Abstract
The global demand for energy has been growing tremendously. A term of hydrogen economy were coined by Prof. John OʼM. Bockris during a talk he gave at General Motors (GM) Technical Center in 1970. The hydrogen economy is a proposed system of delivering energy and generating electrical power using hydrogen instead of traditional fossil fuel. Hydrogen is a non-emission and pollution free energy carrier which can be used as a fuel to produce electricity via chemical reactions such as fuel cell. However, the major issues are how to establish the efficient and cost-effective way to produce clean hydrogen gas and how to improve the utilization efficiency of hydrogen energy. It is the purpose of this research to explore nanocomposite materials as the highly active catalysts by atomic layer deposition (ALD) for photocatalysis and photocatalytic water splitting. We have employed the novel nanolamination of ALD process instead of conventional solution-based method to fabricate TiO2-based materials. It was attempted to achieve homogeneous doping distribution and to inhibit unwanted phase segregation. Furthermore, Pt and Cu3N nanoparticles as the anode and cathode electrocatalysts for application in fuel cell were deposited on innovative heterostructures by ALD. It not only reduced the usage amount of catalysts but also enhanced the efficiency of fuel cell. Therefore, this dissertation is divided two parts, which give insight into the fundamental understanding of correlation between lattice structure, morphology, surface chemistry, and electrochemical and photocatalytic properties of those novel nanocomposite catalysts. In the first section, highly homogeneous Al- and Zn-doped TiO2 nanotubes were fabricated by ALD via nanolaminated stacks of binary layers of Al2O3/TiO2 and ZnO/TiO2, respectively. The bilayers were alternately deposited on the polycarbonate (PC) membrane template by ALD with various cyclic sequences. The nanotubes in a length of 20 μm and a diameter of 220 nm were obtained after removal of the PC membrane by annealing at 450 oC. The doping concentrations of Al2O3 and ZnO in TiO2 depended on the precursor cycle ratios of Al2O3 and ZnO to TiO2. From the depth profiles measured by secondary ion mass spectrometry, Al and Zn are uniformly distributed across the thickness. With the precursor cycle ratios of Al2O3 and ZnO to TiO2 at 0.04, uniform bulk solubilities of ~7-8 at. % were obtained, and the surface concentrations were even higher, ~16-18 at. %. From the transmission electron microscopic observation, the highly doped anatase TiO2 exhibited some regions of severe deformation that resulted in localized solid-state amorphization. In addition to characterizations, the effects of doping composition on the photocatalytic and photoelectrochemical (PEC) activities were investigated. Increasing the Al doping reduced the photocatalytic activity of TiO2 due to formation of charge recombination sites and reduction of hydroxide radicals. In contrast, there was an optimal range of Zn doping to get enhanced photocatalytic activity and higher PEC efficiency. With a doping ratio of 0.01, the hydrogen production rate from water splitting was 6 times higher than that of commercial P25 TiO2. The photoinduced trapped electrons and holes were detected in Zn-doped TiO2 by in-situ electron paramagnetic resonance spectroscopy, which revealed that Ti3+ sites on the surface and surface oxygen vacancies played a key role in promoting the photocatalytic process. We have demonstrated a photocatalytic Au@ZnO@PC nanoreactor composed of monolayered Au nanoparticles chemisorbed on conformal ZnO nanochannel arrays within the PC membrane. Commercial PC membrane was used as the template for deposition of ZnO shell into the pores by ALD. Thioctic acid with sufficient steric stabilization was used as molecular linker for functionalization of Au nanoparticles in a diameter of 10 nm. High coverage of Au nanoparticles anchored on the inner wall of ZnO nanochannels greatly improved the photocatalytic activity for degradation of rhodamine B. The membrane nanoreactor achieved 63% degradation of rhodamine B within only 26.88 ms of effective reaction time owing to its superior mass transfer efficiency based on Damköhler number analysis. Mass transfer limitation could be eliminated in the present study due to extremely large surface-to-volume ratio of the membrane nanoreactor. Second part is to develop highly active catalysts for fuel cell application. A heterostructured electrocatalyst consisting of ZnO nanorods in a diameter of 25 nm on carbon cloth (CC) was synthesized by combining ALD and hydrothermal methods. Platinum nanoparticles were then deposited on photo-induced hydrophilic surface of ZnO nanorods by ALD. Electrochemical performance of the nanocomposite catalyst (Pt@ZnO@CC) for methanol oxidation reaction with or without UV irradiation was evaluated. The surface of ZnO nanorods rich in hydroxide species was more favorable for removal of CO via the so-called bi-functional mechanism. Additionally, the charge transfer occured between the ZnO nanorods and the Pt nanoparticles. UV light irradiation on the catalyst surface increased the chronoamperometric response by 62%, which was attributed to a synergistic effect of large surface area and strong light absorption in the UV region by the presence of ZnO nanorod arrays. A hybrid electrocatalyst consisting of copper (I) nitride (Cu3N) nanoparticles grown on carbon nanotubes (CNTs) by plasma enhanced ALD is presented as well. Island growth mechanism during ALD led to the formation of uniformly distributed Cu3N nanoparticles on the surface of CNTs. The size of copper nitride particles strongly influenced the electrocatalytic properties, and it could be precisely tuned by controlling the cycle number of ALD. The Pt-free non-precious nanocrystals coupling with CNTs exhibited pronounced electrocatalytic activity for oxygen reduction reaction (ORR). Koutecky–Levich analysis on the ORR current densities indicated that the Cu3N@CNT electrodes in alkaline media followed a mixed two- and four-electron transfer ORR pathway, whose mass activities were comparable to that of typical Pt/C electrode. A facile process to fabricate well-dispersed metal nitride on a selected support material as an ORR catalyst could raise the catalytic activity by synergistic chemical coupling effects.