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TiO2 photocatalysis: progress from fundamentals to

modification technology
a b
P. Pattanaik & M.K. Sahoo
a
Department of Chemistry, Government College, Angul, 759 122, India, Tel. +91
9437059431
b
Department of Chemistry, North-Eastern Hill University, Shillong, 793 022, India, Tel. +91
364 2722632, Fax: +91 364 2551634
Published online: 20 Aug 2013.

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To cite this article: P. Pattanaik & M.K. Sahoo (2014) TiO2 photocatalysis: progress from fundamentals to modification
technology, Desalination and Water Treatment, 52:34-36, 6567-6590, DOI: 10.1080/19443994.2013.822187

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 Desalination and Water Treatment 52 (2014) 6567–6590
www.deswater.com October

doi: 10.1080/19443994.2013.822187

TiO2 photocatalysis: progress from fundamentals to modification

technology

P. Pattanaika,*, M.K. Sahoob,*

a
Department of Chemistry, Government College, Angul 759 122, India
Tel. +91 9437059431; email: pattanaikpranayini@gmail.com
b
Department of Chemistry, North-Eastern Hill University, Shillong 793 022, India
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

Tel. +91 364 2722632; Fax: +91 364 2551634; email: mksahoo@nehu.ac.in
Received 2 August 2012; Accepted 18 June 2013

ABSTRACT

Heterogeneous photocatalysis is a promising method among advanced oxidation processes,

which can be used for degradation of various organic pollutants in water and air. In heteroge-
neous photocatalysis, illumination of an oxide semiconductor, usually the anatage form of
titanium dioxide, by UV radiation produces photo-excited electrons (e
) and positively
charged holes (hþ ). In the aqueous phase, the illuminated surface is extensively regarded as a
producer of hydroxyl radicals (hþ þ OH
¼ HO
). These hydroxyl radicals, holes, and
conduction-band electrons can degrade organic pollutants directly or indirectly. However, the
massive recombination of these photo-generated charge carriers and large band gap of TiO2
limits its overall photocatalytic efficiency. These limitations can be overcome by changing sur-
face properties of titania by adding suitable electron scavengers in the reaction medium or by
modifying its electronic band structure through strategies like metal ion/nonmetal atom dop-
ing, narrow band-gap semiconductor coupling, sensitization by organic dyes, etc. Based on
recent studies reported in the literature, nonmetal ion doping and dye sensitization are very
effective methods to extend the activating spectrum to visible radiation. This review empha-
sizes on the visible-light activation of TiO2 and its application to environmental remediation.

Keywords: Heterogeneous photocatalysis; Mineralization; Doping; Dye sensitization; Composite

semiconductor

1. Introduction aqueous or nonaqueous medium, simultaneous oxida-

tion and reduction reactions occur. This conversion
Heterogeneously dispersed semiconductor surfaces
accomplishes complete oxidative degradation of an
are advantageous to provide chemical environment to
organic substrate present in the system. Molecular
a wide range of adsorbates and means to initiate
oxygen may act like an oxidizing agent in a few gas
light-induced redox reaction in these weakly
or solid phase reactions. The incident light that initi-
associated adsorbates. Upon photoexcitation of several
ates the reaction is in the visible or low range of UV
semiconductors, heterogeneously suspended, either in
region of spectrum. It is absorbed by the heteroge-
neously dispersed particulate semiconductor leading
*Corresponding authors. to photoactivation. During the process, photo-induced

1944-3994/1944-3986 Ó 2013 Balaban Desalination Publications. All rights reserved.

 6568 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590

molecular transformations or reactions take place at 2. Fundamentals of photocatalysis

the surface of the catalyst.
2.1. Photocatalytic materials
In heterogeneous photocatalysis, two or more
phases are used and a light source with a semicon- Solids that can promote reactions in the presence
ductor material is used to initiate the photoreaction. of light and are not consumed in the overall reaction
UV light of long wavelength or even sunlight can be are referred to as heterogeneous photocatalysts. These
used. This is possibly the best method to remove are invariably semiconductors. An ideal photocatalyst
organic species in the environment [1]. Because, the for photocatalytic oxidation (PCO) is characterized by
process gradually breaks down the contaminant mole- the following attributes [19]:
cule and no residue of the original material remains.
The catalyst itself remains unchanged during the • Photo-stability (i.e. not prone to photocorro-
process and no consumable chemicals are required. sion);
Moreover, the contaminant is attracted strongly to the • Photo-activity and photo-suitability towards
surface of the catalyst, so the process works at very visible or near UV light;
low concentrations. The major factors affecting the pro- • Chemical and biological inertness;
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

cess are: initial organic contaminants (OCs), amount • Availability and low cost;
of catalyst, irradiation time, presence of ions [2], tem- • Nontoxicity; and
perature [3], solution’s pH [4], and light intensity [5]. • Capability to adsorb reactants under efficient
The discovery of photocatalytic splitting of water photonic activation.
on electrodes by Fujishima and Honda [6] marked the
beginning of a new era in heterogeneous photocataly- Many semiconductors such as TiO2 , ZnO, ZrO2 ,
sis. Since then, efforts in understanding the funda- CdS, MoS2 , Fe2 O3 , CdS, SnO2 , ZnS, WO3 , etc. have
mental processes of semiconductor photocatalysis and been examined and used as photocatalysts for the
enhancing its efficiency have gained momentum. In degradation of OCs [20,21]. In nature, TiO2 crystallizes
recent years, applications to environmental clean-up into three polymorphs: anatase, rutile, and brookite.
have become most active areas in heterogeneous pho- Anatase is thermodynamically less stable than rutile
tocatalysis. The potential application of TiO2 -based and exhibits a shorter wavelength absorption edge.
photocatalysis aims at the total destruction of organic Frequently present in nano-sized TiO2 particles, it is
compounds (OCs) in polluted air and wastewater largely recognized to be the most active agent in the
[7,8]. It has been used successfully for the mineraliza- oxidative detoxification reactions. TiO2 is one of the
tion of a wide variety of compounds such as alkanes, most efficient catalysts for the production of hydrogen
alcohols, carboxylic acids, alkenes, phenols [9], simple and oxygen from water in the presence of UV-part of
aromatics, halogenated hydrocarbons, surfactants, solar light [22,23]. Among the semiconductor
pharmaceuticals [10,11], and pesticides [12,13] as photocatalysts available, TiO2 is the most extensively
well as dyes from textile industry wastewater studied photocatalyst owing to its properties like
[14–16]. Photocatalysis has also been extended to resistance to photocorrosion, low cost, ready availabil-
water-splitting technology to produce solar hydrogen ity, nontoxicity, and its applicability at ambient condi-
to support the future hydrogen economy [17]. A novel tions. Of the three common TiO2 crystalline forms,
photo-induced super hydrophilic phenomenon anatase and rutile forms have been investigated exten-
involving TiO2 has also been reported [18]. sively as photocatalysts. Anatase is reported to be
The whole discussion is divided into three parts. more active as a photocatalyst than rutile. Evonik
In the first part, conventional photocatalysis and its Degussa P25 is a titania photocatalyst that is used
fundamentals have been discussed. In the second part, widely because of its high-level photocatalytic activity.
the recent trend of using modified titanium dioxide It is composed of anatase and rutile crystallites; the
and the formulation of simplified mechanism of modi- reported ratio is 70:30 or 80:20 without being sure of
fication technologies have been described. Modified its actual composition. In a typical analysis, crystalline
titanium dioxide is efficient for the photocatalytic deg- composition of P25 was evaluated to be 78% anatase
radation of pollutants under visible-light irradiation. and 14% rutile. Assuming the remaining 8% part
In the third part, important applications of photocatal- corresponds to amorphous phase, the anatase–rutile–
ysis have been discussed. Photocatalytic mineraliza- amorphous ratio is determined to be 78:14:8 [24]. The
tions of contaminants present in water and air and minimum band-gap energy required for a photon to
production of hydrogen from water splitting utilizing cause photogeneration of charge carriers over the
TiO2 have been discussed to show novel applications TiO2 semiconductor (anatase form) is 3.2 eV
of semiconductor photocatalysis.
 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590 6569

corresponding to a wavelength of less than 400 nm spite of this drawback, many other authors have
[25]. Actually, photoactivation in TiO2 takes place in attempted using ZnO in the PCO process [35–37].
the range 300–380 nm. The minimum wavelength Some other metal oxides including CeO2 ; SnO2 ; WO2 ,
required to promote an electron depends upon the and CdS have also been examined for OCs degrada-
band-gap energy of the photocatalyst (Eq. (1)) as sug- tion [38–40].
gested by Mills et al. [26]. The band-gap energy of
various photocatalysts [20,21] is listed in Table 1. 2.2. Principles of PCO process
Numerous studies on the application of PCO for the
removal and mineralization of organic pollutants In the PCO process, organic pollutants are
[3,26–31] in aqueous solutions have been reported. destroyed in the presence of semiconductor photocata-
lysts (e.g. TiO2 and ZnO), an energetic light source,
1; 240 and an oxidizing agent such as oxygen or air. As
Ebg ¼ ð1Þ illustrated in Fig. 1, only photons with energies
kmin
greater than the band-gap energy (E) can result in the
Due to the high band-gap energies of SnO2 and excitation of VB electrons, which then promote the
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

ZnS, the light energy may not be sufficient to activate possible reactions with organic pollutants. The absorp-
the catalyst. Catalysts like CdS and Fe2 O3 have tion of photons with energy lower than E or longer
smaller band-gap energies [21]. The smaller band gap wavelengths usually causes energy dissipation in the
permits rapid recombination of hole and electron, and form of heat. The illumination of the photocatalytic
hence a negligible photocatalytic activity may be surface with sufficient energy (k 6 380 nm for TiO2
observed. However, due to the higher surface area of and ZnO), leads to the formation of an electron-hole
ZnO (10 m2 g
1) over TiO2 (anatase, 8.9 m2 g
1), ZnO pair: a positive hole in the VB and an electron in the
þ
may show greater photocatalytic activity than TiO2 . In conduction band ðe
CB Þ (Eq. (2)). The holes ðhvb Þ and
accordance with the above facts, it has been reported electrons ðe
CB Þ can either undergo recombination and
that the dye Acid Red 18 (AR 18) undergoes maxi- dissipate the input energy as heat or migrate sepa-
mum degradation with ZnO over TiO2 (anatase) cata- rately to the surface of TiO2 particles, get trapped in
lyst, while SnO2 ; Fe2 O3 , CdS, and ZnS have negligible metastable surface states, or react with electron donors
activity on AR18 decolorization and degradation [32]. and electron acceptors adsorbed on the semiconductor
But, ZnO suffers a major drawback in that it corrodes surface or within the surrounding electrical double
during the oxidation process. When ZnO suspensions layer of the charged particles.
are UV-irradiated, the valence-band (VB) holes that In the absence of suitable electron and hole
are photo-generated can thermodynamically oxidize scavengers, the stored energy is dissipated within a
the ZnO semiconductor because its decomposition
potential is located within the band gap [33]. Some
ZnO dissolves forming Zn2þ ions even in the dark on
stirring the aqueous suspension owing to the ampho- .
O2 , H2O2
teric nature of this metal oxide. Further, the quantity
of Zn2þ is seen to increase when irradiated with the Reduction
400-W lamp, relative to the 75-W light source [34]. In CB e-

O2
Table 1 hv> E
Band-gap energy of various photocatalysts Recombination E
(heat) Excitation
Photocatalyst Band-gap Photocatalyst Band-gap by light
energy (eV) energy (eV) H2O / OH
Si 1.10 ZnO 3.20
VB h+
TiO2 (rutile) 3.00 TiO2 3.20 Oxidation
(anatase)
WO3 2.70 CdS 2.40 HO
.
ZnS 3.70 SrTiO3 3.40
SnO2 3.87 WSe2 1.20 Fig. 1. Simplified mechanism of photocatalytic process
Fe2O3 2.20 a
Fe2 O3 3.10 showing photo-excited semiconductor to produce hydroxyl
SiC 3.00 radical and superoxide radical anion from water and
oxygen respectively.
 6570 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590

few nanoseconds by recombination. If a suitable scav- of paramount importance to prevent electron accumu-
enger or surface defect is available to trap the electron lation in an efficient PCO. Further, it is reported that
or hole, recombination is prevented and subsequent the preferred oxidation route is highly compound-
redox reactions occur. The ðhþ vb Þ, being a strong dependent. Those species that adsorb strongly to TiO2
oxidant (+1.0 to + 3.5 V vs. NHE depending on the (highly polar compounds) are more likely to oxidize
semiconductor and pH), either oxidizes the adsorbed via the surface-trapped holes. The compounds which
organic substrates directly or reacts with electron have no hydrogen atoms available for abstraction by
donors like surface-bound water or hydroxide ions OH radical are oxidized by VB holes. On the other
leading to the formation of adsorbed hydroxyl radical hand, it is important for the excited electron in the
[41–43] within a few picoseconds, which is also a conduction band to be scavenged by an external agent
potent oxidizer (Eqs. (3)–(5)). The conduction-band to prevent its recombination with the positive hole
electrons are good reductants (+0.5 to
1.5 V vs NHE) under ambient conditions, which otherwise would
and they react with reducible species to prevent a lead to very low quantum yields. Use of suitable elec-
build-up of charge. Thus, the ðe
CB Þ reduces the oxygen tron scavengers or the presence of a surface defect
adsorbed on the photocatalyst (TiO2) to a superoxide state can trap the electron prolonging the lifetime of
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

anion radical, O

2 (Eq. (6)). We will see later that O2



the hole.
is involved in the generation of hydrogen peroxide In PCO, the catalyst can either provide energy
(HP) and ultimately HO, which is involved in the levels to mediate electron transfer between adsorbate
degradation process (Eqs. (7)–(9)). molecules (by temporarily accommodating an elec-
tron) or behave as both an electron donor (the
þ
TiO2 þ hm ! e

CB þ hvb ð2Þ photo-generated electron in the conduction band) and
an electron acceptor (the photo-generated hole in the
VB). Here, the band structure of the substrate plays a
hþ

vb þ H2 O ! HOsurf þ H
þ
ð3Þ
significant role. A change in the surface and bulk elec-
tronic structure can dramatically alter the chemical
hþ


vb þ HO ! HOsurf ð4Þ events following photoexcitation of the adsorbate
molecules or the catalyst substrate.
hþ

vb or HOsurf þ Organic molecule

! Intermediates ! CO2 þ H2 O ð5Þ 2.3. Role of electron scavengers

In the heterogeneous photocatalytic system, elec-



e

CB þ O2 ! O2 ð6Þ tron scavengers play an important role by preventing
electron-hole recombination. Oxygen acts efficiently as
Photochemical oxidation may occur by either indi- an electron scavenger, preventing the recombination
rect oxidation via the surface-bound hydroxyl radical of photo-generated electrons and holes. When oxygen
(i.e. a trapped hole at the particle surface) or directly is limited, the rapid recombination of electrons and
via the valence-band hole before it is trapped either holes in TiO2 would markedly reduce its photocata-
within the particle or at the particle surface. It is not lytic actions. In lieu of oxygen, inorganic oxidants


possible to distinguish between a trapped hole and an such as IO


4 ; S2 O8 ; BrO3 ; ClO3 ; and H2 O2 can quench
2

adsorbed HO
radical. It is suggested that adsorbed conduction-band electrons and form reactive radical
HO
radical (or surface trapped hþ vb ) is the major oxi-
intermediates, thereby reducing the probability of
dant while free HO
radicals play only a minor role, if recombination of the photo-generated electrons and
any [44]. The HO
attacks a variety of OCs e.g. chlori- holes and enhancing photodegradation of organic sub-
nated aromatics, aniline, nitrophenols, etc., leading to strates by valence holes [45–49]. However, the relative
various reaction intermediates depending on the nat- efficiency of these oxidants has not been reported as it
ure of the compounds. The resulting intermediates varies from system to system.
further react with HO
to produce final degradation
products such as CO2 and H2O. In the photocatalytic
degradation of pollutants, when the reduction process 2.3.1. Oxygen
of oxygen and the oxidation of pollutants do not As described earlier, molecular oxygen (dissolved
advance simultaneously, there is an electron accumu- oxygen in this case) is an efficient electron scavenger
lation in the CB, thereby causing an increase in the and forms O

þ 2 . Thus, oxygen on the catalyst surface
rate of recombination of e
CB and hvb [3,43]. Thus, it is provides a natural sink for the photo-generated
 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590 6571

electrons. The O

2 formed in Eq. (6) may generate HP

HO
2 þ HO
! H2 O þ O2 ð13Þ
(Eq. (7)) in acidic media [50].

O


þ
2 þ eCB þ 2H ! H2 O2 ð7Þ
2.3.3. Peroxydisulphate
The presence of the oxidant peroxydisulphate
Even though the generation of HP during PCO (S2 O2
8 ; e:g: K2 S2 O8 ) also accelerates the PCO process
has been reported long ago, no significant effort has by trapping e
CB and subsequently preventing its
been made to monitor the formation of HP. The HP recombination with hþ vb . At the same time, it produces
thus formed may undergo photo-induced degrada- SO

, a very strong oxidant (E0 = 2.5–3.1 V vs. NHE)
4
tion, mainly on the photocatalyst surface, by direct
[53] (Eq. (14)). An important advantage of using
reaction with photo-generated charged species, i.e.

S2 O2
as e

CB scavenger is that it produces HO
VB holes, hþ

vb , conduction-band electrons, eCB , and/or
8

other reactive species such as hydroxyl radicals and radicals in aquesous solution at various pH values
the superoxide radical anion. A recent study [51] on (Eq. (15)). In fact, SO

4 starts decomposing into HO at




pH > 8.5, and HO becomes the major species at
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photocatalytic degradation of Acid Red 1 (AR1)

reported that the highest levels of HP were attained pH > 10.7 [57].
in the presence of P25 (TiO2). The HP concentration
rapidly increased and reached a maximum value



8 þ eCB ! SO4 þ SO4
S2 O2
ð14Þ
2

corresponding to 80–90% AR1 photodegradation yield
and then decreased. An almost identical HP concen-
tration profile was observed during the photocatalytic SO



4 þ H2 O ! SO4 þ HO þ H
2
þ
ð15Þ
degradation of benzoic acid in the presence of
P25 [52].

2.3.4. Periodates
2.3.2. Hydrogen peroxide Periodate ion has more than two atoms of oxy-
gen per atom of halogen, where I is the central
PCO can be accelerated by HP because it traps e
CB atom. Polarizability differences in the constituent
more efficiently than oxygen. This prevents electron-
atoms of IO
4 make its central atom extremely elec-
hole recombination and generates HO
during the
tropositive. Therefore, IO
4 can capture conduction-
process (Eq. (8)), which has been demonstrated in
band electrons (Eq. (16)) ejected from a photocatalyst
most studies [53–55]. During the reaction, HP also
more efficiently than other oxidants [45,58–60].
produces HO
by reacting with O

2 or by direct
Besides capturing conduction-band electrons, perio-
photolysis (Eqs. (9) and (10)). However, when pres-
date undergoes decomposition under UV-irradiation
ent at high concentration, HP exerts an inhibition
generating a number of highly reactive radical and
effect on PCO by scavenging hþ

vb and HO [53,56] nonradical intermediates [61–63] (Eqs. (17) and (18))
(Eqs. (11)–(13)). Furthermore, H2 O2 can be adsorbed including HO
. It has been reported that the reaction
onto TiO2 particles to modify their surfaces and rate order with respect to periodate is 0.8 times
subsequently decrease their catalytic activity. higher than that for peroxide [58]. The fact that IO
4
is a more effective oxidant than S2 O2
8 and H2 O2 for



H2 O2 þ e

CB ! HO þ HO ð8Þ degradation has been reported in some studies
[61,62,64]. However, the concentration of IO
4 in
such studies should be optimized, because scaveng-
H2 O2 þ O




2 ! HO þ HO þ O2 ð9Þ ing of the valuable hydroxyl radicals
[65] by IO
4 ions takes place (Eq. (19)) at higher
H2 O2 þ hm ! 2HO
ð10Þ concentration.

þ
H2 O2 þ 2hþ
vb ! O2 þ 2H ð11Þ IO

þ

4 þ 8eCB þ 8H ! 4H2 O þ I ð16Þ

H2 O2 þ HO
! H2 O þ HO
2 ð12Þ IO

4 þ hm ! IO3 þ O ð17Þ
 6572 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590

O

þ Hþ $ HO
ð18Þ Ti
OH þ HO
! TiO2 þ H2 O ð22Þ

HO
þ IO



4 ! HO þ IO4 ð19Þ Ti
OH þ Hþ ! TiOHþ
2 ð23Þ

2.3.5. Bromates
2.4.2. pH
Bromate ions as electron scavengers have been
rarely studied even though the photocatalytic degra- On the basis of Eq. (4), one can expect that a higher
dation efficiencies of organic substrates are signifi- pH value can provide higher concentrations of hydro-
cantly improved in the presence of KBrO3 [32,54,66]. xyl ions ðHO
Þ to react with holes to form hydroxyl
The enhancement of the degradation is due to the radicals and subsequent enhancement of PCO. It has
reaction between BrO
been reported that higher pH favors degradation
3 ions and conduction-band
electrons (Eqs. (20) and (21)), which reduces the through functional group substitution, while lower pH
electron-hole recombination. It has been shown that favors direct ring cleavage prior to the mineralization
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

BrO
of organic carbon [74,75]. In addition, it is interesting

3 exhibits detrimental effect on the ZnO-catalyzed
degradation process at higher concentration due to to note that different proton sources, e.g.
the adsorption effect of Br
ion on ZnO surface, HCl; NHO3 ; HNO3 ; H2 SO4 ; H3 PO4 etc: (i.e. different
which affects the catalytic activity of ZnO. counter ions associated with protons used for main-
taining pH), can alter the photocatalysis to different
degrees. It is reported that inorganic anions are capa-
BrO
þ


3 þ 2H þ eCB ! BrO2 þ H2 O ð20Þ
ble of inhibiting the photocatalytic degradation of
dichloroethane [76] and alachlor, a widely used herbi-
BrO
þ


3 þ 6H þ 6eCB ! ðBrO3 ; HOBrÞ cide [12] in an aqueous suspension of TiO2. This may
! Br
þ 3H2 O ð21Þ be due to the fact that anions may adsorb onto the sur-
face of TiO2, and isolate the contaminant molecules
from the reaction. At pH < pHZPC , TiO2 particles are
positively charged and the inorganic anions can be
2.4. Factors influencing the photocatalytic degradation attracted towards them forming a layer of charged bar-
riers around TiO2. This hinders the collision between
2.4.1. Adsorption
the target molecule and the TiO2 particles. Since the
It is well established that PCO proceeds through density of the hydroxyl radical is highest near the sur-
the attack of HO
radicals on the adsorbed surface face of the TiO2 particles and decreases rapidly with
[67,68]. Thus, the rate of degradation is higher at increasing distance from the surface of TiO2 particles,
higher adsorption [69,70]. Adsorption is affected by the inhibition of the coupling of the target molecule
several factors such as effluent composition and pH. with titanium dioxide particles results in rate retarda-
The pH of the reaction medium has a significant effect tion. This period can be considered as an inhibition
on the surface properties of TiO2, which includes the period. After this period, the intermediates start accu-
surface charge of the particles, the extent of aggrega- mulating near the TiO2 particles and may form a
tion of TiO2 particles and the band edge position of bridge or good coupler to couple TiO2 particles with
TiO2. The pH at which the surface of an oxide is unreacted contaminant molecules [77]. As the number
uncharged is defined as the zero-point charge of the couples increases, they compete with the active
(pHZPC ). The pHZPC of anatase TiO2, Degussa P-25, sites on the TiO2 surface with anions and form more
Fe2 O3 , CuO, ZnO, ZnS, and CdO are about 4.0, 6.25, couples, thereby progressively reducing the inhibition
8.6, 9.5, 8.8, 1.7, and 11.6, respectively [21,71–73]. effect of anions. As a result, the effect of rate of retar-
Above and below this value, the catalyst is negatively dation due to the presence of anions is reduced. An
or positively charged according to Eqs. (22) and (23). interesting result is found with surface-bound phos-
Therefore, at pH < pHZPC , the positively charged sur- phate anion [78]. The degradation of substrates (such
face of TiO2 attracts the anionic species, leading to as 4-chloropehenol, phenol, and rhodamine B) with
greater adsorption and hence increased degradation. weak adsorption on the pure TiO2 was markedly accel-
The reverse effect is observed at pH > pHZPC , where erated by phosphate modification, while substrates
the TiO2 surface being negatively charged would (such as dichloroacetic acid, alizarin red, and catechol)
adsorb cationic species, thereby increasing the degra- with strong adsorption exhibited a much lower
dation efficiency of PCO. degradation rate in the phosphate-modified system.
 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590 6573

Phosphate anion has a negative effect on the degrada- 2.4.4. Presence of cations
tion of compounds that can be adsorbed over the
The presence of transition metal ions was found to
catalyst, and a positive effect on the degradation of
increase the TiO2 photocatalytic degradation of
compounds that hardly adsorb on TiO2 by enhancing
organic pollutants [83–88]. The observed rate increase
the formation of HO
radicals in the solution (through
has been attributed to the tendency of the metal ions
the formation of a surface complex), and thus affects
to be reduced at the semiconductor surface by scav-
the oxidation of those compounds.
enging e

CB (Eq. (26)).

ðn
1Þþ
2.4.3. Presence of anions Mnþ þ e

CB ! M ð26Þ
Apart from pollutants, industrial effluents gener- where Mnþ represents the most extensively studied
ally contain different salts like chloride, sulphate, metal ions to enhance the photocatalytic degradation
bicarbonate, and carbonate. The salts are generally
rates, viz. Fe3þ ; Mn3þ ; and Cu2þ . The above reaction
ionized under the conditions accompanying PCO. The
prevents electron-hole recombination and results in an
presence of an anion or cation affects the rate of PCO.
increased rate of formation of HO
radicals. Transition
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

The anions affect the adsorption of the target molecule

metal ions also increase PCO rate by inducing photo-
on TiO2, act as HO
scavengers, and may absorb UV
Fenton type reactions (Eq. (27)) to produce more HO

light. Negative effects of anions on PCO of aromatic
radicals [85].
compounds have been reported by some authors
[67,79,80]. Recently, the negative effects of Cl
and
NO
3 ions on the degradation rate of phorate, the Mðn
1þÞ þ H2 O2 þ Hþ ! Mnþ þ HO
þ H2 O ð27Þ
restricted pesticides used to control sucking and
chewing pests, were reported [81]. It was suggested
The effect of Fe3þ ions on the photodegradation of
that the inhibition effect of Cl
is due to the scaveng-
OCs has been investigated extensively. It is found that
ing of hþ

vb and HO (Eqs. (24) and (25)). The radicals




ferric ions increase the degradation rate up to a
(Cl and ClOH Þ are not as reactive as e
CB and HO certain concentration, beyond which the rate begins to
and hence cannot contribute towards the degradation decrease [83–85,87]. Butler et al. [87] have also
process.
reported the positive effect of Mn2þ ions on the photo-
catalytic degradation of organic pollutants. However,
Cl
þ hþ
vb ! Cl


ð24Þ
there is considerable controversy on the photocatalytic
activity of cupric ions. Some investigators [86–88]
Cl
þ HO
! ClOH

ð25Þ have found that cupric ion behaves as an accelerator,
where as some others [83,84] have reported a decrease
The effect of an anion or, in particular, any scaven- in the apparent rate constant upon the addition of
ger of HO
, will depend upon the extent of availability cupric ions. The negative effect of cupric ions on the
of the scavenger on the surface of the photocatalyst. photocatalytic degradation may be attributed to the
In other words, conditions which favor adsorption of low reduction potential for Cu2þ =Cuþ couple. As a
the scavenger on the photocatalyst will have a nega- result, cupric ions are reduced to cuprous by e
CB
tive effect on the rate of PCO. Kamble et al. [82] have (Eq. (28)). The Cuþ ions thus formed are oxidized to
shown that HCO
2

3 and CO3 practically do not adsorb Cu2þ ions either by hþ vb on the surface of the TiO2
on TiO2 in the presence of aniline. Further, as the lone particles or by HO
radicals (Eqs. (29) and (30)). This
pair of electrons in aniline is dispersed over the causes a decrease in the concentration of HO
and
benzene ring, the site on which aniline is adsorbed hence a decrease in the degradation rate.
develops a negative charge which probably repels
HCO
3 and CO3
2

species. Thus, these species which Cu2þ þ e
þ
CB ! Cu ð28Þ
can scavenge HO
are effectively eliminated from sites
adjacent to those on which aniline is adsorbed. These
sites then become available for the generation of HO
. Cuþ þ hþ
vb ! Cu
2þ
ð29Þ
These factors contribute to the enhanced PCO of
aniline in the presence of HCO
2

3 and CO3 . Cuþ þ HO
þ Hþ ! Cu2þ þ H2 O ð30Þ
 6574 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590

2.4.5. Temperature r0 ¼
dC=dt ¼ kKC ð32Þ

An increase in reaction temperature generally
results in increased photocatalytic activity. However, lnðC=C0 Þ ¼
kKt ¼
Kapp t ð33Þ
reaction temperature greater than 80˚C promotes the
recombination of charge carriers and disfavors the where Kapp (min
1) is the apparent rate constant, and
adsorption of OCs on the TiO2 surface. Normally, a C0 is the initial concentration of OC (mg l
1).
photocatalytic reaction has an optimum range of In addition to the Langmuir–Hinshelwood model
operational temperature between 20 and 80˚C as men- and pseudo-first-order kinetic model [92,93], a
tioned by Hermann [3] as well as Gogate and Pandit pseudo-second-order kinetic model (Eq. (34)) has been
[2]. When working at a low temperature, desorption used to describe the degradation kinetics of different
of the products formed limits the degradation reac- compounds [94].
tion. On the other hand, at a higher temperature,
adsorption of the OC on the TiO2 surface becomes the
dC=dt ¼ kC2 ð34Þ
limiting stage. The optimum range of temperature has
where k (mg
1 l min
1) is the second-order kinetic
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a small effect on reaction rate, as long as the experi-

ment is carried out near the middle of the range of constant.
20–80˚C. When the optimum range of temperature is It is found that the degradation rate depends on
from 40 to 50˚C, the increase from 40 to 50˚C does not the initial concentration of the OC. The kinetics of
affect the reaction rate [89]. A reaction temperature photocatalytic degradation of OC is shown in Fig. 2
below 80˚C favors the adsorption whereas further plotting concentration against time. Since hydroxyl
reduction of reaction temperature to 0˚C results in an radicals have a very short lifetime (only a few nano-
increase in the apparent activation energy. Therefore, seconds), they can react only at or near the location
temperature range between 20 and 80˚C is regarded where they are formed. A high OC concentration
as the desired temperature for effective photomineral- increases the probability of collision between an
ization of OCs. organic matter and an oxidizing species, leading to an
Photocatalytic reactions at the temperature of increase in the mineralization rate. The values of ln
38–100˚C seem to followed pseudo-first-order rate (C0/C) vs. time are plotted in Fig. 2 (in set). The figure
law, and the temperature affects the reaction rate shows that the photocatalytic degradation follows the
highly. The rate constants increase by about six times pseudo-first-order kinetics with respect to OC concen-
from 3.52  10
4 to 2.17  10
3 min
1 when the tem- tration.
perature is adjusted from 38 to 100˚C. Consequently, The effect of initial OC concentrations on the ini-
this photocatalytic course can be accelerated by using tial rate of degradation is shown in Fig. 3. A linear
the infrared light of solar energy to increase the expression can be obtained conventionally by plot-
temperature of the photocatalytic reaction. It should ting the reciprocal apparent rate constant against the
be a potential way to make full use of solar light in initial concentration (plot of 1/Kapp against C0) as
photocatalysis in practice [90].

3. Kinetics of photocatalytic degradation

ln C0/C

Langmuir–Hinshelwood model as defined by

Eq. (31) has been widely applied for the analysis of
Conc (mg l )
-1

heterogeneous photocatalytic degradation kinetics of

pollutant OC in the aqueous phase [91]:
Irradiation Time (min)

r0 ¼
dC=dt ¼
kKC=ð1 þ KCÞ ð31Þ

where r0 is the initial reaction rate (mg l
1 min
1), C is

the concentration of OC (mg l
1), t is the reaction time
(min), k is the Langmuir–Hinshelwood reaction rate
constant (mg l
1 min
1), and K is the Langmuir adsorp- Irradiation Time (min)
tion equilibrium constant (l mg
1). At a dilute concen-
tration of OC (i.e. KC << 1) [10], pseudo-first-order Fig. 2. Kinetics of photocatalytic degradation at initial
concentration. Inset: first-order plot for photocatalytic
kinetic model can be assumed (Eqs. (32) and (33)), degradation of organic compound.
 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590 6575

CB .
O2 , H 2 O2
e- e-
e-
r0 (mg l min )
-1

O2
hvnm
hv
1/K app (min)
-1

hvm
h+ H2O /OH
h+
h+ h+ .
HO
VB
-1
C0 (mg l )
Fig. 4. Simplified mechanism of pure and doped TiO2
-1 photocatalysis: doping reduces band gap facilitating
C0 (mg l ) photoexcitation and production of reactive radicals.
ht—pure TiO2; hmm —metal-doped TiO2; hmm —nonmetal-
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

Fig. 3. Effect of initial concentration on the initial rate of doped TiO2.

degradation. Inset: reciprocal of apparent rate of
degradation against initial concentration.
4.2. Doping
shown in Fig. 3 (inset). In general, experimental
Doping of ions or atoms causes the TiO2 lattice to
data fit well to the second-order kinetic model for
modify its micro and electronic structures, and the
the early minutes of irradiation when the OC con-
introduced defects distort TiO2 lattice and inhibit
centration is high [95]. A possible explanation for
anatage to rutile transformation. Moreover, alternative
second-order photodegradation kinetics is the aggre-
energy levels in between bands and so mid-gap
gation or dimer formation with increasing OC con-
energy levels below conduction band or above VB are
centration [96]. Experiments carried out with low
formed (Fig. 4). Both metal and nonmetal atom
concentrations are usually described by the pseudo-
impurities serve as trapping centers to retard charge
first-order kinetic model [94]. In general, the value
recombination and extend the excitation wave length
of the second-order kinetic constant was lower when
from the UV to the visible-light range.
the amount of OC was increased for the same
amount of catalyst [95].
4.2.1. Metal ion doping
4. Modified photocatalysis The doping of metal ions into the TiO2 lattice
works wonderful as the resulting materials show an
4.1. Modification technology
enhancement of photocatalytic properties. In recent
In heterogeneous photocatalysis, the limitation of years, extensive research focuses on visible-light-
the photocatalytic degradation is attributed to the induced metal ion-doped semiconductor photocataly-
recombination of photo-generated electron-hole pairs. sis. Studies show that the role of metal ion doping to
As discussed earlier, various attempts have been enhance photocatalytic activities is mainly due to the
made to avoid this recombination in photocatalytic following modification mechanisms:
processes. Another limitation of this process is that
TiO2 can absorb UV light effectively but not visible (1) reduction of the band gap in titania, [98]
light. More recently, significant efforts have been (2) improvement in charge carrier separation, [99]
made to develop new techniques or modified semi- and
conductor photocatalysts that are capable of absorbing (3) increase in the level of surface-adsorbed
visible light (400–700 nm). These include metallic and species, e.g. hydroxyl radicals [100].
nonmetallic element doping, sensitization with organic
dyes, and development of small band-gap semicon- The visible-light photoactivity of metal-doped TiO2
ductors. Besides, noble metal ion implantation and can be explained by a new energy level produced in
nano semiconductor composite catalysts are devel- the band gap of TiO2 by the dispersion of metal
oped as other alternatives. The structural modification nanoparicles in the TiO2 matrix. As shown in Fig. 4,
is also extended to separate and recover titania after electron can be excited from the defect state to the
the process of photocatalysis [97]. TiO2 conduction band by photon with energy equals
 6576 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590

to hmm . The additional benefit of metal ion doping is and evaluated the higher activity of V/TiO2 on the
the trapping of electrons to restrain electron-hole degradation of crystal violet and methylene blue
recombination and leave the holes for oxidative degra- under visible-light irradiation. They found that an
dation of OCs. Further, metal ion doping enhances increase in vanadium doping promoted particle
interfacial charge transfer reactions, thereby increasing growth, in which vanadium, as per X-ray absorption
the photoreactivity of TiO2 [101]. The doping of metal spectroscopy (XAS) analysis, is highly dispersed
ion induces a red shift of absorption capacity from inside the titania structure. This results in enhanced
UV to visible region by introducing a localized band “red-shift” in the UV–vis absorption spectra and sub-
of orbital within the band gap. Recently, most of the sequent higher activity of TiO2.
investigations have focused on preparing TiO2 cata- Indeed, special efforts have been dedicated to
lysts which can be activated by visible light because doping TiO2 with Fe3þ ions [111–113]. Amongst a
there is much more energy produced by the sunlight variety of transitional metals, iron is suitable for dop-
in the visible-light region compared to the UV region ing because of the fact that the radius of Fe3þ (0.79 Å)
[18,102,103]. As mentioned, doping or combining of is similar to that of Ti4þ (0.75 Å), so that Fe3þ can be
TiO2 with various metal ions was reported as a good easily incorporated into the crystal lattice of TiO2. This
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

tool to improve the photocatalytic properties [104] and interest is based on the idea that Fe3þ ions act as
for enhancement of visible-light response [105,106]. shallow charge traps in the TiO2 lattice [99,114,115].
Numerous metal ions, including transition metal ions
However, the role of Fe3þ ions in TiO2 is controver-
(e.g. vanadium, chromium, iron, nickel, cobalt, ruthe-
sial. Some authors suggest that Fe3þ behaves as e
; hþ
nium and platinum) and rare earth metal ions (e.g.
recombination center [116], while others have
lanthanum, cerium, and ytterbium), have been investi-
postulated that the role of dopant ion is to favor
gated as potential dopants for visible-light-induced
photocatalysis. However, as will be discussed later, e
=hþ separation, which enhances the photoactivity
metal ion dopant can also serve as a recombination [70,99,111,112]. Nano-sized Fe-doped and undoped
center, resulting in decreased photocatalytic activities. TiO2 particles have been synthesized by hydrothermal
process at low temperature [117]. Doping of TiO2 by
4.2.2. Transition metal ion doping Fe3þ ion decreases the particle size resulting in an
increase in the surface area. They have demonstrated
The sol–gel method has been widely used to pre- higher photocatalytic performance of Fe-doped TiO2
pare titania nanoparticles under controlled conditions thin film than that of the undoped TiO2 film under
[107]. The integration of dopants into the sol during UV and visible lights for the degradation of Malachite
the gelation process facilitates direct interaction with green. They have also suggested that the interaction of
TiO2. Therefore, dopants can be incorporated into the Malachite green with Fe3þ -doped TiO2 thin films
titania lattice, resulting in materials with different follows the pseudo-first-order reaction kinetics. Nano-
optical and catalytic properties. Paola et al. [101] sized titania homogeneously doped with chromium
used a set of TiO2 photocatalysts loaded with various has also been prepared through the sol–gel method
transition metal ions (Co2þ ; Cr3þ ; Cu2þ ; Fe3þ ; Mo5þ ; [107]. Substantial doping can be achieved until 1.0 wt.
V5þ ; and W6þ ) and tried to find a correlation % of chromium content is reached. Chromium in
between photocatalytic behavior and physicochemical titania-doped materials can have other oxidation states
properties of prepared samples. They have reported than that exhibited in its precursor because of the
a descending sequence of photocatalysts depending possibility of redox reactions during the synthesis.
on their activities: W/TiO2 > Mo/TiO2 > Cu/ The visible-light absorption by chromium-doped TiO2
TiO2 > Fe/TiO2 > Co/TiO2 > V/TiO2 > Cr/TiO2. Chen is believed to be due to a different mechanism, i.e.
and Wang [108] have demonstrated that different chromium doping does not bring down the band gap,
metal ion doping exhibits complex effects on the char- but induced visible-light absorption through the
acteristics of titania. Across the investigated formation of color centers.
ions (Zn2þ ; Fe3þ ; Co2þ ; Cu2þ ; Ni2þ ; Mn2þ ; V5þ ; Cr3þ ),
doped TiO2 has shown the higher photoactivity in 4.2.3. Noble metal atom doping
the decoloration of methyl orange compared with
bare TiO2. Sharma et al. [109] found increased cata- Given that charge separation enhances photocata-
lytic activity of titania films on methyl orange degra- lytic activity, one clever way of achieving charge sepa-
dation when doped with 2–10 mol% Ni. Wu and ration, as well as visible-light activity, is to incorporate
Chen [110] directed their research at developing a noble metal nanoparticles such as silver or gold into the
visible-light response catalyst via vanadium doping titanium dioxide material. For example, incorporation
 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590 6577

of a small amount of silver (1–5%) results in increased leading to an effective separation of electron hole and
efficiency in photocatalysis [118]. Silver has a “Fermi resulting in the improvement of photocatalytic effi-
level” or electron-accepting region at an energy level ciency. Preparation of Pt-modified TiO2 loaded on natu-
just below its conduction band. Therefore, after light ral zeolites (Pt–TiO2/zeolites) by sol–gel technique
absorption and charge separation, the electron in the photoreductive deposition method was reported by
conduction band can be effectively trapped by silver, Huang et al. [128]. Their photocatalytic activities were
while the hole oxidizes water and forms hydroxyl radi- examined by photocatalytic decolorization of methyl
cals, without the threat of recombination. Thus, silver orange solution under UV-light irradiation. The results
nano-particles facilitate longer charge separation by show that Pt doping induces enhancement of photocat-
trapping photo-generated electrons as shown in Fig. 5. alytic decolorization.
Various researchers have shown that there is an opti-
mum amount of silver to be added—just enough is
4.2.4. Nonmetal atom doping
needed so that silver sites are dispersed through the
material to rapidly trap electrons. On the other hand There are three main opinions regarding nonmetal
excess of silver may cover the titanium dioxide and pre- doping as modification mechanism:
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vent light absorption. In addition, too much silver may

mean that the silver acts as a recombination site (1) Band gap narrowing [129];
itself—essentially, it will form a bridge between an elec- (2) Impurity energy level [130]; and
tron and a hole [119]. The emission of titanium dioxide (3) Oxygen vacancies [131].
(and of similar studies with zinc oxide) can be inter-
preted as a measure of the recombination efficiency. It is found that cationic or metal dopants usually
Studies examining the emission of these metal oxides induce localized dopant levels deep in the band gap
have demonstrated that the emission intensity reduces of TiO2, which often serve as recombination centers
on increasing the load of silver—indicating that the sil- for photo-generated charge carriers [132]. Thus non-
ver traps electrons and reduces electron-hole recombi- metal dopants, such as carbon (C), nitrogen (N), and
nation. The higher efficiency of Agþ -doped TiO2 has sulphur (S), may be more appropriate for an extension
been reported in the photocatalytic degradation studies of photocatalytic activity into the visible-light region
of C.I. basic violet 3 [120] and Acid Red 88 [118]. Ag– because the related impurity states are supposed to be
AgCl-modified TiO2 has been used for the degradation close to the VB maximum. The studies of visible-light
of 4-Chloro phenol [121] working under visible radia- active semiconductors doped with nonmetallic
tion. Studies have revealed that the photocatalytic activ- elements such as N, S, and C have been extensively
ity of TiO2 can be improved significantly by doping carried out since the first-ever study of N-doped TiO2
with noble metals such as Pt, Au, and Ag [122–127]. by Asahi and co-workers was published in 2001 [129].
Under UV irradiation, the photo-generated electrons Matsumoto et al. [133] prepared an N-doped titania
quickly transfer from TiO2 surfaces to the Pt particles, from a layered titania/isostearate nanocomposite.
N-doping can be achieved by various methods such
as sputtering TiO2 targets in N2–Ar atmosphere [129];
treating of TiO2 powders in an ammonia atmosphere
CB e-
over several hundred centigrades [130]; heating TiO2
powders with urea [134]; and hydrolyzing organic or
Ag inorganic titanium compounds such as titanium tetra-
nanoparticle isopropoxide or titanium tetrachloride in ammonia
UV light
solution, by direct amination of TiO2 nanoparticles
E
with triethylamine [135]. The investigation of the
efficiency of N-doped TiO2 as a photocatalyst in
the degradation of the herbicide mecoprop
VB (C10 H11 ClO3 ) was studied by Abramović et al. [136]
h+
and it was found that the efficiency of N-doped TiO2
was 1.5 times more than that of Degussa P-25.
TiO2 Recently, Nitrogen-doped TiO2 prepared by sol–gel
method [137] has been used to study the effect of
Fig. 5. Simplified mechanism of photoexcitation of
heterostructure formed by incorporation of silver temperature on the nature of band gap to create a
nanoparticles to TiO2 that facilitates longer charge visible-light active photocatalyst. N-doped TiO2 is by
separation by trapping photogenerated electrons. far the most intensively studied system followed by
 6578 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590

carbon-doped titanium dioxide in harvesting solar There has been some discussion in the literature
light for photocatalytic activity [138]. Yang et al. on the mechanism of the enhancement of photocata-
[139] synthesized a carbon-doped TiO2 and both lytic activity by nitrogen doping. It was originally
carbon- and vanadium-doped TiO2 by the sol–gel proposed that N doping of TiO2 can shift its photo-
process. Both the catalysts show higher activity under response into the visible region by mixing of p states
visible light for acetaldehyde degradation. Moreover, of nitrogen with 2p states of lattice oxygen and by
the doped carbon increased the surface area and increasing the photocatalytic activity by narrowing the
improved the dispersion of vanadium. Recently, it has TiO2 band-gap [129]. However, more recent theoretical
been reported that carbon doping has the best photo- and experimental studies have shown that the nitro-
response compared to other nonmetals. Also, carbon gen species result in localized N-2p states above the
present in titanium dioxide particles is assumed to VB and the electronic transitions from localized N-2p
play as a sensitizer in photocatalytic reaction [140]. state to the CB are made in TiO2 under visible-light
Lettmann et al. [141] have obtained photo-stable irradiation [97,130,146–150]. The other mechanism put
carbon-modified TiO2 photocatalyst by the pyrolysis forward by Nakamura et al. [151] and Irie et al. [130]
of titania alcoholic suspension and proved that the counters Asahi’s original explanation that the N-dop-
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

increase of photocatalytic activity of new material ing reduces the gap between the VB and conduction
under visible-light irradiation was due to the effect of band of titania. These researchers propose that N-dop-
carbon presence in TiO2 lattice. Matos et al. [142] and ing introduced new occupied (i.e. electron rich) orbi-
Janus et al. [143] also reported the higher photocata- tals in between the VB (which are comprised
lytic activity of C-doped TiO2 during the methylene primarily of O-2p orbitals) and the conduction band
blue decomposition under visible light. Ohno et al. (which are comprised primarily of Ti-3d orbitals).
[132] have synthesized chemically modified titanium These N-2p orbitals act as a step-up for the electrons
dioxide photocatalysts in which S substitutes for some in the O-2p orbital. Electrons from N-2p orbitals need
of the lattice titanium atoms. They show strong much smaller jump to be promoted into the conduc-
absorption for visible light and high activities for deg- tion band. Once this process occurs, electron from the
radation of methylene blue, 2-propanol in aqueous original VB can migrate into the mid band-gap energy
solution, and partial oxidation of adamantane under level, leaving a hole in the VB. Thus, N-doping results
irradiation longer than 440 nm. Visible-light-induced in a mid-band-gap energy level which reduces the
degradation of phenol using S-doped TiO2 has also energy gap and thus utilizes visible light instead of
been successfully reported by Rockafellow et al. [144]. UV light for photoexcitation as shown in (Fig. 6).
Recently, Se(IV)-doped TiO2 reported by Yelda et al. Unlike metal ion doping, nonmetallic dopants replace
[145] shows the enhanced degradation rate of 4-nitro lattice oxygen and are less likely form recombination
phenol under both UV-A and sunlight irradiation. centers.

CB e-
Ti 3d CB e-
Ti 3d

UV light
Visible light
E E

N 2p

VB h+ O 2p
VB h+ O 2p

TiO 2 N-TiO2
(a) (b)

Fig. 6. (a) Simplified mechanism of photocatalysis with undoped TiO2; it requires UV light due to large band gap. (b)
Simplified mechanism of N-doped TiO2 showing step-by-step photoexcitation in which N-2p orbitals act as a step-up for
the electrons in the O-2p orbitals that results in a mid band-gap energy level which reduces the band gap and utilizes
visible light.
 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590 6579

4.3. Heterojunction composite photocatalyst high photochemical stability. The enhanced photocata-
lytic performance of LaVO4/TiO2 may be attributed to
Synthesis of composite photocatalyst leads to the
not only the matched band potentials but also the
visible-light-sensitive photocatalysis having higher
interconnected heterojunction of LaVO4 and TiO2
photocatalytic activity than pure TiO2. In composite
nanoparticles. The development of heterojunction
semiconductor photocatalyst, the CB electrons photo-
semiconductor extends the photosensitivity of TiO2
generated from a small band-gap semiconductor by
into the visible region [154,155]. This technique has
the absorption of visible light can be injected to the
advantage over metal- and anion-doped TiO2 catalyst
CB of a large band-gap semiconductor, while the
in that the latter becomes impaired by an increase in
photo-generated holes are trapped in the small
carrier-recombination facilities or thermal instability
band-gap semiconductor (Fig. 7). Thus, an effective
because of photocorrosion or rapid recombination of
electron-hole separation can be achieved. A similar
photo-generated electron-hole pairs. It is assumed that
strategy to that described above, in a rapidly evolving
LaVO4 may function as a sensitizer to absorb visible
area, is the idea of incorporating different semicon-
light and the heterojunction of LaVO4/TiO2 may act
ductors which have different conduction-band energy
as an active center for hindering the rapid recombina-
levels. The strategy is as before, to trap the electron so
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

tion of photo-induced electron-hole pairs generated by

that the hole can have more time to react. A simple
LaVO4. When visible light is supplied to the LaVO4/
example is the anatase–rutile heterojunction. Rutile
TiO2 heterojunction, electrons and holes generated by
has a smaller band gap (by about 0.2 eV) than anatase,
LaVO4 are separated. Some electrons are injected into
although its VB levels are at similar energies. There-
TiO2 nanoparticles quickly since the conduction band
fore, in an analogous fashion to the situation with
of LaVO4 is more negative than that of TiO2. More-
silver (Fig. 5) as described earlier, charge separation in
over, the nanostructure heterojunction on LaVO4/TiO2
anatase, followed by electron injection into the less
composite also leads to a more efficient inter electron
positive rutile conduction band [152] means that there
transfer between the two components. Furthermore,
is a hole in the VB of anatase that can freely oxidize
the large specific surface area of LaVO4/TiO2
water. This also reduces the rate of recombination of
nanocomposite was also favorable for photocatalytic
electrons and positive holes in the anatase part.
reaction. The photocatalytic activity of SiO2–TiO2 com-
A nanocrystal heterojunction of LaVO4/TiO2 visi-
posite photocatalyst prepared by sol–gel method with
ble-light photocatalyst has been successfully prepared
the assistance of sodium dodecyl benzene sulphonate
using a simple coupled method by Huang et al. [153].
is found to be very high during the decolorization of
The results show that such nanocomposite catalysts
methyl orange solution [156]. To sum up, the
exhibit strong photocatalytic activity for decomposi-
improvement of charge separation, efficient inter elec-
tion of benzene under visible-light irradiation with
tron transfer, the generation of HO
, and large specific
surface area were supposed to be responsible for the
high efficient photocatalytic activity of the LaVO4/
e- TiO2 nanocomposite as well as SiO2–TiO2 composite.
The photocatalytic activity of a mesoporous TiO2-
H2O Pt e- pillared hexaniobate nanocomposite has been success-
CB
fully studied in the degradation of acid red G [157].
h
H2
4.4. Dye sensitization
+
h A better method to achieve the utilization of
Small band gap
semiconductor visible light for TiO2 is dye sensitization. It is most
popular and successful for solar-cell applications
[158,159] and hydrogen production. It is found that
VB
metal dopants induce localized d-levels deep in the
band gap of TiO2, which often serve as recombination
Large band gap centers for photo-generated charge carrier. The doping
semiconductor
process of the nonmetal elements always involves
thermal treatment at high temperatures [130,151] or a
Fig. 7. Simplified mechanism showing interparticle
electron transfer process from CB of photo-excited small long time of hydrothermal treatment [160], both of
band-gap semiconductor to CB of TiO2 in composite which are energetically unfavorable. Visible-light-
photocatalyst. induced dye-sensitized TiO2 photocatalysts are readily
 6580 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590

prepared under a mild condition through interfacial D þ hm ðvisibleÞ ! D ð35Þ

adsorption of dye molecules on TiO2 in an ambient
environment [161,162]. D þ TiO2 ! Dþ þ e
ð36Þ
TiO2
A visible-light TiO2 photocatalyst was prepared by
a surface chemical modification process with toluene



2,4-diisocyanate (TDI) [163]. The TiO2–TDI photocata- e

TiO2 þ O2 ! TiO2 þ O2 ð37Þ
lyst has an obvious absorption in visible region, which
is proposed as the direct surface electron transfer from
Dþ þ e
! D ð38Þ
the lone pair electrons of N atom (597 nm) and O
atom (469 nm) to the conduction band of TiO2. TiO2–
Dye-sensitized photocatalysis begins with the visi-
TDI photocatalyst exhibits a satisfactory photostability
ble-light absorption of dye and a subsequent electron
and high photocatalytic performance for the degrada-
transfer from the excited dye to the conduction band
tion of phenol, 2, 4-dichlorophenol, fluorescein, and
of TiO2 as shown in Fig. 8. However, the electron
methyl orange [162]. Results also show an enhanced
transfer from excited dye to TiO2 usually depends
photo catalytic efficiency of decomposing methylene
strongly on the adsorption efficiency of dye molecules
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

blue in the presence of dye-sensitized (TiO2)8SBA

and it can be deeply depressed by the competitive
under visible-light and solar-light irradiation [164]. In
adsorption of other co-existing species in the solution.
another study, dye-sensitized TiO2 nanoparticles
Considering that pollutants usually exist at high con-
loaded with Al2O3/TiO2/Pt were synthesized and
centration in the practical wastewaters, dye-sensitized
investigated to show enhanced activity under visible
photocatalysis may face difficulties of keeping valu-
light for the production of hydrogen [165]. The differ-
able electron transfer efficiency. The mechanism is
ent steps of electron transfer/recombination processes
based on the absorption of visible light for exciting an
occurring on a dye-sensitized TiO2 particle are:
electron from the highest occupied molecular orbital
to the lowest unoccupied molecular orbital of a dye.
• Dye excitation;
The excited dye molecule subsequently transfers
• Electron injection /transfer from excited dye to
electrons into the conduction band of TiO2, and is
TiO2 CB;
converted to its cationic radical. The TiO2 acts only as
• Electron trapping on substrate; and
a mediator for transferring electrons from the
• Back electron transfer to oxidized dye (recombi-
sensitizer dye to the substrate on the TiO2 surface and
nation).
the VB of TiO2 remains unaffected. The injected
electrons jump to the surface of titania where they are
scavenged by molecular oxygen to form superoxide
where D stands for sensitizer, D⁄ for the electronically
radical, O


2 (Eq. (37)) and hydrogen peroxide radical,
excited sensitizer, and D⁄+ for the oxidised sensitizer.

OOH. These reactive species, on disproportionation,
The vis-TiO2 photocatalysis involves the following
produce hydroxyl radicals. The subsequent reactions
sequence of reactions:
lead to the degradation of the pollutant.

electron injection
e-
H2 O Pt
CB D*

H2 Induced D*+
by visible Oxidized
light state

e-
electron mediator

VB D

TiO2

Fig. 8. Simplified mechanism of visible-light-induced dye-sensitized photocatalysis showing electron transfer from the
excited dye to CB of TiO2.
 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590 6581

.- H+ . eCB-
O2 HOO HO-2
O2 Electron (e-) available
conduction band for reduction
(-0.1V) H+ Mineralization
+
Hole (h ) available products
Light for oxidation
- -
eCB OH

-
380 nm
H2 O . h Oxidation
HO . H2O2
O2-
Valence band
-H+ intermediates
(+3.1V) Titanium dioxide Photoexited Titanium - O2 -
particles dioxide particle
-OH
Organic contaminant

Fig. 9. Simplified diagram showing TiO2 photocatalytic mineralization of organic contaminants. Hydroxyl radicals oxidize
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

organic contaminants to intermediates leading to mineralization products.

5. Applications of photocatalysis 5.1.1. Water treatment

5.1. Environmental remediation Treatment of water can be accomplished by adding
a powdered form of TiO2 in the water, or it can be
The field of heterogeneous photocatalysis has
immobilized on a substrate. If TiO2 is in solution then
expanded rapidly, undergoing various evolutionary
some sort of recovery system is necessary in order to
phases related to environment and energy. Titanium
reuse the catalyst. Photocatalysis has been proven to
dioxide offers a great potential as a catalyst material
remove not only both organic and inorganic pollutants
for its use in industrial technology in the environmen-
in addition to trace metals from water, but also
tal remediation. Its capacity in oxidizing substances
nuisance color, taste, and odor compounds. Conven-
arises from the generation of highly reactive oxygen
tional as well as modified TiO2 has been extensively
species (ROS) such as OH and superoxide radicals on
studied for water treatment and it is well known to be
the surface of TiO2. The organic pollutants present in
an effective system to treat several hazardous com-
water subsequently react with ROS, holes, or elec-
pounds in contaminated water.
trons, and they undergo a series of redox chemical
Photocatalysis has been used for the destruction of
reactions, eventually leading to mineralization. The
OCs such as alcohols, carboxylic acids, phenolic deriv-
sequence of events leading to the mineralization of
atives, or chlorinated aromatics, into harmless prod-
OCs using TiO2 as a photocatalyst is presented in
ucts as carbon dioxide, water, and simple mineral
Fig. 9. In this process, the pollutants are degraded by
acids [67,167,168]. In addition to OCs, wide ranges of
UV irradiation (k 6 380 nm) of a semiconductor
inorganic compounds are sensitive to photochemical
suspension of titanium dioxide based on the forma-
transformation on the catalyst surfaces. Inorganic
tion of ROS. The preferred oxidation route is highly
species such as bromate, or chlorate, azide, halide
target molecule-dependent. Those species that adsorb
ions, nitrate ions, nitric oxide, palladium, and rho-
strongly to TiO2 (highly polar compounds) are more
dium species, and sulfur species can also be decom-
likely to oxidize via the photo-generated holes.
posed [169,170] by photocatalysis. Metal salts such as
Photo-induced redox reactions on TiO2 can also
AgNO3, HgCl, and organometallic compounds (e.g.
transform a variety of inorganic pollutants such as
CH3HgCl) as well as cyanide, thiocyanate, ammonia,
oxyanions (arsenates, chromate, bromated, etc.),
nitrates and nitrites can be removed from water
ammonia, and metal ions. The photo-induced ROS
[67,171].
generation on C is exploitable for bacterial/viral inac-
Trace metals, such as mercury (Hg), chromium
tivation as well. Photocatalysis is therefore, a potential
(Cr), lead (Pb), cadmium (Cd), arsenic (As), and
tool for the treatment of different types of aquatic pol-
others metals are considered to be highly health
lutants to get remediated. Many of the air pollutants
hazardous. Thus, the removal of these toxic metals is
are also got remediated, exploiting photo-induced
essentially important for human health and water
redox reactions. The development in the field of TiO2
quality [67,172]. The environmental applications of
photocatalsis has been utilized for a wide range of
heterogeneous photocatalysis include the removal of
environmental and energy applications [166].
heavy metals such as (Hg), chromium (Cr), lead (Pb),
 6582 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590

Cadmium (Cd), lead (Pb), Arsenic (As), nickel (Ni), These VOCs are emitted into the atmosphere by a
and cupper (Cu) [171]. It is seen that modified photo- wide variety of industrial processes and cause adverse
catalyst TiO2/Al2O3 is efficient in the removal of toxic effects on human nervous system, via breathing. The
pollutant like surfactant Triton X-100 from model indoor air shows high level of pollutants than that of
wastewater [173]. the nearby outdoor air. Indoor air refers to air of any
Matsunaga et al. [174] were the first to demonstrate confined place having levels of pollutant which are
the photochemical sterilization method. Microbial above the ambient concentrations outside of the con-
cells were killed photochemically with semiconductor fined place. Among the air contaminants, one finds
power of titanium dioxide deposited on platinum formaldehyde, acetaldehyde, aromatic hydrocarbons,
(TiO2/Pt). The cell was photo-electrochemically NOx, and CO. Photocatalysis well suits for the purifi-
oxidized; as a result, the respiration of cell was cation of indoor air as reported by Agrios and Pichat
inhibited leading to its death. Bacteria and viruses like [187]. Titanium dioxide can be used for both VOCs
Streptococcus Streptococcus natuss, Streptococcus cricetus, mineralization and bacterial disinfection, upon the
Escheria coli (E. coli), Scaccharomyces cerevisisas, Lactoba- addition of silver nanoparticles [188]. This acts as
cillus acidophilus, and Poliovirus I have been destructed indoor-light-activated photocatalyst and prevents bac-
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

effectively using heterogeneous photocatalysis [175– terial growth on the surface when used in tiles. The
177]. Photo-disinfection sensitized by TiO2 has some antibacterial action of nanocrystalline ZnS is compared
effect on the degradation of Chlorella vulgaris (Green with Evonik-Degussa P-25 as indoor-light-activated
algae), which has a thick cell wall. It is reported that photocatalyst [189] to prevent bacterial growth on the
photocatalysis removes not only pollutants from water surface.
but also color, taste, and odor from water. With respect Indoor air treatment usually takes place in an
to the algal bloom in fresh water supplies and the apparatus through which air is circulated. Such sys-
consequent possibility of cyanobacterial microcystin tems, which proceed through the following sequence
contamination of potable water, microcystin toxins, of reactions, contain a blower or an air pump, a par-
present in algal water, are reported to be degraded on ticulates filter or an electrostatic precipitator, and a
immobilized TiO2 catalyst [178]. An analysis by light source and a photocatalyst.
electrophoresis has revealed that bacterial DNA and
RNA molecules completely disappear after 7 h of pho- Contaminated air ! A ! B ! C ! D ! E
tocatalytic treatment. The antibacterial activity of TiO2 ! purified air
is related to ROS production, especially hydroxyl free
radicals and peroxide formed under and reductive where A, B, C, D, and E, respectively, represent fan,
pathways, respectively. Some reviews on photocata- particulates filter, photocatalyst, light source, and
lytic disinfection and its mechanism have also been activated carbon filter (optional).
published recently [179–181]. The nano-sized TiO2 was The drawback of this indoor air treatment is the
also reported to kill viruses including polio virus 1, formation of by-products that blocks active sites.
hepatitis B virus, herpes simplex virus, and MS2 The basic concept in outdoor air treatment is to
bacteriophage [182]. use a large area of construction as walls, roofs, roads,
An attractive feature of TiO2 photocatalytic disin- pavements, bridges, and buildings as platforms for air
fection is its potential to be activated by visible light. decontamination. The photocatalyst can be applied in
It has been demonstrated that doping TiO2 with silver various forms including cementitious modules, in-situ-
has greatly improved photocatalytic inactivation of made concrete objects, and over-coated thin layers.
bacteria [183] and viruses [184]. Recently, the antimi- Outdoor air treatment differs from indoor air treat-
crobial activity of silver-deposited TiO2 nanocompos- ment by the type of contaminants (less VOCs and
ites, Ag+/TiO2–TiO2O3, has shown to exhibit a good more NOx, CO, and SOx), by the use of solar light as
bactericidal activity against E. coli under visible light the dominant irradiation source; by the fact that pho-
[185] and the role of ROS in the photocatalytic tocatalytic platforms are to serve for construction
bacterial disinfection process in presence of a visible- (unlike indoor air treatment devices designed for air
light-active photocatalyst, B-N-co-doped TiO2 has cleaning); by the exposure to harsh environment; and
been investigated [186]. by their visibility to general public. The number of
gas-phase pollutants whose photocatalytic degradation
has been studied is quite large. Some of these pollu-
5.1.2. Air treatment
tants such as aromatic compounds, chlorinated olefins,
Air contains avariety of volatile organic hydrocarbons [190], aldehydes [191], and alcohols
compounds (VOCs) which are hazardous to health. [192] can be found indoor. Nitrogen oxides (NOx)
 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590 6583

released mainly from internal combustion engines and Many investigators have tried to overcome these
furnaces can be reduced upon immobilizing TiO2 on barriers by modification technologies like metal load-
activated carbon [193,194]. Here, nitrogen oxides are ing, metal ion doping, dye sensitization, composite
oxidized to HNO3 and washed away by rainfall when semiconductor, anion doping, and metal ion implanta-
the catalyst is used outdoors. The use of zeolite matrix tion. Now a days, a growing interest in hydrogen pro-
hosting TiO2 may lead to the formation of ecofriendly duction using solar energy and water draws attention
products like N2 and O2 instead of nitric acid [195]. as water could be split (simultaneously oxidized and
The oxidation of acetone, however, leads to the forma- reduced to form O2 and H2, respectively) in a photo-
tion of H2O and CO2 as the by-products (Eq. (39)). electrochemical cell upon illuminating a TiO2-single
crystal photoanode and having an inert cathode to
CH3 COCH3 þ 4O2 ! 3CO2 þ 3H2 O ð39Þ which a small electrochemical bias has been applied
[200,201]. However, from the view point of H2
Treatment of polluted air streams is often more production, it is not very attractive due to the position
efficient than that of waste water streams. Here, of its conduction band edge with respect to the redox
gas-phase reactions occur faster than liquid-phase potential of H2/H2O couple and its low visible-light
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

reactions. In the process of treating air streams, TiO2 absorption [18,202]. Splitting of water to produce
must be suspended on some short of surface to allow hydrogen can be explained with the help of following
the gas to pass over it and react. This may be some reactions:
short of matrix with a large surface area illuminated
by UV light. An air treatment system for ethylene TiO2 þ hm ! e
þ
CB þ hvb ð2Þ
removal has been developed at University of
Wisconsin-Madison [196]. This system can be placed At TiO2 electrode:
in grocery stores to remove the naturally occurring
ethylene that causes fruits and vegetables to spoil. H2 O þ 2hþ ! 1=2 O2 þ 2Hþ ð40Þ
Moreover, UV light reduces bacteria, molds, and
odors. The mechanism in oxidizing pollutants arises At platinum electrode:
from highly oxidizing hydroxyl radicals produced on
the catalyst surface. 2Hþ þ 2e
! H2 ð41Þ
Carbon-doped TiO2 has been used to investigate
the PCO of toluene, a common VOC emitted by many Thus, the overall reaction is:
industrial processes in air [197]. This C-doped TiO2
has been synthesized by a sol–gel combustion method H2 O þ 2hm ! 1=2 O2 þ H2 ð42Þ
using carbon nano power. A nitrogen-doped and plat-
inum-modified TiO2 (Pt/TiO2
xNx) photocatalyst is For hydrogen production, the CB level should be
proven effective for the decomposition of benzene and more negative than hydrogen production level
other persistent VOCs under visible-light irradiation (EH2 =H2 O ), while the VB should be more positive than
in a H2
O2 atmosphere [198]. Ethyl benzene and o-, water oxidation level (EO2 =H2 O ) for efficient oxygen
m-, and p-xylenes are removed by employing N–TiO2 production from water by photocatalysis. The photo-
at indoor air level. Composite N–TiO2/zeolite has catalytic hydrogen production by TiO2 is shown in
been investigated for the removal of toluene from Fig. 10.
waste gas [199]. Dye sensitization is widely used to utilize visible
light for energy conversion. Under illumination by
visible light, the excited dyes can inject electrons to
5.3. Hydrogen production
CB of semiconductors to initiate the catalytic reactions
Photocatalytic water splitting using TiO2 in the as illustrated in Fig. 8. Higher hydrogen production
presence of solar light for hydrogen production offers rate can be obtained by efficient absorption of visible
a promising way for clean, low-cost environmentally light. To obtain higher efficiency of hydrogen produc-
friendly production of hydrogen. Presently, the effi- tion using absorbed light, fast electron injection and
ciency of this water-splitting technology for hydrogen slow backward reaction are required. Based on the
production is very low due to various factors. The literature on electron/hole recombination of dyes,
main barriers are as usual the rapid recombination of the recombination times were found to be mostly in
photo-generated electron-hole pairs, backward reac- the order of nanoseconds to microseconds, sometimes
tions, and poor activation of TiO2 by visible light. in milliseconds [203–205], while the electron injection
 6584 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590

H2
Pt
e-
-0.5 eV
2 H+
0 eV EH /H O
2 2

+1.23 eV EO /H O
2 2

+2.7 eV H2O
TiO2 h+
TiO2
E
_1_ O + 2 H+
2 2
Downloaded by [University of Aberdeen] at 15:19 28 December 2014

Fig. 10. Simplified mechanism of TiO2 photocatalytic water splitting showing photo-excited electrons used for hydrogen
production at platinum electrode.

times were in the order of femtoseconds [204,206,207]. as well as efficient. There are reports of photocatalytic
The fast electron injection and slow backward reaction hydrogen production using CdS–TiO2 composite
make dye-sensitized semiconductors feasible for semiconductors [209] and CdS–ZnS composite
efficient transfer of electrons from excited dyes to the semiconductor [210,211].
CB of TiO2. Dhanalakshmi et al. [208] carried out a
parametric investigation to study the effect of using
[Ru(dcbpy)2(dpd)]2+ [where dcbpy = 4,4-dicarboxy2,2- 6. Conclusion
bipyridine and dpq = 2,3-bis-(2-pyridyl)-quinoxaline] This review focuses and reports the recent
as a dye sensitizer on photocatalytic hydrogen advances in the heterogeneous photocatalysis involv-
production from water, under visible-light irradiation. ing TiO2 which can be used for the degradation and
It was found that hydrogen production rate was mineralization of various OCs found in water and air
enhanced by adsorbing dye molecules to TiO2. Semi- and for hydrogen production. A number of modifica-
conductor composition (coupling) is another method tion technologies, such as metal ion doping and metal
to utilize visible light for hydrogen production. When ion implantation, nonmetal doping, dye sensitization,
a large band-gap semiconductor is coupled with a and composite semiconductor, are promising methods
small band-gap semiconductor with a more negative to expand light response of TiO2 to visible region.
CB level, CB electrons can be injected from the small In spite of extensive investigations, the commercial
band-gap semiconductor to the large band-gap semi- exploitation of photocatalysis has not been done sig-
conductor. Thus, a wide electron-hole separation is nificantly. The application of this technique for real
achieved as shown in Fig. 7. The process is similar to wastewaters and water purification for drinking pur-
dye sensitization. The difference is that electrons are pose needs further investigation. Much research is
injected from one semiconductor to another semicon- needed to achieve stable pollutant removal through
ductor, rather than from excited dye to semiconduc- the optimization of process parameters and then only
tor. Successful coupling of the two semiconductors for this technique would make a significant impact on the
photocatalytic water-splitting hydrogen production potential commercial and industrial application in
under visible-light irradiation can be achieved when water treatment. While the advances in TiO2 photoca-
the following conditions are met: (i) semiconductors talysis using doped materials have been tested for rel-
should be free of photocorrosion; (ii) the small band atively simple and clean solutions, the sustainability
gap semiconductor should be able to be excited by of their photocatalytic activity in real wastewaters is
visible light; (iii) the CB of the small band-gap semi- unclear which essentially requires further attention.
conductor should be more negative than that of the On the technical point of view, the development of a
large band-gap semiconductor; (iv) the CB of the large more reliable and low-cost photocatalyst that can be
band gap semiconductor should be more negative activated by visible and solar light, or both, should be
than EH2 =H2 O ; and (v) electron injection should be fast explored further for the potential application in water
 P. Pattanaik and M.K. Sahoo / Desalination and Water Treatment 52 (2014) 6567–6590 6585

treatment. A comparison between water and air treat- [8] M. Fane, TiO2/AC composites for synergistic adsorption-
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understood on a case-by-case basis. Although pure two anionic azo dyes: Reactive red 120 and reactive black 5,
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methods have been employed to improve its efficiency M.C. Gutiérrez-Bouzán, Photocatalysis with titanium
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photocatalysts will be developed for successful com- [16] S.R. Patil, U.G. Akpan, B.H. Hameed, S.K. Samdarshi, A
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Acknowledgments (2012) 188–195.
[17] N. Meng, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, A
The authors gratefully acknowledge the University review and recent developments in photocatalytic water-
Grants Commission, Govt. of India for financial assis- splitting using TiO2 for hydrogen production, Ren. Sust.
tance (F. 40-76/2011 (SR)). The authors wish to Energy Rev. 11 (2007) 401–425.
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