01 Aug 2007
A new method of laser ablation in liquid is producing nanoparticles at a faster rate than ever before.
Highly efficient nanoparticle generation could have applications in coatings, sensors and solar cells, say researchers in the UK. The team is using a continuous wave (cw) fiber laser to produce titanium-oxide nanoparticles with a mean diameter of 30-40 nm at a rate of around 2 grams/hour (g/h). This is a significant improvement on reported values of 4.4 milligrams/hour produced using pulsed laser ablation.
"We believe that our one-step cw laser ablation in liquid (LAL) process is more efficient than the well-established pulsed LAL techniques," Amin Abdolvand, a researcher at North West Laser Engineering Consortium (NWLEC), told optics.org. "When titanium-dioxide nanoparticles are coated on a substrate, they can absorb ultraviolet radiation. The coatings can also be added to make the surface of a substrate hydrophilic."
So far, mainly nanosecond pulsed laser sources have been used for LAL where the pulses play a self-limiting role in the generation of nanoparticles. "The process efficiency and the size of the colloidal particles are influenced by self-absorption of the laser pulses by the particles," commented Abdolvand. "This means that a large number of pulses and long irradiation times are required (typically between 15-120mins)."
The team selected fiber lasers because of their high average power and brightness. "Fiber lasers are robust and easy-to-use," explained Abdolvand. "The output beam can be focused to a very small spot (less than 20 µm) with a relatively large depth of focus. This provides a high and uniform irradiance and means that the material ablation can be directed as desired."
According to Martin Sharp, a co-researcher at NWLEC, another potential benefit of using fiber lasers is their scalability. "CW fiber lasers in excess of 10 kW are now available and although the fiber size may increase at these powers, it should still be possible to focus down to fairly small spot sizes. This means we can scale up the production rate by increasing the laser power," he said.
The experimental setup included submerging a 1mm high-purity titanium plate in 8 ml of either de-ionized water or an aqueous surfactant solution. "We used a 1070 nm Ytterbium-doped cw fiber laser to focus 250 W onto a spot diameter of 40 µm. This equates to a power density of 20 MWcm-2," explained Abdolvand. "Even a very short exposure time of 1 s was enough to remove up to 0.4 mg of the material, this implies a production rate of about 2 g/h and the entire product can be collected in solution."
Having presented its proof of principle study at CLEO Europe 2007, the team plan to investigate the scope for scale-up production of customized nanoparticles. "We are seeking funding for applied development and would welcome the opportunity to address specific applications under collaborative projects with industry," concluded Abdolvand.
This work was conducted by the NWLEC, as a collaborative project between the Universities of Manchester and Liverpool, funded by the Northwest Development Agency of the UK.