Three-dimensional structure of printer toner visualized using cryogenic X-ray diffraction imaging tomography

Printer toner is a fine powder material essential for fixing digital information on paper during the electrostatic digital printing process. To achieve fine printing, toner particles are composed of colored pigments, dyes, polymers, charge control agents and other surfactants. Toner particles have approximate dimensions of 5-10 µm. Therefore, the distribution of the constituents cannot be observed by transmission electron microscopy. In this study, we visualized the three-dimensional structure of a 5 µm-sized printer toner particle using cryogenic X-ray diffraction imaging tomography. A particle adsorbed on a silicon nitride membrane was rotated in the angular range from -78° to +78° against the direction of the incident X-rays at an angular step of 0.5°. From the 313 diffraction patterns collected, we reconstructed the three-dimensional electron density distribution of the toner particle at a resolution of 141 nm. The particle was wedge-shaped. The electron density distribution inside the particle was non-uniform. The high-electron-density regions were distributed near the surface. These formed a V-shaped structure at the tip of the wedge. With the help of powder diffraction, the high-electron-density regions were interpreted as microcrystals of silicon dioxide. Silicon dioxide crystals function as cleavage sites for toner particles prepared by the milling method. Based on the present results, we discuss the implications of the structure and methods to visualize it at a higher resolution.