Photoelectrochemical cells for hydrogen production from solar energy - Solar Energy Sciences and Engineering Applications - page 300

Figure 10.2.6 Energy band positions for various semiconductors at pH 14 (Krol et al., 2008).
oxide (e.g. Fe 2 O 3 , SrTiO 3 ,TiO 2 ,WO 3 , BiVO 4 ,Cu 2 O, etc.) and non metal oxide
semiconductors (e.g. GaAs, CdS, InP, etc.) (Grimes et al., 2008). The photocorro-
sion stability of photoanode or photocathode, its wavelength response (bandgap) and
current-voltage characteristic are important factors that determine the semiconductor
performance in water-splitting - Figure 10.2.6.
10.2.3.1 Metal oxide semiconductor
Concerning the metal-oxide semiconductors, only a few are able to fulfill the bandgap
and band edge requirements for operating at zero bias voltage; SrTiO 3 ,KTaO 3 and
ZrO 2 are among them but only SrTiO 3 has been in fact studied (Memming, 2001).
SrTiO 3 generates hydrogen without any additional bias even if with a barely small
efficiency, less than 1%, which was ascribed to its large bandgap energy (3.4-3.5 eV)
(Mavroides et al., 1976).
Preparing a semiconductor oxide that meets all criteria needed to achieve efficient
water-splitting is a great challenge and the most frequently studied photoelectrode
materials are TiO 2 ,WO 3 ,Fe 2 O 3 , BiVO 4 SnO 2 and Cu 2 O and their modifications.
The well known titanium dioxide (TiO 2 ), with different crystalline structures
(anatase mostly but also rutile) and arrangements as single crystal, polycrystalline
or thin films, have been largely investigated (Nowotny et al., 2007a; Nowotny et al.,
2007b; Nowotny et al., 2007c; Nowotny et al., 2007d; Grimes et al., 2008) mainly due
to its excellent stability over a wide range of pH and applied potentials, low cost and
abundance (Nowotny et al., 2007a). As a major drawback, TiO 2 only absorbs in the
UV light spectrum due to its large bandgap of approximately 3.2 eV. Several attempts
have been made to extend TiO 2 spectral response into the visible light by doping it
with aliovalent ions, such as W, Ta, Nb, Zn, In, Li and Ge (Karakitsou et al., 1993),
Pb (Rahman et al., 1999), Mo and Cr (Wilke and Breuer, 1999), Cr (Bak et al., 2002),
C (Khan et al., 2002) and N (Nakamura et al., 2004; Babu et al., 2012). Wilke and
co-workers showed the important effect of doping TiO 2 with Mo and Cr ions on the