BiVO4-Based Heterojunction Photocathode for High-Performance Photoelectrochemical Hydrogen Peroxide Production

Photoelectrochemical (PEC) cells provide a promising solution for the synthesis of hydrogen peroxide (H2O2). Herein, an integrated photocathode of p-type BiVO4 (p-BVO) array with tetragonal zircon structure coupled with different metal oxide (MOx, M = Sn, Ti, Ni, and Zn) heterostructure and NiNC cocatalyst (p-BVO/MOx/NiNC) was synthesized for the PEC oxygen reduction reaction (ORR) in production of H2O2. The p-BVO/SnO2/NiNC array achieves the production rate 65.46 μmol L-1 h-1 of H2O2 with a Faraday efficiency (FE) of 76.12%. Combined with the H2O2 generation of water oxidation from the n-type Mo-doped BiVO4 (n-Mo:BVO) photoanode, the unbiased photoelectrochemical cell composed of a p-BVO/SnO2/NiNC photocathode and n-Mo:BVO photoanode achieves a total FE of 97.67% for H2O2 generation. The large area BiVO4-based tandem cell of 3 × 3 cm2 can reach a total H2O2 production yield of 338.84 μmol L-1. This work paves the way for the rational design and fabrication of artificial photosynthetic cells for the production of liquid solar fuel.

Direct solar energy conversion on zinc-air battery

A concept of solar energy convertible zinc-air battery (SZAB) is demonstrated through rational design of an electrode coupled with multifunction. The multifunctional electrode is fabricated using nitrogen-substituted graphdiyne (N-GDY) with large π-conjugated carbonous network, which can work as photoresponsive bifunctional electrocatalyst, enabling a sunlight-promoted process through efficient injection of photoelectrons into the conduction band of N-GDY. SZAB enables direct conversion and storage of solar energy during the charging process. Such a battery exhibits a lowered charge voltage under illumination, corresponding to a high energy efficiency of 90.4% and electric energy saving of 30.3%. The battery can display a power conversion efficiency as high as 1.02%. Density functional theory calculations reveal that the photopromoted oxygen evolution reaction kinetics originates from the transition from the alkyne bonds to double bonds caused by the transfer of excited electrons, which changes the position of highest occupied molecular orbital and lowest unoccupied molecular orbital, thus greatly promoting the formation of intermediates to the conversion process. Our findings provide conceptual and experimental confirmation that batteries are charged directly from solar energy without the external solar cells, providing a way to manufacture future energy devices.

Enhancing photoelectrochemical CO2 reduction with silicon photonic crystals

The effectiveness of silicon (Si) and silicon-based materials in catalyzing photoelectrochemistry (PEC) CO2 reduction is limited by poor visible light absorption. In this study, we prepared two-dimensional (2D) silicon-based photonic crystals (SiPCs) with circular dielectric pillars arranged in a square array to amplify the absorption of light within the wavelength of approximately 450 nm. By investigating five sets of n + p SiPCs with varying dielectric pillar sizes and periodicity while maintaining consistent filling ratios, our findings showed improved photocurrent densities and a notable shift in product selectivity towards CH4 (around 25% Faradaic Efficiency). Additionally, we integrated platinum nanoparticles, which further enhanced the photocurrent without impacting the enhanced light absorption effect of SiPCs. These results not only validate the crucial role of SiPCs in enhancing light absorption and improving PEC performance but also suggest a promising approach towards efficient and selective PEC 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.

Enhanced Light Trapping in GaAs/TiO2-Based Photocathodes for Hydrogen Production

Photoelectrochemical cells (PEC) are appealing devices for the production of renewable energy carriers. In this context, III-V semiconductors such as GaAs are very promising materials due to their tunable band gaps, which can be appropriately adjusted for sunlight harvesting. Because of the high cost of these semiconductors, the nanostructuring of the photoactive layer can help to improve the device efficiency as well as drastically reduce the amount of material needed. III-V nanowire-based photoelectrodes benefit from the intrinsically high aspect ratio of nanowires, their enhanced ability to trap light, and their improved charge separation and collection abilities and thus are particularly attractive for PECs. However, III-V semiconductors often suffer from corrosion in aqueous electrolytes, preventing their utilization over long periods under relevant working conditions. Here, photocathodes of GaAs nanowires protected with thin TiO2 shells were prepared and studied under simulated sunlight irradiation to assess their photoelectrochemical performances in correlation with their structural degradation, highlighting the advantageous nanowire geometry compared to its thin-film counterpart. Morphological and electronic parameters, such as the aspect ratio of the nanowires and their doping pattern, were found to strongly influence the photocatalytic performances of the system. This work highlights the advantageous combination of nanowires featuring a buried radial p-n junction with Co nanoparticles used as a hydrogen evolution catalyst. The nanostructured photocathodes exhibit significant photocatalytic activities comparable with previous noble-metal-based systems. This study demonstrates the potential of a GaAs nanostructured semiconductor and its reliable use for photodriven hydrogen production.

Ferri-hydrite: A Novel Electron-Selective Contact Layer for InP Photovoltaic and Photoelectrochemical Cells

Solar energy conversion devices with charge-selective contacts are attracting significant research interest as a cost-effective alternative to homojunction counterparts. This study presents a novel approach for fabricating high-performance solar cells based on InP heterojunctions using a solution-processed ferri-hydrite (Fh) electron-selective contact (ESC). The champion cell efficiency of 16.6% is achieved, which is a significant improvement over those from previous studies using other solution-processed ESC materials. X-ray photoelectron spectroscopy measurements showed that the low conduction band offset at the Fh-InP interface facilitated selective transport of photogenerated electrons from InP. Moreover, the Fh electron-selective contact layer provided an excellent photoelectrochemical half-cell water reduction efficiency of 8.4%. The Fh layer not only selectively extracts photogenerated electrons from InP but also simultaneously serves as a surface protection layer, improving the cell's long-term stability. These results demonstrate the potential of Fh as a low-cost and easily fabricated material for use in high-efficiency photovoltaic and photoelectrochemical devices. Our findings pave the way for further improvements in the efficiency of InP heterojunction solar cells by addressing the losses incurred in the cells.

Impact of Ball Milling on the Hydrogen Evolution Performance of Cu2Sn0.38Ge0.62S3 Photocatalytic Particles Synthesized via a Flux Method

Ball milling has been shown empirically to produce fine photocatalytic particles from large bulky particles but to drastically reduce the photocatalytic activity of such material during water splitting due to mechanical damage to the photocatalyst surfaces. If the damaged photocatalyst surfaces could be removed or reconstructed, the reduced particle sizes resulting from milling would be expected to provide enhanced photocatalytic activity. In the present study, fine particles of crystalline Cu₂Sn_0.38Ge_0.62S₃ (CTGS), which is responsive to long wavelength light up to the near-infrared region, were synthesized by a flux method and subsequent ball milling. A photocathode made of such particles showed significantly enhanced photoelectrochemical (PEC) performance under simulated sunlight while the photocatalytic hydrogen evolution activity of a powder suspension system made from the same material exhibited a typical decrease. The CTGS crystalline particles synthesized using the flux method were found to be highly crystalline but to have relatively large micrometer-scale sizes. Ball milling reduced the particle size but produced an amorphous coating of oxidized species that lowered the photocatalytic activity of the powder suspension system. Typical surface modifications of a photocathode made from this material, consisting of wet chemical processes, also served as an etching treatment to successfully remove the minimally crystalline surface layer and provide greater PEC activity. These data suggest the benefits of combining flux crystal growth with ball milling and the appropriate chemical etching process to obtain high-crystallinity fine photocatalytic particles responsive to long wavelength light with improved PEC hydrogen evolution activity.

BaTiO(3) Functional Perovskite as Photocathode in Microbial Fuel Cells for Energy Production and Wastewater Treatment

Microbial fuel cells (MFCs) provide new opportunities for the sustainable production of energy, converting organic matter into electricity through microorganisms. Moreover, MFCs play an important role in remediation of environmental pollutants from wastewater with power generation. This work focuses on the evaluation of ferroelectric perovskite materials as a new class of non-precious photocatalysts for MFC cathode construction. Nanoparticles of BaTiO₃ (BT) were prepared and tested in a microbial fuel cell (MFC) as photocathode catalytic components. The catalyst phases were synthesized, identified and characterized by XRD, SEM, UV-Vis absorption spectroscopy, P-E hysteresis and dielectric measurements. The maximum absorption of BT nanoparticles was recorded at 285 nm and the energy gap (Eg) was estimated to be 3.77 eV. Photocatalytic performance of cathodes coated with BaTiO₃ was measured in a dark environment and then in the presence of a UV-visible (UV-Vis) light source, using a mixture of dairy industry and domestic wastewater as a feedstock for the MFCs. The performance of the BT cathodic component is strongly dependent on the presence of UV-Vis irradiation. The BT-based cathode functioning under UV-visible light improves the maximum power densities and the open circuit voltage (OCV) of the MFC system. The values increased from 64 mW m^-2 to 498 mW m^-2 and from 280 mV to 387 mV, respectively, showing that the presence of light effectively improved the photocatalytic activity of this ceramic. Furthermore, the MFCs operating under optimal conditions were able to reduce the chemical oxygen demand load in wastewater by 90% (initial COD = 2500 mg L^-1).

Manipulation of Photoelectrochemical Water Splitting by Controlling Direction of Carrier Movement Using InGaN/GaN Hetero-Structure Nanowires

We report the improvement in photoelectrochemical water splitting (PEC-WS) by controlling migration kinetics of photo-generated carriers using InGaN/GaN hetero-structure nanowires (HSNWs) as a photocathode (PC) material. The InGaN/GaN HSNWs were formed by first growing GaN nanowires (NWs) on an Si substrate and then forming InGaN NWs thereon. The InGaN/GaN HSNWs can cause the accumulation of photo-generated carriers in InGaN due to the potential barrier formed at the hetero-interface between InGaN and GaN, to increase directional migration towards electrolyte rather than the Si substrate, and consequently to contribute more to the PEC-WS reaction with electrolyte. The PEC-WS using the InGaN/GaN-HSNW PC shows the current density of 12.6 mA/cm² at -1 V versus reversible hydrogen electrode (RHE) and applied-bias photon-to-current conversion efficiency of 3.3% at -0.9 V versus RHE. The high-performance PEC-WS using the InGaN/GaN HSNWs can be explained by the increase in the reaction probability of carriers at the interface between InGaN NWs and electrolyte, which was analyzed by electrical resistance and capacitance values defined therein.

Photoelectrochemical Enhancement of Cu2O by a Cu2Te Hole Transmission Interlayer

Cuprous oxide (Cu₂O) films are electrodeposited on fluorinated tin oxide (FTO) substrates with controlled crystallographic orientation and optimized film thickness. The Cu₂O films exhibit a (100)-to-(111) texture change and a pyramid-to-cuboidal crystallite morphology transformation by increasing the electrodeposition current density. The cuboidal crystallites enclosed by (100) sidewalls and (111) truncated surfaces demonstrate better photoelectrochemical property than the pyramid crystallites. By introducing a copper(I) telluride (Cu₂Te) layer in between Cu₂O and FTO, the photocurrent density increases 70% for the (111)-textured Cu₂O film in a 1 M Na₂SO₄ solution under AM1.5 G illumination. The enhancement is mainly attributed to the improved separation of photocarriers in the illuminated Cu₂O film by pumping hole carriers to the Cu₂Te layer. In contrast to typical electron pathway management, this study provides an alternative route to improve the photoelectrochemical performance of Cu₂O-based photocathodes through hole pathway modification.

Optoelectrical Regulation of CuBi2O4 Photocathode via Photonic Crystal Structure for Solar-Fuel Conversion

Metal oxide semiconductors have been regarded as ideal candidates for photoelectrochemical (PEC) CO₂ reduction if the contradiction between photon harvesting and photocarrier collection can be resolved. The novel three-dimensional structure provides an available approach to balancing the above-mentioned contradiction. In this work, CuBi₂O₄ photonic crystal photocathodes with different feature sizes were developed to realize the regulation of optoelectrical properties. The resulted photocathode displays promoted PEC activity as the enhanced photocurrent and CO₂ reduction activity. Such an excellent performance was attributed to the improved efficiency of charge carrier generation and collection through extending the optical path and shortening the carrier transport distance inside films. COMSOL simulations and PEC spectroscopy analysis confirmed the promoted photon harvesting capacity and carrier dynamics. This work demonstrates a feasible strategy for developing novel photocathodes with modulated microstructures in solar-fuel conversion.

Solar-Driven Syngas Production Using Al-Doped ZnTe Nanorod Photocathodes

Syngas, traditionally produced from fossil fuels and natural gases at high temperatures and pressures, is an essential precursor for chemicals utilized in industry. Solar-driven syngas production can provide an ideal pathway for reducing energy consumption through simultaneous photoelectrochemical CO2 and water reduction at ambient temperatures and pressures. This study performs photoelectrochemical syngas production using highly developed Al-doped ZnTe nanorod photocathodes (Al:ZnTe), prepared via an all-solution process. The facile photo-generated electrons are transferred by substitutional Al doping on Zn sites in one-dimensional arrays to increase the photocurrent density to -1.1 mA/cm2 at -0.11 VRHE, which is 3.5 times higher than that for the pristine ZnTe. The Al:ZnTe produces a minor CO (FE ≈ 12%) product by CO2 reduction and a major product of H2 (FE ≈ 60%) by water reduction at -0.11 VRHE. Furthermore, the product distribution is perfectly switched by simple modification of Au deposition on photocathodes. The Au coupled Al:ZnTe exhibits dominant CO production (FE ≈ 60%), suppressing H2 evolution (FE ≈ 15%). The strategies developed in this study, nanostructuring, doping, and surface modification of photoelectrodes, can be applied to drive significant developments in solar-driven fuel production.

Construction of Heterostructured Sn/TiO2 /Si Photocathode for Efficient Photoelectrochemical CO2 Reduction

Using renewable energy to convert CO₂ into liquid products, as a sustainable way to produce fuels and chemicals, has attracted intense attention. Herein, a novel heterostructured photocathode composed of Si wafer, TiO₂ layer, and Sn metal particles has been successfully fabricated by combining of a facile hydrothermal and electrodeposition method. The obtained Sn/TiO₂ /Si photocathode shows enhanced light absorption performance by the surface plasmon resonance effect of Sn metal. Especially, the Sn/TiO₂ /Si photocathode together with rich oxygen vacancy defects jointly promote photoelectrochemical CO₂ reduction, harvesting a high faradaic efficiency of HCOOH and a desirable average current density (-4.72 mA cm^-2 ) at -1.0 V vs. reversible hydrogen electrode. Significantly, the photocathode Sn/TiO₂ /Si also shows good stability due to the design of protecting layer TiO₂ . This study provides a facile strategy of constructing an efficient photocathode to improve the light absorption performance and the electron transfer efficiency, exhibiting great potential in the CO₂ reduction.

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.

Overview of the Semiconductor Photocathode Research in China

With the growing demand from scientific projects such as the X-ray free electron laser (XFEL), ultrafast electron diffraction/microscopy (UED/UEM) and electron ion collider (EIC), the semiconductor photocathode, which is a key technique for a high brightness electron source, has been widely studied in China. Several fabrication systems have been designed and constructed in different institutes and the vacuum of most systems is in the low 10⁻⁸ Pa level to grow a high QE and long lifetime photocathode. The QE, dark lifetime/bunch lifetime, spectral response and QE map of photocathodes with different kinds of materials, such as bialkali (K₂CsSb, K₂NaSb, etc.), Cs₂Te and GaAs, have been investigated. These photocathodes will be used to deliver electron beams in a high voltage DC gun, a normal conducting RF gun, and an SRF gun. The emission physics of the semiconductor photocathode and intrinsic emittance reduction are also studied.

Enhanced H2 Evolution at Patterned MoSx-Modified Si-Based Photocathodes by Incorporating the Interfacial 3D Nanostructure of Ag

Photoelectrochemical cells represent one of the promising ways to renewably produce hydrogen (H₂) as a future chemical fuel. The design of a catalyst/semiconductor junction for the hydrogen evolution reaction (HER) requires various factors for high performance. In catalytic materials, an intrinsic activity with fast charge-transfer kinetics is important. Additionally, their thermodynamic property and physical adhesion should be compatible with the underlying semiconductor for favorable band alignment and stability during vigorous H₂ bubble formation. Moreover, catalysts, especially non-noble materials that demand a large amount of loading, should be adequately dispersed on the semiconductor surface to allow sufficient light absorption to generate excitons. One of the methods to simultaneously satisfy these conditions is to adopt an interfacial layer between the semiconductor and active materials in HER. The interfacial layer efficiently extracts the electrons from the semiconductor and conveys those to the catalytically active surface. We demonstrate Ag as a 3D interfacial nanostructure of patterned MoS_x catalysts for photoelectrochemical HER. The nanostructured porous Ag layer was introduced by a simple chemical process, followed by photoelectrochemical deposition of MoS_x to form MoS_x/Ag nanostructures in cross-shaped catalyst pattern arrays. Ag modulated the surface electronic property of MoS_x to improve the reaction kinetics as well as helped a charge transport at the Ag|p-Si(100) junction. The physically stable adhesion of catalysts was also achieved despite the ∼40 nm thick catalysts owing to the interfacial Ag nanostructure. This work contributes to further understand the complex multistep HER from light absorption to charge transfer to protons, helping to develop cost-effective and efficient photocathodes.

An Electrospun Porous CuBi2O4 Nanofiber Photocathode for Efficient Solar Water Splitting

While the CuBi₂O₄-based photocathode has emerged as an ideal candidate for photoelectrochemical water splitting, it is still far from its theoretical values due to poor charge carrier transport, poor electron-hole separation, and instability caused by self-photoelectric-corrosion with electrolytes. Establishing synthesis methods to produce a CuBi₂O₄ photocathode with sufficient cocatalyst sites would be highly beneficial for water splitting. Here, the platinum-enriched porous CuBi₂O₄ nanofiber (CuBi₂O₄/Pt) with uniform coverage and high surface area was prepared as a photocathode through an electrospinning and electrodeposition process for water splitting. The prepared photocathode material was composed of a CuBi₂O₄ nanofiber array, which has a freestanding porous structure, and the Pt nanoparticle is firmly embedded on the rough surface. The highly porous nanofiber structures allow the cocatalyst (Pt) better alignment on the surface of CuBi₂O₄, which can effectively suppress the electron-hole recombination at the electrolyte interface. The as-fabricated CuBi₂O₄ nanofiber has a tetragonal crystal structure, and its band gap was determined to be 1.8 eV. The self-supporting porous structure and electrocatalytic activity of Pt can effectively promote the separation of electron-hole pairs, thus obtaining high photocurrent density (0.21 mA/cm² at 0.6 V vs. RHE) and incident photon-to-current conversion efficiency (IPCE, 4% at 380 nm). This work shows a new view for integrating an amount of Pt nanoparticles with CuBi₂O₄ nanofibers and demonstrates the synergistic effect of cocatalysts for future solar water splitting.

Solution-processed silicon quantum dot photocathode for hydrogen evolution

The photoelectrochemical response of a photocathode made from a colloidal solution of boron (B) and phosphorus (P) codoped silicon (Si) quantum dots (QDs) 2-11 nm in diameters is studied. Since codoped Si QDs are dispersible in alcohol and water due to the hydrophilic surface, a photoelectrode with a smooth surface is produced by drop-coating the QD solution on an indium tin oxide substrate. The codoping provides high oxidation resistance to Si QDs and makes the electrode operate as a photocathode. The photoelectrochemical response of a Si QD photoelectrode depends strongly on the size of QDs; there is a transition from anodic to cathodic photocurrent around 4 nm in diameter. Below the size, anodic photocurrent due to self-oxidation of Si QDs is observed, while above the size, cathodic photocurrent due to electron transfer across the interface is observed. The cathodic photocurrent increases with increasing the size, and in some samples, it is observed for more than 3000 s under intermittent light irradiation.

Molecular Engineering of Photocathodes based on Polythiophene Organic Semiconductors for Photoelectrochemical Hydrogen Generation

Organic semiconductors provide significant potentials for the construction of photoelectrochemical (PEC) cells for solar hydrogen production because of their highly tunable properties. Herein, on carbon fiber paper (CFP) surface, pyridyl (Py), and 4,4'-bipyridin-1-ium (Py₂⁺) groups were introduced into polythiophene (pTH) semiconductor by electrochemical copolymerization, respectively. After assembly with the Co(dmgBF₂)₂ type catalyst (CoB, dmgBF₂ = difluoroboryldimethylglyoximate), the CoB@Py₂⁺-pTH/CFP photocathode displayed nearly twice the photocurrent enhancement (550 μA cm^-2 at 0.15 V vs reversible hydrogen electrode, RHE) comparing to that generated by the CoB@Py-pTH/CFP photocathode (290 μA cm^-2 at 0.15 V vs RHE) for light-driven H₂ generation under AM 1.5 solar illumination. Investigation of the mechanism revealed that the introduction of the positively charged pyridinium groups could improve the intrinsic Co(dmgBF₂)₂ catalyst activity for the H₂ generation reaction. Meanwhile, the positively charged pyridinium groups serve as p-type dopants to increase the semiconductor bulk charge transfer rate and act as electron transfer mediators to promote the interfacial charge transfer kinetics between the catalyst and the pTH-based organic semiconductor.

Surface-Tailored InP Nanowires via Self-Assembled Au Nanodots for Efficient and Stable Photoelectrochemical Hydrogen Evolution

With a band gap close to the Shockley-Quiesser limit and excellent conduction band alignment with the water reduction potential, InP is an ideal photocathode material for photoelectrochemical (PEC) water reduction. Here, we develop facile self-assembled Au nanodots based on dewetting phenomena as a masking technique to fabricate wafer-scale InP nanowires (NWs) via a top-down approach. In addition, we report dual-function wet treatment using sulfur-dissolved oleylamine (S-OA) to remove a plasma-damaged surface in a controlled manner and stabilize InP NWs against surface corrosion in harsh electrolyte solutions. The resulting InP NW photocathodes exhibit an excellent photocurrent density of 33 mA/cm² under 1 sun illumination in 1 M HCl with a highly stabilized performance without needing additional protection layers. Our approach combining large-area NW fabrication and surface engineering synergistically enhances light harvesting and PEC performance and stability, thereby providing a pathway for the development of efficient and durable InP photoelectrodes in a scalable manner.

Electronic structure and optical properties of Na2KSb and NaK2Sb from first-principles many-body theory

In the search for novel materials for vacuum electron sources, multi-alkali antimonides and in particular sodium-potassium-antimonides have been recently regarded as especially promising due to their favorable electronic and optical properties. In the framework of density-functional theory and many-body perturbation theory, we investigate the electronic structure and the dielectric response of two representative members of this family, namely Na₂KSb and NaK₂Sb. We find that both materials have a direct gap, which is on the order of 1.5 eV in Na₂KSb and 1.0 eV in NaK₂Sb. In either system, valence and conduction bands are dominated by Sb states withp- ands-character, respectively. The imaginary part of the dielectric function, computed upon explicit inclusion of electron-hole interactions to characterize the optical response of the materials, exhibits maxima starting from the near-infrared region, extending up to the visible and the ultraviolet band. With our analysis, we clarify that the lowest-energy excitations are non-excitonic in nature and that their binding energy is on the order of 100 meV. Our results confirm the potential of Na₂KSb and NaK₂Sb as photoemissive materials for vacuum electron sources, photomultipliers, and imaging devices.

Porphyrin-Based Covalent Organic Framework Thin Films as Cathodic Materials for "On-Off-On" Photoelectrochemical Sensing of Lead Ions

Currently, cathodic photoelectrochemical (PEC) sensors, which could effectively reduce background interference, are urgently required for ultrasensitive environmental monitoring. Herein, porphyrin-based covalent organic framework (TAPP-COF) thin films were fabricated via a bottom-up growth approach on the liquid/liquid interface and applied as a photocathode material to "on-off-on" PEC sensing of Pb²⁺. Benefitting from the unique charge channels of COFs and the good photoelectric properties of porphyrin, the as-prepared TAPP-COF thin films presented an improved photocathodic current, with a strongly enhanced "signal-on" response with low background. Then, CdSe@SiO₂ quantum dots (QDs), as a quenching agent, were introduced through a hybridization chain reaction (HCR) to obtain a "signal off" PEC response. Afterward, with the introduction of target Pb²⁺, CdSe@SiO₂ QDs were detached from TAPP-COF thin films, and the PEC response transformed into a signal-on state. Benefiting from the multiple-quenching and steric hindrance effect of CdSe@SiO₂ QDs and the photocathodic property of TAPP-COFs, accurate monitoring of Pb²⁺ in a wide detection range from 0.05 to 1000 nM with a lower detection limit of 0.012 nM was realized based on the proposed on-off-on PEC approach. Notably, the methodology provides an efficient platform for ultrasensitive determination of heavy metal ions, which would play a significant role in environmental monitoring and public safety fields.

Photoelectrochemical Behavior and Computational Insights for Pristine and Doped NdFeO3 Thin-Film Photocathodes

Among the different strategies that are being developed to solve the current energy challenge, harvesting energy directly from sunlight through a tandem photoelectrochemical cell (water splitting) is most attractive. Its implementation requires the development of stable and efficient photocathodes, NdFeO3 being a suitable candidate among ternary oxides. In this study, transparent NdFeO3 thin-film photocathodes have been successfully prepared by a citric acid-based sol-gel procedure, followed by thermal treatment in air at 640 °C. These electrodes show photocurrents for both the hydrogen evolution and oxygen reduction reactions. Doping with Mg2+ and Zn2+ has been observed to significantly enhance the photoelectrocatalytic performance of NdFeO3 toward oxygen reduction. Magnesium is slightly more efficient as a dopant than Zn, leading to a multiplication of the photocurrent by a factor of 4-5 for a doping level of 5 at % (with respect to iron atoms). This same trend is observed for hydrogen evolution. The beneficial effect of doping is primarily attributed to an increase in the density and a change in the nature of the majority charge carriers. DFT calculations help to rationalize the behavior of NdFeO3 by pointing to the importance of nanostructuring and doping. All in all, NdFeO3 has the potential to be used as a photocathode in photoelectrochemical applications, although efforts should be directed to limit surface recombination.

Understanding the Performance of NiO Photocathodes with Alkyl-Derivatized Cobalt Catalysts and a Push-Pull Dye

Mesoporous NiO photocathodes containing the push-pull dye PB6 and alkyl-derivatized cobaloxime catalysts were prepared using surface amide couplings and analyzed for photocatalytic proton reduction catalysis. The length of the alkyl linker used to derivatize the cobalt catalysts was found to correlate to the photocurrent with the highest photocurrent observed using shorter alkyl linkers but the lowest one for samples without linker. The alkyl linkers were also helpful in slowing dye-NiO charge recombination. Photoelectrochemical measurements and femtosecond transient absorption spectroscopic measurements suggested electron transfer to the surface-immobilized catalysts occurred; however, H₂ evolution was not observed. Based on UV-vis, X-ray fluorescence spectroscopy (XRF), and X-ray photoelectron spectroscopy (XPS) measurements, the cobalt catalyst appeared to be limiting the photocathode performance mainly via cobalt demetallation from the oxime ligand. This study highlights the need for a deeper understanding of the effect of catalyst molecular design on photocathode performance.

Toward Current Matching in Tandem Dye-Sensitized Solar Cells

The tandem pn-type dye-sensitized solar cells (pn-DSCs) have received much attention in the field of photovoltaic technologies because of their great potential to overcome the Shockley-Queisser efficiency limitation that applies to single junction photovoltaic devices. However, factors governing the short-circuit current densities (J_sc) of pn-DSC remain unclear. It is typically believed that J_sc of the pn-DSC is limited to the highest one that the two independent photoelectrodes can achieve. In this paper, however, we found that the available J_sc of pn-DSC is always determined by the larger J_sc that the photoanode can achieve but not by the smaller one in the photocathode. Such experimental findings were verified by a simplified series circuit model, which shows that a breakdown will occur on the photocathode when the photocurrent goes considerably beyond its threshold voltage, thus leading to an abrupt increase in J_sc of the circuit. The simulation results also suggest that a higher photoconversion efficiency of the pn-DSCs can be only achieved when an almost equivalent photocurrent is achieved for the two photoelectrodes.

Enhancement of Photoemission on p-Type GaAs Using Surface Acoustic Waves

We demonstrate that photoemission properties of p-type GaAs can be altered by surface acoustic waves (SAWs) generated on the GaAs surface due to dynamical piezoelectric fields of SAWs. Multiphysics simulations indicate that charge-carrier recombination is greatly reduced, and electron effective lifetime in p-doped GaAs may increase by a factor of 10× to 20×. It implies a significant increase, by a factor of 2× to 3×, of quantum efficiency (QE) for GaAs photoemission applications, like GaAs photocathodes. Conditions of different SAW wavelengths, swept SAW intensities, and varied incident photon energies were investigated. Essential steps in SAW device fabrication on a GaAs substrate are demonstrated, including deposition of an additional layer of ZnO for piezoelectric effect enhancement, measurements of current-voltage (I-V) characteristics of the SAW device, and ability to survive high-temperature annealing. Results obtained and reported in this study provide the potential and basis for future studies on building SAW-enhanced photocathodes, as well as other GaAs photoelectric applications.

Using Surface Amide Couplings to Assemble Photocathodes for Solar Fuel Production Applications

A facile surface amide-coupling method was examined to attach dye and catalyst molecules to silatrane-decorated NiO electrodes. Using this method, electrodes with a push-pull dye were assembled and characterized by photoelectrochemistry and transient absorption spectroscopy. The dye-sensitized electrodes exhibited hole injection into NiO and good photoelectrochemical stability in water, highlighting the stability of the silatrane anchoring group and the amide linkage. The amide-coupling protocol was further applied to electrodes that contain a molecular proton reduction catalyst for use in photocathode architectures. Evidence for catalyst reduction was observed during photoelectrochemical measurements and via femtosecond-transient absorption spectroscopy demonstrating the possibility for application in photocathodes.

Sandwich-Nanostructured n-Cu2O/AuAg/p-Cu2O Photocathode with Highly Positive Onset Potential for Improved Water Reduction

An n-Cu₂O layer formed a high-quality buried junction with p-Cu₂O to increase the photovoltage and thus to shift the turn-on voltage positively. Mott-Schottky measurements confirmed that the improvement benefited from a positive shift in flat-band potential. The obtained extremely positive onset potential, 0.8 V_RHE in n-Cu₂O/AuAg/p-Cu₂O, is comparable with measurements from water reduction catalysts. The AuAg alloy sandwiched between the homojunction of n-Cu₂O and p-Cu₂O improved the photocatalytic performance. This alloy both served as an electron relay and promoted electron-hole pair generation in nearby semiconductors; the charge transfer between n-Cu₂O and p-Cu₂O in the sandwich structure was measured with X-ray absorption spectra. The proposed sandwich structure can be considered as a new direction for the design of efficient solar-related devices.

Influence of Anchoring Groups on the Charge Transfer and Performance of p-Si/TiO2/Cobaloxime Hybrid Photocathodes for Photoelectrochemical H2 Production

Although hybrid photocathodes built by immobilizing molecular catalysts to the surface of semiconductors through chemical linkages have been reported in recent years, systematic and comparative studies remain scarce about the impact of various anchoring groups on the performance, stability, and charge-transfer kinetics of molecular catalyst-decorated hybrid photocathodes for photoelectrochemical (PEC) H₂ production. In this study, the molecular cobaloxime catalysts, CoPy-4-X (Py = pyridine, X = PO₃H₂, COOH, and CONH(OH)), bearing different anchoring groups were synthesized and covalently immobilized to the surface of the porous TiO₂ layer coated on a p-Si plate or a fluorine-doped tin oxide glass. The influence of the anchoring groups on the performance of p-Si/TiO₂/CoPy-4-X photocathodes was comparatively studied for PEC H₂ evolution. Among the tested hybrid photocathodes, the one with a hydroxamate as an anchoring group displayed higher activity and lower charge-transfer resistance than that observed for the electrode with a carboxylate or a phosphonate as the anchoring group. Notably, the catalytic current of p-Si/TiO₂/CoPy-4-CONH(OH) was attenuated only by 2.9% in the controlled potential photoelectrolysis tests in borate buffer solution at pH 9 at 0 V versus a reversible hydrogen electrode over 6 h. Moreover, the influence of anchoring groups on the interfacial electron transfer from the TiO₂ layer to the immobilized cobaloxime catalyst and electron-hole recombination was studied by transient absorption spectroscopy. These results revealed that the hydroxamate as an anchoring group is superior to the carboxylate and phosphonate groups for speeding up the interfacial electron transfer and firmly immobilizing the molecular catalysts to the metal oxide semiconductors to build efficient and stable hybrid photoelectrodes.

InGaAsP as a Promising Narrow Band Gap Semiconductor for Photoelectrochemical Water Splitting

While photoelectrochemical (PEC) water splitting is a very promising route toward zero-carbon energy, conversion efficiency remains limited. Semiconductors with narrower band gaps can absorb a much greater portion of the solar spectrum, thereby increasing efficiency. However, narrow band gap (∼1 eV) III-V semiconductor photoelectrodes have not yet been thoroughly investigated. In this study, the narrow band gap quaternary III-V alloy InGaAsP is demonstrated for the first time to have great potential for PEC water splitting, with the long-term goal of developing high-efficiency tandem PEC devices. TiO₂-coated InGaAsP photocathodes generate a photocurrent density of over 30 mA/cm² with an onset potential of 0.45 V versus reversible hydrogen electrode, yielding an applied bias efficiency of over 7%. This is an excellent performance, given that nearly all power losses can be attributed to reflection losses. X-ray photoelectron spectroscopy and photoluminescence spectroscopy show that InGaAsP and TiO₂ form a type-II band alignment, greatly enhancing carrier separation and reducing recombination losses. Beyond water splitting, the tunable band gap of InGaAsP could be of further interest in other areas of photocatalysis, including CO₂ reduction.

Enhancing the Performance of Si-Based Photocathodes for Solar Hydrogen Production in Alkaline Solution by Facilely Intercalating a Sandwich N-Doped Carbon Nanolayer to the Interface of Si and TiO2

Photoelectrochemical (PEC) water splitting is a promising but immensely challenging technology for sustainable production of hydrogen. To this end, highly active, durable, and inexpensive photocathodes that operate under conditions compatible with those for photoanodes are desired. Herein, Si-based composite photocathodes were constructed by coating the front surface of Si with an N-doped carbon nanolayer and then a TiO₂ protective layer, followed by decorating the electrode surface with Ni and Ni-Mo catalysts. The carbon nanolayer, denoted as C_PDA, was formed directly on the Si surface by in situ self-polymerization of dopamine, followed by carbonization of the polydopamine (PDA) coating. The performance of the as-fabricated Si photocathodes with and without the C_PDA layer was comparatively studied for PEC hydrogen evolution reaction (HER) in alkaline electrolytes to evaluate the effect of the sandwich C_PDA layer in between the Si substrate and the TiO₂ layer on the photoelectrocatalytic behaviors of Si-based electrodes. The photocathodes containing the C_PDA layer demonstrated lower electron transfer resistance, higher built-in photovoltage, and larger band bending relative to the analogous electrodes without the C_PDA layer. Accordingly, the short-circuit photocurrents of the Ni and Ni-Mo-decorated photocathodes with the C_PDA layer were enhanced by a factor of 2.8-3.3, and their open-circuit photovoltages were enlarged by 0.14-0.22 V, compared to those of the corresponding electrodes without the C_PDA layer in 1 M KOH under simulated 1 sun illumination. Moreover, the photocathodes with the C_PDA layer also exhibited an improved stability for PEC HER in alkaline solutions, with a faradaic efficiency of 90-97% in the initial hour.

ZnSe Nanorods as Visible-Light Absorbers for Photocatalytic and Photoelectrochemical H2 Evolution in Water

A precious-metal- and Cd-free photocatalyst system for efficient H₂ evolution from aqueous protons with a performance comparable to Cd-based quantum dots is presented. Rod-shaped ZnSe nanocrystals (nanorods, NRs) with a Ni(BF₄ )₂ co-catalyst suspended in aqueous ascorbic acid evolve H₂ with an activity up to 54±2 mmol H 2  g_ZnSe ^-1  h^-1 and a quantum yield of 50±4 % (λ=400 nm) under visible light illumination (AM 1.5G, 100 mW cm^-2 , λ>400 nm). Under simulated full-spectrum solar irradiation (AM 1.5G, 100 mW cm^-2 ), up to 149±22 mmol H 2  g_ZnSe ^-1  h^-1 is generated. Significant photocorrosion was not noticeable within 40 h and activity was even observed without an added co-catalyst. The ZnSe NRs can also be used to construct an inexpensive delafossite CuCrO₂ photocathode, which does not rely on a sacrificial electron donor. Immobilized ZnSe NRs on CuCrO₂ generate photocurrents of around -10 μA cm^-2 in an aqueous electrolyte solution (pH 5.5) with a photocurrent onset potential of approximately +0.75 V vs. RHE. This work establishes ZnSe as a state-of-the-art light absorber for photocatalytic and photoelectrochemical H₂ generation.

Hierarchical SnS2/CuInS2 Nanosheet Heterostructure Films Decorated with C60 for Remarkable Photoelectrochemical Water Splitting

Rational architectural design and catalyst components are beneficial to improve the photoelectrochemical (PEC) performance. Herein, hierarchical SnS₂/CuInS₂ nanosheet heterostructure porous films were fabricated and decorated with C₆₀ to form photocathodes for PEC water reduction. Large-size CuInS₂ nanosheet films were first grown on transparent conducting glass to form substrate films. Then, small-size SnS₂ nanosheets were epitaxially grown on both sides of the CuInS₂ nanosheets to form uniform hierarchical porous laminar films. The addition of C₆₀ on the surface of the SnS₂/CuInS₂ porous nanosheets effectively increased visible light absorption of the composite photocathode. Photoluminescence spectroscopy and impedance spectroscopy analyses indicated that the formation of a SnS₂/CuInS₂ heterojunction and decoration of C₆₀ significantly increased the photocurrent density by promoting the electron-hole separation and decreasing the resistance to the transport of charge carriers. The hierarchical SnS₂/CuInS₂ nanosheet heterostructure porous films containing multiscale nanosheets and pore configurations can enlarge the surface area and enhance visible light utilization. These beneficial factors make the optimized C₆₀-decorated SnS₂/CuInS₂ photocathode exhibit much higher photocathodic current (4.51 mA cm^-2 at applied potential -0.45 V vs reversible hydrogen electrode ) and stability than the individual CuInS₂ (2.58 mA cm^-2) and SnS₂ (1.92 mA cm^-2) nanosheet film photocathodes. This study not only reveals the promise of C₆₀-decorated hierarchical SnS₂/CuInS₂ nanosheet heterostructure porous film photocathodes for efficient solar energy harvesting and conversion but also provides rational guidelines in designing high-efficiency photoelectrodes from earth-abundant and low-cost materials allowing widely practical applications.

Rationally Designed Copper-Modified Polymeric Carbon Nitride as a Photocathode for Solar Water Splitting

Polymeric carbon nitride has been considered to be an active photocathode for catalyzing the generation of H₂ through water splitting. However, the application of this material in photoelectrochemical cells remains a challenge owing to the intrinsically sluggish kinetics of charge separation. Herein, a facile salt-melt method is developed for fabricating Cu-modified polymeric carbon nitride as an effective photocathode material for solar water splitting. Various characterization data confirm that Cu-modified polymeric carbon nitride contains both free CuCl, derived from precursors, and coordinated Cu species incorporated into the polymeric carbon nitride, which can generate type-II heterojunctions. This special heterojunction energy structure contributes to a significantly enhanced photocurrent density for hydrogen evolution. The proposed strategy for synthesizing the Cu-modified polymeric carbon nitride can stimulate research for the development of highly efficient visible-light-active photocathodes.

Bright and Ultrafast Photoelectron Emission from Aligned Single-Wall Carbon Nanotubes through Multiphoton Exciton Resonance

Ultrashort bunches of electrons, emitted from solid surfaces through excitation by ultrashort laser pulses, are an essential ingredient in advanced X-ray sources, and ultrafast electron diffraction and spectroscopy. Multiphoton photoemission using a noble metal as the photocathode material is typically used but more brightness is desired. Artificially structured metal photocathodes have been shown to enhance optical absorption via surface plasmon resonance but such an approach severely reduces the damage threshold in addition to requiring state-of-the-art facilities for photocathode fabrication. Here, we report ultrafast photoelectron emission from sidewalls of aligned single-wall carbon nanotubes. We utilized strong exciton resonances inherent in this prototypical one-dimensional material, and its excellent thermal conductivity and mechanical rigidity leading to a high damage threshold. We obtained unambiguous evidence for resonance-enhanced multiphoton photoemission processes with definite power-law behaviors. In addition, we observed strong polarization dependence and ultrashort photoelectron response time, both of which can be quantitatively explained by our model. These results firmly establish aligned single-wall carbon nanotube films as novel and promising ultrafast photocathode material.

High Efficiency Si Photocathode Protected by Multifunctional GaN Nanostructures

Photoelectrochemical water splitting is a clean and environmentally friendly method for solar hydrogen generation. Its practical application, however, has been limited by the poor stability of semiconductor photoelectrodes. In this work, we demonstrate the use of GaN nanostructures as a multifunctional protection layer for an otherwise unstable, low-performance photocathode. The direct integration of GaN nanostructures on n⁺-p Si wafer not only protects Si surface from corrosion but also significantly reduces the charge carrier transfer resistance at the semiconductor/liquid junction, leading to long-term stability (>100 h) at a large current density (>35 mA/cm²) under 1 sun illumination. The measured applied bias photon-to-current efficiency of 10.5% is among the highest values ever reported for a Si photocathode. Given that both Si and GaN are already widely produced in industry, our studies offer a viable path for achieving high-efficiency and highly stable semiconductor photoelectrodes for solar water splitting with proven manufacturability and scalability.

Thickness-dependent photoelectrochemical properties of a semitransparent Co3O4 photocathode

Co₃O₄ has been widely studied as a catalyst when coupled with a photoactive material during hydrogen production using water splitting. Here, we demonstrate a photoactive spinel Co₃O₄ electrode grown by the Kirkendall diffusion thermal oxidation of Co nanoparticles. The thickness-dependent structural, physical, optical, and electrical properties of Co₃O₄ samples are comprehensively studied. Our analysis shows that two bandgaps of 1.5 eV and 2.1 eV coexist with p-type conductivity in porous and semitransparent Co₃O₄ samples, which exhibit light-induced photocurrent in photoelectrochemical cells (PEC) containing the alkaline electrolyte. The thickness-dependent properties of Co₃O₄ related to its use as a working electrode in PEC cells are extensively studied and show potential for the application in water oxidation and reduction processes. To demonstrate the stability, an alkaline cell was composed for the water splitting system by using two Co₃O₄ photoelectrodes. The oxygen gas generation rate was obtained to be 7.17 mL·h^-1 cm^-1. Meanwhile, hydrogen gas generation rate was almost twice of 14.35 mL·h^-1·cm^-1 indicating the stoichiometric ratio of 1:2. We propose that a semitransparent Co₃O₄ photoactive electrode is a prospective candidate for use in PEC cells via heterojunctions for hydrogen generation.

Synthetic Mechanism Discovery of Monophase Cuprous Oxide for Record High Photoelectrochemical Conversion of CO2 to Methanol in Water

Precise control of the oxidation state of transition-metal oxides, such as copper, is important for high selectivity of CO₂ reduction in an aqueous condition to compete with the reduction of water. The phase of copper oxide nanofibers was controlled by predictive synthesis, which controls the nanoscale gas-solid reaction by considering thermodynamics and kinetics. The driving force of the phase transformation between the different oxidation states of copper oxide is calculated by comparing the Gibbs free energy of each of the oxidation states. From the calculation, the kinetically processable window for the fabrication of Cu₂O in which monophase Cu₂O can be fabricated in a reasonable reaction time scale is discovered. Herein, we report the monophase Cu₂O nanofiber photocathode, which photoelectrochemically converted CO₂ into methanol with over 90% selectivity in an aqueous electrolyte, and a hierarchical structure is developed to optimize the photoactivity and stability of the electrode. Our work suggests a rational design of the calcination strategy for precisely controlling the oxidation states of transition metals that can be applied to various applications in which the phase of the materials plays an important role.

Improving the Stability and Efficiency of CuO Photocathodes for Solar Hydrogen Production through Modification with Iron

Cupric oxide (CuO) is considered as a promising photocathode material for photo(electro)chemical water splitting because of its suitable band gap, low cost related to copper earth abundancy, and straightforward fabrication. The main challenge for the development of practical CuO-based photocathodes for solar hydrogen evolution is to enhance its stability against photocorrosion. In this work, stable and efficient CuO photocathodes have been developed by using a simple and cost-effective methodology. CuO films, composed of nanowires and prepared by chemical oxidation of electrodeposited Cu, develop relatively high photocurrents in 1 M NaOH. However, this photocurrent appears to be partly associated with photocorrosion of CuO. It is significant though that, even unprotected, a faradaic efficiency for hydrogen evolution of ∼45% is attained. The incorporation of iron through an impregnation method, followed by a high-temperature thermal treatment for promoting the external phase transition of the nanowires from CuO to ternary copper iron oxide, was found to provide an improved stability at the expense of photocurrent, which decreases to about one-third of its initial value. In contrast, a faradaic efficiency for hydrogen evolution of ∼100% is achieved even in the absence of co-catalysts, which is ascribable to the favorable band positions of CuO and the iron copper ternary oxide in the core-shell structure of the nanowires.

Interface Engineering of Colloidal CdSe Quantum Dot Thin Films as Acid-Stable Photocathodes for Solar-Driven Hydrogen Evolution

Colloidal semiconductor quantum dot (CQD)-based photocathodes for solar-driven hydrogen evolution have attracted significant attention because of their tunable size, nanostructured morphology, crystalline orientation, and band gap. Here, we report a thin film heterojunction photocathode composed of organic PEDOT:PSS as a hole transport layer, CdSe CQDs as a semiconductor light absorber, and conformal Pt layer deposited by atomic layer deposition (ALD) serving as both a passivation layer and cocatalyst for hydrogen evolution. In neutral aqueous solution, a PEDOT:PSS/CdSe/Pt heterogeneous photocathode with 200 cycles of ALD Pt produces a photocurrent density of -1.08 mA/cm² (AM-1.5G, 100 mW/cm²) at a potential of 0 V versus reversible hydrogen electrode (RHE) ( j₀) in neutral aqueous solution, which is nearly 12 times that of the pristine CdSe photocathode. This composite photocathode shows an onset potential for water reduction at +0.46 V versus RHE and long-term stability with negligible degradation. In the acidic electrolyte (pH = 1), where the hydrogen evolution reaction is more favorable but stability is limited because of photocorrosion, a thicker Pt film (300 cycles) is shown to greatly improve the device stability and a j₀ of -2.14 mA/cm² is obtained with only 8.3% activity degradation after 6 h, compared with 80% degradation under the same conditions when the less conformal electrodeposition method is used to deposit the Pt layer. Electrochemical impedance spectroscopy and time-resolved photoluminescence results indicate that these enhancements stem from a lower bulk charge recombination rate, higher interfacial charge-transfer rate, and faster reaction kinetics. We believe that these interface engineering strategies can be extended to other colloidal semiconductors to construct more efficient and stable heterogeneous photoelectrodes for solar fuel production.

Ultrastable Photoelectrodes for Solar Water Splitting Based on Organic Metal Halide Perovskite Fabricated by Lift-Off Process

Herein, we report an integrated photoelectrolysis of water employing organic metal halide (OMH) perovskite material. As generic OMH perovskite material and device architecture are highly susceptible to degradation by aqueous electrolytes, we have developed a versatile mold-cast and lift-off process to fabricate and assemble multipurpose metal encapsulation onto perovskite devices. With the metal encapsulation effectively protecting the perovskite cell and also functioning as electrocatalyst, the high-performance perovskite photoelectrodes exhibit high photovoltage and photocurrent that are effectively inherited from the original solid-state solar cell. More importantly, thus-fabricated perovskite photoelectrode demonstrates record-long unprecedented stability even at highly oxidizing potential in strong alkaline electrolyte. We expect that this versatile lift-off process can be adapted in a wide variety of photoelectrochemical devices to protect the material surfaces from corroding electrolyte and facilitate various electrochemical reactions.

Turning Earth Abundant Kesterite-Based Solar Cells Into Efficient Protected Water-Splitting Photocathodes

CZTS/Se kesterite-based solar cells have been protected by conformal atomic layer deposition (ALD)-deposited TiO₂ demonstrating its feasibility as powerful photocathodes for water splitting in highly acidic conditions (pH < 1), achieving stability with no detected degradation and with current density levels similar to photovoltaic productivities. The ALD has allowed low deposition temperatures of 200 °C for TiO₂, preventing significant variations to the kesterite structure and CdS heterojunction, except for the pure-sulfide stoichiometry, which was studied by Raman spectroscopy. The measured photocurrent at 0 V vs reversible hydrogen electrode, 37 mA·cm^-2, is the highest reported to date, and the associated half-cell solar-to-hydrogen efficiency reached 7%, being amongst the largest presented for kesterite-based photocathodes, corroborating the possibility of using them as abundant low-cost alternative photoabsorbers as their efficiencies are improved toward those of chalcopyrites. An electrical circuit has been proposed to model the photocathode, which comprises the photon absorption, charge transfer through the protective layer, and catalytic performance, which paves the way to the design of highly efficient photoelectrodes.

Role of Bismuth in the Electrokinetics of Silicon Photocathodes for Solar Rechargeable Vanadium Redox Flow Batteries

The ability of crystalline silicon to photoassist the V³⁺ /V²⁺ cathodic reaction under simulated solar irradiation, combined with the effect of bismuth have led to important electrochemical improvements. Besides the photovoltage supplied by the photovoltaics, additional decrease in the onset potentials, high reversibility of the V³⁺ /V²⁺ redox pair, and improvement in the electrokinetics were attained thanks to the addition of bismuth. In fact, Bi⁰ deposition has shown to slightly decrease the photocurrent, but the significant enhancement in the charge transfer, reflected in the overall electrochemical performance clearly justifies its use as additive in a photoassisted system for maximizing the efficiency of solar charge to battery.

Defect Mitigation of Solution-Processed 2D WSe2 Nanoflakes for Solar-to-Hydrogen Conversion

Few-atomic-layer nanoflakes of liquid-phase exfoliated semiconducting transition metal dichalcogenides (TMDs) hold promise for large-area, high-performance, low-cost solar energy conversion, but their performance is limited by recombination at defect sites. Herein, we examine the role of defects on the performance of WSe₂ thin film photocathodes for solar H₂ production by applying two separate treatments, a pre-exfoliation annealing and a post-deposition surfactant attachment, designed to target intraflake and edge defects, respectively. Analysis by TEM, XRD, XPS, photoluminescence, and impedance spectroscopy are used to characterize the effects of the treatments and photoelectrochemical (PEC) measurements using an optimized Pt-Cu cocatalyst (found to offer improved robustness compared to Pt) are used to quantify the performance of photocathodes (ca. 11 nm thick) consisting of 100-1000 nm nanoflakes. Surfactant treatment results in an increased photocurrent attributed to edge site passivation. The pre-annealing treatment alone, while clearly altering the crystallinity of pre-exfoliated powders, does not significantly affect the photocurrent. However, applying both defect treatments affords a considerable improvement that represents a new benchmark for the performance of solution-processed WSe₂: solar photocurrents for H₂ evolution up to 4.0 mA cm^-2 and internal quantum efficiency over 60% (740 nm illumination). These results also show that charge recombination at flake edges dominates performance in bare TMD nanoflakes, but when the edge defects are passivated, internal defects become important and can be reduced by pre-annealing.

Earth-Abundant Tin Sulfide-Based Photocathodes for Solar Hydrogen Production

Tin-based chalcogenide semiconductors, though attractive materials for photovoltaics, have to date exhibited poor performance and stability for photoelectrochemical applications. Here, a novel strategy is reported to improve performance and stability of tin monosulfide (SnS) nanoplatelet thin films for H₂ production in acidic media without any use of sacrificial reagent. P-type SnS nanoplatelet films are coated with the n-CdS buffer layer and the TiO₂ passivation layer to form type II heterojunction photocathodes. These photocathodes with subsequent deposition of Pt nanoparticles generate a photovoltage of 300 mV and a photocurrent density of 2.4 mA cm^-2 at 0 V versus reversible hydrogen electrode (RHE) for water splitting under simulated visible-light illumination (λ > 500 nm, P_in = 80 mW cm^-2). The incident photon-to-current efficiency at 0 V versus RHE for H₂ production reach a maximum of 12.7% at 575 nm with internal quantum efficiency of 13.8%. The faradaic efficiency for hydrogen evolution remains close to unity after 6000 s of illumination, confirming the robustness of the heterojunction for solar H₂ production.

New Antimony Selenide/Nickel Oxide Photocathode Boosts the Efficiency of Graphene Quantum-Dot Co-Sensitized Solar Cells

A novel assembly of a photocathode and a photoanode is investigated to explore their complementary effects in enhancing the photovoltaic performance of a quantum-dot solar cell (QDSC). While p-type nickel oxide (NiO) has been used previously, antimony selenide (Sb₂Se₃) has not been used in a QDSC, especially as a component of a counter electrode (CE) architecture that doubles as the photocathode. Here, near-infrared (NIR) light-absorbing Sb₂Se₃ nanoparticles (NPs) coated over electrodeposited NiO nanofibers on a carbon (C) fabric substrate was employed as the highly efficient photocathode. Quasi-spherical Sb₂Se₃ NPs, with a band gap of 1.13 eV, upon illumination, release photoexcited electrons in addition to other charge carriers at the CE to further enhance the reduction of the oxidized polysulfide. The p-type conducting behavior of Sb₂Se₃, coupled with a work function at 4.63 eV, also facilitates electron injection to polysulfide. The effect of graphene quantum dots (GQDs) as co-sensitizers as well as electron conduits is also investigated in which a TiO₂/CdS/GQDs photoanode structure in combination with a C-fabric CE delivered a power-conversion efficiency (PCE) of 5.28%, which is a vast improvement over the 4.23% that is obtained by using a TiO₂/CdS photoanode (without GQDs) with the same CE. GQDs, due to a superior conductance, impact efficiency more than Sb₂Se₃ NPs do. The best PCE of a TiO₂/CdS/GQDs-nS^2-/S_n^2--Sb₂Se₃/NiO/C-fabric cell is 5.96% (0.11 cm² area), which, when replicated on a smaller area of 0.06 cm², is seen to increase dramatically to 7.19%. The cell is also tested for 6 h of continuous irradiance. The rationalization for the channelized photogenerated electron movement, which augments the cell performance, is furnished in detail in these studies.

Semiconductor quantum dot-sensitized rainbow photocathode for effective photoelectrochemical hydrogen generation

The present study reports the fabrication of CdSe quantum dot (QD)-sensitized photocathodes on NiO-coated indium tin oxide (ITO) electrodes and their H₂-generating ability upon light irradiation. A well-established spin-coating method was used to deposit CdSe QD stock solution onto the surface of NiO/ITO electrodes, thereby leading to the construction of various CdSe QD-sensitized photocathodes. The present report includes the construction of rainbow photocathodes by spin-coating different-sized QDs in a sequentially layered manner, thereby creating an energetically favorable gradient for charge separation. The resulting rainbow photocathodes with forward energetic gradient for charge separation and subsequent electron transfer to a solution-based hydrogen-evolving catalyst (HEC) exhibit good light-harvesting ability and enhanced photoresponses compared with the reverse rainbow photocathodes under white LED light illumination. Under minimally optimized conditions, a photocurrent density of as high as 115 μA⋅cm^-2 and a Faradaic efficiency of 99.5% are achieved, which is among the most effective QD-based photocathode water-splitting systems.

Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes

The hole-injection and recombination photophysics for NiO sensitized with RuP ([Ru^II(bpy)₂(4,4'-(PO₃H₂)₂-bpy)]²⁺) are explored. Ultrafast transient absorption (TA) measurements performed with an external electrochemical bias reveal the efficiency for productive hole-injection, that is, quenching of the dye excited state that results in a detectable charge-separated electron-hole pair, is linearly dependent on the electronic occupation of intragap states in the NiO film. Population of these states via a negative applied potential increases the efficiency from 0% to 100%. The results indicate the primary loss mechanism for dye-sensitized NiO is rapid nongeminate recombination enabled by the presence of latent holes in the surface of the NiO film. Our findings suggest a new design paradigm for NiO photocathodes and devices centered on the avoidance of this recombination pathway.

Quantum Dot Thin-Films as Rugged, High-Performance Photocathodes

Typical use of colloidal quantum dots (QDs) as bright, tunable phosphors in real applications relies on engineering of their surfaces to suppress the loss of excited carriers to surface trap states or to the surrounding medium. Here, we explore the utility of QDs in an application that actually exploits their propensity toward photoionization, namely within efficient and robust photocathodes for use in next-generation electron guns. In order to establish the relevance of QD films as photocathodes, we evaluate the efficiency of electron photoemission of films of a variety of compositions in a typical electron gun configuration. By quantifying photocurrent as a function of excitation photon energy, excitation intensity and pulse duration, we establish the role of hot electrons in photoemission within the multiphoton excitation regime. We also demonstrate the effect of QD structure and film deposition methods on efficiency, which suggests numerous pathways for further enhancements. Finally, we show that QD photocathodes offer superior efficiencies relative to standard copper cathodes and are robust against degradation under ambient conditions.

Efficient and Stable Silicon Photocathodes Coated with Vertically Standing Nano-MoS2 Films for Solar Hydrogen Production

Water splitting in a photoelectrochemical cell, which converts sunlight into hydrogen energy, has recently received intense research. Silicon is suitable as a viable light-harvesting material for constructing such cell; however, there is a need to improve its stability and explore a cheap and efficient cocatalyst. Here we fabricate highly efficient and stable photocathodes by integrating crystalline MoS₂ catalyst with ∼2 nm Al₂O₃ protected n⁺p-Si. Al₂O₃ acts as a protective and passivative layer of the Si surface, while the sputtering method using a MoS₂ target along with a postannealing leads to a vertically standing, conformal, and crystalline nano-MoS₂ layer on Al₂O₃/n⁺p-Si photocathode. Efficient (0.4 V vs RHE onset potential and 35.6 mA/cm² saturated photocurrent measured under 100 mA/cm² Xe lamp illumination) and stable (above 120 h continuous water splitting) photocathode was obtained, which opens the door for the MoS₂ catalyst to be applied in photoelectrochemical hydrogen evolution in a facile and scalable way.