Synthesis of Anatase Phase Titanium Dioxide Using High-Power Nd:YAG LaserFocused on Titanium Wire in Water

Naoyuki Wada1,+1, Chang-Hwan Kim2,+2, Jack J. Yoh2,Hideki Hamashima3 and Kazuyuki Hokamoto4

1Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan2School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Korea3Kumamoto Industrial Research Institute, Kumamoto 862-0901, Japan4Shock Wave and Condensed Matter Research Center, Kumamoto University, Kumamoto 860-8555, Japan

An experimental method to synthesize titanium dioxide (TiO2) using high-power laser in water was performed. A high-power Nd:YAGpulsed laser was used for the synthesis, with the laser energy fixed at 1 J/pulse. This laser was focused on a titanium wire set in water. Thisinvestigation recovered nano-sized anatase phase titanium dioxide with well crystallized structure. Pulsed bubbles generated in the water wereconfirmed by optical measurement, and their collapse may have induced high pressures and temperatures. The bubbles generated wasapproximately spherical in shape, with an estimated maximum size of 3.7mm generated about 200µs later after focusing the laser. Recoveredpowders were confirmed as single anatase phase titanium dioxide by XRD analysis. Effects of bubbles on synthesis and crystallization are alsosuggested. [doi:10.2320/matertrans.M2011183]

(Received June 16, 2011; Accepted October 25, 2011; Published December 25, 2011)

Keywords: laser ablation synthesis, titanium dioxide, pulsed bubble

1. Introduction

Synthesis of various materials under an intensive energeticcondition has been previously investigated by variousresearchers,1­5) some who have synthesized titanium nitrideand titanium aluminum nitride in liquid nitrogen usingan electrical high-energy pulsed power of about 10 kJ4) orusing an explosive5) through the chemical reaction betweenextremely excited elements. The use of such highly-energeticconditions holds high potential to synthesize various newintermetallics possessing nanometer-scale, ultra-fine grainedstructure and offering highly functional performance. Underthese intensive energetic conditions, a shock wave isgenerated, forming a pulsed bubble if the phenomenon isignited in water which induces rapid phase change to gasfrom the liquid media.6­11) This bubble, composed of as muchhigh energy as an ablated point, can generate highly-energeticconditions at the time of its collapse, inducing high pressureand high temperature conditions repetitively.

Similar methods for synthesizing various intermetallics inliquid through high energy such as impulse plasma12) andlaser ablation focused to a metal plate target13­17) have beeninvestigated and reported. Laser ablation can induce rapidexcitation of the materials and can generate shock waves dueto rapid vaporization, boiling, and phase explosion.18­20)

Laser ablation synthesis in liquid has attracted attention asa good candidate for exploring new intermetallics under highpressure and/or high temperature conditions due to phasequenching by rapid cooling through the low intensity of asingle pulse. Potential production of large-volume interme-tallics using continuous pulses is also expected. In the presentpaper, the synthesis of titanium oxide using a high-energyNd:YAG laser in water was investigated. A high energysingle pulse of one J was loaded to a titanium wire set in

distilled water. This research used the titanium wire to clarifythe effects of the spherical bubble on the synthesis ofintermetallics in comparison to the use of the metal plate sofar employed. Synthesis of titanium oxide was expected tooccur during the excited state created by the exposure ofthe laser and/or the bubble collapse, which would inducehigh pressures and temperatures. Bubble formation was alsoconfirmed by capturing images using a camera with anotherpulsed laser.

2. Experiment

Figure 1(a) illustrates the experimental set-up both forsynthesis and for visualization of the pulsed bubble; Fig. 1(b)illustrates in detail the laser focusing on the target. In order tocapture an image of a pulsed bubble, a camera (digital camerawith telephoto lens, Pentax) and Nd:YAG laser (maximumintensity 25mJ of 532 nm wavelength, Minilite PIV Speci-fication, Continuum Inc.) were aligned to the sample targetperpendicular to the ablation beam.18,19) Another Nd:YAGlaser beam of 532 nm with a 10 ns pulse width and arepetition rate of 10Hz (maximum intensity 3 J/pulse,Powerlite Plus, Continuum Inc.) was used as an ablationpulse for focusing on the target to synthesize oxides. In thepresent work, the experiment was performed at 1 J/pulse for5 h for powder recovery. The target material of this experi-ment was titanium wire of 0.5mm in diameter (99.5mass%purity); the wire was hung from top as shown in Fig. 1(b).The diameter of laser beam on target surface was set to beapproximately 0.5mm for loading high energy for recoveringpowders. The fluence on the target surface was estimated tobe about 5.1MJ/m2 by considering 1 J/pulse and diameterof laser beam, which is relatively high in comparison withother reports.9) However, the targeted area of laser changedslightly due to inaccuracy of wire position. This experimentachieved recovery of the reacted powders by lowering thetitanium wire into distilled water several times during the

+1Graduate Student, Kumamoto University+2Graduate Student, Seoul National University

Materials Transactions, Vol. 53, No. 1 (2012) pp. 244 to 247©2011 The Japan Institute of Metals RAPID PUBLICATION

experiment. The amount of powder recovered was about20mg. A pulse generator was used to control the delay timebetween the laser pulse and imaging system. A single imageframe was taken at each pulse. The powders recovered at thebottom of the container were characterized using scannedelectron microscopes (SEM) and X-ray diffraction machine(XRD).

3. Results and Discussion

Figure 2 is a photograph of the recovered powders. Thepowders appear white gray in color. Figure 3 shows an SEMmicrograph of the recovered powders and a magnified imageof the powder. As shown in Fig. 3(a), the powder size wasin the order of 5 µm, with each particle composed of finesecondary particles in the order of several ten nanometers,as seen in Fig. 3(b). This observation seems similar tothat of other researches using the laser ablation synthesismethod.13­17) The X-ray diffraction pattern shown in Fig. 4shows only the peaks of anatase phase TiO2. Other researchusing laser ablation synthesis focused on a titanium platein water showed three major peaks for anatase phase TiO2,and the peaks were not clear with noises;13) in contrast,our investigation found major five peaks of anatase phase,as shown in Fig. 4, which suggests that the powders arecomposed of a well-developed crystalline structure. Thedifference in the results may be attributed to the high energy

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Fig. 1 Schematic illustration of the experimental set-up for synthesis andvisualization of pulsed bubble (a), and detail of synthesis using Nd:YAGlaser focused on titanium wire in distilled water (b).

Fig. 2 Appearance of recovered powder.

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Anatase phase TiO 2

Fig. 4 X-ray diffraction pattern (Cu-K¡) of recovered powder.

Synthesis of Anatase Phase Titanium Dioxide Using High-Power Nd:YAG Laser Focused on Titanium Wire in Water 245

applied to the synthesized area due to the formation of apulsed bubble, unlike previous research conducted underenergy of 35mJ/pulse.13)

Figure 5 shows images of pulsed bubbles at various delaytimes generated by the high energy loading by laser ablation.The bubble generated in this investigation, which was almostspherical in shape, started to appear about 8 µs later after thelaser was focused to the titanium target. After that, the bubbleshrunk and collapsed to induce high-pressure and high-temperature conditions at the center.8­11) The sustained timeof the bubble from formation to collapse was around 400 µs.Since bubble formation and collapse continues multipletimes,8­11) the whole reaction synthesis for one cycle occurswithin 400 µs before the next laser pulse reaches the targetunder the repetition rate 10Hz (100ms interval). Themaximum size of a bubble generated 200 µs after focusingthe laser was estimated at 3.7mm, as seen in Fig. 5. Bubblesgenerated by laser ablation using low pulse intensity (22mJ/pulse) to a metal plate target in liquid has been reportedto have a hemispherical shape of around 1mm radius andan estimated maximum pressure and temperature of around10GPa and 10000K, respectively, during collapse at a singlepoint.8,9) Calculation of the bubble collapse convergencepoint as reported in their research presents difficultycompared with our research due to the movement of bubblesin their experiments. As the energy of the bubble, whichgenerates high pressures and temperatures, depends on itsshape and radius, it is likely that the convergence points ofbubbles induced in this experiment,6,7) and thus the pressureand temperature, are much higher than those of previousexperiments.

According to other research using impulse plasma inliquid, Mashimo and others have clarified from XRD analysisthe existence of several phases of the metastable-phase, non-stoichiometric titanium oxides TinO and TinO2n¹1. Furtherthey have confirmed the amorphous and anatase phase TiO2

from XRD and TEM analysis.12) In their research, metastableoxides such as TinO and TinO2n¹1 were synthesized due tominimal excitation of oxygen and ionized oxygen atoms dueto the loading of less energy (200V, 3A) compared with our

method. Conversely, the synthesized material in this researchshows a well-crystallized structure of anatase phase TiO2.This investigation suggests the effects of high energeticconditions on the induction of well-developed anatasephase TiO2 though synthesis mechanism or process remainsunclarified; further investigation is planned based onmeasurements and numerical simulation results includingthe effect of the pulsed bubble.

4. Conclusions

Synthesis of titanium oxide using a high-energy Nd:YAGlaser in water was investigated. A laser with energy of1 J/pulse was focused on a titanium wire of 0.5mm indiameter set in water. This research successfully recoverednano-sized anatase phase titanium dioxide with well crystal-lized structure. A pulsed bubble, which may induce highpressure and temperature by its collapse, was confirmed byoptical measurement of single bubbles. Effect of the bubbleon synthesis and crystallization is also suggested.

Acknowledgements

The experiment was conducted with the help of Dr.Ardian B. Gojani, School of Mechanical and AerospaceEngineering, Seoul National University. The authors grate-fully acknowledge the support of this work provided byGlobal COE Program “Pulsed Power Engineering”, and thehelp of language editing and comments by Mr. WilliamBaerg, Kumamoto University, Japan.

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