Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode

ZnO nanotube arrays were synthesized by the electrodeposition method and Cu₂O microcrystals with two kinds of morphologies were deposited on ZnO nanotube arrays successfully. At the deposition potential of −0.5 or −0.7 V, the cubic or spherical Cu₂O microcrystals were selectively deposited on ZnO nanotube arrays. By adjusting the deposition time, Cu₂O microcrystals with different sizes were obtained. The optical properties and photo-electrochemical performance of ZnO/Cu₂O were measured. The results showed that the as-prepared ZnO/Cu₂O heterojunction exhibited improved visible light absorption and enhanced photocurrent due to the excellent ability of Cu₂O microcrystals for harvesting visible light, and the effective separation and transfer of photo-generated electrons and holes owing to p-n junction between ZnO and Cu₂O. The experimental results demonstrate that the photo-electrochemical performance of ZnO/Cu₂O heterojunction nanotube arrays can be manipulated by controlling the morphology and the size of Cu₂O microcrystals.     

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.     

A promising CuOx/WO3 p-n heterojunction thin-film photocathode fabricated by magnetron reactive sputtering

A CuO_x/WO₃ thin-film based on p-n heterojunction proposed as a highly performance and stable photocathode. The CuO_x/WO₃ thin-film was deposited by magnetron reactive sputtering layer by layer, followed with slow rate annealing in O₂ ambient. This is an excellent method for high-quality and uniform composite thin-film deposition with large areas at a high growth rate. The optimized CuO_x/WO₃ thin-film photocathode after slow rate annealing at 500 °C in O₂ provides an obviously enhanced photoinduced current density of −3.8 mA cm⁻² at a bias potential of −0.5 V (vs. Ag/AgCl), which value is 1.5 times higher than that of bared CuO_x thin-film. This highly enhanced photoelectrochemical performance is attributed to p-n heterojunction, which accelerates the photogenerated electrons and holes transfer to n-WO₃ and p-CuO_x, thereby accelerate the separation of photogenerated carries. In addition, WO₃ layer covered on the surface of CuO_x thin film can improve the stability of Cu₂O in electrolytes.     

Physical properties of ZnO:F obtained from a fresh and aged solution of zinc acetate and zinc acetylacetonate

Fluorine-doped zinc oxide thin films, ZnO:F, were deposited by the spray pyrolysis technique on sodocalcic glass substrates. Two different zinc precursors were used separately, namely, zinc acetate and zinc pentanedionate. The effect of the zinc precursor type, the aging of the starting solution, the substrate temperature and a vacuum-annealing treatment on the electrical, morphological, structural and optical properties was studied, in order to obtain conductive and transparent ZnO:F thin films. The resistivity values of ZnO:F thin films deposited from aged solutions were lower than those films obtained from fresh solutions. The lowest resistivity values of as-grown films deposited at 500 °C, using a two-day aged starting solution of zinc acetate and zinc pentanedionate, were 1.4×10⁻² and 1.8×10⁻² Ω cm, respectively. After a vacuum annealing treatment performed at 400 °C for 30 min a decrease in the resistivity was obtained, reaching a minimum value of 6.5×10⁻³ Ω cm for films deposited from an aged solution of zinc acetate. The films were polycrystalline, with a (0 0 2) preferential growth orientation in all the cases. Micrographs obtained by SEM show a uniform surface covers by rounded grains. No evident change in the surface morphology was observed with the different precursors used. The transmittance of films in the visible region was higher than 80%.     

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.     

CdO/Cu2O solar cells by chemical deposition

CdO and Cu₂O thin films have been grown on glass substrates by chemical deposition method. Optical transmittances of the CdO and Cu₂O thin films have been measured as 60–70% and 3–8%, respectively in 400–900 nm range at room temperature. Bandgaps of the CdO and Cu₂O thin films were calculated as 2.3 and 2.1 eV respectively from the optical transmission curves. The X-ray diffraction spectra showed that films are polycrystalline. Their resistivity, as measured by Van der Pauw method yielded 10⁻²–10⁻³ Ω cm for CdO and approximately 10³ Ω cm for Cu₂O. CdO/Cu₂O solar cells were made by using CdO and Cu₂O thin films. Open circuit voltages and short circuit currents of these solar cells were measured by silver paste contacts and were found to be between 1–8 mV and 1–4 μA.     

ZnO/Cu2O-decorated rGO: Heterojunction photoelectrode with improved solar water splitting performance

In present work, we report a facile fabrication process to improve the photoelectrochemical (PEC) performance of ZnO-based photoelectrodes. In order to achieve that, the Cu₂O nanocubes are cathodic-deposited on the as-prepared ZnO nanorods. Then rGO nanosheets are electrodeposited on the ZnO/Cu₂O heterostructures. The fabricated photoelectrodes are systematically studied in detail by different characterization techniques such as powder X-ray diffraction, micro-Raman, X-ray photoelectron spectroscopy, ultraviolet diffused reflectance spectroscopy and photoluminescence spectroscopy analysis. Morphologies of the fabricated photoelectrodes are investigated through electron microscopy in scanning and transmission mode. To evaluate the PEC performance of the fabricated photoelectrodes, the line scan voltammetry (LSV) measurement is performed using a three-electrode system in 0.5-M Na₂SO₄ electrolyte solution under stimulated light illumination at 100 mW/cm² from a 300-W Xenon Arc lamp coupled with an AM 1.5G filter using a three-electrode system. The photocurrent measurement demonstrates that the photoelectrodes based on ZnO/Cu₂O/rGO possess enhanced PEC performance compared to the pristine ZnO and ZnO/Cu₂O photoelectrodes. The photocurrent density of ZnO/Cu₂O/rGO-15 photoelectrode (10.11 mA/cm²) is ∼9 and ∼3 times higher than the photoelectrodes based on pristine ZnO (1.06 mA/cm²) and ZnO/Cu₂O (3.22 mA/cm²). The enhanced PEC performance of ZnO/Cu₂O/rGO photoelectrode is attributed to the excellent light absorption properties of Cu₂O and excellent catalytic and charge transport properties of rGO. Experimental results reveal that the proposed functional nanomaterials have a great potential in water splitting applications.     

Structural properties and enhanced photoelectrochemical performance of ZnO films decorated with Cu2O nanocubes

Combination of ZnO and Cu₂O semiconductors is remarkable for efficient photovoltaic cells and enhanced photoelectrochemical (PEC) performance due to the high electronic energy band alignment of these materials and their controllable electronic structure at the interface. This study reports on a systematic analysis of the effects of Cu₂O nanocube doping on the structural properties and PEC performance of ZnO films. ZnO samples doped with Cu₂O were prepared by a practical electrochemical method. Characterization of the materials was performed by XRD, Raman, FTIR spectroscopy and electrochemical techniques. The XRD, Raman, FTIR spectroscopy analyses indicated a single phase of ZnO for the lower Cu₂O deposition time, while a secondary phase of Cu₂O evolved for the 5 min deposition time. This study showed that ZnO doped with Cu₂O grown for 3 min had the best PEC performance. ZnO/Cu₂O photoelectrodes are recommended as an attractive, competitive and alternative candidate for advanced PEC sensing and this may be for the extended field of water splitting into oxygen and hydrogen under sunlight.     

Nano-structured Cu2O solar cells fabricated on sparse ZnO nanorods

Nano-structured Cu₂O/ZnO nanorod (NR) heterojunction solar cells fabricated on indium tin oxide (ITO)-coated glass are studied. Substrate film and NR density have a strong influence on the preferred growth of the Cu₂O film. The X-ray diffractometer (XRD) analysis results show that highly (2 0 0)-preferred Cu₂O film was formed when plating on plain ITO substrate. However, a highly (1 1 1)-preferred Cu₂O film was obtained when plating on sparse ZnO NRs. SEM, TEM and XRD studies on sparse NR samples indicate that the Cu₂O nano-crystallites mostly initiate its nucleation on the peripheral surfaces of the ZnO NRs, and are also highly (1 1 1)-oriented. Solar cells with ZnO NRs yielded much higher efficiency than those without. In addition, ZnO NRs plated on a ZnO-coated ITO glass significantly improve the shunt resistance and open-circuit voltage (V_oc) of the devices, with consistently much higher efficiency obtained than when ZnO NRs are directly plated on ITO film. However, longer NRs do not improve the efficiency due to low short-circuit current (J_sc) and slightly higher series resistance. The best conversion efficiency of 0.56% was obtained from a Cu₂O/ZnO NRs heterojunction solar cell fabricated on a 80 nm ZnO-coated ITO glass with V_oc=0.514 V, J_sc=2.64 mA/cm² and 41.5% fill factor.     

Cu2O as an emerging photocathode for solar water splitting - A status review

Recently, cuprous oxide (Cu₂O) based photocathodes have gained research attention for hydrogen (H₂) production through photoelectrochemical (PEC) water splitting reactions due to marginally lower synthesis cost and low energy intensity fabrication processes. Unique properties of Cu₂O, such as tunable bandgap, appropriate band edge potentials with water redox levels and non-toxic nature makes it beneficial for PEC applications. Cuprite is mainly studied under visible light to facilitate enhanced H₂ gas production upon illumination. However, notoriously photocorrosion degrades the PEC performance and restricts the photoactivity of Cu₂O. Moreover, because of the redox potentials lies within the band gap of Cu₂O; self-photocorrosion or self-oxidation upon illumination is unavoidable. Improvement in the Cu₂O photocathodes was achieved by finding elegant solutions such as forming thin heterojunction layers by atomic layer deposition (ALD) as well other methods, co-catalyst deposition, tuning crystal facets and surface modifications with different synthetic methods. In this review, we discuss the improvements in Cu₂O photocathodes achieved over the years for enhanced H₂ production with recently studied photocathodes.     

Photoelectrochemical characteristics of electrodeposited cuprous oxide with protective over layers for hydrogen evolution reactions

Cuprous oxide is one of the inexpensive options of highly efficient visible light-based photocathode for hydrogen generation in photoelectrochemical cells. Highly photoactive cuprous oxide (Cu₂O) films are obtained by cathodic electrodeposition using lactate stabilized copper sulphate precursor exhibiting a photo-current density of ~1 mA/cm² at ?0.1 V vs. RHE. Although Cu₂O is a decent choice for photoelectrochemical applications, including hydrogen evolution reaction (HER), it faces serious issues related to photodegradation and instability. To address this issue, a comparative study of two types of thin films, Al (2%)-doped ZnO (AZO) and NiO_x (usually, x > 2 at low T to x→1 at high T annealing) as photo-corrosion protective overlayers is made. The improved stability of the protected photoelectrodes is observed as noted from the photocurrent degradation of 3.5%, 0.16% and 0.03% in Cu₂O (bare), Cu₂O/AZO, and Cu₂O/NiO_x photocathodes, respectively. Furthermore, the electrochemical impedance spectroscopy reveals that electrode protected with NiO_x exhibit faster charge transfer kinetics and minimum photocurrent degradation as compare to the Cu₂O/AZO and Cu₂O(bare) photoelectrodes, proving its potential in HER kinetics.     

Synthesis of nanostructured Al-doped zinc oxide films on Si for solar cells applications

Al-doped ZnO thin films have been prepared by a novel successive chemical solution deposition technique. The variation in morphological, structural, electrical, and optical properties of nanostructured films with doping concentration is investigated in details. It was demonstrated that rapid photothermal processing (RPP) improves the quality of nanostructured ZnO films according to the enhancement of resonant Raman scattering efficiency, and the suppression of the visible luminescence with the increase of RPP temperature. It was found from the I-V characteristics of ZnO/Si heterojunction that the average short-circuit current density is about 8 mA/cm². For 1%Al-doped ZnO/SiO₂/Si structure, the short-circuit current density is about 28 mA/cm². The improvement shown in the characteristics may be assigned partially to the reduction of the defect density in the nanostructured Al-doped ZnO films after RPP. The correlations between the composition, microstructure of the films and the properties of the solar cell structures are discussed. The successive chemically deposited Al-doped ZnO thin film offers wider applications of low-cost solar cells in heterojunction structures.     

Effect of the substrate temperature and acidity of the spray solution on the physical properties of F-doped ZnO thin films deposited by chemical spray

F-doped ZnO thin films were prepared by using the spray pyrolysis technique. The dependence of the electrical, optical, structural and morphological properties on the substrate temperature and spray solution acidity was studied. Additionally, aging of the spray solution presents a clear effect on the resistivity of ZnO thin films. The best films obtained show a resistivity, mobility and carrier concentration of the order of 1.5×10⁻² Ω cm, 6 cm²/V s and 2×10¹⁹ cm⁻³, respectively. Wurtzite hexagonal structure, with a preferential growth along the [0 0 2] direction for all substrate temperatures and acidities used, was obtained. From scanning electron microscopy and atomic force microscopy analysis, it was determined that the grain size of the films decreases and its homogeneity increases when the acidity of the starting solution is increased. High optical transmittances, in the order of 90%, were obtained in all the cases.     

Optical transmittance and photoconductivity studies on ZnO : Al thin films prepared by the sol–gel technique

Polycrystalline ZnO : Al thin films have been prepared by the (Sol–gel) chemical deposition method. The ZnO : Al thin films are very transparent (90%) in the near UV, VIS and IR regions. The films are oriented along the c-axis ([0 0 2] direction) in the hexagonal structure. It is known that pure ZnO thin films are not chemically stable in corrosive media, but aluminium stabilizes the ZnO system and increases its electrical conductivity. Finally, the ZnO : Al thin films are reasonably stable under storage in air and in reactive atmospheres like O₂, H₂O, H₂ or in weak acids. Dark- and photo-conductivity of the ZnO : Al films are very high (1–100 Ω⁻¹ cm⁻¹), so that they can be used as transparent conductors in solar cells or in electrochromic devices.     

Electrodeposited cuprous oxide on indium tin oxide for solar applications

Semiconducting cuprous oxide films were prepared by electrodeposition onto commercial conducting glass coated with indium tin oxide deposited by spraying technique. The cuprous oxide (Cu₂O) films were deposited using a galvanostatic method from an alkaline CuSO₄ bath containing lactic acid and sodium hydroxide at a temperature of 60°C. The film's thickness was about 4–6 μm. This paper includes discussion for Cu₂O films fabrication, scanning electron microscopy and X-ray diffractometry studies, optical properties and experimental results of solar cells. The values of the open circuit voltage V_oc of 340 mV and the short circuit current density I_sc of 245 μA/cm² for ITO/Cu₂O solar cell were obtained by depositing graphite paste on the rear of the Cu₂O layer. It should be stressed that these cells exhibited photovoltaic properties after heat treatment of the films for 3 h at 130°C. An electrodeposited layer of Cu₂O offers wider possibilites for application and production of low cost cells, both in metal–semiconductor and hetero-junction cell structures, hence the need to improve the photovoltaic properties of the cells.     

Fully-depleted dual P–N heterojunction with type-II band alignment and matched build-in electric field for high-efficient photocatalytic hydrogen production

In this work, a dual p-n heterojunction of Cu₂O/Ni(OH)₂/TiO₂ with type-II band alignment and matched build-in electric field was fabricated. This dual p-n heterojunction promoted the separation and transfer of charge carriers, which is much more efficient than individual p-n heterojunction. Photocatalytic hydrogen evolution under the simulated sunlight shows a high rate of 6145 μmol g⁻¹ h⁻¹ for Cu₂O/Ni(OH)₂/TiO₂, which is 1.9 and 2.7 times that of Ni(OH)₂/TiO₂ and Cu₂O/TiO₂, respectively. The apparent quantum yield of Cu₂O/Ni(OH)₂/TiO₂ is about 20.2% under the irradiation of monochromatic light (λ = 420 nm). The recycling test of hydrogen evolution also verified a high stability for this dual heterojunction. The type-II band alignment and matched build-in electric field was confirmed, which accelerate the migration of charge carriers. The width of space-charge layer was calculated and proved that the p-n junctions in Cu₂O/Ni(OH)₂/TiO₂ are fully-depleted, which largely reduced the bulk recombination of charge carriers. The synergistic effect of the improved visible-light response, type-II band alignment, matched build-in electric field, and fully-depleted space-charge layer contributes to the enhanced photocatalytic hydrogen evolution.     

High-efficiency μc-Si/c-Si heterojunction solar cells

P-type microcrystalline silicon (μc-Si (p)) on n-type crystalline silicon (c-Si(n)) heterojunction solar cells is investigated. Thin boron-doped μc-Si layers are deposited by plasma-enhanced chemical vapor deposition on CZ-Si and the V_oc of μc-Si/c-Si heterojunction solar cells is higher than that produced by a conventional thermal diffusion process. Under the appropriate conditions, the structure of thin μc-Si films on (1 0 0), (1 1 0), and (1 1 1) CZ-Si is ordered, so high V_oc of 0.579 V is achieved for 2×2 cm² μc-Si/multi-crystalline silicon (mc-Si) solar cells. The epitaxial-like growth is important in the fabrication of high-efficiency μc-Si/mc-Si heterojunction solar cells.     

Hybrid solar cells based on MEH-PPV and thin film semiconductor oxides (TiO2, Nb2O5, ZnO, CeO2 and CeO2–TiO2): Performance improvement during long-time irradiation

Performance improvement of hybrid solar cells (HSC) applying five different thin film semiconductor oxides has been observed during long-time irradiation in ambient atmosphere. This behavior shows a direct relation between HSC and oxygen content from the environment. Photovoltaic devices were prepared as bi-layers of thin film semiconducting oxides (TiO₂, Nb₂O₅, ZnO, CeO₂–TiO₂ and CeO₂) and the polymer MEH-PPV, with a final device configuration of ITO/Oxide_{thin film}/MEH-PPV/Ag. The oxides were prepared as thin transparent films from sol–gel solutions. The photovoltaic cells were studied in ambient atmosphere by recording the initial values of open circuit voltage (V_oc) and current density (I_sc). Solar decay curves presented as the measurement of the short circuit current as a function of time, IV curves and photophysical analyses were also carried out for each type of device. Solar cells with TiO₂ thin films showed the best performance with maximum V_oc as high as −0.74 V and I_sc of 0.4 mA/cm². Solar decay analyses showed that the devices require a stabilization period of several hours in order to reach maximum performance. In the case of TiO₂, Nb₂O₅ and CeO₂–TiO₂, the maximum current density was observed after 15 h; for CeO₂, the maximum performance was observed after 30 h. The only exception was observed with devices applying ZnO in which the current density decreased drastically and degraded the polymer in just a couple of hours.     

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.     

Preparation and characterization of copper oxide particles/polypyrrole (Cu2O/PPy) via electrochemical method: Application in direct ethanol fuel cell

The electrocatalytic performance of Polypyrrole-Copper oxide particles modified carbon paste electrode (Cu₂O/PPy/CPE) for electrocatalytic oxidation of ethanol was reported for the first time in alkaline media. The composite Cu₂O/PPy was prepared using a facile approach consisting on the deposition of Polypyrrole film on CPE using galvanostatic mode then followed by the deposition of Copper particles at a constant potential. Scanning electron spectroscopy (SEM), infrared spectroscopy (FTIR), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the structural and electrochemical properties of the Cu₂O/PPy/CPE and to explain the mechanism of electrooxidation of ethanol. The experimental parameters that influence the electrooxidation of ethanol were investigated and optimized. Our findings suggest that the electrodeposition of Copper particles on Polypyrrole film enhanced the catalytic activity towards the ethanol oxidation with a peak current density of 2.25 mA cm⁻² at 0.8 V vs Ag/AgCl, which is 2.6 times higher than the peak current density obtained by PPy/CPE electrode. It important to note that the saturation limit reaches a value of 5 M. To summarize, the good catalytic activity, stability and easy preparation make the Cu₂O/PPy composite as an excellent electrocatalyst for ethanol oxidation.