Stacking Faults and Polytypes for Layered Double Hydroxides: What

Dec 2, 2016 - In this paper we present an comprehensive study on a possible structural disorder in LDH. We show how X-ray powder diffraction (XRPD) ca...
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Stacking Faults and Polytypes for Layered Double Hydroxides: What Can We Learn from Simulated and Experimental X‑ray Powder Diffraction Data? Wojciech A. Sławiński,*,†,‡ Anja Olafsen Sjåstad,† and Helmer Fjellvåg† †

Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, PO Box 1033, N-0315 Oslo, Norway ‡ ISIS Facility, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire, OX11 0QX United Kingdom ABSTRACT: Layered double hydroxides (LDH) are a broad group of widely studied materials. The layered character of those materials and their high flexibility for accommodating different metals and anions make them technologically interesting. The general formula for the LDH compound is n− II [MII1−xMIII x (OH)2][A ]x/n·mH2O, where M is a divalent metal III cation which can be substituted by M trivalent cation, and An− is a charge compensating anion located between positively charged layers. In this paper we present a comprehensive study on possible structural disorder in LDH. We show how X-ray powder diffraction (XRPD) can be used to reveal important features of the LDH crystal structure such as stacking faults, random interlayer shifts, anion−molecule orientation, crystal water content, distribution of interlayer distances, and also LDH slab thickness. All calculations were performed using the Discus package, which gives a better flexibility in defining stacking fault sequences, simulating and refining XRPD patterns, relative to DIFFaX, DIFFaX+, and FAULTS. Finally, we show how the modeling can be applied to two LDH samples: Ni 0.67 Cr 0.33 (OH) 2 (CO 3 ) 0.16 ·mH 2 O (3D structure) and Mg0.67Al0.33(OH)2(NO3)0.33 (2D layered structure).

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INTRODUCTION Layered double hydroxides (LDH) belong to a widely studied class of materials that has given rise to ∼3000 peer reviewed papers during the past three years.1 Their layered character and the high flexibility for accommodating different metallic cations and anions make them suited toward technological applications in several fields including (photo) catalysis, gas sorption and separation, medicine, pigments, thermal barriers, polymer fillers, and fire retardants.2−10 The LDH crystal structure consists of a stacking of twodimensional (2D) positively charged brucite-like layers, separated by organic or inorganic anions (An−) and crystal II water, having the composition [M1−x MxIII(OH)2][An−]x/n· II III mH2O where M and M are cations with ionic radius similar to Mg2+, and m is the amount of crystal water and is often found to be 97%), 0.0334 mol of Cr(NO3)3·9H2O (>99%), 0.33 mol of CO(NH2)2 (>99%), all from Sigma-Aldrich] were mixed in a round-bottom flask under magnetic stirring and refluxed at 100 °C for 48 h before being transferred to a Teflon lined steel autoclave for hydrothermal post treatment at 220 °C for 48 h. The obtained product was thereafter washed four times in deionized water and freeze-dried. Thermal gravimetric analysis indicated m ≈ 0.5. Mg0.67Al0.33(OH)2(NO3)0.33·mH2O was synthesized by coprecipitation at 60 °C and constant pH (10.0) under inert conditions.37 Reactants were Mg(NO3)2·6H2O (99%), Al(NO3)3·9H2O (98%), KNO3 (≥99%), and KOH (≥85%) from Sigma-Aldrich. Deionized type II water was boiled under He bubbling to remove any dissolved CO2; pellets of KOH were stored under inert conditions; and KOH solutions were prepared in an argon tent. The reaction vessel was typically filled with 100 mL of 0.15 M KNO3. A few milliliters of ∼0.85 M KOH was added with a peristaltic pump until the desired pH

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RESULTS AND DISCUSSION Simulations of X-ray Powder Patterns. In the following we present a few series of simulated XRPD patterns where different parameters describing the LDH structure such as the probability of stacking faults (pABC), probability of random xyshifts (prandom), H2O content (m), interlayer anion orientation, LDH slab thickness, and width of Gaussian distribution of interlayer distances (csigma) are systematically varied in order to 12883

DOI: 10.1021/acs.inorgchem.6b02247 Inorg. Chem. 2016, 55, 12881−12889

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Figure 3. Series of XRPD patterns of LDH with probability of stacking faults pABC = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1.0. The (hkl) indices and Bragg reflection positions correspond to the hexagonal unit cell of the ideal ABC... stacked structure. The probability of stacking faults pABC = 0.5 describes fully random stacking of LDH layers of ABC or CC′ type.

Figure 4. Series of XRPD patterns of LDH with probability of random xy-shif t prandom = 0.0, 0.25, 0.5, 0.75, and 1.0. The (hkl) indices and Bragg reflection positions correspond (for purpose of comparison) to the hexagonal unit cell of an ideal ABC... stacked structure. The probability of xy-shif t prandom = 1.0 describes fully disordered structure of LDH layers with no xy-interlayer correlations; prandom = 0.0 corresponds to the CC′ stacking type (2H1). pABC = 0.0 in all simulations.

with significant levels of stacking faults. Figure 3 reveals that a change in the stacking fault probability pABC does not affect (0 0 l) Bragg reflections, whereas reflections with mixed h, k, and l indices show significant peak broadening and asymmetry. The most disordered structure with pABC = 0.5 contains the fewest Bragg reflections, mainly belonging to the (00l) and (11l) families. Probability of Random xy-shif t (prandom). The second type of disordered 3D structures to consider is when LDH layers are stacked in a constant distance along the z-axis but show no interlayer xy-correlation. The degree of this type of disorder is defined by the probability of random xy-shif t (prandom). We assume here that the probability of stacking faults pABC = 0.0. For prandom = 0.0 the structure is the CC′ stacking type (2H1) whereas for prandom = 1.0 a structure with 100% randomly xy-shifted layers is obtained with the interlayer distance conserved. Figure 4 shows a series of calculated XRPD

investigate the effect these parameters have on the appearance of the powder diffractograms. Stacking Fault Probability pABC. One of the main issues with modeling the LDH crystal structure is the frequently arising stacking faults. Figure 3 shows a series of XRPD patterns where the effect of implementing stacking faults to the 3R1 (pABC = 1.0) and the 2H1 (pABC = 0) polytypes is examined by assuming different probability of stacking faults (pABC). For example, pABC = 0.0 and pABC = 1.0 describe three-dimensional well-defined periodic structures that can be modeled by standard crystallographic methods such as Rietveld refinements. The XRPD patterns of these two polytypes are characterized by narrow Bragg reflections with symmetric peak shapes. On the other hand, all XRPD patterns for structures with stacking fault probability 0.0 < pABC < 1.0 show asymmetry and broadening of the diffraction peaks. This is the main reason why the interpretation of XRPD patterns becomes difficult for structures 12884

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Figure 5. Series of XRPD patterns of LDH with variable water content H2O, m = content rate of 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5. The (hkl) indices and Bragg reflection positions correspond to the hexagonal unit cell of ideal ABC... stacked structure. m = 0 gives the water free variant whereas m = 0.5 gives the water content reported for hydrotalcite. The break in x-scale is done to show a significant change of the intensity ratio of (003) and (006) Bragg reflections. pABC = 0.5 in all simulations.

Figure 6. Series of XRPD patterns of LDH for orientation of NO3− anions: randomly rotated along 3-fold axis and placed vertically, tilted by 45° and horizontally, and lying horizontally, ordered as shown in Figure 1. pABC = 0.5 in all simulations.

patterns for structures described by probability of random xyshift 0.0 ≤ prandom ≤ 1.0 and pABC = 0.0. One can observe that the Bragg reflections of (00l) type remain unchanged since the interlayer distance cdist is conserved. Also the family of reflections (hkl) with h − k = 3n is not affected by stacking faults in the structure. On the other hand the reflections from the (10l) and (11l) families merge together giving a Warren type peak shape (a sharp left-hand side slope and a very long tail on the right).40 Interlayer H2O Content (m). It is well-known that the LDH structure reversibly absorbs and desorbs water depending on temperature and water vapor pressure.41 When simulating the effect of interlayer water content (m) on the appearance of the XRPD patterns, the H2O molecules are located exactly in the middle of the two LDH layers in between OH− groups from the consecutive LDH layers. The H2O content equal to 100% corresponds to the case where every OH− group has a H2O molecule attached (m = 0.5 in the formula). The 0% H2O

content reflects the lack of water in the structure, i.e., the fully dehydrated LDH (m = 0). In the real cases the value of m = 0.5 can be achieved depending on a number of anions present in between the LDH layers. The series of calculated XRPD patterns for 0.0 ≤ m ≤ 0.5 is presented in Figure 5. In the simulations pABC = 0.5 was kept constant for all compositions. From Figure 5 it is clear that the interlayer water content (m) affects the calculated XRPD patterns, but to a moderate extent. Note that the 2θ scale close to the (003) Bragg reflection is expanded. This is because the most significant change between the calculated patterns can be observed on the intensity ratio between (003) and (006) peaks. Also a smaller change in the intensity ratio between other reflections can be observed. This example shows that the interlayer water cannot be omitted if a precise crystal structure description of LDH is required. Here, we assume that the interlayer distance does not depend on the H2O content. In real cases this effect is observed, and so differences are likely to be more significant. 12885

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Figure 7. Series of powder diffraction patterns of LDH for different thickness of the crystalline slabs equal to 1, 2, 5, and 10 LDH layers in comparison with the bulk LDH. The (hkl) indices and Bragg reflection positions correspond to the hexagonal unit cell of ideal ABC... stacked structure. pABC = 1.0 in all simulations.

Figure 8. Series of powder diffraction patterns of LDH for different width of a Gaussian distribution of interlayer spacing csigma equal to 0.0, 0.025, 0.050, and 0.100. The (hkl) indices and Bragg reflection positions correspond to the hexagonal unit cell of ideal ABC... stacked structure. pABC = 1.0 in all simulations.

Anion Molecule Orientation (An−). In this section we present a series of calculated XRPD patterns obtained for different orientations of the anion molecules that are located between the LDH layers. In this case we exchanged CO32− with NO3− in order to make the effect of different anion orientations more pronounced. Note that for monovalent nitrate anions their orientation changes from flat to highly tilted at high anion concentrations in the interlayer gallery space of Mg−Al LDH.41 The probability of stacking faults is set to pABC = 0.5 in all simulations, and the results are presented in Figure 6. The three upper powder diffraction patterns in Figure 6 present results obtained for vertical, tilted 45°, and horizontal NO3− which were randomly rotated in the plane of the molecule. The bottom powder diffraction pattern is calculated for a model structure where the NO3− anions are lying horizontally between the LDH layers and arranged in such a way that N−O bonds are pointing toward OH− groups (Figure 1). A careful inspection of the calculated XRPD patterns shows that there

are no significant changes for different NO3− tilt angles. The most visible change is observed in the ordered/disordered case of horizontal anion orientation (two bottom patterns on Figure 6, the regions of interests are 10.5° ≤ 2θ ≤ 12.0° and 18.5° ≤ 2θ ≤ 19.5°). This is an important difference when analyzing the real samples of LDH (see Discus Refinements of Experimental Powder X-ray Diffraction Patterns). In this example we did not take into account possible change of the interlayer distances nor distributions of tilt angles of interlayer anions. Thickness of the LDH Crystalline Slabs and Gaussian Distribution in Interlayer Distances. Finally, a series of XRPD patterns were calculated for different LDH slab thicknesses and different widths of Gaussian distribution of interlayer distances. The layers within the slab are assumed ordered with the ABC stacking sequence with pABC = 1.0. Figure 7 shows calculated XRPD patterns for LDH slabs containing 1, 2, 5, and 10 layers as well for the bulk structure. One observes the characteristic Warren type peak shape40 for 12886

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Inorganic Chemistry the XRPD pattern of a single LDH layer. In this case no (00l) reflections can be observed since there is no correlation between the layers. This should be compared with the calculated pattern for a 3-dimensional structure with random xy-shifts (Figure 4) (a very broad XRPD maximum is arising when increasing slab thickness). It is also worth noting that in the case of (003) and (006) the 2θ location of the highest intensity within the profile becomes slightly shifted toward lower scattering angles on decreasing slab thickness. This phenomenon can be misleading for those who want to estimate the interlayer distance just by measuring the scattering angle (and derive d-values) of the highest intensity point. It is no longer possible to calculate c lattice parameter from the position of (0 0 3) and (0 0 6) Bragg reflections. Figure 8 presents XRPD patterns calculated assuming different widths of the Gaussian distribution csigma of interlayer distances. csigma = 0.0 means that all LDH layers are separated by exactly equal distances, whereas csigma = 0.1 represents the case of a Gaussian distribution of LDH distances along the zaxis of 0.1 (in z units). In this case one can observe significant broadening of (00l), (10l), and (11l) family of reflections where the broadening rapidly increases with increasing l index.

Figure 9. XRPD pattern for Ni0.67Cr0.33(OH)2(CO3)0.16·mH2O. Points represent measured data, and the line represents the calculated pattern. Below are shown the difference curve, and peak positions for a structure with perfect ABC stacking. The inset shows a zoomed area in order to present most characteristic XRPD features of a stacking faulted structure with the probability of stacking faults pABC = 0.5.

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DISCUS REFINEMENTS OF EXPERIMENTAL POWDER X-RAY DIFFRACTION PATTERNS In this section we use Discus refinements against experimentally observed XRPD patterns to describe the crystal structure of the two newly synthesized Ni0.67Cr0.33(OH)2(CO3)0.16· mH2O and Mg0.67Al0.33(OH)2(NO3)0.33·mH2O LDH samples. Those two examples will demonstrate the ability of Discus refinements to model complex crystal structures. In order to perform those refinements several aspects of structural disorder have to be combined which were discussed separately in the previous sections. Ni0.67Cr0.33(OH)2(CO3)0.16·mH2O at Room Temperature. The crystal structure of Ni0.67Cr0.33(OH)2(CO3)0.16·mH2O is modeled assuming a stacking fault type structure. The CO32− anions are ordered and horizontally placed in the interlayer gallery. Figure 9 shows the observed and calculated XRPD patterns, and there is a very good agreement between the measured and calculated patterns. The observed Warren type peak shape and anisotropic broadening are precisely reproduced assuming a structural model of stacking faults. Table 1 gives a summary of the crystal structure parameters obtained from the refinements. It can be concluded that Ni0.67Cr0.33(OH)2(CO3)0.16·mH2O is fully disordered with refined probability of stacking faults pABC = 0.480(16). The CO32− anions are ordered and horizontally located between the LDH layers. The H2O content (m) is equal to 0.497(38), which is in line with what was determined experimentally (see Simulations and Experimental Details). Mg0.67Al0.33(OH)2(NO3)0.33·mH2O Random xy-shift Type Crystal Structure. In order to demonstrate the possibility of crystal structure modeling in the case of 2D flakes of LDH layers and a random xy-shift type stacking, the water free Mg0.67Al0.33(OH)2(NO3)0.33 LDH sample heat treated at 150 °C was chosen as an example. The XRPD pattern reveals very poor sample crystallinity. However, a crystal structure model can be evaluated and a refinement conducted. In order to be able to study structural details, a specific data treatment was applied to obtain F(Q) (see Simulations and Experimental Details).

Table 1. Summary of the Crystal Structure Parameters of Ni0.67Cr0.33(OH)2(CO3)0.16·mH2O parameter aL [Å] cL [Å] cdist [Å] csigma pABC H2O content m

3.06599(85) 1.975(55) 7.5401(79) 0.0246(40) 0.480(16) 0.497(38)

The final model for Mg0.67Al0.33(OH)2(NO3)0.33 represents a mixture of 2D LDH layers without any 3D nature as the majority phase fraction of 89% and a random xy-shifted type of structure minority phase of 11%. The probability of random xyshif t is prandom ≈ 1.0, and no interlayer H2O was assumed. Figure 10a presents the measured intensity profile for Mg0.67Al0.33(OH)2(NO3)0.33 (solid line) and background capillary (dashed line). This reveals the difficulty in the intensity versus scattering vector refinement. One can conclude that the intensity and position of the (003) low Q peak is very dependent on the background subtraction, therefore it was excluded for the data refinements. Figures 10b and 10c show the measured and simulated structure factors, respectively, along with Bragg positions and difference curve. One can clearly observe characteristic Warren type reflections.40 It is important to note that structural information can be obtained from the high scattering vector Q range part of the structure factor, even though it would not be possible from the Bragg peak intensity alone. Table 2 gives a summary of the crystal structure parameters of Mg0.67Al0.33(OH)2(NO3)0.33 heat treated at 150 °C.

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CONCLUSIONS Due to the high probability of stacking faults for layered compounds, analysis of their XRPD patterns by standard methods such as Rietveld refinements is typically quite insufficient. On the other hand, major insight can be obtained from the total scattering in the XRPD patterns, and in line with 12887

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laboratories according to their published XRPD patterns. This analysis tool may pave the basis for tuning LDH materials with respect to small, yet very significant differences in layer stacking properties. These may in turn correlate with chemical and physical properties, for instance the ability to undergo delamination.36,42,43 Further systematic studies may provide the answers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wojciech A. Sławiński: 0000-0002-9578-0374 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Research Council of Norway, Grant No. 221905 (FRIPRO). The authors acknowledge the assistance from the research team at the Swiss-Norwegian Beamlines (SNBL), ESRF, and Erik Glesne for carrying out some of the synthesis work. We also acknowledge Prof. Reinhard Neder (University of Erlangen-Nurnberg, Germany) for significant improvements of the Discus program.

Figure 10. XRPD pattern for Mg0.67Al0.33(OH)2(NO3)0.33 heat treated at 150 °C (panel a, solid line) together with the background subtracted (dashed line). Panels b and c show measured and refined structure factor F(Q). Below are shown the Bragg peak positions and the difference curve. All patterns are shown in scattering vector Q scale. See text for the structure description details.

Table 2. Summary of the Crystal Structure Parameters of Mg0.67Al0.33(OH)2(NO3)0.33 Heat Treated at 150 °C

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parameter fraction of 2D phase fraction of 3D phase aL [Å] cL [Å] cdist [Å] csigma prandom H2O content m

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0.889(12) 0.111(12) 3.05840(25) 2.022(11) 7.07(15) 0.09(3) 0.98(5) 0.0

this we successfully modeled the LDH crystal structure using the Discus package. We have demonstrated in detail how different types of disorder affect the XRPD patterns. Quantitative parameters describing the level of disorder can be determined for a real compound: probability of ABC stacking (pABC) and probability of random xy-shift (prandom). In particular, we showed how the patterns are sensitive to parameters such as H2O content, interlayer anion orientation, thickness of LDH slabs, and Gaussian distribution of interlayer distances. The approach was benchmarked by refining a structural model for a real LDH from experimental XRPD data; Ni0.67Cr0.33(OH)2(CO3)0.16·mH2O was described using a stacking fault type disorder model whereas dehydrated Mg0.67Al0.33(OH)2(NO3)0.33 was modeled by assuming 2D LDH flakes as the majority phase. The current simulations of XRPD patterns provide a series of distinct features that can be used as fingerprints in evaluation of which type of stacking fault is dominating in a given synthesized LDH. We demonstrate how such features are observed both in the intensity I(2θ) profile and in the structure factor F(Q). Obviously certain intensity features will also depend strongly on the chemical composition of the LDH; i.e., type of cations (light versus heavy) and type of anions (spherical, triangular, tetrahedral, light, or heavy). The 100% ordered 3R1 and 2H1 polytypes represent ideal situations hardly realized in any synthesized material. Our simulations open up for evaluation of a huge number of synthesized LDH materials in various 12888

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DOI: 10.1021/acs.inorgchem.6b02247 Inorg. Chem. 2016, 55, 12881−12889