Synergistic effect of binary ligands on nucleation and growth/size effect of nanocrystals: Studies on reusability of the
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An attempt to reduce the effect of major toxic components namely phosphine ligands and unsaturated solvents as being used in conventional nanocrystal synthesis, has been made with a new binary ligand, and a reusable solvent N-octadecane for a smokeless and clean synthesis procedure. The optimized effects of the two ligands oleic acid and octadecyl amine on the nucleation rate and growth of CdSe nanocrystals (NCs) are reported and substantiated by AFM analysis. Oleic acid accelerates particle ripening and nuclei growth, but inhibits nucleation whereas octadecyl amine catalyses nucleation and very gradually improves growth to obtain small stable NCs. Another important feature of the present study is the replacement of 1-octadecene by a competitive N-octadecane as a solvent in such ligand mediated nanocrystal synthesis. The GCMS analysis reports a recovery of 95% of solvent after reuse, thus opening a scope for environmental friendly processes.
I. INTRODUCTION
The synthetic methods for preparation of CdSe nanocrystals (NCs) have improved over the past decade by optimizing the precursors, ligands, and solvents. Since Peng’s group1 used an alternative Cd source instead of an unstable, explosive, highly toxic, and pyrophoric dimethyl Cd to prepare high quality cadmium chalcogenides, CdO has been widely used to prepare Cd chalcogenide NCs.1–4 In these methods, an additional preparation process of high temperature above 250 °C was used for the Cd ions to bind to the ligand. However, the expensive and toxic phosphine derivatives were still reported to be used as coordinating ligands in the above synthetic procedures. This often limited the production of QDs in organics to milligram quantities and the need for synthesis of CdSe NCs in phosphine free organics5,6 had gained importance. After a decade, Peng’s group7 reported the replacement of ligand, trioctyl phosphine oxide (TOPO) by oleic acid and oleic acid was used as a capping ligand for the NCs in the presence of a long chain alkene 1-octadecene (ODE) as a solvent. The capping agent’s ability to avoid particle agglomeration and promote nucleation was an important aspect in such synthesis of arrested precipitation. In general, phosphine free ligands like carboxylic acids,5 mercaptans, and amines6 that absorb quickly to bare particle surface to minimize the Van der Wall’s interactions and interactions within crystallites by steric hindrance and solvation forces were identified as efficient a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.180 1556
J. Mater. Res., Vol. 29, No. 14, Jul 28, 2014
http://journals.cambridge.org
Downloaded: 15 Mar 2015
capping agents. Ligands which bind strongly to monomers as soluble complexes would enhance the rate of nuclei growth, but decrease the rate of nucleation. Contrarily, if ligands bind to NCs surface to form insoluble surface precipitate, both the growth of nuclei and dissolution are kinetically hindered to form small sized and more numbered NCs. Hence, a combination of tw
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