Boosting visible-light photocatalysis with MWCNT-modified TiO2/SiO2/g-C3N4: efficient tetracycline removal in pure and hard water

In this study, we present a facile synthesis of a TiO₂/SiO₂/g-C₃N₄/MWCNT composite with dual functionality as both a photocatalyst and adsorbent for the effective elimination of pharmaceutical and dye pollutants from water. The composite was comprehensively characterized using FESEM, EDX, XRD, BET, FTIR, PL, and UV-Vis spectroscopy to elucidate its morphology, crystalline structure, and optical properties. The results revealed a hierarchical flower-like sphere/sheet architecture with an average crystallite size of about 21.3 nm. Multi-walled carbon nanotubes (MWCNTs) acted as visible-light photosensitizers, enhancing the separation efficiency of photogenerated electron-hole pairs. The influence of MWCNT content on photocatalytic activity was systematically investigated. The TiO₂/SiO₂/g-C₃N₄/MWCNT composite with 11 wt% CNT demonstrated superior visible-light degradation performance, achieving 92% efficiency for a 20 mg/L methylene blue (MB) aqueous solution significantly outperforming the MWCNT-free counterpart. Furthermore, the presence of MWCNTs remarkably improved tetracycline (TC) removal in CaCO₃-rich water (20 mg/L TC with 300 mg/L CaCO₃). The high photocatalytic activity is attributed to the formation of abundant local junctions among g-C₃N₄, TiO₂/SiO₂, and MWCNTs, which facilitate efficient charge separation and migration through a direct Z-scheme mechanism under visible light illumination.