Enhanced photocatalytic removal of NO over titania/hydroxyapatite (TiO2/HAp) composites with improved adsorption and charge mobility ability

Effective photocatalysis processes with production of fewer toxic intermediates are highly desirable for air purification. In this study, titania/hydroxyapatite (TiO2/HAp) composites were synthesized by a facile hydrothermal method and employed to decontaminate nitric oxide (NO) in air under simulated solar light irradiation for the first time. It was found that the photocatalytic activity of the as-prepared TiO2/HAp composite (44.61%) was superior to those of the pristine components (TiO2: 38.57%, HAp: 36.73%) and mechanically mixed samples (35.36%). The TiO2/HAp composite with mass ratio of 3 : 1 (75% TiO2/HAp) exhibited the highest NO removal efficiency among them. Moreover, the toxic intermediate NO2 production was significantly inhibited over TiO2/HAp. These synergistically improved properties can be ascribed to the high separation efficiency and faster transfer of the photo-generated charge carriers as evidenced by the experimental results from photocurrent tests and electrochemical impedance spectroscopy (EIS). The results from temperature programmed desorption (TPD) confirmed that the 75% TiO2/HAp sample had stronger chemisorption for NO due to the increased concentration of surface OH groups. Furthermore, the electron spin resonance (ESR) characterization suggested that O-center dot(2)- and (OH)-O-center dot radicals were the major species involved for NO removal over TiO2/HAp composites. The five recycling tests suggested that the TiO2/HAp has superior photocatalytic stability. This study suggests that the combination of TiO2 with HAp is an effective approach for air purification.