- Email: [email protected]
Ionic liquid-stabilized nanoparticles of charge transfer-based conductors
Synthetic Metals 160 (2010) 1223–1227
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Ionic liquid-stabilized nanoparticles of charge transfer-based conductors夽 Dominique de Caro a,b,∗ , Kane Jacob a,b , Christophe Faulmann a,b , Jean-Pierre Legros a,b , Franc¸ois Senocq c , Jordi Fraxedas d , Lydie Valade a,b a
CNRS, LCC (Laboratoire de Chimie de Coordination), 205, route de Narbonne, F-31077 Toulouse, France Université de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France c CIRIMAT-ENSIACET, 4 allée Emile Monso, BP 74233, 31432 Toulouse Cedex 04, France d Centre d’Investigació en Nanociència i Nanotecnologia, CIN2 (CSIC-ICN), Campus de la UAB, Edifici CM-7, 08193 Bellaterra, Catalunya, Spain b
a r t i c l e
i n f o
Article history: Received 11 February 2010 Received in revised form 11 March 2010 Accepted 15 March 2010 Available online 7 April 2010 Keywords: Nanoparticles Molecular conductors Ionic liquids Tetrathiafulvalene Dithiolene complexes
a b s t r a c t Well-dispersed nanoparticles of molecule-based conductors, namely TTF·TCNQ and TTF[Ni(dmit)2 ]2 , have been prepared in organic solution using 1-butyl-3-metylimidazolium tetrafluoroborate as a stabilizing agent. TTF·TCNQ nanoparticles (prepared at room temperature) exhibit sizes ranging from 2 to 5.5 nm, whereas those of TTF[Ni(dmit)2 ]2 (prepared at −80 ◦ C) are larger (sizes in the 16–45 nm range). Nanoparticle powders have been characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and by transport measurements. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Electrically conductive TTF-based (TTF, tetrathiafulvalene) charge transfer salts may provide the basis for a new class of molecular-scale devices with physical and electronic characteristics that can be adjusted by size and shape [1]. Since the molecular structure of organic or metal-organic molecules is synthetically controllable, their organization in the solid, through the balance of competing interactions such as -overlap, van der Waals and hydrogen bonding, can be controlled. Most of such salts have been prepared in the form of single crystals and, only recently, strategies have been developed towards their synthesis in zero and one dimensions [2]. Within this aim, we have previously described the preparation of nanowires of moleculebased charge transfer salts using various techniques such as dip coating [3] and electrodeposition on silicon or on membranefunctionalized silicon electrodes [4]. Nanowires, nanorods, or nanopearl chains of TTF derivatives or of metal dithiolene complexes have also been obtained from organogels [5], using porous alumina templates [6], applying the Langmuir–Blodgett tech-
夽 Dedicated to Bruno Chaudret, Director of the LCC. ∗ Corresponding author at: CNRS, LCC (Laboratoire de Chimie de Coordination), 205, route de Narbonne, F-31077 Toulouse Cedex 4, France. Tel.: +33 561333106; fax: +33 561553003. E-mail address: [email protected] (D. de Caro). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.03.013
nique [7], by electrocrystallization on platinum nanoparticles deposited onto a graphite electrode [8], by electrocrystallization in a nanovolume cell [9], by nanoscale positioning of TTF compounds onto silicon oxide nanopatterns [10] or by a two-phase technique [11]. The wire-like morphology is to be expected for such systems which typically contain quasi-onedimensional segregated stacks of donor and/or acceptor molecules that afford highly anisotropic conductivity. Though rare examples of nanodots of neutral TTF derivatives bearing long alkyl chains are known [12], to our knowledge, nanoparticles of charge transfer quasi-one-dimensional TTF-based conductors have never been described. To our opinion, this constitutes a challenge because, contrary to three-dimensional cyano-bridged moleculebased magnets (Prussian Blue) which can be easily grown as nanoparticles [13,14], the monodimensional character and the absence of strong coordination bonds for TTF-based conductors seem deleterious to a confined growth as spheres. Ionic liquids are good candidates to stabilize metallic [15,16] or Prussian Blue nanoparticles [13]. The ionic liquid which is composed of a substituted imidazolium cation, and of inorganic or organic anions, such as AlCl4 − , PF6 − , BF4 − , CF3 SO3 − , (CF3 SO2 )2 N− , acts as both the solvent and the stabilizing agent. Here, we present results on the preparation of nanoparticles of two molecular metals namely, TTF[Ni(dmit)2 ]2 (dmit2− , 1,3-dithiole-2-thione4,5-dithiolato), and TTF·TCNQ (TCNQ, tetracyanoquinodimethane), precipitated from an organic solution containing 1-butyl-3methylimidazolium tetrafluoroborate [BMIM][BF4 ] as ionic liquid.
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2. Experimental All syntheses have been carried out under an argon atmosphere using freshly distilled and degassed solvents. TTF is purchased from Sigma, TCNQ from Fluka, and [BMIM][BF4 ] from Alfa-Aesar. Other starting compounds are prepared following previously described procedures: [TTF]3 [BF4 ]2 in Ref. [17], and [(n-C4 H9 )4 N][Ni(dmit)2 ] in Ref. [18]. 2.1. TTF[Ni(dmit)2 ]2 nanoparticles A suspension of 26 mg of [TTF]3 [BF4 ]2 in 0.4 mL of [BMIM][BF4 ] and 5 mL of acetonitrile is added dropwise to a solution of 46 mg of [(n-C4 H9 )4 N][Ni(dmit)2 ] in 5 mL of acetone at 25, 0, −20, or −80 ◦ C. Whatever the temperature, a black precipitate is observed as soon as the suspension is added under stirring. Stirring is maintained over a period of 2 h at the addition temperature. The suspension is then allowed to warm to room temperature (when addition is performed at 0, −20, or −80 ◦ C). The black solid is filtered off, washed with 3 × 1 mL of acetonitrile and finally dried under vacuum. The resulting black powder is air stable (yield: 65%). 2.2. TTF·TCNQ nanoparticles A solution of 23 mg of TTF in 0.4 mL of [BMIM][BF4 ] and 5 mL of acetonitrile is added dropwise to a solution of 23 mg of TCNQ in 5 mL of acetonitrile at 25 ◦ C. A black precipitate is observed as soon as the TTF/[BMIM][BF4 ] solution is added under stirring. Stirring is maintained over a period of 2 h at room temperature. The black solid is filtered off, washed with 3 × 1 mL of acetonitrile and finally dried under vacuum. The resulting black powder is air stable (yield: 90%). For TEM observation, powder is dispersed in diethyl ether under very slow stirring for 1 min. The TEM specimens are then prepared by evaporation of droplets of suspension deposited on carbonsupported copper grids. The experiments are performed on a JEOL Model JEM 1011 operating at 100 kV. X-ray diffraction data are collected with a Bruker D8 Advance diffractometer working in the Bragg Brentano configuration (–) using Ni filtered CuK␣ radiation (0.15418 nm) and fitted with a SuperSpeed Vantec Detector. XPS experiments were performed in a PHI 5500 Multitechnique System (from Physical Electronics) with a monochromatic X-ray source (AlK␣ line of 1486.6 eV energy and 350 W), placed perpendicularly to the analyzer axis and calibrated using the Ag 3d5/2 line. The analyzed area was a circle of 0.8 mm diameter. A flood gun of electrons with energy lower than 20 eV was used to compensate the charge. All measurements were made in a ultra high vacuum (UHV) chamber pressure between 5 × 10−9 and 2 × 10−8 Torr. Conductivity measurements were performed on films of nanoparticles deposited on a special holder devoted to conductivity measurements (supplier: Motorola). A few mg of TTF[Ni(dmit)2 ]2 or TTF·TCNQ (10 mg) was suspended in diethylether (ca. 1.5 mL), yielding a highly concentrated suspension. Using a microsyringe, 30–50 L of this suspension was deposited over the full surface of the holder. After evaporation of the solvent, nanoparticles formed a homogeneous black deposit above the probes of the holder. An alternative method has also been performed in depositing the suspension only on and between the gold contacts, filling then totally the step based with the edges of the holder. Doing so, in considering the step totally half-filled with nanoparticles, the total section could be determined (1.57 × 10−3 cm2 ), and the real conductivity of the material could be estimated. The electrical behavior was measured by using the four-probe method between 295 and 77 K.
Fig. 1. Electron micrographs for TTF[Ni(dmit)2 ]2 nanoparticles prepared in ionic liquid medium: (top) precipitation reaction at 25 ◦ C, bar = 500 nm; (bottom) precipitation reaction at −80 ◦ C, bar = 1000 nm.
3. Results and discussion The dropwise addition of an acetonitrile solution of [TTF]3 [BF4 ]2 into an acetone solution of [(n-C4 H9 )4 N][Ni(dmit)2 ] leads to TTF[Ni(dmit)2 ]2 as a black powder [19]. Transmission electron micrographs of this sample exclusively evidence needles (typical sizes: 0.5–2 m wide, >5 m long). The charge transfer complex TTF·TCNQ can also be obtained by slow addition of an acetonitrile solution of neutral TTF to an acetonitrile solution of neutral TCNQ. TEM images are very similar to those obtained for TTF[Ni(dmit)2 ]2 . The dropwise addition of a suspension of [TTF]3 [BF4 ]2 in CH3 CN/[BMIM][BF4 ] (12.5:1 in vol.) into an acetone solution of [(n-C4 H9 )4 N][Ni(dmit)2 ] at 25, 0, −20, or −80 ◦ C leads to a black precipitate. XRD data evidence that the powder corresponds to TTF[Ni(dmit)2 ]2 (vide infra). When the precipitation reaction is performed at 25, 0, or −20 ◦ C, electron micrographs show the presence of homogeneously dispersed nanoparticles (Fig. 1) in addition with ∼5% platelets (typical sizes: 0.5–2 m wide, 2–5 m long). Some nanoparticles are roughly spherical (diameter in the 20–50 nm range) whereas a large number are elongated (20–25 nm × 35–100 nm). At −80 ◦ C, only homogeneously dispersed roughly spherical TTF[Ni(dmit)2 ]2 nanoparticles are observed (Fig. 1). Diameters are in the 16–45 nm range (mean diameter: 25.5 nm, Fig. 2). A size decrease and a preferential growth as spheres are therefore obtained at very low temperatures. These effects have been previously observed on ruthenium nanoparticles stabilized in pure [BMIM][N(CF3 SO2 )2 ] [20]. In our case, the medium consists of the [TTF]3 [BF4 ]2 and [(nC4 H9 )4 N][Ni(dmit)2 ] precursors solubilized in a mixture of the ionic liquid and acetonitrile. The addition of acetonitrile is necessary to
D. de Caro et al. / Synthetic Metals 160 (2010) 1223–1227
Fig. 2. Histogram showing the size distribution for TTF[Ni(dmit)2 ]2 nanoparticles prepared at −80 ◦ C.
(i) solubilize the material precursors and (ii) reach low temperatures (−80 ◦ C), the ionic liquid being a solid at this temperature (melting point: −71 ◦ C). The controlled growth of TTF[Ni(dmit)2 ]2 in [BMIM][BF4 ]/CH3 CN at −80 ◦ C may be first explained by the decrease of the aggregation kinetic. However, this feature would not alone explain the spherical shape of the particles. Indeed the characteristic one-dimensionality of TTF[Ni(dmit)2 ]2 would have produced preferably nanoneedles or nanowires [11]. Therefore, the growth of spherical nanoparticles is controlled by the presence of the ionic liquid. Note also that few roughly spherical nanoparticles are observed at higher temperatures and would not be present without the influence of the ionic liquid. Thus, for a system having no extended coordination bonds, aggregation of TTF and Ni(dmit)2 through -overlap and weak van der Waals interactions in a ionic liquid/solvent stabilizing medium can lead to well-dispersed nanospheres of the TTF[Ni(dmit)2 ]2 metal-organic conductor at −80 ◦ C. The X-ray diffraction patterns of the powders obtained at 25, 0, −20, and −80 ◦ C are recorded at room temperature and are all in good agreement with the simulation calculated using single crystal data for TTF[Ni(dmit)2 ]2 (Fig. 3) [21]. This simulation allows indexing of the experimental powder pattern. The resulting cell parameters: a = 4.621(6); b = 0.3728(4); c = 2.279(2) nm; ˇ = 119.17(1)◦ (for the sample obtained at 25 ◦ C), are in excellent agreement with those obtained in the single crystal X-ray diffraction study: a = 4.621(1); b = 0.3728(3); c = 2.2819(6) nm; ˇ = 119.24(2)◦ , thus confirming unambiguously
Fig. 3. XRD patterns: (top) TTF[Ni(dmit)2 ]2 nanoparticles prepared at 20 ◦ C and (bottom) calculated.
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Fig. 4. XPS S 2p line measured ex situ at room temperature for TTF[Ni(dmit)2 ]2 nanocrystalline powders (continuous black line). A least-square fit using a combination of Gaussians and Lorentzians after a Shirley-type background subtraction is also shown (continuous grey lines). Dark grey lines correspond to the different contributions associated to different chemical environments: C–S–Ni, C–S–C and C S. The small contribution from S–O bonding, due to contamination, is also taken into account. Each line contains two components, 2p3/2 and 2p1/2 , separated by 1.2 eV, with an intensity ratio, 2:1, imposed by the chemical composition of the Ni(tmdt)2 molecule. The light grey line corresponds to the sum of all contributions, which satisfactorily fits the experimental data.
the chemical nature of the phase. Ex situ XPS measurements (Fig. 4) show that the nanoparticles exhibit the expected stoichiometry, TTF[Ni(dmit)2 ]2 , as obtained from a least-square analysis of the S2p line. The experimental lineshape can be satisfactorily decomposed into three contributions. The most intense line, with a binding energy of 163.5 eV, corresponds to C–S–C, while the less intense 161.7 and 164.5 eV lines correspond to C–S–Ni and C S, respectively [22]. The ratios of peak areas give ∼3:2:1 in excellent agreement with the nominal 12 C–S–C, 8 C–S–Ni and 4 C S bonds. The N 1s line (Fig. 5) shows a feature at 401.1 eV, which is ascribed to small amounts of residual ionic liquid from a direct comparison with a [(n-C4 H9 )4 N][BF4 ] reference sample measured under identical experimental conditions. N 1s signals may arise from both butylimidazolium and tetrabutylammonium but the latter exhibits an energy of 402.0 eV [23]. The dropwise addition of a solution of TTF in CH3 CN/[BMIM][BF4 ] (12.5:1 in vol.) into a CH3 CN solution of TCNQ at room temperature leads to a black precipitate. XRD data
Fig. 5. XPS N 1s lines measured ex situ at room temperature for [(nC4 H9 )4 N][BF4 ] (bottom), TTF[Ni(dmit)2 ]2 nanocrystalline powders (centre), and TTF·TCNQ nanocrystalline powders (top).
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Fig. 8. XRD patterns: (top) TTF·TCNQ nanoparticles; (bottom) calculated.
Fig. 6. Electron micrograph for TTF·TCNQ nanoparticles prepared in ionic liquid medium (precipitation reaction at 25 ◦ C; bar = 100 nm).
evidence that the powder corresponds to TTF·TCNQ (vide infra). Existence of charge transfer is revealed by XPS of the N 1s line (Fig. 5). The features at 397.5 and 399.0 eV correspond to charged and neutral TCNQ species, respectively, while S 2p features at 163.5 and 164.5 eV arise from neutral and charged TTF species, respectively [24,25]. Both dynamical configurations, neutral and charged, can be observed with XPS because photoemission is an intrinsic rapid process (10−15 s). As precised above, precipitation in the absence of ionic liquid leads to needles of TTF·TCNQ as shown by electron microscopy. In the presence of ionic liquid, electron micrographs evidence nanoparticles (Fig. 6) in addition with ∼5% nanorods (15–40 nm wide, 250–400 nm long). Some nanoparticles exhibit irregular shapes and small agglomerates are also encountered. However, the smaller the particles are, the more spherical their shape. Diameters of TTF·TCNQ nanoparticles are in the 2.1–5.6 nm range (mean diameter: 3.8 nm, Fig. 7). Similar results are obtained when the precipitation reaction is performed at −80 ◦ C. Particles are as small as Prussian Blue ones in [BMIM][BF4 ] [13]; this is remarkable for a system in which only weak intermolecular interactions are responsible for cohesion. Moreover, TTF·TCNQ nanoparticles are smaller than TTF[Ni(dmit)2 ]2 ones. We suggest that, in nuclei of TTF·TCNQ, nitrogen atoms of TCNQ (bearing a partial negative charge) should favourably associate to the cationic imidazolium cycle of the ionic liquid. Thus, the growth
Fig. 7. Histogram showing the size distribution for TTF·TCNQ nanoparticles prepared at 25 ◦ C.
could be rapidly inhibited by the ionic liquid in close contact with the charge transfer complex even at room temperature. The clear presence of ionic liquid within the TTF·TCNQ nanopowder is confirmed by XPS analysis, thus proving its stabilizing role. The powder X-ray diffraction pattern is recorded and compared with the simulation calculated using single crystal data from the literature [26] (Fig. 8). The Bragg angles of the observed reflections agree well with the calculated ones, however the relative intensities of the lines differ somewhat from those calculated, presumably because of some preferential orientation of the crystallites. The indexation of the experimental powder pattern leads to the following cell parameters: a = 1.2311(8); b = 0.3819(2); c = 1.8430(8) nm; ˇ = 104.36(1)◦ which are consistent with those deduced from the single crystal X-ray diffraction study: a = 1.2298(6); b = 0.3819(2); c = 1.8468(8) nm; ˇ = 104.46(4)◦ , the chemical nature of the phase is thus confirmed. Nanocrystalline TTF[Ni(dmit)2 ]2 and TTF·TCNQ exhibit a semiconducting behavior, which is not surprising for powdered materials (Fig. 9). Their activation energies range between 15 and 40 meV. Considering a thickness of ca. 10 m, the conductivity of the TTF·TCNQ and TTF[Ni(dmit)2 ]2 films are estimated in a range between 0.01–10 and 1–20 S cm−1 , respectively. These values are similar to those for thin films of TTF-based charge transfer compounds processed by various wet or dry preparation methods [1,29]. Whatever the compound (TTF[Ni(dmit)2 ]2 or TTF·TCNQ), it has been noticed that the highest conductivities and lowest activation energies are observed for compounds prepared at −80 ◦ C (homogeneous morphology and smaller size). To our knowledge, there is no relevant study concerning the grain size and the electrical conductivity in molecular conductors such as those reported
Fig. 9. Electrical behavior for TTF[Ni(dmit)2 ]2 and TTF·TCNQ nanocrystalline powders.
D. de Caro et al. / Synthetic Metals 160 (2010) 1223–1227
in this work. Therefore, no conclusion can be drawn from this observation. A possible explanation could be the following: the low activation energy and the high conductivity observed for the smallest particles are probably due to the better homogeneity within the volume of the particles which leads to a better quality of contact between particles within the material. 4. Conclusion In summary, we have isolated the first examples of nanoparticles of conducting molecular materials by using an ionic liquid stabilizing medium. We demonstrated the major role of the ionic liquid in confining the nuclei into nanodomains. The formation of the smallest and homogeneously distributed spherical particles is favoured at low temperatures. The chemical nature of the nanoparticles is confirmed by X-ray diffraction and by X-ray photoelectron spectroscopy. The room temperature conductivity of the nanoparticles is the highest for spherical ones and is only one order of magnitude lower than that of single crystal values (300 and 600 S cm−1 for TTF[Ni(dmit)2 ]2 [27] and TTF·TCNQ [28], respectively). Acknowledgments This work was supported by the Centre National de la Recherche Scientifique (France), by the Ministerio de Ciencia y Tecnologia (Spain), through project FIS2006-12117-C04-01, and by the Generalitat de Catalunya (SGR 00909). We thank the LEA 368 (Laboratoire Trans-Pyrénéen: From Molecules to Materials). References [1] J. Fraxedas, Molecular Organic Materials, Cambridge University Press, Cambridge, 2006. [2] E. Gomar-Nadal, J. Puigmartí-Luis, D.B. Amabilino, Chem. Soc. Rev. 37 (2008) 490. [3] L. Valade, H. Casellas, S. Roques, C. Faulmann, D. de Caro, A. Zwick, L. Ariès, J. Solid State Chem. 168 (2002) 438. [4] J.-P. Savy, D. de Caro, C. Faulmann, L. Valade, M. Almeida, T. Koike, H. Fujiwara, T. Sugimoto, J. Fraxedas, T. Ondarc¸uhu, C. Pasquier, New J. Chem. 31 (2007) 519.
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[5] J. Puigmartí-Luis, V. Laukhin, A. Pérez del Pino, J. Vidal-Gancedo, C. Rovira, E. Laukhina, D.B. Amabilino, Angew. Chem. Int. Ed. 46 (2007) 238. [6] G. Cui, W. Xu, C. Guo, X. Xiao, H. Xu, D. Zhang, L. Jiang, D. Zhu, J. Phys. Chem. B 108 (2004) 13638. [7] H. Miyata, Y. Tatewaki, T. Akutagawa, T. Hasegawa, T. Nakamura, C.A. Christensen, J. Becher, Thin Solid Films 438–439 (2003) 1. [8] F. Favier, H. Liu, R.M. Penner, Adv. Mater. 13 (2001) 1567. [9] K. Kobayashi, H. Koyama, K. Ishikura, T. Mitsui, Appl. Phys. Lett. 93 (2008) 143114. [10] N.S. Losilla, N.S. Oxtoby, J. Martinez, F. Garcia, R. Garcia, M. Mas-Torrent, J. Veciana, C. Rovira, Nanotechnology 19 (2008) 455308. [11] H. Liu, J. Li, C. Lao, C. Huang, Y. Li, Z. Lin Wang, D. Zhu, Nanotechnology 18 (2007) 495704. [12] T. Akutagawa, K. Kakiuchi, T. Hasegawa, S-i. Noro, T. Nakamura, H. Hasegawa, S. Mashiko, J. Becher, Angew. Chem. Int. Ed. 44 (2005) 7283. [13] G. Clavel, J. Larionova, Y. Guari, C. Guérin, Chem. Eur. J. 12 (2006) 3798. [14] L. Catala, A. Gloter, O. Stephan, G. Rogez, T. Mallah, Chem. Commun. 9 (2006) 1018. [15] J. Dupont, D. de Oliveira Silva, in: D. Astruc (Ed.), Nanoparticles and Catalysis, Wiley-VCH, Weinheim, 2007, p. 195. [16] A.I. Bhatt, Á. Mechler, L.L. Martin, A.M. Bond, J. Mater. Chem. 17 (2007) 2241. [17] F. Wudl, J. Am. Chem. Soc. 97 (1975) 1962. [18] G. Steimecke, H.-J. Sieler, R. Kirmse, E. Hoyer, Phosphorus Sulfur 7 (1979) 49. [19] L. Valade, D. de Caro, I. Malfant, in: L. Ouahab, E. Yagubskii (Eds.), Organic Conductors, Superconductors and Magnets: From Synthesis to Molecular Electronics, Kluwer Academic Publishers, 2004, p. 241. [20] T. Gutel, J. Garcia-Antõn, K. Pelzer, K. Philippot, C.C. Santini, Y. Chauvin, B. Chaudret, J.-M. Basset, J. Mater. Chem. 17 (2007) 3290. [21] Room temperature X-ray diffraction single crystal data of TTF[Ni(dmit)2 ]2 were published in Ref. [26], with agreement factors R = 0.097; Rw = 0.117. More accurate data (R = 0.026, Rw = 0.019) were obtained at 295 K by Chasseau and Hebrard-Brachetti (S. Hebrard-Brachetti, Ph.D. Thesis, 1996, Université Bordeaux I, France) and were used in the present work for the simulation of the powder diffraction pattern and taking into account the experimental broadening of the diffraction lines. [22] D. de Caro, J. Fraxedas, C. Faulmann, I. Malfant, J. Milon, J.-F. Lamère, V. Collière, L. Valade, Adv. Mater. 835 (2004) 835. [23] D. de Caro, H. Alves, M. Almeida, S. Caillieux, M. Ejgaddari, C. Faulmann, I. Malfant, F. Senocq, J. Fraxedas, A. Zwick, L. Valade, J. Mater. Chem. 14 (2004) 2801. [24] C. Rojas, J. Caro, M. Grioni, J. Fraxedas, Surf. Sci. 482–485 (2001) 546. [25] M. Sing, U. Schwingenschlögl, R. Claessen, M. Dressel, C.S. Jacobsen, Phys. Rev. B 67 (2003) 125402. [26] T.J. Kistenmacher, T.E. Phillips, D.O. Cowan, Acta Crystallogr. B 30 (1974) 763. [27] M. Bousseau, L. Valade, J.-P. Legros, P. Cassoux, M. Garbauskas, L.V. Interrante, J. Am. Chem. Soc. 108 (1986) 1908. [28] M.J. Cohen, L.B. Coleman, A.F. Garito, A.J. Meyer, Phys. Rev. B 10 (1974) 1298. [29] P. Chaudhari, B.A. Scott, R.B. Laibowitz, Y. Tomkiewicz, J.B. Torrance, Appl. Phys. Lett. 24 (1974) 439.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Ionic liquid-stabilized nanoparticles of charge transfer-based conductors夽 Dominique de Caro a,b,∗ , Kane Jacob a,b , Christophe Faulmann a,b , Jean-Pierre Legros a,b , Franc¸ois Senocq c , Jordi Fraxedas d , Lydie Valade a,b a
CNRS, LCC (Laboratoire de Chimie de Coordination), 205, route de Narbonne, F-31077 Toulouse, France Université de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France c CIRIMAT-ENSIACET, 4 allée Emile Monso, BP 74233, 31432 Toulouse Cedex 04, France d Centre d’Investigació en Nanociència i Nanotecnologia, CIN2 (CSIC-ICN), Campus de la UAB, Edifici CM-7, 08193 Bellaterra, Catalunya, Spain b
a r t i c l e
i n f o
Article history: Received 11 February 2010 Received in revised form 11 March 2010 Accepted 15 March 2010 Available online 7 April 2010 Keywords: Nanoparticles Molecular conductors Ionic liquids Tetrathiafulvalene Dithiolene complexes
a b s t r a c t Well-dispersed nanoparticles of molecule-based conductors, namely TTF·TCNQ and TTF[Ni(dmit)2 ]2 , have been prepared in organic solution using 1-butyl-3-metylimidazolium tetrafluoroborate as a stabilizing agent. TTF·TCNQ nanoparticles (prepared at room temperature) exhibit sizes ranging from 2 to 5.5 nm, whereas those of TTF[Ni(dmit)2 ]2 (prepared at −80 ◦ C) are larger (sizes in the 16–45 nm range). Nanoparticle powders have been characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and by transport measurements. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Electrically conductive TTF-based (TTF, tetrathiafulvalene) charge transfer salts may provide the basis for a new class of molecular-scale devices with physical and electronic characteristics that can be adjusted by size and shape [1]. Since the molecular structure of organic or metal-organic molecules is synthetically controllable, their organization in the solid, through the balance of competing interactions such as -overlap, van der Waals and hydrogen bonding, can be controlled. Most of such salts have been prepared in the form of single crystals and, only recently, strategies have been developed towards their synthesis in zero and one dimensions [2]. Within this aim, we have previously described the preparation of nanowires of moleculebased charge transfer salts using various techniques such as dip coating [3] and electrodeposition on silicon or on membranefunctionalized silicon electrodes [4]. Nanowires, nanorods, or nanopearl chains of TTF derivatives or of metal dithiolene complexes have also been obtained from organogels [5], using porous alumina templates [6], applying the Langmuir–Blodgett tech-
夽 Dedicated to Bruno Chaudret, Director of the LCC. ∗ Corresponding author at: CNRS, LCC (Laboratoire de Chimie de Coordination), 205, route de Narbonne, F-31077 Toulouse Cedex 4, France. Tel.: +33 561333106; fax: +33 561553003. E-mail address: [email protected] (D. de Caro). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.03.013
nique [7], by electrocrystallization on platinum nanoparticles deposited onto a graphite electrode [8], by electrocrystallization in a nanovolume cell [9], by nanoscale positioning of TTF compounds onto silicon oxide nanopatterns [10] or by a two-phase technique [11]. The wire-like morphology is to be expected for such systems which typically contain quasi-onedimensional segregated stacks of donor and/or acceptor molecules that afford highly anisotropic conductivity. Though rare examples of nanodots of neutral TTF derivatives bearing long alkyl chains are known [12], to our knowledge, nanoparticles of charge transfer quasi-one-dimensional TTF-based conductors have never been described. To our opinion, this constitutes a challenge because, contrary to three-dimensional cyano-bridged moleculebased magnets (Prussian Blue) which can be easily grown as nanoparticles [13,14], the monodimensional character and the absence of strong coordination bonds for TTF-based conductors seem deleterious to a confined growth as spheres. Ionic liquids are good candidates to stabilize metallic [15,16] or Prussian Blue nanoparticles [13]. The ionic liquid which is composed of a substituted imidazolium cation, and of inorganic or organic anions, such as AlCl4 − , PF6 − , BF4 − , CF3 SO3 − , (CF3 SO2 )2 N− , acts as both the solvent and the stabilizing agent. Here, we present results on the preparation of nanoparticles of two molecular metals namely, TTF[Ni(dmit)2 ]2 (dmit2− , 1,3-dithiole-2-thione4,5-dithiolato), and TTF·TCNQ (TCNQ, tetracyanoquinodimethane), precipitated from an organic solution containing 1-butyl-3methylimidazolium tetrafluoroborate [BMIM][BF4 ] as ionic liquid.
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2. Experimental All syntheses have been carried out under an argon atmosphere using freshly distilled and degassed solvents. TTF is purchased from Sigma, TCNQ from Fluka, and [BMIM][BF4 ] from Alfa-Aesar. Other starting compounds are prepared following previously described procedures: [TTF]3 [BF4 ]2 in Ref. [17], and [(n-C4 H9 )4 N][Ni(dmit)2 ] in Ref. [18]. 2.1. TTF[Ni(dmit)2 ]2 nanoparticles A suspension of 26 mg of [TTF]3 [BF4 ]2 in 0.4 mL of [BMIM][BF4 ] and 5 mL of acetonitrile is added dropwise to a solution of 46 mg of [(n-C4 H9 )4 N][Ni(dmit)2 ] in 5 mL of acetone at 25, 0, −20, or −80 ◦ C. Whatever the temperature, a black precipitate is observed as soon as the suspension is added under stirring. Stirring is maintained over a period of 2 h at the addition temperature. The suspension is then allowed to warm to room temperature (when addition is performed at 0, −20, or −80 ◦ C). The black solid is filtered off, washed with 3 × 1 mL of acetonitrile and finally dried under vacuum. The resulting black powder is air stable (yield: 65%). 2.2. TTF·TCNQ nanoparticles A solution of 23 mg of TTF in 0.4 mL of [BMIM][BF4 ] and 5 mL of acetonitrile is added dropwise to a solution of 23 mg of TCNQ in 5 mL of acetonitrile at 25 ◦ C. A black precipitate is observed as soon as the TTF/[BMIM][BF4 ] solution is added under stirring. Stirring is maintained over a period of 2 h at room temperature. The black solid is filtered off, washed with 3 × 1 mL of acetonitrile and finally dried under vacuum. The resulting black powder is air stable (yield: 90%). For TEM observation, powder is dispersed in diethyl ether under very slow stirring for 1 min. The TEM specimens are then prepared by evaporation of droplets of suspension deposited on carbonsupported copper grids. The experiments are performed on a JEOL Model JEM 1011 operating at 100 kV. X-ray diffraction data are collected with a Bruker D8 Advance diffractometer working in the Bragg Brentano configuration (–) using Ni filtered CuK␣ radiation (0.15418 nm) and fitted with a SuperSpeed Vantec Detector. XPS experiments were performed in a PHI 5500 Multitechnique System (from Physical Electronics) with a monochromatic X-ray source (AlK␣ line of 1486.6 eV energy and 350 W), placed perpendicularly to the analyzer axis and calibrated using the Ag 3d5/2 line. The analyzed area was a circle of 0.8 mm diameter. A flood gun of electrons with energy lower than 20 eV was used to compensate the charge. All measurements were made in a ultra high vacuum (UHV) chamber pressure between 5 × 10−9 and 2 × 10−8 Torr. Conductivity measurements were performed on films of nanoparticles deposited on a special holder devoted to conductivity measurements (supplier: Motorola). A few mg of TTF[Ni(dmit)2 ]2 or TTF·TCNQ (10 mg) was suspended in diethylether (ca. 1.5 mL), yielding a highly concentrated suspension. Using a microsyringe, 30–50 L of this suspension was deposited over the full surface of the holder. After evaporation of the solvent, nanoparticles formed a homogeneous black deposit above the probes of the holder. An alternative method has also been performed in depositing the suspension only on and between the gold contacts, filling then totally the step based with the edges of the holder. Doing so, in considering the step totally half-filled with nanoparticles, the total section could be determined (1.57 × 10−3 cm2 ), and the real conductivity of the material could be estimated. The electrical behavior was measured by using the four-probe method between 295 and 77 K.
Fig. 1. Electron micrographs for TTF[Ni(dmit)2 ]2 nanoparticles prepared in ionic liquid medium: (top) precipitation reaction at 25 ◦ C, bar = 500 nm; (bottom) precipitation reaction at −80 ◦ C, bar = 1000 nm.
3. Results and discussion The dropwise addition of an acetonitrile solution of [TTF]3 [BF4 ]2 into an acetone solution of [(n-C4 H9 )4 N][Ni(dmit)2 ] leads to TTF[Ni(dmit)2 ]2 as a black powder [19]. Transmission electron micrographs of this sample exclusively evidence needles (typical sizes: 0.5–2 m wide, >5 m long). The charge transfer complex TTF·TCNQ can also be obtained by slow addition of an acetonitrile solution of neutral TTF to an acetonitrile solution of neutral TCNQ. TEM images are very similar to those obtained for TTF[Ni(dmit)2 ]2 . The dropwise addition of a suspension of [TTF]3 [BF4 ]2 in CH3 CN/[BMIM][BF4 ] (12.5:1 in vol.) into an acetone solution of [(n-C4 H9 )4 N][Ni(dmit)2 ] at 25, 0, −20, or −80 ◦ C leads to a black precipitate. XRD data evidence that the powder corresponds to TTF[Ni(dmit)2 ]2 (vide infra). When the precipitation reaction is performed at 25, 0, or −20 ◦ C, electron micrographs show the presence of homogeneously dispersed nanoparticles (Fig. 1) in addition with ∼5% platelets (typical sizes: 0.5–2 m wide, 2–5 m long). Some nanoparticles are roughly spherical (diameter in the 20–50 nm range) whereas a large number are elongated (20–25 nm × 35–100 nm). At −80 ◦ C, only homogeneously dispersed roughly spherical TTF[Ni(dmit)2 ]2 nanoparticles are observed (Fig. 1). Diameters are in the 16–45 nm range (mean diameter: 25.5 nm, Fig. 2). A size decrease and a preferential growth as spheres are therefore obtained at very low temperatures. These effects have been previously observed on ruthenium nanoparticles stabilized in pure [BMIM][N(CF3 SO2 )2 ] [20]. In our case, the medium consists of the [TTF]3 [BF4 ]2 and [(nC4 H9 )4 N][Ni(dmit)2 ] precursors solubilized in a mixture of the ionic liquid and acetonitrile. The addition of acetonitrile is necessary to
D. de Caro et al. / Synthetic Metals 160 (2010) 1223–1227
Fig. 2. Histogram showing the size distribution for TTF[Ni(dmit)2 ]2 nanoparticles prepared at −80 ◦ C.
(i) solubilize the material precursors and (ii) reach low temperatures (−80 ◦ C), the ionic liquid being a solid at this temperature (melting point: −71 ◦ C). The controlled growth of TTF[Ni(dmit)2 ]2 in [BMIM][BF4 ]/CH3 CN at −80 ◦ C may be first explained by the decrease of the aggregation kinetic. However, this feature would not alone explain the spherical shape of the particles. Indeed the characteristic one-dimensionality of TTF[Ni(dmit)2 ]2 would have produced preferably nanoneedles or nanowires [11]. Therefore, the growth of spherical nanoparticles is controlled by the presence of the ionic liquid. Note also that few roughly spherical nanoparticles are observed at higher temperatures and would not be present without the influence of the ionic liquid. Thus, for a system having no extended coordination bonds, aggregation of TTF and Ni(dmit)2 through -overlap and weak van der Waals interactions in a ionic liquid/solvent stabilizing medium can lead to well-dispersed nanospheres of the TTF[Ni(dmit)2 ]2 metal-organic conductor at −80 ◦ C. The X-ray diffraction patterns of the powders obtained at 25, 0, −20, and −80 ◦ C are recorded at room temperature and are all in good agreement with the simulation calculated using single crystal data for TTF[Ni(dmit)2 ]2 (Fig. 3) [21]. This simulation allows indexing of the experimental powder pattern. The resulting cell parameters: a = 4.621(6); b = 0.3728(4); c = 2.279(2) nm; ˇ = 119.17(1)◦ (for the sample obtained at 25 ◦ C), are in excellent agreement with those obtained in the single crystal X-ray diffraction study: a = 4.621(1); b = 0.3728(3); c = 2.2819(6) nm; ˇ = 119.24(2)◦ , thus confirming unambiguously
Fig. 3. XRD patterns: (top) TTF[Ni(dmit)2 ]2 nanoparticles prepared at 20 ◦ C and (bottom) calculated.
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Fig. 4. XPS S 2p line measured ex situ at room temperature for TTF[Ni(dmit)2 ]2 nanocrystalline powders (continuous black line). A least-square fit using a combination of Gaussians and Lorentzians after a Shirley-type background subtraction is also shown (continuous grey lines). Dark grey lines correspond to the different contributions associated to different chemical environments: C–S–Ni, C–S–C and C S. The small contribution from S–O bonding, due to contamination, is also taken into account. Each line contains two components, 2p3/2 and 2p1/2 , separated by 1.2 eV, with an intensity ratio, 2:1, imposed by the chemical composition of the Ni(tmdt)2 molecule. The light grey line corresponds to the sum of all contributions, which satisfactorily fits the experimental data.
the chemical nature of the phase. Ex situ XPS measurements (Fig. 4) show that the nanoparticles exhibit the expected stoichiometry, TTF[Ni(dmit)2 ]2 , as obtained from a least-square analysis of the S2p line. The experimental lineshape can be satisfactorily decomposed into three contributions. The most intense line, with a binding energy of 163.5 eV, corresponds to C–S–C, while the less intense 161.7 and 164.5 eV lines correspond to C–S–Ni and C S, respectively [22]. The ratios of peak areas give ∼3:2:1 in excellent agreement with the nominal 12 C–S–C, 8 C–S–Ni and 4 C S bonds. The N 1s line (Fig. 5) shows a feature at 401.1 eV, which is ascribed to small amounts of residual ionic liquid from a direct comparison with a [(n-C4 H9 )4 N][BF4 ] reference sample measured under identical experimental conditions. N 1s signals may arise from both butylimidazolium and tetrabutylammonium but the latter exhibits an energy of 402.0 eV [23]. The dropwise addition of a solution of TTF in CH3 CN/[BMIM][BF4 ] (12.5:1 in vol.) into a CH3 CN solution of TCNQ at room temperature leads to a black precipitate. XRD data
Fig. 5. XPS N 1s lines measured ex situ at room temperature for [(nC4 H9 )4 N][BF4 ] (bottom), TTF[Ni(dmit)2 ]2 nanocrystalline powders (centre), and TTF·TCNQ nanocrystalline powders (top).
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Fig. 8. XRD patterns: (top) TTF·TCNQ nanoparticles; (bottom) calculated.
Fig. 6. Electron micrograph for TTF·TCNQ nanoparticles prepared in ionic liquid medium (precipitation reaction at 25 ◦ C; bar = 100 nm).
evidence that the powder corresponds to TTF·TCNQ (vide infra). Existence of charge transfer is revealed by XPS of the N 1s line (Fig. 5). The features at 397.5 and 399.0 eV correspond to charged and neutral TCNQ species, respectively, while S 2p features at 163.5 and 164.5 eV arise from neutral and charged TTF species, respectively [24,25]. Both dynamical configurations, neutral and charged, can be observed with XPS because photoemission is an intrinsic rapid process (10−15 s). As precised above, precipitation in the absence of ionic liquid leads to needles of TTF·TCNQ as shown by electron microscopy. In the presence of ionic liquid, electron micrographs evidence nanoparticles (Fig. 6) in addition with ∼5% nanorods (15–40 nm wide, 250–400 nm long). Some nanoparticles exhibit irregular shapes and small agglomerates are also encountered. However, the smaller the particles are, the more spherical their shape. Diameters of TTF·TCNQ nanoparticles are in the 2.1–5.6 nm range (mean diameter: 3.8 nm, Fig. 7). Similar results are obtained when the precipitation reaction is performed at −80 ◦ C. Particles are as small as Prussian Blue ones in [BMIM][BF4 ] [13]; this is remarkable for a system in which only weak intermolecular interactions are responsible for cohesion. Moreover, TTF·TCNQ nanoparticles are smaller than TTF[Ni(dmit)2 ]2 ones. We suggest that, in nuclei of TTF·TCNQ, nitrogen atoms of TCNQ (bearing a partial negative charge) should favourably associate to the cationic imidazolium cycle of the ionic liquid. Thus, the growth
Fig. 7. Histogram showing the size distribution for TTF·TCNQ nanoparticles prepared at 25 ◦ C.
could be rapidly inhibited by the ionic liquid in close contact with the charge transfer complex even at room temperature. The clear presence of ionic liquid within the TTF·TCNQ nanopowder is confirmed by XPS analysis, thus proving its stabilizing role. The powder X-ray diffraction pattern is recorded and compared with the simulation calculated using single crystal data from the literature [26] (Fig. 8). The Bragg angles of the observed reflections agree well with the calculated ones, however the relative intensities of the lines differ somewhat from those calculated, presumably because of some preferential orientation of the crystallites. The indexation of the experimental powder pattern leads to the following cell parameters: a = 1.2311(8); b = 0.3819(2); c = 1.8430(8) nm; ˇ = 104.36(1)◦ which are consistent with those deduced from the single crystal X-ray diffraction study: a = 1.2298(6); b = 0.3819(2); c = 1.8468(8) nm; ˇ = 104.46(4)◦ , the chemical nature of the phase is thus confirmed. Nanocrystalline TTF[Ni(dmit)2 ]2 and TTF·TCNQ exhibit a semiconducting behavior, which is not surprising for powdered materials (Fig. 9). Their activation energies range between 15 and 40 meV. Considering a thickness of ca. 10 m, the conductivity of the TTF·TCNQ and TTF[Ni(dmit)2 ]2 films are estimated in a range between 0.01–10 and 1–20 S cm−1 , respectively. These values are similar to those for thin films of TTF-based charge transfer compounds processed by various wet or dry preparation methods [1,29]. Whatever the compound (TTF[Ni(dmit)2 ]2 or TTF·TCNQ), it has been noticed that the highest conductivities and lowest activation energies are observed for compounds prepared at −80 ◦ C (homogeneous morphology and smaller size). To our knowledge, there is no relevant study concerning the grain size and the electrical conductivity in molecular conductors such as those reported
Fig. 9. Electrical behavior for TTF[Ni(dmit)2 ]2 and TTF·TCNQ nanocrystalline powders.
D. de Caro et al. / Synthetic Metals 160 (2010) 1223–1227
in this work. Therefore, no conclusion can be drawn from this observation. A possible explanation could be the following: the low activation energy and the high conductivity observed for the smallest particles are probably due to the better homogeneity within the volume of the particles which leads to a better quality of contact between particles within the material. 4. Conclusion In summary, we have isolated the first examples of nanoparticles of conducting molecular materials by using an ionic liquid stabilizing medium. We demonstrated the major role of the ionic liquid in confining the nuclei into nanodomains. The formation of the smallest and homogeneously distributed spherical particles is favoured at low temperatures. The chemical nature of the nanoparticles is confirmed by X-ray diffraction and by X-ray photoelectron spectroscopy. The room temperature conductivity of the nanoparticles is the highest for spherical ones and is only one order of magnitude lower than that of single crystal values (300 and 600 S cm−1 for TTF[Ni(dmit)2 ]2 [27] and TTF·TCNQ [28], respectively). Acknowledgments This work was supported by the Centre National de la Recherche Scientifique (France), by the Ministerio de Ciencia y Tecnologia (Spain), through project FIS2006-12117-C04-01, and by the Generalitat de Catalunya (SGR 00909). We thank the LEA 368 (Laboratoire Trans-Pyrénéen: From Molecules to Materials). References [1] J. Fraxedas, Molecular Organic Materials, Cambridge University Press, Cambridge, 2006. [2] E. Gomar-Nadal, J. Puigmartí-Luis, D.B. Amabilino, Chem. Soc. Rev. 37 (2008) 490. [3] L. Valade, H. Casellas, S. Roques, C. Faulmann, D. de Caro, A. Zwick, L. Ariès, J. Solid State Chem. 168 (2002) 438. [4] J.-P. Savy, D. de Caro, C. Faulmann, L. Valade, M. Almeida, T. Koike, H. Fujiwara, T. Sugimoto, J. Fraxedas, T. Ondarc¸uhu, C. Pasquier, New J. Chem. 31 (2007) 519.
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