Modification of TiO2 with Pd NPs for Enhanced Photocatalysis

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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
)(
eVE
nm
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 tnAetA
P
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 tptnAetA
P
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:
lc
I
I
A
t






 0
log
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 (5m, 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
Abs
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
Abs
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
Abs
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
Abs
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
%RB
t (min)
PC10
PC50
PC100
PC500
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
300 400 500 600 700 800
Abs
Wavelength (nm)
Pd-PC100 in UV
0min
3min
6min
9min
12mi
n
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20
%RB
t (min)
PC 500
Pd-PC 500
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20
%RB
t (min)
PC 100
Pd-PC 100
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20
%RB
t (min)
PC 10
Pd-PC 10
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40
%RB
t (min)
PC 50
Pd-PC 50
a b
c d
a
b
ba

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
%RB
t (min)
PC 500
Pd-PC 500
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200
%RB
t (min)
PC 100
Pd-PC 100
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200
%RB
t (min)
PC 50
Pd-PC 50
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200
%RB
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
REFERENCES
Amy L. Linsebigler, G. L. (1995). Phtotocatalysis on TiO2 surface: Principles, Mechanisms,
and selected results. . Chem. Rev , 95, 737-758.
Andrew Mills, S. L. (1997). An overview of semiconductor photocatalysis. Journal of
Photochemistry and Photobiology A: Chemistry 108 , 1-35.
C. Colbeau-Justin, M. Kunst (2003). Structural influence on charge-carrier lifetimes in TiO2
powders studied by microwave absorption. Journal of materials science 38 , 2429-2437.
E. Kowalska, H. R.-J. (2008). Modification of Titanium dioxide with Platinum ions and
clusters: application in photocatalysis. J. Phys. Chem C , 112. 1124-1131.
Kamat, P. V. (2007). Meeting the clean engergy demand: Nanostructure Architectures for
solar energy conversion. J. Phys. Chem C , 111, 2834-2860.
Meng Nan Chong, B. J. (2010). Recent developments in photocatalytic water treatment
technology: A review. Water research , (44) 2997-3027.
Ping Qu, J. Z. (1998). TiO2-assisted photodegradation of dyes: A study of two competitive
primary processes in the degradation of RB in an aqueous TiO2 colloidal solution. . J.
Molecular Catalysis A , 129, 257-268.

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