Tunable structural, optical, and photocatalytic properties of sulfur-doped NiO thin films: influence of doping concentration and scavenger-assisted mechanism analysis

In this study, we systematically investigated the impact of sulfur (S) doping on the structural, microstructural, optical, and photocatalytic properties of nickel oxide (NiO) thin films synthesized via the dip-coating technique. X-ray diffraction (XRD) analysis confirmed the preservation of the cubic NiO phase upon doping, while revealing secondary sulfur-related phases and peak broadening due to lattice strain. Crystallite size decreased from 28 nm (pure NiO) to 12 nm (2% S-NiO), alongside a rise in dislocation density from 0.00127 to 0.0069 and an increase in lattice strain from 0.37% to 0.84%. This reduction in crystallite size originates from the substitution of oxygen by sulfur atoms, generating local lattice distortions that inhibit grain growth and promote microstrain within the crystal lattice. SEM analysis showed that sulfur incorporation transformed the film morphology from uniform grains to porous, crack-filled structures, particularly at higher doping levels. This morphological evolution is mainly attributed to the differential evaporation rate of sulfur-containing precursors during thermal treatment, which induces localized stress and pore formation. EDAX spectra verified sulfur incorporation, with its intensity increasing proportionally to doping concentration. Optical characterization revealed a decline in transmittance (from ~80% in pure NiO to ~55% at 10% S) and a redshift in the absorption edge, confirming bandgap narrowing. The redshift of the optical edge indicates the formation of defect states and impurity levels inside the bandgap, leading to enhanced visible-light absorption. The refractive index rose from 1.78 (5% S-NiO) to 1.94 (10% S-NiO), indicating enhanced optical density and polarizability. Photocatalytic performance, evaluated via methylene blue degradation, improved significantly with doping. The degradation rate increased from 10% (pure NiO) to a maximum of 29% (10% S-NiO) under visible light. Although the overall efficiency remains moderate, this visible-light activation represents a meaningful advancement since NiO generally exhibits higher activity under UV irradiation only. Kinetic analysis showed that 2% S-NiO achieved an R² of 0.9894 with an unconventional high activation energy (−75.364 kJ/mol), suggesting altered charge dynamics. This negative activation energy reflects an inverse temperature dependency, implying a complex balance between adsorption–desorption processes and charge transfer kinetics during photodegradation. This study confirms that sulfur doping is a promising route to tailor NiO’s properties for optoelectronic and photocatalytic applications. NiO thin films were also investigated for their photocatalytic activity under UV light using various scavengers to identify the dominant reactive species. The results revealed that superoxide radicals (•O₂⁻) play the primary role in the degradation process, with ascorbic acid showing the highest inhibition effect. This finding confirms the superoxide-driven degradation mechanism and supports the proposed charge transfer pathway in doped NiO systems.