Urbach Energy as a Probe of Electronic Disorder in Alkali-Modified g-C₃N₄

Urbach energy, derived from the exponential tail of the absorption edge, is a simple parameter that can be used to evaluate structural disorder and localized states in semiconductors used for photocatalysis. It can be estimated from UV-Vis diffuse reflectance data by simple fitting and is therefore useful for comparing related materials. In our previous work on K-modified g-C₃N₄, we found that it changed with precursor type and with the degree of disorder introduced into the carbon nitride framework. Here, the same approach was used for Na-modified graphitic carbon nitride nanosheets prepared by one-step thermal polycondensation. The Na-modified samples retained the g-C₃N₄ structure, while the Urbach energy increased from 71.4 meV for pristine g-C₃N₄ to 81.1 and 82.7 meV for Na(0.01)-CN and Na(0.02)-CN. Photoluminescence and time-resolved fluorescence measurements indicate mild broadening of band-tail states and the formation of shallow electron traps rather than severe deep-defect formation. This was reflected in the photocatalytic performance. Under a single 416 nm LED, Na(0.01)-CN completely removed ofloxacin and tetracycline at 20 ppm within 8 min, with an apparent quantum yield of 5.83%. After Pt photodeposition, the same sample reached an initial hydrogen evolution rate of 8.8 mmol g^-1 h^-1. These results confirm that Urbach energy is useful for following electronic changes in modified carbon nitride and linking them with photocatalytic activity.