J . Phys. Chem. 1991, 95, 493-502
493
Work Function and Spectroscopic Studies of Interactions between Conducting Polymers and Organic Vapors Daniel Blackwood+and Mira Josowicz* Institut f u r Physik, Fakultat f u r Elektrotechnik, Uniuersitat der Bundeswehr Miinchen, Werner Heisenberg Weg 39, 0-8014 Neubiberg, F.R.G. (Received: April 13, 1990)
The mechanism of the interactions which occurs on polypyrrole and p-polyphenylene films during their exposure to various organic vapors was investigated by UV/visible spectroscopy and by the Kelvin probe technique. The spectroscopic method follows the changes in polaron and bipolaron concentrations, which are sensitive to any charge transfer that moves into/out of the films, while the Kelvin probe monitors directy any changes that occurred in the work function of the copolymer. The results indicate a mechanism consisting of the following steps: interaction of the vapor molecules with the polymer, partial chargc transfer to/from a mid-gap state in the film, and lateral dispersion of the charge between all the adsorbed molecules.
Introduction In rcccnt years attention has been given to the possibility of using clcctrochcmically deposited conducting polymers as the sensitive layer in chemical sensors for the detection of organic or the vapors. Usually changes in either the work conductivity4d of the polymers have been monitored as the sensitive parameter. Despite some quite promising qualitative results, until now there has been virtually no attempt made to explain the actual interactions bctwccn the organic chemical vapors and the conducting polymcrs. The results presented in this paper show that certain propcrtics of p-polyphenylene films can be perturbed by organic vapors in a similar manner to polypyrrole films. Since thcrc arc ncithcr any heterogeneous atoms nor any permanent dipoles prcscnt in p-polyphenylene an explanation independent of thcsc fcaturcs for the interaction between conducting polymers and chemical vapors is required. The aim of this paper is to characterize these interactions, using our own observations on polypyrrole and p-polyphenylene and also the results of other authors which have been already published. Methods of investigation used here were the Kelvin probe (vibrating capacitor)’.* and UV/visible optical spectroscopy. We belicvc that thc latter method is presented for the first time as a possible tool for following the chemical sensitivity of conducting polymcr laycrs toward organic vapors. I n ordcr to better comprehend what kind of interactions maybe occurring, it is necessary to summarize briefly the present state of knowledge of the electronic structure of conducting polymers. The electrochemically doped polymers are obtained in their conducting form by the formation of charge-transfer complexes, as for example (P+X-), where the P denotes the polymer and X the doping ion. The counterion is known to affect the conductivity by changing the morphology of the polymer/dopant complex, the concentration of chemical defects, or the oxidation state of the p ~ l y m e r . ~ The * ’ ~ charge on the polymer chain is believed to be mobile moving along the conjugation of the x backbone of the polymer. In thc case of conducting polymers that do not contain degenerate tautomers in their ground state, such as polypyrrole and p-polyphenylene, these defects take the form of either polarons or bipolarons11+12 depending on the extent of the doping. However, it has recently been proved by Waller and ComptonI3 that the mobilitics of these two types of defects are approximately equal and thus both contribute to the conductivity. In the electronic band structure of these organic semiconductors the mobile discontinuities manifest themselves as mid-gap states. The diagram of the predicted electronic structures for a conducting polymer in its neutral state as well as states containing polarons and bipolarons is shown in Figure l.12.14*15 In the caes of a p-type-doped conducting polymer the polaron state contains one *Towhom correspondence should be addressed. ‘Present address: AEA Industrial Technology, Hanuell Laboratory, M&T Division, Oxon OX I I ORA, Great Britain. 0022-3654/91/2095-0493$02.50/0
electron and the bipolaron state is completely empty (Le., contains two holes). Figure 1 also shows the electronic transitions that might be expected to occur in p-type polypyrrole and p-polyphenylene under the irradiation of ultraviolet or visible light. Kaufman et a1.I6 showed that the energy levels for the polaron and bipolaron mid-gap states are close enough to one another that for the purposes of this paper it is safe to assume that wI’= q”,021 w2”. Since the formation of a polaron involves only the removal of one electron from the valence band it can also be assumed w j = 03’. However, it can also be shown that at high doping levels the bipolaron energy states can overlap into bands of their ownI2 thus causing q” to increase due to the fact that the bipolaron states come out of the valence and conduction band edges. Valence effective Hamiltonian (VEH) calculations have been performed by BredasI5 to estimate the electronic energy levels of the n-type polypyrrole and p-polyphenylene systems. However, the author suggested that the electronic structure in the bandgap should be very similar in the p-type materials, which is the form that the polymers are normally obtained when electrochemically grown. From the VEH calculations it is predicted that the electronic transitions for polypyrrole will occur at
(1) Josowicz, M.; Janata, J. Anal. Chem. 1986, 58, 514. (2) Josowicz, M.; Janata, J.; Ashley, K.; Pons, S.Anal. Chem. 1987, 59, 253. (3) Josowicz, M.;Janata, J. In Chemical Sensor Technology;Seiyama, T., Ed.; Elsevier: Amsterdam, 1988; pp 153-177. (4) Bartlett, P. N.; Archer, P. B.; Ling-Chung, S.K. Sensors Actuators 1989, 19, 125, 141. (5) Bartlett, P. N.; Ling-Chung, S.K. Sensors Actuators 1989, 20, 287. (6) Dickert, F. L.; Zeltner, D. Angew. Chem. Ado. Mater. 1989, 101,833. (7) Holzl, J.; Schulte, F. K.; Wagner, H. Solid Surface Physics; Springer-Verlag: Berlin, 1979. (8) Duan-Fu, Hsu; Gratzl, M.; Riley, A. M.; Janata, J. J . Phys. Chem. 1990, 94, 5973. (9) Nalva, H. S.; Rabe, J. G.; Schmidt, W.F.; Dalton, L. R. Macromol. Chem.-Rapid Commun. 1986, 7, 533. (10) Warren, L. F.; Walker, J. A.; Anderson, D. P.; Rhodes, C. G.J . Electrochem. SOC.1989, 136, 2286. (1 I ) Chance, R. R.; Boudreaux, D. S.;Bredas, J. L.; Silbey, R. In Handbook of Conducting Polymers; Skotheim, T. A,, Ed.; Marcel Dekker: New York, 1986; Vol. 2, p 825. (12) Scott, J. C.; Bredas, .I.L.; Kaufman, J. H.; Pfluger, P.; Street, G. B.; Yakushi, J. H. Mol. Cryst. Liq. Cryst. 1985, 118, 163. (13) Waller, A. M.; Compton, R. G. J. Chem. SOC.,Faraday Trans. I 1989, 85, 977. (14). Bredas, J. L.; Silbey, R. In Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol. 2, p 859. (15) Bredas, J. L. Mol. Cryst. Liq. Cryst. 1985, 118, 49. (16) Kaufman, J. H.; Colaneri, N.; Scott, J. C.; Kanazawa, K. K.; Street, G. B. Mol. Cryst. Liq. Cryst. 1985, 118. 171.
0 1991 American Chemical Society
494
The Journal of Physical Chemistry, Vol. 95, No. ut’=
0.5 eV
= 2480 nm
w i = 2.3 eV
I, 1991
Blackwood and Josowicz
(1450 nm)
= 540 nm
= 3.2 eV = 390 nm (490 nm) w3” = 3.6 eV = 345 nm wql = 1.8 eV = 690 nm w5’ = 2.3 eV = 540 nm
w 3 = w3‘
and for p-p~lyphenylene’~ w,’ = 0.7 eV
W
1770 nm
2
NEUTRAL
= 2.8 eV = 440 nm w 3 = w3’ = 3.5 eV E 350 nm w2‘
w3’
> 3.5 eV
w4’
= 2.1 eV
w5‘
= 2.8 eV
590 nm 440 nm
5
The values in parentheses for polypyrrole are experimental values from referen~es.l~.~* A value for the bandgap of strongly doped p-polyphenylene, w3”, does not appear to have been calculated, but it will certainly be larger than that for the undoped polymer for thc rcason stated carlier. Since transitions w , , w2, and w3 involve promoting electrons from the full valence band whereas w4 and w 5 arise from only the half-filled polaron level, it may be expected that the three former transitions will dominate the polymer spectra. In a p-typc conducting polymer the polarons and bipolarons can be considered as cation and dication species, respectively, and thus it can be visualized that a p-type polymer film could be reduccd in thc following ways bipolaron + e polaron (1)
-
(0 expected
to decrease hu polaron
< wq/
+e
+
and to increase hu neutral
>
q’).
(2)
( a expected to decrease hv < w 3 ) . Likewise it could be oxidized as
polaron
+e
( a expected to decrease hu
neutral (a expected to increase hu
+
+e
(3)
bipolaron
-
> wq/
and to increase hv polaron
polaron (no transition in neutral state) w 2 bipolaron
= polaron (no transition in neutral state)
w 3 neutral
> bipolaron = polaron
w4 and w 5 (occur
POLARON
BIPOLARON
Figure 1. Electronic structure of the bandgap of conducting polymers in their neutral, polaron (light doping) and bipolaron (heavy doping) states. Also shown are the transitions that can occur within/across the bandgap for p-type-doped samples.
only in the polaron state)
(17) Tezuka, Y.; Aoki, K. Synth. Met. 1989, 30, 369.
( I 8) Street, G.B. I n Handbook ofConducting Polymers; Skotheim, T. A,, Ed.: Marcel Dekker: New York, 1986; Vol. I , p 265. (19) Bredas, J . L.; Yakushi, K.; Scott, J . C.; Street, G. B. Phys. Reu. B 1984, 30. 1023.
Therefore, the changes in the itensities of the transitions that will occur if the conducting polymer undergoes one of the reactions 1-4 are expected to be reaction 1, w l increases, w4 and w 5 decrease, w2 and w 3 unchanged; reaction 2 , w 3 increases, wI,w2, w4, and w5 decrease; reaction 3, oldecreases, w4 and w 5 increase, w 2 and w 3 unchanged; reaction 4, w 3 decreases, wI,w2, w4, and w5 increase. As the approximate energies for each of the transitions are known, the changes that will occur in the electronic spectra of a polypyrrole or a p-polyphenylene film upon oxidation or reduction have been predicted and are indicated below each of the above reactions. As a consequence of the above assignments it can be seen that if electron density is donated into the polymer films there would always be an increase in the amount of light absorbed at wavelengths with energies less than that required for transition w4 to occur, that is h > 690 nm for polypyrrole and about h > 590 nm for p-polyphenylene. Obviously an equal but opposite change in the absorption coefficient would be expected for the case where electron density is withdrawn from the films. At wavelengths with sufficient energy for transition w4 to occur any observed changes in the absorption coefficients will depend on whether or not polarons are involved in the reaction. From the above discussion it follows that one should be able to monitor the effects of various chemical vapors on polypyrrole and p-polyphenylene by observing any changes in their UV/visible absorption spectrum that occur during exposure. It is proposed here to interpret the changes in terms of increasing/decreasing polaron and bipolaron concentrations, which in turn can be considered as electron density being removed/donated from/into the T backbone of the polymers. However, the optical transmission measurements do not provide information about the distribution of polaron/bipolarons throughout the layer, especially their relative concentrations at the surface and in the bulk. For uncharged materials the work function 4, is given by20 4 = -I+ + ex (6) where ~ is. lthe ~chemical potential of electron which includes the interactions of an electron with ion cores inside the bulk phase (binding energy) and with the surface of the phase (kinetic energy), x, accounts for the surface field due to the presence of surface dipoles and therefore represents a shift of the potential energy. The work function therefore depends on the surface condition of the phase and on the presence of atoms or molecules adsorbed on that surface, which results in the electron distribution at the surface of the material being modified by f6x. Adsorbed molecules can transfer (donor) or receive (acceptor) electron density to or from the sensing layer. The Kelvin probe method relies on the fact that between dissimilar conductors a contact potential difference (CPD) exists which is the result of the difference in work functions and is given by (20) Trasatti, S . In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1977; Vol. IO, p 231.
The Journal of Physical Chemistry, Vol. 95, No. I, 1991 495
Conducting Polymer-Organic Vapor Interactions CPD = (41- + , ) / e (7) where 4, and 9, are the average work functions of the two conductors. By applying this technique it is possible to obtain relative changcs of thc work function of the conducting polymer before and during exposure to a vapor.
SIGNAL VAPOUR
t IN
Experimental Section
The conducting polymer films were electrochemically grown onto either platinum disks of 99.99% purity or on one side of glass slides coated either with indium/tin oxide (ITO) (HOYA, 30 Q / o )or with sputtered platinum films of 100 8, on 15 8, W/Ti. The conducting glass substrates were cut to a glass plate of 0.8 X 5.0 cm. For the optical experiments the transparencies of the substrates were 85% for the IT0 glass and 20% for the Ptsputtered glass throughout the spectral region of interest. The platinum disks (diameter 0.6 cm) were glued with silver epoxy to a cylindrical brass holder and pressed into a Teflon tube (diameter 0.7 cm). These specimens were later mounted onto a metallic rod for the electrochemical depositions and onto a micrometer screw-rod for the Kelvin probe measurements. Acetonitrile (HPLC grade 99.99%) (AcCN), tetraethylammonium p-toluenesulfonate (TOS), pyrrole (98-99%) (P), biphenyl (99%), 7,7,8,8-tetracyanoquinodimethane(98%) (TCNQ), and methanol ((HPLC grade) (MeOH) (all from Aldrich), and AgNO,, LiC104, 2-propanol (i-PrOH), dichloromethane, chloroform, and n-hexane (all from Merck). All chemicals wcrc uscd as received. The polypyrrole films (PP) were deposited from AcCN or dichloromethane containing 0.1 mol dm-, pyrrole monomer with 0.1 mol dm-, doping anion as a supporting electrolyte. The electron-accepting dye 7,7,8,8-tetracyanoquinodimethane(TCNQ) was incorporated into the polypyrrole by simply growing the films in its prcscncc at a concentration of 0.01 mol dm-3. The p polyphenylene films (PPP) were deposited from AcCN containing 0.05 mol biphenyl monomer with 0.1 mol dm-, doping anion as a supporting electrolyte. Stock solutions of biphenyl were found to be stable in air over a period of several weeks; however, solutions of pyrrole had to be made up on the day of use. For thc coating of the platinum disks a titanium (99%) disk elcctrodc (diamctcr 0.8 cm) onto which 2000 A of platinum has been sputtered was used as a counter electrode, while a platinum foil (99.99%) counter electrode was used during deposition onto conducting glass substrates. The counter electrode was always placed facing the working electrode at a distance of 1 cm. The reference electrode used was Ag/O.l M A g N 0 3 in AcCN//O.l M supporting electrolyte to which all potentials stated in this paper havc bccn rcfcrrcd. The potentiostatic deposition of PP and PPP was carried out with the aid of an EG&G PAR 273 potentiostat. For PP films the potentiostatic deposition process involved applying a pulse sequence consisting of switching to the deposition potential, 0.9 V unless otherwise stated, for a period of 0.1 s before returning to open circuit for a period of 1 s. This was repeated until 100 mC cm-, of charge had been passed. Profilometer measurements (Sloan Dektak 11) showed that these films were of the order of 500 nm thick. The potentiostatically prepared p-polyphenylene films were dcpositcd in the manner suggested by Ashley et al.,' in which the working electrodes were cycled between 1 .O and 1.8 V at a scan rate of 100 mV s-'. When perchlorate anions were used for the supporting electrolyte the cycle was continued until the full 100 mC cm-, of charge had been passed; however, if TOSanions were used only -45 mC cm-, of charge could flow before the high resistance of the film blocked the electrode. Profilometer mcasurements showed that the perchlorate films were of the order of 40 nm thick: however, the TOS films were too thin to be mcasurcd by this technique. Attempts to grow the p-polyphenylene films on IT0 substrates proved to be unsuccessful. Since in organic solvents high resistances can appear between the working and reference electrodes, IR compensation (current
-
(21) Ashley, K.; Parry, D. B.; Harris, J. M.; Pons, S.; Bennion, D. N.; LaFollette, R.; Jones, J . Electrochim. Acta 1989, 34, 599.
FILM
VAPOUR OUT
Figure 2. Schematic representation of the single-wavelength apparatus used to monitor the changes induced in the transparency of conducting polymer films by exposure to organic vapors: (1) 150-Wtungsten source, (2) lenses, (3) light chopper, (4) monochromator, (5) sample chamber, ( 6 ) silicon photodiode detector, ( 7 ) current amplifier, (8) lock-in amplifier, and (9) recorder.
interrupt method) was applied in all cases of potentiostatic It is unclear why the polypyrrole films should be a factor of
IO thicker than the p-polyphenylene films when the same charge is passed into each film. One possible explanation is that a large fraction of the formed ppolyphenylene may be lost into the solvent as soluble oligomers. After deposition, all films were rinsed in the plain solvent and left to dry in air for at least 24 h before use. The organic vapors were obtained by bubbling nitrogen gas (99.99% Linde) at a constant rate directly through the liquid phase of the organic compound of interest at room temperature and the resulting vapor flow was then diluted by a second nitrogen line. Samples from the vapor were injected into a gas chromatograph (Philips PU 4900) in order to determine their concentrations, these were as follows: C H 3 0 H 4.6 X IO-, mol dm-,; i-C3H,0H 0.4 X IO-, mol dm-3 (in optical experiments i-C3H70H 0.8 X IO-, mol dm-,); ~ Z - C ~1.6 H ,X~ mol d ~ n - CH,CI, ~; 8.5 X IOd3 mol dm-3; CHCI, 1.2 X IO-, mol dm-,. During experimentation, samples were switched successively between a stream of pure nitrogen and a vapor flow. UV/visible absorption spectra of the polymer films were obtained on a Perkin Elmer 200 spectrometer. The photoelectrochemical experimentation was carried out under potential control in conjunction with a cell based on the design of Tezuka and Aoki" filled with a solution of the background electrolyte from which the films were grown. A greater sensitivity to the reversible changes in the absorption spectra of the polymer films induced by the presence of organic vapours was obtained using the single wavelength apparatus schematically shown in Figure 2 than with the spectrometer. This consisted of a 150-W tungsten source (Osram). The output beam from which was first chopped at a frequency of 27 Hz by a variable-frequency light chopper (EG&G PAR 192) before being passed through a grating monochromator (Jobin Yvon) set to the required wavelength, 800,650, 500, and 400 nm. The transmitted light intensity was recorded by a silicon PN-planar photodiode (Telefunken Electronic BPW 20) placed behind the sample chamber. The current output of this photodiode was passed through a current amplifier (EG&G PAR 181) into a lock-in amplifier (EG&G PAR 5104). The sample chamber consisted of an optical cuvette (Hellma) which had been adapted to allow the passage of the organic vapors. This arrangement directly measures only the intensity of the light transmitted through both the film and the substrate, IT; however, if it is assumed that reflection at the substrate/film interface is negligible, then the amount of radiation actually absorbed by any given film, I,, is given by I , = I , - IT,where I , is the amount of light transmitted through the bare substrate. Therefore, the change that occurs on I , on exposing the film to (22) Britz, D.; Brocke, W.A. J . Electroanat. Chem. 1975, 58, 301.
496
Blackwood and Josowicz
The Journal of Physical Chemistry, Vol. 95, No. I , 1991 0.2
O.O8I 0.06
/
Figure 4. Evaluation of band C for the -1.0 V spectrum of Figure 3 to determine the bandgap of neutral polypyrrole ( n = 1).
1
.
:,
.,
,
, \
-.
*-___
0.0
400
500
600
1"'
, ,,,/
,*
700
800
Wavelength Inml
Figure 3. Change in the absorption spectra of a TOS-doped polypyrrole
film deposited at 0.9 V upon electrochemical reduction to the following potentials: 0.6 V (-), 0.1 V -0.2 V (-.-), -1.0 V (---), and -1.5 V Spectra were recorded in 0.1 mol d d Et,N+TOS-.
the polaron concentration results in an increasing proportion of w4 within band B, which is the cause of the observed red-shifting. The final band C most likely represents the bandgap transition in the neutral polymer. Since the polaron species is a cation radical its concentration can be followed by ESR s p e c t r o ~ c o p y . ' ~ ~ ~ ~ When the electrode potential of a polypyrrole film becomes more anodic than 0.9 V vs Ag/AgN03 it undergoes a further oxidation characterized by a decrease in absorption above about 600 nm and a loss of conductivity. This overoxidation of the film was found to be irreversible so its presence will not significantly affect the interpretation of the rest of the data presented in this paper as only reversible interactions between the polymer films and the chemical vapours were recorded. If parabolic energy bands are assumed, the absorption coefficient, a , is related to the photon energy by24
(ahv)2/"= B(hu - E*)
(8)
(-sa-),
(..e).
a vapor is simply AI,, = -AIT. Thus all the data obtained on the single-wavelengthapparatus has the opposite sign to those obtained on the Perkin Elmer spectrometer. All results were normalized at each wavelength by dividing by the amount of light transmitted through the film in the absence of vapor: hence data is displayed in the form A I T / l T . The relative change of work function was measured with a Besocke Delta-Phi-Elektronik Kelvin Probe S in which the driving electrode is a 2.5 mm diameter gold grid and the contact potential difference (CPD) is picked up directly from the reference and not from the sample as in the conventional Kelvin probe. I n the automatic balancing mode the CPD is compensated automatically and work function changes are plotted directly in calibrating units on the recorder output.
Results fhotoelefrrochemistry. The changes in the absorption spectrum of a polypyrrole film that occur during electrochemical reduction are shown in Figure 3. It can be seen that as the potential is movcd cathodically the band B slightly increases in size and undergoes slight red shifting. When the potential of the film reaches a value of about -0.2 V vs Ag/O.l M A g N 0 3 in AcCN//O.I M TOS in AcCN a strong band C centered around 400 nm appears and at the same time absorption above 500 nm falls. These results are in good agreement with those obtained by Tezuka and Aoki:" however, our results showed that if the potential is reduced yet further to below -1.5 V band A begins to recover at the expense of band C, which was also observed to undergo slight blue shifting. All these changes were found to be reversible. Absorption A almost certainly represents the high-energy tail of transition w I and its decline may represent a decrease of bipolaron concentration in the film. Its recovery below -1.5 V is believed to rcprcscnt the first recorded observation of n-type doped polypyrrole. It should be noted here that Tezuka and Aoki" found that the absorption A continues on to around 1450 nm (0.8 eV); however, this is beyond the range of our spectrophotometer. Band B is bclicvcd to comprise transitions w 2 and w4. An increase in
where h is the Planck's constant, u is the frequency of the incident radiation, E is the bandgap energy, B is a constant, and n is a parameter t i a t takes the value 1 for direct and 4 for indirect transitions. Therefore, by plotting the relative absorption intensity times hu all raised to the power 2 (direct) or 1/2 (indirect) vs hu for the low-energy rise in the absorption of band C for the -1 .O V spectra an estimate of the bandgap in the neutral polypyrrole can be obtained. It is assumed that the foot of the curve represents zero absorption. Figure 4 shows that absorption C evaluated from eq 8 between 400 and 460 nm yields a linear plot with a value of n = 1, indicating transition w 3 to be a direct transition in polypyrrole. From the intercept on the abscissa a value for the bandgap of 2.7 eV was obtained. This is in good agreement with previously reported experimental v a I ~ e s ~ ~but~ 'below * . ~ ~that of the theoretical value of Bredas quoted above.I5 This difference maybe partly due to band tailing caused by the amorphous structure of polypyrrole. The absorption spectra of a p-polyphenylene/perchlorate film showed only minor changes upon electrochemical reduction (Figure 5), with just a slight decrease in absorption being observed at the red end of the spectrum, >530 nm, and a slight increase at the blue end. Two bands were observed in the spectra of p-polyphenylene of which absorption D is again most likely associated with the high-energy tail of transition w , , and band E can be identified with the bandgap transition w3. The band E evaluated beween 350 and 410 nm according to eq 8 results in a linear plot with a value of n = 4 (Figure 6); therefore, transition w3 appears to be an indirect transition in p-polyphenylene. From the intercept on the abscissa a value for the bandgap of 2.7 eV was obtained. This value is smaller than the theoretical value of 3.5 eV,I4 possibly due to the fact that transition w 2 may contribute to the red end of band C; this may also be the reason why transition w! appears to be indirect. However, on replacing the perchlorate anions with (23) Kaufman, J . H.; Colaneri, N.; Scott, J. C.; Kanazawa, K. K.; Street, G. B. Phys. Reo. Lert. 1984, 53, 1005. (24) Pleskov, Yu. V.; Gurevich, Yu. Ya. Semiconductor Photoelectrochemisfry; Bartlett, P. N., Ed.; (translated from Russian); Plenum: New York, 1986. ( 2 5 ) Inoue, T.:Yamase, T. Bull. Chem. Soc. Jpn. 1983, 56, 985.
Conducting Polymer-Organic Vapor Interactions
The Journal of Physical Chemistry, Vol. 95, No. I , 1991 497 (ahv)l Ia.u.1
4
0.4 -
0.3
-
0.2
-
0.1
-
0.0 2.4
,
2.6
2.8
3.0
,hv
IeVl
3.2
Figure 8. Evaluation of the band E of Figure 7 to determine the bandgap of p-polyphenylene which had been deposited in the presence of TOS ( n = I).
400
500
700
600
Wavelength
800
[nml
Figure 5. Absorption spectra of a p-polyphenylene film doped with perchlorate anions 0.1 mol dm-) LiC104 at two electrochemical potentials. Potentials are 1.8 V (-) and 1 .O V (---). 1
(ahv)T
1a.u.l
A
0.8 -
0.6
-
0.4. 0.2
-
0.0 2.4
2.6
2.8
3.0
3.2
3.4
3.6
Figure 6. Evaluation of band E for the 1.8 V spectrum of Figure 5 to determine thc bandgap of p-polyphenylene which had been deposited in the presence of CIOL ( n = 4). Wavelength Inml Figure 9. Absorption spectra of a polypyrrole film doped with TOSanions in air (-) and in the presence of 4.6 X IO-' mol dm-) methanol
w
u z a
(---).
Dm
p-toluenesulfonate anions w3 is found to be a direct transition and a value of 3.6 eV (360 nm) is obtained for the bandgap, which is in very good agreement with the theoretical value (Figures 7 and 8). Thus, it can be concluded that the nature of the anion strongly affects the structure of p-polyphenylene films. No such anion dependency was observed in the optical spectra of polypyrrole films. Chemical Vapor Experiments. Figures 9 and 10 show the absorption spectra of the oxidized uas grown" polypyrrole and ppolyphenylene films in air and in the presence of methanol vapor. On removal of the methanol the absorption spectrum returns to its original shape, thus proving that the processes are reversible. It can be seen that the methanol has caused the absorbance of the polypyrrole film to decrease over the entire sub-bandgap range (>350 nm), which is the type of behavior predicted by reaction 2 or by a combination of reactions 1 and 5, that is, an effective decrease in the concentrations of the polarons. The methanol therefore appears to interact preferentially with polarons rather than with bipolarons, which is opposite to what might be expected from the above electrochemical experiments in which bipolarons are first seen t o be reduced to polarons. This difference possibly arises from the differences that exist between an electrolyte/film
m I
-r: 0.1
400
500
600
700
800
Wavelength Inml
Figure 7. Absorption spectra of an as-grown ppolyphenylene film doped with TOS anions in air.
498
Blackwood and Josowicz
The Journal of Physical Chemistry, Vol. 95, No. 1. 1991
TABLE I: Changes Induced in the Transmission in Polymer Films by Exposure to Vapors change in transmission ~O’AIT/IT polymer films vapor deposited from substrate 800 nm 650 nm 500 nm 400 nm MeOH PP-TOS/AcCN Pt 5.3 20.6 21.3 20.6 2.7 2.1 3.6 i-PrOH PP-TOS/ AcCN Pt 1.8 16.9 7.9 10.0 Pt 3.7 PP-TOS/AcCN CHCI, 17.1 13.4 23.5 PP-TOS/AcCN 4.8 Pt CH2CI2 20.2 21.6 11.4 PP-TOS/AcCN 3.1 Pt AcCN 4.8 6.1 5.9 Pt 0.0 I I - C ~ H I ~ PP-TOS/AcCN 4.5 -24.0 -20.6 Pt 14.4 PPP-CIO4/ AcCN McOH 5.7 -35.6 -41.4 21.1 PPP-CI04/AcCN Pt CHCI, 7.2 -32.3 -40.0 24.5 PPP-CIO,/AcCN Pt CH2CI2 9.3 8.4 6.1 7.3 PP-TOS/AcCN McOH IT0 4.4 12.2 9.0 3.0 i-PrOH P P-TOS / AcCN IT0 -1 0.0 -10.0 -1 5.2 -13.0 CHCI, I TO PP-TOS/AcCN -4.0 -8.0 -5.0 -3.0 PP-TOS/AcCN IT0 CH2CI2 PP-TOS/CH 2CI2 6.0 McOH IT0 8.0 10.0 5.0 2.5 -7.6 -6.4 3.0 PP-TOS/CH2C12 IT0 CHCI, 3.6 4.3 2.5 IT0 I .6 CHZC12 PP-TOS/CH2CI2 6.3 41.9 29.6 -10.2 PP,TCNQ-TOS/AcCN McOH Pt -13.4 -1 2.7 -10.3 -10.0 PP,TCNQ-TOS / AcCN IT0 McOH OFF
-
0.2
-
donor/acceptor donor donor donor
donor donor weak interactions donor
donor donor donor donor acceptor acceptor donor
acceptor donor
donor (see text) donor (see text)
100s
:I I
9
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> E A , and thus the electronegativity is approximately half of the ionization potential bringing it close to the value of the work functions of the conducting polymer films. It is perhaps now possible to explain the linear dependency of the vapor-induced shift in the film’s work function on the original value of this parameter, as it may be expected that the amount of charge transfer that occurs would be linearly dependent on the size of ($ - xM). The intercept of the line on the abscissa represents the electronegativity point at which $ = xM. Unfortunately the absolute work function of gold is also unknown, although it probably lies somewhere between 5.0 and 5.3 (Le., slightly higher than normally quoted Au values due to the oxide layer). Table IV shows that there is relatively good agreement between experimentally determined values and literature values for 1/21p.32 If the coverage is decreased, the lateral interactions would decrease causing the adsorbed molecules to interact with the surface more individually. Thus the electronegativity would slowly be replaced by the ionization potential as the main parameter in determining the size and direction of any charge transfer. Therefore, the above model would predict that the intercept in Figure I5 should increase as the concentration of the vapor is decreased, and preliminary studies have shown this to be the case. The interactions between n-hexane and the polymer films were always far weaker than for the other vapors, possibly due to the fact that its low polarizability would make lateral interactions between adsorbed molecules relatively weak. This would result in greatly reduced electron transfer for the same reasons given for low surface coverages and explains why n-hexane switches from electron-donating properties to electron-accepting properties at a point above Ip/2. It would be interesting to find out whether the surface coverage remains constant as the initial work function varies, as might be expected from the ideas put forward by Ahmed3’ who considered the case where a single molecule can be adsorbed as an electron donor, as an electron acceptor, or as a neutral species depending on the sign and size of the electrostatic field at the interface or if the coverage increases as the magnitude of ($ - xM) increases. Summary
A new optical method of observing the interactions that occur between conducting polymer films and organic vapors has been introduced. This technique clearly shows that these interactions result in reversible changes in the bipolaron and polaron concentrations. Since bipolarons and polarons are charged discontinuities in the K backbone of the polymers, it has been proposed that the changes observed in their concentrations result from (32) Data taken from Handbook of Chemistry and Physics; Weast, R. C., Ed.; CRC Press: Cleveland, OH, 1977; pp 74-80. (33) Ahmed, L. 1. J . Phys. Chem. Solids 1968. 29, 1653.
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The Journal of Physical Chemisfry, Vol. 95, No. I, I991
electron density moving into/out of the K backbone of the conducting polymer from/into the organic vapor. As the bipolaron and polaron states make up the highest occupied molecular orbital (HOMO) of the polymer, changes in their concentrations also lead to shifts in thc polymer work function and this has been followcd by thc Kclvin probe. Sincc the organic vapor is believed to interact directly with the x backbonc of the polymer, the need for the presence of heterogencous atoms in thc polymers structure is eliminated. This explains why p-polyphenylene is capable of interacting with various organic vapors as strongly as polypyrrole. It was found that the size and direction of any vapor-induced shift in the work function of a conducting polymer, 4, is directly related to the relative sizes of 4 and the electronegativity of the vapor xM. It has also been shown that 4 varies with the growth conditions, and thus once a method of accurately controlling the initial work functions of the polymer films is obtained it may be possiblc to prcdict the response of any film to any particular organic vapor. Therefore, it should become possible to produce relatively high sensitivity gas sensors by choosing a film for which the magnitude of 4 - xM is large. However, a lower limit on the valuc of thc work function of the polymer, 4, will be set by the
Blackwood and Josowicz fact that a practical sensor must be stable in its normal environment, which is usually air. Selectivity could be achieved by using an array of films with different work functions, so that the required vapor acts as an electron donor toward some films and as an electron acceptor toward the others, with all other vapors present acting in the same manner toward all films. However, since many organic vapors are likely to have very similar Mulliken electronegativities it may prove difficult to obtain both selectivity and sensitivity at the same time.
Acknowledgment. We thank Jiri Janata for valuable suggestions during course of this work. D.B. thanks the A. v. Humboldt Stiftung for financial support. This work was supported by a contract from the Bundesministerium fur Forschung und Technologie (BMFT Contract No. 13 AS 0107/6/90). The University of Bundeswehr assisted in meeting the publication costs of this article. Registry No. PP, 30604-81-0; TCNQ, 1518-16-7; TOS, 733-44-8; PPP, 251 90-62-9; LiC104, 7791 -03-9; Fe(CN)6, 13408-62-3; MeOH, 67-56-1; i-PrOH, 67-63-0; CHCI,, 67-66-3; CH2CI2, 75-09-2; AcCN, 75-05-8; n-C,H,d, 110-54-3.
