Highly efficient visible-light catalysts are achieved through forming defect-pairs in TiO nanocrystals. This study therefore proposes that fine-tuning the chemical scheme consisting of charge-compensated defect-pairs in balanced concentrations is a key missing step for realizing outstanding photocatalytic performance. This research benefits photocatalytic applications and also provides new insight into the significance of defect chemistry for functionalizing materials.

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Ionic codoping offers a powerful approach for modifying material properties by extending the selection of potential dopant ions. However, it has been a major challenge to introduce certain ions that have hitherto proved difficult to use as dopants (called "difficult-dopants") into crystal structures at high concentrations, especially through wet chemical synthesis. Furthermore, the lack of a fundamental understanding of how codopants are incorporated into host materials, which types of defect structures they form in the equilibrium state, and what roles they play in material performance, has seriously hindered the rational design and development of promising codoped materials. Here we take In (difficult-dopants) and Nb (easy-dopants) codoped anatase TiO nanocrystals as an example and investigate the doping mechanism of these two different types of metal ions, the defect formation, and their associated impacts on high-pressure induced structural transition behaviors. It is experimentally demonstrated that the dual mechanisms of nucleation and diffusion doping are responsible for the synergic incorporation of these two dopants and theoretically evidenced that the defect structures created by the introduced In, Nb codopants, their resultant Ti, and oxygen vacancies are locally composed of both defect clusters and equivalent defect pairs. These formed local defect structures then act as nucleation centers of baddeleyite- and α-PbO-like metastable polymorphic phases and induce the abnormal trans-regime structural transition of codoped anatase TiO nanocrystals under high pressure. This work thus suggests an effective strategy to design and synthesize codoped nanocrystals with highly concentrated difficult-dopants. It also unveils the significance of local defect structures on material properties.

The introduction of defect-pairs into centrosymmetric rutile TiO2 produces a new form of collective nonlinear electrical polarization.

Chemical co-doping and high pressure reaction have been broadly used to synthesize novel materials or tune the physicochemical properties of traditional materials. Here, we take In 3+ and Nb 5+ ions co-doped anatase TiO2 nanocrystals as an example and report that a combination of both chemical and high pressure reaction route is more powerful for the preparation of metastable polymorphs. It is experimentally demonstrated that In 3+ and Nb 5+ co-doping significantly changes the high-pressure reaction behaviours of anatase TiO2 nanocrystals (<10 nm) and leads to their trans-regime structural * Corresponding author: Yun.liu@anu.edu.au.1 transition in terms of in situ Raman analysis, from anatase to a baddeleyite-like phase under compressive pressures and then to an α-PbO2-like structure under decompressive pressures. This abnormal phase transition is attributed to the defect-induced heterogeneous nucleation mechanism. Furthermore, the stiffness of co-doped TiO2 nanocrystals is significantly enhanced due to the synergistic effects of codopants. This research not only proposes a potentially effective strategy to synthesize co-doped metastable polymorphic phases but also suggests one feasible method to improve the mechanical properties of anatase TiO2 nanocrystals.