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Journal of Nanomaterials
Volume 2013 (2013), Article ID 639354, 9 pages
http://dx.doi.org/10.1155/2013/639354
Research Article

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

1Department of Physics, Faculty of Science, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia
2Department of Physics, Faculty of Applied Science, Thamar University, Thamar 87246, Yemen
3Advanced Materials and Nanotechnology Laboratory, Institute of Advanced Technology (ITMA), Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia

Received 25 June 2013; Revised 6 August 2013; Accepted 7 August 2013

Academic Editor: Christian Brosseau

Copyright © 2013 Abdullah Ahmed Ali Ahmed et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The coprecipitation method was used to prepare Zn–Al layered double hydroxide (Zn–Al–NO3-LDH) at pH 7.5 and different Zn2+/Al3+ 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 Zn2+/Al3+ 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 ZnAl2O4 spinel. At low Zn2+/Al3+ molar ratio, the ESR signals were produced from the presence of free nitrate anions in the LDH interlayer. Above Zn2+/Al3+ = 2, the ESR signals were attributed to the existence of ZnO phase and ZnAl2O4 spinel in the samples. Because the nuclear magnetic moment of 67Zn is lower than 27Al, the increasing in Zn2+/Al3+ molar ratio causes a reduction of the magnetic activity of ZnAl2O4 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 Zn2+/Al3+ molar ratio.

1. Introduction

Layered double hydroxides (LDHs), also known as hydrotalcite structure, are a family of lamellar solids. LDHs may be represented by the general formula . and are divalent and trivalent cations, respectively, which are situated at the center of octahedral () units of the brucite-like layers with charge-balancing anions () in the hydrated interlayer regions. is the charge of anions () that lead to the electroneutrality of LDH. The value is defined by , 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 m2 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 m2 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–NO3-LDH has been studied.

In this work, a series of Zn–Al–NO3-LDH samples were prepared by the coprecipitation method at different Zn2+/Al3+ 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 Zn2+/Al3+ molar ratios on ESR spectra and thermal diffusivity of Zn–Al–NO3-LDHs were obtained.

2. Experimental

2.1. Materials and Preparation of Samples

The starting chemicals of zinc nitrate (Zn(NO3)26H2O) (Systerm, 98%), aluminum nitrate (Al(NO3)39H2O) (Hamburg Chemicals Co., 99.4%), and sodium hydroxide (NaOH) (Merck Co., 99%) were used without further purification. Deionized water was used as solvent throughout this study. The preparation of Zn–Al–NO3-LDH precursors with starting value of molar ratio Zn2+/Al3+ = 2, 3, 4, 5, and 6 has been carried out using coprecipitation method and has been presented elsewhere [13]. The samples were labeled as ZnrAl-LDH, where r represents the Zn2+/Al3+ molar ratio.

2.2. 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 CuKα (λ = 1.54187 Å) 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.

3. 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–NO3-LDH samples at Zn2+/Al3+ molar ratio of 2, 3, 4, 5, and 6 are listed in Table 1. The experimental Zn2+/Al3+ 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].

tab1
Table 1: Elemental chemical analysis data and interlayer water content of Zn-Al-NO3-LDH samples and their calculated formulae using ICP analysis.

3.2. Powder X-Ray Diffraction (PXRD) Study

Figure 1 exhibits the PXRD patterns of Zn–Al–NO3-LDH precursors with Zn2+/Al3+ molar ratio of 2, 3, 4, 5, and 6. The characteristic reflections of Zn–Al–NO3-LDH samples show the different planes which are listed in Table 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 ZnAl2O4 spinel [15]. The decreasing of PXRD intensity of the LDH peaks was observed as the Zn2+/Al3+ molar ratio increases. These changes are attributed to the formation of other phases (ZnO phase and ZnAl2O4 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 Zn2+ and Al3+ [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 Al3+ 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 Zn2+/Al3+ molar ratio decreases.

tab2
Table 2: Lattice parameters and surface properties of Zn–Al–NO3-LDH samples.
639354.fig.001
Figure 1: PXRD patterns of (a) Zn2Al-LDH, (b) Zn3Al-LDH, (c) Zn4Al-LDH, (d) Zn5Al-LDH, and (e) Zn6Al-LDH.
3.3. N2 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 adsorption-desorption isotherms of Zn–Al–NO3-LDH prepared at different Zn2+/Al3+ molar ratio. All the isotherms are generally Type III-like with a distinct H3 hysteretic loop in the range of 0.2–1.0 P/P0. These results show the presence of macroporous materials (the volume of pores are greater than 0.05 μm) 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 m2/g (for Zn2+/Al3+ molar ratio = 2.4) to 941 m2/g (for Zn2+/Al3+ molar ratio = 4.8). As seen in Table 2, the BET surface area increases from 0.12 to 8.34 m2/g when Zn2+/Al3+ 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 μm at Zn2+/Al3+ molar ratio of 2.

fig2
Figure 2: N2 adsorption-desorption isotherms of (a) Zn2Al-LDH, (b) Zn3Al-LDH, (c) Zn4Al-LDH, (d) Zn5Al-LDH, and (e) Zn6Al-LDH.
3.4. 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 () from the LDH sheets. This signal is characteristic of ions with D3h symmetry in the LDH interlayer [21]. As seen in our recent paper [13], the FT-IR results showed that the LDH interlayer anions existed with a D3h symmetry. There is also a possibility that this signal is due to a weak interaction of the free electron of 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 Zn2+/Al3+ molar ratio of 3 and above; the first one with a g-factor = 2.18870 is attributed to the formation of ZnAl2O4 spinel, while the second signal with a g-factor = 1.93337 is ascribed to the existence of ZnO phase in the samples.

639354.fig.003
Figure 3: ESR spectra of Zn2Al-LDH, Zn3Al-LDH, Zn4Al-LDH, Zn5Al-LDH, and Zn6Al-LDH at room temperature.

In ZnAl2O4, 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 ZnAl2O4 is due to the presence of an interaction between the oxygen vacancies of ZnAl2O4 and the aluminium nucleus . 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 4s1 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 Zn2+/Al3+ molar ratio for ZnO phase and ZnAl2O4 spinel is plotted. The relative intensity of samples increases until Zn2+/Al3+ molar ratio = 5 which has the highest ESR intensity for ZnO phase, and then it starts to decrease. For the ZnAl2O4 spinel, the highest ESR intensity was at Zn2+/Al3+ molar ratio = 4. Aluminum and zinc cations in ZnAl2O4 have isotopes with nuclear spin of 5/2: 27Al and 67Zn. 27Al is much more abundant (100%) than 67Zn (4.11%), and its nuclear magnetic moment is higher (3.6385) than that of 67Zn (0.8753) [25]. The increase in Zn2+/Al3+ molar ratio causes a reducing of the magnetic activity of ZnAl2O4 because the nuclear magnetic moment of 67Zn is lower than 27Al. 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 ZnAl2O4 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.

639354.fig.004
Figure 4: Variation of the normalized ESR intensity as a function of pH value of the prepared samples.
3.5. Thermal Diffusivity

Thermal diffusivity () 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 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 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 Zn2+/Al3+ molar ratios. The values of thermal diffusivity for samples were found between 2.4 and 3.3 × 10−7 m2 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.

639354.fig.005
Figure 5: Thermal diffusivity versus temperature of Zn2Al-LDH, Zn3Al-LDH, Zn4Al-LDH, Zn5Al-LDH, and Zn6Al-LDH.

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 Zn2+/Al3+ 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 ZnAl2O4 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 Zn2+/Al3+ 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].

639354.fig.006
Figure 6: The thermal diffusivity as a function of Zn2+/Al3+ molar ratio of LDH for several in situ temperatures.
3.6. The Dc Conductivity

The dc conductivity or ionic conductivity () 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 (, , 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.

tab3
Table 3: Values of dc conductivity , preexponential factor and the fractional exponent for Zn–Al–NO3-LDH samples.
fig7
Figure 7: The fitting curves of the experimental data of versus frequency for (a) Zn2Al-LDH, (b) Zn3Al-LDH, (c) Zn4Al-LDH, (d) Zn5Al-LDH, and (e) Zn6Al-LDH.

The obtained values from the fit curves for all samples were plotted against Zn2+/Al3+ molar ratio as shown in Figure 8. This relation illustrated that dc conductivity decreased exponentially with the increase in the Zn2+/Al3+ 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.

639354.fig.008
Figure 8: The variation of dc conductivity versus Zn/Al molar ratio.

4. Conclusions

Powder XRD results showed that the crystallinity of Zn–Al–NO3-LDH phase decreased as Zn2+/Al3+ molar ratio increases due to the distortion of the hydroxide layers networks by the larger difference in ionic radii of Zn2+ and Al3+. Other phases of ZnO and ZnAl2O4 were formed for LDH with Zn2+/Al3+ 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 μm. The characteristic ESR signal of ions with D3h 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 Zn2+/Al3+ molar ratio of 3 and above, two ESR signals with stronger intensity were produced due to the formation of ZnAl2O4 and ZnO phases in LDH samples. Thermal diffusivity of LDH samples was found between 2.4 and 3.3 × 10−7 m2 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 Zn2+/Al3+ molar ratio increased which may be attributed to the decrease in the crystallinity of samples.

Conflict of Interests

The authors declare that they have no conflict of interests in this work.

Acknowledgments

This study was financed by ERGS/1/11/STG/UPM/02/4 (Postdoctoral Grant). Authors would like to thank Universiti Putra Malaysia (UPM) for supporting this work. A. A. A. Ahmed thanks RMC-UPM and Thamar University, Yemen, for the support.

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