Date of Award

May 2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Junhong Chen

Committee Members

Benjamin Church, Woo Jin Chang, Yin Wang, Yongjin Sung

Keywords

Catalysis, Energy conversion, Energy storage, Nanotechnology

Abstract

Transition metal oxides (TMOs) and transition metal dichalcogenides (TMDs) have gained immense interest recently for energy and environmental applications due to their exceptional structural, electronic, and optical properties. For example, titanium dioxide (TiO2) as one of the TMO photocatalysts has been widely studied due to its stability, non-toxicity, wide availability, and high efficiency. However, its wide bandgap significantly limits its use under visible light or solar light. Recent studies also show that semiconducting TMDs could be used as potential supercapacitor electrode materials and platinum (Pt)-free electrocatalysts for economical utilization of renewable energy, because the high cost and scarcity of Pt have impeded the large-scale commercialization of many green technologies.

In this dissertation study, various novel TMO and TMD nanomaterials are designed and synthesized, and their catalytic performance is further investigated. First, a facile route for the controllable synthesis of modified TiO2 is designed to improve its photocatalytic efficiency under the visible/solar light. The resulting Ti3+-doped TiO2 with tunable photocatalytic properties using a hydrothermal method with varying amounts of reductant, i.e., sodium borohydride (NaBH4), showed color changes from light yellow, light grey, to dark grey with the increasing amount of NaBH4. The present method can controllably and effectively reduce Ti4+ on the surface of TiO2 and induce partial transformation of anatase TiO2 to rutile TiO2, with the evolution of nanoparticles into hierarchical structures attributing to the high pressure and strong alkali environment in the synthesis atmosphere; in this way, the photocatalytic activity of Ti3+-doped TiO2 under visible-light can be tuned. The band gap of Ti3+-doped TiO2 based on the Kubelka-Munk function is 3.1 eV, which is smaller than that of pristine TiO2 (3.28 eV), confirming that adding NaBH4 as a reductant causes the absorption edge of TiO2 to shift to a lower energy region. After 20 min of simulated sunlight irradiation of photocatalytic reactions for the degradation of methylene blue (MB) aqueous solution, nearly 97.2% of MB was degraded by the sample TiO2-4 (reduced by 12 g of NaBH4 in the hydrothermal reaction), compared with the degradation efficiency of the pristine TiO2 (23.5%). The as-developed strategy may open up a new avenue for designing and functionalizing TiO2 materials with enhanced visible light absorption, narrowed band gap, and improved photocatalytic activity.

Second, cobalt sulfide-based (CoSx) nanostructures as one of the TMDs are competitive candidates for fabrication of supercapacitor electrodes due to their high specific surface area, high electrical conductivity, and redox-active structures. However, CoSx materials still suffer from relatively low specific capacitances, degradation of performance over long cycling duration, and tedious synthesis and assembly methods. Hence, metallic vertically-aligned cobalt pyrite (CoS2) nanowires (NWs) are prepared directly on current collecting electrodes, e.g., carbon cloth or graphite disc, for high-performance supercapacitors. These vertically-aligned CoS2 NWs have a variety of advantages for supercapacitor applications. Because the metallic CoS2 NWs are synthesized directly on the current collector, the good electrical connection enables efficient charge transfer between the active CoS2 materials and the current collector. In addition, the open spaces between the vertical NWs lead to a large accessible surface area and afford rapid mass transport. Moreover, the robust CoS2 NW structure results in high stability of the active materials during long-term operation. Electrochemical characterization reveals that the CoS2 NWs enable a large specific capacitance (828.2 F/g at a scan rate of 0.01 V/s) and excellent long-term cycling stability (0-2.5% capacity loss after 4,250 cycles at 5 A/g) for pseudocapacitors. This example of vertically-aligned metallic CoS2 NWs for supercapacitor applications expands the opportunities for transition metal sulfide-based nanostructures in emerging energy storage applications.

Third, to combine the advantages of TMOs and TMDs, an aerosol processing method is developed for the facile and green synthesis of reduced graphene oxide (rGO)/tungsten disulfide (WS2)/tungsten trioxide (WO3) ternary nanohybrids, because both TMOs and TMDs are promising candidates for platinum-free electrocatalysts in renewable energy applications. The resulting hybrid material has a spherical structure constructed of crumpled graphene and WS2/WO3 nanorods. The crumpled graphene/WS2/WO3 (CGTH) catalyst showed a superior electrocatalytic activity in the hydrogen evolution reaction (HER), with a Tafel slope of 37 mV/dec and an onset potential of 96 mV. Compared with reported MoS2/WS2-based electrocatalysts, this hybrid material shows one of the highest catalytic activities in HER. The environmentally-friendly synthesis and outstanding performance suggest a great potential of CGTH for noble metal-free electrocatalysts in water splitting.

Next, in order to improve the specific capacity of lithium-ion batteries (LIBs)/ potassium-ion batteries (PIBs) and relieve volume expansion of nanoparticles to fulfill the urgent need of reliable energy storage applications, TMD nanomaterials especially MoS2 quantum dots (QDs) have been considered promising anode materials for LIBs owing to their higher theoretical capacity and better rate capability compared with commercial graphite anodes. An exfoliated mesoporous MoS2 QDs-graphite composite anode was designed and investigated. The MoS2 QDs are located in the void spaces between graphite particles, thereby preventing the graphite particles from losing electrical contact with the current collector and enhancing the cycling performance of the MoS2/graphite composite anode. The optimized MoS2 QDs with graphite composites displayed good charge/discharge characteristics and the capacity maintained at 449.8 mAh g-1 after 300 charge/discharge cycles for LIBs. And the MoS2 QDs for PIB cells exhibited a stable capacity of approximately 409 mAh g-1 for 17 cycles.

Finally, metal-organic frameworks (MOFs) have attracted substantial research attention owing to their tunable pore size, high pore volume, high specific surface area, and highly ordered crystalline porous networks. Previous studies have mostly focused on sensing, drug delivery, batteries, and selective catalysis; however, their application as photocatalysts has not been thoroughly reported. It is well known that bulk MoS2 is unsuitable for photocatalytic applications due to the insufficient reduction and oxidation ability for the photocatalysis. However, exfoliated MoS2 exhibits a direct band gap of 2.8 eV resulting from quantum confinement, which enables it to possess suitable band positions and to retain good visible-light absorption ability. As a result, it is considered to be a promising candidate for photocatalytic applications. Encapsulating exfoliated MoS2 into MOF exhibits enhanced absorption in the visible light range compared with pure MOF and the highest hydrogen production rate could reach 68.4 μmol h-1g-1, which is much higher than that on pure MOF. With suitable band structure and improved light-harvesting ability, exfoliated MoS2@MOF can be a potential photocatalyst for hydrogen production.

This dissertation study suggests that modified TiO2 and exfoliated MoS2@MOF can be efficient photocatalysts with enhanced visible light absorption ability; metallic CoS2 NWs could be active materials with a large specific capacitance and excellent stability; reduced graphene oxide (rGO)/tungsten disulfide (WS2)/tungsten trioxide (WO3) as a ternary nanohybrid offers advantages of TMOs and TMDs, making it an outstanding noble-metal free electrocatalyst in water splitting; and MoS2 QDs with relieved volume expansion are promising anode materials for LIBs/PIBs. The study provides a scientific foundation to design and discover low-cost, efficient and stable TMOs and TMDs candidates for suitable energy and environmental applications.

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