Surface enthalpy and enthalpy of water adsorption of nanocrystalline tin dioxide: Thermodynamic insight on the sensing activity

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 SnO2surfaces and their water adsorption. In this work, the surface energies of anhydrous and hydrated SnO2nanoparticles were measured by combining high-temperature oxide melt solution calorimetry and water adsorption calorimetry. The SnO2nanoparticles 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 m2·mol−1(25–65 m2·g−1). The enthalpy of the anhydrous surface is 1.72 ± 0.01 J·m−2, and that of the hydrated surface is 1.49 ± 0.01 J·m−2. The integral heat of water adsorption is −75 kJ·mol−1, with a chemisorbed maximum coverage of ∼5 H2O·nm−2. SnO2has 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 SnO2may be a consequence of its relatively low affinity for surface H2O molecules that compete with other gases for adsorption.