Identification of Processes Associated with Different Iron Sites in the

ARTICLE pubs.acs.org/JPCC

Identification of Processes Associated with Different Iron Sites in the Prussian Blue Structure by in Situ Electrochemical, Gravimetric, and Spectroscopic Techniques in the dc and ac Regimes Jeronimo Agrisuelas, Jose J. García-Jare~no,* and Francisco Vicente Physical Chemistry Department, University of Valencia, C/Dr. Moliner 50, 46100 Burjassot, Spain ABSTRACT: The physicochemical properties of Prussian blue films are strongly dependent on the ratio Fe3+/Fe2+ in the structure. This ratio can be modulated by electrochemical techniques such as cyclic voltammetry, and some information about physicochemical properties can be extracted from in situ auxiliary techniques. Prussian blue films have been studied by the use of in situ visNIR spectroscopy, quartz crystal microbalance, and electrochemical techniques (cyclic voltammetry and electrochemical impedance spectroscopy). By cyclic voltammetry + absorbance derivative curves + mass derivative curves, it has been possible to identify at least three different processes during the reduction of Prussian blue films to the Everitt’s salt form. By numerical methods, it has been possible to deconvolute these curves and, thus, to obtain information separately from the three identified processes. These processes can be attributed to different Fe3+ sites in the PB structure with the participation of different counterions. By in situ electrochemical impedance + mass impedance + color impedance at a wavelength near 1000 nm, there has been observed a double loop plot with the color impedance where mass and electrochemical impedance show only one loop plot. Color impedance spectra contributions can be separated and identified separately by means of a nonlinear least-squares fitting procedure, and, thus, this double loop may be assigned to at least two different overlapped electrochemical processes. One of these electrochemical processes is related to Fe3+ sites (Fe(NC)5OH) near the ferrocyanide vacancies in the Prussian blue structure, which are strongly affected by the pH of the outer solution.

’ INTRODUCTION Prussian blue, PB, also known as ferric ferrocyanide (KFeFe(CN)6 3 xH2O) is one of the most studied hexacyanometallates due to the easiness to form controlled quality films on the surface of very different kinds of conducting electrodes.14 Electrochemical processes associated with this material are accompanied by color changes at different wavelengths3,59 and also by an ion exchange between the film and the solution to keep the film electroneutrality.1012 The physicochemical properties of this kind of films are very dependent on the ratio Fe3+/Fe2+ in the PB structure, and this ratio can be modulated by electrochemical techniques controlling the oxidation state of the film. Electrochemical processes may be represented by the following simplified reaction schema PBðblue; mixed valence Fe3þ Fe2þ Þ þ K þ þ e a ESðcolorless; reduced form Fe2þ Fe2þ Þ

ð1Þ

PBðblue; mixed valence Fe3þ Fe2þ Þ a PYðyellow oxidized form Fe3þ Fe3þ Þ þ K þ þ e ð2Þ In these reactions, ES represents Everitt’s salt and PY represents Prussian yellow. r 2011 American Chemical Society

Prussian blue films freshly deposited following the procedure described by Itaya et al.13 are in the insoluble form. The films transform into the soluble form after a structural change during a few successive voltammetric cycles around the Prussian blue a Everitt’s salt system.1417 In the past, this structural change has been described as the loss of a part of inner high spin iron atoms replaced by potassium cations.1417 However, recent X-ray observations propose that the soluble and insoluble forms are very similar and that the main change may be the presence of potassium cations filling the vacancies in the PB soluble form.18,19 The role of these vacancies proves of special interest in the interpretation and understanding of the electrochemical behavior of Prussian blue. This face centered cubic structure of PB presents 1/4 of Fe2+(CN)64 vacancies. One consequence of this structure is the presence of at least two different Fe3+ sites in the PB crystalline structure. On the one side, there is Fe3+ coordinated with six (—NtC) groups (75%). On the other side, there is Fe3+ coordinated with five (—NtC) groups and one hydroxyl group or one water molecule (25%). This last situation corresponds to Fe3+ sites surrounding Fe2+(CN)64 vacancies (Figure 1). Received: July 29, 2011 Revised: December 2, 2011 Published: December 19, 2011 1935

dx.doi.org/10.1021/jp207269c | J. Phys. Chem. C 2012, 116, 1935–1947

The Journal of Physical Chemistry C

Figure 1. Schematic representation of the unit cell of Prussian blue. Red balls, Fe2+(CN)6; green balls, Fe3+(NC)6; yellow balls, Fe3+(NC)5OH. Gray balls represent OH or H2O linked to Fe3+ sites. Edges of the Fe2+(CN)64 vacancy are marked with a wider line.

The electrochemical behavior of this soluble form presents some different interesting properties if compared with the insoluble form behavior around the Prussian blue a Everitt’s salt system, eq 1. At first glance, it is observed that voltammetric peaks become narrower and higher for the soluble form and that the stability against the successive cycling of these films increases significantly.14,20 However, there are other differences more difficult to be observed. Among others, the half peak widths of voltammetric peaks may not be explained unless an extra current is considered,21 and the ratio between mass and electrical charge F(dm/dq) during voltammetric experiments presents a local minimum value at potentials near the peak potential that are also explained on the basis of an extra current associated with nonfaradaic processes at these potentials.22 Furthermore, magnetoresistive properties of PB films are manifested at these potentials during electrochemical experiments.23 These observations/experimental results have been related to a structural changeover that takes place at some specific potentials during the electrochemical reduction of Prussian blue to the Everitt’s salt (colorless) form (eq 1).22,23 This situation corresponds to the filling of the vacancies in the structure by potassium or other cations where it should also be noted that potassium cations can occupy different sites in the PB structure.18 From the aforementioned results, it is clear that the detection of some of the most interesting properties of Prussian blue films requires the use of in situ techniques together with the classical electrochemical techniques. Ion exchange may be followed by coupling classical electrochemical and quartz crystal microbalance techniques.2428 Structural changes and magnetic properties can be detected by changes of the motional resistance associated with the acoustic impedance that some modern EQCM can measure.23 Many of these properties of Prussian blue films have proved to be strongly dependent on the experimental conditions. At first, it should not be forgotten that the electrochemical behavior of

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PB films is very different in different solution media.1,20,29 First, it has been known for 20 years that the satability and the shape of voltammetric peaks are very different depending on the nature of the outer solution.1,29 Some finds that can be enumerated concerning this material are that anion participation during electrochemical processes is not very important,11 and also the potassium cation (or generally the alkali cation) inserts or expels without coordination water, the insertion barrier is related to the hydration enthalpy of the alkali cation,25 the participation of different species is strongly dependent on the applied potential, the cations can be inserted in different sites in the Prussian Blue structure,27,28,30 the inner water can occupy different sites in the PB structure, and it is necessary that a certain degree of hydration allows the electrical conduction through the PB film.11,31,32 In other results concerning the pH dependence, it is proved that the peak potential is not very dependent on the outer pH, but the shape and the high of the peaks are very sensitive to smaller changes of the pH of the outer solution.11,25 Electrochromism in Prussian blue films clearly indicates that the physicochemical properties should be also dependent on the redox state of the film.1,5,15,3335,17 It has been proved that the maximum absorption depends on the nature of the inserted alkali cation.34,36 However, there is not only the alkali cation which inserts into the PB structure but also the proton or hydrated protons can participate during redox and electrochromic processes.11,21,30 As of today there are more shadows than lights on the role and also the sites that these protons can occupy in the PB structure.37 The use of spectroscopical techniques together with the electrochemical ones sheds light on discerning the different role that potassium (alkali cations) and hydrated protons can play in the electrochemistry of these materials.33,3840 As commented before, in electrochromic systems such as Prussian blue films, it proves also very interesting to follow color changes at different wavelengths associated with electrochemical processes.3,57,13 Spectroscopic behavior of Prussian blue in the visiblenear-infrared (visNIR) range was analyzed in the past by Robin5 who proposed two main spectroscopic bands centered at 380, 690 nm and another forbidden band centered about 1000 nm. The transition at 690 nm is related to the electronic charge transfer from Fe2+low spin atoms surrounded by cyanide units to Fe3+high spin atoms, so monitoring the amount of Fe2+low spinCNFe3+high spin electronic states (change of color from blue to colorless).40 The transition detected at 380 nm is related to the electronic charge transfer from Fe2+high spin atoms to Fe3+low spin atoms5 so monitoring the amount of Fe2+high spinCNFe3+low spin (ionic trapping states).40 The third interesting and less studied spectroscopic band is centered at about 1000 nm which has been associated initially with a forbidden transition.5Nowadays, the study of electrochemical properties of intrinsically conducting polymers (ICP) needs the use of classical electrochemical techniques together with auxiliary techniques in situ. In recent years the use of cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance has allowed to identify species by means of the ratio mass/electrical charge. In electrochromic materials, there is also the possibility to obtain together the electrochemical response and the spectroscopic response during “classical” electrochemical experiments (voltammetry, chronoamperometry, etc.) This possibility needs the use of transparent electrodes where the ICP is deposited for measuring changes of absorbance. If the absorbance variation at a given wavelength and the current were simultaneously measured, 1936

dx.doi.org/10.1021/jp207269c |J. Phys. Chem. C 2012, 116, 1935–1947

The Journal of Physical Chemistry C this assembly allows the ratio F(dA/dq) to be estimated, which is a measure of the electrochromic efficiency of the polymer. Another possibility is to study changes of reflectance by depositing the ICP on the surface of a high reflectance electrode. This last possibility may be compatible with the use of the gold electrode of the EQCM, allowing the simultaneous measure of current, mass, and color changes during electrochemical experiments in the same experimental conditions. In consequence, it avoids any question of film history effects when sequential experiments and subsequent correlation of the data neglect temporal structural changes. This last strategy has proved to be very interesting for the study of this kind polymers and allows discerning among different electrochemical processes taking place in ICP since commonly there is more than only one electrochemical process. For this purpose, electrochemical impedance spectroscopy (EIS) has been revealed as one of the most powerful tools in recent years. One of the most interesting abilities is the possibility to separate between faster and slower processes by changing the frequency of the electrochemical potential perturbation. In the last years there is also the possibility to synchronize electrochemical response with other signals such as mass (MIS) or color (CIS).38,39,4157 Thus, the possibilities for impedance techniques strongly increase if they are simultaneously applied.2426,38,39,4143,5862 Color impedance was studied at 690 nm in a previous paper39 together with mass impedance and electrochemical impedance. In this work, we continue with the investigation of the theoretical interpretation of the shape of plots in the Nyquist plane of these three impedances associated with electrochemical processes of Prussian blue considered as a model of rigid film electrodeposited on the electrode with the analysis of color impedance at 1000 nm. This wavelength proves very interesting since a different behavior of dA1000/dt curves during voltammetric experiments was detected, showing a minimum and a maximum during the cathodic or anodic scan and not directly related with the main current. As it was commented above, Robin talks about a forbidden spectroscopic transition5 at this wavelength, but it is not enough to explain this double behavior. Thus, this study is focused on the analysis of electrogravimetric, visNIR spectroscopy, and electrochemical in situ techniques for trying to obtain information on the electrochemical role of the structural vacancies that are present the Prussian Blue films in their crystalline structure. In spite of the complexity of the PB behavior, this system may be considered as such a “model” system. Thus, the methodologies here proposed, developed, and tested can be extrapolated to more complicated systems like many organic conducting polymers.

’ EXPERIMENTAL SECTION Specific equipment is needed to obtain simultaneously the electrochemical impedance response, the mass impedance response, and the color impedance response. The analysis of experimental data obtained by this assembly requires the use of complex methodologies and, in some cases, the use of some numerical procedures to obtain a good fitting of experimental data to the proposed theoretical models. The complete description of both the methodologies and the equipment used to obtain the three impedance signals simultaneously is described in recent papers.38,39,41 All experiments were carried out in a three-electrodes electrochemical cell. The auxiliary electrode was a platinum mesh and the

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reference the Ag/AgCl/KCl (sat.) electrode. The working electrode was a high reflectance gold/quartz crystal electrode (AT cut quartz crystal, 6 or 9 MHz, Matel-Fordahl, France) which allows the simultaneous measure of current, mass (from frequency resonance), and reflectance (from light intensity received on the photodiode surface). The cell temperature was controlled by means of Peltier thermoelectric modules, and the cell was a high transmittance glass cell from HELLMA (OG). For the impedance measurements, a small potential perturbation was applied to the polarization potential of the working electrode (25 mV rms). The color response received was a light intensity (I) reflected on the surface of the gold electrode of the EQCM. The photodiode produces a current intensity proportional to the light intensity which is converted by means of a current to voltage converter into an analog signal. This current may be converted into an apparent absorbance (A) by considering the transparent ES form as the blank (100% transmittance). For the color impedance analysis, a small potential perturbation is applied to a steady state system, and, therefore, only small changes of color (reflected light intensity, I) are expected. Thus, an apparent increment of absorbance (ΔA) may be obtained by a first-order Taylor series expansion A ¼ logðI=I0 Þ A þ ΔA ¼ log

ð3Þ

I þ ΔI I0 ≈ logðI=I0 Þ ΔI logðeÞ ¼ A BΔI I0 I

ð4Þ where the absorbance changes (ΔA) may be directly approximated to changes of received light intensity (ΔI) multiplied by a constant factor (B).39,38 The quartz crystal microbalance was built at UPR 15 of CNRS. The potentiostat was a PAR 263 A, and the frequency response analyzer (FRA) was the four-channels FRA SOLARTRON 1254. For the voltammetric experiments, the assembly was the same but using the quartz crystal microbalance RQCM Maxtek Inc. equipment to measure simultaneously changes of resonant frequency, reflectance, and current during the voltammetric experiment. All chemicals used were ScharlauTM Analytical Reagent quality. Prussian blue was deposited from 0.02 M K3Fe(CN)6, 0.02 M FeCl3, and 0.01 M HCl freshly prepared dissolutions by applying a controlled cathodic current of 40 μA cm2 for 150 s. The film thickness was estimated about 115 nm.21,6365 The PB deposits were sufficiently thin to ensure a precise relationship between the frequency variation of the quartz crystal and the mass change without any viscoelastic artifacts.63 PB films freshly prepared (insoluble PB) were stabilized and converted into the soluble form by means of cyclic voltammetry around the PB a ES system14,16,32,17,66 in a KCl solution until narrow and sharp peaks appear (15 cycles). All experiments were carried out in KCl 0.5 M solutions and at the controlled temperature of 25 °C. The pH of the solution was set by the addition of the appropriate 1.0 M HCl solution.

’ RESULTS AND DISCUSSION Voltammetric Analysis. Previous studies of PB films in KCl solutions have pointed to the possibility that hydronium cations play an important role in the electrochemistry of the vacancies in the PB structure.11,25,24,37 Thus, we will study the electrochemical response of these films in KCl solution at different pH values. 1937


The Journal of Physical Chemistry C For this purpose, several PB films were synthetized in the same experimental conditions and then cycled in 0.5 M KCl solutions at pH 2.0, 3.0, 4.0, and 5.0. The first part of this work corresponds to the analysis of mass and color changes (reflectance) at 1000 nm during the cyclic voltammetry of PB around the PB a ES system. In this work, the electrical current recorded during the voltammograms represents the time evolution of the number of the different active Fe3+sites in the PB structure, and it can be expressed as N dq dnFe3þ ¼ nF ð5Þ dt dt i¼1 i

∑

In this equation, it is considered that there are N different electrochemical processes associated with N different Fe3+ sites in the PB structure. F represents the Faraday’s constant, nFe3+ the number of active sites of Fe3+, n = 1 considering one electron involved in the electrochemical reactions, and dq/dt the electrical current passing during the electrochemical experiment. The mass change is mainly caused by the electrical charge balance during the electrochemical processes keeping the electroneutrality of the system. If we consider that each current process is associated with the participation of a counterion whose molar mass is Mi, dm/dt can be written as N dm dnFe3þ ¼ γi Mi ð6Þ dt dt i¼1 i

∑

Here, γi can be +1 for species inserted during the oxidation processes (anions and/or solvent) and 1 for species inserted during the reduction processes (cation and/or solvent). Likewise, changes of absorbance during voltammetric scans are caused by the change of the oxidation state of some specific active sites (chromophores), in this case, Fe3+sites. Thus, according to the LambertBeer’s law, changes of absorbance are directly proportional to the number of these Fe3+ sites multiplied by an apparent electrochromic efficiency, ζi. N dA1000 dnFe3þ ¼ ζi ð7Þ dt dt i¼1 i

∑

As it can be seen, eqs 5, 6, and 7 have a common factor and show the evolution of the number of the different active Fe3+sites, (dnFe3+/dt)i, multiplied by a factor depending on the experimental signal registered. In the simplest case if only one process is considered, then it is easy to obtain a good estimation for the molar mass of the counterion by dividing eqs 6 and 56671 0 1 dm ! dm B C dt C γM ¼ F B ð8Þ @ dq A ¼ F dq dt and, therefore, it is also possible to obtain a good estimation of the electrochromic efficiency by dividing eqs 7 and 5. 0 1 dA ! dA B C dt B C ð9Þ ζ ¼ F@ A ¼ F dq dq dt If there is more than one process, a weighted average of the individual values for each molar mass or electrochromic efficiency is obtained.

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For the analysis of voltammetric peaks, we can suppose as a first and quick approach that PB films are thin enough to not consider diffusion control of counterions or electrons during electrochemical processes. Besides, we can also consider that the electrochemical reactions take place at the electrode/film interface very fast and that the ratio between Fe3+ and Fe2+ sites at this interface always obeys the Nernst law. Assuming also that the total number of Fe sites keeps constant during the voltammetric experiment and that there is no interaction between neighboring active sites, we can obtain an equation which describes the dependence of the number of Fe3+ sites on the applied potential during a voltammetric experiment.21,72 dnFe3þ Ci ¼ ð10Þ 0 2 dE i cosh ðbi 3 ðE Ei 0 ÞÞ ðdnFe3þ Þi ¼

Ci 0

cosh ðbi 3 ðE Ei 0 ÞÞ 2

ðnFe3þ Þmax, i ¼

Z þ∞ ∞

dE

ð11Þ

ðdnFe3þ Þi

bi ðnFe3þ Þmax, i 2Ci ð12Þ where Ci ¼ bi 2 dnFe3þ dnFe3þ dt ¼ dE i dt dE i dnFe3þ 1 dnFe3þ ¼ finally we obtain dt v i dt i vbi ðnFe3þ Þmax, i ¼ ð13Þ 0 2 cosh2 ðbi 3 ðE Ei 0 ÞÞ Equation 10 represents the shape of a peak and Ci corresponds to the maximum value of this peak, E0i 0 represents the formal potential for each one of the electrochemical processes, and b = nαF/RT, where α represents the symmetry factor. Equation 13 is obtained from eqs 10, 11, and 12 and represents the dependence of the number of active sites on the time during a voltammetric experiment, where ν is the scan rate during the voltammetric experiment and (nFe3+)max,i represents the maximum available number of active sites for each one of the electrochemical processes. For a cyclic voltammetric peak due to only one electrochemical process, we can obtain the current (dq/dt)potential dependence by multiplying eq 13 by nF. For the dm/dt or dA/dt curves, eq 13 should be multiplied by the molar mass (Mi) of the counterion or the electrochromic efficiency (ζi) of the chromophore, respectively. Figure 2a shows the differences in the voltammograms for a PB film in a 0.5 M KCl solution at pH 2.0, 3.0, and 5.0 and may be interpreted as the time evolution of the rate of reduction/ oxidation of Fe3+/Fe2+ active sites. However, eqs 5 and 13 which are based on classical models describing the shape of voltammetric curves cannot describe the sharp shape of the voltammetric peak unless an extra current is considered.21 Mass changes in the same experimental conditions are represented in Figure 2b as a derivative curve. This rate of mass change should be directly related to the electrical current since it is due to the entrance or exit of counterions to balance the electrical charge. There is an apparent good correlation between dm/dt and current curves except at potentials between 0.25 and 0.6 V. This behavior has ¼

1938



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Figure 3. F(dm/dq) (a) and F(dA1000/dq) (b) dependence on the potential during the cathodic scan of the experiment of Figure 2.

Figure 2. (a) Voltammogram (dq/dt) and (b) mass derivative (dm/dt) and (c) absorbance derivative at 1000 nm (dA1000/dt) during voltammograms for PB films at different pH values in a 0.5 M KCl solution. Scan rate was 20 mV s1.

been explained on the basis of a different counterion participation at these potentials (proton or hydrated protons instead of potassium cations).11,24,25 Likewise, Figure 2c shows the change of the derivative of absorbance at the wavelength =1000 nm during the same voltammetric experiment. In this case, there is no clear correlation between current and dA1000/dt curves, and the interpretation proves more difficult. This is better understood looking at Figure 3a and Figure 3b where the ratio between the mass and electrical charge, F(dm/dq), and the ratio between the absorbance and the electrical current F(dA/dq) are plotted against the potential during a voltammogram in the cathodic sense. F(dm/dq) gives interesting information on the nature of exchanging species during electrochemical experiments. Thus, for a pure potassium cation participation it is expected a value of (39 g mol1) and smaller values for hydronium cations participation (19 g mol1).

At pH = 5.0, F(dm/dq) reaches values between 30 and 35 g mol1 indicating the main participation of potassium in the range of potentials [0.25, 0.60] V. However, F(dm/dq) varies between 30 and 15 g mol1 at more acidic pH. These results prove that at these potentials both potassium and hydronium cations may participate as counterions11,25 depending on their relative ratio in the aqueous solution. By looking at Figure 3b, one can observe a change from negative to positive values of F(dA/dq) at the same range of potentials ([0.25, 0.60] V) that reinforce the complexity of the processes assumed by F(dm/dq). In particular, no change of absorbance is observed in the same range of potentials at pH = 5.0. It should also be noted that the shape and values of these curves are clearly dependent on the outer pH. In order to quantify these observations, we can try to simulate these curves as a combination of eqs 6 and 13 for the dm/dt curves and eqs 7 and 13 for the dA/dt curves. For the dm/dt case, at least three processes should be used in order to obtain a good simulated curve. Thus, nine parameters were needed: γiMi(nFe3+)max,i which represents the mass change associated with the ith process, bi, and the formal potential, E0i 0 : dm ¼ dt

3

∑ γi Mi bi ðnFe i¼1

0

3þ

Þmax, i υ=2 cosh2 ðbi ðE Ei 0 ÞÞ

ð14Þ

Table 1 shows optimized values obtained for the different parameters by fitting experimental data to eq 14. For this purpose routines of LevenbergMarquard in Mathcad 14TM software were used. Figure 4a shows an example of deconvolution of the dm/dt curve at pH = 4.0. A good correlation between experimental and 1939



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Table 1. Parameters Obtained from the Fitting of Experimental dm/dt Curves (Figure 2b) to eq 14 γ1M1(nFe3+)max,1 pH

(μg cm2)

γ2M2(nFe3+)max,2 b1 (V 1)

γ3M3(nFe3+)max,3

E010 (V)

(μg cm2)

b2 (V1)

E020 (V)

(μg cm2)

b3 (V1)

E030 (V)

2.0

0.41

17

0.27

0.57

32

0.22

1.4

5.9

0.15

3.0

0.39

17

0.28

0.51

44

0.22

1.2

6.4

0.15

4.0

0.33

19

0.28

0.67

32

0.22

1.1

6.2

0.15

5.0

0.29

22

0.27

0.67

36

0.22

1.0

7.1

0.14

and third processes. In both cases there is a decrease with the pH, proving the participation of protons in both processes. In the second process, there is a slight increase due to the fact that for the more acidic pH some protons can occupy sites for the potassium cations. It should be noted that the formal potentials associated with these three processes are in good agreement with those obtained for the simulation of CV of PB films on ITO electrodes in a previous work.21 In a similar way and as a first approach, we can suppose that each one of the peaks appearing in the dA1000/dt curves of Figure 2c is related to the different electrochemical process associated with different electroactive centers in the PB film assumed previously in the dm/dt curves. We can use eqs 7 and 13 to simulate the shape of the dA1000/dt curve. Taking into account these considerations, it is possible to simulate the shape of dA1000/dt curves by overlapping more than one electrochemical process. In order to obtain a good correlation between experimental and simulated curves, at least three processes should be considered in accordance with the results in Figure 4a. Thus, we can write73,74 dA1000 ¼ dt

Figure 4. Deconvolution of the dm/dt peak (a) and deconvolution of the dA1000/dt curve (b) during the cathodic scan of a voltammetric experiment in 0.5 M KCl solution at pH = 4.0. Scan rate was 20 mV s1. The simulated curves for the three processes considered were obtained by using parameters in Table 1 for the dm/dt curves and parameters in Table 2 for the dA1000/dt curves.

simulated curves is well observed. However, there are nine parameters to be calculated, and it proves difficult to interpret it. In spite of that, some conclusions can be extracted. The first one is that three processes could describe the shape of the dm/dt curves at different pH values. The second one is that the potential for these three processes are not or little dependent on the pH in the range of studied pH. The first process at potentials near 0.28 V and the third process near 0.15 V could be associated with the participation of protons as counterions since the F(dm/dq) function (Figure 3a) reaches values between the molar mass of potassium (39 g mol 1) and the hydrated protons (19 g mol1) and the second one centered at 0.22 V should be associated with the potassium participation since F(dm/dq) reaches values near 40 g mol1 at these potentials. The change of mass associated with each one of the electrochemical processes γiMi(nFe3+)max,i shows a clear dependence on the pH for the first

3

∑ ζi bi ðnFe i¼1

0

3þ

Þmax, i υ=2 cosh2 ðbi ðE Ei 0 ÞÞ

ð15Þ

Figure 4b shows an example of this deconvolution for the case of pH = 4.0. From anodic to cathodic potentials, the first process (E010 = 0.25 V) corresponds to positive values of dA1000/dt and the other two (E020 = 0.21 V and E030 = 0.13 V) to negative values. These results confirm that at this wavelength, information on several processes is recovered. Potentials obtained for the three processes are very similar of those used for the dm/dt curves, and then they can be associated with the same counterions participation Table 2 collects parameters obtained from the fitting of experimental data to eq 15 for three processes. It should be noted that the parameter ζi(nFe3+)max,i clearly depends on the pH for the first and third processes. This parameter is directly related to the number of sites available for each one of the three processes considered. At not enough acidic pH (4.0 and 5.0), this parameter clearly decreases for the first and the third processes; however, there is no clear dependence on the pH for the second process. In spite of this fact, the formal potential for the first and third process seems to be not dependent on the pH keeping more or less constant around 0.245 and 0.145 V, respectively. It should be noted that the shape of dA1000/dt curves proves very different from that of current and dm/dt curves. The interpretation and explanation of the spectroelectrochemical response of PB films proves more difficult than the dm/dt curves. Electrochromism in PB films was known from a long time ago. Changes of color associated with the oxidation state of the film at different wavelengths were reported in the scientific literature.5,15,39,40 In particular, color changes at 690 nm have 1940



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Table 2. Parameters Obtained from the Fitting of Experimental dA1000/dt Curves (Figure 2c) to eq 15 pH

ζ1(nFe3+)max,1 (au)

b1 (V1)

E010 (V)

b2 (V1)


E020 (V)


b3 (V1)

E030 (V)

2.0

0.50

6

0.250

0.044

32

0.205

0.91

6

0.155

3.0

0.49

8

0.242

0.10

27

0.209

0.83

7

0.148

4.0

0.15

11

0.246

0.10

25

0.210

0.48

8

0.128

5.0

0.11

11

0.245

0.10

25

0.212

0.38

8

0.141

(NC) groups. This fact causes different electroactive sites, but also different changes of absorbance and mass associated with each one of these electroactive sites.18,19,37,40 Changes of absorbance at 690 and 380 nm during CV experiments have been attributed to the following electroactive centers. At 690 nm: 3þ Fe2þ low spin CN Fehigh spin þ 1e 2þ þ þ K þ f Fe2þ low spin CN Fehigh spin ðK Þ

Figure 5. Voltammogram and dA690/dt curves for a PB film in 0.5 M KCl at pH = 4.0 solution. Scan rate was 20 mV s1.

been studied by color impedance (CIS) together with electrochemical impedance spectroscopy (EIS) and mass impedance (MIS). The transition at 690 nm has been related to the electronic charge transfer from Fe2+low spin atoms surrounded by cyanide units to Fe3+high spin atoms, so monitoring the amount of Fe2+low spinCNFe3+high spin electronic states (change of color from blue to colorless). This process takes place by the main participation of potassium cations as counterions, and dA690/dt curves show a good correlation with dm/dt curves and also with current curves except in the [0.25, 0.60] V potential range40 (Figure 5). A part of these experimental results have been interpreted on the basis of a structural change that takes place at these potentials.22 This structural change together with the appropriate Fe3+/Fe2+ ratio in the PB structure causes nonfaradaic processes in the PB films such as an extra current associated with magnetoresistance.22,75 Magnetism in PB films is detected by changes of the motional resistance at the appropriate potentials.75 This fact may explain the high peak currents obtained for PB films in these experimental conditions (Figure 2a). During a cathodic scan (Figure 4b), dA1000/dt changes from positive in the potential range [0.60, 0.25] V to negative values at potentials smaller than 0.25 V. It should be noted that the 0.15 V peak potential for absorbance derivative curves (1000 nm) is displaced with respect to the current or mass peak potentials and that the more anodic dA1000/dt peak strongly depends on the pH, while the more cathodic dA1000/dt peak seems to be no or less dependent on the studied range of pH. From the aforementioned results, it is clear that it is not possible to explain this behavior by a single spectroscopic band associated with only one electronic transition. On the other hand, the crystal structure of PB causes that not all Fe3+ sites are identical within the crystal structure. Fe3+ sites near the structural vacancies in the PB structure are different since they are coordinated by five (NC) groups and one OH or one water molecule while the other Fe3+ are coordinated by six

ð16Þ

This process was located at potentials near the voltammetric peak potential and corresponds to the Fe3+ sites hexacoordinated by (NC) groups. At 380 nm: 2þ Fe3þ low spin CN Fehigh spin þ 1e þ 2þ þ H3 Oþ f Fe2þ low spin ðH3 O Þ CN Fehigh spin

ð17Þ

This process is located at slightly more cathodic potentials than the peak current potential.40 At more anodic potentials than the peak potential there is an extra current which may not be attributed to these processes and which has been attributed to electrochemical competitive reactions associated with Fe3+ sites located next to ferrocyanide vacancies of the PB structure:40 Fe3þ ðNCÞ5 OH þ K þ þ 1e f Fe2þ ½ðNCÞ5 OHK

ð18Þ

Fe3þ ðNCÞ5 OH2 þ Hþ þ 1e f Fe2þ ðNCÞ5 OH2 ðHþ Þ

ð19Þ

Experimentally, at these potentials, there is no important absorbance change at 690 nm (Figure 5) and the electrochemical processes are also related with the participation of protons as counterions.25,40 The analysis of the 1000 nm derivative absorbance dependence on the potential indicates that the first dA1000/dt peak (E010 = 0.25 V) may be related to reactions 18 or 19 depending on the inner pH. The formal potential for the second process (E020 = 0.21 V) is the same as the potential for the dA690/dt curve, and therefore, it may be supposed that this dA1000/dt peak is also associated with reaction 16. The third dA1000/dt peak obtained at E030 = 0.15 V may be related to reaction 17 and then be similar to the absorbance response at 380 nm40 since it takes place at the same potentials. Even though a great deal of information can be extracted from these potentiodynamic experiments, it is not easy to obtain information on the relative speed of these processes owing to the limitations of the cyclic voltammetry. For this purpose, electrochemical impedance, mass impedance, and color impedance 1941



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Figure 6. Color impedance plots at the polarization potentials 0.300 V (a) and 0.200 V (b) for a PB film in 0.5 M KCl at pH = 3.0. In these figures HF means high frequency (100 Hz) and LF means low frequency (0.01 Hz).

techniques which can obtain separate information for slow and fast processes by changing the potential perturbation frequency in stationary conditions are particularly interesting. Impedance Analysis. Electrochemical impedance spectroscopy (EIS), mass impedance spectroscopy (MIS), and color impedance spectroscopy (CIS) at 1000 nm recorded altogether and simultaneously have been used to analyze the electrochemical behavior of Prussian blue films at the potential range between the fully reduced form (Everitt’s salt) and the mixed valence form (Prussian blue). We will focus on two aspects discussed above: the first one is that the maximum of the dA1000/dt takes place at potentials near 0.15 V (not at the current peak potential) and the second one is that dA1000/dt changes from positive to negative at potentials near the peak potential. This last behavior is also reflected in the color impedance response. At more anodic potentials (0.300 V), the loop of the color impedance function appears on the third quadrant (Figure 6a), and at more cathodic potentials (0.200 V) changes to the first quadrant (Figure 6b) for a PB film at pH = 3. The main question is how to explain this behavior. A band at 1000 nm has been attributed in the literature5 to a forbidden band overlapped to the main electronic transition centered at 690 nm. As it was commented above, there is the

Figure 7. Color impedance at different polarization potentials for a PB film in 0.5 M KCl at pH 2.0 (a), pH 3.0 (b), and pH 5.0 (c). The pH was corrected by HCl or KOH according to the requirement.

possibility to consider that we recover information of more than one electrochemical process at 1000 nm. At these potentials (between 0.400 V and 0.250 V) it is known that the PB films behavior is clearly dependent on the pH of the outer solution.25 The Nyquist plots of color impedance at 1000 nm at different pH values show some clear differences (Figure 7a, 7b, and 7c) that it depends also on the polarization potential applied. For the most acidic pH, there is a mixed behavior: at the more anodic potentials, there is always a third quadrant loop which displaces to the first quadrant at more cathodic potentials. At larger pH (5.0), the third quadrant loop disappears at all the studied 1942



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Figure 8. Low limit of F(Δm/Δq) and F(ΔA/Δq) crossed impedance functions for PB films at different stabilization potentials in 0.5 M KCl solution at pH = 3.0.

potentials. We have attributed this behavior to the electrochemical processes taking place on the active sites in the structural vacancies of the PB structure, reactions 18 and 19. Thus, this change of quadrant on which color impedance appears may be related to the inner pH of the vacancies and the relative weight of reaction 18 instead of reaction 19. From other experimental and simulation results we can estimate that about 25% of electrical charge during a voltammetric experiment corresponds to electrochemical processes taking place at these potentials.21,37,75 Therefore, there may be estimated to be about 25% of vacancies sites for hydronium and also potassium insertion. From Figure 7, it can be corroborated that the electrochemical process associated with the third quadrant loop appears to be faster than this one associated with the first quadrant loop since it appears at higher frequencies. Moreover, the electrochemical process associated with this CIS loop is related to the participation of protons, and thus to reaction 19, since this loop reduces for the largest pH. In order to study in depth this system, the analysis provided by the crossed impedance functions will be helpful.38,39 A first analysis may be to obtain the low frequencies limit of the F(ΔA1000/Δq), F(Δm/Δq) crossed impedance functions at different potentials at pH = 3.0 (Figure 8). Figure 8 shows two clear defined zones. At the more cathodic potentials (near 0.000 V), the ratio change of absorbance/electrical charge, F(ΔA1000/Δq), proves maximum and positive (the oxidized form absorbs at this wavelength). At the more anodic potentials [0.300, 0.600] V, this ratio reaches minimum values (negative). At the peak potentials, this ratio reaches very small values which clearly mean that reactions associated with changes of absorbance at 1000 nm are not the main contribution to changes of the electrical charge. In other words, at this potential, the main process corresponds to reaction 16 responsible for the changes of absorbance at 690 nm. On the other hand, F(Δm/Δq) shows values near 39 g mol1 at potentials in the range [0.000, 0.250] V which proves the main participation of potassium as counterion in the electrochemical processes taking place at these potentials. However, at potentials between 0.100 and 0.150 V this function reaches values slightly different (30, 35 g mol1). This behavior may be explained by the partial participation at these potentials of protons as counterions (reaction 17). However, it should be considered that at these potentials, the only contribution is not from reaction 17, since the F(Δm/Δq) does

Figure 9. Color impedance plots and fitted curves at the polarization potentials 0.280 V (a) and 0.125 V (b) for a PB film in 0.5 M KCl at pH = 3.0. In these figures HF means high frequency (100 Hz) and LF means low frequency (0.01 Hz). Experimental color impedance data were fitted to eq 20. Continuous line represents the deconvoluted response for the first process (fast, Z1) and discontinuous lines for the second (slow, Z2) process. Down triangles correspond to the fitted curve.

not reach a value of 19 g mol1 expected for a pure hydrated proton contribution. The F(ΔA1000/Δq) function may be interpreted as a measure of the electrochromic efficiency, or the ratio between the amount of moles reacting which cause color change and the total amount of moles reacting. At potentials more anodic than the peak current potential, this function reaches negative values, which means that absorbance decreases with the potential. However, at more cathodic potentials than the peak potential the opposite occurs. At the intermediate potential range, the color impedance shows two loops: a high frequency loop corresponding to the faster process placed on the third quadrant and a low frequencies loop placed on the first or second quadrant. The shape of these loops has been explained on the basis of a general model for the impedance (electrochemical, mass, and color) for conducting polymers.24,25,38,39,42,43,62,76 For the color impedance due to the contribution of two processes at a given 1943



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Figure 10. Dependence on the polarization potential of the low frequencies limit (eq 21) for the deconvoluted color impedance spectra. Solution was 0.5 M KCl and pH = 3.0. Continuous line corresponds to the first (fast) process and discontinuous line to the second (slow) process.

wavelength the impedance may be written such as 0 B B ΔAλ 1B ðωÞ ¼ B" jωB ΔE @

1 C C ζλ2 G2 C # " # þ pffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi C K1 cothð jωτ1 Þ K2 cothð jωτ2 Þ C pffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffi A 1 þ 1 þ d d jω=τ1 jω=τ2 ζλ1 G1

ð20Þ where τ represents a time constant associated with the transport of species through the film, K is related to the kinetics of the redox processes, G is related to the insertion of the different species into the electroactive film, and d represents the film thickness. The wavelength is represented by λ, and ζλi represents the apparent molar absorptivity for each one of the processes. ζλi may reach positive values if the reduced form is less colored than the oxidized one and negative values if the opposite occurs.38,39 During the voltammetric study, dA1000/dt curves have been simulated by considering three different processes in all the potential range. For the impedance analysis, only two processes are needed since experimental data are acquired at a given potential and in any case, one of the three contributions may be neglected, depending on the applied potential. Figure 9 shows for two different potentials that fitted curves reproduce quite well the experimental results. However, in the potential range [0.000,0.200] V both contributions (Z1 and Z2) are positive (first quadrant in Figure 9a), while in the potential range [0.260,0.500] V the faster contribution is negative (Z1, third quadrant in Figure 9b). The size of these loops is a measure of the partial contributions of each one of the processes to the global impedance signal. The low frequencies limit for each element in eq 20 corresponds to the size of this loop and may calculated as ! ! ! lim ΔAλ ζλ1 G1 ζλ2 G2 ðωÞ ¼ þ ð21Þ ω f 0 ΔE K1 =d K2 =d Figure 10 shows this low frequencies limit for different potentials. In the potential range [0.250,0.310] V there is always a positive and a negative contribution to the color impedance. The negative one and faster increases in magnitude toward more

Figure 11. F(ΔA/Δq) (a) and F(Δm/Δq) (b) crossed impedance functions for PB films obtained from color impedance and electrochemical impedance and from mass impedance and electrochemical impedance, respectively, in 0.5 M KCl solution at pH = 3.0.

anodic potentials, while the positive one decreases in this sense. In the potential range [0.310, 0.500] V there is only one effective contribution (the negative one). For the more cathodic potentials [0.000, 0.200] V there are always two positive contributions, but one faster than the other. In this figure, the second contribution is always the faster one. These results are in good agreement with the analysis of the voltammetric data and complete the conclusions relative to the PB behavior and the relative speed of the different processes. Figure 7a and b shows that in only 50 mV of the stabilization potential the quadrant on which appears the color impedance changes from the third (E = 0.300 V) to the first (E = 0.250 V). This abrupt change in only 50 mV is interpreted as the overlapping of the signal obtained from at least two electronic transitions. In order to explain this rare behavior, we can start with the color impedance on the third quadrant at potentials near 0.300 V. Electronic transitions associated with the Fe2+low spinCNFe3+high spin centers should appear on the first quadrant since color appears during the oxidation from Fe2+low spinCNFe2+high spin to Fe2+low spinCNFe3+high spin. At potentials very far from the peak current potential, [0.400, 0.600] V, there is no change of absorbance at 690 nm at these potentials, and, therefore, there is no change in the number of Fe2+low spinCNFe3+high spin centers. Besides, at these potentials, protons or hydrated protons are the main species exchanged during electrochemical reactions.5,37 Thus, the spectroscopic response obtained at these potentials should be mainly associated with reactions 18 and 19. Figures 9 and 10 corroborate that in this range of potentials two processes are identified to contribute to the global color impedance response. At potentials in the range [0.250, 0.310] V there are also two processes. On the one side there is the change of the oxidation state of those active centers which cause no color changes at 690 nm but also the electrochemical process involving Fe2+low spinCNFe3+high spin active centers which is associated with the 690 nm spectroscopic band. Another interesting result is related to the order of appearance of color impedance. The high frequencies response corresponds to the faster process; in this case it corresponds to the electronic transition associated with 1944



Figure 12. Dependence on the pH of the crossed impedance F(Δm/Δq) at polarization potential 0.400 V for PB films in 0.5 M KCl solution at different pH.

the proton or hydronium cations participation. This observation is not easy from electrogravimetric results since both potassium and hydronium are cations and, then, the loop appears in both cases on the third quadrant.25 We will now consider the analysis by the shape of the crossed impedance functions F(ΔA1000/Δq) and F(Δm/Δq) which are plotted in parts a and b of Figure 11, respectively. Looking at Figure 11a, one can find that there is a displacement from negative to positive values of the F(ΔA1000/Δq) function as the potential displaces from 0.400 to 0.250 V. At 0.400 V, it is only observed a loop on the third quadrant associated with reaction 19 that takes place with the participation of the electroactive Fe3+ sites near to vacancies. At 0.300 and 0.275 V, it proves clear that this loop corresponds to at least two overlapped processes: the (16) slower and the (19) faster reactions. At 0.250 V, there is no change in the quadrant. It should be noted that in only a few millivolts, the quadrant on which color impedance appears changes from the third to the first quadrant, the same as the F(ΔA1000/Δq) impedance function. Figure 11b shows also what happens with the F(Δm/Δq) transfer function. At 0.250, 0.275, and 0.300 V, a third quadrant loop appears which may be explained by considering that electron conduction proves faster than ion conduction in these experimental conditions.38,39 The low limit value displaces from about 39 g mol1 at 0.250 V to 32 g mol1 at 0.300 V. This behavior may be explained by considering the participation of both potassium and hydrogen cations, increasing the participation of hydrogen cations at more anodic potentials. At 0.400 V, this limit reaches values near 12 g mol1 indicating a large participation of protons. However, at 0.400 V this loop changes from the third to the fourth quadrant indicating now that the electron conduction through the electroactive film proves slower than the cation conduction (mainly proton at these potentials).11,25 This result may be explained by considering that proton motion across the film takes place through the inner water structure, while potassium motion proves more difficult across the zeolitic channels in the PB structure. This conclusion is in good agreement with some hypothesis about the ionic or electronic character of conductivity in these kind films depending on the applied potential or redox state of the film.11,77 If we examine this behavior at 0.400 V at other pH values, we find that at less acidic pH, F(Δm/Δq) displaces to 25 g mol1 at pH = 4.0 indicating a larger participation of potassium cations

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(Figure 12). However, the loop keeps on the fourth quadrant. This means that the proton moves faster through the inner water structure than the electron through the PB structure due the fact that the Fe3+/Fe2+ ratio at 0.400 V is not enough for reaching an intrinsic electron conductor behavior. If the water structure is formed first by vacancies into the PB structure, their distribution could correspond to an ordered way for the protons hopping between neighboring vacancies. This is supported by the 25% of vacancies observed by means of X-ray diffraction, and also it is consistent with the magnetic behavior observed with this kind of films when the ratio of Fe3+/Fe2+ sites are modulated by means of voltammetry.19,75 At pH = 5.0, F(Δm/Δq) ≈ 39 g mol1 indicating that mainly potassium cations participate. In this last case, the loop is not well observed since hydronium ions participation proves small enough. At pH = 2.0, the loop changes to the third quadrant indicating that electron conduction proves faster than proton conduction, since it is possible that chemical equilibrium between the outer solution and the inner active sites for protonation into the film was reached and no more hydrated protons are possible within the PB structure and, thus, there are enough protons in the inner water structure for yielding reaction 19. At highest pH, the potassium can enter into the vacancies and hinders the proton motion through water structure. However, the potassium is the counterion in reaction 16 due to the fact that the transport is possible through the zeolitic channels of the Prussian blue structure.

’ CONCLUSION The use of spectroscopic + EQCM techniques together and in situ with electrochemical techniques represents a great progress in the study of electron and ions transport in conducting polymers. The procedure here presented has allowed to separate the contributions of three different processes to the mass derivative and to the absorbance derivative curves obtained during cyclic voltammetry experiments. The dependence on the applied potential at several pH values of electrochemical impedance, mass impedance, and color impedance plots has allowed to associate these contributions with three different processes. The dependence on the pH of the electrochromic efficiency and also of the apparent molar mass for the three different processes has allowed to associate these processes with different electrochemical reactions and also to the participation of protons and/or potassium as counterions to balance electrical charge. These processes take place at potentials near 0.25, 0.21, and 0.14 V referred to the Ag/AgCl/KCl (sat.) reference electrode. The process at 0.25 V has been associated with the electrochemistry of the electroactive Fe sites near Fe2+(CN)64 vacancies. The different spectroscopic behavior for different pH has allowed concluding that the electrochemistry of vacancies is directly related to the participation of protons as counterions. For the processes at 0.21 and at 0.14 V, the electrochromic efficiency of the oxidized form of the film proves larger than for the reduced form. The opposite occurs for the 0.25 V process, and then dA/dt curves show a change from positive to negative values during a voltammetric experiment. In color impedance experiments, this different electrochromic efficiency is reflected in a change of the quadrant in which the color impedance loop appears (third for the 0.25 V process and first for the 0.21 and 0.14 V processes). At intermediate potentials it is observed a high 1945


The Journal of Physical Chemistry C frequencies loop on the third quadrant at the more anodic potentials and a first or second quadrant loop for the lower frequencies indicating that the 0.25 V process, associated with the electrochemistry of the vacancies, proves faster. These two processes are attributed to two different Fe3+ sites in the PB structure. From these results it has been possible to identify that the process associated with the hydrogen ion insertion into the vacancies of the PB structure is the faster one. Finally, the analysis provided by the crossed impedance functions has allowed to propose that mass transport proves faster than the electron conduction through the PB film at these potentials where there is only the participation of protons as counterions. This is explained by a proton conduction mechanism trough the inner water cluster in the PB crystalline structure. This inner water cluster should run through the vacancies, and some of these water molecules are coordinating different Fe sites near the vacancies.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Part of this work was supported by FEDER-CICyT project CTQ2010-21133/BQU. J.A. acknowledges his position in the Generalitat Valenciana. ’ REFERENCES (1) Itaya, K.; Ataka, T.; Toshima, S. J. Am. Chem. Soc. 1982, 104, 4767–4772. (2) Neff, V. D. J. Electrochem. Soc. 1978, 125, 886–887. (3) Ellis, D.; Eckhoff, M.; Neff, V. D. J. Phys. Chem. 1981, 85, 1225–1231. (4) Rajan, K. P.; Neff, V. D. J. Phys. Chem. 1982, 86, 4361–4368. (5) Robin, M. B. Inorg. Chem. 1962, 1, 337–342. (6) Itaya, K.; Shibayama, K.; Akahoshi, H.; Toshima, S. J. Appl. Phys. 1982, 53, 804–805. (7) Mortimer, R. J. J. Electrochem. Soc. 1991, 138, 633–634. (8) Chen, L. C.; Huang, Y. H.; Ho, K. C. 2nd International Meeting on Advanced Batteries and Accumulators; Brno, Czech Republic, 2001; pp 610. (9) Mortimer, R. J.; Reynolds, J. R. J. Mater. Chem. 2005, 15, 2226– 2233. (10) García-Jare~no, J. J.; Sanmatías, A.; Benito, D.; Navarro-Laboulais, J.; Vicente, F. Int. J. Inorg. Mater. 1999, 1, 343–349. (11) García-Jare~no, J. J.; Sanmatías, A.; Navarro-Laboulais, J.; Vicente, F. Electrochim. Acta 1998, 44, 395–405. (12) García-Jare~no, J. J.; Navarro-Laboulais, J.; Sanmatías, A.; Vicente, F. Electrochim. Acta 1998, 43, 1045–1052. (13) Itaya, K.; Akahoshi, H.; Toshima, S. J. Electrochem. Soc. 1982, 129, 1498–1500. (14) Roig, A.; Navarro, J.; Garcia, J. J.; Vicente, F. Electrochim. Acta 1994, 39, 437–442. (15) Mortimer, R. J.; Rosseinsky, D. R. J. Electroanal. Chem. 1983, 151, 133–147. (16) Lundgren, C. A.; Murray, R. W. Inorg. Chem. 1988, 27, 933– 939. (17) Mortimer, R. J.; Rosseinsky, D. R. J. Chem. Soc., Dalton Trans. 1984, 2059–2061. (18) Bueno, P. R.; Ferreira, F. F.; Gimenez-Romero, D.; Setti, G. O.; Faria, R. C.; Gabrielli, C.; Perrot, H.; Garcia-Jareno, J. J.; Vicente, F. J. Phys. Chem. C 2008, 112, 13264–13271.

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