Figure 1.

Illustration of the particle size dependence of the total free energy of two phases. The diagram is appropriate for the materials studied in this paper. At room temperature and pressure, for TiO₂, rutile is the bulk stable phase, anatase the metastable phase. For ZnS, sphalerite is the bulk stable phase and wurtzite the metastable phase. Solid lines: vacuum environment, dotted lines: aqueous environment. Below particle diameter D_c in vacuum, a phase stability crossover occurs. In the presence of water, the phase stability crossover is shifted to diameter D_c' <D_c.