As one of the main greenhouse gases, the conversion of CO2 into useful chemicals is of great significance to global warming, climate change and energy supply. Herein, a multi-functional synergistic photoelectrocatalytic (PEC) interface is employed to realize enhanced PEC CO2 reduction by using intrinsic Au doped TiO2 as the light harvester, nanotube photonic crystal (NTPC) as optical channels and Cu nanoparticles (NPs) as CO2 electrocatalyst. Formic acid is produced as the main product with a selectivity of 98% and a Faraday efficiency of 82.6% under simulated sunlight illumination and an applied potential of -1.0 V vs. Ag/AgCl. Its yield ratio is up to 1019.3 mu mol L-1 cm(-2) after an 8 h reduction, nearly 6.3 times higher than that on the traditional TiO2 NTs. Such enhanced PEC CO2 conversion performance is due to a synergistic catalytic effect on this multi-functional interface. The extraordinary localized surface plasmon resonance sensitized by the intrinsic Au NPs expands the light absorption region to the visible region. The photonic crystal layer structure raises the multi-reflection of incident light. The electrocatalyst of Cu NPs synergistically promotes PEC activity toward CO2 reduction. The calculated enhancement factors of photonic crystal layer, intrinsic Au NPs, and Cu NPs are 813%, 91%, and 2028%, respectively. This work provides an effective approach for photocathode design to promote the efficiency of PEC CO2 conversion. (C) 2017 Elsevier B.V. All rights reserved.