rapid synthesis and structural characterization of well-defined gold clusters by ...

Published: September 29, 2011

r 2011 American Chemical Society 119 dx.doi.org/10.1021/cg2008528 | Cryst. Growth Des. 2012, 12, 119–123

ARTICLE

pubs.acs.org/crystal

Rapid Synthesis and Structural Characterization of Well-Defined GoldClusters by Solution Plasma SputteringXiulan 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

’ INTRODUCTION

Ultrafine metal nanoparticles of a few nanometers in diameterexhibit size-dependent photonic and electric properties that areof interest for applications such as biosensors, catalysts forchemical and fuel cells, optics, and electronics.1�8 The propertiesof materials change as their size is closer to subnanometers and asthe percentage of atoms at the surface of a material becomessignificantly higher. A metal-to-nonmetal transition occurs as thediameter is decreased below 2.0 nm, and increasing band gapswere detected with the further decrease of clusters size. Thus,particles of diameter less than 2.0 nm are defined as clusters. Thechanges in physical properties are not always predictable. Whengold is prepared as very small particles with diameters less than10 nm and is highly dispersed on metal oxides, it turns out to be ahighly active catalyst for many reactions such as CO oxidationand propylene epoxidation in the gas phase. Additionally, theintrinsic magnetic polarization in gold nanoparticles, which wereprotected by polyallylamine hydrochloride (PAAHC) in the sizerange of 1.0�3.0 nm (However, the relative frequency of clustersof diameter less than 2.0 nm is only about 50%), was revealed bythe X-ray magnetic circular dichroism (XMCD) and element-specific magnetization (ESM) experiments.9 Fluorescent water-soluble gold nanoparticles were synthesize by the reduction of agold salt in the presence of a designed polymer ligand, and the

size and fluorescence of the particles were controlled by thepolymer to gold ratio.10 Therefore, the production of small goldclusters with diameters less than 2.0 nm (<about 300 atoms,>65% of atoms at the surface), which is a critical size for adramatic change in electronic structure, is still an exciting area ofresearch. These small clusters were tuned more pronouncedly bychoosing support materials ranging from metal oxides andactivated carbon to polymers.4,9�13 Typically, in chemical ap-proaches, the formation of clusters is conducted from metal ionsor metal complexes in the presence of reducing agent (e.g.,sodium borohydride, hydrazine hydrate, and citric acid) andstabilizing agents (e.g., thiol compounds and polymers) in orderto obtain the small-nanoscale size. One problem with thechemical methods is the inevitable introduction of impurity,which requires subsequent purification steps after the synthesisfor its potential application.14 On the other hand, vacuum metal-vapor-condensation techniques have been developed for theclean preparation of metal nanoparticles dispersed in inorganic/organic solvents without formation of impurity.15�17 Amongrecent physical methods, sputtering deposition has better

Received: July 7, 2011Revised: September 27, 2011

ABSTRACT: Ultrafine metal nanoparticles of a few nanome-ters in diameter exhibit size-dependent photonic and electricproperties that are of interest for applications such as biosen-sors, catalysts, optics, and electronics. Chemical approaches andvacuum metal-vapor-condensation physical techniques wereused to successfully synthesize gold nanoparticles. While it isdifficult to obtain monodisperse and small sized gold nanopar-ticles without any reductant and polymer stabilizer or under vacuum conditions, in the present study, multiply twinned and nearmonodisperse gold clusters of diameter less than 2.0 nmwere successfully fabricated for the first time by solution plasma sputtering inliquid nitrogen (LN2) without any chemical additions (such as reductant and polymer stabilizer). Gold clusters formed in severalmicroseconds simultaneously with solution plasma sputtering in an open system under atmospheric pressure. Gold clusters areidentified to be well crystalline and multitwin-particles (MTPs) by high-resolution transmission electron microscopy. No surfaceplasmon resonance band was detected in the gold cluster aqueous solutions. Such MTPs with special corners and edges would bebeneficial for tailoring catalytic properties at the nanoscale. The solution plasma sputteringmethod will have potential application inthe future in the design and mass preparation of various multifunctional metal clusters.

120 dx.doi.org/10.1021/cg2008528 |Cryst. Growth Des. 2012, 12, 119–123

Crystal Growth & Design ARTICLE

capability to fabricate refractory metals and intermetallic com-pounds than evaporation and laser ablation. Because the atomvapor is typically generated from targets of pure materials,nanoparticles created by sputtering usually contain fewer impu-rities than those created by chemical methods. Gold nanoparti-cles were obtained via aerosol generation by spark discharge atlow vapor pressure18 and using sputtering deposition of goldonto various ionic liquids.19 However, size controllable and high-dispersibility clusters of diameter less than 2.0 nm have not beenachieved using vacuum methods due to the weak interactionbetween metal and dispersion medium. Thus, it is difficult toobtain monodisperse and small sized gold nanoparticles withoutany reductant and polymer stabilizer or under vacuum conditions.Therefore, a new technique is urgently required to facilely fabricatesize controllable clean clusters for their potential applications.

Solution plasma is defined by nonequilibrium plasma insolutions, which provides us a novel reaction field with highlyexcited energy state. Recently, the rapid synthesis of cleannanoparticles without any reducing agents has succeeded dueto the strong reduction potential of the generated radicals.3,20,21

In the present study, we addressed a novel and one-step route forhighly efficient synthesis of clean gold clusters of diameter lessthan 2.0 nm from metal wire electrodes in an open system underatmospheric pressure by solution plasma sputtering.

’EXPERIMENTAL SECTION

Gold wire with the diameter of 1.0 mm (Aldrich, 99.9%) was used asan electrode. Liquid nitrogen (LN2) and pure water were used assolvent.Solution plasma sputtering is a new scientific technique for synthesis of

nanoparticles. The schematic diagram of the experimetal setup of thesolution plasma sputtering technique is shown in Scheme S1 of theSupporting Information. Plasma was induced to generate by instantcontact using a high voltage pulsed dc power supply (repetitionfrequency, 20 kHz; pulse width, 2 μs; Kurita Co. Ltd., Japan). Thegap between the electrodes was maintained at 0.3 mm with a screwmicrometer 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

selected depending on the actual application. In this study, water wasselected as a medium for the dispersion of gold clusters. When water wasinjected into LN2, water froze very quickly to form ice along with quickevaporation of LN2. Thus, Au clusters were incorporated into the ice.When the ice melted to liquid water at room temperature, a Au colloidsolution was obtained. Then the colloid solution was directly used forcharacterization. In the case of discharge in water, the water temperaturewas kept at about 0 �C with a cooling system.

The optical properties of gold clusters in water were detected with anultraviolet spectrophotometer UV-3600 (UV�vis, Shimadzu, Japan) inthe range of 200�800 nm. Quartz cells (10 mm � 10 mm � 45 mm)were used. Pure water was detected as reference sample before and afterthe measurement of the gold cluster aqueous solution. Transmissionelectron microscopy (TEM) samples were prepared by dropping theaqueous solution containing gold clusters onto a copper grid with anultrathin (about 6 nm) amorphous carbon film without any specialtreatment. The shape and microstructure of the gold clusters wereobserved with annular dark field scanning TEM (ADF-STEM) and highresolution TEM (HR-TEM) in a JEM-2500SE (JEOL, Japan) instru-ment operated at 200 kV. HR-TEM images were recorded closely to theScherzer defocus, and its lattice resolution was 0.14 nm.

Figure 2. Size distributions of gold clusters. (Calculated from STEMmeasurement of gold clusters; the counted numbers are about 1500.).

Figure 1. DF-STEM image of gold clusters.

Figure 3. Typical HR-TEM image of gold clusters in diameter less than2.0 nm. The inset is a well-defined gold cluster. Part of smaller clusters ofdiameter about 1.0 nm were marked by white dashed circles.



’RESULTS AND DISCUSSION

Gold clusters formed simultaneously with solution plasmasputtering in liquid nitrogen (LN2) in several microseconds.The shape andmicrostructure of gold clusters were clearly clarifiedby annular dark field scanning TEM (ADF-STEM) and highresolution TEM (HR-TEM). The annual dark field scanningtransmission electronmicroscopy (DF-STEM) image in Figure 1shows a large amount of isolated gold clusters with sphericalmorphology were fabricated by solution plasma sputtering. Acluster count (about 1500 clusters) taken from many such DF-STEM images, obtained from different regions of the sample,confirmed the mean diameter of spherical gold clusters is1.25 nm( 0.5 nm, as shown in Figure 2. Compared with typicalhigh-resolution TEM images shown in Figures 3 and 4, theseresults clarified it is difficult to obtain well-defined HR-TEMimages in the case of clusters of diameter less than 2.0 nm due tothe limits of our TEM device and the copper scanning grid withcarbon overlayers. Figure 4 shows as-fabricated clusters of diametermore than 2.0 nm have various crystal structures, such as singleface-centered cubic (FCC) (Figure 4a), nanotwin (Figure 4b),multitwin-particles (MTPs) of decahedron (Figure 4c), andicosahedron (Figure 4d). For gold clusters, experimental inves-tigations of about two hundreds of clusters (including of MTPsand single crystalline particles) suggested that clusters fabricatedby solution plasma sputtering favor MTPs. The ratio of MTPs isabout 94%. On the other hand, the thermodynamically stablestructure of Au nanoparticles was theoretically and experimen-tally discussed by Ino,22 Marks,23 and Barnard.24 They clarifiedthat Au MTPs are stable at below 10 nm because Au nano-particles are planar defects such as contact twins and intrinsic or

extrinsic stacking faults that form during growth in materials withlow stacking fault or twin boundary energy and energy anisotro-py. The smaller the size, the more favor to formation of MTPswith low free energy. Therefore, our experimental results sup-ported the previous studies for crystal growth in the initial stagesof the particle growth and/or thin film formation. That is, almostAuMTPs can be readily fabricated by solution plasma sputtering.

Such MTPs would be beneficial for tailoring catalytic proper-ties at the nanoscale due to the surface energy, the elastic energy,and the twin boundary energy. The compositions of the clusterswere investigated by energy dispersive X-ray analysis (EDX)using a STEM mode as shown in Figure S1 in the SupportingInformation. The strong emission peaks of C, O, Cu, and Si werederived from Cu grid and carbon film TEM observations. Thus,the formation of Au clusters was clarified by an obviously Au Memission and Au Lα emission. EDX analysis indicated that allclusters contained Au elements.

It is well-known that moving downward in size, electronicstructure and physical properties begin to change remarkably atabout 3.0 nm, and the change became rapid at about 2.0 nm.Goldclusters below 1.5 nm do not show a surface plasmon band andthe spectrum consists of a continuous increase in absorbancewith decreasing wavelength, because of the presence of a bandgap at the Fermi level.25 Figure 5 shows a representative UV�visspectrum of a gold cluster aqueous solution prepared fromsolution plasma sputtering in LN2. The surface plasmon reso-nance band near 520 nm originated from the gold nanoparticleswas not detected. The transparency of the gold cluster watersolution is almost the same as that of pure water in the visiblerange. In the ultraviolet range, however, the gold cluster aqueous

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 arecrystal structure models of MTPs, respectively.22



solution shows an obvious absorbance. The band gap energy (Eg)of the quartz cell with water or with the gold cluster aqueoussolution can be obtained by extrapolating the straight-lineportion of the plot to the baseline, and those values are foundto be about 5.32 eV (233 nm) and 5.06 eV (245 nm), respec-tively. Thus, the obviously red shift (ΔE = 0.26 eV) for thegold cluster aqueous solution indicated the presence of goldclusters. The inset in Figure 4 is a photograph of the colorless andhighly transparent gold cluster water solution. These results clari-fied 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 aremeasured, a strong surface plasmon resonance band near 520 nm isdetected, and the gold nanoparticles in water are detected by theappearance of a typical red color (inset b) (Figure S2 in theSupporting Information). Inset a shows the size distribution ofthe gold nanoparticles. Thus, comparing these two UV�visspectra, it is well-known that the optical properties of Au particlesare dependent on the particle size. When the particle size of Auparticles decreases to below 1.5 nm, their surface plasmon reso-nance disappears and the colloid solution is well transparent.

The formation mechanism of gold clusters was hypothesizedas follows. When the pulsed voltage was supplied, the gas phasebegan to form due to the Joule heating. Upon increasing up to the

breakdown voltage, sputtering discharge became visible. Thegold electrodes pair is continuously bombarded by the producedenergetic particles in the plasma region. Scheme 1 shows themodel for fabrication of gold clusters in LN2 by solution plasmasputtering. Along with the bombardment of highly energeticplasma particles, gold atoms were ejected from the solid electro-des pair’s tip, with the plasma expanded in the LN2 due to theenormous difference in the temperature and pressure betweenplasma and the surrounding LN2 medium. The expanded plasmaparticles were quickly condensed because of collision with super-low-temperature ambient molecules. Finally, the plasma lost itsexpansive driving force, resulting in the formation of size-smallergold clusters.

It is well-known that gold colloids are stable in water mediumby easily forming hydroxgen bond on their surface, resulting inthe high negative value of the ζ potential.26 Thus, as-fabricateddiameter less than 2.0 nm gold clusters might have much highernegative values of the ζ potential due to facile formation of ahydroxygen bond on their higher special surface area. The goldclusters redispersed in water seemed to be thermodynamicallystable for a long time. The gold clusters in water are stable andwithout any byproduct, giving “clean” clusters that were ideal forcatalytic studies. Our further experimental results indicated thatgold clusters were well deposited on carbon supports. Theirelectrocatalytic activities in a fuel cell are under study.

’CONCLUSIONS

Well-crystalline clean gold clusters with spherical morphology(1.25 nm ( 0.5 nm) were successfully fabricated via a valuableone-step route from metal wire electrodes by solution plasmasputtering in LN2 at atmospheric pressure. The plasma provides anovel reaction field with a highly energetic state for the formationof gold clusters in the liquid medium. Rapid energetic radicals’bombardment, atom vapor diffusion, plasma expansion, andmedium condensation resulted in the formation of gold clusters.Gold clusters fabricated by solution plasma sputtering favormultitwin-particles (MTPs) and would be beneficial for tailoringcatalytic properties at the nanoscale due to the surface energy andthe twin boundary energy. The gold clusters may be easilycollected and well redispersed in various mediums for theirpotential applications. A desired amount of products can becontrollably fabricated by a circulation flow or using multipledischarges. Therefore, solution plasma sputtering will have poten-tial application in the future in the design andmass preparation ofvarious multifunctional metal clusters. This novel process willsurely present a key stepping stone toward the goal of sustainablechemistry.

’ASSOCIATED CONTENT

bS Supporting Information. Schematic diagram of the ex-perimental setup; electron energy spectrum of gold clusters; andUV�vis absorbance spectra of gold colloids. This material isavailable 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.).

Figure 5. UV�vis absorbance spectra of gold clusters in water and ofwater serving as the reference sample. The inset is a photograph of goldclusters in water.

Scheme 1. Fabrication Model of Gold Clusters in LN2

Medium by the Solution Plasma Sputteringa

aNote that strong N2 second positive bands were observed in thewavelength region of 280�410 nm by the photoelectric measurements.



’ACKNOWLEDGMENT

This work was supported by a grant from the JST-CREST,Japan.

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