Anchoring movable Ag2CO3 quantum dots on g-C3N4 with rich oxygen vacancies for enhanced simulated-sunlight CO2 reduction in water medium

•Photocatalytic CO2RR of Z-scheme Ag2CO3-based composites was systematically investigated.•The Ov roles were investigated and discussed in detail.•The movement of Ag2CO3 QDs on g-C3N4 was investigated and discussed in detail during CO2RR process.•Comparing with single g-C3N4 or Ag2CO3, CA composites exhibit enhanced photocatalytic performance of CO2 reduction.•Possible active absorption sites, kinetic CO2RR pathways and interface binding energy were calculated. Z- scheme Ag2CO3 quantum dots (QDs) decorated g-C3N4 photocatalysts (g-C3N4/Ag2CO3, CA) with rich interface oxygen vacancies (Ov) were prepared by calcination and precipitation method. The highest yields of photocatalytic CO2 reduction reaction (CO2RR) over 15 mg of photocatalysts are 7.006 (CO, CA4) and 8.039 μmol·g−1 (CH4, CA3) for 5 h under simulated sunlight irradiation, respectively. The yields in CA composites are 1.90, 1.46 (CO) and 1.85, 1.60 (CH4) times higher than those of pristine g-C3N4 and Ag2CO3, respectively. When the amount of the photocatalysts increases to 30 mg, the yield of CA4 is 17.158 μmol·g−1 (CO). According to density functional theory (DFT) calculations, the key roles of Ov are the decrease of Gibbs-free energy for *COOH and interface binding energy. The Ov of Ag2CO3 QDs exhibit an electron capture effect, and the electrons can be extracted and transferred to surface active sites via synergistic effect of a rich Ov and Ag interface channel (Ag-Ov-Ag-N pathway). Furthermore, rich Ov Ag2CO3 QDs can move, rotate and disperse on g-C3N4 (002) surface through the relaxation and formation of interface Ag-N bonds during CO2RR process. The structure reconstruction of Ag2CO3 QDs on g-C3N4 not only improves the uniformity of Ag2CO3 QDs, but also exhibits a positive influence on photocatalytic efficiency. The DFT results also reveal optimized Lewis sites and possible kinetic CO2RR pathways for CO and CH4, respectively. The CO2RR pathways are CO2 ∼ *COOH ∼ *CO ∼ *+CO and CO2 ∼ *COOH ∼ *CO ∼ *CHO ∼ *CH2O ∼ *CH3O ∼ *CH4 ∼ *+CH4, respectively. The work provides a novel photocatalytic mode via a self-adjusting behavior of QDs to improve photocatalytic activity.