Phase-Transition Mechanism and the Atomic Interface Structure of Brookite to Rutile: Resolving from Machine-Learning Global Pathway Sampling

The rich structural polymorphism of TiO2 provides
an
opportunity to construct a heterophase junction, which reportedly
improves the photocatalyst performance. In the past, using the partial
phase transition to fabricate brookite/rutile biphase materials has
attracted much attention. Although many of the experiments have studied
the phase transition of brookite to rutile, to date, the atomistic
mechanisms and its atomic heterophase junction structure remain unclear.
In this paper, the stochastic surface walking method and neural network
method (SSW-NN) are utilized to map out the local potential energy
surface (PES) of brookite and resolve the lowest energy barrier transition
path. We show that brookite first transforms into the TiO2-II structure, and then the TiO2-II transforms into rutile
with the overall orientation relation (100)B//(100)II, [010]B//[010]II, and (001)II//(101)R, [100]II//[010]R. Anatase
is a byproduct rather than an intermediate phase during the brookite-to-rutile
phase transition. The well-matched interfaces between brookite and
TiO2-II, TiO2-II, and rutile possess spatially
separated CBM and VBM, while the disordered junction between brookite
and rutile shows frustrated electron/hole transport. The proposed
mechanisms not only clarify the role of anatase in the brookite-to-rutile
transition but also help to understand the nature of higher photocatalyst
performance of the biphase.