CERAMICSINTERNATIONAL

Available online at www.sciencedirect.com

0272-8842/$ - sehttp://dx.doi.org/

nCorrespondinE-mail addre

Ceramics International 40 (2014) 7729–7735www.elsevier.com/locate/ceramint

Novel homogeneous gel fibers and capillaries from blend of titaniumtetrabutoxide and siloxane functionalized ionic liquid

Marta Tarkanovskajaa,b, Raul Välbea,n, Kaija Põhako-Eskob,e, Uno Mäeorgb, Valter Reedoa,Andres Hoopc, Kristjan Saala,e, Andres Krummed, Ilmar Kinka,e, Ivo Heinmaaf, Ants Lõhmusa

aInstitute of Physics, University of Tartu, Riia 142, 51014 Tartu, EstoniabInstitute of Chemistry, University of Tartu, Ravila 14A, 50411 Tartu, Estonia

cHaine Paelavabrik OÜ, Tehase 21, 50106 Tartu, EstoniadInstitute of Polymer Materials, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

eEstonian Nanotechnology Competence Center, Riia 142, 51014 Tartu, EstoniafNational Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia

Received 29 November 2013; accepted 24 December 2013Available online 8 January 2014

Abstract

The preparation of ionogels by sol–gel processing has attracted much attention, because the final ceramic materials combine properties of bothinorganic matrix (thermal and mechanical stability) and the ionic liquid (ionic conductivity). The aim of this study was to combine differentimidazolium based ionic liquids (1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4], 1-butyl-3-methyl imidazolium tetrafluoroborate[BMIM][BF4], 1-decyl-3-methylimidazolium tetrafluoroborate [DMIM][BF4] and 1-methyl-3-[30-(triethoxysilyl)propyl]imidazolium chlorideMTICl) with titanium(IV) butoxide to prepare homogenous hybrid fibers through aqueous sol–gel reaction. The study showed that ionic liquidmiscibility with metal alkoxide plays an important role in the preparation of homogenous fibers. Unlike simple imidazolium salts functionalizedionic liquid was dispersed homogenously in fibers, but the main advantage is derived from its chemical structure. New stable ionic liquid can beinvolved in sol–gel processes through ethoxy groups and as a result it associates with titanium alkoxide network by covalent bonding providingnon-leaking ceramic hybrid material. Indirect and direct characterization studies of the product were carried out by energy-dispersive X-rayspectroscopy (EDX), silicon-29 nuclear magnetic resonance spectroscopy (29Si NMR), scanning electron microscopy (SEM) and opticalmicroscopies; also infrared spectra (IR) were recorded. Thermal analyses were performed by differential scanning calorimetry (DSC).& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Sol–gel; Ionogel; Ceramic fibers; Blending

1. Introduction

Ionic liquids are a diverse group of salts, which are liquidbelow 100 1C. Since the synthesis of first air- and moisture-stable imidazolium salts by Wilkes group in 1991 [1], ionicliquids have been in the focus of many investigations becauseof their unique physico-chemical properties like negligiblevapor pressure [2], high ionic conductivity [3], wide electro-chemical window [4] and low flammability [5]. Due to abilityto solvate different compounds ionic liquids have attracted the

e front matter & 2014 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2013.12.114

g author.ss: raul.valbe@ut.ee (R. Välbe).

attention in organic synthesis as an alternative reaction media[6] and in preparing different composites. The immobilizationof ionic liquids within organic or inorganic matrices makes itpossible to take advantage of their unique properties in thesolid state, thus circumventing some drawbacks related toshaping and risk of leakage.In this context, the sol–gel approach is currently of a growing

interest due to its simplicity and flexibility. In sol–gel processesionic liquids play different roles. It is shown that they are able toact as drying control chemical additives, catalysts, structuredirecting agents and even as solvents (or co-solvents) [7]. Todate most of the research in this field has been focused onapproaches where ionic liquid is removed after the sol–gel

ghts reserved.

M. Tarkanovskaja et al. / Ceramics International 40 (2014) 7729–77357730

process. However, in rapidly growing alternative approach ionicliquids are used as an additive in solid composite materials, as itopens many possibilities for functionalization of oxide materials.

Retaining ionic liquids in sol–gel materials results in new typeof hybrid materials, also called ionogels in scientific literature.The material possesses good mechanical and transparency proper-ties of solid network and high conductivity of electrolyte. Due tothese properties ionogels have potential applications in numerousfields in materials science and engineering like optics, sensors,separation techniques and in electrochemical devices such asactuators, lithium batteries, electric double-layer capacitors, dye-sensitized solar cells and fuel cells [8–10].

In current study we prepared ionogels based on imidazoliumsalts and titanium(IV) alkoxide. Our aim was to achieve homo-genous ionogel fibers. Titanium(IV) tetrabutoxide (Ti(OBu)4) waschosen due to its physico-chemical properties and potential appli-cation, which were mentioned in literature previously [11].

In recent years TiO2 has been the object of study for fabricatingfunctional fibers [11]. Excellent charge transfer and separation,unique photoactivity and low toxicity [12–14] of TiO2 make TiO-gels attractive precursor for textile coatings [11]. In our caseprepared ionogel fibers, based on imidazolium salts and titaniumalkoxide, combine properties of both inorganic matrix TiO2

(mechanical stability, non-leaking) and the ionic liquid (ionicconductivity, non-flammability). Such combination of propertiescan allow of obtained hybrids as antistatic textile fibers [11]. Usingalso some different R functional alkylimidazolium salts, instead ofN-methylimidazolium, it is possible to functionalize these materi-als. As it was described before, ionic liquids can act as coordinatingagents and help to prevent cracking, solving one of the commonproblems for sol–gel materials [15].

In the present study four imidazolium based ionic liquidswere investigated and tested for preparation of ionogel fibers.Three of them were simple alkylimidazolium salts withdifferent carbon length alkyl substituents ([EMIM][BF4],[BMIM][BF4] and [DMIM][BF4]) and one was trialkoxysilylfunctionalized 1-methyl-3-[30-(triethoxysilyl)propyl]imidazo-lium chloride (MTICl). In our study the MTICl showed verygood miscibility with titanium(IV) tetrabutoxide which lead tohomogenous sol. By comparison with the other ionic liquids itbecomes clear that good miscibility is crucial for the prepara-tion of homogenous fibers. Also, as it was described in ourprevious work, simple alkylimidazoliums with BF4� anionshowed instability in aqueous environment [16].

2. Experimental

2.1. Materials

1-Methylimidazole (Z99%), 3-(triethoxysilyl)propyl chlor-ide (95%), titanium(IV) tetrabutoxide (97%), 1-ethyl-3-methy-

N N + Cl Si(OEt)3

to

refl

Scheme 1. Synthe

limidazolium tetrafluoroborate ([EMIM][BF4], Z98%) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4],Z98%) were purchased from Sigma Aldrich and used asreceived. 1-decyl-3-methylimidazolium tetrafluoroborate ([DMIM][BF4]) was purchased from Merck KGaA and used withoutadditional purification. Toluene and diethyl ether were obtainedfrom Lachner. n-Butanol (Z99%, Sigma Aldrich) was dried anddistilled over CaH2.

2.2. Characterization

1H-NMR and 13C-NMR spectra were recorded at ambienttemperature on an Avance II 200 spectrometer (Bruker), usingDMSO-d6 as a solvent. The 1H-NMR and 13C-NMR spectrawere measured at 200 MHz and 50 MHz respectively. Thechemical shifts were internally referenced by the residualsolvent signals relative to tetramethylsilane. Infrared (IR)spectra were recorded on a Spectrum BXII FTIR spectro-photometer (Perkin Elmer). The scanning electron microscopy(SEM) was performed on a Vega microscope at 10 kV(Tescan). Carbon tape was used for sample preparation.Energy-dispersive X-ray spectroscopy (EDX) analyses wereperformed on a Helios NanoLab 600 SEM (FEI). Opticalcharacterizations were carried on a BX 51 optical microscope(Olympus). Differential scanning calorimetry (DSC) wascarried out on a DSC analyser DSC 7 (Perkin-Elmer) withheating and cooling rates 10 1C/min. Nitrogen was used as thefurnace purge gas. Temperature and heat flow calibrationswere done with indium and tin standards. 29Si NMR spectrumof solid sample was recorded on the Bruker AVANCE-IIspectrometer at 8.5 T magnetic field, at resonance frequency of71.45 MHz using a home built magic angle spinning (MAS)probe for 10 mm outside diameter zirconia rotors.

2.3. Synthesis of MTICl

1-Methyl-3-[30-(triethoxysilyl)propyl]imidazolium chloridewas synthesized according to the method reported previouslyin the literature (Scheme 1) [17].MTICl was prepared using 1:1 mol ratio of 1-methylimidazole

and 3-chloropropyl triethoxysilane through one step synthesis(Scheme 1). In the experiment, 3-chloropropyl triethoxysilane(36.23 g; 0.15 mol) was added dropwise to 1-methylimidazole(12.29 g; 0.15 mol) dissolved in toluene (150 mL). Then themixture was refluxed at 111 1C for three days in Ar atmosphere.As a result, two layers were formed: brown MTICl at the bottomand toluene at the top. Layers were separated and MTICl layerwas purified from any unreacted starting materials by washingthree times with diethyl ether (50 mL). After drying overnight atroom temperature under vacuum (1 mm Hg) 17.23 g (36%) ofMTICl as yellowish viscous liquid was obtained.

N N+

Si(OEt)3

Cl-

luene

ux, 72h

sis of MTICl.

M. Tarkanovskaja et al. / Ceramics International 40 (2014) 7729–7735 7731

In Fig. 1(a) 1H NMR (200 MHz, DMSO-d6): δ 9.31 (s, 1H,–N–CH–N–), 7.83 (s, 1H, –N–CH¼CH–N–), 7.77 (s, 1H,–N–CH¼CH–N–), 4.17 (t, 2H, J¼7.0 Hz, –N–CH2–), 3.88(s, 3H, –N–CH3), 3.77 (q, 6H, J¼7.0 Hz, –O–CH2–), 1.84(quint., 2H, J¼7.6 Hz, –CH2–CH2–CH2–), 1.16 (t, 9H, J¼7.0Hz, –CH2–CH3), 0.53 (t, 2H, J¼7.6 Hz, Si–CH2–).

In Fig. 1(b) 13C NMR (50 MHz, DMSO-d6): δ 137.21(–N–CH–N–), 124.13 (–N–CH¼CH–N–), 122.69 (–N–CH¼CH–N–), 58.33 (–O–CH2–), 51.49 (–N–CH2–), 36.22(–N–CH3), 24.13 (–CH2–CH2–CH2–), 18.66 (–CH2–CH3),7.20 (Si–CH2–).

2.4. Preparation of [EMIM][BF4]/Ti(OBu)4 and [BMIM][BF4]/Ti(OBu)4 ionogels

Using ionic liquids [EMIM][BF4] and [BMIM][BF4] twoexperimental series were performed with different ionic liquidconcentrations and water/alkoxide ratios (R). In the firstexperiment, 0.03 g of ionic liquids ([EMIM][BF4] in one caseand [BMIM][BF4] in other case) were dissolved in water,water/Ti(OBu)4 ratio R was 1.84. Then water/ionic liquidsolutions were dissolved in butanol (4.10 g) and added portionwise (1 ml after every 5 min) to 1.80 g of Ti(OBu)4 in 50 mLborosilicate glass reaction bulb closed with septum. Themixture was stirred overnight at room temperature.

In the second experiment, the same compounds and methodol-ogy were used only with different concentrations: 1 g Ti(OBu)4,0.10 g ionic liquid, and 3.70 g n-butanol. Water/alkoxide ratio Rwas 5.36.

Fig. 1. (a) 1H NMR spectrum of MTICl and (b) 13C NMR spectra of MTICl.

2.5. Preparation of MTICl/Ti(OBu)4 and [DMIM][BF4]/Ti(OBu)4 ionogels

Experiments with MTICl of different concentrations usingvarious water/Ti(OBu)4 ratios were carried out. Ionic liquids(0.05–0.30 g MTICl in one case and 0.20 g [DMIM][BF4] inother case) were added directly to the Ti(OBu)4 (1.80 g) bystirring. All MTICl/alkoxide and [DMIM][BF4]/alkoxide mix-tures were measured into 50 mL borosilicate glass reactionbulbs and closed with airtight septums. Then water/n-butanolsolutions were added to the ionic liquid/alkoxide mixtureportion wise 1 ml after every 5 min. Water was dissolved inn-butanol (4.30 g in 100 g of water), because ionic liquids usedin the experiment are immiscible with water. All solutionswere stirred for 24 h at room temperature.

2.6. Fiber preparation

Fibers were prepared using the common sol–gel procedurefor preparation different metal oxide or alkoxide fibers utiliz-ing our extensive previous experience on the preparation ofsuch materials [18].After removal of solvents and low molecular mass organics

in vacuum at 2 mm Hg using rotary evaporator and 70 1Cwater bath, all ionic liquid/alkoxide mixtures turned intohighly viscous syrup-like substances. Before detaching thebulbs from the evaporator they were filled with argon untilambient pressure was achieved. Fibers were formed by pullingoligomeric mass in humid air using borosilicate glass stick at

No changes were detected in corresponding NMR spectra, after IL aging.

M. Tarkanovskaja et al. / Ceramics International 40 (2014) 7729–77357732

ambient atmospheric pressure and temperature. Exposing thepulled fibers to air resulted in immediate formation of thinsolid layer on the materials surface due to polymerizationcaused by water vapor in air. Dimensions of the fibers dependgreatly on viscosity of the precursor, humidity of the surround-ing atmosphere and pulling speed. Pulled fibers were left tostand for two days at the ambient temperature (21 1C) andrelative humidity (33%).

3. Results and discussion

The focus of current study was on the preparation of ionicliquid and metal alkoxide blends. The key step in this case isthe formation of ionic liquid containing titanate sol. Miscibilityof components is required for homogenous fibers. We com-pared four different imidazolium based ionic liquids. [EMIM][BF4], [BMIM][BF4] and [DMIM][BF4] did not show anymiscibility with titanium tetrabutoxide. From [EMIM][BF4]and [BMIM][BF4] the formation of homogenous precursorsucceeded by using water as an universal co-solvent, butduring the fiber drawing the separation of phases still occurred.[DMIM][BF4] itself is poorly miscible with water, so thepreparation of sols by described method using ionic liquidswith longer alkyl substituents is not suitable.

Synthesis of MTICl gave good yield and the structure ofproduct was confirmed by NMR (Fig. 1a and b) and FTIR(Fig. 2b). Although the synthesized ionic liquid was yellowish,analysis did not indicate any impurities and the yield ofsynthesis was comparable with the results of other authors[17–20]. Take on color is likely caused by heating the reactionmixture for long time at relatively high temperature. It is wellknown that in the quaternization reaction heating above 40–50 1C is not recommended [6], but here the reaction does notoccur at lower temperatures.

Ionic liquid bearing siloxane moiety mixes very readily withtitanium tetrabutoxide even without water. By mixing MTICland titanium tetrabutoxide an interesting phenomenon was

Fig. 2. FTIR spectrum of (a) MTICl–Ti(OBu)4 fibers and (b) pure MTICl inthe 400–4000 cm�1 spectral range. Only major peaks wavenumbers arenumbered.

observed. Directly after synthesis and a few weeks later thesolvating properties of functionalized ionic liquid were quitedifferent. It could be explained by self organization of ionicliquid. It is well known that ionic liquids tend to formsupramolecular structures, which are harder to solvate thansingle molecules. Observed tendency cannot be explained bychemical changes in ionic liquid (e.g. hydrolysis of ethoxygroup due to moisture), because no changes were detected incorresponding NMR and IR spectra.In the FTIR spectrum (Fig. 2b) the absorption peaks at 2973–

2888 cm�1 are characteristic to different C–H bond strechingvibrations. The peak at 1570 cm�1 can be attributed to C¼Cstretching of the imidazole ring. The peak at 1389 cm�1 isassigned to be stretching vibrations of C–N of imidazole ring[21]. The peak at 1166 cm�1 corresponds to C–O bondstretching. The peaks at 1077 cm�1 and 755 cm�1 appear dueto Si–O and Si–C stretching vibration, respectively [22].In the current study we were able to prepare fibers with

content of MTICl up to 11.7 w/w%. At higher concentrationsof MTICl, materials do not gel. Unlike simple imidazoliumsalts functionalized ionic liquid was dispersed homogenouslyin fibers, but the main advantage is derived from its chemicalstructure. MTICl involves in sol–gel processes through ethoxygroups and as a result it associates with titanium alkoxidenetwork by covalent bonding. Experiments showed that theoptimal R value which is giving suitable viscous MTICl/alkoxide fiber precursor is 0.8. The novel hybrid fibers have

Fig. 3. SEM images of different mechanically cleaved gel fibers: (a) a gelfibers synthesized, using [EMIM][BF4] (9.4% w/w), pores are seen; (b) using[DMIM][BF4] (8% w/w), pores are seen; (c) using [EMIM][BF4] (1.7% w/w),channels are seen.

Fig. 5. Differential scanning calorimetry of gel fibers, using MTICl 1.5% w/w.Peak from the beginning of 282 1C corresponds to the decompositionof MTICl.

M. Tarkanovskaja et al. / Ceramics International 40 (2014) 7729–7735 7733

length of up to 10 cm and diameter of 1 μm up to 100 μm.When drawing speed decreases, diameter of fibers increasesand by drying tubes are formed if diameter is over 100 μm(Fig. 4d) [23].

3.1. Characterization of fibers

The morphology of fibers was characterized by SEM.Depending on the drawing speed [EMIM][BF4], [BMIM][BF4] and [DMIM][BF4] formed channels and pores in thefibers (Fig. 3), as a result the obtained materials are inhomo-genous. Fibers containing MTICl were without any defects andhomogenous (Fig. 4). Presence of ionic liquid in fibers wasdetected and analysed by differential scanning calorimetry,elemental analysis and IR. The formation of Si–O–Ti bondswas confirmed by 29Si NMR and IR. To check the reliability ofthe IR experiment theoretical calculations were carried outusing GAUSSIAN 09 software [24] for determination of Si–O–Ti bond location.

The decomposition peaks in Fig. 5, from the beginning of282 1C presumably correspond to the decomposition of theMTICl. Decomposition of imidazolium salts proceeds throughE2 elimination on the N-substituent, which is the reversiblereaction of SN2 substitution carried out for preparation ofimidazolium cations. Thermal stability of ionic liquids is

Fig. 4. SEM images of different mechanically cleaved gel fibers: (a) a gel fiber syn11.7% w/w; (d) a gel tube using MTICl 2.6% w/w.

influenced by anion nucleophilicity. For imidazolium saltswith Cl� anion this first step of decomposition is reported tooccur between 250 1C and 300 1C. Two maximum in tem-perature range 350–440 1C can be attributed to degradation ofimidazole ring and pyrolysis of all organic matter [6].Elemental analysis detected ionic liquid both inside of fibers

and on its surface (Table 1). EDX were measured using severaldifferent zones of the fibers and using different energies(5–15 kV). The results were coincident and showed micro-homogeneity. From this, conclusion can be made, that ionic

thesized using MTICl 1.5% w/w; (b) using MTICl 9.5% w/w; (c) using MTICl

Scheme 2. Molecular structure used in GAUSSIAN 09 program for Si–O–Tibond IR frequency calculation (Et and Bu are ethyl and butyl groups,respectively).

M. Tarkanovskaja et al. / Ceramics International 40 (2014) 7729–77357734

liquid was homogenously dispersed in the titanium alkoxidesol and thus in prepared fibers.

FTIR spectra indicate the existence of MTICl in fibers(Fig. 1a). In the spectra peaks both characteristic for titanium(IV)butoxide and MTICl are detectable. The bands centered at�600 cm�1 and 745 cm�1 are likely due to the vibration ofthe Ti–O bonds [25]. The peak at 1570 cm�1 can be attributedto C¼C stretching of the imidazole ring and the peak at1077 cm�1 appears due to Si–O vibration as it was describedin the case of MTICl.

To predict Si–O–Ti bond infrared frequency the densityfunctional theory (DFT) calculation was performed by theGAUSSIAN 09 software with B3LYP functional and 6-31Gbasis set. For frequency estimation one of the simplest config-urations of molecular structure, which could hypothetically beformed in our MTICl/Ti(OBu)4/H2O system, was chosen. Thestructure used in calculations is shown in Scheme 2. For suchsystem program predicted intense vibration at 1039 cm�1.According to this result the peak at 1042 cm�1 in MTIClcontaining fibers0 IR spectrum (Fig. 2a) can be attributed to Si–O–Ti bond vibrations [26].

29Si NMR spectrum of 1700 accumulations after single 901pulse excitation with 200 s relaxation delay between the pulsesis given in Fig. 6. The powdered sample was spun at 4.5 kHz

Table 1EDX analysis of MTICl–Ti(OBu)4 hybrid fibers.

Element Weight (%) Atomic (%)

C 40.4 56.5O 31 28.5Si 1.2 0.6Cl 1.2 0.4Ti 24 12N 2.2 2.0

Fig. 6. 29Si NMR spectrum of 1700 accumulations after single 901 pulseexcitation with 200 s relaxation delay between the pulses. Four different Sisignals are presented.

speed and the chemical shift is given relative to TMS. Basedon the model mentioned in literature previously [27] thespectrum signals range at SiC2O2 and SiCO3. Signal at�42 ppm (A1) corresponds to SiCO3 next silicon (three bondsSi–O–Si). Signal at �52 ppm (A2) can be described as thesame, but only one Si–O–Si bond. Signal at �62 ppm (B1)could be the same Si, where two bonds correspond to Si–O–Ti,and the signal at �67 ppm (B2) could be Si, where all thebonds are Si–O–Ti.

4. Conclusions

In current study ionogels based on imidazolium salts andtitanium(IV) alkoxide were prepared. Our results showed thationic liquid miscibility with metal alkoxide plays an importantrole in the preparation of homogenous fibers. Four differentimidazolium based ionic liquids were investigated. [EMIM][BF4], [BMIM][BF4] and [DMIM][BF4] did not show anymiscibility with titanium tetrabutoxide, ionic liquid bearingsiloxane moiety mixes very readily with titanium(IV) alkoxide.In current study we presented a method to prepare fibers withcontent of MTICl up to 11.7 w/w%. Unlike simple imidazo-lium salts functionalized ionic liquid was dispersed homo-genously in fibers, but the main advantage is derived from itschemical structure. MTICl involves in sol–gel processesthrough the ethoxy groups and as a result it associates withthe titanium alkoxide network by covalent bonding. To thebest of authors0 knowledge there is no information about thesehomogeneous fibers being synthesized or described before.

Acknowledgment

This work was supported by Estonian Science Targeted Projectnos. SF0180058s07 and 0180032s12, the Estonian ScienceFoundation Grant nos. 8377, 8794, 9281, and 8420, the EstonianNanotechnology Competence Center, the European ScienceFoundation Fanas program “Nanoparma”, the Graduate Schoolon Functional Materials and Technologies, the EU Social FundsProject 1.2.0401.09-0079, the European Regional DevelopmentFund (Center of Excellence “Mesosystems”, TK114, ERDF“TRIBOFILM” 3.2.1101.12-0028, “IRGLASS” 3.2.1101.12-0027 and “NANO-COM” 3.2.1101.12-0010), European SocialFund0s Doctoral Studies and Internationalisation ProgrammeDoRa, which is carried out by Foundation Archimedes.

M. Tarkanovskaja et al. / Ceramics International 40 (2014) 7729–7735 7735

References

[1] J.S. Wilkes, M.J. Zaworotko, Air and water stable 1-ethyl-3-methylimidazolium based ionic liquids, Chem. Commun. 13 (1992)965–967.

[2] M.J. Earle, J.M.S.S. Esperanca, M.A. Gilea, J.N. Canongia Lopes, L.P.N. Rebelo, J.W. Magee, K.R. Seddon, J.A. Widegren, The distillation andvolatility of ionic liquids, Nature 439 (2006) 831–834.

[3] M. Galinski, A. Lewandowski, I. Stepniak, Ionic liquids as electrolytes,Electrochim. Acta 51 (2006) 5567–5580.

[4] S. Zhang, N. Sun, X. He, X. Lu, X. Zhang, Physical properties of ionicliquids: database and evaluation, J. Phys. Chem. Ref. Data 35 (2006) 4.

[5] D.M. Fox, J.W. Gilman, A.B. Morgan, J.R. Shields, P.H. Maupin,R.E. Lyon, H.C. De Long, P.C. Trulove, Flammability and thermalanalysis characterization of imidazolium-based ionic liquids, Ind. Eng.Chem. Res. 47 (2008) 6327–6332.

[6] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCHVerlag GmbH & Co., 2002.

[7] A. Vioux, L. Viau, S. Volland, J. Le Bideau, Use of ionic liquids in sol–gel; ionogels and applications, C. R. Chim. 13 (2010) 242–255.

[8] J. Le Bideau, L. Viau, A. Vioux, Ionogels, ionic liquid based hybridmaterials, Chem. Soc. Rev. 40 (2011) 907–925.

[9] M.A. Ne´ouze, J. Le Bideau, P. Gaveau, S. Bellayer, A. Vioux, Ionogels,new materials arising from the confinement of ionic liquids within silica-derived networks, Chem. Mater. 18 (2006) 3931–3936.

[10] A. Kavanagh, R. Byrne, D. Diamond, K.J. Fraser, Stimuli responsiveionogels for sensing applications – an overview, Membranes 2 (2012)16–39.

[11] B. Mahltig, T. Textor, Nanosols and Textiles, World Scientific PublishingCo Pte Ltd, London, 2008.

[12] Y.T. Lin, T.W. Zeng, W.Z. Lai, C.W. Chen, Y.Y. Lin, Y.S. Chang,W.F. Su, Efficient photoinduced charge transfer in TiO2 nanorod/conjugated polymer hybrid materials, Nanotechnology 17 (2006)5781–5785.

[13] P. Go´rska, A. Zaleska, E. Kowalska, T. Klimczuk, J.W. Sobczak,E. Skwarek, W. Janusz, J. Hupka, TiO2 photoactivity in vis and UV light:the influence of calcinations temperature and surface properties, Appl.Catal. B 84 (2008) 440–447.

[14] World Health Organization International Agency for Research on Cancer,IARC Monographs on the Evaluation of Carcinogenic Risks to Humans,vol. 93, WHO Press, 2010, pp. 236–251.

[15] M. Järvekülg, R. Välbe, J. Jõgi, A. Salundi, T. Kangur, V. Reedo,J. Kalda, U. Mäeorg, A. Lõhmus, A.E. Romanov, A sol–gel approach toself-formation of microtubular structures from metal alkoxide gel films,Phys. Status Solidi A 12 (2012) 1–6.

[16] R. Välbe, U. Mäeorg, A. Lõhmus, V. Reedo, M. Koel, A. Krumme,V. Kessler, A. Hoop, A.E. Romanov, A novel route of synthesis ofsodium hexafluorosilicate two component cluster crystals using BF4�

containing ionic liquids, J. Cryst. Growth 361 (2012) 51–56.

[17] S. Brenna, T. Posset, J. Furrer, J. Blümel, 14N NMR and two-dimensionalsuspension 1H and 13C HRMAS NMR spectroscopy of ionic liquidsimmobilized on silica, Chem. Eur. J. 12 (2006) 2880–2888.

[18] K. Saal, T. Tätte, M. Järvekülg, V. Reedo, A. Lõhmus, I. Kink, Micro-and nanoscale structures by sol–gel processing, Int. J. Mater. Prod.Technol. 40 (1/2) (2011) 2–14.

[19] Y.S. Chi, J.K. Lee, S. Lee, I.S. Choi, Control of wettability by anionexchange on Si/SiO2 surfaces, Langmuir 20 (2004) 3024–3027.

[20] G. Liu, M. Hou, J. Song, Z. Zhang, T. Wu, B. Han, Ni2þ containingionic liquid immobilized on silica: effective catalyst for styrene oxidationwith H2O2 at solvent-free condition, J. Mol. Catal. A Chem. 316 (2010)90–94.

[21] S.A. Katsyuba, E.E. Zvereva, A. Vidis, P.J. Dyson, The temperature-dependent prediction of the liquid entropy of ionic liquids, J. Phys. Chem.A 111 (2007) 352–370.

[22] L.B. Cappeletti, E Moncada, J. Poisson, I.S. Butler, J.H.Z. Dos Santos,Determination of the network structure of sensor materials prepared bythree different sol–gel routes using fourier transform infrared spectro-scopy (FT-IR), Appl. Spectrosc. 67 (4) (2013) 441–447.

[23] M. Part, K. Riikjärv, K. Hanschmidt, A. Tamm, H. Mändar, G. Nurk,K. Kukli, T. Tätte, Novel method in synthesis of YSZ microtubes andtheir application as ALD substrates, in: TNT International Conference,vol. 13, 2012.

[24] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb,J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson,H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov,J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,H. Nakai, T. Vreven, J.A. MontgomeryJr., J.E. Peralta, F. Ogliaro,M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R.Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S.Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E.Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E.Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W.Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P.Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, Ö. Farkas, J.B.Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Gaussian,Inc., Wallingford CT, 2009.

[25] Y. Gao, Y. Masuda, Z. Peng, T. Yonezawa, K. Koumoto, Roomtemperature deposition of a TiO2 thin film from aqueous peroxotitanatesolution, J. Mater. Chem. 13 (2003) 608–613.

[26] R. Venckates, K. Balachandaran, R. Sivaraj, Synthesis and characteriza-tion of nano TiO2–SiO2: PVA composite – a novel route, Int. Nano Lett.2 (15) (2012) 1–5.

[27] G.D. Sorarù, G. D0Andrea, R. Campostrini, F. Babonneau, G. Mariotto,Structural characterization and high temperature behaviour of siliconoxycarbide glasses prepared from sol–gel precursors containing Si–Hbonds, J. Am. Ceram. Soc. 78 (1995) 379–387.