Preparation and Characterization of Carbon Nitride Film with High Optical Quality

Graphitic carbon nitride (g-CN), as a nonmetal semiconductor material, has been widely used in various fields, such as photocatalysis, electrocatalysis, batteries, light-emitting diodes, and solar cells, owing to its unique electronic and photophysical properties. However, the application of g-CN in practical devices remains limited because of the difficulties in fabricating g-CN films of high quality. In this work, we report a method for preparing a g-CN film with high optical quality on a substrate of indium tin oxide (ITO) glass and/or soda lime (NaCa) glass by using melamine as a precursor. First, we prepared the bulk g-CN from melamine in a muffle furnace via thermal polymerization. Then, we fabricated the g-CN film on the ITO and/or NaCa glass substrate with fine-milled, bulk g-CN in a tube furnace using thermal vapor deposition. With this two-step method, a yellow, transparent g-CN film with high optical quality was successfully fabricated on both the ITO and/or NaCa glass substrates. To check the quality of the film, we used scanning electron microscopy (SEM) to study the morphology of the fabricated gCN film on the ITO glass substrate. Both the high-resolution and low-resolution SEM image results show that the obtained g-CN film on the ITO glass substrate had a homogeneous and dense structure without a corrugated surface, illustrating that it had good surface roughness. Then, we investigated the thickness and surface roughness of the g-CN film via atomic force microscopy (AFM). The AFM results show that the thickness of the g-CN film deposited on the ITO glass substrate was around 300 nm and that the surface roughness of the g-CN film deposited on the ITO glass substrate was less than 40 nm. To verify the chemical composition of the obtained g-CN film on the ITO glass substrate, we performed X-ray photoelectron spectroscopy (XPS) and energy-dispersive spectroscopy (EDS) analyses. Both the XPS and EDS results demonstrate that the chemical composition of the g-CN film deposited on the ITO glass substrate was similar to that of bulk g-CN powder. More importantly, we determined the band structure for the g-CN film deposited on the ITO glass substrate by using a combination of steady-state absorption and high-resolution valence band XPS analysis. It was found that the determined band structure for the g-CN film deposited on the ITO glass substrate was close to that of bulk g-CN powder, also indicating that its chemical composition was similar to that of bulk g-CN. Meanwhile, we also found that the prepared g-CN film on the ITO glass substrate effectively degraded methylene blue dye under Xe lamp irradiation, which was similar to the effect of bulk g-CN powder. All analyses performed demonstrate that the two-step method presented in this study could successfully fabricate a g-CN film with high optical quality. In addition, we also analyzed the fluorescence lifetime of the g-CN film deposited on the ITO glass substrate by using a homemade time-correlated single-photon counting apparatus and found that it was much shorter than that of bulk g-CN.