Wei Li

Monique Body

Christophe Legein

Olaf J Borkiewicz

Damien Dambournet
email: damien.dambournet@upmc.fr

Atomic Insights into Nanoparticle Formation of Hydroxyfluorinated Anatase Featuring Titanium Vacancies

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INTRODUCTION

Titanium dioxide is a material of broad interests particularly in the field of conversion and energy storage. 1 One of the most efficient approach to tune its physico-chemical properties relies on the stabilization of (001) type surfaces due to the occurrence of a large content of under-coordinated Ti atoms featuring enhanced interfacial reactivity. 2 The common route to stabilize a large percentage of (001) surfaces uses solutionbased approach in the presence of a fluorinating agent which acts as a structure directing agent. [3][4][5] The adsorption of fluorine atoms at the (001) surface directs anisotropic growth of anatase yielding crystals with a large content of highly reactive {001} facets. 2 However, the chemical reactivity of fluorine, particularly HF, toward titanium precursor, remains unclear. Consequently, the presence of fluoride into the anatase lattice is questionable. A comparison between the nucleophilic character of HF and H2O yields similar reactivity with a partial charge δ(F) = -0.42 in HF and δ(O) = -0.40 in H2O. 6 Thus, the presence of fluorine very likely induces modifications of the metal precursor at the molecular level and thus its chemical reactivity, 7,[START_REF]Metal Oxide Chemistry and Synthesis: From Solution to Solid State -J[END_REF] eventually yielding new composition and structural features deviating from a well ordered anatase network. Titanium alkoxide precursors, such as titanium tetraisopropoxide, in the presence of aqueous HF, can react through hydrolysis and fluorolysis [START_REF] Rüdiger | [END_REF] reactions. By analogy with the hydrolysis, the fluorolysis consists of the fluorination of a metal alkoxide bond according to: M(OR) n + xHF → M(OR) n-x F x + xROH

Because this process involves the protonation of the oxygen atom of the alkoxide group, it is favored in acidic condition. [START_REF] Rüdiger | [END_REF] Subsequently, the formation of metal-fluorine bridges can result in the incorporation of fluorine within the lattice. [START_REF] Kemnitz | [END_REF]11 Recently, we have shown that the mild solvothermal reaction of titanium alkoxide with aqueous HF yields an oxy-hydroxyfluoride compound Ti1-x-yx+yO2-4(x+y)F4x(OH)4y featuring the anatase type structure with cationic vacancies (). 12 From the obtained general chemical composition, it is shown that both hydrolysis and fluorolysis processes occurred during the synthesis, leading to the stabilization of as much as three types of anions in the vicinity of Ti 4+ . In the present article, we intend to understand the formation mechanism of Ti1-x-yx+yO2-4(x+y)F4x(OH)4y and to provide more insights into the chemical reactivity of titanium alkoxide toward HF. We employ a time dependent study to monitor structural and compositional changes of the as prepared nanoparticles through an ex-situ analysis combining X-ray diffraction (XRD), high-resolution Transmission Electron Microscopy (TEM), pair distribution function (PDF) and 19 F solid-state NMR analyses.

MATERIALS AND METHODS

Synthesis: Titanium tetra-isopropoxide (TTIP, ≥97%, Sigma Aldrich), isopropanol (Reag. Ph. Eur. grade, VMR), aqueous hydrofluoric acid solution (HF, 40 wt%, Prolabo) and ethanol (EtOH, 96% vol, VWR) were used as received. Syntheses via solvothermal process were carried out in a stainless steel autoclave with a 50 mL Teflon liner cup inside. In a typical experiment, 1.2 mL aqueous hydrofluoric acid solution was mixed with 24.8 mL isopropanol. The mixture was then poured into a Teflon liner cup containing 4 mL TTIP under stirring. After sealing the autoclave, the mixed solution was heated inside an oven at 90 °C for a certain time ranging from 1 h to 12 h, and left to cool down to room temperature. The resulting white precipitate was washed twice with ethanol, centrifuged at 4400 rpm for 15 min and, dried at 80 °C under air for 2 h. Further solvents removal was carried out using a tubular furnace under primary vacuum at 50 °C overnight.

Characterization methods: X-ray powder diffraction analysis were carried out using a Rigaku Ultima IV X-ray diffractometer equipped with a Cu Kα radiation source (λ = 1.54059 Å).

Transmission Electron Microscopy analysis were performed using a JEOL 2010 UHR microscope operating at 200 kV equipped with a TCD camera.

Total scattering data were collected at the 11-ID-B beamline at the Advanced Photon Source at Argonne National Laboratory, using high energy X-rays (λ = 0.2128 Å) allowing high values of momentum transfer Qmax = 22 Å-1. 13,14 Onedimensional diffraction data were obtained by integrating the raw 2D total scattering data in Fit2D. 15 PDFs, G(r), were extracted from the background and Compton scattering corrected data following Fourier transform within PDFgetX2. 16 The PDFs were subsequently modeled using PDFgui. 17 PDF peak fitting was performed using Fityk. 18 Titanium was quantified using inductively coupled plasma atomic emission spectroscopy (ICP-AES) performed by the Service Central d'Analyse (Vernaison-CNRS). 19 F solid-state magic angle spinning (MAS) NMR experiments were performed on a Bruker Avance III spectrometer operating at 7.0 T ( 19 F Larmor frequency of 282.2 MHz), using a 1.3 mm CP-MAS probe head. The 19 F MAS spectra were recorded using a Hahn echo sequence with an interpulse delay equal to one rotor period. The 90° pulse length was set to 1.55 μs and the recycle delay was set to 20 s. 19 F spectra are referenced to CFCl3 and they were fitted by using the DMFit software. 19 19 F solid state NMR was also used to quantify the fluorine content on the studied samples by using reference samples. 12 19 F solid-state MAS NMR (Hahn echo) spectra were also recorded for YF3 and LaF3 and the masses of each sample in the rotor were measured. The fits of the spectra allow to determine the integrated intensities (I) for each sample. Since, for each sample, the recycle delays were chosen to ensure that the amount of signal detected is maximum (420 s for YF3 and 120 s for LaF3), we assume that the integrated intensities are proportional to the number of scans (256 for the studied samples and 16 for YF3 and LaF3) and to the molar quantity of fluorine atoms (n) in the rotor. This assumption is verified since the calculated I/n ratio for YF3 and LaF3 are equal. The intensities per scan of the NMR signals of the studied samples, I1, and of YF3 (or LaF3), I2, allow to calculate the fluorine wt. % in the studied samples using the following formula where m and M are the mass and the molar mass, respectively:

𝐼 1 𝐼 2 = 𝑛 𝐹 3𝑚 𝑌𝐹 3 𝑀 𝑌𝐹 3 𝐹 𝑤𝑡. % = 𝑚 𝐹 𝑚 = 𝑛 𝐹 𝑀 𝐹 𝑚 = 3𝑚 𝑌𝐹 3 𝑀 𝑌𝐹 3 I 1 I 2 M F m

RESULTS AND DISCUSSION

The solvothermal process performed at 90 °C for 12-hours of a solution containing titanium isopropoxide, isopropanol and aqueous HF allows to precipitate nanoparticles of anatase having the following chemical composition Ti0.780.22O1.12F0.4(OH)0.48. 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 1-10 hours. A transition from a gel-state to precipitates starts to occur after 3-hours of reaction. Figure 1a shows the X-ray powder diffraction patterns of the dried gels and precipitates obtained from 1 to 5-hours of reaction. At the early stage of the reaction (1 h), the XRD shows a poorly crystallized pattern with a weak peak located at ca. 23.7° and a shoulder (2θ) assigned to the (100) and (101) planes of TiOF2 and anatase, respectively. As the reaction proceeds for up to 5-hours, the peak characteristic of TiOF2 progressively decreases and vanishes yielding a single phase of anatase. The particle size changes occurring during the 1-5 hours treatments were monitored by TEM. PDF analysis, which permits to study both amorphous and nanostructured materials at the atomic scale, 20 was carried out on samples obtained at different stages of the process. Interatomic distances (r) can be visualized over a r-range comprising local structural features and the length scale of ordered structural domains. Figure 2 shows the evolution of the interatomic distances at the local range order, i.e. 1-4 Å, for the samples prepared from 1 to 5 hours. The position of the first peak located at ca. 1.93 Å corresponding to Ti-O/F bond length does not vary significantly while the reaction time increases (Figure S2). Moreover, the peak area which reflects the coordination number of Ti 4+ remains constant along the reaction, i.e. 6-fold coordinated. The most pronounced deviations in the local structure occur at peaks located at ca. 2.7 Å and 3.1 Å characterizing the first anion coordination sphere of anion (X-X with X = F -, OH -and O 2-) and the second nearestneighbor Ti•••Ti distance, respectively. According to partial PDFs analysis of references samples (Figure S3), these peaks were assigned to anion-anion distance in TiOF2 and edgesharing Ti-Ti in TiO2 anatase, respectively. Accordingly, local structural changes visualized by PDF point out the progressive dissolution of a TiOF2-like phase and the concomitant growth of anatase. This structural change is accompanied by an increase of the length scale of the PDF features (Figure S4) indicating a progressive ordering upon reaction. To identify structural changes and evaluate the evolution of the coherent domain (particle size and/or length scale of the range order) during the solvothermal process, PDF data were refined against structural models using a real-space refinement (Figure 3). 17 Structural changes occurring at the early stage of the reaction were followed by considering a two phases analysis including a TiOF2-like and anatase models. The refinements showed that the TiOF2-like phase features a range ordering limited to 8 Å. This phase can thus be related to the amorphous 3-4 nm particles observed by TEM. The difference between the local ordering and the particle size highlights the amorphous nature of this phase. A quantitative phase analysis (Figure S5) allowed following the progressive appearance (from ca. 40% to 100%) of anatase at the expense of the amorphous TiOF2-like phase. The growth of anatase nanoparticles yields an increase of the PDF peaks intensity. Consequently, the accuracy of the PDF refinement characterized by the reliability factor Rw, largely improves which allows extracting reliable structural parameters. Note that for the sample obtained after 1-hour of reaction, the limited r-range prevents detailed structural analysis. Figure 3. PDF refinements of the samples prepared at 1, 2 and 5-hours reaction time. Blue and red curves refer to the experimental and modeled data, respectively. The green curve refers to the difference curve between the experimental and calculated data. For t=1h, the contribution of the TiOF2-like phase to the PDF data is shown.

The evolution of the anatase structural features extracted from PDF refinements was monitored upon reaction and up to the formation of Ti0.780.22O1.12F0.4(OH)0.48 (12h). More specifically, the titanium vacancies' concentration deduced by refining the Ti rate occupancy and the particle size (coherence length of ordered structural domains) were reported in Figure 4 as a function of the reaction time. Most notably, the particle size increases sparsely while the cationic vacancies' content decreases significantly. This structural re-arrangement is particularly pronounced from 2 to 6 hours. Since the particle sizes increase sparsely upon reaction, the origin of the decrease of the titanium vacancy concentration can be related to a solid-state transformation rather than other processes (particle growth or dissolution/recrystallization process). Similar process has been shown to occur during the formation of anatase TiO2 under hydrothermal conditions. 21 To identify the reactions underpinning the solid-state transformation of anatase, we monitored the evolution of the chemical composition upon reaction. Chemical compositions were assessed by combining elemental analyses obtained by 19 F NMR and ICP-AES as well as PDF data. For the samples obtained after 2-4 hours of reaction, elemental analyses gave a quantity corresponding to the element contribution in both TiOF2-like and anatase phases. Using the quantitative phase analysis obtained by PDF refinements (molar % of TiOF2 and anatase), the F/Ti molar ratio in anatase was assessed according to:

𝐹 𝑇𝑖 (𝑎𝑛𝑎𝑡𝑎𝑠𝑒) = 𝐹 𝑇𝑖 (𝑠𝑜𝑙𝑖𝑑) -2 * 𝑚𝑜𝑙𝑎𝑟 % 𝑇𝑖𝑂𝐹 2
𝑚𝑜𝑙𝑎𝑟 % 𝑎𝑛𝑎𝑡𝑎𝑠𝑒 where F/Ti (solid) is the F/Ti molar ratio obtained by elemental analyses. Note that the F/Ti molar ratio in TiOF2 was set to two. The hydroxyls content in anatase was deduced using the general formula Ti1-x-yx+yO2-4(x+y)F4x(OH)4y. The results are gathered in Table 1. At the early stage of the reaction, a composition featuring 50 % of cationic vacancies was obtained. Considering the general formula Ti1-x-yx+yO2-4(x+y)F4x(OH)4y, it implies the absence of oxide anions due to the sole presence of monovalent anions, i.e. Ti0.50.5F0.47(OH)1.53. The composition reveals the presence of 76.5 % and 23.5 % of OH groups and fluorine, respectively. From Table 1, it can be shown that upon reaction the hydroxyfluoride evolves into an oxy-hydroxyfluoride composition. The appearance of O 2-indicates a solid state transformation induced either by oxolation or defluorination reactions. The variation of the relative proportions for the three anions F -, OH -and O 2-(Figure 5) shows that the relative proportion of F -only slightly varies showing that the solidstate transformation of anatase is induced by oxolation reactions yielding to an oxide-rich hydroxyfluoride phase with lower titanium vacancies content. Note that a slight parallel hydrolytic cleavage of Ti-F-bonds contributing to a lowering of the titanium vacancies' concentration cannot be ruled out. Deeper insight into the reaction mechanism was studied by following the evolution of the fluorine local environment upon reaction using high resolution 19 F solid-state MAS NMR. Spectra (Figure 6) of the samples prepared at 1, 2, 3, 4, 6, 8 and 10 hours reaction time show three distinct lines which were previously assigned to fluorine in the vicinity of different numbers of titanium atoms and/or titanium vacancies. The species Ti3-F, Ti2-F and Ti2-F have been previously identified in anatase Ti0.780.22O1.12F0.4(OH)0.48 at -88 ppm, -4 ppm and 98 ppm, respectively. 12 Within pure TiOF2, an anion is surrounded by two titanium atoms. Nevertheless, oxide anions can be substituted by OH groups yielding titanium vacancies 22 so that it exists an additional coordination mode, Ti-X (X = O,F), for the anions. In hydroxylated TiOF2, i.e. Ti0.900.10O0.60(OH)0.74F1.66, the species Ti2-F have been identified at 12 and 20 ppm and the species Ti-F at 146 and 182 ppm. 23 Based on these previous studies, the NMR lines used for the fits of the 19 F solid-state MAS NMR spectra have been assigned to Ti3-F, Ti2-F, Ti2-F, Ti2-F and Ti-F species (see Supporting Information, Figures S6-11 and Tables S1-7). For a reaction time of 1 hour, the 19 F isotropic chemical shifts (iso) of the two main contributions are in-between those of Ti21-F and Ti2-F and those of Ti2-F and Ti-F, respectively, in agreement with the simultaneous occurrence of TiOF2like and anatase phases. The reconstruction of the spectrum pointed out a 19 F isotropic chemical shift (iso) located at 144.2 ppm which can be assigned to Ti-F species. The occurrence of such an environment indicates that the TiOF2-like phase features titanium vacancies due to the substitution of oxide by mono-valent anions, i.e. Ti1-xxO1-4x(OH,F)2+4x.

For longer reaction time (2, 3 and 4h), the 19 F iso values of the NMR lines assigned to F atoms coordinated to one Ti atom are smaller. The occurrence of Ti-F species is then doubtful in these samples. The 19 F iso values of the NMR lines assigned to Ti2-F and Ti2-F species decrease when the reaction time increases up to 6 h and then remain almost constant. Moreover, the relative intensity of the NMR line at ~ -88 ppm (or less for a reaction time of 1 hour) assigned to Ti3-F species increases with the reaction time up to 6 h and then remains constant. All these statements confirm the progressive dissolution of a TiOF2-like phase and the concomitant growth of anatase. Finally, the 19 F solid-state MAS NMR spectra of the samples prepared at 6, 8 and 10 hours reaction time are similar showing that the local environment of fluorine doesn't evolve anymore. The later are formed by oxolation reaction which decreases the content of titanium vacancies. The observed solid-state transformation occurring in anatase has been previously mentioned to occur during the hydrothermal synthesis of nanoparticles. 21 The ageing process of an amorphous TiO2•1.6H2O compound yielded anatase TiO2 which according to Sauvage et al occurred via a slow solid dehydration process. Very likely this process implies oxolation reactions. Interestingly, sol-gel processes excluding fluorine have shown to yield anatase with titanium vacancies which are due to the presence of OH groups substituting O 2-, i.e. Ti0.810.19O1.24(OH)0.76. 25 Overall, the formation of anatase implies the structural re-arrangement of an hydroxide (and hydroxyfluoride in the presence of fluorine) featuring a large content of titanium vacancies which can largely impact on physico-chemical properties. 12 CONCLUSION The solvothermal reaction of titanium tetraisopropoxide, in the presence of aqueous HF, has been investigated to shed lights upon the formation mechanism of highly fluorinated anatase phase featuring titanium vacancies. It is shown that HF reacts readily with titanium isopropoxide yielding both an amorphous phase related to TiOF2 and a fluorinated anatase. At the early stage of the reaction, the anatase anionic sublattice was shown to contain only monovalent fluoride and hydroxide anions. During reaction, TiOF2 completely dissolves while the anatase nanoparticles re-arrange through a solidstate transformation. This solid-phase transformation implies the condensation of OH groups by oxolation reaction yielding oxide anions and a concomitant decrease of the cationic vacancies. The combined uses of 19 F solid-state NMR and PDF analyses were decisive in understanding the formation mechanism of fluorinated anatase. Finally, this work confirms that titanium alkoxide precursors can react with HF via a fluorolysis process yielding fluorinated molecular precursors which further condense producing new composition and structural features deviating from a well ordered anatase network. Supporting Information. Additional information such as TEM images, PDF analysis, fits of 19 F MAS NMR spectra.

  Figure 1b shows the high resolution TEM images of the dried gel obtained after 1 h of reaction. It points out the presence of two types of nanoparticles: (i) 34 nm amorphous particles (circled in yellow in Fig 1b) and (ii) ca. 7 nm nanoparticles featuring crystal lattice spacing of 0.19 nm corresponding to anatase (200) plans. Prolonging the reaction time yields to an increase of the population of the anatase crystallized nanoparticles and concomitantly reduces the amorphous one (Figure S1 in Supporting Information) highlighting a dissolution-recrystallization process.

Figure 1 .

 1 Figure 1. (a) XRD patterns of the samples synthesized within the first 5 hours. (hkl) planes of anatase are indicated. Red and blue dashed lines correspond to the (100) and (101) planes of TiOF2 and anatase, respectively. (b) HRTEM images of the sample obtained after 1 h of reaction.

Figure 2 .

 2 Figure 2. Evolution of the PDFs of the samples prepared from 1 to 5-hours. Arrows indicate peak intensity evolutions upon increasing reaction time.

Figure 4 .

 4 Figure 4. Evolution of the titanium vacancy concentration and particle size of the anatase phase upon reaction.

Figure 5 .

 5 Figure 5. Evolution of the relative proportions of anions in the anatase phase.

Figure 6 .

 6 Figure 6. 19 F solid-state MAS (64 kHz) NMR spectra of the samples prepared at 1, 2, 3, 4, 6, 8 and 10 hours reaction time. The dashed lines indicate the isotropic chemical shifts of Ti3-F, Ti2-F and Ti2-F environments in Ti0.780.22O1.12F0.4(OH)0.48 12 and of Ti2-F and Ti-F environments in Ti0.900.10O0.60(OH)0.74F1.66. 23 Spinning sidebands of the main contribution are marked with an asterisk. The arrow indicates an unidentified impurity. At the early stage of the reaction, the simultaneous presence of two phases having two different structural features is due to the extent of fluorolysis over hydrolysis reaction. The predominance of the TiOF2-like phase indicates that fluorolysis occurs faster than hydrolysis. Highly fluorinated molecular precursors likely assemble to form Ti1-xxO1-4x(OH,F)2+4x while lower fluorine containing species lead to the formation of the anatase network. The presence of a large content of titanium vacancies indicates the preponderance of OH groups over O 2-anions.The later are formed by oxolation reaction which decreases the content of titanium vacancies. The observed solid-state transformation occurring in anatase has been previously mentioned to occur during the hydrothermal synthesis of nanoparticles.21 The ageing process of an amorphous TiO2•1.6H2O compound yielded anatase TiO2 which according to Sauvage et al occurred via a slow solid dehydration process. Very likely this process implies oxolation reactions. Interestingly, sol-gel processes excluding fluorine have shown to yield anatase with titanium vacancies which are due to the presence of OH groups substituting O 2-, i.e. Ti0.810.19O1.24(OH)0.76.25 Overall, the formation of anatase implies the structural re-arrangement of an hydroxide (and hydroxyfluoride in the presence of fluorine) featuring a large content of titanium vacancies which can largely impact on physico-chemical properties.12 CONCLUSION The solvothermal reaction of titanium tetraisopropoxide, in the presence of aqueous HF, has been investigated to shed

Table 1 :

 1 Estimated chemical composition of the anatase phase at different reaction times of the solvothermal process.

	Time (h)	F/Ti ratio a molar	Calculated anatase F/Ti in	Composition of anatase
	2	1.33	0.94	Ti0.500.50F0.47(OH)1.53
	3	1.24	0.97	Ti0.540.46O0.16F0.53(OH)1.31
	4	1.04	0.88	Ti0.570.43O0.28F0.50(OH)1.22
	5	0.75	0.75	Ti0.630.37O0.52F0.51(OH)0.97
	12	0.51	0.51	Ti0.780.22O1.12F0.4(OH)0.48

a obtained by ICP-AES (Ti) and NMR (F)

The authors declare no competing financial interests. The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7/2007-2013) under REA grant agreement n°[321879] (FLUOSYNES). S. Casale is acknowledged for HRTEM measurement. The work done at the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.

For Table of Contents Only.

Structural re-arrangement occurring in anatase nanoparticles synthetized in a solvofluorothermal conditions


Wei Li

Monique Body

Christophe Legein

Damien Dambournet

Sol-gel Chemistry of Titanium alkoxide towards HF: impacts of reaction parameters

Keywords: Fluorolysis, hydrolysis, anatase, hexagonal-tungsten-bronze, oxyhydroxyfluoride, titanium vacancies, 19 F solid state NMR

The sol-gel chemistry of titanium-based oxide prepared in a fluorinated medium leads to oxy-hydroxyfluorinated framework owing to the occurrence of both hydrolysis and fluorolysis reactions. Here, a systematic study was carried out to investigate the impact of the sol-gel synthesis parameters on the reaction of titanium alkoxides and HF. This study provides comprehensive insights of the sol-gel process in a fluorinated medium. Depending on the extend of fluorination, two types of structures can be stabilized including an anatase Ti1-x-yx+yO2-4(x+y)F4x(OH)4y and a fluoride-rich HTB Ti1-xxO1-4x(F,OH)2+4x phases, where  refers to titanium vacancies. The reactivity of titanium alkoxide in alcohol toward HF has been shown to mostly depend on the solvent characteristics highlighting an alcohol interchange reaction between the titanium precursor and the solvent. Increasing the fluorine concentration leads to a structural change from the anatase to the fluoride-rich HTB phase which is rationalized by the preferential linking modes of Ti octahedral subunits in relation with its anionic environment, i.e. corner-sharing configuration is favored for Ti 4+ in a fluorinated environment. Finally, 19 F MAS NMR spectroscopy has allowed to identify the different species involved in the stabilized phases providing spectroscopic signatures to further investigate these complex compounds.

Introduction

The chemistry of the sol-gel process relies on the hydroxylation and condensation of molecular precursors. 1 The reactivity of metal alkoxide toward solvents or additives can yielded new molecular precursors and thus to a large panel of composition and structures. [2][3][4] The chemistry of titanium is a good example of the versatility of the sol-gel process to produce new materials with tailored architecture and properties. 5,6 One of the approaches used to tailor crystal's properties is to play on the complexing ability of anions. For instance, fluoride anions have been shown to play many roles such as driving the nucleation/crystallization toward a specific crystal's structure. 7,8 They can also adsorb onto surfaces thus orienting, in a particular direction, the growth of particles. 9,10 More recently, the reaction of titanium alkoxide with aqueous HF has yielded to the precipitation under mild solvothermal conditions to an oxy-hydroxy-fluoride compound Ti1-x-yx+yO2-4(x+y)F4x(OH)4y featuring the anatase type structure with titanium vacancies (). 11 This result has highlighted the occurrence of a fluorolysis 12,13 reaction between titanium alkoxide and HF according to Ti(OR) n + xHF → Ti(OR) n-x F x + xROH. 11,14 Hence, the functionalization of titanium alkoxide by fluoride can open new opportunities in the sol-gel chemistry of titanium-based compounds providing that a good understanding of the impact of the synthesis parameters is achieved.

Herein, we investigate the effects of synthesis parameters (nature of solvent, alkoxide precursor, HF concentration) on the structure and chemical composition of the precipitated phase obtained by mild solvothermal synthesis. It provides a better understanding on how the synthesis parameters impact on the chemical reactivity of titanium alkoxide toward HF. Two types of phases were synthetized including an anatase Ti1-x-yx+yO2-4(x+y)F4x(OH)4y and a fluoride-rich hexagonal-tungsten-bronze (HTB) Ti1-xxO1-4x(F,OH)2+4x phase. This work provides also guidelines to tune the chemical composition of these phases.

Experimental Section

Synthesis. Titanium ethoxide Ti(OCH2CH3)4 (Ti(OEt)4), titanium isopropoxide Ti(OCH(CH3)2)4, (Ti(OiPr)4), titanium butoxide Ti(OCH2CH2CH2CH3)4 (Ti(OBu)4), ethanol (EtOH), isopropanol (PriOH) and 2-butanol (2-BuOH) were purchased from Sigma-Aldrich and used as received. Aqueous HF (40%) was purchased from Rectapur. Caution: HF solution is highly hazardous and special protective equipments are required.

In a typical synthesis, 13.5 mmol of titanium alkoxide precursor were poured inside a Teflon liner cup, then a solution containing aqueous hydrofluoric acid and alcohol was added under stirring. The volume of HF added depends on the fixed molar ratio of R = F/Ti. The volume of the alcohol was adjusted so that the total volume of the solution equals 30 mL. The Teflon liner cup was placed inside a stainless steel autoclave and then heated in an oven at 90 °C for 12 h.

After the reaction, the autoclave was left to cool down to room temperature. The resulting white precipitate was washed twice with ethanol and subsequently recovered by centrifugation at 4400 rpm for 15 min. Finally, the powder was dried at 80 °C for 2 h. Characterization methods. X-ray powder diffraction analysis was carried out using a Rigaku Ultima IV X-ray diffractometer equipped with a Cu Kα radiation source (λ = 1.54059 Å).

Transmission Electron Microscopy analysis was performed using a JEOL 2010 UHR microscope operating at 200 kV equipped with a TCD camera. 19 F solid-state magic angle spinning (MAS) NMR experiments were performed on a Bruker Avance III spectrometer operating at 7.0 T ( 19 F Larmor frequency of 282.2 MHz), using a 2.5 and/or a 1.3 mm CP-MAS probe head. The 19 F MAS spectra were recorded using a Hahn echo sequence with an interpulse delay equal to one rotor period. The 90° pulse length was set to 1.55 μs (2.5 s) and the recycle delay was set to 20 s (10 s) for the spectra recorded with 1.3 mm (2.5 mm) probe head. 19 F spectra are referenced to CFCl3 and were fitted by using the DMFit software. 15 19 F solid state NMR was also used to quantify the fluorine content on some of the studied samples by using the 1.3 mm probe head and reference samples. 11,14 19 F solid-state MAS NMR (Hahn echo) spectra were also recorded for YF3 and LaF3 and the masses of each sample in the rotor were measured. The fits of the spectra allow to determine the integrated intensities (I) for each sample. Since, for each sample, the recycle delays were chosen to ensure that the amount of signal detected is maximum (420 s for YF3 and 120 s for LaF3), we assume that the integrated intensities are proportional to the number of scans (256 for the studied samples and 16 for YF3 and LaF3) and to the molar quantity of fluorine atoms (n) in the rotor. This assumption is verified since the calculated I/n ratio for YF3 and LaF3 are equal. The intensities per scan of the NMR signals of the studied samples, I1, and of YF3 (or LaF3), I2, allow to calculate the fluorine wt. % in the studied samples using the following formula where m and M are the mass and the molar mass, respectively:

𝐼 1 𝐼 2 = 𝑛 𝐹 3𝑚 𝑌𝐹 3 𝑀 𝑌𝐹 3 𝐹 𝑤𝑡. % = 𝑚 𝐹 𝑚 = 𝑛 𝐹 𝑀 𝐹 𝑚 = 3𝑚 𝑌𝐹 3 𝑀 𝑌𝐹 3 I 1 I 2 M F m

Results and Discussion

Effect of the nature of the solvent. In a previous study, we have demonstrated that the solvothermal reaction of titanium isopropoxide dissolved in its parent alcohol with aqueous HF (R= F/Ti =2) yields to an oxy-hydroxyfluoride Ti1-x-yx+yO2-4(x+y)F4x(OH)4y (the accurate chemical composition being Ti0.780.22O1.12F0.40(OH)0.48) featuring the anatase type structure with titanium vacancies (). 11 Therein, the influence of the solvent was investigated by using other solvents featuring shorter and longer alkyl chains, i.e. ethanol and 2-butanol. The other operating conditions were fixed as defined previously. Figure 1 shows the XRD patterns of the samples obtained using different solvents.

Figure 1. XRD patterns of the samples prepared using ethanol (black), isopropanol (red) and 2butanol (blue) as solvent. Titanium isopropoxide was used as precursor and a molar ratio R was set to 2. 'A' and 'HTB' refer to anatase and hexagonal-tungsten-bronze type phases, respectively and asterisk refers to (200) planes of TiO2-B.

Samples prepared with ethanol and isopropanol display similar XRD patterns indexed with a tetragonal unit cell (space group I41/amd) characteristic of TiO2 anatase. As discussed later, detailed analyses performed on these anatase phases revealed a complex composition Ti1-x-yx+yO2-4(x+y)F4x(OH)4y with a degree of fluorination, i.e. vacancies, that increases from ethanol to isopropanol. Moreover, the use of 2-butanol yields a completely different XRD pattern with peaks characteristic of a fluoride-rich phase crystallizing in the HTB type structure (space group P63/mmc). 16 A peak located at 15.1° (2θ) indicates the presence of a minor phase identified as TiO2-B (space group C2/m). Figure 2 shows the structural representation of the three phases.

Thus, it can be concluded that the chemical reactivity of HF toward titanium alkoxide is strongly influenced by the nature of the solvent, particularly the length of the alkyl chains. It is known that when titanium alkoxides are dissolved into alcohol solvent, an exchange of alkoxy groups takes place by substitution reaction, modifying the molecular structure of the Ti precursor and consequently its chemical reactivity. 2,3,17 This reaction is known as an alcohol interchange reaction: 𝑇𝑖(𝑂𝑅) 𝑛 + 𝑥𝑅 ′ 𝑂𝐻 ⇆ 𝑇𝑖(𝑂𝑅) 𝑛-𝑥 (𝑂𝑅 ′ ) 𝑥 + 𝑥𝑅𝑂𝐻. While the high electrophilic character of titanium favors the reaction, the steric hindrance of the alkoxy group decreases ligand interchange. 2 The predominant effect between the alcohol interchange reactions and the initial molecular structure of the alkoxide precursor was also investigated by varying the nature of Ti precursors while fixing the other operating conditions. Figure 3 shows the XRD patterns of the samples obtained by using Ti(OBu)4 and Ti(O i Pr)4 as precursor. The solvent was butanol and the molar ratio R was fixed to 2. In both cases, XRD patterns showed the main presence of the fluoriderich phase having the HTB type structure. Thus, the nature of the precursor has little influence on the structure of the final products. The above results indicate that due to ligand interchange reactions between the solvent and the titanium precursor, it is the solvent characteristics and particularly the length of the alkyl chains that rules the composition/structure of the final stabilized phase. Thus, ethoxide, isopropoxide and butoxide ligands can be stabilized in the vicinity of Ti 4+ when ethanol, isopropanol and butanol are used as solvents, respectively. Moreover, the stabilization of a higher fluorine concentration in the precipitated phase using solvent with longer alkyl chains indicates that the competition between hydrolysis and fluorolysis reaction is ruled by the structure of the molecular precursor. It has been reported that the reactivity of alkoxides towards water increases when the length of the alkyl chains decreases: OEt -> OiPr -> OBu -. 1,18 Shorter alkyl chains lead to the increase of the positive partial charge of Ti, thus Ti atoms become more reactive towards nucleophilic attack. Moreover, long alkyl chains exert steric hindrance during the nucleophilic substitution. Since HF and H2O display similar nucleophilic characters, i.e., the partial charges are δ(F) = -0.42 in HF and δ(O) = -0.40 in H2O, 1 the difference in reactivity can be related to higher steric hindrance of water molecule as compared to HF. For longer alkyl chains, the fluorination is thus favored over hydrolysis. The rate of hydrolysis reaction decreases while the size of alkoxy group increases.

To further understand the reactivity of titanium alkoxide toward HF, 19 F solid state MAS NMR spectra were recorded on two reference samples having the HTB and anatase type structure (Figure 4). A pure HTB phase was synthetized using the following conditions R = 3.7, solvent:

Effect of the fluorine content.

The aforementioned results have shown that the reactivity of titanium alkoxide toward hydrolysis and fluorolysis is dictated by the length of the alkyl chains. In this part, the reactivity of titanium alkoxide toward increasing concentration of HF, i.e. increasing the R=F/Ti molar ratio, was investigated in the above mentioned three solvents.

Figure 5 displays the evolution of the XRD patterns of the samples obtained using ethanol as solvent while the molar ratio R = F/Ti was varied from 1.0 to 3.0. All the samples show similar diffraction patterns indexed with a tetragonal cell characteristic of anatase. Further increasing the molar ratio R to 3.5 did not result in any precipitate, which is due to the formation of soluble titanium fluorides complexes. 9 Accordingly, the yield of the reaction decreases upon increasing the HF concentration, i.e. from 50 % to 30 % for R=1 and 3, respectively.

Figure 5. XRD patterns of the samples prepared using various molar ratio R = F/Ti. The precursor and solvent used were Ti(OEt)4 and ethanol, respectively. 'A' refers to anatase phase. 19 F solid-state NMR spectra were recorded using a 2.5 mm probe head for all the samples of this series (see SI, Figure S2) and using a 1.3 mm probe head for three samples (R = 1.0, R = 2.2 and R = 3, Figure 5). The fits of these three spectra are given as SI. These spectra are similar to those of anatase Ti0.780.22O1.12F0.40(OH)0.48 (R = 2.0, solvent: isopropanol, precursor : titanium isopropoxide, Figure 3). 11 Fluorine quantification using 19 F solid-state NMR spectroscopy was performed on these three samples yielding 7.1, 7.8 and 9.7 wt. % for R= 1.0, 2.2 and 3.0, respectively. This indicates that increasing the molar ratio R yields higher fluoride concentration and thus higher titanium vacancies concentration in Ti1-x-yx+yO2-4(x+y)F4x(OH)4y, considering the Four domains can be distinguished: (i) R < 2.2, (ii) 2.2 ≤ R ≤ 2.4, (iii) 2.6 ≤ R ≤ 3.5, and (iv) R = 3.7. For R < 2.2, XRD patterns are indexed with a tetragonal cell (space group: I41/amd) characteristic of TiO2 anatase. The increase of R to 2.2 leads to the appearance of a peak located at 2θ = 27.8°. This peak becomes more pronounced at R = 2.4 and can be indexed with the HTBtype structure (space group: P63/mmc). 16 The HTB phase continuously grew while R increased.

The anatase phase disappeared at R = 2.6. Concomitantly, a peak located at 2θ = 15.1°, assigned to (200) plans of TiO2-B, first appeared and tended to vanish while R increased. An almost pure HTB-type phase was obtained at R = 3.7. Further increasing R to 4.0 led to the formation of soluble complexes. 9 The yield of the reaction decreases upon increasing the HF concentration, i.e., from 70 % to 20 % for R=1 and 3.7, respectively. It should be noted that the powder morphology strongly evolved upon increasing R (Figure S6). The evolution of the local structure of fluorine atoms during the structural change from the anatase phase Ti1-x-yx+yO2-4(x+y)F4x(OH)4y to the fluoride-rich HTB phase Ti1-xxO1-4x(F,OH)2+4x was investigated using 19 F solid-state NMR. Spectra were recorded for the samples of the series with R ranging from to 2 to 3.7 (Figure 7). The fits of these spectra are gathered in SI (Figures S8-S14, Tables S5-S11). In agreement with XRD patterns, when R increases from 2.0 to 2.4, the spectra show the decrease of the proportion of the anatase phase as highlighted by the decrease of the relative intensity of the line assigned to Ti3F species. These species disappear for R = 2.6 confirming the disappearance of the anatase phase for this R value. When R increases from 2.0 to 2.2, whereas the anatase phase remains largely predominant, the 19 F NMR spectrum evolves: the proportion of Ti3F species decreases (from 4.6% to 3.2%) and those of Ti2-F species increases (from 35% to 38%), i.e. the vacancy rate (x+y) increases, presumably in relation with an increase of the amount of fluorine (x), in the Ti1-x-yx+yO2-4(x+y)F4x(OH)4y anatase phase. Further increase of R destabilizes the anatase phase in favor of the HTB one which can accommodate larger amount of fluorine. At R = 2.4, the sample consists of a more balanced mixture of both the anatase and HTB phases, complicating the analysis, since on the one hand, Ti2-F species from anatase and Ti-F species from HTB, and on the other hand, Ti2-F species from anatase and Ti2-F species from Ti1-xxO1-4x(F,OH)2+4x HTB have similar chemical shift values. Nevertheless, when R increases from 2.2 to 2.4, the decrease of the proportion of Ti3F species (from 3.2% to 0.9%) is more pronounced than the decrease of the proportion of fluorine atoms bounded to one titanium atom (Ti2-F for R = 2.2 and both Ti2-F and Ti-F for R = 2.4, from 38% to 22%). Therefore, the vacancy rate and the amount of fluorine continue to increase in the anatase phase and/or the vacancy rate is especially high in the HTB phase, this second assumption being not supported by the moderate and quite stable proportion of Ti-F species in the HTB phase for R ≥ 2.6 (in between 7% and 10%). The rates of titanium vacancy (x) in these HTB type Ti1-xxO1-4x(F,OH)2+4x phases, constituting mainly or exclusively the samples for R ≥ 2.6, can be estimated in between 0.04 and 0.05. The observed structural changes from anatase to the HTB phase occurred at a lower fluorine concentration in the case of 2-butanol than with isopropanol indicating an enhanced reactivity of the formed molecular precursor towards fluorine. Moreover, the use of 2-butanol enabled the stabilization of a well-crystallized HTB phase featuring a low content of titanium vacancies (cf.

Figure 4). 19 F solid-state NMR spectra were recorded for the HTB phase obtained at R = 3.7 (Figure 3) and for the anatase phase obtained at R = 1.0 (Figure 10). This last spectrum is typical of Ti1-x-yx+yO2-4(x+y)F4x(OH)4y anatase phase but compared to those of Ti0.780.22O1.12F0.40(OH)0.48 (Figure 3), the proportion of Ti3F species is higher (11%) and those of Ti2-F species is lower (27%), i. e., the vacancy rate (x+y) is lower, in relation with a lower amount of fluorine (x), in this phase. This result shows that even at low fluoride concentration, titanium alkoxide readily reacts with HF to stabilized fluoride in the vicinity of Ti 4+ . Moreover, depending on the synthesis conditions, the proportions of the different Ti3F, Ti2-F and Ti2-F species can be tuned. Upon increasing the fluoride concentration, the observed structural changes (for isopropanol and 2-butanol) from anatase to the HTB phase along with the minor presence of TiO2-B can be rationalized by the connection mode of TiX6 octahedral subunits (X= O 2-, F -, OH -). At low fluoride concentration, the anatase phase having the general chemical formula Ti1-x-yx+yO2-4(x+y)F4x(OH)4y is stabilized. Upon increasing fluoride concentration, fluorolysis increases yielding to the progressive destabilization of the anatase network. The higher content of fluoride anions in the vicinity of Ti 4+ favors octahedral subunits shared by corners as found in other titanium oxy and hydroxyfluoride compounds. 16,22,23 This is in good agreement with the observed phase appearance upon increasing fluorine content from edge-sharing (anatase) to both edge and corner-sharing in TiO2-B and to finally a corner-sharing configuration as found in the HTB type structure (Figure 2).

Conclusion

In summary, a systematicstudy was carried out to investigate structural and compositional effects of the sol-gel synthesis parameters on the reaction of titanium alkoxides and HF. This study provides comprehensive insights of the sol-gel process in fluorinated medium. The impact of the solvent on the reactivity of titanium alkoxide toward increasing concentration of HF showed that alcohol with longer alkyl chains such isopropanol and butanol favor fluorolysis over hydrolysis as compared to ethanol. This confirms that the solvent characteristics drastically impact on the composition/structure of the stabilized phase. Using butanol, the phase evolution

Figure 2 .

 2 Figure 2. Structural representation of (a) TiO2 anatase, (b) TiO2-B and (c) hexagonal tungsten

Figure 3 .

 3 Figure 3. XRD patterns of the samples prepared using two different alkoxide precursors, Ti(O i Pr)4 (black) and Ti(OBu)4 (red). The solvent and fluorine content are 2-butanol and R = 2,

  2-butanol, precursor: titanium butoxide (see later). The anatase phase Ti0.780.22O1.12F0.40(OH)0.48 was synthetized using previously established conditions: R = 2.0, solvent: isopropanol, precursor : titanium isopropoxide.11 Fluorine quantification of these samples yielded 27.4 and 10.8 wt. % for the HTB and anatase type phase, respectively.

Figure 4 .

 4 Figure 4. 19 F MAS (64 kHz and 60 kHz, respectively) solid state NMR spectra of the pure HTB

  preferential location of F close to vacancies (the stabilization of vacancies close to fluorine).11 Moreover, when the molar ratio R increases, the relative intensities of the NMR lines assigned to Ti2-F and Ti3-F decrease whereas the relative intensities of the NMR lines assigned to Ti2-F increase (see FiguresS3-S5and TablesS2-S4). For R = 3.0, the fluorine wt. % and the relative intensities of the NMR lines are close to those of Ti0.780.22O1.12F0.40(OH)0.48 (i. e. 10.8 wt.% and 4%, 64% and 32% for Ti3-F, Ti2-F and Ti2-F, respectively) indicating a rather close formula.

Figure 6 .

 6 Figure 6. 19 F MAS (64 kHz) solid state NMR spectra of the samples prepared using various

Figure 7

 7 Figure7shows the evolution of the XRD patterns of the samples obtained by adjusting the

Figure 7 .

 7 Figure 7. XRD patterns of the samples prepared using various molar ratio R = F/Ti. The

Figure 8 .

 8 Figure 8. 19 F MAS (34 kHz) solid state NMR spectra of the samples prepared using various

Figure 9

 9 Figure9shows the evolution of the XRD patterns of the samples obtained in 2-butanol at

Figure 9 .

 9 Figure 9. XRD patterns of the samples prepared using various molar ratio R = F/Ti. The

Figure 10 .

 10 Figure 10. Experimental (blue) and fitted (red) 19 F MAS (34 kHz) solid state NMR spectra of

  upon increasing fluoride concentration enabled to prepare a pure oxyhydroxyfluoride phase featuring the HTB-type structure by favoring fluorolysis over hydrolysis reaction. The observed structural change from the anatase phase Ti1-x-yx+yO2-4(x+y)F4x(OH)4y to the fluoride-rich HTB phase Ti1-xxO1-4x(F,OH)2+4x upon increasing fluoride concentration has been rationalized by the geometry and linking mode of the Ti building blocks. Overall, the competition between hydrolysis and fluorolysis is a paramount parameter that dictate the stabilization of oxide or fluoride-rich compounds. Finally, this work provides novel insights to control the composition of the anatase Ti1-x-yx+yO2-4(x+y)F4x(OH)4y phase. ASSOCIATED CONTENT Supporting Information. Additional 19 F MAS NMR spectra, fits of 19 F MAS NMR spectra, SEM and TEM images and additional XRD patterns.

Funding Sources

This research was supported by European Union's Seventh Framework Programme (FP7/2007-2013) under REA grant agreement n°[321879] (FLUOSYNES).

ACKNOWLEDGMENT

We kindly acknowledge S. Casale for transmission electron microscopy measurement.

AUTHOR INFORMATION

Corresponding Author

*Corresponding author: damien.dambournet@upmc.fr