Thermal analysis studies of oxygen chemisorption on nanocrystalline SnO 2

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The oxygen chemisorption on nanocrystalline SnO2 at temperature range between 100 and 450 °C was studied with differential thermal analysis (DTA) and electrical conductivity measurement. The O2−, O−, and O2− ionosorptions were observed and could be distinguished from each other only on nanocrystalline SnO2 with grain size less than 5 nm. Assuming steady-state adsorption, the heats of adsorption for O2− and O− (or O2−) on nanocrystalline SnO2 are 1.09 and 1.50 eV respectively from the results of DTA.

I. INTRODUCTION

II. EXPERIMENTAL

SnO2 is an n-type semiconductor with wide energy gap (Eg ⳱ 3.6 eV).1 Because the conductivity of SnO2 varies sharply with the variation of gas species present in the environment, it is one of the most important gas-sensing materials.2 Semiconductor gas sensors operate on the principle that gas molecules react with the O2− and O− (or O2−) ions previously adsorbed onto the sensor surfaces.3–7 These reactions lower the surface Schottky barrier formed by the ionosorbed oxygen species and consequently the electrical resistance decreases. It is therefore very important to know the oxygen adsorption and desorption conditions in order to choose proper working temperatures for gas sensors and their exact processing parameters. Unfortunately, although the influence of oxygen on electronic properties of SnO2 has been widely studied,4–7 concrete experimental data about the heats of adsorption for O2− and O− (or O2−) on surfaces of SnO2 are still lacking. On the other hand, nanocrystalline (nc) materials have recently attracted widespread interest. Nanometer-sized crystallites give rise to a larger number of interfaces and a substantial fraction of atoms are located on the interfaces which have significant influence on the adsorption properties. Therefore nc-SnO2 is expected to exhibit different and potentially better gas-sensing properties. In this paper we report novel differential thermal analysis (DTA) results of oxygen chemisorption on ncSnO2 and calculate the heats of adsorption for O2− and O− (or O2−) ionosorption in steady-state adsorption condition from DTA data.

Nanocrystalline SnO2 powder with average grain size of about 4 nm was prepared by the sol-gel method described in one of our previous papers.8 The bulk specimen, 8 mm in diameter and 0.2 mm thick, was prepared by compacting the powder under a uniaxial pressure of 0.4 GPa, and annealed at various temperatures between 300 and 900 °C for 2 h. The grain sizes of specimens were determined from x-ray diffraction (XRD) results using the Scherrer formula.9 The average grain sizes (D) and volume fractions of interface (or surface) (Ct) of specimens varied with annealing temperature (Ta) as shown in Table I. DTA in air or pure nitrogen atmospheres was conducted with a Seiko SSC 5200H Thermal Analysis System (Chiba, Japan). The weight of the samples was about 20 mg. In order to avoid the interference from the adsorbed water on the sample surfaces, the nanocrystalline SnO2 powder was kept in a desiccator for one month before the DTA measure

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