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COMPOSITIONAL MAPPING OF CHEMICALLY MODIFIED GLASSY CARBON ELECTRODES WITH TAPPING-MODE SCANNING FORCE MICROSCOPY
COMPOSITIONAL MAPPING OF CHEMICALLY MODIFIED GLASSY CARBON ELECTRODES WITH TAPPING-MODE SCANNING FORCE MICROSCOPY
G.K. Kiema, J.K. Kariuki and M.T. McDermott* Department of Chemistry University of Alberta Edmonton, Alberta T6G 2G2 Canada
1 INTRODUCTION
Glassy carbon (GC) has enjoyed extensive usage as an electrode material due to its wide potential window, solvent compatibility and relatively low cost. As with many solid electrode materials, a number of schemes have been developed to ensure highly reactive and reproducible GC electrode surfaces. Procedures such as polishing, electrochemical pretreatment (ECP), vacuum heat treatment and laser activation have been extensively characterized in terms of their influence on electron transfer kinetics and background current.’ Efforts to enhance the selectivity and sensitivity of GC surfaces have led to pathways for chemically modifying these interfaces. ECP was one of the first methods used to activate GC electrodes and effectively transforms compositionally the surface with a layer of graphitic oxide.24 This procedure involves the application of one of several potential waveforms to the GC electrode in some suitable solvent and generally includes anodization of the electrode surface at potentials of 1.6 to 2.2 V vs. SCE. The resulting graphitic oxide layer formed on the surface can itself enhance electron transfer for some species (e.g. dopamine5) or can be further modified. Although ECP is a facile and flexible modification scheme, its effect on the morphology and surface architecture of GC has not been thoroughly explored. Scanning probe microscopic (SPM) techniques have been successfully employed to characterize the two-dimensional structure of electrode surfaces. Despite its wide usage as an electrode material, however, GC has been the subject of only a handful of SPM ~tudies.~.’This is likely due to the rough and ill-defined surface structure of GC compared with typical SPM substrates which are generally atomically flat. We present here a tapping-mode scanning force microscopy (TM-SFM) study of modified GC electrodes. This technique involves monitoring the amplitude of an oscillating cantilever as its integrated tip lightly “taps” the surface. TM-SFM is more suitable for probing relatively rough surfaces than the traditional contact mode SFM because the intermittent contact between the tip and sample in TM-SFM significantly reduces lateral forces experienced by the tip. We demonstrate here that TM-SFM is useful for tracking both morphological and compositional changes of GC surfaces induced by chemical modification. We have recently demonstrated that the phase shift of the oscillating cantilever relative to its driving waveform is sensitive to the adhesive interactions between the tip and sample.* Because adhesion depends on the chemical groups at the interface, phase contrast TM-SFM is sensitive to surface composition. In the present study, we use both topographic and phase contrast images to describe variations in GC
Compositional Mapping of Chemically Modified Glassy Carbon Electrodes
28 1
electrodes induced by ECP and also probe the structure of covalently bound films to GC deposited by a recently reported technique?
2 EXPERIMENTAL
2.1 Reagents Eu3'(aq) in 0.2 M NaC104 solution was prepared at 5 mM from Eu(N03)~*5H20 (Aldrich). 4-Diazo-NJ-diethylaniline (DDEA) and tetrabutylammoniumtetrafluoroborate (TAT) were obtained from Aldrich and used as received. Reagent grade acetonitrile (ACN) was used.
2.2 Electrode Preparation and Electrochemical Measurements GC-20 (Tokai) electrodes were prepared by polishing with successive slurries of 1.O, 0.3 and 0.05 pm alumina in nanopure water on microcloth (Buehler). The GC electrodes were sonicated in nanopure water for 10 min between each polishing step. Polished GC electrodes were patterned by two methods. In some cases a freshly polished GC electrode was sprayed with small droplets of polystyrene (PS) dissolved in CCL. In most cases standard photoresist-based microfabrication techniques were utilized to create patterned surfaces. A freshly polished GC electrode was spin-coated with photoresist. A TEM grid was used as a mask through which regions of the photoresist were exposed to UV light and then developed. ECP was performed by poising a patterned electrode at +1.80 V vs. Ag/AgCl for 10 s to 120 s in either acidic electrolyte (1.0 M H2S04) or basic solution (0.1 M NaOH). The patterned GC electrodes were then sonicated in acetone for cu. 10 min to remove the photoresist or PS. A three-electrode cell was used with a Ag/AgCl reference electrode and a Pt wire counter electrode. Cyclic voltammetry was performed at a scan rate of 0.2 Vls. All solutions were purged with NZ gas for 5 min prior to electrochemical experiments. Attachment of NJ-diethylaniline was carried out in 5 mM DDEA in 0.1 M TATIACN solutions. A Ag/AgCl reference electrode (in saturated LiC104) was used in these modifications.
2.3 SFM Conditions TM-SFM images were obtained in air using Nanoscope I11 (Digital Instruments, Santa Barbara CA). The Si cantilever was oscillated at cu. 300 kHz. Scanning was carried out with a constant oscillation amplitude. The scanning rate was between 0.5 and 1.0 Hz. The images presented here are representative of many images taken at different points on each sample. All topographic and phase contrast images presented in this study were collected simultaneously. 3 RESULTS AND DISCUSSION
The following sections describe our TM-SFM and electrochemical investigations of modified GC electrodes. In an effort to track the morphological and chemical alterations of GC surfaces with TM-SFM following chemical modification, it was necessary to compare directly the initial polished surface with the modified region in the same image. In most cases we used standard photoresistlmask patterning techniques to produce
282
Fundamental and Applied Aspects of Chemically Modified Surfaces
surfaces with segregated unmodified and modified regions. We also utilized small droplets of polystyrene (PS) which were nebulized from a carbon tetrachloride solution and sprayed on a polished GC surface to mask some regions. In all cases, the photoresist or the PS was dissolved from the GC substrate after modification and before imaging. We first present results for GC electrodes following ECP in basic and acidic solutions. We then describe the morphology of covalently bound aryl films on GC electrodes. We conclude by discussing the utility of TM-SFM for probing compositional changes of relatively rough electrode surfaces.
3.1 Electrochemical Pretreatment of GC in 0.1 M NaOH The morphological changes induced by stepping the potential of a patterned GC electrode from 0 to 1.8 V vs. Ag/AgC1 in 0.1 M NaOH for 2 min are illustrated in Figure 1A. Contained in the 100 x 100 pm topographic TM-SFM image are a series of depressions corresponding to the areas exposed to the pretreatment procedure by the pattern. From the cross-sectional profile the measured depth of the depressions is 330 nm for a 2 min oxidation. The plot in Figure 1B shows that the depth of the depressions produced by ECP in basic solutions is controllable and dependent on anodization time. These results indicate that significant alterations in the morphology of the GC surface accompany ECP in basic solutions likely as a result of electrochemically-induced etching. A mechanism proposed by Sherwood et al. for the oxidation of carbon fibers in basic solutions is shown in Scheme 1 and is consistent with our results.''
Scheme 1 Compositional changes of GC surfaces induced by ECP in basic solutions were monitored with both TM-SFM and cyclic voltammetry (CV). Figure 2 contains 4Ox40pm topographic (Figure 2A) and phase contrast (Figure 2B) images of a patterned GC electrode oxidized at 1.8 V in 0.1 M NaOH for 60 s. As mentioned above, the phase lag of the oscillating cantilever tapping the surface is sensitive to the interaction between the tip and sample. In addition, it has been shown that variations in sample mechanical properties as well as tipsample adhesive differences can generate phase contrast." Because the adhesion at a microscopic contact depends on interfacial chemical groups, any chemical alterations affected by ECP should be observed in phase images. Depressions due to etching are again evident in Figure 2A, and are similar to those in Figure 1A. In Figure 2B a difference in phase lag is observed between the modified region inside the pattern and the original polished surface, a significant finding that implies a compositional difference between these two areas. A comparison with chemistry-induced phase contrast reported previously implies that the darker contrast observed at the polished region (outside the depressions) results from a greater adhesive interaction with the tip relative to that in the modified region (inside the depressions).* We believe that the Si-OH groups on the tip surface interact more strongly with the ubiquitous layer of polishing debris known to exist on GC electrodes than with the surface resulting from the ECP procedure. Although ill-defined, this layer is believed to consist of abraded carbon particles and contaminants originating from the polishing
283
Compositional Mapping of Chemically Modified Glassy Carbon Electrodes
B
“
I
0
.
20
.
I
.
40
60
80
.
.
100 120
OxldallonTtme (a)
Figure 1
(A) 100 x 100 p TM-SFM image (2-scale = 500 nm) of the topography of a GC electrode following ECP at 1.8 V in 0.1 M NaOH for 2 min through a TEM pattern. ( B ) Plot of depression depth vs. oxidation time f o r ECP at 1.8 VinO.l MNaOH.
Figure 2
40 x40 p TM-SFM images of a GC sugace oxidized at 1.8 V in 0.1 M NaOH for 60 s through a TEM grid pattern. (A) Topography (2-scale = 500 nm). ( B )Phase contrast (z-scale = 30 deg).
284
Fundamental and Applied Aspects of Chemically Modified Surfaces
slurries. Experiments indicate that this polishing layer is etched away in the first few seconds of base oxidation. As shown in Figure 3A, a 10 s ECP in 0.1 M NaOH through a PS mask (see the Experimental section) removes the top cu. 30 nm of the GC electrode. Because the resulting surface is free from the ill-defined film of polishing debris it is seemingly imaged at higher resolution as evidenced by the more pronounced polishing scratches. Interestingly, a minimal phase contrast is observed in Figure 3B between the polished and modified regions implying little chemical alterations are induced by a 10 s oxidation. However, this also argues a lack of mechanical differences between the two regions. This observation supports the conclusion that the phase contrast observed in Figure 2B is driven by ECP-induced chemical changes.
t5P' Figure 3
'5w'
30 x 3 0 p TM-SFM images of a GC sur$ace oxidized at 1.8 V in 0.1 M NaOH for 10 s through a polystyrene mask. (A) Topography (2-scale = 30 nm), (B) Phase contrast (z-scale = 20 deg).
The electron transfer kinetics of several redox systems are sensitive to surface oxide s ecies, particularly carbonyl groups, on GC electrodes. The aquated ions Fe3+"+ and E$12+ are among these systems.'* It has been shown that electron transfer, as measured by the peak separation in cyclic voltammograms (&), is more facile to these systems at GC electrodes after ECP in acidic solutions. Table 1 shows that AEp for Eu3+/2+decreases with oxidation time in basic solutions indicating that electron transfer for Eu3+"+ at GC electrodes is also facilitated by ECP in this media. Because of the known dependence of hE, for Eu3+"+ on surface oxides, this correlation also implies that the chemistry of the GC surface is transformed during ECP in base via the deposition of a graphitic oxide.3 As shown above, this chemical change can be tracked with phase contrast TM-SFM. Also listed in Table 1 is the observed phase contrast (A@) measured from images on patterned surfaces. The direction of the contrast is always consistent with that in Figure 2B where the phase lag on the polished region is greater than that at the oxidized region. A5 shown in Table 1, the compositional changes induced by oxidation in base as tracked by TM-SFM correlate with electron transfer to the E u ~ + ' ~ + redox system.
Compositional Mapping of Chemically Modified Glassy Carbon Electrodes
Table 1
285
Voltammetric Peak Separations (AE,) for and TM-SFM Phase Contrast (A@)for GC Electrodes Following ECP at I .8 V in 0.I M NaOH for Various Times.
10 30 60
259 f 8 223 f 17 184k 19
1.3-2.9 5.0-7.5 6.6-9.0
3.2 Electrochemical Pretreatment of GC in 1 M H2S04 Figure 4 illustrates the effect of ECP in acidic solutions on the morphology and chemistry of GC. Figure 4A is the topographic image of a patterned GC substrate after oxidation in 1 M HzS04 for 90 s. In contrast to Figure 2A, relatively little topographic changes are observed. Several rings are apparent likely corresponding to an enhanced oxidation at the boundary of the photoresist pattern. However, a significant contrast is observed in the phase image of Figure 4B. In this image the circular regions that were exposed through the patterned photoresist are easily observed, implying a notable compostional change. A chemical transformation is also supported by the correlation between the phase contrast and the electrochemical results in Table 2. Similar to the phase contrast for the base oxidized surfaces, the phase lag at the polished GC is greater (dark contrast) than that at the oxidized regions. This is again consistent with a greater adhesion between the tip and polished surface than between the tip and oxidized surface.
' 14pm ' Figure 4
' 14m'
55 x55 ,um TM-SFM images of a GC surface oxidized at 1.8 V in 1 M HzS04 for 90 s through a TEM grid pattern. (A) Topography (z-scale = 60 nm), (B) Phase contrast (2-scale = 20 deg).
286
Fundamental and Applied Aspects of Chemically Mod$ed Surfaces
The exact chemical nature of this observation is presently under investigation. It is clear from Figure 4 that a morphological change in the GC surface does not accompany the change in surface chemistry induced by ECP in acidic solutions.
Table 2
Voltammetric Peak Separations (AE,,)for and TM-SFM Phase Contrast (A@)for GC Electrodes Following ECP at 1.8 V in 1 M HzSO4 for Various Times.
11
1-
Oxidation Time (s) Polished 30 60 90
I AE, (mV) for E u ~ +I~ +AP(deg) 11 265 f 35 130f9 105 f 5 8W 6
NA 4.0-6.8
-
9.3-1 1
3.3 Reductive Attachment of Nfl-Diethylaniline to GC Surfaces A relatively new method to covalently bind functional groups to the surface of GC electrodes involves the electrochemical reduction of diazonium salts as illustrated in Scheme 2. It is thought that monolayers of aryl moieties are attached to GC electrodes via linear potential sweep or potential step in a solution of the appropriate diazonium salt? Our interest in modified carbon electrodes prompted us to investigate the structure of these films with TM-SFM.
Scheme 2 Figure 5 contains images recorded over a boundary between a region modified with N,N-diethylaniline on the left side (R= -N(CHzCH&) and the original polished surface (right side). A droplet of PS was used to create the boundary. The layer was deposited by stepping the potential of a GC electrode from 0 to -0.9V in a 5 mM solution of 4-diazo-NJV-diethylaniline(DDEA) for 30 min. The boundary is apparent in Figure 5A due to the increased height of the film relative to the polished surface. A slight phase contrast is also observed in Figure 5B,highlighting chemical or mechanical differences. The film architecture appears to be discontinuous and comprised of closely spaced spherical structures. Surprisingly, the height of the layer measured from the polished regions is ca. 30 nm, which is much larger than expected for a single NJV-diethylaniline molecule.
Compositional Mapping of Chemically Modijed Glassy Carbon Electrodes
Figure 5
287
15 x 15 pm TM-SFMimages of the boundary between unmodified, polished GC (right side) and GC modified with N,N-diethylaniline. (A) Topography (z-scale = 60 nm). ( B ) Phase contrast (z-scale = 20 deg).
In an effort to investigate further the anomalous height of the film in Figure 5, we examined N,N-diethylaniline films on highly oriented pyrolytic graphite (HOPG) surfaces. HOPG can be considered nearly single crystal graphite and exhibits an atomically flat basal plane as shown in the TM-SFM image in Figure 6A. Parts C and D of Figure 6 are TM-SFM images following N,N-diethylaniline deposition from a single linear voltage sweep between 0 and -0.9 V in a solution of 0.5 mh4 DDEA. As shown in the topography (Figure 6C) an array of segregated spherical structures are observed exhibiting heights of 5 to 15 nm. The phase contrast image (Figure 6D), however, reveals that these structures are not surrounded by purely basal plane graphite. Compared to the phase image of unmodified HOPG (Figure 6B) the areas surrounding the spheres appears textured. We interpret this textured pattern as being the initial N,N-diethylaniline monolayer. We believe that the higher spherical structures result from attachment of electrochemically produced aryl radicals to the already deposited monolayer. As depicted in Scheme 3, this polymerization can result in multilayers which can grow to yield the measured height in Figure 5A. b
Scheme 3
288
Fundamental and Applied Aspects of Chemically Modified Surfaces
0.5 m
0.5 m
Figure 6 (A) 10 x 10 p n topographic (z-scale = 10 nm) TM-SFM image of unmodified HOPG. ( B ) Corresponding phase contrast image (z-scale = 20 deg). (C) 2 x 2 pm topographic (z-scale = 20 nm) image of HOPG modified with a layer of N,N-diethylaniline. (0) Corresponding phase contrast image (z-scale = 20 ded.
4 CONCLUSIONS We have shown that TM-SFh4 can provide a great deal of information concerning interfacial transformations accompanying the chemical modification of electrode surface. The quality of the images on these relatively rough surfaces (i.e. not atomically flat) leads us to believe that TM-SFM with phase contrast can be utilized to track compositional changes in a variety of technologically relevant interfaces (e.g. polymer degradation, biomaterial compatibility). For GC electrodes, ECP in basic solutions results in both morphological and compositional alterations from the original polished surface. The etching effect of anodization in aqueous NaOH opens pathways for the microfabrication of GC surfaces. These possibilities are currently being explored in our laboratory. The same pretreatment in acidic media produces only chemical changes. These solvent dependent variations should be considered when choosing an ECP
Compositional Mapping of Chemically Modified Glassy Carbon Electrodes
289
procedure. Our experiments have also indicated that the deposition of covalently bound aryl groups may lead to multilayer formation although the generality of this proposal is still under investigation. 5 ACKNOWLEGMENTS
This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Department of Chemistry, University of Alberta. We thank Dr. Glen Fitzpatrick of the Alberta Microelectronics Center (AMC) for the patterning of GC electrodes. We also thank Dr.Ken Westra of AMC for the gift of tapping mode cantilevers. The HOPG substrate was a gift from Dr. Author Moore of Advanced Ceramic Materials, Inc.
References R. L. McCreery, ‘Electroanalytical Chemistry’, A. J. Bard, ed., Marcel Dekker, New York, 1991, Vol. 17, p. 221. 2. R. C. Engstrom, Anal. Chem., 1982,61,23 10. A. L. Beilby and A. Carlsson, J. Electroanal. Chem., 1988,248,283. 3. 4. L. J. Kepley and A. J. Bard, Anal. Chem., 1988,60, 1459. 5. D. M. Anjo, M. Kahr, M. M. Khodabakhsh, S. Nowinski and M. Wagner, Anal. Chem., 1989,61,2603. 6. See, M. T. McDermott, C. A. McDermott and R. L McCreery, Anal. Chem., 1993, 65,937, and references therein. 7. P. Heiduschka, A. W. Munz and W. Gopel, Electrochim. Acta, 1994,39,2207. 8. M. 0.Finot and M. T. McDermott, J. Am. Chem. SOC., 1997,119,8564. 9. M. Delamar, R. Himi, J. Pinson and J. M. Saveant, J. Am. Chem. SOC., 1992, 114, 5883. 10. C. Kozlowski and P. M. Sherwood, J. Chem. SOC.,Faraday Trans., 1985,81,2745. 11. A. J. Howard, R. R. Rye and J. E. Houston, J. Appl. Phys., 1996,79, 1885. 12. C. A. McDermott, K. R. Kneten and R. L. McCreery, J. Electrochem. SOC., 1993, 140,2593. 1.
G.K. Kiema, J.K. Kariuki and M.T. McDermott* Department of Chemistry University of Alberta Edmonton, Alberta T6G 2G2 Canada
1 INTRODUCTION
Glassy carbon (GC) has enjoyed extensive usage as an electrode material due to its wide potential window, solvent compatibility and relatively low cost. As with many solid electrode materials, a number of schemes have been developed to ensure highly reactive and reproducible GC electrode surfaces. Procedures such as polishing, electrochemical pretreatment (ECP), vacuum heat treatment and laser activation have been extensively characterized in terms of their influence on electron transfer kinetics and background current.’ Efforts to enhance the selectivity and sensitivity of GC surfaces have led to pathways for chemically modifying these interfaces. ECP was one of the first methods used to activate GC electrodes and effectively transforms compositionally the surface with a layer of graphitic oxide.24 This procedure involves the application of one of several potential waveforms to the GC electrode in some suitable solvent and generally includes anodization of the electrode surface at potentials of 1.6 to 2.2 V vs. SCE. The resulting graphitic oxide layer formed on the surface can itself enhance electron transfer for some species (e.g. dopamine5) or can be further modified. Although ECP is a facile and flexible modification scheme, its effect on the morphology and surface architecture of GC has not been thoroughly explored. Scanning probe microscopic (SPM) techniques have been successfully employed to characterize the two-dimensional structure of electrode surfaces. Despite its wide usage as an electrode material, however, GC has been the subject of only a handful of SPM ~tudies.~.’This is likely due to the rough and ill-defined surface structure of GC compared with typical SPM substrates which are generally atomically flat. We present here a tapping-mode scanning force microscopy (TM-SFM) study of modified GC electrodes. This technique involves monitoring the amplitude of an oscillating cantilever as its integrated tip lightly “taps” the surface. TM-SFM is more suitable for probing relatively rough surfaces than the traditional contact mode SFM because the intermittent contact between the tip and sample in TM-SFM significantly reduces lateral forces experienced by the tip. We demonstrate here that TM-SFM is useful for tracking both morphological and compositional changes of GC surfaces induced by chemical modification. We have recently demonstrated that the phase shift of the oscillating cantilever relative to its driving waveform is sensitive to the adhesive interactions between the tip and sample.* Because adhesion depends on the chemical groups at the interface, phase contrast TM-SFM is sensitive to surface composition. In the present study, we use both topographic and phase contrast images to describe variations in GC
Compositional Mapping of Chemically Modified Glassy Carbon Electrodes
28 1
electrodes induced by ECP and also probe the structure of covalently bound films to GC deposited by a recently reported technique?
2 EXPERIMENTAL
2.1 Reagents Eu3'(aq) in 0.2 M NaC104 solution was prepared at 5 mM from Eu(N03)~*5H20 (Aldrich). 4-Diazo-NJ-diethylaniline (DDEA) and tetrabutylammoniumtetrafluoroborate (TAT) were obtained from Aldrich and used as received. Reagent grade acetonitrile (ACN) was used.
2.2 Electrode Preparation and Electrochemical Measurements GC-20 (Tokai) electrodes were prepared by polishing with successive slurries of 1.O, 0.3 and 0.05 pm alumina in nanopure water on microcloth (Buehler). The GC electrodes were sonicated in nanopure water for 10 min between each polishing step. Polished GC electrodes were patterned by two methods. In some cases a freshly polished GC electrode was sprayed with small droplets of polystyrene (PS) dissolved in CCL. In most cases standard photoresist-based microfabrication techniques were utilized to create patterned surfaces. A freshly polished GC electrode was spin-coated with photoresist. A TEM grid was used as a mask through which regions of the photoresist were exposed to UV light and then developed. ECP was performed by poising a patterned electrode at +1.80 V vs. Ag/AgCl for 10 s to 120 s in either acidic electrolyte (1.0 M H2S04) or basic solution (0.1 M NaOH). The patterned GC electrodes were then sonicated in acetone for cu. 10 min to remove the photoresist or PS. A three-electrode cell was used with a Ag/AgCl reference electrode and a Pt wire counter electrode. Cyclic voltammetry was performed at a scan rate of 0.2 Vls. All solutions were purged with NZ gas for 5 min prior to electrochemical experiments. Attachment of NJ-diethylaniline was carried out in 5 mM DDEA in 0.1 M TATIACN solutions. A Ag/AgCl reference electrode (in saturated LiC104) was used in these modifications.
2.3 SFM Conditions TM-SFM images were obtained in air using Nanoscope I11 (Digital Instruments, Santa Barbara CA). The Si cantilever was oscillated at cu. 300 kHz. Scanning was carried out with a constant oscillation amplitude. The scanning rate was between 0.5 and 1.0 Hz. The images presented here are representative of many images taken at different points on each sample. All topographic and phase contrast images presented in this study were collected simultaneously. 3 RESULTS AND DISCUSSION
The following sections describe our TM-SFM and electrochemical investigations of modified GC electrodes. In an effort to track the morphological and chemical alterations of GC surfaces with TM-SFM following chemical modification, it was necessary to compare directly the initial polished surface with the modified region in the same image. In most cases we used standard photoresistlmask patterning techniques to produce
282
Fundamental and Applied Aspects of Chemically Modified Surfaces
surfaces with segregated unmodified and modified regions. We also utilized small droplets of polystyrene (PS) which were nebulized from a carbon tetrachloride solution and sprayed on a polished GC surface to mask some regions. In all cases, the photoresist or the PS was dissolved from the GC substrate after modification and before imaging. We first present results for GC electrodes following ECP in basic and acidic solutions. We then describe the morphology of covalently bound aryl films on GC electrodes. We conclude by discussing the utility of TM-SFM for probing compositional changes of relatively rough electrode surfaces.
3.1 Electrochemical Pretreatment of GC in 0.1 M NaOH The morphological changes induced by stepping the potential of a patterned GC electrode from 0 to 1.8 V vs. Ag/AgC1 in 0.1 M NaOH for 2 min are illustrated in Figure 1A. Contained in the 100 x 100 pm topographic TM-SFM image are a series of depressions corresponding to the areas exposed to the pretreatment procedure by the pattern. From the cross-sectional profile the measured depth of the depressions is 330 nm for a 2 min oxidation. The plot in Figure 1B shows that the depth of the depressions produced by ECP in basic solutions is controllable and dependent on anodization time. These results indicate that significant alterations in the morphology of the GC surface accompany ECP in basic solutions likely as a result of electrochemically-induced etching. A mechanism proposed by Sherwood et al. for the oxidation of carbon fibers in basic solutions is shown in Scheme 1 and is consistent with our results.''
Scheme 1 Compositional changes of GC surfaces induced by ECP in basic solutions were monitored with both TM-SFM and cyclic voltammetry (CV). Figure 2 contains 4Ox40pm topographic (Figure 2A) and phase contrast (Figure 2B) images of a patterned GC electrode oxidized at 1.8 V in 0.1 M NaOH for 60 s. As mentioned above, the phase lag of the oscillating cantilever tapping the surface is sensitive to the interaction between the tip and sample. In addition, it has been shown that variations in sample mechanical properties as well as tipsample adhesive differences can generate phase contrast." Because the adhesion at a microscopic contact depends on interfacial chemical groups, any chemical alterations affected by ECP should be observed in phase images. Depressions due to etching are again evident in Figure 2A, and are similar to those in Figure 1A. In Figure 2B a difference in phase lag is observed between the modified region inside the pattern and the original polished surface, a significant finding that implies a compositional difference between these two areas. A comparison with chemistry-induced phase contrast reported previously implies that the darker contrast observed at the polished region (outside the depressions) results from a greater adhesive interaction with the tip relative to that in the modified region (inside the depressions).* We believe that the Si-OH groups on the tip surface interact more strongly with the ubiquitous layer of polishing debris known to exist on GC electrodes than with the surface resulting from the ECP procedure. Although ill-defined, this layer is believed to consist of abraded carbon particles and contaminants originating from the polishing
283
Compositional Mapping of Chemically Modified Glassy Carbon Electrodes
B
“
I
0
.
20
.
I
.
40
60
80
.
.
100 120
OxldallonTtme (a)
Figure 1
(A) 100 x 100 p TM-SFM image (2-scale = 500 nm) of the topography of a GC electrode following ECP at 1.8 V in 0.1 M NaOH for 2 min through a TEM pattern. ( B ) Plot of depression depth vs. oxidation time f o r ECP at 1.8 VinO.l MNaOH.
Figure 2
40 x40 p TM-SFM images of a GC sugace oxidized at 1.8 V in 0.1 M NaOH for 60 s through a TEM grid pattern. (A) Topography (2-scale = 500 nm). ( B )Phase contrast (z-scale = 30 deg).
284
Fundamental and Applied Aspects of Chemically Modified Surfaces
slurries. Experiments indicate that this polishing layer is etched away in the first few seconds of base oxidation. As shown in Figure 3A, a 10 s ECP in 0.1 M NaOH through a PS mask (see the Experimental section) removes the top cu. 30 nm of the GC electrode. Because the resulting surface is free from the ill-defined film of polishing debris it is seemingly imaged at higher resolution as evidenced by the more pronounced polishing scratches. Interestingly, a minimal phase contrast is observed in Figure 3B between the polished and modified regions implying little chemical alterations are induced by a 10 s oxidation. However, this also argues a lack of mechanical differences between the two regions. This observation supports the conclusion that the phase contrast observed in Figure 2B is driven by ECP-induced chemical changes.
t5P' Figure 3
'5w'
30 x 3 0 p TM-SFM images of a GC sur$ace oxidized at 1.8 V in 0.1 M NaOH for 10 s through a polystyrene mask. (A) Topography (2-scale = 30 nm), (B) Phase contrast (z-scale = 20 deg).
The electron transfer kinetics of several redox systems are sensitive to surface oxide s ecies, particularly carbonyl groups, on GC electrodes. The aquated ions Fe3+"+ and E$12+ are among these systems.'* It has been shown that electron transfer, as measured by the peak separation in cyclic voltammograms (&), is more facile to these systems at GC electrodes after ECP in acidic solutions. Table 1 shows that AEp for Eu3+/2+decreases with oxidation time in basic solutions indicating that electron transfer for Eu3+"+ at GC electrodes is also facilitated by ECP in this media. Because of the known dependence of hE, for Eu3+"+ on surface oxides, this correlation also implies that the chemistry of the GC surface is transformed during ECP in base via the deposition of a graphitic oxide.3 As shown above, this chemical change can be tracked with phase contrast TM-SFM. Also listed in Table 1 is the observed phase contrast (A@) measured from images on patterned surfaces. The direction of the contrast is always consistent with that in Figure 2B where the phase lag on the polished region is greater than that at the oxidized region. A5 shown in Table 1, the compositional changes induced by oxidation in base as tracked by TM-SFM correlate with electron transfer to the E u ~ + ' ~ + redox system.
Compositional Mapping of Chemically Modified Glassy Carbon Electrodes
Table 1
285
Voltammetric Peak Separations (AE,) for and TM-SFM Phase Contrast (A@)for GC Electrodes Following ECP at I .8 V in 0.I M NaOH for Various Times.
10 30 60
259 f 8 223 f 17 184k 19
1.3-2.9 5.0-7.5 6.6-9.0
3.2 Electrochemical Pretreatment of GC in 1 M H2S04 Figure 4 illustrates the effect of ECP in acidic solutions on the morphology and chemistry of GC. Figure 4A is the topographic image of a patterned GC substrate after oxidation in 1 M HzS04 for 90 s. In contrast to Figure 2A, relatively little topographic changes are observed. Several rings are apparent likely corresponding to an enhanced oxidation at the boundary of the photoresist pattern. However, a significant contrast is observed in the phase image of Figure 4B. In this image the circular regions that were exposed through the patterned photoresist are easily observed, implying a notable compostional change. A chemical transformation is also supported by the correlation between the phase contrast and the electrochemical results in Table 2. Similar to the phase contrast for the base oxidized surfaces, the phase lag at the polished GC is greater (dark contrast) than that at the oxidized regions. This is again consistent with a greater adhesion between the tip and polished surface than between the tip and oxidized surface.
' 14pm ' Figure 4
' 14m'
55 x55 ,um TM-SFM images of a GC surface oxidized at 1.8 V in 1 M HzS04 for 90 s through a TEM grid pattern. (A) Topography (z-scale = 60 nm), (B) Phase contrast (2-scale = 20 deg).
286
Fundamental and Applied Aspects of Chemically Mod$ed Surfaces
The exact chemical nature of this observation is presently under investigation. It is clear from Figure 4 that a morphological change in the GC surface does not accompany the change in surface chemistry induced by ECP in acidic solutions.
Table 2
Voltammetric Peak Separations (AE,,)for and TM-SFM Phase Contrast (A@)for GC Electrodes Following ECP at 1.8 V in 1 M HzSO4 for Various Times.
11
1-
Oxidation Time (s) Polished 30 60 90
I AE, (mV) for E u ~ +I~ +AP(deg) 11 265 f 35 130f9 105 f 5 8W 6
NA 4.0-6.8
-
9.3-1 1
3.3 Reductive Attachment of Nfl-Diethylaniline to GC Surfaces A relatively new method to covalently bind functional groups to the surface of GC electrodes involves the electrochemical reduction of diazonium salts as illustrated in Scheme 2. It is thought that monolayers of aryl moieties are attached to GC electrodes via linear potential sweep or potential step in a solution of the appropriate diazonium salt? Our interest in modified carbon electrodes prompted us to investigate the structure of these films with TM-SFM.
Scheme 2 Figure 5 contains images recorded over a boundary between a region modified with N,N-diethylaniline on the left side (R= -N(CHzCH&) and the original polished surface (right side). A droplet of PS was used to create the boundary. The layer was deposited by stepping the potential of a GC electrode from 0 to -0.9V in a 5 mM solution of 4-diazo-NJV-diethylaniline(DDEA) for 30 min. The boundary is apparent in Figure 5A due to the increased height of the film relative to the polished surface. A slight phase contrast is also observed in Figure 5B,highlighting chemical or mechanical differences. The film architecture appears to be discontinuous and comprised of closely spaced spherical structures. Surprisingly, the height of the layer measured from the polished regions is ca. 30 nm, which is much larger than expected for a single NJV-diethylaniline molecule.
Compositional Mapping of Chemically Modijed Glassy Carbon Electrodes
Figure 5
287
15 x 15 pm TM-SFMimages of the boundary between unmodified, polished GC (right side) and GC modified with N,N-diethylaniline. (A) Topography (z-scale = 60 nm). ( B ) Phase contrast (z-scale = 20 deg).
In an effort to investigate further the anomalous height of the film in Figure 5, we examined N,N-diethylaniline films on highly oriented pyrolytic graphite (HOPG) surfaces. HOPG can be considered nearly single crystal graphite and exhibits an atomically flat basal plane as shown in the TM-SFM image in Figure 6A. Parts C and D of Figure 6 are TM-SFM images following N,N-diethylaniline deposition from a single linear voltage sweep between 0 and -0.9 V in a solution of 0.5 mh4 DDEA. As shown in the topography (Figure 6C) an array of segregated spherical structures are observed exhibiting heights of 5 to 15 nm. The phase contrast image (Figure 6D), however, reveals that these structures are not surrounded by purely basal plane graphite. Compared to the phase image of unmodified HOPG (Figure 6B) the areas surrounding the spheres appears textured. We interpret this textured pattern as being the initial N,N-diethylaniline monolayer. We believe that the higher spherical structures result from attachment of electrochemically produced aryl radicals to the already deposited monolayer. As depicted in Scheme 3, this polymerization can result in multilayers which can grow to yield the measured height in Figure 5A. b
Scheme 3
288
Fundamental and Applied Aspects of Chemically Modified Surfaces
0.5 m
0.5 m
Figure 6 (A) 10 x 10 p n topographic (z-scale = 10 nm) TM-SFM image of unmodified HOPG. ( B ) Corresponding phase contrast image (z-scale = 20 deg). (C) 2 x 2 pm topographic (z-scale = 20 nm) image of HOPG modified with a layer of N,N-diethylaniline. (0) Corresponding phase contrast image (z-scale = 20 ded.
4 CONCLUSIONS We have shown that TM-SFh4 can provide a great deal of information concerning interfacial transformations accompanying the chemical modification of electrode surface. The quality of the images on these relatively rough surfaces (i.e. not atomically flat) leads us to believe that TM-SFM with phase contrast can be utilized to track compositional changes in a variety of technologically relevant interfaces (e.g. polymer degradation, biomaterial compatibility). For GC electrodes, ECP in basic solutions results in both morphological and compositional alterations from the original polished surface. The etching effect of anodization in aqueous NaOH opens pathways for the microfabrication of GC surfaces. These possibilities are currently being explored in our laboratory. The same pretreatment in acidic media produces only chemical changes. These solvent dependent variations should be considered when choosing an ECP
Compositional Mapping of Chemically Modified Glassy Carbon Electrodes
289
procedure. Our experiments have also indicated that the deposition of covalently bound aryl groups may lead to multilayer formation although the generality of this proposal is still under investigation. 5 ACKNOWLEGMENTS
This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Department of Chemistry, University of Alberta. We thank Dr. Glen Fitzpatrick of the Alberta Microelectronics Center (AMC) for the patterning of GC electrodes. We also thank Dr.Ken Westra of AMC for the gift of tapping mode cantilevers. The HOPG substrate was a gift from Dr. Author Moore of Advanced Ceramic Materials, Inc.
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