Preparation and characterization of single-phase α-Fe2O3 nano-powders by Pechini sol–gel method
Graphical abstract
Research highlights
► Single-phase α-Fe2O3 nano-particles could be synthesized via Pechini sol–gel method. ► In the polymeric intermediates, citric acid combined with metal ions mainly for the two-tooth complexing and bridge complexing. ► The phase transfer at different temperature shows that single-phase α-Fe2O3 could be obtained calcined at 600 °C and above. ► And the lowest particle size was found to be 30 nm.
Introduction
Hematite (α-Fe2O3) is the most stable iron oxide and the most environmentally friendly semiconductor (e.g., 2.1 eV) [1]. It is traditionally used for red pigment [2], catalysts [3], electrodes [4], gas sensors [5], [6], magnetic materials [7], photocatalytic [8] and anticorrosion protective paints. The properties of nano-powders greatly depend on their phase, microstructure and surface characteristics. The importance of single phase iron oxide cannot be ignored as it is crucial for the accurate measurement of electricity and magnetism. In order to prepare homogenous nano-particles of iron oxide, researchers have employed in different routes to facilitate single-phase iron oxide nano-particles such as sol–gel processes [9], w/o microemulsion [10], combustion [11], solvothermal [12], hydrothermal [13], precursor [14], solvent evaporation etc. However, these methods usually involve special equipment, high temperatures, and the tedious removal of impurities, which are all time-consuming and come at a high monetary cost.
The Pechini process has been used for the preparation of nano or sub-micro powders in a variety of metal oxides using inorganic salts as precursors, citric acid as a chelating agent and polyethylene glycol (PEG) as cross-linking agent [15], [16].The principle of the Pechini process is based on the ability of weak polybasic acids to chelate the metal ions. The chelates can undergo polyesterification with polyhydroxyl alcohols to form a solid polymer resin with a homogeneous distribution of cations at molecule level [17]. The highly branched polymer can reduce the cation mobility during heat treatment, so that the product with a good dispersion could be prepared. And Pechini method has better stoichiometric control, lower toxicity and cost.
Hence, in the present work, we have carried out a systematic study on Pechini sol–gel method using polyethylene glycol-6000 with citric acid for the synthesis of single-phase α-Fe2O3 nano-powders. The thermal decomposition, microstructure and the crystallinity were respectively investigated by FTIR, TG-DTG-DSC, XRD and FESEM measurements.
Section snippets
Materials
Citric acid, FeSO4·7H2O and polyethylene glycol (PEG)-6000 were of analytical grade and used without further purification.
Chemical synthesis
The 0.025 mol citric acid, 0.005 mol FeSO4·7H2O and 1.2 g of polyethylene glycol (PEG)-6000 were added to 50 ml of distilled water and ethanol (99.7%). The volume ratio of distilled water to ethanol was 3:2. The mixture was vigorously stirred to obtain a clear yellow-coloured solution. The solution was evaporated at 85 °C for 5 h, and it turned into a viscous yellow-coloured
Results and discussion
Fig. 1 shows the FTIR spectra of the as prepared polymeric intermediates. The peak observed at 3433 cm− 1 is due to the presence of –OH groups in the citric acid and polyethylene glycol derivatives [18]. The peak observed at 2964 cm− 1 is attributed to the asymmetric stretching of –CH2, –CH3 groups present in the organic derivatives [19]. The appearance of the band at 1741 cm− 1 for –COOH groups [15], the bands at 1645 and 1448 cm− 1 respectively for the asymmetric and symmetric stretching of C=O (△ ν =
Conclusions
In summary, single-phase α-Fe2O3 nano-particles have been synthesized via the Pechini sol–gel method in this paper. The thermal behavior observed in the TG-DTG and DSC curves shows the characteristic decomposition steps of the organic precursors whereas FTIR and XRD spectra are determinants for the detection of traces of Fe2O3. The α-Fe2O3 and γ-Fe2O3 phases began to form when the dried gel calcined at 450 °C. The single phase α-Fe2O3 nano-powders were obtained while calcined at 600 °C and above.
Acknowledgement
This study was supported by the Special Foundation of Educational Department of Shaanxi Province (No. 09JK362) and the Natural Science Foundation of Shaanxi Province (No. 2010JQ6006, No. 2009JM6008).
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