10688
J. Phys. Chem. C 2008, 112, 10688–10691
Low-Temperature Synthesis of Superparamagnetic Nanocomposite Particles Composed of Platinum and Maghemite Junling Zhang,*,† Hua Ji,†,‡ Yongge Wei,*,†,§ Yuan Wang,*,† and Nianzu Wu† State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Physical Chemistry, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, P.R. China, Department of Chemistry and Biochemistry, Texas Tech UniVersity, Lubbock, Texas 79409, USA, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, P. R. China ReceiVed: March 9, 2008; ReVised Manuscript ReceiVed: May 22, 2008
Superparamagnetic nanocomposite particles composed of maghemite nanocrystal (ca.15 nm) and deposited Pt nanoclusters (ca. 2.5 nm) were synthesized by a low temperature catalytic method. The magnetic properties of iron oxides species can be easily tuned with different redox atmosphere in the presence of Pt nanoclusters. In this convenient preparation, Pt nanoclusters were first captured on Fe(OH)3 nanoparticles by electrostatic interaction. Then at 80 °C the Fe(OH)3 nanoparticles was catalytically reduced with glycol acid to Fe3O4 nanophase which were finally catalytically oxidized to maghemite (γ-Fe2O3) nanophase in the air at 80 °C. The as-synthesized Pt/Fe3O4 and Pt/γ-Fe2O3 nanocomposites were well characterized by chemical analysis, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and magnetization measurements. Reversible and efficient transformation between the Pt/Fe3O4 and Pt/γFe2O3 nanocomposites was realized at 80 °C. The Pt/γ-Fe2O3 nanocomposite particles modified with oleic acid were easily dispersed in nonpolar solvents such as hexane, resulting in stable ferrofluids. Introduction Much attention has been paid to nanosized maghemite (γFe2O3) particles with superparamagnetic properties due to their potential applications in information storage devices,1 controlled drug delivery,2 stable ferrofluids,3 highly effective magnetic separation and catalysis.4 Several techniques have been developed for the synthesis of γ-Fe2O3 nanocrystalline, including electrochemical5 and sonochemical synthesis,6 mechanism method,7 and wet chemical route.8–13 While most of them need an annealing process at a temperature higher than 200 °C to yield the high quality γ-Fe2O3 nanocrystalline, there is also an interesting report on the direct preparation of monodisperse γ-Fe2O3 nanocrystallites at elevated temperature using Fe(CO)5 as the starting material and oleic acid as the protective agent.8 The annealing process at high temperature usually causes the agglomeration of the particles, and the sintered γ-Fe2O3 particles are difficult to be dispersed in a liquid media, which is harmful to technological applications. To avoid the agglomeration of the nanoparticles, strategies of lowering the requisite annealing temperature or using a protective agent to cover the renascent nanocrystallites during the synthesis process are usually adopted. However, developing a low-temperature route to synthesize γ-Fe2O3 nanoparticles is still a challenge in chemistry of materials. Platinum is an important noble metal with unique catalytic properties.14–18 We have a long interest in developing novel catalysts composed of noble metal nanoclusters.19–21 In this paper, followed the widely used industrial route to prepare * Corresponding authors: (J.Zhang)
[email protected]; (Y. Wang)
[email protected]; Y. Wei)
[email protected]. † State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Physical Chemistry, College of Chemistry and Molecular Engineering, Peking University. ‡ Department of Chemistry and Biochemistry, Texas Tech University. § Department of Chemistry, Tsinghua University.
γ-Fe2O3 starting from Fe(OH)3, we develop a convenient lowtemperature catalytic procedure for the synthesis of Pt/γ-Fe2O3 nanocomposite particles with superparamagnetic properties. The Pt/γ-Fe2O3 nanoparticles can be easily converted into Pt/Fe3O4 nanoparticles by reduction with hydrogen, and reversible transformation between them has been successfully realized at 80 °C. This property and the catalytic activity of Pt nanoclusters, together with the good dispersibility of the nanocomposite, provide us potential opportunities to develop new catalysts or magnetic materials for sensor and bioseparation. Experimental Section Materials and Instruments. H2PtCl6 · 6H2O was purchased from Shenyang Research Institute of Nonferrous Metals. Other reagents used in this work had a level of GR and were used as received. XRD patterns were carried out by a Rigaku Dmax 2500PC diffractometer with Cu radiation at 40 kV and 300 mA. The XPS measurements were conducted via an Axis Ultra photoelectron spectrometer. Raman spectra were recorded on a Renishaw Raman imaging microscope (2000) with a HeNe laser (633 nm). TEM images were taken on a Philips Tecnai F30 at 300 kV and a combined EDX analysis was conducted using an electron beam of 0.8 nm in diameter. The saturation magnetization was measured on a MicroMag Model 2900 instrument at room temperature. Preparation of the Pt/γ-Fe2O3 Nanacomposite. A colloidal solution of Pt nanoclusters stabilized with simple ions and ethylene glycol (Pt: 3.75 g/L), average diameter in 2.5 nm, was prepared according to the method we developed previously:21 50 mL of ethylene glycol solution containing 1 g of H2PtCl6 · 6H2O was added into 50 mL of NaOH ethylene glycol solution (0.5 M), forming a yellow stable platinum hydroxide colloid. The above colloid solution was heated for 3 h at 160 °C to form the stable metal Pt nanoclusters. The prepared Pt nanoclusters (0.0381 g) were purified by adding an aqueous solution of HCl (1 mol/L) into the Pt colloidal
10.1021/jp8020694 CCC: $40.75 2008 American Chemical Society Published on Web 06/28/2008
Superparamagnetic Nanocomposite Particles
J. Phys. Chem. C, Vol. 112, No. 29, 2008 10689
Figure 2. XRD pattern of the Pt/γ-Fe2O3 nanocomposite. Figure 1. Raman spectra of (a) the as-resulted Pt/γ-Fe2O3 nanocomposite and (b) the intermediate Pt/Fe3O4 nanocomposite.
solution and then centrifuging to obtain a black precipitate, which was “re-dissolved” into 8 mL of ethylene glycol solution containing 0.16 g of NaOH. For preparing a colloidal solution of Fe(OH)3, an aqueous solution of ammonia (10%) was added into 100 mL aqueous solution of FeCl3 (4%) to adjust the pH to ca. 7.5, producing a precipitate which was separated by a filter, washed with water and peptized in an aqueous solution of FeCl3 (30 mL, 1.2%), resulting in a transparent colloidal solution of Fe(OH)3. The Pt/γ-Fe2O3 nanocomposite with 3 wt% of Pt was prepared as follows. The purified Pt nanoclusters and Fe(OH)3 nanoparticles in the colloidal solutions were mixed under stirring and then 1.2 g of glycol acid was added into the complex sol. The mixture was heated in a Teflon-lined autoclave at 80 °C for 72 h. A magnetic precipitate was produced during this process, which was separated by centrifugation and dried in air at 80 °C for 48 h, resulting in a brown-red solid (Pt/γ-Fe2O3). Separation of the Superparamagnetic Pt/γ-Fe2O3 Nanoparticles. The Pt/γ-Fe2O3 nanocomposite was covered with oleic acid using the method described by Markovich,22 but with some modification. Typically, the Pt/γ-Fe2O3 nanocomposite (0.1 g) was redispersed in an aqueous solution of tetramethylammonium hydroxide (30 mL, 10%), excess oleic acid (2 mL) was added, and the suspension was vigorously stirred for 2 h. Then the suspension was acidified with an aqueous solution of HCl until the pH was about 5 and an oily precipitate appeared. This precipitate was redispersed in 10 mL of n-hexane and the resulted dispersion was centrifuged to form a homogeneous magnetic liquid of oleic acid-protected Pt/γ-Fe2O3 nonoparticles. This oleic acid-modified Pt/γ-Fe2O3 nonoparticles were further size-selected by adding a suitable amount of ethanol (1 mL) causing the larger particles to precipitate. After removal of the large particles via centrifugation, a stable colloidal solution of the superparamagnetic and narrowly dispersed Pt/γ-Fe2O3 composite nanoparticles was obtained in about 50% yield. Results and Discussion Preparation and Characterization of the Pt/γ-Fe2O3 Nanoparticles. The present procedure for preparing the Pt/γ-Fe2O3 nanoparticles consists of the following steps. First, the Pt nanoclusters stabilized with ethylene glycol and simple ions,21 with an average particle size of 2.5 nm, were electrostatically adsorbed on the surface of Fe(OH)3 colloidal particles to give
Figure 3. XPS spectrum of Fe2p energy level of the as-resulted Pt/γFe2O3 nanocomposite. The signals were calibrated by assigning a value of 284.8 eV to the C1s peak of the contaminant carbon.
a stable and homogeneous complex sol.19,20 Then, the Fe(OH)3 nanoparticles in this sol were reduced with glycol acid at 80 °C in a Teflon-lined autoclave to produce a Pt/Fe3O4 nanocomposite. During the process of drying in the open air for 48 h at 80 °C, the Fe3O4 nanoparticles in the Pt/Fe3O4 composite were transformed to γ-Fe2O3 nanocrystallites by catalytic oxidation over the Pt nanoclusters in the nanocomposite. To narrow the size distribution and get a ferrofluid of superparamagnetic Pt/γ-Fe2O3 nanoparticles, the prepared Pt/ γ-Fe2O3 nanocomposite was dispersed into an aqueous solution of tetramethylammonium hydroxide, modified with oleic acid, and transferred into n-hexane. After the large particles were removed via size-selected precipitation by adding ethanol to the resulted colloidal solution, a stable ferrofluid of Pt/γ-Fe2O3 nanoparticles with an average diameter of 14.8 nm and narrow size distribution was obtained. The γ-Fe2O3 phase in the prepared Pt/γ-Fe2O3 nanocomposite have been definitely corroborated by Raman spectrum study, which is often applied as a powerful tool to differentiate various ferrites.5,23,24 As can be seen from Figure 1a, the Raman spectrum of the present product is very close to that of standard maghemite,24 which shows three characteristic broad bands around 375, 503, and 700 cm-1, confirmedly indicating that it is a nanocomposite of γ-Fe2O3. On the other hand, the Raman spectrum of the Pt/Fe3O4 nanocomposite, shown in Figure 1b, with two characteristic bands at 292 and 665 cm-1, matches
10690 J. Phys. Chem. C, Vol. 112, No. 29, 2008
Zhang et al.
Figure 4. TEM images of the Pt/γ-Fe2O3 nanocomposite particles capped by oleic acid and the particle size distribution.
Figure 5. Raman spectrum of the Pt/Fe3O4 nanocomposite derived from reducing the Pt/γ-Fe2O3 nanocomposite with hydrogen at 80 °C.
well with that of magnetite reported in the literature,23 which definitely confirmed the Fe3O4 nature of the intermediate product. The XRD pattern (Figure 2) of the Pt/γ-Fe2O3 nanocomposite proved the highly crystalline nature of the γ-Fe2O3 nanoparticles in the composite. The observed diffraction peaks for the iron oxide matched well with standard γ-Fe2O3 reflections (99+%, powder, Alfa Aesar) and those of γ-Fe2O3 reported in the literature.8,10 According to the width of half-height peak of (311) plane, the average crystal grain size of the γ-Fe2O3 particles is estimated, by the Scherrer equation, to be about 15.0 nm in diameter. Diffraction signals of the Pt nanoclusters could not be clearly observed in this diffraction pattern due to the low metal loading and small particle size. In the X-ray photoelectron spectrum of the Pt/γ-Fe2O3 nanocomposite (Figure 3), the binding energies of the Fe 2p3/2 and Fe 2p1/2 levels in the Pt/γ-Fe2O3 nanocomposite are 710.9 and 724.4 eV, respectively, which are also in good agreement with the values reported for γ-Fe2O3 in the literature.8,11 TEM images of the Pt/γ-Fe2O3 nanocomposite capped by oleic acid are displayed in Figure 4. As it is illustrated, the γ-Fe2O3 nanoparticles in the composite have an average diameter of 14.8 nm, which is consistent with the above XRD results. In the high-resolution TEM image shown in Figure 4b, the Pt/γ-Fe2O3 nanoparticles are seen to be quite crystalline with obviously parallel lattice fringes. The distance between the adjacent fringes is measured to be 0.48 nm, corresponding to the (111) planes of γ-Fe2O3 phase. A combined area-selected EDX analysis revealed that the signals of Pt could be detected at the small darker spots, while that of Fe were widely present. This implied that Pt nanoparticles are randomly distributed on the surface of the larger γ-Fe2O3 particles without obvious aggregation. Pt nanoclusters play the most key role in the present lowtemperature synthesis of maghemite. Pt nanoclusters can
Figure 6. Magnetization curves of the oleic acid-capped nanocomposite particles with an average particle size of 14.8 nm. (a) The Pt/γ-Fe2O3 nanocomposite. (b) The Pt/Fe3O4 nanocomposite obtained by reduction of the Pt/γ-Fe2O3 nanocomposite with H2.
significantly lower the reaction temperature for the formation of the intermediate Fe3O4 nanoparticles. When the reaction mixture contained no Pt nanoclusters, higher temperature over 150 °C was required for reducing the Fe(OH)3 nanoparticles to Fe3O4 nanoparticles, otherwise, only R-Fe2O3 could be obtained. On the other hand, the oxidation of Fe3O4 is usually believed to occur through the outward diffusion of Fe(II) cations, which react with O2 adsorbed onto the surface of magnetite and form a thin layer of epitaxial γ-Fe2O3.25–27 Pt nanoparticles at the surface of Fe3O4 nanoparticles would facilitate the dissociation and diffusion of oxygen,28,29 therefore, could speed up the oxidation of Fe(II) cations diffused to the surface of Fe3O4 nanoparticles at relatively low temperature. Another key factor for the present low temperature procedure is the size of Fe(OH)3 colloidal particles. In our study, we can achieve the different size of magnetite nanoparticles through tuning the preparation conditions of Fe(OH)3 colloid. We also found that it was difficult for the Fe3O4 nanoparticles with the average diameter of more than 25 nm in the Pt/Fe3O4 nanocomposite to be completely oxidized to γ-Fe2O3 at 80 °C within a week. This may be because the diffusion process is slower than the catalytic oxidation during the oxidation of larger magnetite nanoparticles. According to the Fick’s law with boundary conditions (1) C )0, r ) a, t > 0, in our case, because Pt nanoparticles are involved in the oxidation, it is rational for us to assume that the surface concentration of Fe(II) cations on
Superparamagnetic Nanocomposite Particles
J. Phys. Chem. C, Vol. 112, No. 29, 2008 10691
the magnetite nanoparticles is zero on the surface of magnetite; (2) at the initial time, all Fe(II) distribution in magnetite particles is uniform, then CdC0; (3) due to the sphere symmetry of magnetite, ∂r ) 0, r ) 0, t > 0
(
∂C ∂2C 2 ∂C )D 2 + ∂t r ∂r ∂r
)
(1)
we can obtain t ) where t is the oxidation time, the particle size of nanomagnetite is denoted as a, and K is a function of fractional conversion, temperature and diffusion coefficient. This indicates that, the larger the size is, the longer the oxidation time is, due to the diffusion restriction of Fe(II).25–27 This explained why our larger magnetite nanoparticles needs longer time to be completely oxidized to maghemite at 80 °C, compared with Brus’s 8.7 nm magnetite nanoparticles which can completely be oxidized for 3 h at the same temperature.25 In addition, it is also pointed out in calculation that we did not consider the formation of the epitaxial layers during the oxidation of magnetite; however, this may become relatively serious in high oxidation fraction of large particles. Properties of the Pt/γ-Fe2O3 Nanocomposite Particles. The present Pt/γ-Fe2O3 nanoparticles having an average diameter of 14.8 nm could be easily reduced to the Pt/Fe3O4 nanocomposite in hydrogen atmosphere at 40-80 °C. Figure 5 shows the Raman spectrum of a sample obtained by treating the Pt/ γ-Fe2O3 nanoparticles with H2 at 80 °C. Compared to Figure 1, the characteristic broad bands of γ-Fe2O3 nanoparticles have disappeared, and correspondingly, a characteristic peak of the Fe3O4 nanoparticles appears, indicating the reduction of Pt/γFe2O3 under H2 to Pt/Fe3O4. The as-resulted Pt/Fe3O4 nanocomposite could be reoxidized to Pt/γ-Fe2O3 nanocomposite in the dry air at 80 °C as confirmed by Raman spectrum measurements. The easy interconversion between the Pt/γ-Fe2O3 and Pt/Fe3O4 nanocomposites may be important in magnetic separation because the saturation magnetization of Fe3O4 is larger than that of γ-Fe2O3, but it reduces gradually due to the slow oxidation of Fe3O4 in the air. Preliminary magnetic measurements on the oleic acidprotected Pt/γ-Fe2O3 and Pt/Fe3O4 nanoparticles were performed with a MicroMag 2900 instrument. Their magnetization curves (Figure 6) show that both the nanocomposites are supraparamagnetic at room temperature. The saturation magnetizations of Pt/γ-Fe2O3 and Pt/Fe3O4 are 52.7 and 80.1 emu/g, respectively, which are both a little lower than that of the corresponding bulk γ-Fe2O3 and Pt/Fe3O4 materials, due to surface effects24 and also the existence of nonmagnetic Pt. The difference in the saturation magnetization between the nanocomposites is in agreement with that between the corresponding bulk samples. Ka2,
Conclusion In summary, we have developed a novel protocol for the preparation of easily dispersed Pt/γ-Fe2O3 nanocomposite with superparamagnetic properties using Fe(OH)3 as the starting material under milder conditions. Reversible and efficient conversion between the as-resulted Pt/γ-Fe2O3 and Pt/Fe3O4
nanocomposites was realized at 80 °C. The size and shape control of the Pt/γ-Fe2O3 nanocomposites, and their applications in catalysis and bioseparation are underway in our laboratory. At the same time, we will also develop this synthetic method into other noble metal-iron oxides systems. Acknowledgment. This work was financially funded by the Major State Basic Research Development Program (Project No. G2000077503) from the CMST and grants from NSFC (Project Nos. 29925308, 90206011, 20373001, and 20433010). References and Notes (1) Varanda, L.; Goya, G.; Morales, M.; Marques, R.; Godoi, R.; Jafelicci, M., Jr.; Serna, C. IEEE Trans. Magn. 2002, 38, 1907. (2) Euliss, L.; Grancharov, S.; O’Brien, S.; Deming, T.; Stucky, G.; Murray, C.; Held, G. Nano. Lett. 2003, 3, 1489. (3) Massart, R.; Dubois, E.; Cabuil, V.; Hasmonay, E. J. Magn. Magn. Mater. 1995, 147, 1. (4) Dyal, A.; Loos, K.; Noto, M.; Chang, S.; Spagnoli, C.; Shafi, K.; Ulman, M.; Gross, R. J. Am. Chem. Soc. 2003, 125, 1684. (5) Shafi, K.; Ulman, A.; Dyal, A.; Yan, X.; Yang, N.; Estourne`s, C.; Fourne`s, L.; Wattiaux, A.; White, H.; Rafailovich, M. Chem. Mater. 2002, 14, 1778. (6) Pascal, C.; Pascal, J.; Favier, F.; Moubtassim, M.; Payen, C. Chem. Meter. 1999, 11, 141. (7) Randrianantoandro, N.; Mercier, A.; Hervieu, M.; Grene`che, J. Mater. Lett. 2001, 47, 150. (8) Hyeon, T.; Lee, S.; Park, J.; Chung, Y.; Na, H. J. Am. Chem. Soc. 2001, 123, 12798. (9) Sun, S.; Zeng, H.; Robinson, D.; Raoux, S.; Rice, P.; Wang, S.; Li, G. J. Am. Chem. Soc. 2004, 126, 273. (10) Hou, D.; Nie, X.; Shao, S.; Lu, P.; Luo, H. Phys. Stat. Sol. (A) 1997, 161, 459. (11) Teng, X.; Black, D.; Watkins, N.; Gao, Y.; Yang, H. Nano. Lett. 2003, 3, 261. (12) Teng, X.; Yang, H. J. Am. Chem. Soc. 2003, 125, 14559. (13) Bourlinos, A. B.; Simopoulos, A.; Petridis, D. Chem. Mater. 2002, 14, 899. (14) Huang, J.; Jiang, T.; Han, B.; Gao, H.; Chang, Y.; Zhao, G. Y.; Wu, W. Chem. Commun. 2003, 1654. (15) Lamy-Pitara, E.; Belegridi, I.; Barbier, J. Catal. Today 1995, 24, 151. (16) Carlsson, P.; Osterlund, L.; Thormahlen, P.; Palmqvist, A.; Fridell, E.; Jansson, J.; Skoglundh, M. J. Catal. 2004, 226, 422. (17) Mallat, T.; Baiker, A. Catal. Today 1995, 24, 143. (18) Rachmady, W.; Vannice, M. J. Catal. 2002, 209, 87. (19) Zuo, B.; Wang, Y.; Wang, Q.; Zhang, J.; Wu, N.; Peng, L.; Gui, L.; Wang, X.; Wang, R.; Yu, D. J. Catal. 2004, 222, 493. (20) Zhang, J.; Wang, Y.; Ji, H.; Wei, Y.; Wu, N.; Zuo, B.; Wang, Q. J. Catal. 2004, 229, 119. (21) Wang, Y.; Ren, J. W.; Deng, K.; Gui, L L.; Tang, Y. Q. Chem. Mater. 2000, 12, 1622. (22) Fred, T.; Shemer, G.; Markovich, G. AdV. Mater. 2001, 13, 1158. (23) de Faria, D.; Silva, S.; de Oliveira, M. J. Raman. Spectrosc. 1997, 28, 873. (24) Morales, M.; Veintemillas-Verdaguer, S.; Montero, M.; Serna, C.; Roig, A.; Casas, L.; Martinez, B.; Sandiumenge, F. Chem. Mater. 1999, 11, 3058. (25) Tang, J.; Myers, M.; Bosnick, K.; Brus, L. J. Phys. Chem. B 2003, 107, 7501. (26) Gallagher, K. J.; Feitknecht, W.; Mannweiler, U. Nature 1968, 217, 1118. (27) Sidhu, P. S.; Gilkes, R. J.; Posner, A. M. J. Inorg. Nucl. Chem. 1977, 39, 1953. (28) Bowker, M.; Bowker, L.; Bennett, R.; Stone, P.; Ramirez-Cuesta, A. J. Mol. Catal. A. 2000, 163, 221. (29) Holmgren, A.; Duprez, D.; Andersson, B. J. Catal. 1999, 182, 441.
JP8020694
