Combined photoelectrochemical conditioning and photoelectron spectroscopy analysis of InP photocathodes. I. The modification procedure

Photoelectrochemical modification procedures at p-InP (111)A-surfaces for improved photocurrent-voltage behaviour in acidic vanadium^2+/3+ redox electrolyte are presented. Preconditioning in H₂SO₄ and HCl results in marked differences of the I-V characteristics in the redox—as well as in the supporting electrolytes: in H₂SO₄, the photocathodic reaction of hydrogen evolution is accompanied by photocorrosion leading to metallic indium at the surface. Cyclic polarisation in HCl, however, results in an improved photovoltage and fill factor. A new two-step conditioning procedure is presented yielding a solar-to-electrical conversion efficiency of 11.6% in a two-electrode non-optimised configuration. Electrochemical analyses of the dark current-voltage curves and Mott-Schottky plots show shifts of the flatband potential due to the surface conditioning. In HCl-cycled electrodes, an increased barrier height Φ_bh, is observed whereas upon cyclic polarisation in H₂SO₄, Φ_bh decreases. In HCl, the formation of a passivating interfacial film of thickness of 5-7 Å is assumed, the thickness estimate depending on the assumed corrosion reaction. The film allows efficient charge transfer and also improves the anodic protection of the p-InP electrodes in forward direction. The chloride ion appears to strongly influence the surface chemistry with respect to film formation.     

Electrochemical and photoelectrochemical behaviour of passivated niobium electrodes during hydrogen evolution

The electrochemical and photoelectrochemical behavior of niobium electrodes passivated in 0.5 M H₂SO₄ and 1 M HNO₃ has been investigated. High intensity pulse lasers were used as light sources. This technique allows photoelectrochemical measurements with light wavelengths smaller than the band gap of the semiconducting passive film. The donor concentration and the flat band potential of the passive films were calculated from capacity measurements. The effect of cathodic hydrogen evolution on the behaviour of the oxide film formed was found to depend on the time of the cathodic treatment of the electrode. The results showed that the behaviour of the passive film formed on niobium in nitric acid is different from that formed in sulphuric acid. The calculated donor concentrations and the extrapolated flat band potentials indicate that the nature of the passive film depends on the formation medium. The adsorption of hydrogen on the passivated Nb-electrode up to a time limit of 1 ms could be traced using photocharge measurements with excitation energies less than the band gap energy of the semiconducing oxide film.     

Kinetics of the surface reaction Had ↔ OHad on n-Ge(1 1 1)

Different surface states (GeOH and GeH) were found on n-Germanium. These two surface states can be controlled by potential (GeH at U<−0.2 V, GeOH at U>0.1 V in 0.5 M H₂SO₄). The surface reaction H_ad↔OH_ad is reversible and can be described by:

(1)     

Fabrication and centeracterization of ordered CuIn(1?x)GaxSe2 nanopore films via template-based electrodeposition

Ordered CuIn_(1−x)Ga_xSe₂ (CIGS) nanopore films were prepared by one-step electrodeposition based on porous anodized aluminum oxide templates. The as-grown film shows a highly ordered morphology that reproduces the surface pattern of the substrate. Raman spectroscopy and X-ray diffraction pattern show that CIGS nanopore films had ideal chalcopyrite crystallization. Energy dispersive spectroscopy reveals the Cu-Se phases firstly formed in initial stage of growth. Then, indium and gallium were incorporated in the nanopore films in succession. Cu-Se phase is most likely to act as a growth promoter in the growth progress of CIGS nanopore films. Due to the high surface area and porous structure, this kind of CIGS films could have potential application in light-trapping CIGS solar cells and photoelectrochemical water splitting.     

Corrosion inhibition of iron in 1 M HCl by some gemini surfactants in the series of alkanediyl-α,ω-bis-(dimethyl tetradecyl ammonium bromide)

Three new gemini surfactants in the series of alkanediyl-α,ω-bis-(dimethylalkyl ammonium bromide) were synthesised and tested as corrosion inhibitors of iron in hydrochloric acid medium using gravimetric, electrochemical polarisation and electrochemical impedance spectroscopy (EIS) measurements. Results obtained show that the surfactants studied are good cathodic inhibitors and act on the cathodic hydrogen reaction without modifying its mechanism. EIS results show that the changes in the impedance parameters (R_T and C_dl) with concentration of surfactants studied is indicative of the adsorption of molecules of surfactant leading to the formation of a protective layer on the surface of iron. The effect of the temperature on the iron corrosion in both 1 M HCl and 1 M HCl with addition of various concentrations of 1,2-ethane bis-(dimethyl tetradecyl ammonium bromide) in the range of temperature 20–60 °C was studied. The associated apparent activation corrosion energy has been determined.     

Single frequency electrochemical impedance investigation of zero charge potential for different surface states of Cu–Ni alloys

Adsorption of sulphate anions onto polycrystalline pure copper, pure nickel and two copper–nickel alloys was explored by performing zero charge potential E _PZC measurements. E _PZC measurements were performed by differential capacitance analysis using single?frequency electrochemical impedance spectroscopy. E _PZC and surface charge of the studied materials were analysed for different surface states: bare—immediately after cathodic polarisation and after 5 h of immersion in a 0.1 M Na₂SO₄ solution. According to the surface state and nature of the studied materials, the choice of the appropriate measuring frequency was investigated. Specific methodologies dedicated to the study of electrode/electrolyte interfaces are then presented for bare metal, metals covered by corrosion products and metals covered by passive film. In the presence of passive film, the space charge capacitance interferes with the differential capacitance measurements, and its influence is discussed in the framework of semiconductor electrochemistry.     

Aerosol assisted chemical vapour deposited (AACVD) of TiO2 thin film as compact layer for dye-sensitised solar cell

Compact TiO₂ has been introduced onto the surface of an indium tin oxide glass slide (ITO), using an aerosol-assisted chemical vapour deposition method. This serves as a blocking layer for a dye-sensitised solar cell (DSSC). The thickness of the compact TiO₂ could be controlled by deposition time. X-ray diffraction and Raman spectroscopy analyses reveal that the compact TiO₂ is made up of mixed anatase and rutile phases. The field emission scanning electron microscopy image displays a pyramidal morphology of the compact TiO₂. A layer of P25 paste was then smeared onto the compact TiO₂-modified ITO, using the doctor's blade method. A post-treatment procedure was applied to remove the contaminants from the prepared hybrid film, by immersing in a hydrochloric acid solution. The photoelectrochemical measurements and J–V characterisation of the hybrid film show an approximately fourfold increase in photocurrent density generation (114.22 µA/cm²), and approximately 25% enhancement of DSSC conversion efficiency (4.63%), compared to the acid-treated P25 paste alone (3.68%).     

The use of 2,6-bis (N-pyrazolyl) pyridine as an efficient dopant in conjugation with poly(ethylene oxide) for nanocrystalline dye-sensitized solar cells

Poly(ethylene oxide) (PEO)/2,6-bis (N-pyrazolyl) pyridine (BNPP) polymer electrolyte based photoelectrochemical cells have been fabricated with [cis-dithiocyanato-N, N-bis (2,2′ bipyridyl-4, 4′ dicarboxylic acid)ruthenium(II)] dihydrate (N3 dye) dye complex as the sensitizer and nanoporous TiO₂ film as photo anode. The introduction of 2,6-bis (N-pyrazolyl) pyridine into the poly (ethylene oxide) matrix reduces the crystallinity of the polymer and enhances the mobility of I⁻/I₃⁻ redox couple resulting in an improved performance with a higher conversion efficiency of 8.8% in terms of light energy to electric energy when compared to that of the corresponding dye-sensitized nanocrystalline TiO₂ solar cell.     

Electrochemical hydrogen reactions on tungsten carbide

The kinetics of hydrogen ionisation and electrochemical evolution on a smooth rotating tungsten carbide electrode have been investigated. Polarisation curves in 1 N H₂SO₄, H₃PO₄ and HCl solutions, in the temperature range of 20?80°C, have been obtained. For hydrogen ionisation at ?_r=0·3 V the apparent activation energies (Kcal/moles) were 8·1 in H₂SO₄, 7·4 in H₃PO₄ and 12·8 in HCl. At potentials of ?_r>0·4V a decrease in the hydrogen ionisation rate was observed, explained as a decrease in the number of active surface sites due to surface oxidation and adsorption of anions. However, after a moderate anodic polarisation not leading to the formation of a thick tungsten carbide oxides layer, reiterated recording of the polarisation curve yields an increase in electrode activity.In all acids the first order of reaction with respect to molecular hydrogen was found. The hydrogen ionisation rate was independent of the pH-value of the solution. These results showed adsorption, accompanied by molecular hydrogen dissociation, to be the rate-determining step.Modifying the concentration of sulphuric acid in the presence of excess normal salt or without it, as well as passing from acidic to alkaline solutions caused no change in the course of the polarisation curves. This suggests that recombination of adsorbed hydrogen atoms into molecules on tungsten carbide is the rate determinating step, ie, the same step as in the reverse process of hydrogen ionisation.     

Corrosion inhibition of stainless steel by polyaniline,poly(2-chloroaniline), and poly(aniline-co-2-chloroaniline) in HCl

Polyaniline (PANi), poly(2-chloroaniline) (PClANi), and poly(aniline-co-2-chloroaniline) (co-PClANi) films were synthesized by electrochemical deposition on 304-stainless steel (SS) from an acetonitrile solution. The structural properties of these polymer films were characterized by spectroscopic (FTIR and UV–vis) and electrochemical (cyclic voltammetry) methods. Open circuit potential–time (E_ocp–time) curves, potentiodynamic polarization, and electrochemical impedance (EIS) measurements showed that these films have significant protective performance against corrosion of SS in 0.5 M HCl solution. It was found that co-PClANi film has acted as a passivator as well as barrier for cathodic reduction reaction in a similar manner as PANi film. However, PClANi film has behaved only as barrier for corrosion protection of SS in 0.5 M HCl.     

Hydrogen evolution on RuxTi1−xO2 in 0.5 M H2SO4

Hydrogen evolution from 0.5 M H₂SO₄ on Ti electrodes coated with a Ru_xTi_1−xO₂ (x=0.04-0.5) layer has been studied by potentiostatic polarisation, cyclic voltammetry and ac-impedance spectroscopy. The results indicate that after a certain activation period the performance of such an electrode coating is comparable to platinum. The addition of small amounts of sodium molybdate increased the capacitance of the electrode and a reduction of the performance was observed. Increasing the temperature of the pure electrolyte from 20 to 75 °C caused an increase in the rate of the hydrogen evolution and in addition an increase of the electrode capacitance. The electrodes have been found to be rather tolerant to chloride and Fe²⁺ ions, and could hence be promising candidates as catalyst materials for solid polymer water electrolysis systems. From steady state measurements the Tafel slopes were found to change from −105 mV/decade for pure titanium to about −40 mV/decade for the (RuTi)O₂ coated electrodes. The exchange current densities were calculated from the steady state curves, as well as from impedance data, to be about 10⁻⁴ A cm⁻² after activation.     

Characterization of electrodeposited CuInSe2 (CIS) film

CuInSe₂ thin film was grown by one-step cathodic electrodeposition on Pt-coated glass using amperometry mode (fixed potential), in a three-electrode potentiostatic system containing the precursor solutions, 10 mM CuSO₄, 50 mM InSO₄, 30 mM SeO₂, and 0.1 M K₂SO₄ as a supporting electrolyte, at a pH of 1.5 (±0.1) adjusted with 0.1 M H₂SO₄. The structure, chemical composition, morphology, optical properties, and uniformity of the electrodeposited CuInSe₂ film were characterized by X-ray diffraction (XRD), electron probe micro analysis (EPMA), UV-vis-NIR spectrophotometry, field-emission scanning electron microscopy (FE-SEM), and Auger electron spectroscopy (AES), respectively. Several experiments were conducted in which the deposition voltage and post-annealing conditions were varied. As the applied deposition voltage was increased from −0.6 V to −0.7 V versus Ag/AgCl, a CuInSe₂ thin film with a chalcopyrite structure came to be predominantly formed, in accordance with the chemical composition, while the formation of the binary compounds (CuO and Cu_xSe) which influence the degradation of the performance in the application of CIS-based solar cells, rapidly decreased. The optimum conditions consisted of an annealing temperature of about 350 °C for 30 min under nitrogen ambient. At this temperature, a CuInSe₂ thin film, in the form of ternary compound, is formed with the required conditions, namely good crystallinity, good stoichiometry, a suitable bandgap, and depth uniformity.     

The photoelectrochemical behaviour of polycrystalline AgIn5Se8

Polycrystalline samples of n-type AgIn₅Se₈ were prepared from the constituent element and employed as electrodes in photoelectrochemical cells. These semiconductor electrodes were characterized by spectral response, Mott—Schottky plots, current—voltage measurements and complex impedance analyses. The polycrystalline n-AgIn₅Se₈ electrodes show a very encouraging photoelectrochemical behaviour, since capable of offering solar to electrical conversion efficiencies of the order of 3%. However, stability tests run both under illumination and in the dark, revealed that this interesting photoelectrochemical performance worsened upon time of illumination. This was interpreted in terms of photocorrosion which leads to the formation of a selenium film on the surface of the semiconductor electrode.     

Electrochemically grown passive films on carbon steel (SAE 1018) in alkaline sour medium

Corrosion films were prepared by applying cyclic potential pulses to the 1018 carbon steel-sour medium interface (1 M (NH₄)₂S, 500 ppm CN⁻) for 1 min. Electrochemical behavior and surface morphology of these films were determined using electrochemical impedance spectroscopy (EIS), scanning electron microscopy, and scanning photoelectrochemical microscopy (SPECM). EIS diagrams and SPECM images show the passive properties and homogeneity of the films. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to characterize their chemical nature and structure. XPS results show that different oxide and sulfur structures were developed during the electrochemical oxidation of carbon steel in concentrated sour media. The analysis of O 1s data indicated that, during film growth, H₂O and/or hydroxyl groups are incorporated into the film structure. The XPS spectra of Fe 2p show iron bonds with S as iron sulfide (FeS₂ and FeS) and the corresponding peak of O 1s shows those bonds with oxygen as Fe₂O₃ and/or FeO. XPS depth profile analyses for the film showed that the ratio of FeS and FeO increases from film surface to film-carbon steel interface. This corroborates the diffusion of iron ions through the film during its electrochemical growth. The chemical shift through the film for the peak associated with Fe 2p signal proves that transport mechanism of iron ions through the film is carried out by chemical diffusion.     

Cathodic Film formation during Chromium Electrolysis on Low-carbon Steel using short duration current steps

Short duration galvanostatic electrolyses of chromium plating solutions (1.0 M CrO₃ and 0.01 M H₂SO₄) were performed on rotating disk electrode of low-carbon steel. The influence of sulphate concentration (0.01 and 0.02 M), cathodic current density (30, 60 and 90 A dm^–2), temperature (25, 40 and 55 °C), electrode rotation rate (25 to 1000 rpm) and Na₂SiF₆ addition (0.01 M) on E/t curves and cathodic current efficiency was investigated. Electrical charge was always consumed to form a cathodic film and effective metallic chromium deposition only occurred when the potential reached a low and stable value. The determination of cathodic current efficiency after short duration electrolyses corroborated these results. The time required to form the cathodic film decreased with current density and addition of Na₂SiF₆ and increased with sulphate concentration, temperature and electrode rotation rate, suggesting that this film is not completely compact. A passive film is formed at the onset of the electrolysis process and a viscous layer is formed on the surface of the first film.     

Platinum-free photoelectrochemical water splitting

Hydrogen can be produced by photoelectrochemical water splitting in the presence of S^2 −/SO₃ ²⁻ sacrificial agent by using a Cu₂S electrocatalyst grown on brass by chemical treatment in HCl and polysulfide solution. Photoelectrochemical behavior of Cu₂S/brass was equivalent to that of Pt nanoparticles mixed with carbon black and deposited on a carbon cloth electrode, i.e. the electrocatalyst typically used in fuel cells and electrolyzers. Thus photoelectrochemical water splitting can be achieved in a Pt-free device. The photoanode was made of nanocrystalline titania on FTO sensitized by quantum dots. Combined ZnS/CdSe/CdS quantum dots were by far more effective for hydrogen production than single-component CdS sensitizer.     

Thiophene derivatives as effective inhibitors for the corrosion of steel in 0.5 <Emphasis Type="SmallCaps">m</Emphasis> H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

The influence of thiophene and five of its substituted derivatives on the corrosion inhibition of steel in 0.5 m H₂SO₄ solution was studied using weight-loss, electrochemical polarisation, and impedance measurements. 5-tert-butoxythiophene-2-carbaldehyde phenylhydrazone (TBCP) is the best inhibitor and its inhibition efficiency increases with increase in concentration to attain 87% at 5×10⁻³ m. Potentiodynamic polarisation studies clearly reveal that it acts essentially as a cathodic inhibitor. The partial π-charge on atoms has been calculated. A correlation between the highest occupied molecular orbital E _HOMO and inhibition efficiencies was sought. The inhibition efficiency of TBCP is not affected by rise in temperature in the range 298–353 K. E% values obtained from weight-loss and electrochemical methods were in good agreement. Adsorption of TBCP on steel has an S-shaped adsorption isotherm.     

Disbondment Mechanisms for a Heat Shrink Polyethylene Coating on a Cathodically Protected Hot Pipeline

Interfacial adhesive failure of a pipeline coating was found to be related to the operating temperature of the pipeline, the presence of moisture at the coating/steel interface and cathodic polarisation of the steel. The application of cathodic protection was found to be more detrimental to a pipeline coating than was the immersion of the coating specimens in alkaline environments without polarisation. It is suggested that in the system examined cathodic disbondment is initiated at a coating holiday by the electrochemical reduction of Fe₃O₄ in the interfacial oxide film and that propagation of the disbondment is associated with electrocapillary action which reduces the surface tension between the steel and the crevice solution. This process increases the thermodynamic disbonding force between the adhesive and the steel in the aqueous environment.     

Flüssig/Flüssig‐Extraktion von Indium aus sauren Lösungen

The extraction of indium from a synthetic sulfate‐containing solution using commercial reagents (Cyanex 272, DEHPA, and Cyanex 923) is evaluated on a comparative basis. The extraction profiles of indium (III) were examined with regard to the reagent concentration, the pH value of the aqueous solution, and the indium concentration in a low phase ratio of 1:10. DEHPA and Cyanex 272 are, in contrast to Cyanex 923, very well suited for the extraction of indium. Re‐extraction with HCl and H₂SO₄ is compared.     

Studies of anodic dissolution and film growth on iron in acid chloride solutions

The rate of dissolution of Armco Fe has been measured as a function of time for deaerated solutions of constant ionic strength (HCl + NaCl).For pH < 1·5, no appreciable change of the measured polarisation resistance was observed. For 1·5 < pH < 3 the polarisation resistance gave straight lines when plotted against time. The slope of the lines is represented by a = 2·0√(C_OH^?.10¹⁴ ? 45) Ωcm²/min, where C_OH^? is the concentration of the bulk.The Tafel slopes were 64–70 mV when 0 < pH < 1·2.The experimental data are explained by two very similar reaction mechanisms. The theory presented involves the formation of two surface complexes denoted by FeOH Fe·H₂O and FeO Fe·H₂O. It is suggested that the complex FeOH Fe·H₂O serves as nuclei for film formation when pH > 1·5. This would explain the change of the reaction mechanism at pH = 1·5 from a 60 mV mechanism when pH < 1·5 to a 30 mV mechanism when pH > 1·5.The theory is supported by data from the literature. It gives a reasonable explanation of the nonstationary 60 mV Tafel slopes. The mechanisms are in harmony with a hyperbolic relation between anodic current density and time. The logarithmic law of film growth may be derived from the same principles.