Gap Structure Research Articles

High-entropy (FeCoNiCuZn)WO4 photocatalysts-based fibrous membrane for efficient capturing and upcycling of plastic

Catalytic upcycling of plastics is regarded as a promising way to alleviate environmental pollution and produce value-added products, while still facing huge challenges. Herein, we report a feasible strategy for synthesizing high-entropy photocatalysts-based fibrous membranes with a dual function of capturing and upcycling plastic. Polyacrylonitrile (PAN) nanofibers templates enable the growth of highly arranged high-entropy metal tungstates (FeCoNiCuZn)WO4, denoted XWO4. The effect of lattice distortion in XWO4 modulates the band gap structure and contributes to the upshift d-band center of XWO4. As a result, the modified anti-bonding state facilitates the upcycling of captured polylactic acid (PLA) towards acetic acid production, with an enhanced yield rate of 38.51 mg gcat.−1 h−1 and improved selectivity of 73 %. Thus, the developed NFs template strategy, a tunable electronic structure and exploration of the structure–function relationship provide insight into tailoring the structure and composition of HEMs for photocatalytic upcycling of plastic.

High-throughput calculation screening for new silicon allotropes with monoclinic symmetry.

A total of 87 new monoclinic silicon allotropes are systematically scanned by a random strategy combined with group and graph theory and high-throughput calculations. The new allotropes include 13 with a direct or quasi-direct band gap and 12 with metallic characteristics, and the rest are indirect band gap semiconductors. More than 30 of these novel monoclinic Si allotropes show bulk moduli greater than or equal to 80 GPa, and three of them show even greater bulk moduli than diamond Si. Only two of the new Si allotropes show a greater shear modulus than diamond Si. The crystal structures, stability (elastic constants, phonon spectra), mechanical properties, electronic properties, effective carrier masses and optical properties of all 87 Si monoclinic allotropes are studied in detail. The electron effective masses ml of five of the new allotropes are smaller than that of diamond Si. All of these novel monoclinic Si allotropes show strong absorption in the visible spectral region. Taken together with their electronic band gap structures, this makes them promising materials for photovoltaic applications. These investigations greatly enrich the current knowledge of the structure and electronic properties of silicon allotropes.

Synthesis of TiO2/g-C3N5 heterojunction for photocatalytic degradation of methylene blue wastewater under visible light irradiation: Mechanism analysis

TiO2/g-C3N5 heterojunction is synthesized by the simple method, which is used in photocatalytic degradation methylene blue (MB) under visible light irradiation. The characterization analysis indicates that TiO2/g-C3N5 has good crystallinity, and the heterojunction between TiO2 and g-C3N5 is formed with good MB photocatalytic degradation performance. The MB photocatalytic degradation removal of the TiO2/g-C3N5 is 97.4 % at the concentration of 20 mg/L, which is significantly improved compared to the TiO2 and g-C3N5. The visible light adsorption ability and the separation of photogenerated carriers of the TiO2/g-C3N5 are improved, which is beneficial to the MB photocatalytic degradation. TiO2/g-C3N5 has good reusability with 95.8 % of the initial removal after 5 cycles. The capture experiment and electron paramagnetic resonance spectroscopy analysis indicate that the existence of the h+, O2− and OH involves in MB photocatalytic degradation process. MB photocatalytic mechanism of the TiO2/g-C3N5 is consistent with the direct Z scheme mechanism based on the band gap structure calculations.

α/β phase junction Ga2O3 based high-performance self-powered deep ultraviolet photodetectors with Ti3C2/Ag nanowire hybrid conductive electrode

Gallium oxide (Ga2O3) semiconductor has attracted increasingly interested in the optoelectronic field, due to the wide bandgap of 4.9 eV. However, the fabrication of Ga2O3 pn junction is still an enormous challenge since p-Ga2O3 doping technology is in its infancy. Taking into consideration the α-Ga2O3 and β-Ga2O3 with comparable band gap structure, a solar-blind photodetector was constructed by the vertical α/β-Ga2O3 junction nanorod arrays (NRAs) with a top electrode for a Ti3C2-silver nanowires (Ag NWs). Compared to a standard metal electrode, the Ti3C2-Ag NWs electrode with tight coupling of the α/β-Ga2O3 demonstrated outstanding conductivity and optical transparency, optimizing the photodetector with excellent photoelectric performance. Meanwhile, the photogenerated carriers were easier to automatically separate and transfer to their corresponding electrodes because of the internal electric field of the α/β-Ga2O3 phase junction could drive the photogenerated charge transfer, promoting the photogenerated electrons transfer. Even without external power, the photodetector demonstrated improved performance, large detectivity of 1.45 × 1014 Jones, and high responsivity of 5.2 mA/W. The solar-blind photodetector with self-powered shows high photoelectric performance, which has prospective applications in space missile detection, UV communication, and deep ultraviolet light monitoring.

3D ZnO hollow spheres-dispersed CsPbBr3 quantum dots S-scheme heterojunctions for high-efficient CO2 photoreduction

Efficient electron transfer process and excellent CO2 adsorption capacity are of great significance for improving CO2 photoreduction efficiency. 3D-0D ZnO/CsPbBr3 S-scheme heterojunction hollow spheres were successfully prepared via a simple self-assembled process for efficient CO2 photoreduction. Photocatalytic CO2 reduction experiments showed that the ZnO/CsPbBr3 had the excellent CO2 photocatalytic conversion activity to CO, which was about 2.61 times higher than that of ZnO hollow micro-spheres and 1.9 times higher than that of pure CsPbBr3 quantum dots (QDs). Photoelectrochemical characterization confirmed that the ZnO/CsPbBr3 composite had good photoelectric conversion ability and carrier transfer ability, which was beneficial to the photocatalytic reaction. The suitable band gap structure and DFT calculation results showed that the formation of built-in electric field at the ZnO/CsPbBr3 interface can greatly improve the S-scheme electron transfer ability, which can foster the CO2 photoreduction. Besides, the introduction of 3D structure can improve the dispersion of CsPbBr3 QDs and effectively enhance the CO2 capture ability of the catalyst, which greatly promoted the CO2 photoreduction reaction. The current work can present a novel S-scheme photocatalyst, which is looked forward to providing a certain perception into the advance of novel highly efficient CsPbBr3 QDs-based photocatalysts for CO2 resource utilization.

Compact hybrid EBG microstrip antenna for wearable applications

Abstract A novel strip line fed, circularly polarized (CP), annular slotted dual band antenna using hybrid electromagnetic band gap (EBG) structure for Global Positioning System (GPS) has been herein designed, analyzed, and investigated for wearable applications. Adjusting the radii of the annular slots on the radiating patch excellent circular polarized (CP) radiation and impedance matching is achieved. The design demonstrates Kapton based flexible, robust, and low-profile solution with permittivity of 3.4 to meet the requirements of wearable applications. Due to the high losses of the animal body, the electromagnetic band gap (EBG) structure is used to reduce back radiation and the effect of frequency detuning. The proposed antenna structure also enhances the front-to-back ratio (FBR) by 10 dB. This antenna with dimensions 0.56λ 0 × 0.4913λ 0 × 0.002λ 0 analyzed using a flexible Kapton substrate. Optimized hybrid EBG structure provides an excellent Specific Absorption Rate (SAR) along with all other antenna parameters, within acceptable for GPS-based wearable applications at 1.13 GHz and 1.157 GHz frequency band. Therefore the proposed antenna is a suitable candidate for GPS-based tracking and wireless body area network (WBAN) applications. The proposed antenna was also tested upon fabrication and the measured results agree with simulated results.

Gradient band gap CZTSSe prepared via sputtering from quaternary ceramic targets followed with annealing under different atmospheres

CZTSSe is a thin-film solar cell absorber material with long-time application potential, but its development is limited as its severe open-circuit voltage(Voc) deficit. To further improve the Voc, gradient band gap structure of absorber is expected to achieve this aim without large change of its overall band gap. In this work, a stack structure precursor was deposited by sputtering from quaternary ceramic targets. The top gradient band gap CZTSSe was achieved by annealing the precursor under pure Se or S/Se mix atmosphere. The effects of the atmosphere and the layers-stack precursor structure on the film properties and device performance were studied. Compared with selenization process, sulfo-selenization could effectively suppressed the severe reaction at the back contact interface. It would reduce Mo(S,Se)2 and secondary phases formation and improve solar cell efficiency. The Voc of the double-layers precursor solar cell successfully improved under both atmospheres as expected, representing this method could be a feasible direction to solve the large Voc,def problem. However, for higher efficiency sulfo-selenization cells, the Voc improvement in stack precursor samples did not bring obvious increase of efficiency due to the deterioration of fill factor, indicating that further optimization of device electrical properties should be applied in future study.

Andreev Reflection in Scanning Tunneling Spectroscopy of Unconventional Superconductors.

We evaluate the differential conductance measured in an STM setting at arbitrary electron transmission between STM tip and a 2D superconductor with arbitrary gap structure. Our analytical scattering theory accounts for Andreev reflections, which become prominent at larger transmissions. We show that this provides complementary information about the superconducting gap structure beyond the tunneling density of states, strongly facilitating the ability to extract the gap symmetry and its relation to the underlying crystalline lattice. We use the developed theory to discuss recent experimental results on superconductivity in twisted bilayer graphene.

Filling in the gaps: can gravitationally unstable discs form the seeds of gas giant planets?

ABSTRACT Circumstellar discs likely have a short window when they are self-gravitating and prone to the effects of disc instability, but during this time the seeds of planet formation can be sown. It has long been argued that disc fragmentation can form large gas giant planets at wide orbital separations, but its place in the planet formation paradigm is hindered by a tendency to form especially large gas giants or brown dwarfs. We instead suggest that planet formation can occur early in massive discs, through the gravitational collapse of dust which can form the seeds of giant planets. This is different from the usual picture of self-gravitating discs, in which planet formation is considered through the gravitational collapse of the gas disc into a gas giant precursor. It is familiar in the sense that the core is formed first, and gas is accreted thereafter, as is the case in the core accretion scenario. However, by forming a ∼1 M⊕ seed from the gravitational collapse of dust within a self-gravitating disc there exists the potential to overcome traditional growth barriers and form a planet within a few times 105 yr. The accretion of pebbles is most efficient with centimetre-sized dust, but the accretion of millimetre sizes can also result in formation within a Myr. Thus, if dust can grow to these sizes, planetary seeds formed within very young, massive discs could drastically reduce the time-scale of planet formation and potentially explain the observed ring and gap structures in young discs.

Developing Insoluble Polyoxometalate Clusters to Bridge Homogeneous and Heterogeneous Water Oxidation Photocatalysis.

Cluster catalysts are attractive for their atomically precise structures, defined compositions, tunable coordination environments, uniform active sites, and their ability to transfer multiple electrons, but they suffer from poor stability and recyclability. Here, we report a general approach to the direct insolubilization of a water soluble polyoxometalate (POM) [{(B-α-PW9 O34 )Co3 (OH)(H2 O)2 (O3 PC(O)-(C3 H6 NH3 )PO3 )}2 Co]14- (Co7 ) and formation of a series of POM-based solid catalysts with the counter-cations Ag+ , Cs+ , Sr2+ , Ba2+ , Pb2+ , Y3+ , and Ce3+ . They exhibit improved catalytic activities for visible-light-driven water oxidation following the trend CsCo7 >SrCo7 >AgCo7 >CeIII Co7 >BaCo7 >YCo7 >PbCo7 . While CsCo7 exhibits mainly homogeneous catalysis, the others are predominantly heterogeneous catalysts. An optimal oxygen yield of 41.3 % and a high apparent quantum yield (AQY) of 30.6 % for SrCo7 is obtained, which is comparable to that of the parent homogeneous POM. Band gap structures, UV/Vis spectra, and real-time laser flash photolysis experiments collectively suggest that easier electron transfer from the solid POM catalyst to the photosensitizer promotes photocatalytic water oxidation performance. These solid POM catalysts exhibit good stability, which is directly confirmed by a combination of Fourier-transform infrared spectroscopy, electron microscopy, X-ray diffraction patterns, Raman spectroscopy, X-ray photoelectron spectroscopy, five cycles of tests, and poisoning experiments.

Finite-temperature dynamical quantum phase transition in a non-Hermitian system

We investigate the interplay between the non-Hermiticity and finite temperature in the context of mixed state dynamical quantum phase transition (MSDQPT). We consider a $p$-wave superconductor model, encompassing complex hopping and non-Hermiticity that can lead to gapless phases in addition to gapped phases, to examine the MSDQPT and winding number via the intra-phase quench. We find that the MSDQPT is always present irrespective of the gap structure of the underlying phase, however, the profile of Fisher zeros changes between the above phases. Such occurrences of MSDQPT are in contrast to the zero-temperature case where DQPT does not take place for the gapped phase. Surprisingly, the half-integer jumps in winding number at zero-temperature are washed away for finite temperature in the gapless phase. We study the evolution of the minimum time required by the system to experience MSDQPT with the inverse temperature such that gapped and gapless phases can be differentiated. Our study indicates that the minimum time shows monotonic (non-monotonic) behavior for the gapped (gapless) phase.

Developing Insoluble Polyoxometalate Clusters to Bridge Homogeneous and Heterogeneous Water Oxidation Photocatalysis

AbstractCluster catalysts are attractive for their atomically precise structures, defined compositions, tunable coordination environments, uniform active sites, and their ability to transfer multiple electrons, but they suffer from poor stability and recyclability. Here, we report a general approach to the direct insolubilization of a water soluble polyoxometalate (POM) [{(B‐α‐PW9O34)Co3(OH)(H2O)2(O3PC(O)‐(C3H6NH3)PO3)}2Co]14− (Co7) and formation of a series of POM‐based solid catalysts with the counter‐cations Ag+, Cs+, Sr2+, Ba2+, Pb2+, Y3+, and Ce3+. They exhibit improved catalytic activities for visible‐light‐driven water oxidation following the trend CsCo7>SrCo7>AgCo7>CeIIICo7>BaCo7>YCo7>PbCo7. While CsCo7 exhibits mainly homogeneous catalysis, the others are predominantly heterogeneous catalysts. An optimal oxygen yield of 41.3 % and a high apparent quantum yield (AQY) of 30.6 % for SrCo7 is obtained, which is comparable to that of the parent homogeneous POM. Band gap structures, UV/Vis spectra, and real‐time laser flash photolysis experiments collectively suggest that easier electron transfer from the solid POM catalyst to the photosensitizer promotes photocatalytic water oxidation performance. These solid POM catalysts exhibit good stability, which is directly confirmed by a combination of Fourier‐transform infrared spectroscopy, electron microscopy, X‐ray diffraction patterns, Raman spectroscopy, X‐ray photoelectron spectroscopy, five cycles of tests, and poisoning experiments.

Synthesis of BiFeO3/Bi25FeO40 heterojunction structure and precise adjustment of forbidden band width

An effective strategy to enhance the performance of perovskite oxides is to precise adjust their sizes and forbidden bandwidth. However, it is challenging to synthesize perovskites with the desired morphology and structure because perovskites typically require harsh synthesis conditions, such as high pressure and high temperature. Herein, we designed and synthesized BiFeO3/Bi25FeO40 heterojunction nanoparticles with adjustable energy-band structure using the simple sol-gel method by adjusting the concentration of Fe3+ and calcination temperature. As a result, the BiFeO3/Bi25FeO40 heterojunction exhibits a lower band gap than BiFeO3, Bi2Fe4O9, Bi25FeO40, and BiFeO3/Bi2Fe4O9. The reduced band gap of BiFeO3/Bi25FeO40 is due to the formation of heterojunction structure. Meanwhile, the three strategies of heterojunction construction, temperature controlling, and Fe3+ doping can precise regulate the band gap structure of bismuth ferrite (BFO). These findings not only provide guidance for the construction of perovskite heterostructures but also promote the application of BFO in photocatalytic and other redox reactions.

Gas‐Phase Anion Exchange for Multisegment Heterostructured CsPb(Br1−xClx)3 Perovskite Nanowires

AbstractMetal‐halide perovskites (MHPs) are promising as active optoelectronic materials for a diverse range of devices. Anion exchange is a post‐growth modification of MHP materials that allows tuning of the band gap and crystal structure by exposure to alternative halides, normally using solution methods. Here, low temperature gas‐phase anion exchange for the conversion of CsPbBr3 nanowires (NW) into CsPb(Br1−xClx) NWs using two media, fuming HCl and Cl2 gas, is systematically investigated. It is found that both methods can be used to tune the composition in the full range with excellent control. While fuming HCl is the simplest process, Cl2 gives similar results with no surface damage and better process control. Based on a simple solid diffusion model, an average diffusivity of 1.4 × 10−12 cm2s−1 is extracted for Cl‐anions inside CsPbBr3. By combining the Cl2 exchange process with electron‐beam lithography patterning, heterojunction NWs with varying halide compositions are produced, including complex barcode‐like NWs with segment lengths as short as 500 nm. Designed heterostructures provide an important basis for optoelectronic device applications of MHPs, and gas‐phase anion exchange should be suitable for any MHP morphology.

Y(HSeO3)(SeO3)(H2O)·(H2O) and Y2(SeO3)2(SeO4)(H2O)2·(H2O)0.75: Two yttrium selenites with a short UV cut-off edge explored from pure selenite compounds

Two UV inorganic yttrium selenite optical materials, namely, Y(HSeO3)(SeO3)(H2O)·(H2O) (1) and Y2(SeO3)2(SeO4)(H2O)2·(H2O)0.75 (2), have been synthesized successfully by hydrothermal reactions. Of these, Y2(SeO3)2(SeO4)(H2O)2·(H2O)0.75 (2) is a new mixed valent selenium oxide containing Se(IV) and Se(VI) cations simultaneously. Y(HSeO3)(SeO3)(H2O)·(H2O) (1) shows a 3D structure composed of 2D yttrium selenite layers linked by O−H···O bonds. Y2(SeO3)2(SeO4)(H2O)2·(H2O)0.75 (2) exhibits a 3D structure composed of 2D yttrium selenite layers connected by SeO4 tetrahedron. Y(HSeO3)(SeO3)(H2O)·(H2O) (1) crystallized in a chiral space group P212121 and could display a moderate SHG intensity about 0.7 times of the commercial KH2PO4 (KDP). Its powder laser damage threshold quantity was estimated to be 120.2 MW/cm2, which is about 46.2 times that of AGS (2.6 MW/cm2). UV–vis–NIR spectra reflectance spectra showed that these solids are wide band gap compounds with Eg of 5.5 and 5.1 eV respectively. Theoretical calculations disclosed that Y(HSeO3)(SeO3)(H2O)·(H2O) (1) is an indirect band gap structures and its bandgaps is determined by O and Se atoms.

Design and analysis of microstrip patch antenna with photonic band gap (PBG) structure for high-speed THz application

Design and analysis of microstrip patch antenna with photonic band gap (PBG) structure for high-speed THz application

Finned Tubular Air Gap Membrane Distillation.

Finned tubular air gap membrane distillation is a new membrane distillation method, and its functional performance, characterization parameters, finned tube structures, and other studies have clear academic and practical application value. Therefore, the tubular air gap membrane distillation experiment modules composed of PTFE membrane and finned tubes were constructed in this work, and three representative air gap structures, including tapered finned tube, flat finned tube, and expanded finned tube, were designed. Membrane distillation experiments were carried out in the form of water cooling and air cooling, and the influences of air gap structures, temperature, concentration, and flow rate on the transmembrane flux were analyzed. The good water-treatment ability of the finned tubular air gap membrane distillation model and the applicability of air cooling for the finned tubular air gap membrane distillation structure were verified. The membrane distillation test results show that with the tapered finned tubular air gap structure, the finned tubular air gap membrane distillation has the best performance. The maximum transmembrane flux of the finned tubular air gap membrane distillation could reach 16.3 kg/m2/h. Strengthening the convection heat transfer between air and fin tube could increase the transmembrane flux and improve the efficiency coefficient. The efficiency coefficient (σ) could reach 0.19 under the condition of air cooling. Compared with the conventional air gap membrane distillation configuration, air cooling configuration for air gap membrane distillation is an effective way to simplify the system design and offers a potential way for the practical applications of membrane distillation on an industrial scale.

Preparation of S- scheme heterojunction Bi28O32 (SO4)10/NiAl LDH for efficient photocatalytic degradation of tetracycline

At present, the construction of high-efficiency photocatalytic degradation system of antibiotic pollutants has become a research hotspot. In this paper, Bi28O32(SO4)10/NiAl LDH photocatalyst with three-dimensional spherical morphology was successfully prepared by hydrothermal method followed by calcination, thus applying to the degradation of tetracycline. The characterization and photochemical analysis of the resulted material were used to determine the type of formed heterojunction. Bi28O32(SO4)10 and NiAl LDH build a close contact interface. The matching band gap structure makes S-scheme heterojunction formed between the two single component. Benefited from this structure, the Bi28O32(SO4)10/NiAl LDH composite with the mass ratio of 1:1 exhibited 95% efficiency in degradation of tetracycline after irradiation for 120 min, and it is stable, reusable and universal. The apparent rate constant of TC degradation by heterojunction catalyst is greatly increased, which is 5.35 and 4.91 times that of Bi28O32(SO4)10 and NiAl LDH. Overall, this paper provides a way of thinking for the design of new bismuth based photocatalytic materials, and thus providing a reference for the rational design of S-scheme heterojunction.

Triple-Band Notched Ultra-Wideband Microstrip MIMO Antenna with Bluetooth Band.

In this paper, a novel ultra-wideband UWB antenna element with triple-band notches is proposed. The proposed UWB radiator element operates from 2.03 GHz up to 15.04 GHz with triple rejected bands at the WiMAX band (3.28-3.8 GHz), WLAN band (5.05-5.9 GHz), and X-band (7.78-8.51 GHz). In addition, the radiator supports the Bluetooth band (2.4-2.483 GHz). Three different techniques were utilized to obtain the triple-band notches. An alpha-shaped coupled line with a stub-loaded resonator (SLR) band stop filter was inserted along the main feeding line before the radiator to obtain a WiMAX band notch characteristic. Two identical U-shaped slots were etched on the proposed UWB radiator to achieve WLAN band notch characteristics with a very high degree of selectivity. Two identical metallic frames of an octagon-shaped electromagnetic band gap structure (EBG) were placed along the main feeding line to achieve the notch characteristic with X-band satellite communication with high sharpness edges. A novel UWB multiple-input multiple-output (MIMO) radiator is proposed. The proposed UWB-MIMO radiator was fabricated on FR-4 substrate material and measured. The isolation between every two adjacent ports was below -20 dB over the FCC-UWB spectrum and the Bluetooth band for the four MIMO antennas. The envelope correlation coefficient (ECC) between the proposed antennas in MIMO does not exceed 0.05. The diversity gains (DG) for all the radiators are greater than 9.98 dB.

Competition between nonlinear processes excited by picosecond laser pulses in a disodium ditungstate Raman crystal for two excitation polarizations

Two nonlinear processes, namely, two-photon absorption (TPA) and stimulated Raman scattering (SRS) in Na2W2O7 crystal at a wavelength of 523.5 nm, are studied theoretically and experimentally depending on the polarization of the incident picosecond laser radiation. It was found that, due to the anisotropy of the band gap structure, the TPA coefficient at 523.5 nm strongly depends on the pump radiation polarization. For radiation along the c axis with polarization parallel to the a axis, the TPA coefficient was measured to be 1.28 cm GW−1, and no SRS process was registered. However, for polarization parallel to the b axis, no measurable TPA was detected and SRS with a gain of 6.5 cm GW−1 was observed. To the best of our knowledge, this is the first demonstration of the competition of two nonlinear processes in one crystal depending on its orientation.