Synthesis and thermal stability of new inorganic-organic perovskite-like hybrids based on layered titanates HLnTiO4 (Ln = La, Nd)

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Synthesis and thermal stability of new inorganic-organic perovskite-like hybrids based on layered titanates HLnTiO4 (Ln = La, Nd) Sergey A. Kurnosenko, Oleg I. Silyukov∗, Anton S. Mazur, Irina A. Zvereva Institute of Chemistry, St. Petersburg State University, 198504, St. Petersburg, Russia

A R T I C LE I N FO

A B S T R A C T

Keywords: A - powders: solid-state reaction D – perovskites D - transition metal oxides C - thermal properties Inorganic-organic hybrids

Protonated forms of layered perovskite-like titanates HLnTiO4 (Ln = La, Nd), relating to the Ruddlesden-Popper phases, have been used to yield inorganic-organic hybrids with the simplest representatives of amines (methylamine), alcohols (methanol), aminoalcohols (monoethanolamine) as well as n-butylamine. The possibility of the synthesis of the hybrids is studied in a wide range of conditions using both standard laboratory techniques and solvothermal/solvothermal-microwave methods. It is established that only methylamino derivatives may be synthesized in a single-phase form by a direct reaction between protonated titanates and the corresponding organic compound whereas pure n-butylamino, methanolic and monoethanolamino hybrids may be obtained on the basis of methylamino ones. For all the hybrids synthesized structure, quantitative composition, morphology and a type of a bond between inorganic and organic parts are discussed by means of powder XRD, Raman, IR and NMR spectroscopy, STA, elemental CHN analysis and SEM.

1. Introduction Inorganic-organic hybrids attract great attention as materials that combine physical and chemical properties of both inorganic and organic components in a single complex compound. The inorganic part usually can be associated with electronic and magnetic properties combined with mechanical and thermal stability while the organic part can provide additional organo-specific properties and structural diversity [1]. Recently, a large number of works have been devoted to the study of metal halide inorganic-organic layered perovskites exhibiting interesting optoelectrical properties [2,3], while much less attention has been paid to hybrids based on layered perovskite-like oxides. Ion-exchangeable layered perovskite-like oxides are solid crystalline substances with the block-type structure, in which perovskite slabs with a thickness of n octahedra BO6 (B = Ti, Nb, Ta, etc.) alternate with socalled interlayer spaces containing alkali cations [4,5]. According to the charge of the slabs and, correspondingly, the number of alkali cations per slab, ion-exchangeable layered perovskite-like oxides may be classified into two groups: the Dion-Jacobson phases [6,7] and the Ruddlesden-Popper phases [8,9]. In the Dion-Jacobson phases each of the interlayer cations is surrounded by 4–8 oxygen anions (depending on the cation size) and there is no relative shift of adjacent perovskite slabs, that is, the octahedra vertices of the slab are located directly above those of another one. The Dion-Jacobson phases demonstrate

∗

high reactivity in ion exchange, intercalation and exfoliation processes [10–13], some of them could have practically significant photocatalytic [14–16], electrophysical [17,18] and luminescent [19] properties. In the Ruddlesden-Popper phases perovskite blocks alternate with layers having the structure of rock salt. Octahedra BO6 have unequal distances B–O due to the different local surrounding of oxygen anions and the adjacent slabs, as a rule, are shifted relative to each other by half a and b lattice parameters. Recent studies of Ruddlesden-Popper phases were devoted to their thermal-depending luminescence [20], heat capacity [21], thermodynamic [22] and sorption characteristics [23]. Some of them have also proven to be effective photocatalysts for carbon dioxide reduction [24] and hydrogen evolution from water and water-alcohol solutions [25–28] that is of high importance for the development of the hydrogen power engineering and environmentally friendly technologies. During acid treatment of ion-exchangeable perovskites, the substitution of interlayer alkali cations by protons occurs. Protonated forms («solid acids»), being produced in this way, are able to react with some organic compounds giving so-called inorganic-organic hybrids – substances consisting of chemically bonded inorganic and organic parts, in which the inorganic part serves as a spatial frame [29]. According to the type of a chemical bond between inorganic and organic parts, there are two ways of forming hybrids: intercalation and grafting [30]. Intercalation is a reversible non-covalent introduction of organic bases

Corresponding author. E-mail address: [email protected] (O.I. Silyukov).

https://doi.org/10.1016/j.ceramint.2019.10.249 Received 15 August 2019; Received in revised form 2 October 2019; Accepted 26 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Sergey A. Kurnosenko, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.249

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the literature. We report here reactions of HLnTiO4 (Ln = La, Nd) with the simplest representatives of amines, alcohols, and aminoalcohols as well as with n-butylamine in order to reveal general patterns of the formation of the inorganic-organic hybrids and find optimal conditions of their synthesis. Characterization of the products is carried out with an emphasis on their structure, quantitative composition, and thermal stability.

into the interlayer space proceeding by the acid-base or ion exchange mechanism. Covalent inorganic-organic hybrids may be produced by grafting reactions – condensation of protonated forms and appropriate organic substances (alcohols [31], alkoxysilanes [32], carbohydrates [33], carboxylic and organophosphorus acids [34]) accompanied by the formation of covalent bonds Metal–O–C. The hybrids are of great interest for study in connection with the possibility of combining and fine-tuning of useful properties of the inorganic and organic parts in one material [35]. Furthermore, inorganic-organic derivatives may be exfoliated into monolayers – nanoscale platelets with perovskite structure having high specific surface area and small thickness [36]. They may become a basis for creating nanoscale electronics [37], heterogeneous catalysts [38] and photocatalysts owing to effective reagents adsorption and decrease in the effect of volume electron-hole pairs recombination on the photocatalytic reaction rate [39,40]. Due to the lower charge density of the interlayer space [7], the Dion-Jacobson phases demonstrate higher reactivity in intercalation and grafting reactions [41,42]. In particular, many covalent hybrids may be synthesized directly on the basis of protonated forms bypassing the stage of the intercalation compounds possessing increased interlayer distances [31,43]. As a consequence, Dion-Jacobson hybrids are more thoroughly studied compared with derivatives of the RuddlesdenPopper phases. The latter are reported to be able to react directly with some n-alkylamines. There are data on intercalation of amines into tantalates H2SrTa2O7 [44], H2CaNaTa3O10 [45], H2CaTa2O7 [46] and H2La2Ti3O10 [47]. Nevertheless, the number of papers devoted to the detailed study of the formation of the corresponding hybrids and their characterization is small. Layered perovskite-like titanates HLnTiO4 (Ln = La, Nd) are protonated forms of Ruddlesden-Popper phases ALnTiO4 related to compounds with K2NiF4 type of structure [48–50]. The structure of compounds HLnTiO4 (Fig. 1) is characterized by the complete ordering of H+ and Ln3+ cations in two interlayer spaces with the rock salt type of structure separating perovskite blocks (n = 1). Due to the different polarizing ability of single-charged and triple-charged cations, octahedra TiO6 undergo noticeable vertical distortion. As a result, interlayer cations of these compounds show high mobility in the interlayer space [51] and corresponding substances demonstrate the ability to undergo low-temperature reactions such as ion exchange, topochemical dehydration and acid leaching [52–54]. Reactivity of titanates HLnTiO4 with organic compounds is not sufficiently investigated. Until recently [55], no information on the possibility of their inorganic-organic hybrids formation was available in

2. Materials and methods 2.1. Preparation Ion-exchangeable layered perovskites NaLnTiO4 were synthesized by the standard ceramic method [56] in the air atmosphere at atmospheric pressure using Ln2O3, TiO2, and Na2CO3 as initial materials. Oxides were taken in stoichiometric amounts, sodium carbonate was taken with a 30% excess to compensate for losses during calcination. All components were mixed and ground in the planetary ball mill under a layer of n-heptane. The powder was pelletized into 2 g tablets which were calcined at 825 °C for 12 h. Products obtained were identified by X-ray diffraction (XRD) patterns. Protonated forms HLnTiO4 were prepared by acid treatment of NaLnTiO4 with an excess of 0.1 M HNO3 (200 ml per 1 g of the titanate) for 12 h. After centrifugation, the products were dried under ambient pressure. The lattice parameters calculated from XRD data in the tetragonal system were a = 3.707, c = 12.21 Å for HLaTiO4 and a = 3.684, c = 12.05 Å for HNdTiO4 which showed good consistency with the reported values [27]. The elimination of Na+ was established by thermogravimetric analysis (TG). TG data processing by the reported technique [57–59] showed that in all cases not less than 95% of the alkali cations were substituted by protons. 2.2. Instrumentation The XRD data were obtained on the Rigaku Miniflex II diffractometer (CuKα radiation, angle range 2θ = 3–60°, scanning rate 10°/min, step 0.01°). The lattice parameters were calculated on the basis of all the reflections observed using DiffracPlus Topas software. Raman spectra were obtained on the Bruker Senterra spectrometer (spectral range 50–4000 cm−1, laser 488 nm 20 mW, spectrum accumulation time 10 s). Infrared (IR) absorption spectra were recorded on the Shimadzu IRAffinity-1 Fourier-transform spectrometer (spectral range 400–4000 cm−1, step 1 cm−1) using the KBr tableting technique.

Fig. 1. Schematic structure of protonated compounds HLnTiO4. 2

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dried under ambient pressure. Formulae of hybrids below are merely conventions and do not show true compositions and types of bonding.

The amounts of carbon, hydrogen, and nitrogen in the hybrids were determined during the elemental CHN analysis on the Euro EA3028-HT analyzer. The simultaneous thermal analysis combined with the mass spectrometric detection of gases evolved (STA-MS) was carried out on the Netzsch STA 409 CD-QMS 403/5 Skimmer system using air-containing (oxidative) atmosphere. Calculation of quantitative compositions of the inorganic-organic hybrids from STA-MS data was based on the direct proportionality of ion currents to the quantities of gases evolved and matching integrated ion currents with corresponding mass losses. The final compositions were determined using results of the CHN analysis, and STA-MS in total. The morphology of the samples was investigated by SEM on the Zeiss Merlin scanning electron microscope. The nuclear magnetic resonance (NMR) spectra of the samples were obtained on the Bruker Avance III 400 WB spectrometer (operating frequencies 400.23 MHz and 100.64 MHz for 1H and 13C nuclei, respectively). Solid-phase samples were placed in a rotor with an external diameter of 4 mm, made of zirconium oxide, and rotated at a frequency of 12.5 kHz at a magic angle to the direction of a constant magnetic field. To register the spectra on 13C nuclei, a cross-polarization excitation pulse sequence was used (CP/MAS technique). For the CP/MAS technique, the contact time was 2 ms, the relaxation delay time was 5 s, the number of accumulations was 12000. When recording spectra on 1H nuclei using the direct excitation method (DE), the relaxation delay time was 30 s, the duration of the exciting pulse was 2.5 μs, the number of savings was 8. Tetramethylsilane (TMS) was used as an external reference. Diffuse reflectance spectroscopy was performed on the Shimadzu UV-2550 spectrometer with ISR-2200 integrating sphere attachment.

3. Results and discussion 3.1. Methylamino hybrids According to the experimental data obtained on synthesis of HLnTiO4×MeNH2, direct low-temperature intercalation of methylamine proceeds quickly enough that is indicated by appearance of new reflections (002) at 5.5° in the XRD patterns of the products corresponding to their interlayer distances and a strong decrease in the intensity of the reflections (001) at 7.4° corresponding to interlayer distances of initial protonated forms HLnTiO4 (Supporting information Fig. S1). Thereby, the bulk of HLnTiO4 converts into methylamino hybrids in an hour and the rest of HLnTiO4 (low-intensity reflections at 7.4°) reacts with methylamine much slower. This fact, apparently, is connected with the statistical particle size distribution: methylamine intercalation into the largest of them should proceed under strong diffusion control. So, preparation of pure (not containing appreciable amounts of initial protonated forms) hybrids HLnTiO4×MeNH2 at 25 °C is possible in 7 d. Increasing the temperature to 60° speeds up intercalation but in the case of HLaTiO4 leads to the formation of impurity phases when the experiment duration is 3 days or greater. Synthesis of the hybrids under solvothermal conditions also requires the same duration (7 d) and leads to the uncontrolled formation of by-product phases (Supporting information Fig. S2). The solvothermal-microwave method, which demonstrates high efficiency in a number of intercalation and grafting reactions [47,60], allows obtaining pure hybrids HLnTiO4×MeNH2 at 100 °C in 3 d. Increasing the temperature to 150° results in a decrease in the hybrids yield because of their thermal instability at this temperature. Thus, the suitable ways of obtaining HLnTiO4×MeNH2 (25 °C, 7 d or solvothermal-microwave set, 100 °C, 3 d) are established. Fig. 2 and Fig. 3 show XRD patterns of initial protonated titanates (a) and single-phase methylamino hybrids (b). All the reflections observed are amenable to indexing in the tetragonal system. The formation of the hybrids is accompanied by a noticeable increase in the c lattice parameter and directly related to it interlayer distance d whereas the a lattice parameter is almost unchanged (Table 2).

2.3. Reactions with organic substances Table 1 summarizes the conditions of experiments conducted in order to establish suitable ways for obtaining single-phase inorganicorganic hybrids. Before syntheses, weighed protonated forms were thoroughly ground in an agate mortar. Amino hybrids, which were used as precursors, were taken without preliminary grinding. Low-temperature syntheses were carried out in sealed glass tubes with stirring, solvothermal experiments – in laboratory autoclaves, solvothermalmicrowave ones – using a Berghof Speedwave 4 system with PTFE vessels. All the products were centrifuged, washed with acetone and Table 1 Conditions of experiments on optimization of the hybrids synthesis.

Synthesis of methylamino hybrids HLnTiO4×MeNH2 (Me = methyl) Method Low-temperature synthesis Solvothermal synthesis Precursors HLnTiO4 (0.2 g) HLnTiO4 (0.2 g) Reaction medium 38% methylamine in water (10 ml) 38% methylamine in water (30 ml) Temperatures, °C 25, 60 100 Duration 1 h–14 d 1, 3, 7 d Synthesis of n-butylamino hybrids HLnTiO4 × BuNH2 (Bu = n-butyl) Method Low-temperature synthesis Solvothermal synthesis Precursors HLnTiO4, HLnTiO4×MeNH2 (0.2 g) HLnTiO4 (0.2 g) Reaction medium 90% n-butylamine in water (10 ml) 90% n-butylamine in water (30 ml) Temperatures, °C 25, 60 100 Duration 1–7 d 7d Synthesis of methanolic hybrids HLnTiO4 × MeOH Method Low-temperature synthesis Solvothermal synthesis Precursors HLnTiO4, HLnTiO4, HLnTiO4×MeNH2, HLnTiO4×BuNH2 (0.2 g) HLnTiO4×MeNH2, HLnTiO4×BuNH2 (0.2 g) Reaction medium 90% methanol in water (10 ml) 90% methanol in water (30 ml) Temperatures, °C 60 100 Duration 5, 7 d 5d Synthesis of monoethanolamino hybrids HLnTiO4×MEA (MEA =HO–CH2–CH2–NH2) Method Low-temperature synthesis Solvothermal synthesis Precursors HLnTiO4, HLnTiO4×MeNH2, HLnTiO4×BuNH2 HLnTiO4, HLnTiO4×MeNH2, HLnTiO4×BuNH2 (0.2 g) (0.2 g) Reaction medium 90% monoethanolamine in water (10 ml) 90% monoethanolamine in water (30 ml) Temperatures, °C 60 100 Duration 1–14 d 7d

3

Solvothermal-microwave synthesis HLnTiO4 (0.2 g) 38% methylamine in water (30 ml) 100, 150 1 h–3 d Solvothermal-microwave synthesis HLnTiO4 (0.2 g) 90% n-butylamine in water (30 ml) 100, 150 1 h–3 d Solvothermal-microwave synthesis HLnTiO4, HLnTiO4×MeNH2, HLnTiO4×BuNH2 (0.2 g) 90% methanol in water (30 ml) 75–200 1–3 d Solvothermal-microwave synthesis HLnTiO4 (0.2 g) 90% monoethanolamine in water (30 ml) 150, 200 1d

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(1030 cm−1) and C–H fragments (2800–3000 cm−1). Bands of external modes located in (LnO)2 layer (270 and 320 cm−1) undergo a change in relative intensities whereas bands of asymmetric stretching mode (ν3) of distorted TiO6 octahedra (500–700 cm−1) do not experience noticeable changes during hybrids formation, indicating preservation of the perovskite structure [61]. However, the band relating to the symmetric stretching mode (ν1) of axial Ti–O bonds (840 cm−1 for the protonated forms) [62] splits into two bands (800, 880 cm−1) during methylamine intercalation that points at the existence of two types of octahedra with unequal axial Ti–O distances. As well, in the low-frequency region, the initial band at 135 cm−1 transforms into two overlapping bands at about 115 cm−1 characterizing complex vibrations of interlayer components linked to the oxygen of octahedra. IR absorption spectra of methylamino hybrids (Supporting information Fig. S3) also demonstrate latitudinal vibrations of water (1630 cm−1) and stretching of its O–H fragments (wide band at 2800–3500 cm−1). Thereby, IR spectroscopy indicates joint intercalation of methylamine and water that may be due to the formation of strong hydrogen bonds between them. Besides, bands at 2500 cm−1 relating to protonated amino groups –NH3+ show that interlayer methylamine is presented as methylammonium ions. This fact is well consistent with data from the earlier report [63] where the presence of interlayer n-butylamine in the cationic form was confirmed using NMR spectroscopy. For the La-containing sample the evidence of the formation of the methylamino hybrid was also confirmed by 1H and 13С NMR data. On the 13С spectrum of HLaTiO4×MeNH2 (Fig. 5a) a band at about 26 ppm can be observed, which refers to the carbon nuclei of the methyl group of methylammonium. Additionally, 1H NMR data (Supporting information Fig. S11b) show that in the case of HLaTiO4×MeNH2 relative intensity of a wide band at about 15 ppm corresponding to H atoms of the initial protonated form (similarly to the previously reported data for H2La2Ti3O10 compounds [64]) drastically decreases, suggesting that initial hydrogen atoms either exist in the form of MeNH3+ or are highly affected by intercalated methylamine molecules. Thermal decomposition of methylamino hybrids in the oxidizing atmosphere (Fig. 6) proceeds mainly as deintercalation. The evolution of intercalated water (ion current m/z = 18) starts at temperatures above 50 °C, methylamine (ion current m/z = 30 for the MeNH− fragment) – above 75 °C giving deintercalated protonated forms that decay at 250–300 °C with the shape of DSC and TG curves similar to previously reported [27], except the mass loss at temperatures above 700 °C, associated with evolution of carbon dioxide and water, suggesting that some organic species are still remaining after the first deintercalation step. Quantitative compositions of the samples calculated from CHN analysis and STA-MS data are presented in Table 3. As can be seen from Table 3, both methylamino hybrids contain the amount of intercalated water comparable to the amount of interlayer methylamine and, besides, the amount of the amine is approximately 0.4–0.5 per proton of the initial compound. Similar results were reported for other amino derivatives of protonated titanates [63,65].

Fig. 2. XRD patterns of (a) HLaTiO4 (HLT) and corresponding inorganic-organic hybrids with (b) methylamine (HLT×MeNH2), (c) n-butylamine (HLT×BuNH2), (d) methanol (HLT×MeOH) and (e–f) monoethanolamine (HLT×MEA – α- and β-forms).

Fig. 3. XRD patterns of (a) HNdTiO4 (HNT) and corresponding inorganic-organic hybrids with (b) methylamine (HNT×MeNH2), (c) n-butylamine (HNT×BuNH2), (d) methanol (HNT×MeOH) and (e–f) monoethanolamine (HNT×MEA – α- and β-forms). Table 2 Lattice parameters (a and c indexed in tetragonal system) and interlayer distances d of initial protonated forms and inorganic-organic hybrids. Sample

a, Å

c, Å

d, Å

HLaTiO4 HLaTiO4×MeNH2 HLaTiO4×BuNH2 HLaTiO4×MeOH α-HLaTiO4×MEA β-HLaTiO4×MEA HNdTiO4 HNdTiO4×MeNH2 HNdTiO4×BuNH2 HNdTiO4×MeOH α-HNdTiO4×MEA β-HNdTiO4×MEA

3.707 3.767 3.767 3.733 3.772 3.770 3.684 3.744 3.747 3.714 3.760 3.752

12.21 33.34 48.08 16.16 38.25 36.38 12.02 33.13 47.73 15.95 38.22 36.17

12.21 16.67 24.04 16.16 19.13 18.19 12.05 16.57 23.87 15.95 19.11 18.09

3.2. n-butylamino hybrids Direct low-temperature preparation of pure n-butylamino hybrids HLnTiO4×BuNH2 on the basis of protonated forms, apparently, is not possible. The appearance of new reflections (002) at 3.5° in the XRD patterns of the products (Supporting information Fig. S4) reveals that nbutylamine intercalation does occur but all the samples contain significant amounts of initial protonated titanates. The rise in temperature to 60° does not lead to a noticeable increase in the yield of the hybrids. However, difficulties in the direct obtaining of n-butylamino derivatives are quite natural because of the larger n-butylamine size (~6.7 Å) compared to methylamine (~2.9 Å). Direct synthesis of the hybrids under solvothermal and solvothermal-microwave conditions also does not result in the formation of single-phase products (Supporting

Raman spectra of protonated forms and methylamino hybrids are shown in Fig. 4, a. The fact of the formation of the hybrids is confirmed by appearance of characteristic bands which are not observed in the spectra of the initial titanates: latitudinal vibrations of methyl (1430, 1475 cm−1) and amino (1580 cm−1) groups, stretching of C–N 4

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Fig. 4. Raman spectra of HLaTiO4 (HLT) and HNdTiO4 (HNT) in comparison with corresponding hybrids with (a) methylamine, (b) n-butylamine, (c) methanol and (d) monoethanolamine.

presence of water (1625 cm−1) in the interlayer space. On the 13C spectrum of HLaTiO4×BuNH2, four main spectral lines of about 14 ppm, 21 ppm, 31 ppm, and 39 ppm correspond to structural elements of the n-butylammonium D, C, B and A, respectively (Fig. 5b). At the same time, low intensity lines corresponding to the structural elements of the n-butylamine molecule (A and B about 42 ppm and 37 ppm, respectively) are also seen. This suggests that the majority of intercalated n-butylamine molecules exist in the n-butylammonium form. The latter is also confirmed by 1H NMR data (Supporting information Fig. S11c) as in the case of HLaTiO4×MeNH2. All n-butylamino hybrids undergo thermal decomposition (Fig. 7) mainly in the same way as methylamino ones do. The evolution of intercalated water (ion current m/z = 18) occurs at temperatures above 60 °C, n-butylamine (ion current m/z = 73) – above 75 °C, giving deintercalated protonated forms which decay at 250–300 °C. Quantitative compositions of the samples calculated from CHN analysis and STA-MS data are shown in Table 3. Formation of n-butylamino hybrids HLnTiO4×BuNH2 from methylamino ones is accompanied by an increase in the amount of intercalated water while maintaining the same number of amine molecules per proton of the initial compound.

information Fig. S5). We found that pure hybrids HLnTiO4×BuNH2 can be prepared on the basis of methylamino derivatives HLnTiO4×MeNH2 at 25 °C in 1 d. The increase in the duration of synthesis to 3 days does not lead to noticeable changes on XRD patterns of the products. Figure 2 and 3 show XRD patterns of initial methylamino derivatives (b) and single-phase n-butylamino hybrids (c). All the observed reflections are amenable to indexing in the tetragonal system. The samples possess significantly larger interlayer distances d that is consistent with larger n-butylamine size compared with methylamine. Despite a perceptible increase in the c lattice parameter, the a parameter is still almost unchanged (Table 2) owing to the conformational stability of perovskite-like slabs. Raman spectra of protonated forms and n-butylamino derivatives are presented in Fig. 4b. The hybrids formation is indicated by appearance of characteristic bands relating to latitudinal vibrations of C–C–H (1330 cm−1), methylene (1465 cm−1) and amino (1580 cm−1) fragments, as well as stretching of С–N (1100 cm−1) and C–H fragments (2860–3000 cm−1). As in the case of the aforementioned methylamino hybrids, intercalation of n-butylamine is accompanied by a redistribution of some band intensities (270 and 320 cm−1). As well, the band of the symmetric stretching mode (ν1) of Ti–O (840 cm−1) undergoes bifurcation due to changes in the composition of the interlayer space. In the low-frequency region, a new medium intensity band appears, wavenumber of which (95 cm−1) is 15–20 cm−1 less than in the case of methylamino hybrids. IR spectra of the samples (Supporting information Fig. S6) also indicate successful n-butylamine intercalation and the

3.3. Methanolic hybrids The impossibility of direct reactions between protonated forms and methanol, earlier reported only for H2La2Ti3O10 [63], now has been also established in our research for HLnTiO4 (Ln = La, Nd) using 5

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Table 3 Data on composition of initial protonated forms and inorganic-organic hybrids. Sample

Quantitative composition

Total mass loss on the TG curve, %

HLaTiO4 HLaTiO4×MeNH2 HLaTiO4×BuNH2 HLaTiO4×MeOH β-HLaTiO4×MEA HNdTiO4 HNdTiO4×MeNH2 HNdTiO4×BuNH2 HNdTiO4×MeOH β-HNdTiO4×MEA

HLaTiO4(H2O)0.3 HLaTiO4(MeNH2)0.35(H2O)0.4 HLaTiO4(BuNH2)0.45(H2O)0.5 H0.5LaTiO3.5(MeO)0.5(H2O)0.1a HLaTiO4(MEA)0.45(H2O)0.2a HNdTiO4(H2O)0.05 HNdTiO4(MeNH2)0.45(H2O)0.40 HNdTiO4(BuNH2)0.45(H2O)0.55 H0.45NdTiO3.45(MeO)0.55(H2O)0.1 HNdTiO4(MEA)0.4(H2O)0.2

6.00 9.92 17.1 8.15 14.1 3.98 10.9 17.3 8.78 12.9

a Sample also contains residual MeNH2 (HLaTiO4 × MeOH) or BuNH2 (βHLaTiO4 × MEA) accordingly to the13C NMR data.

Fig. 7. STA-MS data for the n-butylamino hybrid HLaTiO4×BuNH2.

precursors. Solvothermal-microwave reactions lasting 1 d result in the formation of two-phase samples consisting of amino and methanolic derivatives that is indicated by bifurcation of reflections at 4.9–5.1° in the XRD patterns of the products (Supporting information Fig. S7). Pure methanolic hybrids may be prepared at 60 °C in 7 d in sealed tubes or at 100 °C in 5 d in solvothermal autoclaves. Figs. 2 and 3 present the XRD patterns of initial methylamino derivatives (b) and single-phase methanolic hybrids (d). The latter have slightly smaller interlayer distances d (Table 2) compared with methylamino hybrids that is in consistency with sizes of methylamine (~2.9 Å) and a methoxy group of methanol (~2.4 Å). Using n-butylamino derivatives as initial compounds leads to the formation of the same products. Raman spectra of protonated forms and methanolic hybrids are shown in Fig. 4c. The fact of the formation of the hybrids is confirmed by latitudinal vibrations of H–C–O (1170 cm−1) and methyl groups (1450 cm−1) as well as stretching of C–H (2830, 2920 cm−1) fragments. For the methanolic hybrids obtained, the band at 840 cm−1, which is characteristic to protonated compounds, does not exist; instead of it a new one at 690 cm−1 emerges which may be related to the formation of covalent bonds Ti–O–C. A similar band also appears in IR absorption spectra of the hybrids (Supporting information Fig. S8). Moreover, bands of the O–H fragment vibrations, presented in spectra of pure methanol, are not observed. Additionally, a band at about 540 cm−1 referred to the symmetric stretching mode of TiO6 octahedra shifts to 515 cm−1 suggesting a strong influence of methoxy groups on perovskite octahedra. These facts indicate that the interlayer space of methanolic hybrids does actually contain not molecular methanol but its methoxy groups chemically bound to the inorganic frame. 13 C NMR spectrum of HLaTiO4×MeOH synthesized at 60 °C (Fig. 5c) also confirms the covalent bonding of methoxy groups. The spectrum shows an intense signal from methoxy carbon nuclei in the

Fig. 5. 13C NMR spectra of inorganic-organic hybrids of HLaTiO4 with methylamine (a), n-butylamine (b), methanol (c) and monoethanolamine (d). * indicates bands of a residual organic compound (MeNH2 for HLaTiO4×MeOH and BuNH2 for HLaTiO4×MEA), A+ and B+ indicate carbon in butylammonium cation, Bc indicates carbon in covalently bonded MEA.

Fig. 6. STA-MS data for the methylamino hybrid HLaTiO4×MeNH2.

solvothermal and solvothermal-microwave conditions. XRD patterns of the samples obtained (not shown) demonstrate only reflections due to initial protonated compounds indicating the absence of the alcohol introduction. Preparation of methanolic hybrids HLnTiO4×MeOH is possible using amino derivatives HLnTiO4×RNH2 (R = Me or Bu) as 6

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practically does not affect the appearance of the XRD patterns. For both titanates existence of two different forms of monoethanolamino derivatives is established (Figure 2 and 3e, f). The first forms abbreviated as α-HLnTiO4×MEA are direct products of reactions between amino hybrids and aqueous monoethanolamine solution. After centrifugation, they are stable only in a paste state. During washing with acetone and drying, they turn into other forms, β-HLnTiO4×MEA, which are air-stable powders suitable for further use. Because of the pasty state, α-forms were characterized only using XRD. Conversion of α-forms into β-forms, apparently, consists in partial deintercalation of interlayer molecules (probably water, dehydrating effect of acetone) since α-forms have smaller interlayer distances d than β-ones (Table 2). Overall, interlayer distances of monoethanolamino hybrids occupy an intermediate position between those of methylamino and n-butylamino hybrids that is consistent with the monoethanolamine molecule size (~5.2 Å). Raman spectra of protonated forms and monoethanolamino hybrids are presented in Fig. 4d. The hybrids formation is confirmed by appearance of Raman bands relating to stretching of C–N (1055–1070 cm−1), C–H (2860–3000 cm−1) and N–H (3250 cm−1) bonds as well as latitudinal vibrations of amino group (1570–1590 cm−1), C–N–H (1095–1110 cm−1), C–O–H (1340–1360 cm−1) and C–C–H (1470–1490 cm−1) fragments. The C–O–H vibrations indicate the partial or complete presence of interlayer monoethanolamine in a non-grafted form. The change of axial Ti–O bonds vibrations is seen to be mostly the same as in the case of amino derivatives but with appearance of additional low-intensity bands at about 480–520 cm−1 and 700 cm−1 similar to those of HLnTiO4×MeOH, which may point out that at least a part of monoethanolamine exists in a grafted state. Similarly to amino derivatives, intensities of the (LnO)2 layer vibrations at 270 and 320 cm−1 undergo redistribution which is seen to be typical of all the hybrids excluding methanolic ones. In the low frequency region, a new medium intensity band appears at about 110 cm−1 that is between Raman shifts of the same bands of HLnTiO4×MeNH2 (115 cm−1) and HLnTiO4×BuNH2 (95 cm−1). In general, the data obtained allows to deduce that wavenumber of this band is inversely proportional to the size (or mass) of intercalated amine. IR spectra of the samples (Supporting information Fig. S10) also indicate the successful formation of the derivatives and presence of intercalated water in their interlayer space. 13 C NMR data of HLaTiO4×MEA (Fig. 5d) show weak peaks from the carbon nuclei of residual n-butylammonium in the region of 14 ppm, 21 ppm, 31 ppm, and 39 ppm. At about 43 ppm, the line corresponding to A position of the MEA molecule is visible. In this case, one can observe two non-uniformly broadened peaks of comparable intensities at about 60 ppm and 70 ppm. The first one can be associated with MEA carbon nuclei at B position, while the second at about 70 ppm can be associated with the same carbon nuclei in grafted MEA by analogy with HLaTiO4×MeOH. This fact suggests that a part of MEA in the interlayer space exists in the form of the grafted compound while the other comparable part does not. This is to some extent consistent with 1H data (Supporting information Fig. S11e) where the relative intensity of the band at about 15 ppm, corresponding to structural protons of HLaTiO4 compound, is quite big compared to other compounds obtained. The problem of synthesis of chemically pure HLnTiO4×MEA, i.e. not containing residual amine from the initial compound, has not been solved yet. As in the case of methanolic hybrids, extra flushing does not remove impurities. Elongation of the synthesis time results in the formation of non-single-phase products. STA-MS data (Fig. 9) reveal that monoethanolamino hybrids are thermally stable substances compared with methylamino and n-butylamino ones. Monoethanolamine deintercalation (m/z = 30 for its fragment) starts at relatively high temperatures (approximately 225 °C) which coincide with temperatures of the protonated forms decomposition and is accompanied by an exothermal effect. In the same region (250–400 °C) evolution of carbon dioxide proceeds (m/z = 44)

Fig. 8. STA-MS data for the methanolic hybrid HLaTiO4×MeOH.

region of 66 ppm (shifted from 51 ppm in liquid methanol). Also, peaks around 26 ppm and 30 ppm are visible, which can correspond to the carbon atoms of the residual methylammonium and methylamine molecules, respectively. Since these impurities cannot be removed by extra washing of the samples, their minimization in the future should be achieved by using the solvothermal preparation method (100 °C) instead of conventional benchtop (60 °C). On the other hand, the 1H NMR data (Supporting information Fig. S11d) show that for HLaTiO4×MeOH compound a broad band at 15 ppm corresponding to structural protons of HLaTiO4 is presented, suggesting that at least not all available axial sites are grafted. Data on thermal decomposition of methanolic derivatives in the oxidizing atmosphere (Fig. 8) also reveal that they are covalent hybrids because the deintercalation of molecular methanol does not take place (ion current m/z = 31 is not observed). After interlayer water is removed, organic parts of the hybrids do not degrade to relatively high temperatures (approximately 275 °C). After these temperatures are reached, oxidation of methoxy groups slowly starts that is indicated by carbon dioxide (m/z = 44) and water (m/z = 18) evolution. At a temperature about 400 °C a fast mass loss process associated with water release starts, accompanied by a strong exothermal effect. After a major amount of water is released, a rapid mass gain process starts up to temperatures about 600 °C, where it begins to slow down because of carbon dioxide evolution. These two processes probably may be explained as the reaction between methoxy groups and lattice oxygen with water release, the formation of some carbon compounds on the first stage and their subsequent oxidation by air on the second stage. Quantitative compositions of the methanolic samples calculated from CHN analysis and STA-MS data are shown in Table 3. 3.4. Monoethanolamino hybrids As in the case of n-butylamino derivatives, direct preparation of pure monoethanolamino hybrids HLnTiO4×MEA using low-temperature (60 °C) or solvothermal-microwave methods (150 °C, 200 °C), apparently, is not possible. The appearance of new reflections (002) at 4.5–4.8° in the XRD patterns of the products (Supporting information Fig. S9) reveals that partial hybrids formation does occur but the yield of the target products is too low. Variation of temperature and synthesis duration weakly affects the result. Mainly pure hybrids HLnTiO4×MEA may be prepared on the basis of amino derivatives HLnTiO4×RNH2 (R = Me or Bu) at 25 °C in 1 d. The increase in the duration of synthesis of HLnTiO4×MEA to 7 days leads to the formation of impurity phases. Adding water (10%) instead of using pure monoethanolamine was motivated by its need for intercalation (protonation of an amino group) and grafting (possibly as a catalyst) proceeding reported earlier [63]. The decrease in the monoethanolamine concentration to 50% and 10% 7

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Table 4 Absorption characteristics of initial protonated forms and inorganic-organic hybrids. Sample

Eg, eV

λmax, nm

HLaTiO4 HLaTiO4×MeNH2 HLaTiO4×BuNH2 HLaTiO4×MeOH β-HLaTiO4×MEA HNdTiO4 HNdTiO4×MeNH2 HNdTiO4×BuNH2 HNdTiO4×MeOH β-HNdTiO4×MEA

3.45 3.59 3.59 3.47 3.53 3.48 3.43 3.51 3.41 3.43

360 346 346 358 352 357 362 354 364 362

with interlayer oxygen atoms. At temperatures higher than 400 °C hybrids HLnTiO4×MEA show the same behavior as HLnTiO4×MeOH (mass gain with subsequent carbon dioxide evolution). Quantitative compositions of the monoethanolamino samples calculated from CHN analysis and STA-MS data are shown in Table 3. In the calculations, monoethanolamino hybrids were considered to be non-covalent derivatives.

Fig. 9. STA-MS data for the monoethanolamino hybrid β-HLaTiO4×MEA.

4. Discussion We have worked out reproducible techniques for the preparation of 10 new inorganic-organic hybrids based on protonated titanates HLnTiO4 (Ln = La, Nd) and suggested ways to optimize their synthesis. The hybrids of HLnTiO4, and especially their covalent alcoholic derivatives, are fundamentally new objects in the chemistry of corresponding titanates since they were not reported to be able to form any inorganic-organic hybrids. Methylamino derivatives may be produced by direct reactions and, due to enlarged interlayer distances, they are convenient for use as precursors in the synthesis of hybrids with larger interlayer amines as well as alcoholic and aminoalcoholic derivatives. The impossibility of direct preparation of n-butylamino and monoethanolamino hybrids in a pure form may be owing to steric effects impeding the complete course of the reaction. Data on the kinetics of amino hybrids formation reveal that direct intercalation into protonated forms proceeds under marked diffusion control. Furthermore, an increase in temperature in most cases does not lead to a noticeable rise in the target products yield. In the case of methanolic hybrids, the impossibility of their direct synthesis is, evidently, due to other causes. According to earlier reports, the Dion-Jacobson phases HLaNb2O7 and HCa2Nb3O10, capable of direct reacting with alcohols, are able to form stable hydrated compounds HLaNb2O7(H2O)x and HCa2Nb3O10(H2O)x [39,40], and interlayer water molecules play a key role in an alcohol introduction [31]. Hydrated forms of HLnTiO4 appear to be unstable compounds. Being taken in an almost dehydrated state, titanates HLnTiO4 cannot be rehydrated in aqueous solutions that may impede the grafting proceeding. The same behavior was observed for the layered crystalline silicate H-magadiite which did not react directly with silylation agents, but the use of intermediate hybrids with larger interlayer distances enabled the silylation [32,66]. Overall, in spite of a different lanthanide cation, both titanates HLnTiO4 (Ln = La, Nd) demonstrate mostly the same reactivity with considered organic compounds and similar features of corresponding hybrids. Thermal stability of the hybrids obtained increases in a sequence of organic compounds: methylamine/n-butylamine→monoethanolamine→methanol. Data on thermal decomposition of the hybrids coupled with vibrational spectroscopy allow us to conclude that methanolic derivatives, unlike amino ones, are covalent hybrids and monoethanolamino derivatives are semi-covalent and there is no evidence of formation of totally covalent monoethanolamino derivatives as in the case of the reaction between protonated Bi2SrTa2O9 and 5amino-pentan-1-ol [67]. Due to the high thermal stability of its organic

Fig. 10. Dependence of the interlayer distance d of obtained compounds compared on the length of intercalated or grafted organic molecules.

Fig. 11. Schematic representation of proposed HLnTiO4×BuNH2 (a) and HLnTiO4×MEA (b) structures.

that may be due to covalent bonding of a part of monoethanolamine (–O–CH2–CH2–NH2) with the perovskite-like frame or just oxidation of deintercalating monoethanolamine. In the latter case, the heightened thermal stability of the hybrids may take place owing to low volatility of monoethanolamine and its ability to form strong hydrogen bonds 8

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Fig. 12. SEM images of HLaTiO4 (a), HLaTiO4×MeNH2 (b), HLaTiO4×BuNH2 (c), HLaTiO4×MeOH (d).

According to the presented spectra, all the samples mainly absorb the radiation of the near-ultraviolet region (although the HLaTiO4×MEA absorbs in the visible region and has a yellowish color) and the modification of the interlayer space by organic molecules does not significantly affect the Eg value. According to SEM data (Fig. 12), during the intercalation and grafting reactions, sizes and the lamellar form of the particles are predominantly retained. Thus, all the considered reactions of the hybrids formation follow the topochemical mechanism, that is, the structure of inorganic parts of hybrids stays almost native and, consequently, saves all the morphological characteristics of initial perovskite-like particles that may be combined with those of an organic part giving new interesting materials.

parts, methanolic and monoethanolamino hybrids, apparently, may be obtained in an anhydrous (dehydrated) form. The correlation between the interlayer distance d of the hybrids and a length of organic molecules is presented as a bar chart in Fig. 10. During intercalation and grafting reactions, expansion of the layered structure proceeds perpendicularly to the perovskite-like slabs since a lattice parameters stay almost unchanged, and all the La-containing samples have slightly larger lattice parameters compared with Ndcontaining analogs that is, evidently, predetermined by a smaller ionic radius of Nd3+. For methanolic and methylamino hybrids the interlayer space expansion is nearly the same despite the different nature of organic molecules bonded and is higher than the corresponding length of organic molecules that, probably, is mainly caused by intercalated water molecules. For n-butylamino hybrids lattice expansion (11.83 Å and 11.85 Å for La and Nd compounds correspondingly) is nearly twice higher than the length of the single n-butylamine molecule (~6,7 Å) suggesting the existence of bilayer arrangement of organic molecules similar to other reported compounds with n-alkylamines [44,63,65,68–70]. On the basis of n-butylammonium size and an assumption that during intercalation n-butylammonium cations occupy cavities on the (100) surface of the perovskite-like slabs as it was previously reported for other layered perovskite compounds [65,69], a simple structural model can be proposed (Fig. 11a). In the case of monoethanolamino hybrids lattice expansion (5.98 Å and 6.07 Å for La and Nd compounds correspondingly) can be associated with one MEA molecule length (~5.2 Å). That suggests the existence of a pillaring arrangement being similar to previously reported results for aminoalcohols [67]. Taking into account 13C NMR, Raman and IR data and suggesting that not all MEA molecules are bonded to perovskite slabs covalently and that MEA exists in the interlayer space in a non-cationic form, as well as 1H NMR data suggesting the existence of remaining structural protons, a structural model with eclipsed conformation of perovskite slabs octahedra can be proposed for the monoethanolamino compounds (Fig. 11b). Table 4 contains optical band gaps Eg of the samples, calculated from their diffuse reflectance spectra (Supporting information Fig. S12), and corresponding maximum wavelengths λmax of absorbed light.

5. Conclusions In this study we have shown that protonated perovskite-like titanates HLnTiO4 (Ln = La, Nd) are able to form inorganic-organic hybrids with the simplest representatives of amines (methylamine), alcohols (methanol), aminoalcohols (monoethanolamine) as well as nbutylamine. Methylamino derivatives, which can be obtained by a direct reaction on the basis of protonated forms, proved to be convenient for use as precursors in preparation of other hybrids considered. The hybrids formation is studied in a wide range of conditions using both standard laboratory techniques and solvothermal/solvothermal-microwave methods and suitable ways of their preparation are proposed. It is shown that only methylamino derivatives may be synthesized in a single-phase form by a direct reaction on the basis of protonated titanates while pure n-butylamino, methanolic and monoethanolamino hybrids may be obtained using methylamino ones as precursors. It was estimated that methylamino and n-butylamino hybrids are intercalated compounds with amine molecules existing in the interlayer space in cationic form, while methanolic derivatives are covalent hybrids and monoethanolamino derivatives are semi-covalent ones. Since the formation of the hybrids is accompanied by an expansion of the interlayer space, the compounds synthesized become potential precursors for the synthesis of new hybrid derivatives and exfoliation into perovskite9

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structured monolayers. Declaration of competing interestCOI

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgment The study was supported by Russian Science Foundation (grant № 19-13-00184). Oleg I Silyukov thanks to the Russian Foundation for Basic Research for personal financial support (grant № 16-33-60082). Authors also are grateful to Saint Petersburg State University Research Park: Center for X-ray Diffraction Studies, Center for Optical and Laser Research, Center for Chemical Analysis and Materials Research, Center for Magnetic Resonance, Center for Thermal Analysis and Calorimetry, Educational Resource Center for Chemistry, Interdisciplinary Center for Nanotechnology and SDBSWeb: https://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, date of access).

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.10.249.

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