Polymerization Improvement of Graphitic Carbon Nitride Films Derived from Melamine and Thiourea

Deposition of low-cost, efficient, and environmentally friendly graphitic carbon nitride (g-CN) films as photoanodes is a crucial step for constructing photoelectrochemical (PEC) cells and exploring their PEC performance. Currently, the improvement of the photocurrent density of g-CN films is badly needed for their practical applications in PEC water splitting. Enhancing the g-CN crystallinity by optimizing their synthesis conditions only through screening appropriate reactant precursors is insufficient for this purpose. Herein, using melamine and thiourea precursors with mass ratio 5:1, the degree of polymerization of g-CN thin films is successfully improved by a thermal vapor condensation method. The obtained pure g-CN exhibits a remarkably enhanced photocurrent density of 404.4 µA cm^-2 at 1.23 V versus reversible hydrogen electrodes. Theoretical calculations reveal that the continuous attachment of small carbodiimide (HN═C═NH) mainly generated by thiourea to the melamine matrix facilitates the formation of large-area conjugated structure, which fundamentally determines better charge carrier separation and transfer thereby enhancing the PEC performance. This work realizes the synthesis of well-polymerized g-CN films with improved PEC activity and offers a computational understanding for the nucleation and growth mechanism of the polycrystalline g-CN.