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.     

Application of diamond electrodes to electrochemical processes

Conducting diamond thin film is a new electrode material that has received great attention recently because it possesses several technologically important characteristics such as an inert surface with low adsorption properties, remarkable corrosion stability, even in strong acidic media, and an extremely wide potential window in aqueous and non-aqueous electrolytes. Thanks to these properties diamond electrodes meet the requirements for a wide range of electrochemical applications. The object of this article is to summarise and discuss the recent results available in the literature concerning the application of diamond electrodes to electrochemical processes such as water treatment and electro-synthesis of organic and inorganic compounds.     

Microstructural and biological properties of nanocrystalline diamond coatings

In this study, the microstructural, mechanical, adhesion, and hemocompatibility properties of nanocrystalline diamond coatings were examined. Microwave plasma chemical vapor deposition (MPCVD) was used to deposit nanocrystalline diamond coatings on silicon (100) substrates. The coating surface consisted of faceted nodules, which exhibited a relatively wide size distribution and an average size of 60 nm. High-resolution transmission electron microscopy demonstrated that these crystals were made up of 2–4 nm rectangular crystallites. Raman spectroscopy and electron diffraction revealed that the coating contained both crystalline and amorphous phases. The microscratch adhesion study demonstrated good adhesion between the coating and the underlying substrate. Scanning electron microscopy and energy dispersive X-ray analysis revealed no crystal, fibrin, protein, or platelet aggregation on the surface of the platelet rich plasma-exposed nanocrystalline diamond coating. This study suggests that nanocrystalline diamond is a promising coating for use in cardiovascular medical devices.     

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.     

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.     

Fabrication of boron doped diamond microband electrodes for electrochemical detection in a microfluidic channel

The manufacturing and electrochemical characterisation of an array of 20 boron doped nanocrystalline diamond (BNCD) microband electrodes for use in a poly(dimethylsiloxane) (PDMS) based microfluidic system are described. The electrodes were fabricated by plasma etching of a silicon oxide- and BNCD thin film coated silicon wafer and the resulting surface structured silicon wafer was subsequently bonded to the PDMS so that the BNCD microband electrodes were located within the PDMS microchannel. The electrochemical performance of the BNCD electrodes was studied and the electrodes were found to exhibit significantly better stability than previously employed gold microband arrays.     

Growth rate improvements in the hot-filament CVD deposition of nanocrystalline diamond

The hot-filament CVD, a less used technique for NCD films growth using Ar/H₂/CH₄ gas mixtures, is optimized for the coating of silicon nitride ceramics. Parameters such as gas composition (Ar/H₂ and CH₄/H₂ ratios), total gas pressure, total mass flow and substrate and filament temperatures, are studied to assess their effect on NCD growth kinetics as well as on film quality and morphology. The smallest diamond crystallite sizes (8 nm) were recorded for the slowest growth rate of 0.1 μm h^− 1. A remarkable result is the very high growth rate of 1.6 μm h^− 1 of continuous NCD coatings with 28 nm of crystallite size, obtained in selected deposition conditions.     

Enhanced performance of HFCVD nanocrystalline diamond self-mated tribosystems by plasma pretreatments on silicon nitride substrates

Nanocrystalline diamond (NCD) coatings were grown by the hot-filament chemical vapour deposition (HFCVD) method on hydrogen plasma pretreated silicon nitride (Si₃N₄) substrates. The friction and wear behaviour of self-mated NCD films, submitted to unlubricated sliding and high applied loads (up to 90 N), was assessed using an oscillating ball-on-flat configuration in ambient atmosphere. The reciprocating tests revealed an initially high friction coefficient peak, associated to the starting surface roughness of NCD coatings (R_q = 50 nm). Subsequently, a steady-state regime with low friction coefficient values (0.01–0.04) sets in, related to a smoother (R_q = 17 nm) tribologically modified surface. A polishing wear mechanism governing the material loss was responsible for mild wear coefficients (k  10^− 7 mm³ N^− 1 m^− 1). The hydrogen etching procedure notably increased the film adhesion with respect to untreated surfaces as demonstrated by the high threshold loads (60 N; 3.5 GPa) prior to film delamination.     

Tribological investigation of nanocrystalline diamond films at low load under different tribosystems

The mechanical and frictional properties of hydrogen- and oxygen-terminated nanocrystalline diamond films (NCD) grown by hot-filament chemical vapor deposition (HFCVD) have been investigated in the present work.The structure and morphology of the NCD films have been characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman-effect spectroscopy. In addition, X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) have been used to investigate the surface chemical groups on the NCD surface. Mechanical and frictional properties are determined using atomic force microscopy (AFM), nano-indentation, nano-scratching and micro-tribometer. The friction behavior of these films in the load range of 25 to 200 mN under reciprocating sliding conditions, using steel counter-body material has been thoroughly studied.It is noted that these films are highly crystalline with nanometer size grains and contain a very high fraction of sp³ carbon bonds. They exhibit high hardness and high elastic modulus. The friction coefficient of the film is lower under unidirectional scratch with diamond indenter than the friction coefficient under low load reciprocating sliding against steel ball. Transfer of the film from the counter-body, oxidation of transfer film and mixing of transfer film with carbonaceous layer on the worn surfaces are responsible for such behavior. Although, the friction responses of H-terminated and O-terminated films are similar under unidirectional scratch with diamond indenter, the friction coefficient of O-terminated film is always higher than the friction coefficient of H-terminated film under reciprocating sliding condition against steel counter-body material.     

Nitrogenated nanocrystalline diamond films: Thermal and optical properties

Ultrananocrystalline diamond films have been grown by microwave plasma CVD using CH₄/H₂/Ar mixtures with N₂ added in plasma in amounts up to 25%. The films were characterized with AFM, Raman, XRD, and UV–IR optical absorption spectroscopy mainly focusing on optical and thermal properties. In comparison with polycrystalline CVD diamond the UNCD are very smooth (R_a < 10 nm), have low thermal conductivity ( 0.10 W/cm K), high optical absorption ( 10³ cm^− 1 at 500 nm) and high concentration of bonded hydrogen ( 9 at.%). The nitrogen presence in the plasma has a profound impact on UNCD structure and properties, particularly leading to a decrease in resistivity (by 12 orders of magnitude), thermal conductivity, Tauc band gap, optical transmission and H content. The UNCD demonstrated rather good thermal stability in vacuum: the diamond phase still was present in the films subjected to annealing to 1400 °C.     

Electrochemical preparation of peroxodisulfuric acid using boron doped diamond thin film electrodes

We have investigated the electrochemical oxidation of sulfuric acid on boron-doped synthetic diamond electrodes (BDD) obtained by HF CVD on p-Si. The results have shown that high current efficiency for sulfuric acid oxidation to peroxodisulfuric acid can be achieved in concentrated H₂SO₄ (>2 M) at moderate temperatures (8-10 °C). The main side reaction is oxygen evolution. Small amounts of peroxomonosulfuric acid (Caro's acid) have also been detected. A reaction mechanism involving hydroxyl radicals, HSO₄⁻ and undissociated H₂SO₄ has been proposed. According to this mechanism electrogenerated hydroxyl radicals at the BDD anode react with HSO₄⁻ and H₂SO₄ giving peroxodisulfate.     

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.     

Grain size effect on the electrical properties of nanocrystalline ceria

The total and partial electronic conductivities of gadolinium doped ceria (Ce_0.95Gd_0.1O_1.95−δ: GDC) with nanometer grain size have been evaluated in an attempt to identify the nanosize effect in heavily doped ceria. Nanocrystalline GDC bulk specimens with relatively high densities (≥96% of theoretical density) and various grain sizes (70, 100, 170 nm) were successfully fabricated by a conventional solid-state sintering method. According to the measurements of total and partial electronic conductivity via AC-impedance and DC polarization methods, respectively, no significant grain size dependence appeared for either type of conductivity. Furthermore, both total and partial electronic conductivity were not significantly different from those of microcrystalline GDC, which indicated that, upon nanostructuring within the examined grain size range, nanostructured bulk GDC was not affected by any nanosize effect: either space charge layer effect or grain boundary blocking effect.     

Effects of CCl4 concentration on nanocrystalline diamond film deposition in a hot-filament chemical vapor deposition reactor

The effect of CCl₄ concentration on the nanocrystalline diamond (NCD) films deposition has been investigated in a hot-filament chemical vapor deposition (HFCVD) reactor. NCD films with a thickness of few-hundred nanometers have been synthesized on Si substrates from 2.0% and 2.5% CCl₄/H₂ at a substrate temperature of 610 °C. Polycrystalline diamond films and nanowall-like films with higher formation rates than those of the NCD films were deposited from lower and higher CCl₄ concentrations, respectively. The grain sizes of the diamond film grown using 2.0% CCl₄ increased with film thickness while a diamond film with uniform nanocrystalline structure all over a thickness of 1 μm can be deposited in the case of 2.5% CCl₄. We suggest that both the primary nucleation and the secondary nucleation processes are crucial for the growth of the NCD films on Si substrates.     

The microstructure and electrochemical properties of boron-doped nanocrystalline diamond film electrodes and their application in non-enzymatic glucose detection

Boron-doped nanocrystalline diamond (BDND) films were deposited on Si(100) by microwave plasma chemical vapor deposition using trimethyl boron as boron source. The surface morphology, microstructure, and electrochemical properties of the BDND films were investigated. Cyclic voltammograms indicated that the BDND film electrode exhibited good reversibility and repeatability of electrode reaction using [Fe(CN)₆]^3?/4? as redox couple. The non-enzymatic glucose sensor based on the as-prepared BDND film electrode without any modification was developed, and the selective detection of glucose in alkaline solution containing interference species of ascorbic acid and uric acid was demonstrated. The results showed that glucose can be directly oxidized with a wide linear range and high sensitivity, and selectively detected in the presence of uric acid and ascorbic acid in alkaline solution using the as-prepared BDND film electrode.     

Viewing nanocrystalline TiO2 photoelectrodes as three-dimensional electrodes: Effect of the electrolyte upon the photocurrent efficiency

Nanocrystalline TiO₂ films are widely investigated both as photoelectrodes to carry out photocatalytic reactions under band-gap near-UV illumination or, alternatively, as an electronically conducting matrix of dye-sensitised electrodes as a part of a liquid-junction solar photovoltaic cell. The main features of these photoelectrodes are: (i) significant thickness to allow effective absorption of incident light—the films consist typically of 100-1000 superimposed layers of individual nanoparticles; (ii) large porosity—ca. 50% of the film volume is normally filled with electrolyte; (iii) large surface-to-volume ratio resulting from the small size of TiO₂ nanoparticles (with diameters in the range of 10-30 nm) forming the film. This work re-examines the question of charge and mass transport in nanocrystalline titanium dioxide electrodes which are among the major factors affecting the efficiency of TiO₂-based dye-sensitised solar cells and photocatalytic devices. In contrast to most of the reports published in the literature, which analyse of the behaviour of such films under transient conditions, we focus here on the steady-state operation of nanocrystalline TiO₂ electrodes, directly relevant to the efficiency of cells used in real-world applications. Understanding of the charge and mass transport across these nanocrystalline films is greatly facilitated by considering them as three-dimensional electrodes including two coincidental, superimposed continua, i.e., the solid matrix that conducts electrons towards the back contact and the electrolyte—an ionic conductor carrying another part of the current through the pores of the nanostructured film. It is shown that the composition and conductivity of the chosen electrolyte and, in particular, the nature of the ionic species which act as hole scavengers or redox mediators largely affect the current distribution within the film and thus the final photon-to-current conversion efficiency of the photoelectrodes.     

Influence of cathodic overpotential on grain size in nanocrystalline nickel deposition on rotating cylinder electrodes

Nanocrystalline (nc) nickel was electrodeposited using rotating cylindrical electrodes (RCD) from a Watt’s bath containing saccharin. The effects of cathode rotation speed and saccharin concentration on the cathodic overpotential and grain size were studied. The grain sizes are presented as a function of the cathodic overpotential. All reported cathodic overpotentials were corrected for ohmic overpotentials. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to determine the deposit grain size. In addition, the influences of cathode rotation speed and current density on the morphology of nanocrystalline nickel were observed by scanning electron microscopy (SEM).     

钎料合金对钎焊金刚石界面性能的影响

在钎焊金刚石工具制造过程中,金刚石/钎料/基体之间的界面反应产物和微观结构决定了钎料钎焊金刚石与基体之间的结合强度.因此,对界面反应产物及组织微观结构进行研究,进而探讨钎焊工艺的优化十分重要.文章对钎料与基体界面显微组织形貌,高频感应钎焊界面微观,钎焊层的微观组织,钎焊金刚石微结构,钎焊接头界面组织,激光钎焊界面微结构...     

Influence of thin film properties on the electrochemical performance of diamond electrodes

The electrochemical properties of nanocrystalline diamond films grown by microwave-enhanced chemical vapour deposition from a helium–hydrogen–methane gas phase mixture on Ti substrates are explored. A range of important redox systems are examined in aqueous solution including the oxidation of hydroquinone and ascorbic acid, and the electrodeposition and stripping of Au and Cu. Compared to boron-doped diamond materials, the nanodiamond is found to be a highly active electrode material, with low overpotentials and high adherence of metallic electrodeposits, for the redox systems studied.