ARTICLE pubs.acs.org/JPCC
Synthesis of Porous Fe3O4 Nanospheres and Its Application for the Catalytic Degradation of Xylenol Orange Maiyong Zhu and Guowang Diao* College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, P. R. China
bS Supporting Information ABSTRACT: Porous magnetite (Fe3O4) nanospheres composed of primary nanocrystals have been successfully synthesized by solvothermal method with FeCl3 3 6H2O serving as the single iron resource, polyvinylpyrrolidone (PVP) as the capping agent, and sodium acetate as the precipitation agent. To understand the formation mechanism of the porous Fe3O4 nanospheres, the reaction conditions such as the concentration of the precursor, capping agent, precipitation agent, the reaction temperature, and reaction time were investigated. The characterization of the asprepared product was identified with transmission electronic microscopy (TEM), field emission scanning electronic microscopy (FE-SEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM), N2 adsorptiondesorption technique, and Fourier transform infrared spectroscopy (FTIR). The results indicate that the porous Fe3O4 nanospheres display excellent magnetic properties at room temperature, which allows them to be easily separated from the reaction system with the help of external magnet when they serve as catalysts. Catalytic activity studies show that the as-prepared porous Fe3O4 nanospheres are highly effective catalysts for the degradation of xylenol orange (XO) in aqueous solution with H2O2 as oxidant. The degradation reaction is first-order, its rate constant at room temperature being 0.056 min1. Furthermore, the catalytic activity of Fe3O4 nanospheres decreases very slightly after seven cycles of the catalysis experiment. Therefore, porous Fe3O4 nanospheres can serve as effective recyclable catalysts for the degradation of XO.
1. INTRODUCTION During the past few decades, magnetic nanoparticles with peculiar size and structure have attracted wide attention because of their unique physical and chemical properties.15 Many strategies have been developed to get a variety of magnetic nanomaterials, such as iron, cobalt, nickel, alloys, iron oxide, metal ferrite, and so on.68 Of all magnetic materials, magnetite (Fe3O4) is the most important and most widely used one in many fields. When prepared into well-defined nanoscale structures, Fe3O4 may be practically or potentially applied in a wide range of fields, such as biomolecular separation,9 chemical sensor,10 energy storage,11 catalysis,1214 microwave absorption,15 biomedicine/biotechnology,1623 and environmental remediation.2429 Generally speaking, Fe3O4 nanoparticles could be prepared with different methods including coprecipitation of Fe2+ and Fe3+, thermal decomposition, microemulsion, and hydrothermal/ solvothermal.3035 Of all these methods, coprecipitation is a preferred choice for most researchers because the reaction could be performed under a mild condition by using water as solvent. However, the shape of the outcome product is hard to control.7,36 Thermal decomposition is considered to be the best route in obtaining nanomaterials with controllable size and morphology. However, the precursors of the product are organometallic compounds, such as Fe(CO)5, metal cupferronates,37 and metal acetylacetonates,38 which are poisonous. In addition, thermal decomposition of organometallic compound often requires high r 2011 American Chemical Society
temperature, which can be a great challenge for many groups. Microemulsions can also be used to synthesize nanoparticles with specific morphology and uniform size. Unfortunately, this method consumes a large amount of organic solvent; besides, organic surfactants are needed to control the size and shape of the product.39 Compared with other options, solvothermal is often characterized by low yield, yet with this method almost all materials can be solubilized by heating and pressurizing the solvent system to its critical point, allowing the synthesis of highquality nanostructured materials.40 Hydrothermal/solvothermal has become one of the most popular methods to get inorganic nanostructured materials, especially metals and their oxides, despite low yield.41 Higher pressure and higher temperature (sometimes even above critical point of the solvent) occurred in this method and can increase the solubility of solids and accelerate reactions between solid species. Regarding magnetic nanomaterials, many research groups have employed solvothermal method. Gao et al. prepared water-soluble magnetite nanocrystals with different sizes, that is, 4, 12, and 60 nm, by refluxing 2-pyrrolidone solution of FeCl3.42 Li and his coworkers reported a generalized hydrothermal method for synthesizing various nanocrystals by liquidsolidsolution Received: January 14, 2011 Revised: August 24, 2011 Published: August 26, 2011 18923
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The Journal of Physical Chemistry C reaction.43,44 Pinna et al. reported nonaqueous solvothermal synthesis of monocrystalline magnetite particles with sizes ranging from 12 to 25 nm.45 Huang et al. synthesized different kinds of functionalized magnetic microspheres for different bioapplications by hydrothermal reduction. The typical procedure is as follows: mixture consisting of FeCl3, ethylene glycol, sodium acetate, and a polymer, such as dextran, chitosan, or poly(acrylic acid) (PAA), was stirred vigorously to form a clear solution, sealed in a Teflonlined stainless-steel autoclave, and heated to and maintained at 200 °C for 830 h.46 In Yin’s research group, Fe3O4 colloidal nanocrystal clusters (CNCs), with uniform sizes ranging between ∼30 and ∼180 nm, have been successfully synthesized with diethylene glycol solvent. These CNC structures were designed with two-stage growth process in which primary nanocrystals nucleate first and then aggregate uniformly into larger secondary structures.47,48 Although a variety of iron oxide nanocrystals with different morphologies, such as nanorods,4953 fractal,54 pea-pod-like,55 dumbbell-like,56 cubes,57 wires,58 tubes,59 spindles,60 flakes,61 flower-shaped,27 and hollow spheres,62,63 have been successfully synthesized, to the best of the authors’ knowledge, there are few reports on the synthesis of porous Fe3O4 nanoparticles.52,53,64 Compared with the solid counterparts of the same size, porous nanoparticles have their particular merits, such as high surface area, less resistance of mass transfer in catalytic system, controllable pore size, and adjustable framework. In this Article, an effective route for the synthesizing of monodispersed porous Fe3O4 nanospheres could be adjusted by controlling the reaction conditions, although solvothermal method was reported. The average diameter of the porous Fe3O4 nanospheres was ∼250 nm. In this route, ethylene glycol solution of FeCl3 3 6H2O containing PVP and NaAc was heated to produce porous Fe3O4 nanospheres, where ethylene glycol acted as both solvent and reducing agent, NaAc and PVP as the precipitation and capping agents, respectively. In general, many reports indicate that template methods can be used to get hollow and porous nanostructures. However, in doing so, the presynthesized templates and the removal of them without destruction of the product are required, which make the procedures complex.65,66 Compared with that method, the one reported in this study is more convenient. The effects of temperature, time, as well as the concentration of FeCl3 3 6H2O, NaAc, and PVP on the crystal phase of the product and its final structure were investigated. On the basis of the investigation, a possible mechanism for the formation of porous Fe3O4 nanospheres was suggested. The as-prepared porous Fe3O4 nanospheres show excellent magnetic behavior at room temperature, which makes it suitable to work as a recyclable heterogeneous catalyst because they can be separated easily from the reaction system by an external magnet. The degradation of xylenol orange, using H2O2 as the oxidation reagent, was chosen as a model reaction to investigate its catalytic activity. The results indicate that the porous Fe3O4 nanospheres synthesized in this Article exhibit high catalytic activity for the degradation of xylenol orange by H2O2. The porous Fe3O4 nanospheres could be separated conveniently from the reaction system with an external magnet field and can be used in recycling.
2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were analytical grade and purchased from Sinopharm Chemical Reagents Company. The
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water used in this study was deionized by milli-Q Plus system (Millipore, France), whose electrical resistance is 18.2 MΩ. 2.2. Syntheses of Porous Fe3O4 Nanospheres. Porous Fe3O4 nanospheres were prepared with the solvothermal. We added 1.5 g FeCl3 3 6H2O, 1 g PVP, and 2 g NaAc to 30 mL of ethylene glycol. The mixture was stirred vigorously for 2 h to ensure all materials dissolve completely. Then, the mixture was transferred to a Teflon-lined stainless-steel autoclave and sealed for heating at 200 °C for 8 h. The precipitated black products were collected from the solution with an external magnet and washed with ethanol for a few times. Finally, the black products were dried in a vacuum at 60 °C for 24 h. The sizes of the asprepared porous Fe3O4 nanospheres could be controlled by changing the concentration of FeCl3 3 6H2O, PVP, and NaAc, or the reaction temperature. 2.3. Characterization. Transmission electron microscopy (TEM: Tecnai-12, HRTEM: Tecnai-G2 F30 S-TWIN, Philips) and field emission scanning electron microscopy (FE-SEM; Hitachi S-4800) were used to observe the morphologies of the product. X-ray powder diffraction (XRD) was performed on a D8 Advance (Super speed) X-ray diffractometer (Bruker). Raman spectra were taken on a Renishaw InVia spectrometer at room temperature with an argon-ion laser at an excitation wavelength of 532 nm. The X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo Escalab 250 system using Al Kα radiation (hν = 1486.6 eV). The test chamber pressure was maintained below 2 109 Torr during spectral acquisition. The Fourier transform infrared (FT-IR) spectra were recorded with a Tensor 27 spectrometer (Bruker) using a KBr wafer with the wavenumber ranging 4000400 cm1. Magnetic properties of the products were investigated using a vibrating sample magnetometer (VSM, EV7, ADE) with an applied field between 8000 and 8000 Oe at room temperature. The surface area and pore size distribution were measured by N2 adsorptiondesorption technique in an automated surface area and porosity analyzer (ASAP 2020, Micromeritics) at 77 K after the sample was dried at 200 °C for 4 h. The BrunauerEmmett Teller (BET) surface area and the pore size distribution plots were calculated by applying the linear part of the BET plot and the BarrettJoynerHalenda (BJH) model, respectively. 2.4. Catalytic Activity of Porous Fe3O4 Nanospheres. To investigate the catalytic activity of the as-prepared porous Fe3O4 nanospheres, the degradation of XO with H2O2 oxidant under ultrasound were chosen as a test reaction model. We mixed 10 mg porous Fe3O4 nanospheres, 20 mL of 1 104 M XO mother solution, and 10 mL of 30% H2O2 aqueous solution were mixed together. The concentration of XO at different reaction time was determined by UV spectroscopy (Shimadazu UV-2500 spectrophotometer). The content of total organic carbon (TOC) was analyzed by using a multi N/C 2100 analyzer (Analytik Jena AG).
3. RESULTS AND DISCUSSION 3.1. Morphology, Structure, and Properties of Porous Fe3O4 Nanospheres. The size and morphology of Fe3O4 nano-
spheres were characterized via TEM and FE-SEM. The TEM images of magnetic Fe3O4 nanospheres in different magnifications are shown in Figure 1A,B. The diameter of those Fe3O4 nanospheres is ∼250 nm. A typical HRTEM image of a Fe3O4 nanosphere is depicted in Figure 1C, where the polycrystalline nature of the product can be confirmed by electron-diffraction pattern (see the inset). From Figure 1D, the lattice spacing 18924
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Figure 1. (A,B) TEM images of Fe3O4 nanospheres with different magnifications, (C) representative HRTEM image and the electron-diffraction pattern (inset) of a Fe3O4 nanosphere, (D) HRTEM image of the boxed region of part C, and (E,F) FE-SEM images with different magnifications of Fe3O4 nanospheres. The synthesis conditions: 1.5 g FeCl3 3 6H2O, 1 g PVP, 2 g NaAc, 30 mL ethylene glycol, 200 °C, and 8 h.
distance is calculated to be 0.29 nm, in good agreement with the (220) spacing of the Fe3O4 structure. Figure 1E,F shows the FESEM images of the nanospheres, which clearly reveal that each magnetic microsphere consists of many magnetic grains. Unfortunately, the size distribution of them is very wide. Figure 2A shows the XRD patterns of the porous Fe3O4 nanospheres. The diffraction peaks locating at 2θ = 30.1, 35.8, 43.1, 53.8, 57.3, and 63.0° can be indexed to (220), (311), (400), (422), (511), and (440) planes of Fe3O4 in a face-centered cubic (fcc) Fe3O4 (JCPDS card no. 19-629), respectively. The broadened peak centered at a small angle (2θ < 30°) indicates the presence of amorphous materials. The crystal size determined by Debye-Scherre equation from XRD is 11.8 nm, which further confirms that the porous Fe3O4 nanospheres were composed of the primary grains with a diameter of ca. 12 nm.
It is widely accepted that the XRD pattern of Fe3O4 nanostructures is very similar to that of γ-Fe2O3.67 To confirm further the crystal phase of our product, laser micro-raman spectra were recorded. Figure 2B shows the Raman spectra of the as-prepared porous Fe3O4 nanospheres and the commercial Fe3O4 powder. In both curves, the positions of main peaks are almost identical. No peaks at either 1430 or 1580 cm1, typical for γ-Fe2O3,46 were observed, further confirming that there is no γ-Fe2O3 in the as-prepared porous Fe3O4 nanospheres. XPS as in Figure 2CE also comes to the same conclusion. Figure 2C shows XPS of the porous Fe3O4 nanospheres and the inset the expanded spectra of N1s. Figure 2 D shows the high-resolution XPS spectra of Fe2p, and Figure 2E shows the high-resolution XPS spectra of O1s. It is clear that in the as-prepared porous Fe3O4 nanospheres, some PVP exists as N 1s and C 1s are observed from Figure 2C. Two distinct peaks at 712.3 and 726.2 eV appear in Figure 2D. The 18925
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Figure 2. (A) XRD patterns of Fe3O4, (B) Raman spectra of Fe3O4 obtained by this work (a) and commercial (b), (C) XPS of Fe3O4 and the inset is expanded spectra of N1s, (D) high-resolution XPS spectra of Fe2p, (E) high-resolution XPS spectra of O1s, (F) magnetic hysteresis and enlarged partial (inset) curves of Fe3O4, (G) typical BET isotherm of Fe3O4, and (H) BJH isotherm and enlarged partial (inset) curves of the Fe3O4. All Fe3O4 are prepared under same conditions as those described in Figure 1.
former is attributed to Fe2p3/2, and the latter to Fe2p1/2. In Figure 2E, The O1s centered at 529.9 eV belongs to O2. The above results confirm the formation of Fe3O4 in the system. It is well known that the magnetic properties of the material are related to its size, structure, and morphology. Figure 2F is the hysteresis loop of the as-prepared porous Fe3O4 nanospheres measured at room temperature. It can be found that the magnetization saturation value (Ms) of the Fe3O4 nanospheres is 79.8 emu/g, which is close to that of the bulk sample of Fe3O4, 92 emu/g,68 and the coercivity value (Hc) is 35.5 Oe, indicating
that the porous Fe3O4 nanospheres display weak ferromagnetism. The experimental phenomena can be explained by the fact that many primary Fe3O4 particles assemble together to form larger Fe3O4 nanoparticles as in Figure 1, making the porous Fe3O4 nanospheres look like a block magnet and exhibit higher saturation magnetization than individual primary particles. Normally, when the diameter is smaller than 20 nm, the superparamagnetic property of Fe3O4 can be observed.69 However, the magnetic property of the porous Fe3O4 nanospheres consisting of small Fe3O4 grains is different from that of the 18926
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Figure 3. TEM images of Fe3O4 prepared with different initial concentrations of FeCl3 3 6H2O: (a) 0.25, (b) 0.5, (c) 0.75, (d) 1.0, (e) 1.25, and (f) 1.5 g. Other conditions: 0.5 g PVP, 1.0 g NaAc, 30 mL of ethylene glycol, 200 °C, and reaction time: 8 h.
Table 1. Magnetic Parameters of Fe3O4 Nanoparticles Prepared at Different Amount of FeCl3 3 6H2Oa sample
a
FeCl3 3 6H2O (g) Ms (emu/g)
77.8
80.4
82.1
82.2
83.2
87.9
Hc (Oe)
68.3
85.6
72.3
72.3
85.6
73.0
0.25
b 0.5
c 0.75
d 1.0
e 1.25
f 1.5
a
Other conditions: 0.5g PVP, 1.0g NaAc, 30 mL of ethylene glycol, 200 °C, reaction time: 8 h.
Figure 4. Magnetic hysteresis curves and enlarged partial curves (inset) of Fe3O4 prepared under the same conditions as those described in Figure 3.
individual particles. This interesting phenomenon may be related to the interaction among the small particles, which is subject to further study. The surface area and porosity of Fe3O4 nanospheres were determined by measuring the adsorption and desorption isotherms of N2. As is shown Figure 2G, the BET surface area of Fe3O4 nanospheres was calculated to be 47.7 m2/g. Figure 2H displays the distribution of the pores in Fe3O4 nanospheres. A main peak centered at ∼4 nm. Most of the pores range from 3 to 10 nm. The result shows that Fe3O4 nanospheres are a type of mesoporous materials. Such porous structure provides efficient transport pathways to the interior cavities, which would enhance the catalytic activity of catalyst.70 3.2. Mechanism for the Formation of Porous Fe3O4 Nanospheres. In our reaction system, the formation of Fe3O4 nanospheres includes nucleation, growth of nucleus to form primary
crystal, and self-assembly of the primary crystals to form larger nanoshperes. The initial concentration of precursor is a most important factor that affects the formation of nanocrystals. Figure 3 shows the TEM of Fe3O4 prepared under different initial concentrations of FeCl3 under the given conditions. As is displayed in Figure 3 A, no regular porous Fe3O4 nanosphere was observed at low concentration of FeCl3. The concentration of FeCl3 determines the regularity and the morphology of the product. The higher the concentration of FeCl3, the more regular the morphology of porous Fe3O4 nanospheres. The diameter of Fe3O4 nanospheres increases with the concentration of FeCl3. However, the particles are polydispersed except for those prepared, whereas the initial concentration of FeCl3 is >0.0417 g 3 mL1, where uniform, regular, and monodispered Fe3O4 nanospheres will be obtained, whose diameter of the nanoparticles is ∼250 nm. The above phenomenon can be explained by the theory of Von Weimarn rules.71 At the low concentration of FeCl3, although the rates of both nucleation and growth are slow, plenty of precursors can be nucleated with the help of precipitation agent. Therefore, there is not enough FeCl3 for the 18927
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Figure 5. TEM images of Fe3O4 obtained at different temperature of (A) 200, (B) 220, (C) 230, (D) 235, (E) 240, and (F) 250 °C. Other conditions: 1.5 g FeCl3 3 6H2O, 1 g PVP, 2 g NaAc, 30 mL of ethylene glycol, and reaction time: 8 h.
self-assembly step in the system, and thus no porous Fe3O4 nanospheres are observed in Figure 3A. When the concentration of precursors slightly increases, some of them could not be nucleated at the initial period for the limitation of precipitation agent. Therefore, some porous Fe3O4 nanospheres can be obtained (Figure 3BD). As for further increasing the precursor concentration, the rate of nucleation will not increase. However, the growth and self-assembly steps will take longer, so large-sized porous Fe3O4 nanospheres can form in the system as in Figure 3E,F. The magnetic properties of all samples were shown in Figure 4. It is observed that all samples exhibit weak ferromagnetic behavior at room temperature. Their Ms values increase with the concentration of FeCl3, which might be related to size. In another word, the larger the size of Fe3O4 nanospheres, the higher the value of Ms. Table 1 lists the values of the main magnetic parameters (Ms and Hc) of the as-prepared samples. Temperature is another important parameter in the formation. Figure 5 shows TEM images of Fe3O4 prepared at 200, 220, 230, 235, 240, and 250 °C. Figure 5AD displays the fact that temperature has a strong impact on size and morphology. The higher the temperature is, the smaller the particles are. Below 235 °C, Fe3O4 nanospheres formed of primary particles can be obtained. However, when the temperature is >235 °C, the morphologies of Fe3O4 changed significantly. From Figure 5E,F, the shapes of Fe3O4 obtained at 240 and 250 °C are not regular, the size was 95%) in the first four cycles; then, the degradation of XO decreased from the fifth cycle to the seventh cycle. At the end of seven cycles, the percentage degradation remained steady at >75% of the initial value. These results show that the porous Fe3O4 nanospheres can be recycled for catalyzing degradation of XO with H2O2 solution. Therefore, the as-prepared Fe3O4 nanospheres can be used as ideal catalysts in practical applications.
Figure 14. First-order kinetic plot of XO degradation using H2O2 as oxidizing agent under ultrasound in with Fe3O4 nanospheres as catalyst.
4. CONCLUSIONS In conclusion, porous Fe3O4 nanospheres have been prepared with solvothermal method. The products exhibit weak ferromagnetic properties at room temperature. The quality of the product is determined by the experimental variables, such as the concentration of the precursor, capping agent, and precipitation agent as well as the reaction temperature and reaction time. The disadvantage of the method reported here is the wide size distribution of the product. The as-prepared porous Fe3O4 nanospheres show highly catalytic degradation ability toward degradation of organic dyes in aqueous solution. Although the catalytic activity decreases slightly with increasing the cycles of tests, such nanostructured catalyst is expected to find application in industrial applications, where separation and recycling are crucially required in terms of cost and environmental protection. ’ ASSOCIATED CONTENT
bS Figure 15. Catalytic activity of Fe3O4 nanospheres in different cycling numbers.
Supporting Information, the size of the commercial Fe3O4 can be determined to be >1 μm, which is very large compared with that prepared in this work. Meanwhile, the porosity of the materials is another most important factor influencing the catalytic activity. Figure S2B of the Supporting Information shows the N2 adsorptiondesorption isotherm of the commercial Fe3O4, determining that the BET surface area of the commercial Fe3O4 is 1.41 m2/g, whereas the value for the prepared Fe3O4 in this study was determined to be 47.7 m2/g (seen in Figure 2G). The degradation kinetic of XO was investigated by adding excess H2O2. Therefore, the degradation can be considered to be a pseudo-first-order reaction. The concentration, ct, of XO at different reaction time, t, could be described as follows ct ¼ c0 expðktÞ where c0 is the initial concentration of XO and k is the pseudofirst-order rate constant. Figure 14 shows the logarithmic plot of the concentration of XO as a function of degradation time. A well-behaved linear straight line shows that the degradation of XO is a pseudo-first-order reaction. The rate constant k can be calculated from the slope of the straight line. The value of k is 0.056 min1, which is much larger than the literature reported 0.013 min1 using Pd/TiO2 as the catalyst.73 For practical recyclable catalysts, keeping high catalytic activity in each cycle is necessary. The effect of recycling times of the porous Fe3O4 nanospheres on the catalytic performance was
Supporting Information. UV spectra of XO catalyzed by the commercial Fe3O4; TEM image and BET isotherm of the commercial Fe3O4. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: +86-514-87975436. Fax: +86-514-87975244. E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge the financial support from the National Natural Science Foundation of China (grant no. 20973151, 20901065), the Natural Science Key Foundation of Educational Committee of Jiangsu Province of China (grant no. 07KJA 15015), the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 20093250110001), the Foundation of Jiangsu Provincial Key Program of Physical Chemistry in Yangzhou University, and the Foundation of Jiangsu Key Laboratory of Fine Petrochemical Technology. We also acknowledge the Foundation of the Educational Committee of Jiangsu Provincial General Universities Graduate Student Scientific Research Invention Plan. ’ REFERENCES (1) David, H.; Gracias, J. T.; Tricia, L. B.; Carey, H.; George, M. W. Science 2000, 289, 1170–1172. (2) Park, S.; Lim, J. H.; Chung, S. W.; Mirkin, C. A. Science 2004, 303, 348–351. (3) Service, R. F. Science 2005, 309, 95. 18933
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