Environmental Engineering Reference
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temperature in ammonia gas [28]. The UV-visible diffuse reflectance spectra
of Ta
2
O
5
and TaON show that the absorption edge of TaON was around
500 nm with an estimated bandgap of 2.5 eV, which was red shifted by about
170 nm from that of Ta
2
O
5
after nitridation. The time course of H
2
evolution
on TaON under irradiation of visible light (λ > 420 nm) showed that H
2
production was preceded continuously without obvious decrease in activity
[28].
Furthermore, metal phosphides such as GaP also have been studied for
photocatalytic hydrogen generation. Compared with Ga
2
O
3
(4.8 eV), GaP
has a smaller bandgap of 2.79 eV (direct energy gap) and 2.3 eV (indirect
energy gap), and suitable band edge positions for water reduction. Recently,
Sun et al. reported a surfactant free, self-seeded solution-liquid-solid (SLS)
approach for large-scale synthesis of high quality colloidal GaP nanowires
[18]. These high quality GaP nanowires with clean surface hold great poten-
tials as visible light photocatalyst for water splitting. Transmission electron
microscopy (TEM) images proved that the as-prepared GaP nanowires are
uniform single crystals with a length of 1-2 μm and diameter of 30 nm (Fig.
3.2a and Fig. 3.2b). Importantly, the prepared GaP nanowires can be assem-
bled into a large nanowire membrane by filtration (Fig. 3.2c). The capability
of using GaP nanowires as photocatalyst to drive hydrogen evolution from
water reduction under visible light illumination with methanol as hole scav-
enger was demonstrated. To improve hydrogen evolution, GaP nanowires
were further decorated with Pt nanoparticles. As shown in Figure 3.2d, the
hydrogen evolution rate of Pt-modified GaP (with a low Pt loading of 2 wt%)
nanowire was an order of magnitude higher than that of GaP nanowire. The
linear profiles suggest the hydrogen generation rate is constant and GaP
nanowires are stable for photocatalytic hydrogen generation within 12 hours
(Fig. 3.2d).
Metal oxynitrides, nitrides, and phosphides have been demonstrated to be
useful as visible light photocatalysts for hydrogen generation. However, the
long-term stability is still a concern for these materials due to the self-
oxidation during photocatalytic reactions.
3.2.4 Metal Chalcogenides
CdS and CdSe nanoparticles are two common metal chalcogenides used as
photocatalysts for hydrogen generation [9,29]. They have suitable band edge
positions for water splitting, and their bandgap energies are tunable via con-
trolled variation of particle size. For example, Li et al. reported CdS cluster-
modified graphene nanosheets for visible light-driven photocatalytic
hydrogen production [30]. The layered graphene as a supporting matrix can
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