Molybdenum Vacancies Research Articles

Constructing molybdenum vacancy defect for MoP with optimized p-band center towards high-efficiency hydrogen evolution

Vacancy engineering has been widely used to enhance the hydrogen evolution reaction (HER) performance due to its effective electronic modulation effect. Herein, the molybdenum (Mo) vacancies were introduced into molybdenum phosphide (MoP) towards high-efficiency hydrogen evolution for the first time. Benefiting from the charge redistribution around the Mo vacancies, the exposed P sites can serve as ideal catalytic sites with a free energy of hydrogen adsorption close to zero. Meanwhile, an internal polarization field from terminal Mo atoms to exposed P atoms can offer a more efficient “H delivery” mechanism, achieving fast HER kinetics with a dominant Volmer-Heyrovsky mechanism at a wide range of current density and ultimately exceeding the commercial Pt/C catalyst for high-efficiency hydrogen evolution. More importantly, we first report that the P p-band center can be used as an alternative descriptor to evaluate the HER performance of the electrocatalysts which utilize P atoms as the main active sites. Furthermore, this work also demonstrates that metal vacancies provide an effective tactic to optimize catalytic activity for metal compound-based electrocatalysts.

Effect of Introducing Defects and Doping on Different Properties of Monolayer MoS2

Herein, the comprehensive study of different properties of undoped MoS2, MoS2 lattice with sulfur (S) and, molybdenum (Mo) vacancy, and MoS2 with substitutional doping of niobium (Nb), vanadium (V), and zinc (Zn) atoms is done. The density functional theory (DFT) is used and the electronic properties like density of states, band structure, electron density, and optical properties like dielectric function, optical conductivity, and refractive index are studied. It is observed that undoped MoS2 monolayer shows direct bandgap semiconductor characteristics with a bandgap of around 1.79 eV. P‐type characteristics are observed for Nb‐, V‐, and Zn‐doped MoS2 lattices. The real part and imaginary parts of all optical parameters along x and z directions for different MoS2 supercells are found to be anisotropic in nature up to a photon energy of almost 11 eV and thereafter they show nearly isotropic nature. Finally, it is found that the obtained properties of MoS2 monolayer as per literature are suitable for next‐generation MOSFET application.

Oriented adsorption and efficient sensing of NO2 on MoSe2 monolayer: a comparative study with WSe2 monolayer

Oriented adsorption is very rare on two-dimensional inorganic thin layers. Based on preliminary study, density functional theory (DFT) was used to explore the adsorption and sensing behavior of NO2 on MoSe2 perfect and defective monolayers including selenium vacancy (VSe), molybdenum vacancy (VMo), and antisite defect of selenium on molybdenum site (SeMo) by large-scale sampling. The adsorption energy can reach 0.29 eV with 0.182 e charge transfer on the perfect monolayer. The adsorption ability is improved largely on VSe, VMo, and SeMo monolayers (3.27, 0.93, and 1.20 eV) with considerable charge transfer (0.583, 0.152, and 0.139 e, respectively), demonstrating the potential of MoSe2 monolayers as NO2 sensing materials. Meanwhile, similar to WSe2 monolayer, oriented adsorption of NO2 is disclosed. NO2 molecule extends mainly along the Se–Mo–Se trough and the Se–Mo bond on all the four MoSe2 monolayers. Whereas, the oriented adsorption ability is weaker than the WSe2 monolayer, which can be confirmed by the relatively small proportion of oriented adsorption conformations. This kind of double oriented adsorption behavior (vertical adsorption and adsorption along specific direction of the two-dimensional material) was considered to be due to the good polarity match between molecule and monolayer. Molecular dipole moment and material bond population are believed to be two important parameters that determine the oriented adsorption capacity of molecular-material systems. The successive discovery of the unique oriented adsorption and the deepening of understanding provide chance for molecular design of oriented adsorption of inorganic small molecules on two-dimensional layered materials. The strong dissociation ability of the anionic vacancy monolayer to NO2 molecule indicates that MoSe2 monolayer has a very high adsorption, capture, and activation potential for nitrogen oxides. Comparative study proved that the Mo site activity was less than the W site activity on the perfect monolayer, and defects could help the Mo site activity overtake. These findings provide pivotal guidance for creating advanced surface regulated electronic systems, such as sensors.

Basal Planes Unlocking and Interlayer Engineering Endows Proton Doped-MoO2.8F0.2 with Fast and Stable Magnesium Storage.

Molybdenum trioxide has served as a promising cathode material of rechargeable magnesium batteries (RMBs), because of its rich valence states and high theoretical capacity; yet, it still suffers from sluggish (de)intercalation kinetics and inreversible structure change for highly polarized Mg2+ in the interlayer and intralayer of structure. Herein, F- substitutional and H+ interstitial doping is proposed for α-MoO3 materials (denoted HMoOF) by the intralayer/interlayer engineering strategy to boost the performance of RMBs. F- substitutional doping generates molybdenum vacancies along the Mo-O-□ or Mo-F-□ configurations (where □ represents the cationic vacancy) for unlocking the inactive basal plane of the layered crystal structure, and it further accelerates Mg2+ diffusion along the b-axis. Interstitial-doped H+ can expand interlayer spacing for reducing Mg2+ energy barrier along the ac plane and serve as a "pillar" to stabilize the interlayer structure. Moreover, anion and cation dual doping trigger shallow impurity levels (acceptors levels and donor levels), which helps to easily acquire the electrons from the valence band and donate the electrons to the conduction band. Consequently, the HMoOF electrode exhibits a high reversible capacity (241 mA h g-1 at 0.1 A g-1), an excellent rate capability (137.4 mAh g-1 at 2 A g-1), and a long cycling stability (capacity retention of 98% after 800 cycles at 1 A g-1) in RMBs. This work affords meaningful insights in layered materials for developing high-kinetics and long-life RMBs.

Synergistic effect of diatomite and Bi self-doping Bi2MoO6 on visible light photodegradation of formaldehyde

Good indoor air environment is of great significance for the health and comfort assurance of occupants. In this work, a series of bismuth self-doping Bi 2 MoO 6 /diatomite (BD-X) composites with different proportions of bismuth and molybdenum source were prepared via a mild solvothermal process. Under visible light irradiation, the BD-3 sample exhibited the optimal formaldehyde (HCHO) mineralization performance, and the corresponding calculated rate constant was up to 3.2 times higher than that of bare Bi 2 MoO 6 (B). The introduction of diatomite as catalyst carrier could not only contribute to the dispersion of nano-scale Bi 2 MoO 6 photocatalyst, but also promote the exposure of (001) crystal plane of Bi 2 MoO 6 . In particular, the molybdenum vacancy generated with increasing the proportion of bismuth source, which effectively facilitated the separation of photogenerated carriers. Moreover, the photogenerated holes (h + ) played the major role in the photocatalytic degradation process. Overall, this research would provide a novel binary composite photocatalyst derived from natural mineral for the high-efficient mineralization of formaldehyde under visible light. • An efficient adsorption-photocatalysis system was constructed with diatomite as carrier. • The molybdenum vacancy generated with increasing the proportion of bismuth source. • Diatomite and Bi self-doping Bi 2 MoO 6 promoted the synergistic photodegradation of HCHO. • Excellent photocatalytic mineralization of HCHO was obtained under visible light.

Mo‐Vacancies Defect Engineering of One‐Dimensional Porous Mo2C Nanowires for Enhanced High‐Efficiency Hydrogen Evolution

AbstractThe large‐scale application of efficient water‐splitting greatly promote the development of hydrogen economy which benefit both in alleviating the energy crisis and reaching the goal of carbon neutral. To realize considerable hydrogen evolution, rational design of catalysts with controllable structure and surface composition become crucial. In this work, we proposed an enhanced active site originated from Mo‐vacancies defect in Mo2C crystal which was fabricated by the evaporation of Zn content in well‐designed one‐dimensional ZnMoO4 precursor. Density functional theory calculation and experimental results demonstrated that the formation of molybdenum vacancies in molybdenum carbide promoted the water adsorption and H2 desorption effectively, resulting in an excellent HER reaction dynamics. Moreover, one‐dimensional porous nanowires also ensured rapid mass transfer and contributed strong support for superexcellence HER performance. As expected, V−Mo2C‐900@NF exhibited extremely low overpotential (η10=43 mV) at the current density of 10 mA cm−2 and rapid reaction kinetics (Tafel slope 77.89 mV dec−1).

Visualization of defect induced in-gap states in monolayer MoS2

Atomic-scale intrinsic defects play a key role in controlling functional electronic properties of two-dimensional (2D) materials. Here, we present a low-temperature scanning–tunneling microscopy and spectroscopy investigation of a common point-defect in monolayer molybdenum disulfide (MoS2). We employ a sample preparation method in which the film surface is never exposed to air so that the native dangling bonds surrounding the defects in the film are preserved. Molybdenum vacancies are identified by their three characteristic in-gap resonances by combining scanning–tunneling measurements with parallel Green’s function-based theoretical modeling. The relative energy shifts between the various in-gap states allow us to identify a relative charge difference between two of the observed vacancies. The role of the substrate on the band structure of the defective MoS2 monolayer is unveiled. Our study highlights the effects of the substrate on the in-gap states of common defects found in MoS2 providing a pathway in designing and optimizing 2D materials for electronic applications.

First principles study on the cadmium adsorption behaviour of MoS2 with structural defects and doping

To explore the application of MoS2 in the treatment of cadmium air pollution and Cd-containing wastewater treatment, the energy band structure and density of states of five different systems of monolayer MoS2 (without defects, with sulfur vacancies, with molybdenum vacancies, with manganese doping and with iron doping) were calculated. The contribution of the d orbital of the Mo atom near the band gap was greater than that of the p orbital of the S atom. The defect energy level was mainly due to the Mo d-state in the intrinsic DOS system and the doped atom in the doping system. Then, the adsorption capacity of these five systems for cadmium was evaluated. The adsorption capacity of MoS2 for cadmium was significantly enhanced upon introduction of defects, and Fe-doped MoS2 showed the best Cd adsorption performance among the samples. This study is expected to be helpful for understanding the energy band structure and adsorption properties of MoS2-based materials, which will aid in the exploration of environmental protection problems of MoS2 and the identification of efficient cadmium removal materials.

The First Principles Calculation on the Raman Spectrum and Optical Properties of the Defect Monolayer MoS2

In the process of preparing monolayer MoS2 by chemical vapor deposition, the peak value of Raman spectrum will be different due to the thickness of deposited film. After irradiated by fast neutron, the two modes of E2g and A1g in the Raman spectrum of MoS2 are red shifted and broadened, and a new peak appears near 440–460 cm−1 (Tang. G, Pam. M. E, Zhang. H, et al. Applied Physics Express. 12(5), 056,001 2019). In this paper, two types of defects are constructed and their Raman spectra are calculated by first principles. To compare with the experimental Raman spectra of monolayer MoS2, we confirm the defect configurations in the experiment. Through the calculation of optical properties of monolayer MoS2 with defects, the calculation results show that the imaginary part of the dielectric function of ideal monolayer MoS2 has a high intensity peak near 3.0 eV, and the imaginary part of the dielectric function of the defect monolayer MoS2 also has a peak of the similar shape; due to the localized effect of vacancy defects, the peak value appears a little red shift. A new peak value of the imaginary part of the dielectric function of the monolayer MoS2 with molybdenum vacancy defects appears near 0.6~1.2 eV.

Electron and hole trapping in Li2MoO4 cryogenic scintillator

Abstract The origin and thermal stability of charge trapping centers were studied in Li2MoO4 cryogenic scintillators by correlated electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL) measurements. Up to five electron and three hole trapping centers were detected in the crystals X-ray irradiated at 77 K. The electron traps were ascribed to MoO43− molecular complexes, unperturbed and perturbed by neighboring defects, while the hole traps – to O− lattice ions perturbed by lithium and molybdenum vacancies. The thermal stability of the trapping centers was studied and the depths of electron and hole traps were determined. The TSL peaks observed in the 77–300 K temperature range were related to the depletion of the detected paramagnetic centers. Influence of the revealed centers on the scintillation light yield is discussed.

Defects Engineering Induced Ultrahigh Magnetization in Rare Earth Element Nd‐doped MoS2

AbstractVarious concentrations (0, 0.5, 1, and 5 at%) of Nd are doped into MoS2 single crystals via ion implantation. Experimental results reveal that Nd exists in the form of trivalent state when the doping concentration is below 5 at% and a variety of defects, such as sulfur and molybdenum vacancies, are formed in Nd‐doped MoS2. Compared to pure MoS2 that only shows diamagnetism, Nd doping successfully induces room‐temperature ferromagnetic ordering. Extremely high magnetization (1640 emu cm−3) is observed in 1 at% Nd‐doped MoS2. First‐principles density functional theory calculations suggest that the various structural defects, including substitutions, vacancies, interstitials, antisites, and their complexes, are magnetic possessing large spin moments. The defects coupled with Nd dopants ferromagnetically may form the bound magnetic polarons to induce ferromagnetic ordering. The work has demonstrated that through defects engineering and rare earth element doping, extremely high magnetization materials can be achieved in layered structured materials. On the other hand, though the experiment work is done by implanting MoS2 single crystals, theoretical calculations indicate that 2D MoS2 with bilayers or a few layers can also result in ultrahigh magnetization.

Reconfigurable photo-induced doping of two-dimensional van der Waals semiconductors using different photon energies

Two-dimensional semiconductors have a range of electronic and optical properties that can be used in the development of advanced electronic devices. However, unlike conventional silicon semiconductors, simple doping methods to monolithically assemble n- and p-type channels on a single two-dimensional semiconductor are lacking, which makes the fabrication of integrated circuitry challenging. Here we report the reversible photo-induced doping of few-layer molybdenum ditelluride and tungsten diselenide, where the channel polarity can be reconfigured from n-type to p-type, and vice versa, with laser light at different frequencies. This reconfigurable doping is attributed to selective light–lattice interactions, such as the formation of tellurium self-interstitial defects under ultraviolet illumination and the incorporation of substitutional oxygen in tellurium and molybdenum vacancies under visible illumination. Using this approach, we create a complementary metal–oxide–semiconductor (CMOS) device on a single channel, where the circuit functions can be dynamically reset from a CMOS inverter to a CMOS switch using pulses of different light frequencies. Few-layer molybdenum ditelluride and tungsten diselenide field-effect transistors can be reversibly doped with different carrier types and concentrations using pulses of ultraviolet and visible light, allowing reconfigurable complementary metal–oxide–semiconductor circuits to be created.

Synergetic effect of defects rich MoS2 and Ti3C2 MXene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2

The biggest challenging issue in photocatalytic hydrogen production is to efficiently separate the photoinduced electron-hole pairs and requires the enrichment of photoinduced electrons on the surface of photocatalysts. Herein, TiO2 nanosheets (NSs) are in situ grown on highly conductive Ti3C2 MXene and then MoS2 rich in molybdenum vacancies are uniformly distributed on TiO2@Ti3C2 composite through a two-step hydrothermal method. Thus, a unique structure of MoxS@TiO2@Ti3C2 composite with molybdenum vacancies and double co-catalysts (Ti3C2 and MoS2) is achieved. The photocatalytic H2 production rate of MoxS@TiO2@Ti3C2 composite with an optimized hydrothermal treatment temperature (160 °C) is nearly 193 and 6 times higher than that of pure TiO2 NSs and MoS2@TiO2@Ti3C2 composites (160 °C). A large number of active sites with the enhanced specific activity for H2 generation are induced by the introduction of molybdenum vacancies to MoS2 in MoxS@TiO2@Ti3C2 composites. The presence of molybdenum vacancies can suppress carrier recombination, which is beneficial to the reaction. Besides, the presences of Ti3C2 MXene and MoS2 are found to as double co-catalysts to enhance the electronic conductivity, resulting in an increase in efficiency for electron transfer.

Annealing effects on sulfur vacancies and electronic transport of MoS2 films grown by pulsed-laser deposition

We have synthesized high quality and large area MoS2 films on flexible fluorophlogopite substrates using the pulsed-laser deposition (PLD) technique. Annealing in a sufficient sulfur atmosphere was adopted to eliminate oxide molybdenum and sulfur vacancies introduced during the growth in the vacuum chamber. X-ray photoelectron spectroscopy results demonstrate the advantages benefitted from the annealing process. The S/Mo ratio of the annealed MoS2 film was 1.98:1, which was much closer to the theoretical value. Raman spectroscopy, Photoluminescence spectroscopy, and X-ray diffraction spectroscopy provided direct evidence for the crystallinity improvement. Due to the elimination of molybdenum oxide, the Fermi level was shifted by 0.175 eV, and the conductive type changes from the Ohmic contact to the Schottky contact. The optimized method in this paper makes the PLD-derived MoS2 films promising candidates for microelectronic device application.

Defect engineered MoSSe Janus monolayer as a promising two dimensional material for NO2 and NO gas sensing

Gas sensing mechanism of H 2 S, NH 3 , NO 2 and NO toxic gases on transition metal dichalcogenides based Janus MoSSe monolayers is investigated using the density functional theory. Three types of defects (i) molybdenum vacancy, (ii) selenium vacancy, and (iii) sulfur/selenium vacancy are considered and their formation energy is computed to predict the stability. We noticed that selenium vacancy is the most stable among other defects. The maximum adsorption energy for H 2 S, NH 3 , NO 2 and NO molecules on pristine Janus MoSSe monolayer are ~ −0.156 eV, −0.203 eV, −0.252 eV, and −0.117 eV, respectively. NO 2 gas molecule dissociates and forms oxygen doped NO adsorption in selenium and sulfur/selenium defect included MoSSe Janus monolayer. The adsorption energy values are ~ −3.360 eV and −3.404 eV for Se and S/Se defects included MoSSe layer, respectively. Further, the adsorption of NO 2 molecule induced about 1μ B magnetic moment. In contrast, NO molecule showed chemisorption, whereas H 2 S and NH 3 molecules showed physisorption with their adsorption energies in the range of −0.146 to −0.238 eV and − 0.140 to −0.281 eV, respectively. The adsorption of H 2 S, NH 3 , NO 2 and NO molecule on the pristine and defected monolayers suggest that selenium and sulfur/selenium vacancy defects are more prominent for NO 2 and NO gas molecule adsorption. • The higher adsorption energy of NH 3 and NO 2 in S/Se V defect included monolayer may provide an efficient sensing platform. • Se V and S/Se V defects exhibit chemisorption for NO, whereas dissociation of NO 2 in oxygen and doping with NO adsorption. • NH 3 and H 2 S molecules have physisorption for pristine and defects included monolayers. • The onset of 1μ B magnetic moment is noticed after the adsorption of NO 2 and NO molecules.

The effect of growth conditions and vacancies on the electronic and mechanical properties of cubic Mo2N; A DFT study

In this study, the first-principles method was applied to study the effect of growth conditions and vacancies on the properties of cubic Mo2N. The obtained results show that vacancies play an important role in the control of the electronic and mechanical properties of this material. From all kinds of vacancies considered in this work, the molybdenum vacancy has the lowest formation energy under N-rich conditions while the nitrogen vacancy has the lowest formation energy under N-poor conditions. Moreover, we found that the formation of vacancies decreases the elastic constants, the Young's modulus, the bulk modulus, and the Vickers Hardness (Hv) of cubic Mo2N. The most noticeable change is observed in the case of Mo-deficient material.

Temperature-Controlled Vapor Deposition of Highly Conductive p-Type Reduced Molybdenum Oxides by Hydrogen Reduction.

Reduced molybdenum oxide, generally MoO3- x, becomes increasingly metallic as the oxygen level decreases during the reduction of MoO3 to MoO2. Its interesting properties have recently intrigued research on MoO3- x in electrical and electrochemical areas. Lacking effective tools to control the oxygen level is one of the research difficulties for MoO3- x. Herein, we report facile temperature-controlled synthesis of triangular MoO2 and square Mo4O11. The triangular and square flakes showed metallic behavior with conductivity as high as ∼940 and ∼28 S/cm in DC measurement, respectively. The decrease in oxygen level from Mo4O11 to MoO2 affected the density of states mapped in Mo 4d orbitals, leading to higher conductivity for triangular MoO2. Further Mott-Schottky analysis on MoO3- x regrown on carbon fiber paper (CFP) revealed hole mobility of 105-108 cm2 V-1 s-1. The hole carriers at high frequencies are attributed to potential oxygen acceptors, and molybdenum vacancies resulted from limited reduction power of hydrogen.

Band edge optical properties of defected MoS2 nanotubes

Using time dependent density functional theory (TDDFT) calculations, we investigate the impact of atomic defects on the band edge optical properties of MoS2 nanotubes. Our findings demonstrate the sensitivity of the optical properties of MoS2 nanotube to the details of the defect type. Transition density plot corresponding to the principal peak in the absorption spectrum allowed us to understand the differences in the band edge optical excitation of defected MoS2 nanotubes with respect to the pristine MoS2 nanotube. We found that pristine MoS2 nanotube supports collective plasmon-like excitation and this excitation can be strongly spatially localized depending on the defect type. More precisely, mono-carbon atom defect and/or sulfur vacancy defects favour strong degree of localization near the defected region. While in contrast, in the case of mono-silicon atom defect and/or molybdenum vacancy defects, the collective plasmon-like excitation of the MoS2 nanotube is only marginally affected. The strong degree of localization in combination with the entrance of in-gap states in the case of mono-carbon atom defect and/or sulfur vacancy defects could have wide implications in the field of transition metal dichalcogenides based nanodevices.

Resonance Raman scattering and ab initio calculation of electron energy loss spectra of MoS2 nanosheets

The presence of electron energy loss (EELS) peak is proposed theoretically in molybdenum disulfide (MoS2) nanosheets. Using density functional theory simulations and calculations, one EELS peak is identified in the visible energy range, for MoS2 nanosheets with molybdenum vacancy. Experimentally, four different laser sources are used for the Raman scattering study of MoS2 nanosheets, which show two distinct Raman peaks, one at 385 cm−1 (E2g1) and the other at 408 cm−1 (A1g). In the cases of three laser sources with wavelengths 405 nm (3.06 eV), 632 nm (1.96 eV) and 785 nm (1.58 eV), respectively, the intensity of E2g1 Raman peak is more than the A1g Raman peak, while in the case of excitation source of 532 nm (2.33 eV), the intensity profile is reversed and A1g peak is the most intense. Thus a resonance Raman scattering phenomenon is observed for 532 nm laser source.

Localized defect states in MoS2monolayers: Electronic and optical properties

Defects usually play an important role in tuning and modifying various properties of semiconducting or insulating materials. Therefore we study the impact of point and line defects on the electronic structure and optical properties of MoS2 monolayers using density-functional methods. The different types of defects form electronic states that are spatially localized on the defect. The strongly localized nature is reflected in weak electronic interactions between individual point or line defect and a weak dependence of the defect formation energy on the defect concentration or line defect separation. In the electronic energy spectrum the defect states occur as deep levels in the band gap, as shallow levels very close to the band edges, as well as levels in-between the bulk states. Due to their strongly localized nature, all states of point defects are sharply peaked in energy. Periodic line defects form nearly dispersionless one-dimensional band structures and the related spectral features are also strongly peaked. The electronic structure of the monolayer system is quite robust and it is well preserved for point defect concentrations of up to 6%. The impact of point defects on the optical absorption for concentrations of 1% and below is found to be very small. For higher defect concentrations molybdenum vacancies were found to quench the overall absorption and sulfur defects lead to sharp absorption peaks below the absorption edge of the ideal monolayer. For line defects, we did not find a considerable impact on the absorption spectrum. These results support recent experiments on defective transition metal chalcogenides.