Growth of Anodic Films on Compound Semiconductor Electrodes: InP in Aqueous (NH4)2S

Summary.  Film formation on compound semiconductors under anodic conditions is discussed. The surface properties of InP electrodes were examined following anodization in an (NH₄)₂S electrolyte. The observation of a current peak in the cyclic voltammetric curve was attributed to selective etching of the substrate and a film formation process. AFM images of samples anodized in the sulfide solution revealed surface pitting. Thicker films formed at higher potentials exhibited extensive cracking as observed by optical and electron microscopy, and this was explicitly demonstrated to occur ex situ rather than during the electrochemical treatment. The composition of the thick film was identified as In₂S₃ by EDX and XPS. The measured film thickness varies linearly with the charge passed, and comparison between experimental thickness measurements and theoretical estimates for the thickness indicate a porosity of over 70%. Cracking is attributed to shrinkage during drying of the highly porous film and does not necessarily imply stress in the wet film as grown. During the growth of the thick porous film, spontaneous current oscillations have been observed. The frequency of oscillation was found to be proportional to the current density, regardless of whether the measurements were carried out during a potential sweep or at constant potential. Thus, the charge passed per oscillation remained constant. A characteristic value of approximately 0.3 C · cm⁻² was measured under potential sweep conditions, and a similar value was obtained at constant potential. Received October 16, 2001. Accepted (revised) December 21, 2001     

Solution thermolysed tungsten oxide-based electrochromic devices: thickness-dependent step potential analysis

The characterisation of electrochemical behaviour of electrochromic (EC) devices based on solution thermolysed (ST) tungsten oxide (WO₃) thin films was carried out using the step potential excitation method. The method, based on generating plots of current density (J) as a function of passed charge (ΔQ), has been applied for the characterisation of EC-WO₃ thin films in proton-containing aqueous electrolyte. EC devices have been fabricated by employing WO₃ thin films with variable thickness (T) ranging from 0.04 to 0.52 μm. The J vs time (t) responses (chronoamperometry) of these devices were recorded at a fixed applied potential (±0.7 V vs S.C.E.) and values of total passed H⁺ charges (ΔQ) into the WO₃ host lattice during the coloration process are calculated. The J-ΔQ curves corresponding to films of different thickness were plotted as a function of the passed charge volume density, ΔQ /T, and an intercalatable film thickness is calculated to be 0.13 μm. The modulation in optical transmittance after coloration and bleaching was studied in the wavelength range between 350 and 850 nm and an optical efficiency (ξ_λ) is calculated at λ=700 nm. It is found that the ξ_λ wanes with increasing intercalation. Electronic Publication     

Ni-Co/RuO2复合电极的制备及其电催化析氢性能

采用快速凝固结合去合金化的方法制备纳米多孔Ni-Co合金,利用RuO_2对Ni-Co合金进行表面修饰,通过X射线衍射(XRD)、X射线光电子能谱(XPS)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)对多孔材料进行物相分析和形貌表征,并通过线性扫描伏安法、多电位阶跃法、交流阻抗法和恒电流电解法测试多孔电极的电催化析氢性能。结果表明,Ni-Co/RuO_2复合电极材料在50 mA·cm~(-2)电流密度下析氢过电位为180 mV,析氢过程由Volmer-Heyrovsky步骤控制,交换电流密度为4.42 mA·cm~(-2),经10 h恒电流电解后电位仅增加20 mV,表现出良好的析氢稳定性。     

Electrical characterization of poly(3-methylthiophene) electrosynthesized onto a tin-oxide substrate

We have investigated poly(3-methylthiophene) (PMeT) thin films electrochemically synthesized directly onto a tin-oxide (TO) electrode. We find that the PMeT film thickness depends linearly on the charge density used during the electropolymerization. We have demonstrated that the current transport in PMeT films (solid phase) is space-charge limited or controlled by thermionic emission, depending on the electrode material. Using TO/PMeT/Ni devices we estimate the positive charge carrier mobility in PMeT to be around 4 × 10⁻⁴ cm² V⁻¹ s⁻¹, and the potential barrier height for positive charge carrier injection at the Al/PMeT interface to be 0.17 eV. Received: 6 December 1999 / Accepted: 24 February 2000     

Direct electron transfer of cytochrome C and its electrocatalytic properties on multiwalled carbon nanotubes/ciprofloxacin films

In this study, stable and homogenous thin films of multiwalled carbon nanotubes (MWCNTs) were obtained on conducting surface using ciprofloxacin (CF, fluoroquinolone antibiotic) as an effective-dispersing agent. Further, MWCNTs/CF film modified electrodes (glassy carbon and indium tin oxide-coated glass electrode) are used successfully to study the direct electrochemistry of proteins. Here, cytochrome C (Cyt-C) was used as a model protein for investigation. A MWCNTs/CF film modified electrode was used as a biocompatible material for immobilization of Cyt-C from a neutral buffer solution (pH 7.2) using cyclic voltammetry (CV). Interestingly, Cyt-C retained its native state on the MWCNTs/CF film. The Cyt-C adsorbed MWCNTs/CF film was characterized by scanning electron microscopy (SEM), UV–visible spectrophotometry (UV-vis) and CV. SEM images showed the evidence for the adsorption of Cyt-C on the MWCNTs/CF film, and UV–vis spectrum confirmed that Cyt-C was in its native state on MWCNTs/CF film. Using CV, it was found that the electrochemical signal of Cyt-C was highly stable in the neutral buffer solution and its redox peak potential was pH dependent. The formal potential (−0.27 V) and electron transfer rate constant (13 ± 1 s⁻¹) were calculated for Cyt-C on MWCNTs/CF film modified electrode. A potential application of the Cyt-C/MWCNTs/CF electrode as a biosensor to monitor H₂O₂ has been investigated. The steady-state current response increases linearly with H₂O₂ concentration from 2 × 10⁻⁶ to 7.8 × 10⁻⁵ M. The detection limit for determination of H₂O₂ has been found to be 1.0 × 10⁻⁶ M (S/N = 3). Thus, Cyt-C/MWCNTs/CF film modified electrode can be used as a biosensing material for sensor applications.     

Silicomolybdate doped polypyrrole film modified glassy carbon electrode for electrocatalytic reduction of Cr(VI)

Electrically conducting polypyrrole (PPy) film doped with silicomolybdate (SiMo₁₂ or SiMo₁₂) was synthesized by electrochemical polymerization. The synthesized film is capable of fast charge propagation during redox reactions in strong acid medium 0.2 M H₂SO₄ solution. Electrochemical quartz crystal microbalance was used to study the mechanism and amount of SiMo₁₂ doped in the PPy matrix. The modified electrode surface was characterized by using atomic force microscope technique, and it was found that the minimum and maximum globule size were estimated to be in the range of 50–200 nm. The thickness of film was measured to be approximately 30 ± 10 nm. The modified electrode shows electrocatalytic activity towards reduction of Cr(VI) and periodate. The rate constant and optimal film thickness were determined for electrocatalytic reduction of Cr(VI) by using rotating disc electrode experiment. Analytical characterization of the SiMo₁₂ doped PPy film modified electrode was demonstrated by flow injection analysis (FIA) technique and shows good stability for 16 continuous injections for Cr(VI) reduction with RSD of 1.6%.     

Electrostatic interaction between a cylinder and a planar surface

 An exact analytical expression for the potential energy of the electrostatic interaction between a plate-like particle 1 and a cylindrical particle 2 of radius a ₂ immersed in an electrolyte solution of Debye–Hückel parameter κ is derived on the basis of the linearized Poisson–Boltzmann equation without recourse to Derjaguin's approximation. Both particles may have either constant surface potential or constant surface charge density. In the limit of κa ₂→0, in particular, the interaction between a plate with zero surface charge density and a cylinder having constant surface charge density becomes identical to the usual image interaction between a line charge (a charged rod of infinitesimal thickness) and an uncharged plate. Received: 22 September 1998  Accepted in revised form: 27 January 1999     

Electropolymerized composite film of polypyrrole and functionalized multi-walled carbon nanotubes: effect of functionalization time on capacitive performance

The composite film of polypyrrole and functionalized multi-walled carbon nanotubes (PPy/F-MWNTs) was prepared by electropolymerization. MWNTs were functionalized by sonicating with a concentrated solution of H₂SO₄/HNO₃ (3/1, volume ratio) in a water bath for different times. The carbon nanotubes (CNTs) are cut into smaller portions with more functional groups introduced on their surface when the sonicating time (nominated as functionalization time hereafter) is increased. However, the specific capacitance of the composite film reaches a maximum of 240 F g⁻¹ at the scanning rate of 10 mV s⁻¹ when MWNTs are functionalized for 24 h, which is about 205 F g⁻¹, 225 F g⁻¹ and 232 F g⁻¹, respectively, when MWNTs are functionalized for 6 h, 12 h and 48 h. At a current load of 1.0 A g⁻¹, PPy/F-MWNT composite film functionalized for 24 h (PPy/F-MWNTs (24 h)) retains 93.49% of its initial capacitance after 1,000 cycles of galvanostatic charge/discharge, and the discharge efficiency is higher than 98.15% during cycling. High specific capacitance, good rate performance, fast charge/discharge ability and long cycling life are ascribed to the synergistic effect of the two components to form a porous composite film as well as the easy accessibility of counter ions into the film. Therefore, PPy/F-MWNT (24 h) composite film is a kind of promising electrode material for supercapacitors. The mechanism of underfunctionalization and overfunctionalization of carbon nanotubes is also discussed.     

Photostimulated changes in WO3 nanosized films

During the irradiation of WO₃ films d = 7–160 nm thick by light at λ = 320 nm (I = (1.5–7) × 10¹⁵ quantum cm⁻² s⁻¹), absorption band at λ = 850 nm appeared along with absorption band edge shift to shorter waves. The subsequent irradiation of samples at λ = 850 nm caused the disappearance of the longwave absorption band. The intrinsic absorption edge of WO₃ films was determined (λ = 320 nm). The degree of transformations of WO₃ films increased under atmospheric conditions as the intensity of incident light and the time of irradiation (1–140 min) grew and as film thickness decreased. A mechanism of photochemical transformations of WO₃ films was suggested. This mechanism included the generation of electron-hole pairs, the recombination of part of nonequilibrium charge carriers, the formation of [eV_a²⁺e] centers, and the isolation of photolysis products.     

Simultaneous observation of current oscillations and porous film growth during anodization of InP

The observation of spontaneous oscillations in current during the anodization of InP in relatively high concentrations of KOH electrolytes is reported. Oscillations were observed under potential sweep and constant potential conditions. Well-defined oscillations are observed during linear potential sweeps of InP in 5 mol dm(-)(3) KOH to potentials above approximately 1.7 V (SCE) at scan rates in the range of 50 to 500 mV s(-)(1). The oscillations observed exhibit an asymmetrical current versus potential profile, and the charge per cycle was found to increase linearly with potential. More complex oscillatory behavior was observed under constant potential conditions. Periodic damped oscillations are observed in high concentrations of electrolyte whereas undamped sinusoidal oscillations are observed in relatively lower concentrations. In both cases, the anodization of InP results in porous InP formation, and the current in the oscillatory region corresponds to the cyclical effective area changes due to pitting dissolution of the InP surface with the coincidental growth of a thick porous In(2)O(3) film.     

Electrochemistry and electrocatalytic activity of polypyrrole/ferrocyanide films on a glassy carbon electrode

Functionalized polypyrrole films were prepared by incorporation of Fe(CN)₆ ³⁻ as doping anion during the electropolymerization of pyrrole at a glassy carbon electrode from aqueous solution. The electrochemical behavior of the Fe(CN)₆ ³⁻/Fe(CN)₆ ⁴⁻ redox couple in polypyrrole was studied by cyclic voltammetry. An obvious surface redox reaction was observed and dependence of this reaction on the solution pH was illustrated. The electrocatalytic ability of polypyrrole film with ferrocyanide incorporated was demonstrated by oxidation of ascorbic acid at the optimized pH of 4 in a glycine buffer. The catalytic effect for mediated oxidation of ascorbic acid was 300 mV and the bimolecular rate constant determined for surface coverage of 4.5 × 10⁻⁸ M cm⁻² using rotating disk electrode voltammetry was 86 M⁻¹ s⁻¹. Furthermore, the catalytic oxidation current was linearly dependent on ascorbic acid concentration in the range 5 × 10⁻⁴–1.6 × 10⁻² M with a correlation coefficient of 0.996. The plot of i _p versus v ^1/2 confirms the diffusion nature of the peak current i _p. Received: 12 April 1999 / Accepted: 25 May 1999     

Kinetics of the Formation of the Blue Complex CrO(O2)2 Formed by Dichromate and H2O2 in Acid Solutions. A Stopped-Flow Investigation Using Rapid-Scan UV-VIS Detection

 The kinetics of the CrO(O₂)₂ formation by H₂O₂ and Cr₂O₇ ²⁻ in aqueous acidic media was measured at 293 ± 2 K in a pH range between 2.5 and 3.3. Using the stopped-flow method with rapid scan UV-VIS detection, the rate law of the formation of CrO(O₂)₂ was determined. For the media HClO₄, HNO₃ and CH₃COOH, the reaction order in the Cr₂O₇ ²⁻ concentration was found to be 0.5. For [H₂O₂] as well as for [H⁺], the reaction was first order in all acids used. In HCl and H₂SO₄ media the reaction was first order in Cr₂O₇ ²⁻. At T = 293 ± 2 K the rate constant for the formation of Cr(O)(O₂)₂ was found to be (7.3 ± 1.9) · 10² M^−3/2 s⁻¹ in HClO₄.     

Analysis of photocurrent spectra at polybithienyl film coated on Pt

Photoelectrochemical measurements have been performed at a polybithienyl (PBT) film (doping level of 1 × 10¹⁸/cm³) deposited on a platinum electrode. The cathodic photocurrents and negative slope of the Mott-Schottky plot indicate that the PBT film has the features of a p-type semiconductor. The cathodic photocurrents are interpreted in terms of the Gaertner-Butler model on the basis of the theory of the semiconductor|solution interface. The (i _ph hν)^2/n vs. hν plots taken from the photocurrent spectra show two linearities for n=1 in the wavelength range from 460 nm to 490 nm and for n=4 in the wavelength range λ > 490 nm. The band gaps of the PBT film were determined to be 2.05 ± 0.05 eV for n=1 and 1.55 ± 0.05 eV for n=4. The flat-band potential is 0.33 V (vs SCE). From the slope of the Mott-Schottky plot at the modulation frequency of 3 kHz, the dielectric constant ɛ of the film and the thickness of the depletion layer W ₀ of the PBT film were determined to be 7.4 and 0.29 μm, respectively. Received: 6 January 1999 / Accepted: 6 June 1999     

Application of a Single-Wall Carbon Nano-Tube Film Electrode to the Determination of Trace Amounts of Folic Acid

Single-wall carbon nano-tubes were used to modify the surface of a glassy carbon electrode (GC) and applied in the determination of folic acid with voltammetry. The experiments demonstrated that the presence of a carbon nano-tube film on the electrode greatly increased the reduction peak current of folic acid. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were used in a comparative investigation of the electrochemical reduction of folic acid with the film electrode. Effects of pH on the peak current and the peak potential were studied in the pH range of 4.0–8.0 with Britton-Robinson buffer solution. The reduction peak current was found to be linearly related to folic acid concentration over the range of 1 × 10⁻⁸ to 1 × 10⁻⁴ mol L⁻¹ with a detection limit of 1 × 10⁻⁹ mol L⁻¹ after 5 min accumulation. The film electrode provides an efficient way for eliminating interferences from some inorganic and organic species in the solution. The high sensitivity, selectivity and stability of the film electrode demonstrate its practical application from a simple and rapid determination of folic acid in tablets.     

Preparation and dielectric properties of highly preferred-(100) orientation (Pb,La)(Zr,Ti)O<Subscript>3</Subscript> antiferroelectric thick films by sol–gel processing

Pb_0.97La_0.02Zr_0.95Ti_0.05O₃ (PLZT) antiferroelectric thick films of highly preferred-(100) orientation with different thickness were successfully deposited on Pt(111)/Ti/SiO₂/Si(100) substrates depending on the sol–gel process technique. The effects of the PLZT thick films in the preparation and electric properties are investigated. The films show polycrystalline perovskite structure with a (100) preferred orientation by X-ray diffractometer analyses. The antiferroelectric nature of the thick films is demonstrated by P (polarization)–E (electric field). The temperature dependence of the dielectric constant and dielectric loss displays the similar behavior in both cases at 100 kHz while the values of polarization characteristic are decreased with the increase of the film thickness. The phase switching current are studied as a function of a gradually change dc electric field and the voltage dependent current density of the most highly (100)-oriented PLZT film is 1.49 × 10⁻⁸ A/cm² over electric field range from 0 to ±261 kV/cm. The film at 2,498 nm exhibits excellent dielectric properties and highly preferred-(100) orientation.     

Preparation and performance comparison of LiMn2O3.95Br0.05 and LiMn2O3.95Br0.05/SiO2 cathode materials for lithium-ion battery

LiMn₂O_3.95Br_0.05 and LiMn₂O_3.95Br_0.05/SiO₂ cathode composites for lithium-ion battery are prepared by solid-state reaction methods. The crystalline structures of the as-synthesized samples are investigated by X-ray diffraction and transmission electron microscope; at the same time, the electrochemical performances are tested by cyclic voltammetry and galvanostatic cycling. The results reveal that the sample of LiMn₂O_3.95Br_0.05/SiO₂ has more excellent electrochemical performance than the sample of LiMn₂O_3.95Br_0.05. It delivers an initial discharge capacity of 145.3 mA h g⁻¹ at ambient temperature, and 138.9 mA h g⁻¹ at the higher temperature of 55 °C with good capacity retention with the voltage range of 3.0–4.35 V (vs. Li) at a current density of 0.5 C; while the sample of LiMn₂O_3.95Br_0.05 only deliver initial discharge capacity 136.5 mA h g⁻¹ at ambient temperature, and 119.2 mA h g⁻¹ at 55 °C in the same conditions; in addition, the rate performance of LiMn₂O_3.95Br_0.05/SiO₂ is excellent too, so the SiO₂ layer has improved the electrochemical behaviors of LiMn₂O_3.95Br_0.05 availably.     

The Thermal Dehydration of Magnesium Aluminum Pentafluoride Dihydrate: Crystal Structures of MgAlF5(H2O)2 and MgAlF5

 Small plate-like single crystals of MgAlF₅(H₂O)₂ have been obtained during hydrothermal treatment (270°C) of microcrystalline material prepared by precipitation of stoichiometric solutions of Al₂(SO₄)₃ ·  18H₂O and Mg(NO₃)₂ · 6H₂O with diluted hydrofluoric acid. The crystal structure of MgAlF₅(H₂O)₂ has been refined from single crystal data (Imma (# 74), Z = 4, a = 7.0637(7), b = 10.1308(10), c = 6.7745(7) ?, 398 structure factors, 33 parameters, R(F² > σ(F ²)) = 0.0245, wR(F² all) = 0.0525). Main features of the inverse weberite type structure are infinite chains of trans-bridged [AlF₆] octahedra which are connected via common fluorine atoms by isolated [MgF₄(H₂O)₂] octahedra. MgAlF₅(H₂O)₂ dehydrates at temperatures above 300°C to give MgAlF₅. XRPD analysis of this phase has revealed isotypism with FeAlF₅. The crystal structure of MgAlF₅ (Immm (# 71), Z = 2, a = 7.268(1), b = 6.123(2), c = 3.543(1) ?) is built of infinite chains of edge-sharing [MgF₆] octahedra and chains of corner-sharing [AlF₆] octahedra along [001]. Upon further heating to temperatures above 500°C, MgAlF₅ decomposes to MgF₂ and α − AlF₃.     

Preparation and Crystal Structure of K2Ag12Te7

 Single crystals of K₂Ag₁₂Te₇ (a = 11.460(2), c = 4.660(1) ?; V = 530.01 ?³; space group: P6₃/m; Z = 1) were synthesized under hydrothermal conditions at 250°C in concentrated aqueous KOH solution from elementary silver and tellurium. The crystal structure is characterized by trigonal prismatic KTe₆ polyhedra, connected via two common faces to KTe₃ rods parallel to [001]. These rods are combined by two crystallographically independent Ag atoms, each coordinated to four Te and three Ag atoms (Ag–Te and Ag–Ag < 3.1 ?) to a framework of the formula (K₂Ag₁₂Te₆)^2 + and with channels parallel to the sixfold axis. These channels are statistically occupied by one further Te atom per unit cell, distributed over two independent positions.     

The effect of pre-carbonization of mesophase pitch-based activated carbons on their electrochemical performance for electric double-layer capacitors

The activated carbons (ACs) are prepared from mesophase pitch which is pre-carbonized by using potassium hydroxide as an activating agent. Nitrogen adsorption at 77 K is used to characterize the surface area and the porous structure of the ACs. By changing the pre-carbonization temperature from 560 to 750 °C, the arrangement of the ACs’ microcrystallines might be controllable, and the pore size could be adjusted between 1.5 and 2.4 nm. The electrochemical performance of the ACs in organic solvent (1 M Et₄NBF₄ in propylene carbonate) is investigated by voltage sweep cyclic voltammetry and constant current charge–discharge cycling tests. The results show that the ions can transfer rapidly and freely in the pore larger than or equal to 1.85 nm, even when the current density increases to 25 A g⁻¹.     

Electrochemical investigations of the reaction mechanism and kinetics between NADH and riboflavin immobilised on amorphous zirconium phosphate

Electrochemical investigations of the reaction mechanism and kinetics between riboflavin immobilised on zirconium phosphate (ZPRib) in carbon paste and NADH showed results yielding reliable information about aspects on the mechanism of the electron transfer reaction between the flavin and NADH. The formal potential (E°′) of the adsorbed riboflavin was −220 mV versus SCE at pH 7.0. A shift about 250 mV towards a more positive potential compared with its value in solution was assigned to the interaction between the basic nitrogen of riboflavin and the acid groups of ZP. The invariance of the E°′ with the pH of the contacting solution and the effect of different buffer constituents were attributed to the protection effect of ZP over the riboflavin. The electrocatalytic oxidation of NADH at the electrode was investigated using cyclic voltammetry and rotating disk electrode methodology using a potential of −50 mV versus SCE. The heterogeneous electron transfer rate constant, k _obs, was 816 M⁻¹ s⁻¹ and the Michaelis-Menten constant, K _M, was 1.8 mM (confirming a charge transfer complex intermediate in the reaction) for an electrode with a riboflavin coverage of 6.8 × 10⁻¹⁰ mol cm⁻². This drastic increase in the reaction rate between NADH and the immobilised riboflavin was assigned to the shift of the E°′. A surprising effect with addition of calcium or magnesium ion to the solution was also observed. The E°′ was shifted to −150 mV versus SCE and the reaction rate for NADH oxidation increased drastically. Received: 22 February 1999 / Accepted: 10 March 1999