Phase Evolution and Growth of Iron Oxide Nanoparticles: Effect of

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Phase Evolution and Growth of Iron Oxide Nanoparticles: Effect of Hydrazine Addition During Sonication Debabrata Maiti,† Unnikrishnan Manju,# Srihari Velaga,§ and Parukuttyamma Sujatha Devi*,† †

Nano-Structured Materials Division, and #Materials Characterization Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata 700032, India § Indian Beamline at Photon Factory (BL 18B), Photon Factory, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan S Supporting Information *

ABSTRACT: The phase evolution of iron oxide was monitored by carefully controlling the addition of hydrazine monohydrate (N2H4·H2O) during ultrasonication. The manner in which hydrazine was added affected the hydrolysis of iron(III) nitrate resulting in two different phases of iron oxides such as maghemite (γ-Fe2O3) or goethite (α-FeOOH) as the major products. The formation of ferric hydroxide and Fe2+ during the addition of hydrazine monohydrate to iron salt solution at low pH was confirmed by structure analysis and 1,10-phenanthroline tests, respectively. Synchrotron X-ray diffraction analysis confirmed the formation of spherical maghemite (γ-Fe2O3) during the dropwise addition of hydrazine, whereas the formation of rod-shaped goethite (α-FeOOH) was confirmed by the instantaneous addition of hydrazine. Through these experiments, we were able to unequivocally establish the importance of hydrazine addition in controlling the phase formation and growth of iron oxide nanoparticles during sonication.

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INTRODUCTION Iron oxide (Fe2O3) exists in different oxides such as γ-Fe2O3 (maghemite), α-Fe2O3 (hematite), β-Fe2O3, δ-Fe2O3, η-Fe2O3, ε-Fe2O3, and oxyhydroxides such as α-FeOOH (goethite) and γ-FeOOH (lepidocrocite).1,2 Among the oxides, the semiconducting α-Fe2O3, commonly known as hematite, is the most stable oxide finding many useful applications as pigments and adsorbents,3,4 photocatalysts,5 and in solar energy conversion, lithium ion batteries, and sensors.6−8 The other most commonly explored oxide is γ-Fe2O3, known as maghemite, which is one of the most extensively explored magnetic oxides for applications in magnetic recording and storage,8,9 magnetooptical device,10−12 magnetic refrigeration,13 magnetic resonance imaging,14−16 controlled drug delivey,15−19 cell targeting,20 and magnetic hyperthermia.21−23 The ferromagnetic, biocompatible γ-Fe2O3 is our choice of magnetic oxide for developing multifunctional oxide for drug delivery applications. A large number of chemical routes are available for the preparation of γ-Fe2O3 nanoparticles such as the sol−gel method,24,25 microemulsion method,26 combustion process,27 and microwave-assisted synthesis.28 Recently, the sonochemical method has emerged as a highly competent and comparatively better method for the preparation of magnetic nanoparticles.29−33 Through ultrasonic irradiation, the reaction rate could be enhanced making the reaction at low temperatures due to “acoustic cavitation”, which causes the formation, growth, and rapid collapse of bubbles in a liquid.34 In this work, we have used the sonochemical method to prepare nano© XXXX American Chemical Society

particles of iron oxides starting from ferric nitrate as a source of iron and hydrazine monohydrate as a reducing agent. Hydrazine was used due to its strong reducing power, ability to scavenge oxygen, low cost, and capability to control impurities in the final product, since hydrazine decomposes to N2 and H2O under normal conditions.35,36 Though there are many articles on the synthesis of iron oxide nanoparticles using hydrazine,37−39 there is, still, a lack of information on the phase evolution of such nanoparticles at various stages of addition of hydrazine during synthesis. Therefore, the main objective of the present work was to optimize the experimental conditions pertaining to the formation of iron oxide nanoparticles from an aqueous solution of ferric nitrate and hydrazine hydrate under sonication without any stabilizing or capping agent at room temperature. The manner in which hydrazine was added during sonication was a very crucial step in controlling the formation of the final product. The formation of maghemite (γ-Fe 2O 3 ) was confirmed by dropwise addition of hydrazine to the ferric nitrate solution, whereas goethite (α-FeOOH) was formed by simultaneous addition of hydrazine to Fe(NO3)3 solution. Owing to the influence of various reaction parameters on the final product formation, it was difficult to understand and predict a reasonable reaction mechanism. By following the Received: April 25, 2013 Revised: June 16, 2013

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dx.doi.org/10.1021/cg400627c | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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carried out in a PHI 5000 Versaprobe II Scanning XPS microprobe manufactured by ULVAC-PHI, USA. The measurements were performed at room temperature and at a base pressure better than 6 × 10−10 mbar. All spectra were recorded with monochromatic Al Kα (hv = 1486.6 eV) radiations with a total resolution of about 0.7 eV and a beam size of 100 μm. The specific surface area of the calcined samples was measured by Brunauer−Emmett−Teller (BET) method on a Quantachrome Instrument (NOVA 4000 E series). UV−visible analysis was carried out on a Shimadzu UV−vis-NIR spectrometer UV-3600. Magnetic measurements have been carried out on a vibrating sample magnetometer (VSM), Lakeshore, model-7400, unit over ±1 T at room temperature.

different hydrazine addition steps and studying the products formed thereof, a probable mechanism for the formation of the products has been predicted.

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EXPERIMENTAL SECTION

Materials. Ferric nitrate nonahydrate [Fe(NO3)3·9H2O] (98%, Merck), hydrazine monohydrate (N2H4·H2O) (99 to 100%, Merck), ethanol (absolute for synthesis, Merck), 1,10-phenanthroline GR (Merck), and NaOH GR (Merck) have been used for the synthesis. Preparation of Iron Oxide and Oxyhydroxide Nanoparticles. Initially, 100 mL of 0.1 M [Fe(NO3)3·9H2O] solution was prepared. The solution was equally divided into two parts. Hydrazine hydrate was added dropwise to the first portion of the ferric nitrate solution, a red gelatinous precipitate was formed, and the pH became 3.30. Hydrazine hydrate was further added to it and sonicated (ultrasonic power 250 W, ultrasonic frequency 30 kHz and probe diameter 25 mm) for 15 min till the pH reached 3.76. A further addition of hydrazine hydrate and sonication for 15 min changed the solution to brownish black and the pH to 5.12. Hydrazine was added dropwise again to the brownish black solution and sonicated for 2 h till the pH reached 7.12. The flow rate used for the addition of hydrazine was 0.5 mL/s. The precipitate was washed with water followed by ethanol and centrifuged at 10 500 rpm. The centrifuged precipitate was dried at 70 °C for 24 h in a hot air oven and then ground using mortar and pestle, and the product was denoted as “A”. No red coloration was observed after adding 1,10-phenanthroline to the filtrate part, thus confirming the complete precipitation of Fe(III) present in the solution, and no formation of Fe(II) during hydrolysis. To the second part of the ferric nitrate solution, hydrazine that was required to raise the pH ∼ 7 was added instantaneously and sonicated for 2 h. The light yellow precipitate formed was collected as described above and denoted as “B”. In order to make sure that the hydrolysis of Fe(NO3)3 was completed in both cases, we have added NaOH to the filtrate, and no precipitation was noticed confirming complete hydrolysis after the addition of hydrazine. Charaterization of Iron Oxide Nanoparticles. The phase analysis of the synthesized iron oxide particles has been carried out by powder X-ray diffraction (XRD) analysis. The X-ray patterns were collected between 5° and 80° (2θ) in a Philips X-ray diffractometer with Cu Kα radition (λ = 1.5406 Å) at a 2θ scan rate of 2° /min. Synchrotron X-ray diffraction measurements were carried out with 14 KeV X-ray beam (λ = 0.883 Å) at BL-18B (Indian beamline), Photon Factory, KEK, Tsukuba, Japan. All the measurements were carried out in Bragg−Brentano geometry with a divergence slit (200 μm), an antiscattering slit (250 μm), and a receiving slit (200 μm). Wellgrounded powder samples were loaded on quartz glass plate with groove. Dwell time for each measurement was adjusted such that a good signal-to-noise ratio was obtained. High temperature measurements were carried out with Anton Paar DHS 1100 heat cell, where the sample temperature was ±0.5 °C of the set point during the measurements. Fourier transform-infrared (FT-IR) spectra have been measured between 4000 cm−1 and 400 cm−1 on a NICOLET 380 FTIR spectrometer. The pellet was prepared by mixing the asprepared powder in KBr in a ratio 1:4 by weight and compressing the mixture. Particle size and shape were studied by the transmission electron microscopy (TEM) using a Tecnai G2 30ST (FEI) transmission electron microscope operating at a voltage of 300 kV. The local crystallographic structure was studied by high resolution transmission electron microscopic (HRTEM) studies. Samples for TEM analysis were prepared by dispersing a small amount of the nanopowder in ethanol and sonicating for 20 min. One drop of a dilute suspension was dropped on the 300 mesh carbon coated copper grid and dried at room temperature. The thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) analysis were carried out on the as-prepared samples on a NETZSCH STA409C instrument to determine the decomposition nature of the precursor powder samples. The analysis was performed between room temperature and 900 °C at a heating rate of 10 °C/min in an air flow. X-ray photoemission spectroscopy (XPS) measurements were

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RESULTS AND DISCUSSION Effects of Hydrazine Addition Step on the Phase and Morphology of Nanoparticles. In order to identify the factors that facilitated the formation of nanoparticles, we have carried out several ways of adding hydrazine to the ferric nitrate solution during sonication. Finally, the dropwise addition of hydrazine monohydrate, N2H4·H2O to 0.1 M Fe(NO3)3 solution resulted in a brownish black precipitate at a pH of ∼7, whereas instantaneous addition of the same amount of hydrazine formed a light yellow precipitate. By analyzing the powder X-ray diffraction patterns collected from the powders (Figure 1), it was confirmed that the brownish black precipitate

Figure 1. The XRD patterns recorded on a lab diffractometer: (a) JCPDS file no. 39-1346 of γ-Fe2O3, (b) as-prepared γ-Fe2O3, (c) JCPDS file no. 81-0463 of α-FeOOH, (d) as-prepared α-FeOOH.

obtained by dropwise addition of hydrazine consisted mainly of maghemite (γ-Fe2O3) as the room temperature (RT) XRD pattern compared well with the JCPDS file (No. 39-1346) of γFe2O3, whereas the product formed by instantaneous addition of hydrazine confirmed the formation of goethite (α-FeOOH, JCPDS No. 81-0463) as the major phase (Figure 1d). The lattice parameters of goethite has been calculated to be a = 4.681 Å, b = 9.792 Å, and c = 3.024 Å, respectively. The major oxides of iron, namely, maghemite (γ-Fe2O3) and magnetite (Fe3O4), are both cubic inverse spinels and structurally very similar to one another. The distinct feature of maghemite is the presence of iron vacancies in the sublattice. The characteristic peaks for magnetite (Fe3O4) are (220), (311), (400), (422), (511), and (440). Maghemite (γ-Fe2O3) on the other hand contains all the peaks of magnetite with additional low intensity peaks of (110), (210), and (211), respectively, at lower 2Θ B



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heating at 300 °C, the entire product has converted to γ-Fe2O3 as evident from the XRD data collected at 300 °C (Figure 2c). On further heating, the γ-Fe2O3 converted to α-Fe2O3, and at 600 °C, the product was 100% α-Fe2O3 (Figure 2f). We have also ex-situ heated the as-prepared nanoparticles at various temperatures in a muffle furnace. Figure 3a clearly shows that

values. In order to confirm that the phase formed is Fe2O3 and not Fe3O4, a spot test was conducted initially with 1,10phenanthroline. Addition of 1,10-phenanthroline to an aqueous solution of γ-Fe2O3 did not give any red coloration confirming the absence of any Fe2+ in the prepared iron oxide nanoparticles. Since Fe3O4 is a mixture of FeO·Fe2O3 (i.e., Fe in both +2 and +3 states), this spot test indirectly confirmed the absence of Fe3O4 and the presence of γ-Fe2O3. However, in order to reconfirm the product formed as γ-Fe2O3 and not Fe3O4 as they are structurally very similar, we have carried out RT synchrotron measurements on the as-prepared samples (Figure 2).The XRD reflection from the as-prepared sample

Figure 3. The XRD patterns of the calcined samples of γ-Fe2O3 and αFeOOH nanoparticles: γ-Fe2O3 heated at (a) 350 °C, (b) 400 °C, and (c) α-FeOOH heated at 350 °C, respectively.

on external heating maghemite was stable up to 350 °C but at 400 °C it converted to α-Fe2O3 whereas α-FeOOH completely converted to α-Fe2O3 after calcination at 350 °C for 4 h. The phase transformation of γ-Fe2O3 and α-FeOOH was also confirmed by the color changes observed from brownish black to red and light yellow to red, respectively. Contrary to our work, an earlier work on hydrothermal synthesis of iron oxides reported the formation of goethite (α-FeOOH) and a mixture of goethite and magnetite (Fe3O4) during the addition of N2H4·H2O to iron salt solutions.37,38 In Figure 4, the FT-IR spectra of γ-Fe2O3 and α-FeOOH samples are presented. There are two common IR bands at 632 and 342−3450 cm−1, respectively, present in these samples. The band at 632 cm−1 was attributed to the Fe−O stretching

Figure 2. The synchrotron X-ray diffraction data (λ = 0.883 Å) collected at various temperatures (a) JCPDS card no. 39-1346 of γFe2O3, (b) as-prepared γ-Fe2O3, (c) as-prepared γ-Fe2O3 heated at 300 °C, (d) JCPDS card no. 33-0664 of α- Fe2O3, (e) as-prepared γ-Fe2O3 heated at 500 °C, (f) as prepared γ-Fe2O3 heated at 600 °C.

mainly consisted of γ-Fe 2O 3 with a lattice parameter corresponding to 8.335(2) Å. Though evidence of low intensity peaks was not very clear in the RT laboratory XRD data (Figure 1), they are evident in the synchrotron data. The strain analysis using the Williamson plot exhibited a microstrain of 0.00207, and a particle size of around 35 nm was estimated. In addition, the synchrotron XRD data clearly showed the presence of a small amount of α-FeOOH in the as-prepared sample which has not been identified in the lab XRD data (Figure 1b). Because of the large background noice, we could not accurately calculate the weight fractions of α-FeOOH, though it is roughly estimated to be below 5%. From the synchrotron XRD analysis, it has become imperative that the as-prepared nanoparticles contain also a small amount of α-FeOOH in addition to γFe2O3. We have carried out TGA/DSC analysis on the asprepared samples to evaluate their thermal stability and to understand the phase transition temperatures. The DSC study indicated a transition around 470 °C corresponding to the conversion of γ-Fe2O3 to α-Fe2O3 (Supporting Information, Figure S1). The TGA data indicated a weight loss of less than 3% corresponding to the conversion of α-FeOOH to γ-Fe2O3 (Supporting Information, Figure S2). This data further confirms that the amount of α-FeOOH present in the asprepared sample is very low. On the basis of the DSC study, during synchrotron measurements, we have in situ heated the samples above and below the transition temperature. On

Figure 4. The FTIR spectra of iron oxide nanoparticles prepared via sonication route (a) γ-Fe2O3 formed by dropwise addition of hydrazine and (b) α-FeOOH formed by instantaneous addition of hydrazine. C



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mode and the band at 3438 cm−1 was assigned to the vibrational stretching mode of surface O−H. The surface hydroxyl band was more intense and shifted to 3150 cm−1 for α-FeOOH due to intermolecular hydrogen bonding between αFeOOH and adsorbed water.40 The band at 3150 cm−1 strongly indicates the presence of α-FeOOH in our samples. The peak at 2359 cm−1 indicated the presence of adsorbed CO2 in both samples. All the samples had a very narrow peak at 1380−1390 cm−1 that can be assigned to NO¯3 from residual nitrate ions adsorbed on the as-prepared sample.41 The IR bands at 583 and 463 cm−1 are assigned to the FeO stretching and bending vibration mode, respectively, of iron oxide. These two peaks were completely absent for α-FeOOH. The peaks at 798 and 890 cm−1 (Figure 4b) can be assigned to δOH in plane bending and γOH out-of-plane bending of α-FeOOH, respectively.41,42 A weak band around 798 cm−1 is present on the spectrum of γFe2O3 (Figure 4a). In addition, a minor peak at 890 cm−1 is also present, both confirming the presence of a small amount of αFeOOH in the as-prepared γ-Fe2O3 sample. This observation strongly supports the synchrotron measurements confirming the presence of small amount of α-FeOOH in the as-prepared nanoparticles. On heating α-FeOOH at 350 °C, the bands at 798, 890, and 3150 cm−1 completely disappeared (Supporting Information, Figure S3) indicating the conversion of α-FeOOH to Fe2O3. In order to evaluate the size and shape of the prepared nanoparticles, we have carried out TEM studies on ethanol dispersion of the prepared nanoparticles. TEM analysis (Figure 5a−d) confirmed the presence of spherical particles with a

particles. The distance between adjacent lattice fringes measured as 2.55 Å in Figure 5b corresponds to the 311 reflection, and the one in Figure 5d corresponds to the 220 reflection of γ-Fe2O3. Well grown long rods are evident in Figure 6a, confirming the formation of rod-shaped α-FeOOH

Figure 6. The TEM-bright field (a and c) and HRTEM (b and d) images of α-FeOOH nanoparticles formed by the instantaneous addition of hydrazine under sonication.

during instantaneous addition of hydrazine hydrate with an aspect ratio of ∼14.14 (histogram in Supporting Information, Figure S5). The HRTEM image of a single rod (Figure 6b,d) shows well developed lattice fringes that have been extended throughout the rod, confirming the single crystalline nature of the individual rods. The distance between adjacent lattice fringes in Figure 6b measured as 4.19 Å corresponds to the 110 reflection and that measured as 2.40 Å (Figure 6d) corresponds to the 111 reflection of α-FeOOH. We have also performed X-ray photoelectron spectroscopy (XPS) studies investigating the Fe 2p and O 1s core levels to identify the oxidation states of iron present in γ-Fe2O3maghemite and α-FeOOH-geothite (Figure 7). It is well-known that the XPS spectra of iron oxides exhibit distinct “shakeup” satellite structures which are sensitive to the electronic structure of the compounds and hence could be used as fingerprints to identify the iron oxide phases. The Fe2p3/2 and Fe2p1/2 main peaks of both the systems are clearly accompanied by satellite structures on their higher binding energy side, at about 8 eV (Figure 7a). The binding energy of about 711.0 eV for the Fe2p3/2 main peak is consistent with typical values for γ-Fe2O3 reported previously.43 Moreover, the overall Fe 2p core level spectral shape and the binding energy positions of the samples investigated here match well with that reported for the maghemite and geothite phases of iron oxides in the literature with Fe in the 3+ state.44 It could also be observed that the binding energy position of Fe2p3/2 of goethite is shifted by about 0.5 eV to the higher energy side as compared to maghemite. The O 1s core level spectrum clearly shows the different environments/coordinations of oxygen in maghemite and goethite. The spectrum from maghemite is sharper, while

Figure 5. The TEM-bright field (a and c) and HRTEM from the corresponding images (b and d) of γ-Fe2O3 nanoparticles formed by the dropwise addition of hydrazine under sonication.

mean particle size of ∼21.19 nm (histogram in Supporting Information, Figure S4) with a mean deviation of ±0.20 in samples formed under dropwise addition of hydrazine (Figure 5a,c). The HRTEM shows (Figure 5b,d) well developed lattice fringes, and the fringes extend throughout the particle confirming the monocrystalline nature of the individual D



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Figure 7. X-ray photoemission spectra of γ-Fe2O3-maghemite and α-FeOOH−geothite nanoparticles. Panels (a) and (b) correspond to Fe 2p and O 1s core levels, respectively.

Figure 8. TEM-bright field, HRTEM, and SAED images of γ-Fe2O3 heated at 350 °C (a, b, and c, respectively) and α-Fe2O3 formed upon heating of α-FeOOH at 350 °C (d, e, and f, respectively).

UV−visible absorption study was carried out on the asprepared and annealed samples as shown in Figure 9 after dispersing them in ethanol. A strong well-defined band in the 360−390 nm with maxima around 372 nm for the γ-Fe2O3 and α-FeOOH corresponds to the 6A1 → 4E (4D) ligand field transition.46 The transition 6A1 → 4T2 (4D) occurs very close to 6 A1 → 4E (4D) transition with a similar energy, and hence in the observed spectra these transitions are not resolved. It is interesting to note the nearly similar visible to near-ultraviolet spectrum of maghemite and FeOOH phases even though the former contain some tetrahedrally coordinated Fe3+ along with octahedrally coordinated Fe3+. As such the bands attributable to Fe3+ in octahedral and tetrahedral coordination are not resolved in Figure 9. The spectrum of hematite (d) formed on heating (b) at 350 °C exhibits a broad absorption in the visible region, with a tail extending to 570 nm. The threshold of absorption at 570 nm (2.18 eV) is in agreement with the bulk 2.2 eV band gap value for α-Fe2O3.46 The magnetic measurements have been carried out at room temperature, and the data are shown in Figure 10, on the asprepared and heated γ-Fe2O3. From the recorded M−H loop, it

that from goethite is broader, suggesting the multiple components embedded in them which could be coming from oxide, hydroxide, and some adsorbed water that remains on the surface of the goethite as reported previously in the literature.45 In Figure 8a−f, the TEM images of heated samples are presented. It is clear that on heating at 350 °C, the well separated spherical particles of γ-Fe2O3 get agglomerated (Figure 8a) but with an improved crystallinity. In Figure 8b,c, the HRTEM image and the SAED pattern corresponding to the particles shown in Figure 8a are presented. The HRTEM corresponds to the 220 reflection of γ-Fe2O3 and in the SAED pattern, 220 and other higher order reflections of γ-Fe2O3 are indicated. The α-FeOOH, on the other hand, on heating retains the rod shape as clear from the image shown in Figure 8d. The HRTEM shown in Figure 8e corresponds to the 012 reflection of α-Fe2O3, and the SAED pattern shown in Figure 8f corresponds to the higher order reflections of α-Fe2O3. The surface area of the as-prepared γ-Fe2O3 sample was 43 m2/g, which decreased to 40 m2/g due to particle agglomeration on heating. The surface area of the as-prepared α-FeOOH was 38 m2/g, which decreased to 32 m2/g on heating at 350 °C. E



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under sonication. The probable mechanism of the formation of γ-Fe2O3 from Fe(NO3)3 during the dropwise addition of hydrazine is proposed below: Formation of γ-Fe2O3. The hydrazine, a clear, colorless, and very hydroscopic liquid at room temperature, has basic (alkali) chemical properties comparable to those of ammonia: N2H4 + H 2O → [N2H5]+ + OH−

(1)

−6

with a Kb = 1.3 × 10 . However, hydrazine is difficult to deprotonate like ammonia. [N2H5]+ + H 2O → [N2H6]2 + + OH− Kb = 8.4 × 10−16 (2)

Anhydrous hydrazine is a very strong reducing agent with typical byproducts as nitrogen gas and water. In addition, hydrazine is a very reactive molecule, which can decompose exothermically at an incredible speed. In aqueous media, Fe(NO3)3 remained as Fe3+ and the pH of the solution was approximately 1.90, i.e., the solution was acidic in nature.

Figure 9. UV−visible absorption spectra of (a) γ-Fe2O3, (b) αFeOOH, (c) a mixture of α-Fe2O3 and γ-Fe2O3 formed on heating (a) at 400 °C, and (d) α-Fe2O3 formed on heating (b) at 350 °C.

Fe(NO3)3 + H 2O → Fe3 + + H+ + OH− + 3NO3−

(3)

When N2H4 was added dropwise, a red gelatinous precipitate of Fe(OH)3 was formed. Hydrolysis of N2H4 in water produces hydrazinium (N2H5+) and OH− at room temperature as shown in eq 1. N2H5+ is a reducing agent and could reduce some of the Fe3+ to Fe2+ as per the reaction shown in eq 4. In the presence of OH−, Fe3+ could be precipitated as Fe(OH)3 as shown in eq 5.

Figure 10. Magnetic hysteresis curve of (a) as-prepared γ-Fe2O3 and (b) γ-Fe2O3 heated at 300 °C, and (c) the response of γ- Fe2O3 nanoparticles dispersed in water toward a bar magnet.

Fe3 + + N2H5+ + O2 → Fe2 + + N2 ↑ + 3H 2O

(4)

Fe3 + + 3OH− → Fe(OH)3 ↓

(5)

2+

At pH 5.12, Fe that was formed as per eq 4 transformed into geothite (α-FeOOH) in the presence of dissolved oxygen, and hydroxyl ions formed as a result of the interaction of hydrazine with atmospheric oxygen. The α-FeOOH thus formed could readily react with unreacted Fe(OH)3, resulting in a brownish black precipitate. At pH 7.2, there was no color change with 1,10-phenanthroline indicating the absence of Fe2+ in solution. Also, the complete precipitation of Fe3+ was confirmed upon addition of NaOH to the solution. The XRD analysis confirmed that the major component during the dropwise addition of hydrazine was γ-Fe2O3. Hence, it could be concluded that at pH ∼ 7, all of Fe3+ has been converted to γFe2O3 upon dropwise addition of N2H4. The formation of γFe2O3 started at pH 5 and continued up to pH ∼ 7. The reaction of Fe2+ with Fe (OH)3 could also have resulted in the formation of Fe3O4. But our experimental evidence did not indicate the formation of any Fe3O4 in any stage of the reactions.

is clear that both the as-prepared and heated samples are ferromagnetic at RT. The saturation magnetization of the asprepared γ-Fe2O3 was 52 emu/g (Figure 10a), and it increased to a value of 65 emu/g (Figure 10b) on heating at 300 °C. The coercivity values of the as-prepared and calcined γ-Fe2O3 were 133.9 and 56.7 Oe, respectively. The low magnetization value exhibited by the as-prepared sample compared to the heated one could be due to the presence of impurities/defects in the as-prepared γ-Fe2O3. On heating, due to the existence of 100% γ-Fe2O3, we observed a substantial increase in the saturation magnetization value. Though this value is higher than the RT processed sample, it is still much lower than the reported bulk value 80−90 emu/g of γ-Fe2O3.47−49 The ferrimagnetism in γFe2O3 arises from the exchange coupling between different number of spins in the two sublattices. The reduction in the saturation magnetization value could be due to the surface defects arising from finite size effects which could lead to a disordered spin configuration near the surface and reduce the average net moment relative to the bulk material. Under 15 nm, γ-Fe2O3 is reported to exhibit superparamagnetism, which is essential for using it for drug delivery applications.50 Since the size of γ-Fe2O3 nanoparticles synthesized by us lies in the range of 20−30 nm, they are still ferromagnetic.The response of γFe2O3 nanoparticles dispersed in water toward a bar magnet is shown in the inset of Figure 10 as (c). Probable Mechanism of Phase Formation. From the experimental results, it is evident that the way hydrazine was added turned out to be a crucial step in determining the phase formation during the reaction of ferric nitrate and hydrazine

2Fe2 + + O2 + 2OH− → 2(α‐FeOOH)

(6)

α‐FeOOH + Fe(OH)3 → γ‐Fe2O3 + 2H 2O

(7)

Formation of α-FeOOH. Instantaneous addition of hydrazine on the other hand resulted in the formation of αFeOOH as confirmed through various characterization techniques. When the same amount of N2H4 (i.e., N2H4 required to raise pH ∼ 7 during dropwise addition) was added instantaneously to Fe3+, the entire amount of Fe3+ ion gets reduced to Fe2+ (eq 4) and gets converted into goethite as F



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Figure 11. Schematic of proposed synthetic mechanism of the formation of γ-Fe2O3 and α-FeOOH.

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per eq 6, and it became the final product during instantaneous addition. In either case, on heating, both the products get converted to α-Fe2O3. The schematic representation of the various steps involved in the formation of γ-Fe2O3 or αFeOOH are shown in Figure 11. Wiogo et al. reported the formation of a mixture of goethite and magnetite by reduction of Fe(III) chloride by hydrazine during hydrothermal synthesis under highly alkaline conditions.37 Bahadur et al., on the other hand, reported the formation of amorphous iron oxide nanoparticles during sonication of iron(III) citrate with NaOH at pH 10 or more.51 Unlike the report of Ray et al.,38 where the importance of high power ultrasonic has been shown to be the key factor to induce the formation of γ-Fe2O3, through a series of experimental conditions and analytical tools, we were able to demonstrate that the process of hydrazine addition is the most important factor in controlling the phase formation and growth of nanoparticles of iron oxide or oxyhydroxide.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1−S5, DSC curve of as-prepared γ-Fe2O3, TGA curve of as-prepared γ-Fe2O3, FT-IR spectra of α-Fe2O3 formed on heating γ-Fe2O3 at 400 °C and α-FeOOH at 350 °C, histograms for γ-Fe2O3 and α-FeOOH particles obtained from dropwise and instantaneous addition of N2H4, respectively. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Telephone: + 91 33 2483 8082. Fax: +91 33 2473 0957. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.S.D. thanks the Director, CSIR-Central Glass and Ceramic Research Institute (CGCRI) for partial financial support through the OLP project. D.M. is indebted to the Council of Scientific and Industrial Research (CSIR), Govt. of India, for the award of Junior Research Fellowship through the National Eligibility Test. This work was partially supported by CSIR Engineering Science Cluster program on BIOCERAM, ESC 0103. Experiments at BL-18B (Indian beamline), Photon Factory, KEK, Japan, were supported by the Indo-Japanese programme of Department of Science and Technology, Govt. of India, for P.S.D. P.S.D. acknowledges Ms. Kajari Das Gupta of the Electron Microscopy Section of CSIR-CGCRI for assistance in recording the TEM pictures and Mr. Ujjal Chowdhury of NSMD for help in magnetic measurements.

CONCLUSION

Maghemite (γ-Fe2O3) nanoparticles with a spherical shape have been formed by dropwise addition of hydrazine hydrate to ferric nitrate solution, whereas rod-shaped goethite (αFeOOH) has been formed by the instantaneous addition of hydrazine hydrate to the same initial solution during sonication. The synchrotron X-ray diffraction and XPS measurements corroborate the above findings. On heating above 350 °C, the maghemite retained its phase and shape, but goethite completely converted into hematite. Through these experiments, we were able to unequivocally establish the importance of hydrazine addition in controlling the phase formation and growth of iron oxide nanoparticles during sonication. The formation of spherical superparamagnetic maghemite nanoparticles with a uniform size between 10 and 15 nm is under investigation for drug delivery applications.

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ABBREVIATIONS XRD, X-ray diffraction; FT-IR, Fourier transform-infrared; TEM, transmission electron microscopy; HRTEM, high G



Article

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resolution transmission electron microscopy; TGA, thermogravimetric analysis; DSC, differential scanning calorimetry; XPS, X-ray photoelectron spectroscopy; BET, Brunauer− Emmett−Teller; VSM, vibrating sample magnetometer

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Phase Evolution and Growth of Iron Oxide Nanoparticles: Effect of

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