Bonding MnO 2 /Fe 3 O 4 Shell Core Nanostructures to Catalyze H 2 O 2 Degrading Organic Dyes

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1 Copyright 2010 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 10, 1 6, 2010 Bonding MnO 2 /Fe 3 O 4 Shell Core Nanostructures to Catalyze H 2 O 2 Degrading Organic Dyes Shu Zhang, Shulin Wang, and Shengjuan Li Institute of Chemical Engineering, University of Shanghai for Science and Technology, Shanghai , China Shell core nanostructures with both high catalytic activation and recyclability have been becoming hot property in nano-catalysis. By respectively using co-precipitation method, sol gel method, and homogeneous precipitation method we manufactured shell core nano-particles of Fe 3 O 4 core and MnO 2 shell. The Bonding mechanism of the composite is discussed in detail, and the efficiency and nature of the particles to degrade methyl orange by catalyzing H 2 O 2 is also demonstrated. We show that by using homogeneous precipitation method one can obtain morphologically uniform nano-particles of about 5 6 nm MnO 2 shell and nm Fe 3 O 4 core. The characteristic peak of Fe 3 O 4 in the Infrared spectra of the composite particles was blue shifted, and a novel peak appears at cm 1 referring to occurrence of new bond. X-ray Photoelectron Spectroscopy analysis showed that the bonding energy of Fe2p and O1s was increased due to the combination of the MnO 2 shell and the Fe 3 O 4 core, suggesting a new bond of Fe O Mn occurred in the composite. The MnO 2 shell has abundant hydroxyl radicals and exhibits high chemical activity in catalyzing H 2 O 2 and degrading methyl orange with a degree of greater than 95%. On the other hand, the shell core nanostructures are super-paramagnetic, and the saturated magnetization reaches 33.5 eum/g, which is sufficient for the catalyst to be recycled. Keywords: 1. INTRODUCTION The use of MnO 2 nanostructures in removing pollutant in water has been becoming hot property of nano-catalysis, because it possesses high absorbency activity and redox ability 1 in dealing with phenol, 2 5 organic dyes, 6 8 and heavy metals However, it may be impeded by the difficulties to recycle the catalyst. If the catalyst cannot be recycled, the negative would be dual: it wastes materials on the one hand, and on the other hand it causes secondary pollution of the water. That is why the nano-particles with catalytic shell and magnetic core are attracting more and more attention of environmental scientific research. Super-paramagnet shell core nanostructures are also of significance to target drug delivery and solve seal and lubrication problems in industry. In the literature 14 -Fe 2 O 3 particles were coated respectively with amorphous manganese (II) 2,4 pentanedionate (MP), crystalline MnO/MnO 2, and MnCO 3. And literature 15 prepared spinel type Fe 3 O 4 nano-particles of nm and coated with MnO 2 as the shell, but it lacks systematic research and applications, and the bonding Author to whom correspondence should be addressed. mechanism for the combination remained open. This paper compares the results and efficiency of different ways to manufacture MnO 2 /Fe 3 O 4 shell core nano-particles including co-precipitation method (CPM), sol gel method (SGM), and homogeneous precipitation method (HPM), discusses the bonding mechanism of the composite in detail, and demonstrates the application of the particles in catalyzing H 2 O 2 and degrading methyl orange. 2. MATERIALS AND METHODS 2.1. CPM Dissolve g KMnO 4 in 75 ml water and 4.6 g manganese (II) acetate tetrahydrate in 125 ml water respectively. Prepare 5% polyethylene glycol (PEG) solution 100 ml and put the prepared Fe 3 O 4 nano-particles 15 of1g into the solution, and use 30 min ultrasonic stirring. Then mix the KMnO 4 solution and the manganese (II) acetate tetrahydrate solution directly with the magnetic fluid, and the mixture was stirred violently for 6 hours at 60 C. The sediment was purified by magnetic field separation, washed several times with ethanol, dried for 12 hours at 110 C, and finally, calcined for 3 hours at 280 C. J. Nanosci. Nanotechnol. 2010, Vol. 10, No. xx /2010/10/001/006 doi: /jnn

2 Bonding MnO 2 /Fe 3 O 4 Shell Core Nanostructures to Catalyze H 2 O 2 Degrading Organic Dyes Zhang et al SGM Prepare 5% polyethylene glycol (PEG) solution 100 ml and put the Fe 3 O 4 nano-particles of 1 g into the solution, and scatter it 30 min by ultrasonic stirring to get suspension named A. Dissolve 2.45 g manganese (II) acetate tetrahydrate and citric acid 1.05 g respectively in 100 ml water, and then mix the two solutions and scatter it by ultrasonic stirring, add ammonia water to the mixed solution until the ph = 6, and we can obtain a light yellow sol named B. Then drip the suspension A into the sol B and stir for 2 hours at 60 C. Dry the resultant at 80 C until the it is dry, and finally, calcine it for 10 hours at 280 C HPM It has been described elsewhere. 15 In brief, disperse Fe 3 O 4 nano-particles of 0.5 g into 5% PEG solution of 100 ml by 30 min ultrasonic stirring, and then mix the solution with MnSO 4 solution of 100 ml and some urea solution. Keep the mixture reacted for 12 hours at 60 C, and then separate it magnetically, wash it with distilled water for 3 times and dry at 80 C, and finally, calcine for 3 hours at 280 C Utilizing the Nano-Composite to Catalyze H 2 O 2 Degrading Methyl Orange (MO) Put5mLH 2 O 2 solution of 30% concentration and certain nano-composite particles into 50 ml MO solution of 20 mg/l concentration, stir it with a glass rod for a while, take out 5 ml every 10 min and use centrifugation with 3000 r/min for 2 min, then take out the upper layer of the solution and use SHIMADZU UV-1700 spectrophotometer to record the absorption spectra at 465 nm. And finally calculate the degradation according to the formula: A 0 A 1 100% A 0 Where A 0 and A 1 is the absorption at 465 nm before and after degradation. JEM-2100F Field Emission Transmission Electron Microscopy (TEM) and D/max- B X-ray diffractometer (XRD) with CuK radiation were used to characterize the size, morphology, and structures of the particles. And we used Nicolet 200SXV Fourier Transform Infrared spectroscopy (IR) and Thermo ESCALAB 250 X-ray Photoelectron Spectroscopy (XPS) with AlK radiation to analyze the molecular electronic structure and the bonding state. 3. RESULTS AND DISCUSSION 3.1. Morphology and Structure of the Composite Particles Figure 1 illustrates the XRD pattern of the composite particles synthesized by the 3 methods mentioned above, each Fig. 1. XRD pattern of the particles synthesized by: SGM; CPM; HPM. involving peaks of Fe 3 O 4 and MnO 2. But the diffraction intensity of MnO 2 by CPM and SGM is obviously stronger than that by HPM, implying that the enrolled MnO 2 mass for the former two cases is larger than that for the latter. And MnO 2 is of mixed crystal for all the experiments according to the handbook. 16 From the TEM images (Fig. 2) we can see that the light area in the particles is MnO 2, and the deep domain is Fe 3 O 4. Because surface defects such as steps, ledges, and kinks are often the sites where the deposition prefers to occur, the precipitation process tends to lower the surface energy of the structures, as a result, the nano-particles terminated at round shape (Figs. 2(a and c)). But the particles by SGM look rather rough (Fig. 2); it may be that the evolution speed of the sol gel process affects the particle morphology. Furthermore, there was an excess of Fe 3 O 4 particles not coated in the CPM process (Fig. 2), it may be ascribed to that the cation ratio in the precursors was not reasonable, so that a fractional precipitation occurred rather than co-precipitation. It follows that the amount of Fe 3 O 4 and MnO 2 participated in the reaction needs to be adjusted. The particle size produced by CPM, SGM, and HPM is successively 200 nm, nm, and nm. The thickened MnO 2 mass is the main reason for the enlargement of the particle size, as by CPM and SGM (Figs. 2(a and b)). This statement is consistent with the XRD pattern in Figure 1. Besides, agglomeration may be another factor for the enlargement of the particles. Agglomeration is related to the homogeneity of the shell. As shown in Figure 2, the synthesized particles by HPM have a uniform MnO 2 shell of about 5 6 nm thick andafe 3 O 4 core of about nm wide (Fig. 2(d)). Because of the uniformity of the shell thick, the magnetic force can be balanced, making the particles better dispersed in the field. Inversely, the shell thickness of the product by CPM and SGM is very divergent, and thus the 2 J. Nanosci. Nanotechnol. 10, 1 6, 2010

3 Zhang et al. Bonding MnO 2 /Fe 3 O 4 Shell Core Nanostructures to Catalyze H 2 O 2 Degrading Organic Dyes (d) Fig. 2. TEM images of the particles synthesized by: CPM; SGM; (c d) HPM. particle agglomeration is also very evident (Figs. 2(a and b)). Another factor for agglomeration by SGM may be the calcine time in the synthesis, which was much longer than that by HPM and CPM Combination Mechanism and Bonding Nature of the Structure The IR spectra in Figure 3 shows that the featured absorption peak of Fe 3 O 4 in the composites produced by the three methods lies respectively in cm 1, cm 1, and cm 1, but that of the starting Fe 3 O 4 lied in cm 1, 15 therefore, all the peaks are blue-shifted. It is especially significant that in the response of HPM a new peak appeared at cm 1, indicating that new bond may be generated between the shell and the core of the structure. On the other hand, all the responses have hydroxyl absorption peaks at near 1400 cm 1, 1600 cm 1, and 3400 cm 1. The absorption at near 1400 cm 1 is due to bending vibration. And that at near 1600 cm 1 and Fig. 3. IR spectra of the particles synthesized by: CPM; SGM; HPM. J. Nanosci. Nanotechnol. 10, 1 6,

4 Bonding MnO 2 /Fe 3 O 4 Shell Core Nanostructures to Catalyze H 2 O 2 Degrading Organic Dyes Zhang et al cm 1 reflect the bending and stretching vibration of the hydroxyls, which is the featuring peaks for the association of O H and H 2 O, the stronger the peaks, the more active the particles. And the response for HPM is the strongest compared to the others, demonstrating that the MnO 2 shell of the composite by HPM possesses the highest chemical absorption activity. Figure 4 shows the XPS of the materials, where 4 is for the starting Fe 3 O 4, 4 is for the composite by HPM, and 4c and 4d represents the binding energy of Fe2p and O1s in the starting Fe 3 O 4 and in the particles composited by HPM respectively. From Figure 4 one can see the Mn2p peak at ev. According to the handbook, 16 the Mn in the composite exists as Mn 4+. Comparing the binding energy of Fe2p and O1s in the materials before and after the coating, we can find that it is increased from ev and ev to ev and ev respectively, that is, a chemical shift occurred in the process. Chemical shift comes from the change of potential energy due to the transfer of the valence electrons, and the transfer direction of the electrons depends on the electro-negativity of the elements. In general, there are strong Coulomb interactions between inner electrons and the nucleus of the atoms, which determine the binding energy. Meanwhile, it may be affected or shielded by outer electrons. When valence electrons move toward the atoms of lower electron-negativity, the shielding effect may decrease because the electron density in the outer shell decreases. It follows that the binding energy increases, and vice versa. Because the electron-negativity of Mn (1.55) is lower than that of Fe (1.8) and O (3.44), so that the valence electrons move to Mn, and the electron density of Fe and O decreases, decreasing the electron shield and increasing the binding energy of Fe and O, finally, a new bond of Fe O Mn between the shell MnO 2 and the core Fe 3 O 4 can be formed, this claim is in agreement with that of the IR spectra analysis (Fig. 3) Magnetic Behavior of the Composite The hysteresis curves of the composites obtained by the three methods are shown in Figure 5, where 5 for CPM, 5 for SGM, and 5 for HPM. They are all superparamegnitic, but the saturated magnetization is different as 24.7 eum/g, 16.5 eum/g, and 33.5 eum/g successively. And with HPM we obtained the highest value because the (d) Fig. 4. XPS of the starting Fe 3 O 4 and the composite particles by HPM. XPS of the starting Fe 3 O 4 ; XPS of the composited particles; binding energy of Fe2p in the starting Fe 3 O 4 and the composited particles; (d) binding energy of O1s in the starting Fe 3 O 4 and the composited particles. 4 J. Nanosci. Nanotechnol. 10, 1 6, 2010

5 Zhang et al. Bonding MnO 2 /Fe 3 O 4 Shell Core Nanostructures to Catalyze H 2 O 2 Degrading Organic Dyes M 2 /emng H/Oe a b c 1000 Fig. 5. Hysteresis curves of the composites obtained by the three methods. CPM; SGM; HPM. precipitation process has strong selectivity; so that impurities may be blocked to enter the system. And the saturated magnetization is sufficient for the catalyst to be recycled. 4. USING THE SHELL CORE NANOSTRUCTURES TO CATALYZE H 2 O 2 DEGRADING ORGANIC DYES Figure 6 represents the experimental study of catalyzing H 2 O 2 degrading MO by using the synthesized MnO 2 /Fe 3 O 4 nano-composite with HPM. We can see that when only 25 mg composite nano-particles are put into the solution of MO (curve a), the degradation is only a little and almost not changed with time. And it is even smaller and runs almost parallel to the horizon when only 30% H 2 O 2 solution of 5 ml is used to the system (curve b). But when the 25 mg composite nano-particles and the 5 ml 30% H 2 O 2 solution present simultaneously in the MO solution, the degradation line becomes very steep Degradation/% a b c t/min Fig. 6. Degradation of MO with H 2 O 2 catalyzed by MnO 2 /Fe 3 O 4 nanostructures. MO + MnO 2 /Fe 3 O 4 (25 mg); MO + H 2 O 2 ; MO + H 2 O 2 +MnO 2 /Fe 3 O 4 (25 mg). Degradation/% mg 50 mg 75 mg t/min Fig. 7. Influence of the dosages of the catalyst on the degradation of MO. (curve c), and it reaches 95% in about 90 min from the beginning of the reaction, demonstrating that the MnO 2 shell of the particles has very strong catalytic activity. To study further the catalytic performance of the shell core nanostructures, we put different dosages of it into the mixture of MO and H 2 O 2 solution, the curves shown in Figure 7 represent the degraded degree of MO by the catalyst of 25 mg, 50 mg, and 75 mg respectively, which approaches 93.88%, 95.66%, and 95.56% correspondingly. It is interesting that the degradation created with 75 mg catalyst is evidently lower than that with 25 mg and 50 mg at the starting period, but in about one hour it increases and stabilized to a higher level relative to the others. OH, HOO, and O 2 are considered to be free hydroxyls decomposed by H 2 O 2, they can oxidize the molecules and destroy the structures of organic dyes. 17 However, organic dyes can be degraded only if they adsorb on the surface of the catalyst. 18 Based on these assumptions, the reaction mechanism for the shell core nanostructures to catalyze H 2 O 2 and degrade the dyes may be a process of adsorption degradation desorption. First of all, H 2 O 2 and MO are adsorbed on the shell surface of the composite nano-structures; secondly the H 2 O 2 is decomposed into OH, HOO, and O 2 hydroxyls by the MnO 2 catalyst. Part of these newly generated hydroxyls diffuses on the surface and reacts directly with the adsorbed MO molecules and degrades them. And the other hydroxyls are desorbed from the surface, dispersed into the solution, and decompose the MO in the solution. And finally, the degraded small molecules of MO are desorbed from the catalyst surface and enter into the solution, recovering the active potential of the catalyst surface. Large amount of O 2 escaping from the system can be observed in the early time of the reaction if 75 mg catalyst were used. Because excessive catalyst may change H 2 O 2 of great quantity into OH, HOO, and O 2 hydroxyls in a short period, which in turn may increase the hydroxyl density greatly in the solution. Because of the high activity of the hydroxyls, they may react on each other and create J. Nanosci. Nanotechnol. 10, 1 6,

6 Bonding MnO 2 /Fe 3 O 4 Shell Core Nanostructures to Catalyze H 2 O 2 Degrading Organic Dyes Zhang et al. O 2, making the degradation degree decreased because O 2 cannot degrade MO. This is why the degradation with 75 mg nano-particles is even lower than that with 25 mg and 50 mg at the beginning. Fortunately, the density of the H 2 O 2 and the O 2 produced by the hydroxyls are reduced gradually with time, and the degradation of MO increases again (Fig. 7). Therefore, an optimal amount of the catalyst may exist for the process. 5. CONCLUSIONS By using HPM we can synthesize morphologically uniform shell core nano-particles of nm with MnO 2 as the shell and Fe 3 O 4 as the core, where the shell is about 5 6 nm thick, and the core is about nm wide. The characteristic peak of Fe 3 O 4 in the infrared spectra of the composite particles was blue shifted, and a novel peak appears at cm 1 referring to occurrence of new bond. X-ray Photoelectron Spectroscopy analysis showed that the bonding energy of Fe2p and O1s was increased due to the combination of the MnO 2 shell and the Fe 3 O 4 core, in other words, a chemical shift is created, suggesting a new bond of Fe-O-Mn occurred in the composite. The MnO 2 shell has abundant hydroxyl radicals characterized by the strong absorption at 1600 cm 1 and 3400 cm 1 in the IR, and exhibits high chemical activity in catalyzing H 2 O 2 and degrading MO with decomposition degree greater than 95%. An optimal amount of the catalyst may exist for the process. On the other hand, the shell core nanostructures are super-paramagnetic, and the saturated magnetization reaches 33.5 eum/g, which is sufficient for the catalyst to be recycled. References and Notes 1. E. R. Stobble, A. B. Boer, and G. W. Geus, Catalysis Today 47, 161 (1999). 2. L. Ukrainczyk and M. B. Mcbride, Clays and Clay Minerals 40, 157 (1992). 3. A. T. Stone and J. J. Morgan, Environ. Sci. Technol. 18, 617 (1984). 4. A. T. Stone, Environ. Sci. Technol. 21, 979 (1987). 5. H. J. Ulrich and A. T. Stone, Preprints of Papers Presented at the 194th ACS National Meeting, Division of Environmental Chemistry, ACS, Washington, DC (1987), Vol. 27, p Z. Ma, L. Dong, and Y. Kang, Technique and Equipment for Environmental Pollution Control 3, 19 (2002). 7. E. R. Stobble, A. B. Boer, and G. W. Geus, Catalysis Today 47, 161 (1999). 8. R. X. Liu and H. X. Tang, Water Research 34, 4029 (2000). 9. S. R. Randall, D. M. Sherman, and K. V. Ragnarsdottir, Chemical Geology 151, 95 (1998). 10. P. Loganathan and R. G. Burau, Geochimica et Cosmo-chimica Acta 37, 1277 (1973). 11. J. W. Murray, Geochimica et Cosmo-chimica Acta 39, 635 (1975). 12. J. W Murray and J. G. Dillard, Geochimica et Cosmo-chimica Acta 43, 781 (1979). 13. J. W. Murray, Geochimica et Cosmo-chimica Acta 39, 505 (1975). 14. I. Haq and E. Matijević, J. Colloid Interface Sci. 192, 104 (1997). 15. S. Zhang and S. Wang, Journal of Nanomaterials 2009, 5 (2009). 16. C. D. Wagner, W. M. Riggs, L. E. Davis, and J. Moulder, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, New York (1996). 17. W. X. Zhang, Z. H. Yang, X. Wang, Y. C. Zhang, X. G. Wen, and S. H. Yang, Catal. Comm. 7, 408 (2006). 18. C. Ye, Y. Bando, G. Shen, and D. Golberg, J. Phys. Chem. B 110, (2006). Received: 4 August Accepted: 11 September J. Nanosci. Nanotechnol. 10, 1 6, 2010