NANO LETTERS
Fabrication of Polymer−Nanoparticle Composite Inverse Opals by a One-Step Electrochemical Co-deposition Process
2004 Vol. 4, No. 1 177-181
Aimin Yu,†,§ Felix Meiser,†,§ Thierry Cassagneau,† and Frank Caruso*,‡ Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany, and Department of Chemical and Biomolecular Engineering, The UniVersity of Melbourne, Victoria 3010, Australia Received October 1, 2003; Revised Manuscript Received November 2, 2003
ABSTRACT We describe a one-step electrochemical co-deposition method to prepare nanoparticle (NP)-containing semiconducting polymer inverse opals with well-defined pore structure. Gold and cadmium telluride NPs were electrodeposited along with pyrrole in the interstitial voids of colloidal crystals of polymer spheres, and following template removal, composite inverse opals were obtained. The optical characteristics (position of the optical stopband) of the resulting composite films can be tuned through variation of the type and concentration of the NPs embedded in the film.
Recently, considerable interest has been focused on the template-assisted assembly of three-dimensional (3D) macroporous materials, such as inverse opals, because of their potential applications in photonics, catalysis, sensing, and separations.1-6 By templating opaline arrays of colloid spheres (typically silica or polystyrene beads), various types of porous materials with precisely controlled pore sizes and highly ordered 3D porous structures have been prepared. In a typical procedure to prepare such materials, first a 3D template is constructed from self-assembled colloid particles. Then, the template is filled with desired materials such as metals,1 metal oxides,2 inorganic semiconductors,3 ceramics,4 or polymers.5 Finally, the template is removed by dissolution or calcination, resulting in macroporous arrays that reflect the inverse structure of the template. Various methods have been used to deposit materials inside the voids of the colloidal crystals for constructing macroporous materials, e.g., liquidphase reactions,1c,5d sol-gel chemistry,2b-2e chemical vapor deposition,3a,3d,4a nanoparticle infiltration,1a,1h,3c and electrochemical deposition.1d-g,3b,3c,5b,5d,6 Among them, electrodeposition of materials in colloidal crystal templates is a relatively simple and attractive method for producing macroporous systems with unique advantages. This technique enables a spatially controlled deposition of material on various conductive surfaces (including nanoarea patterning) by filling the * To whom correspondence should be addressed. Fax: +61 3 8344 4153. E-mail:
[email protected]. † Max Planck Institute. ‡ The University of Melbourne. § Current address: Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia. 10.1021/nl0348443 CCC: $27.50 Published on Web 11/26/2003
© 2004 American Chemical Society
voids “from the bottom up”, and enables a higher volume fraction of solid than that obtained by other methods.7 In addition, electrodeposition also has the advantages of allowing control over the thickness (by controlling the amount of the material deposited) and the structure (by varying the synthesis conditions, e.g., applied voltage) of the resulting materials.6 To date, ordered macroporous materials such as metals,1d-1g semiconductors,3b,3e and conductive polymers5b,5d have been successfully prepared using this method. Since the first report of electrical conductivity in a conjugated polymer in 1977,8 conductive polymers have attracted much interest due to their combined properties of organic polymers and electronic properties of semiconductors. They have been utilized in various areas, including antistatic coatings, sensors, light emitting diodes, batteries, and electrochromic devices.9 Conducting polymers can be prepared via chemical or electrochemical polymerization. The latter is generally preferred because it provides better control of film thickness and morphology. Among the numerous conducting polymers prepared to date, polypyrrole (Ppy) is one of the most extensively investigated. Although the exact mechanism for electrodeposition of Ppy is not fully understood,10 during the process of electrochemical polymerization, negatively charged molecules present in the electrolytic solution such as anions, polymers, and enzymes can be embedded into the positively charged backbone as dopants.11 This provides an attractive route to trap functional molecules within organic polymers during their electrogeneration on electrodes, and offers a new means to fabricate a range of novel composite materials with new properties. For example,
Figure 1. Schematic illustration of the electrochemical codeposition procedure used for the preparation of NP-containing Ppy inverse opals on ITO substrates by using PS colloidal crystals as templates.
films prepared by electrochemical co-deposition of enzymes and conducting polymers on a conductive substrate have been used to fabricate biosensors.11c,11d,12,13 Anions doped in the conducting polymer can cause a change in the refractive index of the polymer and thus can be used to tune the optical properties of the materials.14 Gold (Au) and cadmium telluride (CdTe) nanoparticles (NPs) are entities of choice to design functional materials. When used as building blocks, their organization into diverse structures is essential for their application in catalytic, optical, or electronic devices.15 Various methods, including layerby-layer electrostatic adsorption, sputtering, electrodeposition, and sol-gel processing have been reported for preparing NP-containing films.16 However, there are very few reports on NPs as dopants trapped within conductive polymers by electrochemical co-deposition.17 In our recent work, we reported the preparation of pure Ppy inverse opals,5d creatinine deiminase-doped Ppy inverse opals11d and biotinylated poly(thiophene-co-3-thiophenemethanol) copolymer inverse opals13 for biosensing by using the electrodeposition technique. Here we demonstrate the use of a one-step electrochemical co-deposition method to fabricate NP-containing semiconducting polymer inverse opals by using preformed negatively charged Au and CdTe NPs acting as doping materials in the interstitial volume of an ordered colloidal crystal. The optical characteristics of the resulting composite films are also investigated. Because the type and concentration of the NPs used for co-deposition can be varied, a diverse range of new composite materials with tailored properties (e.g., optical, structural) can be synthesized. Figure 1 illustrates the scheme for the preparation of NPcontaining inverse opals. Polystyrene (PS, Microparticles GmbH, Berlin) colloid particles were first crystallized on an optically transparent indium tin oxide (ITO, Rs ) 10 ( 178
2Ω, Delta Technologies, USA) conductive substrate.18 After passivation (via epoxy gluing) of the remaining uncovered surface, the crystal formed on the ITO substrate (working electrode) was placed in a solution containing 0.25 M pyrrole and 0.7 or 2.0 × 1014 Au NPs mL-1. The electropolymerization was performed under a voltage of 0.8 V (for Au NPs) or 1.0 V (for CdTe NPs) for an appropriate time (typically a total charge of 0.1 C).19 After mechanically removing the glue, the substrate with the colloidal crystal film and PpyNPs in its interstices was exposed to a tetrahydrofuran (THF) solution for 24 h to remove the PS template. The resulting structures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX), and fluorescence spectroscopy. Figure 2A shows a SEM image of 270 nm PS colloidal crystals on an ITO surface. It can be seen that the crystal comprised (111) oriented domains parallel to the substrate. The cleaved edges of the samples reveal the 3D ordered arrays of the colloid particles. Following electropolymerization and removal of the PS template using THF, the films exhibit a dark blue color. SEM images (Figures 2B and 2C) show the Ppy-Au inverse opals obtained from templating 270 nm PS colloidal crystals. It can be seen that the pores are highly ordered in a hexagonal array, consistent with a (111) plane arrangement of a fcc structure that mimics the orientation of the original latex templates (Figure 2B). The higher magnification SEM image (Figure 2C) clearly reveals that the pores are interconnected with small channels. These channels form because the electrochemical deposition is unable to completely fill in the regions of contact points between the spheres in the template. A previous study on the electropolymerization of Ppy into sintered silica opals has demonstrated that the size of these channels can be tuned via changing the applied voltage.5b As calculated from Figure 2C, the wall thickness is 45 ( 7 nm. The center-to-center distance between pores in the Ppy-Au inverse opal is 260 ( 10 nm, corresponding to a shrinkage of 4% with respect to the initial 270 nm colloidal crystal. This shrinkage is smaller than that observed for inverse opals of pure Ppy (13%)5d prepared by electropolymerization and inverse opals prepared by the sol-gel method (25∼30%).2b,2c We also prepared a Ppy-Au inverse opal from a 925 nm PS colloidal crystal template (SEM image not shown). Due to the larger particle size used, the center-to-center distance (910 ( 10 nm) of the resulting inverse opal is much larger than that obtained from 270 nm PS colloidal crystal templates. However, it is interesting to note that despite the increase in the size of the pores, the wall thickness of the inverse opal remains essentially constant (46 ( 5 nm). This may be due to the same deposition conditions (including concentration of pyrrole and NPs, applied voltage etc.) used in the electropolymerization process for the different template sizes. TEM images were taken to verify doping of the Ppy inverse opal with Au nanoparticles. The inset of Figure 2C shows well-separated Au NPs in the film. Energy-dispersive X-ray (EDX) analysis (data not shown) of the Au NP-doped inverse opals confirmed the presence of Au NPs. The amount Nano Lett., Vol. 4, No. 1, 2004
Figure 3. Fluorescence spectra of a CdTe NP dispersion (solid spectrum) and a Ppy-CdTe inverse opal (dotted spectrum). The excitation wavelength used was 400 nm.
Figure 2. SEM image of a colloidal crystal of 270 nm PS spheres (A). SEM images of Au NP-containing Ppy inverse opals prepared from colloidal crystals of 270 nm PS spheres (B, C). The inset in C is a TEM image of a Au NP-containing Ppy inverse opal (scale bar represents 10 nm).
of gold in the composite film was measured via thermogravimetric analysis (TGA).20 TGA results showed that for the inverse opals prepared from a solution containing 0.7 × 1014 Au NPs mL-1, the weight percentage of Au NPs is ∼18%, while for inverse opals prepared from a solution with Nano Lett., Vol. 4, No. 1, 2004
2.0 × 1014 Au NPs mL-1, the weight percentage of Au NPs increased to ∼28%. We also prepared Ppy inverse opals containing CdTe NPs.21 Fluorescence spectroscopy was used to confirm the incorporation of CdTe NPs in the film. Figure 3 shows the fluorescence spectra of CdTe NPs dispersed in solution (solid line) and a CdTe NP-containing Ppy inverse opal film (dotted line). Both the CdTe NP dispersion and the inverse opal film show strong emission around 650 nm. The fluorescence spectrum of the inverse opal film shows a red-shift of ca. 10 nm compared with that for the CdTe NPs dispersed in solution, which may be due to the increase in refractive index of the surrounding matrix22 and partial particle aggregation of the CdTe NPs in the Ppy film. EDX analysis (data not shown) further confirmed the presence of CdTe NPs in the inverse opal film. Reflectance spectra of the inverse opal films were measured to monitor the influence of the nanoparticles on their optical properties.23 Figure 4 shows the reflectance spectra of a 270 nm PS colloidal crystal (a), a Ppy inverse opal (b), and the NP-containing inverse opals (c-e). The peaks in the reflectance spectra correspond to Bragg reflections arising from the ordered structures.24 The reflectance spectrum of the 270 nm PS crystal exhibits a strong reflectance peak with an optical stopband centered at 556 nm. Upon filling the interstices of the PS colloidal crystal with Ppy (formed in 0.25 M pyrrole and 1.0 M KCl at an applied voltage of 0.8 V) and removal of the PS cores, the optical stopband shifts to 376 nm (spectrum b); that is, a blue-shift of about 180 nm, compared with the PS colloidal crystal template, is observed. The blue-shift of the stopband for the Ppy inverse opal is due to the smaller lattice parameter of the Ppy inverse opal (i.e., the center-to-center distance decreases from 270 nm for the PS crystal to ∼240 nm), as well as the lower refractive index of the Ppy inverse opal compared with that of PS colloidal crystals (i.e., 1.14 vs 1.4625). The relative broadness of the reflectance spectra of the Ppy inverse opal is likely due to the lower crystallinity of the inverse opal, compared with the PS opal template. 179
pore structure by using preformed nanoparticles, electropolymerization, and colloidal crystal templating. This method is attractive in that it allows tuning of the optical properties of the resulting inverse opals (the position of the optical stopband) through variation of the type and concentration of the NPs used. In addition, the strategy of doping the conductive polymer with negatively charged NPs by electropolymerization is expected to be readily adaptable to a host for other NPs. Thus it presents new opportunities to fabricate a range of novel, advanced composite inverse opals for potential applications in the optical, electronic, sensing, and catalytic fields.
Figure 4. Reflectance spectra of a colloidal crystal of 270 nm PS spheres (a), a Ppy inverse opal (b), a Ppy-Au inverse opal prepared with pyrrole solutions containing 0.7 × 1014 Au NPs mL-1 (c) or 3.0 × 1014 Au NPs mL-1 (d), and a Ppy-CdTe inverse opal (e). Reflectance spectra were collected at an incidence angle of 20° to the (111) crystal plane.
Spectrum c in Figure 4 shows the reflectance spectrum of a Au NP-containing Ppy inverse opal (formed from 0.25 M pyrrole and 0.7 × 1014 Au NPs mL-1 at an applied voltage of 0.8 V). The stopband shifts to 418 nm, corresponding to a 42 nm red-shift (as compared with the pure Ppy inverse opal). This red-shift may be due to the increase of the centerto-center distance in the Ppy-Au NP inverse opals (as observed from SEM, 260 nm vs 240 nm) and a higher refractive index as a result of doping with Au NPs (ca. 1.658 around 400 nm26). The influence of the Au NPs on the optical properties of the resulting film was further proved by using a higher concentration of Au NPs in the electropolymerization process. Spectrum d (Figure 4) shows the reflectance spectrum of a Ppy-Au inverse opal prepared using a solution of 2.0 × 1014 Au NPs mL-1 instead of 0.7 × 1014 Au NPs mL-1. The stop band for this inverse opal further red-shifted to 432 nm, reflecting the higher loading of Au NPs in the film. Although we observed a broadened and weak absorption peak centered at ca. 570 nm for the Au NP-doped inverse opal film in absorption mode (data not shown), the absence of such a peak in the reflectance spectra (spectra c and d in Figure 4) may be due to the relatively low concentration (and aggregation) of the Au NPs in the film. The CdTe NP containing Ppy inverse opal (formed from 0.25 M pyrrole and a 3.0 × 1014 CdTe NPs mL-1 solution at an applied voltage of 1.0 V) (spectrum e, Figure 4) shows a stopband centered at 390 nm, corresponding to a 14 nm red-shift compared with the peak position of the pure Ppy inverse opal. In comparison with the spectra of the AuNP-containing inverse opals, the CdTe-Ppy inverse opal shows another reflectance band centered at 650 nm, which is most likely associated with the absorption edge of CdTe NPs in the film. In summary, we have demonstrated a facile route for preparing composite inverse opals incorporating metal (Au) and semiconductor (CdTe) nanoparticles with well-defined 180
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(19)
(20) (21) (22) (23) (24) (25)
(26) (27) (28)
of liquid. Colloidal crystals were obtained by adding 100 µL of the PS dispersion (0.5 wt.-%) into the chamber on ITO glass and allowing the colloids to crystallize by gravity sedimentation and solvent evaporation. Negatively charged Au NPs were capped with triphenylphosphine3,3,3-trisulfonic acid (TPP) (6 ( 1 nm in size, ca. 1 × 1015 particles mL-1). Electrochemical measurements were performed with a threeelectrode system comprising a platinum foil as the auxiliary electrode, a Ag/AgCl (3 M KCl) electrode as reference electrode, and the colloidal crystal-coated electrode as the working electrode. The electrodes were connected to an Autolab PGSTAT30 electrochemical instrument (Netherlands). A Netzsch TG 209 apparatus was used for thermogravimetric analysis of the films, which were heated between 25 and 800 °C in O2. Thioglycolic acid capped CdTe NPs (5∼6 nm in diameter, ca. 3 × 1015 NPs mL-1) were synthesized according to a literature method described elsewhere.27 Schmitt, J.; Ma¨chtle, P.; Eck, D.; Mo¨hwald, H.; Helm, C. A. Langmuir 1999, 15, 3256. The reflectance spectra were recorded on a Cary 500 spectrophotometer and were taken at an angle of incidence of 20° to the (111) crystal plane. Tarhan, I. I.; Watson, G. H. Phys. ReV. B 1996, 54, 7593. The refractive indices of the PS colloidal crystals (1.46) and Ppy inverse opals (1.14) were calculated using the following equation for multicomposite materials28: n2composite ) Σ n2i Vi. For calculation, we assumed that the pure PS crystals arrange in an ideal fcc array (the packing density of PS beads, i.e., volume ratio is ∼0.74, air ∼0.26) with a refractive index of PS particles of 1.59 and 1.50 for Ppy. Lynch, D. W.; Hunter, W. R. In Handbook of optical constants of solids; Palik, E. D., Ed.; Academic Press: Washington D.C., 1985. Rogach, A. L.; Katsikas, L.; Kornowski, A.; Su, D.; Eychmuller, A.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1772. Yamasaki, T.; Tsutsui, T. Appl. Phys. Lett. 1998, 72, 1957.
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