Synthesis And Electrochemistry Of Functionalized Graphene And Molybdenum Disulfide Nanosheets

Since 2004, graphene has attracted growing research interest owing to large theoretical special surface area, excellent electronic mobility and mechanical properties. In decades, researchers have new acquaintance and comprehension to 2D layered materials, leading to a great development of synthesis and application of atomically thin 2D materials. A series of 2D layered materials has been discovered sequentially, such as carbon nitride, hexagonal boron nitride and transition metal dichalcogenides(TMDs). Due to the unique electronic structure and energy-band structure, an increasing attention has also been paid to mono- and few-layer TMDs materials. However, 2D materials with single component, such as graphene and TMDs, are hard to satisfy the requirement of properties tuning in modern technology and appliance. An effective way to regulate these properties is chemical doping and loading, which may not only bring a change in optical and electrical properties to pristine materials, but also provide abundant active sites for catalysis. This paper aims at preparing various doping and functionalized 2D materials, and using them as cathode ORR catalysts of fuel cells to instead of Pt-based materials.Chapter 1 not only briefly introduced the synthesis and application of 2D layered materials(graphene and TMDs), but also elaborated the research process and developing direction of fuel cells. In Chapter 2, a material comprised of regularsized Fe3 C nanoparticles and 3D porous N-doped graphene was in situ pyrolytically synthesized by using Fe-MIL-88 b nanocrystal as the size-controlled Fe precursor. The Fe3C/NGr exhibited extra ORR catalytic performances in alkaline media, even better than the commercial Pt/C catalyst. Moreover, the Fe3C/NGr also showed excellent HER catalytic property, manifesting low onset overpotential and a small Tafel slope.Thus, the Fe3C/NGr material is a novel multi-functional catalyst, which may open upmore potential applications. In chapter 3, via a facile pyrolytical approach and by using layered C3N4 sacrificial template, a novel Mo C-based ORR catalyst comprising of ultrasmall Mo C nanoparticles and 3D porous N-doped graphene is achieved. Due to the unique structural features(i.e., preferential O2 absorbing sites, good accessibility, and favorable 3D porous conductive framework), the proposed catalyst displays great ORR performance in alkaline media with good four-electron selectivity(n ≈ 4.0). In chapter 4, high-quality and water-soluble monolayer molybdenum disulfide quantum dots(MQDs) with unusual up-conversion PL were achieved by a general hydrothermal approach, which may also be instructive to fabricate some other monolayer QDs of TMDs similarly. The synthetic MQDs exhibit activated stronger luminescence at room temperature with manifest blue-shift of the absorption and emission wave-length, as well as enhanced PLQY than the monolayer Mo S2 nanosheets. Significantly, the MQDs exhibited unusual up-conversion PL. NIR excitation is particular attractive for researchers who are searching for efficient biolabeling reagents for bioimaging and medical diagnosis. Detection of Tc by using MQDs as a highly selective and sensitive fluorescent reagent was exemplified based on the fluorescence quenching in the presence of Tc. In chapter 5, ultrathin Mo S2 nanosheets were pyrolytic synthesis by using ammonium molybdate, thiourea and layered g-C3N4 template. In this synthetic process, the g-C3N4 sacrificial template played an important role in forming ultrathin nanosheets and porous structure. The physical characterization indicated that Mo S2 ultrathin nanosheets had good crystallinity, wrinkle, multilevel porous structure and large specified surface area. In chapter 6, phosphorus-doped Mo S2 ultrathin nanosheets(P-Mo S2) are pyrolytically achieved. Dramatically enhanced catalytic activity for ORR---positive potentials,increased ORR current density, four-electron selectivity, extra methanol tolerance and improved stability was obtained. The study illustrated that doping of phosphorus atom increases the frontier orbital energy of the nanosheets, endowing with better capability of donating electrons during the ORR process. Meantime, the lower electronegativephosphorus atoms in the plane of Mo S2 nanosheets lead to an abundance of active sites, which may prefer to absorb oxygen molecules in the electrolyte and further accelerate the subsequent reduction processes. In chapter 7, controllable engineering of oxygen atoms at defect sites(including the edges) of Mo S2 ultrathin nanosheets is simply realized via post-treatmemt of pyrolytic synthesed Mo S2 ultrathin nanosheets with H2O2. Characterization on composition, morphology and the state of elements preliminary explains the replaced process of O heteroatoms. Similar to the P-Mo S2,O-Mo S2 showed dramatically enhanced ORR catalytic activity superior to its pristine form, and O heteroatoms play a significant role in ORR catalytic process. The functionalized NGR and 2D TMDs nanosheets are promising alternatives to the Pt/C catalyst in fuel cells and metal-air batteries.