Electrochemical Roughening of Thin-Film Platinum Macro and Microelectrodes

Summary

This protocol demonstrates a method for electrochemical roughening of thin-film platinum electrodes without preferential dissolution at grain boundaries. The electrochemical techniques of cyclic voltammetry and impedance spectroscopy are demonstrated to characterize these electrode surfaces.

Abstract

This protocol demonstrates a method for electrochemical roughening of thin-film platinum electrodes without preferential dissolution at grain boundaries of the metal. Using this method, a crack free, thin-film macroelectrode surface with up to 40 times increase in active surface area was obtained. The roughening is easy to do in a standard electrochemical characterization laboratory and incudes the application of voltage pulses followed by extended application of a reductive voltage in a perchloric acid solution. The protocol includes the chemical and electrochemical preparation of both a macroscale (1.2 mm diameter) and microscale (20 µm diameter) platinum disc electrode surface, roughening the electrode surface and characterizing the effects of surface roughening on electrode active surface area. This electrochemical characterization includes cyclic voltammetry and impedance spectroscopy and is demonstrated for both the macroelectrodes and the microelectrodes. Roughening increases electrode active surface area, decreases electrode impedance, increases platinum charge injection limits to those of titanium nitride electrodes of same geometry and improves substrates for adhesion of electrochemically deposited films.

Introduction

Nearly five decades ago, the first observation of surface enhanced Raman spectroscopy (SERS) occurred on electrochemically roughened silver¹. Electrochemical roughening of metal foils is still attractive today because of its simplicity over other roughening methods^2,3 and its usefulness in many applications like improving aptamer sensors⁴, improving neural probes⁵, and improving adhesion to metal substrates⁶. Electrochemical roughening methods exist for many bulk metals^1,5,7,8,9,10. Until recently, however, there was no report on the application of electrochemical roughening to thin (hundreds of nanometers thick) metal films⁶, despite the prevalence of microfabricated thin-film metal electrodes in a number of fields.

Established methods to roughen thick platinum (Pt) electrodes^5,8 delaminate thin-film Pt electrodes⁶. By modulating the frequency of the roughening procedure and the electrolyte used for the for the roughening, Ivanovskaya et al. demonstrated Pt thin-film roughening without delamination. That publication focused on using this new approach to increase the surface area of platinum recording and stimulation electrodes on microfabricated neural probes. The roughened electrodes were demonstrated to improve recording and stimulation performance and improve adhesion of electrochemically deposited films and improve biosensor sensitivity⁶. But this approach also likely improves surface cleaning of microfabricated electrode arrays and enhances the capabilities of thin-film electrodes for other sensor applications (e.g., aptasensors) as well.

The approach to roughen thin-film macroelectrodes (1.2 mm diameter) and microelectrodes (20 µm diameter) is described in the following protocol. This includes preparation of the electrode surface for roughening and how to characterize the roughness of the electrode. These steps are presented along with tips on how to optimize the roughening procedure for other electrode geometries and the most important factors to ensure an electrode is roughened nondestructively.

Protocol

​CAUTION: Please consult all relevant safety data sheets (SDS) before use. Several of the chemicals used in this protocol are acutely toxic, carcinogenic, oxidizing and explosive when used at high concentrations. Nanomaterials may have additional hazards compared to their bulk counterpart. Please use all appropriate safety practices when carrying out this protocol including the use of engineering controls (fume hood) and personal protective equipment (safety glasses, gloves, lab coat, full length pants, closed-toe shoes).

1. Cleaning the Pt electrode(s) before initial characterization and surface roughening

1. Chemically clean the electrodes under ozone with a laboratory UV-ozone cleaner at 80 °C for 10 min.
2. Soak the portion of the probe containing the electrode(s) in a solvent (e.g., a 30 min soak in acetone for the microelectrodes demonstrated in this protocol).
   NOTE: Other methods may be more effective for removing organics from the electrodes depending on electrode housing and geometry, but this solvent soaking works well for the electrodes in the protocol.
3. Electrochemically clean the surface of all electrodes by repetitive potential cycling in an acidic solution of perchloric acid. The perchloric acid solution does not need purging to change the concentration of any gasses present.
   1. Load settings onto the potentiostat to apply cyclic voltammograms (CVs) to the electrodes. Scan from 0.22 V to 1.24 V vs Ag|AgCl (or -0.665 V to 0.80 V vs mercury sulfate reference electrode (MSE), the reference used for roughening) at a scan rate of 200 mV/s.
      NOTE: Regardless of reference material used, all potentials in this paper are given with respect to Ag|AgCl (saturated with KCl) reference electrode. The potential offset between the MSE (containing 1.0 M H₂SO₄) used in this study and Ag|AgCl (saturated with KCl)^{is 0.}44 V¹¹.
      1. In the EC-Lab Software, under the Experiment tab, press the + sign to add electrochemical technique. In the pop-up window, Insert techniques will appear.
      2. Click on Electrochemical techniques. When it expands, click on Voltamperometric techniques. When that expands, double click on Cyclic Voltammetry - CV. 1-CV line will appear in the Experiment window.
      3. In the Experiment window, fill in the following parameters:
         Ei = 0 V vs Eoc
         dE/dt = 200 mV/s
         E1 = -0.665 V vs Ref
         E2 = 0.8 V vs Ref
         n = 200
         Measure <I> over last 50% of the step duration
         Record <I> averaged over N = 10 voltage steps
         E Range = -2.5; 2.5 V
         Irange = Auto
         Bandwidth = 7
         ​End scan Ef = 0 V vs Eoc
   2. Submerge the electrode tip of the device in a 500 mM perchloric acid (HClO₄) solution that also contains a Pt wire counter electrode and MSE reference.
      NOTE: To avoid alterations in the electrochemical processes from chloride ion contamination, a chloride-free reference electrode (e.g., leakless Ag|AgCl or MSE, etc.) must be used for all tests performed inside acidic electrolytes in this protocol.
   3. Connect one electrode or short several electrodes of a multielectrode device together as the working electrode.
   4. Connect the working, counter, and reference electrodes to the potentiostat.
   5. In the EC-Lab Software, in the Experiment window, press Advanced settings on the left.
   6. Under Advanced settings, select Electrode configuration = CE to ground. Connect the working, counter and reference electrode to the instrument leads as shown on the Electrode connection diagram.
   7. Press the Run button (green triangle under Experiment window) to begin the experiment.
   8. Perform repetitive potential cycles until the voltammograms visually appear to overlap from one cycle to the next. This typically occurs after 50-200 CVs.

2. Electrochemical characterization of the electrode surface before roughening

1. Perform all electrochemical characterizations in the 3-electrode configuration described above in steps 1.3.2 - 1.3.4. All potentials in the following steps are given with respect to an Ag|AgCl reference electrode. Use a Pt wire as the counter electrode. Use a conventional Ag|AgCl electrode for characterization performed in phosphate buffered saline (PBS), but use a leakless Ag|AgCl or MSE as the reference for all tests performed in acidic solutions.
   1. Load settings on the potentiostat for the application of CVs from -0.22 to 1.24 V vs Ag|AgCl (or -0.665 V to 0.80 V vs MSE) at a scan rate of 50 mV/s. Submerge the electrode tip of the device in a beaker of deoxygenated 500 mM HClO₄ (deoxygenated with N₂ gas for ≥10 min) that also contains a Pt wire counter electrode and MSE reference.
      1. In the EC-Lab Software, under the Experiment tab, press the + sign to add electrochemical technique. In the pop-up window, Insert techniques will appear.
      2. Click on Electrochemical techniques. When it expands, click on Voltamperometric techniques. When that expands, double click on Cyclic Voltammetry - CV. 1-CV line will appear in the Experiment window.
      3. In the Experiment window, fill in the following parameters:
         Ei = 0 V vs Eoc
         dE/dt = 50 mV/s
         E1 = -0.665 V vs Ref
         E2 = 0.8 V vs Ref
         n = 10
         Measure <I> over last 50% of the step duration
         Record <I> averaged over N = 10 voltage steps|
         E Range = -2.5; 2.5 V
         Irange = Auto
         Bandwidth = 7
         End scan Ef = 0 V vs Eoc
         NOTE: The only differences between this setup and that described previously in step 1.3 are the use of deoxygenated 500 mM HClO₄ and ensuring that only one electrode is used as the working electrode. In the EC-Lab Software, in the Experiment window, press Advanced settings on the left.
      4. Under Advanced settings, select Electrode configuration = CE to ground. Connect the working, counter and reference electrode to the instrument leads as shown on the Electrode connection diagram.
      5. Press the Run button (green triangle under Experiment window) to begin the experiment.
      6. Perform repetitive potential cycles until the voltammograms visually appear to overlap from one cycle to the next.
   2. Calculate the electrode surface area from the hydrogen adsorption peaks of the highly reproducible (overlapping) CVs using the method of J. Rodríguez, et al.¹¹.
      1. Determine the charge associated with adsorption of a hydrogen monolayer (Q) to the electrode surface by integrating the two cathodic peaks of a CV between the potentials where the cathodic current deviates from the double layer current () and the hydrogen evolution starts () after subtracting the charge associated with monolayer charging (). Scan rate (ν) also effects this adsorption. Use the equation below to determine Q.
           
         Graphical representation of integrated area can be found in J. Rodríguez, et al.¹¹.
      2. Calculate the effective surface area (A) of an electrode by dividing Q by the charge density of the formation of hydrogen monolayer (k). For an atomically flat polycrystalline Pt surface, k = 208 µC/cm².
         A = Q / k
   3. If the two cathodic peaks of a Pt CV are poorly resolved, estimate the electrode surface area from the double layer capacitance at the electrode-solution interface. Use of the approach described in step 2.1.1 when hydrogen peaks are poorly resolved will lead to inaccurate results.
      1. Measure the impedance spectra of a single electrode under open circuit conditions in PBS (pH 7.0, 30 mS/cm conductivity). Submerge the electrode tip of the device in PBS that also contains a Pt wire counter electrode and MSE reference. Connect one electrode at a time as the working electrode. Next, use a potentiostat to apply an impedance sign wave with an amplitude of 10 mV over the frequency range 1 Hz - 100 kHz.
         1. In the EC-Lab Software, under the Experiment tab, press the + sign to add electrochemical technique. In the pop-up window, Insert techniques will appear.
         2. Click on Electrochemical techniques. When it expands, click on Impedance Spectroscopy. When that expands, double click on Potentio Electrochemical Impedance Spectroscopy. 1-PEIS line will appear in the Experiment window.
      2. In the Experiment window, fill in the following parameters:
         Ei = 0 V vs Eoc
         fi = 1 Hz
         ff = 100 kHz
         Nd = 6 points per decade
         In Logarithmic spacing
         Va = 10 mV
         Pw = 0.1
         Na = 3
         nc = 0
         E Range = -2.5; 2.5 V
         Irange = Auto
         Bandwidth = 7
      3. In the EC-Lab Software, in the Experiment window, press Advanced settings on the left.
      4. Under Advanced settings, select Electrode configuration = CE to ground. Connect the working, counter and reference electrode to the instrument leads as shown on the Electrode connection diagram.
      5. Press the Run button (green triangle under Experiment window) to begin the experiment.
   4. Determine the double layer capacitance from the electrode's impedance spectra (collected in step 2.1.4.1) by fitting the spectra with an equivalent circuit model using impedance analysis software.
      ​NOTE: Analysis in representative results and in Ivanovskaya, et al.⁶ was carried out with the impedance analysis fitting tool Z Fit.
      1. In the EC-Lab Software, click Load data file under Experiment list menu.
      2. Select Nyquist Impedance plot type at the top menu bar.
      3. Click Analysis, then select Electrochemical Impedance Spectroscopy, and click Z Fit.
      4. When then Z-Fit Bio-Logics pop-up window appears, click the Edit button
      5. Select Display circuit with 2 elements and choose R1 + Q1 from the list of equivalent circuit models. Click OK.
      6. Expand the Fit section of the pop-up window and make sure that the settings are Randomize + Simplex, stop randomize at 5,000 iterations, and stop fit on 5,000 iterations.
      7. Press the Calculate button and observe initial fit spectra added to the plot. Press Minimize and observe finalized fit.
      8. Adjust fit boundaries (green circles) to exclude noisy or distorted data from the fit. Estimated fit parameters will appear under Results section.
   5. Ensure that the calculated equivalent circuit model fits a Nyquist plot of the data that includes ohmic resistance (R) in series with a constant phase angle (CPE).
      1. Take note of the double layer capacitance value (Q) that is part of CPE in the equivalent circuit model.
      2. Estimate the change in surface area as a ratio of Q measured before and after roughening since double layer capacitance (Q) increases linearly with active surface area¹².

3. Electrochemical roughening of a macroelectrode

NOTE: Electrochemical roughening is driven by series of oxidation/reduction pulses that result in oxide growth and dissolution. In the case of a weakly adsorbing anion (like HClO₄), this dissolution is accompanied by Pt crystallite redeposition while in the case of strongly adsorbing anions (like H₂SO₄) this process results in preferential intergrain Pt dissolution that creates microcracks in the electrode surface⁶. Therefore, usage of high purity HClO₄ electrolyte is essential to prevent microcracks in the electrode surface.

1. Use a potentiostat able to apply voltage pulses with the 2 ms pulse width to roughen macroelectrodes. This procedure can be done with either potentiostat on the accompanying materials list.
2. Program the following parameters into the potentiostat to roughen a 1.2 mm diameter Pt disk macroelectrode.
   1. Begin the roughening protocol with a series of oxidation/reduction pulses between -0.15 V (V_min) and 1.9 - 2.1 V (V_max) at 250 Hz with a duty cycle of 1:1 for 10 - 300 s. The duration of pulse application determines the extent of roughening, the longer the pulsing the more roughening occurs. Use Figure 1A and the discussion as a guide to help determine the specific parameters required to achieve a particular surface roughness.
      1. Open the VersaStudio program.
      2. Expand the Experiment menu and select New.
      3. In the Select Action pop-up window that appears, choose Fast potential pulses and enter the desired file name when prompted. Fast potential pulses line will then appear under Actions to be performed tab.
      4. Fill out the following under the Properties of Fast Potential Pulses/Pulse properties. Enter Number of pulses = 2, Potential (V) 1 = -0.39 vs Ref for 0.002 s, and Potential (V) 2 = 1.56 vs Ref for 0.002 s.
      5. Under Scan properties, fill out: Time per point = 1 s, number of cycles: 50,000 (for 200 s duration).
      6. Under Instrument properties, enter Current range = Auto.
   2. Program the potentiostat to immediately follow the series of pulses with a prolonged application of a constant reduction potential (-0.15 V (or -0.59 V vs MSE) for 180 s) to fully reduce any oxides produced and stabilize the electrode surface.
      1. In the VersaStudio Software, press the + button to insert a new step.
      2. Double click on Chronoamperometry.
      3. Enter Potential (V) = -0.59, Time per point (s) = 1, and Duration (s) = 180.
   3. Use the visual representation of the paradigm described in steps 3.2.1. and 3.2.2 ( Figure 2) to aid in programming the potentiostat.
      NOTE: Specific parameters will vary for different electrode geometries but using the parameters above as a starting point and then varying V_max and pulse duration is the recommended method to optimize roughening parameters for other geometries. Using a high purity HClO₄ solution is essential for this step.
3. Submerge the electrode containing the tip of the device in 500 mM HClO₄ that also contains a Pt wire counter electrode and MSE reference electrode. Then connect an individual electrode as the working electrode and apply the pulsing paradigm to roughen the electrode.
4. In VersaStudio, press the Run button at the menu to start roughening.

4. Electrochemical roughening of a microelectrode

1. Use a potentiostat that can apply voltage pulses with the 62.5 µs pulse width to roughen microelectrodes. The VMP-300 potentiostat on the materials list is not capable of applying these short pulses, while the VersaSTAT 4 potentiostat can apply the rapid pulses required to roughen thin-film microelectrodes.
2. Program the following parameters into the potentiostat to roughen a 20 µm diameter Pt disk microelectrode fabricated flush with its insulating material. The roughening protocol can be applied to a single electrode or several electrodes shorted together (see additional explanation in step 4.3).
   1. Begin the roughening protocol with a series of oxidation/reduction pulses between -0.25 V (V_min) and 1.2 - 1.4 V (V_max) at 4,000 Hz with a duty cycle of 1:3 (oxidation:reduction pulse widths) for 100 s. Use guidance in the discussion to help determine the specific parameters required for other electrode geometries.
      1. Open the VersaStudio program.
      2. Expand the Experiment menu and select New.
      3. In the Select Action pop-up window that appears, choose Fast potential pulses and enter the desired file name when prompted. Fast potential pulses line will then appear under Actions to be performed tab.
      4. Fill out the following under the Properties of Fast Potential Pulses / Pulse properties, enter Number of pulses = 2, Potential (V) 1 = -0.49 vs Ref for 0.0625 ms, and Potential (V) 2 = 1.06 vs Ref for 0.1875 ms.
      5. Under Scan properties, fill out: Time per point = 1 s, and number of cycles: 400,000 (for 100 s duration).
      6. Under Instrument properties, enter Current range = Auto.
   2. Program the potentiostat to immediately follow the series of pulses with a prolonged reduction potential (-0.20 V for 180 s) to fully reduce any oxides produced and stabilize the chemistry of the electrode surface.
      1. In the VersaStudio Software, press the + button to insert a new step.
      2. Double click on Chronoamperometry.
      3. Enter Potential (V) = -0.64, Time per point (s) = 1, and Duration (s) = 180.
         NOTE: Using a high purity HClO₄ solution is essential for this step.
3. Submerge the electrode containing tip of the device in 500 mM HClO₄ that also contains a Pt wire counter electrode and MSE reference. Then connect an individual electrode or several shorted electrodes as the working electrode and apply the pulsing paradigm. In potentiostatic mode, electrodes can be shorted when trace resistance within the device is small. In that situation, ohmic drop through a device is negligible so all shorted electrodes will experience the applied potential.
4. In VersaStudio, press the Run button at the menu on the top of the screen to start the roughening.
   NOTE: Roughening of microelectrodes may require adjustment of the pulsing parameters depending on the electrode geometry, Pt composition, and topology (e.g., well depth for an electrode recessed in insulating material). Start with the parameters listed here and modify the V_max value to begin optimization of roughening parameters for different electrode geometries. The different pulsing parameters for three different geometries are summarized in Table 1.

5. Characterization of electrode surface after roughening

1. Determine the increase in effective surface area of macroelectrodes using steps 2.1.1-2.1.5.
2. Determine the increase in effective surface area of microelectrodes using steps 2.1.1-2.1.5.
3. Observe the changes in electrode appearance after roughening in optical microscopy as a loss of metal shininess (see Representative Results) and in scanning electron microscopy (SEM)⁶ as an obvious decrease in surface smoothness.

Results

A schematic showing the voltage application for roughening both macroelectrodes and microelectrodes is shown in Figure 2. Optical microscopy can be used to visualize the difference in the appearance of a roughened macroelectrode (Figure 3) or microelectrode (Figure 4). In addition, electrochemical characterization of the Pt surface using impedance spectroscopy and cyclic voltammetry can readily show ...

Discussion

The electrochemical roughening of thin-film macroelectrodes and microelectrodes is possible with oxidation-reduction pulsing. This simple approach does require several key elements to nondestructively roughen thin-film electrodes. Unlike foils, roughening of thin metal films may lead to sample destruction if parameters are not properly chosen. Critical parameters of the roughening procedure are pulse amplitude, duration and frequency. Additionally, ensuring electrode cleanliness and perchloric acid purity prior to the pr...

Materials

Name	Company	Catalog Number	Comments
Acetone	Fisher Scientific, Sigma Aldrich or similar	n/a	Laboratory grade
EC-Lab Software	Bio-Logic Science Instruments	n/a	For instrument control and data analysis
Leakless Silver/Silver Chloride Reference	eDAQ Company, Australia	ET069-1	Free from chloride anion contamination (or other type of chloride free electrode e.g. Mercury sulfate electrode)
Mercury Sulfate & Acid Electrode Kit	Koslow, Scientific Testing Instruments	5100A	glass, 9mm version
Milipore DI water	MilliporeSigma	n/a	Certified resistivity of 18.2 MΩ.cm (at 25°C)
Perchloric acid, 99.9985%	Sigma Aldrich	311421	High Purity
Phosphate-buffered saline	Teknova	P4007	10mM PBS with 100mM NaCl, pH 7 or similar product from elsewhere
Platinum Wire Auxiliary Electrode (7.5 cm)	BASi	MW-1032	Counter electrode
Pt macroelectrodes	Lawrence Livermore National Laboratory	n/a	1.2 mm diameter, 250 nm thick Pt disc electrodes insulated in polyimide. More information in Reference 9.
Pt microelectrode arrays	Lawrence Livermore National Laboratory	n/a	20 µm diameter 250 nM thick Pt disc electrodes insulated in polyimide. More information in Reference 9.
Sulfuric acid, 99.999%	Sigma Aldrich	339741	High Purity
UV & Ozone Dry Stripper	Samco	UV-1	for cleaning electrodes
VersaSTAT 4 Potentiostat	AMETEK, Inc.	n/a	Good time resolution for pulsing tests
VersaStudio Software	AMETEK, Inc.	n/a	For instrument control
VMP-200 Potentiostat	Bio-Logic Science Instruments	n/a	Low current resolution option is preferable for measurements with microelectrodes