| Rutile titanium dioxide(Ti O2),as an outstanding white inorganic filling material,was extensively used in paint,cosmetic,plastic,papermaking,and pharmaceutical industries because of its unique physicochemical properties,such as high refractive index,high chemical stability,and nontoxicity.However,bare Ti O2 exhibits strong photoactivity.Under UV or sunlight irradiation,the separation and migration of electron-hole pairs on the surface of bare Ti O2 result in the generation of reactive groups,including holes(h+),hydroxyl(·OH),and superoxide(O2·-)radicals.These reactive groups can cause the degradation of surrounding polymer binders or other organic colored pigments in practical application,which further shorten the life span of Ti O2-based products.In addition,the relative high surface energy of bare Ti O2 resulted in the serious agglomeration,which decreases their dispersion stability in water-based system.The lower dispersion stability of bare Ti O2 causes uneven colouring and decreases weather durability of Ti O2-based pigment.Moreover,the change of temperature and the erosion of acid rain can obviously decrease the weather durability and the resistance to chalk-fade reaction of bare Ti O2,which would shorten the life span of Ti O2-based products.It is noted that the formation of inorganic coating layer on the surface of Ti O2 could improve its photocatalytic stability,dispersion stability and weather resistance.Actually,the practical application performance of inorganic shell-Ti O2 core products is largely relied on the micro morphology of the inorganics shell.However,there is no precise control technology for the microstructure and structure-activity relationship of inorganic shell.Therefore,it is of great significance to precisely control the microstructure of the inorganic shells.In this paper,the alumina,silica,zirconia and aluminum phosphate coated rutile Ti O2 samples were synthesized using the chemical liquid deposition method.The optimal preparation conditions of alumina,silica,zirconia and aluminum phosphate coated rutile Ti O2 were obtained by using the dispersion stability,weatherability,and photostability as evaluationindex,respectively.On the basis of the characterications for the interface structure and properties of samples,the formation process of inorganic shells and the relationship between the coating process and the microstructure of inorganic shells were systematically investigated.The relationship among the microstructure of inorganic shells,physical and chemical properties,and practical application performance of inorganic shell-Ti O2 core products was investigated.The enhancement mechanism of the practical application performance was studied as well.The main works and achievements of this paper are listed as follows:(1)The optimal preparation conditions of alumina coated rutile Ti O2 were determined:the reaction p H value was 9,the reaction temperature was 70°C,the amount of alumina coating was 3.2%(m(Al2O3):m(Ti O2)),the reaction time was 60minutes,the aging time was 120 minutes,the slurry concentration was 25%(the mass of rutile Ti O2 accounted for the mass percentage of pre-dispersion suspension),and the pre-dispersant was PEG-1000(m(PEG-1000):m(Ti O2)=0.3%).Based on the results of microstructure control and optimization mechanism of practical application performance for alumina coated rutile Ti O2,it is showed that the flocculent and highly continuous boehmite layer on the surface of rutile Ti O2 was benefit for its dispersion stability in aqueous system.During the coating process,the dehydration and condensation between hydrated alumina and the surface hydroxyl of rutile Ti O2resulted in the formation of the thin and continuously dense layer on the surface of rutile Ti O2.Then,the loose and flocculent layer was adsorbed on the surface of rutile Ti O2 by electrostatic adsorption.The formation of above two layers significantly increased the zeta potential,the surface hydroxyl and wettability of rutile Ti O2,which further enhanced the space resistance between rutile Ti O2 particles and hindered the agglomeration and sedimentation of particles.In this case,above characteristics of alumina coated rutile Ti O2 significantly improved the dispersion stability of rutile Ti O2in aqueous system.(2)The optimized preparation conditions of silica coated rutile Ti O2 were determined:the reaction p H value was 9,the reaction temperature was 85°C,the amount of silica coating was 2.5%(m(Si O2):m(Ti O2)),the reaction time was 90minutes,the aging time was 120 minutes,the slurry concentration was 25%(the mass of rutile Ti O2 accounted for the mass percentage of pre-dispersion suspension),and the pre-dispersant was Na2Si O3(m(Si O2):m(Ti O2)=0.5%).Based on the results of microstructure control and optimization mechanism of practical application performance for silica coated rutile Ti O2,it is showed that the formation of continuous and uniform thick silica layer on the surface of rutile Ti O2 can significantly improve its weather resistance.During the coating process,a thin and dense layer was firstly generated on the surface of rutile Ti O2 by chemical bonding(Si-O-Ti bond).Then,a thick and uniform silica layer was adsorbed on the surface of rutile Ti O2 by electrostatic adsorption.The generation of above silica shell-Ti O2 core structure can resist the direct erosion for inner rutile Ti O2 core and improve the structural stability.Above characteristics further contributed to the enhancement of weatherability of rutile Ti O2.(3)The optimum preparation conditions of zirconia coated rutile Ti O2 were determined:the reaction p H value was 5,the reaction temperature was 55°C,the amount of zirconia was 5.0%(m(Zr O2):m(Ti O2)),the reaction time was 60 min,the aging time was 120 min,the slurry concentration was 20%(the mass of rutile Ti O2accounted for the mass percentage of pre-dispersion suspension),and the pre-dispersant was sodium hexametaphosphate(SHMP)(m(P2O5):m(Ti O2)=0.3%).Based on the results of microstrucutre control and optimization mechanism of practical application performance for zirconia coated rutile Ti O2,it is showed that the formation of continuous and uniform zirconia layer is favor for the improvement of the photostability of rutile Ti O2.The photostability optimization mechanism of zirconia coated rutile Ti O2 can be listed as follows:(i)The continuous and uniform zirconia layer on the surface of rutile Ti O2 can block its sunlight absorption,resulting in the reduce of the generation and separation rate of photo generated electron-hole pairs;(ii)Zirconia layer can hinder the migration of photo generated electron to the particle surface and accelerate the recombination of photo generated electron-hole pairs;(iii)Zirconia layer can reduce the hydroxyl content on the particle surface,which hinder the formation of reactive species;(iv)Zirconia layer can prevent the contact between the generated reactive species on the surface of inner rutile Ti O2 core and the organic resin in the surrounding environment,which slowed down the degradation of the organic resin.(4)The optimum preparation conditions of aluminum phosphate coated rutile Ti O2 were determined:the reaction p H value was 9,the reaction temperature was50°C,the amount of aluminum phosphate was 3.0%(m(Al PO4):m(Ti O2)),the reaction time was 60 min,the aging time was 120 min,the slurry concentration was25%(the mass of rutile Ti O2 accounted for the mass percentage of pre-dispersion suspension),the pre-dispersant was SHMP(m(P2O5):m(Ti O2)=0.2%).Based on the results of microstructure control and optimization mechanism of practical application performance for aluminum phosphate coated rutile Ti O2,it is showed that thicker and continuous aluminum phosphate layer is beneficial for the improvement of the dispersion stability and photostability.The photostability enhancement mechanism of aluminum phosphate coated rutile Ti O2 can be listed as follows:(i)The formation of Ti-O-Al bond accelerated the annihilation of electrons,which hindered the migration of photogenerated electrons to the surface of prepared particles;(ii)The aluminum phosphate layer on the surface of rutile Ti O2 can block its sunlight absorption,resulting in the reduce of the generation and separation rate of photo generated electron-hole pairs;(iii)The aluminum phosphate layer can hinder the contact between recative species and the organic resin in the surrounding environment.Above characteristics hindered the formation of the reactive species(hydroxyl radicals,superoxide radicals,etc.),further slowing down the degradation of the surrounding organic resin and improving the photostability of rutile Ti O2.In addition,the dispersion stability ehnacement mechanism of aluminum phosphate coated rutile Ti O2 can be listed as follows:(i)The formation of aluminum phosphate layer could reduce the surface energy of rutile Ti O2,which slowed down the agglomeration between rutile Ti O2particles;(ii)Aluminum phosphate layer could increase the surface wettability of particles,which further accelerated its dispersion in the aqueous system;(iii)Aluminum phosphate layer could enhance the electronegativity of rutile Ti O2 and increase the electrostatic repulsion between rutile Ti O2 particles,which further hinder the agglomeration and precipitation between rutile Ti O2 particles. |
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