Recent Progress in Metal Oxide-Based Photocatalysts for CO2 Reduction to Solar Fuels: A Review

One of the challenges in developing practical CO₂ photoconversion catalysts is the design of materials with a low cost, high activity and good stability. In this paper, excellent photocatalysts based on TiO₂, WO₃, ZnO, Cu₂O and CeO₂ metal oxide materials, which are cost-effective, long-lasting, and easy to fabricate, are evaluated. The characteristics of the nanohybrid catalysts depend greatly on their architecture and design. Thus, we focus on outstanding materials that offer effective and practical solutions. Strategies to improve CO₂ conversion efficiency are summarized, including heterojunction, ion doping, defects, sensitization and morphology control, which can inspire the future improvement in photochemistry. The capacity of CO₂ adsorption is also pivotal, which varies with the morphological and electronic structures. Forms of 0D, 1D, 2D and 3DOM (zero/one/two-dimensional- and three-dimensional-ordered macroporous, respectively) are involved. Particularly, the several advantages of the 3DOM material make it an excellent candidate material for CO₂ conversion. Hence, we explain its preparation method. Based on the discussion, new insights and prospects for designing high-efficient metallic oxide photocatalysts to reduce CO₂ emissions are presented.

A Study on Doping and Compound of Zinc Oxide Photocatalysts

As an excellent semiconductor photocatalyst, zinc oxide is widely used in the field of photocatalysis and is regarded as one of the most reliable materials to solve environmental problems. However, because its band gap energy limits the absorption of visible light and reduces the efficiency of catalytic degradation, it needs to be doped with other substances or compounded with other substances and precious metal. This paper summarizes the research on this aspect at home and abroad in recent years, introduces the doping of transition metal ions by zinc oxide, the compounding of zinc oxide with precious metals or other semiconductors, and the prospect of further improving the catalytic efficiency of zno photocatalyst is also put forward.

Solution Processed Zn1-x-ySmxCuyO Nanorod Arrays for Dye Sensitized Solar Cells

Cu- and Sm-doped ZnO nanorod arrays were grown with 1 wt% of Sm and different weight percents (0.0, 0.5, 1.0 and 1.5 wt%) of Cu by two-step hydrothermal method. The influence of Cu concentration and precursor of Sm on the structural, optical and photovoltaic properties of ZnO nanorod arrays was investigated. An X-ray diffraction study showed that the nanorod arrays grown along the (002) plane, i.e., c-axis, had hexagonal wurtzite crystal structure. The lattice strain is present in all samples and shows an increasing trend with Cu/Sm concentration. Field emission scanning electron microscopy was used to investigate the morphology and the nanorod arrays grown vertically on the FTO substrates. The diameter of nanorod arrays ranged from 68 nm to 137 nm and was found highly dependent on Cu concentration and Sm precursor while the density of nanorod arrays almost remains the same. The grown nanorod arrays served as photoelectrodes for fabricating dye-sensitized solar cells (DSSCs). The overall light to electricity conversion efficiency ranged from 1.74% (sample S₁, doped with 1 wt% of Sm and 0.0 wt% of Cu) to more than 4.14% (sample S₄, doped with 1 wt% of Sm and 1.5 wt% of Cu), which is 60% higher than former sample S1. The increment in DSSCs efficiency is attributed either because of the doping of Sm³⁺ ions which increase the absorption region of light spectrum by up/down conversion or the doping of Cu ions which decrease the recombination and backward transfer of photo-generated electrons and increase the electron transport mobility. This work indicates that the coupled use of Cu and Sm in ZnO nanorod array films have the potential to enhance the performance of dye-sensitized solar cells.

GdFeO3 Perovskite Oxide Decorated by Group X Heterometal Oxides and Bifunctional Oxygen Electrocatalysis

Bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are necessary in the renewable energy systems. However, the kinetically slow and large energy-demanding procedures of oxygen electrocatalysis make the preparation of bifunctional catalysts difficult. In this work, we report a novel hierarchical GdFeO₃ perovskite oxide of a spherelike nanostructure and surface modification with the group X heterometal oxides. The nanostructured GdFeO₃ layer behaved as a bifunctional electrocatalyst in the oxygen electrocatalysis of OER and ORR. Moreover, the surface decoration with catalytically active PtO_x + Ni/NiO nanoparticles enhanced the electrocatalytic performances substantially. Incorporation of mesoporous PtO_x + Ni/NiO nanoparticles into the porous GdFeO₃ nanostructure enlarged the electrochemically active surface area and provided the interconnected nanostructures to facilitate the OER/ORR. The nanostructures were visualized by scanning electron microscopy and transmission electron microscopy images, and the surface area and pore size of nanoparticles were analyzed from N₂ adsorption/desorption isotherms. Tafel analysis indicates that surface modification effectively improves the kinetics of oxygen reactions and accordingly increases the electrocatalytic efficiency. Finally, the 2 wt % PtO_x + NiO|GdFeO₃ (x = 0, 1, and 2) electrode achieved the enhanced OER performance with an overpotential of 0.19 V at 10 mA/cm² in an alkaline solution and a high turnover frequency of 0.28 s^-1 at η = 0.5 V. Furthermore, the ORR activity is observed with an onset potential of 0.80 V and a half-wave potential (E_1/2) of 0.40 V versus reversible hydrogen electrode.

ZnO@CdS heterostructures: an efficient photoanode for photoelectrochemical water splitting

CdS nanoparticles attached to ZnO 2D sheets help to improve light absorbance, leading to enhanced photoelectrochemical water splitting performance.