Acid–Base Site Detection and Mapping on Solid Surfaces by Kelvin Force Microscopy (KFM)

Abstract

Electrostatic potential at the surface of acidic or basic solids changes under higher relative humidity (RH), as determined by using Kelvin force microscopy (KFM). 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. Therefore, Kelvin microscopy, under variable humidity, is a rigorous but convenient alternative to determine the acid-base character of solid surfaces, with a great advantage: it uses only one amphoteric and simple reagent to determine both the acid and base sites. Moreover, this technique provides information on the spatial distribution of acid-base sites, which is currently inaccessible to any other method.

Friction, tribochemistry and triboelectricity: recent progress and perspectives

“…Charging by this mechanism depends mostly on the Brønsted acid-base character of solid surfaces: hydronium ions adsorb on basic sites while hydroxide 15 ions adsorb on acidic sites. 77 This suggests that the behavior of any material tested in a triboelectric series is dependent on the actual state of its surface and especially on the detailed surface chemical composition that is often ignored.…”

“…35 Recent work from the authors' group shows the participation of OHand H + ions derived from atmospheric water on charge pattern formation and suppression, in many materials. 24,77,[94][95][96][97][98][99][100] and its dependence on the acid-base characteristics of the solid surface was referred to in the previous section. Hogue et al 101 40 considered that surfaces may have regions with more or less available sites for a given ionic species, causing charge imbalances and giving rise to differences in electric potential.…”

“…145,146 Another mechanism for charge dissipation is its coupling to water vapor adsorption and desorption events, under dynamic equilibrium conditions, considering the strong independent 20 evidence on the role of atmosphere as a charge reservoir. 24,75,77,[94][95][96][97][98][99][100] In this case, a surface carrying excess negative charge, for instance, desorbs [OH(H 2 O) n ] − ions and/or adsorbs [H(H 2 O) n ] + ions, exchanging them with the atmosphere and thus showing a net decrease in the negative charge. 25 The rate of charging increases with the humidity, in contact electrification experiments of polystyrene with gold or stainless steel.…”

Kelvin-probe force microscopy of the pH-dependent charge of functional groups

“…Especially, the time a sample was exposed to humid air between drying and the actual KFM measurement could cause a significant, unwanted alteration/dissipation of the charge of ionizable groups, for example, through adsorption/ desorption of H þ -ions. 29 Furthermore, the surface charge measured by KFM of dielectric solids such as oxides, sulfates, or phosphates has been shown to be clearly dependent on ambient humidity, which was attributed to the acid or base character of the surfaces and corresponding adsorption/ desorption of H þ or OH À ions from ambient water. 29 These observations underpin our hypothesis and show that also the surface charge of other chemical groups (e.g., SiO À ) measured by KFM in the dry state is strongly dependent on the environmental conditions of the measurement.…”

Sensitive Gas-Sensing by Creating Adsorption Active Sites: Coating an SnO2 Layer on Triangle Arrays

“…It is anticipated that, a large surface-potential increase would occur on the TCAAS where more ethanol gaseous molecules were adsorbed than those on the flat area. In this regard, we use KFM, a well-accepted characterization for the surface potential, [31][32][33] to estimate a surface-potential increase on the TCAAS.…”