Controlling Interfacial Transfer Processes For Improved Photoelectrochemical Performance

As the Earth's population continues to grow exponentially and become more technologically advanced, fossil fuel prices rise concurrently, a reflection of the diminishing resources available at a reasonable price and with reasonable effort. A significant part of the solution to this energy problem is to utilize renewable energy resources' wind, hydroelectric, geothermal, and solar, among others. The amount of solar energy reaching the surface of the Earth each day is orders of magnitude more than the energy needs of all the people of the world combined. An important question, however, immediately arises: how can excess energy be stored to power homes when the sun goes down? Hydrogen is an excellent candidate as a storage medium. It can be used in current internal combustion engines, and it can be stored in pressurized tanks or metal hydrides for ease of access and transportation. Currently, the vast majority (approximately 95%) of hydrogen is produced at a net energy loss from steam-methane reforming. To be useful as an energy carrier, hydrogen must be produced using a 'free' energy input like sunlight. Following Fujishima and Honda's demonstration of photolysis of water on TiO2 using simulated sunlight in 1972, the field of water photolysis and hydrogen generation has grown quickly.

In this dissertation, improvement of photoelectrochemical performance by intercalation of Li+ to passivate Ti4+ trap states have been demonstrated. This passivation increases both the photovoltage and photocurrent generated by increasing the rate of collection of photogenerated electrons. Pulsed laser deposition was used to synthesize metal oxide heterostructures while retaining an excellent electron conducting substrate has also been demonstrated, and this SrTiO3-TiO2 heterostructure was observed to enhance photoelectrochemical performance due to an increase in charge separation. IrO2, a widely studied water oxidation catalyst that has the lowest overpotential for the oxygen evolution reaction, was shown to catalyze an undesirable side reaction on TiO2. The scavenging of trapped holes by reduced oxygen radicals was enabled only in the presence of IrO2, and this scavenging occurred on a timescale approximately 1000 times faster than that of water oxidation, which means that it represents a serious obstacle to developing a water photolysis system that does not rely on external power input. Continuous hydrogen generation in a reverse fuel cell has been demonstrated using CdS on TiO2, and the quantum efficiency of the reaction has been determined using chemical actinometry, demonstrating that calculation of efficiency based on current-voltage characteristics is insufficient. Based on the research presented in this dissertation, future directions to pursue are also discussed.