The tightly embedded La2O3@TiO2 composite films with cracked surface and their enhanced electro-wetting performance

Abstract

The composite structures of tightly embedding the rare-earth oxide La2O3 nanoparticles into TiO2 film are designed and have been successfully prepared by using the sol-gel and spin-coating method. Appearance of a large amount of nano-cracks on the La2O3@TiO2 composite film surface supplies a textured surface to study the effect of nanostructured surface on the electrowetting on dielectric (EWOD) performance. The characterizations of chemical composition and crystal phase structure indicate the La2O3 nanoparticles with polycrystalline nature have been grown and distribute uniformly in TiO2 film. Valuably, plenty of intimate contact heterogeneous La2O3/TiO2 interfaces are formed, and supply an ideal space for charge accumulation and electric energy storage. All data of EWOD response test show the applied superlow voltage can induce a large contact angle (CA) change, but the breakdown voltage is still high. The EWOD response does not agree with the well-known Young-Lippmann equation, especially obvious in low voltage range. Moreover, the optimal parameters of layer number of 8 and La3+ doping concentration of 0.9% are obtained to enhance EWOD response for superlow applied voltage and the lower saturation CA. Detailed investigations and analysis reveal that the electric absorption has important roles on EWOD response under superlow applied voltage for the nanostructured surface. Furtherly, the space charge polarization theory is proposed to illustrate the enhancement of EWOD performance. When the voltage is applied, plenty of charges would be accumulated on the heterogeneous La2O3/TiO2 interfaces and the space charge polarization is formed, this significantly heightens polarizability of the La2O3@TiO2 composite film, and further increases their effective dielectric constant and improves EWOD performance. But the electric absorption of water on the nanostructured surface causes the inconsistency of EWOD response with the tested and calculated capacitance. The present study shows a new insight to design and fabricate structured dielectric for the solution of EWOD difficulties.