J . Phys. Chem. 1992, 96, 3782-3788
3782
of only one set of methyl vibrational modes. The values of [&J,~~~(C-H)]/I$J~~,,,(C-H)]] are 5.9 and 5.7 for the neat liquid and surface forms, respectively. Thus, the multiplier value is ca. 0.97. The orientation of 2-butanethiol shown in Figure 7b is proposed to explain this value. In this picture of 2-butanethiol bonding, the Ag-S bonding is assumed to be analogous to that observed for 1-butanethiol. One methyl group is proposed to be at a large tilt angle with respect to the surface and the other essentially perpendicular to the surface. This leads to the average value of the multiplier observed, which is close to an average apparent tilt angle of ca.45". This orientation is consistent with a multiplier value that represents an average value for the two different methyl group orientations, a very small value for the perpendicular methyl group and a value greater than unity for the methyl group largely parallel to the surface. This orientation is similar to one previously proposed by Joo and co-workers in the existence of the molecule in a gauche conformation on the surface.21 The coincidence of the orientation proposed for this molecule on the basis of this method and the orientation proposed by Joo lends further support to the validity of this method. One final test of the proposed method is the study of l-butanethiol adsorbed at Au. Independent studies have previously established the orientation of alkanethiols with respect to Au surfaces.20J2 On the basis of IR spectral data, Porter and coworkers have estimated the tilt of the methyl group to be ca. 26" with respect to the surface normaLZ0 The liquid and Au surface spectra for 1-butanethiol are shown in Figure 8a. The relative intensity ratios are reported in Table I. The value of [flvaSym(CH)]/qvSym(C-H)]]for the Au surface is 1.4, resulting in a value of the multiplier value of ca. 0.82. This value is slightly less than that measured for the Ag surface and is consistent with the (21) Joo, T. H.; Kim, M. S.; Kim, K. J . Mol. S t r u t . 1987, 160, 81. (22) Nuzzo, R.G.; Dubois, L. H.; Allara, D. L. J . Am. Chem. SOC.1990, 112, 558.
orientation proposed previously for alkanethiols at Au surfaces in which the alkane chain is tilted at an angle of ca. 25-30° with respect to the surface normal with a chain rotation of ca. 50°.20-22 As can be seen by comparison of the proposed orientations in Figures 6b and 8b for the two metals, the methyl group for these alkanethiols on Au is more perpendicular than found on Ag surfaces despite the greater tilt angle of the alkane chain on Au. This effect is a consequence of the greater degree of rotation about the alkane chain on Au than on Ag.
Summary The results presented for all of the molecules considered here suggest that the proposed method for determination of adsorbate orientation relative to the axis at 45" with respect to the surface normal based on previously described surface selection rules is valid. In fact, no molecule containing a methyl group has been investigated in this laboratory which has not adhered to this method. The largest fact limiting better quantitative accuracy of this method is knowledge of the relative radiation fields normal and tangential to the surface. This situation may be rectified through the use of surfaces prepared with roughness features of absolutely controlled size and shape. It should be noted that this approach is not limited to use with methyl groups. Although the methyl functionality provides a nice demonstration of the concept, this approach is valid for any functional groups which have multiple vibrational modes whose spatial relation are known. Thus, molecules containing such functional groups as methylene, nitro, amino, carboxy, and benzene rings may be amenable for study. Acknowledgment. We are grateful for support of this work by the National Science Foundation (CHE-8614955). Registry No. Silver, 7440-22-4; methanol, 67-56-1; 1-butanol, 7136-3; isobutyl alcohol, 78-83-1; 2-butano1, 78-92-2; 1-butanethiol, 10979-5; 2-butanethiol, 513-53-1;gold, 7440-57-5.
Polymerization Mechanism and Physicochemical Properties of Electrochemically Prepared Polyindole Tetrafluoroborate Kyung Moon Choi,+Chung Yub Kim,$and Keu Hong Kim*,+ Department of Chemistry, Yonsei University, Seoul 120, Korea, and Polymer Materials Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Dongdaemun, Seoul, Korea (Received: July 12, 1991)
Polyindole tetrafluoroborate [(PI)BF4]was obtained from a acetonitrile/benzonitrile solution containing 0.2 M indole and 0: 1 M tetraethylammonium tetrafluoroborate, supplying a potential of 0.7 V. Cyclic voltammetry measurements suggested that the electrode reaction in the indole solution was a reversible reaction and the number of related electrons was 2. The measurements suggested that an electropolymerization of indole proceeded by the formation of radical cations as intermediates. Thermal analyses of polyindole- and polyaniline-based systems were performed at 25-800 "C. The TGA results indicated that most polyindole-based polymers were mainly decomposed at higher temperatures than those of the polyaniline-based systems. Also, the maximum values of the reaction rate of thermal decomposition (Rmx)for the polyindole-based systems were not seriously affected by the kind of dopants. On the other hand, the R,, values of the polyaniline-based ones depended on the kind of electron acceptor. The electrical conductivity for (PI)BF4was measured in the temperature range from -150 to 25 "C. The resulting values of log u at 25 "C and E, value as calculated from the Arrhenius plot were -2.43 S/cm and 0.4057 eV, respectively. The conductivity measurements suggested that possibly the conduction mechanism and charge carriers were hopping conduction and polarons, respectively, which then caused the BF4- anion to act as an electron acceptor. The ESR measurements for the (PI)BF4 powder were performed at 25 O C , so the values of AHw, g,and spectral ratio were obtained.
Introduction For the last century or so, the application of polymers in various fields of electronics, daily life, industry, agriculture, and archi+
Yonsei University. Institute of Science and Technology.
1 Korea
0022-3654/92/2096-3782%03.00/0
tecture has been immense. As the demand for polymeric materials with sp.cifiCfunctions grew, SO did the research for new materials. First, syntheses of new functional polymers were Performed in the field of medical materials, which subsequently contributed to the development of artificial internal organs. Also, polymers with high conductivity, such as metals or inorganic compounds, 0 1992 American Chemical Society
Electrochemically Prepared Polyindole Tetrafluoroborate TABLE I: Anodic Peak Potential nod Number of Electron Related Electrode Reaction for Ektrocbcmically Polymerized C o m w ~ n d s ~ * ~ anodic peak no. of anodic peak no. of monomers potential, V electrons monomers potential, V electrons ani1ine 0.73 2 pyrene 1.23 2.3 1 azulene 0.91 2.2 pyrrole 1.20 2.2 carbazole 1.30 2.45 thiophene 2.07 2.06 indole 0.90 2
have been developed in response to the demands in the fields of automobile manufacturing, electronics, and industry. These demands encouraged our scientific study of conducting polymers and their possible applications in daily life. There are two synthetic methods of conducting polymers reported in previous works. In the first method, the polymeric composite systems can be obtained by the injection of filler in the system. In the second, polymer systems having conjugated double bonds can be prepared by chemical and electrochemical oxidations. In this work, polymers obtained from electrooxidation were studied owing to their application in electrochemical fields and their stability in air. Recently, research into aromatic compound-based conducting polymers prepared by electrooxidation has drawn much attention. Results of polypyrrole-,'.2 polyaniline-,3 polya~ulene-,~ polycarba~ole-,~ and polythiophenebased6 systems have been reported. Pfluger' reported from spectroscopic measurements that the conductivity of polypynole perchlorate increases only in the early stages of oxidation, whereas significant changes in the optical and ESR properties occur in later stages of the oxidation when no further changes in the conductivity take place. Bargons reported that polyazulene, polythiophene, polycarbazole, polypyrene, and polytriphenylene were obtained by electrochemical polymerization. Pfluger and Bargon also reported that the NMR, ESCA, SEM, and cyclic voltammetry (CV) measurements for these aromatic compound-based conducting polymers doped with various dopants indicated the polymerization mechanisms of these systems. In order to obtain products as forms of anodic precipitates, we must consider the following electrochemical conditions. First, the polymeric materials obtained on the anode have been only prepared from aromatic monomers which have anodic peak potentials of below 2 V. With potentials above 2 V, the possibility of oxidation of solvents and of supporting electrolytes increases with the increase in anodic potential. Second, we have to consider the stability of radical cations formed around the anode. These anodic products as precipitates can be obtained from electropolymerization of radical cations formed around the anodic electrode. Thus, radical cations must not be diffused to bulk solution until they are polymerized. The proper stability of radical cations is related to the solvent effect. That is, in order to obtain the products in the form of precipitates, the selection of proper solvents is important. Accordingly, the determination of proper solvents and of electrochemical conditions for various aromatic compounds has been carefully considered by many researchers. Third, we also consider the suitable electrode composition. Generally, a three-electrode cell system has been adopted for obtaining conducting polymers in an electrooxidation system. It is well-known that the physical properties of polymers prepared from the three-electrode system are better than those of polymers obtained from the two-cell system. (1) Diaz, A. Z.; Hall, B. IBM J . Res. Deu. 1983, 27, 342. (2) Mermilliod, N.; Tanguy, J.; Petiot, F. J . Electrochem. SOC.1986, 133,
1073. (3) Somasiri, N. L. D.; MacDiarmid, A. G. J . Appl. Electrochem. 1988, 18, 92. (4) Bargon, J.; Mohmand, S.;Waltman, R. J. Mol. Cryst. Liq. Cryst. 1983, 93, 279. ( 5 ) Ambrose, J. F.; Carpenter, L. L.; Nelson, R. F. J . Electrochem. SOC. 1975, 122, 876. (6) Roncali, J.; Garnier, F. J . Phys. Chem. 1988, 92, 833. (7) Pfluger, P.; Krounbi, M.; Street, G. B.; Weiser, G. J . Chem. Phys. 1983, 78, 3212. ( 8 ) Bargon, J.; Mohmand, S.;Waltman, R. J. I B M J . Res. Deu. 1983, 27, 330.
The Journal of Physical Chemistry, Vol. 96, No. 9, 1992 3783 TABLE 11: Elemental Analysis for Polyindole Tetrnfluorobornte polymer chains, % C
(PI)BFd
67.6
H 3.9
N
doDant. %
10.2
18.3
From the values of anodic peak potential listed in Table 1,8,9 we expected that indole and its derivatives may be polymerized by electrooxidation. The study of electropolymerization of indole was reported by Tourillon.lo But, they reported that polyindole films were very brittle and cracked easily when removed from the anode, that is, their physical properties were no good. Therefore, we are trying to obtain polyindole systems which have good quality based on the control of the most suitable solvent and electrochemical conditions. From this try, we obtained polyindole tetrafluoroborate [(PI)BF4] as an anodic precipitate which had good quality from indole in a [acetonitrile (AN)/benzonitrile (BN)/ water] solution containing tetraethylammonium tetrafluoroborate [(TEA)BF,I. The (PI)BF4 polymer was obtained in the form of a powder. Most previous workss-" on conducting polymers concentrated on the property of conducting polymers in the form of film. However, film-type samples have some disadvantages for mass production, reproducibility, handling, and shaping which polymer samples in powder form can overcome. In this paper, the (PI)BF4 was obtained as a powder form from electrooxidation. From electrochemical and spectroscopic measurements and results of morphology and thermal analyses, the mechanisms of conduction and electropolymerization of (PI)BF4 were determined, and the physical properties of the (PI)BF, were studied.
Experimental Section Materiab. Acetonitrile (AN) and benzonitrile (BN) as solvents and tetraethylammonium tetrafluoroborate [ (TEA)BF4] as a supporting electrolyte were obtained from Merck and Aldrich Chemical Co., respectively. For its hygroscopic nature, (TEA)BF, was fully dried in a vacuum oven and then used in every experiment. Indole was obtained from Kanto Chemical Co. and also dried in a vacuum oven due to its deliquescent property. Acetonitrile was purified by vacuum distillation, and the solvent was passed through a column packed with alumina and trifluoroacetic anhydride to remove the water. Sample Preparations. Polyindole tetrafluoroborate [(PI)BF4] was electrochemically polymerized from 0.2 M indole in a (AN/BN/water) solution containing 0.1 M (TEA)BF4 as a supporting electrolyte. The three-electrode cell composition was equipped with Pt plates as a working and a counter electrode shaped like a sheet and a Ag/AgCl electrode as a reference electrode. The temperature of the solution in the reaction cell was also maintained a t 25 "C, using a circulator (Lauda Co.). A nitrogen stream was passed through the solution for 30 min before each measurement in order to remove the dissolved oxygen. The (PI)BF, sample was obtained as insoluble precipitates using a potential of 0.7 V on the anode. With a supplied external potential of 0.7 V, the color of the solution around the anode changed from dark green to maroon black. As oxidation progressed, the solution in contact with the anode was extracted, and the UV measurement for this extracted solution was performed. The anodic precipitates were removed from the electrode, rinsed with acetonitrile, and dried in a vacuum oven for 2 days until constant weight was achieved. The (PI)BF4obtained was a black conducting powder which in its oxidized state was stable in air. Elemental Analysis. Conductivity in conducting polymers depended on the content of the dopants. Thus, an analysis of the dopant content for the conducting polymers obtained was necessary. For this reason, elemental analysis for the (PI)BF, powder was performed by using an elemental analyzer (Perkin Elmer, (9) Diaz, A. F.; Crowley, J. I.; Bargon, J.; Gardini, G. P.;Torrance, J. B. J . Electroanal. Chem. 1981, 121, 355. (10) Tourillon, G.; Garnier, F. J. Electroanal. Chem. Inrerfacial Electrochem. 1982, 135, 173. (1 1) Waltman, R. J.; Bargon, J.; Diaz, A. J . Phys. Chem. 1983, 87, 1459.
3784 The Journal of Physical Chemistry, Vol. 96, No. 9, 1992 240-C), and the result is listed in Table 11. Electrochemical Measurement. Sample preparations and electrochemical measurements were performed using a potentiostat (Hokuto Denko Co., HA-301) and a function generator (Hokuto Denko Co., HB-104). The results of this electrochemical system were obtained using a plotter (Omega, DAP-780). Conductivity Measurements. Electrical conductivity for the (PI)BF4 pellet was measured by the four-probe technique at a temperature range of -150-25 "C. The (PI)BF, was prepared by electropolymerization by supplying a potential of 0.7 V during a synthesis time of 10 h. The resultant anodic precipitates were fully dried in a vacuum oven. Finally, the precipitates obtained were in the form of a fine powder. The (PI)BF, powder was made into a pellet under a pressure of 98.06 MPa, resulting in a diskshaped pellet with a diameter of 12 mm and a thickness of 1.7 mm. For conductivity measurements, a quarz probe was placed in a temperature-controlled chamber, and the temperature of the sample in the specimen basket of the probe was measured by using a digital thermometer (Seoul Control Co., SR-6200, G-116) connected to a ceramic thermocouple. The low temperature was measured using liquid nitrogen. The current and potential through the sample were measured with a digital electrometer (Keithley-616) and a digital multimeter (Keithley-642), respectively. The conductivities for the (PI)BF4 pellet were calculated from the measurements of current and potential at every temperature with a heating rate of 1 OC/min. Thermal Analysis. The thermogravimetric analysis (TGA) of the (PI)BF, powder was performed with a thermal analyzer (Rigaku-8150). TGA under a nitrogen stream was carried out over a temperature range of 25-800 OC with a heating rate of 10 OC/min. The TGA analyzer system was m ~ e c t e dwith a IBM computer. Thus, the rate of thermal decomposition ( R )was directly computed from the result of the TGA measurements. Scanning Electron Microscope (SEM). A (PI)BF, film coated on the anode was prepared for the morphology analysis with a synthesis time of 10 min. This coated film was rinsed with acetonitrile and dried in a vacuum oven at 25 OC. For the SEM analysis, the surface of the (PI)BF4 film was covered with gold using an ion coater (Eiko, IB-3), and the SEM measurements were performed using a scanning electron microscope (Hitachi, S-510). ESR Measurements. Electron spin resonance (ESR) measurements of the (PI)BF, powder at 25 OC were performed using an EPR spectrometer (Bruker, ER 200 E-SRC). The (PI)BF4 powder was placed in an ESR tube at 25 OC. The ESR spectrum was obtained under the following conditions: scan range, 100 G; microwave frequency, 9.45 GHz; microwave power, 20 dB, 2 mW; modulation frequency, 100 KHz; modulation time constant 1 s; modulation amplitude, 4 Gpp;receiver gain, 5 X lo2. ESCA Analysis. An ESCA analysis for the (PI)BF4 pellet was performed using an X-ray photoelectron spectroscopic apparatus (Shimadzu ESCA-750). The source of the X-ray beam was Mg Kor with an energy of 1253.6 eV. IR Measurements. IT-IR analysis for the (PI)BF4 was performed (Biorad, FTS-80).The measurements were carried out by the KBr pellet method. The sample pellet was pressed in a vacuum to protect from the air-mixing phenomenon. UV Measurements. We considered the color change of the solution around the anode during the electrochemical polymerization. As the potential was supplied the dark green solution was initially produced on the anode and then diffused to a bulk solution. The color of bulk solution then changed to light green. We extracted this light green bulk solution with a microsyringe and labeled this extracted solution as BF-1 solution. As the polymerization progressed, the color of the products around the anode darkened, and the products settled on the cell bottom. We extracted this maroon solution with a microsyringe and labeled the solution as BF-2. When the reaction fully progressed, the color of the solution in the reaction cell had totally changed to maroon black. We also extracted this maroon black solution and labeled it as BF-3. The
Choi
I
ai.
10.1 mA
- 500
0
500
E (mv) Figure 1. Cyclic voltammogram of a polyindole tetrafluoroborate film on a Pt electrode in an AN/BN solution at 25 "C.
UV measurements of the BF- 1, BF-2, and BF-3-labeled solutions were performed using a UV spectrometer (Shimadzu UV-240).
Results and Discussion Electrochemical Measurements. In order to define the electropolymerization mechanisms of aromatic compound-based conducting polymers obtained through electrooxidation, a determination of kinetic parameters related to this electrooxidation, besides spectroscopic analysis, should be performed. In electrodics, the typical equation between currents and the corresponding potentials in the case of an irreversible reaction can be written as the Nernst equation E = E l j 2+ (RT/anF) In [ ( i l - i ) / i ]
(1)
where n is the number of electrons transferred in the electrode reaction, i l is the limiting current in a polarographic wave, a is the transfer coefficient, and El12is the half-wave potential. That is, the electrochemical parameters of E I p ,a,and n can be obtained from a plot of potential vs In [ ( i l - i ) / i ] . In cyclic voltammetry, we must consider that peak current (i,) and peak potential (E,) are related to the scan rates of swept potential (u). As for a relationship between the peak current and scan rate, the current must be computed from a base line of charging current (i,) induced by the continuously changing potentials. Then, the faradaic current is proportional to the square of the scan rate, and the charging current is proportional to the scan rate. Also, as for the relationship between peak potential and scan rate, the peak potential is not dependent on the scan rate in the case of a reversible reaction. On the other hand, peak potential changes with the scan rate in an irreversible system. This fact is important in determining whether or not an electrode system is reversible. There is a method to check for system reversibility simply. In a reversible system, the peak potential width (AE ) of anodic and cathodic peak potentials (E,,, EF) reaches 587n (mV) at 25 "C. Figure 1 shows a cyclic voltammogram of a (AN/BN/water) solution containing 0.2 M indole and 0.1 M (TEA)BF4 in the potential ranges from -500 to 500 mV at 25 O C . The anodic and cathodic peak potentials are not affected by the change in scan rate. Thus, we suggest that this reaction is reversible. Figure 1 also shows that the values of E , and E , are 320 and 220 mV, respectively, at a scan rate of 100 mV/s. So, the peak potential is 120 mV. From these results, we suggest that this width (Up) reaction is reversible, and the number of electrons (n) tranderred in this electrode reaction is 2. The result agrees with the value listed in Table I. SEM Analysis. In order to analyze the morphology of the (PI)BF,, a sample film coated on the anode was obtained in a (AN/BN/water) solution containing 0.2 M indole and 0.1 M (TEA)BF4. The result of the morphology analysis of the (PI)BF, film is shown in Figure 2. As shown in this SEM result, the surface of the (PI)BF, film resembles a growth of aggregates shaped as blossoms. Also, irregular granules are densely crowded
Electrochemically Prepared Polyindole Tetrafluoroborate
The Journal of Physical Chemistry, Vol. 96, No. 9, 1992 3785 SCHEME I: Polymerization Mechanism of Polyindole
Indole
1.3
-
polymerized polyindole
Figure 2. Scanning electron micrograph of a polyindole tetrafluoroborate film. dimer
W
350
600
650
500
5 50
Wavelength(nm)
Figure 3. UV spectra of (a) BF-I, (b) BF-2, and (c) BF-3 solutions.
on the film. The scattered parts shaped as blossoms are regarded as dopants, and good conductivity is predicted from this compact structure. UV Measurements. Electrooxidation in the reaction solution containing 0.2 M indole and 0.1 M (TEA)BF4 gradually progressed on the anode, supplying the external potential. As men-
Polyindole
tioned in the Experimental Section, the solutions labeled BF-I, -2, and -3 were extracted using a microsyringe, and UV measurements for these solutions were performed. The result is shown in Figure 3. In the W spectrum of the BF-2 solution (Figure 3), triple peaks around 395 nm were obtained. From the bulk solution labeled BF- 1, a peak at 373 nm was observed. From the final solution labeled BF-3, the main and shoulder peaks were observed at 375 and 425 nm, respectively. The triple peaks shown in (b) of Figure 3 seem to the result from the various species produced around the anode during electrooxidation. We also note that an appearance of a shoulder peak shown in (c) of Figure 3 seems to result from a red shift owing to electropolymerization. FT-IR Measurements. The (PI)BF, sample obtained was an anodic precipitate, and the product was insoluble in aqueous
60
n U u 0 0
t
$= 0 40
.-€ e3
C
El
I-
20
4c
0
I
I
3000
2000
1
1500
Wovenumber (cm-l) Figure 4. FT-IR spectra of polyindole tetrafluoroborate in KBr pellet.
1
1000
500
3786 The Journal of Physical Chemistry, Vol. 96, No. 9, 1992 a
Choi et al.
b
284 6
-0.
1.0
1.1
1.1
1.4
lo00
/
1.U
Z.0
1.1
-4
T (K-')
Figure 6. Temperature dependencia of the reaction rates for various polyaniline and polyindole based-conducting polymers. (Reprinted with permission from ref 15. Copyright 1992 Wiley.)
oxidized around the anode, two radical cations formed into a dimer, and the dimers also oxidized afound the anode. Thus, the electropolymerization continued. TGA Measurements. Thermogravimetric analysis (TGA) for the (PI)BF, powder revealed useful information regarding thermal stability and physical properties. The TGA measurements were performed in a temperature range of 25-800 OC under a nitrogen atmosphere. The reaction rate for thermal decomposition, R (dW/dt), was directly calculated by an IBM computer connected to a thermal analyzer. For a comparison of thermal characteristics of the (PI)BF4 and that of other polyindole-based polymers, the polyindole hexafluorophosphate (PI)PF6 and polyindole perchlorate (PIP) samples were obtained from proper solutions containing indole and tetraethylammonium hexafluorophosphate [(TEA)PF6] or tetraethylammonium perchlorate (TEAP), respectively. Also, for a comparison between the reaction rates (R) of the polyindole-based polymers and the polyaniline-based materials, the final result is shown in Figure 6 from a previously cited report.Is Figure 6 also shows the maximum peaks of the reaction rate (Rmx)for polyindole- or polyaniline-based polymers. The R,,, values of the (PI)BF4, PIPF6, PIP, (PA)BF4, PAPF6, and PAP samples obtained were 0.143 (at 601 "C),0.126 (at 702 "C), 0.198 (at 591 "C), 0.366 (at 324 "C), 0.092 (at 300 "C), and 0.166 (at 327 "C) mg/min, respectively. On the basis of these results, the thermal characteristics for the polyindole- and polyaniline-based conducting polymers can be explained as follows. First, the bulk of the polyindole-based systems thermally decomposed at higher temperature than the polyaniline-based polymers. The thermal stability of polyindole systems was greater than that of polyaniline-based materials. Thus, polyindole systems can be utilized in industrial applications requiring higher thermal stability. Second, the R,,, values of polyindole-based systems were not seriously affected by the kind of dopant anion. On the other hand, the values of the polyaniline systems were mainly affected by dopant anions. Thus, these distinguishable thermal characteristics can be used in various applications because the conductivities of polyindole systems are similar to those of polyaniline ones. Conductivity. Most conducting polymers have characteristics of a semiconductor in which conductivity increases with temperature. The conductivity of these amorphous materials is affected by many factors such as the structure of the polymer chains, degree of crystallinity, temperature, pressure, shape, kinds and amount of dopants, morphology, density, thickness, potential, and solvent conditions. Therefore, a determination of possible conduction mechanisms for conducting polymers should be considered carefully. Many researchers have performed experiments on various conduction mechanismsibMfor these amorphous materials, leading to possible conduction models and related equations. (15) Choi, K . M.; Kim, K. H. J . Appl. Polym. Sci. 1992, 44, 751
The Journal of Physical Chemistry, Vol. 96, No. 9, 1992 3181
Electrochemically Prepared Polyindole Tetrafluoroborate c
I -2.8
d
-3.5
Y
i-! J
-4.8 -5.5
10
-0.8
-7.s.k
Oaurr
Figure 9. ESR spectrum of a polyindole tetrafluoroborate at 25 "C (microwave frequency, 9.44 GHz).
3.5
4.5
5.8
lo00
/ I
7.8
0.8
(K-'
1
0.5
)
Figure 7. Electrical conductivity as a function of temperature for a polyindole tetrafluoroborate. h
TABLE III: Various Analytical Parameters for Tetrafluoroborate log u (at 25 "C), S/cm E,, eV WPP>G g value dopant, 5% R,,, (at 601 "C), mg/min
Polyindole -2.43 0.4057 2.8 2.004 86 18.3 0.143
TABLE I V Comparison of ESR Parameters and log Conductivity at 25 "C for Various Polvindole-Based Polvmers
log polymers
PIP (PWF4 PIPF6
AHpp,G 2.6 2.8 3.3
g value 2.004 15 2.004 86 2.005 74
(7
(at 25 "C), S/cm -1.63 -2.43 -4.15
were 0.4057 eV and -2.43 S/cm, respectively. The conductivity measurements changing in relation to the temperature resulted in the plots of temperature dependence of electrical conductivity based on eqs 2-4. Among these plots, the one based on eq 2 for hopping conduction shows better linearity than that based on eqs 3 and 4. Thus, we submit that the possible conduction mechanism of the (PI)BF4 pellet is hopping conduction: The plot based on hopping conduction is shown in Figure 8. The conductivity results predict that the polarons act as charge carriers which had formed in the polyindole doped with BF4- and act as an electron acceptor hopping from state to state. ESR Measurements. Previous w o r k ~ ~ have l - ~ ~reported the results of ESR studies for polypyrrole-based conducting polymers. JozefowiczZ2reported that various ESR parameters such as peak-to-peak line width (AHpp),g value, and each ratio of the spectrum were related to conductivity. On the other hand, Scottz4 reported that the ESR parameters for polypyrrole-based polymers had nothing to do with conductivity. For this reason, there are many studies of the comparison between ESR parameters and conductivity for aromatic compound-based conducting polymers. Figure 9 shows an ESR spectrum of (PI)BF4 powders at 25 "C.A single ESR peak such as results from other polypyrroleor polyaniline-based polymers was obtained. From these results, the values of ESR parameters were calculated, as summarized in Table 111. Table IV lists a comparison of ESR parameters and log u in various polyindole systems. The results listed in Table IV confirm that the AHH,and g value decrease as the conductivity increases. Thus, we suggest that ESR parameters obtained from polyindole systems are related to conductivity. O C
d 3
0.44
0.h 0.k 0.h 0.h
0.b
0.b
$1
-114(~ -414~
Figure 8. Temperature dependence of electrical conductivity for a polyindole tetrafluoroborate based on the hopping conduction.
Mott16suggested that a related equation based on the hopping conduction in amorphous semiconductors had a form of exp( - P I 4 ) . Mott's suggestion was based on the fundamental assumption that the concentration of charge carriers was not affected by the temperature. Sheng" also reported that an equation based on the hopping conduction in sputtered granular metal films had a form of exp(-T-1/2). Mott's equation applies only in temperature ranges of 60-300 K. Greaves'* reported the following equation for variable range hopping conduction:
uT112= exp(-T1I4) (2) MatarElg also reported that the electronic conduction which includes the grain boundary potential (E,) is given in the following equation: u = AT'/2 exp(-E,/kT) (3) where A is a constant related to the electric field strength and effective mass of electrons and E, is the height of the potential barrier. Zeller20 also gave the equation for the tunneling conduction mechanism u = uo e x p ( - P / 2 ) (4) The electrical conductivity for the pressed pellet of (PI)BF4 was measured by the four-probe method in a temperature range from -150 to 25 OC under a low applied field to ensure Ohmic behavior. Figure 7 shows the results of temperature dependence on conductivity. As shown in Figure 7, the conductivity for the pressed pellet of (PI)BF4increases linearly with the temperature, satisfying the Arrhenius equation, u = uo exp(-E,/kT). The values of E, obtained from the slope of the plot and log u at 25 (16) Mott, N. F. Philos. Mag. 1969, 19, 835. (17) Sheng, P.;Abeles, B.; Ark, Y. Phys. Reu. Lett. 1973, 31, 44. (18) Greaves, G. N. J . Non-Cryst. Solids 1973, 11, 427. (19) Matare, M. F. J. Appl. Phys. 1984, 56, 2605. (20) Zeller, H. R. Phys. Reu. Lett. 1972, 28, 1452.
Conclusions (PI)BF4 polymers were obtained from a (AN/BN/water) solution containing indole and (TEA)BF4. The electrochemical measurements of this solution conclude that the electrooxidation (21) Chung, T. C.; Feldblum, A.; Heeger, A. J.; MacDiarmid, A. G. J. Chem. Phys. 1981, 74, 5504. (22) Jozefowicz, M.; Yu,L. T.; Perichon, J.; Buvet, R. J . Polym. Sci. C 1969, 22, 1187. (23) Scott, J. C.; Pfluger, P.; Clarke, T. C.; Street, G. B. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1982, 23, 119. (24) Scott, J. C.; Pfluger, P.; Krounbi, M. T.; Street, G. B. Phys. Reo. B 1983, 28, 2140.
3788
J . Phys. Chem. 1992, 96, 3788-3795
of indole is a reversible reaction, and the number of related electrons is 2. Structure analysis of anodic precipitates suggests that the polymerization of indole is based on the formation of radical cations as intermediates. When the thermogravimetric analysis for the (PI)BF4 powder was conducted, the reaction rates for thermal decomposition were calculated by a computer connected to a thermal analyzer. A comparison between thermal characteristics of the (PI)BF4 and other polyindole- and polyaniline-based polymers, such as PIP, PIPF6, PAP, PAPF6, and (PA)BF4, revealed that the polyindole-based polymers were thermally decomposed at higher temperatures than the polyaniline-based ones. The maximum values of the reaction rate (R,) in the polyindole-based systems were not seriously affected by the kind of dopants. However, the values of the polyaniline-based polymers were affected by the kind of electron acceptors.
Electrical conductivity for the pressed pellet of (PI)BF4 was measured in the temperature range from -150 to 25 OC. The value of log CT at 25 O C was -2.43 S/cm, and the activation energy calculated from an Arrhenius plot was 0.4057 eV. The conductivity result suggests that a possible conduction mechanism for (PI)BF4 is hopping conduction and that the charge carriers are polarons. We suggest that the conductivity for (PI)BF4 is induced from doping with BF4- anions as electron acceptors. A single ESR peak for (PI)BF4 powder at 25 OC was obtained. The values of AHpp,g, and spectral ratio were also calculated.
Acknowledgment. We are grateful to Doctor H. W. Rhee, Korea Institute of Science and Technology, for cyclic voltammetry measurements. Registry NO.(TEA)BFd, 429-06-1; (PI)BF,, 82451-55-6;(TEA)PFs, 429-07-2; TEAP, 2567-83-1; indole (homopolymer), 120-72-9.
Activation of Carbon Dioxide on Potassium-Modified Ag( 111) Single Crystals Pilar Herrera-Fierro,t Kuilong Wang,* Frederick T.Wagner,s Thomas E. Moylan,$ Gary S. Chottiner,*p*and Daniel A. Scherson*.+ Case Center for Electrochemical Sciences and the Departments of Chemistry and Physics, Case Western Reserve University, Cleveland, Ohio 441 06, and Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090-9055 (Received: September 23, 1991; In Final Form: January 10, 1992)
The adsorption of carbon dioxide on potassium-dosed Ag(ll1) has been investigated with temperature-programmed desorption (TPD), work function measurements, and Auger electron (AES), X-ray photoelectron (XPS),and high-resolution electron energy loss (HREELS) spectroscopies. Unlike the behavior observed for other K-modified single-crystal metal surfaces, the TPD spectra of near-saturation coverages of C 0 2 on K/Ag( 111) for K coverages in the range 0.13 C C 0.47, where exhibit a sharply defined m/e = 44 peak at 796 f 6 K with no evidence the close-packed monolayer corresponds to OK = ]I3, for the desorption of CO at any temperature. Similar TPD experiments involving mixtures of natural and 180-labeledC02 indicate that the oxygen atoms undergo partial scrambling, suggesting that the overall process cannot be represented in terms of a simple adsorption/desorption of C02. The HREELS spectra of C02-saturated K/Ag( 111) show, in addition to very minor features, a sharp peak at -1480 cm-l, and XPS spectra of the same interface display a C(1s) peak with a binding energy characteristic of an electron-rich carbon species. This information is consistent with the presence of a carbon-bound CO species on the surface. Evidence against the complete dissociation of C02was obtained from TPD, which failed to reveal features associated with carbonate (decomposition) expected to be formed via the reaction of C02and adsorbed 0. On the basis of these results, it is proposed that C 0 2 on K/Ag( 1 1 1) binds through the carbon to the surface, leading to the "partial" dissociation (or activation) of each C 0 2 molecule into adsorbed CO and 0. Within this model, such adsorbed 0 would serve as a bridge between the carbon atoms of neighboring -activated" C 0 2 molecules and therefore undergo exchange prior or during thermal desorption. Adventitious water or oxygen in the system and/or defect sites on the surface give rise to an additional m / e = 44 TPD peak at a much higher temperature. The height of this new feature is increased significantly by predosing the K/Ag( 11 1) surface with O2or H 2 0 at coverages as low as 0.05 L. The XPS spectra for these purposely contaminated surfaces reveal features very different from those observed in the absence of such impurities, but consistent with the presence of an ordinary form of carbonate. Ag( 11 1) surfaces which had been damaged prior to K deposition and subsequent C 0 2 adsorption were found to yield significant amounts of CO in the TPD spectra at lower temperatures.
Introduction The activation of carbon dioxide represents an essential step in the synthesis of methanol and other valuable organic compounds from H2
