Effect of Adding TiO2 Nanoparticles in AC-Operated Electrochemiluminescencent Device Using Ruthenium(II) Complex

Abstract

Electrochemiluminescence (ECL) is a phenomenon in which light is emitted from the excited state of a redox-active material generated by electrochemical reactions. Among light-emitting device, ECL devices have various advantages in terms of structure and ease of fabrication, and therefore, they are expected as next generation emitting devices. Concerning to the ECL device, we have reported the improvement of emission response, ECL intensity and stability of the ECL device by employing alternating current (AC)-driven method.[1] Nevertheless, we found that the ECL intensity and long-term stability of the system were not enough, and improvement should be required for practical use. In terms of improving the ECL intensity, several researchers have reported that the brightness of the Ru(bpy)3 2+-based ECL could be improved by adding metal oxide nanoparticles (NPs) into the ECL solution. However, the underlying mechanisms of the enhancements were not clarified.[2], [3] The purpose of this study is to elucidate the mechanism of this ECL enhancement. We first investigated the change in the ECL intensity and long-term stability of the ECL device upon addition of the TiO2 NPs to the Ru(bpy)3 2+-based ECL solution. With the addition of the TiO2 NPs into the solution, the emission luminescence of the ECL device increased to 165 cd/m2, which was 1.4 times higher than the device without the addition of TiO2 NPs (120 cd/m2). Without the TiO2 NPs, the half-life time of the ECL device was ~250 s, in contrast to that of the ECL device with the TiO2 NPs, which was expended by a factor of 4 to ~1000 s. Thus, the ECL intensity and long-term stability of the device were greatly improved by adding TiO2 NPs in the Ru(bpy)3 2+-based ECL solution. To reveal the influence of the TiO2 NPs on achieving the high emission luminescence and long-term stability, we measured the optical and electrochemical properties of the Ru(bpy)3 2+-based ECL solution in detail. Increases in the photoluminescence for Ru(bpy)3 2+ were observed by adding the TiO2 NPs. This indicated that the suppression of the radiationless quenching of the excited states would improve the ECL intensity. In terms of the long-term stability, electron transfer between the Ru(bpy)3 2+ and TiO2 NPs was suggested by the detailed electrochemical measurements. Electron transfer occurred from the reduced species of Ru(bpy)3 2+ to the TiO2 NPs, and subsequently, from the TiO2 NPs to the oxidized species of Ru(bpy)3 2+. This kind of electron transfer is thought to improve the balance between the redox reactions in the ECL device, leading to long-term stability. Reference [1] T. Nobeshima et al., J. Mater. Chem., 20, 10630 (2010). [2] N. Itoh, J. Electrochem. Soc., 156, J37 (2009). [3] N. Itoh, Matrials, 3, 3729 (2010).