Abstract
Several conflicting reports have suggested that heat capacities and the thermodynamic properties of materials change as their particle size decreases into the nanoscale. To further investigate this, we have measured the constant pressure heat capacities of three 7nm TiO2 anatase samples containing varying amounts of surface-adsorbed water using a combination of adiabatic and semi-adiabatic calorimetric methods. These samples have a high degree of chemical, phase, and size purity determined by rigorous characterization. Molar heat capacities were measured from T=(0.5 to 320)K, and data were fit to a sum of theoretical functions in the low temperature (T<15K) range, orthogonal polynomials in the mid temperature range (10>T>75K), and a combination of Debye and Einstein functions in the high temperature range (T>35K). These fits were used to generate Cp,m°, Δ0TSm°, Δ0THm°, and φm° values at smoothed temperatures between (0.5 and 300)K for all hydrated samples. Standard molar entropies at T=298.15K were calculated to be 73.868, 66.072, and 63.845J·K−1·mol−1 all with a standard uncertainty of 0.002·Δ0TSm° for samples TiO2·0.677H2O, TiO2·0.532H2O, and TiO2·0.379H2O, respectively. These and other thermodynamic values were then corrected for water content to yield bare nano-TiO2 thermodynamic properties at T=298.15K, and we show that the resultant thermodynamic properties of anhydrous TiO2 anatase nanoparticles equal those of bulk TiO2 anatase within experimental uncertainty. Thus we show quantitatively that the difference in thermodynamic properties between bulk and nano-TiO2 must be attributed to surface adsorbed water.
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