The synthesis of hierarchical porous iron oxide with wood templates
Introduction
Biological materials are organized by hierarchical architecture with exquisite designs at several discrete length scales. These hierarchical structures are assembled by molecular units and their aggregates embedded or intertwined with other phases [1]. The astonishing arrays of structures and a broad range of functions are evolved from millions of years of natural selection. As a consequence, these biological systems possess unique characteristics different from the man-made materials.
However, biological materials have some limitations for engineering applications because of their thermolabile constituent. Therefore, combining of the heat-resistant components and biological hierarchical structures together could remedy the performance deficiencies. The break-thought came in 1992, when researchers of Mobil Oil presented a new concept in the synthesis of mesoporous materials with liquid crystal template [2]. This templating method has succeeded in bridging the materials-biology gap and recently been extended to various natural bio-templates ranging from microorganisms (bacterial for SiO2 and zeolite [3], [4]; tobacco mosaic virus for SiO2 [5], DNA for metallic nanowires [6], [7], [8]), plants (woods for SiO2 [9], TiO2 [10], [11], SiC [12], [13], [14] and zeolite [15]; cotton for SiC [16]; cloth or cotton for TiO2 [17]), to animals (butterfly for SiO2 [18]; cuttlebone for chitin–silica composites [19]; eggshell membrane for TiO2 tube [20]). However, for applications of bio-templates, the precise control of pore size distribution (PSD) is still a challenge.
Wood provides an excellent template for the natural hierarchical porous structure. The scales range from millimeter scale of growth ring and vessel pore, via micrometer scale of transverse ray parenchyma and longitudinal tracheid or fiber, to nanometer scale of molecular fiber and cell membrane [1], [13].
This paper reports the preparation of wood-templated iron oxide with mesopores (2–50 nm) and macropores (>50 nm) together by sequential impregnation/calcination of inorganic nitrate solution as precursor. Similar macromolecular and inorganic components constitute different wood species with diverse structures. Four kinds of woods, Paulownia, Lauan, Pine and Fir, were used as templates for synthesizing iron oxide to investigate the relationship between template’s structures and product’s PSDs. The Paulownia and Lauan woods are hardwoods while the Pine and Fir woods are softwoods. Hardwood contains several types of cells, including largest and visible vessel elements, fibers and multiseriate rays, while the structure of softwood is much simpler, of which a single cell type—longitudinal tracheid accounts for over 90% of the softwood volume. On the other hand, calcination temperature is speculated to influence the PSD of the wood-templated samples too. In the present study, three kinds of calcination temperatures, 600 °C, 800 °C and 1000 °C, were postulated. After investigation, not only the inheritance of hierarchical porous structures from the wood templates, but also the presence of controllable porous structure of products to the nanometer scale is demonstrated. The different templates or calcination conditions can lead to corresponding wood-templated iron oxide with special pore types from small mesopores to macropores.
Section snippets
Synthesis method
The specimens (20 × 10 × 3 mm3) of Paulownia, Pine, Lauan and Fir woods were heated in boiling 5% dilute ammonia for 6 h. By using this extraction procedure one can get rid of the wood extractive compounds, e.g. gums, tropolones, fats and fatty acids, and enhance the connectivity among pores and cellular affinity for the precursor. The extracted wood templates were washed by deionized water and dried at 80 °C for 24 h. The 15 mmol ferric nitrate (analytically pure, >98.5%) was added to a mixture solvent
Scanning electron microscopy
The inheritance structures of the wood templates were first investigated. Fig. 1(a) and (b) shows the cross-sectional configuration of Paulownia-templated Fe2O3 calcined at 800 °C taken by SEM and FESEM individually. The well-aligned porous framework is observed in μm scale. In Fig. 1(a), a SEM image from the original structure of Paulownia carbon template is inserted for comparison. These two kinds of structures in Fig. 1(a) show that our iron oxide samples really retain the hardwood’s porous
Conclusion
Iron oxides prepared from wood templates have been proved hierarchically here, which are porous at different scales from about 20 nm up to 50 μm. By adjusting the calcination temperature and wood templates, it is possible to control the PSD in nanometer scale. There are millions of trees all over the world and they all contain unique structures. So by making good use of these natural treasures, different metal oxides with a wide range of pore size can be made.
The synthesis scheme proposed here is
Acknowledgments
The authors wish to express their thanks to the financial support of National Natural Science Foundation of China (No. 50271041 and No. 50371055), Excellent Young Teachers Program of MOE of China, Key Project on Basic Research of Shanghai (No. 03JC14044), Major Project on Basic research of Shanghai (No. 04DZ14002), Program for New Century Excellent Talents in university, Nano-research Program of Shanghai (No. 0452nm045), Basic Research Program of China (No. 2006CB601200), Research Program of
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