Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Structural Analysis and Conduction Mechanisms in Polycrystalline Zinc Hydroxide Nitrate Christhy V. Ruiz,†,‡,§ Enrique Rodríguez-Castelloń ,⊥ and Oscar Giraldo*,‡,§,∥ Departamento de Física y Química, Facultad de Ciencias Exactas y Naturales, †Departamento de Ingeniería Química, Facultad de Ingeniería y Arquitectura, ‡Laboratorio de Materiales Nanoestructurados y Funcionales, Facultad de Ciencias Exactas y Naturales, and §Grupo de Investigación en Procesos Químicos, Catalíticos y Biotecnológicos, Universidad Nacional de ColombiaSede Manizales, Kilometro 9 vía al aeropuerto La Nubia, 170003 Manizales, Colombia ⊥ Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain Downloaded via UNIV OF NEW ENGLAND on July 12, 2018 at 06:40:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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ABSTRACT: The conduction and dielectric properties in zinc hydroxide nitrate (Z5HN) were studied in detail as a function of the temperature and relative humidity by impedance spectroscopy, and the structure was investigated by X-ray diffraction (XRD). Elemental analysis indicated a layered material containing carbonate anions [Zn5(OH)8(NO3)1.6(CO3)0.2·1.7H2O] due to the high capability of adsorption of Z5HN, which makes this material appropriate for applications in real conditions. The water content affected the interlayer distance, conductivity, and dielectric response of the layered material. An electrostatic repulsive interaction after reduction of the water content as a function of the temperature causes an increase of the interlayer distance and a decrease in the conductivity response and dielectric behavior. The highest conductivity, 10−7 Ω−1 cm−1, was obtained at a shorter interlayer distance for the sample heat-treated at 25 °C. The Z5HN synthesized was also characterized at different temperatures using thermogravimetric analysis and Fourier transform infrared and Raman spectroscopy. Multipeak analysis of the XRD patterns at various relative humidity levels showed the formation of a most hydrated phase and an increase of the interlayer distance related with the adsorption of water molecules. This layered material presented a conductivity of 10−5 Ω−1 cm−1 at high relative humidity (92%). The dipole−dipole interaction appeared to be the dominant mechanism for the dielectric behavior at the lowest temperatures and highest humidity due to the high water content in the Z5HN structure. Taking into account its crystallization water and high adsorption of water molecules in the interlayer region, a conduction pathway in the Z5HN structure was proposed, which provides the route for proton transport by hydrogen-bonding networks on the basis of a Grotthuss-type mechanism in facilitating the long-range proton hopping at 25 °C. The results for high relative humidity imply that a vehicular conduction mechanism also may contribute to the electrical response.
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INTRODUCTION The potential of layered compounds lies in their ability to exchange chemical species with electrical charges compatible with those of the layers to obtain functional materials and composites with improved properties, such as electrical conductivity. The layer electrical charge classifies the layered compounds. Some representative materials with negatively charged layers are clay minerals such as montmorillonite, which can exchange cationic species. In the group of layered materials with positively charged layers are found the layered double hydroxides (LDHs) and layered hydroxide salts (LHSs), which also are known as anionic clays because of their capability of anion exchange. The structures of the LHSs and LDHs arise from structural modification of the neutral brucite-like structure [Mg(OH)2]. When Mg2+ of the layers is partly replaced by M3+ and the excess of charge is neutralized by interlayer anions, LDHs are formed, whereas upon isomorphic substitution of cations into the layers or © XXXX American Chemical Society
substitution of hydroxide groups by suitable anion or water molecules, LHSs are formed. The ease of modification of these materials by ion exchange expands the functionality of these materials.1,2 Evaluation of the physical properties in the anionic clays such as the conduction mechanism and dielectric behavior has only been explored in the LDH family. However, the ion conduction and charge-carrier species in these materials are not well understood yet. In some studies on LDHs, the ion conductivity of Mg−Al LDH pellets has been evaluated by intercalating inorganic anions, using different Mg/Al ratios3,4 and different humidity and temperature conditions,5 and dielectric measurements at different molar ratios6 of Zn−Al LDHs, with anionic nitrate ions,7 have been performed. Nevertheless, the origin of the conduction mechanism from Received: April 18, 2018
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DOI: 10.1021/acs.inorgchem.8b01074 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Rietveld refinement to justify the origin of the conductivity response in the layered Z5HN material that can be extended to similar materials.
structural characterization is not well elucidated in LDHs. Also, the effect of Zn2+ substitution in Mg−Al LDHs on the ionic conductivity considering different conditions such as the Mg2+/Al3+ and Zn2+/Al3+ ratios, adsorbed water content, temperature, and humidity was carried out by Yamaguchi et al.,8 but no mechanism was proposed to explain the anionic conductivity. Furthermore, it is not clear if the anions or protons cause the dielectric and electrical responses of the LDHs. To our knowledge, studies about the electrical properties have not been performed in the subgroup of inorganic layered materials called LHSs. One representative LHS is zinc hydroxide nitrate (Z5HN). The structure of layered Z5HN with composition [Zn5(OH)8(NO3)2·2H2O] is shown in Figure 1. This layered material consists of infinite brucite-like
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EXPERIMENTAL SECTION
Materials and Methods. Zinc hydroxide nitrate (Z5HN) was prepared with zinc nitrate tetrahydrate [Zn(NO3)2·4H2O; Merck, 98.9%] and zinc oxide (ZnO; Sigma-Aldrich, 99−100%), considering the volume and concentration of method B described by Moezzi et al.14 The samples were synthesized by the addition, with constant stirring, of an aqueous suspension of ZnO (7.398 g of ZnO/60.0 mL of H2O) to 60.0 mL of an 1.50 M Zn(NO3)2·4H2O aqueous solution at room temperature. The mixture was kept under stirring for 24 h. After such a procedure, the slurry was centrifuged, washed with deionized water three times, and dried at room temperature. Structural Refinement. The Rietveld method was performed with the use of a GSAS program through the EXPGUI interface for structural refinement of the Z5HN material. The software allows several profiles and preferred orientation functions and was included with EXPGUI visual inspection to review the fit.15 The refinement was made stepwise until the convergence of χ2 to a minimum value. The background parameters for the histogram were fitted using the Chebyschev polynomial function (type 1) with eight terms, followed by refinement of the lattice parameters for the phase. The Crystallographic Open Database was used to obtain the CIF files of the crystalline phases, which were present in the sample.16 For the phase, the occupancies of each element in the structure as well as the thermal parameters Uiso were refined one time. Next, the pseudoVoigt function (type 4) was used for modeling of the peak shapes of the XRD pattern. The terms GW, GV, GU, LX, and LY were refined one at a time until convergence was achieved. The scaling factor was also refined using a damping of 5 for the scale. The preferred orientation of the Z5HN layered structure (200) was applied, and the intensity ratio of Cu Kα/Cu Kβ radiation was also considered during the refinement. Each step was repeated until the lowest value of χ2 was obtained and the difference between the calculated and observed patterns matched that of the correct crystal structure.17 The polyhedral model of the crystal structure was made with CrystalMaker software.18 Elemental Analysis. The composition of Z5HN was determined on a Leco TruSpec Micro CHNSO elemental analyzer based on the quantitative combustion technique of carbon, hydrogen, and nitrogen. The zinc content was determined using atomic absorption spectroscopy on a Thermo Scientific iCE 3000 AA spectrometer. Thermal Analysis. Thermogravimetric analysis/differential thermogravimetry (TGA/DTG) and differential scanning calorimetry (DSC) curves were recorded on TGA Q500 and DSC Q100 analyzers, respectively, from 25 to 250 °C at a heating rate of 5 °C min−1 to analyze the dehydroxylation and dehydration processes in Z5HN. Fourier Transform Infrared (FTIR) and Raman Spectroscopy. The FTIR spectra were measured in transmittance mode to powder samples using a Thermo Fisher mid-infrared FTIR Nicolet iS5 spectrometer. The spectra were recorded from 4000 to 400 cm−1 at a 4 cm−1 resolution. For preparation of the KBr pellet, powder samples were ground with KBr in an agate mortar to a fine powder and later pressed into a disk under high pressure. Raman vibrational spectroscopy was carried out with the 1 × 1 camera of a Bruker Senterra Raman microscope by averaging spectra for 60−90 min with a resolution of 3−5 cm−1. A CCD camera operating at −50 °C was used for Raman detection from 3500 to 200 cm−1. An Nd:YAG laser with an output power of 2 mW was used as the excitation source. PXRD. PXRD was carried out to identify the crystalline phases, changes in the interlayer distance, and crystallinity quality of the samples using a Rigaku Miniflex II diffractometer with Cu Kα radiation at 30 kV and 15 mA and in the range between 2θ = 3 and 70°. Heat treatment of the samples performed made from 25 to 225 °C with steps of 25 °C for 1 h in an oven. The XRD patterns were
Figure 1. Structure of Z 5 HN with the composition Zn5(OH)8(NO3)2·2H2O.
layers of zinc atoms surrounded by six OH groups in octahedral coordination, with one-quarter of those octahedral sites empty. These octahedral vacancies are occupied on either side by zinc atoms tetrahedrally coordinated by three OH groups (forming the base) and one water molecule occupying the apex of the tetrahedron. Unbonded nitrate ions that enable a charge balance in the structure and water molecules are located in the interlayer region.2,9 A high anion-exchange capacity and accessible routes of synthesis, comparable to those of LDHs, are attractive aspects of the LHS materials. These materials have been used in anion exchange,10 capture of CO2,11 and encapsulation and controlled delivery of bioactive molecules,12,13 but there is still a vast area to be explored to design new materials with adjustable properties. On the basis of the potential applications of LHSs as matrixes for the intercalation of organic and inorganic anions, several physicochemical characterizations have been done to explain the interaction between the layers and interlayer anions. However, the nature of the electric response has not been explored in the LHS family yet. Therefore, this study aims to determine the effect of the water content on the electrical and dielectric responses of a layered Z5HN material as a function of the temperature and percentage relative humidity (%RH) by analyzing the structural changes. In this context, a detailed structure of the layered Z5HN was obtained by using the Rietveld method analysis of the powder X-ray diffraction (PXRD) data. The electrical response and dielectric behavior, derived from impedance spectroscopy, were used to evaluate the effect of the water content inside the lamellar structure. According to the experimental results, we proposed a transport mechanism with the help of the structural information obtained by the B
DOI: 10.1021/acs.inorgchem.8b01074 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry measured at atmospheric conditions and room temperature (20 °C) after the heat treatment. The XRD measurements were conducted in three samples at each temperature, and the error of the interlayer distance and full width at half-maximum (fwhm) were calculated in terms of the standard deviation. The interlayer distance of the d200 diffracted plane was calculated using Bragg’s equation from the (200) reflection, and single-peak analysis of the PXRD pattern was applied to determine the fwhm to the same reflection. Analysis of the crystalline quality of the samples was done following the methodology ́ proposed by Rodriguez et al.19,20 For the samples exposed to various humidities, saturated salt solutions at controlled temperature, in a stove at 25 °C, were used to hydrate the powders following the procedure of the method based on the hygroscopicity of the salts.21 The weight control and relative humidity were measured daily until there was no substantial variation. The XRD patterns were measured at atmospheric conditions, room temperature (20 °C), after the weight of the sample reached equilibrium. The saturated salt solutions employed in this study and the %RH values achieved are summarized in Table 1.
Figure 2. XRD pattern for Z5HN with refined data obtained by the Rietveld method. The experimental points are shown as crosses (×), and the theoretical data are shown as a red solid line. The solid black line represents the difference between the theoretical and experimental data. The dashed lines (green) represent the 200 and 400 diffracted planes. (inset) Unit cell obtained from the pattern for Z5HN with refined data by the Rietveld method.
Table 1. Relative Humidity (%RH) Obtained under Various Saturated Salts saturated salt solution
%RH
BaCl2 KCl NaBr Mg(NO3)2 CH3COOK
92.7 86.0 71.5 69.5 58.0
parameters of elements for Z5HN achieved through structural refinement are presented in Table A1-1. Table 2. Structural Refinement Data for Z5HN cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z density (g cm−3) R χ2
Microscopy Characterization. Scanning electron microscopy (SEM) was carried out using a JEOL JSM-6490LV scanning electron microscope operating at an accelerating voltage of 0.3−30 kV and the maximum magnification of which was 300000×, with such a process used to determine the morphology of the samples with heat treatment. Electrical Properties and Dielectric Behavior. Impedance spectroscopy provides a correlation between the microstructure, defects, transport mechanism, and chemical composition among other properties and the performance, behavior, or quality of the electrochemical and electronic processes in different systems.22 In this study, the conductivity and dielectric properties of powders was measured as a function of the temperature and %RH with a 1296-4A test cell by using Solartron SI 1260 impedance/gain phase analyzer with the Dielectric Interface 1296 in the frequency range of 10 MHz to 0.1 Hz. The measurements as a function of the temperature were carried out from 25 to 125 °C at an applied voltage amplitude of 100 mV rms. Measurements as a function of the relative humidity were taken at 20 ± 1 °C with a voltage amplitude of 800 mV rms. The experimental results of the electrical response at various temperatures and humidities were fitted through the circuit equivalent using ZView (Scribner Association) software.23
C-centered monoclinic C2/m 19.491 6.248 5.525 90.00 93.34 90.00 671.7 8 2.987 0.1391 8.488 (13 variables)
Elemental Analysis. The elemental composition of Z5HN is given in Table 3. The value of the nitrate is lower than that Table 3. Elemental Analysis of Z5HN Calculated Formula Zn5(OH)8(NO3)1.6(CO3)0.2·1.7H2O
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theoretical values (wt %)
RESULTS AND DISCUSSION Structural Refinement. The PXRD pattern of Z5HN was identified as belonging to a monoclinic cell and the C2/m space group with cell parameters a = 19.491 Å, b = 6.248 Å, c = 5.525 Å, and β = 93.345°, coinciding 99.99% with the previously reported data.24,25 The Rietveld method was used to refine and determine the structure of the material. The refinement gave acceptable statistical measures and a good visual fit with χ2 = 8.488 and an R factor of
