Interaction of hydrogen with ultrathin titanium layers on tungsten

Abstract

Hydrogen adsorption on titanium layers of an average thickness from 0.03 to 50 monolayers deposited on a tungsten substrate was investigated at liquid nitrogen temperature using the field electron emission microscopy techniques. It has been found that the interaction between titanium and hydrogen depends on the thickness and atomic structure of the Ti/W layer. For tightly arranged titanium layers isomorphous with the (011)W face the H–Ti interaction is weak and does not change the morphology and atomic structure of the layer. In the case of titanium layers isomorphous with the (001)W the H–Ti interaction dominates over the Ti–W interaction, beginning from the thinnest titanium layers deposited on the tungsten substrate. This results in a 3-dimensional nucleation of TiH x at the steps of the terraces of the (001)W face. For the titanium layers isomorphous with the (111)W and (112)W faces, the Ti–W interaction is stronger than the H–Ti interaction up to the thickness of 3 to 4 monolayers of titanium. Measurements of the average work function changes as a function of the H 2 exposure indicate that, apart from the hydrogen adsorption state which raises the work function, there exists another one which decreases the work function. The latter state occurs in titanium layers isomorphous with the (111)W face for the coverage in the range from 0.65 to 2.7 titanium monolayers. Measurements of the local work function changes carried out for H 2 adsorption on Ti/(111)W and Ti/(016)W indicate that this adsorption state cannot be recognized as the unstable β + adsorption state observed during the hydrogen adsorption on a surface of bulk titanium [R. Duś, E. Nowicka, Z. Wolfram, Surf. Sci. 269/270 (1992) 545]. The one observed in this experiment is stable and is believed to result from the immersion of the adsorbed hydrogen atoms into the electron gas of the Ti–W interface.