Titanium Nanoparticles Produced by Laser Ablation in Liquid Are Studied

Daniel S. Burgess

As applications of nanoscale materials continue to be developed, a fundamental understanding of how changes in the processes that create those materials affect their resulting properties is required. To that end, A. Welford Castleman Jr. and Justin S. Golightly of the department of chemistry and physics at Pennsylvania State University in University Park have investigated how the size, size distribution and composition of nanoparticles generated by the laser ablation of a titanium rod in a liquid depend on the type of environment and the intensity of the laser.

Nanoparticles generated by the laser ablation of titanium immersed in liquid differ in size, size distribution and composition, depending on the environment and intensity of the laser. Here, electron micrographs illustrate particles created in n-hexane under illumination with 50-mJ (top) and 100-mJ (bottom) pulses of 532-nm light from an Nd:YAG laser operating at a repetition rate of 10 Hz. The insets display the respective electron diffraction patterns from the generated particles. Reprinted with permission of the Journal of Physical Chemistry B.

As a solid in a liquid environment is exposed to high-energy laser pulses, both the solid and the surrounding liquid are vaporized. Bubbles of a superheated mixture form and act as miniature reaction vessels that yield nanoparticles. The method, which the scientists note has been studied largely for silver and gold, produces particles coated with molecules of the solution so that they resist clumping without requiring additional surface-modification steps. The particles largely escape oxidation unless they are produced in an oxygen-rich environment.

In their work, the investigators focused 20- to 100-mJ pulses of 532-nm light from a Spectra-Physics frequency-doubled Nd:YAG laser operating at a repetition rate of 10 Hz onto titanium rods immersed in water, n-hexane, ethanol and 2-propanol.

During the 30- to 60-minute-long experimental runs, they rotated the rods at 30 rpm. Analysis of the resulting nanoparticles was performed using electron microscopy and micro-Raman spectroscopy, for which they employed a Renishaw plc Raman spectroscope with a Laser Physics UK Ltd. 514-nm argon-ion and a JDS Uniphase Corp. 633-nm HeNe laser as excitation sources.

They found that the size of the nanoparticles and their size distribution was directly proportional to the intensity of the laser. The composition depended on the solution used, with particles created in water incorporating oxygen and those in n-hexane, ethanol and 2-propanol incorporating carbon. The composition of the suspended particles was stable over a period of months.

The researchers have concluded that, although the laser ablation of a solid in a liquid environment will yield nanoparticles that exhibit a relatively large size distribution, it is a promising approach for the generation of unique materials for a variety of applications.

Journal of Physical Chemistry B, online July 26, 2006, doi:10.1021/jp062123x.