Effect of Vacancy Defects on the Properties of CoS_(2) and FeS_(2)

In order to explore the effect of vacancy defects on the structural,electronic,magnetic and optical properties of CoS_(2) and FeS_(2),first-principles calculation method was used to investigate the alloys.The calculated results of materials without vacancy are consistent with those reported in the literatures,while the results of materials with vacancy defect were different from those of literatures due to the difference vacancy concentration.The Co vacancy defect hardly changes the half-metallic characteristic of CoS_(2).The Fe vacancy defect changes FeS_(2) from semiconductor to half-metal,and the bottom of the spin-down conduction band changes from the p orbital state of S to the d(t_(2g))orbital state of Fe,while the top of the valence band remains the d orbital d(eg)state of Fe.The half-metallic Co vacancy defects of CoS_(2) and Fe vacancy defects of FeS_(2) are expected to be used in spintronic devices.S vacancy defects make both CoS_(2) and FeS_(2) metallic.Both the Co and S vacancy defects lead to the decrease of the magnetic moment of CoS_(2),while both the Fe and S vacancy defects lead to the obvious magnetic property of FeS_(2).Vacancy defects enhance the absorption coefficient of infrared band and long band of visible light obviously,and produce obvious red shift phenomenon,which is expected to be used in photoelectric devices.

Effect of vacancy defects on electronic properties and activation of sphalerite(110) surface by first-principles

The electronic properties of sphalerite(110) surface with Zn-vacancy and S-vacancy were calculated by using density-functional theory,and the effects of vacancy defect on the copper activation of sphalerite were investigated.The calculated results indicate that surface state occurs in the band gap of Zn-vacancy sphalerite,which is from the contribution of S 3p orbital at the first layer of the surface.The presence of S-vacancy results in surface state appearing near the Fermi level and the bottom of conductor band,which are composed of S 3p and Zn 4s orbital,respectively.The surface structure of Zn-vacancy sphalerite is more stable than S-vacancy surface due to the occupation of Zn-vacancy by Cu atoms;hence,the substitution reaction of Cu for Zn vacancy is easier than the substitution of Cu for Zn atoms with S-vacancy surface.

Effect of vacancy defect clusters on the optical property of the aluminium filter used for the space solar telescope

We investigate the microstructures of the pure aluminium foil and filter used on the space solar telescope, irradiated by photons with different doses. The vacancy defect clusters induced by proton irradiation in both samples are characterized by transmission electron microscopy, and the density and the size distribution of vacancy defect clusters are determined. Their transmittances are measured before and after irradiating the samples by protons with energy E = 100 keV and dose φ = 6 × 10^11/mm^2. Our experimental results show that the density and the size of vacancy defect clusters increase with the increase of irradiation doses in the irradiated pure aluminium foils. As irradiation dose increases, vacancies incline to form larger defect clusters. In the irradiated filter, a large number of banded void defects are observed at the agglomerate boundary, which results in the degradation of the optical and mechanical performances of the filter after proton irradiation.

Tunable vacancy defect chemistry on free-standing carbon cathode for lithium-sulfur batteries

The defect chemistry is successfully modulated on free-standing and binder-free carbon cathodes for highly efficient Li-S redox reactions.Such rationally regulated defect engineering realizes the synchronization of ion/electron-conductive and defect-rich networks on the threedimension carbon cathode,leading to its tunable activity for both relieving the shuttle phenomenon and accelerating the sulfur redox reaction kinetics.As expected,the defective carbon cathode harvests a high rate capacity of 1217.8 mAh g^(-1)at 0.2 C and a superior capacity retention of61.7%at 2 C after 500 cycles.Even under the sulfur mass loading of 11.1 mg cm^(-2),the defective cathode still holds a remarkable areal capacity of 8.5 mAh cm^(-2).

Role of vacancy-type defects in magnetism of GaMnN

Role of vacancy-type（N vacancy（VN） and Ga vacancy（VGa）） defects in magnetism of GaMnN is investigated by first-principle calculation.Theoretical results show that both the VNand VGainfluence the ferromagnetic state of a system.The VNcan induce antiferromagnetic state and the VGaindirectly modify the stability of the ferromagnetic state by depopulating the Mn levels in GaMnN.The transfer of electrons between the vacancy defects and Mn ions results in converting Mn3＋（d4） into Mn2＋（d5）.The introduced VNand the ferromagnetism become stronger and then gradually weaker with Mn concentration increasing,as well as the coexistence of Mn3＋（d4） and Mn2＋（d5） are found in GaMnN films grown by metal–organic chemical vapor deposition.The analysis suggests that a big proportion of Mn3＋changing into Mn2＋will reduce the exchange interaction and magnetic correlation of Mn atoms and lead to the reduction of ferromagnetism of material.

Effects of vacancy defect on the tensile behavior of graphene

Graphene is the strongest material but its performance is significantly weakened by vacancy defects. We use molecular dynamics simulations to inves- tigate the tensile behavior of a graphene which contains a single vacancy defect. Our results suggest that because of the single vacancy, the fracture strength of graphene losses about 17.7%. The stress concentration around the vacancy defect leads to the destruction of nearby six-member rings structure, which forms the initial crack. The propagation direction of this crack in defective graphene is at an angle of 60° to the tensile direction initially, but then becomes perpendicular to the tensile direction.

Effects of vacancy and external electric field on the electronic properties of the MoSi_(2)N_(4)/graphene heterostructure

Recently,the newly synthesized septuple-atomic layer two-dimensional(2D)material MoSi_(2)N_(4)(MSN)has attracted attention worldwide.Our work delves into the effect of vacancies and external electric fields on the electronic properties of the MSN/graphene(Gr)heterostructure using first-principles calculation.We find that four types of defective structures,N-in,N-out,Si and Mo vacancy defects of monolayer MSN and MSN/Gr heterostructure are stable in air.Moreover,vacancy defects can effectively modulate the charge transfer at the interface of the MSN/Gr heterostructure as well as the work function of the pristine monolayer MSN and MSN/Gr heterostructure.Finally,the application of an external electric field enables the dynamic switching between n-type and p-type Schottky contacts.Our work may offer the possibility of exceeding the capabilities of conventional Schottky diodes based on MSN/Gr heterostructures.

Influences of vacancy defects on thermal conductivities of Ge thin films

The effects of vacancy defects on the thermal conductivity of Ge thin films were investigated by employing molecular dynamics （MD） simula- tions and theoretical analysis based on the Boltzmann equation. Both the MD and theoretical results show that the lattice thermal conductivity dramatically decreases with the increasing of vacancy concentration at 400 and 500 K. In addition, the dependence of vacancy concentration on the thermal conductivity of Ge thin films becomes less sensitive as the temperature increases. Theoretical results also confirm that the major part of the lattice thermal conductivity reduction is associated with the point-defect scattering and phonon-phonon scattering processes.

Quantum plasmons in the hybrid nanostructures of double vacancy defected graphene and metallic nanoarrays

We study the plasmonic properties of hybrid nanostructures consisting of double vacancy defected graphene(DVDGr)and metallic nanoarrays using the time-dependent density functional theory. It is found that DVDGr with pure and mixed noble/transition-metal nanoarrays can produce a stronger light absorption due to the coherent resonance of plasmons than graphene nanostructures. Comparing with the mixed Au/Pd nanoarrays, pure Au nanoarrays have stronger plasmonic enhancement. Furthermore, harmonics from the hybrid nanostructures exposed to the combination of lasers ranged from ultraviolet to infrared and a controlling pulse are investigated theoretically. The harmonic plateau can be broadened significantly and the energy of harmonic spectra is dramatically extended by the controlling pulse. Thus, it is possible to tune the width and intensity of harmonic spectrum to achieve broadband absorption of radiation. The methodology described here not only improves the understanding of the surface plasmon effect used in a DVDGr-metal optoelectronic device but also may be applicable to different optical technologies.

State-of-the-art advances in vacancy defect engineering of graphitic carbon nitride for solar water splitting

Developing low-cost,efficient,and stable photocatalysts is one of the most promising methods for large-scale solar water splitting.As a metal-free semiconductor material with suitable band gap,graphitic carbon nitride(g-C_(3)N_(4))has attracted attention in the field of photocatalysis,which is mainly attributed to its fascinating physicochemical and photoelectronic properties.However,several inherent limitations and shortcomings—involving high recombination rate of photocarriers,insufficient reaction kinetics,and optical absorption—impede the practical applicability of g-C_(3)N_(4).As an effective strategy,vacancy defect engineering has been widely used for breaking through the current limitations,considering its ability to optimize the electronic structure and surface morphology of g-C_(3)N_(4) to obtain the desired photocatalytic activity.This review summarizes the recent progress of vacancy defect engineered g-C_(3)N_(4) for solar water splitting.The fundamentals of solar water splitting with g-C_(3)N_(4) are discussed first.We then focus on the fabrication strategies and effect of vacancy generated in g-C_(3)N_(4).The advances of vacancy-modified g-C_(3)N_(4) photocatalysts toward solar water splitting are discussed next.Finally,the current challenges and future opportunities of vacancy-modified g-C_(3)N_(4) are summarized.This review aims to provide a theoretical basis and guidance for future research on the design and development of highly efficient defective g-C_(3)N_(4).

Formation Enthalpy Calculation of Oxygen Vacancy Defect in Doped Lithium Niobate Crystals

The relationship between temperature and oxygen vacancy concentration is deduced in this paper. Based on the data of thermal weight-loss experiment, the formation enthalpies of congruent and several doped LN crystals have been calculated. It was found that the formation enthalpy of oxygen vacancies can be decreased evidently by doping valence-changeable ions. The experimental results were discussed and a new reduction process of the photorefractive LN crystal at a relatively low temperature was proposed, and the reduced crystals showed a good effect in practical use.

Thermal transport properties of defective graphene:A molecular dynamics investigation

In this work the thermal transport properties of graphene nanoribbons with randomly distributed vacancy defects are investigated by the reverse non-equilibrium molecular dynamics method. We find that the thermal conductivity of the graphene nanoribbons decreases as the defect coverage increases and is saturated in a high defect ratio range. Further analysis reveals a strong mismatch in the phonon spectrum between the unsaturated carbon atoms in 2-fold coordination around the defects and the saturated carbon atoms in 3-fold coordination, which induces high interfacial thermal resistance in defective graphene and suppresses the thermal conductivity. The defects induce a complicated bonding transform from sp2 to hybrid sp--sp2 network and trigger vibration mode density redistribution, by which the phonon spectrum conversion and strong phonon scattering at defect sites are explained. These results shed new light on the understanding of the thermal transport behavior of graphene-based nanomaterials with new structural configurations and pave the way for future designs of thermal management phononic devices.

Filling the in situ-generated vacancies with metal cations captured by C-N bonds of defect-rich 3D carbon nanosheet for bifunctional oxygen electrocatalysis

Nitrogen-doped carbon materials with vacancies/defects have been developed as highly efficient ORR electrocatalysts but with poor activity for OER,which limits their application in rechargeable metal-air batteries.Filling the vacancies/defects with heteroatoms is expected to be an effective strategy to obtain surprising catalytic activities and improve their stability especially under the strongly oxidizing conditions during the OER process.Herein,we successfully transformed the defect-rich 3 D carbon nanosheets(DCN)into a bifunctional ORR/OER electrocatalyst(DCN-M)by utilizing the in-situ generated vacancies to capture metal cations via a modified salt-sealed strategy.By varying the metal(Fe,Ni)content,the captured metal cations in DCN-M existed in different chemical states,i.e.,metal atoms were stabilized by CàN bonds at low metal contents,while at high metal contents,bimetal particles were covered by graphene layers,taking responsibility for catalyzing the ORR and OER,respectively.In addition,the in-situ formed graphene layers with an interconnected structure facilitate the electron transport during the reactions.The Janus-feature of DCN-M in structures ensures superior bifunctional activity and good stability towards ORR/OER for the rechargeable Zn-air battery.This work provides an effective strategy to design multifunctional electrocatalysts by heteroatom filling into vacancies of carbon materials.

Prediction and observation of defect-induced room-temperature ferromagnetism in halide perovskites

The possibility to induce a macroscopic magnetic moment in lead halide perovskites(LHPs),combined with their excellent optoelectronic properties,is of fundamental interest and has promising spintronic applications.However,these possibilities remain an open question in both theory and experiment.Here,theoretical and experimental studies are performed to explore ferromagnetic states in LHPs originated from lattice defects.First-principle calculations reveal that shallow-level Br vacancies in defective CsPbBr3 can produce spin-splitting states and the coupling between them leads to a ferromagnetic ground state.Experimentally,ferromagnetism at 300 K is observed in room-temperature synthesized CsPbBr3 nanocrystals,but is not observed in hot-injection prepared CsPbBr3 quantum dots and in CsPbBr3 single crystals,highlighting the significance played by vacancy defects.Furthermore,the ferromagnetism in the CsPbBr3 nanocrystals can be enhanced fourfold with Ni2+ion dopants,due to enhancement of the exchange coupling between magnetic polarons.Room-temperature ferromagnetism is also observed in other LHPs,which suggests that vacancy-induced ferromagnetism may be a universal feature of solution-processed LHPs,which is useful for future spintronic devices.

Investigation of Mechanical Behavior of Defective Carbon Nanotubes Using Molecular Dynamics Simulation

Carbon nanotubes （CNTs） having pristine structure （i.e., structure without any defect） hold very high mechanical properties. However, CNTs suffer from defects ＇which can appear at production stage, purification stage or be deliberately introduced by irradiation with energetic particles or by chemical treatment. In this article, mechanical properties of single-walled nanotubes with defects are studied under both compressive and tensile loads using molecular dynamics （MD） simulations. Two types of defectStone-Wales and vacancy defects with different defect densities are considered for present investigation. Molecular simulations are carried out using the classical MD method. The Brenner potential is used for carbon-carbon interaction in the CNT. Temperature of the system is controlled by velocity scaling. Simulation results show that the defects have negligible effect on the modulus of elasticity of nanotubes. However, they have significant effect on the failure stress and strain of the nanotubes.

Thermal stable Pt clusters anchored by K/TiO_(2)—Al_(2)O_(3)for efficient cycloalkane dehydrogenation

Catalytic dehydrogenation of cycloalkanes is considered a valuable endothermic process for alleviating the thermal barrier issue of hypersonic vehicles.However,conventional Pt-based catalysts often face the severe problem of metal sintering under high-temperature conditions.Herein,we develop an efficient K_(2)CO_(3)-modified Pt/TiO_(2)—Al_(2)O_(3)(K—Pt/TA)for cycloalkane dehydrogenation.The optimized K—Pt/TA showed a high specific activity above 27.9 mol·mol^(-1)·s^(-1)(H_(2)/Pt),with toluene selectivity above 90.0%at 600℃with a high weight hourly space velocity of 266.4 h^(-1).The introduction of alkali metal ions could generate titanate layers after high-temperature hydrogen reduction treatment,which promotes the generation of oxygen vacancy defects to anchored Pt clusters.In addition,the titanate layers could weaken the surface acidity of catalysts and inhibit side reactions,including pyrolysis,polymerization,and isomerization reactions.Thus,this work provides a modification method to develop efficient and stable dehydrogenation catalysts under high-temperature conditions.

Regulating Oxygen Vacancy Defects in Heterogeneous NiO-CeO_(2)−δ Hollow Multi-shelled Structure for Boosting Oxygen Evolution Reaction

CeO_(2) with excellent oxygen storage-exchange capacity and NiO with excellent surface activity were used to construct a heterogeneous NiO-CeO_(2)−δhollow multi-shelled structure(HoMS)by spray drying.It turned out that as the proportion of CeO_(2) increases,the overpotential and Tafel slope of NiO-CeO_(2)−δHoMSs first decreased and then increased.This is mainly because the construction of the NiO-CeO_(2)−δHoMSs not only increases the specific surface area,but also introduces oxygen vacancy defects,thus improving the interface charge transfer capability of the materials and further improving the oxygen evolution reaction(OER)performance.However,the increase of the calcination temperature will induce the decay of the OER performance of NiO-CeO_(2)−δHoMSs,which is mainly due to the decrease of the specific surface area,the reduction of oxygen vacancy defects,and the weakening of interface charge transfer capability.Furthermore,a series of heterogeneous composite HoMSs,such as Ni/Co,Mo/Ni,Al/Ni and Fe/Ni oxides was successfully constructed by spray drying,which enriched the diversity of HoMSs.

Highly efficient Z-scheme WO_(3-x) quantum dots/TiO_2 for photocatalytic hydrogen generation

Z-scheme semiconductors are a promising class of photocatalysts for hydrogen generation.In this work,Z-scheme semiconductors composed of WO3-x quantum dots supported on TiO2（WO3-xQDS/TiO2） were fabricated by solvothermal and hydrogen-reduction methods.Characterization by transmission electron microscopy and X-ray diffraction indicated that the amount and size of the WO3-x QDs could be tuned by modulating the addition of the W precursor.Evidence from X-ray photoelectron spectroscopy and photoluminescence spectroscopy suggested that the hydrogen reduction of the composite induced the formation of oxygen vacancy（W^5＋/Vo） defects in WO3.These defects led to ohmic contact between WO3-x and TiO2,which altered the charge-transfer pathway from type Ⅱ heterojunction to Z-scheme,and maintained the highly reductive and oxidative ability of TiO2 and WO3-x,respectively.Therefore,the Z-scheme sample showed 1.3-fold higher photoactivity than pure TiO2 in hydrogen generation.These results suggest that the formation of W^5＋/Vo defects at the interface is highly beneficial for the fabrication of Z-scheme photocatalysts.

Phosphorus-doped MoS_(2) with sulfur vacancy defects for enhanced electrochemical water splitting

MoS_(2)is a promising electrocatalyst because of its natural abundance and outstanding electrochemical stability.However,the poor conductivity and low activity limit its catalytic performance;furthermore,MoS_(2)is unable to satisfy the requirements of most industrial applications.In this study,to obtain a P-doped MoS_(2)catalyst with S vacancy defects,P is inserted into the MoS_(2)matrix via a solid phase ion exchange at room temperature.The optimal P-doping amount is 11.4 wt%,and the resultant catalyst delivers excellent electrocatalytic properties for the hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)with the corresponding overpotentials of 93 and 316 mV at 10 mA cm^(-2) in an alkaline solution;these values surpass the overpotentials of most previously reported MoS_(2)-based materials.Theoretical calculations and results demonstrate that the synergistic effect of the doped P,which forms active centers in the basal plane of MoS_(2),and S vacancy defects caused by P doping intensifies the intrinsic electronic conductivity and electrocatalytic activity of the catalyst.Density functional theory calculations demonstrate that P optimizes the free energy of the MoS_(2)matrix for hydrogen adsorption,thereby considerably increasing the intrinsic activity of the doped catalyst for the HER compared with that observed from pristine MoS_(2).The enhanced catalytic activity of P-doped MoS_(2)for the OER is attributed to the ability of the doped P which facilitates the adsorption of hydroxyl and hydroperoxy intermediates and reduces the reaction energy barrier.This study provides a new environmentally friendly and convenient solid-phase ion exchange method to improve the electrocatalytic capability of two-dimensional transition-metal dichalcogenides in largescale applications.

Oxygen vacancies enriched nickel cobalt based nanoflower cathodes: Mechanism and application of the enhanced energy storage

The rational design of oxygen vacancies and electronic microstructures of electrode materials for energy storage devices still remains a challenge. Herein, we synthesize nickel cobalt-based oxides nanoflower arrays assembled with nanowires grown on Ni foam via the hydrothermal process followed annealing process in air and argon atmospheres respectively. It is found that the annealing atmosphere has a vital influence on the oxygen vacancies and electronic microstructures of resulting NiCo_(2)O_(4) (NCO-Air) and CoNiO_(2) (NCO-Ar) products, which NCO-Ar has more oxygen vacancies and larger specific surface area of 163.48 m^(2)/g. The density functional theory calculation reveals that more oxygen vacancies can provide more electrons to adsorb –OH free anions resulting in superior electrochemical energy storage performance. Therefore, the assembled asymmetric supercapacitor of NCO-Ar//active carbon delivers an excellent energy density of 112.52 Wh/kg at a power density of 558.73 W/kg and the fabricated NCO-Ar//Zn battery presents the specific capacity of 180.20 mAh/g and energy density of 308.14 Wh/kg. The experimental measurement and theoretical calculation not only provide a facile strategy to construct flower-like mesoporous architectures with massive oxygen vacancies, but also demonstrate that NCO-Ar is an ideal electrode material for the next generation of energy storage devices.