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Integrated Scanning Kelvin Probe-Scanning Electrochemical Microscope System: Development and First Applications Artjom Maljusch,† Bernd Sch€onberger,§ Armin Lindner,‡ Martin Stratmann,§ Michael Rohwerder,§ and Wolfgang Schuhmann*,† †
Analytische Chemie-Elektroanalytik & Sensorik and ‡Mechanical Workshop of the Faculty of Chemistry and Biochemistry, Ruhr-Universit€at Bochum, Universit€atsstrasse 150, D-44780 Bochum, Germany § Department of Interface Chemistry and Surface Engineering, Max-Planck-Institute for Iron Research, D-40237 D€usseldorf, Germany ABSTRACT: The integration of a scanning Kelvin probe (SKP) and a scanning electrochemical microscope (SECM) into a single SKP-SECM setup, the concept of the proposed system, its technical realization, and first applications are presented and discussed in detail. A preloaded piezo actuator placed in a grounded stainless steel case was used as the driving mechanism for oscillation of a Pt disk electrode as conventionally used in SECM when the system was operated in the SKP mode. Thus, the same tip is recording the contact potential difference (CPD) during SKP scanning and is used as a working electrode for SECM imaging in the redox-competition mode (RC-SECM). The detection of the local CPD is established by amplification of the displacement current at an ultralow noise operational amplifier and its compensation by application of a variable backing potential (Vb) in the external circuit. The control of the tip-to-sample distance is performed by applying an additional alternating voltage with a much lower frequency than the oscillation frequency of the Kelvin probe. The main advantage of the SKP-SECM system is that it allows constant distance measurements of the CPD in air under ambient conditions and in the redox-competition mode of the SECM in the electrolyte of choice over the same sample area without replacement of the sample or exchange of the working electrode. The performance of the system was evaluated using a test sample made by sputtering thin Pt and W films on an oxidized silicon wafer. The obtained values of the CPD correlate well with known data, and the electrochemical activity for oxygen reduction is as expected higher over Pt than W.
S
canning electrochemical microscopy was introduced by Bard et al.1 as a scanning probe technique based on monitoring of changes in faradaic current during movement of an ultramicroelectrode (UME) across the surface of a sample. Generally, the SECM setup consists of a positioning unit that scans an UME as a local probe in close distance to a sample surface. A typical electrochemical cell for the SECM consists of an UME as a working electrode, a reference electrode, and an auxiliary electrode. By means of a bipotentiostat, the sample can be connected as a second working electrode and thus polarized at a userdefined potential. SECM experiments can be carried out in different modes of operation such as feedback mode (FB) or generation-collection mode (GC), the alternating current mode (AC-SECM),2 or the redox-competition mode (RC-SECM).3 RC-SECM was introduced as a tool for visualization of the local catalytic activity of a sample surface toward the oxygen reduction reaction (ORR). In a bipotentiostatic experiment, the tip competes with the sample for the dissolved oxygen within the gap between the positioned SECM tip and the sample. If the tip is positioned above a catalytically active site for ORR, a current decay at the SECM tip is observed. In order to avoid complete oxygen depletion in the gap between the tip and sample, a multipotential pulse profile is applied at the tip. RC-SECM was successfully applied for visualization of the local electrocatalytic activity at biofuel cell cathodes,4 microstructured metal r 2011 American Chemical Society
hexacyanoferrates,5 and different metallo-porphyrins.6 Recently RC-SECM was used for evaluation of the electrocatalytic activity of novel Pt-based catalysts7 and for the visualization of the local electrocatalytic activity of catalysts for HCl electrolysis under industrial conditions.8 The scanning Kelvin probe (SKP) is a noncontact, nondestructive vibrating capacitor technique, which measures the contact potential difference (CPD) between an electrically conductive vibrating reference probe (Kelvin probe) and an electrically conductive sample. The main principles and some aspects of the scanning Kelvin probe have been discussed in the detailed reviews.9 11 In brief, the conventional SKP consists of a planeended cylindrical electrode vibrating perpendicular to a stationary sample. In this way, both electrodes form a planar capacitor. If an external electrical contact between the two capacitor plates is formed, their Fermi levels start to equalize and the resulting charge flow causes a potential gradient (VCPD). Periodic variation of the distance between the electrode and the sample leads to changes in the capacitance thereby causing an alternating current to flow through the external circuit. Applying a variable backing potential (Vb) permits the complete compensation of Received: April 13, 2011 Accepted: June 15, 2011 Published: June 15, 2011 6114
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Analytical Chemistry the alternating current. When Vb is equal to VCPD, the electric field between the capacitor plates vanishes and a zero output signal is recorded (nulling technique). Since its introduction by Stratmann et al.12 for the study of samples covered by an ultrathin layer of electrolyte in corrosion science, SKP has been increasingly used. However, because of the comparatively low lateral resolution and the considerable effort for reliable operation, it has not found very broad application yet. Integration of the SKP into an AFM leading to scanning Kelvin probe force microscopy (SKPFM) provides a resolution in the nanometer range and thus can be even used for resolving the CPD of intermetallic compounds (IMC) in different alloys. Information obtained about the local CPD on the sample surface was often interpreted as indication for the formation of galvanic elements during immersion of the sample in a corrosive solution. However, SKPFM shows frequent topographical artifacts and was hence mainly applied for previously polished samples. Furthermore, the contribution to the signal by the cantilever affects the measured CPD values13 even leading to an inversion of the contrast.14 The correlation between the corrosion potential of an aluminum alloy and the potential measured by the Kelvin probe technique after its immersion in an electrolyte was in depth discussed by Frankel et al.15 A number of different magnesium,16 aluminum,17 and steel alloys18 have been investigated by means of the SKPFM. Possible corrosion mechanisms were suggested based on mapping of the Volta potential difference on freshly polished surfaces. However, the corrosion process is highly sensitive to the local conditions (pH, composition of the electrolyte, concentration, temperature, etc.) and is also influenced by the kinetics of the involved reactions. For instance, comparison of the Volta potential difference measured by the scanning Kelvin probe at pH 4 and pH 7 would wrongly indicate the corrosion activity of Nb and Fe.19 Obviously, a correlation between the CPD measured by SKPFM on freshly prepared alloy surfaces and its later corrosion behavior is not necessarily straightforward, and additional techniques have to be used for unequivocally elucidating the corrosion mechanism. In the case of alloys containing small intermetallic compounds (IMC) of micrometer or even nanometer dimensions, the elucidation of the corrosion mechanism is even more complicated. Often, pure bulk phase synthesis is required to enable studies on the corrosion mechanism of IMC. Thus, it becomes evident that in addition to a high-resolution mapping of the CPD distribution on the sample surface as it is already possible using SKP or SKPFM, further information on the electrochemical or electrocatalytic activity at a similar lateral resolution is required. In this contribution, we describe the development of an integrated SKP-SECM system allowing sequential SKP followed by SECM measurements of the same sample area using a single tip as both the vibrating SKP capacitor and the SECM tip. Thus, information about the local CPD and the local electrocatalytic activity with respect to the ORR can be obtained for the same area of a sample surface.
’ EXPERIMENTAL SECTION Working Electrode. A glass-insulated Pt-disk microelectrode (conventional SECM tip) fabricated as described previously20 using a 125 or a 25 μm diameter Pt-wire (Goodfellow, Bad Nauheim, Germany) was used as the working electrode. Before fabrication, all glass capillaries (L = 100 mm, Dout = 1.5 mm)
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were sorted to ensure optimal fitting to the brass guide on top of the measurement head. With the use of a special homemade setup, the SECM tip was fixed with two-component epoxy glue in a cylindrical holder (L = 2 cm, Dout = 6 mm) made of polyvinylchloride (PVC). Before each experiment, the working electrode was cleaned by polishing it with different grades of alumina paste (3, 1, 0.3, and 0.05 μm, LECO, Kirchheim, Germany) and subsequently ultrasonciated in 2-propanol and deionized water for 5 min. After cleaning, the electrode was dried with argon. Test Sample. A piece of an oxidized silicon wafer (1.2 cm 1.2 cm) covered partly with Pt and partly with W was used as a test sample due to the expected substantial differences in the CPD as well as the activity for the ORR. An oxidized (500 nm SiO2) 4 in. silicon wafer was covered with 200 nm of W by sputtering. Half of the wafer surface was covered by a mask, and a 100 nm Pt film was deposited. After metal deposition, the wafer was cut in 1.2 cm 1.2 cm pieces in such a way that one-half of the sample is covered by Pt and the other one by W. Before each experiment, the test sample was cleaned by subsequent ultrasonication in acetone, 2-propanol, and deionized water for 5 min. After cleaning, the active area of the test sample was dried with argon. Scanning Kelvin Probe (SKP). For SKP experiments, a new SKP-SECM system was used (for more technical information see the Results and Discussion). The test sample was fixed on the sample holder with conductive carbon filled glue. An additional drop of the conductive glue was placed on the edge of the test sample in order to establish direct electrical contact between the metal layer and the sample holder. On top of the test sample, an O-ring (D = 7 mm) made of rubber was fixed with a nail lacquer as a cell for later SECM experiments. During SKP measurements, a piece of pure Ni (99.999%, Goodfellow, Bad Nauheim, Germany) fixed on the sample holder with the same conductive glue was used as a reference sample. The upper side of the reference sample was ground with 1500 grit abrasive paper and polished with different grades of alumina paste (3, 1, 0.3, and 0.05 μm, LECO, Kirchheim, Germany) to obtain a mirror finish. Before each experiment, the reference sample was cleaned using the same procedure as described for the test sample. Redox Competition Mode of the SECM (RC-SECM). For SECM experiments, the same SKP-SECM setup was used. All experiments were carried out in a four-electrode cell configuration with the test sample acting as working electrode 1 (WE 1), the SECM tip acting as working electrode 2 (WE 2), a Pt-wire as counter electrode, and Ag/AgCl/3 M KCl as the reference electrode. The electrochemical cell comprised of an O-ring placed on the test sample and filled with 150 μL of 50 mM phosphate buffer, pH 7, was used. A software was developed in Visual Basic 6.0 for fast data acquisition and control of all settings. All SECM experiments were performed with x- and y-increments of 150 μm in the case of a 125 μm Pt-disk electrode or 10 μm in the case of a 25 μm Pt-disk electrode at a tip-to-sample distance of 5 μm. In order to avoid complete oxygen depletion at the tip, a multipotential pulse profile was applied at the tip while the sample was continuously polarized to its predefined constant potential. The first potential pulse (P1, 0 mV) is a conditioning potential applied for 500 ms during which no oxygen reduction takes place at the tip. The main function of P1 is the restoration of the diffusional equilibrium after movement of the SECM tip during scanning. The second potential pulse (P2, 600 mV) is the measurement pulse applied for 400 ms which is sufficiently 6115
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cathodic to invoke oxygen reduction. In order to avoid a background current shift caused by an uncompensated tilt between the scanning plane of the SECM tip and the test sample, a software based tilt correction procedure was used.
’ RESULTS AND DISCUSSION System Development. The main objective of the newly developed SKP-SECM system is seen in performing SKP and SECM measurements without changing of the tip or replacement of the sample. Due to the fact that for SECM measurements an insulated disk electrode is required while the demand for the SKP tip is less, we decided to use a conventional SECM tip, namely, a glass-insulated Pt disk microelectrode as the tip in both the SKP and the SECM measurements. In conventional SKP systems, the null output condition is achieved by means of a lock-in amplifier (LIA) and a compensation unit utilizing a proportional21 or an integral feedback.22 The main disadvantage of these systems is a small signal-to-noise ratio at the balance point mainly caused by electromagnetic pickup.23 In order to overcome this limitation, the use of an off-null mode of operation was proposed.24 A number N of discrete values of Vb ranging from 2.5 V to +2.5 V are imposed on the system and the corresponding values of the Kelvin current (iKelvin) are recorded. On the basis of the statistical linear correlation-regression analysis of the Kelvin current as a function of the applied backing potential, the CPD can be derived generally with a good linear correlation coefficient r2 of higher than 0.9999 and errors of less than 10 mV.24 Both approaches for determining the null output condition were implemented in the proposed SKPSECM system. The choice of the driving mechanism for the scanning Kelvin probe plays an important role especially considering attempts to minimize stray capacitance.23 Different driving mechanisms based on an electromagnetic solenoid,25 a voice coil,26 a piezoelectric actuator,27 or an electrostatic system28 were proposed. Presently, only voice coil or piezo based oscillating systems are used. In the case of voice coil based oscillating systems, an alternating voltage is applied to the voice coil thus creating an alternating magnetic field. The interaction of this field with a magnetic field of the permanent magnet generates a mechanical force causing the coil and thus the attached Kelvin probe to move back and forth. Very low off-axis displacements of the vibrating electrode can be achieved by means of the probe suspension system based on two thin circular disks vibrating in a perfect plane-parallel fashion.29 The main advantages are the large amplitudes of oscillation (up to 10 mm), much smaller electromagnetic pickup, and the use of long mechanical transducers for placement of the driver far away from the sample. In contrast, piezoelectric based oscillating systems are used in commercial SKPFM devices. Piezo-driven oscillation systems offer a very high precision, a possibility to work at high oscillating frequencies, and an easy and fast control. However, their main disadvantage is a strong electromagnetic pickup due to the relatively high driving voltage for the piezo element in close proximity to the sample. This limits the CPD resolution and requires additional effort for shielding such as among others the integration of a μ-metal case around the driver. In order to enable the operation of vibrating electrodes at high frequencies and with high precision, we decided to use the preloaded piezo actuator (PSt 150/5/60 VS10, Piezosystem Jena, Jena, Germany) designed for dynamic applications. For very low off-axis displacement of the vibrating
Figure 1. Schematic representation of the system connection for SKP measurements.
electrode, a suspension system was implemented in the measurement head. In order to minimize the electromagnetic pickup caused by the piezo actuator, it was mounted as far away as possible from the operational amplifier and attention was paid to sufficient shielding. The lateral distribution of the CPD on solids, i.e. the value of the displacement current, is directly dependent on the surface area of the Kelvin probe, the tip oscillation frequency and indirectly on the distance between the Kelvin probe and sample. Reduction of the size of the Kelvin probe should lead to a substantial decrease in the measured Kelvin current and consequently to lower sensitivity and accuracy. Thus, for high-resolution measurements, a small tip has to be placed very close to the sample surface. Two different approaches for the regulation of the distance between the Kelvin probe and the sample have been reported. One possibility to control the tip-to-sample distance is taking advantage of the independence of the quotient of two harmonics of the measured displacement current on the value of the CPD.30 An alternative possibility is proposed to add to the backing voltage (Vb) an additional signal (Vm sin ω2t) where the frequency ω2 is very different from the oscillation frequency of the Kelvin probe (ω2. ω1).31 It was shown that the corresponding signal is only dependent on the distance between the probe and the sample. Thus, keeping this signal constant by varying the position of the Kelvin probe with an integrated z-positioning system can be used for feedback loop regulation of the tip-tosample distance. By this, the sample topography can be derived during the SKP scanning making the CPD independent from the sample topography. The latter distance regulation approach was integrated into the proposed SKP-SECM system. In order to perform sequentially SKP and topography scanning followed by SECM imaging without moving the sample and using the same tip, the SKP-SECM system was designed in that way that it is possible to choose between the SKP and the SECM mode from the software together with actuating two reed relays. A scheme of the system connections for SKP measurements is shown in Figure 1. A conventional SECM microelectrode, a preloaded piezo actuator, a high-frequency power supply (LE 150/100 EBW, Piezosystem Jena, Jena, Germany), an oscilloscope for the signal control, a lock-in amplifier (EG&G 5210, AMETEK, Meerbusch, Germany), and a home-build integrator (MPIE, D€usseldorf, Germany) for measurement of the CPD were used. A second lock-in amplifier (EG&G 5210, AMETEK, 6116
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Figure 2. Scheme of the measurement head: main body (1, 2, 3), electrode guide (4), SECM tip (5), stainless steel membrane (6), PVC electrode holder (7), translator (8), holder for the preloaded piezo actuator (9), preloaded piezo actuator (10), insulating plate (11), connector for the power supply (12).
Meerbusch, Germany) was further integrated for the feedback regulation of the tip-to-sample distance. Connection to the PC is established via a 16 bit analog-to-digital conversion card. The Pt-wire inside the SECM tip is connected via a reed relay to the high-quality ground of the first LIA while the sample is connected via a second reed relay to the ultralow noise operational amplifier (AD 549LH, Analog Devices, Munich, Germany). The alternating Kelvin current will be amplified by the operational amplifier, converted to an alternating voltage, and used by the lock-in amplifier and the integrator for measurement of the CPD. Figure 2 shows the heart of the instrument, namely, the measurement head of the SKP-SECM system. It is made of stainless steel (DIN 1.4571), has 3 mm thick walls for effective shielding of the preloaded piezo actuator, and consists of three parts (1, 2, 3) for a facilitated exchange of the tip electrode. Inside the housing, the preloaded piezo actuator (10), a cylindrical holder for the piezo actuator (9), a translator (8), a PVC electrode holder (7), a conventional SECM tip (5), and a 0.1 mm thin stainless steel membrane (6) for stabilization of the electrode oscillation orthogonal to the sample surface were placed. The preloaded piezo actuator is mounted on an insulating plate (11) made of PEEK in order to enable its electrical isolation from the grounded measurement head. Additionally, the preloaded piezo actuator is centered inside of the measurement head by means of the cylindrical holder made of fiber-glass reinforced Teflon. Connection between the preloaded piezo actuator and the working electrode is established via a cylindrical translator made of a stiff material (PEEK) in order to ensure orthogonal electrode oscillation. The outer diameter of the translator is fitting well to the opening in the upper part (1) of the measurement head and is specially made for stabilization of the electrode oscillation. An additional tool for the stabilization of the tip oscillations is a brass guide (4) with an opening that fits well to the working electrode outer diameter. To avoid unwanted friction between moving parts during operation, a drop of penetration oil is used. Connection of the working electrode to the high-quality ground of the bipotentiostat is established via reed relay placed in a small aluminum case at the side of the measurement head. For prepositioning of the measurement head above the sample, it is mounted in a special holder fixed on a translation stage (z-axes) driven by a stepper motor (Owis, Staufen, Germany). The power supply to the piezo actuator is established via a double shielded cable and a two pin connector (12).
Figure 3. Scheme of the sample holder: sample holder (1), shielding plate (2), insulating plate (3), upper (4) and lower part (5), P-611.3 NanoCube positioning unit (6), circuit board (7), contact pin (8), and cylindrical guide (9).
In order to minimize the time dependent stray influence coefficients and thus the stray capacitance, we designed the SKP-SECM system in such a way that the tip electrode is at low impedance (earth potential) and the sample under investigation is connected to an ultralow noise operational amplifier (OA).32 The sample holder is shown in Figure 3. Because of the small value of the Kelvin signal, the amplifier has to have high input impedance. For this purpose, the use of a field effect transistor was proposed.25 However, we decided to use an input resistor of 150 MΩ. The OA and all the electrical components of the amplifier are located on one single circuit board (7) placed in a grounded aluminum case (DIN 3.1645) of 5 mm thick walls in order to enable effective shielding by low weight. The aluminum 6117
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Figure 6. Scanning area on the sample surface (top). Line scan over the marked area on the sample surface (bottom) performed in the SKP mode (triangles) and in the RC-SECM mode (circles). SKP probe/ SECM tip: 125 μm Pt-disk electrode. Scan increment: 150 μm.
Figure 4. Photograph of the SKP-SECM system.
Figure 5. Scheme of the system connections for SECM measurements.
case consists of two parts. On top of the upper part (4), an insulating plate made of PVC (3), a grounded shielding plate made of brass (2), and a sample holder made of stainless steel (1, DIN 1.4571) were placed. The contact between the OA and the
sample holder is established via a thin contact pin (8) isolated from the aluminum case with a cylindrical guide made of PVC (9). For very precise control of the distance between the tip and the sample, a P-611.3 NanoCube positioning unit (6, Physik Instrumente, Karlsruhe, Germany) with nanometer resolution was placed inside the lower part (5). Figure 4 shows a photograph of the assembled SKP-SECM system. For electrical noise elimination, the SKP-SECM system was placed inside a Faraday cage on a vibration damping table. For measurements in the redox-competition mode of the SECM subsequent to the measurements in the SKP mode, both reed relays have to be switched, the electrochemical cell has to be filled with an electrolyte, and a reference as well as an auxiliary electrode has to be connected to the bipotentiostat. Switching of both reed relays results in the electrical connection of the tip electrode to the bipotentiostat as working electrode 2 and connection of the sample to the bipotentiostat as working electrode 1 (Figure 5). Test Measurements. After controlled approach of the tip electrode (here a 125 μm diameter glass-insulated Pt electrode) to the test sample, a scan was performed over the marked sample area (Figure 6), during which the CPD was recorded as a function of the x- and y-axes positions of the tip. Control of the tip-to-sample distance was performed by monitoring the magnitude of an additional alternating voltage (E = 200 mV, F = 10 Hz) and keeping it at a constant value via a computer based feedback loop and displacement of the z-axes position. Since SKP is a very sensitive technique and the results of the measurements may be affected by many factors such as acoustic noise or electromagnetic fields caused by the piezo actuator, by electromagnetic and electrostatic fields arising from the stepper motors and cables, the measured CPD have to be validated against a 6118
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triple shielded. The position of the operational amplifier inside the sample holder was optimized to enable as short as possible connection between the sample and the input of the operational amplifier. Additionally, all conductive parts inside of the Faraday cage were single grounded. After all optimization procedures, the noise level was lowered by up to 30 μV and thus a sufficiently good signal-to-noise ratio was achieved for the study with a 25 μm Pt-disk electrode. An area scan on the same test sample (see Figure 7, top) was performed in the SKP mode and subsequently in the RCSECM mode using a 25 μm Pt-disk electrode with a displacement increment of 10 μm. Single line scans over the marked area (Figure 7 bottom) crossing the W to Pt coated edge clearly demonstrated the increased resolution. The width of the transition between the CPD of W and Pt was determined to be about 120 μm in the SKP mode and 20 μm in the RC-SECM mode. The difference in the obtained resolution was attributed to the insulating sheath of tip electrode. Optical microscopy revealed an outer diameter of the glass sheath of about 120 μm. We assume that the resolution in the SKP mode is influenced by the outer diameter of the glass sheath of the Pt-disk electrode. In contrast, the resolution in the RC-SECM mode is dominated by the size of the Pt-disk itself.
Figure 7. Scanning area on the sample surface (top). Line scan over the marked area on the sample surface (bottom) performed in the SKP mode (triangles) and in the RC-SECM mode (circles). SKP probe/ SECM tip: 25 μm Pt-disk electrode. Scan increment: 10 μm.
standard. The CPD of different metals measured vs Ni as a reference sample are available.15 Thus, in order to compare the measured CPD values obtained with the SKP-SECM system, a Ni plate was added as a reference sample. A ΔCPD between W and Pt of about 350 mV was detected. Figure 6 represents a line scan over the marked sample area performed with the SKPSECM setup in the SKP mode with the reed relay switched as shown in Figure 1. The measured ΔCPD between W and Pt is around 330 mV and agrees well with the literature data. Immediately after measurements in the SKP mode, the reed relays were switched to a position enabling the connections as shown in Figure 5 for SECM measurements. The electrochemical cell formed by an O-ring was filled with 150 μL of phosphate buffer (pH 7.0). During the measurement, the sample and the tip were polarized at 600 mV vs Ag/AgCl (3 M KCl) and the oxygen reduction current measured at the tip was monitored as a function of the x- and y-axes position. In order to avoid complete consumption of the dissolved oxygen in the gap between tip and sample, a multipotential pulse profile was applied at the tip (see the Experimental Section). Control of the tip-to-sample distance was performed using a software based tilt correction procedure described elsewhere.5 Figure 6 (bottom) shows a line scan over the marked sample area from the W coated area to the Pt coated area performed by means of the RC-SECM. Two areas with different catalytic activity toward the ORR are clearly visible. The transition of the current correlate well with the transition of the CPD values from the SKP measurement. After the initial functioning of the SKP-SECM system was proven using a combined 125 μm Kelvin probe/SECM tip, optimization was mainly focused on increasing the lateral resolution. For this, all power supply cables were additionally shielded (double shielding) and especially the power supply cable for the piezo actuator was
’ CONCLUSIONS Design and technical realization of an integrated SKP-SECM system was demonstrated. The performance of the system was tested and it could be shown that the determination of the CPD in air under ambient conditions and visualization of the local catalytic activity of the same area on the sample surface in phosphate buffer was possible without replacement of the sample or exchange of the tip electrode. Further development of the SKP-SECM system is focused on improvement of the lateral resolution and increase of the signal-to-noise ratio. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors are grateful to Dr. Thomas Erichsen (Sensolytics GmbH, Bochum, Germany) for technical support and the team of the mechanical workshop of the Faculty of Chemistry and Biochemistry of Ruhr University Bochum for building some of the SKP-SECM system components. The International MaxPlank Research School SurMat and the DFG (Grant Schu959/ 9-1) are acknowledged for financial support. ’ REFERENCES (1) Bard, A. J.; Fan, F.-R. F.; Kwak, J.; Lev, O. Anal. Chem. 1989, 61 (2), 132–138. (2) Eckhard, K.; Schuhmann, W. Analyst 2008, 133, 1486–1497. (3) Eckhard, K.; Chen, X.; Turcu, F.; Schuhmann, W. Phys. Chem. Chem. Phys. 2006, 8, 5359–5365. (4) Karnicka, K.; Eckhard, K.; Guschin, D. A.; Stoica, L.; Kulesza, P. J.; Schuhmann, W. Electrochem. Commun. 2007, 9, 1998–2002. (5) Guadagnini, L.; Maljusch, A.; Chen, X.; Neugebauer, S.; Tonelli, D.; Schuhmann, W. Electrochim. Acta 2009, 54, 3753–3758. (6) Okunola, A.; Nagaiah, T. C.; Chen, X.; Eckhard, K.; Schuhmann, W.; Bron, M. Electrochim. Acta 2009, 54, 4971–4978. 6119
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