Pointed Carbon Fiber Ultramicroelectrodes: A New Probe Option for

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Pointed Carbon Fiber Ultramicroelectrodes: A New Probe Option for Electrochemical Scanning Tunneling Microscopy Jiyapa Sripirom,† Sonja Kuhn,‡ Ulrich Jung,‡ Olaf Magnussen,‡ and Albert Schulte*,† †

Biochemistry-Electrochemistry Research Unit, Schools of Chemistry and Biochemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand ‡ Institute for Experimental and Applied Physics, University of Kiel, Leibnizstraße 19, D-24118 Kiel, Germany S Supporting Information *

ABSTRACT: Carbon tips for in situ scanning tunneling microscopy studies in an electrochemical environment were prepared by electrochemical etching of carbon fibers and subsequent coating with electrodeposition paint and a silicone elastomer. The tips obtained were stable in acidic electrolyte and allowed high-resolution in situ imaging of the bare Au(111) electrode surface and of Au(111) covered by monolayers of the octyl-triazatriangulenium molecule.

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ver the last 25 years in situ scanning tunneling microscopy (STM) has become one of the most powerful methods for atomic-scale structural studies of electrochemical interfaces, and it is now an essential experimental technique for interfacial electrochemistry.1−7 As well as from probing the structure of the electrode surface and of chemisorbed adlayers, it provides direct insight into electrode reactions such as deposition and dissolution, surface phase transitions, and surface restructuring processes. Examples of modern applications of in situ STM include the identification of active sites on electrocatalytic surfaces,8 the quantitative study of electrode surface dynamics,9−11 and investigations of complex molecular adsorbate species.12,13 In addition to its use as an imaging method, STM can be employed for local spectroscopic studies.14 In the field of electrochemistry, scanning tunneling spectroscopy (STS) has been used to study the structure of the electrochemical double layer15,16 and the reactivity of redox-active molecules17 and has permitted studies of (potential-controlled) molecular conductance.18,19 Furthermore, STM can be used as a powerful tool for nanoscale modification of liquid/solid interfaces.20 For all the studies mentioned above, the chemical, structural, and mechanical properties of the STM tip are critical. Probes for STM and STS are most often metallic (e. g., W or Pt/Ir tips) and made by electrochemical etching or mechanical cutting. Graphite and boron-doped diamond (BDD) were thought of as substitute tip materials because they exhibit high electrical conductivity and are inert to chemical degradation. Furthermore, for STM in electrochemical applications (ECSTM), carbon offers a much larger accessible potential window than, for instance, noble metals such as Pt, for which © 2013 American Chemical Society

electrolysis of the aqueous electrolyte produces interference at a rather moderate electrode polarization.21 In particular for in situ STS measurements, a larger potential window would be of substantial interest, since many applications, such as studies of the electronic properties of organic molecules, require variation of the tunneling bias voltage over a range of at least 2−3 V. Carbon-based STM probes include tips of tapered pencil leads,22 electron beam23- or chemical vapor-deposited (CVD)24 pyrolytic carbon nanofilaments grown on metal STM tips, carbon nanotubes extending from metal STM tips,25−28 CVDdeposited BDD nanofibers on metal tips,29−31 and atomic projections at the end of electrochemically etched rods of carbon fiber composites or pyrolytic carbon.32 Furthermore, carbon fibers attached to a quartz tuning fork force sensor have been used not only for shear force-controlled tip positioning and constant height mode STM imaging but also for molecular junction-breaking experiments.33,34 Our previous work on the fabrication of carbon fiber STM tips focused on monofilaments, which were glued with conductive carbon paste to a metal wire and then electrochemically etched. These tips proved their capability as atomic resolution STM probes in vacuum STM measurements.35 Furthermore, by surface modification with an insulating layer of electrodeposition paint (EDP) they could be converted into conical carbon ultramicroelectrodes, similar to electrochemical STM tips.35,36 In the above-mentioned studies, conical carbon fiber tips with nanometer tip curvatures proved to be valuable probes for Received: September 30, 2012 Accepted: December 11, 2012 Published: January 3, 2013 837

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Technical Note

the solution. A potential was applied, large enough to generate a current of 15−20 μA per mm of the on average about 6.5-μmdiameter fiber at the beginning of the etching, which corresponds to an effective etching current density of 75−100 mA cm−2. Carbon oxidation at the solution/air interface etched the fiber perpendicular to the axis, forming a very sharp tip. These fibers were then rinsed with water. Subsequently, tipsparing cathodic Clearclad precipitation (at 5 V for 4 min) provided a uniform and thin polymer coating to confine the exposure of the carbon fiber to the very apex of the tip, thus establishing the conical microelectrode geometry. In the new step 4, the tungsten wire/carbon fiber junction was electrically insulated. For this purpose, Sylgard, a mixture of a viscous silicone and a catalytic cross-linking agent, was employed. Gentle heat treatment of a mixture with a 9:1 weight ratio of the two components facilitates the chemical reaction to form a rubber-like material. With a strategy that is explained in the Results and Discussion, a thin Sylgard layer was placed carefully on the target area of the conical carbon fiber microelectrode, avoiding deposition on the uniformly EDP-insulated protruding part of the carbon fiber and its electroactive conical tip. Heat curing was facilitated with a stream of warm air from a heat gun. Finally, the completed carbon fiber tips for electrochemical STM were stored in a dark container to avoid damage and contamination. Microscopic and Voltammetric Checks of the Carbon Fiber Tips for Electrochemical STM. The carbon fiber tips for the electrochemical STM measurements were checked with an optical stereomicroscope, with a scanning electron microscope (SEM), and by cyclic voltammetry. For the latter experiments, a computer-controlled potentiostat (model Reference 300, Gamry Instruments, Warminster, PA) was used, with a Pt wire as the counter-electrode and a homemade Ag/AgCl/3 M KCl electrode as the reference. Electrolytes were neutral or acidic solutions, i.e., 0.1 M KCl or 0.1 M HClO4, with or without 1 mM [Ru(NH3)6]Cl3. STM Imaging with Carbon Fiber EC-STM Tips. Electrochemical STM measurements were performed using a PicoPlus STM (Agilent, Inc.) in 0.1 M HClO4. Pt wires were used as the counter and reference electrodes. All potentials are referenced against Ag/AgCl/3 M KCl. The measurements were performed in constant current mode. Lateral drift in the STM images was corrected with a dedicated software.

STM measurements. Because of their large overpotentials for H2 and O2 evolution, the most promising application of these novel probes are in situ STM studies in aqueous media. However, to our knowledge this application has not yet been reported for carbon (fiber) STM tips. This technical note describes the preparation of tapered carbon fiber probes and their application in electrochemical STM measurements. We describe a simple and effective method for achieving the required insulation of the junction between etched and EDPinsulated carbon fibers and demonstrate the value of these probes for electrochemical STM studies by submolecularresolution imaging of organic adsorbate layers on Au(111) electrodes in perchlorate solution.

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EXPERIMENTAL SECTION Chemicals and Materials. For fabrication of the electrochemical STM tips, polyacrylonitrile-based carbon fibers of the type E/XAS (SGL Technik GmbH, Meitingen, Germany), conductive carbon paint (SPI Supplies, West Chester, PA), cathodic electrodeposition paint (EDP) Clearclad HSR C/F (L/D) (LHV Coatings Limited, Birmingham, England), twocomponent silicone elastomer Sylgard 184 (Dow Corning Corporation, Midland, MI), and tungsten wire (250 μm diameter, Goodfellow Cambridge Ltd., Huntingdon, England) were used. KCl, [Ru(NH3)6]Cl3, and HClO4 (all p.a. grade or better), NaOH (anhydrous pellets, 97% purity), H2SO4 (ACS reagent grade), and ethanol (p.a. grade) were bought from various suppliers and used as received. Solutions for electrochemical etching of the carbon fibers and the Au(111) substrates as well as electrolytes for voltammetry and electrochemical STM were prepared with ultrapure water (18.2 MΩ cm, Elga Labwater, Celle, Germany). Au (111) single crystal samples (MaTecK GmbH, Jülich, Germany) exhibited a diameter of 10 mm and were oriented within 0.3°. They were cleaned before use by electrochemical polishing in 0.1 M H2SO4 at 4 V for 20 s (with a Pt wire counter electrode) and subsequently immersed in 0.1 M HCl for 5 min to improve the size of the (111)-oriented terraces. After extensive rinsing in ultrapure water, the substrates were annealed in a butane gas flame for 4 min and then allowed to cool to room temperature in air (∼5 min). Monolayers of octyltriazatriangulenium (TATA) molecules (Figure 5a, inset) were prepared by self-assembly in solution. The Au(111) substrates were immersed in 10 μM solutions of the molecules in ethanol for 30 min at room temperature.37−39 Finally, the samples were thoroughly rinsed in ethanol to remove excess molecules. Fabrication of Carbon Fiber Tips for Electrochemical STM. The steps for preparation of the carbon fiber probes are (1) attachment of a carbon fiber to a tungsten wire, (2) electrochemical etching of the protruding carbon fiber in alkaline solution to obtain a sharp tip, (3) insulation of all except the extreme end of the carbon fiber tip with the electrodeposition paint (EDP) Clearclad, and (4) insulation of the tungsten wire/carbon fiber junction with the silicone elastomer Sylgard for protection against leakage currents. The details of steps 1−3 have been described in our previous study.35 Briefly, the carbon fiber was glued with carbon paste to the end of a ∼4 cm long piece of tungsten wire and trimmed to leave a fiber extension of length ∼2 mm. Electrochemical etching of the fiber was performed in 0.1 M NaOH, with the carbon fiber placed as a central anode within a circular platinum wire cathode. Special care was taken that as much as possible of the carbon fiber, but not the tungsten wire, was immersed in

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RESULTS AND DISCUSSION Conical carbon fiber ultramicroelectrodes, used here as precursors for EC-STM tip fabrication, have a pointed carbon fiber attached by conductive carbon paste to a tungsten holding wire, with a very short length of fiber protruding in the direction of the wire axis. To establish the conical microelectrode geometry, both the wire/paste/carbon fiber junction and the entire carbon fiber surface, apart from the apex of the tip, are thinly coated with electrodeposition paint. Checking the insulation integrity by their partial (fiber only, with increasing immersion depth) or full immersion into a solution of 1 mM [Ru(NH3)6]Cl3 in 0.1 M KCl and subsequent acquisition of cyclic voltammograms (CVs) revealed for full immersion a major disturbance of the initial current response of the conical ultramicroelectrode. This suggested that the electrodeposition paint coating acted as a good electrical insulator of the smooth, cylindrical carbon fiber stem, but not of the wire, with its more irregular surface. In the small-volume electrochemical cell of the STM, however, immersion of the entire electrode cannot be 838

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Figure 1. (a) A scheme illustrating the strategy for insulating the carbon fiber/metal wire junction with a silicone rubber coating. With the aid of course manipulators, a thinly polymer-coated carbon fiber is pulled into a liquid silicone rubber precursor, which is held in a tiny ∪-shaped plastic container. Only the tungsten wire is immersed at the start, while at the end the entire stem apart from the extreme tip of the insulated fiber is covered. Careful lifting of the carbon fiber/metal wire with its adherent Sylgard surface film, followed by gentle curing with a hot air gun, leaves the wire covered with a solid, insulating silicone polymer. (b) Drawing of a pointed and electropainted carbon fiber attached to a tungsten wire, after the application of a protecting silicone layer to the carbon−tungsten junction. (c) Low magnification scanning electron microscope images of a structure as schematically shown in part b. (d) Higher resolution scanning electron microscope image of the front part of the electropainted carbon fiber in part c that has a paint film all over but an invisible area of electroactive graphitic carbon exposed at the end as a functional tunneling tip.

avoided; therefore, additional insulation of the junction between the fiber and the holding wire is necessary. The coating of this part without affecting the tip apex, which must remain free from insulation, is illustrated in Figure 1a. A 10 mm long ∪-shaped container was mounted on a holder and filled with a mixture of the two components of the silicone elastomer Sylgard. The conical carbon fiber ultramicroelectrode was positioned above this tubular basin using an x,y,zmicromanipulator and stereomicroscope and was introduced into the liquid Sylgard precursor mixture, while carefully avoiding immersion of the polymer-insulated fiber and its connection to the wire. Horizontal movement pulled the entire tungsten wire into the Sylgard but not the protruding carbon fiber or its tip. By slow vertical movement, the fiber/wire junction was covered with a layer of the viscous rubber precursor, which was then converted into an electrically insulating silicone rubber coating by exposure to a gentle stream of warm air from a heat gun. Figure 1b shows a diagram of the final carbon-fiber ultramicroelectrode with improved electrical insulation of the fiber/wire junction and the expected electroactivity just at the tip. Imaging by SEM (Figure 1c,d) confirmed the embedding of the holding wire into Sylgard as well as the physical integrity of the electrodeposition paint-coated carbon fiber, with no sign of contamination with the silicone material. The electrochemical performance of the completed tip was checked by cyclic voltammetry in 1 mM [Ru(NH3)6]Cl3 in 0.1 M KCl, immersing either the paint-insulated carbon fiber or the entire ultramicroelectrode, including the silicone-coated end of the tungsten wire. The acquired CVs for the two cases are very similar (Figure 2), confirming the efficient insulation of the tip. The effective radius, reff, of thinly EDP insulated tapered carbon ultramicroelectrodes can be calculated from the limiting currents in the cyclic voltammograms according to the following equation for a hemispherical geometry:40

Figure 2. Two cyclic voltammograms (CVs) of a conical carbon fiberbased ultramicroelectrode either fully immersed into electrolyte (red trace) or with only the tip of the smoothly insulated tapered carbon fiber in the solution. The almost perfect overlap of the two sigmoidal curves indicates efficient insulation of the entire wire/fiber structure.

where n is the number of electrons transferred during the electrochemical reaction at the electrode surface (here, n = 1), F is Faraday’s constant, D and c are the diffusion coefficient and concentration of the electroactive species (here, D = 7.2 × 10−6 cm2 s−1 (ref 41), and c = 1 mM). For the representative example of an ultramicroelectrode that produced the voltammograms in Figure 2, reff was calculated as 2.3 μm, respectively. Since STM imaging experiments are very time-consuming, the stability of the entire insulation of the carbon fiber EC-STM tips was also evaluated electrochemically. For this, CVs were recorded every 10 min over a period of 3 h, with the probe fully immersed in 1 mM [Ru(NH3)6]Cl3 in 0.1 M HClO4, a typical acidic supporting electrolyte for in situ STM measurements. No major changes in the CVs and the diffusion-limited cathodic current occurred during this period, indicating that the

ilim = 2πnFDcreff 839

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Figure 3. Cyclic voltammogram (CV) recorded with (A) a conical carbon fiber-based ultramicroelectrode and (B) a polyethylene-insulated W ECSTM tip in bare 0.1 M HClO4 in the electrochemical cell of the STM setup used in this study. The regions between the arrowed vertical lines represent the expected potential window for electrochemical STM. The inset in part A is the CV recorded with the carbon ultramicroelectrode immersed in 1 mM Ru3+/0.1 M KCl and the potential scanned from +0.1 V (right) to −0.4 V (left) and back. Note: The potentiostat used with the STM system is saturated at currents exceeding ±1000 pA.

electroactive area of the tip did not change over this time (Figure S-1, Supporting Information). This observation confirms that the combined electrodeposition paint/Sylgard insulation was resistant to degradation, even in strongly acidic electrolytes. Figure 3A shows the CVs of an EDP/Sylgard-insulated conical carbon fiber ultramicroelectrode with an reff of 3.6 μm during a measurement in the electrochemical cell of the STM setup containing only 0.1 M HClO4 and from a voltammetric study of the uncoated electrode surface area in 1 mM Ru3+containing electrolyte. When scanning in the positive direction, starting at about 0.7 V vs Ag/AgCl, a small, gradually increasing anodic current is visible, which is probably produced by slow oxidation of the carbon fiber surface. At around 1.4 V vs Ag/ AgCl, a sharp current onset is observed, which corresponds to oxygen evolution. In the negative direction, a considerable current increase occurs at about −0.3 V vs Ag/AgCl. This potential is too low for the current to be related to cathodic water electrolysis. More likely, it is due to the reduction of surface oxides formed during the anodic scan toward high overpotentials. The potential range in the CV between −0.25 and +0.8 V vs Ag/AgCl represents the tip potential window suitable for electrochemical STM in the electrolyte of choice. The residual (leakage) current in this region was about 50 pA or below and, as evidenced in the following imaging experiments, small enough in magnitude not to hinder the STM imaging process with about 10-fold larger tunneling currents in the order of 500 pA or more. Figure 3B shows the CV of a tungsten EC-STM tip as routinely used for imaging experiments in perchloric acid electrolyte. It was prepared from a freshly etched tungsten wire with the routine procedure of manual tip-sparing polyethylene insulation. Apparently, the measured tip current is much smaller than the ones seen for the carbon ultramicroelectrode in Figure 3A. A direct comparison of the signals in- and outside of the plateau region is not possible here as the uncoated electroactive surfaces of the two different type of probes are likely not the same but larger for the carbon variant. It is, however, visible that for the insulated tungsten tip during the anodic scan in the positive direction, an oxidation current arises already at about 0 V, slowly increases till about 0.5 V, and then distinctively increases thereafter. In contrast to carbon, which does not passivate during surface

oxide formation, tungsten is known to form an insulating oxide layer. This significant difference may explain why, as demonstrated below, carbon tips can achieve STM imaging at positive potentials that are not assessable with tungsten tips as scanning probes. With the isolated carbon tips stable high-resolution, in situ STM imaging was possible, as demonstrated for Au(111) electrodes in 0.1 M HClO4 solution. As shown in Figure 4a, the characteristic surface structure with atomically smooth terraces, separated by monoatomically high steps is as visible as in the literature.42 The lateral width of the steps can be as low as 0.7

Figure 4. In situ STM images of a clean Au(111) electrode surface in 0.1 M HClO4, showing (a) an image of the characteristic surface topography, exhibiting steps and the herringbone surface reconstruction (69 × 69 nm2, IT = 0.5 nA, Usample = 0.1 V, Utip = −0.11 V), (b) cross sections along the lines indicated in part a. Parts (c) and (d) are examples of images recorded at variable tip potentials (95 × 95 nm2, IT = 0.6 nA, Usample = 0.2 V, Utip is displayed in the subfigures). 840

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Figure 5. In situ STM images of an octyl-TATA adlayer on Au(111) demonstrating high resolution imaging. (a) Long-range structural order (35 × 35 nm2, IT = 0.1 nA, Usample = 0.4 V, Utip = −0.05 V). (b) Submolecular resolution imaging of the TATA molecules (12.5 × 12.5 nm2, IT = 0.1 nA, Usample = 0.3 V, Utip = −0.06 V). The inset in part a shows the structure of a TATA molecule and that in part b is a magnification of one of the many adsorbed TATA molecules.

of the tip quality with time may occur under ambient conditions. The fiber tip preparation procedure offers distinct advantages but has a limited rate of success so this procedure, and/or the subsequent handling of the carbon fiber tips needs to be further improved for routine use, and suitable strategies are currently being sought. Furthermore, planned work will aim at an exploration of the novel in situ STM probes for imaging and spectroscopy studies at unusual tip potentials.

nm (Figure 4b), an excellent resolution that indicates that the carbon tips can be atomically sharp. On the terraces, the characteristic stripe pattern of the herringbone reconstruction with a vertical modulation of 0.02 nm can be observed in the usual manner.43 STM images of the Au surface could be obtained with tip potentials between 0 and 0.6 V, whereas at lower or higher potentials the imaging became unstable. Although this shows that with carbon tips STM measurements can be performed at substantially more positive tip potentials than with metal tips, a significantly larger potential window should be accessible according to the cyclic voltammetry data (Figure 3). Specifically, while the positive potential limit for STM imaging is in good agreement with the onset of the anodic current in the corresponding voltammogram, the negative limit should be much smaller. The origin of this discrepancy is unclear at present. Within the accessible range of tip potentials, high-resolution imaging is possible, as illustrated by in situ STM measurements of octyl-triazatriangulenium (TATA) adlayers. These molecules form well-ordered hexagonal adlayers with a (√19 × √19) R23.4° superstructure on the Au (111) surface, which could be clearly observed in measurements with the carbon fiber tips (Figure 5a). Typical tip currents for such molecular resolution images are 0.1 nA, which is about 3 times larger than those in STM studies employing conventional tungsten tips. In some small-scale images even higher, submolecular resolution could be achieved, revealing the 3-fold symmetry of the TATA molecules (Figure 5b). The quality of the STM images is similar to those obtained with metal tips for this adsorbate system.37−39

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ASSOCIATED CONTENT

* Supporting Information S

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +66 44 22 6187. Fax: +66 44 22 4185. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support through a Thailand Research Fund (TRF) Strategic Basic Research Grant ‘‘Nanoscience and Nanotechnology’’ (Grant DBG52), a Ph.D. scholarship from The Office of the Higher Education Commission, Thailand, and the Deutsche Forschungsgemeinschaft via Grant SFB 677. A.S. also would like to express his gratitude to Peter Hope and LHV Coatings Limited, Birmingham, England, for providing a free research sample of the electrodeposition paint Clearclad. And last, but not least, Dr. David Apps, Centre for Integrative Physiology, University of Edinburgh, Scotland, is thanked for his critical manuscript reading and language improvements.

CONCLUSION

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The results of this study demonstrate the potential of carbon fiber tips for in situ STM studies in electrochemical environment. However, in the present experiments only ∼10% of the carbon fiber tips could be successfully used for STM measurements, the majority of these tips exhibiting low lateral resolution. Since a similar success rate was also obtained for STM measurements with blank carbon fiber tips in air, this problem seems to be caused by the tip etching procedure, rather than the coating process. Furthermore, slow degradation

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