High Substitution Rate in TiO₂ Anatase Nanoparticles with Cationic Vacancies for Fast Lithium Storage

Abstract

Doping is generally used to tune and enhance the properties of metal oxides. However, their chemical composition cannot be readily modified beyond low dopant amounts without disrupting the crystalline atomic structure. In the case of anatase TiO 2 , we introduce a new solution-based chemical route allowing the composition to be significantly modified, substituting the divalent O 2− anions by monovalent F − and OH − anions resulting in the formation of cationic Ti 4+ vacancies (□) whose concentration can be controlled by the reaction temperature. The resulting polyanionic anatase has the general composition Ti 1−x−y □ x+y O 2−4(x+y) F 4x (OH) 4y , reaching vacancy concentrations of up to 22%, i.e., Ti 0.78 □ 0.22 O 1.12 F 0.4 (OH) 0.48 . Solid-state 19 F NMR spectroscopy reveals that fluoride ions can accommodate up to three different environments, depending on Ti and vacancies (i.e. Ti 3 -F, Ti 2 □ 1 -F, and Ti 1 □ 2 -F), with a preferential location close to vacancies. DFT calculations further confirm the fluoride/vacancy ordering. When its characteristics were evaluated as an electrode for reversible Li-ion storage, the material shows a modified lithium reaction mechanism, which has been rationalized by the occurrence of cationic vacancies acting as additional lithium hosting sites within the anatase framework. Finally, the material shows a fast discharging/charging behavior, compared to TiO 2 , highlighting the benefits of the structural modifications and paving the way for the design of advanced electrode materials, based on a defect mediated mechanism. ■ INTRODUCTIONTransition-metal oxides are an important class of materials, whose properties are dependent on many factors, including their composition, structure, and morphology. Among them, titanium dioxide (TiO 2 ) is a multifunctional material used for a broad range of applications. The low toxicity and the abundance of titanium have favored the emergence of Tibased compounds for photocatalytic hydrogen production by water splitting, rechargeable batteries/supercapacitors, dyesensitized solar cells, sensors, and biomedical devices. 1−6 Over the years, several approaches have been developed to improve its properties. For instance, crystal facets engineering and structural modifications have been widely employed. 7−9 The latter approach comprises the introduction of reduced titanium or heteroatoms within the lattice. Although the use of dopants has been proved as an effective way to tune this material's properties, changing the chemical composition of TiO 2 with a degree of substitution that goes beyond the doping level appears as a challenging and promising way to tailor its properties. On more general grounds, knowing how strongly a metal oxide can be modified while maintaining its original network is of fundamental interest.With the goal of modifying the composition of anatase, which is one of the polymorphs of TiO 2 , we propose an approach where divalent oxide anions are substituted by monovalent ones such as fluoride and hydroxide. The anatase structure...

Atomic Insights into Nanoparticle Formation of Hydroxyfluorinated Anatase Featuring Titanium Vacancies

“…12 To gain further insights into its formation mechanism, we employed a time-dependent study performed at the early stage of the process, i.e., from 1 to 10 h.…”

“…25 Overall, the formation of anatase implies the structural rearrangement of a hydroxide (and hydroxyfluoride in the presence of fluorine) featuring a large content of titanium vacancies that can have a large impact on the physicochemical properties. 12 …”

“…10,11 We recently showed that the mild solvothermal reaction of titanium alkoxide with aqueous HF yields an oxyhydroxyfluoride compound, Ti 1−x−y □ x+y O 2−4(x+y) F 4x (OH) 4y , featuring the anatase-type structure with cationic vacancies (□). 12 From the obtained general chemical composition, it was shown that both hydrolysis and fluorolysis processes occurred during the synthesis, leading to the stabilization of as many as three types of anions in the vicinity of Ti 4+ . In the present article, we intend to understand the formation mechanism of Ti 1−x−y □ x+y O 2−4(x+y) F 4x (OH) 4y and to provide more insights into the chemical reactivity of titanium alkoxide toward HF.…”

“…19 19 F solid-state NMR spectroscopy was also used to quantify the fluorine content of the studied samples on the basis of reference samples. 12 19 F solid-state MAS NMR (Hahn echo) spectra were also recorded for YF 3 and LaF 3 , and the mass of each sample in the rotor was measured. The fits of the spectra allowed the integrated intensities (I) for each sample to be determined.…”

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

“…It is generally considered that fluoride anions are not stabilized within the anatase crystal lattice because X‐ray diffraction patterns refer to “TiO 2 ” anatase and also because of inadequate characterization techniques; the fluorination is often characterized by X‐ray photoelectron spectroscopy (XPS) which probes only the surface of the particles. 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 .…”