Nanoparticle size effects on methanol electrochemical oxidation on carbon supported platinum catalysts

The particle size effect observed on the performance of Pt/C electrocatalysts toward the methanol oxidation reaction (MOR) has been investigated with differential electrochemical mass spectrometry (DEMS). The investigation has been conducted under both potentiodynamic and potentiostatic conditions as research on methanol electrochemical oxidation is closely related to interest in direct methanol fuel cells. The particle size effect observed on the MOR is commonly regarded as a reflection of different Pt-CO and Pt-OH bond strengths for different particle sizes. This work focuses mainly on the mechanism of methanol dehydrogenation on platinum which is central to the problem of the optimization of the efficiency of methanol electro-oxidation by favoring the CO(2) formation pathway. It was found that the partitioning of the methanol precursor among the end products on supported platinum nanoparticles is strongly dependent on particle size distribution. Also, it is postulated that the coupling among particles of different sizes via soluble products must be considered in order to understand the particle size effects on the observed trends of product formation. An optimum particle size range for efficiently electro-oxidizing methanol to CO(2) was found between 3 and 10 nm, and loss in efficiency is mostly related to the partial oxidation of methanol to formaldehyde on either too small or too large particles. The possible reasons for these observations are also discussed.     

Magnetic properties of cobalt ferrite-silica nanocomposites prepared by a sol-gel autocombustion technique

The magnetic properties of cobalt ferrite-silica nanocomposites with different concentrations (15, 30, and 50 wt %) and sizes (7, 16, and 28 nm) of ferrite particles have been studied by static magnetization measurements and Mossbauer spectroscopy. The results indicate a superparamagnetic behavior of the nanoparticles, with weak interactions slightly increasing with the cobalt ferrite content and with the particle size. From high-field Mossbauer spectra at low temperatures, the cationic distribution and the degree of spin canting have been estimated and both parameters are only slightly dependent on the particle size. The magnetic anisotropy constant increases with decreasing particle size, but in contrast to many other systems, the cobalt ferrite nanoparticles are found to have an anisotropy constant that is smaller than the bulk value. This can be explained by the distribution of the cations. The weak dependence of spin canting degree on particle size indicates that the spin canting is not simply a surface phenomenon but also occurs in the interiors of the particles.     

Correlation between crystallite size and photocatalytic performance of micrometer-sized monoclinic WO3 particles

The correlation between crystallite size and the characteristics of micrometer-sized photocatalyst particles was investigated. As a model of photocatalyst, monoclinic tungsten trioxide particles with controllable crystallite sizes were used. The crystallite size was controlled independently in the constant particle outer diameters to comprehend the crystallite size parameters precisely. To minimize the misleading photocatalytic measurements due to the over-dominancy of other catalytic parameters (such as excessive surface area and quantum confinement effect), the present study utilized micrometer-sized particles. The results revealed that in the constant process condition, the photocatalytic properties were strongly dependent on the material crystallinity. Increases in the crystallite sizes had a strong influence to the enhancement of the photodecomposition rate of organic material. The tendency for the impact of crystallite size was also confirmed by varying the number of catalysts in the photocatalytic process. To confirm the analysis of photocatalysis, the study was completed with the theoretical consideration and the proposal of the particle formation as well as the phenomena that happen during the photocatalytic process.     

Liquid-phase catalytic degradation of polyethylene wax over MFI zeolites with different particle sizes

The liquid-phase catalytic degradation of polyethylene wax into fuel oil was studied over MFI catalysts with different particle sizes (0.2 to 4.0 μm). Although structures and acidities of these catalysts were almost the same, the degradation activity varied greatly according to their particle size. The small particle catalyst showed higher activity due to its larger external surface, due to the low restriction on the mass transfer of large polymer molecules. Product distribution was also dependent on the particle size, producing higher hydrocarbons in the liquid product on the catalyst of small particles. The particle size effect of MFI zeolite on its activity and product composition in the liquid-phase degradation was discussed.     

Understanding the effect of cobalt particle size on Fischer-Tropsch synthesis: surface species and mechanistic studies by SSITKA and kinetic isotope effect

Co/γ-Al(2)O(3) catalysts with particle sizes in the range of 4-15 nm were investigated by isothermal hydrogenation (IH), temperature programmed hydrogenation (TPH), and steady-state isotopic transient kinetic analysis (SSITKA). Kinetic isotope effect experiments were used to probe possible mechanisms on Co/γ-Al(2)O(3) with different particle size. It was found that CO dissociated on Co/γ-Al(2)O(3) catalysts at 210 °C. The total amount of CO(2) formed following the dissociation depends on the cobalt crystal size. O-Co binding energy was found to be highly dependent on the Co metal particle size, whereas similar C-Co binding energy was found on catalysts with different Co particle size. Very strongly bonded carbon and oxygen surface species increased with decreasing particle size and acted as site blocking species in the methanation reaction. SSITKA experiments showed that the intrinsic activity (1/τ(CH(x))) remained constant as the particle size increased from 4 to 15 nm. The number of surface intermediates (N(CH(x))) increased with increasing particle size. The apparent activation energies were found similar for these catalysts, about 85 kJ/mol. D(2)-H(2) switches further confirmed that the particle size did not change the kinetically relevant steps in the reaction. The reactivity of the active sites on the 4 nm particles was the same as those on the 8, 11, and 15 nm particles, and only the number of total available surface active sites was less on the 4 nm particles than on the others.     

Size-dependent surface CO stretching frequency investigations on nanodiamond particles

In this work, the spectroscopic properties of surface functionalized nanodiamond particles are investigated via Fourier transform infrared spectroscopy. The functionalization of the nanodiamond surface was achieved chemically using strong acid treatment method. The size dependent C=O stretching frequency (between 1680 and 1820 cm(-1)) are studied for particle diameter sizes from the 5 to 500 nm range. The surface C=O stretching frequencies at approximately 1820 cm(-1), for large particle size (500 nm), down shifted to 1725 cm(-1) (5 nm) with decreasing particle sizes. We attributed the shift as a result of hydrogen bond formation between the COOH groups in the carboxylated nanodiamond surfaces. Particle size was characterized with dynamic light scattering method and surface morphology of the particles was investigated with scanning electron microscopy. The influence of pH value on C=O stretching frequency is also analyzed. This finding affords useful information for the studying of surface functionalized nanodiamonds with implications for their interaction with biomolecules.     

A rotating disk electrode study of the particle size effects of Pt for the hydrogen oxidation reaction

By using a catalyst-lean thin-film RDE method, the fast kinetics of the hydrogen oxidation reaction (HOR) on highly dispersed Pt nanoparticle electrocatalysts can be determined, free from the interference of the mass transport of H(2) molecules in solution. Measurements with carbon-supported Pt nanoparticles of different sizes thus allow revealing the particle size effect of Pt for the HOR. It is shown that there is a "negative" particle size effect of Pt on the kinetics of HOR, i.e., the exchange current density j(0) decreases with the increased dispersion (i.e. decreased mean particle size). A maximum mass activity of Pt for the HOR is found at particle sizes of 3-3.5 nm. The observed particle size effect is interpreted in terms of the size dependent distribution of surface atoms on the facets and edges, which is implied by the voltammetric responses of Pt/C catalysts with differently sized Pt particles. The accompanied decrease in the HOR activity with the increase in the edge atom fraction suggests that the edge atoms on the surface of Pt nanoparticles are less active for the HOR than those on the facets.     

Preparation of chitosan nanoparticles using methacrylic acid

In this work the preparation of chitosan nanoparticle was investigated using methacrylic acid in different conditions and studied by particle size analyzer, zeta-potential, Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM). The particle size was dependent on the chitosan concentration used during the preparation method. Nanoparticles with sizes as small as 60 nm were achieved, that can be extremely important for several applications. The nanoparticles solution was also pH-sensitive, due to swelling and aggregation of the nanoparticles. The nanoparticles obtained presented a very homogeneous morphology showing a quite uniform particles size distribution and a rather spherical shape.     

Synthesis and Characterization of Gold Nanoparticle‐Functionalized Ordered Mesoporous Materials

We report the synthesis and characterization of three different ordered mesoporous materials, labeled MCM‐48, SBA‐155, and SBA‐16 type materials, which were functionalized with gold nanoparticles using three different strategies. The functionalization strategies can be categorized as (i) in situ growth of gold nanoparticles, (ii) template loading, and (iii) diffusion loading of prefabricated gold nanoparticles. Two different particle sizes were employed in the latter two strategies, 5 nm and 10 nm. For all mesoporous structures, functionalization strategies, and particle sizes attempted, the materials retained their long‐range order upon incorporation of nanoparticles. From the adsorption isotherms, incorporation of gold nanoparticles altered the pore structure of the mesoporous support of some of the SBA‐15 and SBA‐16 type materials, with the effect on incorporation on the pore structure being particle size dependent in most cases. The majority of gold nanoparticles were found to reside on the external surface of the materials regardless of substrate and functionalization strategy; however, for the in situ synthesis and the template loading strategies, a significant fraction of the particles was determined to reside within the pore system of the material. In situ growth resulted in the highest content of gold nanoparticles in the solid phase. The relative effectiveness in retaining gold nanoparticles in the solid phase for each functionalization strategy was determined to be, in descending order, in situ synthesis, template loading, and diffusion loading.     

Polarization behavior of polystyrene particles under direct current and low‐frequency (<1 kHz) electric fields in dielectrophoretic systems

The relative polarization behavior of micron and submicron polystyrene particles was investigated under direct current and very low frequency (<1 kHz) alternating current electric fields. Relative polarization of particles with respect to the suspending medium is expressed in terms of the Clausius–Mossotti factor, a parameter of crucial importance in dielectrophoretic‐based operations. Particle relative polarization was studied by employing insulator‐based dielectrophoretic (iDEP) devices. The effects of particle size, medium conductivity, and frequency (10–1000 Hz) of the applied electric potential on particle response were assessed through experiments and mathematical modeling with COMSOL Multiphysics^®. Particles of different sizes (100–1000 nm diameters) were introduced into iDEP devices fabricated from polydimethylsiloxane (PDMS) and their dielectrophoretic responses under direct and alternating current electric fields were recorded and analyzed in the form of images and videos. The results illustrated that particle polarizability and dielectrophoretic response depend greatly on particle size and the frequency of the electric field. Small particles tend to exhibit positive DEP at higher frequencies (200–1000 Hz), while large particles exhibit negative DEP at lower frequencies (20–200 Hz). These differences in relative polarization can be used for the design of iDEP‐based separations and analysis of particle mixtures.     

Size‐controllable synthesis of polymeric iodine‐carrying nanoparticles for medical CT imaging

Iodine‐carrying nanopolymers with particle sizes ranging from 30 to 930 nm were synthesized by microemulsion polymerization. 2‐Methacryloyloxyethyl(2,3,5‐triiodobenzoate) was used as the monomer. Different surfactants were tested to control the particle size. Cetyltrimethylammonium bromide and cetyltrimethylammonium chloride were used as cationic surfactants, and sodium oleate and sodium dodecyl sulfate acted as anionic surfactants. The influences of various reaction parameters, e.g. the amount of surfactant, amount of initiator, and the reaction temperature, were investigated. The particle size was highly adjustable through variation of the reaction parameters. The particles were imaged with an atomic force microscope. In addition, particle workup for further medical application was explored. The particles provided good visible computer tomography contrast. Copyright © 2017 John Wiley & Sons, Ltd.     

Synthesis of large,high‐solid‐content latexes by miniemulsion polymerization

Multiple and diverse applications have been recently found for miniemulsions and miniemulsion polymerization. In this work, miniemulsion polymerization is presented as a suitable technique for the preparation of high‐solid‐content latices with large particle sizes. Monomer miniemulsions were prepared with a high‐pressure homogenizer, and droplet sizes of 200–700 nm were obtained. Latexes with particle sizes larger than the sizes commonly accepted for miniemulsion polymerization were obtained. With fixed operational conditions of the homogenizer, the type of stabilizer was the key parameter determining the droplet size and the droplet size distribution. The particle size of the latices obtained by miniemulsion polymerization indicated that the particles were mainly formed by droplet nucleation. Latexes obtained by this process have multiple applications, including use as seeds in the polymerization of high‐solid‐content latices. This article shows that potential new applications for miniemulsion polymerization are far from being exhausted. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4222–4227, 2004     

Particle size analyses of porous silica and hybrid silica chromatographic support particles. Comparison of flow/hyperlayer field-flow fractionation with scanning electron microscopy, electrical sensing zone, and static light scattering

Porous silica and hybrid silica chromatographic support particles having particle diameters ranging approximately from 1 microm to 15 microm have been characterized by flow/hyperlayer field-flow fractionation (FFF). The particle size accuracy has been improved significantly in this work by a second-order polynomial calibration. Very good agreement between the FFF data and scanning electron microscopic (SEM) results has been achieved. The effects of particle porosity, pore sizes, and particle sizes on the particle size accuracy in electrical sensing zone (ESZ) analyses have been discussed. It has been demonstrated by computer simulation and experimental measurements that false peaks can be generated in certain particle size regions when the static light scattering (SLS) technique is applied to tightly distributed spherical chromatographic support particles.     

Ostwald ripening of binary alloy particles

Classical Lifshitz-Slyozov-Wagner theory is generalized for Ostwald ripening of particles composed of random binary alloy. We show that the steady state ripening process is characterized by self-similar particle size and composition distributions. The shape of particle size distribution depends on whether the process is diffusion controlled (Lifshitz-Slyozov) or reaction controlled (Wagner) and is consistent with the predictions of classical theory for one-component materials. The steady state composition distribution, in contrast, has the same functional form in both extreme cases featuring a universal dependence of the composition upon particle size. We also found that transients in particle's composition can be very quick resulting in a steady state distribution well before it is reached by particles sizes. These transients involve significant changes in particle sizes and open an opportunity for producing metastable particle size distributions of required shape.     

Impact of Hydrogenolysis on the Selectivity of the Fischer–Tropsch Synthesis: Diesel Fuel Production over Mesoporous Zeolite‐Y‐Supported Cobalt Nanoparticles

Selectivity control is a challenging goal in Fischer–Tropsch (FT) synthesis. Hydrogenolysis is known to occur during FT synthesis, but its impact on product selectivity has been overlooked. Demonstrated herein is that effective control of hydrogenolysis by using mesoporous zeolite Y‐supported cobalt nanoparticles can enhance the diesel fuel selectivity while keeping methane selectivity low. The sizes of the cobalt particles and mesopores are key factors which determine the selectivity both in FT synthesis and in hydrogenolysis of n‐hexadecane, a model compound of heavier hydrocarbons. The diesel fuel selectivity in FT synthesis can reach 60 % with a CH₄ selectivity of 5 % over a Na‐type mesoporous Y‐supported cobalt catalyst with medium mean sizes of 8.4 nm (Co particles) and 15 nm (mesopores). These findings offer a new strategy to tune the product selectivity and possible interpretations of the effect of cobalt particle size and the effect of support pore size in FT synthesis.     

Design and FESEM/EDX investigation of functional magnetic nanocomposite particles

Recently, magnetic nanoparticles and nanocomposite microspheres have attracted great interest for biomedical and technical application. Magnetic metal nanoparticles are of special interest due to their beneficial, size‐dependent magnetic properties. Superparamagnetic metal nanoparticles and mesoscale nanocomposite particles (viz. Co nanoparticles, Co@SiO₂, and Co@SiO₂@TiO₂ particles) were obtained by a three‐step synthesis, involving consecutive steps of thermolysis and sol–gel procedures. A high‐resolution Schottky‐type field emission scanning electron microscope (FESEM) equipped with an energy dispersive X‐ray spectrometer was used to characterize intermediate and final products at the successive stages of synthesis. The samples were deposited on carbon‐coated transmission electron microscopy (TEM) grids (thin film technique) which afforded enhanced specimen contrast and reduced X‐ray background contribution in microanalysis. The FESEM was equipped with a special mounting device for these grids with an appropriate detector beneath. By this method, the samples, covering sizes from the nanometer to micron scale, could be characterized and analyzed by several imaging modes, viz. with standard SE and BSE detection mode and supplementary with low‐voltage scanning transmission mode (STEM‐in‐SEM) and fundamental information about particle size, morphology, and elemental distribution was obtained. Copyright © 2012 John Wiley & Sons, Ltd.     

Encapsulation of inorganic particles via miniemulsion polymerization

The encapsulation of TiO₂ particles via miniemulsion polymerization is strongly dependent on the size and stability of the inorganic particles in the monomer medium in which they are initially dispersed. It was found from XPS and FT‐IR studies that both the hydrophilic and hydrophobic TiO₂ particles, which were studied, have hydroxyl groups present on their surfaces, which can strongly interact with the amine end‐groups of the polymeric stabilizer, OLOA370 (polybutene‐succinimide diethyl triamine). It was found from the dispersion and adsorption studies that the amount of OLOA370 retained on the TiO₂ particles is strongly dependent on the area exposed by the sonification that is applied to break up the aggregates in the dispersion process. The TiO₂ dispersions in styrene monomer were themselves dispersed as miniemulsion droplets and subsequently polymerized. It was concluded from the density gradient column (DGC) analysis of the latexes obtained from the encapsulation polymerizations, that the stability of the inorganic particles in the monomer, as well as their particle size, significantly influence the encapsulation efficiencies. The use of the hydrophilic titanium dioxide particles in combination with the stabilizer, OLOA370, resulted in a good dispersibility, dispersion stability, and small TiO₂ particle size. This lead to better encapsulation efficiencies compared to the hydrophobic particles. The poorer results obtained with the hydrophobic TiO₂ particles were attributed to their larger particle size, which resulted from the reduced adsorption of the OLOA370. Fewer hydroxyls and the presence of the trimethoxy octyl silane (TMOS) groups, which themselves are unable to provide sufficient steric stability, are proposed to explain these findings.     

Surface-enhanced fluorescence of fluorescein-labeled oligonucleotides capped on silver nanoparticles

Tiopronin monolayer-protected silver nanoparticles with different core sizes (average diameter = 2, 5, 20 nm) were prepared by using different mole ratios of silver nitrate/tiopronin. Ligands on the silver particles were partially displaced by fluorescein-labeled thiolate single-stranded oligonucleotides or their complementary unlabeled oligonucleotides through ligand exchange. The fluorophores on silver particles showed a surface-enhanced fluorescence (SEF) dependent on the size of metallic cores. The particles could be coupled through hybridizations of oligonucleotides bound on the particles. The coupled particles were aggregated due to multiple displacements of oligonucleotides on each particle, resulting in stronger SEF. The dye-labeled oligonucleotides were assembled on the silver islands on the solid substrate, and the complementary oligonucleotide-displaced particles were coupled via oligonucleotide hybridization. The couplings between particles and islands resulted in an obvious fluorescence enhancement.     

SAXS and TEM Investigations on Thermoplastic Nanocomposites Containing Functionalized Silica Nanoparticles

Transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) were used to characterize the morphology of thermoplastic nanocomposites. These materials were based on a thermoplastic matrix of a copolymer of methylmethacrylate (MMA) and 2-hydroxyethylmethacrylate (HEMA) with spherical 10 nm silica particles as a filler (filler content 2, 5 and 10 vol%, respectively). Depending on the surface modification of the particles, it was possible to control the aggregation tendency of the primary filler particles. With uncoated particles large aggregates about 100 nm in size could be observed by TEM. For nanocomposites containing particles coated with methacryloxypropyltrimethoxysilane (MPTS), TEM showed that the particles were better dispersed in the polymer matrix only forming aggregates comprised of two or three primary particles. In comparison to the TEM results, the volume weighted particle size distribution calculated from SAXS for the systems with uncoated particles is monomodal and shows particle sizes in the range of primary particles whereas the systems with MPTS coated particles revealed a bimodal size distribution with particle sizes comparable to those measured with TEM. To obtain complete information about the morphology of the nanocomposites above the critical upper limit of detectable scattering vectors (particle sizes >50 nm) SAXS has to be supported by TEM, whereas in the nanosize range below the critical limit both methods exhibit an excellent correspondence.     

Determination of Band Structure Parameters and the Quasi‐Particle Gap of CdSe Quantum Dots by Cyclic Voltammetry

Band structure parameters such as the conduction band edge, the valence band edge and the quasi‐particle gap of diffusing CdSe quantum dots (Q‐dots) of various sizes were determined using cyclic voltammetry. These parameters are strongly dependent on the size of the Q‐dots. The results obtained from voltammetric measurements are compared to spectroscopic and theoretical data. The fit obtained to the reported calculations based on the semi‐empirical pseudopotential method (SEPM)—especially in the strong size‐confinement region, is the best reported so far, according to our knowledge. For the smallest CdSe Q‐dots, the difference between the quasi‐particle gap and the optical band gap gives the electron–hole Coulombic interaction energy (J_e1,h1). Interband states seen in the photoluminescence spectra were verified with cyclic voltammetry measurements.