Langmuir 1990,6, 1356-1359
1356
Acknowledgment. We thank the National Science Foundation (CHE 8911906) and Miles Inc. for support of this work.
Registry No. TMPyP, 50391-14-5;calcium chloride, 1004352-4; tetra-n-butylammonium, 10549-76-5;tetramethylammonium, 51-92-3.
Crystal Growth of Polycrystalline a-CdS on Conducting Polymers Evangelos Dalas,+ Sotirios Sakkopoulos,i Jannis Kallitsis,' Evangelos Vitoratos,t and Petros G. Koutsoukos*l§ Department of Chemistry, University of Patras, Greece, Department of Physics, University of Patras, Greece, and Department of Chemical Engineering and the Research Institute of Chemical Engineering and Processes at High Temperatures, P.O. Box 1239, Patras, Greece Received December 8, 1989. In Final Form: February 23, 1990 Conducting polypyrrole polymers were doped during polymerization with a-CdS crystals. Doping made the polymers effective nucleators of a-CdS. Once the doped polypyrrole specimens were introduced into supersaturated cadmium sulfide solutions, a-CdS precipitated immediately without any precursor phase or induction period. The rates of precipitation were found to depend on the solution supersaturation and were maximum at an optimum dopant concentration of 6.4 % w/w. Increasing dopant concentrations decreased the polymer conductivity and increased its p-semiconductingcharacter, possibly through structural changes induced by doping. Introduction Organic polymers are increasingly replacing metals and other inorganic materials, in numerous applications ranging from photoconductors and piezoelectric devices to car parts. This trend is primarily based on advantages shown by the organic polymers, such as performance, durability, processability, and relatively low cost. The field of applications has recently been widened tremendously with the discovery of the first conducting polymers.'-5 Polymers such as polyacetylene and poly(pheny1ene chalcogenide^)^.^ have acquired semiconducting or metallic conductivities through doping with electron donors or electron acceptors. It is believed that the polymer chain structure, both in the presence and in t h e absence of dopants, is of key importance for the electronic properties of the polymer t
Department of Chemistry, University of Patras.
* Department of Physics, University of Patras.
JI Department of Chemical Engineering and the Research Institute of Chemical Engineering and Processes at High Temperatures. (1) Mac Diarmid, A. G.; Heeger, A. J. J. Synth. Met. 1980,1, 101. (2)Kanazawa, K. K.; D i u , A. F.; Geiss, R. H.; Gill, W. D.; Kwak, J. F.; Logan, J. A.; Rabolt, J. F.; Street, G. B. J. Chem. Soc., Chem. Commun. 1979.854. (3)Yam'amoto, T.;Sanechika,K.; Yamamoto,A. J.Polym. Sci., Polym. Lett. Ed. - 1980. ~... , 18. - - ,9. Sibson, H. W.; Bailey, F. C.; Pochan, J. M.; Epstein, A. J.; Rom-
cawa, H.; Ikeda, S. J. Polym (6)Shacklette, L. W.; Elsenbeumer, R. L.; Chance, R. R.; Eckhardt, H.; Frommer, J. E.; Baughman, R. H. J. Chem. Phys. 1981,75,1919. (7)Rabolt, J.F.; Clarke, I. C.; Kanazawa, K. K.; Reynolds, J. R.; Street, G.B. J . Chem. SOC.,Chem. Commun. 1980,348.
0743-7463/90/2406-1356$02.50/0
dopant systems.8 Besides polyacetylene, other polymers with interesting electronic properties include polythiophene, polypyrrole, polydiacetylene, and polyparaphenylene. Doped polypyrrole has advantages over other polymers, the most important of which are that it may be obtained as filmsgJ0 and it is the most environmentally stable conducting polymer. An interesting prospective application of polypyrrole would be the construction of photovoltaic cells by using cadmium sulfide deposits as windows. Polypyrrole, doped with perchlorate, has a band gap of 3.0 eV," and cadmium sulfide with a band gap of 2.43 eV would make a good window material for a CdS/polypyrrole heterojunction solar cell. It has recently been shown that modification of polymers by introducing the appropriate active groups enables them to serve as substrates which nucleate sparingly soluble salts selectively. The nature of the active groups determines the salt which may nucleate.I2-l4 In the present work, we have doped polypyrrole with polycrystalline cadmium sulfide, instead of modifying the polymer. This resulted (8)Baughman, R. H.; Bredas, J. L.; Chance, R. R.; Elsenbauma, R. L.; Shacklette, L. W. Chem. Reu. 1982,82, 209. (9)Dall'Olio, A.; Dascola, Y.; Varacco, V.; Bocchi, V. C. R. Seances Acad. Sci., Ser. C 1968,267, 433. (10)Soga, K.; Kobayashi, Y.; Ikeda, S.; Kawakami, S. J.Chem. Soc., Chem. Commun. 1980,931. (11)Kaufman, J. H.; Colaneri, M.; Scott, J. C.; Street, G. B. Phys. Rev. Lett. 1984, 53,1005. (12)Addadi, L.; Moradian, J.; Shay, E.; Maroudas, N. G.;Weiner, S. Proc. Natl. Acad. Sci. U.S.A. 1987,84, 2732. (13)Dalas, E.;Kallitsis, J.; Koutsoukos, P. G . J. Cryst. Growth 1988, 89,287. (14) Dalas, E.; Koutsoukos, P. G. J . Colloid Interface Sci. 1989,127, 273.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 8,1990 1357
Crystal Growth of a-CdS on Polymers Table I. Polypyrrole Preparations and Their Characteristicsa conductivity, SSA, CdS, Scm-* mag--' oxidant sample electrolyte % w/w 17.3 511A KC103 (NHdzSaOs 0.4 N58 (NHd~Sz08 0.3 22.0 N9 6.4 (NHMaOs N24 FeCl3 (pH 2.3) 30.6 0.1 20.0 0.16 FeC13 (pH 2.0) N59 4.9 21.0 N57 1.6 FeC13 (pH 2.0) 7.1 19.0 16.0 FeC13 (pH 2.0) N56 a Molar ratio oxidankpyrrole = 19.
time\min 20
15
~~~
in selective and rapid cadmium sulfide overgrowth when CdS-doped polypyrrole was introduced in aqueous supersaturated solutions of cadmium sulfide. The effects of doping on the electronic behavior of the polypyrrole substrates were also investigated.
Experimental Section Preparation of the Polymers. Polypyrrole was prepared by polymerization of the monomer, as described in the literature.15 Ammonium persulfate and ferric chloride solid reagents (Fluka) were used as oxidants during the polymerization, which was done a t p H 2.0 adjusted with standard hydrochloric acid (Merck, Titrisol). In addition, during the polymerization, cadmium sulfide particles, prepared by spontaneous precipitation in ammonium sulfide and cadmium nitrate solutions, were suspended. The polymer obtained was doped with cadmium sulfide, as its dry weight increased, while the filtrates from the extensive washing of the polymer on membrane filters (millipore, 0.22 pm) were free from cadmium sulfide particles. The doped polypyrrole thus prepared, was obtained as a black powder, and its conductivity was found to depend strongly on the nature of the oxidant used and also on whether cadmium sulfide powder was added in the polymerization medium. Conductivity Measurements. The specimens in the shape of disks, 13 mm in diameter and about 1.5 mm thick, were made by pressing powder of the conducting polymer in an IR press. For the conductivity measurements, a centered square fourprobe array of the electrical contacts, made by pressing tungs t e n wires against t h e specimen, was employed.16 T h e measurements of the dc conductivity as a function of temperature were made in a cryostat containing liquid Nz. Temperature was stabilized by a feedback procedure a t any value between 80 and beyond 300 K. F u r t h e r Characterization of the Powders. The polymeric powders were further characterized by measurements of their specific surface areas (SSA) by a multiple-point, dynamic BET method using He/N2 mixtures, powder X-ray diffraction (Siemens), and scanning electron microscopy (JEOL JSM-35). The polymers obtained and their properties are summarized in Table I. Crystallization Experiments. Crystallization of cadmium sulfide was done a t 25 OC in a 600-mL, double-walled Pyrex vessel thermostated by circulating water from a thermostat. Cadmium chloride (Merck, Puriss) and ammonium sulfide (Riedel de Haen, Puriss) solids were used for the preparation of the respective stock solutions with triply distilled, carbon dioxide free water. Cadmium solutions were standardized by atomic absorption spectroscopy (Varian 1200) and the ammonium sulfide solutions by the iodine method." Then 250 mL of each of the cadmium and ammonium sulfide solutions, prepared from the stock solutions, was mixed in the reactor, and the solution conductivity and pH were monitored through a conductivity cell (Radiometer P E 104) with a conductivity meter (Radiometer CDM 2e) and a combination glass/saturated calomel electrode (Metrohm) with a pH meter (Radiometer, P H M 26c), respectively. The standardization of the combination electrode was (15) Rapi, S.; Bocchi, V.; Gardini, G. P. Synth. Metals 1988,24,217. (16) Wieder, H. H. Laboratory Notes of Electrical and Galuanomagnetic Measurements; Elsevier: Amsterdam, 1979. (17) Karchnier, J . H. The Analytical Chemistry of Sulphur and its Compounds, part I; Wiley International: New York, 1970.
-
.v
,b
28
33
I
C o n d u c t i v i t y \a rbi t r a r y un P t s
F i g u r e 1. Plot of the change of solution conductivity with supersaturation with respect to a-CdS: (0) change of supersaturation u as a function of time, t , during precipitation; ( 0 ) calibration curve for solution conductivity as a function of total cadmium concentration, Cdt, in equimolar cadmium sulfide solutions. done with NBS standard buffer solutions.'* The p H adjustment was done by the addition of standard hydrochloric acid (Merck, Titrisol), and the stability of both solution conductivity and p H for over 5 h showed that they were stable. Next, a quantity of the cadmium sulfide doped polypyrrole was suspended in the supersaturated solutions, and precipitation started immediately, without the lapse of any appreciable induction period. The solution conductivity, dropping with the progressing crystallization, was recorded by a strip chart recorder (GOERZ R E 541). Since conductivity changes are of interest, in order to achieve maximum sensitivity the initial potential recorded by the recorder was offset by a constant potential supplied from a constant potential source. Variations of solution conductivities as small as 10-2 pS cm-l could thus be recorded. The recordings of the change of the solution conductivity were converted into supersaturation through the appropriate calibration curve. Calibration curves were constructed by measuring conductivities of stable supersaturated solutions, a t various degrees of s~persaturation,~9 thus simulating the precipitation process. The rates of precipitation were computed as initial rates from the slopes of the conductivity vs time data obtained from the recorder, and they were normalized per unit surface area of the substrate. The reproducibility was in all cases better than 5 % (mean of five measurements). During the course of the precipitation process, samples were withdrawn and filtered through membrane filters (Gelman 0.1 pm), and the liquid phase was analyzed for cadmium, while the solids were characterized by powder X-ray diffraction using aluminum as internal standard, infrared spectroscopy (Perkin-Elmer 477), and scanning electron microscopy. The results of analysis of cadmium during the course of reaction confirmed our calculations of supersaturation, from the solution conductivity changes. The agreement was in all cases examined better than 2%. In Figure 1, a typical plot of the variation of the solution conductivity measured with supersaturation and the corresponding calibration line is shown. It should be noted that even at pH 3.0, a t which the experiments were done, the H2S loss was negligible.20
Results and Discussion As already mentioned, precipitation started immediately upon introduction of the CdS-doped polypyrrole powder in the stable supersaturated solutions. T h e initial conditions of the experiments in the present work are summarized in Table 11. The driving force for the crystallization of a-CdS is the difference in chemical (18) Bates, R. G. pH Determination; Wiley: New York, 1972. (19) Nielsen, A. E.; Toft, J. M. J. Cryst. Growth 1984, 67, 278. (20) Peters, R. W.; Ku, Y.; Bhattacharya, D. AIChE S y m p . Ser. No. 243 1985,81, 165.
1358 Langmuir, Vol. 6,No. 8, 1990
Dalas et al.
l!
15
h
?E TC
.-
16
E
t
-
/
K C
d
16
5
15
10
% CdS W/W I
I
1.8
1.2
0.6
Ln a
Figure 2. Kinetics of a-CdS growth on a-CdS-doped polypyrrole substrate N9. Dependence of the rate of precipitation on the relative solution supersaturation: pH 3.0,25 "C.
potentials, Ap, of this salt in the supersaturated solution, and at equilibrium, pw
p,
Figure 3. Conductivity of a-CdS-dopedpolypyrrole specimens (0) and the corresponding thermal activation energies ( 0 )as a function of a-CdS dopant content in the polymer.
I
m
(1)
AP=P-P,
.
since CdS = CdZt + S2pt = C(Cd2+ + pSZ-
(2)
+
= po RT In (Cdzt)(S") (3) In eq 3, parentheses denote t h e activities of the corresponding ions, R is the gas constant, and T is the absolute temperature. Similarly, at equilibrium pt
pt = po pt
+ RT In (Cdzt),(SZ-),,
= po
+ RT In Ko8,a-CdS
(4)
From eqs 1, 2, and 4 it follows that Ap = -RT In
(CdZt)(S2-) 8,a-CdS
Ap = -RT In Qa.CdS (6) In eq 6, Qa-CdS is the supersaturation ratio with respect to a-CdS and Ua-CdS, the relative supersaturation with respect to this phase: 0a.CdS
= (D&dS)1'2 - 1
(7)
per mole of ions AG = -RT In (8) The average free energy change per ion shall then be
Ac = -(RT/2) In Da.CdS
(9)
Calculation of Qa-CdS requires the computation of the activities of free Cd2+ and S2-. Thus, all equilibria involving calcium and sulfide species were taken into account along with the mass balance equations for total cadmium, Cdt, total sulfide, St, and the electroneutrality condition. For the activity coefficients, the Davies
I
ow
*
*
0 0
310
320
330
340
T\ K
Figure 4. Thermopowerof a-CdS-dopedpolypyrrole specimens at various temperatures: (m)N24,0% a-CdS w/w; (*) N57,0.16% a-CdS W/W;(A)N56,1.6% a-CdS W/W; ( 0 )N59,16% a-CdS w/w. Table 11. Precipitation of a-CdS on Polypyrrole Polymers. doped Cdt, AGwcds, R,, expt polymer 10-6 M kJ mol-' mol min-l m-2 1 N9 7.5 -11.1 20.4 2 3 4 5 6 7 8 9 10 11 12
N9 N9 N9 N9 N9 N24 N58 511A N57 N56 N59
5.0 2.5 2.0 1.5 1.0 5.0
5.0 5.0 5.0 5.0 5.0
-10.2 -9.6 -9.1 -7.8 -6.9 -10.2 -10.2 -10.2 -10.2 -10.2 -10.2
Total cadmium, Cdt = total sulfide, S,;pH 3.0
16.8 13.8 10.5 9.2 6.7
15.5 6.7 12.6
* 0.2,25 "C.
formulationZ1was used, and the system of equations was solved by successive approximations for the ionic strength.22 As may be seen from Tables I and 11, only the a-CdSdoped polypyrrole specimens could induce the precipitation (21)Davies, C. W. Ion Association; Butterworths: Washington, DC,
1962. (22) Nancollas, G. H. Interactions in Electrolyte Solutions; Elsevier: Amsterdam, 1966.
Crystal Growth of a-CdS on Polymers
Langmuir, Vol. 6, No. 8, 1990 1359
of this salt. Apparently, the dopant provided the active sites for the overgrowth of a-CdS on the polymer. The highest rates were observed with a dopant concentration 6.4% (w/w). It should be noted, however, that the structure of the polymeric substrate depends not only on the dopant but on the compound used as oxidant as well. Thus, preparation N9 seems to induce a-CdS formation a t significantly faster rates, compared to the other substrates. Within each series of polymeric substrates, the rates of precipitation, R,, depend linearly on the relative supersaturation, as may be seen in Figure 2, in which a logarithmic plot is shown. The fit of experimental data is eq 10
R, = ka"
(10) where k is the precipitation rate constant and n the overall order of the precipitation process. From the slope of the linear kinetics plot, a value of n = 1was obtained for the reaction order, suggesting a spiral growth mechanism,23 which has been found in seeded growth studies of several sparingly soluble salts.24 Polypyrrole lacks the fibrillary structure of other conducting polymers,26and it seems reasonable to assume metallic islands in a semiconducting (insulating) matrix, as in polyaniline. In such a system, the electrical conductivity is determined by the conduction in the metallic islands on the one hand and the contacts between them on the other. Thermal activation energies deduced from curves In Y = f ( l / T ) (11) concern the conduction between the islands. As may be seen in Figure 3, thermal activation energies E, decreased (23) Burton, W. K.; Cabrera, N.; Frank, F. C. Philos. Trans. R. SOC. 1951. A243. 299. .. -, . ._ .., . .. (24) Kazmierczak, T. F.; Schuttringer, E.; Tomazic, B.; Nancollas, G. H. Croat. Chem. Acta 1981,54,277. (25) Heeaer, A. J.; Kivelsa, S.; Schrietter. J. R.; Su, W. P. Reu. Mod. Phys. 1988,-60, 781.
(26) Kivelson, S. Phys. Reu. Lett. 1981, 46, 1344. (27) Kivelson, S. Phys. Rev. 1982, B25, 3798. (28) Kivelson, S. Mol. Cryst. Liq. Cryst. 1983, 77, 65.
