Fluoride additive in epoxide-initiated sol–gel synthesis enables thin-film applications of SnO 2 aerogels† Juan-Pablo Correa-Baena,‡ * ab David A. Kriz,‡ c Marcus Giotto, d Steven L. Suib cd and Alexander G. Agrios * ab Aerogels of SnO 2 were synthesized by an epoxide-initiated sol–gel method. Using ammonium fluoride in the precursor solution allowed for tunability of the aerogel morphology while no change in the conductivity was measured. In particular, aerogel shrinkage was decreased dramatically by the addition of the fluoride precursor. Unfluorinated aerogels showed severe shrinkage of 43% volume change upon supercritical drying compared to the original alcogel volume. Fluorinated samples exhibited a much less pronounced shrinkage at 7%. Multiple characterization methods converged to reveal the mechanism by which fluoride enables the morphological tunability. These findings enable the casting of SnO 2 aerogels as thin films (which in the absence of fluoride these crack and delaminate due to shrinkage), opening potential uses in many optoelectronic devices including solar cells. Introduction Sol–gel chemistry has become widely used for the synthesis of metal oxide nanoparticles that are cross-linked into a gel, and used ultimately either as a dry gel or a powder. Epoxides have been proven to be effective at initiating the formation of the gel by acting as mild proton scavengers, maintaining an elevated pH to promote hydrolysis of metal precursors. While the hydrated metal is deprotonated, it is linked with other hydrated metals via olation and oxolation to form metal oxide particles. 1,2 This facile sol–gel process has been widely used in metal oxide nanoparticle synthesis 2,3 due to its ease of preparation and relatively low cost of the metal salt precursors. Tin oxide (SnO 2 ) is an n-doped, wide bandgap (3.6 eV at room temperature) semiconductor widely used in solar cells, 4–6 water splitting, 7 optoelectronic devices, 8 gas sensors, 9 and transparent conducting oxides (TCOs). 10 Heat treatment of mesoporous tin oxides is required for improving conductivity or crystallinity. 11–15 Heating SnO 2 materials made by the epoxide-initiated sol–gel method induces undesirable morphological features, in particular, shrinkage resulting in cracking and, in the case of lms, delamination. 16,17 The morphology of nanostructures is known to affect critical material properties and has become a key component in the synthesis of nanoparticulate mate- rials. 18,19 In order to take full advantage of such materials one must be able to manipulate the pore structure, surface area, particle size and crystallinity. Here, we report on the use of uorine to control SnO 2 aerogel morphology. We prepared SnO 2 alcogels, by the epoxide- assisted sol–gel process modied by the inclusion of ammo- nium uoride (NH 4 F), and dried them using supercritical CO 2 to form aerogels. The uoride profoundly affected aerogel properties such as shrinkage, density, porosity and surface area. These changes were extensively characterized, and the mecha- nism that triggers this was unraveled. Critically, uoride allows the aerogels to be cast as thin lms on glass substrates by greatly reducing the shrinkage of the gel during supercritical drying, which in unorinated samples results in cracking and delamination. Thin-lm aerogels open a wide array of applica- tions. For example, we have shown that thin-lm SnO 2 aerogels, when also doped with Sb(V) for electrical conductivity, function as mesoporous TCOs, which we have used in dye-sensitized solar cells. 16 Results and discussion The effect of the precursor F : Sn ratio on gel time and gel volume change is shown in Table 1. The increase in F : Sn consistently decreased the gel time from 300 s for unuorinated samples to 105 s for samples containing a 1 : 1 F : Sn ratio. Similar trends have been observed in silica sols, which gel faster a Civil and Environmental Engineering, University of Connecticut, Storrs, Connecticut 06269, USA. E-mail: agrios@engr.uconn.edu; juan.correa@eplf.ch; Tel: +1 860 486 1350; +41 788 135459 b Center for Clean Energy Engineering, University of Connecticut, Storrs, Connecticut 06269, USA c Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269, USA d Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01015f ‡ JPCB and DAK contributed equally to this work. Cite this: RSC Adv. , 2016, 6, 21326 Received 12th January 2016 Accepted 17th February 2016 DOI: 10.1039/c6ra01015f www.rsc.org/advances 21326 | RSC Adv. , 2016, 6, 21326–21331 This journal is © The Royal Society of Chemistry 2016 RSC Advances PAPER Published on 17 February 2016. Downloaded by ECOLE POLYTECHNIC FED DE LAUSANNE on 19/07/2016 09:40:42. View Article Online View Journal | View Issue