Formaldehyde adsorption on a hydrogenated gallium nitride monolayer: A density functional theory study

Based on first principles calculations, we have studied the adsorption of formaldehyde (CH2O) on a hydrogenated gallium nitride (H-GaN) monolayer with a H vacancy. On the N side of the monolayer, the CH2O molecule is physisorbed on top of the vacancy, with a low energy gain of 0.03 eV. On the other hand, the molecule is chemically adsorbed on the Ga atom, with the H vacancy, gaining 1.06 eV. In this intermediate state, the double CO bond is partially broken, leaving an unpaired electron in the C atom. To complete the reaction, a H atom is abstracted from the nearest site, resulting in the formation of a stable methoxyl group (CH3O) plus a new vacancy on the surface. Initial, intermediate, transition, and final states were characterized to understand the proposed reaction. The minimum energy path (MEP) is described using the climbing image nudged elastic band method (CI-NEB). Our calculations show that the molecule chemically adsorbs on the monolayer, on an exothermic reaction. A low activation energy of 0.28 eV is needed to hydrogenate the adsorbed CH2O molecule and generate a new H-vacancy, indicating that a self-propagating reaction is feasible. Adsorption of the following molecules results in a two dimensional growth.