Influence of Metallic Molar Ratio on the Electron Spin Resonance and Thermal Diffusivity of Zn – Al Layered Double Hydroxide

The coprecipitation method was used to prepare Zn–Al layered double hydroxide (Zn–Al–NO 3 -LDH) at pH 7.5 and different Zn/Al molar ratios of 2, 3, 4, 5, and 6. The elemental, structural, and textural properties of prepared samples were studied. The crystallinity of prepared LDH nanostructure decreases as Zn/Al molar ratio increases. The electron spin resonance (ESR) spectroscopy of different LDH samples showed new ESR spectra. These spectra were produced due to the presence of different phases with formed LDH such as ZnO phase and ZnAl 2 O 4 spinel. At low Zn/Al molar ratio, the ESR signals were produced from the presence of free nitrate anions in the LDH interlayer. Above Zn/Al = 2, the ESR signals were attributed to the existence of ZnOphase and ZnAl 2 O 4 spinel in the samples. Because the nuclearmagneticmoment of Zn is lower than Al, the increasing in Zn/Al molar ratio causes a reduction of the magnetic activity of ZnAl 2 O 4 spinel. Thermal diffusivity versus in situ temperature showed nonlinear relation for different samples due to the changing in the water content of LDH as temperature increases. The dc conductivity of samples decreased as Zn/Al molar ratio.


Introduction
Layered double hydroxides (LDHs), also knownas hydrotalcite structure, are a family of lamellar solids.LDHs may be represented by the general formula − .M II and M III are divalent and trivalent cations, respectively, which are situated at the center of octahedral (OH − ) units of the brucite-like layers with chargebalancing anions (A − ) in the hydrated interlayer regions. is the charge of anions (A − ) that lead to the electroneutrality of LDH.The value  is defined by [M III /(M II + M III )], and  is the amount of LDH interlayer water molecules [1,2].The coprecipitation method is a common method to prepare the LDHs [1,3,4] and is done by the mixing of salts of divalent and trivalent cations under certain parameters (such as pH value, stirring, and cations molar ratio).Electron spin resonance (ESR) spectroscopy is an important tool in research due to its ability to investigate several features of examined samples.The geometry and the arrangement components of intercalated guests into LDH interlayer could be identified using ESR spectroscopy [5,6].ESR has also been used to study the electronic structure of metallic centers of a mixed LDH and oxide material to explain its interaction with cations of LDH layers [7].In a recent study, ESR has been used to investigate the isolation of the sebacate when this material was intercalated into LDH [8].In our recent observation [9], ESR spectra have been used to explain the effect of temperature treatment on the nature of nitrate anions in LDH interlayer.
Thermal properties (including thermal diffusivity) of clays (soils, sands, LDHs, etc.) are very vital parameters in the heat transfer from or to buildings, in the distribution rate of electric cables and in the design of heating systems [10].Thermal diffusivity of sands (Utah tar sands) has been measured [11] and had a range of 5 × 10 −7 -9 × 10 −7 m 2 s −1 over the temperature range of 27-147 ∘ C using a constant applied heat flux method.Thermal diffusivity of soils has been measured using the line heat source transient method [10] between the temperature -10 ∘ C and 35 ∘ C. The effect of moisture content up to 40% on diffusivity was also studied.Hinkel [12] has estimated thermal diffusivity of soils from two different regions which was between 1.5 and 2.1 × 10 −7 m 2 s −1 .In our recent paper [9], the effect of in situ temperatures in the range of 27-210 ∘ C on thermal diffusivity of Zn-Al-NO 3 -LDH has been studied.
In this work, a series of Zn-Al-NO 3 -LDH samples were prepared by the coprecipitation method at different Zn 2+ /Al 3+ molar ratios of (2, 3, 4, 5, and 6) and at constant pH value of 7.5.The structural and textural properties of samples were studied.The effect of Zn 2+ /Al 3+ molar ratios on ESR spectra and thermal diffusivity of Zn-Al-NO 3 -LDHs were obtained.

Characterizations. Elemental analyses for Zn and Al
were measured by inductively coupled plasma emission spectroscopy (ICP) (Perkin-Elmer, Optima 2000 DV) after dissolving the samples in hydrochloric acid.Powder X-ray diffraction (PXRD) patterns of the samples were recorded on a X-ray diffractometer (X'pert-PRO Panalytical) using CuK  ( = 1.54187Å) at 40 kV and 30 mA.The surface areas and pore volumes of the samples were characterized using the nitrogen gas adsorption-desorption technique at 77 K using a Micromeritics ASAP 2000 instrument (Norcross, GA).Electron spin resonance (ESR) spectra were recorded using a JEOL ESR spectrometer (JES-FA200).Thermal diffusivity versus temperature was performed using NETZSCH model LFA 457 MicroFlash.The dielectric measurements of samples have been obtained using novocontrol high resolution dielectric analyzer.

Results and Discussion
3.1.Elemental Chemical Analysis.In this section, in addition to ICP analysis, we have used TGA data from our published paper [13] to calculate the chemical formulae of different samples.The chemical compositions of prepared Zn-Al-NO 3 -LDH samples at Zn 2+ /Al 3+ molar ratio of 2, 3, 4, 5, and 6 are listed in Table 1.The experimental Zn 2+ /Al 3+ molar ratio is close to that in the starting solutions.As seen in Table 1, the formula considers that nitrate is the only compensating anion in the LDH interlayer region.Carbonate impurities do not take into account the possible presence in the interlayer gallery.These results are in good agreement with that reported by Rojas Delgado et al. [14].2. For the samples Zn4Al-LDH, Zn5Al-LDH, and Zn6Al-LDH, the PXRD patterns show the characteristic reflections of ZnO phase and the patterns of ZnAl 2 O 4 spinel [15].The decreasing of PXRD intensity of the LDH peaks was observed as the Zn 2+ /Al 3+ molar ratio increases.These changes are attributed to the formation of other phases (ZnO phase and ZnAl 2 O 4 spinel) with low crystallinity.This caused the distortion of the hydroxide layers networks of the LDH crystal by the larger difference in ionic radii of Zn 2+ and Al 3+ [16].Yan et al. have employed the density functional theory to study the influence of cations ratio on the host layer structure of Zn-Al-LDHs [17].They have shown that the formation or bonding stability of the corresponding ZnrAl-LDH (r: molar ratio = 2, 3, 4, 5, and 6) clusters decreases with increasing the molar ratio (r).Another study has also demonstrated that the decreasing of Al 3+ concentration in Zn-Al-LDH solution causes the decreasing of the crystallinity of LDH phase [18].Our results are in good agreement with all of those literatures which exhibited significant increase in crystallinity of ZnrAl-LDH as Zn 2+ /Al 3+ molar ratio decreases.

N 2 Adsorption-Desorption Study.
The pore size distribution of the prepared samples was studied using the Barrett-Joyner-Halenda (BJH) method from the desorption branch of the isotherms as shown in Table 2.The observation is supported by the analyses of the pore structure and surface area of the samples.Figure 2 shows the nitrogen adsorptiondesorption isotherms of Zn-Al-NO 3 -LDH prepared at different Zn 2+ /Al 3+ molar ratio.All the isotherms are generally Type III-like with a distinct H3 hysteretic loop in the range of 0.2-1.0P/P 0 .These results show the presence of macroporous materials (the volume of pores are greater than 0.05 m) according to IUPAC classifications [19].Results from Rojas Delgado et al. on the total layer specific area for Zn-Al-Cl-LDH [14] reflect an increase from 924 m 2 /g (for Zn 2+ /Al 3+ molar ratio = 2.4) to 941 m 2 /g (for Zn 2+ /Al 3+ molar ratio = 4.8).As seen in Table 2, the BET surface area increases from 0.12 to 8.34 m 2 /g when Zn 2+ /Al 3+ molar ratio increases   from 2 to 6 due to the increasing of electrostatic repulsion between adjacent trivalent metals in the layers [14,20].On the other hand, the BJH average pore diameter has the opposite behavior of BET surface area with the highest value of 18.86 m at Zn 2+ /Al 3+ molar ratio of 2.

Electron Spin Resonance (ESR) Spectra.
New ESR spectra of the LDH samples exhibit signal with different intensities and sharpness at room temperature as shown in Figure 3.
For Zn2Al-LDH, a broad and weak signal with a g-factor = 2.01105 is observed, which is attributed to the interaction between nitrate anions from the interlayer region and a close aluminum nucleus ( = 5/2) from the LDH sheets.This signal is characteristic of NO 3 − ions with D 3ℎ symmetry in the LDH interlayer [21].As seen in our recent paper [13], the FT-IR results showed that the LDH interlayer NO 3 − anions existed with a D 3ℎ symmetry.There is also a possibility that this signal is due to a weak interaction of the free electron of NO 3 − radical with the nuclear magnetic moment of protons of the water matrix in LDH interlayer [22].Two signals with stronger intensity begin to appear for samples at Zn 2+ /Al 3+ molar ratio of 3 and above; the first one with a g-factor = 2.18870 is attributed to the formation of ZnAl 2 O 4 spinel, while the second signal with a g-factor = 1.93337 is ascribed to the existence of ZnO phase in the samples.
In ZnAl 2 O 4 , the most probable centers that can be observed are the V centers (a hole trapped in a cation vacancy) and F + centers (an electron trapped in an anion vacancy) [23].The signal of ZnAl 2 O 4 is due to the presence of an interaction between the oxygen vacancies of ZnAl 2 O 4 and the aluminium nucleus ( = 5/2).For the ZnO signal, the oxygen vacancies on the ZnO surface are responsible for the generation of the ESR signals.The Zn + ion, which has a 4s 1 orbital, will also appear on the ZnO surface and produces a parametric signal [24].
As seen in Figure 4, the evolution of the normalized ESR intensity against Zn 2+ /Al 3+ molar ratio for ZnO phase and ZnAl 2 O 4 spinel is plotted.The relative intensity of samples increases until Zn 2+ /Al 3+ molar ratio = 5 which has the highest ESR intensity for ZnO phase, and then it starts to decrease.For the ZnAl 2 O 4 spinel, the highest ESR intensity was at Zn 2+ /Al 3+ molar ratio = 4. Aluminum and zinc cations in ZnAl 2 O 4 have isotopes with nuclear spin of 5/2: 27 Al and 67 Zn. 27 Al is much more abundant (100%) than 67 Zn (4.11%), and its nuclear magnetic moment is higher (3.6385) than that of 67 Zn (0.8753) [25].The increase in Zn 2+ /Al 3+ molar ratio causes a reducing of the magnetic activity of ZnAl 2 O 4 because the nuclear magnetic moment of 67 Zn is lower than 27 Al.On the other hand, for ZnO phase, the magnetic activity increases which may be attributed to the increase in parametric of oxygen in the excess of Zn cations of LDH.The rapidly drop in the intensity of ZnO and stability of ZnAl 2 O 4 may refer to the low crystallinity of whole system due to the distortion of the hydroxide layers networks of the LDH crystal as seen in PXRD results.

Thermal Diffusivity. Thermal diffusivity (𝛼) measures
the ability of a material to conduct thermal energy relative to its ability to store thermal energy.The relation between thermal diffusivity (under transient conditions) and thermal conductivity () (under steady-state conditions) described the heat transport in materials using the following equation [26]: where   is the specific heat capacity and  is the density.In this work, LFA-457 NETSCH equipment has been used to measure the thermal diffusivity.The setup consists of a laser source which radiates a short-time energy pulse that strikes the front of the cylindrical sample (diameter 10 mm and thick between 1 and 2 mm).The transient temperature at the opposite side, which increases over time, is measured by infrared detector triggered simultaneously with laser beam.The mathematical thermal model of the experiment is represented by differential equation whose solution of this equation can be expressed as where  is the thickness of the sample and  max is the highest temperature at the rear surface of sample.The mathematical  analysis of the temperature versus time plot by (2) enables one to determine the thermal diffusivity as the following [27]: where  1/2 is the time corresponding to 50% of the increase in the temperature measured on the opposite surface of the sample.
Figure 5 shows a nonlinear relation of thermal diffusivity versus in situ temperatures for Zn-Al-LDH at different Zn 2+ /Al 3+ molar ratios.The values of thermal diffusivity   From TGA results [13], water molecules were completely removed and LDH starts to collapse at 180 ∘ C and above.For the samples Zn3Al-LDH, Zn4Al-LDH, Zn5Al-LDH, and Zn6Al-LDH, in general, the behavior of thermal diffusivity increases as in situ temperature increases until around 180 ∘ C and then drops rapidly as temperature increases.Thus thermal behavior in this range is strongly influenced by the water content in LDH [29].Above 180 ∘ C, ZnO phase is purely responsible for the decrease in the thermal diffusivity [9].
Figure 6 shows a nonlinear relation of the thermal diffusivity as a function of Zn 2+ /Al 3+ molar ratio of LDH for all measured temperatures.The thermal diffusivity decreases as the molar ratio increases which can be attributed to the presence of ZnO and ZnAl 2 O 4 phases.At temperature 210 ∘ C, the behavior is different due to the collapse of LDH structure [30,31].In addition to the decrease in the crystallinity as Zn 2+ /Al 3+ molar ratio increases, the presence of Zn-O and Al-O in an octahedral coordination state in LDH brucite-like sheet causes decreasing of thermal conductivity as temperature increases which may result from the random disorder phonon scattering induced by the Al deficient sites [32].
3.6.The Dc Conductivity.The dc conductivity or ionic conductivity ( dc ) can be calculated using the Jonscher power law [33]: where  is the preexponential factor and the value of  is between 0 and 1 for this material.The equation (4) has been used to fit the experimental data of real part of conductivity   () which were presented in the previous work [13] as shown in Figure 7.The solid line denotes the fit to power law expression of data, and the values ( dc , , and ) can be achieved using Origin nonlinear curve fitting software and listed in Table 3.The disagreement in the fit curves with experimental data at low frequency was attributed to the polarization effect of the electrode.The obtained  dc values from the fit curves for all samples were plotted against Zn 2+ /Al 3+ molar ratio as shown in Figure 8.This relation illustrated that dc conductivity decreased exponentially with the increase in the Zn 2+ /Al 3+ molar ratio of the prepared samples.This may suggest that the dc conductivity decreased due to the decrease in the crystallinity of samples as shown in PXRD results.

Conclusions
Powder XRD results showed that the crystallinity of Zn-Al-NO 3 -LDH phase decreased as Zn 2+ /Al 3+ molar ratio increases due to the distortion of the hydroxide layers networks by the larger difference in ionic radii of Zn 2+ and Al 3+ .Other phases of ZnO and ZnAl 2 O 4 were formed for LDH with Zn 2+ /Al 3+ molar ratio above 2.The prepared LDH samples were macroporous materials due to their volume diameter of pores which was found between 0.28 and 18.86 m.The characteristic ESR signal of NO 3 − ions with D 3ℎ symmetry in the LDH interlayer was obtained due to the interaction between nitrate anions from the interlayer region and a close aluminum nucleus in brucite-like layer of LDH.At Zn 2+ /Al 3+ molar ratio of 3 and above, two ESR signals with stronger intensity were produced due to the formation of ZnAl 2 O 4 and ZnO phases in LDH samples.Thermal diffusivity of LDH samples was found between 2.4 and 3.3 × 10 −7 m 2 s −1 .The nonlinear behavior of thermal diffusivity against the examined in situ temperature is attributed to the presence of water molecules in the LDH below 180 ∘ C. The dc conductivity which found from the fit curves of experimental data decreased as Zn 2+ /Al 3+ molar ratio increased which may be attributed to the decrease in the crystallinity of samples.
Figure 1 exhibits the PXRD patterns of Zn-Al-NO 3 -LDH precursors with Zn 2+ /Al 3+ molar ratio of 2, 3, 4, 5, and 6.The characteristic reflections of Zn-Al-NO 3 -LDH samples show the different planes which are listed in Table

Figure 4 :Figure 5 :
Figure 4: Variation of the normalized ESR intensity as a function of pH value of the prepared samples.

Figure 6 :
Figure 6: The thermal diffusivity as a function of Zn 2+ /Al 3+ molar ratio of LDH for several in situ temperatures.

Figure 8 :
Figure 8: The variation of dc conductivity versus Zn/Al molar ratio.

Table 1 :
Elemental chemical analysis data and interlayer water content of Zn-Al-NO 3 -LDH samples and their calculated formulae using ICP analysis.
[13]ese results were obtained by TGA data from our work[13].

Table 2 :
Lattice parameters and surface properties of Zn-Al-NO 3 -LDH samples.Average crystallite size in c direction value calculated from the values of (003) and (006) diffraction peaks using the Scherrer equation.

Table 3 :
[9,28] of dc conductivity  dc , preexponential factor  and the fractional exponent  for Zn-Al-NO 3 -LDH samples.× 10 −7 m 2 s −1 which are close to the values that have been reported[9,28].For Zn2Al-LDH, the thermal diffusivity is almost constant as temperature increases until around 150 ∘ C. Above 150 ∘ C, the values of thermal diffusivity drop rapidly as temperature increases because the water molecules start to release from LDH.The thermal diffusivity of this sample in this temperature range may be attributed to the high crystallinity of Zn2Al-LDH, and this sample is almost free from other phases as shown in PXRD results.