Formation mechanisms of interfaces between different TinO2n−1 phases prepared by carbothermal reduction reaction
The carbothermal reduction reaction (CRR) is the most commonly used method for synthesizing Magnéli-phase reduced titanium oxide nanocrystals. Though the reaction process has been investigated from thermodynamics and kinetics perspectives, research focused on the detailed formation mechanism of the interfaces from reactant to product is still limited. In this work, the structures and formation mechanisms of the interfaces between different TinO2n−1 phases were investigated using comprehensive XRD, Raman spectroscopy, and TEM characterization. Both XRD and Raman spectroscopy results indicated that the formation sequence of the TinO2n-1 phases during the CRR was TiO2 → TinO2n−1 (n = 4–9) → Ti3O5 → Ti2O3 → TiO, regardless of the method used to introduce the carbon source. Furthermore, the surface encapsulation of polydopamine (PDA) on TiO2 precursor effectively reduced the reaction temperature compared with samples prepared by ball-mill mixing of TiO2 and carbon. The interfaces of Ti4O7/Ti5O9 and Ti3O5/Ti4O7 were obtained from the CRR using different carbon source introduction methods. TEM results suggested the different coherency of these interfaces, which might be due to different formation mechanisms. By calculating the Gibbs free energies in the temperature range of 298–1598 K for some involved reactions, it was noted that the reaction between TiO2 and carbon proceeds spontaneously to form TinO2n−1 phases (n = 2–10) over the whole temperature range. In contrast, the reaction between TinO2n−1 phases with different n values was not spontaneous when n ≤ 3 from 298 K to 1598 K, which further validated the oriented attachment growth mechanism of the Ti3O5/Ti4O7 interface rather than direct CRR. Finally, the growth mechanisms of the two types of interfaces were summarized at both atomic level and nanoscales.