A Z-scheme Heterojunctional Photocatalyst Engineered with Spatially Separated Dual Redox Sites for Selective CO2 Reduction with Water: Insight by in-situ μs-transient Absorption Spectra

Solar driven CO2 reduction by water with a Z-scheme heterojunction affords an avenue to access energy storage and to alleviate greenhouse gas (GHG) emission, yet the separation of charge carriers and the integrative regulation of water oxidation and CO2 activation sites remain challenging. Here, a BiVO4/g-C3N4 (BVO/CN) Z-scheme heterojunction as such a prototype has been constructed by spatially separated dual sites with CoOx clusters and imidazolium ionic liquids (IL) towards CO2 photoreduction. The optimized CoOx-BVO/CN-IL delivers a ca. 80-fold CO production rate without H2 evolution compared with urea-C3N4 counterpart, together with nearly stoichiometric O2 gas produced. Experimental results and DFT calculations unveil the cascade Z-scheme charge transfer and subsequently the prominent redox co-catalysis by CoOx and IL for holes-H2O oxidation and electrons-CO2 reduction, respectively. Moreover, in-situ μs-transient absorption spectra clearly show the function of each cocatalyst and quantitatively reveal that the resulting CoOx-BVO/CN-IL reaches up to the electron transfer efficiency of 36.4% for CO2 reduction, far beyond those for BVO/CN (4.0%) and urea-CN (0.8%), underlining an exceptional synergy of dual reaction sites engineering. This work provides deep insights and guidelines to the rational design of highly efficient Z-scheme heterojunction with precise redox catalytic sites toward solar fuel production.