Defective MgO Research Articles

Enhanced antibacterial effect with MgO nanoplates: Role of oxygen vacancy and alkalinity

The rational design of MgO nanomaterials with rich oxygen vacancies has garnered significant attention. However, the influence of oxygen vacancies and alkalinity on the generation and stability of active oxygen, particularly regarding the antibacterial activity of MgO nanomaterials, remains uncertain. In this work, the defective MgO nanoplates with adjustable oxygen vacancy and alkalinity were successfully designed by thermal treatment in different atmospheres (air, vacuum, and N2). MgO nanoplates calcined in N2 atmosphere exhibited superior antibacterial activity against Escherichia coli (ATCC 25922), reducing colony counts from 571.5 to 53.5 CFU/mL at 750 μg/mL compared with MgO nanoplates calcined in air. At a low concentration (100 μg/mL), MgO nanoplates calcined in N2 atmosphere exhibited remarkable antibacterial performance, achieving a colony survival ratio of only 6.8 %. This enhanced performance was correlated with a higher oxygen vacancy (OA) content (51.3 %) in MgO nanoplates calcined in N2 compared to those calcined in air (26.5 %). Advanced characterization techniques and alkalinity measurements revealed that calcination in a N2 atmosphere promoted oxygen vacancy formation on the MgO surface, accelerated MgO surface hydrolysis owing to oxygen vacancies, and increased OH− production. Consequently, this process enhances the participation of adsorbed oxygen in surface reaction, leading to the increased generation and stability of active oxygen in an alkaline environment. These findings provide valuable insights into the design of oxygen vacancies and offer a potential approach for constructing highly antibacterial nanomaterials. Such materials could find applications in personal protection and public health.

Terahertz Spin-Conductance Spectroscopy: Probing Coherent and Incoherent Ultrafast Spin Tunneling.

Thin-film stacks | consisting of a ferromagnetic-metal layer and a heavy-metal layer are spintronic model systems. Here, we present a method to measure the ultrabroadband spin conductance across a layer between and at terahertz frequencies, which are the natural frequencies of spin-transport dynamics. We apply our approach to MgO tunneling barriers with thickness d = 0-6 Å. In the time domain, the spin conductance Gs has two components. An instantaneous feature arises from processes like coherent spin tunneling. Remarkably, a longer-lived component is a hallmark of incoherent resonant spin tunneling mediated by MgO defect states, because its relaxation time grows monotonically with d to as much as 270 fs at d = 6.0 Å. Our results are in full agreement with an analytical model. They indicate that terahertz spin-conductance spectroscopy will yield new and relevant insights into ultrafast spin transport in a wide range of spintronic nanostructures.

Crystallinity regulation of ZIF-67 via defective MgO layer made by high voltage-assisted oxidation approach for optimal 4-nitrophenol reduction

Due to their unique structural characteristics, metal-organic frameworks (MOFs) have emerged as favorable contenders for photocatalytic applications. In this study, the crystallization of ZIF-67, a subclass of MOFs, on Mg and MgO substrates via dip coating was explored, and the resulting materials were employed as a photocatalyst for the 4-nitrophenol reduction. Due to the defective surface resulting from plasma activity during the high-voltage-assisted oxidation of AZ31 Mg alloy, MgO significantly impacts the crystallization of ZIF-67. This influence promotes the formation of a stable foundation and strong bonds, contributing to the emergence of a regular and symmetrical structure. This differs from the asymmetrical and irregular structure observed in ZIF-67@Mg when ZIF-67 is grown directly on the Mg substrate. Hence, the ZIF-67@MgO catalyst demonstrated remarkable enhancements in the reduction of 4-nitrophenol to 4-aminophenol, achieving an efficiency of approximately 97.12 % within 12 min under visible light. Importantly, the catalyst exhibited no stability decay even after ten consecutive cycles, surpassing the performance of previously reported catalysts. This research sheds light on the superior capabilities of ZIF-67@MgO in the context of visible-light-driven reduction of 4-NP. The results suggest potential for future progress in creating effective and eco-friendly photocatalytic materials for addressing environmental challenges.

Utilizing a defective MgO layer for engineering multifunctional Co-MOF hybrid materials with tailored leaf-like and polyhedral structures for optimal electrochemical and photocatalytic activities

The hybridization of metal-organic framework (MOF) with inorganic layers would lead to the discovery of novel hybrid materials that can provide a compelling strategy for enhancing its photocatalytic and electrochemical response. In the present study, a highly efficient multifunctional hybrid material was developed by exploiting the defective layer formed on AZ31 Mg alloy through plasma electrolytic oxidation (PEO) as a nucleation and growth site for Co-MOF. The concentrations of the organic linker 2-Methylimidazole (2,MIm) and cobalt nitrate as a source of Co2+ ions were varied to control the growth of the obtained Co-MOF. Lower concentrations of the 2, MIm ligand favored the formation of leaf-like MOF structures through an anisotropic, two-dimensional growth, while higher concentrations led to rapid, isotropic nucleation and the creation of polyhedral Co-MOF structures. The sample characterized by polyhedral Co-MOF structures exhibited superior electrochemical stability, with the lowest corrosion current density (3.11 ​× ​10−9 A/cm2) and the highest top layer resistance (2.34 ​× ​106 ​Ω ​cm2), and demonstrated outstanding photocatalytic efficiency, achieving a remarkable 99.98 ​% degradation of methylene blue, an organic pollutant, in model wastewater. To assess the active adsorption sites of the Co-MOF, density functional theory (DFT) was utilized. This study explores the changes in morphologies of the coatings of Co-MOF with the change of solution concentration to form coatings with enhanced properties on the metallic substrate, which could establish the groundwork for the development of next-generation multifunctional frameworks with diverse applications.

Disentangling photoexcitation and photoluminescence processes in defective MgO

Oxygen vacancies are ubiquitous in oxides and strongly influence the material's electronic structure and catalytic and transport properties. Here we focus on a seemingly simple defective oxide, MgO, and on the electronic properties of oxygen vacancies, which remain controversial in spite of numerous studies. We present an ab initio investigation of the photoexcitation and photoionization process of these defects, using a newly developed embedding Bethe-Salpeter equation approach, implemented in the WEST code. We find absorption and emission energies in good agreement with experiments. Our results provide a detailed, microscopic understanding of the absorption and emission processes of the neutral and positively charged oxygen vacancy, reconciling different views present in the chemistry and condensed-matter physics communities.

SiC Monolayers as Promising Substrates for the Development of Highly Stable Palladium Single Atom Catalysts: A Density Functional Theory Study

Single-atom catalysts have been touted as highly efficient catalysts, but the catalytic single-atom sites are unstable and tend to aggregate into nanoparticles during chemical reactions. In this study, we show that SiC monolayers are promising substrates for the development of highly stable single-atom catalysts (Pd1 /SiC) within the density functional theory. In presence of a Si-vacancy, the diffusion barrier energy of a Pd1 atom embedded in the SiC monolayer is substantially enhanced from 2.3 to 7.8 eV, which is much higher than the reported diffusion barrier energies of graphene, boron nitride and defective MgO of the same catalytic system. Ab initio molecular dynamic calculations at 500 K also confirm the enhanced stability of Pd1 /SiC monolayer (Si-vacancy) such that the Pd1 atom remains embedded in the vacancy. Additionally, the Pd1 /SiC monolayer (Si-vacancy) catalysts show a ∼34 % reduction of activation barrier energy for CO oxidation as compared to pristine catalysts. This work implies that nanostructured SiC materials are promising substrates for the synthesis of highly stable single-atom catalysts.

Scalable and tunable Y2O3‐MgO composite for infrared transparency applications

AbstractThe process‐structure‐property correlationships in yttria‐magnesia (YM) composite have been investigated. YM composite was synthesized using commercial powders via ball‐milling route with three different grinding balls (Si3N4, Al2O3, ZrO2) having two different sizes (2 and 5 mm diameter). The alteration in grinding ball material and size produces sintered ceramic having different grain sizes (420–560 nm) and degree of phase mixing homogeneity (0.40–0.70). The contamination induced by the milling ball resulted in changes in Y2O3 and MgO defect chemistry, which influenced the grain growth behavior in the YM composite. The hot‐pressed composite prepared using 2‐mm Si3N4 ball‐milled powders exhibited the finest grain size (420 nm) and better phase mixing homogeneity (0.63). The subsequent impact was seen on transmittance efficiency (71%) over the 3–7‐μm wavelength range, which is ∼85% of the theoretical limit. The findings show that the selection of the right size and type of grinding ball for milling commercial powder is a simple and cost‐effective way for scalable production of YM composite with high transmittance efficiency for infrared windows and dome applications.

Exploring anchoring performance of defective MgO nanotubes for lithium–sulphur batteries: A density functional theory (DFT) study

AbstractIn recent years, lithium–sulphur batteries have received great attention due to their high theoretical specific capacity. In the exploration of improving battery performance, developing suitable anchoring materials is one of the ways to suppress the shuttle effect which is one of the main problems of lithium–sulphur batteries. In this work, we investigated the anchoring ability of MgO nanotubes (MgONT) and defective MgO nanotubes (MgONTv) to lithium polysulphides (LiPSs) by density functional theory (DFT). The defect formation energy, the HOMO and LUMO of the defective MgONT, the energy difference (ΔE), and the adsorption energy were calculated. The optimized structures of LiPSs adsorbed on the MgONT and MgONTv were also obtained. The calculation results show that MgONTv1 has a strong adsorption effect on LiPSs, and its adsorption energy ranges from −1.78 to −4.51 eV. The adsorption of LiPSs narrows the bandgap of MgONTv. In other words, the conductivity of MgONTv is better than the pristine one. Our study demonstrates that MgONTv has more robust adsorption performance for LiPSs, which is an effective addictive material for the cathode of lithium–sulphur batteries. It can provide a theoretical basis for exploring the application of new one‐dimensional anchoring materials in the cathode of lithium–sulphur batteries.

Adsorption of SO2 on pristine and defective single-walled MgO nanotubes: a dispersion-corrected density-functional theory (DFT-D) study

The dispersion-corrected density functional theory (DFT-D) has been used to study the interactions of SO2 on the pristine and defective single-walled MgO nanotubes (MgONTs). A geometric optimization was performed for MgONTs and adsorbents/MgONTs. The adsorption energies of SO2 molecules at different layers of MgONTs were considered. The adsorption property of SO2 was analyzed in terms of the adsorption energy, the electron donation (basicity), and the atomic charges on the adsorbed materials. The densities of states (DOS) have been calculated and used for examining the adsorption properties. In addition, the presence of vacancy defects increases the adsorption energies of the SO2molecules. The band gaps of the defective MgONTs are sensitive to SO2 molecules.

O2 activation by AuAg clusters on a defective (100)MgO surface

In the present work, we discuss the electronic properties of supported dispersed bimetallic clusters with respect to their size, geometry, and Aun/Agm (n + m = 6) composition. We have studied with supercell-density functional theory calculations the role of the charge transfer from the MgO defective support toward the cluster in the activation of O2 by AunAgm clusters. We first considered gas-phase clusters with different atomic compositions; then, we deposited all of them on a pristine (100)MgO surface and finally on a more realistic (100)MgO F-center. We performed a global and unrestricted search of the (cluster + surface) geometry. The Mexican enhanced genetic algorithm has been used to exhaustively explore the potential energy surface. Our results show that O2 activation depends on the Aun/Agm ratio. It has been found that both metals involved play different and important roles toward (a) the actual O2 dissociation and (b) weakening of the oxygen-cluster bond, which, in turn, may promote the possibility of a catalytic process to take place, such as the oxidation process of CO and NOx among others.

Catalytic conversion of CO2 to propylene carbonate over Pt-decorated Mg-substituted metal organic framework

Porous coordination polymers of Mg (II) dihyroxyterephtalate (Mg-MOF-74) at various Ptx loadings (x = 0–10 wt%) were synthesized, characterized and tested for the catalytic cycloaddition reaction between CO2 and propylene oxide (PO). The reaction was studied over a range of temperatures between (100–170 °C), pressures (9.1–19.5 bar) and reaction times (4–15 hours) to produce propylene carbonate (PC). The results show that the Pt loading on the catalyst surface improves the selectivity toward PC in the presence of dimethylformamide (DMF) and dichloromethane (DCM) which serve as solvents and promoters for the reaction. Pt loading is shown to be concomitantly related to the number of uncoordinated MgO defect sites produced during synthesis. As Pt loading increases, the number of uncoordinated MgO sites in the Mg-MOF-74 framework structure also increases. During reaction, PO conversion and PC selectivity were shown to systematically increase with Pt loading and the number of uncoordinated MgO defect sites. A synergistic effect is observed where the combined use of Ptx/Mg-MOF-74, DMF and DCM exhibit performance benefits beyond their use individually. The catalysts we characterized before and after reaction using XRD, EDX, ICP, BET, SEM and TGA. The catalysts show a relatively high stability and reusability as confirmed by reaction studies, XRD and FE-SEM.

Surface Strain Effects on the Adsorption of Au Adatoms on MgO(001) Surfaces with Surface O Vacancies

By employing ab-initio total-energy and electronic-structure calculations based on the density-functional theory, we studied the effects of surface strain es on the adsorption properties of a Au adatom on defective MgO(001) surfaces with surface oxygen vacancies (Fs centers). The formation energy of the Fs center on MgO(001) varied very slightly in the region of es from −6% to −4% and monotonically decreased with the increase in es, from −4% to +6%. As es increased, the adsorption energy ( $$E_{ads}^{{F_S}}$$ ) of Au on the es center of strained MgO(001) monotonically decreased and, in particular, showed a much larger decrease in $$E_{ads}^{{F_S}}$$ for a tensile surface strain of es > +4%. The surface strain dependence on the physical properties, such as the charge states, the spatial charge rearrangement, for Au on the Fs center of strained MgO(001) surfaces was also analyzed. These results provide important physical information on the effects of surface strain on the adsorption of Au on MgO(001) surfaces with Fs centers.

Effect of surface vacancies on the adsorption of Pd and Pb on MgO(100)

Theoretical quantum mechanical calculations have been carried out to establish the effect of surface vacancies on the adsorption of Pd and Pb atoms on the defective MgO(100) surface. The investigated defects included neutral, singly and doubly charged O and Mg vacancies on the (100) surface of MgO. These vacancies played the role of Fsn+ and Vsn− (n = 0, 1, 2) adsorption centers for a single Pd or Pb atom. From the results of calculations, it is clear that the Pd- and Pb-atom adsorption at the Fsn+ and Vsn− centers shows different characteristics than at the regular O2− and Mg2+ centers. Drastic changes in geometric, energetic, and electronic parameters are evident in Pd/Vsn− and Pb/Vsn−. The effect of Fs0 and Fs+, which in practice are the most important vacancies, is smaller, yet the adsorption of Pd and Pb at these centers is more energetically favorable than at the regular O2− center. Of the two metals studied, the atom of Pd is bound by the Fs0 and Fs+ centers with higher adsorption energies.Graphical abstract

Insight into the influence of addition of a second metal Fe and supports with different morphology on H2 dissociation over Ni/MgO catalysts

The effect of addition of a second metal to activation component and the support with different morphology on H2 dissociation on Ni/MgO catalyst was investigated using density functional theory. The results show that after addition of a second metal Fe, the metal-support interaction is increased in perfect MgO supported Ni catalyst while it is decreased in defective MgO supported Ni catalyst. Although the change is difference between metal-support interaction, the ability of H2 dissociation is increased on the two surfaces. Analyzing those results from addition of Fe, Co and Cu to Ni/MgO, one can conclude that it is neither too strong nor too weak (i.e., medium) interaction between metal and support that favors to improve the catalyst performance.

Insight into carbon deposition associated with NiCo/MgO catalyzed CH4/CO2 reforming by using density functional theory

A density-functional theory method has been conducted to investigate carbon deposition associated with NiCo/MgO catalyzed CH4/CO2 reforming. In the process of carbon deposition formation, CH4 dissociation to produce C, further C accumulates to form carbon deposition. In addition, C and its precursor can be eliminated by oxygen species from CO2 dissociation. Based on above analysis about carbon deposition, three possible ways are considered to contribute to resist carbon deposition (1) reducing CH4 dissociation to decrease C produced; if not, (2) accelerating CHx(x=0∼3) oxidation to eliminate C, (3) blocking C accumulation, further suppressing carbon deposition, that is, the rate of CHx oxidation is more than that of C accumulation. The calculation results show that the dissociation of CH4 has almost equal activation energy on perfect and defective NiCo/MgO, and they are lower than those on Ni/MgO and NiCo(111), indicating that it is impossible to reduce C produce through reducing CH4 dissociation when addition a second metal Co to Ni/MgO or using different support MgO; CH oxidation is favorable to produce CHO compared to CH dissociation into C and H, further it is favorable for CHO dissociation into CO and H on perfect and defective NiCo/MgO catalyst. In addition, C oxidation is easier on defective NiCo/MgO than that on perfect NiCo/MgO, meanwhile it is easier on NiCo/MgO than that on NiCo(111), indicating that using defective MgO support accelerates C oxidation, further elimination carbon deposition. Carbon accumulation is easier on perfect NiCo/MgO than that on defective NiCo/MgO. The results indicate that the defective MgO surface support NiCo bimetal catalyst is more resistant to carbon deposition than perfect one. One can modify the Ni-based catalyst behavior via addition a second metal Co or using MgO support with oxygen-vacancy. This might be given a clue for experimental catalyst to develop Ni-based catalyst with the property of resisting carbon deposition.

Oxide Growth Mechanism on Mg AZ91 Alloy by Anodizing: Combination of Electrochemical and Ellipsometric In-Situ Measurements

The sparking initiation during the anodizing of metals in passivating electrolytic media is highly dependent on the physico-chemical properties of the evolving dielectric interface. The present paper focuses on magnesium alloy AZ91 and describes its anodizing in KOH 3 M over the potential range 0 to 40 V by combining in situ ellipsometric and electrochemical methods. In a first step, a thin and compact MgO anodic film is formed by ionic migration with a rate of 0.4 nm V−1. At 4 V to 5 V, the growth stresses lead to a sharp cracking of this MgO layer and the precipitation of a porous external Mg(OH)2 layer. Under this overlayer acting as a membrane, a defective MgO film grows with a higher growth rate (1.1 nm V−1), inducing a new passivation state. This inner MgO layer is characterized by a relatively high dielectric constant and a large thickness (100 nm at 40 V) and is probably responsible for the low dielectric breakdown voltage observed during the anodizing process of magnesium alloy.

A DFT study of (WO3)3nanoclusters adsorption on defective MgO ultrathin films on Ag(001)

The structures and electronic properties of (WO3)3nanocluster adsorption on defective MgO ultrathin films supported on Ag(001) metal surfaces have been investigated by means of density functional theory (DFT) calculations including dispersion interactions.

Tuning the charge states of CrW2O9 clusters deposited on perfect and defective MgO(001) surfaces with different color centers: A comprehensive DFT study

The structures and electronic properties of bimetallic oxide CrW2O9 clusters supported on the perfect and defective MgO(001) surfaces with three different color centers, FS (0), FS (+), and FS (2+) centers, respectively, have been investigated by density functional theory calculations. Our results show that the configurations, adsorption energies, charge transfers, and bonding modes of dispersed CrW2O9 clusters are sensitive to the charge states of the FS centers. Compared with the gas-phase configuration, the CrW2O9 clusters supported on the defective surfaces are distorted dramatically, which exhibit different chain structures. On the perfect MgO surface, the depositions of clusters do not involve obvious charge transfer, while the situation is quite different on the defective MgO(001) surfaces in which significant electron transfer occurs from the surface to the cluster. Interestingly, this effect becomes more remarkable for electron-rich oxygen vacancies (FS (0) center) than that for electron-poor oxygen vacancies (FS (+) and FS (2+) centers). Furthermore, our work reveals a progressive Brønsted acid sites where spin density preferentially localized around the Cr atoms not the W atoms for all kinds of FS-centers, indicating the better catalytic activities can be expected for CrW2O9 cluster on defective MgO(001) surfaces with respect to the W3O9 cluster.

Effect of Surface Site on the Spin State for the Interaction of NO with Pd<sub>2</sub>, Rh<sub>2</sub> and PdRh Nanoparticles Supported at Regular and Defective MgO(001) Surfaces

An attempt has been made to analyze the effect of surface site on the spin state for the interaction of NO with Pd2, Rh2 and PdRh nanoparticles that supported at regular and defective MgO(001) surfaces. The adsorption properties of NO on homonuclear, Pd2, Rh2, and heteronuclear transition metal dimers, PdRh, that deposited on MgO(001) surface have been studied by means of hybrid density functional theory calculations and embedded cluster model. The most stable NO chemisorption geometry is in a bridge position on Pd2 and a top configuration of Rh2 and PdRh with N-down oriented. NO prefers binding to Rh site when both Rh and Pd atoms co-exist in the PdRh. The natural bond orbital analysis (NBO) reveals that the electronic structure of the adsorbed metal represents a qualitative change with respect to that of the free metal. The adsorption properties of NO have been analyzed with reference to the NBO, charge transfer, band gaps, pairwise and non-pairwise additivity. The binding of NO precursor is dominated by the E(i)Mx-NO pairwise additive components and the role of the support was not restricted to supporting the metal. The adsorbed dimers on the MgO surface lose most of the metal-metal interaction due to the relatively strong bond with the substrate. Spin polarized calculations were performed and the results concern the systems in their more stable spin states. Spin quenching occurs for Rh atom, Pd2, Rh2 and PdRh complexes at the terrace and defective surfaces. The adsorption energies of the low spin states of spin quenched complexes are always greater than those of the high spin states. The metal-support and dimer-support interactions stabilize the low spin states of the adsorbed metals with respect to the isolated metals and dimers. Although the interaction of Pd, Rh, Pd2, Rh2 and PdRh particles with Fs sites is much stronger than the regular sites O2-, the adsorption of NO is stronger when the particular dimers are supported on an anionic site than on an Fs site of the MgO(001). The encountered variations in magnetic properties of the adsorbed species at MgO(001) surface are correlated with the energy gaps of the frontier orbitals. The results show that the spin state of adsorbed metal atoms on oxide supports and the role of precursor molecules on the magnetic and binding properties of complexes need to be explicitly taken into account.

Structural and textural characteristics of systems based on layered Mg-Al hydroxides

Hydrotalcite-type layered Mg-Al hydroxides with Mg/Al ≥ 2 and related oxide systems have been synthesized and characterized. During the synthesis of the Mg-Al hydroxides with Mg/Al = 2 and 3, the interplanar spacings decrease because of the replacement of the interlayer NO 3 − anion by the CO 3 2− anion. Morphologically, the initial materials are aggregates of ∼1-μm oblong planar particles with a layered structure. The transverse dimension of the plates is 50–100 nm, and their thickness is 10–30 nm. As a rule, these planar particles are heavily distorted. Heat treatment of the Mg-Al hydroxides in the temperature range from 450 to 600°C causes the separation of the layered structures into planar domains of epitaxial MgAl2O4/MgO structures, which survive heat treatment at 900°C. During this heat treatment at 900°C, the greater part of the sample breaks down into two-dimensional particles of defective MgO and MgAl2O4 phases whose size is up to 100 nm. The oxide compositions synthesized from the layered Mg-Al hydroxides have a large pore volume and a fairly high specific surface area, which are retained upon heat treatment at 1000–1100°C.