Self-sacrificial templating synthesis of self-assembly 3D layered double hydroxide nanosheets using nano-SiO₂ under facile conditions

Abstract

Self-assembly 3D layered double hydroxide (LDH) nanosheets were synthesized using nano-SiO₂ as self-sacrificial templates via a facile bottom-up strategy. It is an environmentally friendly and economic strategy to prepare LDH nanosheets in comparison with the conventional method by exfoliation of bulk LDHs.

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Layered materials such as graphene, layered metal oxides, clays and layered double hydroxides (LDHs), can be produced as nanosheets, which are promising candidates for a variety of applications in catalysis, biomedicine and nanocomposite research because of their high surface area, nanostructure, and surface activity.^1–4 With the aim of obtaining LDH nanosheets, exfoliation of layered materials has attracted considerable interest.^5–8 Over the past few years, LDHs have been exfoliated for many applications, including pollution control, catalysis and functional materials.^9–13 However, LDHs are difficult to exfoliate because of the high charge density on the layers and strong interlayer electrostatic interactions. Modification of the LDH interlayer environment and then selection of an appropriate solvent has been shown to be an effective method to produce exfoliated LDH nanosheets.^14–16 For example, anionic surfactants such as dodecyl sulfate were used to exchange the interlayered anions, which could enlarge the interlayer spacing and weaken the brucite interlayer force, followed by dispersion in a highly polar solvent.¹⁷ However, anionic surfactants are generally harmful to the environment and human health. Meanwhile, modification of the LDH interlayer environment effects a change in the LDH surface properties. Moreover, the method of exfoliation of bulk LDHs in liquid requires appropriate solvent, many of which are harmful for environmental and human health, such as formamide.^15,17 Although the LDH nanosheets could be exfoliated in water,¹⁸ an additional step to change interlayer environment must be carried out. Resultly, it is difficult to obtain exfoliated solid LDH nanosheets from dispersion, because exfoliated nanosheets spontaneously tend towards restacking when the dispersed media is removed.¹⁹ The standing-nanosheets depend on dispersed media. The process suffers from a low production rate that is far from the demand for large-scale application.

One of the promising solutions is to synthesize nanosheets from the bottom up, that is from nucleation to nanosheets instead of from bulk to nanosheets. Up to now, many studies have been reported to prepare LDH nanosheets by bottom-up methods.^20–23 Hu et al.²¹ reported that LDH platelets with a 40–50 nm diameter and 10 nm thickness were produced in a NaDDS (sodium dodecyl sulfate)–water–isooctane reverse microemulsion system. This work is interesting and valuable, but it should be noted that it requires a water-in-oil microemulsion system, as well as the toxic NaDDS. Liang et al.²² reported unilamellar MgAl-LDH nanosheets can be synthesized in formamide directly. In these studies, part of the raw material is harmful to the environment, and the cost and production rate could not meet the wide requirement of applications. Thus, it is highly desirable to synthesize LDH nanosheets in a facile and cost-effective method without any harmful additives.

In this paper, we report a facile and economic strategy to fabricate self-assembly three-dimensional LDH nanosheets using nano-SiO₂ as an environmentally friendly inorganic additive in a bottom-up method. The key role of such strategy is that the nano-SiO₂ (TEM images in Fig. S1, ESI†) with size distribution of 20–40 nm provides templates and a silicate environment for LDH nucleation and growth. The resulting LDH nanosheets are self-assembly, three-dimensional and approximately 30 nm in platelet size and approximately 10 nm in thickness. In order to investigate the role of nano-SiO₂ throughout the entire process, the nano-SiO₂ was replaced by sodium silicate and micron-sized SiO₂ (TEM images in Fig. S1, ESI†) with the same silica molar ratio to synthesize Na₂SiO₃–MgAl-LDH, M-SiO₂–MgAl-LDH, respectively. Synthesis of SiO₂–MgAl-LDH-H (the growth process was in hydrothermal conditions, details in ESI†) was also carried out. Moreover, SiO₂–MgAl-LDH at different pHs, SiO₂–MgAl-LDH at different initial concentrations of nano-SiO₂, and the pristine MgAl-LDH were also prepared for comparison. The full experimental details can be found in the ESI.†

A schematic illustration of synthesis of self-assembly 3D LDH nanosheets is proposed (Scheme 1). Based on the experimental results, it is believed that the nano-SiO₂ provides a template, silicate environment and is incorporated into the LDH composition. In the first stage, the nano-SiO₂ provides templates for LDH nucleation and reacts with OH⁻ to transform to silicate. Because the nano-SiO₂ is a loosened template, which is different from a shape template, Mg²⁺ and Al³⁺ react with OH⁻ to yield the LDH nucleus around the surface of the nano-SiO₂. As the reaction continues, the LDH nucleus grows with nano-SiO₂ gradually decomposing, following the chemical reaction: SiO₂ + 2OH⁻ → SiO₃²⁻ + H₂O. The nascent silicate forms a high concentration area around the nano-SiO₂, with concentration decreasing from inside to outside, part of which formed [SiO₄] tetrahedrons on the broken bonds of the brucite-like layers. Subsequently, all of the nano-SiO₂ was transformed to silicate, and the solid nano-SiO₂ disappeared. It is convincing that the structure of 3D LDH nanosheets is a product coming from nano-SiO₂ and the silicate.

Scheme 1 Schematic illustration of the synthesis of self-assembly 3D LDH nanosheets.

The XRD patterns of MgAl-LDH, Na₂SiO₃–MgAl-LDH and SiO₂–MgAl-LDH are shown in Fig. 1A. The basal spacing of the samples is 0.78 nm calculated from (003) reflection using the Bragg equation. As for the SiO₂–MgAl-LDH and MgAl-LDH, although both basal spacing were 0.78 nm, the reflections of MgAl-LDH are much sharper than that of SiO₂–MgAl-LDH. The full-width at half-maximum (FWHM) of the main reflections of SiO₂–MgAl-LDH is wider than that of MgAl-LDH, revealing that the nano-SiO₂ and silicate lead to nanocrystallization and disordering for LDH. When the nano-SiO₂ was replaced by Na₂SiO₃ in the coprecipitation system, the obtained Na₂SiO₃–MgAl-LDH had lower diffraction intensity, which indicates that the silicate has a negative influence on crystallinity, leading to stacking faults and defects in the ab plane and along the c-axis of LDH. Nano-SiO₂, as an environmentally friendly nanomaterial, can react with OH⁻ to transform to silicate, so that the crystallinity of SiO₂–MgAl-LDH is lower than that of MgAl-LDH. This feature suggests that not only the silicate but the solid nano-SiO₂ had a negative influence on crystallization of LDH. It is believed that the silicate is the driving force to adjust the direction for the growth of LDH crystals. But the most interesting thing is that the reflections of Na₂SiO₃–MgAl-LDH are also sharper than that of SiO₂–MgAl-LDH, revealing that the nano-SiO₂ had a more pronounced effect on the crystallinity of the LDH, which leads to a poorer crystal structure. In the coprecipitation process, although both nano-SiO₂ and Na₂SiO₃ provide a silicate environment, the nano-SiO₂ is a solid additive and can gradually react with NaOH to transform to silicate, which breaks the original dynamic equilibrium between nutrients (Mg²⁺, Al³⁺) and OH⁻. Accordingly, the self-sacrificial template role of nano-SiO₂ is confirmed. Fig. 1B presents the Si_2p XPS spectra of the Na₂SiO₃, SiO₂–MgAl-LDH and SiO₂. The binding energies of Si_2p of Na₂SiO₃ and SiO₂ are 101.08 eV and 102.98 eV, respectively. The Si_2p XPS spectrum of SiO₂–MgAl-LDH shows the silicon atoms in two different functional groups: the silicate between nanosheets (binding energy of 101.31 eV) and the nascent Si–O bonds (binding energy of 102.07 eV). The binding energy at 102.07 eV is attributed to the Si_2p in [SiO₄] tetrahedron structure at the broken bonds of the brucite-like layers, which confirms the presence of nano-SiO₂ incorporated in the composition of the 3D LDH.

Fig. 1 (A) XRD patterns of (a) MgAl-LDH, (b) Na₂SiO₃–MgAl-LDH and (c) SiO₂–MgAl-LDH. (B) Si_2p XPS spectra of (a) Na₂SiO₃, (b) SiO₂ and (c) SiO₂–MgAl-LDH.

Fig. 2 shows the microstructure and morphology of the self-assembly 3D LDH. From the TEM and SEM (Fig. 2a and c) images of SiO₂–MgAl-LDH, it is observed that LDH layers stack in an end-face way to form large-scale nanosheets. These SiO₂–MgAl-LDH nanosheets are approximately 30 nm in platelet size and approximately 10 nm in thickness. It is noteworthy that there are some centres for LDH nanosheets (seen in the dashed circles) in which the side face of the LDH nanosheets is observed, especially seen prominently in Fig. 2b, in which system the initial concentration of nano-SiO₂ is 0.2 mol L⁻¹. These centres of LDH nanosheets may be caused by the nascent silicate around the solid nano-SiO₂, which concentration decreases from inside to outside. The yield of the SiO₂–MgAl-LDH (the amounts of the raw materials were given in ESI†) is more than 1 gram (shown on a scale at the lower left corner of Fig. 2d). Schematic illustration of the nanostructure of SiO₂–MgAl-LDH is given at the right of Fig. 2d. [SiO₄] tetrahedrons were formed on the broken bonds of the brucite-like layers, which weakens the charge density of the layers and strong interlayer electrostatic interactions of LDH. It results in stacking faults and defects in the ab plane and along the c-axis of LDH, which leads to the decease of crystallinity. This can be confirmed by the decease of the XRD intensity of SiO₂–MgAl-LDH in comparison with the pristine MgAl-LDH. The different wavenumbers of Si–O stretching vibrations in nano-SiO₂ and SiO₂–MgAl-LDH in FTIR spectra (Fig. S2, ESI†) provide additional evidence for the transformation of the nano-SiO₂.

Fig. 2 TEM images of (a) SiO₂–MgAl-LDH (initial concentration of nano-SiO₂ is 0.1 mol L⁻¹) and (b) SiO₂–MgAl-LDH (initial concentration of nano-SiO₂ is 0.2 mol L⁻¹); SEM image of (c) SiO₂–MgAl-LDH (initial concentration of nano-SiO₂ is 0.1 mol L⁻¹); (d) schematic illustration of the nanostructure of SiO₂–MgAl-LDH.

SEM and TEM images (Fig. S3, ESI†) of the MgAl-LDH and Na₂SiO₃–MgAl-LDH were also characterized for comparison. The as-synthesized pristine MgAl-LDH (Fig. S3a and b, ESI†) crystals exhibit an irregular morphology with laminar structure. The LDH layers stack together as aggregates. The morphology and structure of Na₂SiO₃–MgAl-LDH (Fig. S3c and d, ESI†) are similar to that of SiO₂–MgAl-LDH, but it is noted that the size of the LDH nanosheets are more than 100 nm, which is larger than that of SiO₂–MgAl-LDH. The reason is that the nano-SiO₂ provided templates and silicate environment for LDH nucleation and growth, while the Na₂SiO₃ could only provide silicate environment in the coprecipitation system.

The comparison of XRD patterns and TEM images of SiO₂–MgAl-LDH, M-SiO₂–MgAl-LDH and SiO₂–MgAl-LDH-H are shown in Fig. S4, ESI.† Even though both nano-SiO₂ and micron-sized SiO₂ provided templates and reacted with NaOH in the system, the reaction speed was different because of the different size of SiO₂ and the hydrothermal condition. As for M-SiO₂–MgAl-LDH and SiO₂–MgAl-LDH, because nano-SiO₂ templates decomposed gradually with LDH crystals growth, the resulting LDH nanosheets tended to be self-assembly three-dimensional structure (shown in Fig. S4a and b, ESI†), but the size of M-SiO₂–MgAl-LDH was much larger than that of SiO₂–MgAl-LDH. The micron-sized SiO₂ remained in the product, which was evidenced by the reflection of SiO₂ (marked by club in Fig. S4, ESI†) in the XRD pattern of M-SiO₂–MgAl-LDH. It is because micron-sized SiO₂ decomposed slower than nano-SiO₂ and did not decompose completely. As for the synthesis of SiO₂–MgAl-LDH-H, because the nano-SiO₂ decomposed rapidly in the autoclave pressure vessel due to the high temperature and pressure condition, the nano-SiO₂ templates was transferred to silicate rapidly. No constant nano-SiO₂ templates were supplied in the process of LDH growth, yielding bulk SiO₂–MgAl-LDH-H aggregates with large-sized sheets. It is revealed that nano-SiO₂ played as templates for LDH nucleation and growth. Accordingly, the nano-SiO₂ played the role of self-sacrificial templates in the synthesis of LDHs. The XRD reflection at 19.5° is the (100) reflection (marked by spade in Fig. S4b, ESI†) of saponite, which could be generated under hydrothermal condition of metal cations (Mg²⁺, Al³⁺), silicon and alkali source. Moreover, the upper principal by-product in the synthesis of SiO₂–MgAl-LDH was detected to be NaNO₃ and Mg₂SiO₄ (XRD pattern in Fig. S5, ESI†). The silicates transferred by SiO₂ in saponite and Mg₂SiO₄ provided an additional support for the self-sacrificial templates role of nano-SiO₂.

EDS spectrum of SiO₂–MgAl-LDH and mapping images of Mg, Al and Si are given in Fig. S6, ESI.† The presence of Si (weight content of 5.79%) in EDS spectrum and Si mapping image clearly reveal chemical composition and distribution, which indicates that the components of the LDH nanosheets containing silicon. From the FTIR spectrum of SiO₂–MgAl-LDH (Fig. S2b, ESI†), the Si–O stretching vibrations were detected at 1025 cm⁻¹, which is related to SiO₃²⁻ in the SiO₂–MgAl-LDH. The weak band around 3000 cm⁻¹ is attributed to CO₃²⁻–H₂O bridging mode of carbonate. It indicates that CO₃²⁻ existed between interlayers.

The pore structure of SiO₂–MgAl-LDH was investigated by nitrogen adsorption–desorption with the BET specific surface area of 110.21 m² g⁻¹, which is much higher than that of MgAl-LDH of 82.82 m² g⁻¹. The pore size distribution of SiO₂–MgAl-LDH and MgAl-LDH is given in Fig. S7, ESI.† The pore size of SiO₂–MgAl-LDH is in the 3–4 nm with the average pore size of 3.77 nm, while the pore size of MgAl-LDH is in a wider range with the average pore size of 5.99 nm. The homologous structure of SiO₂–MgAl-LDH was attributed to the uniform nucleation and growth in an end-face way.

The XRD of SiO₂–MgAl-LDHs at different pHs are shown in Fig. S8, ESI.† The main phase is still LDH at different pH conditions. However, as the pH increases from 9 to 13, the crystallinity of LDH decreases, as evidenced by the intensity of main reflections and superposition of (110) and (113) (Box II). The small intensity increase at 2-theta values around 20° is the trace of saponite (Box I), indicating that more [SiO₄] tetrahedrons are generated as the pH increases. In a high pH environment, the nano-SiO₂ decomposes faster than in the low pH condition, yielding more silicate in the system, which is suitable for formation of saponite. TEM images of LDH nanosheets prepared at different pHs are also given in Fig. S9, ESI.† The thickness and the size of LDH became thicker and larger as the pH increases from 9 to 13. Further experiments show the influence of the amount of nano-SiO₂ in the base solution on the morphologies of the resulting LDHs (Fig. S10, ESI†). When the amount of nano-SiO₂ increases from 0.04 mol L⁻¹ to 0.2 mol L⁻¹, the LDH nanosheets grows more regularly. When the amount of nano-SiO₂ increases to 0.2 mol L⁻¹, individual LDH nanosheets clusters could be observed, as shown in the dashed circles.

SiO₂–MgAl-LDH was used as a stabilizer to prepare Pickering emulsions. The influence of different pHs on stability of emulsions was studied. The emulsion picture is given in Fig. S11, ESI.† SiO₂–MgAl-LDH exhibits effective stabilization for the emulsions. The stability of the emulsion was independent of pH. It indicates that SiO₂–MgAl-LDH is a potential environmental friendly stabilizer to be used where pH fluctuates.

In conclusion, the present study successfully develops an environmental friendly bottom-up strategy for the green synthesis of LDH nanosheets based on the self-sacrificial nano-SiO₂. The whole process does not require additional polar solvents or any toxic raw materials. The resulting LDH nanosheets are three-dimensional, high-yield (grams), self-assembly and approximately 30 nm in platelet size and approximately 10 nm in thickness. This study may shed light on green synthesis of other novel nanosheets by the bottom-up method with great promise for various applications.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 41272064, 41572034, 51462007), the Scientific Research Project of Guangxi Education Department (No. KY2016LX132) and Guilin University of Technology, Collaborative Innovation Centre for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details; characterization; FTIR spectra of the SiO₂, SiO₂–MgAl-LDH and MgAl-LDH; pore size distribution of MgAl-LDH and SiO₂–MgAl-LDH; SEM and/or TEM images of nano-SiO₂, micron-sized SiO₂, MgAl-LDH, Na₂SiO₃–MgAl-LDH, M-SiO₂–MgAl-LDH, SiO₂–MgAl-LDH-H, SiO₂–MgAl-LDHs at different pHs and SiO₂–MgAl-LDHs at different initial concentrations of nano-SiO₂; XRD patterns of M-SiO₂–MgAl-LDH, SiO₂–MgAl-LDH-H, SiO₂–MgAl-LDHs at different pH conditions. EDS spectrum of SiO₂–MgAl-LDH and mapping images of Mg, Al and Si. Picture of emulsions stabilized by SiO₂–MgAl-LDH at different pHs. See DOI: 10.1039/c6ra20121k

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