Metal oxide nanomaterials for photocatalytic and photovoltaic applications

Abstract

Metal oxide nanomaterials are versatile functional materials for various applications. Among these materials, zinc oxide (ZnO) and titanium dioxide (TiO2) have been intensively investigated for their photocatalytic and photovoltaic applications due to their excellent electronic properties and photoactivities. The distinctions in their properties have led to different research focuses regarding the applications of ZnO and TiO2.

This thesis consists of two major research topics, with the first part focusing on the photocatalytic applications of ZnO, and the second part on the rapidly developing photovoltaic applications based on perovskite materials, in which TiO2 is commonly employed as charge transporting layers and/or mesoporous scaffolds. Research efforts were made to acquire fundamental understanding of the materials properties and their influence on the photocatalytic and photovoltaic behaviors of the materials.

Regarding the photocatalytic applications of metal oxide nanomaterials, detailed understandings of processes and species involved in the photocatalytic degradation is necessary for rational design of high efficient photocatalysts. A comprehensive investigation of photocatalytic activities was conducted with various ZnO nanostructures exhibiting distinct material properties. The correlations between photocatalytic activities and the material properties including native defects, surface adsorbates, and luminescence properties were elucidated. Effects of native defects on the dye adsorption were discussed. In addition, various post-fabrication treatments were applied to modify different material properties resulting in changed photocatalytic performance.

Regarding the photovoltaic applications of metal oxide nanomaterials, this thesis mainly focuses on the highly promising photovoltaic devices based on organic-inorganic hybrid perovskite materials. Predominant research efforts have been concentrated on the optimization of synthesis approaches and device architectures to achieve higher energy converting efficiency, while the fundamental understanding of the perovskite material properties is considerably inadequate. In particular, stability of perovskite materials is a critical issue regarding practical applications of perovskite solar cells. TiO2 as the most commonly employed electron transporting and mesoporous scaffold materials is considered causing photodecomposition of the perovskite films due to its excellent photoactivity. Based on this perspective, the stability of CH3NH3PbI3 perovskite films is investigated, as well as the role of TiO2 compact layer in degradation of perovskite films. The detrimental effect of excess PbI2 on the intrinsic stability of perovskite films is highlighted. Stability testing of perovskite based devices was conducted both at room temperature and 85 °C (accelerated aging) with light soaking.