Anal. Chem. 2005, 77, 1916-1919
Electrochemical Cell for Surface Analysis Roger Bowler, Trevor J. Davies, Michael E. Hyde, and Richard G. Compton*
Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom
We present a simple cost-effective design for an HOPG electrode that is well suited to voltammetric experiments accompanied by surface analysis. The utility of the electrode is demonstrated by an AFM study on the morphology of electrodeposited MoO2 nanowires. The design can be applied to a wide range of electrode materials. The relationship between surface structure and electrochemical performance is of fundamental importance in electrochemistry and highlights the need for electrodes with well-defined surfaces. For this reason, basal plane highly ordered pyrolytic graphite (HOPG) is an attractive electrode material. Not only is a clean surface relatively easy to prepare, freshly cleaved HOPG can possess as few as ∼0.5% surface defects.1,2 McCreery and co-workers have made large advances in understanding carbon electrochemistry by comparing the voltammetry of HOPG in simple redox systems with that of other carbon materials and then combining these results with spectroscopic analyses of the different surfaces.1-6 Recently, we developed a two-dimensional simulation method that takes into account surface defects and were able to show that basal plane HOPG voltammetry resembles that of a nanoband arraysthe nanobands being the edge plane steps on the HOPG surface.7,8 Because the surface is so well defined, HOPG has also proved to be a popular surface on which to conduct electrodeposition experiments. Even more promising is the potential application in nanotechnology: a number of groups have reported the synthesis of nanowires and nanoribbons along edge plane steps.9-15 * To whom correspondence should be addressed: E-mail: richard.compton@ chemistry.ox.ac.uk. Tel.: +44 (0) 1865 275 413. Fax.: +44 (0) 1865 275 410. (1) McCreery, R. L. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Decker: New York, 1990; Vol. 17, p 221. (2) McDermott; M. T.; McCreery, R. L. Langmuir 1994, 10, 4307. (3) Bowling, R. J.; Packard, R. T.; McCreery, R. L. J. Am. Chem. Soc. 1989, 111, 1217. (4) Cline, K. K.; McDermott, M. T.; McCreery, R. L. J. Phys. Chem. 1994, 98, 5314. (5) Kneten, K. R.; McCreery, R. L. Anal. Chem. 1992, 64, 2518. (6) McCreery, R. L.; Cline, K. K.; McDermott, C. A.; McDermott, M. T. Colloids Surf., A 1994, 93, 211. (7) Davies, T. J.; Moore, R. R.; Banks, C. E.; Compton, R. G. J. Electroanal. Chem. 2004, 574, 123. (8) Banks, C. E.; Davies, T. J.; Wildgoose, G. G.; Compton, R. G. Chem. Commun. 2005, 7, 829. (9) Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120. (10) Zach, M. P.; Inazu, K.; Ng, K. H.; Hemminger, J. C.; Penner, R. M. Chem. Mater. 2002, 14, 3206. (11) Walter, E. C.; Zach, M. P.; Favier, F.; Murray, B. J.; Inazu, K.; Hemminger, J. C.; Penner, R. M. ChemPhysChem, 2003, 4, 131. (12) Noll, J. D.; Nicholson, M. A.; Van Patten, P. G.; Chung, C.-W.; Myrick, M. L. J. Electrochem. Soc. 1998, 145, 3320. (13) Atashbar, M. Z.; Bliznyuk, V.; Banerji, D.; Singamaneni, S. J. Alloys Compd. 2004, 372, 107.
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Despite the benefits, the high cost of good quality HOPG, and the fact that it is only available in square pieces make it a difficult material to work with. Because HOPG is cleaved rather than polished, the approach used for most electrode materials, where the electrode (in this case the HOPG slab) is sealed in epoxy or shrink tubing, can lead to problems. As illustrated in Figure 1, after a number of cleaves with adhesive tape, the HOPG electrode surface forms a concave shape that gets progressively deeper, exposing a successively greater number of edge defects. An alternative approach is the inverted drop cell, Figure 2, in which a drop of solution is placed on the HOPG surface and the reference and counter electrodes touch the sides of the drop forming the electrochemical cell.16 Although this avoids the cleaving problem in Figure 1, there are a number of disadvantages: the potential of a wire reference has a tendency to drift when touching the drop; the electrode area is unknown and not reproducible; and this method only works for aqueous solutions (organic solvents tend to wet the HOPG surface far better than water). To obtain a reproducible electrode area, silicone or Viton O-rings can be placed on the HOPG surface to hold the drop, but this method cannot be used for organic solvents.3 The inverted drop cell is therefore undesirable to work with for many reasons. A novel solution to this problem of “housing” HOPG was proposed by Noll et al., who effectively pressed the HOPG slab against a silicone O-ring attached to a Teflon sheath with a hole in it.12 This ensured that only the basal plane surface of the HOPG was exposed to solution and allowed the basal plane HOPG surface (which now had a far more accurate and reproducible electrode area than the inverted drop cell) to be placed in nonaqueous solutions. Between scans, the housing could be dismantled and the HOPG slab cleaved. However, the particular design of their cell made the HOPG slab vulnerable to damage.12 In addition to the above, when using HOPG it can prove difficult to obtain a reproducible surface. The combination of cleaving (with a blade or adhesive tape) and mechanical strain caused by the electrode housing can lead to a large variability in performance. In this technical note, we report on a design for basal plane HOPG electrodes similar to that of Noll et al. but with a number of modifications. Not only can the electrode housing be used for voltammetry in a wide range of solvents, it is well suited to experiments where solution electrochemistry is accompanied by surface analysis. An example of this is the study of electrodeposited materials on HOPG using atomic force microscopy (14) Mukhopadhyay, I.; Freyland, W. Langmuir 2003, 19, 1951. (15) Li, Q.; Newberg, J. T.; Walter, E. C.; Hemminger, J. C.; Penner, R. M. Nano Lett. 2004, 4, 277. (16) McDermott, M. T.; Kneten, K.; McCreery, R. L. J. Phys. Chem. 1992, 96, 3124. 10.1021/ac048443z CCC: $30.25
© 2005 American Chemical Society Published on Web 02/12/2005
Figure 1. Illustration of the damage caused to a basal plane HOPG surface sealed in epoxy after being cleaved with adhesive tape several times.
Figure 2. Schematic diagram of the inverted drop cell.
(AFM), which we discuss below. Furthermore, the cell design is also found to improve reproducibility when electrochemical experiments are performed at a basal plane HOPG surface. EXPERIMENTAL SECTION HOPG Electrode Housing. As illustrated in Figure 3a, the HOPG electrode housing is composed of four parts: the PTFE cap; a PTFE body; an electrode connector; and the HOPG slab itself. With the exception of HOPG, all the raw materials involved are readily available at low cost and can be machined using tools available to most research groups through their associated workshops. The HOPG slab is attached to a mild steel plate (of dimensions similar to the HOPG piece) using a conductive epoxy, in our case, silver epoxy (RS, part 186-3616). This sits in an indent slightly larger than the steel plate at the top of the PTFE body. The PTFE cap is then screwed on the body until tight. The silicone O-ring (RS part 245-0637), concealed in the top of the cap, is now positioned just above the HOPG surface (the circular hole at the top of the cap is slightly smaller than the silicone O-ring). The electrode connector is then slowly screwed into the body, allowing the spring and steel contact to gently push the HOPG against the silicone O-ring and establish a tight seal without any damage to the electrode surface. This method avoids the rotational damage that would result from screwing the cap directly onto the electrode. Because the electrode connector is made entirely of steel, an electrical contact is established between the HOPG and the bottom of the connector, to which the lead for the working electrode can be attached. A cross section view of the assembly illustrating the novel features of the design is shown in Figure 3b.
Figure 3. (a) Schematic diagram of the four-part HOPG electrode housing. (b) Cross section view of the assembled electrode.
The electrode housing is very easy to assemble/disassemble, and the HOPG slab is in no danger of damage. Between voltammetric experiments, the slab can be removed easily, analyzed via an appropriate technique, cleaved, and then placed back in the housing for further experiments. Furthermore, attaching a mild steel plate to the back of the HOPG has a number of advantages. First, the steel plate protects the HOPG slab from any damage that might be caused by the steel connector. Second, a number of surface analysis techniques (for example, AFM) require the sample to be attached to a magnetic stub. Finally, adhesive cleaving of the basal plane HOPG surface can be made considerably easier by using a switchable magnet to immobilize the HOPG during the cleaving process. Recently we have used the HOPG housing for a number of applications including both voltammetry with simple redox couples and more complicated electrodeposition experiments using both aqueous and acetonitrile solutions. In all cases, the new cell has been much easier to use than the alternatives discussed in the introduction and has resulted in much improved reproducibility. In particular, the arrangement is ideally suited to voltammetric experiments followed by surface analysis, an example of which is the deposition of MoO2 nanowires on basal plane HOPG, as studied by Penner and co-workers.9-11 Instrumentation. Electrochemical experiments were conducted in a conventional three-electrode cell, employing a platinum wire counter electrode, a saturated calomel reference electrode, and a basal plane HOPG working electrode as described above. Electrochemical data were recorded using a commercially availAnalytical Chemistry, Vol. 77, No. 6, March 15, 2005
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Figure 6. Chronoamperometric response of three freshly cleaved basal plane HOPG surfaces in 1 mM Na2MoO4, 1.0 M NaCl, and 1.0 M NH4Cl (at pH 8.5), where the HOPG is encased in the electrode housing described in Figure 3.
Figure 4. AFM images of 2 µm × 2 µm sections of the basal plane HOPG surface (a) before and (b) after electrolysis in the sodium molybdate solution.
(Aldrich, 98+%), 1.0 M sodium chloride (BDH, AnalaR), and 1.0 M ammonium chloride (BDH, GPR) made up to pH 8.5 with sodium hydroxide (Acros, g98%), as used by Penner and coworkers.9-11 The water used to make the electrolyte solutions was taken from an Elgastat system (USF, Bucks., U.K.) and had a resistivity of not less than 18 MΩ cm. RESULTS AND DISCUSSION Penner and co-workers have shown that under the right conditions the reduction of sodium molybdate in alkaline solutions at a basal plane HOPG electrode can lead to the formation of MoO2 nanowires along the edge plane steps.9-11 The reaction responsible for MoO2 deposition is outlined in pathway A:
MoO42- + 2H2O + 2e- f MoO2 + 4OH-
Figure 5. Topographic AFM image of a 300 nm × 600 nm section of the basal plane HOPG surface after the electrodeposition of MoO2 nanowires.
able computer-controlled potentiostat (Autolab PGSTAT30, EcoChemie, Utrecht, The Netherlands). The AFM used was a Digital Instruments (now a division of Veeco) Multimode SPM, operating in tapping mode. A model J scanner was used, having a lateral range of 125 × 125 µm and a vertical range of 5 µm. Standard silicon probes (Nascatec GMBH part NST-NCHF) were used. Reagents and Materials. The HOPG was purchased in the form of a 10 mm × 10 mm × 2 mm block from SPI supplies and was of the highest grade available: SPI-1, equivalent to Union Carbide’s ZYA grade, with a lateral grain size, La, of 1-10 mm and 0.4 ( 0.1° mosaic spread.17 MoO2 electrodeposition experiments were conducted in a solution of 1-2 mM sodium molybdate (17) See: http://www.2spi.com/catalog/new/hopgsub.shtml.
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(A)
SEM analysis has confirmed the presence of nanowires, but their three-dimensional shape has rarely been investigated.10 In the following section, we demonstrate the utility of the new HOPG electrode with a short study on the morphology of MoO2 nanowires deposited on basal plane HOPG. Figure 4 illustrates AFM images of a 2 µm × 2 µm section of a basal plane HOPG surface (a) before and (b) after a 60-s deposition period during which the potential of the HOPG working electrode was held at -1.0 V versus SCE in a solution of 1.5 mM Na2MoO4, 1.0 M NaCl, and 1.0 M NH4Cl (at pH 8.5). It should be noted that, in a similar experiment, Penner and coworkers used a rudimentary method for electrodeposition where they suspended the whole HOPG slab in solution using stainless steel forceps, thus exposing both the edge and basal plane surfaces of the HOPG to the plating solution.10 Although this method does not affect the nature of the deposit formed on the basal plane surface, a charge-nanowire growth study is not possible, in contrast to the cell design discussed above. Due to the resolution of the AFM used, only large steps are observed on the electrode surface in Figure 4a. As seen in Figure 4b, under the given conditions, nanowires are formed fairly exclusively.
Figure 5 illustrates an AFM image of a 300 nm × 600 nm section of basal plane HOPG surface after electrolysis that clearly shows the morphology of the deposited nanowire, and the inset confirms the wires are approximately hemicylindrical. The same morphology was observed for over 25 randomly selected nanowires deposited on the electrode surface. The new cell design and controlled adhesive cleaving were also found to improve reproducibility when HOPG was used as an electrode material. Figure 6 illustrates three chronoamperometric responses for the electrodeposition of MoO2 at a basal plane HOPG surface encased in the electrode housing described above (surface area 0.32 cm2). In each case, the freshly cleaved HOPG surface was held at -1.0 V versus SCE in a solution of 1 mM Na2MoO4, 1.0 M NaCl, and 1.0 M NH4Cl (at pH 8.5). The dashed curve was recorded 6 days after the other two voltammograms, and the fact that all three voltammograms are similar demonstrates the affect of the cell design on obtaining a reproducible basal plane HOPG surface; i.e., the three HOPG surfaces corresponding to the three voltammograms in Figure 6 must possess a similar defect density if they give similar signals.7
CONCLUSION In this technical note, we have presented a simple and inexpensive design for a HOPG electrode housing that addresses all the problems previously encountered. The cell is ideally suited to voltammetric experiments coupled with surface analysis and results in a HOPG surface that is easy to adhesively cleave (with the aid of a switchable magnet). In addition, this design can be usefully applied to other electrode materials that are purchased in awkward shapes, such as boron doped diamond, or which require removal from a waterproof housing for analysis. ACKNOWLEDGMENT The authors thank the EPSRC for studentships for T.J.D. and M.E.H. T.J.D. thanks Lincoln College and the Lord Crewe’s Society for a scholarship 2003-2004.
Received for review January 6, 2005.
October
21,
2004.
Accepted
AC048443Z
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