Heterostructures of ZnO Microrods Coated with Iron Oxide

50702013, 50725205), the Analysis and Testing Foundation of Northeast Normal University, and the Science Foundation for Young Teachers of Northeast ...
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15980

J. Phys. Chem. C 2008, 112, 15980–15984

Heterostructures of ZnO Microrods Coated with Iron Oxide Nanoparticles X. Y. Chu, X. Hong, X. T. Zhang, P. Zou, and Y. C. Liu* Center for AdVanced Optoelectronic Functional Materials Research, Northeast Normal UniVersity, Changchun 130024, P. R. China ReceiVed: May 24, 2008; ReVised Manuscript ReceiVed: August 3, 2008

Heterostructured composites of ZnO/Fe3O4 and ZnO/Fe2O3 were respectively prepared by depositing magnetic nanoparticles on ZnO microrod templates through low-temperature hydrothermal procedures. The formation of the heterostructures was evidenced by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and Raman scattering analysis. Hexamethylenetetramine, a pH adjustor, was found to have important influence on the crystal phase of iron oxide in the heterostructured composites and greatly influenced the properties of the heterostructured composites. The heterostructured composites simultaneously possessed the magnetic properties of iron oxides and the optical properties of ZnO. Introduction As bioscience and biotechnology rapidly develop, nano- or microscale self-powering systems have gained much attention recently in the research community.1,2 The pivot of such systems is the conversion from common energies (such as solar,3 thermal,4 or mechanical5) to electrical energy. Significant advances have been made by Wang’s group.5-7 The piezoelectric and semiconducting characters of ZnO nanowires were utilized to form nanogenerators. Atomic force microscope (AFM) tips,5 ultrasonic vibrations,6 and low-frequency vibrations/frictions7 were introduced to provide driving forces, respectively. More opportunities could be provided for the applications of nanogenerators via a noninvasive driving method, such as energy harvesting both in vivo and in vitro. Magnetic response, a remote operated approach, can effectively control the dynamic behaviors of the magnetic materials. Therefore, contactless control of onedimensional ZnO structures can be realized by depositing magnetic particles on their surface. It is worth noticing that the particles should be superparamagnetic, meaning that they are only magnetic when placed in an extra magnetic field. If they were ferromagnetic, the remnant magnetic field would lead each particle to act as a small dipole magnet, resulting in aggregates or chains. This is not convenient to the desired system. Among the already known superparamagnetic materials, iron oxides are most studied owing to their strong magnetism.8-10 There have been several studies devoted to the fabrication of composites of ZnO and iron oxide, using iron oxide nanoparticles as seeds.11,12 Gedanken et al. prepared superparamagnetic hollow ZnO hexagons rods filled with Fe3O4 particles by a sonochemical process.11 Li et al. prepared Fe2O3/ZnO core-shell nanorods for use in gas sensors.12 However, we consider that depositing iron oxide nanoparticles on ZnO is more interesting and important in the materials aspect since ZnO particles with various morphologies or even ZnO nanorod arrays can be employed for the fabrication of heterostructured composites.13,14 The challenge for the fabrication of such ZnO/iron oxides heterostructured composites, however, lies in the instability of ZnO in solution chemistry, that is, it may be dissolved along * To whom correspondence should be addressed. Phone: 86-43185099168. Fax: 86-431-8568-4009. E-mail: [email protected].

with the change of pH value or ionic strength of the solution.15 Thus, milder experimental conditions are of notable significance to avoid the dissolution of ZnO during depositing iron oxides nanoparticles on it. In the present work, a mild hydrothermal method was developed to prepare ZnO/iron oxide heterostructured composites. Hexamethylenetetramine (HMT), a weak organic alkaline, was found to play an important role in the control of the composition and the magnetic properties of the composites. The properties of the heterostructured composites were studied in both optical and magnetic aspects. Experimental Section Chemicals. Zinc acetate dihydrate and HMT were obtained from Beijing Chemical Reagent Company. Ferrous chloride tetrahydrate was purchased from Guangdong Xilong Chemical Company. Milli-Q water (18.4 MΩ · cm-1) was used in all experiments. All chemicals were analytical grade and used as received without purification. Preparation of ZnO Microrods. In a typical preparation, an HMT aqueous solution (20 mmol · L-1) was mixed with a zinc acetate aqueous solution of the same volume under vigorous magnetic stirring at room temperature. The mixture solution was then transferred into a Teflon-lined tube reactor and was kept at 90 °C for 5 h. The resultant white solid product was centrifuged, washed with distilled water, and dried at 40 °C in a vacuum. Preparation of ZnO/Iron Oxide Heterostructures. Ferrous chloride aqueous solution (25 mmol · L-1) was added into an equal volume ZnO suspension either containing HMT (20 mmol · L-1) or not. Then, the mixture solution was transferred into a Teflon-lined tube reactor and was kept at 90 °C for 5 h. The resultant precipitates were separated by a permanent magnet and washed with deionized water three times. Characterization. The morphology of the as-prepared particles was characterized with a field emission scanning electron microscope (FESEM, Philips XL-30) operated at an accelerating voltage of 20 kV. The high-resolution transmission electron microscope (HRTEM) images were acquired using a ZEOL JEM-2100 (acceleration voltage of 200 kV). The compositions of the particles were analyzed with an X-ray diffractometer (Rigaku D/MAX2500, Cu KR line, λ ) 0.1541 nm) and X-ray

10.1021/jp804590y CCC: $40.75  2008 American Chemical Society Published on Web 09/18/2008

Preparation of ZnO/Iron Oxide Heterostructures

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SCHEME 1: Schematic Illustration of the Growth Mechanism of the Heterostructured Composites

photoelectron spectroscopy (XPS, VG ESCALAB LKII using a binding energy reference of 284.6 eV for a C1s line), respectively. Resonant Raman scattering spectra were recorded at room temperature with a JY HR-800 LabRam confocal Raman microscope in a backscattering configuration, with an excitation wavelength of 325 nm and power of 2-3 mW. Photoluminescence (PL) emission spectra were recorded with a JY HR-800 LabRam Infinity Spectrophotometer excited with a continuous He-Cd laser (λexc ) 325 nm, 2-3 mW). Magnetization characteristics of the ZnO/Fe2O3 and ZnO/Fe3O4 heterostructured composites were measured with a Quantum Design MPMS-XL SQUID magnetometer. The field dependence of the magnetization was studied over the range of -50 to +50 kOe at 300 K. Results and Discussion Hydrothermal method has long been used for preparation of inorganic oxides materials. In the present experiments, it was used to fabricate the ZnO templates and the heterostructures as schematically outlined in Scheme 1. Iron oxide nanoparticles were deposited on ZnO rods in two different conditions. It was found that black ZnO/iron oxide precipitate was obtained in the presence of HMT, and red precipitate was obtained in the absence of HMT. In other words, the component of product can be controlled by introducing HMT or not. The morphologies of these heterostructures were investigated by FESEM. The ZnO templates, as shown in Figure 1a,b, are dumbbell-shaped and about 6 µm long and 2 µm wide. After the deposition, the shape of ZnO is retained, but the surface of ZnO templates (Figure 1c) was roughened (as shown in Figure 1d,e,g,h). Under higher magnification, the nanoscaled particles were observed to exist on the surface of ZnO microrods (Figure 1f,i). Similar results were obtained by the corresponding TEM observation. Figure 2a shows the TEM image of a single heterostructured composite microrod fabricated with HMT, and the selected rectangular area was enlarged (as shown in Figure 2b). The HRTEM image, which was recorded from the edge of the ZnO template, exhibits good crystallinity, as shown in Figure 2c. The fast Fourier transmission (FFT) patterns displayed (insets) suggest that the ZnO templates are hexagonal-wurtzite structured and are 〈0002〉 direction-oriented. Figure 2d,e shows the HRTEM images of the nanoparticles on the ZnO templates with and without HMT. Both panels show parallel lattice planes. Corresponding FFT patterns show they were obtained with beam directions along the [1j22] and [013] directions of a facecentered-cubic lattice, respectively. XRD data further confirmed the crystal structure information and the chemical components of the heterostructures. As shown in Figure 3, the diffraction lines of the template (curve 1) coincide with the (100), (002), (101), (102), (110), and (103) planes of the hexagonal-wurtzite ZnO (PDF #36-1451). After the heterostructured composites were formed, new peaks located at 2θ values of 30.16°, 35.52°, 43.14°, 53.52°, and 62.64° appeared, which were marked by asterisks in Figure 3 curves 2

Figure 1. Low-magnification FESEM images of (a) as-prepared ZnO microrods and (b) an individual ZnO microrod. (c) Enlarged image of ZnO microrod from the selected area in panel b. (d) Low-magnification image of heterostructured composites synthesized on the condition of using HMT aqueous solution. (e) Individual composite rod in panel d. (f) Enlarged image of the selected area in panel e. (g) Low-magnification image of the heterostructured composites gained without using HMT. (h) Individual composite rod in panel g. (i) Enlarged image of the selected area in panel g.

Figure 2. (a) TEM image of a single ZnO/Fe3O4 microrod. (b) Enlarged TEM image of the selected rectangular area in panel a. HRTEM recorded from (c) the edge of the ZnO template, (d) a nanoparticle on the ZnO/Fe3O4 microrod, and (e) a nanoparticle on the ZnO/Fe2O3 microrod. The inset images are the FFT patterns.

and 3. These peaks can all be assigned to cubic inverse spinelphase iron oxides (PDF #88-0315). It suggested that iron oxide particles had been formed on the ZnO templates by hydrothermal procedures. However, it was difficult to identify the exact phase of the deposited iron oxide nanoparticles, since the XRD patterns

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Figure 3. XRD patterns of (1) ZnO microrods, (2) ZnO/Fe3O4, and (3) ZnO/Fe2O3. The diffraction lines of curve 1 coincide with the (100), (002), (101), (102), (110), and (103) planes of the hexagonal-wurtzite ZnO, and the marked new peaks in curves 2 and 3 can all be assigned to cubic inverse spinel-phase iron oxides.

Figure 4. (a) Core-level Fe 2p photoelectron spectra for (1) ZnO/ Fe3O4 and (2) ZnO/Fe2O3. (b) Core-level Zn 2p spectra for (1) ZnO microrods, (2) ZnO/Fe3O4, and (3) ZnO/Fe2O3.

of cubic Fe3O4 and γ-Fe2O3 were quite similar and the peaks were nearly emerged by those of ZnO templates. To identify the composition of the composites, a sensitive surface analysis technique, XPS, was used to measure the composition and chemical bonding configurations. As shown in Figure 4a, only two peaks could be observed in core-level Fe 2p spectrum for the heterostructures fabricated with HMT: 724.7 and 710.8 eV (curve 1). They could be assigned to Fe 2p3/2 and Fe 2p1/2 for Fe3O4.17 In Figure 4a curve 2, three peaks could be observed. The peaks of Fe 2p3/2 and Fe 2p1/2 were obviously narrower than those in curve 1, and the peak around 719 eV is a characteristic peak of a ferric cation,17 which demonstrated that the dominant phase of iron oxide was Fe2O3 for the heterostructured composites formed without HMT. Figure 4b shows the Zn 2p spectra of the ZnO templates and the heterostructured composites. The peaks shift toward the lower energy side after deposition of the magnetic nanoparticles, indicating the chemical interaction of iron oxide nanoparticles with surface Zn atoms of the ZnO templates. In order to understand the fabrication of the two different kinds of heterostructures, the mechanism of the growth needs to be discussed. Alkalines such as ammonia are usually stimulative for the formation of iron oxides. However, as an amphoteric oxide, the morphology of ZnO templates sometimes may be destroyed during the composite procedure owing to the

Chu et al.

Figure 5. (a) Resonant Raman scatterings of (1) ZnO microrods, (2) ZnO/Fe3O4, and (3) ZnO/Fe2O3. (b) Enlarged view of selected area in panel a.

change of pH value or ionic strength of the solution.16 The significant effect of HMT, a weak organic alkaline, on retaining the rodlike morphology is noticeable in this condition. HMT is gradually decomposed into amine, formaldehyde, and other intermediates in aqueous solution at elevated temperature.16 This results in the increase of the pH value of the solution and the heterogeneous nucleation and growth of iron oxide on the surface of ZnO templates. In addition, the effect of environmental oxygen should not be neglected because the ferrous cations are apt to be oxidized. Thus, the following reactions might occur:

C6H12N4 + 6H2O f 6CH2O + 4NH3

(1)

NH3 + H2O f NH4+ + OH-

(2)

6Fe2+ + 12OH- + O2 f 2Fe3O4 + 6H2O

(3)

When HMT was not introduced, deionized water was directly used as solvent in the hydrothermal composite procedure. Some hydroxyl anions might exist on the surface of ZnO, as they were synthesized by the hydrothermal method.18 But the quantity of the hydroxyls was limited since no extra alkaline was added. The mineralizing process of iron oxide would be much slower compared with that of ZnO/Fe3O4. Therefore, much more time was provided for the oxidation of the ferrous cations, which resulted in the formation of Fe2O3 but not Fe3O4. The deposition of the two kinds of iron oxide nanoparticles on ZnO would lead to changes not only in the valence state but also in the force constants,19 considering the differences in crystal lattice constants between ZnO (a ∼ 0.325 nm, c ∼ 0.520 nm) (PDF #36-1451) and iron oxide (Fe3O4 ∼ 0.839 nm, γ-Fe2O3 ∼ 0.833 nm) (PDF #88-0315 and 3-862). For the frame stretching result of the interfacial ZnO crystal, phonon peak shifts at the lower-energy side should be observed in the resonant Raman spectra of ZnO. Figure 5a shows the resonant Raman spectra of the ZnO templates ZnO/Fe3O4 and ZnO/Fe2O3. The incident power density of the laser was attenuated to 10% of the full output to reduce the heating effect. As a result of multiphonon processes, two bands at 575 and 1138 cm-1 were observed. They can be assigned to the A1 (1LO) and A1 (2LO) modes,20 respectively. An enlarged image of the 1LO peaks is shown in Figure 5b. The frequencies of the composites were indeed decreased from 575 to 569 cm-1, which is analogous to the result of XPS Zn 2p spectrum.

Preparation of ZnO/Iron Oxide Heterostructures

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Figure 6. Normalized PL spectra for ZnO (black line), ZnO/Fe3O4 (blue line), and ZnO/Fe2O3 (red line). The normalization was followed by the excitonic luminescence.

The differences in the component would lead to great differences in the PL properties of the heterostructures, as shown in Figure 6. Average data were used according to the results of the repeating measurements to increase the reliability. Excitonic luminescence (∼380 nm) could be obtained in both ZnO templates and the heterostructured composites, although the light emission efficiencies were degraded (see the inset picture of Figure 6). An ∼4 nm red shift occurs to the excitonic emission in ZnO/Fe2O3 compared with that of the ZnO templates. A possible reason is that the consumption of hydroxyls attached to the surface of ZnO may increase the surface defects, as described in Scheme 1, since no extra alkaline was introduced. This can be confirmed by the obvious enhanced defect-relative luminescence (420-700 nm) of ZnO/Fe2O3, as shown in the normalized PL spectra. For the ZnO/Fe3O4 heterostructures, an opposite phenomenon occurred, which could be attributed to the passivation of the black Fe3O4 nanoparticles and HMT introduced. The ZnO/Fe3O4 and ZnO/Fe2O3 heterostructures also made sensitive responses to applied magnetic fields. The magnetic properties of the heterostructured composites were strong enough to separate and collect them using a commercial magnet shelf (Figure 7a). Room-temperature magnetization measurement exhibited that there was no hysteresis for both ZnO/Fe3O4 and ZnO/Fe2O3 heterostructures, as shown in Figure 7b, indicating that they are superparamagnetic. However, the differences in the components also lead to differences in the magnetic properties. The saturation magnetization of ZnO/Fe3O4 is about 20 emu · g-1, and that of ZnO/Fe2O3 is 1.3 emu · g-1. The difference might originate from the following aspects. First, the ZnO templates of the heterostructures are nonmagnetic, which will ultimately influence the magnetic properties. Second, the saturation magnetization of bulk Fe3O4 (90 emu · g-1)21 is larger than that of γ-Fe2O3 (75 emu · g-1).22 Finally, structural distortions at the surface and finite size effects have been reported as being responsible for the anomalous magnetic behavior manifested by nanoparticles.23,24 Surface spin canting and interatomic exchange coupling might be decreased at the interface between the iron oxide nanoparticles and the ZnO templates.25,26 Thus, the differences of the effects on the two heterostructures would lead to different reductions of the saturation magnetization. Either way, the results indicate that the heterostructures are able to become reliable magnetic responsive materials needed for noncontact operable nanogenerators.

Figure 7. (a) Magnetic separation photographs of (1) ZnO/Fe3O4 and (2) ZnO/Fe2O3. The left one was taken as soon as they were put onto the magnetic shelf, and the right one was taken within 3 min. (b) Roomtemperature magnetization curves of ZnO/Fe3O4 (dashed line) and ZnO/ Fe2O3 (solid line).

Conclusion In summary, multifunctional ZnO/Fe3O4 and ZnO/Fe2O3 heterostructured composites were successfully fabricated on the condition of preserving the rodlike ZnO templates. FESEM, XRD, and XPS measurements confirmed that heterostructures were obtained, and HRTEM observation demonstrated that the components were good crystalline. The presence or absence of HMT strongly effected the components of the heterostructures and ultimately made a difference in their properties. The PL emissions, Raman signal, and superparamagnetic properties can coexist in both heterostructured composites. Such multifunctional heterostructured composites can expand the applications of ZnO to more fields, such as magnetic driving nanogenerator, magneto-optic devices, biological detection, and so on. Acknowledgment. This work is supported by the National Natural Science Foundation of China (No. 50702013, 50725205), the Analysis and Testing Foundation of Northeast Normal University, and the Science Foundation for Young Teachers of Northeast Normal University (No. 20070204). References and Notes (1) Patolsky, F.; Timko, B. P.; Yu, G.; Fang, Y.; Greytak, A. B.; Zheng, G.; Lieber, C. M. Science 2006, 313, 1100. (2) Katz, E.; Bu¨ckmann, A. F.; Willner, I. J. Am. Chem. Soc. 2001, 123, 10752. (3) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (4) Kuriyama, K.; Ooi, M.; Matsumoto, K. Appl. Phys. Lett. 2006, 89, 242113. (5) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (6) Wang, X. D.; Song, J. H.; Liu, J.; Wang, Z. L. Science 2007, 316, 102. (7) Qin, Y.; Wang, X. D.; Wang, Z. L. Nature 2008, 451, 809. (8) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167. (9) Yavuz, C. T.; Mayo, J. T.; Yu, W. W.; Prakash, A.; Falkner, J. C.; Yean, S.; Cong, L. L.; Shipley, H. J.; Kan, A.; Tomson, M.; Natelson, D.; Colvin, V. L. Science 2006, 314, 964. (10) Sˇafaøı´k, I.; Sˇafaı´kova´, M. Monatsh. Chem. 2002, 133, 737.

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