Potential dependent growth of Cu(OH)2 nanostructures on Cu and their thermal conversion to mixed-valent copper oxides p-type photoelectrode

Herein, we report the anodic growth of nanostructured Cu(OH)₂ thin films on the copper surface by potentiostatic and potentiodynamic methods. The phase formation, crystalline feature, morphology and the progressive growth of Cu(OH)₂ thin films were controlled by the applied potential and concentrations of alkali utilized in anodization process. Electrochemical investigations suggest the rapid growth of Cu(OH)₂ and passivation at higher anodization potentials. In contrast, the lower anodization potentials favours the progressive growth of Cu(OH)₂ nanorod like features. The thermal treatment of Cu/Cu(OH)₂ reveals the formation of crystalline mixed copper oxide film with predominantly Cu₂O phase at 673 K and 773 K. Photoelectrochemical investigations of these copper oxide thin films exhibit the p-type behavior with repeatable photovoltage (55 mV) and stable photocurrent responses (20–60 μA cm⁻²). The hydrogen evolution studies show better activity with nanostructured Cu(OH)₂ and copper oxides than with aggregated thin films and bare substrate.     

Nanostructured CuWO4/WO3-x films prepared by reactive magnetron sputtering for hydrogen sensing

High-purity films consisting of copper tungstate (CuWO₄) and sub-stoichiometric tungsten oxide (WO_3-x) were prepared by reactive sputter deposition. An original two-step deposition process was applied for their synthesis. First, a tungsten oxide thin film was deposited by dc magnetron sputtering from a W target in an Ar + O₂ gas mixture, afterward, rf sputtering of a Cu target in an Ar + O₂ gas mixture was employed to form a discontinuous CuWO₄ layer at the top. This results in a formation of nanostructured branched islands of the tungstate. Bilayers with various layer thicknesses were investigated for the sensitivity to hydrogen gas as a conductometric sensor. The sensitivity changes remarkably with the thicknesses of individual layers. The maximum sensitivity was observed for the films with a layer thickness ratio of 5 nm/20 nm. The response was enhanced more than eight times compared to a 20 nm-thick tungsten oxide alone film. An explanation based on the formation of nano-sized n-n junctions is provided. In addition, a microscopy study of the bilayer growth is presented in detail.     

Imaging the hydrogenation of Mg thin films

Among the metal hydride materials, magnesium (Mg) and its alloys show excellent performance for hydrogen storage. The main drawback is the slow hydrogen absorption and desorption kinetics, the sole barrier to commercial adoption. In this work we use Mg thin films as model materials in order to study these kinetics, and observe the growth process of the hydride. Palladium (Pd) is used as a catalyst coating for improving the conditions of hydrogenation. The hydride formation is followed by in-situ X-ray diffraction. Microscopic imaging of the co-existence of Mg and MgH₂ is presented. The microstructure change is clearly visible in the micrographs, despite the fact that sample preparation damages the hydride phase. The transformation from columnar grains of the as-deposited Mg thin film, to a grainy equi-axed structure film indicate that the hydride is observed. The hydride is immediately formed at the interface between the Pd and the Mg thin film and grows in a layer-like reaction towards the substrate (SiO₂). These combined techniques provide an efficient methodology to follow the kinetics of hydride formation within the layer, and study further the diffusion coefficients and mechanism of hydrogenation.     

Thin film n-titanium oxide photoanodes for photoelectrochemical production of hydrogen

The semiconductor photoanodes made of thin film titanium oxide were prepared at room temperature by anodization of titanium plates in the hydrofluoric acid solution at direct bias. The influence of the change of the titanium oxide film growth conditions (concentration of hydrofluoric acid, voltage, duration of anodization process) and subsequent heat treatment of films on the photocurrent and current–voltage characteristics of photoelectrodes were investigated. The influence of the electrolyte concentration change on photoelectrochemical behaviour of thin film titanium oxide photoanodes was investigated.     

Mechanistic investigation on tuning the conductivity type of cuprous oxide (Cu2O) thin films via deposition potential

The conductivity type of cuprous oxide (Cu₂O) thin films is tuned by controlling the deposition potential of an electrochemical process in an acid cupric acetate solution containing sodium dodecyl sulfate. The morphology and chemical composition of the deposited Cu₂O films are studied by SEM, XRD and XPS. The change of the conductivity type of Cu₂O films is further studied through zero-bias photocurrent and Mott-Schottky measurements. The results indicate that the Cu₂O films behave as n-type semiconductors when the overpotentials are low (potentials higher than ?0.05 V) and p-type semiconductors when the overpotentials are high (potentials lower than ?0.10 V). The transformation of conductivity from n-type to p-type comes from the competition reactions between forming Cu₂O and forming metallic Cu from Cu²⁺. When the potential is lower than ?0.10 V, most of Cu²⁺ are consumed by the growth of metallic Cu at the film/solution interface, so that the Cu²⁺ provided to grow Cu₂O film are insufficient and copper vacancies form in the film, leading to the p-type conductivity.     

CuGaxSey chalcopyrite-related thin films grown by chemical close-spaced vapor transport (CCSVT) for photovoltaic application: Surface- and bulk material properties, oxidation and surface Ge-doping

Device-grade ternary Cu-Ga-Se chalcopyrite thin films used for photovoltaic energy conversion have been prepared by a novel chemical close-spaced vapor transport (CCSVT) technique developed for a deposition on areas of up to 10×10 cm². A two-step process has been developed which allows the fine tuning of the film composition and the electronic properties. The extension of deposition times in the two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 0.9 to 5.7, allowing the study of the structural phase transitions. In this paper the main focus of interest is related to the material properties of the device-grade thin films prepared by CCSVT technique. We present our recent studies on (i) the growth, compositional, structural and electronic structural properties, (ii) the degradation under ambient conditions and (iii) the feasibility of n-type doping this p-type semiconducting material by germanium. Thin films were grown with chalcopyrite (1:1:2) and CuGaSe₂-related defect compound structures (DC) with stoichiometries of CuGa₃Se₅ and CuGa₅Se₈. In order to derive the DC structure, X-ray and neutron powder diffraction investigations have been carried out on powders of these CuGaSe₂-related compounds grown by elemental synthesis (powder) and CCSVT (thin films), respectively. We found no hints for an ordering of defects, as proposed in the past and giving name to the so-called Ordered Defect Compounds (ODC) in this and related structures. From our results a growth model is presented for CuGa₃Se₅ formation in gallium-rich CCSVT-grown CuGa_xSe_y films. The chemical and electronic surface and interface structure of CuGaSe₂ thin films with bulk [Ga]/[Cu] ratios between 0.94 and 1.39 is investigated by X-ray and UV-excited photoelectron spectroscopy (XPS and UPS, respectively). A transition of the Cu:Ga:Se surface composition from 1:1:2 for the Cu-rich bulk sample to 1:3:5 for the sample with the highest bulk [Ga]/[Cu] ratio is observed. Simultaneously, a downward shift of the valence band maximum position with respect to the Fermi energy is found. The comparison of the estimated conduction band minimum with that of CdS reveals the formation of a pronounced “cliff-like” conduction band offset at the respective interface.Furthermore, the CuGaSe₂ thin film degradation under ambient as well as under thermal conditions of CuGaSe₂ thin films has been studied by XPS. During thermal oxidation, the formation of predominantly Ga₂O₃ and some amount of SeO₂ were observed, but no copper oxides could be detected in the near-surface region of the thin films. The same oxides are found after native oxidation in air under ambient conditions. An additional sodium oxide compound formed at the thin film surface, Na_xO and Na₂CO₃ after thermal and native oxidation, respectively.Germanium ion implantation technique of the near-surface region of CuGaSe₂ thin films has been used in order to prove the feasibility of n-type doping. In photoluminescence (PL) studies, the occurrence of a new emission line is identified as Ge related and explained as a donor-acceptor-pair (DAP) recombination. The precise role the Ge is playing in this doping of CuGaSe₂ is revealed by X-ray absorption spectroscopy (XANES and EXAFS) and ab initio calculations based on the density functional theory. The studies indicate that the incorporated Ge atoms preferentially occupy Ga sites when relaxation around the dopant is taken into account. Additionally, our corresponding theoretical band structure model predicts the existence of additional localized electronic acceptor and donor defect bands within the band gap of CuGaSe₂ originating from a strong covalent interaction between Ge 4s and Se 4p states for Ge atoms tetrahedrally surrounded by the Se nearest-neighbor atoms. A theoretically predicted anti-bonding Ge-Se4sp³ defect band appearing well above the Fermi level for the Ge¹⁺_Ga point defect system can be directly linked to a Ge-dopant-related donor-acceptor-pair transition as observed in our photoluminescence spectra.     

Lithium-ion transfer on a LixCoO2 thin film electrode prepared by pulsed laser deposition—Effect of orientation-

LiCoO₂ thin films with different orientations were fabricated by pulsed laser deposition, and Li-ion transfer at the interface between the electrolyte and a LiCoO₂ thin film electrode was investigated. This study particularly focused on the effect of orientation on Li-ion transfer. The thin films were shown to be highly crystallized by X-ray diffraction. Charge transfer resistance ascribed to Li-ion transfer at the interface was observed by ac impedance spectroscopy. While charge transfer resistance was strongly influenced by the preferred orientation of LiCoO₂ thin film, the activation energy evaluated from the temperature-dependence of Li-ion transfer resistance appeared to be independent of the orientation.     

CuInSe2 thin film preparation through a new selenisation process using chemical bath deposited selenium

Copper indium selenide thin films were prepared through a novel and an eco-friendly selenisation process. In this method, selenium film required for selenisation was prepared using chemical bath deposition technique, at room temperature. Thus, totally avoided usage of highly toxic H₂Se or selenium vapour. Here, the process involved annealing the Stacked layer, Se/In/Cu in which Cu and In were deposited using vacuum evaporation technique. Investigations on the solid-state reaction between the layers were done by analysing structural and optical properties of films formed at different annealing temperatures. Optimum annealing condition for the formation of copper indium selenide thin film was found to be 673 K for 1 h in high vacuum. Compositional dependence of the growth process was also studied using various Cu/In ratios. Optical band gap was decreased with increase in Cu/In ratio. Carrier concentration and hence conductivity were found to be increased with increase in Cu/In ratio. The films obtained were p-type and highly Cu-rich films were degenerate.     

Role of electronically coupled in situ grown silver sulfides (Ag2S) nanoparticles with TiO2 for the efficient photoelectrochemical H2 evolution

A solution-processed in situ grown synthesis method has been developed to grow electronically coupled silver sulfide (Ag₂S) nanoparticles (NPs) inside a titanium oxide (TiO₂) thin film. Taking the advantage of better charge transport between Ag₂S and TiO₂, this composite thin film has been utilized for electro-photocatalytic H₂ generation. This thin film growth requires three successive steps, including sol-gel derived ion-conducting thin film fabrication containing loosely bound light ion (Li⁺) followed by ion-exchange (with Li⁺↔Ag⁺) and subsequent sulfurization process. This entire solution-processed deposition technique is capable to fabricate cost-effective large area Ag₂S–TiO₂ thin film containing Ag₂S NPs ranging ~10–70 nm. Since, Ag₂S has a lower band gap and consider as a promising material for photoelectrochemical H₂ generation, therefore Ag₂S (NPs)-TiO₂ thin film is grown on three different substrates, including fluorine-doped tin oxide (FTO), FTO/TiO₂ (sol-gel), and FTO/TiO₂ (NPs) to fabricate photoanode for this study. A comparative photo-electrocatalytic measurement of these three different Ag₂S(NPs)-TiO₂ thin film coated photoanodes showed that sample on FTO/TiO₂ (NPs) substrate generate highest photocurrent of density ~50 mA cm⁻² at 0.5 V vs NHE in 1 M KOH solution which is three orders higher than pure TiO₂ and stable for more than 1.5 h, indicating it's excellent potential application for photoelectrochemical water splitting. The photocurrent generation of this Ag₂S–TiO₂ thin film is significantly higher than earlier reported Ag₂S–TiO₂ system, which is originated due to the reduced carrier recombination from electronically coupled Ag₂S/TiO₂ interface state of such in situ grown Ag₂S NPs.     

Application of WO3 thin films for enhancement of photolysis in AgCl

A double-layer AgCl–WO₃ structure was employed to produce photochemical hydrogen for doping of an AgCl film. Atomic photochemical hydrogen, detached under the action of light from hydrogen donor molecules, previously adsorbed on the WO₃ surface, migrated through the WO₃ film into the AgCl film, which provided doping of the AgCl surface and yielded hydrogen sensitization simultaneous to illumination and yielded the enhancement of photochromism in the AgCl films. The atomic hydrogen played the role of a reducing agent and triggered the formation of sensitization centers on the halide surface, which in turn facilitated the growth of silver clusters and colloids under the action of light. The double-layer AgCl–WO₃ structure realizes the idea of two-stage catalysis: first the oxide surface catalyses hydrogen production under the action of light, then the photochemical hydrogen atoms act as catalysts during the photolysis of the halide.     

Metal silicide-mediated microcrystalline silicon thin-film growth for photovoltaics

Microcrystalline thin Si films were grown by the metal-induced growth method. The metal catalyst (Co, Ni, or Co-coated Ni) first reacted to sputtered Si forming a silicide layer. Then a Si film was epitaxially grown above the silicide seed template. The crystallinity of Si films was investigated by X-ray diffraction (XRD) confirming Si film growth with CoSi₂ or NiSi₂ as an intermediate step. The grown Si films were fabricated into Schottky photodiodes. The Co-coated Ni modulated the silicide formation and gave a short-circuit current density (J_sc) of 10.6 mA/cm², which is one order higher than that for the single Co catalyst case.     

MgH2 thin films deposited by one-step reactive plasma sputtering

The morphology, crystal structure, hydrogen content, and sorption properties of magnesium hydride thin films prepared by reactive plasma-assisted sputter deposition were investigated. Few micrometers-thick films were deposited on Si and SiO₂/Si substrates, at low pressure (0.4 Pa) and close to room temperature using (Ar + H₂) plasma with H₂ fraction in the range 15–70%. The microstructure and hydrogen content of the films are closely related to the surface temperature and hydrogen partial pressure during the deposition process. Operating in pulsed-plasma mode allows the hydrogenation rate of the MgH₂ thin film to top up to 98%, thereby producing a nearly fully hydrogenated film in a single-step process. The positive effect of the pulsed process is explained by the significant decrease in the whole energy flux incident on the surface and the favourable impact of the transient process for the rearrangement/relaxation of the materials. As for the hydrogen storage properties, desorption experiments and cycling of the films show the destabilizing effect of Mg₂Si formation at the interface between the film and the Si substrate resulting in a drastically increased desorption kinetics compared to less reactive SiO₂ substrate. However, the reaction is regrettably not reversible upon hydrogenation and the hydrogen storage capacity is consequently reduced upon cycling. Nevertheless, the deposition process carried out on inert substrates would offer true potential for reversible storage. Finally, our experimental results, which show the possibility to preferentially grow the metastable medium pressure γ-MgH₂ phase, open possibilities for the synthesis of more complex metastable phases such as magnesium-based ternary compounds.     

Electrodeposited zinc oxide thin films: Nucleation and growth mechanism

The nucleation and growth mechanism of the electrodeposited zinc oxide thin films on fluorine-doped tin oxide (FTO) coated (10–20 Ω/cm²) glass substrates from acetate solution, without and with ex situ oxygen bubbling, has been studied by cyclic voltammetry (CV), chronoamperometry (CA) and scanning electron microscopy (SEM) techniques. Ethylene diamine tetra acetic acid (EDTA) was used as a complexing agent. The cyclic voltammograms exhibit crossover, a characteristic of nucleation process on FTO-coated conducting glass substrates for all the baths bubbled with oxygen. The current transients were analyzed by fitting chronoamperometric data into the Scharifker–Hills nucleation model. The plausible nucleation and growth mechanism is proposed. For mother bath and lower oxygen bubbling time, the nucleation and growth mechanism follows 3D progressive nucleation and growth, which became instantaneous in case of baths for higher oxygen bubbling time. The SEM study showed that the films become compact when the oxygen bubbling time was increased. The thin films were further characterized by X-ray diffraction technique for structural studies and the ZnO film formation was confirmed. With the increase in oxygen bubbling time, the shift in band gap energies from 3.2 to 3.3 eV is observed.     

Ellipsometric spectroscopy study of cobalt oxide thin films deposited by sol–gel

Due to their unique optical properties, solar selective coatings enhance the thermal efficiency of solar photothermal converters. Hence it seems to be interesting to study the optical properties of promising materials as solar selective coatings. In an earlier work, it was demonstrated that sol–gel deposited cobalt oxide thin films possess suitable optical properties as selective coatings. In this work, cobalt oxide thin films were prepared by same technique and their optical properties were analyzed as a function of the dipping time of the substrate in the sol, using the spectroscopy ellipsometry, atomic force microscopy and X-ray photoelectron spectroscopy techniques. The optical constants (n and k) for these films, in the 200–800 nm range, are reported as a function of the dipping time. The fitting of ellipsometric data, I_s and I_c, for the glass substrate and the cobalt oxide thin film, as modeled with the Lorentz and Tauc–Lorentz dispersion relations, indicated that the film microstructure resembles a multilayer stack with voids. From these results, the Co₃O₄ and void percentages in the film were estimated. Both, thin film thickness and void/Co₃O₄ percentage ratio, were determined to be strongly dependent on the immersion time. Furthermore, the total thickness of a multilayered film was found to be the sum of thickness of each individual layer.     

Silver: high performance anode for thin film lithium ion batteries

Among metals and intermetallic compounds, silver exhibits a high specific capacity according to the formation of different Ag–Li alloys (up to AgLi₁₂) in a very low voltage range versus lithium (0.250–0 V). Electrochemical results including Galvanostatic Intermittent Titration Technique (GITT) as well as cycling behaviour experiments confirmed the interesting characteristics of silver thin film electrodes prepared by radio frequency (r.f.) sputtering.XRD patterns recorded at different electrochemical stages of the alloying/de-alloying processes showed the complexity of the silver–lithium system under dynamic conditions. Cycling life depends on several parameters and particularly of the careful choice of cut-off voltages. In very well monitored conditions, galvanostatic cycles exhibited flat reversible plateaus with a minimal voltage value (0.050 V) between charge and discharge, a feature of great interest in the use of an electrode. The first results of a lithium ion battery with both silver and LiMn_1.5Ni_0.5O₄ thin films are presented.     

Pulsed electrodeposition of CdTe thin films: effect of pulse parameters over structure, stoichiometry and optical absorption

CdTe thin films were electrochemically deposited using unipolar current pulses of high magnitude between 2.5 and 30 mA/cm² in an aqueous solution. Parametric study of the effect of periodic current pulse magnitude, average current and ON and OFF duration was undertaken to understand the effect of pulse variables on CdTe film properties. Increasing pulse deposition current modifies crystalline growth phase from single cubic to mixed cubic and hexagonal growth phases. In addition to the modification in CdTe growth phases, there is an increasing tendency of the oxide formation particularly CdTeO₃. Increase in pulse current density or average current yields Cd rich CdTe films. The optical absorption coefficient decreases with the decrease in pulse current density, whereas an increase is observed as the OFF time decreases. The optical energy gap is found to increase with OFF time. A systematic study on the effect of pulse variables over the structure, compositional and optical properties of CdTe film is described.     

Effect of native oxide film on interaction between stainless steels and liquid Sn

Abstract

The effect of native oxide films on the reaction of liquid Sn with stainless steels has been investigated by analysing the growth of intermetallic layers. The diversity of the oxide layers has been employed by exposure of various stainless steels [martensite stainless steel (MSS), austenite stainless steel (ASS) and duplex stainless steel (DSS)] in air for different times. Liquid tin was prone to metallurgically interact with MSS by formation of interfacial (Fe, Cr)Sn₂ intermetallics. The native oxide film on MSS decreased (Fe, Cr)Sn₂ layer thickness growth but cannot prevent the intermetallic formation. The reaction of Sn with ASS was hindered by the native oxide layer, and the thickness and coverage ratio of the (Fe, Cr)Sn₂ intermetallics decreased with the increasing exposure time in air. The oxide film grown on DSS was stable enough to prevent the formation of (Fe, Cr)Sn₂ and changed the tin from metallurgical interaction to physical attachment.     

Effects of surface modification by MgO on interfacial reactions of lithium cobalt oxide thin film electrode

Magnesium oxide (MgO)-modified lithium cobalt oxide (LiCoO₂) thin film electrodes were prepared by pulsed laser deposition (PLD) and effects of surface modification by MgO on interfacial reactions of LiCoO₂ were studied. The modification by MgO was carried out by PLD on LiCoO₂ thin film electrode successively after the deposition of LiCoO₂ thin film by PLD. Auger electron spectroscopy suggested that Mg dispersed uniformly in nano-scale on the film electrode. Cyclic voltammetry measurements clearly showed that MgO modification suppresses the increase of resistances caused by repetition of lithium-ion insertion–extraction reaction charged up to 4.2 V versus Li/Li⁺. Moreover, MgO modification decreased the activation energy of lithium-ion transfer reaction at LiCoO₂ thin film electrode–electrolyte interface, indicating that the modification by MgO affects the kinetics of lithium-ion transfer reaction at LiCoO₂–electrolyte interface.     

Optimal Ag concentration for H2 production via Ag:TiO2 nanocomposite thin film photoanode

TiO₂ thin films containing different concentrations of Ag nanoparticles have been synthesized by sol-gel method. According to UV–visible spectra, presence of an intense surface plasmon resonance peak at 490 nm of wavelength indicated formation of silver nanoparticles in the TiO₂ films. Based on atomic force microscopy (AFM) analysis, the surface roughness and the effective surface ratio increased by increasing the Ag mol%. Moreover, scanning electron microscopy (SEM) images showed formation of Ag nanoparticles on the surface for the samples containing high Ag concentration. X-ray diffraction (XRD) patterns revealed that the size of Ag nanocrystals increased by increasing the Ag content in the films while the nanocrystalline size of TiO₂ reduced in the presence of silver nanoparticles. Based on x-ray photoelectron spectroscopy (XPS) data, a stoichiometric chemical composition was detected for TiO₂ while, Ag presented in a combination a metal/oxide states on the surface. Studying photoresponse of the samples showed that the highest photocurrent was obtained for the sample containing 1 mol% Ag. By measuring the photovoltage versus time, it was found that addition of silver nanoparticles to the TiO₂ layer resulted in reduction of the transient time of the photogenerated carriers in the samples. Impedance spectroscopy determined a slight decrease in charge transfer resistance by addition of Ag to the films. Moreover, measuring the amount of hydrogen produced during water splitting reactions verified that the highest quantum yield of 9.6% was obtained for the sample with 1 mol% Ag.     

Colloidal synthesis of CuInS2 nanoparticles: Crystal phase design and thin film fabrication for photoelectrochemical solar cells

This study reports the colloidal synthesis of copper indium disulfide (CuInS₂) nanoparticles in different crystal phases to be employed as thin film photoanodes in photoelectrochemical water splitting process. First, CuInS₂ nanoparticles with chalcopyrite-, zincblende-, wurtzite-as well as polytypic-phases have been synthesized using hot injection method. The effects of solvent, temperature and type of precursors on the phase design have been thoroughly investigated via various spectroscopic techniques such as XRD, SEM, HRTEM, UV-Vis and PL spectroscopy and Zeta particle size analysis. The XRD spectra have been revealed that the all the targeted nanoparticles had good crystallinity and free from undesired binary sulfides. The synthesized nanoparticles have been re-dispersed in N, N-dimethylformamide (DMF) to form nanoink paste and applied on fluorine doped tin oxide coated glass substrate by doctor blade technique. DMF has been found to be an enviable solvent for thin film fabrication since it could lead to the crack free and uniform surface formation. The chalcopyrite thin film has shown the best photoelectrochemical performance with the photocurrent density of ∼15 mA cm⁻² and conversion efficiency of 6.7%. Howbeit, thin films photoanodes bearing wurtzite, zincblende and polytypic CuInS₂ nanoparticles have been investigated to compare the performance of different crystal phases for photoelectrochemical solar cell applications. Moreover, it should be emphasized that all thin film electrodes have been investigated under 1-sun condition without any surface modification, chemical treatment and etching. Additionally, the thin films except wurtzite structure exhibited good stability along 2 h under dark and illuminated conditions.