Research Activity on Supercritical Fluids Technology at Kumamoto
University
Material Synthesis using Pulsed Laser Ablation in Supercritical CO2
Pulsed laser ablation has been widely employed in industrial and biologicalapplications and in other fields. The environmental conditions in which pulsed laser
ablation is conducted are important parameters that affect both the solid particle cloud(plume) and the deposition (debris) produced by the plume. In most studies, particles
are generated either in a vacuum or in ambient conditions. Some studies, however, use a
different ambient medium such as a gas, a surfactant solution [43, 44], organic solvent[45], liquid N2 [46], and supercritical CO2 [47, 48]. Saitow et al. have reported thatsilicon nanoclusters and gold nanonecklaces could be generated by performing pulsed
laser ablation in supercritical CO2 [47, 48]. However, they did not discuss the synthesis
behavior in much detail. Moreover, only those two investigations of pulsed laser
ablation in supercritical CO2 have been reported.
Recently, our laboratory has investigated pulsed laser ablation of copper insupercritical CO2 [49]. We also conducted ablation at atmospheric condition with airand liquid hexane. We obtained that laser ablation of a metal in high pressure wasaffected by pressure and surrounding environment. The ablation of copper plate resulted
deeper crater in supercritical CO2 compared with in atmospheric condition. Furthermore,
we develop pulsed laser ablation of gold substrate in supercritical CO2 to generate
synthesized gold nanoparticles.
MirrorPulsed Nd:YAG Laser
10 Hz
532 nm

CO2 cylinder

Safety valve

MirrorPump
Chiller

PValveAperture

Valve

FilterBPR

TargetHigh pressure cellwith sapphire window (SUS 316)
Pmax:30 MPa
Tmax:100 oC
Vmax:0.11 L

Flow meter(a)(b)
Figure 8. Experimental apparatus of pulsed laser ablation in supercritical CO2 formaterial synthesis. (a) Schematic diagram; (b) Photograph.

(a)

(b)

(c)
(d)
Figure 9. Laser scanning microscopy image of Au plate after ablation at 40oC for 3000s
by Nd:YAG laser. (a) in air at ambient pressure; (b) in liquid CO2 at 6.5 MPa; (c) insupercritical CO2 at 10 MPa; (d) in supercritical CO2 at 20 MPa.
Experiment of laser ablation in supercritical field for material synthesis was carriedout in a high-pressure cell with three sapphire windows as shown in Figure 8. The
second harmonic of a Q-switched pulsed Nd:YAG laser (Spectra-Physics Quanta-Ray
INDI-40-10, wavelength: 532 nm, pulse energy: maximum 200 mJ/(cm2.pulse), pulse
duration: 8 ns, repetition frequency: 10 Hz) was used. Target was fixed in ahigh-pressure cell (AKICO). Incident angle of the laser beam was 30o and the laser waslocated 1 m from the target. A gold plate (Nilaco, purity: 99.99%, thickness: 0.03 mm)
was used as the target. Liquid CO2 was pressurized and pumped into the cell using ahigh-performance liquid chromatography (HPLC) pump (Jasco PU-1586).
Surface morphology of gold plate after ablation at 40oC and various pressures for3000 s is shown in Figure 9. A circle crater was clearly observed in the irradiated gold
plate at all conditions. The crater was more clearly generated as increasing pressure. In
addition, the surface around irradiation region was slightly rough due to the solidparticle cloud (plume) deposited.
Figure 10 shows the crater depth generated by PLA at various pressures. As
expected, the increasing pressure causes increasing the crater depth due to CO2
properties change in the system. However, the deepest crater was observed at criticalpressure of CO2 and it has similar trend line with constant volume heat capacity of CO2.

The craters depth at critical pressure and 10 MPa were thicker than initial gold plate
thickness because the plume deposited around the hole was generated.
12000.016
0.014

1000

0.012

900

800700600
3000510152025250

Depth [mm]200

Absorbance [-]Constant volume
heat capacity (J/kg K)

1100

0.01
0.008
0.006
0.004

1500.002
100050400
00510152025450

500

550 600 650
Wavelength [nm]

700

750

800

P res s ure [MP a]

Figure 10. Depth of crater generated bypulsed laser ablation at various pressures and40oC for 3000 s.

Figure 11. Extinction spectra of gold
nanoparticles deposited in glass slide at
40oC and 20 MPa for 3000 s

Extinction spectra of gold nanoparticles deposited in glass slide was also measured
for confirmation of generated gold nanoparticles. Figure 11 shows the spectra ofgenerated gold nanoparticles in glass slide, measured after CO2 depressurizing. The
spectra contain bands near 530 and 650. The peak near 530 nm has been known tocorrespond to the plasmon band of gold nanospheres with diameters <50 nm [48].
References[43] C. He, T. Sasaki, Y. Shimizu, N. Koshizaki, Appl. Surf. Sci., 254, 2008, p. 2196.
[44] C. Liang, Y. Shimizu, M. Masuda, T. Sasaki, N. Koshizaki, Chem. Mater., 16, 2004,p. 963.
[45] V. Amendola, G. A. Rizzi, S. Polizzi, M. Meneghetti, J. Phys. Chem., 109, 2005, p.
23125.
[46] N. Takada, H. Ushida, K. Sasaki, J. Phys.: Conf. Ser., 59, 2007, p. 40.
[47] K. Saitow, J. Phys. Chem. B, 109, 2005, 3731-3733.
[48] K. Saitow, T. Yamamura, T. Minami, J. Phys. Chem. C, 112, 2008, p. 18340.
[49] Y. Kuwahara, T. Saito, M. Haba, T. Iwanaga, M. Sasaki, M. Goto, Japanese J. Apl.
Phys., 48, 2009, p. 040207-1.