Effect Of Crystallite Size Research Articles

Micro‐Raman investigation of nanosized zinc ferrite: effect of crystallite size and fluence of irradiation

AbstractZinc ferrite nanoparticles of different sizes ranging from 12 to 62 nm were synthesized by using the nitrate route. These nanoparticles were irradiated by a 100 MeV oxygen beam at two fluences: 1 × 1013 and 5 × 1013 ions/cm2. It was observed that modes corresponding to cubic spinel phase were retained after the irradiation in all the systems. The variation in the parameters of various modes follows phonon confinement, while this effect seems to violate in irradiated specimen. It was found that the irradiation‐induced changes in the modes F2g(2) and F2g(3) depend on whether the crystallite size of the pristine sample is less than, equal to or greater than the phonon confinement length, while this length is not dominant for the irradiation‐induced changes in the mode A1g. The changes in various parameters of the modes are attributed to the combined effect of the restructuring of the chemical species and ion‐induced defects. Copyright © 2011 John Wiley & Sons, Ltd.

Synthesis and characterization of (Na 0.85K 0.15) 0.5Bi 0.5TiO 3 ceramics by different methods

The (Na 0.85K 0.15) 0.5Bi 0.5TiO 3 (BNKT) powders were synthesized by solid-state method, sol–gel method and stearic acid method. Microstructure, piezoelectric and dielectric properties of the ceramics were investigated. Attempts had been made to understand the reaction processes by using thermo gravimetric (TG) and differential scanning calorimetry (DSC). The BNKT powders have a perovskite structure with average crystallite sizes of 168 nm, 85 nm and 79 nm, corresponding to the solid-state method, the sol–gel method and the stearic acid method, respectively. The ceramics derived from the powder synthesized by sol–gel method presents the most homogeneous microstructure and largest grain size (5–7 μm). The effects of average crystallite size on microstructures and electric properties of the BNKT ceramics were investigated. Both the piezoelectric properties and dielectric properties were enhanced with the increase of grain size.

Nanoscale Effects on Structural and Giant Dielectric of PZT Synthesized by High Energy Ball Mill

We report the formation of nanostructure Pb(Zr0.53Ti0.47)O3 (PZT) ceramic powder having cubic crystal structure under ambient conditions that has been found to be stable up to 800°C. The cubic structure of nanocrystalline PZT under normal condition is attributed to the crystallite size effect. The crystallite size effect has also reflected its crucial role in substantial enhancement of the dielectric permittvity at Tc that may be termed as a novel “giant dielectric permittivity” (GDP) effect. The origin of such a high permittivity (ϵ∼34600 at 10 kHz), not reported so far, has been traced to the changes occurring in the material microstructure, interface and grain boundary effects predominantly controlled by the crystallite size.

Distinguishing crystallite size effects from those of structural disorder on the powder X-ray diffraction patterns of layered materials

Both crystallite size effects and structural disorder contribute to the broadening of lines in the powder X-ray diffraction (PXRD) patterns of layered materials. Stacking faults, in particular, are ubiquitous in layered materials and aside from broadening also induce peaks due to select reflections to shift away from the Bragg positions. The effect of structural disorder has to be suitably discounted before the application of the Scherrer formula for the estimation of crystallite size.

Effects of Polymorphism and Crystallite Size on Dipole Reorientation in Poly(vinylidene fluoride) and Its Random Copolymers

Poly(vinylidene fluoride) (PVDF) and poly(VDF-co-hexafluoropropylene) [P(VDF−HFP)] films having different polymorphisms and crystallite sizes but a similar crystal orientation (i.e., c-axes parallel to the film surface) were prepared by different film processing methods. Effects of polymorphism and crystallite size on the dipole reorientation behavior and electric energy storage/release were studied by electric displacement−electric field (D−E) loop measurements. Experimental results suggested that coupling interactions among ferroelectric domains, which could be adjusted by different polymorphisms and/or crystallite sizes, determined dipole reorientation/switching behaviors. Note that the ferroelectric domain coupling is realized via induced compensation polarizations from the media (either amorphous or crystalline PVDF) between aligned ferroelectric domains. A high β rather than α/δ content and a large crystallite size facilitated the coupling interactions among ferroelectric domains, and thus dipoles i...

Effects of HZSM-5 crystallite size on stability and alkyl-aromatics product distribution from conversion of propanal

article i nfo The conversion of propanal on large (2-5 µm) and small (0.2-0.5 µm) crystallite HZSM-5 at 400 °C and atmospheric pressure has been studied. Improved catalyst stability was observed on small crystallites due to faster removal of products with the shorter diffusion path length, reducing the formation of coke precursors. As previously shown, C9 aromatics are the initial aromatics produced from propanal via aldol condensation followed by cyclization and these have less opportunity to crack to lighter aromatics on the small crystallites. Thus, a higher ratio of C9/(C8+C7) aromatics was observed on the small crystallites. The main isomer of the C8 aromatic products observed on small crystallites was the initial cracking product of the C9 aromatics, and thermodynamically preferred, meta-xylene, while the shape-selective preferred para-xylene was the predominant product on large crystallites. The higher internal diffusion rate of the para isomer results in greater shape selectivity with the longer path of the large crystallite zeolite. It is concluded that the use of smaller crystallite HZSM-5 improves results for production of alkyl aromatics from light oxygenates at mild conditions that may prove useful for bio-oil upgrading.

Fast Li-Ion Insertion into Nanosized LiMn2O4 without Domain Boundaries

The effect of crystallite size on Li-ion insertion in electrode materials is of great interest recently because of the need for nanoelectrodes in higher-power Li-ion rechargeable batteries. We present a systematic study of the effect of size on the electrochemical properties of LiMn(2)O(4). Accurate size control of nanocrystalline LiMn(2)O(4), which is realized by a hydrothermal method, significantly alters the phase diagram as well as Li-ion insertion voltage. Nanocrystalline LiMn(2)O(4) with extremely small crystallite size of 15 nm cannot accommodate domain boundaries between Li-rich and Li-poor phases due to interface energy, and therefore lithiation proceeds via solid solution state without domain boundaries, enabling fast Li-ion insertion during the entire discharge process.

Colloidal synthesis and blue based multicolor upconversion emissions of size and composition controlled monodisperse hexagonal NaYF4 : Yb,Tm nanocrystals

Monodisperse beta-NaYF4:Yb,Tm nanocrystals with controlled size (25-150 nm), shape (sphere, hexagonal prism, and hexagonal plate), and composition (Yb: 20-40%, Tm: 0.2-5%) were synthesized from the thermolysis of metal trifluoroacetates in hot surfactant solutions. The upconversion (UC) of near-infrared light (980 nm) to ultra-violet (360 nm), blue (450 and 475 nm), red (650 and 695 nm) and infrared (800 nm) light in the beta-NaYF4:Yb,Tm nanocrystals has been studied by UC spectroscopy. Both the total intensity of UC emissions and the relative intensities of emissions at different wavelengths have shown a strong dependence on different particle sizes and different Tm3+ and Yb3+ concentrations. As a result, different overall output colors of UC emissions can be achieved by altering sizes and Yb3+/Tm3+ doping concentrations of the beta-NaYF4:Yb,Tm nanocrystals. The intensity-power curves of a series of samples have proved that emissions at 360 and 450 nm can be ascribed to four-photon process (1D2 to 3H6 and 1D2 to 3H4, respectively), while emissions at 475 and 650 nm are three-photon processes (1G4 to 3H6 and 1G4 to 3H4, respectively) and emissions at 695 and 800 nm are two-photon ones (3F2 to 3H6 and 3F4 to 3H6, respectively). A UC saturation effect would occur under a certain excitation intensity of the 980 nm CW diode laser for the as-obtained beta-NaYF4:Yb,Tm nanocrystals, leading to the decrease of the slopes of the I-P curves. The results of our study also revealed that the successive transfer model instead of the cooperative sensitization model can be applied to explain the UC behaviors of the beta-NaYF4:Yb,Tm nanocrystals. Further, an unexpected stronger emissions of four-photon process at 360 and 450 nm for approximately 50 nm beta-NaYF4:Yb,Tm nanocrystals than those for the bigger (approximately 150 nm) nanocrystals was observed and explained in terms of the effects of crystallite size, surface-to-volume ratio and homogeneity of the doping cations.

Cyclohexane selective photocatalytic oxidation by anatase TiO2: influence of particle size and crystallinity

A systematic study is presented on the effect of crystallite size of Anatase (Hombikat, Sachtleben), varied by calcination at different temperatures up to 800 degrees C, on photocatalytic activity in cyclohexane selective oxidation. Two different reactors were used to test the materials: a top illumination reactor and an in situ ATR-FTIR cell. Properties such as crystallinity and associated availability of holes and electrons for surface reactions, as well as the amount of surface OH-groups, are shown to have a significant influence on TiO(2) activity, (surface) selectivity, and stability. Upon increasing the crystallite size, productivity (g(-1)(catalyst)) decreases, while (i) the TOF (moles of cyclohexanone formed per minute per OH-site), (ii) the rate of cyclohexanone desorption, (iii) catalytic site stability, and (iv) the cyclohexanol/cyclohexanone ratio increase. The results are discussed on the basis of a reaction scheme, and a simple reaction rate equation.

Synthesis and electrical, optical and phonon properties of nanosized Aurivillius phase Bi 2WO 6

Nanoparticles of bismuth-layered ferroelectric and photocatalytic material, Bi 2WO 6, have been prepared by mechanochemical synthesis and firing of amorphous precursor prepared in metathesis reaction. The effect of different synthesis methods and crystallite size on the structure and properties of Bi 2WO 6 nanoparticles have been investigated using the XRD, TEM, DTA, DTG, diffuse reflectivity, conductivity measurements, Raman and infrared spectroscopy. The results show that the smallest particles could be obtained by mechanochemical synthesis. However, this method introduced large number of crystal defects. The conductivity of this sample is higher by half an order of magnitude, in comparison with conductivities measured for the other samples studied in this paper or previously reported in the literature for Bi 2WO 6. Our results also show that firing the amorphous precursor with NaNO 3 by-product yields material composed of small, well-crystallized particles containing small number of crystal defects. This result shows that this synthesis method is the best route to enhance photocatalytic activity of Bi 2WO 6. Raman and IR studies also suggest that the orthorhombic distortion diminishes with decreasing particle size.

Nanostructured aluminium oxide powders obtained by aspartic acid–nitrate gel-combustion routes

In this work, two new gel-combustion routes for the synthesis of Al 2O 3 nanopowders with aspartic acid as fuel are presented. The first route is a conventional stoichiometric process, while the second one is a non-stoichiometric, pH-controlled process. These routes were compared with similar synthesis procedures using glycine as fuel, which are well-known in the literature. The samples were calcined in air at different temperatures, in a range of 600–1200 °C. They were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and BET specific surface area. Different phases were obtained depending on the calcination temperature: amorphous, γ (metastable) or α (stable). The amorphous-to-γ transition was found for calcination temperatures in the range of 700–900 °C, while the γ-to-α one was observed for calcination temperatures of 1100–1200 °C. The retention of the metastable γ phase is probably due to a crystallite size effect. It transforms to the α phase after the crystallite size increases over a critical size during the calcination process at 1200 °C. The highest BET specific surface areas were obtained for both nitrate–aspartic acid routes proposed in this work, reaching values of about 50 m 2/g.

Comparative Study of Lithium Transport Kinetics in Olivine Cathodes for Li-ion Batteries

Crystallite size and composition effects on the kinetics of lithium storage in olivines are studied using potentiostatic intermittent titration tests (PITT). Here we compare undoped Li1-xFePO4 of 113 nm, 42 nm, and 34 nm average particle size, and three aliovalent solute doped compositions formulated for dopant substitution on the Li (M1) site with charge compensation by Li vacancies: Li0.90Mg0.05FePO4, Li0.80Zr0.05FePO4, and Li0.70Zr0.075FePO4. The results first show that a diffusive component can be measured with reasonable accuracy in all samples, allowing determination of the lithium chemical diffusion coefficient D as a function of potential or state-of-charge. In addition, a method is illustrated for the separation of capacity due to diffusive transport from that due to the first-order phase transition. Using the combined analyses, the effects of particle size reduction and aliovalent doping are readily understood. Both effects contribute to a reduced lithium miscibility gap and a greater contribution to stored capacity of the (faster) diffusive process. Simultaneously, the rate of phase transformation within the miscibility gap is also increased. Highly doped samples exhibit a complete lithium solid solution at room temperature, and have 1−2 orders of magnitude higher D at potentials where significant capacity is stored. This translates to improved capacity retention at high cycling rates, albeit at the expense of reduced absolute capacity due to Li diffusion channel blocking.

Effect of crystallite size of SAPO-34 catalysts on their induction period and deactivation in methanol-to-olefin reactions

Methanol-to-olefin (MTO) reactions were investigated over three kinds of SAPO-34 catalysts with similar acidities but different crystallite sizes to elucidate the effect of crystallite size on their catalytic performance, especially in terms of the induction period and deactivation. The prepared SAPO-34 catalysts showed similar physical and chemical properties, except for their crystallite sizes. The catalyst composed of large crystallites (∼7 μm) was activated after a definite induction period and deactivated rapidly, while that composed of small crystallites (∼0.4 μm) achieved complete conversion without any appreciable induction period and retained its activity for a relatively long time. This variation in catalytic performance according to the crystallite size of the SAPO-34 catalysts was attributed to the number of accessible cages near their external surface and to the diffusion limitation of the materials. Simulated conversion profiles in the MTO reactions over different crystallite sizes supported this explanation.

Retention at room temperature of the tetragonal t″-form in Sc 2O 3-doped ZrO 2 nanopowders

Synchrotron X-ray powder diffraction was applied to the study of the effect of crystallite size on the crystal structure of ZrO 2–10 mol% Sc 2O 3 nanopowders synthesized by a nitrate-lysine gel-combustion route. Nanopowders with different average crystallite sizes were obtained by calcination at several temperatures, ranging from 650 to 1200 °C. The metastable t″-form of the tetragonal phase, exhibiting a cubic unit cell and tetragonal P4 2/ nmc spatial symmetry, was retained at room temperature in fine nanocrystalline powders, completely avoiding the presence of the stable rhombohedral β phase. Differently, this phase was identified in samples calcined at high temperatures and its content increased with increasing crystallite size. The critical maximum crystallite size for the retention of the mestastable t″-form resulted of about 35 nm.

Crystallite Size Dependence of Lithium Intercalation in Nanocrystalline Rutile

An electrochemical investigation has been undertaken into the dependence of particle size on the mechanism of lithium intercalation in rutile. A thoroughly characterized suite of rutile nanoparticles with diameters in the broad size range 4−50 nm was used in the study. Variation in the cyclic voltammetry of rutile over cycling is presented for the first time and numerous new electrode reactions identified. While a crystallite size effect was witnessed during fast voltammetry experiments, this effect disappeared as the rate was reduced and no variation in capacity was witnessed in galvanostatic cycling experiments. The intercalation potential was seen to increase with decreasing crystallite size, the opposite of what was predicted from quantum confinement. Enhanced lithium intercalation into the smaller nanoparticles has been ascribed to the slightly expanded structure of smaller nanocrystals relative to larger ones taking the dimensions of the tunnels into which Li inserts beyond a threshold value. This l...

The 390 cm−1 feature of polycrystalline hematite—An optical crystallite size effect

Abstract The 390 cm −1 spectral feature of polycrystalline hematite is shown to be caused by a crystallite size effect which is caused by the anisotropic nature of single crystalline hematite. This effect occurs if the probing light is able to resolve the heterogeneous nature of a polycrystalline material and if the corresponding single crystalline material is optically anisotropic. Therefore it does not depend on whether the material of interest is a powder or consolidated. As a consequence, the resulting macroscopic reflectance and transmittance of the polycrystalline material is an average of the microscopic reflectance and transmittance of the individual crystallites. Therefore, randomly oriented polycrystalline materials with large crystallites show characteristic changes in their spectral profile compared to the spectral profiles of polycrystalline materials consisting of crystallites that are small in comparison with the wavelength. The extent of the spectral changes depends on the degree of optical anisotropy of the corresponding single crystalline material. The spectral changes also comprise non-zero cross-polarization terms despite of random orientation. Therefore a characterization of a polycrystalline material with a scalar dielectric function is possible in general only if the material consists of randomly oriented crystallites small compared to the wavelength.

High-Resolution 89Y and 45Sc NMR Spectroscopic Study of Short-Range Structural Order in Nanocrystalline Y- and Sc-doped CeO2 and ZrO2

The effect of crystallite size on cation coordination environments and oxygen vacancy ordering has been investigated in micro- and nanocrystalline Y- and Sc-doped ZrO2 and CeO2 by using high-resolution 89Y and 45Sc magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Our results indicate that irrespective of crystallite size the vacancies are preferentially associated with the host cation (i.e., Zr) in Y-doped ZrO2 while they display a preference for the dopant cation (i.e., Sc) in Sc-doped ZrO2. On the other hand, vacancies prefer to be associated with the dopant cation in both Y- and Sc-doped CeO2. However, the reduction of crystallite size to a few nanometers shows an unexpected and remarkable effect of increasing randomness in the vacancy distribution in all materials. Such an effect is hypothesized to result from a higher degree of short-range structural disorder in the cation coordination environments in nanocrystals compared to that in their microcrystalline counterparts that con...

Effect of morphology and crystallite size on solar photocatalytic activity of zinc oxide synthesized by solution free mechanochemical method

Zinc oxide crystallites were synthesized by two steps, solution free mechanochemical method. In order to obtain zinc oxide of different morphology and crystallite size, calcination temperature was varied from 400 to 900 °C. These photocatalysts are then characterized by X-ray diffraction (XRD), Scanning Electron Micrograph (SEM), Energy dispersive X-ray spectra (EDXS), Fourier transform infrared spectra (FT-IR) and UV–Visible spectrophotometer. X-ray diffraction data suggest that the obtained ZnO crystallites are of wurtzite structure. The zinc oxide crystallite growth rate is found to be different in different calcination temperature range. Photocatalytic activity of ZnO was checked by means of oxidative photocatalytic degradation (PCD) of resorcinol a potent endocrine disrupter in water under irradiation of sunlight in a batch photoreactor. The PCD efficiency was found to be dependent on crystallite growth rate and morphology of zinc oxide. The zinc oxide calcined from 400 °C to 550 °C exhibit same crystallite growth rate and showed maximum photocatalytic degradation of resorcinol. The PCD efficiency of zinc oxide was found to decrease with increase in calcination temperature as the particle size was increased. In addition to effect of calcination temperature, the influence of various other parameters such as photocatalyst amount, initial concentration of resorcinol and pH was also examined for maximum PCD of resorcinol. Neutral and basic pH is found to be favorable for chemical oxygen demand (COD) removal of resorcinol.

Evolution of the electrochemical processes vs. Li in RuO 2 as a function of crystallite size

Rutile-type RuO 2 powders with different crystallite sizes were obtained by annealing of a nanometric precursor, prepared by a precipitation method, at different temperatures. The effect of crystallite size on the electrochemical Li-intercalation reactivity has been studied. For this type of material the size decrease is detrimental as parasitic reactions become prominent. Furthermore, for samples with crystallite size from 3 to 40 nm, the reaction still proceeds through a triphasic process as underlined by PITT and in situ XRD measurements.

Crystallite size effects in stacking faulted nickel hydroxide and its electrochemical behaviour

β-Nickel hydroxide comprises a long range periodic arrangement of atoms with a stacking sequence of AC AC AC—having an ideal composition Ni(OH) 2. Variation in the preparative conditions can lead to the changes in the stacking sequence (AC AC BA CB AC AC or AC AC AB AC AC). This type of variation in stacking sequence can result in the formation of stacking fault in nickel hydroxide. The stability of the stacking fault depends on the free energy content of the sample. Stacking faults in nickel hydroxide is essential for better electrochemical activity. Also there are reports correlating particle size to the better electrochemical activity. Here we present the effect of crystallite size on the stacking faulted nickel hydroxide samples. The electrochemical performance of stacking faulted nickel hydroxide with small crystallite size exchanges 0.8e/Ni, while the samples with larger crystallite size exchange 0.4e/Ni. Hence a right combination of crystallite size and stacking fault content has to be controlled for good electrochemical activity of nickel hydroxide.