Yasuji Muramatsu

Graphitic carbon nitride (g‐C ₃ N ₄ ) is a promising visible‐light‐driven photocatalyst that can be applied in various fields. However, its intrinsically low photocatalytic efficiency limits practical applications. In this study, g‐C ₃ N ₄ was structurally modified by ultraviolet (UV) irradiation to improve its photocatalytic performance. Both pristine and UV‐irradiated g‐C ₃ N ₄ photocatalysts were characterized by X‐ray diffractometry, X‐ray photoelectron spectroscopy, X‐ray absorption near‐edge structure spectroscopy, electron spin resonance spectroscopy, diffuse reflectance spectroscopy, photoluminescence spectroscopy, and nitrogen adsorption measurement. The photocatalytic efficiency was evaluated by measuring the decomposition rate of rhodamine B (RhB) in water. Our results demonstrated that short‐term UV irradiation for 45 min caused the elimination of pyridinic nitrogen (the N abundance ratio changed from 74 to 67 mol% after UV irradiation), leading to the formation of surface‐deficient sites and new defects in the band structure, which could accelerate the photocatalytic RhB decomposition (the rate constant increased by 1.23 times after UV irradiation). Prolonged UV irradiation resulted in destructive structural changes involving the elimination of carbon, which negatively affected the photocatalytic reactivity due to a decreased number of reactive sites, reduced specific surface area, and increased electron–hole recombination. Therefore, appropriate UV irradiation conditions should be adopted to improve the photocatalytic performance of g‐C ₃ N ₄ .

X-ray absorption near-edge structure (XANES) provides element-specific insights into local electronic and structural environments, but quantitative interpretation of molecular XANES under periodic boundary conditions (PBC) remains challenging due to finite-size effects and core-hole treatments. In this work, we systematically investigate how core-hole approximations and charge compensation schemes affect transition energies, energy alignment, and chemical-shift reproducibility in PBC-density functional theory-based molecular XANES calculations. Using ethane as a model system, we show that the full core-hole (FCH) approach exhibits a pronounced supercell-size dependence originating from interactions between background charge and charged molecules, with transition energies largely changed by leading-order finite-size terms. In contrast, the excited core-hole (XCH) method rapidly converges owing to its neutral final state. We further demonstrate that most finite-size effects in FCH can be removed by Makov–Payne corrections based on multipole expansion of the electrostatic energy of charged supercells under PBC. Furthermore, we propose a simple Fermi-level-based energy correction (EF/2) that provides comparable improvement using only a single supercell. Extending the analysis to an n-alkane series reveals that while intrinsic electronic-structure changes govern peak shifts for small molecules, systematic energy drifts persist in FCH for larger molecules, whereas XCH and FCH + EF/2 remain stable. Finally, for small molecules at the C and N K-edges, XCH and FCH + EF/2 accurately reproduce experimental chemical shifts, whereas uncorrected FCH fails. These results provide practical guidelines for reliable energy alignment and chemical-shift analysis in molecular XANES under PBC, supporting robust applications to molecular, adsorption, and interfacial systems.