Mapping of Electrocatalytic Sites on a Single Strand of Carbon Fiber

(5, 6) Having all of these advantages, a fast-scan cyclic voltammetry on SSCF .... The substrate along with the fiber was then transferred under the o...
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Mapping of Electrocatalytic Sites on a Single Strand of Carbon Fiber Using Scanning Electrochemical Microscopy (SECM) Vrushali S. Joshi,† Santosh K. Haram,*,† Arindam Dasgupta,‡ and G. V. Pavan Kumar‡ †

Department of Chemistry, University of Pune, Ganeshkhind, Pune 411007, India Division of Physics and Chemistry, Indian Institute of Science Education and Research (IISER), Pune 411008, India

‡

S Supporting Information *

ABSTRACT: A novel use of scanning electrochemical microscopy (SECM) to study electrocatalytic heterogeneities present on a single strand of carbon fiber (SSCF) is presented. The SECM investigations on an SSCF have been carried out in a feedback mode using α-methyl ferrocene methanol as a redox mediator. Array scan investigations revealed substantial enhancement in the positive feedback response at specific locations on the SSCF. These locations appeared as high-contrast morphological heterogeneities in the optical micrographs. The probe-approach curves, recorded near a variety of predefined locations on the SSCF, were fitted in the analytical expressions. From this analysis, the apparent pseudo-first-order rate constant keff for the electron-transfer reaction was estimated. The electroactive locations resulted in a keff as high as 0.1 cm s−1. The confocal micro-Raman spectra recorded on the identical portions of the fiber indicated a larger proportion of the distorted sp2-hybridized structure near the electroactive sites and thus implied enhanced local electrocatalytic activity on the fiber. strand of single-walled carbon nanotubes (SWCNTs).13,14 SSCF has also been used as an SECM tip.15,16 However, to our knowledge, there is no report that discusses the application of SECM to image the electrocatalytic microheterogeneities present on the surface of carbon fibers. Recently, we have suggested the use of SSCF as a sensor for the detection of a micromolar quantity of dopamine in the presence of ascorbic acid.1 Herein, we report the application of SECM to map and quantify the spatial distribution of distinct electrocatalytically active sites present on SSCFs. The approach curves recorded on various sites on the SSCF were fitted to the pseudo-firstorder electron-transfer reaction, and the effective rate constant, keff, for those sites was estimated. With the help of confocal micro-Raman spectroscopy, the role of the distorted sp2hybridized structure in the electrocatalytic activity was inferred.

1. INTRODUCTION Carbon fibers demonstrate unique conducting, mechanical, and surface properties, which make them a promising candidate for applications in sensors,1 fuel cells,2 batteries,3 and supercapacitors.4 Therefore, the electrochemical characterization of carbon fibers as electrode materials is the focus of interest in recent years. One of the advantages of carbon fibers is their existence as a single strand, which can be isolated and used to fabricate a self-standing and stand-alone electrode. As a result of the well-defined crystalline surface, mechanical strength, and conducting properties, a single strand of carbon fiber (SSCF) can act as an excellent electrode material for in vivo measurements, at the microscopic level. Moreover, due to the small surface area, an SSCF gives the current response, which is relatively free from the charging component, thus facilitating measurements at very high scan rates.5,6 Having all of these advantages, a fast-scan cyclic voltammetry on SSCF microelectrodes7 has been reported to investigate the electrophysiological events in the brain8 and dynamics of neurochemical signaling in a single cell.9,10 These electrodes have also been used as an amperometric detector for neurotransmitter molecules released by the single cell.11 However, the electronic and thus electrocatalytic properties of the fiber are subject to variation due to synthetic routes of preparation and post heattreatment.12 Thus, the knowledge of surface heterogeneities associated with carbon fibers is of utmost importantespecially in the context of single-strand electrodes. Among various scanning probe techniques, scanning electrochemical microscopy (SECM) is being used extensively to study electrochemical heterogeneities on an electrode surface. SECM has been reported to be employed in the detection of an individual © 2012 American Chemical Society

2. EXPERIMENTAL METHODS 2.1. Materials. α-Methyl ferrocene methanol (97%), sodium dihydrogen phosphate, and disodium hydrogen phosphate were procured from Sigma-Aldrich (USA) and used as received. All the solutions were prepared in Milli-Q water. Carbon fibers were prepared by the catalytic chemical vapor decomposition (CCVD) method. It's detailed method of preparation and characterization by XRD, XPS, and TGA/DTA analyses have been published elsewhere1 and also briefly presented in the Supporting Information. 2.2. Scanning Electrochemical Microscopy (SECM) on a Single-Strand Carbon Fiber (SSCF). 2.2.1. Sample Received: April 6, 2012 Published: April 11, 2012 9703

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Figure 1. (A) Homemade SECM sandwich cell. (B) Ray diagram of SECM setup used: (a) SECM cell, (b) mirror through which SSCF image and tip movement were monitored and recorded with the help of a (c) CCD camera having a (d) digital interface. (e) An image of SSCF and Pt tip on the screen (cross section). (f) Computer interface for the Sensolytic SECM workstation and Autolab-PGSTAT-128 N. (w) Pt tip working electrode.

the position of the fiber was first confirmed by recording the feedback response. Prior to the array scan, the tip was withdrawn along the +Z direction by 3 μm from the SSCF surface and rastered along the X−Y direction with the scan rate of 1−5 μm s−1, at the fixed Z distance. Utmost care was taken to avoid accidental brushing of the tip to the fiber, which was judged by noting an abrupt increase and fluctuations in the current response. In such situations, the tip was withdrawn and repolished and experiments were repeated. To ensure the exact comparison between the optical image and the SECM response, the relative positions of the fiber were marked on the monitor (through the CCD camera). The substrate along with the fiber was then transferred under the optical microscope to record the images. To estimate the rate constants keff, the approach curves were obtained on the SSCF at predetermined locations. These locations in the form of x and y coordinates were based on positions of the tip response noted in the array scans. The tip was lifted along the +Z direction, adjusted to specific x, y coordinates, and the approach curves were acquired near those locations. The data were fitted into the analytical expressions for the probe-approach curve with keff as a fitting parameter for the redox reaction of α-methyl ferrocene methanol. 2.3. Micro-Raman Spectroscopy on SSCF Substrate. Micro-Raman measurements were carried out with the help of a confocal Raman microscope (Horiba Jobin Yvon, France). All the spectra were recorded at a laser excitation wavelength of 632.81 nm and 1.7 mW power. Focusing of the incident light and collection of the backscattered light were accomplished using a 100× objective lens (0.9 NA). This resulted in a laser spot size of 1 μm, which further determined the resolution of the Raman excitation. The image of an individual fiber was

Preparation. The product obtained from the CCVD method was in the form of wool-like fibrils. From that, an individual carbon fiber was pulled out manually and transferred on an optically flat silica substrate (1.0 cm × 1.0 cm), with the help of a microscope (Leica DM2500 M, reflecting type 10×/50× objectives and dark-field mode). Two ends of the fiber were secured on the substrate by dots of silver-epoxy paint. The substrate carrying the SSCF was fixed in the homemade SECM cell such that the fiber strand would be sealed with the “O” ring. The essential components of the cell are schematically shown in Figure 1A. The cell has a provision to enter the SECM tip from the top. The tip movement was monitored with the help of a digital CCD camera (Hik Vision, 13× objectives), through an acrylic window from the bottom. The setup is schematically represented in Figure 1B. 2.2.2. SECM Measurements on SSCF Substrate. A Sensolytics interface (Germany) with Autolab-PGSTAT-128 N workstations were used to carry out the SECM measurements. The SECM tip made of a Pt disk (diameter = 25 μm) sealed in a glass capillary (Rg = 3.01), Ag/AgCl, and Pt wire were used as a working, reference, and counter electrodes, respectively. Prior to the experiments, the tip was manually polished with 0.05 μm alumina powder, and its surface quality was monitored under an optical microscope. The electrochemical performance of the tip was checked periodically with the help of steady-state voltammograms. All the electrochemical measurements were carried out in 3 mM α-methyl ferrocene methanol in phosphate buffer (pH 7.4) solution. The tip was biased at +400 mV vs Ag/AgCl, and the probe approach curves were recorded on the SSCF along the Z direction. For the array scan measurements, the SECM tip was aligned vertically above the SSCF with the help of a CCD camera, and 9704

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heterogeneous rate constant of the electron-transfer reaction at the substrate, which may lead to an intermediate response. When the SECM tip is rastered at a fixed z distance (a few μm) over the electrochemically heterogeneous surface (array-scan mode), a mixed response due to positive and negative feedbacks will be observed. Thus, the “faradaic contrast” is generated in the form of a 3D image that essentially represents a map of electrochemical heterogeneities on the substrate. Figure 3 shows a typical SECM array scan and the corresponding optical micrograph recorded on an identical

captured with a CCD camera and a white light source connected to the Raman microscope. Prior to the measurements, the unique morphology patterns in the optical image of the sample were compared with the previously recorded SECM and optical micrographs. Upon identification of the relevant regions, the Raman spectral measurements were performed. The Raman spectra were recorded with 1800 grooves/mm grating that resulted in a spectral resolution of 0.1−0.5 cm−1. Each spectrum was recorded for 10 s with two accumulations.

3. RESULTS AND DISCUSSION 3.1. Material Characterization. Figure 2A depicts the scanning electron micrograph (SEM) recorded on an as-

Figure 3. (A) SECM array scan and (B) corresponding optical micrograph obtained on the single strand of carbon fiber, resting on a silica substrate. Both the micrographs were recorded on the same regions of the fiber. The points “p”, “q”, “r” and “s” indicate the spatial correlation.

Figure 2. (A) SEM image and (B) powder X-ray diffractogram recorded for an as-prepared carbon fiber sample. The scale bar for SEM is 100 μm.

portion of the fiber. In the SECM image, the fiber appeared to be slightly enlarged (full width at half-maxima, fwhm = 24.4 μm) as compared with the size noted in the optical image (13 μm). Similar observations were reported previously in the case of SECM measurements on the single strand of carbon nanotube13 and band electrodes,18 where the fwhm's of the images were found to be comparable with the diameter of the SECM tip used. Besides these topographical features, some of the portions (marked as p, r, and s in Figure 3) on the SSCF resulted in substantial elevation in the response of the probe current, suggesting a higher local electrochemical activity. The same locations in the optical micrographs appeared as dark-contrast regions, suggesting the presence of topographical heterogeneities. To confirm these correlations, experiments were repeated on several samples of SSCF and compared with their optical images. Figure 4 shows two such typical repetitions. In Figure 4A, these heterogeneities are present along the length of the fiber in the form of rows having a gap between them. The corresponding SECM image also indicates similar features, where the gap between two rows of elevations in the probe current was noted. In Figure 4B, the higher density of defects seen in the optical image was correlated to the enhancement in the probe response in the array scan (marked as arrows).

prepared sample of carbon fibers. We observed horizontally aligned, well-separated fibers, with diameters in the range of 10−13 μm and lengths of a few millimeters. Figure 2B shows an X-ray diffractogram recorded on the sample. The strong reflection at 26.5° was attributed to the (002) plane reported for a layered graphene structure, analogous to multi-walled carbon nanotubes.17 3.2. SECM Measurements on SSCF Substrate. In a typical measurement of probe approach, the Pt ultramicroelectrode (SECM tip) is brought vertically down toward the substrate (in the present case, SSCF), immersed in the redox mediator. The tip is biased in such a way that the mediator gets oxidized/reduced with a steady state. When the tip under biased conditions arrives very close and is in the close proximity (approximately of the order of diffusion layer thickness) to the substrate, two types of responses are noted: (1) If the substrate is electrochemically inactive, then the tip current decreases upon the probe approach due to a hindered diffusion. This response is termed as negative feedback. (2) If the substrate facilitates the electron-transfer reaction, then the mediator gets reverted back to its original oxidation state in the tip−substrate gap. This leads to an increase in the tip current upon the probe approach and is termed as positive feedback. The extent of positive feedback is governed by the dependence on the 9705

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Figure 5. (A) SECM array scan shown in Figure 4B is represented here in the form of a contour map. The numbers in the brackets indicate the x and y coordinates at which (B) SECM approach curves were recorded. Lines in the plots indicate the corresponding fitting in the analytical expressions from which keff values are estimated and presented in the inset. Figure 4. Spatial correlation between SECM array scans and the corresponding optical micrographs for different pieces of a single strand of carbon fiber, resting on a silica substrate. The arrows indicate the correlation between the defects seen in the optical images and corresponding SECM array scans.

Isk

=

(

L 1+

1 Λ

⎡ 0.68 + 0.3315 exp − 1.0672 L ⎢ +⎢ 1 + F (L , Λ ) ⎣

(

0.78377

)

⎛ I ins ⎞ ITk = Isk ⎜1 − TC ⎟ + ITins IT ⎠ ⎝

To further confirm this aspect, the array scans were recorded on the Pt wire of 40 μm in diameter. The results are shown in the Supporting Information. No such heterogeneities in the tip−current response were observed in the case of Pt and thus ruled out the possibility of any kind of artifacts. All of the above-described experiments strongly suggested the direct correlation between surface heterogeneities on the SSCF and the corresponding enhancement in the probe response. 3.3. Measurement of Heterogeneous Kinetics at SSCF. To estimate the pseudo-first-order heterogeneous rate constant keff, the approach curves were recorded at various locations on the fiber, and the data were fitted to an analytical expression with keff as the fitting parameter. Figure 5A represents the array scan shown previously in Figure 4B, but in the form of a contour map. The points “p”, “q”, “r”, “s”, “t”, and “u” indicate the (x, y) coordinates at which the approach curves were acquired. In Figure 5B, the circles indicate the experimental points of the probe approach measured at those specific coordinates. The lines indicate the corresponding fitting in the analytical expressions, as shown below.19,20

) ⎤⎥ ⎥ ⎦

(1a)

(1b)

Here, in eq 1a, Iks is the kinetically controlled substrate current; L = d/a is the normalized tip−substrate separation; Λ = keffd/D, where keff is the apparent heterogeneous rate constant (cm s−1) and D is the diffusion coefficient of the reduced mediator in aqueous solution; and F(L, Λ) is given as 11 + 7.3) ( Λ F (L , Λ ) =

110 − 40L

(2)

In eq 1b, ICT, IkT, and Iins T represent the normalized tip currents for the diffusion-controlled regeneration of a redox mediator, finite substrate kinetics, and insulating substrate, respectively. The analytical expressions for ICTand Iins T are as follows: ITC = 9706

⎛ 1.0672 ⎞ 0.78377 + 0.3315 exp⎜ − + 0.68⎟ ⎝ ⎠ L L

(3a)

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1 0.292 + 1.5151/L + 0.6553 exp(− 2.4035/L)

collectively, all the above experiments successfully revealed that the distorted sp2-hybridized defects were responsible for the higher electrocatalytic activities depicted by those sites. A redox molecule, such as α-methyl ferrocene methanol, is not expected to undergo molecular rearrangement during electron transfer and, in general, is viewed as a typical outersphere electron transfer. Therefore, the surface-site-dependent electrocatalysis is not expected to be observed. However, we believe that α-methyl ferrocene methanol may interact with the surface of the carbon fiber, through π−π stacking between the cyclopentadienyl ring and the graphitic surface. The microRaman measurements at various locations revealed varied proportions of distorted sp2-hybridized carbon on the surface. This perhaps has impacted on π−π stacking interactions, and in the present case, it demonstrated the response analogous to the inner-sphere electron transfer.

(3b)

The rate constant values keff were estimated from the fitting of the experimental approach curves to the analytical expressions, as shown in the inset of Figure 5B. The locations showed high tip responses in the array scans (for example, refer to point “s” in Figure 5B), which yielded keff = 0.1 cm s−1, while locations that delivered a weak tip−current response (for example, refer to point “p”) yielded values as low as 2.5 × 10−3 cm s−1. The rest of the values were distributed between these two limits. The approach curve recorded at location “u” could not be fitted faithfully in the realistic values of keff. Nevertheless, it passed above the curve fitted for the keff = 0.1 cm s−1, which suggested that the electron-transfer reaction was much faster than this value. 3.4. Micro-Raman Spectroscopy on SSCF. Raman spectroscopy is a powerful tool to characterize carbon-based materials. There is a one-to-one correspondence between the Raman vibrational modes and the hybridization characteristics of carbon materials.21 With this insight, we performed Raman microspectroscopy measurements on the carbon fibers. Figure 6 shows the Raman spectra collected near the dark spots

4. CONCLUSIONS For the first time, we have demonstrated the applications of SECM measurements to quantify the electrocatalytic heterogeneities associated with the single strand of carbon fiber. Direct correlations among the optical micrographs, local SECM response, and Raman measurements were established. The SECM approach curves were recorded on the specific locations on the fiber. These curves were fitted into the analytical expressions, and from that, the effective pseudo-first-order rate constants as a function of the locations on the fiber were estimated. These sets of experiments pave the way to map the spatial distribution of electrocatalytic centers, which will have great relevance in the development of nanobiosensors based on a single strand of carbon fiber. In the future, the SECM tool can also be used to generate specific electrocatalytic centers on the single strand of carbon fiber, which would sense various biologically active molecules, simultaneously.

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Figure 6. Micro-Raman spectra recorded on an SSCF at various locations. (a) Near the location marked as “q” and (b) near the location marked as “p” in Figure 3A. The spectrum (c) was recorded near the edge of the fiber (not shown). For the calculation of D/G ratios, the corresponding reference zero for the individual spectrum is considered.

ASSOCIATED CONTENT

S Supporting Information *

Preparation and characterization of carbon fibers, steady-state cyclic voltammogram recorded at Pt-UME, methodology adapted to correlate the optical micrograph, SECM array scan and micro-Raman spectra, micro-Raman spectra and corresponding optical micrograph repeated on SSCF, and controlled SECM array scan recorded on 40 μm diameter cylindrical Pt wire. This material is available free of charge via the Internet at http://pubs.acs.org.

(marked as “p” in Figure 3), and the defect-free portions (marked as “q” in Figure 3). For the sake of comparison, the Raman data were also collected near the edge of the SSCF. All three spectra show the two characteristic vibrational modes near 1345 and 1600 cm−1, which were assigned to the distorted sp2-hybridized carbon (D band) and the tangential acoustic mode for the graphitic structure (G band), respectively. The higher “D”-to-“G” intensity ratio suggested the presence of a greater proportion of the distorted sp2-hybridized structure.21 In the present case, the D/G intensity ratios were found to be 1.15 and 1.04 near the dark spots (position “p” in Figure 3) and away from the dark spots (position “q” in Figure 3), respectively. This ratio for the edge of the SSCF was found to be 1.17, which was close to the ratio noted for the dark spots. The Raman measurements, repeated on the other sample of SSCF, and its comparison with the optical image are presented in the Supporting Information. This data again revealed the correlation between the surface heterogeneities and a corresponding higher D/G ratio at those specific sites. Thus,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +912025691373. Fax: +912025693981. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The financial support from the DBT, Government of India, is acknowledged. V.S.J. thanks CSIR, Government of India, for the Senior Research Fellowship and Prof. Nanda Haram for the fruitful discussion. G.V.P.K. acknowledges the DST Ramanujan fellowship. 9707

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REFERENCES

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