List of Tables
1.9. Applications of tin oxide
The surface and materials properties of SnO2 (and impurity doped SnO2) should be discussed in context of its three major applications. These applications are (i) as a transparent conducting oxide (TCO), (ii) as an oxidation catalyst, and (iii) as a solid state gas sensing material. For the latter two applications the surface of the material is where the „„action‟‟ is and thus surface science investigations are of direct relevance.
For the first application it is the bulk properties that are responsible for making SnO2 a TCO. However, many applications of TCOs require interfacing them with a dissimilar
Introduction
material. Thus the surface and interface properties of SnO2 are also important in the use of SnO2 in TCO applications.
In the next three subsections we try to give a brief overview of the current understanding of the key properties of SnO2 and related materials that make them suitable for these applications. All three applications involve complex mechanisms that are topics of ongoing research. Thus this introduction cannot give a complete review; it is rather an attempt to place surface science studies on SnO2 into a framework where it can benefit from research done in other fields and vice versa.
1.9.1. Transparent conductors
SnO2 belongs to the important family of oxide materials that combine low electrical resistance with high optical transparency in the visible range of the electromagnetic spectrum. These properties are sought in a number of applications;
notably as electrode materials in solar cells, light emitting diodes, flat panel displays, and other optoelectronic devices where an electric contact needs to be made without obstructing photons from either entering or escaping the optical active area and in transparent electronics such as transparent field effect transistors [59-62]. Another property of SnO2 and other TCOs is that although they are transparent in the visible they are highly reflective for infrared light.
This property is responsible for today‟s dominant use of SnO2 as an energy conserving material. SnO2-coated architectural windows, for instance, allow transmitting light but keeping the heat out or in the building depending on the climate region. More sophisticated architectural windows, so-called smart windows, rely on TCOs to electrically contact electrochromic films that are changing their coloring and transparency by applying a voltage across the films [63–65].
Introduction
Many of the binary TCOs already possess a high conductivity due to intrinsic defects, i.e. oxygen deficiencies. This is also the case for SnO2, which, as a wide band- gap semiconductor, is in its stoichiometric form a good insulator. Non-stoichiometry, in particular oxygen deficiency, makes it a conductor, however. Kilic¸ and Zunger showed that the formation energy of oxygen vacancies and tin interstitials in SnO2 is very low and thus these defects form readily, explaining the often observed high conductivity of pure, but non-stoichiometric, SnO2. In all applications of the material the charge carrier concentration and thus the conductivity is further increased by extrinsic dopants.
1.9.2. Heterogeneous catalysis
Many oxides mainly act as a support material for dispersed metal catalysts; tin oxide, however, is an oxidation catalyst in its own right. Tin-oxide based catalysts exhibit good activity towards CO/O2 and CO/NO reactions [66–73]. As in most oxide catalysts the oxidation reactions are supposed to follow the Mars–van Krevelen mechanism. In this mechanism the molecules are oxidized by consuming lattice oxygen of the oxide catalyst which in turn is re-oxidized by gas-phase oxygen. This is possible because transition and post transition oxides have multivalent oxidation states that allow the material to easily give up lattice oxygen to react with adsorbed molecules and can be subsequently re-oxidize by gas-phase oxygen. It is shown that for different oxygen chemical potentials surfaces with Sn4+ or Sn2+ are stable. This indicates that an easy reduction and re-oxidation of SnO2 surfaces can be expected in catalytic oxidation reactions.
Introduction
1.9.3. Solid state gas sensors
Materials that change their properties depending on the ambient gas can be utilized as gas sensing materials [74]. Usually changes in the electrical conductance in response to environmental gases are monitored. Many metal oxides are suitable for detecting combustible, reducing, or oxidizing gases. For instance all the following oxides show a gas response in their conductivity: Cr2O3, Mn2O3, Co3O4, NiO, CuO, CdO, MgO, SrO, BaO, In2O3,WO3, TiO2, V2O3, Fe2O3, GeO2, Nb2O5, MoO3, Ta2O5, La2O3, CeO2, Nd2O3. However, the most commonly used gas sensing materials are ZnO and SnO2 [75]. The gas sensitivity of oxides is often divided into bulk- and surface- sensitive materials. TiO2 for example increases its conductivity due to the formation of bulk oxygen vacancies under reducing conditions and thus is categorized as a bulk sensitive gas sensing material. SnO2 on the other hand, although bulk defects affect its conductivity, belongs to the category of surface sensitive materials. It has not been addressed quantitatively why many TCO materials like In2O3, ZnO, and SnO2 are also excellent gas sensing materials. However, the dispersing conduction band with its minimum at the C-point and the high mobility of the charge carriers ensures that a change in charge carrier concentration results in a strong change in electrical conductance of the material. Consequently, adsorbate induced band bending as the potential to result in strong conductivity changes in these materials and thus triggera gas response signal. In contrast TiO2 has an indirect band gap with the conduction and minimum not at the C-point. Consequently band bending does not have a huge impact on the conductivity of TiO2.