Flame synthesis and doping of metal oxide nanowires and their application in solar water splitting [electronic resource]
Keywords: Mechanical Engineering.

Considering the increasing energy and environmental problems associated with the exhaustible fossil fuels, renewable energy conversion devices have attracted tremendous attention, which hold the promise to supply the fuel and electricity in a sustainable way. For many of these devices, such as batteries, fuel cells, solar cells and solar water splitting cells, metal oxides are very important functional materials due to their earth abundance, good stability and diverse properties. Recently nanowire-based metal oxides have enabled revolutionary advances in various energy conversion devices, because of their unique physical and chemical properties resulting from the high aspect ratio and large surface area. Despite their advantages, practical applications of metal oxide nanowires are hindered, as conventional synthesis methods have limitations for large scale production. Flame synthesis can potentially solve this large-scale production issue for metal oxide nanowires, given its demonstrated scalability in the industrial production of nanoparticles. However, only until very recently has flame synthesis been applied to metal oxide nanowires. More research is needed to develop advanced flame synthesis method for metal oxide nanowires, to understand the mechanism to well control the size, shape and compositions for reliable manufacture, and to evaluate their quality and functionalities in real devices. This thesis presents a novel flame vapor deposition method for the synthesis of metal oxide nanowires with the capabilities of rapid rate, good uniformity over large area and broad substrate choice. Through the investigation of growth mechanism, good control over the morphology and composition was achieved by tuning the process parameters such as fuel/air ratio, source temperature, substrate material and temperature. In addition to synthesis, flame-based doping method (sol-flame doping) was innovated for controllable doping of metal oxide NWs to modify the properties of host materials at the nanometer scale. This sol-flame doping method not only preserves the morphology and crystallinity of the host NWs, but also allows fine control over the dopant concentration by simply varying the concentration of dopant precursor solution. With this method, significant enhancement of the electrocatalytic activity towards oxygen evolution reaction was achieved for TiO2 NWs (up to 760 mV reduction of the overpotential), attributing to simultaneously improved surface charge transfer kinetics and increased bulk conductivity by doping. Finally, the flame-synthesized metal oxide nanowires were implemented as a photoanode in photoelectrochemical water splitting. By rational design and scalable fabrication, the WO3/BiVO4/Ni:FeOOH composite nanowire photoanode generated a high photocurrent of 4.5 mA/cm2 at a potential of 1.23 VRHE under simulated sunlight, which is among the highest produced by any WO3/BiVO4 based photoanodes. With the demonstrated rapid rate, good controllability and superior performance of the flame-produced metal oxide nanowires, these flame synthesis and doping methods can potentially enable future generation of energy devices by removing the barrier for large-scale production of tailored metal oxide nanowires.