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 TiO 2 /SiO 2 /g-C 3 N 4 /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 TiO 2 /SiO 2 /g-C 3 N 4 /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 CaCO 3 -rich water (20 mg/L TC with 300 mg/L CaCO 3 ). The high photocatalytic activity is attributed to the formation of abundant local junctions among g-C 3 N 4 , TiO 2 /SiO 2 , and MWCNTs, which facilitate efficient charge separation and migration through a direct Z-scheme mechanism under visible light illumination.