Photoelectrochemical Techniques Research Articles

Formation of CuSbS 2 and CuSbSe 2 thin films via chalcogenisation of Sb–Cu metal precursors

Due to the availability and low cost of the elements, the ternary Cu–Sb–S and Cu–Sb–Se semiconductor systems are being studied as sustainable alternative absorber materials to replace CuIn(Ga)(S,Se) 2 in thin film photovoltaic applications. Simple evaporation of the metal precursors followed by annealing in a chalcogen environment has been employed in order to test the feasibility of converting stacked metallic layers into the desired compounds. Other samples have been produced from aqueous solutions by electrochemical methods that may be suitable for scale-up. It was found that the minimum temperature required for the complete conversion of the precursors into the ternary chalcogen is 350 °C, while binary phase separation occurs at lower temperatures. The new materials have been characterised by structural, electrical and photoelectrochemical techniques in order to establish their potential as absorber layer materials for photovoltaic applications. The photoactive films consisting of CuSbS 2 and CuSbSe 2 exhibit band-gap energies of ~ 1.5 eV and ~ 1.2 eV respectively, fulfilling the Shockley–Queisser requirements for the efficient harvesting of the solar spectrum.

Optical Analysis of Nanocrystalline ZnO Films Coated on Porous Silicon by Radio Frequency (RF) Magnetron Sputtering

Zinc oxide thin films have been deposited onto porous silicon (PSi) substrates at high growth rates by radio frequency (RF) sputtering using a ZnO target. The advantages of the porous Si template are economical and it provides a rigid structural material. Porous silicon is applied as an intermediate layer between silicon and ZnO films and it contributed a large area composed of an array of voids. The nanoporous silicon samples were adapted by photo electrochemical (PEC) etching technique on n-type silicon wafer with (111) and (100) orientation. Micro-Raman and photoluminescence (PL) spectroscopy are powerful and non-destructive optical tools to study vibrational and optical properties of ZnO nanostructures. Both the Raman and PL measurements were also operated at room temperature. Micro-Raman results showed that the A1(LO) of hexagonal ZnO/Si(111) and ZnO/Si(100) have been observed at around 522 and 530 cm–1, re- spectively. PL spectra peaks are distinctly apparent at 366 and 368 cm–1 for ZnO film grown on porous Si(111) and Si(100) substrates, respectively. The peak luminescence energy in nanocrystalline ZnO on porous silicon is blue-shifted with regard to that in bulk ZnO (381 nm). The Raman and PL spectra pointed to oxygen vacancies or Zn interstitials which are responsible for the green emission in the nanocrystalline ZnO.

Phthalocyanine−Pyrene Conjugates: A Powerful Approach toward Carbon Nanotube Solar Cells

In the present work, a new family of pyrene (Py)-substituted phthalocyanines (Pcs), i.e., ZnPc-Py and H(2)Pc-Py, were designed, synthesized, and probed in light of their spectroscopic properties as well as their interactions with single-wall carbon nanotubes (SWNTs). The pyrene units provide the means for non-covalent functionalization of SWNTs via π-π interactions. Such a versatile approach ensures that the electronic properties of SWNTs are not impacted by the chemical modification of the carbon skeleton. The characterization of ZnPc-Py/SWNT and H(2)Pc-Py/SWNT has been performed in suspension and in thin films by means of different spectroscopic and photoelectrochemical techniques. Transient absorption experiments reveal photoinduced electron transfer between the photoactive components. ZnPc-Py/SWNT and H(2)Pc-Py/SWNT have been integrated into photoactive electrodes, revealing stable and reproducible photocurrents with monochromatic internal photoconversion efficiency values for H(2)Pc-Py/SWNT as large as 15 and 23% without and with an applied bias of +0.1 V.

Investigation of growth mechanism of nano-scaled cadmium sulfide within titanium dioxide nanotubes via solution deposition method

The growth mechanism of cadmium sulfide nanomaterials, including nanodots, nanotubes, and nanorods, within titanium dioxide nanotubes via solution deposition method was investigated. The materials obtained were characterized by field emission scanning electron microscopy, UV–visible spectroscopic and photoelectrochemical techniques. The results revealed that: (1) the concentration of ions introduced into the tubes influenced the morphology of the cadmium sulfide obtained: at low concentration, defects on the tube walls induce heterogeneous nucleation hence cadmium sulfide was observed attaching to the walls; at high concentration, particle aggregation occur due to negligible repulsion between the nuclei resulting in sedimentation of cadmium sulfide particles; (2) cadmium sulfide prefers to grow on seeds formed initially, so that nanodots or nanotubes and nanorods were formed at low and at high concentrations respectively; (3) the order of ions introduction also influences the morphology of cadmium sulfide formed within the tubes, (4) the photoresponse of the obtained nanomaterials was extended efficiently; and (5) the photoelectrochemical properties were strongly influenced by both the amount and the morphology of the deposited CdS sensitizer.

Photoelectrochemical study of the growth of the passive film formed on Fe-20Cr-15Ni in a pH 8.5 buffer solution

The structural change of the passive film on Fe-20Cr-15Ni at its growth stage was examined in-situ using a photoelectrochemical technique. The photocurrent spectra showed that the passive film on Fe-20Cr-15Ni for 0.5 h ∼ 25 h in a pH 8.5 buffer solution was composed of (Cr, Ni)-substituted γ-Fe2O3 mixed with NiO. Photocurrent spectral analysis suggested that a crystalline film of (Cr, Ni)-substituted γ-Fe2O3 formed before 30 min, with its thickness exceeded that of the space charge layer after 1 h of passivation. NiO particles appeared to have gradually precipitated from (Cr, Ni)-substituted γ-Fe2O3 film in the early stage of passivation of 1 h.

Passivity and Pitting Corrosion of X80 Pipeline Steel in Carbonate/Bicarbonate Solution Studied by Electrochemical Measurements

This work investigated the effects of chloride ions and hydrogen-charging on the passivity and pitting corrosion behavior of X80 pipeline steel in a bicarbonate-carbonate solution by electrochemical and photo-electrochemical techniques. It was found that a stable passivity can be established on the steel in the absence and presence of chloride ions. The hydrogen-charging does not alter the transpassive potential, but increases the passive current density. When chloride ions are contained in the solution, pitting corrosion will be initiated. The pitting potential is independent of the hydrogen-charging. Hydrogen-charging would enhance the anodic dissolution and electrochemical activity of the steel, but does not affect the pitting potential, which indicates that the charged hydrogen is not involved in the pitting initiation. However, hydrogen may accelerate the pit growth. Photo illumination could enhance the activity of the steel electrode, resulting in an increase of photo-induced anodic current density.

Photoelectrochemical evaluation of anatase TiO2 polycrystalline aggregation layers with different crystalline orientations

In order to evaluate the characteristics of photocatalysts such as TiO2, it is important to separately estimate the oxidation and reduction reaction rates, since the overall reaction rate is limited by the rate-determining step. In this study, photoelectrochemical techniques were applied to thin films of crystalline oriented anatase TiO2 with polycrystalline aggregations deposited on the transparent conductive oxide (TCO) glass substrate, fabricated by the electrophoretic deposition (EPD) in a strong magnetic field. The influence of the plane orientation on the photocatalytic reaction rates was discovered for both oxidation and reduction with respect to current through the electrochemical measurements. The maximum photocurrent for the (001) plane orientation is three times higher than that for the (100) plane orientation, and is comparable with that of the random orientation. The rate of the anodic reaction determines the rate of the overall photocatalytic reaction, therefore affecting the photopotential.

Accelerating materials development for photoelectrochemical hydrogen production: Standards for methods, definitions, and reporting protocols

Photoelectrochemical (PEC) water splitting for hydrogen production is a promising technology that uses sunlight and water to produce renewable hydrogen with oxygen as a by-product. In the expanding field of PEC hydrogen production, the use of standardized screening methods and reporting has emerged as a necessity. This article is intended to provide guidance on key practices in characterization of PEC materials and proper reporting of efficiencies. Presented here are the definitions of various efficiency values that pertain to PEC, with an emphasis on the importance of solar-to-hydrogen efficiency, as well as a flow chart with standard procedures for PEC characterization techniques for planar photoelectrode materials (i.e., not suspensions of particles) with a focus on single band gap absorbers. These guidelines serve as a foundation and prelude to a much more complete and in-depth discussion of PEC techniques and procedures presented elsewhere.

Studies on growth and characterization of CdS1−xSex (0.0≤x≤1.0) alloy thin films by spray pyrolysis

Abstract The n-CdS1−xSex thin films of variable composition have been deposited on amorphous glass and FTO-coated glass substrate by simple and cost effective spray pyrolysis technique. The various deposition parameters have been optimized by using photoelectrochemical technique. The structural, surface morphological, compositional, optical and electrical properties have been studied. The X-ray diffraction studies indicated that all the films are polycrystalline in nature with hexagonal structure irrespective of the composition. The lattice parameters ‘a’ and ‘c’ vary from 4.1034 to 4.2615 A and 6.6664 to 6.9243 A respectively with change in composition parameter from x = 0.0 to x = 1.0. Polycrystalline texture with nearly smooth surface and clearly defined grains has been observed for all samples from scanning electron microscopy (SEM). EDAX studies confirmed that CdS1−xSex films have approximately same stoichiometry both initially and finally. The absorption coefficient ‘α’ is of the order of 104 cm−1. The optical absorption studies reveal that direct allowed transition with band gap energy between 2.44 and 1.74 eV. It is found that resistivity of the films decreased with increase in ‘x’ up to 0.8 and further it increases for x = 1.0. Semiconducting behavior has been observed from resistivity measurements.

Enhanced Photoelectrocatalytic Activity of Methanol Oxidation on TiO2-Decorated Nanoporous Gold

Au−TiO2 nanocomposites have been widely investigated for their potential applications in solar energy conversion, CO oxidation, and methanol reforming reactions. In this study, commercial TiO2 nanoparticles were assembled on the surface of nanoporous gold (NPG) to fabricate novel TiO2/NPG nanocomposite electrodes. Electrochemical and photoelectrochemical techniques were used to investigate the characteristics of the electrodes. Large photocurrent and nearly reversible voltammetric responses were observed for methanol photoelectrocatalysis under UV radiation, indicating an effective elimination of gold surface passivation due to a pronounced synergistic effect between TiO2 and NPG. A possible mechanism was proposed to elucidate such a synergistic effect, which is based on the reaction of the photogenerated reactive intermediates on the surface of NPG. Kinetic studies showed that the coupling of TiO2 with NPG in our system could lead to about a 30% decrease of apparent activation energy for methanol electro...

Photoelectrochemical Study of Nanostructured ZnO Thin Films for Hydrogen Generation from Water Splitting

AbstractPhotoelectrochemical cells based on traditional and nanostructured ZnO thin films are investigated for hydrogen generation from water splitting. The ZnO thin films are fabricated using three different deposition geometries: normal pulsed laser deposition, pulsed laser oblique‐angle deposition, and electron‐beam glancing‐angle deposition. The nanostructured films are characterized by scanning electron microscopy, X‐ray diffraction, UV‐vis spectroscopy and photoelectrochemical techniques. Normal pulsed laser deposition produces dense thin films with ca. 200 nm grain sizes, while oblique‐angle deposition produces nanoplatelets with a fishscale morphology and individual features measuring ca. 900 by 450 nm on average. In contrast, glancing‐angle deposition generates a highly porous, interconnected network of spherical nanoparticles of 15–40 nm diameter. Mott‐Schottky plots show the flat band potential of pulsed laser deposition, oblique‐angle deposition, and glancing‐angle deposition samples to be −0.29, −0.28 and +0.20 V, respectively. Generation of photocurrent is observed at anodic potentials and no limiting photocurrents were observed with applied potentials up to 1.3 V for all photoelectrochemical cells. The effective photon‐to‐hydrogen efficiency is found to be 0.1%, 0.2% and 0.6% for pulsed laser deposition, oblique‐angle deposition and glancing‐angle deposition samples, respectively. The photoelectrochemical properties of the three types of films are understood to be a function of porosity, crystal defect concentration, charge transport properties and space charge layer characteristics.

Effect of thickness of ALD grown TiO 2 films on photoelectrocatalysis

Atomic layer deposited TiO 2 films were used for photoelectrocatalysis for the first time. Films with different thicknesses were grown and comprehensively characterized by structural and photoelectrochemical techniques. Methylene blue was used as a model substance in photocatalytic and photoelectrocatalytic degradation studies. In both cases, degradation rate was observed to saturate above certain film thickness. The reason for saturation could not be explained by the electrical parameters alone, and it was suggested that film morphology and crystalline structure are of major importance in this case.

Anomalous Photocathodic Behavior of CdS within the Urbach Tail Region

A series of cadmium sulfide materials have been prepared using microwave-assisted hydrolysis of cadmium−thiourea complexes in water and nonaqueous solvents. Materials are characterized using X-ray diffraction, scanning electron microscopy, diffuse reflectance spectroscopy, and various photoelectrochemical techniques. Sulfur-doped samples of CdS exhibit a pronounced photoelectrochemical photocurrent switching effect within low-energy tails of their absorption spectra. A mechanism of the photocurrent switching process is presented on the basis of experimental investigations and quantum−chemical calculations.

Haematite and chromia dissolution in the zirconia matrix during thermal oxidation of Laves-phases γ–Zr(Fe,Cr)2 on Zircaloy-4

The life time of the Zircaloy-4 alloy is linked to the role that intermetallic particles play during the growth of the zirconia scale. In the present work, a photoelectrochemical technique (PEC), has been used to study the phenomena of oxidation of intermetallic particles but also of dissolution of thermal oxidised particles. Both phenomena have been addressed in the literature and studied in particular with TEM; in this work, they were clearly imaged by photoelectrochemical microscopy at a mesoscopic view. Our results do contribute to a better understanding of the major impact that intermetallic precipitates could have on the oxidation rate.

Photoelectrochemical analysis of the effects of pH and sulfate ions on the structure and the composition of the passive film formed on Fe-20Cr-15Ni alloy

The effects of pH and sulfate ions on the structure and compositions of the passive film formed on Fe-20Cr- 15Ni were examined using a photoelectrochemical technique and Mott-Schottky analysis. The photocurrent spectra for the passive film formed on Fe-20Cr-15Ni in the buffer solutions are composed of two spectral components, one of which is generated from Cr-substituted γ-Fe2O3 and the other of which is generated from NiO. However, the passive film formed in sulfate solutions showed only the photocurrent spectrum of Cr-substituted γ-Fe2O3, suggesting that the formation of NiO in the passive film is suppressed because of a severe selective dissolution of Ni in the presence of sulfate ions. Mott-Schottky plots confirmed that the base structure of the passive film on Fe-20Cr-15Ni is n-type (Cr, Ni)-substituted γ-Fe2O3 regardless of solution pH and sulfate ions. The photocurrent intensity, flat band potential, and donor density for the passive film varied depending on the solution pH or the presence of sulfate ions in the solution, due primarily to the Cr enrichment in the film caused by the preferential dissolution of Fe and Ni that is more appreciable in highly acidic solutions containing sulfate ions.

A study of the behavior of copper electrodes in buffered borax solutions containing BTA‐type inhibitors by photoelectrochemical methods

PurposeThe purpose of this paper is to provide useful information pertaining to the corrosion inhibition mechanism of BTA and its derivatives on copper.Design/methodology/approachThe photoelectrochemical behavior of copper electrodes in buffered borax solutions (pH 9.2) containing BTA and its derivatives was comparatively studied by using a photoelectrochemical technique. It was possible to analyze the inhibition mechanism of the derivatives of BTA for copper corrosion from the photoelectrochemical results. The photoresponse of the Cu electrode in buffered borax solutions containing BTA and its derivatives was measured. Different concentrations and different kinds of inhibitors may result in different photoresponses on the Cu electrode in buffered borax solutions.FindingsThe photoresponse for copper electrodes in solutions containing a certain amount of BTA exhibits an n‐type response during anodic polarization and, the greater the n‐type photoresponse, the better the performance of the inhibitor. The photoresponse for copper electrodes in solutions containing 4CBTA, or 5CBTA, or CBT‐1, always exhibited p‐type behavior during anodic polarization, but the photoresponse changed very evidently during cathodic polarization. The larger the maximum cathodic photocurrent, then the greater was the effectiveness of the corrosion inhibitor. In consequence, it is possible to evaluate inhibitors according to ΦV and iph at more negative potentials. The more negative the ΦV and iph, the better is the inhibition. It was shown that the inhibition mechanism of the derivatives of BTA with a −COOH group was different from that occurring with ester groups. The former could make the Cu2O film on the Cu electrode thicker. The photocurrent was observed to increase when the potential was scanned to more negative potentials in the presence of certain concentrated inhibitors. It is therefore possible to evaluate the performance of inhibitors according to the value of the cathodic photocurrent. The larger the cathodic photocurrent, the better is the inhibition effect of the compound. The latter could increase the density of the polymer film on the copper electrode and prevent O2− in the solution from entering the copper surface and changing the stoichiometric ratio of Cu2O. The photocurrent type could transfer from p‐ to n‐type according to the action of certain concentrated inhibitors when the potential was scanned to more positive potentials. The value of the anodic photocurrent can be used to evaluate the effectiveness of inhibition. The larger the anodic photocurrent, the greater is the inhibition effect.Originality/valueThe paper provides useful information pertaining to the corrosion inhibition mechanism of BTA and its derivatives on copper. The photoelectrochemical technique is an effective method with which to evaluate the effectiveness of corrosion inhibitors and to investigate the mechanism of corrosion inhibition on copper.

Imaging by Photoelectrochemical Techniques of Laves-Phases γ Zr(Fe,Cr)<sub>2 </sub> Thermally Oxidized on Zircaloy-4

The oxidation of γ-Zr(Fe,Cr)2 intermetallic particles during the thermal exposition of Zircaloy-4 at 470°C in oxygen was investigated with PhotoElectroChemical techniques (PEC). Via the measurement of bandgap, haematite Fe2O3 (2.2 eV), rhomboedric solid solution (FexCr1-x)2O3 (2.6 eV) and chromia Cr2O3 (3.0 eV) phases were identified as components of oxidised particles. Evolution of size, lateral distribution and density of these particles was studied in function of zirconia scale thickness. During the first stage of oxidation, the density of oxidised particles increased with thickness but decreased during a second stage, highlighting in an innovative way the phenomenon of haematite and chromia dissolution in the zirconia matrix. It is concluded that PEC techniques represent a sensitive and powerful way to locally analyse the various semiconductor phases in an oxide scale at the micron scale.

Electrochemical Preparation and Characterization of Surface-Fluorinated TiO2 Nanoporous Film and Its Enhanced Photoelectrochemical and Photocatalytic Properties

In recent years, surface fluorination of TiO2 has been demonstrated to be an efficient method for improving its photocatalytic (PC) reactivity toward some pollutants. In this paper, a new and simple method for surface fluorination and porosity-creating of TiO2 is presented. The sample was prepared by anodization of TiO2 in HF aqueous solutions formed by direct thermal oxidization of titanium sheet. The preparation conditions were optimized. The prepared samples were characterized by scanning electron microscopy (SEM), Raman spectroscopy, UV–vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL). The photoelectrochemical (PEC) and photocatalytic (PC) properties of the samples were also investigated. SEM and Raman showed that the anodization made the TiO2 nanoporous, which consequently increased markedly the specific surface area, but did not change the bulk crystal structure under a certain preparation condition. DRS analysis revealed that the light absorption was decreased after electrochemical etching. The band gap energy narrowed from 3.15 to 3.04 eV. The surface fluorination of the etched-TiO2 was evidently supported by XPS. Highly enhanced PEC performance and PC activities for the degradation of target pollutants, phenol, methylene blue, and reactive brilliant red on the etched-TiO2 were observed. The possible reasons for such an improvement were studied in detail by a combination of the above-mentioned methods and (photo) electrochemical techniques. It is mainly attributed to the enhanced specific surface area, negative-shifted appearing energy band edges, decreased surface recombination centers, and/or favorable charge transfer rate. The F-containing TiO2 electrode has an excellent stability against fluoride desorption.

Structural, Optical, and Photoelectrochemical Properties of Sprayed TiO2 Thin Films: Effect of Precursor Concentration

Thin films of titanium dioxide (TiO2) have been deposited from methanolic titanyl acetylacetonate precursor solution onto the preheated amorphous and fluorine‐doped tin oxide (F:SnO2)‐coated glass substrates at optimized substrate temperature of 470°C by a spray pyrolysis technique. The objective of the study is to investigate the effect of film thickness on the structural, optical and photoelectrochemical (PEC) properties of TiO2 thin films by varying the precursor concentration from 0.05M, at the interval of 0.025M, to 0.125M. X‐ray diffraction shows that the films are polycrystalline with anatase phase having tetragonal crystal structure. Scanning electron microscopy reveals uniform and compact distribution of spherical clusters comprising nanoplatelets of size ∼200–400 nm. The films are nearly stoichiometric as confirmed by energy‐dispersive X‐ray analysis. Atomic force microscopy study also reveals uniform distribution of grains of size ∼180–370 nm. The films are transparent (with %T>70) and film thickness varies over 162–758 nm range with increase in precursor concentration. The band gap energy varies from 3.31 to 3.36 eV. Thickness of the TiO2 films is shown to have an influence on the PEC performance and plays an important role in determining the efficiency of the PEC cell. The precursor concentration of 0.1M is optimized using PEC technique. Incident photon to current conversion efficiency of 71.24% is obtained for 600 nm thick TiO2 film.

Preparation and properties of spray-deposited ZnIn 2Se 4 nanocrystalline thin films

Zinc indium selenide (ZnIn 2Se 4) thin films have been deposited onto amorphous and fluorine doped tin oxide (FTO)-coated glass substrates using a spray pyrolysis technique. Aqueous solution containing precursors of Zn, In, and Se has been used to obtain good quality deposits at different substrate temperatures. The preparative parameters such as substrate temperature and concentration of precursors solution have been optimized by photoelectrochemical technique and are found to be 325 °C and 0.025 M, respectively. The X-ray diffraction patterns show that the films are nanocrystalline with rhombohedral crystal structure having lattice parameter a=4.05 Å. The scanning electron microscopy (SEM) studies reveal the compact morphology with large number of single crystals on the surface. From optical absorption data the indirect band gap energy of ZnIn 2Se 4 thin film is found to be 1.41 eV.