Abstract
The surface acidity of aluminum oxide (AlOx) films was evaluated for the first time by inelastic electron tunneling (IET) spectroscopy using the Hammett indicators as probe molecules. The AlOx films were formed by oxidation of the aluminum films by DC glow discharge in oxygen atmosphere, and the indicators with pKa values ranging from -8.2 to +4.8 were used. The indicators, methyl red (MR) with pKa=+4.8 and methyl yellow (MY) with pKa=+3.3 contain the N=N bond and do not have the N–H bond. The IET spectrum of MR showed the peak related to the N–H stretching vibration at 3295 cm-1, while the spectrum of MY did not show the peak. These results indicate that the MR molecule chemisorbs at the N atom on the hydroxyl group acting as the Brønsted acid site and the MY molecule does not chemisorb on the site. Thus, the acid strength of the Brønsted site was determined to be approximately +3.3≦H0(B)≦+4.8.
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