We use amorphous titania nanoparticle networks produced by pulsed laser vaporization at room temperature as a model system for understanding the mechanism of formation of black titania. Here, we characterize the transformation of amorphous nanoparticles by annealing in pure Ar at 400 °C, the lowest temperature at which black titania was observed. Atomic resolution electron microscopy methods and electron energy loss spectroscopy show that the onset of crystallization occurs by nucleation of an anatase core that is surrounded by an amorphous Ti2O3 shell. The formation of the metastable anatase core before the thermodynamically stable rutile phase occurs according to the Ostwald phase rule. In the second stage the particle size increases by coalescence of already crystallized particles by a self-organized mechanism of crystallization by particle attachment. We show that the Ti2O3 shell plays a critical role in both black titania transformation and functionality. At 400 °C, Ti2O3 hinders the agglomeration of neighboring particles to maintain a high surface-to-volume ratio that is beneficial for enhanced photocatalytic activity. In agreement with previous results, the thin Ti2O3 surface layer acts as a narrow bandgap semiconductor in concert with surface defects to enhance the photocatalytic activity. Our results demonstrate that crystallization by particle attachment can be a highly effective mechanism for optimizing photocatalytic efficiency by controlling the phase, composition, and particle size distribution in a wide range of self-doped defective TiO2 architectures simply by varying the annealing conditions of amorphous nanoparticles.
Keywords: Ostwald rule of stages; amorphous nanoparticle network; black anatase; crystallization by particle attachment; pulsed laser vaporization; transmission electron microscopy.