title: Study of Low-dimensional Nanomaterials and the Applications on Energy Conversion and Storage Devices
Creators: name: Wang, Yazhou
Publisher: name: Griffith University
Year: 2018
Subjects: subject: Low-dimensional nanomaterials
subject: Energy conversion
subject: Storage devices
subject: Photoelectrochemical sensors
subject: Batteries

description: Low-dimensional nanomaterials have attracted much attention due to their unique morphology, including zero, one and two-dimensional structures. These unique morphologies introduce excellent optical, electrical and magnetic properties, which make these materials promising active materials in high-efficiency energy conversion and storage devices. In this thesis, a series of low dimensional nanomaterials have been designed and fabricated. Their uses in photoelectrochemical and electrochemical devices were then investigated in several studies. In the first study, one-dimensional plasmonic titanium dioxide nanorod arrays (TiO2) photoelectrodes were modified with zero-dimensional plasmonic gold (Au) nanoparticle deposition and subsequent hydrogenation treatment (i.e. Au@H-TiO2). The photoelectrode delivered a tunable oxidation capability to selectively detect organic compounds in water bodies as PEC sensors. Under visible light, the Au@H-TiO2 electrode exhibits selective detection capability to labile organic compounds due to surface plasmonic resonance effects. Furthermore, the modified electrode can also indiscriminately and rapidly detect all kinds of organic compounds under UV irradiation due to the strong oxidation capability. This unique tunable oxidation capability enables the use of Au@H-TiO2 photoelectrodes in new generation PEC sensors for selective and collective degradation of organic compounds. The hydrogenation process was further explored by the synthesis of two-dimensional nanomaterials. A top-down strategy using high-pressure hydrogen surface treatment and controlled etching to reduce the thickness of Fe2O3 nanoplates was proposed. As a result, the thickness of the two-dimensional Fe2O3 materials could be further reduced to ca. 4 nm. Such an ultrathin structure can enhance internal diffusion of lithium ions, shorten electron transport length and increase tolerance to large volume changes. When employed as an anode material in rechargeable lithium-ion batteries (LIBs), it delivers a super-high reversible capacity of ca. 1043 mAh g-1 after 100 cycles at a current denisty of 0.1 A g-1 and ca. 578 mAh g-1 after 500 cycles at a current denisty of 5.0 A g-1. The zero-dimensional Fe3C nanoparticles embedded in two-dimensional nitrogen-doped porous carbon sheets (Fe3C@NPCS) were synthesised by a facile one-step carbonizing process. Fe3C nanoparticles are not only able to strongly adsorb polysulfides via strong Fe-S chemical bonds, but also accelerate the electrochemical reactions of the polysulfides through fast electron transportation. The two-dimensional carbon sheets prevent the aggregation of Fe3C nanoparticles in the synthesis, and accommodate sulfur and its volume expansion during charge/discharge. With these features, the resultant sulfur cathodes (Fe3C@NPCS-S) deliver an excellent cycling performance with a capacity decay of 0.036% per cycle, and high rate performance of ca. 1127, 1020, 907, 802, 731 and 647 mAh g-1 under current density of 0.3, 0.5, 1, 2, 3 and 5 C, respectively. Through a green scalable, and one-step calcination process, two-dimensional holey Fe, N co-doped graphene (HFeNG) was synthesized. Diverging by the holey structures, the Fe atoms are anchored by four N atoms (Fe-N4) or two N atoms (Fe-N2) localized on the graphene sheets and edge of holes, respectively, which was confirmed by X-ray absorption spectroscopy. The holey structures not only provide additional channels for the mass transport of lithium ions, but also adsorb polysulfide species and facilitate their transformation to electrochemical products. Density function theory calculation suggests that the Fe-N2 moiety at the edges are responsible for the strong adsorption forces. The as-obtained HFeNG delivered a high rate capacity of 810 mAh g-1 at 5 C and a stable cycling performance with capacity decay of 0.083% per cycle at 0.5 C for 300 cycles. In summary, the proposed strategies successfully manipulated the morphological and chemical structures of low-dimensional materials. The obtained results demonstrated the advantages of low-dimensional materials for high-efficiency performance photoelectrochemical sensors and lithium sulfur batteries. These strategies can be further developed as generic methods to synthesise and modify low-dimensional nanomaterials for other energy conversion and storage devices such as supercapacitors, alkali-ion batteries, metal-air batteries and metal-halogen batteries.