Unraveling Multiple Roles of N-Doped Graphene Quantum Dots Anchoring on 2D α-SnWO ₄ Photoanode for Visible-Light Driven Unbiased Water Oxidation

Tin tungstate (SnWO4) is a promising photoanode material for visible light-driven photoelectrochemical (PEC) water splitting. Still, its performance is limited due to surface state-mediated charge recombination and sluggish oxygen evolution reaction (OER) kinetics. Herein, a unique surface modification strategy has been adopted to tackle both challenges of the SnWO4 photoanode through synergistic effects of incorporating nitrogen-doped graphene quantum dots (N-GQDs). A strong interaction between N-GQDs and the uncoordinated metal ions on the SnWO4 surface results in electron transfer from N-GQDs to SnWO4, which suppresses the Sn2+ to Sn4+ conversion and related interfacial defect state formation, acting as an effective surface passivator. The net charge redistribution at the N-GQD/SnWO4 heterointerface and surface state passivation depins the surface Fermi level, enlarging the built-in electric field and promoting photocarrier separation and transfer for OER. Thus, serving as an efficient hole transport layer, N-GQDs enhance both the fill factor and photovoltage, while protecting the surface, thereby improving the stability of the photoanode. N-GQDs/SnWO4 photoanode exhibits enhanced visible light harvesting efficiency and promotes low-bias visible light-driven (AM 1.5 G, 100 mW cm–2, 400–950 nm) PEC water splitting activity, with a record photocurrent density of 1.1 mA cm–2 at 1.23 V versus RHE, (reversible hydrogen electrode). The density functional theory (DFT) calculations also revealed that N-GQDs, as a metal free catalyst, anchoring on the surface of SnWO4 create midgap states, enhancing light harvesting efficiency and reducing the energy barrier (ΔG*OOH) of OER by 1.52 eV. The combination of the photoanode with a commercial Si solar cell, in a photovoltaic (PV)-PEC tandem device, exhibits an unassisted solar-to-hydrogen conversion efficiency of 1.57% with improved stability. This work provides insights into the multiple roles of N-GQDs in the visible light-driven solar energy conversion of SnWO4 photoanodes.