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J. Phys. Chem. C 2009, 113, 6056–6060
Formation of Hexabranched GeO2 Nanoparticles via a Reverse Micelle System Yi-Wen Chiu and Michael H. Huang* Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan ReceiVed: January 11, 2009; ReVised Manuscript ReceiVed: February 12, 2009
We report the first synthesis of GeO2 nanoparticles with six symmetrically arranged branches running along the long axis of each particle. The nanoparticles have a shape resembling that of a star fruit. A reverse micelle system with Triton X-100 serving as the capping surfactant for the aqueous phase and n-hexanol as the cosurfactant was adopted. Ge(OEt)4 was selected as the germanium source. Using the optimal synthesis procedure by reacting the mixture at room temperature for 3 h, hexabranched GeO2 particles with an average length of 185 nm were produced. The products have been examined by FE-SEM, TEM, X-ray diffraction, and FT-IR techniques. GeO2 nanoparticles with structually well-developed branches were gnereated only with solution pH values in the range of 0.9-1.1. At a low [H2O]/[Ge(OEt)4] molar ratio of 45, particles having a hexagonal bipyramidal shape but without branch formation were observed. Increasing this ratio to 90, branches begin to appear from the six side edges of the particles. By simply varying the reaction time, the sizes of the branched GeO2 nanoparticles can be adjusted from around 100 nm in length to as large as 300 nm in length. Introduction Compared to the vast number of studies on the preparation of silica-based mesostructures and nanostructures, far fewer reports have addressed the formation of germanium oxide nanostructures. GeO2 does possess some interesting and potentially useful properties for optoelectronic applications, such as a higher refractive index value in the range of 1.6 to 1.65, as compared to around 1.45 for the silica glasses,1 and a high dielectric constant (k).2 Hence, preparation of novel GeO2 nanostructures should continue to be explored. Growth of GeO2 nanowires have been reported by using a thermal vapor deposition method.3-6 More frequently, GeO2 nanostructures and mesostructures are synthesized via solution-phase approaches. Germanium(IV) ethoxide, or Ge(OEt)4, and GeCl4 are the most common sources of germanium used,7-11 although Ge and GeO2 powder have also been chosen in some studies.12,13 Nanocubes and spindle-shaped particles are the most common GeO2 nanostructures prepared. Ge(OEt)4 is a good germanium source for synthesizing GeO2 nanostructures, because the same hydrolysis and condensation reactions of sol-gel process are involved as for Si(OEt)4, typically used to make silica mesostructures.14 However, unlike a more moderate rate of reactions with Si(OEt)4, Ge(OEt)4 undergoes these reactions so rapidly even in the absence of an acid catalyst that the control of final product morphology can present a problem. The amount of water used in a reaction involving the Ge(OEt)4 species must be minimized and controlled. Thus, reverse micelle or microemulsion systems have been adopted in some studies to make GeO2 nanostructures.7,9,10 A reserve micelle system using the anionic surfactant AOT [sodium bis(2-ethylhexyl)sulfosuccinate] is most common. Other nanostructures such as PbWO4 crystals,15 dyedoped silica nanoparticles,16 and Ni complex particles17 have also been made using the AOT-based microemulsion method. In our investigation of the formation of novel GeO2 nanostructures, a reverse micelle system was also employed. * To whom correspondence should be addressed. E-mail: hyhuang@ mx.nthu.edu.tw.
Here we present the first preparation of hexabranched GeO2 nanoparticles via a reverse micelle system using Triton X-100 as the capping surfactant. Ge(OEt)4 was chosen as the germanium source. Structural and infrared spectroscopic characterizations of the nanoparticles were performed. Reaction conditions needed for the formation of these unique GeO2 nanostructures and their size control have been identified. Through these experiments and the consideration of the crystal structure of GeO2, the growth mechanism of these hexabranched nanoparticles is revealed. The preparation of GeO2 nanoparticles with this peculiar shape extends our knowledge of the known morphologies of GeO2 nanostructures. Experimental Section Monodispersed hexabranched GeO2 nanoparticles were synthesized by a nonionic reverse micelle system, which contains germanium(IV) ethoxide, cyclohexane, Triton X-100, n-hexanol, water as precursor, oil phase, surfactant, cosurfactant, and water phase, respectively. The reverse micelle solution was prepraed by first mixing 1.56 g of Triton X-100 (4-octylphenol polyethoxylate, J. T. Baker), 5.60 mL of cyclohexane (99.9%, TEDIA), 1.48 mL of n-hexanol (98.9%, TEDIA), and 0.60 mL of HCl (minimum 37%, Riedal-de Hae¨n). The pH of the HCl solution is 1.0 ( 0.1. The mixture was stirred for about 1 h until the solution became transparent. Then 0.50 mL of 0.56 M germanium(IV) ethoxide (99.995%, Alfa Aesar) cycloxehane solution was added dropwise to the above mixture under nitrogen atmosphere by a 1.0 mL syringe. The final reagent molar ratios are 1.0:118.9:0.2:8.9:52.3:185.0 Ge(OEt)4/H2O/HCl/ TX-100/n-hexanol/cyclohexane. The resulting solution became turbid after 5 min and was vigorously stirred for 3 h at room temperature. The particles formed were immediately centrifuged at 7000 rpm for 7 min and redispersed in 2-propanol in an ultrasonic bath. This centrifugation process was repeated six times in order to completely remove the surfactant and unreacted reagents. Finally the products were collected and redispersed in 1.5 mL of 2-propanol for characterization.
10.1021/jp9002615 CCC: $40.75 2009 American Chemical Society Published on Web 03/25/2009
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Figure 1. (a) SEM image of the synthesized hexabranched GeO2 nanoparticles. (b) A close-up view of the nanoparticles. (c) TEM image of the nanoparticles. (d) TEM image of a single hexabranched GeO2 nanoparticle. (e) TEM image of the nanoparticle after prolonged electron beam irradiation. The branches have disppeared. (f and g) Illustrations of a hexabranched GeO2 nanoparticle viewed from the side and an end.
tion (XRD) patterns were obtained on a Shimadzu XRD-6000 diffractometer with Cu KR radiation. Fourier transform infrared (FT-IR) spectra were recorded on a Perkin-Elmer Spectrum RX I spectrometer. The pH of the HCl solution was measured using a JENCO model 60 portable digital pH meter. Results and Discussion
Figure 2. TEM image of a single hexabranched GeO2 nanoparticle and its corresponding SAED pattern. The lighter oval-shaped region in the center of this particle appears as a result of the effect of prolonged electron beam irradiation.
Figure 3. (a) XRD pattern of the hexabranched GeO2 nanoparticles. (b) Standard diffraction pattern of GeO2 from JCPDF pattern no. 361463.
The morphology of the hexabranched GeO2 nanoparticles was examined by using field-emission scanning electron microscopes (FE-SEM, JEOL JSM-7000F and Hitachi S4700) and a transmission electron microscope (JEOL JEM-2100). X-ray diffrac-
The GeO2 nanoparticles synthesized via the Triton X-100 reverse micelle system and with the use of Ge(OEt)4 as the germanium source were characterized by SEM and TEM techniques. Figure 1 shows some typical SEM and TEM images of the products formed under a HCl solution pH of 1.0 ( 0.1. The GeO2 nanoparticles all exhibit a prolate spheroidal shape with six symmetrical branches running along the long axis of each particle. This unique particle morphology resembles that of a star fruit, or carambola, but with six branches. A star fruit normally possesses five side arms. This peculiar shape of GeO2 nanoparticles has not been reported before. The average length and width of the particles are determined to be 185 ( 16 nm and 147 ( 12 nm from the SEM images. Histograms of the length and width distributions are given in the Supporting Information. Figure 1d is a representative TEM image of a single GeO2 nanoparticle with a smaller than average size. Side branches can be clearly seen. Because of the high sensitivity of the GeO2 nanoparticles to direct electron beam irradiation, the branches can become disappeared within seconds (see Figure 1e). For this reason, it is not possible to obtain high-resolution TEM images of the side branches. To collect selected-area electron diffraction (SAED) patterns of individual hexabranched GeO2 nanoparticles, one must work quickly before the diffraction spots disappear. Figure 2 gives a TEM image of a single hexabranched GeO2 nanoparticle and its corresponding SAED pattern. Because of the inherent structural complexity of the hexabranched nanoparticle, diffraction spots cannot be unambiguously assigned. Only diffraction spots perpendicular to the long axis of the hexabranched GeO2 nanoparticle were determined to arise from the (102) lattice planes of hexagonal GeO2. Further characterization of the hexabranched GeO2 nanoparticles was conducted. Figure 3 provides a typical XRD pattern
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Figure 4. FT-IR spectrum of the hexabranched GeO2 nanoparticles. The right panel displays the lower-wavenumber section of this spectrum.
Figure 5. SEM images of the GeO2 nanoparticle products obtained using different h values, or the [H2O]/[Ge(OEt)4] molar ratios, for the particle growth. A plot of the particle diameters vs the h values used is also shown here.
of the hexabranched GeO2 nanoparticles. The XRD pattern matches well with that of the pure hexagonal phase structure of R-GeO2 (JCPDF pattern no. 36-1463 with unit cell constants a ) 4.985 Å and c ) 5.648 Å). The observation of strong (100) and (101) reflection peaks is consistent with the model of an ideal shape of a hexagonal GeO2 particle consisting of two hexagonal pyramids at the ends and a hexagonal prism in the middle.7,13 The middle hexagonally faceted faces are the {100} faces, and the end hexagonal pyramidal faces contain the {101} faces. The FT-IR spectrum of the hexabranched GeO2 nanoparticles is shown in Figure 4. Six characteristic stretching vibration bands of R-GeO2 located at 518, 553, 586, 883, 935, and 960 cm-1 can be identified.7,9 The weak band at 756 cm-1 may be assigned to the Ge-O stretching mode of hydrolyzed species of Ge(OEt)4.9 This band is only observable in GeO2 particle samples prepared at a solution pH of 1.1 or lower. Additionally, there is a band centered at around 3450 cm-1, which should correspond to the O-H stretching mode of GeOH species. One can exclude the adsorption of water contributing to this band because of the lack of a peak at ∼1635 cm-1.18 This O-H stretching band of the Ge-OH species at 3450 cm-1 is closer to that of the free Si-OH species at 3400 cm-1 than that for the hydrogen-bonded Si-OH species at 3250 cm-1.18
This suggests that the Ge-OH species are less involved in hydrogen bonding, if band positions of Si-OH and Ge-OH species in these chemical environments are similar. Because of their unique structure, reaction conditions necessary for the formation of hexabranched GeO2 nanoparticles were examined. Experiments with variations in the relative concentrations of several different reagents used in this reaction system have been performed. The pH value of the added aqueous solution was found to be the most important factor in governing the formation of the hexabranched GeO2 nanoparticles. Solutions with different pH values were prepared by using HCl or NH4OH. Hexabranched GeO2 nanoparticles can only be synthesized under highly acidic solution conditions with pH values of less than 1.9 (see the Supporting Information). In fact, structurally welldeveloped hexabranched GeO2 particles were obtained only at a solution pH of 0.9-1.1. At a solution pH of 1.9, many particles exhibit a quasicubic morphology with larger dimensions than particles made at a solution pH of 1.1. As the solution pH increases to 7.0, extremely large quasicubic particles with an edge dimension of 600-700 nm were formed (data not shown). Besides pH, one of the most interesting parameters affecting the growth of branches in the reverse micelle system used in this study is the [H2O]/[Ge(OEt)4] molar ratio, or the h value,
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Figure 6. SEM images of the GeO2 nanoparticle products obtained after reaction times of (a) 2 min, (b) 10 min, (c) 3 h, and (d) 12 h. The solutions were constantly stirred. The inset in panel a shows an enlarged view of the nanoparticles formed.
used for the GeO2 nanoparticle growth. Figure 5 presents the SEM images of the GeO2 nanoparticles prepared with different h values at a solution pH of 1.1. At h values of 45 to 90, average particle lengths are in the range of 330 to 350 nm. Increasing the h value to 135-180, average particle lengths decrease dramatically to ∼160 nm. The results imply that a higher water content can increase the rate of hydrolysis reaction, leading to more rapid growth of the GeO2 nanoparticles with smaller particle sizes. In addition to changes in the particle dimensions as a function of the h value, one notes that the side branches become more apparent at h values higher than 110. In this study, an h value of ∼119 was used to make the structurally welldefined hexabranched GeO2 nanoparticles. At an h value of 45, the GeO2 nanoparticles formed display a hexagonal bipyramidal shape without the growth of side branches. By increasing the h value to 90, side branches begin to appear, although they are still not so wide and well-developed. From the SEM images shown for these two conditions, it is confirmed that the branches grow out from the six side edges along the length of a hexagonal bipyramid, rather than from its side faces. At higher h values, the smaller nanoparticles formed all possess distinct side branches. Thus, a higher [H2O]/[Ge(OEt)4] molar ratio at a solution pH of 1.1 can promote the growth of side branches. To further understand the growth mechanism of the hexabranched GeO2 nanoparticles, products formed after different reaction times at a solution pH of 1.1 and under vigorous stirring were examined (see Figure 6). With a reaction time of just 2 min, the GeO2 particles collected appear to be composed of numerous irregularly shaped particles agglomerated together. An overall prolate spherical particle shape is still discernible. Particle sizes range mostly from 80 to 130 nm. The results indicate the formation of GeO2 nanostructures by an extremely rapid nucleation and multipoint growth mechanism. The particles are still reasonably monodisperse in size despite their rapid growth presumably because the TX-100/n-hexanol reverse micelle system is effective at confining the size of the aqueous droplets. After reaction for 10 min, essentially all the GeO2 particles have transformed into hexabranched nanoparticles with smooth surfaces. Most particles are more than 100 nm in length, although their size distribution is relatively wide. Thus, within 8 min the initially formed GeO2 particles with an irregular shape
have undergone a ripening and surface reconstruction process to evolve into the final observed hexabranched particle morphology. It is believed that GeO2 nanoparticles develop into this hexagonally symmetrical shape because it possesses a hexagonal crystal structure. Since the particle size has not increased significantly over this period, the reverse micelles should still play a key role in controlling the aqueous droplet size, and intermicellar collisions proceed at a slower rate. After 3 h of reaction, the particles have grown to become more uniform in size. The products are the same as those shown in Figure 1. By extending the reaction period to 12 h, the hexabranched GeO2 particles have grown to a size range of mainly 200 to 300 nm in length. However, some of them have an appearance that is less perfectly symmetrical than those synthesized using a reaction time of 3 h possibly due to the significant growth of the central cores. Interestingly, we found that relatively large hexabranched GeO2 nanoparticles with sizes in the range of 220-300 nm and the maintenance of a highly symmetrical geometry can be generated by simply vigorously stirring the reaction mixture for 10 min as before and then aging the solution for 2 h (see the Supporting Information). Particle growth still continues without disturbing the reaction mixture after the initial formation of hexabranched GeO2 nanoparticles. Therefore, the use of these stirring and aging periods provides a convenient route to the preparation of larger and well-developed branched GeO2 particles. Conclusion Unique symmetrically hexabranched GeO2 nanoparticles have been synthesized in a reverse micelle system using Triton X-100 as the capping surfactant and n-hexanol as the cosurfactant. Ge(OEt)4 was chosen as the germanium source. The products have been characterized by FE-SEM, TEM, X-ray diffraction, and FT-IR techniques. Reaction conditions have been varied to identify key parameters necessary to promote the growth of branches. Solution pH is the most impoartant factor in governing the particle morphology. A solution pH of 0.9-1.1 in the aqueous phase was found to be optimal for the formation of structurally well-developed hexabranched GeO2 nanoparticles. A higher [H2O]/[Ge(OEt)4] molar ratio can also facilitate the
6060 J. Phys. Chem. C, Vol. 113, No. 15, 2009 development of branches. The sizes of the hexabranched GeO2 nanoparticles can be controlled simply by varying the reaction time. The successful preparation of this new GeO2 nanostructure enriches our knowledge of the structural variety possible to GeO2 nanocrystals. Acknowledgment. We thank the National Science Council of Taiwan for the support of this research (Grant NSC95-2113M-007-031-MY3). Supporting Information Available: Size distribution histograms of the hexabranched GeO2 nanoparticles, SEM images of the particles prepared under different aqueous solution pH values, and a SEM image of the particles synthesized by aging the initially formed products. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Patwardhan, S. V.; Clarson, S. J. Polymer 2005, 46, 4474. (2) Phani, A. R.; Di Claudio, D.; Passacantando, M.; Santucci, S. J. Non-Cryst. Solids 2007, 353, 692. (3) Wu, X. C.; Song, W. H.; Zhao, B.; Sun, Y. P.; Du, J. J. Chem. Phys. Lett. 2001, 349, 210. (4) Hidalgo, P.; Me´ndez, B.; Piqueras, J. Nanotechnology 2005, 16, 2521.
Chiu and Huang (5) Su, Y.; Liang, X.; Li, S.; Chen, Y.; Zhou, Q.; Yin, S.; Meng, X.; Kong, M. Mater. Lett. 2008, 62, 1010. (6) Gu, Z.; Liu, F.; Howe, J. Y.; Paranthaman, M. P.; Pan, Z. Cryst. Growth Des. 2009, 9, 35. (7) Wu, H. P.; Liu, J. F.; Ge, M. Y.; Niu, L.; Zeng, Y. W.; Wang, Y. W.; Lv, G. L.; Wang, L. N.; Zhang, G. Q.; Jiang, J. Z. Chem. Mater. 2006, 18, 1817. (8) Adachi, M.; Nakagawa, K.; Sago, K.; Murata, Y.; Nishikawa, Y. Chem. Commun. 2005, 2381. (9) Kawai, T.; Usui, Y.; Kon-No, K. Colloids Surfaces A: Physicochem. Eng. Aspects 1999, 149, 39. (10) Chen, X.; Cai, Q.; Zhang, J.; Chen, Z.; Wang, W.; Wu, Z.; Wu, Z. Mater. Lett. 2007, 61, 535. (11) Lu, Q.; Gao, F.; Li, Y.; Zhou, Y.; Zhao, D. Microporous Mesoporous Mater. 2002, 56, 219. (12) Liu, P.; Wang, C. X.; Chen, X. Y.; Yang, G. W. J. Phys. Chem. C 2008, 112, 13450. (13) Jing, C.; Hou, J.; Zhang, Y. J. Cryst. Growth 2008, 310, 391. (14) Chang, S.-C.; Huang, M. H. Inorg. Chem. 2008, 47, 3135. (15) Chen, D.; Shen, G.; Tang, K.; Liang, Z.; Zheng, H. J. Phys. Chem B 2004, 108, 11280. (16) Bagwe, R. P.; Yang, C.; Hilliard, L. R.; Tan, W. Langmuir 2004, 20, 8336. (17) Chen, M.; Wu, Y.; Zhou, S.; Wu, L. J. Phys. Chem. B 2008, 112, 6536. (18) Wu, S.-Y.; Hsueh, H.-S.; Huang, M. H. Chem. Mater. 2007, 19, 5986.
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