Surface Of Microcrystals Research Articles

Microstructural features of the La1−xCaxFeO3−δ solid solutions prepared via Pechini route

Solid solutions with La1−xCaxFeO3−δ (0≤x≤0.7) perovskite-like structure prepared via Pechini route have been investigated by using high resolution transmission electron microscopy and X-ray diffraction. Extended planar defects lying in (101) crystallographic planes and α-Fe2O3 nanoparticles on the surface of perovskite microcrystals are characteristic of the samples under investigation. It was found that testing of the samples in catalytic deep CH4 oxidation process results in partial destruction of solid solutions with formation of planar defects in the bulk and α-Fe2O3 particles on the surface.

Impact of Layer-by-Layer Self-Assembly Clay-Based Nanocoating on Flame Retardant Properties of Sisal Fiber Cellulose Microcrystals

The renewable cationic polyelectrolyte chitosan (CH) and anionic nanomontmorillonite (MMT) layers were alternately deposited on the surface of sisal fiber cellulose microcrystals (SFCM) via layer-by-layer (LBL) self-assembly method. The structure and properties of the composites were characterized by zeta potential, thermal gravimetric analysis (TGA), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectrometer (FTIR), microcalorimeter (MCC), and so forth. The zeta potential results show that the cellulose microcrystalline surface charge reversed due to the adsorption of CH and MMT nanoplatelets during multilayer deposition. MMT characteristic diffraction peaks appear in XRD patterns of SFCM(CH/MMT)5and SFCM(CH/MMT)10composites. Additionally, FESEM reveals that the SFCM(CH/MMT)10surface is covered with a layer of material containing Si, which has been verified by elemental analysis. TGA results show that the initial decomposition (weight loss of 5%) temperature of SFCM(CH/MMT)5is increased by 4°C compared to that of pure SFCM. On the other hand, carbon residue percentage of SFCM(CH/MMT)10is 25.1%, higher than that of pure SFCM (5.4%) by 19.7%. Eventually, it is testified by MCC measurement that CH/MMT coating can significantly reinforce the flame retardant performance of SFCM.

Preparation and characterization of konjac glucomannan microcrystals through acid hydrolysis

Konjac glucomannan (KGM) was treated by a facile acid hydrolysis to fabricate KGM microcrystals. At the first day of hydrolysis, KGM microcrystals were formed. With the extension of hydrolysis time, the particle size of KGM microcrystals decreased from 45μm to 15μm. In addition, compared with native KGM, the morphological, physicochemical, crystalline, and thermal properties of KGM microcrystals changed significantly. SEM images showed the irregular shapes and rough surfaces of KGM microcrystals as well as the smooth surface of native KGM. FTIR measurements revealed the cleavage of carbonyl groups in KGM microcrystals. XRD curves clearly presented the crystalline structure of KGM microcrystals, and the relative crystallinity increased to approximately 50%. DSC analysis showed that microcrystals had a better thermal stability than native KGM, which could be preferably used as reinforcement in the biocompatible material at high temperature.

Facile deposition of gold nanoparticles on C60 microcrystals with unique shapes

Gold nanoparticles were densely deposited on the surface of C60 microcrystals having a variety of unique shapes. C60 microcrystals were prepared by a conventional liquid–liquid reprecipitation method. The shapes of C60 microcrystals, such as bipyramid, belt, disc, and rod, were achieved by carefully adjusting the solvent species in the liquid–liquid reprecipitation process. Gold nanoparticles were directly deposited on C60 microcrystals without adding any conventional reducing agent but only heating HAuCl4 in C60 microcrystals ethanol dispersion containing CS2. Scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, powder X-ray diffraction, absorption spectroscopy, and X-ray photoelectron spectroscopy studies were conducted to characterize the gold–C60 heterostructure. The gold–C60 heterostructure showed charge transfer behaviour where gold was an electron donor and C60 was an electron acceptor. C60 microcrystals-supported gold nanoparticles also showed catalytic activity in reduction of p-nitrophenol at room temperature. A facile reduction mechanism was suggested for gold deposition on the surface of C60 microcrystals.

Interaction of dyes with nanoclusters adsorbed on the surface of AgBr microcrystals

We have performed a comparative luminescence investigation into spectral sensitization with dyes of holographic emulsions that contain AgBr microcrystals and are subjected to chemical sulfur or reducing sensitization. We show that, upon spectral sensitization of silver sulfide (Ag2S)n nanoclusters located on AgBr microcrystals, the polylayer adsorption of dyes on the surface of nanoclusters is observed, which is caused by van der Waals forces in the J-aggregated state. For silver oxide (Ag2O)m nanoclusters located on AgBr micro-crystals, the adsorption of dyes on their surface occurs only if chlorine atoms of heterocyclic residues of the dye interact with the nanocluster surface, which determines the adsorption of the dye as its chemisorption. In the remaining investigated cases, the polylayer adsorption of dyes during the spectral sensitization occurs not on the surface of nanoclusters but rather on the surface of AgBr microcrystals, initially in the J-aggregated state and then in the molecular- and H-aggregated states.

A designed multiscale hierarchical assembly process to produce artificial nacre-like freestanding hybrid films with tunable optical properties

In this work, we propose a multiscale hierarchical assembly process to realize the nacre-like layered structural arrangement of functional nanoparticles in a polymer matrix aiming to simultaneously achieve tensile strength enhancement and tunable optical properties in the hybrid materials. Through the designed fabrication route, functional nanoparticles (NPs) were firstly attached onto the surface of brick-like silicate-1 zeolite microcrystals as functional building blocks. Then, NP–zeolite functional microbricks were feasibly assembled with polyvinylalcohol (PVA) to form layered, functional hybrid films. Finally, a 50–100% tensile stress enhancement in the hybrid film is observed when compared to that of pure PVA film. Beside the tensile strength enhancement, the optical properties of the hybrid film can also be tuned by the incorporation of different functional nanoparticles. Our fabrication approach is an example of an alternative strategy to prepare strong hybrid materials with flexible tailoring of functionalities.

Iron/dextran sulfate multilayered microcapsules for controlled release of 10-hydroxycamptothecin

Stable 10-hydroxycamptothecin (HCPT) microcrystals with a length of about 5–10 μm and a ζ-potential of −38.5 mV were produced by pH-induced reprecipitation in presence of a stabilizer hydroxypropylmethylcellulose. Sequential layer growth was achieved by the layer-by-layer (LbL) assembly of Fe 3+ and dextran sulfate (DS) on the surface of HCPT microcrystals via both electrostatic interaction and chemical complexation process. The satisfactory drug loading content (67.2 ± 0.82%) as well as high encapsulation efficiency (60.56 ± 0.82%) for four bilayers of Fe 3+/DS coating was achieved. Both in vitro and in vivo release study revealed that the release time increased as the number of deposited Fe 3+/DS bilayers increased. These results indicated that such iron–polysaccharide multilayered microcapsules can be a promising approach for the construction of an effective controlled release delivery system of HCPT as well as other drugs with potential cytotoxicity or short half-life time.

The formation of oxide nanoparticles on the surface of natural clinoptilolite

Nanoparticles of NiO, ZnO and Cu 2O crystallize when the Ni-, Zn- and Cu-exchanged natural clinoptilolite, respectively, are dehydrated by heating in air at 550 °C. The dehydration of Mn-exchanged clinoptilolite does not lead to the crystallization of manganese oxide but affects the crystallinity of the host clinoptilolite lattice, which becomes amorphous. The NiO, ZnO and Cu 2O nanoparticles are found to be randomly dispersed in the clinoptilolite matrix. The particle size varies from 2 to 5 nm and exceeds the aperture of the clinoptilolite channel (approximately 0.4 nm), suggesting that the crystallization of the oxide phases takes place on the surfaces of clinoptilolite microcrystals.

Fluctuation theory of microheterogeneous systems in the molecular-field approximation

Principles of the fluctuation theory of microheterogeneous systems are considered based on the example of adsorption on a heterogeneous surface of microcrystals and spherical liquid droplets in the vapor phase. Equations that describe the equilibrium distributions of molecules are derived within the lattice gas model with allowance for lateral interactions in the mean-field approximation. This approximation does not take into account the correlation effects of interacting particles. We discuss the structure of entropy and energy contributions in the derived equations and the relationship between the molecular chemical potential and the expansion pressure, which is analogous to the equation of the state, as well as the molecular interpretation of the concept of pressure in an ensemble of small particles and its relation to the mechanical interpretation of the surface tension.

Sonochemical synthesis and characterization of disk-like copper microcrystals

Disk-like Cu microcrystals were successfully synthesized with assistance of ultrasonic irradiation. The products were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), and thermogravimetric/differential thermal analysis (TGA/DTA). It was found that as-prepared Cu microcrystals were single crystals in nature and had a high crystallinity. They were mainly composed of disk-like and triangular Cu microcrystals with an average diameter of about 1 μm. There existed CuO on the outermost surface of the products, while a thin layer of Cu 2O was found between CuO and inner Cu microcrystals. Moreover, CTAB molecules were adsorbed on the surface of Cu microcrystals and they were desorbed while heated during TGA analysis. In addition, the influences of magnetic stirring, stillness, concentration of hexadecyltrimethylammonium bromide (CTAB) surfactant, and starting Cu salts on the morphology and size distribution of the final products have also been studied.

Cholesterol microcrystals and cochleate cylinders: Attachment of pyolysin oligomers and domain 4

Using an established organic solvent injection procedure for the preparation of aqueous cholesterol microcrystal suspensions, it has now been shown that a new, hollow, cylindrical, tightly-coiled, multi-bilayer form of cholesterol can be generated, termed the cochleate cylinder. Cholesterol cochleate cylinders are formed in larger numbers at intermediate temperatures (40-75°C) but are not formed at 100°C. The structure of the cholesterol microcrystals and cochleate cylinders is shown in negatively stained electron micrographs. Oligomerization and attachment of pyolysin to cholesterol microcrystals and cochleate cylinders is shown, as is the attachment of the pyolysin "cholesterol-binding" domain 4 (D4) fragment. The bound D4 domain forms a linear array on the two planar surfaces and edges of the cholesterol microcrystals and a quasi helical array on the surface of the cochleate cylinders. Little evidence has been obtained to support the possibility that interaction or hetero-oligomerization can occur between intact pyolysin and the pyolysin D4 fragment on the surface of cholesterol microcrystals. Using immobilized cholesterol crystals attached to a carbon support film, single-sided linear labelling of the cholesterol surface with pyolysin D4 has been achieved, which correlates well with the images from the microcrystal suspensions and our earlier data using non-cytolytic streptolysin O mutants.

Visible-light-induced photoelectric gas sensing to formaldehyde based on CdS nanoparticles/ZnO heterostructures

A visible-light-induced gas sensing element based on the heterostructures of CdS nanoparticles (NPs)/ZnO microcrystals has been successfully developed. Commercial ZnO microcrystals were coated with CdS nanoparticles by a sonochemical method to form the heterostructures. Compared with native CdS NPs, the as-obtained CdS NPs/ZnO showed enhanced photocurrent intensity. And the heterostructures-based element exhibited an excellent performance of photoelectric gas sensing to formaldehyde induced by visible-light. Moreover, the photocurrent intensity and sensitivity to formaldehyde were controllable by tuning the amount of CdS NPs attached on the surface of ZnO microcrystals. A possible explanation on the sensing mechanism has been proposed. In addition, the gas sensing element possessed good reproducibility. Our results demonstrate that the heterostructures of CdS NPs/ZnO is an efficient and promising material for visible-light-induced photoelectric gas sensors working at room temperature.

Anthraquinone catalysis in the glucose-driven reduction of indigo to leuco-indigo

Anthraquinone immobilised onto the surface of indigo microcrystals enhances the reductive dissolution of indigo to leuco-indigo. Indigo reduction is driven by glucose in aqueous NaOH and a vibrating gold disc electrode is employed to monitor the increasing leuco-indigo concentration with time. Anthraquinone introduces a strong catalytic effect which is explained by invoking a molecular "wedge effect" during co-intercalation of Na+ and anthraquinone into the layered indigo crystal structure. The glucose-driven indigo reduction, which is ineffective in 0.1 M NaOH at 65 degrees C, becomes facile and goes to completion in the presence of anthraquinone catalyst. Electron microscopy of indigo crystals before and after reductive dissolution confirms a delamination mechanism initiated at the edges of the plate-like indigo crystals. Catalysis occurs when the anthraquinone-indigo mixture reaches a molar ratio of 1 : 400 (at 65 degrees C; corresponding to 3 microM anthraquinone) with excess of anthraquinone having virtually no effect. A strong temperature effect (with a composite EA approximately 120 kJ mol(-1)) is observed for the reductive dissolution in the presence of anthraquinone. The molar ratio and temperature effects are both consistent with the heterogeneous nature of the anthraquinone catalysis in the aqueous reaction mixture.

Measurements of Thermal Neutron Diffraction and Inelastic Scattering in Reactor-Grade Graphite

High-resolution Bragg-edge transmission measurements were conducted on granular as well as solid samples of graphite to understand the basis for a bulk measurement of the diffusion length 24% larger than predicted by MCNP5 for bulk reactor-grade graphite. High resolution enabled a measurement of the total diffraction cross section from 1 to 200 meV. This was subtracted from the total cross section to find the inelastic cross section in the same energy range. Small-angle scattering, which has been thought to contribute to the total cross section in the region of the lowest Bragg edge, is shown not to be present in our measurement or in those of others claiming to find it. Instead, neutron total reflection from the surface of graphite microcrystals is shown to contribute to the cross section at low energies. Reactor-grade graphite is shown to have an inelastic scattering cross section over most of the energy range larger by at least 10 than the nearly perfect crystal structure of pyrolytic graphite. The ratio of inelastic scattering to diffraction at 25 meV for our graphite is inferred to be twice as large as that of graphite manufactured 50 yr ago, and we believe that our larger diffusion coefficient is rooted in this difference. The distortions in the microcrystalline structure introduced in the manufacturing of the graphite studied here at 24°C are found to be equivalent to the uncertainty in atom positions seen in heating perfect crystal graphite to a temperature of ~1800°C.

Structure and electronic states of gold species in mordenites

Au-zeolites with SiO 2/Al 2O 3 molar ratios 10 and 206 were prepared by ion exchange of the H-mordenites with [Au(NH 3) 4](NO 3) 3 complex solution followed by hydrogen reduction or calcination at different temperatures. The samples were characterized by UV–visible DRS, XPS, XRD, TEM and TPR in order to understand the effect of zeolite acidity and pretreatment conditions on formation of different gold states. Three kinds of gold species were observed in the samples: (1) Au 3+cations, (2) partly reduced species with the diameter less than 1 nm assigned to charged clusters Au n δ + (with n proposed ⩽8), suggested to be stabilized inside zeolite channels, and (3) Au 0 nanoparticles with diameter varied within interval of 1.5–20 nm. The third type is located on the external surface of zeolite microcrystals. Au n δ + clusters were observed in as-prepared samples just after ion-exchange. Acid site strength did not change significantly amount of the clusters but influenced the electronic state of gold in these clusters and redox properties of gold cations. The contribution of the clusters was not changed significantly with temperature treatment even up to 500 °C. Au 0 nanoparticles were formed by reduction of gold ions at temperatures starting from 50 to 200 °C depending on the medium of treatment and the molar ratio of the zeolite. The obtained results are important for understanding the effect of multiplicity of catalytically active gold species in CO oxidation.

Study of the selection mechanism of heavy metal (Pb 2+, Cu 2+, Ni 2+, and Cd 2+) adsorption on clinoptilolite

The study was carried out on the sorption of heavy metals (Ni 2+, Cu 2+, Pb 2+, and Cd 2+) under static conditions from single- and multicomponent aqueous solutions by raw and pretreated clinoptilolite. The sorption has an ion-exchange nature and consists of three stages, i.e., the adsorption on the surface of microcrystals, the inversion stage, and the moderate adsorption in the interior of the microcrystal. The finer clinoptilolite fractions sorb higher amounts of the metals due to relative enriching by the zeolite proper and higher cleavage. The slight difference between adsorption capacity of the clinoptilolite toward lead, copper, and cadmium from single- and multicomponent solutions may testify to individual sorption centers of the zeolite for each metal. The decrease of nickel adsorption from multicomponent solutions is probably caused by the propinquity of its sorption forms to the other metals and by competition. The maximum sorption capacity toward Cd 2+ is determined as 4.22 mg/g at an initial concentration of 80 mg/L and toward Pb 2+, Cu 2+, and Ni 2+ as 27.7, 25.76, and 13.03 mg/g at 800 mg/L. The sorption results fit well to the Langmuir and the Freundlich models. The second one is better for adsorption modeling at high metal concentrations.

Cesium salt of a heteropolyacid in nanotubular channels and on the external surface of SBA-15 crystals: preparation and performance as acidic catalysts

The Cs-salt of heteropolyacid with stoichiometry Cs 2.5H 0.5PW 12O 40 (CsHPW) was deposited selectively at the external surface of the SBA-15 silica microcrystals, inside its mesoporous channels and simultaneously at both location modes. The structure, texture and performance of these CsHPW/SBA-15 composites were compared with that of a reference bulk salt of the same composition. Location of CsHPW salt on the external surface of SBA-15 microcrystals leads to disintegration of its agglomerates increasing acidity of the catalytic phase. A novel preparation strategy consisting of grafting the basic Cs-species at the internal pores surface of SBA-15 stabilized the CsHPW phase inside the channels in form of 5–8 nm nanocrystals at 30–70 wt.% loadings. The catalytic tests demonstrated that insertion of the CsHPW catalytic phase inside the nanotubular channels of SBA-15 in combination with location of an additional amount of this phase at the external surface of SBA-15 microcrystals allows to increase the specific activity of this phase in MTBE synthesis, propionylation of anisole and alkylation of catechol with t-butanol by a factor of 1.5–3. This level of specific activity in combination with high total loading of catalytic phase >60 wt.% permit to get composite catalytic materials with catalytic activity higher by a factor of 1.2–1.5 with respect to the bulk CsHPW catalyst and stabilizing the catalytic phase against colloidization in polar media.

Esterase activity of biocomposites constituted by lipases adsorbed on layered zirconium phosphate and phosphonates: selective adsorption of different enzyme isoforms

Aqueous solutions of crude extract of lipase from Candida rugosa (C-CRL) and of a semipurified material obtained by treating C-CRL with propan-2-ol (PT-CRL) were used as a source of biomaterial for its adsorption onto the surface of layered micro-crystals of α-zirconium phosphate Zr(HPO 4) 2 and phosphonates such as Zr(C 6H 5PO 3) 2, Zr(HPO 4)(C 6H 5PO 3) and Zr(HOOCCH 2PO 3)(HPO 4), which possess groups with different hydrophobic character anchored to the inorganic matrix. Several biocomposites have been obtained by changing the temperature and the time of equilibration of the various supports with the C-CRL and PT-CRL solutions. The biocomposites have shown different esterase activities and enantio-selectivities in the hydrolysis of p-nitrophenylacetate ( p-NPA), ethyl butyrate and (±)-methyl-2-(4-chlorophenoxy) propionate as a function of the nature of the support and of the time and temperature of equilibration. These results have been interpreted on the basis of a selective adsorption of different isoforms of the enzyme. The biocomposites can be stored for more than 1 month at 4 °C and can be used for several cycles without a significant decrease in catalytic activity.

Photophysics of Sulfur-Containing Centers on AgBr Surfaces

Silver halide microcrystals, the basic element in photographic products, are commonly reacted with labile sulfur-containing compounds to increase their photographic efficiency (speed). Such AgBr microcrystals have been examined with computational and photophysical techniques. Density functional and lattice-relaxation computations have given estimates of the optical transition energies and the electron- and hole-trapping abilities of a number of small sulfur-containing clusters that may be deposited on, or buried in, an AgBr surface. These data indicate that single sulfides, whatever their structure, are not deep traps for electrons or holes. Disulfides, in general, can trap holes and certain structures; for example, a disulfide at a double-kink site can trap electrons. Computations indicate that single sulfides substituted for a halogen in the (001) surface and charge compensated with an interstitial silver ion in the surface or in the bulk do not trap electrons; neither do disulfides in the bulk. A single sulfide substituted for a bromide at a positive kink and compensated with an interstitial silver ion appears to be capable of initiating the formation of small, stable silver clusters (Ag n , n 4) that become latent image, the centers that eventually lead to a visible photographic image. Photographic data from sulfur-sensitized AgBr microcrystals indicate that the optimum formal sulfur level is about 10 000 S atoms/μm 2 . Radio-frequency photoconductivity measurements show that deposited sulfur centers trap photoelectrons. The optimum and higher levels of sulfur deposited on the surface of AgBr microcrystals produce an absorption band (∼525 nm) and associated low-temperature luminescence (∼725 nm). The luminescence lifetime behavior and ODMR data clearly indicate that this sulfur-related absorption and emission is due to a donor-acceptor processes. ODMR spectra obtained by monitoring the intrinsic AgBr luminescence (590 nm) have a resonance that is assigned to a Ag 2 + center. This Ag 2 + species is thought to arise from a silver dimer associated with a sulfur species at the surface.

Mössbauer study of the Impact of Hydrogen Fluoride on Tin Probe Ions Located on the Surface of Cr2O3 microcrystals

Room-temperature exposure of tin-doped Cr2O3 surfaces to HF induces in the 119Sn Mössbauer spectra changes that depend on the valence state of the tin probe and show differences between the Sn(II)- and Sn(IV)-containing Cr2O3 crystallite surfaces. In the Sn(II)-doped samples (Sn(II)/Cr2O3), the Sn and Cr surface cations are coordinatively unsaturated, and upon exposure to HF chemical corrosion occurs. Preliminary contact of Sn(II)/Cr2O3 with oxygen results in oxidation of Sn(II) to Sn(IV) and concomitant saturation of the coordination sphere of the surface cations; the apparent activity of HF then drastically decreases (passivation effect). Similar phenomena were previously observed for Sn(II)/ and Sn(IV)/Cr2O3 samples exposed to HCl or H2S, i.e., acid gases containing anions much larger than F−. Thus, the behavior of the dopant cations in the presence of acid gases should not be imputed to size incompatibility between oxygen vacancies and foreign anions coming onto the surface from its gaseous surrounding.