Visible Light Responsive Ni-Doped Hierarchical SnS₂ Nanoflowers for Photocatalytic Removal of Atmospheric NO_x Gas

Efficient removal of nitrogen oxides (NO_x) from urban air is a pressing environmental need. Here, Ni-doped SnS₂ nanoflowers are synthesized via a microwave-assisted solvothermal method and their visible-light-driven NO_x abatement performance is evaluated. X-ray diffraction and Raman confirm phase-pure hexagonal SnS₂, while X-ray photoelectron spectroscopy verifies successful Ni²⁺ incorporation without secondary phases. Ni doping slightly distorts the marigold flower-like morphology observed in pristine SnS₂ and narrows the bandgap (UV–vis), enhancing visible-light absorption and charge separation. Photoluminescence spectra reveal suppressed carrier recombination, with 5 wt% Ni-doped SnS₂ showing the lowest emission intensity. Photocatalytic tests demonstrate that this composition achieves the highest NO_x removal efficiency (14.2%) within 60 min, 1.6× higher than pristine SnS₂, while maintaining 70% activity after ten cycles. Theoretical calculations reveal Ni-3d/S-p hybridization and the introduction of mid-gap states, which lower the optical transition threshold and support the experimentally observed bandgap narrowing and enhanced light harvesting. A mechanism involving Ni-induced electron trapping and reactive oxygen species generation is proposed to explain the enhanced activity. This work establishes Ni doping as an effective strategy to tailor the structural, optical, and electronic properties of SnS₂, delivering a low-cost, stable, and highly active photocatalyst for sustainable NO_x mitigation.