High-Performance Photoelectrochemical Water Oxidation with Phosphorus-Doped and Metal Phosphide Cocatalyst-Modified g-C ₃ N ₄ Formation Through Gas Treatment

Graphitic carbon nitride (g-C ₃ N ₄ ) has been widely explored as a photocatalyst for water splitting. The anodic water oxidation reaction (WOR) remains a major obstacle for such processes, with issues such as low surface area of g-C ₃ N ₄ , poor light absorption, and low charge-transfer efficiency. In this work, such longtime concerns have been partially addressed with band gap and surface engineering of nanostructured graphitic carbon nitride (g-C ₃ N ₄ ). Specifically, surface area and charge-transfer efficiency are significantly enhanced through architecting g-C ₃ N ₄ on nanorod TiO ₂ to avoid aggregation of layered g-C ₃ N ₄ . Moreover, a simple phosphide gas treatment of TiO ₂ /g-C ₃ N ₄ configuration not only narrows the band gap of g-C ₃ N ₄ by 0.57 eV shifting it into visible range but also generates in situ a metal phosphide (M=Fe, Cu) water oxidation cocatalyst. This TiO ₂ /g-C ₃ N ₄ /FeP configuration significantly improves charge separation and transfer capability. As a result, our non-noble-metal photoelectrochemical system yields outstanding visible light (>420 nm) photocurrent: approximately 0.3 mA cm ⁻² at 1.23 V and 1.1 mA cm ⁻² at 2.0 V versus RHE, which is the highest for a g-C ₃ N ₄ -based photoanode. It is expected that the TiO ₂ /g-C ₃ N ₄ /FeP configuration synthesized by a simple phosphide gas treatment will provide new insight for producing robust g-C ₃ N ₄ for water oxidation.