Mobility of black pigments for electrophoretic display depending on the characteristics of carbon sphere

A carbon sphere can be used as a black pigment in an electrophoretic display, substituting the convectional carbon black. The carbon sphere is synthesized through a hydrothermal reaction of glucose or fructose, and its characteristics are strongly dependent on the reaction parameters. The size of the produced carbon sphere simply increases with the reaction time, while the surface state related to the outermost functional group is not linearly changed. Since the electrophoretic motion of the carbon sphere is determined by the surface zeta potential of the black pigment, the impact of both size and surface chemistry on the surface zeta potential should be investigated. In this study, the changes in the size and surface state of carbon sphere according to the reaction time are examined, and their influences on the surface zeta potential before and after the polymerization step are clarified. It is confirmed that the surface zeta potential and the electrophoretic motion of black pigment under the electric field are obviously affected by the surface chemistry as well as the size of the carbon sphere. The results suggest that the accurate control of the size and surface state of the carbon sphere during the hydrothermal reaction is important to improve the performance of the electrophoretic display.