tio2 nanotube
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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 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
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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
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
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
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
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
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
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
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
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
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
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
[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
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
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
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
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.
202 Photocatalytic Materials & Surfaces for Environmental Cleanup-II
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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