Surface Structure of Protonated R-Sapphire (11̅02) Studied by Sum-Frequency Vibrational Spectroscopy

Abstract

Sum frequency vibrational spectroscopy was used to study the protonated R-plane (1102 ) sapphire surface. The OH stretch vibrational spectra show that the surface is terminated with three hydroxyl moieties, two from AlOH 2 and one from Al 2 OH functional groups. The observed polarization dependence allows determination of the orientations of the three OH species. The results suggest that the protonated sapphire (1102 ) surface differs from an ideal stoichimetric termination in a manner consistent with previous X-ray surface diffraction (crystal truncation rod) studies. However, in order to best explain the observed hydrogenbonding arrangement, surface oxygen spacing determined from the X-ray diffraction study requires modification.

Effect of pH on the Water/α-Al2O3 (11̅02) Interface Structure Studied by Sum-Frequency Vibrational Spectroscopy

“…5. 21,32 This surface has C 1v symmetry with respect to the ( 1 120 [18][19][20] provide estimated pK values for protonation/deprotonation reactions of the different relevant groups:…”

“…[18][19][20] The band at 3490 cm -1 appears to arise mainly from the hydrogenbonded (H-bonded) hydroxyl groups of AlOH 2 at the surface, as it has a relatively narrow linewidth, a fixed peak position, and a polarization dependence that closely resembles the band observed on the dry -Al 2 O 3 (1102 )surface . 21 The sudden decrease of this feature's amplitude with increase of pH from 9 to 10 indicates that the pK value for deprotonation of AlOH 2 is around 9.5. One would anticipate that the - However, in the sum-feqequency vibrational spectrum, the OH stretch mode due to Al 2 OH is relatively weak and may be masked by the OH band of the interfacial water.…”

Surface-Induced Anisotropic Orientations of Interfacial Ethanol Molecules at Air/Sapphire(11̅02) and Ethanol/Sapphire(11̅02) Interfaces

“…Accordingly, the molecules are strongly bonded to the surface with the methyl group protruding out and assuming an azimuthal orientation distribution that reflects the effective C 1V symmetry of the α -Al 2 O 3 (1102 ) surface. [9] In the case of the α -Al 2 O 3 (1102 )/ethanol liquid interface, the spectra indicate that while the first molecular monolayer on the sapphire surface is still well-ordered, orientational ordering of molecules in the second monolayer, though very much randomized thermally, is still appreciable, with an average polar orientation opposite to that of the first monolayer. The overall orientation distribution of the interfacial ethanol molecules still exhibits C 1V symmetry.…”

“…To remove the remaining water and organic contaminates on the surface, the sample was heated at ~350 o C for 1 hour. [9,12] To prepare an ethanol monolayer on the cleaned α -Al 2 O 3 (1102 ) surface, we exposed the sample to ethanol vapor using the scheme described in Ref. [13,14] Our SFVS setup has been described elsewhere.…”

Photonic detection and characterization of DNA using sapphire microspheres

“…The degradation of silicon and glass interfaces in aqueous solutions limits their use as biosensors, 33,34 and therefore, we consider synthetic sapphire as a promising alternative. 35,36 Afterward, the sapphire sphere was rinsed with ultrapure water and dried with flowing nitrogen gas.…”