The Wide Spectrum Modification Of Nanowire Arrays For Multi-bandgap Photoelectrochemical Hydrogen Generation

In recent years, the introduction of nanomaterials and nanostructures has attracted much attention for the development of novel solar energy conversion devices, which have high performance and novel functionalities. A number of photovoltaic device based on one dimensional nanomaterial(nanowires and nanotubes) have been proposed and studied, for they have ideal geometrical structures to provide a direct pathway for charge transport. Furthermore, the advantage of low reflectance in one-dimensional nanostructure arrays over the wide spectrum range endows them with excellent light absorption characteristics compared with their thin film counterparts. On account of their facile synthesis and superior optical properties, ZnO nanowire arrays have been widely studied in the photovoltaic device. However, due to its wide band gap energy(3.37 e V at room temperature), ZnO is disadvantageous in the absorption of energy in the visible-light region of the solar spectrum. To generate electron-hole pairs efficiently through visible-light excitation, some narrow band-gap materials have been loaded on ZnO nanowires. CdS and CdSe are two kinds of well-known semiconductor material among these sensitizing materials, which have been used as visible-light photocatalyst for quantum dot solar cells, and also as photosensitizer for various wide band-gap semiconductor photoanodes in photoelectrochemical application. In this paper, the preparation of TiO2 and ZnO nanowires is reported, followed by the multi-sensitization by chalcogenide to improve their photoelectrochemical performances. The main contents are as follows:1. Firstly, ZnSe/CdS/CdSe triple-sensitized ZnO nanowire arrays were successfully fabricated on FTO substrates for multi-bandgap photoelectrochemical hydrogen generation via a facile hydrothermal and anion exchange reaction, followed by a chemical bath deposition approach. The anion exchange reaction and chemical bath deposition approach show a remarkable controllability of the sensitizing layer thickness, which affects the visible-light absorption properties and photoelectrochemical performances of the heterostructures directly. The experiment found that a remarkable enhancement in photoelectrochemical hydrogen generation performances was achieved by ZnO@ZnSe/CdS/CdSe composite nanowire arrays photoelectrode compared with other photoelectrodes(such as Zn O@ZnSe and ZnO@ZnSe/CdS) through the effective synergistic light absorption of the solar energy and the multi-type-II graded bandgap level between the core ZnO nanowire and the different photosensitization out layers in this composite nanostructures. The ZnO@ZnSe/CdS/CdSe nanowire arrays photoelectrode shows a signicant enhanced photocurrent intensity of 5.3 mA/cm2 at zero bias versus Ag/AgCl, exceeding that of the ZnO@ZnSe(1.1 mA/cm2) or ZnO@ZnSe/CdS photoelectrode(2.6 mA/cm2) and was more than 12 times higher than that of the bare ZnO nanowires.2. In addition, CdS/CdSe co-sensitized bilayer TiO2/ZnO nanowire arrays were also assembled on FTO substrates for photoelectrochemical hydrogen generation. The bilayer design represents a simple structure which the dense TiO2 nanowire arrays were grown on FTO substrate followed by the assembling of ZnO nanowire arrays on the surface of the first-layer wires. The visible light harvesting was effectively enhanced through the sensitization by the low bandgap CdS and CdSe quantum dots, successively. The result of photoelectrochemical performances test suggests that the TiO2/ZnO@CdS/CdS nanowire arrays photoelectrode has a signicant enhanced photocurrent intensity of 9.8mA/cm2 at zero bias versus Ag/AgCl, and was 1.5 times than that of the TiO2/ZnO@CdS nanowires. In addition,the TiO2/ZnO@CdS/CdSe nanowire arrays photoelectrode shows a saturation photocurrent intensity of 11.5mA/cm2 at 0.6V bias versus Ag/AgCl.