Kinetic Control and Thermodynamic Selection in the Synthesis of Atomically Precise Gold Nanoclusters

Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, USA.
Journal of the American Chemical Society (Impact Factor: 12.11). 06/2011; 133(25):9670-3. DOI: 10.1021/ja2028102
Source: PubMed


This work presents a combined approach of kinetic control and thermodynamic selection for the synthesis of monodisperse 19 gold atom nanoclusters protected by thiolate groups. The step of kinetic control allows the formation of a proper size distribution of initial size-mixed Au(n)(SR)(m) nanoclusters following the reduction of a gold precursor. Unlike the synthesis of Au(25)(SR)(18) nanoclusters, which involves rapid reduction of the gold precursor by NaBH(4) followed by size focusing, the synthesis of 19-atom nanoclusters requires slow reduction effected by a weaker reducing agent, borane-tert-butylamine complex. The initially formed mixture of nanoclusters then undergoes size convergence into a monodisperse product by means of a prolonged aging process. The nanocluster formula was determined to be Au(19)(SC(2)H(4)Ph)(13). This work demonstrates the importance of both kinetic control of the initial size distribution of nanoclusters prior to size focusing and subsequent thermodynamic selection of stable nanoclusters as the final product.

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    • "In this work, we have identified several important factors for the synthesis of nanoclusters Au39 and Au40, including the stirring speed of the reaction mixture, the addition speed, and the amount of NaBH4 solution to reduce Au(I) into clusters. The synthetic conditions reported in this work differ from the previous syntheses of Au19(SC2H4Ph)13, Au20(SC2H4Ph)16 and Au24(SC2H4Ph)20 (see ‘Methods’ section) [50-52]. Specifically, in the present work, our major modification lies in the stirring speed of the Au(I) intermediate solution when reduced by NaBH4. "
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    ABSTRACT: This work presents a controlled reduction method for the selective synthesis of different sized gold nanoclusters protected by thiolate (SR = SC2H4Ph). Starting with Au(III) salt, all the syntheses of Au n (SR) m nanoclusters with (n, m) = (20, 16), (24, 20), (39, 29), and (40, 30) necessitate experimental conditions of slow stirring and slow reduction of Au(I) intermediate species. By controlling the reaction kinetics for the reduction of Au(I) into clusters by NaBH4, different sized gold nanoclusters are selectively obtained. Two factors are identified to be important for the selective growth of Au20, Au24, and Au39/40 nanoclusters, including the stirring speed of the Au(I) solution and the NaBH4 addition speed during the step of Au(I) reduction to clusters. When comparing with the synthesis of Au25(SC2H4Ph)18 nanoclusters, we further identified that the reduction degree of Au(I) by NaBH4 also plays an important role in controlling cluster size. Overall, our results demonstrate the feasibility of attaining new sizes of gold nanoclusters via a controlled reduction route.
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    ABSTRACT: The assembly of nanoparticles (NPs) into complex structures is a fundamental topic in nanochemistry. Although progress has been made in this respect, the classical treatment of NPs as hard building blocks that lack the ability to anisotropically “bond” to each other limits the construction of more complex assemblies. More importantly, extension of methods of assembly of robust NPs to the assembly of ultrasmall ones with size below 2 nm is still challenging. Here we report the controlled self-assembly of ∼2 nm gold NPs into one-dimensional (1D) nanochain, two-dimensional (2D) nanobelt and three-dimensional (3D) nanocomet architectures by kinetically controlling the diffusion of ultrasmall gold NPs with anisotropic surfaces in solution. This process is presumed to allow selective ligand exchange with linkers at different binding sites on the NP surface, and results in “multivalent” interactions between NPs. Different from the assembly of “hard building blocks”, our results demonstrate the significance of surface effects for ultrasmall NPs (or clusters) in determining the structure of complex self-assemblies, and suggest the possibility of assembling these “molecule-like” ultrasmall nanocrystals into novel complex materials on a nanoscale approaching that of real atoms or molecules.
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    ABSTRACT: We report the controlled growth of Au25(SR)18 and Au38(SR)24 (where R = CH2CH2Ph) nanoclusters of molecular purity via size-focusing from the same crude product that contains a distribution of nanoclusters. In this method, gold salt was first mixed with tetraoctylammonium bromide (TOAB), and then reacted with excess thiol to form Au(I)-SR polymers in THF (as opposed to toluene in previous work), followed by NaBH4 reduction. The resultant crude product contains polydisperse nanoclusters and was then used as the common starting material for controlled growth of Au25(SR)18 and Au38(SR)24, respectively. In Route I, Au25(SR)18 nanoclusters of molecular purify were produced from the crude product after 6 h aging at room temperature. In Route II, the crude product was isolated and further subjected to thermal thiol etching in a toluene solution containing excess thiol, and one obtained pure Au38(SR)24 nanoclusters, instead of Au25(SR)18. This work not only provides a robust and simple method to prepare both Au25(SR)18 and Au38(SR)24 nanoclusters, but also reveals that these two nanoclusters require different environments for the size-focusing growth process.
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