| Nanodispersions are colloidal systems formed by well-dispersed nanoparticles and dispersion mediums.Generally,when the secondary particle sizes of nanoparticles are less than 40 nm,the nanodispersion can be transparent.Recent years,transparent nanodispersions are of great interests due to its much better optical,mechanical,and catalytic properties than nano powders.Among inorganic nano materials,ZrO2 nanocrystals are one of the most promising functional materials that have wide spread applications in the field of high-refractive-index,catalytic,ceramic,and photoelectric materials,owing to its chemical inertness,extremely high hardness,high refractive index,and high dielectric constant.It is desired to achieve transparent nanodispersions with excellent stability,high solid contents,controllable shapes,crystal forms and sizes of ZrO2 nanoparticles for high-end application.Nowadays,several methods have been developed for the preparation of ZrO2 nanocrystals.However,there are still challenges in the complex preparation process,such as the large particle size,the complicated preparation process,the expensive raw materials,the difficulty of storage and the difficulty to scale up.Therefore,it is urgent to develop a facile way to prepare transparent and stable nanodispersions of ZrO2 nanocrystals in various solvents.In this work,novel routes in an acid surrounding,taking the inexpensive inorganic zirconium salt as raw material,combined with hydrothermal/solvothermal process were presented to prepare ZrO2 nanodispersions.The as-prepared ZrO2 nanocrystals and clusters with tunable crystal forms,sizes and morphologies can be readily dispersed in various solvents,thereby forming the highly concentrated,stable and transparent nanodispersions.The effects of preparation conditions on the structure and the refractive-index,photocatalysis and thermal conductive properties of the ZrO2 nanocrystals are studied.The main contents and innovative points are as follows:1.Highly transparent aqueous dispersions of ZrO2 nanocrystals with tunable crystalline phase were prepared without modification under an precipitation method combined with hydrothermal treatment.The as-prepared ZrO2 nanodispersions prepared at 170℃ for 10 h had an average particle size of 7 nm with a mixture of coexisting monoclinic and tetragonal phases.There was an increase of the monoclinic ratio upon increasing the hydrothermal temperature.Pure monoclinic ZrO2 nanocrystals were obtained at 230℃ and pure tetragonal ZrO2 nanocrystals were obtained at 240℃ and in the water-ethylene glycol mixed solvent with a volume ratio of 1:2 as a hydrothermal medium.High-gravity precipitation-hydrothermal method was further performed to continuously prepare transparent ZrO2 nanodispersions.The optimal process conditions are:the flow fate was 100 mL/min,and the rotate speed of the rotating packed bed(RPB)was 1800 rpm.The average particle size of the ZrO2 nanocrystals prepared by the RPB was 6 nm,which was smaller than that by the flask.Furthermore,highly transparent PVA/ZrO2 nanocomposite films with high refractive index were fabricated using a simple solution mixing method by adding transparent dispersions of ZrO2 nanocrystals with high solid content into PVA solution.The refractive index can be altered in the range of 1.528 to 1.754(@589 nm)by changing the mass fraction(0～80 wt.%)of ZrO2 nanocrystals in transparent nanocomposite films.2.ZrO2 colloidal nanocrystal clusters(CNCs)were prepared via a hydrothermal method.The effects of hydrothermal temperature,time,and inorganic ions on the morphology,particle size and structure of ZrO2 CNCs were mainly studied.The as-prepared ZrO2 CNCs were the monoclinic phase.ZrO2 nanocrystals were aggregated with preferred orientation to form cube-like and star-like shapes of CNCs.Hydrothermal time has a significant effect on the size of ZrO2 CNCs:when the hydrothermal time increased from 6 h to 10 h,the size of ZrO2 CNCs increase from 74 nm to 170 nm.The shape of ZrO2 CNCs can be tuned by using different alkaline sources:cube-like CNCs can be obtained only with Cl-,star-like CNCs can be obtained with the coexistence of Na+and Cl-,and removing inorganic ions ended with nanodispersion.The photocatalytic activity of ZrO2 CNCs for the photodegradation of RhB was further studied.The CNCs obtained with hydrothermal temperature of 200℃ and hydrothermal time of 6 h without modification had better photocatalytic performance.The degradation performance was that:star-like ZrO2 CNCs>P25>cube-like ZrO2 CNCs>ZrO2 nanodispersions.The degradation rate of RhB over star-like ZrO2 CNCs was 1.98 times faster than that of commercial P25.In addition,ZrO2 CNCs also had good applicability for the degradation of Methyl violet and Congo red dyes and for the adsorption of Methyl orange.3.Transparent dispersion of monodispersed oleophilic ZrO2 nanocrystals with tunable shapes,sizes and crystal forms can be efficiently prepared via a phase transfer-solvothermal method.Different shapes of ZrO2 nanocrystals including spindle,cobblestone,half spindle half cobblestone,branch,rod,sphere,and cube can be achieved by tuning the surfactants,organic solvents and precursors.The average size of ZrO2 nanocrystals was less than 10 nm and the lenth of nanocrystals was 5～20 nm.Density functional theory(DFT)calculations were used to elucidate the root causes of underpinning growth mechanisms.Furthermore,this approach was also applied in the preparation of ZrO2:Yb3+/Er3+nanocrystals with near spherical shape,4 nm particle size,and tetragonal crystal form.The ZrO2:Yb3+/Er3+nanocrystals with up-conversion luminescence showed bright visible red(678 nm)and blue(486 nm)fluorescence under 980 nm irradiation.4.Transparent dispersion of monodispersed olephilic ZrO2 nanocrystals were prepared by the solvothermal method.The as-prepared ZrO2 nanocrystals had a average particle size of 3 nm,a tetragonal crytal form and can be transparently dispersed in nonpolar solvents for weeks without modification.ZrO2 nanocrystals which were modified by long chain carboxylic acids or polymers can be monodispersed in low-polarity solvents and modified by silane coupling agents can be monodispersed in medium-polarity solvents.The effects of the solvothermal temperature on the the size and thermalsensitive property of ZrO2 nanocrystals were studied.The average size of ZrO2 nanocrystals was 3 nm when the solvothermal temperature was 210～230℃;while the size was 5 nm when the solvothermal temperature was 240～260℃.There was thermalsensitive property in nanocrystals obtained at relatively high solvothermal temperature(240～260℃).That is,at high temperature(>40℃)the particles can be dispersed and the nanodispersions were transparent,at low temperature(<25℃)the particles were aggregated and the nanodispersions became cloudy.Monodispersed TiO2,CeO2,ZnO and Fe3O4 nanocrystals were further prepared by the similar method.The thermal conductivity was explored based on ZrO2 nanodispersion in toluene phase which was regarded as a nanofluid.With the increase of ZrO2 volume fraction and temperature,the thermal conductivity was significantly improved.In particular,when the ZrO2 volume fraction was 13.21 vol.%,and the temperature was 60℃,the thermal conductivity enhancement achieved by about 60%. |
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