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 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 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 H ₂O·nm^-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.