Tailoring the Surface Properties of Bi 2 O 2 NCN by in Situ Activation for Augmented Photoelectrochemical Water Oxidation on WO 3 and CuWO 4 Heterojunction Photoanodes.

Bismuth(III) oxide-carbodiimide (Bi ₂ O ₂ NCN) has been recently discovered as a novel mixed-anion semiconductor, which is structurally related to bismuth oxides and oxysulfides. Given the structural versatility of these layered structures, we investigated the unexplored photochemical properties of the target compound for photoelectrochemical (PEC) water oxidation. Although Bi ₂ O ₂ NCN does not generate a noticeable photocurrent as a single photoabsorber, the fabrication of heterojunctions with the WO ₃ thin film electrode shows an upsurge of current density from 0.9 to 1.1 mA cm ^-2 at 1.23 V vs reversible hydrogen electrode (RHE) under 1 sun (AM 1.5G) illumination in phosphate electrolyte (pH 7.0). Mechanistic analysis and structural analysis using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM EDX) indicate that Bi ₂ O ₂ NCN transforms during operating conditions in situ to a core-shell structure Bi ₂ O ₂ NCN/BiPO ₄ . When compared to WO ₃ /BiPO ₄ , the in situ electrolyte-activated WO ₃ /Bi ₂ O ₂ NCN photoanode shows a higher photocurrent density due to superior charge separation across the oxide/oxide-carbodiimide interface layer. Changing the electrolyte from phosphate to sulfate results in a lower photocurrent and shows that the electrolyte determines the surface chemistry and mediates the PEC activity of the metal oxide-carbodiimide. A similar trend could be observed for CuWO ₄ thin film photoanodes. These results show the potential of metal oxide-carbodiimides as relatively novel representatives of mixed-anion compounds and shed light on the importance of the control over the surface chemistry to enable the in situ activation.