Facile in situ synthesis of graphitic carbon nitride (g-C3N4)-N-TiO2 heterojunction as an efficient photocatalyst for the selective photoreduction of CO2 to CO

A series of composites of graphitic carbon nitride and in situ nitrogen-doped titanium dioxide (g-C3N4-N-TiO2) were prepared by a simple pyrolysis process of urea and Ti(OH)4. The obtained products were characterized by means of X-ray diffraction, FT-IR transmission spectroscopy, electron microscopy, UV–vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, etc. Compared with g-C3N4 and commercial P25, the as-prepared photocatalysts exhibit enhanced photocatalytic performance for photoreduction of CO2 in the presence of water vapor at room temperature. It was found that the mass ratios of urea to Ti(OH)4 in precursors play a role in formation of the composites, and the high ratios of urea to Ti(OH)4 result in the composites of g-C3N4 and N-doped TiO2, while low ratios only result in N-doped TiO2. An interesting selectivity of photocatalytic products displayed that N-doped TiO2 samples were related to CH4 and CO generation, while g-C3N4 and N-TiO2 composites were related to CO generation, and the product selectivity may originate from the formed g-C3N4. The highest amount of CO (14.73 μmol) was obtained on the optimized photocatalyst under 12 h light irradiation, which is four times of that over commercial P25. Based on these results, a possible mechanism for the enhanced photocatalytic performance was proposed.