Growth of vanadium oxides nanostructures by chemical vapour deposition
Sim, Dao Hao
Date of Issue2015
School of Materials Science and Engineering
Among the transition metal oxides, vanadium oxide (V2O5) has attracted attention in various industrial applications such as electrochromic, optical switching and energy storage devices. However, the commercial application of V2O5 in energy storage devices is limited due to its poor cycling stability, Li ion diffusion and electric conductivity. Two-dimensional (2-D) nanostructures have demonstrated better stability, more active sites for lithium insertion/deinsertion than zero-dimensional and onedimensional (1-D) structure, making them attractive in energy storage devices. The formation of V2O5 nanosheets have been challenging because V2O5 tends to grow into 1- D structure because of its preference to grow in  direction. Although several works have been done on the formation of nanosheets, the synthesis requires long duration (~12- 48 hours). This thesis investigates the growth of 2-D V2O5 nanostructures arrays on various current collectors such as aluminium, nickel and stainless steel using chemical vapour deposition process. This includes various approaches to form 2-D V2O5 nanostructures arrays on current collectors, namely self-growing and self/wet-etching. Aluminium foil was wet etched to create a 2-D hierarchical current collector using lithium hydroxide alkaline solution. Al foil etched with 0.15 M LiOH solution shown larger amount of nanosheets and etched surface area among various concentrations tested. V2O5 thin film grown on etched Al foil has an average particle size of ~205, ~121, ~70 and ~24 nm when placed with a substrate distance of 7, 10, 13, 25 cm away from the center of the tube furnace. The growth of the nanoparticles thin film follows the island growth mechanism where the adatom adsorbed on the nuclei. Supercapacitor measurements were made suggests that V2O5 on hierarchical Al perform better with Cs of ~289 F g-1 and retention of ~91% and V2O5 on untreated Al foil with Cs of ~152 F g-1 and retention of ~76% after 1000 cycles at a rate of 5 A g-1 in 1 M LiClO4 in PC. The capacitance of the electrode is also dependent on the thickness or mass loading of the film. By increasing the amount of V2O5 deposition, the Cs is reduced from ~295 to ~189 F g-1. v Catalyst-free growth of nanowires and nanosheets were synthesized directly on Ni foam. V2O5 nanowire averages a diameter of ~126 nm and a few micrometers in length. The growth is suggested to follow the vapour-solid growth and the formation is due to the anisotropic growth as  is the fastest growth direction. Interconnected V2O5 nanosheet arrays on Ni foam have an average diameter of ~13.5 µm and thickness of ~198 nm. The growth of nanosheets arrays on Ni foam is due to the high substrate temperature which leads to fast diffusion of adatoms and crystal growth rate, and allows side deposition, forming the nanosheets structure. V2O5 nanosheets on Ni foam perform better with Cs of ~1081 F g-1 and retention of ~96% and V2O5 nanowire on Ni foam with Cs of ~833 F g-1 and retention of ~75% after 1000 cycles at a rate of 2 A g-1 using 2 M KOH. Vanadium deposition on stainless steel foils results in the formation of amorphous iron vanadate (FeVO4) nanosheets arrays. The width and thickness of nanosheets are ~100 – 500 nm and 10 – 40 nm, respectively. The growth of the FeVO4 thin film initiates from the oxidation of stainless steel foil forming Fe2O3 on the surface and interacting with the vanadium precursor. Annealing the sample to improve crystallinity in Ar atmosphere leads to the collapse of nanosheet arrays and evaporation of vanadium from the as-grown film. The electrochemical property of the as-grown film achieved an initial discharge capacity of 1693 mAh g-1 on the initial cycle at a rate of 0.2 A g-1 (0.15 C). The Coulombic efficiency is ~73% for the first cycle and shows a reversible capacity of 1237 mAh g-1 after 100 cycles. The formation of two-dimensional nanosheets arrays on various current collectors have shown good performance and improved cyclic stability. Various morphologies such as nanoparticles thin film and nanowires were also synthesized, demonstrating the flexibility of chemical vapour deposition process.