The presence of highly dispersed metal particles on solid supports with well-defined microstructure is important in the field of functional materials, active catalysts as well as bionanomaterials for medical applications. Noble metal nanostructures, in particular silver, palladium, and platinum nanoparticles were formed from ammine complexes ([Pt(NH₃)₄]Cl₂, [Ag(NH₃)₂]OH, and [Pd(NH₃)₄]Cl₂) and supported on high ordered mesoporous silica (SBA-15) and aluminosilica matrix. In this work, the distribution, composition and crystal structure of supported noble metal nanoparticles were determined and characterized. Finally the stability of incorporated nanostructures was confirmed. The microstructures of the obtained samples were analyzed by high resolution transmission electron microscopy. Obtained results indicated that developed procedures of synthesis and modification of mesoporous ordered silica or their derivate by proposed nanostructures are effective and allow to obtain new nanocomposites and nanocatalysts in repeatable and controlled way.
The influence of precipitation temperature on structural and magnetic properties of iron/iron-oxide nanoparticles is investigated. Nanoparticles were prepared by precipitation of γ-Fe precipitates in Cu-Fe solid solution and subsequently isolated by matrix dissolution. Precipitation annealing temperatures were 773, 873, and 973 K. Nanoparticles core-shell structure and morphology were characterized by X-ray diffraction, high-resolution transmission electron microscopy, and selected area electron diffraction. These measurements showed that average diameter of nanoparticles increases with precipitation temperature from 8.5 nm to 20.5 nm. The measurements of magnetization as a function of temperature and applied field have been performed by SQUID magnetometer in temperature range from 5 K to 200 K.
We report on an incorporation of self-assembled templates of superparamagnetic Fe-O nanoparticles into tunnel magnetoresistance devices. We fabricated a multilayer stack composed of the following layer sequence: Cr/Au/Co/NP/Co/Cu on Si(100) substrate where NP stands for a self-assembled layer of nanoparticles deposited by the Langmuir-Blodgett technique. The X-ray reflectivity and grazing-incidence small angle X-ray scattering were employed to study the layers thicknesses and interface morphology in each preparation step. In particular, the grazing-incidence small angle X-ray scattering was measured before and after the nanoparticle incorporation as well as on the complete tunnel magnetoresistance stack. In this way, in-depth morphology profile during subsequent preparation steps was obtained. We demonstrate that X-ray analysis of the deposited tunnel magnetoresistance stack is essential for successful fabrication of novel hybrid devices consisting of self-assembled nanoparticles.
Low-frequency Raman scattering from small spherical particles is analyzed. Frequencies of vibrational modes are calculated in elastic continuum approximation, which considers one nanoparticle as homogeneous elastic sphere. Parameters of this model are transverse (v_T) and longitudinal (v_L) sound velocities of material, i.e. elastic properties of bulk material. Frequencies of vibrational modes are scaled as function of mentioned bulk parameters for symmetric l=0 and quadrupolar l=2 spheroidal modes, in the case of stress-free boundary conditions. Calculated values are compared with the low-frequency Raman experimental results from literature (Ge, Si, CdS, CdSe, CeO_2, ...). These calculated relations can be practically used to examine nanoparticles of any bulk material. We presented also a procedure how to establish v_L and v_T of material from low-frequency Raman spectra and dimension d of particles.
The morphology and other physical properties of ZnO nanopowders synthesized by glycine-nitrate gel combustion process were investigated and characterized by scanning electron microscopy, transmission electron microscopy, nanosizer and X-ray diffraction. Glycine, NH_2CH_2COOH, and zinc nitrate Zn(NO_3)_2·6H_2O were dissolved in distilled water and the solution was coagulated by mixing at 90°C. The viscous gel prepared during glycine-nitrate mixing was heated at ≈220C to initiate the exothermic reactions by self-combustion where the temperature reached up to 1200°C. The glycine-nitrate ratio had a significant effect on the reaction temperature and final particle morphology. Therefore the synthesized powders have a different morphology like formless and spherical tufa ash. The particle size distribution was 50-1200 nm as measured using a nanosizer.
The aim of the research presented is to investigate the effect of pH value on the structural and morphological properties of nanostructured ZnO products. Zinc acetate dihydrate (Zn(CH_3COO)_2·2H_2O) has been used as precursor whereas distilled water is used as a solvent. The pH value of the sol was adjusted with monoethanolamine (MEA) and it changed from acid to base in nature. X-ray diffractometer has been used to determine preferred crystal orientation and particle size of the thin films. Film morphologies have been examined by using JEOL JSM 6060 LV scanning electron microscope equipped with energy-dispersive spectroscopy.
Iron-oxide nanoparticles were nanocasted in the periodic mesoporous silica matrix, consisting of two-dimensional hexagonally ordered channel system with the mean diameter of the channels about 7 nm. The magnetic measurements of dc magnetization confirm behavior typical of a superparamagnetic system, such as the irreversibility of the zero-field-cooled and field-cooled curves, presence of a maximum in zero-field-cooled curve related with blocking temperature T_C and revealing of coercivity H_C below T_C. The existence of negative exchange bias effect below T_C was confirmed in our system represented by value of exchange bias field H_{EB}=-970 Oe measured at the temperature 2 K.
The magnetic properties of iron oxide nanoparticles prepared by the chemical decomposition of the iron chlorides with the ratio Fe^{3+}/Fe^{2+}=2.25 were studied by means of transmission electron microscopy, X-ray diffraction and Mössbauer spectroscopy in- and without external magnetic field. The transmission electron microscopy studies show that the nanoparticles have spherical shape with diameter about 13 nm. The transmission electron microscopy and X-ray diffraction patterns are composed of lines that could be indexed within the cubic spinel structure. The room temperature Mössbauer spectrum shows the coexistence of the broad magnetically split part and superparamagnetic doublet. The in-field Mössbauer measurements carried out in the temperature range from room temperature down to 13 K show a gradual increase in the spin canting on the surface of the nanoparticles and decrease in the intensity of the superparamagnetic doublet. The sample was subjected to annealing process performed at elevated temperature in air atmosphere in order to change the microstructure of the nanoparticles and in consequence to change the magnetic properties of the sample. The annealing process leads to the decrease in the value of quadrupole splitting of the superparamagnetic doublets.
Crystalline nanocomposite KDP/Al₂O₃ was obtained by growth of KDP nanocrystals inside nanopores of amorphous alumina matrix (Al₂O₃) with pores diameter of 35 nm. Performed atomic force microscopy and X-ray diffraction analysis confirmed that Al₂O₃ matrix is filled up with a tetragonal phase KDP nanocrystals in preferred crystallographic orientation [100]. The nonlinear optical response was studied by means of second harmonic generation via the Maker fringe technique employing picosecond laser pulses at wavelength of 1064 nm. The polarization dependent second harmonic generation response was observed mainly due to the macroscopic crystalline structure anisotropy of KDP/Al₂O₃ nanocomposite. The investigation of such type of nanocomposites which combine nanoscale nonlinear optical materials has a great importance since they may improve the performance of entire system.
Nanosized iron-gold magnetic nanoparticles with an average particle size 10 nm were prepared by a reverse micelle method. The magnetic properties measurements of DC and AC magnetization confirm behaviour typical of a superparamagnetic system, such as the irreversibility of the zero-field-cooled and field-cooled curves, the frequency dependence of a blocking temperature T_B, and revealing of coercivity H_C below blocking temperature. The quantitative analysis of AC susceptibility due to value of parameter C_1 = ΔT_B/(T_B Δ log f)=0.0242 confirming the existence of inter-particle interaction in our system.
Effects of nanoparticle size (2.0-6.0 nm) and shape (spherical and cubic) on structural characteristic of atomic ordering processes and order-disorder transformation in B2-type ordered equiatomic-FeCo nanoalloys have been studied by combining electronic theory of alloys in the pseudopotential approximation with Ising-type Hamiltonian site exchange Monte Carlo simulation method. Structural evolutions in amorphous nanoparticles (2-6 nm) of Fe_{50}Co_{50} alloy have been utilized via molecular dynamic simulations from room temperatures to 1700 K temperatures. It has been shown that disordering starts at surface and propagates into volume of nanoparticles with increasing temperature. FeCo nanoparticles with critical dimensions more than 5 nm have order-disorder transformation behavior almost similar with bulk B2-FeCo alloys irrespective of their shape. Molecular dynamic analyses indicate that short- and medium-range ordered atomic structures exist in quenched Fe_{50}Co_{50} nanoparticles at room temperature. Deformed bcc structures and deformed icosahedron structures are most probable atomic configurations for 2, 4, 6 nm particles of Fe_{50}Co_{50} nanoalloy.
Oxide nanoparticles embedded in a polymer matrix produce nanocomposites which are useful for optics and electronic applications. Yttrium oxide nanoparticles have received much attention due to their various properties and are significantly used in fundamental and application oriented fields. The present paper reports the influence of annealing temperature on the Y_2O_3:SiO_2 nanocomposite prepared by sol-gel process. Y(NO_3)_3 ·4H_2O and tetraethoxysilane were used as precursors and obtained powdered form of Y_2O_3:SiO_2 composite. The powder sample was annealed at 500C and 900C for 6 h which were characterized by X-ray diffraction, Fourier transform infrared and transmission electron microscope. X-ray diffraction data described that the broadening of peaks decreases with increase in annealing temperature which may be due to the increase in particle size. Sample analyzed by Fourier transform infrared and transmission electron microscopy confirmed the grain size dependence on annealing temperature. Cubic phase of yttrium oxide crystal structure was obtained within the silica matrix. The nanocrystallites size has been calculated using Debye-Scherrer formula, Williamson-Hall plot and transmission electron micrographs and compared at two different temperatures (a) 500C and (b) 900C.
We report on an experimental study of the thermal annealing processes under air and nitrogen atmospheres of colloidal Au nanoparticles deposited onto SiO₂/Si(100) samples. It was shown that Au nanoparticles during annealing under ambient conditions could penetrate inside silicon dioxide layers forming pores at that their lengths were found to be dependent on the annealing time. The influence of oxygen on the penetration process is discussed. At the same time, the annealing of Au nanoparticles under nitrogen conditions did not result in the formation of pores.
The effect of annealed (0001) α -Al_2O_3 surfaces on heteroepitaxial growth of silver nanoparticles were analysed by reflection high-energy electron diffraction, transmission electron microscope and selected area electron diffraction. Ag nanoparticles were deposited on 1× 1 stoichiometric and reconstructed (111)Al//(0001) α -Al_2O_3 with the Knudsen cell. The maximum cluster density method and the Lifethenz theory of Van der Waals energy were used to investigate the Ag//(0001)α -Al_2O_3 interface parameters. The growth modes, lattice parameters, nanoparticle forms and sizes are strongly dependent on the substrate surface structures. Initially, three-dimensional islands of Ag nanoparticles grow on both kinds of surfaces with partial hexagonal shapes. Ag nanoparticles on stoichiometric surface create the (111)Ag//(0001)α -Al_2O_3 interface without any preferred epitaxial direction. On this surface, Gaussian distribution is characteristic of an atom-by-atom growth mode with density of Ag nanoparticles lower than saturation density while a coalescence growth mode appears due to binary collisions between Ag nanoparticles accompanied by a liquid-like behaviour after saturation density. In case of reconstruction substrates, the epitaxial relationships between Ag nanoparticles and the surface are formed (111)Ag//(0001)α -Al_2O_3, 〈01\bar(1)〉Ag//[12\bar(3)0]α -Al_2O_3 or 〈01\bar(1)〉Ag//[1\bar(1)00]α-Al_2O_3. The Ag nanoparticles make rotation with angles between ± 6° around the epitaxial orientations 〈1\bar(1)00〉 or 〈12\bar(3)0〉. Only the atom-by-atom growth mode were found at all Ag nanoparticles growth processes.
Reduction processes of WO_{3} nanopowder either with carbon or with hydrogen were observed using X-ray powder diffraction and transmission electron microscope. The phase transformations, separation, grain size and electrical conductivity of WO_{3-x} nanopowder during reductions via partial pressure high energy ball-milling have been studied. During the carbon-reduction process the monoclinic WO_{3} structure transforms to nonstoichiometric Magneli phases W_{40}O_{118}, WO_{2.9} and finally to WO_{2} and W mixed phases. The Magneli WO_{3-x} phases exhibit specific fringe contrast imaging of well-ordered crystallographic shear planes. In comparison, the monoclinic WO_{3} structure transforms to hydrate WO_{3}·1/3H_{2}O, hexagonal WO_{3}, non-stoichiometric WO_{2.7} and finally to WO_{2} and W mixed phases during the hydrogen-reduction process. The inclusion of hydrogen atoms between the WO_{6} octahedral structure shifts the reduction steps to lower milling times. It demonstrates that the formation of hydrate WO_{3} phases enhances the amenability of the system to reduction. The activation energy for conduction was deduced from the Arrhenius equation and was found to depend on oxygen partial pressure or presence of the hydrogen atoms. The defect band model was used for interpretation of these behaviors. It supposes that the surface oxygen vacancies introduce donor levels in the gap of semiconductor, so free electrons are produced by reduction.
Using solgel method Nd_2O_3-SiO_2 binary oxide systems were prepared. The binary oxide transformed from the amorphous phase to nanocrystalline phase upon heat treatment in air. Characterization of the Nd_2O_3-SiO_2 was carried out by using X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. The effect of the sintering temperature and time on the evolution of the binary oxide system was discussed. It is found that sintering time plays a pivotal role to obtain Nd_2O_3-SiO_2 nanocomposite. At temperature of 900°C, the sample was sintered for 12 h and monoclinic Nd_2O_3 nanocrystallites, with average crystallite size ≈12 nm, were obtained.
Microstructure and surface morphology of AgO nanocrystallites (25 nm) supported on SiO_2 and subjected to heat treatment in air up to 500C have been studied by transmission electron microscopy, scanning electron microscopy, and the Fourier transform infrared spectroscopy measurements. It has been found that heating at 500C resulted in spreading of AgO over silica and formation of quasi two-dimensional amorphous phase (silicate precursor) exhibiting very weak band at 566 cm^{-1} have appeared in the Fourier transform infrared spectroscopy. Nanocrystalline silver silicate of undefined structure formed at 300-500C.
In this work, electrosynthesis of Fe_3O_4 nanoparticles was carried out potentiostatically in an aqueous solution of C_4H_{12}NCl which acts as supporting electrolyte and electrostatic stabilizer. γ-Fe_2O_3 nanoparticles were synthesized by controlling oxidation of the electrooxidized Fe_3O_4 nanoparticles at different temperature. Finally the phase transition to α-Fe_2O_3 nanoparticles was performed at high temperatures using sintering treatment. The synthesized particles were characterized using X-ray diffraction, Fourier transformation, infrared scanning electron microscopy with energy dispersive X-ray analysis, and vibrating sample magnetometry. Based on the X-ray diffraction results, the transition from Fe_3O_4 to cubic and tetragonal γ-Fe_2O_3 was performed at 200C and 650°C, respectively. Furthermore, phase transition from metastable γ-Fe_2O_3 to stable α-Fe_2O_3 with rhombohedral crystal structure was approved at 800°C. The existence of the stabilizer molecules at the surface of Fe_3O_4 nanoparticles was confirmed by Fourier transformation infrared spectroscopy. According to scanning electron microscopy images, the average particles size was observed around 50 nm for electrooxidized Fe_3O_4 and γ-Fe_2O_3 nanoparticles prepared at sintering temperature lower than 900°C, however by raising sintering temperature above 900C the mean particles size increases. Energy dispersive X-ray point analysis revealed that the nanoparticles are almost pure and composed of Fe and O elements. According to the vibrating sample magnetometry results, saturation magnetization, coercivity field, and remnant magnetization decrease by phase transition from Fe_3O_4 to Fe_2O_3.
Undoped and Mn doped SnO_2 prepared by co-precipitation method exhibits nanocrystalline nature with prominent peaks along (110), (101), (211), and (310) planes. All the prepared samples are nanocrystalline with crystallite size lying in the range of 4.8-5.6 nm. The prepared SnO_2 nanoparticles exhibit single tetragonal crystalline phase. The high resolution transmission electron microscopy images show that the particles are nanocrystalline in nature. The composition of the prepared samples have been analyzed using energy dispersive analysis of X-rays spectra. The photoluminescence spectroscopy shows the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band. Broad UV emission at 426 nm is observed in photoluminescence. UV-vis absorption spectral studies showed a peak at 385 nm. Magnetic measurements revealed that all the doped samples exhibit room temperature ferromagnetism, which is identified as an intrinsic characteristic obtained on doping. Pure SnO_2 nanoparticles showed diamagnetism, SnO_2 with lower Mn content show larger magnetization and with increasing Mn content the retentivity and coercivity are found to decrease.
The present paper investigates the temperature/frequency dependences of admittance Z in the granular Cu_x(SiO₂)_{1-x} nanocomposite films around the percolation threshold x_{C} in the temperature range of 4-300 K and frequencies of 20-10⁶ Hz. The behavior of low-frequency ReZ(T) dependences displayed the predominance of electrons hopping between the closest Cu-based nanoparticles for the samples below the percolation threshold x_{C} ≈ 0.59 and nearly metallic behaviour beyond the x_{C}. The high-frequency curves ReZ(f) at temperatures T > 10 K for the samples with x < x_{C} exhibited behavior close to ReZ(f) ≈ f^{-s} with s ≈ 1.0 which is very similar to the known Mott law for electron hopping mechanism. For the samples beyond the percolation threshold (x > x_{C}), the frequency dependences of ReZ(f) displayed inductive-like (not capacitive) behaviour with positive values of the phase shift angles.
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