Self-assembly of highly-dispersed phosphotungstic acid clusters onto graphitic carbon nitride nanosheets as fascinating molecular-scale Z-scheme heterojunctions for photocatalytic solar-to-fuels conversion

[Display omitted] •The molecular-scale hetero-interface is achieved by assembling ultra-small HPW clusters onto g-C3N4 nanosheets.•Abundant light-harvesting and catalytic sites expose on the HPW/g-C3N4 hetero-interface.•The Z-scheme mechanism-enhanced photocatalytic solar-to-fuels conversion is demonstrated in the HPW/g-C3N4 heterojunction.•The photocatalytic activity of HPW/g-C3N4 heterojunction nanosheets is much higher than those of g-C3N4 nanosheets. Rational assembly of small-sized photosensitizers onto 2D semiconductors can effectively promote the photocatalytic activity of the formed 2D heterojunction photocatalyst due to the hetero-interfacial charge-transfer process. However, the achievement of the 2D semiconductor-based heterojunction photocatalyst with the exposures of both abundant light-harvesting and catalytic sites at the hetero-interface region is still a huge challenge. Herein, we synthesized phosphotungstic acid/graphitic carbon nitride (HPW/g-C3N4) heterojunction nanosheets (NSs) with the highly-dispersed distribution of ultra-small HPW clusters (1∼2 nm) by using a facile self-assembly method based on the static adsorption-deposition process. The formed molecular-scale hetero-interface between HPW clusters and g-C3N4 NSs resulted in the exposure of abundant active-sites on the highly-dispersed HPW/g-C3N4 hetero-interface. By combining the steady-state and transient photoluminescence spectra with the wavelength-controlled experiments, we demonstrated that the hetero-interfacial charge-transfer process occurring in the HPW/g-C3N4 heterojunction NSs obeyed the Z-scheme mechanism rather than the common “type-II” heterojunction mechanism. In this way, the lifetimes of photoinduced electrons on the conduction band of g-C3N4 could be prolonged for initiating the photocatalytic solar-to-fuels conversion. Upon interband excitations of both the two hetero-components in the HPW/g-C3N4 heterojunction NSs, the photocatalytic activities of H2 generation and CO2 reduction could be enhanced by ∼2.2 and ∼6.7 times as compared to the pure g-C3N4 NSs, even though the HPW component was photocatalytic-inert for either H2 generation or CO2 reduction upon UV–vis light irradiation.