Metallic Co nanoparticles of 24-110 nm diameters are prepared by electroless deposition ͑chemical reduction͒ in an aqueous solution at room temperature. The reduction process is monitored by an in situ measurement of a mixed potential. The mixed potential, which is above the redox potential of a Co͑II͒/Co redox pair, drops by the addition of the nucleating agent H 2 PtCl 6 and also decreases with an increase in the H 2 PtCl 6 concentration. Smaller Co nanoparticles are formed. In the smaller particle size, the fraction of the face-centered cubic Co phase increases, and the hexagonal close-packed Co phase decreases.Co nanoparticles have great scientific and technological importance due to their unique properties that are different from the bulk. 1-4 The formation of Co nanoparticles has been extensively studied over the years, leading to a wide variety of synthesis methods, such as thermal decomposition, micellar reduction, sonochemical reduction, and simple chemical ͑solution͒ reduction. 1-9 Most of the methods are conducted in an organic solvent at an elevated temperature because the fabrication of Co nanoparticles at room temperature is still challenging, specifically in an aqueous solution. This is due to the difficulty of formation of size-controlled Co nanoparticles without agglomeration and oxidation. The reducing agent, such as sodium borohydride, can reduce Co͑II͒ ions at room temperature. 3-5 However, the synthesized Co nanoparticles are usually polydispersed and amorphous containing boron. 3,4 In this article, we present a method of preparation of Co nanoparticles with diameters of 24-110 nm by electroless deposition ͑chemical reduction͒ with hydrazine monohydrate ͑N 2 H 4 ·H 2 O͒ as a reducing agent in an aqueous solution at room temperature. Chloroplatinic acid hexahydrate ͑H 2 PtCl 6 ·6H 2 O͒ is used as a nucleating agent. The effects of the nucleating agent on the particle morphology and deposition rates are studied by electron microscopy and in situ mixed potential measurements, respectively.
ExperimentalAll chemical reagents were analytical grade ͑Nacalai Tesque͒. A Co͑II͒ aqueous solution was prepared by dissolving 0.01 mol Co͑II͒ acetate tetrahydrate ͓Co͑C 2 H 3 O 2 ͒ 2 ·4H 2 O͔ and 1.3 ϫ 10 −4 mol poly͑ethylene glycol͒ ͓H͑OCH 2 CH 2 ͒nOH, M w = 20,000͔ in 50 mL deoxygenated water. Then, 1 M sodium hydroxide ͑NaOH͒ aqueous solution of 25 mL was added to provide OH − ions. Next, 2.5 ϫ 10 -6 to 2.5 ϫ 10 −4 mol H 2 PtCl 6 ·6H 2 O in a propylene glycol ͑C 3 H 8 O 2 ͒ solution of 10 mL was added as the nucleating agent. The reducing agent solution was prepared by mixing 0.1 mol N 2 H 4 ·H 2 O in 25 mL deoxygenated C 3 H 8 O 2 . The N 2 H 4 ·H 2 O-C 3 H 8 O 2 solution was then added into the Co͑II͒ aqueous solution at room temperature, while the solution was continuously stirred and nitrogen gas ͑N 2 ͒ was bubbled at 50 mL/min. The pH value of the total solution was about 12. The total volume was 115 mL, and the final concentrations were 0.087 M Co͑II͒ acetate tetrahydrate, 1.1 mM poly͑eth-ylene glyco...
Ferromagnetic Co nanoparticles with diameters of about 40-400 nm are synthesized by electroless deposition in boiling propylene glycol. The Co particle size is decreased to a certain degree by varying the concentration of starting materials and by adding nucleating agents. The electrochemical behavior of propylene glycol is investigated by in situ measurements of mixed potential to understand the formation of Co nanoparticles in polyol systems. The mixed potential decreases with an increase in temperature and in the presence of NaOH, which suggests the faster decomposition of propylene glycol. It also shifts abruptly to a more negative value when nucleating agents are added. This indicates that nucleating agents catalyze both the oxidation reaction of propylene glycol and the reduction reaction of Co͑II͒ species, as well as aid in the formation of Co nanoparticles as heterogeneous nucleation sites.
Hollow tubular structured kapok fibers (Ceiba pentandra) were coated with polyaniline (PANI) molecules using an in situ oxidative polymerization technique. The tubular morphology of the kapok fibers was retained after PANI coating. The Fourier transform infrared (FT-IR) spectrum of the PANI-coated kapok fibers illustrated the vibration modes associated with the presence of PANI molecules. The PANI-treated kapok fibers achieved complete wettability with water molecules (zero water contact angle) from initially being highly hydrophobic (contact angle = 120°). In the present work, the removal of contaminants such as methyl orange dye and Cu(II) from aqueous solution using polyaniline-coated kapok fibers was investigated. Isotherm studies show that the removal of methyl orange dye (R2 ≥ 0.959) and Cu(II) (R2 ≥ 0.972) using PANI-coated kapok fibers follow the Langmuir isotherm model with maximum sorption capacities determined to be 75.76 and 81.04 mg/g, respectively. Based from thermodynamic studies, the sorption of methyl orange dye and Cu(II) are endothermic, feasible and spontaneous. Furthermore, kinetic studies show that the both processes follow a pseudo-second-order model, implying that the rate-determining step is chemisorption.
Abstract. Future electronics devices are not only smaller and thinner, but are also flexible, bendable and even wearable. This evolution in technology requires direct printing of patterns onto any substrate using conductive inks made of a dispersion of metallic nanoparticles. In this study, Cl-ions was used to induce spontaneous sintering of silver nanoparticles (Ag NPs). Ag NPs with an average diameter of 56 nm were synthesized by polyol method using silver nitrate (AgNO3) and ethylene glycol (EG) as precursor and solvent, respectively. Poly(vinyl pyrrolidone) was used as the capping agent. Water-based inks were formulated containing different Ag NP loading (10-25 wt %). Using 50 mM NaCl aqueous solution as the dispersing medium, an ink with 15 wt % Ag exhibited a sheet resistance of about 2.85 Ω/sq. This very low sheet resistance was attributed to sintering of Ag NPs, which was accompanied by an increase in average diameter of nanoparticles from 56 to 569 nm.
Metallic Co nanowires with a mean diameter of about 190 nm and lengths up to 160 mm are prepared by electroless deposition (solution reduction) in propylene glycol at room temperature under external magnetic field. Co deposition behavior in propylene glycol is investigated by in situ monitoring of mixed potentials in conjunction with linear sweep voltammetry and the oxidationreduction potential of the Co(II)/Co redox pair is determined to be about À0.54 V vs Ag/AgCl. When the mixed potential drops below the oxidation-reduction potential of the Co(II)/Co redox pair, small Co nanoparticles are generated in the solution and are magnetized in the presence of external magnetic field. Strong attractive dipolar interactions are induced along the magnetic field direction, which results in the assembly of Co nanoparticles to nanowires. The Co nanowires exhibit ferromagnetic properties at room temperature, with an enhanced coercivity of about 700 Oe possibly due to shape anisotropy.
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