LETTER pubs.acs.org/ac
Monodisperse Au11 Clusters Prepared by Soft Landing of Mass Selected Ions Grant E. Johnson,*,† Chongmin Wang,‡ Thomas Priest,†,§ and Julia Laskin*,† †
Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MSIN K8-88, Richland, Washington 99352, United States ‡ W. R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States ABSTRACT: Preparation of clean monodisperse samples of clusters and nanoparticles for characterization using cutting-edge analytical techniques is essential to understanding their size-dependent properties. Herein, we report a general method for the preparation of high surface coverage samples of monodisperse clusters containing an exact number of atoms. Polydisperse solutions of diphosphine-capped gold clusters were produced by reduction synthesis. Electrospray ionization was used to introduce the clusters into the gas phase where they were filtered by mass-tocharge ratio allowing clusters of a selected size to be deposited onto carbon coated copper grids at well controlled kinetic energies. Scanning transmission electron microscopy (STEM) analysis of the soft landed clusters confirms their monodispersity and high coverage on the substrate. The soft landing approach may be extended to other materials compatible with an array of available ionization techniques and, therefore, has widespread utility as a means for controlled preparation of monodisperse samples of nanoparticles and clusters for analysis by transmission electron microscopy (TEM).
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he chemical and physical characteristics of nanoparticles and clusters are known to be highly size and shape dependent,1 4 resulting in distinctive optical,5 electronic,6 and catalytic properties7 that emerge at the nanometer length scale. Therefore, a new generation of materials is forthcoming that will employ nanoparticles and clusters either as unique functional units or as building blocks in cluster-based materials.8 For example, nanoparticles are already strong candidates for use in photothermal therapeutic treatments,9 as contrast agents in biological and cellular imaging applications,10 as composite devices for chemical sensing and threat detection,11 and as catalysts.12 Due to the pervasive nature of these materials, recent years have seen a major expansion in research efforts to synthesize uniform nanoparticles of a particular size and shape,4,13 and in many cases, scalable solution-phase methods have been developed.4,14 In a similar vein, efforts are also being undertaken to prepare monodisperse subnanometer metal clusters containing an exact number of atoms so that their size-dependent properties may be understood on an atom-by-atom basis. Through reduction synthesis in the presence of capping ligands such as thiols,15,16 phosphines,17 and diphosphines,18,19 it is possible to tightly focus the distribution of cluster sizes obtained in solution, as has been shown previously using electrospray ionization (ESI)18,20 26 and matrix assisted laser desorption ionization (MALDI)27,28 mass spectrometry. However, ESI-MS, which provides valuable insight into the ionizable components of a solution, does not account for neutral clusters and nanoparticles which may also be present. For this reason, complementary characterization tools such as transmission electron microscopy (TEM) are desirable to ensure the monodispersity of clusters produced using a given synthesis technique. r 2011 American Chemical Society
TEM has become a workhorse technique for the characterization of the structural properties of small metal nanoparticles and clusters.29,30 With the advent of aberration corrected31 instruments, it is now possible to image features such as single metal atoms on crystalline supports in catalysts,32 35 the atomic constituents of larger catalytic nanoparticles,36,37 and grain boundaries in nanotwinned metals4,32 with unprecedented spatial resolution. Despite these impressive capabilities, TEM analysis of complex specimens can still be challenging and timeconsuming due to a lack of sample homogeneity. For example, TEM imaging of small catalytic nanoparticles dispersed randomly on metal oxide supports often requires a lengthy period of searching for the species of interest in the overall sample. In the case of scanning transmission electron microscopy (STEM) imaging, the analysis may be complicated further by the fact that the tightly focused high-energy electron beam may rapidly degrade or destroy the small particles. Due to these factors, it is desirable to prepare clean homogeneous samples of clusters and nanoparticles with high surface coverage that will facilitate rapid identification and enable repeated imaging of identical cluster species. Herein, we demonstrate that soft landing38 44 of massselected ions may be used to prepare clean high coverage samples of monodisperse clusters on surfaces for analysis by TEM and STEM. Mass selection offers precise control over the size and charge state of the deposited clusters. Moreover, unprecedented Received: September 22, 2011 Accepted: October 4, 2011 Published: October 04, 2011 8069
dx.doi.org/10.1021/ac202520p | Anal. Chem. 2011, 83, 8069–8072
Analytical Chemistry
Figure 1. (a) Typical positive mode electrospray mass spectrum of DPPP capped gold clusters in methanol; (b) TEM image of a droplet of the nanoparticle solution cast onto a carbon coated copper grid (scale bar = 50 nm).
sample cleanliness is obtained using ion soft landing as molecular precursors, reaction intermediates, counterions, and solvent molecules associated with the solution-phase synthesis are removed so that only the desired cluster is delivered to the substrate.
’ RESULTS AND DISCUSSION Gold clusters were synthesized in solution according to literature procedures.18,45 Briefly, chloro(triphenylphosphine)gold(I) was dissolved in a 1/1 mixture of methanol and chloroform to create 100 mL of a 0.1 mM solution. 1,3-Bis(diphenylphosphino)propane (DPPP) was then added at a concentration of 0.1 mM. After mixing of the gold precursor and capping ligand, a borane-tert-butylamine reducing agent was added to a final solution concentration of 0.5 mM. The solution was stirred rapidly at room temperature for 3 h until it turned a deep orange color indicating the reduction of Au(I) and the formation of gold nanoparticles. Figure 1a shows a typical electrospray mass spectrum (m/z = 50 1500) of the gold nanoparticle solution that was obtained in the positive ion mode using a Bruker HCT ion trap mass spectrometer. A spray potential of 4 kV was employed, and the potential gradient in the source region of the instrument was kept at
