Fabrication And Applications Of Natural Cellulose/Inorganic Nanocrystals Composite Materials

With the rapid development of modern technology, lots of researchers found that the mirco-and nano-scaled materials have unique properties compared with bulk materials even with the same composition. For example, semiconductor nanocrystals have very different optoelectronic properties such as size-tuned fluorescence which benefits from the rather small size in the nanoscale. Noble metallic nanoparticles show great potentials in catalysts. However, bulk noble metals are known to be extremely inert. As a result, making better use of these inorganic nanocrystals has become a focus in the field of fabricating new functional composite materials.Herein, several kinds of inorganic nanocrystals like quantum dots, bimetallic nanoparticles and calcium carbonate were assembled or directly grown on natural cellulose substrates. After the appropriate treatments, the original unstable inorganic nanocrystals had been immobilized on the polymer substrates to generate multi-dimensional functional composite materials. The extraordinary properties of the inorganic nanocrystals were reserved. And because of the existence of the polymer substrates the practical applications of the composite materials have also been extended. The details are described as follows.(1) Bulk luminescent cellulose/CdSe QDs composite material:Each cellulose nanofiber of the filter paper was firstly coated with nanometrethick titania film by means of the surface sol-gel process, followed by alternative deposition of self-assembled layers of stearic acid (SA) and TOPO/HDA capped CdSe nanoparticles, giving a filter paper/titania/SA/(CdSe/SA) composite luminescent sheet possessing stable and well-defined green fluorescence. SEM and TEM observations showed that the resulting luminescent sheet retained the hierarchical structures and morphologies of the initial cellulose substance; meanwhile, the CdSe nanoparticles were anchored on the nanofiber surfaces within SA thin layers through the hydrophobic interaction between the alkyl chains of the surface ligands of CdSe nanoparticles and SA molecules. UV-vis and photoluminescence spectra indicated that the resulting luminescent sheet showed similar characteristic absorption and emission properties as those of the original CdSe nanoparticles. The luminescent property of this composite material can be kept for over months.(2) One-step fabrication of cellulose/ZnS:Mn QDs composite material:A bulk piece of cellulose/ZnS:Mn QDs composite material has been generated through a facile one-step hydrothermal method. TEM results show that the employment of PEI improves the dispersibility of the virginal ZnS:Mn QDs. With the help of hydrogen interaction between PEI and cellulose substances, there was no need of extra modification of the filter paper. XPS results further comfirm the success of immobilization of the ZnS:Mn QDs onto cellulose substrate. The ZnS:Mn QDs were found to be uniformly immobilized on the fibers of the filter paper, which gives the obtained bulk composite material stable and well-defined luminescent property.(3) Catalytic cellulose/bimetallic nanoparticels composite materials:Employing titanium (IV) n-butoxide as precursor, ultrathin titania gel films were deposited on cellulose nanofiber in bulk filter paper by the surface sol-gel process. Five kinds of bimetallic nanoparticles (BNPs) were synthesized by the reduction of NaBH4and sodium citrate. These BNPs were immobilized onto the fibers of the titania-modified filter paper through the electrostatic interaction between the positively charged poly(diallyldimethylammonium chloride)(PDDA) and negatively charged BNPs. The electron microscopes results revealed that the fabricated bulk composite materials inherited the original three-dimensional morphology of the initial filter paper. Moreover, these five kinds of composite materials showed good efficient catalytic activities on the reduction of4-nitrophenol by NaBH4by means of simply filtration three times of the feed solution through the BNPs-related composite materials. The catalytic activities maintained even after ten cycles of the three-time filtration, which indicated the stability and reusability of the BNPs-related composite materials.(4) Cellulose/aragonite crystals composite material:Employing titanium (IV) n-butoxide as precursor, ultrathin titania gel films were deposited on cellulose nanofiber in bulk filter paper by the surface sol-gel process. After prenucleation with calcium carbonate nanoparticles, the titania-modified filter paper was put into classic geothermal water for the growth of needle-like aragonite calcium carbonate crystals, and the needle-like calcium carbonate crystals were guaranteed to be pure aragonite. The mechanism of the formation of the pure aragonite crystals is that the calcium carbonate crystals may undergo a dissolution and recrystallization process. After the surface modification with a monolayer of sodium stearate, this composite material was endowed with a superhydrophobic property showing a water contact angle of153°.