J. Phys. Chem. 1996, 100, 17041-17049
17041
Nanometer-Scale Electropolymerization of Aniline Using the Scanning Tunneling Microscope Ralph M. Nyffenegger and Reginald M. Penner* Institute For Surface and Interface Science, Department of Chemistry, UniVersity of California, IrVine, IrVine, California 92717-2025 ReceiVed: July 17, 1996X
The platinum tip of a scanning tunneling microscope is employed to direct the electropolymerization of aniline on nanometer-scale regions of a graphite surface which is immersed in an aqueous, aniline-containing electrolyte. The electropolymerization of aniline is initiated by two bias voltage pulses which are applied in rapid succession: The firsts(tip-negative) 6.0 V × 5 µsscauses the formation of a pit in the graphite surface, and the seconds(tip-negative) 3.0 V × 50 µssinduces the oxidation and the subsequent polymerization of aniline at this pit to form a particle of polyaniline (PANI). These PANI particles varied from 100 to 600Å in diameter and 10 to 200 Å in height. Nanoscopic PANI particles prepared by this method are shown to be electrochemically responsive: A comparison of in-situ STM images of individual PANI particles acquired at sample potentials which are positive and negative of the PANI oxidation potential reveals that the volume of a particle is larger by ≈30% in the oxidized emeraldine form than in the reduced leucoemeraldine form as a consequence of the higher solvent and anion content of the oxidized PANI particle. The results of in-situ STM experiments, ex-situ electrochemical measurements, and Monte Carlo simulations of transport collectively provide information on the mechanism of PANI particle growth. On the basis of these data, it is postulated that the aniline involved in PANI particle synthesis is derived from an adsorbed monolayer at the graphite surface.
I. Introduction In the past five years, the modification of surfaces on either the atomic or nanometer scale using scanning probe microscopes (SPM) has been reported in more than 300 publications.1-3 While a wide variety of mechanisms have been proposed to account for the modifications which are observed in various experiments, surprisingly, polymerization has been invoked as a mechanism in just a handful: Sasano et al.,4 observed the localized polymerization of pyrrole with submicrometer scale spatial selectivity on graphite substrates using the scanning tunneling microscope (STM); Kranz et al.5 polymerized pyrrole using a scanning electrochemical microscope (SECM), producing lines 50 µm in width and 1 mm in length; Wuu et al.6 used SECM to polymerize aniline in a anilinium sulfate-containing layer of Nafion spin-coated on a Pt electrode; Borgwarth et al.,7 oxidized bromide to bromine at the tip of an SECM, with the bromine then diffusing to a conductive substrate covered with a thiophene derivative causing the oxidative polymerization of this monomer to form polythiophene. A line having a 20 µm width was obtained using this approach. Finally, the localized polymerization of pyrrole was reported by Yang et al.8 In all of these cases, SPM-induced polymerization has purportedly involved the electrochemical oxidative polymerization of monomers (e.g., pyrrole, aniline) from an electrolyte solution (or an ionomer film6) using the tip of a STM or a SECM. This oxidation is expected to yield cation radicals which couple to yield the corresponding conductive polymer, e.g., polypyrrole or polyaniline. Consistent with this expectation, protrusions on surfaces such as mounds or lines have been generated, and based on the deposition conditions, these protrusions have been assumed (very reasonably) to be composed of conductive polymer. * Address correspondence to this author at
[email protected]. X Abstract published in AdVance ACS Abstracts, October 1, 1996.
S0022-3654(96)02145-4 CCC: $12.00
This previous work has demonstrated an important concept: namely, that a STM can be employed to locally modify surfaces under conditions which favor a modification mechanism involving a rather specific chemical transformation. Beyond the demonstration of proof of concept, these observations provoke important, and as yet largely unanswered, questions: Is the mechanism of polymerization truly electrochemical? Is the material which is deposited a conductive polymer (i.e., are characteristics of conductive polymers discernible)? What is the mechanism of polymerization, especially as concerns the disposition of precursor species in the system (e.g., is monomer adsorbed at the tip, on the surface, dissolved in solution, etc.). In this paper, we seek answers to these three questions for the STM-directed oxidative polymerization of aniline in aqueous HCl solutions at highly oriented pyrolytic graphite (HOPG) surfaces. Additionally, using this system as a starting point, an even more general question is addressed: What are the capabilities and limitations of the scanning tunneling microscope (STM) for effecting highly localized electrochemical transformations at surfaces? The technique we employ for aniline polymerization has evolved from a method we first reported in 1992 for the STM-directed deposition of nanoscopic metal particles on graphite surfaces.9,10 In addition to in-situ STM data, the results of conventional electrochemical measurements and Monte Carlo computer simulations have been employed to elucidate the mechanism responsible for the formation of polyaniline nanostructures. II. Experimental Section A standard one-compartment, three-electrode cell was used for bench-top electrochemical experiments. Electrolytes were all certified ACS grade from Aldrich and were used without further purification. Nanopure water was used for the preparation of all aqueous solutions. Either a saturated calomel electrode (SCE) or a saturated mercurous sulfate electrode © 1996 American Chemical Society
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(SMSE) was employed as references; however, for purposes of consistency, we have expressed the potentials in all diagrams versus the SCE. Both platinum and basal plane-oriented graphite working electrodes were employed. Platinum disks with diameters from 0.5 to 2 mm were prepared by embedding platinum wire of the appropriate diameter into Kel-F. These surfaces were prepared by polishing with successively finer alumina powders: 1.0, 0.3, and 0.05 µm. Freshly polished electrodes were rinsed and shortly ultrasonicated and then immersed in deareated 0.1 M H2SO4, and the potential was cycled between -650 and +650 mV vs MSE until voltammetry characteristic of clean polycrystalline platinum was observed. Highly oriented pyrolytic graphite surfaces were freshly cleaved prior to use. A 0.45 cm2 region of the basal plane was then exposed in an electrode holder by means of an O-ring seal to the solution of interest. Conventional voltammetry measurements were obtained using an EG&G 273 potentiostat. For capacitance-voltage measurements, an EG&G M5310 lock-in amplifier was employed in conjunction with the 273 and Princeton Headstart software. Capacitance-voltage (C-V) curves were obtained using an ac modulated voltage having a 3-4 mV root-mean-square amplitude. Frequencies between 20 and 50 Hz were employed as well as potential scan rates of 5 mV/s.11,12 Before acquiring C-V data, 3-4 complete voltage cycles were collected, allowing the voltammetry to attain steady state. Scanning probe experiments were performed with a commercial instrument which, in the STM mode, permitted fourelectrode, in-situ measurements (AutoProbe CP from Park Scientific Instruments (PSI), Sunnyvale, CA). The microscope was also operated in a two-electrode mode during aniline polymerization; in this case, voltage pulses outputted from an arbitrary function generator (Hewlett-Packard 33120A) were coupled into the sample. The cell employed in the STM was constructed from Plexiglas and is based on earlier designs;13,14 an Ag/AgCl reference and a Pt counter electrode were used for the four-electrode setup. Platinum STM tips were etched and insulated with a polystyrene derivative as described earlier.15 III. Results and Discussion Primarily because of its air stability and compatibility with aqueous solutions, polyaniline is an excellent candidate for investigations of STM tip directed electropolymerization. PANI is readily synthesized by the oxidative electropolymerization16 of aniline in acidic electrolytes via an overall three-electron oxidation:17 n
NH2 + (n/2)A– H N
N
+ 3ne– + 3nH+
(1)
+
A-
where A- is a charge compensating anion, and the polymer is shown in its protonated and electronically conductive “emeraldine” form. Emeraldine form PANI may be reversibly reduced at ≈0.15 V vs SCE to yield leucoemeraldine, which is not electronically conductive. At graphite electrode surfaces, the sequence of reactions summarized by eq 1 is initiated by the oxidation of aniline to a cation radical at ≈+0.7 V vs SCE (0.2 M aniline, 1.0 M HCl). These cation radicals couple to produce aniline dimers, oligomers, and eventually PANI. Spatially Selective Aniline Polymerization and PANI Nucleation and Growth. In principle, the reaction indicated by eq 1 can be accomplished with nanometer-scale spatial
Figure 1. Bias voltage pulse applied between the STM tip and sample (and plotted as the sample voltage versus time) used to generate nanoscopic polyaniline particles in this paper.
selectivity at a graphite surface using the platinum tip of a scanning tunneling microscope (STM) as a cathode. In order that the polymerization occur with spatial control, however, two criteria must be satisfied. First, the proximity of the platinum cathode to the graphite surface and its small size must combine to generate a current density distribution on the graphite surface which is sharply peaked under the tip. Second, the insoluble oligomers resulting from radical coupling must nucleate on the graphite surface at the location of the tip on the surface to yield a polymer particle. On highly oriented pyrolytic graphite (HOPG) surfaces, this latter requirement is potentially problematic since the HOPG basal plane has a low defect density, and preexisting defects on the surface are likely to be active sites both for monomer oxidation and for nucleation of PANI. Previously, we have shown that both of these design criteria can be satisfied in the case of metal nanostructure deposition at graphite surfaces using the platinum tip of an STM.18,10,9,19 A key feature of this deposition scheme is that the tip-sample bias pulse responsible for metal nanostructure deposition also causes the formation on the graphite surface of a shallow pit. This pit formation event precedes the deposition of metal by a few microseconds, and the step edges associated with the pit provide high coordination sites for the nucleation of a metal deposit. We have adapted this strategy to the localized polymerization of aniline using the pulse sequence plotted in Figure 1: First, a pit in the graphite surface is generated using a tip-(-), 6.0 V × 5 µs initial voltage “spike”,20 and then the tip-sample bias is lowered to -3.0 V for an additional 45 µs to effect the polymerization of monomer at the pit generated by the initial voltage spike. The amplitude of the initial voltage spike is approximately 2.0 V higher than the damage threshold for HOPG in water in this type of bias pulse experiment of ≈4.0 V.20 This extra voltage improves the probability of pit formation from ≈50% at the threshold to >90%. The STM image of Figure 2 provides a demonstration of this fact: This in-situ STM image shows a 0.7 µm2 region of the graphite surface where seven pits (one of these immediately adjacent to the step edge crossing the bottom of the image) have been produced by the application of seven separate bias pulses in aniline-free 0.1 M HCl. Each pit produced in this experiment has a diameter of approximately 200 Å and an apparent depth of 1 or 2 atomic layers. In other experiments, the pit diameter varied from 100 to 500 Å. Pit formation occurs with equal reliability for tip(+) bias pulses, and the success rate is unaffected by the exclusion of the 3.0 V × 45 µs portion of the waveform of Figure 1. On the basis of these characteristics and on the welldefined 4.0 V threshold for this process in water, we have
Electropolymerization of Aniline
Figure 2. STM image of a region of the graphite basal plane acquired in aniline-free, aqueous 0.1 M HCl following the application of seven separate applications of the bias pulse of Figure 1. Successive bias pulses were spaced by ≈8 min intervals. VB ) 0.10 V, It ) 2.0 nA.
J. Phys. Chem., Vol. 100, No. 42, 1996 17043 The cross section of Figure 3B (taken diagonally through the three larger nanostructures at bottom) shows that the height of each is approximately 15 Å, and the diameter is about 300 Å. In other experiments conducted under identical conditions of aniline and electrolyte concentrations, nanostructure dimensions varied from 200 to 1000 Å in diameter and from 10 to 100 Å in height. The nanostructures produced under these conditions are stable in size and position when imaged with currents smaller than ≈5 nA but can be removed by the tip when higher set points are employed. That particles that those shown in Figure 3 are associated with the presence of aniline in the electrolyte is demonstrated by control experiments conducted in anilinefree electrolytes where only pits are generated, as in the experiments of Figure 2. However, this control does not establish that the particles obtained in the presence of the monomer are the corresponding polymer. STM Observations of Electrochemical Response. We have attempted to demonstrate that the particles generated by this method are PANI by probing an oxidation-state-dependent property of PANI which should be readily observable from STM images of a PANI particlesthe apparent particle volume. The word “apparent” here recognizes the fact that both the electronic conductivity of PANI and the real volume of this material are known to change with the oxidation state of PANI. A real volume effect is caused by the influx and efflux of charge compensating anions and solvent as the fixed ion content of the polymer changes with its oxidation state. This switching reaction is indicated in the mechanism shown below:21 H N
H N (y – x)
–ye– – xH+ + (y – x)A–
H N
Figure 3. (A) STM image obtained acquired in 10 mM aniline, 0.1 M HCl after applying a series of five bias pulses. Successive depositions were spaced by ≈8 min intervals. VB ) 0.35 V, It ) 2.8 nA. (B) Amplitude trace taken diagonally through the three nanostructures at the bottom of (A).
previously postulated20 that pit formation does not involve an electrochemical mechanism but instead is electric field driven. The mechanistic details of this process, however, are unresolved. With the addition of aniline to this electrolyte at a concentration between 1 and 100 mM, the same bias pulse produces particles in addition to pits. Figure 3A shows an in-situ STM image of a region of a graphite surface following the application of five bias pulses. In this particular experiment, one larger nanostructure was produced by each bias pulse, and in addition, a smaller particle was generated just below the first as well.
H N+ A-
H N+ H A-
H N+ H
x
(2)
–ye– + xH+ – (y – x)A–
H N+ A-
H N 0.5y
In addition to this real volume effect, the electronic conductivity of PANI is also strongly oxidation-state-dependent: PANI has a high electronic conductivity (σ > 100 Ω-1 cm-1) in the oxidized emeraldine form (eq 2, bottom) and a lower conductivity (σ < 0.1 Ω-1 cm-1) in the reduced leucoemeraldine form. We have found that the electronic conductivity of an individual PANI particle is difficult to directly measure using the STM, and in any case, the effect of the conductivity on the STM image of the particle is not readily deduced. Qualitatively, however, reduction of the PANI film causes a decrease in the density of states and a reduction in the conductivity, and both of these effects are likely to cause an increase in the barrier for tunneling, thereby causing the tip of a tunneling microscope to move somewhat closer to the reduced PANI particle as compared with an oxidized particle. Therefore, the electronic and the real volume effects should operate in tandem to bring about a reduction in the size of a reduced PANI particle as compared with an oxidized one. This expectation has been borne out in the STM measurements of PANI films on electrode surfaces reported by Nyffenegger et al.22 On the basis of this previous work, the apparent PANI particle volume measured by STM should provide a convenient means by which to determine whether or not the nanoscopic particles prepared using the method described above are electrochemically responsive. Particles generated in aniline-containing solutions were imaged at substrate potentials where PANI is oxidized (+0.35
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Figure 4. Plot of the nanostructure volume obtained from integrating the line scans of a PANI particle having a diameter of 800 Å, as a function of time obtained from a series of in-situ STM images. In this experiment, the potential of the graphite surface during STM image acquisition was adjusted to be either -0.10 or +0.4 V vs SCE as shown. These two potentials bracket the oxidation potential of PANI.
Figure 5. Cyclic voltammograms of HOPG in 0.5 MHCl (ν ) 50 mV s-1). The dashed trace was obtained for a freshly cleaved HOPG. In the solid trace, the electrode was first exposed for ≈5 min to the same electrolyte containing, in addition, 10 mM aniline. At the initial in these scans of +0.6 V, the adsorbed aniline is polymerized to form PANI. Voltammetric waves for the oxidation and reduction of the resulting PANI, centered at +0.4 V, are evident in this voltammogram.
V vs SCE) and reimaged at potentials (e-200 mV) which are negative of the polymer reduction potential. This cycle was repeated several times, and the volume of the particle was calculated from each image by integrating the backgroundcorrected line scans.23 Results for a typical series of experiments are shown in Figure 4. In Figure 4A is shown an oxidized PANI particle which was subjected to three reduction-oxidation cycles in the STM. Following the reduction of this particle at -0.2 V vs SCE, two images yield volumes of 750 and 790 nm3. The particle was then oxidized for 5 min at +0.35 V vs SCE, and two additional STM images were acquired which yielded identical volumes of 950 nm3, corresponding to a 28% increase. The reversibility of this volume change was demonstrated by repeating this redox cycle three timessin these subsequent cycles, the volumes of the reduced and oxidized particles observed in the first redox cycle were essentially reproduced, as shown in the plot of Figure 4B. As already noted, this redoxstate-dependent volume effect is qualitatively the same as that which has been seen for electrochemically synthesized PANI films on graphite or gold surfaces.22 This fact leads to the conclusion that the particles deposited from aniline-containing HCl are PANI. Disposition of Precursor Aniline Monomer: Electrochemical Detection of Adsorbed Aniline at the STM Tip and on the Sample Surface. As a necessary prerequisite to understanding PANI nanostructure growth in the STM, we wish to determine the location of “precursor aniline”, that is, aniline directly involved in PANI nanostructure formation. Three types of monomer can be distinguished based on the location it occupies in the system prior to the application of a bias pulse. Aniline may be (1) adsorbed on the graphite surface, (2) adsorbed on the platinum STM tip, and (3) dissolved in the electrolyte. There is little doubt that dissolved aniline is present at approximately the bulk concentration in the electrolyte present between the STM tip and sample. It is less clear, however, whether aniline is adsorbed on either or both the platinum STM tip and the graphite sample surfaces. Two different bench-top electrochemical measurements have been implemented at macroscopic HOPG and platinum electrodes surfaces to look for the presence of this adsorbed monomer under conditions of potential and electrolyte composition which are identical to those existing in the STM-directed polymerization experiment. The first of these involves the
indirect detection of adsorbed aniline monomer using a scheme in which the adsorbed monomer is first polymerized to form PANI, and the quantity of PANI is then assayed electrochemically. As described in the Experimental Section, this experiment involves equilibrating an electrode surface (either HOPG or platinum at open circuit) in the 10 mM aniline, 0.10 M HCl solution used for STM-directed polymerization, removing the electrode from this “loading” solution, rinsing it with water, transferring the electrode to an aniline-free HCl solution, and polymerizing the monomer using a single, positive-going voltammetric scan of the surface over the potential where aniline polymerization occurs. Once the aniline was polymerized, cyclic voltammetry was used to determine the quantity of monomer initially adsorbed. This indirect assay has two limitations: First, because this procedure involves rinsing of the electrode surface, only aniline which was strongly adsorbed to the surface is detected. Second, the polymerization of adsorbed aniline may not occur with perfect efficiency, and short-chain aniline oligomers may not remain on the electrode surface following polymerization. For both of these reasons, the coVerage of aniline estimated from this indirect assay must be considered a lower limit to the actual aniline coVerage. Although we have performed identical experiments at both HOPG and platinum surfaces, we illustrate only the HOPG results with figures since our results obtained with platinum are essentially in agreement with prior work on adsorption of aniline on metals.24-27 A voltammogram for an HOPG electrode treated according to this procedure and acquired following voltammetric polymerization is shown in Figure 5 which shows the first redox peak obtained on HOPG following the polymerization potential step. Integration of this peak yields a charge of 47 ( 11 µC cm-2 corresponding to (4.9 ( 1.2) × 10-10 mol cm-2 whereas for platinum electrodes a charge of 35 ( 6 µC cm-2 or (3.6 ( 0.6) × 10-10 mol cm-2 was obtained. It was also attempted to determine the effect of the electrode potential on the quantity of aniline adsorbed by controlling the potential of the HOPG or platinum surface during the loading step; however, these measurements yielded no statistically significant variations in the aniline loading. Unfortunately, the standard deviation for the aniline coverage observed at each potential are substantial (≈20%) at both HOPG and platinum surfaces, and this irreproducibility prevents subtle variations in the aniline coverage from being measured by this in direct method. It is nevertheless apparent that, on both HOPG and
Electropolymerization of Aniline platinum surfaces, aniline is adsorbed at potentials both positive and negative of the pzc, i.e., ca. -250 mV vs SCE for graphite and around -150 mV for platinum. In addition to providing an estimate of the absolute monomer coverage on these two surfaces, these numbers yield some information on the orientation of the monomer. Two limiting cases exist for the adsorption geometry: flat and edge-oriented. If aniline is adsorbed flat, monolayer coverage corresponds to 3.4 × 10-10 mol cm-2 (33 µC cm-2, assuming one electron per molecule) or 49 Å2 molecule-1, whereas in an “edge-on” adsorption geometry, a coverage of 8.8 × 10-10 mol cm-2 (85 µC cm-2) or 19 Å2 molecule-1 is found.26 At platinum electrode surfaces, then, the observed coverage (35 ( 6 µC cm-2) is consistent with a flat adsorption geometry for aniline (giving 33 µC cm-2). This is especially true when it is considered that the roughness factor for the platinum surface obtained from H adsorption measurements is 1.2. At HOPG surfaces, in contrast, the observed coverage of 47 ( 11 µC cm-2 is consistent with a mixed monolayer in which both the flat and edge-on adsorption geometries are present. However, the reproducibility of this number is poorer than in the platinum case, reflecting, we believe, the intrinsic irreproducibility of the HOPG basal plane surface which is obtained from cleave to cleave. A second measurementsof the capacitance versus the applied voltage (CV)saffords a means of directly detecting the presence of the adsorbed aniline monomer. The sensitivity of the C-V measurement to the presence and structure of an adsorbate layer derives from the fact that this adsorbate layer substantially alters the structure of the electrochemical double layer (relative to its structure in the absence of this layer). Because the orientation of the monolayer also effects the capacitance of the electrode surface, C-V measurements are able to detect transitions of the monomer layer between different adsorption states at different potentials (edge-on adsorption and flat adsorption, for example). For organic molecules, a maximum adsorption is expected at the potential of zero charge (pzc); at positively charged electrodes (i.e., electrodes poised at potentials positive of the pzc), a mixture between the flat and the “edge-on” arrangement is expected with a potential-dependent ratio.25 On the negative side of the pzc on the other hand, a “head-on” configuration is expected; i.e., the amine moiety of the molecule is closest to the surface. Experimentally, C-V data were acquired by measuring the capacitance at 20-50 Hz by superimposing a small (3-4 mV) sinusoidal signal on the dc working electrode voltage. HOPG and platinum electrode surfaces were prepared the same way as for the “loading” experiment, and C-V data were acquired in the monomercontaining HCl electrolyte. In addition, “control” C-V curves were obtained for both electrodes in aniline-free HCl to establish the base line capacitance of the surface and to locate the potential of zero charge (pzc). In Figure 6 are shown typical C-V curves for both control (A) and aniline-exposed (B) HOPG surfaces. Two differences between these curves are evident: First, the capacitance is lower than in the aniline-free case. Second, in the case of graphite in the monomer-free electrolyte a clear minimum in the capacitance curve is visible (indicating the location of the pzc28), whereas in the monomer-containing solution a relative maximum occurs at the same potential. This peak in the C-V curve is indicative of a transformation between adsorption states or adsorption/desorption of the monomer. When the analogous experiments were performed with platinum, the same clear distinctions between C-V curves in the presence and absence of aniline were not obtained. In part, this may be explained by the fact that aqueous HCl is a rather unfavorable electrolyte in which to measure the pzc of platinum: Chloride
J. Phys. Chem., Vol. 100, No. 42, 1996 17045
Figure 6. Capacitance-voltage curves, acquired using a voltage scan rate of 5 mV s-1, for HOPG immersed in (A) 0.05 M HCl. A minimum in the capacitance in observed at ca. -250 mV vs SCE. (B) 10 mM aniline, 0.05 M HCl. A peak in the capacitance curve at the pzc is attributed to the transformation of the aniline adsorption state.
adsorbs rather strongly on platinum, and hydrogen evolution gives raise to a pseudocapacitance in the C-V measurement. In summary, the results of the indirect assay clearly indicate the presence of approximately one monolayer of adsorbed aniline at both the platinum STM tip and at the HOPG basal plane surface under conditions mimicking those of the STMdirected polymerization experiment. Capacitance-voltage measurements provide a second, and experimentally direct, confirmation of the presence of adsorbed aniline at the HOPG surface. Disposition of Precursor Aniline Monomer: Lower Site Selectivity for the STM-Directed Aniline Polymerization Reaction at the HOPG Substrate Surface. In the case of STM-directed deposition of metal, we have shown that nanoscopic particles can be deposited with nearly perfect site selectivity. That is, a metal particle will be deposited at the position of the STM tip on the graphite surface irrespective of the proximity of this site to preexisting defects such as step edges on the surface. It is important to note that metal is clearly not adsorbed on HOPG under the conditions of these experiments. (The graphite surface is positively charged.18) The same sort of site selectivity is not observed for the STM directed polymerization of aniline and this fact leads to the conclusion that aniline adsorbed at the tip is not primarily responsible for PANI nanostructure growth. The experiment shown in Figure 7 provides a clear demonstration. In this experiment, four PANI particle were first deposited using four successive bias pulses. Then the PANI particles were stripped from their nucleation sites on the graphite surface by “imaging” these particles with a high tunneling current of 10 nA. The in-situ STM image of Figure 7A shows the pits which remain following the removal of PANI. Finally, after 5 min was allowed to elapse, a single pulse was applied at a point in the center of the four pits which is marked by the letter “X” in Figure 7B. This single pulse generates PANI at all four of the nucleation sites as shown in Figure 7B. An analogous effect is observed when the synthesis of a PANI particle is attempted in close proximity (within ≈1000 Å) to a step edge on the surfacesin this case, PANI particles (usually one or two) are generated on the step edge as well as at the intended synthesis site. These observations are readily understandable if it is postulated that the primary source of precursor monomer is the adsorbed monolayer at the graphite surface. A related effect which we have frequently observed is documented in the STM image shown in Figure 8. Here, four bias pulses were applied and four nanostructures were generated. The first bias pulse applied generated the nanostructure at far
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Figure 8. STM image of four nanostructures formed by the application of four successive bias pulses starting from far right and proceeding to the left. The first two attempts were separated in time by ≈40 s, whereas the second and third attempts were spaced by ≈5 min. The fourth nanostructure was generated withing ≈40 s of the third.
Figure 7. (A) STM image obtained in 10 mM aniline, 0.05 M HCl after the application of four bias pulses and following STM imaging at a high tunneling current set point of It ) 10 nA. The PANI produced by the initial pulse has been removed by the tip. (B) STM image of the same area as shown in (A) after the application of a single bias pulse at the location marked with a “X” showing the formation of polymer at all four of the preexisting pits. VB ) 0.2 V, It ) 0.5 nA.
right (a protrusion), and the second bias pulse was applied within a few seconds of the first and generated only the pit shown immediately to the left of the first. Then several minutes was permitted to elapse, and the third bias pulse was applied, again generating a protrusion. Once again after a wait of only a few seconds, the fourth bias pulse was applied, and again a pit was produced. Our interpretation of this result is that the reservoir of adsorbed aniline at the graphite surface is depleted in the vicinity of a pit when a PANI nanostructure is generated. For this reasonsuntil the aniline monolayer is regenerated by the diffusive flux of aniline on the surface and by readsorption from solutionsPANI nanostructures cannot be generated. Disposition of Precursor Aniline Monomer: Dependence of the Success Rate for PANI Particle Generation on the Substrate Potential. The success rate for the aniline polymerization experiment in the STM was measured in many experiments as a function of the electrochemical potentials of the tip and surface, and the statistics of these measurements were compiled. The procedure involved was as follows: The STM tip and a freshly cleaved HOPG sample were conditioned
at the desired initial potential, Einitial, for 5-10 min in the anilinecontaining HCl electrolyte. For each experiment, Einitial was specified for the HOPG surface, and the tip potential was offset from Einitial, usually by -100 mV. The bias pulse of Figure 1 was then applied at a region of the HOPG basal plane which was well-separated from other defects on the surface, and a second STM image was acquired in order to determine the nature of the modification which was produced. On the basis of the result, each attempt was placed into one of three classes: (i) a structure was formed (regardless of size), (ii) a pit only was formed (if there was no extra material that could be related to PANI, but a pit in the graphite was observed) and (iii) neither a pit nor a structure was formed. The statistics for over 400 attempts are compiled as a function of Einitial in Figure 9. In this graph, the upper limit for Einitial was ≈+0.50 V vs SCE since this is close to the potential at which aniline polymerization would even occur in the absence of an applied bias pulse. The major trend evident in these data is the increase in the probability of PANI particle formation with increasing potential up to the +0.50 V limit. At potentials above ≈+0.40 V, the PANI particles were generated with near 100% probability, and the particles which were obtained were also much larger than structures formed at lower potentials. Conversely, at the most negative potential range examined (-0.6 to -0.4 V), PANI was obtained in only 25% of all attempts. Since the amplitude of the voltage pulse used to generate a particle was always the same (i.e., that shown in Figure 1), this trend is explainable based purely on the thermodynamics of aniline polymerization. However, a second recognizable feature is the local maximum at potentials close to the pzc of graphite, where the adsorption of aniline is expected to be a maximum. The existence of this peak is consistent with the postulated involvement of adsorbed aniline at the graphite surface in PANI nanostructure growth. If the statistics for these 400 pulses are averaged without regard to the value of Einitial, the following statistics are obtained: PANI nanostructures, 61%; pits only; 22%, and no modification, 17%. However, with the control of the monomer concentration on the surface provided by Einitial, the success rate for PANI particle formation can readily be increased to nearly 80%. We have independently investigated the effect the tip potential on the probability of PANI deposition (at various constant values
Electropolymerization of Aniline
J. Phys. Chem., Vol. 100, No. 42, 1996 17047
Figure 9. Statistical analysis of the modifications produced at a graphite surface in 10 mM aniline, 0.05 M HCl by the bias pulse of Figure 1 in 400 experiments. The modification produced by each attempt was classified as a PANI particle (hatched bar), a pit without visible PANI (white bar), or no modification (black bar). Also plotted for purposes of comparison are the C-V curve HOPG (B) shown in Figure 6 and a cyclic voltammogram of PANI obtained in the same electrolyte (C).
of the sample potential) and observed no statistically significant effect. It is important to note, however, that the effect of the tip potential cannot be isolated by exploring various tip potentials at a constant sample potential since the tip-sample bias, and therefore the tip-sample distance, is also affected. Disposition of Precursor Aniline Monomer: Monte Carlo Simulations of Monomer Transport from Solution and on the Surface. The simulation of nanostructure deposition using a Monte Carlo computational method allows the exploration of several questions relating to the mechanism of PANI nanostructure growth for which answers are not experimentally accessible. These questions are the following: (1) Can the flux of physisorbed aniline at a 100 Å radius reaction site be large enoughswhen integrated over the 50 µs duration of our experimentsto supply the monomer necessary for PANI nanostructure growth? (2) To what extent will dissolved aniline contribute to the total flux of monomer at the reaction site? (3) How will the relative importance of these two sources of monomer vary with experimental parameters such as the STM tip-sample distance? Monte Carlo simulations of the nanostructure deposition experiment assumed an impermeable hemispherical tip with a radius of 1.0 µm and a 200 Å diameter reaction site which is located on axis with the STM tip on a planar and impermeable surface, as shown in Figure 10A. The boundaries of the simulation are also indicated in this figure. The other parameters employed for these simulations are collected in Table 1. Calibration of the diffusion algorithm was carried out by comparison with analytical solutions to planar, hemispherical, and cylindrical diffusion experiments, and the algorithm was found to be equally accurate in predicting diffusive transport for any of these geometries. In order to determine the relative importance of growth from dissolved and adsorbed precursor material, two types of simulations were performed: Simulations with adsorbed precursor involved an initial, closest packed monolayer on the graphite surface. The nearest-neighbor
Figure 10. (A) Schematic diagram indicating the parameters employed in the Monte Carlo simulation of the diffusive transport of dissolved aniline. (B) Summary of Monte Carlo simulation results for the diffusive transport of aniline to the reaction site. Top plots show the integrated reaction progress as a function of time for five STM tip-sample distances. At bottom are plotted the reaction rate as a function of time for STM tip-sample distances of 10 and 80 Å. (C) Summary of Monte Carlo simulation results for the two-dimensional diffusion of adsorbed aniline molecules on the graphite surface. Shown are integrated reaction progress (top plots) and reaction rate (bottom plots) as a function of time for four diffusion coefficients.
distance was assumed to be 12 Å (center-to-center), corresponding to ≈40% of a monolayer (assuming a flat adsorption state). The monolayer contained ≈82 900 particles and covered a circular region having a radius of 200 nm centered on the 100 Å diameter reaction site. This monolayer was “activated” at time ) 0, and particles were then propagated using the Monte
17048 J. Phys. Chem., Vol. 100, No. 42, 1996
Nyffenegger and Penner
TABLE 1: Summary of Parameters Employed for Monte Carlo Simulation of Aniline Transport in the STM parameter
value or description
time step step amplitude
80 ps variable, bounded by limits of (6.8 Å in x, y, and z 1.0 × 10-5 cm2 s-1 1.0 × 10-4 cm 1.0 × 10-6 cm
Daniline tip radius pit radiusa
a The diameter of the reaction site at the sample surface, centered at (x ) 0, y ) 0). The transport of a diffuser to this circular region was counted as a reactive trajectory.
Carlo algorithm described above. Simulations with dissolved precursor were carried out using a particle concentration of 0.5 mM. For dissolved precursor, the reaction site flux scaled linearly with concentration as expected. In both types of simulation, a “reactive trajectory” was counted as one where a particle intercepts the plane of the surface within the confines of the 200 Å diameter reaction site. Two simplifications were made to achieve computational speed. First, the diffusional motion of the particles was not self-avoiding, and second, the physical growth of the nanostructure and motion of the STM tip during deposition were not modeled. In other words, the boundary of the reaction site in the simulations discussed here does not change over the course of the simulation. The diffusive transport of dissolved aniline monomer was simulated for five different STM tip-sample distances, d, over the range from 10 to 80 Å, and representative data are collected in Figure 10B. At the top of this figure is plotted the integrated reaction progress for each tip-sample distance for experiments having a duration of 50 µs. Except for the noise inherent to this type of simulation, these growth curves are nearly linear as a function time, indicating that the transport of monomer is steady state over the entire 50 µs duration of the experiment. This result is consistent with the fact that a 100Å radius ultramicroelectrode (UME) requires only ≈0.1 µs to achieve a steady-state voltammetric response following a step-wise perturbation of the potential. The steady-state reaction rate increases from ≈1.5 to ≈6 µs-1 as d is increased from 10 to 80 Å, as shown at the bottom of Figure 10B. The transport rate for larger STM tip-sample separations will be bounded by the flux which occurs in the complete absence of the tip. This upper limit can be readily calculated from the expression for an ultramicrodisk electrode having the same radius, r, as the reaction site:
R ) 4DCrNA
(3)
For r ) 100 Å and C ) 0.5 mM, eq 3 yields a rate of 12 µs-1. The integrated reaction progress for the largest STM tip-sample separation of 80 Å was ≈260, and for a more realistic separation of 20 Å, the value was 70 for d ) 80 Å and too small by a factor of more than 200 for d ) 20 Å. Of course, the flux of dissolved aniline at the reaction site will be directly proportional to the concentration as indicated, for example, by eq 3. Therefore, the 1.0 and 10.0 mM aniline electrolytes employed for PANI nanostructure formation experiments described above will yield respectively 2 and 10 times the total flux of the simulations
shown in Figure 9B. For d ) 20 Å and 10 mM aniline, however, the integrated aniline flux of
