Indium–tin nanoscaled oxides synthesized under hydrothermal supercritical and postannealing pathway: Phase dynamics and characterization
Introduction
In the last decade much attention has been paid to the synthesis and study of mixed nanoscaled semiconducting oxides due to their applications in sensing, optoelectronic devices or catalysis [1], [2], [3], [4]. A huge number of oxides such as SnO2, TiO2, In2O3, WO3, Fe2O3, and mixed oxides were synthesized and studied. In particular, the mixed system tin oxide–indium oxide is of great importance mainly due to the representative transparent conducting oxide called ITO (usually 10% SnO2–90% In2O3 by weight). ITO combines the property of optical transparency in the visible region with high electrical conductivity being an essential component of modern optoelectronic devices.
It is known that the electrical, catalytic and optical properties of nanoparticles sensitively depend on their shapes and size that are mainly determined by the physico-chemical parameters of the synthesis method. Among the chemical methods, the solvothermal technique has attracted a great interest because it permits to prepare particles with desirable characteristics by controlling the reaction parameters (temperature, time, pressure, solution pH, nature and concentration of precursors, type of solvent, presence of surfactants etc.) [5]. In a solvothermal process, the physico-chemical properties (viscosity, density and polarity) of the solvent strongly influence the solubility and transport behavior of solutes [6], [7]. By the conventional hydrothermal process in subcritical water as solvent (temperatures and pressures lower than 374 °C and 22.1 MPa respectively), many inorganic materials can be synthesized at temperatures much lower than in solid-phase reactions [8]. In supercritical regime, due to low dielectric constant, low density and high ionic product of water, the reaction rate increases considerable. Nanoscaled In2O3/Fe2O3 solid solutions were obtained in an extended concentration range by synthesis in supercritical water, starting with indium (III)/iron (III) nitrate solutions [9].
In the SnO2–In2O3 system, the phase equilibrium and solubility limits are still under debate [10], [11], [12], [13]. However, it was accepted that the physico-chemical synthesis parameters, mainly the temperature, have an essential role in the formation of solid solutions. The pseudo-binary In2O3–SnO2 phase diagram determined in the temperature range of 1000–1650 °C revealed the formation of intermediate phases In4Sn3O12 and In2SnO5 [14]. The solubility limits of SnO2 in cubic indium oxide (In2O3–C) varied from 1.3 mol % at 1000 °C to maximum 13.1 mol % at 1650 °C. In2O3–C nanoparticles with tin oxide contents up to 15 wt % have been synthesized by a nonaqueous sol–gel procedure involving the solvothermal treatment of indium acetylacetonate and tin tert-butoxide [15]. High-energy in situ and ex situ synchrotron X-ray diffraction studies in the 900–1375 °C temperature range revealed that tin solubility in indium oxide is about 2 cation % at temperatures below 1200 °C and reaches 3 cation % at 1375 °C [16]. It was suggested that tin doped indium oxides obtained by different methods result in unstable materials and need thermal treatment at high temperatures to reach equilibrium. This assumption could be one explanation for the discrepancy in the literature data.
With the aim to obtain nanoscaled indium–tin oxides in an extended solubility range, this paper proposes the study of indium–tin hydrogel (obtained starting with InCl3–SnCl4 water solution) under hydrothermal supercritical conditions and post annealing pathway. The nanophase dynamics in the synthesis of the nanoparticle system xIn2O3–(1−x)SnO2, over all molar concentration range x (0 ≤ x ≤ 1, Δx = 0.1), is presented. The solubility limits in both synthesized composite nanoparticle systems SnO2–InOOH and SnO2–In2O3 are evidenced. To our best knowledge, this is the first study on dynamics and solubility in the synthesis of nanoscaled indium–tin oxide system, under hydrothermal supercritical and post annealing pathway, over all indium oxide molar concentration range x. A particular attention has been given to the optical properties of the rhombohedral ITO obtained under hydrothermal supercritical condition and post annealing pathway. Results on hydrogen production tests, performed in the presence of synthesized ITO phases, are also discussed.
Section snippets
Synthesis
A series of samples with general formula xIn2O3–(1−x)SnO2 (0 ≤ x ≤ 1, Δx = 0.1) have been synthesized starting with an aqueous solution containing In(III) and Sn(IV) chlorides in different molar ratios. The initial precursor ratios corresponding to the preselected molar concentrations are presented in Table 1 together with sample codes. In the first synthesis step a 25% ammonium hydroxide solution was added at room temperature (RT) under stirring to an aqueous solution containing In(III) and
Results and discussion
For a better understanding of the obtained results we have to highlight the main structural characteristics of indium and tin oxides. Indium oxide has two crystalline structures: cubic (known as bixbyite structure) with lattice parameter a = 10.1170 Å, space group (S.G. 206, Ia-3) [17] and rhombohedral (corundum structure) with a = 5.4900 Å and c = 14.5200 Å, space group (S.G. 167, R-3cH) [18]. The rhombohedral In2O3 is usually described as a high-pressure In2O3 polymorph which is metastable
Conclusions
Indium–tin nanoscaled oxides, xIn2O3–(1−x)SnO2 (0 ≤ x ≤ 1), were synthesized under hydrothermal supercritical conditions and postannealing pathway, starting with InCl3–SnCl4 water solution. In the concentration range 0.1 ≤ x ≤ 0.7, by hydrothermal reaction at 400 °C, mixtures of phases isostructural with InOOH and SnO2, were obtained. At higher indium contents (x = 0.8 and x = 0.9) only one phase, isostructural with InOOH, is observed. In the absence of tin (x = 1) the InOOH hydrothermal
Acknowledgments
This paper was prepared with the support of the Romanian National Authority for Scientific Research under the Core project PN09-450102.
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