Intermediate C=O Formation Is the Bottleneck of Overall Water Splitting on Carbon Nitride

Abstract Graphitic carbon nitride (g-C3N4) was long considered incapable of decomposing pure water molecules into hydrogen and oxygen without the addition of small molecule organics, albeit the superior visible-light response and proper band structure that fulfills the demand of oxygen evolution reaction (OER). Herein, we unexpectedly observed a collective C = O bonding during continuous photocatalytic overall water splitting on single-phased g-C3N4 catalyst (denoted CN) by isotopic-labelled (16O/18O) in-situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) and in-situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). Such an inert C = O bonding directly hinders the further OER steps, resulting in a negligible O2 production on CN. As carbon sites on CN were occupied via the surface fluorination, intermediate C = O bonding was vastly minimized on surface fluorinated CN catalyst (denoted F-CN). As a result, the resulting champion F0.1-CN catalyst exhibited excellent overall water splitting activity with the order-of-magnitude improved H2 evolution rate compared to the pristine CN catalyst and continuous O2 evolution upon both white light and AM1.5G simulated solar irradiation. Density functional theory (DFT) calculations further suggest an optimized OER pathway on neighboring N atoms by C-F interaction, which effectively avoids the excessively strong C-O interaction or weak N-O interaction on the pristine CN and enhances the stability of formed *OH on the N site of F-CN.