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Energy band engineering of metal oxide for enhanced visible light absorption 59

Anatase
Total DOS

VB max CB min
E g
O p
Ti s Ti p Ti d

–7 –6 –5 –4 –3 –2 –1 0 1 2 3 4 5
(A) Energy (eV)

Rutile

Total DOS
VB max CB min
E g
O p
Ti s Ti p Ti d

–7 –6 –5 –4 –3 –2 –1 0 1 2 3 4 5
(B) Energy (eV)
Fig. 4.6 The electronic structure of anatase and rutile TiO 2 . Comparison of the total and ion-
decomposed electronic density of states of anatase (A) and rutile (B) TiO 2 calculated using the
HSE06 hybrid density functional.
Reproduced with permission from D.O. Scanlon, C.W. Dunnill, J. Buckeridge, S.A. Shevlin,
A.J. Logsdail, S.M. Woodley, C.R.A. Catlow, M.J. Powell, R.G. Palgrave, I.P. Parkin, G.W.
Watson, T.W. Keal, P. Sherwood, A. Walsh, A.A. Sokol, Band alignment of rutile and anatase
TiO 2 , Nat. Mater. 12 (2013) 798–801. Copyright © Nature Publishing group.

oxide semiconductors as well. Using either transition metal cations with d/n electronic
configurations, such as Fe 2 O 3 , or transition metal cations with occupied high binding
s states, such as SnO (2.4 eV) and PbO (2.1 eV), could raise the valence band and thus
narrow the band gap for visible light absorption [26,51]. Another approach is to intro-
duce anionic species such as N and S that are less electronegative than O, which will
reach the same goal for visible light absorption but work in a different way by having
new energy levels within the forbidden band region. We will discuss these effects in
detail in Section 4.5.

4.4.2 Representative metal oxide photocatalysts

4.4.2.1 TiO 2
TiO 2 is the best-known wide band gap metal oxide semiconductor. It has three crys-
tal structures: anatase, rutile, and brookite. The commonly used structures for pho-
tocatalysis are anatase and rutile, as well as their mixture P25, which has the best