Graphite Oxide Template Based Synthesis and Characterization of Metal Oxide Nanosheets
Kyle B. Tom
Doctor of Philosophy in
Engineering – Materials Science and Engineering
University of California, Berkeley
Professor Jie Yao, Chair
Two dimensional materials and their composite structures show unique and favorable properties for many different kinds of applications and physical studies. Isolated 2D materials have shown a unique array of physical phenomena, including the room temperature observation of the quantum Hall effect, as well as enhanced physical properties, including large exciton energies and extremely high carrier mobilities. Composites have shown enhanced performance in traditional applications, including enhanced photocatalysis and battery performance, that stems from a synergistic effect between the individual materials in the composite. However, there is a limited number of naturally layered materials, an even smaller subset that is stable in ambient conditions. As such, it is beneficial to find 2D materials with better properties that may originate from other types of crystals. It has been predicted certain types of crystal structures, such as wurtzite, will form a two-dimensional phase similar to αBN below a particular thickness threshold. ZnO, and its graphitic form (gZnO) is of particular interest due to its favorable optical properties and oxidation resistance. gZnO has also been shown to have superior piezoelectric performance over similarly structured most other 2D materials, including BN and transition metal dichalcogenides (TMDCs). It also has shown excellent properties for many other applications that range from magnetism to catalysis. However, forming such confined systems is difficult and has limited the lateral size significantly. This has limited experimental work on this system, prohibiting application and basic understanding.
This dissertation explores the use of a graphite oxide template technique that confines the growth direction to synthesize multiple metal oxide nanosheets, with specific focus on sub-nm ZnO. Graphite oxide, a chemically functionalized form of graphene, has many properties that are favorable for use as a template. The interlayer distance ranges from 0.6 – 1.2 nm, creating a nanosized reactor. Additionally, graphite oxide templates are hydrophilic; this allows templates to be easily to synthesized in water by sonication and helps aqueous precursors be drawn in between layers. Because of these advantages, as well as the large template size and uniformity, graphite oxide offers a unique opportunity for nanosheet synthesis.
This method shows growth of nanosheets in the tens of microns size. The structure of these composites and the nanosheets are presented here, including TEM side view images of the stacking and fine geometry verification of the gZnO transition using XANES and thorough simulation work. Changes in the optoelectronic properties are determined through various measurements including XAS, EELS, XPS, and electronic transport. In particular, gZnO has a massive increase in the bandgap and a strong enhancement in the Zn 4s state availability. Improvements and advantages of this method are shown, including the compatibility with plasma cleaning and doping.
In addition to van der Waals materials and thickness dependent structures, a significant amount of work has been focused on taking typical 3D crystals and growing them into two dimensions. This technique offered a way to synthesize some of these nanosheets. Fe2O3 was used as a test case and showed similar lateral sizes to the gZnO. Composites of the synthesized Fe2O3 nanosheets and the rGO showed strong interactions between the two, leading to some unique properties. The rGO transfers a significant amount of charge to the Fe2O3, most likely as a way to passivate the dangling bonds in the crystal. The XAS spectra of composites show significant formation of Fe+2 like states, making the hematite appear more like magnetite or maghemite. The magnetic properties of Fe2O3 are measured using SQUID and XMCD. Composites show enhanced coercivity and saturation, but no change in the overall magnetic structure.
Other materials were also grown using this method, including doped and alloyed gZnO, NiO, and MgO. Various changes were observed, including strong changes in the core level shifts by XPS and the absence of an interlayer between SiO2 and rGO. Some limits of this technique were also found, as Al2O3 and CuO showed distinct nanoparticle formation instead of nanosheet formation. Finally, a rule of thumb is presented for materials that are compatible with this technique in hopes that it can be further refined and used as a guide for future work in this direction.