In order to improve the electrochemical performance of the NiCo₂O₄ material, Ni ions were partially substituted with Cu²⁺ ions having excellent reducing ability. All of the electrodes were fabricated by growing the Ni_1−xCu_xCo₂O₄ electrode spinel-structural active materials onto the graphite felt (GF). Five types of electrodes, NiCo₂O₄/GF, Ni_0.875Cu_0.125Co₂O₄/GF, Ni_0.75Cu_0.25Co₂O₄/GF, Ni_0.625Cu_0.375Co₂O₄/GF, and Ni_0.5Cu_0.5Co₂O₄/GF, were prepared for application to the oxygen evolution reaction (OER). As Cu²⁺ ions were substituted, the electrochemical performances of the NiCo₂O₄-based structures were improved, and eventually the OER activities were also greatly increased. In particular, the Ni_0.75Cu_0.25Co₂O₄/GF electrode exhibited the best OER activity in a 1.0 M KOH alkaline electrolyte: the cell voltage required to reach a current density of 10 mA cm⁻² was only 1.74 V (η = 509 mV), and a low Tafel slope of 119 mV dec⁻¹ was obtained. X-ray photoelectron spectroscopy (XPS) analysis of Ni_1−xCu_xCo₂O₄/GF before and after OER revealed that oxygen vacancies are formed around active metals by the insertion of Cu ions, which act as OH-adsorption sites, resulting in high OER activity. Additionally, the stability of the Ni_0.75Cu_0.25Co₂O₄/GF electrode was demonstrated through 1000th repeated OER acceleration stability tests with a high faradaic efficiency of 94.3%.

One dimensional (1D) metal sulfide nanostructures are one of the most promising materials for photocatalytic water splitting reactions to produce hydrogen (H₂). However, tuning the nanostructural, optical, electrical and chemical properties of metal sulfides is a challenging task for the fabrication of highly efficient photocatalysts. Herein, 1D CdS nanorods (NRs) were synthesized by a facile and low-cost solvothermal method, in which reaction time played a significant role for increasing the length of CdS NRs from 100 nm to several micrometers. It is confirmed that as the length of CdS NR increases, the visible photocatalytic H₂ evolution activity also increases and the CdS NR sample obtained at 18 hr. reaction time exhibited the highest H₂ evolution activity of 206.07 μmol.g⁻¹.h⁻¹. The higher H₂ evolution activity is explained by the improved optical absorption properties, enhanced electronic bandstructure and decreased electron-hole recombination rate.

In this study, we examined whether catalysts with many defects have excellent photoactivity. We prepared ZnO nanoplates with varying degrees of defects in a short time of 4 h by varying the crystal growth temperature at 50, 100, 150, and 200 °C under a strong alkali NaOH atmosphere of 4.0 M. During high-temperature preparation of ZnO, crystal defects were reduced and crystallinity was further increased. In crystallized systems over 100 °C, rhombic nanoplates were used to control particle shape and induce growth in only two axes. The PL, Raman, and XPS analyses confirmed the presence of strong oxygen vacancies in all ZnO nanoplates, and the vacancies decreased with increasing crystallization temperatures. Methylene blue (MB) dye was initially fixed at 50 mg/L with a peak decrease in absorption at 600–700 nm, confirming its decomposition over time. For the 5 h reaction, the MB removal concentration follows the following order: ZnO-50 < ZnO-100 < ZnO-150 < ZnO-200. The study confirms that ZnO-200 nanoplates with fewer oxygen vacancies decompose MB more quickly. ZnO-200 nanoplates synthesized at 200 °C provided the best sterilization performance when tested against gram-positives and gram-negatives, Escherichia coli and Staphylococcus aureus, respectively. ZnO-200 nanoplates after 3 h showed a high sterilization performance of 96.95% (86.67% in a dark room) for staphylococcus aureus and 95.82% (74.66% in a dark room) for Escherichia coli when irradiated with light. Particularly noteworthy in this study is that ·OH and ·O²⁻ radicals are generated more strongly in ZnO-200 than in ZnO-50 nanoplates. These results show that too-strong oxygen vacancies rather inhibit the antibacterial performance, and that the virtue of moderation also exists in the catalytic activity.