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.
Boron doped diamond (BDD) electrodes have shown considerable promise as an electrode material where many of their reported properties such as extended solvent window, low background currents, corrosion resistance, etc., arise from the catalytically inert nature of the surface. However, if during the growth process, non-diamond-carbon (NDC) becomes incorporated into the electrode matrix, the electrochemical properties will change as the surface becomes more catalytically active. As such it is important that the electrochemist is aware of the quality and resulting key electrochemical properties of the BDD electrode prior to use. This paper describes a series of characterization steps, including Raman microscopy, capacitance, solvent window and redox electrochemistry, to ascertain whether the BDD electrode contains negligible NDC i.e. negligible sp2 carbon. One application is highlighted which takes advantage of the catalytically inert and corrosion resistant nature of an NDC-free surface i.e. stable and quantifiable local proton and hydroxide production due to water electrolysis at a BDD electrode. An approach to measuring the local pH change induced by water electrolysis using iridium oxide coated BDD electrodes is also described in detail.
Choice of electrode material is of great importance when conducting any electroanalytical study. In recent years, sp3 carbon (diamond) doped with sufficient boron to render the material "metal-like" has become a popular choice for a wide range of electroanalytical applications due to its excellent electrochemical (and thermal and mechanical) properties1,2,3. These include corrosion resistance under extreme solution, temperature and pressure conditions4 ultra-wide solvent windows, low background currents, and reduced fouling, in comparison to other commonly used electrode materials5-7,3. However, increasing non-diamond-carbon (NDC: sp2) content results in a decreasing solvent window, increasing background currents7,8, changes in both structural integrity and sensitivity towards different inner sphere redox species, e.g. oxygen9-12.
Note for some applications, NDC presence is seen as advantageous13. Furthermore, if the material does not contain sufficient boron it will behave as a p-type semi-conductor and show reduced sensitivity to redox species in the reductive potential window, where the material is most depleted of charge carriers7. Finally, the surface chemistry of boron doped diamond (BDD) can also play a role in the observed electrochemical response. This is especially true for inner sphere species which are sensitive to surface chemistry and lower doped diamond where a hydrogen (H-)-terminated surface may make a semi-conducting BDD electrode appear "metal-like"7.
To take advantage of the superior properties of BDD, it is often essential the material is sufficiently doped and contains as little NDC as possible. Dependent on the method adopted to grow the BDD, the properties can vary14,15. This paper first suggests a materials and an electrochemical characterization protocol guide for assessing BDD electrode suitability prior to use (i.e. sufficient boron, minimal NDC) and then describes one application based on locally changing pH electrochemically using the protocol-verified electrode. This process takes advantage of the surface resilience of NDC-free BDD towards corrosion or dissolution under application of extreme applied potentials (or currents) for long periods of time. In particular the use of a BDD electrode to generate stable proton (H+) or hydroxide (OH–) fluxes due to electrolysis (oxidation or reduction respectively) of water in close proximity to a second (sensor)16,17 is described herein.
In this way it is possible to control the pH environment of the sensor in a systematic way, e.g. for pH titration experiments, or to fix the pH at a value where the electrochemical process is most sensitive. The latter is especially useful for applications where the sensor is placed at the source, e.g. river, lake, sea and the pH of the system is not optimal for the electrochemical measurement of interest. Two recent examples include: (i) generation of a localized low pH, in a pH neutral solution, for the electrodeposition and stripping of mercury17; note BDD is a favored material for electrodeposition of metals due to the extended cathodic window9,18,19. (ii) Quantification of the electrochemically detectable form of hydrogen sulfide, present at high pH, by locally increasing the pH from neutral to strongly alkaline16.
NOTE: BDD electrodes are most commonly grown using chemical vapor deposition techniques, attached to a growth substrate. They leave the growth chamber H-terminated (hydrophobic). If grown thick enough the BDD can be removed from the substrate and is termed freestanding. The freestanding BDD growth surface is often polished to significantly reduce surface roughness. Cleaning the BDD in acid results in an oxygen (O)-terminated surface.
1. Acid Cleaning BDD
2. Contact Angle Measurement
3. BDD Material Characterization
4. Electrochemical Characterization
5. pH Generation: Preparation of pH Sensitive Electrode and pH Generation
Raman spectra and electrochemical characteristics were obtained for representative BDD macrodisc electrodes with different dopant densities, and both significant and negligible levels of NDC, Figures 1 and 2. Figures 1A and B show typical Raman data for NDC-containing thin film microcrystalline BDD and larger grain freestanding BDD, doped above the metallic threshold, respectively. The presence of NDC is identifiable by the labeled broad peaks between 1,400 and 1,600 cm-1; there is no such peak visible in Figure 1C, which shows the typical Raman signature of NDC-free, freestanding BDD. In all three spectra in Figure 1 it is possible to observe a sharp peak at 1,332 cm-1, this is the signature peak of sp3 carbon (diamond); asymmetry of the baseline around this peak is known as a "Fano resonance" and if present indicates that the sample is suitably doped (1020 B atoms cm-3) for use in electrochemical studies. This is the case for all three electrodes shown here.
In Figure 2 example data for electrochemical studies (capacitance, solvent window and CVs recorded in the redox mediator Ru(NH3)63+) are presented for both NDC-containing and NDC-free BDD, doped above the metallic threshold. The capacitance curves in Figure 2A clearly indicate that NDC-containing BDD exhibits a greater capacitive current than NDC-free BDD. The capacitances for each has been calculated as described in the text and are quoted in Figure 2A as 10.8 µF cm-2 (NDC-containing) and 6.3 µF cm-2 (NDC-free) BDD. High quality, low NDC-content, BDD electrodes are expected to have a capacitance <<10 µF cm-2. Similarly, Figure 2B compares the solvent windows of exemplar NDC-containing and NDC-free BDD electrodes. It can be seen that for an NDC-containing electrode the onset of H2O oxidation and reduction has been brought in significantly, narrowing the solvent window. Also of note is the appearance of anodic peaks due to the oxidation of NDC and a cathodic peak due to ORR which is catalyzed on NDC but not on sp3 carbon. For a high quality BDD electrode with negligible NDC the solvent window is expected to be >>3 V in aqueous KNO3 solution. In Figure 2C the CV response of BDD electrodes with a variety of doping levels are investigated using the redox mediator Ru(NH3)63+. For BDD electrodes doped above the metallic threshold, the voltage separation between the anodic and cathodic current peaks is expected to be close to 59 mV, in accordance with the Nernst equation; however, as the dopant level decreases the material becomes depleted of charge carriers resulting in an increase in the peak to peak separation.
A BDD macrodisc, coated in IrOx, was used to record the data in Figure 3, while all diamond (BDD insulated in diamond)39 dual electrodes and an epoxy sealed BDD ring disc electrode were used for the pH generation experiments in Figure 4A. The data in Figure 3 illustrates the deposition and characterization process for a pH sensitive IrOx film on BDD. In Figure 3A a typical CV recorded in the IrOx deposition solution is shown. The potential employed for subsequent IrOx deposition can be identified from the position of the oxidative current peak, as illustrated here. Figure 3B is an exemplar CV in sulfuric acid of an IrOx film electrodeposited on BDD. The shape of the CV is characteristic of a successfully deposited film with the peak current density providing information on film thickness. A higher current density indicates a thicker film. The stability of the film is thickness dependent; too thin and the pH response will drift, too thick and the film response time will be slow and the film can flake off. A value for peak current density ~0.7 mA cm-2 has been shown to indicate a stable film with an excellent pH response. The OCP response of the IrOx layer on a BDD electrode towards different pH buffers is shown in Figure 3C. The drift between measurements is small as evidenced by the size of the error bars and the slope is super-Nernstian (>59 mV) as expected for this type of film.
Finally, Figure 4 illustrates the use of a BDD electrode for pH generation. In Figure 4A the pH change measured at an IrOx coated BDD electrode is presented for a range of currents applied to the pH generation BDD electrode placed nearby, either in ring or band format, as illustrated in Figure 4. For different applied currents, the pH can be changed locally and quantifiably from a starting value (near neutral) to either acidic or alkaline. This process can be observed visually as illustrated in Figure 4B, where a suitable current density is applied to a BDD electrode to change the pH from close to neutral to >10.5. In the presence of phenolphthalein (pH indicator) this results in the solution going from colorless to pink, in the vicinity of the electrode.
Figure 1. Typical Raman data recorded with a 514 nm laser on (A) NDC containing thin film microcrystalline BDD attached to the growth substrate (dopant density 1.9 × 1020 boron atoms cm-3) and (B, C) larger grain freestanding BDD, average dopant density 1.9 × 1020 and 3 × 1020 B atoms cm-3 respectively. NDC is evident in (A) and (B) due to the presence of the labeled NDC peaks between 1,400 and 1,600 cm-1, (C) contains negligible NDC. All three electrodes show a "Fano resonance" and thus are suitably boron doped for electrochemical studies7. Reproduced in part from reference [4c] with permission. Please click here to view a larger version of this figure.
Figure 2. Electrochemical characterization. All representative data in (A, B) has been recorded on insulating diamond encased O-terminated BDD electrodes doped above the metallic threshold i.e. 1020 B atoms cm-339. (A) Capacitance curves for NDC-free BDD where C = 6.3 µF cm-2 (black), and for NDC-containing BDD where C = 10.8 µF cm-2 (red). (B) Representative solvent windows for NDC-free BDD, solvent window > 3.95 V (black) and for NDC-containing BDD, solvent window = 3.22 V (red). (C) CVs recorded in 1 mM Ru(NH3)63+ at 0.1 V sec-1 for glass sealed freestanding BDD macrodisc electrodes of different boron dopant densities in the range 9.2 × 1016 – 3 × 1020 B atoms cm-3. Reproduced in part from reference [4c] with permission. Please click here to view a larger version of this figure.
Figure 3. Characterization of IrOx film deposition on BDD and pH response. (A) CV in IrOx solution prior to deposition. The maximum oxidation current provides a value for the deposition potential, Edep, where film formation is found to be most efficient. Using potentials > Edep, results in an unstable deposited film. (B) Characteristic CV for an electrodeposited IrOx film in 0.1 M H2SO4 recorded at 0.1 V sec-1; ip,a is typically ~ 0.7 mA cm-2. (C) Representative pH calibration curve (R2 = 0.997) for electrodeposited IrOx on a freestanding BDD electrode. The slope shows a super-Nernstian response (65.4 mV) to pH. The small error bars (n=3) indicate film stability and reproducibility in the measurements. Please click here to view a larger version of this figure.
Figure 4. Use of a freestanding BDD ring disc and dual band electrodes for in situ pH control. BDD ring disc electrode, disc diameter = 0.922 mm, separation = 0.262 mm, and ring width = 0.150 mm; BDD band electrode generator = 0.460 × 3 mm, detector = 0.09 × 3 mm, and separation = 0.2 mm. (A) Experimentally measured pH versus time profile over the detector electrodes as a function of applied galvanostatic current (+10 to +50 μA at the ring disc electrode and -0.5 to -8 µA for the dual band electrode). Note the stable pH generated over long periods of time. Modified reproduction of references [9a] and [9b]. (B) Simple visualization of in situ pH generation using phenolphthalein indicator solution; a current of -4.55 µA (-0.58 mA cm-2) was applied to a 1 mm diameter glass sealed BDD macroelectrode. The pink color indicates pH≥10.5, colorless solution indicates pH≤8.4 38. Please click here to view a larger version of this figure.
Starting with an O-terminated surface is advocated because the H-terminated surface is electrochemically unstable, especially at high anodic potentials7,40,41. Changing surface termination can affect the electron transfer kinetics of inner sphere couples, such as water electrolysis (used herein to change the local solution pH). Furthermore, if the BDD contains significant NDC at grain boundaries it is also possible that upon application of the extreme anodic/cathodic potentials advocated in this article for pH generation, etching could occur at these weaker points. This would cause the film to corrode and for thin films, eventually delaminate, manifesting itself in an unstable pH generation profile, as seen with thin film Au and Pt electrodes17. Hence a stringent protocol for assessing the quality of the electrode prior to use is adopted to assess NDC content as discussed in Figures 1 (Raman) and 2 (capacitance and solvent window).
Also of importance is the boron content. If the material is doped below the metallic threshold (< 1020 B atoms cm-3), it will be charge depleted, at potentials negative of the flatband potential, resulting in a decrease in electrochemical performance7,42. The easiest way to qualitatively assess metallic doping levels is to look for the presence of a Fano signature which causes asymmetry in the sp3 peak, in the Raman spectra, as shown in Figure 1(A-C). This is due to interference between the discrete phonon state and the electronic continuum and is seen at boron doping levels > 1020 B atoms cm-343. Secondary ion mass spectrometry (SIMS) ultimately quantifies boron content but is destructive and more intensive to use. Note as SIMS provides total boron content it does not account for possible reductions in the number of freely available charge carriers due to compensation or passivation of boron acceptors with suitable donors such as nitrogen44 or hydrogen45 respectively.
Electrochemically, dopant density differences can be visualized by employment of an outer sphere fast electron transfer redox couple whose formal potential lies within the band-gap of O-terminated semi-conducting BDD, such as Ru(NH3)63+/2+ 46. For example, as shown in Figure 2C, as the doping levels of the BDD electrode increase, and the material moves from semi-conducting to metallic the current increases and the peak to peak separation decreases as electron transfer becomes more facile. At metallic dopant levels the electrode should show behavior similar to a classical electrode where for a mediator such as Ru(NH3)63+, reversible diffusion limited CVs are recorded at a macroelectrode in stationary solution. Note, at boron dopant levels ~ 1019 close to reversible behavior has been recorded but only for H-terminated surfaces. This is due to an interesting peculiarity of this surface where H-terminated causes the energy levels of the valence and conduction bands in diamond to be raised. This means electron transfer from the valence band to H3O+ is now possible, resulting in surface transfer doping and a measurable surface conductivity. However, due to the electrochemical instability of the H-terminated surface, especially at high anodic potentials, working with H-terminated lower dopant density electrodes is not a long-term viable approach7,40,41.
The ability to modify the local pH of the measurement electrode has many different applications, for example local pH titration experiments now become possible where the pH can be systematically modified and the impact on the system electrochemically assessed in situ. Bound metal ions can be freed by decreasing pH enabling the sensor electrode to both assess free metal content at the natural pH and total metal content by locally decreasing to very acidic values, in situ47-50. This is very useful for at the source measurements. Additionally, species can be switched from not being electrochemically detectable to detectable by virtue of changing the local pH, e.g. dissolved hydrogen sulfide completely converts to the electrochemically detectable sulfide form at pH values > 9 16. In the example given, for the electrode geometries employed, pH changes over 4 units (from 6.4 to 2.0 and 6.0 to 10.8) were demonstrated. Larger changes are possible by increasing the galvanostatic current and changing the electrode geometries. For example, decreasing the separation between the generator and detector electrodes and reducing the relative size of the detector will allow lower/higher pH values to be attained. The feature size of the BDD electrode will be limited by the resolution of the fabrication technique employed. Note, there is also an upper limit to the size of the current able to be passed for stable pH generation. This is dictated by the current at which significant gas evolution and bubble formation at the generating electrode is observed.
The authors have nothing to disclose.
We would like to thank Dr. Jonathan Newland for the photograph in Figure 4B and for processing optical microscope images for the video, Miss Jennifer Webb for advice and visuals on contact angle measurements, Miss Sze-yin Tan for the solvent window data in Figure 2B, Dr Maxim Joseph for advice on Raman spectroscopy, and also members of the Warwick Electrochemistry and Interfaces Group who have helped to develop the protocols described herein. We would also like to thank Max Joseph, Lingcong Meng, Zoe Ayres and Roy Meyler for their part in filming the protocol.
Pt Wire | Counter Electrode | ||
Saturated Calomel Electrode | IJ Cambria Scientific Ltd. | 2056 | Reference Electrode (alternatively use Ag|AgCl) |
BDD Electrode | Working Electrode | ||
Iridium Tetrachloride | VWR International Ltd | 12184.01 | |
Hydrogen Peroxide | Sigma-Aldrich | H1009 | (30% w/w) Corrosive |
Oxalic Acid | Sigma-Aldrich | 241172 | Harmful, Irritant |
Anhydrous Potassium Chloride | Sigma-Aldrich | 451029 | |
Sulphuric Acid | VWR International Ltd | 102765G | (98%) Corrosive |
Potassium Nitrate | Sigma-Aldrich | 221295 | |
Hexaamine Ruthenium Chloride | Strem Chemicals Inc. | 44-0620 | Irritant |
Perchloric Acid | Sigma-Aldrich | 311421 | Oxidising, Corrosive |
2-Propanol | Sigma-Aldrich | 24137 | Flammable |
Nitric Acid | Sigma-Aldrich | 695033 | Oxidising, Corrosive |
Sputter/ Evapourator | With Ti & Au targets | ||
Raman | 514.5 nm laser | ||
Annealing Oven | Capable of 400°C | ||
Ag paste | Sigma-Aldrich | 735825 | or other conductive paint |
Potentiostat | |||
pH Buffer solutions | Sigma-Aldrich | 38740-38752 | Fixanal buffer concentrates |
Phenolphthalein Indicator | VWR International Ltd | 210893Q | |
Methyl Red Indicator | Sigma-Aldrich | 32654 |