Acid-basic site detection and mapping on solid surfaces at the nanoscale, by Kelvin force microscopy (KFM)

R.F. Gouveia, J.S. Bernardes, T.R.D. Ducati, F. Galembeck
University of Campinas, BR

Keywords: KFM, Kelvin probe, Brønsted acid-base, solid surfaces, relative humidity


The present work shows, for the first time, that the electric potential pattern of acid and basic solid surfaces is modified under variable relative humidity, as determined by using Kelvin force microscopy (KFM). This allows mapping acid and basic sites at the nanoscale. The potential on acid surfaces becomes more negative as the water vapor pressure increases, while it becomes more positive on basic solids. These results verify the following hypothesis: OH- or H+ ions associated with atmospheric water ion clusters are selectively adsorbed on solid surfaces, depending on the respective Brønsted acid or base character. KFM under variable humidity is thus a rigorous but convenient alternative to determine acid-base character of solid surfaces, with a great advantage: it uses only one amphoteric and simple reagent for determining both acid and base sites. To conclude, this technique provides information on the spatial distribution of acid-base sites at nanoscale, thus contributing to detailed knowledge of particulate matter, which is currently inaccessible to any other method.