Drying cellulose-based materials containing copper nanoparticles
Carboxymethyl cellulose and TEMPO nanofibrillated cellulose were used as substrates to synthesize copper nanoparticles from copper sulfate and they were subsequently dried using spray-, freeze- and modified freeze-drying processes. Morphological characterization, particle size distribution, copper quantification and the oxidation state of copper were evaluated. The morphology and the size distribution of the dried particles were characterized using scanning electron microscopy-energy dispersive X-ray spectroscopy; particle size distribution was evaluated using laser diffraction; copper content was determined by inductively coupled plasma-optical emission spectroscopy; and finally, the oxidation state of copper was determined using X-ray photoelectron spectroscopy and X-ray diffraction. Plate-like structures of cellulose and cellulose-copper nanoparticles were formed after the freeze-drying process, with length and width over hundreds of microns. Most of the spray dried particles exhibited spherical shapes with the particle size ranging from around 300 nm to several microns. The spray drying process caused more copper loss compared with freeze drying, most probably because of the additional steps required to perform the spray drying. XPS results showed that the state of oxidation of the final dried material depends on the specific substrate. CMC surfaces exhibit mostly Cu0 and/or Cu+. On the contrary TNFC surfaces present mostly Cu2+. Solvent exchange process using ethanol and butanol creates more porous structures on the CMC substrates. A solvent exchange process using ethanol and tert-butanol sequence facilitates the formation of a more porous structure on TEMPO nanofibrillated cellulose.