N.G. Ferreira

Kinetics study of diamond electrodes at different levels of boron doping as quasi-reversible systems

Abstract Electrochemical reversibility was investigated for diamond electrodes with different boron concentrations for two redox couple. Diamond electrodes presented quasi-reversible electrochemical behavior for both electrolytes. In the range of scan rate and doping level analyzed diamond electrodes presented heterogeneous electron-transfer rate constant k 0 values of approximately 10 −4 –10 −3 cm/s for ferri/ferrocyanide and of approximately 10 −6 –10 −5 for quinone/hydroquinone. It was observed a maximum k 0 value when the electrode reaches a semi metallic behavior, associated to its higher crystalline quality and lower impurity incorporation. The acceptor concentrations were evaluated in the range of 10 18 –10 21 cm −3 from the Mott–Schottky plot curve slope. The results were compared with platinum and glassy carbon electrodes. Scanning electron microscopy (SEM) has also shown diamond crystallites decreasing with boron doping increasing. Raman spectroscopy has evidenced a drastic decrease of 1332 cm −1 peak, characteristic of boron-doped diamond. Atomic force microscopy (AFM) measurements have also demonstrated the electrode topography and its real active area for electrochemical response.

Anodic oxidation of wastewater containing the Reactive Orange 16 Dye using heavily boron-doped diamond electrodes.

Boron-doped diamond (BDD) films grown on the titanium substrate were used to study the electrochemical degradation of Reactive Orange (RO) 16 Dye. The films were produced by hot filament chemical vapor deposition (HFCVD) technique using two different boron concentrations. The growth parameters were controlled to obtain heavily doped diamond films. They were named as E1 and E2 electrodes, with acceptor concentrations of 4.0 and 8.0 × 10(21)atoms cm(-3), respectively. The boron levels were evaluated from Mott-Schottky plots also corroborated by Ramans spectra, which characterized the film quality as well as its physical property. Scanning Electron Microscopy showed well-defined microcrystalline grain morphologies with crystal orientation mixtures of (111) and (100). The electrode efficiencies were studied from the advanced oxidation process (AOP) to degrade electrochemically the Reactive Orange 16 azo-dye (RO16). The results were analyzed by UV/VIS spectroscopy, total organic carbon (TOC) and high-performance liquid chromatography (HPLC) techniques. From UV/VIS spectra the highest doped electrode (E2) showed the best efficiency for both, the aromaticity reduction and the azo group fracture. These tendencies were confirmed by the TOC and chromatographic measurements. Besides, the results showed a direct relationship among the BDD morphology, physical property, and its performance during the degradation process.

Analysis of residual stress in diamond films by x-ray diffraction and micro-Raman spectroscopy

We investigate the residual stress in diamond films grown on (001) silicon substrates as a function of film thickness. The diamond films were deposited at 1070 K by the conventional hot filament technique using a gas mixture of methane (1.0% vol) and hydrogen (99.0% vol). The film thickness, obtained from cross section scanning electron micrographs, varied from 3.0 to 42 μm as the growth time increased from 1 to 10 h. These images evidenced that the columnar growth is already established for films thicker than 10 μm. Top view micrographs revealed predominantly faceted pyramidal grains for the films at all growth stages. The grain size, obtained from these images, was found to vary linearly with film thickness. Using a high resolution x-ray diffractometer, the residual stress was determined by measuring, for each sample, the (331) diamond Bragg diffraction peak for Ψ values ranging from −60° to +60°, and applying the sin2 ψ method. For the micro-Raman spectroscopy, we used the summation method, which consi...

Morphological and optical characteristics of porous silicon produced by anodization process in HF-acetonitrile and HF-ethanol solutions

Porous silicon (PS) samples were obtained by anodization etching process of n-type silicon wafer phosphorus-doped. Electrochemical oxidation of PS was investigated in aqueous hydrofluoric acid (HF) containing additive such as ethanol or acetonitrile. Pore formation was studied with the variation of type and resistivity of the silicon wafer, taking into account the most important anodization process parameters such as: acid concentration, current density and anodization time. Scanning Electron Microscopy (SEM) and Raman Scattering Spectroscopy measurements were used to characterize the macropore morphology changes and sample photoluminescense responses, respectively. PS layer formed in HF-acetonitrile solution showed more uniform and homogeneous macropore distributions with different shapes and sizes. Behavior may be explained because acetonitrile surface tension is greater than that of ethanol. Therefore, acetonitrile molecules might passivate the silicon surface dissolved during the anodization process.

Efficiency study of perforated diamond electrodes for organic compounds oxidation process

Abstract The electrochemical oxidation of organic compound by bulk electrolysis has been analyzed for diamond electrodes used as anode. Boron-doped diamond thin films were grown on titanium alloy substrates (Ti6Al4V) with a geometric area of 12.2 cm 2 at 870 K and at 6.5×10 3 Pa. By using hot filament chemical vapor deposition technique and perforated and non-perforated substrates, the films were grown in both sample sides. The electrodes were characterized from their electrochemical behavior as a function of film quality, morphology and surface area, for boron concentrations of approximately 10 18 /cm 3 . The electrode performance was determined by cyclic voltammetry measurements in acid and neutral solutions and the reversibility behavior of the Fe(CN) 6 3−/4− at the Ti6Al4V/diamond electrode were also studied. High performance liquid chromatography (HPLC) and total organic carbon (TOC) techniques were used to study the oxidation products. These techniques are important to quantify the different compounds in the oxidation process steps permitting to observe the diamond electrodes efficiency. The results have shown a TOC decrease of approximately 85% and for HPLC it was observed almost complete phenol combustion to CO 2 . Scanning electron microscopy and Raman scattering spectroscopy were also used for morphology and diamond quality evaluation, respectively.

Electrochemical activity of boron-doped diamond electrodes grown on carbon fiber cloths

Electrochemical activity of doped CVD-diamond electrodes was investigated as a function of boron content connected with the films surface characteristics and morphology. A textured cloth of carbon fibers was used as a substrate with a discontinuous surface. It was treated using 0.25-μm diamond powder by ultrasound dispersion in hexane. The nucleation and coalescence of the diamond film around the fibers was evident. Boron-doped diamond electrodes have been analyzed in the range of acceptor concentrations of 10 18 and 10 21 cm -3 . Their electrochemical behavior was studied by cyclic voltammetry measurements in neutral (KCl, Na 2 SO 4 and NaNO 3 ) and acidic (HCl, H 2 SO 4 and HNO 3 ) solutions. The anodic and cathodic currents increase with the doping level and present a maximum when the electrodes reach a metallic behavior. For comparison, films grown on silicon substrate in similar conditions were also studied as standard diamond electrodes. Surface morphology and film quality was analyzed by scanning electron microscopy (SEM) and Raman scattering spectroscopy.

Electrochemical characterization on semiconductors p-type CVD diamond electrodes

Semiconductor boron-doped CVD diamond films were prepared on Si substrates by the hot filament technique. The surface morphology analysis by SEM presented continuous and well faceted films. The samples were grown with different boron concentrations by controlling the B/C ratio in the feeding gas. Raman results showed a drastic change of diamond films for different doping levels. The characteristic line at 1332 cm-1decreases and shifts to lower energy as a function of the film resistivity. It was also observed a broad peak around 1220 cm-1 caused by the incorporation of boron in the diamond lattice. Photocurrent-voltage behaviour of undoped and boron-doped diamond electrodes was investigated in dark and UV visible irradiation. The voltammograms showed that doped electrodes illuminated with a xenon lamp exhibited currents significatively higher because of the increased conductivity. For undoped electrodes it was observed a small photocurrent for anodic and cathodic polarization in the order ofmA for the potential range of +1.0 V and -1.0 V (Ag/AgCl), for 1.0 M KCl. Mott-Schottky plots studied the interfacial processes at diamond-electrolyte junction. The flatband potential Uf b was found between 0.6 and 0.8 V (Ag/AgCl) which varies with the presence of sp2 -type carbon as an impurity. From the curve slope the acceptor concentrations were found in the range of 1018 and 1021 cm-3 .These values agree with the estimated concentration obtained by Raman measurements.

Stress study of HFCVD boron-doped diamond films by X-ray diffraction measurements

Abstract Stress analysis on chemical vapor deposition (CVD) diamond films has demonstrated an apparent disagreement among various researchers in recent works even for similar deposition conditions. The type and the value of stress have shown a strong dependence on film thickness, which can be attributed to columnar growth and grain size and boundaries. X-Ray diffraction techniques appeared to be more suitable to study these effects and permit the evaluation of the average stress in larger sample areas when compared with micro-Raman spectroscopy, which feels a local strain inside the grains. In the case of boron-doped diamond films, boron incorporation on substitucional or interstitial sites can produce stresses according to the doping level. In order to investigate these effects, a series of diamond films were deposited on silicon (001) substrate in a hot filament (HF)-assisted CVD reactor at 800°C. The CH 4 flow is kept at 0.5 sccm for all experiments and the H 2 and B 2 O 3 /CH 3 OH/H 2 flows are controlled in order to obtain the desired B/C ratios. Stress behavior in HFCVD boron-doped diamond films has been investigated by X-ray diffraction measurements using the sin 2 ψ technique. Tensile and compressive stresses have been observed and the thermal and intrinsic components have been calculated. The diamond films were characterized by scanning electron microscopy and Raman spectroscopy.

X-ray investigation of nanostructured stain-etched porous silicon

The structure of porous silicon layers was accurately investigated by diverse x-ray methods. A series of samples with etching times varying from 1 to 10 min was produced by chemical etch using a HF∕HNO3-based solution assisted with NaNO2. The porosity determined from low-angle x-ray reflectivity spectra was found to fluctuate from 35% to 55% as the etch proceeds. Reciprocal space mapping around the (004) Si lattice point revealed that the Si crystallites are deformed due to a distribution of in-plane compressive strain caused by the neighboring pores, which leads to an expansion of the perpendicular lattice parameter. No signature of mosaicity was found. The perpendicular strain could be precisely determined by fitting the x-ray-diffraction spectra, measured in the triple-axis configuration, to a set of Voigt and Gaussian distributions. These strain distributions are certainly associated with the different population of crystallite sizes formed during the stain etching process. We were able here to determ...

Lead detection using micro/nanocrystalline boron-doped diamond by square-wave anodic stripping voltammetry

Monitoring heavy metal ion levels in water is essential for human health and safety. Electroanalytical techniques have presented important features to detect toxic trace heavy metals in the environment due to their high sensitivity associated with their easy operational procedures. Square-wave voltammetry is a powerful electrochemical technique that may be applied to both electrokinetic and analytical measurements, and the analysis of the characteristic parameters of this technique also enables the mechanism and kinetic evaluation of the electrochemical process under study. In this work, we present a complete optimized study on the heavy metal detection using diamond electrodes. It was analyzed the influence of the morphology characteristics as well as the doping level on micro/nanocrystalline boron-doped diamond films by means of square-wave anodic stripping voltammetry (SWASV) technique. The SWASV parameters were optimized for all films, considering that their kinetic response is dependent on the morphology and/or doping level. The films presented reversible results for the Lead [Pb (II)] system studied. The Pb (II) analysis was performed in ammonium acetate buffer at pH 4.5, varying the lead concentration in the range from 1 to 10 μg L(-1). The analytical responses were obtained for the four electrodes. However, the best low limit detection and reproducibility was found for boron doped nanocrystalline diamond electrodes (BDND) doped with 2000 mg L(-1) in B/C ratio.