Structure and Reactivity of α-Al₂O₃(0001) Surfaces: How Do Al–I and Gibbsite-like Terminations Interconvert?

Abstract

The α-Al 2 O 3 (0001) surface has been extensively studied because of its significance in both fundamental research and application. Prior work suggests that in ultra-high-vacuum (UHV), in the absence of water, the so-called Al−I termination is thermodynamically favored, while in ambient, in contact with liquid water, a Gibbsite-like layer is created. While the view of the α-Al 2 O 3 (0001)/H 2 O(l) interface appears relatively clear in theory, experimental characterization of this system has resulted in estimates of surface acidity, i.e., isoelectric points, that differ by 4 pH units and surface structure that in some reports has non-hydrogen-bonded surface aluminol (Al−OH) groups and in others does not. In this study, we employed vibrational sum frequency spectroscopy (VSFS) and density functional theory (DFT) simulation to study the surface phonon modes of the differently terminated α-Al 2 O 3 (0001) surfaces in both UHV and ambient. We find that, on either water dosing of the Al−I in UHV or heat-induced dehydroxylation of the Gibbsite-like in ambient, the surfaces do not interconvert. This observation offers a new explanation for disagreements in prior work on the α-Al 2 O 3 (0001)/liquid water interface�different preparation methods may create surfaces that do not interconvert�and shows that the surface phonon spectral response offers a novel probe of interfacial hydrogen bonding structure.

Computational Investigations of the Water Structure at the α-Al2O3(0001)–Water Interface

“…62 This result is in agreement with the obvious broad peak centered at 3000 cm −1 in the experimental spectra and the previous studies on alumina terminal interconversion. 23,62 To further investigate the spectral signatures of the vSFG spectra and connect it to the first layer water structure, the resonant imaginary component, Imχ xxz (2) , was examined for the two systems at different surface hydroxylation levels. The sign of the imaginary part reflects the direction of the transition dipole of the O−H stretch with respect to the interface, where a positive sign corresponds to the OH bond with the H atom pointing away from the interface, and a negative sign corresponds to an OH bond with H pointing toward the interface.…”

Computational Investigations of the Water Structure at α-Al2O3(0001)-Water Interfaces

“…47 This result is in agreement with the obvious broad peak centered at 3000 cm -1 in the experimental spectra and the previous studies on alumina terminal interconversion. 23,47 To further investigate the spectral signatures of the SFG spectra and connect it to the first layer water structure, the resonant imaginary component, 𝐼𝑚𝜒 qqr (() was examined for the two systems at different surface hydroxylation levels. The sign of the imaginary part reflects the direction of the transition dipole of the O-H stretch with respect to the interface, where a positive sign corresponds to OH bond with the H atom pointing away from the interface, and a negative sign corresponds to an OH bond with H pointing towards the interface.…”

“…[20][21][22] Recent work by Yue et al pointed out that the experimental investigations employed different sample preparation techniques, which in turn can modify the surface structure of the alumina slab. 23 In addition, they revealed that once the alumina surface is created, the Al-terminated surface and the Gibbsite-like surface do not interconvert. 23 Therefore, it is of great importance to characterize the surface structure of these systems.…”

“…23 In addition, they revealed that once the alumina surface is created, the Al-terminated surface and the Gibbsite-like surface do not interconvert. 23 Therefore, it is of great importance to characterize the surface structure of these systems.…”

Atomic-scale modelling of organic matter in soil: adsorption of organic molecules and biopolymers on the hydroxylated α-Al 2 O 3 (0001) surface

“…The (0001) surface of α-Al 2 O 3 is the most energetically stable surface of this mineral. A number of studies investigated the properties of this surface, such as the surface structure [10][11][12][13][14][15], energy [10,16] and adsorption of water [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] and other small molecules [29][30][31][32]. The single aluminium terminated surface is accepted as the most stable form of the bare α-Al 2 O 3 (0001) surface [10,11,14,16].…”