Low Precursor Concentration Research Articles

In Situ and Ex Situ Studies of Platinum Nanocrystals: Growth and Evolution in Solution

In situ studies on the growth and evolution of platinum nanocrystals in solution were carried out using synchrotron-based X-ray diffraction (XRD) techniques. Ex situ low- and high-resolution transmission electron microscopy (TEM) were used to investigate the nanocrystal morphologies through the different growth stages. In a reaction with low precursor concentration, both XRD and TEM results show that growth occurs at a relatively slow rate and yields faceted morphologies, which are characteristic of a thermodynamically controlled regime. In contrast, the platinum nanocrystals in the high-concentration reaction form branched structures and grow at much greater rates under a kinetically controlled regime. Additionally the growth mechanism of the high-concentration reaction involves a morphology transformation from octapod-like shapes to porous nanostructures, which is brought about by a novel mechanism involving selective growth and etching processes that occur simultaneously and at comparable rates.

Template-free synthesis and gas sensing properties of well-controlled porous tin oxide nanospheres

High yield of tin dioxide (SnO 2) nanospheres with various degrees porosity have been successfully synthesized without any physical template via an economical, easily scalable hydrothermal process. The porosity or the specific surface area of SnO 2 nanospheres can effectively be tuned via precursor concentration, temperature and the removal of additive urea. Higher reaction temperature, low precursor concentration and elimination of urea additive favorably increase the porosity of the nanospheres. The gas sensing properties of the nanospheres show that the operating temperature has a strong influence on the response of the sensor whereby a higher operating temperature leads to greater change in conductance and hence, greater response. Other than the specific surface area, pore diameter of the nanospheres has shown to have an effect on the sensing behaviour. The response time is dependent on the operating temperature, 90% of resistance change ( R air– R gas) was achieved within the first 1.3–3.0 min.

Spray-Coated Fluorine-Free Superhydrophobic Coatings with Easy Repairability and Applicability

The present paper reports a very simple and low-cost fluorine-free superhydrophobic coating prepared by spray-coating metal alkylcarboxylates, for example, Cu[CH(3)(CH(2))(10)COO](2), onto virtually any substrates. Superhydrophobicity with a static water contact angle of about 160 degrees and a sliding angle of 5 degrees was achieved from the proper precursor concentration. The advantages of the present approach include the cheap and fluorine-free raw materials, environmentally benign solvents, an industrial implementation method, and easy repairability and applicability so as to make a great application potential in practice. The hydrophobicity of coatings and the adhesion to water were found to be dependent on the surface morphology that was governed by the precursor concentrations from which coatings were prepared. The static wetting behavior of water droplets with different sizes gentlly deposited on the coatings was studied in more detail and correlated to theories, i.e., Wenzel's and Cassie's models. The results indicated that nanoribbon-textured coatings prepared from low precursor concentration (0.02 M) exhibited a transition from the metastable Cassie-Baxter state to the Wenzel state with increments in the droplet volume, and eventually droplets firmly stick to the surface even when the droplet was gently deposited on the surface. Surface coatings with dual roughness at both microscale and nanometer scale were formed as the concentration (0.04 M) was increased and conferred a stable Cassie state, even for increased droplet size and increased droplet deposit speed.

Trioctylphosphine as Both Solvent and Stabilizer to Synthesize CdS Nanorods.

High quality CdS nanorods are synthesized reproducibly with cadmium acetate and sulfur as precursors in trioctylphosphine solution. The morphology, crystalline form and phase composition of CdS nanorods are characterized by transmission electron microscopy (TEM), high-resolution TEM and X-ray diffraction (XRD). CdS nanorods obtained are uniform with an aspect ratio of about 5:1 and in a wurtzite structure. The influence of reaction conditions on the growth of CdS nanorods demonstrates that low precursor concentration and high reaction temperature (260 °C) are favorable for the formation of uniform CdS nanorods with 85.3% of product yield.

A proper amount of carbon nanotubes for improving the performance of Pt–Ru/C catalysts for methanol electro-oxidation

Abstract This study aims to improve the performance of the anode catalyst in a direct methanol fuel cell by using carbon black (XC) and mesoporous carbon (MC) as supporting materials for preparing Pt–Ru/XC and Pt–Ru/MC catalysts. This study investigates the effect of adding different amounts of bare carbon nanotubes (CNTs) or carbon nanotubes impregnated with Pt and Ru (abbreviated as Pt–Ru/CNT, containing 10 wt.% Pt and Ru) to the prepared catalysts. Experimental results reveal that 10 wt.% Pt–Ru/C with carbon black and mesoporous carbon prepared by the multiple impregnation method had smaller Pt–Ru grain sizes and a better dispersion or carbon supports due to low precursor concentrations in each impregnation. These, in turn, achieved better electro-catalytic performance for methanol oxidation. Adding CNTs or Pt–Ru/CNT to Pt–Ru/XC and Pt–Ru/MC obviously improves their electro-catalytic characteristics. The appropriate amounts of bare CNT and Pt–Ru/CNT added to Pt–Ru/XC and Pt–Ru/MC catalysts are 5% and 20%, respectively. The resulting catalysts (both containing 10 wt.% Pt and Ru) produce activities similar to those of the E-TEK Pt–Ru/C catalyst containing 20 wt.% Pt and Ru.

Seed layer-free electrodeposition of well-aligned ZnO submicron rod arrays via a simple aqueous electrolyte

A potentiostatic electrodeposition technique was used to directly fabricate large-scale, well-aligned, and single-crystalline submicron ZnO rod arrays on tin doped indium oxide glass substrate without a pre-prepared seed layer of ZnO from an aqueous solution only containing zinc nitrate. The effects of electrochemical parameters, such as electrodeposition potential, electrodeposition duration, solution temperature, and precursor concentration, on the orientation, morphology, aspect ratio, and growth rate of ZnO rod arrays were systematically investigated. Results show that submicron ZnO rod arrays with (0 0 0 2) preferred orientation and perfect crystallization were obtained when electrodeposition potential was in the range from −0.6 to −1.1 V and solution temperature was controlled above 60 °C. Both high solution temperature and low precursor concentration resulted in the decrease in rod diameters. Photoluminescence measures showed that small diameter and nanotips of ZnO rod arrays should be responsible for strong and sharp ultraviolet emission in the room temperature photoluminescence spectra.

Micropatterning of Hydrogels by Soft Embossing

Conventional in situ hydrogel micropatterning techniques work successfully for relatively stiff hydrogels, but they often result in locally damaged surfaces upon demolding in the case of soft and fragile polymer networks formed at low precursor concentration. To overcome this limitation, we have developed a versatile method, termed soft embossing, for the topographical micropatterning of fragile chemically cross-linked polymer hydrogels. Soft embossing is based on the imprinting of a microstructured template into a gel surface that is only partially cross-linked. Free functional groups continue to be consumed and upon complete cross-linking irreversibly confine the microstructure on the gel surface. Here we identify and optimize the parameters that control the soft embossing process and show that this method allows the fabrication of desired topographies with good fidelity. Finally, one of the produced gel micropatterns, an array of microwells, was successfully utilized forculturing and analyzing live single hematopoietic stem cells. Confining the stem cells to their microwells allowed for efficient quantification of their growth potential during in vitro culturing.

Growth of zirconia particles by heterocoagulation, self-assembly, and convection-induced coagulation during forced hydrolysis in a refluxed reaction vessel

In literature related to forced hydrolysis of zirconia, heterocoagulation between large particulates and primary crystallites is generally accepted as the principal route of growth. Our dynamic light scattering (DLS) results revealed an S-shaped growth curve with the growth rate (slope) varying notably from one stage to the other, implying that other coagulation mechanisms may be present. Time-sequenced side-by-side electron microscopy and DLS analyses were conducted along the curves alongside with field emission scanning electron microscopy of full-grown particles. We find that particles growth follows a two-stage heterocoagulation process with primary crystallites and small particles produced from the first-stage serving as building blocks for the first and second stages, respectively. It interesting to note that growth during the second stage occurs by self-assembly and a hypothesis is proposed. Convection-induced coagulation leads to enhanced particle growth for a low precursor concentration but minimal growth for a higher one, both of which conform to the predictions of Melis et al. [AIChE J. 45, 1383 (1999)].

Thermolytic Growth of ZnO Nanocrystals: Morphology Control and Optical Properties

ZnO nanostructures with triangular and rod-like morphologies could be grown by thermolysis of zinc acetate in oleylamine solvent using oleic acid as surfactant. Concentration of the zinc precursor in the reaction mixture had a strong effect on the final morphology of the ZnO nanostructures. While a low precursor concentration results in triangular morphology, high precursor concentrations favor rod-like morphologies of the nanostructures. The room temperature photoluminescence (PL) spectra of both nanostructures are dominated by the green emission attributed to the oxygen vacancy (VO) related donor−acceptor transition. Presence of several infrared (IR) inactive vibrational modes in the Fourier transform infrared (FT-IR) absorption spectra of the samples indicates a breakdown of translational symmetry in the nanostructures induced by native defects. Possible mechanisms of the morphology evolution and the origin of observed visible emissions in the nanostructures are discussed.

A Comprehensive Study of Titanium-Substituted SBA-15 Mesoporous Materials Prepared by Direct Synthesis

The influence of different synthesis parameters on the physicochemical properties of titanium-substituted SBA-15 silicas prepared by direct one-step synthesis was systematically studied by a combination of N2 physisorption at −196 °C, X-ray diffraction (XRD), diffuse reflectance UV−vis, and elemental analysis. The results showed that for low titanium precursor concentrations used in the initial synthesis gel, incorporation of titanium in the final material is not complete. Under these synthesis conditions, Ti atoms are well-dispersed in the silica framework and mainly exist in tetrahedral coordination. When the isolated titanium species present in the gel reach a critical concentration, an increase in incorporation ratio is observed due to the formation of anatase TiO2 clusters on the materials surface. It was also shown that this critical titanium loading is mainly influenced by synthesis temperature, hydrothermal treatment time, and silicon precursor concentration and does not depend on acid concentrati...

Size-tunable near-infrared PbS nanoparticles synthesized from lead carboxylate and sulfurwith oleylamine as stabilizer

High quality PbS nanocrystals are synthesized reproducibly through lead stearate andsulfur stabilized by oleylamine in a non-coordinating solvent. The morphology,crystalline form and phase composition of PbS nanocrystals are examined bytransmission electron microscopy (TEM), high-resolution TEM, x-ray diffraction(XRD), energy-dispersive x-ray spectroscopy (EDS) and x-ray photoelectronspectroscopy (XPS). The as-synthesized PbS nanocrystals have strong absorptionand photoluminescence emissions in the near-infrared region. The size of PbSnanocrystals from 5 to 13 nm can be adjusted through the optimization of the synthesisconditions. The smaller PbS nanoparticles are obtained at the lower reactiontemperature, lower precursor concentration, larger oleylamine quantity and larger leadprecursor/sulfur ratio. The basic oleylamine enhances the reactivity of both lead stearate precursor andsulfur precursor in the reaction.

Supersensitivity of Transition-Metal Nanoparticle Formation to Initial Precursor Concentration and Reaction Temperature: Understanding Its Origins

A supersensitivity of the formation and stabilization of transition-metal nanoclusters to the initial nanocluster concentration and temperature synthesis conditions is reported, then probed, herein for the specific case of prototype Ir(O)n nanoclusters prepared from the organometallic precursor [Bu4N]5Na3[(1,5-COD)Ir x P2W15Nb3O62] by reduction with H2 in propylene carbonate solvent. Fully isolable, redissolvable, near-monodisperse (i.e., < or = +/- 15% size distribution) and thus excellent Ir(O)n nanoclusters are formed using low temperature (22 degrees C) and moderate precursor concentration (1.2 mM) in propylene carbonate solvent. However, inferior, polydisperse (+/- 40% size distribution), non-redispersable nanoclusters are formed at the seemingly only moderately different conditions of 38 degrees C higher temperature (i.e., 60 degrees C) and 5-fold lower precursor concentration (0.24 mM). Investigation of this supersensitivity to the nanocluster synthesis conditions reveals that it derives from the dissociation of (1,5-COD)Ir(solvent)2+ from the P2W15Nb3O62(9-) polyanionic ligand/ stabilizer, subsequently resulting in a too fast, kinetically uncontrolled reduction of the quickly reduced (1,5-COD)Ir(solvent)2+ as the cause of the inferior synthesis of polydisperse, non-isolable, non-redissolvable nanoclusters. The results are significant in that they illustrate that understanding the mechanism of nanocluster formation, and then performing the nanocluster synthesis under kinetically carefully controlled, understood conditions, is necessary for the formation of superior nanoclusters in this, and by implication probably many other, cases.

The effect of processing conditions on the photoluminescence properties of Ca(Y 0.94Eu 0.06)BO 4 phosphors synthesized by ultrasonic spray pyrolysis

The effect of processing conditions on the photoluminescence properties of the Ca(Y 0.94Eu 0.06)BO 4 phosphors that were synthesized by ultra-sonic spray pyrolysis was investigated. The morphology and size of the Ca(Y 0.94Eu 0.06)BO 4 phosphors were controlled by changing processing conditions, including precursor concentration, organic additives content, and air flow rate, and substantially affected their luminescent properties. Lower precursor concentration yielded higher emission intensity. High emission intensity was obtained for an air flow rate of 5–20 l/min. The magnitude of the emission intensity on organic additive concentration followed the order of 0.5 M>0.3 M>0.7 M.

An evaluation of reaction probabilities of sulfate and nitrate precursors onto East Asian dust particles

We investigate the chemical evolution of dust particles and examine the magnitudes of reaction probability (γ) of sulfate and nitrate precursors (such as H2SO4, SO2, N2O5, and HNO3) onto dust particles in East Asia. For this investigation, three data sets from ACE‐Asia U.S. National Science Foundation (NSF) C130 flights 6, 7, and 8 and three data sets from measurements in Seoul were analyzed. During the selected dust storm periods, large amounts of CO32− still remained in fine‐mode dust particles (Dp &lt; ∼1.3 μm). The average fractions of CO32− (based on [Ca2+] + [Mg2+] equivalences) for the three C130 flights were 0.87, 0.68, and 0.39, respectively, and the average CO32− fractions for the three data sets in Seoul ranged from 0.43 to 0.86. The degrees of chemical evolution of mineral dust indicated by the CO32− fractions were significantly smaller than those reported in previous dust chemistry transport modeling studies conducted with the assumption of aerosol internal mixing. Our analysis suggests that this could be due to the use of excessively high γ values in the model simulations, as further confirmed in this study with Lagrangian photochemical model simulations conducted with both the constraints observed by ACE‐Asia C130 flights and the initial concentrations obtained by U.S.‐EPA Models‐3/Community Multi‐scale Air Quality (CMAQ) modeling over East Asia. It is also found in this study that the magnitudes of γs are closely related with aerosol mixing state. In order to confirm this we conducted Lagrangian model simulations for an example case under the assumption of aerosol external mixing. Under this assumption the formations of sulfate and nitrate on/in fine‐mode mineral dust are greatly limited because of small γs onto mineral dust (γDust) and low precursor concentrations. However, it is also recommended that for more precise evaluations of γDust, a more sophisticated Lagrangian type of photochemical modeling is necessary. In addition, in this study a possible dependence of the γ values on relative humidity (RH) is also investigated. On the basis of our analysis we suggest that particular attention should be paid to the issue of the RH‐dependent γ in future dust chemistry transport modeling studies with the issue of aerosol mixing state.

Influence of porous morphology on optical dispersion properties of template free mesoporous titanium dioxide (TiO 2) films

This paper focuses the influence of porous morphology on the microstructure and optical properties of TiO 2 films prepared by different sol concentration and calcination temperatures. Mesoporous TiO 2 thin films were prepared on the glass substrates by sol–gel dip coating technique using titanium (IV) isopropoxide. Porous morphology of the films can be regulated by chemical kinetics and is studied by scanning electron microscopy. The optical dispersion parameters such as refractive index ( n), oscillator energy ( E d), and particle co-ordination number ( N c) of the mesoporous TiO 2 films were studied using Swanepoel and Wemple–DiDomenico single oscillator models. The higher precursor concentration (0.06 M), films exhibit high porosity and refractive index, which are modified under calcination treatment. Calcinated films of low metal precursor concentration (0.03 M) possess higher particle co-ordination number ( N c = 5.05) than that of 0.06 M films ( N c = 4.90) due to calcination at 400 °C. The lattice dielectric constant ( E ∞) of mesoporous TiO 2 films was determined by using Spintzer model. Urbach energy of the mesoporous films has been estimated for both concentration and the analysis revealed the strong dependence of Urbach energy on porous morphology. The influence of porous morphology on the optical dispersion properties also has been explained briefly in this paper.

Structure and electrical properties of selectively chemically vapor deposited vanadium oxide films from Vanadium tri-i-propoxy oxide vapors

Vanadium oxide films were chemically vapor deposited (CVD) on oxidized Si substrates covered with patterned Cu films using Vanadium tri-i-propoxy oxide (OV(OC 3H 7) 3) vapors. It was shown that at atmospheric pressure and at temperatures varying between 135 and 350 °C deposition was selectively occurring on the patterned Cu films only and not on the silicon oxide, which covered the Si substrate. The dependence of the selectivity on the composition of the gas phase was investigated and it was shown that low precursor concentrations enhance selectivity. X-ray diffraction measurements have shown that the deposited films were composed of a mixture of vanadium oxides; the composition dependent on the deposition temperature. At temperatures up to 150 °C films were mostly composed by monoclinic VO 2. At higher temperatures other peaks corresponding to various vanadium oxides were also observed. The well-known metal-insulator transition was observed near 70 °C for films mostly composed by monoclinic VO 2. Films deposited at 200 and 300 °C exhibited two transitions one near 47 and the other near 65 °C possibly related to the presence of V 6O 11, or of important stresses in them.

Sporangiospore lipids of the mycelial fungus Mucor ramannianus incapable of dimorphic growth

Members of the species Mucor ramannianus are believed to be monomorphic. They grow only as a mycelium and are not capable of growth as budding cells, i.e., of dimorphic growth. In our study, we investigated the lipid composition of M. ramannianus sporangiospores, which retained the capability of initiating mycelial growth in the course of long-term cultivation of the spore-forming mycelium. It was demonstrated that sporangiospores contained high concentrations of triacylglycerides (TAG) in their reserve lipids and high concentrations of phosphatidylcholine (PC) in their membrane lipids; low concentrations of methylated ergosterol precursors were detected among sterols. On the basis of the data presented, in order to evaluate the potential of mucor fungi for yeastlike growth, it has been suggested to analyze the qualitative and quantitative characteristics of their sporangiospore lipids and to consider the following criteria as the criteria of sporangiospore capacity for giving rise to yeastlike growth upon spore germination: (1) the phosphatidylethanolamine/phosphatidylcholine (PE/PC) ratio; (2) the level of ergosterol and the ratio between the methylated and demethylated sterols; as well as (3) the ratios between phospholipids and glycolipids (PL/GL) and (4) between etherified and free sterols (ES/FS).

Synthesis of titania films by combustion flame spray pyrolysis technique and its characterization for photocatalysis

The phase composition, texture and roughness of titania films deposited using combustion flame pyrolysis technique are studied as a function of substrate temperature, deposition time and precursor concentration. In this process, a liquid precursor feedstock that results in TiO 2 film is injected into a combustion flame and the film is deposited onto a silica substrate. The resulting film has anatase at low substrate temperature, rutile at high substrate temperature and a mixture of these two phases at intermediate temperatures. A large decrease in the average size of the primary particles that constitute the film is observed with low precursor concentration. The band gap energy of titania films with different phase compositions is measured using UV spectrophotometer. ZnO films are deposited from sprayed droplets of an aqueous zinc acetate solution by using this process and comparison of TiO 2 and ZnO films on the photocatalytic degradation of remazol brilliant blue is investigated.

Formation of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans from a Mixture of Chlorophenols over Fly Ash: Influence of Water Vapor

Recent efforts have been made to establish readily measurable surrogate compounds, such as chlorophenols, for polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), that would enable plant operations to limit formation. Despite the extensive studies conducted on PCDDs/Fs formation from chlorophenols, very few studies have been carried out in real combustion conditions with a realistic concentration of precursors and the presence of water. In the present study, low (10(-9) M), stable concentrations of chlorinated phenols that are representative of concentrations of such compounds in municipal waste incinerator (MWI) raw flue gas were used in experiments investigating the formation of PCDDs/Fs over fly ash. Different mixtures of the chlorophenols (CPs) studied (2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and 2,3,4,6-tetrachlorophenol) were passed through a bed of oxidized fly ash (carbon-free) and glass beads with and without the presence of water. The chlorophenol reactants used in this study were found to favor PCDD over PCDF formation. The presence of water was observed to considerably reduce the yields of all PCDD/F formed (< 0.3% phenol conversion). The PCDD homologue and isomer distributions were not affected by the presence of water, unlike the PCDF compounds. The major PCDD homologue groups formed were tetra- and penta-, both with or without water in the gas stream. The major PCDF homologue groups were mostly the lower chlorinated ones in the experiments performed in the presence or absence of water. These results contribute to the understanding of PCDD/Fs formation in realistic combustion conditions, including very low concentrations of precursors and the presence of water in the flue gas.

GROWTH CONTROL OF ZnO FILMS ON GLASS BY CHEMICAL BATH DEPOSITION

The effects of substrate-placing manner, deposition temperature and solution concentration on the preparation of ZnO films by chemical bath deposition were investigated in this work. The structures and the morphologies of as-deposited ZnO films were characterized by X-ray diffraction and scanning electron microscope, respectively. The results show that compared with vertically insertion, floating the substrates on the solutions could greatly improve the c-axis orientation of the films. The growth of ZnO films with orientation would also be favored at lower precursor concentrations and temperatures. When the concentration of Zn ( CH 3 COO )2·2 H 2 O is 0.033 M/L and the deposited temperature is 60°C, ZnO films which are well crystallized and highly c-axis orientated were obtained.