Page 93 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin
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Solvent-Free Nickel Nanoparticles Synthesis and Engineering ‒Controllable Magnetic Properties
5.2.2 Characterization
Optical emission spectroscopy (HR4000, Ocean Optics, Inc.) with a spectral resolution of 0.91 nm was employed to analyze the Ni(cp)2 dissociation process and to identify specific intermediate radicals existing in the plasma. To characterize the morphology of the products, sample images were recorded by a Quanta 3D FEG (FEI) scanning electron microscopy (SEM). The particles size and shape were further characterized using a FEI Tecnai 20 (type Sphera) transmission electron microscope, with selected-area electron diffraction analysis to identify crystalline phases. XRD measurements were performed using a Rigaku Geigerflex X-ray diffractometer (Cu-Kα1 radiation, λ=1.54056). The average grain size of the Ni nanoparticles was estimated from the highest intensity peak (111) at 44.5◦ using the Scherrer formula. Magnetic properties of the products were evaluated using a VSM-SQUID magnetometer from Quantum Design (MPMS 3). Hysteresis loops and the saturation magnetization were measured at room temperature. Zero field cooled (ZFC) measurement were performed using the following procedure: the samples were cooled to 5 K in the absence of an external magnetic field. Then a field of 10 mT was applied and the magnetization was measured as a function of temperature up to 300 K. For the field cooled (FC) measurement, samples were cooled to 5 K in the presence of a constant magnetic field of 10 mT, then the magnetization was measured as a function of temperature up to 300 K, whilst keeping the 10 mT field present.
5.3 Results and Discussions
The precursor dissociation process is characterized in-situ through OES measurement by recording the emitted spectra and correlating the spectral features with emission peaks of Ni(cp)2 originated radicals. Based on preceding reports18–22 and NIST Atomic Spectra Database,23 detailed radiative transition information of species resulted by Ni(cp)2 dissociation process is summarized in Table S1. Figure 5.2 shows a representative spectrum recorded from a discharge operated at condition 4 to identify the excited species and radiating intermediate radicals in plasma. Apart from the dominant argon atomic transitions (Ar I) between highly excited electronic states in the region of 690-850 nm, prominent emission peaks of carbonaceous species corresponding to C2 Swan bands [467-474 nm (∆v=-1), 512- 520nm (∆v=0), 550-565nm (∆v=+1)], C2 Fox-Herzberg bands (285-315 nm) and CH bands [380-390 nm (3900 Å system), 430-435 nm (4300 Å system)] are found in all spectra. Furthermore, low intensity peaks at 657 nm and 340-355 nm are observed in all spectra, which are ascribed to Hα and Ni lines respectively.24,25 The presence of emission lines related to Ni(cp)2 fragments proves the decomposition of Ni(cp)2 vapors in the argon discharge.
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