-Fe3+O(OH

J. Phys. Chem. C 2008, 112, 20469–20480

20469

Preparation and Characterization of PEDOT/β-Fe3+O(OH,Cl) Nanospindles with Controllable Sizes in Aqueous Solution Hui Mao,† Xiaofeng Lu,† Danming Chao,† Lili Cui,† Yongxin Li,† and Wanjin Zhang*,† Alan G. MacDiarmid Institute, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: September 9, 2008; ReVised Manuscript ReceiVed: October 20, 2008

By using FeCl3 · 6H2O as an oxidant, a novel organic/inorganic complex nanostructure, poly(3,4-ethylenedioxythiophene)/β-akaganeite (PEDOT/β-Fe3+O(OH,Cl)) nanospindle, was successfully synthesized in aqueous solution in the presence of cetyltrimethylammonium bromide (CTAB) and poly(acrylic acid) (PAA). The lengths and widths of these nanospindles were in the range of 350-370 and 80-90 nm, respectively, which had good monodispersity. PEDOT/β-Fe3+O(OH,Cl) nanospindles were composed of PEDOT and singlecrystalline β-akaganeite, which was confirmed by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction pattern (XRD) measurements, analysis of the X-ray photoelectron spectra (XPS), and thermogravimetric analysis (TGA). They had the dual properties of both a conducting polymer (PEDOT) and an inorganic single crystal (β-akaganeite). The morphology and crystallinity were afforded by the part of inorganic β-akaganeite, and the electrical and electrochemical properties were attributed to the part of PEDOT, which were investigated by using a four-probe method and cyclic voltammetry (CV). The formation of PEDOT/ β-Fe3+O(OH,Cl) nanospindles may be due to the concurrence of the polymerization of EDOT and the hydrolyzation of FeCl3 in aqueous solution at 50 °C and the interactional result of them. 1. Introduction Since 1977 when the conjugated polymers were discovered and could be made to possess the ability of conducting electricity by doping,1 a considerable number of researches have focused on the field of conducting polymers. Recently, many attentions have been paid on the synthesis of nanostructured conducting polymers for their potential applications in electrical nanodevices. For example, the one-dimensional (1D) micro/nanostructure of conducting polymers, which may be used as molecular wires due to their long conjugation length and metallike conductivity, can act as well candidates in the preparation of electrical and optoelectronics devices, such as rechargeable batteries2 and light-emitting diodes.3 Simultaneously, threedimensional (3D) solid or hollow micro/nanospheres showed a broad range of applications for encapsulation of products (cosmetics, inks, and dyes), drug delivery, medical imaging, protein and enzyme transplantation, and contaminated waste removal.4-10 In order to obtain multifunctional materials, conducting polymer/inorganic composite nanomaterials have also been widely studied.11-16 Recently, poly(3,4-ethylenedioxythiophene) (PEDOT) has become more and more important in the family of conducting polymers, due to its high electric conductivity, moderate band gap,goodenvironmentalstability,andhighopticaltransparency.17-20 It will be applied in various fields, such as polymer light-emitting diodes, sensors, and transparent antistatic coatings.21-23 Up to now, a few methods have been developed for preparing nanostructures of PEDOT, including the template method (for hollow particles),24 surfactant-mediated interfacial polymerization (for nanocapsules and mesocellular foams),25 dispersion polymerization in alcoholic media (for nanoparticles and vesicles),26 the reverse microemulsion system (for nanorods and * To whom correspondence should be addressed. Phone and Fax: 86431-85168924. E-mail: [email protected]. † Jilin University.

nanotubes),23,27 the self-assembled micellar soft-template approach (for nanofibers),28 the V2O5 seeding approach (for nanofibers),29 interfacial polymerization-crystallization (for single-crystal nanoneedles),30 and electrochemical synthesis (for nanofibers).31 However, there are only a few reports related to the composites of conductive PEDOT and inorganic crystalline materials. Singh et al. synthesized the PEDOT-γ-Fe2O3 composite via microemulsion oxidative polymerization in aqueous medium.32 By in situ and postsynthesis method, Zhang et al. made Ag nanoparticles with a diameter of 18-22 nm uniformly distributed along the walls of PEDOT nanotubes which were prepared through reverse microemulsion polymerization.27 Lu et al. successfully obtained Au-PEDOT nanocables by onestep interfacial reaction at room temperature.33 Herein, we reported a very simple method for the fabrication of a new nanostructure, PEDOT/β-Fe3+O(OH,Cl) nanospindles, which were prepared using FeCl3 · 6H2O as an oxidant in aqueous solution in the present of cetyltrimethylammonium bromide (CTAB) and poly(acrylic acid) (PAA). These monodisperse nanospindles are composed of PEDOT and highly ordered single crystals of β-akaganeite (β-Fe3+O(OH,Cl)) which were caused by the hydrolyzation of FeCl3 in aqueous solution. PEDOT/β-Fe3+O(OH,Cl) nanospindles had the dual properties of both a conducting polymer (PEDOT) and an inorganic singlecrystal β-akaganeite. The morphology and crystallinity were afforded by the part of inorganic β-akaganeite, and the electrical and electrochemical properties were attributed to the part of PEDOT. 2. Experimental Section 2.1. Materials. 3,4-Ethylenedioxythiophene (EDOT) monomer (g99.0%) was purchased from Beili Pharm Raw Material (Suzhou) Co., Ltd. and used without further purification. All the other reagents were analytical grade and used without further purification, including FeCl3 · 6H2O (Sinopharm Chemical Reagent Co., Ltd., g99.0%), CTAB (Sinopharm Chemical Reagent

10.1021/jp807988f CCC: $40.75  2008 American Chemical Society Published on Web 11/24/2008

20470 J. Phys. Chem. C, Vol. 112, No. 51, 2008

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Figure 1. (a) SEM and (b) TEM images of PEDOT/β-Fe3+O(OH,Cl) nanospindles; (c) TEM image of a single PEDOT/β-Fe3+O(OH,Cl) nanospindle; (d) high-resolution (HR)TEM image of a PEDOT/β-Fe3+O(OH,Cl) nanospindle; (e) electron diffraction data. Conditions: [EDOT] ) 0.025 M, [EDOT]/[FeCl3 · 6H2O] ) 1:3, [CTAB] ) 0.0075 M, [PAA] ) 0.02 mg/mL, 50 °C, 20 h.

Co., Ltd., g99.0%), PAA (Tianjin Kermel Chemical Reagent Co., Ltd., solid content is 30%), Fe(NO)3 · 9H2O (Tianjin Guangfu Fine Chemical Research Institute, g98.5%), ammonium persulfate (APS) (Beijing YILI Fine Chemical Co., Ltd., g98.0%), HCl(aq) (Beijing BEIHUA Fine Chemicals Co., Ltd., 36-38%), ethanol (Tianjin TIANTAI Fine Chemical Co., Ltd., g99.7%), tetrahydrofuran (THF) (Tianjin TIANTAI Fine Chemical Co., Ltd., g99.0%), dimethylformamide (DMF) (Tianjin TIANTAI Fine Chemical Co., Ltd., g99.5%), and dimethyl sulfoxide (DMSO) (Beijing Chemical Works, g99.0%). 2.2. Preparation of PEDOT/β-Fe3+O(OH,Cl) Nanospindles. PEDOT/β-Fe3+O(OH,Cl) nanospindles were synthesized by chemical oxidation using FeCl3 · 6H2O as an oxidant in aqueous solution. In a typical procedure, 1 mmol of EDOT and 0.3 mmol of CTAB were dispersed in 24 mL of deionized water and 8 mL of 0.1 mg/mL PAA aqueous solution was added into it. They were kept at room temperature under magnetic stirring for 1 h. Then, 3 mmol of FeCl3 · 6H2O dissolved in 8 mL of deionized water was added into the above mixture system. After magnetically stirring for another 5 min, the mixture was kept without stirring at 50 °C for 20 h. The dark blue flocculent precipitates were collected by centrifugation and then washed with water and ethanol for several times to remove unreacted chemicals and outgrowths. Finally, they were dried in vacuum at 45 °C for 24 h. The molar ratio of monomer to oxidant ([EDOT]/[FeCl3 · 6H2O]) was adjusted from 1:1 to 1:9, and the concentration of 3,4-ethylenedioxythiophene ([EDOT]) was changed from 0.0125 to 0.05 M. The reactions were also carried out under other different temperatures (20 and 80 °C) and times (0.5, 1, 3, 6, 9, and 12 h) or using different oxidants (APS and Fe(NO)3 · 9H2O). The concentrations of CTAB and PAA ([CTAB] and [PAA]) and the kinds of organic solvent/H2O mixture solvents were also changed in order to investigate their influence to the morphologies of the products and the sizes of PEDOT/ β-Fe3+O(OH,Cl) nanospindles, respectively.

Figure 2. XRD scattering patterns of PEDOT/β-Fe3+O(OH,Cl) nanospindles (a and b) and the bulk PEDOT (c and d) obtained under different conditions: (a) [EDOT] ) 0.025 M, [CTAB] ) 0.0075 M, [PAA] ) 0.02 mg/mL; (b) [EDOT] ) 0.025 M, without CTAB and PAA; (c) [EDOT] ) 0.05 M, without CTAB and PAA; (d) [EDOT] ) 0.025 M, APS as an oxidant, without CTAB and PAA. Other conditions: [EDOT]/[FeCl3 · 6H2O] or APS ) 1:3, 50 °C, 20 h.

2.3. Characterization. The images of PEDOT/βFe3+O(OH,Cl) nanospindles were obtained by scanning electron microscopy (SEM) measurements which were performed on a Shimadzu SSX-550 microscope. Transmission electron microscopy (TEM) experiments were performed on a Hitachi H-8100 electron microscope with an acceleration voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) experiments were performed on a JSM-3010 electron microscope (JEOL, Japan) with an acceleration voltage of 300 kV. Fourier transform infrared spectroscopy (FTIR) spectra of KBr powder-pressed pellets were recorded on a Bruker VECTOR22 spectrometer. Transmission spectra of PEDOT/β-Fe3+O(OH,Cl)

PEDOT/β-Fe3+O(OH,Cl) Nanospindles

J. Phys. Chem. C, Vol. 112, No. 51, 2008 20471

Figure 3. XPS spectra of (i) PEDOT/β-Fe3+O(OH,Cl) nanospindles after being washed with 1 M HCl(aq) and (ii) the bulk PEDOT: (a) survey spectra; (b) C1s; (c) S2p; (d) O1s.


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Figure 4. SEM images of PEDOT/β-Fe3+O(OH,Cl) nanospindles after being washed with different concentrations of HCl(aq): (a) 1 M; (b) 2 M; (c) 2.1 M; (d) 2.2 M; (e) 2.6 M; (f) 3 M.

nanospindles were recorded on a Shimadzu UV-3101 PC spectrometer. X-ray diffraction patterns (XRD) were obtained with a Siemens D5005 diffractometer using Cu KR radiation. Analysis of the X-ray photoelectron spectra (XPS) was performed on an ESCLAB MKII using Al as the exciting source. A Perkin-Elmer PYRIS 1 TGA was used to investigate the thermal stability of the nanospindles powder in the temperature range from 100 to 750 °C under condensed atmosphere at a rate of 10.0 °C/min. The electrochemical performance of PEDOT/β-Fe3+O(OH,Cl) nanospindles was investigated by using cyclic voltammetry (CV) with a CHI660A electrochemical station (CH Instrument, U.S.A.). In a three-electrode system, a modified glassy carbon electrode, a platinum wire, and a saturated calomel electrode (SCE) were used as the working

electrode, the counter electrode, and the reference electrode, respectively. The electrical conductivity of the both nanospindles at room temperature (RT) was measured by a four-probe method using a 2182 nanovoltmeter and 2400 source meter as the current source. 3. Results and Discussion Parts a and b of Figure 1 show typical SEM and TEM images of PEDOT/β-Fe3+O(OH,Cl) nanospindles which were synthesized using FeCl3 · 6H2O as an oxidant in the presence of CATB and PAA in aqueous solution at 50 °C. [EDOT] was 0.025 M, and [EDOT]/[FeCl3 · 6H2O] was kept to 1:3. It is found that the obtained PEDOT/β-Fe3+O(OH,Cl) nanospindles are homogeneous and monodisperse, and other morphologies of the products



Figure 5. TGA thermograms of (a) PEDOT/β-Fe3+O(OH,Cl) nanospindles as-synthesized; (b) after being washed with 1 M HCl(aq); (c) after being washed with 3 M HCl(aq); (d) the bulk PEDOT. Conditions: (a-c) [EDOT] ) 0.025 M, [CTAB] ) 0.0075 M, [PAA] ) 0.02 mg/ mL, 50 °C, 20 h; (d) [EDOT] ) 0.05 M, [EDOT]/[FeCl3 · 6H2O] ) 1:3, 50 °C, 20 h.

hardly exist. The lengths and widths are in the range of 350-370 and 80-90 nm, respectively. HRTEM image and the electron diffraction (ED) examinations verify that the β-Fe3+O(OH,Cl) in a single nanospindle is the single-crystalline nature (Figure 1, parts d and e). But the lattice construction inside the white ring of Figure 1d is not clear as the other parts, which may be attributed to the imperfectness of the crystallization, and the part of polymer (PEDOT) is dominant in this area. The ED pattern shows the well-defined regular diffraction spots, which further proves that β-Fe3+O(OH,Cl) are highly ordered single crystals. When CTAB and PAA are not added into the reaction system, it is clear that nanospindles were still dominant in the products, though some of them conglutinated with each other (Supporting Information Figure S1a). Supporting Information Figure S1b gives the TEM image of them. It is found that the sizes of PEDOT/β-Fe3+O(OH,Cl) nanospindles are inhomogeneous. The lengths and widths are in the range of 250-390 and 60-120 nm, respectively. And some irregular coagula like several nanofibers exist among nanospindles, which may be the pure PEDOT. The XRD patterns of PEDOT/β-Fe3+O(OH,Cl) nanospindles and the bulk PEDOT are presented in Figure 2. According to Matsui’s report,30 single-crystal PEDOT nanoneedles (50 nm in length and 15 nm in width) were prepared from an interfacial polymerization-crystallization of EDOT. But in our case, the powder XRD patterns of PEDOT/β-Fe3+O(OH,Cl) nanospindles indicate a number of diffraction peaks (Figure 2, parts a and b) which are consistent with β-akaganeite (PDF 13-157). According to those data, β-Fe3+O(OH,Cl) in the nanospindles composite is attributed to a tetragonal system and body-centered lattice. The relevant a, b, and c lattice parameters are 10.48, 10.48, and 3.023 Å, respectively. When parts a and b of Figure 2 are compared, the diffraction peaks of β-Fe3+O(OH,Cl) in the nanospindles composite obtained without CTAB and PAA are some weaker than those obtained with CTAB and PAA, because the irregular coagula are amorphous PEDOT. When [EDOT] is 0.05 M without CTAB and PAA, no nanospindles exist. The bulk PEDOT is amorphous, while the diffraction peak at 26.2° is due to the interchain planar ring-stacking distance (Figure 2c).34 When APS is used to be as an oxidant without CTAB and PAA, no nanospindles exist and the products are also

Figure 6. (a) FTIR spectra and (b) UV-vis spectra of (curve a) PEDOT/β-Fe3+O(OH,Cl) nanospindles as-synthesized; (curve b) after being washed with 1 M HCl(aq); (curve c) after being washed with 3 M HCl(aq); (curve d) the bulk PEDOT. Conditions: (curves a-c) [EDOT] ) 0.025 M, [CTAB] ) 0.0075 M, [PAA] ) 0.02 mg/mL, 50 °C, 20 h; (curve d) [EDOT] ) 0.05 M, [EDOT]/[FeCl3 · 6H2O] ) 1:3, 50 °C, 20 h. (They were as-synthesized by the above method and dispersed in deionized water in the UV-vis measurement.)

amorphous (Figure 2d). In a word, only PEDOT/βFe3+O(OH,Cl) nanospindles have crystallinity. The more regular the morphologies of PEDOT/β-Fe3+O(OH,Cl) nanospindles are, the stronger the crystallization capacity of them is. X-ray photoelectron spectra data have also been used to characterize the chemical structure of PEDOT/β-Fe3+O(OH,Cl) nanospindles. In order to eliminate other iron oxides which may be formed due to the hydrolyzation of FeCl3 in aqueous solution, the products were washed by 1 M HCl(aq) before XPS measurement. Parts i and ii of Figure 3 indicate the XPS spectra of PEDOT/β-Fe3+O(OH,Cl) nanospindles and the bulk PEDOT, respectively. The elements of C, S, O, Fe, and Cl are detected in both of them, but Fe and Cl are not very obvious in the survey spectra of bulk PEDOT (Figure 3ii-a). According to Jonas’ report, the actual dopant may be either [FeCl4]- or Cl- in the bulk PEDOT,35 and the content of Fe and Cl is very little in bulk PEDOT, so the doping is not completely for bulk PEDOT. C1s core-line spectra of PEDOT/β-Fe3+O(OH,Cl) nanospindles are similar to that of the bulk PEDOT, but the binding energy of C1s in the bulk PEDOT is a little higher than that in PEDOT/ β-Fe3+O(OH,Cl) nanospindles, which may be due to the interaction between PEDOT and β-Fe3+O(OH,Cl). For PEDOT/


Figure 7. Cyclic voltammogram of (a) PEDOT/β-Fe3+O(OH,Cl) nanospindles and (b) the bulk PEDOT in 1 M H2SO4 solution with scanning rate at 50 mV/s. Conditions: [EDOT]/[FeCl3 · 6H2O] ) 1:3, [CTAB] ) 0.0075 M, [PAA] ) 0.02 mg/mL, 50 °C, 20 h; (a) [EDOT] ) 0.025 M, (b) [EDOT] ) 0.05 M.

β-Fe3+O(OH,Cl) nanospindles, three peaks have been found at 284.86, 286.22, and 287.58 eV in Figure 3i-b, which are associated with C-C/C-H, C-S, and C-O, respectively. For the bulk PEDOT, the main peak is fitted with four components due to C-C/C-H bonds (284.90 eV), C-S (286.39 eV), and C-O (287.11, 288.34 eV), which are similar to the earlier report.36 Parts i-c and ii-c of Figure 3 show the S2p core-line spectra of PEDOT/β-Fe3+O(OH,Cl) nanospindles and the bulk PEDOT, respectively. The main peaks have been found at 164.55 and 166.23 eV in Figure 3i-c, 164.21 and 166.23 eV in Figure 3ii-c, which are associated with neutral S and cationic S+,36,37 respectively. Unlike C1s and S2p spectra, there is much difference in O1s core-line spectra between PEDOT/βFe3+O(OH,Cl) nanospindles and the bulk PEDOT. The main peaks have been found at 530.75 and 532.99 eV in Figure 3i-d, which are associated with lattice oxygen (OI) and C-O-C, respectively. It is proved that the part of β-akaganeite exists in nanospindles. But for the bulk PEDOT, two peaks are found at 532.64 eV (C-O-C) and 534.36 eV (C-O). Figure 4 indicates SEM images of PEDOT/β-Fe3+O(OH,Cl) nanospindles after being washed with different concentrations of HCl(aq) ([HCl]), and their XRD patterns are presented in Supporting Information Figure S2. When [HCl] was lower than 2 M, the spindle-like morphology was not changed (Figure 4,

Mao et al. parts a and b). After being washed with 1 M HCl(aq), their XRD patterns are the same as that of PEDOT/β-Fe3+O(OH,Cl) nanospindles as-synthesized (Supporting Information Figure S2, parts a and b). But when [HCl] was changed from 2.1 to 2.6 M, the sizes of PEDOT/β-Fe3+O(OH,Cl) nanospindles were diminished gradually (Figure 4c-e). After being washed with 3 M HCl(aq), β-Fe3+O(OH,Cl) in PEDOT/β-Fe3+O(OH,Cl) nanospindles disappeared completely and the PEDOT that was left was amorphous, while the diffraction peaks at 6.4° and 26.2° were due to the interchain packing along a pseudo-orthorhombic a axis and the interchain planar ring-stacking distance,28 respectively (Supporting Information Figure S2c). Similarly, we can use a calcination method to remove PEDOT. Supporting Information Figure S3 gives SEM and TEM images of PEDOT/ β-Fe3+O(OH,Cl) nanospindles after calcination at 400 °C for 6 h until constant weight. The spindle-like morphology was not changed except that the sizes were decreased a little. The lengths and widths were in the range of 250-300 and 60-90 nm, respectively. Supporting Information Figure S3c shows the TEM image and ED pattern of a single PEDOT/β-Fe3+O(OH,Cl) nanospindle after calcination, which indicates that the surface of the obtained nanospindle is rough but it is still singlecrystalline. The XRD patterns of PEDOT/β-Fe3+O(OH,Cl) nanospindles after calcination (Supporting Information Figure S2d) are consistent with hematite (PDF 24-72) which is attributed to a rhombohedral system and rhomb-centered lattice. The relevant a, b, and c lattice parameters are 5.083, 5.083, and 13.772 Å, respectively. Thermogravimetric analysis (TGA) thermograms of PEDOT/ β-Fe3+O(OH,Cl) nanospindles (as-synthesized, washed with 1 and 3 M HCl(aq)) and the bulk PEDOT in the temperature range from 100 to 750 °C under condensed atmosphere at a rate of 10 °C/min are shown in Figure 5, which further prove that PEDOT/β-Fe3+O(OH,Cl) nanospindles are composed of PEDOT and β-akaganeite. Curves a and b exhibit the typical changes of polymer embedded in an inorganic compound. From 265 to 350 °C, there is a continuous and rapid loss of 38.5%, which may be due to the degradation of polymer in the PEDOT/βFe3+O(OH,Cl) nanospindles. The parts left, about 40%, are hematite. After being washed with 3 M HCl(aq), the spindlelike morphology disappears completely and only amorphous PEDOT is left. Therefore, the thermogram (Figure 5c) is similar to that of the bulk PEDOT (Figure 5d). Because of the influence of 3 M HCl(aq), β-akaganeite was almost removed, the remaining dopant FeCl4- was very little. So the two parts of weight loss were more obvious, and the remaining weight (0.454%) was less than that of the bulk PEDOT (7.117%). The chemical structure of PEDOT in PEDOT/βFe3+O(OH,Cl) nanospindles has also been characterized by FTIR and UV-vis absorption spectra. Figure 6a gives typical FTIR spectra of PEDOT/β-Fe3+O(OH,Cl) nanospindles (assynthesized, washed with 1 and 3 M HCl(aq)) and the bulk PEDOT. All of the curves are similar and exhibit the typical curve of PEDOT. The vibrational bands associated with PEDOT are routinely seen at 1600 cm-1, and other lower vibrational bands are also observed.38 For instance, the bands at 1536-1472 cm-1 are due to CRdCβ asymmetric stretching. The peaks at 1390 and 1347 cm-1 are assigned to Cβ-Cβ stretching. The absorption at 1322 cm-1 is due to C-C or CdC stretching of quinoid structure of thiophene; the bands at 1205, 1186, 1140, 1090, and 1054 cm-1 are associated to C-O-C band stretching in the ethylene dioxy group. The peaks at 982, 842, and 695 cm-1 are due to C-S stretching. The absorption at 1645 cm-1, which is assigned to the doped level of PEDOT, is also observed.



Figure 8. SEM images of PEDOT/β-Fe3+O(OH,Cl) nanospindles synthesized under different reaction times: (a) 0.5, (b) 1, (c) 3, (d) 6, (e) 9, and (f) 12 h.

The bands at 2860-2980 cm-1 are associated to the weak characteristic CH2 stretchings of the dioxyethylene bridge. All of these data are consistent with those previous reports for PEDOT.23,24 Figure 6b presents the UV-vis spectra of them dispersed in deionized water. The absorption peak near 850 nm is observed, which is the feature typically seen in the partially doped (oxidized) state of PEDOT.28 The electrochemical performance of PEDOT/β-Fe3+O(OH,Cl) nanospindles is investigated by using CV. Parts a and b of Figure 7 give the CV curves of PEDOT/β-Fe3+O(OH,Cl) nanospindles and the bulk PEDOT in 1 M H2SO4 solution with scanning rate at 50 mV/s, respectively. Figure 7a presents a pair of weak redox peaks with oxidation peak at 0.608 V and reduction peak at 0.281 V referenced to a SCE, which indicates a typical behavior

of conducting polymers. The bulk PEDOT CV curves also show a pair of weak redox peaks with oxidation peak at 0.626 V and reduction peak at 0.264 V (Figure 7b), respectively. In comparison with the bulk PEDOT CV curves, the changes of the redox peaks in PEDOT/β-Fe3+O(OH,Cl) nanospindles CV curves may be due to the existence of β-akaganeite in nanospindles, which suppresses the further oxidation of PEDOT during the formation of PEDOT/β-Fe3+O(OH,Cl) nanospindles. Similarly, because of the existence of β-akaganeite in PEDOT/ β-Fe3+O(OH,Cl) nanospindles, the electrical conductivity of them at room temperature is about 1.96 × 10-2 S/cm using a four-probe method, which was about 50 times lower than that of the bulk PEDOT (9.61 × 10-1 S/cm). When CTAB and PAA are not added into the reaction system, the electrical conductivity


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SCHEME 1: Growth Process of PEDOT/β-Fe3+O(OH,Cl) Nanospindles in Aqueous Solution: [EDOT] ) 0.025 M, [EDOT]/[FeCl3 · 6H2O] ) 1:3, [CTAB] ) 0.0075 M, [PAA] ) 0.02 mg/mL, 50 °C, 20 h

of the products is 9.17 × 10-2 S/cm due to the presence of the irregular PEDOT coagula among nanospindles. The decreasing of electrical conductivity in the PEDOT/β-Fe3+O(OH,Cl) nanospindles might link to interactions at the interfaces of the singlecrystalline inorganic species and the PEDOT polymers. In a word, PEDOT/β-Fe3+O(OH,Cl) nanospindles are a kind of novel composite which consist of PEDOT and β-akaganeite, and they have the dual properties of both a conducting polymer and an inorganic single crystal. The morphology and crystallinity are afforded by the part of inorganic β-akaganeite, and the electrical and electrochemical properties are attributed to the part of PEDOT. The mechanism of PEDOT/β-Fe3+O(OH,Cl) nanospindles is due to the simultaneity of the polymerization of EDOT and the hydrolyzation of FeCl3 in aqueous solution at 50 °C and the interactional result of them. According to the report of Imai and Oaki,39 CTAB, PAA, and PEDOT obtained from certain concentrations of EDOT act as gelling agents during the formation of PEDOT/β-Fe3+O(OH,Cl) nanospindles and suppress the diffusion of Fe3+, Cl-, OH- (caused by the hydrolyzation of FeCl3 in water) and forms a diffusion field. In order to observe the growth process of them along with the increase of reaction time, the reactions are carried out under different times. Figure 8 gives the SEM images of the growth process of PEDOT/β-Fe3+O(OH,Cl) nanospindles at the reaction times of 0.5, 1, 3, 6, 9, and 12 h, respectively. At first, CTAB, a cationic surfactant, and PAA, an anionic polyelectrolyte, can form enhanced micelles with EDOT monomer under magnetic stirring. When it encountered Fe3+ in water, the polymerization occurred and PEDOT formed. At the same time, the hydrolyzation of FeCl3 was also carried out. PEDOT and Fe3+O(OH,Cl) which might be the products of the hydrolyzation of FeCl3 were aggregated and nucleated gradually by themselves. These PEDOT/β-Fe3+O(OH,Cl) nuclei became bigger and bigger. As shown in Figure 8a, when the reactor was displaced to the thermostat water bath at 50 °C for 0.5 h, PEDOT/β-Fe3+O(OH,Cl) nanodots appeared with the diameter less than 50 nm. These nanodots conglutinated with each other (Figure 8b). After 3 h, small PEDOT/β-Fe3+O(OH,Cl) nanospindles appeared (Figure 8c). PEDOT/β-Fe3+O(OH,Cl) nanospindles with lengths of 180-270 nm and widths of 50-75 nm were observed at the reaction time of 6 h (Figure 8d). They

continued to grow along with the increase of reaction time. After 9 h, the lengths of PEDOT/β-Fe3+O(OH,Cl) nanospindles increased to 250-290 nm and the widths of 60-90 nm (Figure 8e). Figure 8f reveals the SEM image of them synthesized for 12 h. The sizes of them became bigger than that obtained for short time. They seemed as those synthesized for 20 h with the lengths of 270-310 nm and the widths of 80-90 nm. Finally, after polymerization for 20 h, dark blue precipitates of PEDOT/ β-Fe3+O(OH,Cl) nanospindles were obtained in the reaction system. The scheme of the growth process of PEDOT/βFe3+O(OH,Cl) nanospindles is shown in Scheme 1. In order to investigate the influence of the reaction conditions to the morphologies of PEDOT and the sizes of PEDOT nanospindles, the [EDOT], [EDOT]/[FeCl3 · 6H2O], reaction temperature, and the kind of oxidant are adjusted, respectively. Figure 9 shows the SEM images of PEDOT nanospindles synthesized under different reaction conditions. When [EDOT] decreases to 0.0125 M, the lengths and widths of PEDOT/βFe3+O(OH,Cl) nanospindles shorten to 200-250 and 60-80 nm, respectively (Figure 9a). However, when [EDOT] increases to 0.05 M, no nanospindles exist (Figure 9b). The sizes of PEDOT/β-Fe3+O(OH,Cl) nanospindles are also changed with [EDOT]/[FeCl3 · 6H2O], when [EDOT] is still 0.025 M. When [EDOT]/[FeCl3 · 6H2O] is 1:1, the lengths and widths of nanospindles shorten to 170-190 and 50-70 nm, respectively. However, most of them are conglutinate together with each other (Figure 9c). When [EDOT]/[FeCl3 · 6H2O] is changed to 1:9, PEDOT nanospindles disappeared, and the morphology seemed to be porous nanofiber-network coagula (Figure 9d). Parts e and f of Figure 9 reveal the morphologies of the products synthesized under different temperatures (20 and 80 °C), respectively. It is found that nanospindles are not obtained at higher temperature. But the morphology obtained at 20 °C was most like that synthesized at 50 °C for 3 h (Figure 8c). Interestingly, PEDOT/ β-Fe3+O(OH,Cl) nanospindles are obtained only using FeCl3 · 6H2O as an oxidant, and no nanospindles formed when we used other oxidants, such as Fe(NO3)3 · 9H2O and APS (Figure 9, parts g and h). As a result, Fe3+ and Cl- play important roles in the formation of PEDOT/β-Fe3+O(OH,Cl) nanospindles, and they are obtained only in the presence of them Fe3+ and Cl- at the special temperature. Because β-akaganeite is composed of four elements which are Fe, O, H, and Cl, in



Figure 9. PEDOT/β-Fe3+O(OH,Cl) nanospindles synthesized under different conditions, and the following conditions were changed, respectively: (a) [EDOT] ) 0.0125 M; (b) [EDOT] ) 0.05 M; (c) [EDOT]/[FeCl3 · 6H2O] ) 1:1; (d) [EDOT]/[FeCl3 · 6H2O] ) 1:9; (e) 20 °C; (f) 80 °C; (g) Fe(NO)3 · 9H2O as the oxidant, [EDOT]/[Fe(NO)3 · 9H2O] ) 1:3; (h) APS as the oxidant, [EDOT]/[APS] ) 1:3. Other conditions were not adjusted and consisted of those of Figure 1a except (h) (panel h was without CTAB and PAA).


Mao et al.

Figure 10. SEM images of PEDOT/β-Fe3+O(OH,Cl) nanospindles synthesized in different kinds of organic solvent/H2O mixture solvents (v/v): (a) ethanol/H2O ) 5/35; (b) ethanol/H2O ) 10/30; (c) THF/H2O ) 5/35; (d) THF/H2O ) 7.5/32.5; (e) DMF/H2O ) 2.5/37.5; (f) DMF/H2O ) 5/35. Other conditions: [EDOT] ) 0.025 M, [EDOT]/[FeCl3 · 6H2O] ) 1:3, 50 °C, 20 h.

TABLE 1: Details of the Changes of PEDOT/β-Fe3+O(OH,Cl) Nanospindles Morphologies Synthesized under Different [CTAB] and [PAA]: [EDOT] ) 0.025 M, [EDOT]/[FeCl3 · 6H2O] ) 1:3, 50 °C, 20 h sample

[CTAB] (M)

[PAA] (mg/mL)

lengths (nm)

widths (nm)

remarks

a b c d e f g h

0.00375 0.0075 0.015 0.03 0 0 0.0075 0.0075

0 0 0 0 0.02 0.08 0.005 0.08

300-375 390-410 220-360 220-380

-Fe3+O(OH

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