Nanowire Photocathode Research Articles

Novel dual-photoelectrode Z-scheme PEC system for efficient TBBPA removal and debromination: Performance and promoting mechanism

Herein, a novel dual-photoelectrode Z-scheme photoelectrocatalytic system is created to achieve efficient synergistic anodic oxidation and cathodic reduction for TBBPA degradation and debromination. Based on the polydopamine-modified N-doped TiO2 nanotubes (PN-TNTs) photoanode and polydopamine-modified Cu2O nanowires (PC-CNWs) photocathode, the degradation, debromination and mineralization were 99.4%, 77.7% and 52.1% at 0.7 V under visible light for 2 h. The introduction of PDA enhances the visible light absorption by the TiO2 photoanode and protects the Cu2O photocathode from photocorrosion. The main active species identified by electron paramagnetic resonance (EPR) and scavenger experiments as •OH and e− were found to promote the degradation and debromination of TBBPA by oxidation and reduction, respectively. The degradation pathway of TBBPA was determined by Density Functional Theory (DFT) calculations and HPLC-MS analyses. Comparing the intermediates of single and dual chambers, it demonstrated that the photocathode was able to achieve the continuous debromination of TBBPA and intermediates, which reduced the toxicity. Finally, the stability, reusability and energy consumption were evaluated, confirming the feasibility of the system.

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.

Enhanced Photoemission Performance of InGaN Inclined Nanowire Array.

We propose to improve the solar energy utilization by using InGaN inclined nanowire array photocathodes. We first study vertical nanowire array. On the basis of vertical nanowire array, we study inclined nanowires by changing the inclination angle of nanowires. The inclined nanowires exhibit higher quantum efficiency at larger period and larger inclination angle. However, the infinite expansion of period will cause its performance to degrade. The quantum efficiency of inclined nanowires with a period of 175 nm and an inclination angle of 5.35° is as high as 80.2% when the incident light angle is irradiated at 5°. In addition, applying an electric field can improve the collection efficiency of inclined nanowires and help them maintain a high collection efficiency over a longer wavelength range. The design principles proposed in this work will provide a theoretical reference for the performance improvement of InGaN photocathodes.

Nanoscale heterojunctions of InGaN/GaN photocathodes for electron sources

The design of the photocathode is crucial for generating high-quality electron beam in electron sources. In this study, InGaN/GaN heterojunction nanowire photocathodes were proposed, and the photoemission theoretical model was developed. The results demonstrate that the built-in electric field along the axis enables the heterojunction nanowire photocathode to achieve higher collection efficiency across the response spectrum compared to In0.5Ga0.5N nanowire photocathodes. The variation of incidence angle results in distinct peaks in quantum efficiency and collection efficiency for the heterojunction nanowire photocathode. Meanwhile, the “additional electric field” has the potential to decrease the number of laterally emitted electrons from the nanowires, consequently reducing the shielding effect of adjacent nanowires. With an incidence angle of 15° and an “additional electric field” of 2 V/μm, the collection efficiency of photoelectrons at the collection end can be maximized to 47.6 %. This indicates that the heterojunction nanowire array photocathode has potential for use in high-performance vacuum electron sources.

Emission surface improvement of In0.5Ga0.5N tilted nanowire array photocathode for vacuum electron sources

Photocathode has received increasing attention in scientific devices that require high-quality electron beams. A theoretical photoemission model for enhancing the performance of In0.5Ga0.5N tilted nanowire array photocathodes with negative electron affinity surface is proposed. Finite Element and finite difference methods are employed in simulation experiments to study the In0.5Ga0.5N tilted nanowire array photocathode. The results demonstrate that the photoelectric conversion performance of the In0.5Ga0.5N tilted nanowire array photocathode can be significantly enhanced through the combined impact of oblique incident light and an additional electric field. Specifically, when θ = 20°, β = −40°, and Eout = 4 V/μm, the total quantum efficiency and collection efficiency are 2.79 times and 4.27 times higher than compared to vertical nanowires without external assistance, respectively.

Carbon coated Cu2O/CuO nanowire photocathodes for enhanced photoelectrochemical water splitting performance

ABSTRACT We herein report the fabrication of mixed oxide Cu2O/CuO nanowire electrodes through the anodisation of Cu-mesh. These synthesised electrodes are then coated with a protective carbon layer to enhance hydrogen production via photoelectrochemical (PEC) water splitting. Our study addressed Cu2O/CuO nanowire surface modification via anodisation voltage adjustment and analyzed annealing effects on carbon-coated electrodes under controlled temperatures in a nitrogen environment. Optimisation at 5V anodisation potential yielded enhanced photocurrents, with the Cu2O/CuO mixed phase proving crucial for improved PEC activity. XRD, SEM and TEM analyses were conducted to evaluate the structural and morphological characterisation of the synthesised samples. A thin carbon coated sample annealed at 550 °C enhances the photoelectrochemical efficacy of Cu2O nanowires, obtaining a peak photocurrent density of ∼4 mA cm−2 at 0 V (vs. RHE). In contrast, as prepared pristine optimised Cu2O/CuO nanostructured electrodes deliver a photocurrent density of ∼ 2 mA cm−2 at 0 V (vs. RHE) under AM 1.5G light solar irradiation with an intensity of 100 mWcm-2. The Mott–Schottky analysis exhibits p-type semiconducting nature of carbon coated Cu2O/CuO electrodes. The photocurrent stability test alsoconfirms that carbon coated Cu2O/CuO electrodes are one of the best candidates for improving the photoelectrochemical water splitting reaction.

Quantitative study on the photoemission of AlGaN nanoarrays based on the three-dimensional transportation within a four-step process.

Photocathodes play a crucial role in photoelectronic imaging and vacuum electronic devices. The quantum efficiency of photocathodes, which determines their performance, can be enhanced through materials engineering. However, the quantum efficiency of conventional planar photocathodes remains consistently low, at around 25%. In this paper, we propose what we believe is a novel structure of AlGaN nanowire array to address this issue. We investigate the photoemission characteristics of the nanowire array using the "four-step" process, which takes into account optical absorption, electron transportation, electron emission, and electron collection. We compare the quantum efficiency of nanowire arrays with different structure sizes and Al components. After studying the effect of incident light at various angles on the nanowire array photocathode, we identify the optimal dimensional parameters: a height of 400∼500 nm and a wire width of 200∼300 nm. Furthermore, we improved the collection efficiency of the photocathode by introducing a built-in/external electric field, and obtained a 104.4% enhancement of the collection current with the built-in electric field, meanwhile the photocurrent was increased by 87% compared to the case without the external electric field. These findings demonstrate the potential of optimizing photocathode performance through the development of a novel model and adjustment of parameters, offering a promising approach for photocathode applications.

Photoemission of InGaN nanowire array photocathode assisted by external electric field

An external electric field-assisted method is proposed to improve the electron collection capability of InGaN photocathode. We use the diffusion drift equation to calculate the photoemission performance of the nanowire arrays. The results show that nanowire array has better electron collection ability with the aid of external electric field. When the incidence angle is 88∘ and 70∘, the quantum efficiency and collection efficiency reach the maximum respectively. When the external electric field is 0.6[Formula: see text]V/[Formula: see text]m, the maximum collection efficiency is 34.2%. Compared with no electric field, the electron collection ratio increased by 69.4%. The external electric field- assisted InGaN photocathode can significantly improve the photoelectric emission performance, which is expected to be realized in future experiments.

High-Photovoltage Silicon Nanowire for Biological Cofactor Production.

Photocathodic conversion of NAD+ to NADH cofactor is a promising platform for activating redox biological catalysts and enzymatic synthesis using renewable solar energy. However, many photocathodes suffer from low photovoltage, consequently requiring a high cathodic bias for NADH production. Here, we report an n+p-type silicon nanowire (n+p-SiNW) photocathode having a photovoltage of 435 mV to drive energy-efficient NADH production. The enhanced band bending at the n+/p interface accounts for the high photovoltage, which conduces to a benchmark onset potential [0.393 V vs the reversible hydrogen electrode (VRHE)] for SiNW-based photocathodic NADH generation. In addition, the n+p-SiNW nanomaterial exhibits a Faradaic efficiency of 84.7% and a conversion rate of 1.63 μmol h-1 cm-1 at 0.2 VRHE, which is the lowest cathodic potential to achieve the maximum productivity among SiNW-sensitized cofactor production.

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.

Photoemission enhancement of InxGa1-xN nanowire array photocathode

The semiconductor material InxGa1-xN can be used in optoelectronic devices to achieve a tunable wide spectral response. Based on “Spicer's model” and “Andachi's model”, a complete theoretical model of photoemission of reflective InxGa1-xN nanowire array photocathode is established. The results show that the photocathode has a sharp cut-off characteristic, and the increase of the nanowire height is conducive to enhancing the quantum efficiency in the response band. Both oblique light and additional electric field can improve the electron collection efficiency of the anode. With an incidence angle of 0° to 40°, the collection efficiency of photocathodes with H = 800 nm is improved, with a maximum increase of 50.21%. In addition, the electron collection efficiency of nanowire photocathode with H = 400 nm, β = 20°and Eout = 0.4 V/μm can be enhanced by 121% compared to the traditional planar photocathode. This work is expected to provide theoretical guidance for researching the InGaN photocathode electron source.

Photochemical Diodes for Simultaneous Bias-Free Glycerol Valorization and Hydrogen Evolution.

Artificial photosynthesis offers a route to producing clean fuel energy. However, the large thermodynamic requirement for water splitting along with the corresponding sluggish kinetics for the oxygen evolution reaction (OER) limits its current practical application. Here, we offer an alternative approach by replacing the OER with the glycerol oxidation reaction (GOR) for value-added chemicals. By using a Si photoanode, a low GOR onset potential of -0.05 V vs RHE and a photocurrent density of 10 mA/cm2 at 0.5 V vs RHE can be reached. Coupled with a Si nanowire photocathode for the hydrogen evolution reaction (HER), the integrated system yields a high photocurrent density of 6 mA/cm2 with no applied bias under 1 sun illumination and can run for over 4 days under diurnal illumination. The demonstration of the GOR-HER integrated system provides a framework for designing bias-free photoelectrochemical devices at appreciable currents and establishes a facile approach to artificial photosynthesis.

Photoelectrocatalysis Synthesis of Ammonia Based on a Ni-Doped MoS2/Si Nanowires Photocathode and Porous Water with High N2 Solubility

The synthesis of ammonia through photocatalysis or photoelectrochemistry (PEC) and nitrogen reduction reaction (NRR) has become one of the recent research hotspots in the field, where the catalyzed materials and strategies are critical for the NRR. Herein, a Ni-doped MoS2/Si nanowires (Ni-MoS2/Si NWs) photocathode is prepared, where the Si NWs are formed on the surface of a Si slice by the metal-assisted chemical etching method, and the hydrothermally synthesized Ni-MoS2 nanosheets are then cast-coated on the Si NWs electrode. Porous water with high solubility of N2 is prepared by treating a hydrophobic porous coordination polymer with hydrophilic bovine serum albumin for subsequent aqueous dispersing. The relevant electrodes and materials are characterized by electrochemistry, UV-vis spectrophotometry, scanning electron microscopy/energy dispersive spectroscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller method, and zeta potential method. The uses of the Ni-MoS2/Si NWs photocathode and the porous water with high nitrogen solubility for PEC-NRR give a yield of NH3 of 12.0 mmol h-1 m-2 under optimal conditions (e.g., at 0.25 V vs RHE), and the obtained apparent Faradaic efficiency higher than 100% is discussed from the inherent photocurrent-free photocatalysis effect of the photoelectrodes and the suggested classification of three kinds of electrons in PEC, which may have some reference value in understanding and improving other PEC-based processes.

Comparative study on structure and electronic properties of Al0.5Ga0.5N thin film and nanowire photocathode under surface oxidation

Oxidation will generate higher surface barriers and new surface states, resulting in declined electron escape probability and electron emission performance, which seriously affects surface activation effect of the photocathode. In order to explore the effects of oxidation on Al0.5Ga0.5N thin film photocathodes and nanowire photocathodes, we have analyzed structure and electronic properties of Al0.5Ga0.5N thin film and nanowire under oxidation. The results show that formation energies of Al0.5Ga0.5N film oxidation surface is negative in both O-rich and O-poor conditions under O adsorption, indicating that surface of Al0.5Ga0.5N film is easily oxidized. However, Al0.5Ga0.5N nanowires are easily oxidized only when O concentration is higher. The formation energies of oxygen substitution model are all positive, indicating that this form of oxide is not easy to form on the surface. The results of band structure and PDOS indicate that pristine Al0.5Ga0.5N film has abundant surface states near the Fermi level. After oxygen adsorption, Ga atoms form stable Ga–O bonds with O atoms and the surface states are weakened. For oxygen substitutional oxidation, compared to Al0.5Ga0.5N thin film, conduction band of Al0.5Ga0.5N nanowire splits to form more energy levels and CBM moves downward, forming an n-type surface, which is not conducive to photoelectric emission.

InGaN nanowire array photocathode with high electron harvesting capability

With the rise of III-V semiconductor materials, InGaN materials began to be widely concerned. Nanowire arrays have excellent light trapping ability and generate a large number of photogenerated electrons. However, the adjacent nanowires will absorb these electrons twice, so the quantum efficiency cannot perfectly measure the emission performance of the nanowire array photocathode. We introduce the collection efficiency to evaluate the photoemission performance. It is based on this background that we study the photoemission performance of InGaN nanowire array under the assistance of height, incident light Angle and applied electric field. Finite-difference time-domain (FDTD) method is used to compare the quantum efficiency and collection efficiency of InGaN nanowire. We find that when the height of the nanowire is 600 nm, the angle of incident light is 65°, and the electric field strength is 0.6V/μm, InGaN nanowire arrays have the highest collection efficiency, with the collection efficiency up to 34% and the collection ratio up to 71%. The collection efficiency is 66% higher than that without external electric field. Therefore, the research in this paper can provide a theoretical reference for the preparation and performance improvement of InGaN nanowire array photocathode.

NEA surface AlGaN heterojunction tilted nanowire array photocathode for vacuum electron sources

As the core component of the electron accelerator, the electron source is responsible for providing high-quality electron beams. GaN has a wide bandgap and must be driven by an ultraviolet laser, making it a strong candidate for photocathode electron source research. We established a theoretical model of photoemission for field-assisted negative electron affinity (NEA) AlGaN heterojunction tilted nanowire array photocathodes. The top-down built-in electric field facilitated the transport of photogenerated carriers within the nanowire. The inclination of the nanowires effectively improved the quantum efficiency of the photocathode. With the critical angle of incidence of ultraviolet, the quantum efficiency was up to 81.2%, and the collection efficiency was up to 51%. This work could provide theoretical support for the design and fabrication of high-quality photocathode electron sources.

Alkali Cation Engineered Chemical Self-Oxidation of Copper Oxide Nanowire-Based Photocathodes.

Hydrogen energy production through photoelectrochemical (PEC) water splitting has great potential in the field of renewable energy. This study focuses on the hydration enthalpy difference of cations (Li+ , Na+ , and K+ ) in an aqueous solution for the chemical self-oxidation process without an external applied bias. The thickness of the cation/H2 O double layer is controlled. The starting materials are low-cost copper foil and the synthesis uses alkali cation-engineered chemical self-oxidation. Li+ ions are strongly attracted to water molecules. This forms a sufficient OH- layer on the Cu foil surface. By accelerating the oxidation reaction, a large surface area of Cu(OH)x nanowires (NWs) with high purity and a uniform shape are obtained. This optimal p-type Cu2 O NWs photocathode is CuO-free, has the highest conductivity, and is fabricated through phase transition using precise vacuum annealing. The other alkali cations produce the Cu2 O/CuO mixed or CuO phases that degrade the PEC performances with severe corrosive reactions. The Cu/Li : Cu2 O/AZO/TiO2 /Pt photocathode has a 50 h stability with a photocurrent density of 8.4 mA cm-2 at 0 VRHE . The fabricated photoelectrode did not structurally collapse after stability measurements during this period. The captured hydrogen production was in agreement with the calculated faradaic efficiency.

Photoabsorption and quantum efficiency of multi-diameter combined AlxGa1-xN nanostructure UV photodetectors

In this article, we numerically studied the influence of the geometric structure of the variable Al component AlxGa1-xN nanowires. In nanowire array photocathodes, the geometrical parameters of the nanowires together with the cross-sectional shape influence the photon trapping effect of the nanostructure and likewise the photon absorption properties of the cathode. We aim to design a variable Al component AlxGa1-xN nanostructure to obtain optics response optimization for UV photocathode. The simulation results show that the uneven-sized rectangle nanostructure array achieves the best light absorption spectrum of from 300 to 450 nm with an optimal absorption efficiency of 97.47%.

Electronic Behavior of GaN Nanowire Photocathode Surface Coated with n‐Type GaN Capping Layer via First Principles

Conventional negative electron affinity photocathodes require alternate activation of Cs/O, and the instability of Cs makes the cathode surface easily inactive. A Cs‐free GaN photocathode structure is reported, in which band engineering of the photocathode surface induced by Si doping eliminates the need to use Cs for photocathode activation. This heterojunction structure can obtain a low‐barrier surface without activation. GaN nanowires with n‐type capping layers are established using first principles, and their stability, work function, and optoelectronic structures are analyzed. Si atoms replace Ga atoms more easily, thereby forming an n‐type capping layer. The formation energy of the n‐type capping layer of the nanowires surface is lower than that of the surface capping layer of the thin film. The addition of the n‐type capping layer reduces the work function of the GaN surface, which will help to enhance photoemission capability. The results clearly illustrate the positive effect of the n‐type capping layer on the electronic properties of the device, so this Cs‐free activation structure can be further considered.

Hydrogen-substituted graphdiyne encapsulated cuprous oxide photocathode for efficient and stable photoelectrochemical water reduction

Photoelectrochemical (PEC) water splitting is an appealing approach for “green” hydrogen generation. The natural p-type semiconductor of Cu2O is one of the most promising photocathode candidates for direct hydrogen generation. However, the Cu2O-based photocathodes still suffer severe self-photo-corrosion and fast surface electron-hole recombination issues. Herein, we propose a facile in-situ encapsulation strategy to protect Cu2O with hydrogen-substituted graphdiyne (HsGDY) and promote water reduction performance. The HsGDY encapsulated Cu2O nanowires (HsGDY@Cu2O NWs) photocathode demonstrates a high photocurrent density of −12.88 mA cm−2 at 0 V versus the reversible hydrogen electrode under 1 sun illumination, approaching to the theoretical value of Cu2O. The HsGDY@Cu2O NWs photocathode as well as presents excellent stability and contributes an impressive hydrogen generation rate of 218.2 ± 11.3 μmol h−1cm−2, which value has been further magnified to 861.1 ± 24.8 μmol h−1cm−2 under illumination of concentrated solar light. The in-situ encapsulation strategy opens an avenue for rational design photocathodes for efficient and stable PEC water reduction.