Page 74 - Multifunctional Photocatalytic Materials for Energy

Energy band engineering of metal oxide for enhanced visible light absorption 63

Fig. 4.8 Scheme for engineering the electronic band structure of semiconductors. The band
structure and optical absorption curves of (A) a pristine semiconductor, (B) doping-induced
shallow-level and deep-level states and (C) doping-induced band gap narrowing.
Reproduced with permission from J. Li, N. Wu, Semiconductor-based photocatalysts and
photoelectrochemical cells for solar fuel generation: a review, Cat. Sci. Technol. 5 (2015)
1360–1384. Copyright © Royal Society of Chemistry.

intrinsic band gap of the semiconductor, and apparently generate the light absorp-
tion extension to a longer wavelength. A representative feature of this kind of defect-
induced light absorption shown in a light absorption spectrum is an add-on shoulder
on the edge of an absorption curve (Fig. 4.8B) [6]. The first observation of doping-
induced visible light absorption for TiO 2 was reported by Asashi and coworkers with
N doping [69]. Since then, extensive studies have been done either by using transition
metal cations to replace metal sites or by inserting nonmetal anions such as N, C, S,
P, and B at the O sites and/or interstitial sites. The fact is that doping could really im-
prove the light absorption (increasing η abs ); however, this is realized by inserting defect
states within the band gap, which has negative effects on the charge carrier transport
by acting as a charge carrier recombination center (decreasing η sep ). Deep-level energy
states are especially effective recombination centers, such as the Shockley–Read–Hall
recombination shown in Section 4.2.3.
The ideal case is done by doping to narrow the band gap while not introducing
any local states within the band gap. This has been occasionally observed in the
nitrogen-doped La 2 Ti 2 O 7 . Different from other doping cases, N doping into La 2 Ti 2 O 7
lifts the VB and pushes a parallel shift of the optical absorption edge to the longer
wavelength, as shown in Fig. 4.8C [6,71]. This does produce a benefit by increasing
the light absorption in the visible light region (increasing η abs ), but does not insert
local states to consume charge carriers (not decreasing η sep ), and thus improves the