ZIF-8 derived bimodal carbon modified ZnO photocatalysts with enhanced photocatalytic CO2 reduction performance

The use of ZnO photocatalysts is usually limited by their low specific surface area, low visible light absorption capacity and inferior photochemical stability. In this report, visible-light responsive porous ZnO photocatalysts were fabricated by calcining pre-synthesized ZIF-8 (a kind of zeolitic imidazolate framework) polyhedra as solid precursor in air at suitable calcination temperature. It was showed that the crystallographic structure of ZIF-8 precursor was basically remained up to 300 degrees C upon calcination, while phase transformation from ZIF-8 to wurtzite ZnO nanocrystals progressively proceeded above 400 degrees C. Interestingly, special bimodal carbon modifications, that is, simultaneous carbon doping and surface carbon coating, were achieved simultaneously during pyrolysis, which were confirmed by X-ray photoelectron spectroscopy, Fourier transform Raman spectroscopy and electron microscopy. The nitrogen sorption analysis indicated that the high specific surface area and mesoporous texture was partially inherited from highly porous ZIF-8. As a consequence of cooperative textural and carbon modifications, the as-prepared mesoporous ZnO photocatalyst was efficient in both CO2 capture and photocatalytic CO2 reduction towards CH3OH as a typical solar fuel under full-spectrum Xe lamp irradiation. Especially, the 500 degrees C-calcined sample manifested the highest photocatalytic CO2 reduction activity (0.83 mu mol h(-1) g(-1)), which was about six folds higher than that of hydrothermally synthesized ZnO nanorods as reference. Specifically, the superior photocatalytic CO2 reduction performance was associated with the bimodal carbon modification, high specific surface area and porous framework, which contributed to enhanced visible light harvesting, charge transport and CO2 capture.