Indoor solar-light-driven heterogeneous photocatalysis enabled by a Fresnel-lens/fiber-optic sunlight delivery system: CuOx/graphene-oxide decorated silicon nanowires for aqueous emerging pollutant degradation

Heterogeneous photocatalysis is often evaluated with artificial lamps, which do not reproduce the spectral profile and short-term variability of natural solar irradiation, a significant barrier to translating laboratory results into deployable water-treatment systems. Here, we experimentally integrate (i) Si NW -based immobilized photocatalytic surfaces and (ii) a solar-harvesting module that collects outdoor sunlight with a Fresnel lens and delivers part of it to a compact indoor reactor through an optical-fiber bundle. Si NWs were fabricated by metal-assisted chemical etching, decorated with CuO x nanoparticles, and coated with graphene oxide (GO) by atmospheric-pressure plasma jet deposition. Photocatalytic activity was assessed for methyl orange (20 ppm, 3 h) under three conditions: darkness, direct sunlight, and sunlight delivered via a Fresnel lens and an optical fiber, while irradiance and spectra were monitored to capture solar fluctuations. Under direct sunlight, Si NWs –CuO x achieved the highest degradation (≈86.9%), comparable to Si NWs (≈85.4%) and consistent with competitive performance against TiO₂ under realistic solar exposure. When sunlight was delivered through the Fresnel–fiber pathway, the irradiance at the reactor plane decreased markedly, and endpoint removal decreased for Si NWs , Si NWs –CuO x , and TiO₂ relative to direct sunlight. Nevertheless, substantial MO removal was still obtained under guided sunlight, especially for Si NWs –GO (≈64.5%), followed by Si NWs (≈59.2%), and Si NWs –CuO x (≈56.7%). Spectral analysis reveals a significant reduction in the UV fraction after light-guiding. At the same time, the visible range is comparatively preserved, which is consistent with the apparent optical response and low-reflectance behavior of the Si NW -based surfaces under genuine, dynamically varying solar irradiation.