Two-dimensional condensation anisotropic crystallization: H2/Ni(1 1 0)

The effects of hydrogen adsorption and desorption on the structure and morphology of Ni(1 1 0) have been investigated with STM over a wide range of temperatures and pressures. The adsorption of hydrogen at moderate coverage results in the formation of –Ni–H– strings and missing rows of Ni atoms oriented along the [1 1̄ 0], in agreement with previous work. At low hydrogen coverage only missing rows of Ni atoms are observed; no added rows are seen, suggestive of unresolved, mobile Nix–Hy, precursors to the –Ni–H– chains. The appearance and accumulation of the added –Ni–H strings is concomitant with the sharp, nearly vertical rise in work function reported by others, suggesting that the formation of the added row, Ni–H overlayer is due to the two-dimensional condensation of isolated –Ni–H– moieties at a critical pressure dictated by the surface temperature. During H2(g) desorption, the Ni liberated from the Ni–H overlayer fills the Ni missing rows, migrates along the [1 1̄ 0] to [0 0 1] steps or agglomerates into anisotropic islands of monoatomic step height (i.e. anisotropic 2D crystallization). The islands coalesce in time to yield isotropic structures which serve as sources of Ni for further Ni–H adsorption.