Controlling Gold Nanoparticles with Atomic Precision: Synthesis and Structure Determination

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1 Controlling Gold Nanoparticles with Atomic Precision: Synthesis and Structure Determination Huifeng Qian Department of Chemistry, Carnegie Mellon University, USA Advisor: Prof. Rongchao Jin Background and Objective Gold nanoparticles have drawn significant research and technological interests due to their unique catalytic, optical, and electronic properties. 1,2 Control of nanoparticles with atomic precision poses major challenges and has long been a major dream of nanoparticle chemists. Atomically precise nanoparticles allow the total structure (metal core plus surface ligands) to be revealed and attain fundamental understanding of the structure (in particular the surface atom arrangement), which is of paramount importance in order to study the catalytic properties of gold nanoparticles and for better design of highly efficient catalysts. This dissertation focuses on the development of new methodologies for the synthesis of atomically precise gold nanoparticles and structural determination by X-ray crystallography. A universal size focusing strategy has been successfully established, which fulfills the major goal of this thesis. Contributions and Results 1. Developed a universal size-focusing methodology to synthesize atomically precise Au nanoparticles A new size focusing methodology has been developed in my thesis research for the synthesis of atomically precise gold nanoclusters. 3 Using this methodology, a number of molecular purity gold nanoclusters (such as Au 25 (SR) 18, [Au 25 (PPh 3 ) 10 (SR) 5 Cl 2 ] 2+, Pd 1 Au 24 (SR) 18, Pt 1 Au 24 (SR) 18, Au 38 (SR) 24, Au 40 (SR) 24, Au 55 (SR) 31, Au 144 (SR) 60 and Au 333 (SR) 79 ) have been made with metal core diameter spanning from 1.0 to 2.2 nanometer The major chemical principle involved in the size-focusing methodology is that different sized Au n (SR) m nanoclusters possess vastly different stability, which leads to the survival of the robustest nanoparticle in the harsh size-focusing process, much like natural law Survival of the Fittest.

1,2 Control of nanoparticles with atomic precision poses major challenges and has long been a major dream of nanoparticle chemists.

2 Scheme 1. Size-focusing synthesis of atomically precise gold nanoparticles (SG: glutathione). Figure 1. ESI-MS (left panel) and UV-vis absorption (right panel) spectra of Au 25 (SR) 18, Au 38 (SR) 24 and Au 144 (SR) 60 nanoclusters, respectively (R: C 2 H 4 Ph). The key condition for this methodology is to control the size distribution of the starting size-mixed nanoclusters (prior to the size-focusing step). A too broad size distribution would

60 nanoclusters, respectively (R: C 2 H 4 Ph).

3 result in several robust nanoclusters after size-focusing, and isolation of them would be very difficult, albeit in some special cases it might be possible. Therefore, in an ideal situation one would like to control the initial size distribution in a proper range and render it favorable for size focusing into one size of robust nanoclusters. (Scheme 1) After mapping out the important parameters that influence the size distribution of nanoclusters, we chose appropriate conditions for size focusing synthesis of nanoclusters with size control. (Figure 1) 2. Determined the total structure (core and surface) of Au 38 (SR) 24 nanoparticles by single crystal X-ray crystallography The obtained atomically precise nanoclusters allow for the determination of their total structures (core plus surface). Understanding the total structure, in particular the surface atom arrangement, is of paramount importance in order to study the fundamental properties, for example, the catalytic properties of metal nanoclusters for future design of highly efficient catalysts. 12 In order to determine the total structure, the growth of single crystals of nanoclusters is the critical step, but unfortunately crystallization of nanoclusters in general has long been a huge challenge. Significantly, the Au 38 (SR) 24 nanoclusters with molecular purity achieved by the size focusing methodology have led to successful crystallization. 13 The unit cell of Au 38 (SCH 2 CH 2 Ph) 24 crystals surprisingly reveals a pair of enantiomeric nanoparticles (left-handed and right handed). (Figure 2) Here we choose the left-handed enantiomer for a detailed analysis of its structure. (Figure 3) The particle shows a prolate shape (diameter~ 0.8 nm, length~1.1 nm, Au center-to-center distances). The core of Au 38 (SR) 24 comprises a face-fused biicosahedral Au 23 motif ( =23, inner core or kernel), Figure 3A-B. The fusion of the two icosahedra occurs along a common C 3 axis. The rod-like Au 23 kernel is structurally strengthened by three monomeric SR-Au-RS staples (Figure 3C). Then, the top icosahedron is further capped by three SR-Au-SR-Au-RS dimeric staples (Figure 3D), which are arranged in a rotative fashion, resembling a tri-blade propeller. A similar arrangement of the other three staples is found on the bottom icosahedron, but the bottom propeller rotates by ~60 o relative to the top one, forming a staggered dualpropeller configuration (Figure 3E). The entire nanoparticle becomes chiral due to the rotative arrangement of the dimeric staples. The chiral structure of Au 38 (SR) 24 is potentially very useful in chiral catalysis, chiral separation, and other applications.

(Scheme 1) After mapping out the important parameters that influence the size distribution of nanoclusters, we chose appropriate conditions for size focusing synthesis of nanoclusters with size

(SR) 24 structure (color lables: yellow: S, other colors: Au). 3.

4 Figure 2. Two chiral isomers of Au 38 (SC 2 H 4 Ph) 18 revealed by X-ray crystallography (color labels: magenta, Au; yellow, S; grey: C; white, H). Figure 3. Anatomy of the Au 38 (SR) 24 structure (color lables: yellow: S, other colors: Au) Au n (SR) m Catalysts for Selective Oxidation and Hydrogenation Reactions We have also explored the catalytic applications of the synthesized well-defined Aun(SR) m nanoparticles in selective oxidation and hydrogenation processes. 12,14 On the basis of their crystal structures, these Au particles permit a correlation of particle structure with their excellent catalytic properties and an identification of catalytically active sites on the particle; the latter has long been pursued in nanocatalysis, but conventional polydisperse nanoparticles preclude the

5 identification of active sites and deep understanding of catalysis since the surface structure of polydisperse nanoparticles is impossible to determine. Thus, the atomically monodisperse nanoparticles we have made are expected to significantly advance the catalysis science. Summary My doctoral research has successfully establish the first-ever size focusing methodology for preparing a series of perfect Au n (SR) m nanoparticles with molecular purity. Growth of single crystals has revealed the intriguing chiral structure of Au 38 (SCH 2 CH 2 Ph) 24. The range of sizes attained, including Au 25 (SR) 18, Au 38 (SR) 24, Au 40 (SR) 24, Au 55 (SR) 31, Au 144 (SR) 60, and Au 333 (SR) 79, has explicitly demonstrated the evolution of the structural, optical and electronic properties with the number of gold atoms in the particle. These well defined nanoparticles hold great promise in many applications such as catalysis, optics, and sensing. Reference (1) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, (2) Qian, H.; Zhu, M.; Wu, Z.; Jin, R. Acc. Chem. Res. 2012, 45, (3) Jin, R.; Qian, H.; Wu, Z.; Zhu, Y.; Zhu, M.; Mohanty, A.; Garg, N. J. Phys. Chem. Lett. 2010, 1, (4) Qian, H.; Zhu, Y.; Jin, R. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, (5) Qian, H.; Jiang, D.-e.; Li, G.; Gayathri, C.; Das, A.; Gil, R. R.; Jin, R. J. Am. Chem. Soc (6) Qian, H.; Jin, R. Nano Lett. 2009, 9, (7) Qian, H.; Zhu, Y.; Jin, R. J. Am. Chem. Soc. 2010, 132, (8) Qian, H.; Zhu, Y.; Jin, R. ACS Nano 2009, 3, (9) Qian, H.; Eckenhoff, W. T.; Bier, M. E.; Pintauer, T.; Jin, R. Inorg. Chem. 2011, 50, (10) Qian, H.; Jin, R. Chem. Mater. 2011, 23, (11) Qian, H.; Zhu, M.; Gayathri, C.; Gil, R. R.; Jin, R. ACS Nano 2011, 5, (12) Zhu, Y.; Qian, H.; Jin, R. J. Mater. Chem. 2011, 21, (13) Qian, H.; Eckenhoff, W. T.; Zhu, Y.; Pintauer, T.; Jin, R. J. Am. Chem. Soc. 2010, 132, (14) Zhu, Y.; Qian, H.; Zhu, M.; Jin, R. Adv. Mater. 2010, 22,

Summary My doctoral research has successfully establish the first-ever size focusing methodology for preparing a series of perfect Au n (SR) m nanoparticles with molecular purity.

Chemical Synthesis. Overview. Chemical Synthesis of Nanocrystals. Self-Assembly of Nanocrystals. Example: Cu 146 Se 73 (PPh 3 ) 30

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