Continuous synthesis of metal nanoparticles in supercritical methanol

Metal nanoparticles were synthesized continuously in supercritical methanol (scMeOH) without using reducing agents at a pressure of 30 MPa and at various reaction temperatures ranging 150-400 °C. Wide angle X-ray diffraction (WAXD) analysis revealed that metallic nickel (Ni) nanoparticles were synthesized at a reaction temperature of 400 °C while mixtures of nickel hydroxide (α-Ni(OH)₂) and metallic Ni were produced at lower reaction temperatures of 250-350 °C. In contrast, metallic silver (Ag) nanoparticles were produced at reaction temperatures above 150 °C while metallic cupper (Cu) nanoparticles were produced at reaction temperatures above 300 °C. Mixtures of copper oxide (CuO and Cu₂O) and metallic Cu were produced at lower reaction temperatures of 250 °C. Scanning electron microscopy (SEM) showed that the particles size and morphology changed drastically as the reaction temperature increased. The average diameters of Ni, Cu and Ag particles synthesized at 400 °C were 119 ± 19 nm, 240 ± 44 nm, and 148 ± 32 nm, respectively. The scMeOH acted both as a reaction medium and a reducing agent. A possible reduction mechanism in scMeOH is also presented.     

碳纳米管表面铜和银纳米粒子的修饰

利用非共价修饰的方法,先在碳纳米管表面包裹上表面活性剂十六烷基三甲基溴化铵（CTAB）和聚丙烯酸钠（PAA）,然后原位修饰上铜和银纳米粒子,制备出MWCNT/CTAB/PAA/M（M：Cu或Ag）纳米复合材料,最后通过XRD、SEM和TEM等技术对其进行表征。结果表明,利用这种简单的层层自组装方法能够在碳管上均匀地修饰金属纳米粒子,并且这两种金属纳米粒子的尺寸都小于5 nm。     

Influence of nickel on the electrochemical activity of PtRu/multiwalled carbon nanotubes electrocatalysts for direct methanol fuel cells

This paper presents the effect of Ni in PtRu electrocatalysts over multiwalled carbon nanotubes (MWCNT) utilized for the electro-oxidation of methanol with the purpose of increasing reaction activity and tolerance to carbon monoxide. Two kinds of MWCNT were prepared using the same technique but different catalytic agents, ferrocene, and nickelocene. MWCNT obtained from ferrocene were treated after the synthesis to eliminate amorphous carbon and Fe excess, while MWCNT from nickelocene were used as synthesized to leave the nickel nanoparticles formed during the synthesis. PtRu particles were deposited over the surface of both types of MWCNT in order to study the effect of the Ni presence. The structure of the electrocatalysts was analyzed by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Chemical elemental microanalysis was carried out by X-ray energy dispersive spectroscopy (EDS). The synthesized MWCNT had an average diameter in the order of 60 nm and an average length of about 30 microns. Metallic nanoparticles deposited had a particle size in the order of 10 nm each. The electrochemical surface area (ESA) was measured using CO stripping curves and the activity toward the methanol oxidation reaction was evaluated. The ESA was improved with the presence of Ni, achieving an activity and onset potential similar to a commercial electrocatalyst (20 wt% PtRu/C, ETEK) with a lower PtRu loading (10 wt% PtRu).     

Catalytic formation of carbon phases in metal modified, porous polymer derived SiCN ceramics

A novel method for the simultaneous formation of catalytic active metal nanoparticles, multiwall carbon nanotubes (MWCNTs) and/or turbostratic carbon and porous M@SiCN (M = Fe, Co, Pt, Cu, Ag, Au) ceramics during pyrolysis of metal modified polysilazanes and polyethylene (PE) particles as sacrificial filler is described. The thermal decomposition of the polyethylene leads not only to the generation of the porosity but also to an in situ reduction of the metal compounds to the metal nanoparticles, due to the reductive atmosphere. Depending on the metal, carbon nanotubes as well as turbostratic carbon were formed in different amounts, due to the chemical vapor deposition (CVD) like conditions. The resulting carbon phases, ceramics and metal nanoparticles were investigated using the combination of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) measurements, giving evidence for the presence of the carbon phases and the metal particles.     

Binary [Cu2O/MWCNT] and ternary [Cu2O/ZnO/MWCNT] nanocomposites: formation, characterization and catalytic performance in partial ethanol oxidation

Cuprous oxide agglomerates composed of 4-10 nm Cu2O nanoparticles were deposited on multiwalled carbon nanotubes (MWCNTs) and on ZnO/MWCNTs to give binary [Cu2O/MWCNT] and ternary [Cu2O/ZnO/MWCNT] composites. Di-aqua-bis[2-(methoxyimino)propanoato]copper Cu[O2CCCH3NOMe](2)·2H2O 1 in DMF was used as single source precursor for the deposition of nanoscaled Cu2O. The precursor decomposes either in air or under argon to yield CuO2 by in situ redox reaction. Thermogravimetric coupled mass spectroscopic analysis (TG-MS) of 1 revealed that methanol formed during the decomposition of 1 acts as a potential in situ reducing agent. Scanning electron microscopy (SEM) of the binary [Cu2O/MWCNT] nano-composite shows an increase of cuprous oxide loading depending on the precursor amount, along the periphery of the MWCNTs as well as formation of larger particle agglomerates. Transmission electron microscopy (TEM) of the sample shows crystalline domains of size 4-10 nm surrounded by an amorphous region within the larger particles. SEM and TEM of ternary [Cu2O/ZnO/MWCNT] clearly reveal that Cu2O nanoparticles are primarily deposited on ZnO rather than on MWCNTs. The catalytic activities of the [Cu2O/MWCNT] and [Cu2O/ZnO/MWCNT] binary and ternary composites were studied for the selective partial oxidation of ethanol to acetaldehyde with molecular oxygen. While using binary [Cu2O/MWCNT] (13.8 wt% Cu) as catalyst, acetaldehyde was obtained with a yield of 87% at 355 °C (selectivity 96% and conversion 91%). When nanoscale ZnO is present, the resulting [Cu2O/ZnO/MWCNT] composite shows preferential hydrogen and CO2 formation due to the fact that the dehydrogenation and total oxidation pathway is more favoured compared to the binary composite. Significant morphological changes of the catalyst during the catalytic process were observed.     

Growth of carbon nanotube forests on metallic thin films

Growing carbon nanotube (CNT) forests on metals for integrated circuits interconnections or active layers applications is currently a challenge. CNT forests easily develop on insulators but their growth on metallic substrates is subject to interdiffusion and wettability effects that hamper the formation of the catalyst nanoparticles. This paper reports the successful growth of dense CNT forests on some metallic layers (Mo, Ta, W, and Ir) in comparison to other metallic films (Au, Cu, and Ti) over which CNTs are hardly achieved. The CNT forests are grown by thermal decomposition of C₂H₂ diluted in NH₃ and characterized by Raman spectroscopy and scanning electron microscopy. Stabilizing Al thin films placed between the metallic substrates and the Fe catalyst promote the formation of Fe nanoparticles. Metallic substrates, thickness of the Al stabilizer, and temperature and raise time during nanoparticles formation are all instrumental parameters in the growth and final structure of the CNT forests.     

Dispersion of gold nanoparticles into thiol-functionalized carbon nanotubes by γ-radiation

Gold nanoparticles are dispersed into thiol-functionalized multi-wall carbon nanotubes (MWCNTs). The thiol groups were utilized as linker to hold the gold nanoparticles without agglomeration. γ-Radiation was used as source to reduce gold metal ions without having any additional reducing agents. Field emission transmission electron microscopy, UV–visible spectroscopy and X-ray diffraction analysis were used to confirm the existence of Au metallic particles in the MWCNT matrix.     

Ultrasonic synthesis of highly dispersed Pt nanoparticles supported on MWCNTs and their electrocatalytic activity towards methanol oxidation

A new method of synthesis of highly dispersed Pt nanoparticles with large catalytic surface area on multi-walled carbon nanotubes (MWCNTs) under high-intensity ultrasonic field was developed. The method, with low processing temperature at 25 °C, took only about 5 min. The surface characterization of MWCNTs was carried out by fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy methods. The electrochemical surface area and pore volume of MWCNTs were also examined. The result showed that functional groups of the MWCNTs which favored the high loading and high dispersion of particles and electrochemical surface area of MWCNTs were reinforced in the case of high-intensity ultrasonic field. The Pt/MWCNT catalysts were characterized by energy dispersion X-ray spectra analysis (EDX), transmission electron microscopy (TEM) and X-ray diffraction (XRD) measurements. The prepared Pt nanoparticles were uniformly dispersed on the MWCNT surface. The mean size of Pt particles was 3.4 ± 0.2 nm. The electrocatalytic properties of Pt/MWCNT composites and kinetic characterization for methanol electro-oxidation were investigated by cyclic voltammetry. The Pt/MWCNT catalysts prepared for 5 min in ultrasonic field present excellent electrochemical activities. The schematic of the reaction was also introduced.     

A simple method for the synthesis of PtRu nanoparticles on the multi-walled carbon nanotube for the anode of a DMFC

We report the synthesis of PtRu nanoparticles on the multi-walled carbon nanotubes (MWCNTs) by a simple sodium borohydride reduction method. Transmission electron microscopy (TEM) analysis indicated that well-dispersed small (2-3 nm) PtRu particles were formed on the MWCNTs. X-ray diffraction (XRD) analysis confirmed the formation of the PtRu alloy on the MWCNTs. X-ray photoelectron spectroscopy (XPS) measurements revealed that 70.4% Pt and 61.0% Ru are present in their metallic states. Cyclic voltammetry (CV) and chronoamperometry results demonstrated that the PtRu/MWCNT synthesized by this method exhibited a higher methanol oxidation current than did the PtRu/MWCNT synthesized by the more complex method using sodium borohydride as the reducing agent and tetraoctyl ammonium bromide as the stabilizer. Finally, the direct methanol fuel cell (DMFC) performance test showed that the PtRu/MWCNT nanocatalyst used at the anode of the fuel cell yielded higher performance than did the commercial E-TEK PtRu/C catalyst.     

Direct atomic scale analysis of the distribution of Cu valence states in Cu/γ-Al2O3 catalysts

In this paper, the microstructure of a 1 wt.% Cu/γ-Al₂O₃ catalyst that was reduced in a 4% hydrogen/argon atmosphere at temperatures of 523, 773 and 1073 K, is studied by Z-contrast imaging and electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Results show that the copper species are well dispersed when the catalyst is reduced below 523 K. At 773 K, separated Cu(I) and Cu(0) species are found existing as ring-like and bulk-like particles. This appears to indicate that the copper has not been reduced to its metallic form due to the interaction between the copper oxide and the support. Large spherical particles having core-shell structures with Cu(I) in the shells and Cu(0) in the cores are generated when the catalyst is reduced at 1073 K. The formation of partially oxidized copper species upon reduction at 1073 K is attributed to the metallic copper interaction with the alumina support. This study also demonstrates that high-spatial resolution Z-contrast imaging and EELS performed simultaneously can provide unique information on the morphology and chemistry of metal species in supported metal catalysts.     

In situ formation of noble metal nanoparticles on multiwalled carbon nanotubes and its implication in metal–nanotube interactions

A simple method to decorate multiwalled carbon nanotubes (MWCNTs) with Au, Ag and Cu nanoparticles is illustrated. The method consists in directly depositing the selected metals by thermal evaporation on the carbon nanotubes. Comparative measurements carried out on samples that differ in the quantity and type of the deposited metal, reveal that isolated discrete particles form on the nanotube outer wall for all three metals. The CNT-based composites have been investigated by scanning and transmission electron microscopy to determine the size, shape and distribution of the nanoparticles. The results indicate that the quantity of evaporated metal only affects the nanoparticle size and not the average particle density. Particle composition was determined by X-ray photoelectron spectroscopy study. The results are discussed in terms of metal nanoparticle–tube interactions, an important issue for the fundamental and practical applications of similar MWCNT based composites.     

Functionalization of multi-walled carbon nanotubes with furan and maleimide compounds through Diels-Alder cycloaddition

Functionalization of multi-walled carbon nanotubes (MWCNTs) is performed through Diels-Alder (DA) reaction, using either furfuryl alcohol (a diene) or N-(4-hydroxyphenyl)maleimide (NHMI, a dienophile) as a functionalization agent. The structures of the functionalized MWCNTs are characterized with Fourier-transform infrared spectroscopy and Raman spectroscopy. Thermogravimetric analysis results also demonstrate the presence of organic portions of the functionalized MWCNTs. The DA reaction is further applied to incorporate polymer chains onto MWCNT surfaces, using a polyamide with maleimide pendent groups. Transmission electron microscopy images show the presence of a polymer layer of about 2-5 nm around the polymer modified MWCNTs.     

Comparison of the electrocatalytic performance of PtRu nanoparticles supported on multi-walled carbon nanotubes with different lengths and diameters

To investigate the electrocatalytic performance of PtRu nanoparticles supported on multi-walled carbon nanotubes (MWCNTs) with different lengths and diameters, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry experiments were conducted. It is demonstrated that the length and diameter of MWCNTs play an important role in the electrocatalytic performance of PtRu catalysts. The co-existence of amorphous carbon impurities on the MWCNT10-2 support lowered the accessible surface area of the PtRu nanoparticles, hampered the dispersion of the PtRu nanoparticles, and induced the formation of a low degree of PtRu alloy, thus lowered the electrocatalytic performance of the PtRu/MWCNT10-2 catalyst for methanol oxidation. The highest mass-specific activity of PtRu/MWCNT3050-2 results from a highly accessible PtRu surface and a good dispersion of PtRu particles. Our experimental results also demonstrate that the tube length of MWCNT samples has little effect of the surface area specific activity of the PtRu/MWCNT catalyst, whereas the PtRu nanoparticles supported on the MWCNT samples with large tube diameter tends to exhibit a higher surface area specific activity for methanol oxidation reaction. This result is suggested to be the combined effects of a high degree of PtRu alloying and the high electronic conductivity of these MWCNT samples.     

Hydrogen bond containing multiwalled carbon nanotubes in polyurethane composites

Introduction of hydrogen bonding sites onto multi‐walled carbon nanotubes (MWCNTs) included carboxylic acid, amide‐amine, and novel amide‐urea MWCNTs for the formation of homogenous polyurethane composites. Acid oxidation and subsequent derivatization introduced hydrogen bonding functionality onto MWCNTs to reveal the effect of surface functionalization on mechanical properties in a 45 wt% hard segment polyurethane matrix. Raman spectroscopy showed an increase in the D/G peak ratio, which indicated successful oxidation, and X‐ray photoelectron spectroscopy also revealed elemental compositions that supported each step of the functionalization strategy. Thermogravimetric analysis supported functionalization with an increase in percent weight loss for each functionalization, and the MWCNT surface functionalization determined pH‐dependent dispersibility. The nonfunctionalized MWCNT composites showed poor dispersion with transmission electron microscopy, and in sharp contrast, the functionalized composites displayed homogenous dispersions. Tensile testing revealed improved stress at break in the functionalized MWCNT composites at low loadings due to homogenous dispersion. POLYM. COMPOS., 37:1425–1434, 2016. © 2014 Society of Plastics Engineers     

Ion beam-induced shaping of Ni nanoparticles embedded in a silica matrix: from spherical to prolate shape

Present work reports the elongation of spherical Ni nanoparticles (NPs) parallel to each other, due to bombardment with 120 MeV Au⁺⁹ ions at a fluence of 5 × 10¹³ ions/cm². The Ni NPs embedded in silica matrix have been prepared by atom beam sputtering technique and subsequent annealing. The elongation of Ni NPs due to interaction with Au⁺⁹ ions as investigated by cross-sectional transmission electron microscopy (TEM) shows a strong dependence on initial Ni particle size and is explained on the basis of thermal spike model. Irradiation induces a change from single crystalline nature of spherical particles to polycrystalline nature of elongated particles. Magnetization measurements indicate that changes in coercivity (H _c) and remanence ratio (M _r/M _s) are stronger in the ion beam direction due to the preferential easy axis of elongated particles in the beam direction.     

Transmission Electron Microscopic Observations on Technegas and Pertechnegas

ABSTRACT

We examined the features of technegas, pertechnegas, and related nonradioactive aerosols using transmission electron microscopy. Samples of technegas, pertechnegas and unlabeled saline aerosols were created and collected within the Technegas Generator. The samples were examined by transmission electron microscopy and electron energy-loss spectroscopy (EELS). Technegas, pertechnegas, and the unlabeled saline aerosol particles have the same ultrastructure. Heating by the electron beam produces damage in some of these particles, revealing an opaque core material that is water soluble, and a thin surface layer. Disappearance of chlorine is noted by EELS during heating. We conclude that all of the examined aerosols have the same ultrastructure, regardless of the presence of small concentrations of oxygen during particle formation, and regardless of the presence of the technetium-99m label. We also note that some particles are susceptible to damage by heating; these consist of a thin surface layer and an opaque core substance that is dissipated upon heating.     

Deposition of Size-Selected Cu Nanoparticles by Inert Gas Condensation

Nanometer size-selected Cu clusters in the size range of 1–5 nm have been produced by a plasma-gas-condensation-type cluster deposition apparatus, which combines a grow-discharge sputtering with an inert gas condensation technique. With this method, by controlling the experimental conditions, it was possible to produce nanoparticles with a strict control in size. The structure and size of Cu nanoparticles were determined by mass spectroscopy and confirmed by atomic force microscopy (AFM) and scanning electron transmission microscopy (STEM) measurements. In order to preserve the structural and morphological properties, the energy of cluster impact was controlled; the energy of acceleration of the nanoparticles was in near values at 0.1 ev/atom for being in soft landing regime. From SEM measurements developed in STEM-HAADF mode, we found that nanoparticles are near sized to those values fixed experimentally also confirmed by AFM observations. The results are relevant, since it demonstrates that proper optimization of operation conditions can lead to desired cluster sizes as well as desired cluster size distributions. It was also demonstrated the efficiency of the method to obtain size-selected Cu clusters films, as a random stacking of nanometer-size crystallites assembly. The deposition of size-selected metal clusters represents a novel method of preparing Cu nanostructures, with high potential in optical and catalytic applications.     

Porous Co/Co–Ni–Pt nanostructures prepared by galvanic replacement towards methanol electro-oxidation

The nanostructured Co/Co–Ni–Pt catalyst were synthesized by electrodeposition process and galvanic replacement reaction. The alloy prepared on a copper electrode (Cu/Co/Co–Ni–Zn) was dipped in platinum containing alkaline solution to produce a porous Cu/Co/Co–Ni–Pt catalyst. The catalyst was characterized by energy dispersive X-ray and scanning electron microscopy techniques and its electrocatalytic properties were evaluated using cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry techniques. The results showed that the Co/Co–Ni–Pt coatings are porous, and composed of discrete Pt nanoparticles with the crystallite size of about 66 nm. It was shown from cyclic voltammograms in alkaline solutions that the oxidation current of methanol on the nanostructured Cu/Co/Co–Ni–Pt electrode was much higher than that on flat platinum. Electrochemical impedance spectra on the Co/Co–Ni–Pt electrode reveal that the charge transfer resistance decreases with the increase of anodic potentials. All results show that the Co/Co–Ni–Pt catalysts can be potential anode catalysts for the direct methanol fuel cell.     

Antibacterial surfaces obtained through dopamine and fluorination functionalizations

Self-polymerized dopamine was used to form a thin layer onto stainless steel (SS) and poly(ethylene terephthalate) (PET) sheets followed by covalent grafting of pentadecafluorooctanoyl chloride by esterification and amidation reactions. The surface functionalization was characterized at each step by contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The anti-adhesive properties of native surfaces, polydopamine-coated surfaces and hydrophobic fluorinated surfaces were tested against Gram-negative (Pseudomonas aeruginosa) and Gram-positive bacteria (Listeria monocytogenes and Staphylococcus aureus). The results reveal an inhibition of bacteria growth towards Gram-negative bacteria on fluorinated surfaces. This work proposes a novel method to easily fluorinate in two steps both metallic and organic surfaces using “universal” polydopamine coating as a key step.     

An evaluation of cell proliferation and adhesion on vertically-aligned multi-walled carbon nanotube films

Biocompatibility tests were performed on vertically-aligned multi-walled carbon nanotube (MWCNT) films produced by microwave plasma chemical vapor deposition on titanium substrates with iron (Fe) and nickel (Ni) as catalysts. The cell adhesion and morphology of L-929 mouse fibroblast cells were studied by high resolution scanning electron microscopy, after up to 7 days incubation periods. Cell viability and proliferation were evaluated by two “in vitro” tests: (1) 2-(4,5-dimethyl-2-thiazolyl)-3,5-diphenyl-2H-tetrazolium bromide (MTT), and (2) lactate dehydrogenase (LDH) assays. Low level of bioavailable Fe and Ni was determined by inductively coupled plasma optical emission spectrometry. Neither functionalization nor purification of MWCNT films was necessary to obtain good response to the biocompatibility tests. Efficient cell growth and non-toxicity suggest the use MWCNTs in tissue regeneration. The MWCNT films stimulated the cell growth, showing a proliferation 20% higher than on Ti.