A reverse slipping strategy for bulk-reduced TiO2−x preparation from Magnéli phase Ti4O7

Abstract

Black TiO_2−x is an attractive material due to its adjustable band structure, useful in different applications, and has been heavily investigated and developed. The current reduction of TiO₂ involves a process from surface to center: thus, the issue of either insufficient or excessive reduction seems inevitable. To the best of our knowledge, it is rarely reported that a homogeneous defect distribution can be facilely achieved inside black TiO_2−x. In this study, a bulk-reduced rutile TiO_2−x was obtained via a circuitous two-step approach, with an intermediate Magnéli phase (Ti_nO_2n−1). With the decomposition of a solid atmosphere creator (e.g. KClO₄ grains) at a high temperature, the missing oxygen ions in Ti₄O₇ could be replenished quantitatively. The TEM and SAED results reveal that the oxidation process is not just a surface reaction, but it involves reverse displacement or structural rearrangement inside the crystal. In particular, the periodic variation of crystallographic sheer planes was the direct evidence of the above bulk reaction, i.e. unified long period for Ti₄O₇ and varied long period for critical ratio sample. The as-prepared samples showed different band gaps and colors, based on their oxygen content. KClO₄, with a critical mixing ratio of around 17.5 mol%, could almost oxidize Ti₄O₇ to black rutile TiO_2−x. Furthermore, the sample formed at the critical ratio showed a considerable photo-thermal response speed of 1.7 °C s⁻¹ and an equilibrium temperature of up to 70 °C under irradiation with light above 400 nm, which can be an evidence for the enhancement of non-intrinsic absorption. In short, this study provides a new route for the controllable preparation of black TiO_2−x, and a possibility for further development of other solid atmosphere creators.