Engineering nanostructured tantalum nitride photoanodes for solar water splitting [electronic resource]
Keywords: Chemical Engineering.

The current global energy consumption of 549 quadrillion British thermal units (BTU) is projected to increase by nearly 50% to 815 quadrillion BTU by 2040. The majority of this energy consumption will be needed for the economic development of countries that are not in the Organization for Economic Co-operation and Development that will primarily rely on fossil fuels for their energy needs. A continued increase in fossil fuels will contribute to greenhouse gas emissions which are associated with global climate change that can adversely affect natural life on Earth. Therefore, renewable energy such as wind and solar is required to offset the increasing energy needs and having a way to store that energy is necessary for making renewables competitive with fossil fuels. Photoelectrochemical water splitting is a technology that can store solar energy in the form of chemical bonds. Tantalum nitride (Ta3N5) is a promising photoanode material for photoelectrochemical water splitting due to its nearly ideal band energetics. However, high water splitting performance with this material is difficult to achieve due to inadequate photogenerated current and voltage and poor stability in aqueous oxidizing conditions. Thus, in this dissertation, we try to circumvent and engineer around these limitations to produce an efficient photoanode with Ta3N5. The first part of this dissertation aims to gain a better understanding of the optoelectronic properties of Ta3N5 using experimental ellipsometry and computational density functional theory. We find good agreement between theory and experiment, and that the optical band gap of Ta3N5 is direct. However, the material is highly anisotropic and has high effective masses of charge carriers which could lead to poor mobilities. With this understanding, we design a new Ta3N5 device architecture that aims to absorb a large portion of incident light while maintaining short diffusion lengths for photogenerated charge carriers within Ta3N5. The device is a nanostructured core-shell Si-Ta3N5 composed of Si nanowires coated with a highly conformal thin film of Ta3N5. The Si nanowire scaffold not only provides long optical path lengths and light trapping effects to aid with light absorption in the ultra-thin Ta3N5, but also generates a photovoltage that is additive to the photovoltage of Ta3N5, thereby making a tandem light-absorber. We studied the effect thickness of Ta3N5 in these structures to better understand minority carrier diffusion lengths in this material. With the addition of CoTiOx and NiOx oxygen evolution co-catalysts to the tandem structures, we achieved highly active and stable photoelectrochemical water splitting. To improve upon the tandem Si-Ta3N5 performance, we studied the effect of synthesis conditions, primarily the nitridation temperature, on the photocurrent and photovoltage of the devices. By using numerous characterization techniques, we found that intermediate nitridation temperatures allow high crystalline Ta3N5 to be formed without chemically reduced tantalum nitride phases and oxygen impurities that occur at higher temperatures. Understanding the underlying phenomena that occur at different synthesis conditions enables us to improve the Ta3N5 performance. We have learned that forming a tandem junction between Ta3N5 and n-type Si enables an earlier onset of photocurrent. However, the increased surface area of nanostructured devices can impact device photovoltage and photocurrent due to surface recombination. To study the effect of nanostructuring on the performance of Si-Ta3N5 we vary the length of the Si nanowires, and find that there is a trade off in device performance with increasing nanowire length. As the nanowires become longer, the photocurrent increases due to increased light trapping and electrochemically active surface area, but simultaneously the photovoltage decreases due to increased surface recombination. Understanding the interplay of these effects is crucial to optimizing device performance. Finally, to further improve the tandem Si-Ta3N5 device photovoltage, we incorporate a p-n junction within the Si. We fabricate nanostructured p+n Si core-Ta3N5 shell photoanodes and improve photocurrent onset by 300-350 mV compared to n-Si devices. The addition of a cobalt titanate, CoTiOx, oxygen evolution co-catalyst enables water oxidation photocurrent to onset at low applied bias and achieves a high photocurrent density at the water oxidation potential. The photovoltages of these tandem devices are among the highest of Ta3N5-based photoanodes reported to date. Overall, this dissertation covers fundamental studies on the optoelectronic properties of Ta3N5 and uses knowledge of the material's limitations to engineer highly efficient Ta3N5-based photoanodes that overcome these obstacles. The results presented herein are contributions towards making scalable photoelectrochemical water splitting a reality.