Low-density nanoporous iron foams synthesized by sol-gel autocombustion
© Hua et al; licensee Springer. 2012
Received: 19 October 2011
Accepted: 14 February 2012
Published: 14 February 2012
Nanoporous iron metal foams were synthesized by an improved sol-gel autocombustion method in this report. It has been confirmed to be pure phase iron by X-ray diffraction measurements. The nanoporous characteristics were illustrated through scanning electron microscope and transmission electron microscope images. Very low density and quite large saturation magnetization has been performed in the synthesized samples.
Porous nanostructured materials possess many good properties including high surface area, ultralow density, and high strength-to-weight ratio. So they are attractive materials for use in a number of catalytic [1–3], gas-sensing , optical [5, 6], and mechanical applications [7, 8]. Many kinds of porous nanostructured metal oxide foams have been synthesized and adopted in a wide range of applications, such as V2O5, TiO2, SiO2, iron oxides, and other oxides [9–14]. Additionally, nanoporous metal foams combine with properties characteristic of metals, such as good electrical and thermal conductivity, selected catalytic activity, and malleability, resulting in its desirability for acoustical insulation, electromagnetic shielding, fuel cell, catalytic applications, and plasmatic resonance [15, 16], which further distinguish the potential of bulk forms of metals. This makes the synthesis of metal foams at the forefront of materials science. Approaches used to synthesize such porous metal foam nanostructures include selective etching ('dealloying') of metal alloys [1, 17], self-organization of ultrathin nanowires , and deposition onto porous templates via physical vapor, chemical vapor, or wet chemical routes [16, 19–21]. Transition metal foams of nickel, copper, cobalt, and Ni-Cu and Ni-Co alloys have been synthesized with the controlled combustion method by Peter et al. . Otherwise, the synthesis of porous iron, the most important magnetic transition metal, remains a very difficult work . Tappan et al. have recently reported a cyanogel-based synthesis of macroporous refractory metals well below their melting point [23, 24]. Several transition metal porous foams have been obtained by heating the cyanogel under an inert atmosphere. However, as mentioned in the references, there are several disadvantages in their synthesis metrology: firstly, the by-products in their thermal processing contain very toxic hydrogen cyanide and cyanogen; secondly, the metal complexes with energetic ligand bistetrazolamine used in the process are expensive and complex to be synthesized; and thirdly, elemental analysis confirmed that the Fe foams contain only approximately 50% Fe, and the iron foams are not perceptibly magnetic prior to heat treatment under a flow of Ar or H2 gas [23, 25]. In this communication, we demonstrate a method, which is extended from the sol-gel autocombustion route , for the synthesis of nanoporous iron foams. The synthesis method is inexpensive and very convenient, while the obtained iron foams present quite large saturation magnetization at room temperature.
The iron foams were synthesized by an improved sol-gel autocombustion method. We have just introduced a sol-gel autocombustion method in the preparation of several metals and alloys recently . However, this method met some difficulties in the synthesis of metal iron. The metal iron is more active than other metals such as Co and Ni; thus, it is more difficult to reduce the metal from the iron oxide(s) than the cobalt oxide(s) and nickel oxide(s). In this study, we found that the reduction ability of the sol-gel combustion process can be improved by the addition of a suitable amount of ethanol in the preparation of the sol; thus, the metal iron can be reduced from its dried gel by the improved sol-gel autocombustion process.
In short, a sol-gel approach was applied in the preparation by using iron nitrate (Fe(NO3)3·9H2O) and citric acid (C6H8O7·H2O) as the starting materials, and ethanol and distilled water (rather than only distilled water in our previous report ) as the dissolvent. In a typical experiment, 12.5 mmol citric acid and 10 mmol iron nitrate were dissolved in 50 ml distilled water. The solution was ultrasonic agitated for about 10 min after adding 8 ml ethanol. Then the pH value of the solution was adjusted to 5 to approximately 6 by ammonia. The resultant solution was poured into a beaker and then boiled for about 2 min by an electrical furnace to drive off the air in the solution before transferring into a baking box heated at 95°C to develop a dried gel. Then the dried gel was put into a quartz tube and washed by pure nitrogen gas for about 30 min. After that, the nitrogen gas was cut down, and then the tube with the dried gel was transferred into a tube furnace heated to the preset temperature to activate the combustion. The gel burned violently, and a large amount of gas was released. After the reaction, the product, loose iron foam, was cooled down to room temperature under the protection of nitrogen. The dried gel can be ignited at different temperatures above its ignition point (little higher than 200°C as measured below). We have synthesized several samples with different ignition temperatures of 300°C, 400°C, 500°C, 600°C, and 700°C, respectively. For comparison of magnetic properties, we further prepared a sample by annealing the 600°C-ignited product for 3 h with the protection of the hydrogen gas just after the combustion.
The resulting materials were characterized by X-ray diffraction [XRD] with CuKα radiation, scanning electron microscope [SEM], transmission electron microscope [TEM], and vibration sample magnetometer [VSM]. Thermogravimetry [TG] and mass spectrometry were applied for the analysis of the combustion of the dried gel under the protection of the argon gas.
Results and discussion
The moderate addition of ethanol is as important as the ratio of citric acid and iron nitrate. We have tried many synthesis parameters and found that, without the ethanol in the preparation of the sol, no iron peaks can be observed in the XRD patterns of the synthesized samples. Although the detailed reason is not clear, the ethanol plays a very important role in the synthesis of iron foams. All the samples mentioned below are obtained with the ratio (citric acid, iron nitrate, and ethanol) of 12.5 mmol:10 mmol:8 ml.
The obtained silvery gray Fe foams are very active and can even burn violently in air and form brownish red powder because of the dumping friction during collection from the quartz tube. The samples can be easily attracted by a magnet, while the burned brownish red powder cannot. This may reveal the formation of the iron with very fine particles in the combustion synthesis of gels, while the brownish red antiferromagnetic α-Fe2O3 formed in the burning of the produced iron.
A convenient sol-gel autocombustion method was employed in the synthesis of iron foams. Moreover, the iron foams are characterized by very low density and quite large saturation magnetization at room temperature. The XRD and VSM measurements illustrated the formation of the metal iron in the combustion. SEM and TEM studies confirmed the nanoporous structure of the samples. This kind of ferromagnetic porous iron foams may find wide applications in the fields of catalysis, fuel cells, hydrogen storage, unique insulation, and electromagnetic absorption.
This work was supported by the Natural Science Foundation of Jiangsu Province (BK2009245), the Foundation of National Laboratory of Microstructures (2010ZZ15), the Natural Science Foundation of Education Bureau of Jiangsu Province (09KJD430004), the Priority Academic Program Development of JiangsuHigher Education Institutions, and China Postdoctoral Science Foundation (20100471290).
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