Initial adsorption of water molecule on HfC and TaC (001) surfaces from density-functional study

We investigated the initial adsorption of molecular water on HfC and TaC (0 0 1) surfaces using first-principles density-functional theory. In our investigation, the revised Perdew-Burke-Ernzerhof functional (RPBE) generalized gradient approach (GGA) was used to treat the exchange and correlation potential. We modeled the (0 0 1) surfaces with (root 2 x root 2) R45 degrees supercell. Every supercell contains five atomic planes. Our calculations show that the adsorption energies of H2O are low. The energies are -0.44 eV/molecule and -0.65 eV/molecule on HfC and TaC (0 0 1) surfaces, respectively. On both the (0 0 1) surfaces, molecular water prefers to be adsorbed on top of surface metal atom. However, only the oxygen atom of water molecule is bound with surface metal atom. And the plane of water molecule tends to be parallel to the surfaces. Compared with the oxygen and hydrogen atoms of an isolated water molecule, the oxygen and hydrogen atoms in H2O/HfC (0 0 1) and H2O/TaC (0 0 1) have less negative and positive charges, respectively. Our results show that water molecule is adsorbed molecularly on HfC and TaC (0 0 1) surfaces at initial adsorption stage. Moreover, the interaction of water molecule with the (0 0 1) surfaces is weak. Charge density differences and density of states show that O-Ta bond is slightly stronger than O-Hf bond. Due to the strong O-Ta bond, the oxidation resistance of TaC may stronger than HfC in water vapor atmosphere. (C) 2013 Elsevier B.V. All rights reserved.