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Oxide Materials in Photocatalytic Processes

Richard P.K. Wells

755

Metal Oxide Catalysis. Edited by S. David Jackson and Justin S. J. Hargreaves

ISBN: 978-3-527-31815-5

19

19.1

Introduction

In 1972, Fujishima and Honda reported the photocatalytic splitting of water using

TiO

2

electrodes [1] . This event marked the beginning of a new era in the study of

photocatalysis. Since then, considerable research effort has been invested in

understanding the physicochemical processes occurring, and in the continued

development of active heterogeneous photocatalysts based on metal oxide semi-

conductors such as TiO

2

[2] . The breadth of the subject, in terms of both the

materials used and their applications, is immense, and it is not the aim of this

chapter to deal with all aspects of the science; rather it is to present the basic

principles of the process and to review progress in the modiﬁ cation of metal oxides

in order to enhance their photoreactivity in the visible part of the spectrum. There

are many excellent reviews in the literature dealing with the basic concepts of the

photocatalytic process and the reader is referred in particular to those by Hoff-

mann and coworkers [3] , Mills and coworkers [4] , and Kamat [5] . By far the majority

of research has focused on TiO

2

as the material of choice and much effort has

gone into enhancing the ability of visible light to activate the material rather than

merely the ultraviolet component of the spectrum [6, 7] . In terms of application,

environmental remediation has been the target of most research, owing to the

ability of TiO

2

to function as a slurry in aqueous media, the annual rate of publica-

tion in this area being of the order of four hundred reports [8, 9] . More recent uses

include the abatement of contamination within gaseous environments, with par-

ticular emphasis on the removal of chlorinated and sulfurous compounds in air

[10] , and the incorporation of photocatalytically mediated steps in synthetic organic

chemistry [11, 12] .

756 19 Oxide Materials in Photocatalytic Processes

19.2

Basic Principles of Heterogeneous Photocatalysis

Heterogeneous photocatalysis was deﬁ ned by Palmisano and Sclafani [13] as “ a

catalytic process during which one or more reaction steps occur by means of

electron – hole pairs photogenerated on the surface of semiconducting materials

illuminated by light of a certain energy. ” The term photocatalysis in general, refers

to any chemical process in which the external energy source is derived from radia-

tion in the ultraviolet - visible range [14] . The basic mechanisms of heterogeneous

photocatalysis have been investigated by many researchers [3, 9, 15 – 17] and can

be represented schematically by the band gap model (Figure 19.1 ).

The band gap is characteristic of the electronic structure of the semiconductor

and is deﬁ ned as the energy difference ( ∆ E

g

) between the valence and conduction

bands; that is the highest energy band with all electronic levels occupied and the

lowest energy band without electrons [18] . Table 19.1 shows some typical metal

oxide and chalcogenide photocatalysts, together with their respective band gaps

[9] .

When energy greater than ∆ E

g

is applied to the semiconductor surface, valence

band electrons are promoted to the conduction band, creating electron – hole pairs

( 1 ). Migration of the pairs to the semiconductor surface ( 2 ) allows the occurrence

of redox reactions with adsorbates with suitable redox potentials ( 3 ). Thermo-

dynamically, oxidation will occur if the redox potential of the valence band is more

positive than that of the adsorbates. Similarly, conduction band electrons can

reduce adsorbed species if their redox potential is more negative than that of the

adsorbates. Clearly, the most likely outcome of the formation of electron – hole

pairs is their simple recombination, with subsequent release of thermal energy

and/or light ( 4 ). Recombination proceeding within the same timescale as the redox

Figure 19.1 Schematic representation of the “ band gap

model ” . (1) Photoinduced electron – hole creation;

(2) charge migration to the surface; (3) redox reactions;

(4) recombination. VB and CB represent valence and

conduction band respectively.

19.3 Traditional Photocatalysts 757

processes being promoted reduces photocatalytic activity considerably. Therefore,

for electron – hole pairs, whose lifetime can normally be considered to be of the

order of femtoseconds, separation is a vital parameter when determining activity

[19] . Before any reaction or recombination step can take place the charge carriers

must be trapped, and defects play an important role in this process. Defects can

be either intrinsic, for example oxygen vacancies in nanostructured reducible

oxides, or extrinsic, such as dopants or impurities. Minimizing the likelihood of

recombination has led to the preparation of nanostructured oxide materials [14]

together with the development of materials capable of enhanced photocatalytic

activity through spatial structuring [20] and encapsulation within host materials

such as zeolites [21] .

19.3

Traditional Photocatalysts

TiO

2

is by far the most widely investigated metal oxide, mainly because of its

chemical stability, non - toxicity, and well - positioned valence and conduction bands.

Two crystalline phases have been investigated, rutile and anatase. As shown in

Figure 19.2 , the valence band redox potential of both forms are more positive than

that of the • OH/

−

OH redox couple, resulting in the oxidation of adsorbed water

and hydroxyl groups to highly reactive hydroxyl radicals on both irradiated forms

of the oxide. This reaction is the basis behind the immense research effort into

the oxidation of organic contaminants in aqueous waste streams [3, 9, 22 – 24] . Of

the two phases of TiO

2

, anatase is widely regarded as being the most photocatalyti-

cally active [25, 26] ; this is due to a variety of factors, the main one being the more

negative redox potential of the anatase conduction band, making it a more efﬁ cient

reducing agent than rutile. For example, molecular oxygen can be reduced to

superoxide radicals by illuminated anatase, but not by illuminated rutile. This also

results in a greater amount of electron – hole recombination and hence lowers the

efﬁ ciency in the latter material.

Table 19.1 Band gap energy of various photocatalysts.

Photocatalyst Band gap energy

(eV)

Photocatalyst Band gap energy

(eV)

Si 1.1 ZnO 3.2

TiO

2

(rutile) 3.0 TiO

2

(anatase) 3.2

WO

3

2.7 CdS 2.4

ZnS 3.7 SrTiO

3

3.4

SnO

2

3.5 WSe

2

1.2

Fe

2

O

3

2.2

α - Fe

2

O

3

3.1

758 19 Oxide Materials in Photocatalytic Processes

The origin of the differing band gaps between the various polymorphs of TiO

2

is in their structure. Anatase is the least thermodynamically stable, although from

energy calculations this is the predominant phase when the particle size is small

( < 10 nm) [27] . The crystalline structure is made up of TiO

6

octahedral chains dif-

fering by the distortion of each octahedron and the assembly pattern of octahedral

chains [14] . The Ti

–

Ti bond lengths in anatase are greater than those in rutile,

whereas Ti

–

O distances are shorter [28] . These structural differences lead to dif-

fering mass densities and hence differing electronic structures of the bands.

Anatase is 9% less dense than rutile, resulting in a greater degree of delocalization

of the Ti 3d states, and therefore a narrower 3d band. Also the O 2p

–

Ti 3d hybrid-

ization of the two structures differs, with rutile exhibiting a greater degree of

covalent mixing and hence anatase exhibiting a valence and conduction band with

more pronounced O 2p

–

Ti 3d character [29] . These structural and electronic dif-

ferences are responsible for the different reactivities of the materials.

Although anatase presents the highest photoactivities reported, a commercial

material, Degussa P25, is commonly used as a reference catalyst. This product is

a mixture of anatase and rutile in a ratio of 80 : 20 and displays an unexpectedly

high photoactivity, the source of which is still a matter of some debate. Bickley

and coworkers reported a model based upon there being a small amount of amor-

phous TiO

2

present in the mix [30] . The coexistence of the phases leads to a syn-

ergistic effect that improves the photoreactivity of the catalyst. A similar junction

Figure 19.2 Band gaps, together with valence and conduction band edges of common

semiconductors, placed alongside the standard redox potentials (versus normal hydrogen

electrode ( NHE )) of the

OO

22

i−

and • OH/

−

OH redox couple.

19.3 Traditional Photocatalysts 759

effect has been reported for an anatase – brookite mixture, which presents a high

activity for methanol photo - oxidation [31] .

The principal use of TiO

2

as a photocatalyst has been in the degradation of

organic material in either aqueous or gaseous waste streams. Two oxidation reac-

tions can take place at the TiO

2

valence band. Firstly, adsorbed organic material

(RH) can be directly oxidized by TiO

2

(h

+

) producing cationic radicals RH

–

+

(1).

Secondly, the oxidation of water and/or hydroxyl groups can give rise to the forma-

tion of highly reactive hydroxyl radicals, • OH (2 - 3) that can subsequently initiate

further compound oxidation.

TiO h RH TiO RH TiO R H

ads ads ads222

+++

()

+→+ ++

ii



(19.1)

TiO h H O TiO H O TiO OH H

ads ads ads22 22 2

+++

()

+→+ ++

ii



(19.2)

TiO h OH TiO OH

ads ads22

+−

()

+→+

i

(19.3)

The main reaction occurring at the TiO

2

conduction band is the reduction of

adsorbed molecular oxygen, resulting in the formation of superoxide radicals

O

2

i−

()

. Such superoxide radicals, can give rise to the formation of additional • OH

radicals through a series of reactions (Equations 19.5 to 19.7 ).

TiO e O TiO O

ads ads22 22

−−

()

+→+

,

i

(19.4)

OHHO

ads ads22

ii−+

+ 

(19.5)

2

2222

HO H O O

ads ads ads

i



,,

+

(19.6)

TiO e H O TiO OH OH

ads ads ads222 2

−−

()

+→++

,

(19.7)

However, photocatalytic reduction of organic compounds is generally less

important than their oxidation because the reduction potential of a TiO

2

(e

−

)

is lower than the oxidation potential of a TiO

2

(h

+

). Also, most of the reducible

substrates cannot compete kinetically with oxygen as an electron scavenger [15] .

Of the other potential photocatalysts listed in Table 19.1 , ZnO has limited use

as it is relatively unstable when illuminated in aqueous solution, particularly at

low pH, tending to form Zn

2+

(aq) after oxidation by the valence band positive

holes. However, at higher pH it has been shown to be as active as TiO

2

for the

degradation of phenol [32] . Also, for applications in the gas phase, such as the

degradation of ethanal, ZnO thin ﬁ lms have been shown to be highly active [33] .

WO

3

, although active in a portion of the visible spectrum is considered in general

to be less active than TiO

2

[34] . CeO

2

presents many of the characteristics of

a suitable photocatalyst, having a band gap of 3.1 eV [35] and being relatively

inexpensive and non - toxic; it is also highly absorbing in the UV spectrum and has

been shown to have signiﬁ cant activity for the photocatalytic oxidation of water

[36] .

Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

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