Historic Prussian blue (PB) pigment is easily obtained as an insoluble precipitate in quantitative yield from an aqueous mixture of Fe 3+ and [Fe II (CN) 6 ] 4− (Fe 2+ and [Fe III (CN) 6 ] 3−). It has been found that the PB pigment is inherently an agglomerate of 10-20 nm nanoparticles, based on powder x-ray diffraction (XRD) line broadenings and transmission electron microscopy (TEM) images. The PB pigment has been revived as both organic-solvent-soluble and water-soluble nanoparticle inks. Through crystal surface modification with aliphatic amines, the nanoparticles are stably dispersed from the insoluble agglomerate into usual organic solvents to afford a transparent blue solution. Identical modification with [Fe(CN) 6 ] 4− yields water-soluble PB nanoparticles. A similar ink preparation is applicable to Ni-PBA and Co-PBA (nickel and cobalt hexacyanoferrates). The PB (blue), Ni-PBA (yellow), and Co-PBA (red) nanoparticles function as three primary colour inks.

We developed the electrically color-switchable glass and information displays with electrochromic (EC) nanoparticle thin films. The devices were fabricated with liquid processes with the “ink” of multi-colored EC nanoparticles of Prussian blue and its analogues. The color switchable glass with spin-coated uniform EC films exhibits its color change between stained and colorless under applied voltage less than ±1.5 V. Serious degradation of the device was not observed even after the 10,000 times operations. We also made EC nanoparticle thin film with fine patterns. An EC device with such patterned films shows clear change of displayed image electrically.

We dispersed nanoparticles of Prussian blue (PB) in water using surface modification of the PB nanoparticles. Using spincoating method, a thin film is easily fabricated with water dispersible ink of PB nanoparticles. The PB film thickness was estimated as 40 -430 nm. The PB nanoparticle film on an indium tin oxide glass substrate showed a stable electrochromic response: the color changed repeatedly between blue and colorless without any post-treatment.