One-Pot Hydrothermal Synthesis of ZnSe Hollow Nanospheres from

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One-Pot Hydrothermal Synthesis of ZnSe Hollow Nanospheres from an Ionic Liquid Precursor Xiaodi Liu,†,‡ Jianmin Ma,† Peng Peng,† and Wenjun Zheng*,† †

Department of Materials Chemistry, College of Chemistry, Nankai University, Tianjin 300071, P. R. China, and ‡ College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Henan 473061, P. R. China Received January 4, 2010. Revised Manuscript Received March 13, 2010

Ionic liquids (ILs) have been gradually used to synthesize nanomaterials; however, it is rare that “tailoring” task-specific ILs to guide the synthesis pathway toward desirable nanostructures and morphologies. In this paper, a Se-containing ionic liquid 1-n-butyl-3-methylimidazolium methylselenite ([BMIm][SeO2(OCH3)]) was used as a new Se precursor to purposely prepare ZnSe hollow nanospheres with bubble templating through a facile one-pot hydrothermal method. The as-prepared ZnSe hollow nanospheres with good dispersity are relatively uniform with an average diameter of about 100 nm and a wall thickness range of 10-20 nm. More importantly, it was found that [BMIm][SeO2(OCH3)] not only serves as Se source but also acts as stabilizer for the ZnSe hollow nanospheres. In addition, the UV-vis spectrum of the products displayed adsorption maxima at 278 and 426 nm; therefore, the obtained ZnSe hollow nanospheres might have promising applications in blue emitters, catalysts, and gas sensors. It would be expected that [BMIm][SeO2(OCH3)] could be used to prepare other nanoscale metal selenides with special morphologies and improved properties on a large scale.

Introduction Ionic liquids (ILs), consisting of organic cations and inorganic anions, have aroused increasing interest owing to their unique properties such as negligible vapor pressure, good dissolving ability, low melting point, and high ionic conductivity.1-4 More importantly, they are referred as “designer liquids” with tunable physicochemical properties and demanded functions by changing cations and anions; furthermore, the possible combination number of cations and anions is uncountable (high to ∼1018).5,6 Therefore, it is reasonable to conceive that ILs with special cations and anions can be tailored, according to the compositions, initial crystalline structures, and crystal growth habit of the materials, to purposefully synthesize inorganic nanomaterials with novel morphologies and improved properties. Nevertheless, the explorations of ILs for the fabrication of inorganic nanomaterials, such as metal selenides, have just begun;7-9 moreover, there are only few reports on the application of task-specific ILs to guide the synthesis pathway toward desirable structures and morphologies.10-14 Thus, intensive investigations into this strategy are considered to be essential. *To whom correspondence should be addressed: Ph þ86-22-23507951; Fax þ86-22-23502458; e-mail [email protected].

(1) Varma, R. S.; Namboodiri, V. V. Chem. Commun. 2001, 643–644. (2) Dupont, J.; Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667–3692. (3) Morris, R. E. Angew. Chem., Int. Ed. 2008, 47, 442–444. (4) Parnham, E. R.; Morris, R. E. Acc. Chem. Res. 2007, 40, 1005–1013. (5) Antonietti, M.; Kuang, D.; Smarsly, B.; Zhou, Y. Angew. Chem., Int. Ed. 2004, 43, 4988–4992. (6) Fuller, J.; Carlin, R. T.; Osteryoung, R. A. J. Electrochem. Soc. 1997, 144, 3881–3885. (7) Taubert, A.; Li, Z. Dalton Trans. 2007, 7, 723–727. (8) Jiang, Y.; Zhu, Y. J.; Cheng, G. F. Cryst. Growth Des. 2006, 6, 2174–2176. (9) Green, M.; Rahman, P.; Boyle, D. S. Chem. Commun. 2007, 574–576. (10) Zheng, W.; Liu, X.; Yan, Z.; Zhu, L. ACS Nano 2009, 3, 115–122. (11) Taubert, A. Angew. Chem., Int. Ed. 2004, 43, 5380–5382. (12) Li, Z.; Geβner, A.; Richters, J. P.; Kalden, J.; Voss, T.; K€ubel, C.; Taubert, A. Adv. Mater. 2008, 20, 1279–1285. (13) Dobbs, W.; Suisse, J. M.; Douce, L.; Welter, R. Angew. Chem., Int. Ed. 2006, 118, 4285–4288. (14) Zhu, H.; Huang, J. F.; Pan, Z. W.; Dai, S. Chem. Mater. 2006, 18, 4473– 4477.

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Recently, nanoscale metal selenides have attracted much attention due to their remarkable properties and potential applications; therefore, much research has focused on the synthesis, morphology and phase control, and physics of these nanomaterials.15,16 Up to now, only a few Se sources have been developed for fabricating metal selenides. Thereinto, Na2SeO3 was widely used due to its high activity and good water solubility;17-22 nevertheless, Na2SeO3 could react with metal ions (Mnþ) to form precipitates in some systems. To obtain uniform reaction conditions, Mnþ ions should be transformed into stable complexes,20-22 which would make the systems complicated, affect the thermodynamics and kinetics in the nanocrystal nucleation stage, and further influence the morphology of the products. Fortunately, we have recently found that an IL 1-n-butyl-3-methylimidazolium methylselenite ([BMIm][SeO2(OCH3)], Supporting Information, Figure S1) may represent a new Se precursor. As to this ionic liquid precursor (ILP), the reactivity of the anion ([SeO2(OCH3)]- ions) is similar to SeO32- ions (see the Supporting Information, Figure S2). More importantly, one oxygen among SeO32- ions is replaced by a methoxy; thus, [SeO2(OCH3)]- has weaker polarizing capability, and accordingly Mnþ ions can exist as free ions in the solution. On the other hand, the cation ([BMIm]þ) can serve as stabilizer.23 Motivated by these potentials, we considered that [BMIm][SeO2(OCH3)] can be used as a novel Se precursor to synthesize nanoscale metal selenides with special morphologies and novel properties. (15) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664–670. (16) Hagfeldt, A.; Gr€atzel, M. Chem. Rev. 1995, 95, 49–68. (17) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (18) Yang, Y. A.; Wu, H.; Williams, K. R.; Cao, Y. C. Angew. Chem., Int. Ed. 2005, 44, 6712–6715. (19) Chen, O.; Chen, X.; Yang, Y.; Lynch, J.; Wu, H.; Zhuang, J. Y.; Cao, C. Angew. Chem. 2008, 120, 8766–8769. (20) Peng, Q.; Dong, Y.; Deng, Z.; Li, Y. Inorg. Chem. 2002, 41, 5249–5254. (21) Zhuang, Z.; Peng, Q.; Zhuang, J.; Wang, X.; Li, Y. Chem.;Eur. J. 2006, 12, 211–217. (22) Xiong, S.; Xi, B.; Wang, C.; Xi, G.; Liu, X.; Qian, Y. Chem.;Eur. J. 2007, 13, 7926–7932. (23) Itoh, H.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 2004, 126, 3026–3027.

Published on Web 04/16/2010

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Zinc selenide (ZnSe), as an important group II-VI semiconductor with a bulk band gap of 2.69 eV (460 nm) at room temperature, possesses superior photovoltaic and optical properties and great potential applications.24 As far as the applications in the fields of photonic crystals, catalysts, photonic bandgap crystals, and delivery vehicle systems are concerned, ZnSe with hollow spherical structures are of particular importance for their special tailored structural, optical, and surface properties.25-27 However, to the best of our knowledge, there have been few reports regarding the synthesis of ZnSe hollow spheres until now, especially the nanosized hollow spheres.28-30 For example, ZnSe hollow spheres in the micrometer size range (1-15 μm) have been fabricated by chemical methods, including CVD method and hydrothermal/solvothermal method; Jiang and co-workers have reported a hydrothermal synthesis of hollow ZnSe nanospheres (200-300 nm in diameter). Additionally, it is well-known that size and shape control has been traditionally important and necessary to tune the properties of nanomaterials.31 As to the hollow spherical nanomaterials, their physical and chemical properties are strongly dependent on the morphological characteristics, that is, exterior shapes, interior spaces, and shell structures.32,33 In these regards, in spite of the previous successes, it still remains a great challenge to explore simple and effective methods to synthesize ZnSe hollow nanospheres with small size and good dispersibility for desired application and to develop novel characteristics. Herein, we report a facile, effective, and one-pot hydrothermal route to successfully synthesize ZnSe hollow nanospheres with excellent optical property from ILP [BMIm][SeO2(OCH3)]. The synthetic system is simple, and all the processes are carried out in aqueous solution without the use of any hard templates. Most specifically, we demonstrate that [BMIm][SeO2(OCH3)] is favorable for the synthesis of high-quality ZnSe hollow nanospheres due to its critical characteristics, low surface tension, and weak polarizing capability, by controlling the nucleation and growth of N2 bubble templates and ZnSe nanocrystals. Furthermore, we hope that this new approach can pave the way for the purposive synthesis of other metal selenide nanomaterials and will, in turn, motivate the developments of ILs.

Experimental Section Materials. All the reagents were analytical grate and used without further purification. The ILP [BMIm][SeO2(OCH3)] was prepared according to the literature procedures,34,35 and its general structural feature is shown as follows:

Synthesis of ZnSe Hollow Nanospheres. In a typical synthesis, Zn(NO3)2 3 6H2O (0.20 mmol) and [BMIm][SeO2(OCH3)] (24) Nirmoal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Nature 1996, 383, 802–804. (25) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425–2427. (26) Peng, Q.; Xu, S.; Zhuang, Z.; Wang, X.; Li, Y. Small 2005, 1, 216–221. (27) Yuan, J.; Laubernds, K.; Zhang, Q.; Suib, S. L. J. Am. Chem. Soc. 2003, 125, 4966–4967. (28) Peng, Q.; Dong, Y.; Li, Y. Angew. Chem., Int. Ed. 2003, 42, 3027–3030. (29) Geng, B.; You, J.; Zhan, F.; Kong, M.; Fang, C. J. Phys. Chem. C 2008, 112, 11301–11306. (30) Jiang, C.; Zhang, W.; Zou, G.; Yu, W.; Qian, Y. Nanotechnology 2005, 16, 551–554. (31) Mokari, T.; Habas, S. E.; Zhang, M. J.; Yang, P. D. Angew. Chem., Int. Ed. 2008, 47, 5605–5608. (32) Shiomi, T.; Tsunoda, T.; Kawai, A.; Matsuura, S.; Mizukami, F.; Sakaguchi, K. Small 2009, 5, 67–71. (33) Sun, X.; Li, Y. Angew. Chem., Int. Ed. 2004, 43, 3827–3831. (34) Kim, S. K.; Kim, J. K.; Lee, H.; Park, K. Y.; Lee, C.; Chin, C. S. Angew. Chem., Int. Ed. 2002, 41, 4300–4302. (35) Kim, S. K.; Kim, J. K.; Lee, H.; Park, K. Y.; Chin, C. S. J. Catal. 1998, 176, 264–266.

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Figure 1. XRD pattern of the as-prepared ZnSe hollow nanospheres.

(0.25 mmol) were dissolved into deionized water (19 mL) under vigorous magnetic stirring to form a clear solution, followed by adding of N2H4 3 H2O (80%, 2.5 mL). Then the mixture was transferred into a Teflon-lined stainless steel autoclave (30 mL) and maintained at 150 °C for 12 h. The resulting yellow powders were centrifuged, washed with deionized water and anhydrous ethanol several times, and finally dried at 60 °C for 4 h under vacuum. Instruments and Characterizations. The crystallographic information on the obtained ZnSe hollow nanospheres was established using X-ray diffraction (Rigaku D/max 2500 V/PC, Cu KR radiation, λ = 1.540 56 A˚). The morphology and nanostructure of the products were observed by scanning electron microscopy (JSM 6700F), transmission electron microscopy (Hitachi H-7650, 100 kV), and high-resolution TEM (Hitachi H-7650, 200 kV). The compositional analysis for the as-prepared sample was performed with energy-dispersive spectrometer, an accessory of SEM (JSM 6700F). The UV-vis adsorption spectrum of the hollow nanospheres suspension in glycol was recorded on a UV-vis spectrophotometer (Hitachi-U3010) in the wavelength range of 250-800 nm.

Results and Discussion Structural Characterization and Morphology of ZnSe Hollow Nanospheres. Figure 1 shows the X-ray diffraction (XRD) pattern of the products. All of the diffraction peaks can be indexed as zinc-blende ZnSe with lattice parameter a = 5.670 A˚, which is in good agreement with the value reported in the literature (JCPDS Card No. 88-2345). No characteristic peaks corresponding to impurities are found, showing the high purity of the sample; moreover, the relatively broadened diffraction peaks indicate that the ZnSe crystals constituting hollow spheres are in small sizes. The morphology and nanostructure of the obtained ZnSe hollow nanosperes are investigated by TEM. The products are dispersed in ethanol by ultrasonic treatment (30 min), dipped onto the copper grids, and then characterized by TEM. Figure 2a presents an overview TEM image of the sample. It can be seen that the products have relatively uniform spherical shape; moreover, the strong contrast between the dark edge and pale center proves the hollow structure of the sample.36 The average diameter of these ZnSe hollow nanospheres is about 100 nm, and the shell thickness is in the range of 10-20 nm. The high-magnification TEM image of a representative hollow sphere (inset of Figure 2a) (36) Braun, P. V.; Stupp, S. I. Mater. Res. Bull. 1999, 34, 463–469.

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Figure 2. (a) TEM image of the obtained ZnSe hollow nanospheres; the inset is the high-magnification TEM image of a representative hollow nanosphere. (b) HRTEM image taken on the shell of one hollow nanosphere; the inset is the corresponding HRTEM lattice image.

Figure 3. (a) SEM image of the large-scale ZnSe hollow nanospheres. The arrows indicate some imperfect and hemispherical spheres; the inset is the high-magnification SEM image. (b) EDS spectrum of the ZnSe hollow nanospheres.

indicates that the shells of hollow spheres are composed of nanoparticles with the size of 10-15 nm. The HRTEM image taken at the shell of one hollow sphere (shown in Figure 2b) further reveals that the sizes of the primary ZnSe nanoparticles are in the range of 10-15 nm, and the sphere shells are well-crystallized single crystals. As displayed in the inset of Figure 2b, the interplanar spacing is about 0.32 nm, which corresponds with the lattice spacing of the (111) d-spacing of cubic ZnSe. Furthermore, it is worthwhile to mention that few collapsed ZnSe hollow spheres can be found, which indicates that the walls of the hollow nanospheres have relatively high compactness and stability;37 therefore, the compact ZnSe layer can be able to retain its hollow spherical structure even after the complete washing procedure and subsequently drying process and ultrasonic treatment. The SEM image, as shown in Figure 3a, also gives a large-scale view of the ZnSe hollow nanospheres. We can see that the diameters of products are in good agreement with the results observed in TEM images, and the hollow nanospheres have relatively good dispersity. As arrowed in SEM image, a few imperfect or hemispherical hollow spheres are found, thus indicating the hollow structure of the nanospheres. In order to further confirm the composition of the sample, EDS analyses are

recorded for the nanospheres under a N2 atmosphere, and the result is shown in Figure 3b. In the spectrum, besides the Au signals from the Au film which coated on the sample for good electrical conductivity, only Zn and Se are observed. Based on the relative areas of the peaks of Zn and Se, the atomic ratio of Zn to Se is calculated to be about 45.96:50.12, which is close to 1:1.1. The superfluous Se may be caused by the oxidation of Se2- on the hollow spheres surfaces into amorphous Se in the air.28 Formation Mechanism of ZnSe Hollow Nanospheres. The synthesis was carried out in aqueous solution just containing [BMIm][SeO2(OCH3)], Zn(NO3)2, and N2H4. In the synthesis, when Zn(NO3)2 and [BMIm][SeO2(OCH3)] are dissolved into the deionized water, a clear solution can be obtained. As illustrated in Scheme 1, [SeO2(OCH3)]- ions can be first reduced by N2H4 to Se atoms, which have high reactivity and are easy to be further disproportionated into Se2- ions; meanwhile, lots of N2 bubble nuclei are formed (step A). Then, Se2- ions can directly react with Zn2þ ions and ZnSe monomers can be quickly formed in the solution (step B). It is known that the morphology of the asprepared products can be determined by their crystal structures to some extent.38 The isotropic unit cell structure generally results in isotropic growth of particles and accordingly leads to spherical

(37) Zheng, X.; Xie, Y.; Zhu, L.; Jiang, X.; Yan, A. Ultrason. Sonochem. 2002, 9, 311–316.

(38) Du, W. M.; Qian, X. F.; Niu, X. S.; Gong, Q. Cryst. Growth Des. 2007, 7, 2733–2737.

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Scheme 1. Schematic Illustration of the Formation Process of ZnSe Hollow Nanospheres

morphology of the products.39 Therefore, after the initial nucleation, owing to the cubic crystal structure of zinc-blende ZnSe (see Supporting Information, Figure S3), the ZnSe monomers can grow into grainlike nanocrystals (step C). Moreover, N2 bubble nuclei gradually grow into N2 gas bubbles, which have high surface energies for their small diameters (high curvatures) and can serve as the heterogeneous nucleation centers for the newly formed ZnSe nanocrystals. Driven by the minimization of interfacial energy, therefore, the original ZnSe nanocrystals have a tendency to aggregated around the gas-liquid interface between N2 bubble and water, resulting in the formation of ZnSe hollow nanospheres (steps D and E).28 The overall reaction can be formulated as follows: 2Zn2þ þ 2½SeO2 ðOCH3 Þ - þ 3N2 H4 þ 2OH - f 2ZnSeV þ 3N2 v þ 2CH3 OH þ 6H2 O Effect of [BMIm][SeO2(OCH3)] on the Morphology of ZnSe Hollow Nanospheres. Most recently, various gaseous bubbles have been used as soft templates for hollow spherical structures.28,40-42 In these synthetic systems, the inner diameters of hollow structures are solely determined by the sizes of bubble templates;28,43,44 the wall thicknesses, on the other hand, depend on the sizes of the primary nanocrystals and the number of the bubble templates. Hence, in order to obtain hollow spheres with relatively small sizes and large interior-cavity sizes, primary nanocrystals and gas bubbles should have small diameters and the amount of bubbles should be increased. Then how to achieve these targets? First, on the basis of the classical theory of bubble nucleation, the formation of bubbles in solutions includes nucleation stage and growth process.45,46 Bubble nucleation is initiated by the formation of a bubble of critical size, while a rather large energy is required to form the critical bubble owing to the macroscopic surface energy; subsequently, the gas nucleus gradually grow up with the prolonging of the reaction time. Therefore, the number and size of bubbles can be adjusted by controlling the nucleation and growth environment. Traditionally, Na2SeO3 was used to prepare ZnSe hollow spheres because SeO32- ions can be reduced into highly reactive (39) Lee, S. M.; Cho, S. N.; Cheon, J. Adv. Mater. 2003, 15, 441–444. (40) Li, X.; Xiong, Y.; Li, Z.; Xie, Y. Inorg. Chem. 2006, 45, 3493–3495. (41) Wang, J. G.; Li, F.; Zhou, H. J.; Sun, P. C.; Ding, D. T.; Chen, T. H. Chem. Mater. 2009, 21, 612–620. (42) Yang, J.; Sasaki, T. Chem. Mater. 2008, 20, 2049–2056. (43) Guo, L.; Liang, F.; Wen, X.; Yang, S.; He, L.; Zheng, W.; Chen, C. Adv. Funct. Mater. 2007, 17, 425–430. (44) Agrawal, M.; Pich, A.; Gupta, S.; Zafeiropoulos, N. E.; Simon, P.; Stamm, M. Langmuir 2008, 24, 1013–1018. (45) Delale, C. F.; Hruby, J.; Marsı´ k, F. J. Chem. Phys. 2003, 118, 792–806. (46) Wang, Z. J. J. Phys. Chem. B 2009, 113, 3776–3784.

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Se and N2 templates can generate synchronously (2SeO32- þ 3N2H4 f 2Se2- þ 3N2v þ 6H2O). Unfortunately, as discussed above, SeO32- ions can react with Zn2þ ions to form a white ZnSeO3 precipitate; thereby, Zn2þ ions are converted into other stable forms in the previous successes (i.e., Zn(NH3)42þ and ZnO22-). Finally, Zn2þ ions should slowly release from the complexes to generate ZnSe, leading to the fact that N2 bubbles grow up and the hollow spheres have relatively large diameters. In comparison, [BMIm][SeO2(OCH3)] with high reaction activity is selected in our synthesis to provide Se source. It is found that this ILP has influence on the sizes of the products: (1) Free Zn2þ ions can be presented due to the weak polarizing capability of [SeO2(OCH3)]-; that is, Zn2þ ions do not need to be converted into other forms any more and ZnSe can be quickly generated. Actually, this speculation can be demonstrated by the experimental results. Therefore, while the formation of N2 bubble templates and ZnSe nanocrystals are harmonious, once the nanoscale N2 bubbles are formed, they are surrounded by ZnSe and have no time to grow up. (2) ILs show polar features, but they have lower surface tensions compared with water.5 Therefore, N2 bubbles nucleate easily with the deceasing of the surface tension of the microenvironment containing ILP, which results in a large number of bubble nuclei and also a relatively high nucleation rate. As a result, compared with the previous successes, N2 bubble templates have smaller diameters, and ZnSe hollow nanospheres with smaller interior sizes can be obtained. We have chosen Na2SeO3 to carry out a control experiment to further substantiate this proposed mechanism. In the synthesis, when Na2SeO3 was added into the solution containing Zn2þ, lots of white precipitates were formed. Eventually, the as-formed products have irregular morphologies, and the present hollow nanostructures cannot be obtained (see Supporting Information, Figure S4a). Second, it is well-known that nucleation stage and growth process are involved in the synthesis of crystals.47 When nucleation rate is faster than growth rate, the average crystal size can be decreased, and vice versa.48 As mentioned above, ZnSe nuclei can be quickly formed because of the employment of [BMIm][SeO2(OCH3)], which would cause the ZnSe nanocrystals have smaller diameters.21 Actually, the diameters of the primary ZnSe nanocrystals, ranging between 10 and 15 nm, is much smaller than that of ZnSe nanocrystals synthesized in other hydrothermal systems.49,50 Finally, the smaller size of ZnSe nanocrystals and a larger number of N2 bubbles led to the wall thickness of the hollow spheres thinner. Most interestingly, it is found that the dispersibility of the ZnSe hollow nanospheres is affected by [BMIm]þ. As reported in some literatures, some ILs have been chose to serve as stabilizers based on the selective adsorption of ILs on the surfaces of the samples.9,23,51 In our experiments, we speculate that some Se2ions locate on the surfaces of the ZnSe hollow nanospheres for the superfluous use of ILP, which can be indicated by the EDS result (Figure 3b). Along with the Se2-, [BMIm]þ will also adsorb on the ZnSe spheres surfaces possibly driven by the electrostatic attractions between [BMIm]þ and Se2-.52 As a result, as illustrated in Figure 4, [BMIm]þ ions have a large steric hindrance, which would hinder the agglomeration of the obtained ZnSe nanospheres in the solution, and accordingly the dispersibility of the ZnSe hollow nanospheres can be improved. Moreover, in order to (47) Mott, N. F. Nature 1950, 165, 295–297. (48) Ren, G.; Lin, Z.; Gilbert, B.; Zhang, J.; Huang, F.; Liang, J. Chem. Mater. 2008, 20, 2438–2433. (49) Peng, Q.; Dong, Y.; Deng, Z.; Sun, X.; Li, Y. Inorg. Chem. 2001, 40, 3840– 3841. (50) Gong, H.; Huang, H.; Wang, M.; Liu, K. Ceram. Int. 2007, 33, 1381–1384. (51) Zhou, Y.; Schattka, J. H.; Antonietti, M. Nano Lett. 2004, 4, 477–481. (52) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 6386–6387.

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Figure 4. Adsorption of [BMIm]þ on the surfaces of the ZnSe hollow nanospheres.

Figure 5. (a) SEM image of the sample synthesized with 5 mL of N2H4. (b, c) SEM image and XRD pattern of the sample synthesized under the ratio of Zn/Se = 1:1 (ZnO, JCPDS No. 79-0208). (d) SEM image of the sample synthesized under the ratio of Zn/Se = 1:2.

prove our opinion, we employed SeO2, which also has high reactivity and does not react with Zn2þ to generate precipitate, to synthesize ZnSe hollow nanospheres. In the control experiment, [BMIm][SeO2(OCH3)] was replaced by SeO2, and all other parameters were unchanged. As a result, the product is composed of ZnSe microspheres agglomerated of hollow nanospheres (see Supporting Information, Figure S4b). Effect of Reaction Parameters on the Morphology of ZnSe Hollow Nanospheres. According to the overall reaction, 9972 DOI: 10.1021/la1000182

it can be speculated that some reaction parameters, such as N2H4 and moral ratio of Zn/Se, may have influences on the morphology and purity of the ZnSe hollow nanospheres. A series of experiments have been performed to study their effects, and the results demonstrated that the amount of N2H4 and the ratio of Zn/Se in the precursors were influencing the growth of ZnSe hollow nanospheres. For example, when the amount of N2H4 was increased to 5 mL, congeries with irregular morphologies existed in the sample (Figure 5a), which indicated the excess of N2H4 Langmuir 2010, 26(12), 9968–9973

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Figure 6. UV-vis spectrum of the as-synthesized ZnSe hollow

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optical property of the obtained ZnSe hollow nanospheres, the UV-vis adsorption spectrum of the product is measured, and the result is shown in Figure 6. The adsorption spectrum was recorded by dispersing the powder sample in glycol through ultrasonic (3 min) treatment. It can be seen that the sample display adsorption maxima at 278 and 426 nm, and the corresponding adsorption energy is 4.46 and 2.91 eV, respectively. The adsorption peaks are obviously blue-shifted relative to the bulk ZnSe (460 nm, 2.69 eV),54 which may be attributed to the small dimension of the primary ZnSe nanoparticles or well-ordered arrays of the ZnSe hollow-sphere walls.55 Moreover, the features indicated the quantum-confined effects of the ZnSe nanoparticles of which the nanosphere wall is made up.56,57 It is expected that the novel property of the ZnSe hollow nanospheres may offer exciting opportunities for the potential application in blue emitters, gas sensors, and catalysts.

nanospheres.

Conclusions could accelerate the oxidation of N2H4 and the disproportionation of Se. Finally, the number of the formed ZnSe nanocrystals is so large that most of them have no time to aggregate on the surfaces of N2 bubbles but with each other. That the generation of N2 bubbles cannot be harmonized with the growth of ZnSe nanocrystals may be the main reason for the formation of the irregular morphology. In addition, in the case when all other reaction parameters were kept constant and only the moral ratio of Zn/Se was varied, it could be observed that the ratio of Zn/Se in the precursors played a curial role in controlling the purity of the ZnSe hollow nanospheres. For instance, when the moral ratio of Zn/Se was 1:1, as displayed in the SEM image and XRD pattern (Figure 5b,c), the products were composed of ZnO microplates and ZnSe hollow nanospheres. We speculated that Zn2þ can react with OH- to form Zn(OH)2 under alkaline conditions and subsequently transformed into ZnO under high temperature; that is to say, there existed a competition between Se2- and OH- for reacting with Zn2þ. Therefore, Se should be in excess of Zn in the precursors to obtain pure ZnSe. However, the products composed of yellow and red powders were obtained from a Zn/Se ratio of 1:2. As shown in Figure 5d, ZnSe spheres with different diameters (0.1-1.0 μm) and Se microrods with different aspect ratios coexisted in the products. Therefore, a relatively narrow moral ratio of Zn/Se was suitable to control the purity of ZnSe hollow nanospheres, and the ideal moral ratio of Zn/Se in the precursors was determined to be 4:5. Optical Characterization. It is known that the ability to control the size of nanomaterials affords an opportunity to testify the theory of “quantum confinement” and obtains samples with desirable optical characterization.53 In order to investigate the (53) Peng, X. G.; Manna, L.; Yang, W. D.; Wichham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59–61.

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In summary, we have used a reasonable ILP [BMIm][SeO2(OCH3)] to purposefully synthesize ZnSe hollow nanospheres. The significance of the present study is fourfold. First, this method provides a facile, effective, one-pot route to the nanosized ZnSe hollow spheres. Second, the as-synthesized ZnSe hollow nanospheres with good dispersity have an average diameter of about 100 nm, and the shell thicknesses varies from 10 to 20 nm. Third, most importantly, as to [BMIm][SeO2(OCH3)], it not only serves as Se precursor but also has crucial influence on the morphology of the products. Fourth, this task-special ILP can also be applied as a new Se source to fabricate other metal selenides (i.e., CdSe, CuSe) with novel morphology and improved properties, and further studies are underway to broaden its applicability; furthermore, we hope that this new approach can pave the way for the purposive synthesis of other inorganic nanomaterials. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grants 20571044 and 20971070). Supporting Information Available: Structure of [BMIm][SeO2(OCH3)]; XRD pattern of Se from [BMIm][SeO2(OCH3)]; schematic illustration of the crystal structure of zinc-blende ZnSe; SEM images of ZnSe from other Se precursors (Na2SeO3 and SeO2). This material is available free of charge via the Internet at http://pubs.acs.org. (54) Klude, M.; Hommel, D. Appl. Phys. Lett. 2001, 79, 2523–2525. (55) Hu, Y.; Chen, J. F.; Chen, W. M.; Lin, X. H.; Li, X. L. Adv. Mater. 2003, 15, 726–729. (56) Panda, A. B.; Acharya, S.; Efrima, S.; Golan, Y. Langmuir 2007, 23, 765– 770. (57) Xie, Y. J.; Huang, X.; Li, B.; Liu, Y.; Qian, Y. T. Adv. Mater. 2000, 12, 1523–1525.

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