g-C3N4 microtubes@CoNiO2 nanosheets p–n heterojunction with a hierarchical hollow structure for efficient photocatalytic CO2 reduction

Carbon nitrides (g-C3N4) have emerged as the promising photocatalysts for CO2 conversion. However, they suffer from inefficient charge transfer and a lack of metal sites. Herein, the p-type CoNiO2 nanosheets are anchored on the n-type g-C3N4 microtubes to construct p–n heterojunction photocatalysts. The CoNiO2 nanosheets provide metal sites to bind and activate CO2 molecules, and the configuration of p–n heterojunction impels electron–hole separation. The CoNiO2 nanosheets that are vertically aligned on the surfaces of g-C3N4 microtubes build a hierarchical hollow structure and an intimate contact interface, which also contributes to the CO2 adsorption and rapid migration of charged carriers. As a result, the g-C3N4@CoNiO2 p–n heterojunction photocatalysts exhibit outstanding yields (3645 μmol h−1 g−1 of CO and 943 μmol h−1 g−1 of H2) from CO2 reduction under visible light irradiation, much higher than that of pure CoNiO2 nanosheets and g-C3N4 microtubes. In situ DRIFT spectra are performed to monitor the intermediates of CO2 reduction on the surfaces of g-C3N4@CoNiO2, the results of which confirm the adsorption and activation of CO2 molecules. It is expected that this work would provide a feasible strategy for designing high-efficiency g-C3N4-based heterojunction photocatalysts.