S-Scheme Heterojunction Photocatalyst

The appropriate interfacial contact of heterojunction photocatalysts plays a critical role in transfer/separation of interfacial charge carriers. Design of two-dimensional (2D)/2D surface-to-surface heterojunction is an effective method for improving photocatalytic activity since greater contact area can enhance interfacial charge transfer rate. Herein, ultrathin 2D/2D WO3/g-C3N4 step-like composite heterojunction photocatalysts were fabricated by electrostatic self-assembly of ultrathin tungsten trioxide (WO3) and graphitic carbon nitride (g-C3N4) nanosheets. The ultrathin WO3 and g-C3N4 nanosheets were obtained by electrostatic-assisted ultrasonic exfoliation of bulk WO3 and a two-step thermal-etching of bulk g-C3N4, respectively. The thickness of ultrathin WO3 and g-C3N4 nanosheets are 2.5–3.5 nm, which is equivalent to 5–8 atomic or molecular layer thickness. This ultrathin layered heterojunction structure can enhance surface photocatalytic rate because photogenerated electrons and holes at heterogeneous interface more easily transfer to surface of photocatalysts. Therefore, the obtained ultrathin 2D/2D WO3/g-C3N4 step-scheme (S-scheme) heterojunction photocatalysts exhibited better H2-production activity than pure g-C3N4 and WO3 with the same loading amount of Pt as cocatalyst. The mechanism and driving force of charge transfer and separation in S-scheme heterojunction photocatalysts are investigated and discussed. This investigation will provide new insight about designing and constructing novel S-scheme heterojunction photocatalysts.

Ultrathin 2D/2D WO3/g-C3N4 step-scheme H2-production photocatalyst

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

To solve the problem of the global energy shortage and the pollution of the environment, in recent years, semiconductor photocatalytic technology that converts solar energy into chemical fuel has been widely studied. Regarding semiconductor-based photocatalysts, CdS has attracted extensive attention due to its relatively narrow bandgap for visible-light response and sufficiently negative potential of the conduction band edge for the reduction of protons. Studies have shown that CdS-based photocatalysts possess excellent photocatalytic performance in terms of solar-fuel generation and environmental purification. This critical review presents the recent advances and progress in the design and synthesis of various CdS and CdS-based photocatalysts. The basic physical and chemical properties of CdS and the related growth mechanism have been briefly summarized. Moreover, the applications of CdS-based photocatalysts have been discussed such as in photocatalytic hydrogen production, reduction of CO2 to hydrocarbon fuels and degradation of pollutants. Finally, a brief perspective on the challenges and future directions for the development of CdS and CdS-based photocatalysts are also presented.

CdS-Based photocatalysts

Direct Z-scheme photocatalysts: Principles, synthesis, and applications

Direct Z-scheme TiO2/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity

Co3 O4 /nitrogen-doped carbon hollow spheres (Co3 O4 /NHCSs) with hierarchical structures are synthesized by virtue of a hydrothermal method and subsequent calcination treatment. NHCSs, as a hard template, can aid the generation of Co3 O4 nanosheets on its surface; while SiO2 spheres, as a sacrificed-template, can be dissolved in the process. The prepared Co3 O4 /NHCS composites are investigated as the electrode active material. This composite exhibits an enhanced performance than Co3 O4 itself. A higher specific capacitance of 581 F g-1 at 1 A g-1 and a higher rate performance of 91.6% retention at 20 A g-1 are achieved, better than Co3 O4 nanorods (318 F g-1 at 1 A g-1 and 67.1% retention at 20 A g-1 ). In addition, the composite is employed as a positive electrode to fabricate an asymmetric supercapacitor. The device can deliver a high energy density of 34.5 Wh kg-1 at the power density of 753 W kg-1 and display a desirable cycling stability. All of these attractive results make the unique hierarchical Co3 O4 /NHCS core-shell structure a promising electrode material for high-performance supercapacitors.

Core–Shell Nitrogen-Doped Carbon Hollow Spheres/Co3O4 Nanosheets as Advanced Electrode for High-Performance Supercapacitor

Graphitic carbon nitride (g-C3N4) is an attractive photocatalyst which has appealing visible light absorption, outstanding layered porous structure, high stability and nontoxicity. Many experimental methods have been developed to modify the pristine g-C3N4 and enhanced photocatalytic activities have been witnessed. First-principle calculation based on density functional theory is an effective approach to investigate the structural, electronic, optical and thermodynamic properties of molecules and crystals, which provides important information to elucidate the improved photocatalytic activity of modified g-C3N4 at atomic or unit-cell levels, or even further, to predict the property and photocatalytic activity of experimentally un-synthesized g-C3N4-based photocatalysts. This review is dedicated to this important material, i.e. tri-s-triazine-based g-C3N4 and summarized a panorama of the major advances in the first-principle investigation. The existing challenges and future directions at the forefront of this emerging research hotpot have also been discussed.

First-principle calculation study of tri- s -triazine-based g-C 3 N 4 : A review

Self-assembled hierarchical direct Z-scheme g-C3N4/ZnO microspheres with enhanced photocatalytic CO2 reduction performance

Reasonable design of efficient hierarchical photocatalysts has gained significant attention. Herein, a step-scheme (S-scheme) core-shell TiO2 @ZnIn2 S4 heterojunction is designed for photocatalytic CO2 reduction. The optimized sample exhibits much higher CO2 photoreduction conversion rates (the sum yield of CO, CH3 OH, and CH4 ) than the blank control, i.e., ZnIn2 S4 and TiO2 . The improved photocatalytic performance can be attributed to the inhibited recombination of photogenerated charge carriers induced by S-scheme heterojunction. The improvement is also attributed to the large specific surface areas and abundant active sites. Meanwhile, S-scheme photogenerated charge transfer mechanism is testified by in situ irradiated X-ray photoelectron spectroscopy, work function calculation, and electron paramagnetic resonance measurements. This work provides an effective strategy for designing highly efficient heterojunction photocatalysts for conversion of solar fuels.

Liuyang Zhang

Papers

Direct Z-scheme TiO2/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity

Core–Shell Nitrogen-Doped Carbon Hollow Spheres/Co3O4 Nanosheets as Advanced Electrode for High-Performance Supercapacitor

First-principle calculation study of tri- s -triazine-based g-C 3 N 4 : A review

Self-assembled hierarchical direct Z-scheme g-C3N4/ZnO microspheres with enhanced photocatalytic CO2 reduction performance

In situ Irradiated XPS Investigation on S-Scheme TiO2@ZnIn2S4 Photocatalyst for Efficient Photocatalytic CO2 Reduction