Langmuir 2007, 23, 3409-3417
3409
Dispersibility, Stabilization, and Chemical Stability of Ultrathin Tellurium Nanowires in Acetone: Morphology Change, Crystallization, and Transformation into TeO2 in Different Solvents Wen-Jie Lan, Shu-Hong Yu,* Hai-Sheng Qian, and Yong Wan DiVision of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, School of Chemistry and Material Science, UniVersity of Science and Technology of China, Hefei 230026, P. R. China ReceiVed NoVember 8, 2006. In Final Form: December 20, 2006 The dispersibility and stabilization of freshly synthesized ultrathin tellurium nanowires with diameters of 4-9 nm using poly(vinyl pyrrolidone) (PVP) as a capping agent can be well controlled through an easy acetone-addition process. Ultrathin Te nanowires synthesized by a hydrothermal method using PVP as a capping agent will aggregate in a water/acetone system, and their aggregation state strongly relies on the volume of water and acetone in this mixed solution. This phenomenon is due to the different solubility of PVP in water and acetone, which has significant influence on the dispersibility and stabilization of the nanowires. The results also demonstrate that the freshly prepared Te nanowires are not stable after being stored for a prolonged time in contact with air, ethanol, and water. Ultrathin Te nanowires can be oxidized easily with various final morphologies, which are core-shell structures in contact with air, amorphous nanoparticles and nanoplatelets in ethanol, and large square flakes in water. The entire conversion process from crystalline Te nanowires to amorphous TeO2 nanoparticles or single-crystal paratellurite (TeO2) at room temperature was carefully studied, implying that tellurium nanowires synthesized by other chemical methods and other nanomaterials after synthesis could also not be stable, and their storage methods require special attention.
1. Introduction During the past decades, numerous nanostructured materials have been synthesized.1 From the viewpoint of applications, the stabilities of these materials and the storing methods are key issues. Recently, Kotov and his co-workers found that nanoparticles of CdTe can spontaneously reorganize into crystalline nanowires upon controlled removal of the protective shell of an organic stabilizer.2 Also, in the presence of EDTA, stabilizerdepleted CdTe nanoparticles were found to go through a chemical decomposition and form highly crystalline Te nanowires with a diameter of 100 nm spontaneously.3 Moreover, nanoparticles were found to have various responses, including structural changes toward external physical and chemical stimuli.4 However, few reports have been published concerning the stabilities of onedimensional nanostructures in solution. Tellurium, a narrow band gap semiconductor material, exhibits many important applications, such as being used for nonlinear optical response devices, photoconductive detectors, and other applications concerning its electronic and optical electronic properties.5 Especially, due to its anisotropic growth tendency * To whom correspondence should be addressed. Fax: + 86 551 3603040. E-mail:
[email protected]. (1) (a) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (b) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (b) Lieber, C. M. Solid State Commun. 1998, 107, 607. (c) Yang, P. D.; Wu, Y. Y.; Fan, R. Int. J. Nanosci. 2002, 1, 1. (d) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayer, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 5, 353. (2) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (3) Tang, Z. Y.; Wang, Y.; Sun, K.; Kotov, N. A. AdV. Mater. 2005, 17, 358. (4) (a) Zhang, H.; Gilbert, B.; Huang, F.; Banfield, J. F. Nature 2003, 424, 1025. (b) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (c) Aguirre, C. M.; Kaspar, T. R.; Radloff, C.; Halas, N. J. Nano Lett. 2003, 3, 1707. (5) (a) Kudryavstev, A. A. The Chemistry and Technology of Selenium and Tellurium; Collet’s, Ltd.: London, 1974. (b) Beauvais, R.; Lessard, A.; Galarneau, P.; Knystautas, E. J. Appl. Phys. Lett. 1990, 57, 1354. (c) Shih, I.; Champness, C. H. J. Cryst. Growth 1978, 44, 492. (d) Tangney, P.; Fahy, S. Phys. ReV. B. 2002, 14, 279.
and potential applications, synthesis of tellurium nanowires has attracted more and more attention.6 In addition, as an oxide of tellurium, paratellurite (TeO2) is a significant acousto-optical material with elastic behavior, high refractive index, and good optical quality.7 Moreover, in paratellurite, different regimes of anisotropic Bragg scattering with unequal selectivity and sensitivity to changes in acoustic frequency, optical wavelength, and angle of incidence may be observed owing to its inner anisotropic interaction between sound and light.8 Large crystals of paratellurite can be grown both by resistive heating and induction heating methods based on Czochralski technology with different temperature gradients.9 More recently, we have demonstrated that Te nanoribbons with a width of 200-300 nm tend to be destroyed and dissolve in deionized water and pure ethanol with prolonged storing time.10 This kind of Te nanoribbon, synthesized using amino acids as additives, is not as stable as that produced in ethylene glycol solvent.11 Very recently, our group has developed a new largescale selective synthesis of uniform ultrathin tellurium nanowires with diameters of 4-9 nm by a hydrothermal method using poly(vinyl pyrrolidone) (PVP) as a capping agent,12 and these (6) (a) Zhu, Y. J.; Wang, W. W.; Qi, R. J.; Hu, X. L. Angew. Chem., Int. Ed. 2004, 43, 1410. (b) Yu, H.; Gibbons, P. C.; Buhro, W. E. J. Mater. Chem. 2004, 14, 595. (c) Gautam, U. K.; Rao, C. N. R. J. Mater. Chem. 2004, 14, 2530. (d) Mayers, B.; Xia, Y. N. J. Mater. Chem. 2002, 12, 1875. (e) Lu, Q. Y.; Gao, F.; Komarneni, S. AdV. Mater. 2004, 16, 1629. (f) Tang, Z. Y.; Wang, Y.; Sun, K.; Kotov, N. A. AdV. Mater. 2005, 17, 358. (7) Uchida, N.; Ohmachi, N. J. Appl. Phys. 1969, 40, 4692. (8) (a) Voloshinov, V. B. Opt. Eng. 1992, 31, 2089. (b) Kumaragurubaran, S.; Krishnamurthy, D.; Subramanian, C.; Ramasamy, P. J. Cryst. Growth 2000, 211, 276. (9) (a) Kumaragurubaran, S.; Krishnamurthy, D.; Subramanian, C.; Ramasamy, P. J. Cryst. Growth 1999, 197, 210. (b) Grabmaier, J.G.; Plattner, R. D.; Schieber, M. J. Cryst. Growth 1973, 20, 82. (c) Lukasiewicz, T.; Majchrowski, A. J. Cryst. Growth 1992, 116, 364. (10) He, Z. B.; Yu, S. H. J. Phys. Chem. B 2005, 109, 22740. (11) (a) Mayers, B.; Xia, Y. AdV. Mater. 2002, 14, 279. (b) Mayers, B.; Xia, Y. J. Mater. Chem. 2002, 12, 1875. (12) Qian, H. S.; Yu, S. H.; Gong, J. Y.; Luo, L. B.; Fei, L. F. Langmuir 2006, 22, 3830.
10.1021/la063272+ CCC: $37.00 © 2007 American Chemical Society Published on Web 02/13/2007
3410 Langmuir, Vol. 23, No. 6, 2007
Lan et al.
synthesized by a hydrothermal method using PVP as a capping agent. In addition, the transformation of ultrathin Te nanowires to amorphous TeO2 nanoparticles and crystalline paratellurite TeO2 plates at room temperature by just changing the solvents has been discovered. The entire conversion process from crystalline Te nanowires to amorphous TeO2 nanoparticles or paratellurite single crystals (TeO2) in different solvents has been carefully studied. 2. Experimental Section
Figure 1. The dispersibility state of freshly prepared ultrathin tellurium nanowires in a mixed solution made of water and acetone. The volume ratios of deionized water and acetone were (a) 1:1, (b) 1:2, (c) 1:2.5, and (d) 1:3.
nanowires can act as excellent templates for the production of Te@C nanocables and carbon nanofibers13 or carbon nanofibers and silica nanotubes loaded with noble metal nanoparticles.14 In the process of synthesis and storage, it has been found that the as-prepared ultrathin Te nanowires can easily be oxidized in water and transformed to paratellurite nanocrystals at room temperature. Thus, the investigation on the chemical stabilities of ultrathin Te nanowires in different environments is very important from the viewpoint of applications. In this work, we systematically examine the dispersibility and chemical stability of as-prepared ultrathin Te nanowires with diameters of 4-9 nm in different environments, which are
All chemicals were analytical grade, commercially available from Shanghai Chemical Reagent Co., Ltd., and used in this study without further purification. Dispersibility Behavior and Aggregation of Ultrathin Te Nanowires in Acetone. Ultrathin tellurium nanowires with diameters of 4-9 nm were prepared using a hydrothermal technique reported previously.12 Briefly, a mixture solution containing 0.25 mmol of sodium tellurite, 1 mL of hydrazine hydrate (85%, w/w %) and 2 mL of aqueous ammonia solution (25-28%, w/w %) was added into 15 mL of PVP solution (0.6 g PVP). Then, 6 mL of doubledistilled water was added again to 80% capacity of the total volume of Teflon vessel (the total volume capacity of a Teflon vessel is 30 mL). The container was closed and maintained at 180 °C for 4 h. After that, the autoclave was cooled down to room temperature naturally. The final produced dispersion in the autoclave was poured into a beaker with a volume of 250 mL, and then the acetone was added into the solution gradually with continuous shaking. The volume ratio of water and acetone (VH2O/Vacetone) was respectively varied as 1:1, 1:2, 1:2.5, and 1:3. Chemical Stability of Ultrathin Te Nanowires in Different Solution Systems. The as-prepared nanowires were precipitated by the addition of 60 mL of acetone. After that, the product was centrifuged and washed with absolute alcohol and double-distilled water several times. Then a quantity of nanowires was sequentially
Figure 2. TEM images of freshly prepared tellurium nanowires in a mixed solution made of water and acetone, showing the aggregation behavior in the water/acetone mixed solution. The volume ratios of deionized water and acetone were (a) 1:1, (b) 1:2, (c) 1:2.5, and (d) 1:3. The TEM samples were made by dipping the dispersion solution on a Cu grid, then they were observed as soon as possible after 1 h.
Stability of Ultrathin Te Nanowires in Acetone
Figure 3. TEM images of the samples. (a) Ultrathin Te nanowires synthesized in water in the presence of PVP without being washed after being stored for 2 weeks. (b) After adding acetone into the dispersion in panel a, the sample was collected after the dispersion of nanowires was stored in a mixed solution with a volume ratio of VH2O/Vacetone ) 1:2 for 1 h. dispersed in ethanol and deionized water. The nanowires were separated from the different media after different time intervals for examination of their structures and morphologies. Characterization. The obtained samples were characterized on an (Philips X’Pert Pro Super) X-ray powder diffractometer with Cu Ka radiation (λ ) 1.541874 Å). The morphology was examined with a JEOL JSM-6700F scanning electron microscope (SEM), a Hitachi (Tokyo, Japan) H-800 transmission electron microscope (TEM) at an accelerating voltage of 200 kV, and a high-resolution TEM (HRTEM) (JEOL-2010) operated at an acceleration voltage of 200 kV. X-ray photoelectron spectra (XPS) were measured on an ESCALab MKII X-ray photoelectron spectrometer, using a Mg KR radiation as the exciting source. UV-vis spectra were recorded on a UV-2501PC/2550 at room temperature (Shimadzu Corporation, Japan).
3. Results and Discussion 3.1. Dispersibility and Stabilization Behavior of Te Nanowires in a Water/Acetone System. As-synthesized ultrathin Te nanowires display excellent dispersibility in pure water. The addition of acetone to such a solution results in a dramatic change in the dispersibility. The dramatic change in the macroscopical aggregation state of Te nanowires upon the addition of acetone to the initial dispersion is shown in Figure 1. It can be seen that the initial solution remained homogeneous, and the nanowires dispersed quite well in the system with the volume ratio between water and acetone being 1:1 and 1:2 (Figure 1a,b). However, as the volume ratio VH2O/Vacetone increased to 1:2.5, the dispersion started to become heterogeneous, and there was a large amount
Langmuir, Vol. 23, No. 6, 2007 3411
Figure 4. TEM images of the samples that were obtained after the freshly prepared nanowires were dipped on a copper grid and were exposed to air for (a) 2 months, and (b) 3 months.
of small floc dispersed widely in the solution, which started to affix on the glass wall (Figure 1c). Once the volume of acetone increased to 3 times that of the water, the nanowires aggregated to a large floc and precipitated at the bottom of the flask (Figure 1d). The corresponding morphologies of the nanowires in different solutions with different volume ratios of VH2O/Vacetone were examined as shown in Figure 2. When the volume ratio of VH2O/ Vacetone is 1:1, the dispersibility state of the nanowires is almost the same as that in pure water (Figure 2a). When the volume ratio VH2O/Vacetone increased to 1:2, although the digital photo did not show any change, the TEM image (Figure 2b) clearly indicates that the tellurium nanowires began to aggregate, accompanying the formation of a lot of nanoparticles. This aggregation state became more evident in the case of VH2O/Vacetone ) 1:2.5, and a lot of nanoparticles appeared (Figure 2c). When the volume ratio VH2O/Vacetone increased to 1:3, more densely aggregated nanowires were observed (Figure 2d). It has been proven that PVP, as a polymer surfactant, can play an important role in the synthesis of one-dimensional nanorods and nanowires of tellurium. The PVP molecules covered on Te nanowires are of great significance in the dispersibility and stability in a water/acetone system. As a good solute, PVP can be easily dissolved in many solvents, such as water, ethanol, glycerol, and so on. However, it shows poor solubility in some solvents, for example, in acetone. There is a large amount of PVP in the as-prepared dispersion of ultrathin Te nanowires, which selectively absorbed on the specific planes of nanowires and also stabilized the nanowires to form homogeneous dispersion. Once the acetone was introduced to this dispersion, due to the poor solubility of PVP in acetone, PVP molecules became unstable
3412 Langmuir, Vol. 23, No. 6, 2007
Lan et al.
Figure 5. TEM images of the samples being stored in ethanol after (a) 3 days, (b) 15 days, (c,d) 70 days, and (e,f) 3 months. The inset pictures shown in panels b and e are the ED patterns taken on the nanoparticles shown in panels a and e, respectively, showing the amorphous feature of the nanoparticles. (g) A magnified TEM image of a part of one nanoplatelet.
in the solution and tended to be desorbed from the nanowires and undergo aggregation. Lacking the absorption and protection of a polymer, the tellurium nanowires could no longer be dispersed well in the solution and began to aggregate owing to their large surface energy. This trend was accelerated with the addition of acetone, and, finally, when the ratio increased to VH2O/Vacetone ) 1:3, a separate PVP phase began to flock, a lot of PVP white particles separated out from the solution, and the wires became aggregated. During the experiments for the examination of dispersibility in different solvents, many small nanoparticles appeared on the (13) Qian, H. S.; Yu, S. H.; Luo, L. B.; Gong, J. Y.; Fei, L. F.; Liu, X. M. Chem. Mater. 2006, 18, 2102. (14) Qian, H. S.; Antonietti, M.; Yu, S. H. AdV. Funct. Mater. 2007, 17, 10.1002/adfm.200600657.
nanowires after being stored in the mixed solution for only a short time (Figure 2b-d). From Figure 3, a clear contrast in the morphologies under these two different storing conditions can be observed. When the sample was stored without any washing, the nanowires could maintain their morphology and chemical stability for at least 2 weeks (Figure 3a). However, the addition of acetone can largely accelerate the oxidization speed of these ultrafine nanowires. Within only 1 h, many nanoparticles appeared on the surface of tellurium nanowires. This observation underlies the idea that the decreasing stability of the nanowires and the desorption of PVP from the nanowires will make the nanowires bare to the oxygen in the solution, thus making them more easily oxidized. Energy-dispersive X-ray (EDX) quantitative analysis results showed that the molar ratio between Te and O was 43.05:56.95.
Stability of Ultrathin Te Nanowires in Acetone
Figure 6. (a) TEM image of the samples after being stored in ethanol after 3 days. (b) An HRTEM image obtained from the edge of an individual nanowire and one nanoparticle shown in panel a. The fringe spacings of 5.9 Å and 4.0 Å correspond to the interplanar distances of the (001) and (100) planes, respectively. The growth direction of this nanowire is along the [001] direction.
This indicated that the sample, after being stored in the mixed solution for one hour, consisted of tellurium and some oxide of tellurium, that is, TeO2. Moreover, TEM images (shown in Figure 3b) indicate that the process of oxidization under the help of acetone occurred spontaneously on the surface of the nanowires. An electron diffraction (ED) pattern showed that the nanowires, which have nanoparticles on the surface, are still single-crystalline (data not shown). Small amorphous nanoparticles are merely absorbed on the surface of the nanowires. The contrast between the two different conditions is due to the partial lost of PVP in the second system as more acetone was added into the nanowire dispersion, which largely decreased the stability of tellurium nanowires. 3.2. Chemical Stability of Ultrathin Tellurium Nanowires in Contact with Air. The nanowires were not stable when exposed to air. Figure 4 shows the TEM images of tellurium nanowires after being stored for a long time in contact with air. After being stored for 2 months, oxidized nanoparticles formed around the nanowires, indicating that the nanowires can easily be oxidized in contact with air. EDX quantitative analysis indicated that only two elementssTe and Oswere detected, and the molar ratio between Te and O was 37.22:62.78, suggesting the existence of TeO2 nanoparticles as in the same initial oxidization process in other media. Another month of exposure to air resulted in the formation of many nanoparticles, which appeared on the nanowires; also, some relatively larger nanoparticles were
Langmuir, Vol. 23, No. 6, 2007 3413
Figure 7. XPS spectra for the sample after being eroded in ethanol for 20 days: (a) survey of the Te3d region in the sample; (b) survey of the O1s region in the sample.
observed (Figure 4b). Most of the nanowires still kept their onedimensional structures; however, these nanowires will be completely destroyed if the original Te nanowires are stored in a solution, and the detailed results will be discussed in the following. 3.3. Chemical Stability of Ultrathin Tellurium Nanowires in Ethanol and Water. The chemical stability of ultrathin tellurium nanowires in ethanol and water was carefully examined. Figure 5 reflects the morphology change and chemical oxidization process of tellurium nanowires in ethanol. In an initial stage, amorphous TeO2 nanoparticles appeared around the nanowires (Figure 5a). This is similar to that in a water/acetone mixed solution, but the oxidization speed is relatively slow. It may be due to the stabilization ability provided by PVP in ethanol, which protects the nanowires from being quickly oxidized. However, the succeeding oxidization process is quite different from that in the water/acetone system or in contact with air. The nanowires in ethanol disappeared completely, accompanying the formation of near-spherical white TeO2 nanoparticles with an average size of 60-70 nm (Figure 5b). An ED pattern confirmed that all these particles were amorphous (inset in Figure 5b). Such uniform nanoparticles existed stably in the ethanol for a long time. However, after aging for 70 days, the dispersion was still stable and contained many nanoparticles with similar particle size as in the very early stage, accompanied by the presence of a few nanoplatelets with a size of 400-500 nm formed in the solution (Figure 5d). These nanoplatelets found in ethanol are not single crystalline, but are still amorphous. If the aging time is prolonged
3414 Langmuir, Vol. 23, No. 6, 2007
Figure 8. XRD pattern of as-prepared product after the oxidization of Te nanowires in water. The reaction was performed at room temperature for 7 days.
up to more than 3 months, the dispersion is still stable, but, again, few platelets and aggregates formed in the solution (Figure 5e,f). An ED pattern indicated that both the small nanoparticles and the few individual platelets were still amorphous (inset in Figure 5e). The magnified TEM image shown in Figure 5g indicates that the platelet was porous. The number of those flakes is no more than 5%. The reason for the appearance of these nanoplatelets is still not clear and needs more careful investigation. Because the large nanoplatelets are amorphous and the small nanoparticles exist stably in the solution, this cannot be well explained by the “Ostwald ripening” theory15 or the “oriented attachment” mechanism16 based on lowering the total energy of a system. The presence of aggregates with platelet-like morphology could be due to the mutual interaction forces between the nanoparticles. It is also more likely that the few PVP molecules
Lan et al.
existing in the solution directed the small amorphous nanoparticles to assemble into the larger platelets, and the detailed aggregation mechanism is still not clear at this stage. The intermediate product after the ultrathin Te nanowires were eroded in ethanol for 3 days was collected for TEM and HRTEM examination. TEM and HRTEM images taken on an individual nanowire and an individual nanoparticle were produced as shown in Figure 6. The HRTEM image in Figure 6b clearly shows interplanar distances of 5.9 and 4.0 Å, corresponding to the (001) and (100) planes, respectively, which are consistent with the result observed previously,12 showing that the incompletely eroded/oxidized Te nanowires still kept their crystalline structures. However, the nearly spherical nanoparticles were amorphous, which can be further confirmed by the ED pattern. The XPS data measured on the sample after being eroded in ethanol for 20 days is shown in Figure 7. The spectrum (Figure 7a) demonstrates the presence of oxidized tellurium (576 eV), accompanied by the presence of an O1s peak (531 eV) in the spectra (Figure 7b). These are consistent with the data for TeO2 reported by Bahl et al.17 However, the oxidization process and morphology change of tellurium nanowires in water are dramatically different from that observed in other media, as discussed above. Figure 8 shows an X-ray diffraction (XRD) pattern of the products obtained from the oxidization of tellurium nanowires in a water solution; all peaks can be indexed to the orthorhombic phase of paratellurite (TeO2) with a cell constant of a ) 12.0 Å, b ) 5.46 Å, and c ) 5.61 Å, which is in good agreement with the standard literature data (JCPDF card number: 74-1131). The enhanced relative intensities of the (h 0 0) diffraction peaks compared with those for the standard indicated the possible preferential orientation of the crystals. The SEM image in Figure 9a,b shows that the white product after storing the ultrathin Te nanowires in water for 10 days is composed of square flakes of TeO2 with edge lengths of about
Figure 9. SEM images of the square flakes obtained from the oxidization of ultrathin tellurium nanowires in water at room temperature for 10 days: (a) a general view of the flakes; (b) a magnified SEM image of the flakes; (c) a general view of the sides of several flakes obtained; (d) a magnified SEM image of one flake.
Stability of Ultrathin Te Nanowires in Acetone
Langmuir, Vol. 23, No. 6, 2007 3415
Figure 10. (a) TEM image of an individual square flake. (b) A lattice-resolved HRTEM image showing that lattice spacings of 5.9 Å and 2.8 Å were observed, which, when taken from the edge of the flake, implies that the flake grew along the [100] and [001] directions. (c) SAED pattern taken on the square flake. The electron beam was focused along the [010] axis.
Figure 11. TEM images of time-dependent morphology evolution during the oxidization of ultrathin tellurium nanowires in water at room temperature: after being stored for (a) 6 h and (b) 10 h in a solution that had about 10 wt % of the prepared nanowires described in the Experimental Section dispersed in 30 mL of water; (c) 6 days in a more dense solution, which had about half of the prepared nanowires dispersed in 50 mL of water; (d) 5 days with a whole bundle of nanowires immersed in water without shaking in order to achieve a heterogeneous solution.
30 µm and thicknesses of 5-15 µm (Figure 9c,d). The unusually strong (h 0 0) reflection peaks observed in the XRD pattern are due to the higher amount of exposure of the (h 0 0) face of the square flakes. The side view of the flakes (Figure 9c) shows that the thick edge consists of several thinner flakes. As shown in Figure 9c,d, the TeO2 square flakes with multilayered structure could be formed through a hierarchical aggregation process or a stacking process.
HRTEM and selective area electron diffraction (SAED) were performed on an individual TeO2 flake. The HRTEM image in Figure 10b was taken from the edge of one single flake, showing lattice spacings of ca. 5.9 and 2.8 Å, corresponding to the lattice spacings of the (200) and (002) planes, respectively, which are perpendicular to each other. The corresponding ED pattern of the TeO2 square flake shown in Figure 10c, which was taken along the [010] axis, confirmed that the flake is single-crystalline.
3416 Langmuir, Vol. 23, No. 6, 2007
Lan et al.
Figure 12. Digital photos of a fresh sample (a) and the samples after being stored in water for (b) 1 day, (c) 2 days, and (d) 6 days.
Figure 14. Digital photos of a fresh sample (a) and the samples after being stored in ethanol for (b) 1 day, (c) 2 days, and (d) 6 days.
Figure 13. Time-dependent UV-vis spectrum change during the oxidization of tellurium nanowires in water at room temperature: (a) fresh sample (b) sample after being stored in water for 2 h, (c) 6 h, (d) 1 day, (e) 2 days, and (f) 6 days.
Figure 15. Time-dependent UV-vis spectra during the oxidization of tellurium nanowires in ethanol at room temperature: (a) fresh sample, (b) sample after being stored in water for 1 day, (c) 2 days, (d) 3 days, (e) 6 days, and (f) 12 days.
Such square-like shape is related to the intrinsic growth habit of paratellurite (TeO2) and the differences in the growth rates along the [100], [001], and [010] directions. Meanwhile, the flake piled up along the [010] axis to form stacked square flakes with multilayers. The time-dependent growth of flakes was investigated. The nanowires became partially eroded after 6 h (Figure 11a). The width was no longer uniform, and some nanowires became fractured due to the attack of water molecules as well as the oxidization, which is similar to that previously found for the erosion of Te nanoribbons with diameters of 200-300 nm synthesized in tetraethylene pentamine aqueous solution.10 After 10 h, amorphous nanoparticles formed around the backbone of the nanowires. A similar process was found in a more concentrated solution (shown in Figure 11c). In order to check the detailed oxidation reaction, after centrifugation and purification, a bundle of nanowires was put at the bottom of a test tube and immersed in water without further shaking in order to achieve a heterogeneous solution. Figure 11d shows the TEM image of the product from the interface at which the nanowires and the water touched. The product was amorphous nanoparticles, as that found in
ethanol. This demonstrates that the initial stage of oxidization in water is the same as the process in ethanol; that is, the white amorphous nanoparticles formed first. However, the following stage is quite different. The amorphous white TeO2 nanoparticles can be sustained in ethanol for a long time, while the same nanoparticles formed in water are no longer stable. The next step was the crystallization of these amorphous TeO2 particles through a classical crystallization process18 accompanying the Ostwald ripening process.15 In the Ostwald ripening process, the formation of tiny crystalline nuclei in a supersaturated medium occurs first, followed by the growth of larger crystals from smaller crystals due to the fact that the smaller particles have larger solubility than the larger ones. The intrinsic growth habit of TeO2 discussed above and the inhibition of PVP molecules may be the possible reason for the formation of the square-like flakes. (15) (a) Ostwald, W. Z. Phys. Chem. 1897, 22, 289. (b) Ostwald, W. Z. Phys. Chem. 1900, 34, 495. (16) (a) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (b) Banfield, F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751. (17) Bahl, M. K.; Watson, R. L.; Irgolic, K. J. J. Chem. Phys. 1977, 66, 5526. (18) (a) Co¨lfen, H. Top. Curr. Chem. 2007, 271, 1. (b) Yu, S. H. Top. Curr. Chem. 2007, 271, 79.
Stability of Ultrathin Te Nanowires in Acetone
Langmuir, Vol. 23, No. 6, 2007 3417
Scheme 1. Schematic Illustration of the Oxidization and Transformation of Te Nanowires in Ethanol and Water
3.4. Optical Property of Ultrathin Tellurium Nanowires during Oxidization in Water and Ethanol. Figure 12 shows digital photos of the samples after being stored in water for different periods. As the time was prolonged, the color of the sample became gradually thinned (Figure 12b) and even showed a little hint of purple after 2 days (Figure 12c). Finally, accompanying the formation of square flakes at the bottom of the flask, the solution became clear and colorless (Figure 12d). Time-dependent UV-vis spectra were measured to follow such changes in the solution. The UV-vis spectra for the nanowires dispersed in water showed two broad typical absorption peaks (Figure 13). The absorption peak at 278 nm (4.46 eV), which is due to the valence band (p-bonding triplet) to the conduction band (p-antibonding triplet), weakened gradually during the oxidization in water. However, another absorption peak centered from 550 to 640 nm decreased in intensity with a relatively high speed, and it blue-shifted from 640 to 550 nm. Such a timedependent UV-vis spectrum change clearly reflected the oxidization process of the ultrathin Te nanowires during the storage after synthesis, in which the dissolution of Te nanowires and the formation of amorphous TeO2 nanoparticles occurred as discussed before. In comparison with the change of optical properties in water, the transformation process experienced in ethanol is different. The oxidization occurring in ethanol did not proceed as fast as that in water. The changes in optical property were rather slow, but a similar process, especially for the UV-vis spectra, was observed (Figures 14 and 15). Because of the formation of a large number of amorphous TeO2 nanoparticles during the reaction, the final solution is not transparent as is that in water (Figures 12 and 14). In addition, the white TeO2 nanoparticles are small enough as to be able to disperse widely in ethanol. In contrast, the final product in water precipitated at the bottom of the bottle due to the large size of the TeO2 crystalline flakes. On the basis of the all-about results, the scenario of the entire conversion process from crystalline Te nanowires to amorphous TeO2 nanoparticles or single-crystal paratellurite (TeO2) after storage in ethanol and water at room temperature is schematically shown in Scheme 1.
4. Conclusions In summary, a systematic investigation of the dispersibility and chemical stability of freshly synthesized ultrathin tellurium nanowires with diameters of 4-9 nm using PVP as a capping agent has been carefully performed. Upon the addition of acetone in a stable dispersion solution of Te nanowires synthesized by a hydrothermal process, the ultrathin Te nanowires will aggregate, and their aggregation state is strongly dependent on the volume of water and acetone in this mixed solution. The freshly prepared ultrathin Te nanowires are not stable after being stored either in contact with air or in various solvents such as water, ethanol, and acetone. The results showed that the ultrathin Te nanowires can transform into amorphous TeO2 nanoparticles and single-crystalline TeO2 flakes in ethanol and water, respectively, at room temperature. The crystallization and transformation process can be intentionally controlled by controlling the storage conditions of the Te nanowires, which can provide an efficient route for the production of stable amorphous TeO2 nanoparticles and crystalline TeO2 plates at ambient conditions. This research represents a detailed example for the study on the dispersibility and stability of freshly prepared Te nanowires in a solution system, and emphasizes that attention should be given to storing freshly prepared nanomaterials from the viewpoint of applications. In addition, further study on how to enhance the stability of freshly prepared nanomaterials and achieve ideal storing conditions for nanomaterials, which are, in fact, not stable at all under some circumstances, is still urgently needed. Acknowledgment. This work is supported by the special funding support from the Centurial Program of the Chinese Academy of Sciences, the Natural Science Foundation of China (Grant Nos. 20325104, 20621061, and 20671085), the 973 project (2005CB623601), the Anhui Development Fund for Talent Personnel (2006Z027), the Scientific Research Foundation for the Returned Overseas Chinese Scholars supported by the State Education Ministry, the Specialized Research Fund for the Doctoral Program (SRFDP) of Higher Education State Education Ministry, and the Partner-Group of the Chinese Academy of Sciences, the Max Planck Society. LA063272+
