Nitric acid-assisted growth of InVO4 nanobelts on protonated ultrathin C3N4 nanosheets as an S-scheme photocatalyst with tunable oxygen vacancies for boosting CO2 conversion

[Display omitted] •An S-scheme protonated C3N4 (p-C3N4)/InVO4 photocatalyst is constructed.•Nitric acid assists in-situ growth of InVO4 nanobelts with tunable Vo on p-C3N4.•Effects of protonation on CO2 photoreduction are reported for the first time.•p-C3N4/InVO4 displays greatly enhanced photoactivity compared to bare components. Engineering bulk defects and surface modifications are effective strategies to facilitate carriers separation and achieve high-efficiency photocatalysis, but it is challenging to realize the integrated regulation of the two aspects. Herein, we report an S-scheme heterojunction with tunable oxygen vacancies (Vo) via in-situ growth of InVO4 nanobelts on protonated ultrathin C3N4 nanosheets (p-C3N4/InVO4) assisted by nitric acid for efficient CO2 photoreduction. The nitric acid plays three roles in the synthetic process: providing protonation sources for the p-C3N4, assisting the growth of InVO4 nanobelts on p-C3N4, and facilitating the formation of Vo in InVO4. Intriguingly, the Vo content is tuned by varying the amount of C3N4 which reduces free H+ concentration and increases the electron density around V in VO43-, leading to a tade-off effect on Vo formation and thus a volcano-shaped evolution profile of Vo content. The introduction of Vo reduces the band gap of InVO4 and enhances the n-type conductivity, expediting the interfacial charge transfer in terms of the S-scheme pathway. Besides, the protonation of C3N4 improves electrical conductivity, promotes the adsorption and activation of CO2 molecules, and thermodynamically favours the conversion to CO. Due to the composite effect of Vo and protonation in the S-scheme system, the optimized p-C3N4/InVO4 hetero-nanosheet displays dramatically boosted photocatalytic CO2 reduction activity with a CO production rate of 14.05 μmol g-1 h−1, 6.03 and 3.23 times higher than that of bare InVO4 and p-C3N4, respectively. This work provides new platforms for the development of efficient photocatalysts by integrated refining of structure defects and surface modification in constructing heterostructures.