Characterization of boron-doped diamond electrodes by electrochemical impedance spectroscopy

Charge transfer on boron doped diamond (BDD) electrodes was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The diamond films of 5 μm thickness and boron content between 200 ppm and 3000 ppm were prepared by the hot filament CVD technique on niobium substrate and mounted in a Teflon holder as rotating disk electrodes. The electrochemical measurements were carried out in aqueous electrolyte solutions of 0.5 M Na₂ SO ₄ + 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆]. Significant deviation in the redox behaviour of BDD and active Pt electrodes was indicated by a shift of the peak potentials in the cyclic voltammograms with increasing sweep rate and lower limiting diffusion current densities under rotating disk conditions. In the impedance spectra an additional capacitive element appeared at high frequencies. The potential and rotation dependence of the impedance spectra can be described quantitatively in terms of a model based on diffusion controlled charge transfer on partially blocked electrode surfaces. Direct evidence for the non-homogeneous current distribution on the diamond surface was obtained by SECM measurements.     

The impedance of fast charge transfer reactions on boron doped diamond electrodes

Reversible charge transfer on boron doped diamond (BDD) electrodes was studied using cyclic voltammetry and electrochemical impedance spectroscopy. Polycrystalline diamond films of 5 μm thickness with 200 and 3000 ppm boron content were prepared by chemical vapour deposition on niobium substrate. The samples were mounted in a Teflon holder and used as rotating disk electrodes (RDE) with rotation frequencies between 0 and 4000 rpm. The electrochemical measurements were carried out in aqueous electrolyte solutions of 0.5 M Na₂SO₄ + 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] and 0.1 M KCl + 5 mM [Ru(NH₃)₆]Cl₂/[Ru(NH₃)₆]Cl₃. The electrochemical redox behaviour of the BDD electrodes was found to differ significantly from that of an active Pt electrode. The deviations are indicated by a large peak potential difference and a shift of the peak potentials in cyclic voltammograms with increasing sweep rate. At rotating electrodes lower limiting current densities are found and the impedance diagrams exhibit an additional capacitive impedance element at high frequencies. The results are described quantitatively by an impedance model which is based on partial blocking of the diamond surface.     

Electrochemical behavior of high-pressure synthetic boron doped diamond powder electrodes

Boron doped diamond (BDD) was synthesized under high pressure and high temperature using B-doped graphite intercalation compositions (GICs) as carbon sources. The electrochemical characteristics of high-pressure synthetic BDD powder electrodes were investigated by measuring the cyclic voltammetry curves and AC impedance spectrum. For the [Fe(CN)₆]^3−/4− redox couple, the electrode reaction process is reversible or quasi-reversible at the scan rates of 0.01-1.0 V/s. At the low scan rate the linear relation between peak current and square root of scan rate indicates that the electrode process was a diffusion-controlled mass transport process. The electrochemical behavior is similar to a planar electrode. With the increasing of the scan rate the electrode process is controlled by the mass transport plus kinetic process. AC impedance spectra exhibit the porous structure characteristic of BDD powder electrode.     

Voltammetric and electrochemical impedance spectroscopy characterization of a cathodic and anodic pre-treated boron doped diamond electrode

The effect of boron doped diamond (BDD) surface termination, immediately after cathodic and anodic electrochemical pre-treatments, on the electrochemical response of a BDD electrode in aqueous media and the influence of the different supporting electrolytes utilized in these pre-treatments on the final surface termination was investigated with [Fe(CN)₆]^4−/3−, as redox probe, by cyclic and differential pulse voltammetry and electrochemical impedance spectroscopy. The cyclic voltammetry results indicate that the electrochemical behavior for the redox couple [Fe(CN)₆]^4−/3− is very dependent on the state of the BDD surface, and a reversible response was observed after the cathodic electrochemical pre-treatment, whereas a quasi-reversible response occurred after anodic electrochemical pre-treatment. Differential pulse voltammetry in acetate buffer also showed that the potential window is very much influenced by the electrochemical pre-treatment of the BDD surface. Electroactivity of non-diamond carbon surface species (sp² inclusions) incorporated into the diamond structure was observed after cathodic and anodic pre-treatments. Electrochemical impedance spectroscopy confirmed the cyclic voltammetry results and indicates that the BDD surface resistance and capacitance vary significantly with the electrolyte and with the electrochemical pre-treatment, caused by different surface terminations of the BDD electrode surface.     

Evaluation of boron doped diamond electrodes for organic electrosynthesis on a preparative scale⋆

We have investigated some anodic and cathodic transformations using boron doped diamond (BDD) electrodes. The oxidation of a propargylic alcohol as well as the aromatic side chain oxidation in water as electrolyte did not␣yield the desired products in high yield and selectivity and led mainly to the formation of CO₂ due to␣electrochemical incineration of the starting material. With methanol as electrolyte, however, the reactivity of BDD electrodes is similar to graphite in most anodic methoxylation reactions, but the inactive behaviour of BDD electrodes leads to a different reaction pathway possibly involving methoxyl radicals with charge transfer from the electrolyte. It has been found that at BDD anodes benzylic single and double bonds can be split yielding aromatic acetals and esters. With phenanthrenes as starting material, o,o′-disubstituted biaryls were obtained. So the use of BDD electrodes provides an efficient and environmentally friendly access to this interesting class of compounds. The high H₂ overpotential of BDD cathodes enables smooth and selective reduction of functional groups like oximes. Due to the high chemical and mechanical stability of the diamond layer of today’s electrodes, electrode lifetime as well as reproducibility of the electrosyntheses has improved markedly. Aqueous basic conditions, however, must be avoided for BDD anodes. These conditions result in degradation of the diamond surface. ^⋆Dedicated to Professor G. Kreysa on the occasion of his 60th birthday.     

Direct electrochemistry of blue copper proteins at boron-doped diamond electrodes

Boron-doped diamond (BDD) is a promising electrode material for use in the spectro-electrochemical study of redox proteins and, in this investigation, cyclic voltammetry was used to obtain quasi-reversible electrochemical responses from two blue copper proteins, parsley plastocyanin and azurin from Pseudomonas aeruginosa. No voltammetry was observed at the virgin electrodes, but signals were observed if the electrodes were anodised, or abraded with alumina, prior to use. Plastocyanin, which has a considerable overall negative charge and a surface acidic patch which is important in forming a productive electron transfer complex with its redox partners, gave a faradaic signal at pre-treated BDD only in the presence of neomycin, a positively charged polyamine. The voltammetry of azurin, which has a small overall charge and no surface acidic patch, was obtained identically in the presence and absence of neomycin. Investigations were also carried out into the voltammetry of two site-directed mutants of azurin, M64E azurin and M44K azurin, each of which introduce a charge into the protein's surface hydrophobic patch. The oxidizing and cleaning effects of the BDD electrode pre-treatments were studied electrochemically using two inorganic probe ions, Fe(CN)₆³⁻ and Ru(NH₃)₆³⁺, and by X-ray photoelectron spectroscopy (XPS). All of the electrochemical results are discussed in relation to the electrostatic and hydrophobic contributions to the protein/diamond electrochemical interaction.     

Electrochemical oxidation of aniline at boron-doped diamond electrodes

The electrochemical oxidation of aniline at boron-doped diamond (BDD) electrodes was investigated by cyclic voltammetry, steady-state polarization measurements and bulk electrolysis under potentiostatic control. It was found that acidic media is suitable for efficient electrochemical oxidation of aniline, because at low pH, the potential required for avoiding electrode fouling is lower than in neutral and alkaline media. The results of the longtime polarization measurements suggested that more anodic potentials ensure slightly higher efficiency for the conversion of aniline to CO₂, while the direct oxidation process does not play a prominent part in the overall electrochemical incineration of aniline. The current efficiencies (44%) and the efficiency of aniline conversion to CO₂ (80%) favourably compare with those reported for other electrochemical methods for aniline destruction. The results demonstrate the possibility of using BDD as an electrode material for electrochemical wastewater treatment, mainly when very high anodic potentials are required.     

Electrochemical behavior of carbon electrodes in organic liquid electrolytes containing tetrafluoroborate and hexafluorophosphate anionic species in different non-aqueous solvent systems

Electrochemical behavior of tetrabutylammonium salts containing tetrafluoroborate (BF₄⁻) and hexafluorophosphate (PF₆⁻) anionic species in different non-aqueous solvents had been investigated on glassy carbon (GC), boron-doped diamond (BDD) and Pt electrodes. Though both BF₄⁻ and PF₆⁻ ionic species are considered to be inert, they are found to undergo electrochemical oxidation only on GC electrode rather than BDD and Pt as found out from their anodic peaks in linear sweep and cyclic voltammograms (LSV and CV). The voltammetric peak is influenced by the sweep rate as well as by the concentration of the ionic species and the electron transfer process appears to be diffusion controlled one. The formation of an inhibitory C-F film on the electrode surface during anodic polarization either by potentiostatic or potentiodynamic techniques was clearly established by X-ray photoelectron spectroscopy (XPS) analysis and the charge transfer resistance (θ) is higher under latter conditions than the former. The inhibitory effect of this surface film towards the electron transfer reaction of Fe(CN)₆]⁴⁻/[Fe(CN)₆]³⁻ redox couple using impedance technique reveals that among the high permittivity non-aqueous solvents investigated in this work, CH₃CN shows maximum θ value and produces C-F film of optimum thickness than the others. A direct correlation was also found out from the plot of θ versus peak potential E_p and peak current density (i_p) obtained from LSV. The mechanism of film formation on GC electrode and the absence of such phenomenon on BDD were explained from the product analysis using ¹⁹F nuclear magnetic resonance (NMR) spectra resulting from the constant current electrolysis of BF₄⁻ and PF₆⁻ ionic species on both electrodes in CH₃CN medium.     

Electron Transfer Kinetics on Boron-Doped Diamond Part I: Influence of Anodic Treatment

Boron-doped diamond electrodes, both as-grown and polarized anodically under different conditions, were prepared in order to study the chemical and electrochemical changes of diamond and clarify the role played by the surface-state density. Many different treatments were employed: as-grown (BDD_ag), mildly polarized (BDD_mild), strongly polarized in perchloric acid (BDD_severerpar;, and strongly polarized in a sulphuric acid-acetic acid mixture (BDD_AcOHrpar;. Charge transfer processes at the electrode surface were studied by cyclic voltammetry. Simple electron transfer processes such as the outer-sphere redox system ferri/ferrocyanide (Fe^III/II;(CN)₆rpar; and complex charge transfer reactions such as the inner-sphere 1,4-benzoquinone/hydroquinone (Q/H₂Q) redox reaction were chosen to test the electrochemical properties of the electrodes. The properties of the diamond electrodes were found to undergo strong modification as a function of surface treatment. The active surface area and the reaction rate constants decreased significantly upon anodic polarization. Important drops in the charge carrier concentration on the surface and in true surface area led to hindrance of electron transfer at the electrode.     

The effect of CVD-diamond film thickness on the electrochemical properties of synthetic diamond thin-film electrodes

The electrochemical behavior of polycrystalline diamond films of different thickness (0.5–7 μm), grown by hot-filament CVD method, was studied by electrochemical impedance spectroscopy and cyclic voltammetry. The differential capacitance, background current, and potential window were measured in supporting electrolyte solution; the electrochemical kinetics, in [Fe(CN)₆]^3−/4− model redox system. With the increasing of the films thickness, the crystallite size increased; both the differential capacitance and background current in the indifferent electrolyte, as well as the transfer coefficients in the redox system, decreased; thus, the diamond electrode becomes as if less reversible. The effect of the films’ thickness is reduced to that of nondiamond (amorphous) carbon contribution from intercrystalline boundaries on the electrochemical behavior of the polycrystalline diamond electrodes.     

On the changing electrochemical behaviour of boron-doped diamond surfaces with time after cathodic pre-treatments

The electrochemical response of the Fe(CN)₆^4−/3− redox couple on boron-doped diamond (BDD) electrodes immediately after a cathodic pre-treatment and as a function of time exposed to atmospheric conditions is reported here. After this pre-treatment the electrode exhibits a changing electrochemical behaviour, i.e., a loss of the reversibility for the Fe(CN)₆^4−/3− redox couple as a function of time. Raman spectra showed that neither important bulk structural differences nor significant changes in the sp²/sp³ content are introduced into the BDD film by the cathodic pre-treatment indicating that H-terminated sites play an important role in the electrochemical response of the electrodes. Thus, the changing behaviour reflected by a progressive decrease of the electron transfer rate with time must be associated to a loss of superficial hydrogen due to oxidation by oxygen from the air, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis. Moreover, it was also found that this changing electrochemical behaviour is inversely proportional to the doping level, suggesting that the boron content has a stabilizing effect on the H-terminated surface. These results point out the necessity of doing the cathodic pre-treatment just before the electrochemical experiments are carried out in order to ensure reliable and reproducible results.     

Plasma etching treatment for surface modification of boron-doped diamond electrodes

Boron-doped diamond (BDD) thin film surfaces were modified by brief plasma treatment using various source gases such as Cl₂, CF₄, Ar and CH₄, and the electrochemical properties of the surfaces were subsequently investigated. From X-ray photoelectron spectroscopy analysis, Cl and F atoms were detected on the BDD surfaces after 3 min of Cl₂ and CF₄ plasma treatments, respectively. From the results of cyclic voltammetry and electrochemical AC impedance measurements, the electron-transfer rate for Fe(CN)₆^3−/4− and Fe^2+/3+ at the BDD electrodes was found to decrease after Cl₂ and CF₄ plasma treatments. However, the electron-transfer rate for Ru(NH₃)₆^2+/3+ showed almost no change after these treatments. This may have been related to the specific interactions of surface halogen (C-Cl and C-F) moieties with the redox species because no electrical passivation was observed after the treatments. In addition, Raman spectroscopy showed that CH₄ plasma treatment of diamond surfaces formed an insulating diamond-like carbon thin layer on the surfaces. Thus, by an appropriate choice of plasma source, short-duration plasma treatments can be an effective way to functionalize diamond surfaces in various ways while maintaining a wide potential window and a low background current.     

Covalent modification of boron-doped diamond electrodes with an imidazolium-based ionic liquid

An ionic liquid (IL, 1-(methylcarboxylic acid)-3-octylimidazolium-bis (trifluoromethylsulfonyl)imide) was covalently coupled onto a boron-doped diamond (BDD) surface through an esterification reaction. The resulting surface was characterized by X-ray photoelectron spectroscopy, water contact angle and electrochemical measurements. Selective electron transfer towards positively and negatively charged redox species was recorded. While the presence of Fe(CN)₆⁴⁻ could be detected on the IL-modified BDD interface, no surface-immobilized Ru(NH₃)₆³⁺ was recorded. The IL-modified BDD electrode showed in addition changes in surface wettability when immersed into aqueous solution containing different anions.     

Electrochemical characteristics of boron doped polycrystalline diamond electrode sintered by high pressure and high temperature

The bulk B-doped polycrystalline diamond (PCD) electrode in this study was prepared by high-pressure, high-temperature (HPHT) technology. The PCD was sintered under HPHT conditions, using B-doped diamond powders and a metal catalyst as raw materials, then the metal solvent phase was dissolved by aqua regia. The morphology and composition of the PCD were investigated with a scanning electron microscope (SEM), X-ray diffraction (XRD), and energy dispersion spectrum (EDS). The results show that the sintered body possesses a polycrystalline structure with direct diamond–diamond bond and irregularly shaped pores of 1–10 μm distributed on the grain boundaries after the metal solvent phase was removed. The cyclic voltammogram and electrochemical impedance spectroscopy of this B-doped electrode have been investigated. The B-doped PCD electrode exhibits stable electrochemistry in a KCl support solution over a wide potential range. The quasi-reversible reaction occurs on the electrode for the [Fe(CN)₆]^3−/4− couples. The electrode process combines the diffusion-controlled mass transport plus the kinetic process. The electrochemical impedance spectroscopy (EIS) analysis shows the porous structure characteristic of the PCD electrode.     

Electrochemical reactivity at graphitic micro-domains on polycrystalline boron doped diamond thin-films electrodes

This paper deals with the electrochemical reactivity of boron doped diamond (BDD) electrodes. A comparative study has been carried out to show the influence of the presence of graphitic micro-domains upon the surface of these films. Those graphitic domains are sometimes present on as-grown boron doped diamond electrodes. The effect of doping a pure Csp³ diamond electrode is established by highly oriented pyrolytic graphite (HOPG) abrasion onto the diamond surface. In order to establish the effect of doping on a pure Csp³ diamond electrode, the amount of graphitic domains was increased by means of HOPG crystals grafted onto the BDD surface. Indeed that method allows the enrichment of the Csp² contribution of the electrode.The presence of graphitic domains can be correlatively associated with the presence of kinetically active redox sites. The electrochemical reactivity of boron doped diamond electrodes shows a distribution of kinetic constants on the whole surface of the electrode corresponding to different active sites. In this paper, we have studied by cyclic voltammetry and electrochemical impedance spectroscopy the kinetics parameters of the ferri/ferrocyanide redox couple in KCl electrolyte. A method is proposed to diagnose the presence of graphitic domains on diamond electrodes, and an electrochemical “pulse cleaning” procedure is proposed to remove them.     

Comparison of glassy carbon and boron doped diamond electrodes: Resistance to biofouling

Carbon based electrodes are widely used for in vivo and in vitro electrochemical studies. In particular, monoamine neurochemistry has been investigated using carbon microfibre electrodes. Similarly, glassy carbon (GC) is the preferred material for many biochemical applications, such as electrochemical detection in chromatography. More recently, boron doped diamond (BDD) has been utilized for biosensing, as its carbon sp³ structure is expected to provide better resistance to analyte fouling. However, the main factor limiting the use of electrochemical sensors for biological studies is the effect of the biological matrix. Indeed, in vivo or in situ measurements expose the sensor to a complex matrix of proteins, which adsorb on the sensing surface and interfere with the electrochemical measurements.Here, we compare the performance of three carbon based electrodes: GC, GC with low surface oxides and BDD. The redox species ruthenium(III) hexaammine (outer-sphere), ferrocyanide (surface sensitive) and the biologically significant dopamine have been investigated in protein and blood-mimicking matrices. Cyclic voltammetry and electrochemical impedance spectroscopy have been used to examine the effect of spectator molecules and reaction products on electrode mechanisms.Our results show that BDD generally exhibits the best performance for most conditions and reactions and should therefore be preferred for measurements in biologically fouling environments. Furthermore, surface oxides seem also to improve resistance of the GC electrode to biofouling.     

Synthetic diamond electrodes: The effect of surface microroughnesson the electrochemical properties of CVD diamond thin films on titanium

The electrochemical behavior of B-doped diamond films on Ti substrates subjected to different pretreatment procedures (annealing, sand-blasting, and etching in HCl) is evaluated as a function of surface microroughness. Generally, the differential capacitance follows the true surface area of the electrodes. The width of the potential window also increases, but slightly, with the roughness. The electrode reversibility in the [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ redox system increases with increasing surface roughness. The apparent increase in the reversibility of the reaction may be also explained by the decrease in the true current density. Although the variations in the electrochemical parameters are not strongly pronounced, the tendencies observed can be used to optimise the electrode properties.     

Electrochemical oxidation and determination of antiretroviral drug nevirapine based on uracil-modified carbon paste electrode

A novel uracil covalently grafted carbon paste electrode (Ura/CPE) based on electro-deposition of uracil on CPE was prepared for the quantitative determination of nevirapine. The records of electrochemical impedance spectroscopy (EIS) and cyclic voltammograms (CV) in K₃Fe(CN)₆/K₄Fe(CN)₆ solution illustrated that uracil grafted on CPE efficiently decreased the charge transfer resistance value of electrode and improved the electron transfer kinetic between analyte and electrode. The electrochemical properties of Ura/CPE towards the oxidation of nevirapine were investigated by cyclic voltammetry and differential pulse voltammetry (DPV) in 0.1 M NaOH. The effects of pH and scan rates on the oxidation of nevirapine were studied. The results indicated the participation of the same protons and electrons in the oxidation of nevirapine, and the electrochemical reaction of nevirapine on Ura/CPE is an adsorption-controlled process. Under optimized conditions, the linearity between the oxidation peak current and nevirapine concentration was obtained in the range of 0.1–70.0 μM with detection limit of 0.05 μM and the sensitivity of 2.073 μA mM^?1 cm^?2 (S/N = 3). The proposed method was also successfully applied to detect the concentration of nevirapine in human serum samples.     

Electrochemical behavior of K<Subscript>4</Subscript>Fe(CN)<Subscript>6</Subscript> in [bmim]PF<Subscript>6</Subscript>/TX-100/H<Subscript>2</Subscript>O based microemulsions

The electrochemical behavior of potassium ferrocyanide [K₄Fe(CN)₆] at Pt/ionic liquid (IL) microemulsion interfaces was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). H₂O/TX-100/bmimPF₆ was used to prepare three IL microemulsions: water in 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆) (W/IL), bicontinuous (WIL) and bmimPF₆ in water (IL/W). The results show that the IL microemulsion systems have relatively narrower potential windows compared with the pure IL system. The redox potential gap is about 100 mV in the pure water and the three IL microemulsions. The redox potentials of K₄Fe(CN)₆/K₃Fe(CN)₆ and the redox peak currents decrease in the order pure water, IL/W, WIL, W/IL. Furthermore, the peak currents increase linearly with the square root of the scan rate, while the diffusion coefficient increased in the order W/IL, WIL, IL/W. The Nyquist plots obtained in the WIL and IL/W systems show capacitive resistance arcs at high frequencies and 45° straight lines at low frequencies, implying that the electrochemical reactions are controlled by charge transfer and diffusion steps. For the W/IL system there is only a 45° straight line in the Nyquist plot, indicating that diffusion is the controlling step at all frequencies.     

n-Type nitrogenated nanocrystalline diamond thin-film electrodes: The effect of the nitrogenation on electrochemical properties

Nitrogenated nanocrystalline diamond thin-film electrodes with controlled conductivity are grown from microwave- or arc-plasma in CH₄-Ar-H₂-N₂ gas mixtures. Their electrochemical behavior is studied using cyclic voltammetry and electrochemical impedance spectroscopy techniques. It is concluded from Mott-Schottky plots that the studied material has n-type conductance; the donor concentration is estimated. The character of electrode behavior is controlled by the degree of nitrogenation of the material. In particular, with the increasing of nitrogen concentration in the feeding gas (0-25%) supplied to plasma-chemical reactor, the potential window in the supporting electrolyte (2.5 M H₂SO₄) becomes somewhat narrower, the reversibility of electrochemical reactions in the [Fe(CN)₆]^3−/4− redox couple becomes more pronounced. Kinetic parameters of redox reactions in this couple are determined. By and large, with the increasing of the nitrogenation the electrochemical behavior of “poor conductor” gives way to that of metal-like conductor.