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Chemical Physics Letters 389 (2004) 58–63
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Pulsed-laser ablation of Mg in liquids: surfactant-directing
nanoparticle assembly for magnesium hydroxide nanostructures
Changhao Liang, Takeshi Sasaki, Yoshiki Shimizu, Naoto Koshizaki
*
Nanoarchitectonics Research Center (NARC), National Institute of Advanced Industrial Science and Technology (AIST),
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Received 27 February 2004; in final form 27 February 2004
Published online:
Abstract
Ultrafine brucite Mg(OH)2 nanostructures exhibiting wormhole-, tube-, rod- or platelet-like morphologies were prepared by
pulsed laser ablation of an Mg plate target immersed in deionized water or aqueous solutions of sodium dodecyl sulfate (SDS)
surfactant. The structure, morphology, and composition of the resultant Mg(OH)2 nanostructures were investigated by X-ray
diffraction (XRD), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). A possible formation mechanism of the Mg(OH)2 nanostructures is proposed based on the initial laser-induced reactive quenching process and
subsequent surfactant-directing growth.
Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction
Nanocrystalline alkaline earth metal oxides, such as
MgO, have been efficiently used as adsorbents for acidic
gases and decomposition of hazardous chemicals [1,2].
MgO nanocrystals with high concentrations of corner/
edge sites, small particle size, and unusual crystalline
shapes would provide high specific surface reactivity [3].
Conventional chemical methods for synthesis of MgO
often involve decomposition of magnesium hydroxides
(Mg(OH)2 ). The size and morphology of the final MgO
particle depend greatly on the precursor of Mg(OH)2 in
the process of topochemical decomposition. For example, 4 nm size MgO particles were prepared by utilizing
Mg(OH)2 aerogel and supercritical drying techniques,
and exhibited strong adsorbent properties [4]. In addition, magnesium hydroxide itself is commonly used as
flame retardant filler that does not evolve into toxic and
corrosive substances upon combustion [5]. It is of great
interest to explore novel synthetic routes for Mg(OH)2
nanocrystallines of ultrafine size and particular morphologies. Brucite Mg(OH)2 , whose basic layered unit
*
Corresponding author. Fax: +81-29-861-6355.
E-mail address: [email protected] (N. Koshizaki).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.03.056
possesses hexagonal symmetry (C6-type), can be grown
into a peculiar one-dimensional structure or platelet-like
structure. For example, Mg(OH)2 nanorods have been
synthesized with the assistance of an ethylenediamine
ligand via a hydrothermal technique [6]. Ding et al. [7]
synthesized Mg(OH)2 nanocrystallines with rod-, tube-,
needle-, or lamella-like morphologies by a hydrothermal
reaction using different magnesium precursors and solvents as the reactants. Mg(OH)2 nanorods were prepared by a liquid–solid arc discharge technique using
NaCl as an electrolyte [8]. We investigate here the
growth of Mg(OH)2 nanocrystallines in a special liquid
environment generated by pulsed-laser ablation of the
Mg target in liquids.
There have been an increasing number of reports in
the past few years on the formation of nanostructures
via laser ablation/irradiation of a solid target immersed
in liquids or of specially prepared absorbing solutions/
colloids [9]. Most studies have focused on the preparation and shape/size modification of noble metal nanoparticles [10–13]. Patil et al. [14] developed a method
known as pulsed-laser-induced reactive quenching
(PLIRQ). New metastable phases such as oxides, nitrides, and even diamond nanoparticles, could be formed
due to the transient extreme high temperature and
C. Liang et al. / Chemical Physics Letters 389 (2004) 58–63
Fig. 1a–d presents the XRD spectra of the precipitates formed by the PLD of Mg in different liquids. All
diffraction peaks can be assigned to brucite-like
Mg(OH)2 (JCPDS File. No. 44-1482) with a hexagonal
and c ¼ 4:777 A).
However, the relacell (a ¼ 3:144 A
tive intensities and widths of the peaks varied in the
different products. The diffraction peaks from samples A
and B were broader than those in samples C and D,
indicating a smaller particle size in samples A and B.
Furthermore, the (0 0 1) peak in sample A was the most
intense and the broadest among the other Bragg peaks.
The non-uniform broadening of the Bragg peaks was
more clearly presented in sample B, although the (1 0 0)
and (1 1 0) peaks were exclusively narrow. These results
indicate that the crystallite dimensions in samples A and
B were preferentially restricted along the crystallographic c-axis, leading to broadening of the (0 0 1) reflection peak. Furthermore, various disorders, such as
stack faults, turbostraticity, or interstratifications, are
considered to account for non-uniform broadening in
layered materials such as hydroxides [20]. In fact, the
(0 0 1) reflection from sample A exhibits two split sub and d2 ¼ 4:625 A,
peaks with d-spacings of d1 ¼ 5:101 A
and the center of the whole broad peak with a d-spacing
(103)
(201)
(110)
(111)
(012)
(101)
(100)
An Mg metal plate (99.99% pure) was fixed on the
bottom of a glass vessel filled with 6 cm3 deionized water
and an aqueous solution of sodium dodecyl sulfate
(SDS, 99.5% pure) with a variety of concentrations
(0.001, 0.01, 0.05 M). The plate surface was ablated with
an output of the third harmonic (355 nm) of a Nd:YAG
laser operated at 10 Hz with a maximum output of
about 100 mJ/pulse and pulse duration of 7–8 ns. The
laser beam was focused on the plate surface with a spot
size about 1 mm in diameter by a lens with a focal length
of 250 mm. Laser ablation lasted for 60 min. All solutions became turbid with a white color after ablation.
Remarkable powder-like precipitates were found in the
aqueous solution of the surfactants after sitting for
about 6 h, while cloud-like precipitates existed in the
deionized water. The clear supernatant liquid was
carefully removed, the precipitates were repeatedly
centrifuged at 3500 rpm, and the sediments were washed
with deionized water several times to remove as many
SDS molecules as possible. A small amount of the precipitate was re-dispersed in deionized water, ultrasonically separated for 2 min, and deposited onto a holey
carbon-coated copper grid for TEM observation. The
remaining wet precipitate was transferred to a quartz
substrate for powder X-ray diffraction (XRD) investigation. A transparent gel-like film formed from the
product ablated in deionized water after drying at room
temperature in an auto-dry desiccator, while white
powder-aggregated films formed from the products ablated in the surfactant solutions. For clarity, we refer to
the products obtained from the deionized water, 0.001,
0.01, and 0.05 M SDS solutions as samples A, B, C,
and D.
3. Results and discussion
(001)
2. Experimental
Powder XRD measurements were performed on a
Rigaku powder diffractometer using Cu Ka (filtered)
at 40 kV and 35 mA in a
radiation (k ¼ 1:54056 A)
2h h scanning mode. A transmission electron microscopy (TEM) investigation was conducted on a JEOL
2000 FXII with an energy-dispersive X-ray spectrometer
(EDS) (EDAX, DX-4) attached and a JEOL-2010 highresolution TEM (HRTEM), all operated at 200 kV.
Intensity (a.u.)
pressure of liquid-confined laser ablation plumes and
subsequent rapid quenching [15–17]. We recently performed a series of experiments on the formation of
nanostructures by using active main or transient group
metals as the target, and ultrafine metal oxide nanocrystals were successfully produced [18,19]. Furthermore, an appropriate surfactant could also be used to
efficiently control the nanostructure growth in our lasergenerated liquid environment, similar to conventional
soft chemical synthesis strategies. We present here our
experiments on the formation of Mg(OH)2 nanostructures by pulsed-laser ablation of an Mg plate in an
aqueous solution of anionic surfactants as well as in
deionized water. Tube-like nanofibers, nanorods with
ultrafine diameters, nanoplatelets, and wormhole-like
crystalline porous gel made from Mg(OH)2 were produced in different liquids. The pulsed-laser-induced reactive-quenching process and the surfactant effects in
directing the growth of Mg(OH)2 nanostructures will be
discussed.
59
(a)
(b)
(c)
(d)
20
30
40
50
60
70
80
2θ (deg)
Fig. 1. X-ray powder diffraction spectra of products prepared by
pulsed-laser ablation of an Mg target in: (a) deionized water, (b) 0.001
M SDS solution, (c) 0.01 M SDS solution, and (d) 0.05 M SDS
solution.
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C. Liang et al. / Chemical Physics Letters 389 (2004) 58–63
The (0 0 1) peak from sample B also displays
of 4.771 A.
a clear shoulder at a low angle, probably resulting from
superstructures or some disorder along the c-axis. The
reflections from samples C and D were also non-uniform. The (0 0 l) and (h 0 l) reflection peaks were slightly
broader than that of the (h k 0) reflection, and the (0 0 1)
peak was more intense than the (1 0 1) peak, which is
generally the strongest peak. In addition to the formation of ultrafine Mg(OH)2 crystallites by PLIRQ in our
experiment, the H2 O and SDS molecules were involved
in the crystallization process of Mg(OH)2 , possibly resulting in the formation of disorders within the layered
structures along c-axis of Mg(OH)2 .
Fig. 2 depicts representative TEM images of the assynthesized Mg(OH)2 nanostructures. Bulk porous
Mg(OH)2 gel was obtained in deionized water with a
wormhole-like morphology after drying (Fig. 2a). The
pore or crystallite size was 1–2 nm. However, the crystallite morphologies produced by ablation in aqueous
solutions of SDS were completely different. Large-scale
curled fiber-like crystallites formed with uniform diameters of 4–6 nm and lengths of several hundred nanometers in a 0.001 M SDS solution (Fig. 2b). The TEM
observation revealed that small Mg(OH)2 crystallites
were produced in deionized water and 0.001 M SDS
solutions, which corresponds with the broadening of
XRD peaks in samples A and B. Many short rod-like
and platelet-like Mg(OH)2 crystallites were observed
with increase in the SDS concentration to 0.01 or 0.05 M
(Fig. 2c,d), while fiber-like crystallites disappeared. The
rods had diameters of 10–30 nm and a length similar to
that of the fibers. The platelets had diameters in the
range of several tens to hundreds of nanometers. The
amounts of Mg(OH)2 nanorods were relatively decreased in a solution with a higher SDS concentration of
0.05 M, whereas the platelets remarkably increased
(Fig. 2d). The crystallite size exhibited no obvious
change compared with that in 0.01 M SDS solutions. In
addition, Figs. 2b, c also depict morphologies consistent
with turbostratic disorder, as stated in previous reports
[20,21]. The detailed morphology and structure were
further investigated by high-resolution TEM.
Fig. 3 displays typical HRTEM images of Mg(OH)2
nanofibers, rods, and platelets that formed in SDS solutions with different concentrations. Fig. 3a indicates
that the ultrafine fibers shown in Fig. 2b were indeed
tube-like structures with a channel spacing of 3–4 nm.
The rods shown in Fig. 3b have stripe-like fringes
Fig. 2. Low-magnification TEM images of Mg(OH)2 nanocrystals generated by pulsed-laser ablation of an Mg target in: (a) deionized water, (b)
0.001 M SDS solution, (c) 0.01 M SDS solution, and (d) 0.05 M SDS solution.
C. Liang et al. / Chemical Physics Letters 389 (2004) 58–63
61
Fig. 3. High-resolution TEM images of the as-synthesized Mg(OH)2 nanocrystals with different morphologies. (a) Tube-like nanofibers. (b) Stripelike rods. (c) Single needle-like rods. (d) Single nanoplatelet. Insets depict the corresponding SAED patterns from a (b) single stripe-like rod and
(d) platelet.
equally spaced approximately by 3 nm in parallel along
the rod axis, and the corresponding ED pattern (inset in
Fig. 3b) indicates the polycrystalline nature of the rods,
resulting from the assembling of small particles. Fig. 3c
depicts a needle-like rod that was often observed in
samples C and D. The rods were not well-crystallized,
but presented some parallel lattice fringes of Mg(OH)2
and some signs of aggregation from small particles.
Fig. 3d depicts a typical Mg(OH)2 platelet. Obscure
lattice fringes were observed, similar to needle-like rods.
The ED pattern exhibited bright spots and weak diffuse
rings. The spots could be indexed according to hexagonal Mg(OH)2 structures. The amorphous-like diffuse
ring indicates that the platelets were not well crystallized. The compositions of these nanostructures were
simultaneously analyzed by attached energy-dispersive
spectrometer (EDS). Only Mg, O, Cu (from the copper
grid), and small C peaks appeared; no S or Na elements
could be detected. Therefore, no SDS-related molecules
were included in the formed nanostructures after repeated washing, centrifugation, and final drying. However, we cannot preclude the possibility of a very small
number of water molecules being intercalated between
the layers of Mg(OH)2 . A previous report demonstrated
that interstratification of water molecules could result in
turbostratic disorder between the layers [20]. The XRD
and TEM results indicate that the splitting and nonuniform broadening of the XRD peaks arose from a
combined effect of particle size and turbostratic disorders, particularly in the products from water and
0.001M SDS solutions.
We demonstrated the formation of Mg(OH)2 nanostructures by pulsed-laser ablation of Mg metal in water
and SDS solutions. We can tentatively describe the
formation process based on the experimental results as
having the following three stages: (1) laser-induced formation of Mg species (atoms, clusters, or nanoparticles);
(2) interfacial reaction between Mg species and water,
resulting in the formation of a magnesium hydroxide
species (molecules, clusters, or particles), and (3) surfactant-directing growth of magnesium hydroxide
structures with selected morphologies. Most of the laser
energy (355 nm) would be absorbed by Mg metal in our
synthetic strategy, due to the complete transparency of
pure water and SDS solutions in the visible and ultraviolet regions. The strong excitation of the metal surface
by the laser shot can lead to rapid ejection of Mg species
(e.g., atoms or clusters), resulting in a hot plasma plume
above the metal surface, similar to that in vacuum or
diluted gases. The plume is confined in a liquid; previous
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C. Liang et al. / Chemical Physics Letters 389 (2004) 58–63
studies indicated a further increase of plasma pressure
and temperature due to adiabatic expansion and creation of a shock wave at the plasma–liquid interface
[22,23]. However, it is known that excited electrons will
also transfer energy to the metal lattice by typical electron–phonon coupling. The electron-electron and electron-phonon relaxation times are usually <1 ps [24].
This time-scale is much shorter than the pulse duration
of 7–8 ns in our study. Thus, in addition to the direct
ejection of metal species, the metal lattice will be heated,
leading to melting of the metal surface. However, the
target was slowly rotated in our experiment, and the
laser spot was 1 mm in diameter. Therefore, the rapid
melting region would soon re-solidify upon fast heat
diffusion before the next laser pulse. Almost no largespherical Mg droplet-like particles could be found in the
final product. The strong reaction between the Mg
species and water molecules at the interface between the
plasma and liquid will lead to the formation of magnesium hydroxides (Mg + 2H2 O!Mg(OH)2 + 2H ). The
increase in the amount of Mg(OH)2 also caused the
free hydroxyl ions in the liquid to increase gradually through relatively slow dissolution of Mg(OH)2
(Mg(OH)2 () Mg2þ + 2OH ), leading to an increase
in the solution pH. The isoelectric point (IEP) of
Mg(OH)2 particles in water is reportedly about 12 [25].
The pH of different solutions in our study increased
after ablation from the initial neutral to basic with pH of
12.2, 12.6, 12.8, and 13.2 in water and 0.001, 0.01, and
0.05 M SDS solutions. The particles formed were
probably positively charged before the solution pH exceeded the IEP. The negatively charged anionic DS
chain molecules tended to adsorb onto the surface of the
Mg(OH)2 particles. The morphologies of the Mg(OH)2
crystallites indicate that the surfactant molecules can
direct the growth either along one direction to yield
fibrous, rod-like morphologies, or along two directions
to yield a platelet-like shape, depending on the concentration of surfactant, the electric-static interactions
between inorganic species and DS ions, and the
hydrophobic nature of the DS chains. This one-step
assembling of Mg(OH)2 materials presented their own
crystallite structure and preferred growth directions in
our experiment, which were probably formed based on a
surfactant-directing mechanism, as suggested by the
formation of alumina nanofibers [26] and other unidirectional alumina nanostructures [27]. The surfactants
do not act simply as a template, but also serve as a
shape-inducing reagent. In addition, the bulk XRD investigation and EDS analysis of single nanostructures in
our products did not indicate any sign of DS molecules
included in the final product. The IEP of Mg(OH)2 , the
net residual electric charge on the particle surface, is
expected to be negative when the pH in SDS solutions is
greater than 12. The adsorption of anions is no longer
favored. In contrast, desorption of the anionic DS
molecules and adsorption of sodium ions probably occurs on the surface of particles for electric charge
equilibration. Thus, only pure Mg(OH)2 nanocrystals
were obtained in the final product after washing.
4. Conclusion
In summary, we investigated the formation of brucite
Mg(OH)2 nanostructures by pulsed-laser ablation of an
Mg target in liquids. SDS surfactants were used to direct
the growth of Mg(OH)2 crystallites with concentrationdependent morphologies. A wormhole-like bulk gel was
obtained in deionized water after drying at room temperature, but it exhibited crystallized pore walls without
any postcalcination. Ultrafine tubular-like fibers were
formed in lower concentrations of SDS solution. Stripelike rods and large platelets grew preferentially with
increasing surfactant concentration. A growth mechanism based on laser-induced reactive quenching and
surfactant-directing aggregation was proposed for the
formation of Mg(OH)2 nanostructures.
Acknowledgements
C.H.L. acknowledges support by the Japan Society
for the Promotion of Science (JSPS) fellowship at the
National Institute of Advanced Industrial Science and
Technology (AIST), Tsukuba, Japan.
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