Preparation of the carbon sphere coated with iron oxide and its application for electronic paper

•Carbon spheres were applied to electronic paper as pigments.•Blackness of carbon sphere increased after attaching iron oxide nanoparticles.•Modified pigments (CE/FEO) exhibited fast response and high blackness.

Carbon sphere is a candidate for black pigment in electronic paper (ePaper) due to the fast and reliable response time based on narrow size distribution and spherical shape. However, its dark brown color, which reduces the contrast ratio between black and white pigments, needs to be manipulated. To overcome the poor blackness of the carbon sphere, iron oxide nanoparticles that have high blackness were coated on its surface. From a reflectance analysis, the increase of blackness was verified and was then optimized by controlling iron salts concentration. After fabricating the carbon sphere coated with iron oxide nanoparticle, 2-ethylhexyl acrylate (EHA, monomer) was grafted on the carbon sphere to match the density of that of tetrachloroethylene (TCE) for high dispersibility as well as bistability. This approach enables us to make the carbon sphere with narrow size distribution, high blackness, high dispersibility and bistability. A monochrome operation was successfully conducted with high contrast ratio and fast response time of 128.5 ms.