Yubin Chen

Nanoparticles enwrapped with nanotubes: A unique architecture of CdS/titanate nanotubes for efficient photocatalytic hydrogen production from water

CdS/titanate nanotubes (CdS/TNTs) photocatalysts with a unique morphology were successfully synthesized via a simple one-step hydrothermal method. Compared with traditional CdS@TNTs composite photocatalysts prepared by the common two-step method, CdS/TNTs exhibited much higher activity for photocatalytic hydrogen evolution under visible light irradiation. Transmission electron microscopy (TEM) revealed that the CdS nanoparticle was intimately enwrapped by the surrounding TNTs. This unique architecture resulted in the appropriate dispersion of CdS nanoparticles and the intimate multipoint contacts between the CdS nanoparticle and TNTs, which led to significant enhancement of charge separation in CdS/TNTs. Accordingly, the photoactivity was improved. Meanwhile, X-ray powder diffraction (XRD) demonstrated that the highly crystalline hexagonal CdS was obtained in CdS/TNTs, which was also essential for the enhanced photocatalytic performance. The unique morphology and photocatalytic activity of CdS/TNTs were influenced by the Cd/Ti molar ratio with an optimal value of 0.05. Under this condition, the CdS amount was only 6 wt% of the total photocatalyst, which was important from an environmental point of view. The influence of loaded Pt on the activity of CdS/TNTs had also been investigated. The hydrogen production rate of 2.0 wt% Pt-loaded CdS/TNTs reached 353.4 μmol h−1, with the apparent quantum yield of 25.5% at 420 nm. This study provides a potential way to synthesize highly efficient composite photocatalysts with a novel architecture.

Highly efficient visible-light-driven photocatalytic hydrogen production from water using Cd0.5Zn0.5S/TNTs (titanate nanotubes) nanocomposites without noble metals

Semiconductor based nanocomposites are quite promising in the areas of photocatalysis and photovoltaics due to their efficient charge separation. Herein, we demonstrated a simple and green one-step method to prepare novel Cd0.5Zn0.5S/TNTs (titanate nanotubes) nanocomposites with low-priced metallic elements. Transmission electron microscopy (TEM) images revealed that an enwrapped architecture was achieved for Cd0.5Zn0.5S/TNTs nanocomposites. Cd0.5Zn0.5S nanoparticles of ca. 90 nm were tightly surrounded by the network of titanate nanotubes, which led to the high dispersity of Cd0.5Zn0.5S nanoparticles and the intimate multipoint contacts between Cd0.5Zn0.5S and TNTs. Highly efficient charge separation was finally achieved in the hybrid Cd0.5Zn0.5S/TNTs through the enwrapped structure. Under visible light irradiation Cd0.5Zn0.5S/TNTs displayed improved activities compared to the single Cd0.5Zn0.5S for hydrogen evolution. The effect of sacrificial reagents on the photocatalytic activity of Cd0.5Zn0.5S/TNTs was discussed. The highest apparent quantum yield of 38.1% at 420 nm was achieved. This value is among the highest efficiencies for the noble-metal free photocatalysts ever reported. Meanwhile, Cd0.5Zn0.5S/TNTs showed good stability for hydrogen production, and the content of toxic cadmium was as low as 4.0 wt% of the nanocomposites. These factors are of great significance for their application in the field of solar energy conversion.

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.

General applicability of nanocrystalline Ni2P as a noble-metal-free cocatalyst to boost photocatalytic hydrogen generation

To replace the role of noble-metal cocatalysts (e.g. Pt) in photocatalytic hydrogen generation, low-cost alternatives with earth-abundant elements should not only possess high catalytic activities, but also have general applicability. Herein, nanocrystalline Ni2P cocatalysts are used to modify CdS, TiO2, and C3N4 host photocatalysts. It is observed that the Ni2P cocatalyst boosts hydrogen generation over all the host photocatalysts, which demonstrates its good catalytic property and general applicability. To investigate its action mechanism, nanocrystalline Ni2P was successfully integrated with TiO2 nanorods (TiNR) for the first time. The optimized Ni2P/TiNR sample exhibits an 85 times higher activity compared to single TiNR, and its apparent quantum efficiency was calculated to be 11.6% at 360 nm. Among the varied nickel-based semiconductor cocatalysts, Ni2P is also proven to be the best cocatalyst. Photoluminescence and electrochemical results reveal that the Ni2P cocatalyst promotes the charge transfer both in the photocatalyst and at the photocatalyst/solution interface, as well as accelerates the surface reaction. The enhanced charge transfer efficiency and improved surface reaction rate finally result in a dramatically improved performance. It is believed that the present work can provide basic principles for the development of noble-metal-free cocatalysts with high catalytic activity and general applicability.

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.

Towards efficient solar-to-hydrogen conversion: Fundamentals and recent progress in copper-based chalcogenide photocathodes

Abstract Photoelectrochemical (PEC) water splitting for hydrogen generation has been considered as a promising route to convert and store solar energy into chemical fuels. In terms of its large-scale application, seeking semiconductor photoelectrodes with high efficiency and good stability should be essential. Although an enormous number of materials have been explored for solar water splitting in the last several decades, challenges still remain for the practical application. P-type copper-based chalcogenides, such as Cu(In, Ga)Se2 and Cu2ZnSnS4, have shown impressive performance in photovoltaics due to narrow bandgaps, high absorption coefficients, and good carrier transport properties. The obtained high efficiencies in photovoltaics have promoted the utilization of these materials into the field of PEC water splitting. A comprehensive review on copper-based chalcogenides for solar-to-hydrogen conversion would help advance the research in this expanding area. This review will cover the physicochemical properties of copper-based chalco-genides, developments of various photocathodes, strategies to enhance the PEC activity and stability, introductions of tandem PEC cells, and finally, prospects on their potential for the practical solar-to-hydrogen conversion. We believe this review article can provide some insights of fundamentals and applications of copper-based chalco-genide thin films for PEC water splitting.

Surface treatment effect on the photocatalytic hydrogen generation of CdS/ZnS core-shell microstructures

CdS/ZnS core-shell microparticles were prepared by a simple two-step method combining ultrasonic spray pyrolysis and chemical bath deposition. The core-shell structures showed enhanced photocatalytic properties compared with those of CdS or ZnS spherical particles. CdS/ZnS photocatalysts with different amount of ZnS loaded as shells were prepared by adjusting the concentrations of Zn and S precursors during synthesis. The optical properties and photocatalytic activity for hydrogen production were investigated and the amount of ZnS loaded as shell was optimized. Thermal annealing and hydrothermal sulfurization treatments were applied to the core-shell structure and both treatments enhanced the materials photocatalytic activity and stability by eliminating crystalline defects and surface states. The result showed that thermal annealing treatment improved the bulk crystallinity and hydrothermal sulfurization improved the surface properties. The sample subjected to both treatments showed the highest photocatalytic activity. These results indicate that CdS/ZnS core-shell microspheres are a simple structure that can be used as efficient photocatalysts. The hydrothermal sulfurization treatment may also be a useful surface treatment for metal sulfide photocatalysts. The simple two-step method provides a promising approach to the large-scale synthesis of core-shell microsphere catalysts.

Functionalized nanostructures for enhanced photocatalytic performance under solar light.

Summary Photocatalytic hydrogen production from water has been considered to be one of the most promising solar-to-hydrogen conversion technologies. In the last decade, various functionalized nanostructures were designed to address the primary requirements for an efficient photocatalytic generation of hydrogen by using solar energy: visible-light activity, chemical stability, appropriate band-edge characteristics, and potential for low-cost fabrication. Our aim is to present a short review of our recent attempts that center on the above requirements. We begin with a brief introduction of photocatalysts coupling two or more semiconductors, followed by a further discussion of the heterostructures with improved matching of both band structures and crystal lattices. We then elaborate on the heterostructure design of the targeted materials from macroscopic regulation of compositions and phases, to the more precise control at the nanoscale, i.e., materials with the same compositions but different phases with certain band alignment. We conclude this review with perspectives on nanostructure design that might direct future research of this technology.