Silica-layered double hydroxide nanoarchitectured materials

Silica-layered double hydroxide nanoarchitectured materials

Applied Clay Science 171 (2019) 65–73 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay ...

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Applied Clay Science 171 (2019) 65–73

Contents lists available at ScienceDirect

Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research paper

Silica-layered double hydroxide nanoarchitectured materials Meriem Djellali

a,b

a

, Pilar Aranda , Eduardo Ruiz-Hitzky

a,⁎

T

a

Instituto de Ciencia de Materiales de Madrid, CSIC, C/Sor Juana Inés de la Cruz 3, Cantoblanco, 28049 Madrid, Spain Laboratoire de Physico-Chimie des Matériaux, Catalyse et Environnement, Université des Sciences et de la Technologie d'Oran Mohamed Boudiaf (USTOMB), BP 1505, El M'naouar, 31000 Oran, Algeria b

A R T I C LE I N FO

A B S T R A C T

Keywords: Layered double hydroxides Silica Sol-gel Delamination Clay nanoarchitectures

This work shows the viability for assembling layered double hydroxides (LDH) containing surfactant anions in its interlayer region, with silicon alkoxides through a sol-gel processes, leading to SiO2-LDH nanoarchitectured materials consisting in delaminated LDH particles assembled to silica nanoparticles. Tetrametoxysilane (TMOS) has been used as silica precursor being hydrolyzed and polycondensed in presence of MgAl-, NiAl- and NiCoFeLDH containing dodecylsulphate ions as interlayer anions. The resulting materials were further heated to remove the organic matter and to consolidate the final nanoarchitecture, being the LDH re-constructed in a final step in the presence of a salt solution (e.g., Na2CO3, sodium dodecysulphate, …), producing LDH-SiO2 materials. These resulting materials have being characterized using XRD, FTIR, 29Si and 27Al NMR spectroscopy, N2 adsorptiondesorption isotherms, FE-SEM and TEM amongst other physical-chemical characterization techniques. The obtained results confirm the formation of silica nanoparticles within the nanosheets generated by delamination of the pristine LDH, producing functional materials of elevated specific surface area of potential interest in diverse fields of applications.

1. Introduction Clay minerals and related solids, such as layered double hydroxides (LDH), are attracting increasing interest in the development of functional materials for applications within the scope of Nanotechnology (Ruiz-Hitzky et al., 2012; Ruiz-Hitzky and Fernandes, 2013; Yang et al., 2016; Lvov et al., 2017). Their typical dimension below the micrometer, with the possibility of going towards materials at the nanometer scale when LDH reached exfoliation into individual nanosheets, together with their rich chemistry, render these inorganic solids very attractive for the bottom-up development of advanced nanoarchitectured materials (Roth et al., 2016; Mao et al., 2017; Zhao et al., 2017). In this way, and going beyond classical polymer-clay nanocomposites, clay minerals of different nature (smectites, kaolinites, halloysite, sepiolite and palygorskite) as well as LDH of diverse composition are more and more employed as constitutive elements of building-blocks of nanoarchitectures by its assembly with a large variety of nanoparticles (NP), as for instance magnetite, silica, titania, etc. (Ruiz-Hitzky and Aranda, 2014; Massaro et al., 2017; Aranda and Ruiz-Hitzky, 2018; Dedzo and Detellier, 2018). Under the concept of clay-based nanoarchitectures (Ruiz-Hitzky and Aranda, 2014; Aranda and Ruiz-Hitzky, 2018; Aranda et al., 2015 and 2018), the use of organoclays in combination with sol-gel approaches ⁎

represents a very convenient methodology to prepare functionalized based-silicates. In this way, the organic-inorganic interface of intermediate organoclays may facilitate the association and stabilization of alkoxysilanes, and other alkoxides, providing a suitable environment for the further controlled hydrolysis and polycondensation of the alkoxide (Aranda et al., 2018). In the case of layered clays, this type of approach has been used to produce both organized pillars separating the silicate layers as in the so-called porous clay heterostructures (PCH) firstly developed by Pinnavia's group (Galarneau et al., 1995) or even to reach delamination of the silicate platelets by the action of the growing polycondensated species in the interlayer spacing, as occurs in the formation of silica-clay nanocomposites reported by Ruiz-Hitzky's group (Letaïef and Ruiz-Hitzky, 2003; Letaïef et al., 2006; Zapata et al., 2013). Interestingly, this last methodology can be employed involving not only alkoxysilanes but also diverse metal-alkoxides, such as for instance the Al and Ti based alkoxides, as well as other type of metalorganic precursors useful for introducing selected functionalities (acidity, photoactivity, etc) (Manova et al., 2010; Belver et al., 2012, 2015 and 2016; Bouna et al., 2012; Ruiz-Hitzky and Aranda, 2014; Rhouta et al. 2015a and b; Perez-Carvajal et al., 2016). Moreover, this sol-gel route has been applied to organoclays derived from fibrous clays, sepiolite and palygorskite, producing highly stable nanostructured materials where the clay nanofibers became assembled

Corresponding author. E-mail address: [email protected] (E. Ruiz-Hitzky).

https://doi.org/10.1016/j.clay.2019.02.004 Received 31 October 2018; Received in revised form 30 January 2019; Accepted 5 February 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.

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2.2. LDH-silica materials preparation

to diverse metal oxide NP (SiO2, TiO2, TiO2-SiO2, TiO2-Pt, TiO2-Pd, ZnO, Al2O3, Al2O3-SiO2, …) (Aranda et al., 2008; Bouna et al., 2011 and 2013; Stathatos et al., 2012; Gómez-Avilés et al., 2013; Belver et al., 2013; Ruiz-Hitzky and Aranda, 2014; Perez-Carvajal et al., 2016; Akkari et al., 2018; Aranda and Ruiz-Hitzky, 2018). The motivation of the present work consists in the potential application of this procedure to other solids related to clay minerals, such as LDH. In fact, nanostructured materials based on LDH attract increasing interest for applications in catalysis, energy storage, environmental remediation and nanomedicine amongst others. Methodologies for production of such nanomaterials involve the exploration of new strategies for production of delaminated LDH, co-assembly of nanoparticles and nanostructuration procedures. In this way, exfoliated LDH are used to produce diverse nanocomposite thin films by assembly with graphene materials, metal oxides and other nanoparticles using layer-bylayer methodologies of interest in preparation of active phases of supercapacitors, electrochromic devices and controlled drug delivery applications (Liu et al., 2015; Kim and Kim, 2015; Lv et al., 2015). Diverse type of core-shell nanoarchitectures can be produced for example by co-precipitation of LDH nanoparticles in the presence of spherical silica particles (Kwok et al., 2018) or on the top of [email protected] magnetite nanoparticles (Gilanizadeh and Zeynizadeh, 2018). Alternatively, heterostructured materials have been produced using in this case the LDH particles as template for the growing of nanoporous silica heterostructures (Shimura and Ogawa, 2007). In this last approach, tetraetoxysilane (TEOS) was used as precursor of the silica which is generated in a sol-gel process in the presence of the hydrotalcite particles dispersed in a mixture of water, methanol, hexadecyltrimethylammonium chloride and ammonia solution. On the other hand, SiO2-LDH heterostructured materials have been also produced via sol-gel synthesis but combining here the generation of silica NP from TEOS and Ni-Cr/Mg-Cr LDH produced from the corresponding acetylacetonate precursors (Saikia et al., 2016). In this last case, it is produced a continuous silica matrix in which the LDH remains embedded. Other authors have recently proposed the synthesis of the LDHsilica based heterostructured materials by co-precipitation of the LDH in presence of a surfactant, e.g. Na-dodecylsulfate, and organosilanes, e.g. aminoproypyltrimethoxysilane, which leads to the formation of silane-grafted materials of interest as adsorbents for removal of organic pollutants, additive in the preparation of polymer nanocomposites, or CO2 adsorption (Tao et al. 2009a and b; Tang et al., 2018). In this context, the present work introduces an initial study on the use of the sol-gel route firstly applied to the preparation of silica-clay nanocomposites to produce silica-LDH nanostructured materials. So, intermediate organic-LDH materials and tetrametoxysilane (TMOS) have been used as starting components leading to new LDH-nanoarchitectures.

All LDH, either with chloride or dodecylsulphate ions (DS) as compensating interlayer anions, were prepared by co-precipitation from the corresponding metal chloride solutions at controlled pH to produce a 3:1 MII/MIII stoichiometry. In the case of the LDH incorporating DDS ions the co-precipitation was accomplished in an aqueous solution of sodium dodelcysulphate (SDS) instead of pure water. The general protocol of synthesis is based on the one described by Leroux and co-workers (Leroux et al., 2001) for the direct preparation of the LDH-DS hybrid material at constant pH and under nitrogen atmosphere. For instance, 100 mL of a solution prepared by dissolving 6.10 g MgCl2·6H2O and 6.41 g AlCl3·6H2O (3:1 Mg:Al molar ratio) was dropwise added to 100 mL of a solution that contains 2.88 g SDS while the pH is automatically maintained constant at 9 by adding NaOH 1 M. The addition was completed after 6 h and then the reaction is kept under N2 flow for other 24 h aging the precipitated solid. The resulting suspension was washed several times with de‑carbonated water and dried at 60 °C in air to produce the final MgAl-LDH-DS. A similar procedure was used to prepare the NiAl-LDH-DS using NiCl2·6H2O instead of MgCl2·6H2O salt, maintaining the pH at 8.5 and aging the suspension for 72 h in this case. The NiCoFe-LDH-DS material was prepared using the corresponding metal nitrate salts as precursors and a 3:1 MII/MIII stoichiometry with equimolecular amounts of the two divalent cations (1:1 Ni:Co ratio), maintaining this time the pH at 8.0 and aging the suspension for 18 h. On the other hand, MgAl-, NiAl- and NiCoFe-LDH incorporating chloride in the two first and nitrate in the latest one as compensating anions, were prepared with the same protocols but precipitating them into pure water instead of the SDS solution. Additionally, a NiAl-LDH-DS material (noted as NiAl-LDH-DSAE) was prepared by ion-exchange reaction of 1 g of freshly prepared NiAl-LDHCl with a 0.1 M SDS solution (1 g of surfactant) kept at pH 8.5 by adding NaOH 1 M. The reaction was kept for 72 h under N2 flux, then washed several times with decarbonated water and finally dried at 60 °C. The LDH-silica materials were prepared adapting the protocol reported for the preparation of silica-clay nanocomposites by Letaïef and co-workers (Letaïef and Ruiz-Hitzky, 2003; Letaïef et al., 2006). In the present case, 1 g of a selected LDH-DS intermediate organic-inorganic hybrid was dispersed in 20 mL of isopropanol and stirred for 15 min, then sonicated for 15 min in a ultrasound bath, and finally kept under stirring while adding 2.46 mL of TMOS (calculated for theoretically produce 1 g SiO2, and so a 1:1 LDH:SiO2 w/w ratio). Later on it is slowly added 1.2 mL of water dispersed in 2.54 mL of isopropanol keeping the proportion 4:2:1 (H2O/alcohol/alkoxysilane), with also a few drops of HCl 1 N as catalyst of the reaction. The system is kept at 50 °C under stirring while the hydrolysis and polycondensation of TMOS take place and a sol-gel transition is produced, which is visualized by the sudden stopping of the magnet stirrer (aprox. 50 min) due to the high viscosity developed by the sample at the end of the process. The resulting solid was dried at 60 °C to produce the LDH-SDS/SiO2 hybrid intermediate. For the MgAl-LDH system it was also tested a 1:0.5 LDH/SiO2 ratio following the same approach but using in this case a half of the TMOS amount and so H2O/alcohol reagents for the hydrolysis. The LDH-DS/ SiO2 hybrid intermediates were submitted to a thermal treatment heating first from room temperature to 400 °C (10 °C/min) under N2 for 2 h and then under air flow for other 3 h, to remove the organic matter and consolidate the SiO2 network. In this process it is also produced the reversible dehydroxylation of the LDH, which is transformed in a layered double oxide (LDO) system, i.e. leading to LDO/SiO2 systems. These last LDO/SiO2 systems were further reconstructed to LDH/SiO2 materials by treatment of the solids with appropriated saline solutions (Tao et al., 2010), such as Na2CO3 0.1 M and SDS 0.1 M to produce the corresponding LDO/SiO2-CO3 and LDO/SiO2-DS materials, respectively. Typically, 0.5 g of LDO/SiO2 was dispersed in 100 mL of the 0.1 M saline solution kept at pH 9 under stirring for 18 h, then collected by centrifugation, washed 4 times with deionized water and finally

2. Experimental 2.1. Materials Reagents used in this work were all of analytical grade and included: AlCl3·6H2O (> 98%, Sigma-Aldrich), MgCl2·6H2O (99%, Panreac), NiCl2·6H2O (98%, Riedel de Haën), Ni(NO3)2·6H2O (> 99%, Panreac), Fe(NO3)2·9H2O (> 98%, Sigma-Aldrich), Co(NO3)2·6H2O (98.5%, Riedel de Haën), NaOH (98%, Fluka), CrCl3·6H2O (Carlo Erba, RP), and sodium dodecylsulphate (> 99%, Sigma-Aldrich). Solvents and other reagents included: tetramethoxysilane (TMOS) (> 98%, Fluka), isopropanol (iPrOH) (99.5%, Sigma-Aldrich) and hydrochloric acid (37%, Fluka). All the aqueous solutions were prepared using deionized water (resistivity > 18.2 MΩ cm) obtained with a Maxima Ultrapure Water system from Elga.

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dried at 60 °C. 2.3. Characterization studies X-ray diffraction (XRD) patterns from 2°–75° (in 2θ reflection angle) were collected from powdered samples using a Bruker D8-Advance diffractometer (Cu-Kα radiation). Fourier transform infrared spectra (FTIR) were reordered in a BRUKER IFS 66 V-S spectrophotometer in the 4000–400 cm−1 spectral range at 2 cm−1 resolution, using pellets of samples dispersed in KBr. CHN chemical microanalysis was used for determining the organic content in diverse samples using a Perkin Elmer 2400 CHN analyzer. Thermogravimetric and thermal behaviors were analyzed from the simultaneously recorded thermogravimetric (TG) and differential scanning calorimetry (DSC) curves using a Q600 SDT Q600-TA Instruments equipment, heating (10 °C/min) from room temperature to 1000 °C in air atmosphere (100 mL/min flux). Textural properties were analyzed from the N2 adsorption/desorption isotherms at −196 °C determined in a Micromeritics ASAP 2010 analyzer. The analyzed samples (150–200 mg) were previously outgassed under dynamic vacuum for 12 h at 120 °C. The BET specific surface area was calculated from the nitrogen adsorption data in the relative pressure range from 0.05 to 0.2. The external surface area and the micropore volume were obtained by means of the t-plot according to De Boer's method (Lippens and De Boer, 1965). The total pore volume (Vp) was estimated from the amount of nitrogen adsorbed at a relative pressure of 0.99. Materials were visualized by FE-SEM using a Philips XL30S-FEG microscope after putting the powdered sample on a carbon tape and sputter it with a conducting coating of Cr. EDX analysis of samples was carried out in a Hitachi S-3000 N microscope. Samples were also visualized by TEM using a JEOL 2100F STEM microscopy (200 kV) equipped with an EDX detector (INCA x-sight of Oxford Instruments). Solid state 27Al and 29Si MAS-NMR spectra were collected at room temperature using a Bruker AVANCE-400 spectrometer in samples spun at 10 kHz. 27Al spectra were recorded operating at 104.26 MHz, using a single-pulse sequence of 2 μs and a cycle delay of 5 s for 400 accumulations, using as reference a solution of aluminum trichloride. The 29Si spectra were recorded at 79.49 MHz, using a single-pulse sequence of 6 μs with a cycle delay between accumulations of 10 s for a number of scans of 800, and the chemical shifts were evaluated in relation to tetramethylsilane as reference.

Fig. 2. TG and DSC curves of MgAl-LDH and the MgAl-LDH-DS/SiO2 intermediate.

LDH with interlayer dodecylsulphate anions (DS−) to also confer an equivalent organophilic interface with the aim to facilitate the access of alkoxysilanes such as TMOS to the interlayer space. It has been proved that direct treatment of the MgAl-LDH with TMOS without previous SDS intercalation does not afford delaminated materials according to XRD results (data not shown). As reported for clay minerals, TMOS is incorporated within the organophilic intracrystalline region being slowly hydrolyzed after addition of small amounts of water. Then it is spontaneously started a polycondensation process that through a sol-gel transition gives rise to a silica matrix, resulting here in LDH-DS/SiO2 intermediates (Fig. 1B). In the present case the gelation is attained after 50–60 min from the addition of water observing a sudden increase of the viscosity till the turning of the stirrer magnetic bar is stopped. In the present case the removal of the organic matter (mainly DS− species) and consolidation of the formed polysiloxane matrix has been carried out by heating at 400 °C in two steps, under N2 and air flux respectively, taking into account that thermal treatments at more elevated temperatures may lead to the irreversible transformation of the LDH in

3. Results and discussion 3.1. Experiments involving MgAl-LDH systems Fig. 1 shows the scheme of the general procedure used to prepare the silica/LDH nanoarchitectured materials based on a protocol previously developed in our group for the preparation of silica-clay nanocomposites (Letaïef and Ruiz-Hitzky, 2003; Letaïef et al., 2006). In the case of clay minerals belonging to 2:1 phyllosilicates such as smectites and vermiculites, the organoclays derivatives prepared by intercalation of hexadecyltrimethylammonium cations were used as intermediates in the preparation of delaminated layered silicates dispersed within a silica matrix. Here the LDH have been accordingly modified by treatment with sodium dodecylsulphate (SDS) leading to

Fig. 1. Scheme of the general procedure employed in the preparation of LDH/SiO2 nanoarchitectured materials. 67

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Fig. 3. XRD patterns of the starting MgAl-LDH-DS intercalation compound and the materials obtained at different steps of the MgAl-LDH/SiO2 nanoarchitecture synthesis, (A) for 1:1 and (B) 1:0.5 LDH:SiO2 series.

Fig. 4. FE-SEM images of (A) MgAl-LDH-DS, (B) MgAl-LDH-DS/SiO2, (C) MgAl-LDO/SiO2, (D) MgAl-LDH/SiO2-CO3, (E) MgAl-LDH/SiO2-DS and (F) MgAl-LDH/SiO2DS (1:0.5).

oxide phases. TG-DSC study supports the elimination of the organic anion when the LDH-DS/SiO2 intermediate is heated in air with a strong exothermic process that overlaps typical dehydroxilation

processes of the LDH component, which is transformed in the oxide phase (LDO) (Fig. 2). The resulting layered double oxide-silica (LDOSiO2) materials (Fig. 1C) can be re-constructed to the final LDH/SiO2 68

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Fig. 5. TEM images of (A) MgAl-LDH-DS, (B) MgAl-LDH-DS/SiO2, (C) MgAl-LDO/SiO2, and (D) MgAl-LDH/SiO2-DS.

starting MgAl-LDH-DS intercalation compound to the final MgAl-LDH/ SiO2 materials at the different steps of the synthesis process. It is clearly observed that under the adopted experimental conditions, the prepared MgAl-LDH-DS (Fig. 4A) shows the presence of typical shape of LDH platelet particles of submicrometer size. Once incorporated the TMOS and after its hydrolysis and polycondensation the produced MgAl-LDHDS/SiO2 intermediate (Fig. 4B) shows the presence of globular-like submicrometric particles in a certain degree of agglomeration. This agglomeration and cemented aspect of particles takes place after the thermal treatment that leads to the LDH dihydroxylation to the LDO oxide phase with formation of the MgAl-LDO/SiO2 material (Fig. 4C). After reconstruction, the platelet aspect of the particles is recovered, especially in the nanoarchitecture produced after treatment with SDS solution compared to the one treated with Na2CO3 (Fig. 4D and E), though in both cases the re-constructed LDH particles remain quite agglomerated and cemented by the formed SiO2 particles. There is not big differences between the aspect of materials based on the LDH composition (i.e., MgAl-LDH/SiO2-DS (1:1) and MgAl-LDH/SiO2-DS (1:0.5) materials), but in both systems it is clearly observed the presence of globular nanoparticles agglomerated on the surface of the LDH particles (Fig. 4E and F). From the TEM images shown in Fig. 5, it is confirmed some of the abovementioned insights. Thus, the starting MgAl-LDH-DS is composed of very small platelet particles typically of around 100 nm diameter (Fig. 5A). The aspect of the MgAl-LDH-DS/SiO2 intermediate compound clearly shows the presence of the LDH particles embedded into a polymeric matrix that maintains cemented the particles, some of them probably rolled-up due to a partial delamination favored by the presence of polysiloxane entities formed within the hydroxide layers (Fig. 5B). The presence of these rolled-up platelets is more clearly

samples (Fig. 1D) by treatment at room temperature with saline solutions, for instance sodium carbonate or sodium dodecylsulphate, incorporating then the carbonate or dodecylsulphate anions as counterions in the resulting nanoarchitectures, respectively. Following the general protocol various MgAl-LDH/SiO2 nanocomposite materials were prepared using two different LDH-DS:TMOS ratio with the aim to produce 1:1 and 1:0.5 LDH:SiO2 nanoarchitectures and re-constructed with incorporation of DS− and carbonate anions. Fig. 3 shows the XRD patterns of the different materials obtained in each step of the preparation process. The incorporation of TMOS into the MgAl-LDH-DS intercalation compound as indicated in the step i) in the Fig. 1 (A- > B) followed of the silica formation (step ii) is associated with an slight increase in the basal spacing from around 2.6 nm (characteristic of intercalation of DS− anions in a monolayer with partially tilted hydrocarbon chains) to 2.8 nm in the MgAl-LDH-DS/ SiO2 intermediates prepared with 1:1 (Fig. 3A) and 1:0.5 (Fig. 3B) LDH:SiO2 theoretical ratio, signaling that at least partially the hydrolysis-polycondensation of the alkoxide occurs within the interlayer region of the LDH. The stacking of layers is lost once the LDH is transformed by thermal treatment in LDO (B- > C in Fig. 1). The reconstruction (C- > D in Fig. 1), either in presence of carbonate or DS anions, maintains the high disordered stacking of layers in the regenerated LDH monosheets. The formation of the LDH can be clearly observed in both the MgAl-LDH-DS/SiO2-DS (1:1) and MgAl-LDH-DS/ SiO2-DS (1:0.5) materials where the characteristics (012) and (110) reflections at around 35 and 65° in 2θ angle are detectable within the high background (X-ray diffusion) afforded by the amorphous silica matrix, which also shows a typical scattering in the 20–30° 2θ angle region. Fig. 4 shows FE-SEM images of the different materials from the 69

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Fig. 6. (A)

27

Al and (B)

29

Si NMR spectra of several MgAl-LDH-SiO2 series samples.

observed in the agglomerates formed in MgAl-LDO/SiO2 upon the thermal treatment (Fig. 5C). After re-construction of the LDH the platelet particles appear covered by very small particles of silica (ca. 10 nm) which procures a certain degree of cementation between the reconstructed LDH particles in the MgAl-LDH/SiO2-DS material (Fig. 5D). NMR spectroscopy shows evidences of the interaction of the created SiO2 nanoparticles to the LDH matrices. When compared the 27Al NMR spectra of pure MgAl-LDH, either containing Cl− or DS− species as interlayer anions, with the other materials of the series, i.e., MgAl-LDHDS/SiO2 intermediate, MgAl-LDO/SiO2, or the re-constructed MgAlLDH/SiO2-CO3 and MgAl-LDH/SiO2-DS nanoarchitectures, it is clearly evidenced the presence of two signals in the later instead of the one characteristic of Al in octahedral coordination present in the pure LDH (Fig. 6A). This signal appears at around 9 ppm and it is clearly asymmetric in MgAl-LDH-DS, which could be related to the presence of an heterogeneous organization of the interlayer anions. The 27Al NMR spectrum of the MgAl-LDH-DS/SiO2 intermediate shows the development of a new signal centered at around 55 ppm that can be ascribed to a distortion in the coordination of polyhedral Al as consequence of the

Fig. 7. N2 adsorption-desorption isotherm at 196 °C for the MgAl-LDH/SiO2CO3 and MgAl-LDH/SiO2-DS nanoarchitectures.

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gel approach to smectites and vermiculite clay minerals, where the formed silica or metal-oxide nanoparticles originated from alkoxides within the silicate layers provoke the disorganization of the stacked platelets (Letaïef et al., 2006; Belver et al., 2012; Perez-Carvajal et al., 2016). As occurs in these last solids, the loss of LDH layers stacking together with the presence of the created silica nanoparticles result in an increase of the BET surface area in the final nanoarchitectures and the presence of porosity, which can be mainly ascribed to mesopores with a low contribution of microporosity (Table 1).

Table 1 Textural characteristics of various LDH/SiO2 nanostructured solids, calculated from their N2 adsorption–desorption isotherms. Sample

SBET (m2/ g)a

Sext (m2/ g)b

Vp (cm3/g)c

VMP (cm3/g)d

MgAl-LDH-Cl MgAl-LDH-DS MgAl-LDH-DS/SiO2 MgAl-LDO/SiO2 MgAl-LDH/SiO2-CO3 MgAl-LDH/SiO2-DS MgAl-LDH/SiO2-DS (1:0.5) NiAl-LDH/SiO2-CO3 NiCoFe-LDH/SiO2-CO3

23 18 21 106 352 284 332

n.d. n.d. n.d. n.d. 334 275 332

n.d. n.d. n.d. n.d. 0.8156 0.6604 0.5512

n.d. n.d. n.d. n.d. 0.0068 0.0003 –

622 371

570 312

0.1591 0.4651

0.0197 0.0239

3.2. Experiments involving other LDH systems This sol-gel route has been successfully extended to the preparation of other silica-LDH nanostructured materials using LDH containing transition metal ions (e.g., Ni, Co, Fe) due to their potential interest in applications related to catalysis, energy storage and sensing devices. In this way, NiAl-LDH/SiO2 and NiCoFe-LDH/SiO2 nanoarchitectures with a 1:1 LDH:SiO2 theoretical composition have been prepared from the corresponding NiAl-LDH-DS and NiCoFe-LDH-DS intercalation compounds prepared by the co-precipitation method. Fig. 8 shows the XRD patterns of the resulting materials observing, in a similar way than reported for the MgAl-LDH/SiO2 materials, the loss of the layers stacking due to the silica generation in the intracrystalline space of the pristine solid. FE-SEM images of the NiAl-LDH/SiO2-CO3 nanoarchitecture (Fig. 9A) show the presence of agglomerated particles apparently with a higher degree of agglomeration than that observed in the materials based on MgAl-LDH. Interestingly, TEM images of this material (Fig. 9B) show the presence of clusters of metal nanoparticles homogeneously and highly dispersed within the LDH nanosheets. This observation is very interesting as both NiAl-LDH/SiO2-CO3 and NiCoFeLDH/SiO2-CO3 nanoarchitectures present also an elevated specific surface area associated with both mesoporosity and microporosity, (Table 1). It is noteworthy the elevated surface area of the NiAl-LDH/ SiO2-CO3 nanoarchitecture with values above 600 m2/g, which could be relevant for diverse applications, as for instance catalysis.

n.d.: not determined (“one-point BET” measurements). a Specific surface area from BET method b Specific external surface area c Total pore volume at P/P0 equal to 0.99 d Micropore volume calculated by the t-method.

interactions with the growing silica matrix. The presence of Al in tetrahedral coordination when bonded to Si has been reported in silicaalumina materials due to the interaction between Si and Al atoms shearing an hydroxyl group [≡Al−-O+(H)-Si≡], the relative intensity of Al(IV) signal increasing with the content in Si (Eckert, 1994). In the present study, the relative intensity of Al(VI) and Al(IV) signals is reversed after the further treatments of calcination and re-construction giving rise to the MgAl-LDO/SiO2 and MgAl-LDH/SiO2 materials. This observation supports the interpretation that the agglomeration and cementation of lamellar particles due to the presence of the formed SiO2 nanoparticles in the consolidation of the nanoarchitectures took place. On the other hand, 29Si NMR spectra show in all the studied cases the presence of asymmetric signals with the most intense component centered at around −110 ppm that corresponds to the Q4 sites of fully condensed silicon atoms (Brinker and Scherer, 1990). There are other components at around −100 ppm or even at lower downfield, which may be ascribed to silicon atoms in interaction with the layered inorganic solid. However, the possibility of numerous Si(OSi)n(OAl)4-n environments and the poor resolution of the obtained spectra even after long period of accumulations preclude a deeper analysis of the formed silica-based structures. Fig. 7 shows as a selected example the N2 adsorption/desorption isotherm at −196 °C of the final MgAl-LDH/SiO2-CO3 and MgAl-LDH/ SiO2-DS materials, characteristic of this type of nanoarchitectures. The observed isotherms can be classified as type IIb (Rouquerol et al., 1999) with a type H3 hysteresis loop according to Rouquerol et al. (Sing et al., 1985; Rouquerol et al., 1999). This behavior is typical of aggregates of plate-like particles with non-rigid slit-shaped pores and it has been previously observed in other nanoarchitectures based on a similar sol-

4. Concluding remarks This work constitutes a preliminary study on the possibility to apply a sol-gel approach for the preparation of silica nanoparticles assembled to the elemental layers (nanosheets) of delaminated LDH solids to produce nanoarchitectured materials showing elevated specific surface area. The methodology here applied is related to that previously reported using layered clay minerals instead, now requiring the use of intermediate LDH previously intercalated with long-chain surfactant species such as dodecylsulfate anions. This represents an adequate interface to facilitate the access and further hydrolysis of alkoxides generating polysiloxane networks while takes place the LDH delamination.

Fig. 8. XRD patterns of NiAl-LDH/SiO2 and NiCoFe-LDH/SiO2 nanoarchitectures (1:1 LDH:SiO2 theoretical composition) and intermediates at the different steps of the synthesis process from the starting LDH-DS intercalation compounds prepared by co-precipitation. 71

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Fig. 9. FE-SEM (A) and TEM (B) images of NiAl-LDH/SiO2-CO3 nanoarchitecture.

This study can be considered as a proof of concept of a methodology that could be further extended to the production of other LDH-based nanoarchitectures using diverse alkoxide precursors. In this context, it has been proved that the procedure initially applied to MgAl- LDH can be also applied to LDH of different compositions which may result specially interesting in the case of those containing transition metal ions in their composition to produce nanoarchitectured materials of interest in catalysis or for application in energy storage devices. Considering the interest of LDH in pharmacology, the resulting materials (e.g., derived from MgAl-LDH) may be also of interest for adsorption of biomolecules and other compounds provided of biological activity in view to diverse biomedical applications. Moreover, the resulting nanoarchitectures containing silica nanoparticles could be further functionalized by treatment with organosilanes, affording multifunctionality to the final materials. Briefly, the current work represents a first step towards new nanoarchitectured solids derived from layered double hydroxides with positive results pointing out to the possibility of preparing a wide variety of materials that may show tunable functionality introduced by the nature of the LDH as well as by the involved alkoxides and, predictably, by post-synthetic grafting or ion-exchange reactions.

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