Surface Science Studies of Metal Oxide Gas Sensing Materials

In this chapter we present recent advances in the study of metal oxide surfaces and put them in relation to gas sensing properties. A reoccurring scheme is the dependence of chemical surface properties on the crystallographic orientation of the surface. This dependence will become more important in gas sensing applications as nanomaterials with controlled crystal shapes are being designed. In particular we focus on differences of the surface properties of the two polar surfaces of ZnO and the two most abundant bulk terminations of rutile TiO2, i.e. the (110) and (011) crystallographic orientations. On the example of these metal oxides, we describe the use of vacuum based surface science techniques, especially scanning tunneling microscopy and photoemission spectroscopy, to obtain structural, chemical, and electronic information. 2.1 Relation Between Metal Oxide Gas Sensors and Surface Science Semiconducting metal oxides can exhibit a conductivity change due to the adsorption or reactions of molecules from the gas phase with the surface. Monitoring this conductivity change enables the use of this information as a gas response signal. The change in conductivity is brought about by an upward or downward shift of the Fermi-level within the band-gap of these predominantly n-type materials. The Fermi-level shift may be induced by charge transfer from the gas sensing material to an adsorbate. For macroscopic materials this induces a J. Tao M. Batzill (&) Department of Physics, University of South Florida, Tampa 33620, USA e-mail: [email protected] M. A. Carpenter et al. (eds.), Metal Oxide Nanomaterials for Chemical Sensors, Integrated Analytical Systems, DOI: 10.1007/978-1-4614-5395-6_2, Springer Science+Business Media New York 2013 35 band bending at the surface while for microscopic particles (smaller than the Debye screening length) the Fermi-level within the entire particle shifts. Such adsorbate induced shifts of the Fermi-level are dominant for surface sensitive gas sensing materials such as SnO2 and ZnO. Another mechanism that can result in the shift of the Fermi-level is a variation of the bulk dopant concentration. For example, Ti-interstitials and oxygen vacancies in the bulk of TiO2 act as intrinsic n-type dopants. The concentration of these dopants depends on the oxidation potential of the surrounding gas phase and the surface of the TiO2 may act as a source or sink of Ti-interstitials for the bulk. Clearly, gas sensing with semiconducting metal oxides is initiated by molecular interaction with surfaces and surface science studies therefore have played an important role in the understanding and describing of the fundamental mechanisms [1]. Recent advances in surface science now allow a molecular scale description of metal oxide surfaces and this has provided many new fundamental insights on the properties of these materials. Traditional surface science studies make two important simplifications to the general complex morphology of gas sensing materials; (1) macroscopic single crystalline materials (usually bulk samples but sometimes also high quality epitaxial thin films may be used) are studied, and (2) the surfaces are prepared and investigated under well-controlled ultra high vacuum (UHV) conditions. The consequences of these simplifications are that in surface science experiments on macroscopic single crystal samples, no size effects and no interface effects are observed. Furthermore, because the surfaces are generally prepared in vacuum, they are exposed to a reducing environment. In return for the loss of the ‘real’ gas sensing environment we gain control over crystallographic orientation and surface composition. Furthermore, we have the full arsenal of surface science techniques at our disposal for a detailed surface analysis. Therefore, surface science studies can address critical fundamental questions like: What role does the surface structure play in the adsorption of certain molecules, i.e. do different crystallographic orientations of the same material exhibit different gas sensitivities? What are the sites for molecule adsorption? and consequently, can these sites be controlled to obtain better sensitivity and selectivity? What is the electronic response of the surface upon molecule adsorption or reaction with the surface? Modern surface science studies can provide rich information on the atomic scale surface properties, which is not easily accessible by other methods. Detailed reviews on surface science studies of SnO2 have been recently published by one of the authors of this article [2–4] and interested readers are referred to those texts. Here we focus on recent studies mainly done in the authors’ laboratory on the two common metal oxide gas sensing materials ZnO and TiO2. On ZnO we demonstrate the use of high energy-resolution photoemission studies to obtain information on the fundamental stabilization mechanisms of polar surfaces and in particular investigate the role of hydrogen adsorption on different surface orientations. We also illustrate the power of photoemission spectroscopy to gain information on gas sensing reactions on the example of ZnO reacting with H2S, a common gas sensing application. In the second part we study two surfaces of rutile TiO2 and show that the surface structure strongly affects the adsorption of 36 J. Tao and M. Batzill