Layer-By-Layer Assembly of Core-Shell Magnetite Nanoparticles: Effect of Silica Coating on Interparticle Interactions and Magnetic Properties
✍ Scribed by Farkhad G. Aliev; Miguel A. Correa-Duarte; Arif Mamedov; John W. Ostrander; Michael Giersig; Lius M. Liz-Marzán; Nicholas A. Kotov
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 913 KB
- Volume
- 11
- Category
- Article
- ISSN
- 0935-9648
No coin nor oath required. For personal study only.
✦ Synopsis
can also be molded into macroporous structures by the templating method presented here. By appropriate modification and optimization of the templating procedure we expect that it will be possible to vary the pore-size range significantly. The macroporous metals may themselves be used as templates on which other materials could be deposited or grown.
Experimental
Monodisperse PS spheres were synthesized and packed into colloidal crystals by centrifugation as described previously [6]. Sphere diameters in various batches ranged from 420 ± 50 nm to 660 ± 30 nm. To prepare macroporous nickel oxide, nickel(II) acetate (2 g) was dissolved in either acetic acid (5 mL) or a mixture of H 2 O (5 mL)and ethanol (5 mL) at 60 C. After cooling to room temperature, any undissolved solid was removed by filtration. Centimeter-scale, close-packed, colloidal polystyrene crystals (1 g) were soaked in this solution for 3±5 min. Excess solution was removed from the impregnated colloidal crystals by vacuum filtration. The samples were dried at 60 C for 2 h. The dried composites were soaked in 10 mL of a saturated solution of oxalic acid in ethanol (ca. 25±30 g in 100 mL) for 3±5 min. After additional vacuum filtration and drying, the PS spheres were removed by calcination in flowing air at 360±575 C for 7±10 h (heating rate: 1 C/ min). Macroporous Ni was prepared by heating macroporous NiO in flowing H 2 at 300 C for 2±5 h (heating rate: 1 C/min) or by heating the nickel oxalate/PS composite in flowing nitrogen at 450±500 C for 10 h. Preparation of partially reduced samples containing both NiO and Ni was possible by decreasing the reduction time or temperature, or by carrying out a fast calcination, for example at 360 C for 1 h in fast flowing air.
Electrochemical tests were performed in two electrode cells using 1 M aqueous KOH as the electrolyte and nickel pellets as both electrodes. Pellets were made by pressing a mixture of 10 wt.-% poly(vinylidene fluoride) binder with 90 wt.-% nickel to 1500 psi for 30 s. An SEM of the material indicated that a porous structure was still maintained. No carbon powder was needed to enhance the pellet conductivity. Impedance analysis was performed using a Solartron 1260 Impedance/Gain-Phase Analyzer along with a Solartron 1287 Electrochemical Interface. Other product characterization was carried out as described previously [6].