Decorating Cu2O with Ni-doped metal organic frameworks as efficient photocathodes for solar water splitting

Severe photocorrosion and fast photoexcited charges recombination hinder the application of Cu₂O in photoelectrochemical (PEC) water splitting. In this work, Ni-doped metal-organic frameworks is firstly applied to improve the performance of electrodeposited Cu₂O. A decorative layer of Ni-doped Cu₃(BTC)₂ (Ni-CuBTC) was in-suit constructed on Cu₂O through solvothermal followed by ion-exchange. Cu₂O/Ni-CuBTC photocathodes increase absorption edge to ∼800 nm, positive shift flat band position to 0.4 V, and decrease Tafel value to 74 mV/dec. These results confirm the decorative layer can extend light absorption, facilitate photoexcited charge separation and transfer, and enhance HER activity. A photocurrent density of −1.51 mA/cm² at 0 V_RHE is obtained with the decoration of Ni-CuBTC, which is 3.4 times of pristine Cu₂O photocathode. Here, the PEC water splitting performance of electrodeposited Cu₂O has been significantly improved with noble-metal-free decorations, which provides a new idea for solving the defects of Cu₂O based photocathode.     

Atom doping in α-Fe2O3 thin films to prevent hydrogen permeation

Prevention of hydrogen (H) penetration into passive films and steels plays a vital role in lowering hydrogen damage. This work reports effects of atom (Al, Cr, or Ni) doping on hydrogen adsorption on the α-Fe₂O₃ (001) thin films and permeation into the films based on density functional theory. We found that the H₂ molecule prefers to dissociate on the surface of pure α-Fe₂O₃ thin film with adsorption energy of −1.18 eV. Doping Al or Cr atoms in the subsurface of α-Fe₂O₃ (001) films can reduce the adsorption energy by 0.03 eV (Al) or 0.09 eV (Cr) for H surface adsorption. In contrast, Ni doping substantially enhances the H adsorption energy by 1.08 eV. As H permeates into the subsurface of the film, H occupies the octahedral interstitial site and forms chemical bond with an O atom. Comparing with H subsurface absorption in the pure film, the absorption energy decreases by 0.01–0.22 eV for the Al- and Cr-doped films, whereas increases by 0.82–0.96 eV for the Ni-doped film. These results suggest that doping Al or Cr prevents H adsorption on the surface or permeation into the passive film, which effectively reduces the possibility of hydrogen embrittlement of the underlying steel.     

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.     

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.     

Photoelectrochemical hydrogen generation with nanostructured CdS/Ti–Ni–O composite photoanode

Composite photocatalysts have aroused great interest due to combination of favorable electronic and optical properties. Herein, novel CdS/Ti–Ni–O composite photoanodes were constructed through anodic fabrication of nanostructured Ni-doped TiO₂ (Ti–Ni–O) oxide films and CdS deposition by successive ionic layer adsorption and reaction (SILAR). The morphology and composition evolution, optical properties and photoelectrochemical (PEC) performance of the photoanodes were investigated. The composite nanofilms mainly consisted of micropores and nanotubes. The CdS/Ti–Ni–O composite photoanode demonstrated remarkable PEC hydrogen generation properties with a high photocurrent density (6.72 mA·cm^?2 at 0 V vs Ag/AgCl) which was 18.2 times to that of the bare Ti–Ni–O photoanode. The synergy of Ni-doping and CdS-coupling on the enhancement of PEC performance offers useful ideas to the exploitation of effective photocatalysts and contributes to the development of solar-driven PEC hydrogen generation.     

Chemically grown Bi2S3 nanorod films for hydrogen evolution

Bi₂S₃ nanorod films were grown on ITO-coated glass substrates through chemical bath deposition (CBD) and annealing in a sulfur atmosphere. The as-deposited films were amorphous/nanocrystalline, with a particle size of 20 nm and a direct optical band gap of 1.87 eV. Upon annealing at 350 °C, the films exhibited a nanorod morphology with a length of 300 nm. Further increasing the temperature from 400 to 450 °C resulted in an increased diameter of nanorods. The direct optical band gap decreased from 1.68 to 1.47 eV upon increasing the annealing temperature from 350 to 400 °C. Photoelectrochemical (PEC) measurements showed that the nanorod films grown on ITO-coated glass substrates exhibited significantly increased PEC activity owing to their nanorod structures. The Bi₂S₃ nanorod films formed at 400 °C exhibited a maximum photocurrent density of 6.1 mA/cm² at 1 V, which was 2.5 times higher than that of the as-deposited films. The enhancement in the photocurrent density could be due to the effective visible-light absorption of Bi₂S₃ nanorods as a result of the increased crystallinity and decreased band gap. This study demonstrates the synthesis route involving a simple and inexpensive CBD method of Bi₂S₃ nanorod films for the optimized PEC water-splitting applications.     

Influence of chromium and lanthanum incorporation on the optical properties,catalytic activity,and stability of IrOx nanostructured films for hydrogen generation

Hydrogen production (HP) by photocatalytic water splitting (PWS) is becoming more and more popular on a global scale. The world's largest and most accessible renewable energy source—the Sun—as well as widely accessible metal oxide-based photoelectrodes are both utilized in this process. The preparation of pure and doped iridium oxide (IrO_x) films is attempted in this work in an effort to better understand how Cr and La affect optical and HP efficiency as well as electrode stability. By using FE-SEM, the films' varying thicknesses and nanorod-like morphologies were detected. UV–Vis spectra reveal that the composition has an impact on the films' absorption and reflectance. IrO_x has an optical band gap (E_g) of 2.9 eV, and this value decreased/increased after Cr doping/La codoping. The micro-Raman spectra, which showed that the E_g mode of Ir–O stretching was red-shifted from 563 to 553 cm⁻¹, validate the films' amorphous nature. The resultant (IrO_x) films were utilized in the HP via the solar photoelectrochemical (PEC) process. The codoped film, which has a solar-to-hydrogen conversion efficiency of 2.32% and a hydrogen evolution rate of 23.5 mmol h⁻¹cm⁻², is the most efficient and stable photoelectrode among the electrodes under examination. The highest absorbed photon-to-current conversion efficiency (APCE%) values for pure and codoped IrO_x photoelectrodes were 3.62%@460 nm and 5.54%@490 nm, respectively. With enhancement factors of 2.77, 1.89, and 2.90 for pure IrO_x, IrO_x:5% Cr, and IrO_x:Cr,2.5% La, respectively, the J_ph increased to 1.58, 1.70, and 1.83  mA cm⁻² at 90 °C. After ten runs, the codoped photoelectrode still has 99.2% of its initial photocurrent, compared to 80.8% and 82.8% for pure and Cr-doped IrO_x. Calculated Tafel slopes, corrosion rates, and PEC thermodynamic parameters show how codoping and doping affect photoelectrode performance and stability.     

Thin film deposition of Cu2O and application for solar cells

Deposition conditions of cuprous oxide (Cu₂O) thin films on glass substrates and nitrogen doping into Cu₂O were studied by using reactive radio-frequency magnetron sputtering method. The effects of defect passivation by crown-ether cyanide treatment, which simply involves immersion in KCN solutions containing 18-crown-6 followed by rinse, were also studied. By the crown-ether cyanide treatment, the luminescence intensity due to the near-band-edge emission of Cu₂O at around 680 nm was enhanced, and the hole density was increased from 10¹⁶ to 10¹⁷ cm⁻³. Finally, polycrystalline p-Cu₂O/n-ZnO heterojunctions were grown for use in solar cells. Two deposition sequences were studied, ZnO deposited on Cu₂O and Cu₂O deposited on ZnO. It was found that the crystallographic orientation and current–voltage characteristics of the heterojunction were significantly influenced by the deposition sequence, both being far superior for the heterojunction with structure Cu₂O on ZnO than for the inverse structure. We successfully obtained a photoresponse for the first time in the deposited thin film of Cu₂O/ZnO.     

CuO/Pd composite photocathodes for photoelectrochemical hydrogen evolution reaction

CuO has been considered as a promising photocathodic material for photoelectrochemical (PEC) hydrogen evolution reaction (HER). In this work, CuO films were prepared by a facile and cost-effective method that involves solution synthesis, spin-coating and thermal treatment processes. The resulting CuO films had a monoclinic crystal structure with bandgap energy of 1.56 eV and a conduction band position of 3.73 eV below the vacuum level in borate buffer solution. The CuO films exhibited good PEC activity toward HER and the preparation conditions had great effect on the activity. The photoactivity of the CuO film decayed to approximately 19% of its original value after reaction for 10 h under illumination. The reduction of CuO to Cu₂O has been confirmed to be a parallel competitive reaction against HER. The mismatched band structure of the resulting CuO/Cu₂O heterojunction was believed to be the main cause of the decay of photoactivity. The photo-assisted electrodeposition method was developed to prepare CuO/Pd composite photocathode. The presence of Pd on CuO greatly increased the photocurrent especially at low overpotentials. In addition, the CuO/Pd composite exhibited significantly improved photostability compared to CuO. This work demonstrates the feasibility of increasing PEC activity and stability of CuO-based photocathodes by combining CuO with noble metal nanoparticles.     

An efficient tandem photoelectrochemical cell composed of FeOOH/TiO2/BiVO4 and Cu2O for self-driven solar water splitting

An integrated solar water splitting tandem cell without external bias was designed using a FeOOH modified TiO₂/BiVO₄ photoanode as a photoanode and p-Cu₂O as a photocathode in this study. An apparent photocurrent (0.37 mA/cm² at operating voltage of +0.36 V_RHE) for the tandem cell without applied bias was measured, which is corresponding to a photoconversion efficiency of 0.46%. Besides, the photocurrent of FeOOH modified TiO₂/BiVO₄–Cu₂O is much higher than the operating point given by pure BiVO₄ and Cu₂O photocathode (∼0.07 mA/cm² at +0.42 V_RHE). Then we established a FeOOH modified TiO₂/BiVO₄–Cu₂O two-electrode system and measured the current density-voltage curves under AM 1.5G illumination. The unassisted photocurrent density is 0.12 mA/cm⁻² and the corresponding amounts of hydrogen and oxygen evolved by the tandem PEC cell without bias are 2.36 μmol/cm² and 1.09 μmol/cm² after testing for 2.5 h. The photoelectrochemical (PEC) properties of the FeOOH modified TiO₂/BiVO₄ photoanode were further studied to demonstrate the electrons transport process of solar water splitting. This aspect provides a fundamental challenge to establish an unbiased and stabilized photoelectrochemical (PEC) solar water splitting tandem cell with higher solar-to-hydrogen efficiency.     

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.     

Characterization and electrochromic properties of CuxNi1−xO films prepared by sol–gel dip-coating

Cu_xNi_1−xO electrochromic thin films were prepared by sol–gel dip coating and characterized by XRD, UV–vis absorption and electrochromic test. XRD results show that the structure of the Cu_x Ni_1−xO thin films is still in cubic NiO structure. UV–vis absorption spectra show that the absorption edges of the Cu_xNi_1−xO films can be tuned from 335 nm (x = 0) to 550 nm (x = 0.3), and the transmittance of the colored films decrease as the content of Cu increases. Cu_xNi_1−xO films show good electrochromic behavior, both the coloring and bleaching time for a Cu_0.2Ni_0.8O film were less than 1 s, with a variation of transmittance up to 75% at the wavelength of 632.8 nm.     

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.     

Molybdenum doped bilayer photoanode nanotubes for enhanced photoelectrochemical water splitting

Conversion of solar energy into hydrogen energy via photoelectrochemical (PEC) water splitting is one of the most promising approaches for generation of clean and sustainable hydrogen energy in order to address the alarming global energy crisis and environmental problems. To achieve superior PEC performance and solar to hydrogen efficiency (STH), identification, synthesis, and development of efficient photoelectrocatalysts with suitable band gap and optoelectronic properties along with high PEC activity and durability is highly imperative. With the aim of improving the performance of our previously reported bilayer photoanode of WO₃ and Nb and N co-doped SnO₂ nanotubes i.e. WO₃-(Sn_0.95Nb_0.05)O₂:N NTs, herein, we report a simple and efficient strategy of molybdenum (Mo) doping into the WO₃ lattice to tailor the optoelectronic properties such as band gap, charge transfer resistance, and carrier density, etc. The Mo doped bilayer i.e. (W_0.98Mo_0.02)O_3-(Sn_0.95Nb_0.05)O₂:N revealed a higher light absorption ability with reduced band gap (1.88 eV) in comparison to that of the undoped bilayer (1.94 eV). In addition, Mo incorporation offered improvements in charge carrier density, photocurrent density, with reduction in charge transfer resistance, contributing to a STH (～3.12%), an applied bias photon-to-current efficiency (ABPE ～ 8% at 0.4 V), including a carrier density (N_d ～ 7.26 × 10²² cm^?3) superior to that of the undoped bilayer photoanode (STH ～2%, ABPE ～ 5.76%, and N_d ～5.11 × 10²² cm^?3, respectively). The substitution of Mo⁶⁺ for W⁶⁺ in the monoclinic lattice, forming the W–O–Mo bonds altered the band structure, realizing further enchantments in the PEC reaction and charge transfer kinetics. Additionally, doped bilayer photoanode revealed excellent long term PEC stability under illumination, suggesting its robustness for PEC water splitting. The present work herein provides a simple and effective Mo doping approach for generation of high performance photoanodes for PEC water splitting.     

Enhanced solar hydrogen generation using Cu–Cu2O integrated polypyrrole nanofibers as heterostructured catalysts

Nanostructured conducting polymeric materials are beneficial for electron conduction and mass transport, showing high photocatalytic performance under visible light. Herein, we report a colloidal synthesis of copper and copper oxides (Cu₂O) modified polypyrrole nanofibers (PPy) heterostructures, which demonstrates significantly high photocatalytic H₂ generation under visible light. The presence of Cu nanoparticles (NPs) of 50 nm and cubic shaped Cu₂O nanoparticles of size 200 nm endows the heterostructures with a large specific surface area as well as good dispersion of nanoparticles on PPy nanofibers allows the migration of electron during catalysis. Cu₂O/PPy exhibits excellent H₂ production (67 mmol h⁻¹) which is 12 times higher than pure PPy (5.7 mmol h⁻¹). The high catalytic activity of Cu₂O/PPy heterostructure provides a fervent alternative to noble metal-based catalysts for the hydrogen generation and water splitting.     

Outstanding stability and photoelectrochemical catalytic performance of (Fe,Ni) co-doped Co3O4 photoelectrodes for solar hydrogen production

In this work, pure and (Fe, Ni) co-doped Co₃O₄ nanostructured photoelectrodes of different doping levels and thicknesses were manufactured at constant substrate temperature (450 °C) using the spray pyrolysis technique. In addition to the chemical compositions; the structural, optical, electrical, and photoelectrochemical (PEC) properties were investigated through the use of various analysis techniques. By increasing the codpoing ratio to 6%, the low energy band gap is decreased from 1.43 to 1.3 eV and the high energy bandgap is increased from 2.63 to 2.87 eV, in addition to the reduction in particle size from 30.2 to 12.0 nm. The high energy gap vanishes by increasing the codoped film's spread volume to 60 ml. X-ray photoelectron spectroscopy of 6%(Fe, Ni)-60ml Co₃O₄ confirms the existence of Ni^2+,3+ and Fe^2+,3+. Among the studied photoelectrodes, the 6%(Fe, Ni)-60ml Co₃O₄ photoelectrode displays a photocatalytic hydrogen output rate of 150 mmol/h.cm² @-1V in 0.3M Na₂SO₄ electrolyte. The photocurrent density of 6%(Fe, Ni)-60ml photoelectrode reached up to 13.6 mA/cm²@-1V with an IPCE (incident photon to current conversion efficiency) of ～42%@405 nm and STH (solar to hydrogen conversion efficiency) of ～11.37%, which are the highest values yet for Co₃O₄-based photocatalysts. The value of ABPE(applied bias photon-to-current efficiency) is 0.34%@(-0.28V and 636 nm). Interestingly, this photoelectrode shows a photogenerated current density of ?0.14 mAcm^?2 at 0 V and a PEC current onset over 0.266V. The thermodynamic parameters, corrosion parameters, PEC surface areas, Tafel slopes, and impedance spectroscopies are also being studied to confirm and classify the PEC H₂ production mechanism. The 6%(Fe, Ni)-60ml Co₃O₄ photoelectrode stability/reusability shows only a 6.6% reduction in PEC performance after ten successive runs at -1V with a corrosion rate of 1.2 nm/year. This work offered a new codoping strategy for the design of a highly active Co₃O₄ based photocatalyst for the generation of solar light-driven hydrogen.     

Bandgap control of p-n heterojunction of Cu–Cu2O @ ZnO with modified reduced graphene oxide nanocomposites for photocatalytic hydrogen evolution

Here, we describe the in-situ synthesis of multicomponent ZnO-based photocatalysts for hydrogen production. We fabricated ZnO coupled with Cu–Cu₂O nanoparticles and modified reduced graphene oxide (mRGO) to ameliorate hydrogen production. The simultaneous introduction of mRGO and Cu–Cu₂O enhanced the generation rate of photocatalytic hydrogen to 3085.02 μmol g^?1 h^?1 due to significant alteration of the electronic structure. The bandgap energy of the prepared catalysts decreased from 3.2 eV for pristine ZnO to 2.64 eV for a composite containing 15% Cu–Cu₂O. The optimal designed heterostructure efficiently separates photo charge carriers and prevents charge carriers’ recombination by accelerating charge transfer with the help of mRGO and metallic Cu and as a result leading to efficient hydrogen yields.     

Precursor self-derived Cu@TiO2 hybrid Schottky junction for enhanced solar-to-hydrogen evolution

Solar-to-hydrogen production has attracted increasing attention since it possesses great potential in alleviating energy and environmental crises to some extent. The key issue is to develop efficient photocatalysts exhibiting superior hydrogen production capability. In this work, Cu@TiO₂ hybrid (Cu nanoparticles encapsulated in TiO₂) has been successfully prepared by Cu₂O self-template reduction through solvothermal treatment in ethylene glycol-water mixed solvent. When octahedral Cu₂O is involved in the reaction system, the Cu₂O@Ti-precursor octahedral structure is first formed and subsequently the Cu@TiO₂ hybrid is prepared with the reduction of ethylene glycol (EG). The Cu@TiO₂ hybrid derived with different mass of Cu exhibits improved photocatalytic hydrogen production performance compare to pure TiO₂ and P25. Among those photocatalysts, the Cu@TiO₂-10% (the copper content is 10 wt%) shows the highest hydrogen evolution rate of 4336.7 μmol g^?1 h^?1, and it is twice as much as the pure TiO₂ and 1.9 times as much as P25, respectively. Based on the photo/electrochemical results, an efficient photo-generated electron-hole separation contributes to the enhancement of photocatalytic H₂ evolution upon the Cu@TiO₂ hybrid. When replacing octahedral Cu₂O with cubic and truncated octahedrons ones, the Cu@TiO₂ hybrid photocatalysts are also obtained and they also display superior solar-to-hydrogen evolution than pure TiO₂ and P25. It is expected this work could develop an approach to design Cu-encapsulated hybrid photocatalysts for hydrogen generation.     

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.     

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.