Preparation of Uniformly Dispersed ITO Nanoparticles Via Direct Oxidation of in-Sn Alloy in Ionic Liquids

Abstract

Introduction Transparent and electrically conductive metal oxide nanoparticles (NPs) prepared by solution phase synthetic methods have attracted much attention due to their applicability to wet processing, such as ink-jet printing or spin coating, for fabrication of densely packed particulate films, which can be used to improve the performance of solar cells, transparent electrode, and sensors. Among various kinds of metal oxide particles, indium tin oxide (ITO) NPs are useful nanomaterials because of their high electronic conductivity, chemical stability, or unique optical properties induced by their surface plasmon resonance (LSPR) in the near-infrared region. So far, well-dispersed ITO NPs have been prepared by sol-gel technique or solvothermal method, though these conventional techniques required well-trained skills for obtaining particles with well-controlled size and shape. Recently a facile preparation method of colloidal metal particles in organic solvent has been reported with use of low-melting-point metals such as gallium, indium, and tin,[1] in which molten metals were mechanically dispersed in organic solvent containing surfactant molecules by ultrasonication and/or magnetic stirring, resulting in the formation of metal nanoparticles. On the other hand, room temperature ionic liquids (RTILs) with extremely low vapor pressure and thermal stability have been reported to be a useful medium for the preparation and stabilization of noble metal NPs. For example we have reported that the sputter deposition of noble metals such as Au, Ag, or Pt into RTILs produced corresponding ultrafine metal nanoparticles uniformly dispersed in the solution.[2]RTILs are also expected to act as a good medium not only for the preparation of low-melting-point metal NPs by mechanically dispersing but also for the oxidation of resulting metal particles with heat treatment due to high thermal stability of RTILs. However such attempt has not been carried out. In this study, we report a facile preparation of uniformly dispersed ITO NPs via direct oxidation of In-Sn alloy as a precursor in RTILs. Experimental Indium (0.5 g) was added in RTILs (2.0 cm3) of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate (HyEMI-BF4), 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (HyEMI-TFSA), N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide (TMPA-TFSA), or 2-hydroxyethyl-N,N,N-trimethylammonium bis(trifluoromethanesulfonyl)amide (Ch-TFSA). The mixture was heated at 523 K for 10 h in air with vigorous stirring. Thus-obtained NPs dispersed in the solution were collected by a centrifugation and re-dispersed in acetonitrile for characterization. ITO NPs with different Sn content were prepared in the same method except for the use of an In-Sn alloy (Sn content; 3, 5, 10, 20, 30 wt%) as a precursor in the Ch-TFSA. Results and discussions The particles prepared by the oxidation of pure In metal as a precursor in RTILs of EMI-BF4, HyEMI-TFSA, TMPA-TFSA and Ch-TFSA, exhibited broad diffraction peaks in XRD pattern assignable to cubic In2O3. In contrast, the XRD pattern of particles prepared in HyEMI-BF4 was assignable to orthorhombic InOF. These results indicated that In metal was oxidized with oxygen molecule dissolved in TFSA-based RTILs to produce In2O3 or reacted with fluoride ion produced by the decomposition of BF4 - anion in HyEMI-BF during heat treatment, resulting in the formation of InOF. TEM observation revealed that uniformly dispersed spherical particles with average diameter of 27 and 39 nm were observed in hydroxyl-functionalized RTILs of HyEMI-TFSA and Ch-TFSA, respectively, while largely aggregated In2O3 particles appeared in EMI-BF4 and TMPA-TFSA. These indicated that hydroxyl group in cationic species of RTILs played an important role in the prevention of In2O3NPs in RTILs. To prepare ITO NPs, we carried out the similar preparation protocol, in which In-Sn alloy containing different Sn content (3-30 wt%) were oxidized in Ch-TFSA with vigorous stirring. Thus-obtained particles exhibited the XRD pattern assignable to ITO crystal structure. Figure 1 shows a TEM image of ITO NPs prepared with alloy containing 30 wt% Sn. Polygonal particles with average diameter ca. 40 nm were observed. With an increase in the Sn content from 3 to 30 wt% in alloy as a precursor, the amount of Sn doping in ITO NPs was enlarged from 0.67 to 10 wt%. ITO NPs exhibited a LSPR peak in the near-IR wavelength region in the extinction spectra, the peak wavelength being blue-shifted from 2860 to 2400 nm with an increase in the amount of Sn doping in NPs from 0.67 to 10 wt%.