# Correlation between Electrochemical Impedance Spectroscopy and Structural Properties of Amorphous Tunisian Metanacrite Synthetic Material

Authors: Nouha Jaafar; Hafsia Ben Rhaiem; Abdessalem Ben Haj Amara
Journal: Advances in Materials Science and Engineering
 (2014)

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## Abstract

In the present work, we report the structural and electrochemical properties of metanacrite. Metanacrite is a synthetic material originated by heating (550°C) of a clay mineral (Tunisian nacrite) belonging to the kaolin group. The structure of the amorphous synthetic product was corroborated by X-ray diffraction (disappearing of nacrite peaks) and infrared spectroscopy (disappearing of Al–OH and water bands). The decomposition of the silicate framework was confirmed by transmission electron microscope (TEM). The obtained metanacrite synthetic material was also examined by electrochemical impedance spectroscopy (EIS). Accordingly, the electronic conduction is followed by the correlated barrier hopping (C.B.H.) model. Therefore, by combining ac and dc electrical conductivity, a semiconductor behavior is evidenced. The dependence of the dielectric constant (ε′) and dielectric loss (ε″) on both temperature and frequency is also discussed.


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## Body

## 1. Introduction

Clay materials are the most common components of all sediments and soils in the Earth’s crust. Most of the researches and developments in clay materials come from technological needs for its low cost, easy availability, nontoxicity, bulk properties, and its surface chemical properties [1].Among a wide variety of clay resources,kaolin is considered as one of the most promising candidates due to its engineering and industrial applications in several sectors such as porcelain, pottery manufacturing, ceramics, nuclear waste treatment, paper manufacturing, and pharmaceutical and cosmetic industries [2–4]. Kaolin, with the chemical formula Al2Si2O5(OH)4, is a dioctahedral clay mineral of 1 : 1 layer type. One side of the layer is gibbsite-like with aluminium atoms octahedrally coordinated to corner oxygen atoms and hydroxyl groups. The other side of the layer constitutes a silica-like structure in which the silicon atoms are tetrahedrally coordinated to oxygen atoms. The adjacent layers are linked via hydrogen bonds (O–H⋯O) involving aluminol (Al–OH) and siloxane (Si–O) groups. The kaolin group is divided into three polytypes (i.e.,nacrite,kaolinite, anddickite) in addition tohalloysite, their hydrated analogue [5].Recently, several studies focused on the physicochemical characteristics ofmetakaolin (MK) materials derived from dehydroxylation of kaolin at 823 K. This phenomenon is accompanied by loss of water and decomposition into a disordered metastate [6], which undergoes massive structural changes of the octahedral layer: aluminium changes its coordination from six to four and five [7–9]. This metastable state yields highly activated aluminosilicate and environmentally friendly synthetic material used as follows.(i)
An inexpensive adsorbent for the removal and recovery of uranium, in particular for the treatment of wastewater [10].(ii)
A relatively new material in several research projects concerning cement and construction materials [11–13]: for example, in the concrete industry, MK is very effective in increasing strength, reducing sulphate attack, and improving air-void network; high strength concrete (HSC) with metakaolin as admixtures offers then many advantages, so it can be used for (a) dams, (b) bridges, (c) water retaining structures, (d) high rise buildings, (e) off shore structures, (f) industrial flooring, (g) warehouses, (h) container depots, (i) roads, (j) lining, (k) mass concreting, (l) aqueducts, (m) nuclear power stations, and (n) structural members where cross section is required to be small and so forth [14].(iii)
A good pozzolanic source suitable for the preparation of performed geopolymers: these geopolymers are known as high ionic conductors used for all solid-state batteries, electrochemical sensors, and so forth [15].In the present study, a similar calcination process is adopted to a Tunisian “nacrite” sample in order to elaborate a novel metastable synthetic material, commonly named “metanacrite.”Nacrite belongs to the kaolin polymorph and was previously described in the works of Ben Haj Amara et al. [16–20] and recently functionalized in the work of Jaafar et al. [21]. Bailey [5] described nacrite by a 2 M stacking mode which crystallized in the monoclinic lattice with a Cc space group. The structural parameters of nacrite are a=8.906 Å, b=5.146 Å, c=15.669 Å, and β=113.58° with a main basal distance d002=c sin β/2=7.18 Å [17–19].The dehydroxylation process may be presented by the following simple equation:(1)Al2O3·2SiO2·2H2Onacrite⟶Al2O3·2SiO2metanacrite+2H2Owater
The obtainedmetanacrite was then analyzed via X-ray diffraction (XRD), infrared spectroscopy (IR), and transmission electron microscopy (TEM) in order to highlight the structural features of the calcined material at different scales. This allows achievement of better applicability of Tunisian metanacrite, as a new synthetic source of pozzolan, for producing composite building materials.So far, no study has been reported on the electrical and dielectric properties of Tunisian metanacrite. This is the main point addressed in this paper. The study of the electrochemical characteristic of metanacrite is based on the structural analysis in order to correlate the material response to defects and disorder ratio induced by dehydroxylation of nacrite.


## 2. Experimental Procedure

Nacrite raw material used in this work was collected from the mine of Jbel Slata located in the region of Kef in North Tunisia [16]. The sample shows large particles and minor impurities and represents an available raw material for the synthesis of metanacrite. The sample of about 20 g was heat treated in the laboratory furnace. The optimal calcination parameters, for which complete dehydroxylation of the material was achieved, are temperature of 823 K and heating time of 120 min. The conversion of the nacrite to metanacrite was followed by XRD, IR, TEM, and EDXS.The qualitative XRD investigation consisted in a global description based on the profile geometry (peaks symmetry and/or asymmetry, shape, angular width at half height,d00l basal spacing values…, etc.) in order to acquire a preliminary idea about the analyzed sample. The X-ray diffraction (XRD) patterns were obtained by reflection setting with a D8 Bruker installation monitored by the EVA-version Diffrac plus software (Bruker AXS GmbH, Karlsruhe, Germany) and using CuKα1 radiation (λ=1.54186 Å). Usual scanning parameters were 0.02°2θ as step size and 6 s as counting time per step over the angular range 2–50°2θ.IR spectroscopy is used as a valuable tool to determine the frequencies evolution of metanacrite bands. IR spectra are recorded at room temperature using a Thermo Scientific Nicolet IR 200 FT-IR with ATR Spectrometer, equipped with a diamond crystal operating in the frequency range 4000–400 cm−1.The morphology, size, and composition of the synthesized sample were characterized via local chemical microanalysis performed using energy dispersive X-ray spectroscopy (EDXS) coupled to the transmission electron microscope (TEM) (FEI Tecnai G2) operating at 200 kV.The sample preparation process and the microstructure of the resultant amorphous synthetic material play a major role in the transport properties. For this purpose, we carried out electrochemical impedance spectroscopy (EIS) using a Hewlett-Packard (HP) 4192 analyzer. The impedance measurements were taken in an open circuit with a signal amplitude of 50 mV and a frequency band ranging from 10 Hz up to 13 MHz at different temperatures. The examined sample is pressed into pellet using a hydraulic press. To ensure good electrical contact between the sample and the electrical junctions, the pellet was sandwiched between two platinum electrodes to form a symmetrical cell. The cell was placed into a programmable oven coupled with a temperature controller. The resulting data were fitted using the equivalent circuit of theZview software.


## 3. Results and Discussion

The experimental XRD patterns of both nacrite and metanacrite are reported in Figures1(a) and 1(b), respectively. Results show a broad characteristic reflection extending from approximately 17 to 39°2θ attributed to the overlapping of the 00l  reflections of the well crystallized natural nacrite [16–20]. The XRD analysis represents then the amorphous structure of the obtained metanacrite synthetic phase.(a) Experimental X-ray pattern of nacrite raw-clay material. (b) Experimental X-ray pattern of metanacrite synthetic material. (c) Infrared spectra in the 4000–400 cm−1 region for nacrite and metanacrite samples.
(a)
(b)
(c)By comparing the IR spectra of metanacrite and nacrite recorded in Figure1(c), it is possible to follow the modifications of the stretching and deformation vibrations obtained after heating nacrite above 823 K: they shift from their positions in the spectrum of untreated nacrite, and their shapes change.Indeed,at high frequencies, it should be mentioned that no bands were detected in the infrared spectrum of metanacrite, indicating the disappearance of the OH lattice stretching band since it is located in the envelope of water band corresponding to natural nacrite at 3714 and 3647 cm−1 [16–20]. Figure 1(c) reveals that,at low frequencies, the Si–O stretching vibrations ν(Si–O) located at 1073 cm−1 and the Si–O deformation vibrations located at 459 cm−1 are commonly present with a slight change in their shape and intensity, while the Al–O deformation vibrations are shifted to high frequencies from 524 to 813 cm−1 and the Al–OH deformation vibrations placed at 753, 800, and 911 cm−1 in the spectrum of nacrite are omitted in the spectrum of metanacrite.All these changes in the infrared spectrum between metanacrite and nacrite imply that the starting aluminosilicate loses water during calcinations. The layered structure is then destroyed and transformed into a disordered metastate characterized by major changes in the octahedral sheet with minor changes in the tetrahedral sheet. In other words, metanacrite phase possesses amorphous structure very different from the nacrite matrix.Figure2(a) provides the TEM observations micrographs of Tunisian metanacrite. At first glance, the studied sample exhibits a disordered granular structure with various shapes and sizes. We note the presence of defects, mostly dislocations, which are generated by the distortion around the particle. The contributions of defects, grain size, distribution, and morphology confirm that the amorphous behavior dominates the whole structural composition of metanacrite synthetic material.(a) TEM micrographs of metanacrite synthetic material. (b) EDXS spectra of metanacrite.
(a)
(b)Such defects seem to be produced during the heating of the starting nacrite clay material and play an important role in the second part of this paper. The larger the number of defects and disorder, the higher the mobility of the free charge carriers, which results in an improvement of the conduction behavior.In order to obtain more detailed information about the microstructure and the chemical composition of metanacrite, we performedTEM coupled withEDXS analysis. The obtained spectra (Figure 2(b)) show the presence of Si and Al atoms with a major proportion, constituting the fundamental elements of the metanacrite phase, with a minor contribution of Fe and Cu atoms corresponding to the presence of a trace amount of impurities in the metanacrite material. However, the presence of Ni atoms belongs to the membrane on which the sample is placed.The preliminary physicochemical analysis of metanacrite reveals that the sample is mainly in high amorphous aluminosilicate phase with a disordered polymerized silicon/aluminum framework.In this section, we will discover the influence of the heat treatment of metanacrite from room temperature to 873 K on the enhancement of the electrical conductivity and the dielectric permittivity.Accordingly, the complex impedance plots of metanacrite in the temperature range 298–873 K are displayed in Figure3(a).(a) Nyquist diagram of metanacrite at the temperature range from 298 to 873 K. (b) The equivalent circuit of metanacrite compound.Rs and Rp represent electric resistance elements. CPE represent constant phase element: 1/ZCPE=Y0  jωβ, where Y0 is the admittance (1/Z) at ω=1 rad/s and β a value between 0 and 1.
(a)
(b)The Nyquist plots describe the transport properties which are strongly affected by the microstructure. From Figure3(a), two domains are identified: the first one is observed at low temperature (298 K–673 K) describing the insulator behavior and the second one is detected at high temperature (673–873 K) for which a significant increase in conductivity is observed. The increase of the electrical conductivity may be attributed to an increase of the disorder degree as well as defects density in the sample. The electric response of the sample was fitted in the frequency domain by a simple equivalent circuit involving constant phase element Rs (CPE, Rp) (Figure 3(b)), where Rs and Rp represent series and parallel connection of grain resistance, respectively.In general, at fixed temperature and over a wide range of frequencies, the total conductivity can be expressed as [22]
(2)σt=σdc+Aωs=σdc+σac,
where σac is the ac conductivity, σdc is the bulk conductivity, A is a temperature dependent parameter which determines the strength of polarisability, and the exponent s represents the degree of interaction between mobile charge carriers of the specimen.Using the empirical relation given by (3)  σac is calculated:
(3)σac=gR,
where g represents the geometric factor given by
(4)g=dS=dπr2,
where d corresponds to the thickness of the pellet, r is the radius of the pellet, S is the area of the pellet, and R is the resistance.The values of the frequency exponents as determined are presented in Figure 4 as a function of temperature. As it can be seen, the frequency exponent values decrease with temperature from 0.69 to 0.36.Figure 4
Temperature dependence of frequency exponent(s) for metanacrite sample.Different models have been proposed to describe and draw the ac conduction model corresponding to metanacrite such as quantum mechanical tunneling (Q.M.T.) [23, 24], nonoverlapping small-polaron tunneling (N.S.P.T), overlapping large-polaron tunneling (O.L.P.T.), and the correlated barrier hopping (C.B.H.) [23–28].According to Q.M.T. model, the ac conductivity originates from the tunneling of carriers between different valence states. In this case the expressions of the ac conductivity and frequency exponent are, respectively, given by the following equations [29]:
(5)σacω=Ce2akTNEf2ωRω4,s=1-4ln⁡1/ωτ0,
where [N(Ef)] is the density of states at Fermi level, Rω is the hopping length at frequency ω, τ0 is the atomic vibration period, and a is the localized radius. The last equation predicts that s=0.8 and is temperature independent. This implies that the Q.M.T. model is in disagreement with our results (see Figure 4).In the case of the N.S.P.T model, it is usually associated with an increase ins with temperature. Concerning the O.L.P.T model, the frequency exponent s depends on both frequency and temperature and it drops to a minimum value with rising temperature and then increases as temperature rises. Finally, in the classical hopping model, the exponent s is a constant value and is equal to unity [25].All these models were deemed to be in disagreement with our results. Therefore, the experimental data will be discussed in the frame of the C.B.H. model. Thus, the conduction occurs via the hopping carriers over a potential barrier between two different valence states. The ac conductivity and frequency exponent expressions due to the C.B.H. model are given by the following equations:(6)σacω=124π3ε′N2ε0ωRω6,(7)s=1-6kTWM+kTln⁡1/ωτ0,
where ε0 is the free-space dielectric permittivity, ε′ is the dielectric constant, and WM is the maximum barrier height. Equation (7) predicts that s decreases with increasing temperature. Therefore the C.B.H. is the involved conduction mechanism for the investigated metanacrite synthetic sample [30, 31].Concerning theac conductivity of metanacrite, it is found to obey the universal Arrhenius power law [32]:
(8)σac=σ0exp⁡-EaackBT,
where σ0 is the preexponential factor, kB is the Boltzmann constant, and Eaac is the activation energy that controls the jump of charge carriers from one site to another neighboring site. The Eaac values have been calculated at four fixed frequencies and are listed in Table 1.Table 1
ac activation energiesEa(ac) for metanacrite sample.
Temperature range:773→873K
Frequency
E
a
(
ac
) (eV)(±0.01eV)
100 Hz
0.24
1 MHz
0.21
5 MHz
0.20
10 MHz
0.17Indeed, the activation energy varies between0.24±0.01 eV (at low frequencies) and 0.17±0.01 eV (at high frequencies). These results are specific to the temperature range 773–873 K. Thus, the increase of the applied frequency enhances the electronic jumps between the localized states. Consequently, the activation energy decreases with increasing frequency. Moreover, at high frequency, the ac activation energies are found to be lower than those found at low frequencies regions (see Figure 5). Therefore, at high frequencies the mobility of charge carriers over short distances needs lower energy than that necessary for mobility over longer distances at low frequencies.Figure 5
Evolution of the ac conductivity of metanacrite between 773 K and 873 K at 100 Hz, 1 MHz, 5 MHz, and 10 MHz.The ac conductivity increases as a function of frequency at a fixed temperature (773 K) from3.40·10-6 S·m−1 at f=100 Hz to 1.27·10-2 S·m−1 at f=10 MHz (Table 2). Besides the ac conductivity increases as a function of temperature at a fixed frequency (f=10 MHz), it increases from 1.27·10-2 S·m−1 at T=773 K to 1.98·10-2 S·m−1 at T=873 K. The small values of the ac activation energy Ea(ac) and the increase of ac conductivity σac with the increase of frequency confirm the dominant hopping conduction mechanism.Table 2
Variation of the ac conductivity as a function of temperature and frequency.
Temperature range
σ
ac (S·m−1)
f= 100 Hz
f= 1 MHz
f= 5 MHz
f= 10 MHz
773 K
3.40
·
10
-
6
1.14
·
10
-
3
6.30
·
10
-
3
1.27
·
10
-
2
783 K
3.45
·
10
-
6
1.20
·
10
-
3
6.45
·
10
-
3
1.31
·
10
-
2
793 K
3.56
·
10
-
6
1.66
·
10
-
3
6.76
·
10
-
3
1.37
·
10
-
2
803 K
3.62
·
10
-
6
2.05
·
10
-
3
6.91
·
10
-
3
1.43
·
10
-
2
813 K
3.73
·
10
-
6
2.33
·
10
-
3
7.24
·
10
-
3
1.52
·
10
-
2
823 K
3.84
·
10
-
6
2.54
·
10
-
3
7.59
·
10
-
3
1.61
·
10
-
2
833 K
3.92
·
10
-
6
3.14
·
10
-
3
7.87
·
10
-
3
1.66
·
10
-
2
843 K
4.08
·
10
-
6
3.57
·
10
-
3
8.17
·
10
-
3
1.70
·
10
-
2
853 K
4.16
·
10
-
6
4.23
·
10
-
3
8.45
·
10
-
3
1.78
·
10
-
2
863 K
4.30
·
10
-
6
4.81
·
10
-
3
8.79
·
10
-
3
1.86
·
10
-
2
873 K
4.38
·
10
-
6
5.83
·
10
-
3
9.12
·
10
-
3
1.98
·
10
-
2The conductivity is thermally activated; therefore the electrical conduction follows a process in which the electron or hole hops from one localized site to another. The electron resides at one site; when it is thermally activated it migrates to another site. Moreover, we demonstrated in the first part of this paper that the defects constitute the active sites in the conduction process. Therefore, the electron or hole tends to associate with local defects, so the activation energy for charge transport may also include the energy of freeing the hole from its position next to the defect [33]. Otherwise, the electronic conduction takes place by hopping between two charge-defect states over the barrier separating them.As mentioned above, the ac conductivity increases with increasing both temperature and frequency. Simultaneously, the dc conductivity increases with increasing temperature (Figure6).Figure 6
Evolution of the dc conductivity of metanacrite between 773 K and 873 K.Indeed, the dc conductivity(σdc) fits the well-known Arrhenius relation [32]. We found Eadc=0.24±0.01 eV (see Figure 6):
(9)σdc=σ0Texp⁡-EadckBT,
where σ0 is the preexponential factor, Eadc is the activation energy, and kB is the Boltzmann constant.As a consequence of the increased dc conductivity with temperature for small activation energies we conclude that the amorphous metanacrite sample behaves like a semiconductor material. To add a more comprehensive picture about the transport mechanism of the present synthetic sample, dielectric investigations were carried out. Results are reported in Figure7.(a) Variation of real part (ε′) of dielectric permittivity with frequency at temperature range from 298 to 873 K for metanacrite. (b) Variation of imaginary part (ε′′) of dielectric permittivity with frequency at temperature range from 298 to 873 K for metanacrite. Frequency dependence of imaginary (ε′′) part of dielectric permittivity in the (773–873 K) temperature range on a log-log scale. (d) Temperature dependence of frequency exponent, m, for metanacrite sample.
(a)
(b)
(c)
(d)The complex permittivity, the dielectric constantε′ω, and the dielectric loss ε′′(ω) are determined according to the following expressions [34]:
(10)ε*ω=ε′ω-jε′′ω,ε′=-Z′′C0Z′2+Z′′2ω,ε′′=Z′C0Z′2+Z′′2ω,
where Z′ is the real part of the electric impedance, Z′′ is the imaginary part of the electric impedance, j2=-1, C0=ε0S/d is the capacitance of the empty cell, ω is the angular frequency, ε0 is the dielectric permittivity of free space (8,85418782 × 10−12 F·m−1), S is the electrode surface, and d is the distance between the two plane electrodes.Upon heat treatment of metanacrite from room temperature to 873 K, the disorder degree and the number of defects rise more significantly which influences the dielectric permittivity considerably.From Figure7(a), it is found that heating of metanacrite undergoes a gradual decrease in the dielectric constant. The observed dielectric dispersion atlow frequencies may be associated with the thermally activated conduction of mobile current carriers and with the increase of disorder degree and defects density in the sample. Simultaneously, we mark alow frequency dispersion and increase of the dielectric loss (ε′′) in the temperature domain ranging from 773 K up to 873 K (Figure 7(b)). Indeed, according to the structural analysis, metanacrite sample is characterized by a significant decrease of grain dimensions caused by the destruction and decomposition of the layered nacrite material. Therefore, metanacrite exhibits defects that liberated a small distance movement. So, the charge carriers are assumed to migrate easily. This allows increasing the polarization with temperature.However,at high frequencies, the dielectric constant remains invariant with temperature (Figure 7(a)); the dielectric loss drops and becomes almost temperature independent for f>103 Hz (Figure 7(b)); this may be due to the fact that, in the high frequency range, the charge carriers cannot follow the electric field.The behavior ofε′′ as a function of both frequency and temperature can be analyzed according toGiuntini equation [35]:
(11)ε′′=εs-ε∞2π2Nne2εs3kTωmτmWM-4,
where ε′′ should follow a power law with frequency:
(12)ε′′=Bωm,m=-4kTWM,
where WM is the maximum barrier height, εs is the static dielectric constant, ε∞ is the dielectric constant at infinite frequency, B is a constant depending on temperature, τ is the relaxation time for electrons to hop over a barrier, and k is Boltzmann’s constant.The variation ofε′′ with frequency on double logarithmic scale at different temperatures for metanacrite presents a series of straight lines with different slopes (Figure 7(c)).The power,m, represents the value of the slope corresponding to each temperature. m decreases with increasing temperature showing a linear behavior which is observed in Figure 7(d); the maximum barrier height WM was found to be 0.13 eV.


## 4. Conclusions

The structural and electrical properties of metanacrite synthesized from Tunisian nacrite raw clay were investigated. The amorphous character of metanacrite was identified using XRD, IR, TEM, and EDXS analysis. Results of the electrochemical impedance spectroscopy show that the sample behaves like a semiconductor material. On the other hand, an enhancement of the electrical conductivity is clear with increase in temperature and frequency. Application of the C.B.H. model reveals that the electronic conduction mechanism is governed by hopping between two charge-defect states over a barrier heightWM around 0.13 eV. The electrical/dielectric properties agree very well with the structural analysis.The present work gives possible valorization of Tunisian metanacrite as an interesting precursor to produce a new class of cementitious materials possessing good electrical properties. Further studies are in progress to point up the use of this amorphous semiconductor material in diverse electronic applications with a cost-benefit focus.



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