Acetic acid functionalized TiO2/kaolinite composite photocatalysts with enhanced photocatalytic performance through regulating interfacial charge transfer

In this study, sol-gel method modified with acetic acid (CH 3 COOH) was employed to facilely synthesize hydrogen functionalized TiO 2 /kaolinite composite. Weak acid (CH 3 COOH) was used to modify the surface of TiO 2 /kaolinite composite in order to activate TiO 2 surface and generate more hydroxyl groups. The modified surface can act as Lewis acid sites to allow the physisorption of uncharged carboxyl acid groups, which is believed to be beneficial for electron injection and transfer. Using ciprofloxacin (CIP) as the target pollutant, in comparison with the use of strong acid such as hydrochloric acid (HCl) or sulfuric acid (H 2 SO 4 ), it was found that CH 3 COOH functionalized TiO 2 /kaolinite composite possessed the strongest adsorption ability and exhibited highest degradation efficiency under broad spectrum (200–800 nm). Moreover, the concentration of CH 3 COOH required is low (0.1 mol/L) in this work. The results show that the CH 3 COOH treated sample exhibited much higher degradation rates than bare TiO 2 or TiO 2 /kaolinite composite under both UV light and visible light. The kinetic constants under UV light were almost 2.61 and 1.57 times higher than those of TiO 2 and TiO 2 /kaolinite, respectively. According to the X-ray diffraction (XRD), N 2 adsorption-desorption isotherms, X-ray photoelectron spectroscopy (XPS), Fourier Transform infrared spectroscopy (FTIR) as well as Raman characterization results, it is revealed that the improved photocatalytic activity can be mainly attributed to the CH 3 COOH functionalization towards the TiO 2 /kaolinite composite. Furthermore, density functional theory (DFT) calculations were employed to explore the interface effect between TiO 2 and kaolinite as well as the function of newly introduced hydrogen on the surface of the composite, which significantly contributes to better understanding the function of CH 3 COOH in the constructed system. On the other hand, electron spin resonance (ESR) analysis suggests that the main oxidation species in the degradation process under broad light spectrum illumination was the holes. This study also provides information about the enhancement mechanism. New insights in photocatalysts synthesis and properties adjustment based on natural minerals for the elimination of organic contaminants of emerging concern in water was successfully introduced in the present study.