Tailoring the Surface Properties of BiONCN by Activation for Augmented Photoelectrochemical Water Oxidation on WO and CuWO Heterojunction Photoanodes.

Bismuth(III) oxide-carbodiimide (BiONCN) 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 BiONCN does not generate a noticeable photocurrent as a single photoabsorber, the fabrication of heterojunctions with the WO thin film electrode shows an upsurge of current density from 0.9 to 1.1 mA cm 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 BiONCN transforms during operating conditions to a core-shell structure BiONCN/BiPO. When compared to WO/BiPO, the electrolyte-activated WO/BiONCN 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 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 activation.