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.