UC San Diego

It is believed that the solar energy is the ultimate clean energy source to meet global human energy consumption demand. However, The harvesting of solar energy in a clean and sustainable fashion and the storage and transport of the electricity could be challenges precluding practical scaling up of solar energy applications. Photoelectrochemical (PEC) water splitting using semiconducting materials is the most attractive approach of the solar energy applications because it efficiently converts solar energy, with high efficacy, to storable and transportable hydrogen fuel through an environmentally benign process (reaction with water and with hydrocarbon and oxygen as by-products). However, the good chemical and electrochemical stability and high overall energy conversion efficiency, in addition to low cost, are current challenges for the use of large scale PEC for practical and sustainable solar fuel production. The focus of this thesis is to develop economically competitive and efficient PEC water splitting cells by selecting low cost and earth abundant semiconductors and developing cheap and facile scalable processing for photoelectrode fabrication. Two systems, three dimensional branched nanowire heterostructures and metal coating enabled planar Si structures, are studied in details. This dissertation is structured in the following: after a briefly introduction of the principle of photoelectrochemical (PEC) water splitting cells in chapter 1, TiO₂ nanowires and TiO₂/Si branched nanostructures are discussed in chapters 2 and 3. As a low cost material, TiO₂ nanowire structure is prepared by a hydrothermal method and further modified by different post growth treatments to improve light absorption and kinetic properties and investigated their effects on PEC water splitting performances. The results show that types and sequences of post growth treatments should be carefully considered to improve the properties and performances of TiO₂ NWs. Also, hierarchically heterogeneous integrated TiO₂/Si nanostructures such as core/shell and multibranched nanowire structures are fabricated by a combination of nanoimprint lithography, reactive ion etch, and hydrothermal reactions. The structures have increased surface area for enhanced light absorption and more reaction sites and short diffusion length of minority carriers for higher reaction rate. Their PEC performances and the associated charge transfer at heterojunction interface are studied showing that photo current of TiO₂/Si heterojunction structure is limited by the recombination at the TiO₂/Si junctions or the properties of TiO₂. In chapter 4, metal-Si (MS) and metal- insulator-Si (MIS) structures, which have been studied in photovoltaic cells, are employed to developed cost- effective and efficient Si-based photoelectrodes. Earth abundant Ni film is selected as an oxygen evolution electrocatlayst for PEC photoanode, which can provide a junction voltage and protect Si surface, and applied to MS and MIS structures to compare their PEC performances and investigate the effects of insulating layer. Furthermore, MS and MIS structures are used to fabricate PEC photocathode with bimetal layer and the study is discussed in chapter 5. Bimetal layer can decouple catalytic reaction part from photovoltaic part. We design different patterned bimetal layers to improve the amount of light absorption of Si substrate and investigate the effects of MS or MIS contact, differently prepared insulator and bimetal thickness ratio on PEC water splitting performances. Finally, a summary on the major accomplishments and perspectives on future improvements are presented in Chapter 6