J. Phys. Chem. C 2008, 112, 14965–14972
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Structure and Phase Behavior of Gold Nanocondensates: Effects of Laser Ablation Parameters and Carbon Catalysis Chang-Ning Huang,† Shuei-Yuan Chen,‡ Yuyuan Zheng,† and Pouyan Shen*,† Institute of Materials Science and Engineering, Department of Materials and Optoelectronic Science, Center for Nanoscience and Nanotechnology, National Sun Yat-sen UniVersity, Kaohsiung, Taiwan, Republic of China, and Department of Mechanical and Automation Engineering, I-Shou UniVersity, Kaohsiung, Taiwan, Republic of China ReceiVed: June 15, 2008; ReVised Manuscript ReceiVed: July 24, 2008
Au nanocondensates in the form of amorphous clusters, multiply twinned particles (MTPs), and face-centered cubic (fcc) structures with progressively larger particle size, as characterized by transmission electron microscopy (TEM), were fabricated by pulsed laser ablation on Au in vacuum with or without argon gas flow at 1.5 × 108 versus 1.4 × 1012 W/cm2 power density for a rapid heating/cooling effect. The observed critical size, ca. 2 nm, for the amorphous to MTP transformation implies a minimum {111} surface area of ca. 4 nm2 for MTP. Coarsening and {111}-specific coalescence of the MTPs and fcc to form defects are competitive under the influence of laser parameters and Ar gas flow. In situ TEM observations further indicated that the MTP f fcc transformation occurred for a surprisingly small particle size of 5 nm due to the catalytic effect of graphite-like materials (G) with varied extent of rolling following the habit plane (0001)G/(111)fcc. The ultimate relationship would be (21j1j0)G//(21j1j)fcc; [01j10]G//[01j1]fcc for a minimum misfit strain with perfectly crystallized graphite. I. Introduction The interest in Au nanoparticles stems largely from their assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.1 Bonding and structure changes of Au with size reduction, in particular hollow golden cages consisting of a specific number of atoms according to spectroscopic experimental results and theoretical evidence,2-4 are thus of great interest. Small Au clusters fabricated by a vacuum-evaporation process typically form multiply twinned particles (MTP) with a pentagonal arrangement of decahedral (Dh, 10-faced) and icosahedron (Ih, 20-faced) types.5,6 The Ih and Dh particles prepared by this method under purified helium gas were reported to be 3-14 nm in size.7 These MTP particles were found to remain as Ih and Dh types at high temperatures due to a high activation energy of MTP f fcc (face-centered cubic) transformation.7 MTPs prepared via an alternative wet chemistry route with stabilizers (surfactants) can be even up to micrometer size.8,9 [Refer to Elechiguerra et al.9 for a thorough review of twinningcontrolled shape of noble metal (Au, Ag and Pt) nanoparticles synthesized by wet method with surfactant.] Here, pulsed laser ablation (PLA) condensation with a very rapid heating/cooling and hence pressure effect, as in previous syntheses of dense dioxide nanocondensates with considerable residual stress,10-13 was used to fabricate amorphous clusters, MTPs, and fcc Au nanocondensates. We focused on the combined effects of excitation power density and Ar gas flow on the size distribution and phase selection of the Au nanoparticles as condensed in vacuum, and the underlying growth mechanism via a coarsening versus {hkl}-specific coalescence * To whom correspondence should be addressed: fax +886-7-5254099; e-mail
[email protected]. † National Sun Yat-sen University. ‡ I-Shou University.
process to form defects. Our in situ transmission electron microscopy (TEM) observations also showed that the MTP f fcc transformation of Au occurred for a surprisingly small particle size of 5 nm due to the catalytic effect of graphite-like materials. Such phase transformation would otherwise not occur upon prolonged electron irradiation in the absence of such a catalyst. II. Experimental Procedures Au (99.99% pure, 0.3 mm in thickness) foil was subjected to energetic Nd:YAG laser (Lotis, 1064 nm in wavelength, beam mode: TEM00) pulse irradiation. The laser beam was focused to a spot size of 0.03 mm2 on the target inside the ablation chamber in vacuum (3.5 × 10-5 Torr) with or without argon gas (99.999% purity) at a flow rate of 5 sccm. A power density of 1.5 × 108 at 1100 mJ/pulse under free run mode and pulse time duration of 240 µs at 10 Hz were found to have a good yield of Au nanocondensates with various structures depending on the particle size. Alternatively, a higher power density (1.4 × 1012 W/cm2 at 650 mJ/pulse under Q-switched mode and nanosecond pulse duration) was adopted for a comparison study. The laser ablation parameters that significantly affected the morphology and size of the condensed Au phases are compiled in Table 1. Copper grids overlaid with a carbon-coated collodion film and fixed in position by a plastic holder at a distance of 25-100 mm from the targets were used to collect Au condensates. The composition and crystal structures of the condensates were characterized by field emission TEM (FEI Tecnai G2 F20 at 200 kV) with selected area electron diffraction (SAED), and point-count energy dispersive X-ray (EDX) analysis at a beam size of 1 nm. Point-count EDX analysis of local areas of interest was performed with K-shell counts for Au and the principle of ratio method without absorption correction.14 Bright-field images (BFIs) taken by TEM were used to study the general morphol-
10.1021/jp805254h CCC: $40.75 2008 American Chemical Society Published on Web 08/30/2008
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TABLE 1: Laser Ablation Parameters and Resultant Phase Assemblages of Aua 1064 nm excitation pulsed energy (mJ/pulse) pulse duration beam size (mm2) fluence (kJ/cm2) frequency (Hz) power density (107 W/cm2) Ar gas flow (sccm) phases mean particle size (nm)
FR-vac FR-vac 1100 0.24 ms 0.03 3.7 10 15 0 MTP/fcc 8.0
1100 0.24 ms 0.03 3.7 10 15 5 MTP/fcc 8.4
QS-vac
QS-vac
650 16 ns 0.03 2.2 10 1.4 × 105 0 C/MTP/fcc 5.0
650 16 ns 0.03 2.2 10 1.4 × 105 5 C/MTP 1.8
a Abbreviations: MTP, multiply twinned particle; C, cluster; fcc, face-centered cubic.
ogy and agglomeration of the nanocondensates. Optimas 6.1 software was used to calculate the mean particle size in a typical BFI with more than 500 particles with their aspect ratios taken into consideration. Lattice imaging coupled with two-dimensional (2-D) Fourier transform and inverse transform were used to characterize the crystal structure of the individual nanoparticles and their phase transformation, if any, upon electron irradiation with a spot size of 1 nm at 200 kV and probe current slightly larger than 5 nA for a time period up to 60 min. The d spacings measured from SAED patterns taken with a selected area aperture of 1 µm were used for least-squares refinement of the lattice parameters. The CASTEP calculation module in Materials Studio software was used to depict the atom disposition of graphite and fcc gold with a specified crystallographic relationship. III. Results As-Condensed Nanoparticles. SAED patterns, TEM BFIs, and size-distribution histograms of the as-condensed Au nanoparticles produced by PLA under specified laser ablation conditions are compiled in Figure 1. The laser ablation parameters and resultant phase assemblages of the Au nanocondensates are also summarized in Table 1. In general, the results show that the shape of the particles is more heavily influenced by the Ar flow and that the size is obviously impacted only by the laser power. In the case of free run mode at 1.5 × 108 W/cm2 (Figure 1a), the nanocondensates typically gave strong (111), (200), and (220) diffractions and have a normal distribution of particle diameter predominating at 6-8 nm under the combined effects of particle coarsening and coalescence. Argon purging at 5 sccm under the same laser ablation condition (Figure 1b) caused much sharper diffraction spots and a considerably larger particle diameter predominating at 8-10 nm due to a more pronounced coalescence process of the Au nanocondensates. There is, however, a better yield of the fine particles less than 2 nm in diameter due to the nucleation event in such a case. The condensates produced by laser ablation under Q-switched mode for a rather short pulse time duration of nanoseconds and hence a very high power density of 1.4 × 1012 W/cm2, either without (Figure 1c) or with argon gas flow (Figure 1d), were poorly crystallized, finer in particle diameter, and more spherical in shape than the case of free run with a much lower (4 orders of magnitude difference) power density (Figure 1a,b). This indicates that coarsening is a more effective growth process than coalescence for the nanocondensates fabricated at very high power density. The introduction of argon gas flow at such a high power density has caused a bimodal size distribution of
the condensates having finer particles predominating at 1.5-2.0 nm as shown in the histogram in Figure 1d. Lattice images of the representative Au nanocondensates produced under various laser ablation conditions (Figure 2) indicated that the amorphous clusters are less than 2 nm in diameter, and the quasi-crystalline Ih and Dh and crystalline fcc are progressively larger in size. (The amorphous particles were carefully tilted to make sure that they indeed lack lattice fringes, although small size clusters are not necessarily amorphous.) In general, the observed critical size of MTP f fcc transformation is consistent with that reported (ca. 6 nm) for the Au particles prepared by a vacuum-evaporation process7 but much smaller than that prepared via a wet chemistry route with stabilizers as mentioned.8,9 The condensates of the same or different structures tended to undergo {hkl}-specific coalescence to form defects according to lattice images coupled with 2-D Fourier transform. In the sample fabricated at 1.5 × 108 W/cm2, such a coalescence process typically caused formation of an assembly of MTP (Figure 3a), {111} stacking fault of fcc in the necking area with stress concentration (Figure 3b), and dislocations near the {111} interface of the MTP and fcc particles (Figure 3c). Electron Irradiation-Induced Phase Change. The SAED patterns coupled with intensity line profiles and the corresponding images of the nanocondensates were taken at a specified electron irradiation time (t) from a specified area (ca. 1300 nm2) of a typical sample fabricated at 1.4 × 1012 W/cm2 as compiled in Figure 4. In the as-condensed state, that is, t ) 0 min (Figure 4a), the Au nanocondensates with poor Bragg diffraction contrast gave diffuse diffraction rings for the nonstable amorphous phase and metastable MTP phases, rather than sharp diffraction spots for fcc crystallites. The intensity line profile indicated the broadened diffractions can be reasonably assigned as (111) and (311) of MTP/fcc. At t ) 30 min, the nanocondensates moved apparently from their original positions and became better crystallized with facets. In such a case, there is stronger Bragg diffraction contrast for the nanoparticles to show sharp diffractions including (111) and a well-resolved shoulder close to (200) in the intensity line profile (Figure 4b). At t ) 60 min, the nanocondensates further shifted in position and orientation to have a strong (220) diffraction of MTP yet with negligible impingement (Figure 4c). Finally, at t ) 90 min, the nanocondensates became much larger in size via {hkl}-specific coalescence and transformed into fcc structure (Figure 4d). In such a case, there are strong {111}, {200}, and {220} diffractions, as shown in the intensity line profile, due to Brownian rotation and hence preferred orientation of the cuboctahedral fcc crystallites. Our independent lattice imaging showed further details of the movement/rotation, impingement, and phase transformation of the individual amorphous Au clusters (Figure 5). It was observed in real time that the Au clusters changed into Ih by a migration-coalescence process within 12 min and then transformed into Dh (Figure 5a-f). It was also confirmed that the MTP (Ih and/or Dh) did not change into fcc unless catalyzed by graphite-like flake with varied extent of rolling due to electron heating of the carbon-coated collodion film. The graphitelike (G) flake and fcc particle follow the habit plane (0001)G/ (111)fcc for such a catalytic event, as indicated by Figure 5g and another independent observation in Figure 6. [Note additional observations by through-focusing indicated that the conclusion about the shape and habit plane of the Au particles is not affected by image delocalization (i.e., Fresnel fringes)15 that appeared in Figures 5 and 6.] On the basis of such a habit plane,
Structure and Phase Behavior of Gold Nanocondensates
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Figure 1. SAED pattern, TEM BFI, and histogram of the size distribution of Au nanocondensates produced by laser ablation on Au target: (a) at 1.5 × 108 W/cm2 in vacuum; (b) at 1.5 × 108 W/cm2 in vacuum under 5 sccm argon gas flow; (c) at 1.4 × 1012 W/cm2 in vacuum; (d) at 1.4 × 1012 W/cm2 in vacuum under 5 sccm argon gas flow.
the ultimate relationship (21j1j0)G//(21j1j)fcc; [01j10]G//[01j1]fcc would be reached for the well crystallized graphite and fcc particle in order to minimize the misfit strain as discussed later. IV. Discussion Effect of Quenching Rate on Crystallization. The formation of amorphous Au clusters and MTP nanocondensates, rather than pristine truncated octahedra as produced by conventional vacuum-evaporation techniques,6 can be attributed to very rapid cooling of the condensates via the laser irradiation technique. The radiant cooling rate, after the pulse was extinguished, was estimated to be 1 × 107 K/s for 2 nm particles by use of an
expression16 pertinent to laser pulses without or with background gas effect17 (Appendix) and with the assumption that the density and heat capacity of the molten state are similar to those of its solid-state counterpart fcc18 with a common radiant emissivity of noble metals.16 This cooling rate is about 1 order of magnitude higher than the threshold (1.6 × 106 K/s) for metastable phase formation in pulsed laser deposition process17 and formation of Au-Si alloy bulk metallic glass,19 but much lower than the theoretical threshold of cooling rate (1.5625 × 1013 K/s) for formation of amorphous Au nanoclusters.20 The discrepancy may be due to underestimation of radiant emissivity and/or temperature of the particulate in the present calculation
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Figure 2. TEM lattice images showing size-dependent structure change of the nanoparticles as condensed by laser ablation on Au target: (a) Amorphous
