Generation of Alumina nanoparticles by a nanosecond laser-induced
liquid plasma in water
S. A. Al-Mamun1, Nakajima R.1, Zhu X.1, Ishigaki T.1,2
1
Department of Chemical Science and Technology, Hosei University, Tokyo, Japan
2
Nano Ceramic Center, National Institute for Materials Science, Tsukuba, Japan
Abstract: Nano-size particles of alumina were prepared by laser ablation of bulk
α-alumina immersed in water using nanosecond laser pulses of 1064 nm
wavelength. Effect of laser fluence and water level above the corundum alumina
target has been investigated in this study. Particle shape and size distributions
observed with scanning electron and transmission electron microscopy suggest
that spherical and irregular shaped submicron particles tend to diminish below a
critical laser power or fluence. At sufficiently low laser fluence, spherical nanosize particles with average particle size below 10 nm were formed. XRD analysis
showed that ablated nano-size particles of thermodynamically stable phase of αalumina are present.
Keywords: Laser ablation, Nanosecond Laser, Alumina, Liquid Plasma, Size
distributions
1. Introduction
For a couple of decades, alumina nanoparticles
has found extensive applications in microelectronics,
ceramics, catalyst, abrasive agent and many more
fields for its unique physical and chemical properties.
For commercial production, aqueous chemical
synthesis of nanoparticles (metals, ceramics,
semiconductors, etc.) is well established though this
method offers some disadvantages like high cost of
chemicals, long processing time, high impurities and
lower crystallinity of ceramics due to the presence of
residual hydroxyls which requires post annealing.
Since the pioneering work by Henglein and coworkers [1,2] recently, laser ablation technique is
increasingly drawing attention in producing
comparatively stable phase nanoparticle formation
for its high grade of purity [3] enabling size and
stability control with controlling processing
parameters; surfactant and nanoparticle post
processing [4]. Yeh and co-workers [5] and later
Tsuji and co-workers [6,7] have shown that the
formation efficiencies and size of particles were
changed with power of laser pulses. Recently, Sajti
and co-workers found that laser ablation in water
yields greater material removal rate than in air [8].
Thus, experimental studies in various ablation
conditions in water should be necessary.
A plasma plume is induced at the surface of target
by laser pulse mainly as a result of interband
transition, multiphoton absorption and inverse
Brehmstrahlung in liquid phase and nanoparticles
are formed after condensation of plume which is
thought to be the driving mechanism in nanosecond
laser ablation apart from femtosecond laser ablation.
2. Experimental details
Experiments were carried out with Q-switched
Nd:YAG laser source (Quantel Brilliantb) which
provides 5-6 ns pulse of wavelength 1064 nm
(maximum pulse energy 900mJ), 532nm and 355 nm
with repetition rate of 10 Hz. Figure 1 demonstrates
the schematic diagram of experimental setup with
laser source along with laser guiding and focusing
mirrors and lens system. The laser beam which was
in the TEM00 mode was focused onto a laser spot
size approximately 225µm. The laser spot size was
measured by taking burned spots on a specially
made burn paper at a distance where the target was
placed. Figure 2 shows laser spots on the burn paper.
Figure1 Schematic diagram of experimental setup
target which differs from amount of laser power
emitting from the source due to loss associated with
mirrors and air-dust.
Laser-generated particle size, size distribution and
morphology
of
water-dispersed
ceramic
nanoparticles were determined by scanning electron
microscopy (SEM; Jeol JSM-5310) and transmission
electron microscopy (TEM; Hitachi H-7650, 120 kV,
Emission current 12 μ A). For the SEM sample
preparation, few drops containing colloidal particles
are placed on a small, flat copper plate and dried and
finally sputter-coated with a layer of Au by a twin
coater (Jeol JEC-550). To prepare samples of TEM
analysis, immediately after experiment, a few drops
of colloidal solution were dropped on carbon-coated
copper micro-grid.
Crystal phase was analyzed with Rigaku Smartlab
X-ray diffractometer with CuKα radiation. After the
laser ablation, the particles produced form colloidal
solutions, which were micro-centrifuged at 10,000
rpm for 5 minutes, resulting in a very small of
visible sediment which was then collected on a thin
glass substrate and dried for several times.
Fgure2 Snapshots of laser spots taken on burn paper
The irradiated corundum crystal target has a
dimension of 10mm×10mm×4mm with 99.9%
purity. All ablation experiments were performed at
room temperature at atmospheric pressure in the
distilled water environment. The target crystal was
immersed in water contained in a small 20 ml beaker
which was mounted on a motor-driven rotator (0100 rpm). The purpose of rotation was to ensure the
uniform irradiation on target and the movement of
water that can enhance ablated particle diffusion.
The rotation speed was set to 40 rpm. The laser
beam entered the solution from above at a normal
incident angle to the target. The beam irradiated the
target for 1-2 hours.
Laser power was measured with low power
thermal sensor (Ophir; Model 7Z01560) equipped
with laser power meter (Ophir Vega Model). Power
measurements were done before and after each
experiment and measurement point was below the
lens after the laser guiding equipments to ensure the
amount of actual laser power colliding with the
3. Results and discussion
Laser fluence, liquid layer height above target, and
laser spot size play an important role in ablated
particle`s shape, size, structure, phase, and ablation
efficiency. In experiments, laser fluence and liquid
layer were varied to investigate particle size
distribution and morphology with 1064nm
irradiation.
Effect of laser fluence:
Keeping the water level constant at 4mm above
the target and same laser spot by keeping the same
height of lens above the target, emitted laser power
was varied which ultimately varied the fluence.
Figure 3 shows electron micrographs of
nanomaterials produced at different laser fluences.
Based on the particle morphology and crystallinity,
it is evident that ablated particles fall into three
categories as also observed for metals like platinum
by Nichols W.T. et. al. [9]. At high fluence above 35
J/cm2, both nanoparticles and submicron particles
present with both spherical and irregular shapes (a).
Relative Particle number frequency [%]
40
100
35
30
80
25
60
20
15
40
10
Cumulative frequency [%]
Thermal stress and mechanical defragmentation
play an important role in this regime. At 19.5 J/cm2,
isolated nanoparticles are predominant (b) and at
13.8 J/cm2, nanoparticles tend to stick together (c)
while at low fluence of 7.5 J/cm2, amorphous,
globular and gel-like particles tend to be present (d).
Among the globes, there are small, spheroidal
nanomaterials also present indicating that they lack
sufficient energy absorbed to form complete and
isolated particles. Figure 4 indicates that the average
particle size is about 9.3 nm whereas d50 is about 6
nm at laser fluence of 19.5 J/cm2 and liquid layer of
4mm. It is also evident that for a ceramic material
like alumina, the ablation regime has a very narrow
range as far as the laser fluence is concerned. Above
that range, thermal stress and defragmentation is
predominant resulting irregularly shaped micron and
submicron sized particles and below that range,
incident energy is not sufficient enough to ablate the
target.
20
5
0
0
0
10
20
30
40
50
Diameter [nm]
Figure 4 TEM image and corresponding size distribution at
laser fluence of 19.5 J/cm2 and liquid layer of 4mm
Effect of water height above target:
Figure 3 Electron micrographs of ablated alumina particles at
different laser fluences; (a) 35.25 J/cm2 [SEM], (b) 19.5
J/cm2[TEM], (c) 13.8 J/cm2[TEM] and (d) 7.5 J/cm2[TEM] and
a liquid layer of 4mm
To investigate the effect of water level on ablation
rate, experiments were operated at constant power of
80mW and water level was varied from 2mm to
10mm. Figure 5 shows ablation weight per hour at
different water level. Ablation rate is comparatively
very high at water level of only 2mm and it was
found that irregular-shaped crater particles of
submicron and micron size tend to dominate causing
a high ablation weight. At water level of 4mm,
ablation rate sharply decreased indicating that
mechanical stress was no longer present and all
ablated particles were nano-sized, spherical resulting
from evaporation and condensation. At higher water
levels, ablation rate gradually decreased probably
due to increases in loss associated with incident
power absorbed by water and already ablated
particles present on the laser path.
Figure 6 demonstrates X-ray diffraction pattern of
(a) bulk α-Al2O3, (b) Al2O3 nanoparticles
synthesized by laser ablation in water for a laser
power of 80 mW and liquid layer of 4mm indicating
stable α-Al2O3 phase. Clearly it is evident that laser
generated nanoparticles have identical trigonal
Bravais lattice structure as the solid target,
corresponding to corundum or alpha phase alumina
with space group R3c . McHale et. al. [10] predicted
that γ-Al2O3 should become the energetically stable
crystal structure for a specific particle surface area
exceeding 125 m2g-1 at room temperature. Specific
surface area can be calculated by the following
Ablation weight [mg/h]
0.8
0.6
0.4
0.2
0.0
0
2
4
6
8
10
Water level above target [mm]
Figure 5 Al2O3 nanoparticle production rate at different water
thickness above the target with a constant laser power of 80 mW
at 10 Hz repetition rate i.e. pulse energy of 8 mJ
(1 0 4)
Intensity [a.u.]
20000
(0 1 2)
(3 0 0)
15000
(0 2 4)
(1 1 0)
(2 1 4)
10000
total ablated mass, ρ is the density of corundum,
of a single spherical particle. Assuming ablated mass
of 1 gram and particle size of 9.3 nm, calculated
specific surface area was approximately 160 m2g-1.
So, although it was expected to synthesize γ-Al2O3 at
nanoparticle`s size below 12 nm, XRD observation
demonstrated the stable alpha phase of alumina.
Further investigation should be done behind this
unusual but interesting observation.
(1 1 6)
25000
M Abl
A p [8] where M Abl is the
Vp ⋅ ρ
V p and A p are the volume and effective surface area
(a)
(1 1 3)
30000
equation: ATot =
5000
0
20
30
40
50
60
70
80
2 Theta [degree]
References
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5000
[2] A. Henglein, J. Phys. Chem. 97 (1993) 5457
(0 1 2)
(b)
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4000
Intensity [a.u.]
[4] Mafune et. al. J. Phys. Chem. B, 105, (2001) 5114
[5] J.S. Jeon, C.H Yeh, J. Chin. Chem. Soc. 45 (1998) 721
3000
[6] T. Tsuji, K. Iryo, H. Ohta, Y. Nishimura, Jpn. J. Appl.
Phys. Part 2 39 (2000) 981
(1 0 4)
2000
(1 1 3)
(1 1 0)
1000
(1 1 6)
(0 2 4)
[7] T. Tsuji, K. Iryo, Y. Nishimura, M. Tsuji, J.
Photochem. Photobiol. A 145 (2001) 201
(3 0 0)
(2 1 4)
0
20
30
40
50
60
70
80
2 Theta [degree]
Figure 6 Crystal structure: X-ray diffraction pattern of (a) bulk
α-Al2O3, (b) Al2O3 nanoparticles synthesized by laser ablation
in water for a laser power of 80 mW and liquid layer of 4mm
indicating stable α-Al2O3 phase
[8] C.L. Sajti, R. Sattari, B. Chichkov, S. Barcikowski,
Appl. Phys. A 100 (2010) 203-206
[9] W.T. Nichols, T. Sasaki, N. Koshizaki, J. Appl. Phys.
100 (2006) 114913
[10] J.M. McHale, A. Navrotsky, A.J. Perrotta, J. Phys.
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