Density functional theory based studies on the adsorption of rare-earth ions from hydrated nitrate salt solutions on g-C3N4 monolayer surface

This article represents density functional theory (DFT) based comparative analysis on six trivalent rare-earth ions (RE3+; RE: Y, La, Ce, Sm, Eu and Gd) absorption, from the respective nitrate-hexahydrate salts, on graphitic carbon nitride (g-C3N4) 2D monolayer, and the photocatalytic properties of the RE3+ adsorbed g-C3N4 systems (g-C3N4/RE3+) based on the ground-state electronic structure calculations. Structure, stability and coordination chemistry of two configurations of each hydrated RE-salt system are discussed in detail. Both DFT (B3LYP/SDD) and semi-empirical (Sparkle/PM7) calculations identify the central N6 vacancy of pristine g-C3N4 as the most suitable site for RE3+ adsorption. Bader’s QTAIM, Mayer bond order and charge population analyses (ADCH, CHELPG and DDEC) are performed to describe the bond characteristics within the systems under study. Thermochemical calculations suggest that the adsorption process is thermodynamically more feasible for higher atomic number (Z) RE3+ [Sm3+, Eu3+ and Gd3+], compared to lower-Z RE3+ [Y3+, La3+ and Ce3+] ions. Besides, the better photocatalytic properties of higher-Z RE3+ adsorbed g-C3N4 systems are revealed from better HOMO-LUMO delocalization, decreased HOMO-LUMO gap, increased softness, higher electrophilicity and electron transfer parameter, compared to pristine or lower-Z RE3+ adsorbed g-C3N4 systems, as obtained from Hirshfeld orbital compositions, density of states and condensed Fukui function analyses.