Electronic Perspective on the Electrochemistry of Prussian Blue Films

Abstract

The derivative of the voltabsommetric scans, together with previous nano-electrogravimetric and X-ray diffraction results, allow different electrochemical processes to be distinguished during the Prussian blue ͑PB͒ voltammetric scan. Potassium, proton, and hydrated proton counterions involved in PB electrochemistry are related here to the electrochemical reactions of specific Fe sites. Potassium counterions show two different sites for their insertion: one located in the crystalline framework and another in ferrocyanide vacancies. From the monitoring of electroactive Fe sites, the covalent-exchange model is suggested as one of the first approaches to explain the origin of the PB magnetic ordering observed at room temperature during voltammetric scanning. Systems that undergo reversible and controlled changes of their properties offer appealing perspectives for the fabrication of electronic switch devices. Thus, Prussian blue ͑PB͒-like materials have raised intense interest because of their electrochromic, ionexchange, ion-sensing, electrocatalytic, or photomagnetic properties, 1-5 because the PB Everitt's salt ͑ES͒ voltammetric scan is accompanied by reversible changes in these physicochemical properties.6-10 Likewise, they also display a wide range of important magnetic ͑e.g., room temperature magnetic ordering͒ and magnetooptical ͑e.g., ferromagnetism, photoinduced ͑de͒-magnetization͒ properties. [11][12][13][14] In particular, the PB electromagnetic properties can also be tailored by external conditions, such as the magnetic field, 15-17 light, 15,18,19 and by electrochemical methods. 18,[20][21][22][23][24] X-ray studies showed that the PB three-dimensional ͑3D͒ structure is face-centered cubic 25 with a general formula equal to Fe 4 ͓Fe͑CN͒ 6 ͔ 3 ·mH 2 O. Trivalent and divalent iron ions are in highand low-spin sites, Fe͑III͒ high-spin and Fe͑II͒ low-spin , respectively. 26Enclosed in this structure, high-and low-spin sites are both octahedral and surrounded by -NC and -CN units, respectively. Thus, the PB lattice is composed of repetitive units of Fe low-spin -CN-Fe high-spin in the three spatial directions. Nonetheless, 1 4 of low-spin Fe͑II͒ sites are missing and occupied by water molecules coordinated to Fe͑III͒ high-spin ions. The PB structure is completed with water molecules occupying interstitial positions. 27-29The above crystalline structure corresponds to the freshly deposited PB film following the electrochemical methodology described by Itaya et al. 22 and it is known as insoluble. The insoluble PB is electrochemically converted into the soluble PB by means of successive voltammetric cycles between the PB and ES forms 30,31 until a stable electrochemical response is achieved. 32 The soluble PB has been described as similar to the insoluble PB 3D structure. 33,34 Repetitive structural units of Fe low-spin -CN-Fe high-spin also compose its rigid framework, where 1 4 of the low-spin sites are missing. The position of the trivalent iron ions can be drawn as Fe͑II͒ low-spin -CN-Fe͑III͒ high-spin a...

Identification of electroactive sites in Prussian Yellow films

“…One possible explanation is to attribute this process to the trapped ionic sites. It has been proposed that these trapped sites are responsible of the absorbance at 380 nm in s-PB films [24,34]. If s-PB films are reduced to the ES form, the absorbance decrease at 380 nm takes place when Fe 3+ in these units is reduced to Fe…”

“…For the case of only one electro-optically active species, In the case of voltammetric peaks due to more than one electrochemical proces, the information provided by electro optical signals may be useful to separate the whole response into individual responses [23,24,34,35]. [28].…”

Interfacial Role of Cesium in Prussian Blue Films

“…5a, time-derivative absorbance at 670 nm ( 670 ⁄ ) for the first cycle shows the color evolution of the chromophores Fe(II)low spinCNFe(III)high spin which require the alkaline cation participation 13 . This chromophore is the responsible of the blue/white electrochromism of PB.…”

“…form stabilized in presence of K + is 13,14 : Crystalline PB structure has a face-centered cubic lattice that shows two different types of water molecules: coordinated water into vacancies and uncoordinated water in interstitial positions 15,16 . Consequently, the content of inner water affects directly to all physical properties of the material.…”

“…This band is caused by the electron charge transfer from the Fe(II) atoms surrounded by -CN units to the Fe(III) atoms surrounded by -NC units 13,18 . Furthermore, this band and its electrochromic efficiency remain unaltered during the transformation of insoluble PB to soluble PB in the stabilization process 19 .…”