Preparation, Optical Spectroscopy, and Electrochemical Studies of Novel

Studies of Novel π-Conjugated Polymer-Protected Stable. PbS Colloidal ... stable colloidal form were of cubic rock-salt structure with narrow size di...
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Langmuir 2002, 18, 5287-5292

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Preparation, Optical Spectroscopy, and Electrochemical Studies of Novel π-Conjugated Polymer-Protected Stable PbS Colloidal Nanoparticles in a Nonaqueous Solution Yong Zhou, Hideaki Itoh, Takashi Uemura, Kensuke Naka,* and Yoshiki Chujo* Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan Received November 5, 2001. In Final Form: April 15, 2002 A stable colloidal form of novel π-conjugated polydithiafulvene (PDF)-protected PbS nanoparticles was first prepared in a nonaqueous solution at room temperature. The results of X-ray diffraction (XRD) and transmission electron microscopy (TEM) images demonstrated that the synthesized PbS nanoparticles of stable colloidal form were of cubic rock-salt structure with narrow size distribution. The average diameters of PDF-protected PbS nanoparticles were controllable by varying the initial concentrations of the lead and sulfur sources in the reaction systems. The absorption spectra and electrochemical properties of the protected PbS nanoparticles have been investigated. It was found that the PDF influenced strongly the excitonic absorption peaks of the protected PbS nanoparticles.

Introduction Recently, nanocomposites of π-conjugated polymers and inorganic semiconductor nanoparticles are particularly interesting materials in the study of electrical transports.1 Modulation of different physical properties of poly(pphenylene vinylene) (PPV) by incorporation of n-TiO2 nanoparticles into the polymer has been first reported by Baraton et al.2 Charge separation at the interface between organic molecules and nanocrystals is currently of great interest particularly since the report by O’Regan and Gra¨tzel on efficient photovoltaic devices based on organic dyes adsorbed on a TiO2 nanocrystalline film.3 Inorganic semiconductor clusters are potentially useful classes of polymer photosensitizers in many applications.4-7 As electron acceptors, the dispersion of inorganic semiconductor nanoparticles in conjugated polymers is energetically favorable for separation of the photogenerated carriers, and achieving the high quantum efficiency for charge separation approaches unity because of the fact that the forward electron-transfer process occurs in the subpicosecond time domain, faster than the radiative and nonradiative decays of the singlet exciton.6 Until now, the largest focus on the nanocomposite consisting of π-conjugated polymers and semiconductor nanoparticles is mainly the incorporation of inorganic semiconductor nanoparticles in conjugated polymer solid matrixes either by some suitable chemical routes or by an electrochemical incorporation technique. However, to the best of our knowledge, there is no report on the stable colloidal form of π-conjugated polymer-protected semiconductor nano(1) Artemyev, M. V.; Sperling, V.; Woggon, U. J. Appl. Phys. 1997, 81, 6975. (2) Baraton, M. I.; Merhari, L.; Wang, J.; Gonsalves, K. E. Nanotechnology 1998, 9, 356. (3) O’Regan, G.; Gra¨tzel, M. Nature 1991, 353, 737. (4) Winiarz, J. G.; Zhang, L. M.; Lal, M.; Friend, C. S.; Paras, P. N. J. Am. Chem. Soc. 1999, 121, 5287. (5) Cassagneau, T.; Mallouk, T. E.; Fendler, J. H. J. Am. Chem. Soc. 1998, 120, 7848. (6) (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (c) Greenham, N. C.; Peng, X. G.; Alivisatos, A. P. Phys. Rev. B 1996, 54, 17628. (d) Colvin, V. L.; Alivisatos, A. P. J. Chem. Phys. 1992, 97, 730. (7) Heeger, P. S.; Heeger, A. J. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12219.

particles, since these conjugated polymers are generally insoluble in most common solvents.8 Nanocrystalline PbS is a semiconductor that has wide application, and a number of studies related to the synthesis and optical characterization of this material have been reported in the literatures.9-22 PbS (galena) is the major ore mineral of lead and an important semiconductor material with a rather small bulk band gap (0.41 eV at 300 K) and a larger exciton Bohr radius of 18 nm.11 PbS has been used in several applications, for example, Pb2+ ion-selective sensors,12 photography,13 an IR detector,14 and a solar absorber.15 PbS nanoparticles have been found to have exceptional third-order nonlinear optical properties that may be useful in optical devices such as an optical switch.16 It was predicted that the nonlinearities of PbS with a given particle size will be 30 times as large as those of GaAs and 3 orders of magnitude larger than those of CdS.17 Many methods have been developed for the preparation of PbS nanoparticles. The general techniques employed were based on traditional precipitation techniques in aqueous media or crystallization in structures such as micelles,18 amphiphilic block (8) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (9) Arango, A. C.; Carter, S. A.; Brock, P. J. Appl. Phys. Lett. 1999, 74, 1698. (10) Wang, Y.; Herron, N. Chem. Phys. Lett. 1992, 200, 71. (11) Machol, J. L.; Wise, F. W.; Patel, R. C.; Tanner, D. B. Phys. Rev. B 1993, 48, 2819. (12) Hirata, H.; Higashiyama, K. Bull. Chem. Soc. Jpn. 1971, 44, 2420. (13) Nair, P. K.; Gomezdaza, O.; Nair, M. T. S. Adv. Mater. Opt. Electron. 1992, 1, 139. (14) Gadenne, P.; Yagil, Y.; Deutscher, G. J. Appl. Phys. 1989, 66, 3019. (15) Chaudhuri, T. K.; Chatterjes, S. Proc. Int. Conf. Thermoelectr. 1992, 11, 40. (16) Kane, R. S.; Cohen, R. E.; Silbey, R. J. Phys. Chem. 1996, 100, 7928. (17) (a) Banyai, L.; Hu, Y. Z.; Lindberg, M.; Koch, S. W. Phys. Rev. B 1988, 38, 8142. (b) Olshavasky, M. A.; Goldstein, A. N.; Alivisators, A. P. J. Am. Chem. Soc. 1990, 112, 9438. (18) Fendler, J. H. Chem. Rev. 1987, 87, 877. (19) Forster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195. (20) Mukherjee, M.; Datta, A.; Chakravorty, D. Appl. Phys. Lett. 1994, 64, 1159. (21) Trindade, T.; O’Brien, P.; Zhang, X. M.; Motevalli, M. J. Mater. Chem. 1997, 7, 1011. (22) Trindade, T.; O’Brien, P. Adv. Mater. 1996, 8, 161.

10.1021/la011642i CCC: $22.00 © 2002 American Chemical Society Published on Web 06/01/2002

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copolymers,19 and zeolites.20 However, most of the methods were involved the treatment of poisonous H2S gas. Recently, O’Brien et al. reported the synthesis of PbS nanoparticles using the thermolysis of single molecule precursors.21,22 In that case, a higher temperature (100150 °C) was required for the thermolysis and the synthetic process of the single molecule precursor was complicated. Thus, we used thioacetamide (TAA) as the sulfur source, which can decompose into S2- slowly in dimethyl sulfoxide (DMSO) solvent at room temperature.23 In this paper, we report the first example of a stable colloidal form of novel π-conjugated polymer-protected PbS nanoparticles with narrow size distribution at room temperature in a nonaqueous solution. The π-conjugated polymer used, polydithiafulvene (PDF), has a dithiafulvene (DF) unit in the main chain, which was recently synthesized by our group.24 The PDF can stabilize the formed PbS nanoparticle in a stable form. The absorption spectrum and electrochemical properties of the π-conjugated polymer-protected PbS nanoparticle have been investigated. Experimental Section Chemical Reagents. All the chemicals were provided by Wako Pure Chemical Industries, Ltd., Osaka, Japan. Unless stated otherwise, all reagents and chemicals in this study were reagent grade without further purification. Pb(OAc)2 and thioacetamide (TAA) were used as the Pb source and S source, respectively. The π-conjugated poly(dithiafulvene) (PDF) was synthesized by the cycloaddition polymerization of bis(aldothioketene) derived from 1,4-diethynylbenzene, which has been reported in detail in our previous paper.24 The number-average molecular weight (Mn) of the PDF used here was 5440, determined by 1H NMR. Preparation Procedures. In a typical preparation of the π-conjugated PDF-protected PbS nanoparticle (sample 1), Pb(OAc)2‚3H2O (3.79 mg, 0.01 mmol) was dissolved into 5 mL of a dimethyl sulfoxide (DMSO) solution of PDF (10 mg, 0.0526 mmol by repeating unit). To the above solution was added slowly dropwise (1.0 mL/s) 5 mL of a DMSO solution of TAA (0.75 mmg, 0.01 mmol) under ambient conditions. Under vigorous stirring at 2000 rpm for 6 h, the resulting solution [1.0 mM Pb(OAc)2, 1.0 mM TAA, and 1 mg/mL PDF] changed gradually from yellow to brown red and transparent with no evidence of precipitation, indicating the formation of PbS nanoparticles. Analogous mixture solutions with varied initial Pb2+ and TAA concentrations [2.0 mM Pb(OAc)2, 2.0 mM TAA ,and 1 mg/mL PDF, sample 2; and 3.0 mM Pb(OAc)2, 3.0 mM TAA, and 1 mg/mL PDF, sample 3] were also stirred for 6 h under ambient conditions. Characterization. The X-ray powder diffraction (XRD) pattern for the resulting PbS nanoparticle was determined at the scanning rate 0.02 deg s-1 in 2θ ranging from 10° to 70°, using a Shimadzu X-ray diffractometer-6000 with high-intensity Cu KR radiation (λ ) 0.151 478 nm). The sample powder for the XRD measurement was prepared with addition of vast amounts methanol to the DMSO solution of the PDF-protected PbS nanoparticle. The precipitate thus obtained was washed with distilled water and methanol several times and dried in vacuo at 60 °C for 12 h. Transmission electron microscopy (TEM) was performed using a JEOL JEM-100SX operated at an accelerating voltage of 100 kV. Several drops of the DMSO solution of the sample measured were placed for 2 min on a 200 mesh copper grid covered with a carbon film. The excess of the solution was removed with filter paper. The grid was dried in vacuo at 60 °C for 2 h. The size distribution histogram of the PbS nanoparticles was obtained by averaging the sizes of 300 particles directly from the TEM image. The UV-vis absorption spectra of the PbS nanoparticles were recorded on a JASCO-530 spectrophotometer using pure DMSO solution as a reference. The cyclic voltammetry (CV) measurement was carried out with a BAS CV-50W (23) Sugimoto, T. D.; Dirige, G. E. J. Colloid Interface Sci. 1995, 716, 442. (24) Naka, K.; Uemura, T.; Chujo, Y. Macromolecules 1999, 32, 4641.

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Figure 1. Schematic illustration of the formation of the PDFprotected PbS nanoparticles in a DMSO solution under ambient conditions. electrochemical analyzer. A thin PbS nanoparticle-dispersed π-conjugated PDF film (10-20 µm in thickness) was deposited on an indium-tin-oxide (ITO)-coated glass electrode by casting from a DMSO solution. The electrode was dried under vacuum. A platinum wire auxiliary electrode and a Ag/AgCl RE-5 reference electrode were used in all CV measurements.

Results and Discussion (1) Formation of the π-Conjugated PDF-Protected PbS Nanoparticle. The formation process of the PDFprotected PbS nanoparticles can be formulated as following eqs 1 and 2.23 DMSO

CH3CSNH2 98 CH3CN + H2S DMSO

Pb(OAc)2 + H2S 98 PbS + H(OAc)

(1) (2)

The schematic illustration was presented in Figure 1. The π-conjugated PDF protected the formed PbS nanoparticle as a stable colloidal form in DMSO solution. The resulting DMSO solution of the PDF-protected PbS nanoparticle was stable without any precipitates for >3 months at room temperature under air. The sample exhibited a film-forming property when the DMSO solution was cast onto glass slides. A similar solution in the absence of the PDF was also stirred for 6 h at room temperature. Dark suspensions were obtained, and the precipitation appeared quickly. These results indicate that the π-conjugated PDF can protect the formed PbS nanoparticle as highly stable colloidal forms in DMSO solution. It is well-known that general surface ligands and polymers can stabilize colloidal nanoparticles by either steric or electrostatic stabilization. In the case of nanocomposites consisting of π-conjugated polymers and inorganic nanoparticles, it has been reported that conjugate moieties of polymers can coordinate or form complexes with incorporated inorganic nanoparticles.25,26 It is known that the sulfur atoms of the thioether moieties (R-S-R) of poly(thioether), acting as a soft Lewis base, can react with many metal ions, such as Cu(I), Ag(I), and Hg(II) and so on in solutions.27 Recently, Ng et al. reported the polythiophene-deposited quartz crystal microbalance (QCM) sensor for analysis of heavy metal ions.28 They (25) Wong, H. P.; Dave, B. C.; Leroux, F.; Harreld, J.; Dunn, B.; Nazar, L. F. J. Mater. Chem. 1998, 8, 1019. (26) Cioffi, N.; Torsi, L.; Losito, I.; Franco, C. D.; Bari, I. D.; Chiavarone, L.; Scamarcio, G.; Tsakova, V.; Sabbatini, L.; Zambonin, P. G. J. Mater. Chem. 2001, 11, 1434. (27) (a) Chayama, K.; Sekido, E. Anal. Chim. Acta 1991, 248, 511. (b) Chayama, K.; Sekido, E. Bull. Chem. Soc. Jpn. 1990, 63, 2420. (28) Ng, S. C.; Zhou, X. C.; Chen, Z. K.; Miao, P.; Chan, H. S. O.; Li, S. F. Y.; Fu, P. Langmuir 1998, 14, 1748.

Polymer-Protected Stable PbS Colloidal Nanoparticles

Figure 2. X-ray diffraction (XRD) pattern of the π-conjugated PDF-protected PbS nanoparticle produced with 1.0 mM Pb(OAc)2, 1.0 mM TAA, and 1 mg/mL PDF under ambient conditions.

found that the sulfur atoms of the polythiophenes can complex with the metal ions. The coordination selectivity of the polymer to the first-row transition metal ions was Cu2+ > Ni2+ > Co2+ > Zn2+ > Fe2+, which agrees well with the Irving-Williams order based on ionic potentials and size. Ag+ and Hg2+ ions exhibited intense complexation to the polymer, due to the strong affinity of these metal ions to the sulfur atoms. In the present case, with the size on the nanometer scale, the produced PbS nanoparticles were of large surface/volume ratios. The surfaces of the nanoparticles contain a large number of unsaturatedly coordinated Pb2+ ions. Therefore, owing to the strong Pb-S ionic bond, we believe that the surface unsaturatedly coordinated Pb2+ of the PbS nanoparticle may interact weakly with the S moiety, which renders the stabilization of the produced PbS nanoparticles. This proposed mechanism of stabilization is currently confirmed further by virtue of X-ray photoelectron spectroscopy (XPS), IR, NMR, and Raman spectroscopy, and detailed results will be published elsewhere. Figure 2 shows the X-ray diffraction (XRD) pattern of the π-conjugated PDF-protected PbS nanoparticle (sample 1) produced under ambient conditions. All the diffraction peaks can be indexed to the cubic rock-salt structure of the PbS phase with the cell parameter a ) 0.5947 nm, which is in good agreement with the reported data for PbS (JCPDS card No. 5-592, a ) 0.5936 nm). The average size of the PbS particles is about 4.2 nm, calculated by the Scherrer formula, much smaller than the exciton Bohr radius of 18 nm.11 Figure 3 shows the transmission electron microscopy (TEM) images and the corresponding size histograms of the π-conjugated PDF-protected PbS nanoparticles of sample 1-3 produced under ambient conditions. It can be seen that all of the PbS nanoparticles in these three samples were of spherical shape and separated from each other and that no particle aggregation was observed, indicating that the π-conjugated PDF provided high dispersity of the formed PdS nanoparticles. It was clear that the sizes of the formed π-conjugated PDF-protected PbS nanoparticles increased with the initial concentrations of Pb2+ and TAA at the constant PDF concentration. The size histograms show that the PbS nanoparticles in three samples were of a narrow size distribution. The nanoparticle in sample 1 was generally in the range 2-6 nm in size with the average diameter 4 nm, well consistent

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with the result of the XRD pattern. The PbS nanoparticles in samples 2 and 3 have the average sizes 5 and 7 nm, respectively. The average sizes of all the PbS nanoparticles were much smaller than the exciton Bohr radius. (2) UV-vis Absorption Spectra of the π-Conjugated PDF-Protected PbS Nanoparticles. Figure 4a shows the room-temperature UV-vis absorption spectrum of a DMSO solution of the pure PDF. The pure PDF shows a strong absorption peak around 398 nm, assigned to the π-π* transition of the conjugate molecular chain.23 Figure 4b presents the room-temperature UV-vis absorption spectrum of a DMSO solution of sample 1. It is obvious that the protected PbS nanoparticle in sample 1 has an absorption onset around 725 nm, displaying the dramatic optical blue shift of the band gap energy compared with that of the bulk material at 3020 nm,29 due to the quantum confinement effect. This blue shift of the absorption onset of the PbS nanoparticle was first observed by Gallardo et al. in 1989 in the study of its absorption and fluorescence properties.30 The absorption spectrum of the PbS nanoparticle in sample 1 also showed a peak around 600 nm, which can be assigned to the excitonic absorption, associated with the 1 Se-1 Sh electronic transition of the particle.11 The appearance of the excitonic absorption peak indicates the narrow size distribution31 and the direct band gap of the protected PbS nanoparticle.11 The average size of the PDF-protected PbS nanoparticle in sample 1 is about 3.86 nm, calculated by the Brus method on the basis of the blue shift of the UV-vis absorption spectrum,32 well consistent with the results of the XRD pattern and TEM observation. The band gap of the protected PbS nanoparticle in sample 1 was calculated to be about 1.51 eV, according to the hyperbolic model33 as shown in the following eq 3, exhibiting a strong quantum confinement effect compared with that for the bulk PbS (0.41 eV).

∆E ) [Eg2 + 2h2Eg(π/R)2/m*]1/2

(3)

In the above equation, ∆E is the band gap of the PbS nanoparticle, Eg ) 0.41 eV for bulk PbS, m*/me ) 0.085 (m* is the actual electron mass), and R is the radius of the nanoparticle. This calculated value is in good agreement with an observed value (1.71 V) from the UV-vis absorption spectrum of sample 1. Figure 4c and d show the room-temperature UV-vis absorption spectra of the PDF-PbS nanoparticles of samples 2 and 3. The inset in Figure 4 is the magnified spectra of Figure 4b-d in the wavelength range 500-700 nm. From the inset, it is clear that the absorption onset gradually shifts toward longer wavelength with an increase of the initial Pb2+ concentrations. The shift can be attributed to the increasing sizes of the PbS nanoparticles.11 The absorption onsets of both samples 2 and 3 started from the wavelength longer than 800 nm, extending into a near-infrared region. It can also be seen that, with the increase of the initial Pb2+ concentrations, the exciton absorption peak of the PbS nanoparticle gradually red shifted and weakened in sample 2 compared to that in sample 1, and completely disappeared in sample 3. The red shift can be due to the size variation.11 It is wellknown that the excitonic absorption peaks of semiconduc(29) Nenadoviæ, M. T.; E Å omor, M. I.; Vasiæ, V.; Miæiæ, O. I. J. Phys. Chem. 1990, 94, 6390. (30) Gallardo, S.; Gutierrez, M.; Henglein, A.; Janata, E. Ber. BunsenGes. Phys. Chem. 1989, 93, 1080. (31) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302. (32) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (33) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. J. Chem. Phys. 1987, 87, 7315.

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Figure 3. Transmission electron microscopy (TEM) images and corresponding size distribution histograms of the π-conjugated PDF-protected PbS nanoparticles produced under ambient conditions with 1.0 mM Pb(OAc)2, 1.0 mM TAA, and 1 mg/mL PDF (a); 2.0 mM Pb(OAc)2, 2.0 mM TAA, and 1 mg/mL PDF (b); and 3.0 mM Pb(OAc)2, 3.0 mM TAA, and 1 mg/mL PDF (c). The size distribution histograms were obtained by averaging the sizes of 300 particles from the TEM images.

tor nanoparticles are strongly related with the surfacerelated charge separation and polarization effects in semiconductor nanoparticles, which were suggested to be

sensitive to the charges on the surfactant molecules.34 The electron-hole pair generated by the light absorption can be easily trapped at the surface defect sites.34 Exciton

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absorption bleaching experiments indicated that, in the presence of the trapped electron-hole pairs, the strong interaction between the trapped electron-hole pair and the exciton causes the reduction of spatial overlap of the electron and hole wave functions and the oscillator strength of the exciton, which results in the bleaching of exciton absorption.35 Furthermore, it has been reported that the surface defect sites for the nonradiation recombination of charge carriers can be destroyed chemically by covering the surface with capping materials.36 Particularly, for PbS nanoparticles, the appearance of the excitonic absorption peak at 600 nm has been demonstrated to rely strongly on the surfactants used, preparation techniques, and the shape and size distribution of synthesized PbS nanoparticles. Antonietti et al. reported the synthesis of needle-shaped and spherical PbS nanoparticles in the presence of amphiphilic block copolymer micelles composed of polystyrene-b-poly(4)vinylpyridine (PS-P4VP).37 They found that the needle-shaped PbS nanoparticles exhibited the excitonic peak at 580 nm, which disappeared in the case of the spherical PbS nanoparticles. In contrast, later work by Wang et al. reported that the rod-shaped PbS nanoparticle prepared in a poly(vinyl butyral) (PVB) film with a functionalized lead(II) salt of the surfactant anion AOT-, Pb(AOT), as the precursor showed a featureless absorption spectrum.38 However, the spherical PbS nanoparticle prepared under the same conditions in solution displayed the excitonic peak at 600 nm. One of the proposed reasons for the absence of the exciton absorption peak in the PbS nanorod was that the nanorod was confined only in two directions, and the third dimension much more than the Bohr radius of bulk PbS may wash out the exciton absorption peaks. An alternative explanation was that the surface of the

PbS nanorod may be capped by the SO3- group of AOT-. Because of the polarization of the charged capping group, the trapped electron density was reduced significantly, leading to the disappearance of the exciton absorption peaks. For the spherical PbS nanoparticle in solution synthesis, AOT- ions on the surface of the particle may be diffused away as the particle grew, leaving the OH group of PVB to attach to the nanoparticle surface, having no influence on the absorption of the excitons. Recently, Patel and co-workers studied the optical spectroscopy of the PbS nanoparticles stabilized with a number of surface capping agents including poly(vinyl alcohol) (PVA), poly(vinylpyrrolidone) (PVP), gelatin, DNA, polystyrene (PS), and poly(methyl methacrylate) (PMMA).39 They proposed that it was not the particle shape correlating with the apparent discrepancy but the crystal structure of the nanoparticles that determined the absorption features. In the present studies, we believe that the weakening and disappearance of the excitonic peaks of the PDF-protected PbS nanoparticles in samples 2 and 3 is not due to the size distribution, since the forementioned TEM images have shown that the PDF-protected PbS nanoparticles in three samples have a narrow size distribution and all average diameters were much smaller than the exciton Bohr radius. It is known that the electrical properties of π-conjugated polymers exhibit both metallic and semi behaviors.40 The electronic structures of the π-conjugated polymer chains can strongly influence the characteristic of embedded nanoparticles.41,42 Various π-conjugated polymers have been used in combination with semiconductor nanoparticles to optimize the transport properties of electrons and holes in organic/inorganic hybrid devices.43 We suppose that the possible complexation of PDF with the surface unsaturatedly coordinated Pb2+ of the PbS nanoparticle and the π-conjugated electronic structure of the PDF molecular chain may vary the surface charge state of the PbS nanoparticles, favoring the destruction of the surface defect sites and enlargement of the spatial overlap of the electron and hole wave functions, both of which facilitate the exciton absorption.34,35 With increasing concentration of the initiate lead and sulfur sources in the reaction systems, more of the PbS nanoparticle was formed, expectedly followed by an increasing amount of surface defect sites. These increasing surface defect sites may result in the bleaching of the exciton peak, since even one trapped electron-hole pair can lead to the exciton absorption bleaching of a whole cluster.40 Further results showed that the exciton peak in sample 3 appeared, 6 h after the addition of an excess of PDF to the DMSO solution of the synthesized sample 3 even though the particle size was not changed. This result further confirms that the presence of the π-conjugated electronic structure of the PDF polymer chain indeed influenced the excitonic peaks of the protected PbS nanoparticles. (3) Cyclic Voltammetry of the π-Conjugated PDFProtected PbS Nanoparticle. Prior research on the nanocomposites consisting of inorganic semiconductor nanoparticles and π-conjugated polymers mainly focused

(34) Gao, M. Y.; Yang, Y.; Yang, B.; Shen, J. C. J. Chem. Soc., Faraday Trans. 1995, 91, 4121. (35) (a) Wang, Y. Acc. Chem. Res. 1991, 24, 133. (b) Wang, Y.; Suna, A.; McHugh, J.; Hilinski, E. F.; Lucas, P. A.; Johnson, R. D. J. Chem. Phys. 1990, 92, 6927. (36) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (37) Schneider, T.; Haase, M.; Kornowski, A.; Naused, S.; Weller, H.; Forster, S.; Antonietti, M. Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 1997, 101, 1654. (38) Wang, S. H.; Yang, S. H. Langmuir 2000, 16, 389.

(39) Patel, A. A.; Wu, F. X.; Zhang, J. Z.; Torres-Martinez, C. L.; Mehra, R. K.; Yang, Y.; Risbud, S. H. J. Phys. Chem. B 2000, 104, 11598. (40) Singh, R.; Tandon, R. P.; Chandra, S. J. Appl. Phys. 1991, 70, 243. (41) Sarathy, K. V.; Narayan, K. S.; Kim, J.; White, J. O. Chem. Phys. Lett. 2000, 318, 543. (42) Hertel, T.; Knoesel, E.; Wolf, M.; Ertl, G. Phys. Rev. Lett. 1996, 76, 535. (43) Gao, M. Y.; Richter, B.; Kirstein, S.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 4096.

Figure 4. Room-temperature UV-vis absorption spectra of DMSO solutions of the pure PDF (a) and the PDF-protected PbS nanoparticles produced with 1.0 mM Pb(OAc)2, 1.0 mM TAA, and 1 mg/mL PDF (b); 2.0 mM Pb(OAc)2, 2.0 mM TAA, and 1 mg/mL PDF (c); and 3.0 mM Pb(OAc)2, 3.0 mM TAA, and 1 mg/mL PDF (d). The inset is the three-times magnified spectra of parts b-d in the wavelength range 500-700 nm.

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cannot be reduced by adding electrons into the π-conjugated system of the polymer chain in the applied potential range. In the anodic oxidation direction, the cast film of the pure PDF gave a single broad oxidation peak at about 0.61 V versus Ag/Ag+. Figure 5b presents the cyclic voltammogram of the cast film of the PDF-protected PbS nanoparticles (sample 1) under the same measurement conditions. For the anodic oxidation scanning, besides the oxidation peak of PDF at about 0.60 V, another obvious peak was observed at around 1.1 V. This peak was associated with the oxidative dissolution of the protected PbS nanoparticle.44,47,48 The oxidation reaction process has been proved to be a two-electron process, as shown in eq 4.44,47,48

PbS f Pb2+ + S + 2e

Figure 5. Cyclic voltammograms) of the cast films of the pure PDF (a) and the PDF-protected PbS nanoparticles (b) produced with 1.0 mM Pb(OAc)2, 1.0 mM TAA, and 1 mg/mL PDF under ambient conditions.

on photoluminescence and electroluminescence studies. However, to the best of our knowledge, the electrochemical property of the nanocomposite has never been reported yet. Earlier electrochemical studies of PbS were primarily concentrated on the anodic dissolution of bulk galena (main component PbS) for the extraction of lead metal.44,45 Davis and Huang have studied the electrochemical oxidation of PbS by using a rotating ring PbS disk electrode.46 Recently, Bard et al. reported the electrochemical and photoelectrochemical studies of films of quantum size PbS particles incorporated in a self-assembled monolayer on gold substrates.47 Chen et al. studied the electrochemical properties of the PbS nanoparticles passivated by a monolayer of alkanethiolates in a nonaqueous system.48 Figure 5a shows the cyclic voltammogram of the cast film of the pure PDF by initially scanning the potential in the cathodic direction from 0 V versus Ag/Ag+ in a CH3CN solution of 0.1 M [NEt4]BF4 at 300 mV s-1 at room temperature. In the cathodic reduction direction, the curve displayed a featureless structure, suggesting that PDF (44) Gardner, J. R.; Woods, R. J. Electroanal. Chem. Interfacial Electrochem. 1979, 100, 447. (45) (a) Paul, R.; Nicol, M. J.; Diggle, J. W.; Saunders, A. P. Electrochim. Acta 1978, 23, 625. (b) Paul, R.; Nicol, M. J.; Diggle, J. W. Electrochim. Acta 1978, 23, 635. (46) Davis, A. P.; Huang, C. P. Langmuir 1991, 7, 803. (47) Ogawa, S.; Hu, K.; Fan, F. F.; Bard, A. J. J. Phys. Chem. B 1997, 101, 5707. (48) Chen, S. W.; Truax, L. A.; Sommers, J. M. Chem. Mater. 2000, 12, 3864.

(4)

For cathodic reduction of the PDF-protected PbS nanoparticle, the particle displayed a reduction peak at about -0.9 V. The reaction process can be described in eq 5.44,47,48

PbS + 2e f Pb + S2-

(5)

The cathodic reduction potential of PbS nanoparticles has been demonstrated to be dependent on the particle size, that is, to shift to the cathodic direction with decreasing particle size.47 It is likely that the shift is due to the increase of the band gap energy as the particle size decreases, moving the conduction band level to a more negative position,47 which results in it being more difficult to inject electrons into the conduction band, presumably the initial step in PbS reduction. Conclusions We report the preparation of novel π-conjugated polydithiafulvene (PDF)-protected stable PbS colloidal nanoparticles with a narrow size distribution at room temperature in a nonaqueous solution. The synthesized PbS nanoparticles displayed a dramatic optical blue shift of the band gap energy compared with that of the bulk material, exhibiting a strong quantum confinement effect. The possible complexation of the PDF with the surface unsaturatedly coordinated Pb2+ of the PbS nanoparticle, and the π-conjugated molecular chain of the PDF were found to influence strongly the excitonic absorption peak of the protected PbS nanoparticle. The electrochemical results revealed that the present PDF-protected PbS nanoparticle displayed an oxidation peak at 1.1 V and a reduction peak at about -0.9 V. The present stable nanocomposite in colloidal form consisting of the PDF and PbS nanoparticles may find applications in the fabrication of novel optical and electronic devices. LA011642I