Synthesis And Electrochemical Applications Of Low-Dimensional Functional Nanomaterials

The goal of this dissertation is to design and develop novel chemical reaction routes for the controllable fabrication of low-dimensional functional nanomaterials by analyzing the crystal structural characteristics of the target products. On the basis of the intrinsic properties of the materials, these low-dimensional functional nanomaterials have been applied for various electrochemical area. The corresponding morphology-structure-property relationships have been also investigated in this dissertation. The details are summarized briefly as follows:1. For the first time, we report a facile route to prepare the hierarchical LiV3O8nanofibers constructed by interleaving two dimensional nanosheets with exposed {100} facets using electrospinning combined with calcination. PVA is proved to play a dual role in both electrospinning and calcination process:besides it acts as the template for forming the fibers, it could prevent effectively the aggregation of LiV3O8nanoparticles, which makes them grow into small nanosheets with exposed{100} facets owing to growth characteristic habit of LiV3O8. In addition, the calcination time also affects the formation of hierarchical LiVO8nanofibers. Significantly, the fabricated LiV3O8nanofibers with hierarchitecture and exposure of the specific facets exhibit higher discharge capacity and better cycling stability than those reported in previous literature, suggesting that the hierarchical LiV3O8nanofibers have promising potential for application in ALIBs. Meanwhile, we extend the electrospinning technique to fabricate the electrode material for aqueous lithium ion batteries. The electrospun hierarchical nanofiber described in this study has been confirmed as a unique submicrostructure that will probably have potential applications in sensors, catalysis, chemical power sources, and so on.2. We firstly synthesize large-area nanosheets of tungsten oxide dihydrate with a thickness of only about1.4nm via ultrasonic exfoliation of lamellar inorganic-organic hybrid intermediate. Compared with the bulk counterpart, WO3·2H2O ultrathin nanosheets possess more flexible feature, better interface contact with the substrate and higher Li+diffusion coefficient. The flexible electrochromic device based on WO3-2H2O ultrathin nanosheets has been successfully assembled, which exhibits large transparency contrast (△T=48%), fast coloration/bleaching response time (tc,90%=5.1s, tb,90%=9.7s), a high coloration efficiency of120.9cm2 C-1, good cycling stability and excellent flexibility. These superior electrochromic performances are strikingly higher than those of WO3-2H2O bulk-based ECD and the previously reported flexible ECDs in full consideration of various parameters, which can be ascribed to the synergetic effect between the unique atomic structure of WO3-2H2O and the morphological features of ultrathin nanosheets. The calculated density of states for WO3·2H2O and LixWO3·2H2O indicate that WO3·2H2O undergoes a transition from semiconducting to metallic state accompanied by Li+intercalation, which is the reason of electrochromism. The relationship between the structural features of ultrathin nansoheets and color-switching time is further researched, and the results manifest that WO3-2H2O ultrathin nanosheets can effectively facilitate the semiconductor-to-metal transition and increase the coloration/bleaching response speed, especially for the coloration process. It will provide plenty of valuable information for ultrathin2D nanosheets to develop the high-performance flexible electrochromic devices. The excellent electrochromic performances with the combination of flexibility make our device a prominent candidate in flexible and portable electronics.3. We firstly report an inorganic graphene analogue, WO3·H2O ultrathin nanosheets with only2-3nm thickness, as a promising material to construct a flexible resistance switching random access memory (RRAM) device. The material possesses abundant vacancy associates V""[OWOH2] as revealed by the positron annihilation spectra, providing some new insights into switching mechanisms in the oxide-based RRAM. By virtue of the synergic advantages of metal Cu ions and vacancy associates V""[OWOH2], the as-established flexible Cu/WO3·H2O ultrathin nanosheets/ITO-PET device achieves low operating voltage (+1.0V/-1.14V), large resistance OFF/ON ratio (>105), long retention time (>105s), good endurance (>5000cycles), and excellent flexibility. The finding of the existence of distinct defects in ultrathin nanosheets undoubtedly leads to a deep understanding of the underlying nature of the resistance switching (RS) behavior, which may serve as a guide to improve the performances and promote the rapid development of RRAM.4. We firstly synthesize porous WO3thin nanosheets with4nm thickness using liquid exfoliation combined with calcination. For comparison, nonporous WO3thin nanosheets and nonporous bulk WO3were also obtained and investigated by various characterization methods. The evolution from monoclinic WO3·2H2O ultrathin nanosheets with (010) preferred orientation to monoclinic WO3thin nanosheets with (001) preferred orientation can be eadily explained by topotactic transformation process. The porous WO3thin nanosheets, nonporous WO3thin nanosheets and nonporous bulk WO3are assembled into thin films on the conductive indium-tin-oxide (ITO) coated glasses, respectively. The fabricated thin films as the photoanodes carry out photoelectrochemical tests in a standard three-electrode configuration. The results indicate that the porous WO3thin nanosheets exhibit higher photocurrent density, lower interfacial charge-transfer resistance and better photostability than nonporous WO3thin nanosheets and nonporous bulk WO3, which attributes to the synergic effect of two-dimensional morphology and porous structures. The porous WO3thin nanosheets will be a promising photoanode material for photoelectrochemical water splitting.