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Chemical Physics Letters 389 (2004) 58–63 www.elsevier.com/locate/cplett Pulsed-laser ablation of Mg in liquids: surfactant-directing nanoparticle assembly for magnesium hydroxide nanostructures Changhao Liang, Takeshi Sasaki, Yoshiki Shimizu, Naoto Koshizaki * Nanoarchitectonics Research Center (NARC), National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 27 February 2004; in final form 27 February 2004 Published online: Abstract Ultrafine brucite Mg(OH)2 nanostructures exhibiting wormhole-, tube-, rod- or platelet-like morphologies were prepared by pulsed laser ablation of an Mg plate target immersed in deionized water or aqueous solutions of sodium dodecyl sulfate (SDS) surfactant. The structure, morphology, and composition of the resultant Mg(OH)2 nanostructures were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). A possible formation mechanism of the Mg(OH)2 nanostructures is proposed based on the initial laser-induced reactive quenching process and subsequent surfactant-directing growth. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Nanocrystalline alkaline earth metal oxides, such as MgO, have been efficiently used as adsorbents for acidic gases and decomposition of hazardous chemicals [1,2]. MgO nanocrystals with high concentrations of corner/ edge sites, small particle size, and unusual crystalline shapes would provide high specific surface reactivity [3]. Conventional chemical methods for synthesis of MgO often involve decomposition of magnesium hydroxides (Mg(OH)2 ). The size and morphology of the final MgO particle depend greatly on the precursor of Mg(OH)2 in the process of topochemical decomposition. For example, 4 nm size MgO particles were prepared by utilizing Mg(OH)2 aerogel and supercritical drying techniques, and exhibited strong adsorbent properties [4]. In addition, magnesium hydroxide itself is commonly used as flame retardant filler that does not evolve into toxic and corrosive substances upon combustion [5]. It is of great interest to explore novel synthetic routes for Mg(OH)2 nanocrystallines of ultrafine size and particular morphologies. Brucite Mg(OH)2 , whose basic layered unit * Corresponding author. Fax: +81-29-861-6355. E-mail address: [email protected] (N. Koshizaki). 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.03.056 possesses hexagonal symmetry (C6-type), can be grown into a peculiar one-dimensional structure or platelet-like structure. For example, Mg(OH)2 nanorods have been synthesized with the assistance of an ethylenediamine ligand via a hydrothermal technique [6]. Ding et al. [7] synthesized Mg(OH)2 nanocrystallines with rod-, tube-, needle-, or lamella-like morphologies by a hydrothermal reaction using different magnesium precursors and solvents as the reactants. Mg(OH)2 nanorods were prepared by a liquid–solid arc discharge technique using NaCl as an electrolyte [8]. We investigate here the growth of Mg(OH)2 nanocrystallines in a special liquid environment generated by pulsed-laser ablation of the Mg target in liquids. There have been an increasing number of reports in the past few years on the formation of nanostructures via laser ablation/irradiation of a solid target immersed in liquids or of specially prepared absorbing solutions/ colloids [9]. Most studies have focused on the preparation and shape/size modification of noble metal nanoparticles [10–13]. Patil et al. [14] developed a method known as pulsed-laser-induced reactive quenching (PLIRQ). New metastable phases such as oxides, nitrides, and even diamond nanoparticles, could be formed due to the transient extreme high temperature and C. Liang et al. / Chemical Physics Letters 389 (2004) 58–63 Fig. 1a–d presents the XRD spectra of the precipitates formed by the PLD of Mg in different liquids. All diffraction peaks can be assigned to brucite-like Mg(OH)2 (JCPDS File. No. 44-1482) with a hexagonal and c ¼ 4:777 A). However, the relacell (a ¼ 3:144 A tive intensities and widths of the peaks varied in the different products. The diffraction peaks from samples A and B were broader than those in samples C and D, indicating a smaller particle size in samples A and B. Furthermore, the (0 0 1) peak in sample A was the most intense and the broadest among the other Bragg peaks. The non-uniform broadening of the Bragg peaks was more clearly presented in sample B, although the (1 0 0) and (1 1 0) peaks were exclusively narrow. These results indicate that the crystallite dimensions in samples A and B were preferentially restricted along the crystallographic c-axis, leading to broadening of the (0 0 1) reflection peak. Furthermore, various disorders, such as stack faults, turbostraticity, or interstratifications, are considered to account for non-uniform broadening in layered materials such as hydroxides [20]. In fact, the (0 0 1) reflection from sample A exhibits two split sub and d2 ¼ 4:625 A, peaks with d-spacings of d1 ¼ 5:101 A and the center of the whole broad peak with a d-spacing (103) (201) (110) (111) (012) (101) (100) An Mg metal plate (99.99% pure) was fixed on the bottom of a glass vessel filled with 6 cm3 deionized water and an aqueous solution of sodium dodecyl sulfate (SDS, 99.5% pure) with a variety of concentrations (0.001, 0.01, 0.05 M). The plate surface was ablated with an output of the third harmonic (355 nm) of a Nd:YAG laser operated at 10 Hz with a maximum output of about 100 mJ/pulse and pulse duration of 7–8 ns. The laser beam was focused on the plate surface with a spot size about 1 mm in diameter by a lens with a focal length of 250 mm. Laser ablation lasted for 60 min. All solutions became turbid with a white color after ablation. Remarkable powder-like precipitates were found in the aqueous solution of the surfactants after sitting for about 6 h, while cloud-like precipitates existed in the deionized water. The clear supernatant liquid was carefully removed, the precipitates were repeatedly centrifuged at 3500 rpm, and the sediments were washed with deionized water several times to remove as many SDS molecules as possible. A small amount of the precipitate was re-dispersed in deionized water, ultrasonically separated for 2 min, and deposited onto a holey carbon-coated copper grid for TEM observation. The remaining wet precipitate was transferred to a quartz substrate for powder X-ray diffraction (XRD) investigation. A transparent gel-like film formed from the product ablated in deionized water after drying at room temperature in an auto-dry desiccator, while white powder-aggregated films formed from the products ablated in the surfactant solutions. For clarity, we refer to the products obtained from the deionized water, 0.001, 0.01, and 0.05 M SDS solutions as samples A, B, C, and D. 3. Results and discussion (001) 2. Experimental Powder XRD measurements were performed on a Rigaku powder diffractometer using Cu Ka (filtered) at 40 kV and 35 mA in a radiation (k ¼ 1:54056 A) 2h h scanning mode. A transmission electron microscopy (TEM) investigation was conducted on a JEOL 2000 FXII with an energy-dispersive X-ray spectrometer (EDS) (EDAX, DX-4) attached and a JEOL-2010 highresolution TEM (HRTEM), all operated at 200 kV. Intensity (a.u.) pressure of liquid-confined laser ablation plumes and subsequent rapid quenching [15–17]. We recently performed a series of experiments on the formation of nanostructures by using active main or transient group metals as the target, and ultrafine metal oxide nanocrystals were successfully produced [18,19]. Furthermore, an appropriate surfactant could also be used to efficiently control the nanostructure growth in our lasergenerated liquid environment, similar to conventional soft chemical synthesis strategies. We present here our experiments on the formation of Mg(OH)2 nanostructures by pulsed-laser ablation of an Mg plate in an aqueous solution of anionic surfactants as well as in deionized water. Tube-like nanofibers, nanorods with ultrafine diameters, nanoplatelets, and wormhole-like crystalline porous gel made from Mg(OH)2 were produced in different liquids. The pulsed-laser-induced reactive-quenching process and the surfactant effects in directing the growth of Mg(OH)2 nanostructures will be discussed. 59 (a) (b) (c) (d) 20 30 40 50 60 70 80 2θ (deg) Fig. 1. X-ray powder diffraction spectra of products prepared by pulsed-laser ablation of an Mg target in: (a) deionized water, (b) 0.001 M SDS solution, (c) 0.01 M SDS solution, and (d) 0.05 M SDS solution. 60 C. Liang et al. / Chemical Physics Letters 389 (2004) 58–63 The (0 0 1) peak from sample B also displays of 4.771 A. a clear shoulder at a low angle, probably resulting from superstructures or some disorder along the c-axis. The reflections from samples C and D were also non-uniform. The (0 0 l) and (h 0 l) reflection peaks were slightly broader than that of the (h k 0) reflection, and the (0 0 1) peak was more intense than the (1 0 1) peak, which is generally the strongest peak. In addition to the formation of ultrafine Mg(OH)2 crystallites by PLIRQ in our experiment, the H2 O and SDS molecules were involved in the crystallization process of Mg(OH)2 , possibly resulting in the formation of disorders within the layered structures along c-axis of Mg(OH)2 . Fig. 2 depicts representative TEM images of the assynthesized Mg(OH)2 nanostructures. Bulk porous Mg(OH)2 gel was obtained in deionized water with a wormhole-like morphology after drying (Fig. 2a). The pore or crystallite size was 1–2 nm. However, the crystallite morphologies produced by ablation in aqueous solutions of SDS were completely different. Large-scale curled fiber-like crystallites formed with uniform diameters of 4–6 nm and lengths of several hundred nanometers in a 0.001 M SDS solution (Fig. 2b). The TEM observation revealed that small Mg(OH)2 crystallites were produced in deionized water and 0.001 M SDS solutions, which corresponds with the broadening of XRD peaks in samples A and B. Many short rod-like and platelet-like Mg(OH)2 crystallites were observed with increase in the SDS concentration to 0.01 or 0.05 M (Fig. 2c,d), while fiber-like crystallites disappeared. The rods had diameters of 10–30 nm and a length similar to that of the fibers. The platelets had diameters in the range of several tens to hundreds of nanometers. The amounts of Mg(OH)2 nanorods were relatively decreased in a solution with a higher SDS concentration of 0.05 M, whereas the platelets remarkably increased (Fig. 2d). The crystallite size exhibited no obvious change compared with that in 0.01 M SDS solutions. In addition, Figs. 2b, c also depict morphologies consistent with turbostratic disorder, as stated in previous reports [20,21]. The detailed morphology and structure were further investigated by high-resolution TEM. Fig. 3 displays typical HRTEM images of Mg(OH)2 nanofibers, rods, and platelets that formed in SDS solutions with different concentrations. Fig. 3a indicates that the ultrafine fibers shown in Fig. 2b were indeed tube-like structures with a channel spacing of 3–4 nm. The rods shown in Fig. 3b have stripe-like fringes Fig. 2. Low-magnification TEM images of Mg(OH)2 nanocrystals generated by pulsed-laser ablation of an Mg target in: (a) deionized water, (b) 0.001 M SDS solution, (c) 0.01 M SDS solution, and (d) 0.05 M SDS solution. C. Liang et al. / Chemical Physics Letters 389 (2004) 58–63 61 Fig. 3. High-resolution TEM images of the as-synthesized Mg(OH)2 nanocrystals with different morphologies. (a) Tube-like nanofibers. (b) Stripelike rods. (c) Single needle-like rods. (d) Single nanoplatelet. Insets depict the corresponding SAED patterns from a (b) single stripe-like rod and (d) platelet. equally spaced approximately by 3 nm in parallel along the rod axis, and the corresponding ED pattern (inset in Fig. 3b) indicates the polycrystalline nature of the rods, resulting from the assembling of small particles. Fig. 3c depicts a needle-like rod that was often observed in samples C and D. The rods were not well-crystallized, but presented some parallel lattice fringes of Mg(OH)2 and some signs of aggregation from small particles. Fig. 3d depicts a typical Mg(OH)2 platelet. Obscure lattice fringes were observed, similar to needle-like rods. The ED pattern exhibited bright spots and weak diffuse rings. The spots could be indexed according to hexagonal Mg(OH)2 structures. The amorphous-like diffuse ring indicates that the platelets were not well crystallized. The compositions of these nanostructures were simultaneously analyzed by attached energy-dispersive spectrometer (EDS). Only Mg, O, Cu (from the copper grid), and small C peaks appeared; no S or Na elements could be detected. Therefore, no SDS-related molecules were included in the formed nanostructures after repeated washing, centrifugation, and final drying. However, we cannot preclude the possibility of a very small number of water molecules being intercalated between the layers of Mg(OH)2 . A previous report demonstrated that interstratification of water molecules could result in turbostratic disorder between the layers [20]. The XRD and TEM results indicate that the splitting and nonuniform broadening of the XRD peaks arose from a combined effect of particle size and turbostratic disorders, particularly in the products from water and 0.001M SDS solutions. We demonstrated the formation of Mg(OH)2 nanostructures by pulsed-laser ablation of Mg metal in water and SDS solutions. We can tentatively describe the formation process based on the experimental results as having the following three stages: (1) laser-induced formation of Mg species (atoms, clusters, or nanoparticles); (2) interfacial reaction between Mg species and water, resulting in the formation of a magnesium hydroxide species (molecules, clusters, or particles), and (3) surfactant-directing growth of magnesium hydroxide structures with selected morphologies. Most of the laser energy (355 nm) would be absorbed by Mg metal in our synthetic strategy, due to the complete transparency of pure water and SDS solutions in the visible and ultraviolet regions. The strong excitation of the metal surface by the laser shot can lead to rapid ejection of Mg species (e.g., atoms or clusters), resulting in a hot plasma plume above the metal surface, similar to that in vacuum or diluted gases. The plume is confined in a liquid; previous 62 C. Liang et al. / Chemical Physics Letters 389 (2004) 58–63 studies indicated a further increase of plasma pressure and temperature due to adiabatic expansion and creation of a shock wave at the plasma–liquid interface [22,23]. However, it is known that excited electrons will also transfer energy to the metal lattice by typical electron–phonon coupling. The electron-electron and electron-phonon relaxation times are usually <1 ps [24]. This time-scale is much shorter than the pulse duration of 7–8 ns in our study. Thus, in addition to the direct ejection of metal species, the metal lattice will be heated, leading to melting of the metal surface. However, the target was slowly rotated in our experiment, and the laser spot was 1 mm in diameter. Therefore, the rapid melting region would soon re-solidify upon fast heat diffusion before the next laser pulse. Almost no largespherical Mg droplet-like particles could be found in the final product. The strong reaction between the Mg species and water molecules at the interface between the plasma and liquid will lead to the formation of magnesium hydroxides (Mg + 2H2 O!Mg(OH)2 + 2H ). The increase in the amount of Mg(OH)2 also caused the free hydroxyl ions in the liquid to increase gradually through relatively slow dissolution of Mg(OH)2 (Mg(OH)2 () Mg2þ + 2OH ), leading to an increase in the solution pH. The isoelectric point (IEP) of Mg(OH)2 particles in water is reportedly about 12 [25]. The pH of different solutions in our study increased after ablation from the initial neutral to basic with pH of 12.2, 12.6, 12.8, and 13.2 in water and 0.001, 0.01, and 0.05 M SDS solutions. The particles formed were probably positively charged before the solution pH exceeded the IEP. The negatively charged anionic DS chain molecules tended to adsorb onto the surface of the Mg(OH)2 particles. The morphologies of the Mg(OH)2 crystallites indicate that the surfactant molecules can direct the growth either along one direction to yield fibrous, rod-like morphologies, or along two directions to yield a platelet-like shape, depending on the concentration of surfactant, the electric-static interactions between inorganic species and DS ions, and the hydrophobic nature of the DS chains. This one-step assembling of Mg(OH)2 materials presented their own crystallite structure and preferred growth directions in our experiment, which were probably formed based on a surfactant-directing mechanism, as suggested by the formation of alumina nanofibers [26] and other unidirectional alumina nanostructures [27]. The surfactants do not act simply as a template, but also serve as a shape-inducing reagent. In addition, the bulk XRD investigation and EDS analysis of single nanostructures in our products did not indicate any sign of DS molecules included in the final product. The IEP of Mg(OH)2 , the net residual electric charge on the particle surface, is expected to be negative when the pH in SDS solutions is greater than 12. The adsorption of anions is no longer favored. In contrast, desorption of the anionic DS molecules and adsorption of sodium ions probably occurs on the surface of particles for electric charge equilibration. Thus, only pure Mg(OH)2 nanocrystals were obtained in the final product after washing. 4. Conclusion In summary, we investigated the formation of brucite Mg(OH)2 nanostructures by pulsed-laser ablation of an Mg target in liquids. SDS surfactants were used to direct the growth of Mg(OH)2 crystallites with concentrationdependent morphologies. A wormhole-like bulk gel was obtained in deionized water after drying at room temperature, but it exhibited crystallized pore walls without any postcalcination. Ultrafine tubular-like fibers were formed in lower concentrations of SDS solution. Stripelike rods and large platelets grew preferentially with increasing surfactant concentration. A growth mechanism based on laser-induced reactive quenching and surfactant-directing aggregation was proposed for the formation of Mg(OH)2 nanostructures. Acknowledgements C.H.L. acknowledges support by the Japan Society for the Promotion of Science (JSPS) fellowship at the National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan. References [1] Y.X. Li, O. Koper, M. Atteya, K.J. Klabunde, Chem. Mater. 4 (1992) 2468. [2] S. Rajagopalan, O. Koper, S. Decker, K.J. Klabunde, Chem. Eur. J. 8 (2002) 2602. [3] K.J. Klabunde, J.V. Stark, O. Koper, C. Mohs, D.G. Park, S. Decker, Y. Jiang, I. Lagadic, D.J. Zhang, J. Phys. Chem. 100 (1996) 12142. [4] R. Richards, W.F. Li, S. Decker, C. Davidson, O. Koper, V. Zaikovski, A. Volodin, T. Rieker, K.J. Klabunde, J. Am. Chem. Soc. 122 (2000) 4921. [5] R.N. Rothon, P.R. Hornsby, Polym. Degrad. Stabil. 54 (1996) 383. [6] Y. Li, M. Sui, Y. Ding, G. Zhang, J. Zhuang, C. Wang, Adv. Mater. 12 (2000) 818. [7] Y. Ding, G. Zhang, H. Wu, B. Hai, L. Wang, Y. Qian, Chem. Mater. 13 (2001) 435. [8] L. Hao, C. Zhu, X. Mo, W. Jiang, Y. Hu, Y. Zhu, Z. Chen, Inorg. Chem. Commun. 6 (2003) 229. [9] S. Georgiou, A. Koubenakis, Chem. Rev. 103 (2003) 349. [10] S. Lin, C. Burda, M.B. Mohamed, B. Nikoobakht, M.A. ElSayed, J. Phys. Chem. B 104 (2000) 6152. [11] F. Mafune, J. Kohno, Y. Takeda, T. Kondow, H. Sawabe, J. Phys. Chem. B 104 (2000) 8333. [12] F. Mafune, T. Kondow, Chem. Phys. Lett. 372 (2003) 199. [13] 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. C. Liang et al. / Chemical Physics Letters 389 (2004) 58–63 [14] P.P. Patil, D.M. Phase, S.A. Kulkarni, S.V. Ghaisas, S.K. Kulkarni, S.M. Kanetkar, S.B. Ogale, V.G. Bhide, Phys. Rev. Lett. 58 (1987) 238. [15] S.B. Ogale, P.P. Patil, D.M. Phase, Y.V. Bhandarkar, S.K. Kulkarni, S. Kulkarni, S.V. Ghaisas, S.M. Kanetkar, Phys. Rev. B 36 (1987) 8237. [16] J.B. Wang, G.W. Yang, C.Y. Zhang, X.L. Zhong, Zh.A. Ren, Chem. Phys. Lett. 367 (2003) 10. [17] S.B. Ogale, A.P. Malshe, S.M. Kanetkar, S.T. Kshirsagar, Solid State Commun. 84 (1992) 371. [18] C.H. Liang, Y. Shimizu, T. Sasaki, N. Koshizaki, J. Phys. Chem. B 107 (2003) 9220. [19] C.H. Liang, Y. Shimizu, T. Sasaki, N. Koshizaki, Appl. Phys. A, DOI:10.1007/s00339-003-2489-6. 63 [20] A.V. Radha, P.V. Kamath, G.N. Subbanna, Mater. Res. Bull. 38 (2003) 731. [21] P. Oliva, J. Leonardi, J.F. Laurent, C. Delmas, J.J. Braconnier, M. Figlarz, F. Fievet, A. de Guibert, J. Power Sources 8 (1982) 229. [22] L. Berthe, R. Fabbro, P. Peyre, L. Tollier, E. Bartnicki, J. Appl. Phys. 82 (1997) 2826. [23] S. Zhu, Y.F. Lu, M.H. Hong, X.Y. Chen, J. Appl. Phys. 89 (2001) 2400. [24] R.H.M. Groeneveld, R. Sprik, A. Lagendijk, Phys. Rev. B 51 (1995) 11433. [25] G.A. Parks, Chem. Rev. 65 (1965) 177. [26] H.Y. Zhu, J.D. Riches, J.C. Barry, Chem. Mater. 14 (2002) 2086. [27] H.C. Lee, H.J. Kim, S.H. Chung, K.H. Lee, H.C. Lee, J.S. Lee, J. Am. Chem. Soc. 125 (2003) 2882.