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This paper presents a comprehensive investigation into the quantum efficiency (QE) of metallic photocathodes used in modern high-performance radio frequency (RF) and superconducting radio frequency (SRF) guns. The study specifically examines how laser cleaning treatment impacts the QE of these photocathodes, providing detailed insights into their performance and potential improvements for accelerator applications, and assesses the chemical and environmental factors affecting the surface composition of metallic laser-photocathodes used in modern high-performance radio frequency (RF) and superconducting radio frequency (SRF) electron guns. This paper overviews the photocathode rejuvenation effects of laser cleaning treatment. Laser cleaning removes the oxides and hydrides responsible for the deterioration of photocathodes, increases the photoelectron emission quantum efficiency (QE) and extends the operational lifetime of high-brightness electron injectors. QE enhancement is analyzed with the aim of parametric cleaning process optimization. This study excludes semiconductor and thermionic cathodes, focusing solely on the widely used bulk and thin-film photocathodes of Cu, Mg, Y, Pb and Nb. Laser cleaning enhancement of QE in Cu from 5 × 10−5 to 1.2 × 10−4, in Mg from 5.0 × 10−4 to 1.8 × 10−3, in Y from 10−5 to 3.3 × 10−4, in Pb from 3 × 10−5 to 8 × 10−5, and in Nb from 2.1 × 10−7 to 2.5 × 10−5 is demonstrated. The analysis concludes with a specialized practical guide for improving photocathode efficacy and lifetime in RF and SRF guns.

The technology of creation the photocathode with quantum efficiency at the level of 5% based on the InP/InGaAs heterostructures is given. The effect of decreasing the quantum efficiency of the photosensitive structure on radiant sensitivity is considered. Several variants of realization of vacuum photoelectronic device with InP/InGaAs photocathode for special purposes are represented.

We developed, constructed and commissioned novel photocathode lasers for the X-ray FELs European XFEL and FLASH which allow emittance optimization via beam shaping. Both facilities operate continuously with those lasers since Jan. 2024 with high uptime and excellent performance.

This paper presents an innovative exploration of advanced configurations for enhancing the efficiency of metallic and superconducting photocathodes (MPs and SCPs) produced via pulsed laser deposition (PLD). These photocathodes are critical for driving next-generation free-electron lasers (FELs) and plasma-based accelerators, both of which demand electron sources with improved quantum efficiency (QE) and electrical properties. Our approach compares three distinct photocathode configurations, namely: conventional, hybrid, and non-conventional, focusing on recent innovations. Hybrid MPs integrate a thin, high-performance, photo-emissive film, often yttrium or magnesium, positioned centrally on the copper flange of the photo-injector. For hybrid SCPs, a thin film of lead is used, offering a higher quantum efficiency than niobium bulk. This study also introduces non-conventional configurations, such as yttrium and lead disks partially coated with copper and niobium films, respectively. These designs utilize the unique properties of each material to achieve enhanced photoemission and long-term stability. The novelty of this approach lies in leveraging the advantages of bulk photoemission materials like yttrium and lead, while maintaining the electrical compatibility and durability required for integration into RF cavities. The findings highlight the potential of these configurations to significantly outperform traditional photocathodes, offering higher QE and extended operational lifetimes. This comparative analysis provides new insights into the fabrication of high-efficiency photocathodes, setting the foundation for future advancements in electron source technologies.

Photoelectrochemical (PEC) artificial leaves hold the potential to lower the costs of sustainable solar fuel production by integrating light harvesting and catalysis within one compact device. However, current deposition techniques limit their scalability 1 , whereas fragile and heavy bulk materials can affect their transport and deployment. Here we demonstrate the fabrication of lightweight artificial leaves by employing thin, flexible substrates and carbonaceous protection layers. Lead halide perovskite photocathodes deposited onto indium tin oxide-coated polyethylene terephthalate achieved an activity of 4,266 µmol H 2  g −1  h −1 using a platinum catalyst, whereas photocathodes with a molecular Co catalyst for CO 2 reduction attained a high CO:H 2 selectivity of 7.2 under lower (0.1 sun) irradiation. The corresponding lightweight perovskite-BiVO 4 PEC devices showed unassisted solar-to-fuel efficiencies of 0.58% (H 2 ) and 0.053% (CO), respectively. Their potential for scalability is demonstrated by 100 cm 2 stand-alone artificial leaves, which sustained a comparable performance and stability (of approximately 24 h) to their 1.7 cm 2 counterparts. Bubbles formed under operation further enabled 30–100 mg cm −2 devices to float, while lightweight reactors facilitated gas collection during outdoor testing on a river. This leaf-like PEC device bridges the gulf in weight between traditional solar fuel approaches, showcasing activities per gram comparable to those of photocatalytic suspensions and plant leaves. The presented lightweight, floating systems may enable open-water applications, thus avoiding competition with land use. This work introduces lightweight, leaf-like photoelectrochemical devices for unassisted water splitting and syngas production, which could be used in the fabrication of floating systems for solar fuel production.

Formic acid (or formate) is suggested to be one of the most economically viable products from electrochemical carbon dioxide reduction. However, its commercial viability hinges on the development of highly active and selective electrocatalysts. Here we report that structural defects have a profound positive impact on the electrocatalytic performance of bismuth. Bismuth oxide double-walled nanotubes with fragmented surface are prepared as a template, and are cathodically converted to defective bismuth nanotubes. This converted electrocatalyst enables carbon dioxide reduction to formate with excellent activity, selectivity and stability. Most significantly, its current density reaches ~288 mA cm −2 at −0.61 V versus reversible hydrogen electrode within a flow cell reactor under ambient conditions. Using density functional theory calculations, the excellent activity and selectivity are rationalized as the outcome of abundant defective bismuth sites that stabilize the *OCHO intermediate. Furthermore, this electrocatalyst is coupled with silicon photocathodes and achieves high-performance photoelectrochemical carbon dioxide reduction. Carbon dioxide can be electrochemically reduced to form valuable chemical feedstocks, but efficiency of electrocatalysts should be improved. Here the authors report nanotube-derived bismuth for electrocatalytic reduction of carbon dioxide to formate, with performance that is enhanced by defects.

Photoelectrocatalytic (PEC) production of fuels and chemicals by using solar energy, water, and CO 2 paves a promising avenue toward carbon neutrality. Over the past decades, for accelerating this process, a variety of photocathodes have been explored. Among them, the hybrid of GaN nanowires (NWs) and planar silicon has appeared as a disruptive platform for this grand topic, owing to their distinctive structural, optoelectronic, and catalytic properties. This review illustrates the most recent advances in GaN NWs/Si-based photocathodes for CO 2 reduction reactions powered by simulated sunlight, beginning with a discussion of the critical requirements and fundamental challenges of PEC CO 2 reduction. The characteristics of GaN NWs/Si are then discussed, showing its great potential in precisely controlling the behavior of photons, charges, and chemical species. As the focus of this review, the progress on the PEC CO 2 reduction reactions toward different products over GaN NWs/Si-based photocathodes is highlighted. In the end, the challenges and prospects of GaN NWs/Si-based photocathodes for the practical synthesis of solar fuels and chemicals are proposed.

Solar fuels offer a promising approach to provide sustainable fuels by harnessing sunlight 1 , 2 . Following a decade of advancement, Cu 2 O photocathodes are capable of delivering a performance comparable to that of photoelectrodes with established photovoltaic materials 3 , 4 – 5 . However, considerable bulk charge carrier recombination that is poorly understood still limits further advances in performance 6 . Here we demonstrate performance of Cu 2 O photocathodes beyond the state-of-the-art by exploiting a new conceptual understanding of carrier recombination and transport in single-crystal Cu 2 O thin films. Using ambient liquid-phase epitaxy, we present a new method to grow single-crystal Cu 2 O samples with three crystal orientations. Broadband femtosecond transient reflection spectroscopy measurements were used to quantify anisotropic optoelectronic properties, through which the carrier mobility along the [111] direction was found to be an order of magnitude higher than those along other orientations. Driven by these findings, we developed a polycrystalline Cu 2 O photocathode with an extraordinarily pure (111) orientation and (111) terminating facets using a simple and low-cost method, which delivers 7 mA cm −2 current density (more than 70% improvement compared to that of state-of-the-art electrodeposited devices) at 0.5 V versus a reversible hydrogen electrode under air mass 1.5 G illumination, and stable operation over at least 120 h. A study introduces a novel method to grow single-crystal Cu 2 O thin films with selected crystal orientations, highlighting enhanced bulk carrier mobility and carrier diffusion length along the [111] direction that yields Cu 2 O photocathodes with improved performance.

Determining cost-effective semiconductors exhibiting desirable properties for commercial photoelectrochemical water splitting remains a challenge. Herein, we report a Sb 2 Se 3 semiconductor that satisfies most requirements for an ideal high-performance photoelectrode, including a small band gap and favourable cost, optoelectronic properties, processability, and photocorrosion stability. Strong anisotropy, a major issue for Sb 2 Se 3 , is resolved by suppressing growth kinetics via close space sublimation to obtain high-quality compact thin films with favourable crystallographic orientation. The Sb 2 Se 3 photocathode exhibits a high photocurrent density of almost 30 mA cm −2 at 0 V against the reversible hydrogen electrode, the highest value so far. We demonstrate unassisted solar overall water splitting by combining the optimised Sb 2 Se 3 photocathode with a BiVO 4 photoanode, achieving a solar-to-hydrogen efficiency of 1.5% with stability over 10 h under simulated 1 sun conditions employing a broad range of solar fluxes. Low-cost Sb 2 Se 3 can thus be an attractive breakthrough material for commercial solar fuel production. While photoelectrochemical water splitting offers an integrated means to convert sunlight to a renewable fuel, cost-effective light-absorbers are rare. Here, authors report Sb 2 Se 3 photocathodes for high-performance photoelectrochemical water splitting devices.