Jinzhan Su

Nanostructured WO3/BiVO4 Heterojunction Films for Efficient Photoelectrochemical Water Splitting

We report on a novel heterojunction WO(3)/BiVO(4) photoanode for photoelectrochemical water splitting. The heterojunction films are prepared by solvothermal deposition of a WO(3) nanorod-array film onto fluorine-doped tin oxide (FTO) coated glass, with subsequent deposition of a low bandgap, 2.4 eV, visible light responding BiVO(4) layer by spin-coating. The heterojunction structure offers enhanced photoconversion efficiency and increased photocorrosion stability. Compared to planar WO(3)/BiVO(4) heterojunction films, the nanorod-array films show significantly improved photoelectrochemical properties due, we believe, to the high surface area and improved separation of the photogenerated charge at the WO(3)/BiVO(4) interface. Synthesis details are discussed, with film morphologies and structures characterized by field emission scanning electron microscopy and X-ray diffraction.

Vertically Aligned WO3 Nanowire Arrays Grown Directly on Transparent Conducting Oxide Coated Glass: Synthesis and Photoelectrochemical Properties

Photocorrosion stable WO(3) nanowire arrays are synthesized by a solvothermal technique on fluorine-doped tin oxide coated glass. WO(3) morphologies of hexagonal and monoclinic structure, ranging from nanowire to nanoflake arrays, are tailored by adjusting solution composition with growth along the (001) direction. Photoelectrochemical measurements of illustrative films show incident photon-to-current conversion efficiencies higher than 60% at 400 nm with a photocurrent of 1.43 mA/cm(2) under AM 1.5G illumination. Our solvothermal film growth technique offers an exciting opportunity for growth of one-dimensional metal oxide nanostructures with practical application in photoelectrochemical energy conversion.

Fabrication and Properties of a Branched (NH4)xWO3 Nanowire Array Film and a Porous WO3 Nanorod Array Film

We describe the successful fabrication of a three-dimensional branched (NH4)xWO3 nanowire array film on fluorine-doped tin oxide coated glass by a facile one-step hydrothermal method. The porous WO3 nanorod array film formed after heat treatment and recrystallization. Specifically, the branched (NH4)xWO3 nanowire array film has very thin nanowires that were about 10 nm in diameter. The results of an optical and photoelectrochemical test show that the branched (NH4)xWO3 nanowire array film could be used as a near-infrared shielder, while the porous WO3 nanorod array film can be used as a photoanode for water splitting. Moreover, the morphology, structure, and composition of the as-prepared films are revealed, and the related changes caused by heat treatment are discussed in detail.

Spontaneous photoelectric field-enhancement effect prompts the low cost hierarchical growth of highly ordered heteronanostructures for solar water splitting

In this study, a potentially universal new strategy is reported for the large-scale, low-cost fabrication of visible-light-active highly ordered heteronanostructures based on the spontaneous photoelectric-field-enhancement effect inherent in pyramidal morphology. The hierarchical vertically oriented arrayed structures comprise an active molecular co-catalyst at the apex of a visible-light-active large band gap semiconductor for low-cost solar water splitting in a neutral aqueous medium without the use of a sacrificial agent.

High aspect ratio TiO2 nanowires tailored in concentrated HCl hydrothermal condition for photoelectrochemical water splitting

TiO2 nanowire/nanorod arrays grown on FTO substrate by hydrothermal reaction have attracted great attention because of favorable applications in dye-sensitized solar cells and quantum dot solar cells. In this paper, discrete vertically aligned TiO2 nanowire arrays as long as 7.2 μm and diameter of less than 100 nm were successfully synthesized on FTO substrate via the hydrothermal method. The influence of hydrothermal precursor composition on morphologies of the grown wires was investigated. Photoelectrochemical water-splitting performance for nanowires with different lengths was studied; the best performance was observed for the longest array without a compact layer. The growth mechanism in concentrated HCl was also examined. The TiO2 nanowire array developed in this paper provides an optimal structure for energy-harvesting applications requiring long and discrete nanowires as electron collectors.

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.

Comparison of sandwich and fingers-crossing type WO3/BiVO4 multilayer heterojunctions for photoelectrochemical water oxidation

WO3 and BiVO4 thin films were spin-coated on FTO in a intersecting way with the WO3 and BiVO4 overlapping only in the center part, resulting in multilayer fingers-crossing thin films (fingers-crossing heterojunction). Samples with only an overlapped area (sandwich heterojunction) were obtained by cutting off the non-overlapped area of both the WO3 and BiVO4 layers (the nodes) as a counterpart for comparison. The influence of layer number, layer thickness and junction type on the photoelectrochemical (PEC) water splitting performance under simulated solar light for the obtained heterojunctions was investigated. XRD, SEM, XPS and UV-vis absorption measurements were conducted to investigate the structural properties and morphology. The photocurrent density of the sandwich heterojunction is higher than that of the fingers-crossing heterojunction at a low applied bias. The IPCE shows that both the fingers-crossing heterojunction and sandwich heterojunction can effectively utilize light up to 510 nm. Mott–Schottky plots show that the sandwich heterojunction has a more negative flat band potential, which is desirable for water splitting, than the fingers-crossing heterojunction. In addition, the mechanism of photogenerated carrier separation for these two heterojunctions was discussed.

Morphology engineering of WO3/BiVO4 heterojunctions for efficient photocatalytic water oxidation

Four types of nanostructural WO3/BiVO4 heterojunctions were designed by depositing a BiVO4 layer onto WO3 scaffolds with different morphologies and investigated to understand the influence of heterojunction morphology on their photoelectrochemical performances. The architecture of WO3 scaffolds was controlled by adjusting the solvents used in their solvothermal synthesis. The solvent was found to exert a great influence on the architecture of the obtained WO3 scaffolds. Methanol leads to the formation of block-like nanostructures and ethanol facilitates the formation of nanorods, while isopropyl alcohol and ethylene glycol give rise to the formation of triangular nanowalls and 3D hierarchical nanostructures, respectively. Monoclinic WO3 nanostructures were obtained via removal of ammonium or structural water by annealing the samples obtained at 510 °C in air. A BiVO4 layer was then deposited on the monoclinic WO3 scaffolds with different morphologies by spin-coating to form WO3/BiVO4 heterojunction films, which show significantly enhanced photocurrents compared to their corresponding bare WO3 scaffolds. Among these WO3/BiVO4 nanostructure heterojunctions, the one based on a rod-like WO3 nanostructure shows the best photoelectrochemical (PEC) performance, which can be ascribed to its optimal rod-like morphology, for the construction of WO3/BiVO4 heterojunction regarding efficient charge carrier transfer and collection, while the other one based on WO3 nanowalls exhibits better stability and a higher photocurrent in a long-run test, indicating that morphology engineering significantly influences the activity and stability of WO3-based heterojunctions.

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.