J . Phys. Chem. 1992, 96, 7824-7830
1824
Characterization of the Interaction between Poly(pyrrole) Films and Methanol Vapor Patrice Topart and Mira Josowicz* Institut fiir Physik, Fakultat fiir Elektrotechnik, Universitdt der Bundeswehr, Werner Heisenberg Weg 39, 0-8014 Neubiberg, Germany (Received: October 16, 1991; In Final Form: May 26, 1992) A combination of mass, optical spectroscopy, and work function measurements has been used to study the interaction of methanol vapor with poly(pyrro1e) thin films. The steady-state measurements carried out in this study open the way for qualitative and quantitative evaluation of the charge-transfer and sorption interactions of organic vapors with the ‘network solids”. It is shown that the chemical nature of the dopant anion incorporated in the poly(pyrro1e) matrix strongly influences the reversible doping by the vapor.
1. Introduction The use of organic semiconductors as sensitive layers in chemical gas sensors requires a firm understanding of the sorption processes of molecules, by which the chemical interaction is transformed into a measurable signal.’ In our previous paper,2 we addressed the problem of the chemical doping, in a thermodynamically reversible manner, of conducting polymers exposed to various vapors. It was suggested that a transfer of electrons between the organic semiconductor and the electrically neutral molecule occurs. The electron-donor or -acceptor character of the polymer was monitored by changes in the work function and optical absorption at selected wavelengths. Different responses were observed depending on the difference between the ionization potential of the guest molecule(s) and the work function of the organic semiconductor (film). These interactions result in sorption and the formation of charge-transfer complexes. The sorption of the guest molecules can be seen as a two-step process in which the molecules first adsorb on the surface of the polymer and then absorb in the bulk of the material. When sorption equilibrium is reached, the presence of the guest molecules in the film may result from two phenomena: the solubility of the molecule in the film,as govemed by Henry’s law, and the partial charge transfer between the molecule and some specific sites in the film.’ The overall magnitude of the bulk and surface interactions results in a modulation of the work function.’ The former is affected by the presence of donor/acceptor sites inside the film, whereas the latter is govemed by the presence of a surface density of states. Thus, the physical and chemical nature of the polymer bulk and its surface will play a major role in the electron distribution. For instance, the polarizability of the species contained in the molecular solid will affect the electrostatic interactions with the dopant molecule and consequently drive the energetics of the sorption process. Upon exposure of the film to a vapor, electroneutrality must be maintained in the polymer. This is achieved by recombination of the electron-hole pairs and involves a concentration change of the redox active sites (polaron and bipolaron). Relative changes in the concentration of the electronically active sites in the polymer are measurable by optical spectroscopy and by the Kelvin probe technique. The relative intensity changes of the electronic transitions are modulated by changes of the concentration of occupied sites in the film. The Kelvin probe method relies on the contact potential difference that exists between dissimilar conductors. Thus,optical measurements are absolute, while Kelvin probe measurements are always relative. The present paper focuses on a quantitative study of the interactions responsible for work function changes. It is thought that the study of similar materials can help our understanding of these interactions. Poly(pyrroletosy1ate) (PPTOS) and poly(pyrroletetrafluoroborate)(PPTFB) films differ only in the nature of their anion. It is known4that both films grown under the same conditions are in the same oxidation state and display approximately the same conductivity which is only slightly affected by To whom correspondence should be addressed.
0022-3654/92/2096-7824%03.00/0
exposure to air over long periods of time (months). PPTOS films are hygroscopic and take up ca. 3% water at room temperature within 12 hn5This water uptake is reversible. The mechanical properties of both f i differ. Dry PPTOS films are characterized by greater tensile strength and Young’s moduli than thoseof dried PPTFB films which can be qualified as soft polymers? The mechanical properties of conducting polymers strongly affect the solubility properties of the analyte.7.8 Thus, these properties are also important in the study of polymer-vapor interactions. 2. Experimental Section
Acetonitrile (99.99%) (AcCN), methanol (99.9%) (MeOH), pyrrole (98-99%) (P), tetraethylammonium 4-toluenesulfonate (TEATOS), tetrabutylammonium tetrafluoroborate (99%) (TBATFB), all from Aldrich, and silver nitrate (99.99%) (AgN03), from Merck, were of analytical grade and used as received. The substrates used in the QCM experiments consisted of 1-in.-diameter overtone polished 5-MHz AT-cut and 6.19-MHz BT-cut quartz crystal plates (Valpey-Fisher, CA). Both types of crystal cuts had the same thickness shear mode of vibration and were temperature compensated. They had the same mass sensitivity of 17.7 ng Hz-I cm-2. On both sides of the crystals, gold film electrodes of about l0OO-A thickness were sputtered on top of a Ni/Cr layer (glue metal). An asymmetric electrode pad design was used as described in ref 9. The region of maximum mass sensitivity was restricted to the overlap area between the two opposing electrodes (0.31 an2).This arrangement minimized nonuniform edge effects. The larger pad with an area of 0.50 cm2 served as the working electrode. The electrical contact to the gold pads was made by soldering thin copper wires with low-temperature indium solder. The electrochemical deposition of the poly(pyrro1e) films (PP) on the QCM substrates was carried out in an electrochemical quartz crystal microbalance (EQCM) cell, which allowed the simultaneous evaluation of the frequency shift, Ah,corresponding to a mass increase of the deposited film, Am,, and the charge, Q. The PP films were grown on the EQCM gold working electrode (0.50 an2)from AcCN containing 0.05 mol dm-’pyrrole monomer in the presence of 0.1 mol dm-’ TEATOS or TBATFB as a supporting electrolyte. A stainless steel rod (with a diameter of 1.2 cm) onto which 2000 A of platinum had been sputtered was used as the counter electrode. The latter was always placed facing the working electrode at a distance of approximately 0.8 cm.This kind of electrode arrangement in the cell used for the electrochemical deposition has been shown to eliminate edge effect problems at the working electrode.I0 The substrates used in the Kelvin probe experiments were stainless steel rods having a diameter of 1.20 cm on which metal thin f i i were sputtered: 200 A of Ti/W (glue metal) and 3000 A of platinum. These substrates were mounted onto a metallic rod holder for electrochemical deposition of organic films or onto a micrometer screwrod for the Kelvin probe measurements. During the deposition, the substrate was tightly pressed into a surrounded Teflon ring (0.2 cm thick and 1.0 cm high) which was placed approximately 0.5 cm above the edge of the platinum 0 1992 American Chemical Society
Poly(pyrro1e) and Methanol Vapor Interaction surface (1.1 1 cm2)of the metallic rod. The counter electrode was a platinum grid with an active area of 1.5 cm2. The substrates used in the optical measurements were microscope glass slides cut to 2.0-cm X 0.5-cm dimensions. Twenty angstroms of Ti/W (a glue metal) followed by 100 A of platinum was sputtered on one side of these slides. The cell arrangement used for the organic film electrochemical deposition was similar to that described above. All polymer layers were deposited from freshly prepared electrolyte. They were grown potentiostatically with a pulse sequence mode which was generated by an EG&G PAR 273 potentiostat. The pulses were applied between the initial opencircuit potential, 0.6 V (measured with the “cell off“ setting) for 5 s and 0.9 V with “cell on” for 100 ms. The thickness of the deposited PP layers was controlled coulometrically. This control was p i b l e kcause of the known stoichiometry of the polymer.” Two electrons are required for the oxidation of the pyrrole molecule into a radical cation, and an additional fraction of electron, between 0.2 and 0.5, is required for the further oxidation of the polymer backbone. The reference electrode for all electrochemical experiments was Ag/O.l M AgNO, in AcCN. The reference electrode was separated from the electrolyte by a Luggin capillary containing the background electrolyte. The freshly deposited PP film was rinsed with AcCN and left to equilibrate for about 1 h in the background electrolyte until the open cell potential reached a stable value ( f l mV).I0 The polymers were dried at 80 OC in a vacuum oven and their thicknesses measured by a profilometer (Sloan Dektak 11). Since the solvent used for the electrochemical growth strongly affects the viscoelastic properties of the synthesized PP films,12 a conditioning pretreatment was used. This pretreatment was applied to all PP layers immediately after their deposition and after repeated exposure to methanol vapor. The entrapped solvent was removed by vacuum-drying in the oven at 400 mbar and 80 OC for 12 h. The films were then exposed for a few hours to ambient air before they were used again in further experiments. Methanol vapors within a concentrationrange of 3.0-15.0 vol Sb were obtained by bubbling nitrogen gas (99.99% Linde) at a constant flow rate of 50-400 cm3min-’ directly through a bubbler containing methanol maintained at room temperature. The resulting vapor s t ” was further quantitatively diluted by a second nitrogen line. For the vapor concentrations within the range of 3Ck15000 ppm, a capillary diffusion tube system, Dynacalibrator 230 (VICI Metronix), was used. The switching between an alternating vapor stream consisting of the vapor of interest and nitrogen was performed by automatically driven miniature solenoid valves (Asco/Angar GmbH). At the end of the gas/vapor line, the measuring devices (QCM, Kelvin probe, spectrophotometer) were connected in a series so that they were exposed to the same vapor concentrations. Teflon tubing was used with stainless steel (Swagelok) or Teflon (Asco/Angar) adaptors. The flow rates were monitored by mass flow controllers (Wigha GmbH, Germany). The vapor generation system was calibrated by a gas chromatograph (Philips PU 4900) which was connected to an integrator (Spectra Physics). The deposition rate of the PP layer on the quartz crystal was monitored by an electrochemical quartz crystal microbalance (EQCM) equipped with an oscillator circuitry similar to that given in ref 13. The side of the crystal which was used as the working electrode in the electrochemical cell was grounded and connected to the EG&G PAR 273 potenti0~tat.I~ For the measurements in the gas phase, the crystals were made to oscillate at their resonant frequency using a high-stability feedback oscillator circuit (CD 4049). The resonant frequency of the crystals was measured using an HP 5335 or a Siemens B2032 frequency counter with a resolution of 0.1 and 1.0 Hz, respectively. The data were stored in an HP Vectra computer via an IEEE 488 interface bus at a minimum sampling frequency of 45 Hz. The quartz crystal plate was sealed between two O-rings recessed in a Teflon mount cell. This cell was either a batch-type or a flow type cell when used for the electropolymerization or the exposure to vapors, respectively. The volume of that cell was 1.40
The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7825 cm3. Furthermore, it was found that the resonant frequency of the PP-coated QC depended on the flow rate. Therefore, all films were tested at the same flow rate, typically between 100 and 400 cm3 m i d . It is well-known that the validity of Sauerbrey’s equation relies on the continuity of the thickness shear wave propagation at the quartz/polymer interface. The analysis of the frequency domain response of the admittance of the quartz/polymer mechanical resonat~r’~ proved to be a useful technique to monitor the damping of the thickness shear wave. Acoustic losses can be directly evaluated by monitoring the conductance maximum at resonance G,, as loading changes. The details of the method have already been discussed.16 The attenuation due to the overlayer can be also evaluated by the broadening of the maximum/minimum bandwidth (AQ) of the susceptance in the resonance regi0n.I’ Such experiments were carried out on a 5000-A PP film deposited on a BT-cut quartz crystal. The crystal was either connected to an impedance analyzer (Hewlett-Packard 4192 A) or was part of a feedback oscillator circuit. The same vapor concentrations were used for the admittance and the mass measurements. The resonator was repeatedly subjected to an alternating stream consisting of the methanol vapor and the carrier gas until the resonant frequency had reached a constant value. The conductance, G,, and the bandwidth, AQ, were then recorded using a chart recorder (SE 780 BBC, Goerz Metrawatt, Germany). The frequency was scanned with a 10-Hz step around the resonant frequency, i.e., 5 f 0.002 MHz. The oscillation levels were 0.10 and 0.01 V. The measurement accuracy for AG/G was 0.6% and for AQ was f10 Hz. The obtained AQ and G, values of the resonator were not affected by the flow rate of nitrogen in the range 50-400 cm3 mid. A gas flow-through cell containing the Delta-Phi-Elektronik Kelvin Probe S (Besocke GmBH, Jiilich, Germany) was used for the work function measurements. The work function of PP-coated substrates was obtained from the contact potential difference (CPD) measured vs a vibrating gold grid reference electrode (diameter 2.5 mm) which was facing the center of the sample.2 In the automatic balancing mode, the CPD was automatically compensated and the work function changes monitored by a Keithley 199 System DMM/scanner which was connected to an HP VECTRA through the IEEE 488 interface. The changes in work function were obtained when the signal had reached a stable value ( f l mV). All optical experiments were performed on a Cary I/Varian spectrophotomer equipped with a gas flow cuvette (3.5 cm3) in which the substrate was placed. The UV/vis absorption spectra were recorded once the PP film was in equilibrium with the vapor. The obtained spectra were reproducible to repeated exposure to methanol vapors. The absorbance of the unmodified Pt substrate was always subtracted. For the simultaneous measurements of changes of mass, work function, and optical absorption, identical PP films were grown from the same solution on the substrates used in the various techniques. All films underwent simultaneously the same pretreatments. Before each experiment, the films were equilibrated under flowing nitrogen until the frequency and the work function signal remained constant. 3. Results Quartz Crystal Microbalance Studies: (i) Characterizationof the Rimpry Doping of t h e E k ” i c n u y Sydmi7.d P P h From the EQCM experiments, the mass of the deposited PP film and the amount of charge which was used during the electropolymerization were obtained. These data were used for the calculation of the number of electrons transferred during the film growth. This was essential to determine the doping level of the film. Figure 1 demonstrates the linear dependence between the charge used for the electrosynthesisof the PPTOS and PPTFB films and their mass. This dependence shows that the PP films behave as rigid layers within the thickness range studied, 30015000 A. These results agree with those of ref 18. The total number of electrons transferred to each pyrrole molecule during
Topart and Josowicz
7826 The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 REGION I
[REGION I I
o PPTFB PPTOS
50
0
20
40
60
EO
A m s [fig C l 8 1
Figure 1. EQCM experiment: correlation between the mass of the deposited PPTOS and PPTFB films and the charge passed during their deposition.
MeOH i n N2 [vol
XI
Figure 3. Combined admittance and mass measurements for a 5OOO-Athick PPTOS film deposited on a 5-MHz BT-cut quartz crystal plate. The mass sensitive area equals 0.31 em2, and the mass sensitivity is 27.1 ng/(Hz em2). - A f y IHzl
2150
B
o
o
}
]
4500-60008, ~
2000-30008,
50
}
0
0
SCAN LENGTH [pml
2
500-10008,
4 6 E 1 0 1 2 MeOH i n Np lvol XI
Figure 4. Influence of the mass of the PPTOS film on the methanol sorption equilibrium. The mass sensitivity is 17.7 ng/(Hz em2). 1 , RRR
A
SCAN LENGTH [pm]
Figure 2. Profilometric surface scans of as-grown PPTFB (A) and PPTOS (B) deposited on 1500-A Au/NiCr after equilibration and vacuum drying. The films are about 2000 A thick. The same surface morphology was obtained for the films deposited on Pt substrates.
film growth can be obtained from Faraday's law and the measured mass of the polymer. By carrying out a calculation similar to that presented in ref 18, n values of 2.27 and 1.70 electrons/pyrrole in the polymer chain were obtained for PPTOS and PPTFB, respectively. The particularly low n value for PPTFB is attributed to solvent incorporation in the film.19 Supporting evidence comes from profdometer traces where the roughness and the morphology of the dry films clearly depends on the nature of the counterion, Figure 2. The greater roughness of the PPTFB films is due to the smaller size of the counteranion, which influences the packing density of the polymer matrix during polymerization.20 The equilibration of the films in the background electrolyte may result in insertion or removal of ionssolvent in/from the film in order to maintain electroneutrality.2' With medium-sized TOS- anions, no variation of the film thickness was observed after equilibration. However, the PFWB film thickness decreased upon equilibration in the supporting electrolyte. This is in agreement with ref 21 where the small BF,- anion was shown to move easily out of the film. The PPTOS films are denser and harder than the PPTFB films.2oIn spite of their morphology difference, the two films were obtained in the same oxidation state, bccause the oxidation state is an intrinsic characteristic of the polymer and is not sensitive to the nature of the incorporated anions.
(ii) Characterization of the Methanol-Doping Process. The validity of the linear frequency to mass relation, as formulated by Sauerbrey's equation, must be verified when quartz crystal microbalance (QCM) measurements are acquired. When a polymer-film-coated QCM is in contact with methanol vapors, it is important to ascertain that the overlayer behaves as a rigid film and that contributions to the frequency shift other than mass loading are negligible. These arguments are directly related to the dependence of the viscoelastic properties of the polymer on the vapor concentration. In order to characterize the viscoelastic properties of the polypyrrole films, combined measurements of admittance and mass were carried out as previously de~cribed.'~ Combined QCM and admittance measurements on a 5000-hhick PPTOS film are shown in Figure 3. It is seen that the amount of absorbed vapor varies linearly with the concentration in the gas phase as long as PP behaves as a rigid layer. Departure from linearity signals the onset of plasticization, which translates into a decrease of G,,, and an increase of AQ; see Experimental Section. In Figure 3, two regions can be distinguished. In region I, the bandwidth and the conductance maximum remain unchanged. The good reproducibility of the data points in this range of methanol concentration confirms a rigid behavior of the PP film. Region I1 is characterized by the departure from linearity. This is accompanied by a substantial increase of AQ (8.4%) and decrease of G,, (7%) which occurs at about 12 vol % of MeOH in nitrogen. It is obvious that there is a damping of the thickness shear wave propagating from the quartz crystal into the PP film. Viscous losses have occurred in the polymer. The boundary between regions I and I1 actually corresponds to the onset of plasticizatioin of the PP film by the methanol vapor. The polymer switches from an elastic to a plastic behavior for an uptake of about 12 vol % of methanol in the vapor phase. The film viscosity also increases as demonstrated by the decrease of the conductance maximum. The thermodynamicsorption equilibrium between the methanol vapor and the poly(pyrro1e)-coated piezoelectric crystal studied in concentration region I always resulted in a resonant frequency decrease, A&, corresponding to a mass increase due to methanol incorporation in the film. The frequency shifts of several PPTOS films (500-6000 A) exposed to various methanol vapor concen-
~
The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7827
Poly(pyrro1e) and Methanol Vapor Interaction
0
- A f v IHZl
1150 - 1
1
,
I
,
,
,
I
,
I
,
I
,
I
,
I
.
,
,
I
,
-0.01 PPTFB
/I
-0.02
->,
-0.03
-0.04
U
50
ul -0.05
I 0
0.5
1.0
1.5
2.0
1
-0.06
-0.07
ReOH i n Nq I v o l XI
PPTOS
-0.08
Figure 5. Methanol sorption isotherm for 4000-A-thick PPTOS and
-0.09
PPTFB films in the concentration range 20-15000 ppm. The mass sensitivity of the QCM is 17.7 ng/(Hz cm2).
10
0
20
30
50
40
70
60
90
80
100
elapsed time ( s e d
TABLE I: Dependence of the Partition Coefficient on the Methanol Vapor Concentration, C,O
C,, W c m '
log KlPprOS
log KZPprFB
0.13 0.65 1.30 4.55 6.50 13.0
4.77 4.49 4.29 3.89 3.79 3.61
4.99 4.73 4.54 4.26 4.14 3.94
--
.A -0.01
->
-0.02 -0.03
,-0.04
Y
"log Kl was calculated for ALfr(ppros)= 1582 Hz 28 pg/cmZ, and = 1524 Hz 26.9 pg/cm2. log KZ was calculated for
trations are shown in Figure 4. The shapes of the curves suggest a Type I1 BET sorption isotherm, where the enthalpy of vaporization of methanol is much lower than that required for the desorption of one monolayer.22 A comprehensive study of the sorption mechanism is the subject of another publication.u Similar results have been obtained for PPTFB films. In order to investigate the influence of f h morphology on the sorption equilibrium, frequency shifts caused by vapor sorption, Ah, were recorded for PPTOS and PPTFB films. In Figure 5, the sorption isotherms deviate from linearity and depend on the nature of the film. Henry's law is only valid for the low y n centrations, where fugacities can safely be replaced by concentrations. From the relationship between frequency shifts caused and the frequency shifts due to the presence by vapor sorption, 4, of the film alone, AA, a vapor partition coefficient, K, can be calculated:8
K=- ALP AACV where p is the density of the coating material and Cvthe vapor concentration. A density of 1.4g cm-3 was used for both films. This value was obtained from electrochemical quartz microbalance (EQCM) measurements and agrees with that given in the literat~re.~~ More quantitative information about the solubility interaction can be obtained from log K value, which is a function of the solvation parameters of the solute.8 Values of log K calculated for various concentrations of methanol vapor decrease with increasing concentration (Table I). This dependence emphasizes the importance of vapor solubility in the film. Kelvin Robe Studies. In order to explain the dependence of log K on the concentration of the charged species present in the poly(pyrro1e) layer, changes in the work function, A@, resulting from the exposure of the thin film to the methanol vapor were investigated. The work functions, measured in nitrogen, of PPTOS and PPTFB were qual, as expected for films in the same oxidation state. Upon exposure to methanol, the changes in work function of the two PP layers were different in sign and in magnitude, Figure 6A. Revenibility of these changes in work function is shown in Figure 6B. This supports the charge-transfer model, which at the equilibrium state can be described as follows: MeOHpp + be MeOHd
-
-
\
PPTOS
'
-0.09 0
"
I
200
400
'
I
600
'
I
I
800
1000
'
I
1200
'
I
'
1400 1600
elapsed time ( s e d
Figure 6. Work function transients for 4ooo-A-thick PPTOS and PPTFB films during (A) the absorption and (B) the desorption of 1.0% MeOH in N2.For the desorption, a nitrogen flow rate of 200 cm3/min was used. A*
ImeVl
4
1 I
40 20
__
-70
-16
-14
-12
-8
-10
ReOH i n N2 I n
-6
mol/ll
Figure 7. Change in work functions of the PROS and PPTFB layers measured during their exposure to various methanol vapor concentrations.
Both layers were deposited using 68.6 mC/cm2.
where MeOHpp refers to the concentration of methanol in the matrix and tie to the exchanged partial charge density of the methanol molecule entering the solid? Note also the 20-fold time increase between desorption and sorption of the methanol vapor. The changes in work function, A@, resulting from the exchanged partial electron density between the layers and various concentrations of methanol are shown in F m 7. The sensitivity slopes (A@ vs In CMaH)for the PPTOS and PPTFB layers were -3 and +14, respectively. In the context of a charge-transfer model, the charge-transfer parameters, 6, defined by ref 3, were evaluated as 6 = 4for PPTOS and 6 = +0.9 for PPTFB. The 1st < 1 for the PPTFB layer strongly suggests the transfer of electrons from the matrix to methanol, forming a charge-transfercomplex. The increase of the work function indicates that the methanol acts as an oxidizing agent. The experimentally determined ltil > 1 value for the PPTOS film suggests that some other electron-transfer mechanism competes with the charge-transfer interaction. Although [til > l cannot be explained by the charge-transfertheory, a negative sign of 6 would account for the acceptor behavior of
7 8 1 The Journal of Physical Chemistry, Vol. 96, No. 19, 1992
'
-
I
2700
D D ~
1'
Oo0
Wm
Topart and Josowicz 0.54
I
0.52
0.3
92
-
0.5
0
uU
0.48
2
0.24
5 n
0.21
n
