Phase-field modeling of TiO2 nanocarving via reaction with hydrogen-bearing gas

TiO 2 nanocrystals can be fabricated by carving the TiO 2 bulk polycrystals using reductive H 2 -bearing gases, yielding single-crystal [001] nanowire arrays. However, the origin of the strongly anisotropic nanowire morphology during nanocarving remains largely unexplored. In this study, we formulate a 2-D phase-field model to investigate the TiO 2 morphology evolution during nanocarving processes. The model incorporates TiO 2 reduction reaction, Ti 3+ diffusion, and anisotropies in surface energy, diffusivity and reaction rate. Through systematic simulations in both single- and poly-crystals, we elucidate the roles of different anisotropy factors in nanocrystal morphologies at different carving stages, identifying the strong reaction rate anisotropy as the dominant factor for experimentally observed nanowire morphologies. We further explore the effect of grain misorientation angle, generating a nanocarving morphology map to guide grain orientation control. This study provides insights into microstructure evolution mechanisms during nanocarving and guidances for related microstructure control.