Constructing a crystalline–amorphous hydrated niobium pentoxide homojunction for superior photocatalytic CO2 reduction into CH4 with high selectivity

The photocatalytic reduction of carbon dioxide (CO2) into renewable fuels holds great promise as a sustainable energy production approach, addressing both the imminent energy crisis and global warming simultaneously. However, the poor CO2 adsorption and rapid recombination of photogenerated charge carriers on semiconductors greatly limit the CO2 conversion efficiency along with poor selectivity towards the desired product. Herein, we propose that the construction of a homojunction in a crystalline–amorphous photocatalyst can greatly enhance its photocatalytic performance. In this study, we developed a hydrated niobium pentoxide (HNBO) homojunction with co-existing crystalline and amorphous structures through a facile hydrothermal method. The resulting crystalline–amorphous HNBO demonstrated exceptional photocatalytic activity for CO2 reduction into methane (CH4) and carbon monoxide (CO), in which the selectivity for CH4 reached 85.7%. The superior performance of HNBO can be attributed to the following factors: (i) the presence of a large specific surface area (355 m2 g−1) due to the aggregation of HNBO nanoparticles with abundant “ink bottle” mesopores, (ii) the abundant OH− groups on HNBO promoting the adsorption and activation of CO2, (iii) the enhanced separation of charge carriers resulting from the crystalline–amorphous homojunction interface, and (iv) a narrowed bandgap along with an elevated conduction band potential to give strong reduction power. This work presents a remarkable instance of a solitary crystalline–amorphous material for CO2 photoreduction into CH4 with superior activity and high selectivity. Notably, this was achieved without the need for any dopants or co-catalysts, underscoring the elegance of this photocatalyst.