Structures And Properties Of Semiconductor Oxide Nanomaterials Via Flame Combustion Synthesis

Flame technology with advantages of scalable, continuous, without post-treatment and low cost, is by far the most widely used for manufacture of commercial quantities of nanoparticles. However, flame-made nanomaterials are always solid particles instead of-nanostructures with different shapes, which can not satisfied multifunctional development of nanomaterials. So developing nanostructures with controllable tailoring by the designation of flame reactors and precursors is still a big challenge for flame-made nanomaterials. In addition, flame synthesis is a high temperature and milliseconds process, involving nanoparticles nucleation, growth, agglomeration and aggregation, which have been called as "black box", limiting the research of flame-made nanostructures. Therefore, more and more attentions have been put on studying the formation mechanism during this process. In this thesis, based on flame reactor designing, nanostructured TiO2 microspheres, SnO2 nanorods and nanowires and core/shell nanostructured Fe2O3@SiO2 and TiO2@SiO2 semiconductor oxide nanomaterials were synthesized. The preparation method, structures and properties were studied systematically, and the formation mechanisms were also proposed. The major contents have been summarized as follows.1. Well-defined TiO2 solid microspheres, hollow microspheres and ball-in-shell microspheres with diameters of 100-300 nm have been successfully synthesized by controlling temperature profile of a facile diffusion flame. Meanwhile, the rutile fraction and crystallite size increased as flame temperature rised. The production rate in lab-scale is up to 30 g/h. Comparing with TiO2 solid microspheres, the UV-vis absorbance peak of TiO2 hollow and ball-in-shell microspheres have a red shift, and the band gap decreased observably, showing more efficient use of the light source, which can be attributed to multiple reflections of light within the interior voids. The formation mechanism of speres nanostructured TiO2 was investigated, depending on the competition between the chemical reaction rate and diffusion rate during the flame process. Lower temperature favors reaction controlled process, forming solid microspheres. Higher temperature favors diffusion controlled process, forming hollow microspheres. At the middle temperature, ball-in-shell structured microspheres were formed. More importantly, this approach also provides a new pathway for continuous and large-scale engineering of spheres nanostructures. 2. By controlling the dopant concentration and flame residence time, SnO2 nanorods and nanowires were successfully synthesized by self-designed diffusion flame reactor and flat flame deposition reactor, respectively. The as-prepared 2.5 at.%Fe doped SnO2 nanorods with uniform length up to 200 nm and diameter around 20 nm are smooth and single crystal rutile structures, growing along the [001] direction. At Fe concentration of 0.0-2.5 at.%, Fe dopant is incorporated into the SnO2 lattice and selectively effects a specific SnO2 crystal plane, promoting the further crystal oriented growth into nanorods. On the other side, At Fe concentration of 2.5-20.0 at.%, Fe dopant would aggregated to second phase, inhibiting crystal growth. Meanwhile, the photoluminescence (PL) spectrum of such SnO2 nanorods exhibits a broad, stronger orange-emission peak around 620 nm, which would be enhanced by increasing Fe dopant concentration and flame residence time, suggesting potential applications in optoelectronics. Further magnetic property research showed that 2.5at.%Fe doped SnO2 nanorods had good room temperature ferromagnetic property. Further increasing the flame residence time by flat flame deposition method, SnO2 nanowires with uniform length of 0.4μm-4μm and diameter around 30 nm are smooth and single crystal rutile structures, were synthesized, growing along the [001] direction. Meanwhile, a growth mechanism, combining both vapor-solid (VS) growth mechanism and particles formation mechanism in flame, is proposed. Furthermore, the optical property is investigated by photoluminescence (PL) spectroscopy, indicating that such SnO2 nanowires exhibit a much stronger emission peak at 620nm, which is attributed to oxygen defect.3. For the first time, flame synthesis, surface coating and surface functionalization were combined together into a gas-phase continuous process to successfully prepare core/shell Fe2O3@SiO2, MEMO-modified core/shell Fe2O3@SiO2, MPTMS-modified core/shell Fe2O3@SiO2 and core/shell TiO2@SiO2 nanocomposites. Firstly, core/shell Fe2O3@SiO2 nanocomposites were prepared, meanwhile, the effect of exit velocity of quench ring and burner to quench ring distance(BQD) on coating quality is studied as follows:Uniform and smooth coating can be formed at the gas rate of quench ring above 2.0 m3/h, which can ensure mixture intensity between Fe2O3 aerosol and TEOS vapor. In addition, lower BQD would form dispersed Fe2O3 in amorphous SiO2, and higher BQD may cause Fe2O3 aggregation before coating. So when BQD=30 cm, high quality coating can be formed. By spraying modification agent downstream, nerwork-like MEMO-modified core/shell Fe2O3@SiO2 nanocomposites were synthesized, and core-Fe2O3 nanoparticles with count mean diameter of 23 nm are coated hermetically and homogeneously with an average thickness of about 3.9 nm. Magnetic liquid marbles were assembled by the above network MEMO-modified core/shell Fe2O3@SiO2, and showed super mechanical stability, high magnetic driven ability and adjustable inner liquid property. In order to prove the versatility of our method, core/shell TiO2@SiO2 were also prepared with 0wt%-35wt%SiO2 content. Structural and morphological characterizations have shown that core-TiO2 about 25 nm in diameter can be hermetically and uniformly coated by SiO2 with accurately controlled shell thickness of 1.5-3.3 nm. The crystallite size, rutile fraction of TiO2 and shell thickness of SiO2 could be independently controlled. By means of structure optimization, we found that core/shell TiO2@SiO2 with shell thickness of about 2.2 nm would show complete suppressing photo-catalytic activity and better UV-shielding activity, suggesting their tremendous potential for UV-filter applications.