Mid-gap states induced by sulfur substitution in ZnO nanoparticle via low solvothermal temperature for enhanced photocatalytic NO_x removal under visible light irradiation

The photocatalytic efficiency of ZnO nanoparticles is limited by their wide band gap energy, which restricts activation to UV light irradiation. To address this limitation, sulfur atoms were incorporated into the crystal structure of ZnO using a low temperature solvothermal process to introduce new energy states into the band structure. The study revealed that sulfur incorporation significantly affects the crystallite size, particle size, and surface area of ZnO nanoparticles. More importantly, sulfur incorporation reduces the band gap of ZnO, attributed to mid-gap states generated by the S2p orbitals, thereby extending light absorption into the visible light region. Photocatalytic experiments using NO_x demonstrated that sulfur-doped ZnO exhibits enhanced overall photocatalytic performance under both UV and visible light irradiation. However, the improvement under UV light (approximately 42 %) is primarily attributed to the increased surface area of the S-doped ZnO sample. In contrast, the visible light photocatalytic activity of S-doped ZnO showed a remarkable enhancement compared to pure ZnO, achieving a 386 % improvement. This significant enhancement is ascribed to the formation of mid-gap states in the band structure, which lowers the activation energy and improves photocatalytic performance. This research demonstrates the potential for developing a visible light-active photocatalyst with low energy consumption by sulfur doping, paving the way for further practical applications.