Facile Fabrication Of Novel Visible-Light-Driven Nanocrystal With Highly Photocatalytic Application

It is highly desirable to develop a facile method to synthesize novel functional nanomaterial with various morphologies that could result in a remarkable enhancement in catalytic activity, while keeping the preparation process relatively simple and practical. Driven by increasing environmental pollutions and growing threat of current energy crisis, the search for cost-effective, sustainable and green energy sources to meet the global energy demands has attracted considerable research attention. Utilization of solar energy to complete degradation of organic pollutants over highly effective photocatalysts has long been considered the ultimate solution because semiconductor-based photocatalysis is an ambient temperature and green chemical process. Due to its intrinsic large band gap, most of the traditional metal oxide response to UV light, while the visible light accounts for43%of the total sunlight. Recently, enormous efforts have been dedicated to tailor the electronic structures and physical properties of semiconductors or develop novel nanocrystals which can absorb visible-light. In this thesis, several nanomaterials have been prepared and the photocatalytic activities are also measured.1. Tailored fabrication of non-stoichiometric semiconductor nanocrystals with tunable electronic structures has attracted considerable attention owing to the scientific and technological importance. In this work, we have developed a novel and facile approach to prepare stable Sn2+self-doped SnO2-x nanocrystals with a large surface area via a synproportionation reaction of Sn4+with metal tin under mild conditions. The effects of Sn2+doping concentration in SnO2-x lattice on the nanoparticle size, band structure, and photodegradation of methyl orange (MO) were investigated in detail. It is found that the obtained deep-yellow colored Sn2+self-doped SnO2-x sample shows exceptionally higher visible-light photocatalytic performance than stoichiometric Sno2, which is only sensitive to UV light due to its intrinsic large band gap. To the best of our knowledge, this is the first experimental example that self-doped metal oxide nanocrystals have been utilized as an effective photocatalyst for the degradation of pollutants within15min under visible-light irradiation (λ≥400nm). The superior photodegradation activity of the Sn2+self-doped SnO2-x can be ascribed to the incorporation of Sn2+into the lattice matrix and accompanying oxygen vacancies, which can result in significant narrowing of the band gap and enhancement in the visible-light absorption capability. Notably, the efficient separation of the photogenerated electron-hole pairs in SnO2-x, which has been further confirmed by remarkable enhancement of the photocurrent response. Moreover, strong photo-oxidation capability for high content·OH radical formation over SnO2-x (ca.25times higher than SnO2sample) also contributes to the improvement of photocatalytic performance. Our synthetic approach could be extended to design other non-stoichiometric semiconductor nanostructures with tunable band structure, highly efficient visible-light photocatalytic activity and enhanced photoelectric conversion properties in the future.2. Ternary Ⅰ-Ⅲ-Ⅵ2semiconductors (where Ⅰ=Cu, Ag; Ⅲ=Ga, In; Ⅵ=S, Se) have been gaining considerable interest in nanoscale research in recent years primarily due to their band structures, which are highly suitable for light-harvesting and light-emitting applications. For instance, nanocrystals based on CuInS2and CuGaSe2have been found to be useful in the areas of optoelectronics, photovoltaics and photocatalysis. While a great deal of attention has been given to the Cu-based systems, there are relatively few reports on their Ag-based counterparts. Much of the studies on the Ag-based systems have been limited to AgInS2(AIS), which has been known to exist in the tetragonal as well as in the orthorhombic form. Meanwhile, AgGaS2(AGS), an attractive material for optoelectronics and photocatalytic applications, is not as well-explored. Unlike in the case of AIS, the orthorhombic phase has never been previously observed for AGS. Using a simple non-injection colloidal synthetic approach, we were able to obtain AGS nanocrystals that have the wurtzite-derived orthorhombic crystal structure. The synthetic strategy involves the co-thermal decomposition of Ag(Ⅰ) and Ga(Ⅲ) dithiocarbamates in suitable coordinating solvents. It was found that the formation of the orthorhombic polymorph of AGS can be promoted with the use of long-chain alkanethiols and primary alkylamines as coordinating solvents. Using the crystallographic data for orthorhombic AIS as reference, we have carried out Rietveld analysis of our diffraction data and determined the lattice parameters for orthorhombic AGS. From optical absorption data, we have determined the band gap of orthorhombic AGS to be-2.7eV, which lies in the visible spectrum. The photocatalytic potential of the orthorhombic AGS nanocrystals for organic dye degradation was investigated under visible-light illumination. We have found that93%of RhB dye molecules could be degraded within90min, demonstrating the effectiveness of orthorhombic AGS nanocrystals as a visible-light-responsive photocatalyst.3. The orthorhombic polymorph of Cu2ZnGeS4(CZGS) is a metastable wurtzite-derived phase that can only be prepared in the bulk form by extensive heating at high temperatures (≥790℃) when using the conventional solid-state reaction route. By employing a facile solution-based synthetic strategy, we were able to obtain phase-pure orthorhombic CZGS in nanocrystalline form at a much lower reaction temperature. Prior to this work, the colloidal synthesis of single-phase orthorhombic CZGS in the nanoscale has never been reported. We find that the use of an appropriate combination of coordinating solvents and precursors is crucial to the sole formation of this metastable phase in solution. A possible formation mechanism is proposed on the basis of our experimental results. Because CZGS consists of environmentally benign metal components, it is regarded as a promising alternative material to the technologically useful yet toxic cadmium-containing semiconductors. The orthorhombic CZGS nanocrystals display strong photon absorption in the visible spectrum and are photocatalytically active in dye degradation under visible-light illumination.4. Direct fabrication of core-shell or yolk-shell functional nanomaterials via a facile template-free method remains a challenge. In this work, we present a novel approach that involves straightforward chemical transformation and thermal treatment of the infinite coordination polymer particles to obtain composition-tunable CeO2yolk-shell structures. Uniform CeO2yolk-shell hollow spheres with a high surface area are promising support materials for tiny gold nanoparticles (ca.4nm), forming Au-CeO2nanocomposites which exhibit a remarkable catalytic activity and high stability for the reduction of p-nitrophenol. A possible mechanism for the formation of CeO2yolk-shell microspheres is also proposed.