skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Atomistic siumulations of amorphous alumina surfaces.

Abstract

The surface structure of amorphous Al{sub 2}O{sub 3} has been studied using atomistic molecular dynamics simulations. The density profiles indicate that oxygen is preferred at the surface causing Al enrichment just below (<2 {angstrom}) the surface. Distributions of coordination numbers, bondlengths and bond angles indicate that edge sharing Al tetrahedra configurations are more preferred at the surface than in the bulk. Structural differences of amorphous and crystalline alumina surfaces are discussed.

Authors:
; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
935628
Report Number(s):
ANL/MSD/JA-56991
TRN: US200816%%638
DOE Contract Number:
DE-AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: Phys. Rev. B; Journal Volume: 74; Journal Issue: 2006
Country of Publication:
United States
Language:
ENGLISH
Subject:
36 MATERIALS SCIENCE; ALUMINIUM OXIDES; AMORPHOUS STATE; SURFACE PROPERTIES; MOLECULAR DYNAMICS METHOD; BOND ANGLE; COORDINATION NUMBER; CRYSTAL STRUCTURE

Citation Formats

Adiga, S. P., Zapol, P., and Curtiss, L. A. Atomistic siumulations of amorphous alumina surfaces.. United States: N. p., 2006. Web. doi:10.1103/PhysRevB.74.064204.
Adiga, S. P., Zapol, P., & Curtiss, L. A. Atomistic siumulations of amorphous alumina surfaces.. United States. doi:10.1103/PhysRevB.74.064204.
Adiga, S. P., Zapol, P., and Curtiss, L. A. Sun . "Atomistic siumulations of amorphous alumina surfaces.". United States. doi:10.1103/PhysRevB.74.064204.
@article{osti_935628,
title = {Atomistic siumulations of amorphous alumina surfaces.},
author = {Adiga, S. P. and Zapol, P. and Curtiss, L. A.},
abstractNote = {The surface structure of amorphous Al{sub 2}O{sub 3} has been studied using atomistic molecular dynamics simulations. The density profiles indicate that oxygen is preferred at the surface causing Al enrichment just below (<2 {angstrom}) the surface. Distributions of coordination numbers, bondlengths and bond angles indicate that edge sharing Al tetrahedra configurations are more preferred at the surface than in the bulk. Structural differences of amorphous and crystalline alumina surfaces are discussed.},
doi = {10.1103/PhysRevB.74.064204},
journal = {Phys. Rev. B},
number = 2006,
volume = 74,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • The effect of hydroxylation on the surface structure of amorphous alumina is investigated using classical molecular dynamics simulations. It is found that the hydroxylated amorphous alumina surface is terminated by hydroxyl groups singly and doubly coordinated to aluminum. Root-mean-square roughness calculations and density profiles across the film indicate that hydroxylated surfaces are rougher than non-hydroxylated surfaces. The power spectrum identifies different vibrational stretching frequencies for the singly and doubly coordinated surface OH groups. The role of the surface OH groups in surface reactivity is discussed.
  • Cited by 3
  • Adsorption isotherms of H/sub 2/ and of D/sub 2/ at 78 K have been determined for alumina and modified alumina surfaces. In all cases plots of P/sub H/sub 2// vs. P/sub D/sub 2// for constant amounts adsorbed were linear. This behavior has been predicted by statistical thermodynamics and is shown to follow from equations used to describe chemisorption data. Values of separation factors (S) were derived from the slopes of these curves. This was justified by the earlier statistical thermodynamic formulation and by derivations from Henry's law and Langmuir adsorption isotherms. The adsorption data, however, obeyed accurately the Freundlich lawmore » and it is shown that this does not lead to the usual definition of S, i.e., a ratio of adsorption equilibrium constants. The surfaces could also be characterized in terms of fractional coverage (theta) or the amount adsorbed (n/sub H/sub 2//) at constant pressure. When the parent aluminas were dehydroxylated at increasingly higher temperature, both theta and S increased, but when surfaces were fluorided, S increased while theta decreased. Impregnation with Ga/sup 3 +/ greatly increased S with little change in theta, but when a molybdenum monolayer was grown onto the surface the changes in these properties were relatively small. Reduction of the molybdenum-alumina surface with H/sub 2/, however, effected increases in both theta and S, whereas the latter was substantially decreased when the reducing agent was CO. Comparison is made of the present values of S with those obtained by the chromatographic method. 6 figures, 2 tables.« less
  • The adsorption of ammonium heptamolybdate (AHM) on phosphated alumina and of P{sub 2}Mo{sub 5}O{sub 23}{sup 6{minus}} and PMo{sub 12}O{sub 40}{sup 3{minus}} on {gamma}-Al{sub 2}O{sub 3} has been studied by using FTIR, Raman, and {sup 31}P solid-state NMR spectroscopies, and TPR. Adsorption of phosphate prior to contacting of {gamma}-Al{sub 2}O{sub 3} with AHM leads to a deactivation of the alumina surface still remaining. Phosphate reacts with {gamma}-Al{sub 2}O{sub 3} to form an AlPO{sub 4}-type surface phase, which itself is capable of adsorbing molybdate, with the formation of an irreducible surface molybdophosphate, its adsorption capacity being lower, however, than that of themore » original alumina. The primary phosphomolybdate adsorption reaction on alumina is shown to be the same as that observed in the case of AHM, viz., a reaction with the basic surface OH groups, leading to the decomposition of the adsorbing species.« less
  • This high-resolution transmission electron microscopy (HRTEM) study revealed changes due to calcium segregation in the atomistic structure of the ({bar 1}104) twin boundary in {alpha}-Al{sub 2}O{sub 3}. Calcium grain-boundary doped and undoped twins were prepared using ultrahigh vacuum (UHV) diffusion bonding of two highly pure single crystals. Two distinct boundary configurations--each having mirror and glide mirror symmetry--were found in the calcium-doped bicrystal. Thus far, only the mirror-related configuration could be identified in the undoped boundary. To estimate the atomistic structure of the grain boundary, simulated images of four supercells with different calcium contents were compared to experimental HRTEM micrographs. Themore » amount of calcium found at the ({bar 1}104) twin boundary using this method was consistent with results that were obtained by energy-dispersive X-ray spectroscopy.« less