Surface characterization of silica fine powder prepared by microwave cold plasma heating

Abstract

Silica fine powders have been prepared by means of the microwave cold plasma heating technique, and their surface characterizations have been investigated. The plasma heating, performed under suitable conditions of heating time and microwave output power, produced water-free samples with large surface areas after only a few minutes. Observation of the samples via atomic force microscopy (AFM), using a technique devised by the authors, revealed an uneven surface covered by fine, bubble-like elements of > 1 nm in height and thickness. The surface valences of the samples were also examined by X-ray photoelectron spectrometry (XPS), and it was found that Si²⁺ low-valence sites exist on the surface of the silica fine powder, probably owing to reduction activity by hydrogen radicals within the plasma. The solid acidic properties of the prepared silica fine powder have been investigated, revealing acidic sites having a strength of –8.2 < H₀⩽–5.6. These are shown, by IR absorption spectroscopy and by the results of catalytic reactions, to be Lewis acid sites.