Sol Gel Synthesis and Characterization of Tin Oxide and Doped-Tin Oxide Nanosized Materials Used for Gas-Sensing
Date of Award
Dissertation - Restricted
Doctor of Philosophy (PhD)
Wiellio, Charles A.
Tin dioxide is an n-type, wide band gap semiconductor "formula" transparent to visible light. The tin dioxide-based materials have many potential or demonstrated applications in various fields, such as solar energy conversion, catalysis, gas sensing, antistatic coating, and transparent electrode preparation. Among these applications, semiconductor gas sensors based on Sn02 have been shown to be convenient tools for detecting inflammable or toxic gases diluted in air, such as H2, "formula" methanol, and ethanol. The sensor properties of SnO2 (such as sensor sensitivity and selectivity) are strongly dependent on the crystallite/grain size, and grain size reduction is one of the main factors in enhancing sensor properties of semi-conducting oxides. Therefore, making SnO2 nanocrystalline materials as small as possible is one of our goals in this work. The presence of mixed tin oxidation states has been suggested to influence sensor response, so the second goal of this work is to develop methods for reproducibly controlling and varying the SnOx stoichiometry. Gas-sensing applications of metal oxide semiconductor-based materials are based on the change of the material resistance when exposed to different atmospheres, which is due to the charge carrier exchange between the adsorbed gas and the oxide surface. The first step in characterizing potential sensor materials is to determine the band gap, Eg, and the Urbach energy, Eu. The latter is a parameter describes the width of the exponential absorption edge, which originates from the sub-bandgap photon assisted optical transitions. So the third goal of this work is to characterize the optical properties of doped and undoped tin oxide. Addition of certain metals can suppress the growth of the grain size and also has been demonstrated to increase the sensitivity towards specific gases. In addition, the presence of CuO in SnO2 has been shown to improve sensitivity due to the p-n junction, so the fourth goal of this work is to extend some of synthesis and characterization methods described in this dissertation to more complex mixed metal oxide materials. The dissertation consists of two parts, Part A and Part B. Part A focuses on synthesis and characterization of doped and undoped tin oxide nanocrystalline powders. Chapter 1 describes our efforts in making nanometer sized powders with variable tin oxide stoichiometry by sol-gel method. Chapter 2 describes the phase composition and optical properties of mixed tin(II)/tin(IV) oxide powders after sintering at different temperatures. Chapter 3 describes the phase composition and optical properties of doped tin oxide powders as a function of sintering temperature, mainly focusing on Cu-doped materials. Part B (Chapter 4) presents our preliminary results on testing thick film gas sensors fabricated from nanocrystalline metal oxides dispersed in polypyrrole, a conducting polymer. The summary and the suggested future work are described in Chapter 5.