Photocatalytic Degradation of Organic Dye Pollutants in Water Using La-Doped ZnO Nanocomposites Under Visible and Solar Light Irradiation

Abstract ZnO stands out as one of the most widely employed photocatalysts among n-type semiconductors, owing to its favorable physicochemical attributes including its band gap, binding energy, room-temperature optical gain, non-toxicity, biocompatibility, and cost-effectiveness. The f-orbital interactions of rare earth elements with functional groups present in dyes enable complex formation and effectively reduce the photocatalyst band gap upon doping. In this investigation, nano ZnO was functionalized by incorporating Lanthanum (La) at varying concentrations from 1 to 5 wt.% via the sol-gel technique. The physicochemical properties—including crystal structure, morphology, surface area, absorption maximum, band gap, emission characteristics, and surface composition—of the synthesized nano ZnO and La-doped nano ZnO were thoroughly analyzed using a range of characterization techniques. XRD data confirmed retention of the ZnO Wurtzite hexagonal phase across all La-doped samples; the average crystallite size progressively diminished from 19.5 to 9.8 nm as La content rose from 1 to 5 wt.%, while the band gap (determined by UV-DRS) decreased correspondingly. Photoluminescence (PL) intensity also declined with increasing La content. The photodegradation performance of the synthesized photocatalysts was evaluated against organic dye solutions. All tested catalysts retained consistent activity across five successive reaction cycles. Radical trapping experiments using appropriate scavengers confirmed that photogenerated electron-hole pairs, superoxide radicals, and hydroxyl radicals participate in the degradation pathway.