Anisotropic Amorphization of Black Titania

The amorphous-shell/crystalline-core architecture of black titania is central to its exceptional visible-light absorption and catalytic properties, yet the underlying physical principles governing the surface amorphization remain a "black box", being particularly intriguing with the rippled contrast at the interface from recent TEM. Here, by using machine-learning amorphous structural search and long-time molecular dynamics, we discover an unprecedented crystallographic anisotropy that dictates black titania amorphization under aluminum reduction: the amorphous front preferentially advances along rutile(100) facets, driven by a collective Ti migration, leading to a distinctive wedge-shaped interface. This anisotropic amorphization creates interstitial Ti³⁺ trapping at the buried amorphous-crystalline interface, rationalizing the "anomalous" Ti³⁺ signatures observed in electron energy-loss spectroscopy (EELS). We demonstrate that the structural transition is triggered uniquely by a critical high concentration of oxygen vacancies (>1 monolayer) under aluminum reduction, but no appreciable Ti migration occurs under hydrogen treatment due to a lower concentration of oxygen vacancy. This work not only establishes a predictive methodology framework for exploring amorphization phase engineering of oxide materials in general, but also offers profound insights on the reduction-induced surface enrichment of low-coordinated oxide metal cations that plays important roles in heterogeneous catalysis, known as strong metal-support interaction (SMSI).