Unveiling an inverted S-scheme pathway in nickel oxide/titanium dioxide for enhanced photocatalytic oxidation of gaseous formaldehyde

Abstract

A heterojunction is constructed by integrating p-type NiO with n-type TiO₂. The resulting NiO@TiO₂ composite (referred to as x-NT, where x denotes the NiO/TiO₂ molar ratio) is experimentally and theoretically validated to function as an inverted p-to-n step (S)-scheme photocatalyst with NiO and TiO₂ serving as the reduction and oxidation photocatalysts, respectively. The x-NT is coated onto a honeycomb filter and integrated into an air purifier (AP) for the photocatalytic oxidation (PCO) of gaseous formaldehyde (FA) in air. 4-NT exhibits superior optoelectronic properties compared to TiO_2, such as a higher photocurrent density (153.71 μA vs. 38.62 μA) and lower charge transfer resistance (83.7 Ω vs. 99.73 Ω). Under optimized conditions, AP (4-NT) achieves 100 % degradation of FA (1 ppm) with a clean air delivery rate of 10.35 L min⁻¹ and an apparent quantum yield of 6.34 × 10⁻² %. In situ diffuse reflectance infrared Fourier transform spectroscopy and electron paramagnetic resonance analyses reveal that FA is oxidized to yield H₂O and CO₂ through a sequence of intermediates (DOM → HCOO⁻ → CO₃²⁻), driven by reactive oxygen species (•O₂⁻ and •OH). This work introduces a novel inverted S-scheme design in a practical air purification platform, bridging material innovation with scalable VOC control.