Construction And Properties Of Wood-based Functional Carbon Dots-Cellulose Nanofibril Gel Material

As one of the most abundant forestry resources in nature,cellulose nanofibrils（CNF）which can be extracted and dispersed from wood can be obtained with excellent properties such as low-carbon environmental protection,recyclable regeneration,high aspect ratio,rich in hydrophilic active groups,etc.At present,most of the gel materials based on CNF have the disadvantages of low cross-linking degree,poor mechanical strength,and single function.On the other hand,carbon quantum dots prepared from various wood-based biomass have unique structural features,low biotoxicity,and excellent performance.However,there are also defects such as difficulty in recycling,easy agglomeration and deposition,and aggregation-caused quenching in solid state or when the concentration is too high.In this study,we considered the construction of a wood-based carbon quantum dot/nanocellulose self-assembled gel material based on CNF,so that the carbon quantum dots were uniformly embedded in the CNF gel network.Based on the gel material,the functional expansion of the gel material is realized by further introducing other functional nanomaterials,and its feasibility in the fields of photocatalytic degradation of pollutants,detection and adsorption of heavy metal ions in water,and electrochemical energy storage is explored.The main research results of this paper are as follows:（1）Based on CNF,a wood-based carbon quantum dot-nanocellulose hydrogel（NCDs-CNFgel）with fluorescence emission and certain mechanical strength was prepared by a facile and environmentally friendly one-step hydrothermal method.The NCDs with active groups are fluorescent sources and small molecular cross-linking agents.NCDs-CNFgel is self-assembled through chemical cross-linking and hydrogen bonding between the active groups on the surface of NCDs and CNF.Under the excitation of 365 nm ultraviolet light,NCDs-CNFgel can emit blue-green fluorescence with a wavelength of 440～480 nm,which has the characteristics of acid resistance and alkali sensitivity.The compressive strength of NCDs-CNFgel can reach to75.44 k Pa,while retaining the shear thinning characteristic and injectability.（2）Based on the NCDs-CNFgel prepared in the previous part,after further solvent replacement and low-temperature hydrothermal treatment,the Ti O₂sol was modified by NCDs in the gel network,and the NCDs@Ti O₂nanosheets were grown in situ in the gel network.As a result,the wood-based carbon quantum dots@Ti O₂nanosheet-nanocellulose gel films（NCDs@Ti O₂-CNFm）were obtained.The modification of NCDs can effectively improve the photocatalytic activity of NCDs@Ti O₂.The adsorption and degradation rate of methyl orange by NCDs@Ti O₂-CNFm can reach 94.92%,and it can still reach 89.60%after 5 cycles.The loading rate of NCDs@Ti O₂in NCDs@Ti O₂-CNFm is higher,about 59.503%,and its loading on CNF gel film also makes the recycling of photocatalyst easier.（3）A wood-based carbon quantum dots-nanocellulose/chitosan interpenetrating network hydrogel（NCDs-CNF/CSgel）with fluorescence detection and adsorption properties for heavy metal ions was prepared by a mild and environmentally friendly low-temperature hydrothermal treatment combined with solvent replacement.NCDs-CNF/CSgel exhibits an interpenetrating three-dimensional network structure,where NCDs and CNF are cross-linked to form the first layer of chemical gel network,and chitosan forms the second layer of physical gel network.The NCDs-CNF/CSgel exhibited a wide linear fluorescence response range（50-1000 mg/L）for Cu（II）,and excellent sensitivity and selectivity to the fluorescence response for Cr（VI）（detection limit was 0.7093 mg/L）.Due to the electrostatic attraction and chemical interaction between the N,O-riched active groups of NCDs-CNF/CSgel and heavy metal ions,the hydrogel exhibited a high adsorption capacity of Cu（II）and Cr（VI）at 148.30 mg/g and 294.46mg/g,respectively.（4）Based on the mentioned synthesis method of NCDs-CNFgel,NCDs-MWCNT/CNFgel was prepared by one-step hydrothermal treatment by introducing carboxylated MWCNT into it.Subsequently,the NCDs-modified,MWCNT-complexed CNF-derived carbon aerogel NCDs-MWCNT/CNFcag was obtained by ice-templated freeze-drying and high-temperature calcination.During high-temperature calcination,a large number of NCDs were induced by MWCNT to in situ grow in carbon particles and carbon microvillis containing a large amount of nitrogen and oxygen heteroatoms on the carbon aerogel.It significantly increased the specific surface area and the number of defect state structures of carbon aerogels,providing faster and more convenient electron/ion transport channels,excellent electrical conductivity,and higher specific capacity for energy storage.The specific surface area of NCDs-MWCNT/CNFcag can reach up to 377.9 m²g^-1,and the specific capacitance can reach to 121.8F g^-1under the three-electrode test system.At a current density of 2 A g^-1,the capacitance retention rate can reach more than 131.7%after 5000 charge-discharge cycles.