Environmental Engineering Reference
In-Depth Information
Electrochemical impendence studies proved that the donor density of TiO 2
was substantially increased after hydrogen treatment (Fig. 3.7d). More
importantly, hydrogenation has been demonstrated to be a general strategy
for increasing donor density of metal oxides. Likewise, enhanced donor
density and improved photocurrent density have been observed in other metal
oxides, including WO 3 and ZnO [24,25].
Visible light absorption of wide bandgap metal oxides can be enhanced
by chemical doping using elements such as nitrogen and carbon [8, 21, 43].
For example, Park et al. reported carbon-doped, vertically aligned TiO 2
nanotube arrays for PEC water splitting by annealing TiO 2 nanotube in the
mixed gas of CO and CO 2 [43]. The carbon-doped TiO 2 showed substantially
increased photocurrent density under visible light illumination (>420  nm)
than pristine TiO 2 nanotubes. Similarly, Hoang et al. reported nitrogen-doped
TiO 2 nanowire arrays for PEC water splitting under visible light [8]. A sub-
stantial amount of nitrogen (up to 1.08 atomic %) can be incorporated into
the TiO 2 lattice via nitridation in ammonia gas low at a relative low tem-
perature. After nitrogen doping, the white-colored pristine TiO 2 became
yellow. The absorption spectrum confirmed visible light absorption of TiO 2 .
IPCE analysis shows that nitrogen-doped TiO 2 exhibits visible light photo-
activity up to 500 nm. Hoang et al. also combined hydrogen treatment and
nitrogen doping of TiO 2 nanowire arrays and found a synergistic effect
between oxygen vacancies and nitrogen dopant in the PEC performance [21].
The hydrogen-treated and nitrogen-doped TiO 2 showed the best performance
with visible light photoactivity, compared with TiO 2 samples treated with
hydrogen and nitrogen alone.
As mentioned earlier, hematite ( α -Fe 2 O 3 ) exhibits poor PEC performance
for water splitting, due to its poor electrical conductivity and short charge
diffusion length. Element doping using elements such as Si [44], Ti [41], and
Sn [45] have been explored to increase the donor density and electrical con-
ductivity of hematite. Recently, Ling et al. found that hematite can be acti-
vated via the incorporation of oxygen vacancies, which function as shallow
donors in hematite [46]. The formation of oxygen vacancy was achieved by
annealing β -FeOOH nanowire arrays in an oxygen-deficient condition
(mixture of nitrogen and air), whereas Fe 3+ can be reduced to Fe 2+ . X-ray
photoelectron spectroscopy (XPS) Fe2p confirmed the existence of Fe 2+ in
the hematite after the treatment (Fig. 3.8a). Electrochemical impedance
studies supported the electrical conductivity and donor density being signifi-
cantly enhanced, which was attributed to the incorporation of oxygen vacan-
cies (Fig. 3.8b). As a result, the photocurrent density of hematite was
increased by orders of magnitude to more than 3 mA cm −2 at 1.4 V versus
NHE. The substantial enhancement was believed to be due to the improved
Search WWH ::

Custom Search