Enhanced dielectric and photocatalytic properties of Li-doped ZnO nanoparticles for sustainable methylene blue degradation with reduced lithium environmental impact

Lithium−doped zinc oxide nanoparticles (LZO NPs) were synthesized via a facile co-precipitation method to enhance the photocatalytic performance of ZnO NPs. The effects of Li doping on the structural, morphological, optical, vibrational, electrical, and dielectric properties of ZnO were systematically investigated using various characterization techniques. Structural analysis, employing the Williamson-Hall method, revealed a reduction in crystallite size from 90.7 nm for ZnO to 53.7 nm for LZO upon Li incorporation. Transmission electron microscopy (TEM) confirmed a spherical nanoparticle morphology with increased agglomeration in the doped samples, while Fourier transform infrared spectroscopy (FTIR) verified the hexagonal wurtzite structure of LZO. UV–Vis spectroscopy indicated a decrease in the bandgap from 3.18 eV (ZnO) to 3.12 eV (LZO). Photoluminescence (PL) spectroscopy exhibited a redshift in UV emission and enhanced oxygen vacancy-related emissions in LZO, reflecting increased defect concentrations. Electrical and dielectric studies demonstrated improved conductivity and dielectric permittivity in LZO. The photocatalytic activity was assessed through the degradation of methylene blue (MB) under UV irradiation, with LZO achieving approximately 90 % degradation within 90 min and a first-order rate constant of 0.018 min −1 , compared to 0.008 min −1 for ZnO. This enhanced performance is attributed to the decreased crystallite size, reduced bandgap, increased defect concentration (notably oxygen vacancies), higher specific surface area (from 4.13 m 2 /g for ZnO to 38.12 m 2 /g for LZO), and elevated dielectric permittivity. Furthermore, LZO NPs exhibited excellent reusability, retaining approximately 82 % photocatalytic activity over six MB degradation cycles. These findings, coupled with the use of low Li-doping concentrations, demonstrate the potential of Li-doped ZnO for sustainable wastewater treatment and environmental remediation with minimized secondary pollutant generation.