Enhancing Z-scheme photocatalytic CO₂ methanation at extended visible light (>600 nm): Insight into charge transport and surface catalytic reaction mechanisms

This article reports Z-scheme photocatalytic CO2 methanation at the extended visible light (>500 nm) in absence of sacrificial electron donor/acceptor and noble metal cocatalysts. This Z-scheme photocatalyst consists of reduced graphene oxide (rGO) sandwiched between two different redox active photocatalysts (Ca-doped Bi2O3 and Cu2O). The rGO acts as a mediator by relaying electrons from Ca-doped Bi2O3 and holes from Cu2O that enables Z-scheme spatial confinement of electrons in Cu2O while holes in Ca-doped Bi2O3. This charge carrier delocalization enforces the enhanced oxidation strength of holes in Ca-doped Bi2O3 and reduction strength of electrons in Cu2O. As a result, it has exhibited methane (CH4) production rate under wider range of visible light (up to 700 nm) absorption. This Z-scheme photocatalyst therefore advents a new '600 nm-class photocatalyst' for CO2 methanation. With the help of C-13 isotopic measurements, first principles calculations and photoelectrochemical measurements, we have revealed the atomistic insight into the charge transport and surface catalytic reactions mechanisms to understand the rate limiting factors towards unassisted PCR.