Cite this: RSC Advances, 2013, 3, 13869 Studies on mesoporous silica ionogels prepared by sol– gel method at different gelation temperatures Received 12th April 2013, Accepted 7th June 2013 DOI: 10.1039/c3ra41774c www.rsc.org/advances Abhishek Kumar Gupta, Rajendra Kumar Singh* and Suresh Chandra Ionogels (IGs), a novel group of materials having an ionic liquid entrapped in a mesoporous silica matrix, have been synthesized by a non-hydrolytic sol–gel process at different gelation temperatures (30, 0, 210 uC) using 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF 4 ] ionic liquid (IL), tetraethylortho- silicate (TEOS) as a metallic precursor and formic acid as a catalyst. The rate of gelation was lower at lower temperatures. Gelation temperature has also been found to affect (i) the densities of IGs, (ii) pore parameters of IGs like average pore diameter, pore surface area, pore volume and porosity, (iii) phase transition temperatures T m , T c , and T g , (iv) thermal stability of IL and (v) vibrational spectra. The porosity of IG ingots, prepared at lower gelation temperatures (0, 210 uC) was higher. Unexpectedly, their densities were found to be higher than the IGs prepared at higher temperatures (say 30 uC). Possibly, one of the reaction products of the sol–gel process (namely, ethyl formate) could not escape the matrix due to its slow evaporation at low temperatures which resulted in their higher densities. The presence of ethyl formate in the IGs was confirmed by FTIR. The variation of the other properties of the IGs, listed above, synthesized at different gelation temperatures is also discussed and explained in the paper. Introduction Recently, ionic liquids have aroused much interest in the field of material application research due to some of its inherent properties like high ionic conductivity, thermal stability, high solvability, chemical stability, negligible vapour pressure, wide electrochemical window, non-flammability and wide liquidus range etc. 1–4 Some applications of ILs are in electrochemical devices such as Li-ion batteries, 5 supercapacitors, 6 sensors, 7 fuel cells, 8 actuators, 9 electro-chromic display devices 10 and dye sensitized solar cells. 11 In most devices, the IL is incorporated in either a polymer matrix to give modified polymer electrolytes 12 or porous matrices. 13–16 ILs entrapped in mesoporous materials are often termed as ionogels. 14,16,17 Some commonly used porous matrices are silica, TiO 2 , SnO 2 , controlled pore glass and carbon nanotubes etc. 18–22 Out of these, most studied IGs are based on a mesoporous SiO 2 - matrix prepared by a sol–gel route. 13–16 Incorporation of ILs in the sol–gel pot during synthesis modifies the pore-structure of the matrix because the IL acts as a catalyst and affects the rate of gelation and consequently, the nature of the matrix formed. 14,23 Earlier, we have synthesized IGs by adding different mol% of an ionic liquid (1-ethyl-3-methylimidazolium tetrafluorobo- rate) in the sol–gel reaction vessel containing tetraethyl orthosilicate as a metallic precursor and formic acid as a catalyst. 14 The gelation was allowed to occur at ambient temperature (y30 uC). The gelation occurred rapidly (y50 s) 14 and the density of the IGs so obtained was small (y0.3 g cm 23 ) as compared to that of the SiO 2 matrix without IL (y1.48 g cm 23 ). However, the IGs obtained under the above conditions were unstable and became flaky in less than two weeks because of strains/voids built in the matrix due to the fast gelation. The motivation of the present study was to increase the stability of the ingots by lowering the gelation temperature and hence, slowing down the gelation rate. Two temperatures (0 and 210 uC) were chosen for this study. The ionogels prepared at low gelation temperatures had higher stability and density than those synthesized at y30 uC. 14 Unexpectedly, the measured fractional porosity of the ingots prepared at lower temperatures was higher which should have resulted in a lower density but the density was found to be higher. These self-contradictory results are explained later in this paper on the basis of a retention of one of the sol–gel reaction products (ethyl formate) in the SiO 2 matrix. This paper also reports changes in the phase transition temperatures (T g , T c , T m ), thermal stability and vibrational frequencies of the IL confined in the mesoporous silica matrix. These changes are correlated with the average pore diameter of the different mesoporous matrices. Department of Physics, Banaras Hindu University, Varanasi, 221005, India. E-mail: rksingh_17@rediffmail.com; sureshchandra_bhu@yahoo.co.in; Fax: +91 542 2368390; Tel: +91 542 2307308 RSC Advances PAPER This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 13869–13877 | 13869 Published on 07 June 2013. Downloaded by Banaras Hindu University on 23/01/2014 09:40:37. View Article Online View Journal | View Issue