Investigation Of Solution-Phase Synthesis And Properties Of Sulfide And Selenium Microstructures/Nanostructures

In this dissertation, various solution-based routes including biomolecule-assisted method, visible-light-assisted technique, solution-phase deposition approach, refluxing, were developed for the morphology-controlled synthesis of metal sulfides and trigonal selenium (t-Se) materials. The formation mechanism and the electrochemical hydrogen storage behaviors of the as-prepared samples were investigated in detail. A facile biological approach was developed to fabricate Bi₂S₃ and Ni₃S₂ nanostructures using L-cysteine as both the sulfur source and the directing molecule under hydrothermal conditions. The electrochemical hydrogen storage behaviors of those sulfides were studied systematically and the dependence of these nanostructures' capacities of electrochemical hydrogen storage on their shapes was discussed; The biomolecule beta-carotene-assisted approach was developed, for the first time, to synthesize t-Se nanowires and nanoribbons with high crystallinity. The one-dimensional (1D) Se nanostructures formed via a novel flack-cracking mechanism rather than a typical sphere-wire process; The author developed a one-step solution method to generate ultra-long single-crystalline t-Se submicrotubes with good electrochemical capacity of hydrogen storage; A novel visible-light-assisted solution-phase technique was developed to prepare t-Se microstructures with abundant morphologies and was extended to the morphology-controlled synthesis of trigonal tellurium (t-Te); A series of crystalline multiarmed tubular t-Se nanostructures with unique electrical properties were successfully synthesized using a facile refluxing approach. Applications of these multi-armed Se materials in preparing functional composite nanomaterials were discussed, and first-principles studies were also adopted to predict the as-prepared materials' electronic properties; A novel and facile solution-phase deposition approach was developed, for the first time, to synthesize porous t-Se materials with various morphologies; The bio-activity of inorganic nano Se was investigated on the level of cell. Such t-Se materials' applications in electrochemical hydrogen storage were studied. The details are summarized as follows.1. A simple L-cysteine-assisted method was developed to prepare Bi₂S₃ flowerlike patterns with well-aligned nanorods. L-cysteine, a small biomolecule, was found to serve as both the S source and the directing molecule in the formation of bismuth sulfide nanostructures. The author further extended this facile biomolecule-assisted approach to generate nanothread-based porous spongelike nanosturctures on Ni foil with a high yield at 120°C. These novel nickel sulfide nanomaterials grown on Ni foils could electrochemically charge and discharge with a maximum capacity of 380 mAh/g (corresponding to 1.4 wt% hydrogen in SWNTs). By varying the experimental parameters, other Bi₂S₃ and Ni₃S₂ nanostructures were obtained. It was demonstrated that these sulfides nanostructures' morphologies had a noticeable influence on the capacity of electrochemical hydrogen storage. The formation mechanism of these sulfides nanostructures was discussed and the dependence of their electrochemical properties on the products' morphologies was explained in detail.2. A simple beta-carotene-assisted method was developed for the first time to synthesize t-Se nanowires and nanoribbons with high crystallinity. It was demonstrated that beta-carotene served as not only the reducing agent, but also an in situ template in the preparation of t-Se 1D nanostructures. It is found that the growth mechanism of Se nanomaterials is different from the familiar sphere-wire process. A novel flake-cracking mechanism is proposed. By this biomolecule-assisted route, Te 1D nanostructures and Pd nanowires were also fabricated. Thus, this method is a new, facile, general method to prepare 1D nanomaterials.3. Ultra-long single-crystalline t-Se submicrotubes with the length of over 100μm) were synthesized using a hydrothermal approach with the assistance of nonionic surfactant Polyoxyethylene(20)sorbitan monolaurate (Tween-20). The author demonstrated that the as-prepared ultra-long t-Se submicrotubes could charge and discharge with the high capacity of 265 mAh/g. It was observed that the morphology of the synthesized Se products had a remarkable influence on their capacity of electrochemical hydrogen capacity. The tubular or porous structure was considered to be the main factor for the high electrochemical capacity. Previous investigations on electrochemical hydrogen storage mainly restricted to layered-like materials (e.g. CNT, BN nanotubes, MoS₂ nanotubes, TiS₂ nanotubes), and this is the first report for the helix-chain-like materials. Thus, this work undoubtedly extends the study of materials with the function of hydrogen storage.4. A novel and facile visible-light-assisted solution-phase technique was successfully developed to synthesize t-Se and t-Te 1D nanostructures with various morphologies. By varying the kind and relative amount of the reacting agent, Se submicrotubes, Se nanorods, Se shuttles, urchin-like assembly of Se nanorods, Te nanowires, Te nanobelts, Te nanotubes, thornlike 1D assembly of Te nanothreads can be fabricated on a large scale. Both light and thermal effects play significant roles in this visible-light-assisted technique. It was found that the light from a lawn lamp could accelerate the transformation of amorphous Se/Te to trigonal crystalline Se/Te and enhance the directing role of water-soluble polymers in the current method. The as-prepared Te nanowires were chosen as the example to investigate the products' applications in fabricating various functional nanomaterials. Furthermore, using the Te nanowires as the template, Pt-Te nanochains with modulated Pt dopants, Te@carbon-rich nanocables, carbonaceous nanotubes, Pt-Te@carbon-rich nanomaterials could be successfully generated.5. A series of crystalline multiarmed tubular selenium nanostructures were successfully synthesized using a facile refluxing approach at 100°C. Such mutiarmed tubes could be utilized as the template to prepare CdSe-Se composite nanomaterials. In addition, different arms had different growth directions, and some new directions (e. g. the directions perpendicular to (102) and (0-12) crystal planes) were observed through TEM, SAED and HRTEM analyses. Furthermore, first-principles studies demonstrated that arms with different directions in the branched tubes exhibited different unusual electronic properties and some arms had a quasi-metal character.6. A novel and facile solution-phase deposition approach was developed, for the first time, to prepare porous t-Se materials. Porous flower-like Se patterns, porous apple-like Se microstructures, porous lotus-root-like microspheres, porous walnut-like spheres and ear-like materials could be fabricated on a large scale by choosing Zn foil as the deposition substrate, and either N₂H₄H₂O or EDTA (ethylenediaminetetraacetic acid) as both the reducing and coordinating agents. It was demonstrated that both the zinc ions released on the Zn substrate and the soft template formed in situ through the coordination between the Zn²⁺ and organic molecules are significant in controlling the porous structures and morphologies of the final products in this solution-phase deposition method. The porous t-Se materials had been shown to electrochemically store hydrogen with good capacity, which was considered to be associated with the porous structure of the obtained samples.7. The bio-activity of inorganic nano Se was investigated on the level of cell. The results demonstrated that the toxicity of inorganic nano Se was lower than that of the familiar inorganic selenium (e.g. SeO₂, SeO₃^2-), similar to that of organic selenium.