The energetics of the TiO2 polymorphs (rutile, anatase, and brookite) were studied by high temperature oxide melt drop solution calorimetry. Relative to bulk rutile, bulk brookite is 0.71 ؎ 0.38 kJ͞mol (6) and bulk anatase is 2.61 ؎ 0.41 kJ͞mol higher in enthalpy. The surface enthalpies of rutile, brookite, and anatase are 2.2 ؎ 0.2 J͞m 2 , 1.0 ؎ 0.2 J͞m 2 , and 0.4 ؎ 0.1 J͞m 2 , respectively. The closely balanced energetics directly confirm the crossover in stability of nanophase polymorphs inferred by Zhang and Banfield (7). An amorphous sample with surface area of 34,600 m 2 ͞mol is 24.25 ؎ 0.88 kJ͞mol higher in enthalpy than bulk rutile.

Here we report the synthesis of a series of new nanocrystalline TiO2-(MoO3) core−shell materials whose photoabsorption energy (PE, the energy required to excite TiO2-core valence band electrons to MoO3-shell conduction band states) properties are correlated with both the nanoparticle size and the degree of chemical interaction between the TiO2 core and the MoO3 shell. The TiO2-(MoO3) nanoparticle size can be readily adjusted from 80 to 40 Å, and in this series, the PE decreases from 2.88 to 2.60 eV with decreasing particle size. The systematic PE red-shift exhibited by the core−shell materials is ascribed to the change in the relative position of the MoO3-shell conduction band as it evolves from less than a monolayer to a two monolayer shell.