J. Phys. Chem. C 2009, 113, 15343–15345
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Nanocrystallization of Anatase or Rutile TiO2 by Laser Treatment O. Van Overschelde,*,† G. Guisbiers,‡ and M. Wautelet† Physics of Condensed Matter, UniVersity of Mons-Hainaut, AVenue Maistriau 23, 7000 Mons, Belgium, and IEMN, CNRS-UMR8520, Scientific City, AVenue Henri Poincare´ BP60069, 59652 VilleneuVe d’Ascq, France ReceiVed: June 2, 2009; ReVised Manuscript ReceiVed: July 22, 2009
We present an original way to produce anatase titanium dioxide (TiO2) nanoparticles from amorphous nanofilm. TiO2 nanofilm has been deposited by reactive magnetron sputtering on glass and subsequently irradiated by ultraviolet (UV) radiation using a krypton-fluor (KrF) excimer laser. X-ray diffraction and transmission electron microscopy reveal that the film has been nanostructured. Experimental results are compared with theoretical predictions concerning the structural phase stability with the size of nanostructures. The size distribution of spherical nanoparticles confirms that the most stable phase is anatase for particle sizes smaller than ∼40 nm and rutile for sizes bigger than ∼40 nm. On a technological point of view, TiO2 material presents a lot of interest because of its wide range of applications.1,2 Furthermore, TiO2 exhibits interesting properties at the nanoscale3,4 and plays then a significant role in nano- and microtechnologies (including microelectronics,5 highly efficient catalysts,6 self-cleaning coatings,7 gas sensors,8 and photovoltaic cells9). Focusing on photocatalytic applications, anatase TiO2 has the highest specific surface area among the TiO2 structural phases (rutile, anatase, brookite) and is therefore the most interesting.10 To get large surface to volume ratio, we have to create a structured film made of nanosized particles. Indeed, by reducing the grain size to the nanometer length scale, it increases the specific surface area.11 Unfortunately, conventional deposition techniques of TiO2 thin films such as magnetron sputtering12 lead most of the time to relatively smooth film which is not the optimal morphology for photocatalytic applications. Postannealing treatment has been often used to modify topography and crystallographic constitution of initially amorphous TiO2,13,14 but classic annealing methods (taking minutes to hours to obtain transformations15) have difficulties to create anatase TiO2 in comparison to rutile TiO2.12 Therefore, efforts have still to be realized to optimize the structural phase of TiO2 thin films. It has been shown that excimer laser is a powerful technique to process the surface after deposition.16,17 In this letter, we report on the anatase TiO2 nanostructures generated by excimer laser irradiation. TiO2 film is deposited by reactive magnetron sputtering in an Ar/O2 mixture. The deposition chamber consists of an industrial system (TSD 400-CD HEF R&D). TiO2 coating (208 nm thick) is deposited on 180 cm2 glass substrate. No intentional heating of the substrate is performed. After deposition, the film is irradiated in air using a Lambda Physik (model Compex 205) excimer laser (248 nm wavelength, 25 ns pulse duration). The crystallization of the TiO2 film is examined by X-ray diffraction (XRD) using a D8 Discover goniometer equipped with HI-STAR 1024 × 1024 pixels 2D detector (Bruker-AXS Inc.). The Cu source is used with an acceleration voltage and current of 40 kV and 40 mA, respectively. Transmission electronic microscopy (TEM) * Corresponding author. E-mail:
[email protected]. † University of Mons-Hainaut. ‡ IEMN, CNRS-UMR8520.
Figure 1. XRD pattern (intensity, I, in arbitrary units vs twice Bragg’s angle θ) obtained for TiO2 thin film (deposited on glass by the magnetron sputtering technique) before and after excimer laser irradiation. The laser irradiation was done at a fluence equal to 0.1 J/cm2. Inset: Cross-sectional TEM image of irradiated (fluence ) 0.1 J/cm2) TiO2 thin film. Anatase nanoparticles are clearly visible in the film with sizes between 7 and 37 nm.
investigation was carried out on cross-sectional thin foil by means of a Philips CM200 apparatus using an acceleration voltage of 120 kV. Thin foil was prepared by an ultramicrotomy method using a Leica Ultracut UCT system. The as-deposited film is amorphous18 as confirmed by the diffraction diagram for the TiO2 where no peaks appear (Figure 1). Moreover, spectra from the irradiated sample (fluence ) 0.1 J/cm2) have been superimposed in Figure 1, where the crystalline structure of anatase TiO2, (100) and (112) peaks, appears. The inset of Figure 1 displays a crosssectional TEM image of our irradiated TiO2 sample. It appears from this picture that spherical nanoparticles, with diameter values from ∼7 to ∼37 nm, were created. From the inset of Figure 1, it can be seen that the density of nucleated particles is bigger at the film-substrate interface than in the film itself. This can be understood as the smallest particles nucleate from the film-substrate interface and expand their sizes naturally in the direction where there is
10.1021/jp905163j CCC: $40.75 2009 American Chemical Society Published on Web 08/04/2009
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J. Phys. Chem. C, Vol. 113, No. 34, 2009
Figure 2. Melting temperature, Tm, versus the diameter, D, of the TiO2 spherical nanoparticles. Inset: Cross-sectional TEM image of TiO2 thin film irradiated with fluence equal to 2 J/cm2. The nanoparticles (rutile structure) exhibit sizes between 50 and 150 nm.
less pressure stress (in this case, to the film-air interface). Although nucleation induced by nanosecond laser irradiation (which is a nonequilibrium process) could not be explained by a purely thermal model,18 the film-substrate interface as a starting point of nucleation is not surprising seeing that this is the place where the thermal stress (due to thermal expansion coefficient mismatch between TiO2 and glass) is the most elevated.19 Indeed, it has been shown by Delogu20 that anatase to rutile phase transformation could occur under mechanical processing conditions. Moreover, we underlined that initially amorphous TiO2 irradiated with fluence equal to 2 J/cm2 displays a rutile structure.19 The TEM image of that last sample (inset of Figure 2) shows particles with “pseudo” spherical shape with sizes between 50 and 150 nm. The origins of experimental observations described above could be explained by laser-matter interaction theory. Laser irradiation could lead to two main processes: photochemical or photothermal. These processes are the starting point of several mechanical stresses in the film. The magnitude of these mechanical stresses depends on the laser fluence. In the case of our experiments, the mechanisms responsible for the material’s transformation involve electronics effects. On the other hand, a photothermal process means that the response of the surface is much longer than the heating time. Our results could be classified into two categories: low fluence irradiation (amorphous to anatase transformation) and high fluence irradiation (amorphous to rutile transformation). From previous work,21 it has been confirmed that the structural phase of irradiated TiO2, with a fluence between 0.075 and 0.350 J/cm2, is anatase. The 0.1 J/cm2 fluence is too low to reach the melting temperature of TiO2, which means that crystallization occurs here in the “partial” melting regime. It has been shown18 that although the nucleation theory explains qualitatively the transformation it disagrees with the experimental results. The transformation induced by the laser is about 10 orders of magnitude faster than a classic process. The origin of this discrepancy is attributed to electronic effects. They are responsible for direct bond breaking, and then the amorphous to anatase transition is achieved via creation, migration, and recombination of dangling bonds sites. Following these last arguments, we could deduce that the amorphous to anatase transition occurs
Overschelde et al. here by means of mechanical stresses induced by a low fluence photochemical process. On the other hand, the amorphous to rutile transition requires 2 J/cm2. It has been argued22 that when irradiation fluence is equal to two times the value of the ablation threshold, the photochemical mechanisms are negligible toward photothermal ones. The ablation threshold in the case of TiO2 thin films is about ∼0.65 J/cm2.23 This demonstrates that the amorphous to rutile transition is conducted by mechanical stresses generated by a high fluence irradiation where thermal effects are predominant. In a previous paper, we have discussed the size and shape effects on the melting temperature of the three different phases (rutile, anatase, brookite) of TiO2 nanostructures.4 It has been predicted that the most stable phase for sizes smaller than ∼40 nm is anatase and rutile for sizes bigger than ∼40 nm (Figure 2). Indeed, thermodynamically, the most stable structure is the one with the lowest Gibbs’ free energy G ) H - TS; therefore, it means the one which exhibits the highest melting temperature. This last result is clearly confirmed by our experiment. Indeed, after irradiation, the film is composed of anatase spherical nanoparticles with diameters ranging from 7 to 37 nm (inset Figure 1). Moreover, the TEM image (inset Figure 2) showing size particles ranging between 50 and 150 nm is in good agreement with theoretical prediction showing that rutile is the most stable phase for spherical nanoparticles featuring sizes bigger than 40 nm. Moreover, our results are in agreement with the phase map of TiO2 nanocrystals published by Barnard and Xu.24 To conclude, we successfully investigated the possibility to transform initially amorphous TiO2 film into an anatase and rutile nanostructured one. Classic annealing treatments take minutes to hours to obtain transformation of the crystal structure of the film. Here, we propose a fast method to nanostructure the film with a process time no longer than 25 ns. Moreover, we used our experimental results to support with success our theoretical predictions concerning the relation between structural phase stability and size. Acknowledgment. The authors thank Rony Snyders and Yoan Paint from Inorganic and Analytical Chemistry Laboratory (University of Mons-Hainaut, Belgium) for XRD and TEM measurements. References and Notes (1) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33. (2) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (3) Luca, V. J. Phys. Chem. C 2009, 113, 6367. (4) Guisbiers, G.; Van Overschelde, O.; Wautelet, M. Appl. Phys. Lett. 2008, 92, 103121. (5) Burns, G. P. J. Appl. Phys. 1989, 65, 2095. (6) Carlson, T.; Griffin, G. L. J. Phys. Chem. 1986, 90, 5896. (7) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (8) Zakrzewska, K. Vacuum 2004, 74, 335. (9) Vigil, E.; B., G.; Zumeta, I.; S., D.; Peiro, A. M.; Gutie´rrez-Tauste, D.; Domingo, C.; Dome`nech, X.; Ayllon, J. A. J. Cryst. Growth 2004, 262, 366. (10) Kaneko, M.; Okura, I. Photocatalysis: Science and Technology; Springer-Verlag: Berlin, 2002. (11) Manera, M. G.; Cozzoli, P. D.; Leo, G.; Curri, M. L.; Agostiano, A.; Vasanelli, L.; Rella, R. Sens. Actuators, B 2007, 126, 562. (12) Choi, Y.; Yamamoto, S.; Umebayashi, T.; Yoshikawa, M. Solid State Ionics 2004, 172, 105. (13) Sankapal, B. R.; Lux-Steiner, M. C.; Ennaoui, A. Appl. Surf. Sci. 2005, 239, 165. (14) Negishi, N.; Takeuchi, K. Mater. Lett. 1999, 38, 150. (15) Zhang, H.; Banfield, J. F. J. Mater. Res. 2000, 15, 437.
Nanocrystallization of Anatase or Rutile TiO2 (16) Mariucci, L.; Pecora, A.; Fortunato, G.; Spinella, C.; Bongiorno, C. Thin Solid Films 2003, 427, 91. (17) Ishihara, R.; van der Wilt, P. C.; van Dijk, B. D.; Burtsev, A.; Metselaar, J. W.; Beenakker, C. I. M. Thin Solid Films 2003, 427, 77. (18) Van Overschelde, O.; Snyders, R.; Wautelet, M. Appl. Surf. Sci. 2007, 254, 971. (19) Van Overschelde, O.; Dinu, S.; Guisbiers, G.; Monteverde, F.; Nouvellon, C.; Wautelet, M. Appl. Surf. Sci. 2006, 252, 4722. (20) Delogu, F. J. Alloys Compd. 2009, 468, 22.
J. Phys. Chem. C, Vol. 113, No. 34, 2009 15345 (21) Van Overschelde, O.; Guisbiers, G.; Hamadi, F.; Hemberg, A.; Snyders, R.; Wautelet, M. J. Appl. Phys. 2008, 104, 103106. (22) Ba¨uerle, D. Laser processing and chemistry, 3rd ed.; Springer: Berlin, 2000. (23) Van Overschelde, O. Treatment and structuration of TiO2 thin films by excimer laser, Ph. D. Thesis, University of Mons-Hainaut, 2009. (24) Barnard, A. S.; Xu, H. ACS Nano 2008, 2, 2237.
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