Spatially Resolved Product Formation in the Reaction of Formic Acid with Calcium Carbonate (101̅4): The Role of Step Density and Adsorbed Water-Assisted Ion Mobility.

The reaction of calcium carbonate (101̅4) single-crystal surfaces with formic acid (HCOOH) vapor was investigated using at. force microscopy (AFM) and XPS. AFM images indicate the reaction produces rather well-defined crystallites, preferentially at step edges and at distinct angles to one another and mirroring the rhombohedral structure of the calcite surface, while exposing unreacted carbonate surface. The size and surface d. of the crystallites depend upon substrate step d., exposure time, and relative humidity. XPS data confirmed the crystallite compn. as the expected calcium formate product. The AFM images show erosion and pit formation of the calcite surface in the vicinity of the product crystallites, clearly providing the spatially resolved characterization of the source of Ca ions. AFM expts. exploring the effects of water vapor on the reacted surface show that the calcium formate crystallites are mobile under conditions of high relative humidity, combining to form larger crystallites and nanometer-sized crystals with an orthorhombohedral habit consistent with the α form, as confirmed by X-ray diffraction. The implications for the reactions described here are discussed. [on SciFinder(R)]