High-efficient Synthesis Of Nanostructure Coatings Via Pre-seeding Combined With Microwave Hydrothermal Method On Wood Surface

In the background of mature synthesis process, nanomateirals were introduced into the field of wood modification, and their special performances had played an important role in wood protection. Based on the idea of surface engineering, fabrication of functional nanomaterials on wood surface using the hydrothermal method was one of the most important methods for wood modification in recent years.However, nanomaterials which synthesized by hydrothermal method had a series of inevitable problems, such as particles aggregation, uneven dispersion, poor combination with wood surface,and even splitting off wood.Moreover, the treated time of hydrothermal process was relatively longer, it also could make the wood in a certain degree of degradation. In order to solve the aboving problems, this paper presented two solutions. The first solution was an idea of growing nanocrystal seeds on wood surface before hydrothermal synthesis, which could overcome the shortcoming of poor combination with wood. Take these nanocrystal seeds as growing sites, nanomaterials were grown in-situ on wood surface through the hydrothermal energy induction and surfactant self-assembly. It could effectively prevent nanoparticles aggregation. In consideration of the long time hydrothermal treatment, the second solution was using microwave irradiation instead of thermal transmission for heating. The reaction time was grealy reduced and the efficiency of synthesis was also enhanced.From the aspect of adsorption, this paper had designed two pathways of nanocrystal seeds grown on wood surfaces in advance. The first, based on the idea of colloidal chemisty, the colloid has the strong ability of adsorption because of its huge surface area, so it could combine with lots of OH- on wood surface quickly through repeated dipping. The second, cationic polyelectrolyte was adsorbed on wood surface firstly, and then the anionic polyelectrolyte and nanoparticles with opposite charges were layer-by-layer assembled in turn. These two methods were simply operated at room temperature, and solve the technical problems in nanocrystal seeds pre-treatments. Subsequently, setting ZnO as representative growths unite, ZnO nanostructure coatings were fabricated on wood surface through microwave-assisted hydrothermal method. The conditions of nanocrystal seeds, the precursor concentration, reaction temperature, microwave radiation time and the stage-heating setted on the crystal structure, morphology, size and growth were systematically discussed in this paper. And the controllable growth mechanisms of ZnO nanostructure at the corresponding conditions were also analyzed.Finally, the properties of functional modified wood and the special properties derived from the ZnO nanostructure coatings were examined and analyzed. The major research contents and conclusions of this paper were as follows.1. With repeated dipping colloid solution as the method of nanocrystal seeds grown on wood surfaces in advance, ZnO nanostructure coatings were facbricated on wood surface by the conventional hydrothermal method. In the mild alcohol chemical environment, the reaction precursors firstly formed a supersaturated solution, then hydrolyzed to nucleation, and finally dispersed in the gel network evenly. Wood samples were dipped into the colloid solution repeatedly, and the nanocrystal seeds could combine with lots of OH- on wood surface. Takeing these nanocrystal seeds as growing sites, ZnO nanostructure coatings were grown in-situ on wood surface. During the conventional hydrothermal process, the structure, morphology, dimension and orientability of ZnO nanocrystals could be controlled by adjusting the reaction temperature, treated time and surfactants. The detailed testings showed that the typical ZnO hexagonal wurtzite rod-structures were grown on wood surface. The average diameter of nanorods was between 150-200nm, and the average length was about 900nm. This paper systematically illuminated the mechanisms of the formation of ZnO nanocrystal seeds and crystal epitaxial growth. Furthermore, the influence of the pre-treatment of nanocrystal seeds grown on wood surfaces, hydrothermal reaction time and the choice of surfactant on the growth of ZnO nanostructure coatings were also discussed.2. With repeated dipping colloid solution as the method of nanocrystal seeds grown on wood surfaces in advance, ZnO nanostructure coatings were facbricated on wood surface by microwave-assisted hydrothermal method. The wood samples were pre-treated as same as before, ZnO nanostructures were fabricated using microwave-assisted hydrothermal method, which would reduce the reaction time from 2-10h to 30-90min, with different precursor solution concentration, microwave radiation time, reaction temperature and the times of hydrothermal growth. The influence of the various factors on the crystal structure, morppology, size and distribution of ZnO were systematically analyzed. The results showed that:the increasing concentration of solution would result in a rise in average length-diameter ratio of ZnO nanostructures and better morphology. The growth and crystallinity of ZnO nanostructures could be controlled by higher temperature. It seemed that the longer growth time would synthesize larger average length of ZnO nanostructures. But the final length was limited with the consumption of precursor solution. Lager length-diameter ratio of ZnO nanorods would be obtained by multi-repeated hydrothermal growth, and even appear a’new’ growth at the top of the nanorods. We tried to carry out’two/three periods staged’microwave heating technique compared to direct microwave heating in the same total reaction time and final holding time. Using this heating method, ZnO nanorods had a lager length-diameter ratio and better crystallinity.3. With layer-by-layer self-assembed polyelectrolyte/ZnO nanoparticles as the method of nanocrystal seeds grown on wood surfaces in advance, ZnO nanostructure coatings were facbricated on wood surface by microwave-assisted hydrothermal method. Firstly, poly (allylamine hydrochloride) as polycation was attached to the wood surface due to electrostatic adsorption. And then the polycation adsorbed polyanion and ZnO nanoparticles with positive charge in turn. On the basis of this nanocrystal seeds pre-treatment, lager length-diameter ratio and good crystallinity of ZnO nanorods were grown on wood surface by microwave-assisted hydrothermal method. We focused on the analysis of the mechanisms of the layers-by-layer self-assembled polyelectrolyte/ZnO nanoparticles and discussed the influences of the concentration of polyelectrolyte and adsorbent layers on the combination between nanocrystal seeds and wood surface. Various kinds of nanostructures were fabricated on wood surface by pre-treatment different nanoparticles (Fe2O3、Al2O3、CuO) using this method.4. This paper would carry out property tests for wood which ZnO nanostructures were grown on, including stability, wettability, ultraviolet resistance and antimicrobial property tests. In order to verify the stability of the nanostructure and the bonding strength to the wood surface, extreme chemical conditions (acid rain, alkaline rain, seawater, polar/nonpolar solution) and physical environment (temperature, ultrasonic) were simulated to analyze the macro-morphology of wood surface and the existence of ZnO nanostructure coatings. Wood surface which grown on ZnO nanorods could improve the hydrophobicity and then modified with stearic acid of low surface energy, it could be kept long time hydrophobic effect and prevent droplets gradually infiltrated. Meanwhile, it was found that the ZnO nanostructure coatings could also effectively prevent the common reagents (glycerol) and common household solutions (milk, coke, soy sauce, coffee) infiltrated. The range of application would be expanded. ZnO nanostructure coatings had the strong absorption and reflection to ultraviolet. After 1000h of UV irradiation, the wood surface which grown on ZnO nanorods had little change in color, but slight degradation. The color of original wood became darker obviously. In addition, after 12h of UV irradiation, the modified wood samples had a certain antibacterial effect on escherichia coli and staphylococcus aureus.