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...
