Doctor of Philosophy
Mechanical and Materials Engineering
Dr. Sun, Xueliang
This thesis presents the fabrication of a series of novel nanostructured materials using atomic layer deposition (ALD). In contrast to traditional methods including chemical vapor deposition (CVD), physical vapor deposition (PVD), and solution-based processes, ALD benefits the synthesis processes of nanostructures with many unrivalled advantages such as atomic-scale control, low temperature, excellent uniformity and conformality. Depending on the employed precursors, substrates, and temperatures, the ALD processes exhibited different characteristics. In particular, ALD has capabilities in fine-tuning compositions and structural phases. In return, the synthesis and the resultant nanostructured materials show many novelties.
This thesis covers ALD processes of four different metal oxides including iron oxide, tin oxide, titanium oxide, and lithium titanium oxide. Four different substrates were used in the aforementioned ALD processes, i.e., undoped carbon nanotubes (CNTs), nitrogen-doped CNTs (N-CNTs), porous templates of anodic aluminum oxide (AAO), and graphene nanosheets (GNS). In practice, owing to their distinguished properties and structural characters, the substrates contributed to various novel nanostructures including nanotubes, coaxial core-shell nanotubes, and three-dimensional (3D) architectures. In addition, the surface chemistry of the substrates and their interactions with ALD precursors also were considered.
The ALD process of iron oxide (ALD-Fe2O3) was the first one studied and it was fulfilled on both undoped CNTs and N-CNTs by using ferrocene and oxygen as precursors. It was found that N-CNTs are better than undoped CNTs for the ALD-Fe2O3, for they provide reactive sites directly due to their inherent properties. In contrast, undoped CNTs need pretreatment via covalent acid oxidation or non-covalent modification to create reactive sites before the ALD-Fe2O3 could proceed on their surface. This work resulted in different CNT-Fe2O3 core-shell structures with controlled growth of crystalline α-Fe2O3.
Another metal oxide, tin dioxide (SnO2) was performed using tin chloride (SnCl4) and water as ALD precursors. It was synthesized into different nanostructures based on N-CNTs, AAO, and GNS. The work on N-CNTs disclosed that the ALD-SnO2 is favored by doped nitrogen atoms but the effects of different nitrogen-doping configurations vary with growth temperatures. In comparison, the ALD-SnO2 on AAO and GNS mainly relies on hydroxyl groups. A common finding from the studies is that growth temperatures influence the resultant SnO2, leading to amorphous, crystalline phase, or the mixtures of the aforementioned two. In addition, the cyclic nature of ALD contributes to controlled growth of SnO2. Based on the results from the ALD-SnO2 on AAO, it was concluded that the ALD-SnO2 experience three different growth modes with temperature, i.e., layer-by-layer, layer-by-particle, and evolutionary particles. The layers are in amorphous phase while the particles are in crystalline rutile phase. The aforementioned understandings on ALD-SnO2 led to pure SnO2 nanotubes based on AAO, CNT-SnO2 core-shell coaxial nanotubes, and GNS-based SnO2 3D architectures with controlled growth and structural phases.
The third metal oxide, titanium dioxide (TiO2) was deposited using titanium isopropoxide (TTIP) and water as ALD precursors. It was found that the ALD-TiO2 is tunable from amorphous to crystalline anatase phase with temperature while the resultant deposition is controllable from nanoparticles to nanofilms as well. Based on different substrate, i.e., AAO, acid-pretreated CNTs, and GNS, TiO2 was fabricated with different nanostructures including nanotubes, core-shell coaxial nanotubes, and 3D architectures. In particular, the resultant nanostructures are distinguished with controlled phases and morphologies of TiO2.
Different from the above binary metal oxides, the last metal oxide, lithium titanium oxide (Li4Ti5O12, LTO) is a ternary compound. The route for ALD-LTO is based on combining and tuning two sub-ALD systems. One sub-ALD system is for TiO2 using TTIP and water, and another sub-ALD system is for lithium-containing films using lithium tert-butoxide (LTB) and water as precursors. It was revealed that, through suitably matching the ratios of the two sub-ALD systems and annealing the resultant films, LTO is successfully synthesized on N-CNTs. However, this pioneering work shows a bit rutile TiO2 with LTO, and thus further effort is needed in future work.
In summary, the discoveries in this thesis contribute to a better understanding on various ALD systems and provide a series of novel nanostructured materials for various potential applications. In particular, these materials are promising candidate materials for energy-related devices, such lithium-ion batteries, fuel cells, and solar cells.
Meng, Xiangbo, "On Developing Novel Energy-Relates Nanostructured Materials by Atomic Layer Deposition" (2011). Electronic Thesis and Dissertation Repository. 247.