© Copyright 1999 American Chemical Society
OCTOBER 26, 1999 VOLUME 15, NUMBER 22
Letters Platinum Microelectrodes with Unique High Surface Areas Joanne M. Elliott,* Peter R. Birkin, Philip N. Bartlett, and George S. Attard Department of Chemistry, University of Southampton, Southampton, U.K. SO17 1BJ Received July 9, 1999. In Final Form: September 8, 1999 Nanostructured platinum films were electrodeposited onto microelectrodes from an hexagonal lyotropic liquid crystalline plating mixture. Cyclic voltammetry in dilute sulfuric acid and copper underpotential deposition confirmed that the resulting platinum films had high surface areas (with roughness factors of ca. 210) as a direct consequence of their electrochemically accessible nanostructure. The microelectrodes were shown to combine a high real surface area with efficient mass transport characteristics. This combination of properties is unique and may be of significance to the fields of electrochemistry and electroanalysis.
It has been shown previously that the normal topology hexagonal (HI) liquid crystalline phase can be used as a template for the synthesis of mesoporous platinum powders (HI-Pt) by the chemical reduction of hexachloroplatinic acid.1 More recently we reported the production of nanostructured platinum films (HI-ePt) by electrodeposition.2 The resulting films were adherent, shiny, and shown to have a nanostructure that was identical to that of the HI-Pt powders. This consisted of cylindrical pores distributed on a hexagonal lattice. For films deposited from mixtures containing octaethylene glycol monohexadecyl ether (C16EO8), the pore diameter and wall thickness were found to be ∼2.5 nm. Figure 1 shows a transmission electron micrograph of a typical nanostructured platinum film; the regular array of pores running through the metal can be seen in this cross section through the structure. Cyclic voltammetric analysis and impedance spectroscopy confirmed that the films had high surface areas and concomitantly large double layer capacitances and thus
Figure 1. TEM image of nanostructured platinum (obtained using a JEOL 2000FX transmission electron microscope operating at a voltage of 200 kV). A side view of the pore structure is shown. Platinum was deposited at -0.1 V vs SCE and at 25 °C with a charge density of ∼6.37 C cm-2.
* Author for correspondence: Fax: 01703 676960. Tel: 01703 594991. E-mail:
[email protected].
could be of interest for a wide range of applications (for example, in batteries, fuel cells, and sensors).3-6
(1) Attard, G. S.; Goltner, C. G.; Corker, J. M.; Henke, S.; Templer, R. H. Angew. Chem., Int. Ed. Engl. 1997, 36, 1315. (2) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838.
(3) Leroux, F.; Koene, B. E.; Nazar, L. F. J. Electrochem. Soc. 1996, 143, L181. (4) Ye, S.; K Vijh, A.; Dao, L. H. J. Electrochem. Soc. 1996, 143, L7.
10.1021/la9908945 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/09/1999
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Figure 2. Cyclic voltammograms recorded in 2 mol dm-3 aqueous sulfuric acid (aerobic conditions) at 200 mV s-1 between -0.2 and +1.2 V vs SCE for (a) a nanostructured platinum microelectrode (RF ) 219, 25 µm diameter), deposition temperature was 25 °C, deposition charge density was 6.37 C cm-2, and deposition potential was -0.1 V vs SCE, and (b) a polished platinum macroelectrode (RF ) 2.8, 0.5 mm diameter).
Among previously reported methods for preparing nanostructured electrodes is membrane-based synthesis, where the pores of a microporous or nanoporous membrane are used as templates. Polymers, metals, and semiconductors with either a micro- or nanoscale structure have been electroplated within the pores of the host membrane; in addition both nanofibrils (solid cylinders) or nanotubules (hollow cylinders) have been prepared.7,8 Of particular relevance is the preparation of a gold nanoelectrode ensemble where the diameter of each gold microdisk was only ∼10 nm; however, the overall geometric area was still quite large. The approach described here differs significantly as the preparation of a single nanostructured platinum microelectrode of small geometric area but with a high surface area is described. The electrodes, despite their high real surface areas, are shown to retain efficient mass transfer characteristics. Thus the liquid crystal templating approach described here, when applied to microelectrodes, produces a unique combination of properties which are not achievable by any other method. The surface area of the electrodes prepared in this study was characterized by two methods: first by cyclic voltammetry in sulfuric acid (i.e., hydrogen underpotential deposition) and second by copper underpotential deposition. Underpotential deposition (UPD)9 describes the formation of a monolayer of material at potentials more positive than those required for bulk deposition. The charge associated with this process is intrinsically related to the real surface area of the electrode. UPD of sub- and monolayer surface coverages has been widely reported, and the deposition of copper on platinum, in particular, has been studied in some detail.10-13 The diffusional
characteristics of the nanostructured microelectrodes were investigated by making voltammetric measurements of an aqueous redox couple. Prior to use microelectrodes were cleaned by polishing and by cyclic voltammetry in dilute sulfuric acid.14 The electrodes were then rinsed with deionized water and allowed to dry under ambient conditions prior to deposition. Electrodeposition of the platinum films was carried out under potentiostatic and thermostatic control.15 Postdeposition treatment of all films was identical. The electrodes were removed from the electrochemical cell and allowed to soak in copious quantities of water in order to remove the surfactant. The surface area of each electrodeposited film was determined by cyclic voltammetry in 2 mol dm-3 sulfuric acid assuming a conversion factor of 210 µC cm-2.16 This surface area measurement was divided by the geometric area of the electrode to produce values of the roughness factor (RF), values of ca. 210 ( 13 (95% confidence interval, n ) 5) were obtained. The roughness factor of the platinum films was found to decrease as the size of the electrodes used was reduced, indicating that the efficiency of the deposition process was decreasing. As the electrodes become smaller, more efficient radial diffusion of the intermediate Pt2+ species away from the electrode surface occurs before reduction to platinum metal can take place, and the deposition efficiency is decreased; hence the apparent real surface area of the 25 µm diameter electrodes used here corresponds to a value of only ∼6.45 m2 g-1. Figure 2a shows a typical cyclic voltammogram for a 25 µm diameter nanostructured platinum microelectrode. Comparison with a typical cyclic voltammogram obtained
(5) Pell, W. G.; Conway, B. E. J. Power Sources 1996, 63, 255. (6) Wang, J.; Agnes, L. Anal. Chem. 1992, 64, 456. (7) Hulteen, J. C.; Martin, C. R. J. Mater. Chem. 1997, 7, 1075. (8) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920. (9) Leiva, E. Electrochim. Acta 1996, 41, 2185. (10) Watanabe, M.; Uchida, H.; Ikeda, N. J. Electroanal. Chem. 1995, 380, 255. (11) Abe, T.; Swain, G. M.; Sashikata, K.; Itaya, K. J. Electroanal. Chem. 1995, 382, 73. (12) Sashikata, K.; Furuya, N.; Itaya, K. J. Electroanal. Chem. 1991, 316, 361. (13) Machado, S. A. S.; Tanaka, A. A.; Gonzalez, E. R. Electrochim. Acta 1991, 36, 1325.
(14) The electrodes were polished with alumina (Buehler), 25, 1.0, and 0.3 µm grades. Gold microelectrodes were then cycled in 2 mol dm-3 sulfuric acid (Fisher Scientific) between the limits -0.2 and +1.8 V vs SCE at 200 mV s-1. This cycling process results in the sequential formation/removal of a layer of gold oxide at the electrode surface and is an effective method of cleaning the electrode surface. Platinum microelectrodes were electrochemically cleaned by cycling between -0.2 and +1.2 V vs SCE at 200 mV s-1. (15) Electrodeposition procedure: the potential was stepped from +0.6 to -0.1 V vs SCE, at 25 °C. In each case a charge density of 6.4 C cm-2 was passed. Plating mixtures consisted of 42% (w/w) C16EO8, 29% (w/w) water, and 29% (w/w) platinic acid hexahydrate. (16) Trasatti, S.; Petrii, O. A. Pure Appl. Chem. 1991, 63, 711.
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Figure 3. SEM image of a nanostructured platinum microelectrode (25 µm diameter) obtained using a JEOL JSM-6400 scanning electron microscope operating at a voltage of 20 kV.
for a large area polished platinum electrode (RF value ∼2.8, 0.5 mm diameter) as illustrated in Figure 2b shows that the microelectrode cyclic voltammograms were of very good quality and exhibit the characteristic features expected for a polycrystalline platinum electrode. Due to the small size of the electrode, transmission electron microscopy could not be carried out on the platinum deposited at the microelectrode surface. However, it was possible to obtain scanning electron micrographs of the platinum deposit; the nanostructure of the platinum films (pores ∼2.5 nm diameter) could not be resolved, but the surface morphology of the film could be examined. Figure 3 shows a typical scanning electron micrograph of a nanostructured platinum microelectrode. The surface morphology of the platinum deposit was very smooth. The high roughness factors obtained from cyclic voltammetry in sulfuric acid were therefore assigned to the presence of a nanostructure not observable on the scanning electron microscope scale. The electrodes were then placed in a copper sulfate solution and held at +50 mV vs SCE for up to 5 min to allow copper UPD. The current due to copper stripping/ redeposition was then measured. Figure 4a shows a typical cyclic voltammogram obtained for copper stripping/redeposition on nanostructured platinum. A clear signal on the forward scan was observed for copper stripping, but no signal was apparent on the backward scan for copper redeposition. However, a second scan, recorded sequentially (and therefore with no preconcentration step at +50 mV), does show a stripping peak, albeit smaller in size, indicating that incomplete coverage of the electrode surface occurred. The responses of the HI-ePt microelectrodes were compared with the response of a polished platinum macroelectrode, as shown in Figure 4b. The macroelectrode exhibited both stripping and redeposition processes. The charge associated with the stripping process can be obtained by integrating the current on the forward scan (found to be ∼0.35 µC for Figure 4a), this may be converted to a surface area by using a conversion factor for copper UPD on polycrystalline platinum of 410 µC cm-2.13 In the case of Figure 4a the surface area was determined to be 8.5 × 10-4 cm2, this agrees reasonably with the surface area of 1.1 × 10-3 cm2 determined by cyclic voltammetry in dilute sulfuric acid (i.e., hydrogen UPD). This suggests
Figure 4. (a) Cyclic voltammogram, recorded at 100 mV s-1, of a nanostructured platinum microelectrode (25 µm diameter) in a solution of 10 mmol dm-3 copper sulfate in 2 mol dm-3 sulfuric acid. The electrode was held initially at +50 mV vs SCE for 5 min, and two sequential scans labeled 1 and 2, respectively, are shown. (b) Cyclic voltammogram, recorded at 100 mV s-1, of a platinum macroelectrode (0.5 mm diameter) in a solution of 10 mmol dm-3 copper sulfate in 2 mol dm-3 sulfuric acid. The electrode was held initially at +50 mV vs SCE for 5 min, and two sequential scans labeled 1 and 2, respectively, are shown. (c) Plot to show the effect of copper deposition time (time held at +50 mV vs SCE) upon the charge density of the copper stripping peak (obtained by taking the real surface area of the platinum electrodes into account). The symbol (b) refers to a nanostructured microelectrode (25 µm diameter) and the symbol (O) refers to a polished platinum macroelectrode (0.5 mm diameter).
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Figure 5. Cyclic voltammograms, recorded at 5 mV s-1, of two platinum microelectrodes (50 µm diameter) in a solution of 10 mmol dm-3 potassium ferricyanide in 1.0 mol dm-3 potassium chloride: (a) nanostructured platinum microelectrode and (b) polished platinum microelectrode.
that nearly complete coverage of the platinum surface by copper was achieved after a 5-min deposition time. Figure 4c shows how the charge density due to copper stripping varied with time held at +50 mV vs SCE for both the nanostructured microelectrode and the macroelectrode. As can be seen, both electrodes exhibit approximately the same stripping charge density after a 5-min deposition time. The response of the normal macroelectode is unaffected by the deposition time; however at shorter deposition times the stripping charge density for the nanostructured electrode decreases indicating incomplete coverage of the electrode. One possible explanation for this unusual decrease in stripping charge density at the nanostructured microelectrode as deposition time was reduced (and also for the absence of a redeposition peak for the nanostructured microelectrode) is a Donnan exclusion effect,17 which would limit the number of copper ions present within the nanostructure at any one time. The scale of the nanostructure (∼2.5 nm pores) falls within the definition of a fine pore membrane18 whose pore electrolyte composition is governed by the electric charge on the pore walls and the concentration of the bulk electrolyte. The electrical double layer formed inside the pores can, under the right conditions, result in counterions (ions of opposite charge to the pore wall) having a greater concentration in the pores than co-ions (ions of the same charge as the pore wall). When the pores are of the scale described in this work, it is possible that the diffuse electrical layer would have an effective thickness comparable with the pore radius and that inside the pore structure counterions would be in clear excess of co-ions. Thus after stripping, the electrode potential (∼+700 mV vs SCE) is positive with respect to the potential of zero charge (pzc) of platinum,19 any Cu2+ ions formed will tend to be expelled from the nanostructure, and hence one might expect redeposition to be inhibited. The established equilibrium is perturbed during redeposition by electrochemical (17) Kemery, P. J.; Steehler, J. K.; Bohm, P. Langmuir 1998, 14, 2884. (18) Koryta, J.; Dvorak, J.; Kavan, L. Principles of Electrochemistry; John Wiley & Sons: Chichester, 1993; p 416. (19) Bockris, J. O’M.; Jeng, K. T. J. Electroanal. Chem. 1992, 330, 541.
depletion of the number of Cu2+ ions within the pore structure; thus a steady influx of Cu2+ ions from the bulk electrolyte ensues. Only by holding the electrode at a potential negative of the platinum pzc can sufficient Cu2+ ions be forced into the pore structure allowing complete coverage to take place. This explanation is further supported by the behavior observed with increasing scan rate. At high scan rates there is insufficient time for all Cu2+ to be expelled, significant redeposition can occur during the return scan, and hence a cathodic current peak becomes apparent. Our data show that the nanostructured microelectrodes have very high electrochemically accessible surface areas as a consequence of the very fine pore structure produced by the liquid crystal templating. It is well-known that under normal conditions diffusion to and from microelectrodes is quite different to that at macroelectrodes.20 Diffusion to a large surface area macroelectrode is planar and hence mass transport efficiency will be low and non steady state. However, diffusion to microelectrodes is radial ensuring a high mass transport coefficient, km, where km ) 4D/πa and D is the diffusion coefficient and a the electrode radius. Figure 5 compares the diffusional characteristics of a nanostructured platinum microelectrode and of a polished platinum microelectrode of the same diameter. The nanostructured platinum microelectrode was seen to have a greater double layer capacitance (due to its high surface area); this is observed as greater hysteresis in the plateau regions of the cyclic voltammogram. In both cases the diffusion-limited current, ilim, was approximately the same, and D may be determined using ilim ) 4nFaDc, where n is the number of electrons transferred, F is the Faraday, a is the electrode radius, and c is the concentration of the redox species. D was found to be ∼9.2 × 10-6 cm2 s-1, using a scan rate of 5 mV s-1. Both electrodes exhibited characteristics of a typical mass transfer limited reaction, despite the real surface area of the nanostructured electrode being 0.01 cm2 (as determined by hydrogen UPD in sulfuric acid). In conclusion it has been shown that nanostructured platinum films may be successfully electrodeposited onto microelectrodes. High real surface area electrodes are (20) Forster, R. J.; Chemical Society Reviews 1994, 23, 289.
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obtained which retain the efficient mass transport characteristics of the underlying microelectrode. This combination of properties is unique, and these electrodes may be of benefit to many branches of electrochemistry, such as electroanalysis, where, for example, the amperometric detection of organic species with poor electrode kinetics may be possible. Another related field of application is adsorptive stripping analysis. In addition the nanostruc-
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tured electrodes appear to have some interesting properties inherently related to their unique architecture, and ultimately it may prove possible to exploit such charge exclusion characteristics, for example, in the field of ion selective membranes. LA9908945
