Fabrication and characterization of ZnO-based water splitting devices by atomic layer deposition and hydrothermal synthesis

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Kegel, Jan
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University College Cork
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The urgent need for clean and storable energy drives many currently topical areas of materials research. Metal oxides like zinc oxide (ZnO) are particularly promising materials to be used in core technologies such as batteries, photovoltaics or solar fuel production. The latter photo- (electro)chemical application is considered as the ‘holy grail’ of energy research by some. This high consideration stems from the possibility to convert solar energy directly into chemical bonds and thus into a storable energy carrier. In this regard, the generation of hydrogen via solar water splitting forms a key building block related to this field of research. Governed by its low-cost, environmentally friendly and earth-abundant nature, ZnO represents a promising material for this application. However, the material possesses some natural limitations, which are hindering the use of ZnO for photo-(electro)chemical applications. Therefore, this study aims at tailoring the material properties of ZnO towards these applications by addressing the key issues of: low visible-light absorption, fast charge carrier recombination over intrinsic defects and the deterioration of the material due to photo-corrosion. Employing a combination of atomic layer deposition (ALD) for the growth of functional layers and low-temperature hydrothermal methods for the deposition of ZnO nanorod arrays, strategies are presented as to how to use defect-engineering to enhance the photo-(electro)chemical performance and how to increase the visible-light- absorption and activity by substitutional cobalt doping. First, a synthetic approach for the growth of ZnO nanorod arrays, which allows for the induction of strong defect-related orange luminescence upon rapid thermal annealing, is introduced. The nature of this defect center is thoroughly investigated and the oxygen vacancy–zinc interstitial defect-complex is identified as a possible origin. When used as a photo-anode for the splitting of water or as photo-catalyst material for the degradation of model pollutants, the orange luminescent nanorod arrays are shown to outperform samples that do not exhibit this emission. The mechanism leading to the defect-promoted enhancement of these key properties is discussed based on a possible charge carrier lifetime enhancement ascribed to this defect center. In regard to an improvement of the visible-light absorption of ZnO, a low temperature solution-based growth method was developed, which enables a rapid deposition (1 h) of cobalt-doped ZnO nanorod arrays with controllable visible-light absorption. Upon optimizing this method, a onepot synthesis is introduced, which allows for the growth of high surface-area cobalt doped ZnO nanorod-arrays decorated with catalytic cobaltic over-coatings. Control over the visible light absorption and the nature of the catalytic cobaltic over-coating (Co(OH)2 and/or Co3O4) can be easily achieved by changing the growth parameters used during the one-pot synthesis. The proposed growth mechanism may enable the controlled deposition of functional metal-oxide based material systems for various applications. In addition to the aforementioned solution-based approaches, which aim at altering the material properties of ZnO, ultrathin ALD titanium dioxide (TiO2) layers are investigated as chemical barriers for the protection of ZnO against photocorrosion. The resulting core-shell structures exhibit an improved chemical stability and the photo-current obtained is sustained for a longer time when compared to unprotected ZnO nanorod arrays. Pinholes in the TiO2 shell, particularly found at the rod tips, are identified as source for the photo-current and material degradation which nevertheless occurred after prolonged measurements. The experimental studies presented in this thesis will potentially accelerate the incorporation of ZnO-based materials into photo-(electro)chemical applications. This is not only achieved due to the introduction of tailored synthetic approaches, but also arises from the in-depth analysis and interpretation of the resulting material properties, which will deepen our fundamental understanding of ZnO as semiconductor.
ZnO , Doping , Defects , Solar water splitting , Oxygen evolution reaction , Water oxidation , Photo-corrosion
Kegel, J. 2018. Fabrication and characterization of ZnO-based water splitting devices by atomic layer deposition and hydrothermal synthesis. PhD Thesis, University College Cork.