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H 2 production under the same measurement conditions [30]. This work dem-
onstrated the potential of graphene as a support for CdS nanoparticles for
photocatalytic hydrogen generation.
Recently, Han et al. recently reported a robust and highly active system
of Ni catalyst-modified CdSe quantum dots for solar hydrogen generation
[29]. The photoexcited electrons on CdSe nanocrystals are shuttled to Ni
catalyst and reduce protons to hydrogen while the photoexcited holes are
used to oxidize ascorbic acid [29]. The hydrogen evolution performances of
CdSe nanocrystals with and without Ni-catalyst modification were measured
under light illumination. The linear profiles indicate that the hydrogen gen-
eration rate is constant for over 360 hours, suggesting high stability over a
long period of time. In comparison with the system without Ni catalyst, the
Ni-modified CdSe (denoted as Ni-CdSe) nanocrystals exhibited more than
one order of magnitude enhancement in hydrogen generation rate. The results
suggested that the Ni catalyst facilitates the electron transfer for proton
reduction and further boosts the photoactivity of CdSe. The Ni-CdSe system
achieved an optimal turnover number over 600,000  mol of H 2 per mole
of catalyst, which is a benchmark value for photocatalytic H 2 generation
among different photocatalysts such as metal oxides, metal nitrides, and
metal chalocogenides. The quantum yield obtained is around 36% at the
wavelength of 520 nm with ascorbic acid as hole sacrificial reagent. Impor-
tantly, the homogenous nickel catalyst is from just common inexpensive
nickel salts, such as nickel nitrate, nickel chloride, and nickel acetate [29].
3.2.5 Conclusion
Meal oxides, oxynitrides, and chalcogenides have been extensively explored
for photocatalytic hydrogen generation. The recent breakthroughs in
developing catalyst-modified nanostructured photocatalysts suggested the
feasibility of large-scale production of solar hydrogen. Nevertheless, there
are still several outstanding questions that need to be addressed. For instance,
although these photocatalysts have suitable electronic band structure for
water splitting, most of these photocatalytic systems require the addition of
sacrificial reagents, due to large overpotential for water oxidation and their
instability in oxidative environment. Moreover, spontaneous water reduction
requires the conduction band of photocatalyst more negative than water
reduction potential. Semiconductors such as α-Fe 2 O 3 has a favorable bandgap
for visible light absorption but relatively positive conduction potential that
limits its application as photocatalyst for hydrogen generation. Alternatively,
these semiconductors can be used as electrode materials for PEC hydrogen
generation, by applying an external bias, as discussed in the next section.
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