Jinwen Shi

Visible-light-driven photocatalytic water splitting on nanostructured semiconducting materials

In view of the unlimited resource of solar energy and the abundance of water on earth, producing hydrogen through photocatalytic splitting of water under solar irradiation has the great potential to offer a low cost, environmentally friendly green fuel that does not contribute to greenhouse gas emissions. Since the pioneering work of Fujisima and Honda in 1972, tremendous research on semiconductor photocatalysis has yielded better understanding of the processes involved in photocatalytic water splitting, as well as notable enhancement of energy conversion efficiency for solar hydrogen generation. However, the solar-to-hydrogen energy conversion efficiency is still too low to be viable for practical applications, primarily due to the limitation of electronic band structure of semiconductor photocatalysts and rapid recombination of photogenerated charges. Thus, ideal photocatalysts are the key for the realisation of high efficiency hydrogen generation system. Fortunately, various kinds of effective semiconductors have been developed as good candidates for photocatalytic hydrogen generation, and high-efficiency photocatalysis systems in lab scale have also been constructed. This paper provides an overview of the common approaches that have been used in the search for high efficiency photocatalysts (i.e., photocatalyst itself and co-catalyst) and matched reaction systems of sacrificial reagents for water splitting, especially under visible light irradiation. From this review, one can also observe that nanotechnology plays an important role in the design of novel nanostructured or heterogeneous photocatalysts for the establishment of high-efficiency photocatalytic solar hydrogen system. These may offer a general guide to those who are interested in tackling the challenges.

Site-selected doping of upconversion luminescent Er3+ into SrTiO3 for visible-light-driven photocatalytic H2 or O2 evolution.

A series of upconversion luminescent erbium-doped SrTiO(3) (ABO(3)-type) photocatalysts with different initial molar ratios of Sr/Ti have been prepared by a facile polymerized complex method. Er(3+) ions, which were gradually transferred from the A to the B site with increasing Sr/Ti, enabled the absorption of visible light and the generation of high-energy excited states populated by upconversion processes. The local internal fields arising from the dipole moments of the distorted BO(6) octahedra promoted energy transfer from the high-energy excited states of Er(3+) with B-site occupancy to the host SrTiO(3) and thus enhanced the band-to-band transition of the host SrTiO(3). Consequently, the erbium-doped SrTiO(3) species with B-site occupancy showed higher photocatalytic activity than those with A-site occupancy for visible-light-driven H(2) or O(2) evolution in the presence of the corresponding sacrificial reagents. The results generally suggest that the introduction of upconversion luminescent agents into host semiconductors is a promising approach to simultaneously harnessing low-energy photons and maintaining redox ability for photocatalytic H(2) and O(2) evolution and that the site occupancy of doped elements in ABO(3)-type perovskite oxides greatly determines the photocatalytic activity.

CdS/CdSe core-shell nanorod arrays: energy level alignment and enhanced photoelectrochemical performance.

Novel CdS/CdSe core-shell nanorod arrays were fabricated by a chemical bath deposition of CdSe on hydrothermally synthesized CdS nanorods. The CdS rods were hexagonal phase faced and the top of the rod was subulate. After the chemical bath deposition approach, CdS nanorod arrays were encapsulated by a uniform CdSe layer resulting enhanced absorbance and extended absorption edges of the films. A tandem structure of the energy bands of CdS/CdSe was also formed as a result of the Fermi level alignment, which is a benefit to the efficient separation of photogenerated charges. CdS/CdSe core-shell arrays gave a maximum photocurrent of 5.3 mA/cm(2), which was 4 and 11 times as large as bare CdS and CdSe, respectively.

Hydrothermal Synthesis of Na0.5La0.5TiO3–LaCrO3 Solid-Solution Single-Crystal Nanocubes for Visible-Light-Driven Photocatalytic H2 Evolution

Cubic perovskite structure photocatalysts of Na(0.5)La(0.5)TiO(3) and (Na(0.5)La(0.5)TiO(3))(1.00)(LaCrO(3))(0.08) solid solution that consisted of well-defined single-crystal nanocubes were successfully prepared by means of facile and surfactant-free hydrothermal reactions for the first time. The results from different instrumental characterizations and theoretical calculations consistently confirmed the formation of nanocubic single-crystal solid solution of (Na(0.5)La(0.5)TiO(3))(1.00)(LaCrO(3))(0.08), and clearly revealed the modification of its physicochemical properties compared with those of Na(0.5)La(0.5)TiO(3). In particular, the effective narrowing of the bandgap (from 3.19 to 2.25 eV) by Cr(3+) in the solid solution made it possible to utilize visible light. The solid-solution configuration maintained the charge balance to preserve the valence of Cr(3+) rather than Cr(6+), and accommodated Cr(3+) with high content to form new energy bands instead of localized impurity levels. The hydrothermal preparation strategy ensured the formation of single crystals with high purity, few defects, and regulated morphology; it also guaranteed the valences of Ti(4+) and Cr(3+) in the solid solution. Consequently, the recombination of photogenerated carriers could be effectively suppressed to benefit photocatalytic H(2) evolution. (Na(0.5)La(0.5)TiO(3))(1.00)(LaCrO(3))(0.08) nanocubic single-crystal solid solution showed stable photocatalytic activity, and thus was proved to be a promising candidate for visible-light-driven photocatalytic H(2) evolution.

WO3/g-C3N4 composites: one-pot preparation and enhanced photocatalytic H2 production under visible-light irradiation

A series of WO3/g-C3N4 composites with different WO3 contents were prepared via a facile one-pot pyrolysis method, and showed notably enhanced visible-light-driven photocatalytic H2-evolution activities, with the highest rate of 400 μmol h-1 gcat-1 that was 15.0 times of that for pristine g-C3N4. Contents and sizes of WO3 crystallites in the composites were easily adjusted by changing the molar ratios of (NH4)2WS4 to C3H6N6 in the feed reagents, thereby successfully optimizing the Z-scheme system constructed by WO3 and g-C3N4 and thus effectively reducing the recombination of photogenerated charge carriers in g-C3N4. Moreover, pore volumes and surface areas of the composites were gradually enlarged by introducing WO3 into g-C3N4 via the one-pot preparation strategy, therefore promoting the redox reactions to evolve H2. This work presented an effective route to simultaneously optimize the phase compositions and textural structures of photocatalysts for enhanced H2 evolution.

A novel Sn2Sb2O7 nanophotocatalyst for visible-light-driven H2 evolution

AbstractA novel pure cubic-phase pyrochlore structure tin(II) antimonate nanophotocatalyst, stoichiometric Sn2Sb2O7, has been prepared by a modified ion-exchange process using an antimonic acid precursor, and employed in visible-light-driven photocatalytic H2 evolution for the first time. The physicochemical properties (crystal phase, chemical composition and state, textural properties, and optical properties) of the material were investigated by different instrumental techniques. Compared with the antimonic acid precursor, the as-prepared Sn2Sb2O7 had a narrower bandgap, smaller crystal size, and larger BET surface area. The as-prepared Sn2Sb2O7 was validated as a promising candidate for visible-light-driven photocatalytic H2 evolution with a constant rate of 40.10 μmol·h−1·gcat−1.

Tin(II) Antimonates with Adjustable Compositions: Effects of Band‐Gaps and Nanostructures on Visible‐Light‐Driven Photocatalytic H2 Evolution

A series of tin(II)–antimonate photocatalysts with varied Sn content were prepared by altering the ion‐exchange time and reaction temperature to control their physicochemical properties, especially their band‐gaps and nanostructures. Furthermore, the effect of these catalysts on visible‐light‐driven photocatalytic H2‐evolution was also investigated. With an increase in Sn content, the narrowed band‐gaps enhanced the absorption of photons to excite the photogenerated charge carriers. A decrease in nanocrystal size approaching stoichiometric compositions impeded the recombination of the photogenerated charge carriers; the increased surface areas and pore volumes, owing to the nanostructural transformation, accelerated the redox reactions. Consequently, the photocatalytic activities gradually improved and the highest rate was observed for stoichiometric Sn2Sb2O7. As a result, the as‐prepared tin(II) antimonates—especially Sn2Sb2O7—were confirmed to be stable and efficient photocatalysts for visible‐light‐driven H2 evolution. Moreover, the activities of these photocatalysts could be improved by tuning their physicochemical properties to jointly optimize all of the processes in the photocatalytic reaction.

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.

Visible light-driven photocatalysis of doped SrTiO 3 tubular structure

SrTiO3 tubular structures co-doped with Cr and Ta were synthesized through a combination of solvothermal-hydrothermal processes. X-ray photoelectron spectroscopy (XPS) measurements of the oxidation state of Cr ions reveal that the formation of Cr6+ ions, which would serve as the non-radiative recombination centers for photogenerated electrons and holes, was suppressed without the process of high temperature hydrogen reduction. Compared to similar co-doped materials synthesized by solid-state reaction, (Cr, Ta) co-doped SrTiO3 tubular structures have significantly higher photocatalytic activity for hydrogen evolution as measured in an aqueous methanol solution under visible light irradiation.