Covalent Grafting of Ferrocene to Vertically Aligned Carbon

Sep 24, 2008 - Ferrocene was used as a model system to understand the electron-transfer properties of redox-active molecules covalently linked to the ...
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J. Phys. Chem. C 2008, 112, 16910–16918

Covalent Grafting of Ferrocene to Vertically Aligned Carbon Nanofibers: Electron-transfer Processes at Nanostructured Electrodes Elizabeth C. Landis and Robert J. Hamers* Department of Chemistry, UniVersity of Wisconsin-Madison, 1101 UniVersity AVenue, Madison, Wisconsin 53706, USA ReceiVed: July 13, 2008; ReVised Manuscript ReceiVed: July 30, 2008

Ferrocene was used as a model system to understand the electron-transfer properties of redox-active molecules covalently linked to the surface of vertically aligned carbon nanofibers. Ultraviolet-initiated grafting of organic alkenes was used to prepare carboxylic acid-terminated layers, and ferrocene was then linked to these layers via amide groups. The electrical properties of the resulting layers were measured using cyclic voltammetry and electrochemical impedance spectroscopy. Standard rate constants for electron transfer (kapp) of approximately 1.0-1.3 s-1 were found for ferrocene covalently linked to the nanofiber surfaces, compared to about 3 s-1 on glassy carbon surfaces. Measurements of the electron-transfer rates through molecular layers of different length show no significant change. Similarly, no significant changes were observed in electron-transfer rate constants or peak width upon dilution of the ferrocene-containing molecules. Our results show that molecular layers grafted to carbon nanofibers are sparse and disordered compared with those commonly studied on planar surfaces. A model based on preferential grafting to exposed graphitic edge planes is proposed to explain the results. 1. Introduction Nanostructured carbons are interesting materials because although carbon has extremely good chemical stability, the electronic properties of carbon are highly dependent on the atomic-scale structure. The excellent chemical, electrochemical, and thermal stability of carbon have long made it useful for applications in electroanalysis, chemical and biological sensing, energy storage, and catalysis.1-4 In recent years various forms of nanostructured carbon, such as carbon nanotubes, have been a subject of particularly intense study because of their unusual properties. Among these nanostructured carbons, vertically aligned carbon nanofibers (VACNFs) are especially interesting because of their geometric alignment and molecular structure.5-7 Although the more commonly studied single- and multiwalled carbon nanotubes expose primarily basal-plane graphite,8-10 VACNFs consist of nested cones of graphene that expose large amounts of edge-plane graphite along their sidewalls.5 Because electron-transfer rates at edge-plane graphite are ∼105 times faster than those at the basal plane,11 this implies that VACNFs may have outstanding properties as supports for electrocatalytic reactions. VACNFs are also aligned with each fiber approximately perpendicular to the underlying growth substrate, providing each nanofiber with a direct electrical connection to an underlying electrode. Although carbon has excellent stability, the chemical and physical properties of the nanofibers can be more precisely and flexibly controlled by grafting molecular layers to the surface.12,13 We have recently shown that molecular layers can be grafted to the sidewalls of VACNFs using ultraviolet-initiated grafting of organic alkenes, yielding modified VACNFs exhibiting excellent stability and, when functionalized with appropriate biomolecules, good biomolecular recognition properties.12-14 In these studies we found that grafting of molecular layers to VACNFs enhanced the subsequent electrostatic binding of the * Corresponding Author e-mail: [email protected].

redox-active protein cytochrome c, and that the resulting layers showed clear redox peaks.13 This result suggests that grafted molecular layers may act as tunable interfaces linking VACNFs to a wider range of electrocatalytic materials. However, it remains unclear what molecular and structural factors control the electron-transfer properties at chemically modified VACNFs. In this paper, we report studies of the electron-transfer properties of ferrocene covalently linked to VACNFs via molecular layers, and we explore the connections between surface structure and electron-transfer kinetics at VACNFs. Because ferrocene has been widely used as a model system for investigating electron-transfer processes on surfaces of silicon,15,16 glassy carbon,17 and gold,18-24 we use it here as a model system for comparison with other materials and for understanding the behavior that might be expected when more complex redox-active or electrocatalytically active molecules are linked to VACNFs. Measurements using Ru(NH3)2+/3+ and 1 mM Fe(CN)63-/4- provide information about the importance of edge-plane sites on the redox behavior.11,25,26 Our results provide fundamental insights into the nature of monolayers on VACNFs, the impact of these monolayers on electron-transfer processes, and the factors controlling the potential utility of VACNFs as a substrates for more complex electrocatalytic reactions. 2. Experimental Methods 2.1. Substrates. Vertically aligned carbon nanofibers were grown in a custom-built plasma-enhanced chemical vapor deposition system.5,6,27 The nanofibers were grown on highly doped silicon substrates coated with a film of 20 nm of molybdenum, 20 nm of titanium, and, finally, 20 nm of nickel forming the top layer. The nanofibers were grown at a total chamber pressure of 4 torr with 100 standard cubic centimeters per minute (sccm) ammonia and 36 sccm acetylene in a DC plasma at a power of 360 W. The nanofiber length is determined by the growth time. The nanofibers reported here were typically grown for 14 min, which yields fibers 1.0 ( 0.3 µm long.

10.1021/jp806173d CCC: $40.75  2008 American Chemical Society Published on Web 09/24/2008

Grafting of Ferrocene to Carbon Nanofibers

Figure 1. (a) SEM image of the as-grown VACNF surfaces. (b) Reaction scheme of the ferrocene attachment to the VACNF surface.

Nanofibers used for infrared spectroscopy experiments were grown for a shorter time of 5 min in order to retain sufficient reflectivity. Figure 1a shows an SEM image of the fibers as grown; the dark region near the top of each nanofibers is the nickel catalyst. Because the nickel catalyst becomes coated with a thin film of amorphous carbon, it is entrapped and has no electrochemical activity under normal conditions; experiments conducted before and after removal of the nickel (not shown) showed no significant difference in electrochemical response. Glassy carbon samples (Type 2, Alfa Aesar) were polished using 1.0 µm Buehler Alpha alumina micropolish, then cleaned in piranha solution made from a 3:5 volumetric ratio of 30% H2O2 and concentrated H2SO4, and rinsed with water. CAUTION: Piranha solution reacts Violently with organic materials. Before photochemical functionalization, the samples were heated to ∼600 °C in 20 torr H2, then hydrogen-terminated in a very weak 13.65 MHz inductively coupled hydrogen plasma for 5 min, cooled in the plasma for 15 min, and cooled to room temperature in 5 torr H2. Raman spectra of the VACNFs and glassy carbon are presented in the Supporting Information. 2.2. Substrate Functionalization. The VACNF substrates were functionalized using ultraviolet light (254 nm, 10 mW/ cm2) to covalently graft alkenes to the surface. This method has previously been used to create covalently bonded molecular monolayers on other forms of carbon, including VACNFs,12,13 diamond,2,28 and amorphous carbon.29 The as-grown carbon nanofiber samples were placed in a quartz-covered reaction chamber with a drop of the alkene on the surface and covered with a quartz window to prevent evaporation. The chamber was then purged with nitrogen before being illuminated at 254 nm for 16 h in N2. Samples were then rinsed in alternating portions of methanol and chloroform. Glassy carbon electrodes were functionalized using the same method immediately after Hterminating the surfaces in a H2 plasma as described above.

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16911 Ferrocene molecules were bound to the surface using the method outlined in Figure 1b. We first grafted alkenes bearing a terminal carboxylic acid group to the VACNF surface.30 Three different tethering chain lengths were achieved by grafting 1-undecenoic acid, 6-heptenoic acid, or vinyl acetic acid to the surface. To link ferrocene to the exposed carboxylic acid groups, the samples were first immersed in 0.2 M 1-ethyl-3-(3dimethylaminopropyl)carbodiimide hydrochloride (EDC) and 0.1 M N-hydroxysuccinimide (NHS) in nitrogen-purged water for 2 h to activate the carboxylic acid groups.31,32 After rinsing in water and then ethanol, the samples were immersed in a solution of 0.05 M aminoferrocene (TCI Chemicals) in ethanol in a humid chamber containing ethanol for 16 h. 2.3. Surface Characterization. X-ray photoelectron spectroscopy (XPS) experiments were performed in an ultrahigh vacuum system with a monochromatized Al KR source and a hemispherical analyzer with a 16 channel detector array. All spectra were recorded at 45° photoelectron takeoff angles. Atomic area ratios for core-level spectra were calculated by fitting the raw data to baseline-corrected Voigt functions and correcting the values using atomic sensitivity factors (C ) 0.296, O ) 0.711, Fe ) 2.686).33 Fourier transform infrared reflection-absorption spectroscopy was performed on a Bruker Vertex70 Fourier transform infrared (FTIR) spectrometer with a variable angle reflectance accessory (VeeMaxII, Pike) and a liquid nitrogen-cooled HgCdTe detector. Spectra presented here were collected at 60° incidence angle relative to the surface normal using p-polarized light and 100 scans at 4 cm-1 resolution for both the sample spectra and the background of unmodified nanofibers. The spectra were baseline corrected using commercial software (Winfirst). 2.4. Electrochemical Characterization. The electrochemical properties were studied using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). EIS measurements were performed using a Solartron 1260 potentiostat and Solartron 1287 impedance analyzer using Corrware and Zplot software (Scribner Associates). Cyclic voltammetry was performed using a Gamry Series G 300 Potentiostat with Gamry Framework 4.21 software, which provided positive feedback iR compensation. Electrochemical measurements were performed using a threeelectrode Teflon electrochemical flow cell. The samples, either carbon nanofibers or glassy carbon samples, were used as working electrodes with a surface area of 0.196 cm2 defined by a Viton O-ring with a Ag/AgCl junction reference electrode and platinum wire counterelectrode. Experiments were performed at room temperature, with electrolyte flowing through the cell at 0.1 mL/min. EIS experiments were performed in 4.4 mM tetrabutyl ammonium hexafluorophosphate in acetonitrile, and cyclic voltammetry experiments were performed in 1.0 M HClO4 made from water purified with a Nanopure filtration system with 18 MΩ resistivity. Redox systems used were 1 mM Ru(NH3)63+/2+ and Fe(CN)63-/4-, both in 1 M KCl. All solutions were purged with N2 or argon before use. 3. Results 3.1. Substrate Functionalization. Figure 1b shows the schematic of the functionalization method used to covalently link ferrocene to the VACNFs. In the first step, 1-undecenoic acid was photochemically grafted to the surface and characterized using infrared reflection-absorption spectroscopy (IRRAS). The spectrum, shown in Figure 2a, exhibits peaks at 2922 and 2851 cm-1, which we attribute to the symmetric and asymmetric CH2 stretching modes, and a peak at 1702 cm-1 from the CdO

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Figure 3. Cyclic voltammograms of the VACNF surface before and after ferrocene attachment, in 1.0 M HClO4, 1 V s1-, vs Ag/AgCl.

Figure 2. Characterization of chemical modification of VACNFs. (a) IRRAS of VACNFs after grafting of 10-undecenoic acid. (b) XPS survey spectrum of VACNFs after ferrocene attachment. (c) Highresolution XPS spectrum of VACNFs after ferrocene attachment.

stretching. These peaks, combined with the absence of peaks above 3000 cm-1 (where vinyl C-H stretching modes would be observed), demonstrate that the 1-undecenoic acid binds to the carbon nanofibers through the vinyl group, in agreement with prior work on diamond.30 After ferrocene was bound through an amide linkage using EDC/NHS coupling, the ferrocene attachment was characterized using XPS. Figure 2b shows the XPS survey spectrum after ferrocene attachment. The main features of the survey spectrum are the carbon peak at 283 eV and a peak from oxygen at 531 eV, and smaller features from iron are visible between 710 and 740 eV. It was not possible to differentiate the carbon in the linked chain from the bulk carbon peak in the high-resolution carbon spectrum (not shown). Figure 2c shows the iron region at higher resolution: two peaks at 707.8 and 720.5 eV, which correspond to the Fe 2p3/2 and Fe 2p1/2 signals, respectively, and are consistent with previously reported values for ferrocene.34,35 3.2. Electrochemical Response of Ferrocene on VACNFs. Figure 3 shows cyclic voltammograms (1 V/s scan rate) of a bare carbon nanofiber sample and a nanofibers sample after covalent grafting of ferrocene. Although the sample of bare nanofibers shows no significant peaks, the ferrocene-modified sample shows clear oxidation and reduction peaks at 0.235 and 0.176 V versus Ag/AgCl, demonstrating that the covalently linked ferrocene is electrochemically active. A significant capacitive contribution is evidenced by the trapezoidal background in both bare and functionalized nanofibers. Similar measurements were also performed on samples of glassy carbon before and after functionalization in an identical manner.

Figure 4. (a) Cyclic voltammograms of ferrocene attached to VACNF and glassy carbon surfaces. (b) Peak-to-peak splitting for ferrocene redox couple attached to VACNF and glassy carbon as a function of scan rate.

Because glassy carbon is widely used as a carbon electrode material, we used cyclic voltammetry to compare the number of redox-accessible ferrocene molecules and the interfacial capacitance of ∼1 µm long nanofibers compared with glassy carbon. Figure 4a shows two representative cyclic voltammograms of ferrocene attached to VACNF and glassy carbon surfaces. The glassy carbon curve shows a significantly lower capacitance compared to the VACNF surface, as evidenced by the smaller separation between anodic and cathodic curves at potentials away from the redox peaks. On carbon nanofibers, the transferred charge, computed by integrating the area under the oxidation peak after subtracting a linear background to remove the capacitive charging current, was 5.3 ( 0.7 × 10-5 C, whereas the charge transferred from a similarly composed monolayer on glassy carbon was 6.3 ( 0.8 × 10-6 C; thus, the nanofibers yield an 8-fold increase in redox-accessible ferrocene molecules. Measurements of the interfacial capacitance yielded an 11-fold increase, from 1.3 × 10-8 F for glassy carbon to 1.5 × 10-7 F on nanofibers. The 11-fold increase in capacitance with only an 8-fold increase in redox current suggests that the ferrocene groups are slightly less tightly packed on nanofibers compared with glassy carbon. Because the ferrocene groups are linked to the surface, the integrated charge corresponds to q*N, where N is the number of redox-active ferrocene groups on the surface, and q is the electron charge. Using the microscopic sample area, determined by the capacitance measurement, and

Grafting of Ferrocene to Carbon Nanofibers

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16913

TABLE 1: Parameters Measured for Ferrocene Attached to VACNF Surfaces through Undiluted Monolayers of Varying Length at a Scan Rate of 1 V s-1a linker

E° (mV)

∆Ep (mV)

k0 (s-1)

undecanoic acid heptanoic acid vinyl acetic acid

222 ( 5 215 ( 2 212 ( 4

90 ( 18 92 ( 20 89 ( 5

1.2 ( 0.4 1.1 ( 0.3 1.15 ( 0.10

a

All potentials reported vs Ag/AgCl.

assuming that the microscopic area of the glassy carbon corresponds to the geometric area of 0.20 cm2 exposed to the solution, we calculate that the effective area per ferrocene molecule on the VACNFs is 2.5 nm2, or equivalently, that the ferrocene molecules are separated on average by a relatively large distance of ∼1.6 nm. 3.3. Electron-transfer Rates and the Role of Chain Length. Previous studies have shown that for strongly adsorbed, reversible systems, the anodic and cathodic waves have the same peak potential in the limit as the scan rate decreases to zero. As a result, the peak-to-peak splitting for a fast redox couple is expected to collapse to zero at slow scan rates.36,37 We obtained CVs over a wide range of scan rates; Figure 4b summarizes the peak-to-peak splittings on the CVs of ferrocene on VACNF and glassy carbon as a function of scan speed. From these data it is clear that VACNFs have a larger peak-to-peak splitting (implying a slower electron-tranfer rate) over the entire range of scan rates. At the lowest scan rates the peak-to-peak splitting of ferrocene linked to carbon nanofibers through undecenoic acid was 90 ( 18 mV at 1 V s-1, which decreased to around 50 mV at 100 mV s-1. However, further reducing the scan rate had no effect on the peak splitting. This remaining ∼50 mV splitting implies some significant nonideality in the electrochemical response. To understand how the organic monolayers impact the electron transfer process, we calculated the electron-transfer rate constants from the peak-to-peak separation of CVs using the method of Laviron, who used numerical integration of the rate equations to correlate the standard electron-transfer rate constant k0 with the separation between cathodic and anodic peaks ∆Ep at different scan rates, V.36 This method uses a dimensionless rate constant m ) [k0(RT/nF)]/V, where R (universal gas constant), T (temperature), n (number of electrons transferred), and F (Farady’s constant) have their usual definitions. At peak-to-peak splittings below 200 mV, the rate constants are fairly insensitive to R, the charge transfer coefficient, so the rate constants were calculated directly from Laviron’s working curves of m-1 versus n∆Ep without first finding R.20 Using the values measured at 1 V/s, the electron transfer rate for ferrocene attached to VACNF electrodes through undecenoic acid was 1.2 ( 0.4 s-1. As expected from the large separation of oxidation and reduction peaks, this value is lower that for ferrocene layers prepared in the same way on glassy carbon, which yielded a rate constant of 3.3 ( 0.6 s-1. A second important variable in monolayer-tethered redox centers is the influence of the chain length of the organic monolayers. To test how the molecular length affects the electron-transfer process, we also obtained CV data using similar molecules having alkyl chains of lengths varying between 10 and 3 CH2 groups. These data were analyzed as described above, and the results are summarized in Table 1. Surprisingly, the data show that there is no significant change in the electrontransfer rate constant as the length of the carbon chain is varied from 3 CH2 groups (1.15 ( 0.10 s-1) to 6 CH2 groups (1.1 ( 0.3 s-1) to 10 CH2 groups (1.2 ( 0.4 s-1.) The length of the

Figure 5. (a) Cyclic voltammograms of ferrocene on monolayers diluted with dodecene. (b) Peak-to-peak splitting of attached ferrocene redox couple on diluted monolayers as a function of scan rate.

tethering chain slightly changed the redox potential of the ferrocene groups from 212 ( 2 mV for the shortest chain to 222 ( 5 mV versus Ag/AgCl for the longest, but otherwise had no significant effect. 3.4. Monolayer Dilutions and Molecular Heterogeneity. The width of the oxidation and reduction peaks can provide information on the degree of homogeneity of the monolayers.22 Previous theoretical work has shown that a full width at halfmaximum (fwhm) of 90.6 mV is expected for a homogeneous layer of noninteracting redox centers with reversible electron transfer.37-39 This value has been experimentally observed for ferrocene attached to self-assembled monolayers on gold surfaces.19 Peak broadening beyond the 90.6 mV value has commonly been attributed to interactions between neighboring redox-active molecules, leading to a range of redox potentials, or inhomogeneity in the molecular layer and variations in the local chemical environment of different ferrocene molecules.18,19,40 In cases where these interactions are important, peak widths are generally narrowed upon dilution of the monolayers.19,41 For carbon nanofiber surfaces with ferrocene attached to undiluted carboxylic acid groups, we typically observe a fullwidth at half-maximum (fwhm) of 128 ( 3 mV, substantially larger than the ideal limit of 90.6 mV. To determine whether the broadening we observe arises from interactions between nearby ferrocene molecules or chemical inhomogeneities, we prepared diluted monolayers by using 1:1 and 1:5 mixtures of 1-undecenoic acid and 1-dodecene to form mixed monolayers via photochemical grafting and then linking ferrocene to the undecenoic acid sites. Figure 5a shows two representative voltammograms obtained for ferrocene linked to nanofibers using 1:1 and 1:5 dilutions of the carboxylic acid with dodecene. Although a fwhm of 128 ( 3 mV was obtained on the dense layers, diluting the carboxylic acid groups with 1-dodecene in a 1:1 mixture yields 130 ( 6 mV and in a 1:5 mixture yields a peak width of 128 ( 11 mV; all three values are identical within experimental error. The redox potentials were also unchanged by dilution, yielding values of 222 ( 5 mV for the undiluted monolayer, 223 ( 4 mV using the 1:1 dilution, and 223 ( 14 mV for the 1:5 dilution. Finally, measurements at different scan rates, shown in Figure 5b, show that all three layers exhibit identical behavior. Using the Laviron method to calculate the electron transfer rates yielded 1.2 ( 0.4 s-1 for undiluted monolayers, and the 1:1 dilution was 1.3 ( 0.3 s-1,

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Figure 6. Cyclic voltammetry of glassy carbon and carbon nanofibers in (a) 1 mM Ru(NH3)2+/3+ and (b) 1 mM Fe(CN)63-/4- in 1 M KCl at 100 mV/s, vs Ag/AgCl.

and the 1:5 dilution was 1.01 ( 0.14 s-1. Although there was some variation in rate at the lowest scan rates, the electron transfer rates did not significantly vary between the three dilutions from scan speeds of 0.6-3.5 V/s. To verify that the use of mixed layers of 1-undecenoic acid and 1-dodecene did indeed dilute the ferrocene groups, we measured the total charge transferred in each CV scan by integrating the area under the CV peaks. Oxidation of ferrocene grafted to a layer formed from pure 1-undecenoic acid transferred 5.3 ( 0.7 × 10-5 C, whereas the 1:1 mixture of 1-undecenoic acid and 1-dodecene yielded 3.5 ( 0.3 × 10-5 C, and the 1:5 ratio of 1-undeceonic acid to 1-dodecene transferred 1.8 ( 0.3 × 10-5 C. These data show that the number of electrically active ferrocene molecules significantly decreases as the number of carboxylic acid-containing molecules in starting mixture is reduced and confirm that the use of mixed monolayers effectively dilutes the ferrocene molecules. Thus, our electrical measurements show that there are no significant changes in peak widths, electron-transfer rates, or redox potentials as the ferrocene monolayers are diluted. This clearly shows that interactions between nearby ferrocene groups do not have any significant effect on the electron-transfer kinetics or on the unexpected large peak widths observed. 3.5. Solution-phase Redox-active Probes. To probe the electron-transfer properties of the VACNF electrodes, we make use of prior work showing that the electron transfer rate constants of many redox couples are sensitive to the surface chemistry of carbon electrodes.11,42,43 These previous studies showed that outer-sphere electron transfer systems such as Ru(NH3)62+/3+ are sensitive to the electronic density of states but are insensitive to changes in the functional groups exposed at the surface. In contrast, the redox kinetics of Fe(CN)63-/4are very sensitive to the ratio of edge-plane-to-basal-plane graphite on the glassy carbon substrates.11 Thus, the electrochemical response of the VACNF electrodes in these two redox couples can probe the total surface area and the amount of edgeplane graphite exposed.42,43 Figure 6a shows CVs of bare (nonfunctionalized) VACNF and glassy carbon surfaces using the Ru(NH3)62+/3+ redox couple. The peak-to-peak splitting of 67 ( 6 mV measured on carbon nanofibers was not significantly different from the 61 ( 5 mV splitting observed on glassy carbon, demonstrating that the intrinsic electron transfer properties of the two substrates do not differ greatly. However, Figure 6b shows that when using

Figure 7. Impedance spectra of the bare, COOH-modified, and ferrocene-modified VACNF surfaces in 4.4 mM tetrabutylammonium hexafluorophosphate; lines are fit results: (a) absolute impedance; (b) phase angle; (c) equivalent circuit used in fitting the data in Figure 7.

Fe(CN)63-/4- (the redox couple sensitive to edge-plane graphite) a peak-to-peak splitting of 83 ( 13 mV was observed on VACNFs, compared with only 63 mV for glassy carbon. Although this difference is small, it is outside the error calculated from repeated experiments on at least three samples. The larger peak-to-peak splitting on VACNFs indicates that although VACNF samples have higher total surface area than planar glassy carbon samples, the VACNFs have a smaller fraction of the exposed area consisting of edge-plane graphite. 3.6. Electrochemical Impedance Spectroscopy. Although the electrical properties of the VACNF samples are strongly affected by the local atomic structure, the three-dimensional nature of the material also influences its electrical response. To more completely characterize the electrical response, we used electrochemical impedance spectroscopy (EIS) measurements. Those reported here were obtained at 0 V versus the Ag/AgCl reference electrode and are representative of spectra obtained at other potentials near the E° value for ferrocene. Figure 7 shows impedance data for bare VACNFs and VACNFs functionalized with 1-undecylenic acid before and after linking ferrocene through the amide linkage described above. Figure 7a shows the magnidue of the impedance vs frequency, and Figure 7b shows the phase shift. At the highest frequencies, the behavior of all three systems is almost entirely resistive, with a constant impedance of ∼2 × 103 Ω and the phase angle close to 0°; this limit represents the uncompensated solution resistance. As the frequency decreases, the system becomes more capacitive, with a phase angle decreasing to about -60° as the impedance rises. The bare VACNF surface reaches the highest total impedance, followed by the COOH-terminated sample and the ferrocene-terminated sample. At frequencies between 1 and 0.1 Hz, all samples reach a maximum phase angle of around -60° (compared with the ideal value of -90° for a perfect

Grafting of Ferrocene to Carbon Nanofibers

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16915

TABLE 2: Parameters Measured for Ferrocene Attached to VACNF Surfaces through Monolayers of Undecenoic Acid, Diluted with Dodecene in Varying Ratios at a Scan Rate of 1 V s-1+a

a

dilution

E° (mV)

∆Ep (mV)

Efwhm (mV)

area (C)

k0 (s-1)

undiluted 1:1 1:5

222 ( 5 223 ( 4 223 ( 14

90 ( 20 89 ( 12 114 ( 14

128 ( 3 130 ( 6 128 ( 11

5.3 ( 0.7 × 10-5 3.5 ( 0.3 × 10-5 2.2 ( 0.6 × 10-5

1.2 ( 0.4 1.3 ( 0.3 1.01 ( 0.14

All potentials reported vs. Ag/AgCl.

TABLE 3: Results of Fitting Data in Figure 7a and 7b to the Circuit Model Shown in Figure 7c surface

Rs

bare COOH FcNH2

2200 ( 300 2200 ( 100 2200 ( 20

Rct 1.7 × 10 ( 0.8 1.49 ×105 ( 0.08 8.6 × 104 ( 0.2 5

capacitor). Below this frequency the phase angle shifts back toward zero, reflecting Faradaic processes associated with charge transfer at the interface that is in parallel with the double-layer capacitance at the interface. The response in Figure 7 can be interpreted based on the work of deLevie, who investigated the response of surfaces containing cylindrical pores.44 Although our samples consist of cylindrical fibers, the interstices between the fibers are roughly equivalent to the pores of the deLevie model, enabling a similar qualitative interpretation of the data. DeLevie showed that for an array of pores, the electrical response can be modeled using the simple Randles cell model, similar to that shown in Figure 7c. In a porous sample, the variation in effective solution resistance along the length of the pores leads to a continuous distribution of time constants, such that the overall response can be described using a constant phase element (CPE). The CPE is defined through the relationship of the complex impedance Zˆ to the angular frequency ω and two additional real parameters, P and T, via the equation Zˆ ) 1/T(iω)P. The exponent P is a dimensionless number related to the porosity of the material, whereas T is a real number that has units defined such that Zˆ is in ohms. For P ) 1, this equation reduces to the expression for the impedance of a perfect capacitor, with T equal to the capacitance, whereas P ) 0.5 is characteristic of pores of infinite length.44,45 In the model shown in Figure 7c, the CPE represents the double-layer capacitance, and the parallel resistor (Rct) accounts for interfacial charge transfer resistance, and Rs represents the uncompensated solution resistance. Fitting the data in Figure 7a and 7b to the circuit in Figure 7c results in fits that are indistinguishable from the experimental data, demonstrating that this circuit is a good model for the system. Table 3 shows the actual values obtained for the four different fit parameters to the three surfaces. We first note that the two fit parameters associated with the solution resistance (Rs) and the CPE exponent (P) do not significantly change between the three different samples. The essentially constant value of Rs is in agreement with the expectation that the uncompensated solution resistance should only change with variations in the reference electrode placement and should be independent of surface characteristics. Similarly, the nearly constant value of P ≈ 0.83 demonstrates that all three samples are similar in their physical structure; the value of P ≈ 0.83 is appropriate for a forest of nanofibers that are significantly shorter than the length of ∼30 µm required to reach deLevie’s “infinite limit” (P ) 0.5) but exhibit much greater microscopic roughness than a perfectly planar surface (P ) 1). This leaves the charge transfer resistance (Rct) and the T parameter of the CPE to account for the changes in the impedance spectra between the surfaces with different functionalization. Table 3 shows that the grafting of the carboxylic

T

P

capacitance (µF/cm2)

5.4 × 10 ( 0.6 3.2 × 10-5 ( 0.2 3.3 × 10-5 ( 0.8

0.83 ( 0.01 0.81 ( 0.02 0.83 ( 0.02

410 ( 30 260 ( 10 250 ( 40

-5

acid layer to the nanofibers decreases the charge-transfer resistance (Rct) by only a small amount, from (1.7 ( 0.8) × 105 Ω (bare VACNFs) to (1.49 ( 0.08) × 105 Ω (acidterminated), whereas linking of ferrocene onto the acidterminated surface significantly decreases Rct to (8.6 ( 0.2) × 104 Ω. Grafting the molecular layer to the bare VACNFs decreases T from (5.4 ( 0.6) × 10-5 to (3.2 ( 0.2)x10-5, but linking ferrocene to the surface leads to no further change, maintaining T at (3.3 ( 0.8) × 10-5. Again, these results lead to the surprising conclusion that the grafting of the carboxylic acid molecular layer to the VACNFs does not significantly impact the overall charge-transfer kinetics. An effective capacitance (Ceff) can be calculated from the P and T parameters of the CPE using the equation Ceff ) T(ωmax)P-1, where ωmax is the frequency at which the imaginary component of the impedance reaches its maximum value.46 These results are included in Table 3. It is notable that the capacitance of 410 ( 30 µF/cm2 for the bare surface is reduced to 260 µF/cm2 upon grafting of the 1-undecylenic acid layer. The impedance spectroscopy results show that grafting the carboxylic acid layer to the surface has little effect on the interfacial resistance but significantly decreases the capacitance. Linking of the ferrocene to the acid-terminated surface significantly decreases the interfacial resistance, with little or no further change to the capacitance. Grafting of molecular layers to surfaces is usually modeled as an additional capacitance in series with the double-layer capacitance, such that the total capacitance (CT) is related to the monolayer capacitance (CM) and the double-layer capacitance (CDL) by CT-1) (CM-1 + CDL-1).47,48 In this case, the decrease from 410 to 260 µF/cm2 would correspond to a molecular layer having a capacitance of 710 µF/cm2. This value is unphysically high even after accounting for the increased surface area of the VACNFs. Previous work with monolayers of similar length have reported values of ∼1.5 µF/cm2 on gold surfaces48 and 2.3 µF/cm2 on silicon surfaces;49 even accounting for a ∼10-fold increase in microscopic surface area for the VACNFs, the value of ∼710 µF/cm2 is unphysically large. However, in this series capacitance, a small change in total capacitance is manifest as a large apparent interfacial capacitance. The large apparent value we observe is consistent with a sparse molecular layer in which the 1-undecylenic acid molecules link to only a fraction of the total area. 4. Discussion The ability to use molecular layers as tunable interfaces that will facilitate electron-transfer processes at the surfaces of nanofibers and other forms of nanostructured carbon ultimately depends on the electron-transfer properties of the functionalized interfaces. Consequently, we focus our attention on understand-

16916 J. Phys. Chem. C, Vol. 112, No. 43, 2008 ing how the molecular structure of the VACNFs and of the monolayers linked to the VACNF surfaces impact the electrical properties. Our data show that ferrocene molecules covalently grafted to VACNF surfaces exhibit standard electron-transfer rates of ∼1.0 to 1.3 s-1. These values are quite low compared with prior work for ferrocene tethered to gold surfaces.19,22,50 Recent studies linking ferrocene to self-assembled monolayers on gold using an amide linkage identical to ours yielded standard rate constants of ∼200 s-1,50 whereas ferrocene moieties linked without any intervening amide group yielded values of ∼104 s-1.22 Although the rates on gold are substantially lowered upon dilution, the values we observe are substantially lower than values previously measured on gold surfaces. One significant contribution to this substrate dependence is that electron-transfer rates are strongly dependent on the density of states of the electrode. Gold is an excellent metal with a high density of states near the Fermi energy, whereas VACNFs are based on graphite, a semimetal with a much lower Fermi-level density of states. The density of states at the basal plane of highly oriented pyrolitic graphite (HOPG) is ∼50 times smaller than that of gold;51 thus, the relatively slow electron-transfer that we observe at VACNF electrodes is in large part a consequence of the low density of states of graphitic carbons. One surprising result from our work is that there is that the electron-transfer rates do not depend on the length of the molecules constituting the grafted monolayer; this result is quite distinct from most previous work. Previous studies of selfassembled monolayers on gold have typically studied dense, close-packed layers that are very effective at blocking electrontransfer into solution.19-21,52,53 Intercalation of redox-active moieties into these insulating layers leads to electron-transfer rates that are strongly dependent on the chain length of the tethered molecules and redox peaks (in CV measurements) whose width increases with increasing surface density due to interactions among neighboring probes. The length dependence is usually attributed to tunneling of electrons through the molecular layers,20-22,40 leading to electron-transfer rates that exponentially decrease with increasing layer thickness. The absence of any length dependence on VACNFs (Table 1) suggests that the mechanism of electron transfer is not a simple through-bond tunneling processes, but instead suggests a more complex molecular structure. Our data show that the electron-transfer properties of ferrocene tethered to VACNFs is consistent with a structure consisting of monolayers that are not fully dense and that have a relatively high degree of structural disorder. As noted above, a comparison of glassy carbon with VACNFs shows that although there is an ∼11-fold increase in capacitance, there is only an 8-fold increase in the density of electrically active ferrocene molecules. The impedance data in Figure 7 show that grafting 1-undecylenic and then ferrocene to the surface leads to a decrease in capacitance from 420 to 260 µF/cm2, but this change is significantly smaller than what would be expected from a densely packed molecular layer. Ultimately, our data show that the ferrocene molecules linked to VACNFs have a structure like that depicted in Figure 8, in which the undecylenic acid molecules are sparsely bonded to the nanofibers, most likely with the molecules preferentially bonding at the graphitic edge-plane sites. We note that unlike most single- and multiwalled carbon nanotubes, the VACNFs used in our studies have a structure (see Figure 1) in which graphene cones intersect the VACNF walls at an angle.5,7 Transmission electron microscopy measurements of our nanofi-

Landis and Hamers

Figure 8. Space-filling model of VACNF electrode with exposed edgeplane sites 2 nm apart. This model depicts undecylenic acid molecules preferentially bonded at the exposed edge-plane sites. The topmost molecule is shown fully extended, and the lower ones show moretypical configurations that are expected.

bers (not shown) show an angle of intersection of approximately 8°; consequently, the VACNFs studied are comprised of narrow rings of exposed edge-plane graphite, separated by approximately 2-3 nm of basal plane graphite.54 Figure 8 depicts a space-filling, side-view model of a VACNF with edge-plane sites separated by 2 nm, with ferrocene undecylenic acid grafted at these edge-plane sites. Although the molecule at the top is shown in fully extended form, in such a loose layer there will a high degree of conformational flexibility such that molecules will adopt a variety of conformations like the lower two molecules shown. Support for this model comes from several experiments. First, it explains the relatively small change in interfacial capacitance observed in the impedance spectroscopy data upon grafting of the undecylenic acid. Further support comes from integrating the charge transferred during cyclic voltammetry peaks; using the 11-fold increase in capacitance between glassy carbon and VACNFs allows us to estimate the microscopic surface areas of the exposed fibers and allows us to determine that the average separation between ferrocene groups of ∼1.6 nm. This is somewhat larger than the distance between ferrocene molecules within self-assembled monolayers on gold, which approximated ferrocene molecules as spheres 6.6 Å in diameter, and suggests that the ferrocene coverage on the VACNF surfaces is ∼40% of a densely packed monolayer.

Grafting of Ferrocene to Carbon Nanofibers The structure in Figure 8 is also consistent with our observation that the ferrocene molecules linked to nanofibers have some significant heterogeneity and that the rates are essentially independent of the length of the molecular chain. Previous studies of ferrocene groups linked to gold surfaces19 showed that at high densities, electron exchange between ferrocenes leads to enhanced electron-transfer rates dominated by a small number of high-conductivity defects; as the monolayers are diluted, the electron-transfer rates decrease to ∼5.1 s-1 at 10% mole fraction ferrocene. At high concentrations, interactions among the ferrocene groups broadens the peak width, reaching an ideal fwhm of 90 mV when the dilution was 1:3 or higher.19 Although the rates of ∼1.3 s-1 we observe on nanofibers are somewhat smaller than values on gold, the larger fwhm of 128-130 mV and the absence of change upon dilution demonstrates that the unusually large peak width on nanofibers cannot be attributed to ferrocene-ferrocene interactions but most likely arises from other inhomogeneities in the local environment. The model in Figure 8 shows that such inhomogeneity likely arises from the fact that the sparse packing of the layers leads to a high degree of conformational flexibility within the chains. Thus, the average distance from ferrocene molecules to the surface likely does not scale simply with length. Although the present work shows no direct evidence for preferential grafting of the molecular layers at the edge-plane sites, in recent studies we have found that photochemical grafting of molecular layers to carbon-based materials is initiated by the UV-excited photoemission of electrons, yielding positively charged “holes” at the surface that react with the alkene (CdC) groups.29,55,56 Because graphitic edge-plane sites have a much high density of states near the Fermi energy and higher electron-transfer rates11 than basal-plane sites, it is reasonable to assume that edge-plane sites are likely to be preferential sites of photoelectron emission and also more reactive locations for grafting of the molecular layers. We anticipate that the molecular layers and the associated ferrocene molecules will bind tightly around the rings of exposed edge-plane sites, but since the edgeplane sites are separated by more than the ∼6.6 Å diameter of an individual ferrocene molecule19 there will be gaps between the edge-plane sites that are not functionalized or occluded by ferrocene molecules. 5. Conclusions Our results demonstrate the molecular layers tethered to vertically aligned carbon nanofibers can serve as good attachment points for linking electrochemically active molecules to the VACNF surfaces. Although most prior studies of electrontransfer have focused on self-assembled monolayers on gold, the inherent instabilities resulting from the gold/thiol bond lead to electrochemical desorption between -0.7 and -1.4 V20 as well as chemical instability. The very high chemical stability and high surface area of VACNF electrodes makes them an attractive platform for linking complex electrocatalytic molecules to electrodes. Our results show that the molecular layers formed on VACNFs are loose and disordered due to the molecular structure of the nanofibers. These properties are intrinsically linked to the molecular structure of the nanofibers, which presents significant amounts of edge-plane graphite along the sidewalls. This behavior is quite distinct from the more commonly observed carbon nanotubes, which present edge-plane sites only at the ends. Ultimately, our results demonstrate that the ability to chemically functionalize VACNFs with redoxactive molecular species makes them an attractive platform for investigating more complex electrocatalytic reactions.

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