Surface enthalpy and enthalpy of water adsorption of nanocrystalline tin dioxide: Thermodynamic insight on the sensing a
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Tin dioxide (SnO2) is an important base material for a variety of gas sensors and catalysts. However, there is a lack of experimental data on the energetics of SnO2 surfaces and their water adsorption. In this work, the surface energies of anhydrous and hydrated SnO2 nanoparticles were measured by combining high-temperature oxide melt solution calorimetry and water adsorption calorimetry. The SnO2 nanoparticles were synthesized through oxidation of metallic tin using nitric acid followed by heat treatment at different temperatures to achieve surface areas ranging from 4000 to 10,000 m2mol 1(25–65 m2g 1). The enthalpy of the anhydrous surface is 1.72 6 0.01 Jm 2, and that of the hydrated surface is 1.49 6 0.01 Jm 2. The integral heat of water adsorption is 75 kJmol 1, with a chemisorbed maximum coverage of ;5 H2Onm 2. SnO2 has a lower surface energy and less exothermic enthalpy of water adsorption than the isostructural TiO2 (rutile) reported previously. This comparison suggests that the excellent sensing properties of SnO2 may be a consequence of its relatively low affinity for surface H2O molecules that compete with other gases for adsorption.
I. INTRODUCTION
Tin dioxide is one of the most important base materials for a variety of semiconductor gas sensors. The sensitivity and selectivity of these sensors are mostly determined by the surface properties of SnO2.1–5 Possibly as a result of changes in surface properties, SnO2 modified by different metal oxides or metals as cocatalysts can show higher selectivity toward specific gases.6–16 SnO2 nanoparticles have attracted considerable attention as sensors because of their improved sensitivity to various gases.1–5 Such improvement presumably arises from the availability of a higher surface area compared with the traditional polycrystalline oxide thin films. However, the high surface area may increase instability due to both kinetic and thermodynamic factors. Thus, knowledge of the surface thermodynamics is of key importance in predicting nanosensor stability during processing or operation. Another challenge in the development of highly selective/sensitive nanosensors arises because water molecules (commonly present in the environment) often influence the measured signals. That is, any gas being sensed is usually competing with H2O for adsorption on the surface of the sensor. Hence, one may expect that the strength of the water adsorption on the surface of the sensor will affect its sensitivity and selectivity, and since a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.97 848
J. Mater. Res., Vol. 26, No. 7, Apr 14, 2011
http://journals.cambridge.org
Downloaded: 14 Mar 2015
nanoparticles generally have more adsorbed water than do films, such competition may be more important for nanoparticle-based sensors. Thermodynamically, this can be analyzed by providing data on the energetics of water adsorption and evaluating the differences between the surface energies of hydrated and anhydrous SnO2 nanoparticles. The avera
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