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ZnO/ZnFe2O4 Magnetic Fluorescent Bifunctional Hollow Nanospheres: Synthesis, Characterization, and Their Optical/Magnetic Properties Hai-Sheng Qian,*,† Yong Hu,† Zheng-Quan Li,† Xing-Yun Yang,† Liang-Chao Li,† Xian-Ting Zhang,† and Rong Xu*,‡ Department of Chemistry, College of Chemistry and Life Science, Institute of Physical Chemistry, Zhejiang Normal UniVersity, Jinhua 321004, People’s Republic of China, and School of Chemical & Biomedical Engineering, Nanyang Technological UniVersity, 62 Nanyang DriVe, Singapore 637459 ReceiVed: June 17, 2010; ReVised Manuscript ReceiVed: August 24, 2010
Uniform ZnO/ZnFe2O4 fluorescent magnetic composite hollow nanopsheres can be successfully fabricated using carbonaceous nanospheres as templates, which are synthesized via a hydrothermal approach. The ZnO/ ZnFe2O4 composited nanoarchitectures show fluorescent and magnetic properties for improved physical and chemical properties for electronics, magnetism, optics, and catalysis compared with their single counterparts and may find potential applications in many fields, such as catalyst supports, catalysis, drug delivery, chemical/ biological separation, sensing, and so on. This method may be extended to synthesize some other binary/ ternary metal oxide composite hollow nanoarchitectures. 1. Introduction Nanostructured nanomaterials with a controllable size and shape are of great scientific and technological importance due to their unique chemical and physical properties.1,2 In the past decades, the synthesis and application of hollow nanostructures have received much more attention owing to their wide applications in many fields, such as catalysis, drug delivery, chemical/biological separation, and sensing,3–8 especially for composite hollow nanostructures due to their improved physical and chemical properties over their single-component counterparts, which are of great importance to a potentially broader range of applications in optics, electronics, magnetism, and catalysis.9–11 Zinc oxide is an important direct and wide bandgap semiconductor material (3.37 eV) and possesses unique optical, acoustic, and electronic properties, and as a result, this stimulates a wide range of research interest.12–16 Meanwhile, spinel ferrites, including ZnFe2O4, are among the most important magnetic materials due to important application in electronic devices, information storage, magnetic resonance imaging, and drug delivery technology.17–20 It is still a challenging topic to synthesize composites consisting of the two kinds of materials with fine nanostructures and excellent optical and magnetic properties. For this purpose, much effort has been made to synthesize some functional material with optical and magnetic properties. Recently, Fe2O3/ZnO core/shell nanorods have been synthesized successfully by a solution phase controlled hydrolysis method.21 Novel molecular precursors Fe2Zn(OR)8 (R ) tBu, Pr-i) have been used to synthesize nanostructured ZnFe2O4 and zinc oxide-iron oxide composites by sol-gel processing of a heterobimetallic alkoxide.22 ZnFe2O4/TiO2 nanocomposite films have been fabricated successfully by a radio frequency magnetic technology.23 To date, there are few reports about the synthesis of ZnO/ZnFe2O4 composite hollow nanospheres. * To whom correspondence should be addressed. E-mail:
[email protected] (H.-S.Q.),
[email protected] (R.X.). † Zhejiang Normal University. ‡ Nanyang Technological University.
In the present study, we report a simple templating approach for the preparation of ZnO/ZnFe2O4 magnetic-fluorescent bifunctional hollow nanostructures by a coprecipitation method, in which carbon nanospheres prepared by hydrothermal carbonization from glucose are used as hard templates. The optical and magnetic properties of the ZnO-ZnFe2O4 composite hollow nanospheres are investigated. 2. Experimental Section 2.1. Chemicals. All chemicals are of chemical grade and used without purification further. 2.2. Synthesis of ZnO/ZnFe2O4 Hollow Architectures. The synthesis of carbon nanospheres was carried out in a Teflonlined stainless steel autoclave. The synthesis method has been described elsewhere previously in detail.24,25 The synthesis of ZnO/ZnFe2O4 hollow nanospheres was as follows. In a typical synthesis, 1 mmol of ZnCl2 and 1 mmol of FeCl3 · 6H2O was added into a 100 mL flask and dissolved with 20 mL of ethanol to form a clear solution with strong stirring; 1 g of the as-prepared carbon nanospheres was added and dispersed into this solution. Ammonia-ethanol solution (10 mL, volume of ammonia solution is 1 mL) was added into the previous solution drop by drop. Finally, the solution was aged at room temperature for 3 h and collected by centrifugation. The products were washed with absolute alcohol and distilled water separately three times to remove possible remaining cations and anions before being dried at 60 °C for 4 h. The final products were obtained after calcination at 500 °C in static air for 3 h with a heating rate of 1 °C/min. 2.3. Characterization. The phases of the final products were characterized by X-ray powder diffraction (XRD) analyses, which were carried out on a Bruker D8 Advance X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54056 Å), and the operation voltage and current were maintained at 40 kV and 40 mA, respectively. Morphologies of these products were investigated by field emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM), which were carried out with a field emission scanning electron microscope (JEOL-6700F) and a
10.1021/jp105583b 2010 American Chemical Society Published on Web 09/28/2010
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Figure 1. Schematic illustration of the formation of magnetic-fluorescent ZnO/ZnFe2O4 hollow nanospheres.
Figure 3. FESEM and TEM images of the samples obtained after calcination at 500 °C for 3 h of a mixture solution of 1.0 mmol of ZnCl2 and 1.0 mmol of FeCl3 · 6H2O reacting with 1.0 mL of ammonia solution at room temperature in the presence of 1.0 g of carbon nanospheres.
our previous study, some other reactions could happen for the formation of ZnFe2O4 during the calcination process and could be presented by reaction 1 as follows
Zn(OH)2 + 2Fe(OH)3 S ZnFe2O4 + 4H2O Figure 2. XRD patterns of the samples obtained after calcination at 500 °C for 3 h of a mixture solution of 1.0 mmol of ZnCl2 and 1.0 mmol of FeCl3 · 6H2O reacting with 1.0 mL of ammonia solution at room temperature in the presence of 1.0 g of carbon nanospheres: 4, cubic ZnFe2O4 (JCPDS File No. 22-1012); /, hexagonal ZnO (JCPDS File No. 36-1451).
JEOL 3010 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The photoluminescence analysis was carried out on a Fluorolog3-TAU-P. The magnetic measurements were carried out with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-5s). 3. Results and Discussion A schematic drawing is given in Figure 1 to illustrate how to synthesize the ZnO/ZnFe2O4 fluorescent and magnetic bifunctional hollow nanospheres. A amorphous layer of Zn(OH)2 and Fe(OH)3 is coated onto the carbonaceous nanospheres to form carbon@M(OH)n (M: Zn and Fe) nanoparticles in ethanol solution at room temperature and, after removal of the hard templates, to form composited hollow nanostructures. Carbonaceous materials fabricated from hydrothermal carbonization of saccharide carbohydrate was an excellent platform for controlled synthesis of metal oxide hollow nanoarchitectures.26–28 Figure 2 shows the X-ray patterns of the samples after calcination of the product obtained from 1 mmol of ZnCl2 and 1 mmol of FeCl3 · 6H2O in the presence of 1.0 g of carbon nanospheres. The peaks located at 31.58, 34.30, 36.08, 47.58, 56.44, 62.40, 66.98, 68.08, and 68.84° are ascribed to the characteristic peaks of the hexagonal phase of ZnO with cell constants a ) 3.25 Å and c ) 5.20 Å (JCPDS No. 36-1451). The peaks located at around 18.20, 29.93, 35.30, 36.82, 42.90, 46.95, 53.17, 56.69, 62.25, and 73.50° can be readily ascribed to the characteristic peaks of the cubic phase of ZnFe2O4 (spinel ferrite) with the cell constant a ) 8.44 Å. Here, comparing to
(1)
Reaction 1 also could be interpreted as reactions 2-4 as follows:
Zn(OH)2 S ZnO + H2O
(2)
2Fe(OH)3 S Fe2O3 + 3H2O
(3)
ZnO + Fe2O3 S ZnFe2O4
(4)
The morphologies and size of the products have been examined by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Figure 3a,b shows SEM images of the as-prepared ZnO and ZnFe2O4 composites, which are uniform and spherical-like with diameters of ca. 300 nm. The hollow architectures with a shell width of 20 nm formed by removal of the hard templates used, and the shell consisted of well-closed small nanoparticles, which have been confirmed by TEM (Figure 3c,d). No some single small nanoparticles were found in the TEM images. That is to say, the as-prepared samples were with a shell composition of ZnO and ZnFe2O4. In a previous study, there were only small nanoparticles of ZnFe2O4 synthesized in the absence of carbon nanospheres via a coprecipitation process.29 The chemical composition of the composites was also characterized by energy-dispersive X-ray analysis (EDX), which is shown in Figure 4, in which Si was detected due to the use of a silicon plate as the samples’ substrate. C was from the absorption CO2 of the surface of samples, and Pt was from the sputtering system for FESEM observation. The molar ratio of ZnO and ZnFe2O4 was ca. 6:5, calculated according to the data of atomic percent. X-ray photoelectron spectra (XPS) were also used to measure and examine the chemical composition of the hollow composites and is shown in Figure 5. Peak values at 1050.5, 1027.4, 730.7, 719.9, 535.6, and 285.8 eV of binding energies can be indexed to the binding energy of Zn 2p1/2, Zn
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Figure 4. Energy-dispersive X-ray analysis (EDX) of the composite hollow architectures.
Figure 5. XPS spectra of the samples of ZnO and ZnFe2O4: (a) survey of the spectra, (b) Zn 2p spectra, (c) Fe 2p spectra.
2p3/2, Fe 2p1/2, Fe 2p3/2, O 1s, and C 1s, respectively, which confirmed that Zn and Fe elements had been formed by this coprecipitation process. The as-prepared ZnO/ZnFe2O4 composited hollow spheres show fluorescent and magnetic properties. Previous studies
reveal that the optical properties of ZnO is strongly dependent on the parameters conducted.30,31 The room-temperature photoluminescence spectrum of the composites (shown in Figure 6) was measured using a He-Cd laser as the excitation wavelength (325 nm). The ultraviolet emission band around 378
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Figure 8. ZFC and FC curves of ZnO/ZnFe2O4 hollow nanospheres with the field of 500 Oe. Figure 6. PL spectrum of as-prepared ZnO/ZnFe2O4 hollow nanospheres obtained at room temperature.
a ferromagnetic behavior that indicates a nonzero inversion degree due to the mixed distribution of Zn2+ and Fe3+ cations along the A and B sites in the spinel structure when the temperatures are lower than the TB (TB ) 20 K).36 In a word, the as-prepared ZnO/ZnFe2O4 hollow nanoarchitectures show fluorescent and magnetic bifunctional properties, which is of great importance to apply in drug delivery, chemical/ biological separation, etc. 4. Conclusion
Figure 7. Hysteresis loops for ZnO/ZnFe2O4 hollow nanospheres obtained at (a) 300 K and (b) 4 K.
nm was detected, which is attributed to the near band edge emission of the wide band-gap ZnO as observed in the nanowires fabricated by a gas evaporation growth method.32 A strong and broad green-yellow emission was located around 606 nm, which originated from the recombination of a delocalized electron close to the conduction band with a deeply trapped hole in the single ionized oxygen vacancy and the single negatively charged interstitial oxygen ion centers.33 Figure 7 is the magnetic hysteresis loops of the composites. It is commonly recognized that the methods of material preparation and their relations to the morphology, surface condition, and strains are important factors determining the materials’ physical properties. The ZnO/ZnFe2O4 hollow nanoarchitectures exhibit a very weak paramagnetic performance at room temperature (shown in Figure 7, curve a) as the bulk zinc ferrite due to that it belongs to the normal spinel type and behaves paramagnetic at room temperature.34,35 A previous study revealed that Zn ferrite, in the nanometric range, presents ferromagnetic behavior below a blocking temperature (TB) as it has a mixed spinel structure. It can be seen that the composited hollow nanoarchitectures show weak ferromagnetic properties when the temperature is down to 4 K.20 Magnetic susceptibility versus temperature measurements for both zero-field-cooled (ZFC) and field-cooled (FC) processes are shown in Figure 8, which shows that the ZnO/ZnFe2O4 hollow nanospheres present
In summary, ZnO/ZnFe2O4 fluorescent-magnetic hollow nanospheres have been synthesized successfully by a coprecipitation process in the presence of carbonaceous nanospheres. Most of these nanospheres consist of a shell of closely packed nanoparticles with a shell wall thickness of a few tens of nanometers. The ZnO/ZnFe2O4 composited nanoarchitectures show fluorescent and magnetic properties for improved physical and chemical properties for electronics, magnetism, optics, and catalysis compared with their single counterparts and may find potential applications in many fields, such as catalyst supports, catalysis, drug delivery, chemical/biological separation, sensing, and so on. Acknowledgment. H.-S.Q. thanks the the National Science Foundation of China (Grant No. 20901068), the Zhejiang Qianjiang Talent Project, the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and Zhejiang Normal University for support. R.X. acknowledges the research funding support from A-Star, Singapore (PSF0521010016). References and Notes (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayer, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (2) Wang, Z. L.; Kong, X. Y.; Ding, Y.; Gao, P. X.; Hughes, W. L.; Yang, R. S.; Zhang, Y. AdV. Funct. Mater. 2004, 14, 943. (3) Davis, M. E. Nature 2002, 417, 813. (4) Zhu, Y. F.; Shi, J. L.; Shen, W. H.; Dong, X. P.; Feng, J. W.; Ruan, M. L.; Li, Y. S. Angew. Chem., Int. Ed. 2005, 44, 5083. (5) Liu, S. H.; Zhang, Z. H.; Han, M. Y. AdV. Mater. 2005, 17, 1862. (6) Qian, H. S.; Antonietti, M.; Yu, S. H. AdV. Funct. Mater. 2007, 17, 637. (7) Zhou, J.; Liu, J.; Yang, R. S.; Lao, C. S.; Gao, P. X.; Tummala, R.; Xu, N. S.; Wang, Z. L. Small 2006, 2, 1344. (8) Sun, Y.; Mayers, B.; Xia, Y. AdV. Mater. 2003, 15, 641. (9) Caruso, F. AdV. Mater. 2001, 13, 11. (10) Van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 3027. (11) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. A. Chem. Mater. 2001, 13, 109.
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