Influence of Particle Size and Water Coverage on the Thermodynamic Properties of Water Confined on the Surface of SnO2 Cassiterite Nanoparticles

Inelastic neutron scattering (INS) data for SnO 2 nanoparticles of three different sizes and varying hydration levels are presented. Data were recorded on five nanoparticle samples that had the following compositions: 2 nm SnO 2 · 0.82H 2 O, 6 nm SnO 2 · 0.055H 2 O, 6 nm SnO 2 · 0.095H 2 O, 20 nm SnO 2 · 0.072H 2 O, and 20 nm SnO 2 · 0.092H 2 O. The isochoric heat capacity and vibrational entropy values at 298 K for the water confined on the surface of these nanoparticles were calculated from the vibrational density of states that were extracted from the INS data. This study has shown that the hydration level of the SnO 2 nanoparticles influences the thermodynamic properties of the water layers and, most importantly, that there appears to be a critical size limit for SnO 2 between 2 and 6 nm below which the particle size also affects these properties and above which it does not. These results have been compared with those for isostructural rutile-TiO 2 nanoparticles [TiO 2 · 0.22H 2 O and TiO 2 · 0.37H 2 O], which indicated that water on the surface of TiO 2 nanoparticles is more tightly bound and experiences a greater degree of restricted motion with respect to water on the surface of SnO 2 nanoparticles. This is believed to be a consequence of the difference in chemical composition, and hence surface properties, of these metal oxide nanoparticles.