Appl Phys A (2009) 95: 373–380 DOI 10.1007/s00339-008-5068-z
Synthesis and studies of growth kinetics of monodispersed iron oxide nanoparticles using ferrocene as novel precursor G.M. Bhalerao · A.K. Sinha · Himanshu Srivastava · A.K. Srivastava
Received: 26 September 2008 / Accepted: 15 December 2008 / Published online: 9 January 2009 © Springer-Verlag 2009
Abstract We have used ferrocene and paraffin wax as novel precursor and solvent for the growth of iron oxide nanoparticles. The proposed method of growth has several advantages over existing methods of growth using iron pentacarbonyl a precursor. Highly crystalline and monodispersed particles are obtained which assemble in two- and three-dimensional hexagonal closed packed superlattices. Growth kinetics has been studied by varying concentration of the precursor and time of growth. A phenomenological model has been proposed to explain the growth kinetics. PACS 81.07.-b · 81.07.Bc · 81.16.Be · 68.37.Lp
1 Introduction Iron oxide particles are being used in various applications like magnetic memories [1], ferro-fluids [2], industrial catalyst [3, 4], bio-medical application [5] and in carbon nanotube synthesis [6, 7]. For catalysis, iron oxide is used as precursor to form iron, which has catalytic properties but cannot be stored because of its high reactivity with oxygen. Size dependent properties are exhibited when the size of the particles is reduced to few tens of nanometers or lower. Control on the diameter and diameter distribution of nanoparticles is required to control the properties. Thermal decomposition method is an efficient route to synthesize iron oxide nanoparticles of controlled diameter and diameter distribution [8–10]. In this method, an G.M. Bhalerao · A.K. Sinha () · H. Srivastava · A.K. Srivastava Indus Synchrotrons Utilization Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India e-mail:
[email protected] Fax: +91-731-2442125
iron precursor is thermally decomposed in a mixture of organic solvent and surfactant. There are many reports of synthesis of iron oxide nanoparticles of controlled diameter using iron pentacarbonyl (IPC) as the precursor. Although use of IPC gives good control on the nanoparticle size and distribution, it is highly volatile, toxic and dangerous to handle. Due to these constraints, supply of IPC to small laboratories is prohibited. Therefore, there is a need to search for an alternate precursor to synthesize iron/iron oxide nanoparticles of controlled size and with a narrow distribution. Ferrocene Fe(C5 H5 )2 , which is far less dangerous and more ecofriendly [11], is also easily available. In our previous work, it has been used for the first time, to grow iron oxide nanoparticles with a narrow diameter distribution [12]. But the very important issue of diameter control had remained unaddressed. In addition, identification of relevant growth parameters and studies on growth kinetics, which are unique to a given synthesis route, had to be carried out for understanding and better control of synthesis. In the present work, we report studies on growth kinetics of iron oxide nanoparticles, using ferrocene as a precursor and paraffin wax as solvent. In addition to being safe and ecofriendly, the use of ferrocene and wax has advantages that the reaction can be frozen at any time to study the kinetics of the growth of iron oxide nanoparticles and the crystallinity of nanoparticles is better because of considerably high synthesis temperature. Effects of precursor concentration and reaction time on size and size distribution of the nanoparticles have been investigated. We show that the growth kinetics of ferrocene route is significantly different from IPC route. We identify that by varying the reaction time, we can control the size of the nanoparticles.
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2 Experiment 2.1 Synthesis Thermal decomposition of ferrocene requires temperature >400°C [12]. This temperature cannot be achieved by using conventional organic solvents, e.g. octyl ether or kerosene [8–10], because of their low boiling point. Therefore, we use paraffin wax as solvent, which boils at ∼400°C. Oleic acid was used as a surfactant. Ferrocene dissolved in a mixture of surfactant and solvent was refluxed up to the completion of reaction. During reflux, ferrocene sublimates and appears as vapors of yellow-orange color. We identify two indicators to determine the progress of reaction. The first indicator is the absence of ferrocene in the refluxing vapors, which is indicative of conversion of ferrocene into a complex (unidentified compound) with oleic acid, in solution form. We observe that the duration at which this indicator appears is longer for higher concentration of ferrocene. At this stage, the solution becomes dark brown, but remains clear. The second indication is the appearance of turbidity in the solution. Turbidity appears because the solid nanoparticles scatter light. Appearance of the second indicator was used to decide the completion of growth process. Reaction was brought to halt by slow natural cooling of the reaction mixture, or by fast cooling in water (20°C). Ferrocene-to-oleic acid mole ratio was kept constant at ∼0.1 for all the experiments. Four samples, A, B, C and D, were prepared in order to investigate the effect of ferrocene concentration and duration of growth. Details of the parameters are given in Table 1, which is self-explanatory. Ferrocene of two limiting concentrations, viz. 0.01–0.02 g and 0.50 g, respectively, was decomposed in 5.0 g of paraffin wax. A lower ferrocene concentration results in nanoparticle concentration too low to separate from the solution. On the other hand, concentration higher than the maximum requires large amount of oleic acid, negating the role of paraffin wax. Duration of growth was varied between ∼15 and 65 min. It Table 1 Details of the nanoparticle samples. Amount of ferrocene is in 5.0 g of paraffin wax. A mole ratio of ferrocene to oleic acid is kept fixed at 0.1 for all the samples Sample
Ferrocene (g)
Growth duration (min)
Diameter (nm)
Phase
A
0.02
65
20 ± 2
γ -Fe2 O3
B
0.01
15
14 ± 4
γ -Fe2 O3
C
0.50
40
12 ± 2
γ -Fe2 O3
D
0.50
20
25 ± 2 Irregular flakes
Amorphous
should be noted that the completion of reaction and formation of nanoparticles was judged by the appearance of turbidity in the reaction mixture and was found to be strongly dependent on concentration of the precursor. Typically it was found to be about 12–15 min for a low concentration (samples A and B) and about 25–30 min for a high concentration (sample C). Sample D was prepared with the aim to freeze the reaction at an early stage, prior to nucleation of nanoparticles in the solution. The reaction was stopped at a moment between the two indicators of growth. 2.2 Purification No size-selective separation technique was used. The solidified samples A, B and C containing the nanoparticles were dissolved in toluene and the nanoparticles were separated using a permanent magnet. Solid state of paraffin wax makes separation of nanoparticles a tricky matter. High viscosity of molten wax makes the dispersion of nanoparticles very stable. Therefore, the solidified samples (1 part) were dissolved in toluene (4 parts), in order to reduce viscosity. Toluene, which is a very good solvent of nanoparticles as well, makes the separation still difficult. In order to reduce the solubility of nanoparticles, methanol, which is a polar solvent, was added (1–2 parts) in the above solution (4 parts). Now the nanoparticles were separated in the field of a permanent magnet. The separated nanoparticles were got rid of free solvent (wax) and surfactant by repeated washing with the mixture of toluene and methanol. It was difficult to purify the sample D (which contains amorphous material, as we would see later) with the procedure described above. This is because the ferromagnetic domain size in amorphous materials is infinitesimally small, as compared to the crystalline materials, and the magnetic force does not supersede the solvation forces (chemical forces, which hold the nanoparticles in solution). Therefore, a different approach has been adopted. The solidified sample D was dissolved in hot propanol. We observe that propanol dissolves the molten wax, but not the solid. Hence the temperature of this solution was kept more than the melting point of wax (58–60°C) but less than the boiling point of propanol (100°C). Propanol is a polar solvent, hence it reduces the solubility of the amorphous material. This mixture was then kept in the magnetic field applied from the bottom of the container, for several hours, until any appreciable precipitation was observed. The precipitate was repeatedly washed with hot propanol and then with methanol. The separated materials disperse well in toluene on ultrasonic agitation. The dispersion is stable for several days. Addition of ∼5% oleic acid as a surfactant, in toluene, increases the stability of dispersion to several weeks. The as-grown particles were characterized by x-ray diffraction
Synthesis and studies of growth kinetics of monodispersed iron oxide nanoparticles using ferrocene
(XRD), transmission electron microscopy (TEM), high resolution TEM (HRTEM) and energy dispersive x-ray spectroscopy (EDS). For XRD, solidified mixture of nanoparticles in wax was used. Nanoparticle solution in methanol with a typical concentration of ∼0.2 mg/ml was prepared by ultrasonic agitation. Formvar and carbon coated TEM copper grid was dip-coated in the solution and viewed in TEM CM200. For EDS characterizations, purified solid material was supported on a silicon wafer and examined in scanning electron microscope.
3 Characterization results 3.1 Size distribution of nanoparticles Figures 1(a), 1(b), 1(c) and 1(d) show the representative TEM images of samples A, B, C and D, respectively. The reaction mixtures for the samples A and B contain low concentration of ferrocene, whereas those for the samples C and D contain about 25–50 times higher concentration of ferrocene. Well separated round-shaped nanoparticles are observed in samples A, B and C. We measured diameter of individual nanoparticles from several such TEM images and the statistical analyses are plotted as histograms, in Fig. 2, (a), (b) and (c), respectively. The peaks observed in these histograms were fit with Gaussian peak profile, in order to find out the mean and spread of nanoparticle size. Diameters of the nanoparticles was found to be 20 ± 2 and 14 ± 4 nm Fig. 1 TEM images of nanoparticles obtained on thermal decomposition of ferrocene in paraffin wax for samples A (a), B (b), C (c) and D (d). Ferrocene-to-oleic acid mole ratio is 0.1 in all the samples
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for the samples A and B, respectively. Few nanoparticles of larger diameter, ∼25 nm, are also present in the sample B (see Figs. 1(b) and 2(b)). On the other hand, two peaks are seen in the diameter distribution of nanoparticles in the sample C (Fig. 2(c)). The dominant peak, accounting for about 65% of the total observed particles in TEM, shows diameter distribution to be 12 ± 2 nm. The second peak at 25 ± 4 nm accounts for remaining 35% of particles. Sample D, obtained by freezing the reaction before the reaction is complete, does not show any nanoparticles, as seen in the TEM image (Fig. 1(d)). It possesses flake-like deposition, identical to that of amorphous iron obtained on sonochemical synthesis [13]. On the TEM grid, most of the nanoparticles (sample A) form 3D hcp superlattices, as shown in Fig. 3. However, a few of the agglomerates are oriented in such a way that the basal planes are either exactly parallel or perpendicular to the electron beam path for the hcp superlattice to be clearly visible. There are many such 3D hcp superlattices, two of which are shown in Fig. 3, (a) and (b). Figure 3(a) is a view along the basal planes which are clearly resolved. From these images, the pitch () of the hcp structure was measured. The pitch is the distance between two adjacent (A and B) planes (Fig. 3(a)). Mean value of was found to be 19 nm. Value of in terms of the distance (a) between the centers of the two adjacent spheres √ is given by 6a/3, from geometrical considerations. Using this relationship, the value of a was calculated to be 23 nm. Mean diameter obtained from 2D assembly of nanoparticles
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in sample A (Fig. 1(a)) is found to be 20 ± 2 nm (Fig. 2(a)), in close agreement with 23 nm, estimated above from 3D assembly. The difference may be attributed to the gap between nanoparticles due to surfactant coating and also to the blurring of boundaries in the 3D structures. Figure 3(b) is a view along the principle c-axis of the superlattice. The figure
G.M. Bhalerao et al.
shows hexagonal arrangement in the basal planes. To guide the eyes, a hexagon is drawn in the figure, with its vertices superimposing the centers of the nanoparticles arranged in the hexagonal plane. Side of the hexagon equals the particle diameter, as illustrated in Fig. 3(b). A measured value of a (∼23 nm) is in agreement with the calculated value from the measured pitch () of the hcp superlattice. The excellent agreement in diameter of the nanoparticles calculated from the pitch of the superlattice and from the side of the hexagon shows that all the superlattices are made of nanoparticles, with a very lesser spread in their diameters. It has earlier been reported that nanoparticles arrange into hcp 2D and 3D superlattices, only if their size distribution is narrow enough (
