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

Energy storage devices that can deliver high powers have many applications, including hybrid vehicles and renewable energy. Much research has focused on increasing the power output of lithium batteries by reducing lithium-ion diffusion distances, but outputs remain far below those of electrochemical capacitors and below the levels required for many applications. Here, we report an alternative approach based on the redox reactions of functional groups on the surfaces of carbon nanotubes. Layer-by-layer techniques are used to assemble an electrode that consists of additive-free, densely packed and functionalized multiwalled carbon nanotubes. The electrode, which is several micrometres thick, can store lithium up to a reversible gravimetric capacity of approximately 200 mA h g(-1)(electrode) while also delivering 100 kW kg(electrode)(-1) of power and providing lifetimes in excess of thousands of cycles, both of which are comparable to electrochemical capacitor electrodes. A device using the nanotube electrode as the positive electrode and lithium titanium oxide as a negative electrode had a gravimetric energy approximately 5 times higher than conventional electrochemical capacitors and power delivery approximately 10 times higher than conventional lithium-ion batteries.

High-Power Lithium Batteries from Functionalized Carbon Nanotube Electrodes

As one important component of sulfur cathodes, the carbon host plays a key role in the electrochemical performance of lithium-sulfur (Li-S) batteries. In this paper, a mesoporous nitrogen-doped carbon (MPNC)-sulfur nanocomposite is reported as a novel cathode for advanced Li-S batteries. The nitrogen doping in the MPNC material can effectively promote chemical adsorption between sulfur atoms and oxygen functional groups on the carbon, as verifi ed by X-ray absorption near edge structure spectroscopy, and the mechanism by which nitrogen enables the behavior is further revealed by density functional theory calculations. Based on the advantages of the porous structure and nitrogen doping, the MPNC-sulfur cathodes show excellent cycling stability (95% retention within 100 cycles) at a high current density of 0.7 mAh cm −2 with a high sulfur loading (4.2 mg S cm −2 ) and a sulfur content (70 wt%). A high areal capacity (≈3.3 mAh cm −2 ) is demonstrated by using the novel cathode, which is crucial for the practical application of Li-S batteries. It is believed that the important role of nitrogen doping promoted chemical adsorption can be extended for development of other high performance carbon-sulfur composite cathodes for Li-S batteries.

Nitrogen-Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of High-Areal-Capacity Sulfur Cathode with Exceptional Cycling Stability for Lithium-Sulfur Batteries

A novel graphene-bridged Ag3PO4/Ag/BiVO4 (040) Z-scheme heterojunction with excellent visible-light-driven photocatalytic performance was fabricated using a facile in situ deposition method followed by photo-reduction. The as-obtained nanocomposite was employed to degrade tetracycline (TC) in water under visible light irradiation. Compared to pure BiVO4, Ag3PO4 and other nanocomposites, Ag/Ag3PO4/BiVO4/RGO displayed more superior photodegradation efficiency with 94.96% removal of TC (10 mg/L) in 60 min, where the optimal conditions was catalysis dosage 0.50 g/L and initial pH at ca. 6.75. The influences of TC concentrations, light irradiation condition, coexistence ions and water sources were also investigated in details. The enhanced photocatalytic activities could be attributed to the suppression of charge recombination, high specific surface area and desirable absorption capability of Ag/Ag3PO4/BiVO4/RGO, which were in sequence confirmed by PL, PC, EIS, BET and DRS tests. The synergistic effects of RGO and Ag/Ag3PO4 in the hybrid could also contribute to the improved photo-stability and recyclability towards TC decomposition. In addition, radical trapping experiments and ESR measurement revealed that the photo-induced active species superoxide radical ( O2−) and holes (h+) were the predominant active species in the photocatalytic system. The Ag/Ag3PO4/BiVO4/RGO nanocomposite also possessed desirable photocatalytic performance on the degradation of TC from real wastewater, further verifying its potential in practical industries. This work provides a promising approach to construct visible-light response and more stabilized nanocomposite photocatalysts applied in efficient treatment of persistent pollutants in wastewater.

Hierarchical assembly of graphene-bridged Ag3PO4/Ag/BiVO4 (040) Z-scheme photocatalyst: An efficient, sustainable and heterogeneous catalyst with enhanced visible-light photoactivity towards tetracycline degradation under visible light irradiation

Room-temperature in situ fabrication of Bi 2 O 3 /g-C 3 N 4 direct Z-scheme photocatalyst with enhanced photocatalytic activity

MoS2/CdS-TiO2 nanofibers have been successfully prepared by an electrospinning mediated photodeposition method for improving the interfacial contact between MoS2 and CdS-TiO2 nanofibers. The structural features, morphologies, and photo-absorption performances of the as-prepared samples were investigated in detail. The photocatalytic activity of a typical sample 1%MoS2/CdS-TiO2(60) nanofibers for H2 evolution under visible light irradiation exhibits 3.0 and 4.0 times higher than that of 1%MoS2/CdS-P25(60) and 1%Pt/CdS-TiO2(60), respectively. Based on the above results, it reveals that one dimensional TiO2 nanofibers can serve as excellent substrates for improving the dispersion of CdS and MoS2. Moreover, the intimate interaction among MoS2, CdS and TiO2 may be suggested by HRTEM images and elements mapping patterns. Further photoelectrochemical analyses show that the effective separation of photogenerated carriers can be markedly accelerated. Finally, a tentative photocatalytic reaction mechanism has also been investigated. This work demonstrates that the method of combining electrospinning and photodeposition is feasible for the preparation of an effective MoS2/CdS-TiO2 nanofibers composite photocatalyst.

Highly efficient photocatalytic H2 evolution over MoS2/CdS-TiO2 nanofibers prepared by an electrospinning mediated photodeposition method

Active, stable, and cost-effective electrocatalysts are attractive alternatives to the noble metal oxides that have been used in water splitting The direct nucleation and growth of electrochemically active LDH materials on chemically modified MWCNTs exhibit considerable electrocatalytic activity toward oxygen evolution from water oxidation CoMn-based and NiMn-based hybrids were synthesized using a facile chemical bath deposition method and the as-synthesized materials exhibited three-dimensional hierarchical configurations with tunable Co/Mn and Ni/Mn ratio Benefiting from enhanced electrical conductivity with MWCNT backbones and LDH lamellar structure, the Co5Mn-LDH/MWCNT and Ni5Mn-LDH/MWCNT could generated a current density of 10 mA cm–2 at overpotentials of ∼300 and ∼350 mV, respectively, in 1 M KOH In addition, the materials also exhibited outstanding long-term electrocatalytic stability

Formation of Hierarchical Structure Composed of (Co/Ni)Mn-LDH Nanosheets on MWCNT Backbones for Efficient Electrocatalytic Water Oxidation.

A highly effective Iron Metal-Framework photocatalyst (MIL-68(Fe)) has been successfully prepared via a facile solvothermal method under acidic condition. The UV–vis diffuse reflectance spectrum reveals that the absorption edge of MIL-68(Fe) is 440 nm. The flat-band potential of MIL-68(Fe) is −0.6 V vs. NHE at pH = 6.8, which is more negative than the redox potential of Cr(VI)/Cr(III) (+0.51 V, pH = 6.8). Consequently, it is thermodynamically permissible for the transformation of photogenerated electrons to the Cr(VI) to produce Cr(III). Moreover, MIL-68(Fe) could perform as an efficient photocatalyst towards the reduction of Cr(VI) aqueous with a wide pH range. After 5 min of visible light illumination (λ > 420 nm), almost 100% Cr(VI) can convert to Cr(III) with (NH 4 ) 2 C 2 O 4 as a scavenger (pH = 3), which is also higher than that of N-doped TiO 2 (50%) and ZnO (7.6%) under identical conditions. Furthermore, MIL-68(Fe) is proved to perform as a highly efficient photocatalyst for remove of different aqueous contaminant with malachite green (MG) as a scavenger. Finally, a possible reaction mechanism has also been investigated in detail.

Shiying Zhang

Papers

Room-temperature in situ fabrication of Bi 2 O 3 /g-C 3 N 4 direct Z-scheme photocatalyst with enhanced photocatalytic activity

Highly efficient photocatalytic H2 evolution over MoS2/CdS-TiO2 nanofibers prepared by an electrospinning mediated photodeposition method

Formation of Hierarchical Structure Composed of (Co/Ni)Mn-LDH Nanosheets on MWCNT Backbones for Efficient Electrocatalytic Water Oxidation.

MIL-68(Fe) as an efficient visible-light-driven photocatalyst for the treatment of a simulated waste-water contain Cr(VI) and Malachite Green

Bi 2 O 3 cocatalyst improving photocatalytic hydrogen evolution performance of TiO 2