ZnO nanoparticles (NPs) are synthesized by deoxidizing ZnO powder in a vacuum drying process. This process reduces the size of the NPs and increases the concentration of oxygen vacancies on their surfaces. ZnO NPs with sufficient oxygen vacancies are highly effective for the photodecomposition of methylene blue (MB) dye in water under ultraviolet irradiation. The MB degradation efficiency exceeds 99 percent after 50 min of light irradiation, and the catalytic property of the NPs remains stable over several complete degradation cycles. It is revealed that the concentration of oxygen vacancies on the surface, and the photocatalytic activity, are both higher for smaller NPs. Oxygen vacancies reduce the recombination rate of photo-generated charge carriers by capturing the electrons and hence, improve the efficiency of redox reactions. In addition, a smaller particle size leads to a larger specific surface area and a higher photonic efficiency for the ZnO NPs.

Practical, efficient synthesis of metal oxide nanocrystals with good crystallinity and high specific surface area by a modified polymer-network gel method is demonstrated, taking ZnO nanocrystals as an example. A novel stepwise heat treatment yields significant improvement in crystal quality. Such nanophase materials can effectively degrade common organic dyes under solar radiation and can perform very well in photo-assisted detection of NO₂ gas. Other typical metal oxide nanocrystals with good crystallinity and high specific surface area were also synthesized successfully under similar conditions. This work provides a general strategy for the synthesis of metal oxide nanocrystals, balancing the crystallinity and specific surface area.

Heterogeneous photocatalytic processes have exhibited tremendous potential in the field of solar energy and environmental remediation. Next to TiO₂, ZnO has been widely used as a promising photocatalyst material in recent years. However, several noticeable drawbacks make it less effective, such as poor solar-light harvesting and rapid recombination of photo-excited charge carriers. To overcome these drawbacks, the decoration of noble metal nanoparticles on ZnO surfaces is a feasible strategy due to its localized surface plasmon resonance absorption, which aggrandizes the visible absorption of ZnO. In addition, the formation of the Schottky barrier at the metal-semiconductor interface can effectively separate photo-induced charge carriers. Among theses noble metals, due to the high conductivity, antimicrobial activity, mechanical thermal strength, and electrical double layer capacitance of silver (Ag), Ag-decorated ZnO (ZnO/Ag) nano-photocatalysts have exhibited remarkable photocatalytic efficiency and high cost performance for the degradation of organic pollutants, endowing them with growing potential in applications. In this review, we focus on the recent advancements (including the fundamental degradation mechanisms, synthesis methods, corresponding applications, and optimized strategies ) in ZnO/Ag-based photocatalysts for visible light-driven photocatalytic degradation, and conclude with a perspective on the future direction and prospects for these types of nanocomposite photocatalysts.

The construction of heterostructured photocatalyst with an appropriate energy band structure will help realize highly efficient photo-excited charge separation. In this study, ternary CuO/CeO₂/ZnO nano-particle (NP) composites were synthesized by a facile two-step sol-gel method, which exhibit significantly enhanced photocatalytic degradation performance for various organic pollutants under UV and visible light excitation. The photo-responses to both UV and visible light, as well as the visible light absorption and utilization rates of ZnO are found to be synergistically intensified by CeO₂ and CuO co-coupling. The first-order kinetic constants (K) of 3%CuO/CeO₂/ZnO for methylene blue (MB) degradation are ~3.9, ~4.1 and ~4.8 times higher than ZnO under UV light, visible light and simulated sunlight illumination, respectively. The roles of CuO and CeO₂ in optical properties and photo-degradation under UV and visible light were explored. Besides, the photogenic holes (h⁺) of ZnO, CeO₂, and the produced hydroxyl radicals (·OH) are proved to be the main active species under UV light. Dissimilarly, under visible light, the superoxide radicals (·O₂⁻) formed by the reactions between oxygen molecules and the photo-generated electrons (e⁻) of CuO moving towards the catalysts surface are also found to be important for promoting dye decomposition. The improved photo-responses, the well-matched band structure that facilitates charge transfer processes, and the highly efficient utilization of the photo-excited carriers of the ternary nano-heterostructure are suggested to be the key factors for the remarkable enhancement of photocatalytic performance of ZnO nano-photocatalyst. This work offers a low-cost strategy to acquire highly active UV and visible light-driven photocatalyst.

Selectively harnessing photo-induced carriers to control surface photo-redox reactions can enable currently limited specificity in photocatalytic applications. By using a new approach to switching between dominant electron and hole charge transfer on the surfaces of metal oxide nanocrystals, depending on the optimal carrier for specific application functionality in photocatalytic pollutant degradation, H₂ production, CO₂ reduction, and gas sensing. The approach is based on the surface redox properties of custom-designed p-n hetero-structured hybrid nanoparticles (NPs) containing copper oxide, and wide-gap metal oxide semiconductors (MOSs). The customized Cu_xO/ZnO (CXZ) heterostructures ensure effective charge separation and surface reactions driven by UV-vis excited highly reactive holes and show high performance in the photo-oxidative degradation of organic dyes and NO₂ gas sensing. By switching the dominant surface carrier type from holes to electrons, the hybrids exhibit excellent performance in photocatalytic H₂ evolution and CO₂ reduction. This work offers a generic approach to engineering multipurpose photocatalytic materials.