In situ ALD growth of ZnO thin films incorporating Cu nanoparticles for Solar-driven photocatalysis

This study investigates ZnO thin films incorporating copper nanoparticles (Cu NPs) as efficient photocatalysts for solar-driven wastewater treatment. The ZnO@Cu composite films were fabricated by in-situ integration of Cu NPs into the ZnO during atomic layer deposition (ALD) growth. Structural characterization by X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy confirmed the preservation of the wurtzite ZnO structure, with metallic Cu NPs of an average size of around 80 nm embedded into the semiconductor matrix. UV–vis spectroscopy demonstrated enhanced visible-light absorption by the ZnO@Cu films - which is attributed to the localized surface plasmon resonance in Cu nanoparticles, while Kelvin probe force microscopy revealed local variations in ZnO surface potential in the vicinity of Cu NPs, indicating electron transfer from ZnO to Cu and the formation of a Schottky-type barrier at ZnO/Cu interface. Photocatalytic degradation of methylene blue (MB), Rhodamine B (RhB), and caffeine under simulated solar irradiation demonstrated significantly enhanced photocatalytic performance of the ZnO@Cu films compared to pristine ZnO. The ZnO@Cu photocatalyst achieved degradation efficiencies of 78.2%, 89.6%, and 38.5% for MB, RhB, and caffeine, respectively, in contrast to pure ZnO, which showed degradation efficiencies of 44.4% (MB), 42.4% (RhB), and 14.2% (caffeine), confirming the effectiveness of the ZnO@Cu composite toward both dye and emerging organic pollutants. The improved photocatalytic performance is attributed to the injection of hot-electrons from Cu NPs into the ZnO conduction band, as revealed from our photoconductivity measurements, which show nearly two orders of magnitude larger photoinduced carrier generation in ZnO@Cu films under simulated sunlight illumination. These results demonstrate that Cu NPs effectively enhance charge-carrier generation and utilization in ZnO, leading to improved solar-driven photocatalytic activity.