Plasmonic and magnetic ZnO-based nanocomposites for enhanced photocatalysis and ultrasensitive SERS detection of malachite green

Abstract Water pollution arising from persistent organic dyes constitutes a critical environmental and public health concern. In this work, ZnO nanorods (NRs), Fe 3 O 4 nanoparticles (NPs), and their corresponding Ag@ZnO and Fe 3 O 4 @ZnO nanocomposites were successfully fabricated via a facile and cost-effective microwave-assisted hydrothermal approach. The photocatalytic activities of ZnO NRs, Fe 3 O 4 NPs, and the as-prepared nanocomposites, including Ag@ZnO, were systematically evaluated for the degradation of malachite green (MG) dye. In addition, Ag@ZnO was further employed as an efficient active substrate for surface-enhanced Raman spectroscopy (SERS) sensing, demonstrating its dual functionality in both photocatalytic degradation and sensing applications. Structural, morphological, and optical properties were comprehensively characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman spectroscopy, diffuse reflectance spectroscopy (DRS), photoluminescence (PL), transmission electron microscopy (TEM), and dynamic light scattering (DLS) analysis. The XRD analysis confirmed the hexagonal wurtzite structure for both ZnO NRs and Ag@ZnO nanocomposites, in addition to the cubic structure for Fe 3 O 4 NPs. The FTIR and Raman studies confirmed the formation of ZnO, Ag@ZnO, and Fe 3 O 4 NPs by revealing their distinct characteristic vibrational bands. Zeta potential results display the negative surface charge of Fe 3 O 4 NPs, indicating their strong adsorption affinity toward positively charged malachite green (MG) dye. The DRS and PL results reveal the effect of Ag NPs on improving the optical property of ZnO and reducing the (e–h) recombination rate. Photocatalytic experiments under visible-light irradiation showed that ZnO achieved 55% degradation of MG within 90 min, whereas complete degradation was achieved using Ag@ZnO and Fe 3 O 4 @ZnO nanocomposites, highlighting their superior photocatalytic performance. In addition, Ag@ZnO exhibited excellent SERS activity, enabling the detection of MG down to 2 ppm, attributed to strong localized surface plasmon resonance and interfacial charge-transfer effects. The combined photocatalytic efficiency, adsorption capability, and SERS sensitivity demonstrate that Ag@ZnO and Fe 3 O 4 @ZnO nanocomposites are promising multifunctional materials for wastewater treatment and trace-level pollutant detection.