Preparation And Applications Of Chromatic Hollow Nanospheres

Electrophoretic display（EPD） was a reflective digital display, which was often considered to be suitable for reading because of its stable image, wide viewing angle, good contrast ratio and low power consumption. Currently, black white electronic books（e-books） had been commercialized and had achieved mass market success. Therefore, the most common development for electrophoretic display techniques was the achievement of full-color E-paper and it would bring great economic profits. Even though the RGB tricolor was achieved through the placement of a color filter array on the electrophoretic film, the color filter not only limited the brightness to less than a third of the incident light, it also sacrificed the saturation of the colors. On the other hand, the control of electrophoresis rate and a pixel composed of tricolor ink particles were two promising approaches to achieve chromatic EPD. Hence, the preparation of high-quality tricolor electrophoretic particles was necessary and irreplaceable. Inorganic pigment seemed to be a suitable alternative to organic pigments due to its superior abilities in color properties, light stability and heat resistance. However, the weakness in high density as colored electrophoretic particles limited their practical application, resulted in severe sedimentation and a slow response speed in dielectric media. Hollow nanostructures could effectively overcome the disadvantages, prevent the nanoparticles from agglomeration and improve the stability of the particles in the electrophoretic dispersion because of its characteristics, such as high surface-to-volume ratio, low coefficients of thermal expansion and a low refractive index. Therefore, the preparation of inorganic pigment-based hollow particles offered some distinct advantages to fabricate full-color electrophoretic display. In this paper, a general and facile method was developed to prepare inorganic pigment color hollow particles and the hollow particles were employed for electrophoretic display.At first, PMMA-BA-TiO₂ nanospheres with a core-shell structure were successfully prepared by using PMMA-BA which was synthesized via emulsifier-free emulsion polymerization as the core template, and tetrabutyl titanate as the precursor. Next, Fe（OH）₃ was deposited onto the surface of the composite core-shell microsphere via boiling hydrolysis. After freeze drying and thermal treatment of the precursor powder, the prepared hollow TiO₂ coated with Fe₂O₃ nanospheres exhibited a bright color, uniform size, obvious hollow structure and low breakage rate. The average diameter of the hollow spheres coated with a layer of α-Fe₂O₃ was approximately 300 nm and the thickness of the layer was roughly 50 nm. The multicomponent composite hollow Fe/Co/Al nanospheres were produced with Co and Al as tinting metal ions so as to endow them with higher color saturation and brightness. Its x, y and Y parameter values were 0.5074, 0.2958 and 14.67, respectively, which was closer to the red region and brighter in colour. The electrophoretic mobility and zeta potential of two kinds of hollow particles in Isopar-L were about-1.0×10^-5cm²v^-1s^-1 and-100 mV, respectively.Hollow TiO₂ coated with Cr₂O₃ nanospheres, multicomponent composite hollow Cr/Co/Al green particles and composite hollow Co/Al blue hollow particles had been successfully prepared by using Pechini sol-gel process, in which PMMA-BA at TiO₂ core–shell nanoparticles were synthesized, and then mixed with metal oxide in the solution, followed by calcination in the atmosphere. For hollow TiO₂ coated with Cr₂O₃ particles, the optimal color and perfect surface morphology of hollow nanoparticles was achieved at 3mmol CrCl₃.6H₂O. For multicomponent composite hollow Cr/Co/Al green particles, when the molar ratio of CrCl₃: Co（NO₃）₂: Al（NO₃）₃ was 3:1:1, the green hollow nanoparticles showed bright color and its surface morphology was perfect. The blue hollow nanospheres（Co: Al=1:3） calcination at 700 oC were rarely displayed fragmentation and exhibited hollow structures with uniform size of 280 nm and shape.The electrophoretic inks prototype device was successfully assembled using dispersion of the obtained hollow RGB colour nanospheres in a mixed dielectric solvent with TiO₂ white particles as contrast. Under an applied bias voltage of 30 V, the response time of the simple red/white EPD device was 1121 ms and the max contrast ratio was 3.173. The highest contrast ratio of the green/white and blue/white electrophoretic display prototype device was 2.826 and 1.816, respectively, which was close to the level of organic pigments and had shown great potential for practical application in a vivid chromatic electrophoretic display.Futhermore, the composite hollow titania-carbon particles were applied to electrophoretic display as black particles and photocatalytic degradation of methylene blue as photocatalysts. The electrophoresis particles could monochrome dual display at 30 V, the contrast ratio and the response time was 7.2 and 1100 ms, respectively, which presented the hollow TiO₂/C particles could be used as displays particles in electrophoretic displays. The TiO₂/C hollow nanostructures exhibited good photocatalytic activity for the degradation of methylene blue under ultraviolet irradiation and the degradation rate was 96%.