A porous ZnO nanosheet with a near-rectangular morphology has been successfully prepared through a simple solvothermal-annealing method using Zn₅(OH)₆(CO₃)₂ as a pore-directing agent. Moreover, the features of ZnO can be easily tuned by changing the annealing temperature. The evolution of defects along with the increase of annealing temperature has been revealed as follows: the content of surface oxygen vacancy of the as-prepared samples first increases and then decreases, however, the content of impurities decreases gradually. A clear relationship between the defects and photoluminescence/photocatalytic characteristics of ZnO is observed. The defect-related emission mechanism of the visible photoluminescence (PL) for the as-prepared ZnO samples has been proposed. In addition, the samples also show good activities for photo-degradation of phenol under UV light irradiation. ZnO-500 °C (annealed at 500 °C) exhibits the best photocatalytic activity, which is superior to that of commercial ZnO. The photocatalytic activity can be greatly influenced by the relative concentration ratio of surface defects to bulk defects, crystallization performance and specific surface area.

The oxygen reduction reaction (ORR) is a key component for many clean energy technologies and other industrial processes. However, the low selectivity and the sluggish reaction kinetics of ORR catalysts have hampered the energy conversion efficiency and real application of these new technologies mentioned before. Recently, tremendous efforts have been made in mechanism understanding, electrocatalyst development and system design. Here, a comprehensive and critical review is provided to present the recent advances in the field of the electrocatalytic ORR. The two-electron and four-electron transfer catalytic mechanisms and key evaluation parameters of the ORR are discussed first. Then, the up-to-date synthetic strategies and in situ characterization techniques for ORR electrocatalysts are systematically summarized. Lastly, a brief overview of various renewable energy conversion devices and systems involving the ORR, including fuel cells, metal–air batteries, production of hydrogen peroxide and other chemical synthesis processes, along with some challenges and opportunities, is presented.