Design and fabrication of high efficiency metal oxide and silicon-based photoelectrodes for solar water splitting
Keywords: Mechanical Engineering.

Renewable energies have attracted significant attention in recent years due to the increasing energy demand and critical environmental problems. Among all the renewable energy sources, solar energy is the most promising one with the largest capacity (12,000 TW), minimal environmental impact, and inexhaustible usage. Photoelectrochemical (PEC) water splitting, which can directly convert the solar energy into chemical bonds (hydrogen and oxygen) through splitting water molecules, is one of the most promising ways to solve those issues. This technique has several advantages: First of all, since it can directly convert the solar energy to chemical bonds (e.g. H2), so the solar energy can be appropriately stored, which can the solve the intermittent supply of sunlight. Second, the stored hydrogen can be used freely based on the demands. Third, the whole solar to hydrogen converting process is completely carbon free, which can ultimately solve the environmental issues by reducing the greenhouse gas emissions. Thus, researchers have spent tremendous efforts on exploring and developing efficient and stable photoelectrodes for effective photocatalytic/electrocatalytic solar conversion. Metal oxides and silicon are two promising candidates due to their unique material properties. However, both metal oxides and silicon have their own drawbacks that limiting their performance for solar fuel generation. This thesis presents some novel methods developed towards improving metal oxide and silicon based photoelectrodes performance. The mismatch between the light absorption depth and charge carrier diffusion length is the biggest challenge for metal oxide photoanodes. Fabricating ultrathin light-absorbers on textured substrates offers an effective way to solve this issue. Such ultrathin films with the thickness smaller than the minority charge carrier diffusion length, can effectively enhance the charge collection efficiency. Additionally, textured substrates would greatly enhance both light absorption and surface reactions of photoanodes, and thus reduce the total amount of light-absorbers needed. Furthermore, the overall PEC performance can be further enhanced by properly control the interfacial layer morphologies. An amphiphilic graft copolymer self-assembly method was developed to create a mesoporous metal oxide interfacial layer to significantly reduce the interfacial charge recombination. The biggest challenges about Si are the instability when operating under photoanodic conditions in aqueous electrolyte and surface kinetics for water splitting reaction. To date, the most effective protection layer for Si photocathodes in alkaline solutions is the atomic layer deposited (ALD) dense TiO2 layer combined with noble metal-based catalysts on top. However, those high vacuum based techniques are expensive and hard to scale up for practical applications. Herein, two novel solution-based deposition methods were developed to replace the high-vacuum based techniques for efficient Si photoelectrode protection. The first method used a modified hydrothermal method to deposit earth-abundant NiFe-layered double hydroxide (LDH) to simultaneously protect and catalyze Si photocathodes in alkaline solutions. The second method used a modified electroless deposition (ELD) method to uniformly deposit protective and catalytic Ni films on Si wafers, resulting in an efficient and stable Si photoanode for solar water oxidation.