Vacuum-annealed undoped polycrystalline CVD diamond: a new electrode material

Vacuum-annealing imparts conductivity to initially insulating undoped polycrystalline chemical-vapor-deposited diamond, thus turning it to a possible electrode material. The diamond film annealed at 1775 K appeared to be practically not conducting. With further increase in the annealing temperature above 1825 K, the film effective resistivity decreased from initial value of 10¹¹ to 10¹² Ω cm down to less than 0.1 Ω cm; the differential capacitance increased from ∼10⁻³ to ∼50 μF per 1 cm² of geometrical surface; the transfer coefficients for electrochemical reactions in the [Fe(CN)₆]^3−/4− redox solution increased from ∼0.2 to 0.5; and the degree of reversibility of the electrochemical reaction increased. The observed changes in the electrode properties are attributed to gradual change in the thickness and/or properties (first and foremost, conductivity) of the nondiamond carbon phase formed along the intercrystallite boundaries upon the annealing; the conducting phase is outcropping at the film surface as an array of microelectrodes (“active sites”).     

AC impedance studies of anodically treated polycrystalline and homoepitaxial boron-doped diamond electrodes

The electrochemical properties of several types of diamond electrodes, including polycrystalline and homoepitaxial films, that underwent anodic treatment were examined with the electrochemical impedance spectroscopic (EIS) technique, as well as with capacitance-potential measurements. From an analysis of the impedance behavior, it was found that an additional capacitance element, which is apparent in the relatively high-frequency range (100-1000 Hz), was generated on the polycrystalline and (1 0 0) homoepitaxial diamond electrodes after anodic treatment. This capacitive element can be characterized as being non-Faradaic, because it has negligible dependence on the applied potential. Acceptor densities and depth profiles were calculated from the Mott-Schottky plots, and the acceptor densities in the near-surface region of the anodically treated surfaces were found to be extremely low. These results indicate that passive layers were generated on the diamond surfaces by the anodic treatment. The capacitance-potential behavior was also consistent with a model consisting of a semiconductor with a passive surface film. The passive film is proposed to arise as a result of the removal of hydrogen acting as an acceptor in the subsurface region, leaving hydrogen that is paired essentially quantitatively with the boron dopant, effectively neutralizing it.     

Electrochemical impedance spectroscopy of oxidized and hydrogen-terminated nitrogen-induced conductive ultrananocrystalline diamond

We have studied the electrochemical impedance spectroscopy of conductive ultrananocrystalline diamond (UNCD) modified by either oxidation or hydrogenation surface treatments. The impedance was measured in the frequency range from 0.1 Hz to 40 kHz at different DC voltages and the results fitted to an equivalent electrical circuit. Despite the complexity of the conductive UNCD surface, composed of sp³-bonded grains and grain boundaries with a high content of sp²-bonded carbon atoms, a Randles circuit with a constant phase element (CPE) for the capacitive element provided a reasonable model for both terminations. However, the parameters of the CPE were very different for each termination. Taking into account the results obtained, we propose that the interfacial impedance of oxidized UNCD is dominated by the oxidized sp²-bonded carbon atoms present at the grain boundaries, and the interfacial impedance of hydrogen-terminated UNCD is governed by both the grain boundaries and the grains.     

Chemical surface characterization of electrochemically and thermally oxidized boron-doped diamond film electrodes

In situ Cyclic Voltammetric analysis and ex situ X-ray Photoelectron Spectroscopy investigations have been carried out on highly boron-doped diamond film electrodes subjected to different oxidative treatments. Both the electrochemical and thermal oxidation modify, at different extent, the qualitative and quantitative chemical surface composition of the as prepared samples by partially converting the non diamond carbon species, for instance graphitic-like and hydrogenated carbon forms, to oxygenated ones. These modifications are relatively stable: less oxidized (e.g., alcoholic) groups can be converted into more oxidized ones (e.g., carboxyls, ethers, esthers) under extreme polarization conditions, resulting in a loss of electrochemical activity for the surface species. Surface silicon enrichment has been observed after a thermal treatment at 400 °C, as a result of the silicon diffusion, along the columnar structure of the diamond layers, from the substrate.     

The effects of nitrogenation on the electrochemical properties of nanocrystalline diamond films

The effect of the nitrogenation on the electrochemical properties of nanocrystalline diamond films produced by microwave plasma CVD in CH₄–Ar–H₂–N₂ gas mixtures was studied systematically, using cyclic voltammetry and electrochemical impedance spectroscopy measurements, for the first time. Differential capacitance, kinetic parameters of reactions in [Fe(CN)₆]^3-/4-redox system and potential window were found to be sensitive to the nitrogen concentration in the process gas. With its increase (from 0 to 25%), a transition of the NCD film behavior from “poor conductor” to metal-like character takes place. The heavily N-doped nanocrystalline diamond films have satisfactory electrochemical properties to be used as electrodes.     

Capacitance and photoelectrochemical studies for the assessment of anodic oxide films on aluminium

Photoelectrochemical spectroscopy and capacitance measurements were used in this work to assess the electronic properties of the oxide films formed on 99.5% aluminium and 2024-T3 aluminium alloy by anodising in a sulphuric-boric bath. The morphology of these films was also studied by transmission electron microscopy cross-section observations.The results obtained indicate that the oxide films formed on aluminium show a n-type semiconductive behaviour, with bandgap energies that are identical for the oxides studied, despite their different characteristics.It was found out that capacitance measurements may be used as a valuable technique for the assessment of the quality of anodised layers, allowing the distinction between an efficient and an inefficient sealing. Therefore, they may be used to predict the corrosion resistance of these materials.     

The phosphorous level fine structure in homoepitaxial and polycrystalline n-type CVD diamond

The application of very sensitive photocurrent-based spectroscopic techniques have led to the detection of new levels for the electronic structure of the phosphorous donor in n-type CVD diamond. By combining quasi-steady-state photocurrent measurements (PC), photothermal ionisation spectroscopy (PTIS) and the highly sensitive Fourier transform photocurrent spectroscopy (FTPS) technique at different temperatures, ranging from liquid nitrogen temperature to 170 K, the resulting spectra point to a richer structure than assumed up to now. This is the consequence of the improved sample quality over the last years, opening up to a much larger attainable doping window. By using doping levels, ranging from 10¹⁹ cm⁻³ down to 10¹⁶ cm⁻³ on 111 -oriented Ib HPHT substrates, still giving rise to measurable n-type conductivity, spectra showed less line broadening and more fine structure. Finally, the results will be compared with spectra measured on active P-doped polycrystalline n-type films.     

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.     

Cadmium cathodic deposition on polycrystalline p-selenium: Dark and photoelectrochemical processes

Cathodic reduction of Cd²⁺ on p-Se proceeds at low overpotential in the dark and results in bulk Cd, while the underpotential deposition is kinetically inhibited. Cadmium adlayer is photoelectrochemically deposited on illuminated electrode 0.7 V above E(Cd²⁺/Cd). The adlayer cathodic deposition under illumination proceeds with simultaneous formation of CdSe nanoparticles. Potentiodynamic electrochemical impedance spectroscopy has discriminated the two products of the photoelectrochemical reaction both by their potentials of anodic oxidation and by characteristic dependences of impedance on potential. Anodic oxidation of CdSe nanoparticles gives a sharp peak of real impedance in low frequencies close to the corresponding anodic current peak in cyclic voltammogram. The impedance peak appears below a threshold frequency f_t. The latter separates two modes of diffusion in anodic dissolution of CdSe nanoparticles. The diffusion proceeds independently at different particles above f_t and turns to cooperative mode below the threshold frequency. Due to this effect, information on spatial distribution of growing nuclei on electrode surface in early stages of electrodeposition can be obtained from potentiodynamic impedance spectra.     

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.     

Systematic studies of Bi2O3 hierarchical nanostructural and plasmonic effect on photoelectrochemical activity under visible light irradiation

The effect of Ag plasmonic nanowire layers on the hierarchical nanostructure of Bi₂O₃ photoelectrodes for water splitting under visible light irradiation was studied for the first time. In addition, the impact of various Bi₂O₃ nanostructures on light harvesting and generation of relative photocurrent have been investigated. The cubic structure of Bi₂O₃ was confirmed using X-ray diffraction analysis. Optical bandgaps of 2.14 and 2.30 eV have been achieved for the Bi₂O₃ nanoparticles and nanoflowers photoelectrodes (BP and BF), respectively. The photocurrent density (J) of the BP featuring Ag plasmonic layer photoelectrode (Ag/BP) was 6.47 mA cm⁻², and was higher than that of the BF featuring Ag plasmonic layer photoelectrode (Ag/BF), which was 4.33 mA cm⁻². These values were approximately 647 and 2165 times higher than those of BP and BF, respectively. However, the J value of BP was 2.13 mA cm⁻² higher than that of BF. The superior J values of Ag/BP and Ag/BF were attributed to the increased light absorption and reduced electron-hole recombination rate at the time scale beyond a few 10⁻¹² s, owing to the Ag nanowires. In addition, the plasmonic field was able to reduce the charge recombination rate of the nanostructured electrodes in reactor cells.     

Au dissolution during the anodic response of short-chain alkylthiols with polycrystalline Au electrodes

The electrochemical characteristics of polycrystalline Au in LiClO₄ electrolyte solutions containing 3-mercaptopropionic acid (MPA) or meso-2,3-dimercaptosuccinic acid (DMSA) were studied with linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) over a wide range of positive potentials vs. Ag/AgCl. The EIS data exhibited linear capacitive behaviour at 0.0 V with either MPA or DMSA added directly to the electrolyte suggesting the formation of an adsorbed layer of the alkylthiol on the electrode surface. Above this potential, a single well-defined impedance loop appeared for electrolyte solutions containing DMSA or MPA, an observation indicative of a charge transfer reaction that could be related to several processes including oxidative desorption, oxidation of the alkylthiol, or Au oxidation/dissolution. To test for Au dissolution, the electrode was held at 0.8 V vs. Ag/AgCl for 12 h in electrolytes containing MPA or DMSA followed by surface analysis with Atomic Force Microscopy and solution analysis with Atomic Absorption Spectroscopy. When the electrolyte contained MPA, the extended potential holding procedure resulted in significant roughening of the electrode with no detectable quantities of Au in the electrolyte. X-ray photoelectron spectroscopy (XPS) analysis of the Au surface revealed an additional species in the Au 4f_7/2 spectrum indicating the presence of an insoluble electrochemically generated Au(I)–MPA species. When the electrolyte contained DMSA, the Au electrode appeared smoother, 56.6 ± 9.6 ppb of Au was detected in the electrolyte and the XPS analysis displayed a single species in the Au 4f_7/2 spectrum indicative of metallic Au after the potential holding procedure. Both observations with MPA and DMSA support the charge transfer resistance to be at least partially related to the corrosion of Au, but also suggest that an electrochemically generated Au–DMSA species is soluble and of potential industrial relevance.     

Electrochemical impedance studies of chitosan-modified electrodes for application in electrochemical sensors and biosensors

Graphite-epoxy resin composite (GrEC) electrodes were modified with chitosan (Chit) films and characterised using electrochemical impedance spectroscopy (EIS). Several film modifications were made using different crosslinking agents: glutaraldehyde (GA), glyoxal (GO), epichlorohydrin (ECH) and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) together with N-hydroxysuccinimide (NHS) and the characteristics of each of them evaluated in the presence of model electroactive compounds potassium hexacyanoferrate(III) and hexaammineruthenium(III) chloride. Immobilisation of functionalised carbon nanotubes into chitosan matrices (Chit-CNT) using the same crosslinking agents was also investigated. The impedance of the electrode with the best performance (GrEC/Chit-CNT/EDC-NHS) was characterised as a sensor for dipyrone and hydroquinone and for a glucose biosensor by immobilisation of glucose oxidase (GOx) on top of Chit-CNT using GA. Modelling and equivalent circuit analysis was carried out, with emphasis on diffusion characteristics and the significant features of the spectra are discussed.     

Micro- and nanotopographies for photoelectrochemical energy conversion. I: The photovoltaic mode

The development of an efficient photovoltaic electrochemical solar cell with n-Si (1 0 0) based on the nanoemitter concept is described. The corresponding structure results from the self-organized formation of nanopores in about 10 nm thick silicon oxide during photocurrent oscillations in dilute fluoride containing solutions. The oscillations occur at higher anodic potentials and, due to the stress between Si and its anodic oxide, nanopores form in the oxide. Site selective metal electrodeposition (Pt) into those pores that have contact to the Si substrate and immersion into an acidic iodine-iodide redox electrolyte gives efficiencies in the range of 10%. Model experiments on the electrodeposition of Pt nanoparticles on n-Si (1 1 1) using photoelectron spectroscopy, performed with synchrotron radiation and atomic force microscopy in the contact- and tapping mode show that upon Pt deposition, silicon oxide is formed and step-bunching is observed. Mechanistic models are presented that explain the observed features. Scanning tunnelling experiments are performed with the tip on top of Pt nanoparticles. The resulting local I–V characteristics are interpreted based on a modified MOS (metal-oxide-semiconductor) model.     

Influence of the substrate type on CVD grown homoepitaxial diamond layer quality by cross sectional TEM and CL analysis

To assess diamond-based semiconducting devices, a reduction of point defect levels and an accurate control of doping are required as well as the control of layer thickness. Among the analyses required to improve such parameters, cross sectional studies should take importance in the near future. The present contribution shows how FIB (focused ion beam) preparations followed by electron microscopy related techniques as TEM or CL allowed to perform analysis versus depth in the layer, doping and point defect levels. Three samples grown along the same week in the same machine with identical growth conditions but on different substrates (CVD-IIIa (110) oriented, CVD-optical grade (100) oriented and a HPHT-Ib (100) oriented) are studied. Even though A-band is observed by CL, no dislocation is observed by CTEM. Point defect type and level are shown to substantially change with respect to the substrate type as well as the boron doping levels that vary within an order of magnitude. H3 present in the epilayer grown on HPHT type of substrate is replaced by T1 and NE3 point defects for epilayers grown on the CVD type one. An increase of excitonic transitions through LO phonons is also shown to take place near the surface while only TO ones are detected deeper in the epilayer. Such results highlight the importance of choosing the correct substrate.     

High pressure and high temperature treatment of chemical vapor deposited polycrystalline diamond: From opaque to transparent

The enhanced optical properties of chemical vapor deposited (CVD) diamond have been pursued in academia and industry for many applications. However, the barrier of CVD technology limits the application field of diamond. Herein, the performance of CVD polycrystalline diamond thick films was improved by high-pressure and high-temperature (HPHT) treatment. The microstructures of CVD polycrystalline diamond films before and after HPHT treatments were thoroughly examined using optical microscope, UV–visible and infrared absorption, Raman spectroscopy, scanning electron microscope (SEM) and transmission electron microscope (TEM). It is found that the transparency of the CVD samples at 10 GPa increases dramatically with processing temperatures, from the original opacity to almost full optical transparency. Through spectroscopic and microstructural analyses, the modification mechanism of CVD polycrystalline diamond under HPHT conditions is proposed. The results show that the HPHT treatment can significantly enhance the optical properties of the starting CVD polycrystalline diamond films.     

Electrochemical studies of ceramic carbon electrodes for fuel cell systems: A catalyst layer without sulfonic acid groups

Ceramic carbon electrodes (CCEs) have been produced via the sol-gel process using 20% Pt on Vulcan XC72 carbon black and tetra ethyl orthosilicate (TEOS) as the organosilane precursor. This process produces a homogenous distribution of SiO₂ and carbon supported Pt catalyst. Electrochemical experiments (cyclic voltammetry, electrochemical impedance spectroscopy) were performed to determine the effect of SiO₂ loading on the active area of Pt in the catalyst layer. A volcano-type dependence was observed with the maximum active area of Pt occurring with an SiO₂ loading of 45% by mass. Pt utilization was lower than that achieved with Nafion-based catalyst layers and was explained in terms of the lower proton conductivity of SiO₂ compared to Nafion. These CCE structures may be useful for high temperature fuel cell systems.     

Reduced influences of the HPHT substrates on the electronic quality of homoepitaxial CVD diamond layers and on their ultraviolet detector performance

We have carried out a detailed estimation of the influences of the high-pressure/high-temperature-synthesized (HPHT) Ib substrate on the crystalline quality of the homoepitaxial diamond and on the performance of the ultraviolet (UV) detector. The H3 center related luminescence peaks were observed even from the homoepitaxial diamond film having a thickness of 250 μm on a HPHT Ib substrate, suggesting that carriers excited in the epitaxial diamond layer can diffuse over a rather long distance to the HPHT substrate when the quality of the epitaxial layer is sufficiently high. Furthermore, we have attempted to efficiently reduce the long-distance carrier diffusion phenomenon by inserting a boron-doped layer between the epitaxial layer for the detection and the HPHT Ib substrate. The electrically-floating B-doped layer inserted between the homoepitaxial layer and the HPHT substrate efficiently reduced the long-distance carrier diffusion phenomenon, and substantially improved the performance of the UV detector fabricated on a low-quality HPHT Ib substrate.     

Electrochemical oxidation of alcohols and carboxylic acids with diamond anodes: A comparison with other advanced oxidation processes

In this work, the conductive-diamond electrochemical oxidation of several short-chain organics (2-propanol, ethanol, butyric acid and chloroacetic acid) has been studied. The results obtained have been compared with those obtained in the oxidation of these compounds with Fenton reagent and with ozone at pH 12. Significant differences have been observed among the three technologies and also between the two ranges of pollutant concentrations studied. However in both cases, the performance of the CDEO overcomes the results obtained by the oxidation with ozone and with Fenton reagent. Likewise, the oxidation with ozone seems to be more effective that the oxidation carried out by the Fenton reagent. Except for ethanol, the nature of the organic does not seem to influence on the performance of the electrochemical technology. However, the efficiencies were found to strongly depend on the concentration of pollutant. In spite of that, the CDEO is able to reduce completely the COD content of the wastes. On the contrary, important concentrations of oxidation-refractory compounds were accumulated during ozonation and Fenton treatments. The mean oxidation state of carbon of the final products obtained by the three technologies is different and this suggests significant differences in the oxidation mechanisms.