Semiconductor Photoelectrodes For Photocatalytic Energy Conversion & Semiconductor Nanorods For Solid State Lighting
Open Access
- Author:
- Fanghanel, Julian
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 13, 2020
- Committee Members:
- Raymond Edward Schaak, Thesis Advisor/Co-Advisor
Ismaila Dabo, Thesis Advisor/Co-Advisor
Venkatraman Gopalan, Committee Member
John C Mauro, Program Head/Chair - Keywords:
- Photocatalysis
Photocatalytic
Hydrogen
Water Splitting
Quantum Wells
Solid State Lighting
Nanorods
Optoelectronics
Heterojunctions
Energy
Photoelectrodes - Abstract:
- Energy is the currency of the physical world; it is what allows us to perform any action. The development of denser and more usable forms of energy has supported the emergence of our technological civilization. From animal labour to the industrial revolution to the use of fossil fuels, the progressive acquisition of more powerful sources of energy has enabled humanity to increase agricultural and industrial productivity, and to raise the standards of living for all. Our current dependence on fossil fuels stems from their relative abundance and ease of use; however, as has become undoubtfully apparent over the past decades, this dependence has taken an immense toll on the planet and the ecosystem. For this reason and because of the accelerating depletion of these nonrenewable resources, there exists a compelling need to advance alternative sources of energy. Ultimately, most sources of energy available on Earth originate from a single main source, namely, solar energy. Winds arise from the uneven heating of the planet’s surface; biomass is created by the conversion of solar energy into organic molecules; even fossil fuels are the concentrated traces of ancient biological systems that once derived their sustenance from sunlight. Solar energy is the most abundant source of energy accessible to humankind, but it is variable and affected by climatic, seasonal, and diurnal intermittency. These limitations make it inadequate to fulfill the growing and delocalized energy needs of our society. A possible option to circumvent these disadvantages would be to store solar energy for when it is unavailable or insufficient to meet the consumer demand. One solution to achieve this goal is to convert solar radiation into chemical energy that can be stored for later use. In a manner similar to biological photosynthesis, it is possible to use semiconductors to harness the power of the Sun for storing sunlight into chemical energy. Photocatalytic semiconductors have the potential to convert abundant chemical substances such as water into fuels such as molecular hydrogen, which can be stored and utilized in internal combustion engines or in fuel cells, with the key benefit that the only by-product of this reaction is water. This process is carbon neutral and, if scalable, could reduce our reliance on fossil fuels by accelerating the deployment of renewable energy into the electric grid and into the transportation sector. In this thesis we have develop a high-throughput method to search for photocatalytic semiconductors using ab-initio DFT calculations and materials databases. The resulting candidates are screened by considering their band gap, band edges, abundance, toxicity, and synthesizability. Refined computational calculations are performed on the resulting candidates to improve the prediction of the band gaps (the Hubbard U correction). This search which initiated with a pool of over 66,000 materials was narrowed down to 68 candidates out of which 14 were successfully synthesized and characterized based on their optical, chemical, electrochemical and photocatalytic properties. Much of this work is dedicated to the synthetic pathways taken to produce these materials and the characterization of its optical properties. We were able to identify, synthesize, and validate 9 photocatalytic materials. Out of these materials, four are previously known water splitting photocatalysts, suggesting that our screening criteria are a valid way of approaching the problem. Three of these nine materials – NaInO2, SrIn2O4 and PbTiO2 – have been previously screened as potential candidates for water splitting, and we were able to validate this result experimentally. Three previously screened plumbates – Ca2PbO4, Ba2PbO4 and Cu2PbO2 – have never been experimentally validated as photocatalysts, even though they have been screened in multiple occasions. One of our materials, Na3Fe5O9, has been determined as a good photoelectrode for the production of hydrogen but is thermodynamically not suitable for producing oxygen while being a potential candidate for hydrogen evolution. Nb2Co4O9 is the most promising material obtained from our search as it has never been described as a potential candidate computationally or experimentally and has been determined to be a successful photocatalyst for water splitting. These results demonstrate the importance of coupling experimental and computational results. It also demonstrates a viable path for the discovery of novel photocatalytic materials that could ultimately provide renewable fuels from solar energy. The second part of this thesis relates to the development of optoelectronic materials utilizing nanoparticle semiconductors. Lighting has become a commodity of the modern world that has allowed human activity and productivity to go past that established by natural daylight. This important aspect of our lives, accounts for 16-20% of all energy use worldwide. As we further increase our development in this world, more and more people will require the use of lighting to allow for improved lives. It is thus important that we transition away from the use of inefficient lightbulbs like incandescent lights and instead transition to the most efficient lighting system that we know of so far, solid state lighting. Solid state lighting is done using semiconductors in light emitting diode (LED) devices. These LEDS are 7 times more efficient than incandescent light and could contribute significantly in the reduction of energy we devote to lighting worldwide. In this thesis, we propose the use of quantum nanorods to further improve solid state lighting. Quantum nanorods of particular compounds such as selenides and sulfides are susceptible to chemical modifications, through a process called cation exchange, where the intrinsic anion structure of the particle, the morphology, the shape and size of the particle are retained while the native cations of the particle are swapped for those of another element. This process through careful tailored conditions can create distinct regions in these nanoparticles with different chemical compositions, creating heterostructures in the nanoparticle. The formation of heterostructures at such scales we can create the formation of quantized systems such as quantum wells that are capable of emitting and absorbing lights at very particular frequencies with very high efficiencies. Using this high level of colloidal chemistry, we suggest the study of the possible quantum systems we can synthesize to develop optoelectronic devices. Though the use of computational studies and experimental values of bandgaps and band edge positions of possible cation exchange products in nanorod particles, we were able to design 42 new quantum well nanostructures that could be of use in applications such as: Solid state lighting, lasers, photocatalysts and solar panels. Additionally, we propose troubleshooting approaches to modify the injection rates of carriers by modifying the well barriers to increase efficiency. Lastly, we propose a plan of action for the development and testing of a basic system and how to observe quantum confinement in the structure. These nanoparticle systems could ultimately be use for solid state lighting, flexible displays and solar panels, and could also enable high efficiency multijunction photocatalysts and photovoltaics.