Peering into the mechanism of low-temperature synthesis of Bronze

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Peering into the mechanism of low-temperature synthesis of Bronze-type TiO2 in Ionic Liquids Pascal Voepel, Christoph Seitz, Jan M Waack, Stefan Zahn, Thomas Leichtweiß, Aleksandr Zaichenko, Doreen Mollenhauer, Heinz Amenitsch, Markus Voggenreiter, Sebastian Polarz, and Bernd M Smarsly Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01231 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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Crystal Growth & Design

Peering into the mechanism of low-temperature synthesis of Bronzetype TiO2 in Ionic Liquids Pascal Voepela, Christoph Seitza, Jan M. Waacka, Stefan Zahna,b, Thomas Leichtweißc, Aleksandr Zaichenkoa, Doreen Mollenhauera,c, Heinz Amenitsche, Markus Voggenreiterd , Sebastian Polarzd and Bernd M. Smarslya,c* a

Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff Ring 17, 35392 Giessen, Germany. Wilhelm-Ostwald-Institute, University of Leipzig, Linnéstr. 2, 04103 Leipzig, Germany c Center for Materials Research (LaMa),Justus-Liebig-University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany d Department of Chemistry, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany e Institute of Inorganic Chemistry, Graz University of Technology, Stremayergasse 6/V, 8010 Graz, Austria KEYWORDS Ionic Liquids, Low-Temperature, Metastable Titania, Phase determination b

ABSTRACT: In this work, we present detailed investigations on the influence of binary ionic liquid (IL) mixtures on sol-gel syntheses of metastable metal oxide phases. The synthesis of the metastable TiO2 bronze phase and anatase as well as the rutile modification is followed via in-situ diffraction methods coupled with thermal gravimetric analysis. The controlled variation of the IL composition allows for the adjustment of TiO2 phase composition at low temperatures, whereas competing reactions take place subsequently. Based on these results the synthesis of the hexagonal tungsten bronze (HTB)-like titanium hydroxyl oxy fluoride was also possible. Our results pave the way for a deeper understanding of IL participation in the syntheses of inorganic nanomaterials, going further than treating them as solvents and thus, contributing to the advancement in IL research.

Introduction In recent years, the usage of Ionic liquids (ILs) has introduced an additional methodology for the synthesis of metal oxides, especially in nanostructured form. In the year 2000, as a first system silica aerogels were prepared using [C4mim][NTf2] by S. Dai and coworkers.1 Based on these investigations, several syntheses for nanostructured SiO2 were reported.2,3 Later, the mild conditions of IL-mediated synthesis were transferred to nanostructured TiO2, and as the first TiO2 material anatase nanosponges were synthesized by the Antonietti group using [C4mim][BF4] and a low reaction temperature.4 Such studies represent the starting point of using ILs for synthesizing metal oxide nanostructures by modified sol-gel reactions within IL-solvent mixtures. Also, ILs were applied on purpose to cause specific polymorph formations or to limit crystal growth.5,6 In general, imidazolium-based ILs have been applied in several studies for the synthesis of different titania polymorphs.6–10 As common feature these syntheses were shown to achieve crystallinity of the metal oxides applying comparably low synthesis temperatures, while other syntheses require temperatures of several hundred °C to induce high crystallinity. There are several review articles dealing with IL application in inorganic syntheses trying to reflect their influence.11–14 The option of synthesizing metal oxides with complex composition and especially of polymorphs of one particular oxide under mild conditions represents a remarkable facet of ILbased syntheses. For instance, the application of certain ILs enable the variable synthesis of nanocrystalline anatase or rutile.15 Yet, the influence of the ILs on the nucleation and crystallization of metal oxide species in IL-based solutions is still a matter of discussion. Recent studies have proven the beneficial impact of the ILs, such as oriented growth,16 phase

determination,15,16 and influencing the crystallization process .17,18 Titanium dioxide bronze phase (TiO2(B) has gained a lot of interest in the last years. The extended unit cell of TiO2(B) is depicted in Figure 1. In contrast to the most common TiO2 modifications, namely anatase, rutile and brookite, TiO2(B) has a sheet-like structure, the sheets being interconnected by corner-sharing TiO6 octahedra. As hollandite and ramsdellite it belongs to the metastable modifications of titania and exhibits after hollandite the second lowest density of all titania modifications.19,20 The first reported synthesis of TiO2(B) was based on alkaline titanates such as K2Ti8O17 in the year 1980 and comprises harsh reaction steps such as a calcination step at 1000 °C for 2 days for the formation of the precursor material adding several steps for dehydration or longterm treatment in acidic conditions.21,22 In recent years the pre-templating of TiO2(B) by layered alkali titanates was shown as a proper way to synthesize TiO2(B).23 In comparison to the other modifications TiO2(B) exhibits a very high theoretical storage capacity for lithium ions.24–26 Moreover, TiO2(B) is also very interesting for photocatalytic applications. The bandgap is in the range of 3 - 3.2 eV and the character of TiO2(B) is n-type.27,28 Nanostructuring TiO2(B) has been investigated with high interest, providing syntheses for nanotubes, nanosheets, nanobelts, mesoporous nanowires or microspheres as well as nanoparticles.29–35 A huge improvement was achieved by IL-based synthesis, enabling shorter synthesis times being in the range of few hours compared to days for solvothermal syntheses. In the synthesis developed in our group, TiCl4 is hydrolyzed in a controlled way with water in an IL-mixture of [C4mim][BF4] and [C16mim][Cl], followed by treating this solution at only 95 °C

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under reflux, which results in precipitation of phase-pure TiO2(B).35

3.

4.

5.

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Figure 1: a) 2x3 supercell of TiO2(B). Oxygen atoms are depicted in red, titanium atoms are depicted in gray (embedded in blue octahedral) b) all four possible oxygen positions in the unit cell (OBr, O3f1, O3f2 and O4f)

First investigations showed that the synthesis of the TiO2(B) in ILs depends on an appropriate water concentration in the reaction medium and also on the type of the used ILs, where [BF4]--based ILs seemed to be crucial.35,36 In a different approach Mansfeldova et al. investigated the influence of the ILs’ cations. It was reported that the ratio of anatase and TiO2(B) was influenced by the concentration of the [C16mim]+ cation. However, a phase-pure TiO2(B) sample could not be obtained in these studies.37 In summary, important factors and molecular processes in the IL-based synthesis of TiO2(B) are not yet fully understood, which is the case for other IL-mediated syntheses, too. We thus regard the formation of an unusual crystal structure such as TiO2(B) under surprisingly moderate conditions as an opposite model for elucidating the role of ILs in such syntheses in general. Several important questions regarding our recently developed TiO2(B) synthesis constitute the motivation of the present study: 1. The role of F-containing ILs, in particular [BF4]-, has remained unclear. It can be assumed that [BF4]- is partially hydrolyzed, which needs direct experimental evidence. If HF is released, fluorine might both act as ligand for Ti and also might impact the crystallization of TiO2. In essence, one needs to address which chemical reactions (if any) occur in the IL at 95 °C in the reaction mixture. This issues pertains to the question if the ILs are solvent and/or reaction partner. 2. Using an IL mixture with one IL possessing a long hydrocarbon chain is crucial for obtaining TiO2(B). IL cations such as [C16mim]+ can influence the reaction in different potential ways. If dissolved in

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C4mim-based IL, ILs such as [C16mim][Cl] might form IL-in-IL phase-separated domains on the nanometer scale, which might direct the nucleation/crystallization of TiO2(B) through the charge distribution at the interface. In general, the impact of the used cations ([C4mim]+/[C16mim]+) and anions ([BF4]-, Cl-) on the crystallization needs to be clarified, i.e. which composition results in the modifications anatase, rutile and TiO2(B). Even if a molecular understanding is perhaps not yet possible, experimental data are needed to assess the role of each of the IL constituents. The importance of using a special amount of water has remained unclear, in light of the amount needed to spur the hydrolysis of TiCl4. It is inevitable to establish a relationship between Ti species (complexes) and the occurrence of the solidstate structure. In-situ studies are needed to clarify if TiO2(B) forms directly from solution, or if anatase nucleates first and transforms later into TiO2(B).

Here, we present an in-depth study to clarify the impact of the ILs on this highly interesting synthesis. The investigations are based on both detailed ex- and in-situ X-ray diffraction methods at a synchrotron facility. Also we invented an in-situ Raman spectroscopic cell to monitor the temporal behavior of the ILs in reaction conditions. Thermal stability tests have been carried out on the reaction medium and the product to settle the post-synthetic influence of the IL on the TiO2 material.

Experimental Section In a typical synthesis according to our previous studies, a mixture of ILs (3.85 mmol) was heated to 95 °C and stirred with 250 rpm (see ESI Figure S 1). After homogenization of the mixture 0.2 mL (1.8 mmol) of TiCl4 was added dropwise. Subsequently, after 5 minutes, 0.45 mL of dist. water was carefully added dropwise over a period of 20 seconds to the solution in order to prevent excessive heating. The reaction was carried out over 4 hours under reflux with additional 8 hours at 80 °C in excess of ethanol.35 The composition of the IL mixture was varied within the experiments using [C16mim][Cl], [C16mim][BF4], [C4mim][Cl] and [C4mim][BF4] (See ESI Table S1 for detailed information). The ILs were purchased by IoLiTec and were used without further purification. Wide-angle X-ray powder diffraction (XRPD) experiments at ambient conditions were conducted with an X’Pert PRO diffractometer (PANalytical instruments) using CuKα radiation. The instrument was operated at 40 kV and 40 mA using a 1° divergence slit for the incident beam. Temperature-depending measurements were carried out on an Empyrean diffractometer (PANalytical instruments) using a XRK 900 reactor (Anton Paar). Thermogravimetric measurements were performed on a Netzsch STA 409 PC at a heating rate of 5 °C/min. The thermobalance was coupled to a Balzers QMG 421 quadrupole mass spectrometer. The ionization energy was 70 eV. In-situ Small-Angle X-ray Scattering (SAXS) and WideAngle X-ray Diffraction (WAXD) experiments were performed at the SAXS Beamline (5.2) at the Elettra Synchrotron facility (Elettra-Sincrotrone Trieste; AREA Science Park; 34149 Basovizza, Trieste; Italy) using a monochromatic wavelength of 0.077 nm. The combined SAXS and WAXD data were rec-


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orded with a Pilatus3 1M (SAXS) and a Pilatus 100k (WAXD) detector (both Dectris, Baden-Daetwill Switzerland). The synthesis were carried out in an 80 mm x 1.5 mm glass capillary with 0.1 mm wall thickness (Hilgenberg GmbH, Malsfeld Germany) and heated to 95 °C. Raman spectroscopy was performed using a Senterra Raman microscope from Bruker equipped with a 532 nm Laser (o.2 W), MPlan 20x lense and 90° mirror adapter from Bruker. Transmission electron microscopy has been carried out on a Philips CM 30 operating at 300 kV using carbon covered copper grids. X-ray photoelectron spectra (XPS) were acquired on a PHI VersaProbe II Scanning ESCA Microprobe (Physical Electronics) with a monochromatic Al Kα X-ray source with a system base pressure of < 10-7 Pa. The X-ray power was 50 W and the pass energy of the analyzer was set to 23.5 eV for detail spectra and to 187.6 eV for survey scans. The built-in argon ion gun (2 kV) was used for ion etching. The C1s signal from adventitious hydrocarbons was set to 284.8 eV to correct for charging effects. Elemental quantification was performed after background subtraction (Shirley method) using the relative sensitivity factors provided by the instrument manufacturer. Computational Details: Non-periodic calculation: The programs provided by the Turbomole-suite 6.638 were applied for all structure optimizations. Structures were optimized with the B3LYP density functional , a dispersion correction of Grimme with Becke-Johnson dumping was applied.39–42 The 631++G** basis set43,44 was employed for all elements except for Ti for which the def2-TZVP basis set45 was used. It was shown that interaction energies of ionic liquid ion pairs are reproduced very well by a 6-31++G** basis set in combination with a dispersion corrected DFT functional and that the accuracy can be hardly improved by post Hartree-Fock methods with large basis sets and a counter poise correction.46–48 The convergence criterion of the iteration cycle was increased to 10-8 Hartree in all calculations. Frequency calculations were done numerically by NumForce in combination with the derivatives of quadrature weights for more accurate gradients. Thermodynamic data were calculated by freeh, a tool provided by Turbomole. Solvation effects were approximated by the conductor-like screening model (COSMO)49,50 for which a dielectric constant of 40 was selected to approximate a mixture of water and imidazolium-based ionic liquids. Periodic calculations: The Vienna ab initio simulation package (VASP) version 5.4.1.51–54 was employed for the periodic density functional theory (DFT) calculations. The generalized gradient approximation (GGA) functional of Perdew, Burke, and Ernzerhof (PBE) was applied for all calculations.55,56 For the DFT+U calculations the PBE functional was supplemented with the +U term of the Dudarev approach acting on Ti 3d states.57 The +U value was chosen to be 2.5 eV according to Ref. Zhu et al.58 for which a good description of relative energies of several TiO2 polymorphs was obtained. Furthermore, the D3 dispersion correction of Grimme with Becke-Johnson dumping was applied.42,59 We refer the approach to DFT+U-D3. Projector augmented wave potentials and a plane wave basis set with an energy cutoff of 700 eV were used. The Monkhorst-Pack scheme was used for the sampling of the k-points in the first Brillouin zone. A 3x10x5 k-point grid was chosen for TiO2(B), and a 2x6x4 k-point grid for TiO2-xFx(B). Test calculations using different k-points grids were performed. Structure optimization was done in four steps. First, the atomic positions and super cell parameters

were optimized using the conjugate-gradient algorithm. Second, the atomic positions were re-optimized using the quasiNewton algorithm. Third, the grid-points of the FFT mesh were adjusted and the atom positions and super cell parameters re-optimized by using the conjugate-gradient algorithm. Fourth, the atomic positions were again re-optimized using the quasi-Newton algorithm. For these four steps the MethfesselPaxton approximation (σ=0.05 eV) was applied. Finally, single point calculation was carried out employing the tetrahedron method with Blöchl corrections. A Bader charge analysis was done to study the charge distribution in the system.60–63

Results and Discussion X-Ray Diffraction In this study we present an in-depth investigation on the influence of composition of IL mixtures on the outcome of the synthesis scheme depicted in Figure S 1. The combination of the two cations ([C16mim+], [C4mim+]) and two anions ([BF4]-, [Cl]-) is varied in systematic manner to point out the influence of the respective ions isolated from each other. The investigations are based on both in-situ and ex-situ Xray diffraction and Raman spectroscopic techniques. Figure 2 depicts the WAXD pattern of a representative TiO2(B) sample reproducing the protocol developed by Wessel et al.35 All the measured reflections can be assigned to TiO2(B). The (001), (002) and (003) reflections possess a higher relative intensity than predicted by reference data, see Feist et al.22

Figure 2: WAXD pattern of a typical TiO2(B) powder sample with theoretical reference signals based on Ref. 21 (JCPDS- 00046-1238) and 22. A 2D diagram is inserted to clarify the composition of the respective cations and anions used.

Based on hydrothermal synthesis it was shown that surface hydroxylation will introduce surface relaxation and the particle will exhibit ellipsoidal appearance in c-direction which explains the intensity differences in (00l) direction.64 Based on the Scherrer equation the crystallite size could be calculcated to approx. 5 nm which matches the crystalline domains observed in HRTEM-studies (See Figure S 2). Although the particles seem to aggregate (See Figure S 2 a)), th (001) spacing is observed in high magnifications (see Figures S 2 b) and c)). In order to assess the thermal stability and transitions of TiO2(B), a sample was heated to 900 °C and studied by in-situ WAXD measurements. From Figure 3 it can be seen that at



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approx. 530 °C a phase transformation to TiO2 anatase takes place. After thermal annealing at 900 °C for 4 hours a second phase transition can be observed, corresponding to the transformation of anatase into the thermodynamically stable rutile modification. It is known from previous studies that anatase particles possess a size-dependent transformation temperature. Smaller particles tend to transform into rutile at higher temperatures.65,66 In our case, we observe the anatase modification even for temperatures above 650° C. The average crystallite size was calculated via the Scherrer equation to approx. 20 nm at 650 °C, and an increasing crystallite size for rising temperature is obtained.67 At 900 °C a grain size of approx. 45 nm can be observed. Thus, this TiO2(B)-derived anatase shows a higher thermal stability against phase transition compared to

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though the IL cation was slightly different ([C4mim]+ compared to [C2mim]+ in the literature). On the other hand, high [BF4]- concentrations (compositions A and B in Fig. 4) lead to a WAXD pattern which is barely described in literature and known to be related to the hexagonal tungsten bronze (HTB) structure which possesses a high fluorine content.68,69

Figure 3: Temperature-dependent WAXD patterns of a TiO2(B) sample, heated from 25 °C to 900 °C. Rutile traces are marked with an asterisk (*)

literature values. In accordance with Figure 3 we can state that TiO2(B) is present in traces up to elevated temperatures of 650 °C. It was also shown for ribbon-like structures that TiO2(B) is transformed into anatase between 600 °C and 800 °C.31 Since the [BF4]- anion concentration was previously shown to possess a significant influence on the phase composition of the TiO2 samples,35 the concentration of the [BF4]anion in the solution was varied by partial exchange of [C4mim][BF4] by [C4mim][Cl] in this study. The total amount and the ratio of the two different cations was kept constant, implying that only the anion ratio is changed. Note that the indicated percentages of the anion concentrations in Figure 4 are only related to the IL- based ions, while the concentration of TiCl4 was not changed in these mixtures. From WAXD data after synthesis it is found that the formation of TiO2(B) is aggravated with decreasing concentration of [BF4]-(see Figure 4a)). Accordingly, the (101) reflection of anatase gets more pronounced with decreasing [BF4]- concentration and TiO2(B) was not present for [BF4]- concentrations below 50 mol% (corresponding to a Ti:[BF4]-ratio of approx. 1), but can be obtained without WAXD-detectable impurities for mixtures of approx. 50 mol% [BF4]- and above. Without any [BF4]- present, the resulting sample consisted of a mixture of anatase and rutile. It was reported that rutile formation in ILs is highly sensitive to the HCl concentration in the reaction medium.16 Since the [BF4]- anions were substituted by [Cl]anions, the formation of rutile case is understandable, even

Figure 4: a) WAXD patterns of synthesis products with different [BF4]- concentrations.(A) 100 mol%, (B) 90 mol%, (C) 79 mol% (D) 69 mol%, (E) 60 mol% (F) 50 mol%, (G) 33 mol%, (H) 30 mol%, (I) 22 mol%, (K) 17 mol%, (L) 0 mol% [BF4]-, respectively. (Ref. 21 (JCPDS- 00-035-0088) and 22 (JCPDS 00-0461237), rutile signals are marked with an asterisk (*) b) WAXD pattern of alternative synthesis approaches with a different se-


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quence of mixing the reactants and varying IL concentrations during the reaction (see text): (M) addition of ILs solely, (N) addition of ILs and TiO2 precursor. (anatase JCPDS 00-021-1272 and rutile JCPDS 00-021-1276)

Figure 5: WAXD patterns of synthesis products with variation of the IL cation composition. (A) 100 mol%, (B) 90 mol%, (C) 79 mol%, (D) 69 mol%, (E) 47 mol%, (F) 23 mol% [C4mim]+ in the reaction medium, with [C16mim]+ being the complementary cation. The percentages refer to the concentration of [C4mim]+ in relation to the sum of the concentrations of [C4mim]+ and [C16mim]+. The [BF4]- concentration was set to 70 mol% of the total anion concentration.35.

Our experiments indicate that the synthesis of phase-pure TiO2(B) is highly sensitive to the amount of IL anions. While the upscaling by a factor of 2 still leads to the desired product (not shown here), the ratio of titania precursors to the IL anions seems to be essential for this synthesis. To gain a deeper understanding of the subsequent mechanistic steps during the synthesis and the role of the different compounds involved, we changed the sequence of adding the single compounds and studied the resulting material by WAXD. (see Figure 4 b). First, we started from a [BF4]--free reaction solution (i.e. only [C16mim][Cl] and [C4mim][Cl] were used) and after two hours [BF4]- anions were added in two different ways. At this point it was made sure that titania crystallization already took place, but also there was enough time for further crystallization until the end of the synthesis. In the first approach, a solution of TiCl4 in the [C4mim][Cl]-[C16mim][Cl] mixture was mixed with [C4mim][BF4] after 2 hours in order to establish the standard [BF4]- concentration of approx. 70 mol%.

In the second approach, only the half of all educts for a [BF4]--free equivalent experiment was heated for 2 hours and then a mixture of [C16mim][BF4], [C4mim][BF4] and TiCl4 was added to guarantee the standard concentration of both the [BF4]- and cations in the solution. In both cases no TiO2(B) was observed, but only rutile and anatase are formed (see Figure 4 b). Although the composition of the mixture is different in the two cases, the absence of TiO2(B) proves that the [BF4]- anion determines the nucleation and crystallization of the final composition of TiO2 already in the very beginning of the synthesis. A delayed addition of [BF4]--based IL cannot provide the formation of TiO2(B) after hydrolysis of the TiCl4 has already started and TiO2 particles are already present. Therefore, we further conclude that the initial concentration of [BF4]- in the reaction is the phase-determining factor. A subsequent formation of TiO2(B) seems not to be possible, or is very small and not detectable. Since the IL cation was reported as crucial factor found by Mansfeldova et al.,37 we also investigated the composition of the final titania material in dependence on the cation variation (see Figure 5). The reaction was carried out at a molar fraction of 100 mol%, 90 mol%, 79 mol%, 69 mol%, 47 mol% and 23 mol% [C4mim]+, with respect to the total concentration of [C4mim]+ and [C16mim]+ cations, using an anion content of [BF4]- of 70 mol%. With exception of the lowest and the highest [C4mim]+ content (100 mol% and 23 mol%), phase-pure TiO2(B) is formed, whereas the optimized [C4mim]+ content of 69 mol% leads to the most pronounced TiO2(B) WAXD pattern. In the case of a low concentration of [C4mim]+ the resulting product consists of a mixture of mainly anatase and TiO2(B). This effect may be partially attributable to the high increase in viscosity due to the [C4mim]+ substitution with [C16mim]+, and not to substitution itself. Also, this trend seems to be in conflict with the observations of Mansfeldova et al.37 The large range of [C4mim]+ concentrations resulting in TiO2(B) indicates a minor influence of the cation species on the final TiO2 composition, whereas the cation-induced viscosity increase seems to be detrimental for the formation of TiO2(B). On the other hand, a total lack of [C16mim]+ does also inhibit the phase pure formation of TiO2(B). The increase in viscosity might be caused by a phase separation of the ILs as already reported by Kaper et al.36 As these ex-situ investigations show a competing formation of anatase and TiO2(B), the formation of TiO2(B) was investigated by in-situ X-ray scattering/diffraction experiments to get further insights into the time-dependent nucleation and crystallization mechanism, using the SAXS Beamline (BL5.2) at Elettra Synchrotron facility (Trieste, Italy). The coupled WAXD and SAXS set-up gives information about the time dependence of particle growth and crystallization (Figure 6). It is noteworthy that the IL-based medium is a very strong absorber. Therefore an energy of 16keV was chosen instead of commonly used 8keV (please note that for comparison the 2 Theta values are calculated for an energy of 8 kV). Figure 6 a) depicts the WAXD patterns of a synthesis resulting in phase-pure TiO2(B) particles and indicated as the “standardsynthesis”. It can be seen that within the first hour the intensities of the (110) and (002) reflections increase significantly originating from the nucleation of TiO2(B). The corresponding SAXS patterns (Figure 6 c)) reveal a marked correlation maximum at q = 1.15 nm-1 which is found throughout the entire synthesis. The slope of the SAXS patterns is strongly increased in the Guinier region for small scattering vectors. The intensity of the described correlation maximum declines dur-



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ing the synthesis, and its position is shifted towards lower scattering vectors q indicating that the signal is based on chemical processes and is not related to a set-up based influence. This feature may be attributable to a species which is directly involved in and consumed during the reaction, e.g. a self-aggregated, micellar object. For longer times, the effects ease and after the completed synthesis, the SAXS maximum is almost vanished after the total reaction time of 4 hours. Using Guinier’s law and expanding the analysis to the Beaucage model,70 the calculated radius of gyration Rg (Figure 6 f)) increases with ongoing reaction. The SAXS analysis reveals that the nucleation of the TiO2(B) phase takes place within the first hour and is saturated after approx. three hours, reaching an average Rg of ca. 28 nm. These observations are in accordance with WAXD patterns, showing that the TiO2(B) phase is nucleated directly from solution and does not form via transformation from a different crystalline TiO2 polymorph, as reported for classic bulk material phases.21 Additional in-situ WAXD-SAXS experiments were performed using IL-mixtures, with both lower [BF4]- and lower [C16mim]+ concentrations. In this case the concentration of [BF4]- anions was set to approx. 50 mol% and the concentration of the [C16mim]+ was set to approx. 10 at%, as the ex-situ

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WAXD investigations indicated that such concentrations should result in a mixture if TiO2(B) and anatase. Figure 6 b) depicts the time-dependent WAXD patterns. Analogue to the phase-pure case discussed above, TiO2(B) is generated within 50 minutes. The (110) reflection is superimposed by the (101) signal of anatase. However, the growth of anatase happens subsequently after the growth of TiO2(B), confirming that TiO2(B) is not generated by a transformation from anatase, but nucleates directly from the reaction solution. Also, the experiments support the aforementioned assumption that a certain concentration of [BF4]- has to be present in the reaction mixture. The in-situ SAXS data acquired from this mixture exhibit multiple features which point to a mixture of several species on the nanometer-scale, contrary to the pure TiO2(B) particles. Analogue to the SAXS pattern recorded on the reaction mixture leading to pure TiO2(B) a maximum at approx. q = 1.15 nm-1 occurs. Both, the intensity and the width of this signal do not go in line with the observed data for the first case also indicating that this data is not related to capillary scattering. Nevertheless, the broader appearance of this correlation maximum can be attributed to a higher degree of disorder, introduced


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Figure 6: a) Time-dependent WAXD pattern of the in-situ reaction for TiO2(B). Each pattern represents a time advancement of 5 minutes; b) Time-dependent WAXD pattern of the in-situ reaction for TiO2(B)/anatase mixture. Each pattern represents a time advancement of 5 minutes; c) Respective SAXS pattern of the in-situ reaction leading to pure TiO2(B). Each pattern represents a time advancement of 5 minutes. The green graph represents the beginning, the red one the end of the respective synthesis; d) Respective SAXS pattern of the insitu reaction for TiO2(B)/anatase mixture. Each pattern represents a time advancement of 5 minutes. The green graph represents the beginning, the red one the end of the respective synthesis. e) Time- dependence of the different WAXD-signal intensities with background correction: (A) anatase (101), (B) TiO2(B) (110), (C) signal intensity of the TiO2(B) (110) peak position in the phase pure case, (D) anatase intensity with respect to the TiO2(B) intensity in the mixture; f) variation of the (110) peak integral width (■) and the peak area (■) and the calculated radius of gyration (■).

by a decreased concentration of [C16mim]+. Here, we propose a micelle-like formation of the long alky chain ILs which

directly leads to the detection of the correlation peak and might be disturbed by decrease of the concentration of these



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species. In order to exclude an anion-based effect, different approaches were analyzed. The concentration of [C16mim]+ was kept constant at 10 mol% whereas the concentration of [BF4]- was varied in the range of 70 mol% to 30 mol%. However, the SAXS pattern remains unchanged upon anion variation which clearly indicates a cation dependence of the SAXS pattern (see ESI Figure S 3 in comparison to 30 mol% [C16mim]+). The pronounced correlation peak is only found in the standard approach (using 70 mol% [C4mim][BF4] and 30 mol% [C16mim][Cl] ). In combination with the ex-situ investigations (see Figure 5A) this also explains why the presence of [C16mim]+ is necessary to form TiO2(B). We think that a certain concentration of [C16mim]+ has to be reached in order to start micelle formation which has already been shown for the used ILs.36,71 These micelles might increase the local effective [BF4]- concentration and therefore promote the TiO2(B) formation. A second prominent feature is obtained at q = 3.7 nm-1 which vanishes completely within the first hour (no positive curvature). Similar to phase-pure TiO2(B), the SAXS maximum at q = 1.15 nm-1 diminishes with time, which is partially due to the superposition with other SAXS contributions. A distinct signal appears after 50 minutes in the region of q = 0.4 nm-1 which is in fair accordance with the emergence of anatase, as the corresponding WAXD pattern also starts to increase after the same time. By comparing the in-situ WAXD patterns in Figure 6 a) and Figure 6 c) the primary growth of the TiO2(B) nanoparticles can be observed, followed by the emergence of a shoulder at the position of the anatase (101) reflection, the intensity of which is continuously enhanced until the end of the synthesis. Monitoring the respective WAXD signal intensity over the reaction time shows similar trends for the TiO2(B) (110) reflection for both reaction mixtures (Figure 6e)). The shown WAXD data are backgroundcorrected based on the first diffraction patterns recorded. After approx. one hour the intensity of the TiO2(B) (110) reflection reaches a plateau (Figure 6 e) and the anatase (101) signal is further increased. The evolution of the relative intensity of the TiO2(B) (110) reflection is in fair agreement with the signal intensities extracted from the WAXD patterns of the phasepure case, showing that the kinetics of the TiO2(B) phase formation is not related to the crystallization of anatase. In conclusion, the study of these two solutions indicates that the changed concentration of [BF4]- anions leads to the formation of anatase after the formation of TiO2(B), which further indicates a critical minimum concentration of [BF4]- in the reaction solution. Please note that the absolute intensity of a certain X-ray reflection is based on various parameters such as anisotropy, atomic occupation numbers or Debye-Waller-factors. But in this case the absolute intensity of the two respective peak positions gives a fair qualitative hint for the phases emerging during the synthesis. To estimate the presence of a compound quantitatively, the integrated intensity has to be considered. Deconvolution of the diffraction pattern into TiO2(B) and anatase contributions would give rise to big errors due to the almost complete superposition of the respective diffraction peaks. However, for the phase-pure sample the comparison of the integrated peak intensity of TiO2(B) and the temporal behavior of the Guinier radii and the integral width indicates

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that the radius of gyration already reaches a maximum and stays constant after approx. 2.5 hours. This is not the case for the integral intensity or the integral width. A further slight increase in the integrated intensity describes further formation of respective scattering species. The analogue further decrease of the integral width indicates a slight growth of the crystalline domains. As the Guinier radii stay constant, this means that the crystalline TiO2(B) parts are surrounded by titania precursors or early stages of hydrolyzed species which do contribute to the small-angle scattering, but do not contribute to the crystalline scattering yet. The formation of TiO2(B) therefore has to carried out from the inside of this agglomerates to the outside. The width of the reflections of TiO2(B) determined from insitu WAXD data after a period of 60 min. is comparable to the width obtained via ex-situ WAXD. This finding implies that the TiO2(B) nanocrystals, within the TiO2(B) nanoparticles, reach their final size of ca. 5 nm, calculated from the integral width of the reflections, already after ca. 60 min. Taking into account a radius of gyration Rg of 28 nm, as determined from SAXS, the TiO2(B) particles are thus composed of several nanocrystals.

Scheme 1: IL-dependent coordinate system, depicting the ILbased phase dependence of the different titania samples (R: rutile, A: anatase, B: Bronze, F: titanium hydroxyl-oxy-fluoride).

Summarizing the diffraction analysis, a coordinate system can be derived indicating the influence of the IL mixture’s composition on the formation of the different titania polymorph (Scheme 1). It can be seen that TiO2(B) can be synthesized in a quite substantial range of concentrations of the involved IL cations and anions. In particular, a minimum concentration of [BF4]- is inevitable for obtaining phase-pure TiO2(B). Furthermore, a certain minimum content of the longchain cation [C16mim]+ is required to achieve phase-pure TiO2(B). Further WAXD data on TiO2 using other ILcombination are shown in the ESI Figure S 4 which were used to develop the following coordinate system. Thermal gravimetric analysis Previous reports indicate a post-synthetic influence of the IL on the crystallization behavior of amorphous TiO2 samples.18 By thermogravimetric analysis of as-prepared TiO2(B) the IL residues inside or on the sample were investigated, also addressing their role in the aforementioned transition from


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TiO2(B) to anatase at elevated temperatures. In Figure 7 a comparison of thermal decomposition of the pure ILs as well as of a 3:2 weight mixture of [C4mim][BF4] and [C16mim][Cl] is shown in addition to the TG plot of as-prepared TiO2(B) (Figure 7 (A)). The mass of the TiO2(B) sample decreases almost linearly up to a temperature of 500 °C. Afterwards, the TG-plot exhibits a significant drop at approx. 530 °C followed by a remaining mass of approx. 75 wt%. This mass drop is in accordance with the phase transformation from TiO2(B) to anatase, as observed by WAXD (Figure 3). As depicted in Figure 7, the weight loss at 530 °C is not directly related to the decomposition of residual ILs, as they exhibit a decomposition at lower temperatures (Figure 7(B), (D)). Also, mixing of the ILs does not elevate the decomposition temperature (Figure 7 (C)). A stepwise decomposition of IL mixtures has also already been investigated.72,73 Thus, the weight loss observed for as-prepared TiO2(B) has to be attributed to species bound or incorporated to/in TiO2(B). The decomposition fragments of the samples were analyzed via mass spectrometry coupled with thermogravimetric analysis (TGA-MS). A detailed listing of the findings can be seen in Figure 8, where the mass transients of different mass-tocharge ratios (m/z) within the regarded temperature range are depicted. Signals recorded for temperatures below 400 °C can be explained by the decomposition of residual ILs as indicated in Figure 7. For as-prepared TiO2(B) it can be seen that at 530 °C significant peaks for several channels/fragments can be observed, which corresponds to the transition from TiO2(B) to anatase. The signals at m/z = 15 and 18 indicate a mass loss based on methyl groups or water, respectively. An m/z value of 19 might be related to H3O+. However, water desorbs at lower temperatures up

Figure 7: TGA of different IL mixtures and the synthesized TiO2(B) sample in the range of 50 °C to 1000 °C. (A) as synthesized TiO2(B) sample, (B) [C16mim][Cl], (C) 3:2 mass mixture of the [C4mim][BF4] and [C16mim][Cl], (D) [C4mim][BF4].

to 250 °C, thus the signal observed at 530 °C probably corresponds to fluorine. If H3O+ contributed to this signal, there should be a correlation with m/z = 18. Therefore, it is unlikely to attribute m/z = 19 to H3O+. The broader release pattern of fluorine starting at a temperature of approx. 300 °C is due to the decomposition of the ILs, as it can also be found in the TGA profile found for pure [C4mim][BF4]. Besides fluorine also fluorine-containing compounds such as BF+ (m/z = 30), BF3O+ (m/z = 84) and [BF4]+ (m/z = 87) were detected in the MS transients at a temperature of 530 °C. As these signals are also present in the TGA data of [C16mim][Cl], they are unlikely to represent only [BF4]- derivatives. These signals might also be attributable to organic residuals such as C5H10N+ (m/z = 84), C5H11O+ (m/z = 87) or CH2NH2+(see Figure 8 middle)). Again, these sharp peaks neither occur in the case of the pure IL samples nor the comparative titania sample showing rutile and anatase modification. Chlorine (m/z = 35) shows a very broad desorption range with an onset at approx. 190 °C which is in accordance with the observed pattern of the pure IL decomposition. Figure 8 c) - e) depicts the MS transients of the used IL mixture. A comparison between Figure 8 a) and b) reveals the different behavior of TiO2(B) upon thermal treatment compared to an anatase/rutile mixture obtained via synthesis variations. Whereas some transients such as m/z = 18 or m/z = 30 show similar behavior, there are other signals which exhibit completely different features. Water adsorbed on the TiO2 surface shows in both cases a desorption temperature of approx. 150 °C and 300 °C. For signals with m/z = 19 no peaks are observable for the



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Figure 8: Mass spectra in the region of m/z = 15 to 87 of a TiO2(B) sample (a), TiO2 (Anatase/Rutile) sample (b), the used ILs mixture (c) and the pure ILs (d,e).

anatase/rutile case, as no fluorine is present in the synthesis. In case of TiO2(B) the same transient exhibits a broad signal starting at approx. 300 °C and shows a distinct peak at 530 °C which must be related to fluorine extraction. Also, the findings fit well with the decomposition signals of the pure IL shown in Figure 8 d). Another correlation of the two presented oxides can be seen in m/z = 44. This signal should be attributed to the oxidative decomposition of organic matter to CO2. In Figure 8 a) the signals show two broad and one sharp maxima. The first two broad maxima can be found in the anatase/rutile sample as

well at the same position (approx. 350 °C and 450 °C). Under consideration of the thermal decomposition of the IL mixture and the pure IL phases (shown in Figure 8 c), d) and e)) it can be stated that the first peak is based on the decomposition of [C16mim][Cl] and the latter on the decomposition of [C4mim][BF4], while the longer alkyl chains decompose at lower temperatures. However, the distinct peak for m/z = 44 occurring at 530 °C cannot be found in any other MS transient and therefore must be related to trapped IL residuals inside the channel-like structure of TiO2(B), which is destroyed upon


Page 11 of 19

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phase transition towards anatase generating CO2. The assignment of the signals for higher m/z ratios becomes more uncertain, as several fragments (esp. hydrocarbon chain fragments) can lead to the same m/z ratio. For instance, signals such as m/z = 84 or 87 cannot exclusively assigned to hydrocarbon derivatives, but also to tetrafluoroborate derivatives or fragments such as BF3O+ or BF3+, respectively for the case of the [BF4]--based ILs. Figure 9 shows a direct comparison of the TG of a TiO2(B) sample, the sample consisting of a mixture of rutile and anatase, which was synthesized in a [BF4]--free solution under otherwise equal parameters, and the HTB-like sample synthesized in [BF4]--based ILs. It can be seen that the decay in mass below ca. 500 °C is very similar in all three cases.

Figure 9: TG-plot for the HTB-like sample (A), a representative TiO2(B) sample (B) and the rutile/anatase mixture based on a [BF4]-free synthesis.

This decomposition step is caused by the thermal decomposition of the residual ILs on the surface of the different TiO2 samples and desorption of water. The sudden drop of mass at ca. 530 °C is absent in the case for the rutile – anatase mixture which also indicates that it has to be related to the transformation of TiO2(B) into anatase.

The decomposition profiles of the samples indicate fluorine release at approx. 530 °C. At this temperature the stability of fluorinated titania samples seems to be exceeded. For fluorinated anatase particles it was shown that the decomposition temperature was found to be between 525 °C and 538 °C without a significant dependence on the crystal sizes, and a similar behavior was observed for other titanium hydroxfluorides.74,75 The decomposition profile of the HTB-like sample is similar to previously reported results and only differs in the decomposition of IL residues in the temperature region below 400 °C. Thus, we can conclude from WAXD and TG analysis that the rapid mass drop and phase transformation of TiO2(B) to anatase goes in line with the release of fluorine derivatives from the TiO2(B) nanoparticles. Together with the synthesis modification showing that [BF4]- needs to be present in the beginning of the synthesis, [BF4]- exhibits both a structuredirecting effect and results in the fluorination of the growing TiO2 nanoparticles. XPS Detailed XPS analysis of phase-pure TiO2(B) does not show a detectable amount of boron in the sample, even if sample sputtering was applied to get insight into higher depths of the particles (See ESI Figure S 5 for survey spectra). A significant amount of fluorine can be detected by XPS, but TGA-MS indicates residues of [BF4] derivatives. The lack of boron in this XPS study shows the amount of these residues is very low as the sensitivity for boron is approx. only 10 % of the sensitivity for fluorine. This fact implies that the intensity of the F1s signal is directly related to fluorine which is not bound to boron anymore. Also the position of the F1s peak is shifted with respect to the position in the IL (see Figure S 7 a)). The observed position is described in literature for Ti-F-Ti-Bonds in TiOF2.76,77 However, a slight asymmetry of the F1s signal can be detected. This finding can be explained by a small contribution of a second maximum which corresponds to the [BF4]--based signals in the IL and represents a small residue of the ILs in TiO2(B). The low sensitivity for boron explains the lack of B1s signal, but the slight asymmetric peak of the F1s signal goes in line with the TGA-MS-data and solid state 19FNMR measurements which indicate two fluorine species (see Figure S 6), a small amount of [BF4]-derivatives. Figure S 7 b) depicts the Ti2p region and shows that only a very small contribution of Ti3+ species is necessary (2%) in order to fit the Ti2p 3/2 peak reasonably. Note that the fit only involves the Ti2p 3/2 peak, as a deconvolution of the Ti2p 1/2 peak into the contributions from Ti4+ and Ti3+ is not possible due to the very low intensity of Ti3+. Table 1: Summary of the atomic concentrations of the respective elements in the standard TiO2(B) sample as determined via XPS. Ti2p

C1s

O1s

F1s

B1s

N1s

Cl2p

%conc.

22.2

25

41.6

9.4

0

1.6

0.2

STD

0.12

0.33

0.24

0.11

0.11

0.16

0.05



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The amount of nitrogen emerging from IL residuals is approx. 1.6 at%, but is decreased by sputtering. The amount of oxygen seems to be too low in order to form TiO2 (see Table 1). However, the sum of the fluorine and oxygen fractions is too high for a hypothetical TiO2-xFx compound. This deviation might be due to titanium vacancies, interstitial fluorine or ILrelated residues. These XPS studies might also explain the aforementioned high thermal stability of the TiO2(B)-derived anatase sample as residual fluorine in the sample and the interfacial contact might increase the thermal stability as already shown in literature.78 In-situ Raman spectroscopy In order to gain a deeper understanding of the influence and the temporal change of the ILs within the reaction mixture, we conducted in-situ Raman spectroscopic experiments. The specially constructed cell is depicted in Figure 10. Reference experiments with P25 particles revealed that below a mass concentration of approx. 5 wt% of TiO2 in ILs the Raman modes of TiO2 cannot be detected. The setup did not allow for stirring the reaction mixture, which however is not required for a successful synthesis outcome, as revealed by the in-situ WAXD experiments. In the course of the succession of the reaction, formed TiO2(B) particles continuously precipitate and therefore will not be in the laser focus anymore, thus minimizing absorption of the Raman laser beam. Therefore this technique and designed cell is only appropriate to investigate the behavior of the ILs in the reaction. The time-resolved Raman spectra (Figure 10) measurements reveal an increasing intensity of a Raman mode at approx. 350 cm-1. Calculating the Raman modes of [BF4]- and its hydrolysis products indicates that this mode can be explained by the partial hydrolysis of [BF4]-, the Raman mode of which is located at 336 cm-1. This value is in good agreement with previously published results.79,80 The modes in the region of 1300-1550 cm-1 can be described by cation vibrations, as it was already shown for the [C4mim] and [C2mim]+ cation.81,82 The relatively high intensity in the region of 150 cm-1 can be explained by both the BF3(OH)- and the BF(OH)3-

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Figure 10: a) In-situ Raman-cell; b) In-situ time-dependent Raman spectra recorded with the standard approach for TiO2(B); c) temporal evolution of the signal corresponding to BFx(OH)y species; d) Intensity of the signals corresponding to BFx(OH)y and [BF4]-.

anions. Also, next to the [BF4]- mode at approx. 325 cm-1 a second mode is formed at 350 cm-1, corresponding to the transition from [BF4]- to BF3(OH)-. The hydrolysis of [BF4]- can also be observed by the intensity decrease of the mode at 1025 cm-1. This mode originates from the [BF4]- anion, and therefore the decreasing intensity indicates the transformation into other species. Hence, the marked shifts in the Raman signals indicate the hydrolysis of the [BF4]- anion. Therefore, these experiments prove the release of a fluorine anion and thus support the proposed crystallization mechanism. Fluorine has already been proven as beneficial for the TiO2(B) formation.83 Theoretical calculations As indicated by the X-ray diffraction methods, the very first moments in the synthesis are crucial for the phase formation of the TiO2 material. Therefore, we think that the reactions taking place in these first moments take an essential influence on the TiO2 modification. Inspired by the results obtained from the insitu experiments theoretical calculation were carried out in order to rank the thermodynamic stability of each complex and, hence, the possibility to find it in the respective solution. Different types of solutions were considered in order to monitor the application order correctly and to simulate the reaction conditions as effectively as possible. Initially, diverse reactions in the mixture

without water were investigated, see Table S 2. The addition of [Cl]- to TiCl4 should be only slightly exergonic due to the possible error of the calculation. Overall, an octahedral coordination of Ti4+ by [Cl]- does not seem to be


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preferred under the respective conditions. Furthermore, the weakly coordinating [BF4]- anion binds significantly weaker to Ti4+ than [Cl]- even if [BF4]- acts as a twofold chelate ligand. Thus, it seems unreasonable that the [BF4]- ion coordinates at Ti4+ as long as [Cl]- is in the mixture. However, the [F]- anion binds significantly stronger to Ti in TiCl4 than [Cl]- and results in a Ti complex which prefers an octahedral coordination, similar to synthesized TiO2 compound. Thus, [F]- generated from [BF4]- might play a crucial role in the reaction. However, neither a halogen exchange of TiCl4 and [BF4]- nor a [F]- anion transfer from [BF4]- to TiCl4 are an exergonic reaction in a water-free system. The situation changes, if water is added to the mixture, see the results in Table S 3. H2O is overall a weak ligand, and thus, similar as [Cl]-, water does not seem to prefer an octahedral coordination, if added to TiCl4. Unfortunately, calculated reaction energies in which the number of positive and negative charges changes during the reaction are very prone to large errors, especially if small ions participate. This can be seen for example in the reaction of B(OH)3 with water, which is strongly endergonic. The error results mainly from a too weak stabilization of the formed ions. The hydrolysis of [BF4]- is also a strongly endergonic reaction, although it is known that [BF4]is not stable in the presence of water and partially hydrolyzes.84 Also, the findings in TGA-MS confirm the hydrolysis. Therefore, we will discuss only those results in the following, in which the number of positive and negative charges are conserved on both sides of the reaction. It seems reasonable that HCl is removed via evaporation during the reaction and a hydroxide anion is formed. To allow for a charge conservation of the investigated reactions, we will use OH- in the following discussion. The most essential pathways are depicted in Scheme 2. As can be seen in Table S 4, [Cl]- can be replaced by [F]- at 4+ Ti as well as at B3+. [OH]- is the anion which binds strongest to Ti4+ and B3+ and, thus, can replace both halogenide anions. Therefore, it seems reasonable that initially HCl is removed while HF should be removed with ongoing reaction as well. Interestingly, the addition of one [OH]- or [F]- to Ti(OH)4 is exergonic while the addition of a second anion is endergonic (see Table S 4). Furthermore, [OH]- does not seems to be capable of removing the last [F]- anion coordinated at Ti4+, if at least four [OH]- ligands are coordinated at Ti4+ as well. These issues finally result in an overall ligand exchange between Ti4+ and B3+: As soon as the coordination of Ti increases beyond four anions, the hydroxide anion prefers a coordination at B3+ instead of Ti4+ and an [F]- anion is transferred to Ti4+. This anion inhibits the formation of Ti-O-Ti bonds and, therefore, the unusual TiO2 structure is observed in the titania product, because exactly one coordination side of Ti4+ is blocked by [F]- (Please note: the titanium atoms in TiO2(B) are connected to one twofold coordinated oxygen atom). The origin of the [OH]-/[F]- exchange between Ti4+ and B3+ might be attributed to a larger amount of space requested by the OHgroup. The reaction energy of an OH-/F- exchange between Ti4+ and B3+ is negligible as long as B and Ti are tetrahedral coordinated, see Table S 4. Thus, the nature of the bond be-

tween ligand and central atom should be comparable in the OH-/F- systems with the same central atom.

Scheme 2: Sketch of fundamental processes regarded from the theoretical point of view.

A significant reaction energy of an OH-/F- exchange between Ti4+ and B3+ can be only observed if at least five ligands are bonded to Ti4+(see Table S 4). Since OH- prefers lower coordination number and a larger bond angle between each ligand, it seems reasonable that OH- requests more space than F-. Please note that reactants and products are very similar and the number of particles is not changed in the reactions shown at the end Table S 4. Therefore, the calculated free reaction enthalpy should possess an overall small error for these reactions. The subsequent formation of anatase found in the in-situ WAXD experiments is reasonable, as the [BF4]- concentration decreases with time by the proposed reactions. Hence, at a certain point the amount of [BF4]- is insufficient for an ongoing formation of TiO2(B). For higher concentrations, this critical value is not obtained within the synthesis times used. In order to understand fluorine substitutional of oxygen in the TiO2(B) bulk phase, first principles calculations with a DFT+U-D3 approach were performed. We assumed a TiO2xFx(B) composition with x = 0.125, considering one fluorine substitional of oxygen in a unit cell to determine the preferred position of fluorine. The higher fluorine content in the XPS data can either originate from a small amount of [BF4]-, interstitial or surface fluorine ions. The formation of surface fluorine ions that substitutes hydroxyl groups was for example reported for the fluorinated anatase phase.85 The TiO2(B) bulk structure contains four different oxygen sites,22 namely a nearly linear two-fold coordination (OBr), a three-fold coordination parallel to the ac-plane (O3f1) or parallel to the ab-plane (O3f2) and a tetrahedral four-fold coordination (O4f) (see Figure 1). The Ti-Ox bond length as well as the charges for oxygen is as



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follows O4f < O3f1/2 < Obr in agreement with previous theoretical studies.86,87 Substituting one of these oxygen’s by fluorine leads to a small increase in the lattice constants and volume of the super cell (see Table 2). Relative energies of the four fluorinated bulk phases (x = 0.125) indicate the preference of fluorine occupying a two-fold coordinating oxygen (0.0 eV) position instead of three-fold (0.3 eV and 0.4 eV, respectively) or fourfold (0.7 eV) coordinating one. The substitution of oxygen by fluorine goes ahead with an elongation of the Ti-X bond from 1.786 Å to 1.957 Å. A two-fold coordination was also found for the hexagonal TiOF2 structure.76,77

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anion [BF4]-. A preliminary, plausible mechanism based on the various experiments and basic theoretical evaluation is sketched in Scheme 3. As verified experimentally, [BF4]releases fluoride upon hydrolysis which then coordinates TiCl4 in an apical position of Ti4+. Based on basic theoretical evaluation we propose that this coordination inhibits an isotropic condensation of Ti oxyhydroxy clusters and thus enforces the condensation of the Ti clusters into the layered structure of TiO2(B) where the former ligand fluorine occupies regular oxygen lattice positions resulting in Ti vacancies for charge neutrality, as Ti3+ species are not found in the same order of magnitude as the fluorine content.

Table 2: Lattice constants and volume of the TiO2(B) and different TiO2-xFx(B) (x=0.125) bulk structures calculated at PBE+U-D3/pw(PAW P) level of theory. For the TiO2-xFx(B) structures the positions which were substituted by fluorine are given corresponding to Figure 1b). The experimental determined lattice constants were taken from Feist et al.22. Exp.

TiO2(B)

br

3f1

3f2

4f

TiO2(B)22 a/ Å

12.18

12.30

12.32

12.28

12.41

12.36

b/ Å

3.74

3.78

3.80

3.82

3.81

3.79

c/ Å

6.52

6.59

6.61

6.60

6.58

6.64

β°

107.1

107.4

106.8

106.9

106.8

106.9

349

351

352

353

353

V/Å3

The fluorine substitutional of oxygen introduces one excess electron which can reduce Ti4+ to Ti3+. However, our DFT+UD3 calculations indicate that the excess electron is delocalized in the supercell. Experimentally, the presence of reduced Ti3+ could be ruled out. Although a recent study of a fluorinated anatase phase identified the presence of a reduced Ti3+ species by electron paramagnetic resonance and DFT calculations applying hybrid functionals.85 Our further studies of the fluorinated bulk phase will consider additional defects and concentrations of fluorine and focus on the changes in geometric and electronic structure as well as charge distribution.

Conclusion In this study the formation of a special polymorph of TiO2, namely the Bronze-type TiO2(B), in certain mixtures of ILs was studied by various experimental techniques. While this synthesis had already been published in previous studies, the formation mechanism had remained unclear. In-depth X-Ray diffraction methods, supported by in-situ Raman spectroscopy, revealed that the formation of this exotic phase strongly depends on the concentration of the usually weakly coordinating

Scheme 3: Scheme of the reaction mechanism of TiCl4 towards TiO2(B) of the IL-mediated reaction.

The proposed fluorination on an apical position suggests that the fluorine will occupy the Obr-position (see Figure 1) which solely are two-fold coordinated. Calculations confirm that [BF4]- ions are crucial for the fluoride ions supply and suggest a “shuttle” effect which occurs between the Ti4+ and the B3+ ions. We propose that the coordination of F- to TiCl4 results in a less favorable coordination position for hydrolysis, which explains the remarkable stability of the solution of TiCl4 in the IL mixture against hydrolysis after the addition of water, although the ratio of TiCl4:H2O is quite high (approx. 1:2 v/v).


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The interaction of the titanium precursor and water might be disturbed by the ILs used, possibly leading to a deactivation of either the water molecules or otherwise highly active titanium ion. The concentration dependence of the SAXS patterns indicating a micelle formation might be a clue for this unusual behavior. However, this issue has to be addressed on a molecular level involving theoretic calculations to clarify the deactivation of the reaction educts by the IL cations. Yet, the reaction mechanism on the level of the involved molecular chemical species needs to be corroborated by more elaborate theoretical approaches, which will be the focus of a separate study. One of the key questions underlying the present study was the preference for the formation of TiO2(B) rather than anatase, which is said to be thermodynamically favorable. Using in-situ and ex-situ diffraction methods we showed that the competing formation of anatase is suppressed above a certain concentration of [BF4]- anions. Also, a transition of TiO2(B) to anatase can be inducted by treatment at approx. 520 °C and is accompanied by the release of fluorine. These findings are in fair agreement with XPS data which prove that fluorine is in a different chemical environment than in the pristine IL. The metastable TiO2(B) structure is thus stabilized by fluorine incorporation, resulting in phase transition into anatase upon fluorine extraction. In this respect, the synthesis does not generate phase-pure TiO2(B), but a material containing a few percent of fluorine stabilizing the bronze-type crystal structure. In-situ analysis revealed that in case of lower [BF4]concentrations, the anatase modification is obtained as a byphase or even as main phase. Our study thus stimulates further thought-provoking questions regarding the preference for generating TiO2(B) by the quite simple reaction components and procedure: is the nanoscopic, F-containing TiO2(B) material indeed thermodynamically favored, in comparison to nanoscopic anatase and also pure TiO2(B)? Such considerations will be addressed by suitable theoretical approaches in a separated study. As a surprising outcome of our study, an increase of the fluorine source leads to the formation of a fluorine-rich titanium hydroxyl oxyfluoride with hexagonal tungsten bronze structure, in which only two-fold coordinated anion lattice sites are present. Therefore, in this crystal structure, the amount of twofold coordinated sites is further increased compared to the TiO2(B) as a consequence of increasing fluorine in the reaction medium. Obviously, the formation of this fluorine-rich structure is equally interesting as the nucleation of TiO2(B) and will be subjected to further investigations as well, again with respect to the stability of this structure in competition with other TiO2 polymorphs. The concentration of the IL cations used ([C4mim]+ and [C16mim]+) exerts a minor impact on the formation of TiO2(B) if a mixture of these ILs is present. Upon a micelle-like structuring with a correlation length in region of approx. 5 nm the formation of TiO2(B) is promoted. These aspects on the interplay within a ternary mixture of two ILs and water have to be addressed in the future to expand the knowledge already gained in this study further and transfer it to other material syntheses. The application of the presented materials in photocatalytic investigations and battery research

will be part of future research projects as TiO2(B) and biphasic systems have already been reported for photocatalytic investigation. 27,88–90

ASSOCIATED CONTENT Supporting Information. Summary of the synthesis protocols; SAXS data for different IL mixtures; XPS and solid state 19FNMR of TiO2(B); calculated reaction energy ∆E and free reaction enthalpy ∆G. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Bernd M. Smarsly

[email protected]

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

Funding Sources SPP1708 DFG Grant No. SM 199/11-1 DFG Grant No. ZA 606/4-1 Acknowledgement PV, MV, SP and BMS acknowledge financial support within the SPP1708 of the German Research Foundation (DFG). SZ acknowledges financial support of the DFG by grant ZA 606/4-1. This work was strongly supported by the Laboratory of Material Research (LaMa) of the Justus-Liebig University Giessen (Germany). We kindly acknowledge the Elettra Synchrotron Radiation Facility (Trieste, Italy) for provision and support on the work at SAXS-Beamline 5.2. Also, we would like to acknowledge Hubert Wörner and Vanessa Zimmermann for TGA-MS measurements.

ABBREVIATIONS C4mim: 1-Butyl-3-Methylimidazolium; C16mim: 1Hexadecyl-3-Methylimidazolium; WAXD: wide-angle x-ray diffraction; SAXS: short angle X-ray diffraction; TGA-MS: thermogravimetric analysis with mass spectrometry;

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The formation of bronze-type TiO2 in mixtures of ionic liquids is elucidated by a combined experimental and theory approach. 224x167mm (150 x 150 DPI)


Peering into the mechanism of low-temperature synthesis of Bronze

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