Water-Driven Assembly of Laser Ablation-Induced Au Condensates as Mesomorphic Nano- and Micro-Tubes
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Reddish Au condensates, predominant atom clusters and minor amount of multiply twinned particles and fcc nanoparticles with internal compressive stress, were produced by pulsed laser ablation on gold target in de-ionized water under a very high power density. Such condensates were self-assembled as lamellae and then nano- to micro-diameter tubes with multiple walls when aged at room temperature in water for up to 40 days. The nano- and micro-tubes have a lamellar- and relaxed fcc-type wall, respectively, both following partial epitaxial relationship with the co-existing multiply twinned nanoparticles. The entangled tubes, being mesomorphic with a large extent of bifurcation, flexibility, opaqueness, and surface-enhanced Raman scattering, may have potential encapsulated and catalytic/label applications in biomedical systems.
KeywordsGold Nanocondensates Nanotubes Self-assembly Water
Under ambient pressure condition, Au atoms condense in the order of increasing particle size, as structural motifs of atom clusters with planar, cage or pyramid structures [1, 2, 3], an anomalous multiply twinned particle (MTP) of decahedral (Dh) and icosahedron (Ih) types [4, 5] and a face-centered cubic (fcc) structure. On the other hand, helical gold rolling into multi-shell nanowire  and nanotube  was demonstrated via a top–down approach such as electron beam thinning.
Pulsed laser ablation (PLA) technique with a very rapid heating/cooling and hence pressure effect, as in the syntheses of dense dioxide nanocondensates with considerable internal stress , has been used to fabricate Au nanocondensates . A high laser power density was found to cause atom clusters in addition to much larger sized MTP/fcc nanocondensates .
Here we used an alternative route of pulsed laser ablation in liquid (PLAL) to form more Au atom clusters for further formation of lamellae and then multiple-walled tubes (MWT) in a subsequent water-driven assembly process. This stabilizer-free approach is analogous to the fabrication of carbon onions via arc discharge in water , but different from surfactant/copolymers or other template-assisted assembly of Au nanoparticles in a desired manner and quantum-size-related properties for applications toward biology, catalysis, and nanotechnology [11, 12]. The self-assembled Au tube has potential biomedical applications in view of its good bending flexibility, partial epitaxial filling of MTP/fcc particles, and surface-enhanced Raman scattering (SERS) effect useful for the detection of molecules or nanoparticles adsorbed on rough metal surfaces [13, 14].
PLAL Synthesis and Further Aging of Au Nanocondensates
Laser ablation parameters and resultant phase assemblages of Au via PLAL
1,064 nm excitation
Pulsed energy (mJ/pulse)
Beam size (mm2)
Power density (107 W/cm2)
1.1 × 105
1.4 × 105
Ablation time (min)
MTP > fcc
C > MTP > fcc
C > MTP > fcc > T
Mean particle size (nm)
Residual stress (GPa)*
The optical absorbance of the as-deposited nanocondensates and further developed MWTs in solution with specified dwelling times was acquired by a UV–vis spectrophotometer (U-3900H, Hitachi) operating at an instrumental resolution of 0.1 nm in the range of 300–800 nm. The powders recovered from such samples were dried for microstructure observations using optical polarized microscopy and scanning electron microscopy (SEM, JEOL JSM-6700F, 10 kV, 10 μA). The crystal structure of the MWTs was determined by X-ray diffraction (XRD, Siemens D5000, Cu Kα, 40 kV, 30 mA, at 0.05° and 3 s per step from 2θ angle for 30° up to 90°). The d-spacings measured from XRD trace were used for least-squares refinement of the lattice parameters with an error ±0.0001 nm using bulk gold reflections as a standard. Field emission transmission electron microscopy (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 was used to study the composition and phases of the tubular walls. Lattice imaging coupled with two-dimensional (2-D) Fourier transform and inverse transform were used to study the rolling planes of the MWT and their partial epitaxy relationship with the associated MTP/fcc nanoparticles. Powdery sample mixed with KBr was studied by FTIR (Bruker 66v/S) for the extent of OH−signature on MWT. As for the SERS effect, the powdery MWTs settled on a vitreous SiO2substrate were studied by micro-Raman in a backscattering geometry by a Jobin Yvon T64000 system working in the triple-subtractive mode.
Optical Absorbance of the Condensate Solution
Structural Development of Nanocondensates upon Aging in Water
Optical Properties and Surface-Enhanced Raman Scattering of Au Microtubes
The hollow core and optically isotropic wall of the Au microtubes were manifested by optical polarized microscopic observation under a single polarizer and crossed polarizers, respectively (not shown). The wall of such microtubes showed significant light reflection despite the dark appearance of the sediments under the naked eye. The absence of metallic luster for the microtubes can be attributed to the surface roughness and defects of the wall.
Comparison of Au Condensates via PLAL vs. PLA
In comparison with that fabricated by PLA , The present Au condensates via PLAL have much more atom clusters than MTP and fcc. This can be attributed to an order-of-magnitude higher heating-cooling rate (ca. 1 × 1010 K/s) for PLAL  than PLA process and hence a pressure effect to form smaller sized nanocondensates. Base on Birch–Murnaghan equation of state and relevant bulk modulus B0 = 166.7 GPa as well as its pressure derivative B0′ = 5.48 for bulk fcc Au at zero pressure and 298 K [24, 25], the internal compressive stress of the as-formed fcc nanocondensates was calculated to increase up to ~8 GPa with the increase of PLAL power density (Table 1).
A high quenching rate of laser-induced plasma in confining liquid was known to cause metastable phase, such as diamond, that is thermodynamically stable in high-temperature high-pressure regime . The estimated compressive stress level up to ca. 8 GPa is not high enough to stabilize the high-pressure hcp phase, which typically occurs above ~240 GPa at ambient temperature . The reasons to retain a high internal stress for the fcc nanocondensates may include a very high quenching rate, the capillarity force under nanosize effect, and the constraint exerted by partially epitaxial MTPs. It should be noted that the internal stress remained nearly unchanged for the fcc nanocondensates after room-temperature aging for 40 days in water (not shown). The relaxed fcc-type wall of the microtubes thus has nothing to do with such nanocondensates. Instead, they were derived from atom clusters through an intermediate lamellar phase. The driving force for the formation of such a mesomorphic phase was likely total surface reduction under the influence of water for hydrogen-bonding related coordination change of Au atoms.
Water-Driven Rolling and Transformation of Lamellae in the Absence of Surfactant
The present Au nano- and micro-tubes were able to develop from atom clusters and then lamellae for further rolling in aqueous solution without the assistance of artificial surfactant for the following reasons. The atom clusters were likely self-assembled into lamellae under the influence of hydration effect, as indicated by OH stretching mode in the FTIR spectrum (Fig. 6c), in order to reduce the exposed surface area and to modify the structure of nanoparticles as the case of other materials . The mesomorphic rolling of the Au lamella was then triggered by the mismatch strain of its structure units with considerable distortion and residual stress due to a dynamic PLAL process. The lamellae could possibly be made of atom clusters in the form of trigonal planar and pentagonal pyramid with electron positive ligands that donate electron density to the relativistically contracted and stabilized Au 6s orbital, and thus enhance the aurophilic interactions , or alternatively distorted hexagon as a precursor of fcc structure motif. In any case, the anisotropic strain among the randomly distributed atom clusters with directional ligands throughout the network would cause the lamella to roll up in order to minimize the dangling bonds analogous to the case of carbon nanotubes . Such energetic favorable rolling of the structural units is also adopted by viruses, Buckminster fullerene , and chrysotile, a well-known fiber tubular mineral having is curvature determined by the degree of mismatch between the cations occupying the tetrahedral and octahedral layers .
It should be noted that the lamellar wall turned into fcc-type structure when the nanotubes were coarsened up to microns in diameter. This indicates that size dependent chemical free energy is of concern to form a more stable close packed structure for the tube wall. A partial epitaxial nucleation route by (111) or (200)MTP//lamella layer may also lower the activation energy for the lamellar wall to change into fcc structure.
Phase Behavior of Hydrophilic Au Microtubes
The mesomorphic Au microtubes were able to migrate along the humidified wall of the capped bottle especially when desiccated. This indicates that the outer surface of the Au microtube is hydrophilic. This hydration nature is in drastic contrast with the case of carbon nano-onions floating on the water surface or falling to the bottom of the container , and carbon nanotubes by PECVD or pyrolysis in the absence of surfactant [32, 33]. Still, it is not clear if the Au nanotube has hydrophobic channel for tight hydrogen-bonding network facilitated conduction of water analogous to the case of carbon nanotube with the channel occupancy and conductivity tunable by the change in the local polarity and solvent condition .
As a final remark, PLAL is an effective process to fabricate atom clusters for further water-driven assembly of lamellae which rolled into tubular materials under the influence of atom clusters misfit strain and hydration effect. The resultant Au microtubes with a large extent of bifurcation, inner wall exposure, and flexibility as well as SERS effect may have potential encapsulated and catalytic/label applications in biomedical systems. Further study is required to clarify if the PLAL synthesis of semiconducting nano- and micron-tubes can be extended to other noble metals.
(See supplementary material 1)
We thank Dr. R.H. Hsu for helpful discussion on UV–visible spectrum and an anonymous referee for constructive comments. This work was supported by Center for Nanoscience and Nanotechnology at NSYSU and National Science Council, Taiwan, ROC.
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