electrophoretic displays ( epds ) have been the subject of intense research and development for a number of years [ 1 - 5 ] . in such displays , pigments are dispersed as a colloidal suspension in a colored solvent , which is placed in a narrow space between two transparent electrodes . by applying an electric field to the suspension , the charged pigments can be moved through the fluid . when the field is applied in one direction , the pigments move toward the observer and the pixel has the color of the pigments . when the field is applied in the opposite direction , the ito electrode closest to the observer is covered with colored solvent . just like printed paper , epds are lightweight , flexible and easy to read in all lighting conditions [ 6 - 9 ] . above of all , they provide an important improvement in the field of power consumption : due to the bistability of the pigments , an image is retained on the display without the need for constant refreshment . their properties make them the most likely candidate in the search for a display that combines the benefits of printed paper and an image display . epds are now used in consumer products such as the amazon kindle or sony e - reader . nevertheless , problems with the long - term image quality of the displays have prevented their widespread usage . for example , pigments that make up epds tend to settle , resulting in inadequate service life for the displays [ 10 - 13 ] . the change in the appearance of a pixel of the epds is solely due to the change of the position of the pigments within the suspension . thus , the pigments play a vital role on the speed and efficiency of the epds . for a white / black display cell , the white particles are typically titanium oxide because of its relatively high refractive index of 2.7 , which leads to excellent scattering properties . generally , carbon black is the material of choice to produce the black color in most applications . however , carbon black has a complex and poorly understood surface chemistry , which may vary widely with the specific raw material and the exact process used for the carbon black production . it has also been discovered that carbon black has a low density ( 0.8 g / cm ) , which is not matched to that of the usual suspending medium ( about 1.7 g / cm ) . there is thus a need for black particle with proper density and surface chemistry for used in epds that can be compatible with the suspending fluid . herein , we demonstrate that carbon  iron oxide microspheres black pigments ( ciombs ) are formed via ultrasonic spray pyrolysis of the aqueous solutions containing fecl2 and glucose . different from the formation process of carbon powders , in situ - formed iron oxide nanoparticles are introduced as surface chemistry and density modification component to form carbon  iron oxide composites . the density of the ciombs can be easily tuned in the range of 1.52.2 g / cm by acid etching , which resolved the problem of mismatched density between carbon - based black pigment and electrophoretic medium . the ciombs can be well dispersed in various solvents including polar solvents ( water , methanol , ethanol , etc . ) and nonpolar solvent ( e.g. tetrachloroethylene ) due to their surfaces are functionalized with abundant functional groups such as oh , coh and ch2 . ferrous chloride ( fecl2 ) , glucose , and hcl solution ( 12 m ) , titania , diserbyk-161 were purchased and used without further purification . the usp experimental setup is similar to that of suslick s group [ 16 - 19 ] . in a typical experiment , 0.5 mol / l glucose and 0.8 mol / l fecl2 were added to deionized water so as to form a precursor solution under magnetic stirring . for nebulization , a yuyue household ultrasonic humidifier was used to nebulize a precursor solution into a fine mist . the furnace temperature was set as 700c with a nitrogen flow rate of 2 slpm ( standard liters per minute ) . after 1 h of collection into water - filled beaker , the black product was collected with a magnet . after washed with deionized water and then ethanol three more times , the black sample was collected for further analysis . in order to tune the density of ciombs , as - prepared ciombs were immersed in concentrated hcl ( 12 m ) at room temperature for 112 h. the content of iron oxide in the composite dissolve in the concentrated hcl gradually , which leads to decrease in the ciombs density . furthermore , ciombs were washed with deionized water and then ethanol three more times , and the black sample was collected for further analysis . electrophoretic dispersions were prepared via dispersing of ciombs ( 0.05 g / ml ) and titania ( 0.10 g / ml ) in tetrachloroethylene using diserbyk-161 ( 0.01 g / ml ) as a dispersant . it is then introduced in an electrophoretic cell , composed of two ito - coated glasses ( 3.0  7.0 cm ) sealed with 75 m thickness , and the electrode area was prepared by a photoetching technique . x - ray diffraction ( xrd ) patterns were obtained on rigaku dmax2000 with cu k radiation . the morphology of the samples was examined by scanning ( sem , xl30esem feg ) and transmission ( tem , jeol 2010 ) electron microscopy . fourier transform infrared spectrum ( ft - ir ) of the samples was recorded at the ambient temperature on a nigolet 560 fourier transform infrared spectrometer with the resolution factor of 0.1 cm . in the present route , ultrasound is used to produce aerosol droplets dispersed in nitrogen via ultrasonic spray treatment of precursor aqueous solution . the aerosol droplets are carried in nitrogen stream through a quartz - glass tube in a hot furnace , and water evaporation and precursor decomposition occur , generating ciombs . with usp , the aerosol droplets will act as individual micrometer - sized chemical reactors to impose morphology control on the target products . herein , ferrous chloride and glucose are used as precursor of iron oxide and carbon , respectively , to form ciombs via usp . the morphology of the product is studied by both scanning electron microscopy ( sem ) and transmission electron microscopy ( tem ) . 1 , which indicates that the sample is composed of a large quantity of core  shell microspheres with the size in the range of 13 m . from fig . 1 , we can see that the initial concentration of fecl2 has influence on both the size and the morphology of ciombs . as shown in fig . 1a , the size of ciombs is in the range of 3001,400 nm when the concentration of fecl2 is 0.08 m. the surface of as - prepared spheres is smooth . when the concentration of fecl2 is 0.32 m , the size of sample will increase to 5003,000 nm . in addition , the surface becomes rough , which comprises iron oxide nanoparticles with size of about 50100 nm ( the inset of fig . furthermore , microspheres with fe - rich core / c - rich shell ( the size is in the range 5003,000 nm ) are formed when the concentration of fecl2 reaches up to 0.8 m. as shown in fig . 1c , d , as - prepared spheres display two distinct surfaces : bubble - like surface and rough surface . that is , some spheres have bubble - like carbon surface ( consists of many bubble in the carbon surface ) , and fe - rich core comprises the agglomeration of iron oxide nanoparticles . the other spheres have rough carbon - rich shell ( the thickness is about 100 nm ) and fe - rich core ( fig . 1d and the inset of fig . sem and tem images of core / shell cmm obtained via usp of glucose and fecl2 with different initial concentration of fecl2 : a 0.08 m ; b 0.32 m ; c , d 0.8 m. the inset of a c is the high - magnification sem images of corresponding samples the possible formation mechanism of ciombs can be illustrated as follows : ( 1 ) carbonaceous nanoparticles loaded with fe / fe ion are generated via the catalyzing of fe ; ( 2 ) with the furnace temperature increasing , carbon - coated iron oxide nanoparticles will be formed via the oxidization of iron ion , accompanying by the further carbonization of carbonaceous components with the catalysis of in situ - formed iron oxide ; ( 3 ) with the reaction time prolonging , more carbon will be formed in the neighborhood of aforementioned iron oxide nanoparticles via the carbonization of glucose . when the initial concentration of fecl2 is low , as - produced ciombs display smooth surface , in which iron oxide nanoparticles are randomly dispersed in the carbon spheres ( fig . when the initial concentration of fecl2 is high enough , as - produced ciombs display rough surface due to aforementioned iron oxide nanoparticles tend to agglomerate to form iron oxide - rich core ( fig . subsequently generated carbon will be formed carbon shell around the iron oxide - rich core . 2 , the diffraction peaks can be indexed to the mixture of face - centered cubic fe3o4 ( jcpds file no . 48 - 1487 ) and hexagonal - structured -fe2o3 ( jcpds file no.33 - 0664 ) . there are no obvious crystalline peaks of carbon , indicating that the structure of carbon is amorphous . notes : the peaks labeled by * can be indexed to -fe2o3 ( jcpds file no . 33 - 0664 ) the ft - ir spectrum is used to identify the surface chemistry present in the ciombs . overall , in the range of 1,0003,500 cm , the spectrum is similar to that of carbon spheres obtained via the hydrothermal treatment of glucose aqueous solution . as shown in fig . 3 , the bands at 3,400 and 1,625 cm can be assigned to the o  h and c = c , respectively . the bands in the range it should be noted that the bands at 2,925 and 2,855 cm , which can be assigned to hydrogen bonded sp3 carbon , are stronger than those of the pure carbon sphere , while the band at 1,710 ( c = o ) is almost disappear . the reason may lie on the relative higher reaction temperature ( 700 vs. 180c ) , which leads to the more complete carbonization of glucose . in the case of three bands ( 455 , 535 and 570 cm ) in the range of 500700 cm both fe3o4 and fe2o3 exist in the spectrum of sample ciombs , which are consistent with the results of xrd ( fig . ft - ir spectra of sample ciombs as demonstrated in the previous literatures , both crude carbon blacks and crude carbon nanotubes are hard dispersed in either polar or nonpolar solvents . in the case of carbon spheres obtained via the dehydration of polysaccharides aqueous solution under hydrothermal conditions ( < 200c ) it is well known that surface s functional groups play important roles in the process of dispersion . as shown in fig . 3 , there are abundant functional groups ( oh , coh , ch2 , etc . ) on the surface of ciombs , which lead to good dispersibility in various solvents . the ciombs not only can be dispersed in polar solvents ( i.e. water , methanol and ethanol ) , but also can be dispersed in nonpolar solvent , such as tetrachloroethylene . the density of the as - prepared ciombs is about 2.2 g / cm , which makes ciombs gravitational settling in tetrachloroethylene ( density , 1.63 g / cm ) due to the density mismatch ( fig . the iron oxide nanoparticles instead of carbon in the composite can react with hcl , and the product can dissolve in the solution . thus , the density of ciombs decreases during the hcl etching . when the duration of etching is 1 h , the ciombs can be well dispersed in tetrachloroethylene , whose density is about 1.7 g / cm , which is a good density match . when the duration of etching is 12 h , the density of ciombs is too light to be dispersed in tetrachloroethylene . most of the ciombs float on the surface of the suspension . from the tem pictures , a ciombs with different etching time ( from left to right 0 , 1 , 12 h ) are dispersed in tetrachloroethylene . tem images of ciombs with 1 h ( b ) and 12 h etching exploiting their well dispersibility in tetrachloroethylene , ciombs are used as black electrophoretic particles for electrophoretic displays . electrophoretic dispersions are prepared via dispersing of sample ciombs and titania ( served as white electrophoretic particles ) in tetrachloroethylene using diserbyk-161 as a dispersant . after above dispersions are introduced into an electrophoretic cell , an electrophoretic display is formed . 5 presents photographs of an electrophoretic display with dimensions of 75 m ( t )  7 cm ( w )  3 cm ( l ) after applying 30 v. photograph of black  white electrophoretic display cell in summary , using aqueous solutions containing ferrous chloride and glucose as precursors , a simple and continuous strategy is developed to synthesize ciombs via usp . due to the presence of affluent functional groups in their surface , ciombs can be dispersed in water , methanol , ethanol and tetrachloroethylene . by adjusting the concentrated hcl etching duration the surface chemistry and density of as - prepared ciombs are appropriate for epds assembly . this work was financially supported by the national hi - tech 863 programme ( 2009aa03z302 ) , national science foundation of china ( 60736001 , 60907042 ) , and beijing natural science foundation ( 2093044 ) . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
carbon  iron oxide microspheres black pigments ( ciombs ) had been prepared via ultrasonic spray pyrolysis of aqueous solutions containing ferrous chloride and glucose . due to the presence of carbon , 
 ciombs not only exhibited remarkably acid resistance , but also could be well dispersed in both polar solvents and nonpolar solvent . 
 finally , dispersions of hollow ciombs in tetrachloroethylene had successfully been applied in electrophoretic displays .