Chemistry
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Assessment of Boron Doped Diamond Electrode Quality and Application to In Situ Modification of Local pH by Water Electrolysis
Chapters
Summary January 6th, 2016
A protocol is described for the characterization of the key electrochemical parameters of a boron doped diamond (BDD) electrode and subsequent application for in situ pH generation experiments.
Transcript
The goal of this video is to illustrate a standardized method for the electro-chemical assessment of diamond quality through solvent window, capacitance, reversability, and non-diamond carbon presence. Quality checked diamond electrodes are used to electro-chemically control the local pH of the measurement environment. The diamond electrode assessment method can help researchers in the field compare and contrast their results with others, as well as choose the appropriate diamond electrode for a given application.
The main advantages of the work presented are both a standardized assessment approach to diamond electrode quality, and a reliable quantitative methodology for changing local pH. Generally, individuals new to this method will struggle, because electro-chemical characterization of diamond quality does not often take place across the research field, leading to contrasting data. Demonstrating the procedures alongside Tania will be Max Joseph, post-doc from my laboratory, as well as the PhD students Lincong Meng, Roy Meyler, and Zoe Ayres.
Several types of boron doped diamond, or BDD, are available for electro-chemical use. Typical examples include free-standing all diamond substrates, and thin film supported on a non-diamond growth substrate. The thin film substrate-supported BDD should not be acid cleaned to avoid disintegration.
To begin, place a beaker of concentrated sulfuric acid on a hot plate at room temperature, and insert the BDD. Add potassium nitrate until it no longer dissolves. Then, cover with a watch glass and heat to approximately 300 degrees Celsius.
The solution will turn brown as it heats, and the potassium nitrate will dissolve. Leave the solution to heat for at least 30 minutes or until there is no longer any brown color to the solution. Then, turn off the hot plate and leave the solution to cool to room temperature.
Hydrophobic hydrogen terminated electrodes have reported contact angles of 60 to 90 degrees, which significantly reduces as the surface is rendered hydrophilic through oxygen termination. Place the diamond on the sample stage of a contact angle analyzer, ensuring it is flat. Place a one milliliter syringe in the positioner above the sample stage, and secure a needle on the end.
Fill the syringe with de-ionized water. Use the Z controller to lower the syringe to the sample. Then, use the X and Y controllers and the camera illuminator to align the syringe above the center of the diamond.
Using the analyzer software, dispense repeat one microliter volumes of water out of the syringe until a droplet forms at the tip of the needle, as visible on the camera image. Lower the needle to deposit the droplet onto the surface, and adjust the illumination for maximum contrast. Collect an image and apply the drop shape analysis software using the Conic section method.
Click find baseline'in the software, and then click computation'followed by tangent. To perform raman analysis for sp2 sp3 content, turn on the micro raman spectrometer and allow approximately 30 minutes for the ccd detector to cool down. Check that the appropriate lens, defraction grading, and filters are in place for use with the laser of choice.
To calibrate the system using a silicon calibration sample, first place the silicon substrate in the instrument chamber. Focus optically on the sample with the microscope. After shutting the door to the chamber, switch to laser view and check that the laser spot is well-defined and circular.
Calibrate using the software by clicking tools'followed by calibration, quick calibration, and then, okay. Remove the silicon substrate from the chamber and replace it with the BDD electrode. Optically focus the microscope on the area of interest.
Switch to laser view and open the shutter to check that the laser is focused. Close the shutter. Take a raman measurement using the software.
Click measurement, then new, then spectral acquisition. Set the measurement wave number range to cover the features of interest. Set the scan acquisition time to less than 10 seconds, the laser power to 100 percent, and the number of accumulations to five.
Press run'and save the resulting spectrum for analysis. Take a picture of the area raman was performed in using the live video. Save the image as a reference.
Observe the peak at approximately 1, 332 inverse centimeters in the spectrum, which indicates sp3 diamond. The broader the peak, the more defects are present. Observe any non-diamond carbon, or NDC, as indicated by a broad g peak centered at 1, 575 inverse centimeters in the spectrum, originating from the stretching of paired sp2 sites.
The greater the peak intensity, the more NDC present. Clean the electrode prior to use either by aluminum polishing or by electro-chemically cycling in dilute acid. Using a potentiostat, run cyclic voltammograms, or Cvs, at 0.1 volts per second between negative 0.1 volt and 0.1 volt, starting at zero volts.
Use the BDD as the working electrode, versus a platinum counter electrode and a common reference electrode. Analyze the second CV.Measure the total current magnitude at zero volts from the recorded capacitance curve, and divide by two. This value is I.Determine the capacitance C using the value for I as described in the text protocol.
Using a potentiostat, run a CV in 0.1 molar potassium nitrate at 0.1 volts per second from zero volts to negative two volts, and then between negative two volts and positive two volts, and back to zero volts. Use the BDD as the working electrode, versus a common reference electrode and platinum counter electrode. After repeating once, analyze the second CV.Convert the current to current density, taking surface roughness into account.
Quote the solvent window as the potentially window, defined by current limits of plus or minus 0.4 milliamps per centimeter squared in both directions. Observe the evidence of NDC in the solvent window. The oxygen reduction reaction is favored on NDC that is clearly evident in the reductive window.
Using a potentiostat, record Cvs in one millimolar ruthenium hexamine and 0.1 molar potassium nitrate between positive 0.2 volts and negative 0.8 volts versus SCE. Use scan rates in the range of 0.05 volts per second to 0.2 volts per second. Measure the peak current of the forward scan IP and correlate with that expected from the Randles-Sevcik equation.
Assuming that the electrode is disc-shaped in geometry and large enough that linear diffusion dominates. Using a potentiostat, run a CV in the iridium oxide solution between zero volts and positive one volt versus SCE to determine the potential at which the maximum current is recorded. This is the deposition potential, EDep, typically lying between approximately positive 0.6 volts and positive 0.85 volts.
It can vary depending on temperature and electrode material. Using chronoamperometry with a potentiostat, step the potential from zero volts, where no electrolysis occurs, to EDep for a time period of 0.2 seconds per step. Shown here is the electrode surface before and after deposition, displaying film formation.
Next, run a CV between zero volts and positive one volt in 0.1 molar sulfuric acid for the iridium oxide deposited electrode. The characteristic CV shape is shown here. Add five drops of phenolphthalein indicator solution to a 20 milliliter 0.1 molar potassium nitrate solution using a Pasteur pipette, and stir.
Place the BDD working electrode and platinum counter in solution. Apply a sufficiently negative current to the working electrode using a galvanostat such that the solution local to the electrode changes color from colorless to pink. Repeat with five drops of methyl red solution, instead of phenolphthalein, and stir.
Apply a sufficiently positive current such that the solution local to the electrode changes color from yellow to red. Shown here is typical raman data recorded with a 514 nanometer laser on NDC containing diamond. A high-quality boron doped diamond would only show the sharp sp3 peak at around 1, 332 inverse centimeters.
A high-quality NDC free boron doped diamond electrode typically exhibits a wide, featureless solvent window. The capacitance curve should return a capacitance of much less than 10 microfarads per centimeter squared, as shown. A CV recorded with a typical outer sphere redox mediator at a sufficiently doped boron doped diamond shows a close to 59 millivolt separation between the anodic and cathodic current peaks.
Using various geometries of dual boron doped diamond electrodes in either stationary or convective systems, it is possible to generate both acidic and alkaline local pH changes via electrolysis of water on one electrode as seen in these typical, experimentally-measured pH versus time profiles. After watching this video, you should have a good understanding of how to characterize diamond electrodes, and be able to choose the right electrodes for the appropriate application.
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