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J. Phys. Chem. B 2004, 108, 13254-13257
Growth of NaFe4P12 Skutterudite Single Crystalline Nanosprings Synthesized through a Hydrothermal-Reduction-Alloying Method Hong Liu,*,† Hongmei Cui,† Jiyang Wang*,† Lei Gao,† Feng Han,† R. I. Boughton,‡ and Minhua Jiang† State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100 China and Department of Physics and Astronomy, Bowling Green State UniVersity, Bowling Green, Ohio 43403 ReceiVed: April 7, 2004; In Final Form: July 2, 2004
NaFe4P12 nanosprings were synthesized using a hydrothermal method at 170 °C for 24 h. The nanosprings are 1000-5000 nm in length and 200-1000 nm in diameter and consist of coiled nanobelts of 80-150 nm in width and 20-50 nm in thickness. With an increase in the annealing time, the nanospring grows as a macrorod with a multilayered helical tubular structure. X-ray diffraction (XRD) patterns indicate that the nanosprings have the skutterudite crystal structure with cell parameter a ) 7.782 Å. Electron diffraction (ED) and high-resolution transmission electron microscopy (HRTEM) prove that the nanobelts grow along the (111) plane and that their growth direction is not parallel to the [110] direction in the growth plane. A possible growth process for NaFe4P12 nanosprings is suggested.
1. Introduction Over the past decade, wirelike nanostructures, including nanotubes,1-5 nanowires,6-9 nanorods,10,11 and nanocables,12,13 have attracted much attention because of their novel physical properties and their potential application for constructing nanoscale electronic and optoelectronic devices.1,14-16 Recently, a new nanostructure, a beltlike structure (a so-called nanobelt), having a potential for applications in inexpensive, ultrasmall sensors, flat-panel-display components, and other electronic devices, has become one of the most widely investigated nanostructures during the past three years.17-22 Although long semiconductor nanobelts can be obtained by some synthesis methods, it is difficult to construct complex nanostructures using nanobelts as the basic building blocks because nanobelts are too tiny to be manipulated. The fabrication of a nanostructure with fixed regularity that is built from nanobelts for special device applications is one of the greatest challenges in applying them. Most recently, some oxide, nanoring, nanaonanohelixes, and nanosprings have been synthesized successfully by Wang and his group.23-26 These new nanostructures will attract much attention of materials scientists. Skutterudite is one of the most promising thermoelectric semiconductor materials because of its high mobility and large Seebeck coefficient and the marked reduction in lattice thermal conductivity it displays upon filling the interstitial voids in the crystal structure with lanthanum or other ions.27-30 In recent years, some theoretical and experimental research has shown that low-dimensional materials, especially nanostructures such as quantum wires and quantum wells, possess excellent thermoelectric properties compared to conventional bulk thermoelectric materials.31-34 However, for skutterudites, it is difficult to prepare a nanostructure by conventional physical methods because of its complex crystal structure. Recently, we success* Corresponding authors. E-mail: icm.sdu.edu.cn. † Shandong University. ‡ Bowling Green State University.
[email protected]; jywang@
fully synthesized the skutterudite semiconductor NaFe4P12 in whisker and nanowire form by using a hydrothermal-reduction-alloying method.35,36 These materials can form the building blocks for the fabrication of advanced thermoelectric nanostructures. Here, we report on some novel skutterudite nanosprings synthesized through a hydrothermal method similar to that used in previous work under different reaction conditions. The nanosprings consist of coiled skutterudite nanobelts. Because skutterudite is an alloy and a semiconductor material, this new nanostructure may have the potential for application in fabricating nanomachines and nanoelectronic, nanothermoelectric, and nanoelectric transforming devices. 2. Experiments Two grams of FeCl3‚6H2O, 1 g of NaOH, and 1.2 g of white phosphorus were added to distilled water in a 24-mL stainless steel autoclave with a Teflon liner. All the reactant reagents are analytical grade, obtained from Shanghai Chemical Regents Co. The total volume of the reactants in solution was 23 mL. The volume fraction of the reactant solution in the autoclave in this work (95%) was much higher than that in our previously reported work (75%), so during the hydrothermal process the pressure in the autoclave at the same heating temperature was much higher. The autoclave was put into an electric furnace that was preheated to 170 °C. After being heated for 24 h, the autoclave was taken out and cooled to room temperature. To investigate the effect of heating time on the morphology of the product, the reaction was repeated at the same temperature for different heating times. The pH value of the product solution was about 5, which is much lower than the pH value of the reactant solution (pH > 12). The black product powder obtained was first washed carefully with HCl (pH ) 1-2) and distilled water, then washed with CS2 to eliminate excessive white phosphorus, and finally dried at 60 °C in a vacuum oven. The product was characterized and analyzed by X-ray diffraction (XRD) (Rigaku D/max γB with Ni-filled Cu KR radiation), scanning electron microscopy (SEM) (Hitachi S800 FEG), transmission electron microscopy (TEM), and electron diffrac-
10.1021/jp0484736 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/11/2004
Growth of NaFe4P12 Nanosprings
J. Phys. Chem. B, Vol. 108, No. 35, 2004 13255 tion (ED) [Hitachi HF-2000 FEG at 200 kV and Philips Tecnai 20v-Twin high-resolution transmission electron microscope (HRTEM) at 400 kV]. 3. Results and Discussions
Figure 1. XRD patterns of powder products synthesized at 170 °C for different times: (a) 24 h; (b) 40 hours.
Figure 1a and b shows the XRD patterns of the powder product synthesized at 170 °C for 24 h and 40 h, respectively. From Figure 1, we can see that the XRD patterns of the product synthesized at 170 °C for different times are very similar. Almost all the peaks of the pattern correspond to those of the skutterudite LaFe4P12 synthesized by Jeitskcho (JCPD710236).7,37 The calculated cell parameters a of the products synthesized at 170 °C for 24 and 40 h are 7.782 and 7.794 Å, respectively, values which are slightly smaller than that reported for LaFe4P12 (a ) 7.832). From the XRD patterns, we find that the (220) peak is much higher than the other peaks, an indication that the particles of the powder product have grown along a special direction. Figure 2a-c shows the TEM images of the powder synthesized at 170 °C for 24 h. Most of particles of the product are helical nanostructures (Figure 2a) of 1000-5000 nm in length and 200-1000 nm in diameter, so-called nanosprings. Figure 2b and c shows two separate nanosprings. The nanosprings are of helical shape as formed by coiled nanobelts that are about 80-150 nm in width and 20-50 nm in thickness. The inset of
Figure 2. TEM and SEM images of the product powders nanosprings synthesized at 170 °C for (a) 24 h; (b) and (c) separate nanosprings; inset of c: ED pattern of the top of the nanospring; (d) SEM image of product powder synthesized at 170 °C for 40 hours.
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Figure 3. HRTEM image of a nanospring. (a) Morphology of a nanospring, (b) ED pattern of the nanospring, and (c) HRTEM image of the top of the nanospring perpendicular to the nanobelt.
Figure 2c is the electron diffraction pattern perpendicular to the plane of a nanobelt at the top of a nanospring and can be indexed as the [111] zone axis of the skutterudite crystal. We suggest that nanobelt growth occurs along the (111) face of skutterudite. Figure 2d shows the SEM image of the powder product synthesized at 170 °C for 40 h. The exterior shape of the particles remains as a helical rod and marks of the pitch of the spring can still be seen on the surface of the rods. From the SEM image taken of a broken rod, we can see that the rod is nearly solid and is self-assembled by the helical tubes layer by layer. An HRTEM image and ED pattern of a nanospring is shown in Figure 3. A well-shaped nanospring of 1000-2000 nm in width, consisting of a long coiled nanobelt, is shown in Figure 3a. Because the nanobelt is bent into a helix, when an electron beam goes through it diffraction should occur on a series of different atomic planes of the skutterudite structure. Therefore, we can see that the diffraction pattern of the nanospring consists of several sets of diffraction dots (Figure 3b). Figure 3c shows the lattice image of the top of the nanospring perpendicular to the belt surface. The lattice pattern exhibited by the belt indicates that it is a well-crystallized single crystal. The interplanar spacing of 0.54 nm as indicated between arrowheads corresponds to the distance between the (110) planes (5.47 Å). In some regions of the image, a hexagonal two-dimensional image can be found. At these points, the plane of the nanobelt surface is parallel to the (111) plane of skutterudite. The HRTEM images support our suggestion that the nanobelt grows along the (111) face. From Figure 3c, we can see that the growth direction of the nanobelt is not parallel to the [110] direction on the (111) face. The angle between the growth direction and the [110] direction on the growth plane is about 14°. This is the reason the nanospring forms as the nanobelt bends into a ring. From the above experiments performed on the nanospring, we can suggest a possible growth process for skutterudite nanosprings as shown in Figure 4. The formation of skutterudite NaFe4P12 nuclei from the reactant solution has been discussed in previous papers.31,32 Briefly, skutterudite solid nuclei can form in the solution through an alloying reaction with iron from the reduction of iron hydroxide and phosphorus by the deposition of PH3. PH3 vapor is formed through a reaction between white phosphorus and sodium hydroxide solution at high temperature. When the skutterudite nuclei form, they grow along the (111) face and form nanobelts because of the high pressure obtained (Figure 4a). With increasing length of the nanobelts, they become curved because defects form during their formation (Figure 4b). As discussed above, the growth direction of the nanobelt is not parallel to the crystal axis [110] in the growth plane (111), so with the increase in the length of the nanobelts,
Figure 4. A possible growth process of NaFe4P12 nanosprings: (a) A short nanobelt grown from a skutterudite nucleate; (b) a curved belt; (c) an arched nanobelt; (d) a nanospring; (e) and (f) multilayer structures.
the curving belts have difficulty in forming closed nanocircles but easily form nanohelices (Figure 4c). As the growth process continues, the nanohelices become long nanosprings (Figure 4d). With an increase in the synthesis time, the nanobelts that form the nanospring become thick and wide, the pitch of the nanosprings becomes small, and so the morphology of the nanospring becomes tubular. At the same time, some nuclei form and some nanosprings grow on the surface of the initial nanospring to form a second layer (Figure 4f). Then, the third layer forms, and so on. Finally, a multilayered structure of nanosprings can form with the accumulation of NaFe4P12 skutterudite molecules in the solution Figure 4g. In this work, we have demonstrated that skutteudite nanosprings can be synthesized repeatedly. Nanosprings have potential applications in the preparation of nanoelectronic components, such as quantum devices and nanoelectric transformers. They also have potential in use as springs in nanomachines. Therefore, they are attractive to scientists in the nanoelectronics and nanomechanics areas. Of course, because the crystal structure of the nanospring is filled skutterudite, the nanospring can serve as a basic building block for the construction of advanced thermoelectric materials with a quantum structure. 4. Conclusion NaFe4P12 nanosprings of 200-2000 nm in diameter and 1000-5000 nm in length formed by coiled nanobelts of 80150 nm in width and 20-50 nm in thickness were synthesized by a hydrothermal-reduction-alloying method at 170 °C for 24 h. XRD measurements indicate that NaFe4P12 nanosprings are cubic with a cell parameter a ) 7.782 Å. HRTEM images indicate that the nanobelts grow along the (111) plane of the skutterudite structure, but the growth direction of the nanobelt is not parallel to the [110] direction in the growth plane. The growth of the nanosprings is caused by the bending of the nanobelts as they grow and by the difference between the growth direction and the crystalline zone axes in the growth plane. With increasing annealing time, the nanosprings can grow to be rods with a multilayered helical structure. Acknowledgment. This work was supported by the National High Technology Research and Development Program of China
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