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AC impedance and cyclic voltammetry studies on PbS semiconducting film prepared by electrodeposition
Abstract
Semiconducting lead sulfide film was deposited on Stainless Steel (SS) electrode by cyclic voltammetry (CV) at room temperature. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry were used to investigate the electrochemical properties of PbS film in Na2SO4 solution. The voltammetric results showed that oxidative dissolution of PbS film occurred at about 0.21V vs. Ag/AgCl and total film detachment from the surface occurs with increasing potential. It is concluded that the electrode surface was not passivated by sulfur produced from oxidative dissolution of PbS film. Also, cathodic reduction of the film continued to beyond the hydrogen evolution at −1.35V vs. Ag/AgCl. Capacitive parts of impedance spectra showed frequency dispersion and constant phase element (CPE) was used instead of capacitor to simulate the interfacial capacitance in the corresponding equivalent circuit. The flat band potential and carrier concentration were determined from Mott–Schottky plot and are estimated to be −0.32V and 1.6×1023m−3 respectively. The film was p-type semiconductor.
... The CVs and the photoelectrochemical measurements of the prepared electrodes have been collected at the bias voltages of þ0.8 V (SCE) and À0.3 V (SCE), that are respectively above and below the flat band potential of the OLEA-coated PbS NCs (0.057 V), as estimated from the Mott Schottky plot reported in Fig. S7, according to ref. 42. Such a plot shows a linear trend, intercepting the potentials axis at the flat band potential of 0.057 V, with a negative slope, according to the typical p-type behavior of the PbS NC semiconductor [42]. ...
... The CVs and the photoelectrochemical measurements of the prepared electrodes have been collected at the bias voltages of þ0.8 V (SCE) and À0.3 V (SCE), that are respectively above and below the flat band potential of the OLEA-coated PbS NCs (0.057 V), as estimated from the Mott Schottky plot reported in Fig. S7, according to ref. 42. Such a plot shows a linear trend, intercepting the potentials axis at the flat band potential of 0.057 V, with a negative slope, according to the typical p-type behavior of the PbS NC semiconductor [42]. ...
... The lower value of the anodic photocurrent can be explained considering that, the applied external bias of À0.3 V, doping with electrons PCA-RGO, contributes to increase the level mismatch between the Dirac point of RGO and the HOMO level of the PbS NCs (Fig. 6C), thus favoring the transfer of the photogenerated holes from the photoexcited PbS NCs to the PCA-RGO sheets. Additionally, the intrinsic p-type behavior of the PbS semiconductor [42], featuring photogenerated holes as majority carriers, further contributes in enhancing the photocurrent density at such a negative bias voltage. ...
Novel hybrid nanocomposites formed of 1-Pyrene Carboxylic Acid (PCA) functionalized Reduced Graphene Oxide (RGO) sheets, surface decorated with UV-Vis-NIR light-harvester PbS nanocrystals (NCs), are prepared by means of an in situ colloidal chemistry approach, using oleic acid (OLEA) as a surfactant. PCA molecules anchored onto the RGO sheets feature carboxyl groups that serve as coordinating sites for the PbS NCs heteronucleation. Here, the mechanism underlying the NCs decoration of RGO is investigated and the interplay between the concentration of PCA and OLEA and the Pb precursor:OLEA molar ratio are demonstrated able to careful control NCs morphology and size distribution, governing also RGO covering density. Remarkably, the PCA-RGO/PbS NCs modified Indium Thin Oxide (ITO) electrodes show an electrochemical specific capacitance much higher than that characterizing the single components of the composite, that can be further enhanced under illumination in the NIR. The nanocomposite modified electrodes demonstrate to behave as (photo)electrochemical supercapacitors, chemically stable, able to work both as (photo)charging and blocking electrodes, exhibiting a photocathodic behavior. The investigation of the NIR-light enhanced photocapacitive characteristics of the hybrid (photo)cathodes provides useful insights on their future integration in device, including electrochemical photocapacitors, and photoelectrochemical (bio)sensors.
... They also pointed out that electro-dissolution of galena was only partially inhibited at highly oxidizing potentials due to oxidation of S°to porous thiosulfate and lead sulfate species. Aghassi et al. [24] investigated the electrochemistry of PbS semiconducting film deposited on stainless steel in neutral condition by voltammetry tests. Oxidative dissolution was found to occur in mild to moderately oxidizing condition. ...
... Galena was thought to oxidize (reaction 2) during electrode preparation stage just before CV test due to high reactivity of exposed mineral surface in spite of great care given to minimize oxidizing effect of atmospheric oxygen [1,13,22,24]. Metal-hydroxides might also form by anodic decomposition of galena surface (reaction 3) together with the formation of oxysulfur species, such as metal-thiosulfates and sulfates [7]. The redox products of electrode preparation stage were reduced by scanning the electrode surface in cathodic direction. ...
... When anodic scan was reversed from −500 mV, only peak C3 formed (Fig. 1d). Galena is thermodynamically not stable at highly reducing condition, and then reduces to metallic form according to reaction 4 [24]. When cathodic scanning was reversed and continued in the anodic region, an oxidation peak formed weakly at about −625 mV. ...
Electrochemical potential determines types of redox products formed on electrically conducting minerals like galena, which might manipulate process efficiency applied on the target mineral. Therefore, electrochemical behavior of galena has utmost importance for flotation and hydrometallurgical applications. This study was performed to elucidate redox behavior of galena by cyclic voltammetry (CV) technique in a wide potential range. Voltammograms were obtained at pH 9.2 using deoxygenated borate buffer solution in a three-electrode system electrochemical cell. CV tests revealed that redox reactions proceeded reasonably irreversibly on galena electrode. Pb-oxyhydroxides released on the electrode together with sulfoxy species during anodic process while oxygen containing Pb-species reduced to metallic lead at highly reducing potentials. Oxidation product was thought to form a porous layer on mineral surface. Anodic oxidation process of galena obeyed hypothetical polarization diagram. Beyond transpassive surface corrosion, further oxidation of galena proceeded at highly oxidizing potentials at a limit current due to formation of porous Pb-oxide + sulfoxy layer.
... The Nyquist diagrams at −0.2 V exhibit semicircular 30 At selected potential of −0.1 V the impedance spectra present two depressed overlapping capacitive semi-circles in high and low frequencies signifying a non-ideal contact ( frequency dispersion). 31 So far, researchers have not been able to find an exact solution to this problem and several models have been proposed to explain the phenomenon. Surface states, 32 dielectric relaxations in the space charge layer or at the interface electrolyte-semiconductor, 33 the deep level of the donor, 34 the roughness of the electrode surface, 35 the fractal structure of the semiconductor surface, 36 and the interaction of surface states with the electrolyte 37 are asserted as causes of frequency dissipation. ...
This study aims to study and compare the electrochemical properties of variously silver-based carbon paste electrodes towards the catalytic reduction of hazardous nitroaromatic compound p-nitroaniline (PNA). Silver-based carbon paste electrodes have been manufactured and electrochemically characterized by using Cyclic Voltammetry (CV) and electrochemical impedance spectroscopy (EIS) to better define the electrochemical characteristics for their use as electrocatalytic sensors. The morphology and the distribution of silver on the electrode surface were evaluated by Scanning electron microscopy (SEM). The electro-active surfaces area of the as prepared electrodes were estimated to be 0.095, 0.06, 0.12 and 0.10 cm2 for electrode modified with silver via electrodeposition (CPE/Ag-Edp), impregnation (CPE/Ag-Imp), graphite electrode modified with silver nanoparticles (CPE/Ag-NPs) and graphite electrode modified graphite with silver adsorbed onto Chitosan (CS) (CPE/Ag-CS), respectively, using [Fe(CN)6]3-/4- as redox probe. The equivalent circuit might have been used to interpret the achieved impedance spectra which included one resistor in series with parallel circuit comprised of a capacitor and resistor (1R//C). The electrocatalytic activity of silver based electrodes for the reduction of PNA is discussed on the basis of the real electro-active surface area and particle size which results from varying modification modes.
... This technique constitutes a powerful tool for studying the photoelectrode interface properties [36]. It has the potential to provide information about physical and electrical properties of semiconductor materials including the type of semiconductor, charge carriers density (N), and flat band potential (V fb ) [37]. ...
In this paper, we address the effect of shell thickness and core/shell structure on the electrochemical and photoelectrochemical cell (PEC) measurements properties of ZnO/In2S3 core/shell nanowires (NWs). The objective is to elucidate the mechanisms responsible for the extended photoresponse of ZnO/In2S3 core/shell NWs to solar radiation. Well aligned ZnO/In2S3 core/shell NWs were fabricated on indium tin oxide substrates using electrochemically grown ZnO NWs as the cores and electrodeposited In2S3 as the shells. The samples structure was characterized by X-ray diffraction, revealing the mixed wurtzite and tetragonal structures of both ZnO cores and In2S3 shells, and the improvement in the structure with the increases of In2S3 shell thickness. The optical properties were studied through optical absorbance and photoluminescence measurements, showing the optical properties featured with type-II heterogeneous nanostructures constructed from ZnO and In2S3. Electrochemical impedance spectroscopy (EIS) is employed and an equivalent circuit model is designed suggesting that the cell performances were affected by increasing In2S3 shell deposition times. From Mott–Schottky plots, several parameters such as flat-band potential and free carrier concentration were determined. Next, from (PEC) measurements, the highest photocurrent density produced by the In2S3 shell electrode prepared at deposition time of 5 min reached 9.30 mA cm2 at an applied potential of 0.8 V vs. Ag/AgCl. This value was about 6.8 times as much as more than that measured on ZnO NWs one and provided the highest value of solar-to-hydrogen energy efficiency η (%). These results are very encouraging and suggest that In2S3 covered ZnO NWs nanostructures are valuable photo-anodes in order to build cheap devices for solar energy-to-hydrogen generation devices.
... In which, , 0 , e, k and T have their usual meaning [dielectric constant of the ZnS (ε ≈ 10)] [37], permittivity of a vacuum ( 0 = 8.85 × 10 − 14 F/cm), electronic charge (e = 1.602 × 10 − 19 C), Boltzmann constant (k = 1.38 × 10 − 23 J/K) and absolute temperature T. Whereas A is the surface area of the electrode (0.25 cm 2 ), N D the density of donor in the semiconductor, V is the applied potential and V fb is the flat band potential. Mott-Schottky plot is an indicator of the conductivity type of the semiconductor, where, negative sign is for p-type and positive for n-type semiconductor [38]. ...
In this work, Mn²⁺-doped ZnS onto (ITO)-coated glass substrates have been prepared by electrodeposition technique. Various samples of the ZnS with different Mn contents 0, 0.5, 1, 2, and 3 at.% were prepared. XRD analysis shows that all films crystallize in a cubic phase of ZnS with a preferential orientation along (200) direction. The variation in peak positions of X-ray diffraction shows that the Manganese element is well incorporated into ZnS matrix and does not remain in the interstitial sites. Surface morphology studies by Atomic Force Microscopy showed that an increase in the concentration doping causes an increase in the grain size and the mean square roughness. Moreover, the optical analysis reveals that the band gap energy varied between 3.54 and 3.76 eV in terms of Mn content. On the other hand, the electrochemical impedance spectroscopy data have been modeled using an equivalent circuit approach. Finally, from Mott–Schottky plot, the flat-band potential and carrier density of Mn-doping ZnS thin films have been determined. The results reveal that all the films showed n-type semiconductor character with a flat band potential and a carrier density varying from −0.46 V to −0.57 V and 3.20 × 10¹⁶ cm⁻³ to 4.34 × 10¹⁶ cm⁻³, respectively.
... In Fig. 1a, for the solution that contains only Pb 2? (clearer curve), in the cathodic scan one peak is visible at -0.55 V (see inserted curve), which corresponds to the reduction of Pb 2? to Pb [7]. The curve is quite similar to that previously reported by several authors [20,21]. From -0.55 V onwards, the cathodic current density increases, this could be due to several facts, one of which is the ongoing roughening of the growing Pb film [2,22]. ...
Lead (Pb)–cobalt (Co) films have been studied by several researchers regarding their electrocatalytic activity when used as anodes in oxygen evolution reactions. Although several authors have suggested that the electrodeposition technique is the best alternative to Pb–Co film deposition, the process is complex. In many cases, the use of complexing agents facilitates the electrodeposition of multi-element coatings; but this performance has not been reported in the literature for Pb–Co electrodeposition. In this work, the effect of three different complexing agents (tartaric acid, sodium citrate, and ascorbic acid) on Pb–Co electrodeposition was studied. Speciation diagrams and polarization curves have been used to determine the reduction potentials of Pb²⁺ and Co²⁺ ions in the different systems studied. Speciation diagram predictions were incapable of revealing all species present in solution and for an accurate prediction of the actual electrochemical functioning of the system; the information given by the speciation diagrams and polarization curves must be considered together. Additionally, electrodeposition of Pb–Co films was achieved without complexing agent and using the three complexing agents. Morphological and chemical properties of these coatings are shown and discussed in terms of the electrochemical functioning of each system. Finally, the electrocatalytic ability of the Pb–Co films was investigated by means of polarization curves.
... In recent years, various techniques have been used to deposit PbS thin films including microwave-assisted chemical bath deposition (CBD) [2], successive ionic layer adsorption and reaction (SILAR) technique [14][15][16][17], atomic layer epitaxial process [18], pulse electro-deposition [19], spray pyrolysis [20] and chemical bath deposition. ...
PbS thin films were grown on glass substrates by chemical bath deposition (CBD) using lead nitrate, thiourea and sodium hydroxide in aqueous solutions at three different temperatures (22, 36 and 50 °C). The microstructure and morphology evolution of the films were investigated using X-ray diffraction, scanning electron microscopy and atomic force microscopy. Optical properties were studied using UV–Vis–IR spectroscopy. The results indicate that temperature plays an important role in controlling the morphology and optical properties of nanostructured PbS thin films through changing deposition mechanism. The active deposition mechanism changed from cluster to ion-by-ion mechanism with an increase in deposition temperature from 22 to 50 °C, and consequently, film properties such as morphology, optical absorption and preferred orientation changed completely.
... This indicated that the ZnS semiconductor/electrolyte interface was composed of at least two different electrochemical processes. One of them at high frequencies could be associated with a space charge, and the other at low frequencies could be related to the double layer [36]. This enabled us the determination of two capacitive contributions involved in the measured impedance. ...
... MBT and ABT were purchased from Merck and used as inhibitor. Fitting of experimental impedance spectroscopy data to the proposed equivalent circuit was done by means of home written least square software based on the Marquardt method for the optimization of functions and Macdonald weighting for the real and imaginary parts of the impedance (Aghassi et al., 2011;. ...
Corrosion inhibition effect of 2-mercaptobenzothiazole (MBT) and 2-Aminobenzothiazole (ABT) compounds on ST-37 carbon steel in 1M hydrochloric acid solution was investigated by electrochemical impedance spectroscopy (EIS), and it was observed that both of these compounds have corrosion inhibition effect on carbon steel. Evaluation of electrochemical behavior in test solutions showed that by increasing the immersion time from 15 to 300 minutes, corrosion resistance of samples are increased and at the same immersion time MBT have the better corrosion inhibition in comparison to ABT. AFM technique was performed for MBT and ABT. The results of calculations showed superior inhibition efficiency of MBT in comparison to ABT. This can cause easier protonation and consequently adsorption on the metal surface occurs.
... The electrodeposition of p-type PbS thin film on the stainless steel in acidic medium have also been reported elsewhere. 17 It is therefore expected that it can be deposited as n-type PbS thin film with high concentration of carriers producing the sulfur anions slowly from alkaline solution of thiourea and excess Pb cations quickly. The purpose of this present work is the study of n-type PbS by electrodeposition in order to find suitable conditions for deposition of PbS film. ...
Electrochemical deposition and characterization of PbS nanocrystals on Pb electrodes in alkaline solution contain of thiourea are carried out by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods respectively. The diffusion coefficient of electro-active species (Pb+2) in the synthetic potential −0.36 V/SCE is investigated in different alkaline media. The analysis of PbS crystals using X-ray diffraction are shown (XRD) that the average of crystalline size and the lattice constant parameter are approximately 39.04 and 0.5948 nm respectively. Also, the morphology of surface was studied by scanning electron microscope (SEM) technique. This work focuses on EIS extraction of Mott-Schottky plots and calculation of several important parameters including Debye length (LD), carrier density (ND) and flat band potential (Efb) for PbS. The parameters are calculated 3.40×10−8 cm, 8.65×1019 cm−3 and 0.035 V/NHE respectively in the dark condition. The comparison of these values in the dark and under light provides some insights on the charge transfer. The increase in the charge transfer resistance (Rct) in the dark condition with respect to under illumination condition and the confirmation of the presence of surface states in the n-type PbS are some of the obtained results.
... The surface area of the working electrode is about 0.5 cm 2 . All electrochemical measurements were released in an electrolyte solution of 0.5 M Na 2 SO 4 (pH 6.5) [26]. sinusoidal AC perturbation of 10 mV was applied to the electrodes over the frequency range of 0.01-10 5 Hz. ...
... This demonstrates that the sample shows capacitive behavior in this corresponding range [41]. We can also notice from Fig. 5(a) that the phase angle of the impedance is less negative than that of the perfect capacitor (À90°), indicating frequency dispersion [42]. As illustrated in Fig. 5(b) Nyquist diagram (imaginary impedance (ÀZ 00 ) vs. real impedance (Z 0 )) shows an arc in the measurement frequency from 100 kHz to 0.01 Hz. ...
We present a new work on the development of electrodeposition route for synthesis of ternary ZnIn2S4 alloy. These thin films were grown on (ITO)-coated glass substrate from acidic plating bath containing Zinc (II) Chloride (ZnCl2), Indium Chloride (InCl3) and sodium thiosulfate (Na2S2O3) at room temperature. Prior to deposition, a cyclic voltammetry study was performed in binaries (Zn–S, In–S) and ternary (ZnIn2S4) systems. The influence of various deposition potentials on structural, morphological, optical, and electrical properties of samples was investigated. X-ray diffraction patterns of samples demonstrate the presence of major crystalline phase of ZnIn2S4 at an applied potential of −1050 mV versus Ag/AgCl. Energy band gap of samples determined from optical measurements has been estimated in the range of 1.90–2.50 eV. From atomic force microscopy (AFM) and scanning electron microscopy (SEM) analysis, it was found that surface morphology, grain size and roughness were strongly influenced by varying the deposition potentials. Electrochemical impedance spectroscopy data have been modeled using an equivalent circuit approach. Flat-band potential and free carrier concentration have been determined from Mott–Schottky plot and are estimated to be around −0.72 V and 1.46 × 1017 cm−3 respectively. The film was n-type semiconductor.
... which is unstable in the acidic medium (below pH 4) gets reduced to colloidal sulfur and is adsorbed on the electrode surface as per Eq. (1) [29]. In the absence of Pb ions the ...
PbS thin films were deposited on transparent conducting substrates by pulse electrodeposition. Stoichiometric and highly adherent films were obtained by optimizing the deposition parameters such as pulse duration, deposition and dissolution potentials. Structural properties of the films were studied using XRD, and HRTEM techniques. The film consists of nanocrystals with size in the range 2–20 nm. Using atomic resolution images an atomic model with cuboctahedral morphology was proposed for the PbS crystals. The stability of the PbS nanoparticles was attributed to the surface configuration of Pb and S atoms causing low surface energy. SEM and AFM surface analysis indicated a uniform and void free surface with agglomerations of crystals with an average size in the order of 250 nm. Appearance of the 2LO mode in the Raman spectra revealed good crystalline quality of the film, and the characteristic bands were assigned in agreement with the mineral galena. The XPS spectrum in the binding energy region of sulfur shows the presence of S in 2 different environments; S 2p3/2 at 160.4 eV which is due to the sulfur in sulfide (PbS), and a second peak at higher binding energy of 161.64 eV which can be due to Pb–S–O and the Pb 4f7/2 peak at binding energy 138 eV indicate the presence of traces of PbO. However, Raman and XRD analysis showed no evidence of PbO and PbSO3. From the transmittance spectrum the band gap of the film was estimated as 0.74 eV. The resistivity of the film was 2 × 104 ohm-cm and the films were photosensitive.
Novel heterostructure photocatalyst built from titanium dioxide (TiO2), graphene oxide (GO) and indium sulfide (In2S3) has been successfully prepared for photoelectrochemical hydrogen production. The stepwise introduction of three materials on conductive glass substrate has been realized through hydrothermal, electrochimical and spin coating deposition methods, respectively. The structure, morphology, composition, optical and photoelectrochemical properties of the resultant photoanodes were investigated in detail. The presence of GO in the heterostructure film was confirmed by Raman analysis with D and G band intensity. Surface morphology analysis of the GO/In2S3/TiO2 NRs structure reveal the homogenous distribution of graphene oxide on In2S3/TiO2 NRs surface. From UV–Vis analysis, band gap energy of the samples decreases gradually from 3.34 eV (TiO2 NRs) to 3.12 eV, with In2S3 and GO addition. The electrochemical impedance spectroscopy (EIS) further confirmed that GO/In2S3/TiO2NRs heterostructure possessed the lowest charge-transfer resistance, revealing that In2S3 and GO could significantly accelerate the electron mobility compared with bare TiO2. From Mott-Schottky plots, several parameters such as flat-band potential and free carrier concentration were determined. Next, The GO/In2S3/TiO2 NRs electrode achieved remarkably improved current density (0.45 mAcm² at 0.8 V vs Ag/AgCl) compared to pure TiO2 NRs or In2S3/TiO2 NRs electrodes, which attributes to the uniform structure and excellent electrical conductivity of GO, which could reduce the combination rate of the photo electron-hole pairs. These results reveal that GO/In2S3/TiO2 NRs possesses great potential toward the development of newly synthesizable catalysts in the field of photoelectrochemical water splitting.
Lead sulphide (PbS), is an excellent material for optoelectronic devices owing to its tunable optical and electrical properties. PbS is largely synthesized by chemical bath deposition (CBD) method using different complexing agents. We have grown PbS thin films by CBD method using ethylenediamine tetraacetic acid (EDTA) as the complexing agent. The effects of EDTA concentration and the bath temperature were chosen for the investigation to fabricate highly crystalline PbS films. The concentration of EDTA has been varied from 0 to 40 mM and kept the other precursors as constant in the solution. The addition of 10 mM EDTA to the solution has increased the PbS film thickness to 1.3μm. EDTA has promoted the growth of (200) oriented PbS cubic crystals with decreased hole mobilities due to the reduced grain sizes. Its direct optical band gap energy decreases from 0.42 to 0.41 eV on increasing the EDTA concentration. The PbS films deposited at different bath temperatures (40–60 °C) at an optimized EDTA concentration of 10 mM showed an enhancement in the crystallinity and modified the PbS growth direction to (111) plane. The grain size predominantly increased and the shape of the grains has turned into perfect cubic crystals aligned semi-vertical to the substrate. The electrical resistivity of the films decreased from 0.79 to 0.25 Ω cm, and the hole mobilities improved from 18.3 to 46.6 cm² V⁻¹ s⁻¹ on increasing the bath temperature from 30 to 60 °C. In contrast with other complexing agents, EDTA has shown favourable conditions for the growth of large-grained PbS thin films with improved hole mobilities, which is beneficial for its use in IR photodetectors.
Lead selenide (PbSe)‐based nanomaterials have been extensively investigated as building blocks for next‐generation optoelectronic devices owing to their unique properties. In this work, PbSe nanocrystals (NCs) have been successfully fabricated by a facile liquid phase exfoliation approach and directly applied as active materials for photo‐electrochemical (PEC)‐type photodetectors (PDs). Taking advantage of broadband absorption and fast carrier dynamics, the PbSe NCs‐based PDs exhibit excellent photo‐current density (11.88 μA cm−2), photo‐responsivity (12.37 mA W−1), response/recovery time (0.12/0.13 s), and long‐term cycling stability. The working mechanism of PbSe NCs‐based PDs is explored by density functional theory calculations based on their structural and electronic properties under various conditions. It is anticipated that this contribution paves the way to readily fabricate low‐dimensional PbSe NCs and extend their practical applications in PEC‐type PDs.
A multi-step dimethylformamide (DMF)-free green synthesizing method based on (i) initial electrodeposition of lead precursor, i.e. lead sulfide (PbS) on mesoporous TiO2/fluorine-doped tin oxide (FTO) conductive glasses substrates, (ii) subsequent conversion of PbS to PbI2 and (iii) synthesis of methylammonium lead triiodide (CH3NH3PbI3) perovskite film and their microstructural, optical and solar cell performance are described. Different electrodeposition techniques including direct current and cyclic voltammetry deposition were investigated to produce PbS films. We find that the perovskite films produced based on PbS deposited by cyclic voltammetry exhibit compact layer consisting of cuboid grains with an average size of approximately 800 nm and a bandgap of 1.58 eV whose properties are comparable to those of perovskite films generally prepared by conventional methods like spin coating. It was observed that uniform perovskite layers deposited under different conditions as the absorber layer generate a power conversion efficiency (PCE) of up to 7.72 % under the standard AM 1.5 condition in the first attempt by this fabrication approach. PCEs obtained under different electrodeposition conditions were improved by eliminating of pores between the cuboid perovskite crystallites. This approach neglects employing hazardous solvents from the routine perovskite solar cell fabrication method and has potential to enhance its PCE similar to the common strategies by spin-coating methods improved over last decade by further modification of the electrodeposition process of the metal precursors and other steps towards highly efficient green perovskite solar cells.
PbS nanocubes have been produced by a very simple solvothermal procedure that employs a unique molecule, ethylenediamine, as solvent and capping ligand to control the size and shape of nanocrystals. Detailed structural, optical and photoelectrochemical evaluation confirmed the suitability of these nanoparticles for photocapacitive applications, when synergistically combined with spin‐coated BiVO4 photoelectrodes, as derived from the estimated energy diagram. Furthermore, the p‐n junction facilitates the photo‐oxidation of PbS nanoparticles under light irradiation. In the dark, the photogenerated charges are released providing an electric output response with a solar‐to‐current efficiency of 0.042 %, storing energy extra to the H2 produced by water splitting when the BiVO4 photoanode works under illumination. This study highlights the importance of the synthetic route and methodology to ensemble materials for advanced solar energy storage applications.
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Lead and silver sulfides are materials with diverse technological applications, such as photovoltaic dispositives, electrodes and catalysts. More recent applications in the formation of quantum dots and catalysts synthesis for hydrogen evolution reactions involves these sulfides and some similar compounds. It is well known that defects, and consequently the diffusion process, can change some properties and performances of such materials. Herein, a study of defect formation and ionic diffusion process were performed in PbS and Ag2S lattice structure. The activation energies for diffusion of Ag+, Li+, and H+ in PbS and Ag2S lattices were calculated at the DFT level and diffusion coefficients were estimated within the Arrhenius approximation. Pb-doped Ag2S exhibited a very small activation energy and an anomalous high diffusion coefficient for Ag+ diffusion, which is in good agreement with experimental data. Less evident is the fact that Li+ present lower diffusion coefficients than Ag+. For H+ diffusion in PbS lattice, the process is completely different. The hydrogen moves over the lattice from a sulfur site to the other, apparently cleaving and forming H-S bonds. The present work can provide more understanding of the features of defect formation and solid state ionic diffusion mechanism in sulfides from an atomic perspective. Hopefully, this results can help experimentalists in the development of more efficient materials for some important technological applications.
Ternary semiconductor Cu4SnS4 films have been prepared using electrode position method. During the experiment, thin films were studied under different parameters such as temperature, time, pH, concentration of solution. Characterization of films were carried out using various tools such as XRD, UV-Visible spectro photometer, Atomic force microscopy. Lastly, the optimum conditions were reported.
Tin-based chalcogenide semiconductors, though attractive materials for photovoltaics, have to date exhibited poor performance and stability for photoelectrochemical applications. Here, a novel strategy is reported to improve performance and stability of tin monosulfide (SnS) nanoplatelet thin films for H2 production in acidic media without any use of sacrificial reagent. P-type SnS nanoplatelet films are coated with the n-CdS buffer layer and the TiO2 passivation layer to form type II heterojunction photocathodes. These photocathodes with subsequent deposition of Pt nanoparticles generate a photovoltage of 300 mV and a photocurrent density of 2.4 mA cm⁻² at 0 V versus reversible hydrogen electrode (RHE) for water splitting under simulated visible-light illumination (λ > 500 nm, Pin = 80 mW cm⁻²). The incident photon-to-current efficiency at 0 V versus RHE for H2 production reach a maximum of 12.7% at 575 nm with internal quantum efficiency of 13.8%. The faradaic efficiency for hydrogen evolution remains close to unity after 6000 s of illumination, confirming the robustness of the heterojunction for solar H2 production.
Pure and calcium doped lead sulfide (PbS) thin films were prepared by chemical bath deposition (CBD) method. The effects of calcium doping content on their structural, morphological and optical properties were investigated by X-Ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and UV-visible spectroscopy measurements, respectively. According to X-Ray diffraction patterns all films were polycrystalline in nature, with a cubic crystal structure. It was observed from SEM analysis that calcium doping level affected the film morphology and surface roughness. Incorporation of Ca was confirmed from Energy dispersive X-Ray analysis results. Tauc method was used to estimate the optical band gap energies (Eg) of the pure and calcium doped films. UV-visible analysis showed that the transmittance of the samples varied between 15.3 and 20%. Also, it was observed that transmittance of deposited films increased with adding of calcium. Further, the optical energy band gap values were found to enhance (1.92–2.50 eV) with increasing the Ca content in PbS films.
CuInSe2 nanoparticules (CIS-NP) were synthesized on ITO-coated glass substrate by electrodeposition and rapid thermal processing (RTP). The as-deposited films were annealed under argon atmosphere at 250 °C, 350 °C and 450 °C using RTP during a short annealing time. The latter is practicable to avoid further losing of the Se content in CIS films. In order to analyze the effect of annealing temperature, the structural, morphological, optical and electrical properties were investigated by means of X-ray diffraction, scanning electron microscopy, UV–Visible Spectroscopy and Mott-Schottky plots respectively. XRD results show that elaborated films have a tetragonal chalcopyrite CIS with preferential orientation along the (112) orientation. The phase formation of CIS-NP with good crystallinity was observed at low annealing temperature. Optical absorption studies indicate a direct band gap around 1.02 eV at 250 °C. The optical constants such as refractive index n(λ) and extinction coefficient k(λ) were estimated using an appropriate optical model. To determine the doping type of elaborated semiconductor, its flat band potential and the free carrier concentration we used the Mott-Schottky plots. A new attempt to anneal the electrodeposited CIS films by short annealing duration using RTP process was proved to be a useful method to synthesize polycrystalline CIS films for solar cell application.
Deposition of lead sulfide (PbS) nanocrystalline thin films onto conducting fluorine-doped tin oxide (FTO) glass has been performed by cyclic voltammetry (CV) in 1.5 mM solution of lead nitrate and sodium thiosulfate at 100 mV s⁻¹ scan rate in the potential range of −1.0 V to 0.0 V versus saturated calomel electrode. X-ray diffraction analysis and scanning electron microscopy revealed formation of cubic PbS crystals with size of 100 nm to 150 nm after 50 cycles. High electrocatalytic activity of the synthesized PbS film for the S²⁻/Sn²⁻ redox couple, used as a mediator for quantum dot solar cells (QDSCs), was demonstrated by electrochemical impedance spectroscopy and CV measurements. The prepared PbS/FTO was used as a counterelectrode to fabricate PbS-QDSCs with a photoanode consisting of CdS/CdSe quantum dots adsorbed on mesoporous TiO2 film and a polysulfide solution electrolyte. The performance of the PbS-QDSC was compared with a QDSC with a platinum counterelectrode (Pt-QDSC). It was found that, using the same fabrication conditions, the performance of the PbS-QDSC was better than that of the Pt-QDSC. At 1 sun (100 mW cm⁻²) simulated light, average energy conversion efficiency of 2.14%, short-circuit current of 9.22 mA cm⁻², open-circuit potential of 0.50 V, and fill factor of 0.47 were achieved by the fabricated PbS-QDSC.
Preliminary results regarding the electrochemical behavior of Se(IV) in sodium citrate solution on a polycrystalline SnO2 electrode are presented. A schematic diagram of the energy levels for an n-type SnO2 electrode in contact with the working electrolyte was constructed. The results of cyclic voltammetry (CV) show that SeO
32− reduction occurs through a surface state in a complex multistep pathway. After a minute amount of Se is deposited, the electrode is expected to behave locally like a Schottky diode in contact with the solution. CV analysis and current transient results indicate that selenium growth on the polycrystalline SnO2 surface proceeds without nucleation. A thin Se film obtained at a potential of −0.8 V vs. Ag|AgCl, KCl(sat) was analyzed by lateral force and atomic force microscopy.
The effect of thiourea on the electrochemical nucleation of tin on a copper substrate from a sulfate bath was studied using voltammetry, chronoamperometry, electrochemical impedance spectroscopy, and scanning electron microscopy. Without thiourea, electrodeposition of tin showed very
poor surface coverage. However, re-nucleation and growth of tin occurred after the addition of thiourea. In particular, very rapid re-nucleation and growth behavior of tin were observed when up to 6 g/L of thiourea was added. Furthermore, impedance analysis allowed the estimation of the change
in the growth behavior of tin when up to 6 g/L of thiourea was added.
A good deal of information regarding the synthesis and opto–electro-structural properties of thin films of lead chalcogenides have been revealed. The development of laser technology had opened up new application for narrow gap lead salts and their alloys. The polycrystalline thin films were deposited onto optically plane and chemically clean glass substrates by vacuum evaporation technique. The films were thin, uniform, smooth and tightly adherent to the substrates.Optical absorption spectroscopy, X-ray diffraction technique and current–voltage characteristics method were used to characterize the films. The absorption coefficients and optical band gaps of films were determined by using FTIR spectrophotometer. The nature of sample, crystal structure and lattice parameters of films were found from X-ray diffractograms. The dc conductivities and activation energies of films were measured in temperature range 300–380 K. Schottky junctions of PbS, PbSe and PbTe with indium metal were made. The barrier heights and ideality factors of these metal–semiconductor junctions were determined by using I–V characteristics.
The impedance of several nearly ideally polarizable semiconductor electrodes (CdSe, CdS, TiO2) has been studied as a function of applied voltage and of frequency. The differential capacitance and resistance appear to obey to simple frequency laws. A mathematical model accounting for these laws is presented, in which a distribution of time constants is assumed, associated with dielectric relaxation phenomena in the double layer at the semiconductor/electrolyte interface. The experimental results indicate, that the source of the frequency-dependence has to be sought in structural irregularities of the surface region of the electrode. The proposed model appears to be also applicable to results, mentioned in the literature, concerning other semiconductor/electrolyte systems. The possibility of determining the flat-band potential from frequency-dependent capacitance data is discussed.Die Impedanz verschiedener fast ideal polarisierbaren Halbleiterelektroden (CdSe, CdS, TiO2) wurde in Abhängigkeit von der angelegten Spannung und der Frequenz untersucht. Die Differentialkapazität und der Differentialwiderstand zeigen einfache Frequenzabhängigkeiten. Diese Gesetze werden aufgrund eines mathematischen Modells gedeutet, in dem eine Verteilung von Zeitkonstanten angenommen wird, die mit dielektrischen Relaxationserscheinungen in der Doppelschicht an der Halbleiter/Elektrolyt-Phasengrenze in Zusammenhang stehen. Die Ergebnisse deuten darauf hin, Strukturdefekte im Oberflächengebiet der Elektrode seien für die Frequenzabhängigkeit verantwortlich. Es zeigt sich, daß das vorgeschlagene Modell auch auf Literaturergebnisse betreffs anderer Halbleiter/Elektrolyt-Systeme anwendbar ist. Die Möglichkeit, das Flachbandpotential aus frequenzabhängigen Kapazitätsergebnissen zu bestimmen, wird besprochen.
Investigation of the anodic oxidation of galena by cyclic voltammetry in stirred and quiescent solutions indicates that, in acid media, lead ions are dissolved leaving a surface layer of sulphur. It is concluded that the sulphur produced by the electrochemical reaction is of an amorphous or plastic form which inhibits further oxidation of the galena surface and that the sulphur film converts by a nucleation and growth mechanism to a crystalline form which is sufficiently porous to allow continued oxidation of the underlying galena.
PbS films have been prepared by reactive evaporation for the first time. Films prepared onto room temperature substrates are amorphous in nature. Increase in substrate temperature makes the films polycrystalline. These films are p-type. Hall effect measurements show that the films have a carrier concentration of ≈ 6 × 1017cm−3 and that the mobility is temperature activated. These films show high thermoelectric power.
The science describing semiconductor-liquid interfaces is highly interdisciplinary, broad in scope, interesting, and of importance to various emerging technologies. We present a review of the basic physicochemical principles of semiconductor-liquid interfaces, including their historical development, and describe the major technological applications that are based on these scientific principles. 205 refs., 27 figs., 1 tab.
Two lead tert-butoxide complexes (1)[Pb(Obut)2]m(m= 3 or 2 in the solid and gaseous phase, respectively) and (2) Pb4O(OBut)6 were used as precursors for atomic layer epitaxy (ALE) deposition of PbS thin films. The growth on soda lime glass, with and without an alumina coating, was studied by varying the source furnace and the substrate temperatures as well as the total number of cycles. Pb(thd)2(3) and Pb(dedtc)2(4) were used for comparison while H2S served in all experiments as the sulfur source. The films obtained were smooth and generally highly crystalline. The substrate temperature had a strong effect on the growth rate of PbS thin films. Nevertheless, in the self-controlled region of ALE growth the tert-butoxide complexes gave a significantly higher growth rate than the other source chemicals, with a maximum of 0.9 Å per cycle at 150 °C. Upon sublimation 1 is converted to 2, which contains four Pb atoms in a tetrahedral arrangement; this may cause the higher growth rate. Thermogravimetry/differential thermal analysis curves and mass spectrometric data were measured for all precursors. As the butoxide and thd complexes (1–3) are thermally unstable the useful ALE prosessing windows (temperature/pressure) are narrow compared to the much more stable dedtc complex (4).
PbS is successfully synthesized in aqueous solutions by photochemical reactions. The solution contains Pb(CH3COO)2 and Na2S2O3, and pH is controlled in an acidic region by adding H2SO4. The solution is irradiated with light from a high-pressure mercury-arc lamp. The PbS particles formed by the homogeneous nucleation in the solution are micro-crystalline and have nearly stoichiometric composition. PbS thin films are also formed on a Pd-treated glass sheet by the heterogeneous nucleation.
The lead(II) ion-selective ceramic membrane electrode developed by sintering a mixture of lead, silver, and cuprous sulfides showed sensitivity, selectivity, and other response characteristics well suited to analytical utilization. The Nernstian slope was obtained over a concentration range from 10−1 to 10−6M Pb2+ in activity, and the analytical range had a concentration of 10−1–10−7M when the membrane contained less than 30 wt% of cuprous sulfide and more than 1 wt% of lead sulfide. Among the common ions, silver, cupric, mercury(II), ferric, sulfide and iodide ions interfered seriously. About 10 times as many cadmium and bromide ions and more than 1000 times as many alkali metal, alkaline earth metal, zinc, aluminum, nickel, manganese(II), cobalt, and nitrate ions did not interfere with the lead ion, however. The electrode potentials did not change over a pH range from 2 to the pH at which the precipitation of lead hydroxide occurred. The electrode was safely used at temperatures from 0 to 95°C, and the potentials of the membrane satisfied the Nernstian equation within the limits of experimental error. The membrane electrode responded to activity changes very quickly: the rate of those changes was twice that in a lead sulfide-silver sulfide two-component ceramic electrode. A continuous potential measurement for 5 months promised long-term stability and accuracy. The rapid response rate and the long lifetime suggest that the continuous monitoring of some changing systems is feasible.
A theoretical model is developed for evaluating a semiconductor electrode wih a deep impurity level. An expression is derived for the capacitance as an explicit function of the applied bias, and a computer simulation is made to reproduce the experimentally observed C-V characteristics. It is shown in an analytical manner that a relaxation effect of a deep level causes the slope of the Mott-Schottky plot to be frequency-dependent. It is pointed out that, if a deep level lies just below the bulk Fermi level, the resultant C(-2)-V plot is approximated by two inflecting straight lines and a nominal inflecting potential does exist that does not, however, give precise information about the energy level of a deep level. If an extremely low a-c signal is used, the C(-2)-V plot will show an abrupt drop at a critical potential beyond which charges on a deep level can respond to an a-c signal.
It is demonstrated that impedance spectroscopy can be used for the energetic characterization of the semiconductor/electrolyte interface. Furthermore, by means of several examples, it is shown how the mechanisms of electrochemical reactions are studied by the electrical impedance technique. Also the use of the opto-electronic admittance method for the investigation of photoelectrochemical processes is discussed.
Flat band potentials (Vfb) of In2S3 polycrystalline thin films obtained by chalcogenization of electroplated metallic indium films on Ti substrates with a flowing stream of H2S gas have been obtained. The variation of this potential with different redox couples, solution concentration and pH values has been studied. Photoelectrochemical characterization of the electrodes was accomplished in aqueous polysulphide solutions and the application of the Gärtner—Butler model to the semiconductor/electrolyte interface makes it possible to obtain the semiconductor energy gap. The value obtained is 2.06 eV, corresponding to a direct allowed transition.
Unique characteristics of semiconductor electrodes are their large photosensitivity and the fact that an applied potential can penetrate quite sizeably into the electrode material (depletion layer ∼ 1 μm). These peculiar features have favored the development of many specific impedance or transient techniques. The classical electric impedance is mostly dominated by the capacitance of the depletion layer in the semiconductor, allowing for the determination of the flatband potential. More complex behaviors are often associated with the occurrence of surface states in the forbidden gap. Opto-electric impedance techniques include photocurrent and photopotential response to a modulated illumination. They provide additional information on the interface kinetics. Electro-optic impedance techniques include absorption and luminescence response to a potential modulation. Infrared absorption response provides detailed information on the nature of the charge being diplaced upon potential modulation (free carriers or surface states). The concentration of free carriers near the interface may also be probed by the surfrace conductivity, measured at optical, microwave or d.c. frequencies. These “parallel” probes can be used in response to a light modulation or a potential modulation. Since they give a direct measure of the charge (and not the current) they may be especially interesting at the lower frequencies.
The cathodic reduction of natural samples of high-purity galena has been studied in both acidic and alkaline solutions. The products of the reaction are lead metal and, depending on the pH value, the various forms of sulphide ion. Linear-sweep voltammetric measurements in solutions of various pH values have been used to obtain current-potential data from which possible mechanisms for the reaction have been derived. Results are also presented that show that the cathodic reduction of anodically produced sulphur is possible, but that the rate of reduction is strongly dependent upon the amount of sulphur present and its history.
A chalcopyrite CuFeS2 electrode obtained from the “El Teniente” mine has been studied by Electrochemical Impedance Spectroscopy (EIS) in an alkaline solution for different oxidation potentials. The experimental results can be interpreted from a Randles equivalent circuit, Vdc0.4V vs. SCE. From these results, the variation with the d.c. applied potentials of charge transfer electrical resistance of the redox reaction, the double layer capacitance and other characteristic parameters are considered.
The electrodeposition of PbS films from an acidic solution containing Na2S2O3 and Pb(NO3)2 has been investigated. The cathodic deposition mechanism and the composition of the films deposited were examined by cyclic voltammetry, X-ray diffraction measurement and X-ray photoelectron spectroscopy measurement. PbS films have been deposited at −0.60 V vs. Ag / AgCl from an acidic solution with a pH value less than 3.0, containing 1 mM Na2S2O3 and 1 mM Pb(NO3)2. In the cathodic electrodeposition of PbS films from an acidic solution, colloidal sulfur played an important role.
The electroreduction of sulfur dioxide has been studied by chronopotentiometry, controlled-potential coulometry, and cyclic voltammetry. The results of these investigations lead to the conclusion that dissolved sulfur dioxide undergoes a reversible two-electron reduction to sulfoxylic acid. At pH values less than pH 3 the sulfoxylic acid is rapidly decomposed into elemental sulfur and sulfur dioxide. Between pH 3 and pH 7 the sulfoxylic acid reacts with bisulfite ion to form dithionous acid, which is slowly decomposed to sulfur dioxide, thiosulfate ion and sulfur. Hence, the final reduction product of sulfur dioxide is sulfur and the overall reaction is irreversible. Above pH 7 no reduction of sulfur dioxide is observed.
The impedance of the photo-electrochemical cells using TiO2 thin film electrodes was investigated by means of complex admittance spectra within the frequency range 0.01Hz–100kHz, and in the presence of an electrical bias between −0.7 and 0.4V. Plausible equivalent circuits are discussed. It was found that the impedance can be divided into two frequency ranges. Based on the determined capacitance vs. bias voltage, both flat band potential and density of donors of the TiO2 electrode have been estimated. The flat band potential with respect to a saturated calomel electrode and the density of donors were −0.64V and 3.7×1024m−3, respectively. The thickness of the depletion layer corresponding to 1V voltage was equal to 46nm.
The electrodeposition of lead sulphide on polycrystalline lead from aqueous sulphide solutions has been investigated. At low deposition potentials, a unit cell of galena oriented on the (110) plane is deposited through a two-dimensional nucleation and growth mechanism. Non-linear two-dimensional growth rates have been observed, possibly caused by impedements originated in the grain structure of the underlying substrate. High field growth of a thicker lead sulphide film occurs at higher potentials, and dielectric breakdown was observed at fields of ca 4 × 10+6 V cm±1. The electrochemical reactions attending the film formation process are discussed on the basis of variations in the voltammetric peak potentials with sulphide concentration and pH.
The frequency dependence of the semiconductor∣electrolyte interfacial capacitance may be simulated by a parallel connection of a capacitor—representing the semiconductor space-charge layer—and a constant-phase element. In this paper, we present an experimental impedance study of both n- and p-InP and n- and p-GaAs in H2SO4-containing solutions. It is shown that the frequency dependence of the interfacial capacitance is related to the conductivity of the electrolyte and to the pretreatment of the semiconductor surface. These experimental results enable a thorough discussion on the possible origins of the dispersion phenomenon. It is argued that frequency dispersion originates from localized states at the semiconductor surface, interacting with the electrolyte. From the experimental results, it follows that the surface states involved probably originate from surface damage, whereas surface roughness most likely determines the relation between dispersion and electrolyte conductivity.
Simple electrostatic theory describes a blocking semiconductor∣electrolyte contact as a perfect capacitor. This idealized behaviour of the interface does not correspond to the experimental fact that the capacitance of a semiconductor∣electrolyte contact very often depends on the measuring frequency (so-called ‘frequency dispersion’). The nature of this discrepancy remains largely unknown. In this paper, we show for n-InP and n-GaAs that the imperfect capacitor which a semiconductor∣electrolyte contact constitutes may be described as a parallel connection of a perfect capacitor, corresponding to the semiconductor space-charge layer, and a constant-phase element, describing the frequency dependence of the capacitance of the semiconductor∣electrolyte contact. Based upon the proposed equivalent circuit, the various interpretations of the frequency dispersion found in the literature are discussed. It is concluded that surface states may play a major role in the dispersion phenomenon.
Lead sulfide thin films were grown at room temperature by the successive ionic layer adsorption and reaction (SILAR) technique on soda lime glass, ITO and Al2O3 covered glass, SiO2, (100)Si and (111)Si substrates. SILAR utilises sequential treatment of the substrate with aqueous precursor solutions. Dilute solutions of lead acetate and thioacetamide were used as precursors for Pb2+ and S2–, respectively. The lead precursor solution also contained triethanolamine (tea) as a complexing agent, with a Pb: tea mole ratio of 1 : 2. On glass the growth rate was 0.12 nm per cycle with 0.2 mol dm–3 lead and 0.4 mol dm–3 thioacetamide solution. The appearance of the films was metallic. X-Ray diffraction studies revealed a strong [200] orientation of the films. According to the Rutherford back-scattering (RBS) and nuclear reaction analysis (NRA) results the films were stoichiometric PbS and contained small amounts of some lighter impurities, possibly O and H. Scanning electron microscope (SEM) images revealed that the films were rather rough and consisted of grains with a diameter approximately corresponding to the thickness of the film.
Current—voltage and capacitance—voltage measurements at the n- and p-InP single crystal electrode in contact with indifferent electrolyte solution show that, for both types, a depletion region is present over a certain voltage range. The results allow the determination of the flat-band potential, of the energetic position of the band edges at the electrode surface and of the band-bending at zero current. These data lead to energetic considerations on the applicability of the InP electrode in the photoelectrochemical decomposition of water.
Deposition of lead sulfide thin films has been carried out from an aqueous solution using a novel electrochemical method which includes alternate anodic and cathodic, reactions. Na2S and Pb(CH3COO)(2) complexed with ethylenediaminetetraacetic acid were used as precursors. During the deposition the substrate potential was continuously cycled between two potentials. At the cathodic potential elemental Pb was deposited and at the anodic potential it was reoxidized and reacted with sulfide forming a PbS film. From basic solutions at pH 8.5, PbS films were deposited only with the cyclic method, but with acidic solutions at pH 5 also the conventional constant potential method could be used. The films were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometry, wavelength-dispersive X-ray spectrometry, Rutherford backscattering spectrometry, elastic recoil detection analysis, deuteron induced reactions, and profilometry. The electrodeposited PbS films were stoichiometric and crystalline, having nontextured rock salt cubic structure.
PbS thin films were deposited on glass slide substrates using the chemical bath deposition technique. The films were obtained in a reaction bath at temperatures of 10, 15, 20, 25 and 30 °C. The structure and surface morphology of the films were studied by X-ray diffraction and by atomic force microscopy measurements. The optical properties were determined from spectroscopy measurements of ellipsometry, transmission and reflection, in the energy range of 240–840 nm. In order to analyze the ellipsometry measurements, two models for the dielectric function of the films were considered, the Bruggeman's effective medium approximation and the Lorentz oscillator expression. From this analysis, the complex dielectric function ε(E)=ε1(E)+iε2(E), thickness, roughness and void fraction of the films were examined as a function of temperature deposition. With the model obtained in the ellipsometry analysis, the optical spectra of reflection and transmission were calculated and compared with the measured spectra finding a good agreement.
Nickel and nickel–manganese alloy modified graphite electrodes (G/Ni and G/NiMn) prepared by galvanostatic deposition were examined for their redox process and electrocatalytic activities towards the oxidation of methanol in alkaline solutions. The methods of cyclic voltammetery (CV), chronoamperometry (CA) and impedance spectroscopy (EIS) were employed. In CV studies, in the presence of methanol NiMn alloy modified electrode shows a significantly higher response for methanol oxidation. The peak current of the oxidation of nickel hydroxide increase is followed by a decrease in the corresponding cathodic current in presence of methanol. The anodic peak currents show linear dependency upon the square root of scan rate. This behavior is the characteristic of a diffusion controlled process. Under the CA regime the reaction followed a Cottrellian behavior and the diffusion coefficient of methanol was found to be 4 × 10−6 cm2 s−1. A mechanism based on the electro-chemical generation of Ni3+ active sites and their subsequent consumptions by methanol have been discussed and the corresponding rate law under the control of charge transfer has been developed and kinetic parameters have been derived. The charge transfer resistance accessible both theoretically and through the EIS have been used as criteria for derivation of the rate constant.
Nanostructured thin films of lead sulfide have been synthesized by a new electrochemical approach based on the underpotential deposition (UPD) of Pb and S from the saturated solution of PbS containing excess of PbS particles as a source of Pb2+ and S2− at various temperatures.We have demonstrated that this new electrochemical route is a simple method with several advantages, including better control of the growth conditions and a one-step process to obtain the nanostructures of PbS. Scanning probe microscopy studies indicate that the growth of PbS nanofilms follows a two-dimensional layer-by-layer growth kinetics at the beginning of electrodeposition but a three-dimensional growth dominates after the formation of the first few layers. The results of morphological and structural investigations reveal that PbS nanostructures grown by this method are single-crystalline in cubic structure and have a preferential orientation along the [2 0 0] direction. The optical absorption spectra of PbS nanostructures show the blue-shift with respect to those of the bulk counterpart, which are attributed as quantum-size effect.
The electrochemical oxidation of lead sulfide is studied by using a rotating ring-disk electrode. The effect of pH, light intensity, and mass transfer on the dissolution of PbS(s) is examined. The major oxidation reaction is: PbS(s) → Pb2+ + S(s) + 2e-, which is independent of pH. However, the elemental sulfur formed from this reaction inhibits further PbS(s) oxidation. The semiconducting properties of lead sulfide cause the oxidation to be photosensitive. The presence of aqueous lead inhibits the dissolution, and diffusion of released Pb(II) from the surface may control the reaction. The type of anion in the supporting electrolyte influences the dissolution.
In semiconductor electrochemistry there is considerable confusion concerning the potential distribution at the semiconductor/solution interface under weak depletion and accumulation conditions. The applied potential is partitioned between the space charge layer in the semiconductor and the Helmholtz layer on the solution side of the interface. Under deep depletion conditions, a change in the applied potential usually appears across the space charge layer and the band bending can be determined using the Mott−Schottky relation. Under conditions of weak depletion or accumulation, however, the applied potential is partitioned between the two double layers and determination of band bending is not straightforward. In this paper, expressions for the dependence of the band bending on the applied potential are derived and the consequences for charge-transfer processes are discussed.
Films of AOT-capped (AOT = dioctyl sulfosuccinate) lead sulfide nanoparticles (Q-PbS) were prepared by incorporation into a self-assembled monolayer of hexanethiol on Au. The Q-PbS particles showed either cathodic or anodic photocurrents, depending on the existence of a hole scavenger or an electron scavenger in the contacting solution, on the state of the surface, and on the electrode potential. The electrochemistry of Q-PbS indicated that the anodic dissolution of Q-PbS occurred at about 0.25 V vs SCE and was not particle size dependent, while the cathodic corrosion potentials of Q-PbS ranged from −1.1 V vs SCE to beyond the hydrogen evolution potential and depended on the size of the Q-PbS particles.
The deposition of PbS thin films from chemical baths is significantly accelerated when exposed to solar radiation or other sources of illumination. The enhanced deposition rate is seen to be a cumulative effect of an increase in the bath temperature due to photothermal conversion as well as to photoactivated deposition at the PbS film. For deposition employing a 1:1 ratio of Pb2+:thiourea (TU) in the bath, a film thickness of approximately=0.09 mu m, corresponding to an integrated reflectance in the visible region of approximately=17% and producing a purple appearance in reflected daylight, is obtained in 40 min under solar radiation of 750 W m-2 as opposed to 80 min inside the laboratory (26 degrees C) and to 55 min when the bath temperature simulates the variation in temperature (i.e. up to 36 degrees C) observed in the bath under solar radiation. The time lag between the three cases can be varied by varying the bath composition. Novel applications of the photoaccelerated deposition process for producing a variably reflective glaze (golden, purple, bluish, etc.) on the glass surface for decorative coatings and to obtain pleasantly reflective latent images, such as a purple image on a golden background, are presented.
Polycrystalline thin films of lead sulphide were electrodeposited on titanium, aluminium and stainless steel (SS) substrates at a constant potential of −0.7V vs. Ag|AgCl|(sat)KCl electrode using 1mM solutions of Pb(NO3)2 and Na2S2O3 at pH 2.7, 2.8 and 2.9 Except at pH 2.9 (which also gave tetragonal PbS2) all other pHs gave single phase PbS (fee). While deposition on Al gave a crystalline phase of PbS with very prominent (200) and (111) planes, that on SS substrate showed growth of the (200) plane only. Although the deposition of PbS on Ti was good. it showed (111) and (200) planes of low intensity compared to that on Al. Film thickness and grain size were found to be of the order of micrometres. The cyclic voltammetry of the film formation was studied in a potential range −1 to 0V (Ag|AgCl) followed by XRD, SEM and AFM analyses. The mechanism of growth is discussed.
Metal sulphide thin film (MSTF) photography based on photo-accelerated chemical deposition (PACD) of PbS thin films is described. Here an intensity distribution over the surface of a growing PbS thin film produces a thickness variation (0.06–0.15 μm) of the film which, when viewed under daylight, yields a specularly reflective image. Under 800 W m−2 of solar radiation a bluish MSTF photographic image (0.15 μm film thickness) on a coppery-bronze background (0.08 μm) is obtained in the PACD of PbS at the end of 25 min deposition when a high-contrast photographic negative is used as the object. The best contrast of 0.46 in the PbS MSTF photography in the reflection mode is obtained under the above condition of exposure when the optical transmission in a photographic negative in the image area is ∽30% and that in the background is ∽1%. The contrast available in the transmission mode in the MSTF photographic image is considerably less: ∽0.28 (maximum) for optical transmission of 1% and 100% in the background and image areas of the photographic negative respectively.
Preparation of lead sulfide thin films on different substrates by the Atomic Layer Epitaxy process has been studied. Sulfur source was in all experiments H2S, but as lead source the following compounds were tested: bromide, iodide and acetate as well as thd (2,2,6,6-tetramethyl-3,5-heptanedione) and diethyldithiocarbamate chelates. The last complex gave the highest growth rate. The growth experiments were carried out at 300–350°C and the film thickness varied between 0.1 and 1 μm. The films were characterized by XRD, XRF, SEM, and by photoconductivity and Hall measurements. The results showed that the films were polycrystalline and randomly oriented. The conductivity was p-type and the carrier concentration and mobility were comparable with those found in films deposited by traditional chemical methods.
Lead sulfide (PbS) nanoparticle films were chemically grown on glass, quartz and silicon substrates. Structure and size of PbS nanoparticles were characterized by X-ray diffraction and transmission electron microscopy (TEM), respectively. Large optical band gap has been observed in these films. The decreases in dc-conductivity, Hall mobility and carrier concentration with reducing grain size were also examined. Heterojunctions of p-PbS/n-Si were fabricated and photovoltaic effect was observed in these self-assembled heterojunctions.
Electrochemical quartz crystal microbalance combined with cyclic voltammetry was used to study the electrodeposition of PbS thin-films on Au surface. The electrodeposition was carried out by cycling between two potentials with voltage scan rates of 0.1 and 0.01 V/s. In addition, the electrochemical behaviour of PbS film and the related precursors, PbEDTA2− and HS− were studied separately. It was found that the reduction of PbEDTA2− occurred at a more positive potential on the PbS surface than on the Au surface. Similarly, HS− oxidised at a more negative potential on the PbS than on the Au surface. Therefore, the deposition of PbS was proposed to occur by two mechanisms. The first mechanism includes the reduction of PbEDTA2− to elemental lead which during the oxidation reacts with HS−. In the other mechanism, PbS surface induces the chemical reaction between PbEDTA2− and HS. The electrodeposited PbS is stable in the potential range of −1.2 to −0.2 V vs. Ag/AgCl.
The luminescence behavior of a PbS quantum dots (QD) in the near-IR spectral range was investigated. The PbS QDs were prepared using a water-based synthesis. The x-ray diffraction, and transmission electron microscopy (TEM) were used to study the optical properties of the QDs. The quantum dots combined stable, spectrally tunable luminescence from 1000 to 1400 nm. the fluorescent tags were observed to have a high quantum yield of photoluminescence and efficiencies that were largely independent of environment.
The frequency dependent capacitance of semiconductor-electrolyte junction and its relationship to the surface roughness of the semiconductor and the ions in the electrolyte are discussed. Due to very low mobility of the ions, the observed capacitance can be dominated by the Helmholtz double-layer of the electrolyte rather than the space charge layer of the semiconductor. The capacitance will also depend on the frequency. This, often observed power-law frequency dependence of capacitance is ascribed to the contribution of constant phase angle impedance. The power-law exponent can easily be related to the fractal dimension if the semiconductor surface can be described by fractal geometry.
The frequency dependence of the impedance of polarizable semiconductor/metal and semiconductor/electrolyte solution interfaces is reconsidered. No frequency dispersion of the polarization capacitance is found with n-GaAs/Au contacts whereas the capacitance of n-GaAs/electrolyte solution interfaces show considerable dispersion. The frequency dispersion depends on the microroughness of the electrode surface and on the specific conductivity of the electrolyte solution. As for polarizable metal electrodes, the relationship between the capacitance and the frequency is closely related to the relationship between the capacitance and the specific conductivity of the electrolyte solution. It is concluded that the main origin of frequency dispersion should not be sought in the solid but is related to the development of the electric double layer at the electrolyte side of the interface. A model is presented to account for these results.
Single crystal GaInP2 and Ga1−xAlxAs-aqueous electrolyte interfaces were investigated utilizing electrochemical impedance spectroscopy. Both materials have band gaps that are much larger than the theoretical free energy required to decompose water (1.23 eV) and may do so depending on the energetic positions of the band edges. Impedance spectra (500 μHz-100 kHz) were used to model the interface in terms of a space-charge layer capacitance, a constant phase element and oxide film capacitance in some cases, and associated resistances. The flat-band potential and carrier concentrations were determined from Mott-Schottky plots. It was found that the corresponding band edges of both materials fall short of encompassing both the hydrogen and oxygen redox levels and are thus not capable of splitting water in the absence of an external bias.
The significance of the flat-band potential and the energetic position of the band edges at the semiconductor/electrolyte interface in semiconductor electrochemistry and photoelectrochemistry is pointed out. Different methods for determining these parameters experimentally are discussed, such as methods based on the measurement of the photovoltage or photocurrent, as well as the method for determining the flat-band potential from interfacial capacitance measurements. The capacitance-voltage relationship of the ideal semiconductor/electrolyte Schottky barrier is described. Subsequently, possible complications of the capacitance behavior are discussed, and conditions indicated under which the determination of the flat-band potential from non-ideal capacitance results is still possible. A critical survey is then given of flat-band data for some selected semiconductor electrodes (ZnO, CdS, GaP, GaAs, TiO2, SrTiO3), comprising a discussion of problems encountere, factors on which the flat-band potential depends and discrepancies between different results. Attempts to predict the flat-band potential and the position of the band edges from atomic electronegativity data are reviewed. The relationship between flat-band potential or band-edge position and electrochemical behaviour is considered, i.e., as far as the magnitude of the photovoltage as well as the electrochemical and photoelectrochemical reactivity are concerned.
Impedance spectroscopy is employed in a study on the electrical and optical properties of boron phosphide (BP) electrolyte interfaces. With this technique both the space charge capacitance can be determined and the charge trapping in surface states can be investigated. The flatband potential of boron phosphide and the corresponding band-edge positions are found to be considerably more positive than the related III-V semiconductors GaP, GaAs, and InP. As a consequence, BP is found to be more stable for photo-etch reactions than any of these materials. It is shown that the band positions of GaP, GaAs, InP, and BP can be predicted from the atomic electronegativities fairly accurately. In neutral and alkaline electrolytes the surface of BP is free of native oxide and the surface state density is low. In acid electrolytes, however, a native oxide is present on the surface of BP. This oxide is probably hexagonal B2O3 and is observed to be able to cause a large shift of the bandedges. In the latter electrolytes the Mott-Schottky plots of BP are heavily distorted by the presence of additional surface state capacitances.
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