The Effect Of Cellulose Surface Groups On The Photocatalytic Performance Of ZnO/Cellulose Composite

At present,water pollution has become a global environmental problem that needs to be solved urgently.So as an effective tool to degrade organic pollutants,the green and efficient photocatalysis has attracted much attention in many countries.Zinc oxide（ZnO）is an ideal semiconductor photocatalyst due to its safety,non-toxicity,and low cost.It was loaded on degradable cellulose to further improve photocatalytic efficiency.However,most of the current studies were focused on improving the agglomeration and non-recovery problems of nano-ZnO through cellulose substrate.There have been no studies on using characteristics of rich varieties of groups on cellulose surface to explore the interaction between different groups and ZnO,and then modulate the morphology of nano-ZnO to improve their photocatalytic properties.Notably,the scale of fibers also has an effect on the morphology of ZnO,so it needs to be regulated to more accurately study the effect of different groups on ZnO/cellulose composite photocatalytic materials.Meanwhile,fractal dimension was introduced to characterize the size distribution and microstructure of cellulose nanofibers and composites because it can directly describe the complexity and diversity of things.In this study,bleached conifer wood pulp was used as raw material to prepare cellulose nanofibers with the same scale and different surface groups through chemical modification and mechanical treatment.The surface groups and pore structure of cellulose nanofibers were used to control the morphology and loading of ZnO,and the morphology and structure of fibers and composites were characterized by fractal dimension.Finally,an efficient ZnO/cellulose composite photocatalytic system was constructed.The specific results are as follows:1.Firstly,TEMPO-mediated oxidation and ultrafine grinding were used to prepare cellulose nanofibers with the same scale and different carboxyl contents,namely CNF-40,TOCNF-30,TOCNF-20,TOCNF-10,and TOCNF-1.Then ZnO was loaded on them by chemical precipitation method to obtain ZnO/cellulose composites called ZnO/CNF-40,ZnO/TOCNF-30,ZnO/TOCNF-20,ZnO/TOCNF-10,and ZnO/TOCNF-1.When the carboxyl content was 1.58 mmol/g,the Zeta potential was-52.6m V and the electronegativity of fibers was the strongest.The statistical diameter of cellulose nanofibers with different carboxyl content was～23 nm,the fractal dimension of the image was～1.56,and the fractal dimension of the FHH pore structure was～2.48,which all proved that the scale and structure of cellulose nanofibers were regulated to be consistent.With the increase of carboxyl groups content,ZnO flower size,sheet thickness,and sheet number increased.However,the specific surface areas of fibers and composites were more affected by the electronegativity of carboxyl groups,and which of TOCNF-10 and ZnO/TOCNF-10 with the carboxyl content of 1.58 mmol/g were the highest.Similarly,ZnO/TOCNF-10 had the best photocatalytic performance,and the MO photodegradation efficiency was 95.3%under 2 h light,which is confirmed by photocurrent response and impedance spectrum.2.CNF-40,ACNF-38,ACNF-36,ACNF-34,and ACNF-32 with the same scale and different acetyl group contents were prepared by CH₃COOH/（CH₃CO）₂O system to esterize cellulose and ultrafine grinding for different times.Then ZnO was loaded on them to obtain ZnO/CNF-40,ZnO/ACNF-38,ZnO/ACNF-36,ZnO/ACNF-34,and ZnO/ACNF-32.The Zeta potential and the yield of ACNFs decreased with the increase of acetyl group contents.The statistical diameter of cellulose nanofibers with different acetyl content was～22 nm,the fractal dimension of the image was～1.56,and the fractal dimension of the FHH pore structure was～2.5,which all proved that the scale and structure of cellulose nanofibers were regulated to be consistent.With the increase of acetyl group content,the specific surface areas of the fibers and composite materials and the loading content of ZnO increased.The presence of acetyl groups promoted the adsorption of Zn²⁺on fibers and enriched the network space of the nanofibers and complexes.From these two aspects,ZnO self-assembly was promoted to form more completed ZnO flowers（big size,large number of ZnO nanosheets,and thick ZnO nanosheets）.ZnO/ACNF-32 with the highest acetyl group content（28.7%）had the strongest photocatalytic degradation ability in the experiment of photocatalytic degradation of MO,which the degradation rate of ZnO/ACNF-32 was 92.5%when exposed to light for 3 h.3.Based on NaOH/C₂H₅Cl etherifying system and ultrafine grinding,cellulose nanofibers with the same size and different ethoxy content were prepared,named CNF-40,ECNF-38,ECNF-36,ECNF-34,and ECNF-32.Then ZnO/CNF-40,ZnO/ECNF-38,ZnO/ECNF-36,ZnO/ECNF-34,and ZnO/ECNF-32 were prepared by chemical deposition of ZnO.The absolute value of the Zeta potential of nanofibers increased with the increase of ethoxy group content,which meant the electronegativity of fiber increased.The diameter of cellulose nanofibers with different ethoxy content was～22 nm,the fractal dimension of the image was～1.51,and the fractal dimension of the FHH pore structure was～2.43,indicating that the fiber scale and structure were consistent.The specific surface areas of fibers and composites increased with the increase of ethoxy content,and so did the loading content of ZnO.With the increase of ethoxy group content,the proportion of flower-like ZnO with large size,more ZnO nanosheets,and thick nanosheets in the composite increased.What’s more,ZnO/ECNF-32 with the highest ethoxy content（36.2%）had the highest MO photodegradation efficiency under 3 h light,which reached 91.1%.4.TOCNF-30,ACNF-32 and ECNF-34 with the same scale,similar electronegativity and different surface groups were prepared by using the three different chemical modification systems mentioned above and different ultrafine grinding times.Then ZnO was loaded on them to get ZnO/TOCNF-30,ZnO/ACNF-32 and ZnO/ECNF-34.The Zeta potential of cellulose nanofibers with different surface groups is about-35 m V,the diameter is～22nm,and the fractal dimension of the images is 1.58,indicating that the electronegativity and scale of the fibers are regulated to be consistent.The electronegativity of cellulose fibers is the main factor affecting the photocatalytic effect of ZnO/cellulose composites,while the surface groups themselves of fibers have no significant effect on the load and structure of ZnO and overall photocatalytic effect.The load of ZnO on the fibers is actually the effect of Zn²⁺and O on different groups,and the electronegativity of the groups determines the strength of this effect.The results showed that different surface groups of fibers could not only regulate the morphology and structure of ZnO（including the size of flower-like ZnO,thickness and number of ZnO nanosheets）,but also improved and enriched the three-dimensional network structure of cellulose nanofibers and composites,thus improving the photocatalytic performance.The loading of ZnO on fibers is actually the result of the interaction between Zn²⁺and O in different groups,and the electronegativity of the groups determines the strength of this effect.In general,this study provides a new direction for the development of green,efficient and sustainable photocatalysts.