Solar Hydrogen Generation: Photocatalytic and Photoelectrochemical Methods - Hydrogen Generation, Storage, and Utilization - page 30

of TiO 2 in 1972 [6]. TiO 2 and ZnO are two common semiconductor photo-
catalysts, due to their low cost and favorable band edge positions for hydro-
gen generation [10, 11, 14]. However, the photocatalytic performance of
ZnO and TiO 2 are limited by their large bandgap and rapid charge recombi-
nation. In this regard, nanostructured metal oxides have been designed to
improve the charge separation at the interface between semiconductor and
electrolyte [4, 7]. Element doping, such as nitrogen doping, has also been
used to narrow their bandgap and thereby increase the visible light absorp-
tion [8, 13, 21].
Recently, Chen et al. developed an alternative approach to improve visible
and near infrared optical absorption of TiO 2 by introducing surface disorder
through hydrogen treatment [22]. Hydrogen-treated TiO 2 nanoparticle con-
sists of a crystalline TiO 2 core and a highly disordered surface layer [22].
Creation of large amount of lattice disorder yields mid-gap states in TiO 2 .
Instead of forming discrete donor states near the conduction band edge,
these mid-gap states can form a continuum extending to and overlapping
with the conduction band edge, which are often also known as band tail
states [23]. Similarly, the large amount of disorder can result in band tail
states merging with the valence band [23]. Therefore, the bandgap and
optical property of TiO 2 can be significantly changed by creating surface
disorder. The color of TiO 2 changed from white to black, indicating TiO 2 is
able to absorb visible light. Notably, the black TiO 2 nanoparticles achieved
pronounced activity and stability in photocatalytic hydrogen generation. The
solar H 2 generation rate obtained was around 10 mmol·hour −1 ·g −1 , which is
about two orders of magnitude greater than the yields of most semiconduc-
tor photocatalysts [22]. The energy conversion efficiency for solar hydrogen
production, defined as the ratio of the energy of solar produced hydrogen to
the energy of incident light, reached 24% for surface-disordered black TiO 2
nanoparticles [22].
Li and coworkers also developed hydrogen-treated metal oxides such as
TiO 2 [11], ZnO [24], WO 3 [25], and BiVO 4 for PEC and photocatalytic
applications. Figure 3.1a shows the rate of photogeneration of H 2 collected
for air-annealed ZnO and hydrogen-treated ZnO nanorod arrays and ZnO
nanorod powder [24]. Hydrogen-treated ZnO shows significantly higher
hydrogen production rate than pristine ZnO, indicating hydrogen treatment
is effective in enhancing ZnO photoactivity. Importantly, the hydrogen-
treated ZnO exhibits excellent photostability in photogeneration of hydrogen,
without obvious decay in the production rate within 24 hours (Fig. 3.1b).
Moreover, it is noteworthy that there was no visible light photoactivity
observed for hydrogen-treated ZnO, although the color of ZnO changed from
white to black after hydrogen treatment [24]. The enhanced photocatalytic