Electronic structure of alumina surface

Physical review, 1994

Using a tight-binding, total-energy model, we predict the atomic and electronic structure of the relaxed (1X1) corundum (0001) surface. The surface shows a large, bond-length-conserving relaxation, which is allowed by the topology of the surface. The relaxation is driven by a rehybridization of the surface Al atoms to sp, and an accompanying drop in the energy of occupied surface states, during the re-0 laxation. Displacements of surface atoms from bulk positions are as large as 0.7 A, and should be observable using a low-energy electron difFraction intensity analysis. Full relaxed atomic positions are reported, as well as a wavelength-resolved surface band structure, including orbital characters of surface states.

Applied Surface Science, 2008

Electronic and atomic structure of low index polar (0 0 1, 0 1 1, 1 1 1) spinel g-alumina surfaces have been investigated using a modified largescale quantum semi-empirical simulations. Atomic structure optimization and electronic structure calculation were done in the direct space on periodic large unit cells in which random distribution of cationic vacancies close to the surface were included. Predicted electronic structures match more accurate ab initio Density Functional Theory (DFT) results on aluminum oxides. We found that the restructured surfaces behave as amorphous-like shapeless planes provided unit cell dimensions are much greater than primitive translations as periodicity constraints are weakened. Aluminum ions vacancies in the surface region are shown to induce a surface atomic disordering even at 0 K, correlated with the appearance of low coordinated ions, as proven experimentally and theoretically with classical Molecular Dynamics (MD) calculation on some aluminum oxide systems. The lowering of the surface charge density is obtained from two distinct mechanisms involving large ion movements. On the oxygen rich plane termination, this results from the formation of O 2 dÀ entities. On the aluminum rich plane termination, the same effect is related to a loss of coordination number together with a reduction of cationic charge. Furthermore, large-scale simulations allow to statistically quantify specific relaxed surface ion densities for which the Local Density of States (LDOS) is evaluated. Their inferred electronic properties are then compared to available probe molecule adsorption experiments investigated by infrared spectroscopy. We show that surface electronic states are not a simple function of ion coordination. Our large-scale quantum semi-empirical calculation fills the gap between atomic structures predicted by classical MD approach with electronic structure results obtained by DFT on small systems.

The Journal of Physical Chemistry, 1994

The structure and bonding in 8-A1203 has been investigated at the ab initio level with the periodic Hartree-Fock method. The structure optimization reported is in very good agreement with the available crystallographic data. The energy of &A1203 is found to be 42 kJ mol-' higher than that of a-alumina. The bonding in 8-A1203 is discussed from the electron density and electron localization (ELF) functions. The structure is found to be ionic; the main differences between tetrahedral and octahedral sites are due to the polarization of the oxygen dianion. Estimates of cationic radii for tetracoordinated and hexacoordinated A1 are found to be in good agreement with the recent compilation of Feth et al.

Surface and Interface Analysis, 2017

We have studied the evolution of surface basicity as a function of calcination temperature in a range of transition alumina-rich analogues of metallurgical grade aluminas produced by calcination of Bayer gibbsite. Specific basicity, the number of basic sites per unit surface area, was determined by thermometric back-titration and found to increase with calcination temperature up tõ 700°C to 800°C, decreasing dramatically thereafter. The population of tetrahedrally coordinated Al 3+ , which exhibits a qualitatively similar evolution, was determined by Al K-edge near-edge X-ray absorption fine structure (NEXAFS) spectroscopy but found not to correlate with basicity changes. Interestingly, a shift to higher and then lower binding energy of Al 2p photoelectrons by Xray photoelectron spectroscopy and changes in the intensity of O K-edge NEXAFS spectra do appear to correlate with surface basicity. Exploiting the differing surface sensitivities of NEXAFS spectra collected in partial electron and total fluorescence yield modes, we find O K-edge spectra intensities, Al 2p X-ray photoelectron spectrometer binding energies, and surface basicity all reflect the reorganisation of internal surfaces rather than changes in AlO 4 :AlO 6 occupation.

2007

Physical Review B, 2003

First-principles calculations are performed on the stable ␣-Al 2 O 3 and metastable-Al 2 O 3 phases to understand the stability and bonding of the flexible alumina surfaces. The ͑001͒ and (001) surfaces of-Al 2 O 3 are investigated and compared to ␣-Al 2 O 3 (0001). A needed extension of the original formulation of the Tasker's rule for the stability of low-symmetry ion-crystal surfaces is found. Also, use of extended Pauling's rules makes the results applicable to other metastable alumina phases. The most stable termination of (001)/(001) is found to be in the middle of an Al layer, similarly to ␣-Al 2 O 3 (0001). This surface is shown to be nonpolar, even though a Tasker point-charge description implies a polar classification. The asymmetry in atomic and electronic structures introduced by the tetrahedrally coordinated Al (Al T) ions is found to have important consequences for the surface properties. The bulk cation-vacancy lines caused by the Al T make the (001)/(001) surfaces more open than ␣(0001), thus allowing a huge inward relaxation (Ϫ117%) at (001), making this surface O terminated. The charge asymmetry in bulk-Al 2 O 3 causes an excess of electrons at (001), yielding a one-dimensional metallic surface state. Also, the presence of Al T in the near-surface layer is found to be destabilizing.

Journal of the American Ceramic Society, 1991

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The Journal of Physical Chemistry B, 2006

We have studied surface hydroxyls adsorbed onto (001), (011), and (111) γ alumina surfaces using a quantumchemistry approach in order to compare with empirical models proposed in the literature. Local electronic structures and geometries in the low OH coverage limit have been evaluated for both ideal and relaxed surfaces with the help of a large scale periodic quantum-chemical code. Hydroxyl groups are adsorbed onto surfaces, and a study of their local electronic structure, vibrational frequencies, charges, and adsorption energies is performed and analyzed as a function of their adsorption site geometry. Our results show that, even on ideal (nonrelaxed) surfaces, OH local environments are more complicated than those stated by empirical models and strongly influence the hydroxyl stretching vibrational mode. Large scale simulation shows that disorder takes place even at 0 K, and the analysis of the vibrational frequencies leads to a revision of Knözinger's empirical model. Cationic vacancies in the first surface layers have also been taken into account; they have a significant influence on the surface atomic and electronic structures, modifying the physical properties of adsorbed OH entities. This work emphasizes the necessity to perform an electronic structure calculation to better understand adsorbed OH properties on γ alumina surfaces and reveals the difficulty to make a oneto-one correspondence between OH stretching frequencies and their other physical properties. Finally, we show that these results agree with some available experimental studies.

Ultramicroscopy, 1987

Atomic-resolution electron microscopic images cf ruby show (0001) polar surface facets developing under electron irradiation. A ttempts...~. to understand the "dark-line'" c~mtrasts associated with the surface atom profiles, following comparison of the experimental images w~.th computer modelling, leads to the realization that aluminium atom and, xygen atom surface terminations may be distinguished. Hence surface polarity, as well as details of surface step structure and mobility, may be determined. Positive surface polarity is preferred.

Physica B: Condensed Matter, 2012

In this work we performed an ab initio/Density Functional Theory (DFT) study of structural and electronic properties of the (0 0 1) a-Al 2 O 3 surface. For this study we used two methods with different basis set: the Full-Potential Augmented Plane Wave plus local orbital (FP-APW þ lo) and a linear combination of numerical localized atomic orbital basis sets, employing the WIEN2k code and the SIESTA code, respectively. In order to calculate the structural and electronic properties of the reconstructed surface, we calculated the final equilibrium atomic position with the SIESTA code and then the electric-field gradient (EFG) at Al sites was calculated with the FP-APW þlo code using the optimized positions. Using this procedure we found equilibrium structures with comparative lower energy than those obtained using only the FP-APW þlo method. The EFG tensor and the local structure for Al were studied as a function of the depth from the surface for the relaxed structures. We found that distances down to 6Å from the surface are sufficient to converge the EFG and the Al-O distances to bulk values. The predicted bulk EFG at the Al site is in good agreement with available experimental values. These results can be used for local probes purposes, e.g., in the case of doping, with important sensitivity for probes located close to the top of the surface, in particular for distances smaller than 6Å.