1

MASTER’S THESIS

Modification of titanium dioxide with palladium

nanoparticles: Application in photocatalysis

LE QUOC CHON

International Master Physical-Chemistry SERP-Chem

2

Laboratoire de Chimie Physique

CNRS – UMR 8000

Master’s thesis in Physical-Chemistry

Subject

Modification of titanium dioxide with palladium

nanoparticles: Application in photocatalysis

LE QUOC CHON

International Master Physical-Chemistry SERP-Chem

Université Paris-Sud 11

Orsay, 2011

Supervisor: REMITA BOSI Hynd

COLBEAU-JUSTIN Christophe

Co-supervisor: NGUYEN Dinh Lam

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Abstract

Titanium dioxide (TiO2) is a popular semi-conductor that has many different applications. One of them is

photocatalysis, where it is used to treat wastewater and air. The most important drawback of TiO2 is its

limited ability to absorb light. It only absorbs UV light which comprises less than 5 % of the sunlight

spectrum. This disadvantage prevents it from being widely utilized. In our work, we modified the surface

of TiO2 by Palladium nanoparticles to improve its photocatalytic activity both under UV and particularly

under visible light. The morphology of the synthesized photocatalysts was examined by transmission

electron microscopy (TEM). The ability of TiO2 to absorb visible light was studied by diffusion

reflectance spectroscopy (DRS) and to follow the lifetime of the charge-carriers, we used time resolve

microwave conductivity (TRMC) technique. The photocatalytic tests were carried out with two pollutant

models: Rhodamine B and Phenol. High performance liquid chromatography (HPLC) and UV-Vis

spectrophotometer were used to follow the degradation of Rhodamine B and Phenol. The result show that

the effect of Pd nanoparticles on photocatalytic activity of TiO2 is complex and depends on the nature of

TiO2. For some kind of TiO2, surface modification by Pd nanoparticles leads to the improvement of their

photocatalytic activity both under UV and visible light. The explanation is based on the prevention of

charge-carrier recombination and the enhancement in visible light absorption.

Résumé

Le dioxyde de titane (TiO2) est un semi-conducteur très connu et ayant beaucoup d’applications. Une des

applications est la photocatalyse pour la dépollution de l’eau usée et de l’air. Cependant, un important

inconvénient limite ses applications dans l’industrie. Celui-ci concerne la capacité d’absorption de la

lumière. En effet, TiO2 n’absorbe que dans l’UV ce qui constitue moins de 5% de la lumière solaire.

Durant ce stage, nous avons modifié la surface du dioxyde de titane par des nanoparticules de Palladium

pour augmenter l’activité photocatalytique de TiO2. Nous avons utilisé différentes techniques pour

caractériser ces photocatalyseurs: la radiolyse pour synthétiser les nanoparticules métalliques à la

surface de TiO2, la Microscopie électronique à transmission (MET) pour caractériser la morphologie des

photocatalyseurs, la Réflexion diffuse (DRS) pour étudier leur capacité d’absorption de la lumière

visible. La durée de vie de porteurs de charge dans TiO2 après illumination UV a été étudiée par des

expériences d’absorption micro-ondes utilisant la méthode de conductivité miro-ondes résolue en temps

(TRMC). L’activité photocatalytique sous UV-visible et lumière visible a été examinée en solution

aqueuse pour des pollutants modèles (le phénol et la rhodamine B). La photodégradation de ces polluants

modèles a été suivie par spectrométrie d’absorption UV-Vis et Chromatographe liquide à haute

performance (HPLC). Le résultat montre que l’effet du palladium sur l’activité photocatalytique de TiO2

est complexe et varie avec la nature de TiO2. Pour certaines poudres de TiO2, la modification par du Pd

permet d’augmenter son activité sous lumière UV et visible. Dans ce cas, les nanoparticules de palladium

permettent de diminuer la recombinaison électron-trou et d’augmenter l’absorption de TiO2 dans le

visible.

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Preface

This report is my master’s thesis carried out in the Laboratoire de Chimie Physique, Université

Paris-Sud 11.

I am deeply grateful to Université Paris-Sud 11 and the French Government, which gave me

scholarships to attend the International Master Program SERP-Chem.

I would like to thanks Mr. MOSTAFAVI Mehran, director of Laboratoire de Chimie Physique

(LCP), who gave me the permission to do my internship in the LCP.

I am so grateful to Mrs. REMITA BOSI Hynd, research director of CNRS, for her help, her

responsibility, her advice and her empathy to me during my internship.

I want to express my sincere gratitude and special thanks to Mr. COLBEAU-JUSTIN

Christophe, Head of Chemistry Department at IUT d’Orsay, for his generosity, assistance and

advice within my internship.

I would like to say thanks to my professor NGUYEN Dinh Lam, in Vietnam, for his help, his

advice and his encouragement.

My internship would not have been successful without the help and the advice from Ms. TAHIRI

ALAOUI Ouafa, Postdoc at the LCP, who accompanied me during my internship, especially

during the first few months. I am pleased to convey my profound gratitude and appreciation for

her support.

I feel delighted to convey my appreciation and thanks to Mehdi, PhD student at the LCP, who is

always willing to help me with HPLC experiments.

I also want to thank Sébastien SORGUES, Maitre de Conférences at the LCP and Alexandre

HERISSAN, trainee student at the LCP, who helped me to understand and to work with Time

Resolved Microwave Conductivity (TRMC).

I enjoyed the internship so much at the LCP because of the friendly atmosphere and the helpful

advice from other colleagues, particularly to Mamy, Pyranka, Anaïs and Ronan.

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Abbreviations

ACN Acetone nitrile

CNRS Centre national de la recherche scientifique

DRS Diffusion reflectance spectroscopy

EDTA Ethylenediaminetetraacetic acid

HPLC High performance liquid chromatography

LCP Laboratoire de Chimie Physique

NIR Near infrared

NPs Nanoparticles

RB Rhodamine B

TEM Transmission electron microscopy

TRMC Time resolved microwave conductivity

UV Ultra violet

Vis Visible

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Table of contents

INTRODUCTION ....................................................................................................................... 8

Water problem ......................................................................................................................... 8

Photocatalysis .......................................................................................................................... 9

AIMS ......................................................................................................................................... 12

METHOD .................................................................................................................................. 13

TECHNIQUES .......................................................................................................................... 14

MATERIALS ............................................................................................................................ 19

EXPERIMENTAL SECTION ................................................................................................... 21

Radiolysis to synthesize Pd-TiO2 powders ............................................................................ 21

Textural and optical properties .............................................................................................. 21

Photocatalytic properties ........................................................................................................ 22

RESULTS .................................................................................................................................. 23

Characterization of photocatalyst .......................................................................................... 23

DISCUSSIONS ......................................................................................................................... 31

CONCLUSION ......................................................................................................................... 32

REFERENCES .......................................................................................................................... 33

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List of figures

Figure 1: General principle of Photocatalyst based on TiO2 ......................................................... 11

Figure 2: Schematic mechanism of RB degradation under UV & visible irradiation on Pt-TiO2 13

Figure 3: Schematic mechanism of radiolysis ............................................................................... 15

Figure 4: Reflection of light .......................................................................................................... 16

Figure 5: Sample holder design for TRMC technique .................................................................. 17

Figure 6: The quartz cell reactor ................................................................................................... 17

Figure 7: Batch reactor system ...................................................................................................... 18

Figure 8: A beam light travels through a cuvette .......................................................................... 19

Figure 9: HPLC system at Laboratoire de Chimie Physique ........................................................ 19

Figure 10: TEM image of PC50-Pd .............................................................................................. 24

Figure 11: Diffuse reflection signals for PCxx and Pd-PCxx ........................................................ 24

Figure 12: TRMC signal of pure TiO2 in series ............................................................................ 25

Figure 13: TRMC signal of TiO2 series in comparison ................................................................ 26

Figure 14 : Phenol degradation over pure TiO2 ............................................................................ 26

Figure 15 : Effect of surface-Pd on phenol degradation ............................................................... 27

Figure 16 : The evolution of benzoquinone .................................................................................. 28

Figure 17 : The evolution of hydroquinone .................................................................................. 28

Figure 18 : RB photodegradation (a) over pure TiO2 and (b) over Pd-PC100.............................. 29

Figure 19 : Effects of surface-Pd on RB photodegradation under UV ......................................... 29

Figure 20 : Effects of surface-Pd on RB photodegradation under visible .................................... 30

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INTRODUCTION

Water problem

Population growth and economic development are two of the factors adversely increasing the

demand for available water resources. This demand affects not only the quantity but also the

quality of water resources. The United Nations predicts that by 2025, two-thirds of global

population will experience water shortages, with severe lack of water blighting the lives and

livelihoods of 1.8 billion. According to the United Nation World Water Assessment Program, by

2050, 7 billion people in 60 countries may have to cope with water scarcity. At this year's World

Economic Forum, United Nation secretary-general Ban Ki-Moon recommended that water

scarcity should be at the top of the international agenda. "As the global economy grows, so will

its thirst," he said, warning of a future marred by conflicts over water.

Every day, 2 million tons of sewage and industrial and agricultural waste are discharged into the

world’s water, the equivalent of the weight of the entire human population of 6.8 billion people.

The United Nation estimates that the amount of wastewater produced annually is about 1.500

km3, six times more water than that exists in all the rivers of the world.

Worldwide, 2.5 billion people live without improved sanitation, 70 % of these people who live in

Asia.

70 % of industrial wastes in developing countries are disposed of untreated into waters where

they contaminate existing water supplies.

3.1 % of deaths worldwide are the result of unsafe or inadequate water, sanitation and hygiene.

The data above shows a big problem related to freshwater supplies and wastewater treatment.

What we can do to solve the problem is a huge challenge. We need to manage well the

freshwater and make it more available for the user and prevent the contamination from industrial

and domestic wastes.

The widespread disposal of industrial wastewater containing organic dyes onto land and into

water bodies has led to serious contamination in many regions, countries worldwide. Organic

dyes are one of the largest groups of pollutants released into wastewater from textile and other

industrial processes such as paper mill wastewater, olive mill wastewater landfill leachate and

winery and distillery wastewater. About 1% to 20 % of total global production of dyes is lost

during the dyeing process and is released into the environment as textile effluent. These

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wastewaters in natural environment are not only hazardous to aquatic life but also in many cases

mutagenic to human. The toxicity and the visibility of dyes in the water and on the water surface

make us seriously need to remove them out, or to change them from toxic substances to non

toxic and/or biodegradable substances.

Photocatalysis

Photocatalysis is the acceleration of a photoreaction in the presence of a catalyst and light. In

photogenerated catalysis, the photocatalytic activity depends on the ability of the catalyst to

create electron-hole pairs (charge-carrier), which can recombine or migrate to the surface of

catalyst. When they reach the surface, they are scavenged by O2, H2O, and OH which absorb or

are present on the surface of catalyst and lead to the formation of free radicals. These formed

radicals are able to undergo secondary reactions and oxidize organic molecules leading to their

mineralization (the complete degradation of substances to CO2 and H2O).

In recent years, semiconductor photocatalytic process has shown a great potential as a low cost,

environmental friendly and sustainable treatment technology to align with the zero waste scheme

in the water/wastewater industry. Its ability to remove persistent organic compounds and

microorganisms in water has been widely demonstrated. However, it still has some drawbacks

which need to be considered: the post recovery of the catalyst particle after water treatment and

the energy efficiency in terms of UV artificial or just a very small amount of solar energy can be

absorbed by the catalyst.

The most popular material used for photocatalysis nowadays is Titania (Titanium dioxide).

Titania is a naturally occurring oxide mineral (anatase, rutile and bookite). As a bulk chemical it

is produced mainly from ilmenite (FeO.TiO2) and rutile ores (TiO2) by means of the traditional

route or the more recent chloride route. Titania is a large volume of the inorganic chemical

compounds used as a white inorganic pigment with unique properties in painting, printing ink,

plastics, paper, synthetic fibres, rubber, crayons, ceramics, cosmetics and electronic components.

Titania is a semiconductor; its molecules contain two bands (conduction band and valence band)

with different energies. The valence band filled with low-energy electrons, and empty band

called conduction band with higher energy. The difference in energy between the two bands is

called band gap. This gap determines the wavelengths which the semiconductor can absorb.

Titania has three kinds of crystal structure (anatase, brookite and rutile) and the gap varies

depending on the type of crystal, 3.2 eV is the band gap of anatase and 3.0 eV are band gaps of

rutile. Anatase exhibits the highest photocatalytic activity among them. With the band gap of

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energies corresponding to anatase and rutile structure, Titania only absorbs the UV light. The

relationship between the absorbed light wavelength and the gap energy can be express by:

)(

1240)(

eVEnm

gap

abs

Where

- abs is the wavelength corresponding to the band gap of the semiconductor

- Egap is the band gap energy

Mechanism of photocatalytic process

In 1972, Fujishima and Honda discovered the photocatalytic splitting of water on TiO2

electrodes (Amy L. Linsebigler, 1995). This event marked the beginning of a new era in

heterogeneous photocatalysis. Since then, research efforts in understanding the fundamental

processes and in enhancing the photocatalytic efficiency of TiO2 have come from extensive

research performed by chemists, physicists and chemical engineers. The principle can be

summarized as follow: When Titania absorbs UV light; the electron in the valence band will be

excited to conduction band and form a corresponding hole (h+) in the valence band. Then, the

electron-hole generated can recombine or migrate to the surface of TiO2 where they can undergo

sub-reactions with other species. The excited electrons can be transferred to the adsorbed species

like oxygen or water molecules to form radical species (majority is hydroxyl radical) which react

with pollutant molecules and give final products of oxidation CO2 and H2O. The hole can get an

electron from the donator species like water and OH- to generate radicals which react with

pollutant molecules in solution. The mechanism of TiO2 was widely postulated as follow (Meng

Nan Chong, 2010):

Photoexcitation: TiO2 + h e- + h

+

Charge-carrier trapping of e-: e

-CB e

-TR

Charge-carrier trapping of h+: h

+VB h

+TR

Eletron-hole recombination: e-TR + h

+VB (h

+TR) e

-CB + heat

Photoexcited e- scavenging: (O2)ads + e

- O2

.-

Oxidation of hydroxyls: OH- + h

+ OH

.

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Photodegradation by OH.: R-H + OH

. R

. + H2O

Photodegradation by photoholes: R + h+ R

+. Intermediate(s)/Final degradation products

Protonation of superoxides: O2.- + OH

. HOO

.

Co-scavenging of e-: HOO

. + e

- HO2

-

Formation of H2O2: HOO- + H

+ H2O2

The e-TR and h

+TR mention above represent the surface trapped valence band electron and the

conduction-band hole.

The following figure shows the principle of photocatalytic mechanism in a simple way:

Figure 1: General principle of Photocatalyst based on TiO2

Where:

- A is an acceptor

- D is a donator

The recombination processes can occur within the bulk TiO2 (path B in figure 1) or/and on the

surface of TiO2 (path A in figure 1). In the absence of electron scavengers, the photoexcited

electron recombines with the valence band hole in nanoseconds (Meng Nan Chong, 2010), thus

the presence of electron scavengers is vital for prolonging the lifetime of charge carriers and

successfully functioning of photocatalysis.

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AIMS

TiO2 is cheap, abundant and it has the specific characters suitable for photocatalysis (Andrew

Mills, 1997):

Photoactive;

Able to utilize visible and/or near UV-light;

Biologically and chemically inert;

Photostable (not liable to photoanodic corrosion for example);

Inexpensive.

Although, TiO2 has many advantages mentioned above, it still shows two main drawbacks: first,

it can only absorb UV light (less than 5% of the solar spectrum at ground level); second, the

recombination of electrons-holes leads to low photonic efficiency. The objectives of our work

are to develop a photocatalyst based on TiO2 with higher photocatalytic activity under UV and

visible light.

There are many methods to improve the photocatalytic efficiency of TiO2 for the degradation of

organic dyes in wastewater treatment (Fang Han, 2009):

Modification with noble metals

Modification with transition metals

Modification with lanthanide metals

Modification with CdS

Modification with Bi2S3-a comparative study with CdS/TiO2

Modification with nonmetals: N, C, F and S

In 2008, our group published an article about modification of TiO2 by platinum ions and clusters

(E. Kowalska, 2008). The results showed that it is possible to enhance the photocatalytic activity

of TiO2 under UV-Visible and Visible light by doping it with Pt clusters. Pt has the role to

separate the electrons and the holes, prevent them from recombination. Besides, Pt helps to

improve the visible light absorption of the photocatalyst (figure 2). Under visible, Pt absorbs

visible light and becomes excited states Pt* which can get electron from rhodamine B (RB) and

leads RB to autodegradation.

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Figure 2: Schematic mechanism of RB degradation under UV & visible irradiation on Pt-TiO2

Nevertheless, platinum is not only rare, but also expensive; therefore some researchers are

currently working to devote the use of cheaper metals. In this work, in order to improve the

photoactivity of Titania, we modified the surface of Titania by palladium nanoparticles.

METHOD

Palladium nanoparticles (NPs) induced by - radiolysis were used to modify the surface of

anatase Titania (one commercial TiO2 series includes PC10, PC 50, PC 100 and PC 500). The

synthesized phototatalysts were characterized by different techniques (Transmission electron

microscopy (TEM), Diffusion reflectance spectroscopy (DRS), Time resolved microwave

conductivity (TRMC). The photocatalytic activity of the modified Titania was tested in

photoreactors with the pollutant models (Rhodamine B and Phenol). The efficiency of the

photocatalyst was envestigated by following the photodegradation of the pollutant models using

UV-Vis spectrophotometer and High performance liquid chromatography. The effects of surface

modification on the photocatalytic properties were studied.

In this work, different methods were used:

- Synthesis Pd NPs on the surface of TiO2 by -radiolysis

- Characterization of the photocatalysts by TEM, DRS and TRMC

- Photocatalytic tests by using pollutant models such RB and Phenol with photoreactors

- UV-Vis spectrophotometer and HPLC were used to follow the photodegradation of the

pollutant models

- The results obtained from different techniques were evaluated

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TECHNIQUES

Radiolysis

Radiolysis is the interaction of high energy radiation such as -ray, X-ray, and or ion beams

or electron beams with matter. Radiolysis of water is accompanied by the formation of solvated

electron and the dissociation of water molecule in terms of free radical:

2222 ,,,,, HOHHOHHeOH aq

The solvated electrons and H. radical are strong reducing agents (Jacqueline Belloni, 1998) that

can reduce the metal ions from high oxidation numbers to lower, finally until zero-oxidation

number. For a monovalent metal ion:

HMHM

MeM aq

The atoms are formed with a homogenenous distribution in the solution, they will dimerize when

they encounter each other or they can aggregate with the excess metal ions:

2

2

MMM

MMM

The process continues and the metal clusters are homogeneously formed in solution:

z

zp

y

yn

x

xm MMM

Where m, n and p represent the number of nulearities, similarly x, y and z represent the number

of associated ions.

The metal atoms tend to coalesce progressively. To limit the coalescence and to obtain small

metal nanoparticles, ligands (CN-, EDTA…), polymers (polyvinyl alcohol…), surfactants

(sodium dodecylsulfate…) or supports are added to the solution before irradiation.

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Figure 3: Schematic mechanism of radiolysis

Transmission Electron Microscopy (TEM)

TEM is a microscopy technique which uses electron beam transmission through an ultra thin

specimen. An image is formed which will be magnified and focused on an imaging device. This

image gives information about the size and morphology of sample.

The synthesized Pd NPs were observed by TEM.

Diffusion reflectance spectroscopy (DRS)

DRS is an excellent sampling tool for powdered or crystalline materials in the mid-IR and NIR

spectral ranges. It can also be used for analysis of intractable solid samples. Samples to be run by

diffuse reflectance are generally ground and mixed with an IR transparent salt such as potassium

bromide (KBr) prior to sampling. Diffuse reflectance can also be used to study the effects of

temperature and catalysis by configuring the accessory with the heating chamber.

Diffuse reflectance relies upon the focused projection of the spectrometer beam into the sample

where it is reflected, scattered and transmitted through the sample material (figure 4 below). The

back reflected, diffusely scattered light is then collected by the accessory and directed to the

detector optics. Only the part of the beam that is scattered within a sample and returned to the

surface is considered to be diffuse reflection.

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Figure 4: Reflection of light

Time resolved Microwave Conductivity (TRMC)

TRMC method is based on the measurement of the change of the microwave power reflected by

a sample P(t), induced by laser pulsed illumination of this sample. The relative change

(P(t)/P) of the reflected microwave power is caused by a variation of the sample conductivity

(t) induced by the laser. For small perturbations of conductivity, a proportionality between

P(t)/P and (t) was established (C. Colbeau-Justin, 2003):

i

ii tnAetAP

tP )()(

)(

- n(t) is the number of excess charge-carriers i at time t.

- i is the mobility of charge carriers i

- A is the sensitivity factor, which is independent on time, dependent on the microwave

frequency and on the conductivity of sample

The charge-carriers mentioned in the present work are electrons in the conduction band and

holes in the valence band. Then, the above formula can be expressed following:

))()(()()(

hn tptnAetAP

tP

- n(t) is the number of excess electrons, n is the mobility of electrons in the conduction

band

- p(t) is the number of excess holes, h is the mobility of holes in the valence band

For TiO2, the mobility of the holes is so small in comparison with that of electrons such that the

signal TRMC can be attributed to the electrons.

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The TRMC signal obtained by this technique is called microwave photoconductivity, it allows to

follow directly the decay of the number of electrons and of the holes after the laser pulse on the

time scale of nanosecond to microsecond.

The TRMC signal can be characterized by two parameters: the maximum value (Imax) and the

decay I(t). Imax is determined by the electron mobility and by fast decay processes with an

appreciable activity during the excitation (e.g the first 10 ns after excitation).

From the TRMC signal we can get information about the lifetime of charge-carriers and

particularly the lifetime of electrons. This information will give us the clue to predict the role of

palladium adsorbed on surface of TiO2 concerning the prevention of recombination process of

charge-carrier. The sample holder designed for TRMC technique is depicted:

Figure 5: Sample holder design for TRMC technique

Photoreactor

Photocatalytic activity tests were carried out in a photoreactor. We used two kinds of

photoreactors: a xenon lamp with a quartz cell (Figure 6) and a batch reactor (Figure 7)

Figure 6: The quartz cell reactor

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With the quartz cell reactor, we can perform the experiment in two ranges of wavelength: UV-

Visible range (without an optic filter) and visible range (using an optic filter).

Figure 7: Batch reactor system

For the batch reactor system, the experiments were conducted only in the UV range.

UV-Vis Spectrophotometer

We used an HP 845 UV-Visible spectrophotometer. The UV-visible absorption technique is

based on the Beer-Lambert law:

lcI

IA

t

0log

Where:

- A is absorbance

- I0 is the intensity of incident light

- It is the intensity of transmitted light

- is extinction coefficient of the absorber

- l is the path length of incident light

- c is the concentration of the absorber

The pathway of light through the cell containing solution of sample is simply depicted in the

following diagram:

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Figure 8: A beam light travels through a cuvette

High Performance Liquid Chromatography (HPLC)

HPLC is a chromatographic technique which can be used to separate a mixture of compounds, to

quantify, purify and identify the compounds present in the sample. HPLC is one of the most

popular techniques used in biochemistry and analytical chemistry. In Laboratoire de Chimie

Physique, we used the HPLC version of Varian Prostar Series 230 ternary gradient pump

combined with a prostar 330 photodiode array detector (D2 lamp). For elution, an isocratic

mobile phase consisting in 80% of H2O and 20% acetonitrile (ACN), at a 1 mL min-1

flow rate,

was used, with 270-nm detection. The column as Adsorbosphere C18 reverse phase (5 m, 1:

150 mm, ID: 4.6 mm, Alltech). For data acquisition, Star software was used. The HPLC system

is shown in the figure below:

Figure 9: HPLC system at Laboratoire de Chimie Physique

MATERIALS

Metal

Modification of TiO2 by noble metals such as Pt, Au and Ag has been shown to increase the

photonic efficiency and inhibit the electron-hole recombination. We chose Palladium to modify

the surface of TiO2 to enhance the photocatalytic activity of Titania. It is less expensive and

more abundant than platinum and is a very efficient catalyst. It also has many applications such

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as in electrical equipment, dental appliance, jewelry, packaging materials, artificial fibres and

catalyst using in automotive emission control, in production of vinyl acetate monomer, in

production tetraphthalic acid, in hydrogen purification and in groundwater treatment. In 2010,

Palladium-catalysed organic reactions were recognized by the Nobel Prize in Chemistry.

Titanium dioxide (TiO2)

In our studies, we used a series of commercial TiO2 from Millenium with different

characteristics that are cited in the following table.

Type of TiO2 specific

surface

(m2/g)

Pore

diameter

(nm)

Crystal

size

(nm)

Phase

compositions

PC 10 10 24,11 65-67 Anatase

PC 50 50 20,15 20-30 Anatase

PC 100 80-100 15,3 15-25 Anatase

PC 500 317 6,32 05-oct Anatase

Table 1 : Structural data of the Millennium TiO2

Pollutant models

Rhodamine B

As one important representative molecule of xanthenes dyes, Rhodamine B is famous for its

good stability as dye laser materials (Ping Qu, 1998). The formula of rhodamine B is depicted

following:

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Phenol

Phenol has the characteristics that are suitable for a pollutant model:

- An absorption band at 269 nm

- The mechanism of photodegradation is known

- Decomposes only in presence of photon together with photocatalyst

- Degrades completely

- One typical organic pollutant

The formula of phenol is depicted:

EXPERIMENTAL SECTION

Radiolysis to synthesize Pd-TiO2 powders

The Pd-TiO2 powders were prepared by modification of commercial TiO2 Cristal Global PC-

series powders (PC10, PC50, PC100, and PC500) with metal by direct surface adsorption of

palladium (II) acetyl acetonate (99 % purity, purchased from Aldrich) in alcohol solution

(metal/TiO2 = 1% w/w). Palladium (II) acetyl acetonate was dissolved in 2-propanol (purchased

from Aldrich); 1g of TiO2 was added to the solution and dispersed by stirring in the dark for 2

hours. The suspersion was then sonicated for 15 min. The palladium nanoparticles were

synthesized by radiolytic reduction (using a 60

Co panoramic -source of 3000 curies, dose rate

1.7 kGy.h-1

, dose of 3.2 kGy) of Pd (II) in 2-propanol solution (10-3

M) under N2 atmosphere.

The modified TiO2 photocatalysts were separated by centrifugation and dried at 60 °C. The UV-

visible spectra of the supernatant indicate that all the palladium was deposited on the TiO2

powders. In the text, the modified titania will be referenced Pd-PCxx (PCxx refers to PC10,

PC50, PC100 and PC500)

Textural and optical properties

The surface morphology of TiO2 modified and the size of palladium NPs were observed using a

Transmission Electron Microscope (JOEL JEM 100CX II) operating at 100 kV. The irradiated

suspensions were first sonicated for a few minutes. Then a few drops of the suspension were

deposited on copper coated carbon grids for TEM observations.

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The diffusion reflectance spectra of the modified TiO2 samples were obtained using a Cary 5E

spectrophotometer equipped with a Cary 4/5 diffuse reflection sphere. The baseline was recorded

using a poly (tetrafluoroethylene) reference.

Electronic properties

The charge-carrier lifetimes in TiO2 after UV illumination were studied by microwave

absorption experiments using the Time Resolved Microwave Conductivity method (TRMC). The

incident microwaves were generated by a Gunn diode in the Ka band (29-31 GHz). The

experiments were performed at 30.0 GHz, frequency corresponding to the highest microwave

power. Pulsed light source was a Nd:YAG laser providing an IR radiation at = 1064 nm. Full

width at half-maximum of one pulse was 10 ns, repetition frequency of the pulses was 10 Hz.

UV light (355 nm) was obtained by tripling the IR radiation. The light energy density received

by the sample was 1.3 mJ.cm-2

. At energy densities higher than 0.5 mJ.cm-2

, like those used in

this work is needed to take into account the recombination phenomena during the pulse are

important.

Photocatalytic properties

The photocatalytic behavior of the synthesized TiO2 powders under UV-illumination has been

studied via photodegradation of phenol and rhodamine B in water. The photocatalytic reactor

(Heraeus UV-RS1) consisted of a cylindrical reservoir containing 350 mL of a catalyst

suspension and the model compound, in which an ultraviolet mercury lamp (150 W) was dipped.

This lamp was provided with double envelope quartz used to circulate water for isolation and

thermostatization. The lamp provides maximum energy at 365 nm and 254 nm, and the quartz

jacket avoids IR radiation entering the reservoir. Phenol was added at an initial concentration of

5.3 10-4

M (50 mg.L-1

) in deionized water. The photocatalyst concentration was 1 g.L-1

. The

initial measured pH of the suspension was 6, and pH was allowed to vary freely during the

reaction. Before the reaction, the suspension was ultrasonicated for 10 min with stirring in the

dark. The reservoir was magnetically stirred (900 rpm) and oxygen was continuously bubbled

throughout the reaction time (20 mL/min). Samples (4 mL) were withdrawn every 10 min for an

hour and two additional samples were taken at 75 and 90 min. After filtration through a 0.20 m

pore size PTFE membrane (TITAN), the solutions were analyzed by HPLC. Analyses were

carried out by using a Varian Prostar 230 ternary gradient pump combined with a Prostar 330

photodiode array detector (D2 lamp), by a method developed in our laboratory. For elution, an

23

isocratic mobile phase consisting in 80 % of H2O and 20 % ACN, at a 1 mL min-1

flow rate, was

used, with 270-nm detection. The column was Adsorbosphere C18 reverse phase (5m, l: 150

mm, ID: 4.6 mm, Alltech) combined with All-Guard cartridge systemTM

(7.5 4.6 mm, Alltech).

For data acquisition, Star software was used.

The photocatalytic behavior of the synthesized TiO2 powders under UV and visible illumination

has been studied via photodegradation of rhodamine-B (RB - C28H31CIN2O3) in water. The

photodegradation reaction of RB at 10-4

M was carried out in a quartz cell reactor containing 3.5

mL of a model solution and with 1 g/L of photocatalyst. The suspension was magnetically stirred

and irradiated for 20 min (under UV-visible light) or 200 min (under visible light > 450 nm) with

an Oriel 300 W xenon lamp. For each experiment, the aqueous suspensions of model compound

and the photocatalyst were stirred in the dark to ensure that the adsorption equilibrium was

established prior to irradiation; 0.5 mL of aliquots were taken from the reactor at different times

by means of a 0.5 mL single channel pipette and were centrifuged to separate the catalyst of the

water treated. The reactor was operated under mild stirring. For the determination of

concentration of RB, UV–Vis spectra were measured with a Kontron Uvicon 860 UV/Vis

spectrophotometer, using a 2 mm quartz cell. The kinetics of the reaction was obtained by

monitoring the dye maximum absorbance, i.e. at 554 nm.

RB and phenol were obtained from Fluka and Aldrich respectively. Dye and phenol solution

were prepared using ultra pure water (Milli Q with 18.6 M.cm).

RESULTS

Characterization of photocatalyst

The textural properties of modified compounds have been studied by TEM. Figure 10 shows a

TEM picture of Pd-PC50. It is representative of the Pd-PCxx series. The Pd-nanoparticles are

clearly observable and localized at the surface of TiO2 particles. The size of the Pd nanoparticles

is quite homogeneous. A mean size of 3 nm is observed.

24

Figure 10: TEM image of PC50-Pd

The pure TiO2 are white, while all the modified TiO2 are gray indicating a change in the

absorption properties. The optical properties have been studied in details by diffuse reflectance

spectroscopy. Figure 11 shows the spectra of pure and modified TiO2: (a) PC10; (b) PC50; (c)

PC100; (d) PC500. It can be observed that the absorbance in the visible region is always higher

for the modified than for pure TiO2. This point explains the gray color of modified TiO2. But, it

should be pointed out that no shift of the transition was observed. The surface modification with

Pd does not have any influence on the band gap of the photocatalyst.

Figure 11: Diffuse reflection signals for PCxx and Pd-PCxx

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

100 300 500 700 900 1100

Ab

s

wavelength (nm)

PC50

Pd-PC50

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

100 300 500 700 900 1100

Ab

s

wavelength (nm)

PC10

Pd-PC10

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

100 300 500 700 900 1100

Ab

s

wavelength (nm)

PC100

Pd-PC100

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

100 300 500 700 900 1100

Ab

s

wavelength (nm)

PC500

Pd-PC500

a b

c d

The band gap within TiO2

does not change

25

Figure 12 shows TRMC signals after excitation at 355 nm of pure TiO2. The measured signals

are very different for the four compounds. PC50 presents a lowest Imax value than the other

compounds. PC500 presents a very fast decay (75 % of intensity decrease between 0.01 and

0.1 s) while PC10 has the slowest decay (10 % of intensity decreases in the same amount of

time). PC50 and especially PC100 have slightly fastest decays than PC10.

Figure 12: TRMC signal of pure TiO2 in series

Figure 13 shows the influence of surface-Pd on TRMC signals of TiO2. The surface-Pd

increases Imax values for PC10 (a) and PC50 (b), while a weak decrease is observed with

PC100 (c) and PC500 (d). The normalized signals (see insert in Figure 13(a) and (b)) show

that the decay is significantly slowed down in PC10 and more weakly in PC50. On the other

hand, the decay is nearly unchanged in PC100, and slightly accelerated in PC500.

Imax

26

Figure 13: TRMC signal of TiO2 series in comparison

Photocatalytic activity

Figure 14 shows the phenol photodegradation over pure TiO2. It is observed that the

photocatalytic properties are very different for the four types of TiO2. PC10 presents the best

degradation kinetics, followed by PC50, PC100 and PC500 respectively.

Figure 14 : Phenol degradation over pure TiO2

a b

c d

27

Figure 15 shows the influence of surface modification with Pd on the photodegradation of

phenol. It indicates that the surface-Pd is beneficial for PC10 (a) and PC50 (b), by increasing

the kinetics of phenol degradation. The surface-Pd had a hardly effect for PC100 (c), and it is

harmful for PC500 (d).

Figure 15 : Effect of surface-Pd on phenol degradation

The photodegradation of phenol gives some intermediate compounds: hydroquinone,

benzoquinone and catechol were depicted here:

At the end, the photodegradation is finished with CO2 and H2O.

Figures 16 and 17 show the evolution of the intermediate compounds formed during the

phenol photodegradation: the benzoquinone evolution over pure TiO2 16(a) and modified

TiO2 16(b) and the hydroquinone evolution over pure TiO2 17(a) and modified TiO2 17(b).

a b

c d

28

The results confirmed the behavior observed with phenol: the surface-Pd decreased the

amount of intermediate formed and accelerated their degradation in the case of PC10 and

PC50. A complete opposite effect was observed with PC500. Whereas, in the case of PC100

the amount of the formed intermediates increased, and in the same time their degradation was

accelerated. However, the effects of surface-Pd were very weak with PC100.

Figure 16 : The evolution of benzoquinone

Figure 17 : The evolution of hydroquinone

The photodegradation of RB under UV illumination was also studied. This is a

complementary investigation to the phenol photodegradation. Indeed, the photodegradation

mechanisms may be quite different because RB slightly adsorbs at the TiO2 surface while

phenol does not.

The decomposition rate of RB is based on the absorbance at its maximum absorption max =

554 nm (figure 18(b)).

a b

a b

29

The Figure 18(a) shows the RB photodegradation over pure TiO2 under UV illumination.

PC10 also presents the best degradation kinetics, but at this time, followed by PC100, PC500

and PC50 respectively.

Figure 18 : RB photodegradation (a) over pure TiO2 and (b) over Pd-PC100

Figure 19 shows the influence of surface-Pd on RB photodegradation under UV illumination:

(a) PC10; (b) PC50; (c) PC100; (d) PC500. In this case, the surface-Pd has always a positive

effect on the photodegradation, even for PC500. The effect is especially sharpened with

PC50.

Figure 19 : Effects of surface-Pd on RB photodegradation under UV

The experiments with RB have been completed with its photodegradation performed under

visible illumination. Figure 20 shows the influence of surface-Pd on RB photodegradation

under visible illumination: (a) PC10; (b) PC50; (c) PC100; (d) PC500. Under visible light, the

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40

% R

B

t (min)

PC 10

PC 50

PC 100

PC 500

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

300 400 500 600 700 800

Ab

s

Wavelength (nm)

Pd-PC100 in UV

0min

3min

6min

9min

12min

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20

% R

B

t (min)

PC 500

Pd-PC 500

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20

% R

B

t (min)

PC 100

Pd-PC 100

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20

% R

B

t (min)

PC 10

Pd-PC 10

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40

% R

B

t (min)

PC 50

Pd-PC 50

a b

c d

a b b a

30

surface-Pd has always no effect on photodegradation, except a slightly positive effect for

PC50.

Figure 20 : Effects of surface-Pd on RB photodegradation under visible

To summarize those results, all the data concerning the phenol and RB photodegradation

under UV and visible irradiation over pure and modified TiO2 are gathered in Table 2.

Sample % phenol

(30 min)

% RB - UV

(5 min)

% RB - Vis

(100 min) PC10 96 85 86

Pd-PC10 100 95 95

PC50 96 17 40

Pd-PC50 97 90 50

PC100 93 50 98

Pd-PC100 92 58 92

PC500 78 42 75

Pd-PC500 70 63 80

Table 2. Data of phenol and RB photodegradation under irradiation

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200

% R

B

t (min)

PC 500

Pd-PC 500

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200

% R

B

t (min)

PC 100

Pd-PC 100

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200

% R

B

t (min)

PC 50

Pd-PC 50

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200

% R

B

t (min)

PC 10

Pd-PC 10

a b

c d

31

DISCUSSIONS

The results described above show that 3nm Pd NPs had been successfully deposited onto the

surface of the commercial anatase PC-series by gamma radiolysis. The results also revealed a

complex effect of surface-Pd on photocatalytic activity of TiO2. The explanation can be given

concerning the following factors: irradiated light, the mechanism of photocatalysis, the textual

properties, optical properties, electronic properties of the photocatalysts, and the properties of

pollutant compounds.

The photodegradations of phenol and rhodamine B occur on the surface of the photocatalyst.

In other words, this is an interfacial process, where the interaction between the photocatalysts

and the chemical species is vital. If the catalyst has high specific area, it is likely to have more

active sites. From this point of view, if the photocatalyst has a higher specific surface area, the

better is its photocatalytic activity. From table 1, we can see that PC500 has the highest

specific surface area; it should be the best photocatalyst among others. However, it is not the

best but PC10. So, other factors need to be considered: the textual, optical and electronic

properties. These properties determine the absorption of photons, the creation, the dynamic

and the trapping of charge-carrier which affect the light absorption ability of photocatalyst,

the amount and the lifetime of charge carrier.

From the signal of TRMC, PC10 must be the most efficient photocatalyst because of highest

Imax and longest lifetime of charge-carrier. This is confirmed by photodegradation of both

phenol and rhodamine B (figure 14 and 18).

Light absorption ability is also an important factor. The signal of diffusion reflectance

spectroscopy shows that surface-Pd has no effect on the light absorption ability of TiO2. The

pure TiO2 and Pd-TiO2 series mostly still absorb UV light. Consequently, they are not active

under visible illumination.

Under visible light irradiation, our results show that pure TiO2 and Pd-TiO2 exhibit no

photocatalytic activity. However, in case of rhodamine B, Pd-TiO2 shows a slight

photocatalytic activity. This can be explained by the auto-degradation of rhodamine B which

absorbs visible light leading to mineralization.

Under UV light irradiation, Pd-surface has an effect on the photocatalytic activity of TiO2,

particularly with PC50; Pd plays a role in separation of the electrons and holes, preventing

32

them from recombination. Consequently, the photocatalytic activity of TiO2 was noticeably

improved.

CONCLUSION

The presented results show that 3 nm Pd NPs were successfully deposited on the surface of

the commercial anatase PC-series by gamma radiolysis.

The photocatalytic properties of the modified TiO2, followed by phenol and RB

photodegradation under UV and visible illumination, reveal a complex effect of the surface-

Pd. Its influence depends on the properties of TiO2, pollutant and the illumination. The

modification can be favorable, harmful or without effect.

For phenol photodegradation, under UV illumination, the surface-Pd increases the

photoactivity of PC50 and PC10. Those results have been explained in terms of modification

of charge-carrier dynamics by TRMC measurements.

For RB photodegradation, the surface-Pd always promotes the photoactivity under UV

illumination, especially for PC50, whereas it has no effect under visible illumination.

The surface-Pd played a role in charge-carrier separations to increase the activity under UV-

light, but it cannot be used to modify the absorption properties of the photocatalyst to create

an activity under visible light.

Presently, other modifications of TiO2 by metal NPs (Ag, Ag-Au…) are under investigation in

our laboratory.

33

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