Capacitance and photocurrent study of electronic properties of anodic oxide films on Nb and Ta. Evaluation of the ionized donor concentration profile in Nb2O5 film

Electronic properties of electrochemically formed oxide films on Nb were studied by photocurrent and differential capacitance measurements in 0.025 M KH₂PO₄+0.025 M Na₂HPO₄ electrolyte, pH 6.9. Oxide films of n-type conductivity were formed galvanostatically for final potentials ranging from 4 to 230 V. Measurements were performed in two potential regions, which correspond to formation of a depleted layer of variable thickness at relatively low potentials, and to complete depletion of oxide films of electronic charge carriers at higher potentials. In the first potential region the behavior of both capacitance and photocurrent, was governed by a build up of a depleted layer of potential dependent thickness. In the second, high potential, region, which extends up to the oxide film formation potential, the photocurrent and capacitance of oxide films in most features followed the trends typical of dielectric films containing defects and traps. The photocurrent and capacitance measurements on presumably dielectric oxide films formed on Ta were staged for comparison. The capacitance–potential measurements performed in the first potential region enabled us to construct the ionized donor concentration profile across the Nb₂O₅ film width. The limitations on the use of the C–E profiling method for electrochemically formed oxide layers are considered.     

Improvement of dye-sensitized solar cell performance through electrodepositing a close-packed TiO2 film

A simple electrodepositing method was proposed for fabricating a uniform, tight, and close-packed TiO₂ nanocrystalline film on the ITO substrate. The electrode and dye-sensitized solar cell (DSSC) with electrodeposited TiO₂ layer were characterized by scanning electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. The phthalocyanin dye, zinc tetra-carboxyl phthalocyanin complex, was used as a model dye to evaluate the influence of close-packed TiO₂ blocking layer on the photocurrent–voltage property. On the electrodeposition, the close-packed TiO₂ layer could effectively inhibit the recombination of charges, and therefore improve the performances of the corresponding cells. The effects of film thickness on light transmittance and photocurrent density of the corresponding cell were also demonstrated. The optimum film thickness was found to be approximately 400 nm. At the optimum thickness, the photocurrent density greatly increased comparing with that of the screen printing TiO₂ film. These results imply that our proposition was a potential and feasible method for the fabrication of DSSC practically.     

AES- und XPS-Untersuchungen zum Einflu? von Chrom, Nickel und Molybd?n auf das Korrosionsverhalten von rostfreien St?hlen in S?uren

Summary The alloys Fe17.8Cr, Fe16Cr2.4Mo and Fe18Cr14Ni2.5Mo (at%) were polarized in 0.5 mol/l H₂SO₄ or in 0.1 mol/l HC1 + 0.4 mol/l NaCl. The composition of the oxide layer and of the metallic layer beneath the oxide and the kinetics of the passive layer formation were determined by AES and XPS. In the active region, selective dissolution of Fe leads to an enrichment of Cr, Ni and Mo at the metal/electrolyte interface. In the passive region, the thickness of the rapidly formed passive layer is determined by the potential. The chromium content of the passive layer approaches a stationary, high value. The passive layer essentially consists of the anions O^2- and OH^– and of the cations of Cr, Fe, Mo, whereas Ni — and less pronounced Mo — are enriched below the layer.     

Electrochemical impedance spectroscopy study of cuprous oxide films formed on copper: effect of pH and sulfate and carbonate ions

The semiconducting properties of anodic passive films formed potentiostatically on polycrystalline copper in aqueous borax solutions were studied using electrochemical impedance spectroscopy, photocurrent spectroscopy and ellipsometric measurements. The semiconducting nature of the cuprous passive layer was analysed as a function of pH (9.2>pH>7.4), electrode potential and with the addition of 3.5 mM Na₂CO₃ and 8 mM Na₂SO₄. The different growth conditions change the compactness and the defect or excess of cations accumulated in the compact inner cuprous layer, leading to different semiconducting properties. Electronic Publication     

Effect of Titanium Dioxide Film Thickness on Photocatalytic and Bactericidal Activities Against Listeria monocytogenes

Structural, microstructural and bactericidal surface properties of TiO₂‐coated glass substrates elaborated by reactive Radiofrequency sputtering are investigated. As pathogenic bacteria in biofilms are a major concern in food industries due to their growing resistance to cleaning and sanitizing procedures, the development of photoactive surfaces exhibiting bactericidal properties is acknowledged as an effective approach to tackle bacterial contaminations. Our principal aim concerns the study of the photoactive top‐layer thickness impact (from 80 nm to ~500 nm) on Listeria monocytogenes. Structural characterization of the TiO₂ layers demonstrates that anatase and rutile phases are both present, depending on the film thickness. Photocatalytic activity of the samples has been evaluated through the degradation of aqueous methylene blue (MB) solutions under UVA light illumination for various time periods. The results show an efficiency rating increase according to TiO₂ film thickness up to a threshold value close to 400 nm. Moreover, a significant decrease of the adherent bacteria number is observed after 20 min of UVA illumination. The quantitative study of the bactericidal activity associated with scanning electron microscopy observations of the postprocess bacteria damaged cells demonstrates the efficiency of the 240‐nm‐thick TiO₂ coating sample. The results are correlated with the production of hydroxyl radicals during the process of photocatalysis.     

Evolution of an iron passive film in a borate buffer solution (pH 8.4)

The evolution under open-circuit conditions of iron passive films formed at 0.8 V_SCE in a borate buffer solution at pH 8.4 was investigated with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry. The composition of the freshly formed passive film as determined by X-ray photoelectron spectroscopy (XPS) was found to be in agreement with a bilayer model, where the inner layer is composed mainly of iron oxide and the outer layer consists of a hydrated material. Results of XPS measurements also showed that the open-circuit breakdown of passive films was consistent with a reductive dissolution mechanism. When the iron electrode reached an intermediate stage in the open-circuit potential decay (approximately −0.3 V_SCE), the oxide film, containing both Fe(II) and Fe(III), was still protective. The impedance response in this stage exhibited a mixed control by charge transfer at the metal/film and film/solution interfaces and diffusion of point defects through the film. At the final stage of the open-circuit potential decay (approximately −0.7 V_SCE), the oxide film was very thin, and the ratio of Fe³⁺/Fe²⁺ and O²⁻/OH⁻ had decreased significantly. The impedance response also exhibited a mixed charge-transfer–diffusion control, but the diffusion process was related to transport of species in the electrolyte solution resulting from dissolution of the oxide film.     

Study of ion adsorption at passive iron by using the impedance and photoadmittance methods

Intensity-modulated photocurrent (photoadmittance) and electrochemical impedance of anodicoxidized iron electrode in neutral nitrate solutions and in the presence of Ba²⁺, Ca²⁺, Cl^?, and C₆H₅COO^? (benzoate) are studied. It is shown that the ion adsorption at passive iron affects but slightly the system’s impedance; by contrast, it affects the photocurrent value significantly: when adsorbing, the anions increase the photoeffect, while the cations decrease it. These effects are associated with the potential drop redistribution in the Helmholtz layer and the film. The dissimilar changes of the generation current in the presence of similarly charged ions at their equal concentration evidence their different adsorption activity. The correlation between the generation current and surface-active ion concentration in solution is found. The photoelectrochemical spectroscopy allows evaluating qualitatively the surface-active ion adsorption at the passive iron and judging on the ion adsorption by the dependence of the generation current on the ion concentration.     

Photoelectrochemical studies on gold electrodes with surface oxide layers: Part I. Photocurrent measurement in the visible region

By illumination with visible light at wavelengths of 340–730 nm, anodic photocurrents were observed at a gold electrode in contact with a 0.2 M NaClO₄ aqueous solution (pH 6.0) in the potential range from +1.0 to +2.1 V vs. RHE. Based on the results of potentiodynamic and photocurrent measurements, the following three characteristic potential ranges have been distinguished: (1) +1.0–+1.4 V, where possibly low-coverage surface oxides (or chemisorbed OH) other than Au₂O₃ are present; (2) +1.5–+1.8V where the predominant surface oxide is Au₂O₃ up to ca. 1 nm in thickness; and (3) above +1.9 V, where a thicker surface layer, absorbing mainly in the UV wavelength range, is formed on the Au₂O₃ under-layer. The photocurrent quantum yield at the Au₂O₃ layer 0.3–0.8 nm thick was estimated to be on the order of 20% at 490 nm, assuming an absorption coefficient of 10⁵ cm^?1.     

Photoelectrochemical investigation of ultrathin film iron oxide solar cells prepared by atomic layer deposition

Atomic layer deposition was used to grow conformal thin films of hematite with controlled thickness on transparent conductive oxide substrates. The hematite films were incorporated as photoelectrodes in regenerative photoelectrochemical cells employing an aqueous [Fe(CN)(6)](3-/4-) electrolyte. Steady state current density versus applied potential measurements under monochromatic and simulated solar illumination were used to probe the photoelectrochemical properties of the hematite electrodes as a function of film thickness. Combining the photoelectrochemical results with careful optical measurements allowed us to determine an optimal thickness for a hematite electrode of ～20 nm. Mott-Schottky analysis of differential capacitance measurements indicated a depletion region of ～17 nm. Thus, only charge carriers generated in the depletion region were found to contribute to the photocurrent.     

Extraction and Purification of Purple Membrane for Photochromic Thin Film Development: Application in Photoelectrochemical Investigation

Purple membrane (PM) has been extracted and purified from archaebacteria for thin film development. The purified purple membrane is isolated in 1?% polyvinyl alcohol solution for making thin film within gelatin and organically modified silicate matrices. For thin film within gelatin matrix, homogenized purple membrane suspension is mixed with 8?% gelatin and poured into a specially designed block with desired thickness of spacer having hydrophobicity followed by gelatinization of the same over home-made thermostatic control unit at 38?°C. The gelatinized matrix is then allowed to dry under controlled conditions of humidity and temperature. The films of varying thicknesses ranging between 40, 50, and 60??? are used for photo-electrochemical measurements. The results on photo-electrochemistry of non-oriented purple membrane film provides valuable information on the generation of forward (light on) and backward (light off) photocurrent as a function of: (a) applied potential and (b) film thickness. An increase in applied negative potential increases the amplitude of photocurrent whereas decrease in film thickness facilitates the reversibility of photocurrent response.     

Preparation and photoelectrochemical performance of TiO<Subscript>2</Subscript>/Ag<Subscript>2</Subscript>Se interface composite film

Coupling TiO₂ with a narrow band gap semiconductor acting as the photosensitizer has attracted much attention in solar energy exploitation. In this work, the porous TiO₂ film was first formed on the conducting glass plate (CGP) substrate by the decomposition of polyethylene glycol (PEG) mixing in titanium hydroxide sol at 450°C. Then, the TiO₂/Ag²Se interface composite film was fabricated by interface reaction of AgNO₃ with NaSeSO₃ on the activated surface of porous TiO₂ film. The results of SEM and XRD analyses indicated that the porous TiO₂ layer was made up of the anatase crystal, and the Ag₂Se layer was made up of congregative small particles that have low-temperature α-phase structure. Due to its efficient charge separation for the photo-induced electron-hole pairs, the TiO₂/Ag₂Se interface composite film as-prepared has good photovoltaic property and high photocurrent response for visible light, which have been confirmed by the photoelectrochemical measurements.     

Interface analysis of pure Sn,pure Ag and Sn?Ag binary alloys in H2SO4

The electrochemical and passivation properties of three selected binary xSn? Ag (x = 26, 50 and 70 wt%) alloys were studied by means of open‐circuit potential variation, potentiodynamic curves and a.c. impedance spectroscopy techniques.The specimens were polarized between ?1.0 and 0.5 V versus saturated calomel electrode (SCE) in naturally oxygenated sulfuric acidsolutions of different concentrations The experimental results indicate that i_corr increases with increasing either the acid concentration or the Sn content in the solid phase. Electrochemical impedance spectroscopyresults measured at the free corrosion potential confirm that alloy I (26Sn? Ag) characterizes by thicker passive film with higher protective ability compared to the other two samples richer in tin component. The exponential variation of the relative thickness of the surface film on any of the tested samples assumes an almost constant thickness for a thin barrier layer and a much larger outer porous layer that dominates the total film thickness on the alloy. Copyright © 2010 John Wiley & Sons, Ltd.     

TiCl4 assisted formation of nano-TiO2 secondary structure in photoactive electrodes for high efficiency dye-sensitized solar cells

A new kind of photoactive electrodes with nanocrystalline TiO2(nano-TiO2)secondary structure is successfully prepared via a simple method of adding a small amount of TiCl4 2-propanol solution in conventional nano-TiO2 paste to form micro-sized nano-TiO2 aggregates.The benefits of this special structure include improved optical absorption,increased light scattering ability,and enhanced electron transport and collection efficiency.Dye-sensitized solar cells(DSCs)based on these photoactive electrodes show improved performance.The power conversion efficiency of the cells can be increased from 5.03%to 7.30%by substituting 6μm conventional nano-TiO2 thin film with the same thickness of as-prepared nano-TiO2 aggregates film in the photoactive electrodes.A higher power conversion efficiency of the cells can be obtained by further increasing the thickness of the nano-TiO2 aggregates film.     

Aqueous solution-processed NiOx anode buffer layers applicable for polymer solar cells

A green and facile method has been developed for the room temperature and aqueous solution preparation of NiO_x film as anode buffer layers for polymer solar cells (PSCs). The NiO_x buffer layer is prepared simply by spin-coating nickel acetylacetonate precursor-based aqueous solution onto ITO substrate at room temperature in air. UV-ozone post-treatment promotes the formation of dipolar NiOOH species on the film surface, resulting in the anode buffer layer with suitable work function. PSCs have been fabricated with the device structure of ITO/NiO_x/photoactive layer/PFN/Al. The power conversion efficiencies of the PSCs based on PTB7:PC₇₁BM blends (8.43%) and P3HT:PC₇₁BM blends (3.04%) with NiO_x anode buffer layer are comparable to those with the commonly used PEDOT:PSS anode buffer layer. In addition, the devices made with NiO_x buffer layer exhibit much better air stability than those with PEDOT:PSS. These results indicate that the water solution processed NiO_x film at room temperature in air is a promising anode buffer layer for efficient and stable PSCs. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 747–753     

A photoelectrochemical study of passive copper in alkaline solutions

The photoelectrochemical behaviour of copper covered with a passivating Cu₂O layer has been studied in alkaline solution. Cu₂O shows the characteristics of a p-type semiconductor with a band gap of 2.3 eV and a flatband potential of −0.28 V (SHE). Its photocurrent spectrum shows the characteristics of the absorption spectrum of Cu₂O films. Several redox systems have been tested, including a CuO layer of the duplex film formed at sufficiently positive potentials. The cathodic photocurrent leads to a reduction of the CuO overlayer to Cu₂O rather than to a self-reduction of Cu₂O to Cu. For the duplex film a decrease of the band gap and an increase of the flatband potential is found, suggesting a participation of CuO in the generation of photoelectrons.     

Photoelectrochemical Study of Anodic Oxide Films on Titanium–Cobalt Alloys

Properties of anodic oxide films as a function of the composition of Ti _x –Co (x= 0, 10, 20, 30, 50, 67, 75 at. %) alloys in solutions of a borate buffer and sodium sulfate are studied by the voltammetric technique combined with photocurrent measurements. The oxide film on the alloys is shown to contain TiO₂and Co₂O₃. In a borate buffer, the oxide film presumably comprises two layers: an outer layer of cobalt oxides and an inner layer of a mixture of cobalt and titanium oxides.     

Electrochemically promoted photocatalytic oxidation of nitrite ion by using rutile form of TiO2/Ti electrode

The photocatalytic oxidation of nitrite ion in a NaCl aqueous solution using the rutile form of TiO₂/Ti as the working electrode was studied. Experimental results indicate that the rutile form of TiO₂/Ti film electrode has excellent photoactivity by applying a bias potential and irradiation simultaneously. The incident photo-to-current conversion efficiency (IPCE) of this working electrode is a function of the applying bias potential. The photocurrent efficiency of nitrite ion oxidation was 33–40% at a pH of about 7. The oxidation rate of the nitrite ion in brine wastewater using the rutile form of TiO₂/Ti electrode can be estimated by photocurrent measurements. The applying bias potential, light power and pH value were the major factors affecting the oxidation rate and the photocurrent efficiency of nitrite ion oxidation, while the concentrations of nitrite ion was minor.     

Modelling the formation and growth of anodic passive films on metals in concentrated acid solutions

Two model approaches to the formation of passive films as adsorbed layers during the active anodic dissolution of a metal in acid and their subsequent growth are presented. The first depicts passivation as proceeding in parallel to active dissolution. Adsorption of water on active surface sites leads to passivation, whereas adsorption of acid leads to active dissolution of the metal. The model is consistent with the impedance response during passivation of Fe and an Fe-20%Mo alloy in concentrated H₃PO₄. The second model is an updated version of the so-called surface charge approach to the mechanism of conduction of anodic passive films. It is based on the assumptions that oxygen vacancies are the main ionic charge carriers and the field strength in the barrier layer is constant. A negative surface charge built up at the film/solution interface via accumulation of metal vacancies accelerates oxygen vacancy transport, thus explaining the pseudoinductive behaviour of the metal/film/electrolyte system under small amplitude a.c. perturbation. The model describes the growth of thin anodic films on Fe, Mo and an Fe-20%Mo alloy in concentrated H₃PO₄. Received: 24 January 1997 / Accepted: 18 April 1997     

Peculiarities of mass transfer and natural convection in the redox system [Fe(CN)<Subscript>6</Subscript>]<Superscript>3−</Superscript>/[Fe(CN)<Subscript>6</Subscript>]<Superscript>4−</Superscript> with current flowing through a microorifice in insulating coating on the electrode

Variations in the current in the [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ system flowing through a vertical microorifice in the insulating film on the electrode are shown. Steady- and nonsteady-state conditions of electrolysis are studied for different insulating film thicknesses. The obtained results suggest that at steady-state electrolysis, in an insulator channel, near the electrode, a “stagnant zone” is formed in which the natural convection of electrolyte is weak. Mass transfer in this zone preferentially occurs due to the reagent diffusion. The length of this zone increases with the increase in the channel length. A zone with the natural convection of electrolyte is located at a certain distance from the electrode, closer to the insulator surface. A part of this zone is located in the solution bulk and its thickness is independent of the channel length. The mass transfer in this zone is realized by both the reagent diffusion and the natural convection of electrolyte. Voltammetric measurements show that at sufficiently high potential scanning rates, the peak currents on a planar electrode and on an electrode placed on the bottom of the channel in the insulating film virtually coincide. This result points to the possibility of using potentiodynamic methods for analyzing the electrolyte composition inside the channel and in the solution bulk irrespective of the thickness of the electrode-insulating film.     

Investigation on Electronic Property of Passive Film on Nickel in Bicarbonate/Carbonate Buffer Solution

The electronic properties of passive film formed on nickel in bicarbonate/carbonate buffer solution were studied by electrochemical impedance spectra (EIS) and Mott‐Schottky plot. The film composition was analyzed by X‐ray photoelectron spectroscopy (XPS). The results showed that passive film exhibited p‐type semi‐conductive character, and the acceptor density (N_A) decreased with increasing potential, prolonging time, decreasing temperature, increasing pH value and decreasing chloride/sulfur ions concentration. The transfer resistance and film resistance increased with the above factors changing. XPS results showed that passive film was composed of NiO and a little amount of Ni₂O₃.