Abstract
Very fine nanosized metal oxide, namely, iron oxide (α-Fe2O3) has been synthesized by precipitation method using ammonia as precipitating agent and characterized by using XRD (X-ray diffraction), TGA/DTA, surface area measurement, transmission electron microscopy (TEM), and magnetic measurements techniques. XRD studies show that iron oxide was formed as α-Fe2O3 instead of the commonly formed magnetite nanoparticles (Fe3O4) or a mixture of magnetite (Fe3O4) and maghemite (γ-Fe2O3, cubic), and it has rhombohedral structure. Magnetic measurements showed that iron oxide has five unpaired electrons and is ferromagnetic in nature, Ms value being 1.7 emu/g. The particle size of the synthesized iron oxide was determined by TEM. TEM images show that the size of particles of Fe2O3 varied from 15 nm to 49 nm with average crystallite size 35 nm.
1. Introduction
Transition metal oxides have many applications as catalysts [1–5], sensors [6–9], superconductors [10, 11], and adsorbents [12, 13]. Metal oxides constitute an important class of materials that are involved in environmental science, electrochemistry, biology, chemical sensors, magnetism, and other fields. One of their most important applications is heterogeneous catalysis. Iron oxides belong to the most abundant minerals and occur with a large variety of stoichiometries, structures, and properties. The more important ones are FeO (wustite), -Fe2O3 (maghemite), -Fe2O3 (hematite), and Fe3O4 (magnetite) with rock-salt, vacancy rich inverse spinel, corundum, and inverse spinel structures, respectively; the two former ones being thermodynamically less favorable and -Fe2O3 being the most oxidized one. Iron oxides are widely used in industry as catalysts or catalyst supports. Nanosized iron oxide particles within various ordered mesoporous silicas (SBA-15, SBA-16, Fm3m, and Ia3d) have been prepared from the corresponding nitrate and acetylacetonate precursors and studied for their catalytic behavior in methanol decomposition [14]. Nanosized catalyst (iron oxide) into the pores of a mesoporous material (titania) has been deposited using ultrasound radiation and the resulting catalyst is used for the oxidation of cyclohexane under mild conditions [15]. Nanosized iron and mixed iron-cobalt oxides supported on activated carbon materials and their bulk analogues have been synthesized and their catalytic behavior in methanol decomposition to H2, CO, and methane is tested [16]. A series of nanosized gold/iron-oxide catalysts has been prepared and tested for CO oxidation [17]. Catalytic oxidation of PCDD/Fs (polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans) with ozone (catalytic ozonation) over nanosized iron oxides (denoted as ) has been carried out at temperature of 120–180°C [18]. Iron oxide nanoparticles (IOnPs) as solid catalyst were prepared using a biotic method, that is, biomineralization, and abiotic methods, that is, thermal decomposition and electrochemical methods, for use as solid catalysts in the heterogeneous catalytic ozonation of para-chlorobenzoic acid (pCBA) [19]. Nanosized iron oxide has applications in waste water treatment and as sensors. Role of nanosized colloidal iron oxides in microbial iron reduction has been studied [20]. Implications of heat treatment on the properties of a magnetic iron oxide-titanium dioxide photocatalyst have been studied [21]. Nanosized iron oxide particles were intercalated into the interlayer of layered compounds HTiNb(Ta)O5 by a successive ion-exchange reaction and studied for photocatalytic water splitting [22]. Adsorption and desorption properties of arsenate [As (V)] on nano-sized iron-oxide-coated quartz (IOCQ) through batch experiments were conducted to investigate the coating of nano-sized iron oxide on the quartz surface using the heat treatment process which aimed to utilize the adsorption properties of the nano-sized iron oxide and the filtration properties of the quartz [23]. Removal of phosphate from solution using nanosized FeOOH-modified anion resin was studied in fixed bed column. Effect of bed height and flow rate on the breakthrough curves was investigated [24]. Nanosized particles of -Fe2O3 in the range of 17–64 nm were synthesized and were used for LPG sensing [25]. A simple and reproducible method to obtain TiO2 and Fe2O3 mixed oxide thin films by reactive RF sputtering has been presented and investigated for the gas sensing properties toward CO [26]. Different methods have been used for the synthesis of nanosized iron oxide nanoparticles. The raw material Lauha churna (iron filings) that has been taken as raw material for the synthesis of nanosized iron oxide and phase transformation from - to -phase has been studied [27]. Highly crystallized iron oxide nanorods have been fabricated by hydrothermal synthesis in the cavity of carbon-coated nanochannels with a diameter of 25 nm [28]. Nanosized iron oxide powder with average crystallite sizes 35, 100 and 150 nm was prepared by thermal evaporation and coprecipitation techniques and tested as catalyst for the photocatalytic decomposition of Congo red dye [29]. Nanosized iron oxides have considerable attention due to their unique magnetic properties (superparamagnetism, high coercivity, low curie temperature, high magnetic susceptibility, nontoxicity, biocompatibility, and low cost of production), which allow their usage in various nanotechnology applications in a broad range of disciplines. Magnetic nanoparticles are also important in biomedical applications, for example, magnetic bioseparation [30], magnetic target drug delivery [31], hyperthermia [32], magnetic resonance imaging [33], and magnetofection [34]. In the present paper we have synthesized -Fe2O3 nanoparticles by simple aqueous precipitation using ammonia as precipitating agent and their magnetic properties have been studied. This method involves a simple, cheap, and one-step process for synthesis of Fe2O3 nanoparticles. Iron oxide was formed as -Fe2O3 instead of the commonly formed magnetite nanoparticles (Fe3O4) or a mixture of magnetite (Fe3O4) and maghemite (-Fe2O3, cubic). The obtained particles of Fe2O3 have size from 15 to 42 nm. The synthesized nanoparticles were characterized by XRD, TGA/DTA, magnetic susceptibility, and TEM.
2. Methods and Materials
2.1. Chemicals
All chemicals used in the experiment are analytic reagent grade. Ferric nitrate, Fe(NO3)3, was purchased from Merck, India. Ammonium hydroxide (liquor ammonia) was purchased from SRL. Deionized water was used throughout the experiment.
2.2. Synthesis of Iron Oxide
500 mL of 0.1 M solution of Fe(NO3)3 was taken and aqueous ammonia was added dropwise with constant stirring until the pH of the solution reached 10. The precipitates thus obtained were filtered by Buckner funnel and were washed several times with distilled water. The precipitates were dried in oven at 70°C for 24 hrs and were calcined at 500°C in a muffle furnace for 5 hrs. Obtained material was ground and sieved through 100 mesh size sieve.
2.3. Characterization Techniques
The microstructure of the particles was characterized by X-ray diffraction (XRD), Philips PW 11/90 diffractometer using nickel filtered CuK ( Å) radiations. The average diameter () of the iron oxide nanocrystals has been calculated from the broadening of the XRD peak intensity after K corrections using the Debye-Scherrer equation. Transmission electron microscopy (TEM) measurements of the sample were taken on Hitachi H7500 with a 70 kV accelerating voltage. The dispersions of nanoparticles in water were placed on carbon-coated 400 mesh copper grids, allowed to dry at room temperature before taking measurement. The obtained micrographs were then examined for particle size and shape. The magnetic property of the solid was measured at 300 K using a Vibrating sample Magnetometer Model 155. TGA/DTA studies were carried out using Perkin Elmer Pyris Diamond. The BET surface area of the samples was measured by nitrogen adsorption isotherms on micromeritics ASAP 2010 (UK).
3. Results and Discussions
3.1. X-Ray Studies
X-ray diffraction of synthesized oxide is shown in Figure 1. X-ray diffraction pattern of pure iron oxide indicated that iron oxide was in the form of -Fe2O3 (Figure 1). The X-ray diffraction plot, shown in Figure 1, shows peaks only due to -Fe2O3 and no peak is detected due to any other material or phase indicating a high degree of purity of the as-synthesized sample. The broadening of the X-ray diffraction lines, as seen in the figure, reflects the nanoparticle nature of the sample. In X-ray diffraction, some prominent peaks were considered and corresponding -values were compared with the standard [JCPDS file no. 85-0987] (Table 1). X-ray diffraction shows that metal oxide is pure -Fe2O3 having rhombohedral structure.
Sharpness of the peaks shows good crystal growth of the oxide particles. Average particle size () of the particles has been calculated from high intensity peak using the Debye-Scherrer equation where is the average crystallite size of the phase under investigation, is the Scherrer constant (0.89), is the wave length of X-ray beam used, is the full width at half maximum (FWHM) of diffraction (in radians), and is the Bragg’s angle.
The average crystallite size calculated is 35 nm which is in close agreement with the TEM results.
3.2. Magnetic Measurements
The magnetic moment for iron oxide was carried out at room temperature and was observed as 5.68 B.M. This value of magnetic moment supports the fact that the synthesized iron oxide is in the form of Fe2O3 with actual magnetic moment 5.92 B.M. This indicates the presence of 5 unpaired electrons in Fe2O3. Magnetic measurements were also carried out at temperatures ranging from 300 K to 100 K to determine the temperature of Morin transition. The results are shown in Figure 2(a) and have been reported in Table 2. VSM studies were carried out at 300 K to show hysteresis behavior of nanosized particles and it has been observed that Fe2O3 show ferromagnetic behavior in nanocrystalline form, Ms value being 1.7 emu/g (Figure 2(b)).
(a)
(b)
3.3. TGA/DTA Studies
TGA/DTA transition shows an endothermic peak at 364°C (Figure 3). It simply indicates that when FeO(OH) is heated, it takes an amount of energy and 1.5 water molecules are removed. So, for the formation of iron oxide temperature above 364°C is required.
3.4. Surface Area Measurement
The BET surface area of the samples was measured by nitrogen adsorption isotherms. Surface area of the metal oxide was 27 m2/g. Samples were activated at 473 K for 4 h prior to the measurement.
3.5. TEM Studies
TEM studies were carried out to find out exact particle size of synthesized Fe2O3. Figure 4 shows the TEM image of the synthesized Fe2O3 nanoparticles. TEM images show that Fe2O3 nanoparticles are having particle size in the range of 15 nm–49 nm (Figure 4). The size distribution histograms for nanoparticles provided their respective sizes as 29.8 ± 8.4 nm (Figure 4(a)), 30.6 ± 7.0 nm (Figure 4(b)), 26.4 ± 4.7 nm (Figure 4(c)), and 32.4 ± 6.6 nm (Figure 4(d)), respectively.
(a)
(b)
(c)
(d)
4. Conclusion
-Fe2O3 nanoparticles with rhombohedral structure are synthesized successfully by aqueous precipitation method using ammonia as precipitating agent. From TEM study, it is found that particles are with average size of 15–49 nm. Magnetic measurements show that Fe2O3 has five unpaired electrons. VSM studies show ferromagnetic behavior of synthesized oxides. XRD studies show that iron oxide was formed as -Fe2O3 instead of the commonly formed magnetite nanoparticles Fe3O4 or a mixture of magnetite and maghemite.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.