Nanoparticles of ATO (antimony doped tin oxide) were used to produce thick conductive, free standing mats of nanofibers via electrospinning. These fibrous mats were incorporated into polymer films to produce a transparent conducting polymer foil. Moreover, the fiber mats can serve as porous electrodes for electrodeposition of Prussian Blue and TiO2 and were tested in dye-sensitized solar cells

Ostermann, Rainer

Rudolph, Melanie

Schlettwein, Derck

Smarsly, Bernd M.

Zieba, Roman

Giessener Elektronische Bibliothek

This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 12119–12121 12119Cite this: Chem. Commun., 2011, 47, 12119–12121Electrospun antimony doped tin oxide (ATO) nanoﬁbers as a versatileconducting matrixwRainer Ostermann,a Roman Zieba,b Melanie Rudolph,c Derck Schlettweinc andBernd M. Smarsly*aReceived 22nd June 2011, Accepted 3rd October 2011DOI: 10.1039/c1cc13724gNanoparticles of ATO (antimony doped tin oxide) were used toproduce thick conductive, free standing mats of nanoﬁbers viaelectrospinning. These ﬁbrous mats were incorporated intopolymer ﬁlms to produce a transparent conducting polymer foil.Moreover, the ﬁber mats can serve as porous electrodes forelectrodeposition of Prussian Blue and TiO2 and were tested indye-sensitized solar cells.TCO (transparent conducting oxide) materials are widely usedin opto-electronic applications as transparent electrodes,commonly as thin, compact ﬁlms deposited on glass or quartzsubstrates.1,2 These TCO layers of 200–300 nm thickness possesssheet resistances typically below 20 O/& and transparenciesabove 85% in the visible range, making them also suitable astwo-dimensional electrodes for electrochromic devices3 and solarcells.4 TCO ﬁlms with 3D porosity with pore sizes beyond100 nm are supposed to be useful for diverse applications. Asproposed by Zaban et al., a 3D TCO electrode theoreticallyoﬀers advantages for electron collection in DSSCs (dye-sensitizedsolar cells),5 due to a higher surface area and a shorter electrondiﬀusion pathway to the back electrode. The practical realizationof such DSSCs with TiO2 on ITO (indium tin oxide) nano-powders6 and nanowires generated by laser-ablation7 showedonly low conversion eﬃciency, but recently a more elaborateapproach based on a poly(carbonate) template yielded around4% eﬃciency.8 In addition, porous TCO electrodes mightprove suitable in other applications with non-conductingpolymers or oxides with low intrinsic conductivities, as forexample oxide cathodes for Li-batteries.9Here we demonstrate a novel procedure allowing the productionof ATO (antimony doped tin oxide) nanoﬁber mats and their usageto endow intrinsically insulating polymer ﬁlms with conductivity,and as highly porous electrodes for electrodeposition and devicefabrication. To the best of our knowledge, this is the ﬁrst workingexample of a nanoﬁber-based ATO electrode. For ordinaryTCO electrodes, typical deposition techniques are CVD andPVD (chemical and physical vapor deposition), includingspray pyrolysis, PLD (pulsed laser deposition) and magnetronsputtering, resulting in compact layers. Various attemptshave been made to employ sol–gel processing, yielding goodtransparency, but generally much lower conductivity. Mainbeneﬁts of sol–gel approaches are the possibility of solution-basedcoating technology and the facile generation of porosity throughsuitable polymer templates, as demonstrated for mesoporous andmacroporous ITO.10,11 An alternative concept to endow TCOswith porosity is the generation of nanoﬁbers by electrospinning.The combination of a suitable polymer as a carrier withconventional sol–gel processing thus can be applied forthe electrospinning of metal oxide ﬁbers, including ITO.8,15However, for other n-type TCO materials like ATO, FTO(ﬂuorine doped tin oxide) or AZO (aluminum doped zinc oxide)only low conductivities have been reported, even for nonporousﬁlms, while these TCOs are desirable substitutes of the costlyITO.12 In contrast to ITO, for these ternary oxides a sol–gelroute leads to a non-ideal stoichiometry and a phase-separationinto the two corresponding oxides, thus resulting in a lowerconductivity. These shortcomings can be overcome to someextent by using preformed nanoparticles.13,14In this work we extend the electrospinning of TCO nanoﬁbersby using a dispersion of colloidal ATO nanoparticles instead ofclassical sol–gel precursors to obtain more stable electrospinningsolutions. Further advantages of this approach are the control ofstoichiometry and crystallinity of the preformed nanoparticles,which are especially beneﬁcial for ternary oxides like TCOs.A main challenge was to develop ATO nanoparticle dispersionsenabling electrospinning of high-quality nanoﬁbers. A precipitationmethod from the literature13 was modiﬁed to create highlycrystalline ATO nanoparticles (Fig. S1 and S2, ESIw) ofca. 3–6 nm in size (Fig. S2 and S3, ESIw) that did not containionic stabilizers and could thus be redispersed in solventssuitable for electrospinning. In a typical electrospinningexperiment, a dispersion of ATO nanoparticles (20 wt%)was spun with PVP (poly(vinyl pyrrolidone) as a carrierpolymer (see ESIw for details) to produce a non-woven mat.As an important step to increase the packing density ofthe ﬁbers, this mat was soaked with hexane and dried atroom temperature, before calcination at 550 1C for 10–20 min.a Justus Liebig University, Institute of Physical Chemistry,Heinrich-Buﬀ-Ring 58, D-35392 Giessen, Germany.E-mail: bernd.smarsly@phys.chemie.uni-giessen.de;Fax: +49 6419934509; Tel: +49 6419934590b BASF SE, GVP/E, D-67056 Ludwigshafen, Germanyc Institut fu¨r Angewandte Physik, Heinrich-Buﬀ-Ring 16,D-35392 Giessen, Germanyw Electronic supplementary information (ESI) available. See DOI:10.1039/c1cc13724gChemComm Dynamic Article Linkswww.rsc.org/chemcomm COMMUNICATIONDownloaded by UNIVERSITAT GIESSEN on 30/04/2013 07:05:49. Published on 14 October 2011 on http://pubs.rsc.org | doi:10.1039/C1CC13724GView Article Online / Journal Homepage / Table of Contents for this issue12120 Chem. Commun., 2011, 47, 12119–12121 This journal is c The Royal Society of Chemistry 2011The free-standing ﬁber mat could then be ﬁxed onto a glass orcommercial FTO (Haller Glas, Germany) substrate by wettingthe substrate with a drop of 5 wt% Sn(IV)Cl4 solution inethanol. This procedure might seem cumbersome, but takinginto account the 15–20% shrinkage of the ﬁbers duringcalcination, it is the only way to avoid extensive rupture anddelamination of the nanoﬁbers, as it occurs for nonfree-standing mats on substrates. We believe this problem to bethe main reason that TCO nanoﬁber mats have not been usedso far for electrodeposition or device fabrication (other thansingle ﬁber sensing), although there are several reports on ITOnanoﬁbers,16 showing good conductivity for an individualnanoﬁber. Actually, the sol–gel approach in these worksleads to even more pronounced shrinkage, but our ﬁxationprocedure is equally applicable for such sol–gel derived nano-ﬁber mats, while we only found the sol–gel derived ITO ﬁbersto be highly conductive. For other TCO materials includingATO, the nanoparticle approach seems more viable.To evaluate the macroscopic conductivity, the resistance ofﬁbers mats on glass was measured over a distance of 1 cmbetween contacts made from conducting silver paste. For a matelectrospun for 120 s typically the resistance is approx.400 O cm1 (see Fig. S4, ESIw), being only one order ofmagnitude higher than for commercial TCO-coated glass.The nanoﬁber mats could be generated with variable thickness(controlled by the spinning time), all possessing reasonableconductivities. This was demonstrated by depositing PrussianBlue (Iron(II,III) hexacyanoferrate).17As shown in Fig. 1, deeply colored ﬁlms were obtained, andPrussian Blue was homogeneously deposited onto the ﬁbers.In an aqueous solution of 0.1 M KCl as an electrolyte, thecyclovoltammogram shows oxidation and reduction peaks atthe same potentials as for a Prussian Blue ﬁlm deposited oncommercial FTO glass (see Fig. S5, ESIw). Similarly, theelectrochromic behavior of Prussian Blue, i.e. the reversiblecoloration and bleaching, was observable with a stableresponse for at least ten cycles. Whereas the electrodepositionof Prussian Blue on ATO nanoﬁbers proves the applicabilityof the methodology, the concept of ﬁbrous electrodes ismore universal and oﬀers advantages in other systems likeTiO2-based dye-sensitized electrodes. TiO2 can be anodicallydeposited from acid TiCl3 solution and the addition of surfactantsFig. 1 ATO nanoﬁber electrode before (left) and after (right) Prussian Blue electrodeposition: scheme (top), SEMs and digital photographs(lower inset) of the ﬁber mat used as an electrode on a glass slide.Fig. 2 Digital photographs of ATO nanoﬁber mats (a) in air and (b) inside poly(styrene). The latter composite poly(styrene) (PS) ﬁlm containingthe ATO nanoﬁbers imaged with a laser microscope (c) and the macroscopic resistivity through the PS-ATO ﬁlm (d).Downloaded by UNIVERSITAT GIESSEN on 30/04/2013 07:05:49. Published on 14 October 2011 on http://pubs.rsc.org | doi:10.1039/C1CC13724GView Article OnlineThis journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 12119–12121 12121like SDS (sodium dodecylsulfate) or BTB (bromothymol blue)aﬀords the generation of porous titania.18,19 However, when thethickness of the titania layer exceeds 500 nm on ﬂat substratescracks appear and the ﬁlm tends to peel oﬀ, thus severely limitinglight absorption and the solar cell eﬃciency (see Fig. S6, ESIw).In contrast, a deposit of 500 nm of TiO2 on each nanoﬁber in athick ﬁber mat results in an overall thickness of several microns(see Fig. S7, ESIw). Consequently, the dye-sensitized solar cellsbased on our nanoﬁbrous ATO electrodes generate a higherphotocurrent of 3.9 mA cm2 than solar cells similarly preparedon ﬂat FTO substrates showing a value of 0.9 mA cm2 (Fig. S8,ESIw). Although the cell displayed a ﬁll factor of only 33%,possibly due to defects in the blocking layer on the ﬁbers, weobserved conversion eﬃciencies of up to 1% for a standardGraetzel-cell test without optimization (other than using a ﬂatFTO substrate instead of a glass slide as back contact to avoid ahigh internal resistance).Another ﬁeld of application for the ATO nanoﬁbers matscan be found in polymer ﬁlms that are usually insulating. Wedemonstrated a hybrid polymer–TCO ﬁlm with antistaticproperties and good transparency (unlike CNT hybrids20)by simply hot pressing our ATO nanoﬁber mats intopoly(styrene) or poly(vinylpyrrolidone) (Fig. 2, see ESIw forthe synthesis). Since the average indices of refraction of thepolymer and the ATO ﬁbers were similar, the hybrid ﬁlmsshowed high transparency, in contrast to the nanoﬁber matsthemselves. The resistivity was determined by 2-probemeasurement to be 20 kO cm1 in-plane and 100–300 kOperpendicular to the composite ﬁlm. As seen by a lasermicroscope (Fig. 2), the ﬁbers were homogeneously distributedwithin the polymeric matrix. The optical transmission wasca. 75% (at 500 nm). Thus, high conductivity can be achievedat moderate loadings of ATO ﬁbers owing to a quite lowpercolation threshold. Such hybrids take advantage of thenanoﬁbers’ conductivity while oﬀsetting their fragility andtherefore represent an interesting type of antistatic and trans-parent polymer ﬁlm. Such conducting polymer hybrids canalso be achieved with carbon nanotubes,20 but evidently thetransparency is a signiﬁcant advantage of the TCO alternative.In conclusion, we demonstrated a novel approach for thefabrication of nanostructured electrodes based on electrospunATO nanoﬁbers and their use as porous electrodes and for thegeneration of conducting composite ﬁlms with insulatingpolymers of high industrial impact. We are further convincedthat our approach of generating and using TCO ﬁber matsrepresents a novel conceptual methodology with substantialpotential in diverse ﬁelds requiring a three-dimensionalelectrode.Notes and references1 T. Minami, Semicond. Sci. Technol., 2005, 20, S35–S44.2 P. P. Edwards, A. Porch, M. O. Jones, D. V. Morgan andR. M. Perks, Dalton Trans., 2004, 2995–3002.3 C. G. Granqvist, Appl. Phys. A: Solids Surf., 1993, 57, 19–24.4 E. Fortunato, D. Ginley, H. Hosono and D. C. Paine, MRS Bull.,2007, 32, 242–247.5 S. Chappel, L. Grinis, A. Oﬁr and A. Zaban, J. Phys. Chem. B,2005, 109, 1643–1647.6 L. Grinis, A. Oﬁr, S. Dor, S. Yahav and A. Zaban, Isr. J. Chem.,2008, 48, 269–275.7 E. Joanni, R. Savu, M. de S. Go´es, P. R. Bueno, J. N. de Freitas,A. F. Nogueira, E. Longo and J. A. Varela, Scr. Mater., 2007, 57,277–280.8 H.-W. Wang, C.-F. Ting, M.-K. Hung, C.-H. Chiou, Y.-L. Liu,Z. Liu, K. R. Ratinac and S. P. Ringer, Nanotechnology, 2009,20, 055601.9 M. S. Whittingham, Chem. Rev., 2004, 104, 4271–4302.10 D. Fattakhova-Rohlﬁng, T. Brezesinski, J. Rathousky,A. Feldhoﬀ, T. Oekermann, M. Wark and B. M. Smarsly, Adv.Mater., 2006, 18, 2980–2983.11 X. Zhang, Y. Man, J. Wang, C. Liu andW.Wu, Sci. China, Ser. E:Technol. Sci., 2006, 49, 537–546.12 E. Shanthi, V. Dutta, A. Banerjee and K. L. Chopra, J. Appl.Phys., 1980, 51, 6243–6251.13 C. Goebbert, R. Nonninger, M. A. Aegerter and H. Schmidt, ThinSolid Films, 1999, 351, 79–84.14 M. M. Munir, F. Iskandar, K. M. Yun, K. Okuyama andM. Abdullah, Nanotechnology, 2008, 19, 145603.15 D. Li and Y. Xia, Adv. Mater., 2004, 16, 1151–1170.16 D. Lin, H. Wu, R. Zhang and W. Pan, Nanotechnology, 2007,18, 465301.17 J. J. Garcia-Jareno, D. Benito, J. Navarro-Laboulais andF. Vicente, J. Chem. Educ., 1998, 75, 881–884.18 L. Kavan, B. O’Regan, A. Kay and M. Graetzel, J. Electroanal.Chem., 1993, 346, 291–307.19 K. Wessels, A. Feldhoﬀ, M. Wark, J. Rathousky andT. Oekermann, Electrochem. Solid-State Lett., 2006, 9, C93–C96.20 J. K. W. Sandler, J. E. Kirk, I. A. Kinloch, M. S. P. Shaﬀer andA. H. Windle, Polymer, 2003, 44, 5893–5899.Downloaded by UNIVERSITAT GIESSEN on 30/04/2013 07:05:49. Published on 14 October 2011 on http://pubs.rsc.org | doi:10.1039/C1CC13724GView Article Online

Electrospun antimony doped tin oxide (ATO) nanofibers as a versatile conducting matrix

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Electrospun antimony doped tin oxide (ATO) nanofibers as a versatile conducting matrix

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