FTIR characterization of layered double hydroxides and modified layered double hydroxides

FTIR characterization of layered double hydroxides and modified layered double hydroxides

2

Meisam Shabanian1, Mohsen Hajibeygi2 and Ahmad Raeisi3 1 Faculty of Chemistry and Petrochemical Engineering, Standard Research Institute (SRI), Karaj, Iran, 2Faculty of Chemistry, Kharazmi University, Tehran, Iran, 3Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Arak University, Arak, Iran

2.1

Introduction

Infrared (IR) spectroscopy especially Fourier transform infrared (FTIR) spectroscopy is a technique that has been used over the years in both academia and industry for the structural and compositional analysis of organic, organometallic, polymeric, and inorganic materials, in addition to quality control of raw materials and commercial products. FTIR spectroscopy is a useful tool for functional group identification and quantification. Certain functional groups of an organic or inorganic structure can be identified easily using the FTIR technique. Also, FTIR spectroscopy can be used to confirm a pure compound or to detect the presence of specific impurities. The term “infrared” generally refers to any electromagnetic radiation falling in the region from 0.7 to 1000 μm. However, the region between 2.5 and 25 μm (4000
400 cm21) is the most attractive for chemical analysis. The relationship of the infrared region to other electromagnetic radiations is represented in Fig. 2.1. The “mid-IR” region includes the frequencies corresponding to the fundamental vibrations of virtually all of the functional groups and different bonds of metals in organic and inorganic compounds (Rives, 2001). The absorption bands in FTIR spectra are typically narrow and distinguished, making it possible to identify and monitor an absorption band related to the specific structural feature that is to be modified with a reaction. When a sample is exposed to an infrared beam, various wavelengths of radiation corresponding to the energies of the possible vibrational transitions in the molecule or crystal will be absorbed by the bonds of the sample. The remaining signals are recorded as an absorption band in the spectrum. In this process, those frequencies related to the infrared beam that match the natural vibrational frequencies of the bonds in the molecule are absorbed, and the energy absorbed serves to increase the amplitude of the vibrational motions related to the bonds. Note that a molecule can absorb only selected energies (frequencies) of infrared radiation, and not all bonds in a molecule are capable of absorbing infrared energy, even if the frequency of the Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00002-1 © 2020 Elsevier Ltd. All rights reserved.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 2.1 Electromagnetic spectrum and relationship of vibrational infrared to other radiations.

radiation exactly matches that of the bond motion (Pavia et al., 2008). Only the dipole bonds are capable of absorbing infrared radiation. Some molecules such as H2 or Cl2 are symmetric bonds and cannot absorb infrared radiation (Pavia et al., 2008). An electrical dipole must be present in an asymmetric bond in a molecule that is changing at the same frequency as the incoming energy of radiation to be transferred. Then the changing electrical dipole from the dipole bond can couple with the sinusoidally changing electromagnetic field of the incoming radiation. Thus a nonpolar bond (symmetric bond) that has identical or nearly identical groups on each end will not absorb in the infrared (Pavia et al., 2008). Near-infrared (NIR) spectroscopy is also known as “proton” infrared spectroscopy, as it covers the spectral region in which all the overtone and combination bands of vibrations involving hydrogen appear. The NIR spectral region has been defined by Kaye (1954, 1955) to extend from 700 to 3500 nm (14,285
2860 cm21). The only fundamental vibrations in the NIR region between 4000 and 10,000 cm21 are those associated with hydrogen atoms existing in hydroxyl groups or water in the case of minerals and inorganic compounds like layered double hydroxides (LDH). Whittet et al. (1997) reported average band positions for hydroxyl group and water in the NIR region around 4200 cm21 due to M
OH motions, 5200 cm21 as the H2O combination mode (bending 1 stretching), and around 7100 cm21 as the first OH stretching overtone. It is clear that the main structure can be obtained by mid-IR but NIR spectroscopy could be a suitable technique to study many compounds such as LDH, which contain both water and OH groups in their structure, to obtain more information about the local environments involved. By using the FTIR method, the structures and relative quantities of modifier molecules in the LDH surfaces can be analyzed. However, in some cases the low concentrations and aqueous environment for synthesized LDH can complicate the interpretation of measurement results. The absorbance of trace impurities or background noise can influence the FTIR absorbance result at low

FTIR characterization of layered double hydroxides and modified layered double hydroxides

79

concentrations of LDH. Since water strongly absorbs infrared light, the removal of water, as well as contamination, in the LDH and the nanocomposite films is necessary.

2.2

Fourier transform infrared spectra of layered double hydroxides

LDH or hydrotalcite-like compounds belong to the anionic clay family. The structures of these materials are made on the layers with a brucite-like structure carrying a net positive charge that is balanced by the anions intercalated between the positively charged layers. Positive charge on the electrostatically neutral brucite was created through the substitution of octahedral M21 by M31 cations (Qu et al., 2016a,b; Takehira, 2017; Chubar et al., 2017). One of the ways to identify the structures of LDH and intercalated anions between LDH layers is the FTIR technique. Mumpton et al. (1965) represented for the first time the FTIR spectrum of hydrotalcite like Mg
Al LDH. Also, Ross and Kodama exhibited the characteristic absorption bands of Mg
Al LDH (Ross and Kodama, 1967). In all FTIR spectra of prepared LDH, a broad absorption band was observed around 3480 cm21 with a shoulder band around 3000 cm21, which was related to OH stretching vibration. In general, the structure of LDH can be identified by different characteristic absorption bands in a typical FTIR spectrum. These bands were assigned to four series, as listed here. (1) The OH stretching vibration related to water molecules in the interlayer LDH and metal hydroxide layers, which usually appeared around 3300
3600 cm21. (2) The absorption band around 1620 cm21 related to the OH bending vibration. (3) The characteristic absorption bands in the region of 400
1100 cm21 can be attributed to the metal
oxygen and oxygen
metal
oxygen bands (Shabanian et al., 2014, 2016a,b). (4) The characteristic absorption related to anions in the interlayer LDH usually appeared in the range of 800
1700 cm21.

2.2.1 Fourier transform infrared characteristic absorption bands of layered double hydroxides with different anions 2.2.1.1 Mg
Al LDH
CO322 The OH stretching frequency of the Mg
Al LDH appeared in a broad absorption band in the range of 3300
3500 cm21. The absorption band at about 1600
1650 cm21 was attributed to the bending motion of interlayer water. Generally, the absorption bands related to carbonate anion for asymmetric and symmetric stretching vibration appeared at about 1450 and 880 cm21, respectively. Three different absorption bands at 590, 637, and 667 cm21 were attributed to the M
O and O
M
O (M: Mg or Al) stretching vibration. In the Mg
Al LDH

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Layered Double Hydroxide Polymer Nanocomposites

spectrum a medium absorption band appears at 450 cm21 due to the Al
O bond related to the [AlO6]32 structure (Valcheva-Traykova et al., 1993). Also some weak absorption bands around 3000 cm21 can be related to the OH hydrogen bonded stretching of water molecules intercalated in the LDH layer (Acharya et al., 2007). The main region in FTIR spectrum of Mg
Al LDH
CO322 as a typical LDH is illustrated in Fig. 2.2. In Fig. 2.2, for O
H stretching vibration a broad absorption band as well as a recognizable shoulder can be seen around 3300
3600 cm21. This

Figure 2.2 The approximate region of absorption bands of Mg
Al LDH
CO322.

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81

strong band is broad due to the overlapping of two and/or three possible absorption vibrations of the interlayer water. Due to a combination of lattice or vibrational modes, the brucite-type hydroxides can appear in this area as a strong and broad absorption band (Duan and Evans, 2006). Also due to O
H and CO322 interaction in the interlayer of LDH, the broad absorption band of hydroxyl group can appear as a broad shoulder band. In some FTIR spectra of Mg
Al LDH, a weak band appeared around 3040
3060 cm21, which could be related to water molecules that were solved in the microporosity area of the LDH structure (Chˆatelet et al., 1996). The O
H bond in LDH is shorter than in brucite, and its effect can shift the IR absorption band of the main O
H stretching vibration around 3550
3570 cm21 for brucite to around 3470 cm21 for Mg
Al LDH
CO322 (Kagunya et al., 1998).

2.2.1.2 Mg
Al LDH
NO32 Mg
Al LDH with nitrate anions has been reported many times (Shabanian et al., 2016a,b; Lennerova´ et al., 2015; Xu and Zeng, 2001; Zhao et al., 2012; Zhang et al., 2014; Wang et al., 2011; Nyambo et al., 2008). The characteristic absorption band around 1380 cm21 observed in the FTIR spectrum of all Mg
Al LDH
NO32 samples was attributed to the NO32 group. This intensive sharp absorption band was related to v3 vibrational mode with D3h symmetry in NO32 structure. In many FTIR spectra of Mg
Al LDH
NO32 a sharp and strong characteristic band appeared around 450 cm21, which was related to the metal
oxygen bond in the brucite-like lattice. The FTIR spectrum of Mg
Al LDH
NO32 prepared by the coprecipitation method from aluminum and magnesium nitrate is presented in Fig. 2.3. The characteristic absorption band centered at 3441 cm21 was attributed to the O
H stretching of the  metal
hydroxide layer and interlayer water molecules. The bending vibration of the water interlayer was reflected at 1621 cm21. Also, the NO32 stretching vibration appeared at 1383 cm21. The appeared absorption bands in the range of 580
870 cm21 were related to Al
O and Mg
O stretching modes. The shoulder bands at 2917 and 2856 cm21 were attributed to H2O
NO3 bridging vibration (Hajibeygi et al., 2017).

2.2.1.3 Mg
Al LDH
SO422 The Mg
Al LDH containing SO422 in its interlayer was prepared and the characteristic absorption band related to SO422 was reported. The characteristic absorption band related to sulfate ions in the interlayer LDH appeared at 1000
1300 cm21 (Acharya et al., 2007; Fahami and Beall, 2016). Also, OH as a weak absorption band appeared as two shoulder bands at 2920 and 2852 cm21 due to OH hydrogen bond stretching vibration of intercalated water molecules. In addition, the bands in the range of 500
1000 cm21 are related to M
O, O
M
O, and M
O
M lattice vibrations (M 5 Mg and Al) (Acharya et al., 2007; Obadiah et al., 2012).

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Figure 2.3 FTIR spectrum of Mg
Al LDH
NO32.

2.2.1.4 Mg
Al LDH
PO432 and Mg
Al LDH
HPO422 The Mg
Al LDH with phosphate in its interlayer as well as its corresponding FTIR absorption bands was also reported (Shimamura et al., 2012). The absorption bands related to HPO422 appeared at 1085, 995, and 860 cm21 which can be related to antisymmetric stretching of P
O, symmetric stretching of P
O, and antisymmetric stretching of P
OH, respectively (Dartiguelongue et al., 2016). Three absorption bands related to HPO422 are transformed into a single broad absorption band at 1056 cm21. Also, a shoulder absorption band was obvious at 870 cm21 close to the antisymmetric stretching of P
OH. The characteristic absorption band related to phosphate in the interlayer of Zn
Al LDH was reported by other authors at 1056 cm21 (Costantino et al., 1997; He et al., 2010; Cheng et al., 2010).

2.2.1.5 Mg
Al LDH
Cl2 The preparation of Mg
Al LDH
Cl2 was reported via a coprecipitation method (Yue et al., 2017). The broad bands at 3466 and 1636 cm21 are associated with the stretching and bending vibrations of the 2 OH group of LDH layers and interlayer water molecules. The sharp band observed at 1372 cm21 is due to antisymmetric stretching of the CO322 ion, which may be introduced into the interlayer of Mg
Al LDH by absorption of CO2 during the preparation procedure (Li et al., 2009, 2014). The bands observed below 1000 cm21 (400
850 cm21) correspond to the characteristic lattice vibrations of Mg
O and Al2O3. Chloride did not have a significant and clear absorption band in the FTIR spectrum of LDH.

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Figure 2.4 The approximate region of absorption bands of typical interlayer anions.

By comparison of FTIR of LDH with different anions, the approximate region of absorption bands for some common anions are obtained and represented in Fig. 2.4. The absorption band related to the anions such as carbonate, nitrate, phosphate, and sulfate can be found in two regions (Fig. 2.4). Due to the presence of O
H bonds in the hydrogen phosphate anion, its absorption bands appeared in four or more regions.

2.2.2 Fourier transform infrared characteristic absorption bands of layered double hydroxides with different metals The LDH metals could be replaced with different metals (Hajibeygi et al., 2015; Rastin et al., 2017). The FTIR spectra in the range of 400
2000 cm21 related to some LDH with different M21 and Al31 are illustrated in Fig. 2.5. The Ca
Al and Cu
Al LDH were prepared from nitrate salts of pristine metals and for preparation of Ni
Al LDH, nickel chloride and aluminum nitrate solution salts were used. By comparison of these three spectra, it is clear that with changing metals in the LDH structure, their absorption bands were also changed. Some absorption bands in all three spectra are held in common such as the absorption band around 1375
1450 cm21, which is related to the presence of the nitrate ion and the CO322 group because of possible adsorption of CO2 during aging processes (Qu et al., 2016a,b). Also, a broad and weak band was observed at 1644 cm21 in all FTIR spectra related to the bending vibration of water (Zhong et al., 2017). The main differences between these three FTIR spectra are observed in the range of 400
900 cm21 related to typical stretching vibrations of metal oxides and metal hydroxide as well as O
M
O bonds in the LDH structure. In the FTIR spectrum of Ca
Al LDH, three shoulder and individual broad absorption bands appeared around 525, 765
870, and 1023 cm21. All of these bands were related to the typical stretching vibrations of M
O and M
OH (M 5 Ca and Al) in the LDH (Plank et al., 2006; Chen et al., 2015; Perioli et al., 2006).

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Layered Double Hydroxide Polymer Nanocomposites

In FTIR spectrum of Cu
Al the mentioned three absorption bands related to metal oxides and metal hydroxide can be seen, although the bands around 780 cm21 appeared as a broad band. The absorption bands which appeared in the range of 400
1040 cm21 were due to the pulsation of metal
oxygen and oxygen
metal
oxygen as well as metal hydroxide bonds in the brucite type (Chakraborty et al., 2015). Also, the absorption band at 1384 cm21 can be related to the nitrate ion vibration bands (Fig. 2.5) (Sahu and Pugazhenthi, 2011; Chakraborty et al., 2014). In the FTIR spectrum of Ni
Al LDH, a clear difference can be seen at 429 cm21. It can be related to presence of Ni
O bonds in the LDH structure (Chakraborty et al., 2014). Other absorption bands appeared as a broad and shoulder band in the region 500
1050 cm21 attributed to metal
oxide (Ni and Al) in the LDH structure that is typical of this kind of layered solids (Fig. 2.5) (Chakraborty et al., 2014). The absorption band of NO32 was observed as broad and sharp in Ni
Al as compared to Cu
Al and Ca
Al LDH. The bending vibration of interlayer H2O molecules (H\OH) appeared at 1637 cm21 confirming the presence of water molecules as bending modes. Also, other absorption bands in the region of 500
1000 cm21 as broad overlapped bands can be related to metal
oxide vibration modes (Costa et al., 2008). The FTIR spectra of Cu
Fe, Ni
Fe, and Ca
Fe LDH are presented in Fig. 2.6. All of the metals were used as their nitrate salts except for Ni which was used as a chloride salt. The absorption bands related to water bending vibration and nitrate anions are obvious around 1623 and 1355 cm21, respectively (Li et al., 2004).

Figure 2.5 FTIR spectra of Ca, Cu, and Ni
Al LDH.

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85

Figure 2.6 FTIR spectra of Cu
Fe, Fe
Ca, and Ni
Fe LDH.

In FTIR spectra of Cu
Fe and Ni
Fe LDH, the absorption band around 500 cm21 appeared in both spectra and can be related to M
O and M
OH lattice mode vibration (Nejati and Rezvani, 2012; Nejati et al., 2013; Zhang et al., 2010; Li et al., 2010). Due to the presence of chloride and nitrate anions, the absorption band around 1480 cm21 related to carbonate was decayed (Iwasaki et al., 2012). In FTIR spectrum of Ca
Fe, the absorption bands at 467, 588, and 853 cm21 were related to stretching vibration of M
O (M: Ca or Fe) and Ca
Fe
O in the LDH structure (Fig. 2.6) (Ferraro, 2012; Frost et al., 2009; Wu et al., 2012). The absorption band at 588 cm21 can be related to the Fe
O bond (Shabanian et al., 2015, 2016a,b).

2.2.3 FTIR spectra of layered double hydroxides containing three metals In recent years the preparation of ternary metal LDH has been investigated due to their unique application in electronic, magnetic, and optical areas. Ternary metal LDHs in general have better properties as compared to two-metal LDHs. These materials have better crystallinity and a well-defined hexagonal shape (Ma et al., 2010; Han et al., 2008, 2009; Yang et al., 2013). Some ternary LDH derivatives have been prepared by reaction of different M21 cations with M31 cations with different concentrations. FTIR characterizations of these ternary LDH derivatives are reviewed in the following sections.

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Layered Double Hydroxide Polymer Nanocomposites

It should be mentioned that all infrared spectra of LDH typically showed similar absorption bands, especially at high wavenumber regions such as the O
H stretching mode of the basal layer, the interlayer water, and stretching vibration of the anion (e.g., CO322). Moving to the wavenumber region below 1000 cm21, which showed the information on the absorption bands of the lattice, dual LDH materials showed bands of HO
M
OH and M
O, but in the ternary system usually the absorption bands shifted to higher wavenumbers with increasing concentration of the third ion. It should be noted that these shifts depended on the nature of the third ion. For example a broad absorption band of Cu substitution was found to be comparable to Co substitution. The possible explanation may be due to a Jahn
Teller distortion in Cu21 octahedral compounds that causes c-axis elongation (Fahel et al., 2016). The absorption bands of third cations are usually observed at lower wavenumbers as compared to the corresponding free hydroxide anions. The absorption bands related to different metals in LDHs may appear at different frequencies in the FTIR spectrum. Some ternary LDHs were prepared by different metals and their FTIR spectra were reported (Pe´rez et al., 2012). The characteristic absorption bands related to metal
oxide and O
M
O (M: Zn, Al, and Cr) appeared at around 428, 553, 608, 780, and 938 cm21. Also, for LDH containing Cd, Al, and Cr, the absorption bands appeared at around 419, 489, and 531 cm21. The preparation of ternary metal LDH containing Co, Ni, and Fe with carbonate anion has been reported (Ehlsissen et al., 1993). The FTIR of the above-mentioned LDH had some characteristic absorption bands that confirmed its structure. The OH related to water molecules in the interlayer of LDH appeared at around 3503 cm21 as a broad band due to hydrogen bonding (Ehlsissen et al., 1993). The antisymmetry vibration of carbonate anions in the interlayer was observed at around 1366 cm21 (Hernandez-Moreno et al., 1985). Also, some absorption bands appeared at 1385 and 1525 cm21, which can be attributed to the vibration mode of carbonate. The absorption band in the region 480
800 cm21 was related to metal
oxide vibration and the band at 646 cm21 was related to symmetric bending of carbonate that overlapped with absorption of metal
oxide and shifted to low frequency (Zhang et al., 2008). The FTIR spectrum of Mg
Zn
Al LDH that was prepared by the coprecipitation method has been reported (Eshaq and ElMetwally, 2016). The broad band in the range of 3445
3500 cm21 is ascribed to the O
H stretching vibration of the water molecule and metal hydroxide in the brucite-like layers (Yang et al., 2002). The band at 1640 cm21 is attributed to the O
H bending mode in the water molecule. The weak shoulder band appeared at approximately 3000 cm21 owing to the OH stretching mode of interlayer water molecules hydrogen bonded to interlayer anions. The characteristic bands in the low-frequency region (400
1000 cm21) are related to the metal
oxygen stretching vibrations (Zn
O, Mg
O, and Al
O). In addition, a strong band at 1370 cm21 indicates the presence of CO322 anions in the interlayer region. The FTIR spectra of Ni
Mg
Al LDH and Co
Mg
Al LDH have also been considered. The bands at 3478 and 1663 cm21 were ascribed to the stretching vibrations of the OH group of LDH layers and bending vibration of water molecules in

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87

the interlayer (Parida et al., 2012). The sharp band appeared at 1388 cm21 due to antisymmetric stretching of the CO322 ion and the broad bands between 500 and 800 cm21 are attributed to the characteristic M
O vibrations (Bharali et al., 2015).

2.3

FTIR spectra of organo-modified layered double hydroxides

The use of LDH as a nanofiller to improve thermal, mechanical, and flameretardancy properties of a polymer matrix is one of the recent applications being investigated in both academia and industry (Costa et al., 2007; Manzi-Nshuti et al., 2008; Raeisi et al., 2017). However, the modification of LDH is necessary for preparation of uniform polymer LDH nanocomposites. Different methods have been used in the modification of LDH such as anion exchange (Choy et al., 2000) and regeneration in situ synthesis (Desigaux et al., 2006), etc. These methods have some drawbacks and so can be replaced by a onestep synthesized method (Wang et al., 2009). Accordingly, a one-step method as the correct approach to synthesize organo-modified LDH was reported from solution of metal salts and the anionic surfactant in a reactor (Wang et al., 2009). Many organic compounds that include an anionic segment in their structures have been used for the modification of LDH. In FTIR spectra of organo-modified LDH the presence of characteristic absorption bands related to functional groups of organic modifier can be helpful in investigating their structures. For the FTIR spectra of the organo-modified LDH two types of bands would be expected, one corresponding to the intercalated anionic modifier and the other corresponding to the host LDH material. The approximate region of absorption bands of some functional groups that can be presented in organo-modifiers are represented in Fig. 2.7. It is

Figure 2.7 The approximate region of absorption bands in organo LDH modifiers.

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Layered Double Hydroxide Polymer Nanocomposites

noticeable that the FTIR absorption bands can slightly shift due to different factors such as hydrogen bonding, intramolecular interaction, and chemical structure. Characteristic P
O
C stretching vibration bands are usually observed about 1140 cm21 (symmetric) and 1037 cm-1 (antisymmetric). The P 5 O stretching vibrations are indicated by a strong band about 1225 cm21. The antisymmetric and symmetric stretching vibrations of COO2 (carboxylate anion) have usually been observed about 1570 and 1440 cm21, respectively. The corresponding FTIR bands of organic modifiers are expected to shift toward a lower wavenumber in comparison to their free-state absorption bands, as more energy is required for executing such vibrations due to the presence of restriction between the layers. The bands in the range 1000
1800 cm21 are mostly due to the functionalities of the modifier and also due to interlayer water molecules. The appearance of characteristic bands for CO32
(γ) means that some CO32
still exists in the interlayer region of the modified LDH. This is perhaps caused by partial free movement of the CO32
ions due to enlargement of the interlayer region after organic modification. Most of the modified LDH materials have exhibited strong absorption bands in the range 2850
3100 cm21, corresponding to the
CH
stretching vibration arising from the hydrocarbon tail present in each modifier. In FTIR spectra of the modified LDH, the presence of interlayer water is not clear. In this regard, the only difference observed is the disappearance of a weak band (in the form of a shoulder) in the region 3000
3100 cm21, which originates from the interaction between O
H groups and CO32
ions. One of the common organic compounds used as an organo-modifier of LDH is sodium dodecylbenzene sulfonate (SDBS). This organic sodium salt can change with the anions in the interlayer of LDH via an ion exchange reaction (Wang and O’Hare, 2012). The organo-modified LDH can be prepared by a one-step reaction from metal salt in the presence of organic modifier salt in sodium hydroxide solution with pH 5 10 (Wang et al., 2009). The structure of SDBS is represented in Fig. 2.8. In LDH
SDBS, the characteristic OQSQO stretching vibration bands have appeared about 1040 cm21 (antisymmetric) and 1070 cm21 (symmetric), whereas the corresponding bands in some modifiers appear at 1230 and 1186 cm21, respectively. The C
S stretching vibration band is also observed in the range of 610
630 cm21. SDBS additionally has shown multiple bands corresponding to the aromatic ring C
C vibrations in the range 1450
1610 cm21. In general, the presence of absorption bands of SDBS in FTIR spectrum of the modified LDH indicates a good intercalation anionic structure in the interlayers of

Figure 2.8 Molecular structure of SDBS.

FTIR characterization of layered double hydroxides and modified layered double hydroxides

89

LDH. The important bonds of SDBS are aromatic C
H and aliphatic C
H, double bond carbon
carbon groups, which showed the bands around 3020
3100, 2930
2990, and 1550
1650 cm21, respectively. The bands related to SO32 appeared at 1037 cm21 as well as 1182 cm21 (symmetric and antisymmetric). The mentioned region absorption bands for SDBS in the FTIR spectrum of a typical LDH
SDBS are illustrated in Fig. 2.9. The FTIR spectrum of a typical SDBS-modified Zn
Al LDH is illustrated in Fig. 2.10. The characteristic absorption bands can be observed at 2858, 2927, and 2962 cm21 which are related to the antisymmetric and symmetric CH3 and CH2 group vibration modes resulting from the long alkyl chains of the SDBS anion. The absorption bands at 3063, 1600, and 1131 cm21 are attributed to the aromatic CH

Figure 2.9 The approximate region of absorption bands of SDBS in a typical LDH
SDBS.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 2.10 FTIR spectrum of SDBS-modified Zn
Al LDH.

stretching, double bond aromatic carbon
carbon and C
H aromatic in-plane bending of SDBS, respectively. The two strong absorption bands at 1036 and 1172 cm21 are attributed to symmetric and antisymmetric stretching vibration of SO32 bands in the SDBS structure. Also, the absorption band around 1398 cm21 is related to NO32 bands which are maintained from metal salt in the preparation processes of LDH (Xu et al., 2013; Pavel et al., 2012; Hajibeygi et al., 2015). Other absorption bands in FTIR are related to the LDH structure. One of the other common organic modifiers for LDH is sodium dodecyl sulfate (SDS). The FTIR spectrum of SDS is similar to that of SDBS with only slightly modification. In SDS molecular structure there is no aromatic ring, therefore there are no absorption bands related to aromatic C
H and double bond carbon
carbon of the aromatic ring (Xu et al., 2013). The structure of SDS is presented in Fig. 2.11. The aspartic acid-modified Li
Al LDH was prepared and used for preparation of poly(ethylene terephthalate) LDH nanocomposites (Bunekar et al., 2016). In this work the organo-modified LDH was prepared using a two-step reaction. At first neat Li
Al LDH was synthesized from LiNO3  3H2O and Al(NO3)3  9H2O, and then organo-modified LDH was prepared via an ion exchange reaction with aspartic acid solution salt. The structure of aspartic acid is presented in Fig. 2.12. The FTIR spectra of LDH and aspartic acid-modified LDH were reported. The aspartic acid-modified Li
Al LDH spectrum exhibited some bands, included an absorption band related to the neat LDH as well as absorption bands related to functional groups of aspartic acid as modifier. A broad absorption band around

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91

Figure 2.11 Molecular structure of SDS.

Figure 2.12 Molecular structure of aspartic acid.

Figure 2.13 Molecular structure of lauric acid.

3200
3500 cm21 appeared which can be ascribed to the hydrogen-bonded hydroxyl groups from both the hydroxide layers and interlayer water molecules. The antisymmetric vibration bands of NO32 or CO322 appeared at 1354 cm21. The absorption bands at 530 and 740 cm21 can be attributed to the metal
oxide stretching modes. Characteristic absorption bands related to alkyl C
H stretching vibration were observed in the region 2800
3000 cm21. The antisymmetric and symmetric stretching modes of the carboxylate group appeared at 1527 and 1404 cm21, respectively. The lauric acid-modified Mg
Al LDH was prepared by Katiyar et al. (2010). In this research work, laurate-modified LDH was prepared and used in the preparation of nanocomposite based on polylactic acid. The structure of lauric acid is illustrated in Fig. 2.13. The FTIR spectra of LDH, organo-modified LDH, and lauric acid are considered. In the FTIR spectrum of the neat LDH, the broad absorption band in the range of 3200
3700 cm21 related to OH stretching vibration as well as a shoulder band at 3000 cm21 indicating that the hydrogen bonding between water molecules and carbonate ions was observed (Komarewsky et al., 1953). The absorption bandrelated to bending vibration of water molecules appeared at 1638 cm21. The characteristic absorption bands attributed to the carbonate ion in the interlayer also appeared. Two absorption bands of the remaining nitrate ions were also observed at 1384 and 830 cm21 (Hansen et al., 1994) and the absorption band of lattice vibration of metal
oxide was observed at 411 cm21. In the FTIR spectrum of laurate-modified LDH some new absorption bands appeared and some absorption bands disappeared. The disappearance of the

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Layered Double Hydroxide Polymer Nanocomposites

absorption band related to carbonate ion at 1373 cm21 and the appearance of absorption bands related to carboxylate groups indicated the lauric acid salt presence in the interlayer of LDH. The associated absorption bands of the carbonyl group stretching vibration of carboxylate appeared at 1561 and 1411 cm21 related to antisymmetric and symmetric stretching vibration of laurate COO2 in the anionic form (Borja and Dutta, 1992; Venkataraman and Vasudevan, 2000). Also, the absorption band for stretching vibrations related to antisymmetric and symmetric stretching of CH2 groups that appeared in the range of 2953
2858 cm21, while the CH2 bending vibration and CH2 rocking vibration appeared as single bands at 1467 and 721 cm21, respectively. Lauryl alcohol phosphoric acid ester potassium also was used as an organic modifier of LDH (Xie et al., 2016). The FTIR spectrum of the mentioned organomodified LDH revealed absorption bands at 438 and 3434 cm21 due to the presence of metal
oxide bonds and water molecules in the interlayer of LDH. Also, the absorption band at 2952 cm21 was related to symmetric and antisymmetric vibration of aliphatic C
H groups in the organic modifier structure. Manzi-Nshuti et al. (2009) reported the preparation of oleate-modified Zn
Al LDH. The oleate-modified LDH was prepared by coprecipitation method (Wang et al., 2005). The molecular structure of oleic acid is presented in Fig. 2.14. The FTIR spectrum of oleate-modified Zn
Al LDH revealed the absorption bands related to LDH as well as absorption bands related to oleate carboxylate salt as an organic modifier (Xu et al., 2004; Hibino, 2004). The symmetric and antisymmetric mode attributed to C
H aliphatic groups appeared around 2800
3000 cm21. Two strong bands appeared around 1400
1600 cm21, which was attributed to symmetric and antisymmetric carboxylate bands in oleate salt. An absorption band that appeared at 3006 cm21 as a weak band is related to C
H attached to a carbon
carbon double bond (Simons, 1978). The molecular structure of taurine (2-aminoethanesulfonic acid) is presented in Fig. 2.15. This organic compound was used in the preparation of organo-modified Mg
Al LDH (Lennerova´ et al., 2015).

O OH

Figure 2.14 Molecular structure of oleic acid. O

HO

S O

Figure 2.15 Molecular structure of taurine.

NH2

FTIR characterization of layered double hydroxides and modified layered double hydroxides

93

In this work the FTIR spectra of neat LDH, modified LDH, as well as taurine as modifier, were compared. In the FTIR spectrum of modified LDH, the vibrational absorption bands related to taurine can be found. Three characteristic absorption bands appeared at 1241, 1184, and 1046 cm21, which were related to the SO32 group in the modified LDH structure. The stretching vibration of C
N appeared at 1046 cm21. Also, the absorption bands at 1512, 1305, and 1114 cm21 were attributed to different vibrations of the NH2 group. In some cases the bands attributed to 2 NH31 that were observed in the pure powders decreased significantly or vanished in the corresponding modified LDH. This can be attributed to the interaction between the N atom and the metal ions in the layers of LDH. The stretching vibration of CH2 groups in the taurine structure appeared at 1305, 963, 894, and 741 cm21 (Ohno et al., 1992). A sharp absorption band was observed in the FTIR spectrum of the neat LDH at 1383 cm21, which was related to nitrate anions in the interlayer of LDH. This absorption band appeared as a weak band in the FTIR spectrum of modified LDH. It can be related to a trace of nitrate anions remaining in the rehydrated LDH. Two organic compounds included 2-naphthalene sulfonate and 2,6-naphthalene disulfonate, which contain a naphthalene ring with one and two sulfonate groups being used for modification of LDH (Kameda et al., 2006). The structures of two aromatic modifiers are presented in Fig. 2.16. The FTIR spectrum of modified LDH indicated the naphthalene containing organic modifier anions in the interlayer region of LDH. In FTIR spectra of 2naphthalene sulfonate- and 2,6-naphthalene disulfonate-modified LDH, the absorption bands related to the organic modifier appeared as well as absorption bands related to the LDH structure. The absorption bands related to metal
oxide bonds appeared in the region of 500
1000 cm21. Also, the OH hydroxyl group in water molecules appeared as a broad band centered at 3500 cm21. The aromatic CH stretching vibration related to the naphthalene ring appeared around 3000
3100 cm21 and the stretching vibrations of the SO32 group have been observed at around 1120
1150 cm21 (Kameda et al., 2008). For the preparation of chiral organo-modified LDH, N,N0 -(pyromellitoyl)-bis-Lisoleucine diacid was used for modification (Mallakpour and Dinari, 2013). The molecular structure of N,N0 -(pyromellitoyl)-bis-L-isoleucine diacid as an organic modifier is shown in Fig. 2.17. The FTIR spectrum of organo-modified LDH showed two types of absorption bands: one related to the anionic organic modifier intercalated between LDH layers and the other attributed to the LDH structure.

SO3H

SO3H

HO3S

Figure 2.16 Molecular structures of naphthalene derivative modifiers.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 2.17 Molecular structure of a chiral organic modifier.

Figure 2.18 Molecular structure of diacid-diimide as an organic modifier of LDH.

An absorption broad and shoulder band was observed in modified LDH in the range 1600
1640 cm21, indicating the presence of H2O molecules as a bending vibration appeared in this region (Kloprogge et al., 2004). The characteristic absorption bands at 2930
3100 cm21 related to the aliphatic and aromatic C
H modes in the dicarboxylate salt have also been observed. FTIR spectra of Mg
Al LDH and diacid-diimide-modified Mg
Al LDH were reported in a research work (Hajibeygi et al., 2017). A diacid-diimide organic compound containing imide heterocyclic ring and aliphatic long chains was synthesized and used for modification of LDH. The molecular structure of diacid-diimide is shown in Fig. 2.18. The neat Mg
Al LDH was prepared via the coprecipitation method from magnesium and aluminum nitrate. The modified LDH was prepared by an ion exchange reaction between the neat LDH and carboxylate dianion salt of organic modifier. The FTIR spectra of neat LDH (A) and modified LDH (B) are shown in Fig. 2.19. In the FTIR spectrum of neat LDH (LDH
NO32), a broad and strong absorption band related to OH stretching vibration due to the presence of interlayer water molecules and metal hydroxide layers appeared at 3466 cm21. The bending vibration of the interlayer water molecules was reflected as a broad band centered at 1618 cm21. Also, the characteristic absorption band was observed at 1378 cm21 related to nitrate interlayer anions stretching vibrations. The Al
O and/or Mg
O as well as M
O
M (M: Al and/or Mg) stretching modes appeared as broad and shoulder absorption bands around 590
890 cm21. The FTIR spectrum of modified LDH revealed the absorption bands related to neat LDH as well as absorption bands attributed to an organic modifier. The shoulder absorption bands, which appeared around 2850
2930 cm21, were related to stretching vibration of CH aliphatic groups in the organic modifier structure. Two clear absorption bands at 1774 and 1711 cm21 were related to antisymmetric and symmetric stretching vibration of carbonyl in an imide heterocyclic ring in the

FTIR characterization of layered double hydroxides and modified layered double hydroxides

95

Figure 2.19 FTIR spectra of (A) neat LDH and (B) modified LDH.

organic modifier. The absorption band at 1383 cm21 was related to C
N vibration mode of the imide group. It is clear that to understand FTIR spectra of organo-modified LDH, comparison of FTIR spectra of neat LDH, neat modifier, and organo-modified LDH would be a promising method. However, some shift would be expected in the FTIR bands of organo-modified LDH as compared to the neat structures. These can be attributed to the new van der Waals forces, including repulsion and attraction between the atoms.

2.4

Conclusion

The infrared spectroscopic method is an excellent technique to study the structure of LDH and modified LDH. The infrared absorption bands identify molecular components and structures. This technique measures the absorption of infrared radiation by the sample material versus the wavelength. The presence of different polar bonds, LDH sheet structures, and anions in the interlayers (inorganic and organic compounds) can easily be detected using FTIR spectroscopy. To achieve good results it is necessary to compare FTIR spectra of the neat LDH, modifier, and modified LDH. The main functional groups and sharp absorption bands can be helpful in investigating the structures. All LDH showed some similar bands in FTIR spectra, such as a broad absorption band in the range 3300
3500 cm21

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Layered Double Hydroxide Polymer Nanocomposites

attributed to O
H stretching mode of the basal layer and the interlayer water, the band at 1620
1650 cm21, assigned to the bending mode of the interlayer water, the anion bands, and the band related to M
O and M
OH between 400 and 800 cm21. For modified LDH two types of bonds are expected, one corresponding to the intercalated anionic modifier and the other corresponding to the host LDH material. Both the anionic modifier and LDH bands showed some shift in the FTIR bands of organo-modified LDH which can be attributed to the formation of new van der Waals forces in organo-modified LDH.

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