Wang, Fuxian: Multinary Metal Oxides for Solar Water Splitting: Towards High Efficiency and High Stability = Multinäre Metalloxide zur solaren Wasserspaltung: auf dem Weg zu hoher Effizienz und hoher Stabilität. , 2017
Technischen Universität Berlin
Open Accesn Version
The world has experienced an exponential increase in the demand for energy primarily coming from the consumption of fossil fuels, which is accompanied by the inevitable emission of carbon dioxide, causing global warming. Developing renewable and carbon-free sources is essential to reduce carbon dioxide emission and mitigate global warming. Among the renewable energy sources, solar energy has the greatest potential to meet our future energy demands. One crucial problem of solar energy utilization is its intermittency due to day and night cycles as well as fluctuations in weather. Solar water splitting provides a possible route to convert solar energy into more storable chemical energy—hydrogen. To ensure good efficiency and stability in photoelectrochemical water splitting, the photoelectrode materials must satisfy critical thermodynamic and kinetic requirements. Unfortunately, so far no material in nature can meet all these requirements. Therefore, exploring new materials and modifying the properties of the existing materials are the crucial tasks ahead for materials scientists. In this thesis, we demonstrate the development of CuBi2O4 as a photocathode material for hydrogen production and BiVO4 as a photoanode material for water oxidation. A new spray pyrolysis recipe is developed to deposit homogeneous CuBi2O4. The key to the synthesis is to use additives including (1) triethyl orthoformate to avoid rapid hydrolysis of the bismuth precursor in the spray solution and (2) polyethylene glycol to improve the spreading behavior of the droplets over the substrates. A comprehensive investigation of the structure, optical, electrical, and morphological properties of CuBi2O4 demonstrates its potential to be used as a photocathode material with the main limitations being poor charge separation efficiency and photo-corrosion. To address these limitations, we explore a new strategy of gradient self-doping by introducing a Cu vacancy gradient using a two-step-diffusion assisted spray pyrolysis process. The flat-band of the CuBi2O4 photocathodes can be tailored by varying the Cu : Bi ratio. Introducing a Cu : Bi gradient inside the film leads to a gradient in Cu vacancies and therefore a built-in electric field, which in turn enhances or reduces the photoelectrochemical performance depending on the direction of the gradient. Applying CdS/TiO2 as protection layers and Pt as a catalytic layer significantly improves the stability of the forward gradient CuBi2O4 photocathode. We also explore direct current magnetron sputtering as a potential scaling-up technique for the economical deposition of BiVO4 photoanodes. The role of stoichiometry on structure, grain size, diffusion length, and photoelectrocatalytic performance is investigated, revealing a strong relationship between the grain size and the electronic properties. Our self-designed solvent capture technique combined with attenuated total reflection infrared (ATR-IR) spectroscopy can be useful for in-situ analysis of the reaction mechanisms in solution. The strategies we utilizedto improve the solution chemistry by adding additives might be applicable for developing solution-based recipe for the synthesis of other metal oxides. The concept of using self-gradient-doping to improve the charge separation efficiency in CuBi2O4 can be easily applied to other multinary metal oxide photoelectrodes. In addition, we demonstrate the potential of using direct current magnetron sputtering, a highly scalable technique, to produce multinary oxide photoelectrodes at high deposition rate and low cost.