title: Molten Salt Synthesis of TiO2 and Na2Ti6O13 Crystals with Tailored Habit and Morphology
Creators: name: De Carolis, Dario Mariano
Publisher: name: UNSPECIFIED
Year: 2023

description: Molten salt synthesis (MSS) is an efficient way to produce dispersed materials with a well-defined structure and morphology, ranging from the nano- to the micro-scale. This thesis presents a thorough analysis of the crystal growth of TiO2 polymorphs (i.e. anatase and rutile) and sodium titanates by MSS, also known as flux growth. Metal chlorides and sulfates are investigated as high-temperature growth media. Two preparation methods of the precursor of the desired crystals have been followed: i) direct mixing of nanosized anatase with a salt (dry-mixed preparation (DMP)) and ii) wet-chemical preparation with TiOSO4 and TiCl4 as starting materials (wet chemical preparation (WCP)). The thesis has been concentrated on two subjects: i) tuning of the crystal habit and morphology of TiO2 (rutile) and ii) the synthesis of highly pure Na2Ti6O13 (NTO) nanorods. The samples obtained at temperatures ranging from 700 °C to 1200 °C are analyzed by means of X-ray diffraction (XRD), Raman spectroscopy, electron back-scatter diffraction (EBSD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to investigate the correlation between the synthesis conditions and the obtained crystal habit, crystal size, composition and morphology. The variation of the structure and morphology of rutile is rationalized by the impact of the MSS parameters like the choice of the salt matrix, the precursor preparation and concentration, the temperature and atmosphere (air, argon, O2). Hence, a central topic of this work is a thorough analysis of the MSS parameters influencing the anatase to rutile phase transition (ART) and the growth of NTO nanocrystals. The MSS parameter salt matrix has a strong influence on the rutile morphology. The results mainly depend on the salt basicity, given by the O2– activity in the salt (Lux-Flood theory). Chlorides generally favor the formation of rod-shaped rutile crystals. Their growth can be accelerated by an increase of the precursor concentration and of the synthesis temperature. The ART rate is fast in the chloride melt. The use of an inert atmosphere, e.g. synthesis under argon, increases the ART rate. The addition of NaF changes the rutile growth direction, hindering the formation of rod-shaped crystals at the expense of facetted bipyramidal octahedra. The replacement of chlorine by sulfate anions retards the ART, rutile being dominant only at temperatures ≥ 1000 °C. The ART is faster for the lowest precursor concentration (1 wt. % at 900 °C). Moreover, in the presence of sulfate anions, rutile crystals are not assuming the rod-shaped morphology as in chlorides. A study performed by varying the chlorine/sulfate anion ratio in a NaCl-Na2SO4 mixture further confirms these correlations. The WCP confirms the general trends observed by the DMP. However, WCP with TiCl4 (or TiOSO4) provides a faster ART rate. The addition of Na3PO4 — a strong Lux-Flood acid — to the TiOSO4 precursor solution strongly retards the ART at 1000 °C (and hinders the titanate formation). The retardation effect increases with Na3PO4 concentration, leading to an anatase quantity greater than rutile. WCP with 5 % Na3PO4 and treatment at 1100 °C under pure oxygen provides the most useful crystal habit for the TiO2 application as pigment (pearl luster effect): ”mosaic tile”-like rutile crystals. Higher Na3PO4 concentrations only lead to an increased formation of V-shaped rutile crystals. WCP with Na2SO4 as salt component strongly enhances the formation of NTO in contrast to the DMP. Increasing the pH in the precipitation step by adding NaOH, promotes the formation of pure NTO. Therefore, this innovative MSS approach to obtain exceptionally pure Na2Ti6O13 nanorods in high yield, opens a promising route for new applications, notably sodium batteries in stationary energy storage. In contrast to all the synthesis approaches reported in literature, no TiO2 is found in the final specimen achieving an overall yield of Na2Ti6O13 &gt; 80 wt. %. The elaborated synthesis is fast, cost efficient and suitable for industrial up-scaling. The Na2Ti6O13 nanorods generated at 900-1100 °C have an average length and diameter of about 30 μm and 200 nm, respectively, providing an average aspect ratio of 150. X-ray diffraction and Raman studies confirm the high purity of the material. The crystal growth direction of the nanorods has been determined by EBSD and selected area electron diffraction techniques, revealing a &lt;010&gt; growth direction. Moreover, Raman and electron energy-loss spectroscopy hint at slight changes in the Ti–O and Na bonding in the tunnel structure of NTO at &gt; 900 °C. Electrochemical tests in Li and Na-half cells reveal stable performance providing capacities of about 100 mA h g−1 (Li) and 40 mA h g−1 (Na). Increase of the synthesis temperature of Na2Ti6O13 to 1100 °C leads to a capacity decrease of ≈ 20 % most likely resulting from i) the morphology/volume change with temperature and ii) distortion of the Na2Ti6O13 tunnel structure. Carbon coating and other activation processes enhance the performance establishing NTO nanorods as promising material for energy applications.