Hierarchical S-scheme titanium dioxide@cobalt-nickel based metal–organic framework nanotube photocatalyst for selective carbon dioxide photoreduction to methane

Efficient photocatalysts are of great importance for the photochemical conversion of CO2 into fuels. Herein, S-scheme titanium dioxide@cobalt-nickel based metal-organic framework (TiO2@CoNi-MOF) heterojunction photocatalysts with high surface area and porosity are designed and fabricated by a multi-step controllable strategy. The photocatalytic activity of the composites can be optimized by adjusting the loading content of CoNi-MOF in TiO2@CoNi-MOF and molar ratios of Co2+ and Ni2+ in CoNi-MOF. The optimized hybrid photocatalyst showed a much higher CO2 photoreduction activity than the control single-component samples (TiO2 and CoNi-MOF) with a high CH4 yield (41.65 μmol g-1 h-1) and selectivity (93.2%). The accelerated charge carrier separation induced by the S-scheme heterojunction significantly promoted the photocatalytic performance of TiO2@CoNi-MOF NTs. Meanwhile, the introduction of bimetallic CoNi-MOF nanosheets significantly resulted in the increase of active sites, CO2 adsorbability, visible-light utilization, and CH4 selectivity. Moreover, the S-scheme photoinduced charge transfer model of the TiO2@CoNi-MOF NTs photocatalyst was confirmed by photoluminescence spectroscopy, free radical trapping tests, and work function calculated from Kelvin probe. The work aims to design and fabricate heterojunction photocatalysts with high efficiency for solar fuel production.