Surface and photoelectrochemical studies of semiconducting MoS2

Redox reactions with Cu(NH₃)^2+/1+_x, Fe(CN)^3?/4? and I^?₃/I₂ were studied on |-C surfaces of MoS₂ using rdes. From electrophoretic measurements, the zpc was found to occur at pH 2. The role of potential-determining ions and the effects of specific adsorption of ions on the electrode behaviour have been discussed. The change of potential with pH was ? 30 mV/pH unit for MoS₂ electrode in contact with a solution saturated with molybdenum and S^2? ions. From the information available, an energy level diagram has been constructed. This diagram is in agreement with the observed behaviour that electron exchange for the first three redox couples occurs with the conduction band, except that the electron transfer from the reduced species, Fe(CN)^4?₆ and Fe²⁺, in the reverse direction (oxidation) is partly and totally restricted, respectively. The charge transfer behaviour in the cse of I^?₃/I₂ couple, appears to be much more complex, most probably involving adsorption of I^? on the surface.     

The mechanism of the nitrate (platinum) electrode in molten alkali nitrates

The mechanism for the nitrate electrode, (Pt)NO₂, O₂/NO^?₃, is shown to be: NO^?₂ = NO₂ + e; NO₂ = NO + $\text{1}\text{2}$O₂; NO^?₃ + NO = NO^?₂ + NO₂; with the overall reaction: NO^?₃ = NO₂ + $\text{1}\text{2}$O₂ + e.     

Electrode processes of bismuth in weakly acidic,neutral and alkaline solutions

The electrode process Bi(III)+ 3e = Bi(Hg) has been investigated in weakly acidic, neutral and alkaline solutions by means of amalgam polarography and cyclic voltammetry and chronopotentiometry with the use of hanging amalgam drop electrode. It has been found that in weakly acidic and neutral solutions, oxidation of bismuth amalgam goes through aquocomplexes of bismuth. In the cathodic step on cyclic voltammetry curves four peaks of reduction are observed, with magnitude and shape strongly dependent on pH of the solution. These peaks have been ascribed to the reduction of Bi³⁺, Bi(OH)⁺₂, Bi(OH)⁺² and Bi(OH)₃.In alkaline solutions, oxidation of bismuth amalgam leads to Bi(OH)₃. In cathodic scan on cyclic voltammetry curves only one peak is observed, which corresponds to Bi(OH)₃ reduction. Formal potentials of electrode process and solubility product of Bi(OH)₃ have been calculated and discussed. The step-wise nature of the overall electrode reaction has been ascertained and discussed. At higher concentrations of OH^? ions, Bi(OH)₃ dissolves.     

Photoinduced layer phenomenon caused by iodine formation in MoSe2: electrolyte (iodide) junctions

When iodide is photooxidized at a n-type MoSe₂ electrode at a high rate, a dim film is observed to spread over the metallic bright electrode and the reflected light intensity throughout the visible and near ir spectral region drops by 70–80%. The phenomenon can — in a reversible way — be controlled by small potential variations (0 ? +0.5 V) and beyond +1.0 V produces electrochemical oscillations, which are paralleled by an additional reflection change which can be seen to propagate across the electrode surface.The phenomenon could be reproduced in absence of light at semiconducting p-type MoSe₂, at metallic NbSe₂ and at Pt-electrodes which excluded the possibility that the pronounced optical changes are produced by a photoelectrochemical or electrochemical modification of layer-type electrode material in presence of iodide. The reflection anomalies can be explained by physical properties of an iodine layer, which is formed in the iodide-depleted diffusion region at the electrode surface.Evidence is given that the spreading of the dim film is actually a phase transition from dispersed to crystalline iodine with an asymmetrical size distribution within the I^? depleted diffusion layer. The resulting gradient of the refraction index works like an anti-reflecting coating. The described phenomenon will have to be considered in attempts to optimize solar cells based on the I^?/I₂ redox couple and it might be of some practical interest for antireflecting layer-technology and electro-optical applications.     

Potential of the Pb,PbSO4/H2SO4 electrode at temperatures up to 240°C

Standard lead—lead sulphate electrode potential was determined over the temperature range 20–240°C from emf measurements of the Pb, PbSO₄H₂SO₄ (0.05M)K₂SO₄KClHCl(0.1M)/AgCl, Ag and Pb, PbSO₄H₂SO₄(m)K₂SO₄H₂SO₄(0.05M)PbSO₄, Pb cells where m = 0.005, 0.01, 0.1 and 0.5 M. To this effect lead—lead sulphate electrode potential was calculated using the temperature relationship of the standard silver—silver chloride electrode potential and activity coefficients of hydrochloric acid determined by Greeley et al. at temperatures up to 260°C. Diffusion potentials occurring at the phase boundaries in the cells under investigation were calculated using the Henderson's equation. Values of the standard lead—lead sulphate electrode potential were determined by extrapolation of the E°′ function to the zero ionic strength which was calculated using the second sulphuric acid dissociation constant determined by Lietzke et al. at temperatures up to 300°C. The standard electrode potential was described in the temperature range 20–240°C by the following relationship: E°_{Pb, PbSO4/SO^2?4}(V) = 0.040-0.00126T. A change in entropy ΔS° of the electrode reaction Pb + SO^2?₄ = PbSO₄ + 2e^? is constant in this temperature range and is ?243 JK^?1 mol^?1 (?1018 cal K^?1 mol^?1).     

The applicability of a nitrate melt as a cell electrolyte in a flue gas desulphurization cell

This paper presents equilibrium and kinetic studies of the O₂ and O₂/SO₂ electrodes in molten NaNO₃-KNO₃ eutectic at 380°C. For the O₂ electrode the overall reaction is given by the equation: $\text{1}\text{2}$O₂ + 2e^? = O^2?.The exchange currents for this reaction are higher with platinum than with gold and are 1–2 orders of magnitude higher than those previously measured in molten sulphates.The overall reaction of the O₂/SO₂ electrode is: SO₂ + O₂ + 2e^? = SO^2?₄.Gold is a better catalyst than platinum for this reaction, quite similar to the case in molten sulphates. Molten NaNO₃-KNO₃ seems to be an appropriate electrolyte for use in a new electrochemical flue gas desulphurization process in the temperature range of 350–450°C.     

Supramolecular assembly of one-dimensional channels and two-dimensional brick-wall networks from asymmetric dithioether ligands and copper(I) iodide

Two asymmetric dithioether ligands with cyclohexyl (L¹) and phenyl (L²) end-groups were synthesized. Reaction of L¹ with copper(I) iodide afforded a 1D channel-type coordination polymer {[Cu₄I₄(L¹)₂](CH₃CN)_0.5}_n (1) interconnected by cubane-type tetranuclear Cu₄I₄ cluster units. Whereas, a 2D brick-wall-type coordination polymer [CuI(L²)]_n (2) with rhomboid dinuclear Cu–I₂–Cu nodes was isolated from the reaction of L² with copper(I) iodide.     

Studies with chromium ferricyanide epoxy resin-based membranes

The solid membrane of chromium ferricyanide in epoxy resin has been used as a silver sensitive electrode. The electrode response to silver ions is non-Nernstian, but a linear plot has been obtained in the concentration range 10^?1-10^?3M with a slope of 46 mV per decade of activity. The response time of the electrode is a few seconds and potentials generated are reproducible. The electrode is not specific to silver ions but can nevertheless be used in presence of Hg²⁺, Zn²⁺ and Cd²⁺. Anions show no interference. Silver can be estimated even in the presence of interfering ions by potentiometric titration using the membrane as an end point indicator electrode.     

A study of Ga3+ Reduction in the drop mercury electrode

A kinetic study is carried out of the Ga^3? + 3e → Ga electrode reaction in relation to the existing ionic species in the media used: LiClO₄, NaCl, KBr, NaNO₃, Na₂SO₄ and NH₄OH. It is found that the predominant species in equilibrium at the electrode have a coordination index of 6 ligands for the SO^?₄, the Cl^? and NO^?₃ ions, 5 for Br^? and 4 for NH₃. It is also noted that Ga³⁺ does not form complex compounds with ClO^?4 ions. The specific rate constant of electrode reaction in the equilibrium potential, k_O calculated by Koutecky's method, is in all cases between 2·10^?5 and 1·10^?8 cm s^?1, and the transfer coefficient, α, is about 0·15.By microcoulometric methods it is confirmed that the number of electrons in the electron transfer is three.A study is also made of interaction of the Ga³⁺ reduction wave, which in a 0·1 M LiClO₄ medium presents a half wave potential (E_{$\text{1}\text{2}$}) of ?1·270 V with reference to sce, with the H⁺ wave. It is concluded that the optimum pH of study is between 2·7 and 3·1. The ionic intensity of the medium is also taken into account.     

Reduction electrochimique de sels de fer(III) et de leurs complexes chlores en solution dans le, N, N-dimethylformamide

The Fe(III) electrochemical reduction, Fe³⁺ + e^? → Fe²⁺ was studied, for the first step, in DMF with or without Cl^? ions. The results obtained with anhydrous and hydrated salts are compared and show the fast ligand water—DMF exchange when the salts are solubilized. Without Cl^? ions, Fe(III) shows two equilibrated solvation states. When Cl^? ions exist in the solutions, the species reduced at the electrode are generally more complexed than those existing in the bulk of the solution.     

Electrode reactions of nickel(II) at mercury electrodes in aqueous solutions of pyridine at high ionic strengths

At high ionic strength the ion pair (NiPy²⁺₄, nX^?) or complex (NiPy₄X₂), n = 0, 1, 2; X^? = Cl^?, Br^?, SCN^?, N^?₃, F^?, NO^?₃, ClO^?₄; is adsorbed at the surface of mercury electrode. Under specified conditions in chloride, bromide, and thiocyanates solutions the electroreduction is preceded by a crystallization of a complex on the electrode surface. The inductive role of specifically coadsorbed Cl^? ions is discussed.     

Automated serum chloride analysis using the Apple computer

Chloride analysis employing a coulometric technique is a wellestablished method. However, the equipment needed is specialized and somewhat expensive. The purpose of this paper is to report the development of the hardware and software to perform this analysis using an Apple computer to control the coulometric titration, as well as to automate it and to print out the results.The Apple computer is used to control the flow of current in a circuit, which includes silver and platinum electrodes where the following reactions take place:Ag → Ag⁺ + le^????(at silver anode) 2H₂O + 2e^? → 2OH^? + H₂???(at platinum cathode) The generated silver ions then react with the chloride ion in the sample to form AgCl.Ag⁺ + Cl^? → AgCl_(s) When all of the chloride ion has been titrated, the concentration of silver ions in solution increases rapidly, which causes an increase in the current between two silver microelectrodes. This current is converted to a voltage and amplified by a simple circuit. This voltage is read by the analogue-to-digital converter. The computer stops the titration and calculates the chloride ion content of the sample. Thus, the computer controls the apparatus, records the data, and reacts to the data to terminate the analyses and prints out the results and messages to the analyst.Analysis of standards and reference sera indicate the method is rapid, accurate and precise. Application of this apparatus as a teaching aidfor electronics to chemistry and medical students is also described.     

On the electrochemical behavior of H2O2 AT Ag in alkaline solution

Electrochemical oxidation and reduction of H₂O₂ on Ag were studied in alkaline solution of 10^?3?0.3 M H₂O₂ and 2 × 10^?3 ?1.0 M KOH under N₂ bubbling. Steady i-φ curves obtained by a cyclic potential sweep method in a potential range where no electrode oxidation takes place, lead to the following results: (1) i^c_d (A cm^?2) (cathodic limiting current density) = 1.0 × [H₂O₂]^1.0_T (M), (2) i¹_d (A cm^?2 (anodic limiting one) = i^c_d ([KOH] ? [H₂O₂]_T) or 1.0 × [KOH] < [H₂O₂]_T), (3) φ_m (V) (mixed potential) = 0.126-0.060 log [KOH]^1.0 and (4) (?φ/?i)_φ=φm (Ωcm²) (reaction resistance at φ = φ_m) = 0.057 × [H₂O₂]^?1.0_T (M^?1), where [H₂O₂]_T designates a total H₂O₂ concentration and the others have their usual meanings.The above results are explained by the following mechanism; HO^?₂ formed by the reversible chemical reaction, H₂O₂ + OH ? HO^?₂ + H₂O, is oxidised in anodic reaction by two steps: HO^?₂

HO₂ (a) + e^? and HO₂(a) + OH^? → O₂ + H₂O + e^?, whereas in cathodic reaction, H₂O₂ is reduced by H₂O₂ + e^?

OH(a) + OH^?, OH(a) + e^? → OH^?. Here,

designates a rate determining step,Catalytic decomposition of H₂O₂ on the electrode is also discussed.     

Kinetics of armco iron dissolution in an anhydrous methanol and hydrochloric acid solution

The polarization behaviour of Armco iron in anhydrous solutions of CH₃OH+HCl and CH₃OH+HCl+LiCl was studied by the potentiostatic method. The following reaction orders were found for the anodic process: Z_a(Cl^?) = 1.7 –1.82 and Z_a(H⁺) = 0. The following mechanism of dissolution is proposed: Fe+Cl^? ? FeCl^?_ads, FeCl^?_ads+Cl^? → FeCl₂+2e (rate-determining step), FeCl₂?FeP²⁺ + 2Cl^?.     

Studies on the mechanism of the hydrogen ionization reaction on rhodium in alkaline solution by means of deuterium tracer

The mechanism of the hydrogen ionization reaction on Rh in alkaline solution was investigated by means of a deuterium tracer technique for a wide range of anodic polarization. The reaction route consists of two steps, H₂? 2 H(a) and H(a) + B?H⁺B + e(B = H₂O or OH^?). The free energy decrease associated with the former step was around 5% of that of the overall reaction somewhat depending on the electrode pretreatment. The individual step rates are evaluated.     

Etude des phenomenes electrochimiques et des transports de matiere d'un systeme metal electrolytique: cas d'un disque tournant en cuivre dans des solutions aqueuses d'acide chlorhydrique

In the first two areas of the curves I = F(E) corresponding to Cu/CuCl_x^1?x and Cu/CuCl/CuCl_x^1?x redox systems we study the diffusion of Cl^?, Cu⁺ Cu²⁺ and CuCl_x^1?x species: in acidic chloride media (0,1 Ml^?1 ?HCl ? 3 Ml^?1) the diffusion of Cl^? does not exist because the amount of ions consumed is negligible compared to the large quantities contained in the solution, these HCl solutions behave like supporting electrolyte. Cu⁺ and Cu²⁺ does not exist in the diffusion layer on account of the high Cl^? concentration. At all events the diffusion of CuCl_x^1?x complex is the rate determining step. Experimental current is a pure diffusion current because the transference number tCuCl_x^1?x is negligible compared to one and the charge transfer step is very fast. The electrode surface seems to be uniformly reactive although for the Cu/CuCl/CuCl_x^1?x system we can assume there may be a partially blocked electrode surface constituted by independent sites.     

Synthesis,crystal structure and second harmonic generation of (DMAPM+)[Cd(DMAP)I3]−

A new inorganic–organic hybrid, (DMAPM⁺)[Cd(DMAP)I₃]⁻, (DMAPM⁺I⁻=4-N,N-dimethylamino-1-methyl-pyridinium iodide, DMAP = 4-N,N-dimethylamino- pyridine) 1, has been prepared, showing a moderate second harmonic generation (SHG) of about three times as strong as that of potassium dihydrogen phosphate (KDP).     

Acceleration of the redox kinetics of VO2+/VO2 + and V3+/V2+ couples on carbon paper

The redox kinetics of VO²⁺/VO₂ ⁺ and V³⁺/V²⁺ couples on a carbon paper (CP, HCP030 N, Shanghai Hesen, Ltd., China) electrode were investigated in terms of their standard rate constant (k ₀) and reaction mechanism. The values determined for k ₀ for VO²⁺ ?? VO₂ ⁺ and V³⁺ ?? V²⁺ using the CP electrode are 1.0 × 10^?3 and 1.1 × 10^?3 cm s^?1, respectively. The value of k ₀ increases by one or two order(s) of magnitude compared with values obtained using electrodes composed of pyrolytic graphite and glassy carbon. The acceleration of the redox kinetics of vanadium ions is a result of the large surface area of the CP electrode. An inner-sphere mechanism for the reaction on the surface of the electrode is proposed. The kinetic features of vanadium redox reactions on the CP electrode reveal that CP is suitable for use as the electrodes in vanadium redox-flow batteries.     

The kinetics of silver iodide film formation on the silver anode

The potentiodynamic formation of AgI films by reaction (1) on an Ag rotating disc electrode was studied and the results were compared to previous work regarding the formation of anodic AgBr and Ag₂S films,

In dilute iodide solutions, a very porous AgI film is formed at a rate controlled by the diffusion of I^? to the electrode surface. At higher iodide concentrations, and particularly in high conductivity supporting electrolytes, a porous AgI film forms at a rate initially limited by ionic migration in solution followed by diffusion of I^? in the pores of the AgI film.In 1 M NaI solutions, when AgI becomes rather soluble, thick granular AgI films form at a rate limited by the solid-state migration of ions in the bulk of the film. On the basis of the low-field model of film growth, the ionic conductivity of these AgI films has been determined to be 8.7 × 10^?5 Scm^?1     

A chromotropic acid modified SBA-15 as a highly sensitive fluorescent probe for determination of Fe<Superscript>3+</Superscript> and I<Superscript>−</Superscript> ions in water

A novel organic–inorganic hybrid nanomaterial (SBA-15-CA) was prepared by covalent immobilization of chromotropic acid onto the surface of mesoporous silica material SBA-15. Different techniques such as XRD, TEM, FT-IR, N₂ adsorption–desorption and TGA analyses were employed to characterize the grafting process. The data showed that the organic moiety (0.41 mmol g^?1) was successfully grafted to the SBA-15 and the primary hexagonally ordered mesoporous structure of SBA-15 was preserved after the grafting procedure. SBA-15-CA has been realized as a highly sensitive and selective fluorescent probe towards Fe³⁺ and I^? ions in aqueous media. SBA-15-CA exhibited a remarkable fluorescent quenching in the presence of Fe³⁺ ion over other competitive cations including Na⁺, Mg²⁺, Al³⁺, K⁺, Ca²⁺, Cr³⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Pb²⁺ as well as I^? ion among a series of anions including F^?, Cl^?, Br^?, CO₃^2?, HCO₃^?, CN^?, NO₃^?, NO₂^?, SCN^?, SO₄^2?, H₂PO₄^?, HPO₄^2?, and CH₃COO^?. A good linear response was observed between the concentration of the quenchers (Fe³⁺ and I^? ions) and fluorescence intensity of SBA-15-CA with detection limits of 1.5?×?10^?7 M for Fe³⁺ and 0.2?×?10^?7 M for I^?. Moreover, the effects of various pH values on the sensing ability of SBA-15-CA were investigated. Finally, the proposed method was successfully utilized for the determination of Fe³⁺ and I^? ions in river water, well water and tap water samples.