Studies on the preparation of Fe₂O₃ aerogels through sol-gel process and supercritical drying technique

Gan Lihua, Chen Longwu, Li Guangming, Yue Tianyi
(Department of Chemistry, Tongji University, Shanghai 200092, China)

Received Mar. 20, 2001; Supported by the National Natural Science Foundation of China (29973029).

Abstract Fe₂O₃ Aerogels were prepared from ferric trichloride and sodium hydroxide through sol-gel process and supercritical drying technique. By means of the DTA, BET, TEM, XRD, Mössbauer technique and so on , the aerogel samples were characterized. The results showed that initially obtained aerogels were b- FeOOH, which would be completely converted into a-Fe₂O₃ through heating treatment at 450 °C temperature. Fe₂O₃ aerogels, which consist of particles with 60nm in diameters, are with uniform size and low density, nanosized porous materials consist of nearly spherical a- Fe₂O₃ nano-particles. Adjusting molar ratio of ferric trichloride and sodium hydroxide can control the density and network structure of Fe₂O₃ aerogels.
Keywords Aerogels, Fe₂O₃, sol-gel process, supercritical drying technique.

1. INTRODUCTION 
Originally synthesized by Kistler ^[1] in the early thirties a variety of single and multicomponent oxide aerogels have been prepared ^{[2 - 7]}. Aerogels are a kind of structure under control, low density, nanosized porous materials, which consist of nano-particles or self-coalescent high polymers, exhibiting high porosity ( 80 - 98.8% ), surface area as 1000 m²·g^-1, a low refractive index ( 1.01-1.05 ) and sound velocity ( 100 m·s^-1 ), low thermal conductivity ( 0.01 W·m^-1·K^-1 ), and exceeding low density ( as low as 3 kg·m^-3 ). Because of these unique properties aerogels have particularly suitable for a number of application including Cerekov detectors, insulators, acoustic impedance matching, gaseous filters, and catalytic substrates ^{[8 -12]}. The preparation of aerogels is usually divided into two steps ^{[ 3, 5 ]}, the first step is to prepare aqueogels or alcogels by hydrolysis and polycondensation of precursor molecules ( usually monomeric alkoxide ) in host solvent utilizing a suitable catalyst to produce a sol-gel, the second step is to put the gels into a high-pressure vessel to dry supercritically. Many kinds of aerogels have been prepared through this method, however, as it is extremely difficult to prepare the aqueogels of transition metal oxide with network structure, reports about the   preparation of one-component transition metal oxide aerogels are still rare. This paper details synthesis and characterization of Fe₂O₃   one-component aerogels. Structural aspects of the resulting aerogels have been examined using transmission electron microscope ( TEM ), Brunauer-Emmet-Teller ( BET ),differential thermal analysis ( DTA ), X-ray diffraction, and Mössbauer techniques.

2. EXPERIMENTAL PROCEDURES
2.1 The preparation of Fe₂O₃ aerogels samples
Aqueous sodium hydroxide solutions (1.50mol.dm^-3) were added dropwise to aqueous of ferric chloride (1.50mol.dm^-3) with vigorous stirring, the molar ratio of OH ^- / Fe³⁺ in the sols was controlled by the variations of the amount of sodium hydroxide aqueous solutions, and the composition of the aerogels is expressed in terms of the molar ratio of OH^- / Fe³⁺. The sols so obtained were placed into the semipermeable membrane which was first made by collodium, then moved to the distilled water (60-65°C) for thermoosmosis purification to remove the free Fe³⁺ and Cl ^- in the sols until no Fe³⁺ and Cl^- were left in the thermoosmosis solutions. The purified sols were de-hydrated in a vacuum drier contained phosphorus pentoxide, and homogeneous aqueogels can be formed under various molar ratio of OH ^- /Fe³⁺. The wet gels were immersed in pure acetone to exchange the remaining water solvent, the gels so obtained were placed in an autoclave ( Polaron CPD ) in which the solvent was replaced in the gels by liquid carbon dioxide ( at 4-6°C for 48h ), followed by supercritical extraction of the carbon dioxide in critical temperature and pressure ( 32-35°C, 7.5-8.0MPa) drying unit in order to obtain the aerogel initial samples, then the Fe₂O₃ aerogels samples were obtained after heating at 450°C.

2.2 Characterization techniques 
Apparent density was measured from the weight and size of the aerogel samples that were cut into common shape at first. The BET surface areas of the aerogel samples were measured using the BET autosorb instrument (Micrometrics Flow Sorb II 2300), the carrier gas was 30.2% N₂ and 69.8% He. DTA curve was measured using CDR-1 differential thermal analysis apparatus. Powder X - ray diffraction patterns were obtained with a D / MAX - B X - ray diffractometer with Cuk radiation, V=100kV, I=40mA. The crushed samples were placed into distilled water, the shape and size of particles were observed by JEOL JEM 200 CX electron microscope whose point resolving power is about 0.260nm. The Mössbauer spectra were obtained on samples mounted in polyethylene cells using a constant acceleration spectrometer in the accelerator mode at room temperature equipped with a ⁵⁷Co(Rh) source. The resultant spectra were analyzed by a constraint, least-square fit to Lorentzian-shaped lines. The isomer shift values are quoted relative to an a-Fe absorber.

3.RESULTS AND DISCUSSION
3.1 Formation of iron polymer aquogels
During the homogeneous titration process of ferric trichloride solutions by sodium hydroxide, the hydrolysis process of ferric chloride solutions can be divided into four stages^{[ 13, 14 ]}: (1) hydrolysis to mono- and dimers, (2) reversible, rapid growth to small polymers, (3) formation of slowly reacting large polymers, (4) precipitation of a solid phase. This process can also be described as follows:

The hydrolysis process of ferric chloride solutions was controlled at the third stage in this paper, and a hydrosol containing large amount of big polymers was formed, from which homogeneous iron polymer aquogels could be obtained by oxolation. While adjusting the molar ratio of OH ^- / Fe³⁺ of sols, the network structure of the aquogels so obtained could be controlled, and this in turn influence the structure and properties of finally produced aerogel samples.

3.2 Phase composition of initial aerogel samples 
Using the methods described in this paper, aerogel samples prepared by carbon dioxide supercritical drying of the aquogels were red brown solids with higher strength. The phase composition of typical samples could be described as a set of slightly asymmetric quadrupole doublet on the Mössbauer spectrum ( Fig.1 ). The results of computer simulation showed that the quadrupole splitting of the samples ( D= 0.68 mm/s) is quite similar to the quadrupole splitting of b-FeOOH as reported ^{[ 15 ]}, and the as-prepared samples contain tetrahedral framework Fe (III) with an isomer shift of 0.25mm/s^{[ 16 ]}. b-FeOOH is one kind of substances with anti-ferromagnetism pro-perty, whose Mössbauer spectrum using large blocks of it at the room temperature was a typical Lexa-line spectrum with magnetic splitting. However, when the sample particles were very small, the magnetic splitting would disappear completely and what remained was quadrupole doublet. Meanwhile the samples appeared to be super-paramagnetic. Consequently it was confirmed by the Mössbauer spectrum that the initial samples were aerogels with network structure composed of ultrafine b- FeOOH particles. This conclusion is also verified by the measurement results of XRD.

Fig.1 Mössbauer spectrum of initial aerogel samples prepared from the 1.50 molar ratio of OH^- / Fe³⁺ recorded at room temperature.

3.3 Phase transition of aerogels
The initial samples were put into the differential thermal analysis apparatus and the temperature increased according to a set program to obtain the DTA curve ( Fig.2 ). The endothermic peak occurred at 74°C is a desorption peak of adsorbed water from the surfaces. Because the samples were porous with relatively large specific surfaces, the adsorption on them was especially strong and the desorption peak of adsorbed water was quite broad. The other endothermic peak occurred at 170°C was caused by b-FeOOH losing of structural water when heated and converted to a-Fe₂O₃. The exothermic peak, at 285°C, was a phase transition peak caused by the conversion from a- Fe₂O₃ to poorly crystallized a- Fe₂O₃ , while the endothermic peak at 406°C whose form was very regular and sharp was the crystallization peak of the poorly crystallized a-Fe₂O₃^[17]_. So the process of phase transition of aerogel samples could be described as follow:

The DTA study showed that the phase transition to a-Fe₂O₃ had been completed after heating to 406°C.
The results of XRD measurements of initial and after-heating samples confirmed the above-mentioned phase transition.
The position of XRD characteristic peaks of the initial aerogel samples prepared from solutions with different molar ratio of OH^- / Fe³⁺ were almost the same, and the XRD spectrum of typical samples is showed in Fig.3 ( a ). The composition of the samples could be confirmed to be b-FeOOH in accordance with the characteristic peak values of its peak acme (7.55, 5.38, 3.32, 2.54, 2.30, 2.08, 1.95, 1.74, 1.64, 1.51, 1.44, 1.37 ), and the disperse peak lines appearing on the XRD spectrum was evident that the particles of samples were extremely small. Fig.3( b) shows the XRD spectrum of aerogel samples after heating at 450°C, it can be seen that the aerogel samples prepared in this method are crystal form a-Fe₂O₃ from their characteristic peak value of the XRD pattern (3.68, 2.70, 2.51, 2.20, 2.08, 1.84, 1.70, 1.60, 1.48, 1.45 ).This Conclusion is still supported by the research of Mössbauer spectrum ^{[ 18 ]}.

Fig.2 DTA curve of an aerogel prepared from the 1.50 molar ratio of OH^-/Fe³⁺

Fig.3 XRD pattern of aerogel samples prepared from the 1.50 molar ratio of OH^-/Fe³⁺
(a) initial samples after supercritical drying (b) samples after heating at 450°C

3.4 Structure of typical iron oxide aerogel samples 
Typical a-Fe₂O₃ aerogel samples are brown porous solids with higher strength, whose transmission electron micrograph ( TEM ) is shown in Fig.4. These aerogel samples were produced at a solution: molar ratio of OH^- / Fe ^{3 +} = 1.50. Statistical analysis of particle size seen in the TEM, shows that the particle size of the aerogel samples was 60nm in diameter. The particles were nearly spherical and the size distribution was quite narrow. This result indicated that the aerogels prepared by this method were a kind of narrowly distribution, low-density, nanosized porous solid materials consisted of nearly spherical a-Fe₂O₃ nano-particles.

Fig.4 Transmission electron micrograph of an aerogel derived from the 1.5 molar ratio of OH^-/Fe³⁺

3.5 The effects of the preparing conditions on properties of aerogels
Fe₂O₃ aerogel samples produced by the same method with different molar ratio of OH^-/Fe³⁺ showed that the primary particles are similar, but the density and the specific surface area of the aerogel samples changed. The measured density and specific surface area of  the aerogel samples with OH^-/Fe³⁺molar ratio varying from 0.40 to 2.00 are shown as table 1. With increasing molar ratio of OH^-/Fe³⁺, the density of the aerogels increased, and the specific surface area of the aerogels decreased. These observations can be interpreted by considering that shrinkage in our system should occur from condensation between adjacent groups during the long aging and solvent exchange times following thermal dialysis. Since the reaction medium ( water ) is the most probable source of oxo ligands required for condensation, the increase in hydroxide ion ( i. e. increased pH ) increases the potential for deprotonation of ligand water and hydroxo groups leading to a higher degree of oxolation. These will cause the solutions with relatively large molar ratio of OH^- / Fe³⁺ to produce finally aerogels with large degree of cross-linking and dense network, so their density was large and specific surface was small.

Table 1 The result of the density and specific surface area of the aerogel samples

4. CONCLUSIONS 
1. Fe₂O₃ aerogels have been prepared, as low-density porous solid materials, with network structure that consists of spherical nano-particles with sizes of about 60 nm.
2. Adjusting the molar ratio of OH^- / Fe³⁺ can control the network structure of Fe₂O₃ aerogels. The present study shows that the increase of molar ratio of OH^- / Fe³⁺ will make the network structure of aerogels become dense, whence the increase of density.
3. According to the preparing method in this paper, the initial samples are a porous network materials consisted of b-FeOOH ultrafine particles.

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