Promising Photocathode Material Research Articles

Tailoring Phase Transition of Mo-S-Te Ternary System using Heat-Driven Process for Target Functionalities

Although structural engineering and phase modulation of transition metal dichalcogenides (TMDs) can be used to implement high-performance optoelectronic devices or energy cells, their expandability combined with individualized optimization for diverse applications using limited TMD materials is still challenging. Herein, we suggest an intriguing strategy to control the target functionalities of the Mo-S-Te ternary system using heat-driven crystallographic selection. The structure and phase alteration of the Mo-S-Te system can be effectively governed by modulating the thermal decomposition temperature, as evidenced by comprehensive spectroscopic and microscopic evaluations. The dual eligibility of the Mo-S-Te ternary system was validated by ascertaining the temperature-dependent selective enhancement of the optoelectronic properties or photoelectrochemical (PEC) performance as compared with those of bare MoS2. The Mo-S-Te synthesized at 300 °C exhibited superior photoresponse dynamics across various wavelengths, indicating its potential for advanced photodetection applications. In field of PEC, Mo-S-Te synthesized at 500 °C was highlighted as a promising photocathode material, offering enhanced PEC reactivity and stability. This study suggests the importance of material characteristics tailored to the synthetic conditions for target applications, which is a promising path to for the expansion of applicability in various areas using a simple approach that utilizes a limited range of TMD materials.

Aerosol Deposition of CuFeO2 Photocathode Coatings for Hydrogen Production

AbstractPhotoelectrochemical (PEC) water splitting is a viable route for green hydrogen generation. In PEC cells, the electrodes are coated with suitable semiconductor materials, which absorb the sunlight, generating charge carriers that are used to split water molecules into H2 and O2. CuFeO2 is one promising photocathode material for water splitting. However, its performance is limited by electron/hole pairs recombination within the film and at the film/substrate interface. Aerosol deposition (AD) can be employed to minimize charge recombination by spraying dense, thin films and by establishing a good back-contact interface. In this study, CuFeO2 powders were synthesized through a conventional solid-state technique and sprayed by AD under varied parameter sets. The effect of particle size distributions, carrier gas, gas pressure and substrate temperature was investigated. The best spraying parameter set was then tuned to obtain thin coatings (< 1 µm). Single-particle deformation and coatings microstructure were investigated by scanning electron microscopy. Optical properties of CuFeO2 films were analyzed by UV–Vis spectroscopy, while photoelectrochemical performances were estimated through amperometry tests under simulated sunlight. The results of this research show that CuFeO2 photocathodes can be successfully manufactured by AD. Their performance can be optimized by adjusting coating thickness and by annealing in air.

A Visible Light-Responsive TiO2 Photocathode Achieved by a Rh Dopant.

Developing an efficient and stable photocathode material for photoelectrochemical solar water splitting remains challenging. Herein, we demonstrate the potential of rutile TiO2 as a photocathode by Rh doping with visible light absorption up to 640 nm and an onset potential of 0.9 V versus the reversible hydrogen electrode. The dopant transforms the rutile host from an n-type semiconductor to a p-type one, as confirmed by the Mott-Schottky curve and kelvin probe force microscopy. Physical and photoelectrochemical analyses further suggest that the doping mechanism is dependent on concentration. Lower levels of dopants generate localized Rh3+, while higher levels favor Rh4+ that interacts more strongly with the O 2p orbitals. The latter is found not only to extend the visible light absorption range but also to facilitate charge transport. This work elucidates the role of the Rh dopant in adjusting the photoelectrochemical behavior of TiO2, and it provides a promising photocathode material for solar energy conversion.

Photoelectrocatalytic Reduction of CO2 to CO via Cu2O/C/PTFE Nanowires Photocathodes

AbstractThe consumption of fossil fuels releases large amounts of carbon dioxide (CO2) in the atmosphere, causing a serious greenhouse effect. Photoelectrochemical (PEC) reduction of CO2 to chemical fuels is an effective way to alleviate the current energy and environmental crisis. However, it is still difficult to rationally design efficient PEC CO2 reduction photocathodes. Cuprous oxide (Cu2O) is a promising photocathode material, but its surface is susceptible to the accumulation of photogenerated electrons leading to corrosion and activity reduction, and is accompanied by hydrogen evolution reaction (HER), both of which lead to the overall low conversion efficiency of CO2 reduction by Cu2O. In this study, the PEC CO2 conversion efficiency was improved by the synergistic effect of the C electron transport layer to accelerate the electron transfer to alleviate the Cu2O corrosion problem and the polytetrafluoroethylene (PTFE) hydrophobic layer to inhibit the HER. The test showed that the CO yield of Cu2O/C/PTFE at the optimum potential (−0.7 V vs. RHE) was 54.6 μmol cm−2 h−1, which was 3.2 times higher than that of pure Cu2O. This study provides a facile strategy for constructing an efficient photocathode with great potential for CO2 reduction.

Key Strategies on Cu2O Photocathodes toward Practical Photoelectrochemical Water Splitting.

Cuprous oxide (Cu2O) has been intensively in the limelight as a promising photocathode material for photoelectrochemical (PEC) water splitting. The state-of-the-art Cu2O photocathode consists of a back contact layer for transporting the holes, an overlayer for accelerating charge separation, a protection layer for prohibiting the photocorrosion, and a hydrogen evolution reaction (HER) catalyst for reducing the overpotential of HER, as well as a Cu2O layer for absorbing sunlight. In this review, the fundamentals and recent research progress on these components of efficient and durable Cu2O photocathodes are analyzed in detail. Furthermore, key strategies on the development of Cu2O photocathodes for the practical PEC water-splitting system are suggested. It provides the specific guidelines on the future research direction for the practical application of a PEC water-splitting system based on Cu2O photocathodes.

Growth and Photoelectrochemical Characterization of Epitaxial ZnTe Photocathodes for Carbon Dioxide Reduction

Harnessing solar energy to drive photocatalytic carbon dioxide reduction reactions (CO2RR) provides an appealing pathway to generate hydrocarbon and oxygenated fuels without an external power source. Zinc telluride (ZnTe) is a II-VI semiconductor which has been identified as a promising photocathode material due to its suitable band gap alignments to CO2 reduction reaction potentials, chemical stability, and strong p-type character. Using molecular beam epitaxy (MBE), single crystal and epitaxial layers are synthesized to gain a deeper understanding of fundamental charge transport and reactivity mechanisms between the single crystal ZnTe thin film and the CO2-saturated electrolyte. These findings are of fundamental interest and are also critical to the design of efficient tandem solar fuels generators for unassisted photoelectrochemical CO2R. Epitaxy allows for highly controlled doping of the thin films over a large range of carrier concentrations. This work focuses largely on the synthesis and characterization of nitrogen doped p-type ZnTe via MBE. Films grown in the temperature range of 340–360ºC on GaAs of (100) orientation with a 200 nm undoped ZnTe buffer layer and a 100 nm doped ZnTe layer have been characterized by RHEED, XRD, AFM, and Hall effect measurements. Doping concentrations between 1020 cm-3 and 1018 cm-3 have been achieved. Dark current-voltage measurements have been used to indicate stability of the electrode in aqueous conditions in less than -0.5 V vs RHE. Future work will include further investigations into carrier dynamics via transient absorption spectroscopy and continual development of a tandem, ZnTe-based photocathode.

Effect of charge selective contacts on the quasi Fermi level splitting of CuGa3Se5 thin film photocathodes for hydrogen evolution and methylviologen reduction

The defect-copper chalcopyrite CuGa3Se5 is a promising photocathode material for solar hydrogen generation. Here we assess its performance with photoelectrochemical measurements and vibrating Kelvin probe surface photovoltage spectroscopy.

Evaluation of the Efficiency of Photoelectrochemical Activity Enhancement for the Nanostructured LaFeO3 Photocathode by Surface Passivation and Co-Catalyst Deposition.

Perovskite-type lanthanum iron oxide, LaFeO3, is a promising photocathode material that can achieve water splitting under visible light. However, the performance of this photoelectrode material is limited by significant electron-hole recombination. In this work, we explore different strategies to optimize the activity of a nanostructured porous LaFeO3 film, which demonstrates enhanced photoelectrocatalytic activity due to the reduced diffusion length of the charge carriers. We found that surface passivation is not an efficient approach for enhancing the photoelectrochemical performance of LaFeO3, as it is sufficiently stable under photoelectrocatalytic conditions. Instead, the deposition of a Pt co-catalyst was shown to be essential for maximizing the photoelectrochemical activity both in hydrogen evolution and oxygen reduction reactions. Illumination-induced band edge unpinning was found to be a major challenge for the further development of LaFeO3 photocathodes for water-splitting applications.

Insights into Boosting Photoelectrochemical Performance Over Cu3(BTC)2 Passivated Cu2O Nanorod Arrays

AbstractEarth‐abundant Cu2O is a promising photocathode material for photoelectrochemical (PEC) water splitting. However, the serious photocorrosion and low efficiency retard its practical applications. Herein, a porous Cu‐based metal–organic framework material, Cu3(BTC)2, as surface passivation layer is grown on Cu2O nanorod arrays (NRARs) photocathode by in situ surface/interfacial growth. Compared to conventional Cu2O photocathodes, the optimized Cu2O/Cu3(BTC)2 NRARs exhibit two times higher photocurrent density, ≈2.73 mA cm−2 at 0 VRHE under visible light irradiation, and the maximum incident photon to converted electron (IPCE) performance reaches 45% (450 nm). It is demonstrated that the Cu3(BTC)2 layer synergistically passivates interfacial defects, forms p–n junctions at the Cu2O/Cu3(BTC)2 interface, and introduces active sites catalyzing the hydrogen evolution reaction (HER), improving charge separation, transfer, and utilization efficiency. Moreover, the oxidative Cu2O photocorrosion is inhibited owing to the accelerated charge dynamics at the Cu2O/Cu3(BTC)2 interface and the hole‐scavenging ability of Cu3(BTC)2, meanwhile, reductive Cu2O photocorrosion at Cu2O/Cu3(BTC)2 is also attenuated by catalyzing the water reduction reaction that competes for photoelectrons and sustaining balanced “Cu2+(Cu+)/Cu0” cycles.

Theoretical analysis and experimental research of photon-enhanced thermionic emission solar energy converters with InN photocathode

In this study, an InN photocathode consisting of ZnO nanowire arrays was proposed as a cathode for a photon-enhanced thermionic emission (PETE) solar energy converter. To optimize the performance of the device, we applied InN with a phonon bottleneck effect to the PETE converter. The theoretical model for PETE conversion with this complex photocathode was developed based on one-dimensional continuous equations and boundary conditions. We simulated and analyzed the conversion efficiency as a function of operating temperature, solar concentration, surface electron affinity, and operating voltage. Furthermore, we verified the feasibility of the proposed cathode through experiments on the current-voltage characteristics of the InN photocathode at different temperatures. The results of simulations and experiments indicated that InN is a promising photocathode material and may enable high-performance PETE devices. • An InN photocathode consisting of ZnO nanowire arrays is proposed and applied to a PETE solar converter. • By using one-dimensional continuity equations, the theoretical model with this complex photocathode is deduced. • The relationships between PETE conversion efficiency of InN photocathode and various parameters are simulated and analyzed. • The feasibility of the proposed cathode is verified by experiments on the I-V characteristics at different temperatures.

Stabilizing CuO photocathode with a Cu3N protection shell

CuO, as a promising photocathode material, suffers from severe photocorrosion in photoelectrochemical water splitting applications. Herein, a Cu3N protection shell was used to protect the CuO photocathode for the first time to effectively suppress the photocorrosion of CuO. Consequently, the Cu3N-protected CuO photocathode shows improved stability, retaining 80% of its initial current density in a 20-min test, while only 10% of the initial current density can be retained for the bare photocathode. This work may provide an important strategy for using Cu3N shells to stabilize unstable photocathodes.

Structural and Compositional Investigations on the Stability of Cuprous Oxide Nanowire Photocathodes for Photoelectrochemical Water Splitting.

Cuprous oxide (Cu2O) is a promising photocathode material for photoelectrochemical (PEC) water splitting. Recently, the PEC performances of Cu2O-based devices have been considerably improved by introducing nanostructures, semiconductor overlayers, and hydrogen evolution reaction (HER) catalysts. However, Cu2O devices still suffer from poor stability in aqueous solution, especially in strong acidic or alkaline conditions, despite the use of an intrinsically stable oxide overlayer as a protection layer. Thus, it is essential to fully understand the stability of the entire Cu2O photocathodes in these conditions for establishing suitable protection strategies to achieve durable PEC water splitting. In this work, the stability of bare and protected Cu2O nanowire (NW) photocathodes was evaluated in detail using microscopy techniques and compositional analyses. The insights gained in this work will guide the design and synthesis of durable photoelectrodes for PEC water splitting.

CuFeO2–Water Interface under Illumination: Structural, Electronic, and Catalytic Implications for the Hydrogen Evolution Reaction

CuFeO2 is a p-type semiconductor that has been recently identified as a promising photocathode material for photoelectrochemical water splitting. CuFeO2 can absorb solar light and promote the hydrogen evolution reaction (HER), even though the photocurrents achieved so far are still well below the theoretical upper limit. While several experimental and theoretical works have provided a detailed characterization of the bulk properties of this material, surfaces have been largely unexplored. In this work, we perform first-principles simulations based on DFT to investigate the structure, electronic properties, and thermodynamic stability of CuFeO2 surfaces both in vacuum and in an electrochemical environment. To estimate the alignment of the band edges on the electrochemical scale, we perform ab initio molecular dynamics in explicit water, unraveling the structure of the solid/liquid interface for various surface terminations. We consider the system both in the dark and under illumination, showing that light absorption can induce partial reduction of the surface, giving rise to states in the gap that can pin the Fermi level, in agreement with recent measurements. Using the free energy of adsorption of atomic hydrogen as a descriptor of the catalytic activity for the HER, we show that hydride species formed at oxygen vacancies can be highly active and could therefore be an intermediate of reaction.

Controllable Distribution of Oxygen Vacancies in Grain Boundaries of p‐Si/TiO2 Heterojunction Photocathodes for Solar Water Splitting

AbstractSilicon is a promising photocathode material in photoelectrochemical water splitting for hydrogen production, but it is primarily limited by photocorrosion in aqueous electrolytes. As an extensively used protective material, crystalline TiO2 could protect Si photoelectrode against corrosion. However, a large number of grain boundaries (GBs) in polycrystalline TiO2 would induce excessive recombination centers, impeding the carrier transport. This paper describes the introduction of oxygen vacancies (Ovac) with controllable spatial distribution for GBs to promote carrier transport. Two kinds of Ovac distribution, Ovac along GBs and Ovac inside grains, are compared, where the latter one is demonstrated to facilitate carrier transport owing to the formation of tunneling paths across GBs. Consequently, a simple p‐Si/TiO2/Pt heterojunction photocathode with controllable Ovac distribution in TiO2 shows a +400 mV onset potential shift and yields an applied bias photon‐to‐current efficiency of 5.9 %, which is the best efficiency reported among silicon photocathodes except for silicon homojunction.

Controllable Distribution of Oxygen Vacancies in Grain Boundaries of p-Si/TiO2 Heterojunction Photocathodes for Solar Water Splitting.

Silicon is a promising photocathode material in photoelectrochemical water splitting for hydrogen production, but it is primarily limited by photocorrosion in aqueous electrolytes. As an extensively used protective material, crystalline TiO2 could protect Si photoelectrode against corrosion. However, a large number of grain boundaries (GBs) in polycrystalline TiO2 would induce excessive recombination centers, impeding the carrier transport. This paper describes the introduction of oxygen vacancies (Ovac ) with controllable spatial distribution for GBs to promote carrier transport. Two kinds of Ovac distribution, Ovac along GBs and Ovac inside grains, are compared, where the latter one is demonstrated to facilitate carrier transport owing to the formation of tunneling paths across GBs. Consequently, a simple p-Si/TiO2 /Pt heterojunction photocathode with controllable Ovac distribution in TiO2 shows a +400 mV onset potential shift and yields an applied bias photon-to-current efficiency of 5.9 %, which is the best efficiency reported among silicon photocathodes except for silicon homojunction.

High-performance bulky crystalline copper bismuthate photocathode for enhanced solar water splitting

Copper bismuthate (CuBi2O4, CBO) has attracted attention as a promising photocathode material for photoelectrochemical (PEC) water-splitting because of its small bandgap (1.6–1.8 eV), high internal photovoltage, and moderate stability in aqueous media. Herein, we report a novel solution synthesis method to fabricate bulky crystalline CBO photocathodes via an evaporation decomposition controlled process. The CBO photocathode synthesized under optimal conditions exhibited a uniform, dense film with intimate substrate contact and large-grains, allowing an enhanced charge transport and photocurrent stability in comparison to porous CBO counterparts. Consequently, the dense CBO achieved a photocurrent density of − 1.16 mA/cm2 at 0.4 V versus the reversible hydrogen electrode (RHE) without a scavenger addition under simulated sunlight irradiation (AM 1.5 G, 100 mW/cm2). Significantly, the coupling of the CBO with a cupric oxide (CuO) underlayer and a Pt electrocatalyst increased the photocurrent density further to − 2.8 and − 3.5 mA/cm2 at 0.4 V versus RHE, respectively, which is the highest value to date among all reported CBO based photocathodes.

Magnetron sputtered copper bismuth oxide photocathodes for solar water reduction

There is an urgent need for new materials that can catalyze or drive the photoelectrochemical (PEC) conversion of solar energy into chemical energy, i.e. solar fuels. Copper bismuth oxide (CBO) is a promising photocathode material for the photochemical reduction of water. Here, we systematically control the stoichiometry of CBO thin films prepared by reactive, direct-current magnetron co-sputtering from metallic Bi and Cu targets. The intrinsic photophysical and PEC material properties are investigated and evaluated in order to determine the optimum composition for hydrogen formation. Changing the stoichiometry of the films reveals a dramatic change in the optical band gap and crystal structure of CBO. The largest photocurrent density was achieved for a copper-to-bismuth ion ratio of 0.53, close to the CuBi2O4 stoichiometry, which yielded Jph = − 0.48 mA cm−2 at 0 VRHE (RHE = reversible hydrogen electrode). This is the highest value to date for the photochemical reduction of water with CuBi2O4 without an externally applied bias. The absorbed photon-to-current efficiency and the photostability of the films in neutral and alkaline electrolytes were also investigated.

All-Electrochemically Grown Sb2Se3/a-MoSx Photocathodes for Hydrogen Production: The Effect of the MoSx Layer on the Surface Recombination and Photocorrosion of Sb2Se3 Films

Sb2Se3 is considered a promising photocathode material for hydrogen production via solar water splitting. Nevertheless, losses caused by photocorrosion and slow charge transfer at the semiconductor...

Boosting Solar Hydrogen Production of Molybdenum Tungsten Sulfide-Modified Si Micropyramids by Introducing Phosphate

Si is regarded as a promising photocathode material for solar hydrogen evolution reaction (HER) because of its small band gap and highly negative conduction band edge. However, bare Si electrodes have high overpotential because of sluggish HER kinetics on the surface. In this study, molybdenum tungsten sulfide (MoS2-WS2) was decorated on Si photocathodes as the co-catalyst to accelerate HER kinetics. The catalytic performance of MoS2-WS2 was further enhanced by introducing phosphate materials. Phosphate-modified molybdenum tungsten sulfide (PO-MoWS) was deposited on Si photoabsorbers to provide an optimal current of -15.0 mA cm-2 at 0 V. Joint characterizations of X-ray photoelectron and X-ray absorption spectroscopies demonstrated that the phosphate material dominantly coordinated with the WS2 component in PO-MoWS. Moreover, this phosphate material induced a large number of sulfur vacancies in the PO-MoWS/Si electrodes that contributed to the ideal catalytic activity. Herein, TiO2 thin films were prepared as the protective layer to improve the stability of photocathodes. The PO-MoWS/2 nm TiO2/Si electrode maintained 83.8% of the initial photocurrent after chronoamperometric measurement was performed for 8000 s.

Promoting photoelectrochemical hydrogen evolution activity of CuBi2O4 photocathode through ramping rate control

P-type CuBi2O4 represents a very promising photocathode material with visible light response for dual absorber photoelectrochemical (PEC) water splitting device. However, its photocurrent reported is limited by its inefficient utilization of photogenerated carriers, which greatly hinders its practical application. Herein, CuBi2O4 films were prepared by a simple spin-coating method and four distinct ramping rates were applied to investigate the influence of annealing rate on its hydrogen production activity. Among four different ramping rates, the CuBi2O4 film annealed with the rate of 2 °C/min successfully achieved the highest photocurrent of 0.68 mA/cm2 at 0.25 VRHE, corresponding to 38%–83% improvement when compared to those prepared with other rates. To reveal the working mechanism of this simple but effective heating control, structural and electrochemical characterizations were performed. It was found that, the electronic structure of the best performing CuBi2O4 film was notably modified with the largest amount of low valence Cu+ species. Meanwhile, the highest carrier concentration was measured through electrode impedance investigation.