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Physics Procedia 41 (2013) 787 – 793
Lasers in Manufacturing Conference 2013
Synthesis of titanium oxide nanoparticles by Ytterbium fiber laser ablation M. Boutinguizaa, J. del Vala, A. Riveiroa,b, F. Lusquiñosa, F. Quinteroa, R. Comesañac, J. Poua* a
Applied Physics Department, University of Vigo, EEI, Lagoas-Marcosende, Vigo, E- 36310, Spain. Centro Universitario de la Defensa, Escuela Naval Militar, Plaza de España, Marín, E-36920, 0 Spain c Materials Engineering, Applied Mechanics and Construction Dpt., University of Vigo, EEI, Lagoas-Marcosende, Vigo, E- 36310, Spain. b
Abstract Nanosized titanium particles have recently received a special attention due to their applications in many different fields, such as catalysis, biomedical engineering, etc. Pulsed laser ablation in liquid media allows obtaining metallic and metallic oxide nanoparticles in colloids. This technique has been used in the present work to prepare titanium colloids from a solid piece immersed in liquid media. A monomode Ytterbium doped fiber laser has been focused onto the upper surface of the titanium target in de-ionized water or ethanol. Crystalline phases, morphology and optical properties of the obtained colloidal nanoparticles were characterized by XRD, HRTEM, and UV/VIS absorption spectroscopy. The produced titanium oxide crystalline nanoparticles show spherical shape and are polycrystalline, exhibiting anatase as well as rutile phases. accessB.V. under CC BY-NC-ND license. © 2013 The Published byPublished Elsevier B.V. © Authors. 2013 The Authors. by Open Elsevier Selection and/or peer-review under responsibility the German Scientific Laser Society (WLTLaser e.V.) Society (WLT e.V.) Selection and/or peer-review under of responsibility of the German Scientific
Keywords: nanoparticles; titanium oxide; laser ablation.
* Corresponding author. Tel.: +34-986-812-216; fax: +34-986-812-201. E-mail address:
[email protected].
1875-3892 © 2013 The Authors. Published by Elsevier B.V. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of the German Scientific Laser Society (WLT e.V.) doi:10.1016/j.phpro.2013.03.149
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1. Introduction Pulsed laser ablation of targets in liquids has recently become a promising technique for nanoparticle production due to several advantages such as simplicity, low production cost and nanoparticle collecting in liquid avoiding the use of vacuum equipment. Pulsed laser ablation in aqueous media has been used to obtain metallic and metallic oxide nanoparticles in colloids [1-3]. Titanium dioxide is being an important subject of investigation due to its wide range of applications, such as, photocatalysis , photoelectric conversion in solar cells , antimicrobial coating or gas sensor [4-8]. Its high refractive index makes it interesting for antireflection devices, optical wave guides, etc. At the nanoscale, surface properties play a crucial role in changing the bulk material properties, thus many of the mentioned applications depend on the crystallographic structure, the crystal size as well as morphology. The main crystallographic phases of titanium dioxide are rutile (tetragonal), anatase (tetragonal) and brookite (orthorhombic), being rutile the most stable in bulk form and ambient conditions. However, the crystallographic phase is influenced by the synthesis method. There is a diversity of methods for producing titania nanoparticles, such as sol-gel, hydrothermal, radiofrequency induction plasma, laser pyrolysis, etc. Some of these methods of production use precursors and solvents which can contaminate the obtained nanoparticles, others produce typically amorphous nanoparticles and an annealing treatment at high temperatures is required to improve their crystallinity. Laser ablation of solids in liquids has been developed as a promising method for producing nanoparticles because of some advantages, such as: direct formation of nanoparticles in solutions, the absence of contamination, all particles are collected, easiness of preparation, low costs of processing, many kind of colloidal nanoparticles can be obtained etc [10-12]. Resizing of colloidal nanoparticles are also possible trough melting and fragmentation mechanism. In most of the works reporting the production of nanoparticles by laser ablation in liquids, pulsed lasers have been used [13-15]. Nevertheless, nanoparticles can be obtained in aqueous media not only by the use of pulsed laser, but also using continuous wave lasers (CW). In this work the synthesis and characterization of dioxide nanoparticles with this technique using a CW fiber laser to produce controllable size nanoparticles is reported. The formation mechanism of nanoparticles as well as their size reduction is discussed. 2. Experimental Ti plate targets were cleaned and sonicated to be ablated by laser in liquid media. The targets were attached to a bottom of a glass vessel and filled with liquid up to 1 mm over the upper surface of the Ti plate. Ethanol and de-ionized water were employed as liquid media. The laser source system used was a monomode Ytterbium doped fiber laser (YDFL). This laser works in continuous wave mode delivering a maximum average power of 200 W. Its high beam quality allowed setting the irradiance range between 2×10 5 and 106W/cm2. The laser beam was coupled to an surface of the target. The laser beam was kept in relative movement with respect to the metallic plate at a scanning speed of 5 mm/s. After each experiment, the obtained colloidal suspensions were dropped on carbon coated copper microgrids and on Si substrates for examination of particle morphology and microstructure. Transmission electron microscopy (TEM), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL-JEM 210 FEG transmission electron microscope equipped with a slow digital camera scan, using an accelerating voltage of 200 kV, to reveal their crystalline phases. The morphology as well as the composition were described by Field Scanning Electron Microscopy (FSEM) using a JEOL-JSM-6700F unit. Identification of phases was achieved by comparing the diffraction patterns and distances from SAED of samples with ICDD (JCPDS) standards.
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3. Results and discussion The ablated particles in de-ionized water and in ethanol produced stable colloids of titanium oxides. The observed morphology of the particles was almost a perfect sphere, and in all experiments the TEM images of the particles exhibited the same perfect rounded appearance (Figure 1). The size of the obtained particles was comprised mainly between 5 nm and 25 nm. Only in few cases some large particles had settled out, showing deposits on the bottom of the vessel. Figure 2 shows the histogram of more than 300 particles obtained in deionized water and in ethanol respectively. Despite the presence of some occasional large particles, the particle size distribution is quite narrow.
Fig. 1. TEM images of the TiO2 nanoparticles produced by ablating Ti submerged in: a) De-ionized water. b) Ethanol.
The size and the morphology of the particles are governed by laser parameters and the surrounding liquid. The interaction between the laser beam and the Ti target submerged in solvent leads to heating up a very thin layer of metal above its melting point. The adjacent solvent layer reaches approximately the same temperature due to heat transfer from the metal to liquid, which is very much higher than the boiling point of the liquid at normal pressure. This provokes the bubble formation. The nanoparticles are formed as result of the reaction between the melted metal and oxygen from the vaporized solvent to give titanium oxide. Using both solvents, the produced particles were titanium dioxide with different crystalline structure, some were almost single-crystal with the presence of defects, and others were clearly polycrystalline; which is a strong indication of the rapid cooling of the particles that possibly cause lattice imperfections, twinning and dislocations.
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Fig. 2. Histograms of more than 300 nanoparticles obtained by laser ablation of Ti target immersed in: a) De-ionized water. b) Ethanol.
This is an important point to take into account in the formation mechanism because it is strongly related to the fabrication process. Some authors reported the formation of nanoparticles containing exclusively metallic titanium, others also have obtained particles with a range of oxygen content by ablating a titanium target submerged in water with a pulsed Nd:YAG laser [16-17]. In our work a continuous wave laser was used to ablate the titanium target enabling larger exposing time for the evaporated liquid and the metallic species to complete the chemical reaction and to produce stoichimetric TiO 2 nanoparticles [18-19]. Several EDS microanalysis performed on individual particles confirmed that the obtained particles were TiO2. The semiquantitative analysis gives an O/Ti ratio of 2.20 in atomic percentage for the particles obtained in de-ionized water and 2.07 for the particles obtained in ethanol, which agrees quite well with the stoichiometry of titanium dioxide. The HRTEM images of obtained nanoparticles in both solvents revealed that they are crystalline, as can be seen from figures 3 and 4 which show clearly visible lattice fringes and their corresponding Fast Fourier Transform (FFT). The calculated interplanar distances from the FFT for particles obtained in each solvent are compared with those of TiO2 phases in tables 1 and 2.
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Fig. 3. HRTEM image of the obtained nanoparticles in de-ionized water with clearly visible lattice fringes and their corresponding FFT.
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Fig. 4. HRTEM image of the obtained nanoparticles in ethanol with clearly visible lattice fringes and their corresponding FFT.
In the case of particles obtained in de-ionized water, there are some interplanar distances that could be assigned to both anatase as well as rutile phases, but there are others which agree only with anatase while others agree only with rutile, as collected in Table 1. This fact confirms the existence of both phases in the particles. In table 2, measured interplanar distances from the FFT corresponding to Figure 4, are compared to their possible indexation. The presence of different phases of TiO 2 is still observed, but brookite is the predominant phase. Under ambient conditions, the rutile is the most stable phase in titanium oxide as a bulk material. However, some researchers reported that the phase stability depends on the size of particles, being anatase the more stable phase for spherical nanoparticles smaller than 40 nm [20]. Brookite is most stable for crystal sizes between 11 and 35 nm [21]. This is in agreement with the presence of brookite phase in our work. Table 1. Measured interplanar distances from the FFT of figure 3 compared to those of TiO2 phases.
Measured d- (nm)
Rutile, dhkl (nm)
0.2349 0.2479
0.2487
0.3162
0.3247
Anatase, dhkl (nm)
Brookite, dhkl (nm)
0.2378
0.2344 0.2476
Table 2. Measured interplanar distances from the FFT of figure 4 compared to those of TiO2 phases.
Measured d- (nm)
Rutile, dhkl (nm)
Anatase, dhkl (nm)
0.2116 0.2136 0.3645
Brookite, dhkl (nm) 0.2133
0.2187
0,2133 0.3520
0.3512
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In our work, the use of the CW laser with at moderate irradiance contributes to control the size distribution of the obtained nanoparticles, since the CW mode does not produce high peaks power which can provoke deep holes. This reduces the formation of particles with wide size dispersion. Furthermore, the laser beam was kept in movement relative to the target in order to reduce the thickness of the melted layer. The reduction of the interaction time by increasing the scanning speed can lead to a reduction of the extracted layer from the metal plate and the formation of smaller particles. This effect has been reported by other authors obtaining bigger nanoparticles when the interaction time was increased by increasing the pulse width [22] 4. Conclusions In conclusion, we have obtained colloidal nanoparticles of TiO 2 in de-ionized water and ethanol using a CW laser to ablate metallic Ti target. The use of CW contributes to a complete reaction between the metallic species and the evaporated liquid due to long interaction time. The obtained nanoparticles have a stoichiometric composition corresponding to TiO2. The particles are present mainly in anatase and rutile phases, being the rutile the predominant one. Some particles are almost single-crystals, while others are polycrystalline. The size distribution can be controlled by the scanning speed with mean diameter ranging from 5 to 25 nm. The obtained nanoparticles are almost perfect spheres in shape. Acknowledgements This work was partially funded by the European Union (MARMED - 2011-1/164), the Spanish government (CICYT DPI2009-14412) and Xunta de Galicia (CN2012/292) References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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