanoparticles will, in general, tend to aggregate in the fluidized
framework or in a fixed bed, bringing about extreme activity loss and pressure drop [14].
Secondly, it is as yet a testing assignment to recover the vast majority of the depleted
nanoparticles (except for magnetic nanoparticles) from the treated water for reuse [15,16].
3
Thirdly, the conduct and destiny of the nanomaterials in water and wastewater treatment
process are not completely comprehended, and the effect of nanomaterials on the aquatic
environment and human wellbeing is a noteworthy issue that could upset the utilization of
nanotechnology [17,18].
To evade or relieve the potential adverse effect brought by the utilization of
nanotechnology, it is attractive to create material or a device that could limit the release of
nanomaterials while keeping up their high reactivity. The generation of metal-oxide
nanotubes on the surface of valve metals is proved to be an effective and promising approach.
The nanotubes formed on the surface of these valve metals (semiconductors) can possibly
degrade the contaminants and microorganisms from the water and wastewater, through a
process called heterogeneous photocatalysis. Heterogeneous photocatalysis uses photon
energy and converts it into chemical energy and offers extraordinary potential for numerous
applications particularly water and wastewater treatment. This method is extremely powerful
in degrading a wide range of organic pollutants, in the long run mineralizing them into carbon
dioxide and water [19]. Hence, in contrast to other water treatment methods, for example,
coagulation and flocculation, heterogeneous photocatalysis totally remove the contaminants,
instead of changing them from one stage to another.
1.3 Photocatalysis
1.3.1 Principles of Semiconductor Photocatalysis
From the perspective of semiconductor photochemistry, the role of photocatalysis is
to start or accelerate the particular reduction and oxidation reactions in the presence of
illuminated semiconductors. When the energy of incident photons exceeds/matches the
bandgap, the photoexcitation of electron-hole pair takes place [20,21,22]. The chemical
potential of conduction band electrons in semiconductors is +0.5 to -1.5 V versus the normal
hydrogen electrode (NHE), so they can act as reducing agents. The valence band holes have a
high oxidation potential of +1.0 to +3.5 V versus (NHE) [20]. In semiconductors, the energy
of incident photons is amassed by photoexcitation and then by series of surface reactions and
electronic processes it is converted into chemical energy.
4
Generally, a semiconductor photocatalytic cycle contains three stages (Figure 1.1):
First, the transition of electrons from valence band to conduction band by irradiation, thereby
leaving an equal number of holes in the valence band; in the second step these excited
electrons and holes migrate to the surface and in the third step these electrons and holes
react with the absorbed electron donors and electron acceptors respectively.
During the second step, a high amount of electron-hole pair recombination takes place
and the input energy is dissipated in the form of heat or emitted light. This can be prevented
by inputting the co-catalysts such as Pt, Pd and NiO on the surface of the semiconductor. The
formed hetero-junction provides an internal electric field, facilitates the electron-hole pair
separation and also provides a faster carrier migration.
Figure 1.1 The basic mechanism of Semiconductor Photocatalysis [23]
5
1.3.2 Heterogeneous Photocatalysis
The photocatalysis with metal oxides is called heterogeneous photocatalysis because
of the different physical states of catalyst and reactants. The metal oxide substrate which acts
as a catalyst is in the solid state and the pollutants are generally in the liquid or gaseous phase.
Commonly used heterogenous photocatalysts are valve metal oxides and semiconductors as
they have distinctive properties because of their band gap. The most appealing features of
this process include [24]
• The contaminants are completely degraded into CO2, H2O and other inorganic
substances.
• The reaction takes place at room temperature and atmospheric pressure.
• The primary requirements for the reaction to start are the availability of
oxygen and ultra-bandgap energy and they are obtained directly from the sun
and air.
• The catalyst is non-expensive, non-toxic and long-lasting.
Figure 1.2 The potential application of heterogenous photocatalysis [25].
6
1.3.3 Mechanism
The basic heterogenous photocatalysis mechanism is explained below (Figure 1.3).
As above mentioned, when the photons with energy (=hν) equal to or greater than the
bandgap of the photocatalyst hit the photocatalyst surface then the transfer of electrons from
valence band to conduction band takes place and leaving a hole in the valance band.
Substrate + hν h+(VB) + e-(CB) (1)
Some proportion of these electrons and holes recombine and the energy is released as heat.
h+(VB) + e-(CB) Heat (energy) (2)
Highly unstable and powerful radicals are formed as the result of the reaction between the
photo-generated electrons, holes with the available oxidants and reductants respectively.
H2O + h+(VB) OH• + H+ (3)
O2 + e-(CB) O2• - (Superoxide radical species) (4)
The generated hydroxyl and superoxide radicals further react with the contaminants and
mineralize them to carbon dioxide and water. A number of intermediate species are formed
during the reaction process.
OH• + contaminants intermediates H2O + CO2 (5)
O2• - + contaminants Intermediates H2O + CO2 (6)
7
Figure 1.3 Representation of steps involved in heterogenous photocatalysis [26].
The electron scavenging by oxygen is observed from the reaction equation (4). In the absence
of oxygen, the photo-excited electron in the conduction band recombines with the hole in the
valence band and dissipates the absorbed photon energy in the form of heat, which can be
seen from the reaction equation (2). Thus, the presence of oxygen is very important for the
photocatalytic reaction. Oxygen when reacted with the excited electron it forms high reactive
species such as superoxide radical or the singlet oxygen [27].
Finally, we can summarize the heterogeneous photocatalysis into 5 independent steps:
1. Mass transportation of contaminants in the liquid phase to the substrate surface.
2. Adsorption of contaminants on the activated substrate surface.
3. Photocatalysis reaction of the adsorbed phase on the substrate surface.
4. Desorption of the intermediate products and the final products from the substrate
surface.
5. Mass transfer of the intermediate and final products into the liquid phase [28].
8
The rate of reaction depends on the reaction rate of the slowest step. The mass transfer rates
are very fast compared to the rates of adsorption, photocatalysis and desorption. The overall
rate of reaction is limited by the steps 2,3 and 5 [29].
The use of nanomaterials as photocatalysts is increasing recently and most of the current
research is focused in this area. The efficiency of nanomaterials must be improved to meet
the engineering requirements. It is very important to detect and design novel semiconductor
materials that are firm, ample and efficient.
1.3.4 Effect of pH
In aqueous environments, the photocatalytic reactions have several pathways depending on
the pH.
The photocatalytic reactions can change the surface pH over time because of the alkaline
environment which is induced by the reduction of oxygen or H+ when an electron is ejected
from the conduction band. The main aim of photocatalytic reactions is to oxidize the organic
pollutants by photo-holes. This depends on the ability of pollutants to be specifically adsorbed
onto the surface of the photocatalyst or not. Many organic compounds containing
carboxylate groups are strongly adsorbed on the photocatalyst surface at sufficiently low pH
values. And thus, we have a higher probability for oxidation by direct proton transfer.
In this report, we discuss the metal oxide nanotubes for heterogenous photocatalysis that are
formed by electrochemical anodization of titanium, iron and tungsten.
1.4 Titanium and Titanium Oxides
Titanium is one of the most abundant metals (exceeded only by Al, Fe and Mg). It was
discovered in 1971 in England by Reverend William Gregor and after few years it was
rediscovered by German chemist Heinrich Klaproth in rutile ore who named it after Titans.
Titanium dioxide has fascinated the attention of the scientific world thanks to the wide range
of applications in which it can be used. TiO2 is a transition metal oxide and is widely studied
with more than 40000 publications over the past decade. TiO2 received that consideration
because of its chemical inertness, non-toxicity, low cost and other beneficial properties. For
9
example, it is used as an anti-reflection coating in silicon solar cells because of its high
refractive index [30]. It is used as a gas sensor because its electrical conductivity depends on
the gas composition [31,32,33]. It is also used as a biomaterial (bone substituent) [34-41].
The nanostructured TiO2 is expansively considered in the field of solar cells,
photochemical and photophysical applications like photolysis of water, photodegradation of
pollutants, specific catalytic reactions and photo-induced superhydrophilicity.
1.4.1 Structural Properties
TiO2 exhibits three polymorphs: anatase (tetragonal), rutile (tetragonal), brookite
(orthorhombic)
Figure 1.4 crystalline structures of anatase (a), rutile (b) and brookite (c)
The thermodynamic calculations expect that rutile is the most stable phase at all
temperatures and the small differences in the Gibbs free energy (4-20 kJ/mole) between the
anatase, TiO2 (B) and brookite propose that the metastable states are almost as stable as
rutile at normal temperatures and pressures. Rutile is investigated as a dielectric gate material
for MOSFET because it has a high dielectric constant (> 100) [42,43]. Anatase structure is
favoured over the rutile structure for solar cell and photocatalytic applications because of its
higher electron mobility, lower density and lower dielectric constant. Brookite is the rarest
form and cannot be produced easily.
10
Some of the key properties of rutile brookite and anatase are listed in table 1.2
Table 1.2 some physical properties of TiO2 polymorphs [44]
Rutile is the densest phase and has a high refractive index, anatase has the widest band gap
with low density and refractive index. The properties of brookite are in between of anatase
and rutile [44].
1.4.2 Photocatalytic properties
A photocatalyst is considered by its ability to adsorb reactants which can be reduced
and oxidized at the same time by a photonic activation. The capability of a semiconductor to
undergo photoinduced electron transfer to an adsorbed particle is ruled by the energy
positions of the semiconductor and the redox potentials of the adsorbates. The energy level
at the bottom of conduction band (CB) is the reducing ability of photo-electrons, whereas the
energy level at the top of the valence band (VB) governs the oxidation potential of the photo-
holes. The energy values reflect the ability of the photocatalyst to endorse oxidation and
reduction reactions. For a semiconductor to be an effective photocatalyst, the energy position
of the conduction band must be more negative than the potential of a single electron to
reduce oxygen. The consumption of electrons is necessary to prevent the electron-hole
recombination. This allows the holes in the valence band to react with the pollutants and
oxidize them to carbon dioxide and water.
11
Figure 1.5 Band positions of semiconductors with selected redox potentials [45]
Titanium dioxide has different band structures with the different band gaps for anatase, 3.2
eV and for rutile, it is 3.02 eV. The band gap is suitable for the degradation of pollutants. TiO2
can absorb the UV – A radiation for activation of photocatalytic behaviour. TiO2 is capable to
absorb only 5 to 8% of solar energy, which is the percentage UV portion in the sunlight.
Current research is focused on developing the methods and techniques that can utilize a
larger portion of solar energy.
Figure 1.6 relative positions of various red-ox couples and work functions of various metals
relative to the band edges of TiO2 [46]
12
1.5 Iron and Iron oxides
Iron is the second most abundant element available in the earth crust exceed only by
aluminium. The chemical name of iron is Ferrum and is denoted by Fe. Ferrum is the Latin
word meaning ‘firmness’. The date of discovery is still unknown but is approximately 3500 BC,
one of the most ancient metal. Iron oxide occurs in various forms in nature. The most common
forms are magnetite (Fe3O4), maghemite(ϒ-Fe2O3) and hematite (α-Fe2O3). Recently the
synthesis and use of iron oxide nanomaterials have been widely investigated because of their
nano-range superparamagnetism and very high surface availability.
Hematite is investigated in applications like catalysis, gas sensors, supercapacitors,
environmental remediation and photocatalysis because of its low bandgap, low cost,
availability, eco-friendliness and bio-compatibility. In addition, it is being used as electrode
for lithium-ion batteries, storage media and spin electronics. While the magnetite is used
mainly in the medical field for drug delivery because of its magnetic property. These
nanoparticles are also used in bioseparation [47].
Figure 1.7 overview of Iron oxide nanomaterial applications [47].
13
1.5.1 Structural Properties
Iron oxide is available in different oxide states; hematite, magnetite and maghemite
are the more prominent oxides.
Figure 1.8 crystalline structures of hematite (a), magnetite (b) and maghemite (c)
Table 1.3 some physical properties of Iron oxides
The thermodynamic calculations expect that magnetite is the most stable state at all
temperatures. In some literature, it is mentioned that by high-temperature annealing we
obtain only hematite.
Table 1.4 Iron oxides crystalline states at different temperatures [48]
Bulk Crystal system Band gap (eV)
hematite Rhombohedral 2.1 - 2.3
Magnetite Cubic 0.1
Maghemite Cubic / Tetragonal 2.0
14
1.5.2 Photocatalytic properties
Figure 1.9 Energy level diagram indicating the CB and VB potentials of Titanium dioxide and
Iron oxides
Hematite (α-Fe2O3) possesses a suitable band gap of (2.1 to 2.3 eV) and the n-type
semiconducting property makes it suitable for the photocatalysis and photoelectrocatalysis
applications. As its band gap is less than that of TiO2, it can able to absorb light up to 600 nm,
which collects about 40% of the solar energy. However, there are some limitations for using
hematite (α-Fe2O3), such as poor electron mobility, which results in the high electron-hole
recombination rate resulting in the decreased photocatalytic efficiency. The hole diffusion
length of the hematite (α-Fe2O3) is small (2-4 nm) which further limits its application in
photocatalysis. But recent advances in nanostructuring proved that there is an increase in the
photoresponse for hematite (α-Fe2O3). The nanotubular structure has proven effective
because of the increased surface area and reduced free electron scattering.
Literature review on iron oxide nanotubes
A literature review on iron oxide nanotubes states different anodization conditions for
obtaining nanotubes. Each and every parameter in the anodization process strongly effects
the morphology of the nanotubes mainly anodization voltage, anodization time and the
electrolyte composition- the amount of water, amount of fluorine. Bianca et al., 2018 [49]
15
said that iron oxide nanostructures were obtained with anodizing iron rods in ethylene glycol
solution containing 0.1 M ammonium fluoride and 3 % vol. of water for 15 mins at 50 V. Jin
Kim et al., 2018 [50] mentioned in their paper that iron oxide nanostructures were obtained
with anodizing iron foils in ethylene glycol containing 0.25 wt.% ammonium fluoride 2 vol.%
water for 5 mins at 50 V. Yuyun Yang et al., 2017 [51] says that iron oxide nanostructure arrays
were obtained in 150 s when anodized at 60 V with ethylene glycol solution containing 0.8
wt.% ammonium fluoride and 2 M water. Raghu R. Rangaraju et al., 2009 [52] said that iron
nanotubes with a uniform diameter of 85-100 nm were obtained with iron foils anodized at
50 V for 15 mins in ethylene glycol solution containing 0.1 M ammonium fluoride and 3 vol.%
water. Keyu Xie et al., 2014[53] said that highly ordered iron oxide nanotubes were obtained
when anodized in ethylene glycol solution with 0.5 wt.% ammonium fluoride and 3.0 wt.%
water for 300 s at 50 V. Anna Pawlik et al., 2017[54] in their paper mentioned that iron oxide
nanostructures were obtained in ethylene glycol solution containing 0.2 M ammonium
fluoride and 0.5 M water when anodized at 40 V for 1 hour. Zhonghai Zhang et al., 2009 [55]
said that self-assembled hematite nanotube arrays were obtained when anodized in ethylene
glycol solution containing 0.3 wt.% ammonium fluoride and 2 vol.% water for 5 mins at 50 V
followed by sonication in water for 10 mins and then again anodizing in same conditions.
16
1.6 Tungsten and Tungsten oxides
Tungsten is one of the rare elements that can be found in the earth crust. Tungsten
never occurs as a free element. The most common ores are scheelite and wolframite. The
chemical name of tungsten is Wolfram and is denoted with the symbol W. It was first
identified as a novel element in 1781 and isolated as a metal in 1783. Nanostructures of
tungsten are gaining more attention recently because of their ability to perform
photocatalysis in the visible spectrum.
Tungsten oxide is a multipurpose metal oxide with a vast number of polymorphs, sub-
stoichiometric compositions. Tungsten oxide is a striking candidate for optoelectronic
applications, electrochromism, photochromism, photocatalysis and photothermal therapy.
1.6.1 Structural properties
Tungsten oxide (WOx) is a series of complicated materials with complex polymorphism. which
makes it a wide platform for both theoretical and application studies. The ideal crystalline
structure of WO3 is cubic. However, because of its instability, cubic WO3 is never observed
experimentally. Various polymorphs of WO3 are observed. Depending on the tilt pattern of
lattice five different polymorphs of WO3 are recognized including monoclinic WO3, triclinic
WO3, monoclinic WO3, orthorhombic WO3 and tetragonal WO3. At room temperature, WO3
shows mixed polymorphs of monoclinic WO3 and triclinic WO3.
The amorphous WO3 has a large band gap of 3.4 eV, while the band gap of the monoclinic
crystalline structure is 2.5 to 2.8 eV.
Figure 1.10 Stable monoclinic WO3 [57]
17
1.6.2 Photocatalytic Properties
For a semiconductor to be an effective photocatalyst, the energy position of the conduction
band must be more negative than the potential of a single electron to reduce oxygen. The
consumption of electrons is necessary to prevent the electron-hole recombination. This
allows the holes in the valence band to react with the pollutants and oxidize them to carbon
dioxide and water.
Figure 1.11 Representation of band positions of various semiconductors at NHE
Tungsten oxide has been developed as a promising photocatalytic material because of its
ability to capture approximately 12% of the solar spectrum (visible spectrum up to 500
nanometers). WO3 is an n-type semiconductor with an indirect band gap of 2.5 to 2.8 eV. The
maximum theoretical conversion efficiency of tungsten oxide is approximately 6.3% for the
photons with energies higher than 2.6 eV [56]. Tungsten oxide also has a moderate hole
diffusion length of 150 nm which is higher than that of the iron oxide 2-4 nm and titanium
oxide 100 nm. WO3 can reduce the oxygen and hence it is good photocatalytic material.
Literature review on tungsten oxide nanotubes
A literature review on tungsten oxide nanotubes states different anodization conditions for
obtaining nanotubes. Each and every parameter in the anodization process strongly effects
the morphology of the nanotubes mainly anodization voltage, anodization time and the
18
electrolyte composition- the amount of water, amount of fluorine. Chin Wei Lai et al., 2013
[58] said that tungsten oxide nanotubes were synthesized in an aqueous electrolyte
containing 1 M sodium sulphate and 0.5 wt.% ammonium fluoride when anodized for 15 mins
at 40 V. Chin et al., 2016 [59] mentioned that tungsten oxide nanotubes were obtained when
anodized in aqueous electrolyte (pH 3) consisted of 1 M sodium sulphate and 0.7 wt.%
ammonium fluoride when anodized for 15 mins at 40 V. R.M. Fernandez-Domene et al., 2018
[60] mentioned that tungsten oxide nanoplatelets/nanosheets shows better
photoelectrodegradation when anodized in aqueous electrolyte containing 1.5 M sulfuric acid
and 0.005 M hydrogen peroxide.
19
1.7 Metal Oxide Nanostructures
Since the discovery of carbon nanotubes, the field of nanotechnology has become popular
and interesting because of the unique properties exhibited by the material at the nanoscale
such as high electron mobility, quantum confinement, high surface area to volume ratio and
very high mechanical strength. The nanostructures have provided us a range for improvement
in the fields of bio-medical, photochemical, electrical and electronics.
The use of nanostructures has been rapidly increasing in the past decade, so the synthesis of
these nanostructures also gains a lot of attention. There are various techniques such as sol-
gel, template-assisted, hydro/solvothermal and electrochemical. Each technique has its own
advantages and drawbacks. By electrochemical anodization, we can obtain a homogenous
array of oxide nanotubes. In the following chapter, we discuss the anodization process.
Figure 1.12 Overview of different synthesis techniques for metal oxide nanotubes [64].
20
1.7.1 Electrochemical anodization and Self-Organization
Electrochemical anodization approach leads to the synthesis of a homogeneous array of oxide
nanotubes aligned perpendicular to the surface of the substrate with a well defined and
controllable tube length. The nanotubes grown by this technique are electrically connected
to the substrate surface and are easy to handle for different applications. This method allows
to grow nanotubes on any shapes of valve metals. The main advantage of this technique is it
is very low cost and its very similar to traditional electrochemical anodization but in controlled
conditions. The process parameters can deeply affect the resulting nanostructures.
Overview of the anodizing and nanotube growth process
Anodization is an electrolytic oxidation process used to generate an oxide layer on the metal
surface. This technique helps in growing the uniform layer. Here the metal to be coated is
connected as the anode in the electrical circuit and hence the name anodization is derived
from this.
Figure 1.13 schematic setup for anodization with Ti as an anode.
The anodization process can be summarized in a few reaction steps that are happening
simultaneously at the surface of the anode and cathode in the electrochemical cell.
Reactions at anode
M Mn+ + ne-
Reactions at cathode
H+ + e- H2
21
The metal ions that are released at the anode react with the OH- and O2- species provided by
water (field-assisted dissociation) to form the metal oxide and the reactions are shown below;
Mn+ + OH- M(OH)n
Mn+ + O2- MO2
M(OH)n MO2 + n/2 H2O
In the acidic environment at cathode, hydrogen reduction takes place and in neutral/alkaline
environment reduction of oxygen will take place. In real situations, the reactions are more
complicated because of the formation of hydroxides and also because of the presence of
different species in the electrolyte used for anodization.
This is the general description of the anodization process and does not allow us to grow oxide
nanotubes. In order to grow oxide nanotubes, we have to create a situation of high field oxide
formation and partial dissolution of formed oxide: these conditions are generally achieved in
the electrolyte types listed in Table 1.4
Table 1.5 Historic development of oxide nanotubes by anodization [44]
The presence of fluorine is very important in growing the nanotubes as they dissolve the
metal oxide forming by anodization by forming complex metal fluorides. For anodization, we
need to expose the metals to sufficiently high anodic voltage and time for the metal oxidation
reaction to take place. We are using the second and third generation techniques for the
synthesis of nanotubes in this report. Apart from anodic voltage and anodization time; the
amount of fluoride content, amount of water content and annealing conditions to crystallize
nanotubes play a key role in the final nanotubes formed. During the growth of the layer in an
22
electrolyte containing fluorides the current-time curve derivates from the classical high field
growth. Once reached the desired voltage and potentiostatic conditions onset, in the first
stage the current decreases exponentially as a result of the oxide layer formation. After the
initial decay, the increases again with a time lag that is shorter, the higher fluoride
concentration; the rise of current is probably caused by the formation of pores on the
substrate that increases the surface area of the electrode. The individual pores start
interfering with each other and start competing for the available current. This leads under
optimized conditions to a situation where the pores equally share the available current and
self-ordering under steady state condition is established. The current reaches a virtually
constant value reflecting the establishment of the steady-state situation between dissolution
and formation of oxide, current value increases with increasing fluoride concentration
[61,62,63]. The anodic conditions and annealing conditions differ for different metals. The
parameters for the synthesis of metal oxide nanotubes are explained in the next chapters.
23
2. METHODOLOGY
In this chapter, we discuss the materials and methods that are used in this study. The first
section describes the surface treatment method that was used in the laboratory on Ti, Fe and
W foils. The second section is about the general methods that are used either in organic or
aqueous electrolytes to perform the electrochemical anodization. The third section describes
the characterization analyses performed on the titanium, iron and tungsten sheets while the
last one discusses the measuring method of photocatalytic activity, the pollutant used and its
degradation pathways.
2.1 Surface treatments
The surface texture is an important parameter when speaking of nucleation and growth of a
new layer and so the same holds for the growth of nanotubes on the metal surface. The
surface treatment methods that were used on titanium in previous experimental works
include a polishing treatment with 800 SiC grinding paper followed by a degreasing step by
sonicating in alcohol to generate uniform site of growth on the bare titanium surface. In this
case, only the sonication in alcohol was applied, due to the high quality of surface finishing of
as-received mental sheets.
Titanium and tungsten samples are immersed in ethanol and ultrasonication treatment is
performed for five minutes, followed by rinsing with distilled water and drying with air flow.
In the case of iron, cleaning with acetone was performed before immersing it ethanol because
of the adhesive sheet present on the iron foil.
All samples were cut to a dimension of 2 cm x 2 cm from foils of different thickness, all below
0.5 mm.
Figure 2.1 Samples pictures of a) Titanium; b) Iron; c) Tungsten after sonication.
24
2.2 Samples production
2.2.1 Titanium
The anodizing process is carried out in an organic electrolyte consisting of 0.2M ammonium
fluoride (NH4F), 2M water in ethylene glycol (C2H6O2). i.e, 1.85 g of NH4F + 9 g of water +
265.45 g of EG, following conditions chosen a previous experimental work.
The anodization is performed with a custom pre-industrial anodizing plant by LTC Caoduro
working in potential control (either potential ramp or potentiostatic).
A titanium mesh is used as the cathode.
The voltage applied is 45 V in potentiodynamic conditions (voltage ramp) and ramp duration
of 2 seconds, then it is maintained constant for 30 min. with such a small ramp time we can
assume that the voltage application is instantaneous.
Once the sample is anodized, it is rinsed with distilled water and dried with air flow to remove
any trace of organic electrolyte on the sample.
In the second type of experiment, the anodizing process is carried out in an aqueous
electrolyte consisting of 0.5 wt.% ammonium fluoride (NH4F), 1M sodium sulfate anhydrous
(Na2SO4). i.e, 1.36 g of NH4F + 35.51 g of Na2SO4 + 235.48 g of H2O.
The anodization is performed with the conditions of voltage 40 v reached with a ramp 40
seconds, 15 minutes. After that, the samples are rinsed with water and dried.
2.2.2 Iron
In the first type of experiment, the anodizing process is carried out in an organic electrolyte
consisting of 0.8 wt.% ammonium fluoride (NH4F), 2M water in ethylene glycol
The anodization is performed with the conditions of voltage 60 V obtained with a ramp of 2
seconds, then maintained for 150 seconds. After that, the samples are rinsed with water and
dried.
25
In the second type of experiment, the organic electrolyte consists of 0.3 wt.% ammonium
fluoride (NH4F), 2 vol% water.
The anodization is performed with the conditions of voltage 50 V reached with a ramp of 2
seconds and maintained for 5 minutes. Once the anodization is completed the samples are
rinsed with water and sonicated with distilled water for 5 minutes, then rinsed with water
and then again sonicated for 5 minutes. This step helps removing the nanotubes that are
formed during the anodization; on the sample surface only the barrier layer formed at the
bottom of nanotubes remains. Then samples are anodized for the second time with the same
parameters of the first step to grow nanotubes. Lastly, the samples are rinsed with water and
dried.
2.2.3 Tungsten
In the first type of experiment, the anodizing process is carried out in an aqueous electrolyte
consisting of 0.5 wt.% ammonium fluoride (NH4F), 1M sodium sulfate anhydrous (Na2SO4). i.e,
1.36 g of NH4F + 35.51 g of Na2SO4 + 235.48 g of H2O.
The anodization is performed with the conditions of voltage 40 V reached with a ramp
40seconds, then maintained for 15minutes, which is analogous to the second type of
experiments used on titanium. After that, the samples are rinsed with water and dried.
In the second type of experiment, the anodizing process is carried out in an organic electrolyte
consisting of 0.2M ammonium fluoride (NH4F), 2M water in ethylene glycol (C2H6O2). i.e, 1.85
g of NH4F + 9 g of water + 265.45 g of EG.
The anodization is performed with the conditions of voltage 45 V reached with a ramp of 2
seconds, then maintained for 15 minutes. This condition is similar to the first anodizing
experiment on titanium, with a difference maintenance time which is half of that used on
titanium. After that, the samples are rinsed with water and dried.
26
2.2.4 Annealing
Annealing is the last step of sample production. It is performed in a GEFRAN 1200 oven. The
samples of titanium and tungsten are put inside in the oven when it has reached the desired
temperature and after a predetermined annealing time, the samples are taken out of the
oven and cooled down to room temperature. When it comes to annealing of iron samples,
different heating and cooling conditions were tested, like inserting the sample inside the oven
from room temperature to the desired temperature and leaving the samples inside the oven
till room temperature to reduce the kinetics of heating and cooling of samples for the better
stability of oxide layer.
Table 2.1 summarizes the sample production with the different anodization and annealing
conditions for different metals used in this study.
Table 2.1 Anodizing and annealing conditions of different metal oxide nanotubular samples.
S.No. Metal Anodizing parameters Annealing parameters
Electrolyte ramp voltage time Heating
in oven
Cooling
in oven
Temperature
&time in oven
1 Ti 0.2M NH4F,
2M H2O in EG
2s 45V 30min - - 500 ֯C, 2 hr
2 Ti 0.5wt% NH4F,
1M Na2SO4 in
H2O
40s 40V 15min - - 500 ֯C, 2 hr
3 W 0.5wt% NH4F,
1M Na2SO4 in
H2O
40s 40V 15min - - 500 ֯C, 2 hr
4 W 0.2M NH4F,
2M H2O in EG
2s 45V 15min - - 500 ֯C, 2 hr
5 Fe 0.8wt%
NH4F, 2M
H2O in EG
2s 60V 150s - - 400 ֯C, 1 hr
27
6 Fe 0.8wt%
NH4F, 2M
H2O in EG
2s 60V 150s - 400 ֯C, 45min
7 Fe 0.3wt%
NH4F, 2vol%
H2O in EG
2s 50 V for 5 min,
sonication, then
50 V for 5 min
- - 400 ֯C, 1 hr
8 Fe 0.3wt%
NH4F, 2vol%
H2O in EG
2s 50 V for 5 min,
sonication, then
50 V for 5 min
- 400 ֯C, 45min
9 Fe 0.3wt%
NH4F, 2vol%
H2O in EG
2s 50 V for 5 min,
sonication, then
50 V for 5 min
400 ֯C, 1 hr
10 Fe 0.3wt%
NH4F, 2vol%
H2O in EG
2s 50 V for 5 min,
sonication, then
50 V for 5 min
- - 500 ֯C, 2 hr
11 Fe 0.3wt%
NH4F, 2vol%
H2O in EG
2s 50 V for 5 min,
sonication, then
50 V for 5 min
- 600 ֯C, 2 hr
2.3 Nanostructures characterization techniques
The samples produced from the above step have been analysed from the perspectives of
morphology and crystalline structure with SEM and XRD analysis. The instrumentations are
described below.
2.3.1 Scanning Electron Microscopy (SEM)
SEM is an electron microscope which produces images of a sample by scanning it with a
focused beam of electrons. The information about the surface is obtained by the interactions
of electrons with the atoms of the sample, detailing the topography and composition. With
SEM we can achieve resolutions better than 1 nanometer.
Inside of an SEM microscope (Figure 2.2), the following elements can be found:
28
• an electron beam which is accelerated down the column;
• a series of lenses (condenser and objective) for controlling the diameter of the beam as
well as for focusing the beam on the specimen;
• a series of apertures (micron-scale holes in the metal film) from which the beam passes
through and which affects its properties;
• controls for specimen position (x, y, z -height) and orientation (tilt, rotation);
• an area of beam/specimen interaction that generates the types of signals that can be
detected and processed to produce an image or spectra.
Figure 2.2 Schematic illustration of SEM apparatus
The instrumentation used is a field emission scanning electron microscopy (FESEM Zeiss
Supra40) that is able to produce clearer, less electrostatically distorted images with spatial
resolution down to less than 1 nanometre.
2.3.1 X-Ray Diffraction (XRD)
X-ray diffraction spectroscopy is a tool used for identifying the atomic and molecular structure
of a crystal, in which the crystalline atoms cause a beam of incident X-rays to diffract into
many specific directions. The interaction of the incident rays with the sample produces
constructive interference (and a diffracted ray as in figure 4.3) when conditions satisfy Bragg's
law (nλ=2dsinθ).
29
Figure 2.3 Schematic illustration of the plane diffraction in a crystal used for Bragg’s law [65].
This law relates the wavelength of electromagnetic radiation to the diffraction angle and the
lattice spacing in a crystalline sample. The diffracted X-rays are then detected, processed and
counted. By scanning the sample through a range of 2θangles, all probable diffraction
directions of the lattice are attained. Conversion of the diffraction peaks to d-spacings allows
identification of the crystal phase because each crystal structure has a set of unique d-
spacings. This is achieved by comparison of d-spacings with standard reference patterns.
2.4 Photocatalytic Activity
The organic dye that is used in this work is Rhodamine B (figure 2.4).
Figure 2.4 Rhodamine B
To evaluate the photocatalytic efficiency of the metal oxide nanostructures in the degradation
of Rhodamine B, the absorbance of the organic dye is measured at regular intervals i.e, every
hour for six hours by a spectrophotometer SPECTRONIC 200.
30
The degradation of the dye is then calculated from the change in the absorbance of the
solution. This is possible because of the proportional relation between the concentration of
the pollutants and the absorbance which can be seen from the Beer-Lambert equation.
Absorbance is the direct measure of light which is absorbed by the sample and can be written
as
A = − 𝑙𝑙𝑙𝑙𝑙𝑙 𝐼𝐼𝐼𝐼𝐼𝐼
The Beer-Lambert law gives a direct relationship between the concentration of a solution and
its absorbance. The higher the concentration, the higher is the absorbance and is expressed
by the following equation
A = l * ɛ * C
Where
l = Optical path length
ɛ = Molar absorptivity of absorbing chemical species
C = Concentration of absorbing chemical species
If we consider l and ɛ are constant, we can able to plot a calibration curve for the absorbing
species that allows us to determine its concentration by measuring the absorbance.
Figure 2.5 shows the calibration curve of RhB in water solution
31
Figure 2.5 calibration curve of RhB in Water solution.
The RhB is not only degraded by the photocatalysis of metal oxide present in the solution but
also by the absorption of RhB on metal oxide surface and by photolysis. Hence we get only
apparent photocatalytic activity. The amount of the dye absorbed by the sample or adsorbed
at its surface would not appear in absorbance measure, but it is not degraded, hence it should
be subtracted from the measured absorbance decrease before converting it into
photodegradation.
To do so, an anodized sample is left immersed in the solution for six hours in the dark and the
variation of absorbance is calculated every hour.
To remove the effect of photolysis we expose the solution for six hours to UV light without
the sample and the variation of the absorbance is calculated every hour.
In this study, we have done dark test for each sample as well as photolysis test; the actual
photocatalytic activity of the samples is then measured as the variation of concentration of
solution exposed to UV light in presence of the photocatalyst, after eliminating photolysis and
absorbance contributions.
The photocatalytic degradation pathway of RhB is shown in figure 2.6
32
Figure 2.6 Photocatalytic degradation pathway of RhB in presence of water [66].
The photocatalytic tests are performed on all the samples that are mentioned in table 2.1 and
the rate of the reaction during these tests are compared and discussed in the next chapter.
33
3. RESULTS
In this chapter, we will discuss the results obtained in the laboratory. This study of metal oxide
nanostructures for water purification is divided into two parts, the first part is concentrated
mainly on the synthesis of nanotubular structures on the different metals like titanium, iron
and tungsten. The second part deals with the use of these samples in the photocatalytic
application for purification of water.
3.1 Microstructural Analysis of Titanium oxide nanotubes
Figures 3.1 and 3.2 show the SEM analysis of TiO2 nanotubes obtained in the organic
electrolyte with and without annealing treatment. From both the images, the presence of
nanotubes on the surface of the sample is visualized. The anodic conditions of the sample are
0.2M NH4F, 2M H2O in EG with a ramp of 2s, the voltage of 45V for 30 min. The annealing of
the sample is done at 500 ֯C for 2 hours. Figure 3.3 shows the XRD analysis performed on the
sample. From the XRD spectrum, we can see a high content of crystalline anatase phase which
means high potential for photocatalytic efficiency, while the presence of rutile expected from
some literature works in the bottom compact layer is not detected by the instrument.
Figure 3.1 SEM image of the annealed TiO2 sample (1)
34
Figure 3.2 SEM image of the not annealed TiO2 sample
Figure 3.3 XRD results of the annealed TiO2 sample
35
3.2 Microstructural Analysis of Iron oxide nanotubes
Figures 3.4 and 3.5 show the SEM analysis of the iron sample anodized in the organic
electrolyte with and without annealing treatment. The anodic conditions of the sample are
0.8 wt% NH4F, 2 M water in EG with a ramp of 2s, the voltage of 60V for 150 seconds. The
annealing of the sample is done at 400°C for 1 hour. From the images, the presence of
nanotubes on the surface of the annealed sample is visualized, whereas the presence of
nanotubes on the surface of the sample without annealing is not visualized because of
limitations in SEM analyses resolution. This is due to the poor conductivity of the iron oxide
coating, which limits the resolution of the imaging.
Figure 3.4 SEM image of the annealed iron oxide sample (400°C for 1 hour)
Figure 3.5 SEM image of the not annealed iron oxide sample
36
Figure 3.6 shows the SEM analysis of the sample with the same anodizing conditions but
annealed at 400°C for 45 minutes and cooled inside the oven.
Figure 3.6 SEM image of the iron oxide sample (400°C for 45 minutes and cooled in the oven)
Figures 3.7, 3.8 show the XRD analyses performed on these two samples. From the XRD
spectrum, we can see in both cases the presence of crystalline phases of hematite and
magnetite.
Figure 3.7 XRD results of the annealed iron oxide sample (400°C for 1 hour)
37
Figure 3.8 XRD results of the iron oxide sample (400°C for 45 minutes and cooled in the oven)
SEM and XRD analyses were also performed on iron samples anodized in different conditions,
specifically, 0.3 wt % NH4F + 2 vol% water in EG with a ramp of 2s, anodizing at 50 V for 5 min,
followed by ultrasonication in deionized water for 10 min, then anodized again at 50 V for 5
minutes. The annealing of the sample is done at 400°C for 1 hour.
Figures 3.9 and 3.10 show the SEM analysis of the anodized iron sample with and without
annealing treatment. From the images, the presence of nanotubes on the surface of the
annealed sample is visualized, whereas the presence of nanotubes on the surface of the
sample without annealing is difficult to visualize because of the resolution limitations
previously commented.
Figure 3.9 SEM image of the annealed iron oxide sample (400°C for 1 hour)
38
Figure 3.10 SEM image of the not annealed iron oxide sample
Figure 3.11 shows the SEM analysis of the sample with the same anodic conditions but
annealed at 400°C for 45 minutes and cooled inside the oven.
Figure 3.11 SEM image of the iron oxide sample (400°C for 45 minutes and cooled in the oven)
Figures 3.12, 3.13 show the XRD analysis performed on the samples. From the XRD spectrum,
we can see the presence of crystalline phases of hematite and magnetite.
39
Figure 3.12 XRD results of the annealed iron oxide sample (400°C for 1 hour)
Figure 3.13 XRD results of the iron oxide sample (400°C for 45 min and cooled in the oven)
Because iron oxide is not stable in neutral pH solutions it tends to dissolve and the substrate
to further corrode. In this study, the iron oxide samples are kept in water and in the alkaline
solution of pH 11 for 24 hours to check the stability of the produced oxides. In water, the
results of the test showed that iron oxide is not stable. In alkaline solution, the samples
obtained by double anodization showed better stability.
40
3.3 Microstructural Analysis of Tungsten oxide nanotubes
Figures 3.14 and 3.15 show the SEM analysis of the anodized tungsten sample with and
without annealing treatment. From both the images, the presence of nanotubes on the
surface of the sample is visualized. The anodic conditions of the sample are 0.5wt% NH4F, 1M
Na2SO4 in H2O with a ramp of 40 s, the voltage of 40V for 15 min. The annealing of the sample
is done at 500°C for 2 hours.
Figure 3.14 SEM image of the annealed WO3 sample
Figure 3.15 SEM image of the not annealed WO3 sample
41
Figure 3.16 shows the XRD analysis performed on the annealed sample. From the XRD
spectrum, we can see the presence of the crystalline phase of WO3.
Figure 3.16 XRD results of the annealed WO3 sample
To conclude this first section on different metals anodizing, the presence of nanotubes on the
metal surfaces of titanium, iron and tungsten was actually obtained. But from the SEM
images, it can be concluded that homogeneity is higher in the titanium oxide nanotubes
compared to iron. The length, diameter of nanotubes and the thickness of the oxide layer
were not analysed in details since it is beyond the scope of this study. Given the instability of
iron oxide in neutral pH solutions, tests were performed both at pH 11.
Interesting results are observed in the second part of the study, during photocatalysis of these
samples. In order to avoid confusion, the samples that are used in the photocatalysis are
named and tabulated below.
Table 3.1 Names of different samples used for photocatalysis
S.No. Name Anodization parameters
1 Titanium 1 0.2M NH4F, 2M H2O, 2s, 45V, 30min, 500°C, 2hrs
2 Titanium 2 0.5wt% NH4F, 1M Na2SO4, 40s, 40V, 15min, 500°C, 2hrs
3 Iron 3 0.8wt% NH4F, 2 M H2O, 2s, 60V, 150s, 400°C, 1hr / O.C.
42
4 Iron 4 0.3wt% NH4F+2vol% H2O, 2s, 50V, 5min, ultrasonication
10 min, again at 50 V for 5 minutes, 400°C, 1hr / O.C.
5 Tungsten 2 0.5wt% NH4F, 1M Na2SO4, 40s, 40V, 15min, 500°C, 2hrs
O.C. Oven Cooling
3.4 Adsorption tests and Photolysis tests
Table 3.2 shows the results of adsorption tests carried out with no light source applied onto
the solution, the initial absorbance is measured at the beginning of the test and the final
absorbance is recorded again after six hours.
Table 3.2 Absorbance values of different solutions for adsorption tests in the dark.
Photocatalyst Solution Initial
absorbance
Final
absorbance
Percentage
variation
Titanium 1 RhB (pH 6) 0.495 0.507 2.3
Titanium 1 RhB (pH11) 0.499 0.504 1
Titanium 2 RhB (pH 6) 0.495 0.492 0.6
Titanium 2 Not annealed RhB (pH 6) 0.482 0.477 1
Iron 3 Oven cooled RhB (pH 11) 0.483 0.490 1.4
Iron 3 RhB (pH 11) 0.487 0.489 -0.4
Iron 4 Oven cooled RhB (pH 11) 0.47 0.48 2.1
Iron 4 RhB (pH 11) 0.48 0.49 2.1
Tungsten 2 RhB (pH 6) 0.499 0.497 0.4
Tungsten 2 Not annealed RhB (pH 6) 0.491 0.486 1
Table 3.3 shows the results of the photolysis tests carried out without photocatalyst to see
the degradation of RhB under direct UV light
Table 3.3 Absorbance values of different solutions for photolysis test
Solution Initial absorbance Final absorbance Percentage variation
RhB (pH 6) 0.488 0.488 0
RhB (pH11) 0.488 0.494 1.2
43
From the data, it is evident that the UV light-induced degradation of RhB (pH6 & pH 11), as
well as dye adsorption on the surface of nanotubes is negligible.
Since data collected after six hours of photocatalysis tests (in presence of both UV and
photocatalyst) show a decrease in absorbance from data collected at zero time, it can be
deduced that all the results obtained during photocatalysis are generated from pure
photodegradation and not by adsorption phenomena & photolysis.
In order to show in a clear way photocatalytic activity, a graphical representation is used. The
typical trend of absorbance measurement during the photodegradation test is shown in figure
3.17
Figure 3.17 Typical trend of absorbance during the photodegradation test
Since the photodegradation process of Rhodamine B (RhB) follows the first order kinetics and
Beer-Lambert law directly correlates the absorbance and concentration of the solution, it is
convenient to represent the test results in absorbance vs time plots, which can then be
further processed to obtain the logarithm ratio of the current concentration (C) to the initial
concentration (C0) vs time.
In this kind of plots, the slope of the obtained line is the apparent reaction rate constant ‘k’.
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1 2 3 4 5 6 7
ABSO
RBAN
CE
TIME [h]
44
The equations below show the mathematical steps that allowed us to use this approach
The reaction rate is given by the equation:
−𝑑𝑑𝐶𝐶𝑑𝑑𝑡𝑡
= 𝑘𝑘𝑘𝑘 𝑑𝑑𝐶𝐶𝐶𝐶
= −𝑘𝑘 𝑑𝑑𝑡𝑡
By integrating we get
� 1𝐶𝐶𝑑𝑑𝑘𝑘
𝐶𝐶
𝐶𝐶0= −𝑘𝑘 ∫ 𝑑𝑑𝑡𝑡𝑡𝑡
𝑡𝑡0
Considering time t0 = 0, we get
𝑙𝑙𝑙𝑙 𝑘𝑘 − 𝑙𝑙𝑙𝑙 𝑘𝑘0 = −𝑘𝑘𝑡𝑡
𝑙𝑙𝑙𝑙 � 𝐶𝐶𝐶𝐶𝐼𝐼� = −𝑘𝑘𝑡𝑡
3.5 Photocatalysis tests on titanium oxide nanotubes
From a theoretical point of view, the pH of the solution can influence the rate of reaction
during photocatalysis. The higher the pH the lower is the reaction rate expected, as the low
pH values are favourable for the availability of organic pollutants on the photocatalyst
surface, allowing the photo-generated holes to react with these molecules and thereby
decreasing the unnecessary electron-hole recombination.
The photocatalytic test at pH 11 is performed to know the effect of basic conditions on the
reaction rate, as the tests on ‘Iron 3’ and ‘Iron 4’ are performed only in pH 11 because of the
fact that iron oxide is stable in alkaline medium.
The first series of tests are performed on the ‘Titanium 1’ sample in RhB in pH 6 and 11 under
the UV lamp to find experimental evidence of this consideration.
The results of photocatalytic tests on the ‘Titanium 1’ sample in RhB in pH 6 and 11 are shown
the figure 3.18
45
Figure 3.18 Degradation curves of Titanium 1 sample in RhB at pH 6 and pH 11
The apparent rate constants are shown in figure 3.19
Figure 3.19 influence of pH on apparent rate constant
The photocatalysis performed on RhB in pH 6 and pH 11 shows more or less the same reaction
rate and the influence of pH in case of ‘Titanium 1’ sample is considered to be negligible.
Once obtained this result, the RhB in pH 6 and pH 11 solutions are used to determine the
reaction rates during photocatalysis with samples of tungsten and iron.
0.375
0.38
0.385
0.39
0.395
0.4
0.405
pH 6 pH 11
k[S
-1]
46
The second series of tests are performed on the ‘Titanium 2’ samples (anodized with the same
conditions of tungsten). In this step, the effect of annealing of the samples on the degradation
of RhB is studied. The second series of tests are performed in pH 6 condition.
The results of photocatalytic tests on the ‘Titanium 2’ sample in RhB with and without
annealing of the samples are shown in figure 3.20
Figure 3.20 Degradation curves of ‘Titanium 2’ samples with and without annealing
The apparent rate constants are shown in figure 3.21.
Figure 3.21 Influence of annealing on apparent rate constant
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
annealed not annealed
k[S
-1]
47
The photocatalysis performed on ‘Titanium 2’ samples in RhB shows different reaction rates.
The rate of reaction of the annealed sample is higher compared to that of the not annealed
sample and this can be justified by the presence of crystalline phases in the annealed
‘Titanium 2’ sample.
3.5.1 Comparison of reaction rates of titanium oxide samples
The reaction rate during the photocatalysis of both ‘Titanium 1’ sample and ‘Titanium 2’
sample in RhB (pH 6) is compared and is shown in figure 3.22
Figure 3.22 Degradation curves of ‘Titanium 1’ & ‘Titanium 2’ samples (with and without
annealing)
The apparent rate constants are shown in figure 3.23.
48
Figure 3.23 Influence of annealing and anodization parameters on apparent rate constant
The reaction rates during the photocatalysis with ‘Titanium 1’ & ‘Titanium 2’ samples are
different and the reaction rate with samples anodized in the organic electrolyte is higher
compared to that of the aqueous electrolyte. This result is expected because with organic
electrolyte we obtain a more homogenous array of nanotubes with thin walls and large
diameter and higher length, which means more surface area.
3.6 Photocatalysis tests on iron oxide nanotubes
Iron oxide is not as stable as titanium oxide and hence the rate of cooling from annealed
temperatures affects the stability of the oxide. All the photocatalysis tests on iron samples
are done in dye solutions that are modified to reach pH 11. In the first step, ‘Iron 3’ samples
are anodized and photocatalysis tests are performed on ‘Iron 3’ samples with oven cooling
and without oven cooling(quenched). Since with quenched samples no photocatalytic activity
was observed, only results concerning samples cooled in the oven are shown in figure 3.24
0.02
0.07
0.12
0.17
0.22
0.27
0.32
0.37
0.42
0.47
Titanium 1 Titanium 2 Titanium 2 not annealed
k[S
-1]
49
Figure 3.24 Degradation curves of ‘Iron 3’ sample with oven cooling
The degradation curve of ‘Iron 3’ sample without oven cooling is not present in figure 3.24
because the reaction rate is zero. The ‘Iron 3’ sample without oven cooling did not show any
photocatalytic degradation.
In the second step, the ‘Iron 4’ samples are anodized and photocatalytic tests are performed
on ‘Iron 4’ samples with oven cooling and without oven cooling(quenched).
The results of photocatalytic tests on the ‘Iron 4’ sample with oven cooling and without oven
cooling(quenched) in are shown the figure 3.25
50
Figure 3.25 Degradation curves of ‘Iron 3’ sample with oven cooling and without oven cooling(quenched)
The apparent rate constants are shown in figure 3.26
Figure 3.26 Influence of cooling rate on the apparent rate constant
By using the double anodization technique as mentioned earlier in chapter 2 the oxide is more
stable compared to the traditional anodization method. Reaction rates during photocatalysis
are more or less the same for oven cooled and not oven cooled samples, but the stability of
the oxide in pH 11 is more for the one which is cooled inside the oven. (It was observed that
there is very less rust formed on the sample after immersed in pH 11 solution for 24 hours).
0.002
0.0025
0.003
0.0035
0.004
0.0045
oven cooled not oven cooled
k[S
-1]
51
3.6.1 Comparison of reaction rates of iron oxide samples
The reaction rate during the photocatalysis of both ‘Iron 3’ samples and ‘Iron 4’ samples in
RhB (pH 11) is compared and is shown in figure 3.27
Figure 3.27 Degradation curves of ‘Iron 3’and ‘Iron 4’ samples
The apparent rate constants are shown in figure 3.28
Figure 3.28 Influence of annealing and anodization parameters on apparent rate constant
The reaction rates obtained during the photocatalysis are all similar, except – as previously
mentioned – in the case of ‘Iron 3’ not oven cooled, which did not show any activity. Yet, as
emerges from figure 3.27, curves if lnC/C0 do not show a good linearity, which means that
0
0.002
0.004
0.006
0.008
Iron 3 oven cooled Iron 3 not oven cooled Iron 4 oven cooled Iron 4 not oven cooled
k[S
-1]
52
there may be a large impact of experimental errors – such as instrument sensitivity – on the
reaction rate calculated from the linear regression of data. This has to be ascribed mainly to
the poor reactivity of the samples, which causes a small variation of absorbance and therefore
a larger impact of experimental errors on single measurements.
In fact, on a whole, the dye degradation rates are very low this is explained by the presence
of magnetite in the Iron oxide crystalline samples. As mentioned in chapter 1, magnetite has
a very low bandgap value (0.1 eV) causing the recombination of photo-generated electrons
and holes Future studies are necessary to obtain the nanotubular structures on iron with the
presence of only hematite phase. One of the most challenging problems is to maintain the
nanotubular structure at high temperatures during annealing as an increasing amount of
hematite phase is present in samples annealed over 600̇͘°C.
3.7 Photocatalysis tests on tungsten oxide nanotubes
Tungsten oxide is one of the very few metal oxide semiconductors which can generate the
photoexcited electron and holes in the solar visible spectrum. The application of these metal
oxide in the water purification has not been investigated widely in literature. In this study, the
first set of experiments is used to determine the photocatalytic activity of the tungsten oxide
nanotubes in RhB (pH 6) solution. The sample used for the test is ‘Tungsten 2’. The anodizing
and annealing parameters are selected based on the previous literature, as reported in
previous sections.
The results of photocatalysis tests on the ‘Tungsten 2’ sample in RhB with and without
annealing of the samples are shown in figure 3.29. The apparent rate constants are shown in
figure 3.30.
53
Figure 3.29 Degradation curves of ‘Tungsten 2’ samples with and without annealing
Interestingly, the reaction rate observed was higher in the case of the non- annealed sample
compared to the annealed sample. This is the reverse situation when compared to that of
titanium.
Figure 3.30 Influence of annealing on apparent rate constant
0.006
0.026
0.046
0.066
0.086
0.106
0.126
0.146
0.166
0.186
0.206
annealed not annealed
k[S
-1]
54
One of the possible explanations to this is the shift to higher energies of the conduction band
when passing from an amorphous structure to a crystalline one, thereby decreasing the
capability of the photo-excited electrons to reduce oxygen. Further analyses are necessary to
determine the band gap of the amorphous and crystalline tungsten oxides obtained. Given
this anomalous inverted trend of photocatalytic and its hypothesized dependence on the
oxide crystal structure and consequent band position, it could also be interesting to analyse
the photocatalytic behaviour of tungsten oxide in the visible light spectrum.
One drawback of this finding is that the most effective tungsten oxide sample is the non-
annealed one, while annealing is useful not only to modify the crystal structure, but also to
increase the adhesion of the nanotubular oxide layer to the metal surface). Hence, reusability
tests were performed to investigate the behaviour of these oxide layers in repeated use.
The results of photocatalytic tests on the ‘Tungsten 2’ sample in RhB with same sample
(reusability) and different samples (repeatability) are shown in figure 3.31. In the first case,
three photocatalytic tests were performed on the sample. In the second case, only one test
is performed on each sample of a set of three samples anodized in identical conditions.
Figure 3.31 Degradation curves of ‘Tungsten 2’ on the same and different samples
55
The average apparent rate constants of all tests performed on one same sample and on three
different samples are shown in figure 3.32.
Figure 3.32 Influence of reuse/repeatability on apparent rate constant
The results make evident that the non-annealed ‘Tungsten 2’ samples have oxide stability
issues. There is a huge decrease in the rate constant from Test 1 to Test 3(same sample used
for 3 tests Test 1, Test 2 and Test 3). The detachment of the oxide layer can not be seen with
naked eye as the samples of tungsten with and without coating are visually the same and the
decrease in photocatalytic activity justifies the detachment of the oxide coating. One way to
overcome this problem may be to increase the adhesion of the oxide layer by annealing the
samples at low temperatures, thereby not changing the crystalline structure drastically.
3.8 Comparison of Photocatalytic test performed on different
metal oxide samples
The reaction rates during the previous experiments are collected together and plotted in two
different graphs.
Figure 3.33 shows the photocatalytic activity of the samples that were tested in RhB pH 6
solutions.
0.0040.0240.0440.0640.0840.1040.1240.1440.1640.1840.2040.2240.2440.2640.284
Test 1 Test 2 Test 3 Sample 1 Sample 2 Sample 3
k[S
-1]
56
Figure 3.34 shows the photocatalytic activity of the samples that were tested in RhB pH 11
solutions.
Figure 3.33 apparent rate constants of different samples in the degradation of RhB in pH 6
Figure 3.34 apparent rate constants of different samples in the degradation of RhB in pH 11
00.030.060.090.120.150.180.210.240.27
0.30.330.360.390.420.45
Titanium 1annealed
Titanium 2annealed
Titanium 2 Tungsten 2differentsamples
Tungsten 2same samples
Tungsten 2annealed
k[S
-1]
0
0.03
0.06
0.09
0.12
0.15
0.18
0.21
0.24
0.27
0.3
0.33
0.36
0.39
Titanium 1 Iron 3 oven cooled Iron 3 not ovencooled
Iron 4 oven cooled Iron 4 not ovencooled
k[S
-1]
57
From the above figures, it can be deduced that
1. Titanium (produced in the organic electrolyte) has the highest degradation rate.
2. Titanium in pH 6 and pH 11 shows more or less the same degradation rate.
3. Titanium anodized in organic electrolyte shows better degradation rate compared to
that of titanium anodized in aqueous electrolyte.
4. Annealing on titanium increases photocatalytic activity.
5. All iron oxides obtained in this work produce a degradation rate much lower than that
of titanium, actually they show an almost negligible activity.
6. Anodic tungsten oxides in absence of annealing show good photoactivity, although
lower than that of the optimized titanium grown in an organic electrolyte.
7. Tungsten oxides without annealing have a high degradation rate compared to that of
annealed one.
8. Tungsten oxides need further optimization in order to obtain oxide stability and avoid
the need to discard a sample right after its first use.
58
4 CONCLUSIONS
In this work, the photocatalytic degradation of Rhodamine B using different metal oxide
nanotubes is investigated, starting from the production of nanotubes on titanium, iron and
tungsten. Titanium oxide nanotubes are produced by anodizing the sample in either organic
or aqueous electrolyte, iron oxide nanotubes are produced only in the organic electrolyte and
tungsten oxide nanotubes are produced only in the aqueous electrolyte. Different annealing
treatments were used for different samples. All the photocatalytic experiments are
performed in a batch under irradiation with a UV LED with a fixed stirring speed. In the entire
work, two different solutions are used one is RhB in pH 6 and the other is RhB in pH 11. The
degradation of RhB in pH 6 is studied with titanium oxide samples and tungsten oxide
samples. The degradation of RhB in pH 11 is studied with titanium oxide samples and iron
oxide samples.
The best results were obtained with titanium anodized in the organic electrolyte and
annealed at 500 0 C for 2 hours and with tungsten anodized in the aqueous electrolyte without
annealing treatment. The reason for the low activity obtained with annealed tungsten oxide
is not clearly understood. The probable explanation is the change in position of the bandgap
edges of the annealed tungsten oxide sample, which may have limited its oxidating or
reducing power. It is difficult to determine a priority the exact bandgap as tungsten oxide has
many polymorphs in its crystalline state. Future work needs to be done in the annealing
parameters of tungsten oxide and in the use of different light sources in order to better
understand the photocatalytic behaviour of WO3.
The results obtained with iron oxide nanotubes are not satisfying because of their very low
degradation rates. The advantageous low bandgap value which is capable of generating
photo-electrons and photo-holes in visible light is not sufficient to improve iron oxide
photocatalytic behaviour, probably because of the presence of magnetite in the oxide
crystalline structure. Annealing at high temperatures is one way to reduce the amount of
magnetite but it may lead to the destruction of the nanotubular structure.
59
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