Amorphous Titania-Coated Magnetite Spherical ... - ACS Publications

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Amorphous Titania-Coated Magnetite Spherical Nanoparticles: Sonochemical Synthesis and Catalytic Degradation of Nonylphenol Ethoxylate Sambandam Anandan,*,†,‡ Gang-Juan Lee,‡ Shu-Han Hsieh,‡ Muthupandian Ashokkumar,*,§ and Jerry J. Wu*,‡ †

Nanomaterials & Solar Energy Conversion Laboratory, Department of Chemistry, National Institute of Technology, Trichy 620 015, India ‡ Department of Environmental Engineering and Science, Feng Chia University, Taichung 407, Taiwan § School of Chemistry, University of Melbourne, Vic 3010, Australia

bS Supporting Information ABSTRACT: Well-defined magnetically separable, amorphous titania-coated magnetite spherical nanoparticles were successfully synthesized by a sonochemical method using 20 kHz ultrasound. Amorphous magnetite particles were generated by the pyrolysis of Fe(CO)5 within the cavitation bubbles. The simultaneous formation of titania particles from titanium isopropoxide in the bulk liquid led to the incorporation of the magnetite nanoparticles into the titania particles. Transmission electron microscopy images of the titania-coated magnetite nanoparticles showed that they were relatively uniform in size with an average size of ∼150 nm. The M
H hysteresis loop for titania-coated magnetite particles indicated that the composite spherical nanospheres exhibit superparamagnetic characteristics at room temperature. The titania-coated magnetite nanoparticles were found to be useful for the removal of a wastewater pollutant (nonylphenol ethoxylate) by a combined process of heterogeneous catalysis and ozonation.

’ INTRODUCTION Many natural materials, such as seashell and lotus leaf, are composed of ordinary molecules but exhibit fascinating properties owing to their unique structures.1 Such intricate natural designs have inspired extensive research in the synthesis of materials with controlled structure and morphology, with expectations of achieving novel or enhanced properties.2,3 Therefore, in recent years, considerable effort has been devoted to the design and controlled fabrication of nanostructured materials with specific functional properties, with the creation of core
shell nanomaterials with tailored structural, optical, and surface properties4
9 being of interest from both fundamental and academic points of view. The shell can alter the charge, functionality reactivity, stability, and dispersibility of the core. Such core
shell nanoparticles can be prepared mostly by controlled precipitation of inorganic precursors onto the core particles or by deposition of small particles of the coating material on the core by heterocoagulation. Similarly, several preparation methods are also available for bimetallic core
shell nanoparticles and for the particular case of Au
Ag nanoparticles, and a number of proposals have been put forward to explain the nanoparticle formation processes.10
15 In continuation, the preparation of nanoparticles with a low-index core and a high-index shell, such as titania, are suitable building blocks for photonic crystals, provided that they can be made with high monodispersity and smooth coating.16
18 However, particles coated with titania are exceptionally difficult to synthesize because the titania precursors are highly reactive, which causes the core particles to aggregate. r 2011 American Chemical Society

Recent methods offer new alternatives for the controlled synthesis of novel, stable, and functional core
shell-type materials.19
23 Among these methods, the sonochemical technique has been found to generate nanomaterials of a much smaller size range and higher surface area than those reported by other methods.24 Acoustic cavitation—the formation, growth, and collapse of bubbles in a liquid medium—generates extreme reaction conditions. The extremely high temperatures and very high cooling rates (>109 K/s) attained during cavitation have been exploited in this method to create different nanostructured materials.25,26 In the current work, we used the sonochemical method to synthesize titania-coated magnetite nanoparticles with a spherical geometry. The reason for preparing such titania-coated magnetite nanoparticles is their efficiency in the removal of wastewater pollutants by a process that combines heterogeneous catalysis with ozonation. Furthermore, this work is mainly concerned with enhanced settling and reuse of suspended catalysts. That is, such magnetic catalysts permit recovery of the catalyst from the treated water by magnetism without the need for complex removal process.

’ EXPERIMENTAL DETAILS All chemicals were of the highest purity available and were used as received without further purification. Iron pentacarbonyl Received: November 30, 2010 Accepted: May 30, 2011 Revised: May 30, 2011 Published: May 30, 2011 7874

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Industrial & Engineering Chemistry Research [Fe(CO)5], titanium(IV) isopropoxide [(i-C3H7O)4Ti], and decalin were purchased from Aldrich. Using a 20 kHz (23
47 W cm
3) horn sonifier (Sonics 750), titania-coated magnetite nanoparticles with a spherical geometry were made as follows: A controlled amount of Fe(CO)5 (40 mL) and 4 mL of (i-C3H7O)4Ti were dissolved in decalin (51 mL) and sonicated at room temperature under O2 atmosphere for 1 h. Then, 2 mL of a solution of isopropanol and water (isopropanol/H2O = 1:1) was added dropwise into the sonication cell, and the system was continuously sonicated for another 5 h, during which time the initially yellow solution turned brown. After sonication, the mixture was filtered, and the products were washed twice with n-pentane and acetone. The surface morphology and particle size of the sonochemically prepared titania-coated magnetite nanoparticles were analyzed by transmission electron microscopy (JEOL JEM 2100F model). Energy-dispersive X-ray analysis was used to determine the elemental composition of the spherical nanoparticles. Raman spectra were recorded on a Bruker Raman spectrometer with a 1064-nm argon ion laser as the excitation source. The activity of the titania-coated magnetite particles was tested for the degradation of an organic pollutant (nonylphenol ethoxylate, NPE) by a process combined with ozonation. A 500 mL capacity borosilicate glass reactor was used in all experiments. The degradation of NPE was carried out under atmospheric conditions (25 °C) at neutral pH. The amounts of catalyst and NPE used for all experiments were the same (250 mg per 250 mL of 2.0  10
4 M NPE in aqueous solution). For the ozone-assisted processes, ozone was introduced through a porous frit that can produce fairly fine bubbles with diameters of less than 1 mm, which was determined using a camera with a close-up lens and Matrox Inspector 2.0 image analysis software. Ozone was produced from pure oxygen by corona discharge using an ozone generator (Ozonia) that produced a maximum ozone concentration of ca. 6% (by volume) in the oxygen-enriched gas stream. The gas flow rate was regulated at 200 mL min
1 by a gas flow controller (Brooks 5850E), and the input ozone concentration was adjusted to 40 mg/L. The gaseous ozone concentrations were determined spectrophotometrically by the absorbance of ozone measured in a 2-mm flow-through quartz cuvette at the wavelength of 258 nm. An extinction coefficient of 3000 M
1 cm
1 was used to convert absorbance into concentration units.27 NPE was not adsorbed on titania-coated magnetite particles. To ensure this, the solution was stirred for about 20 min prior to the beginning of the experiments. High-performance liquid chromatography (HPLC) analysis before and after stirring indicated that there was no change in the concentration of NPE, implying that NPE was not adsorbed on titania-coated magnetite particles.

’ RESULTS AND DISCUSSION Sonication of Fe(CO)5 in a nonaqueous medium leads to the formation of nanosized clusters of amorphous iron nanoparticles under argon atmosphere.28 However, sonication in O2 atmosphere yields iron oxides.29
31 In both cases, the volatile iron complex evaporates into the cavitation bubbles and decomposes to form iron/iron oxide amorphous nanoparticles under the extreme temperature conditions generated within the cavitation bubbles. In the current study, the overall synthetic procedure of titaniacoated magnetite nanoparticles can be described as follows:

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Initially, the system is in a nonaqueous medium for up to 1 h. Upon ultrasonic irradiation, Fe(CO)5 in O2 atmosphere yields Fe2O3. The presence of a small amount of (i-C3H7O)4Ti in the system acts as a stabilizing agent. After 1 h, the addition of isopropanol and water (isopropanol/H2O = 1:1) to the reaction mixture hydrolyzes (i-C3H7O)4Ti to hydrophobic Ti
OH, which can interact with the Fe2O3 nanoparticles formed and get emulsified as droplets under the physical forces generated in the acoustic field. At the same time, a high-speed collision driven by high-intensity ultrasound irradiation can generate localized high-temperature regions. This can enhance the condensation reactions among hydroxyl groups and adjacent Fe2O3 nanoparticles to produce agglomerates of spherical nanoparticles (titania-coated magnetite nanoparticles). During this period, the conversion of Fe2O3 into Fe3O4 takes place (reactions 1 and 2). This might be due to the continuous sonication in the presence of O2 atmosphere, Ti
OH group interactions, or cavitation-induced chemical reactions32
35 that can occur in the hot interfacial region of the collapsing bubbles.36 FeðCOÞ5 f Fe2 O3

ð1Þ

Fe2 O3 þ ði-C3 H7 OÞ4 Ti f Fe3 O4 þ TiO2

ð2Þ

As-prepared titania-coated magnetite spherical nanoparticles were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption/desorption isotherms. TEM images of the titania-coated magnetite nanoparticles (Figure 1a
c) show that they are relatively uniform in size with an average size of ∼150 nm. The TEM image of the dense titania-coated magnetite nanoparticles reveals that the titania coating is uniform, with a thickness of about 20 nm (Figure 1d). The energy dispersive X-ray (EDX) spectrum of the titania-coated magnetite particles shows that the magnetite species were successfully immobilized in the nanoporous shell (Figure 1e). Further, the identified elements were Fe with a weight percentage of 85.56%, originating from iron oxide, and Ti with a weight percentage of 14.44%. A diffuse ring pattern (Figure 1f) is detected in the selected-area electron diffraction (SAED) pattern, indicating that the product is amorphous in nature. The XRD pattern of the product shown in Figure 2aA supports the conclusion that the as-prepared product is amorphous in nature. On the other hand, when the amorphous sample was calcined at 500 °C, it showed a peak for crystalline Fe3O4 (Figure 2aB). The 2θ value at 35.82° (311) belongs to magnetite structure (JCPDS card no. 89-4319). However, some authors have stated that XRD data cannot discriminate between Fe2O3 and Fe3O4 nanoparticles because of poorly resolved peaks caused by the small size of the nanoparticles.37 The XPS spectrum (Figure 2b) shows main peaks around 709 and 722 eV, which are characteristic for the formation of typical Fe 2p peaks of Fe2þ ions. In addition, a shakeup satellite line at 717 eV is noticed, which is characteristic for Fe3þ ions in Fe2O3.38 The Ti 2p3/2 and Ti 2p1/2 binding energies for titania-coated magnetite (Fe3O4) nanoparticles were found to be 456 and 462 eV, which indicates that Ti existed as Ti4þ (Figure 2c). To further support the observed XPS profile, we performed Raman spectroscopic analysis of the titania-coated magnetite (Fe3O4) nanoparticles. In the Raman spectra of titania-coated magnetite (Fe3O4) nanoparticles, the three clear peaks at 667, 521, and 306 cm
1 can be indexed to 7875

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Figure 1. (a
d) High-resolution TEM images of titania-coated magnetite spherical nanoparticles. (e) EDX and (f) SAED patterns of titania-coated magnetite spherical nanoparticles.

the A1g, T2g, and Eg modes, respectively, of Fe3O4 (see Figure S1, Supporting Information).39 This also confirms that the asprepared product mainly contained titania-coated magnetite (Fe3O4) nanoparticles. Representative nitrogen adsorption/desorption isotherms and the corresponding pore size distribution of the titania-coated magnetite spherical particles obtained from the analysis of the adsorption branch using the BJH (Barrett
Joyner
Halenda) method are shown in Figure 2d,e. The BET surface area and single-point total pore volume were calculated to be 388.64 m2 g
1 and 0.43 cm3 g
1, respectively. The pore size distribution (PSD) curve, shown in the Figure 2e, exhibits a single peak centered at 3.52 nm. Several magnetic nanostructured materials have recently been used as catalyst supports to facilitate the recovery and recycling of catalysts from the reaction mixture. The magnetic behavior of the titania-coated magnetite spherical particles was investigated using

a superconducting quantum interference device (SQUID) magnetometer (Figure 2f). The observed magnetization data were fitted with the Langevin function incorporating the particle size distribution, as described elsewhere40
42 (see Figure S2, Supporting Information, for Langevin the fit.) The average particle size found from the fitting was 2.9 nm. Particles with sizes in this range will behave as completely superparamagnetic, as the size limit is much below the critical superparamagnetic size limit of magnetite. The superparamagnetic behavior is clearly evident from the nearzero coercivity value. Furthermore, it is important to note that (i) no saturation of magnetization occurs as a function of the field, (ii) the observed magnetization values are much lower than the 92 emu/g value of the corresponding bulk magnetite, and (iii) no hysteresis was observed for the titania-coated magnetite nanoparticles, which is further evidence that the product is a superparamagnetic material.19,43 7876

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Figure 2. (a) XRD, (b,c) XPS, (d,e) BET, and (f) magnetization spectra of titania-coated magnetite spherical nanoparticles.

The activity of the titania-coated magnetite particles was tested for the catalytic degradation of an organic pollutant (i.e., nonylphenol ethoxylate, NPE) in combination with ozonation. The HPLC chromatograms recorded during the ozone degradation of nonylphenol ethoxylate with and without titania-coated magnetite particles

are shown in a three-dimensional plot in Figure 3a,b. Under the HPLC conditions used here, NPE eluted at 19.27 min. The corresponding peak area decreased as the reaction time progressed, and a new peak appeared at a retention time of 7 min representing a shorter-chain NPE.44
47 The change in the concentration of NPE as 7877

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Figure 3. Three-dimensional plots showing the evolution of the HPLC chromatogram as a function of (a) ozonation and (b) catalytic ozonation of nonylphenol ethoxylate (2  10
4 M). The mobile phase was acetonitrile/water (60/40 v/v), and the detection wavelength was 226 nm. (c) Degradation plot for NPE ozonation with (2) and without (9) catalyst. (d) Kinetics of NPE degradation with (2) and without (9) catalyst.

a function of reaction time was calculated by integrating the area under the entire peak of NPE eluted at 19.27 min. Figure 3c shows the change in the concentration as a function of reaction time for ozonation of NPE with and without the catalyst. The degradation was completed in about 1 h of ozonation without any catalyst. However, upon the addition of titania-coated magnetite nanoparticles, the degradation was completed in 40 min. Furthermore, plots of ln [NPE] versus time for the ozonation of NPE with and without catalyst are shown in Figure 3d. The linear relationship between ln(C/C0) and irradiation time indicates that the degradation follows first-order kinetics. From the slopes of ln(C/C0) versus time, the first-order rate constants were calculated for the ozonation of NPE with (3.49  10
3 s
1) and without (1.86  10
3 s
1) catalyst. The decrease in the concentration of NPE with time during ozonation is due to the direct oxidation of nonylphenol ethoxylate by O3 molecules. The ability to oxidize NPE is closely related to the oxidation potential (reaction 3) of the oxidant.48 O3 þ 2Hþ þ 2e
f O2 þ H2 O

E0 ¼ 2:07 V

ð3Þ

The acceleration observed when ozonation was applied together with the nanoparticles is as a consequence of the dual functionality of the nanoparticles. Their presence causes both an

increase in ozone dissolution and the initiation of the ozone decomposition reaction.49 As a consequence, more hydroxyl radicals are generated in the solution. Hydroxyl radical is one of the most reactive free radicals and one of the strongest oxidants,50 and it oxidizes NPE effectively (reactions 4 and 5). HO• þ Hþ þ e
f H2 O

E0 ¼ 2:33 V

nonylphenol ethoxylate þ HO• f oxidized products

ð4Þ ð5Þ

In addition to the above degradation processes, additional reactions/processes are also involved in the presence of nanoparticles. Both ozone and organic molecules are transported to the surface of the nanoparticles. Generally, the proposed mechanism of catalytic ozonation on nanoparticles as catalysts assumes that the adsorptions of both organic molecules and ozone take place simultaneously on the surface of the nanoparticles.51
53 As a result of ozone adsorption and its conversion, O
radicals51 or OH• radicals54 are generated (the O2•
transfers an electron to another ozone molecule to form an ozonide anion (O3•
), which is the chain-reaction promoter and produces HO• radicals). Free radicals can initiate a chain reaction both on the surface of the catalyst and in the bulk of the aqueous phase. Oxidation proceeds stepwise through several intermediates, as radicals are continuously 7878

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Figure 4. Proposed pathway of degradation of nonylphenol ethoxylate.

generated by the dissolved ozone that is transferred onto the catalyst surface. To identify several minor products, mass spectrometric analysis was performed. The mass spectrum (see Supporting Information, Figure S3) obtained for NPE before ozonation exhibited various ionization peaks (m/z 287 [Mn=1 þ Na]þ, m/z 551 [Mn=7 þ Na]þ, m/z 595 [Mn=8 þ Na]þ, m/z 639 [Mn=9 þ Na]þ, m/z 683 [Mn=10 þ Na]þ). In the typical fragments, nonylphenol (m/z 287) and shorter-chain NPE adducts (m/z 551) were observed as a result of fragmentation. That is, cleavage took place in ethers either at an internal position to yield nonylphenol, polyethylene glycols, polyglycolaldehyde, and polyglycol carboxylate or at a terminal position to yield glycol, glycolic acid, glycolaldehyde, and acetaldehyde.55 In addition, hydroxyl shift took place in ethers to yield hemiacetal, oxidative cleavage took place in ethers at the R position to yield glycolic acids,

β-elimination cleavage took places in ethers to yield polyglycols, and finally ω-carboxylation also took place (Figure 4).56 Several aspects of the adduct formation processes that occur in the mass analysis of NPE, such as the formation of the doubly charged adducts or dimers, have been discussed in the literature.57 However, the mass spectrum obtained for NPE after 30 min of ozonation exhibits various ionization peaks in addition to the above-mentioned peaks. Also, enhancement in the m/z 44 CO2 peak was noticed. Almost similar mass spectrum was noticed in the presence of titania-coated magnetite nanoparticles after 30 min but the intensities of the above-mentioned peaks were much lower compared to those observed in the ozonation process alone. However, the observed intensity of the CO2 peak was very high compared to that for ozonation alone, indicating that the mineralization of NPE was accelerated when ozonation was applied together with nanoparticles. 7879

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’ CONCLUSIONS This research leads to several conclusions: (1) A simple sonochemical reduction method has been successfully developed to synthesize titania-coated magnetite spherical nanoparticles for the enhanced removal of nonylphenol ethoxylate from water using catalytic ozonation. (2) The titania-coated magnetite nanoparticles synthesized in this study were relatively uniform in size with an average size of 150 nm. (3) The M
H hysteresis loop for titania-coated magnetite particles indicates that the composite spherical nanoparticles have superparamagnetic characteristics at room temperature, which reveals the possibility of recovering nanoparticles from aqueous mixtures using an appropriate magnetic field. (4) The combination of TiO2 and superparamagnetic Fe3O4 nanoparticles endows this material with a bright perspective in the purification of polluted wastewaters because of the additional reactions/processes occurring at the surface of the composite nanoparticles. Further, the magnetic properties of the hybrid composite are useful for the efficient recovery and recycling of magnetic catalysts in liquid-phase reactions. (5) This methodology could be used to synthesize a number of nanoparticles involving other semiconductors as well as to degrade wastewater pollutants effectively by combining it with the ozonation process. ’ ASSOCIATED CONTENT

bS

Supporting Information. Raman spectra of titaniacoated magnetite spherical nanoparticles, Langevin fit for magnetization spectra of titania-coated magnetite spherical nanoparticles, and mass spectra of nonylphenol ethoxylate before and after ozonation and catalytic ozonation. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (S.A.), [email protected] (M.A.), [email protected] (J.J.W.). Tel.: þ91-431-2303639 (S.A.), þ61-3-83447090 (M.A.), þ886-4-24517250 ext. 5206 (J.J.W.). Fax: þ91-431-2300133 (S.A.), þ61-3-94785180 (M.A.), þ886-4-24517686 (J.J.W.).

’ ACKNOWLEDGMENT The research described herein was financially supported by the National Science Council (NSC), Taiwan, under Contract 98-2221-E-35-12-MY3. S.A. thanks Feng Chia University, Taiwan, for the Visiting Professor appointment during May
June 2010. The authors thank Dr. R. Justin Joseyphys, Department of Physics, NIT, Trichy, India, for Langevin fit analysis. ’ REFERENCES (1) Douglas, T. A Bright Bio-Inspired Future. Science 2003, 299, 1192–1193. (2) Lu, Y. F.; Fan, H. Y.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Aerosol-Assisted Self-Assembly of Mesostructured Spherical Nanoparticles. Nature 1999, 398, 223–226.

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