Silica Colloidal

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© Copyright 2003 American Chemical Society

MAY 27, 2003 VOLUME 19, NUMBER 11

Letters Synthesis of Poly(3,4-ethylenedioxythiophene)/Silica Colloidal Nanocomposites Moon Gyu Han and Steven P. Armes* School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, East Sussex BN1 9QJ, U.K. Received September 3, 2002. In Final Form: March 19, 2003 The synthesis of colloidally stable nanocomposites comprising poly(3,4-ethylenedioxythiophene) (PEDOT) and a 20 nm diameter silica sol is described for the first time. Raspberry-shaped PEDOT-silica nanocomposites of submicrometer dimensions were obtained, with silica contents ranging from 19% to 80% by mass and with electrical conductivities as high as 0.2 S cm-1.

Introduction Poly(3,4-ethylenedioxythiophene) (PEDOT) is an electrically conductive polymer with excellent long-term stability and relatively high transparency.1,2 Over the past decade, PEDOT has been widely used by various groups as a hole injection layer in the manufacture of polymer LED devices.3,4 However, its efficient chemical synthesis in aqueous solution is somewhat problematic due to the relatively low solubility of the EDOT monomer. Unlike polypyrrole5 and polyaniline,6 it has proved relatively * To whom correspondence should be addressed. E-mail: [email protected]. (1) Heywang, G.; Jonas F. Adv. Mater. 1992, 4, 116. (2) Pei, Q.; Zuccarello, G.; Ahlskog, M.; Inganas, O. Polymer 1994, 35, 1347. (3) (a) Kim, J. S.; Granstrom, M.; Friend, R. H.; Johansson, N.; Salaneck, W. R.; Daik, R.; Feast, W. J.; Cacialli, F. J. Appl. Phys. 1998, 84, 6859. (b) De Jong, M. P.; Van Ijzendoorn, L. J.; De Voigt, M. J. A. Appl. Phys. Lett. 2000, 77, 2255. (4) We are, of course, aware of the important work by the Bayer group on the development of a water-soluble formulation for PEDOT, which is based on polyelectrolyte complex formation with poly(sodium 4-styrenesulfonate). In contrast, the present manuscript is focused on particulate colloidal forms of PEDOT. (5) (a) Bjorklund, R. B.; Liedberg, B. J. Chem. Soc., Chem. Commun. 1986, 1293. (b) Armes, S. P.; Vincent, B. J. Chem. Soc., Chem. Commun. 1987, 288. (c) Simmons, M. R.; Chaloner, P. A.; Armes, S. P. Langmuir 1995, 11, 4222. (d) DeArmitt, C.; Armes, S. P. Langmuir 1993, 9, 652. (6) (a) Armes, S. P.; Aldissi, M. J. Chem. Soc., Chem. Commun. 1989, 88. (b) DeArmitt, C.; Armes, S. P. J. Colloid Interface Sci. 1992, 150, 134.

difficult to prepare PEDOT in the form of colloidal dispersions, which is a useful approach for alleviating the otherwise poor processability exhibited by these intractable materials. We have succeeded in coating micrometersized polystyrene latex particles with ultrathin PEDOT overlayers,7 but we are aware of only one report of the preparation of PEDOT particles of submicrometer dimensions.8 This involved the polymerization of EDOT in dilute aqueous solution in the presence of a short-chain alcohol ethoxylate surfactant and a large excess of ammonium persulfate (APS) oxidant. The APS apparently oxidizes the surfactant micelles, generating an unspecified species in situ that leads to PEDOT particles of 150-200 nm diameter. Over the last 10 years we have developed an alternative approach to the synthesis of colloidal dispersions of conducting polymers that involves the use of an ultrafine aqueous silica sol in order to prepare nanocomposite particles.9 Both polyaniline-silica and polypyrrolesilica nanocomposites have been synthesized using this route, but our preliminary attempts to prepare PEDOTsilica nanocomposites using the same aqueous silica sol were disappointing.10 Recently we have shown that the use of an ultrafine methanolic silica sol in place of the (7) (a) Khan, M. A.; Armes, S. P. Langmuir 1999, 15, 3469. (b) Khan, M. A.; Armes, S. P.; Perruchot, C.; Ouamara, H.; Chehimi, M. M.; Greaves, S. J.; Watts, J. F. Langmuir 2000, 16, 4171. (8) Henderson, A. M. J.; Saunders, J. M.; Mrkic, J.; Kent, P.; Gore, J.; Saunders, B. R. J. Mater. Chem. 2001, 11, 3037.

10.1021/la020753u CCC: $25.00 © 2003 American Chemical Society Published on Web 05/01/2003

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Figure 1. Schematic representation of the formation of PEDOT-silica nanocomposites by the oxidative polymerization of 3,4ethylenedioxythiophene (EDOT) in the presence of an ultrafine methanolic silica sol (MA-ST-M; donated by Nissan Chemicals, Japan).

aqueous silica sol offers significant advantages for the synthesis of poly(methyl methacrylate)-silica nanocomposite particles.11 Herein we show that this protocol also allows the efficient synthesis of colloidally stable PEDOT-silica nanocomposite particles; see Figure 1. In a typical synthesis, the PEDOT-silica nanocomposites were prepared as follows: 2.50 g methanolic silica sol (MA-ST-M; provided by Nissan Chemicals, Japan as a 40% w/w methanolic dispersion, thus 2.50 g of solution contains 1.0 g of dry weight silica; the manufacturer’s nominal particle diameter is 22 nm) was added to a water/ methanol mixture so as to obtain the desired composition (varying quantities of 4-toluenesulfonic acid (TSA) were also added to most of the syntheses at this stage). Then, 1.67 g of ammonium persulfate (APS) was added to this solution (total solvent volume ) 100 mL), which was stirred for 1 h at room temperature. Finally, 1.00 g of 3,4-ethylenedioxythiophene (EDOT; APS/EDOT molar ratio ) 1.00) was injected via syringe into this stirred solution, and the polymerization was allowed to proceed for 24 h. The supernatants were carefully decanted and discarded, and the resulting dark blue sediments were redispersed in a 1:1 methanol/water mixture. In subsequent centrifugation/redispersion cycles (three to four cycles were employed overall) the aqueous methanol supernatants were replaced with deionized, doubly distilled water. This cleanup procedure was repeated until colorless supernatants were obtained so as to ensure complete removal of the excess, nonaggregated silica sol and any (in)organic byproducts (unreacted monomer, excess oxidant, etc.). In preliminary PEDOT bulk powder syntheses prepared in the absence of any silica sol, we found that the addition of TSA led to significant improvements in both the yield and the conductivity of the isolated PEDOT. In the absence of TSA, a 98:2 water/methanol mixture yielded only 60 mg of PEDOT with a conductivity of 2.2 × 10-2 S cm-1. In contrast, addition of 0.05 M TSA under otherwise (9) (a) Maeda, S.; Armes, S. P. J. Colloid Interface Sci. 1993, 159, 257. (b) Maeda, S.; Armes, S. P. J. Mater. Chem. 1994, 4, 935. (c) Maeda, S.; Gill, M. T.; Armes, S. P.; Fletcher, I. W. Langmuir 1995, 11, 1899. (d) Gill, M.; Mykytiuk, J.; Armes, S. P.; Edwards, J. L.; Yeates, T.; Moreland, P. J.; Mollett, C. J. Chem. Soc., Chem. Commun. 1992, 189. (e) Terrill, N. J.; Crowley, T.; Gill, M.; Armes, S. P. Langmuir 1993, 9, 2093. (f) Stejskal, J.; Kratochvil, P.; Armes, S. P.; Lascelles, S. F.; Riede, A.; Helmstadt, M.; Prokes, J.; Krivka, I. Macromolecules 1996, 29, 6814. (10) Corradi, R. DPhil Thesis, University of Sussex, U.K., 1998. (11) Percy, M. J.; Armes, S. P. Langmuir 2002, 18, 4562.

identical conditions gave a PEDOT yield of 520 mg and its conductivity was approximately 5 S cm-1. The use of TSA led to a dramatic increase in the aqueous solubility of the EDOT monomer, which most likely becomes protonated by this strong acid. This leads to faster rates of polymerization and hence higher yields of PEDOT. In addition, the TSA is presumably incorporated as a dopant anion in preference to the SO42- or HSO4- anions derived from the APS oxidant, which would account for the higher conductivities obtained from the syntheses involving TSA. Larger amounts of methanol led to reduced yields, and excess APS oxidant (APS/EDOT molar ratio g2.0) produced overoxidized PEDOT of inferior quality and reduced conductivity. These studies were used to optimize the reaction conditions employed for the subsequent nanocomposite syntheses. A summary of the synthesis conditions, silica contents, particle densities, weight-average particle diameters, and pressed pellet conductivities of various PEDOT-silica nanocomposites is presented in Table 1. Several parameters were varied in these syntheses, including the water/ methanol composition, the use of TSA, and the silica sol concentration. As for the bulk powder syntheses above, the addition of TSA led to faster rates of polymerization and hence higher yields of nanocomposite particles. Higher conductivities and larger nanocomposite particles were also obtained in the presence of TSA (compare entries 1 and 3). Higher silica sol concentrations led to nanocomposites with greater silica contents, higher particle densities, and better colloidal stabilities (compare entries 2-5). Increasing the proportion of methanol led to a minimum of 19% in the silica content (and hence also particle density) at 30 vol % methanol but relatively little change in the nanocomposite particle size and electrical conductivity (compare entries 3 and 6-10). We have no satisfactory explanation for the surprising observation of an apparent minimum in silica content at the present time. Increasing the TSA concentration from 0.05 to 0.20 M led to lower silica contents, larger nanocomposite particles, but little further increase in conductivity (compare entries 7, 11, and 12). A representative transmission electron micrograph of the PEDOT-silica nanocomposite particles prepared using the methanolic silica sol in the presence of 20 vol % methanol (entry 7 in Table 1) is shown in Figure 2a. The distinctive “raspberry” particle morphology is evident, and the number-average particle diameter is around 180

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Table 1. Summary of the Synthesis Parameters, Silica Contents, Weight-Average Particle Diameters, Particle Densities, and Conductivities of PEDOT-Silica Nanocomposites Prepared in Various Methanol/Water Mixtures Using the MA-ST-M Methanolic Silica Sol and APS Oxidant and 4-Toluenesulfonic Acid (TSA)a entry no.

H2O/MeOH composition (v/v)

initial silica amt (g)

TSA concn (mol dm-3)

colloid formation?

silica contentb (wt %)

particle densityc (g cm-3)

wt av particle diameter (nm)d

pressed pellet conductivity (S cm-1)

1 2 3 4 5 6 7 8 9 10 11 12

98/2 98/2 98/2 98/2 98/2 90/10 80/20 70/30 60/40 50/50 80/20 80/20

1.0 0.5 1.0 2.0 3.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

nil 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.10 0.20

yes no partially yes yes yes yes yes yes yes yes yes

80 47 59 78 80 40 39 19 29 37 32 30

2.04 1.82 1.94 2.02 1.99 1.80 1.81 1.66 1.72 1.78 1.72 1.73

95 ( 60

3.1 × 10-4 3.6 × 100 1.3 × 100 2.0 × 10-1 5.2 × 10-2 1.5 × 10-1 2.6 × 10-2 1.5 × 10-2 2.4 × 10-3 6.3 × 10-2 1.4 × 10-2 1.6 × 10-1

750 ( 290 150 ( 60 230 ( 90 230 ( 90 240 ( 110 280 ( 80 290 ( 100 505 ( 130 360 ( 130 510 ( 180

a In each case 1.00 g of EDOT monomer was used and the APS/EDOT molar ratio was 1.00. b Determined by thermogravimetry. c Measured by helium pycnometry (the densities of PEDOT bulk powder and the ultrafine silica sol were 1.60 and 2.15 g cm-3, respectively). d Determined by disk centrifuge photosedimentometry.

Figure 2. (a) Typical transmission electron micrograph of PEDOT-silica nanocomposite particles (entry no. 7 in Table 1) prepared using the methanolic silica sol. (b) Scanning electron micrograph of PEDOT bulk powder prepared under similar conditions in the absence of any silica sol. Note the distinctive “raspberry” particle morphology of the well-separated nanoparticles obtained in the former case.

nm. The silica content of this nanocomposite was around 39% by mass as judged by thermogravimetry. The apparent interparticle aggregation is merely an artifact of the TEM sample preparation protocol: particle size analysis of the same nanocomposite using a disk centrifuge (Brookhaven instrument) indicated a weight-average particle diameter of approximately 240 ( 110 nm. For comparison, a scanning electron micrograph of PEDOT bulk powder prepared under similar conditions but in the absence of any silica sol is shown in Figure 2b. The dimensions of the pseudospherical, globular features of this precipitated PEDOT are superficially similar to the PEDOT-silica nanocomposite particles shown in Figure 2b, but there is no evidence for any raspberry morphology. Selected PEDOT-silica nanocomposites were subjected to a HF etch treatment in order to dissolve the silica component, as described previously.12

Subsequent transmission electron microscopy (TEM) examination (not shown) revealed etched particles that were slightly smaller than the original nanocomposite particles. Thermogravimetric analyses confirmed that all of the original silica had been removed by the HF treatment and close inspection of the TEM images showed that the remaining PEDOT particles contained light spherical domains that corresponded to the original ultrafine silica sol. Thus these HF experiments provide quite good evidence for a predominantly PEDOT core-silica shell nanomorphology. Similar results were obtained with HF-treated polypyrrole-silica and polyaniline-silica nanocomposites. The aqueous electrophoresis data obtained for one of the PEDOT-silica nanocomposites (entry 12 in Table 1) (12) Han, M. G.; Armes, S. P. J. Colloid Interface Sci., in press.

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Figure 3. Zeta potential vs pH curves for the original ultrafine silica sol (9), pure PEDOT powder (b), and a PEDOT/silica nanocomposite (entry 12 in Table 1) ([).

are presented in Figure 3. The zeta potential curve for this nanocomposite, which has a silica content of 30 wt %, exhibits no isoelectric point and is quite similar to the electrophoretic data obtained for the original ultrafine silica sol. As a comparison, the electrophoretic data obtained for one of the PEDOT bulk powders is also presented: this whole curve is shifted to smaller negative zeta potentials, regardless of the solution pH. Overall, the zeta potential data suggest that the surface composition of this nanocomposite is silica rich, which is consistent with the HF etching experiments described earlier and also the X-ray photoelectron spectroscopy (XPS) data (see below). The surface compositions of selected PEDOT-silica nanocomposite dispersions were assessed using XPS. This technique is highly surface specific, with a typical sampling depth of around 5-10 nm. The unique elemental marker approach previously reported9c was employed to quantify the relative proportions of the inorganic and organic components: the silicon signal acted as a marker for the

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silica sol and the sulfur signal was taken to be diagnostic of the PEDOT. Bulk Si/S atomic ratios were calculated for four nanocomposites, and these data were compared to the surface Si/S atomic ratios determined directly using XPS. The surface Si/S atomic ratios were in the range of 29-47, whereas the corresponding bulk atomic ratios were in the range of 1.0-3.3. This confirmed that each of these nanocomposites has a highly silica-rich surface composition, which is consistent with their good longterm colloidal stability and suggests an electrostatic stabilization mechanism.13 Similar XPS results were reported by Maeda and co-workers for both polypyrrolesilica and polyaniline-silica nanocomposites.9c In summary, using a methanolic ultrafine silica sol in combination with 4-toluenesulfonic acid facilitates the synthesis of colloidally stable, electrically conductive PEDOT-silica nanocomposites. This is the first convenient synthesis of colloidal dispersions of this technologically important conducting polymer. The mean nanocomposite particle diameter could be varied from 150 to 510 nm and the silica content ranged from 19% to 80% by mass. Four-point probe measurements on pressed pellets indicated conductivities as high as 0.2 S cm-1. Acknowledgment. This work was supported by the Postdoctoral Fellowship Program of the Korean Science and Engineering Foundation (KOSEF). Nissan Chemicals, Japan are thanked for the donation of the methanolic ultrafine silica sol. Prof. J. F. Watts’s group at the University of Surrey, U.K., are thanked for their assistance with the XPS analyses. LA020753U (13) Healey, T. M. Stability of Aqueous Silica Sols. In The Colloid Chemistry of Silica; Bergna, H. E., Ed.; Advances in Chemistry Series 234; American Chemical Society: Washington, DC, 1994.