Formation of ZnO,MgO and NiO Nanoparticles from Aqueous Droplets in Flame Reactor

Nanoparticles of ZnO, MgO and NiO were produced from droplets of aqueous salt solution in the flame spray pyrolysis reactor. Conventional spray pyrolysis, in which electrical furnace reactor is used, is reported to produce nanoparticles only from acetate precursor. If the reactor pressure is low (60torr), nitrate salt precursor is also known to produce nanoparticles. In this paper, we report that nanoparticles are produced from nitrate as well as acetate salt precursor solution when propane–oxygen diffusion flame is used to decompose aqueous aerosol droplets. At low flame temperature, however, nanoparticles are not formed and the particle morphology is similar to the morphology produced by the conventional spray pyrolysis. At high flame temperature, nanoparticles are formed, regardless of the salt type. Nanoparticles are formed at lower flame temperature from acetate salts than from nitrate salts. All nanoparticle prepared in this work were fully crystallized and the size measured from transmission electron microscopy images was 30nm. This size agreed well with the particle size calculated from X-ray diffraction and specific surface area data.     

Numerical simulation of water vapor nucleation on electrically neutral nanoparticles

Atomic-level Monte Carlo simulations are performed to calculate the free energy, entropy, and work of nucleation for clusters of more than 6 × 10³ water molecules growing on silver iodide crystalline particles of size up to 4 nm at a temperature of 260 K. The Hamiltonian of the system includes explicit expressions for hydrogen bonding energy and Coulomb, dispersion, exchange, and polarization interactions. The work of nucleation exhibits complex behavior depending on the nucleation-site size. With increasing nanoparticle size, clusters become less stable and the probability of crystallization increases. Mutual polarization enhances the bonding between a cluster and a crystalline particle. Cluster growth on relatively large nanoparticles involves two stages characterized by two critical sizes: monolayer growth on the surface and growth normal to the surface. Spontaneous microdroplet polarization involving domain formation is found to occur at the crystal surface. The dependence of the ice-forming activity of an aerosol on particulate size observed in experiments is explained by combined effects of several competing factors, the dominant ones being the stabilizing and destabilizing effects of the nanoparticle electric field.     

Nanoparticle Microreactor: Application to Synthesis Of Titania by Thermal Decomposition of Titanium Tetraisopropoxide

The nanoparticle microreactor (NPMR) is a new concept that we have introduced to describe a very small-scale system capable of converting an aerosol precursor to solid particles. The liquid precursor of about 1 µl is injected by a syringe through a septum into a tubular evaporator of 1.0 cm³ in volume with stopcocks at both ends. The evaporator has been preheated by a heating tape to a temperature sufficiently high for vaporization to occur in half a minute. By opening the stopcocks, the vaporized precursor is transported by a carrier gas stream into a quartz tube which is mounted along the axis of a tubular furnace. The nanoparticle aggregates produced in the reactor are sampled by deposition on an electron micrograph grid at the reactor exit. The NPMR was applied first to the synthesis of TiO₂ particles by thermal decomposition of titanium tetraisopropoxide (TTIP) in a nitrogen carrier gas, with TTIP concentrations varying from 1.0 to 7.0 mol% or 2.35×10^–6 to 1.65×10^–5 in TiO₂ volume loading, and decomposition temperatures from 300°C to 1000°C. Studies were made with a 2 mm reaction tube and a 4 mm tube with sheath gas. With the 2 mm tube, a considerable fraction of the TTIP precursor was consumed at the wall by surface reaction, resulting in very small particles. With the 4 mm tube, the primary particle size was comparable to that reported in the literature for steady flow experiments using a 22.2 mm tube. Primary particle sizes ranged from 200 to 400 nm. Depending on TTIP concentration and reactor temperature, the particles exhibited a bimodal size distribution, probably due to a two-stage nucleation. A fourfold increase in the gas flow rate had little effect on particle size, indicating that particle growth ended early, within one-fourth the tube length. Residence time in the reactor was between 0.35 and 1.4 s, and total run time about 1 min. The NPMR has potential for rapid assembly of large databases and is adaptable to combinatorial discovery of nanoparticles with novel properties. Design requirements for an ideal aerosol microreactor are discussed briefly.     

Buoyancy induced limits for nanoparticle synthesis experiments in horizontal premixed low-pressure flat-flame reactors

Premixed low-pressure flat-flame reactors can be used to investigate the synthesis of nanoparticles. The present work examines the flow field inside such a reactor during the formation of carbon (soot) and iron oxide (from Fe(CO)₅) nanoparticles, and how it affects the measurements of nanoparticle size distribution. The symmetry of the flow and the impact of buoyancy were analysed by three-dimensional simulations and the nanoparticle size distribution was obtained by particle mass spectrometry (PMS) via molecular beam sampling at different distances from the burner. The PMS measurements showed a striking, sudden increase in particle size at a critical distance from the burner, which could be explained by the flow field predicted in the simulations. The simulation results illustrate different fluid mechanical phenomena which have caused this sudden rise in the measured particle growth. Up to the critical distance, buoyancy does not affect the flow, and an (almost) linear growth is observed in the PMS experiments. Downstream of this critical distance, buoyancy deflects the hot gas stream and leads to an asymmetric flow field with strong recirculation. These recirculation zones increase the particle residence time, inducing very large particle sizes as measured by PMS. This deviation from the assumed symmetric, one-dimensional flow field prevents the correct interpretation of the PMS results. To overcome this problem, modifications to the reactor were investigated; their suitability to reduce the flow asymmetry was analysed. Furthermore, ‘safe’ operating conditions were identified for which accurate measurements are feasible in premixed low-pressure flat-flame reactors that are transferrable to other experiments in this type of reactor. The present work supports experimentalists to find the best setup and operating conditions for their purpose.     

Preparation of luminescent Si nanoparticles by tailoring the size,crystallinity and surface composition

The preparation of sizeable quantities of luminescent Si nanoparticles (Si-np) with controlled morphology is a challenging task. Here, we describe two strategies aiming at size reduction of the Si-np produced in a laser-assisted silane pyrolysis reactor without detrimental effects on the process yield and on the nanoparticle structural and compositional properties. The first method is based on the addition of a sensitizer gas to dilute silane and consequently reduce the nucleation centres density without decreasing the reaction temperature. The second consists in the introduction of a collector at a variable distance from the irradiated region to freeze the particle growth and decrease the inter-particle sintering probability. We report on the characterization of the produced Si-np, and we show that by combining the two methods, we are able to prepare 4 nm crystalline core size nanoparticles with a productivity of 1 g per hour. We also describe the enhancement effect of the wet-chemical oxidation processes on the luminescence emission intensity of the Si-np.     

The Effect of Boundary Conditions on Gas-phase Synthesised Silver Nanoparticles

We have prepared spherical non-agglomerated silver nanoparticles by an evaporation–condensation–dilution/cooling technique. Silver was evaporated from a crucible in a tubular flow reactor. A porous tube diluter was used to quench the carrier gas at the outlet of the reactor to enhance the formation of small particles and to suppress agglomeration and other particle growth mechanisms. The number size distribution of the prepared particles was measured with a differential mobility analyser–condensation nucleus counter combination and the size and the shape of the particles were analysed with transmission electron microscope. The system was modelled using a sectional aerosol dynamics computer code to estimate the importance of different aerosol processes. In all conditions the particles obtained were non-agglomerated and spherical. The mean particle diameter varied from 4 to 10-nm depending on boundary conditions. From the modelling studies it can be concluded that the nucleation rate is the most important parameter controlling the final particle size.     

Influence of synthesis parameters on iron nanoparticle size and zeta potential

Zero valent iron nanoparticles are of increasing interest in clean water treatment applications due to their reactivity toward organic contaminants and their potential to degrade a variety of compounds. This study focuses on the effect of organophosphate stabilizers on nanoparticle characteristics, including particle size distribution and zeta potential, when the stabilizer is present during nanoparticle synthesis. Particle size distributions from DLS were obtained as a function of stabilizer type and iron precursor (FeSO₄·7H₂O or FeCl₃), and nanoparticles from 2 to 200 nm were produced. Three different organophosphate stabilizer compounds were compared in their ability to control nanoparticle size, and the size distributions obtained for particle volume demonstrated differences caused by the three stabilizers. A range of stabilizer-to-iron (0.05–0.9) and borohydride-to-iron (0.5–8) molar ratios were tested to determine the effect of concentration on nanoparticle size distribution and zeta potential. The combination of ferrous sulfate and ATMP or DTPMP phosphonate stabilizer produced stabilized nanoparticle suspensions, and the stabilizers tested resulted in varying particle size distributions. In general, higher stabilizer concentrations resulted in smaller nanoparticles, and excess borohydride did not decrease nanoparticle size. Zeta potential measurements were largely consistent with particle size distribution data and indicated the stability of the suspensions. Probe sonication, as a nanoparticle resuspension method, was minimally successful in several different organic solvents.     

Controlled synthesis of colloidal silver nanoparticles in capillary micro-flow reactor

In this study, using a polytetrafluoroethylene (PTFE) capillary tube as a micro-flow reactor, well-dispersed colloidal silver nanoparticles were controllably synthesized with different flow rates of precursory solution. Scanning transmission electron microscopy images and UV–visible absorbance spectra showed that silver nanoparticles with large size can be prepared with slow flow rate in the PTFE capillary reactor. The effects of tube diameters on the growth of colloidal silver nanoparticles were investigated. Experiment results demonstrated that using tube with small diameter was more propitious for the controllable synthesis of silver nanoparticles with different sizes.

     

Experimental study of nanoparticles penetration through commercial filter media

In this study, nanoparticle penetration was measured with a wide range of filter media using silver nanoparticles from 3 nm to 20 nm at three different face velocities in order to define nanoparticle filtration characteristics of commercial fibrous filter media. The silver particles were generated by heating a pure silver powder source via an electric furnace with a temperature of 870°C, which was found to be the optimal temperature for generating an adequate amount of silver nanoparticles for the size range specified above. After size classification using a nano-DMA, the particle counts were measured by an Ultrafine Condensation Particle Counter (UCPC) both upstream and downstream of the test filter to determine the nanoparticle penetration for each specific particle size. Particle sampling time continued long enough to detect more than 10⁵ counts at the upstream and 10 counts at the downstream sampling point so that 99.99% efficiency can be detected with the high efficiency filter. The results show a very high uniformity with small error bars for all filter media tested in this study. The particle penetration decreases continuously down to 3 nm as expected from the classical filtration theory, and together with a companion modeling paper by␣Wang et al. in this same issue, we found no significant evidence of nanoparticle thermal rebound down to 3 nm.     

Computer simulation of water vapor nucleation on charged nanoparticles

The Monte Carlo method is applied to the study of the formation of condensed-phase nuclei from water vapor on electrically charged silver iodide nanocrystals. This study is a continuation of the investigations carried out earlier in [1] with electrically neutral nucleation centers. Nanoparticles with a size of up to 4 nm and flat nanoparticles with a size of up to 10 nm are investigated. The free energy, entropy, and the work of formation of nuclei with a size of up to 6729 molecules are calculated at the atomic level by the bicanonical statistical ensemble (BSE) method at a temperature of 260 K. Thermodynamic stability of nuclei is investigated depending on the size, shape, and charge of nanocrystal nucleation centers, as well as depending on the presence of crystal defects and the degree of spatial localization of charge on the surface of nanoparticles. The excess charge has a crucial effect on the work of formation of a nucleus only in the case of strong spatial localization of the latter near a point crystal defect; however, this effect is restricted to a relatively small size of the nuclei and therefore cannot substantially enhance the ice-forming activity of nanoparticles. A nucleus that grows on the surface of a nanoparticle evolves through three stages that differ in molecule retention mechanism and thermodynamic stability. The charge of a nanoparticle has a small effect on these factors. The leading factor that determines the ice-forming activity of ion nanocrystals is their intrinsic electric field due to the nonuniform distribution of charge within a unit cell of the crystal lattice.     

Preparation of AgBr Nanoparticles in Microemulsions Via Reaction of AgNO<Subscript>3</Subscript> with CTAB Counterion

Nanoparticles of AgBr were prepared by precipitating AgBr in the water pools of microemulsions consisting of CTAB, n-butanol, isooctane and water. An aqueous solution of AgNO₃ added to the microemulsion was the source of Ag⁺ ions. The formation of AgBr nanoparticles in microemulsions through direct reaction with the surfactant counterion is a novel approach aimed at decreasing the role of intermicellar nucleation on nanoparticle formation for rapid reactions. The availability of the surfactant counterion in every reverse micelle and the rapidity of the reaction with the counterion trigger nucleation within individual reverse micelles. The effect of the following variables on the particle size and size distribution was investigated: the surfactant and cosurfactant concentrations, moles of AgNO₃ added, and water to surfactant mole ratio, R. High concentration of the surfactant or cosurfactant, or high water content of the microemulsion favored intermicellar nucleation and resulted in the formation of large particles with broad size distribution, while high amounts of AgNO₃ favored nucleation within individual micelles and resulted in small nanoparticles with narrow size distribution. A blue shift in the UV absorption threshold corresponding to a decrease in the particle size was generally observed. Notably, the variation of the absorption peak size with the nanoparticle size was opposite to those reported by us in previous studies using different surfactants.     

Temperature‐driven directional coalescence of silver nanoparticles

Silver nanoparticles were synthesized with a chemical reduction method in the presence of polyvinylpyrrolidone as stabilizing agent. The thermal stability behavior of the silver nanoparticles was studied in the temperature range from 25 to 700°C. Thermal gravimetric analysis was used to measure the weight loss of the silver nanoparticles. Scanning electron microscopy and high‐resolution transmission electron microscopy were used to observe the morphology and the change in shape of the silver nanoparticles. In situ temperature‐dependent small‐angle X‐ray scattering was used to detect the increase in particle size with temperature. In situ temperature‐dependent X‐ray diffraction was used to characterize the increase in nanocrystal size and the thermal expansion coefficient. The results demonstrate that sequential slow and fast Ostward ripening are the main methods of nanoparticle growth at lower temperatures (<500°C), whereas successive random and directional coalescences are the main methods of nanoparticle growth at higher temperatures (>500°C). A four‐stage model can be used to describe the whole sintering process. The thermal expansion coefficient (2.8 × 10^?5 K^?1) of silver nanoparticles is about 30% larger than that of bulk silver. To our knowledge, the temperature‐driven directional coalescence of silver nanocrystals is reported for the first time. Two possible mechanisms of directional coalescence have been proposed. This study is of importance not only in terms of its fundamental academic interest but also in terms of the thermal stability of silver nanoparticles.     

平面射流场中纳米颗粒的成核与凝并

采用大涡模拟和直接积分矩方法,数值模拟了在Reynolds数为8300的平面射流中,水蒸气（相对湿度φ=70%）和硫酸蒸气（质量分数为5×10^-6）二元体系中纳米颗粒的成核与凝并,详细分析了颗粒数密度、体积密度和平均粒径的分布.计算结果表明.射流场混合动量厚度的增长和实验结果一致;射流场的拟序结构导致了涡核中心处硫酸蒸气浓度的明显减小,而纳米颗粒数密度则明显增加;拟序结构的出现导致颗粒碰撞概率增大,提高了颗粒凝并效率;在颗粒数密度较大的涡核中心,颗粒成核作用增强,从而加 关键词： 纳米颗粒 成核 凝并 平面射流     

Effect of silver addition on the formation and deposition of titania nanoparticles produced by liquid flame spray

In this study, liquid flame spray (LFS) was used to produce titania, silver and silver–titania deposits of nanoparticles. Titanium(IV)ethoxide (TEOT) and silver nitrate in ethanol solutions were used as precursors and sprayed into turbulent hydrogen–oxygen flame. Production rates of 1.5–40 mg/min of titania were used with silver additions of 1, 2, 4, and 8 wt% compared to titania. Nanoparticle deposits were collected by thermophoretic sampling at six different axial distances from the flame torch head: 3, 5, 10, 12, 15, and 20 cm, of which the all but the last one occurred inside the flame. The deposit samples were analysed by TEM and SAED analysis. The powder samples of the particles were also collected by electric precipitator to XPS and specific surface area analysis. Particle size and effective density after the flame in the aerosol were analysed with SMPS and ELPI. The results from the previous studies i.e. controlling the particle size by setting the production rates of the particles were seen to apply also for this binary system. Characterisation of the deposits showed that when the substrate is inserted into the flame, in the beginning of the flame the deposit is formed by gas phase deposition whereas further down the flame the particles are first formed in the gas phase and then deposited. The location of the transition from gas phase deposition to gas phase nucleation prior to deposition depends on chemical/physical properties (e.g. thermodynamics and gas phase interactions) of the precursor, precursor concentration in the flame and also flame temperature profile. Therefore, the deposit collection distance from the burner also affected the collected particle size and degree of agglomeration. The two component deposits were produced in two different ways: one-step method mixing both precursors in the same solute, and two-step method spraying each precursor separately. The particle morphology differs between these two cases. In one-step method the primary (d _TEM) and agglomerate particle size (d _SMPS) decreased with the amount of silver addition, verifying the fact that when present, the silver has a clear effect on the titania nanoparticle formation and growth.     

Fabrication and characterization of silver/titanium dioxide composite nanoparticles in ethylene glycol with alkaline solution through sonochemical process

This paper aims to study fabrication and characterization of silver/titanium oxide composite nanoparticle through sonochemical process in the presence of ethylene glycol with alkaline solution. By using ultrasonic irradiation of a mixture of silver nitrate, the dispersed TiO₂ nanoparticle in ethylene glycol associated with aqueous solution of sodium oxide yields Ag/TiO₂ composite nanoparticle with shell/core-type geometry. The powder X-ray diffraction (XRD) of the Ag/TiO₂ composites showed additional diffraction peaks corresponding to the face-centered cubic (fcc) structure of silver crystallization phase, apart from the signals from the cores of TiO₂. Transmission electron microscopy (TEM) images of Ag/TiO₂ composites, which average particle size is roughly 80 nm, reveal that the titanium oxide coated by Ag nanoparticle with a grain size of about 2–5 nm. Additionally, the formation of silver nanoparticles on TiO₂ was monitored by ultraviolet visible light spectrophotometer (UV–Vis). As measured the optical absorption spectra of as-synthesized Ag nanoparticle varying with time, the mechanism of surface formatting silver shell on the cores of TiO₂ could be explored by autocatalytic reaction; the conversion of Ag particle from silver ion is 98% for the reaction time of 1000 s; and the activity energy of synthesizing Ag nanoparticles on TiO₂ is 40 kJ/mol at temperature ranging from 5 to 25 °C. Hopefully, this preliminary investigation could be used for mass production of composite nanoparticles assisted by ultrasonic chemistry in the future.     

Agglomeration,isolation and dissolution of commercially manufactured silver nanoparticles in aqueous environments

The increasing use of manufactured nanoparticles ensures these materials will make their way into the environment. Silver nanoparticles in particular, due to use in a wide range of applications, have the potential to get into water systems, e.g., drinking water systems, ground water systems, estuaries, and/or lakes. One important question is what is the chemical and physical state of these nanoparticles in water? Are they present as isolated particles, agglomerates or dissolved ions, as this will dictate their fate and transport. Furthermore, does the chemical and physical state of the nanoparticles change as a function of size or differ from micron-sized particles of similar composition? In this study, an electrospray atomizer coupled to a scanning mobility particle sizer (ES-SMPS) is used to investigate the state of silver nanoparticles in water and aqueous nitric acid environments. Over the range of pH values investigated, 0.5–6.5, silver nanoparticles with a bimodal primary particle size distribution with the most intense peak at 5.0 ± 7.4 nm, as determined from transmission electron microscopy (TEM), show distinct size distributions indicating agglomeration between pH 6.5 and 3 and isolated nanoparticles at pH values from 2.5 to 1. At the lowest pH investigated, pH 0.5, there are no peaks detected by the SMPS, indicating complete nanoparticle dissolution. Further analysis of the solution shows dissolved Ag ions at a pH of 0.5. Interestingly, silver nanoparticle dissolution shows size dependent behavior as larger, micron-sized silver particles show no dissolution at this pH. Environmental implications of these results are discussed.     

微乳液法制备纳米银粒子的结构及其荧光现象研究

采用微乳液法合成了不同粒径的纳米银粒子,考察了环己烷和甲苯作为油相对制备纳米银的影响.对纳米银粒子的尺寸与结构进行了表征,观察到近球形多晶粒子,并有孪晶结构存在,对晶体结构的分析表明银粒子存在不同程度的点阵畸变,晶面间距增大.不同粒径的纳米银粒子氯仿体系可呈现荧光光谱,而纳米银甲苯体系则无荧光发射.结合紫外—可见吸收光谱和电子自旋共振谱对该体系的荧光发射机理进行了分析 关键词： 微乳液 纳米银粒子 纳米晶结构 荧光     

Nucleation of Polypropylene Crystallization with Gold Nanoparticles. Part 2: Relation between Particle Morphology and Nucleation Activity

The influence of gold nanoparticle morphology on nucleation of isotactic polypropylene (PP) crystallization was investigated. Previous experiments indicated certain nucleation activity of gold nanoparticles, varying with their size. In this work, eight types of gold micro/nanoparticles were used: vacuum-sputtered nanostructures (nanoparticles, nanoislands, and nanolayers), chemically prepared isometric gold nanocrystals (5, 20, and 100 nm diameters), and two types of gold microcrystals with well-developed crystal facets [with (100) and (111) facets, respectively]. To minimize the effect of particle agglomeration, we used our recently introduced sandwich method, in which the nucleating agent was deposited between thin PP films and the nucleation was evaluated by polarized light microscopy (PLM), X-ray scattering (WAXS), and differential scanning calorimetry (DSC). The nucleation activity of Au particles in PP was lower than it might be expected from the previous studies and depended on their morphology. The nucleation activity of Au microcrystals with well-developed facets was higher than the activity of non-faceted Au nanocrystals.     

Size controlled synthesis of semiconductor nanocrystals in a continuous-flow mode microcapillary reactor

A microcapillary reactor with 320 μm inner diameter was utilized for CdSe nanoparticle synthesis. The influence of the reaction temperature and flow rate of precursors on the size and size distribution of prepared CdSe nanoparticles was systematically studied. The as-prepared nanoparticles exhibit sharp excitonic absorption and photoluminescence peak (FWHM 30 nm) with a quantum-yield around 10–40%. The microcapillary reactor was also used for CdSe/ZnS core-shell nanoparticle synthesis in continuous-flow mode. The quantum yield of the core-shell nanoparticles was found to be considerably influenced by the reactor temperature and have a close correlation with the thickness of ZnS shell under growth. An optimized quantum yield up to 70% was obtained for the CdSe/ZnS core-shell nanoparticles.     

An aerosol model to predict size and structure of soot particles

An aerosol model to simulate soot formation and growth was developed using moving- and fixed-sectional methods. The new model is composed of a set of subroutines that can be easily combined with the Chemkin package. Using the model, we have simulated soot formation and growth in plug flow reactors.

Our model was compared with a previously published method of moments model for a simulation of the plasma pyrolysis of methane in a plug flow reactor. Inclusion of the transition correction factor for the condensation coefficient led to the prediction of a smaller condensation rate compared with the method of moments model. The average coagulation rate calculated by the sectional model was much higher than that by the method of moments model for a broad particle size distribution. The two models predicted significantly different soot precursor concentration and rates of aerosol processes, but substantially similar particle mass and number for the pyrolysis process.

We have also simulated soot formation and growth in a jet-stirred/plug flow reactor (JSR/PFR) system for which soot size distribution measurements are available in the literature. It is shown that the adjusted-point fixed-sectional method can provide comparable accuracy to the moving-sectional model in a simulation of soot formation and growth. It is also shown that the measured surface growth rate could be much higher than the value used in this study. Soot mass concentrations and size distributions for particles larger than 10 nm were well predicted with a surface reaction enhancement. The primary particle size was underpredicted by only about 30% compared with the measurements, without any model adjustments. As the new model can predict both the particle size distribution and structure, and is suitable for application in complex flows, its application to diverse soot formation conditions will enhance our knowledge on the evolution of soot structures.