Covalent and Ionic Functionalization of HLN ... - ACS Publications

Dec 21, 2016 - We present a broad overview of the intercalation of organic molecules in the Dion−Jacobson layered perovskite HLN, whose crystal stru...
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Covalent and Ionic Functionalization of HLN Layered Perovskite by Sonochemical Methods Francesco Giannici,*,† Adriana Mossuto Marculescu,† Alice Silvia Cattaneo,‡ Cristina Tealdi,‡ Piercarlo Mustarelli,‡ Alessandro Longo,§,∥ and Antonino Martorana† †

Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Viale delle Scienze ed. 17, I-90128 Palermo, Italy Dipartimento di Chimica Sezione di Chimica-Fisica, Università degli Studi di Pavia, Via Taramelli 16, I-2700 Pavia, Italy § CNR−Istituto per lo Studio dei Materiali Nanostrutturati, Via U. La Malfa, I-90146 Palermo, Italy ∥ Netherlands Organization for Scientific Research at ESRF, F-38043 Grenoble, France ‡

S Supporting Information *

ABSTRACT: We describe the functionalization of the layered perovskite HLaNb2O7 with n-propanol, n-decanol, 3-mercaptopropyl-trimethoxysilane, imidazole, n-decylamine, and histamine. The use of sonication is found to significantly improve the reaction yield and to reduce the reaction time, compared to conventional thermal treatment under reflux. The obtained intercalates are thoroughly characterized through the use of several complementary experimental techniques (scanning electron microscopy, IR spectroscopy, X-ray diffraction, thermogravimetric analysis, magic-angle spinning NMR), clarifying their structure and chemical bonding. The implications for the design of inorganic−organic composite materials are discussed.

1. INTRODUCTION Layered perovskites show several properties due to their peculiar structure, ranging from superconductivity to photocatalysis and from ion conduction to luminescence.1−6 Their functionalization with organic molecules or biomolecules has attracted significant research efforts devoted to the development of hybrid nanocomposites for various applications in the last years (e.g., catalysis, H2 photogeneration, biosensors), as witnessed by the recent literature.7−9 The Dion−Jacobson layered perovskite RbLaNb2O7 (RLN) is constituted by bidimensional lanthanum niobate sheets, separated by a layer of Rb+ cations. The protonated form HLaNb2O7 (HLN) is usually prepared by treatment of RLN with nitric acid. HLN is a solid acid, and it can intercalate various organic amines via acid−base reaction.10,11 HLN also reacts covalently with several other compounds, forming hybrid composites characterized by the presence of Nb−O−C and Nb−O−P bonds, for example, alcohols,12,13 organophosphonic acids,14 glucose derivatives,15 and trifluoroacetate groups.16 During such functionalization reactions, the structure of the perovskite layers remains largely unmodified, while the interlayer space expands to accommodate the intercalated moieties. This provides an attractive way to prepare highly ordered inorganic−organic hybrid materials, possessing various functional groups, which combine the structural stability and thermal resistance of the inorganic scaffold with the chemical flexibility of the organic moieties. The growing interest in developing innovative or high-performing synthetic approaches for the functionalization of Dion−Jacobson layered perovskites is demonstrated by recent literature.17,18 © XXXX American Chemical Society

The intercalation of organic molecules inside layered perovskites under reflux is generally slow. Indeed, syntheses usually require long times (days or weeks) and high temperatures. Moreover, the intercalation of certain molecules cannot be reached by direct reaction with the protonated form of layered perovskites; instead, the preparation of intermediates is required.13,14 These steps increase the preparation times and the risk of failure of the synthesis procedures. Here we consider the intercalation of six molecules in the HLN layered structure: as it is shown below, three of them can form covalent bonds with the inorganic scaffold, while the other can be included through an ionic interaction. This approach allows to obtain a broad overview concerning (a) the suitability of HLN to be used as a layered support for functionalization; (b) the synthetic procedures suitable to maximize yield and to reduce reaction times; and (c) the analytical techniques that constitute a valuable tool for detecting and quantifying the intercalation of the organic moieties in HLN, and studying their chemical environment. In particular, we found that sonication, using a common laboratory ultrasonic bath, significantly enhances the rates of the intercalation reactions in this system, also reducing the reaction time (by a factor of ∼10) and avoiding intermediate steps. In some cases, we found that sonication is necessary to achieve intercalation, since refluxing at high temperatures alone is not effective. In recent years, sonochemical methods have been extensively applied as an important tool for different types of chemical reactions that can Received: October 24, 2016

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80 °C for 5 d. The product was recovered by filtration on a membrane filter, washed with methanol and acetone, and dried under vacuum. Reactions with Imidazole. Thermal Intercalation. HLN (0.5 g) was dispersed in 20 mL of tetrahydrofuran (THF) containing 0.5 mL of water and 1.8 g of imidazole, and the mixture was stirred under reflux at 60 °C for 10 d. The powder was recovered by filtration on membrane filter, washed with THF and acetone, and dried under vacuum at room temperature. Intercalation by Ultrasound. HLN (0.5 g) was dispersed in 20 mL of THF containing 0.5 mL of water and 1.8 g of imidazole. The mixture was sonicated for 10 h using an ultrasonic bath. The powder was recovered by filtration on membrane filter and washed with THF and acetone. Finally, the solid product was dried under vacuum at room temperature. Reactions with n-Octylamine and n-Decylamine. Thermal Intercalation. HLN (0.5 g) was added to a solution of n-octylamine or n-decylamine in heptane (50 mass %), and the resultant mixture was stirred under reflux at 90 °C for 3 and 4 d, respectively. The product was recovered by filtration on membrane filter, washed with heptane, ethanol, and acetone, and finally dried under vacuum to obtain a white powder. Intercalation by Ultrasound. HLN (0.5 g) was added to a solution of n-octylamine or n-decylamine in heptane (50 mass %), and the suspension was sonicated for 10 h using an ultrasonic bath. The powder was recovered by filtration on membrane filter, washed with heptane, ethanol, and acetone, and dried under vacuum at room temperature. Reactions with Histamine. Thermal Intercalation. n-OctylamineHLN (0.5 g; prepared by sonication) was dispersed in 14 mL of ethanol containing 3 mL of water and 0.6 g of histamine, and the mixture was heated at 66 °C for 6 d. The powder was recovered by filtration on membrane filter, washed with methanol and acetone, and finally dried under reduced pressure. Intercalation by Ultrasound. n-Octylamine-HLN (0.5 g; prepared by sonication) was dispersed in 10 mL of methanol containing 4 mL of dimethyl sulfoxide and 0.6 g of histamine, and the mixture was sonicated for 15 h using an ultrasonic bath. The product was recovered by filtration on membrane filter, washed with methanol and acetone, and finally dried under reduced pressure. 2.2. Characterization. SEM images in secondary-electron mode were acquired with an FEI Quanta 200 microscope equipped with an EDAX energy-dispersive fluorescence analyzer using a 20 kV electron beam. To avoid sample charging, the chamber was operated in environmental mode, with fixed 0.8 mbar pressure. XRD patterns were acquired with a Siemens D500 diffractometer in Bragg−Brentano geometry, using Cu Kα radiation and a graphite monochromator on the diffracted beam. TGA from 5 to 700 °C was conducted in nitrogen atmosphere using a TA Q5000 thermogravimetric analyzer. After mixing the samples with KBr and pressing into a pellet, IR spectra were recorded from 250 to 4000 cm−1 using a PerkinElmer Frontier FT-IR spectrometer. Solid-state NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer at room temperature. 1H and 13C magic-angle spinning (MAS) NMR experiments were obtained using a 4 mm MAS probe operating at a spinning speed of 10 kHz. 1H one-pulse experiments were recorded using a 90° pulse of 4.3 μs and a recycle delay of 8 s. 13C(1H) cross-polarization (CP) MAS experiments were acquired using a 1H 90° pulse of 4.5 μs, 1 or 3 ms as contact time, SPINAL-64 decoupling scheme, and a recycle delay of 4 s. For both nuclei, chemical shifts (δ) are reported relative to tetramethylsilane (TMS), using adamantane as the secondary standard. 29Si(1H) CP MAS NMR spectra were acquired in a 7 mm MAS probe operating at the spinning speed of 5 kHz and using 1H 90° pulse of 5.5 μs, contact time of 3 ms, SPINAL-64 decoupling scheme, and recycle delay of 6 s. Chemical shifts are reported relative to TMS, using tetrakis(trimethylsilyl)silane as a secondary standard, with δCS = −9.8 ppm for the trimethylsilyl groups.

be performed in higher yields, with shorter reaction times or milder conditions.19,20 In this work, HLN was intercalated with n-propanol, ndecanol, 3-mercaptopropyl-trimethoxysilane, imidazole, n-decylamine, and histamine: the resulting products are labeled PRO-HLN, DEC-HLN, MPTMS-HLN, IMI-HLN, DECAMHLN, and HIST-HLN, respectively. For each composition, two sets of samples were prepared using a conventional thermal treatment under reflux (labeled T) or using the sonication method (labeled S). To assess the efficiency of the two synthetic methods, and to explore peculiarities and limits of organic functionalization in HLN, all samples were thoroughly characterized from a structural and a chemical point of view. In particular, scanning electron microscopy (SEM) was used to assess the particle morphology and their average size, while the increase of the spacing between the perovskite layers due to intercalation was measured by X-ray diffraction (XRD). The presence of the desired functional groups was determined with infrared (IR) spectroscopy, and the amount of the intercalated organic moieties was determined by thermogravimetric analysis (TGA). Finally, multinuclear solid-state nuclear magnetic resonance (NMR) was used to study the nature of the chemical bonds between inorganic layers and organic molecules and to explore the modifications induced on the chemical environment of the HLN protons by the organic functionalization.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Preparation of HLN. RbLaNb2O7 was prepared from a mixture of Rb2CO3, La2O3, and Nb2O5 by calcining at 1000 °C for 8 h. A 30% molar excess of Rb was added to compensate for the loss of Rb during calcination. HLN was prepared by treatment of 2 g of RbLaNb2O7 with 40 mL of HNO3 (6M) at 60 ◦C for 72 h.13 The product was centrifuged, washed with distilled water, and filtered under vacuum on a membrane filter. Finally the product was dried at 80 °C overnight, and the phase purity was checked with XRD. Reactions with n-Propanol. Thermal Intercalation. HLN (1 g) was stirred in 25 mL of n-propanol containing 3.6 mL of water under reflux at 80 °C for 6 d.13 The resultant product was recovered by filtration on membrane filter and dried under vacuum to obtain a white powder. Intercalation by Ultrasound. The mixture of HLN, n-propanol, and water was irradiated with ultrasonic wave at room temperature for 8 h using an ultrasonic bath. Subsequently the mixture was stirred under reflux at 80 °C for 3 d. The product was recovered by filtration on membrane filter and dried under vacuum. Reactions with n-Decanol. Thermal Intercalation. PRO-HLN (1 g) was stirred in 30 mL of n-decanol containing 1 mass % of water under reflux at 80 °C for 7 d.13 The resultant product was recovered by filtration on membrane filter and dried under vacuum to obtain a white powder. Intercalation by Ultrasound. PRO-HLN (0.6 g) was dispersed in 20 mL of decanol containing 2 mass % of water. The mixture was irradiated with ultrasonic wave at room temperature for 15 h using an ultrasonic bath. Subsequently the mixture was stirred under reflux at 80 °C for 3 d. The product was recovered by filtration on membrane filter, washed with acetone, and dried under vacuum. Reactions with 3-Mercaptopropyl Trimethoxysilane. Thermal Intercalation. DEC-HLN (0.4 g) was dispersed in 5 mL of 3mercaptopropyl trimethoxysilane (MPTMS), and the mixture was stirred under reflux at 80 °C for 6 d. The powder was recovered by filtration on membrane filter, washed with methanol and acetone, and dried under vacuum. Intercalation by Ultrasound. DEC-HLN (0.4 g) was dispersed in 5 mL of MPTMS, and the mixture was sonicated for 12 h using an ultrasonic bath. Subsequently the mixture was stirred under reflux at B

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3. RESULTS AND DISCUSSION The total amount of intercalated molecules in all samples, derived from TGA and expressed as molecules per HLN formula unit, is shown in Table 1. Table 1. TGA Weight Loss from 100 to 700 °C and Amounta of Organic Molecules Intercalated in the Samples

PRO-HLN_T PRO-HLN_S DEC-HLN_T DEC-HLN_S MPTMS-HLN_T MPTMS-HLN_S DECAM-HLN_T DECAM-HLN_S HIST-HLN_T HIST-HLN_S IMI-HLN_T IMI-HLN_S

weight loss (%)

intercalated molecules (mol/mol HLN)

4.03 6.01 5.38 11.21 4.88 11.16 13.55 13.13 2.85b 6.17b 10.13 7.71

0.205 0.419 0.107 0.313 0.083 0.300 0.360 0.346 0.032 0.169 0.566 0.386

Figure 2. XRD pattern of HLN. The most intense Bragg peaks are labeled with their Miller indices.

In the following discussion, a shift of the (001) peak from 8° 2θ to lower angles is used to detect the enlargement of the layers following the intercalation of organic molecules. The HLN platelets have a diameter between 500 nm and 5 μm, with most particles in the 2−3 μm range. The TGA trace of HLN (see Supporting Information) shows a small loss of water at 100−200 °C (amounting to ∼0.1 H2O molecules per HLN formula); at 400−550 °C, a characteristic loss of 2.05% corresponds to the structural collapse of the layered structure with loss of water, according to the reaction 2HLaNb2O7 → La2Nb4O13 + H2O. This same weight loss is also observed in all HLN derivatives, being due to the thermal degradation of the inorganic perovskite layers. The 1H MAS NMR spectrum of HLN shows three main peaks at 5.7, 8.8, and 10.3 ppm (see Figure 3a). The broad peak

a

Expressed as Moles per HLN unit formula. bFor the HIST-HLN samples, the weight loss from 170 to 700 °C is reported, as there is substantial weight loss in the 100−170 °C region due to adsorbed histamine.

In general, the overall particle morphology is the same in both HLN and intercalated derivatives, as it derives from the layered perovskite structure: the powders are composed of very thin discs with diameters ranging from 100 nm to 8 μm (see Supporting Information). The size distribution is found to be dependent on the preparation route; in particular, all samples show a substantial fraction of small particles (100−200 nm), while particles up to 5 μm are not found, or are very rare, in samples prepared by sonication. Representative micrographs of samples PRO-HLN are shown in Figure 1.

Figure 1. SEM images of PRO-HLN_T (left) and PRO-HLN_S (right). Magnification: 18 000×.

3.1. HLN. HLN crystallizes in the P4/mmm tetragonal space group. The Bragg peaks (see Figure 2) show some anisotropic broadening due to the oblate geometry of the crystallites: reflections with l = 0 have a lower broadening, since the c axis corresponds to the short axis of the ellipse. For instance, applying the Scherrer formula to calculate the crystallite size gives 40 nm for the (102) peak and 140 nm for the (110) peak. This estimate does not account for instrumental broadening, so it is clearly related to the anisotropic nature of the HLN crystal structure, while the real crystallite size may be larger.

Figure 3. 1H NMR MAS spectra of HLN (a), DEC-HLN_T (b), and DEC_HLN_S (c).

at 5.7 ppm is attributed to water between the layers, while the other peaks arise from the interlayer H+. The splitting in two signals of the H+ resonance arises from a different chemical environment, with the proton at 8.8 ppm being coordinated by water molecules.21 The IR spectrum of HLN (Figure 4a) shows the presence of two strong bands at ∼3396 and 1625 cm−1, which correspond C

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Figure 4. IR spectra of samples prepared with the sonochemical method. (a) HLN; (b) PRO-HLN; (c) DEC-HLN; (d) MPTMS-HLN; (e) IMIHLN; (f) DECAM-HLN; (g) HIST-HLN.

∼300 °C (DEC-HLN) or 340 °C (PRO-HLN), regardless of the preparation methods. A representative TGA trace is shown in Figure 6. As noted above, the particle size in the sonicated

to the O−H stretching (with hydrogen bonding) and O−H bending vibrations, respectively. These bands are also a common feature in all spectra of HLN derivatives, since only a fraction of the pristine H+ ions between the layers react during functionalization. 3.2. PRO-HLN and DEC-HLN. With the thermal method, intercalation of HLN with either n-propanol or n-decanol is incomplete even after 6 and 7 d of treatment at 80 °C, respectively. On the contrary, substantially better results were obtained after only 3 d at 80 °C when the reaction mixture was pretreated with ultrasounds for 8 h. Figure 5 shows such improvement in the XRD pattern for PRO-HLN, as the intensity of the (001) peak shifted at lower angle compared to the peak found in HLN and, indicative of the presence of the intercalated species, is much higher after sonication. A corresponding improvement can be seen in the amount of alcohol bound to the interlayer surfaces, as shown in Table 1. The organic parts of the intercalates decompose at

Figure 6. TGA of DEC-HLN_S. Weight and weight derivative are plotted in black and red, respectively.

samples is slightly lower; in particular, in these samples the larger platelets (up to 5 μm in diameter) are not detected. Smaller particles (100−200 nm) are always present in both samples, and their size is not reduced further by repeated sonication cycles. 1 H MAS NMR spectra recorded for DEC-HLN-T (b) and DEC-HLN-S (c) are compared with the spectrum of pristine HLN in Figure 3. The signals in the 4−0 ppm range arise from the methyl and methylene protons of the organic moieties. DEC-HLN-T shows a larger amount of residual HLN protons with respect to DEC-HLN-S. As already reported, 13C MAS NMR spectra can be used to observe the formation of Nb−O−C bonds between the perovskite internal surfaces and the alkoxyl groups.12,13 The signal related to the carbon α to the −OH group resonates at

Figure 5. XRD patterns of PRO-HLN_S (a) and PRO-HLN_T (b). D

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Inorganic Chemistry ∼64 ppm in free n-alcohols, and it is shifted downfield at ∼80 ppm when the Nb−O−C bond is formed. The 13C CP MAS NMR spectra recorded for both DEC-HLN samples are very similar, and both show the signal of the −CH2O− group at 80 ppm (see Figure 7a). The intercalation of the organic moieties

Figure 8. XRD pattern of MPTMS-HLN_S. Figure 7. 13C(1H) CP MAS NMR spectra of DEC-HLN_S (a) and MPTMS-HLN_S (b).

is further demonstrated by the splitting in different signals of all the other resonances arising from the pendant (35−20, and 15−10 ppm). Such a multiplicity arises from the presence of different conformations of the alkoxyl chain, induced by the restricted mobility into the HLN interlayer space. As shown in Figure 4b,c, the IR spectra of PRO-HLN and DEC-HLN show characteristic bands of C−O (stretching at 1122 cm−1) and CH2 groups (stretching between 2850 and 2950 cm−1, bending at 1460 and 1467 cm−1). 3.3. MPTMS-HLN. Direct reaction of HLN with MPTMS does not take place with either thermal or ultrasonic treatment. For this reason, the MPTMS intercalation was performed on the DEC-HLN intermediate. In this last case, the intercalation was successful with ultrasonic treatment, while with thermal reaction the amount of silane anchored to the HLN internal surface is very low according to TGA (Table 1). As a consequence, in the following, we only consider the MPTMSHLN_S sample. Most platelets are smaller than 400 nm, with very few particles ∼2 μms. The energy-dispersive spectroscopy (EDS) maps show a homogeneous distribution of all elements (La, Nb, Si, and S). EDS spot analysis in different points of the sample shows a Si/S ratio of 1, a Nb/La ratio of 2.1, and a Si/ La ratio of ∼0.8. The XRD pattern of MPTMS-HLN_S, shown in Figure 8, features the same interlayer distance as DEC-HLN. The complete removal of n-decanol, and its exchange with MPTMS, is confirmed by the 13C CP MAS NMR spectrum in Figure 7b. Only the signal at 12 ppm accounting for the CH2 group α to the silicon, and the signal at 28 ppm related to the other methylene groups of the mercaptopropyl chain of MPTMS can be observed. There are no residual signals that can be attributed to the DEC-HLN intermediate, nor is the signal at 50 ppm attributable to −OCH3 groups of unreacted MPTMS. It can be concluded that MPTMS is completely hydrolyzed. The 29Si CP MAS NMR spectrum reported in Figure 9 sheds light on the Si bonding in the interlayer space. The line shape is composed by a minor contribution, labeled T1 (−53 ppm), and two major contributions: T2 (−59 ppm) and T3 (−66 ppm).

Figure 9. 29Si(1H) CP MAS NMR spectrum of MPTMS-HLN_S, with T1, T2, and T3 contributions.

These are attributed to RSi(OH)2(OSi) (T1), RSi(OH)(OSi)2 (T2), and RSi(OSi)3 (T3) units.22,23 Since Si−O−Si and Si−O−Nb contributions cannot be clearly distinguished with this technique, we cannot directly observe the formation of Si−O−Nb bonds.24,25 In any case, a silicon atom should not form more than one Si−O−Nb linkage with the HLN surface, since the oxygen surface sites are too widely spaced (3.9 Å). In the 1H MAS NMR spectrum, shown in Figure 10a, methylene and S−H protons give rise to a broad peak centered around 2.1 ppm, while the peak at 5.9 ppm can be attributed to both interlayer water and terminal Si−O−H interacting with water. The peak at 8.5 ppm accounts for the H+ ions of HLN, most likely coordinated by water. In the IR spectrum (see Figure 4d), the S−H stretching is visibile at 2557 cm−1. The bands at 2929 and 2857 cm−1 are ascribed to the asymmetric stretching and symmetric stretching of methylene groups, respectively, while the band at 1453 cm−1 is ascribed to the bending of the methylene group. Two bands at 1111 and 1023 cm−1, assigned to νas Si−O−Si modes, further confirm the formation of a cross-linked siloxane structure. The different characterization techniques provide multiple and independent evidence that (a) n-decanol is deintercalated completely as a result of treatment with MPTMS; (b) MPTMS enters the enlarged interlayer space left by n-decanol; (c) MPTMS molecules are hydrolyzed, and react with the internal E

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Figure 10. 1H NMR MAS spectra of MPTMS-HLN_S (a), DECAM-HLN_S (b), HIST_HLN_S (c), and IMI_HLN_S (d).

From XRD and 13C MAS NMR results, some information about the arrangement of the alkyl chain between the HLN layers can be derived. A bilayer of n-decylamine molecules with all-trans conformation has a spacing ∼34 Å, while for a single layer the spacing is ∼21 Å. One possible way to reduce the interlayer spacing is by interdigitation of the chains forming the bilayer, providing stabilization of the chains to form a hydrophobic environment. Considering that the experimentally determined coverage is one decylammonium chain for 2.8 HLN unit formulas, such an interdigitation is likely. The presence of additional peaks in the 13C CP MAS NMR spectrum of DECAM-HLN corroborates the presence of several conformations in the alkyl chains, so that their length cannot be rigidly predicted with molecular modeling. This would also explain the variability of the interlayer distances obtained experimentally. Furthermore, HLN intercalated with n-octylamine and with n-decylamine feature a very similar (001) interlayer distance of ∼30 Å,21 confirming that the organization of long alkyl chains between the HLN layers is complex, and the perovskite layers still provide a structural framework that is not pulled apart easily. We discussed in detail the 1H MAS NMR spectrum of DECAM-HLN (shown in Figure 10b) in a previous paper.21 The signals in the 0−3 ppm range are assigned to the alkyl chains, and the peak at 5 ppm is assigned to −NH3+, while the interlayer protons resonate at 7−8 ppm. The IR spectrum (see Figure 4f) shows the characteristic stretching (3180 cm−1) and bending (1506 cm−1) vibrations of alkylammonium, further confirming the acid−base reaction between HLN and decylamine. The C−H stretching (2960− 2850 cm−1), CH2 bending (1468 cm−1), and C−N stretching of the alkyl chain (1146 cm−1) are also observed. 3.5. HIST-HLN. Direct intercalation of histamine in HLN does not occur with either thermal reaction or sonication. For this reason, we used the n-octylamine-HLN intercalate as an intermediate, achieving successful intercalation of histamine. HIST-HLN_S presents a markedly higher amount of intercalate with respect to HIST-HLN_T, even with a much shorter reaction time. This can be correlated with the more homogeneous distribution of platelet diameters, which are between 200 and 500 nm for HIST-HLN_S; on the contrary, HIST-HLN_T presents a large amount of particles between 1 and 5 μms. The absence of residual n-octylamine after the inclusion of histamine could be confirmed by 13C CP MAS NMR (see Figure 12a). The peaks at 38 and 25 ppm are attributed to the methylene groups in α and β position to −NH3+, respectively, while the peaks at ∼123, 127, 137, and 139 belong to the aromatic carbons. Even if the spectrum was recorded with a very large number of scans, it results to be rather noisy.

HLN surfaces, forming dimers, trimers, and more complex oligomers inside the 2D interlayer space. 3.4. DECAM-HLN. To obtain the n-decylamine derivative, the methods described previously in the literature involved the use of an intermediate step (reaction of HLN with nhexylamine), with a combined time of reaction of ∼20 d.26,27 Here, we found that intercalation of n-decylamine can be achieved directly by reaction of the amine with HLN with either thermal treatment (4 d) or sonication (10 h). In this latter case, effective intercalation is achieved within a shorter reaction time. The synthesis method does not influence the sample size: in both cases most particles have 200−600 nm diameter; larger particles, ∼2−3 μms, are very rare (see Supporting Information). In the literature, the spacing of the (001) Bragg peak of DECAM-HLN was reported between 29 and 36 Å.26,27 In our case, for the T sample the (001) spacing is 26.6 Å, while for the S sample it is 31 Å (see Supporting Information). The 13C CP MAS NMR spectrum of DECAM-HLN_S is reported in Figure 11. As in the case of DEC-HLN-S, the sharp

Figure 11. 13C(1H) CP MAS NMR spectrum of HLN-DECAM_S. The arrows indicate the additional peaks due to gauche-like conformations of the alkyl chains.

signal at 15 ppm accounts for the methyl group, and its intensity indicates a strongly reduced mobility of the alkyl chains. Furthermore, the presence of multiple signals for several methylene groups suggests that the decylammonium chains assume different arrangements in the interlayer space of HLN (for a complete assignment see ref 21). The peak at 40 ppm accounts for the CH2 in α position with respect to the −NH3+ group. It is worth noting that this carbon in free decylamine resonates at ∼42 ppm, and the upfield shift to 40.6 ppm confirms the protonation of the alkylamine chains inside the interlayer space of HLN.28 F

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histamine molecules are intercalated per HLN unit formula, that is, one histamine molecule for each three H+ sites. 3.6. IMI-HLN. Effective direct intercalation of imidazole in HLN occurs in the presence of water. With the thermal method, the intercalation takes ∼10 d. The same result is achieved, using sonication, in ∼10 h. The experimentally determined coverage of imidazole is 0.6 and 0.4 molecules of imidazole per HLN unit formula for IMI-HLN_T and IMIHLN_S, respectively. The XRD pattern (Figure 14) confirms the substantial disappearance of the HLN peaks, and the enlargement of the layered structure along the c axis, with the resulting periodicity being 14 Å.

Figure 12. 13C(1H) CP MAS NMR spectra of HIST-HLN_S (32 768 scans) (a), IMI-HLN_S (20 480 scans) (b).

However, the splitting of all the signals in at least two contributions can be observed. This evidence suggests that the imidazole ring could be present in both protonated and unprotonated forms, as already reported in biological systems.29 In the 1H MAS NMR spectrum reported in Figure 10c, most of the signals related to the organic moieties lie in the broad peak around 6.5 ppm. The residual amount of HLN protons should be relatively small, but their signal is blurred at the base of the broad peak together with the N−H signals of the imidazole ring (possibly also hydrogen bonded with water), which presumably resonate around 12 ppm. The IR spectrum (Figure 4g) features all distinctive vibrations of histamine, with imidazole N−H stretching (3270 cm−1), stretching and bending of alkylammonium (3120, 1583, and 1442 cm−1), stretching and bending of the alkyl chain (2980, 2920, 1464 cm−1), ring stretching (1647, 1543, 1465 cm−1), and stretching and bending of the aromatic C−H (3149 and 1099 cm−1). The XRD peaks of HIST-HLN_S (Figure 13) cannot be assigned to either pristine HLN or to the n-octylamine-HLN intermediate, confirming the substitution of n-octylamine with histamine in the HLN interlayer. The main (001) spacing appears at 20.1 A, which is roughly compatible with a bilayer of histamine molecules, and the TGA results indicate that ∼0.3

Figure 14. XRD pattern of IMI-HLN_S.

The IR spectrum (Figure 4e) shows all distinctive vibrations of imidazole, with N−H stretching (3270 cm−1), stretching and bending of C−H (3154, 2980, 1090, and 1051 cm−1), C−N stretching (1311 and 1185 cm−1), coupled vibrations of N−H, C−C, and C−N (1584 and 1441 cm−1). The 1H MAS NMR spectrum recorded for IMI_HLN_S (Figure 10d) shows a very broad peak at ∼8 ppm. Such a broad line shape could be the overlap of different signals: acid water at ∼5 ppm, aromatic protons at 6.5 ppm, HLN protons at 10 or 8 ppm, and possibly −NH units involved in H-bonds with water resonating between 10 and 14 ppm.30 The 13C(1H) CP MAS NMR spectrum of IMI-HLN_S (Figure 12b) shows one peak at 134 ppm and a splitted peak at ∼120 ppm. The 13C MAS NMR spectrum of solid imidazole (i.e., unprotonated) shows three peaks at 116, 128, and 137 ppm, assigned to C2, C4, and C5, respectively. Instead its protonated form shows only two signals at 121 and 135 ppm, because C4 and C5 are then equivalent.31 As a consequence, it can be assumed that the imidazole ring inside HLN is mostly protonated. The experimentally determined interlayer distance (14 Å) is consistent with an imidazolium ion placed crossways between the perovskite layers, stabilized by N−H···O hydrogen bonds with both upper and lower slabs.

4. CONCLUSIONS In this paper, we present a broad overview of the intercalation of organic molecules in the Dion−Jacobson layered perovskite HLN, whose crystal structure and chemical bonding were analyzed using XRD, MAS NMR, IR, and TGA. In particular,

Figure 13. XRD pattern of HIST-HLN_S. G

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the intercalation of imidazole, histamine, and MPTMS into a layered perovskite is described for the first time. We also demonstrate that the use of sonication to perform intercalation reactions in layered perovskites is effective in both increasing reaction yields and reducing reaction times. The structural models presented here, as well as the proton conduction mechanism between imidazole groups intercalated in HLN, are currently being investigated computationally with ab initio methods.32 After encouraging preliminary results, further investigation is underway on the possibility to use the IMI-HLN composite as a proton conductor in the intermediate temperature range or as a filler in proton-conducting polymer membranes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02581. TGA analysis, SEM images, XRD patterns (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Francesco Giannici: 0000-0003-3086-956X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding through Innovative Ceramic and hYbrid materials for Proton conducting fuel cells at Intermediate Temperature project financed by Italian Ministry of University and Research (FIRB (“Futuro in Ricerca”) 2012, RBFR12CQP5). We thank F. Giordano (ISMN-CNR, Palermo) for the XRD measurements.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b02581 Inorg. Chem. XXXX, XXX, XXX−XXX