Photocathode Surface Research Articles

The CuS-coated CuO nanostructures grown by ion exchange reaction for efficient photoelectrochemical water splitting

CuO is a promising solar photocathode for hydrogen production. However, it suffers serious photocorrosion in the electrolyte, which hinders the efficiency of photoelectrochemical (PEC) water splitting. In the study, Cu(OH)2 nanowires were in situ grown on copper foils through a straightforward wet chemical oxidation process, followed by annealing in air to synthesize CuO photocathode. To enhance their performance, the surface of CuO photocathode was modified by loading CuS with the ion exchange reaction (IER). Under AM 1.5 G, the CuO photocathode modified by CuS showed a significant improvement in the photo-current density, achieving the highest photo-current density was −4.83 mA·cm−2 at 0 V vs. RHE, which was far superior to the photo-current density (-2.02 mA·cm−2) of pristine CuO photocathode at the same potential. Furthermore, the maximum applied bias photon-to-current efficiency (ABPE) of the CuO/CuS photocathode is 0.77 %, which is superior to that of pristine CuO (0.24 %). The enhanced PEC performance of CuO/CuS photocathode is attributed to the improved light absorption and the efficient separation of photogenerated electron-hole pairs.

Electronic structure and properties of activated Al0.5Ga0.5N thin films and nanowires photocathode surface adsorbed with residual gas

To study effect of residual gas in vacuum system on structure and electrical performance of activated AlGaN photocathodes, adsorption models for H2O, CO and CO2 adsorbed on Al0.5Ga0.5N activated surfaces are established and investigated. The results indicate that activated Al0.5Ga0.5N surfaces are not easily adsorbed by residual gases compared to pristine surface. For Al0.5Ga0.5N thin film, H2O and CO exhibit higher affinity for adsorption on Cs/NF3-activated surfaces, while CO2 shows greater tendency to adsorb on Cs-activated surface·H2O, CO and CO2 are most easily adsorbed on Cs/NF3-activated surface for Al0.5Ga0.5N nanowires. Work function of Cs/NF3-activated thin film surface is 2.34 eV, and after adsorbing H2O, CO and CO2, work functions are 1.84 eV, 2.37 eV, and 3.43 eV, respectively. This means that residual gas will increase work function and is harmful to quantum efficiency improvement.

In Situ and Continuous Decoding Hydrogen Generation in Solar Water-Splitting Cells.

Photoelectrochemical (PEC) water splitting is gaining recognition as an effective method for producing green hydrogen. However, the absence of in situ, continuous decoding hydrogen generation tools hampers a detailed understanding of the physics and chemistry involved in hydrogen generation within PEC systems. In this article, we present a quantitative, spatiotemporally resolved optical sensor employing a fiber Bragg grating (FBG) to probe hydrogen formation and temperature characteristics in the PEC system. Demonstrating this principle, we observed hydrogen formation and temperature changes in a novel cappuccino cell using a BiVO4/TiO2 photoanode and a Cu2O/CuO/TiO2 photocathode. Our findings demonstrate that FBG sensors can probe dynamic hydrogen formation at 0.5 s temporal resolution; these sensors are capable of detecting hydrogen concentrations as low as 0.6 mM. We conducted in situ and continuous monitoring of hydrogen and temperature to ascertain various parameters: the rate of hydrogen production at the photocathode surface, the time to reach hydrogen saturation, the distribution of hydrogen and temperature, and the rate of hydrogen transfer in the electrolyte under both external bias and unbiased voltage conditions. These results contribute valuable insights into the design and optimization of PEC water-splitting devices, advancing the in situ comprehensive monitoring of PEC water-splitting processes.

Characterisation of a Cs–Implanted Cu Photocathode

The generation of high-brightness electron beams is a crucial area of particle accelerator research and development. Photocathodes which offer high levels of quantum efficiency when illuminated at visible wavelengths are attractive as the drive laser technology is greatly simplified. The higher laser power levels available at longer wavelengths create headroom allowing use of manipulation techniques to optimise the longitudinal and transverse beam profiles, and so minimise electron beam emittance. Bi–alkali photocathodes which offer quantum efficiency ∼ 10 % under illumination at 532 nm are an example of this. Another solution is the use of modified photoemissive surfaces. Caesium has a low work function and readily photoemits when illuminated at green wavelengths (∼532nm). Caesium oxide has an even lower work function and emits at red wavelengths (∼635nm). We present data on our work to create a hybrid copper photocathode surface modified by implantation of caesium ions, measuring the surface roughness and probing its structure using MEIS. We measure the energy spread of photoemitted electrons, the QE as a function of illumination wavelength, and the practicality of this surface as a photocathode by assessing its lifetime on exposure to oxygen.

Effects of H+ ion bombardment on GaAs photocathode surface with Cs-O and Cs-F activation layers

To characterize the degree of damage to the GaAs photocathode surface caused by H+ ion back bombardment in the electron-bombarded complementary metal–oxide–semiconductor (EBCMOS), Stopping Range of Ions in Matter software based on the Monte Carlo method was used to investigate the effect of H+ ions with different incident energies on the surface of Cs-O (Cs-F) activated GaAs photocathode. During the simulations, different Cs/O (Cs/F) ratios ranging from 1:1 to 4:1 were considered. The sputtering rates, backscattering electrons, and longitudinal and lateral displacements along with vacancies/ions were investigated. According to the analysis of sputtering rates and vacancies, the optimal Cs/O ratio and Cs/F ratio are 3:1 and 4:1, respectively. With the increase in the incident energy, the backscattering rates decrease, the peak value of the H+ ion distribution decreases, while the corresponding peak position increases, and the peak value of the vacancy distribution increases first and then decreases, while the corresponding peak position increases. In addition, the projected ranges, and lateral and longitudinal displacements increase with the increase in incident energies, while the projected ranges may far exceed the straggle lengths and make the ion trajectory become more and more concentrated in the high incident energy region. This work helps to understand the degeneration mechanism of the GaAs photocathode operating in EBCMOS.

A cryogenically cooled 200kV DC photoemission electron gun for ultralow emittance photocathodes.

Novel photocathode materials like ordered surfaces of single crystal metals, epitaxially grown high quantum efficiency thin films, and topologically non-trivial materials with dirac cones show great promise for generating brighter electron beams for various accelerator and ultrafast electron scattering applications. Despite several materials being identified as brighter photocathodes, none of them have been tested in electron guns to extract electron beams due to technical and logistical challenges. In this paper, we present the design and commissioning of a cryocooled 200kV DC electron gun that is capable of testing a wide variety of novel photocathode materials over a broad range of temperatures from 298 to 35K for bright electron beam generation. This gun is designed to enable easy transfer of the photocathode to various standard ultra-high-vacuum surface diagnostics and preparation techniques, allowing a full characterization of the dependence of beam brightness on the photocathode material and surface properties. We demonstrate the development of such a high-voltage, high-gradient gun using materials and equipment that are easily available in any standard university lab, making the development of such 200kV electron guns more accessible.

Exploration on structural stability, electronic and optical properties of Cs-activated and Cs/O-activated Al0.5Ga0.5N thin film and nanowire photocathode surface

To explore effects of surface activation on AlGaN-based photocathode, this paper analyzes in detail the structural stability, charge transfer, band structure, density of states, absorption coefficient and reflectivity of Cs-activated and Cs/O-activated Al0.5Ga0.5N thin films and nanowires by using first-principles. Our results reveal that adsorption energy of Al0.5Ga0.5N thin films and nanowires adsorbed by Cs will gradually increase as Cs coverage increases, and structural stability will be weakened. Cs-adsorbed thin film surfaces are more stable than nanowire when Cs coverage is same. Cs/O co-adsorbed Al0.5Ga0.5N systems are more stable under high Cs coverage. And Cs/O co-adsorbed Al0.5Ga0.5N possess the most stable structure when the ratio of Cs to O is 2:1. Band structure and density of states imply that Cs and O adsorption introduce new energy levels, which are derived from s, p orbitals of Cs and s orbitals of O, respectively. Furthermore, only when the Cs/O ratio is 2:1, the work function of Al0.5Ga0.5N thin film is lower than that of Cs-only adsorption, which is conducive to electron escape and improving quantum efficiency. Results of optical properties show that Cs activation and Cs/O activation can greatly improve the optical performance of Al0.5Ga0.5N.

Surface Engineering of Cu2O Photocathodes via Facile Graphene Oxide Decoration for Improved Photoelectrochemical Water Splitting.

Copper oxide (Cu2O) has attracted significant interest as an efficient photocathode for photoelectrochemical (PEC) water splitting owing to its abundance, suitable band gap, and band-edge potential. Nevertheless, a high charge recombination rate restricts its practical photoconversion efficiency and reduces the PEC water-splitting performance. To address this challenge, we present the facile electrodeposition of graphene oxide (GO) on the Cu2O photocathode surface. To determine the effect of varying GO weight percentages on PEC performance, varying amounts of GO were deposited on the Cu2O photocathode surface. The optimally deposited GO-Cu2O photocathode exhibited a photocurrent density of -0.39 to -1.20 mA/cm2, which was three times that of a photocathode composed of pristine Cu2O. The surface decoration of Cu2O with GO reduced charge recombination and improved the PEC water-splitting performance. These composites can be utilized in strategies designed to address the challenges associated with low-efficiency Cu2O photocathodes. The physicochemical properties of the prepared samples were comprehensively characterized by field-emission scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, Raman spectroscopy, UV-visible spectroscopy, and X-ray photoelectron spectroscopy. We believe that this research will pave the way for developing efficient Cu2O-based photocathodes for PEC water splitting.

Cs and O co-adsorption on p-type Al0.5Ga0.5N (0001) UV photocathode surface

In the study, the co-adsorption of Cs and O atoms on p-type Al0.5Ga0.5N (0001) surface was firstly calculated. The adsorption became unstable and the surface structure was deformed when the adsorption of Cs was at 0.75 monolayer coverage. The adsorption stability and surface structure were improved after oxygen adsorption on Cs-covered surface. In the stage of Cs and O co-adsorption, after the adsorption of 2Cs2O, two O atoms form dioxygen. After the adsorption of 3Cs2O, two O atoms were independent from each other. One O atom was in the outermost layer and the dipole moment pointing to the outside of the surface was formed and thus hindered the escape of photoelectron. In Cs and O co-adsorption, after Cs was further adsorbed, the adsorbed atoms of 2Cs2O1Cs and surface atoms formed the three-staged dipole moment. Both the largest dipole moment and the smallest work function were achieved for the surface. The negative electron affinity was realized due to the downward shift of the conduction band and the upward shift of the valence band. The surface adsorption peak at 177.16 nm met the requirements for ultraviolet detection.

Self-powered photoelectrochemical/visual sensing platform based on PEDOT/BiOBr0.8I0.2 organic-inorganic hybrid material and MWCNTs/SnS2 heterojunction for the ultrasensitive detection of programmed death ligand-1

Programmed death ligand-1 (PD-L1) can enhance the immune tolerance of tumor cells by suppressing the activity of T-cells, and is one of the culprits that lead to the immune escape of tumor cells. Thus, the sensitive and portable detection of PD-L1 levels is essential for many types of tumor prognosis. Herein, a novel dual-mode analytical device for the ultrasensitive detection of PD-L1 has been developed. In this configuration, an advanced organic-inorganic hybrid material of poly(3,4-ethylenedioxythiophene) -BiOBr0.8I0.2 is designed as photocathode to enhance the photogenerated electron migration efficiency of the MWCNTs/SnS2-photoanode by external circuit, amplifying cathodic photocurrent without extra energy supply. The PD-L1 aptamer is loaded on the photocathode surface to ensure selectivity. The obtained sensing platform can achieve highly sensitive and specific detection of PD-L1 in complex environment, with a low detection limit of 0.29 pg mL-1. On the other hand, electrochromic material Prussian blue (PB) and MWCNTs/SnS2 are integrated to fabricate a portable sensing chip for PD-L1. Under illumination, photogenerated electrons of MWCNTs/SnS2 are injected into Prussian blue, and the blue PB is reduced to white product, indicating the concentration of PD-L1, without need of other instrument. This self-powered photoelectrochemical and visual analysis system has good practicability and is a promising clinical diagnosis tool.

Enhanced blue-green response of nanoarray AlGaAs photocathodes for underwater low-light detection.

Underwater optical communication and low-light detection are usually realized via blue-green laser sources and blue-green light-sensitive detectors. Negative-electron-affinity AlGaAs photocathode is an ideal photosensitive material for ocean exploration due to its adjustable spectrum range, long working lifetime, and easy epitaxy of materials. However, compared with other photocathodes, the main problem of AlGaAs photocathode is its low quantum efficiency. Based on Spicer's three-step photoemission model, nanoarray structures are designed on the surface of AlGaAs photocathode to improve its quantum efficiency from two aspects of optical absorption and photoelectron transport. Through simulation, it is concluded that the cylinder with diameter of 120 nm and height of 600 nm is the best nanoarray structure, and its absorptance is always greater than 90% in the 445∼532 nm range. Moreover, the absorptance and quantum efficiency of the cylinder nanoarray AlGaAs photocathode are less affected by the incident angle. When the angle of incident light reaches 70°, the minimum absorptance and quantum efficiency are still 64.6% and 24.9%. In addition, the square or hexagonal arrangement pattern of the nanoarray has little effect on the absorptance, however, a reduction in the overall emission layer thickness will decrease the absorptance near 532 nm.

Mixed‐phase WO3 Cocatalysts on Hierarchical Si‐based Photocathode for Efficient Photoelectrochemical Li Extraction

AbstractPhotoelectrochemical lithium (Li) extraction can be expected to provide a useful recycle of Li+ from waste Li‐containing battery, but the process is limited by the photocathodes with poor Li+ absorption and low yield rate. Here, we have designed a hierarchical silicon (Si)‐based photocathode with mixed‐phase tungsten oxide (WO3) cocatalysts for photoelectrochemical Li extraction under 1 sun illumination, achieving a high Li yield rate of ≈223.0 μg cm−2 h−1 and an excellent faradaic efficiency of 91.9 % at 0.0817 V versus Li0/+ redox couple. The WO3 cocatalysts with the mixture of amorphous and crystalline phase accelerates the Li+ insertion and precipitation and enriches the concentration of Li+ at the photocathode surface. This robust photoelectrochemical Li extraction system provides a new insight on designing green and efficient route for cyclic utilization of Li resources in the sustainable energy field.

Mixed-phase WO3 Cocatalysts on Hierarchical Si-based Photocathode for Efficient Photoelectrochemical Li Extraction.

Photoelectrochemical lithium (Li) extraction can be expected to provide a useful recycle of Li+ from waste Li-containing battery, but the process is limited by the photocathodes with poor Li+ absorption and low yield rate. Here, we have designed a hierarchical silicon (Si)-based photocathode with mixed-phase tungsten oxide (WO3 ) cocatalysts for photoelectrochemical Li extraction under 1 sun illumination, achieving a high Li yield rate of ≈223.0 μg cm-2 h-1 and an excellent faradaic efficiency of 91.9 % at 0.0817 V versus Li0/+ redox couple. The WO3 cocatalysts with the mixture of amorphous and crystalline phase accelerates the Li+ insertion and precipitation and enriches the concentration of Li+ at the photocathode surface. This robust photoelectrochemical Li extraction system provides a new insight on designing green and efficient route for cyclic utilization of Li resources in the sustainable energy field.

Charge Separation Enhancement Enables Record Photocurrent Density in Cu2ZnSn(S,Se)4 Photocathodes for Efficient Solar Hydrogen Production (Adv. Energy Mater. 19/2023)

Solar Hydrogen Production The image representing article number 2300215 by Shuo Chen and co-workers depicts photo-electrochemical water splitting that converts solar energy into hydrogen energy, presenting obvious hydrogen bubbles emerging from the photocathode surface to the electrolyte under light illumination. The blue columnar rods indicate high-quality Cu2ZnSn(S,Se)4 with large crystal grains and benign growth orientation. The yellow spheres denote photoelectrons, indicating efficient charge carrier generation, separation and transport.

Spectral emission properties of a nitrogen-doped diamond (001) photocathode: Hot electron transport and transverse momentum filtering

The electron emission properties of a single-crystal nitrogen-doped diamond(001) photocathode inserted in a 10kV DC photoelectron gun are determined using a tunable (235-410nm) ultraviolet laser radiation source for photoemission from both the back nitrogen-doped substrate face and the front homo-epitaxially grown and undoped diamond crystal face. The measured spectral trends of the mean transverse energy (MTE) and quantum efficiency (QE) of the emitted electrons are both anomalous and non-monotonic, but are shown to be consistent with (i) the known physics of electron photoexcitation from the nitrogen substitution states into the conduction bands of diamond, (ii) the energy position and dispersion characteristics of the conduction bands of diamond in the (001) emission direction, (iii) the effective electron affinity of the crystal faces, (iv) the strong electron-(optical)phonon coupling in diamond, and (v) the associated hot electron transport dynamics under energy equipartition with the optical phonons. Notably, the observed hot electron emission is shown to be restricted parallel to the photocathode surface by the low transverse effective masses of the emitting band states - a transverse momentum filtering effect.

Optoelectronic Properties of In0.87Ga0.13As0.25P0.75(001)β2(2×4) Surface: A First-Principles Study.

InGaAsP photocathode surface affects the absorption, transport and escape of photons, and has a great influence on quantum efficiency. In order to study InGaAsP photocathode surface, the electronic structure, work function, formation energy, Mulliken population and optical properties of In0.87Ga0.13As0.25P0.75(001)β2(2×4) reconstruction surface were calculated from first principles. Results show that stabilized the In0.87Ga0.13As0.25P0.75(001)β2(2×4) surface is conducive to the escape of low-energy photoelectrons. The narrow bandgap and emerging energy levels of the reconstruction surface make the electron transition easier. Under the action of the dipole moment, the electrons transfer from inner layers to the surface during the surface formation process. By contrast to the bulk, the surface absorption coefficient and reflectivity considerably decrease, and the high-reflection range becomes narrower as the falling edge redshifts. On the contrary, the surface transmissivity increases, which is conducive for the photons passing through the surface into the bulk to excite more photoelectrons. Meanwhile, the higher absorption coefficient of surface in low-energy side is favorable for long-wave absorption. The dielectric function peaks of the surface move toward the low-energy side and peak values decrease.

Photoelectrochemical properties of Cu2O/PANI/Si-based photocathodes for CO2 conversion

The present study was aimed at converting carbon dioxide (CO2) into value-added products such as methanol, which can provide not only a potential solution for controlling the carbon dioxide concentration level in the atmosphere but also can offer an alternative approach for the production of renewable energy sources. From this perspective, various hybrid photoelectrocatalysts were synthesized, characterized and used as photocathodes for photoelectrochemical (PEC) reduction of carbon dioxide to methanol in an aqueous medium under visible light irradiation. Flat silicon (Si) and pyramidal textured silicon (SiPY) substrates, covered with polyaniline (PANI) with or without sensitization with copper (I) oxide (Cu2O) particles, were investigated. It was observed that the combination of PANI and copper (I) oxide greatly increased PEC carbon dioxide reduction to methanol owing to the enhancement in the carbon dioxide chemisorption capacity by the photocathode surface and at the same time facilitated the separation of photogenerated electron–hole (e−/h+) pairs. The PEC results demonstrated that the applied potential impacts the photocurrent stability. Sensitization with copper (I) oxide effectively separated the photogenerated e−/h+ pairs and, therefore, enhanced the PEC carbon dioxide reduction activity of the hybrid photocatalyst. The best faradaic efficiency for methanol formation reached 57.66%, which was recorded when the copper (I) oxide/PANI/SiPY heterostructure was used as a photocathode at an applied potential of −1.2 V against the saturated calomel electrode.

First-principles study on electronic and optical properties of O-adsorbed Al0.5Ga0.5N photocathode surface

In this article, to explore effects of oxygen adsorption on Al0.5Ga0.5N film photocathodes surface, we have studied stability, electronic and optical properties of Al0.5Ga0.5N (0001) surface with different O adsorption positions and different O coverage by using the first-principles. Our results indicate that TN is the most stable adsorption site for O coverage with 0.25 ML. The adsorption energies increase and stability of Al0.5Ga0.5N decreases when O coverage is increased. O atoms obtain electrons from Al0.5Ga0.5N surface and are chemically adsorbed on Al0.5Ga0.5N (0001) surface. The band structure and DOS imply that abundant surface states appear near the Fermi level, which originates from the Ga-s, Ga-p and Al-p orbitals of surface dangling bonds. The O-p electronic states of O adatom induce shallow levels near the Fermi level, and the O-s electronic states form new deep levels. For O coverage with 0.25 ML, work function is reduced. However, when O coverage continues to increase, the work function will be greater than pristine Al0.5Ga0.5N surface, which is harmful to electron emission of photocathode and is not conducive to improve quantum efficiency of photocathode.

Influence of surface carbon on the performance of cesiated p-GaN photocathodes with high quantum efficiency

This study shows residual surface carbon’s influence on photocathodes’ quantum efficiency based on p-GaN grown on sapphire by metal organic chemical vapor deposition. An X-ray photoelectron spectrometer (XPS) built in an ultrahigh vacuum system allowed the in-situ monitoring of the photocathode surface beginning immediately after their cleaning and throughout the activation and degradation processes. An atomically clean surface is necessary to achieve a negative electron affinity, which is the main prerequisite for high quantum efficiency. The p-GaN samples were cleaned with ethanol and underwent a sub-sequential thermal vacuum cleaning. Although carbon and oxygen contaminations are expected to be undesired impurities from the metal organic chemical vapor deposition, which remained on the surface, p-GaN could still form a negative electron affinity surface when exclusively activated with cesium. After the activation with cesium, a shift to a higher binding energy of the photoemission peaks was observed, and a new species, a so-called cesium carbide, was formed, growing over time. The XPS data elucidated the critical role of these cesium carbide species in photocathode degradation. The X-ray damage to the p-GaN:Cs photocathodes, especially the influence on the cesium, was additionally discussed.

Study on Denoising Method of Photoionization Detector Based on Wavelet Packet Transform

Aiming at the task of noise suppression caused by the photoionization detector (PID) monitoring signal of volatile organic compounds (VOCs) due to local non-uniformity of the photocathode surface of PID in the ionization chamber, this paper proposes an analytical method of a PID signal with the adaptive weight of the small wave package decomposition node. The PID signal is transmitted to the upper machine software through the single-chip microcontroller. The appropriate wavelet packet decomposition level is determined according to the time frequency characteristics of the original signal of the PID, and the optimal wavelet packet base is selected through the polynomial fitting of the signal quality evaluation index. By comparing the quality of signals processed by the traditional wavelet packet denoising method and the denoising method presented in this paper, the superiority of the proposed method in the denoising signals of PID was verified. This method can eliminate the noise generated by local non-uniformity on the photocathode surface of the PID ionization chamber in a high humidity environment, which lays a foundation for the accurate monitoring of VOCs in a high humidity environment.