Finding new energy solutions have been the focus of recent research in the fields of science and engineering. Nanotechnology could provide decisive technological breakthroughs and have a considerable impact on harvesting the renewable energy supply that is required to make the transition from fossil fuels. During my PhD study, I have worked on nanostructured inorganic materials with applications in energy conversion and storage. This thesis includes several different topics.
First, we have developed a facile method to prepare free-standing Co3O4 nanowire arrays in solution via the ammonia evaporation induced growth. These nanowires are hollow, mesoporous, and single-crystalline. We have carefully studied their growth mechanism, and discovered that they are from the topotactic oxidation conversion of intermediate brucite Co(OH)2 nanowires. This process is accompanied by the Kirkendall effect: fast-moving outward diffusion of Co2+ ions are balanced by the inward diffusion of vacancies, which eventually condense as the hollow core. More interestingly, we have identified that axial screw dislocation plays a critical role in the 1D growth of intermediate brucite Co(OH)2 crystals. Moreover, we have extended the ammonia evaporation induced growth to the syntheses of other metal hydroxide/oxide nanostructures. Hierarchical architectures, including CuO spheres, ZnO dendrites, Cd(OH)2 or CdO nanofibers, and Ni(OH)2 or NiO nanoplates have been prepared by carefully controlling reaction conditions.
We have demonstrated the use of nanowire arrays for two important electrochemical applications: lithium ion batteries and oxygen evolution reaction (OER). Co3O4 nanowire arrays grown on Ti foils have been directly applied as the anode of Li ion batteries. They have high specific capacity and excellent rate capability at current rates as large as 50C. Also, we are able to dope Co3O4 nanowire arrays with Ni by a similar method. They show high electrocatalytic activity for OER. In both applications, the superior performance of nanowire array electrode compared to their nanoparticle counterparts is resulted from the unique nanowire array configuration and mesoporous microstructures.
On the Li ion battery project, the use of LiFePO4/graphene composite as the cathode material has also been explored. They have higher capacity and dramatically improved rate capability compared to pure LiFePO4. The enhancement results from the fast kinetics of electron and ion transport when graphene is introduced.
In addition to the use of graphene for hybrid materials, we have also discovered that its oxidized analogue – graphene oxide (GO) can act like a 2-D surfactant molecule for aligning nanowires. Na0.44MnO2 nanowires have been successfully prepared by a hydrothermal reaction. They are transformed from intermediate birnessite nanosheets through a stress-induced splitting mechanism. The interesting colloidal property of GO nanosheets and Na0.44MnO2 nanowires has been carefully studied. Upon mixing in aqueous solution, the interaction between them leads to the modification of nanowire surface property from superhydrophilic to less-hydrophilic. Consequently in our droplet test, nanowires are gradually enriched at the solution surface and self-assemble with GO. Unidirectional nanowire alignment can also been achieved on a wafer scale by the meniscus deposition. These phenomena have been understood by modeling nanowire surface potential energy and applying Onsager’s rigid rod theory.