Electrochemical impedance spectroscopy and corrosion behaviour of Al2O3-Ni nano composite coatings

In this paper, the results on the electrochemical impedance spectroscopy and corrosion properties of electrodeposited nanostructured Al₂O₃-Ni composite coatings are presented. The nanocomposite coatings were obtained by codeposition of alumina nanoparticles (13 nm) with nickel during plating process. The coating thickness was 50 μm on steel support and an average of nano Al₂O₃ particles inside of coatings at 15 vol.% was present. The structure of the coatings was investigated by scanning electron microscopy (SEM). It has been found that the codeposition of Al₂O₃ particles with nickel disturbs the nickel coating's regular surface structure. The electrochemical behavior of the coatings in the corrosive solutions was investigated by polarization potentiodynamic and electrochemical impedance spectroscopy methods. As electrochemical test solutions 0.5 M sodium chloride and 0.5 M potassium sulphate were used in a three electrode open cell. The corrosion potential is shifted to more negative values for nanostructured coatings in 0.5 M sodium chloride. The polarization resistance in 0.5 M sodium chloride decreases in 24 h, but after that increases slowly. In 0.5 M potassium sulphate solution the polarization resistance decreases after 2 h and after 30 h of immersion the polarization resistance is higher than that of the beginning value. The corrosion rate calculated by polarization potentiodynamic curves obtained after 30 min from immersion in solution is smaller for nanostructured coatings in 0.5 M potassium sulphate (4.74 μm/year) and a little bit bigger in 0.5 M sodium chloride (5.03 μm/year).     

Tetracarboxylic acid cobalt phthalocyanine SAM on gold: Potential applications as amperometric sensor for H2O2 and fabrication of glucose biosensor

This report describes the applications of cobalt tetracarboxylic acid phthalocyanine (CoTCAPc) self-assembled monolayer (SAM) immobilized onto a preformed 2-mercaptoethanol (Au-ME) SAM on gold surface (Au-ME-CoTCAPc SAM) as a potential amperometric sensor for the detection of hydrogen peroxide (H₂O₂) at neutral pH conditions. The Au-ME-CoTCAPc SAM sensor showed a very fast amperometric response time of approximately 1 s, good linearity at the studied concentration range of up to 5 μM with a coefficient R² = 0.993 and a detection limit of 0.4 μM oxidatively. Also reductively, the sensor exhibited a very fast amperometric response time (∼1 s), linearity up to 5 μM with a coefficient R² = 0.986 and a detection limit of 0.2 μM. The cobalt tetracarboxylic acid phthalocyanine self-assembled monolayer was then evaluated as a mediator for glucose oxidase (GOx)-based biosensor. The GOx (enzyme) was immobilized covalently onto Au-ME-CoTCAPc SAM using coupling agents: N-ethyl-N(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxy succinimide (NHS), and the results demonstrated a good catalytic behavior. Kinetic parameters associated with the enzymatic and mediator reactions were estimated using electrochemical versions of Lineweaver-Burk and Hanes equation, and the stability of the sensor was tested. The biosensor (Au-ME-CoTCAPc-GOx SAM) electrode showed good sensitivity (7.5 nA/mM) with a good detection limit of 8.4 μM at 3σ, smaller Michaelis-Menten constant (4.8 mM from Hanes plot) and very fast response time of approximately 5 s.     

Effect of sulphuric acid and copper sulphate concentrations on the morphology of silver deposit in the cementation process

The silver ion cementation on copper was investigated in the presence or absence of oxygen in solutions containing 1.85 × 10⁻⁴ M Ag⁺ at 25 °C. The influence of sulphuric acid and copper sulphate concentration (0.005-0.5 M) on the silver cement morphology was studied in details and results were linked with the previously determined kinetics data of the process. The morphology of silver deposit was found to be independent of the presence of oxygen in the system as well as the sulphuric acid concentration. Contrary, the concentration of copper sulphate strongly influenced the morphology of silver deposit. At the beginning of the cementation process silver covers uniformly the copper surface. Afterwards, a growth of dendrites is initiated on preferential parts of the surface. The growing dendrite behaves as cathodic sites, with relatively huge surface area and promotes the creation of anodic sites in a close neighbourhood. Finally, the anodic site encloses the dendrite island and develops its area inward the copper material. Copper ions at low concentration modified slightly silver dendrites but the increase in concentration up to 0.5 M Cu²⁺ leads to completely disappearance of dendrites from the surface. The lack of dendrites on the surface is a result of the competitive process that consumes additional silver ions, occurring in the bulk of the solution. The morphology of silver deposit cemented in the deoxygenated solution containing 0.5 M H₂SO₄ + 0.5 M CuSO₄ depends strongly on the mechanism of the process.     

Screening study of spray solution parameters for depositing cerium-based conversion coatings on Al alloy 2024-T3

Cerium-based conversion coatings were deposited on aluminum alloy 2024-T3 by a spray process using a solution containing cerium chloride, hydrogen peroxide, and gelatin. As deposited coatings were composed of hydrated cerium oxide and were post-treated in a phosphate solution to improve corrosion performance. Coating solution parameters, including the pH (1–2.5), cerium chloride concentration (0.025–0.125 M), and hydrogen peroxide content (0–1.2 M), were varied to investigate the effect(s) of solution parameters on the corrosion performance of the post-treated coatings. Results indicated that thickness of coatings deposited from solutions with different pH values were similar, while coating thickness increased with increasing concentration of cerium chloride and hydrogen peroxide in the solutions. Electrochemical impedance spectroscopy and observations of the surface appearances of the coatings indicated that coatings deposited from solutions with a pH 2, a cerium concentration of 0.1 M, and a hydrogen peroxide concentration of 0.8 M exhibited the best corrosion resistance.     

Iron-enriched natural zeolite modified carbon paste electrode for H2O2 detection

This work demonstrates that iron-enriched natural zeolitic volcanic tuff (Paglisa deposit, Cluj county, Transilvania, Romania) resulting from a previous use as adsorbent in wastewater treatment can be recycled into effective electrode modifier applied to the electrocatalytic detection of hydrogen peroxide. After physico-chemical characterization of tuff samples using various techniques such as chemical analysis, X-ray diffraction, scanning electron microscopy, infrared spectroscopy, BET analysis and X-ray photoelectron spectroscopy, the electrochemical response of the iron-enriched zeolites was studied on the basis of solid carbon paste electrodes modified with these samples. The results indicate that iron centers in the zeolite are electroactive and that they act as electrocatalysts in the voltammetric and amperometric detection of H₂O₂. Best performance was achieved in phosphate buffer at pH 7, showing a sensitivity of 0.57 mA M⁻¹ cm⁻², a detection limit down to 60 μM, and a linear domain up to 100 mM H₂O₂.     

Preparation and electrochemical characterization of palladium single crystal electrodes in 0.1 M H2SO4 and HClO4: Part I. Low-index phases

The electrochemical properties of Pd(1 1 1), Pd(1 0 0) and Pd(1 1 0) single crystal bead electrodes, prepared by a novel electron beam heating and inductive annealing technique, have been characterized in 0.1 M sulfuric acid and 0.1 M perchloric acid by cyclic voltammetry and chronoamperometry. Hydrogen and (hydrogen) sulfate adsorption as well as surface oxidation were found to depend strongly on the crystallographic orientation and the nature of the electrolyte. The combination of charge displacement and voltammetric experiments allowed the determination of the potentials of zero total charge (E_pztc) of Pd(1 1 1) and Pd(1 0 0). The values of E_pztc in sulfuric acid were found to be more negative than in perchloric acid. The estimation of E_pztc for Pd(1 1 0) was hampered by the superposition with hydrogen absorption. The electro-oxidation of irreversible adsorbed carbon monoxide monolayers was studied on the three low-index Pd electrodes. The onset potential of the CO oxidation reaction follows the sequence Pd(1 0 0) < Pd(1 1 0) < Pd(1 1 1). Chronoamperometric experiments revealed a pronounced structure sensitivity of the reaction kinetics. The processes involved are determined by nucleation of oxygen-containing species on defect (step) sites and by slow diffusion of CO_ads on (1 1 1) terrace sites.     

Effect of ZrO2 on phase transformation of Al2O3

This study investigates the effect of ZrO₂ on phase transformation of alumina. Alumina and alumina–zirconia composite powders were produced by the precipitation method from aluminum sulfate and zirconium sulfate precursors. Precipitates obtained at 15 °C were dried at 80 °C for 72 h, and then calcinated at four different temperatures; 1000 °C, 1100 °C, 1200 °C and 1300 °C for 1 h. Powders calcinated at 1300 °C were pressed uniaxially and sintered at 1600 °C for 1 h. XRD and DSC analyses showed that the presence of zirconia retarded the transformation to α-alumina. SEM studies on the powders calcinated at 1300 °C revealed that both alumina and alumina–zirconia particles were 100–300 nm in size and of near spherical shape. Zirconia additions inhibited abnormal grain growth of alumina and provided homogeneous, equaxied grain structure.     

Synthesis, microstructure, and photocatalysis of In2O3 hollow particles

Indium oxide (In₂O₃) microspheres with hollow interiors have been prepared by a facile implantation route which enables indium ions released from indium-chloride precursors to implant into nonporous polymeric templates in C₂Cl₄ solvent. The templates are then removed upon calcination at 500 °C in air atmosphere, forming hollow In₂O₃ particles. Specific surface area (0.5-260 m² g⁻¹) and differential pore volume (7 × 10⁻⁹ to 3.8 × 10⁻⁴ m³ g⁻¹ Å⁻¹) of the hollow particles can be tailored by adjusting the precursor concentration. For the hollow In₂O₃ particles with high surface area (260 m² g⁻¹), an enhanced photocatalytic efficiency (up to ∼one-fold increase) against methylene blue (MB) dye is obtained under UV exposure for the aqueous In₂O₃ colloids with a dilute solids concentration of 0.02 wt.%.     

Stable spinel type cobalt and copper oxide electrodes for O2 and H2 evolutions in alkaline solution

The stability of one material, Ti/Cu_xCo_3−xO₄, as anode and also cathode was investigated for electrolysis of alkaline aqueous solution. The electrodes were prepared by thermal decomposition method with x varied from 0 to 1.5. The accelerated life test illustrated that the electrodes with x = 0.3 nominally showed the best performance, with a total service life of 1080 h recorded in 1 M NaOH solution under alternating current direction at 1 A cm⁻² and 35 °C. The effects of copper content in electrode coating were examined in terms of electrode stability, surface morphology, coating resistivity and coating compositions. The presence of Cu in the spinel structure of Co₃O₄ could significantly enhance the electrochemical and physicochemical properties. The trends of crystallographic properties and surface morphology have been analyzed systemically before, during and after the electrodes were employed in alkaline electrolysis. The oxygen evolution would lead to the consumption of the coating material and the progressive cracking of the coating. Along with hydrogen evolution, cobalt oxide could be reduced to metal Co and Co(OH)₂ with particle sizes changed to smaller values in crystal and/or amorphous form at the cathode. The formation of Co is the key process for this electrode to serve as both anode and cathode. It is also the main reason leading to the eventual failure of the electrodes.     

Measurement of thermal residual stresses in ZrB2-SiC composites

Neutron diffraction, Raman spectroscopy, and x-ray diffraction were employed to measure the stresses generated in the ZrB₂ matrix and SiC dispersed particulate phase in ZrB₂-30 vol% SiC composites produced by hot pressing at 1900 °C. Neutron diffraction measurements indicated that stresses begin to accumulate at ∼1400 °C during cooling from the processing temperature and increased to 880 MPa compressive in the SiC phase and 450 MPa tensile in the ZrB₂ phase at room temperature. Stresses measured via Raman spectroscopy revealed the stress in SiC particles on the surface of the composite was ∼390 MPa compressive, which is ∼40% of that measured in the bulk by neutron diffraction. Grazing incidence x-ray diffraction was performed to further characterize the stress state in SiC particles near the surface. Using this technique, an average compressive stress of 350 MPa was measured in the SiC phase, which is in good agreement with that measured by Raman spectroscopy.     

Gold nanoparticles mediate the assembly of manganese dioxide nanoparticles for H2O2 amperometric sensing

Aggregates of gold nanoparticles (AuNPs) that mediate the assembly of manganese dioxide nanoparticles (nano-MnO₂) for hydrogen peroxide (H₂O₂) amperometric sensing have been developed. The aggregates were prepared by directly mixing citric-capped AuNPs and poly(allylamine hydrochloride) (PAH)-capped nano-MnO₂ using an electrostatic self-assembly strategy. The prepared sensor exhibited excellent electrochemical behaviors and a wide linear range from 7.80 × 10⁻⁷ to 8.36 × 10⁻⁴ M with a detection limit of 4.68 × 10⁻⁸ M (S/N = 3) because of the synergistic influence of excellent catalytic ability of MnO₂ and good electrical conductivity of AuNPs. In addition, its applicability to practical samples for measuring H₂O₂ in toothpastes has obtained a satisfactory result. Due to the ease of preparation and excellent properties of the sensor, indicating the MnO₂-AuNP material may be a potential H₂O₂ sensor.     

Adsorption behavior of acetonitrile on platinum and gold electrodes of various structures in solution of 0.5 M H2SO4

The variation of electrode nature and surface structure (the use of stepped single crystal faces with controlled width of (1 1 1) terraces and monoatomic steps of (1 0 0) or (1 1 0) orientation) allows to determine peculiarities of co-adsorption of acetonitrile molecules, hydrogen adatoms and (bi)sulfate anions. It has been shown that first of all acetonitrile blocks adsorption sites at the steps. Anion adsorption at terraces of stepped platinum surfaces in 0.5 M H₂SO₄ solution with additions of acetonitrile depends on terrace width and the step orientation. This demonstrates the important role of structural factors in competitive adsorption processes. The decrease in adsorption of hydrogen and anions on narrow terraces is substantially due to the influence of acetonitrile molecules placed at the steps or nearby sites. At E < 1.0 V, electrochemical conversion of acetonitrile has not been detected at single crystal Pt surfaces. However, acetonitrile oxidation might proceed on polycrystalline platinum followed by product desorption. On Au(1 1 1) surface acetonitrile adsorption is considerably weaker than that on platinum electrodes.     

Electrochemical behavior of an Ag/TiO2 composite surfaces

Silver in a form of randomly distributed submicron-sized particles was electrodeposited, and simultaneously, titanium dioxide layer was formed, on the surface of mechanically polished titanium, by its potentiodynamic polarization in an aqueous AgNO₃ solution. In such obtained Ag/TiO₂ composite layer, the particle size distribution, and silver loading were determined by means of scanning electron microscopy. The electrochemical response of this composite layer was studied in both oxygen-free and oxygen saturated aqueous 0.1 M NaOH solution. The catalytic activity toward oxygen reduction reaction of this and other forms of silver- and platinum-based materials was compared.     

Parametric study of the gel-combustion synthesis of nanocrystalline ZrO2-based powders

In this paper, we report a study of the effect on the powder morphology of zirconia based solid solutions of several parameters of the synthesis by gel-combustion routes, such as fuel, metal/fuel ratio, composition of the nanopowder, pH of the precursor solution, etc. We evaluated the average crystallite size, BET specific surface area and the degree of agglomeration of the final ceramic nanopowders. The intermediate stages of the synthesis were characterized by Fourier Transform Infrared Spectroscopy, Small Angle X-ray Scattering, X-ray Powder Diffraction, X-ray Absorption Spectroscopy and Infrared Thermography. We found that the precursor gel remained compositionally homogeneous up to the self-combustion step and that in order to obtain a low average crystallite size (< 10 nm) and a low degree of agglomeration, the combustion temperature must be tailored to be as high as possible. Finally, a novel gel-combustion route assisted with hydrogen peroxide is reported.     

Relationship between microstructure and abrasive wear resistance of Al2O3-FeAl2O4 nanocomposites produced via solid-state precipitation

The work investigates the correlation between the microstructure and wear behaviour of novel Al₂O₃-FeAl₂O₄ nanocomposites, developed by precipitation of FeAl₂O₄ particles through reduction aging of Al₂O₃-10 wt.% Fe₂O₃ solid solutions in N₂/4%H₂. Reduction aging at 1450 °C for 10 and 20 h resulted in considerable improvements in abrasive wear resistance. The nanocomposites developed from solid solutions doped additionally with ∼250 ppm of Y₂O₃ contained finer intergranular second phase particles (by a factor of ∼2) and showed further improvements in the wear resistance. Doped nanocomposites reduction aged for 20 h at 1450 °C exhibited the minimum wear rate (reduced by a factor of ∼2.5 with respect to monolithic Al₂O₃). The suppression of fracture-induced surface pullout in the presence of intragranular nanosized second phase particles was the major factor responsible for the improved wear resistance of the nanocomposites with respect to monolithic alumina; microstructures without these intragranular nanoparticles showed no improvement. Higher aging temperature led to the presence of coarse (>2 μm) intergranular FeAl₂O₄ particles which had a detrimental effect on the wear resistance.     

Adsorption of thiourea on monocrystalline silver electrodes in neutral solution

Impedance spectroscopy and radiometric method have been used in the study of thiourea (TU) adsorption on monocrystalline silver electrodes of basal indices: (1 1 1), (1 0 0) and (1 1 0) in neutral solution. The dependence of the surface concentration of TU on the electrode potential and on the bulk concentration was determined for each studied surface. From radiometric measurements it follows that adsorption of TU on silver electrodes takes place in the entire range of applied potentials. The process of adsorption is practically reversible with respect to the electrode potential (in the range of the double layer) and the bulk concentration of TU. Differential capacity of silver electrodes in 0.01 M NaClO₄ solution containing TU of concentrations from 10⁻⁶ to 5 × 10⁻⁴ M has been measured. The isotherms of TU adsorption, determined from the capacitance and radiometric measurements have been compared and the Gibbs energy of adsorption was calculated. The values of limiting surface concentration of adsorbed TU as well as the Gibbs energy of adsorption depend on the plane of Ag electrode and follow the sequence: Ag(1 1 1) > Ag(1 0 0) > Ag(1 1 0) which is in agreement with the surface density of Ag atoms.     

Fabrication of graphene/prussian blue composite nanosheets and their electrocatalytic reduction of H2O2

In this work, graphene/prussian blue (PB) composite nanosheets with good dispersibility in aqueous solutions have been synthesized by mixing ferric-(III) chloride and potassium ferricyanide in the presence of graphene under ambient conditions. Transmission electron microscopy (TEM) shows that the average size of the as-synthesized PB nanoparticles on the surface of graphene nanosheets is about 20 nm. Fourier-transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) patterns have been used to characterize the chemical composition of the obtained graphene/PB composite nanosheets. The graphene/PB composite nanosheets exhibit good electrocatalytic behavior to detection of H₂O₂ at an applied potential of −0.05 V. The sensor shows a good linear dependence on H₂O₂ concentration in the range of 0.02-0.2 mM with a sensitivity of 196.6 μA mM⁻¹ cm⁻². The detection limit is 1.9 μM at the signal-to-noise ratio of 3. Furthermore, the graphene/PB modified electrode exhibits freedom of interference from other co-existing electroactive species. This work provides a new kind of composite modified electrode for amperometric biosensors.     

Soft template synthesis of mesoporous Co3O4/RuO2·xH2O composites for electrochemical capacitors

Co₃O₄/RuO₂·xH₂O composites with various Ru content (molar content of Ru = 5%, 10%, 20%, 50%) were synthesized by one-step co-precipitation method. The precursors were prepared via adjusting pH of the mixed aqueous solutions of Co(NO₃)₂·6H₂O and RuCl₃·0.5H₂O by using Pluronic123 as a soft template. For the composite with molar ratio of Co:Ru = 1:1 annealed at 200 °C, Brunauer-Emmet-Teller (BET) results indicated that the composite showed mesoporous structure, and the specific surface area of the composite was as high as 107 m² g⁻¹. The electrochemical performances of these composites were measured in 1 M KOH electrolyte. Compared with the composite prepared without template, the composite with P123 exhibited a higher specific capacitance. When the molar content of Ru was rising, the specific capacitance of the composites increased significantly. It was also observed that the crystalline structures as well as the electrochemical activities were strongly dependent on the annealing temperature. A capacitance of 642 F/g was obtained for the composite (Co:Ru = 1:1) annealed at 150 °C. Meanwhile, the composites also exhibited good cycle stability. Besides, the morphologies and textural characteristic of the samples were also investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM).     

Phase transitions in hydrothermal K2HPO4 solutions

A 0.1 M potassium phosphate (K₂HPO₄) solution was reacted in a flow-through cell pressurized to 22 MPa. Reduced light transmission through the cell windows was observed at a setpoint temperature ≥400 °C, along with a decrease in effluent conductivity, but with no effect on flow. These observations suggest solution separation at ∼360 °C, with accumulation of a salt-concentrated liquid in the cell body and transition of a dilute liquid to a supercritical fluid at temperature >374 °C. High-pressure differential scanning calorimetry experiments confirm an onset temperature of 354 °C with an endothermic transition at 377 °C and 22 MPa. For apparent density, ρ = 150-500 kg/m³, the average transition temperature for 0.1 M solutions, 375 ± 5 °C, is slightly elevated relative to that of water at 371 ± 4 °C. Highest deviation for 1.0 M solutions, 365 ± 15 °C, is attributed to increased K₂HPO₄ hydrolysis and polymerization reactions.     

Oxidative leaching kinetics of molybdenum-uranium ore in H2SO4 using H2O2 as an oxidizing agent

The processing of molybdenum-uranium ore in a sulfuric acid solution using hydrogen peroxide as an oxidant has been investigated. The leaching temperature, hydrogen peroxide concentration, sulfuric acid concentration, leaching time, particle size, liquid-to-solid ratio and agitation speed all have significant effects on the process. The optimum process operating parameters were: temperature: 95°C; H₂O₂ concentration: 0.5 M; sulfuric acid concentration: 2.5 M; time: 2 h; particle size: 74 μm, liquid-to-solid ratio: 14 ∶ 1 and agitation speed: 600 rpm. Under these experimental conditions, the extraction efficiency of molybdenum was about 98.4%, and the uranium extraction efficiency was about 98.7%. The leaching kinetics of molybdenum showed that the reaction rate of the leaching process is controlled by the chemical reaction at the particle surface. The leaching process follows the kinetic model 1 ? (1?X)^1/3 = kt with an apparent activation energy of 40.40 kJ/mole. The temperature, concentrations of H₂O₂ and H₂SO₄ and the mesh size are the main factors that influence the leaching rate. The reaction order in H₂SO₄ was 1.0012 and in H₂O₂ it was 1.2544.