Self-Assembled Fabrication of Superparamagnetic ... - ACS Publications

J. Phys. Chem. C 2010, 114, 22493–22501

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Self-Assembled Fabrication of Superparamagnetic Highly Stable Mesoporous Amorphous Iron Oxides Manickavachagam Muruganandham,*,† Ramakrishnan Amutha,† Bashir Ahmmad,‡ Eveliina Repo,† and Mika Sillanpaä¨†,§ Laboratory of Applied EnVironmental Chemistry, Department of EnVironmental Science, UniVersity of Eastern Finland, Patteristonkatu 1, FI-50100, Mikkeli, Finland, Department of Fundamental Material Science, Graduate School of Natural Science and Technology, Okayama UniVersity, 3-1-1 Tsushima, Okayama 700-8530, Japan, and Faculty of Technology, Lappeenranta UniVersity of Technology, Patteristonkatu 1, FI-50100 Mikkeli, Finland ReceiVed: September 14, 2010; ReVised Manuscript ReceiVed: NoVember 16, 2010

In this article, we report the fabrication of highly stable amorphous mesoporous iron oxides (AMIOs) with surface tunable properties by a template-free method. AMIOs with surface areas from 264 to 43 m2/g and mesopore sizes from 5 to 11 nm were synthesized. The synthesized AMIOs were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectra, and nitrogen adsorption analysis. The XRD patterns indicated that the thermal decomposition of the ferric oxalate complex under open atmospheric conditions at low temperature (200-275 °C) and in inert atmospheric conditions at high temperature (400 °C) facilitates formation of crystallographically amorphous iron oxides. ζ-potential analyses of AMIOs indicated that they have almost similar points of zero charge. Magnetic measurements revealed superparamagnetic behavior, with magnetization values around 20 emu/g. The capacities of various amorphous iron oxides to remove As(V) and Cr(VI) by adsorption were investigated. The efficiencies of removal of Cr(VI) and As(V) were found to be 24 and 81 mg/g, respectively. We propose a plausible mechanism for the stability of AMIOs. 1. Introduction Fabrication of mesoporous amorphous materials is of considerable interest from basic as well as applied points of view. Amorphous metal oxides have important applications in many fields, including solar-energy transformation, magnetic storage media, electronics, and catalysis.1-4 Iron oxides are particularly important due to their positive qualities, such as low cost, good stability, nontoxicity and environmentally friendly properties, and their applications in magnetic storage, catalysis, sensors, adsorption, and rechargeable lithium batteries.5-10 Another important class of materials are those that are mesoporous. However, the synthesis of mesoporous transition-metal oxides is much more difficult than the synthesis of mesoporous silica.9 Interestingly, mesoporous materials exhibit great potential for many applications due to their large and uniform pore size and high surface area, their multidimensionality, and their easy recyclability.11-13 However, the need for templates and the associated high fabrication costs may limit large-scale production, although this may be overcome by developing a simple methodology for synthesis. In addition, template removal requires additional energy and may damage the porous structure.14 Earlier studies confirmed that amorphous Fe2O3 nanoparticles are more active than nanocrystalline polymorphic Fe2O3 in catalyst-based processes. This is due to the formation of dangling bonds and a higher surface-bulk ratio for the amorphous * To whom correspondence should be addressed. E-mail: mmuruganandham@ yahoo.com. Tel: +358403553415. Fax: +35815336013. (M.M.); mika.sillanpaa@ lut.fi (M.S.). † University of Eastern Finland. ‡ Okayama University. § Lappeenranta University of Technology.

phase.15 Similarly, amorphous iron oxide possesses a higher arsenic adsorption capacity than other iron oxide phases due to the formation of surface complexes.16,17 The magnetic properties of the iron oxides are also very important and are influenced by various experimental parameters during synthesis. Generally, crystallization of amorphous materials results in a more stable crystalline state, and therefore, the controlled fabrication of stable AMIOs is a challenging task for materials scientists and chemists. The presence of a mesoporous structure in amorphous iron oxides is expected to substantially increase the catalytic and adsorption capacity. The synthesis of iron oxides by thermal decomposition of various iron-containing precursors is an attractive method due to its simplicity and high yield. One such method is the thermal decomposition of an iron-oxalate complex, and most studies have been done using the commercially available iron(II) and iron(III) oxalates under various experimental conditions.18-22 Similarly, studies have also been done using iron salts with oxalic acid as a coordination agent in hydrothermal methods.23,24 However, these reports did not study the synthesis of AMIOs. To the best of our knowledge, no attempts have been made to study how ferric oxalate preparation conditions influence the formation of iron oxides and their surface properties. In this article, we report the template-free, industrially applicable fabrication of highly stable AMIOs using thermal decomposition of ferric nitrate-oxalic acid complex under open atmospheric conditions. The influence of various experimental conditions, decomposition time, and temperature on the formation of AMIOs was investigated. The surface properties, stability, ζ-potential, magnetic properties, and heavy metals adsorption of the synthesized AMIOs were studied, and the results are discussed.

10.1021/jp110326m  2010 American Chemical Society Published on Web 12/09/2010

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2. Experimental Section Ferric nitrate nonahydrate, anhydrous ferric chloride, iron(III) oxalate hexahydrate, and oxalic acid were purchased from Aldrich Chemical Co. Ltd. (Helsinki, Finland). All chemicals were of analytical grade and used as received without further purification. For all experimental work, Milli Q-Plus water (resistance ) 18.2 M Ω) was used. The synthesis of iron oxide involves two steps: the first step is to prepare the ferric nitrate-oxalic acid complex (ferric oxalate) by simple mixing of an equal volume (100 mL) of 0.1 M ferric nitrate with 0.17 M oxalic acid. Under magnetic stirring, oxalic acid was added drop by drop (30-45 min) into ferric nitrate solution under a dark environment. The solution was stirred for several hours, and then the ferric oxalate solution was evaporated on the hot plate until it became a dry greenish solid. Care was taken at the final stages of solvent evaporation. Hence, nitric acid may evaporate, and evaporation should be done in the fume cupboard. In the second step, the prepared ferric oxalate complex was decomposed at the desired temperature and time under open atmospheric conditions. After decomposition at the desired time, the oven was allowed to cool to room temperature naturally, and then the samples were washed with plenty of water and ethanol and dried in an oven at 120 °C for 2 h. The adsorption experiments have been performed as follows. A 10 mg portion of adsorbent and 5 mL of the solution containing each metal were mixed for 24 h. After that, adsorbent was filtered using 0.45 µm polypropylene syringe filters; samples were analyzed by an inductively coupled plasma optical atomic emission spectrometer (ICP-OES), model iCAP 6300 (Thermo Electron Corporation, U.S.A.). Cr(VI) and As(V) were analyzed at wavelengths of 267.716 and 189.042 nm, and the quantitation limits were 0.003 and 0.017 mg/L, respectively. The X-ray diffraction (XRD) patterns were recorded using an X’Pert PRO PAN analytical diffractometer, with a scanned angle from 10° to 100°. High-resolution transmission electron microscope images were recorded using an FE-TEM, Philips CM-200 FEG-(S) TEM-Super Twin. Samples for HR-TEM were prepared by ultrasonically dispersing the catalyst into ethanol and then placing a drop of this suspension onto a carboncoated copper grid and then drying in air. The working voltage of the TEM was 80 kV. The morphology of the catalyst was examined using a Hitachi S-4100 scanning electron microscope (SEM). Prior to SEM measurements, the samples were mounted on a carbon platform that was then coated by platinum using a magnetron sputter for 10 min. The plate containing the sample was then placed in the SEM for the analysis with desired magnifications. The Raman spectra were acquired using a Renishaw inVia microscope using a 514.5 nm Argon laser at 50% power with a 50× aperture. Five acquisitions were taken for each sample from 100 to 1000 cm-1 for 10 s. The FTIR spectra were recorded by using a BRUKER VERTEX 70 with a Platinum ATR-QL, Diamond accessory. The X-ray photoelectron spectra were collected on an ESCA-1000 X-ray photoelectron spectrometer (XPS), using Mg KR X-ray as the excitation source. The surface charge (ζ-potential) of the iron oxide was analyzed at different pHs (4-10) using a Malvern Zetasizer (ZEN, 3600), and the pH of the solution was adjusted using 0.1N HCl or 0.1N NaOH by using the autotitration method. The surface properties (surface area, pore size, and pore volume) of the iron oxides were measured with an Autosorb1-C surface analyzer (Quantachrome, U.K.). All the samples were degassed at 120 °C for overnight before measurements. The metal concentrations in the filtrates were analyzed by an

Muruganandham et al. inductively coupled plasma optical atomic emission spectrometer (ICP-OES), model iCAP 6300 (Thermo Electron Corporation, U.S.A.). 3. Results and Discussion Initially, we examined the formation of AMIOs by thermal decomposition of the ferric oxalate complex prepared using various ferric salts as iron precursors with oxalic acid. Our experimental results clearly indicated that neither ferric sulfate nor ferric phosphate is suitable for the preparation of ferric oxalates. The decomposition of the ferric oxalate complex (200 °C for 2 h) prepared using ferric chloride with oxalic acid results in well-crystallized hematite nanoparticles, as shown in the XRD and HR-TEM images presented in Figure S1 (Supporting Information). Moreover, nitrogen adsorption analysis indicated the absence of any characteristic porous surface structure, and the surface area was found to be 13 m2/g. However, decomposition of the ferric oxalate complex prepared using ferric nitrate with oxalic acid yields AMIOs. Therefore, the ferric oxalate complex was prepared using ferric nitrate with oxalic acid under similar experimental conditions. Initially, decomposition results showed that the decomposition temperature is crucial for the formation of characteristic pore structures. Thus, ferric oxalate complex decomposition at low temperatures (up to 275 °C) facilitates formation of AMIOs. However, decomposition at higher temperatures (over 300 °C) yields hematite with macroporous superstructures. In this paper, we report the fabrication of AMIOs, and the formation of hematite superstructures will be addressed in a future publication. The FTIR spectra of the synthesized ferric oxalate have been analyzed, and the results were compared with commercially available ferric oxalate hexahydrate, as shown in Figure S2 (Supporting Information). The υ (OH) and υ (CO) bands appeared at 3541, 3496 and 1730, 1643, 1598 cm-1, respectively. The peaks that appeared at 1345, 1240, 758, and 816 cm-1 are assigned to the υ(C-C) vibration mode. To further confirm the composition, the results were compared with literature reports. Edwards et al. studied the structure of commercial iron(III) oxalate hexahydrate (Aldrich), and the FTIR spectra are quite similar to that of synthesized ferric oxalates presented in Figure S2 (Supporting Information).25 Thermal decomposition of either iron(II) oxalate or iron(III) oxalate is quite complex, and the nature of the decomposition products is influenced by many experimental parameters. The decomposition proceeds via many intermediates, and it depends on the decomposition conditions.18-22,26-30 Earlier reports indicated that the decomposition of both iron(II) oxalates and iron(III) oxalates under air or oxygen atmospheres yields Fe2O3 as a final decomposition product.20,21 Thermal analysis of the synthesized ferric oxalate complex was done under air and nitrogen flow conditions, and the results are presented in Figure 1. Both thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analyses of the ferric oxalate complex decomposition under air flow conditions are shown in Figure 1a. It is apparent that the weight decrease is continuous and does not take place in separate, distinguishable steps. The results confirmed that ferric oxalate starts to decompose at 150 °C and decomposes into Fe2O3 without producing stable intermediates. The results from the DSC analysis, as discussed previously, are in good agreement with those from the TGA analysis. The observed decomposition pattern is in good agreement with the iron(III) oxalate decomposition reported earlier.18 However, under a nitrogen atmosphere, the decomposition pattern slightly differs from that noted

Synthesis of Mesoporous Amorphous Iron Oxides

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Figure 1. TGA and DSC analyses of ferric oxalates under (a) air and (b) nitrogen atmospheric conditions.

under air flow conditions, as shown in Figure 1b. TGA analysis clearly indicated that the first major decomposition starts at 158 °C, which is expected to be dehydration, followed by decomposition of the ferric oxalate complex. The TGA curve indicated that the decomposition may proceed via either stable intermediates or mixed phase iron oxides before Fe2O3 is formed as a final product. It is known that there are many factors that influence the mechanism and kinetics of solid-state decompositions of iron oxalates.18 The DSC analysis revealed a two-step decomposition; one large exothermic peak was observed at 200 °C and another at about 340 °C. The first one clearly indicated the decomposition of ferric oxalate, and the second one is suspected to be the conversion of a mixed phase of iron oxides

to a hematite phase. We did not further explore the relationship between decomposition temperature and iron oxide phase because it is out of the scope of this paper. The above results are quite consistent with the noted XRD and energy-dispersive X-ray analysis (EDX), which confirmed the formation of Fe2O3. The EDX analysis shown in Table 1 clearly indicates that the Fe:O ratio is nearly 2:3, which confirmed the formation of Fe2O3 under the different experimental conditions mentioned in Table 1. The Raman spectra of the AMIOs were investigated, and the results are presented in Figure S3 (Supporting Information). The Raman spectra of all three AMIOs prepared from 200 to 250 °C exhibit two strong peaks at about 210 and 275 cm-1 corresponding to the typical frequencies observed for R-Fe2O3 and a weak peak at 380 cm-1 also noted. No other characteristic peaks were noted in all three AMIOs. A weak peak that appeared at 380 cm-1 is suspected to be either due to the maghemite (γ-Fe2O3) phase or due to lower wavenumber shift of the Eg mode peak (412 cm-1) of R-Fe2O3. Generally, the hematite phase yields seven phonon lines (two A1g modes, 225 and 498 cm-1, and five Eg modes, 247, 293, 299, 412, and 613 cm-1). Thus, compared with the bulk material, a small shift toward lower wavenumber was observed, which might be related to the nanometer scale and due to the presence of amorphous contents. Similar observations were also noted in earlier studies.31,32 As discussed earlier, the decomposition temperature is one of the important factors for controlling the morphology and surface properties of AMIOs. Therefore, to understand the formation and stability (crystallization) of AMIOs, thermal decomposition has been studied at different temperatures (from 200 to 275 °C) and times (2-200 h at 200 °C) under open atmospheric conditions. The XRD results for AMIOs formed at different decomposition times at 200 °C are presented in Figure 2. The XRD patterns shown in Figure 2, patterns A-D, imply the formation of crystallographically amorphous iron oxide even after 200 h. However, crystallized iron oxide peaks appeared at two θ values of 35.6 and 57.4, and these indicate the presence of the hematite phase. The quantification of the relative proportions of amorphous and nanocrystalline fractions is a difficult task.33 Despite the small amount of crystallized hematite phase, the iron oxides possess an amorphous nature even after prolonged calcinations (200 h). These results demonstrated the high stability of the synthesized AMIOs. The synthesized AMIO is one of the first stable amorphous iron oxides reported in the literature. To understand this stability, we have also examined its surface properties at various decomposition times, as shown in the Table 1. Interestingly, increasing the decomposition time from 2 to 200 h decreases the surface area from 232 to 209 m2/g, and the pore size decreases from 6.6 to 5.7 nm. As expected, the decrease in the surface area with increasing time of calcination is due to an increase in the degree of crystallization. The decrease in pore

TABLE 1: Various Properties of AMIOs Prepared under Different Experimental Conditions sample no.

DT (°C)a

DT (h)b

ZPC

Fe (%)d

O (%)d

BET (m2/g)

pore size (nm)

pore volume (cm3/g)

S1 S2 S3 S4 S5 S6 S7 S8c S9c

200 200 200 200 225 250 275 300 400

2 12 99 200 2 2 2 2 2

8.3 8.1 7.9 8.1 7.8 8.0 8.4 7.5 8.0

59.4 60.0 64.7 57.4 59.7 85.7 59.6 60.4 59.7

40.5 39.9 35.2 42.9 40.2 41.2 39.3 39.5 40.2

232 229 215 209 264 233 151 109 43

6.6 6.6 6.1 5.7 5.9 6.9 7.0 4.7 11

0.38 0.37 0.33 0.29 0.39 0.40 0.26 0.12 0.12

a

DT ) decomposition temperature. b DT ) decomposition time. c Decomposition under nitrogen atmospheric conditions. d Atomic weight.

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Figure 2. XRD of iron oxides formed by the thermal decomposition of the ferric nitrate-oxalic acid complex under open air atmospheric conditions: (A) 200 °C for 2 h, (B) 200 °C for 12 h, (C) 200 °C for 99 h, (D) 200 °C for 200 h, (E) 225 °C for 2 h, (F) 250 °C for 2 h, and (G) 275 °C for 2 h.

size with increasing calcination time is due to self-assembly of initially formed aggregates into bundles, as confirmed by FESEM analysis, and the details are discussed in a later section of this paper. These results clearly indicated that increasing the calcination time did not change the surface properties appreciably, and this may be due to the stability of AMIOs. For comparison, we also examined the effect of calcination time on the surface properties of commercially available iron (III) oxalate hexahydrate under similar experimental conditions. Increasing the decomposition time from 2 to 50 h decreases the surface area of iron oxide from 221 to 150 m2/g, and the pore size increases from 6.9 to 10 nm. These results are mainly due to an increase in crystallization of the iron oxides formed as the calcination time increases, as evident from the XRD presented in Figure S4 (Supporting Information). Thus, when compared to the above results, the synthesized AMIOs showed good stability, and this is mainly due to slow crystallization. To understand the influence of decomposition temperatures on the formation of AMIOs, decomposition was studied from 200 to 500 °C. Increasing the decomposition temperatures from 200 to 275 °C results in AMIOs with small percentages of hematite, as shown in the XRD results in Figure 2, patterns E-G, and in Table 1. However, further increasing the temperature from 300 to 500 °C results in well-crystallized macroporous hematite, as evident from the XRD pattern presented in Figure S4 (Supporting Information). The surface properties of AMIOs prepared at various decomposition temperatures are shown in Table. 1. It should be mentioned here that, among the AMIOs prepared under different experimental conditions, the AMIO prepared at 225 °C possesses the highest surface area (264 m2/ g) with a pore size of 5.9 nm. However, we did not optimize the surface area with decomposition temperatures, and such studies may yield high surface area AMIOs. Increasing the decomposition temperature from 200 to 250 °C increases both pore size and pore volume of the AMIOs. However, a further increase in the decomposition temperature decreases the surface area and pore volumes, which may be due to an increase in the degree of crystallization. A representative nitrogen adsorption-desorption isotherm and pore size distribution of the AMIO prepared at 200 °C for 99 h is presented in Figure S5 (Supporting Information). The isotherm displays the typical type IV curve, which is usually attributed to the predominance

Figure 3. HRTEM images and their corresponding SAED patterns (insets) of the synthesized mesoporous iron oxides under open atmospheric conditions at (A) 200 °C for 12 h, (B) 200 °C for 200 h, (C) 225 °C for 2 h, and (D) under nitrogen atmospheric conditions at 400 °C for 2 h.

of mesopores. The narrow pore size indicated a uniform pore size distribution on the AMIO surface. The HR-TEM images and the corresponding selected area electron diffraction (SAED) patterns for the synthesized AMIOs under different experimental conditions are shown in Figure 3. The HR-TEM images clearly revealed the presence of a mesoporous structure, and the SAED pattern confirms the amorphous nature of the iron oxides. Figure 3A-C shows mesoporous structures, and the SAED patterns shown in the insets show the amorphous nature of the iron oxides. However, increasing the decomposition temperature from 200 to 225 °C increases the crystalline nature, as evident from the SAED pattern presented in Figure 3C. As we discussed earlier, the crystallized phase (hematite) also coexists along with the amorphous phase. Therefore, it is reasonable to expect some diffraction spots on the SAED pattern, and the results are consistent with the XRD results. The iron oxide synthesized under nitrogen atmospheric conditions at 400 °C is presented in Figure 3C. The above-discussed results clearly indicated that the iron oxide possesses a more crystalline nature than those that were prepared at 200 °C under open atmospheric conditions, as shown in Figure 3A,B. AMIOs were also prepared by decomposition performed under an inert (nitrogen) atmosphere. As we discussed earlier, ferric oxalate decomposition under open atmospheric conditions up to 275 °C results in AMIOs. Therefore, decomposition has been studied at higher temperatures (300-600 °C) under a nitrogen atmosphere. The XRD results in Figure 4 confirmed the formation of crystallographically amorphous iron oxides up to 400 °C; however, decomposition over 400 °C results in hematite. It is quite interesting to note that the decomposition at 300 °C yields a mesoporous surface structure with a pore size of 4.7 nm and a surface area of 109 m2/g, although crsytallographically amorphous iron oxides were noted at 400 °C that did not possess a mesoporous structure (a pore size of



Figure 4. XRD patterns of iron oxides formed by the thermal decomposition of the ferric nitrate-oxalic acid complex under nitrogen atmospheric conditions at various decomposition temperatures: (A) 200 °C for 2 h, (B) 300 °C for 12 h, (C) 400 °C for 99 h, (D) 500 °C for 200 h, and (E) 600 °C for 2 h.

11 nm). It is unclear why the synthesized iron oxide showed a crystallographically amorphous nature, although HR-TEM images and the SAED pattern clearly reveal a partially crystalline nature, as shown in Figure 3D. However, decomposition at the same temperature under open atmospheric conditions results in a macroporous structure with a surface area of 55 m2/g and pore size of 20 nm. It should be mentioned here that the degree of crystallization of an iron oxide prepared under nitrogen at 400 °C is higher than that of one synthesized at low temperature (200 °C). Despite the varying degrees of crystallization, this research provides an opportunity to synthesize crystallographically amorphous iron oxides with different surface properties. To understand the surface charges of the synthesized iron oxides under different experimental conditions, ζ-potential analysis was also performed. The points of zero charge (PZC) of the AMIOs are presented in Table 1. The results clearly indicate that there is no appreciable difference in the PZC values of the AMIOs prepared under different conditions. The variation in the PZC values of the synthesized AMIOs may be due to the varying degree of crystallization. These PZC values are close to those reported previously.34 The formation of iron oxides has also been confirmed by X-ray photoelectron spectroscopy (XPS) analysis. Figure 5 shows the XPS spectra of an AMIO prepared by thermal decomposition under an open atmosphere at 250 °C for 2 h. Figure 5a,b depicts XPS spectra of Fe 2p and O 1s, respectively. The Fe 2p spectrum indicates the existence of the doublet Fe 2p3/2 and Fe 2p1/2, with binding energies of 711.4 and 725 eV, respectively. The corresponding satellite peak noted at 718.4 eV, a result of charge-transfer screening, can be solely attributed to the presence of the Fe3+ ions of Fe2O3. A high-resolution O 1s spectrum is shown in Figure 5b. The peak at 531.8 eV can be mainly attributed to the Fe-O in Fe2O3. We have also examined the XPS spectra of an AMIO formed at 200 °C for 99 h, and the results are quite consistent with those discussed above, as shown in Figure S6 (Supporting Information). XPS results also confirmed the formation of Fe2O3, which is in good agreement with the earlier reports for Fe2O3.35,36 Because we did not use any templates or structure-directing agents for the fabrication of either ferric oxalates or iron oxide during the decomposition process, AMIOs could form by selfassembly of initially formed nanoparticles. Self-assembled Fe2O3

Figure 5. XPS spectra of a mesoporous iron oxide synthesized under open atmospheric conditions at 250 °C for 2 h: (a) Fe 2p spectrum and (b) O 1s spectrum.

fabrications have been observed in earlier studies.37,38 Recently, we reported the fabrication of self-assembled mesoporous ZnS without using any templates or matrices.39 It should be discussed here why mesoporous iron oxide is not formed if we used ferric chloride instead of ferric nitrate as the iron precursor. Earlier studies also confirmed iron precursor-selective Fe2O3 formation using oxalic acid as a coordination reagent.22,23 The ferric oxalate complex formed after thermal evaporation from ferric nitrate-oxalic acid had an emerald green color, which results from the formation of the iron(III) oxalate complex. However, the complex formed from ferric chloride-oxalic acid had a yellowgreen color, which is quite different from that of the ferric nitrate-oxalic acid mentioned earlier. Similarly, the thermal decomposition pattern of the ferric oxalate formed from ferric chloride-oxalic acid is quite different from that presented in Figure 1. Therefore, the synthetic method of the iron oxalate complex is a very important step for the synthesis of AMIOs. We did not discuss the details of how the iron(III) oxalate preparation method influences the morphology and surface structure of the AMIO. Thus, such an investigation is not within the scope of this paper and will be addressed in our future publication.

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Figure 6. FESEM pictures of amorphous iron oxides prepared at (A) 200 °C for 2 h, (B) 200 °C for 50 h, (C) 200 °C for 200 h, (D) 225 °C for 2 h, (E) 250 °C for 2 h, (F) 275 °C for 2 h, and (G) 300 °C for 2 h.

Finally, we have to discuss why the synthesized AMIO is stable. On the basis of the experimental results, we speculate that the morphology of the AMIO could play an important role in prevention of crystallization of the amorphous phase to give the thermodynamically stable hematite phase. It should be mentioned here that AMIOs prepared under all conditions possess both aggregate and microbundle morphologies. However, time- and temperature-dependent morphological evaluations indicated that increasing the calcination times from 2 to 200 h at 200 °C increased the proportion of aggregate to bundleshaped iron oxides, as evident from the FE-SEM images shown in Figure 6. Similarly, increasing the decomposition temperature from 200 to 300 °C increases the proportion of aggregates to bundles, as shown in Figure 6. Therefore, the aggregate-intobundle formation process may compete with the crystallization process (amorphous to hematite). We also noted a similar selfassembly of iron oxides under nitrogen atmospheric conditions, which indicated that the self-assembly process depended on the temperature. Therefore, at low temperatures (below 300 °C), calcinations may favor self-assembly processes rather than crystallization of amorphous iron oxides into hematite. Moreover, as discussed previously, at 300 °C, the microbundles possess macroporous surface structures and have a crystalline hematite. These results indicate that crystallization of AMIOs results in macroporous hematite. However, such a conversion

required a minimum of 300 °C and an air (oxygen) atmosphere. Therefore, the synthesized AMIO is stable below 300 °C. To support the proposed mechanism, decompositions were also done using a commercial iron(III) oxalate hexahydrate sample. Decomposition under similar experimental conditions (200 °C) showed that there is no microbundle formation, as shown in the FESEM picture in Figure S7 (Supporting Information). These results clearly indicated that the synthesized iron(III) oxalate is quite different from commercially available materials. The magnetic properties of the synthesized AMIOs were studied using a quantum-designed physical property measurement system (PPMS) instrument. Among the various iron oxides prepared under different experimental conditions, the iron oxides synthesized at 200 and 250 °C for 2 h were selected for analysis. Initially, the zero-field-cooled curve (ZFC) measurements were done by cooling the samples to 5 K in a zero field and then raising the field to 10 mT. The magnetic moment was recorded by warming both the samples (warming cycle), and the fieldcooled curve (FC) measurement was done directly afterward by cooling the samples in the 10 mT field (cooling cycle). Figure 7 shows the results of temperature-dependent magnetization of AMIOs. At 100 Oe magnetic fields, ZFC and FC curves of both iron oxides are irreversible below the irreversible temperature, Tirr = 300 K. Moreover, the blocking temperatures, TB, defined as the temperature at the maximum of the ZFC curve, of the



Figure 7. Temperature-dependent magnetization of amorphous iron oxides synthesized (A) at 250 °C for 2 h and (B) at 200 °C for 2 h.

AMIOs prepared at 250 and 200 °C were found to be 94 and 110 K, respectively. At the ZFC condition, the magnetization rapidly increases below Tb, and with a further increase in temperature, the magnetization decreases. In the case of field cooling, above Tb, the magnetization considerably decreases with increasing temperature. A similar ZFC-FC behavior for γ-Fe2O3 NPs with a 12.4 nm average particle diameter has been previously reported.40 The hysteresis loops were measured at 5 and 300 K. Figure 8 shows the magnetization (M) as a function of applied magnetic field (H) for both of the AMIOs mentioned above, and the results indicate that both AMIOs possess superparamagnetic properties. These results clearly indicate that amorphous iron oxides prepared at various conditions possess similar magnetic behavior. Thus, at 5 K, the magnetization values for the two AMIOs are similar, and the maximum magnetization is found to be 26 emu/g. However, at room temperature, the maximum magnetization values for AMIOs prepared at 200 and 250 °C are found to be 21 and 20 emu/g, respectively. These results clearly indicate that, at room temperature, the values of magnetization of both AMIOs were lower than they were at low temperature (5 K). The magnetization values of the AMIOs were compared with those of amorphous iron oxides reported earlier. Cao et al. studied the magnetization of pure amorphous Fe2O3 nanoparticles at room temperature, and the value was found to be 1.44 emu/g.41 The results clearly showed that the AMIOs possess higher magnetization values than the pure amorphous iron oxides. Adsorption is generally an attractive method to remove heavy metals from an economic point of view.42 Therefore, developing low-cost adsorbents has been a current topic of interest.43,44 Moreover, the most promising and widely used adsorbents are iron or iron compounds, such as iron oxides, oxyhydroxides, and hydroxides, because they have a high adsorption capacity at a low cost and are nontoxic in nature.45 Because of the presence of both the mesoporous structure and the high surface

Figure 8. M-H curves of iron oxides prepared at (a) 250 °C for 2 h and (b) 200 °C for 2 h (insets show the close view of the hysteresis loops).

area of the AMIOs, they are expected to have excellent adsorption properties for heavy metal removal. We have examined the ability to remove both As(V) and Cr(VI) using the various synthesized AMIOs as adsorbents. The adsorption isotherms for both As(V) and Cr(VI) at pH 2 are shown in Figure 9. The adsorption studies clearly revealed that the synthesized AMIOs are potential adsorbents. The maximum efficiency of removal of Cr(VI) was found to be 24 mg/g and of As(V) was found to be 81 mg/g. The AMIO formed at 250 °C for 2 h showed maximum removal efficiency, followed by that prepared at 225 °C. Thus, on the basis of the experimental results on adsorption and the surface properties of the AMIOs, it was concluded that AMIOs possessing a high pore volume and pore size have high adsorption removal efficiencies. For the aforementioned reason, the high surface area iron oxide formed at 225 °C (264 m2/g) showed lower efficiency than that formed at 250 °C, as mentioned in Figure 9. Iron dissolution was also investigated, and the ICP analysis showed that about 0.65 mg/L of iron leaching is observed in As(V) adsorption experiments, and 30.12 mg/L of iron leaching is noted in Cr(VI) adsorption studies. These results confirmed the stability of the

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Figure 10. (A) Dispersion of AMIOs in water and (B) magnetic separation of iron oxides by using permanent magnet.

than those of 132 other adsorbents.46 Similarly, among the 53 adsorbents listed in another review, the Cr(VI) removal rate is higher than those of 30 other adsorbents.47 Cr(VI) removal efficiencies of various adsorbents under different conditions of pH and temperature have also been tabulated.48 Among the 270 adsorption capacities reported under different conditions, the synthesized AMIO gives better adsorption than that seen at 141 of the conditions listed in the review. In conclusion, the synthesized AMIO is a good potential candidate for heavy metal removal from wastewater. 4. Conclusions

Figure 9. Adsorption isotherm of (a) As (V) and (b) Cr (VI) removal by using AMIOs at pH 2 (A) 200 °C for 2h, (B) 200 °C for 12 h, (C) 225 °C for 2 h, (D) 250 °C for 2 h, (E) 275 °C for 2 h.

synthesized AMIOs. We have also examined the stability of the AMIO (prepared at 250 °C for 2 h) in water at higher temperature. The As(V) adsorption studies were performed at 22, 35, and 55 °C, and the iron leaching was analyzed by ICP. There was no significant iron leaching noted (0.65 mg/L) at all three temperatures, which clearly indicated the stability of AMIO catalysts in water at higher temperature. The high adsorption rates of the synthesized AMIOs are due to the mesoporous structure, high surface area, and pore volume, which are expected to substantially enhance the adsorption of heavy metals. The adsorption removal efficiency of the AMIOs was compared with commercially available hematite. Under similar experimental conditions, hematite showed 3.4 and 5.1 mg/L of Cr(VI) and As(V), respectively. The lower efficiency of hematite may be due to its low surface area (15 m2/g). The magnetic AMIOs can be dispersed in water to form a suspension (Figure 10A) that gradually settles under gravity but can be immediately separated using a magnet, as shown in Figure 10B. Therefore, the synthesized magnetic adsorbents can be easily separated from wastewater. Given the various mechanisms that have been proposed for heavy metal adsorption onto adsorbents, an attempt has been made to compare the heavy metal removal efficiency of the synthesized AMIOs with efficiencies reported in the literature. A recent review examined the As(V) removal efficiency of various adsorbents under different experimental conditions by Mohan et al.46 Among the 146 adsorbents listed in the review, the As(V) adsorption capacity of synthesized AMIO is higher

We have achieved the large-scale fabrication of AMIOs using thermal decomposition of a ferric oxalate complex without using any templates. The XRD pattern confirms the formation of crystallographically amorphous iron oxide. The AMIO is stable even with prolonged calcinations at low temperature, and the surface properties can be controlled by changing either decomposition times or temperatures. The XPS and EDX analyses confirmed the formation of Fe2O3 as the decomposition product. The influences of decomposition temperature, type of iron precursor, and decomposition time on the formation and surface properties of AMIOs were investigated. The FTIR spectra confirmed the formation of iron oxalates, and the Raman spectra indicated the presence of the hematite phase along with the amorphous phase. The iron oxides possess superparamagnetic behavior, and the magnetization value was found to be around 20 emu/g. Adsorption experiments indicated that the Cr(VI) and As(V) removal efficiencies were 24 and 81 mg/g, respectively. These results showed that the synthesized AMIO is a potential adsorbent for heavy metal removal. A self-assembly process could be competing with the crystallization process during prolonged calcinations, which seems to eventually prevent crystallization of amorphous iron oxide to the thermodynamically stable hematite. Acknowledgment. An EU Transfer of Knowledge Marie Curie Grant MKTD-CT-2006-042637 is acknowledged for financial support of the study. Supporting Information Available: The XRD pattern, Raman and FTIR spectra, nitrogen adsorption isotherm, and TEM and XPS analyses of the AMIOs prepared under various experimental conditions are presented. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Cao, H.; Suib, S. L. J. Am. Chem. Soc. 1994, 116, 5334–5342. (2) Curry-Hyde, H. E.; Musch, H.; Baiker, A. Appl. Catal. 1990, 65, 211–223. (3) Murawski, L.; Chang, C. H.; Mackenzie, J. D. J. Non-Cryst. Solids 1979, 32, 91–104.

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