Single-atom　metal-insulator-semiconductor　(SMIS)　heterojunctions　based　on　Sn-doped　Fe2O3　nanorods　(SF　NRs)　were　designed　by　combining　atomic　deposition　of　an　Al2O3　overlayer　with　chemical　grafting　of　a　RuOx　hole-collector　for　efficient　CO2-to-syngas　conversion.　The　RuOx-Al2O3-SF　photoanode　with　a　3.0　nm　thick　Al2O3　overlayer　gave　a　＞5-fold-enhanced　IPCE　value　of　52.0　%　under　370　nm　light　irradiation　at　1.2　V　vs.　Ag/AgCl,　compared　to　the　bare　SF　NRs.　The　dielectric　field　mediated　the　charge　dynamics　at　the　Al2O3/SF　NRs　interface.　Accumulation　of　long-lived　holes　on　the　surface　of　the　SF　NRs　photoabsorber　aids　fast　tunneling　transfer　of　hot　holes　to　single-atom　RuOx　species,　accelerating　the　O-2-evolving　reaction　kinetics.　The　maximal　CO-evolution　rate　of　265.3　mmol　g(-1)　h(-1)　was　achieved　by　integration　of　double　SIMS-3　photoanodes　with　a　single-atom　Ni-doped　graphene　CO2-reduction-catalyst　cathode;　an　overall　quantum　efficiency　of　5.7　%　was　recorded　under　450　nm　light　irradiation.