Effect of alkaline-doping on photoelectrochemical activity of electrodeposited cuprous oxide films

P-type Cu₂O films with alkaline ions (Li⁺, Na⁺ and K⁺) of unintentional dopants on indium tin oxide coated glass substrate are successfully fabricated via a simple electrodeposition method for photoelectrochemical (PEC) hydrogen generation. The SEM and XRD analysis show the as-grown films with the pyramid-like morphology and cubic structure, and the composition of alkaline-doped Cu₂O films are examined using XPS spectroscopy to demonstrate the substitution of alkaline ions in the Cu₂O lattice. The optical analyses, including the absorbance and low-temperature photoluminescence spectra, confirm a bandgap of 2.3 eV and the presence of structural defects in alkaline-doped Cu₂O films. The Mott-Schottky plot shows the flat band potentials of the alkaline-doped Cu₂O films to be approximately ?0.1 V and the hole concentrations in the order of 10¹⁷ cm^?3. Significantly, the Cu₂O:Li film photocathode exhibits a higher photocurrent of ?2.2 mA cm^?2 at a potential of ?0.6 V vs Ag/AgCl which are greater than those of Cu₂O:K and Cu₂O:Na films due to greater preferred orientation degrees along (111) and less structural defects, because the ionic radii of Cu and Li is similar. These results demonstrate the great potential of alkaline doped Cu₂O films in solar-related applications.     

Preparation of carbon nanotube and graphene doped polyphenylene sulfide flexible film electrodes and the electrodeposition of Cu2O nanocrystals for hydrogen-generation

Through annealing and electrochemical reduction methods, we successfully fabricates reduced graphene oxide layer (RGOL) modified carbon nanotube and reduced graphene oxide (CNT + RGO) doped polyphenylene sulfide (PPS) flexible thin film electrodes. These composite structure films can not only overcome the brittle nature of PPS, but also make good use of the thermal stability of PPS. Furthermore, carbon nanotube and reduced graphene oxide enhance the electrical conductivity of the composite films. Truncated octahedral and cuboctahedral Cu₂O nanocrystals are synthesized on RGOL modified CNT + RGO doped PPS (RGOL@PPS/CNT + RGO) composite film by a facile electrodeposition method without using any surfactants or external heating. RGOL on the PPS/CNT + RGO substrate facilitates the formation of Cu₂O morphology. The obtained Cu₂O composite film shows a superior ability for the hydrogen evolution reaction (HER) compared with other Cu₂O electrocatalysts. The Cu₂O with a smaller loading less than 0.04 mg cm^?2 on the composite film exhibits excellent HER activities with a low onset potential of 0.05 V and large current densities. The results of the HER performance indicates that the RGOL@PPS/CNT + RGO composite film has a potential application in flexible hydrogen-producing devices.     

On the growth and doping of Fe/Ti chalcogenide thin films

Formation of pyrite (FeS₂) thin films by sulphuration of metallic layers (Fe) and bilayers (Fe–Ti) has been investigated. An experimental technique, which allows the measurement of both the electrical resistance (R) and the thermoelectric coefficient (S) of the films during the sulphuration process (in situ), has been used. Sulphurated Fe–Ti bilayers appear to be n-type semiconductors in contrast with sulphurated Fe films, which are always p-type semiconductors. The change of conductivity type has been attributed to the doping effect of FeS₂ films by Ti.     

Electrochemically deposited p-n homojunction cuprous oxide solar cells

The electrical properties of both p- and n-type cuprous oxide (Cu₂O) films electrochemically deposited from two electrolyte solutions were examined by current-voltage measurements. The resistivity of p-type Cu₂O varied from 3.2×10⁵ to 2.0×10⁸ Ω cm, while that of n-type Cu₂O from 2.5×10⁷ to 8.0×10⁸ Ω cm, depending on deposition conditions such as solution pH, deposition potential and temperature. With optimized deposition conditions for minimum resistivity, p-n homojunction Cu₂O solar cells were fabricated by a two-step deposition process. The p-n homojunction Cu₂O solar cells showed a conversion efficiency of 0.1% under AM1 illumination. The low efficiency is attributed to the high resistivity of p- and n-type Cu₂O, which require doping to reduce.     

Evaluation of hydrogen evolution reaction on chemical bath deposited Cu2O thin films: Effect of copper source and triethanolamine content

The chemical bath deposition method was used to deposit thin films of cuprous oxide. The effect of copper source and triethanolamine content on the optical, morphological, structural, electrochemical and photoelectrochemical properties of the thin films for the development of photocathodes for hydrogen production was investigated. Triethanolamine promotes the complexing of Cu⁺ ions independent of the copper source used, its increase promotes thicker films due to better growth control and reduction of rapid Cu₂O precipitation in the bulk solution. The increase in thickness promoted a change in preferential orientation from (111) plane to (200) plane, which also influenced and reduced the conductivity because there is a decrease in disorder (Urbach energy E_U). The thickness also varied due to copper source used, reaching the thickest films with copper nitrate while the thinnest films with copper acetate, this tendency is in agreement with their solubility in water. The lower solubility reduces the complexing of Cu ⁺ ions which promotes the Cu₂O precipitation in the bulk solution, limiting the growth of the film. Also, electrical properties varied (measured as disorder E_U) with copper source. The most conductive being the thin films deposited with copper acetate and nitrate while the most resistive being the films deposited with copper sulphate. Very little variation in optical properties was observed, estimating the band gap in the range of 2.62–2.66 eV, while high absorption coefficient (>10⁵ cm^?1) was calculated below the absorption edge (460–470 nm). All thin films showed p-type semiconducting behavior with a flat band potential in the range of ?0.10 to 0.18 V (Ag/AgCl sat electrode), which confirms their ability to work as photocathodes for hydrogen production. The best photoelectrochemical performance was observed with the thinnest films, which also are the most conductive and present the highest values of absorption coefficient.     

Gold protective layer decoration and pn homojunction creation as novel strategies to improve photocatalytic activity and stability of the H2-evolving copper (I) oxide photocathode

p-type Cu₂O (p-Cu₂O) is a promising candidate for engineering of photocathodes for solar H₂ generation but it suffers from the photo-induced corrosion process. In this work, we report on the two new strategies to overcome the photo-induced corrosion and to enhance the photocatalytic activity of the p-Cu₂O photocathode: (i) a 60-to-300-nm-thick Au layer prepared by sputtering was used as a protective layer to the p-Cu₂O electrode; (ii) a thin layer of n-type Cu₂O was deposited onto the p-Cu₂O electrode surface before sputtering the Au protective layer. For the latter, as a direct result, a pn-Cu₂O homojunction was formed that was then protected by the Au thin layer. The pn-Cu₂O homojunction helps to enhance the charge separation within the Cu₂O electrode, consequently contributes to the enhancement of the photocatalytic activity. Under the standard 1 Sun irradiation, the best Au-protected pn-Cu₂O photocathode showed an onset photovoltage of +0.55 V vs. Reversible Hydrogen Electrode (RHE), and a photocurrent density of 0.76 mA/cm² at an applied potential of 0 V vs. RHE that remained more than 50% after 3 min of operation. Whereas, the bare p-Cu₂O showed a photocurrent density of 0.3 mA/cm² that was completely degraded after 1 min of operation under the identical conditions.     

Application of CuS–ZnS PN junction for photoelectrochemical water splitting

A CuS thin film was prepared by the sulfurization of the electrodeposited copper layer on the FTO substrate using sulfur powder at 400 °C. Surface morphology and structure of the CuS thin film were investigated by scanning electron microscopy and X-ray diffraction. The surface morphology of the CuS thin film was worm-like with the diameter of 70 nm and its crystal structure was hexagonal. Band gap energy of the CuS thin film was obtained as 1.5 eV using absorption spectra. Photoelectrochemical response of the CuS thin film was analyzed under chopped illumination at negative and positive potentials. It showed photoelectrochemical response at negative potentials (ca. 2.6 μA cm^?2 at ?0.4 V vs. Ag/AgCl), but not at positive potentials, which confirmed its p-type semiconductivity. A ZnS thin film was synthesized by spray pyrolysis method and characterized using field emission scanning electron microscopy, X-ray diffraction and UV–vis spectrometer. It was shown that the surface morphology was smooth with the grain size of about 50–150 nm. Also, its crystal structure and band gap energy were hexagonal and 3.72 eV, respectively. In order to obtain PN (positive–negative) junction and increase photoelectrochemical response, the ZnS (n-type semiconductor) thin film was deposited on CuS (p-type semiconductor). Linear scan of elemental composition confirmed the presence of FTO, CuS and ZnS layers. Photoelectrochemical characterization showed more photoresponse than the CuS thin film at negative potentials (13.6 μA cm^?2 at ?0.4 V vs. Ag/AgCl) and no photoresponse at positive potentials. The results confirmed the synthesizing of PN junction at the interface of CuS and ZnS.     

Optical and electrical properties of flash-evaporated amorphous CuInSe2 films

Amorphous films of CuInSe₂ were deposited on glass substrate by flash evaporation of source materials. The films were found to be p-type semiconductors. The direct optical band-gap energy was obtained to be 1.21–1.41 eV. The film DC conductivity ranged from 1.2–5.7 S cm⁻¹ at 285 K for different film thickness with corresponding activation energy of 55.5–301 meV. From temperature dependence of conductivity, the carrier transport was interpreted to be due to band conduction above 270 K.     

Chemical bath deposition synthesis of TiO2/Cu2O core/shell nanowire arrays with enhanced photoelectrochemical water splitting for H2 evolution and photostability

In this study, we have developed a facile chemical bath deposition (CBD) method to grow p-type Cu₂O nanoparticles on n-type TiO₂ nanowire arrays (TiO₂ NWAs) to fabricate TiO₂/Cu₂O core/shell heterojunction nanowire arrays (TiO₂/Cu₂O core/shell NWAs). When used as photoelectrode, the fabricated TiO₂/Cu₂O core/shell NWAs show improved photoelectrochemical (PEC) water splitting activity to pure TiO₂ NWAs. The effects of the CBD cycle times on the PEC activities have been studied. The TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode prepared by cycling 5 times in the CBD process achieves the highest photocurrent of 2.5 mA cm^?2, which is 2.5 times higher than that of pure TiO₂ NWAs. In addition, the H₂ generation rate of this photoelectrode reaches to 32 μmol h^?1 cm^?2, 1.7 times higher than that of pure TiO₂ NWAs. Furthermore, the TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode shows excellent photostability and achieves a stable photocurrent of over 2.3 mA cm^?2 during long light illumination time of 5 h. The enhanced photocatalytic activity of TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode is attributed to the synergistic actions of TiO₂ and Cu₂O for improving visible light harvesting, and efficient transfer and separation of photogenerated electrons and holes.     

Electrodeposited p-type Cu2O as photocatalyst for H2 evolution from water reduction in the presence of WO3

Different p-type Cu₂O powders were prepared from electrodeposition and subjected to analysis of their photocatalytic activity in water reduction. The electrodeposited Cu₂O powders were obtained by scraping the deposited films off the substrate. Under illumination the Cu₂O powders alone were not able to catalyze H₂ generation from water reduction. However, these Cu₂O powders exhibited photocatalytic activity in H₂ generation when they were coupled with n-type WO₃ in suspensions. The coupling was made to avoid back reactions of the photo-induced charges. The electrodeposited Cu₂O powders showed higher photocatalytic activity than a commercially available Cu₂O powder. The suspension containing electrodeposited Cu₂O with a strong [1 1 1] orientation gave a larger amount of H₂ evolution than that containing Cu₂O with a [1 1 0] orientation. Appropriate crystalline-texture tuning, as well as charge delocalization promotion, is looked to as the key issue for efficient H₂ generation from water reduction over p-type Cu₂O photocatalysts.     

Photoelectrochemical characteristics of CuO films with different electrodeposition time

This paper explores the effect of electrodeposition time on microstructure, optical, and photoelectrochemical properties of CuO films. CuO films were electrochemically deposited on tin-doped indium oxide (ITO) substrates using a Cu₂O electrodeposition method followed by annealing at 550 °C for 2 h. The electrochemical deposition was carried out at different times (300, 600, 1200, and 1800 s) utilizing a copper sulfate pentahydrate and lactic acid solution. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) were used to perform phase and microstructure analysis. Photoluminescence (PL) studies confirmed an increase in emission intensities with increasing deposition time. In addition, a significant change was observed in photoelectrochemical properties of the film by varying the deposition time. The film deposited for 600 s showed a high photocurrent density of ?0.55 mA cm^?2 at ?0.5 V. Moreover, a lowest resistance from electrochemical impedance spectroscopy (EIS) was recorded for the films electrochemically deposited for 600 s.     

Nanostructured SrTiO3 thin films sensitized by Cu2O for photoelectrochemical hydrogen generation

We prepared nanostructured thin films of pristine SrTiO₃, Cu₂O and SrTiO₃/Cu₂O heterojunction with varying the thickness of Cu₂O. SrTiO₃ and Cu₂O thin films were deposited on ITO (Sn:In₂O₃) glass substrate using sol–gel spin-coating technique and spray pyrolysis method respectively. Samples were characterized using XRD (X-ray diffractometry), SEM (Scanning electron microscopy), and UV–Visible absorption spectroscopy. Nanostructured thin films of pristine SrTiO₃, Cu₂O and SrTiO₃/Cu₂O heterojunction systems were used as photoelectrode in the Photoelectrochemical (PEC) cell for water splitting reaction. Maximum photocurrent density value of 2.44 mA cm⁻² at 0.95 V/SCE were observed for SrTiO₃/Cu₂O heterojunction photoelectrode with 454 nm thickness, which was approximately 34 times higher than pristine SrTiO₃ thin film. Increased photocurrent density observed for the heterojunction can be attributed to the improved conductivity and better separation of the photogenerated charge carriers at the SrTiO₃/Cu₂O interface.     

Crystal structure,electrical and electrochemical properties of Cu co-doped Pr1.3Sr0.7NiO4+δ mixed ionic-electronic conductors (MIECs)

Room temperature crystal structure, electrical properties and electrochemical properties in the temperature range 25–700 °C of Cu co-doped Pr_1.3Sr_0.7NiO_4+δ prepared by acetate combustion is investigated from intermediate temperature solid oxide fuel cell cathode viewpoint. The Pr_1.3Sr_0.7Ni_1?xCu_xO_4+δ (PSNCO) solid solutions have a tetragonal I4/mmm K₂NiF₄-type structure which consists of a (Pr_1.3Sr_0.7) (Ni_1.xCu_x)O₃ perovskite unit and a (Pr_1.3Sr_0.7)O rock salt unit in the whole compositional range 0 ≤ x ≤ 0.4. A reduction in bond-length of Ni/Cu-O resulting from PSNCO lattice contraction eases hop of small polaron from Ni³⁺ to Ni²⁺/Cu²⁺ in (Ni_1?xCu_x)-O layer with low activation energy, which increases electron conductivity. The maximum electronic conductivity (σ = Ω cm^?1) with minimum activation energy (E_a = eV) is observed at x = 0.3. Lattice expansion along c-direction owing to Cu²⁺ doping facilitates hop of O^2? from its occupied interstitial site (O₃) to nearby equivalent site assisted by anisotropic thermal motion of apical oxygen O2 resulting in increase in ionic conductivity. The minimum polarization resistance value (R_p = 0.13 (2) Ω cm²) and activation energy (E_a = 1.321 (5) eV) at x = 0.3 is attributed to high electronic and ionic conductivities compared to other compositions. Complex impedance spectroscopy studies suggest that the ORR is co-limited by O^2? diffusion and O₂ surface exchange.     

Growth of n- and p-type ZnTe semiconductors by intrinsic doping

Using intrinsic doping, n- and p-type ZnTe thin films have been electrodeposited (ED) on glass/fluorine-doped tin oxide (FTO) conducting substrate in aqueous solutions of ZnSO₄·7H₂O and TeO₂. The intrinsic doping was achieved by simply varying the deposition potential. The films have been characterised for their structural, optical, electrical, morphological and compositional properties using X-ray diffraction (XRD), optical absorption, photoelectrochemical (PEC) cell measurements, scanning electron microscopy and energy-dispersive X-ray analysis techniques, respectively. The XRD results reveal that the electroplated films are polycrystalline and have hexagonal crystal structures. Optical absorption measurements have been used for the bandgap determination of as-deposited and heat-treated ZnTe layers. The bandgap of the as-deposited ZnTe films are in the range (1.70–2.60) eV depending on the deposition potential. PEC cell measurements reveal that the ED-ZnTe films have both n- and p-type electrical conductivity. Using the n- and p-type ZnTe layers, a p-n homo-junction diode with device structure of glass/FTO/n-ZnTe/p-ZnTe/Au was fabricated. The fabricated diode showed rectification factor of 10², ideality factor of 2.58 and threshold voltage of ~0.25 V.     

Conductivity controlling of Cu2O film photoelectrode for water splitting by a novel electrochemical approach - Differential potentiostatic deposition

Cuprous oxide (Cu₂O) is a kind of low-cost and promising material for water splitting to produce hydrogen (p-type Cu₂O) and oxygen (n-type Cu₂O). However, the reason of conductivity transforming from p-type to n-type for Cu₂O films during potentiostatic deposition is waiting to be revealed. In this work, a novel electrochemical technology, differential potentiostatic deposition (DPD), is developed by coupling a 3-electrode setup with a resistor connected in series with the counter electrode circuit through a potentiostat. By this approach, deposition current density is adjusted in a short period to simulate different stages in a traditional potentiostatic deposition (TPD). The result shows that semiconducting conductivity of Cu₂O film changes from p-type to n-type with time during a long-term TPD in basic CuSO₄ solution. Employing the DPD method, conductivity of Cu₂O film transforms from p-type to n-type with current density decreasing. Through characterizing thickness, composition and photoelectrochemical performance of Cu₂O films, the mechanism of semiconducting conductivity transformation for Cu₂O films is proposed. Besides, the results indicate that the DPD is an effective method to tune the conductivity of metal oxide photoelectrodes for water splitting.     

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.     

Electrical conduction studies of hot wall deposited CdSexTe1−x thin films

CdSe_xTe_1−x thin films of different compositions have been deposited on cleaned glass substrates using the hot wall deposition technique under conditions very close to thermodynamical equilibrium with minimum loss of material. The electrical conductivity of the deposited films has been studied as a function of temperature. All the films showed a transition from phonon-assisted hopping conduction through the impurity band to grain-boundary-limited conduction in the conduction/valence band at temperature around 325 K. The conductivity has been found to vary with composition; it varied from 0.0027 to 0.0198 Ω⁻¹ cm⁻¹ when x changed from 0 to 1. The activation energies of the films of different compositions determined at 225 and 400 K have been observed to lie in the range 0.0031–0.0098 and 0.0285–0.0750 eV, respectively. The Hall-effect studies carried out on the deposited films revealed that the nature of conductivity (p or n-type) was dependent on film composition; films with composition x=0 and 0.15 have been found to be p-type and the ones with composition x=0.4, 0.6, 0.7, 0.85 and 1 have been observed to exhibit n-type conductivity. The carrier concentration has been determined and is of the order of 10¹⁷ cm⁻³. The majority of carrier mobilities of the films have been observed to vary from 0.032 to 0.183 cm² V⁻¹ s⁻¹ depending on film composition. The study of the mobility of the charge carriers with temperature in the range of 300–450 K showed that the mobility increased with power of temperature indicating that the type of scattering mechanism in the studied temperature range is the ionized impurity scattering mechanism.     

Temperature dependence of the optical transitions in electrodeposited Cu2O thin films

Cu₂O films on flexible copper and molybdenum (Mo) substrates were prepared by electrodeposition form an alkaline bath. The as-deposited films were p-type and the XRD analysis revealed that the film contains only the Cu₂O phase. The thickness of the films was calculated from the interference fringes in the reflection spectra. The Au/Cu₂O Schottky diodes were prepared by sputtering a 15 nm thick layer of very pure gold onto the Cu₂O films on Mo substrate. The probable optical transitions near the band edge were calculated from the spectral response of the device. The band gap calculated at various temperatures show a linear dependence on temperature and the absolute zero value of the band gap is deduced as 2.206 eV. The 2.493 eV direct transition observed in the room temperature shows a temperature dependence. Evidence of phonon assisted indirect transitions were observed at various temperature regions.     

Highly efficient sputtered Ni-doped Cu2O photoelectrodes for solar hydrogen generation from water-splitting

Hydrogen gas can be converted to electricity through fuel cells and is considered as a friendly energy source. Herein, pure Cu₂O and Ni-doped Cu₂O thin films were deposited on glass substrates using the RF/DC-sputtering technique for hydrogen production via the photoelectrochemical (PEC) water-splitting process. The preferred orientation for pure and Ni-doped Cu₂O films was (111) crystallographic plane. The average nanograins size was decreased from 32.17 nm for pure to 10.40 nm through the doping process with Ni content. Field-emission scanning electron microscopy (FE-SEM) and ImageJ analysis showed that the pure Cu₂O and Ni-doped Cu₂O were composed of normal distribution of nanograins in a regular form. The optical bandgap of the Cu₂O film was decreased from 2.35 eV to 1.9 eV after doping with 2.6 wt% of Ni-dopants. The photoluminescence (PL) spectra for all the sputtered films were recorded at room temperature to examine the effect of Ni-dopants in the Cu₂O lattice. Pure and Ni-doped Cu₂O films were applied for PEC water splitting for hydrogen (H₂) production under white light and monochromatic illumination. The PEC studies displayed that increasing the Ni content up to 2.6 wt% in the pure Cu₂O films led to an increase in the photocurrent density to reach ?5.72 mA/cm². The optimum photoelectrode was studied for reproducibility, stability, and electrochemical impedance. The incident photon to current conversion efficiency (IPCE%) was 16.35% at 490 nm, and the applied bias photon to current conversion efficiency (ABPE%) was 0.90% at 0.65 V. Consequently, Ni-doped Cu₂O photoelectrodes are efficient and low-cost for practical and industrial solar H₂ production.     

Increased SO2 electrooxidation activity on a copper-nitrogen doped catalyst and its active sites analysis

It's a meaningful work to develop a highly active nonprecious SO₂ electrooxidation catalyst, replacing the Pt-based ones. Here, a novel Cu-N doped carbon-based catalyst is synthesized by pyrolyzing the imidazole chelated copper ions on the chitosan modified carbon BP2000. During the preparation, metallic Cu is developed and encapsulated in the carbon lattices, and transformed into the CuN_x structures on the catalyst surface, simultaneously. Metallic Cu plays significant role in the doping and developing of active sites, which have vital effects on the catalysis activity. The prereduction of Cu²⁺ by NaBH₄ during the preparation of Doping(I)-Cu@N-C makes great contribution to the development of metallic Cu, which highly dispersive anchor in the carbon lattices. This as-synthesized Doping(I) -Cu@N-C catalyst exhibits excellent SO₂ electrooxidation activity. Its SO₂ oxidation currents are remarkably increased with the elevation of applied potentials, and the oxidation performances prominently surpass the commercial Pt/C, when the potential is above 0.822 V. The peak SO₂ oxidation current (i_p) of Doping(I)-Cu@N-C is 7.17 mA cm^?2 @ 0.684 V, much higher than the 3.03 mA cm^?2@ 0.584 V of Pt/C with the same mass loading. In the chronoamperometry tests under 1.2 V, the terminal oxidation current of Doping(I)-Cu@N-C was 1.74 times as high as that of Pt/C, indicating that this prepared catalyst also displays much better SO₂ electrooxidation activity than Pt/C under constant applied potentials.