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.
The perovskite prototype is one of the most promising solar cell materials. However, perovskite suffers from a phase transition leading to thermodynamic instability, which tightly influences the solar cell operation performance. Thus, modulating transition dynamics would extend its lifetime, which needs an in-depth understanding of the potential energy surface (PES) and the phase transition kinetics at the atomic level. In this work, taking CsPbI 3 as an example of a perovskite prototype, we map out the PES and resolve the three lowest energy barrier paths of γ-CsPbI 3 degradation by using a stochastic surface walking method integrated with high-dimensional neural-network potential. Path I is γ-CsPbI 3 to hexagonal δ′-CsPbI 3 , a five-step transition with (110) Pv to (001) hex with the energy barrier 0.25 eV/f.u.; Path II is γ-CsPbI 3 to cmcm-CsPbI 3 , a two-step transition with an over all energy barrier 0.22 eV/f.u. and (001) Pv //(110) cmcm + [010] Pv // [001] cmcm ; Path III is γ-CsPbI 3 to δ-CsPbI 3 , a one-step transition without forming an inherent interface, with the highest energy barrier 0.34 eV/f.u. Interestingly, We find that with the substitution of the A-site and/or B-site by other atoms, such as Bi and Te, the γ-CsPbI 3 to δ-CsPbI 3 transition could be extensively hindered. In this work, by resolving the potential energy surface, we not only reveal the degradation mechanism at the atomic level but also find a way to design perovskites with high and long-term stability.
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