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Anal. Chem. 1986, 58, 2756-2761 Sleszynski, N.; Osteryoung J.; Carter, M. Anal. Chem. 1984, 56, 130- 135. Anderson, J. L.: Chesney, D. J. Anal. Chem. 1980, 52,2156-2161. Anderson, J. E.; Tallman. D. E.; Chesney, D. J.; Anderson, J. L. Anal. Chem. 1978, 50, 1051-1056. Chesney, D. J.; Anderson, J. L.; Weisshaar, D. E.; Tallman, D. E. Anal. Chim. Acta 1981, 124, 321-331. Weisshaar, D. E.; Tallman, D. E.;Anderson, J. L. Anal. Chem. 1981, 53, 1809-1813. Caudill, W. L.; Howell, J. 0.; Wightman, R. M. Anal. Chem. 1982, 54, 2532-2536. Tallman, D. E.; Weisshaar. D. E. J . Lip. Chromatogr. 1983, 6, 2157-2172. - . Anderson. J. L.;Whiten, K. K.; Brewster, J. D.; Ou, T. Y.; Nonidez, W. K. Anal. Chem. 1985, 57, 1366-1373. Anderson, J. L.; Ou, T. Y.; Moldoveanu, S. J , Electroanal. Chem. 1985. 196, 213-226. Aoki, K.; Tokuda, K.; Matsuda, H. J. Nectroanal. Chem. 1977, 79, 49-78. Fosdick. L. E.; Anderson, J. L. Anal. Chem., 1988, 58,2481-2465. Cope, D. K.; Tallman, D. E. J. Electroanal. Chem., in press. Anderson, L. B.; Sanderson, J. L. Anal. Chem. 1985, 57, 2388-2393. Thormann, W.; van den Bosch, P.; Bond, A. M. Anal. Chem. 1985, 57, 2764-2770. Kittlesen, G. P.; White, H. S.;Wrighton, M. S. J. Am. Chem. SOC. 1984, 106. 7389-7396. White, H. S.;Kittlesen, G. P.; Wrighton, M. S. J . Am. Chem. SOC. 1984, 106, 5375-5377.
(19) Chidsey, C. E.; Feldman. B. J.; Lundgren, C.; Murray, R. W. Anal. Chem. 1988, 58,601-607. (20) Belal, F.; Anderson, J. L. Analyst (London) 1985, 110, 1493-1496. (21) Siu, W.; Butler J.; CobboM R. S.C. Procwdlngs of the International Conference on Biomedical Transducers, 1975: Vol. 1, pp 319-324. (22) Weber, S.G.;Purdy, W. C. Anal. Chim. Acta 1978, 100, 531-544. (23) Anderson, J. L.; Moldoveanu, S. J. Electroanal. Chem. 1984. 179, 107- 1 17. (24) Moritz, H. I€€€ Trans. Electron Devices 1985, ED-32,672-676. (25) Oesch, U.;Janata, J. Electrochlm. Acta 1983, 28, 1237-1246. (26) Oesch, U.; Janata, J. Electrochlm. Acta 1983, 28, 1247-1253. (27) Weber, S.G. J. Electroanal. Chem. 1983, 145, 1-7. (28) Elbicki, J. M.; Morgan, D. M.; Weber, S. G. Anal. Chem. 1984, 56, 978-985. (29) Meschi, P. L.; Johnson, D. C. Anal. Chim. Acta 1981, 124,303-314. (30) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; Chapter 1. (31) Anderson, J. L. International Symposium on LCEC and Voltammetry, Indianapolis, IN, May 1983, Abstract No. 1.
RECEIVED for review April 28,1986. Accepted July 11, 1986. This work was supported by the U.S. Department of the Interior, Office of Water Research and Technology, EG&G Princeton Applied Research Corporation, and the Alabama Research Institute.
Electrodeposition and Characterization of Platinum Microparticles in Poly(4-vinylpyridine) Film Electrodes Duane E. Bartak,*' Beth Kazee, Katsuaki Shimazu, and Theodore Kuwana2 Department of Chemistry, T h e Ohio State University, Columbus, Ohio 43210
Electrodeposltlon of Pt micropartlcles at microgram levels In three types of M y ( 4-vlnylpyrldine) (PVP) films on glassy carbon (gc) electrodes Is described. The PVP films were formed by (1) electrochemical polymerization, (2) splncoaUng linear PVP on the gc surface, and (3) crob&linWng the llnear PVP on the gc surface. SEM photomicrographs of these flhns revealed the general structure, morphology, and degree of adhesion of the polymer to the carbon surface. Electrochemlcalpolymerlzatlon of 4-vInylpyrldkre was carrled out In dlmethylformamlde by potentlostatic techniques on an anodically pretreated gc surface. Cross-llnklng of PVP was accomplished by heatlng a mixture of llnear polymer, triallyl-substituted cross-linking agent,and radkal bdtlator on the gc surface. Electrochemical reduction of an acidic solution of hexachloroplathate, which was allowed to penetrate the PVP films, produced three-dlmenslonal dispersion of Pt micropatticks. The Pt/PVP/gc electrodes exhiblted good actlvity with regard to the generation of hydrogen. The staMllty of the cross-linked PVP films In acid solution was conskJeraMy better than the linear PVP films.
An important application of polymer-modified electrodes is their utilization in electrocatalysis (1). Wrighton has utilized an electroactive, viologen-based polymer into which Pt or Pd was dispersed to improve hydrogen evolution on semiconductor electrodes (2, 3). In addition, it has been shown that Pd deposition in the viologen-based polymer results in high Present address: Department of Chemistry, University of N o r t h Dakota, G r a n d Forks, ND 58202. *Present address: Center of Bioanalytical Research, 2099 Constant Ave., U n i v e r s i t y of Kansas, Lawrence. K S 66046.
current efficiencies for the reduction of bicarbonate to formic acid ( 4 ) . More recently, Wrighton demonstrated that Rh or Pd deposition in a cobaltocenium redox polymer on a p-type photocathode resulted in improved hydrogen generation (5). Our laboratory recently reported on the electrochemical deposition of metal microparticles into poly(viny1acetic acid) (PVAA) f i i s on glassy carbon electrodes (6,7). In particular, platinum was shown to be effectively dispersed in the film in a manner that resulted in significant catalytic activity with regard to the electrochemical generation of hydrogen. Furthermore, the stability of the platinum microparticles which were dispersed in the film was reported to be considerably better than microparticles deposited on a "baren glassy carbon (gc) surface (6). The PVAA film on the gc surface was produced by refluxing neat vinylacetic acid at 165 "C under nitrogen for a minimum of 16 h. However, the thickness of the PVAA f i i on the gc surface was difficult to control during the reflux and monolayer coverages were often the result of this process (8). This report describes the preparation and utilization of alternative polymer films whose thickness and properties can be better controlled for three-dimensional metal microparticle deposition. The three-dimensionality of metal microparticles in these films is important so as to attain high surface areas without agglomerization of the catalytic metal which should result in improved activities. In addition, it will be shown that cross-linking polymers directly on the gc surface can result in a film with a higher degree of stability. Poly(viny1pyridine) (PVP)has been shown to be an effective matrix for the incorporation of a variety of transition-metal ions into polymer electrodes (9). In particular, Anson and co-workers have shown that protonated PVP films can effectively bind transition-metal complexes by both covalent bonding and electrostatic interactions. Due to our interest
0003-2700/S6/0356-2756$01.50/0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
in the dispersion of metal microparticles in polymer films for electrocatalytic purposes, a study was initiated on the use of PVP films for microparticle metal deposition. The objective of the present study is t o prepare three-dimensional arrays of metal microparticles in the P V P polymer films, which can be easily fabricated and are stable in solution for long periods of time. We herein report on the dispersion of platinum microparticles in both linear and cross-linked PVP films. EXPERIMENTAL SECTION Chemicals. Linear poly(4-vinylpyridine) with a molecular weight of approximately 71 000 to 73 000 was obtained from Polyscience, Inc., and was used as received. 4-Vinylpyridine, triallyl trimesate (triallyl 1,3,5-benzenetricarboxylate),triallyl isocyanurate (triallyl 1,3,5-triazine-2,4,6(lH,3H,5H)-trione), and benzoyl peroxide were obtained and used as received from Aldrich. Azobis(isobutyronitrile) (AIBN) (2,2'-azobis(2-methylpropionitrile) was used as received from Alfa. Electrochemical grade tetrabutylammonium perchlorate (TBAP) (Southwestern Analytical Chemicals) was dried under vacuum (lo-' torr) at 100 "C for at least 24 h and used as the supporting electrolyte in the polymerization reactions. Spectroscopic grade NJV-dimethylformamide (DMF) (Burdick and Jackson) was dried over molecular sieves and was employed as the electrochemicalsolvent in these reactions. All inorganic chemicals were reagent grade and used without further purification. All aqueous solutions were prepared daily with NANOpure water (Sybron Barnstead). Glassware was cleaned in an 2-propanol/KOH bath followed by 1 M H2S04and thoroughly rinsed in NANOpure water. All solutions that were used for electrochemical measurements were purged with prepurified nitrogen prior to use. Electrochemical Cells and Instrumentation. The electrochemical cell has been previously described (IO). A silver/silver chloride (saturated KC1) electrode was used as the reference and all potentials are reported with respect to this reference. Glassy carbon (gc) plates (approximately 1.5 cm2,GC-20 and GC-30 grade, Tokai) were used as the working electrodes. The geometric area exposed to the solution via a Kalrez (Du Pont) gasket was 0.385 cm2. The gc electrodes were hand-polished successively with 600 grit and 1.0-pm alumina (Baikolox) followed by ultrasonication as previously described (10). The auxiliary electrode was a Pt wire which was isolated by a frit. All electrochemical measurements were performed with an in-house-constructed potentiostat or a CV-1A (BAS) potentiostat. An Omnigraphic 2000 X-Y recorder (Houston Instruments) was used for data acquisition. Preparation of Polymer Modified Electrodes. Three types of PVP films were prepared on the previously polished gc electrodes: (1)electrochemically polymerized, (2) spin-coated linear, and (3) cross-linked. The electrochemically polymerized PVP films were prepared by reduction of 4-vinylpyridine in DMF/ TBAP on anodically pretreated gc surfaces (vide infra). The linear PVP films were spin-coated from 1.0% solutions of PVP (Polyscience) in methanol or glacial acetic acid. The spin-coating process was accomplished by adding 25 pL of the above solution on the gc electrode (ca. 1.5 cm2 area) rotated at 250 rpm with a Pine Instruments RDE rotator, which was modified in order to mount the electrode facing up. The cross-linked PVP films were prepared by initially spincoating a mixture of linear PVP, radical initiator, and cross-linking agent onto the gc electrodes (vide ante). The spin-coated electrodes were subsequently placed in an oven at 90-95 "C for a minimum of 3 h in order to effect the cross-linking reaction. The solvents used were methanol, propanol, or glacial acetic acid with concentrations of 0.5% linear PVP, 0.3% cross-linkingagent, and 0.2% radical initiator. The cross-linkingagent was either triallyl trimesate or triallyl isocyanurate. The radical initiator that was used was either benzoyl peroxide or AIBN. An alternative method of cross-linkingthe PVP included refluxing a mixture of the linear PVP, radical initiator, and cross-linking agent for 6 h before spin-coating. The type and concentration of each component were the same as described above in 1-propanol. The resultant solution mixture was immediately spin-coated onto a polished gc electrode. Dispersion of Pt Microparticles in the P V P Films. Two potentiostatic methods, cyclic voltammetry (CV) and double potential step chronoamperometry (DPCA), were used to deposit
2757
Pt particles into the films from 10 mM potassium hexachloroplatinate in 1N sulfuric acid solution. The polymer film electrodes were placed in contact with the above acidic hexachloroplatinate solution for a minimum of 3 h before potentiostatic deposition to allow for maximum penetration of the hexachloroplatinateinto the polymer matrix. Cyclic voltammetric deposition was carried out by previously described procedures (7). Double potential-step chronoamperometry utilized an initial potential of +0.80 V with a potential program of -0.40 to +0.10 V. Typical timing during the process included 60 s at the initial potential with 0.2 s at -0.40 V and 10 s a t +0.10 V but was dependent on the extent of cross-linking of the polymer. The amount of Pt deposited was quantitated from the total charge consumed (i.e., integration of current) during the electrochemical process assuming 100% current efficiency and an overall four-electron reaction for the reduction of Pt(1V) to Pt metal. This amount will be reported with respect to the cross-sectional area of the carbon substrate. Cyclic voltammetric experiments on the resultant Pt microparticle/PVP film electrodes in 1N H2S04produce broad oxidation waves for P t on the anodic scan followed by reduction of the platinum oxide on the cathodic sweep prior to reduction of hydrogen ions. Evaluation of the Pt Microparticle/PVP Films. The catalytic activity of the P t microparticle/PVP films was evaluated with regard to the generation of hydrogen from 1 N sulfuric acid solution. Potentiostatic techniques were utilized to obtain Tafel plot (log of the reduction current for H2 evolution vs. the overpotential (7))data. Current measurements were made by using the above cell with vigorous binding of nitrogen near the electrode surface. The measured current was independent of the rate of bubbling, which indicated an absence of mass-transfer effects. In an alternative approach,the electrode was mounted on a spindle with conducting epoxy and RDE measurements were made at 2000 rpm to improve mass transfer. The electrode surfaces were examined by scanning electron microscopy (SEM) before Pt deposition, before catalytic evaluation, and after catalytic evaluation. A IS1 Model SX-30 scanning electron microscope (International Scientific Instruments) was used with Polaroid type 55 film to obtain photomicrographs of the electrodes. Film thickness measurements were made with a Sloan Dektak I1 surface profiling system. RESULTS AND DISCUSSIONS A. P l a t i n u m Microparticle Deposition in Electrochemically Polymerized 4-Vinylpyridine Films. Polymerization reactions that are initiated by electrochemical techniques have been previously reported (11-18). The utilization of electrochemical techniques offers the possibility of controlling the thickness and homogeneity of the film on the electrode surface. Finklea recently reported on the formation of films on platinum and gold electrodes by the oxidation of divinylbenzene, 4-vinylpyridine, or phenol and by the reduction of N-methyl-4-vinylpyridiniumion in acetonitrile ( I 7).The resultant films were nonelectroactive; however, incorporation of redox molecules in the films was demonstrated. Faulkner and co-workers have prepared films of polystyrene cross-linked with divinylbenzene on carbon in DMF and acetonitrile by electrochemical techniques (18). They also produced poly(viny1pyridine) films by reducing 4-vinylpyridine and divinylbenzene. Incorporation of Fe(CN)63-within the film was demonstrated under acidic conditions. However, no additional data were given for the poly(viny1pyridine) films. In the present study, glassy carbon electrodes were pretreated to improve the bonding between the carbon surface and the electrochemically prepared polymer film. Previous reports by several workers have indicated that oxygen functionalities can be introduced on glassy carbon surfaces by oxidative pretreatment techniques (19-21). The oxygen functionalities introduced by the oxidative technique included hydroxyl, carbonyl, and carboxylate groups which were measured by surface spectroscopic techniques (21). Electrochemical reduction of these groups in an aprotic solvent will
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ANALYTICAL CHEMISTRY, VOL. 58. NO. 13. NOVEMBER 1s
Table 1. Platinum Microparticle Deposition in Electrochemically Polymerized 4-Vinylpyridine Filmsa platinum deposition H2 generation electrode* technique/loading exchange current; mA/cm2 GC-20-2 GC-20-3
CV. > 10 gg/cm2
DPCA, 7 gg/cm2
1.5 1.7
'Film formation was accomplished by reduction of &vinylpyridine in DMF/TElAP at a constant potential of -2.5 V VB. Ag/ Ag+ (0.01 M) far a minimum of 5 min. 'The previously polished Tokai GC-20 electrodes were electrochemically pretreated by oxidation at +l.B V in a 0.1 M H2S01solution (not deaerated) for 15 min prior to polymerization of 4-vinylpyridine. (The exchange currents were determined by mounting the platinum micrcparticle/polymer electrode on a rotating electrode spindle with subseauent rotation at 2Mx) rom in 1 N HSO.. produce anionic centers (i.e., reduction of acidic protons or stable radical anion formation), which will initiate polymerization of vinylic monomers resulting in either ether or ester linkages between the carbon surface and the polymer. Electrochemical pretreatment of freshly polished gc electrodes was carried out by anodization a t 1.8 V vs. Ag/AgCI for 15 min in 0.1 M HzSO,. The electrodes were then dried and placed in deaerated DMFJO.1 M TBAP. In order to reduce the oxygen Cunctionalitieson the electrode surface to their anionic form. the electrode potential was scanned several times between 0 and -2.6 V. 4-Vinylpyridine (0.2 M) was subsequently added to the DMF solution. Cyclic voltammetric data showed an irreversible reduction pat approximately -2.6 V va. AgJAgCI. The potential was subsequently stepped to -2.8 V for 5 min during which time polymerization occurred. Cyclic voltammetric data taken after the potential-step proress showed two waves at -2.6 and -2.8 V which were considerably smaller than the wave observed on the initial scan before the potential step. This type of behavior is consistent with nonconducting film formation on the electrode surface in the aprotic medium, DMF. The electrodes were then washed with DMF and water, respectively, and subsequently dried under nitrogen. During drying it was noted that any water film formed was held very tightly to the treated surface indicating a hydrophilic polymer film. Electrochemical deposition of platinum in the polymer fh was accomplished by either double-potential step or cyclic voltammetric techniques using an acidic chloroplatinate solution. Integration of the resultant current was carried out to determine the metal loading levels in the films. A SEM photomicrograph of a Pt micropanicle/film electrode which was prepared by the above techniques is shown in Figure la. The film of poly(4vinlypyridine) is distinguishable because a amall portion of film has separated from the gc surface after extensive e x p u r e to an acid solution. Close examination of the edge of the film indicates that P t microparticles are dispersed into the polymer matrix. The film is relatively thin, since scratches produced during polishing the electrode before anodization are 'visible'as indentations in the film. A higher magnification photomicrograph (Figure Ib) indicates panicle sizes in the range of 1 0 0 - 1 0 A. The catalytic activities of these electrodes were tested by observing the generation of hydrogen from a 1 N sulhuic acid solution. Exchange currents were obtained from Tafel plots using data obtained at overpotentials less than 50 mV (Table I). In addition, current densities of 100 mA/cmz of carbon area were consistently attained at overpotentials of 100-150 mV using loading levels at less than 10 MJcmz of carbon area. Due to relative thickness of these films. a panicle count was not possible and therefore all current densities are reported on the basis of carbon area. A crude estimate of the ^true" exchange current density could be made based on the surface
Flgure 1. S-heV,30' tin angle SEM photomicrographs: (a) platinum
microparticles dispersed in an electrochemically produced poly(4vlnyipyridine)film on an anodically pretreated glassy carbon surface: (b) same as (a) except h m magnification. The upper4ght whne bar represents 1.0 pn. area of particles that can be 'seen" from the SEM photomicrographs. However, a higher value for the current density would result since the 'observed" area would be less than the carbon area. In addition, an estimate of area based upon 'observed" particles would not have much meaning since the particles are arrayed in a three-dimensional manner and some particles are probably not visible via the SEM photomicrograph. Thus, the reported current densities, which are with respect to carban area, are lower-limit values and the "true" current densities could be higher. Although comprehensive stability testa were not done, the electrode that is shown in Figure 1had been e x p d to a 1 N sulfuric acid solution for several hours. B. P l a t i n u m Microparticle Deposition in L i n e a r Poly(4-vinylpyridine) Films. Films of linear poly(4vinylpyridine) (Polyacience, Inc.) were obtained by spincoating onto a freshly polished gc surface. Both methanol and glacial acetic acid were used as solvents for these procedures. SEM photomicrographs indicate that the general structure of the polymer film is dependent on the solvent used in the coating procedure. Figure 2a shows a photomicrograph of a f h of linear poly(4vinylpyridine) PVP)that was spin-mted from methanol on a freshly polished Tokai GC-20 electrode. This film appears to have a reticulated structure by highcontrast photo techniques with cavities of 1 to 10 Frn diameter. Furthermore, the film exhibited a high degree of "charging" during the SEM analyais. This "charging" phenomenon is the
ANALYTICAL CHEMISTRY, VOL. 58. NO. 13. NOVEMBER 1986
2759
Table 11. Linear PVP Films/Pt Electrodeposition/Activity DataD electrode
Pt deposition
cv cv
GC-20-4, spin-coated from methanal GC-30-V, spin-coated from methanol GC-20-6,spin-coated from methanol GC-20-2,spin-coated from acetic acid
DPCA. 25 pg/cm' DPCA, 10 wg/cm2
exchange current (30 mV slope), mA/cmz 0.7 0.7 0.4
0.5
'Linear PVP (Polyscience, Inc.) was spin-coated on previously polished gc electrodes. 1%Solutions were used in producing the films.
Flgure 3. SEM photomicrograph (5 kV. 30° tilt angle) of platinum microparticles underneath a film 01 linear PVP spin-coated from methanol. The upper-right white bar represents 1.0 pm.
....,..
,:
,,., i :
..~,
",'
Figure 2. 5-keV. 30' tin angle SEM photomicrographs: (a) linear poly(vinylpyridine)(PVP) film which was spin-coated from methanol on heshty p o w gc: (b) same as (a)except PVP spin-coated lrom ace& acid. The upperwight white bar represents 1.0 &m.
apparent result of a nonconductive thick-film structure and/or a film that is not making good electrical contact with the carbon substrate. The films obtained from spin-coating PVP from glacial acetic acid were considerably more uniform, which was apparently due to slower evaporation of solvent during the coating process (Figure 2b). In addition, the films obtained in this manner did not undergo extensive "charging" during the SEM analysis, which indicates improved contact between the film and the carbon substrate. Since the "holes" in the gc substrate (which are produced during production of the gc) beneath the film are discernible, the film is relatively thin (approximately 1 pm) (Figure 2b). Platinum deposition on these electrodes was carried out in acidic hexachoroplatinate solutions using cyclic voltammetric or double potent-ial9tep techniques. Table I I contains representative data for the linear PVP films on gc. Platinum depositions on these electrodes result in suhstantial metal deposition on the carbon suhstrate itself as well as in the film.
This type of deposition is illustrated in Figure 3, which is a SEM microphotograph of Pt microparticles using a linear PVP film spin-coated from methanol. In this case, the Pt deposit was visible on the glassy carbon surface with very little deposition in the polymer matrix which remained. The films that result from acetic acid solutions were more stable than those from methanol; however, peeling of both types of films from the gc substrate occurred after several hours in 1M sulfuric acid. The result was that the catalytic activity with regard to hydrogen evolution of the Pt microparticle/linear PVP film appeared to be somewhat less than the electrochemically polymerized PVP films. In order to increase the stability of the PVP spin-coated films, the linear PVP was cross-linked directly on the gc surface (vide infra). C. Platinurn Microparticle Deposition in Cross-Linked Poly(4-vinylpyridine). Linear poly(4-vinylpyridine) (PVP) was cross-linked by a procedure that was similar to that reported by Durst and co-workers (22). A radical initiator is used to abstract hydrogen atoms from the ethylenic carbons of the linear polymer "backbone". The initiator radicals were produced hy heating either benzoyl peroxide or azohis(isobutyronitrile) (ABN).The phenyl or isobutryonitrile radicals reacted with the polymer "backbone" to produce ethylenic radicals which subsequently reacted with the cross-linking agent, either triallyl trimesate or triallyl isocyanurate. The result was that the PVP could be cross-linked directly on the gc surface. Films produced hy these techniques with either methanol or glacial acetic acid as solvents appeared to be relatively uniform with g o d "binding" between the film and the carbon substrate. Figure 4 is a SEM microphotograph of a cross-linked PVP film on gc suhstrate that has been deliberately "scratched" to remove part of the films. The thickness of these films was found to be 1-3 pm, as measured by a surface profiling system. The cross-linked PVP films exhibited an improved stability over the linear PVP films in 1 N sulfuric acid with no observed peeling of the exposed film after 24 b.
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 13,NOVEMBER 1986
Table 111. Cross-linked PVP Films/Pt Electrodeposition/Activity Data
electrode
cross-linking procedure
Pt loading level, fig/cmz
GC-30-VI1 GC-30-XV GC-30-IV GC-20-11 GC-20-2 GC-30-XVII GC-20-8
heated on electrode* heated an electrode' heated on electrodec heated on electrodec heated on electroded refluxed" refluxed"
GC-30-XVI
refluxed'
DPCA, 25 DPCA, I DPCA, 12 DPCA, 23 cv. 20 SPCA, 20 DPCA, 26 DPCA, 5
exchange current (mA/cm2)/H2 generation 0.9
0.6 0.1
overpotential for 100 mA/cm' 95 130 90
0.8
80
0.1
90 i." n 85 110
0.8 1.0 0.6
*The cross-linking agent was triallyl isocyanurate and the radical initiation was AIBN with propanol as the solvent. 'The cross-linking agent was triallyl trimesate with benzoyl peroxide as the radical initiator in propanol. 'Same as b with methanol as the solvent. dSame as b with elaeial acetic acid as the solvent.
i Flgure 4. SEM photomicrograph (5 kV, 30' tin angle) of a PVP film Uwt has been cross-linked using bialiy bimesate and benzoyl peroxide in memnol on a gc sudace. The film has been mechanicalhl removed hail 01 the photograph.' The upper-righl from the gc surlace on the white bar represents 1.0 fim.
The deposition of Pt microparticles in the cross-linked PVP films was carried out by potentiostatic procedures in acidic potassium hexachloroplatinate. Due to an apparently more compacted polymer film, longer nucleation and growth times were required during deposition in the cross-linked films. Figure 5 illustrates the three-dimensionality of the metal microparticles in a PVP film which was cross-linked with the AIBN initiator and triallyl trimesate cross-linking agent. Figure 5a is a SEM photomicrograph of the Pt/cross-linked PVP film on gc using an accelerating voltage of 5 kV with detection of secondary electrons. Figure 5b is the same area of the same electrode, photographed with a higher accelerating voltage (15 kV) and detection of backscattered electrons. The latter condition permits analysis at greater depths in the polymer matrix. Microparticles of Pt which were partially obscured by the polymer film in the secondary-electron photomicrograph are observable in the backscattered electron photo, which images electrons emitted from greater depths within the sample. These data clearly demonstrate that the Pt microparticles are electrodeposited in a thee-dimensional array in this polymer matrix. The catalytic activities of PtJcross-linked PVP/gc surfaces were examined by current measurements during hydrogen evolution. Exchange current densities based upon the gc geometric area were found to be dependent on Pt loading levels with typical values of 0.5-1.0 mA/cm2 obtained from Tafel plots in the lower overpotential region (Table 111). These data were obtained at overpotentials of less than 50 mV and resulted in slopes of 30 3 mV from the Tafel plots.
Figure 5. SEM photomicrographs 01 the same area on a cross-linked PVP film into which platinum microparticles have been eleclrodeposiled: (a) the accelerating voiiage was 5 keV with detection 01 secondary electrons; (b) the accelerating vonage was 15 keV with detection of backscattered electrons. The upper-righl white bar represents 1.0 !.tin.
However, due to slope changes in the Tafel plots as a result of uncompensated iR drops, larger exchange currents of 1C-54 mA/cm2 were calculated from plots of data obtained at overpotentiak of 100-300mV. However, it should be noted that current densities of 100 mA/cm2 were obtained at overpotentials of 70-130 mV. These current densities were dependent on the amount of Pt deposited and particle size. For example, the sixth electrode entry in Table I11 (GC-30-XVII) had a current density of 100 mA/cm2 at an overpotential of only 70 mV. This electrode had a relatively high loading level
Anal. Chem. lQ86, 58, 2761-2765
of 20 pg/cm2 and a particle size range of 100-1000 A. No attempt was made to directly correlate the exchange currents to the amount of Pt deposited in the film because different deposition techniques (e.g., CV, DPCA, SPCA) will affect the dispersion and size of metal microparticles in the film (6,23). However, i t should be noted that if the same deposition technique (e.g., DPCA) is used on films that are cross-linked in a similar manner (e.g., heated on the electrode), the exchange current increases with increasing Pt loading levels (see first four entries in Table 111). The relatively high activity of these electrodes toward hydrogen generation is the apparent result of a high ratio of the surface area to the loading level of weight of Pt. The long-term stability of the Pt/cross-linked PVP films was examined by continuous exposure to 1 N sulfuric acid solution. There was no appreciable peeling of the film or deterioration of the activity with regard to hydrogen generation on the cross-linked films after more than 24 h in the above acidic solutions. Thus, the cross-linked polymer films appear to be the most promising for long-term stability and good catalytic activity. More extensive testing of these films under a variety of conditions is in progress. In addition, work on multimetal depositions and their catalytic properties in these and other stable polymer films including attempts to covalently link the polymer to the gc surface is currently in progress. Registry No. Pt, 7440-06-4;PVP, 25232-41-1; C, 7440-44-0; H2, 1333-74-0;H2S04,7664-93-9;4-vinylpyridine, 100-43-6;triallyl isocyanurate, 1025-15-6;triallyl trimesate, 17832-16-5.
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RECEIVED for review March 10,1986. Accepted June 12,1986. This work was supported by The National Science Foundation, Koppers Co., Inc, and the Ohio State Material Research Laboratory.
Comparison of Linear Scan and Staircase Voltammetry: Experimental Results &nata Bilewicz,' R. A. Osteryoung, and Janet Osteryoung*
Department of Chemistry, State University of New York, Buffalo, New York 14214
The Influence of characterlstlc staircase parameters on the extent of slmllarlty between staircase and linear scan voitammograms Is Investigated. Condltlons for which voltammograms are in agreement within experimental error are e& tablished. Experiments using ferric oxalate show, as predicted by previous theoretlcal work, that staircase voitammograms are analogues of linear sweep vokammograms ll the current sampling Is done at one-fourth the step length and the density of data points In the staircase curve Is not too low. When the current Is sampled toward the end of the step, shapes of staircase and linear scan voltammograms differ markedly. Peak helghts are dlstlnctly smaller and peak separations larger for staircase voltammograms than for those obtalned by the linear scan technique. Even for potential steps as small as 2 mV current should be sampled at onefourth the step length In order to obtaln cyclic staircase voltammograms that can be treated as linear scan voltammograms. Permanent address: Department of Chemistry, University of Warsaw, 02093 Warsaw, Pasteura 1, Poland.
Staircase voltammetry (SCV) has been widely explored both theoretically and experimentally by many authors (1-9). It appears that this technique will replace linear scan voltammetry (LSV) not only because it discriminates against charging current but also because modern electroanalytical equipment is based on digital electronics employing discrete potential step wave forms. As the usefulness of linear scan (cyclic) voltammetry in mechanistic studies of electrode processes cannot be overestimated, it becomes of considerable interest to determine to what extent the results of staircase voltammetric experiments may be treated as linear scan results. A theoretical description of staircase voltammetry presented by Christie and Lingane (5) shows that voltammograms identical to those predicted by linear (or cyclic) scan theory (10) should be obtained, if the potential step (hE)approaches zero and the current is sampled at the end of each step. However, fulfilling the first condition poses practical difficulties. Interesting observations have been made by Reilley and coworkers (8) and Perone and co-workers (9) regarding the importance of sampling the current at some point during the staircase period to obtain voltammograms with peak current ratios and peak separations close to those predicted by the
0003-2700/S6/0358-2761$01.50/00 1986 American Chemical Society
