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42820-AC5
In Situ Atomic Force Microscopy Studies of Surface Reactions under Ambient Conditions
Scanning probe techniques such as atomic force microscopy (AFM) have been shown to play an important role in understanding the structure and properties of surfaces. However, much less has been done to utilize scanning probe techniques to investigate the reaction chemistry of surfaces under ambient conditions of temperature and relative humidity. The focus of these studies is to investigate fundamental aspects of reactions on oxide and carbonate surfaces using atomic force microscopy. Thus far, we have investigated reactions of HNO3, SO2 and HCOOH on single crystal magnesium oxide and calcium carbonate surfaces. These reactions are important in several environmental processes including environmental remediation, heterogeneous catalysis and heterogeneous atmospheric chemistry. A brief summary of these results is given here.
Water, Sulfur Dioxide and Nitric Acid Adsorption on Single Crystal Calcium Carbonate and Magnesium Oxide Surfaces: In this study, Atomic Force Microscopy (AFM) is used to image freshly cleaved MgO(100) and CaCO3(104) as these surfaces undergo reaction with water and nitric acid under ambient conditions of temperature, pressure and relative humidity. The reaction of water, sulfur dioxide and nitric acid results in the formation of hydroxylated, sulfided and nitrated surfaces, respectively. It is clear from the AFM images that there are spatial inhomogenieties and surface features that form on micron and nanometer length scales as these reactions proceed. These features, which include hillocks, patches, microcrystallites and micropuddles, are due to surface and phase segregation as a result of facile ion mobility in the presence of adsorbed water. In addition, instabilities and oscillations in the AFM images provide an indication of liquid formation and the deliquescence, i.e. a solid to liquid phase transition, of nitrate salts as a function of relative humidity.
SO2 and HCOOH on Single Crystal Calcium Carbonate: The reaction of calcium carbonate single-crystal surfaces with formic acid (HCOOH) vapor was investigated using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM images indicate the reaction produces rather well-defined crystallites, preferentially at step edges and at distinct angles to one another, mirroring the rhombohedral structure of the calcite surface. The size and surface density of the crystallites depend upon substrate step density, exposure time, exposure pressure, and relative humidity. XPS data confirm the crystallite composition 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 indicating the source of Ca ions. AFM experiments 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 longer crystallites and nanometer-sized crystals with an orthorhombohedral habit consistent with the alpha form of calcium formate. XRD confirmed the alpha phase of the product crystallites.
For reaction of SO2 on calcium carbonate single crystal and particle surfaces, Atomic force microscopy following the reaction shows that adsorbed water facilitates surface reactivity by enhancing the mobility of surface ions, giving rise to the formation of nanometer sized product crystallites approximately 1 nm in height. Simultaneous with the formation of these crystallites is pitting and etching of the underlying substrate, which occurs preferentially in the vicinity of monoatomic surface steps. In the absence of water, there is little pitting and no evidence for the formation of crystallites.
As discussed above, it is clear that AFM provides a better understanding of the reaction chemistry of these surfaces under ambient conditions of temperature and relative humidity.
