Letter abstract
Nature Materials 3, 289 - 293 (2004)
doi:10.1038/nmat1106
Subject Categories: Surface and thin films | Computation, modelling and theory
A negative surface energy for alumina
Zbigniew 
odziana1,
Nan-Yu Topsøe2
and
Jens K. Nørskov1
The surface energy of a solid measures the energy cost of increasing the surface area. All normal solids therefore have a positive surface energy1—if it had been negative, the solid would disintegrate. For this reason it is also generally believed that when certain ceramics can be found in a highly porous form, this is a metastable state, which will eventually sinter into the bulk solid at high temperatures. We present theoretical evidence suggesting that for 
-alumina, the surface energy is strongly dependent on the size of the crystallites, and that for some facets it is negative for thicknesses larger than 
1 nm. This suggests a completely new picture of porous alumina in which the high-surface-area, nanocrystalline form is the thermodynamic ground state. The negative surface energy is found to be related to a particularly strongly adsorbed state of dissociated water on some alumina surfaces. We also present new experimental evidence based on infrared spectroscopy, in conjunction with X-ray diffraction and surface-area measurements, that -alumina has indeed very stable surface OH groups at high temperatures, and that this form of alumina does not sinter even at temperatures up to 1,300 K.
- Center for Atomic-scale Materials Physics, Department of Physics, Technical University of Denmark, Building 307, DK-2800 Lyngby, Denmark
- Haldor Topsøe Research Laboratories, Nymøllevej 55, DK-2800 Lyngby, Denmark
Correspondence to: Jens K. Nørskov1 e-mail: norskov@fysik.dtu.dk
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