Effect of Defects on Optical, Electronic, and Interface Properties of NiO/SnO₂ Heterostructures: Dual-Functional...

Photocatalytic H₂ evolution and organic pollutant oxidation have witnessed a radical surge in recent times. However, this integration demands spatial charge separation and unique interface properties for a trade-off between oxidation and reduction reactions. In the current work, defect engineering of NiO/SnO₂ nanoparticles aided in altering the optoelectronics and interface properties and enhanced photocatalytic activity. After annealing the catalysts in a N₂ atmosphere, the hydroxyl groups were replaced by water molecules through surface modification. The photoexcited holes accumulated on SnO₂ break the water molecules and facilitate the reduction of protons on NiO; this is known as spatial separation. Meanwhile, direct hole oxidation, an oxygen reduction reaction, ensures the degradation activity in this 2-fold system. By defect engineering, the limitations of SnO₂ such as higher H₂O adsorption, wide bandgap (reduced from 3.02 to 1.88 eV), and electronic properties were addressed. The H₂ production in the current work has attained a value of 3732 μmol/(g h), which is 2.9 times that of the previous best reported under sunlight. Recyclability tests confirmed the stability of vacancies by promoting the reoxidation of defect states during photocatalytic activity. Additionally, efforts were made to study the effect of defect density on the photocurrent, the electrical resistance, and the mechanism of photocatalytic reactions. Electrochemical characterizations, UPS, XPS, UV-DRS, and PL were employed to understand the influence of defects on the bandgap, charge recombination, charge transport, charge carrier lifetime, and the interface properties that are responsible for photocatalytic activity. In this regard, it was understood that maintaining the optimal defect concentration is important for higher photocatalytic efficiencies, as the defect optimality preserves key photocatalytic properties. Apart from characterizations, the photocatalytic results suggest that excess defect density triggers the undesired thermodynamically favored back reactions, which greatly hampered the H₂ yield of the process.