From hot-injection synthesis to heating-up synthesis of cobalt nanoparticles: observation of kinetically controllable nucleation.

Monodisperse nanoparticles of well-defined size and shape are required in several emerging applications, which take advantage of their size-dependent properties such as the superparamagnetic limit in the case of magnetic nanoparticles. Accurate tuning of the nanoparticle size and shape requires understanding of the mechanisms involved in particle nucleation and growth. In spite of extensive ongoing research, these mechanisms are still not fully understood owing to their complexity and interplay. Moreover, the current small-scale synthesis methods, such as the hotinjection method, can be difficult to scale to industrially relevant levels. Hence, more suitable methods are sought. Herein, we revisit a widely studied hot-injection synthesis of monodisperse cobalt nanoparticles and show that the particle nucleation differs from what is expected for a hotinjection synthesis. Evidence is given that the particles nucleate several tens of seconds or a few minutes after the injection, depending delicately on how the reaction temperature is controlled after the sudden temperature drop caused by the injection. The delayed nucleation is followed by a period during which the cobalt precursor decomposes endothermically, the temperature drops, carbon monoxide evolves, and the nuclei rapidly grow into mature nanoparticles. Particle growth after the endothermic period is negligible, and we show that the final particle size is determined by the rate of temperature increase after the injection-induced temperature drop. A rapid increase results in a higher peak temperature before the endothermic period and more nuclei, hence smaller particles, in comparison to the case of a slower rate of temperature increase. The contribution of the injection to particle nucleation seems minor, and it is shown that injection can be replaced entirely by an accurately controlled heating up of the solution containing all reagents (including the cobalt precursor) from room temperature to the nucleation temperature. This synthetic method, which is often termed either “non-injection synthesis” or “heating-up synthesis”, results in nanoparticles that are nearly identical to those made by the hot-injection method. We synthesized cobalt nanoparticles by injecting dicobalt octacarbonyl, [Co2(CO)8], dissolved in a small amount of ortho-dichlorobenzene (o-DCB, b.p. 181 8C) into a solution of oleic acid and trioctylphosphine oxide (TOPO) in o-DCB at reflux. The injection led to an immediate temperature drop of several tens of degrees, which is characteristic of the hotinjection method in general. It has been shown that [Co2(CO)8] undergoes partial decarbonylation during the injection to form gaseous carbon monoxide and intermediate cobalt carbonyl species (e.g. tetracobalt dodecacarbonyl, [Co4(CO)12], and cobalt tetracarbonyl, [Co(CO)4]) in the solution phase; these species then further decompose more slowly to cobalt atoms. It has been shown that both maintaining the lower temperature and letting the temperature recover to the reflux temperature after the injection can lead to monodisperse nanoparticles. In this study, we concentrated on the latter approach and studied for the first time in detail the kinetics of the temperature recovery to reflux. The recovery rate can be conveniently controlled by tuning the rate of heat transfer from the heat bath to the reaction medium, for example, by using an oil bath at different temperatures or an electric heating mantle with different heating powers. A typical development in the reaction temperature after the injection is shown in Figure 1 for the hot-injection synthesis HI1 (see the Experimental Section for a complete list of syntheses with details). The temperature dropped from 180 to 143 8C during the injection, after which it started to recover, as heat was being transferred from the heat bath to the reaction medium. In contrast to the expected continuous increase until the reflux temperature was reached, one minute after the injection we observed a characteristic endothermic period during which the reaction temperature dropped despite continuous heating. Interestingly, the peak temperature (174 8C) reached just before the endothermic period was very close to the temperature prior to injection (180 8C). Vigorous evolution of carbon monoxide during the endothermic period indicated decomposition of the cobalt carbonyl species and release of cobalt atoms. Further evolution of carbon monoxide after the endothermic period was negligible, even when the reflux temperature was reached. This observation indicated that nearly all cobalt carbonyl species had decomposed during the endothermic period, which was [*] J. V. I. Timonen, Prof. Dr. O. Ikkala, Dr. R. H. A. Ras Department of Applied Physics, Aalto University (formerly Helsinki University of Technology) P.O. Box 15100, FI-02150 Espoo (Finland) E-mail: [email protected] [email protected] Homepage: http://tfy.tkk.fi/molmat/