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Laser ablation synthesis of indium oxide nanoparticles in water
Applied Surface Science 256 (2010) 6918–6922
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Laser ablation synthesis of indium oxide nanoparticles in water N. Acacia a , F. Barreca b,∗ , E. Barletta b , D. Spadaro b , G. Currò b , F. Neri a a b
Dipartimento di Fisica della Materia e Ingegneria Elettronica, Università di Messina, Salita Sperone 31, I-98166 Messina, Italy Advanced and Nano Materials Research s.r.l., Salita Sperone 31, I-98166 Messina, Italy
a r t i c l e
i n f o
Article history: Received 28 October 2009 Received in revised form 30 April 2010 Accepted 1 May 2010 Available online 10 May 2010 Keywords: Indium oxide nanoparticles Laser ablation in liquid Morphology Chemical composition
a b s t r a c t Colloidal solutions of Indium oxide nanoparticles have been produced by means of laser ablation in liquids (LALs) technique by simply irradiating with a second harmonic (532 nm) Nd:YAG laser beam a metallic indium target immersed in distilled water and varying the laser fluence up to 10 J cm−2 and the ablation time up to 120 min. At all the investigated fluences the vaporization process of the indium target is the dominant one. It produces a majority (>80%) of small size (<6 nm) nanoparticles, with a very limited content of larger ones (size between 10 and 20 nm). The amount of particles increases regularly with the ablation time, supporting the scalability of the production technique. The deposited nanoparticles stoichiometry has been verified by both X-ray photoelectron spectroscopy (XPS) and Energy Dispersive X-ray (EDX) analysis. Optical bandgap values of 3.70 eV were determined by UV–vis absorption measurements. All these results confirm the complete oxidation of the ablated material. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Transparent conducting oxides (TCOs) have been studied for many years because of their wide range of applications. Indium oxide (In2 O3 ) is an important TCO material because of its unique properties such as high electrical conductivity and optical transparency in the visible region. For example, it has been used successfully for the conductive top layers in solar cells, flat panel displays and low-emissivity windows [1]. Moreover, the wide band gap tunability, due to quantum confinement effects in very small nanoparticles, boosted its application in UV detectors or as an active layer in chemiresistive sensing of toxic oxidizing gases like ozone, CO and NO2 [2–4]. Many attempts have been made to produce In2 O3 nanoparticles by a variety of techniques, such as thermal hydrolysis [5], microwave irradiation [6], thermal decomposition [7], sol–gel technique [8], microemulsion [9], reactive magnetron sputtering [10], spray pyrolysis [11], mechanochemical processing [12], laser photolysis [13] and pulsed laser ablation [14]. Each of these fabrication techniques has provided well defined systems, with properly tuned stoichiometry and size distribution. However all these methods are more or less demanding in terms of energy consumption, costs, eco-compatibility and safety. The goal being the achievement of a few nanometer sized particles with a narrow size distribution, to be used for building up a very high surface to volume ratio nanostructured material, with the above mentioned physico-chemical properties.
∗ Corresponding author. 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.2010.05.003
Laser ablation in liquids (LALs) provides a new technique for the synthesis of size controlled nanoparticles in colloidal as well as powder phases, usually in a one step top–down procedure [15,16]. In particular, experiments on the formation of oxide nanoparticles by laser ablation of pure metals in water were already reported by several authors, showing as LAL is a reliable alternative to traditional chemical reduction methods [17–20]. Size, shape and morphology of the produced particles depend critically on several ablation parameters such as: laser fluence and frequency, pulse width, repetition rate, temperature, height of the liquid above the target surface and, when used, surfactant/stabilizing agent topology and concentration. The advantage of this technique is that, being it effective, simple and consequently cheap, it will deserve a significative potential for industrial and massive production, once relevant optimization and integration are carried out. As a matter of facts, LAL involves only an appropriate pulsed laser power source and a reactionvessel/pipeline network. The fact that it does not require vacuum conditions makes the LAL technique very convenient, either in terms of production costs and power consumption. Moreover, the intrinsic ability to produce stable colloids without the a-priori need for any aggressive chemicals, like reducing or capping agents, makes LAL particularly attractive as an eco-compatible technique. To our knowledge, only Ganeev et al. reported about the preparation of metallic indium nanoparticles (average size above 40 nm) by ablation in ethanol and water, investigating their optical properties [21]. On the other hand, by using LAL in water, we successfully prepared indium oxide nanoparticles colloids, with good stoichiometry, sizes well below 20 nm and a properly tuned structure. In particular, the influence of the ablation process control parameters on the nanoparticles morphology has been investigated, by
N. Acacia et al. / Applied Surface Science 256 (2010) 6918–6922 Table 1 Samples examined and ablation parameters (laser fluence F, ablation time t) adopted for each of them. Sample label
F (J cm−2 )
t (min)
A B C D E F G H I
0.5 1 3 10 3 3 1 3 5
30 30 30 10 60 120 10 10 10
varying the operative laser fluence up to 10 J cm−2 and testing the ablation time up to 120 min.
2. Experimental Indium oxide nanoparticles in water colloids were produced by irradiating a metallic indium target immersed in distilled water with a pulsed laser beam. The target was pure indium (99.7%) with a thickness of 1 mm, obtained by Koch-Light Laboratories Ltd. Before immersion and irradiation, the indium target was cleaned in acetone. The target was positioned on an off-axis rotating target holder and immersed in the ablation vessel filled with distilled water. The water level above the target was 10 mm at rest. The ablation was performed with the second harmonic (532 nm) of a Nd:YAG laser (New Wave Mod. Tempest 300) operating at 10 Hz repetition rate, with a pulse width of about 5 ns. The beam was focused on the surface of the target through a mirror tilted at 45◦ and a lens with 20 cm of focal length. The spot size was about 1.5 mm in diameter, as confirmed by the measurement of the spot trace with optical microscopy. The laser fluence on the target was determined by measuring both the laser energy per pulse and the beam spot area. Experiments at 0.5, 1, 3, 5 and 10 J cm−2 fluence were carried out with a duration time of 10 or 30 min. The highest fluence experiment lasted 10 min only, due to the boiling effect of the water. A second run of experiments was carried out at 3 J cm−2 with an ablation time of 60 and 120 min. Table 1 summarizes the preparation conditions of the samples examined. Structural and chemical characterization were carried out by depositing and evaporating in air at room temperature some drops of the colloid onto carbon coated copper grids for TEM analysis and onto crystalline silicon wafers for SEM and XPS ones. The size and shape of the indium oxide nanoparticles were examined by TEM analysis, using a JEOL JEM 2000 FX microscope operating at 200 kV. A SEM equipped with EDX was used to measure the presence of larger size oxide nanoparticles and to evaluate the bulk atomic composition. Moreover, XPS spectroscopy was used to determine the samples chemical composition. 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. UV–vis–NIR absorption spectroscopy measurements were carried out on a Perkin-Elmer LAMBDA 2 spectrophotometer, to measure the optical bandgap and to monitor the amount of indium oxide nanoparticles in the water colloids.
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3. Results and discussion The synthesis of nanomaterials by means of the application of laser ablation in liquids is quite recent [22] and a comprehensive understanding of the full process is still lacking. The interaction of a laser beam with a solid involves different physical processes depending on the energy density released to the target material. Vaporization and explosive processes are the ablation mechanisms observed simultaneously to the production of a plasma confined near the target by the liquid. Concerning the nanoparticles formation by LAL, the predominant mechanism consists in the nucleation during the plasma cooling followed by the nuclei growth and coalescence [15,20]. The size of the nanoparticles depends prevalently on the density of atoms (metal atoms in our case), during the nucleation and growth processes, and on the temperature. Consequently the size distributions depend on the ablation mechanisms. In particular, nanoparticles with small diameters (3–6 nm) are generally associated with low laser fluences and originate from the vaporization process of the target. On the contrary, larger nanoparticles (tens of nanometers) are associated with high laser fluences and due to the phase explosion process that occurs on the target surface [20,23,24]. In this contest, to produce nanoparticles with sizes below 20 nm, we choose laser fluences values in the 0.5–10 J cm−2 range where the vaporization process of the target should be the predominant one. Such an hypothesis, in absence of data concerning indium targets, has been experimentally verified in the case of aluminum targets, which can be considered sufficiently similar [25]. A TEM analysis, carried out on sample B and reported in Fig. 1, clearly evidences the presence of the nanoparticles. Generally, the presence of sizeable nanoparticles aggregates was detected, which suggests a relatively poor stability of the colloid during the ablation process. The coexistence of about two different nanoparticles diameters is evident from TEM images, as illustrated in Fig. 1a and b. The particles size distribution histogram, reported in the inset of Fig. 1, was obtained by the analysis of a set of TEM micrographs taken in different places of the same sample. Bimodal size distributions are detected, with a high fraction (85%) of small spherical particles with 2–5 nm diameters and a less frequent population (12%) of big nanoparticles of 10–20 nm. We considered only nanoparticles in the 2–20 nm size range to achieve a reliable result and also because the larger ones were uncommon (about 0.1%). This result confirms the efficiency of the LAL technique to obtain a narrow distribution of small nanoparticles with size comparable or smaller than the ones produced with other preparation methods [26–28]. Under dark field analysis, most of the particles appeared to be amorphous. Only in a few cases, some of the largest nanoparticles showed a crystalline structure. This aspect, typical of the above mentioned [15,20] formation mechanism and of the fast plasma cooling, was not investigated further. However, TEM results, by themselves, do not give informations about the overall qualitative structure and the compositional nature of the observed nanoparticles. Therefore a SEM analysis with EDX probe was carried out on all the samples after the deposition of the colloids on silicon substrates and a natural drying process at room temperature. A nanoporous film is observed in all cases, as shown for example in Fig. 2 in the case of samples C and F (Fig. 2a and b). A qualitative look reveals an evident increase of the larger nanoparticles (10–20 nm size range) number for longer ablation times. The results of the EDX analysis are reported in Table 2 and refer to samples C, E and F, prepared by ablation at 3 J cm−2 , for different times. The presence of oxygen concentrations exceeding the 3:2 ratio, with respect to In (i.e. In2 O3 ), together with significant amounts of carbon and nitrogen is typical of these nanostructured films and may be due to a getter ability of indium oxide nanoparti-
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Fig. 1. TEM images of the sample B showing nanoparticles with a diameter of 2–5 nm (a) and with a diameter of 10–20 nm (b). In the inset an histogram showing the distribution of nanoparticles diameters in the sample B.
cles against volatile species in the atmospheric ambient. A detailed analysis of such a possible getter mechanism is currently under investigation. An evaluation of the deposited nanoparticles composition was obtained from an XPS analysis carried out, in particular, on sample B. The In 3d photoelectron peak, showed in Fig. 3, is characterized by the presence of a spin-orbit doublet, namely In 3d5/2 and 3d3/2 core levels. The doublet peaks positions are located at 444.7 and 452.3 eV, respectively, which correspond to In2 O3 compared with the reported In 3d5/2 peak of metallic indium which occurs below 444 eV [27,29]. The stoichiometric ratio O/In = 1.7, calculated using the peaks area and the appropriate sensitivity factors, is quite close to the stoichiometric value of In2 O3 (O/In = 1.5). The small deviation can be ascribed to the presence of chemisorbed oxygen on the nanoparticles surface, in agreement with the EDX analysis results. For a semi-quantitative analysis of the nanostructures in the colloidal solutions, UV–vis optical absorption measurements were carried out on the samples in colloidal suspension phase. The typical UV–vis absorption spectra of indium oxide nanoparticles are shown in Fig. 4, after the distilled water background subtraction. In Fig. 4a, the optical absorption spectra of the fresh nanocolloids, obtained by increasing the laser fluence from 1 to 10 J cm−2
Fig. 2. SEM images of the sample C (a) and sample F (b), after deposition on a silicon substrate. Table 2 EDX derived chemical composition (atomic %) of samples C, E and F after deposition on silicon substrates. Spectral lines
Atomic % Samples
CK NK OK Si K In L
C
E
F
7 6 51 20 16
7 7 57 10 19
6 8 48 18 20
Fig. 3. X-ray photoelectron spectrum of indium oxide nanoparticles deposited on Si substrate: In 3d (5/2, 3/2) core level of sample B.
N. Acacia et al. / Applied Surface Science 256 (2010) 6918–6922
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Fig. 4. UV–vis absorption spectra of the samples prepared as a function of the laser fluence for 10 min of process (a) and at the fixed laser fluence of 3 J cm−2 in 10, 30, 60 and 120 min (b).
and keeping fixed the ablation time at 10 minutes (samples G, H, I and D), are shown. The spectra are characterized by a progressively increasing absorption going towards shorter wavelengths and by the presence of a shoulder at about 260 nm, in agreement with the presence of In2 O3 nanoparticles in the colloids, although somewhat shorter than values reported in the literature [26,30]. Upon increasing the laser fluence, the lineshape is unchanged unless for an overall rise of the absorption intensity at shorter wavelengths. Taking into account the increase of scattering effects at short wavelengths, this behavior suggests that no significant change has occurred in the size distribution of the nanoparticles, but only an increase of the number of the smallest size ones. An analogous trend was observed in the 30 min ablated samples (A, B and C, not shown) [31]. In Fig. 4b the UV–vis absorption spectra of H, C, E and F samples are reported. In this case, the laser fluence is fixed at 3 J cm−2 , while the ablation time is increased up to 120 min. The characteristic shoulder at 260 nm is visible only for the shorter ablation time cases (10 and 30 min). For longer times, a general growth of the absorption intensity along all the investigated spectral range is observed. This behavior is consistent with the presence of multiple scattering phenomena that mask the intrinsic absorption ones (as the shoulder located at about 260 nm) due to the increased density of nanoparticles per unit volume [31]. When multiple scattering effects can be neglected (i.e. for low ablation times), a closer examination of the spectra can provide a valuable information about the optical gap Eg width modulation, in correlation to possible quantum confinement effects. Indeed, it is well known that bulk indium oxide is a large direct band gap semiconductor (Eg = 3.70 eV [1]) and, in the parabolic bands and constant transition matrix element approximations [32], the absorbance of the material at a given wavelength can be expressed as: ˛(h) = Bd (h − Eg )
1/2
where Bd is a constant of the material, h is the energy of excitation and ˛ is the absorption coefficient (cm−1 ) that is proportional to the measured absorbance. Therefore the intercept on the energy axis of a plot of the square of the absorbance versus the photon energy yields the Eg value for an allowed direct transition. For all the samples the extrapolated band gap is about 3.70 eV. At variance with other reports [1,26], no evidence of a gap width modulation, correlated to the NP size, was found in the samples. This means that in the present case any quantum size effect is absent or, possibly, too weak to be detected. The absence of metallic indium in the deposited nanoparticles, as evidenced by XPS analysis and more indirectly from the
EDX ones, suggests the complete oxidation of the ablated material. These results confirm the predicted enhancement of the species chemical reactivity which takes place near the target, due to the high pressure and high temperature conditions established at the interface between the expanding plasma and the confining liquid. Then, analogously to what proposed for the ablation of Zn and Cu in water [17,19], the plasma cooling stage favors the formation of small metallic clusters, that interacting with the solvent form their hydroxy derivates followed by the decomposition to produce the indium oxide nanoparticles. In the investigated laser fluence range, the vaporization process appears to be the really dominant one, since a very large density of small spherical nanoparticles (<20 nm) were always obtained, whose amount scales progressively with the laser fluence. 4. Conclusion The reported study demonstrates, to our best knowledge for the first time, the effective production of indium oxide nanoparticles by means of pulsed laser ablation in water, for different laser fluence and ablation time values. A large density of spherical nanoparticles is obtained in all cases, with the size falling into two main ranges, 2–5 nm (≈85%) and 10–20 nm (≈12%), respectively. No significant evidence of larger particles is found. SEM–EDX observations on the deposited films reveal the presence of indium, a large content of oxygen and, to a lesser extent, carbon and nitrogen. These findings are presumably due to a getter ability of indium oxide against the atmospheric ambient volatile species. The XPS analysis confirms the oxidized nature of the deposited nanoparticles with a slightly over stoichiometric ratio O/In = 1.7 due to the chemisorbed oxygen on the surface. The UV–vis analysis returns an optical gap extension identical for all the nanoparticle colloids and similar to the bulk In2 O3 value, that is no quantum size effects are evidenced. According with the literature, the morphological results confirm that the vaporization process of the target is the dominant one in the investigated laser fluence range. Moreover, the complete oxidation of the ablated material is obtained thanks to the extreme conditions of the laser plasma produced near the target surface and to its interaction with the surrounding liquid. The total amount of nanoparticles in the colloid unit volume depends on the density of indium atoms produced during the ablation process. This density increases with the laser fluence, whilst a prolonged laser irradiation, at a fixed fluence, seems to favor the formation of a limited number of larger nanoparticles, about 20 nm in diameter. All the above findings point out the strong and promising potentiality of laser ablation in liquids as a scalable technique to produce
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small size indium oxide nanoparticle colloids in a simple and ecosustainable way. Acknowledgments We thank Salvatore Pannitteri of the CNR-IMM institute of Catania (Italy) for his valuable help in TEM analyses. References [1] D.P. Dutta, V. Sudarsan, P. Srinivasu, A. Vinu, A.K. Tyagi, J. Phys. Chem. C 112 (2008) 6781. [2] G. Kiriakidis, M. Bender, N. Katsarakis, E. Gagaoudakis, E. Hourdakis, E. Douloufakis, V. Cimalla, Phys. Status Solidi (A) 185 (2001) 27. [3] G. Neri, A. Bonavita, G. Micali, G. Rizzo, E. Callone, G. Carturan, Sens. Actuators B 132 (2008) 224. [4] H. Steffes, C. Imawan, F. Solzbacher, E. Obermeier, Sens. Actuators B 78 (2001) 106. [5] L.A. Perez-Maqueda, L. Wang, E. Matijevic, Langmuir 14 (1998) 4397. [6] C.R. Patra, A. Gedanken, N. J. Chem. 28 (2004) 1060. [7] W.S. Seo, H.H. Jo, K.Y. Lee, J.T. Park, Adv. Mater. 15 (2003) 795. [8] A. Gurlo, M. Ivanovskaya, Sens. Actuators B 44 (1997) 327. [9] X. Wu, J. Vac. Sci. Technol. 15 (1997) 1889. [10] A.S. Ryzhikov, R.B. Vasiliev, M.N. Rumyantseva, Mater. Sci. Eng. B 96 (2002) 268. [11] J.J. Prince, S. Ramamurthy, B. Subramanian, J. Cryst. Growth 240 (2002) 142. [12] H. Yang, A. Tang, X. Zhang, W. Yang, G. Qiu, Scr. Mater. 50 (2004) 413. [13] T. Tsuchiya, A. Watanabe, H. Niino, A. Yabe, I. Yamaguchi, T. Manabe, T. Kumagai, S. Mizuta, Appl. Surf. Sci. 186 (2002) 173.
[14] A. Murali, A. Barve, V.J. Leppert, S.H. Risbud, I.M. Kennedy, H.W.H. Lee, Nano Lett. 1 (2001) 287. [15] G.W. Yang, Prog. Mater. Sci. 52 (2007) 648. [16] A.V. Simakin, V.V. Voronov, G.A. Shafeev, Phys. Wave Phenom. 15 (2007) 218. [17] H. Zeng, W. Cai, Y. Li, J. Hu, P. Liu, J. Phys. Chem. B 109 (2005) 18260. [18] T. Tsuji, T. Hamagami, T. Kawamura, J. Yamaki, M. Tsuji, Appl. Surf. Sci. 243 (2005) 214. [19] K. Amikura, T. Kimura, M. Hamada, N. Yokoyama, J. Miyazaki, Y. Yamada, Appl. Surf. Sci. 254 (2008) 6976. [20] V. Amendola, M. Meneghetti, Phys. Chem. Chem. Phys. 11 (2009) 3805. [21] R.A. Ganeev, A.I. Ryasnyanskiy, U. Chakravarty, P.A. Naik, H. Srivastava, M.K. Tiwari, P.D. Gupta, Appl. Phys. B 86 (2007) 337. [22] A. Fojtik, A. Henglein, Ber. Bunsen-Ges. Phys. Chem. 97 (1993) 252. [23] A.V. Kabashin, M. Meunier, J. Appl. Phys. 94 (2003) 7941. [24] W.T. Nichols, T. Sasaki, N. Koshizaki, J. Appl. Phys. 100 (2006) 114912. [25] J.M. Fishburn, M.J. Withford, D.W. Coutts, J.A. Piper, Appl. Surf. Sci. 252 (2006) 5182. [26] S. Maensiri, P. Laokul, J. Klinkaewnarong, S. Phokha, V. Promarak, S. Seraphin, J. Optoelectron. Adv. Mater. 10 (2008) 161. [27] Z. Li, X. Tao, Y. Cheng, Z. Wu, Z. Zhang, H. Dang, Mater. Sci. Eng. A 407 (2005) 7. [28] M. Niederberger, G. Garnweitner, J. Buha, J. Polleux, J. Ba, N. Pinna, J. Sol–Gel Sci. Technol. 40 (2006) 259. [29] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1992, p. 125. [30] X. Chen, Z. Zhang, X. Zhang, J. Liu, Y. Qian, Chem. Phys. Lett. 407 (2005) 482. [31] C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by Small Particles, Wiley-Interscience, New York, 1998. [32] J.I. Pankove, Optical Processes in Semiconductors, Dover Publ. Inc., New York, 1975.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Laser ablation synthesis of indium oxide nanoparticles in water N. Acacia a , F. Barreca b,∗ , E. Barletta b , D. Spadaro b , G. Currò b , F. Neri a a b
Dipartimento di Fisica della Materia e Ingegneria Elettronica, Università di Messina, Salita Sperone 31, I-98166 Messina, Italy Advanced and Nano Materials Research s.r.l., Salita Sperone 31, I-98166 Messina, Italy
a r t i c l e
i n f o
Article history: Received 28 October 2009 Received in revised form 30 April 2010 Accepted 1 May 2010 Available online 10 May 2010 Keywords: Indium oxide nanoparticles Laser ablation in liquid Morphology Chemical composition
a b s t r a c t Colloidal solutions of Indium oxide nanoparticles have been produced by means of laser ablation in liquids (LALs) technique by simply irradiating with a second harmonic (532 nm) Nd:YAG laser beam a metallic indium target immersed in distilled water and varying the laser fluence up to 10 J cm−2 and the ablation time up to 120 min. At all the investigated fluences the vaporization process of the indium target is the dominant one. It produces a majority (>80%) of small size (<6 nm) nanoparticles, with a very limited content of larger ones (size between 10 and 20 nm). The amount of particles increases regularly with the ablation time, supporting the scalability of the production technique. The deposited nanoparticles stoichiometry has been verified by both X-ray photoelectron spectroscopy (XPS) and Energy Dispersive X-ray (EDX) analysis. Optical bandgap values of 3.70 eV were determined by UV–vis absorption measurements. All these results confirm the complete oxidation of the ablated material. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Transparent conducting oxides (TCOs) have been studied for many years because of their wide range of applications. Indium oxide (In2 O3 ) is an important TCO material because of its unique properties such as high electrical conductivity and optical transparency in the visible region. For example, it has been used successfully for the conductive top layers in solar cells, flat panel displays and low-emissivity windows [1]. Moreover, the wide band gap tunability, due to quantum confinement effects in very small nanoparticles, boosted its application in UV detectors or as an active layer in chemiresistive sensing of toxic oxidizing gases like ozone, CO and NO2 [2–4]. Many attempts have been made to produce In2 O3 nanoparticles by a variety of techniques, such as thermal hydrolysis [5], microwave irradiation [6], thermal decomposition [7], sol–gel technique [8], microemulsion [9], reactive magnetron sputtering [10], spray pyrolysis [11], mechanochemical processing [12], laser photolysis [13] and pulsed laser ablation [14]. Each of these fabrication techniques has provided well defined systems, with properly tuned stoichiometry and size distribution. However all these methods are more or less demanding in terms of energy consumption, costs, eco-compatibility and safety. The goal being the achievement of a few nanometer sized particles with a narrow size distribution, to be used for building up a very high surface to volume ratio nanostructured material, with the above mentioned physico-chemical properties.
∗ Corresponding author. 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.2010.05.003
Laser ablation in liquids (LALs) provides a new technique for the synthesis of size controlled nanoparticles in colloidal as well as powder phases, usually in a one step top–down procedure [15,16]. In particular, experiments on the formation of oxide nanoparticles by laser ablation of pure metals in water were already reported by several authors, showing as LAL is a reliable alternative to traditional chemical reduction methods [17–20]. Size, shape and morphology of the produced particles depend critically on several ablation parameters such as: laser fluence and frequency, pulse width, repetition rate, temperature, height of the liquid above the target surface and, when used, surfactant/stabilizing agent topology and concentration. The advantage of this technique is that, being it effective, simple and consequently cheap, it will deserve a significative potential for industrial and massive production, once relevant optimization and integration are carried out. As a matter of facts, LAL involves only an appropriate pulsed laser power source and a reactionvessel/pipeline network. The fact that it does not require vacuum conditions makes the LAL technique very convenient, either in terms of production costs and power consumption. Moreover, the intrinsic ability to produce stable colloids without the a-priori need for any aggressive chemicals, like reducing or capping agents, makes LAL particularly attractive as an eco-compatible technique. To our knowledge, only Ganeev et al. reported about the preparation of metallic indium nanoparticles (average size above 40 nm) by ablation in ethanol and water, investigating their optical properties [21]. On the other hand, by using LAL in water, we successfully prepared indium oxide nanoparticles colloids, with good stoichiometry, sizes well below 20 nm and a properly tuned structure. In particular, the influence of the ablation process control parameters on the nanoparticles morphology has been investigated, by
N. Acacia et al. / Applied Surface Science 256 (2010) 6918–6922 Table 1 Samples examined and ablation parameters (laser fluence F, ablation time t) adopted for each of them. Sample label
F (J cm−2 )
t (min)
A B C D E F G H I
0.5 1 3 10 3 3 1 3 5
30 30 30 10 60 120 10 10 10
varying the operative laser fluence up to 10 J cm−2 and testing the ablation time up to 120 min.
2. Experimental Indium oxide nanoparticles in water colloids were produced by irradiating a metallic indium target immersed in distilled water with a pulsed laser beam. The target was pure indium (99.7%) with a thickness of 1 mm, obtained by Koch-Light Laboratories Ltd. Before immersion and irradiation, the indium target was cleaned in acetone. The target was positioned on an off-axis rotating target holder and immersed in the ablation vessel filled with distilled water. The water level above the target was 10 mm at rest. The ablation was performed with the second harmonic (532 nm) of a Nd:YAG laser (New Wave Mod. Tempest 300) operating at 10 Hz repetition rate, with a pulse width of about 5 ns. The beam was focused on the surface of the target through a mirror tilted at 45◦ and a lens with 20 cm of focal length. The spot size was about 1.5 mm in diameter, as confirmed by the measurement of the spot trace with optical microscopy. The laser fluence on the target was determined by measuring both the laser energy per pulse and the beam spot area. Experiments at 0.5, 1, 3, 5 and 10 J cm−2 fluence were carried out with a duration time of 10 or 30 min. The highest fluence experiment lasted 10 min only, due to the boiling effect of the water. A second run of experiments was carried out at 3 J cm−2 with an ablation time of 60 and 120 min. Table 1 summarizes the preparation conditions of the samples examined. Structural and chemical characterization were carried out by depositing and evaporating in air at room temperature some drops of the colloid onto carbon coated copper grids for TEM analysis and onto crystalline silicon wafers for SEM and XPS ones. The size and shape of the indium oxide nanoparticles were examined by TEM analysis, using a JEOL JEM 2000 FX microscope operating at 200 kV. A SEM equipped with EDX was used to measure the presence of larger size oxide nanoparticles and to evaluate the bulk atomic composition. Moreover, XPS spectroscopy was used to determine the samples chemical composition. 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. UV–vis–NIR absorption spectroscopy measurements were carried out on a Perkin-Elmer LAMBDA 2 spectrophotometer, to measure the optical bandgap and to monitor the amount of indium oxide nanoparticles in the water colloids.
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3. Results and discussion The synthesis of nanomaterials by means of the application of laser ablation in liquids is quite recent [22] and a comprehensive understanding of the full process is still lacking. The interaction of a laser beam with a solid involves different physical processes depending on the energy density released to the target material. Vaporization and explosive processes are the ablation mechanisms observed simultaneously to the production of a plasma confined near the target by the liquid. Concerning the nanoparticles formation by LAL, the predominant mechanism consists in the nucleation during the plasma cooling followed by the nuclei growth and coalescence [15,20]. The size of the nanoparticles depends prevalently on the density of atoms (metal atoms in our case), during the nucleation and growth processes, and on the temperature. Consequently the size distributions depend on the ablation mechanisms. In particular, nanoparticles with small diameters (3–6 nm) are generally associated with low laser fluences and originate from the vaporization process of the target. On the contrary, larger nanoparticles (tens of nanometers) are associated with high laser fluences and due to the phase explosion process that occurs on the target surface [20,23,24]. In this contest, to produce nanoparticles with sizes below 20 nm, we choose laser fluences values in the 0.5–10 J cm−2 range where the vaporization process of the target should be the predominant one. Such an hypothesis, in absence of data concerning indium targets, has been experimentally verified in the case of aluminum targets, which can be considered sufficiently similar [25]. A TEM analysis, carried out on sample B and reported in Fig. 1, clearly evidences the presence of the nanoparticles. Generally, the presence of sizeable nanoparticles aggregates was detected, which suggests a relatively poor stability of the colloid during the ablation process. The coexistence of about two different nanoparticles diameters is evident from TEM images, as illustrated in Fig. 1a and b. The particles size distribution histogram, reported in the inset of Fig. 1, was obtained by the analysis of a set of TEM micrographs taken in different places of the same sample. Bimodal size distributions are detected, with a high fraction (85%) of small spherical particles with 2–5 nm diameters and a less frequent population (12%) of big nanoparticles of 10–20 nm. We considered only nanoparticles in the 2–20 nm size range to achieve a reliable result and also because the larger ones were uncommon (about 0.1%). This result confirms the efficiency of the LAL technique to obtain a narrow distribution of small nanoparticles with size comparable or smaller than the ones produced with other preparation methods [26–28]. Under dark field analysis, most of the particles appeared to be amorphous. Only in a few cases, some of the largest nanoparticles showed a crystalline structure. This aspect, typical of the above mentioned [15,20] formation mechanism and of the fast plasma cooling, was not investigated further. However, TEM results, by themselves, do not give informations about the overall qualitative structure and the compositional nature of the observed nanoparticles. Therefore a SEM analysis with EDX probe was carried out on all the samples after the deposition of the colloids on silicon substrates and a natural drying process at room temperature. A nanoporous film is observed in all cases, as shown for example in Fig. 2 in the case of samples C and F (Fig. 2a and b). A qualitative look reveals an evident increase of the larger nanoparticles (10–20 nm size range) number for longer ablation times. The results of the EDX analysis are reported in Table 2 and refer to samples C, E and F, prepared by ablation at 3 J cm−2 , for different times. The presence of oxygen concentrations exceeding the 3:2 ratio, with respect to In (i.e. In2 O3 ), together with significant amounts of carbon and nitrogen is typical of these nanostructured films and may be due to a getter ability of indium oxide nanoparti-
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Fig. 1. TEM images of the sample B showing nanoparticles with a diameter of 2–5 nm (a) and with a diameter of 10–20 nm (b). In the inset an histogram showing the distribution of nanoparticles diameters in the sample B.
cles against volatile species in the atmospheric ambient. A detailed analysis of such a possible getter mechanism is currently under investigation. An evaluation of the deposited nanoparticles composition was obtained from an XPS analysis carried out, in particular, on sample B. The In 3d photoelectron peak, showed in Fig. 3, is characterized by the presence of a spin-orbit doublet, namely In 3d5/2 and 3d3/2 core levels. The doublet peaks positions are located at 444.7 and 452.3 eV, respectively, which correspond to In2 O3 compared with the reported In 3d5/2 peak of metallic indium which occurs below 444 eV [27,29]. The stoichiometric ratio O/In = 1.7, calculated using the peaks area and the appropriate sensitivity factors, is quite close to the stoichiometric value of In2 O3 (O/In = 1.5). The small deviation can be ascribed to the presence of chemisorbed oxygen on the nanoparticles surface, in agreement with the EDX analysis results. For a semi-quantitative analysis of the nanostructures in the colloidal solutions, UV–vis optical absorption measurements were carried out on the samples in colloidal suspension phase. The typical UV–vis absorption spectra of indium oxide nanoparticles are shown in Fig. 4, after the distilled water background subtraction. In Fig. 4a, the optical absorption spectra of the fresh nanocolloids, obtained by increasing the laser fluence from 1 to 10 J cm−2
Fig. 2. SEM images of the sample C (a) and sample F (b), after deposition on a silicon substrate. Table 2 EDX derived chemical composition (atomic %) of samples C, E and F after deposition on silicon substrates. Spectral lines
Atomic % Samples
CK NK OK Si K In L
C
E
F
7 6 51 20 16
7 7 57 10 19
6 8 48 18 20
Fig. 3. X-ray photoelectron spectrum of indium oxide nanoparticles deposited on Si substrate: In 3d (5/2, 3/2) core level of sample B.
N. Acacia et al. / Applied Surface Science 256 (2010) 6918–6922
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Fig. 4. UV–vis absorption spectra of the samples prepared as a function of the laser fluence for 10 min of process (a) and at the fixed laser fluence of 3 J cm−2 in 10, 30, 60 and 120 min (b).
and keeping fixed the ablation time at 10 minutes (samples G, H, I and D), are shown. The spectra are characterized by a progressively increasing absorption going towards shorter wavelengths and by the presence of a shoulder at about 260 nm, in agreement with the presence of In2 O3 nanoparticles in the colloids, although somewhat shorter than values reported in the literature [26,30]. Upon increasing the laser fluence, the lineshape is unchanged unless for an overall rise of the absorption intensity at shorter wavelengths. Taking into account the increase of scattering effects at short wavelengths, this behavior suggests that no significant change has occurred in the size distribution of the nanoparticles, but only an increase of the number of the smallest size ones. An analogous trend was observed in the 30 min ablated samples (A, B and C, not shown) [31]. In Fig. 4b the UV–vis absorption spectra of H, C, E and F samples are reported. In this case, the laser fluence is fixed at 3 J cm−2 , while the ablation time is increased up to 120 min. The characteristic shoulder at 260 nm is visible only for the shorter ablation time cases (10 and 30 min). For longer times, a general growth of the absorption intensity along all the investigated spectral range is observed. This behavior is consistent with the presence of multiple scattering phenomena that mask the intrinsic absorption ones (as the shoulder located at about 260 nm) due to the increased density of nanoparticles per unit volume [31]. When multiple scattering effects can be neglected (i.e. for low ablation times), a closer examination of the spectra can provide a valuable information about the optical gap Eg width modulation, in correlation to possible quantum confinement effects. Indeed, it is well known that bulk indium oxide is a large direct band gap semiconductor (Eg = 3.70 eV [1]) and, in the parabolic bands and constant transition matrix element approximations [32], the absorbance of the material at a given wavelength can be expressed as: ˛(h) = Bd (h − Eg )
1/2
where Bd is a constant of the material, h is the energy of excitation and ˛ is the absorption coefficient (cm−1 ) that is proportional to the measured absorbance. Therefore the intercept on the energy axis of a plot of the square of the absorbance versus the photon energy yields the Eg value for an allowed direct transition. For all the samples the extrapolated band gap is about 3.70 eV. At variance with other reports [1,26], no evidence of a gap width modulation, correlated to the NP size, was found in the samples. This means that in the present case any quantum size effect is absent or, possibly, too weak to be detected. The absence of metallic indium in the deposited nanoparticles, as evidenced by XPS analysis and more indirectly from the
EDX ones, suggests the complete oxidation of the ablated material. These results confirm the predicted enhancement of the species chemical reactivity which takes place near the target, due to the high pressure and high temperature conditions established at the interface between the expanding plasma and the confining liquid. Then, analogously to what proposed for the ablation of Zn and Cu in water [17,19], the plasma cooling stage favors the formation of small metallic clusters, that interacting with the solvent form their hydroxy derivates followed by the decomposition to produce the indium oxide nanoparticles. In the investigated laser fluence range, the vaporization process appears to be the really dominant one, since a very large density of small spherical nanoparticles (<20 nm) were always obtained, whose amount scales progressively with the laser fluence. 4. Conclusion The reported study demonstrates, to our best knowledge for the first time, the effective production of indium oxide nanoparticles by means of pulsed laser ablation in water, for different laser fluence and ablation time values. A large density of spherical nanoparticles is obtained in all cases, with the size falling into two main ranges, 2–5 nm (≈85%) and 10–20 nm (≈12%), respectively. No significant evidence of larger particles is found. SEM–EDX observations on the deposited films reveal the presence of indium, a large content of oxygen and, to a lesser extent, carbon and nitrogen. These findings are presumably due to a getter ability of indium oxide against the atmospheric ambient volatile species. The XPS analysis confirms the oxidized nature of the deposited nanoparticles with a slightly over stoichiometric ratio O/In = 1.7 due to the chemisorbed oxygen on the surface. The UV–vis analysis returns an optical gap extension identical for all the nanoparticle colloids and similar to the bulk In2 O3 value, that is no quantum size effects are evidenced. According with the literature, the morphological results confirm that the vaporization process of the target is the dominant one in the investigated laser fluence range. Moreover, the complete oxidation of the ablated material is obtained thanks to the extreme conditions of the laser plasma produced near the target surface and to its interaction with the surrounding liquid. The total amount of nanoparticles in the colloid unit volume depends on the density of indium atoms produced during the ablation process. This density increases with the laser fluence, whilst a prolonged laser irradiation, at a fixed fluence, seems to favor the formation of a limited number of larger nanoparticles, about 20 nm in diameter. All the above findings point out the strong and promising potentiality of laser ablation in liquids as a scalable technique to produce
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