Structural distinctions of Fe2O3–In2O3 composites obtained by various sol–gel procedures, and their gas-sensing features
Introduction
Composites based on iron and indium oxides are widely used as active layers of semiconducting gas sensors. The gas-sensing behavior of Fe2O3–In2O3-based sensors is essentially determined by the phase composition of the oxides, their dispersity and the Fe2O3 concentration [1], [2], [3], [4], [5].
A substantial literature on this topic has been published; Fe2O3–In2O3 films containing γ-Fe2O3 modification of iron oxide are reported to be more sensitive to O3 than α-Fe2O3-based films [6]. Fe2O3–In2O3 films with α-Fe2O3 as a main phase exhibit an enhanced response to C2H5OH vapors as compared to In2O3 films slightly doped with Fe2O3 [1]. Chibirova and Gutman showed that the functional (in particular, gas-sensing) features of Fe2O3-based oxides strongly depend on their pre-history, i.e. on the preparation procedure and mode of thermal treatment [7].
Nevertheless, a detailed structural distinction between the Fe2O3–In2O3 films in relation to their dissimilar gas-sensing behavior has not been considered before.
In this paper, the structural peculiarities of Fe2O3–In2O3 nanosized composites have been studied depending on the synthesis conditions and annealing temperature in order to assess the deposition parameters, which allow to obtain different structural modifications of Fe2O3 with different gas-sensing features.
The samples were obtained by inorganic modification of the sol–gel technology, which is based on the use of inorganic metal salts as precursors, as opposed to the classical modification using organic metal derivatives. The inorganic approach gives the possibility to prepare oxide composites in a nanosized state and to attain considerable mutual solubility of the components. It is worth noting that the used technology allows also to widely vary the phase composition and fine structure of the samples and therefore to control their gas-sensing characteristics.
Section snippets
Experimental
Fe2O3–In2O3 samples with a Fe:In = 9:1 molar ratio and various phase composition were studied along with individual iron and indium oxides. The 9:1 molar ratio was found to be the most promising for gas-sensing applications [1].
The samples were synthesized as stabilized sols of the corresponding metal hydroxides. The synthesis procedures consisted of the following steps: (i) forced hydrolysis of an inorganic metal salt solution (0.5 mol l−1) with a base agent (water solution of NH3, >99.99% purity,
System α-Fe2O3–In2O3
The α-Fe2O3–In2O3 composite, obtained by combined hydrolysis of Fe(NO3)3 and In(NO3)3 salts and subsequent co-precipitation of the resulting Fe(III) and In(III) hydroxides, remains X-ray amorphous after its thermal treatment at 150–300 °C (Fig. 1). After annealing the composite at 500 °C, broadened reflections of the α-Fe2O3 phase (ref. JCPDS 33-0664) appear in the XRD pattern.
The increase of the experimental lattice parameters compared to the reference data for α-Fe2O3 phase, as well as the
Conclusion
The influence of synthesis conditions on the structural and gas-sensing properties of Fe2O3–In2O3 composites has been carefully analyzed. It was found that co-precipitation of Fe(III) and In(III) hydroxides leads to the formation of an α-Fe2−xInxO3 single-phase solid solution, which is homogeneous and poorly crystalline. The thermodynamic stability of this phase along with high structural homogeneity of the sample causes its poor sensitivity to both reducing and oxidizing gases.
The
Acknowledgement
D. Kotsikau thanks the Belarusian Foundation of Fundamental Investigations for financial support.
Dr. Maria I. Ivanovskaya received her degree in chemistry in 1980 from the Belarusian State University in the field of photochemistry. Since 1989 she has been working at the Research Institute for Physical Chemical Problems of the Belarusian State University. Her main scientific interest is solid state chemistry in applications to catalysis and semiconductor gas sensors, structural features of nanosized oxides (SnO2, MoO3, In2O3, Fe2O3, CeO2, ZrO2, La2O3) and oxide composites.
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Dr. Maria I. Ivanovskaya received her degree in chemistry in 1980 from the Belarusian State University in the field of photochemistry. Since 1989 she has been working at the Research Institute for Physical Chemical Problems of the Belarusian State University. Her main scientific interest is solid state chemistry in applications to catalysis and semiconductor gas sensors, structural features of nanosized oxides (SnO2, MoO3, In2O3, Fe2O3, CeO2, ZrO2, La2O3) and oxide composites.
Dr. Dzmitry A. Kotsikau received his degree in chemistry in 2005 from Belarusian State University in the field of semiconductor gas sensors. Now he is working at the Research Institute for Physical Chemical Problems of the Belarusian State University in the field of semiconductor gas sensors and catalysts. His main scientific interests are Fe2O3–In2O3 and Fe2O3–SnO2 nanosized composites, their structural and gas-sensitive characterization.
Dr. Antonietta Taurino took her PhD in physics in 1999 at the University of Lecce. Since 2001 she has been working as researcher at the Institute for Microelectronics and Microsystems of CNR. Her competences are related to the analysis of the morphological, structural and compositional properties of materials by transmission electron microscopy techniques, scanning transmission electron microscopy (STEM), electron beam induced current (EBIC), as well as by new techniques for nano-manipulation of materials by FIB. The main fields of interest are nanostructured thin films of single and mixed metal-oxides for gas sensors and III–V low dimensional semiconductors for optoelectronics.
Dr. Pietro Siciliano received his degree in physics in 1985 from the University of Lecce. He took his PhD in physics in 1989 at the University of Bari. During the first years of activities he was involved in research in the field of electrical characterization of semiconductors devices. He is currently working in the field of preparation and characterization of thin film for gas sensor and multisensing systems.