Growth Of Nanocrystallites Research Articles

Enhanced Photocatalysis by Doping Cerium into Mesoporous Titania Thin Films

Mesoporous TiO2 thin films doped with varying amounts of cerium have been synthesized via a supramolecular-templated sol−gel route. The cerium dopant strongly affects the mesostructure, photocatalytic, and optical properties of the mesoporous TiO2 films. At an appropriately low level of cerium doping (i.e., Ce/Ti < 10 mol %), the film showed an increase in the degree of mesoscopic orderliness with an obvious degradation in the nanocrystallinity, which is related to the fact that cerium adversely affects the growth of titania nanocrystallites during thermal annealing. The photocatalytic ability of the film with Ce/Ti = 0.3 mol % was highest for degradation of methylene blue under UV irradiation, which is 3.5-fold of that of the undoped mesoporous TiO2 film. This remarkable enhancement in photoreactivity is attributed to the facilitated transferring of electrons to oxygen in association with cerium cations. The cerium-doped mesoporous TiO2 thin films also demonstrate photoluminescence at room temperature, w...

Synthesis of zinc oxide nanocrystalline powders for cosmetic applications

The synthesis of zinc oxide (ZnO) nanocrystalline powders for cosmetic applications by a coprecipitation process has been investigated. When the Zn(OH) 2 precipitates are calcined at 373 K for 10 min, the crystalline phases comprise the major phase of Zn(OH) 2 and the minor phase of ZnO. XRD pattern shows that only ZnO is present and no other phase is detected when the Zn(OH) 2 precipitates calcined at 413 K for 10 min. The nanocrystallite size of ZnO increases slightly from 32.3 to 44.3 nm when the calcination temperature increases from 413 to 873 K. The activation energy of ZnO nanocrystallite growth is 2.02 kJ/mol, which reveals that the nanocrystalline ZnO is easily grown at low temperature. The UV transmission of ZnO nanocrystallites in the wavelength range from 290 to 375 nm is about 35%, indicating that the ZnO nanocrystallites have an excellent UV-absorbing capability.

Reverse precipitation synthesis and characterization of CeO2 nanopowder

In the present work, CeO2 nanopowder was synthesized via a reverse precipitation method using CeCl3 center dot 7H(2)O, NH4OH and sodium dodecyl sulfate as raw materials. The effect of thermal treatment on the crystal growth, surface area and chemical bonds of the powder was discussed. The structural evolutions and morphological characteristics of the nanopowder were investigated using X-ray diffractometery, transmission electron microscopy, scanning electron microscopy, differential thermal analysis and Fourier transform infrared spectroscopy. The results showed that CeO2 with an average particle size of 45 nm is formed. The BET surface area increased to approximate to 41 m(2)/g, with increasing calcination to 300 degrees C; then, it decreased following calcination at higher temperatures. The particles sintered and agglomerateed together after 500 degrees C. The activation energy for CeO2 nanocrystallite growth during calcination was calculated to be about 14.6 kJ/mol. (C) 2009 Elsevier B.V. All rights reserved.

Polyhedral Oligomeric Silsesquioxane (POSS)-Stabilized Pd Nanoparticles: Factors Governing Crystallite Morphology and Secondary Aggregate Structure

Polyhedral oligomeric silsesquioxane (POSS)-based amine ligands are used as capping agents in the reductive growth of Pd nanocrystallites, and the effects of the specific form of the ligand on the structure of both the secondary aggregate and the primary metal nanoparticle are explored. Secondary aggregates of the ligand capped Pd particles are seen to form a uniform structure only if (a) the POSS ligand is totally functionalized with amine groups and (b) the amine groups are in the hydrochloride form. By casting these results in the context of density functional theory calculations, we show that the morphology of Pd core−shell nanomaterials is determined by the ratio of self-interaction potentials of the ligands to their interaction with solvent. The structures of the primary metal crystallites are also shown to be sensitive to the amine ligand in the hydrochloride form. Our results further show that the POSS versions of a ligand will frequently form precipitating capped metal nanoparticle structures, wh...

Effect of Al addition on the crystallization of a-Al xSi 1−x (0.025 ⩽ x ⩽ 0.100) by electron-beam irradiation

Si crystals and nano-rods were formed in Al-added amorphous Si films (a-Al x Si 1− x ; 0.025 ⩽ x ⩽ 0.100) by the irradiation of a focused electron-beam; the films were in situ heated to be kept at 400 °C and the current density of the electron-beam was 15.7 pA/cm 2. The size, shape, and concentration of the Si crystallites were varied sensitively with the Al content as well as the irradiation time. Under the electron-beam irradiation, crystallization occurred to produce polycrystalline phases in the a-Al 0.025Si 0.975 film, while rod-shaped Si nanostructures were formed in the a-Al 0.050Si 0.950 and a-Al 0.100Si 0.900 film. It is evident that the removal of Al and as a result the atomic rearrangements and/or local restructuring in the Al/a-Si film are critically affected by the electron-beam irradiation, which lead to the local crystallization and growth of Si nanocrystallites.

Fine sructure of Na5Lu9F32 nanocrystallites formed at the initial stages of crystallization

X-ray diffraction investigations have demonstrated that the initial stages of the formation and growth of Na5Lu9F32 nanocrystallites from a powder mixture of lutetium fluoride, ammonium fluoride, and sodium fluoride during the solid-phase synthesis are accompanied by complex structural transformations, among which a cyclic transformation of the type single-phase → two-phase → single-phase state can be distinguished. It is shown that the two-phase state consists of two isomorphous phases with a small misfit of the lattice parameters. The isomorphous phases have common extended interphase boundaries, along which a continuous change of the lattice parameters occurs as one phase transforms into the other. A long-term annealing of the two-phase state leads to the final single-phase state, which is characterized by the growth of nanocrystallites to the scale of microcrystallites.

Nickel Ferrite Nanocrystallites Synthesized by Sol-Gel and Coprecipitation Methods: A Comparative Study

The microstructures and magnetic properties of nickel ferrite synthesized by coprecipitation and sol–gel methods are comparatively studied. The coprecipitation-derived samples have Fe/Ni ratios differing from their raw materials because of the precipitation washing process. The stoichiometric metal cations (Fe/Ni=2.0) in the xerogel facilitated the nucleation and growth of nickel ferrite nanocrystallites at lower calcination temperature in sol–gel method. The samples consist of nickel ferrite nanocrystallites, and have superparamagnetic properties at room temperature.

Coexistence of several structural phases in MOCVD TiO2 layers: evolution from nanometre to micrometre thick films

The morphology and the structure of TiO2 films, grown on Si (1 0 0) substrates by metal organic chemical vapour deposition (MOCVD) was investigated in 5–500 nm thick films. It was shown that the TiO2 layer is mainly amorphous at the first stages of deposition. The growth of nanocrystallites begins inside the amorphous TiO2 layer, and it continues at the expense of the amorphous phase until the crystallized grains occupy the whole layer. Then, the film growth continues with a columnar structure. The coexistence of anatase and rutile phases was evidenced from the beginning of the growth by high resolution transmission electron microscopy and grazing incidence x-ray diffraction. However, the anatase growth overcomes that of rutile, leading to an inhomogeneous phase distribution as a function of the film thickness.

Atomic-Scale Structural Evolution upon Crystallite Nucleation and Growth in Amorphous Fe78B13Si9

The structural evolution upon nucleation and growth of nanocrystals was investigated for Fe78B13Si9 alloy. Upon the onset of α-Fe nanocrystallization, free volumes dominantly surrounded by Fe atoms are developed in the intergranular amorphous phase. Along with the growth of Fe-based nanocrystallites, free volumes in the intergranular amorphous phase enlarge and Fe atoms in the surroundings of the free volumes get partially replaced by B and Si. The results suggest that nanocrystallization is associated with a local structural relaxation in the metastable intergranular amorphous phase.

Chitosan/apatite composite beads prepared by in situ generation of apatite or Si-apatite nanocrystals

The objective of this work was to develop nanocrystalline apatite (Ap) dispersed in a chitosan (CHI) matrix as a material for applications in bone tissue engineering. CHI/Ap composites of different weight ratios (20/80, 50/50 and 80/20) and with CHI of different molecular weights were prepared by a biomimetic stepwise route. Firstly, CaHPO 4·2H 2O (DCPD) crystals were precipitated from Ca(CH 3COO) 2 and NaHPO 4 in the bulk CHI solution, followed by the formation of CHI/DCPD beads by coacervation. The beads were treated with Na 3PO 4/Na 5P 3O 10 solution (pH 12–13) to crosslink the CHI and to hydrolyse the DCPD to nanocrystalline Ap. This new experimental procedure ensured that complete conversion of DCPD into sodium-substituted apatite was achieved without appreciable increases in its crystallinity and particle size. In addition, composites with silicon-doped Ap were prepared by substituting Na 3PO 4 by Na 2SiO 3 in the crosslinking/hydrolysis step. Characterization of the resultant composites by scanning electron microscopy, X-ray powder diffraction (XRD), thermal analysis and Fourier transform infrared spectroscopy confirmed the formation, within the CHI matrix, of nanoparticles of sodium- and carbonate-substituted hydroxyapatite [Ca 10–xNa x(PO 4) 6–x(CO 3) x(OH) 2] with diameters less than 20 nm. Relatively good correspondence was shown between the experimentally determined inorganic content and that expected theoretically. Structural data obtained from its XRD patterns revealed a decrease in both crystal domain size and cell parameters of Ap formed in situ with increasing CHI content. It was found that the molecular weight of CHI and silicate doping both affected the nucleation and growth of apatite nanocrystallites. These effects are discussed in detail.

Controlled synthesis of pure and doped ZnS nanoparticles in weak polyion assemblies:growth characteristics and fluorescence properties

A polyelectrolyte multilayer (PEM) fabricated by the layer-by-layer (LbL) self-assembly ofweak polyions of ployacrylic acid (PAA) and polyallylamine (PAH) was applied as a matrixfor the in situ nucleation and growth of pure and Mn-doped ZnS nanocrystallites. Thenucleation and growth is initiated by the adsorption and binding of the metalions to the ionized carboxylic groups of the weak polyions within the matrix,followed by the subsequent precipitation of semiconductor nanocrystallites withNa2S. Transmission electron microscopy (TEM), atomic force microscopy (AFM) andUV–vis spectroscopy were employed to establish the growth characteristics ofthe spherical ZnS nanocrystallites in the polyion matrix. The conformationalarrangement of polyion chains induced by variation in the assembly pH is thekey parameter that affects the structural and morphological characteristics ofZnS nanocrystallites. Repeating the reaction cycle resulted in an increase in thevolume density of ZnS nanoparticles and further growth of the initially formedparticles by the Ostwald ripening mechanism. The surface passivation of theZnS nanocrystallites within the polyion matrix enables the enhanced radiativeemission of ZnS composite films in the UV range, whereas by doping the ZnS,nanocrystallites show emission characteristic of the manganese ions in the visible region.

Effects of annealing in nitrogen atmosphere and HCl-etching on the photoluminescence spectra of spray-deposited CdS:In thin films

The aim of this work is to find the effect of processing on the photoluminescence (PL) of spray-deposited CdS:In thin films. So the PL spectra of the as-deposited, annealed and HCl-etched CdS:In thin films prepared by the spray pyrolysis (SP) technique were recorded at T = 23 K. The yellow and red bands were observed in the spectrum of the as-deposited film beside bands with weaker intensity in the infrared region. The PL signal was weakened by annealing in nitrogen atmosphere at T = 400 °C and HCl-etching. A deconvolution peak fit was established to find the effects of these treatments on the different bands. The spectrum of the as-deposited film was deconvoluted to 12 peaks, which were reduced to 6 peaks after both treatments. It was found that both treatments have approximately the same effects on the PL spectra; that is they removed most of the peaks and shoulders in the red and infrared regions and attenuated most of the peaks in the yellow region. Some peaks were blue-shifted after annealing which was explained by the growth of nanocrystallites due to the thermal stress that results from the different expansion coefficients of the film and the substrate. These changes were accompanied by a phase change from the mixed (cubic and hexagonal) phase to the hexagonal phase as shown in the X-ray diffractograms.

Tailoring the exchange bias of Ni/NiO nanogranular samples by the structure control

The exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by annealing in H 2, at selected temperatures (200≤ T ann≤300 °C), NiO powder previously milled for 5, 10, 20 and 30 h. Both the as-milled NiO powders and the Ni/NiO samples have been analyzed by X-ray diffraction and the exchange bias properties have been investigated in the 5–200 K temperature range. The structure and the composition of the Ni/NiO samples can be satisfactorily controlled during the synthesis procedure by varying both T ann and the milling time of the precursor NiO powders. In particular, by increasing this last parameter, the mean grain size of the NiO phase reduces down to the final value of 16 nm and the microstrain increases, which is consistent with an enhancement of the structural disorder. The structure of the milled NiO matrix strongly affects the process of nucleation and growth of the Ni nanocrystallites induced by the H 2 treatments, so that, T ann being equal, the amount and the mean grain size D Ni of the Ni phase vary substantially in samples having different milling times. Such features of the Ni phase determine the extent of the Ni/NiO interface and consequently the magnitude of the exchange field H ex: the highest value (∼940 Oe) has been measured at T=5 K in a sample containing ∼7 wt% Ni and with D Ni=19 nm. However, in Ni/NiO samples with very different structural characteristics and different values of H ex at T=5 K, the EB effect vanishes at the same temperature (∼200 K) and the same thermal dependence of H ex is observed. We consider that the evolution of the EB effect with temperature is ultimately determined by the microstructure of the Ni/NiO interface, which cannot be substantially modified by changing the synthesis parameters, milling time and T ann.

Epitaxial 3C-SiC nanocrystal formation at the SiO2/Si interface by combined carbon implantation and annealing in CO atmosphere

High quality 3C-SiC nanocrystallites were epitaxially formed on (100) Si wafers covered by a 150 nm thick SiO2 capping layer after low dose carbon implantation and high temperature annealing in CO atmosphere. Carbon implantation is used to introduce nucleation sites by forming silicon-carbon clusters at the SiO2/Si interface acting as nucleation sites for the growth of 3C-SiC nanocrystallites. The formation of the nucleation clusters as well as the morphology, the size, and the density of the nanocrystals were systematically studied by conventional and high resolution transmission electron microscopy. The nanocrystallites were developed following two different modes of growth: The first develops facets along the ⟨100⟩ crystallographic direction giving tetragonal grains and the second facets along the ⟨110⟩ direction resulting in elongated nanocrystallites. The formation mechanism of the nanocrystallites and the strain related with them are also discussed.

Radiation-induced formation of ZnO nanoparticles on the ZnSe single-crystal surface

The possible formation of ZnO nanocrystals was studied as a result of radiolysis of a ZnSe crystal surface exposed to zinc vapor and irradiated with gamma rays and in producing ZnSe-ZnO heterostructures. Under 60Co gamma radiation in air, nanocrystals ∼27 nm in size are formed from nanoscopic ZnO nuclei. Under a mixed flux of gamma rays and thermal-neutron radiation, a twin structure is formed in the host ZnSe lattice and ZnO is removed. The oxide layer is also destroyed under proton irradiation in vacuum. It is found that the growth of ZnO nanocrystallites causes a manyfold increase in the luminescence intensity in the ∼600-nm band and in microhardness and also a decrease in the resistance and blocking and threshold voltages irrespective of polarity. Thus, gamma irradiation brings about the formation of light-emitting ZnSe-ZnO: Zn semiconductor structures with a p-n junction.

Hydrogen Assisted Nano-crystallization in TiO2 Thin Film Prepared by Hot-Wire Chemical Vapor Deposition

Preparations and structural studies of TiO2 thin films using hot-wire chemical vapor deposition (CVD) (hot-filament CVD) are reported for the first time. Titanium tetra-isopropoxide [Ti(OC3H7)4] was used as a source gas and decomposed on a heated rhenium filament. The film deposited at the filament temperature (Tf) of 1300 °C shows amorphous structure with the substrate temperature (Ts) of 300 °C, and X-ray diffraction (XRD) peaks originated from nano-crystalline with anatase structure appeared over Ts of 400–700 °C. The optical band gap energies of the nano-crystalline TiO2 films with anatase structure were ∼3.4 eV. An increase of Ts from 400 to 700 °C enhanced the XRD peak intensity of (112) orientation. Meanwhile, an increase of Tf up to 1500 °C induces nano-crystalline TiO2 with rutile structure. Furthermore, the hydrogen dilution realizes the nano-crystallite growth of rutile structure even in the deposition at Tf = 1300 °C. During this deposition, the actual substrate surface temperature (Tsuf) was 305 °C. In bulk TiO2 materials, the anatase structure changes to the rutile structure by thermal annealing up to about 800 °C. We propose for the first time that atomic hydrogen contributes to the low temperature nucleation of rutile structure in the deposition of oxide system, TiO2 films.

Effect of seeded substrates on hydrothermally grown ZnO nanorods

We report a study on the effect of seeding on glass substrates with zinc oxide nanocrystallites towards the hydrothermal growth of ZnO nanorods from a zinc nitrate hexahydrate and hexamethylenetetramine solution at 95 °C. The seeding was done with pre-synthesized ZnO nanoparticles in isopropanol with diameters of about 6–7 nm as well as the direct growth of ZnO nanocrystallites on the substrates by the hydrolysis of pre-deposited zinc acetate film. The nanorods grown on ZnO nanoparticle seeds show uniform dimensions throughout the substrate but were not homogenously aligned vertically from the substrate and appeared like nanoflowers with nanorod petals. Nanorods grown from the crystallites formed in situ on the substrates displayed wide variations in dimension depending upon the preheating and annealing conditions. Annealing the seed crystals below 350 °C led to scattered growth directions whereupon preferential orientation of the nanorods perpendicular to the substrates was observed. High surface to volume ratio which is vital for gas sensing applications can be achieved by this simple hydrothermal growth of nanorods and the rod height and rod morphology can be controlled through the growth parameters.

Epitaxial 3C-SiC nanocrystal formation at the SiO2/Si interface after carbon implantation and subsequent annealing in CO atmosphere

3C-SiC nanocrystallites were epitaxially formed on a single crystalline Si surface covered by a 150nm thick SiO2 capping layer after low dose carbon implantation and subsequent high temperature annealing in CO atmosphere. Carbon implantation is used to introduce nucleation sites by forming silicon–carbon clusters at the SiO2/Si interface facilitating the growth of 3C-SiC nanocrystallites.

Strong blue excitonic emission from CdS nanocrystallites prepared by LB technique

CdS nanocrystallites formed in ordered fatty acid LB multilayers exhibited strong surface states emission ∼550 nm and weak excitonic emission ∼400 nm. Treatment with aqueous CdCl2 resulted in the suppression of surface states emission and enhancement of the blue excitonic emission. Subsequent annealing in air at 200°C caused an order of magnitude enhancement of excitonic emission. The growth of nanocrystallites during annealing as seen from the red-shift of excitonic absorption and emission is suppressed by the CdCl2 treatment. The hindered growth of nanocrystallites, the significant enhancement of excitonic emission from CdS, and the suppression of surface states emission are attributed to surface passivation of CdS nanocrystallites by surface oxide formation.

Evolution of Surface Morphology, Crystallite Size, and Texture of WO3 Layers Sputtered onto Si-Supported Nanoporous Alumina Templates

Thin Al films sputtered on n-silicon wafers were anodized in two potentiodynamic steps in tartaric acid at and malonic acid at to prepare nanoporous alumina templates of type A and B, respectively. Processes at the interface were closely examined to avoid field-assisted alumina dissolution during anodizing. semiconductor layers were radio-frequency magnetron sputtered onto the as-anodized and pore-widened templates. It was revealed that the sputtered layers covered the entire surface of type A templates while only partly penetrating into the widened pores of type B templates. The layers crystallized at , with monoclinic phase of symmetry and an average crystallite size of (type A) and (type B). Amorphous alumina in the systems did not crystallize up to . The common trend for the systems was a preferential growth of nanocrystallites in the direction, the texture increasing with further pore widening. Besides, the crystal texture in the layer was systematically higher on type B templates. The link between the structural features and the gas-sensing ability of the systems has been discussed.