Abstract
Nanostructured cobalt is one of the key elements in catalysis and therapeutic drug delivery. To design and prepare nanosize-controllable cobalt, a better understanding of its growth mechanism is essential. Growth of Co nanoparticles encapsulated in carbon-shell (Co@C) during temperature-programmed carbonization of the Co2+-β-cyclodextrin (CD) complex at 363–573K was, therefore, studied by in situ synchrotron small-angel X-ray scattering and X-ray absorption near edge structure spectroscopy. The carbon-shell having a thickness of about 2nm can prevent the core Co from being aggregated and oxidized. A relatively slow reduction of Co(II) to Co is observed at 393–423K (stage I) prior to a particle growth transition-state possessing Co of 2.2nm in diameter at 423–483K. At 483–513K (stage II), an increasing Co(II) reduction rate coupled with a rapid fusion and coalescence of Co nanoparticles is found. The average growth rates of Co at stages I and II are about 27 and 98 atoms/min, respectively. The most-probable particle diameter of the ripened Co is 5.9nm. The carbon-shell can be removed by steam reforming to yield the Co nanoparticles. This work also exemplifies the possible temperature-controllable growth of Co@C, especially in the Co size range of 2–6nm in diameter.
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