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The structural features of wood were replicated in silica on all levels of hierarchy from the macroscopic to the nanoscopic level of the cellulose elementary fibrils. This was achieved by a series of processing steps on spruce wood templates. Sodium chlorite was used to partially remove the lignin matrix from the wood cell walls, exposing the cellulose fibrils. These were optionally functionalized with maleic acid anhydride to stabilize the fibrillar structure and reduce the shrinkage of the template. Repeated infiltration with tetraethyl orthosilicate in ethanol deposited silica on the fibrils. Calcination at 500 °C removed the rest of the organic template by oxidation and resulted in the fusion of the deposited material into a positive silica replica. Small-angle x-ray scattering evidenced fibrillar structures parallel to the original cellulose fibrils at length scales in the order of 10 nm, suggesting the successful nanoscopic replication of the cellulose fibrils and their orientation.
Luminescent silica nanotubes and nanowires were fabricated from cellulose whisker templates by sol-gel processing. The cellulose templates were removed by calcination at 650 °C to generate silica nanotubes with diameters of 15 nm and lengths up to 500 nm. At temperatures of 900 °C the core region previously occupied by the cellulose template was closed yielding silica nanowires. Cathodoluminescence spectra of the silica nanotubes and nanowires were measured in the transmission electron microscope during irradiation with 150 keV electrons. A blue emission at 450 nm was observed for the silica nanowires calcined at 900 °C. This luminescence was found to be related to defects induced by electron irradiation and was investigated in situ as a function of irradiation dose. The as-synthesized and 650 °C calcined nanowires and nanotubes showed a fast decay of the signal. The observed irradiation dose dependent changes in the luminescence spectra will be discussed in terms of defect formation and transformation mechanisms.
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