Z-scheme BiFeO3-CNTs-PPy as a highly effective and stable photocatalyst for selective oxidation of benzyl alcohol under visible-light irradiation

Z-scheme BiFeO3-CNTs-PPy photocatalysts were prepared via hydrothermal reaction and oxidative polymerization, and characterized by physico-chemical techniques to detect their structural and optical properties. The formed Z-scheme could efficiently enhance the separation of photo-generated electron-hole pairs, promote holes to directly participate in the oxidation reaction and avoid the formation of non-selective hydroxyl radicals. The photocatalytic performance of Z-scheme BiFeO3-CNTs-PPy photocatalysts was inspected by selective oxidation of benzyl alcohol under visible-light irradiation. In optimal condition, the highest conversion rate of benzyl alcohol was up to 83.2%, much higher than those of PPy (2.3%), BiFeO3 (15.2%), BiFeO3-CNTs (24.6%) and BiFeO3-PPy (29.4%), while the selectivity was above 99%. Simultaneously, BiFeO3-CNTs-PPy also presented good stability and could be recycled up to 5 times without remarkable loss of activity and change in structure. The relationship between solvents, reaction temperature, light wavelength, substituent type, and photocatalytic efficiency was investigated. The results showed that the best solvent and reaction temperature were ethyl acetate and 303 K, respectively. In the conversion of benzyl alcohol, 360-560 nm light played the most important role and the present of electron-withdrawing groups was beneficial to the selective oxidation. Moreover, according to the conclusions of the free radical capture experiments, the coumarin conversion experiments and electron spin resonance (ESR) spectra, the main active species in the photocatalytic process were determined. Based on these, the possible mechanism for photocatalytic selective oxidation of benzyl alcohol over BiFeO3-CNTs-PPy was proposed.