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γ-Fe2O3 composite microspheres
Materials Science and Engineering C 29 (2009) 1128–1132
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
One-step facile fabrication of Ag/γ-Fe2O3 composite microspheres Xian-Ming Liu a,⁎, Ying-Shun Li b a b
College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, Henan, PR China Department of Chemistry, the Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, PR China
a r t i c l e
i n f o
Article history: Received 15 August 2008 Received in revised form 22 September 2008 Accepted 24 September 2008 Available online 7 October 2008 Keywords: Composite Microsphere Solvothermal reduction method Ferromagnetic behavior
a b s t r a c t Ag/γ-Fe2O3 composite microspheres were successfully prepared via a simple solvothermal reduction method under mild conditions. Electron and X-ray diffraction data revealed that these composites consisted of silver and maghemite. Through the optimization of processing conditions, Ag/γ-Fe2O3 composites with a spherical shape were successfully produced. The results from the transmission and scanning electron microscopy revealed that the composites were spherical with a diameter in the range of 200–300 nm. Magnetic measurements showed that the mixed microspheres exhibited a typical ferromagnetic behavior, a specific saturation magnetization of 56 emu/g and an intrinsic coercivity of 38 Oe at room temperature. The presence of Ag nanoparticles dispersed into γ-Fe2O3 microspheres was also confirmed by UV–Vis absorption. These composites with microspherical morphologies can be applied in a variety of areas, including catalysis, medicine, photonics, and new functional device assemblies. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The major interest in recent materials research lies not only in synthesizing new materials but also in controlling the morphology of materials to produce the desired nanostructures, for instance, nanoparticles with controllable sizes and shapes. The developments in nanoparticle synthesis from single component nanoparticles to nanoparticles with two or more components are attractive for advanced applications, such as catalysts[1,2]. Because many physical properties arise from fundamental length scales of a few to a few hundred nanometers, the capability of compositional control at such length scales renders tremendous opportunities for achieving unique combinations of these properties. Recent progress in nanomaterial synthesis has made it possible to uniformly mix nanoscale components in hybrid materials. However, the random nucleation of a second phase or the random mixing of different components provides limited control over the size and the components. These hybrid nanomaterials can exhibit different geometric shapes that include spherical and cubic core-shell [3–5], dumbbell-like [6–8], and more complex structures [9,10]. The resulting hybrid materials not only retain their individual magnetic, semiconducting, and plasmonic properties but also they exhibit enhanced optical, magnetic, and catalytic properties compared with their individual single-component materials. Bifunctional precious metal-iron oxide nanoparticles have been paid great attention to in recent years due to the advances in nanobiotechnology. For example, Pt@Fe2O3 core-shell nanoparticles ⁎ Corresponding author. Tel./fax: +86 379 65523821. E-mail address: [email protected] (X.-M. Liu). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.09.041
were synthesized using a sequential synthetic method. Platinum nanoparticles were synthesized via reduction of platinum acetylacetonate in octyl ether, and layers of iron oxide were subsequently deposited on the surface of Pt nanoparticles through thermal decomposition of iron pentacarbonyl [11]. Dumbbell-like Au–Fe3O4 nanoparticles were synthesized through decomposition of Fe(CO)5 on the surface of Au nanoparticles followed by oxidation in 1-octadecene solvent [12]. Magnetic composite nanoparticles of Au/γ-Fe2O3 were synthesized in an aqueous phase using gamma-ray [13]. A general strategy was also developed to synthesize engineering binary and ternary hybrid nanoparticles based on spontaneous epitaxial nucleation and growth of a second and third component onto the seed nanoparticles in high-temperature organic solutions [14]. Gu et al. reported a general synthetic method for production of size-controlled Ag–Fe3O4 heterodimeric nanoparticles using the Fe3O4 nanoparticles as the seeds [15], whereas Li and coworkers reported the synthesis of bifunctional Au–Fe3O4 nanoparticles that are formed by chemical bond linkage [16]. Pt@Fe2O3 core-shell nanoparticles were produced as precursors to obtain face-centered tetragonal (fct) FePt nanoparticles, where the size of the product was tunable [17,18]. These approaches were focused on the synthesis of hybrid nanoparticles with a diameter below 100 nm and the preparation of nanoparticles involved multi-step processes. However, little attention has been paid to the synthesis of monodisperse and microscale spheres made from magnetic multifunctional materials. Controlling the size, shape, monodispersity, and yield of the desired products has always been a biggest challenge in developing new materials. In this paper, an improved, facile technique is developed to produce near-monodisperse binary hybrid microspheres that are co-assembled with Ag and
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ometer equipped with graphite-monochromated Cu-Kα radiation (λ = 1.54178 Å), employing a scanning rate of 0.02°s− 1 in the 2θ ranging from 10 to 80. The transmission electron microscope (TEM) photographs and the electron diffraction (ED) pattern were recorded on a Hitachi H-800 TEM, using an accelerating voltage of 200 kV. The scanning electron microscopy (SEM) images and energy-dispersive Xray (EDX) were obtained using a HITACHI S-4300 microscope and EMAX Horiba, respectively. X-ray photoelectron spectroscopy data were obtained with an ESCALab220i-XL electron spectrometer (VG Scientific) using 300 W AlKα radiation. The base pressure was about 3 × 10− 9 mbar. Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7307, USA) with a maximum magnetic field of 10 kOe. The UV–Vis absorption spectra were taken at room temperature on a UV–vis spectrophotometer (Perkin Elmer, Lambda 20) equipped with an integrator sphere for diffuse reflectance studies. MgO (100%) was used as the reflectance reference. 3. Results and discussion
Fig. 1. XRD curves of the products. (a) γ-Fe2O3; (b) Ag/γ-Fe2O3 composite.
maghemite nanoparticles in one-step process. The hybrid microspheres developed in this study have spherical morphologies other than simple core-shell and dimer nanostructures.
The XRD pattern of the as-prepared product without silver is shown in Fig. 1a. The powder X-ray diffraction pattern of the material also proved its crystalline nature and the peaks matched well with standard γ-Fe2O3 reflections (JCPDS card no. 39-1346). Fig. 1b shows the representative XRD spectra of the Ag/Fe2O3 composites. Besides the characteristic diffraction peaks of γ-Fe2O3 reflections, the diffraction peaks at 38.06, 44.28, 64.38, and 77.4° 2θ can be indexed to (111), (200), (220), and (311) planes of silver with the face-centered cubic (fcc) structure (space group: Fm3m), respectively (JCPDS card, No. 04-0783). The low strength of iron oxide diffractions was most likely due to the low crystallinity of iron oxide and the heavy atom effect arising from silver [11]. Although the product was brown in color, no α-Fe2O3 phase was shown in the XRD pattern of the samples. The panoramic morphology of the products was obtained by SEM, in which the solid samples were mounted on a conductive resin with
2. Experimental 2.1. Materials Chemical reagents such as ethylene glycol (EG), dimethylformamide (DMF), hydrated iron (III) nitrate (Fe(NO3)3·9H2O), polyvinylpyrrolidone (PVP) and silver nitrate (AgNO3) were purchased from Beijing Chemicals Company. All chemicals were of analytical grade and were used as received. Deionized water was used in the experiments. 2.2. Synthesis of Ag/γ-Fe2O3 composite microspheres Ag/γ-Fe2O3 composite microspheres were synthesized following a procedure described elsewhere [19], using Fe(NO3)3·9H2O instead of FeCl3·6H2O as the iron source materials. In a typical experimental procedure, Fe(NO3)3·9H2O (0.5 g), AgNO3 (0.05 g) and PVP (0.5 g, MW 58,000) were dissolved in EG (35 mL) and DMF (5 mL, 99%) to form a clear yellow solution. After vigorously stirring for 30 min, the mixture was then transferred into a Teflon-lined stainless-steel autoclave with a capacity of 50 mL for solvothermal treatment at 160 °C for 8 h. After the autoclave had cooled down to room temperature, the precipitates were separated by centrifugation, washed several times with deionized water and absolute ethanol, and subsequently dried under a vacuum at 60 °C. Finally, a brown product was obtained. 2.3. Characterizations The crystalline structure of the products was characterized by power X-ray diffraction (XRD) using a Rigaku D/max2500 diffract-
Fig. 2. SEM images of the products. (a) γ-Fe2O3; (b) Ag/γ-Fe2O3 composite.
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The process of forming γ-Fe2O3 sub-microspheres can be used to explain the mechanisms behind the formation of Ag/γ-Fe2O3 composite microspheres [19]. Besides the formation of maghemite sub-microspheres, there is another reaction occurring in the system. Because DMF is an active reducing agent under suitable conditions, Ag nanoparticles can easily be obtained through in situ reduction of silver ions in the aforementioned solvent mixture under the solvothermal conditions. The reaction taking place in forming the Ag nanoparticles is given: HCONMe2 þ 2Agþ þ H2 O→2Ag0 þ Me2 NCOOH þ 2Hþ
Fig. 3. EDX patterns of the composite.
dispersion treatment. Fig. 2a presents the as-synthesized γ-Fe2O3 microspheres, indicating that all the products had almost identical morphology and the proportion of spheres in the whole sample was very high. The diameters of the spheres ranged from 100 nm to 400 nm, which is similar to those described in the literature [19]. Fig. 2b shows the composites obtained via one-step synthesis by the solvothermal method. The composites were spherical with a diameter of 200–300 nm. Compared to γ-Fe2O3 microspheres, the surface of the composites was not glossy, indicating that silver nanoparticles were attached to γ-Fe2O3 microspheres. Moreover, the diameter of the composites had a narrow distribution. The elemental analysis by EDX (Fig. 3) confirmed the presence of Fe, Ag and O elemental signatures in the composite. The analysis also identified that the products had over 98% iron oxides and silver by mass, with a small amount of carbon (b2%). The presence of a small amount of C could be anticipated from the conducting resin used for measurements. In order to further investigate the crystal phase of the samples, XPS was used to examine the shell structure of the composites, because core electron lines of silver, ferrous and ferric ions can both be detected and are distinguishable in XPS. This technique was successfully employed previously to differentiate between Fe2O3 and Fe3O4 [20,21]. Fig. 4a shows representative XPS spectra of the product. Elemental analysis confirmed the presence of Ag, Fe, C and O elemental signatures. No other metallic signals were detected in the XPS spectra. The photoelectron peaks at 711 and 725 eV are the characteristic doublets of Fe 2p3/2 and 2p1/2 core-level spectra of iron oxide, respectively, as shown in Fig. 4b. Fe 2p data are in good agreement with the values reported for γ-Fe2O3 in the literature [21]. The Ag 3d core level spectrum is shown in Fig. 4c, and can be resolved into two spinorbit components, Ag 3d5/2 and Ag 3d3/2. These peaks occurred at BEs of 368 and 374 eV, respectively, which are closed to the values of zerovalent silver (Ag 3d5/2: 367.9 eV; Ag 3d3/2: 373.9 eV) [22]. According to the XRD and XPS results, it can be inferred that the solvothermal products consisted of γ-Fe2O3 and silver. The observed XPS spectra of C 1s (284.8 eV) and O 1s (530.4 eV) arose from the adventitious carbon and iron oxides, respectively. The TEM photograph of the composites shown in Fig. 5 indicates that the composites consisted of microspherical particles, consistent with the SEM observation (Fig. 2b). The diameter of a single microsphere is estimated in the range of 200–300 nm. It is naturally considered that the final product had a spherical structure, which was formed only by self-assembly of γ-Fe2O3 nanoparticles [19], and the Ag nanoparticles were adsorbed into the microspheres. The bright field images of the ED pattern shown in Fig. 5b indicates that the composite consisted of γ-Fe2O3 nanoparticles with a deficient spinel structure and silver with an fcc structure, which are all in agreement with the results from XRD and XPS analyses.
ð1Þ
The mass ratio of Fe oxide to Ag was varied while all other conditions were kept constant to evaluate the effect of mass ratio and the results are shown in Fig. 6. It is noted that the weight ratio of Ag/γ-Fe2O3 had great influence on final morphology of the product. The lower ratio (5:1) exhibited more nanoparticles attached onto the microspheres than the
Fig. 4. XPS spectra of the composites. (a) the composite; (b) Fe 2p; (c) Ag 3d.
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Fig. 5. TEM photograph and ED pattern of the composite.
higher ratio (20:1). The effects of other parameters, such as solvothermal temperature and PVP, on the formation of maghemite submicrospheres, can be found in our previous study [19]. Fig. 7 shows the magnetization plotted as a function of applied field (M versus H curves), which was measured at room temperature. It can be seen that the M–H curves exhibited nonlinear, reversible behavior with observed coercivity and remanence. Magnetic measurements also showed that there were small changes in magnetic moment and coercivity of γ-Fe2O3 microspheres between before and after the introduction of Ag nanoparticles. The magnetization of γ-Fe2O3 microspheres was about 78 emu/g, and that of γ-Fe2O3/Ag microspheres was about 56 emu/g. The 28.2% decrease in magnetization suggests that the silver nanoparticles introduced to the original γ-Fe2O3 microspheres were about 28.2% in weight, which is larger than the theoretical value of 24.3%. The reason may be that the formation of γ-Fe2O3 was not complete. The coercivity of both materials can be found in the enlarged
Fig. 6. SEM images of the composites with different weight ratios of Fe(NO3)3·9H2O/ AgNO3. (a) 20:1; (b) 5:1.
Fig. 7. Magnetization curves of the products at room temperature. (a) γ-Fe 2 O 3 ; (b) γ-Fe 2 O 3/Ag microspheres.
view of the central loop shown in Fig. 7, 52 Oe for the γ-Fe2O3 microspheres and 38 Oe for the γ-Fe2O3/Ag composite microspheres. The reductions in the specific saturation magnetization and coercivity are attributed to the nonmagnetic metal Ag which existed in the composites. It is also found that the magnetization can be saturated at a low applied field, indicating that the as-prepared γ-Fe2O3/Ag microspheres exhibited typical ferromagnetic behavior. Diffuse reflection spectra of as-prepared samples was measured using a UV–Vis spectrophotometer (Perkin Elmer, Lambda 20) and was converted from reflectance to absorbance by the Kubelka– Munk method. UV–Vis absorption spectra have been proved to be quite sensitive to the form of Ag nanoparticles, since Ag nanoparticles exhibit a characteristic absorption peak around 350–400 nm due to the surface plasmon excitation [23]. The UV–Vis diffusereflectance spectra of Ag/γ-Fe2O3, Ag, and γ-Fe2O3 are showed in Fig. 8. Ag particles had two strong and broad absorption peaks at ca. 250–350 nm and 350–400 nm, as shown in Fig. 8a. The strong absorption at ca. 250–350 nm exhibited steep absorption. It is different from the spectra of the as-prepared γ-Fe2O3, the absorption ranging from 400 to 750 nm (Fig. 8b). Compared to the UV–Vis absorption of Ag and γFe2O3, Ag/γ-Fe2O3 had strong adsorption in the visible region and the UV region (Fig. 8c), which indicated the joint action of Ag and γ-Fe2O3. Therefore it is believed that most of the Ag primary nanoparticles are
Fig. 8. UV–Vis spectra of the products. a) Ag; b) γ-Fe2O3; c) Ag/γ-Fe2O3 composite.
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separated in the Fe2O3 microspheres and can not gather together to form Ag-Fe2O3 composite under this circumstance. 4. Conclusions In summary, this work has demonstrated that bi-composite γ-Fe2O3/Ag spherical particles can be synthesized through a direct reduction route and the obtained products were characterized by XRD, SEM, EDS, TEM, XPS, VSM and UV–Vis absorption. These composites with microspherical morphologies can be used in a variety of areas, including catalysis, medicine, photonics, and new functional device assemblies. Moreover, the synthetic strategy proposed in this paper would pave the way for facile preparation of diverse bifunctional and multifunctional hybrid nanomaterials. Acknowledgements The authors wish to thank the financial support from the National Research Project (2008B430019) of Education Department of Henan Province, P. R. China. References [1] H. Zheng, J. Wang, S.E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, et al., Science 303 (2004) 661.
[2] H. Zeng, J. Li, J.P. Liu, Z.L. Wang, S.H. Sun, Nature 420 (2002) 395. [3] X. Peng, M.C. Schlamp, A.V. Kadavanich, A.P. Alivisatos, J. Am. Chem. Soc. 119 (1997) 7019. [4] M.A. Malik, P. O’Brien, N. Revaprasadu, Chem. Mater. 14 (2002) 2004. [5] S. Decker, K.J. Klabunde, J. Am. Chem. Soc. 118 (1996) 12465. [6] H.W. Gu, R.K. Zheng, X.X. Zhang, B. Xu, J. Am. Chem. Soc. 126 (2004) 5664. [7] K.W. Kwon, M. Shim, J. Am. Chem. Soc. 127 (2005) 10269. [8] Y. Li, Q. Zhang, A.V. Nurmikko, S. Sun, Nano Lett. 5 (2005) 1689. [9] S. Kudera, L. Carbone, M.F. Casula, R. Cingolani, A. Falqui, E. Snoeck, et al., Nano Lett. 5 (2005) 445. [10] T. Mokari, E. Rothenberg, I. Popov, R. Costi, U. Banin, Science 304 (2004) 1787. [11] X. Teng, D. Black, N.J. Watkins, Y. Gao, H. Yang, Nano Lett. 3 (2003) 261. [12] H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S.H. Sun, Nano Lett. 5 (2005) 379. [13] S. Seino, T. Kinoshita, Y. Otome, K. Okitsu, T. Nakagawa, T.A. Yamamoto, Chem. Lett. 32 (2003) 690. [14] W. Shi, H. Zeng, Y. Sahoo, T.Y. Ohulchanskyy, Y. Ding, Z.L. Wang, et al., Nano Lett. 6 (2006) 660. [15] L. Zhang, Y.H. Dou, H.C. Gu, J. Colloid. Interface Sci. 297 (2006) 660. [16] J. Bao, W. Chen, T. Liu, Y. Zhu, P. Jin, L. Wang, ACS Nano 1 (2007) 293. [17] X. Teng, H. Yang, J. Am. Chem. Soc. 125 (2003) 14559. [18] V. Tzitzios, D. Niarchos, G. Hadjipanayis, E. Devlin, D. Petridis, Adv. Mater. 17 (2005) 2188. [19] X.M. Liu, S.Y. Fu, H.M. Xiao, J. Solid State Chem. 179 (2006) 1554. [20] N.S. McIntyre, D.G. Zetaruk, Anal. Chem. 49 (1977) 1521. [21] T. Fujii, F.M.F. de Groot, G.A. Sawatzky, F.C. Voogt, T. Hibma, K. Okada, Phys. Rev. B 59 (1999) 3195. [22] C.D. Wagner, W.M. Riggs, C.E. Davis, J.F. Moulder, in: G.E. Muilenberg (Ed.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Eden Praire, MN, 1979, p. 122. [23] A. Henglein, Chem. Mater. 10 (1998) 444.
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
One-step facile fabrication of Ag/γ-Fe2O3 composite microspheres Xian-Ming Liu a,⁎, Ying-Shun Li b a b
College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, Henan, PR China Department of Chemistry, the Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, PR China
a r t i c l e
i n f o
Article history: Received 15 August 2008 Received in revised form 22 September 2008 Accepted 24 September 2008 Available online 7 October 2008 Keywords: Composite Microsphere Solvothermal reduction method Ferromagnetic behavior
a b s t r a c t Ag/γ-Fe2O3 composite microspheres were successfully prepared via a simple solvothermal reduction method under mild conditions. Electron and X-ray diffraction data revealed that these composites consisted of silver and maghemite. Through the optimization of processing conditions, Ag/γ-Fe2O3 composites with a spherical shape were successfully produced. The results from the transmission and scanning electron microscopy revealed that the composites were spherical with a diameter in the range of 200–300 nm. Magnetic measurements showed that the mixed microspheres exhibited a typical ferromagnetic behavior, a specific saturation magnetization of 56 emu/g and an intrinsic coercivity of 38 Oe at room temperature. The presence of Ag nanoparticles dispersed into γ-Fe2O3 microspheres was also confirmed by UV–Vis absorption. These composites with microspherical morphologies can be applied in a variety of areas, including catalysis, medicine, photonics, and new functional device assemblies. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The major interest in recent materials research lies not only in synthesizing new materials but also in controlling the morphology of materials to produce the desired nanostructures, for instance, nanoparticles with controllable sizes and shapes. The developments in nanoparticle synthesis from single component nanoparticles to nanoparticles with two or more components are attractive for advanced applications, such as catalysts[1,2]. Because many physical properties arise from fundamental length scales of a few to a few hundred nanometers, the capability of compositional control at such length scales renders tremendous opportunities for achieving unique combinations of these properties. Recent progress in nanomaterial synthesis has made it possible to uniformly mix nanoscale components in hybrid materials. However, the random nucleation of a second phase or the random mixing of different components provides limited control over the size and the components. These hybrid nanomaterials can exhibit different geometric shapes that include spherical and cubic core-shell [3–5], dumbbell-like [6–8], and more complex structures [9,10]. The resulting hybrid materials not only retain their individual magnetic, semiconducting, and plasmonic properties but also they exhibit enhanced optical, magnetic, and catalytic properties compared with their individual single-component materials. Bifunctional precious metal-iron oxide nanoparticles have been paid great attention to in recent years due to the advances in nanobiotechnology. For example, Pt@Fe2O3 core-shell nanoparticles ⁎ Corresponding author. Tel./fax: +86 379 65523821. E-mail address: [email protected] (X.-M. Liu). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.09.041
were synthesized using a sequential synthetic method. Platinum nanoparticles were synthesized via reduction of platinum acetylacetonate in octyl ether, and layers of iron oxide were subsequently deposited on the surface of Pt nanoparticles through thermal decomposition of iron pentacarbonyl [11]. Dumbbell-like Au–Fe3O4 nanoparticles were synthesized through decomposition of Fe(CO)5 on the surface of Au nanoparticles followed by oxidation in 1-octadecene solvent [12]. Magnetic composite nanoparticles of Au/γ-Fe2O3 were synthesized in an aqueous phase using gamma-ray [13]. A general strategy was also developed to synthesize engineering binary and ternary hybrid nanoparticles based on spontaneous epitaxial nucleation and growth of a second and third component onto the seed nanoparticles in high-temperature organic solutions [14]. Gu et al. reported a general synthetic method for production of size-controlled Ag–Fe3O4 heterodimeric nanoparticles using the Fe3O4 nanoparticles as the seeds [15], whereas Li and coworkers reported the synthesis of bifunctional Au–Fe3O4 nanoparticles that are formed by chemical bond linkage [16]. Pt@Fe2O3 core-shell nanoparticles were produced as precursors to obtain face-centered tetragonal (fct) FePt nanoparticles, where the size of the product was tunable [17,18]. These approaches were focused on the synthesis of hybrid nanoparticles with a diameter below 100 nm and the preparation of nanoparticles involved multi-step processes. However, little attention has been paid to the synthesis of monodisperse and microscale spheres made from magnetic multifunctional materials. Controlling the size, shape, monodispersity, and yield of the desired products has always been a biggest challenge in developing new materials. In this paper, an improved, facile technique is developed to produce near-monodisperse binary hybrid microspheres that are co-assembled with Ag and
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ometer equipped with graphite-monochromated Cu-Kα radiation (λ = 1.54178 Å), employing a scanning rate of 0.02°s− 1 in the 2θ ranging from 10 to 80. The transmission electron microscope (TEM) photographs and the electron diffraction (ED) pattern were recorded on a Hitachi H-800 TEM, using an accelerating voltage of 200 kV. The scanning electron microscopy (SEM) images and energy-dispersive Xray (EDX) were obtained using a HITACHI S-4300 microscope and EMAX Horiba, respectively. X-ray photoelectron spectroscopy data were obtained with an ESCALab220i-XL electron spectrometer (VG Scientific) using 300 W AlKα radiation. The base pressure was about 3 × 10− 9 mbar. Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7307, USA) with a maximum magnetic field of 10 kOe. The UV–Vis absorption spectra were taken at room temperature on a UV–vis spectrophotometer (Perkin Elmer, Lambda 20) equipped with an integrator sphere for diffuse reflectance studies. MgO (100%) was used as the reflectance reference. 3. Results and discussion
Fig. 1. XRD curves of the products. (a) γ-Fe2O3; (b) Ag/γ-Fe2O3 composite.
maghemite nanoparticles in one-step process. The hybrid microspheres developed in this study have spherical morphologies other than simple core-shell and dimer nanostructures.
The XRD pattern of the as-prepared product without silver is shown in Fig. 1a. The powder X-ray diffraction pattern of the material also proved its crystalline nature and the peaks matched well with standard γ-Fe2O3 reflections (JCPDS card no. 39-1346). Fig. 1b shows the representative XRD spectra of the Ag/Fe2O3 composites. Besides the characteristic diffraction peaks of γ-Fe2O3 reflections, the diffraction peaks at 38.06, 44.28, 64.38, and 77.4° 2θ can be indexed to (111), (200), (220), and (311) planes of silver with the face-centered cubic (fcc) structure (space group: Fm3m), respectively (JCPDS card, No. 04-0783). The low strength of iron oxide diffractions was most likely due to the low crystallinity of iron oxide and the heavy atom effect arising from silver [11]. Although the product was brown in color, no α-Fe2O3 phase was shown in the XRD pattern of the samples. The panoramic morphology of the products was obtained by SEM, in which the solid samples were mounted on a conductive resin with
2. Experimental 2.1. Materials Chemical reagents such as ethylene glycol (EG), dimethylformamide (DMF), hydrated iron (III) nitrate (Fe(NO3)3·9H2O), polyvinylpyrrolidone (PVP) and silver nitrate (AgNO3) were purchased from Beijing Chemicals Company. All chemicals were of analytical grade and were used as received. Deionized water was used in the experiments. 2.2. Synthesis of Ag/γ-Fe2O3 composite microspheres Ag/γ-Fe2O3 composite microspheres were synthesized following a procedure described elsewhere [19], using Fe(NO3)3·9H2O instead of FeCl3·6H2O as the iron source materials. In a typical experimental procedure, Fe(NO3)3·9H2O (0.5 g), AgNO3 (0.05 g) and PVP (0.5 g, MW 58,000) were dissolved in EG (35 mL) and DMF (5 mL, 99%) to form a clear yellow solution. After vigorously stirring for 30 min, the mixture was then transferred into a Teflon-lined stainless-steel autoclave with a capacity of 50 mL for solvothermal treatment at 160 °C for 8 h. After the autoclave had cooled down to room temperature, the precipitates were separated by centrifugation, washed several times with deionized water and absolute ethanol, and subsequently dried under a vacuum at 60 °C. Finally, a brown product was obtained. 2.3. Characterizations The crystalline structure of the products was characterized by power X-ray diffraction (XRD) using a Rigaku D/max2500 diffract-
Fig. 2. SEM images of the products. (a) γ-Fe2O3; (b) Ag/γ-Fe2O3 composite.
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The process of forming γ-Fe2O3 sub-microspheres can be used to explain the mechanisms behind the formation of Ag/γ-Fe2O3 composite microspheres [19]. Besides the formation of maghemite sub-microspheres, there is another reaction occurring in the system. Because DMF is an active reducing agent under suitable conditions, Ag nanoparticles can easily be obtained through in situ reduction of silver ions in the aforementioned solvent mixture under the solvothermal conditions. The reaction taking place in forming the Ag nanoparticles is given: HCONMe2 þ 2Agþ þ H2 O→2Ag0 þ Me2 NCOOH þ 2Hþ
Fig. 3. EDX patterns of the composite.
dispersion treatment. Fig. 2a presents the as-synthesized γ-Fe2O3 microspheres, indicating that all the products had almost identical morphology and the proportion of spheres in the whole sample was very high. The diameters of the spheres ranged from 100 nm to 400 nm, which is similar to those described in the literature [19]. Fig. 2b shows the composites obtained via one-step synthesis by the solvothermal method. The composites were spherical with a diameter of 200–300 nm. Compared to γ-Fe2O3 microspheres, the surface of the composites was not glossy, indicating that silver nanoparticles were attached to γ-Fe2O3 microspheres. Moreover, the diameter of the composites had a narrow distribution. The elemental analysis by EDX (Fig. 3) confirmed the presence of Fe, Ag and O elemental signatures in the composite. The analysis also identified that the products had over 98% iron oxides and silver by mass, with a small amount of carbon (b2%). The presence of a small amount of C could be anticipated from the conducting resin used for measurements. In order to further investigate the crystal phase of the samples, XPS was used to examine the shell structure of the composites, because core electron lines of silver, ferrous and ferric ions can both be detected and are distinguishable in XPS. This technique was successfully employed previously to differentiate between Fe2O3 and Fe3O4 [20,21]. Fig. 4a shows representative XPS spectra of the product. Elemental analysis confirmed the presence of Ag, Fe, C and O elemental signatures. No other metallic signals were detected in the XPS spectra. The photoelectron peaks at 711 and 725 eV are the characteristic doublets of Fe 2p3/2 and 2p1/2 core-level spectra of iron oxide, respectively, as shown in Fig. 4b. Fe 2p data are in good agreement with the values reported for γ-Fe2O3 in the literature [21]. The Ag 3d core level spectrum is shown in Fig. 4c, and can be resolved into two spinorbit components, Ag 3d5/2 and Ag 3d3/2. These peaks occurred at BEs of 368 and 374 eV, respectively, which are closed to the values of zerovalent silver (Ag 3d5/2: 367.9 eV; Ag 3d3/2: 373.9 eV) [22]. According to the XRD and XPS results, it can be inferred that the solvothermal products consisted of γ-Fe2O3 and silver. The observed XPS spectra of C 1s (284.8 eV) and O 1s (530.4 eV) arose from the adventitious carbon and iron oxides, respectively. The TEM photograph of the composites shown in Fig. 5 indicates that the composites consisted of microspherical particles, consistent with the SEM observation (Fig. 2b). The diameter of a single microsphere is estimated in the range of 200–300 nm. It is naturally considered that the final product had a spherical structure, which was formed only by self-assembly of γ-Fe2O3 nanoparticles [19], and the Ag nanoparticles were adsorbed into the microspheres. The bright field images of the ED pattern shown in Fig. 5b indicates that the composite consisted of γ-Fe2O3 nanoparticles with a deficient spinel structure and silver with an fcc structure, which are all in agreement with the results from XRD and XPS analyses.
ð1Þ
The mass ratio of Fe oxide to Ag was varied while all other conditions were kept constant to evaluate the effect of mass ratio and the results are shown in Fig. 6. It is noted that the weight ratio of Ag/γ-Fe2O3 had great influence on final morphology of the product. The lower ratio (5:1) exhibited more nanoparticles attached onto the microspheres than the
Fig. 4. XPS spectra of the composites. (a) the composite; (b) Fe 2p; (c) Ag 3d.
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Fig. 5. TEM photograph and ED pattern of the composite.
higher ratio (20:1). The effects of other parameters, such as solvothermal temperature and PVP, on the formation of maghemite submicrospheres, can be found in our previous study [19]. Fig. 7 shows the magnetization plotted as a function of applied field (M versus H curves), which was measured at room temperature. It can be seen that the M–H curves exhibited nonlinear, reversible behavior with observed coercivity and remanence. Magnetic measurements also showed that there were small changes in magnetic moment and coercivity of γ-Fe2O3 microspheres between before and after the introduction of Ag nanoparticles. The magnetization of γ-Fe2O3 microspheres was about 78 emu/g, and that of γ-Fe2O3/Ag microspheres was about 56 emu/g. The 28.2% decrease in magnetization suggests that the silver nanoparticles introduced to the original γ-Fe2O3 microspheres were about 28.2% in weight, which is larger than the theoretical value of 24.3%. The reason may be that the formation of γ-Fe2O3 was not complete. The coercivity of both materials can be found in the enlarged
Fig. 6. SEM images of the composites with different weight ratios of Fe(NO3)3·9H2O/ AgNO3. (a) 20:1; (b) 5:1.
Fig. 7. Magnetization curves of the products at room temperature. (a) γ-Fe 2 O 3 ; (b) γ-Fe 2 O 3/Ag microspheres.
view of the central loop shown in Fig. 7, 52 Oe for the γ-Fe2O3 microspheres and 38 Oe for the γ-Fe2O3/Ag composite microspheres. The reductions in the specific saturation magnetization and coercivity are attributed to the nonmagnetic metal Ag which existed in the composites. It is also found that the magnetization can be saturated at a low applied field, indicating that the as-prepared γ-Fe2O3/Ag microspheres exhibited typical ferromagnetic behavior. Diffuse reflection spectra of as-prepared samples was measured using a UV–Vis spectrophotometer (Perkin Elmer, Lambda 20) and was converted from reflectance to absorbance by the Kubelka– Munk method. UV–Vis absorption spectra have been proved to be quite sensitive to the form of Ag nanoparticles, since Ag nanoparticles exhibit a characteristic absorption peak around 350–400 nm due to the surface plasmon excitation [23]. The UV–Vis diffusereflectance spectra of Ag/γ-Fe2O3, Ag, and γ-Fe2O3 are showed in Fig. 8. Ag particles had two strong and broad absorption peaks at ca. 250–350 nm and 350–400 nm, as shown in Fig. 8a. The strong absorption at ca. 250–350 nm exhibited steep absorption. It is different from the spectra of the as-prepared γ-Fe2O3, the absorption ranging from 400 to 750 nm (Fig. 8b). Compared to the UV–Vis absorption of Ag and γFe2O3, Ag/γ-Fe2O3 had strong adsorption in the visible region and the UV region (Fig. 8c), which indicated the joint action of Ag and γ-Fe2O3. Therefore it is believed that most of the Ag primary nanoparticles are
Fig. 8. UV–Vis spectra of the products. a) Ag; b) γ-Fe2O3; c) Ag/γ-Fe2O3 composite.
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separated in the Fe2O3 microspheres and can not gather together to form Ag-Fe2O3 composite under this circumstance. 4. Conclusions In summary, this work has demonstrated that bi-composite γ-Fe2O3/Ag spherical particles can be synthesized through a direct reduction route and the obtained products were characterized by XRD, SEM, EDS, TEM, XPS, VSM and UV–Vis absorption. These composites with microspherical morphologies can be used in a variety of areas, including catalysis, medicine, photonics, and new functional device assemblies. Moreover, the synthetic strategy proposed in this paper would pave the way for facile preparation of diverse bifunctional and multifunctional hybrid nanomaterials. Acknowledgements The authors wish to thank the financial support from the National Research Project (2008B430019) of Education Department of Henan Province, P. R. China. References [1] H. Zheng, J. Wang, S.E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, et al., Science 303 (2004) 661.
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