Photoelectrochemistry of Langmuir—Blodgett films of a C60 iminodiacetic acid ester derivative on ITO electrodes

The photoelectrochemical response of a C₆₀ iminodiacetic acid ester derivative (C₆₀IDA), deposited on ITO electrodes by means of the Langmuir—Blodgett technique, has been investigated. The anodic photocurrent observed on the modified electrode corresponds to an electron transfer from the electrolyte through the LB film to the electrode. The action spectrum of photocurrent indicates C₆₀IDA as the photoactive species in the photoinduced electron transfer process. Positive bias voltage, reducing agent and higher pH of the solution are beneficial factors for generating higher photocurrent. The quantum yield for photocurrent generation is 0.94% and can be raised to 3.40% under favorable conditions.     

Spectral distribution of photocurrent in poly(6-tert-butyl-3-methyl-3,4-dihydro-2H-1,3-benzoxazine)

The spectral distribution of photoconductivity in poly(6-tert-butyl-3-methyl-3,4-dihydro-2H-1,3-benzoxazine) was studied in the sandwich configuration with indium tin oxide and silver electrodes. The photocurrent was found to originate mainly from the bulk dissociation of excitons with lesser contribution from electrode-mediated exciton dissociation. The molecule was fluorescent, but in the presence of the electron acceptor molecule 2,4,6-trinitrophenol (TNP), the intensity was reduced. The reduction in fluorescence intensity corresponded to an increase in photocurrent. The molecule showed photoconductivity in the visible without any sensitizers, which makes it suitable for stable photorefractive composites.     

Photocarriers generation process and photovoltaic effect in PPHT thin film Schottky barrier devices

Photogeneration process and photovoltaic effect of Al/poly(3-phenylhydrazone thiophene) (PPHT)/ITO and In/PPHT/ITO sandwich devices were investigated by measuring steady state photocurrent resulting from illuminating through the ITO electrode. From the comparison of photoaction spectra of the device with the absorption spectra of the PPHT layer, it was observed that PPHT forms Schottky barrier with In and Al and ohmic contact with ITO. The voltage dependence of the photocurrent in the vicinity of V_bi was measured at 1 mW/cm² of the incident illumination to give the open circuit voltage (V_oc) and short circuit photocurrent (J_sc). The photoaction spectra of the device also suggest that only light absorbed near the blocking contact, i.e., at Al/PPHT, In/PPHT is effective in producing carriers for external circuit. The dependence of the short circuit photocurrent on the illumination intensity was also described in detail. Various photovoltaic parameter was also calculated from the current–voltage (J–V) characteristics of the open device under illumination through ITO.     

Electrocopolymerization of poly(o-toludine-co-metanilic acid) on mild steel surfaces and their physico-electrochemical characterizations

For the first time, poly(o-toludine-co-metanilic acid) (POM) layers have been deposited on mild steel (MS) substrate by galvanostatic (GS), potentiostatic (PS) and potentiodynamic (PD) methods. AC impedance of the electrodes has been analyzed. The shape and impedance parameters of galvanostatically prepared electrodes are similar to those of potentiostatically grown electrodes. The impedance spectra of potentiodynamically prepared electrode, however, are different. The redox behavior of POM copolymer has been studied using cyclicvoltammetric technique. Also, the copolymer prepared by potentiodynamic method has been characterized by UV–vis spectroscopy, Fourier Transform Infrared spectroscopy (FTIR) and Thermogravimetric analysis. It has been observed that polymerization of POM are greatly influenced by the method of preparation.     

Polymer-electrolyte-based photoelectrochemical solar energy conversion with poly(3-methylthiophene) photoactive electrode

A photoelectrochemical cell based on neutral poly(3-methylthiophene) electrochemically coated on indium—tin oxide as a photoactive electrode, an amorphous poly(ethylene oxide) complexed with an I₃⁻/I⁻ redox couple as a solid polymer electrolyte, and thin transparent platinum film vapour deposited on indium—tin oxide as a counter electrode has been constructed. At catholic potentials a cathodic photocurrent was obtained indicating that the neutral poly(3-methylthiophene) is a p-type semiconductor. The short-circuit current and open-circuit voltage obtained with white light illumination at 100 mW cm⁻² are 0.35 μA cm⁻² and 140 mV, respectively. The monochromatic photon to current conversion efficiency obtained under illumination through back and front side is 0.3 and 0.6%, respectively. Studies of the photocurrent action spectra revealed that the poly(3-methylthiophene)/solid polymer electrolyte junction is responsible for the photocurrent generation. The intensity and time dependence of the short-circuit current and open-circuit voltage have been studied.     

Photocurrent spectral responses in poly (2,5-dimethoxy-p-phenylenevinylene) devices

We report the spectral response of the photocurrent in the sandwich configuration of poly (2,5-dimethoxy-p-phenylenevinylene) (DMPPV) devices. The photocurrent spectral responses are dependent on the polarity of the illuminated surface and the electrodes. The polarity-dependent photocurrent is modeled on a basis where charge carrier drift plays a dominant mechanism and hole transport is predominant over electron transport. The model can explain the results for the Au/DMPPV/Au symmetric configuration. The model cannot however explain the results for the indium-tin oxide (ITO)/DMPPV/Al device. The response for the light incident on the Al side is also reported. The spectral dependence is also studied as a function of bias electric field. Mechanisms leading to the behavior of the photocurrent spectral dependence are speculated.     

Experimental and theoretical studies of absorption and photoluminescence excitation spectra of poly(p-phenylene vinylene)

In this paper, we present the experimental absorption spectra and photoluminescence excitation spectra of poly(p-phenylene vinylene) (PPV). We also present the theoretically calculated absorption and photoluminescence spectra of PPV. The singlet-to-singlet transition energies and moments calculated from the intermediate neglect of differential overlap with the spectroscopic parametrization (INDO/S) semi-empirical method were used to analyze the absorption spectra. The symmetrical vibrational modes calculated from the abinitio method were used to obtain the vibronic features in the spectra of PPV. It is demonstrated that the inhomogeneity observed in the absorption spectra of PPV can be modeled by summing up the absorption of PPV oligomers with different chain lengths. The difference between absorption and photoluminescence excitation spectra is explained.     

Studies of the photocurrent generation performances from a series of amphiphilic bis-chromophore zinc complexes and correlation between photocurrent generation performance and molecular structure

A systematic study of the photocurrent generation (PCG) performances from seven analogues of bis-hemicyanine dye chromophore cations ionically combined with a bivalent zinc anion, bis-(2-thione-1,3-dithiol-4,5-dimerapto) zinc, has been made under the same conditions in order to discuss the influence of molecular structure on PCG performance. The coincidence of the action spectra of the PCG with the absorption spectra of the monolayer LB films on indium–tin oxide (ITO) electrodes indicate that the hemicyanine analogue chromophore cations in the bis-chromophore complex are the photoelectric active moieties which is responsible for the PCG for each analogue. The results also demonstrate that these bis-chromophore zinc complexes have larger quantum yield than their corresponding iodide salts. The correlation between PCG capability and the molecular structure of hemicyanine dye chromophore cation has been established. This correlation is valuable in leading to systematic improvement in performance and new molecule design with high quantum efficiency.     

Influence of the counter ion on the properties of poly(o-toluidine) thin films and their Schottky diodes

The influence of various counter ions on the electrochemical, morphological and optical properties of poly(o-toluidine) (POT) thin films and on the electrical properties of POT-based diodes was investigated. The POT thin films were deposited on indium tin oxide (ITO)-coated glass substrates by electrochemical polymerization under cyclic voltammetric conditions from o-toluidine monomer in aqueous solutions of HCl, H₂SO₄ and HNO₃. The deposited films were characterized by cyclic voltammetry, scanning electron microscopy and UV–vis spectrophotometry. It was observed that the current densities and optical absorption spectra are influenced by the counter ions of the electrolyte solutions.On the other hand, ITO/POT/Al devices were fabricated by thermal evaporation of aluminum circular electrodes on the as-deposited POT films. The current–voltage (C–V) characteristics of these devices are non linear. The diode parameters were calculated from current–voltage (I–V) characteristics using the modified Shockley equation. The C–V and capacitance–frequency (C–F) characteristics were also measured. These measurements revealed that the junction electrical parameters depend strongly on the type of supporting electrolytes used in the synthesis of the polymer.     

Internal photoemission in polyaniline revealed by photoelectrochemistry

After self-assembling a p-aminothiophenol (PATP) monolayer on bare Au electrode, polyaniline (PANI) films were prepared by electropolymerization on the modified Au electrode and three forms of PANI (the emeraldine salt, the leucoemeraldine and the pernigraniline) were obtained by electrochemical method. The cyclic voltammograms of the probe (K₃Fe(CN)₆/K₄Fe(CN)₆) for the three forms of PANI show that three different oxidized forms can be found and all of them are stable under our experiment conditions. The potential dependence of magnitude and the spectra of the photocurrent of the PANI films were observed. When the spectra of photocurrent are displayed in a Fowler plot (the square-root of the incident photon-to-current efficiencies versus the photon energy), the functional form appears linear, which is interpreted as indication of internal photoemission of holes or electrons from metal into the appropriate electronic band of the semiconductor or insulator. The band gap energy of insulating matrix in the emeraldine salt is determined as 3.33 eV by the Fowler plots. The flat-band potential, in the order of 0.63 V versus SCE, is obtained from Mott–Schottky plots. A photoelectrochemical process based on internal photoemission in the emeraldine salt, which agrees with the model of granular metal island that assumes metallic polymer particles are embedded in the insulating matrix, is proposed.     

锡电极嵌锂过程的交流阻抗谱

采用交流阻抗谱研究锡电极的首次嵌锂和第二次嵌锂过程,对比锡纳米阵列电极、锡薄膜电极和锡箔3种不同材料微观结构对电极交流阻抗谱特征的影响。用等效电路模型分析交流阻抗谱,得到嵌锂过程电化学特征参数与电位关系。结果表明:锡纳米阵列电极与锡薄膜、锡箔电极具有不同交流阻抗谱特征,锡纳米阵列电极在中频区出现双电层阻抗;首次嵌锂时在1.6~0.8 V之间形成固体电解质膜(SEI);电极材料微观结构显著影响锡电极的SEI膜电阻、Warburg阻抗和锂离子扩散速率;锡纳米阵列电极上的SEI膜电阻和Warburg阻抗最小,锂离子扩散能力最强;锡纳米阵列电极上锂的扩散系数为4.4×10-15~1.4×10-11 cm2/s;锂扩散系数随电位变化显著。     

Photoconductivity in Langmuir—Blodgett multilayer structures of phthalocyaninato-polysiloxane

The electrical and photoelectrical properties of planar photodiodes based on phthalocyaninato-polysiloxane (PcPS) are presented. Sandwich structures consisting of thin PcPS Langmuir—Blodgett films have been prepared between semi-transparent gold and aluminum electrodes. These structures possess pronounced rectifying behavior. The photocurrent spectra exhibit peaks at 550 and 700 nm. The wavelength dependence of the photoconductivity clearly differs from the linear optical properties.     

Photoelectrochemical behaviour of electrogenerated poly(2,3-diaminophenazine) on platinum in aqueous media

The photoelectrochemical behaviour of poly(2,3-diaminophenazine) (PDAPh) on a Pt substrate has been investigated in aqueous media. A p-type photoresponse was observed from both photocurrent and photovoltage measurements, indicating formation of a depletion layer between the polymer and the electrolyte. Analysis of the optical absorption spectrum of the film showed a direct transition, with an optical band-gap energy equal to 2.1 eV. Capacitance measurements gave a flat-band potential equal to 0.54 V versus a saturated mercury sulfate electrode.     

Polyacetylene film: A new electrode material for photoenergy conversion

Polyacetylene films were characterized as an electrode material in aqueous solutions. The trans polyacetylene films behaved as electrodes under reducing conditions. Oxygen gas evolution was greatly retarded under oxidizing conditions, primarily due to the high reactivity with oxygen. Visible light illumination resulted in a positive shift in electrode potential. The electrode developed photopotential reversibly in accord with illumination. The polyacetylene electrode gave cathodic photocurrent when contacted with a deoxygenated acetate buffer. The cathodic photocurrent increased with decreasing pH, ranging from 0 to 5.5 × 10^?6 A/cm². The possible use of polyacetylene electrodes for photoenergy conversion is discussed.     

Photoelectrochemical investigation of passive layers formed on Fe in different electrolytic solutions

The passive films formed on Fe in different electrolytic solutions, spanning almost the overall pH range, and with different growth procedures were investigated systematically by photocurrent spectroscopy. The potentiodynamic growth curves are compared both in aerated and de-aerated electrolytes and a potentiostatic growth procedure is also employed. For high positive formation potentials, similar anodic spectra are recorded in all solutions giving an optical gap very close to that expected for crystalline Fe₂O₃. The origin of photocurrent spikes is also investigated and the effect of the formation potential upon the measured absorption threshold is discussed taking into account the ordered or disordered structure of the films and its Fe(II) species content. Films formed at quite negative potentials display different photocurrent spectra, characterized by cathodic stationary photocurrent, absence of spikes and lower absorption in the visible range, indicating the formation of Fe(II) passive layers. The same behaviour is observed with film formed at positive potentials, after a long negative biasing.     

Electron spin resonance of thin films of organic light-emitting material tris(8-hydroxyquinoline) aluminum doped by magnesium

We have successfully observed electron spin resonance (ESR) signals of radical anions in thin films of tris(8-hydroxyquinoline) aluminum (Alq₃), a compound widely used as electron transporting and luminescent layers in organic light-emitting diodes. To obtain definitely defined radical-anion states in Alq₃, we doped Alq₃ with Mg by co-evaporating these materials. The obtained g value and peak-to-peak ESR linewidth ΔH_pp of Alq₃ radical anions are 2.0030 and 2.19 mT, respectively. Theoretical g value and hyperfine interactions were calculated by density functional theory method, which are in good agreement with the experimental results. A quantitative evaluation of doping concentration was performed. We confirmed that doped charges are localized at deep trapping sites by the lineshape analysis and temperature dependence of the ESR signals. Morphological investigation using transmission electron microscopy clarified that the co-evaporated Mg atoms form clusters.     

Fully oriented cis-(CH)x: Experimental and theoretical analysis of the polarized Raman spectra

New experimental results of polarized Resonant Raman Scattering (RRS) spectra of fully oriented cis-rich (CH)_x films for the excitation wavelengths 676.4 and 600 nm are presented together with the theoretical interpretation. The three Raman active bands due to the totally symmetric vibrational modes of cis-(CH)_x are observed only in the / / / / polarized spectrum, indicating that the films are fully oriented. The contribution of the Raman active vibrational modes of the trans segments is observed in all polarized spectra and for both incident wavelengths. Also, we report the RRS spectra of the same sample thermally isomerized, without stretching, for λ_L = 676.4 nm and λ_L = 457.9 nm. We find that the depolarization ratios of the Raman bands due to the vibrational modes of trans-segments are much higher after isomerization than for an usual stretched trans (CH)_x sample.     

Electrochemical behaviour of stainless steels in media containing iron-oxidizing bacteria (IOB) by corrosion process modeling

Localized corrosion mechanism of stainless steel (SS) types UNS S30403 and UNS 31603 in the presence of iron-oxidizing bacteria Sphaerotilus spp. isolated from rust deposits was studied electrochemically. OCP transient, cyclic anodic and cathodic potentiodynamic polarization curves were measured on steel electrodes through their exposure to 3% NaCl solution supplemented with Sphaerotilus culture. The exposure period was composed of three parts: (a) 5 days incubation of steel electrodes in sterile 3% NaCl solution; (b) addition of 3 days-old Sphaerotilus culture to 3% NaCl at 3:2 v/v ratio with subsequent electrodes exposure for 11 days up to complete sedimentation of ferric oxides and (c) subsequent exposure of electrodes for 14 days in upper and bottom (sediments layer) fractions of the experimental medium. The results revealed an instantaneous gradual shift of the transient potential of both steels towards negative potentials from steady-state value of −0.15 V to −0.35 to −0.42 V (SCE) during the whole exposure interval since IOB culture addition into sterile 3% NaCl solution.No evidence of pitting corrosion was found on SS samples subsequent to their exposure to sterile 3% NaCl solution, though in the presence of IOB culture, numerous pits were revealed on 304 L steels specimens exposed to iron hydroxides sediments layer. Electrochemical characteristics (OCP or corrosion potential - E_CORR, breakdown potential - E_BD, repassivation potential - E_RP, passivation current - i_PASS) periodically measured by cyclic polarization method, allowed monitoring the electrochemical behavior changes of experimental SS and to establish the initiation of pitting corrosion in the presence of IOB, resulting in crevice effect caused by biogenic ferric oxides deposits precipitated on steel surface. Overall, steel 316L demonstrated higher resistance to pitting corrosion compared to 304L.     

Spectroscopy of conducting and insulating ladder-type poly(para-phenylene) device structures

We have applied the electroabsorption (EA) and charge-induced absorption (CIA) spectroscopic techniques to methyl substituted ladder-type poly(para-phenylene) (m-LPPP) sandwich device structures. The device structures have been either of conducting (ITO/m-LPPP/Al) or insulating (ITO/SiO/m-LPPP/Al) type. For both devices we have observed a CIA band with its 0–0 transition at 1.95 eV. From a comparison to doping induced and photoinduced absorption spectra we assign this band to polarons. In addition, in the insulating device we observed a derivative-like band with zero crossing at 1.15 eV. In the conducting device two derivative bands are observed with zero crossing at approximately 0.8 and 1.2 eV, respectively. Their intensity increases linearly with the modulus of the applied electric field and the features are independent of the modulation frequency. We tentatively assign these derivative-like bands to a Stark-effect related to charge transfer transitions at the ITO and Al electrodes. At high currents in the conducting device we also observed a relatively broad, structureless CIA band peaking at 1.3 eV, that we assign to biplarons.