Visible light responsive heterostructure HTDMA-BiPO4 modified clays for effective diclofenac sodium oxidation: Role of interface interactions and basal spacing

In the present study, we investigated comparatively the role of hexadecyltrimethylammonium bromide (HDTMA) in enhancing the adsorption and photocatalytic activity of BiPO4 coated montmorillonite (M-BiPO4), and BiPO4 coated smectite (S-BiPO4). Montmorillonite has large interlayer spacing compared to smectite. Firstly, it was found that the simple hybridization of BiPO4 with clays led to enhance the visible light response. The co-fixation of BiPO4 and HDTMA on clay supports reduces the BET specific surface area, but it enhances significantly the adsorption ability. XPS analysis shows a notable Bi4f high resolution peak shifting in the case of HDTMA-S-BiPO4, while such a shifting was not observed in terms of HDTMA-M-BiPO4. Regarding the adsorption and photocatalytic tests, at lower concentration of diclofenac (20 ppm), M-BiPO4 was several times more effective than S-BiPO4. However, the coating of clays by BiPO4 and HDTMA showed a different manner, wherein HDTMA-S-BiPO4 achieved an oxidation rate of around 88% under solar light within 90 min at a concentration of 140 ppm of diclofenac. On the contrary, HDTMA-M-BiPO4 shows an oxidation rate of only 22% under the same conditions. It was deduced that the strong surface interactions between HDTMA and BiPO4 coated on smectite can form a strong interfacial bridge which boosts the visible light response and the separation of photogenerated charges.