Recent Advances in p-Type Nickel Oxide (NiO) for Solar-Light-Driven Photocatalytic Degradation of Organic Pollutants

The excessive discharge of contaminants into water systems poses significant challenges to water quality, necessitating sustainable and efficient degradation methods. While numerous techniques exist for wastewater treatment, most photocatalytic catalysts face limitations in cost-effectiveness and practical scalability. Nickel oxide (NiO), a p-type semiconductor with strong oxidation capacity and chemical stability, has emerged as a promising candidate for organic pollutant degradation. However, its intrinsic wide bandgap (~ 3.6–4.0 eV)​ restricts pure NiO to UV light absorption​ (< 400 nm), limiting its solar-driven efficiency. Recent advances demonstrate that structural modifications, including heterojunction engineering and doping can effectively address these limitations by tailoring the NiO electronic structure and charge dynamics. Sulphur/Nitrogen (S/N) co-doping​ reduces the bandgap of NiO from 3.6 eV to 2.50 eV, enabling 98.9% degradation of methylene blue (MB) under sunlight within 60 min. Similarly, different heterojunction systems narrow the bandgap via enhanced charge separation. The hybrid carbon composites that create interconnected conductive networks, reduce charge-transfer resistance, and suppress electron–hole recombination, this approach primarily optimize surface reactions and charge migration pathways at the NiO interface. These modifications suppress electron–hole recombination (evidenced by photoluminescence quenching) and expand visible-light absorption into the 450–590 nm range. This review critically evaluates NiO electronic properties, heterojunction, doping, and hybrid carbon composites, highlighting how these strategies overcome intrinsic limitations. By integrating quantitative data on bandgap tuning and degradation kinetics, we provide a mechanistic framework for optimizing NiO-based photocatalysts toward sustainable environmental applications.