ISSN 10876596, Glass Physics and Chemistry, 2010, Vol. 36, No. 3, pp. 345–350. © Pleiades Publishing, Ltd., 2010. Original Russian Text © T.P. Maslennikova, E.N. Korytkova, 2010, published in Fizika i Khimiya Stekla.
Aqueous Solutions of Cesium Salts and Cesium Hydroxide in Hydrosilicate Nanotubes of the Mg3Si2O5(OH)4 Composition T. P. Maslennikova* and E. N. Korytkova Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, nab. Admirala Makarova 2, St. Petersburg, 199034 Russia *email:
[email protected] Received December 19, 2009
Abstract—This paper reports on the results of studies on the filling of nanotubes of the Mg3Si2O5(OH)4 com position with aqueous solutions of hydroxide, chloride, and nitrate of cesium under different temperature– time conditions. It has been established that the nanotubes differently interact with solutions of different chemical nature. Key words: hydrosilicate nanotubes, chrysotile, filling, aqueous solutions, nanoliquid DOI: 10.1134/S1087659610030119
INTRODUCTION Since the discovery of carbon nanotubes [1] in 1991, considerable interest has been expressed by researchers in quasionedimensional nanostructures of different nature, which exhibit unique physico chemical properties and are of great importance for basic research and practical application. Apart from carbon, many inorganic compounds with a layered structure are able to form nanotubes. To date, a large number of inorganic nanotubes of sul fides, selenides, borides, oxides, hydroxides, and sili cates of metals have been synthesized [2–12]. One of the most important problems associated with the application of nanotubes is the potential pos sibility of filling their internal spaces with various sub stances. Nowadays, a large number of works have been devoted to the filling of carbon nanotubes with metals, metal oxides, metal halides, fullerenes, etc. [13, 14] and to the investigation of physicochemical properties of these structures. Similar phenomena have also been investigated in other nanotubes [15]. However, it should be noted that, at present, works concerned with the mass transfer and chemical transformations in liq uids bounded by a nanometer space (liquids inside nanotubes) are almost absent. The investigation into the influence of the chemical composition of nano tubes and their size on the physicochemical properties of the liquid inside nanotubes (“nanoliquid”), the for mation of new chemical compounds in it, and the redistribution of components between the bulk liquid phase and the nanoliquid is of special interest. From this point of view, interesting objects are synthetic hydrosilicate nanotubes of the Mg3Si2O5(OH)4 com position, which represent an analog of the natural
mineral chrysotile and are formed by rolling double layers into the socalled “nanoscrolls” [16]. This is due to the moment of forces induced by the internal stresses that arise as a result of the misfit between the sizes of tetrahedral and octahedral frameworks of the chrysotile structure [17]. The ends of nanoscrolls are opened, which provides the possibility of filling their internal channels with various substances. The incorporation of any substances into nano tubes in order to prepare nanowires with different types of conductivity is of great interest for researchers dealing with nanotubes. It is believed that these wires can provide the basis for nanoelectronics. There are a small number of studies on the filling of hydrosilicate nanotubes, i.e., natural chrysotile asbestos [18–20]. For the most part, these are studies on the filling of internal channels of chrysotile nanotubes with metal melts at high temperatures (700°C) and pressures up to 10 kbar. Kumzerov et al. [19] discussed the possibil ity of using chrysotile nanotubes as a dielectric matrix for investigating the thermal conductivity of thin nanowires formed during the introduction of metals or semiconductors into channels of chrysotile asbestos nanotubes. Studies on the filling of chrysotile nanotubes with liquid substances are not available in the literature. In our earlier works [21, 22], we carried out the investiga tions of heat treatment (at temperatures of 50 and 80°C) of synthetic hydrosilicate nanotubes with aque ous solutions of potassium salts and potassium hydroxide for different times and demonstrated for the first time that these solutions can be intercalated into nanotubes. By using the KOH and KCl compounds, we developed the technique for filling hydrosilicate nanotubes with aqueous solutions of inorganic sub
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Fig. 1. Xray diffraction patterns of (1) the initial nano tubes and the nanotubes treated in a 0.5 M CsCl solution for (2) 2, (3) 4, (4) 6, (5) 8, and (6) 10 h.
stances. However, from the practical point of view, the filling of nanotubes with solutions of cesium salts and cesium hydroxides is of special interest. The experi mental data obtained can be subsequently used at dif ferent stages of cleaning and processing of liquid radioactive wastes for the selective separation of some solution components from them, as well as for the immobilization and storage of gaseous and liquid sub stances. At present, this is one of the important prob lems of the development of nuclear industry. Taking into account the foregoing, the purpose of this work was to investigate the intercalation of aque ous solutions of cesium salts and cesium hydroxide into synthetic hydrosilicate nanotubes with a chryso tile structure. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE Hydrosilicate nanotubes of the Mg3Si2O5(OH)4 composition with a chrysotile structure were synthe sized under hydrothermal conditions according to the technique described in [9]. Mixtures of silicon oxide (KSMG silica gel, GOST (State Standard) 395676) and magnesium oxide (analytical grade) in the MgO : SiO2 = 3 : 2 ratio were used as the initial components. These mixtures were subjected to hydrothermal treat ment by aqueous solutions of NaOH (with a concen tration of 1 wt %) at a temperature of 350°C and a pressure of 70 MPa for 24 h. The products of hydro thermal treatment were washed from alkali compo
nents by decantation, dried in air at a temperature of 110–120°C, and subjected to physicochemical inves tigations with the purpose of controlling the degree of purity, singlephase composition, and the structural state of the product. Xray powder diffraction analysis of nanotubes was performed on a DRON3 diffractometer (CuKα radia tion). The shape and sizes of nanotubes were deter mined with the use of transmission electron microscopy on an EM125 electron microscope (Uacc = 75 kV) and a JEM 2100F highresolution electron microscope (Uacc, up to 200 kV). The elemental composition of the samples was determined using energydispersive Xray analysis with an Oxford Link microprobe attachment to the scanning electron microscope. The error in the deter mination of the element content amounted, on aver age, to 0.3 wt % [23]. The specific surface area of the samples was mea sured from the thermal desorption of nitrogen by the Brunauer–Emmett–Teller method described in [24]. In this case, the error in the determination of the spe cific surface area did not exceed 5%. The filling of the synthesized chrysotile nanotubes with the aqueous solutions of cesium chloride and cesium hydroxide was studied by the technique devel oped in our previous works [21, 22]. The synthesized nanotubes were treated in 0.5 and 1.0 M CsCl, CsNO3, and CsOH solutions, which were prepared from the salts CsCl (specialpurity grade) and CsNO3 (special purity grade), as well as the aqueous solution of CsOH (Aldrich). The initial weighed portions of nanotubes were placed in glass vessels with a cover (0.04 g in each vessel), and the salt or hydroxide solution was added to the vessels. The vessels were held in a preheated ther mostat at a temperature of 80°C for 2–10 h. After the completion of holding, the vessels with the samples were removed from the thermostat and cooled in air. The nanotubes were separated from the solution by decantation. Then, they were poured several times by distilled water (40 ml) and held at room temperature for 1 h. After each treatment, the nanotubes were sep arated from the liquid phase. Thereafter, the samples were dried in air at room temperature, and their ele mental composition and structural parameters were determined. RESULTS AND DISCUSSION The investigation of the products of hydrothermal synthesis with the use of the above methods revealed that the prepared samples, which represent magne sium hydrosilicate Mg3Si2O5(OH)4 with a chrysotile (Fig. 1, pattern 1) nanotubular (Fig. 2a) structure, have a singlephase composition and are structurally and morphologically homogeneous. The electron microscopic examination showed that the synthesized nanotubes have a cylindrical shape with insignificantly
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Fig. 2. Electron micrographs of (a) the initial nanotubes and (b–d) the nanotubes treated in (b) the CsCl solution for 10 h, (c) the CsOH solution for 10 h, and (d) the CsOH solution for 24 h.
varying outside (20–25 nm) and inside (4–5 nm) diameters and a length of 300–500 nm. The specific surface area of the synthesized samples amounted to 50–60 m2/g. The synthesized chrysotile nanotubes were filled using heat treatment with the aqueous solutions of cesium hydroxide and cesium salts (see table). In this case, we observed a different behavior of the solutions under investigation in the nanotubes and revealed the specific features of their transformation under the action of solutions with different chemical natures. If the nanotubes were treated with the 0.5 M CsCl solution at a temperature of 80°C, their filling is rapid for the first two hours of treatment, is somewhat decel erated in subsequent hours, and is sharply accelerated after the treatment for eight hours. This is clearly seen from the plot constructed from the results of energy dispersive Xray analysis of the samples treated for dif ferent times (Fig. 3). This circumstance is also con firmed by the fact that, in the Xray diffraction pat terns of the treated samples, there appear lines of CsCl with the intensity that noticeably increases with the treatment duration (8–10 h) (Fig. 1). This is accom panied by a decrease in the intensity of chrysotile lines. It should also be noted that the dependences of the Cs
and Cl contents in the nanotubes on the duration of holding in the CsCl solution (Fig. 3) almost coincide with each other, which indicates the presence of Cs+ and Cl– ions in the nanotubes in the form of cesium chloride. The investigations carried out with the use of the highresolution electron microscope showed that the transport of the CsCl solution occurs predomi nantly in the internal channel of the nanotubes and in the interlayer space of the hydrosilicate structure (Fig. 2b). As a consequence, the outside diameter of nanotubes increases by a factor of approximately two. The filling of the internal channel of the nanotube can be clearly seen from the darkfield images: the cesium cations as the heaviest atoms reflect electrons most strongly. Furthermore, the calculations per formed from the data of microdiffraction recorded from the internal filled part of the nanotube (the chan nel) confirm the presence of the cubic phase CsCl inside the nanotube (Fig. 2b). The incorporation of the CsCl solution inside the nanotubes is also confirmed by the results of the Xray investigation of the samples: an increase in the value of the (002) reflection indicates an increase in the inter layer space of the nanotube structure (see table). In spite of the aforementioned structural transformation
Influence of the duration of holding of nanotubes at a temperature of 80°C in the CsCl solution and in water on the value of the interlayer spacing Sample
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of the nanotubes, their destruction does not occur. The destruction of nanotubes is also not observed with an increase in the concentration of the CsCl solution to 1 M, which leads to an increase in the intensity of the filling process (Fig. 3). The interaction of the hydrosilicate nanotubes with cesium nitrate proceeds in a similar manner. As in the case of treatment of the nanotubes by the CsCl solu tion, for short treatment times (2–4 h), the filling of nanotubes with the CsNO3 solution occurs and
increases with an increase in the treatment duration (Fig. 4). Already after the holding of the nanotubes in the cesium nitrate solution for 2 h, the Xray diffrac tion patterns contain reflections that correspond to the CsNO3 phase and become more pronounced for longer holdings of the nanotubes in the solution (Fig. 5). The destruction of nanotubes was not observed over the entire time–temperature range of treatments. A more complex character of the behavior of the solutions is revealed by filling the Mg3Si2O5(OH)4 hydrosilicate nanotubes with the CsOH solutions: at a temperature of 80°C, their filling with the hydroxide solution (Fig. 6) and the chemical interaction of the inner silicon oxide layer of nanotubes with the CsOH solution proceed for 2–6 h. This interaction leads to the formation of a number of cesium silicates (Cs2SiO3, Cs2Si2O5, and Cs2Si4O9) identified by Xray powder diffraction analysis (Fig. 7). An increase in the duration of the action of the CsOH solutions to 8 and especially to 10 h results in an increase in the diameter of nanotubes; in this case, the inside diameter increases most significantly and the wall thickness of the nanotubes decreases sharply. In the case of treatment for 8 h, the filling of nano tubes reaches a maximum (Fig. 6). The cesium silicate solutions, which are formed in the chemical reaction with the silicon oxide layer of the nanotubes, have a higher viscosity as compared to water and the CsOH solution [25]; therefore, a further incorporation of the CsOH solution into the nanotubes is hindered in spite of a considerable structural–morphological transfor
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Fig. 5. Xray diffraction patterns of (1) the initial nano tubes and (2–6) the nanotubes treated in a 0.5 M CsNO3 solution for (2) 2, (3) 4, (4) 6, (5) 8, and (6) 10 h. GLASS PHYSICS AND CHEMISTRY
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Fig. 6. Dependence of the amount of cesium contained in the nanotubes of the Mg3Si2O5(OH)4 composition (mol %) on the duration of treatment (t, h) in a 0.5 M CsOH solution.
mation of nanotubes, which manifests itself in the deformation and a sharp decrease in the thickness of their walls, the expansion of the internal channel (especially substantial after the treatment for 10 h), and an increase in the outside diameter (Fig. 2c). Nonetheless, the nanotubes are not destructed under the aforementioned treatment conditions. Even an increase in the duration of the heat treatment of nan otubes by the CsOH solution to 24 h does not lead to their destruction (Fig. 2d). CONCLUSIONS Thus, the performed investigations made it possible to establish the difference between the processes of filling of the hydrosilicate nanotubes of the Mg3Si2O5(OH)4 composition with aqueous solutions of different chemical natures and showed that a varia tion in the time–temperature conditions does not cause the destruction of nanotubes. The results obtained allow us to recommend the use of nanotubes for the selective sorption of individual solution com ponents and their storage in nanotubes. It was demonstrated that, in principle, the nano tubes of the Mg3Si2O5(OH)4 composition can be filled with aqueous solutions of cesium chloride, cesium nitrate and cesium hydroxide. It was revealed that the salt solutions fill both the internal channel of the nanotube and the interlayer spaces of the nanoscroll structure, which results in some expansion of the nanotube structure. The filling of the nanotubes with the CsOH solu tion leads to the chemical interaction of their silicon– oxygen layer with the solutions and, as a result, to the structural–morphological transformation of nano tubes. It was shown that the destruction of nanotubes is absent in the ranges of temperatures, concentrations, GLASS PHYSICS AND CHEMISTRY
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Fig. 7. Xray diffraction patterns of (1) the initial nano tubes and (2–6) the nanotubes treated in a 0.5 M CsOH solution for (2) 2, (3) 4, (4) 6, (5) 8, and (6) 10 h. Peaks unmarked in the diffraction patterns correspond to cesium silicate phases.
and treatment durations under investigation irrespec tive of the chemical nature of the solution. ACKNOWLEDGMENTS We are grateful to I.A. Drozdova and A.A. Sitnik ova for performing the electron microscopy investiga tions. This study was supported by the Russian Founda tion for Basic Research (project no. 080300456a). REFERENCES 1. Iijima, S., Helical Microtubules of Graphitic Carbon, Nature (London), 1991, vol. 354, pp. 56–59. 2. Tenner, R. and Rao, C.N.R., Inorganic Nanotubes, Philos. Trans. R. Soc. London, Ser. A, 2004, vol. 362, pp. 2099–2125. 3. Rao, C.N.R. and Nath, M., Inorganic Nanotubes, Dalton Trans., 2003, no. 1, pp. 1–24. 4. Ivanovskii, A.L., NonCarbon Nanotubes: Synthesis and Simulation, Usp. Khim., 2002, vol. 71, no. 3, pp. 203–225. 5. Zakharova, G.S., Volkov, V.L., Ivanovskaya, V.V., and Ivanovskii, A.L., Nanotubes and Related Nanostruc tures of dMetal Oxides: Synthesis and Computer Design, Usp. Khim., 2005, vol. 74, no. 7, pp. 651–685. 6. Jancar, B. and Suvorov, D., The Influence of Hydro thermalReaction Parameters on the Formation of Chrysotile Nanotubes, Nanotechnology, 2006, vol. 17, pp. 25–29. 7. Falini, G., Foresti, E., Gazzano, M., Gualtieri, A.F., Leoni, M., Lesci, I.G., and Roveri, N., Tubular Shaped Stoichiometric Chrysotile Nanocrystals, Chem.—Eur. J., 2004, vol. 10, no. 12, pp. 3043–3049. 2010
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