Preparation of nanostructured ZnO nanorods in a hydrothermal–electrochemical process

Abstract

An array of ZnO nanorods, each nanorod being covered with a shell of porous ZnO was prepared in two steps by hydrothermal–electrochemical processes. The growth of ZnO nanorods was achieved in a zinc nitrate and hexamethylenetetramine aqueous solution on fluorine-doped tin oxide film. The porous ZnO shell was grown from a similar solution in the presence of eosin Y as nanostructuring agent. A dye-sensitized solar cell was assembled using as photoanode an eosin Y sensitized ZnO nanorod/shell layer.

Introduction

Ordered arrays of 1D ZnO nanowires and nanorods have attracted increasing attention due to promising applications in electronic and optoelectronic devices arising from their physical properties. Some improving in the performance of various ZnO based nanodevices such as nanostructured solar cells [1], [2], field-emission devices [3], short-wave lasers [4], gas sensing [5] are expected. ZnO-based photoanode for dye-sensitized solar cell (DSSC) has attracted considerable interest during the past several years due to the similarity of the energy bandgap and the electron-injection process of ZnO to that of TiO₂, the primary photovoltaic material for DSSCs; in addition, ZnO has higher electron mobility. However, only a maximum conversion efficiency of 5.4% have been obtained with films of well-packed polydisperse ZnO aggregates and Ru complex dye [1] or of 5.56% using indoline dyes [6], while a 11% efficiency has been achieved with small size cells based on TiO₂ photoanode [7]. An array of ZnO nanorods could provide a direct path from the point of photogeneration of carriers to the conducting substrate, thus increasing electron diffusion length and effectively reducing the electron recombination losses. Zinc oxide has been prepared into various nanostructures [8], [9], [10], [11], [12] such as, nanowires, nanotubes, tetrapods, nanoparticles using a wide variety of techniques. The electrodeposition method has advantages over other processes because of its simplicity, low cost equipment and the possibility in making large area thin films. It is a low-temperature synthetic approach which permits use of plastic substrates for the fabrication of flexible and light in weight DSSCs. However, the electrodeposited ZnO is highly crystallized and that means it has a small surface area, so that no appreciable amount of dye could be adsorbed. In order to prepare a nanoporous ZnO film, it was electrodeposited a ZnO/eosinY hybrid thin film [13], [14] and the loaded eosinY molecules were completely removed by the alkaline treatment.

In this paper, we investigated the fabrication of an array of ZnO nanorods, each nanorod being covered with a shell of porous zinc oxide; combined hydrothermal–electrochemical processes were used for the deposition of a ZnO nanorod/shell layer. The feasibility of employing such ZnO layer as photoelectrode for DSSCs was studied.