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DOI: 10.1039/C4RA11688G (Paper) RSC Adv., 2014, 4, 63946-63950

Sol–gel auto-combustion synthesis of PbFe12O19 using maltose as a novel reductant

Fatemeh Ansaria, Azam Sobhanib and Masoud Salavati-Niasari*a
aInstitute of Nano Science and Nano Technology, University of Kashan, P. O. Box. 87317-51167, Kashan, I. R. Iran. E-mail: salavati@kashanu.ac.ir; Fax: +98 361 5552935; Tel: +98 361 5912383
bDepartment of Chemistry, Kosar University of Bojnord, Bojnord, Islamic Republic of Iran

Received 2nd October 2014 , Accepted 19th November 2014

First published on 19th November 2014

Abstract

A new way of preparing lead hexaferrite (PbFe12O19) nanoplates and nanoparticles has been developed using the sol–gel auto-combustion route, without adding external surfactant. For the first time, maltose has been applied as reductant to form PbFe12O19. In this work, we have chosen lead nitrate and iron nitrate as starting reagents. The morphology, phase structure, composition and phase purity of PbFe12O19 can be controlled by adjusting the Pb/Fe and Pb[thin space (1/6-em)]:[thin space (1/6-em)]reductant ratios and also calcination temperature. When the molar ratio of Pb/Fe is 1[thin space (1/6-em)]:[thin space (1/6-em)]6 and calcination temperature is 900 °C, the product is pure and found to be PbFe12O19 nanoplates. The size and agglomeration of the nanoplates increase with decreasing maltose content. On decreasing the calcination temperature to 600 °C, nanoparticles with spherical morphology are obtained. The PbFe12O19 nanostructures are characterized using SEM, TEM, EDS and XRD. Products with a high coercive force of 5609 Oe are obtained when the Pb/maltose and Pb/Fe ratios are 1[thin space (1/6-em)]:[thin space (1/6-em)]26 and 1[thin space (1/6-em)]:[thin space (1/6-em)]6, respectively, and calcination temperature is 900 °C.

Introduction

Detailed reports of all the main hexaferrite phases were published by Philips Laboratories in the 1950s, culminating in Smit and Wijn's excellent book “Ferrites”, published in 1959.1 Since their discovery there has been an increasing degree of interest in the hexaferrites, which is still growing exponentially today.2–7 Most studies in hexaferrites have been focused on those containing barium and cobalt,8,9 but the studies of on lead hexaferrites are less. Our group has been interested in the synthesis and characterization of PbFe12O19 because of its excellent magnetic properties and potential application in various fields.10,11 PbFe12O19 becomes more attractive because of its lower crystallization temperature than barium and strontium hexaferrite when used as films on the glass or silicon substrate.11 The hexaferrites have been become massively important materials commercially and technologically. They are used as permanent magnets, magnetic recording and data storage materials and as components in electrical devices.12–14 Several techniques including coprecipitation, sol–gel, hydrothermal, glass crystallization, microemulsion and ultrasonic spray pyrolysis have been developed to synthesize lead hexaferrite powders.15–17

It is good to know that properties of powders depend on their particle size and morphology.18–23 Therefore, exploring appropriate methods to synthesize ferrites and controlling their particle morphology and size is significant. Herein, we develop the sol–gel auto-combustion route to prepare PbFe12O19. This is a novel way with a unique combination of the chemical sol–gel process and the combustion process based on the gelling and subsequent combustion of an aqueous solution containing salts of the desired metals and reductant. In comparison to other works, our method is simple, convenient and low-cost approach for the synthesis of PbFe12O19 nanoplates and nanoparticles. This process is less complicated than the others. We have synthesized PbFe12O19 from metal nitrates and maltose without adding external surfactant, capping agent or template. To the best of our knowledge, it is the first time that maltose is used as reductant in the synthesis of PbFe12O19. We have used nontoxic reactants and solvent. Water is an interminable, cheap, available solvent.

Experimental

Materials and methods

All the chemicals used in our experiments, Fe(NO3)3·9H2O, Pb(NO3)2 and maltose (C12H22O11), were of analytical grade, were purchased from Merck and were used as received without further purification. XRD patterns were collected from a diffractometer of Philips Company with X'PertPro monochromatized Cu Kα radiation (λ = 1.54 Å). FE-SEM images were obtained on MIRA3 FEG-SEM. TEM images were obtained on a Philips EM208S transmission electron microscope with an accelerating voltage of 100 kV. EDS analysis was obtained on Philips EM208. The magnetic measurement was carried out in a vibrating sample magnetometer (VSM) (BHV-55, Riken, Japan) at room temperature.

Synthetic procedures

The PbFe12O19 nanoplates and nanoparticles were prepared by the sol–gel auto-combustion process. First, 0.0006 mol of Pb(NO3)2 was dissolved in distilled water to form a clear solution. Then an aqueous solution containing 0.0078 mol of maltose was added into the lead nitrate solution drop-wise under strong magnetic stirring at room temperature. The original molar ratio between Pb and maltose was selected to be 1[thin space (1/6-em)]:[thin space (1/6-em)]13, 1[thin space (1/6-em)]:[thin space (1/6-em)]26 and 1[thin space (1/6-em)]:[thin space (1/6-em)]6.5. The solution was heated by stirring at 60 °C. After stirring the solution for 30 min, an aqueous solution containing 0.0036 mol of Fe(NO3)3·9H2O was added to the above solution and was heated at 120 °C by stirring for 1 h. The experiments were carried out with three different molar ratios between Pb and Fe, that including 1[thin space (1/6-em)]:[thin space (1/6-em)]6, 1[thin space (1/6-em)]:[thin space (1/6-em)]9 and 1[thin space (1/6-em)]:[thin space (1/6-em)]12. Evaporation of the mixed solution caused formation of a highly viscous gel. The gel then was dried in an oven at 100 °C. The 0.2 g of the gel dried in oven was calcined at different temperatures (600–900 °C) for 2 h to form the nanoplates and nanoparticles ferrite. We performed the reactions in static air. Table 1 lists the reaction conditions for the synthesis of PbFe12O19.
Table 1 The reaction conditions of lead hexaferrites synthesized in this work
Sample no. Pb[thin space (1/6-em)]:[thin space (1/6-em)]Fe Pb[thin space (1/6-em)]:[thin space (1/6-em)]maltose Temperature Time
1 1[thin space (1/6-em)]:[thin space (1/6-em)]6 1[thin space (1/6-em)]:[thin space (1/6-em)]13 900 °C 2 h
2 1[thin space (1/6-em)]:[thin space (1/6-em)]9 1[thin space (1/6-em)]:[thin space (1/6-em)]13 900 °C 2 h
3 1[thin space (1/6-em)]:[thin space (1/6-em)]12 1[thin space (1/6-em)]:[thin space (1/6-em)]13 900 °C 2 h
4 1[thin space (1/6-em)]:[thin space (1/6-em)]6 1[thin space (1/6-em)]:[thin space (1/6-em)]26 900 °C 2 h
5 1[thin space (1/6-em)]:[thin space (1/6-em)]6 1[thin space (1/6-em)]:[thin space (1/6-em)]6.5 900 °C 2 h
6 1[thin space (1/6-em)]:[thin space (1/6-em)]6 1[thin space (1/6-em)]:[thin space (1/6-em)]26 600 °C 2 h
7 1[thin space (1/6-em)]:[thin space (1/6-em)]6 1[thin space (1/6-em)]:[thin space (1/6-em)]26 700 °C 2 h
8 1[thin space (1/6-em)]:[thin space (1/6-em)]6 1[thin space (1/6-em)]:[thin space (1/6-em)]26 800 °C 2 h
9 1[thin space (1/6-em)]:[thin space (1/6-em)]6 — 900 °C 2 h

Results and discussion

X-ray diffraction

Fig. 1 shows the XRD patterns of as-prepared ferrites. When the ratio of Pb/Fe is 1[thin space (1/6-em)]:[thin space (1/6-em)]6 (sample no. 1), the products have the peaks corresponding to pure hexagonal PbFe12O19 (space group: P63/mmc) with cell constants a = b = 0.587 nm and c = 2.301 nm (Fig. 1a). When the iron content is increased to 1[thin space (1/6-em)]:[thin space (1/6-em)]9 (sample no. 2) or 1[thin space (1/6-em)]:[thin space (1/6-em)]12 (sample no. 3), a mixture of PbFe12O19 and Fe2O3 are obtained, as shown in XRD patterns in Fig. 1b and c. The peaks have been indexed with ▼ indicate the Fe2O3. The peaks of Fe2O3 in this figure are weak, whereas intensity of the peaks of PbFe12O19 is high. In our experimental conditions, the ideal ratio between Pb/Fe is 1[thin space (1/6-em)]:[thin space (1/6-em)]6.
image file: c4ra11688g-f1.tif
Fig. 1 XRD patterns of lead hexaferrites prepared from Pb(NO3)3 and Fe(NO3)3·9H2O at calcination temperature 900 °C for 2 h, with different ratios of Pb/Fe: (a) 1[thin space (1/6-em)]:[thin space (1/6-em)]6 (sample no. 1), (b) 1[thin space (1/6-em)]:[thin space (1/6-em)]9 (sample no. 2), (c) 1[thin space (1/6-em)]:[thin space (1/6-em)]12 (sample no. 3) (▼: Fe2O3), (d) 1[thin space (1/6-em)]:[thin space (1/6-em)]6 (sample no. 9 obtained in the absence of maltose).

For investigating the effect of maltose on the product, XRD pattern of sample prepared in the absence of maltose (sample no. 9) was taken. As shown in Fig. 1d, the sample no. 9 is found to be pure Fe2O3 with JCPDS no. 24–0072. As a result, the expected product, PbFe12O19, could not be obtained in the absence of maltose.

SEM images

In continuation, the effect of the ratio of Pb[thin space (1/6-em)]:[thin space (1/6-em)]reductant on the morphology of the products was investigated, whereas the molar ratio between Pb and Fe was selected to be 1[thin space (1/6-em)]:[thin space (1/6-em)]6. Three ratios were applied and other conditions were constant. Fig. 2 shows the SEM images of the PbFe12O19 obtained with different ratios of Pb[thin space (1/6-em)]:[thin space (1/6-em)]reductant including 1[thin space (1/6-em)]:[thin space (1/6-em)]13, 1[thin space (1/6-em)]:[thin space (1/6-em)]26 and 1[thin space (1/6-em)]:[thin space (1/6-em)]6.5. It can be observed that agglomerated nanoplates with hexagonal shapes and different sizes have been formed. The size and agglomeration of the nanoplates increase with decreasing maltose content. The sample no. 4 with molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]26 (Pb[thin space (1/6-em)]:[thin space (1/6-em)]reductant) has more even distribution than other samples.
image file: c4ra11688g-f2.tif
Fig. 2 SEM images of lead hexaferrites prepared with 1[thin space (1/6-em)]:[thin space (1/6-em)]6 ratio of Pb/Fe and different ratios of Pb[thin space (1/6-em)]:[thin space (1/6-em)]reductant: (a and b) 1[thin space (1/6-em)]:[thin space (1/6-em)]13 (sample no. 1), (c and d) 1[thin space (1/6-em)]:[thin space (1/6-em)]26 (sample no. 4), (e and f) 1[thin space (1/6-em)]:[thin space (1/6-em)]6.5 (sample no. 5).

In other side, the effect of the calcination temperature on the morphology of PbFe12O19 was investigated. With decrease of temperature from 900 °C (Fig. 2c and d) to 800 °C (Fig. 3a and b) size of plates is decreased and their agglomeration increased. By detailed look at the Fig. 3b, it can be understood that the surface of the nanoplates is not smooth and is covered with nanoparticles. Increasing the temperature results in more nucleation sites, leading to smaller grains. However, on decreasing the calcination temperature to 700 °C (Fig. 3c and d), plates coalesce and turn into bulk structures. Decreasing the temperature to 600 °C (sample no. 6), nanoparticles with spherical morphology are obtained, as shown in Fig. 3e and f. This figure shows directional linkage of particles due to magnetic interaction between them. It is observed that the average particle size of the sample no. 6 is about 20 nm.


image file: c4ra11688g-f3.tif
Fig. 3 SEM images of lead hexaferrites prepared with 1[thin space (1/6-em)]:[thin space (1/6-em)]6 ratio of Pb/Fe and 1[thin space (1/6-em)]:[thin space (1/6-em)]26 ratio of Pb[thin space (1/6-em)]:[thin space (1/6-em)]reductant at different calcination temperatures for 2 h: (a and b) 800 °C (sample no. 8), (c and d) 700 °C (sample no. 7), (e and f) 600 °C (sample no. 6).

TEM image

Fig. 4a and b shows the TEM images of the hexaferrite obtained at calcination temperature 600 °C (sample no. 6). This figure shows formation of agglomerated nanoparticles with diameters ranging from 10 nm to about 20 nm. To explore the effect of reductant on the morphology of the products, the reaction was performed in the absence of maltose, when calcination temperature was 900 °C (sample no. 9). SEM images of the sample no. 9 at two magnifications have been displayed in Fig. 4c and d. The images show that coalesced nanoparticles are formed in the absence of the maltose, when calcination temperature is 900 °C. Whereas, hexagonal nanoplatelets are obtained with maltose (Fig. 2). The morphology of the nanoferrite samples gets modified according to the shielding ability of the maltose. It means that the morphology anisotropy of the samples becomes increased with incorporation of maltose.
image file: c4ra11688g-f4.tif
Fig. 4 (a and b) TEM images of lead hexaferrite prepared with 1[thin space (1/6-em)]:[thin space (1/6-em)]6 ratio of Pb/Fe and 1[thin space (1/6-em)]:[thin space (1/6-em)]26 ratio of Pb[thin space (1/6-em)]:[thin space (1/6-em)]reductant at calcination temperature 600 °C for 2 h (sample no. 6), (c and d) SEM images of sample obtained in the absence of maltose (sample no. 9).

EDS analysis

EDS analysis measurement was employed to investigate the chemical composition and purity of as-synthesized lead hexaferrite nanostructures. Fig. 5 shows the EDS spectrum of the PbFe12O19 (sample no. 4). The figure indicates that the elements in the product are Pb, Fe and O only, and the atomic ratio of Pb[thin space (1/6-em)]:[thin space (1/6-em)]Fe[thin space (1/6-em)]:[thin space (1/6-em)]O is 3[thin space (1/6-em)]:[thin space (1/6-em)]38[thin space (1/6-em)]:[thin space (1/6-em)]56.
image file: c4ra11688g-f5.tif
Fig. 5 EDS pattern of PbFe12O19 nanostructures.

Magnetic properties

The hexaferrites are all ferrimagnetic materials, and their magnetic properties are intrinsically linked to their crystalline structures.12 They all have a magnetocrystalline anisotropy, that is the induced magnetization has a preferred orientation within the crystal structure. They can be divided into two main groups: uniaxial hexaferrites (with an easy axis of magnetization) and ferroxplana or hexaplana ferrites (with an easy plane, or cone, of magnetization).12 Fig. 6 shows the magnetization versus applied magnetic field (M − H) curve at 300 K for the as-synthesized lead hexaferrites (samples no. 4 and 9). In ferro- or ferrimagnetic materials, the magnetization will not pass through the origin however, and a symmetric ferroic hysteresis loop, or M − H loop, is formed. The hysteresis loops in this figure exhibit a ferromagnetic behavior for the PbFe12O19 nanostructures. As the M − H loops do not pass through the origin, at zero applied field the material will still retain a magnetization value, and this is the remnant magnetization (Mr). Coercivity (Hc) is a measure of the magnetic field that has to be applied in a reverse direction to coerce the remnant magnetisation back to zero, and it is the H value when the hysteresis loop crosses zero M.12 Fig. 6 shows Mr around 0.205 emu g−1 and Hc about 5609 Oe for sample no. 4. This sample with a high Hc is characterized as a hard ferrite and is appropriate for using permanent magnetic materials. Both Hc (2122 Oe) and Mr (0.13 emu g−1) are lower for the sample obtained in the absence of maltose (sample no. 9).
image file: c4ra11688g-f6.tif
Fig. 6 M − H hysteresis at 300 K for PbFe12O19.

In PbFe12O19 there is five non equivalent sublattices of which three are octahedral (2a, 12k and 4f2), one tetrahedral (4f1) and one trigonal bipyramidal (2b).24 Further it has been found that whereas 12k, 2a and 2b have their spins in upward direction, 4f1 and 4f2 have downward spin and the resulting magnetic moment of PbFe12O19 is due to the upward spins. Yang et al. were prepared PbFe12O19 with molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]6 of Pb[thin space (1/6-em)]:[thin space (1/6-em)]Fe and calcination temperature 900 °C.11 They were measured Hc of the PbFe12O19 as a function of calcination temperatures from 600 °C to 900 °C. The powders derived at 900 °C showed nearly 2500 Oe coercivity11 that is much lower than that obtained by our group in this work.

Conclusions

In summary, lead hexaferrite nanostructures have been successfully synthesized from metal nitrates and maltose as reductant, using a new sol–gel auto-combustion method. XRD shows that the ratio between Pb and Fe has a significant effect on the purity of obtained PbFe12O19 and an optimal ratio to synthesize pure lead hexaferrite is found. By adjusting the Pb/maltose ratio and calcination temperature, we could obtain lead hexaferrite nanoplates and nanoparticles. This method is proved to be a new, simple, efficient and quick way.

Acknowledgements

The authors are grateful to council of University of Kashan for providing financial support to undertake this work.

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