c. School of Environmental Science and Engineering, Shaanxi
University of Science and Technology, Xi’an, 710021, Shanxi, China.
Corresponding author:zl-jin@nun.edu.cn (Z.L. Jin);tsubaki@eng.u-toyama.ac.jp(N. Tsubaki)
Abstract: The design and construction of hierarchical
CoP/ZnCdS/Co3O4quantum dots (QDs) (800>40>4.5 nm)
bi-heterostructure cages as an ultrahigh-performance photocatalyst for
hydrogen evolution with visible light is investigated. Three excellent
photoactive materials that ZnCdS solid solution, high-conductivity CoP
and high-efficiency Co3O4 QDs were
integrated into a all-in-one bi-heterostructure cage architecture.
Presence of the two high-efficiency
electron-transfer pathways in
CoP/ZnCdS/Co3O4 QDs can seriously
facilitate the separation and migration of light-induced electrons while
the unique structure also can offer large reaction surface and expose
abundant active sites for photocatalytic hydrogen evolution reaction.
Because of the distinctively compositional and structural merits, the
hierarchical CoP/ZnCdS/Co3O4 QDs
bi-heterostructure cages without introducing any cocatalysts exhibit
ultrahigh activity and favorable stability for generation of high-purity
hydrogen under visible light irradiation.
Key words: CoP/ZnCdS/Co3O4;
quantum dots; bi-heterostructure; hydrogen generation.
Introduction
Overexploitation and hence the ever growing depletion of fossil fuels,
in addition to the anthropogenic climate change caused by the release of
greenhouse gases by combustion of these nature resources is a matter of
profound concern because more than 80% of the world energy requirement
still is derived from fossil fuels
at present [1-3]. Thus, it is urgent to develop renewable energy
resources. Of the green alternative
energy resources available, hydrogen
energy from solar energy and water is arguably the most promising one
because of its merits of green, high energy density, renewable
[4-7]. However, the low separation and migration efficiency of
charge carriers greatly restrict the water-splitting properties of
semiconductor photocatalysts. Therefore, in order to meet the dawn of
the practical application of hydrogen energy from solar energy and water
earlier, improving seriously hydrogen generation rate via designing and
constructing more efficient functional semiconductor materials still is
a key subject up to now.
Metal chalcogenide semiconductor photocatalysts, such as CdS,
ZnIn2S4,
Mn1-xCdxS,
Zn1-xCdxS, have attracted considerable
attention due to the favorable visible-light response ability
[8-12]. Of all sulfide semiconductors, CdS is a well-established
photocatalyst for hydrogen evolution due to its appropriate band gap
(2.4 eV) and good visible light response performance. However, rapid
electron recombination and serious photocorrosion hinder the
photocatalytic efficiency and stability of CdS [13]. In order to
overcome these shortcomings, CdS-based solid solution semiconductor
photocatalysts were constructed, such as
Mn1-xCdxS, and
Zn1-xCdxS, and they have been widely
investigated in photocatalytic hydrogen production because of them
merits in tunable band gap (2.28~3.94 eV) for light
absorption and optimizeable band edge positions for photo redox
reactions [14,15]. But, while the Cd/Zn molar ratio is 1, that is,
where x is 0.5, the prepared Zn0.5Cd0.5S
nanoparticles possess the best photocatalytic performance among
Zn1-xCdxS
solid solution semiconductors and far exceed alone CdS and ZnS catalysts
as well [16]. Nevertheless, the fact that fast electron-hole
recombination still exists in the
Zn0.5Cd0.5S semiconductor, which would
impede the further enhancement of photocatalytic properties of
Zn1-xCdxS solid solution materials.
Consequently, it is to absolutely accelerate the separation and
migration of charge carriers that is the vital factor to be overcome for
extremely high-efficiency photocatlytic hydrogen evolution.
Transition metal phosphides (TMPs) have arisen as a high-performance
class of candidates in photocatalytic water splitting, supercapacitors,
and electrochemical energy storage owing to their inherent
semi-metallic
nature and high electrical conductivity [17-19]. And metal
phosphides not only are good conductors of electricity and heat, but
also have high chemical stability, so they are kinetically favorable for
the rapid charge transfer required for high power density [20,21].
Not only that, they possess these advantages of environmentally benign
nature, and high abundance [22]. The negatively charged P on the
surface of a metal phosphide could not only trap protons as the base,
but also provide high activity for the dissociation of
H2, thus preventing the system from deactivation caused
by high coverage of strongly bonding hydrogen atoms [23,24]. Cobalt
phosphide (CoP) as one of TMPs has been proven to be high-performance
photocatalyst and cocatalyst owing to the coexistence synergism
conductive to fast charge transport [25,26].
Three-dimensionally confined quantum dots (QDs) have emerged as
new-generation semiconducting materials owing to tunable bandgaps,
narrow emission bandwidth, and high efficiency [27]. And their
typical dimensions range from nanometres to a few microns, and their
size can be precisely controlled. And QDs have these advantages of
higher specific surface areas and shorter paths for charge transport
[28]. QDs are more widely used in the fields of display,
photovoltaic, transistor [29]. Co3O4QDs with a size of about 4.5 nm was synthesized by Shi et al. for the
first time and applied in efficient oxygen evolution [30]. However,
it has few reports in the field of solar-driven water splitting, It was
prepared for improved hydrogen evolution that a series of novel quasi
0D/2D CoP/g-C3N4 derived from 4.5 nm
Co3O4 QDs with
g-C3N4 nanosheets [31]. The
construction of
Co3O4QDs (4.5 nm)/3D hexagonal CdS single crystals p-n heterojunction
enhanced photocatalytic hydrogen production [32].
In view of these merits and the application blank in field of
photocatalytic hydrogen evolution,
4.5 nm
Co3O4 QDs involved hierarchical
CoP/ZnCdS/Co3O4QDs
(800>40>4.5
nm) bi-heterostructure cages was reasonably designed and successfully
constructed via taking their large differences in size. The architecture
aims to build bi-heterojunction structure to greatly accelerate the
separation and migration of electron-hole pairs from
visible-light-induced ZnCdS solid
solution for more high-activity hydrogen evolution reaction, that is, to
rapidly migrate charges on the conduction band of ZnCdS from two paths
make more excited electrons put into photocatalytic hydrogen reaction.
Experimental section