ARTICLE pubs.acs.org/crystal
Rapid Synthesis and Structural Characterization of Well-Defined Gold Clusters by Solution Plasma Sputtering Xiulan Hu,*,†,§ Sung-Pyo Cho,‡ Osamu Takai,†,‡,§ and Nagahiro Saito*,†,‡,§,|| †
EcoTopia Science Research Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan § CREST, Japan Science and Technology Agency (JST), Tokyo 102-0075, Japan Research and Development Center for Green Vehicle Materials, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
)
‡
bS Supporting Information ABSTRACT: Ultrafine metal nanoparticles of a few nanometers in diameter exhibit size-dependent photonic and electric properties that are of interest for applications such as biosensors, catalysts, optics, and electronics. Chemical approaches and vacuum metal-vapor-condensation physical techniques were used to successfully synthesize gold nanoparticles. While it is difficult to obtain monodisperse and small sized gold nanoparticles without any reductant and polymer stabilizer or under vacuum conditions, in the present study, multiply twinned and near monodisperse gold clusters of diameter less than 2.0 nm were successfully fabricated for the first time by solution plasma sputtering in liquid nitrogen (LN2) without any chemical additions (such as reductant and polymer stabilizer). Gold clusters formed in several microseconds simultaneously with solution plasma sputtering in an open system under atmospheric pressure. Gold clusters are identified to be well crystalline and multitwin-particles (MTPs) by high-resolution transmission electron microscopy. No surface plasmon resonance band was detected in the gold cluster aqueous solutions. Such MTPs with special corners and edges would be beneficial for tailoring catalytic properties at the nanoscale. The solution plasma sputtering method will have potential application in the future in the design and mass preparation of various multifunctional metal clusters.
’ INTRODUCTION Ultrafine metal nanoparticles of a few nanometers in diameter exhibit size-dependent photonic and electric properties that are of interest for applications such as biosensors, catalysts for chemical and fuel cells, optics, and electronics.1 8 The properties of materials change as their size is closer to subnanometers and as the percentage of atoms at the surface of a material becomes significantly higher. A metal-to-nonmetal transition occurs as the diameter is decreased below 2.0 nm, and increasing band gaps were detected with the further decrease of clusters size. Thus, particles of diameter less than 2.0 nm are defined as clusters. The changes in physical properties are not always predictable. When gold is prepared as very small particles with diameters less than 10 nm and is highly dispersed on metal oxides, it turns out to be a highly active catalyst for many reactions such as CO oxidation and propylene epoxidation in the gas phase. Additionally, the intrinsic magnetic polarization in gold nanoparticles, which were protected by polyallylamine hydrochloride (PAAHC) in the size range of 1.0 3.0 nm (However, the relative frequency of clusters of diameter less than 2.0 nm is only about 50%), was revealed by the X-ray magnetic circular dichroism (XMCD) and elementspecific magnetization (ESM) experiments.9 Fluorescent watersoluble gold nanoparticles were synthesize by the reduction of a gold salt in the presence of a designed polymer ligand, and the r 2011 American Chemical Society
size and fluorescence of the particles were controlled by the polymer to gold ratio.10 Therefore, the production of small gold clusters with diameters less than 2.0 nm (65% of atoms at the surface), which is a critical size for a dramatic change in electronic structure, is still an exciting area of research. These small clusters were tuned more pronouncedly by choosing support materials ranging from metal oxides and activated carbon to polymers.4,9 13 Typically, in chemical approaches, the formation of clusters is conducted from metal ions or metal complexes in the presence of reducing agent (e.g., sodium borohydride, hydrazine hydrate, and citric acid) and stabilizing agents (e.g., thiol compounds and polymers) in order to obtain the small-nanoscale size. One problem with the chemical methods is the inevitable introduction of impurity, which requires subsequent purification steps after the synthesis for its potential application.14 On the other hand, vacuum metalvapor-condensation techniques have been developed for the clean preparation of metal nanoparticles dispersed in inorganic/ organic solvents without formation of impurity.15 17 Among recent physical methods, sputtering deposition has better Received: July 7, 2011 Revised: September 27, 2011 Published: September 29, 2011 119
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Figure 2. Size distributions of gold clusters. (Calculated from STEM measurement of gold clusters; the counted numbers are about 1500.).
Figure 1. DF-STEM image of gold clusters.
capability to fabricate refractory metals and intermetallic compounds than evaporation and laser ablation. Because the atom vapor is typically generated from targets of pure materials, nanoparticles created by sputtering usually contain fewer impurities than those created by chemical methods. Gold nanoparticles were obtained via aerosol generation by spark discharge at low vapor pressure18 and using sputtering deposition of gold onto various ionic liquids.19 However, size controllable and highdispersibility clusters of diameter less than 2.0 nm have not been achieved using vacuum methods due to the weak interaction between metal and dispersion medium. Thus, it is difficult to obtain monodisperse and small sized gold nanoparticles without any reductant and polymer stabilizer or under vacuum conditions. Therefore, a new technique is urgently required to facilely fabricate size controllable clean clusters for their potential applications. Solution plasma is defined by nonequilibrium plasma in solutions, which provides us a novel reaction field with highly excited energy state. Recently, the rapid synthesis of clean nanoparticles without any reducing agents has succeeded due to the strong reduction potential of the generated radicals.3,20,21 In the present study, we addressed a novel and one-step route for highly efficient synthesis of clean gold clusters of diameter less than 2.0 nm from metal wire electrodes in an open system under atmospheric pressure by solution plasma sputtering.
Figure 3. Typical HR-TEM image of gold clusters in diameter less than 2.0 nm. The inset is a well-defined gold cluster. Part of smaller clusters of diameter about 1.0 nm were marked by white dashed circles. selected depending on the actual application. In this study, water was selected as a medium for the dispersion of gold clusters. When water was injected into LN2, water froze very quickly to form ice along with quick evaporation of LN2. Thus, Au clusters were incorporated into the ice. When the ice melted to liquid water at room temperature, a Au colloid solution was obtained. Then the colloid solution was directly used for characterization. In the case of discharge in water, the water temperature was kept at about 0 °C with a cooling system. The optical properties of gold clusters in water were detected with an ultraviolet spectrophotometer UV-3600 (UV vis, Shimadzu, Japan) in the range of 200 800 nm. Quartz cells (10 mm 10 mm 45 mm) were used. Pure water was detected as reference sample before and after the measurement of the gold cluster aqueous solution. Transmission electron microscopy (TEM) samples were prepared by dropping the aqueous solution containing gold clusters onto a copper grid with an ultrathin (about 6 nm) amorphous carbon film without any special treatment. The shape and microstructure of the gold clusters were observed with annular dark field scanning TEM (ADF-STEM) and high resolution TEM (HR-TEM) in a JEM-2500SE (JEOL, Japan) instrument operated at 200 kV. HR-TEM images were recorded closely to the Scherzer defocus, and its lattice resolution was 0.14 nm.
’ EXPERIMENTAL SECTION Gold wire with the diameter of 1.0 mm (Aldrich, 99.9%) was used as an electrode. Liquid nitrogen (LN2) and pure water were used as solvent. Solution plasma sputtering is a new scientific technique for synthesis of nanoparticles. The schematic diagram of the experimetal setup of the solution plasma sputtering technique is shown in Scheme S1 of the Supporting Information. Plasma was induced to generate by instant contact using a high voltage pulsed dc power supply (repetition frequency, 20 kHz; pulse width, 2 μs; Kurita Co. Ltd., Japan). The gap between the electrodes was maintained at 0.3 mm with a screw micrometer during the discharging time. After discharging in LN2, products were collected by dropping a desired medium into the LN2. The desired medium, such as water, ethanol, or other solutions, may be 120
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Figure 4. Typical HR-TEM images for various structured gold clusters (about 2.0 nm): (a) single FCC crystal; (b) nanotwin crystal; and MTPs of (c) decahedron and (d) icosahedron. The inset in part a is a fast Fourier transform (FFT) pattern of the single FCC crystal; the insets in parts c and d are crystal structure models of MTPs, respectively.22
’ RESULTS AND DISCUSSION Gold clusters formed simultaneously with solution plasma sputtering in liquid nitrogen (LN2) in several microseconds. The shape and microstructure of gold clusters were clearly clarified by annular dark field scanning TEM (ADF-STEM) and high resolution TEM (HR-TEM). The annual dark field scanning transmission electron microscopy (DF-STEM) image in Figure 1 shows a large amount of isolated gold clusters with spherical morphology were fabricated by solution plasma sputtering. A cluster count (about 1500 clusters) taken from many such DFSTEM images, obtained from different regions of the sample, confirmed the mean diameter of spherical gold clusters is 1.25 nm ( 0.5 nm, as shown in Figure 2. Compared with typical high-resolution TEM images shown in Figures 3 and 4, these results clarified it is difficult to obtain well-defined HR-TEM images in the case of clusters of diameter less than 2.0 nm due to the limits of our TEM device and the copper scanning grid with carbon overlayers. Figure 4 shows as-fabricated clusters of diameter more than 2.0 nm have various crystal structures, such as single face-centered cubic (FCC) (Figure 4a), nanotwin (Figure 4b), multitwin-particles (MTPs) of decahedron (Figure 4c), and icosahedron (Figure 4d). For gold clusters, experimental investigations of about two hundreds of clusters (including of MTPs and single crystalline particles) suggested that clusters fabricated by solution plasma sputtering favor MTPs. The ratio of MTPs is about 94%. On the other hand, the thermodynamically stable structure of Au nanoparticles was theoretically and experimentally discussed by Ino,22 Marks,23 and Barnard.24 They clarified that Au MTPs are stable at below 10 nm because Au nanoparticles are planar defects such as contact twins and intrinsic or
extrinsic stacking faults that form during growth in materials with low stacking fault or twin boundary energy and energy anisotropy. The smaller the size, the more favor to formation of MTPs with low free energy. Therefore, our experimental results supported the previous studies for crystal growth in the initial stages of the particle growth and/or thin film formation. That is, almost Au MTPs can be readily fabricated by solution plasma sputtering. Such MTPs would be beneficial for tailoring catalytic properties at the nanoscale due to the surface energy, the elastic energy, and the twin boundary energy. The compositions of the clusters were investigated by energy dispersive X-ray analysis (EDX) using a STEM mode as shown in Figure S1 in the Supporting Information. The strong emission peaks of C, O, Cu, and Si were derived from Cu grid and carbon film TEM observations. Thus, the formation of Au clusters was clarified by an obviously Au M emission and Au Lα emission. EDX analysis indicated that all clusters contained Au elements. It is well-known that moving downward in size, electronic structure and physical properties begin to change remarkably at about 3.0 nm, and the change became rapid at about 2.0 nm. Gold clusters below 1.5 nm do not show a surface plasmon band and the spectrum consists of a continuous increase in absorbance with decreasing wavelength, because of the presence of a band gap at the Fermi level.25 Figure 5 shows a representative UV vis spectrum of a gold cluster aqueous solution prepared from solution plasma sputtering in LN2. The surface plasmon resonance band near 520 nm originated from the gold nanoparticles was not detected. The transparency of the gold cluster water solution is almost the same as that of pure water in the visible range. In the ultraviolet range, however, the gold cluster aqueous 121
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breakdown voltage, sputtering discharge became visible. The gold electrodes pair is continuously bombarded by the produced energetic particles in the plasma region. Scheme 1 shows the model for fabrication of gold clusters in LN2 by solution plasma sputtering. Along with the bombardment of highly energetic plasma particles, gold atoms were ejected from the solid electrodes pair’s tip, with the plasma expanded in the LN2 due to the enormous difference in the temperature and pressure between plasma and the surrounding LN2 medium. The expanded plasma particles were quickly condensed because of collision with superlow-temperature ambient molecules. Finally, the plasma lost its expansive driving force, resulting in the formation of size-smaller gold clusters. It is well-known that gold colloids are stable in water medium by easily forming hydroxgen bond on their surface, resulting in the high negative value of the ζ potential.26 Thus, as-fabricated diameter less than 2.0 nm gold clusters might have much higher negative values of the ζ potential due to facile formation of a hydroxygen bond on their higher special surface area. The gold clusters redispersed in water seemed to be thermodynamically stable for a long time. The gold clusters in water are stable and without any byproduct, giving “clean” clusters that were ideal for catalytic studies. Our further experimental results indicated that gold clusters were well deposited on carbon supports. Their electrocatalytic activities in a fuel cell are under study.
Figure 5. UV vis absorbance spectra of gold clusters in water and of water serving as the reference sample. The inset is a photograph of gold clusters in water.
Scheme 1. Fabrication Model of Gold Clusters in LN2 Medium by the Solution Plasma Sputteringa
a
Note that strong N2 second positive bands were observed in the wavelength region of 280 410 nm by the photoelectric measurements.
solution shows an obvious absorbance. The band gap energy (Eg) of the quartz cell with water or with the gold cluster aqueous solution can be obtained by extrapolating the straight-line portion of the plot to the baseline, and those values are found to be about 5.32 eV (233 nm) and 5.06 eV (245 nm), respectively. Thus, the obviously red shift (ΔE = 0.26 eV) for the gold cluster aqueous solution indicated the presence of gold clusters. The inset in Figure 4 is a photograph of the colorless and highly transparent gold cluster water solution. These results clarified that the diameter of the gold clusters is less than 2.0 nm. When the gold nanoparticles up to about 2.0 nm in diameter are measured, a strong surface plasmon resonance band near 520 nm is detected, and the gold nanoparticles in water are detected by the appearance of a typical red color (inset b) (Figure S2 in the Supporting Information). Inset a shows the size distribution of the gold nanoparticles. Thus, comparing these two UV vis spectra, it is well-known that the optical properties of Au particles are dependent on the particle size. When the particle size of Au particles decreases to below 1.5 nm, their surface plasmon resonance disappears and the colloid solution is well transparent. The formation mechanism of gold clusters was hypothesized as follows. When the pulsed voltage was supplied, the gas phase began to form due to the Joule heating. Upon increasing up to the
’ CONCLUSIONS Well-crystalline clean gold clusters with spherical morphology (1.25 nm ( 0.5 nm) were successfully fabricated via a valuable one-step route from metal wire electrodes by solution plasma sputtering in LN2 at atmospheric pressure. The plasma provides a novel reaction field with a highly energetic state for the formation of gold clusters in the liquid medium. Rapid energetic radicals’ bombardment, atom vapor diffusion, plasma expansion, and medium condensation resulted in the formation of gold clusters. Gold clusters fabricated by solution plasma sputtering favor multitwin-particles (MTPs) and would be beneficial for tailoring catalytic properties at the nanoscale due to the surface energy and the twin boundary energy. The gold clusters may be easily collected and well redispersed in various mediums for their potential applications. A desired amount of products can be controllably fabricated by a circulation flow or using multiple discharges. Therefore, solution plasma sputtering will have potential application in the future in the design and mass preparation of various multifunctional metal clusters. This novel process will surely present a key stepping stone toward the goal of sustainable chemistry. ’ ASSOCIATED CONTENT
bS
Supporting Information. Schematic diagram of the experimental setup; electron energy spectrum of gold clusters; and UV vis absorbance spectra of gold colloids. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected],
[email protected] (X.L.H.);
[email protected] (N.S.). 122
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