Defect, interface and stack engineering of two dimensional materials for energy applications

• Main Author: Yan, Cheng
• Other Authors: Seh Zhi Wei
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Materials
• Online Access: http://hdl.handle.net/10356/75769
• Institution: Nanyang Technological University

The emergence of novel two-dimensional (2D) materials triggered by graphene opens a new window and provides an exceptional platform in a large variety of potential applications such as flexible electronics, photodetector and clean energy due to their intrinsic novel and fascinating physicochemical properties compared with their bulk counterparts. So far, various 2D materials with versatile functions have been reported and synthesized with varied success. For example, insulating hexagonal boron nitride (h-BN), semiconducting and metallic transition metal dichalcogenides (TMDCs), black phosphors (BP) and even non-layered structure 2D materials pure metal nanosheets have aroused tremendous research interests in recent years, further expanding the 2D family. More importantly, the unique 2D crystal structure and high anisotropy hold great promise for endowing them with tunable electronic and catalytic properties. In this regard, the aim of this PhD thesis is to tune novel 2D materials in a broad regime including defect engineering, interface engineering and stacking engineering and then investigate their possibilities and potential applications for high-performance lithium-ion batteries (LIBs), photocatalytic hydrogen evolution reactions (HER) and electrocatalytic oxygen reduction reactions (ORR). Firstly, highly nitrogen-doped and defect-rich porous nanocarbon has been synthesized by in-situ simple pyrolysis of zeolitic imidazolate framework nanoparticles grown on graphene oxide as negative electrode in lithium ion batteries (LIBs), which demonstrates extraordinary capacities, remarkable cycling performances and rate capability and surpasses carbon-based electrodes in literature and most of metal oxide-based anodes from metal organic frameworks (MOFs). With an impressive initial capacity of 1378 mAh g−1 under current density with 100 mA g−1, it could be operated for 100 and 200 cycles under current densities of 500 mA g−1 and 1000 mA g−1, leading to reversible capacities of 1070 mA g−1 and 948 mAh g−1, separately. If current density was up to 5000 mA g−1, it still reversibly presents a decent capacity of more than 530 mAh g−1 after 400 cycles, exhibiting a high rate of retention of 84.4% in terms of capacity. It is considered that the excellent electrochemical output is due to the perfect bond of an extremely conductive platform (graphene), high-level doping of nitrogen (defect-rich) and hierarchically porous structure. Secondly, via interface engineering, solid edge-on heterostructures between few-layered MoS2 and TiO2 were produced on large scale through a facile two-step, which realized the idea that layered MoS2 can be welded laterally to photocatalysts via edges to enhance the catalytic activity toward water splitting owing to the efficient transport of photoexcited electrons along the highly conductive edges and basal planes of MoS2. The unusual photocatalytic activity is attributed to the improved electronic contact and the optimized electron transport pathways between TiO2 and MoS2, where the recombination of photoexcited electron-hole pairs is effectively suppressed. This work represents a strategy, which marries low-cost layered transition-metal dichalcogenides with backboned photocatalyst as a promising heterostructure for high-efficiency water splitting. Lastly, a facile and large-scale approach has been developed to prepare platinum-based binary alloy nanosheets by stacking engineering target platinum-based alloy foil with aluminum foil, in which asymmetric mechanical rolling technique was employed to repeat the folding and rolling process. With the assistance of sodium hydroxide, aluminum metal layers were selectively etched and removed completely. Combining theoretical calculations, rational material design with optimal compositions of Pt3M (M: Ni, Fe, Co, Ir, Y, Au, Pb) was proposed for electrocatalytic oxygen reduction reactions (ORR). Through a series of characterization techniques, the sheet-like morphology was observed with lateral size of 2-3 μm and the thickness ranges from 5.5 nm to 9.7 nm. The activity and stability of ORR were greatly improved. It is widely believed that altered electronic properties by alloying and changed surface by 2D materials induce a variation for adsorption behavior, which were considered to be the origin of the high ORR activity.