Hole Pairs Research Articles

Piezo-photocatalysis synergy in γ-GeSe for highly efficient oxygen evolution reaction

Solar-driven semiconductor photocatalysts are highly appealing in applications of environmental remediation and energy conversion. However, photocatalytic reactions, particularly oxygen evolution reaction (OER), are often constrained by the swift recombination of electron–hole pairs, thereby resulting in low reaction efficiency. Although it is effective to separate charge carriers by constructing heterojunctions to form built-in electric field, the lattice mismatch and inefficient interlayer charge transfer of heterojunctions in the photocatalysts limit their further development. Here, we propose a new strategy by constructing an internal electric field for OER through an individual piezoelectric two-dimensional material. The results indicate that the piezoelectric effect regulates the electronic structure, reduces bandgap, improves light absorption efficiency, and that the displacement of positive and negative charge centers is the key factor in the enhanced OER. This research indicates the feasibility of combining piezoelectric properties of two-dimensional materials with OER (1.19 eV), providing new insights and guidance for applying the piezoelectric effect in the OER and opening up a way to promote efficient separation of charge carriers.

Construction of C3N4/Ag2S p-n semiconductor heterojunction coupled with Ag-induced SPR effect on CF cloth for improving photocatalytic activity

C3N4/Ag2S p-n semiconductor heterojunction coupled with Ag-induced SPR effect on carbon fiber cloth with large macroscopic area, recyclability and excellent photocatalytic performance was prepared by thermal condensation-chemical precipitation. CF/C3N4/Ag/Ag2S cloth showed strong Vis-NIR light absorption and outstanding separation efficiency of photo-generated electron and hole pairs. CF/C3N4/Ag/Ag2S cloth displayed a markedly enhanced catalytic activity (94 % of TC, 85 % of Cr(VI)) compared with CF/C3N4 cloth (52 % of TC, 50 % of Cr(VI)) or CF/Ag/Ag2S cloth (34 % of TC, 19 % of Cr(VI)), attributing to the establishment of p-n junction along with Ag SPR effect. Moreover, degradation pathways for TC were proposed utilizing HPLC-MS and Fukui index analysis, and photocatalytic mechanism was confirmed by active species experiments and DFT computations. This facile strategy provides novel approach to the synthesis of material with large macroscopic areas, recyclability, and excellent photocatalytic property for degrading contaminants from wastewater.

High power deep ultraviolet radiation generation from short pulse laser induced electron–hole pair production in a wide bandgap semiconductor

High power deep ultraviolet radiation generation from short pulse laser induced electron–hole pair production in a wide bandgap semiconductor

Enhanced Visible-Light-Assisted Photocatalytic Removal of Tetracycline Using Co/La@g-C3N4 Ternary Nanocomposite and Underlying Reaction Mechanisms

Graphitic carbon nitride (g-C3N4) is a promising material for photocatalytic applications. However, it suffers from poor visible-light absorption and a high recombination rate of photogenerated electron–hole pairs. Here, Co/La@g-C3N4 with enhanced photocatalytic activity was prepared by co-doping Co and La into g-C3N4 via a facile one-pot synthesis. Co/La@g-C3N4 displayed better performance, achieving 94% tetracycline (TC) removal within 40 min, as compared with g-C3N4 (BCN, 65%). It also demonstrated promising performance in degrading other pollutants, which was ~2–4-fold greater relative to BCN. The improved photocatalytic activity of Co/La@g-C3N4 was associated with improved photogenerated charge separation, reduced charge transfer resistance, a built-in electric field arising from the p-n-p heterojunction, and the synergistic effect of ternary components for the separation and transfer of the photogenerated charge carriers. Superoxide radicals are suggested to be the most notable reactive species responsible for the photocatalytic reaction. Environmental factors, including the pollutant concentration, catalyst dosage, solution pH, inorganic salts, water matrices, and mixture with dyes, were considered in the photocatalytic reactions. Co/La@g-C3N4 showed good reusability for five cycles of the photocatalytic degradation of TC. The facile one-pot co-doping of Co and La in g-C3N4 formed a p-n-p heterojunction with boosted photocatalytic activity for the highly efficient removal of TC from various water matrices.

Dynamics of dissipative gravitational collapse in the Morris–Thorne wormhole metric: One scenario - several outcomes

We consider the dynamical Morris–Thorne metric with radiating heat flow. By matching the interior Morris–Thorne metric with an exterior Vaidya metric we trace out the collapse solutions for the corresponding spherically symmetric inhomogeneous distribution of matter. The solutions obtained are broadly of four different types, giving different end state dynamics. Corresponding to three of the solutions we elaborate the collapsing dynamics of the Morris–Thorne type evolving wormhole. We show that for all those cases where collapse upto zero proper volume is obtained in finite time, the ensuing singularity is always a black hole type. However our solutions can also show other end states, like oscillating wormhole-black hole pair or infinite time contracting universe or a conformal past matter dominated universe. In all the cases we have worked out the background dynamics and physics of the solution. All our solutions are illustrated with appropriate graphical descriptions.

Realizing enhanced photoresponse for self-powered broadband photodetector with asymmetric contacts based MoTe2/WSe2 van der Waals heterostructure

Self-powered photodetectors constructed by two-dimensional (2D) materials are significant in advanced science and technology with the growing focus on green and energy-efficient solutions. The built-in electric field in photodetector drives efficient separation and transfer of photogenerated electron–hole pairs. However, traditional metal-2D material-metal structures have more interface defects and larger Schottky barrier to hinder carrier transport. In this work, an asymmetric electrode material structure is constructed by adding graphene to provide a more efficient transport channel for photogenerated carriers. Graphene electrodes can play a variety of roles in efficient separation and collection of photogenerated carriers, reducing the barrier and improving the contact quality. Compared with the device using symmetric gold electrodes, the device prepared with asymmetric contacts show optimized performance. The MoTe2/WSe2 heterostructure exhibits brilliant self-powered photoresponse (a responsivity of 268 mA/W and a detectivity of 7.48*1011 Jones). And the rise/decay times are up to 145μs/125μs. Additionally, the detector has a broadband detection capability from visible light to near-infrared. These results provide a promising and simple way to fabricate a MoTe2/WSe2 self-powered broadband photodetector with high performance

Quasi-bound states in the continuum induced by in-plane and out-of-plane asymmetry in all-dielectric metasurfaces

In this paper, we propose a periodic structure consisting of a pair of square holes etched through square silicon substrates to achieve a high-Q quasi-BIC resonance peak at 1524 nm. The simulation results indicate that the Q factor of the excited quasi-BIC can reach 12,660. Multipole decomposition and near-field distribution show that the quasi-BIC of the square etched hole structure is primarily dominated by magnetic dipoles, electric quadrupoles, and magnetic quadrupoles. When structural parameters shift in other directions, or when the square hole is rotated or changed to a circle, this quasi-BIC demonstrates good robustness.

Study of leakage current degradation based on stacking faults expansion in irradiated SiC junction barrier Schottky diodes

Abstract A comprehensive investigation was conducted to explore the degradation mechanism of leakage current in SiC junction barrier Schottky (JBS) diodes under heavy ion irradiation. We propose and verify that the generation of stacking faults (SFs) induced by the recombination of massive electron--hole pairs during irradiation is the cause of reverse leakage current degradation based on experiments results. The irradiation experiment was carried out based on Ta ions with high linear energy transfer (LET) of 90.5 MeV/(mg/cm2). It is observed that the leakage current of the diode undergoes the permanent increase during irradiation when biased at 20% of the rated reverse voltage. Micro-PL spectroscopy and PL micro-imaging were utilized to detect the presence of SFs in the irradiated SiC JBS diodes. We combined the degraded performance of irradiated samples with SFs introduced by heavy ion irradiation. Finally, three-dimensional (3D) TCAD simulation was employed to evaluate the excessive electron–hole pairs (EHPs) concentration excited by heavy ion irradiation. It was observed that the excessive hole concentration under irradiation exceeded significantly the threshold hole concentration necessary for the expansion of SFs in the substrate. The proposed mechanism suggests that the process and material characteristics of the silicon carbide should be considered in order to reinforcing against the single event effect of SiC power devices.

Photocatalytic reduction of the uranium(Ⅵ) by ultra-thin porous g-C3N4 nanosheets synthesized via microwave-assisted

Graphitic carbon nitride (g-C3N4) has garnered significant attention in the realm of photocatalytic reduction of U(Ⅵ) from aqueous solutions owing to its favorable bandgap and visible light responsiveness. Nevertheless, conventionally prepared bulk g-C3N4 suffers from limitations such as a small specific surface area and high recombination of photo-generated charges, consequently impeding the efficiency of photocatalytic reactions and hampering practical applications. To address these shortcomings, this study presents a novel approach wherein ultra-thin porous g-C3N4 nanosheets were successfully synthesized through the microwave-assisted calcination of urea in a single step. The resulting g-C3N4 nanosheets exhibit a multi-hollow mesh structure with a significantly enlarged specific surface area, the specific surface area and pore size of MCN90 are 170 m²/g and 17 nm, respectively, which are 2.3 times and 8 times that of the bulk g-C3N4. Photocatalytic experiments revealed that these ultra-thin porous nanosheets demonstrate superior performance in the reduction of U(Ⅵ) to U(Ⅳ), achieving a reduction rate of 100 % within 20 minutes for the optimal sample, as opposed to 78 % in 40 minutes for bulk g-C3N4. This enhanced photocatalytic activity can be primarily attributed to the unique ultra-thin porous nanosheet structure, which facilitates increased specific surface areas, improve photoelectric properties, and substantially shortened electron migration distances which reduce the recombination efficiency of photogenerated electrons and holes pairs. Consequently, these findings not only offer insights into the optimization of g-C3N4 for U(Ⅵ) reduction but also provide a valuable reference for the preparation of other two-dimensional ultra-thin porous materials aimed at similar applications.

Signatures of polarized chiral spin disproportionation in rare earth nickelates

In rare earth nickelates (RENiO3), electron-lattice coupling drives a concurrent metal-to-insulator and bond disproportionation phase transition whose microscopic origin has long been the subject of active debate. Of several proposed mechanisms, here we test the hypothesis that pairs of self-doped ligand holes spatially condense to provide local spin moments that are antiferromagnetically coupled to Ni spins. These singlet-like states provide a basis for long-range bond and spiral spin order. Using magnetic resonant X-ray scattering on NdNiO3 thin films, we observe the chiral nature of the spin-disproportionated state, with spin spirals propagating along the crystallographic (101)ortho direction. These spin spirals are found to preferentially couple to X-ray helicity, establishing the presence of a hitherto-unobserved macroscopic chirality. The presence of this chiral magnetic configuration suggests a potential multiferroic coupling between the noncollinear magnetic arrangement and improper ferroelectric behavior as observed in prior studies on NdNiO3 (101)ortho films and RENiO3 single crystals. Experimentally-constrained theoretical double-cluster calculations confirm the presence of an energetically stable spin-disproportionated state with Zhang-Rice singlet-like combinations of Ni and ligand moments.

Constructing a Type-II CdS/Bi2MoO6 Heterostructure: Promoting Photocatalytic Degradation of Contaminants.

Constructing a heterostructure is regarded as one of the most favorable approaches to attaining the separation ability of photogenerated carriers and strengthening photocatalysis efficiency. In this study, a CdS/Bi2MoO6 type-II heterostructure was constructed through a hydrothermal technique. The photocatalytic test result shows that the degradation efficiency of rhodamine B (RhB) and tetracycline (TC) over CS/BMO-1 was 100 and 92% under visible light, respectively, which is the highest compared to other samples. The exceptional photocatalytic efficiency is principally associated with generating an inherent electric field within a type-II heterostructure, effectively restraining the recombination of photogenerated electron hole pairs. The intermediate products during the photocatalytic degradation of RhB and TC were identified through liquid chromatography-mass spectrometry, and the hypotheses were formulated regarding the corresponding photodegradation mechanisms. Furthermore, the outcomes of capture tests exhibited that the primary active species were •O2- and h+, and a mechanism of the photocatalytic degradation procedure has been proposed.

Mechanochemical synthesis of Ag/TiO2@PANI nanocomposites for enhanced toluene photocatalytic degradation under near-ultraviolet light

Photocatalytic oxidation technology is a promising green technology for degrading volatile organic compounds (VOCs) due to its non-toxic, environmentally friendly, energy-saving, and affordable characteristics. In this paper, Ag/TiO2@PANI-MC with high stability and activity was synthesized by the mechanochemical method. The designed Ag/TiO2@PANI-MC were of high specific surface area, light absorption capacity and low recombination rate of electron–hole pairs, which was demonstrated by various characterizations. When applied in photocatalytic toluene oxidation, the conversion is 17% at 20 °C under 100 W high-pressure mercury lamp. This photocatalytic performance is with less temperature sensitivity, and significantly improved compared with Ag/TiO2 or TiO2 catalysts. Furthermore, the reaction routine was also confirmed by GC-MS and the toluene was mineralized to CO2. More importantly, the Ag/TiO2@PANI-MC indicated good reusability after 3 cycles, which was verified by the FT-IR comparison of fresh and used catalysts. Our work proves a potential way on constructing nanocomposite based on mechanochemical synthesis for enhanced toluene photocatalytic degradation.

Visible-light-driven Bi2WO6/biochar composite photocatalyst efficiently degrades 17α-ethinylestradiol (EE2) and 17β-estradiol (E2) under persulfate synergy

Biochar (BC) composite photocatalysts have attracted much attention by virtue of their high efficiency in removing endocrine disruptors (EDCs). However, there has been a lack of systematic research on the mechanism and constitutive relationship of BC on photocatalysts, which has seriously impeded the development process in this field. In this study, Bi2WO6–BC2% composite photocatalysts were prepared by microwave-assisted hydrothermal method and used for degradation of 17α-ethinylestradiol (EE2) and 17β-estradiol (E2) in conjunction with persulfate synergy (PS). The removal rate of EE2 and E2 by Bi2WO6–BC2% was as high as 99.41 % (12 min) and 97.84 % (15 min), respectively. Systematic studies have shown that BC enhances the catalytic activity mainly through the following ways, increasing the specific surface area of the photocatalytic system, enriching the types of functional groups, enhancing the separation ability of photogenerated electron–hole pairs, and promoting the concentration of active free radicals. The results of the phytotoxicity experiments showed that compared to the EE2 or E2 pollutant solutions, the stem and root lengths of V. radiata increased by 93.63 % and 389.58 % in EE2 metabolites, and by 117.35 % and 160.38 % in E2 metabolites. Similarly, the stem and root lengths of P. vulgaris increased by 80.08 % and 174.91 % in EE2 metabolites, and by 188.92 % and 345.75 % in E2 metabolites. This work not only fills the gap in the study of the mechanism of action of BC, but also provides a green catalyst with very excellent photocatalytic efficiency and detoxification properties.

Improving the efficiency and stability of betavoltaic batteries based on understanding efficiency fluctuations and gaps with theoretical limits

Wide-bandgap semiconductors are regarded as preferred materials for preparing semiconductor conversion devices in betavoltaic batteries due to their high theoretical conversion efficiency (ηc). However, there are a few comprehensive analytical studies on why the experimental values of ηc are generally much lower than the theoretical limit of ηc (ηc-limit) and how to improve ηc and its stability. In this work, combined with the energy deposition distributions of Ti3H2, 63Ni, and 147Pm2O3 radioactive sources in SiC obtained from Monte Carlo simulations, a multi-physical mechanism, multi-parameter coupling numerical model was established. This model can comprehensively analyze the output characteristics of betavoltaic batteries under the influence of actual device structural and material parameter changes. Our results show that changes in structural and material parameters cause significant variations in the collection efficiency (Q) of the radiation-generated electron–hole pair (RG-EHP). Considering structural parameters are easy to control, instabilities in actual SiC material parameters, which include electron diffusion length (Ln), hole diffusion length (Lp), and surface recombination velocity (S), are the main reason that ηc fluctuates significantly and is generally far lower than ηc-limit. Due to differences in the distribution of RG-EHP produced by different radioactive sources in SiC, the dominant parameters causing ηc fluctuations differ. By analyzing differences in recombination loss mechanisms under different radioactive sources, the device structures were designed in a targeted manner to make ηc closer to ηc-limit. Meanwhile, when the SiC material quality fluctuates, the stability of ηc increases by 58.5%, 35.3%, and 48.2% under Ti3H2, 63Ni, and 147Pm2O3, respectively.

(Invited) Simultaneous Use of Electric Fields and Plasmonic Carriers for Catalysis

The visible-light excitation of plasmonic nanostructures is now well known to induce catalytic reactions that are not otherwise observed in the dark. One example we have discovered is the conversion of carbon dioxide to hydrocarbons on gold nanoparticles driven by electron–hole pairs generated by plasmonic excitation. However, conversions driven by plasmonically generated carriers can suffer from thermodynamic and efficiency limits. I will describe how such limits can be overcome by simultaneous use of electric fields and plasmonically generated carriers.

Precursor impurity‐mediated effect in the photocatalytic activity of precipitated zinc oxide

AbstractPhotocatalytic degradation of pollutants using nanoparticles presents a promising method globally. However, effectively harnessing light absorption while mitigating recombination and nanoparticle agglomeration remains challenging. Here, we explore the synthesis and characterization of zinc oxide nanoparticles for photocatalytic dye removal in water. The ZnO catalyst, controlled by impurity amount, is developed, demonstrating a notable impact on photolytic performance. Various zinc precursors, namely, zinc acetate, zinc sulfate, zinc nitrate, and zinc chloride, were used in the precipitation technique. Optical characterization showed distinct band transitions and UV‐dominant absorption peaks, indicating the presence of different impurities in each precursor. Photocatalytic performance is assessed using Rhodamine B decomposition with the sample prepared from zinc acetate, demonstrating enhanced photocatalytic activity attributed to its larger surface area, surface defects, and superior morphology, enabling efficient organic pollutant degradation. Oxygen vacancies aid in charge carrier separation, crucial for effective photocatalysis. The material's intense interaction with pollutants and a high photocurrent density of 5.18 µAcm−2 highlight superior electron–hole pair separation capabilities influenced by morphology and impurity‐generated defects, significantly boosting its overall photocatalytic reaction. These findings emphasize the critical role of precursor selection in designing effective ZnO‐based photocatalysts, water treatment, and environmental remediation applications.

A comprehensive study of laser irradiated hydrothermally synthesized 2D layered heterostructure V2O5(1−x)MoS2(x) (X = 1–5%) nanocomposites for photocatalytic application

Abstract It has been studied that both two-dimensional (2D) MoS2 and V2O5, which are classified as transition metal dichalcogenides and transition metal oxides, are good photocatalyst materials. For this purpose, the hydrothermal method was practiced to synthesize V2O5(1−x)MoS2(x) (X = 1–5% w/w) nanocomposites with different 1–5% w/w weight percent of MoS2 as a prominent photocatalyst under laser irradiation for 2, 4, 6, 8, and 10 min to tune photocatalytic degradation of industrial wastage water. The surface of the 2D molybdenum nanolayered matrix was efficaciously decorated with V2O5 nanoparticles. The crystal phase and layered structures of the V2O5(1−x)MoS2(x) (X = 1–5% w/w) nanocomposites samples were verified by X-ray diffraction and scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy respectively. In the range of the UV visible spectrum, the increment in light absorption from 3.6 to 14.5 Ω−1 cm−1 with an increase of active surface from 108 to 169 μ m 2 {{\rm{\mu }}{\rm{m}}}^{2} with increased MoS2 doping percentage. Furthermore, dielectric findings like the complex dielectric function, tangent loss, electrical conductivity, quality factors, and impedance of V2O5(1−x)MoS2(x) (X = 1–5% w/w) nanocomposites are studied. According to photoluminescence studies, the intensity of peaks decreases when laser irradiation time and doping percentages of MoS2 are increased. As a result, a small peak indicates a decrement rate of electron–hole pair recombination, which increases the capacity for separation. Thermo-gravimetric analysis and differential thermal analysis results revealed that weight loss decreased from 0.69 to 0.35 mg and thermal stability increased with increased doping concentrations. Methylene blue was degraded in 150 min, proving that the prepared MoS2-doped V2O5 material was a stable and economically low-cost nanocomposite for photocatalytic activity.

High-performance GaN ultraviolet polarization-sensitive photodetector based on ferroelectric polarization LiNbO3

High responsivity ultraviolet (UV) photodetectors (PDs) are essential for abundant civilian and military applications. Gallium nitride (GaN) has emerged as an ideal material for UV PD fabrication due to its favorable properties. However, the quality of GaN epitaxial layers significantly impacts device performance and reliability. Sapphire-based GaN epitaxial growth technology enables the realization of high-quality GaN epitaxial layers, making it the preferred choice for GaN substrates. Nonetheless, the thermal expansion coefficient mismatch between sapphire and GaN can lead to crystal mismatch and stress accumulation at high temperatures, affecting device performance and reliability. In contrast, lithium niobate (LiNbO3) exhibits similar coefficients of thermal expansion to GaN, mitigating crystal mismatch and stress accumulation issues. Here, we report the realization of a GaN UV PD by laminating GaN membrane onto ferroelectric LiNbO3 through selective electrochemical etching of the sapphire-based GaN epitaxial film. The LiNbO3-based GaN PD achieves a specific high on/off ratio of 107. At a 5 V bias voltage, the device exhibits a high peak responsivity of 1.712 × 103 A/W under 325 nm laser illumination. Furthermore, the device demonstrates excellent performance for polarization light detection, with a polarization ratio of approximately 54.95. Exploiting the local ferroelectric polarization of x-cut LiNbO3, the photogenerated electron–hole pairs in GaN are efficiently separated by the electrostatic field from the polarization of ferroelectric LiNbO3, resulting in enhanced light-to-electric conversion efficiency. Our work presents a method for fabricating high responsivity GaN-based UV PD, showcasing the potential of integrating ferroelectric LiNbO3 to enhance device performance.

Reduction of the photogenerated electron transport path by modulating the conduction band region to achieve the highest lignin β–O–4 bond photocleavage efficiency under xenon excitation

Photocracking of lignin β–O–4 bonds is a green approach for the high-value modification of lignin. Graphitic carbon nitride (g-C3N4) that can photocleave the β–O–4 bond is presently the only stable, metal-free photocatalyst owing to the residual metal ions generated due to photocorrosion of metal sulfides. Herein, g-C3N4 with a N–C3 vacancy structure is prepared by etching site–engineering strategy based on the protection of C–N=C structure by chlorine atom. The N–C3 vacancy structure can reduce the transport path of the photogenerated electrons by tuning the conduction band (CB) region to the edge of the photocatalyst, which impedes the critical complexation of the photogenerated electron–hole pairs and enhances the photoelectric conversion efficiency five-fold. Combined with the unique chemisorption capacity (137.82 kJ/mol) of N–C3 vacancies for oxygen, the g-C3N4 photocatalysts with N–C3 vacancy structure can generate 9.6-fold higher superoxide radicals and 13-fold higher lignin β–O–4 bond photocleavage efficiency lignin compared with untreated g-C3N4. Based on the reaction enthalpy changes and free energies investigated through simulations, a route of lignin Cα–Cβ bonds being oxidized and dehydrogenated twice by photogenerated cavities is proposed for the first time. This work reveals the relation between the properties of g-C3N4 and the photocleavage efficiency of lignin β–O–4 bonds.

Kilonova Spectral Inverse Modelling with Simulation-based Inference: An Amortized Neural Posterior Estimation Analysis

Kilonovae represent a category of astrophysical transients, identifiable as the electromagnetic (EM) counterparts associated with the coalescence events of binary systems comprising neutron stars and neutron star–black hole pairs. They act as probes for heavy-element nucleosynthesis in astrophysical environments. These studies rely on an inference of the physical parameters (e.g., ejecta mass, velocity, composition) that describe kilonovae-based on EM observations. This is a complex inverse problem typically addressed with sampling-based methods such as Markov Chain Monte Carlo or nested sampling algorithms. However, repeated inferences can be computationally expensive, due to the sequential nature of these methods. This poses a significant challenge to ensuring the reliability and statistical validity of the posterior approximations and, thus, the inferred kilonova parameters themselves. We present a novel approach: simulation-based inference using simulations produced by KilonovaNet. Our method employs an ensemble of amortized neural posterior estimation (ANPE) with an embedding network to directly predict posterior distributions from simulated spectral energy distributions. We take advantage of the quasi-instantaneous inference time of ANPE to demonstrate the reliability of our posterior approximations using diagnostics tools, including coverage diagnostic and posterior predictive checks. We further test our model with real observations from AT 2017gfo, the only kilonova with multimessenger data, demonstrating agreement with previous likelihood-based methods while reducing inference time down to a few seconds. The inference results produced by ANPE appear to be conservative and reliable, paving the way for testable and more efficient kilonova parameter inference.