Protonic Surface Conductivity and Proton Space-Charge Relaxation in

Article pubs.acs.org/JPCC

Protonic Surface Conductivity and Proton Space-Charge Relaxation in Hydrated Fullerol Efstratia Mitsari, Michela Romanini, María Barrio, Josep Lluís Tamarit, and Roberto Macovez* Grup de Caracterització de Materials, Departament de Física, Universitat Politècnica de Catalunya, EEBE, Campus Diagonal-Besòs, Av. Eduard Maristany 10-14, 08019 Barcelona, Spain Barcelona Research Center in Multiscale Science and Engineering, Av. Eduard Maristany, 10-14, 08019 Barcelona, Spain ABSTRACT: The ac dielectric properties of both anhydrous fullerol (C60(OH)24) and hydrated fullerol with 20% water mass content are investigated by means of temperature-dependent dielectric spectroscopy. Anhydrous polycrystalline fullerol exhibits charge transport mediated by hopping of electronic charge carriers. Hydrated fullerol has a dc conductivity higher by more than a factor of 103 than that of the anhydrous sample due to waterinduced proton transport. Four distinct dielectric relaxation processes are observed in hydrated fullerol, two of which lie in the frequency range of the electrode polarization. The fastest relaxation is only observed below the melting point of pure water and is assigned to the migration of hydrogen-bond defects in the physisorbed H2O layers. The other three processes exhibit nonmonotonous temperature dependence upon dehydration by heating. The fastest of the three is present also in the anhydrous powder, and it is assigned to a space-charge relaxation due to accumulation of electronic charge carriers at sample’s heterogeneities such as grain boundaries. By studying the temperature dependence of the two slower relaxations across dehydration, we identify them as separate electrode polarization effects due to distinct charge carriers, namely electrons and protons. The electronic electrode polarization is also present in pure fullerol, while the proton space-charge relaxation is only present in the hydrated material. Our findings help elucidate the hitherto puzzling observation of more than one nonmonotonous relaxation process in hydrated and water-containing systems. sieves,20,21 hygroscopic powders of organic small-molecule semiconductors,11,22 hydrated polymers,23 and water-intercalated layered material such as aluminosilicate clays24,25 or graphite oxide.26,27 The proposed interpretation of these anomalous features varies between different studies: it has been related either to a reduction, upon heating, of the available free volume per defect in the hydrogen-bond network of interstitial water,17,19,21 or to a Maxwell−Wagner-Sillars polarization effect due to the strong increase of the static permittivity of a porous sample when water adsorbs onto inner surfaces,18 or else to a space-charge relaxation associated with accumulation of charge carriers at heterogeneities such as grain boundaries.11 A puzzling aspect of these “anomalous” relaxations is that some samples display more than one loss features with nonmonotonous temperature dependence,21−23 the existence of which is hard to rationalize with existing models. In this contribution we study the dielectric and conductivity response of a hydrated hydrophilic C60 derivative (fullerol), and we show that several distinct charge transport processes and space-charge relaxations take place, due to the simultaneous presence of electronic and protonic charge carriers. We choose

1. INTRODUCTION The interfacial properties of H2O molecules are far from trivial,1 and their understanding is crucial to deepen our knowledge of diverse phenomena ranging from solvation of organic molecules, biopolymers and membranes,2,3 to the function and reactivity of proteins,4−6 and from the doublelayer capacitance effect in supercapacitors with aqueous electrolytes7 to humidity sensing.8 For what concerns electrical conduction, it has been shown in recent years that exposure of nonmetallic materials to humid atmosphere leads to enhanced charge transport due to proton conduction through interfacial hydration layers. The effect has for example been observed in inorganic materials,9 polymers,10 and small-molecule organic solids,11 and it has been suggested that it may lead to the design of a new class of ion conductors.12 One of the main tools that have been employed in such studies is broadband dielectric spectroscopy, a versatile technique that allows the simultaneous study of dipolar relaxation dynamics and charge transport.13,14 Apart from the water-induced conductivity enhancement, the other striking feature observed in hydrated samples with this technique is the existence of water-related relaxations whose characteristic frequency does not display monotonic dependence on temperature. Such “anomalous” relaxations have been detected in hydrated nanostructured inorganic materials,15 watercontaining inorganic porous glasses16−19 and molecular © XXXX American Chemical Society

Received: December 13, 2016 Revised: February 6, 2017 Published: February 7, 2017 A

DOI: 10.1021/acs.jpcc.6b12530 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

imaginary permittivity (known as loss spectrum) are related as σ′( f) = 2πf ε0ε″(f). Several space-charge relaxation features were observed in the loss spectra, each of which was fitted by a model function (Cole−Cole for symmetric loss features and Havriliak−Negami for asymmetric ones)35 to extract the frequency of maximum loss f max, from which the characteristic relaxation time is obtained as τ = (2πf max)−1. (f max appears as a parameter in the analytical expression of the Cole−Cole function and is thus a direct fit parameter, while it must be calculated as a combination of several fit parameters in the case of the Havriliak−Negami function).35 Only the imaginary part of the permittivity was fitted, due to the inherent difficulties of modeling the capacitive contributions to the real part of the permittivity at low frequency (see section 3.2).

a fullerene derivative because on one hand its dipole moment is zero by virtue of the high molecular symmetry, so that no dielectric signal due to molecular motions is present, and on the other hand because such choice allows a clean separation of conduction effects due to the organic matrix and to the water hydration layers: it is known in fact that pristine and functionalized C60 behave as (n-type) electron semiconductors,28−31 while water conducts by proton hopping.32−34 Compared to inorganic samples, where water is mainly present as a surface hydration layer, in the case of the hydrated fullerol powder the water is at least partially present as a structural component. We had previously reported the existence of a nonmonotonic relaxation in hydrated fullerol,22 but were unable to provide a definitive assignment of this dielectric process. This work builds on and extends our prior study: we show that a total of three “anomalous relaxations” are actually present in hydrated fullerol, whose dielectric strength is directly correlated with the water content of the sample. Two of these relaxations are separate electrode polarization effects, stemming respectively from accumulation of electronic and protonic charge carriers at the electrode−sample interface. To the best of our knowledge, ours is the first report of a protonic spacecharge relaxation in a hydrated solid. Our findings help elucidating the peculiar behavior of interfacial water in dielectric spectroscopy and electrical conductivity experiments.

3. RESULTS AND DISCUSSION 3.1. Themodynamic and Structural Characterization. Panels a and b of Figure 1 display the TGA and DSC thermograms of a hydrated fullerol sample. The line shapes of the DSC and derivative TGA graphs are quite similar. As shown in panel a, the water fraction detected in TGA experiments is approximately 20% in weight, in agreement with our previous study.22 Such water content is high enough for (partial) crystallization of the water component upon cooling below 273 K, as visible from the DSC thermogram displayed in the inset to panel (b), where an endothermic melting transition is observed near the freezing point of pure water. The water fraction is observed to desorb upon heating between 350 and 420 K. The dehydration occurs in four main steps, highlighted by corresponding humps in both the DSC thermogram (panel b) and in the derivative of the TGA curve (panel a); of these, the broad process occurring at lower temperature is the desorption of secondary water. The presence of different features in different temperature ranges indicates the existence of water molecules in different molecular environments, bound more or less strongly to one another and to the fullerol matrix. Heating to a temperature higher than 420 K leads to decomposition of fullerol via detachment of the hydroxyl groups covalently attached onto the fullerene cages;22 hence, the temperature was always kept below 420 K during further characterization, and the pure material was obtained by annealing the hydrated powder at 415 K to desorb the water fraction. Figure 1c shows the room-temperature XRPD patterns of hydrated and anhydrous fullerol. The pure (anhydrous) material exhibits relatively sharp Bragg peaks at high scattering angles, and an intense background with somewhat broader diffraction features at low ones, indicating a partially disordered (poly)crystalline structure. Interestingly, the position of the peaks match those of the related C60(ONa)24 fullerene derivative,11 which differs from fullerol in the substitution of the hydroxyl hydrogens for sodium moieties, with no other change in the molecular geometry. The virtually identical lattice structure implies that intermolecular interactions are similar in the anhydrous form of both derivatives, which indicates that no significant intermolecular H-bonding takes place in pristine fullerol. The hydrated powder displays instead only weak and broad diffraction features at all scattering angles. This shows that hydration of fullerol leads to increased disorder and partial loss of crystallinity, contrary to the case of C60(ONa)24 which forms a crystalline hydrate.11 This difference is likely due to the different bonding of both derivatives to water molecules: while the ONa group can only act as acceptor of hydrogen bonds

2. EXPERIMENTAL SECTION The hydrated fullerol powder was synthesized as described in ref 22 and characterized by thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC). TGA scans were acquired while heating the sample under N2 flow between 300 and 620 K at a rate of 2 K min−1, by means of a Q50 thermobalance from TA-Instruments. DSC measurements were carried out in an open vessel between 200 and 600 K at a heating/cooling rate of 2 K min−1, using a Q100 calorimeter from TA-Instruments. The samples were weighed before and after the measurements with a microbalance sensitive to 0.01 mg. Pure fullerol was obtained by annealing the hydrated fullerol powder at 415 K in nitrogen (N2) atmosphere. High-resolution X-ray powder diffraction (XRPD) profiles were recorded with a vertically mounted INEL cylindrical position-sensitive detector (CPS120), on both hydrated and anhydrous fullerol. The generator voltage and current were set to 35 kV and 35 mA, and Cu Kα1 radiation was selected with an asymmetric-focus curved quartz monochromator. The detector was used in transmission mode with 2θ angular step of 0.029°, and the powder samples were placed into a Lindemann capillary tube (0.5 mm diameter). Dielectric measurements were carried out in the frequency (f) range from 10−2 to 106 Hz with a Novocontrol Alpha analyzer, on parallel-plate capacitors consisting of a pellet sandwiched between two stainless steel electrode disks. The pellet form of the fullerol samples was achieved using a hydraulic press (20 kN), with pellet thickness between 0.25 and 0.35 mm. Isothermal frequency scans were acquired in the temperature range between 193 and 413 K (with a temperature stability of 0.3 K) in a N2 flow Quatro cryostat. The isothermal dielectric spectra were displayed and analyzed in several representations, namely as real and imaginary part of the relative permittivity ε*, as imaginary part of the complex modulus, defined as M* = 1/ε*, and as real part of the ac conductivity σ*. The real part of the ac conductivity and the B


Article


3.2. Electric and Dielectric Characterization. Typical dielectric spectra acquired on hydrated fullerol are shown in three representations in Figure 2a, for the measuring temper-

Figure 2. (a) Dielectric response of hydrated fullerol at 313 K, shown as imaginary modulus spectrum (blue squares, left axis) and as ac conductivity (red triangles) and loss (black circles) spectra (right axis). Inset: comparison between the real and imaginary permittivity of the same sample at 313 K. The straight line is a guide to the eye. (b) Arrhenius plot of the dc conductivity (σdc, green circles) and of the “conductivity relaxation” frequency ( fσ, purple triangles) measured upon heating an initially hydrated fullerol sample (empty markers) and subsequent cooling of the same, now anhydrous, sample (filled markers). Data in the temperature range between 283 and 413 K are shown. Inset: σdc data for anhydrous fullerol and fit with the Shklovskii-Efros variable-range hopping model.

Figure 1. (a) TGA thermogram (upper blue curve) of the hydrated fullerol powder and its first derivative (lower gray curve), measured upon heating from room temperature to 605 K. (b) DSC thermogram of the same sample as in panel a between room temperature and 440 K. Inset: DSC thermogram of hydrated fullerol acquired in a cooling− heating cycle between 295 and 235 K. In all thermodynamic measurements the heat/cool rate was 2 K/min. (c) Room-temperature XRPD patterns of hydrated fullerol (black line) and of pure fullerol (sample obtained by annealing at 415 K, red line), between 15° and 55° 2θ-degrees.

ature of 313 K. Focusing first on the ac conductivity spectrum σ′( f) (red triangles in Figure 2a), it is apparent that it may be separated in three distinct frequency regions, namely, a central region displaying a flatter, plateau-like response, which corresponds to the dc regime, a low-frequency region where the conductivity decreases due to accumulation of charges at the metallic electrodes (electrode polarization effect), and a high-frequency region where the conductivity increases, generally known as dispersive region. By comparison with the loss spectrum ε″(f) (black circles), it is seen that the high-

from water, the hydroxyl (OH) group of fullerol may act both as acceptor and as donor. C


Article

The Journal of Physical Chemistry C frequency increase of σ′ is actually due to the presence of a dielectric loss feature, whose origin will be discussed below. As seen in Figure 2a, the modulus spectra exhibit two peaks, one in correspondence with the dielectric loss feature at high frequency and a second one at a lower frequency (marked as fσ) lying at the high-frequency end of the plateau of σ′( f). This correspondence allows identifying unambiguously the lowerfrequency modulus component as the so-called “conductivity relaxation” (see also below)36−38 and confirms the interpretation of the plateau-like portion of the conductivity spectra as the dc contribution. The real and imaginary parts of the complex permittivity are compared in the inset to Figure 2a. At intermediate frequency, i.e., in the region corresponding to the dc conduction regime, both ε″ and ε′ spectra exhibit a linear slope (in logarithmic representation) equal to approximately −0.7. The identical slope implies that real and imaginary parts maintain a constant, frequency-independent ratio in this frequency range. This behavior is known as “low-frequency dispersion”,39 and it is ubiquitous in ionic electrolytes40 and quite common in hydrated samples.15,41,42 The increase is real permittivity is indicative of a capacitive effect associated with storage of ionic charges either in the bulk of the sample or in the proximity of the metal electrodes.40 At lower frequency, as mentioned, the spectra are characterized by a bending associated with electrode polarization effects. Such low-frequency bending is visible in both the σ′ and ε″ representations and even in the ε′ one, while it is absent in the modulus M″(f) representation (blue squares in Figure 2a), as expected for an electrode polarization effect.35,43 The value of σdc for each isothermal spectrum was determined as the value of σ′(f) in the middle of the dc plateau-like contribution, namely at the frequency where the first derivative of σ′( f) was minimum. The so-obtained dc conductivity values are shown as Arrhenius plot in Figure 2b (empty circles). The filled circles are instead the σdc values extracted from the spectra acquired on the same sample upon cooling down from 420 K, i.e., once the anhydrous form of fullerol is obtained. The Arrhenius plot of σdc for pure (anhydrous) fullerol, shown also in the inset to Figure 2b for more clarity, displays a slight positive curvature. In the inset, the data points were fitted assuming a power-law dependence of the form log(σdc) = A − B/Tn. The result of the fit for different samples gave a value of the power n of 0.43 ± 0.05, which is close to the theoretical value of 1/2 that corresponds to the Shklovskii-Efros model of variable-range hopping electronic conductivity,44,45 and which is observed experimentally in a large variety of electron-conducting organic systems such as fullerenes31,46 or conjugated polymers.47 On the other hand, the nonmonotonous temperature dependence of σdc, and the higher conductivity value in hydrated fullerol (by three decades at room temperature) is reminiscent of the conductivity enhancement observed in many materials when they are exposed to humid atmosphere.9−12 The changes observed in the value of σdc upon heating the hydrated fullerol sample occur at basically the same temperatures at which the DSC and derivative TGA peaks are observed in our thermal analysis (Figure 1). Furthermore, no changes are observed in the anhydrous material. These results show that the conductivity decrease upon heating is indeed due to the desorption processes of water molecules. We have shown in our study of hydrated C60(ONa)24 that the extra conductivity contribution due to hydration water is protonic in nature, and

that it is likely due to a proton exchange mechanism between adjacent water molecules.11 Moreover, polyhydroxylated fullerenes have been indeed suggested to act as proton conductors both in powder form, inside membranes, and in aqueous solution.48−50 We conclude therefore that the majority charge carriers in hydrated fullerol are protons. For comparison, in Figure 2b we have also plotted the temperature evolution of the frequency fσ of the low-frequency “conductivity relaxation” in the modulus spectra (see Figure 2a). It is clear that the temperature- and sample-dependences of the two quantities f σ and σ dc are virtually identical, corroborating also our assignment of the low-frequency modulus peak as the “conductivity relaxation”. 3.3. Dielectric Relaxation Processes. Figure 3a shows the dielectric loss spectra of hydrated fullerol in the temperature

Figure 3. (a) Dielectric loss spectra of hydrated fullerol at selected temperatures. The solid lines though the data are fits assuming either a Cole−Cole (processes I, II, and IV) or Havriliak−Negami (process III) line shape for each loss peak (dashed lines). At low temperature a contribution proportional to inverse frequency was added to model the high-frequency tail of slow relaxations centered at frequency below the experimental range. (b) Comparison of the imaginary (main panel) and real (inset) parts of the permittivity for hydrated fullerol at 313 K (triangles) and anhydrous fullerol at 363 K (circles). The straight lines in the inset are guides to the eye. D


Article

The Journal of Physical Chemistry C interval between 193 and 303 K. Two well-separated relaxation processes are observed at low temperatures (at intermediate and high frequency), and two more processes are needed to fit the loss spectra in the low-frequency electrode polarization region at high temperature. In Figure 3a we label the four processes with roman numerals from the fastest (highest frequency) to the slowest (lowest frequency) relaxation. The dielectric strength (spectral intensity) increases from process I to process IV. Process I was not reported in our previous study; process II corresponds instead to process R1 of our previous work.22 The need for two separate electrode-polarization losses (processes III and IV) in the spectra of hydrated fullerol is not only required to obtain a reasonable spectral fit at low frequency, but it is also apparent from the comparison with the data of anhydrous fullerol. This is shown in Figure 3b, where we compare real and imaginary permittivity spectra acquired at different temperatures in order to have a similar vertical scale for the electrode-polarization maximal loss in both samples. It may be observed in the loss spectra shown in the main panel of Figure 3b that the overall width of the electrode polarization region is considerably broader in the hydrated sample than in the anhydrous one. In the logarithmic plot of the real permittivity (inset to the same panel), anhydrous fullerol exhibits a linear increase toward low frequency, in agreement with the expectation for a simple electrode polarization effect,35 whereas hydrated fullerol exhibits a deviation from linearity at low frequency, indicating the presence of separate components. It is also worth noticing that the anhydrous sample does not follow the expected behavior for a low-frequency dispersion feature as discussed in section 3.2, in agreement with the nonionic nature of electrical conduction in the pure material. It may seem unusual that two distinct space-charge relaxation losses are identified in the electrode polarization region. However, it has been shown in a detailed study of electrode polarization effects that, when two different types of charge carrier are present in a sample, separate electrode polarization relaxations may be observed, for each charge carrier.51 In the present case, the two distinct carriers are protons and electrons: protons dominate the dc conduction properties of hydrated fullerol, while electrons are the charge carriers responsible for electrical conduction in the pure material. Upon increasing the temperature, process I moves outside the available experimental frequency range already below 273 K, the melting point of pure water. The three other relaxations are observed to undergo a decrease in dielectric strength, while simultaneously shifting to lower frequency, as the temperature is raised (as shown in our previous study22 for process II). Relaxation I is absent in the spectra acquired on a dehydrated sample, whereas process II is still present (Figure 3b); the electrode polarization is present in both hydrated and anhydrous fullerol, but contrary to the hydrated case where two distinct processes are observed, in the anhydrous material one process is sufficient to model the loss spectrum. To better visualize the temperature evolution of the relaxation processes in hydrated fullerol, in Figure 4, we have plotted their relaxation times against inverse temperature (filled green markers). The nonmonotonic temperature dependences of the relaxation times of the relaxations II, III, and IV are clearly visible in the Arrhenius plot. They are observed in the same temperature range, which coincides with that of dehydration. The onset temperature of this anomalous behavior is roughly the same (∼313 K) for all three processes.

Figure 4. Arrhenius plots of the relaxation times of all four loss processes of hydrated fullerol upon heating (filled markers) and of the surviving processes in anhydrous fullerol upon cooling (empty markers).

The temperature evolution of process II can be easily followed in our loss spectra (see also ref 22). The anomalous behavior of process II is observed up to 360÷370 K, temperature above which the relaxation time resumes the normal temperature dependence (positive slope in the Arrhenius plot). The nonmonotonic shift of the relaxation times is irreversible: upon cooling from high temperature (420 K), i.e., in the anhydrous sample, only one electrode polarization process is visible, together with process II, and both these processes follow the expected temperature dependence for dielectric losses. The corresponding data are displayed as open blue markers in Figure 4. Since in the pure material electrical conduction is due to hopping of electronic charge carriers, the electrode polarization effect must be due to accumulation of electrons at the fullerol−electrode interface, that is, to a Shottky barrier effect.51 The dielectric losses in pure fullerol (process II, electrode polarization) have therefore a purely electronic origin. Both are due to accumulation of electronic charge carriers, either at material’s heterogeneities such as grain boundaries (process II), or at the fullerol-metal interfaces (electrode polarization). The large difference in characteristic frequency between the two reflects the very different length scales involved in these two space-charge processes. 3.4. Discussion. In what follows we analyze in more detail the four relaxation processes in hydrated fullerol, and provide an interpretation for each of them. Given that the observed relaxation processes cannot be intrinsic to fullerol molecules due to their zero dipole moment, the origin of all four processes must be related with the molecular dynamics of the water species or with space-charge effects. The fastest relaxation (process I) is only observed at low temperatures (below the freezing point of pure water), and it is absent in the pure material. Such process is observed at much longer relaxation times than the typical secondary relaxation of water;52 instead, as displayed in Figure 5a, it matches the relaxation times of the dielectric relaxation process of bulk ice.53,54 We conclude that process I has the same origin as the dielectric relaxation of pure ice, namely, the migration of hydrogen-bond defects.55 The presence of bulk-like ice at low temperature is confirmed by our DSC experiments (see the inset to Figure 1a). By fitting together both our experimental data and the data of refs 53 and E


Article


Figure 5. (a) Arrhenius plot of the relaxation time of process I in hydrated fullerol, in the temperature range 198−233 K. For comparison purposes, we have added the relaxation times of bulk ice from two different studies (ref 53, circles, and ref 54, asterisks). The red line through the data is the simultaneous Arrhenius fit of all experimental data. (b) Plot of the relaxation frequency of process II vs the characteristic relaxation frequency ( f BNN) of spatial charge fluctuations in anhydrous fullerol (see the text; data acquired upon cooling between 413 and 283 K). (c,d) Comparison between the temperature dependence of the dc conductivity and of the relaxation frequency of processes III and IV in hydrated fullerol through the dehydration process (data acquired upon heating from 298 to 343 K).

different initial water content. Nonetheless, each and every sample displayed, upon heating, the same qualitative changes in spectral profile (“anomalous” frequency shift, reduction in strength), always reaching the same frequency characteristic of the BNN relaxation of the anhydrous sample. In other words, the continuous evolution from the hydrated process II to the anhydrous process II has been observed in all samples studied, regardless of their initial water content. This is a strong indication that process II has the same origin in all samples, namely, it is a space-charge relaxation due to accumulation of charges at the material’s heterogeneities. The dramatic effect of the presence of water on process II, visible as a change in frequency, strength and width of this spectral feature, may be due both to the impact of water on electron hopping processes and to a possible contribution of protonic charge carriers to this space-charge relaxation. We finally discuss the electrode polarization effect in the hydrated sample. In panels c and d of Figure 5, we display both the dc conductivity and the characteristic frequency of processes III and IV, as a function of temperature between room temperature and 340 K (above this temperature one relaxation feature is sufficient to model the loss spectrum). All the data show a similar profile, with a maximum around 310 K (notice that both vertical scales in each panel span the same number of decades), above which the dielectric strength of both processes decreases (not shown). These observations indicate that the bimodal electrode polarization effect in hydrated fullerol is related to the dc conductivity of the sample, which as discussed above is mainly due to the motion of protonic charge carriers. In light of this, we ascribe the existence of two relaxation features in the electrode polarization regime to the existence of a protonic space-charge relaxation, i.e., to the

54 describing the same process in bulk ice, an activation energy of ∼0.58 eV is obtained for process I. Moving on to process II, in our previous study22 we tentatively assigned process II to the molecular reorientations of confined water molecules in hydration layers. We based this interpretation on that proposed for some inorganic porous materials, in which the anomalous temperature dependence was ascribed to a reduction of the available free volume per defect in the hydrogen-bond network of interstitial water upon heating, a model proposed by Ryabov and co-workers17,19 and often used to explain the nonmonotonous temperature dependence of relaxation times.21,56 However, as visible in Figure 5b, process II in dehydrated fullerol actually fulfills the so-called BartonNakajima-Namikawa (BNN) condition,57 which predicts that the characteristic frequency f max of an electronic space-charge relaxation in a disordered semiconductor should be linearly proportional to the so-called space-charge relaxation frequency defined as f BNN = σdc/2πε0Δε, where Δε is the dielectric strength of the electronic space-charge relaxation. As visible in Figure 5b, the two quantities are perfectly correlated in the anhydrous sample, showing that indeed process II follows the BNN predicament. The origin of process II in pure fullerol is therefore the accumulation of electronic charge carriers at the surface of crystalline domains. Since process II displays a continuous, smooth evolution between the hydrated and anhydrous states, we extend this interpretation also to hydrated fullerol, where the electronic space-charge relaxation is affected by the presence of water, as reported previously for the related C60(ONa)24 compound.11,31 By performing measurements on different samples of hydrated fullerol, we found that, at a given temperature, the frequency of process II is slightly different in each sample, likely due to a F


Article

The Journal of Physical Chemistry C accumulation of protons at the sample−electrode interface. Given that the proton mass is significantly larger than the electronic one, we suggest that the protonic electrode polarization effect corresponds to the slower process IV, while the faster process III might have an electronic character. Regardless of the validity of such assignment, the multicomponent character of the electrode polarization effect and the correlation visible in panels c and d of Figure 5 provide conclusive evidence that there is a protonic contribution to the electrode polarization in hydrated fullerol. In other words, the pronounced heterogeneity of the hydrated fullerol sample, with electron-conducting organic moieties surrounded by protonconducting water species, leads to the existence of two distinct electrode-polarization processes at different frequencies, one due to accumulation of electrons coming from the organic component or the metal electrode, and the other due to accumulation of protons stemming from water. We have shown that the most likely explanation for the nonmonotonic temperature dependence of dielectric losses in our hydrated samples is in terms of space-charge relaxations. The other mechanisms proposed to account for such unusual behavior are likely not applicable to our samples. For example, Maxwell−Wagner-Sillars relaxations are typical of mixed-phase samples58 with domains of mesoscopic dimensions and welldefined values of the static permittivity.35 In the case of hydrated organic powders as our fullerol samples, a description in terms of two well separated and defined phases (fullerol on one side, and water on the other) is probably not appropriate. Also, a Maxwell−Wagner-Sillars scenario may rationalize the presence of at most one relaxation with nonmonotonic temperature dependence, rather than three. As for the model proposed by Ryabov and co-workers,17 it mimics the possible effect of increased pressure (reduced free volume) of dipolar species under confinement, and it has been invoked to explain the behavior of secondary relaxations also in samples without water, where it is apparently unrelated to conductivity effects.59 In our data, however, it would be artificial to ascribe the anomalous behavior of the three processes (all in the same temperature range) to different dipolar processes displaying exactly the same free-volume effect. Also, while free-volume effects may be relevant for molecular species confined inside well-defined pores of a rigid inorganic structure, i.e., interacting with rigid pore surfaces, this is hardly the case in our hydrated organic powder, where there are no well-defined pore geometries and where the shape and size of the intergrain voids are actually affected by the loss of the structural water. To the best of our knowledge, ours is the first identification of a protonic space-charge relaxation in a hydrated material. Such protonic mechanism might actually be common to other hydrated materials. For example, Figure 6 shows the comparison between the nonmonotonic temperature dependence of process III in fullerol with the anomalies reported in other materials, both organic and inorganic.18,22 It is seen that all these anomalies lie in the same temperature window corresponding to water desorption, and that the relaxation times are not far from those of the electronic and protonic space-charge (electrode polarization) processes in fullerol, which suggests that proton space-charge relaxations may have been observed previously but not identified as such in other hydrated solids. Since in general the analysis of the dc conductivity is missing in previous studies reporting anomalous relaxations in hydrated samples, it cannot be concluded whether this interpretation may hold in the latter cases. It is

Figure 6. Arrhenius plot of the slowest relaxation process III of hydrated fullerol upon heating between 278 and 340 K. For comparison, the relaxation times of water-containing systems of rhodamine 6G (ref 22) and microporous silica (ref 18) are also shown.

interesting to notice in this respect that “double” electrode polarization features are reported for example in aqueous ion solutions.51 Most importantly, our interpretation naturally accounts for the existence of several water-induced anomalous relaxations in the same sample,21−23 an observation that was hitherto puzzling. It would be interesting to test whether such multiple space-charge features are also present in mixed electron−ion conductors as many electrolytes used in electrochemical applications.60

4. CONCLUSIONS We have probed anhydrous and hydrated fullerol by means of dielectric spectroscopy. By studying the temperature dependence of the dc conductivity, we find that anhydrous fullerol behaves as a disordered electronic semiconductor, while charge transport in hydrated fullerol is dominated by a water-induced proton conduction mechanism. Two space-charge relaxation losses are observed in pure fullerol, associated with electron accumulation either at material’s heterogeneities (the surface of the crystalline grains) or at the fullerol-electrode interface, respectively. By analyzing the temperature dependence of the relaxation times across dehydration of the hydrated fullerol powder, we are able to observe a continuous evolution of the space-charge relaxation at material heterogeneities between hydrated and anhydrous fullerol. The latter displays in total four relaxation processes, two of which in the electrode polarization regime. Of the two losses intrinsic to hydrated fullerol, the faster one is observed only below the melting point of pure water and is due to the migration of hydrogen-bond defects in the ice layer formed by partial crystallization of the hydration water. The slower process is instead observed above room temperature, and it is ascribed to a space-charge relaxation due to accumulation of the majority charge carriers, namely protons, at the sample−electrode surface. With the exception of the fastest process, whose temperature behavior could not be followed across dehydration, all other three processes in hydrated fullerol exhibit nonmonotonous temperature dependence upon desorption of water. We ascribe the anomalous behavior to the dramatic changes in conduction associated with the loss of surface hydration water and of the protonic charge carriers. Our findings help elucidate the hitherto puzzling observation of more than one nonmonotoG


Article


(13) Zachariah, M.; Romanini, M.; Tripathi, P.; Tamarit, J.Ll.; Macovez, R. Molecular Diffusion and Dc Conductivity Perfectly Correlated with Molecular Rotational Dynamics in a Plastic Crystalline Electrolyte. Phys. Chem. Chem. Phys. 2015, 17, 16053−16057. (14) Tripathi, P.; Gonzalo-Ruiz, J.; Mitsari, E.; Zachariah, M.; Romanini, M.; Tamarit, J.Ll.; Muñoz, F. X.; Macovez, R. Silicon-ChipBased Dielectric Spectroscopy for Conductivity and Molecular Dynamics Studies of Organic Films. J. Phys. Chem. Lett. 2014, 5, 2796−2801. (15) Haspel, H.; Bugris, V.; Kukovecz, Á . Water Sorption Induced Dielectric Changes in Titanate Nanowires. J. Phys. Chem. C 2013, 117, 16686−16697. (16) Jansson, H.; Swenson, J. Dynamics of Water in Molecular Sieves by Dielectric Spectroscopy. Eur. Phys. J. E: Soft Matter Biol. Phys. 2003, 12, S51. (17) Ryabov, Y.; Gutina, A.; Arkhipov, V.; Feldman, Y. Dielectric Relaxation of Water Absorbed in Porous Glass. J. Phys. Chem. B 2001, 105, 1845−1850. (18) Sjöström, J.; Swenson, J.; Bergman, R.; Kittaka, S. Investigating Hydration Dependence of Dynamics of Confined Water: Monolayer, Hydration Water and Maxwell−Wagner Processes. J. Chem. Phys. 2008, 128, 154503−154508. (19) Ryabov, Y. E.; Puzenko, A.; Feldman, Y. Nonmonotonic Relaxation Kinetics of Confined Systems. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 014204. (20) Frunza, L.; Kosslick, H.; Frunza, S.; Schönhals, A. Unusual Relaxation Behavior of Water Inside the Sodalite Cages of Faujasite− Type Molecular Sieves. J. Phys. Chem. B 2002, 106, 9191−9194. (21) Frunza, L.; Kosslick, H.; Pitsch, I.; Frunza, S.; Schönhals, A. Rotational Fluctuations of Water Inside the Nanopores of SBA−Type Molecular Sieves. J. Phys. Chem. B 2005, 109, 9154−9159. (22) Macovez, R.; Mitsari, E.; Zachariah, M.; Romanini, M.; Zygouri, P.; Gournis, D.; Tamarit, J. Ll. Ultraslow Dynamics of Water in Organic Molecular Solids. J. Phys. Chem. C 2014, 118, 4941−4950. (23) Jastrzebska, M.; Kocot, A.; Vij, J. K.; Zalewska-Rejdak, J.; Witecki, T. Dielectric Studies on Charge Hopping in Melanin Polymer. J. Mol. Struct. 2002, 606, 205−210. (24) Cerveny, S.; Schwartz, G. A.; Bergman, R.; Swenson, J. Glass Transition and Relaxation Processes in Supercooled Water. Phys. Rev. Lett. 2004, 93, 245702. (25) Le Caër, S.; Lima, M.; Gosset, D.; Simeone, D.; Bergaya, F.; Pommeret, S.; Renault, J.-Ph.; Righini, R. Dynamics of Water Confined in Clay Minerals. J. Phys. Chem. C 2012, 116, 12916−12925. (26) Cerveny, S.; Barroso-Bujans, F.; Alegrıa, A.; Colmenero, J. Dynamics of Water Intercalated in Graphite Oxide. J. Phys. Chem. C 2010, 114, 2604−2612. (27) Yu, J.; Tian, Y.; Gu, M.; Tang, T. B. Anomalous Dielectric Relaxation of Water Confined in Graphite Oxide. J. Appl. Phys. 2015, 118, 124104. (28) Anthopoulos, T. D.; Singh, B.; Marjanovic, N.; Sariciftci, N. S.; Ramil, A. M.; Sitter, H.; Cölle, M.; de Leeuw, D. M. High Performance n-channel Organic Field-Effect Transistors and Ring Oscillators Based on C60 Fullerene Films. Appl. Phys. Lett. 2006, 89, 213504. (29) Lai, Y.-Y.; Cheng, Y.-J.; Hsu, C.-S. Applications of Functional Fullerene Materials in Polymer Solar Cells. Energy Environ. Sci. 2014, 7, 1866−1883. (30) Mitsari, E.; Romanini, M.; Zachariah, M.; Macovez, R. Solid State Physicochemical Properties and Applications of Organic and Metallo-Organic Fullerene Derivatives. Curr. Org. Chem. 2016, 20, 645−661. (31) Macovez, R.; Zachariah, M.; Romanini, M.; Zygouri, P.; Gournis, D.; Tamarit, J.Ll. Hopping Conductivity and Polarization Effects in a Fullerene Derivative Salt. J. Phys. Chem. C 2014, 118, 12170−12175. (32) Knight, C.; Voth, G. A. The Curious Case of the Hydrated Proton. Acc. Chem. Res. 2012, 45, 101−109. (33) Agmon, N. The Grotthuss Mechanism. Chem. Phys. Lett. 1995, 244, 456−462.

nous relaxation process in hydrated and water-containing systems.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +34 934016568. E-mail: [email protected]. ORCID

Roberto Macovez: 0000-0001-5026-9372 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been supported by the Spanish Ministry MINECO through project FIS2014-54734-P and by the Generalitat de Catalunya under project 2014 SGR-581.

■ ■

ABBREVIATIONS XRPD, X-ray powder diffraction; BNN, Barton-NakajimaNamikawa REFERENCES

(1) Barati Farimani, A.; Aluru, N. R. Existence of Multiple Phases of Water at Nanotube Interfaces. J. Phys. Chem. C 2016, 120, 23763− 23771. (2) Ball, P. Water as an Active Constituent in Cell Biology. Chem. Rev. 2008, 108, 74−108. (3) Mallamace, F.; Corsaro, C.; Mallamace, D.; Vasi, C.; Vasi, S.; Stanley, H. E. Dynamical Properties of Water-Methanol Solutions. J. Chem. Phys. 2016, 144, 064506. (4) Levy, Y.; Onuchic, J. N. Water Mediation in Protein Folding and Molecular Recognition. Annu. Rev. Biophys. Biomol. Struct. 2006, 35, 389−415. (5) Gascoyne, P. R.; Pethig, R.; Szent-Györgyi, A. Water StructureDependent Charge Transport in Proteins. Proc. Natl. Acad. Sci. U. S. A. 1981, 78, 261−265. (6) Mallamace, F.; Corsaro, C.; Mallamace, D.; Vasi, S.; Vasi, C.; Baglioni, P.; Buldyrev, S. V.; Chen, S.-H.; Stanley, H. E. Energy Landscape in Protein Folding and Unfolding. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 3159−3163. (7) Jiang, G.; Cheng, C.; Li, D.; Liu, J. Z. Molecular Dynamics Simulations of the Electric Double Layer Capacitance of Graphene Electrodes in Mono-Valent Aqueous Electrolytes. Nano Res. 2016, 9, 174. (8) Chen, Z.; Lu, C. Humidity Sensors: A Review of Materials and Mechanisms. Sens. Lett. 2005, 3, 274−295. (9) Haspel, H.; Laufer, N.; Bugris, V.; Ambrus, R.; Szabó-Révész, P.; Kukovecz, Á . Water-Induced Charge Transport Processes in Titanate Nanowires: An Electrodynamic and Calorimetric Investigation. J. Phys. Chem. C 2012, 116, 18999−19009. (10) Ogura, K.; Saino, T.; Nakayama, M.; Shiigi, H. The Humidity Dependence of the Electrical Conductivity of a Soluble Polyaniline− Poly(vinyl Alcohol) Composite Film. J. Mater. Chem. 1997, 7, 2363− 2366. (11) Zachariah, M.; Mitsari, E.; Romanini, M.; Zygouri, P.; Gournis, D.; Barrio, M.; Tamarit, J.Ll.; Macovez, R. Water-Triggered Conduction Mediated by Proton Exchange in a Hygroscopic Fulleride and its Hydrate. J. Phys. Chem. C 2015, 119, 685−694. (12) Miyoshi, S.; Akao, Y.; Kuwata, N.; Kawamura, J.; Oyama, Y.; Yagi, T.; Yamaguchi, S. Low-Temperature Protonic Conduction Based on Surface Protonics: An Example of Nanostructured Yttria-Doped Zirconia. Chem. Mater. 2014, 26, 5194−5200. H


Article

The Journal of Physical Chemistry C (34) Agmon, N.; Bakker, H. J.; Campen, R. K.; Henchman, R. H.; Pohl, P.; Roke, S.; Thämer, M.; Hassanali, A. Protons and Hydroxide Ions in Aqueous Systems. Chem. Rev. 2016, 116, 7642−7672. (35) Kremer, F.; Schönhals, A. Broadband Dielectric Spectroscopy; Springer: Berlin, 2003. (36) Rivera, A.; Brodin, A.; Pugachev, A.; Rössler, E. A. Orientational and Translational Dynamics in Room Temperature Ionic Liquids. J. Chem. Phys. 2007, 126, 114503. (37) Sippel, P.; Lunkenheimer, P.; Krohns, S.; Thoms, E.; Loidl, A. Importance of Liquid Fragility for Energy Applications of Ionic Liquids. Sci. Rep. 2015, 5, 13922. (38) Wojnarowska, Z.; Kołodziejczyk, K.; Paluch, K. J.; Tajber, L.; Grzybowska, K.; Ngai, K. L.; Paluch, M. Decoupling of Conductivity Relaxation from Structural Relaxation in Protic Ionic Liquids and General Properties. Phys. Chem. Chem. Phys. 2013, 15, 9205−9211. (39) Jonscher, A. K. Dielectric Relaxation in Solids; Chelsea Dielectrics Press: London, 1983. (40) Jonscher, A. K. Low-Frequency Dispersion in Volume and Interfacial Situations. J. Mater. Sci. 1991, 26, 1618−1626. (41) Ek, R.; Hill, R. M.; Newton, J. M. Low Frequency Dielectric Spectroscopy Characterization of Microcrystalline Cellulose, Tablets and Paper. J. Mater. Sci. 1997, 32, 4807−4814. (42) Suherman, P. M.; Taylor, P.; Smith, G. Low Frequency Dielectric Study on Hydrated Ovalbumin. J. Non-Cryst. Solids 2002, 305, 317−321. (43) Macedo, P. B.; Moynihan, C. T.; Bose, R. The Role of Ionic Diffusion in Polarization in Vitreous Ionic Conductors. Phys. Chem. Glasses 1972, 13, 171−176. (44) Efros, A. L.; Shklovskii, B. I. Coulomb Gap and Low Temperature Conductivity of Disordered Systems. J. Phys. C: Solid State Phys. 1975, 8, L49−L51. (45) Shklovskii, B. I.; Efros, A. L. Electronic Properties of Doped Semiconductors; Springer: Berlin, 1984. (46) Zachariah, M.; Romanini, M.; Zygouri, P.; Gournis, D.; Tamarit, J.Ll.; Barrio, M.; Macovez, R. Variable-Range Electron Hopping, Conductivity Cross-Over and Space-Charge Relaxation in C60Br6. Synth. Met. 2016, 217, 123−128. (47) Capaccioli, S.; Lucchesi, M.; Rolla, P. A.; Ruggeri, G. Dielectric Response Analysis of a Conducting Polymer Dominated by the Hopping Charge Transport. J. Phys.: Condens. Matter 1998, 10, 5595− 5617. (48) Hinokuma, K.; Ata, M. Fullerene Proton Conductors. Chem. Phys. Lett. 2001, 341, 442−446. (49) Rajagopalan, M.; Oh, I.-K. Fullerenol-Based Electroactive Artificial Muscles Utilizing Biocompatible Polyetherimide. ACS Nano 2011, 5, 2248−2256. (50) Sardenberg, R. B.; Teixeira, C. E.; Pinheiro, M.; Figueiredo, J. M. A. Nonlinear Conductivity of Fullerenol Aqueous Solutions. ACS Nano 2011, 5, 2681−2686. (51) Emmert, S.; Wolf, M.; Gulich, R.; Krohns, S.; Kastner, S.; Lunkenheimer, P.; Loidl, A. Electrode Polarization Effects in Broadband Dielectric Spectroscopy. Eur. Phys. J. B 2011, 83, 157. (52) Ngai, K. L.; Capaccioli, S.; Ancherbak, S.; Shinyashiki, N. Resolving the Ambiguity of the Dynamics of Water and Clarifying its Role in Hydrated Proteins. Philos. Mag. 2011, 91, 1809−1835. (53) Chan, R. K.; Davidson, D. W.; Whalley, E. J. Effect of Pressure on the Dielectric Properties of Ice I. J. Chem. Phys. 1965, 43, 2376− 2383. (54) Artemov, V. G.; Volkov, A. A. Water and Ice Dielectric Spectra Scaling at 0°C. Ferroelectrics 2014, 466, 158−165. (55) Popov, I.; Puzenko, A.; Khamzin, A.; Feldman, Y. The Dynamic Crossover in Dielectric Relaxation Behavior of Ice Ih. Phys. Chem. Chem. Phys. 2015, 17, 1489. (56) Grzybowska, K.; Grzybowski, A.; Paluch, M. Role of Defects in the Nonmonotonic Behavior of Secondary Relaxation of Polypropylene Glycols. J. Chem. Phys. 2008, 128, 134904. (57) Namikawa, H. Characterization of the Diffusion Process in Oxide Glasses Based on the Correlation between Electric Conduction and Dielectric Relaxation. J. Non-Cryst. Solids 1975, 18, 173.

(58) Zachariah, M.; Romanini, M.; Tripathi, P.; Barrio, M.; Tamarit, J.Ll.; Macovez, R. Self-Diffusion, Phase Behavior, and Li+ Ion Conduction in Succinonitrile-Based Plastic Co-Crystals. J. Phys. Chem. C 2015, 119, 27298−27306. (59) Grzybowska, K.; Grzybowski, A.; Paluch, M. Role of Defects in the Nonmonotonic Behavior of Secondary Relaxation of Polypropylene Glycols. J. Chem. Phys. 2008, 128, 134904. (60) Patel, S. N.; Javier, A. E.; Stone, G. M.; Mullin, S. A.; Balsara, N. P. Simultaneous Conduction of Electronic Charge and Lithium Ions in Block Copolymers. ACS Nano 2012, 6, 1589−1600.

I


Protonic Surface Conductivity and Proton Space-Charge Relaxation in

Recommend Documents