Zhixiao Qin

Facile Fabrication of Sandwich Structured WO3 Nanoplate Arrays for Efficient Photoelectrochemical Water Splitting

Herein, sandwich structured tungsten trioxide (WO3) nanoplate arrays were first synthesized for photoelectrochemical (PEC) water splitting via a facile hydrothermal method followed by an annealing treatment. It was demonstrated that the annealing temperature played an important role in determining the morphology and crystal phase of the WO3 film. Only when the hydrothermally prepared precursor was annealed at 500 °C could the sandwich structured WO3 nanoplates be achieved, probably due to the crystalline phase transition and increased thermal stress during the annealing process. The sandwich structured WO3 photoanode exhibited a photocurrent density of 1.88 mA cm(-2) and an incident photon-to-current conversion efficiency (IPCE) as high as 65% at 400 nm in neutral Na2SO4 solution under AM 1.5G illumination. To our knowledge, this value is one of the best PEC performances for WO3 photoanodes. Meanwhile, simultaneous hydrogen and oxygen evolution was demonstrated for the PEC water splitting. It was concluded that the high PEC performance should be attributed to the large electrochemically active surface area and active monoclinic phase. The present study can provide guidance to develop highly efficient nanostructured photoelectrodes with the favorable morphology.

Noble-metal-free Cu2S-modified photocatalysts for enhanced photocatalytic hydrogen production by forming nanoscale p–n junction structure

Developing efficient noble-metal-free photocatalysts is of great importance for the large-scale application of photocatalytic hydrogen production. Herein, low-cost and environment-friendly p-type Cu2S was successfully loaded on n-type CdS photocatalyst by an in situ method to achieve efficient Cu2S/CdS hybrid photocatalysts. Cu2S nanoparticles of ca. 50 nm were intimately assembled on the surface of polyhedral CdS nanocrystals, giving rise to the formation of numerous nanoscale p–n junctions between p-type Cu2S and n-type CdS. Compared to single CdS, Cu2S/CdS exhibited increased photocatalytic hydrogen production under visible light irradiation. The generated nanoscale p–n junctions in Cu2S/CdS, leading to the enhanced charge separation efficiency and better utilization of visible light, were crucial to the improved photocatalytic activity. During the photocatalytic reaction, Cu2S nanoparticles captured the photogenerated holes in CdS and served as the active sites for the surface oxidation reaction. The photocatalytic property of Cu2S/CdS photocatalysts was influenced by the Cu/Cd molar ratio, with the optimal one of 0.05. P-type Cu2S could also be utilized for improving the photocatalytic activities of n-type ZnIn2S4 and n-type TiO2 by forming efficient p–n junctions, indicating the general applicability of p-type Cu2S. This work demonstrates that forming p–n junction structure was a useful strategy for developing efficient noble-metal-free hybrid photocatalysts.

Intergrowth of Cocatalysts with Host Photocatalysts for Improved Solar-to-Hydrogen Conversion

In the field of photocatalytic hydrogen generation, cocatalysts play a vital role in enhanced properties. Delicate control of the physicochemical properties of cocatalysts and systematic optimization of the coupling between cocatalysts and host photocatalysts are essential. Herein, a simple one-step hydrothermal method was proposed to synthesize noble-metal-free NiSx/CdS photocatalysts for the first time. Time-dependent growth studies revealed that NiSx cocatalysts and CdS host photocatalysts were intergrown with each other in the one-step hydrothermal process. Compared with NiSx@CdS photocatalysts prepared by the common two-step method, the intergrowth effect induced close contact between NiSx and CdS, as well as smaller size and better dispersity of NiSx nanoparticles. These specific characters of NiSx/CdS finally resulted in efficient charge separation and rapid surface reaction, giving rise to significantly improved photocatalytic activity with the apparent quantum efficiency at 420 nm as high as 60.4%. To our knowledge, this value is the highest efficiency for NiSx modified CdS photocatalysts and is among the best efficiencies for visible-light-driven photocatalysts. It is believed that the present work can provide a general guidance to develop an efficient heterostructured cocatalyst/photocatalyst system for hydrogen generation.

Electron-transfer dependent photocatalytic hydrogen generation over cross-linked CdSe/TiO2 type-II heterostructure

Developing type-II heterostructures with a spatial separation of photoexcited electrons and holes is a useful route to promote photocatalytic hydrogen generation. However, few investigations on the charge transfer process across the heterojunction have been carried out, which can allow us to uncover the reaction mechanism. Herein, CdSe quantum dots (QDs) and TiO2 nanocrystals were synthesized and combined in water yielding CdSe/TiO2 type II heterostructures. It was found that mercaptopropionic acid as bifunctional molecules could bind with CdSe and TiO2 to form a cross-linked morphology. The charge carrier dynamics of bare CdSe and CdSe/TiO2 were detected using femtosecond transient absorption spectroscopy. In the presence of TiO2, the average exciton lifetime of CdSe QDs was apparently decreased, owing to the electron transfer from photoexcited CdSe to TiO2. Particularly, the electron-transfer rate from small CdSe QDs (3.0 nm) was much faster than that from big CdSe QDs (4.2 nm). The improved photocatalytic hydrogen generation was observed for CdSe/TiO2 compared to bare CdSe QDs. The enhancement factor for small CdSe QDs was higher than that for big CdSe QDs, which was in good agreement with the electron-transfer rates. This result indicated that the electron transfer between CdSe and TiO2 played an important role in photocatalytic hydrogen generation on CdSe/TiO2 type-II heterostructure. Our study provides a fundamental guidance to construct efficient heterostructured photocatalysts by delicate control of the band alignment.

A bifunctional NiCoP-based core/shell cocatalyst to promote separate photocatalytic hydrogen and oxygen generation over graphitic carbon nitride

Developing suitable cocatalysts is crucial to promoting photocatalytic hydrogen and oxygen generation using solar energy. Herein, non-precious NiCoP-based cocatalysts were synthesized by a facile solid-state phosphorization reaction, and coupled with metal-free graphitic carbon nitride (g-C3N4) for photocatalytic reactions. It was revealed that NiCoP-based nanoparticles exhibited a core/shell structure, where the NiCoP core was surrounded by amorphous-like nickel cobalt phosphate (NiCo–Pi) shell. The detailed spectroscopic and electrochemical studies demonstrated that NiCoP cores behaved as the active sites for the photocatalytic reductive half-reaction, and NiCo–Pi shells could serve as the active sites for the photocatalytic oxidative half-reaction. As a consequence, the improved surface reaction rate through the bifunctional NiCoP-based cocatalyst, as well as the enhanced charge separation efficiency, cooperatively boosted the separate photocatalytic hydrogen and oxygen generation in the presence of appropriate sacrificial reagents. The apparent quantum efficiency for hydrogen generation over the NiCoP@NiCo–Pi/g-C3N4 photocatalyst can reach 9.4% at 420 nm, which is one of the best values for noble-metal-free g-C3N4-based photocatalysts. To our knowledge, this is the first demonstration of the NiCoP-based cocatalyst to promote both photocatalytic hydrogen and oxygen generation, which is expected to pave a new way to exploit efficient bifunctional cocatalysts for overall water splitting.

Optimization of (Cu2Sn)xZn3(1−x)S3/CdS pn junction photoelectrodes for solar water reduction

Surface modification of p-type Cu2ZnSnS4 films with n-type semiconductors is an efficient way to enhance the properties for photoelectrochemical water reduction. However, the detailed optimization of the pn junction photoelectrodes and examination of the underlying mechanism are seldom reported. Herein, Cu2ZnSnS4-derived (Cu2Sn)0.45Zn1.65S3 (CTZS) nanocrystal films were first fabricated, and subsequently coated with n-type CdS layers to form CTZS/CdS photoelectrodes. To obtain an insight into the pn junction, we have examined the depletion region widths that extended into both CTZS and CdS layers. It was revealed that the CdS layer was fully depleted and the CTZS layer was partially depleted in CTZS/CdS photoelectrodes. Consequently, increased CTZS thickness led to the decreased charge separation, and increased CdS thickness resulted in the improved charge separation. Owing to the balance of light absorption and charge separation, CTZS/CdS films with moderate thicknesses of CTZS and CdS layers showed the highest photocurrent. Meanwhile, the annealing treatment for CTZS/CdS film was indispensable for the improved property. After Pt modification, the incident photon to current conversion efficiency could reach 24.7% at 450 nm, which was among the best values for Cu2ZnSnS4-based photocathodes. This work is expected to provide general guidance for exploring efficient photoelectrodes with pn junction structure.

Novel Cu 3 P/g-C 3 N 4 p-n heterojunction photocatalysts for solar hydrogen generation

Developing efficient heterostructured photocatalysts to accelerate charge separation and transfer is crucial to improving photocatalytic hydrogen generation using solar energy. Herein, we report for the first time that p-type copper phosphide (Cu3P) coupled with n-type graphitic carbon nitride (g-C3N4) forms a p-n junction to accelerate charge separation and transfer for enhanced photocatalytic activity. The optimized Cu3P/g-C3N4 p-n heterojunction photocatalyst exhibits 95 times higher activity than bare g-C3N4, with an apparent quantum efficiency of 2.6% at 420 nm. A detail analysis of the reaction mechanism by photoluminescence, surface photovoltaics and electrochemical measurements revealed that the improved photocatalytic activity can be ascribed to efficient separation of photo-induced charge carriers. This work demonstrates that p-n junction structure is a useful strategy for developing efficient heterostructured photocatalysts.摘要开发高效的异质结光催化剂促进电荷的分离和转移对提高太阳能光催化产氢性能至关重要. 本文采用p型的磷化铜和n型的氮化碳形成p-n结来促进电荷分离和转移, 从而提高光催化产氢性能. 与纯的氮化碳相比, 磷化铜/氮化碳p-n异质结光催化剂的产氢性能提高了95倍, 在420纳米处的量子效率达到2.6%. 我们通过荧光光谱, 表面光电压谱以及电化学测试进一步分析反应机理, 发现显著提高的光催化产氢性能应归因于p-n异质结光催化剂中高效的电荷分离. 本研究表明形成p-n异质结是开发高效光催化剂的一种有效途径.

Facet-selective growth of CdS nanorods on ZnO microrods: intergrowth effect for improved photocatalytic performance

Well‐designed architectures play an important role in accelerating charge transfer between the different components of hybrid photocatalysts. Herein, we report a simple one‐step hydrothermal method to achieve a CdS/ZnO heterostructure with a novel spatial arrangement. The CdS nanorods were found to be attached to the surface of the ZnO microrods with an intimate face‐to‐face contact, and the heterointerfaces corresponded to {1 0 0} facets of CdS and {1 0 0} facets of ZnO. It was discovered that Zn(OH)2 intermediates were formed at first and then grew into hexagonal ZnO microrods through a solid–solid‐phase transformation. Simultaneously, CdS nuclei grew to nanorods on the {1 0 0} facets of ZnO by oriented attachment during the one‐step hydrothermal process. Under visible‐light irradiation, obtained CdS/ZnO exhibited enhanced photocatalytic hydrogen generation owing to improved charge separation from the two‐phase intergrowth effect. This work provides a facile hydrothermal route to construct intergrown heterostructures with desired spatial arrangements for improved photocatalytic properties.

Heterostructured photocatalysts for improved solar hydrogen generation

Photocatalytic hydrogen generation from water over semiconductors is a promising route for the conversion and storage of solar energy, and this methodology has thus been widely studied over the past few decades.1 In a typical process, semiconductor photocatalysts absorb the incident light to generate electrons and holes that are separated and transferred to the surface of the photocatalysts. The photoexcited electrons reduce hydrogen ions to produce hydrogen gas. In addition, the photoexcited holes oxidize hydroxide ions to generate oxygen gas, or they react with sacrificial reagents (e.g., sulfur or sulfite ions). Most semiconductor photocatalysts, however, have poor activity because of rapid charge recombination. Coupling semiconductors that have different energy levels to form heterostructures has been demonstrated as a suitable way to achieve effective charge separation.2 In this approach, the basic requirement for obtaining the improved properties is the formation of a high-quality heterojunction. Generally, such heterostructured photocatalysts are prepared via a twostep method.3 In the first step, one component is initially prepared and is subsequently coupled to the other component by using a physical or chemical treatment. This technique, however, can cause large amounts of individual components to aggregate and thus leads to reduced contacts between the two phases.4 The contact between the phases may also be loose because the individual components are synthesized separately. Such poor contacts are detrimental to the charge separation. In our work, we have therefore introduced a simple ‘one-pot’ method for the synthesis of hybrid photocatalysts.5–7 In this approach, two phases are intergrown with each other. In this way, our technique can provide strong coupling between the Figure 1. Schematic diagram of the photocatalytic process taking place over heterostructured cadmium sulfide/titanate nanotubes (CdS/TNTs). In this process, under visible light (hv) irradiation, hydrogen ions (H+) are converted to hydrogen gas (H2), and sulfur ions (S2–) or sulfite ions (SO2– 3 ) are converted to polysulfur ions (S 2– 2 ) and sulfate ions (SO2– 4 ).