Synthesis of multi-segmented TiO2/Pt nanorods for photocatalytic hydrogen production
Creators: name: Teo Yanru, Gladys
Publisher: name: Imperial College London
Year: 2014

description: Photovoltaic modules are under active consideration as a major contributor to future energy requirements. Coupled with an electrolyzer, this energy system converts energy harvested from the sun into chemical power. As the demand for a sustainable yet efficient and cost effective approach of producing hydrogen increases, researchers are seeking ways to improve the technology of forming solar fuel. Mimicking the idea of how nature collects and stores solar energy in chemicals bonds through photosynthesis, economically viable water splitting cells capable of splitting water directly at the semiconductor surface are being developed. The catalytic semiconductor is designed to be both a light absorber and an energy converter to store solar energy in the simplest chemical bond, H2, thereby eradicating significant fabrication and system costs involved with the use of separate electrolyzers wired to photovoltaic cells. In this work, water-splitting materials have been designed to consist of multi-component nanorods of titanium dioxide and platinum with well-defined nanostructures to function as photocatalytic cell for hydrogen production. As the TiO2-Pt nanorods are irradiated with light in the presence of a water source, oxygen and hydrogen are evolved at the anode TiO2 and cathode Pt segments of the nanorods respectively. The alternating segments of TiO2 semiconductor and Pt metal enable the control of the direction of charge movement and light absorption pathways in the material, thereby presenting a solution to improving the overall efficiency of photocatalytic hydrogen production. By employing templated electrodeposition, homogeneous multi-segmented TiO2/Pt nanorods have been successfully fabricated in anodic aluminium oxide membrane (AAM). This simple method of synthesis permits an easier control of the position and composition of TiO2 and Pt along the length of the nanorods, which allows for a customizable and highly reproducible method of obtaining segmented rods with uniformly distributed active sites for efficient catalytic activity. The morphology and material composition of the as-prepared Pt-TiO2 multi-segmented nanorods were characterized using the scanning electron microscope (SEM), transmission electron microscope (TEM) and the x-ray diffraction (XRD); and the photocatalytic properties of these multi-segmented TiO2/Pt nanorods are then examined by carrying out absorption studies using Rhodamine B (RhB). Two different Ti precursors, TiOSO4 and TiCl3 were employed in the successful fabrication of TiO2 nanorod segments. High potentiostatic conditions of -1.2 V (vs. 3 M Ag/AgCl) using TiOSO4 precursor resulted in TiO2 nanorods, but at the same time had promoted hydrogen evolution, which made it challenging for TiO2 nanorods to be deposited onto noble metal surfaces such as Pt; while lowering the potentiostatic voltages resulted in the growth of TiO2 nanotubes. Cyclic voltammetry and chemical tests were carried out to determine the detailed mechanisms of their respective growth, and the difference in the formation of TiO2 nanorods and nanotubes was attributed to be the regeneration of high amounts of NO3- species occurring at more negative deposition voltages. Using TiCl3, a protocol has been developed for the electrodeposition of TiO2 nanorods, which involved a low deposition voltage of -0.1 V (vs. 1 M Ag/AgCl) that would facilitate the deposition of TiO2 on noble metals such as Pt. This protocol allows both the flexibility of preparing TiCl3 precursor solution and the electrodeposition of TiO2 nanorods to be carried out at ambient conditions. In the presence of TiO2/Pt nanorods, RhB degradation studies reveal several pertinent characteristics of the photoactivities of the TiO2/Pt nanostructures, where nanostructural morphology, addition of Pt metal and TiO2/Pt interfaces were found to enhance TiO2 photoactivity. The effects of varying TiO2 length segments of Pt-TiO2 nanorods on the photoactivities of TiO2/Pt nanorods reveal an interesting trend, demonstrating an interplay between light absorption and the amount of light reaching the TiO2/Pt interface. Lastly, differences in the orientation of bi-segmented TiO2/Pt nanorods to the light source are presented, which may be useful in any future work on the investigation of the photoactivities of nanostructure arrays.