- Email: [email protected]
Surfactant-free small Ni nanoparticles trapped on silica nanoparticles prepared by pulsed laser ablation in liquid
Chemical Physics Letters 591 (2014) 193–196
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Surfactant-free small Ni nanoparticles trapped on silica nanoparticles prepared by pulsed laser ablation in liquid Fumitaka Mafuné a,⇑, Takumi Okamoto b, Miho Ito b a b
Department of Basic Science, School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan DENSO CORPORATION, 1-1, Showa-cho, Kariya, Aichi 448-8661, Japan
a r t i c l e
i n f o
Article history: Received 20 October 2013 In final form 14 November 2013 Available online 21 November 2013
a b s t r a c t Small Ni nanoparticles supported on silica nanoparticles were formed by pulsed laser ablation in liquid. Water dispersing surfactant-free silica particles was used here as a solvent, and a bulk Ni metal plate as a target. The nanoparticles formed by laser ablation in water were readily stabilized by the silica particles, whereas Ni nanoparticles prepared in water without silica were found to be precipitated a few hours after aggregation into 5–30 nm particles. The nanoparticles were characterized by TEM, dark-field STEM and optical absorption spectroscopy, which indicated that small 1–3 nm Ni nanoparticles were adsorbed on the surface of silica. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Single-nanometer-sized particles are of great interest because of their characteristic optical properties, catalytic activities etc. [1–6]. For instance, gold nanoparticles smaller than 5 nm in diameter are known to emit intense photo-luminescence [4], whereas gold nanoparticles smaller than 10 nm exhibit catalytic activities in low-temperature CO oxidation [5,6]. Hence, it is important to develop methods to prepare nanoparticles in a size-controlled manner [7–14]. Fine chemical synthesis has been used to prepare small metal nanoparticles in solutions [9–11]. A metal salt in a solvent is reduced into metal nanoparticles in the presence of a stabilizing reagent such as a ligand. The size of the particles is determined by the strong interaction between the metal particle and the ligand. In the absence of ligand molecules, the particle would grow further. However, when the nanoparticles thus produced are utilized as a catalyst, for instance, one needs to remove ligand molecules from the particles, because an active surface must be exposed. In this regard, an eager requirement in the industrial circle is that nanoparticles should be prepared in surfactant-free conditions. Pulsed laser ablation in liquid (PLAL) is a physical method enabling us to prepare nanometer-sized particles in liquid media [15–22]. Here, atoms and clusters are released from a metal target to the liquid by irradiation of laser pulses onto a target. Particles are formed by aggregation of the atoms and the clusters in the liquid. The size of the particles can be controlled using a stabilizing reagent such as a surfactant [15]. Moreover, surfactant-free particles can be prepared without using any surfactant [23]. ⇑ Corresponding author. Fax: +81 3 5454 6597. E-mail address: [email protected] (F. Mafuné). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.11.034
Figure 1 shows pictures of colloidal solutions of Au, Ag, Pt and Ni nanoparticles prepared by PLAL in pure water. The characteristic colors of the solutions along with TEM images (not shown here) evidently indicate that the nanoparticles are formed in water. However, we noticed that Ni nanoparticles in water were precipitated after gradual aggregation, whereas Au, Ag, and Pt particles were stably dispersed in water for at least a couple of weeks. For Au, a surface of the nanoparticles is known to be partially oxidized and negatively charged [24,25], causing them to be dispersed stably in water. Gradual aggregation of the Ni nanoparticles suggests that they are either uncharged or charged in a lesser extent than other precious metals. In addition, the size control of surfactantfree nanoparticles is in general so hard that the particle size tends to be widely distributed [23]. We have investigated formation of Ni nanoparticles in the present study by PLAL in water which contains surfactant-free silica nanoparticles. Silica nanoparticles provide such a wide surface area in water that Ni particles are expected to be readily trapped. In addition, trapping of Ni nanoparticles on silica nanoparticles may hinder further growth of the nanoparticles by suppressing encounter of the Ni nanoparticles, so that production of small Ni nanoparticles is expected by PLAL in surfactant-free conditions. 2. Experimental The present method of preparation comprises conventional PLAL in combination with surfactant-free silica nanoparticles dispersed in water. Nickel nanoparticles were produced by pulsed laser ablation of a Ni metal plate (see Figure 2a). A metal plate (>99.9%) was placed on the bottom of a glass vessel filled with 10 mL of a solvent and was rotated by using a magnetic rotator to move a laser-irradiated position pulse by pulse. The metal plate
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F. Mafuné et al. / Chemical Physics Letters 591 (2014) 193–196
spectrum of the solution was measured by a Shimadzu UV-1200 spectrometer. 3. Results
Figure 1. Pictures of colloidal solutions prepared by PLAL in pure water. (a) Colloidal solutions, right after preparation. (b) Colloidal solutions, 1 week after preparation. Nanoparticles were produced by laser ablation at 1064 nm with a fluence of 1.6 J/pulse cm2.
(a)
pulsed laser @1064 nm convex lens
target magnetic stirrer
(b)
Figure 3 shows typical electron micrographs of silica nanoparticles. The size of the nanoparticles is rather narrowly distributed with an average size of 10 nm. In addition, they exhibit characteristic features in electron micrographs; they are not coagulated but are separated with a finite distance from one another. In general, surfactant-free nanoparticles tend to coagulate, because they are not covered by a stabilizing reagent. In contrast, surfactant-free silica nanoparticles are stably dispersed in water, as they are known to be negatively charged [26,27]. The repulsive forces among the nanoparticles due to their negative charges are likely to exceed the van der Waals attractive forces leading to coagulation, and hence, the nanoparticles are present in a solution without being coagulated. Surfactant-free small Ni nanoparticles supported on silica particles were formed by pulsed laser ablation in liquid. As the laser ablation of a Ni metal plate was continued, the color of water containing silica nanoparticles turned to brown. Figure 4 shows the optical absorption spectrum of the solution, which exhibits broad absorption in the UV region. The optical absorption spectrum is consistent with that of a colloidal solution containing Ni nanoparticles prepared chemically [28]. This finding proves that Ni nanoparticles are produced in water by laser ablation. The Ni nanoparticles are considered to be non-oxides, since the optical absorption spectrum did not show any absorption peak in the UV region at 350 nm due to the band gap (3.5 eV) [29,30]. Figure 5a shows a TEM image of the Ni nanoparticles thus produced. The whole TEM image resembles that of the silica
(a)
(b)
1
Absorbance
0.8 0.6 0.4 0.2 400 600 800 1000 Wavelength (nm)
Figure 2. (a) Experimental setup of PLAL. (b) Optical extinction spectrum of 20 wt% of silica nanoparticles in water. (inset) A picture of silica nanoparticles dispersed in water.
was irradiated with a focused output of the fundamental of a nanosecond Surelite I Nd:YAG laser operated at 10 Hz with a lens having a focal length of 250 mm. The spot diameter of the laser beam on the surface of the metal plate was typically 1 mm. The pulse energy of the laser, 100 mJ, was monitored by Ophire power meter. Colloidal solution of silica nanoparticles dispersed in water. (Nissan Chemical Co.), pH = 2–4, was used as a solvent amorphous. As shown in Figure 2b, an optical absorption spectrum of the silica nanoparticles dispersed in water does not show any absorption in the IR and visible regions, whereas it exhibits extinction of light in the UV region originating from optical absorption and light scattering. Hence, as far as visible or IR laser pulse is used, the solvent is considered to behave as a totally transparent medium. The nanoparticles thus prepared were analyzed by TEM, darkfield STEM and optical absorption spectroscopy. The absorption
10 nm
50 nm
Figure 3. (a) and (b) TEM images of silica nanoparticles.
Absorbance (arb. units)
0 200
200
400
600
800
1000
Wavelength (nm) Figure 4. Extinction spectra of colloidal Ni nanoparticles supported on silica nanoparticles (black solid line) and silica nanoparticles only (blue solid line). The contribution of silica nanoparticles in the extinction spectrum of Ni nanoparticles on silica is assumed to be negligible. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
F. Mafuné et al. / Chemical Physics Letters 591 (2014) 193–196
(a)
(b)
20 nm
10 nm
(c)
50 nm Figure 5. (a) TEM and (b) dark-field STEM mages of the Ni nanoparticles supported on silica nanoparticles produced by PLAL. (c) TEM image of Ni nanoparticles produced in pure water in the absence of silicon nanoparticles.
nanoparticles, except that a few coagulated nanoparticles are observed and very small particles exist on the silica particles. Note that no large spherical particles are observed in the TEM image. It is highly likely that Ni nanoparticles are attached to the silica surface. A light 3d metal such as nickel gives a weak contrast in the TEM image, and particles are barely recognized on the silica nanoparticles. In order to observe them, a dark-field STEM image was observed as shown in Figure 5b. There are bright spots on silica, which are evidently the images of Ni nanoparticles with sizes as small as 1–3 nm. For comparison, Ni nanoparticles were prepared by pulsed laser ablation in pure water without silica nanoparticles. As shown in Figure 5c, spherical particles with diameters of 5–30 nm were produced.
4. Discussion Our comparison of the images of the Ni nanoparticles indicates that the particles produced in the presence of silica are much smaller than those produced in the absence. Why are such small nanoparticles present on silica nanoparticles? The formation mechanism of nanoparticles by PLAL has not been fully understood [31–33]. It is known that a plasma plume is built over the laser spot on the metal plate right after the laser ablation, followed by the formation of cavitation bubbles from the laser irradiated area [31,32]. The cavitation bubbles grow and shrink in the time scale of 100 ls. Plech and co-workers observed formation of cavitation bubbles using time-resolved small-angle X-ray scattering [33]. They also observed formation of gold nanoparticles in cavitation bubbles with diameters ranging 8–10 and 40–60 nm. However, it is yet unknown whether all the nanoparticles are formed in the cavitation bubbles and how the nanoparticles are released to the liquid phase. Phenomenologically, atoms or small atomic clusters released from a metal plate by laser ablation are known to be nucleated as fast as the atoms collide mutually right after the laser ablation to form primary particles [15]. This initial nucleation continues until available atoms in the close vicinity are consumed almost completely. Then, the primary particles are gradually aggregated in the liquid forming secondary particles. In fact, when nanoparticles
195
were prepared in a liquid containing a surfactant, the size of the nanoparticles was changed with the concentration of the surfactant: The size decreased with an increase in the concentration of the surfactant [15]. These experimental facts were interpreted as such that the growth processes forming the secondary particles and the stabilization processes by the surfactant compete and the gradual growth of secondary particles are suppressed if there are higher numbers of surfactant molecules in the solution [15]. It is highly likely that silica nanoparticles provide such a wide surface area for the Ni nanoparticles that primary particles are readily trapped on the surface. Note that Ni nanoparticles have a tendency to grow in liquid if there is no stabilizing reagent, as evidenced in the TEM image showing particles sized 5–30 nm (see Figure 5c). We were able to estimate from the weight measurement that the irradiation of the single laser pulse onto the Ni plate in water produces 1015 atoms i.e. 1012 nanoparticles with 2 nm in diameter in water. Under the assumption that the Ni atoms are confined in the region of 1 1 1 mm3, which is as large as the cavitation bubbles, there are 1014 silica nanoparticles in the same volume, which exceeds the number of the Ni particles by 2 orders of magnitude. Moreover, silica nanoparticles dispersed in water provide as wide as 10 nm2 surface to the small Ni nanoparticles. Hence, we infer that the collision frequency of the primary nanoparticles with silica is much higher than that between the primary particles. A close analysis of the dark-field STEM image finds that a few silica nanoparticles are contacted each other and small Ni nanoparticles are located in a boundary of the silica nanoparticles. This finding suggests that silica nanoparticles trapping small Ni nanoparticles by the hydrophobic interaction can aggregate each other with the Ni particles as a joint. This may cause the silica nanoparticles to look more aggregated after the laser ablation of Ni metal. Finally, we need to emphasize the fact that surfactant-free small Ni nanoparticles were actually formed in liquid. A variety of surfactant free nanoparticles have so far been prepared by the laser ablation in water, but the size of the nanoparticles was hardly controlled. In contrast, small nanoparticles have been prepared in the size-controlled manner by using a stabilizing reagent. However, the size is uncontrollable if chemicals are removed. The present method has now enabled us to prepare surfactant-free small Ni nanoparticles for the first time and is expected to lead to wide applications.
5. Conclusions Formation of surfactant-free size-controlled Ni nanoparticles in liquid has been achieved. Nanoparticles of precious metals such as Au, Ag, and Pt have so far been prepared by laser ablation in water in surfactant-free conditions, but Ni nanoparticles have been prepared in the present study in the presence of water containing silica nanoparticles. It is highly likely that silica nanoparticles provide such a wide surface area that the primary Ni particles formed right after the laser ablation are readily trapped on the surface. Once the Ni nanoparticles are trapped on the surface, they cannot grow further. Since this physical method of preparation allows us to prepare a variety of small nanoparticles without use of stabilizing chemicals, it is expected to promote a further technical advancement.
Acknowledgments This work is supported by the collaborative feasibility study project between DENSO COPORATION and The University of Tokyo, and by JSPS KAKENHI Grant No. 22350004.
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References [1] M. Takagi, J. Phys. Soc. Jpn. 9 (1954) 359. [2] D.A. Buffat, J.P. Borel, Phys. Rev. A13 (1976) 2289. [3] I. Chakraborty, S. Bag, U. Landman, T. Pradeep, J. Phys. Chem. Lett. 4 (2013) 2769. [4] J.P. Wilcoxon, J.E. Martin, F. Parsapour, B. Wiedenman, D.F. Kelley, J. Chem. Phys. 108 (1998) 9137. [5] Y. Kuwauchi, S. Takeda, H. Yoshida, K. Sun, M. Haruta, H. Kohno, Nano Lett. 13 (2013) 3073. [6] H. Sakurai, M. Haruta, Appl. Catal. A 127 (1995) 93. [7] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, J. Chem. Soc. Chem. Commun. 7 (1994) 801. [8] C. Petit, P. Lixon, M.P. Pileni, J. Phys. Chem. 97 (1993) 12974. [9] A. Desireddy et al., Nature 501 (2013) 399. [10] P.M. Pileni, Langmuir 13 (1997) 3266. [11] B.E. Baker, N.J. Kline, P.J. Treado, M.J.J. Naten, Am. Chem. Soc. 118 (1996) 8721. [12] M. Takesue, T. Tomura, M. Yamada, K. Hata, S. Kuwamoto, T. Yonezawa, J. Am. Chem. Soc. 133 (2011) 14164. [13] H.J. Zhang, N. Toshima, Appl. Catal. A 447 (2012) 81. [14] P.A. Clayborne, O. Lopez-Acevedo, R.L. Whetten, H. Gronbeck, H. Hakkinen, J. Chem. Phys. 135 (2011) 094701. [15] F. Mafuné, J. Kohno, Y. Takeda, T. Kondow, H. Sawabe, J. Phys. Chem. B 104 (2000) 9111. [16] F. Mafuné, J. Kohno, Y. Takeda, T. Kondow, H. Sawabe, J. Phys. Chem. B 105 (2001) 5114.
[17] F. Mafuné, J. Kohno, Y. Takeda, T. Kondow, H. Sawabe, J. Phys. Chem. B 104 (2000) 8333. [18] A. Fojtik, A. Henglein, Ber. Bunsen-Ges. Phys. Chem. 97 (1993) 252. [19] A. Henglein, J. Phys. Chem. 97 (1993) 5457. [20] M.S. Yeh, Y.S. Yang, Y.P. Lee, H.F. Lee, Y.H. Yeh, C.S. Yeh, J. Phys. Chem. B 103 (1999) 6851. [21] J. Neddersen, G. Chumanov, T.M. Cotton, Appl. Spectrosc. 47 (1993) 1959. [22] M.S. Sibbald, G. Chumanov, T.M. Cotton, J. Phys. Chem. 100 (1996) 4672. [23] F. Mafuné, J. Kohno, Y. Takeda, T. Kondow, J. Phys. Chem. B 107 (2003) 4218. [24] J.-P. Sylvestre, S. Poulin, A.V. Kahashin, E. Sacher, M. Meunier, J.H.T. Luong, J. Phys. Chem. 108 (2004) 16864. [25] H. Muto, K. Yamada, K. Miyajima, F. Mafuné, J. Phys. Chem. C 111 (2007) 17221. [26] P.L. He, N.F. Hu, J.F. Rusling, Langmuir 20 (2004) 722. [27] I.J. Bruce et al., Materials 284 (2004) 145. [28] K. Nouneha, M. Oyama, R. Diaz, M. Abd-Lefdil, I.V. Kityk, M. Bousmina, J. Alloys Compd. 509 (2011) 5882. [29] M. Alagiri, S. Ponnusamy, C. Muthamizhchelvan, J. Mater. Sci. Mater. Electron. 23 (2012) 728. [30] K.G. Chandrapp, T.V. Venkatesha, K.O. Nayana, M.K. Punithkumar, Mater. Corros. 63 (2012) 445. [31] K. Sasaki, T. Nakano, W. Soliman, N. Takada, Appl. Phys. Express 2 (2009) 046501. [32] K. Sasaki, N. Takada, Pure Appl. Chem. 82 (2010) 1317. [33] P. Wagener, S. Ibrahimkutty, Phys. Chem. Chem. Phys. 15 (2013) 3068.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Surfactant-free small Ni nanoparticles trapped on silica nanoparticles prepared by pulsed laser ablation in liquid Fumitaka Mafuné a,⇑, Takumi Okamoto b, Miho Ito b a b
Department of Basic Science, School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan DENSO CORPORATION, 1-1, Showa-cho, Kariya, Aichi 448-8661, Japan
a r t i c l e
i n f o
Article history: Received 20 October 2013 In final form 14 November 2013 Available online 21 November 2013
a b s t r a c t Small Ni nanoparticles supported on silica nanoparticles were formed by pulsed laser ablation in liquid. Water dispersing surfactant-free silica particles was used here as a solvent, and a bulk Ni metal plate as a target. The nanoparticles formed by laser ablation in water were readily stabilized by the silica particles, whereas Ni nanoparticles prepared in water without silica were found to be precipitated a few hours after aggregation into 5–30 nm particles. The nanoparticles were characterized by TEM, dark-field STEM and optical absorption spectroscopy, which indicated that small 1–3 nm Ni nanoparticles were adsorbed on the surface of silica. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Single-nanometer-sized particles are of great interest because of their characteristic optical properties, catalytic activities etc. [1–6]. For instance, gold nanoparticles smaller than 5 nm in diameter are known to emit intense photo-luminescence [4], whereas gold nanoparticles smaller than 10 nm exhibit catalytic activities in low-temperature CO oxidation [5,6]. Hence, it is important to develop methods to prepare nanoparticles in a size-controlled manner [7–14]. Fine chemical synthesis has been used to prepare small metal nanoparticles in solutions [9–11]. A metal salt in a solvent is reduced into metal nanoparticles in the presence of a stabilizing reagent such as a ligand. The size of the particles is determined by the strong interaction between the metal particle and the ligand. In the absence of ligand molecules, the particle would grow further. However, when the nanoparticles thus produced are utilized as a catalyst, for instance, one needs to remove ligand molecules from the particles, because an active surface must be exposed. In this regard, an eager requirement in the industrial circle is that nanoparticles should be prepared in surfactant-free conditions. Pulsed laser ablation in liquid (PLAL) is a physical method enabling us to prepare nanometer-sized particles in liquid media [15–22]. Here, atoms and clusters are released from a metal target to the liquid by irradiation of laser pulses onto a target. Particles are formed by aggregation of the atoms and the clusters in the liquid. The size of the particles can be controlled using a stabilizing reagent such as a surfactant [15]. Moreover, surfactant-free particles can be prepared without using any surfactant [23]. ⇑ Corresponding author. Fax: +81 3 5454 6597. E-mail address: [email protected] (F. Mafuné). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.11.034
Figure 1 shows pictures of colloidal solutions of Au, Ag, Pt and Ni nanoparticles prepared by PLAL in pure water. The characteristic colors of the solutions along with TEM images (not shown here) evidently indicate that the nanoparticles are formed in water. However, we noticed that Ni nanoparticles in water were precipitated after gradual aggregation, whereas Au, Ag, and Pt particles were stably dispersed in water for at least a couple of weeks. For Au, a surface of the nanoparticles is known to be partially oxidized and negatively charged [24,25], causing them to be dispersed stably in water. Gradual aggregation of the Ni nanoparticles suggests that they are either uncharged or charged in a lesser extent than other precious metals. In addition, the size control of surfactantfree nanoparticles is in general so hard that the particle size tends to be widely distributed [23]. We have investigated formation of Ni nanoparticles in the present study by PLAL in water which contains surfactant-free silica nanoparticles. Silica nanoparticles provide such a wide surface area in water that Ni particles are expected to be readily trapped. In addition, trapping of Ni nanoparticles on silica nanoparticles may hinder further growth of the nanoparticles by suppressing encounter of the Ni nanoparticles, so that production of small Ni nanoparticles is expected by PLAL in surfactant-free conditions. 2. Experimental The present method of preparation comprises conventional PLAL in combination with surfactant-free silica nanoparticles dispersed in water. Nickel nanoparticles were produced by pulsed laser ablation of a Ni metal plate (see Figure 2a). A metal plate (>99.9%) was placed on the bottom of a glass vessel filled with 10 mL of a solvent and was rotated by using a magnetic rotator to move a laser-irradiated position pulse by pulse. The metal plate
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F. Mafuné et al. / Chemical Physics Letters 591 (2014) 193–196
spectrum of the solution was measured by a Shimadzu UV-1200 spectrometer. 3. Results
Figure 1. Pictures of colloidal solutions prepared by PLAL in pure water. (a) Colloidal solutions, right after preparation. (b) Colloidal solutions, 1 week after preparation. Nanoparticles were produced by laser ablation at 1064 nm with a fluence of 1.6 J/pulse cm2.
(a)
pulsed laser @1064 nm convex lens
target magnetic stirrer
(b)
Figure 3 shows typical electron micrographs of silica nanoparticles. The size of the nanoparticles is rather narrowly distributed with an average size of 10 nm. In addition, they exhibit characteristic features in electron micrographs; they are not coagulated but are separated with a finite distance from one another. In general, surfactant-free nanoparticles tend to coagulate, because they are not covered by a stabilizing reagent. In contrast, surfactant-free silica nanoparticles are stably dispersed in water, as they are known to be negatively charged [26,27]. The repulsive forces among the nanoparticles due to their negative charges are likely to exceed the van der Waals attractive forces leading to coagulation, and hence, the nanoparticles are present in a solution without being coagulated. Surfactant-free small Ni nanoparticles supported on silica particles were formed by pulsed laser ablation in liquid. As the laser ablation of a Ni metal plate was continued, the color of water containing silica nanoparticles turned to brown. Figure 4 shows the optical absorption spectrum of the solution, which exhibits broad absorption in the UV region. The optical absorption spectrum is consistent with that of a colloidal solution containing Ni nanoparticles prepared chemically [28]. This finding proves that Ni nanoparticles are produced in water by laser ablation. The Ni nanoparticles are considered to be non-oxides, since the optical absorption spectrum did not show any absorption peak in the UV region at 350 nm due to the band gap (3.5 eV) [29,30]. Figure 5a shows a TEM image of the Ni nanoparticles thus produced. The whole TEM image resembles that of the silica
(a)
(b)
1
Absorbance
0.8 0.6 0.4 0.2 400 600 800 1000 Wavelength (nm)
Figure 2. (a) Experimental setup of PLAL. (b) Optical extinction spectrum of 20 wt% of silica nanoparticles in water. (inset) A picture of silica nanoparticles dispersed in water.
was irradiated with a focused output of the fundamental of a nanosecond Surelite I Nd:YAG laser operated at 10 Hz with a lens having a focal length of 250 mm. The spot diameter of the laser beam on the surface of the metal plate was typically 1 mm. The pulse energy of the laser, 100 mJ, was monitored by Ophire power meter. Colloidal solution of silica nanoparticles dispersed in water. (Nissan Chemical Co.), pH = 2–4, was used as a solvent amorphous. As shown in Figure 2b, an optical absorption spectrum of the silica nanoparticles dispersed in water does not show any absorption in the IR and visible regions, whereas it exhibits extinction of light in the UV region originating from optical absorption and light scattering. Hence, as far as visible or IR laser pulse is used, the solvent is considered to behave as a totally transparent medium. The nanoparticles thus prepared were analyzed by TEM, darkfield STEM and optical absorption spectroscopy. The absorption
10 nm
50 nm
Figure 3. (a) and (b) TEM images of silica nanoparticles.
Absorbance (arb. units)
0 200
200
400
600
800
1000
Wavelength (nm) Figure 4. Extinction spectra of colloidal Ni nanoparticles supported on silica nanoparticles (black solid line) and silica nanoparticles only (blue solid line). The contribution of silica nanoparticles in the extinction spectrum of Ni nanoparticles on silica is assumed to be negligible. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
F. Mafuné et al. / Chemical Physics Letters 591 (2014) 193–196
(a)
(b)
20 nm
10 nm
(c)
50 nm Figure 5. (a) TEM and (b) dark-field STEM mages of the Ni nanoparticles supported on silica nanoparticles produced by PLAL. (c) TEM image of Ni nanoparticles produced in pure water in the absence of silicon nanoparticles.
nanoparticles, except that a few coagulated nanoparticles are observed and very small particles exist on the silica particles. Note that no large spherical particles are observed in the TEM image. It is highly likely that Ni nanoparticles are attached to the silica surface. A light 3d metal such as nickel gives a weak contrast in the TEM image, and particles are barely recognized on the silica nanoparticles. In order to observe them, a dark-field STEM image was observed as shown in Figure 5b. There are bright spots on silica, which are evidently the images of Ni nanoparticles with sizes as small as 1–3 nm. For comparison, Ni nanoparticles were prepared by pulsed laser ablation in pure water without silica nanoparticles. As shown in Figure 5c, spherical particles with diameters of 5–30 nm were produced.
4. Discussion Our comparison of the images of the Ni nanoparticles indicates that the particles produced in the presence of silica are much smaller than those produced in the absence. Why are such small nanoparticles present on silica nanoparticles? The formation mechanism of nanoparticles by PLAL has not been fully understood [31–33]. It is known that a plasma plume is built over the laser spot on the metal plate right after the laser ablation, followed by the formation of cavitation bubbles from the laser irradiated area [31,32]. The cavitation bubbles grow and shrink in the time scale of 100 ls. Plech and co-workers observed formation of cavitation bubbles using time-resolved small-angle X-ray scattering [33]. They also observed formation of gold nanoparticles in cavitation bubbles with diameters ranging 8–10 and 40–60 nm. However, it is yet unknown whether all the nanoparticles are formed in the cavitation bubbles and how the nanoparticles are released to the liquid phase. Phenomenologically, atoms or small atomic clusters released from a metal plate by laser ablation are known to be nucleated as fast as the atoms collide mutually right after the laser ablation to form primary particles [15]. This initial nucleation continues until available atoms in the close vicinity are consumed almost completely. Then, the primary particles are gradually aggregated in the liquid forming secondary particles. In fact, when nanoparticles
195
were prepared in a liquid containing a surfactant, the size of the nanoparticles was changed with the concentration of the surfactant: The size decreased with an increase in the concentration of the surfactant [15]. These experimental facts were interpreted as such that the growth processes forming the secondary particles and the stabilization processes by the surfactant compete and the gradual growth of secondary particles are suppressed if there are higher numbers of surfactant molecules in the solution [15]. It is highly likely that silica nanoparticles provide such a wide surface area for the Ni nanoparticles that primary particles are readily trapped on the surface. Note that Ni nanoparticles have a tendency to grow in liquid if there is no stabilizing reagent, as evidenced in the TEM image showing particles sized 5–30 nm (see Figure 5c). We were able to estimate from the weight measurement that the irradiation of the single laser pulse onto the Ni plate in water produces 1015 atoms i.e. 1012 nanoparticles with 2 nm in diameter in water. Under the assumption that the Ni atoms are confined in the region of 1 1 1 mm3, which is as large as the cavitation bubbles, there are 1014 silica nanoparticles in the same volume, which exceeds the number of the Ni particles by 2 orders of magnitude. Moreover, silica nanoparticles dispersed in water provide as wide as 10 nm2 surface to the small Ni nanoparticles. Hence, we infer that the collision frequency of the primary nanoparticles with silica is much higher than that between the primary particles. A close analysis of the dark-field STEM image finds that a few silica nanoparticles are contacted each other and small Ni nanoparticles are located in a boundary of the silica nanoparticles. This finding suggests that silica nanoparticles trapping small Ni nanoparticles by the hydrophobic interaction can aggregate each other with the Ni particles as a joint. This may cause the silica nanoparticles to look more aggregated after the laser ablation of Ni metal. Finally, we need to emphasize the fact that surfactant-free small Ni nanoparticles were actually formed in liquid. A variety of surfactant free nanoparticles have so far been prepared by the laser ablation in water, but the size of the nanoparticles was hardly controlled. In contrast, small nanoparticles have been prepared in the size-controlled manner by using a stabilizing reagent. However, the size is uncontrollable if chemicals are removed. The present method has now enabled us to prepare surfactant-free small Ni nanoparticles for the first time and is expected to lead to wide applications.
5. Conclusions Formation of surfactant-free size-controlled Ni nanoparticles in liquid has been achieved. Nanoparticles of precious metals such as Au, Ag, and Pt have so far been prepared by laser ablation in water in surfactant-free conditions, but Ni nanoparticles have been prepared in the present study in the presence of water containing silica nanoparticles. It is highly likely that silica nanoparticles provide such a wide surface area that the primary Ni particles formed right after the laser ablation are readily trapped on the surface. Once the Ni nanoparticles are trapped on the surface, they cannot grow further. Since this physical method of preparation allows us to prepare a variety of small nanoparticles without use of stabilizing chemicals, it is expected to promote a further technical advancement.
Acknowledgments This work is supported by the collaborative feasibility study project between DENSO COPORATION and The University of Tokyo, and by JSPS KAKENHI Grant No. 22350004.
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