Molecular films of thiol-derivatized tetraphenylporphyrins on gold: film formation and electrocatalytic dioxygen reduction

Langmuir 1993,9,3211-3283 3211

Molecular Films of Thiol-Derivatized Tetraphenylporphyrins on Gold: Film Formation and

Electrocatalytic Dioxygen Reduction James E. Hutchison, Timothy A. Postlethwaite, and Royce W. Murray*

Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290

Received May 6,1993. In Final Form: July 26,1993@

A tetraphenylporphyrin derivative with four thiol moieties, 5,10,15,20-tetrakis [o-(2-mercaptoethoxy)- phenyl] porphyrin, Hz(o-TMEPP), has been synthesized and is shown to form molecular f i i via irreversible chemisorption on a variety of gold surfaces. The properties of these molecular films have been investigated by X-ray photoelectron spectroscopy (XPS) and electrochemical methods. XPS confirms the formation of a porphyrin film on the electrode and the presence of thiolate moieties bound to the gold surface. Electrochemical surface coverage determinations, although not conclusive, suggest near monolayer coverage with preferential binding of the porphyrin ring coplanar to the gold surface. Molecular films of Co(o- TMEPP) reduce dioxygen at potentials consistent with two-electron reduction to hydrogen peroxide. These f ibs retain catalytic activity for more than 105 turnovers and are more active and durable than nonspecifically adsorbed cobalt tetraphenylporphyrin. Production of hydrogen peroxide was verified withknew approach based on use of goldlplatinum interdigitated array electrodes where collection efficiency of electrogenerated hydrogen peroxide is as high as 90%. The feasibility of employing thiolate binding to gold to study dioxygen reduction by related, thiolated cofacial diporphyrins is discussed.

Introduction

A large number of chemistries have been identified by which electroactive species can be immobilized on electrode surfaces.' A recent entry into this chemistry, chemisorption of electroactive derivatives of alkanethiols,2s3 offers a special appeal in that (1) such chemisorbed layers can exhibit substantial ordering and (2) the alkane layer imposes separation between the electrode and redox reactant. Self-assembled monolayers of alkanethiols on gold have already been used to form interfaces useful in the study of corrosion prevention, tribology, and sensing electrodes. Electroactive monolayers have been used to study electron-transfer kinetics.4~~ The results of electrochemical studies of these monolayers suggest a need to expand the range of redox species that can be immobilized by thiolate chemisorption.

In addition to the well-studied long-chain alkyl monolayers based on ferrocene4 and pentaa"ine(pyridine)- ruthenium,5 several groups have prepared monolayers that differ in the number and type of linkers between the electrode and the electroactive group. Preliminary electrochemical studies of flavin monolayers bound to gold via two thiourea side chains have been reported? Obeng et al. have reported voltammetry of a monolayer-forming derivative of the pentammineruthenium fragment where the linker group consists of a rigid [21staffane.' Two anthraquinone derivativesa and a surface-attached cat-

@ Abetradpublishedin Advance ACSAbstracts, October15,1993. (1) Murray,R. W. InMolecular Design ofElectrode Surfaces; Murray,

R. W., Ed.; Techniques of Chemistry Series; Wiley-Interscience: New York, 1992; Vol. 22, Chapter 1.

(2) Ulman, A. An Introduction to Ultrathin Organic Fi lm; Academic Press: San Diego, 1991.

(3) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (4) Chidsey, C. E. D. Science 1991,251, 919. (5) Finklea, H. 0.; Hanshew, D. D. J. Am. Chem. SOC. 1992,114,3173. (6) Edwards, T. R. G.; Cunnane, V. J.; Parsons, R.; Gani, D. J. Chem.

SOC., Chem. Commun. 1989,1041. (7) Obeng, Y. S.; Laing, M. E.; Friedli, A. C.; Yang, H. C.; Wang, D.;

Thulstrup, E. W.; Bard, A. J.; Michl, J. J. Am. Chem. SOC. 1992,114,9943. (8) Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993, 9,

786.

enane! each employing dual thiol attachment, have recently been prepared and some their electrochemical properties measured. Alves and Porter have recently discussed scanning tunneling microscopic studies of a dithiolated porphyrin.1°

We are interested in preparing porphyrins possessing multiple thiol binding appendages for use as new electrodeconfined redox probes. In particular, we seek to uncover the orientational dependence of electron transfer to or from the porphyrin ring. Additionally, there is substantial interest in understanding the electrode/porphyrin interactions seemingly required for four-electron dioxygen reduction by bis(cobalt) cofacialdiporphyrins.ll Thus far, these cofacial diporphyrins have catalyzed the four- electron reduction of dioxygen only when adsorbed on edge-plane graphite. Covalent attachment of these porphyrins to a smooth and well-characterized surface such as gold should facilitate spectroscopic studies and surface analyses that elucidate the role of the electrode surface in these electroreductions. A thiol-derivatized cobalt porphyrin could provide an interesting comparison to the previously studied, monomeric porphyrin-based dioxygen electrode catalysts as weU.11-16 This paper describes a major step toward the completion of these goals: the design and preparation of a suitable monomeric, tetrathiolated tetraphenylporphyrin.

Herein we report the synthesis of Co(o-TMEPP), o-TMEPP = 5,10,15,20-tetrakis[o-(2-mercaptoethoxy)-

~ ~~

(9) Lu, T.; Zhang, L.; Gokel, G. W.; Kaifer, A. E. J. Am. Chem. SOC. 1993,115, 2542.

(10) Alves,C. A.;Portar,M.D. 205thNationalMeetingoftheAmerican Chemical Society, Denver, CO, March 1993; Paper COLL 122.

(11) (a) Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.;Koval, C.; Anson, F. C. J. Am. Chem. SOC. 1980,102,6027. (b) Durand, R. R.; Bencosme, C. S.; Collman, J. P.; Anson, F. C. J. Am. Chem. Soc. 1983, 105,2710.

(12) Durand, R. R.; Anson, F. C. J. Electroanal. Chem. 1982,134,273. (13) (a) Bettelheim, A.; White, B. A,; Murray, R. W. J. Electroanal.

Chem. 1987,217,271. (b) Bettelheim, A.; White B. A.; Raybuck, 5. A.; Murray, R. W. Inorg. Chem. 1987,26, 1009.

(14) Van Galen, D. A.; Majda, M. Anal. Chem. 1988,60, 1549. (15) Bettelheim, A.; Chan, R. J. H.; Kuwana, T. J. Electroanal. Chem.

(16) Ni, C.-L.; Anson, F. C. Inorg. Chem. 1985,24,4754. 1979,99,391.

0743-1463/93/2409-3211$04.00/0 0 1993 American Chemical Society

Hutchison et al.

2.48 (m, 8H, -CH2SCOCH,), 1.77 (m, 12H, SCOCH3), -2.70 (br

The acetyl protecting groups were removed from 5,10,15,20- tetrakis[o-[(S-acetyl-2-thio)ethoxy]phenyllporphine by acidic hydrolysis in CHsOH/HCl followed by NdCO3 workup. MS (FAB, p-nitrobenzyl alcohol): m/e 919 (MH+). lH NMR (400 MHz, CDC13, ppm): 6 8.78 (s,8H, @-pyrrole), 8.02 (m, 4H, phenyl rings), 7.76 (m, 4H, phenylrings), 7.36 (m, 8H, phenyl rings), 4.02 (m, 8H, PhOCHZ), 2.09 (m, 8H, -CH2SH), 0.62 (m, 4H, -Sm, -2.63 (br s,2H,-NH). UV/vis (methylene chloride): 418 (Soret), 514 nm.

Metalation with cobaltous acetate in refluxing, deoxygenated CHCWCH30H21gave the desired compound, Co(o-TMEPP). UV/ vis (methylene chloride): 412 (Soret), 528, 558 nm.

Cobalt(I1) tetraphenylporphyrin, Co(TPP), was obtained from Aldrich.

Electode Preparation. Gold films (typically 130-230 nm thick) were evaporated onto piranha-etched22 glass microscope slides or oxidized silicon wafers. Adhesion of the gold to the substrate was enhanced through an underlayer of either chromium (250 A thick) or (3-mercaptopropy1)trimethoxysilane (MPS).% Wire contacts were made to the gold using silver epoxy (Epo-Tek H20E, Epoxy Technology, Billerica, MA). This contact was covered and the electrode's surface area defined by a layer of insulating epoxy (Epoxi-Patch lC, Dexter Adhesives, Pittsburgh, CA). Geometric electrode areas were determined by the cut and weigh method using photographically enlarged images of the electrodes.

Mica-supported gold films (150 nm thick) were evaporated onto freshly cleaved 1-in. X 3-in. pieces of mica (ASTM grade, Ashville Mica, Newport News, VA). The gold films were evaporated using a modification24 of the method of Chidsey et al.26 which has been shown to produce gold with large (on the order of 300 nm), atomically-flat regions. Electrode areas (0.57 cm2) were defined by a Viton O-ring that was used to seal the electrodes to an opening on the side of the electrochemical cell.

The gold interdigitated array (IDA) electrodes used in these studies, a generous gift from Nippon Telegraph and Telephone (Nl") Corp., consist of two seta of 50 coplanar, lithographically defiied gold bands (fingers) 2 mm long, 3 pm wide, and 0.1 pm high. The two seta are interdigitated, resulting in an edge-to- edge separation of 2 pm between each finger and the pair of fingers flanking it from the other set. Electrical contact was made to each set of bands via a gold contact pad using the method described above for the gold f i i electrodes. The fabrication of the sputter-deposited gold IDA electrodes is analogous to the preparation of platinum IDA electrodes that has been described in detail by the suppliers.% For preparation of the hybrid gold/ platinum IDA electrodes, platinum was electrochemically de- positedm on one set of fingers by applying a current of l pA for 200 s in 2 mM KzPtC4 in 0.1 M aqueous KzHPO, using a platinum anode. The other set of fingers was left at open circuit during the platinization. Success of the procedure was confirmed both by optical microscopy and by cycling the platinized set in 0.5 M H2SO4 to observe the characteristic platinum surface waves.28 Film Preparation and Characterization. Porphyrin mono-

layers were deposited from 1 mM methylene chloride (Fisher, optima grade, dried over 4-A molecular sieves) soaking solutions. Typically, gold films and IDAs were rinsed with acetone and 2-propanol, blown dry with a stream of nitrogen gas, and argon plasma cleaned at 300 mTorr for 3 min at about 16-W radio

8,2H, -Nm.

3278 Langmuir, Vol. 9, No. 11, 1993

HS

SH Figure 1. Co(o-TMEPP) structure.

phenyllporphyrin dianion (Figure l), as well as surface voltammetry of and dioxygen electroreduction catalysis by its molecular films on gold surfaces. The design is calculated, via multiple thiol bonding, to produce a coplanar orientation of the porphyrin ring with the electrode surface; however, the evidence for ordering of the porphyrin layer is at this time indirect, from X-ray photoelectron spectroscopy (XPS) and surface coverage results. As a consequence of the short chains, there should be little flexibility of the linkage compared to long-chain derivatives. The short alkanethiol chains of Co(o-TME- PP) may not provide hydrophobically-induced ordering effects found in longer chain thiols, but such ordering may be possible with longer linker chains or coadsorption of alkanethiols to buttress the porphyrin. The use of multiple, independent thiol linkages provides for strong binding to gold as demonstrated by measurements of film stability and persistence of dioxygen electroreduction catalysis.

Experimental Section Co(+TMEPP) Synthesis and Characterization. 240-

Formy1phenoxy)ethyl bromide waa prepared from salicylaldehyde and 1,a-dibromoethane under basic conditions by a modification of Schweizer's preparation of the propyl derivative." Mp: 54- 60 OC. 1H NMR (200 MHz, CDCb, ppm): 6 10.54 (d, lH, aldehydic), 7.86 (dd, lH, aromatic), 7.56 (dt, lH, aromatic), 7.07 (dt, lH, aromatic), 6.98 (d, lH, aromatic), 4.42 (t, 2H, PhOCH2), 3.71 (t, 2H, CH2Br).

Condensation of this benzaldehyde with pyrrole under Lind- sey's conditions's (1 mM each benzaldehyde, pyrrole, and trifluoroacetic acid) gave 5,10,15,20-tetrakis[o-(2-bromoethoxy)- phenyllporphine as a mixture of a t ropis~mers .~~ lH NMR (400 MHz, CDC13, ppm): 6 8.72 (s,8H, @-pyrrole), 7.99 (t, 4H, phenyl rings), 7.74 (t, 4H, phenyl rings), 7.34 (m, 8H, phenyl rings), 4.06 (t, 8H, PhOCH2); 2.74 (m, 8H, -CH2SCOCH3), -2.72 (br s,2H, -NH).

The bromides were displaced from 5,10,15,20-tetrakis[o-(2- bromoethoxy)phenyl]porphine with sodium thiolacetate in refluxing CHCls/CHsCH20H" to yield 5,10,15,20-tetrakis[o- [(S- acetyl-2-thio)ethoxy]phenyllporphine after flash column chro- matography (silica gel/dichloromethane) to remove any residual unreacted bromo compounds. MS (FAB; p-nitrobenzyl alcohol): m/e 1088 (MH+). 1H NMR (400 MHz, CDCl, ppm): 6 8.73 (s,8H, @-pyrrole), 8.00 (m, 4H, phenyl rings), 7.73 (t, 4H, phenyl rings), 7.35 (m, 8H, phenyl rings), 3.94 (t, 8H, PhOCH2),

(17) Schweizer, E. E.; Berninger, C. J.; Crouse, D. M.; Davis, R. A.; Logothetis, R. S. J. Org. Chem. 1969,34,207.

(18) (a) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.; Marguerettaz, A. M. J. Org. Chem. 1987, 52, 827. (b) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989,54, 828.

(19) Gottwald, L. K.; Ullman, E. F. Tetrahedron Lett. 1969,36,3071. (20) Bonner, W. A. J. Am. Chem. SOC. 1951, 73, 2659.

(21) Fuhrhop, J. H.; Smith, K. M. In Porphyrins and Metallopor- phyrins; Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; p 798.

(22) Pirahna solution is a 20% v/v mixture of 30% aqueous HpOp and concentrated HBO,. Warning: Cautioue handling of pirahna solutions is required as it reacta violently with organic materials.

(23) Goes, C. A,; Charych, D. H.; Majda, M. Anal. Chem. 1991,63,85. (24) Goss, C. A,; Bnunfield, J. C.; Irene, E. A.; Murray, R. W. Langmuir

1993,9, 2986. (25) Chidsey, C. E. D.;Loiacono, D. N., Sleator, T.; Nakahara, S. Surf.

Sci. 1988. 200. 45. (26) Aoki, K.; Morita, M.; Niwa, 0.; Tabei, H. J. Electroanal. Chem.

1988. 256. 269. ~ (27) Btkd, A. J.; Crayon, J. A.; Kittlesen, G. P.; Shea, T. V.; Wrighton, M. S. Anal. Chem. 1986,58,2321.

(28) Hubbard, A. T.; Ishikawa, R. M.; Katekaru, J. J. Electroanal. Chem. 1978,86, 271.

Films of Thiol-Derived Tetraphenylporphyrim

frequency (rf) power (Harrick Scientific, Ossining, NY) immediately prior to immersion into the soaking solution. Mica- supported gold electrodes were placed in the soaking solutions immediately after fabrication, without rinsing or plasma cleaning. Deposition times were usually 8-20 h. Soaking the electrodes for longer times did not affect the characteristics of the resulting f i (other details such as variation of concentration in the soaking solution and coadsorption with other thiols remain to be investigated). After soaking, the electrodes were copiously rinsed with methylene chloride, soaked in methylene chloride for 15 min to 1 h, rinsed again, and blown dry with nitrogen gas, a procedure aimed at removing any nonspecifically adsorbed porphyrin. Both the soaking solutions anad the resulting monolayers were protected from unnecessary exposure to light by storing in the dark.

X-ray photoelectron spectroscopy (XPS) was performed with a Perkin-Elmer Phi Model 5400 electron spectrometer equipped with a standard Mg KCXIS radiation source at 1253.6 eV and 400-W (15-kV) power at the anode. The system base pressure was lcFB Torr during the analyses. Spectra were collected from each sample with an X-ray spot size of 1.1 mm, an analyzer pase energy of 35.7 eV, and a 45' takeoff angle. Low-resolution survey spectra over the 0-1000-eV binding energy range were acquired on each sample for about eight scans. Higher resolution scans of each detected element were acquired for about 160 scam each. The highest resolution spectra of the S(2p) regions of the Co(o- TMEPP) crystalline sample and the Co(o-TMEPP) monolayer on gold were acquired for approximately 160 and 500 scans, respectively. Spectra are referenced to C(1s) at 284.7-eV binding energy. The crystalline Co(o-TMEPP) sample was mechanically pressed into clean indium foil to avoid charging effecta. Porphyrin monolayer samples were prepared for XPS analysis as described above for electrode preparation, using freshly evaporated gold on glase slides with a chromium underlayer. Control analyses were performed on gold substrates after argon plasma cleaning followed by soaking in methylene chloride and drying.

Electrochemistry. All reagents and electrolytes were ACS reagent grade or better and used as received. In-house distilled water was further purified by paseage through a Barnstead Nanopure system (>18 MOcm). Most electrochemical experiments were performed with a BAS l00B electrochemical analyzer (Bioanalytical Systems, West Lafayette, IN) using a single- compartment cell. Surface coverages were determined by integration of voltammetric surfam waves using in-how software. IDA electrode experiments were performed using a Pine RDEX bipotentiostat (Pine Instruments, Grove City, PA), a triangle waveform generator of local design, and a conventional three- compartment cell. Counter electrodes were platinum wires or flags in all cases. All potentials are referenced to an SSCE. All solutions were sparged with nitrogen or oxygen gas for at least lominandthen blanketedwith therespectivegaswhenanaerobic solutions or oxygen-saturated solutions were desired. During electrolysis experiments, the stirred solutions were continuously sparged with oxygen gas.

Results and Discussion X-ray Photoelectron Spectroscopy of Monolayer

and Crystalline Co( o-TMEPP): Evidence for Thiol Binding to Gold. Survey and higher resolution spectra conf i ied the presence of the expected elements in monolayer and crystalline Co(o-TMEPP) samples. Cobalt was observed on both samples at 796 eV (2~112) and 781 eV (2~312); peaks at these energies were absent for the gold substrate blank and for a H2(o-TMEPP) f i b on gold. Signals for sulfur (2p) were present for all thiol-derivatized porphyrin samples (vide infra) and absent in the gold substrate blank. Peaks for carbon, nitrogen, and oxygen present in spectra for crystalline Co(o-TMEPP) and for the molecular films on gold were not interpreted owing to impurity peaks on the gold substrate blank.

In order to establish the thiol-gold binding, the S(2p) spectra for Co(o-TMEPP) on gold and crystalline Co(o- TMEPP) samples were examined using extended acqui-

Langmuir, Vol. 9, No. 11,1993 3279

B

170 168 166 164 162 160

binding energy (eV) Figure 2. (A) XPS spectrum of the S(2p) region of Co(o- TMEPP) solid pressed into indium foil: raw data (solid irregular line), fitted 2psp and 2p1p components (solid lines), simulated spectrum (dashed line). (B) XPS spectrum of the S(2p) region of a Co(o-TMEPP) monolayer on gold raw data (solid irregular line), fitted 2pap and 2p1p components (solid and dotted lines) for the two forms of sulfur, simulated spectrum (dashed line).

sition times (Figure 2). In the crystalline sample, the S(2p3p) and S ( 2 p 4 peaks appearzg at 163.8 and 165.0 eV, respectively, as shown in Figure 2A. The S(2psp) energy lies within the range of previously reported values for free-thiol sulfur (163.3,30 163.9 eV31). The S(2p) spectrum of Co(o-TMEPP) chemisorbed on gold was strongly, but not completely, shifted to lower binding energy and was fit to two different binding energies as shown in Figure 2B. The resulting lower energy peaks lie at 162.3 and 163.5 eV for S(2p3p) and S(2p1/2), respectively. The binding energy for the S(2p3p) peak is close to literature values (162.@O and 162.4 eV39 reported for sulfur present as a thiolate bound to gold. The higher binding energy peaks assigned to S(2p3p) at 163.7 eV and S(2p1p) at 164.9 eV in Figure 2B correspond very closely to the free-thiol sulfur seen in the crystalline sample (Figure 2A) and thus represent a population of thiols not attached to the gold surface. The relative areas of peaks for the two types of sulfur indicate that about 70 % of the sulfur atoms (e.g., about three of four) in the monolayer are present as gold-bound thiolates. This result, considering the Co(o- TMEPP) structure (Figure 11, strongly suggests a pre- ponderance of coplanar electrode-porphyrin binding.

(29) The S(2p) fib were performed by constraining the difference between the 2psp and 2pl 2 peaks at 1.2 eV and their area ratio at 2 1 and allowing full width at half-maximum, intensity, and binding energy to V W .

(30) Bain, C. D.; Biebuck, H. A.; Whiteaides, G . M. Langmuir 1989, 5, 723.

(31) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. SOC. 1987,109,133.

(32)Fabianowski, W.; Coyle, L. C.; Weber, B. A.; Granata, R. D.; Castner, D. G.; Sadownik, A.; Regen, S. L. Langmuir 1989,5, 36.

3280 Langmuir, Vol. 9, No. 11, 1993 Hutchison et al.

-20 - 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4

volts vs. SSCE

-20 ‘ I I I I I 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4

volts vs. SSCE

.-. Co(o-TMEPP)

Y

3 -2 bare gold

I I 4 I 0.2 0.0 -0.2 -0.4 -0.6

volts vs. SSCE Figure 3. (A) Cyclic voltammograms in deaerated 1 M HClOh of films of Hz(o-TMEPP) on Au/mica compared to bare Au/ mica: u = 100 mV/s, electrode area 0.57 cm2, initial potential -0.3 V. (B) Cyclic voltammograms in deaerated 1 M HClO, of films of Co(o-TMEPP) on Admica compared to bare Admica: ranodic = 1.7 x mol/cm2 measured from charge below the baseline extrapolation shown, u = 100 mV/s, electrode area 0.57 cm2, initial potential -0.3 V. (C) Cyclic voltammograms in deareated 1 M NaOH of films of Co(o-TMEPP) on Au/mica compared to bare Au/mica: randc = 8 X mol/cm2 measured from charge below the baseline extrapolation shown, u = 100 mV/s, electrode area 0.57 cm2, initial potential 0.1 V.

Electrochemical Studies of Porphyrin Films under Anaerobic Conditions: Determination of Surface Coverage. Voltammograms of bare Au/mica and H2(0- TMEPP)-coated Au/mica are shown in Figure 3A. From these voltammograms it is clear that binding of free-base thiol porphyrin Hz(o-TMEPP) reduces the capacitance of the gold electrode. The film of HAo-TMEPP) is not expected to be electroactive within the range (+700 mV to -300 mV v8 SSCE) accessible at the gold electrode in contact with 1 M HClO4, and no surface waves are observed. While a surface voltammogram appears at = 1.08 V vs Ag quasi-referenced electrode (Up& = 2&70 mV) for Ha(o-TMEPP)-coated Admica in contact with ace- tonitrile/B~NPFs solution, this wave is stable only for 1-3 scans. The instability is likely that of monocationic, chemisorbed Hdo-TMEPP).

A cyclic voltammogram of a film of Co(o-TMEPP) on Admica in contact with deoxygenated 1 M HClO4 is shown as the solid line in Figure 3B. The cathodic branch is enhanced by electrocatalysis of residual dioxygen reduction. We are not always able to observe the Co(o-TMEPP) wave although XPS studies of these same electrodes invariably gave detectable Co(2p) and S(2p) signals for the porphyrin and showed strong dioxygen electroreduction catalysis (vide infra). The difficulty in observing the

Co(II/III) surface wave is perhaps caused by the combi- nation of large capacitive current and the large anodic- cathodic peak potential splitting. The peak splitting for this wave (Up > 150 mV) is large, especially for a surface- confined species, indicating slow kinetics of the couple. In addition, the wave is very broad, as would be caused by site-site interactions or a distribution of formal potentials within the surface film.

It is well-known that the Co(II/III) couple is kinetically slow especially in low-pH solutions.12 Of the previously- studied examples of electrodeconfined cobalt porphyrins, nearly all the observed surface waves were recorded in contact with organic so1vents18*ss or high-pH12 solutions. Our efforts to observe Co(II/III) or Co(II/I) surface waves of Co(o-TMEPP) in contact with either CH3CN or CH3- CN/pyridine/BuNPFa were unsuccessful although solution voltammetry of Co(o-TMEPP) in methylene chloride/ Bu4NPFs gave voltammograms expected for a cobalt tetraphenylporphyrin derivative.%

A cyclic voltammogram of Co(o-TMEPP) on Au/mica in contact with deoxygenated 1 M NaOH is shown as the solid line in Figure 3C. Here again, the surface wave is broad and ill-defined. Comparing the anodic peak potential to that of the voltammogram in acid (Figure 3B), the pH-induced shift in potential is ca. 0.5 V, in good agreement with the pH dependency of previously reported s y ~ t e m s . ~ ~ J ~ * Addition of an axial ligand is known to sometimes sharpen and shift the Co(II/III) wave to more negative potentials; however, adding pyridine (up to 10% v/v) to the NaOH solution did not change the appearance of the surface waves in Figure 3C. Voltammetry in 1 M NaOH at more negative potentials is complicated and has not yet been deciphered. Waves observed near -1 V vs SSCE may arise variously from the Co(II/I) redox couple, reduction of the gold-thiolate bond,35 or reduction of residual dioxygen. The use of reducing potentials under basic conditions has been shown to desorb alkanethiol monolayer^;^^ if all four of the thiolate bonds of the porphyrin become reduced, the porphyrin monolayer may desorb as well, and this is suggested by the voltammetry observed.

Integration of the charge under the anodic surface waves such as shown in Figure 3B,C provides an estimate of the surface coverage, r, of Co(o-TMEPP). The measured values of J? are not highly reproducible; the factors involved include (1) differences in the gold substrates used and in the quality of the individual films, (2) choice of baseline for voltammogram integration, and (3) the presence of electrocatalytic currents arising from trace amounts of oxygen (in the case of the cathodic wave). The appearance of the Co(o-TMEPP) surface wave in Figure 3B is typical (Le., not the “best” result). The results of a number of surface coverage determinations in 1 M using the anodic wave, and without any attempt at microscopic roughness corrections, are given in Table I. Coverages measured on a limited number of electrodes under basic conditions fall within this same range.

Van Galen and Majda have reported expected values of t h e molecular a r e a a n d covera e for coba l t meso-tetrakis(4-pyridy1)porphyrin: 262 fiz and 7.0 X mol/cm2 if the porphyrin is coplanar to the gold surface and 50 A2 and 3.3 X mol/cm2 for the perpendicular

(33) (a) Lennox, J. C.; Murray, R. W. J. Am. Chem. SOC. 1978,100, 3710. (b) Jester, C. P.; Rocklin, R. D.; Murray, R. W. J. Electrochem. SOC. 1980,127,1979. (c) Rocklin, R. D.: Murray, R. W. J. Phvs. Chem. - . 1981,85,2104.

(34) Kadieh, K. M. h o g . Znog. Chem. 1984,34,435. (35) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. SOC.

1992,114,5860.

Films of Thiol-Derived Tetraphenylporphyrins

Table I. Dioxygen Reduction in Dioxygen-Saturated 1 M H c l o 4 by Various Co(o-TMEPP)-Coated and Bare Gold

Electrodes

electrode r.nodic Ep,c(mV) area (cm2) type of gold (mol/cm2) (A)

02 reduction

Langmuir, Vol. 9, No. 11,1993 3281

0.78 0.49 0.78 0.84 0.84 0.57 0.86 0.62

0.57 0.88 0.66 a

(I

Co(o-TMEPP)/Au/MPS 5.80 X lo-" Co(o-TMEPP)/Au/MPS 4.76 X le'' Co(o-TMEPP)/Au/MPS 8.72 X le'' Co(o-TMEPP)/Au/MPS 7.14 X l@" Co(o-TMEPP)/Au/MPS 6.72 X lo-" Co(o-TMEPP)/Au/mica 1.7 X 1&l0 Co(o-TMEPP)/Au/Cr a

Cob-TMEPP)/Au wire a bare Au/mica NAc bare Au/Cr NA bare Au/MPS NA bare Au wire NA

Co(o-TMEPP)/Au/MPS a

a a +75 (513) +lo4 (620) +83 (612) +132 (295) +112 (597) +41 (404) +2 (131) -210 (275) -145 (437) -165 (374) -2006

Not measured. Poorly defined. NA = not applicable.

0rientati0n.l~ Although they were unable to observe surface waves for their monolayers of Cle-tailed porphyrin, they were able to determine 'coverages on the basis of spectrophotometric arrays ((8.6 f 0.6) X lo-" mol/cm2) and Langmuir-Blodgett experiments ((7.2 f 0.2) X mol/cm2). Values of r would, on a metal surface, be inflated by microscopic roughness of the metal, Le., in the present case on the type of gold employed. The surface coverage measurements in Table I, considering that some microscopic surface roughness is present, are consistent with a preference for orientation of the ring coplanar to the and reinforce that inference drawn from the XPS result (vide supra).

Electrocatalysis of Dioxygen Reduction by Co( TMEPP) Molecular Films on Gold. The electrocatalytic reduction of dioxygen is a much more sensitive detector of electrode surface modification with Co(o- TMEPP) than are the surface waves in Figure 3B,C. The catalytic reduction of dioxygen by a monomeric cobalt porphyrin like Co(o-TMEPP) is expected"-l4 to be a two- electron, not a four-electron catalysis; an investigation of this catalysis nonetheless probes the character of the porphyrinic molecular films. The peak potentials and currents of cyclic voltammograms of porphyrin films on gold are keen indicators of the film's dioxygen reduction catalytic activity. Cyclic voltammetry under dioxygen was examined for the cobalt and free-base porphyrins on four different types of gold (Au/MPS, Au/Cr, Au/mica, and Au wire) in contact with 1 M HC104.37 Figure 4 shows an example and Table I compares dioxygen reduction potentials and peak currents for both modified and bare electrodes.

The positive shift (Figure 4) in the peak potential of the dioxygen reduction wave for the Co(o-TMEPP)-coated electrode compared to the bare Au electrode indicates that molecular films of Co(o-TMEPP) catalyze the reduction of oxygen when in contact with 1 M HC10.i. In fact, Co- (0-TMEPP) binds to and catalyzes dioxygen reduction on all four forms of gold used in this study. Analogous to other cobalt porphyrin electrode films,12 molecular films of Co(o-TMEPP) also catalyze the reduction of dioxygen

(36) Reflective IFt andvisible spectroscopicexperiments are in progrees to probe porphyrin ring orientation with respect to the electrode surface.

(37) The four types of gold substratesused in this study are designated as Au wire (gold wire, 0.25-mm diameter, 99.999%, Johnson Matthey), AdCr (evaporated gold with a chromium underlayer on glass), AdMPS (evaporated gold with a (3-mercaptopropy1)trimethoxyailane underlayer on glass), and Admica (evaporated gold on mica). The IDA electrodes uaed consisted of sputtered gold with a chromium underlayer on oxidized silicon substrates.

300

T 3 i? a 100

Y 0

LI

0

0.6 0.4 0.2 0.0 -0.2 -0.4

volts vs. SSCE Figure 4. Cyclic voltammograms in dioxygen-saturated 1 M HClOi of f i i of CO(O-TMEPP), Co(TPP), and Ha(o-TMEPP) on Au/mica compared to bare Admica: u = 100 mV/s, electrode area 0.57 cm2, initial potential +0.7 V.

$ 1 Y E 100

-.- ~

0.2 0.0 -0.2 -0.4 -0.6 -0.8

volts vs. SSCE Figure 6. Cyclic voltammograms in dioxygen-saturated 1 M NaOH of f i i of Co(o-TMEPP) on Admica compared to bare Admica: u = 100 mV/s, electrode area 0.57 cm2, initial potential +0.1 v. when in contact with 1 M NaOH, as indicated by the positive shift of the voltammogram (solid curve) in Figure 5. Co(o-TMEPP)-coated electrodes, in contact with either acid or base, yield linear i, vs v1/2 plota, so the catalyzed reaction rates are sufficiently fast that the process is controlled by dioxygen diffusion. The slopes of these plota, proportional to n (the number of electrons in the reduction), are con~ i s t en t~~ with a two-electron reduction to hydrogen peroxide, which is confmed below. The potential of onset for dioxygen reduction is similar to that previously reported for numerous monomer cobalt por- p h y r i n ~ ~ ~ - ' ~ that are also two-electron catalysts.

Determination of the rate of dioxygen reduction catalysis and catalyst lifetime is another way to elucidate the differences between films of the various porphyrins. Figure 6 shows electrolytic charge vs time plots for the different porphyrin-modified electrodes in stirred, dioxygen-sparged solutions. A value for the total turnovers per cobalt center can be calculated from the surface coverage and the electrolytic charge data. On a 0.86 cm2 Co(o-TMEPP)/Au/Cr film, dioxygen was reduced for just over 1 h (Figure 6), with the current decaying from 760 to 525 "cm2 during that time. The total charge passed

(38) Slopes of iw va ul/* plots for acid give nlDP = 1.7 and for base n = 1.9 on the basis of the dioxygen diffusion coefficient12 of 1.7 X 1W c s s-l and solubilities for dioxygen of 1.3 mM in 1 H HClOl and 0.8 mM in 1 M NaOH.

3282 Langmuir, Vol. 9, No. 11,1993 Hutchison et al.

3.0 r

h 2s t / Co(o-TMEPP)

0 1000 2000 3000 4000

time (sec) Figure 6. Electrolytic charges for dioxygen reduction in stirred, dioxygen-sparged solutions. In all cases the potential was held at 0.0 V vs SSCE. The charge has been normalized for electrode areas. Films of Co(o-TMEPP), Co(TPP), and HAo-TMEPP) on Au/Cr are compared to bare Au/Cr. On Au/MF’S, Co(TPP) catalytic activity decays more rapidly relative to Co(o-TMEPP).

was 2.2 C. This corresponds to 1.3 X lo6 turnovers per porphyrin site assuming r = 1 X 10-lo mol/cm2. This catalyst is quite robust, especially considering the current density remaining after 1 h. Further, sonication of Co- (0-TMEPP) films on gold wire in dichloromethane for 10 min did not change its dioxygen reduction activity, nor did soaking the electrode for an additional 8 h in pure dichloromethane after removal from the soaking solution.

Under anaerobic conditions, H2(o-TMEPP) binds to gold substrates and lowers the double-layer capacitance compared to that seen at bare gold (Figure 3A). The lowered capacitance suggests a well-formed layer on the electrode surface. Whereas the Co(o-TMEPP) film is a good dioxygen electrode catalyst, the free-base porphyrin Hz(o-TMEPP) passivates gold electrodes, thus making dioxygen reduction more difficult than on bare gold (Figure 4). Because by steric considerations the thiols of the free- base porphyrin cannot react with every gold atom on the surface, in order to passivate the electrode, it must either shield all the surface gold atoms from reaction with dioxygen or, more likely, the porphyrins must bind up all of the most active gold sites, presumably at terrace edges and grain boundaries.

Co(TPP) Nonspecific Binding Control Experi- ments. It is important to determine the extent to which porphyrins adsorb to clean gold surfaces without the aid of a thiol linker group14-39 because, in order to gain control over surface order, one must control surface binding and steric interactions. To this end Co(o-TMEPP)-coated electrodes were compared to electrodes treated with the “thiol-less” Co(TPP). In contrast to cobalt meso-tetrakis- (4-pyridy1)porphyrin which reportedly14 does not adsorb on gold, we observed that Co(TPP) tends to adsorb and catalyze dioxygen reduction at every gold surface except gold wire. On gold wire, only a trace of dioxygen catalysis was evident, in contrast to the strong catalysis observed using Co(o-TMEPP) coatings on gold wire. At least on this substrate, it is certain that nonspecific adsorption effects cannot be responsible for the strong binding of Co(o-TMEPP) to gold. The controls were less definitive for the other forms of gold, but did give results that differed for Co(o-TMEPP) and Co(TPP). Thus, on all forms of gold Co(o-TMEPP) has a more positive peak potential (Figure 4) for dioxygen reduction than Co(TPP). Sec-

(39) Ngameni, E.; Laoubnan, A.; L’Her, M.; Hinnen, C.; Hendricks, N. H.; Collman, J. P. J. Electroanal. Chem. 1991,301,207.

ondly, it is clear from the results of the plots in Figure 6 that Co(o-TMEPP) has a higher overall activity which decays more slowly than that of adsorbed Co(TPP).

Detection of Hydrogen Peroxide Employing an Interdigitated Array (IDA) Electrode. In preparation for studies involving ordered and cofacial porphyrins bound to gold,40 there is a need for a method of determining the number of electrons involved in dioxygen reduction, which can be judged from the extent of H202 production. Usually this is accomplished by employing rotating-disk electrode (RDE) and rotating ring-disk electrode (RRDE) ~o l t a”e t ry , l ’*~~ but the clean, well-ordered gold surfaces conductive to the best ordering of thiol-based films like Co(o-TMEPP) would be more difficult to prepare with that electrode arrangement.

We have accordingly conducted a preliminary study of hydrogen peroxide production, using both gold/gold and hybrid gold/ platinum interdigitated array (IDA) electrodes in the generatorlcolledor mode.% The collection efficiency for a reversible redox couple at a multifiiger IDA electrode can be higher than 95 % ,26 much larger than a good RRDE value of about 40%. Theiefore, IDA electrodes should, in principle, be more sensitive for hydrogen peroxide detection.

In an IDA generator/collector experiment, the potential of the generator (one set of fingers) is scanned toward reducing potentials while the collector (the other set of fingers) is held at a constant potential. As a redox species (e.g., dioxygen) is reduced at the generator, the electrode product (e.g., hydrogen peroxide) diffuses toward the collector where it is reoxidized to its original state. The reoxidized species can then diffuse back to the generator and participate in another redox cycle. Due to the small gap between the generator and collector, the redox flux between them can exceed that by diffusion to and from the bulk solution, thus providing for very high collection efficiencies. In addition, repeated transitting of a species between the generator and collector (redox feedback) enhances currents at both electrodes.

The gold/gold IDA electrodes used in this experiment (described in the Experimental Section) gave collection efficiencies for R u ( N H ~ ) ~ ~ + / ~ + of greater than 95%. Efforts to attain collection efficiencies in this range for OdH202 were unsuccessful. The collection efficiencies varied from experiment to experiment with a maximum attained of about 48 % using a collection potential of +1.28 V vs SSCE. If the potential of the bare gold collector is made more positive (+1.4 in an effort to increase collection, the collector currents actually decrease. We attribute this passivation of the collector toward H202 oxidation to the formation of less-active gold oxides on the collector surface.43

In order to improve the efficiency and reproducibility of hydrogen peroxide collection, the figers of the generator set were platinized, yielding a hybrid gold/platinum IDA electrode. Our rationale for platinization is that surface oxides of platinum that may form at the positive potentials

(40) Hutchison, J. E.; Postlethwaite, T. A,; Murray, R. W.; Tyvoll, D. A.; Chng, L. L.; Collman, J. P. Work in progress.

(41) Albery, W. J.; Hitchman, M. L. Ring-Disk Electrodes; Clarendon Press: Oxford, 1971.

(42) We observed some faradaic currents presumably related to oxidation of gold and/or the chromium underlayer upon stepping the gold collector electrodesto potentials more positive than +1.1 V v8 SSCE. However, at the collection potentials used, these currents decayed to baseline prior to beginning a generator/collector experiment. Control generator/collector experiments in deaerated solutions showed no current a t either electrode.

(43) (a) Oesch, U.; Janata, J. Electrochim. Acta 1983,28, 1237. (b) Oesch, U.; Janata, J. Electrochim. Acta 1983, 28, 1247.

Films of Thiol-Derived Tetraphenylporphyrins Langmuir, Vol. 9, No. 11, 1993 3283

collector to generator currents, the collection efficiency, is 0.89 for the experiment shown in Figure 7. This result demonstrates the utility of using IDA electrodes in detecting the catalytically produced product and confirms that the Co(o-TMEPP) acta as a two-electron catalyst.

Conclusions Co(o-TMEPP) chemisorption to gold produces ex-

tremely stable molecular films. Surface coverage measurements suggest that these films consist of approximately one monolayer, with the porphyrin rings preferring a coplanar orientation with respect to the electrode surface. XPS results confirm that, on average, about three of four thiolates per porphyrin become bound to the gold substrates. Films of Co(o-TMEPP) reduce dioxygen at a potential consistent with a two-electron process to form hydrogen peroxide in both acidic and basic media. The effect of pH on the potentials of the Co(II/III) wave and the onset of dioxygen reduction is similar to that observed for cobalt porphyrin films on other electrode surfaces. Production of hydogen peroxide was confirmed using a new approach involving the use of hybrid gold/platinum IDA electrodes in the generator/collector mode. Appli- cation of this technique to the study of dioxygen reduction by f h of Co(o-TMEPP) and related cofuciul diporphyrin derivativesq0 will facilitate study of the mechanism of metalloporphyrin-catalyzed reduction of dioxygen. Ad- ditionally, catalysts bound to gold electrodes should be more amenable to surface analysis and spectroscopic characterization than when they are adsorbed on the traditional edge-plane graphite electrodes.11J2

Acknowledgment. J.E.H. is the recipient of a National Science Foundation Postdoctoral Fellowship (Grant CHE- 9203585). We are grateful to Drs. Masao Morita and Osamu Niwa of NTT for supplying the IDA electrodes. We also thank Roger Terrill for supplying us with the electrochemical analysis software used in this study and Wei Ou for help with the XPS experiments. T.A.P. would like to personally acknowledge the guidance of Dr. Jack E. Fernandez of the University of South Florida. This work is supported by grants from the National Science Foundation and the Office of Naval Research.

0 Y

0 5 -+ -10

-15 1

(collector at generator

generator (collector at open circuit)

.___ __,__ .__..-.

-20 I I I I I J 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4

volts vs. SSCE Figure 7. Generator/colledor voltammograms utilizing a Co- (o-TMEPP)/Au/Pt hybrid IDA electrode in &oxygen-saturated 1 M HC10,. The dashed line ia a cyclic voltammogram (LJ = 10 mV/s) taken with the collector potential at open circuit. During the generator/collector experiment, the generator potential was scanned at 10 mV/s while that of the collector was held at +0.90 V w SSCE.

required for hydrogen peroxide oxidation will not passivate the electrode surface as in the case with a gold collector. Also, the overpotential required for hydrogen peroxide collection is less positive than that on gold. The facile oxidation of hydrogen peroxide at the platinized fingers was demonstrated by tests on hydrogen peroxide solutions.

Platinization of the collector electrodes improved the hydrogen peroxide collection considerably. A cyclic voltammogram of dioxygen reduction at a gold generator (collector at open circuit) of a Co(o-TMEPPbmodified hybrid gold/platinum IDA electrode is shown as the dashed line in Figure 7. When a positive potential is applied to the platinized collector (+0.90 V, sufficient to oxidize the product hydrogen peroxide back to dioxygen) , current resulting from hydrogen peroxide oxidation measured at the collector (lower solid curve) becomes a near mirror image of the generator current (upper solid curve). Simultaneously, the magnitude of the generator current is increased 5-fold due to redox feedback. The ratio of the

Documents

Molecular films of thiol-derivatized tetraphenylporphyrins on gold: film formation and electrocatalytic dioxygen reduction