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J. Phys. Chem. B 2002, 106, 288-292
Raman Spectroscopic and Electrochemical Studies on the Doping Level Changes of Polythiophene Films during Their Electrochemical Growth Processes Gaoquan Shi,* Jingkun Xu, and Mingxiao Fu Department of Chemistry and Bio-organic Phosphorus Laboratory, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: August 6, 2001; In Final Form: NoVember 3, 2001
Polythiophene (PT), poly(3-chlorothiophene) (PCT), and poly(3-methylthiophene) (PMeT) have been electrochemically deposited on mirrorlike stainless steel (SS) electrode surfaces by direct oxidative polymerization of corresponding monomers in boron trifluoride diethyl etherate (BFEE) solution. The doping levels of as-grown thin films have been determined electrochemically. The results demonstrated that PTs were formed nearly in neutral state initially and their doping levels increased during electrochemical growth processes. Monomer structure and electrolyte have strong effects on the doping levels of PTs with a given thickness. Raman spectroscopic studies also confirmed these findings. Furthermore, it is found that the oxidized species of polythiophene and poly(3-chlorothiophene) are mainly presented in radical cations, while dications are favored to be formed in poly(3-methylthiophene) with a doping level higher than ca. 5%.
Introduction Electrochemical polymerization is an useful and widely applied technique for synthesizing conducting polymer (CP) films.1-3 The CP films prepared by this technique are usually formed in an oxidized (doped) state and used without further treatment.4,5 The doping levels of CP films depend strongly on the experimental conditions such as applied potential, electrolyte, supporting salt, and monomer structure.6,7 The color and morphology of the CP film are known to change during the film growth process.8-12 However, little work has concerned the thickness dependence of the doping level of electrochemically deposited CP films. Recently, we found that the CP films such as polypyrrole, poly(para-phenylene) and polythiophenes (PTs) were formed almost in a neutral state initially and then the doping levels increased gradually to constant values. The thickness scales for doping level change are from several tens of nanometers to several micrometers, depending on the structure of the monomer, the properties of the supporting salt, and the conditions of polymerization. In this article, we report the results of polythiophenes and discuss the effects of substituted groups. Vibrational spectra, in particular Raman spectra, can provide structural information on pristine and doped conducting polymers.13-18 Raman spectra of polythiophenes also have been studied extensively,13-18 and most of these works were carried out by using a surface-enhanced Raman scattering (SERS) technique. SERS can identify a PT film whether in the doped (oxidized) or in the neutral (reduced) state, however, cannot sensitively respond to the doping level change of PT. This is because SERS can selectively enhance the Raman bands associated with the oxidized species of polythiophene.13 Furthermore, the SERS technique needs a roughed electrode surface, which does not represent the real condition of polymer synthesis. Therefore, we used the usual resonance Raman technique to carry out this work, and mirrorlike stainless steel (SS) electrodes were applied as substrates for polymer syntheses. Fortunately, * To whom correspondence should be addressed. Tel: 86-10-62773743. Fax: 86-10-62774129. E-mail:
[email protected].
high-quality spectra with high signal-to-noise ratios were obtained. The polymers were synthesized by direct electrochemical oxidation of corresponding monomers in freshly produced boron trifluoride diethyl etherate (BFEE) solution at constant applied potentials. In this medium, compact and flexible polythiophene films with great strengths are obtainable.19-22 Experimental Section Chemical pure grade thiophene was a product of Beijing University of Science and Technology (China) and used after distillation. 3-Chlorothiophene (98%) and 3-methylthiophene (98%) were purchased from Acros and used as received. Boron trifluoride diethyl etherate (BFEE) with a BF3 content of 48.2% and a water content of 0.24% (by volume) was purchased from Beijing Changyang Chemical Factory. It was a fresh colorless product and also used directly. Trifluoroacetic acid (TFA, 99%) was a product of Beijing Hongyu Chemical and Engineering Company and used without further treatment. Electrochemical syntheses and examinations were performed in a one compartment cell with the use of a model 283 potentiostat-galvanostat (EG&G Princeton Applied Research) under computer control. The working and counter electrodes were AISI 430 stainless steel sheets with a surface size of about 1.0 cm2 each and were placed 0.5 cm apart. The electrodes have shiny and mirrorlike surfaces and were cleaned ultrasonically after each synthesis. All potentials were referred to a Ag/AgCl electrode immersed directly in the solution. A correction of 0.069 V was needed to bring the measured potentials in BFEE originally vs Ag/AgCl to potentials vs the standard hydrogen electrode.19 The electrolyte solution was BFEE containing 30 mmol/L thiophene, 100 mmol/L 3-methylthiophene, or 100 mmol/L 3-chlorothiophene. All solutions were deaerated by dry nitrogen stream and maintained under a slight nitrogen overpressure during the experiments. Polythiophene (PT), poly(3methylthiophene) (PMeT), and poly(3-chlorothiophene) (PCT) films were uniformly deposited on the working electrode surfaces by applying a constant potential of 1.30, 1.15, and 1.50 V (vs Ag/AgCl), respectively. For PCT, a mixed electrolyte of
10.1021/jp013023o CCC: $22.00 © 2002 American Chemical Society Published on Web 12/20/2001
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J. Phys. Chem. B, Vol. 106, No. 2, 2002 289
Figure 1. Plots of doping level versus film thickness of PTs: (a) PCT prepared in BFEE; (b) PCT prepared in BFEE + 10% TFA; (c) PT prepared in BFEE; (d) PMeT prepared in BFEE.
Figure 2. Successive cyclic voltammograms of a 150 nm PCT film prepared from the medium of BFEE + 10% TFA in the monomer-free electrolyte at a potential scan rate of 100 mV/s.
BFEE + 10% TFA was also used and the applied potential was 1.35 V (vs Ag/AgCl). The thickness of the polymer film was calculated from the total charges passed in the cell. The charge values were read directly from the i-V curves by computer. The charge efficiencies were determined by weighting the dedoped polymers collected from the electrode surfaces, and the weight densities of the films were measured to be ca. 1.2 g cm-3. As-grown polymer films were washed repeatedly with diethyl ether to remove the electrolyte and monomer before Raman characterizations. Cyclic voltammograms were carried out by using polymer-film-coated stainless steel sheets as working electrodes and a monomer-free solvent as electrolyte. The doping levels (f) of as-grown films of PTs with different thickness were determined electrochemically by using eq 1:23
f ) [2Qo/(Qd - Qo)] × 100%
(1)
where Qd is the total charge used for PT deposition and Qo is the total charge of oxidized species in the PT films. Raman spectra were carried out by using a RM 2000 microscopic confocal Raman spectrometer (Renishaw PLC., England) employing a 514, 633, or 785 nm laser beam and a CCD detector with 4 cm-1 resolution. The spectra were recorded by using a 50× objective and accumulated for 3 times and 30 s each. The power was always kept very low (∼0.1 mW) to avoid destruction of the samples. Some complex Raman peaks are divided into component Lorentzian peaks with proper background subtraction using the “automatic fitting” program provided by the Raman spectrometer. The UV-visible spectra of the poly(3-chlorothiophene) films electrochemically deposited on ITO electrodes were recorded on an Ultra-spec 4000 spectrometer (Bio-tech). Results and Discussion Figure 1 plots the doping level (f) measured electrochemically versus film thickness for the polythiophenes. It is clear from this figure that the PTs are formed almost in a neutral state (f ≈ 0) initially and then their doping levels increase gradually to constant values with film thickening. The film formed in the initial stage is still conductive enough for film growth, while its doping level is fairly low, mainly because the film has an ultrathin thickness and a conjugated chain structure. As shown in Figure 1, the highest doping levels of PMeT, PT, and PCT prepared in pure BFEE medium are found to be about 28%, 23%, and 19%, respectively. However, the thickness scales of doping level change for these three polymers are in an opposite sequence. They were determined to be about 0-600 nm for
Figure 3. The Raman spectra of a 600 nm PCT film prepared from BFEE excited by a 514 nm (a), 633 nm (b), or 785 nm (c) laser beam.
PMeT, 0-800 nm for PT and 0-2000 nm for PCT films. These results indicated that the electron-donating group, methyl, improved the doping ability of polythiophene, while the electronwithdrawing group, chlorine, had an opposite effect. The addition of trifluoroacetic acid (TFA) in BFEE can increase the doping level of PCT as shown in curves a and b of Figure 1. The low doping level of a thin film of PTs also can be confirmed by cyclic voltammetry (CV). Here, take a thin PCT film as an example. Figure 2 presents the successive CV scans of a 150 nm PCT film [prepared from the medium of BFEE + 10% TFA (by volume)] in monomer-free electrolyte. On the first CV cycle, the redox waves of the polymer are very weak, indicating a low doping level of the original film. The current densities of the couple of waves increase gradually as the CV scan continues, demonstrating the doping level increase of the polymer. After 24 CV scans, the total charge of oxidation wave or reduction wave nearly reaches the maximum, which is about 8 times that on the first CV cycle. These results imply that the as-grown thin CP film was much less charged than it could be and successive cyclic voltammetric scanning is an effective technique for further doping the thin CP film. This observation is in good agreement with that of polypyrrole film reported by Zhou et al.24 The thickness dependence of the doping level of CP films also can be studied by Raman spectroscopy. The Raman spectroscopic features of polythiophenes depend strongly on the wavelength of the laser beam used for excitation. Figure 3 shows the Raman spectra of a 600 nm poly(3-chlorothiophene) film. The assignments of Raman bands of PTs, in agreement with
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Shi et al.
TABLE 1: Band Assignments of the Raman Spectra of PTs (R ) Pristine, O ) Oxidized) PMeT assignments CRdCβ ring stretching (anti) CRdCβ ring stretching quinoid (radical cations) quinoid (dications) CH3 in-plane deformation Cβ-Cβ′ ring stretching CR-CR′ stretching Cβ-H bending CR-CR′ stretching (anti) ring-CH3 stretching ring C-S stretching ring deformation C-S-C ring deformation C-S-C ring in-plane bending Cβ-Cl deformation Cβ-Cl deformation ring-CH3 out-of-plane bending
R
O
PT R
PCT O
R
O
notes
1512 1498 1495 1495 1495 1495
ν1
1445
ν2
1455
1455
1420
1420
1420
Q1
1395
1400
1390
Q2
1352 1353 1370 1360 1355 1367
ν3
1206 1217 1221 1223 1211 1213 1182 1188 1045 1056 1137 1148 1146 1160 1153 1151 983 983 872 865 868 865 737 736 736 729 733 727 718 721 693 693 720 712 548 548 433 436 236 232 274 274
ν4 ν5
1383
ν6 ν7
previous reports,13-17 are listed in Table 1. The normal modes are denoted as Q1, Q2, and ν1-ν7. The bands related to distorted parts of PTs resulting from doping are denoted as D and indicated in the figures. As can be seen from Figure 3, the Raman spectrum obtained by excitation at 633 nm is almost the same as that excited at 514 nm. The most intense band appeared at ca. 1450 cm-1, which is assigned to the totally symmetric in-phase vibration of the bulk thiophene rings spreading over the whole polymer chain.13,25 This band is associated with the conjugated polythiophene segments in the neutral state. The overall features of these figures are similar to those of the Raman spectra of neutral polythiophenes. Accordingly, these spectra cannot sensitively respond to the oxidized species and therefore cannot to be applied for studying the doping level and doping level change of the PCT. On the other hand, in the 785 nm excited spectrum, there is a shoulder band at ca. 1420 cm-1 (Q1). This band is attributed to the symmetric stretching mode of the CdC bond ring of radical cations (polaron).26,27 Furthermore, several broad shoulder bands of kinks associated with distorted parts of the polymer chains are found and indicated as D bands in the figure. UV-visible spectral results demonstrated that undoped poly(3-chlorothiophene) films had a very strong visible absorption band, the maximum of which was found to be located at ca. 550 nm. On the contrary, doped species only show a very weak absorption band in this region and have a broad electronic absorption band with a maximum at ca. 850 nm. Therefore, when excited at 514 or 633 nm, the resonance effect enhances more specifically the Raman lines of the undoped species. However, the 785 nm excited Raman spectrum can give the structural information of both neutral and oxidized species simultaneously and thus can be used to study the doping level change of PCT. Figure 4 illustrates the 785 nm excited Raman spectra of the as-grown poly(3-chlorothiophene) films synthesized by electrochemical polymerization of 3-chlorothiophene in pure BFEE. It is clear from this figure that the overall features of the Raman spectrum depend strongly on film thickness. The 45 nm film showed a spectrum of lowly doped PCT, and a sharp and strong ν2 band appeared. The width of this band increases with the increase of film thickness. In the spectrum of the 600 nm film, a new shoulder band (Q1) appeared at ca. 1420 cm-1. On
Figure 4. 785 nm excited Raman spectra of PCT films with different thickness and prepared in BFEE.
processing the band in the region of 1250-1650 cm-1 into components of Lorentzian profiles (Figure 5a), the intensity ratio of Q1 and ν2 is increased from 0.15 for the 45 nm film to 2.97 for the 800 nm film (Table 2). Furthermore, a new band at ca. 1400 cm-1 appeared in the Raman spectra of 300 nm or thicker films. This band is assigned to dications (dipolaron) of doped polythiophene (Q2).26,27 The intensity ratio of Q2 and ν2 bands is also increased with the increase of film thickness (Table 2). The ν3 band at ca. 1360 cm-1 is assigned to the C-C ring stretching. Its intensity and width also increase with the increase of film thickness.13 The well-defined and characteristic sharp and medium strong scattering near 1140 cm-1 is assigned to the totally symmetric in-plane wag of the C-H bonds in the position of β′. The bands of C-S-C deformation appeared at ca. 730 and ca. 710 cm-1. These bands showed a little shift to higher frequencies with the increase of film thickness. The bands at ca. 430, 235, and 180 cm-1 are attributed to C-Cl deformation. The widths of these bands increase with the increase of film thickness. On the basis of the spectral observations described above, it is reasonable to conclude that the doping level of the PCT film increases with the increase of film thickness. Accordingly, the Raman spectrum of the PCT film with thickness higher than 800 nm is kept unchanged and has the features of the spectrum of deeply doped PCT. It is known that BFEE exists in diethyl ether as a polar molecule, [(C2H5)3O+]BF4-, and the existence of the small amount of water complexes BF3 into H+[BF3OH]-, which furnishes a conducting medium.28,29 However, the conductivity of pure BFEE is relatively low (∼960 µS cm-1). The addition of a small amount of TFA (10%, by volume) into BFEE can increase the conductivity of the electrolyte significantly (∼4500 µS cm-1). This supports the doping level improvement of the PCT film. As shown in Figure 6, the 300 nm PCT film prepared from BFEE + 10% TFA showed much stronger Q and D bands than those in Figure 3 of the film with the same thickness, indicating a much higher doping level. In this medium, the smallest thickness at which the Raman spectrum of the PCT film remained unchanged was found to be about ca. 500 nm. This value is much smaller than that observed in pure BFEE medium (>800 nm). The spectroscopic phenomenon described above was also found on polythiophene and poly(3-methylthiophene) films. For these two polymers, the 633 nm laser beam was tested to be the best excitation source for detecting the Raman signals of neutral and oxidized species simultaneously. Figure 7 illustrates
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J. Phys. Chem. B, Vol. 106, No. 2, 2002 291
Figure 5. Decomposition of the Raman bands in the region of 1250-1600 cm-1 into components of Lorentzian profiles for the Raman spectra of PTs: (a) PCT (1800 nm) prepared in BFEE; (b) PCT (1400 nm) prepared in BFEE + 10%TFA; (c) PT (800 nm) prepared in BFEE; (d) PMeT (2100 nm) prepared in BFEE.
TABLE 2: Band Intensity Ratio in Terms of Film Thickness of PTs polymer PCT
PCT
PT
PMeT
film thickness, nm
doping level (f), %
IQ1/Iν2
IQ2/Iν2
IQ2/IQ1
30 70 300 600 800 1800 70 110 300 530 700 1400 50 110 180 240 800 20 50 60 100 600 2100
3.8 4.1 5.7 7.3 9.5 19.43 4.2 4.4 7.4 17.1 21.8 22.6 8.3 9.6 12.7 16.1 22.7 2.5 4.6 5.5 11.3 25.9 28.4
0.152 0.193 0.282 1.120 2.967 3.622 0.139 0.139 0.899 2.807 5.251 8.929 0.180 0.566 0.938 1.124 2.977 0.040 0.271 1.229 1.388 1.560 2.488
0 0 0.123 0.413 1.168 1.232 0 0 0.354 1.728 2.000 3.524 0 0 0.130 0.225 0.105 0 0.203 1.416 2.730 2.991 3.437
0 0 0.436 0.369 0.394 0.340 0 0 0.394 0.616 0.381 0.395 0 0 0.130 0.225 0.177 0 0.749 1.901 1.967 1.917 1.382
the 633 nm excited Raman spectra of polythiophene (A) and poly(3-methylthiophene) (B) films. The band assignments of the Raman spectra of polythiophene and poly(3-methylthiophene) have been reported in detail previously and are also simply listed in Table 1. As can be seen from Figure 7 and Table 2, the Raman bands related to oxidized species such as radical cations, dications, and kinks are increased, while the
Figure 6. 785 nm excited Raman spectra of PCT films with different thickness and prepared from BFEE + 10% TFA.
band associated with neutral species such as the ν2 band decreased with the increase of film thickness, indicating a increase of doping levels. The doping level (f) and the Raman intensity ratio of the Q2 and Q1 bands (IQ2/IQ1) are also listed in Table 2. It is clear from this table that in the spectra of polythiophene and poly(3chlorothiophene), the strength of the Q2 band is always lower than that of the Q1 band (IQ2/IQ1 < 0.5). However, in the spectra of PMeT films with a doping level higher than 5%, the intensity of Q2 band is stronger than that of Q1 (IQ2/IQ1 > 1). Therefore, it is reasonable to conclude that the oxidized species in polythiophene and poly(3-chlorothiophene) are mainly presented
292 J. Phys. Chem. B, Vol. 106, No. 2, 2002
Shi et al. chemically deposited as-grown films of PTs. The Raman spectroscopic features of the PTs depend strongly on film thickness. This is mainly because the doping level increases during the film growth process. This finding suggests that the composition and properties of the electrosynthesized conducting polymer depend strongly on film thickness. In polythiophene and poly(3-chlorothiophene) films, the oxidized species are favored to present in radical cations, while in deeply doped poly(3-methylthiophene) films, the charges are mainly formed in dications. The thickness dependence of doping level of PTs has also been confirmed electrochemically. Acknowledgment. This work was supported by National Natural Science Foundation of China with Grant Numbers 50073012 and 50133010. References and Notes
Figure 7. 633 nm excited Raman spectra of PT (A) and PMeT (B) films with different thickness and prepared in BFEE.
as radical cations and dications are preferred to be formed in poly(3-methylthiophene) films. This is mainly because the electron-donating methyl groups support the formation of dications. Navarrete’s group studied the Raman spectra of R,R′dimethylquinquethiophene and R,R′-dimethylsexithiophene and found that the oxidized species are mainly dications.26 They pointed out that their observations were in contrast to the results reported by Furukawa who studied the polymer and oligomers of polythiophene.30 Our results are in good agreement with those of these two groups, and the answer to the scientific controversy is whether the methyl groups are present on the thiophene rings. Furthermore, at a given doping level, it is found that the IQ2/IQ1 of polythiophene (
