In Situ Synthesis of Polymer Nanocomposite Electrolytes Emitting a High Luminescence with a Tunable Wavelength Mikrajuddin Abdullah, I. Wuled Lenggoro, and Kikuo Okuyama* DiVision of Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima UniVersity, Higashi, Hiroshima 739-8527, Japan Frank G. Shi Department of Chemical Engineering & Materials Science, UniVersity of California, IrVine, California 92697-2575 ReceiVed: October 14, 2002 Zinc oxide (ZnO) nanoparticle-based nanocomposite polymer electrolytes with a very high luminescence intensity were prepared by an in situ method, in which ZnO nanoparticle fillers were grown in the polymer matrix and ion carriers were inserted during the growth of the nanoparticles (contributed by a precursor). By using a high concentration of lithium hydroxide (LiOH) as the precursor, the remaining unreacted LiOH was distributed in the form of an amorphous complex around the produced ZnO nanoparticles, thus preventing the agglomeration of the nanoparticles. This resulted in a high number concentration of ZnO nanoparticles that serve as luminescent centers for inducing a high luminescence intensity. Compared to samples prepared using the usual concentration of LiOH (about 0.14 M), the luminescence intensity was enhanced by about 22 times for the ZnO nanopowder and 6 times for the nanocomposite polymer electrolytes (poly(ethylene glycol)/ ZnO/Li + ) when a LiOH concentration of 0.35 M was used. Introduction ZnO is a semiconductor material that produces an efficient blue-green luminescence. It serves as an efficient material for use in low-voltage phosphors and vacuum fluorescent displays as well as field emission displays (FEDs). 1 Recently, the last application has become even more important because FEDs are one of the promising candidates for use in next generation flat panel displays. 2 The discovery of a quantum size effect in nanometer sized particles, such as a dependence of optical spectrum on particle size has led to even more interest in ZnO materials. In a previous report we described an in-situ method for preparing nanocomposite polymer electrolytes. 3 ZnO nanopar- ticles were grown directly in the polymer matrix and precursor materials containing alkali ions, which did not participate in the formation of nanoparticles were used. 3 Because ZnO nanoparticles emit luminescence, luminescent polymer electro- lytes nanocomposites in which the nanoparticles serve as luminescent centers could be produced. 3 The measured electrical conductivity was found to be comparable with the data for composites made by dispersing preprepared particles in polymer/ salt solutions. 4,5 The luminescence intensities of the resulting composites, however, were still too weak, 3 especially when compared with rare earth based luminescent materials. 6,7 We attempted to use high concentrations of LiOH in the production of ZnO-based polymer electrolyte nanocomposites on the expectation that the excess LiOH woul served as a separator for the nanoparticles so that small sized, nonagglomerated, and high number con- centration ZnO nanoparticles could be produced, generating a strong luminescence intensity. In the first part we focused on the production of ZnO nanopowder (dried colloid) and in the second we applied that approach to the production of ZnO- based polymer electrolyte nanocomposites. Experimental Section In principle, the preparation method was similar to that reported previously. 3 A solution of zinc acetate dihydrate (Zn- (CH 3 COO) 2 ‚2H 2 O) in ethanol was distilled at 80 °C to produce 40 vol % of a hygroscopic solution and 60 vol % of unused condensate. 8 The lithium hydroxide hydrate (LiOH‚H 2 O) was dissolved in ethanol separately and then mixed with the hygroscopic solution of zinc acetate (Zn(CH 3 COO) 2 /LiOH ) 2/3 v/v). The concentrations of the lithium hydroxide used were typically less than 0.14 M. 8,9 An extensive mixing process was required even to dissolve lithium hydroxide at concentrations below 0.14 M. A highly concentrated lithium hydroxide solution was prepared here by dissolving the lithium hydroxide in ethanol with stirring for around 3 h at elevated temperatures (around 50 °C). Several minutes after mixing the precursors, the mixture was then dried at 40 °C for about 3 days to produce a ZnO nanopowder. We used three concentrations of LiOH to produce ZnO nanopowders: (i) 0.075 M, (ii) 0.14 M, and (iii) 0.35 M. On the other hand, to prepare polymer electrolyte nanocom- posites, we used LiOH concentrations of (i) 0.05 M, (ii) 0.075 M, (iii) 0.1 M, (iv) 0.14 M, (v) 0.2 M, (vi) 0.25 M, (vii) 0.3 M, (viii) 0.35 M, and (ix) 0.5 M. Poly(ethylene glycol) (PEG) 500 000 (0.5 g) was dissolved in the lithium hydroxide solution at a temperature of approximately 50 °C before mixing with the zinc acetate precursor. The mixture was then dried at 40 °C for 3 days. X-ray diffraction patterns were obtained using a Rigaku Denki RINT2000 instrument (Cu KR source), electrical conductivities * Corresponding author. E-mail: okuyama@hiroshima-u.ac.jp. 1957 J. Phys. Chem. B 2003, 107, 1957-1961 10.1021/jp022223c CCC: $25.00 © 2003 American Chemical Society Published on Web 02/07/2003