1

CHAPTER 1

INTRODUCTION

1.1 Literature review

Titanium is the world’s fourth most abundant metal and ninth most abundant element. It

was discovered in 1791 in England by Reverend William Gregor, who recognised the

presence of a new element in ilmenite [1]. It was then rediscovered in rutile ore several

years later by a German chemist, Heinrich Klaporth who named it after Titans,

mythological first sons of the goddess Ge (earth in Greek mythology). Titanium has the

symbol Ti, with atomic number 22, and atomic weight 47.90. It is placed in the fourth

group of the periodic table, and its chemistry shows similarities to that of silicon and

zirconium. The outer electronic arrangement is 3d24s2, and the principal valence state is

IV; III, II states are also known, but are less stable. The element burns in air when

heated to give the oxide, TiO2. Titanium is not found in its elemental state, it occurs

mainly in minerals like rutile, ilmenite, leucoxene, anatase, brookite, perovskite and

spene. It is also found in titanates and many iron ores. The metal has been detected in

meteorites and stars. In fact, samples brought back from the moon by Apollo 17

contained 12.1 % TiO2. The primary source and the most stable form of titanium

dioxide is rutile ore. It was discovered in Spain by Werner in 1803. Its name is derived

from the Latin rutilus, red because of the deep colour observed in some specimens when

the transmitted light is viewed. Rutile is one of three main polymorphs of titanium

dioxide (TiO2), the other polymorphs being; anatase and brookite [2, 3]. Brookite was

discovered in 1825 by A. Levy and was named after an English mineralogist, H. J.

Brooke. In 1801 anatase was named by R. J. Hauy from the Greek word ‘anatasis’

meaning extension, due to its longer vertical axis compared to that of rutile. In all three

forms, titanium (Ti4+) atoms are co-ordinated to six oxygen (O2-) atoms, forming TiO6

octahedra [4]. All three forms differ only in the arrangement of these octahedra. The

anatase structure, is made up of corner (vertices) sharing octahedral (figure 1.1a)

resulting in a tetragonal structure.

In rutile the octahedra share edges to give a tetragonal structure (figure 1.1b) and in

brookite both edges and corners are shared to give an orthorhombic structure (figure

1.1c)[5].

2

(a) Anatase (b) Rutile

(c)-Brookite

Figure 1.1 Crystalline structures of titanium dioxide (a)-rutile, (b)-anatase, (c)-

brookite [6]

Figure 1.2 more clearly shows the unit cell structures of the rutile and anatase TiO2 [7].

Both these structures can be described in terms of chains of TiO6 octahedra. The two

crystal structures differ in the distortion of each octahedron and by the assembly pattern

of the octahedra chains. In rutile, the octahedron shows a slight orthorhombic distortion;

in anatase, the octahedron is significantly distorted so that its symmetry is lower than

orthorhombic. The Ti-Ti distances in anatase are larger, whereas the Ti-O distances are

3

shorter than those in rutile. In the rutile structure, each octahedron is in contact with 10

neighbour octahedrons (two sharing edge oxygen pairs and eight sharing corner oxygen

atoms), while, in the anatase structure, each octahedron is in contact with eight

neighbours (four sharing an edge and four sharing a corner). These differences in lattice

structures cause different mass densities and electronic band structures between the two

forms of TiO2.

Figure 1.2 Lattice structures of rutile and anatase TiO2

Titanium dioxide is an n-type semiconductor [8] that has a band gap of 3.2 eV for

anatase,[9-12] 3.0 eV for rutile [13], and ~3.2 eV for brookite[14]. Titanium dioxide

(TiO2) is the most widely investigated photocatalyst due to its strong oxidative

properties, low cost, non-toxicity, chemical and thermal stability [15]. Anatase and

rutile are the most researched polymorphs. Their properties are summarised in Table.1

and Table 1.2.

In the past few decades there have been several exciting breakthroughs with respect to

titanium dioxide. The first major breakthrough was in 1972 when Fujishima and Honda

reported the photoelectrochemical splitting of water (2H2O → 2H2 + O2) using a TiO2

anode and a Pt counter electrode. Titanium dioxide first showed promise for the

remediation of environmental pollutants in 1977 when Frank and Bard investigated the

4

reduction of CN- in water [16]. This led into an increasingly well researched area of

TiO2 because of the potential implications for environmental water and air purification

utilising solar energy [17]. In 1997 Wang et al reported TiO2 surfaces with excellent

anti-fogging and self-cleaning abilities which were attributed to the super hydrophilic

attributes of the TiO2 surfaces [18].

Table 1.1 Physical and structural properties of anatase and rutile

Property Anatase Rutile

Molecular weight ( g/mol) 79.88 79.88

Melting point (°C) 1825

Boiling Point (°C) 2500 ~ 3000 2500 ~ 3000

Specific gravity 3.9 4.0

Light absorption (nm) < 390 < 415

Mohr’s Hardness 5.5 6.5-7.0

Refractive index 2.55 2.75

Dielectric constant 31 114

Crystal structure Tetragonal Tetragonal

Lattice constants (Å) a = 3.78

c = 9.52

a = 4.59

c = 2.96

Density (g/cm3) 3.79 4.13

Ti–O bond length (Å) 1.94 (4)

1.98 (2)

1.95 (4)

1.98 (2)

Nano sized titanium dioxide was employed to excellent use in an efficient solar cell, the

dye sensitised solar cell (DSSC) as reported by Graetzel and O’Regan in 1991[19].

1.2 Anatase to rutile transformation

The anatase to rutile phase transformation in TiO2 is the most studied area of scientific

and technological interest [20]. The anatase to rutile transformation (ART) is kinetically

defined and the reaction rate is determined by parameters such as particle shape/size

5

[21], purity [22], source effects [23], atmosphere [24] and reaction conditions [25]. It is

agreed that the mechanism for phase transformation of titania is one of nucleation and

growth [26]. Anatase nanocrystals coarsen, grow and then transform to rutile only when

a critical size is reached [27]. Therefore, phase transformation is dominated by effects

such as defect concentration [28], grain boundary concentration [29], and particle

packing [30].

Table 1.2 X-Ray Data on TiO2 modifications (Clark, 1968: 268)

Titania

Phase

Space group Z Cell parameters A

a b c

Ti-O

( A)b

Anatase C4h= C4/ amc 8 3.784 9.515 1.934 (4), 1.98 (2)

Brookite D2h= Pbca 8 9.51 5.44 4.593 1.84-2.03

Rutile D4h= P42/mnm 2 4.593 2.959 1.94(4), 1.98(2)

Rutile is the thermodynamically most stable phase, while anatase and brookite are both

metastable, transferring to rutile under heat treatment at temperatures typically ranging

from 873 to 973 K [2]. Anatase is widely regarded as the most photocatalytically active

of the three crystalline structures [31-33]. The generally accepted theory of phase

transformation is that two Ti–O bonds break in the anatase structure, allowing

rearrangement of the Ti–O octahedra, which leads to a smaller volume, forming a dense

rutile phase [34]. The removal of oxygen ions, which generate lattice vacancies,

accelerates the transformation. The transition follows first order kinetics; with activation

energy of ~ 418 k J mol-1[22]. The breaking of these bonds can be affected by a number

of factors, including the addition of dopants, synthesis method and thermal treatment.

1.3 Electronic Structure of a Semiconductor

When molecular orbitals are formed from two atoms, each type of atomic orbital gives

rise to two molecular orbitals. When N atoms are used, N molecular orbitals are formed.

In solids, N is very large, resulting in a large number of orbitals [35]. The overlap of a

large number of orbitals leads to molecular orbitals that are closely spaced in energy and

so form a virtually continuous band (figure 1.3) [36]. The overlap of the lowest

6

unoccupied molecular orbitals (LUMO) results in the formation of a conduction band

and a valence band is formed from overlapping the highest occupied molecular orbitals

(HOMO). The band separation is known as the band gap (Eg), a region devoid of

energy levels. From the illustration shown in figure 1.3, the reduction in the band gap

size with the formation of bands can clearly be seen.

Figure 1.3 Change in the electronic structure of a semiconductor compound with

increasing number of monomer units [37]

If a band is formed from the molecular overlap of s orbitals it is called an s band and

likewise, an overlap of p orbitals, forms a p band and an overlap of d orbitals will give a

d band. Typically, p orbitals have higher energy than s orbitals, resulting in a band gap.

However, if the s and p bands are of similar energy, then the two bands overlap [38]. In

titanium dioxide, the valence band consists of oxygen 2 p orbitals and the conduction

band is made up from the titanium 3 d orbitals [12]. The band gap of TiO2 anatase is 3.2

eV and rutile is 3.0 eV corresponding to an absorbance threshold, λ = 388 and 415 nm

respectively. Figure 1.4 shows the various band positions of different semiconductors.

For a semiconductor to be capable of producing hydroxyl radicals, the potential of the

valence band must be greater than the potential of OH•. From figure 1.4 it can be seen

that ZnO, TiO2, WO3 and SnO2 have the potential to produce hydroxyl radicals.

7

Figure 1.4 Band positions of various semiconductors and relevant redox couples

Titanium dioxide is regarded as an n-type semiconductor due to the presence of oxygen

vacancies in the lattice. These vacancies are formed upon the release of two electrons

and molecular oxygen leaving a positive (+2) oxide ion vacancy [2]. When electrons of

energy lower than the conduction band are present, the result is an n-type semiconductor

(figure 1.5a). Alternatively if a material is added with fewer electrons than the host,

positive holes are added above the valence band resulting in a p-type semiconductor

(figure 1.5b).

(a) (b)

Figure 1.5 Semiconductors, n-type (a), p-type (b)

8

1.4 A brief study of TiO2 as photo catalyst

Photo catalytic degradation of organic compounds on semiconductor surfaces is an

important area of current research with respect to both fundamental understanding [39]

and potential practical applications [40]. Several simple oxide and sulfide

semiconductors have band-gap energies sufficient for promoting or catalysing a wide

range of chemical reactions of environmental interest. The primary criterion for good

semiconductor photocatalysts for organic compound degradation is that the redox

potential of the H2O/·OH (OH- = ·OH + e-; Eº= -2.8 V) couple lies within the band gap

domain of the material (photocatalysts) and that the material is stable over prolonged

periods of time. Among the semiconductors TiO2 (Eg = 3.2 eV), WO3 (Eg = 2.8 eV),

SrTiO3 (Eg= 3.2 eV), a-Fe2O3 (Eg = 3.1 eV), ZnO (Eg = 3.2 eV), and ZnS (Eg = 3.6 eV)

[41], TiO2 is the most promising, low-cost, robust photocatalyst and extensively

investigated material. Semiconductor photocatalysis with a primary focus on TiO2 as a

durable photocatalyst has been applied to a variety of problems of environmental

interest in addition to water and air purification, for the destruction of microorganisms

[42], for the inactivation of cancer ce11s [43], for the photo splitting of water to produce

hydrogen gas [44], and for the clean-up of oil spills [45].

In the light of solid-state physics, semiconductors (and insulators) are defined as

solids in which at 0 K (and without excitations) the uppermost band of occupied

electron energy states is completely filled up with electrons. It is well-known from

solid-state physics that electrical conduction in solids occurs only via electrons in

partially-filled bands, so conduction in pure semiconductors occurs only when electrons

have been excited--thermally, optically, etc.--into higher unfilled bands.

At room temperature, a proportion (generally very small, but not negligible) of

electrons in a semiconductor have been thermally excited from the "valence band," the

band filled at 0 K, to the "conduction band," the next higher band. The ease with which

electrons can be excited from the valence band to the conduction band depends on the

energy gap between the bands, and it is the size of this energy band gap that serves as an

arbitrary dividing line between semiconductors and insulators. Generally,

semiconductors are defined as materials with band gap less than 4 eV at room

temperature. Semiconductor electronic structures are characterized by a filled valence

band (VB), and an empty conduction band (CB). When a photon with energy of hv

9

matches or exceeds the band gap energy (Eg) of the semiconductor, an electron (ecb-), is

promoted from the valence band, into the conduction band, leaving a hole (hvb+) behind.

Excited state conduction-band electrons and valence-band holes can react with electron

donors and electron acceptors adsorbed on the semiconductor surface or within the

surrounding electrical double layer of the charged particles or recombine and dissipate

the input energy as heat; get trapped in metastable surface states. The above process in

the photocatalysis is illustrated in Figure 1.6.

Figure 1.6 Schematic photo excitation in a solid followed by de-excitation events [7]

Once excitation occurs across the band gap there should be a sufficient lifetime (in the

nanosecond regime) for the created electron-hole pair to undergo charge transfer to

adsorbed species on the semiconductor surface from solution or gas phase contact. If the

semiconductor remains intact and the charge transfer to the adsorbed species is

continuous and exothermic the process is termed heterogeneous photocatalysis [46]. The

initial process for heterogeneous photocatalysis of organic and inorganic compounds by

semiconductors is the generation of electron-hole pairs in the semiconductor particles.

Upon excitation, the fate of the separated electron and hole can follow several

pathways. Recombination of the separated electron and hole can occur at the surface

(pathway A) in the volume of the semiconductor particle (pathway B) or with the

release of heat. The photo induced electron/hole can migrate to the semiconductor

surface. At the surface the semiconductor can donate electrons to reduce an electron

acceptor (usually oxygen in an aerated solution) (pathway C); in turn, a hole can

10

migrate to the surface where an electron from a donor species can combine with the

surface hole oxidizing the donor species (pathway D). The electron transfer process is

more efficient if the species are spreadsorbed on the surface [47]. The probability and

rate of the charge transfer processes for electrons and holes depends upon the respective

positions of the band edges for the conduction and valence bands and the redox

potential levels of the adsorb ate species.

1.5 TiO2 nanostructured materials as semiconductor photocatalysts

Titanium (Ti) is a light, strong, lustrous, and corrosion-resistant metal. Titanium dioxide

(TiO2) is the most commonly used compound of titanium. Since its commercial

production in the early twentieth century, TiO2 has been widely used as a pigment in

sunscreens, paints, ointments, and toothpaste. It is also used in cements, gemstones, as

an optical opacifier in paper, and a strengthening agent in graphite composite fishing

rods and golf clubs. TiO2 powder is chemically inert, stable under sunlight, and is very

opaque. This allows it to impart a pure and brilliant white colour to the brown or gray

chemicals that form the majority of household plastics. However, in 1972, Fujishima

and Honda discovered the phenomenon of photocatalytic splitting of water on a TiO2

electrode under ultraviolet (UV) light. Since then, enormous efforts have been devoted

to the research of TiO2, which has led to many promising applications such as

photocatalysis, photovoltaic, photoelectrochromics and sensors. Some potential

applications of TiO2 are mentioned below.

1.6 Photocatalytic Applications

As has been pointed out by Heller, the development of semiconductor photo

electrochemistry during the 1970 and 1980s has greatly assisted the development of

photocatalysis [47, 48]. In particular, it turned out that TiO2 is excellent for

photocatalytically breaking down organic compounds. For example, if one puts

catalytically active TiO2 powder into a shallow pool of polluted water and allows it to

be illuminated with sunlight, the water will gradually become purified. Ever since 1977,

when Frank and Bard first examined the possibilities of using TiO2 to decompose

cyanide in water [16], there has been increasing interest in environmental applications.

These authors quite correctly pointed out the implications of their result for the field of

11

environmental purification. Their prediction has indeed been borne out, as evidenced by

the extensive global efforts in this area [49]. One of the most important aspects of

environmental photocatalysis is the availability of a material such as titanium dioxide,

which is close to being an ideal photocatalyst in several respects. For example, it is

relatively inexpensive, highly stable chemically, and the photogenerated holes are

highly oxidizing. In addition, photogenerated electrons are reducing enough to produce

superoxide from dioxygen. The commonly studied principle of the photocatalysis

reaction mechanisms is given in Figure 1.7.

SEMICONDUCTOR TiO2

Figure 1.7 The principle of TiO2 photocatalysis [50]

When photons with energies larger than the band gap of TiO2 are absorbed, electrons

are excited from the valence band to the conduction band, creating electron-hole pairs

[50]. These photogenerated charge carriers can undergo recombination, become trapped

in metastable states, or can migrate to the surface of the TiO2, where they can react with

adsorbed molecules. In an aqueous environment saturated with air, the photogenerated

electrons and holes participate in reacting with dissolved molecular oxygen, surface

hydroxyl groups, and adsorbed water molecules to form hydroxyl and superoxide

radicals.

The energy band diagram for TiO2 in pH 7 solution is shown in Fig. 1.8.

12

Figure 1.8 The energy band diagram for TiO2

As shown, the redox potential for photo generated holes is + 2.53V [41] versus the

standard hydrogen electrode (SHE). After reaction with water, these holes can produce

hydroxyl radicals (OH), whose redox potential is only slightly decreased. Both are more

positive than that for ozone. The redox potential for conduction band electrons is

−0.52V, which is in principle negative enough to evolve hydrogen from water, but the

electrons can become trapped and lose some of their reducing power, as shown.

However, even after trapping, a significant number are still able to reduce dioxygen to

superoxide O2−., or to hydrogen peroxide H2O2. Depending upon the exact conditions,

the holes, OH radicals, O2−., H2O2 and O2, all can play important role in the

photocatalytic reaction mechanisms. Although the detailed mechanism of TiO2

photocatalysis reactions differs from one pollutant to another, it has been widely

recognized that superoxide and, specifically, •OH hydroxyl radicals act as active

reagents in the degradation of organic compounds. These radicals are formed by

scavenging of the electron–hole pair by molecular oxygen and water, through the

following sequences [51]:

TiO2 + hν → e− + h+

O2 + e− →O2 − •

H2O + h+ → •OH + H+

13

•OH + •OH → H2O2

H2O2 + O2−•→ •OH + OH− +O2

Following a similar mechanism, the TiO2 photocatalyst can reduce CO2 to CH3OH or

produce H2 in a controlled environment (inert or oxygen free). However, the

photocatalytic activity of a semiconductor widely depends on its: (i) light absorption

properties, (ii) surface reduction and oxidation rates by the electron and hole, and (iii)

electron-hole recombination rates. On the other hand, the following three factors

pertaining to the band structure of semiconductors have the greatest effect on the

photocatalytic reactions: (i) band gap energy, (ii) position of the lowest point in the

conduction band, and (iii) position of the highest point in the valence band. In

photocatalytic reactions, the band gap energy principally determines which light

wavelength is the most effective, and the position of the highest point in the valence

band is the main determinant of the oxidative decomposition power of the photocatalyst.

Naturally occurring TiO2 has three polymorphs, i.e. anatase, brookite, and rutile.

Although all three types of polymorphs are expressed using the same chemical formula

(TiO2), their structures are different. Rutile is thermodynamically the most stable phase,

although the anatase phase forms at lower temperatures. Despite the fact that the band

gap values are 3.0 eV for the rutile and 3.2 eV for the anatase phases, both absorb only

UV rays. Hence both of these phases show photocatalytic activity whereas the brookite

phase does not. Characteristics of the rutile phase seem more suitable for use as a

photocatalyst because the rutile phase can absorb light of a wider range. But, the anatase

phase exhibits higher photocatalytic activity [52]. The most prominent reasons are

attributed to the difference in the energy structure between the two phase types and the

surface area. In both phases, the position of the valence band is deep, and the resulting

positive holes show sufficient oxidative power. However, the conduction band in the

anatase phase is closer to the negative position than in the rutile phase. Therefore, the

reducing power of the anatase phase is stronger than that of the rutile phase. Usually,

the anatase crystal phase formed at lower temperatures, show higher surface areas

compared to the rutile phase. A larger surface area with a constant surface density of

adsorbents leads to faster surface photocatalytic reaction rates. In this sense, the higher

the specific surface area, the higher the photocatalytic activity that one can expect.

Therefore, the anatase phase exhibits higher overall photocatalytic activity compared to

14

the rutile phase. However, Hermann et al. [53] hypothesized that the anatase and rutile

mixed phases showed better photocatalytic activity because any kind of solid–solid

interface is a key structural feature that facilitates the charge separation to hinder

recombination. The interface also acts as an active site to initiate the catalytic activity

and enhance photocatalytic efficiency [54]. In addition, TiO2 nanomaterials with high

crystallinity show superior photocatalytic activity. High temperature treatment usually

improves the crystallinity of TiO2 nanomaterials, which can induce the aggregation of

small nanoparticles and decrease the surface area [55]. Therefore, it is very difficult to

predict the photocatalytic activities from the physical properties of TiO2 nanomaterials.

Optimal conditions are sought by taking into account all these considerations, which

may vary from case to case.

1.7 Other Potential Applications

In addition to the photocatalysis and photovoltaic applications, TiO2 nanomaterials can

also be used in biomedical applications [56], functionalized hybrid materials [57], and in

sensors and nanocomposites [58, 59]. The one dimensional TiO2 nanotubes are

promising for reinforcement due to their unique physical properties, i.e. large surface to

volume ratio, low cost and better biocompatibility compared with carbon nanotubes

[60]. These materials can be used as fillers for many applications such as a radio

pacifier in bone cement, a solid plasticizer of poly (ethylene oxide) for lithium batteries,

a dye in a conjugated polymer for photoelectrochemical photoconductive agents, and as

a photocatalyst in a photodegradable TiO2-polystyrene nanocomposite film [61, 62].

Moreover, TiO2 nanocrystalline films have been widely studied as sensors for various

gases. For instance, Varghese et al. observed that TiO2 nanotubes were excellent room-

temperature hydrogen sensors not only with a high sensitivity but also with ability for

self-cleaning after environmental contamination [63]. TiO2 nanomaterials also have

been widely explored as electro chromic devices, such as electro chromic windows and

displays. Electrochromism can be defined as the ability of a material to undergo colour

change upon oxidation or reduction. Electro chromic devices are able to vary their

throughput of visible light and solar radiation upon electrical charging and discharging

using a low voltage.

15

1.8 Water splitting using semiconductor photocatalysis

The importance of hydrogen energy has recently been recognized again, especially for

use in fuel cells with due concern for environmental issues. However, industrially

hydrogen is produced by steam reforming of hydrocarbons such as methane.

Dihydrogen has to be produced from water using natural energy if one is concerned

about environmental issues. Therefore, water splitting using a photocatalyst is a

challenging reaction because it is one of the most important reactions for solving energy

and environmental problems. Water splitting using photocatalysts is not new. Water

splitting has been studied in the fields of catalysis, electrochemistry, photochemistry,

organic and inorganic chemistry, etc., for about 30 years since the Honda–Fujishima

effect was reported using a TiO2 semiconductor electrode. However, the number of

reported photocatalysts that are able to decompose water into H2 and O2 in a

stoichiometric amount with a reasonable activity has been limited. The only active

photocatalysts at present are oxide materials. Many people believe that water splitting is

impossible at a practical level. Against such a background, various types of new

photocatalysts for water splitting have been developed and this research field is taking a

new turn.

H2O cannot be photo decomposed on clean TiO2 surfaces, even though TiO2 can be

effectively photo excited under band-gap irradiation. Figure 1.9 illustrates the band edge

positions of TiO2 relative to the electrochemical potentials of the H2/H2O and O2/H2O

couple [64].

Figure 1.9 Potential energy diagram for the H2/H2O and O2/H2O redox couples

relative to the band-edge positions for TiO2.

16

According to this electron energy diagram, water photolysis is energetically favourable.

However, due to the presence of a large over potential for the evolution of H2 and O2 on

the TiO2 surface, TiO2 alone becomes inactive. Sustained photodecomposition of water

has been achieved under conditions where photogenerated electrons and holes are

separated for maximum photoreaction yield.

1.9 Limitation of TiO2 as an efficient photocatalyst and the modification of TiO2

There are some important drawbacks that severely limit the application of titanium

dioxide photocatalyst as a general tool either to degrade organic pollutants in the gas or

liquid phase or to perform useful transformations of organic compounds [65, 66]. One

of the most important limitations is the lack of TiO2 photocatalytic activity with visible

light [67, 68].

Figure 1.10 Solar spectrum at sea level with the sun at zenith with the absorption

region of TiO2

The reason for this is that the anatase form of TiO2 is a wide band gap semiconductor

with a band gap of 3.2 eV in most media, corresponding to an onset of the optical

absorption band at light photocatalytic activity, since approximately 5% of the solar

17

light energy (figure 1.10) can be absorbed by TiO2. The above comments explain the

continued interest in improving the photocatalytic efficiency of TiO2.

1. 10 Modified Titania systems for improved photocatalytic activity

Two general strategies have been developed to increase the photocatalytic activity of

TiO2, namely the use of an organic dye as photosensitizer or doping TiO2 with metallic

and non-metallic elements [69-71]. The first route (using an organic dye that absorbs

visible light) has worked very well under conditions where oxygen/air is excluded and

the degradation of the dye is minimized by the efficient quenching of the dye oxidation

state with an appropriate electrolyte [72, 73]. Otherwise, particularly the dye becomes

rapidly mineralized and the photocatalytic system loses its response towards visible

light in the presence of oxygen. The mechanism of TiO2 dye sensitization has been

determined using time resolved sub nanosecond laser flash photolysis techniques [74].

In dye sensitization, the most relevant points are the absorption spectrum of the dye in

the visible region and the energy of the electron in the excited electronic state of the

dye, which has to be high enough to be transferred to the semiconductor conduction

band.

A second chemical route to promote titania photoresponse into the visible spectra is the

doping of TiO2 material either with metallic or non-metallic elements [75]. In this case,

doping introduces occupied or unoccupied orbitals in the band gap region leading to

negative or positive doping, respectively. A summary of some novel preparations of UV

and visible light responsive titania photocatalysts developed over the last few years has

been recently compiled [76].The limitations of titania semiconductor as a photocatalyst

for a particular use can be surmounted by modifying the surface of the semiconductor in

the following ways:

1.10.1 Metal semiconductor modification

Among the metal doped titania, Pt doping has recently attracted a great deal of attention

due to promising improvement in photo oxidation rate especially in gas phase. It has

been found that Pt–TiO2 improves the photo oxidation rate of ethanol, acetaldehyde and

acetone in the gaseous phase [77]. Li et al [78] reported a further development of

mesoporous titania as photocatalyst. These authors have prepared a photocatalyst

constituted by mesoporous titania embedding gold nanoparticles (Au/TiO2) whose

preparation requires P-123 as structure directing agent, a mixture of TiCl4 and Ti(OBu)4

18

and AuCl3 as the source of gold using ethanol as solvent. The gel is cast on a Petri dish

to form a thin layer that is subsequently aged at 373 K to form a homogeneous

mesostructured nanocomposite. Calcinations at 350°C in air remove the template while

inducing crystallization of TiO2 and formation of gold nanoparticles. TiO2 has also been

doped with other metals, including V, Cr, Mn, Fe and Ni. The presence of the dopant

was found to cause large shift in the absorption band of titanium dioxide towards the

visible region. However, there are contradictory reports, particularly in the case of metal

doping, describing either an increase or a decrease of the photocatalytic activity [79].

This controversy arises in part from the difficulty to establish valid comparisons

between the photocatalytic activity of various solids testing different probe molecules

and employing inconsistent parameters. Also, the doping procedure and the nature of

the resulting material are very often not well defined and, most probably, controversial

results can be obtained depending on the way in which the metal has been introduced. It

also depends on the final concentration of the dopant. Thus, it has often been reported

that there is an optimum doping level to achieve the maximum efficiency and beyond

this point a decrease in photocatalytic activity is again observed [80]. Nevertheless, a

generalised consensus has been reached with regards to the inappropriateness of metal

doping as valid solution to enhance the photocatalytic activity of TiO2.

1.10.2 Composite semiconductors

Figure 1.11 Photoexcitation in composite semiconductor photocatalyst.

The efficiency of a photocatalytic process can be increased by coupled semiconductor

photocatalysts which provide an interesting way to increase the charge separation, and

extending the energy range of photoexcitation for the system. Figure 1.11 illustrates

19

geometrically and energetically the photoexcitation process for the composite (coupled)

semiconductor photocatalyst CdS-TiO2. The energy of the excitation light is too small

to directly excite the TiO2 portion of the photocatalyst, but it is large enough to excite

an electron from the valence band across the band gap of CdS (Eg = 2.5 eV) to the

conduction band. The hole produced in the CdS valence band from the excitation

process remains in the CdS particle while the electron transfers to the conduction band

of the TiO2 particle. The electron transfer from CdS to TiO2 increases the charge

separation and efficiency of the photocatalytic process. The separated electron and hole

are then free to undergo electron transfer with adsorbates on the surface. The quantum

yield for the reduction of methylviologen drastically increased and approached an

optimum value of 1 when the concentration of TiO2 was increased in a CdS-TiO2

system [81]. The coupling of semiconductors with the appropriate energy levels can

produce a more efficient photocatalyst via better charge separation.

1.10.3 Surface sensitization

Figure 1.12 Excitation steps using dye molecule sensitizer.

The efficiency of the excitation process of wide band-gap semiconductor photocatalyst

(TiO2) can also be increased by surface sensitization of a chemisorbed or physisorbed

dyes. The photosensitization process can also expand the wavelength range of excitation

for the photocatalyst through excitation of the sensitizer followed by charge transfer to

the semiconductor. Erythrosin B, 3 thionine, and analogs of Ru (bpy) 3 2+ are some of

the common dyes which are used as sensitizers. Figure 1.12 illustrates the excitation and

charge injection steps involved for the regenerative dye sensitizer surface process.

Excitation of an electron in the dye molecule occurs to either the singlet or triplet

excited state of the molecule. If the oxidative energy level of the excited state of the dye

20

molecule with respect to the conduction band energy level of the semiconductor is

favourable (i.e. more negative), then the dye molecule can transfer the electron to the

conduction band of the semiconductor. The surface acts as a quencher accepting an

electron from the excited dye molecule. The electron in turn can be transferred to reduce

an organic acceptor molecule adsorbed on the surface.

1.10.4 Transition metal doping

An interesting area of semiconductor modification is the study of the influence of

dissolved transition metal impurity ions on the photocatalytic properties of TiO2. The

advantage of transition metal doping species is the improved trapping of electrons to

inhibit electron-hole recombination during light illumination. Different metals have

been employed to tune the electronic structure of TiO2-based material either by the ion

implantation method or by a wet chemical method [82]. The photocatalytic reactivity of

metal-doped TiO2 depends on many factors, including the dopant concentration, the

energy level pattern of the dopants within the TiO2 lattice, their d-electronic

configuration, and the distribution of the dopants. Enhanced photocatalytic activity has

been reported for Fe (III), Mo (V), Ru (III), Re (V), V (IV), and Rh (III) metals doped

TiO2 at 0.5% atomic ratio. It has been reported that the metal ions physically implanted

have produce significant changes in the TiO2 material. However, metal doping can

result in thermal instability and increased carrier trapping.

1.10.5 Non-metal doped TiO2 for photocatalysis

The elements used as the dopants in TiO2 cover almost the whole periodic table and is

summarized in Figure 1.13. Recent theoretical and experimental studies have shown

that the desired band gap narrowing of TiO2 can also be achieved by using a nonmetal,

although there is controversy regarding the origin of the resulting band gap narrowing.

Asahi and co-workers calculated the electronic band structures of anatase TiO2 with

different substitutional dopants, including C, N, F, P, or S and found that the

substitutional doping of N was the most effective for band gap narrowing. This is

because the p states of N mix with the 2p states of O, while the molecularly existing

species, e.g., NO and N2 dopants, give rise to the bonding states below the O 2p valence

21

bands, while the antibonding states deep within the band gap hardly interact with the

band states of TiO2 [83].

Figure 1.13 The elements used as the dopants in TiO2

Di Valentin et al. reported that the N 2p orbital formed a localized state just above the

top of the O 2p valence band, both for the anatase and rutile phases. However, these two

crystal systems showed the opposite photoresponce. In anatase, these dopant states

caused a red shift of the absorption band edge toward the visible region, while in rutile,

an overall “blue shift” was observed. This experimental evidence confirmed that N-

doped TiO2 formed N induced midgap levels slightly above the oxygen 2p valence band

[84]. Nakano et al. reported that in N-doped TiO2, deep levels located at approximately

1.18 and 2.48 eV below the conduction band were attributed to the O vacancy state and

a band gap narrowing was achieved by mixing with the O 2p valence band, respectively

[85]. N-doped TiO2 normally has a colour from white to yellow or even light gray, and

the onset of the absorption spectra red shifts to longer wavelengths. Table 1.3 shows

different methods used to prepare N-doped and S-doped titania. In N-doped TiO2

nonmaterial, the band gap absorption onset shifted 600 nm from 380 nm for the

undoped TiO2, extending the absorption up to 600 nm. The optical absorption of N-

doped TiO2 in the visible light region was primarily located between 400 and 500 nm,

while that for oxygen deficient TiO2 was mainly above 500 nm. N-F-co-doped TiO2

prepared by spray pyrolysis was shown to absorb light up to 550 nm in the visible

spectrum [102, 103]. Livraghi et al. recently found that N-doped TiO2 contained single

atom nitrogen impurity centres, localized in the band gap of the oxide, which were

responsible for the visible light absorption with promotion of electrons from the band

gap localized states to the conduction band [104].

22

Table 1.3 Various methods of preparation of N and S doped titania

N-Doped titania

Reference

By hydrolysis of TTIP in a water/amine mixture and the post

treatment of the TiO2 sol with amines

86, 87

Directly from a Ti-bipyridine complex 88

By ball milling of TiO2 in a NH3 water solution 89

By heating TiO2 under NH3 flux at 500-600 °C 90, 91

By calcination of the hydrolysis product ofTi(SO4)2

with ammonia as precipitation

92

By decomposition of gas-phase TiCl4 with

an atmosphere microwave plasma torch

93

By sputtering/ion-implanting techniques with

nitrogen or N2+ gas flux.

94, 95

S-Doped Titania Reference

By mixing TTIP with ethanol containing thiourea 96, 97

By heating sulfide powder 98, 99

10. By using sputtering or ion-implanting

techniques with S+ ion flux

100, 101

1.11 Synthetic Methods for TiO2 Nanostructures: Various synthetic methods used for

the preparation of TiO2 Nanostructures are as follows-

1.11.1 Sol-gel methods:

The sol-gel method is a versatile process which is widely used for synthesizing various

oxide materials [105-107]. This synthetic method generally allows control of the

texture, the chemical, and the morphological properties of the solid. This method also

has several advantages over other methods, such as allowing impregnation or co

precipitation, which can be used to introduce dopants. The major advantages of the sol-

gel technique includes molecular scale mixing, high purity of the precursors, and

homogeneity of the sol-gel products with a high purity of physical, morphological, and

chemical properties. In a typical sol-gel process, a colloidal suspension, or a sol, is

23

formed from the hydrolysis and polymerization reactions of the precursors, which are

usually inorganic metal salts or metal organic compounds such as metal alkoxides.

When a metal precursor alkoxide/salt reacts with water, nucleophilic substitution occurs

as shown in Figure 1.14 For the coordinative saturated metals such as the metal

alkoxides, hydrolysis and condensation both occur by nucleophilic substitution (SN)

mechanisms, in which coordination and proton transfer are involved and followed by

removal of either alcohol or water. Complete polymerization and loss of solvent leads to

the liquid sol transforming into a solid gel phase.

Figure 1.14 Hydrolysis and condensation pathways of metal alkoxides

Thin films can be produced on a piece of substrate by spin-coating or dip-coating. A

wet gel will form when the sol is cast into a mold, and the wet gel is converted into a

dense ceramic upon further drying and heat treatment. A highly porous and extremely

low-density material, called an aerogel, is obtained if the solvent in a wet gel is removed

under supercritical conditions. Figure 1.15 shows the conventional scheme to produce

aerogels. This method involves hydrolysis of the metal alkoxide with water and a

catalyst, i.e. an acid or a base, condensation into macromolecules, forming a colloidal

sol and subsequently three-dimensional network, solvent exchange to remove water by

alcohol, and then drying the wet gel using a supercritical fluid to produce the aerogel.

Aerogels can be used for advanced applications including electrochemical devices, thin

coatings, composite biomaterials, catalysts, ceramics, and heat and electric insulation

24

devices due to the aerogels having unique morphological and chemical properties.

Ceramic fibbers can be drawn from the sol when the viscosity is adjusted into the proper

range. Since the precursors react with water quickly and tend to precipitate, many

methods have been used to control the reaction rate in order to obtain desired

nanostructures. However, in binary metal systems, the main difficulty is to control the

hydrolysis rate of the precursors. Modifying the precursors with a suitable complexing

agent is one way to alter the hydrolysis rate, while generating in-situ water (instead of

externally added water) is another possible way to control the hydrolysis rate. In

practice, acetic acid is often used to modify the reactivity of metal alkoxides by

replacing the alkoxy groups bonded to the metal by acetate groups, forming M-OAc

complexes and alcohols.

Figure 1.15- Conventional scheme to produce aerogels

Moreover, acetic acid (HO-Ac) can react with the alcoholic solvent and intermediates,

(produced by modification reaction) to form water via an esterification reaction [108]

However, the properties of the sol-gel products depend on the precursors, processing

temperature, catalyst, solvents, and solvent removal process [109]

1.11.1.1 Mechanism of Sol-Gel Synthesis

The sol-gel process involves hydrolysis and condensation of the metal alkoxide

followed by heat treatment at elevated temperatures which induce polymerisation,

Metal alkoxide Water+ acid (or base)

+alcohol

Aging

Calcinatio

Hydrolysis

25

producing a metal oxide network [110]. In general, transition metals have low

electronegativity’s and their oxidation state is frequently lower than their coordination

number in an oxide network. Therefore, coordination expansion occurs spontaneously

upon reaction with water or other nucleophilic reagents to achieve their preferred

coordination [111]. Metal alkoxides are in general very reactive due to the presence of

highly electronegative OR groups (hard-π donors) that stabilise the metal in its highest

oxidation state and render it very susceptible to nucleophilic attack. The lower

electronegativity of transition metals causes them to be more electrophilic and thus less

stable toward hydrolysis, condensation and other nucleophilic reactions. Controlling the

conditions can be difficult but successful control of the reaction conditions has the

potential to produce materials of consistent size, shape and structure [112-114].

1.11.1.2 Mechanism of hydrolysis

As stated previously the sol-gel reaction involves hydrolysis of the metal alkoxide

followed by condensation. Hydrolysis of titanium alkoxides occurs through a

nucleophilic substitution (SN) reaction. When a nucleophile, such as water is introduced

to titanium alkoxide a rapid exothermic reaction proceeds. The nucleophilic addition

(AN) of water involves a proton from the attacking nucleophile (water) being

transferred to the alkoxide group. The protonated species is then removed as either

alcohol or water (scheme 1.) [107, 115].

HOH+ RO M → HO M + ROH

Scheme 1- Hydrolysis reaction

The nucleophilic substitution reaction that occurs during hydrolysis can be described as

follows [115]:

1. Nucleophilic addition of the H2O onto the positively charged metal atom.

2. Proton transfer, within the transition state from the entering molecule to the

leaving alkoxy group.

1.11.1.3 Mechanism of condensation

Condensation reactions complete the sol-gel process. Condensation can proceed through

either alcoxolation or through oxolation. In both processes an oxo bridge is formed

26

between the metals (M–O–M) but the leaving group differs. During alcoxolation, two

partially hydrolysed metal alkoxide molecules combine and an oxo bridge is formed

between the two metals with alcohol departing as the leaving group (scheme 2) [109,

115].

Scheme 2 Alcoxolation reaction

In oxolation (scheme 3) two partially hydrolysed metal alkides combine to form an oxo

bridge between the metal centres but water is the leaving group.

Scheme 3 Oxolation reaction

Condensation reactions ultimately polymerised to form large chains of molecules.

Solvent used also influence the reaction kinetics of sol-gel reactions. Because if a

suitable solvent is chosen; it may be preferentially hydrolysed over the alkoxide ligands.

This allows for greater control over the reaction kinetics through the correct use of

solvent [116]. Hydrolysis and condensation kinetics are also affected by the organic

ligand. The hydrolysis rate of titanium alkoxides decreases with increasing alkyl chain

length [117, 118]. This is mainly due to the steric effect which is expected for an

associative SN reaction mechanism

1.11.1.4 Role of the catalyst

Acid and base catalysts can have a strong influence over hydrolysis and condensation

rates as well as the structural properties of the final product. The addition of an acid will

27

result in the preferential protonation of the negatively charged alkoxide group (scheme

4). Protonation of the alkoxide group produces good leaving groups which enhance the

reaction kinetics, eliminating any proton transfer that occurs in the transition state and

thus allowing hydrolysis to go to completion upon the addition of water.

M OR + H+ → +

M O

Scheme 4- Acid catalyst reaction

In alkaline conditions, hydroxo ligands are deprotonated, producing strong

nucelophiles:

L–OH +: B → L–O- + BH+

where L = M or H and B = OH- or NH3. The hydrolysis rate of Ti (OBu) 4 was less in

basic conditions than in acidic or neutral conditions. It was postulated that the

nucleophilic addition of OH- may reduce the partial charge of the titanium metal centre

(δ (Ti)), making the hydrolysis reaction less favourable [109, 119]. While hydrolysis

kinetics is retarded in basic conditions, the condensation kinetics of metal alkoxides is

enhanced.

1.11.2 Hydrothermal Method

The hydrothermal method is widely used for the production of small particles in the

ceramics industry. Usually, this process is conducted in steel pressure vessels called

autoclaves under controlled temperature and/or pressure with the reactions occurring in

aqueous solution. The temperature can be elevated above the boiling point of water,

reaching the pressure of vapour saturation. The temperature and the amount of solution

added to the autoclave largely determine the internal pressure produced. The

hydrothermal method has been widely used to prepare TiO2 nanotubes after the

pioneering work by Kasuga et al. in 1998 [120]. Generally, several grams of TiO2

powders are added to concentrated NaOH solution and held at 20-110oC for 20 h in an

autoclave. TiO2 nanotubes were obtained after the products were washed with a dilute

HCl aqueous solution and distilled water. When the raw TiO2 material was treated with

H

R

28

NaOH aqueous solution, some of the Ti-O-Ti bonds were broken and new Ti-O-Na and

Ti-OH bonds were formed. New Ti-O-Ti bonds were formed after the Ti-O-Na and Ti-

OH bonds reacted with acid and water. The bond distance from one Ti atom to the next

Ti atom on the surface was found to decrease. This resulted in the folding of the sheets

and connection between the ends of the sheets, resulting in the formation of a tube

structure. In this mechanism, the TiO2 nanotubes were formed in the stage of the acid

treatment following the alkali treatment. The hydrothermal method is also currently

being used to produce many new hybrid materials with new morphologies and

improved crystallinity.

1.11. 3 Micelle and Inverse Micelle Methods

Micelles and inverse micelles are commonly employed to synthesize TiO2

nanomaterials. A statistical experimental design method was conducted by Kim et al. to

optimize experimental conditions for the preparation of TiO2 nanoparticles [121]. The

values of H2O/surfactant, H2O/titanium precursor, ammonia concentration, feed rate,

and reaction temperature were significant parameters in controlling TiO2 nanoparticle

size and size distribution.

1.11.4 Sol Method: The sol method here refers to the nonhydrolytic sol-gel processes

andusually involves the reaction of titanium chloride with a variety of different oxygen

donor molecules, e.g., a metal alkoxide or organic ether [122-125]

1.11.5 Solvothermal Method: The solvothermal method is almost identical to the

hydrothermal method except that the solvent used here is nonaqueous.

1.11.6 Direct Oxidation Method: TiO2 nanomaterials can be obtained by oxidation of

titanium metal using oxidants or under anodization.

1.11.7 Chemical Vapour Deposition: Vapour deposition refers to any process in which

materials in a vapour state are condensed to form a solid-phase material. Thick

crystalline TiO2 films with grain sizes below 30 nm as well as TiO2 nanoparticles with

29

sizes below 10 nm can be prepared by pyrolysis of TTIP in a mixed helium/oxygen

atmosphere, using liquid precursor delivery [126].

1.12 Characterization Methods for TiO2 Nanostructures

1.12.1 XRD: XRD gives the following information:

i) The nature of materials such as amorphous or crystalline

ii) Types of structure and phases

iii) Degree of crystallinity and thermal stability

iv) In case of metal doping in a particular structure it can tell us whether the metal

atom has been incorporated into the framework by measuring the unit cell volume.

The powdered diffraction pattern of titania semiconductors are generally recorded

between 2θ values of 10-90 0. XRD patterns of nano-TiO2 and microTiO2 in rutile and

anatase phases are shown in Figure 1.16 and Figure 1.17, respectively [127]. In Figure

1.16, XRD patterns exhibited strong diffraction peaks at 27°, 36° and 55° indicating

TiO2 in the rutile phase. On the other hand, in Figure 1.17, XRD patterns exhibited

strong diffraction peaks at 25° and 48° indicating TiO2 in the anatase phase. All peaks

are in good agreement with the standard spectrum (JCPDS no.: 88-1175 and 84-1286).

Figure 1.16 X-ray diffraction of rutile TiO2 (a) micropowders and (b)

nanopowders.

30

Figure 1.17 X-ray diffraction of anatase TiO2 (a) micropowders and (b)

nanopowders.

These results suggested that the nano-TiO2 powder is composed of irregular

polycrystalline. Amorphous revealed a broad pattern with low intensity; however, the

effect of the amorphous materials on the broadening of the XRD patterns of nanosized

TiO2 is negligible. The low-angle X-ray diffraction patterns of the mesostructured

titanium dioxide thin films before and after calcinations (573 or 673 K for 4 h) are

shown in Figure 1.18 [128].

Figure 1.18 Low-angle X-ray diffraction patterns of a mesostructured titanium

dioxide thin film before (––) and after (- - - -) calcination (573 K for 4 h). The insets

show expanded views of the (110) and (200) regions for each.

31

Table 1.4 2 Theta values and hkl planes of anatase, rutile and brookite phase of

titania.

PHASE 2-THETA hkl PLANE JCPDS

ANATASE 25.30 (101) 84-1286

37.80 (004)

540 (105)

BROOKITE 25.40 (120) 29-1360

30.80 (121)

40.20 (022)

RUTILE 27.30 (110) 86-1175

35.90 (101)

54.10 (211)

The XRD of the as-synthesised films display 3 peaks in the low-angle range with d

(interatomic plane) spacing corresponding to 9.59 nm, 5.66 nm and 4.93 nm.

1.12.2 Raman Spectroscopy

Raman spectroscopy is used in condensed matter physics and chemistry to study

vibrational, rotational, and other low-frequency modes in a system. It relies on inelastic

scattering, or Raman scattering of monochromatic light, usually from a laser. The laser

light interacts with phonons or other excitations in the system, resulting in the energy of

the laser photons being shifted up or down. The shift in energy gives information about

the phonon modes in the system. Infrared spectroscopy yields similar, but

complementary information.

32

Figure 1.19 Raman spectra of anatase, rutile and brookite phases of titania

The three crystalline phases of titania; anatase, rutile and brookite, exhibit well-

separated and distinct Raman activities (figure 1.19) following 1064 nm excitation

which allows unambiguous identification of phase composition [129]. Knowledge of

the Raman scattering cross-sections allows analysis of both crystallite size and phase

compositions by comparing the relative intensities of isolated bands such as those

appearing at 320 cm-1

(brookite), 608 cm-1

(rutile) and 519 cm-1

(anatase)[130, 131].

1.12.3 I.R.Study

The Diffuse Reflectance Fourier-Transform Infrared (DRFT-IR) spectroscopy helps to

identify characteristic bands of the mesoporous framework. For example let us consider

the FT-IR spectra of the F-doped TiO2 sol particles shown in Fig. 1.20 [132]. The TiO2

sol particles showed the main bands at 400–700 cm–1, which were attributed to Ti-O

stretching and Ti-O-Ti bridging stretching modes. The small peak at 889.0 cm–1 was

attributed to Ti-F vibration [133]. The peak at 1384.0 cm–1 was produced by NO3–1. The

stronger peak at 1626.0 cm–1 was attributed to bending vibrations of O-H and N-H. The

IR spectra of the sol sample dried at 383 K revealed that Ti-O, Ti-F, N-H, Ti-OH, H-O-

H groups existed in the as-prepared sample by sol-gel-hydrothermal method.

33

Figure 1.20 FT-IR spectra of the F-doped TiO2 sol particles

The forming of Ti-F indicated that F atoms were incorporated into the TiO2 crystal

lattice.

1.12.4 UV-Vis Spectroscopy

Many molecules absorb ultraviolet (UV) or visible light. The absorption of UV or

visible radiation is caused by the excitation of outer electrons, from their ground state to

an excited state. This technique is based on the reflection of light in the ultra violet,

visible and near infrared region by a powdered sample. The DRS study (Fig. 1.21) gives

valuable information on the coordination of the transition metal such as Ti. Titania

samples have in common an intense UV absorption band with a maximum in the range

220–320 nm due to charge transfer from oxygen to titanium (IV) [134]. The position of

this band is affected by the coordination geometry around the titanium atom and by the

presence of adsorbents. More precisely, the bands in the region 210– 240 nm are

attributed to oxygen to tetrahedral Ti (IV) [135], whereas the bands at higher

wavelength (λ>240 nm) are due to octahedral Ti (IV) sites [136]. For TiO2 in the form

of anatase the transition occurs in 315 nm region. Hence Diffuse Reflectance UV-

Visible Spectroscopy is a widely available technique and is a very useful tool for

characterization TiO2 structure.

34

Figure 1.21 DRS of F-doped titania

The absorption spectra of pure undoped TiO2, pure F-doped TiO2 and F- doped TiO2 sol

prepared by the sol-gel-hydrothermal method and dried in a rotatory evaporator and

dried in vacuum at 333 K was shown in Fig.1.21A [132]. It was found that sample F-

TiO2 could cause a new absorption band in the visible range of 400–600 nm apart from

the fundamental absorption edge of TiO2, which was located in the UV region at about

385 nm (shown in Fig. 1.21C); whereas, pure F-TiO2 did not lead to any significant

shift in optical absorption of TiO2 (shown in Fig. 1.21B).

1.12.5 X-ray Photoelectron Spectroscopy (XPS)

XPS is a quantitative spectroscopic technique that measures the elemental composition,

empirical formula, and the chemical state and electronic states of the elements that exist

within a material. XPS spectra are obtained by irradiating a material with a beam of

aluminium or magnesium X-rays while simultaneously measuring the kinetic energy

(KE) and number of electrons that escape from the top 1 to 10 nm of the material being

analysed. Because the energy of a particular X-ray wavelength equals a known quantity,

we can determine the electron binding energy (BE) of each of the emitted electrons by

using an equation that is based on the work of Ernest Rutherford (1914):

Ebinding = Ephoton - Ekinetic – Φ; where E binding is the energy of the electron

emitted from one electron configuration within the atom, E photon is the energy of the

35

X-ray photons being used, E kinetic is the kinetic energy of the emitted electron as

measured by the instrument, and Φ is the work function of the spectrometer.

Fig. 1.22 XPS survey spectra of the 4%Fe doped TiO2 films grown on LAO

substrate. [137].

Fig. 1.23 XPS spectra of (a) N-doped mesoporous titania (1:1), (b) N-doped

mesoporous titania (1:2) and (c) N-doped mesoporous titania (1:3) [138].

Figure 1.22 and 1.23 gives the XPS spectra of Fe-doped and N-doped titania.

36

1.12.6 Thermal Analysis

Thermal analysis is an important technique for the characterization of solid materials, as

it provides information on the physical and chemical changes involving endothermic

and exothermic processes, temperatures for phase transitions, melting points and

crystallization, and the weight loss when the temperature is increased. TGA can

determine: (1) moisture/liquid content and the presence of volatile species, (2)

decomposition temperatures, and (3) the rate of degradation.

Figure 1.24 gives the TGA data of Er-doped luminescent TiO2 nanoparticles [139]. The

results indicate there were about 4% weight loss from room temperature to 773 K and

little weight loss after that. The weight loss from room temperature to 373 K is almost

certainly from removal of water, while it is suspected that the weight loss above 373 K

is due to removal of water, and perhaps hydroxyl groups.

Figure 1.24: TGA data obtained from 3% Er-doped TiO2

Full decomposition of hydroxyl groups requires a heat treatment around 1273 K;

however, it is seen that little weight loss between 773 K and 1073 K, indicating that

only trace structural water is present at 773 K since most would likely be removed by

1073 K.

1.12.7 N2 Physisorption

The textural characterization, such as surface area, pore volume and the pore size

distribution of the titania samples can be obtained by N2 physisorption studies. Prior to

N2 physisorption, the samples have to be degassed at 473 K under vacuum for 2 hours.

From the N2 adsorption isotherms, the specific surface area can be calculated. The

37

mesopore volume (VBJH), the average pore diameters (Dpore), and the pore size

distributions can be estimated by the Barret–Joyner– Halenda (BJH) method applied to

the desorption branch of the isotherm.

BET Surface Area

When a highly dispersed solid is exposed in a closed space to a gas or vapour at some

definite pressure, the solid begins to adsorb the gas. Adsorption on porous materials

proceeds via monolayer adsorption followed by multilayer adsorption and capillary

condensation. At a low relative pressure, a monolayer of adsorbent is absorbed on the

surface like non porous materials. The amount of adsorbent is used to calculate the

surface area. Using the Brunauer, Emmett and Teller (BET) theory, one can calculate

the specific surface area of a solid sample from the number of monolayer gas molecules

required to cover the solid surface, and the cross-sectional area of the gas molecule

being adsorbed.

BJH Pore-Size Distribution and Pore Volume

According to the IUPAC definitions, microporous, mesoporous and macroporous

materials exhibit pore diameters of less than 2 nm, in the range of 2-50 nm and above

50 nm, respectively [140]. During a physisorption process, beyond a monolayer

formation, the amount of adsorbed gas gradually increases as the relative pressure

increases, and then at higher pressures, the amount of gas adsorbed increases steeply

due to capillary condensation in the mesopores. Once these pores are filled, the

adsorption isotherm is complete. The capillary condensation is believed to be

proportional to the equilibrium gas pressure and the size of the pores inside the solid.

The Barrett, Joyner and Halenda (BJH) computational method allows the determination

of pore sizes from the equilibrium gas pressures.

Hysteresis Loops

After an adsorption isotherm, desorption isotherm can be generated by withdrawing the

adsorbed gas by reducing the pressure. However, capillary condensation and capillary

evaporations do not take place at the same pressure, resulting in hysteresis loops. The

resulting hysteresis loops can be mechanistically related to particular pore shapes.

38

Figure 1.25 shows the classification of hysteresis loops denoted as H1, H2, H3 and H4

by IUPAC. Type H1 and H2 loops were obtained from agglomerated spherical particles

and corpuscular systems, respectively, while type H3 and H4 were obtained from slit-

shaped pores or plate-like particles.

Figure 1.25 Hysteresis loops

Figure 1.26 BET surface area of Phosphated titania [141]

Figure 1.26 gives the BET surface area of phosphate titania which demonstrates that the

structure of pure titanium dioxide collapses after calcination at 973 K. In contrast,

hysteresis observed in the case of phosphated titanium dioxide samples is indicative of

porous structure. The specific surface area (as BET) increases with increasing

phosphate content at a given calcination temperature

39

1.12.8 Scanning electron microscopy (SEM)

Manfred von Ardenne pioneered the scanning electron microscope (SEM) and built his

universal electron microscope in the 1930s. In the SEM, a very fine beam of electrons

with energies up to several tens keV is focused on the surface of a specimen, and is

scanned across it in a parallel pattern. The intensity of emission of secondary and

backscattered electrons is very sensitive to the angle at which the electron beam strikes

the surface of the sample. The emitted electron current is collected and amplified. The

magnification produced by the SEM is the ratio between the dimension of the final

image display and the field scanning on the specimen. Usually, the magnification range

of the SEM is between 10 to 222,000 times, and the resolution is between 4-10 nm

(figure 1.27).

Figure 1.27: SEM images of Ni-doped TiO2 spheres: (a) side and (b) top views.

SEM images of Co-doped TiO2 spheres: (c) side and (d) top views [142].

Generally, the TEM resolution is about an order of magnitude greater than the SEM

resolution, however, because the SEM image relies on surface processes rather than

transmission, it is able to image bulk samples and has a much greater depth of view, and

so can produce images that are a good representation of the overall 3D structure of the

sample.

SEM images of the Fe-doped TiO2 agglomerates are presented in Fig. 1.27.

40

1.12.9 Transmission Electron Microscopy (TEM)

In 1931, Knoll and Ruska built the first electron microscope prototype. In 1938, Eli

Franklin Burton built the first practical electron microscope and in 1939, Siemens

produced the first commercial TEM. However, the TEM was not used for material

studies until 40 years ago, when the thin-foil preparation technique was developed.

More improvements of the TEM technique in the 1990s provided 0.1 nm of resolution,

which made TEM an indispensable analysis technique for studying materials in the

micron or nano size region.

TEM bright field images of TiO2 nanopowders in anatase phases are shown in Figure

(1.28a), respectively [127].

Figure 1.26: Images of anatase phase. (a) TEM image of micro-TiO2 powder; (b)

TEM image of nano-TiO2 powder; (c) SAED pattern of nano-TiO2 powder and (d)

HRTEM image of nano-TiO2 powder.

41

It can be estimated that the particle size of powders in Figure (1.28 b) are nanoscale

with the grain size less than 10 nm. The corresponding selected area electron diffraction

(SAED) patterns of nano- TiO2 powder anatase phases are shown in Figure (1.28c),

respectively. SAED patterns of nano-TiO2 powders in anatase phase (Figure (1.28c)

shows that the brightness and intensity of polymorphic ring is weak, so they are poorly

crystallized and partly amorphous. The crystallinity of nano-TiO2 powders can also be

observed by phase-contrast images or Moire patterns. Figure (1.28) show crystal lattice

planes of nano-TiO2 in rutile and anatase phases, respectively. It is seen that, anatase

give many crystal lattice planes with d-spacing of 0.313 nm for the plane (101).

1.12.10 Photoluminescence:

Photoluminescence (PL) emission spectra have been widely used to investigate the

efficiency of charge carrier trapping, migration, and transfer in order to understand the

fate of electron/hole pairs in semiconductor particles, since PL emission results from the

recombination of free carriers. Figure 1.29a [143] shows the room-temperature PL

spectra for C-doped and undoped TiO2 using the excitation light of 280 nm UV light. It

can also be seen that the PL intensity of the undoped TiO2 is much higher than that in

the spectra of C-doped samples, indicating that carbon doping can effectively inhibit

excited electron and hole recombination. The effective inhibition of excited electron and

hole recombination due to carbon doping can be explained by the following two

pathways. First, the surface hydroxyl groups can be trapped by the holes to form

hydroxyl radicals which can suppress electron-hole recombination, thus the PL

intensity. Second, theoretical and experimental investigation indicated that carbon

doping favours the formation of oxygen vacancies. The electron is trapped by the

oxygen vacancy, while the hole is trapped by the doped carbon, which can decrease the

PL intensity. However, no PL signal is observed for undoped TiO2, while a strong PL

peak is excited for C-doped TiO2 under the excitation of 514.5 nm visible light (Figure

1.29b). The energy of the excitation visible light used is not sufficient enough to

promote electronic transitions from the VB to conduction band (CB) for undoped TiO2

according to the UV-vis DRS and VB XPS. As a result, no electron/hole pairs can be

generated to give PL signals for undoped TiO2 under 514.5 nm light irradiation. As for

C-doped TiO2, electron/hole pairs can be generated and recombine radiatively to give

42

broad and strong PL signals. This can be ascribed to the carbon doping which reduces

the band gap and increases visible light absorption.

Figure 1.29: PL spectra of undoped TiO2 and C-doped TiO2 under UV-280 nm (a)

and visible light-514.5 nm (b).

1.12.11 EPR Study

This technique provides the information of oxidation state of the metal atom in the

mesopores and the position of the metal ion in the framework

1.12.12 Photocatalysis of dyes with titania:

The effluents, gaseous or liquid produced by some of our industries are harmful to the

health and general well-being of man. When undesirable substances are present in liquid

effluents, it can be disastrous as their presence pose severe threat to the immediate

recipients. Wastewaters from various industries, factories, laboratories, etc. are serious

problems to the environment. The discharged wastes containing dyes are toxic to

microorganisms, aquatic life and human beings [144]. These deleterious effects of

chemicals on the earth ecosystems are a cause for serious concern. Several of these

chemicals such as azo dyes, herbicides, and pesticides are actually present in rivers and

lakes, and are in part suspected of being endocrine-disrupting chemicals (EDCs) [145,

146]. Konstantinou and Albanis [147] reported that textile dyes and other industrial

dyestuffs constitute one of the largest groups of organic compounds that represent an

increasing environmental danger. About 1–20% of the total world production of dyes is

43

lost during the dying process and is released in the textile effluents [148]. Photocatalysis

may be termed as a photoinduced reaction which is accelerated by the presence of a

catalyst [37]. These types of reactions are activated by absorption of a photon with

sufficient energy (equals or higher than the band-gap energy (Eg) of the catalyst). The

absorption leads to a charge separation due to promotion of an electron (e−) from the

valence band of the semiconductor catalyst to the conduction band, thus generating a

hole (h+) in the valence band. The recombination of the electron and the hole must be

prevented as much as possible if a photocatalyzed reaction must be favoured. The

ultimate goal of the process is to have a reaction between the activated electrons with an

oxidant to produce a reduced product, and also a reaction between the generated holes

with a reductant to produce an oxidized product. The photogenerated electrons could

reduce the dye or react with electron acceptors such as O2 adsorbed on the Ti (III)-

surface or dissolved in water, reducing it to superoxide radical anion O2−• . The

photogenerated holes can oxidize the organic molecule to form R+, or react with OH− or

H2O oxidizing them into OH• radicals. Together with other highly oxidant species

(peroxide radicals) they are reported to be responsible for the heterogeneous TiO2

photodecomposition of organic substrates as dyes. The resulting •OH radical, being a

very strong oxidizing agent (standard redox potential +2.8 V) can oxidize most azo dyes

to the mineral end-products. According to this, the relevant reactions at the

semiconductor surface causing the degradation of dyes can be expressed as follows

[149]:

TiO2 +hv (UV) → TiO2 (eCB− +hVB+) (1)

TiO2 (hVB+) + H2O → TiO2 +H+ +OH• (2)

TiO2 (hVB+) + OH−→ TiO2 +OH• (3)

TiO2 (eCB−) + O2→ TiO2 +O2−• (4)

O2−• + H+→ HO2

• (5)

Dye + OH• → degradation products (6)

Dye + hVB+→ oxidation products (7)

Dye + eCB−→ reduction products (8)

where hv is photon energy required to excite the semiconductor electron from the

valence band (VB) region to conduction band (CB) region.

44

Table 1.5 Photocatalysis of different dyes with titania:

Dye degraded Type of

Catalyst

Reference

no

Methylene Violet(MV), cationic red (X-GRL) Ag–TiO2 150

Acid orange 7 (AO7) Ag–TiO2 151

Sirius Gelb GC (SG-GC) Ag–TiO2 152

Rhodamine B (RB) Ag0–TiO2

nanosol

153

Acid red B (ARB), reactive red (K-2G), cationic red (X-

GRL reactive brilliant red (X-3B )

Ag–AgBr–

TiO2

154

Methylene blue (MB) Au–TiO2 155

Fast Green FCF, Patent Blue VF TiO2 156

Orange G Sn-TiO2 157

BRL K-TiO2 158

Methyl orange Pt-TiO2 159

Acid Red B Ce-TiO2 160

Orange II Zn-TiO2 161

Bromocresol purple TiO2 162

methyl orange (MO), thymol blue (TB), and bromocresol

green (BG)

M-TiO2 , (M =

Ni, Cu, Zn)

163

1.13. Specific aims of this dissertation

Since the discovery of photoinduced splitting of water on single crystal TiO2 electrodes,

the technology of semiconductor-based photocatalysis has shown potential application

in various areas such as sewage disposal, air purification, antibiosis and detoxification,

glass antifogging and self-cleaning, reclamation of precious metal ions, and

development of solar energy. TiO2 is a chemically stable, nontoxic, biocompatible,

inexpensive material with very high dielectric constant and interesting photocatalytic

activities. It is a wide-gap semiconductor, and depending on its chemical composition, it

shows a large range of electrical conductivity. However, the performance of TiO2

45

nanomaterials strongly relies on their crystallinity, crystallite size, crystal structure,

specific surface area, thermal stability and quantum efficiency. Bulk modifications such

as cation or anion doping have been found very effective to improve the properties of

TiO2 to enhance their performance. Several different processing techniques have been

used for the preparation of TiO2-based nanostructures, such as template techniques,

hydrothermal processes, and soft chemical processes. However, each of these methods

has limitations. The main objective of this project was to develop a synthesis route to

produce superior quality TiO2 based nanomaterials by introducing Zr, Fe, Co, N, S, and

P as doping materials.

The following have been identified as the specific objectives of this project:

I. Synthesis of the following nanomaterials using sol-gel technique and hydrothermal

technique.

A. Non-metal doped titania

(i) S-doped TiO2

(ii) P- doped TiO2

(iii) N- doped TiO2

B. Metal doped titania

(i) Fe- doped TiO2

(ii) Zr- doped TiO2

(iii) Co- doped TiO2

II. Characterization of the synthesized nanomaterials using different physical and

chemical techniques in order to,

a. Study the surface characteristics, morphology, composition and electronic

properties.

b. Investigate the effects of different operating variables and doping agents (ions)

on the properties.

III. Evaluation of the performance of selective nanomaterials as photocatalysts for the

degradation of different dyes as well as some pesticides.

46

References

[1] O. Carp, C. L. Huisman, A. Reller, Progress in Solid State Chem. 32 (2004) 33.

[2] Y. Hu, H. L. Tsai, C. L. Huangk, Eur. Ceram. Soc. 23 (2003) 691.

[3] Y. Shao, D. Tang, J. Sun, Y. Lee, W. Xiong, China Particuology, 2 (2004) 119.

[4] D. Nicholls, Complexes and First-Row Transition Elements; MacMillan

Education: Hong Kong, 1974.

[5] X. Chen, S. S. Mao, Chem. Rev. 107 (2007) 2891.

[6] http://staff.aist.go.jp/nomurak/common/STRUCIMAGES/Anatase.gif&imgrefurl=

http://staff.aist.go.jp/nomura; 27-May-2010

[7] A. L. Linsebigler, G. Lu, Jr. J. T. Yates, Chem. Rev. 95 (1995) 735.

[8] A. Wisitsoraat, A. Tuantranont, E. Comini, G. Sberveglieri, W. Wlodarski, Thin

Solid Films. 517 (2009) 2775.

[9] L. Kavan, M. Gratzel, S. E. Gilbert, C. Klemenz, H. I. Scheel, J. Am. Chem.

Soc. 118 (1996) 6716.

[10] N. Hosaka, T. Sekiya, C. Aatoko, S. Kurita, J. Phys. Soc. Japan. 66 (1997) 877.

[11] N. Hosaka, T. Sekiya, M. Fujisawa, C. Satokob, S. Kurita, J. Electron Spectrosc.

Relat. Phenom. 78 (1996) 75.

[12] R. Asahi, R. Y. Taga, W. Mannstadt, A. J. Freeman, J. Phys. Rev. B 61 (2000)

7459.

[13] A. Amtout, R. Leonelli, Phys. Rev. B, 51 (1995) 6842.

[14] M. Koelsch, S. Cassaignon, C. T. Thanh Minh, J. F. Guillemoles, J. P. Jolivet,

Thin Solid Films.86 (2004) 451.

[15] Y. Wang, Y. Huang, W. Ho, L. Zhang, Z. Zou, S. Lee, J. Haz. Mater. 169 (2009)

77.

[16] S. N. Frank, A. Bard, J. Am. Chem. Soc. 99 (1977) 303.

[17] J. Yu, H. Yu, C. Ao, S. C. Lee, J. C. Yu, W. Ho, Thin Solid Films, 496

(2006)273.

[18] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura,

Nature, 388 (1997) 431.

[19] B. O'Regan, M. Gratzel, Nature, 238 (1991) 37.

[20] Q. Zhang, L. Gao, J. Guo, Appl. Catal. B. Environ. 26 (2000) 207.

[21] S. R. Yoganarasimhan, C. N. R. Rao, Trans. Faraday Soc. 58 (1962) 1579.

47

[22] K. J. D. MacKenzie, Trans. J. Br. Ceram. Soc. 74 (1975) 29.

[23] C. Byun, J. W. Wang, L. T. Kim, K. S. Hong, B.W. Lee, Mater. Res. Bull. 32

(1997) 431.

[24] K. J. D. MacKenzie, Trans. J. Br. Ceram. Soc. 74 (1975) 121.

[25] Y. Li, T. J. White, S. H. Lim, J. Solid State Chem. 177 (2004) 1372.

[26] X. Z. Ding, X. H. Liu, J. Mater. Res. 13 (1998) 2556.

[27] H. Zhang, J. F. Banfield, J. Mater. Chem. 8 (1998) 2073.

[28] H. Liu, S. Cheng, M. Wu, J. Zhang, W. Li, C. Cao, J. Phys. Chem. A. 104

(2000) 7016.

[29] Y. U. Ahn, E. J. Kim, H. T. Kim, S. H. Hahn, Mater. Lett. 57 (2003) 4660.

[30] K. P. Kumar, K. Keizer, A. J. Burggraaf, T. Okubo, H. Nagamoto, J. Mater.

Chem. 3 (1993) 1151.

[31] J. Aguado, R. van Grieken, M. J. Lopez-Munoz, J. Marugan, Catal. Today, 75

(2002) 95.

[32] K. N. P. Kumar, K. Keizer, A. J. Burggraaf, Nature, 48 (1992) 358.

[33] U. Diebold, Surf. Sci. Reports, 48 (2003) 53.

[34] A. W. Czanderna, C. N. R. Rao, J. M. Honig, Trans. Faraday Soc. 54 (1958)

1069.

[35] G. L. Miessler, D. A. Tarr, D. Inorganic Chemistry; Prentice-Hall Inc.: New

Jersey, 1999.

[36] D. F. Shriver, P. W. Atkins, Inorganic Chemistry; Oxford University Press:

Oxford, 1999.

[37] A. Mills, S. Le Hunte, J. Photochem. Photobiol. A 108 (1997) 1.

[38] S. U. M. Khan, J. Akikusa, J. Phys. Chem. B, 103 (1999) 7184.

[39] A. Fujishima, T. N. Rao, D. A. Tryk, J. Photochem. Photobio. C 1(2000)1.

[40] D. A. Tryk, A. Fujishima, K. Honda, Electrochimica Acta. 45 (2000) 2363.

[41] M. Grätzel, Nature, 414 (2001) 338.

[42] J. C. Ireland, P. Klostermann, E. W. Rice, R. M. Clark, Appl. Environ.

Microbiol. 59 (1993) 1668.

[43] R. Cai, K. Hashimoto, Y. Kubota, A. Fujishima, Chem. Lett. 3 (1992) 427.

[44] K. E. Karakitsou, X. E. Verykios, J. Phys. Chem. 97 (1993) 1184.

[45] M. Nair, Z. H. Luo, A. Heller, Ind. Eng. Chem. Res. 32 (1993) 2318.

48

[46] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341.

[47] R. W. Matthews, J. Catal. 113 (1988) 549.

[48] G. Li, S. Ciston, Z. V. Saponjic, L. Chena, N. M. Dimitrijevic, T. Rajh, K. A.

Graya, J. Catal. 253 (2008)105.

[49] G. Li, K. A. Gray, Chem. Phy. 339 (2007) 173.

[50] J. C. Sjogren, R. A. Sierka, Appl. Environ. Microbiol. 60 (1994) 344.

[51] A. Dodda, A. McKinleya, T. Tsuzuki, M. Saunders, J. Phys. Chem. Solids. 68

(2007) 2341.

[52] V. M. Cristante, A. G. S. Prado, S. M. A. Jorge, J. P. S. Valente, A. O.

Florentino, P. M. Padilha, J. Photochem. Photobiol. A. 195 (2008) 23.

[53] J. M. Herrmann, Topics in Catal. 34 (2005) 49.

[54] A. J. Nozik, R. Memming, J. Phys. Chem. 100 (1996) 13061.

[55] J. Yu, H. Yu, B. Chenga, C. Trapalis, J. Mol. Catal. A: Chemical, 249 (2006)

135.

[56] C. R. Martin, P. Kohli, Nat. Rev. Drug Discovery, 2 (2003) 29.

[57] Y. Wan, J. Ma, W. Zhou, Y. Zhu, X. Song, H. Li, Appl. Catal. A: General, 277

(2004) 55.

[58] J. Pena, M. Vallet-Regi, J. San Roman, J. Biomed. Mater. Res. 35 (1997) 129.

[59] R. Gangopadhyay, A. De, Chem. Mater. 12 (2000) 608.

[60] S. M. Khaled, R. Sui, P. A. Charpentier, A. S. Rizkalla, Langmuir, 23 (2007)

3988.

[61] L. Zan, L. Tian, Z. Liu, Z. Peng, Appl. Catal. A: General. 264 (2004) 237.

[62] H. Yuvaraj, E. J. Park, Y. S. Gal, K. T. Lima, Coll. and Surf. A: Physicochem.

Eng. Aspects 313 (2008) 300.

[63] O. K. Varghese, M. G. K. C. A. Grimes, M. Paulose, N. Mukherjee, J. Nanosci.

Nanotechnol. 4 (2004) 733.

[64] A. Kudo, Catal. Surv. Asia, 7 (2003) 31.

[65] M. Kitano, M. Matsuoka, M. Ueshima, M. Anpo, Appl. Catal. A. General, 325

(2007) 1.

[66] J. Cunningham, G. Al-Sayyed, P. Sedlak, J. Caffrey, Catal. Today, 53 (1999)

145.

[67] D. B. Hamal, K. J. Klabunde, J. Colloid. Interface. Sci. 311 (2007) 514.

49

[68] Y. L. Kuo, H. W. Chen, Y. Ku, Thin Solid Films. 515 (2007) 3461.

[69] J. Zhu, F. Chen, J. Zhang, H. Chen, M. Anpo, J. Photochem. Photobiol. A, 180

(2006) 196.

[70] E. Borgarello, J. Kiwi, M. Gratzel, E. Pelizzetti, M. Visca, J. Am. Chem.

Soc.104 (1982) 2996.

[71] D. Li, N. Ohashi, S. Hishita, T. Kolodiazhnyi, H. Haneda, J. Sol. State Chem.

178 (2005) 3293.

[72] D. I. Kreller, P. V. Kamat, J. Phys. Chem. 95 (1991) 4406.

[73] Y. Ohsaki, N. Masaki, T. Kitamura, Y. Wada, T. Okamoto, T. Sekino, K.

Niihara, S. Yanagida, Phys. Chem. Chem. Phys. 7 (2005) 4157.

[74] J. E. Kroeze, T. J. Savenije, J. M. Warman, J. Am. Chem. Soc. 126 (2004) 7608.

[75] R. Bacsa, J. Kiwi, T. Ohno, P. Albers, V. Nadtochenko, J. Phys. Chem. B, 109

(2005) 5994.

[76] U. I. Gaya, A. H. Abdullah, J. Photochem. Photobiol. C, 9 (2008) 1.

[77] M. A. Aramendia, J. C. Colmenares, A. Marinas, J. M. Marinas, J. M. Moreno,

J. A. Navio, F. J. Urbano, Catal. Today, 128 (2007) 235.

[78] H. Li, Z. Bian, J. Zhu, Y. Huo, H. Li, Y. Lu, J. Am. Chem. Soc. 129 (2007)

4538.

[79] Y. J. Lin, S. L. Tseng, W. J. Huang, W.J. Wu, J. Environ. Sci. Health, Part B, 41

(2006) 1143.

[80] T. Okato, T. Sakano, M. Obara, Phys. Rev. B, 72 (2005) 115124.

[81] L. Spanhel, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 6632.

[82] X. H. Xu, M. Wang, Y. Hou, W. F. Yao, D. Wang, H. Wang, J. Mater. Sci. Lett.

21 (2002) 1655.

[83] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, K. Y. Taga, Science, 293 (2001)

5528.

[84] C. DiValentin, G. Pacchioni, A. Selloni, S. Livraghi, E. Giamello, J. Phys.

Chem. B, 109 (2005) 11414.

[85] Y. Nakano, T. Morikawa, T. Ohwaki, Y. Taga, Appl. Phys. Lett. 5 (2005) 87.

[86] X. Chen, Y. Lou, C. Burda, Int. J. Nanotechnol. 1 (2004) 105.

[87] J. L. Gole, J. D. Stout, C. Burda, Y. Lou, X. Chen, J. Phys. Chem. B, 108

(2004) 1230.

50

[88] H. Irie, Y. Watanabe, K. Hashimoto, J. Phys. Chem. B, 107 (2003) 5483.

[89] S. Chen, L. Chen, S. Gao, G. Cao, Chem. Phys. Lett. 413 (2005) 404.

[90] O. Diwald, T. L. Thompson, T. Zubkov, E. Goralski, S. D. Walck, Jr. J. T.

Yates, J. Phys. Chem. B, 108 (2004) 6004.

[91] R. Nakamura, T. Tanaka, Y. Nakato, J. Phys. Chem. B, 108 (2004) 10617.

[92] X. H. Huang, Y. C. Tang, C. Hu, H. Q. Yu, C. S. Chen, J. Environ. Sci. (Beijing,

China) 17 (2005) 562.

[93] Y. C. Hong, C. U. Bang, D. H. Shin, H. S. Uhm, Chem. Phys. Lett. 413(2005)

454.

[94] J. T. Chang, Y. F. Lai, J. L. He, Surf. Coat. Technol. 200 (2005) 1640.

[95] O. Diwald, T. L. Thompson, E. G. Goralski, S. D. Walck, Jr. J. T. Yates, J. Phys.

Chem. B 108 (2004) 52.

[96] T. Ohno, Water Sci. Technol. 49 (2004) 159.

[97] T. Ohno, T. Mitsui, M. Matsumura, Chem. Lett. 32 (2003) 364.

[98] T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui, M. Matsumura,

Appl. Catal. A, General, 265 (2004) 115.

[99] T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, Appl. Phys. Lett. 81 (2002) 454.

[100] T. Umebayashi, T. Yamaki, S. Tanaka, K. Asai, Chem. Lett. 32 (2003) 330.

[101] T. Umebayashi, T. Yamaki, S. Yamamoto, A. Miyashita, S. Tanaka, T. Sumita,

K. Asai, J. Appl. Phys. 93 (2003) 5156.

[102] T. Umebayashi, T. Yamaki, T. Sumita, S. Yamamoto, A. Miyashita, S. Tanaka,

K. Asai, Trans. Mater. Res. Soc. Jpn. 28 (2003) 461.

[103] D. Li, H. Haneda, S. Hishita, N. Ohashi, Chem. Mater. 17 (2005) 2596.

[104] S. Livraghi, M. C. Paganini, E. Giamello, A. Selloni, C. D. Valentin, G.

Pacchioni, J. Am. Chem. Soc. 128 (2006) 15666.

[105] A. C. Pierre, G. M. Pajonk, Chem. Rev. 102 (2002) 4243.

[106] Z. L. Lu, E. Lindner, H. A. Mayer, Chem. Rev. 102 (2002) 3543.

[107] A. P. Wight, M. E. Davis, Chem. Rev. 102 (2002) 3589.

[108] C. C. Legrand-Buscema, S. B. Malibert, Thin Solid Films, 418 (2002) 79.

[109] C. J. Brinker, G. W. Scherer, Sol-Gel Science: The physics and Chemistry of

Sol-Gel Processing Academic press: New York, 1990.

51

[110] A. Verma, S. B. Samanta, A. K. Bakhshi, S. A. Agnihotry, Sol. Energy Mater.

Sol. Cells, 88 (2005) 47.

[111] S. Barboux-Doeuff, C. Sanchez, Mater. Res. Bull. 29 (1994) 1.

[112] M. Keshmiri, T. Troczynski, M. Mohseni, J. Haz. Mater. 128 (2006) 130.

[113] D. S. Muggli, J. T. McCue, J. L. Falconer, J. Catal. 173 (1998) 470.

[114] S. I. Nishimoto, B. Ohtani, H. Kajiwara, T. Kagiya, J. Chem. Soc. Faraday

Trans. 81 (1985) 61.

[115] C. Sanchez, J. Livage, M. Henry, F. Babonneau, J. Non. Cryst. Solids, 100

(1988) 65.

[116] J. Livage, M. Henry, C. Sanchez, Prog. Sol. State Chem. 18 (1988) 259.

[117] G. Winter, Oil Colour Chem. Assoc. 36 (1953) 689.

[118] E. Bistan, I. Gomory, Chem. Zvesti. 10 (1956) 91.

[119] D. C. Bradley, Adv. Chem. Ser. 23 (1959) 10.

[120] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, Langmuir, 14 (1998) 3160.

[121] K. D. Kim, S. H. Kim, H. T. Kim, Colloids Surf. A, 254 (2005) 99.

[122] M. Niederberger, M. H. Bartl, G. D. Stucky, Chem. Mater. 14 (2002) 4364.

[123] H. Parala, A. Devi, R. Bhakta, R. A. Fischer, J. Mater. Chem. 12 (2002) 1625.

[124] J. Tang, F. Redl, Y. Zhu, T. Siegrist, L. E. Brus, M. L. Steigerwald, Nano Lett.

5 (2005) 543.

[125] M. Andersson, L. Oesterlund, S. Ljungstroem, A. Palmqvist, J. Phys. Chem. B

106 (2002) 10674.

[126] S. Seifried, M. Winterer, H. Hahn, Chem. Vap. Deposition 6 (2000) 239.

[127] K. Thamaphat, P. Limsuwan, B. Ngotawornchai, J. Kasetsart, Nat. Sci. 42

(2008) 357.

[128] R. Vogel, P. Meredith, I. Kartini, M. Harvey, J. D. Riches, A. Bishop, N.

Heckenberg, M. Trau, H. Rubinsztein-Dunlop, Chem. Phys. Chem, 4 (2003)

595.

[129] B. B. Lakshmi, P. K. Dorhout, C. R. Martin, Chem. Mater, 9 (1997) 857.

[130] H. Cheng, J. Ma, Z. Zhao, L. Qi, Chem. Mater, 7 (1995) 663.

[131] G. A. Tompsett, G. A. Bowmaker, R. P. Cooney, J. B. Metson, K. A. Rodgers

and J. M. Seakins, J. Raman Spectrosc. 26 (1995) 57.

52

[132] D. Huang, S. LiaO, S. Quan, L. Liu, Z. He, J. Wan, W. Zhou, J. Mat. Sci. 42

(2007) 8193.

[133] J. C. Yu, J. G. Yu, W. K. Ho, Z. T. Jiang, L. Z. Zhang, Chem. Mater. 14 (2002)

3808.

[134] C. Berlini, G. Ferraris, M. Guidotti, G. Moretti, R. Psaro, N. Ravasio, Micropor.

Mesopor. Mater. 595 (2001) 44.

[135] L. Marchese, E. Gianotti, V. Dellarocca, T. Maschmeyer, F. Rey, S. Coluccia, J.

M. Thomas, Phys. Chem. Chem. Phys. 1 (1999) 585.

[136] J. Yu, Q. Xiang, M. Zhou, Appl. Catal. B: Environ. 90 (2009) 595.

[137] K. Bapna, R. J. Choudhary, D. M. Phase, Adv. Mat. Lett. 2 (2011) 281.

[138] M. M. Joshi, N. K. Labhsetwar, P. A. Mangrulkar, S. N. Tijare, S. P. Kamble, S.

S. Rayalu, Appl. Catal. A: General, 357 (2009) 26.

[139] S. Jeon, P. V. Braun, Chem. Mater. 15 (2003) 1256.

[140] S. G. Gregg, K. S. W. Sing, Adsorption, surface area, and porosity. 2nd ed.;

Academic Press: London ; Toronto, 1982; p xi, 303 p.

[141] L. Korosi, S. Papp, I. Bertoti, I. Dekany, Chem. Mater. 19 (2007) 4811.

[142] C. Hsieh, M. Lai, C. Pan, J. Chem. Technol. Biotechnol. 85 (2010) 1168.

[143] F. Dong, H. Wang, Z. Wu, J. Phys. Chem. C, 113 (2009) 16717.

[144] P. Borker, A. V. Salker, Mater. Sci. Eng. B, 133 (2006) 55.

[145] H. M. Coleman, B. R. Eggins, J. A. Byrne, F. L. Palmer, E. King, Appl. Catal.

B: Environ. 24 (2000) 1.

[146] C. S. Hong, Y. Wang, B. Bush, Chemosphere, 36 (1998) 1653.

[147] I. K. Konstantinou, T. A. Albanis, Appl. Catal. B: Environ. 49 (2004) 1.

[148] H. Zollinger (Ed.), Color Chemistry: Synthesis, Properties and Applications of

Organic Dyes and Pigments, 2nd revised ed., VCH, Weinheim, Germany, 1991.

[149] M. Sokmen, D. W. Allen, F. Akkas¸, N. Kartal, F. Acar, Water Air Soil Pollut.

132 (2001) 153.

[150] A. K. Gupta, A. Pal, C. Sahoo, Dyes and Pigments, 69 (2006) 224.

[151] V. S. R. Kambala, B. Lavedrine, P. Boule, J. Photochem. Photobiol. A: Chem.

154 (2003) 189.

[152] A. Ozkan, M. H. Ozkan, R. Gurkan, M. Akcay, M. Sokmen, J. Photochem.

Photobiol. A: Chem. 163 (2004) 29.

53

[153] H. M. Sung-Suh, J. R. Choi, H. J. Hah, S. M. Koo, Y. C. Bae, J. Photochem.

Photobiol. A: Chem. 163 (2004) 37.

[154] C. Hu, Y. Lan, J. Qu, X. Hu, A. Wang, J. Phys. Chem. B 110 (2006) 4066.

[155] X. Z. Li, F. B. Li, Environ. Sci. Technol. 35 (2001) 2381.

[156] M. Saquiba, M. A. Tariqa, M. Faisala, M. Muneer, Desalination, 219 (2008)

301.

[157] J. Sun, X. Wang, J. Sun, R. Sun, S. Sun, L. Qiao, J. Mol. Catal. A: Chem. 260

(2006) 241.

[158] L. C. Chen, C. M. Huang, F. R. Tsai, J. Mol. Catal. A: Chem. 265 (2007) 133.

[159] M. Huang, C. Xu, Z. Wu, Y. Huang, J. Lin, J. Wu, Dyes Pigm. 77 (2008) 327.

[160] C. H. Wei, X. H. Tang, J. R. Liang, S. Tan, J. Environ. Sci.19 (2007) 90.

[161] Y. Zhiyong, M. Bensimon, V. Sarria, I. Stolitchnov, W. Jardim, D. Laub, E.

Mielczarski, J. Mielczarski, L. Kiwi-Minsker, J. Kiwi, Appl. Catal. B: Environ.

76 (2007) 185.

[162] W. Baran, A. Makowski, W. Wardas, Dyes Pigm. 76 (2008) 226.

[163] T. K. Ghorai, D. Dhak, S. K. Biswas, S. Dalai, P. Pramanik, J. Mol. Catal. A:

Chemical 273 (2007) 224.