Selective CO₂-to-CO photoreduction over an orthophosphate semiconductor via the direct Z-scheme heterojunction of Ag₃PO₄ quantum dots decorated on SnS₂ nanosheets

Direct Z-scheme heterojunctions are widely used for photocatalytic water splitting and CO₂ reduction due to facilitating well-separated photogenerated charge carriers and spatial isolation of redox reactions. Here, using a facile two-step hydrothermal and ion-exchange method, we uniformly decorate silver orthophosphate (i.e., Ag₃PO₄) quantum dots with an average characteristic size of ∼10 nm over tin(iv) sulphide (i.e., SnS₂) nanosheets to form a 0D/2D heterojunction. The direct Z-scheme mechanism, i.e. charge transport for efficient electron (from SnS₂) and hole (from Ag₃PO₄) recombination, is confirmed by the following experiments: (i) ultraviolet and X-ray photoelectron spectroscopies; (ii) photodeposition of Pt and PbO₂ nanoparticles on reduction and oxidation sites, respectively; (iii) in situ X-ray photoelectron spectroscopy; and (iv) electron paramagnetic resonance spectroscopy. Owing to the photoreduction properties of Ag₃PO₄ with orthophosphate vacancies, Z-scheme charge carrier transfer, and efficient exciton dissociation, an optimized heterojunction shows a high CO₂-to-CO reduction yield of 18.3 μmol g⁻¹ h⁻¹ with an illustrious selectivity of ∼95% under light illumination, which is about 3.0 and 47.8 times larger than that of Ag₃PO₄ and SnS₂, respectively. The carbon source for the CO product is verified using a ¹³CO₂ isotopic experiment. Moreover, by tracing the peak at ∼1190 cm⁻¹ in the dark and under light irradiation, in situ diffuse reflectance infrared Fourier transform spectroscopy demonstrates that the CO₂ reduction pathway goes through the COOH* intermediate.