Preparation of magnetic nanoparticles by pulsed plasma chemical vapor synthesis

FePt nanoparticle is expected as a candidate for the magnetic material of the high density recording media. We attempted to synthesize FePt alloy nanoparticles using 13.56 MHz glow discharge plasma with the pulse operation of a square-wave on/off cycle of plasma discharge to control the size of nanoparticles. Vapors of metal organics, Biscyclopentadienyl iron (ferrocene) for Fe and (Methylcyclopentadienyl) trimethyl platinum for Pt, were introduced into the capacitively coupled flow-through plasma chamber, which consisted of shower head RF electrode and grounded mesh electrode. Synthesis experiments were conducted at room temperature under the conditions of pressure 0.27 Pa, source gas concentration 0.005 Pa, gas residence time 0.5 s and plasma powers 60 watts. Pulse width for plasma duration was chosen from 0.5 to 30 s and plasma off period was 4 s to each pulse operation. Visual observations during the particle growth showed plasma emission in the bulk region was increased with the particle growth. These were theoretically explained by using the model for both transient particle charging in the plasma and single particle behavior in the stationary plasma as well as assuming the similarity between the negative charged particle and negative gas containing plasma. Synthesized nanoparticles were directly collected onto TEM grid, which was placed just below the grounded mesh electrode in the plasma reactor downstream. TEM pictures showed two kinds of particles in size, one of which was nanometer size and isolated with crystal structures and the other appeared agglomerate of nanometer size particles. The size of agglomerated particle was controlled in the 10–120 nm range by varying the plasma-on time from 0.5 to 30 s, although the nanometer size particles did not change. The composition of FePt alloy particles could be altered by adjusting the source gas feed ratio. Also magnetization of FePt nanoparticles was measured by use of SQUID (superconducting quantum interference device) magnetometry measurements. As-synthesized FePt nanoparticles did not exhibit loop-shape characteristic, which indicated superpamagnetic property. Annealed nanoparticles with the composition of Fe₅₈Pt₄₂ at 650°C in atmospheric hydrogen showed clear hysterisis loop with the coercivity as large as 10 KOe.     

Synthesis and characterization of carbon coated nanoparticles produced by a continuous low-pressure plasma process

Core–shell nanoparticles coated with carbon have been synthesized in a single chamber using a continuous and entirely low-pressure plasma-based process. Nanoparticles are formed in an argon plasma using iron pentacarbonyl Fe(CO)₅ as a precursor. These particles are trapped in a pure argon plasma by shutting off the precursor and then coated with carbon by passing acetylene along with argon as the main background gas. Characterization of the particles was carried out using TEM for morphology, XPS for elemental composition and PPMS for magnetic properties. Iron nanoparticles obtained were a mixture of FeO and Fe₃O₄. TEM analysis shows an average size of 7–14 nm for uncoated particles and 15–24 nm for coated particles. The effect of the carbon coating on magnetic properties of the nanoparticles is studied in detail.     

Size dependence of complex refractive index function of growing nanoparticles

The evidence of the change of the complex refractive index function E(m) of carbon and iron nanoparticles as a function of their size was found from two-color time-resolved laser-induced incandescence (TiRe-LII) measurements. Growing carbon particles were observed from acetylene pyrolysis behind a shock wave and iron particles were synthesized by pulse Kr–F excimer laser photo-dissociation of Fe(CO)₅. The magnitudes of refractive index function were found through the fitting of two independently measured values of particle heat up temperature, determined by two-color pyrometry and from the known energy of the laser pulse and the E(m) variation. Small carbon particles of about 1–14 nm in diameter had a low value of E(m)∼0.05–0.07, which tends to increase up to a value of 0.2–0.25 during particle growth up to 20 nm. Similar behavior for iron particles resulted in E(m) rise from ∼0.1 for particles 1–3 nm in diameter up to ∼0.2 for particles >12 nm in diameter.     

Controlled flame synthesis of αFe<Subscript>2</Subscript>O<Subscript>3</Subscript> and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles: effect of flame configuration,flame temperature,and additive loading

Superparamagnetic iron oxide nanoparticles are used in diverse applications, including optical magnetic recording, catalysts, gas sensors, targeted drug delivery, magnetic resonance imaging, and hyperthermic malignant cell therapy. Combustion synthesis of nanoparticles has significant advantages, including improved nanoparticle property control and commercial production rate capability with minimal post-processing. In the current study, superparamagnetic iron oxide nanoparticles were produced by flame synthesis using a coflow flame. The effect of flame configuration (diffusion and inverse diffusion), flame temperature, and additive loading on the final iron oxide nanoparticle morphology, elemental composition, and particle size were analyzed by transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), energy dispersive spectroscopy (EDS), and Raman spectroscopy. The synthesized nanoparticles were primarily composed of two well known forms of iron oxide, namely hematite αFe₂O₃ and magnetite Fe₃O₄. We found that the synthesized nanoparticles were smaller (6–12 nm) for an inverse diffusion flame as compared to a diffusion flame configuration (50–60 nm) when CH₄, O₂, Ar, and N₂ gas flow rates were kept constant. In order to investigate the effect of flame temperature, CH₄, O₂, Ar gas flow rates were kept constant, and N₂ gas was added as a coolant to the system. TEM analysis of iron oxide nanoparticles synthesized using an inverse diffusion flame configuration with N₂ cooling demonstrated that particles no larger than 50–60 nm in diameter can be grown, indicating that nanoparticles did not coalesce in the cooler flame. Raman spectroscopy showed that these nanoparticles were primarily magnetite, as opposed to the primarily hematite nanoparticles produced in the hot flame configuration. In order to understand the effect of additive loading on iron oxide nanoparticle morphology, an Ar stream carrying titanium-tetra-isopropoxide (TTIP) was flowed through the outer annulus along with the CH₄ in the inverse diffusion flame configuration. When particles were synthesized in the presence of the TTIP additive, larger monodispersed individual particles (50–90 nm) were synthesized as observed by TEM. In this article, we show that iron oxide nanoparticles of varied morphology, composition, and size can be synthesized and controlled by varying flame configuration, flame temperature, and additive loading.     

Low temperature synthesis of iron containing carbon nanoparticles in critical carbon dioxide

We develop a low temperature, organic solvent-free method of producing iron containing carbon (Fe@C) nanoparticles. We show that Fe@C nanoparticles are self-assembled by mixing ferrocene with sub-critical (25.0 °C), near-critical (31.0 °C) and super-critical (41.0 °C) carbon dioxide and irradiating the solutions with UV laser of 266-nm wavelength. The diameter of the iron particles varies from 1 to 100 nm, whereas that of Fe@C particles ranges from 200 nm to 1 μm. Bamboo-shaped structures are also formed by iron particles and carbon layers. There is no appreciable effect of the temperature on the quantity and diameter distributions of the particles produced. The Fe@C nanoparticles show soft ferromagnetic characteristics. Iron particles are crystallised, composed of bcc and fcc lattice structures, and the carbon shells are graphitised after irradiation of electron beams.     

Emission characteristics and parameters of a glow discharge in an argon-xenon-chlorine mixture

We present the results of a study of the emission characteristics of a low-pressure longitudinal glow discharge in an Ar-Xe-Cl₂ mixture (p = 0.5–6.0 kPa) and numerical modeling of its plasma parameters as a function of the parameter E/P. We optimized the average emission power of the discharge in the spectral range 160–310 nm as a function of the discharge current, the pressure and composition of the gas mixture. The plasma parameters were calculated for the optimal mixture (according to experimental data) and included obtaining the dependences of the electron transport and energy characteristics, the power losses per unit pressure in the discharge going toward elementary electron processes in the plasma, and also the ionization and attachment coefficients as a function of the parameter E/P. The results of numerical modeling of the glow discharge parameters allow us to provide a qualitative basis for the emission distribution in the spectrum of a plasma based on an Ar-Xe-Cl₂ mixture. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 563–567, July–August, 2008.     

Differential mobility analysis of nanoparticles generated by laser vaporization and controlled condensation (LVCC)

Silicon and iron aluminide (FeAl) nanoparticles were synthesized by a laser vaporization controlled condensation (LVCC) method. The particles generated by the laser ablation of solid targets were transported and deposited in the presence of well-defined thermal and electric field in a newly designed flow-type LVCC chamber. The deposition process of nanoparticles was controlled by the balance of the external forces; i.e., gas flow, thermophoretic and electrostatic forces. The size distributions of generated nanoparticles were analyzed using a low-pressure differential mobility analyzer (LP-DMA). The effect of synthesis condition on the size distribution was analyzed by changing the pressure of the carrier gas (20–200 Torr), the temperature gradient in the LVCC chamber (ΔT=0–190°C) and the electric field applied between the LVCC chamber plates (E=0–3000 V/m). It was found that electrostatic field was effective to selectively deposit small size nanoparticles (about 10 nm) with expelling large droplet-like particles.     

Carbon nanotubes as nanoparticles collector

This communication reports on a new method for the collection of nanoparticles using carbon nanotubes (CNT) as collecting surfaces, by which the problem of agglomeration of nanoparticles can be circumvented. CNT (10–50 nm in diameter, 1–10 μm in length) were grown by thermal CVD at 923 K in a 7 v/v% C₂H₂ in N₂ mixture on electroless nickel-plated copper transmission electron microscopy (TEM) grids and Monel coupons. These samples were then placed downstream of an arc plasma reactor to collect individual copper nanoparticles (5–30 nm in diameter). It was observed that the Cu nanoparticles preferentially adhere onto CNT and that the macro-particles (diameter >1 μm), a usual co-product obtained with metal nanoparticles in the arc plasma synthesis, are not collected. Cu–Ni nanoparticles, a catalyst for CNT growth, were deposited on CNT to grow multibranched CNT. CNT-embedded thin films were produced by re-melting the deposited nanoparticles.     

Characteristic of nanoparticles generated from different nano-powders by using different dispersion methods

A standard rotating drum with a modified sampling train (RD), a vortex shaker (VS), and a SSPD (small-scale powder disperser) were used to investigate the emission characteristics of nano-powders, including nano-titanium dioxide (nano-TiO₂, primary diameter: 21 nm), nano-zinc oxide (nano-ZnO, primary diameter: 30–50 nm), and nano-silicon dioxide (nano-SiO₂, primary diameter: 10–30 nm). A TSI SMPS (scanning mobility particle sizer), a TSI APS (aerodynamic particle sizer), and a MSP MOUDI (micro-orifice uniform deposit impactor) were used to measure the number and mass distributions of generated particles. Significant differences in specific number and mass concentration or distributions were found among different methods and nano-powders with the most specific number and mass concentration and the smallest particles being generated by the most energetic SSPD, followed by VS and RD. Near uni-modal number or mass distributions were observed for the SSPD while bi-modal number or mass distributions existed for nano-powders except nano-SiO₂ which also exhibited bimodal mass distributions. The 30-min average results showed that the mass median aerodynamic diameter (MMAD) and number median diameter (NMD) of the SSPD ranged 1.1–2.1 μm and 166–261 nm, respectively, for all three nano-powders, which were smaller than those of the VS (MMAD: 3.3–6.0 μm and NMD: 156–462 nm), and the RD (MMAD: 5.2–11.2 μm and NMD: 198–479 nm). For nano-particles (electric mobility diameter < 100 nm), specific mass concentrations were nearly negligible for all three nano-powders and test methods. Specific number concentrations of nano-particles were low for the RD tester but were elevated when more energetic VS and SSPD testers were used. The quantitative size and concentration data obtained in this study is useful to elucidate the field emission and personal exposure data in the future provided that particle loss in the generation system is carefully assessed.     

Sticking efficiency of nanoparticles in high-velocity collisions with various target materials

In order to find reliable collector surfaces for the Mesospheric Aerosol – Genesis, Interaction and Composition (MAGIC) sounding rocket experiment, intended to collect atmospheric nanoparticles, the sticking efficiency of nanoparticles was measured on several targets of different materials. The nanoparticles were generated by a molecular beam apparatus in Jena, Germany, by laser ablation (Al₂O₃ particles, diameter 5–50 nm) and by laser pyrolysis (carbon particles, diameter 10–20 nm). In a vacuum environment (>10⁻⁵ mbar) the particles condensed from the gas phase, formed a particle beam, and were accelerated to ∼ ∼1 km/s. The sticking efficiency on the target materials carbon, gold and grease was measured by a microbalance. Results demonstrate moderate to high sticking probabilities. Thus, the capture and retrieval of atmospheric nanoparticles was found to be quantitatively feasible.     

Characterizing the coating and size-resolved oxidative stability of carbon-coated aluminum nanoparticles by single-particle mass-spectrometry

Aluminum nanoparticles are of significant interest in enhancing the rate of energy release from propellants. One of the major impediments to their use is that bare aluminum is highly reactive, while oxide coated aluminum significantly decreases overall performance. We investigate creating aluminum nanoparticles with a thin carbon coating using either a laser induced plasma or a DC plasma-arc. The carbon coating was created by injecting ethylene (C₂H₄) directly downstream of the plasma. The elemental composition of the coated aluminum nanoparticles was measured in real time with a recently developed quantitative single particle mass spectrometer (SPMS). We found that the aluminum nanoparticles were coated with a carbon layer of thickness around 1–3 nm.The thermal and oxidative stability of these particles was determined by passing the aerosols through a heated flow reactor in a carrier flow of either air or argon, and measuring the aluminum, carbon and oxygen content in the particles with the single particle mass spectrometer. We found that below 700°C the coating was stable, but that the coating oxidized above ∼ ∼800°C. In contrast the carbon coating was thermally stable above ∼ ∼900°C. These results indicate that a carbon coating may be a suitable passivating agent.     

Preparation and structure of carbon encapsulated copper nanoparticles

Carbon-encapsulated copper nanoparticles were synthesized by a modified arc plasma method using methane as carbon source. The particles were characterized in detail by transmission electron microscope, high-resolution transmission electron microscopy, selected-area electron diffraction, X-ray diffraction, thermogravimetric and differential scanning calorimetry. The encapsulated copper nanoparticles were about 30 nm in diameter with 3–5 nm graphitic carbon shells. The outside graphitic carbon layers effectively prevented unwanted oxidation of the copper inside. The effect of the ratio of He/CH₄ on the morphologies and the formation of the carbon shell were investigated.     

Generation of nanoparticles by spark discharge

The production of nanoparticles by microsecond spark discharge evaporation in inert gas is studied systematically applying transmission electron microscopy, mobility analysis and BET surface area measurement. The method of spark discharge is of special interest, because it is continuous, clean, extremely flexible with respect to material, and scale-up is possible. The particle size distributions are narrow and the mean primary particle size can be controlled via the energy per spark. Separated, unagglomerated particles, 3–12 nm in size, or agglomerates can be obtained depending on the flow rate. The nanoparticulate mass produced is typically 5 g/kWh. A formula is given, which estimates the mass production rate via thermal conductivity, evaporation enthalpy and the boiling point of the material used. We showed that with gas purified at the spot, the method produced gold particles that were so clean that sintering of agglomerated particles occurred at room temperature. The influence of a number of parameters on the primary particle size and mass production rate was studied and qualitatively understood with a model of Lehtinen and Zachariah (J Aerosol Sci 33:357–368, 2002). Surprisingly high charging probabilities for one polarity were obtained. Spark generation is therefore of special interest for producing monodisperse aerosols or particles of uniform size via electrical mobility analysis. Qualitative observations in the present study include the phenomenon of material exchange between the electrodes by the spark, which opens the possibility of producing arbitrary mixtures of materials on a nanoscale. If spark generation of nanoparticles is performed in a standing or almost standing gas, an aerogel of a web-like structure forms between surfaces of different electrical potential.     

Preparation and characterization of polymer-supported nano-amorphous Ni–B particles

Polymer-supported nano-amorphous Ni–B particles have been prepared by an ion exchange/chemical reduction protocol. The products were characterized by inductively coupled plasma spectroscopy (ICP), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The polymer-supported Ni–B particles prepared at low temperature are 60–70 nm in size with a Ni–B ratio of Ni₁₉B₁₀.The choice of temperature is a crucial factor affecting particle size.     

Effect of surrounding gas temperature on the morphological evolution of TiO2 nanoparticles generated by laser ablation in tubular furnace

Titanium oxide nanoparticles are synthesized by laser ablation of Ti target in oxygen atmosphere under well-controlled temperature profiles in a tubular furnace. The size and the shape of generated nanoparticles are varied by changing the temperature of furnace. The mobility-based size distributions of generated air-borne nanoparticles are measured using a scanning mobility particle sizer, and the size distributions of primary particles are analyzed by a scanning electron microscope. When the particles are generated by laser ablation at the room temperature, the particles are agglomerates in gas phase with the average mobility diameter of 117 nm and the mean diameter of primary particles of 11 nm. The primary particle diameter increases from 11 to 24 nm by raising the furnace temperature up to 800 °C. Since the mass of Ti vapor ablated from a target is found to be constant regardless of the furnace temperature, this particle growth may be attributed to the reduction in nuclei number as a result of mild quenching at higher temperatures. As the temperature reaches higher than 1,000 °C, the mobility diameter suddenly drops and the primary particle diameter increases due to sintering, and at 1,200 °C the mobility diameter coincides with the primary particle diameter. Since the laser oven method offers an independent control of vapor concentration and the temperature of surrounding atmosphere, it is an effective tool to study the formation process of nanoparticles from primary particles with a given size.     

Ultra fine carbon nitride nanocrystals synthesized by laser ablation in liquid solution

Crystalline carbon nitride nanopowders and nanorods have been successfully synthesized at room temperature and pressure using the novel technique of pulsed laser ablation of a graphite target in liquid ammonia solution. High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and Fourier transform infrared spectroscopy (FTIR) were used to systematically study the morphology, nanostructure and chemical bonding. The experimental composition and structure of the nanoparticles are consistent with the theoretical calculations for α-C₃N₄. After 2 h ablation the particles had a size distribution ∼8–12 nm, whereas after 5 h ablation the particles had grown into nanorod-like structures with a crystalline C₃N₄ tip. A formation mechanism for these nanorods is proposed whereby nanoparticles are first synthesized via rapid formation of an embryonic particle, followed by a slow growth, eventually leading to a one-dimensional nanorod structure.     

Output characteristics and parameters of the plasma from a gas-discharge low-pressure ultraviolet source using helium-iodine and xenon-iodine mixtures

The output characteristics and parameters of the plasma of a powerful gas-discharge source of UV radiation are studied. The UV source uses He-I₂ and Xe-I₂ mixtures and is excited by a longitudinal glow discharge. The pressure of the gas mixtures is varied from 100 to 1500 Pa, and the discharge power falls into the range 15–250 W. The source (lamp) emits in the spectral interval 200–390 nm, which covers the spectral line of the iodine atom at 206.2 nm, the spectral band of XeI(B-X) with a maximum at 253 nm, and the spectral band of with a maximum at 342 nm. For He(Xe)-I₂ mixtures at a pressure of 800–1000 Pa (this pressure range is near-optimal according to our experimental data), the electron energy distribution functions and the electron kinetic coefficients as functions of parameter E/N (E is the electric field strength, and N is the particle concentration in the discharge) are calculated. The calculated plasma parameters are used in the qualitative analysis of key electronic processes in the plasma of an exciplex halogen UV source and will be subsequently employed in numerical simulation of the process kinetics and output characteristics of a UV source based on helium-iodine or xenon-iodine mixtures.     

Synthesis of mixed metallic nanoparticles by spark discharge

Short spark discharges (2 μs) were successfully applied to generate mixed particles a few nanometres in diameter by fast quenching. Alloyed Cr–Co electrodes were applied to demonstrate this. Further it was shown that if the anode and the cathode are different materials, the discharge process mixes the vapour of both materials, forming mixed nanoparticles. Electron microscopy (TEM, SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analyses were performed on the collected particles to study their size, morphology, composition and structure. The average compositions of the particles were measured by inductively coupled plasma (ICP). In addition, online measurements of the particle size distribution by mobility analysis were carried out. In the case of alloyed electrodes (Cr–Co), the relative concentration of the elements in the nanoparticulate sample was consistent with the electrode composition. When using electrodes of different metals (Au–Pd and Ag–Pd) the individual nanoparticles showed a range of mixing ratios. No surface segregation was observed in these mixed noble metal particles. Crystalline nanoparticulate mixed phases were found in all cases.     

Fabrication of magnetic gold nanorod particles for immunomagnetic separation and SERS application

The preparation and application of rod-shaped core–shell structured Fe₃O₄–Au nanoparticles for immunomagnetic separation and sensing were described for the first time with this study. To synthesize magnetic gold nanorod particles, the seed-mediated synthetic method was carried out and the resulting nanoparticles were characterized with transmission electron microscopy (TEM), ultraviolet visible spectroscopy (UV–Vis), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD). Magnetic properties of the nanoparticles were also examined. Characterization of the magnetic gold nanorod particles has proven that the resulting nanoparticles were composed of Fe₃O₄ core and the gold shell. The rod-shaped gold-coated iron nanoparticles have an average diameter of 16 ± 2 nm and an average length of about 50 ± 5 nm (corresponding aspect ratio of 3). The saturation magnetization value for the magnetic gold nanorod particles was found to be 37 emu/g at 300 K. Rapid and room temperature reaction synthesis of magnetic gold nanorod particles and subsequent surface modification with E. coli antibodies provide immunomagnetic separation and SERS application. The analytical performance of the SERS-based homogenous sandwich immunoassay system with respect to linear range, detection limit, and response time is also presented.     

Radiation of pulsed and glow discharges in CCl4

The radiation of a pulsed-periodic discharge in CCl₄ has been investigated. Its spectrum is found to differ considerably from the spectrum of the glow discharge. It is found that ‖60% of the energy from the 215–650 nm region falls in the UV spectrum. Moscow Physicotechnical Institute, 9, Institutskii Per., Dolgoprudnyi, 141701, Moscow Region, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 302–305, March–April, 1998.