Jes 156 (10) P149-P156 (2009)

Journal of The Electrochemical Society, 156 �10� P149-P156 �2009� P149

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Insights on the Mechanism of Insoluble-to-Soluble PrussianBlue TransformationJeronimo Agrisuelas, Jose Juan García-Jareño, David Gimenez-Romero,z andFrancisco Vicente*

Departament de Química Física, Universitat de València, 46100 València, Spain

The electrochemical transformation of the soluble form of Prussian blue �PB� material from the insoluble form was monitoredusing electrochemical, gravimetric, acoustic, and spectroscopic techniques simultaneously. The described combination of in situtechniques represents an innovative tool for measurement in electrochemistry, which provides complementary information on theelectrochemical systems. The insoluble-to-soluble PB transformation process takes place during the successive voltammetriccycles between the mixed valence form �PB� and the fully reduced form �Everitt’s salt �ES��. One of the processes that takes placeis the exit of free Fe�CN�6

4− ions occluded in the vacancies of the insoluble PB crystalline framework during the fresh PBelectrodeposit process. In potassium salt solutions, the exit of each ferrocyanide ion is compensated by the entrance of twopotassium ions and six hydroxyl anions. This species exchange increases the manifestation of viscoelastic phenomena of the rigidskeleton from the insoluble PB form to the soluble one, which could facilitate the appearance of an internal magnetic field at roomtemperature during the soluble PB � ES voltammetric cycle.© 2009 The Electrochemical Society. �DOI: 10.1149/1.3177258� All rights reserved.

Manuscript submitted February 12, 2009; revised manuscript received June 16, 2009. Published July 27, 2009.

0013-4651/2009/156�10�/P149/8/$25.00 © The Electrochemical Society

Valence-tautomeric Prussian blue �PB�-like materials with thegeneral formula Mk��M��CN�6�l·mH2O �in which M� �high spin� andM� �low spin� are transition metals� have attracted considerable in-terest owing to their intrinsic properties, such as electrochromic, ionexchange, ion sensing, electrocatalytic, as well as high Curietemperatures,1,2 photomagnetic phenomena,3-10 and magnetoresis-tance phenomena.11 The study of changes in these properties at themolecular level introduces an important route to the development ofswitches for advanced molecular devices12 given that they can betailored by external conditions, such as the magnetic field,13-15

light,12,16,17 as well as by the variation in the oxidation state of thesematerials caused by the electrochemical methods.18-20

PB and some heterometal analogs are examples of well-knowncyanide-bridged metal complexes. X-ray studies21 have shown thatthe PB has a face-centered cubic lattice structure. Accordingly, thecrystalline framework of this material is composed of repetitiveunits of Fe�II�low spin–CN–Fe�III�high spin in the three spatial direc-tions. The Fe�III�–NC sites are ascribed to high spin metallic atoms,whereas the Fe�II�–CN sites are ascribed to low spin atoms whereone-quarter of the low spin Fe�II� sites is missed.20 The first coor-dination spheres of Fe�III�high spin and Fe�II�low spin ions are�Fe�III�high spin�NC�6� and �Fe�II�low spin�CN�6�, respectively. In thisstructure, there are two different types of water molecules: �i� Watermolecules coordinated to octahedral Fe�III�high spin in emptyFe�II�low spin�CN�6 sites and �ii� uncoordinated water molecules ininterstitial positions.22-24

The above-described crystalline structure corresponds to thefreshly deposited PB films following the methodology of Itaya etal.19 and it is known as insoluble structure. The soluble PB form isgenerated by successively cycling the fresh PB electrodeposit be-tween the mixed valence form �PB, 0.60 V� and the fully reducedform �Everitt’s salt �ES�, −0.20 V� in potassium salt solutions.25-28

The processes that take place during this transformation were de-scribed as the loss of one-quarter of the high spin Fe�III� ions of theinsoluble PB films, which are replaced by potassium ions.19,27,29,30

Furthermore, the loss of a great amount of interstitial water occludedduring the electrodeposit process also takes place.31 As a result,soluble PB films have important differences in their electrochemicalresponse with respect to the insoluble one, leading to interestingmolecular switches.11,27,29,32 Once converted, soluble PB films areelectrochemically stable and therefore, they can be cycled reversiblybetween the PB and ES forms.18,19,25,26,33 During this process, cat-

* Electrochemical Society Active Member.z E-mail: [email protected]

ownloaded 15 Sep 2009 to 161.111.100.100. Redistribution subject to E

ions are exchanged between the PB structure and the electrolyticsolution to compensate the electrical charge lost or gain in the elec-troactive film.19,34-37 In potassium salt solutions, the exchanged cat-ions are mainly potassium ions, protons, and hydratedprotons.18,34,36,38,39

The in situ use of gravimetric techniques such as electrochemicalquartz-crystal microbalance �EQCM� is a very powerful tool for thestudy of electrochemical reactions. The analysis of the mass/electrical charge ratio allows active species that take part in thereaction to be identified by the calculation of the molar mass of thespecies exchanged between the reaction substrate and the workingsolution.40-43 For EQCM electrodes, it is also possible to follow insitu changes in the motional resistance �acoustic impedance� that arerelated to structural and viscoelastic changes in electroactive filmsdeposited on the electrode surface,33,44,45 among other things. Thus,the motional resistance increases its value significantly when themanifestation of viscoelastic phenomena increases. The most directinterpretation of this is that the magnitudes of the underlying param-eters �shear modulus components� are increased. However, in thecase where the storage modulus �“stiffness”� was increased, thiswould mean that viscoelastic phenomena were less marked. Further-more, the EQCM device can also be used as a magnetic fieldsensor33,46 where the detectable magnetic field range is similar to therange of the magnetic sensors based on the giant magnetoresistanceeffect.46

Working with high reflectance gold electrodes in the EQCM alsoallows one to record changes in reflectance of the deposited PB film.Thus, spectroscopic techniques provide information about the com-position of the crystalline structure of the PB film through theirchromophore units. In the near-UV/visible spectrum of soluble PBfilms, the main spectroscopic band is centered at 672 nm and it isassociated with Fe�II�low spin–CN–Fe�III�high spin structural units thatcompose to a large extent the PB rigid skeleton.21 This band iscaused by the electron charge transfer from the iron �II� atoms sur-rounded by –CN units �Fe�II��CN�6, �t2g�C

6 electronic configuration�to the iron �III� atoms surrounded by –NC units �Fe�III��NC�6,�t2g�N

3 �eg�N2 electronic configuration�, schematized as

�t2g�C6 �t2g�N

3 �eg�N2 → �t2g�C

5 �t2g�N4 �eg�N

2 .47 In the same manner, theband centered at 1023 nm is also related toFe�II�low spin–CN–Fe�III�high spin structural units because it identifiesthe forbidden electron charge transfer �t2g�C


2

→ �t2g�C5 �t2g�N

3 �eg�N3 .47 Finally, a third spectroscopic band is de-

tected at 383 nm and it is related to Fe�III�low spin–CN–Fe�II�high spinstructural units given that it involves the electron charge transferfrom the iron �II� atoms surrounded by –NC units �Fe�II��NC� ,
6
CS license or copyright; see http://www.ecsdl.org/terms_use.jsp

P150 Journal of The Electrochemical Society, 156 �10� P149-P156 �2009�P150

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�t2g�N4 �eg�N

2 electronic configuration� to the iron �III� atoms sur-rounded by –CN units �Fe�III��CN�6, �t2g�C

5 electronic configura-tion�, schematized as �t2g�C


2 → �t2g�C6 �t2g�N

3 �eg�N2 .47 The

spectroscopic signal at this wavelength is overlapped with the spec-troscopic band associated with the �CN�5Fe–CN–Fe�NC�4OH2structural units,48 which are placed into the ferrocyanide vacanciesof the PB crystalline framework.49,50

Herein, it is emphasized that the simultaneous monitoring of theinsoluble-to-soluble PB transformation process in potassium salt so-lutions by electrochemical, gravimetric, acoustic, and spectroscopictechniques does not agree completely with the above-mentionedtransformation mechanism. Thus, the main goal of this paper is topropose a transformation mechanism that clarifies these results. Fur-thermore, the results obtained exhibit the big advantages obtained bythe combination of in situ techniques, e.g., it is possible to calculatecrossed functions.

Experimental

Insoluble PB films were electrochemically deposited by immer-sion in 0.02 M K3Fe�CN�6 �analytical reagent �AR�, Panreac�,0.02 M FeCl3 �Sigma�, and 0.01 M HCl �AR, R.P. NORMAPUR�solution.19 A controlled cathodic current of 40 �A cm−2 was appliedfor 150 s to obtain the great majority of insoluble PB films. Only thespectroscopic studies of the insoluble-to-soluble PB transformationprocess at 380 and 1000 nm were carried out with films that weredeposited by a controlled cathodic current of 160 �A cm−2 for150 s. As the film extinction coefficients at 380 and 1000 nm weresmall, the film thickness was increased to increase the system sen-sibility at these wavelengths. This thickness could also be increasedby the increase in the deposit time, but the secondary reactionsbecame more important and therefore, the film quality decreased.All PB deposits were sufficiently thin to ensure a precise relation-ship between the frequency variation in the quartz crystal and themass change without any viscoelastic artifacts.51 The soluble PBform was generated by cycling the system between 0.60 V �PBform� and −0.20 V �ES form� in 0.50 M KCl �A.R., R.P. NOR-MAPUR� with pH 3.0 and at 20 mV/s and 298 K, according to Ref.25-28.

The voltammetric cycles were carried out in a typical electro-chemical three-electrode cell. A high surface mesh was used as thecounter electrode and the Ag�AgCl�KClsat electrode was used as thereference one. The working electrode was a high reflectance gold/quartz-crystal electrode �AT-cut quartz crystal, 9 MHz, Matel-Fordahl, France�, which allowed the simultaneous measurement ofcurrent, mass �from frequency changes�, and reflectance �from lumi-nous intensity received on the photodiode surface�. The cell tem-perature was controlled by Peltier thermoelectric modules and thecell was a high transmittance glass cell from Hellma �OG�. An Au-tolab potentiostat–galvanostat �PGSTAT302� was used to realize thevoltammetric measurements, whereas an EQCM �RQCM, Maxtek,Inc.� was used to record the motional resistance and resonance fre-quency changes during successive voltammetric cycles. A conve-niently modified Spectronic 20 spectrometer was used for the spec-troscopic measurements. The light sensor was replaced by a fastresponse and more sensible photodiode �silicon PIN photodiode/OSD5.8-7Q�, which gave a current proportional to the received lu-minous intensity. This current was converted into an analogical po-tential by a current to potential converter �homemade�. Theanalogical potential was converted into an apparent absorbance A byconsidering the potential initial values. It is emphasized that theelectrochemical, gravimetric, acoustic, and spectroscopic measure-ments were recorded simultaneously during the insoluble-to-solublePB transformation process by the homemade assembly of the above-commented equipment.

Near-UV/visible–near–IR absorbance spectra �Fig. 4� were re-corded by a HE�IOS � UV/visible spectroscope �Spectronic Uni-cam� that allowed the electronic transitions between 300 and1100 nm to be studied. In these measurements, PB films were de-


posited by the above procedure on an indium tin oxide transparentelectrode with a covering surface of 1 cm2, and a PalmSens poten-tiostat �Palm Instrument BV� controlled the polarization potential.

Results

Electrogravimetric analysis.— Figure 1 shows the electrochemi-cal response and mass changes in PB films during the insoluble-to-soluble PB transformation process, which takes place by cyclingsuccessively the fresh PB electrodeposit between 0.60 V �PB form�and −0.20 V �ES form� in potassium salt solutions, PB � ES vol-tammetric cycle. During these cycles, the electrochemical responseof PB films changes considerably, mainly during the first cathodicscan.32 In this manner, voltammetric peaks become narrower andhigher at the end of the PB conversion process.

Figure 1 also shows that the mass of PB films decreases duringthe cathodic scans and increases during the anodic scans due to thecation exchange between the PB structure and the electrolytic solu-tion to compensate the electrical charge lost or gained in the elec-troactive film.18,34,36,38,39 Furthermore, the mass of the freshly elec-trodeposited PB film �36.41 �g cm−2� decreases −1.68 �g cm−2

between the beginning �the beginning of the first cycle� and the end�the end of the last cycle� of the insoluble-to-soluble PB transforma-tion process in potassium salt solutions; that is, about 5% of theinsoluble PB film original mass. The global voltammetric chargealso decreases at the end of this conversion process.

Obtaining more information on these results, the record of massand current allows the F��moverall/�Qoverall� function to be calcu-lated through Faraday’s laws.40-43 This function allows the molarmass of species that go into the film or leave it during the electro-chemical reactions to be determined. Thus, this function can be em-ployed during the conversion process to study the change in themass and the electrical charge between voltammetric cycles. Table Ishows that the F��moverall/�Qoverall� value between the beginningand the end of each voltammetric cycle changes mainly in the firstconversion cycle. Considering that, the value of theF��moverall/�Qoverall� function for the first cycle is only taken as areference value because changes are more evident for this cycle.Between the beginning of the first cycle and the beginning of thesecond one, the mass of the PB film decreases −0.60 �g cm−2,whereas the electrical charge decreases −2.4 mC cm−2. As a result,the value of the F��m /�Q � function for the first conver-

Figure 1. �Color online� Voltammetric scan and evolution of the masschange in the working electrode during the voltammetric cycles correspond-ing to the insoluble-to-soluble PB transformation process at 20 mV/s. Theelectrolyte was 0.50 M KCl with pH 3.0. The potentials were referred to theAg�AgCl�KClsat electrode.

overall overall


P151Journal of The Electrochemical Society, 156 �10� P149-P156 �2009� P151

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sion cycle is equal to F��moverall/�Qoverall�experimental= 96,484.56��−0.60 � 10−6�/�−2.4 � 10−3�� = 24 g mol−1. Ascommented in the introduction, one-quarter of the high spin Fe�III�ions of the insoluble PB films are replaced by potassium ions duringthe insoluble-to-soluble PB transformation process.27,29 Therefore,as the iron atoms are expelled as iron–cyanide complexes,32,35 theexpected value of the F��moverall/�Qoverall� function for the firstconversion cycle should be

F��moverall/�Qoverall�

=3 � 39�Mw�K+�� − 211.847�Mw�Fe�CN�6

4−��− 1�decrease in the exchanged electron number�

� 95 g mol−1

This value is different from the experimental value. A possible ex-planation for the difference between both values may be found in apartial expulsion of the Fe�CN�6

4− ions from the insoluble PB filmsafter the insertion of potassium ions and anions �i.e., hydroxyl orchloride anions� that compensate the mass and charge decreasecaused by this expulsion, not only potassium ions as commented inthe introduction.

During the last few years, the mass-electrical charge changesduring electrochemical experiments have been analyzed by the useof the F�dm/dQ� function.40,52 This function allows the differentreaction steps reached during the voltammetric cycle to be charac-terized in situ. A negative value of this function means that cationsare exchanged between the electrolyte and the electroactive film.However, a positive value means an anion exchange. Figure 2 showsthe evolution of this function during both first �insoluble� and last�soluble� PB conversion cycles. More concretely, Fig. 2 shows onlythe values of the F�dm/dQ� function during the cathodic scan giventhat the electrochemical response of PB films changes mainly duringthe first PB → ES cathodic scan, as commented above.

Accordingly, Fig. 2 shows that the values of the F�dm/dQ� func-tion during the PB → ES cathodic scan are negative when thesoluble PB form is already generated �last conversion cycle�, whichindicates that the main mass balance is carried out by cation ex-change during this voltammetric cycle.18,34,36,38,39 In the same way,the values of the F�dm/dQ� function are negative for the most partduring the first PB conversion cycle. Therefore, insoluble PB filmsalso mainly exchange cations during the PB � ES voltammetriccycle. Nonetheless, the F�dm/dQ� function also has positive valuesduring the first cathodic scan, which are not observed during the lastcathodic scan, that reveal the anion exit from the insoluble PB filmduring the conversion process. As previous works established,32,35

these expelled anions correspond to the Fe�CN�64− complexes

�Mw = 211.847 g mol−1� with an expected value of the F�dm/dQ�function equal to F�dm/dQ� = −211.847/−4 � 54 g mol−1 consid-ering the principle of electroneutrality. These iron–cyanide com-plexes are mainly expelled at more cathodic potentials than −0.10 Vbecause the F�dm/dQ� function has positive values at these poten-

Table I. F„�moverallÕ�Qoverall… values between the beginning andthe end of each voltammetric cycle during the insoluble-to-soluble PB transformation process at 20 mVÕs. The electrolytewas 0.50 M KCl with pH 3.0.

F��moverall/�Qoverall� function

CycleValue

�g mol−1�

1 242 146 1110 215 0


tials. In spite of this, the expulsion process of these ions may beginat about 0.10 V because the F�dm/dQ� function during the firstconversion cathodic scan tends to positive values from this potential.Figure 2 shows that the shape of the F�dm/dQ� function changessignificantly in the potential range corresponding to the voltammet-ric peak �zone 1�.

Electroacoustic analysis.— The conversion process was alsomonitored simultaneously by changes in the motional resistance.Figure 3 shows that the motional resistance of the PB film increasesduring the cathodic scans, whereas it decreases during the anodicscans along all the conversion process. During the soluble PB � ESvoltammetric cycle �last cycle�, this variation occurs because thesoluble PB lattice is cubic at 0.60 V, whereas it is tetragonal dis-torted cubic at −0.20 V.33 The insoluble PB � ES voltammetriccycle �first cycle� may also go through this conformational changebecause this variation is also observed during the first conversioncycle. Furthermore, Fig. 3 shows that the motional resistance in-creases irreversibly during the insoluble-to-soluble PB transforma-tion process in potassium salt solutions between the beginning �firstcycle� and the end �last cycle� of the conversion process, indicatingthat the manifestation of viscoelastic phenomena during this processincreases. Finally, it is emphasized that the curve of the motionalresistance during the soluble PB � ES voltammetric cycle �lastcycle� has a characteristic swelling between 0.13 and 0.30 V causedby the appearance of an internal magnetic field at room temperatureduring this cycle in potassium salt solutions.11,33 This swelling ap-pears as the insoluble-to-soluble PB transformation process takesplace. Hence, the PB conversion process may be related to the ap-pearance of the PB internal magnetic field at room temperature dur-ing the soluble PB � ES voltammetric cycle.

Herein, it is emphasized that the Sauerbrey equation is only ap-propriate to interpret quartz-crystal microbalance �QCM� frequencychanges when the film has zero dissipation �signaled by zero resis-tance and changes therein in the acoustic experiment�. At firstglance, Fig. 3 shows changes in resistance from the acoustic experi-ment in PB films, which are clearly inconsistent with this assump-tion. However, and as a previous paper established,51 the error tointerpret QCM frequency changes by the Sauerbrey relationship issmall for these PB thin films.

Figure 2. �Color online� F�dm/dQ� function during the cathodic scan of thefirst �insoluble PB form� and the last �soluble PB form� voltammetric cyclecorresponding to the insoluble-to-soluble PB transformation process at20 mV/s. The electrolyte was 0.50 M KCl with pH 3.0. The potential rangewhere the electric current is equal to 0 A is not shown here because theF�dm/dQ� function cannot be calculated at these potentials.



D

Electrospectroscopic analysis.— Figure 4 shows the near-UV/visible–near-IR absorbance spectrum of a soluble PB film at 0.60 V.At first glance, this spectrum has the three spectroscopic bands men-tioned in the introduction. However, the asymmetric shape of thespectroscopic peak centered at about 700 nm indicates that this maybe composed of at least two overlapped bands. Identifying them, thisspectrum is deconvoluted into four spectroscopic bands described byGaussian functions through a nonlinear least-squares-fitting method:peak I, peak II, peak III, and peak IV as follows

�I� A =6.90

82��/2�e−2��E − 383.0� /82�2

�II� A =105

173��/2�e−2��E − 672� /173�2

�III� A =66

204��/2�e−2��E − 849� /204�2

�IV� A =10.4

85��/2�e−2��E − 1023.8� /85�2

The good correlation index between experimental and theoreticaldata �R2 = 0.99914� denotes effectively that the near-UV/visible–near-IR absorbance spectrum of the soluble PB film is well de-scribed by four spectroscopic bands centered at 383 �peak I�, 672�peak II�, 849 �peak IIII�, and 1029 �peak IV� nm. As established inthe literature, the spectroscopic band centered at 383 �peak I� nm isrelated to the amount of Fe�III�low spin–CN–Fe�II�high spin chro-mophore units47 and to the amount of �CN�5Fe–CN–Fe�NC�4OH2chromophore units,48 whereas the bands centered at 672 �peak II�and 1029 �peak IV� nm are related to the amount ofFe�II�low spin–CN–Fe�III�high spin chromophore units.47 Nevertheless,the spectroscopic band centered at 849 nm �peak III� may be re-garded as photoinduced outer-sphere optical electron transfer fromFe�II��CN�6 to Fe�III��CN�6 accompanied by reorganization in boththe solvent and in one or several nuclear modes that represent ion-pair motion in the Fe�II��CN� ·M+ and Fe�III��CN� ·M+ ion pairs.53

Figure 3. �Color online� Current–motional resistance response during thevoltammetric cycles corresponding to the insoluble-to-soluble PB transfor-mation process at 20 mV/s. The electrolyte was 0.50 M KCl with pH 3.0.Clarifying the experimental data, this figure shows only values of the mo-tional resistance during the cathodic scan of the first and last conversioncycles.

6 6


Considering that band maxima have the optimum signal-noiseratio, the insoluble-to-soluble PB transformation process was moni-tored here by recording the absorbance of the PB film at 383, 672,and 1000 nm. Figure 4 shows that these wavelengths correspondto the three wavelengths of maximum absorbance observed in thenear-UV/visible–near-IR spectrum of soluble PB films. Accordingly,the absorbance at 1000 nm is related simultaneously to theforbidden electron charge transfer associated withFe�II�low spin–CN–Fe�III�high spin structural units �1023 nm� as wellas to the electron charge transfer from Fe�II��CN�6 toFe�III��CN�6 �849 nm�.

Next, the absorbance is used as a population probe. This is fineas long as the proportionality constant �extinction coefficient� is in-variant with film composition. Therefore, the level of uncertaintyassociated with this assumption is very small because the insolubleand soluble PB frameworks are very similar21,49,50 and as a result,both extinction coefficients should be very similar because there aresimilar absorbance environments inside both structures.

Figure 5 shows the evolution of the absorbance at 672 nm duringthe insoluble-to-soluble PB transformation process in potassium saltsolutions. The absorbance of this signal decreases during the ca-thodic scans whereas it increases during the anodic scans. This isbecause the oxidation state of the high spin iron atoms is modulatedduring the PB�blue� � ES�transparent� voltammetric cycle54

�Fe�II�low spin–CN–Fe�III�high spin�signal at 672 nm�PB�

+ 1e− � �Fe�II�low spin–CN–Fe�II�high spin�nonsignal at 672 nm�ES�

�1�Nonetheless, the absorbance at this wavelength does not change

from the beginning �first cycle� to the end �last cycle� of theinsoluble-to-soluble PB transformation process in potassium salt so-lutions. Therefore, the amount of theFe�II�low spin–CN–Fe�III�high spin structural units may remain unal-tered during this transformation process. This proves clearly that theinsoluble and soluble PB structures have a very similar crystallineframework composed of repetitive units ofFe�II�low spin–CN–Fe�III�high spin in the three spatial directions.These results are supported by previous X-ray measurements.49

Figure 4. �Color online� Experimental and fitted �R2 = 0.99914, both spectraare similar� spectra of the soluble PB film at 0.60 V. The electrolyte was0.50 M KCl with pH 3.0.



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In the same way, Fig. 6 shows the electrospectroscopic resultsobtained at 383 nm. The decrease and increase in this signal duringthe PB � ES voltammetric cycle in potassium salt solutions arealso due to the modification of the oxidation states of the trivalentiron atoms.54 Furthermore, Fig. 6 shows that the absorbance at thiswavelength increases between the first and the last conversion cycle.This increase corresponds to about 5% of the absorbance changebetween the PB and ES forms and it is similar to the percentage ofmass decrease observed in Fig. 1 during the conversion process. Asmentioned above, this signal is related to the amount ofFe�III�low spin–CN–Fe�II�high spin chromophore units47 and to the

Figure 5. �Color online� Current–absorbance response during the voltam-metric cycles corresponding to the insoluble-to-soluble PB transformationprocess at 20 mV/s. The electrolyte was 0.50 M KCl with pH 3.0. Theabsorbance values were recorded at 672 nm. Clarifying the experimentaldata, this figure shows only values of the current–absorbance response dur-ing the cathodic scan of the first and last conversion cycles.



amount of �CN�5Fe–CN–Fe�NC�4OH2 chromophore units.48 There-fore, the increase in this signal may be due to the increase in theamount of �CN�5Fe–CN–Fe�NC�4OH2 structural units during thisconversion process given that the amount ofFe�III�low spin–CN–Fe�II�high spin units is not modified during thisprocess. If the increase in the amount ofFe�III�low spin–CN–Fe�II�high spin structural units took place, then theamount of Fe�II�low spin–CN–Fe�III�high spin units should be affectedgiven that both states are in interconversion equilibrium.

The electrospectroscopic results at 1000 nm are shown in Fig. 7.The absorbance of this signal has, in general, the same behaviordescribed for previous signals because it is also altered by the modi-fication of the oxidation states of the trivalent iron atoms. Nonethe-less, the curve at 1000 nm during the soluble PB � ES voltammet-ric cycle has a characteristic swelling between 0.13 and 0.30 V�zone 1 in Fig. 7�. This swelling is not observed during the firstvoltammetric scan, appearing as the insoluble-to-soluble PB trans-formation process takes place in potassium salt solutions. The exis-tence of this swelling during the PB � ES voltammetric cycle canbe most easily observed by the dA/dt function. This function isnegative if there is a decrease in the absorbance whereas it is posi-tive if there is an increase in the absorbance. Consequently, thespectroscopic swelling may be characterized by a sign change in thedA/dt function during the PB → ES cathodic scan. Thus, Fig. 8shows clearly that this sign change that is related to the spectro-scopic swelling is observed during the soluble PB → ES cathodicscan �last conversion cycle� but it is not observed during the in-soluble PB → ES cathodic scan �first conversion cycle�.

Figure 7 shows that the absorbance at 1000 nm decreases be-tween the beginning and the end of the insoluble-to-soluble PBtransformation process in potassium salt solutions. This decreasealso corresponds to about 5% of the absorbance change between thePB and ES forms, which is similar to the percentages of mass de-crease and of absorbance increase at 383 nm mentioned earlier. Asthis spectroscopic signal is related to the Fe�CN�6

4− andFe�II�low spin–CN–Fe�III�high spin absorbers, the amount of one ofthese absorbers should decrease during the insoluble-to-soluble PBtransformation process in potassium salt solutions. Thus, this absor-




D

bance decrease may be related to the decrease in the amount ofFe�CN�6

4− ions because the amount ofFe�II�low spin–CN–Fe�III�high spin structural units is not modified dur-ing the conversion process.49,50

F�dA/dQ� function.— The simultaneous measurement of the cur-rent and absorbance curves allows the F�dA/dQ� function to becalculated, which opens new possibilities for the analysis of thissystem.55-60 This function can be obtained quickly by an easy math-ematical operation from the derivative of the absorbance curve andthe electric current

F�dA/dQ� = F

dA

dt

dQ

dt

= F

dA

dt

i�2�

The F�dA/dQ� function represents a measurement of the electro-chromic efficiency at each wavelength during a voltammetriccycle.55,56 Accordingly, Fig. 9 shows that the main difference be-tween the values of the F�dA/dQ� function at 672 nm during thefirst PB conversion cycle and the last conversion cycle correspondsto an important increase in the electrochromic efficiency in the po-tential range where the voltammetric peak takes place.

Figure 10 shows that the electrochromic efficiency of the PB filmat 383 nm proves to be higher between 0.20 and −0.20 V. This maybe because the electrochemical reactions of the structural unitsmonitored at 383 nm take place mainly at these potentials.54 Fur-thermore, the electrochromic efficiency at these potentials increaseswhen the soluble PB film is generated, which could be related to thefact that the absorbance at 383 nm also increases during the conver-sion cycles. Finally, Fig. 11 shows that the electrochromic efficiencyat 1000 nm increases at about 0 V when the soluble PB film isgenerated, which should be a contradiction with the decrease in theamount of Fe�CN�6

4− ions during the insoluble-to-soluble PB trans-formation process in potassium salt solutions.32

It is emphasized that the combination of dc techniques, electro-chemical, gravimetric, acoustic, and spectroscopic techniques repre-sents an innovative tool for measurement in electrochemistry, pro-viding valuable information on the studied systems. The maincharacteristic of this technique is the calculus at the same time of thecrossed functions �F�dA/dQ� and F�dm/dQ� functions� from the

Figure 8. �Color online� dA/dt curve at 1000 nm during the cathodic scan ofthe first �insoluble PB form� and the last �soluble PB form� voltammetriccycle corresponding to the insoluble-to-soluble PB transformation process at20 mV/s. The electrolyte was 0.50 M KCl with pH 3.0.


original signals, which allows the electrochemical mechanisms to becharacterized quickly. These original functions can be calculated bythis measurement tool because the electrical, mass, acoustic, andspectroscopic signals are measured simultaneously during the elec-trochemical experience. Although this methodology is presentedhere for a particular case, it may be suitable for other similar sys-tems.

Discussion

As mentioned above, the Fe�CN�64− complexes are mainly ex-

pelled from the insoluble PB film at more cathodic potentials than0.10 V during the conversion process in potassium salt solutions.This potential range is closed to the formal potential of the

Figure 9. �Color online� F�dA/dQ� function at 672 nm during the cathodicscan of the first �insoluble PB form� and the last �soluble PB form� voltam-metric cycle corresponding to the insoluble-to-soluble PB transformationprocess at 20 mV/s. The electrolyte was 0.50 M KCl with pH 3.0. Thepotential range where the electric current is equal to 0 A is not shown herebecause the F�dA/dQ� function cannot be calculated at these potentials.

Figure 10. �Color online� F�dA/dQ� function at 383 nm during the cathodicscan of the first �insoluble PB form� and the last �soluble PB form� voltam-metric cycle corresponding to the insoluble-to-soluble PB transformationprocess at 20 mV/s. The electrolyte was 0.50 M KCl with pH 3.0. Thepotential range where the electric current is equal to 0 A is not shown herebecause the F�dA/dQ� function cannot be calculated at these potentials.



D

Fe�CN�64− complexes in solution, 0.145 V.61 For that, the redox pro-

cess of these free complexes may be considered as a secondaryreaction at these potentials during the insoluble PB � ES voltam-metric cycle. However, this redox process may not be considered as

Figure 11. �Color online� F�dA/dQ� function at 1000 nm during the ca-thodic scan of the first �insoluble PB form� and the last �soluble PB form�voltammetric cycle corresponding to the insoluble-to-soluble PB transforma-tion process at 20 mV/s. The electrolyte was 0.50 M KCl with pH 3.0. Thepotential range where the electric current is equal to 0 A is not shown herebecause the F�dA/dQ� function cannot be calculated at these potentials.

a secondary reaction during the soluble PB � ES voltammetric of the second one equal to

cycle because all expelled Fe�CN�64− ions may spread irreversibly

toward the working solution during the conversion given that themass and electric charge of the PB films decrease during this pro-cess, as mentioned above. Thus, the decrease in the amount of theFe�CN�6

4− ions during the insoluble-to-soluble PB transformationprocess in potassium salt solutions would easily explain the increasein the electrochromic efficiency at 383 nm during this conversion.The electrochemical reactions of these ions do not involve a netabsorbance change at 383 nm and therefore, the absolute value ofF�dA/dQ� function decreases when these ions react electrochemi-cally because �A at 383 nm remains invariable whereas �Q in-creases.

Contrary to expectations, the electrochromic efficiency at1000 nm that is related to the Fe�CN�6

4− ions increases at about 0 Vwhen the soluble PB film is generated, although the amount of theseions decreases during the insoluble-to-soluble PB transformationprocess. This apparent controversy could be explained consideringthat these ions can be around the working electrode as ion pairs thatare detected at 1000 nm and as free solitary ions that are not de-tected at 1000 nm.53 For that, the absolute value of the F�dA/dQ�function would be lower during the first conversion cycle becausethere would be free solitary ions around the working electrode andtheir electrochemical reactions do not involve a net absorbancechange at 1000 nm.

The Fe�CN�64− ions eliminated during the conversion process are

not part of the PB crystalline framework given that there are nosignificant differences between the insoluble and soluble PB


structures.49 They may be free ions occluded during the fresh PBelectrodeposition process and, due to their large size, they may beplaced mainly into the vacancies of the insoluble PB crystallineframework. Thus, the expulsion of these free ions during the con-version process should empty these structural vacancies. As previ-ous X-ray measurements showed,49 the vacancies of the soluble PBcrystalline framework are constituted by a water molecule substruc-ture. Hence, the insoluble-to-soluble conversion process in potas-sium salt solutions could be summarized as the substitution of oc-cluded free Fe�CN�6

4− complexes with the water moleculesubstructure that is placed into the soluble PB vacancies. It is em-phasized that the �CN�5Fe–CN–Fe�NC�4OH2 structural units areplaced into the ferrocyanide vacancies of the soluble PB crystallineframework.49,50 Accordingly, and in agreement with the proposedconversion scheme, it is detected above an increase in the�CN�5Fe–CN–Fe�NC�4OH2 structural units �absorbance at 383 nm�and a decrease in the Fe�CN�6

4− ions �absorbance at 1000 nm� dur-ing the insoluble-to-soluble PB transformation process. All theseprocesses may be related to the mass decrease observed during theconversion process because all the change percentages are verysimilar. Furthermore, the amount of theFe�II�low spin–CN–Fe�III�high spin structural units �absorbance at672 nm� remains unaltered during this conversion process.

Previous X-ray diffraction data49 showed that the water moleculesubstructure of the soluble PB vacancies is formed by two potassiumions and six hydroxyl anions when the soluble PB film is generatedin potassium salt solutions. Consequently, all these species wouldreplace each free Fe�CN�6

4− complex expelled during the insoluble-to-soluble PB transformation process. This conversion scheme im-plies a theoretical value of the F��moverall/�Qoverall� function be-tween the beginning of the first conversion cycle and the beginning

F��moverall/�Qoverall� =2 � 39�Mw�K+�� + 6 � 17�Mw�OH−�� − 211.847�Mw�Fe�CN�6

4−��− 1�decrease in the exchanged electron number�

� 32 g mol−1

a value very close to the above experimental value of 24 g mol−1,which confirms the proposed conversion scheme. The proposed con-version scheme also agrees with the movement of water moleculesdetected previously by the literature43 as well as with the fact that aniron atom is replaced by potassium ions during the insoluble-to-soluble PB transformation process in potassium salt solutions.27,29

Furthermore, the proposed scheme preserves the principle of elec-troneutrality because four negative charges �Fe�CN�6

4−� are replacedby a global balance of four negative charges �2K+ + 6OH−�.

As mentioned above, the PB conversion process in potassiumsalt solutions also involves the change in the PB magnetic proper-ties. This fact explains that the shape of the F�dm/dQ� andF�dA/dQ� at 672 nm functions changes in the potential range cor-responding to the voltammetric peak during the conversion processbecause their shapes were previously related to a nonfaradaic extracurrent due to the charge/discharge of the PBmagnetocapacitor.11,22,33,54 The change in the PB magnetic proper-ties during the conversion process could also explain the fact thatthe above-mentioned absorbance swelling �1000 nm� at about0.20 V increases as the conversion process takes place, see Fig. 7,because this is caused by the appearance of an internal magneticfield at room temperature during the soluble PB � ES voltammetriccycle.11,33

Among other things, the increase in the PB internal magneticfield at room temperature during the soluble PB � ES voltammetriccycle in potassium salt solutions is caused by the conformationalchange observed above by the motional resistance.11 Therefore, it is



D

possible that the increase in the film viscoelastic phenomena duringthe insoluble-to-soluble PB transformation process in potassium saltsolutions could facilitate the appearance of this internal magneticfield during the soluble PB � ES voltammetric cycle because thePB framework is easier to modify during the conversion process.The expulsion of the free Fe�CN�6

4− large ions during this conver-sion would easily explain why the soluble PB crystalline frameworkproves more viscoelastic than the insoluble PB crystalline frame-work. As a consequence, all these experimental data allow the pro-posed insoluble-to-soluble PB transformation scheme to be corrobo-rated. These data point out that the PB rigid skeleton remainsunaltered during this conversion process even though the propertiesof this material change to a large extent.

Conclusion

Herein, an insoluble-to-soluble PB transformation mechanism inpotassium salt solutions is proposed as a result of the study of thisprocess simultaneously using electrochemical, gravimetric, acoustic,and spectroscopic techniques. This complex process involves,among other processes, the expulsion of free Fe�CN�6

4− anions oc-cluded in the insoluble PB vacancies during the fresh PB electrode-posit process. In potassium salt solutions, the loss of each free anionis compensated by the entrance of two potassium ions and six hy-droxyl anions, preserving the principle of electroneutrality. The ex-pulsion of this Fe�CN�6

4− large complex from freshly deposited PBvacancies increases the manifestation of viscoelastic phenomena ofthe PB structure during the conversion process, which could facili-tate the appearance of an internal magnetic field at room temperatureduring the soluble PB � ES voltammetric cycle in potassium saltsolutions.

Acknowledgments

This work was supported by FEDER-CICyT project CTQ2007-64005/BQU. D.G.-R. acknowledges his position with the Generali-tat Valenciana. We are grateful to Irene Gimenez-Pastor for her im-mense patience.

University of Valencia assisted in meeting the publication costs of thisarticle.

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