Molecular simulation for gas adsorption at NiO (100) surface.

Density functional theory (DFT) calculations have been employed to explore the gas-sensing mechanisms of NiO (100) surface on the basis of energetic and electronic properties. We have calculated the adsorption energies of NO(2), H(2)S, and NH(3) molecules on NiO (100) surface using GGA+U method. The calculated results suggest that the interaction of NO(2) molecule with NiO surface becomes stronger and contributes more extra peaks within the band gap as the coverage increases. The band gap of H(2)S-adsorbed systems decrease with the increase in coverage up to 0.5 ML and the band gap does not change at 1 ML because H(2)S molecules are repelled from the surface. In case of NH(3) molecular adsorption, the adsorption energy has been increased with the increase in coverage and the band gap is directly related to the adsorption energy. Charge transfer mechanism between the gas molecule and the NiO surface has been illustrated by the Bader analysis and plotting isosurface charge distribution. It is also found that that work function of the surfaces shows different behavior with different adsorbed gases and their coverage. The work function of NO(2) gas adsorption has a hill-shaped behavior, whereas H(2)S adsorption has a valley-shaped behavior. The work function of NH(3) adsorption decreases with the increase in coverage. On the basis of our calculations, we can have a better understanding of the gas-sensing mechanism of NiO (100) surface toward NO(2), H(2)S, and NH(3) gases.