Estudo teórico das propriedades estruturais e eletrônicas de nitretos de carbono grafíticos intercalados com Na⁺ e Mg²⁺ em reações de fotocatálise de evolução de H₂

Abstract

Graphitic carbon nitride (g-C₃N₄) is a semiconductor material with many nitrogen rings (pores) throughout its structure and constituted by several layers stacked on top of each other united by van der Waals forces. One of its potential applications is as a photocatalyst in hydrogen evolution reactions. Recently, g-C₃N₄ intercalated with alkali or alkaline earth metals in its structure, also known as poly(heptazine imide) alkali metal salts (M-PHI), showed superior photocatalytic activity than conventional g-C₃N₄. Given this experimental evidence, the materials Na-PHI and Mg-PHI were studied through classical molecular dynamics simulations and excited state calculations, in order to better understand the organization of cations throughout the structure of this material, their role in photocatalytic hydrogen evolution reactions, and the reason why Mg-PHI has higher photocatalytic activity than Na-PHI. In molecular dynamics simulation, it was found that the preferred position of the Na⁺ cation was always as close as possible to the vertice’s pores of this material, and as the number of layers increased, the amount of Na⁺ confined inside the pores also increased. Furthermore, it was observed throughout the molecular dynamics trajectory that there was always a smaller amount of cations inside the pores of Mg-PHI compared to Na-PHI, thus allowing more space for water molecules to accommodate themselves inside. This first difference between these two systems can be considered one of the reasons why the photocatalytic activity of Mg-PHI is greater than Na-PHI. Another factor that may also contribute to this is the presence of a larger hydration layer around the Mg²⁺, which leads to the hypothesis that the role of cations in these reactions is to stabilize the water molecules inside the pores. The excited state calculations corroborate to this hypothesis, since the analysis of the natural transition orbitals (NTO) around the experimental region of interest (420 nm) showed that in the transition with the highest oscillator strength (most probable), the NTO hole is much more localized in the PHI sheet, while the NTO particle is located both in the PHI sheets and in the water molecules around the Mg²⁺ cations inside the central pore, thus indicating that there is a probability of photoelectrons be transferred from the PHI sheets to the water molecules belonging the Mg²⁺ solvation sphere. A similar pattern was also found in the NTOs for the transition with the second highest oscillator strength in this region, although in this transition there was a higher probability of electron-hole recombination than the photoelectron transfer to the water molecule.