Nanostructuring molybdenum disulfide to engineer its surface and bulk properties for solar hydrogen production [electronic resource]
- Zhebo Chen.
- Physical description
- 1 online resource.
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|3781 2012 C||In-library use|
- Chen, Zhebo.
- Jaramillo, Thomas Francisco, primary advisor.
- Bent, Stacey, advisor.
- Prinz, Friedrich, advisor.
- Stanford University. Department of Chemical Engineering
- The development of readily available and sustainable sources of energy is a key component to ensuring our long-term economic growth and prosperity. Solar technologies, in particular, hold the potential to address a significant portion of our energy demands for decades to come. However, large scale proliferation of solar energy requires further technological advancements to efficiently collect, store, and distribute the energy provided by the sun. Photoelectrochemical (PEC) water splitting is one approach that converts the energy available from solar photons directly into hydrogen (H2) in a manner that is sustainable and carbon-free. In this process, the solar energy absorbed by a PEC device oxidizes water via the oxygen evolution reaction (OER) to produce a sustainable supply of protons (H+) which are concurrently reduced into H2 via the hydrogen evolution reaction (HER). H2 production currently represents a $100 billion/year industry that consumes 2-3% of worldwide energy demand due to the use of H2 in large-scale industrial processes such as ammonia synthesis and petroleum refining. It can also serve as a fuel that can be stored and distributed, and is amenable to future integration into the transportation sector. The successful development of PEC devices requires researchers to engineer high performance materials that can efficiently absorb and convert solar photons into electrons, and that can effectively drive the HER and OER half-reactions that represent overall water splitting. In addition, the materials must be non-precious, simple to manufacture, and stable over many years of operation in order to be economically viable. No material currently meets all of these requirements. Molybdenum disulfide (molybdenite, MoS2) is widely used as an industrial catalyst in hydrodesulfurization reactions. In recent years, it has also gained attention for its ability to efficiently drive the HER and for its unique optical properties as a semiconductor. In this thesis, we discuss how nanoscale engineering of MoS2 enables novel material properties that make this material attractive for implementation into PEC devices. We first present the development of core-shell MoO3-MoS2 nanowires which display high efficiency for the HER and excellent stability in acidic electrolytes. By controlling the length scale of MoS2 to just a few atomic layers, we demonstrate the ability to completely eliminate charge transport limitations in MoS2 while simultaneously imparting a protecting effect upon its underlying support architecture. We further discuss how we overcome thermodynamic limitations in the morphology of MoS2 by carefully engineering a mesoporous architecture with a high radius of curvature of only a few nanometers to selectively expose a higher density of catalytically active edge sites, enabling the production of H2 at solar-relevant current densities with minimal overpotential. MoS2 also displays quantum confinement effects at the nanoscale that allow tuning of its bandgap to potentially achieve greater photovoltages as the absorber material in PEC water splitting. We present the synthesis of quantum confined nanoparticles using an inverse micellar method and elucidate their band structure using PEC measurements. Furthermore, we bridge the gap between the properties of bulk MoS2 and nanostructured MoS2, and discuss how surface state effects impact PEC device performance. Lastly, we present strategies for integration of photoactive materials into hierarchical support structures to simultaneously enable high optical density and efficient charge transport. These examples showcase how nanostructuring can be a powerful tool in the pursuit to develop next generation materials for solar energy conversion and storage. The design principles discussed in this thesis are applicable to the development of a broad range of materials.
- Publication date
- Submitted to the Department of Chemical Engineering.
- Thesis (Ph.D.)--Stanford University, 2012.
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