Multiple Vacancies Research Articles

Hierarchical hollow MnO/carbon fiber@WS2 composite material exhibits strong wideband electromagnetic wave attenuation

Constructing multi-layer heterogeneous interfaces has been studied as one of the important means to optimize electromagnetic wave absorption performance. WS2, as a special layered material with high specific surface area and multiple anionic vacancies, although its multi-layered interface structure cannot meet the requirements of all absorbing materials, composite absorbers designed through composition and structure are undoubtedly not one of the strategies to improve absorption bandwidth. In this study, we utilized the defect rich characteristics of WS2 Nano flowers and designed a multi-layer core–shell MnO/CF@WS2 (H-MCW) hybrid materials using carbon fiber (CF) as a carrier to provide a carbon source. The core cavity of H-MCW is conducive to multiple scattering of electromagnetic waves, with a large number of defects leading to an increase in electrochemical active sites in the material, and enhanced dipoles helping to improve impedance matching. This work provides a new method for designing lightweight thin film absorbing materials.

Constructing boosted charge separation for efficient H2O2 production and pollutant degradation by highly defective ZnIn2S4/ carbon doped boron nitride

Solar-driven photocatalytic production of H2O2 as a promising environmentally-friendly approach has been applied as a chemical reagent and for enhancing photocatalytic remediation. However, this process is still limited by unsatisfactory efficiency and hard separation of photogenerated charge carriers. Herein, we design a dual pathway to improve hydrogen peroxide production by establishing a 2D/2D heterojunction between carbon-doped boron nitride (BCN) and defective zinc indium sulfide (ZIS). The introduction of multiple vacancies and dopants at the interface successfully promoted the interfacial charge transfer and peroxide generation. The Z-scheme heterojunction establishes a built-in electronic field, facilitating continuous electron accumulation and depletion, thereby inducing band bending. As a result, the optimized BCN/ZIS-30 composite (>420 nm wavelength) exhibits 925 μmol/g/h in pure water with no other additions and advances this production to 2006.1 μmol/g/h (2 times as defective ZIS and 12.1 times as BCN) under the existence of isopropanol and aeration. Mechanism investigation indicated that H2O2 is generated via a two-step electron reduction route and water oxidation reaction. The generated H2O2 successfully constructs a self-photo-Fenton system by forming hydroxyl radicals. Accompanied by strong oxygen activation ability, the photo-Fenton system also exhibits rapid degradation of antibiotics with 95 % elimination in 60 minutes. The practical applicability of this photocatalyst was demonstrated through real water remediation tests, exhibiting its effectiveness in removing antibiotics, COD, and total ammonia nitrogen, highlighting its potential for practical water treatment applications. This work provides a novel idea on constructing heterojunction materials for energy saving and water remediation.

Size-Dependent Isovalent Impurity Doping for Ambipolar Control in Cu3N.

Substitutional doping, involving the replacement of a host with an aliovalent impurity ion, is widely used to attain ambipolar controllability in semiconductors, which is crucial for device application. However, its effectiveness for p-type doping is limited in monovalent cation compounds due to the lack of suitable aliovalent (i.e., zerovalent) impurities. We propose an alternative approach for p- and n-type doping, mediated by the sizes of isovalent alkali metal impurities in Cu(I)-based semiconductors, such as copper nitride with an electron concentration of ∼1015 cm-3. Doping of isovalent Li with a smaller size to interstitial positions improves n-type conductivity, and electron concentration is controllable in the range of 1015 to 1018 cm-3. In contrast, larger isovalent Cs and Rb impurities facilitate p-type conversion, resulting in a hole concentration controllability of 1014 to 1017 cm-3. First-principles calculations indicate that Li is placed as an interstitial impurity acting as a shallow donor in conjunction with the formation of a neutral impurity on Cu defects. As the impurity size increases beyond the capacity of the vacant space, the formation of multiple acceptor-type Cu vacancies is enhanced owing to the repulsion between host Cu+ and Cs+/Rb+ impurities. Consequently, the Cs or Rb impurity is located at the sites of the N accompanied by six neighboring Cu vacancies, forming acceptor defect complexes. This size-dependent isovalent impurity doping scheme opens up an alternative avenue for advancement in optoelectronic devices using monovalent cation-based semiconductors.

High-performance sky-blue quasi-2D perovskite light-emitting diodes via synergistic defect passivation and phase narrowing strategies

Quasi-2D perovskite light-emitting diodes (PeLEDs) are considered as one of the most promising candidates for next-generation displays. Yet, the unstable electroluminescent spectra and inferior performance of blue PeLEDs hinder their commercial use. Herein, we demonstrate highly efficient quasi-2D pure bromide (QPB) sky-blue PeLEDs based on an oxygen-rich triethyl 2-acetylcitrate (TEAC) additive. The TEAC additive contains four C = O functional groups, offering several coordination sites to simultaneously passivate multiple bromine vacancy defects. Furthermore, it inhibits the growth of large n (n ≥ 5) phases to achieve narrow phases distribution. As a result, the TEAC-modified PeLEDs show sky-blue emission at 494 nm, with maximum current efficiency and external quantum efficiency up to 29.1 cd/A and 18.5 %, respectively. Moreover, the T50 lifetime of TEAC-modified PeLED is about 25 times higher than that of pristine PeLEDs. Our results provide a reliable approach to realizing high-efficiency blue quasi-2D PeLEDs.

Preparation of SnS2/MoS2 with p–n heterojunction for NO2 sensing

Conventional metal sulfide (SnS2) gas-sensitive sensing materials still have insufficient surface area and slow response/recovery times. To increase its gas-sensing performance, MoS2 nanoflower was produced hydrothermally and mechanically combined with SnS2 nanoplate. Extensive characterization results show that MoS2 was effectively integrated into SnS2. Four different concentrations of SnS2–MoS2 composites were evaluated for their NO2 gas sensitization capabilities. Among them, SnS2–15% MoS2 at 170 °C demonstrated the greatest response values to NO2, 7.3 for 1 ppm NO2, which is about three times greater than the SnS2 sensor at 170 °C (2.58). The creation of pn junctions following compositing with SnS2 was determined to be the primary reason for the composite’s faster recovery time, while the heterojunction allowed for the rapid separation of hole–electron pairs. Because the MoS2 surface has multiple vacancy defects, the adsorption energy of these vacancies is significantly higher than that of other places, resulting in increased NO2 adsorption. Furthermore, MoS2 can serve as active adsorption sites for SnS2 micrometer sheets during gas sensing. This study may help to build new NO2 gas sensors.

Integrating Sulfur Doping with a Multi-Heterointerface Fe7S8/NiS@C Composite for Wideband Microwave Absorption.

Heterointerface engineering is presently considered a valuable strategy for enhancing the microwave absorption (MA) properties of materials via compositional modification and structural design. In this study, a sulfur-doped multi-interfacial composite (Fe7S8/NiS@C) coated with NiFe-layered double hydroxides (LDHs) is successfully prepared using a hydrothermal method and post-high-temperature vulcanization. When assembled into twisted surfaces, the NiFe-LDH nanosheets exhibit porous morphologies, improving impedance matching, and microwave scattering. Sulfur doping in composites generates heterointerfaces, numerous sulfur vacancies, and lattice defects, which facilitate the polarization process to enhance MA. Owing to the controllable heterointerface design, the unique porous structure induced multiple heterointerfaces, numerous vacancies, and defects, endowing the Fe7S8/NiS@C composite with an enhanced MA capability. In particular, the minimum reflection loss (RLmin) value reached -58.1dB at 15.8GHz at a thickness of 2.1mm, and a broad effective absorption bandwidth (EAB) value of 7.3GHz is achieved at 2.5mm. Therefore, the Fe7S8/NiS@C composite exhibits remarkable potential as a high-efficiency MA material owing to the synergistic effects of the polarization processes, multiple scatterings, porous structures, and impedance matching.

Strengthening the Synergy between Oxygen Vacancies in Electrocatalysts for Efficient Glycerol Electrooxidation.

Defect-engineered bimetallic oxides exhibit high potential for the electrolysis of small organic molecules. However, the ambiguity in the relationship between the defect density and electrocatalytic performance makes it challenging to control the final products of multi-step multi-electron reactions in such electrocatalytic systems. In this study, controllable kinetics reduction is used to maximize the oxygen vacancy density of a Cu─Co oxide nanosheet (CuCo2O4 NS), which is used to catalyze the glycerol electrooxidation reaction (GOR). The CuCo2O4-x NS with the highest oxygen-vacancy density (CuCo2O4-x-2) oxidizes C3 molecules to C1 molecules with selectivity of almost 100% and a Faradaic efficiency of ≈99%, showing the best oxidation performance among all the modified catalysts. Systems with multiple oxygen vacancies in close proximity to each other synergistically facilitate the cleavage of C─C bonds. Density functional theory calculations confirm the ability of closely spaced oxygen vacancies to facilitate charge transfer between the catalyst and several key glycolic-acid (GCA) intermediates of the GOR process, thereby facilitating the decomposition of C2 intermediates to C1 molecules. This study reveals qualitatively in tuning the density of oxygen vacancies for altering the reaction pathway of GOR by the synergistic effects of spatial proximity of high-density oxygen vacancies.

Electron transport through the multiple sulfur vacancies in MoS2

We conducted a thorough investigation of the impact of sulfur vacancies on electronic structures and electron transport in MoS2, with a particular focus on neighboring sulfur vacancies by employing density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods. Although pristine MoS2 exhibits a Schottky-like behavior, an intriguing behavior emerged in the presence of aligned vacancies. The current flows through interconnected in-gap states even at low bias region (0.0–0.2 V), but it does through tunneling from defect states to the conduction bands (CBs) at higher voltages. Negative differential resistance (NDR) appears in the middle range of voltage. Through systematic examination of the inter-vacancy distance at which current could flow, we found that MoS2 ribbons could exhibit a random network of vacancies capable of mediating current at feasible vacancy concentration, which means percolation emerges. This research offers valuable insights into the potential of defect-engineered MoS2 for novel electronic applications.

Eco-Friendly Zn-Ag-In-Ga-S Quantum Dots: Amorphous Indium Sulfide Passivated Silver/Sulfur Vacancies Achieving Efficient Red Light-Emitting Diodes.

I-III-VI quantum dots (QDs) and derivatives (I, III, and VI are Ag+/Cu+, Ga3+/In3+, and S2-/Se2-, respectively) are the ideal candidates to replace II-VI (e.g., CdSe) and perovskite QDs due to their nontoxicity, pure color, high photoluminescence quantum yield (PLQY), and full visible coverage. However, the chaotic cation alignment in multielement systems can easily lead to the formation of multiple surface vacancies, highlighted as VI and VVI, leading to nonradiative recombination and nonequilibrium carrier distribution, which severely limit the performance improvement of materials and devices. Here, based on Zn-Ag-In-Ga-S QDs, we construct an ultrathin indium sulfide shell that can passivate electron vacancies and convert donor/acceptor level concentrations. The optimized In-rich 2-layer indium sulfide structure not only enhances the radiative recombination rate by preventing further VS formation but also achieves the typical DAP emission enhancement, achieving a significant increase in PLQY to 86.2% at 628 nm. Moreover, the optimized structure can mitigate the lattice distortion and make the carrier distribution in the interior of the QDs more balanced. On this basis, red QD light-emitting diodes (QLEDs) with the highest external quantum efficiency (EQE; 5.32%) to date were obtained, providing a novel scheme for improving I-III-VI QD-based QLED efficiency.

Investigating the Effect of Structural Modifications on Mechanical Properties of Carbon Nanotubes under Tensile Loading Using Molecular Dynamics Simulations

The discovery of Carbon Nanotubes (CNT) has opened the doors for revolutionary applications in the mechanical, aerospace, and electrical sectors. However, to fully utilize the potential of carbon nanotubes, there is a persisting need to identify all sorts of structural modifications that can be observed in any type of manufacturing procedure for CNTs. Thus, the presented study investigates the mechanical properties of CNTs with variable waviness and defect density. Furthermore, the study is performed using classical Molecular Dynamics simulations (MD). The structures are then characterized with single or multiple vacancy defects along the axis of the nanotube structure, which is modeled as wavy structures to replicate their natural structure. After the simulation results were analyzed, it was observed that the increase in the surrounding temperature from 300K to 1500K reduces the overall tensile strength of the CNT sample from 89-47 GPa. However, introducing a single vacancy defect to the same structures was shown to reduce the tensile strength to 41 GPa at 1500K and 62 GPa at 300K.

Transport properties of phosphorene nanoribbons with defects

We have theoretically studied the electron transport properties in the phosphorene nanoribbons with different kinds of defects, including either single or multiple vacancies and substitutions, in comparison with otherwise perfect lattice. It is found that whether the transport channel would be destroyed or not is dependent on the defects' locations. If the defects locate on the original perfect current paths, the conductivity would decrease to zero at a certain Fermi energy. This is also confirmed by calculating the microscopic currents, which shows how the relative location, the number of defects and its severity affect the conductivity. The results are helpful for its application in the transport devices.

Ion Irradiation Activated Catalytic Activity of MoSe2 Nanosheet for High‐Efficiency Hydrogen Evolution Reaction

AbstractDeveloping highly efficient, low cost, and stable electrocatalysts that work at a large current density is crucial for upgrading the current industrial electrochemical water splitting to produce H2. Molybdenum selenide (MoSe2) is a promising 2D transition metal dichalcogenide (TMD), however, its reported output is inadequate due to its inert basal plane. Herein, the catalytic activity of MoSe2 nanosheet arrays is activated by a novel and controllable method of He+ ion irradiation to introduce multiple vacancies simultaneously into their inert basal planes. The vacancies activated MoSe2 have improved electrocatalytic performance and stability with a minimum overpotential of 90 mV at 10 mA cm−2, a Tafel slope of 49 mV dec−1 and high stability of 650 h at the industry‐level large current density of 1000 mA cm−2 compared to several hours for the pristine sample. The DFT results reveal that single Se and single Mo vacancies on the MoSe2 basal plane can efficiently increase the electrical conductivity and reduce energy barriers for water dissociation and subsequent proton adsorption, thus improving the electrocatalytic capability. This finding proves the application of ion beam in defect engineering for effective hydrogen evolution in TMDs‐based catalysts.

Controlled Synthesis of MOF-Derived Nano-Microstructure toward Lightweight and Wideband Microwave Absorption.

Correlating metal-organic framework (MOF) synthesis processes and microwave absorption (MA) enhancement mechanisms is a pioneer project. Nevertheless, the correlation process still relies mainly on empirical doctrine, which hardly corresponds to the specific mechanism of the effect on the dielectric properties. Hereby, after the strategy of modulation of protonation engineering and solvothermal temperature in the synthesis route, the obtained sheet-like self-assembled nanoflowerswereconstructed.Porous structures with multiple heterointerfaces, abundant defects, and vacancies are obtained by controlled design of the synthesis procedure. The rearrangement of charges and enhanced polarization can be promoted. The designed electromagnetic properties and special nano-microstructures of functional materials have significant impact on their electromagnetic wave energy conversion effects. As a consequence, the MA performance of the samples has been enhanced toward broadband absorption (6.07GHz), low thickness (2.0mm), low filling (20%), and efficient loss (-25dB), as well as being suitable for practical environmental applications. This work establishes the connection between the MOF-derived materials synthesis process and the MA enhancement mechanism, which provides insight into various microscopic microwave loss mechanisms.

Insight into radical-nonradical coupling activation pathways of peroxymonosulfate by CuxO for antibiotics degradation

In this work, a heterogeneous catalyst of CuxO was rationally designed by using Cu-based metal organic frameworks (marked Cu-BDC) as the template, and was used to degrade tetracycline (TC) via activation of peroxymonosulfate (PMS). The optimal CuxO-350 showed excellent catalytic efficiency for TC degradation, and the reaction rate constant (0.104 min−1) was 8 times higher than that (0.013 min−1) of raw Cu-BDC. The characterization observations confirmed that CuxO-350 possessed multiple valence states (CuO and Cu2O) and oxygen vacancies (Ov), both of which were favorable for the activation of PMS, resulting in promoting the generation of active species in the CuxO-350 + PMS system. Different from the free radical pathway in Cu-BDC + PMS system, a radical-nonradical coupling process was detected in the CuxO-350 + PMS system, which was confirmed by quenching experiments and EPR measurements. Moreover, the toxicity prediction showed that the toxicity of degradation intermediates declined compared with TC. This work not only opened up a new strategy for the rational design and preparation of high-efficient catalysts by employing metal organic frameworks precursors, but also offered an insight into the reaction mechanism of PMS activation through a radical-nonradical coupling process catalyzed by CuxO-350 derived from Cu-BDC.

Investigation of unified impact of Ti adatom and N doping on hydrogen gas adsorption capabilities of defected graphene sheets

In this work, we studied the hydrogen adsorption capabilities of functionalized graphene sheets containing a variety of defects (D-G) via molecular dynamics (MD) simulations that govern the mechanisms involved in hydrogen adsorption. Specifically, the graphene sheets containing monovacancy (MV), Stone-Wales (SW), and multiple double vacancy (DV) defects were functionalized with Ti and N atoms to enhance their hydrogen adsorption capacity. We measured the adsorption capacities of the N-/D-G sheets with varying concentrations of Ti adatoms at 300 K and 77 K temperatures and various pressures. Our study revealed that the increasing concentration of Ti adatoms on the D-G sheets led to a significant improvement in the hydrogen adsorption capacity of the graphene sheets. The DV(III)-G sheets showed the maximum adsorption capacity at 300 K because the DV(III)-G sheets had a small number of large-sized pores that bind hydrogen with high binding energy. Thus, hydrogen remained adsorbed even at higher temperatures (300 K). The N doping on the D-G sheets initially reduced their hydrogen adsorption capabilities; however, the N-D-G sheets enhanced their hydrogen adsorption capacity with the increasing concentrations of Ti adatoms. Compared to all other defect types, the Ti–N-DV(III)-G sheet with a Ti concentration of 10.5% showed a hydrogen uptake of 5.5 wt% at 300 K and 100 bar pressure. Thus, the N doping and Ti implantations improved the hydrogen storage capabilities of the graphene sheets, and these findings helped design solid-state hydrogen storage systems operating at ambient conditions and moderate pressure ranges.

Breaking the structural anisotropy of ZnO enables dendrite-free lithium-metal anode with ultra-long cycling lifespan

Crystalline materials have shown great potential for lithium-metal batteries. However, crystalline materials are prone to uncontrollable growth of lithium dendrites due to their anisotropic characteristics. Herein, we break the structural anisotropy of zinc oxide as an amorphous host to efficiently induce a uniform lithium deposition. Theoretical calculations first confirm that amorphous ZnO presents a much larger lithium adsorption energy. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) studies reveal that amorphous zinc oxide possesses both numerous “floppy” bonds and multiple vacancies, resulting in not only an increase in lithiophilic sites but also a more uniform distribution of the electric field. As a result, amorphous zinc oxide delivers an improved Coulomb efficiency of 98.2% even after 500 cycles at 1 mA cm−2 and an ultra-long cycling lifespan over 2,500 h in symmetric cells, outperforming most representative and state-of-the-art lithium-metal anodes. This work opens a new pathway to exploring amorphous materials for lithium-metal batteries.

Synergistic effect of multiple vacancies to induce lattice oxygen redox in NiFe-layered double hydroxide OER catalysts

The role of multiple vacancies containing cation and anion defects in NiFe layered double hydroxide (LDH) is still worth pursuing in electrocatalytic oxygen evolution reaction (OER). Herein, a strategy integrating the oxygen and iron vacancies is proposed to fabricate NiFe LDH with highly efficient OER performance. More Fe vacancies have been introduced with increasing Fe content in NiFe-LDH structure; operando electrochemical characterizations and density functional theory (DFT) calculations reveal that the synergic effect between the iron and the oxygen vacancies promotes the generation of reconstructed active layers, optimizing the adsorption free energy of oxygen-containing intermediates. More importantly, active NiOOH intermediate species induced by oxygen vacancies are associated with OH- intercalation pseudocapacitance, activating lattice oxygen during water oxidation. The as-prepared Ni0.3Fe0.7-LDH@NF showed ultralow overpotentials of 184 and 287 mV to achieve 10 and 400 mA·cm-2, respectively. Our work provides a new paradigm for designing high-efficiency OER electrocatalysts.

Long-Term Evolution of Vacancies in Large-Area Graphene.

Devices based on two-dimensional (2D) materials such as graphene and molybdenum disulfide have shown extraordinary potential in physics, nanotechnology, and electronics. The performances of these applications are heavily affected by defects in utilized materials. Although great efforts have been spent in studying the formation and property of various defects in 2D materials, the long-term evolution of vacancies is still unclear. Here, using a designed program based on the kinetic Monte Carlo method, we systematically investigate the vacancy evolution in monolayer graphene on a long-time and large spatial scale, focusing on the variation of the distribution of different vacancy types. In most cases, the vacancy distribution remains nearly unchanged during the whole evolution, and most of the evolution events are vacancy migrations with a few being coalescences, while it is extremely difficult for multiple vacancies to dissolve. The probabilities of different categories of vacancy evolutions are determined by their reaction rates, which, in turn, depend on corresponding energy barriers. We further study the influences of different factors such as the energy barrier for vacancy migration, coalescence, and dissociation on the evolution, and the coalescence energy barrier is found to be dominant. These findings indicate that vacancies (also subnanopores) in graphene are thermodynamically stable for a long period of time, conducive to subsequent characterizations or applications. Besides, this work provides hints to tune the ultimate vacancy distribution by changing related factors and suggests ways to study the evolution of other defects in various 2D materials.

Effect of Powder Bed Fusion Process Parameters on Microstructural and Mechanical Properties of FeCrNi MEA: An Atomistic Study

In our study, molecular dynamics (MD) simulations of laser powder bed fusion (LPBF) have been conducted on equimolar FeNiCr medium entropy alloy (MEA) powders. With the development of newer LPBF technologies capable of printing at the microscale, an even deeper understanding of the underlying atomistic effects of the process parameters on the microstructural and mechanical properties of the manufactured FeNiCr MEA products is required. In accordance with previous literature, the parameters of the LPBF process have been systematically varied, including layer resolution from 1 to 6, laser power from 100 {\mu}W to 220 {\mu}W, bed temperature from 300 K to 1200 K, and laser scan speed from 0.5 {\AA}/ps to 0.0625 {\AA}/ps. Consistent with prior macroscopic experimental findings, the atomistic results suggest that additive manufacturing using thinner layers imparts higher ultimate tensile strength (UTS) than fabricating with thicker layers. The latter, however, requires a shorter process time but induces keyhole defect formation if the laser-induced temperature is not sufficiently high enough. Increasing the temperature proves useful in mitigating this problem. Enhancement of UTS for the multi-rowed powders has been observed by raising the substrate temperature to 600 K or laser power to 160 {\mu}W during production. Beyond these critical limits, however, the UTS of the product diminishes due to the emergence of multiple vacancies. The results of our present study will help researchers to find a good balance between the production speed and strength of additive manufactured products at the nanoscale.

Carbon nanomaterials in nickel and iron helping to disperse or release He atoms

Experimental and theoretical studies have shown that helium (He) atoms in metal tend to form into nano-sized bubbles which greatly affects mechanical properties of metals. Thus, designing nanostructure to control the growth of He bubbles and increase its radiation resistance is an important task. In this work, we systematically study the atomistic mechanism on how the carbon (C) nanomaterials embedded inside metals with different crystal structures (FCC-Ni and BCC-Fe) disperse or release He atoms using molecular dynamics simulations. The dynamic evolution of He atoms in different Ni/C and Fe/C nanocomposites were observed and compared. It is found that the presence of multiple C72 with vacancy defects and graphene with multiple vacancy defects can suppress the formation of large He bubbles and dislocations by helping to disperse He atoms, while, the C nanotube with vacancy defects can suppress the formation of large He bubbles and dislocations by helping to release He atoms. The formation energies of He in different Ni/C and Fe/C nanocomposites were also calculated to explain why the He atoms tend to diffuse into the region near C structures. The average diffusion coefficients of He in bulk Ni and bulk Fe were calculated to explain the mechanism for the better abilities of C structures in Ni/C nanocomposites than in Fe/C nanocomposites help to disperse or release He atoms.