Angewandte
Communications
Chemie
Hydrogen Evolution
Enhanced Photoexcited Carrier Separation in Oxygen-Doped ZnIn2S4
Nanosheets for Hydrogen Evolution
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Wenlong Yang , Lei Zhang , Junfeng Xie, Xiaodong Zhang,* Qinghua Liu, Tao Yao,*
Shiqiang Wei, Qun Zhang, and Yi Xie*
Abstract: Limited by the relatively sluggish charge-carrier
separation in semiconductors, the photocatalytic performance
is still far below what is expected. Herein, a model of ZnIn2S4
regarded as unavoidable deactivation processes inside the
photocatalyst, which tremendously hampers the catalytic
[
3]
activity of the semiconductors. Thus, to maximize the
hydrogen generation efficiency of the photocatalyst system,
the consumption of the photoexcited electrons should be
avoided as far as possible during the photocatalytic process.
Elemental doping is one of the most effective approaches
to regulate the electronic structure and improve the photo-
(
ZIS) nanosheets with oxygen doping is put forward to obtain
in-depth understanding of the role that doping atoms play in
photocatalysis. It shows enhanced photocatalytic activity
compared with pristine ZIS. The electron dynamics analyzed
by ultrafast transient absorption spectroscopy reveals that the
average recovery lifetime of photoexcited electrons is increased
by 1.53 times upon oxygen incorporation into the ZIS crystals,
indicating enhanced separation of photoexcited carriers in
oxygen-doped ZIS nanosheets. As expected, the oxygen-doped
ZIS nanosheets show a remarkably improved photocatalytic
[4]
catalytic activity. So far, in spite of research on the
optimization of the photocatalytic activity by means of
elemental doping in semiconductors, there still exists con-
troversy on the role that the dopants play in photocatalysis,
because dopants would act as the recombination centers for
the photoexcited electrons and holes, impeding further
activity with
a
hydrogen evolution rate of up to
under visible-light irradiation, which is
À1 À1
[5]
2
4
120 mmolh
g
improvement of the photocatalytic activity. More impor-
.5 times higher than that of the pristine ZIS nanosheets.
tantly, for the bulk system, the external dopants tend to be
randomly located in the interior of photocatalysts, which
increases the recombination probability of photoexcited
electrons and holes. To this end, ultrathin two-dimensional
(2D) materials would provide a favorable platform to realize
surface elemental doping, which could serve as an ideal model
with clear structure–property relationship to achieve in-depth
atomic-level insights into the correlation between the doping
A
s a clean and efficient source of energy, hydrogen is
receiving increasing attention because of its potential in
solving the energy shortage arising from the overuse of fossil
fuels and the resulting serious environmental pollution
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1]
problems. In the past decades, photocatalytic hydrogen
evolution on semiconductors was considered to be a promising
and efficient solution to convert inexhaustible solar energy
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atoms and the corresponding photocatalytic activity.
Recently, ZnIn S (ZIS) with a layered structure has been
[2]
into storable hydrogen. Up to now, although a great variety
of impressive materials have been explored and employed as
photocatalysts for hydrogen production, most of them still
suffer from a quite low photocatalytic activity, far below the
requirements of practical applications. Apart from the light
absorption, charge separation and transport are fundamen-
tally important for the photocatalytic hydrogen-generation
reaction. Nevertheless, the relatively inefficient separation
and transfer for photoexcited electrons and holes are
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regarded as an excellent photocatalyst for hydrogen evolu-
tion, mainly because of its high activity, favorable chemical
stability, and appropriate band gap corresponding to the
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visible-light absorption. Notably, with its thickness reduced
to only a few layers, a large proportion of interior sulfur atoms
in ZIS were exposed on the surfaces, which could be readily
substituted by oxygen atoms under appropriate conditions,
realizing surface oxygen-doped ZIS (O-doped ZIS) nano-
sheets. Accordingly, the ultrathin O-doped ZIS nanosheets
will provide an ideal catalytic model for understanding the
role that the dopants play in the photocatalytic process.
Herein, we first use models of the ZIS ultrathin nanosheet
and its oxygen-doped counterpart with only a few layers as
examples to illustrate the aforementioned considerations.
Density functional theory (DFT) calculations were carried
out to study the effect of oxygen doping on the electronic
structure of ZIS nanosheets. As shown in Figure 1a,b, upon
the substitution of oxygen for lattice sulfur atoms in ZIS
nanosheets, the calculated density of states (DOS) of the O-
doped ZIS exhibit a clearly increased DOS at valence band
maximum (VBM) with respect to that of the pristine ZIS,
revealing the effect of oxygen doping on the electronic
structure, which is further confirmed by their calculated
partial charge density around the VBM in Figure 1c,d,
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[
*] W. L. Yang, L. Zhang, Dr. J. F. Xie, Dr. X. D. Zhang, Prof. Q. Zhang,
Prof. Y. Xie
Hefei National Laboratory for Physical Sciences at the Microscale,
Collaborative Innovation Center of Chemistry for Energy Materials,
University of Science and Technology of China
Hefei, Anhui, 230026 (P.R. China)
E-mail: zhxid@ustc.edu.cn
Prof. Q. H. Liu, Prof. T. Yao, Prof. S. Q. Wei
National Synchrotron Radiation Laboratory, University of Science and
Technology of China
Hefei, Anhui, 230029 (P.R. China)
E-mail: yaot@ustc.edu.cn
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[
] These authors contributed equally to this work.
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 6716 –6720