Synthesis, Characterization And Properties Of Hierarchical Porous Oxides Derived From Wood Templates

Hierarchical porous materials have displayed important researching and application values at the fields of separation and purification, selective adsorption, optical function, and sensor design etc. Some preparation methods have been designed to fabricate porous materials. But these traditional methods have to use specific equipments and complicated techniques, and obtained porous materials have single pore size distributions with single functions. The morph-genetic transformation technology is a simple processing technology to fabricate refined hierarchical porous materials using organisms as template. The organisms in nature are the perfect unities of highly delicate structures and effectively complex functions through millions of years of evolution and natural survival law, which prepare plentiful structural templates for morph-genetic hierarchical porous materials. In the present work, wood was used as template to fabricate hierarchical porous Fe₂O₃, ZnO and NiO to inherit wood's morphology and structure. The synthetic mechanism was studied to optimize the parameters of morph-genetic technology, and the hierarchical porous oxides with wood structure were fabricated successfully. The porous structures in multi-scales, the optical properties and the gas sensing properties of hierarchical porous oxides were researched in detail. The contents and results are summarized as follows:1. The hierarchical porous Fe₂O₃, NiO, and ZnO have been synthesized successfully using wood templates to retain the wood's porous structures fromμm scale to nm scale. The parameters of morph-genetic technology have been optimized on the basis of synthetic mechanism research. Wood template not only provided the structural template for oxides, but also restrained the grain growth of oxides to cause much smaller grains of hierarchical porous oxides than ordinary oxides. 2. The combination of mercury intrusion / electron microscopy / nitrogen adsorption measurement has been used in the present work to characterize hierarchical porous structures in multi-scales. The fractal dimensions of oxides were calculated from mercury intrusion data to research the network connectivity of porous structures. Oxides obtained hierarchical macroporous structures with pore size distributions of 20¹00μm and 0.1¹Î¼m through duplicating wood's radial cells, pits and cell walls etc. It's proved that the parameters of morph-genetic technology have obvious influences on porous structures of oxides. Firstly, wood template type decided the basic characteristics of oxides'structures. But all of hierarchical porous oxides calcined at 600oC have mesopores distributed at 10⁵0nm. Secondly, oxides with different morphologies of pore walls can be obtained through infiltrating different precursor solutions. Thirdly, increases of infiltration rate and calcination temperature could change the morphologies of pore walls, increase pore wall thickness and decrease the quantities of mesopores. Fractal research discovered that Fir-templated ZnO calcined at 600oC had both the highest fractal dimension and porosity to prove its best network connectivity.3. Hierarchical porous oxides have much better UV-IR absorption and luminescence abilities than ordinary oxides. Due to the standing wave resonance absorption ability of pores in various scales, hierarchical porous oxides have much better UV-IR absorption properties. UV absorption abilities of Fir-templated oxides calcined at 600oC have been improved about 5.6%²0% compared with ordinary oxides. IR absorption abilities of Fir-templated oxides calcined at 600oC were 2.0³.9 times higher than ordinary oxides. Photoluminescence abilities of hierarchical porous oxides were 2.4².7 times higher than ordinary oxides due to higher UV excitation wave absorption ability, smaller grains and better surface crystal quality.4. Hierarchical porous ZnO measured with cathodoluminescence was detected both UV emission at 390nm and blue visible emission at 480⁴90nm. Both emissions strongly depended on the calcination temperature of ZnO. On one hand, with the increase of calcination temperature, larger grains and worse crystal quality of ZnO decreased UV emission intensity. On the other hand, with the increase of calcination temperature, more defects including oxygen vacancies and zinc vacancies and smaller pore volume increased the defect transition probability and enhanced the blue emission intensity as the result.5. Hierarchical porous Fe₂O₃ has better gas sensing ability than ordinary Fe₂O₃. Strong oxygen absorption ability made hierarchical porous Fe₂O₃ produce surface inversion layer and display p-type semiconductor, different from n-type of ordinaryα-Fe₂O₃. Moreover, the higher free carrier concentration of hierarchical porous p-type Fe₂O₃ led to better gas sensing properties than ordinary n-type Fe₂O₃.6. Hierarchical porous ZnO showed excellent gas selectivity to H2S. At working temperature of 332oC, Fir-templated ZnO calcined at 600oC has high H2S sensitivity of about 200, high selectivity coefficient of H2S over other gases of 8.5¹98.0, and short response and recovery time of 4s and 12s. In contrast, ordinary ZnO has H2S sensitivity of only 35 with longer response and recovery time (6s and 15s).