Electron transfer in cobalt hexacyanoferrate nanoparticles promoted by reverse microemulsions prepared with Cetyltrimethylammonium surfactants

Reverse microemulsions (RME) formed with the systems CTAX + pentanol/hexane/water (where X = Br−, Cl− or p-toluensulfonate) were used as synthesis media of cobalt hexacyanofarrate nanoparticles (CoHCF-NP). In this work, the effect that these RME have on the final structure of the CoHCF-NP is presented. Starting with a reaction between the reactant ions (RI) Co2+ and [Fe(CN)6]3−, CoHCF-NP with the expected structure based on the chain FeIII-CN-CoII can be obtained. By changing variables that defines the RME, it is possible to promote an electron transfer inside the CoHCF-NP to change the structure from FeIII-CN-CoII to FeII-CN-CoII. Variables as water concentration, distribution of the RI in the RME droplets, the surfactant counterion, and the concentration ratio of the RI showed a remarkable effect on the CoHCF-NP. CoHCF-NP in which the chain FeIII-CN-CoII predominates in the structure can be synthesized using RME formed with: i) CTAB at high RI and water concentration; ii) CTAT, when the concentration of RI is higher than 0.02 M. On the other hand, CoHCF-NP with predominance of FeII-CN-CoII can be synthesized using RME formed with: i) CTAB at low RI and water concentration; ii) CTAC, regardless the RI and water concentrations. This information is summarized in a concentration diagram where it is possible to predict the structure of the CoHCF-NP. To the best of our knowledge, this type of diagrams has not been constructed before for any of the Prussian blue analogues synthesized in RME.