Regulating band alignment in terrace-like tungsten trioxide via Prussian blue analogues deposition for efficient photoelectrocatalytic water splitting

Tungsten trioxide (WO₃) suffers from the rapid recombination of photogenerated charge carriers. For practical photoelectrochemical (PEC) water splitting, effective oxygen evolution catalysts need to be introduced to alleviate electron-hole recombination. Prussian blue analogues, with their open framework structures and adjustable metal centers, have emerged as promising candidates for oxygen evolution catalysts. This study investigates the deposition of Cobalt hexacyanoferrate (CFP) nanoparticles with different iron valence states (III and II) on terrace-like WO₃ (TW) photoanodes via a simple sequential dipping method. Notably, CFP(III) has shown to exhibit a stronger influence on photocurrent response than CFP(II). Based on Mott-Schottky studies and density functional theory calculations, CFP(III) facilitates a reduction in the depletion layer width, improves photoinduced hole kinetics, and induces steeper band bending in CFP(III)-TW, thereby enhancing PEC performance. The CFP(III)-TW photoanode achieves a photocurrent density of 1.64 mA cm⁻² at 1.23 V_RHE under visible light, which is 2 times and 5.4 times higher than that of TW and porous WO₃ films, respectively. Photoinduced holes with longer lifetimes suggest CFP(III)-TW experiences less surface recombination and faster separation of charge carriers compared to that of TW and CFP(II)-TW. The scalability of the CFP(III)-TW is demonstrated through the fabrication of a 25 cm² sheet, attaining a current of 4.6 mA at 1.23 V_RHE under visible light illumination. This study highlights the straightforward synthesis of low-cost, environmentally friendly photoanode materials and establishes CFP(III)-TW as a scalable and efficient oxygen evolution catalyst for practical PEC water splitting applications. These findings underscore the potential of CFP(III) as a promising material for advancing renewable energy technologies.