Article pubs.acs.org/Langmuir
Colloidal Solutions of Organic Conductive Nanoparticles Dominique de Caro,*,†,‡ Matthieu Souque,†,‡ Christophe Faulmann,†,‡ Yannick Coppel,†,‡ Lydie Valade,*,†,‡ Jordi Fraxedas,§,∥ Olivier Vendier,⊥ and Frédéric Courtade# †
CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France Université de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France § ICN2 - Institut Catala de Nanociencia i Nanotecnologia, Campus UAB, 08193, Bellaterra (Barcelona), Spain ∥ CSIC - Consejo Superior de Investigaciones Cientificas, ICN2 Building, 08193 Bellaterra (Barcelona), Spain ⊥ Thalès Alénia Space France, 26 avenue Jean-François Champollion, F-31100 Toulouse, France # CNES, Centre Spatial de Toulouse, 18 avenue Edouard Belin, F-31401 Toulouse Cedex 9, France ‡
S Supporting Information *
ABSTRACT: Although molecular metals have been known for decades, their insolubility, low vapor pressure, and synthesis routes have prevented them from being integrated into electronic devices. We have prepared stable colloidal solutions of the organic metal TTF−TCNQ that overcome such difficulties. The solutions contain well-dispersed nanoparticles stabilized by long alkyl chain amines. They afford soluble powders by evaporation and homogeneous thin films by drop-casting. Powders and films show room temperature conductivities in the 0.01−0.1 S cm−1 range.
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INTRODUCTION The history of molecular conductors was staked by the discovery of the organic metal TTF−TCNQ (TTF = tetrathiafulvalene, TCNQ = tetracyanoquinodimethane; Scheme 1) in 1973, 1 the organic superconductor
production of electronic devices. Neither physical vapor deposition can be widely applied, because molecular conductors exhibit too low vapor pressures, nor spray-coating or inkjet printing, because they are insoluble. Therefore, their preparation as stable and workable solutions was required. Colloidal solutions of conventional metals and metallic oxides can be prepared by adding a stabilizing agent to the reaction medium.17 The stabilizing agent controls the particle growth through coordination to the metal center and is responsible for the solubility of nanoparticles. Conventional metals and metal oxides have three-dimensional structures leading easily to spherical nanoparticles. On the other hand, the 1D packing arrangements of molecular conductors is not favorable to their growth as spheres. Reaction of TTF and TCNQ in solution leads to needle-like microcrystalline TTF− TCNQ powders. Indeed, in the presence of an ionic liquid or a quaternary ammonium salt as stabilizing agent, the reaction affords nanoparticles,7,18 but they are insoluble and rapidly decant. We report here that, by using long chain aliphatic amines as stabilizing agent, soluble TTF−TCNQ nanoparticles are prepared and afford colloidal solutions that can be directly used for film processing or line drawing.
Scheme 1. Molecular Formulas of Tetrathiafulvalene (TTF) and Tetracyanoquinodimethane (TCNQ)
(TMTSF)2PF6 (TMTSF = tetramethyltetraselenafulvalene) in 1980,2,3 the transition metal complex-based superconductor TTF[Ni(dmit)2]2 (dmit = dimercaptoisotrithione or 1,3-dithio2-thione-4,5-dithiolate) in 1986,4 the series of superconductors based on BEDT-TTF salts (BEDT-TTF = bisethylenedithiotetrathiafulvalene) during the 1990s,5 and the single-component conductor Ni(tmdt)2 (tmdt = trimethylenetetrathiafulvalenedithiolate) in 2001.6 These materials are obtained as crystals exhibiting needle-like morphology because of their intrinsic one-dimensional (1D) structural arrangement. Films, nanowires or gels, were successfully grown.7−13 Films, grown on various substrates,14−16 show room-temperature conductivities in the 1−70 S cm−1 range. Metallic behavior and superconductivity were evidenced in electrodeposited films of Ni(tmdt)2 and TTF[Ni(dmit)2]2, respectively.16 However, the applied techniques, double-entry CVD (CVD = chemical vapor deposition) or electrodeposition, are not well suited for the © 2013 American Chemical Society
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EXPERIMENTAL SECTION
Material Synthesis. TTF and n-octylamine were purchased from Sigma-Aldrich, and TCNQ was from Fluka. Colloidal solutions of TTF−TCNQ are typically obtained in 1:1:1 concentration conditions Received: April 18, 2013 Revised: June 13, 2013 Published: June 17, 2013 8983
dx.doi.org/10.1021/la401371c | Langmuir 2013, 29, 8983−8988
Langmuir
Article
Scheme 2. Reaction between n-Octylamine and TCNQ; Formation of Monosubstituted Compounds IIA and IIB and Disubstituted Compound III
Figure 1. Monitoring of reaction by UV−vis spectroscopy. UV−vis spectra of a 0.25 × 10−3 mol L−1 THF solution of TCNQ upon addition of noctylamine at room temperature. of TTF, TCNQ, and n-octylamine. The solution is obtained at room temperature by adding 1 mmol (204 mg) of TTF, 1 mmol (204 mg) of TCNQ, and 1 mmol (160 μL) of n-octylamine to 120 mL of tetrahydrofuran (THF) and stirring for 1 h. Nanoparticle powder can be obtained after evaporation of the THF solvent. The isolated powder is soluble in various solvents: acetone, ethanol, ether, and THF. Characterization Techniques. XPS measurements were performed with a PHOIBOS 150 hemispherical analyzer at the Institut de Tècniques Energètiques of the Politechnical University of Barcelona with a pass energy of 10 eV and using nonmonochromatized Al Kα (1486.6 eV) radiation. AFM images were taken with an Agilent 5500 instrument using tapping mode at ambient conditions. TEM and HRTEM observations have been performed at TEMSCAN (Université Paul Sabatier in Toulouse) on a Jeol JEM-1011 at 100 kV and on a Jeol-JEM-2100F at 200 kV for the high-resolution images. NMR experiments were performed in CD3CN at a temperature of 263 K. 1D and 2D 1H and 13C experiments were recorded on a Bruker Avance 500 spectrometer equipped with a 5 mm triple resonance inverse Zgradient probe (TBI 1H, 31P, BB). All chemical shifts for 1H and 13C are relative to TMS using 1H (residual) or 13C chemical shifts of the solvent as a secondary standard. Elemental analyses were performed by the Microanalysis Service of LCC-CNRS. Infrared spectra were taken
at room temperature on a PerkinElmer Spectrum GX spectrophotometer. UV−vis spectroscopy studies were performed on a PerkinElmer Lambda 35 spectometer. Raman measurements were performed using a LabRAM-HR800 (Jobin Yvon) setup. The spectra are obtained at the temperature of liquid nitrogen using the 632.8 nm line of a He−Ne laser. Powder X-ray diffraction pattern are collected on a XPert Pro (θ−θ mode) Panalytical diffractometer with λ (Cu Kα1, Kα2) = 1.540 59 and 1.544 39 Å. The extraction of peak positions for indexing is performed with the fitting program, available in the PC software package Highscore+ supplied by Panalytical. Pattern indexing is carried out by means of the DICVOL program implemented in the Highscore+ package. The powder X-ray diffraction pattern is compared with the simulated pattern calculated using single crystal data from the TTFTCQ01/IUCr A10935 file. The room-temperature conductivity of the samples was measured on compressed pellets by two-probe contacts using a homemade equipment. Thermal analyses (TGA) were conducted on PerkinElmer diamond equipment.
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RESULTS AND DISCUSSION The growth control of a material as spherical nanoparticles relies upon the efficiency of the interaction between the 8984
dx.doi.org/10.1021/la401371c | Langmuir 2013, 29, 8983−8988
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Article
compound IIA that will be referred to as TCNQ-OA. Upon reaction of TTF with TCNQ and n-octylamine (0.25 × 10−3 mol L−1 each in THF), no precipitation of TTF−TCNQ occurs but a black solution is obtained. A 1:1:1 solution (10−3 mol L−1 in THF) of TTF, TCNQ, and n-octylamine was studied by electrochemistry without supporting electrolyte to avoid formation of additional products. The electrochemical systems are less reversible due to the lack of supporting electrolyte (Figure S8). The half-wave potential of the first oxidation of TTF is 0.53 V vs ECS (0.32 V vs ECS for TTF alone). The half-wave potential of the first reduction of TCNQ species is 0.22 V vs ECS (0.23 V vs ECS for TCNQ alone). The electrochemical properties of TTF and TCNQ in the presence of n-octylamine show electron affinity of TTF and ionization potential of TCNQ in favor of a partial charge transfer between both and therefore the possibility of getting the conductive TTF−TCNQ phase. UV−vis spectroscopy of the solution confirms the presence of TTF and TCNQ-OA species (Figure S9). When the concentration of the solution is increased by evaporation of the solvent (Figure S10), the IR spectrum clearly evidences the presence of TTF−TCNQ within the solution. The νCN region shows three modes: 2203 cm−1 is characteristic of TCNQ0.59− species as in TTF−TCNQ, while 2170 and 2125 cm−1 correspond to the presence of TCNQ-OA species. We then varied the concentration conditions. At low concentration (
