Hydroxyl (OH) groups on the well-characterized ultra-thin (0.5 nm) Al2O3 film grown by oxidation of NiAl(110) [1] dramatically alter the morphology of deposited nano-crystalline transition metal particles (Rh, Pd) [2]. According to STM characterization, the OH groups increase the metal dispersion by promoting uniform nucleation of small metal particles in large numbers on open oxide terraces, whereas on the pristine Al2O3 film without OH the same metal coverage preferentially decorates intrinsic oxide line defects with fewer but larger metal aggregates [2,3]. The introduction of OH groups requires dissociating H2O molecules at pre-deposited aluminum atoms [1,3], because unlike other Al2O3 phases the surface of this particular Al2O3/NiAl(110) film is purely oxygen-terminated and behaves inert against H2O vapor and even condensed ice films. Although the hydroxylation procedure is well established, the resulting OH coverage has never been quantified.
We therefore employ hydrogen-specific 1H(15N,αγ)12C nuclear reaction analysis (NRA) [4] in order to evaluate the absolute OH coverage on the Al2O3/NiAl(110) film after hydroxylation with metallic Al. In particular we study the dependence of the OH coverage on the amount of pre-deposited Al and the thermal stability of the OH groups. The results confirm that the native OH coverage on the pristine oxide film is very low (E13 /cm^2), i.e. in the order of spurious point defects. After hydroxylation, the OH coverage linearly follows the amount of pre-deposited Al in the range up to 2 monolayers (1.2xE15 /cm^2) Al, with one OH group being formed per Al atom. The OH groups are thermally stable up to 450 K, above which they decay and entirely disappear at 600 K. A thermally de-hydroxylated Al2O3 surface retains its ability to dissociate H2O molecules and thus to reversibly regenerate the OH coverage determined by the amount of pre-deposited Al. The results are discussed in comparison to the behavior of OH groups on surfaces of other Al2O3 phases.
[1] J. Libuda et al.: Surf. Sci. 384 (1997) 106.
[2] M. Heemeier et al.: Catal. Lett. 68 (2000) 19.
[3] M. Baeumer, H.-J. Freund: Prog. Surf. Sci. 61 (1999) 127.
[4] M. Wilde et al., J. Appl. Phys. 98 (2005) 023503. |