Size Control of Gold Nanocrystals in Citrate Reduction: The Third Role of Citrate

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• 作者：Xiaohui Ji ; Xiangning Song ; Jun Li ; Yubai Bai ; Wensheng Yang ; Xiaogang Peng
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：November 14, 2007
• 年：2007
• 卷：129
• 期：45
• 页码：13939 - 13948
• 全文大小：723K
• 年卷期：v.129,no.45(November 14, 2007)
• ISSN：1520-5126

文摘

Growth kinetics and temporal size/shape evolution of gold nanocrystals by citrate reduction inboiling water were studied systematically and quantitatively. Results reveal that the size variation and overallreaction mechanism were mostly determined by the solution pH that was in turn controlled by theconcentration of sodium citrate (Na₃Ct) in the traditional Frens's synthesis. This conclusion was furtherconfirmed by the reactions with variable pH but fixed concentrations of the two reactants, HAuCl₄ andNa₃Ct. Two substantially different reaction pathways were identified, with the switching point at pH = 6.2-6.5. The first pathway is for the low pH range and consists of three overlapping steps: nucleation, randomattachment to polycrystalline nanowires, and smoothing of the nanowires via intra-particle ripening to dots.The second pathway that occurred above the pH switching point is consistent with the commonly knownnucleation-growth route. Using the second pathway, we demonstrated a new synthetic route for the synthesisof nearly monodisperse gold nanocrystals in the size range from 20 to 40 nm by simply varying the solutionpH with fixed concentrations of HAuCl₄ and Na₃Ct. The switching of the reaction pathways is likely due tothe integration nature of water as a reaction medium. In the citrate reduction, the solution pH was variedby changing the initial HAuCl₄/Na₃Ct ratio. Consequently, when pH was higher than about 6.2, the veryreactive [AuCl₃(OH)]^- would be converted to less reactive [AuCl₂(OH)₂]^- and [AuCl(OH)₃]^-.