Interactions of water molecule with HfB2 and TaB2 (0 0 0 1) surfaces: A first-principles investigation

• H2O is adsorbed preferentially on top-Hf of HfB2 (0 0 0 1) and top-B of TaB2 (0 0 0 1).
• The interactions of H2O with HfB2 and TaB2 (0 0 0 1) surfaces are very weak.
• There is no dissociation of water molecule on HfB2 and TaB2 (0 0 0 1) surface.
• H2O does not oxidize HfB2 and TaB2 (0 0 0 1) surface at initial adsorption stage.

To investigate the interactions of water molecule with HfB2 and TaB2 (0 0 0 1) surfaces, we calculated the adsorption of molecular water on the (0 0 0 1) surfaces with first principles. In our calculations, we found that the adsorption energies calculated with PW91 (Perdew and Wang) functional are greater than with RPBE (Revised Perdew–Burke–Ernzerhof). Adsorption energies show that water molecule is adsorbed preferentially on top-Hf site of HfB2 (0 0 0 1) surface, whereas the preferential position is top-B site on TaB2 (0 0 0 1). And there exists a tilt angle between the plane of water molecule and HfB2 and TaB2 (0 0 0 1) surfaces, respectively. On the preferential sites, the adsorption energy calculated with PW91 is −0.43 eV on HfB2 (0 0 0 1) and −0.29 eV on TaB2 (0 0 0 1) surface. The H–O bond length and bond angle of water molecule do not change greatly compared with isolated water molecule. The charges of H and O atoms on TaB2 (0 0 0 1) surface are closer to those of isolated water molecule than on HfB2 (0 0 0 1) surface. Furthermore, the charge density differences between water molecule and the (0 0 0 1) surfaces are very small. The differences around O atom on TaB2 (0 0 0 1) are about eight times smaller than those on HfB2 (0 0 0 1). Our results indicate that the interactions between water molecule and the (0 0 0 1) surfaces are very weak. Moreover, the interaction of water molecule with TaB2 (0 0 0 1) is weaker than with HfB2 (0 0 0 1). No dissociation of water molecule is found in our investigations. Therefore, HfB2, especially TaB2, may not be oxidized by water molecule at initial adsorption stage.

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