Nanocrystalline Thin Films as a Model System for Sulfated Zirconia

Introduction and Strategy Discovery of the extraordinary activity of sulfated zirconia for n-butane isomerization below 373K has led to numerous efforts to investigate and correlate structure, acidity, and reactivity of this material. Although a large data set has been obtained from powdered sulfated zirconia no consistent picture has evolved that satisfactorily describes acidity and reactivity. In a new approach to better characterize sulfated zirconia we have developed a model system which consists of a nanocrystalline zirconia film supported on silicon. A preparation procedure for the deposition of various oxide films from aqueous solution has been described in the literature. A key element in this preparation is the use of a self-assembled monolayer (SAM), an ordered array of long chain surfactant molecules which are anchored to the oxidized surface of a silicon wafer. A sulfonic acid group is then introduced as the functional group at the monolayer-air interface. The SAM-coated wafer is immersed into an aqueous deposition medium, containing zirconium (Zr), sulfate, and chloride. Within several hours a sulfate containing film consisting of nanosized particles, some of which are tetragonal zirconia, is formed. Calcination at 773K produces a predominantly tetragonal ZrO2 film with 2-10 nm crystals. We investigated each step of the preparation, using scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), in order to learn how to optimize the preparation procedure for our purposes. We also attempted to identify the mechanism of film growth to be able to tune properties such as film thickness and particle size distribution.