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