Tailoring the Surface Properties of Bi2O2NCN by in Situ Activation for Augmented Photoelectrochemical Water Oxidation on WO3 and CuWO4 Heterojunction Photoanodes.

Zheng Chen,Anna Rokicińska,Jianhong Chen,Caspar De Bruin-Dickason,Alex J Corkett,Adam Slabon,Piotr Kuśtrowski,Richard Dronskowski

doi:10.1021/acs.inorgchem.0c01947

Abstract

Bismuth(III) oxide-carbodiimide (Bi2O2NCN) 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 Bi2O2NCN does not generate a noticeable photocurrent as a single photoabsorber, the fabrication of heterojunctions with the WO3 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 Bi2O2NCN transforms during operating conditions in situ to a core–shell structure Bi2O2NCN/BiPO4. When compared to WO3/BiPO4, the in situ electrolyte-activated WO3/Bi2O2NCN 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 CuWO4 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.

Highlights

The development of clean, renewable, and long-term sustainable energy sources to help prevent impending climate change while sustaining the global population and economic growth is a colossal challenge.[1,2] To this end, harnessing solar energy through energy conversion technologies represents a promising piece of the puzzle.[2−7] One such pathway uses PEC cells to obtain hydrogen from water upon solar illumination.[7−10] Photochemical water-splitting includes the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), which have to be accomplished simultaneously.[11]
Article chemical transport properties which are proposed to originate from the dispersed nature of the Bi states in the vicinity of the valence band edge (VBE) and conduction band edge (CBE), thereby providing efficient electron−hole separation.[38]
An important structural feature of Bi2O2NCN is the presence of fluorite-type lBaiyOerCslo, faletdegrnea-sthinagrinwgit[hBNi4OC]Nt2e−trlaahyeedrsr.a,Meqoureivoavleern,tBtoi2Oth2oNseCiNn is isostructural to Bi2O2Ch oxide chalcogenides (Ch = S, Se, and Te; n.b., the S analogue adopts an orthorhombically distorted low-symmetry modification), thereby highlighting the divalent nitride or pseudochalcogenide nature of NCN2−

Summary

Introduction

The development of clean, renewable, and long-term sustainable energy sources to help prevent impending climate change while sustaining the global population and economic growth is a colossal challenge.[1,2] To this end, harnessing solar energy through energy conversion technologies represents a promising piece of the puzzle.[2−7] One such pathway uses PEC cells to obtain hydrogen from water upon solar illumination.[7−10] Photochemical water-splitting includes the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), which have to be accomplished simultaneously.[11]. When compared to WO3/BiPO4, the in situ electrolyte-activated WO3/Bi2O2NCN photoanode shows a higher photocurrent density due to superior charge separation across the oxide/oxide-carbodiimide interface layer.

Results

Conclusion