MgO Nanostructures Research Articles

Terahertz Dielectric Properties of MgO Nanocrystals

Terahertz properties of chemically synthesized MgO nanocrystals are studied by terahertz time-domain spectroscopy in the frequency range extending up to 3.7 THz. The experimentally determined optical and dielectric properties of the nanocrystalline MgO are compared with those of a bulk MgO single crystal. Three prominent resonance modes, which are essentially responsible for the dielectric properties of the nanostructured MgO in the terahertz regime, are characterized at ω1/(2π) = 4.26 THz, ω2/(2π) = 7.68 THz, and ω3/(2π) = 12.18 THz and are well-described by a multiple-oscillator model through theoretical fitting. The MgO nanocrystals may find intriguing applications in developing terahertz quasi-optic components and terahertz spectroscopy.

Electronic energy transfer on CaO surfaces

We excite low-coordinated surface sites of nanostructured CaO samples using tunable UV laser pulses and observe hyperthermal O-atom emission indicative of an electronic excited-state desorption mechanism. The O-atom yield increases dramatically with photon energy, between 3.75 and 5.4 eV, below the bulk absorption threshold. The peak of the kinetic energy distribution does not increase with photon energy in the range from 3.9 to 5.15 eV. These results are analyzed in the context of a laser desorption model developed previously for nanostructured MgO samples. The data are consistent with desorption induced by exciton localization at corner-hole trapped surface sites following electronic energy transfer from higher coordinated surface sites.

Synthesis of Micrometer-Sized Nanostructured Magnesium Oxide and Its High Catalytic Activity in the Claisen−Schmidt Condensation Reaction.

Micrometer-sized nanostructured flowerlike MgO nanostructures were synthesized by an ethylene-glycol-mediated self-assembly process. The synthesized samples were studied systematically by X-ray powder diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, Fourier-transform IR, and energy-dispersive X-ray analysis. The results show that the flowerlike MgO have uniform morphologies with an average diameter of 600 nm for micrometer-sized spheres and 16 nm for the ribbonlike nano building blocks. Compared to hierarchical structured cobalt oxide, a new morphology evolution process is observed for the MgO precursor, indicating a new kinetic control mechanism for morphology evolution. Because of its micrometer-sized nanostructure, the as-obtained MgO shows high catalytic activity, high stability, and easy recovery for the Claisen−Schmidt condensation between benzaldehyde and acetophenone.

Oxidation of Mg/Ag(1 1 1) investigated using scanning tunneling microscopy: Towards atomically smooth MgO nanostructures

The oxidation of ultra-thin Mg layers deposited on a clean Ag(1 1 1) surface was investigated at room temperature by means of scanning tunneling microscopy (STM). Long-range organized MgO surfaces could be obtained, all of them showing atomically smooth one-dimensional lines with elementary dots inside. These dotted lines with an apparent width of 1 or 2.5 nm most often run parallel and rotated by about 30° with respect to a Ag close-packed axis. The three-fold symmetry in the dot arrangement and the measured apparent heights are consistent with polar-orientated MgO(1 1 1) layers. Two dot periodicities are observed, whose origin is attributed to coincidence lattices between silver and a 30° rotated hexagonal MgO layer.

Synthesis of Micrometer-Sized Nanostructured Magnesium Oxide and Its High Catalytic Activity in the Claisen−Schmidt Condensation Reaction

Micrometer-sized nanostructured flowerlike MgO nanostructures were synthesized by an ethylene-glycol-mediated self-assembly process. The synthesized samples were studied systematically by X-ray powder diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, Fourier-transform IR, and energy-dispersive X-ray analysis. The results show that the flowerlike MgO have uniform morphologies with an average diameter of 600 nm for micrometer-sized spheres and 16 nm for the ribbonlike nano building blocks. Compared to hierarchical structured cobalt oxide, a new morphology evolution process is observed for the MgO precursor, indicating a new kinetic control mechanism for morphology evolution. Because of its micrometer-sized nanostructure, the as-obtained MgO shows high catalytic activity, high stability, and easy recovery for the Claisen−Schmidt condensation between benzaldehyde and acetophenone.

Synthesis and Growth Discussion of One-Dimensional MgO Nanostructures: Nanowires, Nanobelts, and Nanotubes in VLS Mechanism

Three kinds of one-dimensional (1D) MgO nanostructures: nanowires, nanobelts, and nanotubes were synthesized through introducing the low-melting point mental Sn as catalyst in the chemical vapor deposition route. The morphology and crystalline structure were characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The detailed growth processes of these structures were discussed. A conventional VLS mechanism, explicated VLS mechanism, and VLS mechanism combined with second evaporation-oxidation process were proposed to explain the growth of nanowire, nanobelt, and nanotube, respectively. The establishment of growth models on nanobelt and nanotube gave a new annotation of VLS mechanism different with previous reports, and the growth analyses are helpful to comprehensively understand VLS mechanism. These results have important referenced significance in fabrication of various 1D nanostructures. Furthermore, the obtained nanostructures may find var...

MgO nanobelts using a reactive and auto-removed ZnO nanobelt template

A newly developed chemical conversion route has been used to manufacture MgO nanobelts by using prefabricated ZnO nanobelts as a template. The converted MgO nanobelts are composed of small nanocrystals with an average diameter of 12 nm. Room-temperature photoluminescence measurement shows two emission bands centered at 392 and 478 nm which are attributed to various structural defects with different densities in the MgO nanobelts. Our approach here is effective and facile without additional etching agents being introduced. Other novel MgO nanostructures may be further fabricated by this method, depending on the morphologies of ZnO nanostructure templates.

Energy and site selectivity in O-atom photodesorption from nanostructured MgO

We excite low-coordinated surface sites of nanostructured MgO samples using 4.7 eV UV laser pulses and observe dominant hyperthermal O-atom emission. Excitation of the same samples with 7.9 eV photons results in thermal O-atom desorption. These results are analyzed in the context of laser desorption models developed previously for alkali halide crystals and MgO. We detail two multi-step mechanisms for hyperthermal O-atom desorption, under surface selective excitation, based on exciton and hole trapping at three-coordinated (corner) O-atom sites, and evaluate the validity of each based on available experimental data and calculated results. The proposed models are significantly different from and more complex than the surface exciton desorption model established for alkali halides. Nonetheless, the principles of site-specific photo-excitation and exciton and hole localization induced atomic desorption are clearly extendable to a prototypical metal oxide.

Fabrication and structural characterization of Mg2SiO4 nanowires

This article demonstrates the first reported successful synthesis of Mg2SiO4 nanowires. We have thermally heated Au-coated Si substrates, using a quartz tube with its inner surface pre-coated with MgO nanostructures. We have characterized the sample morphologies by using scanning electron microscopy and transmission electron microscopy (TEM). X-ray diffraction analysis and high-resolution TEM observation coincidentally revealed that the nanowires were crystalline with an orthorhombic Mg2SiO4 structure. We have discussed the possible growth mechanism of Mg2SiO4 nanowires.

Tuning the architecture of MgO nanostructures by chemical vapour transport andcondensation

In this paper, we demonstrate the potential of chemical vapour transport and condensationin tuning the architecture of a series of MgO nanostructures through adjustingexperimental parameters. Different oxygen partial pressures lead to correspondingwell-faceted nanostructures, such as nanowires, cubes, orthogonally branched structuresand ‘nanotrees’, while nanotubes and ‘nanoflowers’ were formed through multi-stepprocesses under modified experimental details. The synthesis roadmap may promotethe in-depth understanding of the growth kinetics of nanostructures in vapourroutes, and these intriguing nano-architectures may find various applications innanotechnology.

Site-specific laser modification of MgO nanoclusters: Towards atomic-scale surface structuring

Direct neutral metal emission from MgO nanostructures is induced using laser light tuned to excite specific surface sites at energies well below the excitation threshold of the bulk material. We find that near UV excitation of MgO nanocrystalline films and nanocube samples desorbs neutral Mg atoms with hyperthermal kinetic energies in the range of 0.1-0.4 eV. Our ab initio calculations suggest that metal atom emission is induced predominantly by electron trapping at surface three-coordinated Mg sites followed by electronic excitation at these sites. The proposed general mechanism can be used to control atomic-scale modification of insulating surfaces.

Zn-catalytic growth and photoluminescence properties of branched MgO nanostructures

Abstract Mass production of uniform MgO nanostructures has been achieved by a thermal evaporation method. X-ray diffraction (XRD) analyses show the product is composed of pure single-crystalline MgO. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations show that the MgO branched nanostructures consist of many slim nanowires growing from the thick MgO rods. The as-synthesized nanowires have a length of several tens of microns and a diameter of several tens of nanometers. The preferred growth direction of the nanowires is [001]. Many nanowires are found to have a dendritic structure and temperature grade is thought to be the main cause of the growth of this structure. Zn nanoparticles scattered on the surface of the MgO rods are thought to be the catalyst of the VLS (vapor-liquid-solid) growth of the MgO nanowires. Room-temperature photoluminescence measurements show that the synthesized MgO nanostructures have a strong emission band at 401 nm and a weak emission band at 502 nm.

MgO nanostructures synthesized by thermal evaporation

MgO nanostructures with various morphologies were fabricated by thermal evaporation through controlling the growth temperature and gas flow, including MgO polyhedral shells, nanotubes, cubes and nanowires. MgO polyhedral shells and nanotubes were fabricated when H 2 was introduced to the system in the beginning of the thermal process, MgO cubes and nanowires were fabricated when Ar/O 2 was introduced. MgO polyhedral shells and MgO cubes were collected in the lower temperature zone, while MgO nanotubes and nanowires were collected in the relative higher temperature zone. The growth mechanism was proposed based on the crystal structure and analysis of growth conditions.

Kinetics-Driven Growth of Orthogonally Branched Single-Crystalline Magnesium Oxide Nanostructures

Orthogonally branched single-crystalline magnesium oxide nanostructures were synthesized through a simple chemical vapor transport and condensation process in a flowing Ar/O(2) atmosphere. Other morphologies, such as cubes and nanowires, can also be obtained under different controlled conditions. The formation of different types of nanostructures can be tuned by modifying oxygen partial pressure during the synthesis. All the nanostructures are cubic single-crystalline enclosed by low-index {100} facets. Growth mechanisms for the nanostructures are discussed in detail: different supersaturation ratios, relatively high substrate temperatures, and surface defects in certain crystallographic planes cooperatively take important effects on determining the product morphologies. Structural defect-related blue light emission of the three types of MgO nanostructures was investigated. The MgO nanostructures with abundant morphologies may find applications in various nanodevices, and the kinetics-driven methodology might be exploited to synthesize similar nanostructures of other functional oxide materials.

Growth Mechanism and Ultraviolet-Light Emission of the Self-Assembled Complex of MgO Nanostructures

Bulk-quantity net-like nanodendrites and four-fold hierarchical nanostructures were synthesized by direct thermal evaporation and oxidation of metallic Mg powder. Their formation mechanism is explained using the self-catalytic vapor-liquid-solid mechanism together with dendritic-crystal epitaxial growth mechanism. Four-branch and eight-branch nanodendrites were also detected. The photoluminescence spectrum reveals that the peak with the maximum intensity is centered at about 3.16 eV (392 nm). Through Gaussian fitting, a strong and narrow ultraviolet-light emission peak centered at 3.16 eV (392 nm) and a relatively weak but broad blue-light emission band centered at 2.74 eV (453 nm) were observed in the photoluminescence emission spectrum, which are respectively attributed to the recombination luminescence of the F+ and F centers (belonging to oxygen-vacancy related defect levels) in the MgO nanostructures. In addition, another very weak and broad red-infrared emission band can also be detected, which is probably due to the relaxation luminescence of impurity levels in the MgO nanostructures.

Direct Observation of the Growth Process of MgO Nanoflowers by a Simple Chemical Route

Novel flowerlike nanostructures consisting of MgO nanofibers were successfully synthesized by a simple chemical route with H(2)O at 950 degrees C in an Ar atmosphere. Various durations of heating gave different growth stages that led to varied product morphologies. The synthesized products were systematically studied by X-ray powder diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and energy-dispersive X-ray analysis. The results show that the nucleation and growth process of the nanoflowers seems to be a vapor-solid mechanism, and that the total heating time during the reaction process is a critical factor for the development of MgO nanoflowers. Initially, Mg particles formed on the Si substrate, followed by the formation of MgO clusters as nucleation centers on the magnesium melt surface and the nucleation of short MgO nanofibers, then growth of the MgO nanofibers occurred, and finally MgO nanoflowers were formed. Besides nanoflowers, novel hierarchical MgO nanostructures were also observed. These nanostructures may be used as three-dimensional composite materials and as supports for other materials.

Growth and morphologies of one- and three-dimensional MgO nanostructures

A variety of MgO nanostructures have been fabricated by a direct gas reaction route using Mg as starting material. The evaporation of Mg combined with a mixed oxygen/argon gas provided the constituents for the MgO nanostructure growth. They were characterized by scanning electron microscopy (SEM), selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). SEM observations revealed several MgO morphologies including one-dimensional nanowires, three-dimensional cube-shaped entities and rope-like nanowires. HRTEM and SAED results indicate that they were single-crystalline face-centered cubic MgO.

Bulk synthesis of single-crystalline magnesium oxide nanotubes.

Crystalline tubular magnesium oxide nanostructures were obtained through carbon-thermal evaporation of a MgO powder. Gallium oxide was added into the mixture of MgO and carbon. The reduction of gallium oxide by carbon resulted in gallium vapor at high temperatures. Condensed gallium droplets catalyzed the anisotropic growth of tubular MgO nanostructures in situ. The products were characterized by X-ray powder diffraction technique, scanning electron microscopy, and high-resolution microscopy. All the analyses indicated that the prepared tubular MgO nanostructures are, in fact, single crystals.

Preparation and characterization of networked rectangular MgO nanostructures

Networked rectangular MgO nanostructures have been prepared through direct reaction of Mg with flowing oxygen. The nanostructures consist of the branches that have square cross sections and grow in a right-angled zigzag manner to elongate in three dimensions, which form the networked rectangular nanostructures. The edge dimension of their cross-sections is in the range of 50–150 nm. A growth mechanism based on the vapor–solid process is proposed for the formation of the networked nanostructures.

Uniform MgO nanobelts formed from in situ Mg 3N 2 precursor

MgO nanobelts with a pure morphology and high yield were generated by the direct evaporation of Mg metals under initial N 2 gas at 650 °C and subsequent N 2/O 2 atmosphere at 800 °C. The growth of these MgO nanobelts is explained in a VS route in which the in situ formed Mg 3N 2 slowly decomposes and reacts with oxygen. Mg 3N 2 is therefore a very effective precursor for the fabrication of high-purity one-dimensional (1D) MgO nanostructures.