- Email: [email protected]
Small size TiO2 nanoparticles prepared by laser ablation in water
Applied Surface Science 256 (2010) 6408–6412
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Small size TiO2 nanoparticles prepared by laser ablation in water F. Barreca a,∗ , N. Acacia b , E. Barletta a , D. Spadaro a , G. Currò a , F. Neri b a b
Advanced and Nano Materials Research s.r.l., Salita Sperone 31, I-98166, Messina, Italy Dipartimento di Fisica della Materia e Ingegneria Elettronica, Università di Messina, Salita Sperone 31, I-98166, Messina, Italy
a r t i c l e
i n f o
Article history: Received 25 September 2009 Received in revised form 14 January 2010 Accepted 11 April 2010 Available online 18 April 2010 Keywords: Titanium dioxide nanoparticles Laser ablation in Liquid Morphology Chemical composition Gas sensing devices
a b s t r a c t Titanium dioxide nanoparticles in distilled H2 O solvent were prepared by laser ablation. The experiments were performed irradiating a Ti target with a second harmonic (532 nm) output of a Nd:YAG laser varying the operative fluence between 1 and 10 J cm−2 and for an ablation time ranging from 10 to 30 min. Electron microscopy measurements have evidenced the predominant presence of nanoparticles with diameter smaller than 10 nm together with agglomerations of 100–200 nm whose content increases with the laser fluence. At low laser fluence the particles’ size distribution shows that more than 85% of the nanoparticles have a size smaller than 5 nm while at mid and high fluences the presence of 5–7 nm nanoparticles is predominant. XPS analysis has revealed the presence of different titanium suboxide phases with the prevalence of Ti–O bonds from TiO2 species. The optical bandgap values, determined by UV–vis absorption measurements, are compatible with the anatase phase. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since 1972, when Fujishima and Honda discovered the phenomenon of photocatalytic splitting of water on a titanium dioxide (TiO2 ) electrode under ultraviolet (UV) light [1,2], many efforts have been devoted to the research on TiO2 material which has led to many promising applications ranging from photovoltaics and photocatalysis to photo-electrochromics and sensing. As the most promising photocatalyst, TiO2 materials are expected to play an important role in helping to solve many serious environmental and pollution challenges. For example, under UV illumination, TiO2 is able to oxidize gaseous pollutants in quantum yields ranging from 1% to over unity, which has important applications in cleaning indoor and outdoor air. Analogously, most of the organic pollutants in water, such as alkanes, haloalkanes, aliphatic alcohols, carboxylic acids, alkenes, aromatics, polymers, surfactants, pesticides and dyes, can be completely decomposed and mineralized at the surface of UV-excited TiO2 photocatalysts [3]. Performance of TiO2 based devices is largely influenced by the size of the nanometric TiO2 building units because of the increased surface-to-volume ratio that facilitates reaction/interaction between them and the interacting media, which
∗ Corresponding author at: Advanced and Nano Materials Research s.r.l., Dipartimento di Fisica della Materia e Ingegneria Elettronica, Università degli Studi di Messina, Salita Sperone 31, I-98166, Messina, Italy. Tel.: +39 0 90 6765452; fax: +39 0 90 391382. E-mail address: [email protected] (F. Barreca). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.04.026
mainly occurs on the surface or at the interface. In particular it has been found that a decrease in the size of the TiO2 particles will enhance the total photoreactivity of TiO2 . Moreover, the size of the TiO2 particles is known to alter the width of the band gap and the band bending at the interfaces, and thus to influence their photochemical properties. Therefore, many attempts have been made to make TiO2 nanostructures and nanoparticles by a variety of methods, such as thermal hydrolysis, thermal decomposition, sol–gel technique, microemulsion, chemical vapour deposition [4]. Pulsed laser ablation, in vacuum or in appropriate gas atmosphere, has been widely applied to prepare thin film of different nature included the metal oxide films [5–11]. Recently, Laser Ablation in Liquids (LAL) has provided a new technique for synthesis of size controlled metallic and metallic oxide nanoparticles in colloidal as well as powdered phases. A large number of parameters are involved in the laser ablation in the liquid media technique, regarding both the source (i.e. laser wavelength, laser energy, pulse width, repetition rate) and the liquid media (i.e. composition, viscosity, dipole moment, dielectric constant and height of liquid above the target surface). Thus, a large number of available parameters to control size, shape, and morphology of nanostructured materials is one of the advantages of this very fast and very cheap approach [12]. To our knowledge, there are few reports about the synthesis and the study of the properties of TiO2 nanoparticles produced by LAL [13,14]. Even less work can be found about TiO2 colloidal solutions in distilled water synthesized via LAL using a Ti target. Indeed, no direct evidence of a large yield of small TiO2 nanoparticles with a diameter of a few nanometers (2–3 nm) is reported, unless high laser fluences are used [15–18].
F. Barreca et al. / Applied Surface Science 256 (2010) 6408–6412
In this respect, we report about the synthesis of titanium dioxide nanoparticles in distilled H2 O from a Ti target, studying in a systematic way the effects of the laser fluence up to 10 J cm−2 . The principal aim is to produce a narrow distribution of nanoparticles with very small diameters, suitable to increase their photocatalytic properties for applications in environmental, pollution and chemical resistant gas sensing fields. 2. Experimental TiO2 nanoparticles were produced by laser ablation of a metal titanium disk (99.7%) in distilled water varying the laser fluence between 1 and 10 J cm−2 . The target irradiation time was decreased from 30 min down to 10 min increasing the fluence to minimize solution losses due to boiling. For simplicity we labeled as A, B and C the colloidal solutions produced at lowest, mid and highest fluence, respectively. The metal disk, about 8 mm in diameter and 2 mm thick, was placed on the bottom of a rotary glass vessel filled with 5 mL of the solvent and the thickness of the water layer above the titanium target was about 10 mm. The target was irradiated with the second harmonic (532 nm) output of a 5 ns pulse duration Nd:YAG laser operating at 10 Hz; the produced radiation was reflected with a mirror and vertically supplied to the titanium disk after focusing it by a quartz lens with a focal length of 250 mm.
6409
The particles’ morphology was investigated using both TEM and HRTEM observations on a JEOL JEM 2000 FX and a JEOL JEM 2010 transmission electron microscopes, respectively. Digital images were acquired by a GATAN multiscan camera system. The chemical composition was investigated by means of XPS spectroscopy. The spectra were acquired using a K-Alpha system of Thermo Scientific, equipped with a monochromatic Al K˛ source (1486.6 eV) and operating with an analyzer in CAE mode with a pass energy of 50 eV and with a spot size of 400 m. Surface charging effects were avoided using an electron flood gun and the binding energy shifts were calibrated keeping the C1s position fixed at 285 eV. Electron micrographs’ measurements were carried out through depositing and evaporating in air at room temperature some drops of the colloid onto a carbon coated copper grid (TEM, HRTEM) and onto a c–Si substrate (XPS). The UV–vis spectra of the colloidal solutions were collected by means of a Lambda 2 UV–vis spectrometer in the range 200–1100 nm using quartz cells. 3. Results Transmission electron microscopy measurements of the samples were carried out to investigate the morphology and the size of the nanoparticles grown by ablating the Ti target at different laser fluences. In particular, Fig. 1(a–c) reports TEM views of the samples A–C, recorded 1 week after the production, and Fig. 2 illustrates the particles size distribution histogram obtained by the analysis of a
Fig. 1. (a–c) TEM views of the samples A–C collected after 1 week. (d) TEM views of a typical nanoparticle from sample B.
6410
F. Barreca et al. / Applied Surface Science 256 (2010) 6408–6412
Fig. 3. HRTEM view of the samples C and measurements of the interplanar distances.
Fig. 2. Histograms showing the distribution of nanoparticles diameters in samples A–C.
set of TEM micrographs of the same samples. We considered only nanoparticles in the 2–50 nm size range to achieve a reliable result and also because larger ones were uncommon (about 0.3%). It is important to evidence that all the three samples exhibit high density of nanoparticles smaller than 10 nm in diameter. In fact, the reported histograms show that almost all the nanoparticles in the sample A and about 55% in the samples B and C are less than 10 nm in size. Besides, more than 85% of the nanoparticles in the sample
A have a size smaller than 5 nm while the predominant ones in samples B and C are 5–7 nm diameter nanoparticles (about 30%). Furthermore, as early pointed out, in the sample A it is also evident the presence of some spherical particles agglomerations of about 100–200 nm (not shown), presumably generated by Ostwald ripening phenomena [19]. These agglomerations are more numerous in the samples produced at higher fluences (i.e. B and C), as shown in Fig. 1(b and c). In Fig. 1(d) we show a typical well faceted nanoparticle from sample B. High resolution TEM microscopy was then carried out to investigate the crystalline structure of the produced nanostructures. In Fig. 3 we show a particular HRTEM image taken from sample C, that is the largest fluence case. It evidences better the existence of
Fig. 4. X-ray photoelectron spectra of titanium dioxide nanoparticles on Si substrate: Ti2p (3/2, 1/2) core level on the left, O1s core level on the right.
F. Barreca et al. / Applied Surface Science 256 (2010) 6408–6412
small nanoparticles, about 5 nm in diameter, and their crystalline structure with the relative atomic planes. The interplanar distances of 0.17, 0.19 and 0.23 nm, measured on the nanoparticle on the right of this figure, are consistent with the reflections from the (1 0 5), (2 0 0) and (0 0 4) crystallographic planes of the TiO2 anatase phase. On the contrary, within the experimental resolution, the measured distances of 0.16, 0.17 and 0.21 nm on the nanoparticle shown on the left, could be consistent both with the reflections from the planes (2 2 0), (2 1 1) and (2 1 0) of the TiO2 rutile phase and with the (2 1 1), (1 0 5) and (0 0 4) ones belonging to the anatase phase [20]. In Fig. 4 are reported the XPS spectra of the nanoparticles: the subbands analysis of the Ti2p and O1s core level spectra of the samples is also showed. The Ti2p region may be decomposed with a fitting procedure into three contributions corresponding to the different oxidation states of titanium. Each contribution consists of a doublet of 2p3/2 and 2p1/2 peaks. Gauss–Lorentzian shape functions with the same FWHM for all the peaks were used for the deconvolution. For each doublet, the ratio of the area of the two peaks A (Ti2p3/2 )/A (Ti2p1/2 ) is fixed to 0.5 and the binding energy difference Eb = Eb (Ti2p1/2 ) − Eb (Ti2p3/2 ) is 5.7 ± 0.1 eV. The individual 2p3/2 components, with BE values located at approximately 456.4, 457.6, 458.5 eV, are attributed to Ti2+ (TiO), Ti3+ (Ti2 O3 ) and Ti4+ (TiO2 ) [21]. The O1s region may be decomposed with four contributions at about 528.5, 530.1, 531.2 and 532.2 attributed respectively to O–Ti2+ , O–Ti4+ , O–Ti3+ and OH− [22]. From the XPS analysis it is evident that the peaks due the TiO2 phase are almost always the predominant ones, although some suboxide phases are also significantly present. UV–vis absorption spectra were carried out in order to characterize the optical absorbance of colloidal solutions and to confirm the nature of the nanoparticles taking into account that for anatase, brookite and rutile structures in phase-pure powders optical band gap values of 3.19, 3.11 and 3.0, respectively have been reported [23]. Fig. 5 shows the UV–vis spectra collected immediately after the production of nano-colloids in distilled water. A strong absorption due to the TiO2 nanoparticles dominates the spectrum below 350 nm while a smooth decay towards longer wavelengths is observed. Moreover, the absorption intensity registered in the 200–300 nm wavelength range increases with increasing the laser fluence in spite of the shorter deposition time. This fact points out the effectiveness of an increased laser fluence in producing a larger concentration of the nanoparticles in the colloidal solution.
Fig. 5. Optical absorbance spectra of the TiO2 colloidal solutions deposited at low (dashed-dotted line), mid (dashed line) and high fluence (whole line), respectively.
6411
The onset of the optical absorption can be extrapolated to about 391, 374 and 380 nm for samples A–C respectively, that is 3.17, 3.32 and 3.26 eV as a function of photon energy. According to literature data [24], these values suggest that the predominant nano-species in the colloidal solutions could be anatase TiO2 (Eg = 3.2 eV) and, consequently, the obtained values can be ascribed to 3 → X1b indirect transition [25].
4. Discussion Today, it is well known that titanium dioxide has three most commonly encountered crystalline polymorphs: anatase, rutile and brookite and they are made up of differently distorted TiO6 octahedra. Recently, it has been reported [3] that rutile is the most stable phase for particles above 35 nm in size, anatase is the most stable phase for nanoparticles below 11 nm and brookite has been found to be the most stable for nanoparticles in the 11–35 nm range, although it is also claimed that anatase is the only nanocrystalline phase obtained [3]. Our electron microscopy measurements have evidenced the predominant presence of smaller than 5 nm size nanoparticles in sample A and of 5–7 nm nanoparticles in the remaining samples. Along with them, it was observed the existence of clusters of 100–200 nm whose content increases with the laser fluence. Therefore, ablation at higher fluence led to the creation of larger nanoparticles, larger aggregates of nanoparticles and, consequently, to broader size distributions. Moreover, if we compare their size and shape with the expectations of the thermodynamic model on the phase stability of TiO2 nanoparticles in different environments proposed by Bernard [4,26,27], we can attribute the predominant nanoparticles crystalline phase to the anatase one, in agreement with the results of optical absorption data. In particular, for example, the nanoparticle of Fig. 1(d) is identified as an anatase crystal with oxygenated surfaces. The crystallographic HRTEM analysis also shows the existence of nanoparticles belonging to the anatase phase and a possible presence of a smaller quantity of nanoparticles of the rutile phase, presumably with hydrogenated surfaces. It has been suggested that the anatase phase appears as the majority part of the nanoscale samples and it can transform to the rutile one upon reaching a particular size that depends on the synthesis conditions. However, in the sample produced at the highest laser fluence (C), we have presumably observed nanoparticles of both phases with the same small dimensions, even if only a minority of them belongs to the rutile phase. Such an occurrence could be explained with the existence, along with the predominant presence of 5–7 nm size anatase nanoparticles, of a small quantity of metastable rutile nanoparticles which are produced with such small dimensions (≈5 nm) due to the high energy involved in the ablation process. Considering the samples composition determined by means of XPS, it appears evident that our colloids are hydroxylated via adsorbed hydroxyl moieties from the ambient. This reaction is favored on the Ti3+ sites which, controlling this adsorptive activity [22], have a positive effect on the photocatalytic process due to the electron capturing process that reduces the recombination of electron and holes. Moreover, also the observed different amounts of surface OH groups play an important role in photocatalysis, where an increase in the surface hydroxyl content will enhance the photocatalytic activity. This can partially be explained by the fact that also OH groups can act as capture centers for the photoexcited electrons [24,28]. According to the reported considerations and to the XPS data, our samples should deserve a good photocalytic activity, and in particular the sample A (deposited at low fluence) due to its higher content of the OH groups and a fair presence of the Ti3+ sites.
6412
F. Barreca et al. / Applied Surface Science 256 (2010) 6408–6412
About the optical properties, we must note at first that the light absorption is small in indirect bandgap bulk semiconductors, like TiO2 , where the direct electronic transitions between the band edges are prohibited by the crystal symmetry. However, it has been reported an enhancement of the light absorption in TiO2 nanocrystallites due to indirect transitions with momentum nonconservation at the interface. This interface absorption becomes the main mechanism of light absorption for the crystallites that are smaller than 20 nm [4,29]. 5. Conclusion Titanium dioxide nanoparticles were synthesized ablating a Ti target in distilled H2 O solvent with laser fluences from 1 up to 10 J cm−2 . Electron microscopy imaging measurements have evidenced that low fluence ablation yields nanoparticles of size smaller than 10 nm. Increasing the fluence leads to the formation of larger nanoparticles and of their aggregates with distributions broader in size. In particular, at low laser fluence the particles’ size distribution shows that more than 85% of the nanoparticles have a size smaller than 5 nm while, at mid and high fluence, only about 30% of them have a size of 5–7 nm. In the latter case, the occurrence of 100–200 nm clusters is also evident. A XPS analysis has evidenced that the nanoparticles incorporate different suboxide phases with the prevalence of Ti–O bonds from TiO2 species. Both anatase and rutile crystalline polymorphs were observed, being the first predominant as supported by UV–vis absorption spectra. A higher photocatalytic activity can be supposed for the sample produced at low fluence due to a higher content of small size nanoparticles, in addition to the presence of both OH groups and Ti3+ sites. Then, an evaluation of the photocatalytic properties of the samples will be the next step of our studies. Acknowledgments We thank Salvatore Pannitteri and Corrado Bongiorno of the CNR-IMM institute of Catania (Italy) for their valuable help in TEM analyses.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
A. Fujishima, K. Honda, Nature 238 (1972) 37. A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1. A. Fujishima, X. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515. X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891. F. Neri, F. Barreca, S. Trusso, Diamond Relat. Mater. 11 (2002) 273. S. Trusso, E. Barletta, F. Barreca, F. Neri, Appl. Phys. A: Mater. Sci. Process. 79 (2004) 1997. E. Fazio, E. Barletta, F. Barreca, F. Neri, S. Trusso, J. Vac. Sci. Technol. B 23 (2) (2005) 519. E. Fazio, E. Barletta, F. Barreca, G. Mondio, F. Neri, S. Trusso, Spriger Ser.: Top. Appl. Phys. 100 (2006) 287. F. Neri, F. Barreca, E. Barletta, E. Fazio, G. Mondio, S. Trusso, H. Modrow, B. Brendebach, J. Vac. Sci. Technol. A 25 (1) (2007) 117. S. Yamamoto, T. Sumita, A. Sugiharuto, H. Miyashita, Naramoto, Thin Solid Films 401 (2001) 88. D. Luca, D. Macovei, C.-M. Teodorescu, Surf. Sci. 600 (2006) 4342. S.C. Singh, R.K. Swarnkar, R. Gopal, J. Nanopart. Res. 11 (2009) 1831. T. Sasaki, C. Liang, W.T. Nichols, Y. Shimizu, N. Koshizaki, Appl. Phys. A 79 (2004) 1489. X. Huang, W. Zhang, Appl. Surf. Sci. 254 (2008) 3403. S.I. Dolgaev, A.V. Simakin, V.V. Voronov, G.A. Shafeev, F. Bozon-Verduraz, Appl. Surf. Sci. 186 (2002) 546. C.H. Liang, Y. Shimizu, T. Sasaki, N. Koshizaki, Appl. Phys. A 80 (2005) 819. A.S. Nikolov, P.A. Atanasov, D.R. Milev, T.R. Stoyanchov, A.D. Deleva, Z.Y. Peshev, Appl. Surf. Sci. 255 (2009) 5351. S.C. Singh, R.K. Swarnkar, R. Gopal, J. Nanosci. Nanotechnol. 9 (2005) 5367. Guozhong Cao, Synthesis Properties & Applications, Imperial College Press, London, 2004. http://rruff.geo.arizona.edu/AMS/amcsd.php. J. Yu, X. Zhao, Q. Zhao, Mater. Chem. Phys. 69 (2001) 21. P.M. Kumar, S. Badrinarayanan, M. Sastry, Thin Solid Films 358 (2000) 122. J.G. Li, T. Ishigaki, X. Sun, J. Phys. Chem. C 13 (2007) 111. H. Jensen, A. Soloviev, Z. Li, E.G. Sogaard, Appl. Surf. Sci. 246 (2005) 239. N. Serpone, D. Lawless, R. Khairutdinov, J. Phys. Chem. 99 (1995) 16646. A.S. Barnard, L.A. Curtiss, Nano Lett. 5 (2005) 1261. A.S. Barnard, P. Zapol, L.A. Curtiss, J. Chem. Theory Comput. 1 (2005) 107. J. Yu, X. Zhao, J. Du, W. Chen, J. Sol–gel Sci. Technol. 17 (2000) 163. L. Braginsky, V. Shklover, Eur. Phys. J. D 9 (1999) 627.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Small size TiO2 nanoparticles prepared by laser ablation in water F. Barreca a,∗ , N. Acacia b , E. Barletta a , D. Spadaro a , G. Currò a , F. Neri b a b
Advanced and Nano Materials Research s.r.l., Salita Sperone 31, I-98166, Messina, Italy Dipartimento di Fisica della Materia e Ingegneria Elettronica, Università di Messina, Salita Sperone 31, I-98166, Messina, Italy
a r t i c l e
i n f o
Article history: Received 25 September 2009 Received in revised form 14 January 2010 Accepted 11 April 2010 Available online 18 April 2010 Keywords: Titanium dioxide nanoparticles Laser ablation in Liquid Morphology Chemical composition Gas sensing devices
a b s t r a c t Titanium dioxide nanoparticles in distilled H2 O solvent were prepared by laser ablation. The experiments were performed irradiating a Ti target with a second harmonic (532 nm) output of a Nd:YAG laser varying the operative fluence between 1 and 10 J cm−2 and for an ablation time ranging from 10 to 30 min. Electron microscopy measurements have evidenced the predominant presence of nanoparticles with diameter smaller than 10 nm together with agglomerations of 100–200 nm whose content increases with the laser fluence. At low laser fluence the particles’ size distribution shows that more than 85% of the nanoparticles have a size smaller than 5 nm while at mid and high fluences the presence of 5–7 nm nanoparticles is predominant. XPS analysis has revealed the presence of different titanium suboxide phases with the prevalence of Ti–O bonds from TiO2 species. The optical bandgap values, determined by UV–vis absorption measurements, are compatible with the anatase phase. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since 1972, when Fujishima and Honda discovered the phenomenon of photocatalytic splitting of water on a titanium dioxide (TiO2 ) electrode under ultraviolet (UV) light [1,2], many efforts have been devoted to the research on TiO2 material which has led to many promising applications ranging from photovoltaics and photocatalysis to photo-electrochromics and sensing. As the most promising photocatalyst, TiO2 materials are expected to play an important role in helping to solve many serious environmental and pollution challenges. For example, under UV illumination, TiO2 is able to oxidize gaseous pollutants in quantum yields ranging from 1% to over unity, which has important applications in cleaning indoor and outdoor air. Analogously, most of the organic pollutants in water, such as alkanes, haloalkanes, aliphatic alcohols, carboxylic acids, alkenes, aromatics, polymers, surfactants, pesticides and dyes, can be completely decomposed and mineralized at the surface of UV-excited TiO2 photocatalysts [3]. Performance of TiO2 based devices is largely influenced by the size of the nanometric TiO2 building units because of the increased surface-to-volume ratio that facilitates reaction/interaction between them and the interacting media, which
∗ Corresponding author at: Advanced and Nano Materials Research s.r.l., Dipartimento di Fisica della Materia e Ingegneria Elettronica, Università degli Studi di Messina, Salita Sperone 31, I-98166, Messina, Italy. Tel.: +39 0 90 6765452; fax: +39 0 90 391382. E-mail address: [email protected] (F. Barreca). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.04.026
mainly occurs on the surface or at the interface. In particular it has been found that a decrease in the size of the TiO2 particles will enhance the total photoreactivity of TiO2 . Moreover, the size of the TiO2 particles is known to alter the width of the band gap and the band bending at the interfaces, and thus to influence their photochemical properties. Therefore, many attempts have been made to make TiO2 nanostructures and nanoparticles by a variety of methods, such as thermal hydrolysis, thermal decomposition, sol–gel technique, microemulsion, chemical vapour deposition [4]. Pulsed laser ablation, in vacuum or in appropriate gas atmosphere, has been widely applied to prepare thin film of different nature included the metal oxide films [5–11]. Recently, Laser Ablation in Liquids (LAL) has provided a new technique for synthesis of size controlled metallic and metallic oxide nanoparticles in colloidal as well as powdered phases. A large number of parameters are involved in the laser ablation in the liquid media technique, regarding both the source (i.e. laser wavelength, laser energy, pulse width, repetition rate) and the liquid media (i.e. composition, viscosity, dipole moment, dielectric constant and height of liquid above the target surface). Thus, a large number of available parameters to control size, shape, and morphology of nanostructured materials is one of the advantages of this very fast and very cheap approach [12]. To our knowledge, there are few reports about the synthesis and the study of the properties of TiO2 nanoparticles produced by LAL [13,14]. Even less work can be found about TiO2 colloidal solutions in distilled water synthesized via LAL using a Ti target. Indeed, no direct evidence of a large yield of small TiO2 nanoparticles with a diameter of a few nanometers (2–3 nm) is reported, unless high laser fluences are used [15–18].
F. Barreca et al. / Applied Surface Science 256 (2010) 6408–6412
In this respect, we report about the synthesis of titanium dioxide nanoparticles in distilled H2 O from a Ti target, studying in a systematic way the effects of the laser fluence up to 10 J cm−2 . The principal aim is to produce a narrow distribution of nanoparticles with very small diameters, suitable to increase their photocatalytic properties for applications in environmental, pollution and chemical resistant gas sensing fields. 2. Experimental TiO2 nanoparticles were produced by laser ablation of a metal titanium disk (99.7%) in distilled water varying the laser fluence between 1 and 10 J cm−2 . The target irradiation time was decreased from 30 min down to 10 min increasing the fluence to minimize solution losses due to boiling. For simplicity we labeled as A, B and C the colloidal solutions produced at lowest, mid and highest fluence, respectively. The metal disk, about 8 mm in diameter and 2 mm thick, was placed on the bottom of a rotary glass vessel filled with 5 mL of the solvent and the thickness of the water layer above the titanium target was about 10 mm. The target was irradiated with the second harmonic (532 nm) output of a 5 ns pulse duration Nd:YAG laser operating at 10 Hz; the produced radiation was reflected with a mirror and vertically supplied to the titanium disk after focusing it by a quartz lens with a focal length of 250 mm.
6409
The particles’ morphology was investigated using both TEM and HRTEM observations on a JEOL JEM 2000 FX and a JEOL JEM 2010 transmission electron microscopes, respectively. Digital images were acquired by a GATAN multiscan camera system. The chemical composition was investigated by means of XPS spectroscopy. The spectra were acquired using a K-Alpha system of Thermo Scientific, equipped with a monochromatic Al K˛ source (1486.6 eV) and operating with an analyzer in CAE mode with a pass energy of 50 eV and with a spot size of 400 m. Surface charging effects were avoided using an electron flood gun and the binding energy shifts were calibrated keeping the C1s position fixed at 285 eV. Electron micrographs’ measurements were carried out through depositing and evaporating in air at room temperature some drops of the colloid onto a carbon coated copper grid (TEM, HRTEM) and onto a c–Si substrate (XPS). The UV–vis spectra of the colloidal solutions were collected by means of a Lambda 2 UV–vis spectrometer in the range 200–1100 nm using quartz cells. 3. Results Transmission electron microscopy measurements of the samples were carried out to investigate the morphology and the size of the nanoparticles grown by ablating the Ti target at different laser fluences. In particular, Fig. 1(a–c) reports TEM views of the samples A–C, recorded 1 week after the production, and Fig. 2 illustrates the particles size distribution histogram obtained by the analysis of a
Fig. 1. (a–c) TEM views of the samples A–C collected after 1 week. (d) TEM views of a typical nanoparticle from sample B.
6410
F. Barreca et al. / Applied Surface Science 256 (2010) 6408–6412
Fig. 3. HRTEM view of the samples C and measurements of the interplanar distances.
Fig. 2. Histograms showing the distribution of nanoparticles diameters in samples A–C.
set of TEM micrographs of the same samples. We considered only nanoparticles in the 2–50 nm size range to achieve a reliable result and also because larger ones were uncommon (about 0.3%). It is important to evidence that all the three samples exhibit high density of nanoparticles smaller than 10 nm in diameter. In fact, the reported histograms show that almost all the nanoparticles in the sample A and about 55% in the samples B and C are less than 10 nm in size. Besides, more than 85% of the nanoparticles in the sample
A have a size smaller than 5 nm while the predominant ones in samples B and C are 5–7 nm diameter nanoparticles (about 30%). Furthermore, as early pointed out, in the sample A it is also evident the presence of some spherical particles agglomerations of about 100–200 nm (not shown), presumably generated by Ostwald ripening phenomena [19]. These agglomerations are more numerous in the samples produced at higher fluences (i.e. B and C), as shown in Fig. 1(b and c). In Fig. 1(d) we show a typical well faceted nanoparticle from sample B. High resolution TEM microscopy was then carried out to investigate the crystalline structure of the produced nanostructures. In Fig. 3 we show a particular HRTEM image taken from sample C, that is the largest fluence case. It evidences better the existence of
Fig. 4. X-ray photoelectron spectra of titanium dioxide nanoparticles on Si substrate: Ti2p (3/2, 1/2) core level on the left, O1s core level on the right.
F. Barreca et al. / Applied Surface Science 256 (2010) 6408–6412
small nanoparticles, about 5 nm in diameter, and their crystalline structure with the relative atomic planes. The interplanar distances of 0.17, 0.19 and 0.23 nm, measured on the nanoparticle on the right of this figure, are consistent with the reflections from the (1 0 5), (2 0 0) and (0 0 4) crystallographic planes of the TiO2 anatase phase. On the contrary, within the experimental resolution, the measured distances of 0.16, 0.17 and 0.21 nm on the nanoparticle shown on the left, could be consistent both with the reflections from the planes (2 2 0), (2 1 1) and (2 1 0) of the TiO2 rutile phase and with the (2 1 1), (1 0 5) and (0 0 4) ones belonging to the anatase phase [20]. In Fig. 4 are reported the XPS spectra of the nanoparticles: the subbands analysis of the Ti2p and O1s core level spectra of the samples is also showed. The Ti2p region may be decomposed with a fitting procedure into three contributions corresponding to the different oxidation states of titanium. Each contribution consists of a doublet of 2p3/2 and 2p1/2 peaks. Gauss–Lorentzian shape functions with the same FWHM for all the peaks were used for the deconvolution. For each doublet, the ratio of the area of the two peaks A (Ti2p3/2 )/A (Ti2p1/2 ) is fixed to 0.5 and the binding energy difference Eb = Eb (Ti2p1/2 ) − Eb (Ti2p3/2 ) is 5.7 ± 0.1 eV. The individual 2p3/2 components, with BE values located at approximately 456.4, 457.6, 458.5 eV, are attributed to Ti2+ (TiO), Ti3+ (Ti2 O3 ) and Ti4+ (TiO2 ) [21]. The O1s region may be decomposed with four contributions at about 528.5, 530.1, 531.2 and 532.2 attributed respectively to O–Ti2+ , O–Ti4+ , O–Ti3+ and OH− [22]. From the XPS analysis it is evident that the peaks due the TiO2 phase are almost always the predominant ones, although some suboxide phases are also significantly present. UV–vis absorption spectra were carried out in order to characterize the optical absorbance of colloidal solutions and to confirm the nature of the nanoparticles taking into account that for anatase, brookite and rutile structures in phase-pure powders optical band gap values of 3.19, 3.11 and 3.0, respectively have been reported [23]. Fig. 5 shows the UV–vis spectra collected immediately after the production of nano-colloids in distilled water. A strong absorption due to the TiO2 nanoparticles dominates the spectrum below 350 nm while a smooth decay towards longer wavelengths is observed. Moreover, the absorption intensity registered in the 200–300 nm wavelength range increases with increasing the laser fluence in spite of the shorter deposition time. This fact points out the effectiveness of an increased laser fluence in producing a larger concentration of the nanoparticles in the colloidal solution.
Fig. 5. Optical absorbance spectra of the TiO2 colloidal solutions deposited at low (dashed-dotted line), mid (dashed line) and high fluence (whole line), respectively.
6411
The onset of the optical absorption can be extrapolated to about 391, 374 and 380 nm for samples A–C respectively, that is 3.17, 3.32 and 3.26 eV as a function of photon energy. According to literature data [24], these values suggest that the predominant nano-species in the colloidal solutions could be anatase TiO2 (Eg = 3.2 eV) and, consequently, the obtained values can be ascribed to 3 → X1b indirect transition [25].
4. Discussion Today, it is well known that titanium dioxide has three most commonly encountered crystalline polymorphs: anatase, rutile and brookite and they are made up of differently distorted TiO6 octahedra. Recently, it has been reported [3] that rutile is the most stable phase for particles above 35 nm in size, anatase is the most stable phase for nanoparticles below 11 nm and brookite has been found to be the most stable for nanoparticles in the 11–35 nm range, although it is also claimed that anatase is the only nanocrystalline phase obtained [3]. Our electron microscopy measurements have evidenced the predominant presence of smaller than 5 nm size nanoparticles in sample A and of 5–7 nm nanoparticles in the remaining samples. Along with them, it was observed the existence of clusters of 100–200 nm whose content increases with the laser fluence. Therefore, ablation at higher fluence led to the creation of larger nanoparticles, larger aggregates of nanoparticles and, consequently, to broader size distributions. Moreover, if we compare their size and shape with the expectations of the thermodynamic model on the phase stability of TiO2 nanoparticles in different environments proposed by Bernard [4,26,27], we can attribute the predominant nanoparticles crystalline phase to the anatase one, in agreement with the results of optical absorption data. In particular, for example, the nanoparticle of Fig. 1(d) is identified as an anatase crystal with oxygenated surfaces. The crystallographic HRTEM analysis also shows the existence of nanoparticles belonging to the anatase phase and a possible presence of a smaller quantity of nanoparticles of the rutile phase, presumably with hydrogenated surfaces. It has been suggested that the anatase phase appears as the majority part of the nanoscale samples and it can transform to the rutile one upon reaching a particular size that depends on the synthesis conditions. However, in the sample produced at the highest laser fluence (C), we have presumably observed nanoparticles of both phases with the same small dimensions, even if only a minority of them belongs to the rutile phase. Such an occurrence could be explained with the existence, along with the predominant presence of 5–7 nm size anatase nanoparticles, of a small quantity of metastable rutile nanoparticles which are produced with such small dimensions (≈5 nm) due to the high energy involved in the ablation process. Considering the samples composition determined by means of XPS, it appears evident that our colloids are hydroxylated via adsorbed hydroxyl moieties from the ambient. This reaction is favored on the Ti3+ sites which, controlling this adsorptive activity [22], have a positive effect on the photocatalytic process due to the electron capturing process that reduces the recombination of electron and holes. Moreover, also the observed different amounts of surface OH groups play an important role in photocatalysis, where an increase in the surface hydroxyl content will enhance the photocatalytic activity. This can partially be explained by the fact that also OH groups can act as capture centers for the photoexcited electrons [24,28]. According to the reported considerations and to the XPS data, our samples should deserve a good photocalytic activity, and in particular the sample A (deposited at low fluence) due to its higher content of the OH groups and a fair presence of the Ti3+ sites.
6412
F. Barreca et al. / Applied Surface Science 256 (2010) 6408–6412
About the optical properties, we must note at first that the light absorption is small in indirect bandgap bulk semiconductors, like TiO2 , where the direct electronic transitions between the band edges are prohibited by the crystal symmetry. However, it has been reported an enhancement of the light absorption in TiO2 nanocrystallites due to indirect transitions with momentum nonconservation at the interface. This interface absorption becomes the main mechanism of light absorption for the crystallites that are smaller than 20 nm [4,29]. 5. Conclusion Titanium dioxide nanoparticles were synthesized ablating a Ti target in distilled H2 O solvent with laser fluences from 1 up to 10 J cm−2 . Electron microscopy imaging measurements have evidenced that low fluence ablation yields nanoparticles of size smaller than 10 nm. Increasing the fluence leads to the formation of larger nanoparticles and of their aggregates with distributions broader in size. In particular, at low laser fluence the particles’ size distribution shows that more than 85% of the nanoparticles have a size smaller than 5 nm while, at mid and high fluence, only about 30% of them have a size of 5–7 nm. In the latter case, the occurrence of 100–200 nm clusters is also evident. A XPS analysis has evidenced that the nanoparticles incorporate different suboxide phases with the prevalence of Ti–O bonds from TiO2 species. Both anatase and rutile crystalline polymorphs were observed, being the first predominant as supported by UV–vis absorption spectra. A higher photocatalytic activity can be supposed for the sample produced at low fluence due to a higher content of small size nanoparticles, in addition to the presence of both OH groups and Ti3+ sites. Then, an evaluation of the photocatalytic properties of the samples will be the next step of our studies. Acknowledgments We thank Salvatore Pannitteri and Corrado Bongiorno of the CNR-IMM institute of Catania (Italy) for their valuable help in TEM analyses.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
A. Fujishima, K. Honda, Nature 238 (1972) 37. A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1. A. Fujishima, X. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515. X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891. F. Neri, F. Barreca, S. Trusso, Diamond Relat. Mater. 11 (2002) 273. S. Trusso, E. Barletta, F. Barreca, F. Neri, Appl. Phys. A: Mater. Sci. Process. 79 (2004) 1997. E. Fazio, E. Barletta, F. Barreca, F. Neri, S. Trusso, J. Vac. Sci. Technol. B 23 (2) (2005) 519. E. Fazio, E. Barletta, F. Barreca, G. Mondio, F. Neri, S. Trusso, Spriger Ser.: Top. Appl. Phys. 100 (2006) 287. F. Neri, F. Barreca, E. Barletta, E. Fazio, G. Mondio, S. Trusso, H. Modrow, B. Brendebach, J. Vac. Sci. Technol. A 25 (1) (2007) 117. S. Yamamoto, T. Sumita, A. Sugiharuto, H. Miyashita, Naramoto, Thin Solid Films 401 (2001) 88. D. Luca, D. Macovei, C.-M. Teodorescu, Surf. Sci. 600 (2006) 4342. S.C. Singh, R.K. Swarnkar, R. Gopal, J. Nanopart. Res. 11 (2009) 1831. T. Sasaki, C. Liang, W.T. Nichols, Y. Shimizu, N. Koshizaki, Appl. Phys. A 79 (2004) 1489. X. Huang, W. Zhang, Appl. Surf. Sci. 254 (2008) 3403. S.I. Dolgaev, A.V. Simakin, V.V. Voronov, G.A. Shafeev, F. Bozon-Verduraz, Appl. Surf. Sci. 186 (2002) 546. C.H. Liang, Y. Shimizu, T. Sasaki, N. Koshizaki, Appl. Phys. A 80 (2005) 819. A.S. Nikolov, P.A. Atanasov, D.R. Milev, T.R. Stoyanchov, A.D. Deleva, Z.Y. Peshev, Appl. Surf. Sci. 255 (2009) 5351. S.C. Singh, R.K. Swarnkar, R. Gopal, J. Nanosci. Nanotechnol. 9 (2005) 5367. Guozhong Cao, Synthesis Properties & Applications, Imperial College Press, London, 2004. http://rruff.geo.arizona.edu/AMS/amcsd.php. J. Yu, X. Zhao, Q. Zhao, Mater. Chem. Phys. 69 (2001) 21. P.M. Kumar, S. Badrinarayanan, M. Sastry, Thin Solid Films 358 (2000) 122. J.G. Li, T. Ishigaki, X. Sun, J. Phys. Chem. C 13 (2007) 111. H. Jensen, A. Soloviev, Z. Li, E.G. Sogaard, Appl. Surf. Sci. 246 (2005) 239. N. Serpone, D. Lawless, R. Khairutdinov, J. Phys. Chem. 99 (1995) 16646. A.S. Barnard, L.A. Curtiss, Nano Lett. 5 (2005) 1261. A.S. Barnard, P. Zapol, L.A. Curtiss, J. Chem. Theory Comput. 1 (2005) 107. J. Yu, X. Zhao, J. Du, W. Chen, J. Sol–gel Sci. Technol. 17 (2000) 163. L. Braginsky, V. Shklover, Eur. Phys. J. D 9 (1999) 627.