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Recent development on titania based mixed oxide photocatalysts for environmental application under visible light Noor Aman, Trilochan Mishra* CSE Division, CSIR-National Metallurgical laboratory, Jamshedpur-831007, India E. mail: [email protected] Key words: Titania, mixed oxide, doping, photocatalyst, visible light, environmental, selenium. Abstract: In the recent years most of the studies are confined to the mixing of ZrO 2 , SiO 2 , WO 4 or ceria with titania in different composition so as to stabilize anatase phase, maintain high surface area and increase visible light absorption for better photocatalytic activity. Method of synthesis also helps in effective doping and enhancing surface area of the resultant materials. Nonmetal doping of oxide semiconductor materials facilitates the visible light application of photocatalysis. Based on the recent literature this review elaborately discuss on the development of titania based mixed oxide catalyst with or without different doping for visible light application. In addition this review deals with critical analysis of these materials towards photocatalytic oxidation of organics and reduction of pollutants like toxic metal ions and nitrates. 1. Introduction Since the discovery of photo-induced water decomposition under UV light [1] titania has received much attention as a potential photocatalyst for future environmental application. Due to the presence of a small amount of oxygen vacancies, which are compensated by the presence of Ti 3+ centres, TiO 2 is an n-type semiconductor. The valence band of this is mainly formed by the overlapping of the oxygen 2p orbitals, whereas the lower part of the conduction band is mainly constituted by the 3d orbitals. Among various semiconductors tested till date, TiO 2 is the most promising photocatalyst because of its appropriate electronic band structure, photostability, chemical inertness, and commercial availability [2]. TiO 2 exists in three main crystallographic forms anatase, brookite and rutile [3-5]. All the three polymorphs of titania can be described in terms of distorted TiO 6 octahedra with different symmetries or arrangements [6, 7]. The band gap of anatase and rutile are 3.2 and 3.0 eV, respectively. Anatase has been usually found to be photocatalytically more active than other polymorphs [8, 9]. Fermi level position, electron mobility, surface hydroxyl concentration are the main factors responsible for its higher activity. Rutile has larger crystallite size which increases the electron-hole recombination. However, it is widely accepted that the mixed phase of titania is beneficial in reducing the recombination of photogenerated electrons and holes and it always results in enhancement of photocatalytic activity [10-12]. Commercially available Degussa P 25 TiO 2 containing anatase and rutile mixture in 80:20 ratio possesses an excellent photocatalytic activity [13]. Rutile is the stable phase at high temperatures, but anatase and brookite are common in fine grained (nanoscale) natural and synthetic samples [3, 4, 14-16]. In general the crystallite phase, porosity, surface area and surface hydroxyl groups primarily determines the photocatalytic activity of a material. To improve these parameters, a number of synthetic strategies have been adopted by the researchers. Even the synthetic technique helps in increasing the visible light absorption of photocatalysts. The synthesis methods have a lot of influence in controlling the surface properties as well as activity of a particular photocatalyst. It is now well understood that the surface area of pure titania can be increased through different synthetic techniques. But the stability of anatase phase and porosity at high calcination temperature still remains a concern for the material chemists. Therefore, the concept of another metal ion doping and use of titania based binary oxides have been getting more and more importance to overcome the above shortcomings. In addition to the metal-anatase system, the oxide- Materials Science Forum Vol. 734 (2013) pp 186-214 Online available since 2012/Dec/27 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.734.186 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 103.6.236.38-03/02/13,16:16:22)

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Page 1: tio2 nanotube

Recent development on titania based mixed oxide photocatalysts for

environmental application under visible light

Noor Aman, Trilochan Mishra*

CSE Division, CSIR-National Metallurgical laboratory, Jamshedpur-831007, India

E. mail: [email protected]

Key words: Titania, mixed oxide, doping, photocatalyst, visible light, environmental, selenium.

Abstract: In the recent years most of the studies are confined to the mixing of ZrO2, SiO2, WO4 or

ceria with titania in different composition so as to stabilize anatase phase, maintain high surface

area and increase visible light absorption for better photocatalytic activity. Method of synthesis also

helps in effective doping and enhancing surface area of the resultant materials. Nonmetal doping of

oxide semiconductor materials facilitates the visible light application of photocatalysis. Based on

the recent literature this review elaborately discuss on the development of titania based mixed oxide

catalyst with or without different doping for visible light application. In addition this review deals

with critical analysis of these materials towards photocatalytic oxidation of organics and reduction

of pollutants like toxic metal ions and nitrates.

1. Introduction

Since the discovery of photo-induced water decomposition under UV light [1] titania has received

much attention as a potential photocatalyst for future environmental application. Due to the

presence of a small amount of oxygen vacancies, which are compensated by the presence of Ti3+

centres, TiO2 is an n-type semiconductor. The valence band of this is mainly formed by the

overlapping of the oxygen 2p orbitals, whereas the lower part of the conduction band is mainly

constituted by the 3d orbitals. Among various semiconductors tested till date, TiO2 is the most

promising photocatalyst because of its appropriate electronic band structure, photostability,

chemical inertness, and commercial availability [2]. TiO2 exists in three main crystallographic

forms anatase, brookite and rutile [3-5]. All the three polymorphs of titania can be described in

terms of distorted TiO6 octahedra with different symmetries or arrangements [6, 7]. The band gap of

anatase and rutile are 3.2 and 3.0 eV, respectively. Anatase has been usually found to be

photocatalytically more active than other polymorphs [8, 9]. Fermi level position, electron mobility,

surface hydroxyl concentration are the main factors responsible for its higher activity. Rutile has

larger crystallite size which increases the electron-hole recombination. However, it is widely

accepted that the mixed phase of titania is beneficial in reducing the recombination of

photogenerated electrons and holes and it always results in enhancement of photocatalytic activity

[10-12]. Commercially available Degussa P25 TiO2 containing anatase and rutile mixture in 80:20

ratio possesses an excellent photocatalytic activity [13]. Rutile is the stable phase at high

temperatures, but anatase and brookite are common in fine grained (nanoscale) natural and synthetic

samples [3, 4, 14-16]. In general the crystallite phase, porosity, surface area and surface hydroxyl

groups primarily determines the photocatalytic activity of a material. To improve these parameters,

a number of synthetic strategies have been adopted by the researchers. Even the synthetic technique

helps in increasing the visible light absorption of photocatalysts. The synthesis methods have a lot

of influence in controlling the surface properties as well as activity of a particular photocatalyst.

It is now well understood that the surface area of pure titania can be increased through

different synthetic techniques. But the stability of anatase phase and porosity at high calcination

temperature still remains a concern for the material chemists. Therefore, the concept of another

metal ion doping and use of titania based binary oxides have been getting more and more

importance to overcome the above shortcomings. In addition to the metal-anatase system, the oxide-

Materials Science Forum Vol. 734 (2013) pp 186-214Online available since 2012/Dec/27 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.734.186

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 103.6.236.38-03/02/13,16:16:22)

Page 2: tio2 nanotube

anatase system corresponds to the most broadly applied composite materials in the field of

photocatalysis. It is well known that binary oxide catalysts often exhibit higher catalytic activity and

selectivity than what one can predict from the properties of their components. It is established in the

literature that mixing two dissimilar oxides adds another parameter since they are liable to form

new stable compounds, which can lead to totally different physicochemical properties and catalytic

behaviour. In addition mixed oxides system helps in the development of Z scheme photocatalyst

which helps in increasing the activity by delaying the e/h recombination. Effect of different

semiconductors with anatase has been reviewed recently in relation to solar photocatalysis [17].

Details about the structural interfaces between oxides such as WO3, Cu2O, CeO2 or Fe2O3 with

anatase are essentially unknown from a structural point of view. This is essential to understand the

electronic properties of the mixed oxides. However, pure titania can only work under ultraviolet

(UV) light due to its wide band gap, which means only about 2-4% of the incoming solar energy on

the earth surface can be utilized. Therefore the development of visible-light photocatalysts has

become one of the most important topics in photocatalysis research. In most of the case non-metal

doping helps in increasing the visible light absorption. So it is of interest to look into the

development of non-metal doped binary oxide systems as potential photocatalyst for future

environmental application. Another approach for achieving this objective is to sensitize TiO2 by

using a narrow band gap semiconductor with a higher conduction band than that of TiO2. CdS and

CdSe with band gap energy of .3.0 eV are considered to be potential sensitizers used for large band

gap semiconductors because of the ideal position of its conduction and valence band edges. So CdS

and CdSe sensitized TiO2 system were also investigated by some researchers [18, 19]. CdSe, CdS,

PbS, PdS, and PbSe with titania are among the most used composite systems for visible light

photocatalysis. However, lack stability in specific environments due to anodic photocorrosion by

holes and the subsequent release of cations into the media. [20-25] restrict their prospective use. So

here we will discuss about the recent development on binary oxide photocatalyst. Titania based

binary oxide systems are limited in the literature and mostly talks about the systems like TiO2-SiO2,

TiO2-ZrO2, TiO2-WO3 and TiO2-CeO2 etc. In addition limited studies are carried out on TiO2-NbO

[26], TiO2-Fe2O3 [27] and TiO2-GeO2 [28] systems. Highly transparent TiO2-In2O3 composite film

[29] and hollow sphere [30] systems were also investigated by some researchers as visible light

sensitive photocatalyst. Comparative structure–activity relationship in Ti–M (M = V, Mo, Nb, W)

mixed-metal oxides with anatase structure are studied for the photoelimination of toluene under

sunlight type excitation [31]. The result indicates the suitability of TiO2-WO3 systems as efficient

photocatalyst in comparison to others. However, some of these systems are rarely studied and

reported in the recent time. So we will discuss in detail about the development of only extensively

studied four systems with or without non-metal doping.

2. Binary oxides

2.1 Titania-Silica materials

Mixed metal oxides of TiO2-SiO2 and TiO2-ZrO2 are of special interest because of their

common valency (+4). Titania-silica mixed oxides have a large number of applications in catalysis,

either as catalyst by themselves or as catalyst support [32-34]. Titania-silica mixed oxide is reported

to be more active than pure titania photocatalyst. The unique chemical and physical properties

exhibited by titania-silica binary oxides depend on both the composition and the degree of

homogeneity. Therefore, strategies have been developed to synthesize the mixed oxides in a

uniform manner and typically include co-precipitation, flame hydrolysis and sol-gel hydrolysis.

Synthesis in presence of surfactant and other complexing agents results in the formation of

homogenous distribution having high porosity. Desired arrangement of TiO2 and SiO2 species in the

binary TiO2-SiO2 particles can be achieved by appropriate synthesis conditions. Introduction of

SiCl4 vapour during oxidation of TiCl4 in a laminar diffusion flame reactor at reaction temperatures

of 935 and 1068°C [35, 36] slows down the sintering rate of titania, resulting in decrease of primary

particle size and in increase of anatase phase fraction. In sol-gel process, alkoxides of Ti are

Materials Science Forum Vol. 734 187

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typically hydrolyzed in water at a much faster rate than the standard Si alkoxide which leads to the

formation of separate titania-rich phases. This problem can be mitigated by prehydrolyzing the

tetraethyl orthosilicate before adding the titanium alkoxide [37, 38] and by controlling the

acidity/basicity of the medium [39]. In particular surfactant mediated synthesis of mesoporous

photocatalysts with improved activity are reported [40-43] in the literature. Recently Mishra et al

reported the synthesis of spherical titania silica material through modified sol-gel techniques in

presence of CTAB [44]. CTAB concentration plays an important role in controlling the shape and

size of the material. Only 2mol% of CTAB can form spherical titania particles (Fig-1) with

mesoporosity. Same method can be used to prepare spherical titania based other binary oxides (Fig-

2) also.

Fig.1. SEM micrographs of TiO2 prepared with varied CTAB concentration. A: without CTAB, B:

1mol% CTAB, C: 2mol% CTAB and D: 4mol% CTAB

The crystallinity of titania is a critical factor for the photoactivity. The addition of silica in

titania enhances thermal stability by suppressing phase transformation of anatase to rutile and also

increases the surface area and surface acidity. Suitable addition of silica in titania can stabilise

anatase phase of titania with high degree of crystallinity and smaller crystalline size and reduce the

bulk defects even at high calcination temperature. Addition of 30 mol% silica is found to increase

the phase transformation temperature of anatase to rutile by about 180 °C [84]. At higher silica

content (50 mol%) in the mixed oxides, the activity is, however, significantly dropped down due to

the presence of amorphous titania covered with photocatalytically inactive silica on the surface [39].

Stabilisation of anatase phase up to 900°C is reported with 10-20 wt% silica in the materials

[45]. Interestingly presence of silica not only stabilises the anatase phase at high temperature but

also maintains the porous structures [45]. Whereas pure titania transforms to rutile phase even at

700 °C calcination thus decreasing the surface area and the photocatalytic activity.

A B

D C

188 Photocatalytic Materials & Surfaces for Environmental Cleanup-II

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Fig.2. Materials (A) Titania, (B) Titania-silica and (C) titania-zirconia synthesized in presence of

2mol% of CTAB.

Fig-3 shows the change in shape and size of the spherical material due to heat treatment at

900 °C. Effect of silica on the surface area and crystallite size is presented in Table-1 with respect

to calcinations temperature. Low crystallite size of silica mixed titania even at 900 °C calcination is

well marked indicating the controlling effect of silica on the size of the materials. Stabilization of

the anatase phase by the surrounding SiO2 phase is reported to be through the Ti-O-Si interface

[46]. The SiO2 lattice locks the Ti-O species at the interface with the TiO2 domains preventing the

nucleation. Similar findings have also been reported by Ding et al. [47]. Moreover, smaller ionic

radius of Si4+

(0.042 nm) allows it to enter into the TiO2 lattice easily. At high temperatures, silica

desolutes from the anatase lattice due to its smaller ionic radius. This may also be one of the reasons

for high anatase phase stability of SiO2 mixed titania [48]. The Brønsted acidity, associated with the

formation of Ti–O–Si hetero linkages where tetrahedrally coordinated silica is chemically mixed

with the octahedral titania matrix, may also have important contribution to enhance the

photocatalytic activity of titania– silica mixed oxides. It has been found that the new acidic sites

created by lattice substitution exhibit Brønsted acid character instead of Lewis acid character [49].

The highest Brønsted acidity is achieved in a material having 10 mol% silica in titania [49].

Itoh et al. have also reported a similar trend [50]. In another study, increased acidity with increasing

titania content up to nearly pure titania (99.5 mol%) has been observed for coprecipitated titania-

silica catalysts [51]. Surface area of titania-silica mixed oxide proportionally increases with

increasing the silica content [52]. Silica modified titania prepared by sol-gel process retains its high

surface area even at higher calcination temperature [53].

Fig. 3. (A) TiO2 , (B) Si (10%)TiO2 and (C) Si(20%)TiO2 after calcinations at 900°C.

However, the increase of surface area by making materials porous does not always increases

photoactivity [54]. Hierarchically macro/mesoporous TiO2-SiO2 oxides photonic crystal with highly

efficient photocatalytic activity has been synthesized by combining colloidal crystal template and

amphiphilic tri-block copolymer. It is found that the thermal stability of mesoporous structures in

the composite matrix is improved due to the introduction of silica which acts as glue and links

anatase nanoparticles together. The photocatalytic activity increases due to the increased anatase

A B C

A B C

Materials Science Forum Vol. 734 189

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crystallinity and surface area [55]. A recent study [56] indicates that the physical mixing of silica

with Degussa P25 titania can enhance the photocatalytic activity under UV light to a large extent due

to the change in the electronic property of the material with increased band gap.

Table-1: Textural properties of synthesized TiO2 and binary oxide materials.

Sample

code

Titania

Wt%

Silica

(Wt%)

Zirconia

(wt%)

BET Surface area

(m2/g)

Crystallite Size(nm)

500˚C 700˚C 900˚C 500˚C 700˚C 900˚C

Ti 100 - - 91 48 30 14.85 40.4 48.49

TiSi-10 90 10 - 126 106 85 10.17 12.1 16.05

TiSi-20 80 20 - 138 112 88 9.98 12.02 15.17

TiZr-10 90 - 10 105 84 75 12.6 12.75 24.13

TiZr-20 80 - 20 96 80 68 15.3 21.9 24.15

2.2 Titania-zirconia binary oxide

Titanium and zirconium belong to the same group (IVB) of elements in the periodic table.

Both TiO2 and ZrO2 are n-type semiconductors with similar physicochemical properties. However,

ZrO2 has a wide band gap (ca. 5.0 eV) in comparison to those of TiO2. The more-negative

conduction band (-1.0 V vs NHE) and more-positive valence band potential (4.0 V vs NHE) makes

ZrO2 a promising alternative photocatalyst for the degradation of a great variety of pollutants. In

fact, TiO2-ZrO2 mixed oxides exhibit high surface area, profound acid-base properties, a high

thermal stability and strong mechanical strength. There are two types of combinations between TiO2

and ZrO2: physically mixed (mixture of two oxides) and chemically bonded having Ti–O–Zr type

linkages. Physicochemical and catalytic properties of titania-zirconia mixed oxides vary depending

on the type of bonding takes place in a mixed oxide. The degree of interaction or in other words the

homogeneity largely depends on the preparation techniques and parameters. Several preparation

methods have been employed to synthesize the TiO2-ZrO2 composite oxides. The most widely

employed methods for synthesis of TiO2-ZrO2 mixed oxides are co-precipitation [57-62] and sol-gel

[53, 63-67]. Other less frequently applied procedures include the super critical fluid extraction [68],

non-hydrolytic modified sol-gel [69], citric acid complexation [70], neutral amine [71, 72] and

reverse microemulsion [73] techniques. In sol-gel processes, separate domain formation due to the

difference in the hydrolysis and condensation rates of Ti- and Zr-alkoxides is a major problem in

the preparation of mixed oxides. In homogeneous colloids, zirconia is segregated within the matrix

of the titania crystallites whereas, in the heterogeneous colloids, zirconia is segregated on the

surface of the titania crystallites [74]. In yet another approach, Anderson et al. prepared titania and

zirconia sols separately prior to their combination [53, 67]. Presence of nearby zirconia inhibits

densification and crystallite growth by providing dissimilar boundary. This is also supported by the

fact that anatase TiO2 and tetragonal ZrO2 are not compatible with each other. So zirconium doping

retards the amorphous–anatase and anatase–rutile phase transition processes. For the Zr-doped TiO2

materials with less than 30% of Zr, only anatase phase is observed in the temperature range of 500–

700 °C. The sample containing 30 % of Zr remains amorphous when calcined at 500 °C and

transforms into anatase when the calcination temperature increases to 600 and 700 °C. Chang et al.

[69] have suggested that a higher temperature is required for crystallisation of binary oxides than

the individual oxides. Partial substitution of Ti4+

by Zr4+

has also been reported in Zr-doped TiO2

[73]. The excess Zr4+

ions are accumulated at the surface of the anatase TiO2 because of slow

condensation of Zr4+

ions [65]. Particle size of TiO2-ZrO2 mixed solution is found to be lower than

the Zr-doped TiO2. Recently Mishra et al reported the change in shape and size of the spherical

TiO2-ZrO2 material, prepared in presence of CTAB, due to heat treatment at 900°C (Fig-4) [45]. It

indicates that the spherical shape still retains with partial sintering of particles after calcinations at

190 Photocatalytic Materials & Surfaces for Environmental Cleanup-II

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900°C. Moreover with 10 wt% zirconia samples, no extra phase formation is observed at high

calcinations temperature. Only small zirconium titanate phase is observed particularly at 900°C

calcination in case of 20 wt% zirconia sample. Unlike silica mixed samples zirconia mixed samples

shows mostly uniform particle size.

Fig. 4. (A). Zr(10%)TiO2 and (B) Zr(20%)TiO2 after calcinations at 900°C.

The average particle size increases from 1100nm to 1150nm with the increase in zirconia content.

At 900 °C calcinations only pure TiO2 completely loses the spheroid shape due to sintering of the

particles. However, at 900 °C calcination still spherical particles (Fig-4) are observed with little

sintering effect for all the binary oxide materials. It was concluded that the presence of zirconia not

only helps in anatase phase stabilization but also controls the sharp decrease in surface area by

restricting the particle aggregation. In addition, surface area of the titania increases drastically due

to the presence of 2 mol% CTAB during synthesis. So the presence of CTAB not only controls the

particle size and shape but also increases the porosity of the materials.

Yu et al. [70] have observed a decrease in iso-electric point of the mixed oxide with increase

in the Zr4+

substitution in the TiO2 lattice. The increment of surface acidity in mixed oxides has

been generally attributed to a charge imbalance, associated with heterometallic bonding formation.

Increasing surface acidity of the material helps to improve the photocatalytic activity. UV-Vis

absorption shifts to lower range on increasing the Zr-content. However, Wu et al. [75] observed

slight decreased band gap of the mixed oxide. These study shows the prospect of titania based

binary oxide as stable and effective photocatalysts.

2.3 Non-metal doped TiO2-ZrO2 and TiO2-SiO2 materials

In general, the preparation of N-doped TiO2 powder mainly includes wet chemical methods such

as co-precipitation and sol-gel. These methods are usually performed at relatively low temperature

as the prepared N-doped TiO2 materials usually have a low thermal stability. The introduction of a

second metal oxide (SiO2, ZrO2 etc.) has been proved to be an effective route to improve the

thermal stability and UV light photocatalytic activity of TiO2 [45, 53]. The role of ZrO2 in

enhancing the visible light photocatalysis of TiO2-xNx towards oxidation of gaseous organic

compounds has been evaluated recently [76]. Nitrogen doping in the TiO2 lattice has been done by

the thermal treatment of NH3-adsorbed TiO2 hydrous gels. The introduction of ZrO2 into TiO2-xNx

considerably inhibits the undesirable crystal growth during calcination and thus preserves the lattice

nitrogen. As a result the amount of lattice nitrogen on the surface of TiO2-xNx-ZrO2 is more than

that on the TiO2-xNx. In another study ZrO2 modified S-doped TiO2 visible light photocatalysts has

been successfully prepared by simple sol-gel method [77]. The introduction of ZrO2 could

effectively suppress phase transformation of anatase-to-rutile and stabilize the sulphur in the S-

TiO2-ZrO2 matrix which in turn enhances the visible-light absorption compare to the corresponding

non-modified S-TiO2 sample. Codoping of Zr4+

and F- ions within anatase hollow microspheres

promotes lattice substitution of Ti4+

ions by Zr4+

and facilitates the transformation of surface-

segregated amorphous ZrOx clusters into Zr-F species [78]. Interstitially N doping of TiO2-SiO2

A B

Materials Science Forum Vol. 734 191

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mixed oxide, leads to increased visible light absorption [79]. The reported results revealed a

decrease in the size of TiO2 particles with increasing the SiO2 content, but the rise of SiO2/TiO2

ratio in the mixed oxides led to a decrease in the amount of surface sulphate species. Mainly

addition of SiO2 results in the improvement of thermal stability of the catalystand the formation of

Brønsted acid sites. Decreased particle size due to the presence of silica is also reported earlier [53].

All these factors are responsible for an enhancement in photogenerated electron-hole separation and

thus increased photocatalytic activity. The photoactivity was initially affected by factors such as the

amount of inactive surface SiO2, TiO2 particle size, and surface area.

a b c

Fig.5. TEM micrographs of (a) titania-zircona and (b) & (c) N doped titania-zircona materials at

different magnifications.

Aman et al [80] used a simple approach for the synthesis of high surface area, thermally stable N-

doped zirconium titanium mixed oxide with enhanced photocatalytic activity towards reduction

reactions under visible light employing hydrazine as a source of nitrogen. Synthesis at low pH of 2

results in the formation of nanomaterials (Fig-5). Presence of low amount of zirconium oxide (10

wt.%) helps in phase stabilization and maintains the porous structure even at higher calcination

temperatures in comparison to that of pure titania. Besides, effect of hydrazine to metal oxide ratio

on the overall activity is investigated so as to have better understanding on the optimum

composition. Amount of hydrazine plays an important role in nitrogen doping as well as formation

of porous structure in the material. Surface area and pore volume increases with the increasing

hydrazine amount to 298 m2/g and 0.323 cm

3/g, respectively. Amount of nitrogen doping and the

visible light absorption (Fig-6a) capacity of the material increase with the increasing hydrazine

amount up to an optimum value. Nitrogen doped titania-zirconia material exhibit high surface area

and enhanced visible light mediated photocatalytic activity in comparison to nitrogen doped pure

titania. Fig-6b shows the presence of three different peaks with varying intensity in case of both N

doped materials. Presence of peaks at 397 and 400.5eV signifies the doping in the matrix and the

formation of NO species, respectively in the material. Still another low intensity peak is observed in

the range of 402.5 eV which is somewhat prominent in the case of TiZr-6N material. This peak is

rarely observed in the N doped materials. It is observed in between the standard position of NO and

NO2 (405 eV) so it can be due to the formation of more electropositive NO type species. XPS

analysis of Ti 2p and O1s spectra confirm the presence of Ti3+

and oxygen vacancy in N doped

materials. Enhanced photocatalytic reduction and oxidation activity of N doped titania–zironia

material under visible light was attributed to the synergistic effect of high surface area, oxygen

vacancy and substantial N doping. Even if NF co-doping of binary oxides is found to be beneficial

in comparison to only N doped titania [81].

192 Photocatalytic Materials & Surfaces for Environmental Cleanup-II

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Fig.6. (A) N1s spectra and (B) diffused reflectance spectra of N doped materials. TiZr denote the

zirconium titanium mixed oxide and the number indicates different mole of hydrazine (N) used.

2.4 Titania-tungsten oxide materials

Titania-tungsten oxide system is studied by several researchers due to some inherent interesting

properties of WO3 in combination with titania. Nanosized WO3 is mostly crystallized into

monoclinic or orthorhombic phases on contact with titania. The presence of several W oxidation

states, anion vacancies, and correspondingWO3 surface phases on anatase gives an idea of the

complexity of the situation [82]. The defect chemistry of WO3 is remarkably interesting. This

includes the presence of anomalous oxidation (W5+

) states mostly at the surface layers or defects

sites. Present knowledge of understanding cannot rationalized the defects formation as a function of

size. Typically the monoclinic polymorph exhibits a band gap of 2.7-2.8 eV [83]. Energy diagram

indicates that the bulk contact between anatase and WO3 allows energy transfer between two

components but inefficient in promoting charge separation in case of both UV and sunlight

excitation. For WO3 particles below 10 nm size, the band modification of the solid should allow

efficient hole transfer from WO3 to anatase upon light absorption. But theWO3 band gap energy is

large enough to limit visible photon absorption. However, the presence of gap states may allow

electron transfer from TiO2 to WO3 and adequate charge separation. For particles well below 10

nm, WO3 is considered to be mainly amorphous and hence the capability to absorb visible photons

and handle charge carriers to produce useful chemistry is essentially lost. Thus in case of TiO2-WO3

system, a strong size effect is expected for sunlight excitation and positive effects arising from

interphase charge transfer can only be envisaged in a narrow size domain mainly governed by WO3

characteristics. is therefore in case of WO3, surface properties, particularly acidity constitute one of

the important factors controlling the photocatalytic activity enhancement. However, a full analysis

of surface and electron-hole recombination as a function of the particle size still remains to be

explored. Gao et al prepared a series of N-doped TiO2 coupled with WO3 showing higher

photocatalytic activity under both UV and visible light [84]. However, there should be further

improvement in their works since these preparation processes were too complicated and the TiO2-

N-WO3 material prepared from immerging TiO2-N into tungstic acid may result in most of WO3

adsorbed on TiO2 surface instead of inserted into the TiO2 lattice, which may accelerate the photo-

electron and holes recombination rate and decrease the photocatalytic activity. So there is a need to

look into new method of synthesis for tungsten and nitrogen co-doped TiO2 as a potential catalyst

under visible light.

390 395 400 405 410

B in d in g en erg y (eV )

TiZr

T i-6N

TiZr-6NIn

ten

sit

y/

a.

uA

Materials Science Forum Vol. 734 193

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Fig. 7. TEM image of the N doped titania-tungsten oxide material.

Recently noodle shaped, mesoporous N doped TiO2-WO3 system was reported [85] with increased

visible light activity. The noodles of 150-200nm are composed of nanoparticles of 10-20 nm (Fig-

7). However separate WO3 phase in addition to anatase phase was not detected in the material. It was observed that the visible light absorption increases with the increasing tungsten amount.

Surface area and the photocatalytic activity varies with the variation in WO3 content in the material.

Keller et al [86] reported that tungsten doping of the TiO2 leads to the appearance of Lewis and

Brønsted acid sites and therefore an increase of the surface acidity is observed. Such surfaces have a

higher affinity to the species with unpaired electrons and can easily adsorb the hydroxyl groups and

water, which are necessary for the hydroxyl radical generation thus facilitating the photocatalytic

process.

In addition composite Ti-W oxide nanotubes have been successfully fabricated through

anodization of TiW alloy [87]. Obtained nanotubes show straight wall morphology and are well

vertically aligned on the substrate. Compared to titanium oxide nanotubes, the composite Ti-W

oxide nanotubes showed highly improved ion insertion and electrochromic properties even when

only small amounts such as 0.2 wt% WO3 are present. In particular, Ti-W oxide nanotubes have a

much higher electrochromic contrast and lower onset voltage and exhibit a good cycling stability.

Authors believe that these nanotube structures based on TiW alloy anodization can find

applications, in electrochromic devices, photoelectrodes in photocatalytic devices or

photoelectrochemical solar cells.

2.5 Titania-Ceria materials

Continuous improvement of the photo-reaction rate and the solar efficiency via adjusting the band

structure of TiO2 is a meaningful work at the present stage. To overcome these limitations,

numerous efforts have been made to improve the photocatalytic activity by modifying the surface or

bulk properties of TiO2 through mixing of two semiconductors as described above. Photocatalytic

efficiency can be remarkably enhanced by the introduction of rare-earth metal elements.

Alternatively, the photocatalytic activity of TiO2 can be significantly enhanced by doping with

lanthanide ions/ oxides because lanthanide ions can form complexes with various Lewis bases

through interaction of the functional groups with their f orbital [88-90]. Among the lanthanide

oxides, the catalytic properties of ceria have received much attention due to two features (i) the

redox couple Ce3+

/Ce4+

with the ability of ceria to shift between CeO2 and Ce2O3 under oxidizing

and reducing conditions; and (ii) the easy formation of labile oxygen vacancies with the relatively

high mobility of bulk oxygen species [91]. Even pure ceria nanomaterial is reported recently as an

efficient photocatalyst [92]. Ce-doped TiO2 materials have been preferentially prepared by the sol–

gel and hydrothermal methods [93-98]. In this context hierarchically nanostructured Ce/TiO2 is

found to increase the photodegradation of RhB under visible light [99]. Hierarchically

nanostructured cerium-doped titania materials having anatase phase with a squamalike morphology

100n

20

nm

194 Photocatalytic Materials & Surfaces for Environmental Cleanup-II

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have been reported through a simple O/W emulsion technique using Span-60 as an emulsifier. It

was demonstrated that the oil in water emulsion method is practical to produce a hierarchical

nanocomposite structure of Ce- TiO2 with high surface area, narrow mesopore size distribution and

good thermal stability [99]. The UV-vis diffuse reflectance spectra show that the cerium-doped

samples exhibit obviously enhanced absorbance in the visible light region compared with pure

titania. Moreover, the beneficial effect of Ce doped TiO2 catalysts depends on the Ce content in the

material. Synthesis of a novel thermally stable mesoporous ceria−titania phase using a neutral

templating route was reported [100] with high removal of volatile organic compound like toluene.

The toluene removal performance was further enhanced for Pt impregnated mesoporous

ceria−titania. Recently synthesis and photocatalytic activity of Ce/TiO2 varying the cerium oxide

content from 2-10wt% with respect to TiO2 was studied by Aman et al. [101] Surface properties and

the photocatalytic activity under visible light towards methylene blue oxidative decomposition and

selenium (IV) reductive removal reaction are studied to correlate the surface properties with respect

to optimum cerium concentration. Highest photocatalytic activity is observed for TiCe5 having

5wt% cerium oxide calcined at 600°C. Activity is found to depend on the presence of Ce4+

/ Ce3+

rather than only visible light absorption. Role of oxygen vacancy created due to mixed oxidation

state can not be ruled out. Material calcined at 600ºC shows better activity than that calcined at

400ºC even though surface area decreases. Anatase crystallinity mostly decides the photocatalytic

activity rather than only surface area. Therefore an optimum amount of cerium with appropriate

heat treatment is required so as to have best photocatalytic result under visible light. It is also

understood that the presence of cerium helps in better separation of hole-electron pair. Sol-gel

synthesis of titania doped with 5-mole% CeO2 after calcining to 500°C, possesses specific surface

area of 97 m2 g

–1 and has anatase phase stability up to 900C [102]. The exceptionally high phase

stability, crystallinity and high surface area are due to the extremely fine particle size and effective

doping achieved by the specific synthesis method. Similar to the above observation [101] 5mol%

Ce gives better photocatalytic activity.

2.6 Other binary oxides systems

The interaction of Cu2O/CuO with titania as a photocatalyst has not been well studied till date.

However, the band energy levelling is adequate for enhancing charge separation mostly with UV

light [103]. Change in particle size plays an important role in photocatalysis of these materials.

Modification of the Cu2O band gap and the nanocomposite Fermi level position with 10mm size

provide an effective way to transfer visible light excited electrons from the copper oxide to anatase

and promote electron-hole separation [104-107]. This leads to the development of highly active

sunlight-driven photocatalysts. However, to date studies on copper oxide based photocatalysts is

limited [108, 109]. Below 10 nm size the band gap is expected to reach a value close to 3.0 eV

which limits the absorption of visible light photons.

Combination of Fe2O3 lacks adequate electronic characteristics to efficiently separate the charge

carrier for visible light excitation. The only positive effect might be derived from UV-excited

electrons on anatase which may be efficiently transferred to the Fe2O3 phase. However, systematic

studies of the anatase/hematite material as a function of the particle size are missing from the

literature. Only a few groups studied this nanocomposite system for visible [110] or UV-visible

[111, 112] light excitation. In fact the latter studies indicate that defect rich nanosized iron oxide

transfer visible light excited electrons to the titania. Other oxide like Bi2O3 [113, 114] is useful in

the context of their interaction with titania for improved photocatalytic activity. The latter is

particularly well suited as valence-conduction relative band positions is capable of promoting

visible-photon generated electron transfer from Bi2O3 to titania and UV-photon generated hole

transfer from titania to Bi2O3. Other complex oxides such as LaVO4 or Bi oxyhalides are also very

useful in this context because of similar electron-hole charge separation [115, 116].

Materials Science Forum Vol. 734 195

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3. Photocatalytic applications

Semiconductor photocatalysis appears to be a promising technology with wide applications

in environmental systems such as air purification, water disinfection and hazardous waste

remediation in addition to hydrogen energy generation. Moreover, the photocatalysis allows us to

carry out both oxidation and reduction type of reactions. The option for recovery of valuable metals,

after converting to their less-toxic/nontoxic metallic states through reduction was explored.

Feasibility of a reaction depends on the redox potential of the reactant and the semiconductor used.

Majority of the recent research works have focused on the generation of hydrogen as a clean and

renewable fuel to fulfil the ultimate objective of producing hydrogen in large scale using solar

energy. Photoassisted water electrolysis using UV light was demonstrated over 30 years ago by

Fujishima et al. [1]. However, in the recent year development of various new materials with

enhanced activity for direct water splitting in visible/solar light stimulated the researchers to

improve the process for a prospective technology [117]. The recently developed photocatalysts like

In1-xNiTaO4 [118] and ruthenium chromium modified solid solution of (Ga1-xZnx)(N1-xOx) [119]

with improved efficiency in presence of visible light have opened a new direction and hope for the

direct water splitting. However, our work in this dissertation will mostly focus on the importance of

environmental applications of photocatalysis. The treatment of hazardous wastewater containing

heavy metals and organic compounds by photocatalysis with respect to various parameters such as

pH, light intensity, dissolved oxygen etc. has been reviewed recently [2].

3.1 Photocatalytic oxidation

The major applications investigated for this technology are mostly confined to colour

removal and destruction of dyes, reduction of COD (chemical oxygen demand), mineralization of

hazardous organics, destruction of hazardous inorganics such as cyanides, disinfection of water,

destruction of malodorous compounds, decontamination of soil, purification and decontamination of

indoor air. The complete mineralization of a variety of aliphatic and aromatic chlorinated

hydrocarbons via heterogeneous photo-oxidation on TiO2 has been reported [120]. It is often found

that photocatalytic oxidation process is nearly suppressed in the absence of oxygen, possibly

because of the back electron transfer from the active sites on the photocatalyst surface.

Photodegradation of different common dyes like methylene blue, methyl orange, indigo

carmine, chicago sky blue 6B have been studied as a test reaction in the presence of titanium

dioxide [120, 121]. Titanium dioxide supported on glass has been found most effective in removing

methyl orange while chicago sky blue 6B is most stable against photodegradation process. In

another study, Degussa P25 has been found more effective than ZnO for the photomineralisation of

Chrysoidine Y [122]. Textile dyes, being the major constituent of total dyestuff used, have emerged

as a focus of environmental remediation efforts [123]. Significant works have been reported in the

field of degradation of textile dyestuff in presence of other cations and anions into non-hazardous

end products under visible light radiation [124, 125]. Higher degradation rate of dyes by

combustion synthesized nano-TiO2 in comparison to Degussa P25 was explained on the basis of

reversible adsorption, high surface area, higher degree of water and hydroxyl groups adsorbed on

the surface of nano-TiO2 [126]. Promising degradation of crystal violet and methylene blue has

been reported on the vanadium doped TiO2 [127]. Asahi et al. [128] succeeded in degrading

methylene blue under visible light using N-doped TiO2. Later, acetaldehyde decomposition under

visible light also emphasized the significance of N-doped TiO2 [129].

Ethylene oxidation is found to increase over silica and zirconia mixed TiO2 in comparison to

pure titania [130]. The photodegradation of oil in water was carried out using slightly crystallized

titanates [131]. Photodegradation of chlorinated organics (chlorophenol, tricloroethylene) in

aqueous solution has also been investigated using mixed semiconductors like SiO2-TiO2 [52], ZrO2-

TiO2 [58] and CdS-TiO2 [132] with increased efficiency mostly at higher pH. Similarly, 4-

196 Photocatalytic Materials & Surfaces for Environmental Cleanup-II

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chlorophenol, 2-propanol are completely mineralised by carbon modified TiO2 [133, 134] under

visible light. Decomposition of other organics like methylcyclohexanone [73], salicylic acid and t-

cinnamic acid [88] was studied using titania based mixed oxides. C-doped TiO2 is also found to

increase the oxidation of gaseous benzene, acetaldehyde and carbon monoxide. Brezova et al. have

reported the degradation of phenol over metal (Fe and Pt) doped TiO2 to find out the optimum level

of metal doping [135]. Comparative degradation of acid orange 7, tetrazene and 3-benzene sulfonic

acid by Ag-doped TiO2 photocatalyst has been reported [136]. In another study, the comparative

activity of TiO2, ZnO and Fe2O3 towards degradation of phenol under concentrated solar light has

been studied [137]. Effect of boria doping was also investigated for vapour phase Beckmann

rearrangement of cyclohexanone oxime [62]. Ceria doping has improved the decomposition of 2-

mecaptobenzothiazole in aqueous solution [93]. Dye photooxidation along with simultaneous

photoreduction of Cr(VI) has also been investigated [138]. In this case, photogenerated electrons

are utilized to reduce the toxic Cr(VI) to the environmentally benign and immobile Cr(III) species.

This strategy is particularly relevant to practical remediation scenarios involving wastewater

effluent streams containing both toxic metal ions and organic pollutants. In addition CO oxidation

with appreciable results [99] was reported on cerium doped titania.

3.2 Photocatalytic reduction

For photo-reduction of a metal ion Mn+

to M0, the energy of the cb electron must be more negative

than the E0 of the Mn+

/M couple. These metallic couples are also influenced by the solution pH.

Schematic presentation of cb and vb of TiO2 with respect to different cations is given in Fig-8.

Prairie et al. [139, 140] have demonstrated that only metal ions with redox potentials more positive

than 0.4 V can be reduced on pure TiO2. Accordingly the photoreduction of several inorganic

cations and anions investigated for removal or recovery applications has been discussed below.

Fig. 8. Positions of the redox potentials of various metallic couples related to the energy levels of

the conduction and valence bands of TiO2 at pH ~ 0.

3.2.1 Selenium and copper

The most common selenium species in wastewater exist as mobile ions, such as selenate

(SeO42−

, Se(VI)), selenite (SeO32−

, Se(IV)) or their protonated forms (HSeO4−, HSeO3

−). The

toxicity of Se compounds is related to their oxidation states. Se(VI) is more mobile and stable

towards reduction than Se(IV), hence it is harmful and difficult to remove it from wastewater [141].

The photocatalytic reduction of Se(VI/IV) species using TiO2 has been initiated by Sanuki et al.

V

Materials Science Forum Vol. 734 197

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[142, 143]. They found that the adsorption of Se(VI/IV) species onto titanium dioxide and the

presence of suitable hole scavenger is essential for an effective reduction process. Amal et al. [144-

146] have studied the effects of a range of hole scavengers on photocatalytic reduction of

Se(VI/IV). Photoreduction has been shown only in the presence of methanol, ethanol and formic

acid, out of which latter showed the best result for Se(VI/IV) reduction due to competitive

adsorption of formate anion [144]. However, modification of TiO2 by silver has resulted in

simultaneous generation of H2Se along with elemental selenium [145]. Electron mediation from

TiO2 to selenium via silver greatly enhances the electron density in the selenium particles leading to

the formation of H2Se by reduction of Se(0). Studies on photocatalytic activity of all the

commercially available titania show that Millenium PC-500 with highest surface area as the best

photocatalyst for complete reduction of Se(VI) and Se(IV) into elemental selenium [146].

Selenium reduction is also studied on TiO2-ZrO2 systems. It is reported that 25ppm of Se(VI) can

be reduced to elemental Se in only 45min of reaction under visible light [80]. Increased

photocatalytic activity under visible light is correlated to the synergistic effect of substantial

nitrogen doping, high surface area and presence of oxygen vacancy.

Photochemical copper ion reduction proceeds only in presence of hole scavenger like

sodium formate, EDTA, sodium oxalate or alcohols in the aqueous phase [147, 148]. Generally the

rate of reduction increases with the increase of the concentration of hole scavenger. The presence of

phosphates, sulfates and chlorides has inhibitory effect on copper reduction due to irreversible

adsorption on photocatalyst surface. When the ethyl lactate and phenol contaminated waste-water

contains copper ions, heterogeneous photocatalysis is able to remove both copper and organic

compounds simultaneously [149]. Similarly cyanide degradation along with copper reduction is also

possible [150]. Under aerobic conditions, electron scavenging by O2 is a thermodynamically

favoured process, which interferes with copper reduction. In a mixed Cu(II)/Fe(III)-EDTA system,

Fe(III) has been selectively removed over Cu(II) due to reduction potential difference [151].

However, another group [152] has reported the synergistic photoreduction in mixed solution of

Cu(II) and Cr(VI). Chen et al. [153] have observed a significant enhancement in photoreduction of

copper in presence of alanine over TiO2 nanoparticles.

In addition, Noor et al studied for the first time simultaneous photocatalytic reduction of

Cu(II) in presence of Se(IV) over some spherical binary oxide catalysts [154]. Among a range of

organic additives, only EDTA (ethylene diamine tetraacetic acid) and formic acid are found to be

the most active hole scavengers for the reduction reaction. A comparative study is reported using

both the hole scavengers varying reaction time, concentration, pH etc. For a single contaminant,

EDTA is found to be the best for Cu(II) reduction whereas formic acid is the best for Se(IV)

reduction. In a mixed solution both EDTA and formic acid perform very well under visible light

irradiation. Solution pH plays an important role in the photocatalytic performance as observed in

Fig-9. Highest photocatalytic reduction in a mixed solution is observed at pH 3. Among all the

synthesized materials, titania-zirconia mixed oxide performs as the best photocatalyst for both

Cu(II) and Se(IV) reduction. However under UV light, Degussa P25 performs slightly better than

titanium zirconium mixed oxide. Present study shows that 100ppm of mixed solution can be

removed under visible light in 40minutes of reaction using titanium zirconium mixed oxide as

catalyst. Photodeposited material is found to be copper selenide rather than pure copper and

selenium metal. This indicates that the waste water containing copper and selenium ions can be

efficiently treated under visible or solar light. SEM images in Fig-10 show the catalyst before

reaction, after 15min and 35min reactions. This clearly indicates that deposition of the metal

gradually increases with the reaction time and ultimately completely covers the catalyst surface thus

deactivating the catalyst.

198 Photocatalytic Materials & Surfaces for Environmental Cleanup-II

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Fig. 9: Effect of pH on the photoreduction activity.

Fig. 10: SEM image of the catalyst (a) before reaction, (b) after 15minutes and (c) after 45minutes of

reaction.

3.2.2 Lead and cadmium

Photoreduction of Pb(II), with a relatively negative E0 value (E

0(Pb

2+/Pb

0) = 0.126 V), is

very difficult using TiO2 photocatalyst. Tanaka et al. [155] have found efficient reduction of Pb(II)

under the illumination of Pt–TiO2 suspensions. They have identified PbO2 as the product on use of

Pt-TiO2 photocatalyst while PbO is formed on use of pure TiO2. In a solution containing

nitrobenzene and Pb(NO3)2 at pH 6, the degradation of the organic pollutant and PbO2 deposition

has taken place simultaneously [156]. The reductive process is enhanced on pure TiO2 in presence

of strong hole scavenger such as 2-propanol [157]. Further study shows that formic acid and 2-

propanol exhibit highest effects on lead reduction among other hole scavengers such as methanol,

ethanol and citric acid used.

Cadmium is used in electroplating, alloys, paints, insecticides, batteries and as a neutron

absorber in nuclear reactors. It is highly toxic and can enter the human body through food. The

redox potential of cadmium is -0.403 V, hence the thermodynamic driving force for the

photocatalytic reduction is small. Surface modification of TiO2 particles with adsorbents has been

used for improving the efficiency of the reduction process [158, 159]. In particular, the presence of

formate ions enhances both adsorption and photoreduction of cadmium [160]. Cadmium reduction

is only feasible in presence of selective hole scavengers. Photoreduction of cadmium to its metallic

form is possible in presence of formate whereas there is no reduction with methanol [161]. In a

recent study the surface modified TiO2 with histidine and alanine has shown complete

photoreduction of cadmium into its metallic form [162]. Linkage of alanine and histidine to TiO2

has possibly shifted the conduction band cathodically so as to make electron transfer possible.

Similar results have also been observed by Skubal et al. [163] when TiO2 was modified with

thiolactic acid.

0

10

20

30

40

50

60

70

80

0.5 1.5 2.5 3.5 4.5

Solution pHP

ho

tore

du

cti

on

(m

ol%

)

Cu(II)

Se(IV)

Total

Materials Science Forum Vol. 734 199

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It is reported that mixing of 10wt% silica with titania not only increases the surface area of the

material but also increases the photocatalytic activity in UV light [44, 45]. In particular mixing of

zirconia with titania proved to be beneficial for visible light reaction. However, addition of hole

scavenger increases the activity many folds and complete removal of Pb2+

and Cd2+

was possible in

60 minutes of reaction using synthesized catalysts. Among all the organic hole scavengers used,

sodium formate is found to be the most active one. Interestingly quite high metal removal (89 %) is

also observed in presence of visible light within 60minutes of reaction. In the mixed metal solution

comparatively lead is reduced at a faster rate than that of cadmium which is correlated to the

reduction potential of the corresponding metal ion. Thus the above study indicates that the presence

of certain oxides like silica and zirconia in low quantity with titania can facilitates the

photocatalytic process selectively in UV as well as visible light. Spherical binary oxide

photocatalysts showed high surface area and catalytic activity towards lead reduction even after

calcinations above 700 °C as shown in Fig-11.

0

20

40

60

80

100

Ph

oto

red

uc

tio

n o

f le

ad

(m

ol%

)

Ti TiSi-10 TiSi-20 TiZr-10 TiZr-20

Photocatalysts

500C

700C

900C

Fig.11: Effect of calcinations temperature on photocatalytic removal of lead after 60 min of

reaction in visible light.

3.2.3 Mercury and chromium

Mercury is regarded as serious pollutant, as it can not be degraded either biologically or

chemically. Mercury is toxic at concentrations higher than 0.005 mg/l and included in the list of

priority pollutants [164]. It has been seen that initial formation of elemental mercury by the

photoreduction process increases the activity of TiO2 and thereafter decreases with time [165, 166].

Presence of other metal ions such as Cr(VI) [167], Fe(III) [168] etc. inhibits the photocatalytic

reduction of Hg(II) due to their more positive reduction potential and better adsorption behaviour on

catalyst surface while the presence of hole scavengers increases the Hg(II) photoreduction. Chen et

al. [168] has shown that the presence of EDTA enhances the photoreduction of Hg(II) more than the

4-nitrophenol, methanol and salicylic acid. Arginine modification of TiO2 enhances the adsorption

of Hg(II), facilitate charge transfer and also prevent charge recombination [169]. On the other hand,

thiolactic acid adsorbed TiO2 has detrimental effect on photoreduction of Hg(II) [170]. High affinity

of sulphur for mercury and probable formation of HgS are the main reasons for this inhibition. The

photocatalytic reduction of toxic Cr(VI) species with semiconductors such as TiO2, Pt-TiO2, ZnO,

CdS and WO3 has been widely studied, both in laboratory scale and large scale, for technological

200 Photocatalytic Materials & Surfaces for Environmental Cleanup-II

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applications. It is not possible to reduce Cr(VI) to Cr(0) but can be converted to less toxic Cr(III)

through photocatalytic process. In acidic pH, the Cr(VI) reduction is relatively more than that

occurs in neutral or basic pH [171, 172]. In another study [173], the optimum pH for the adsorption

of Cr(VI) has been found to be in the range 3-6. In the presence of EDTA and other reducing

agents, Cr(VI) reduction is greatly accelerated [174, 175]. In fact the reduction of Cr(VI) is

dependent on the type of organic compound added. For example, low molecular weight acids,

alcohols and aldehydes do not cause any effect on reduction of Cr(VI), while easily oxidizable

organics such as EDTA, succinic acid and citric acid increases the reduction rates.

3.2.4 Other inorganic ions

As(V) or As(III) transformation by a direct reductive pathway is not thermodynamically

possible. However, complete As(V) removal in the presence of methanol under N2 at pH ~ 3

indicates the participation of an indirect reductive mechanism [176]. Zhang et al. [177] has

described a low-cost, environmental friendly mixture of TiO2 and slag-iron oxide for As(III)

removal. Process involves photocatalytic oxidation of As(III) to As(V) in a fast step followed by a

slow adsorption of arsenate. Borgarello et al. [178] has reported the rapid photochemical

deposition of Au(III) from its aqueous chloride solution in the presence of TiO2 (UV light) or WO3

(visible light) at pH ~ 3.1. Air and argon purging yielded the same results, indicating that oxygen

did not affect the rate of deposition. Synthesis of gold having different morphology on TiO2 surface

by photoreduction method is of immense potential in modulating the catalytic processes [179]. Au

clusters (d < 3.0 nm) can be formed rapidly on TiO2 by using physisorbed H2O as a reductant [180].

Addition of H2PtCl6 to degassed aqueous suspensions of TiO2 results in irreversible Pt0 deposition

on the particles with no reoxidation of the metal [181]. Other reports by the same group [182, 183]

show the deposition of Pt from its aqueous acetate solution over TiO2, WO3, SnO2 and SrTiO3

substrates under inert atmosphere which can be recovered quantitatively from the catalyst.

However, some essential aspects of the photodeposition process are yet to be well understood.

0 10 20 30 40 50 60

0

20

40

60

80

100

NO2-

NH4+

N2

Reaction Time (min)

Se

lec

tiv

ity

(%

)

0

20

40

60

80

100

NO

3- c

on

vers

ion

(%

)

Fig.12: Correlation of nitrate photoreduction and product selectivity with reaction time over WN

co-doped TiO2 photocatalyst

Nitrate (NO3-) a major component of fertilizers is toxic to human as per WHO recommendation

with a maximum allowed concentration of 10 mg/l in drinking water. Photocatalytic reduction of

nitrate on bare TiO2 has been reported to be negligible. However, noble metal loading is reported to

increase the nitrate reduction [184]. Role of reductants such as humic acid and oxalic acid on nitrate

reduction have been found encouraging for further research [185, 186]. Though photocatalysis

under visible or solar light is a green process but limited studies were carried out on the

photocatalytic nitrate reduction [187-189]. Metal doped titania is found to reduce nitrate selectively

Materials Science Forum Vol. 734 201

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and efficiently under UV light [188]. Mostly silver doped titania is reported to be the better catalyst

for nitrate reduction. Zinc doped WO3 is reported as a potential photocatalyst under visible light

[190]. Recently Mishra et al reported selective nitrate reduction under visible light over WN co-

doped titania photocatalyst [27]. Among the studied hole scavengers, formic acid is found to be the

best one with 94.6 % nitrogen gas selectivity. Amount of hole scavenger also controls the reaction

efficiency and product selectivity. High nitrate reduction with formic acid is attributed to the

formation of CO2•–

species having high reduction potential. Among all the materials N doped 2% W

containing titania shows the highest surface area and nitrate photoconversion. The change in the

product selectivity with reaction time and surface area is presented in the Fig- 12. Initial few

minutes of reaction shows the high nitrite selectivity and low N2 selectivity. However, after 20 min

of reaction nitrogen selectivity approaches 90 % and thereafter slowly increases to 94.6%.

Ammonia selectivity nearly remains constant throughout the reaction. This confirms that nitrate

reduction to nitrogen gas proceed via nitrite formation. It is observed that chloride ion can not be

used as a hole scavenger but in combination with the formic acid, it can enhance the rate of the

reduction process. Overall the increased nitrate removal with high nitrogen gas selectivity over WN

co-doped titania can be attributed to the synergistic effect of tungsten and nitrogen co-doping,

surface hydroxyl group and mesoporosity.

4. Conclusion and future prospect

This review concludes that mixing of another oxide with titania can improve the phase stabilisation,

porosity and photocatalytic activity. Particulary through doping of another metal oxide brookite or

anatase phase can be stabilized. Further study is warrant to see the comparative effect of anatase

and brookite phase on different the photocatalytic reactions. Overall the improvement of

photocatalytic activity of silica, ceria, tungsten oxide and zirconia mixed titania than titania alone

justifies the need to study the effect of other oxides on titania. This can be looked into seriously

keeping the energy level of different oxides so as to develop more efficient photocatalyst for

specific reactions. Enhanced photocatalytic reduction activity of nitrogen doped titania based mixed

oxides under visible light can be attributed to the synergistic effect of high surface area, presence of

oxygen vacancy and substantial N-doping. Few researchers emphasized that the synergistic N/F co-

doping is better than the pure N doped binary oxides. Again in all the cases it is well observed that

nonmetal doped binary oxides are better photocatalyst than that of doped titania. So it is important

to study further the beneficial effect of binary oxides for future development of solar photocatalyst.

As such, the enhanced visible light photocatalysis of the synthesized materials may generate

significant interest for waste-water treatment under visible/solar light. Formation of copper selenide

instead of pure copper and selenium metal during simultaneous reduction of copper and selenium

indicates the possibility of depositing different alloys on oxide surface through photocatalytic

reduction of multicomponent systems. This could open up an interesting way for material

development. Rapid development of material science and technique in the past few years has

resulted in the creation of various advanced photocatalytic materials. In future these techniques can

be utilized to produce more size and shape controlled composite photocatalytic materials.

Interesting properties may be explored by combining these novel mixed oxide photocatalysts with

graphene or graphene oxide sheets. In addition these mixed oxides can be efficiently used in the

development of Z scheme photocatalyst which is of present importance.

Acknowledgement

Authors acknowledge the encouragements of IPSG members and Director of CSIR-NML

throughout the work. One of the authors NA acknowledges the CSIR for providing SRF fellowship

to carry out the Ph. D work.

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Photocatalytic Materials & Surfaces for Environmental Cleanup-II 10.4028/www.scientific.net/MSF.734 Recent Development on Titania Based Mixed Oxide Photocatalysts for Environmental Application

under Visible Light 10.4028/www.scientific.net/MSF.734.186