Atomic Insights into Nanoparticle Formation of Hydroxyfluorinated Anatase Featuring Titanium Vacancies

Abstract

Anatase TiO2 with exposed highly reactive (001) surface is commonly prepared using solution-based synthesis in the presence of a fluorinating agent acting as a structure-directing agent. Recently, the solvothermal reaction of titanium tetraisopropoxide in the presence of aqueous HF has resulted in the stabilization of an oxyhydroxyfluorinated anatase phase featuring cationic vacancies. In the present work, we have studied its formation mechanism, revealing a solid-state transformation of a highly defective anatase phase having a hydroxyfluoride composition that subsequently evolves through an oxolation reaction into an oxyhydroxyfluoride phase. Importantly, this work confirms that titanium alkoxide precursors can react with HF via a fluorolysis process yielding fluorinated molecular precursors, which further condense to produce new composition and structural features deviating from a well-ordered anatase network.

Solvothermal Temperature Drives Morphological and Compositional Changes through Dehydroxyfluorination in Anatase Nanoparticles

“…At the early stage of the reaction, a large content of titanium vacancies was observed, indicating the predominance of OH – and F – anions over O 2– in Ti 1– x – y ⬜ x + y O 2–4( x + y ) F 4 x (OH) 4 y compounds. This signifies that the titanium alkoxide readily reacted by hydrolysis and fluorolysis reactions, which is similar to what was observed at low temperature ( T = 90 °C) . Thereafter, the vacancy content decreases progressively, indicating a compositional change upon reaction.…”

“…Our previous work showed that fluoride is actually incorporated within the anatase's lattice leading to the formation of titanium vacancies, that is Ti 1– x – y ⬜ x + y O 2–4( x + y ) F 4 x (OH) 4 y . For a reaction temperature of 90 °C, a time‐dependent study revealed that first, an hydroxyfluoride composition forms and further evolves through a solid‐state transformation into an oxy‐hydroxyfluoride phase by oxolation reactions . To understand the effect of temperature on the reactivity of titanium alkoxide and HF, a time‐dependent study was performed at higher temperature, that is T = 150 °C.…”

“…It should be noted that the content of titanium vacancies is too low to be detected by PDF refinements. Nevertheless, 19 F solid‐state NMR spectroscopy was a suitable probe to identify the presence of titanium vacancies , , …”

“…1 H and 19 F spectra are referenced to TMS and CFCl 3 , respectively, and they were fitted by using the DMFit software . 19 F solid‐state NMR was used to quantify the fluorine content by using reference samples , , . 19 F solid‐state MAS NMR (Hahn echo) spectra were also recorded for YF 3 and LaF 3 , and the masses of each sample in the rotor were measured.…”

“…Recently, we showed that the reaction between titanium alkoxide and HF under mild solvothermal conditions leads to the stabilization of nanoparticles of anatase featuring the oxy‐hydroxyfluoride composition Ti 1– x – y ⬜ x + y O 2–4( x + y ) F 4 x (OH) 4 y where ⬜ represents the titanium vacancies . The formation mechanism of this compound revealed that titanium alkoxide undergoes both hydrolysis and fluorolysis (“fluorolysis” was introduced by Kemnitz et al, ) yielding an anatase phase with an hydroxyfluoride composition which further evolved into an oxy‐hydroxyfluoride phase by oxolations reactions . Moreover, the impact of the solvent characteristics and fluorine concentration has been nvestigated with respect to the structure/composition of the stabilized phases, and they were shown to be key parameters .…”