Nanostructured Semiconductors for Electrochemical and Photoelectrochemical Water Splitting

Abstract

Chemical energy storage by water splitting is a promising solution for the utilization of solar energy in numerous applications. Both efficient electrocatalysts and photocatalysts with a facile and scalable synthesis method are indispensable to achieve economic feasibility for water splitting. In the introductory chapter, a comprehensive review of electrochemical and photoelectrochemical (PEC) water splitting is provided and the fundamental concept of the PEC device design is described. Three different systems for water splitting were then explored in the following experimental chapters. In the second chapter, Manganese oxides (Mn3O4, Mn5O8 and Mn2O3) nanoparticles, with a range of specific surface area and crystal structures, were synthesized and compared. The nearly identical morphology of synthesized manganese oxide nanocrystals allows a systematic investigation on the relation between oxidation states and catalytic activities of different manganese oxide nanocrystals. Interesting discoveries regarding surface-specific turnover frequency, mole-specific turnover frequency and catalytic stability were demonstrated. Highly transparent and robust sub-monolayers of Co3O4 nano-islands were subsequently developed, which efficiently catalyse water oxidation with a comparable performance to noble metal-based catalysts. The potential of the Co3O4 nano-islands for photoelectrochemical water splitting has been demonstrated by incorporation of co-catalysts in GaN nanowire photoanodes. The Co3O4-GaN photoanodes reveal significantly reduced onset overpotentials, improved photoresponse and photostability compared to the bare GaN ones. In the last experimental chapter, we developed both physically- and chemically-induced morphology/structure tuning procedures, viz. capillary force-induced self-assembly and corrosion followed by regrowth, drastically increasing the water oxidation photocurrent density of nanostructured hematite photoanode. In addition to morphological changes, structural transformations were obtained by capillary force-induced self-assembly resulting in improved crystallinity of hematite with preferential orientation in the [110] direction. High conductivity of the hematite (001) basal planes contributes to the significantly enhanced photo-electrocatalytic activity. Subsequent dissolution and regrowth of hematite nanostructures further improved the performance, resulting in improved light absorption, more efficient charge separation and surface charge transfer processes.