Cu2O Photoelectrodes Research Articles

Enhanced photoelectrochemical CO2 reduction activity towards selective generation of alcohols over CuxO/SrTiO3 heterojunction photocathodes

Photoelectrochemical reduction (PECR) of CO2 to value-added chemicals and fuels will reduce the dependency on fossil fuels, also it will solve environmental issues that arise due to greenhouse gases. A heterojunction of CuxO/SrTiO3 with varying post-annealing temperature ranges from 300 to 600 °C (CS300-600) was synthesized by overlying the spin-coated SrTiO3 on electrodeposited Cu2O over the FTO glass substrate. The synthesized photoelectrode's crystallinity and phase formation significantly varied by varying the post-annealing temperature, which was characterized via XRD and Raman spectroscopy. Optical, morphological, type of heterojunction formation and elemental surface oxidation states of photoelectrodes were studied through UV–visible DRS spectrum, FE-SEM, and XPS analysis respectively. Electrocatalytic analysis such as Linear Sweep Voltammetry (LSV), and Electrochemical Impedance Spectrometry (EIS) was employed, thus it conforms to the highest photocurrent density (−1.38 mA/cm2 at −0.6V vs. Ag/AgCl), and low charge transfer resistance (RCT=0.412 kΩ) at the electrode-electrolyte interfaces for CS500 photoelectrode as compared to others photoelectrodes. PECR of CO2 to liquid product formation was evaluated by applying different potential ranges (0 to −0.6 V vs. Ag/AgCl). Highest methanol formation of 48.69 μmol.cm−2hr−1, which is approximately 9 times enhanced as compared to the pure Cu2O (5.62 μmol.cm−2hr−1) photoelectrode at 0 V vs Ag/AgCl for CS500 heterojunction. Optimization of overlaying SrTiO3 synthesis temperature for crystallization formation on Cu2O and applied photoelectrode potential findings could pave the way for designing other new heterojunction types for selective liquid alcohol production.

Investigating the effect of Cu underlayer and FTO etching towards photoelectrochemical performance enhancement of Cu2O photoelectrode

Cuprous oxide (Cu2O) exhibits potential as a photoactive material for photoelectrochemical water splitting, owing to its appropriate bandgap, efficient charge carrier separation, and ability to enhance solar-driven hydrogen production. This study investigates the influence of substrate etching, Cu underlayer and Cu2O electrodeposition time, and annealing time on enhancing the photoelectrochemical (PEC) performance. Electrodeposition and thermal oxidation techniques were used to fabricate the Cu2O/Cu/FTOe-A photocathode. It has been observed that FTO etching improves adhesion, light transmission, and efficiency. A Cu underlayer also impacts the PEC performance, wherein an ideal thickness of Cu leads to enhanced PEC performance. This study also focuses on the annealing time that leads to CuO layers and nanowires forming on the Cu2O surface. The structural and chemical changes before and after annealing are confirmed via XRD, XPS, AFM and FESEM analyses. UV–Vis analysis also reveals that the presence of Cu underlayer, FTO etching, and the annealing process affect the electrical properties and light absorption capacities of the Cu2O photoelectrode. Electrochemical impedance analysis (EIS) and Mott-Schottky analysis have provided insights into the enhanced charge transfer properties and band bending in the Cu2O/Cu/FTOe-A, resulting in enhanced PEC performance. Overall, this study provides significant insights into the understanding and enhancement of Cu2O/Cu/FTOe-A photocathodes for potential use in PEC water splitting applications.

Environment-Benign Colloidal Quantum Dots-Modified Dual Photoelectrodes for Self-Biased Photoelectrochemical Water Splitting.

Photoelectrochemical (PEC) water splitting based on colloidal quantum dots (QDs) presents a promising approach for utilizing solar energy to produce green hydrogen energy. Previous research has been mainly focused on the single-photoelectrode QDs-PEC device operated under external bias, while the investigation of dual-photoelectrode configuration for self-biased QDs-PEC system is still lacking. In this work, two types of eco-friendly Cu-AISe/ZnSe:Cu (CZAC) and Mn-AIS/ZnS@Cu (MAZC) QDs were used to respectively sensitize the semiconductor n-type TiO2 and p-type Cu2O photoelectrodes, which acted as the photoanode and photocathode to build a heavy metal-free QDs-based bias-free solar water splitting cell, yielding a maximum photocurrent density of 0.47 mA cm-2 and a solar-to-hydrogen (STH) efficiency of 0.4 % under 1 sun AM 1.5G illumination (100 mW cm-2). Moreover, approximate 692 nmol of H2 and 355 nmol of O2 with molar ratio of ~2 : 1 was detected after two hours of continuous light illumination, demonstrating the effective overall water splitting. This work indicates a significant advancement towards the realization of a cost-effective, efficient and "green" QDs-based artificial solar-to-fuel conversion system.

High carrier mobility along the [111] orientation in Cu2O photoelectrodes

Solar fuels offer a promising approach to provide sustainable fuels by harnessing sunlight1,2. Following a decade of advancement, Cu2O photocathodes are capable of delivering a performance comparable to that of photoelectrodes with established photovoltaic materials3–5. However, considerable bulk charge carrier recombination that is poorly understood still limits further advances in performance6. Here we demonstrate performance of Cu2O photocathodes beyond the state-of-the-art by exploiting a new conceptual understanding of carrier recombination and transport in single-crystal Cu2O thin films. Using ambient liquid-phase epitaxy, we present a new method to grow single-crystal Cu2O samples with three crystal orientations. Broadband femtosecond transient reflection spectroscopy measurements were used to quantify anisotropic optoelectronic properties, through which the carrier mobility along the [111] direction was found to be an order of magnitude higher than those along other orientations. Driven by these findings, we developed a polycrystalline Cu2O photocathode with an extraordinarily pure (111) orientation and (111) terminating facets using a simple and low-cost method, which delivers 7 mA cm−2 current density (more than 70% improvement compared to that of state-of-the-art electrodeposited devices) at 0.5 V versus a reversible hydrogen electrode under air mass 1.5 G illumination, and stable operation over at least 120 h.

Modulation of Cu/Cu2O nanoparticles to promote the photocurrent response for light-enhanced pseudocapacitive charge storage

Bifunctional CC@Cu/Cu2O photoelectrodes are prepared for light-enhanced pseudocapacitive charge storage, and have achieved a high capacitance retention of 92% after 5000 cycles under light irradiation.

The optimization of CuxO microwires synthesis for improvement in photoelectrochemical performance

One of the most important challenges in the fabrication of CuxO microstructures via the electrochemical method is formation of long, regular and well-packed microwires with good adhesion on the Cu substrate and to achieve better photoelectrochemical properties, which can be potentially applied in solar-driven water splitting and CO2 conversion to light hydrocarbons. In this paper, Cu2O photoelectrode has been fabricated by direct anodization of the Cu foil, then CuO microwires (MWs) was formed by calcination process. The effect of the applied potential (10–40 V), sodium fluoride content in the electrolyte (0.2–0.5% wt.) as well as calcination conditions (400–500 °C, 60–360 min) on the morphology, phase composition, optical and photoelectrochemical properties was investigated. The highest photocurrent response was −0.71 mA cm−2 for the sample 0.35%_10V_400 °C_60min with 0.83 μm length and 154 nm diameter. The mechanism of the photocurrent generation process in the presence of CuxO/Cu under UV–Vis irradiation was proposed.

Modulating the Cu2O Photoelectrode/Electrolyte Interface with Bilayer Surfactant Simulating Cell Membranes for Boosting Photoelectrochemical CO2 Reduction.

The low solubility of CO2 molecules and the competition of the hydrogen evolution reaction (HER) in aqueous electrolytes pose significant challenges to the current photoelectrochemical (PEC) CO2 reduction reaction. In this study, inspired by the bilayer phospholipid molecular structure of cell membranes, we developed a Cu2O/Sn photocathode that was modified with the bilayer surfactant DHAB for achieving high CO2 permeability and suppressed HER. The Cu2O/Sn/DHAB photocathode stabilizes the *OCHO intermediate and facilitates the production of HCOOH. Our findings show that the Faradaic efficiency (FE) of HCOOH by the Cu2O/Sn/DHAB photoelectrode is 83.3%, significantly higher than that achieved with the Cu2O photoelectrode (FEHCOOH = 30.1%). Furthermore, the FEH2 produced by the Cu2O/Sn/DHAB photoelectrode is only 2.95% at -0.6 V vs RHE. The generation rate of HCOOH by the Cu2O/Sn/DHAB photoelectrode reaches 1.52 mmol·cm-2·h-1·L-1 at -0.7 V vs RHE. Our study provides a novel approach for the design of efficient photocathodes for CO2 reduction.

Rational design and fabrication of Cu2O film as photoelectrode for water splitting

Cu2O can be n-type and p-type depending on the oxygen and copper vacancies as either photocathode or photoanode for water splitting. However, the synthesis of n-type Cu2O is still a challenge. At the condition of pH= 6 and ΔU= −0.05 V, n-type Cu2O films can be electrodeposited at varied temperatures. The bandgap of n-type Cu2O films is in the range of 1.6–2.0 eV with the impurity level of 1.57 eV. Among them, n-type Cu2O films deposited at 60 °C achieved the highest photocurrent in water splitting (pH=7) as the light on due to the less impurity concentration and more highly texture structure. The n-type Cu2O photoelectrode shows a stable LSV cycling performance in Na2SO4 solution as compared to the buffer acetate and phosphate solution which may be due to the formation of Cu(OH)2 layer. This work can benefit the electrodeposition of n-type Cu2O for future water splitting applications.

Investigation of non-stoichiometric delafossite photoelectrodes composed of cuprous oxide and iron oxide for enhanced water-splitting

Photoelectrochemical energy conversion is the ultimate approach for converting sunlight energy into value-added chemical fuels. The path of achieving economically feasible, highly efficient, and earth-abundant material-based photoelectrodes can be a driving force for realizing PEC on a global scale. Copper-based delafossite photoelectrodes are attractive candidates because they exhibit appropriate absorption properties and structural stability in an aqueous system. In this study, delafossite CuxFe2−xO2 films of varying compositions ranging from 0 ≤ x ≤ 2 are prepared by adjusting the Cu/Fe composition via radio frequency plasma co-sputtering to evaluate the fundamental optoelectronic properties. The optical bandgap of the CuFeO2 films tuned from 1.4 to 1.65 eV, with the maximum carrier concentration of 5.9 × 1017 cm−3 noticed in Cu1.5Fe0.5O2 film. The X-ray photoelectron spectroscopy measurements confirm the existence of a high level of oxygen-deficient sites in CuFeO2 film photoelectrodes that are suitable for PEC performance. The fabricated non-stoichiometric photoelectrode exhibits an excellent hydrogen evolution reaction of 4.3 μmol/hcm2, nearly 1.8 times improvement over Cu2O photoelectrode with a long-term photostability even in an aqueous system. The ability to fabricate tunable electronic structures photoelectrodes along with oxygen-deficient surface defects facilitates to localize band edge closer to the HER of water, and the Fermi level is slightly away from the reduction potential of Cu2O, which supports to improve PEC performance by suppressing photo-induced corrosion to retain greater photostability.

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.

Investigation and mitigation of degradation mechanisms in Cu2O photoelectrodes for CO2 reduction to ethylene

Investigation and mitigation of degradation mechanisms in Cu2O photoelectrodes for CO2 reduction to ethylene

Bundle-Type Columnar Cu2O Photoabsorbers with Vertical Grain Boundaries Fabricated Using Instant Strike-Processed Metallic Seeds and Their Enhanced Photoelectrochemical Efficiency

The design to control grain boundaries (GBs) causing recombination losses as planar defects in the absorbing layers is an essential strategy for the development of highly efficient photoelectrochemical (PEC) reaction systems. We propose a method to design a progressive bundle-type columnar structure as an alternative to single crystals with the least defects, where the GBs are parallel to the charge transport movement and the electric field direction. An instant strike bias (0.01 s and −1 V vs Ag/AgCl) in the same electrolytes induces the formation of island-shaped metallic Cu nanoparticles in the initial stage. These act as seed crystals for controlling Cu₂O growth evolution, resulting in dramatically high-density Cu₂O nuclei. This is followed by the deposition of bundle-type columnar Cu₂O with longitudinal GBs, contrary to the typical randomly crystallized Cu₂O. Metallic Cu seeds with a stronger electric field than that of the exposed indium tin oxide (ITO) region provide selective crystallization sites for Cu₂O growth along the ⟨111⟩ ionic bonding. Despite the instant strike interval, the p-type Cu₂O photoelectrodes retained an outstanding photocurrent density of 5.2 mA cm–² at 0 VRHE and an onset potential of 0.7 VRHE because of the highly improved charge transport and transfer efficiencies inside Cu₂O induced by effectively suppressing charge scattering in the GBs. The impedance measurements depict the overall decrease in the total resistance, charge-transfer resistance, and the photoabsorber/electrode interface resistance of the PEC systems with the introduction of the instant strike process.

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 NiOx (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/NiOx photocathodes, respectively. Furthermore, the electrochemical impedance spectroscopy reveals that electrode protected with NiOx 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.

Bifunctional photoelectrochemical process for humic acid degradation and hydrogen production using multi-layered p-type Cu2O photoelectrodes with plasmonic Au@TiO2

Bifunctional photoelectrochemical (PEC) process for simultaneous hydrogen production and mineralisation of humic acid in water using TiO2-1 wt% Au@TiO2/Al2O3/Cu2O multi-layered p-type photoelectrodes is demonstrated. The newly designed bifunctional PEC system leads to a high degradation efficiency of dissolved humic compounds, the target pollutant, by up to 87% during 2 h reaction time. Simultaneously, humic acid is also served as a sacrificial electron donor in the proposed system, contributing to a high photocurrent density of the multi-layered p-type Cu2O photoelectrodes up to -6.32 mA cm−2 at 0 V vs. Reversible Hydrogen Electrode (RHE) under the AM 1.5 simulated 1-Sun solar illumination. The Z-scheme feature of this bifunctional PEC devices exhibiting a short-circuit photocurrent density of -0.45 mA cm−2 and solar-to-hydrogen conversion (STH) of 0.5 % in the presence of humic acid sheds light on the new bias-free artificial photosynthesis PEC system.

3D-printed Cu2O photoelectrodes for photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting is an alternative to fossil fuel combustion involving the generation of renewable hydrogen without environmental pollution or greenhouse gas emissions. Cuprous oxide (Cu2O) is a promising semiconducting material for the simple reduction of hydrogen from water, in which the conduction band edge is slightly negative compared to the water reduction potential. However, the solar-to-hydrogen conversion efficiency of Cu2O is lower than the theoretical value due to a short carrier-diffusion length under the effective light absorption depth. Thus, increasing light absorption in the electrode–electrolyte interfacial layer of a Cu2O photoelectrode can enhance PEC performance. In this study, a Cu2O 3D photoelectrode comprised of pyramid arrays was fabricated using a two-step method involving direct-ink-writing of graphene structures. This was followed by the electrodeposition of a Cu current-collecting layer and a p–n homojunction Cu2O photocatalyst layer onto the printed structures. The performance for PEC water splitting was enhanced by increasing the total light absorption area (Aa) of the photoelectrode via controlling the electrode topography. The 3D photoelectrode (Aa = 3.2 cm2) printed on the substrate area of 1.0 cm2 exhibited a photocurrent (Iph) of −3.01 mA at 0.02 V (vs. RHE), which is approximately three times higher than that of a planar photoelectrode with an Aa = 1.0 cm2 (Iph = −0.91 mA). Our 3D printing strategy provides a flexible approach for the design and the fabrication of highly efficient PEC photoelectrodes.

FeOOH as hole transfer layer to retard the photocorrosion of Cu2O for enhanced photoelctrochemical performance

Photocorrosion of Cu2O photoelectrode regarded as a serious factor extremely restricts its photoactivity. So far, nonetheless, there are few comprehensive reports on the mechanism affecting the stability of Cu2O photocathode in photoelctrochemical (PEC) water splitting. In this work, we firstly explore the influence mechanism for the stability of Cu2O photocathode, revealing that self-oxidation caused by accumulation of photogenerated holes is the main reason to the occurrence of Cu2O photoelectrode photocorrosion. By knowing the dominant deactivation mode of Cu2O photoelectrode, we firstly select FeOOH as a hole transfer layer (HTL) to promote the extraction of photogenerated holes from the bulk of bare Cu2O photoelectrode and thus suppress the self-photooxidation of Cu2O. Furthermore, restraint of photocorrosion combined with increase in its total photoelectrochemistry activity can be accomplished after the surface of FeOOH/Cu2O photoelectrode activated with Pt co-catalyst. This work provides a novel strategy and method for improving the stability of photoelectrodes due to self-redox reactions.

Femtosecond time-resolved two-photon photoemission studies of ultrafast carrier relaxation in Cu2O photoelectrodes

Cuprous oxide (Cu2O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu2O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu2O surface as well as the interface between Cu2O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu2O and provide guidance for future material development.

Experimental and Theoretical Study of Cu2O Photoelectrode and Cu2O Doped with Ag, Co, Ni and Zn Metals for Water Splitting Application

Experimental and Theoretical Study of Cu2O Photoelectrode and Cu2O Doped with Ag, Co, Ni and Zn Metals for Water Splitting Application

Electrochemical Fabrication of Cu2o-Nanowires-Based Photo-Cathodes for Enhanced Solar Water Splitting

Solar-power driven photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes is one of the most promising approaches for energy conversion, in terms of production of renewable hydrogen with a minimum carbon footprint. There are, however, enormous challenges to be overcome to obtain sufficient solar water-splitting efficiency, such as over-potential losses due to fast recombination rates of charge carriers, electrode photo-degradation, limited light-harvesting capacities, to name a few. In this work, we report an extensive study on Cu2O based photocathode for hydrogen evolution reaction (HER) in terms of enhancing its solar-to-hydrogen (STH) conversion efficiency, while using low-cost fabrication methods. We have addressed the inefficiency of Cu2O which mainly arises due to incompatible light absorption and charge carrier diffusion lengths, by fabricating highly reproducible Cu2O-nanowire arrays on Cu substrate. The reported optimal thickness of Cu2O for efficient absorption of sunlight is around 2-4 µm, while the minority charge carrier diffusion length is limited to 200-250 nm only. The nanostructured Cu2O, especially in the form of nanowire arrays, improves its performance through morphology control, by improving the light-harvesting capacity along the full length of wire and providing a shorter diffusion path length corresponding to the radial distance in the wire, towards the electrolyte. We prepared the photocathode by anodizing Cu foil to Cu(OH)2 nanowires at 10 mA/cm2, followed by oxidation to Cu2O nanowires in an inert atmosphere. Experimental measurements showed that the process was highly reproducible and resulted in an increased current density from 0.5 mA/cm2 for planar Cu2O to 82.3 mA/cm2 for Cu2O nanowires, both at 1 V cathodic bias. The photocurrent density was enhanced from 0.15 mA/cm2 for planar Cu2O to 5.54 mA/cm2 for the nanowire morphology, while using visible light source in all cases. All the photoelectrodes were reproducible in the regime of 4 µm thicknesses of Cu2O nanowires, oxidized over Cu-foil substrates. The STH efficiency calculated for all the fabricated photoelectrodes was as high as 3.54±0.21%, in comparison to 0.24% for the planar Cu2O photoelectrodes. Keywords: Cu2O, nanowires, photocathode, solar water splitting, photoelectrochemistry, STH efficiency, PEC.

Photostable Cu2O photoelectrodes fabricated by facile Zn-doping electrodeposition

Cu2O is a promising semiconductor material for photoelectrochemical (PEC) cells owing to element abundance, nontoxicity, good mobility and broad visible-spectrum light absorption. However, its practical applications are always limited by poor photostability. In this work, we report a facile Zn-doping electrodeposition approach to highly stable Cu2O photoelectrodes, which allows tailoring compositions and crystalline structures by simply altering the usage of ZnSO4 solution. As a result, the optimal Zn-doped photoelectrode demonstrates a high stability of 92.97%. The Zn cation dopant contributes to improving carrier concentration and charge transfer; however, too high concentration of Zn doping may induce charge recombination to lower initial photocurrent density. Meanwhile, the Zn doping can suppress the oxidation of Cu2O to CuO so that the photocurrent density maintains in a limited oscillation mode rather than undergoes an exponential degradation. This work represents a step toward the fabrication of low-cost, stable and visible-responsive photoelectrodes.