Brij-58 template synthesis of self-assembled thermostable lamellar crystalline zirconia via a reflux-hydrothermal hybrid method

Abstract

Using polyoxyethylene-20-cetyl-ether (Brij-58) as a supramolecular template, thermostable lamellar crystalline zirconia (TSLCZ) was synthesized though the reflux-hydrothermal hybrid method (R-HT) with ZrOCl₂·8H₂O as the zirconium source and sodium hydroxide as the precipitating agent. The TSLCZs were characterized by XRD, TEM, SAED, FE-SEM, TG-DTA and FTIR. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analyses showed that zirconia is the only crystal phase in TSLCZ and Brij-58 promoted zirconia nucleation and 2D-growth and induced self-ordered lamellar nanostructured assembly. Transmission electron microscopic (TEM) observations further proved this regular lamellar structure, which was detected by XRD. The repeat periodicity of the structure is about 1.20 nm, as observed by TEM, which is in well accordance with the XRD data (1.19 nm). The selected area electron diffraction (SAED) results indicate that TSLCZ is a polycrystalline structure. XRD, TEM and field emission scanning electron microscopy (FE-SEM) results show that the TSLCZ can be well preserved even after a high temperature thermal treatment at 500 °C for 2 h. Moreover, the R-HT significantly promoted the Brij-58 directed synthesis of TSLCZ in the three aspects of heterogeneous nucleation, controlled 2-D growth and ordered lamellar self-assembly as compared to only the reflux (R) or hydrothermal (HT) method. The possible formation of the TSLCZ could be explained by the use of the surfactant template.

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1. Introduction

Self-assembly is the spontaneous association of an ensemble of molecules into one or more supramolecular structures. Self-assembly allows the construction of complex, adaptable, and highly tunable materials. The use of self-assembly for the construction of functional materials now is a highly promising and exciting area of research. Over the past ten decades, a number of self-assembly materials, such as functional biomaterials,¹ “tree-like” amphiphilic glycopolypeptides,² complex hollow materials,³ triphenylene-containing conjugated macrocycles,⁴ N-isopropylacrylamide,⁵ branched glycopolypeptides,⁶ TiO₂ nanoparticles,⁷ and zirconium titanate,²⁶ have been successfully synthesized.

Zirconia is one of the most important functional metal oxide materials, which can be widely used as catalysts and catalyst supports,^8–10 dielectric materials,¹¹ ceramics,¹² solitary photocatalytic materials,^13,14 and solid oxide fuel cells.¹⁵ Recently, self-assembled highly crystalline thermally stable zirconias have been considered as important materials due to their several unique properties.^16,20 A series of self-assembled ordered superstructured zirconia materials have been successfully synthesized by the use of surfactants such as anionic surfactants,^17–19 blockcopolymers,²⁰ cationic surfactants²¹ and composite surfactants.²² Unfortunately, in most cases, the ordered superstructured zirconia that are synthesized are still far from satisfactory and their crystalline framework and corresponding superstructure collapses during subsequent crystallization or heat treatment; thus the synthesis of thermostable self-assembled ordered lamellar superstructured crystalline zirconia remains a great challenge, which severely hinders practical applications.

Brij-58 (C₁₆H₃₃O(CH₂CH₂O)₂₀H) is a common nonionic surfactant, which has a calculated HLB value of 15.7. This surfactant has been used in solubilizing proteins,²³ porous hydrogels,²⁴ nanoporous carbons,²⁵ and mesoporous zirconium titanium oxide thin films,²⁶ but there are no reports about the Brij-58 assisted synthesis of ordered lamellar materials.

Herein, the Brij-58 template was successfully applied to synthesize thermostable lamellar crystalline zirconia (TSLCZ). The possible synthesis mechanism via this novel Brij-58 template is discussed.

2. Experimental

(1) Materials

ZrOCl₂·8H₂O and NaOH were analytical grade reagents and were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Deionized water was used throughout the experiment. The nonionic surfactant, polyoxyethylene-20-cetyl ether (Brij-58, M_w = 1123.5, SERVA Company), HLB (hydrophile–lipophile balance) = 15.7, was of analytic grade and was used as received without further purification.

(2) Synthesis of TSLCZ via the R-HT route

A typical synthetic procedure is as follows: 0.72 g of NaOH and 0.75 g Brij-58 were dissolved in 50 mL water. Under vigorous stirring, this mixture was slowly added to a 50 mL solution of 0.97 g ZrOCl₂·8H₂O. After refluxing for 2 h under stirring, the resulting suspension was sealed in a PTFE-lined stainless steel autoclave and crystallized at 160 °C for 12 h. After cooling and deposition at room temperature for 12 h, the precipitate was filtered and dried at 60 °C. The white thermostable lamellar crystalline zirconia (TSLCZ) powder was then obtained. The as-synthesized TSLCZ was calcined in air from room temperature to 500 °C at a heating rate of 5 °C min⁻¹, maintained at 500 °C for 2 h, and then cooled to room temperature to obtain the calcined TSLCZ. Samples were prepared using the R-HT, R, and HT methods and the synthesis recipes are mentioned in Table 1.

Table 1 Synthesis recipes of different methods

Method	ZrOCl2	NaOH	Brij-58	Reflux time at 100 °C	Hydrothermal time at 160 °C
R-HT	0.003 mol	0.018 mol	0.75 g	2 h	12 h
R	0.003 mol	0.018 mol	0.75 g	2 h	—
HT	0.003 mol	0.018 mol	0.75 g	—	12 h

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(3) Characterization

The samples prepared using the reflux (R), reflux-hydrothermal hybrid (R-HT), hydrothermal (HT) and calcined methods were analyzed and compared using powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM). XRD data were recorded using a Rigaku D/max 2500PC X-ray diffractometer (Rigaku Corporation, Tokyo, Japan), with a Cu Kα target, voltage of 40 kV, current of 150 mA, scan range of 3–80° and step of 0.02°. TEM data was observed using a JEM-2100 [JEOL Ltd, Tokyo, Japan] with 100 kV. FE-SEM images were observed using a field emission scanning electron microscope (FE-SEM, Hitachi S-4800) at 10.0 kV after samples were sputter coated with gold under vacuum. The thermostable lamellar crystalline zirconia (TSLCZ) was characterized by Fourier transform infrared spectroscopy (FT-IR) and thermal gravimetry and differential thermal analysis (TG-DTA). Fourier transform infrared spectroscopy (FTIR, TENSOR37, German) spectra were recorded over the range from 400 to 4000 cm⁻¹ after the samples were mixed with KBr powder. TG-DTA studies were carried out between room temperature and 1300 °C using a Seiko TG/DTA 6300.

3. Results and discussion

(1) X-ray diffraction (XRD) analysis

In order to investigate the role of the Brij-58 template in the ordered lamellar superstructure and crystallization, (b) calcined, (c) HT, (d) R samples were synthesized for comparison.

The samples were analyzed by XRD (Fig. 1). As shown in the small angle XRD (SAXRD, 2θ = 5–10°) of the four samples synthesized via different routes, significant peaks were detected at 2θ = 7.38° and 8.72° [labelled L(100)] {Fig. 1(a) and (b)}. This SAXRD peak can be attributed to an ordered lamellar nanostructure. According to the Bragg equation (eqn (1)),

2d sin θ = nλ (1)

where, λ = 0.15406 nm, the corresponding calculated repeat period is 1.19 nm. According to the lamellar formula (eqn (2)):^29,30

d_hkl = a/(h² + k² + l²)^1/2 (2)

where a is the lamellar parameter, a₁ = 1.19 nm {Fig. 1(a)} and a₂ = 1.01 nm {Fig. 1(b)}. The above mentioned calculation reflects that the corresponding lamellar period just slightly shrank from 1.19 to 1.01 nm during the calcination at 500 °C (confirmed by Fig. 2(a) and (b)), and the ordered lamellar superstructure is very thermostable.

Fig. 1 XRD patterns of the samples prepared using the (a) R-HT, (b) calcined, (c) HT, and (d) R methods, showing the L ordered lamellar superstructure, M monoclinic ZrO₂, and C cubic ZrO₂.

Fig. 2 TEM images of the samples prepared using the (a) R-HT, (b) calcined, (c) R, and (d) HT methods via the Brij-58 template. Inset shows the SAED image of the calcined TSLCZ in (b).

Furthermore, comparing the SAXRD peaks of R, HT and R-HT, R-HT has the most evident (100) diffraction peak. This indicates that the R-HT method significantly promoted the Brij-58 templating role in the ordered superstructure.

The WAXRD patterns (2θ = 10–70°) of R, HT and R-HT show that the HT and R-HT method can promote crystallization, and as a result, a mixed phase of monoclinic and cubic was obtained, and the latter is the main phase. Samples are mixed with an ordered lamellar crystalline zirconium oxide nanostructure, and it can be seen that the different preparation methods only affect the phase composition ratio of the product and the apparent degree of crystallinity of the product. In addition, the difference in the WAXRD patterns indicated that R-HT not only effectively induced the self-assembly of the TSLCZ, but also significantly controlled the zirconia nucleation and crystal growth.

In summary, R-HT is the best method for the Brij-58 templating synthesis of thermostable lamellar crystalline zirconia.

(2) Transmission electron microscopy (TEM) observations

To further support the XRD analysis, TEM was used to observe the morphologies of the samples synthesized via the different routes (Fig. 2). From Fig. 2(a) and (b), it can be seen that there are 1.20 nm and 1.00 nm of period structures, and the dark and light thickness is about 0.6 nm and 0.5 nm, alternately. Because the XRD (Fig. 1) and FTIR (Fig. 5) analyses showed no other crystal phases, except for pure zirconia, this ordered nano-array should consist of a zirconia layer (dark) and pore layer (light), alternately. These data are in well agreement with the 1.19 nm and 1.01 nm of the calculated results from SAXRD (confirmed by Fig. 1(a) and (b)). The inset in Fig. 2(b) shows the corresponding selected area electron diffraction (SAED) pattern taken from an ordered nanoarray of TSLCZ after calcination. The SAED results indicate that the TSLCZ is polycrystalline. The rings are sharp and continuous, which shows that the TSLCZ is highly crystalline. The TEM and SAED analyses further support that the ordered lamellar structure of TSLCZ exhibits good thermostability at 500 °C.

In contrast, no ordered lamellar structures were found in the samples synthesized by only the R and HT methods.

(3) Field emission scanning electron microscopy (FE-SEM) observations

The samples synthesized using the different methods were observed by FE-SEM. Fig. 3(a) and (b) show that the morphologies of the as-synthesized and calcined TSLCZ have no significant changes, and they show similar multilayer morphologies (confirmed by Fig. 1(a) and (b) and 2(a) and (b)). This reflects that TSLCZ has very good morphology thermostability at 500 °C. In contrast, the products synthesized via the R and HT route exhibit different morphologies (as shown in Fig. 3(c) and (d)), and specifically, none of them exhibit the multilayer morphology. The difference between them is that the latter (Fig. 3(d)) shows a considerably clearer shape and boundary than the former (Fig. 3(c)). This reflects their different crystallinities, as presented in (Fig. 1(c) and (d)).

Fig. 3 FE-SEM images of samples prepared using the (a) R-HT, (b) calcined, (c) R, an (d) HT methods via the Brij-58 template.

(4) Thermal gravimetry and differential thermal analysis (TG-DTA) analysis of TSLCZ

TG-DTA was used to observe the morphologies of the samples, and the TG-DTA results of TSLCZ are shown in (Fig. 4). For the case of TSLCZ, there was one period of weight loss. This period occurred at 0 °C to 200 °C, due to the loss of physically adsorbed water. Subsequently, the weight almost has slightly changed in the temperature range from 200 °C to 1200 °C. On the other hand, in the entire process, there were no evident endothermal and exothermic peaks in the DTA curve, which indicate that the TSLCZ synthesized though R-HT method has high stability.

Fig. 4 TG-DTA curves of TSLCZ via the Brij-58 template.

(5) Fourier transform infrared spectroscopy (FTIR) analysis of TSLCZ

In order to examine the samples prepared by the different methods containing the Brij-58 template, Brij-58 and the samples prepared by the different methods were analyzed by FTIR (Fig. 5). As can be seen from Fig. 5(a), (b) and (d), the absorption peak appearing at 3431 cm⁻¹ could be assigned to the stretching vibration of the sample water-binding group H–O; the absorption bands that occur at 1638 cm⁻¹ and 1352 cm⁻¹ were the bending vibration peaks, and they belonged to the coordinated water and the coordination hydroxyl group, respectively, and the corresponding absorption peaks of Zr–O (970, 758 and 586 cm⁻¹ in Fig. 5(a), (b) and (d)) were also found in the samples. For Brij-58 (Fig. 5(c)), a triple peak appears at 1062–1146 cm⁻¹, which could be attributed to the –CH₂–O–CH₂– stretching vibration. In comparison with Fig. 5(a), (b), and (d), no clear triple peak of Brij-58 was found in the absorption peaks, and thus, Brij-58 acts as a template nanostructure in the preparation of ordered crystalline layered zirconium oxide and is discarded during the hot water washing and drying process. However, only the R-HT method can synthesize the self-assembly thermostable lamellar crystalline zirconia without post-treatments.

Fig. 5 FTIR spectra of the (a) HT, (b) R, (c) Brij-58, (d) R-HT samples.

(6) Template action of Brij-58 in TSLCZ

From these experiments, the role of the Brij-58 template and synthesis mechanism of TSLCZ (Fig. 6) may be described as follows: Brij-58 contains hydrophilic polyoxyethylene and hydrophobic alkyl chain segments that associate with each other through hydrophobic interactions and electrostatic forces to minimize the system's free energy. Thus, Brij-58 forms spherical, cylindrical and layered structures under various concentrations on account of the differences in surface tension. Brij-58's hydrophilic head interacts with ZrO²⁺ and its hydrophobic alkyl chain segments prefer to be close to each other via hydrophobic interactions to form a bilayer template (BLT). In this nanohybrid system, Brij-58 selectively adsorbs on the amorphous zirconia surface, and as a result, the interface energy (σ) of the nucleus decreases. Moreover, amorphous zirconia has higher supersaturation (S) in this micro-phase-separated nanohybrid system than in the solution phase.

Fig. 6 Lamellar formation mechanism of TSLCZ synthesized via the Brij-58 template, and the possible interaction between zirconia and Brij-58.

According to the Gibbs free energy equation of nucleation,^27–30

ΔG* = Bσ³V²/(kT ln S)²

where ΔG* refers to the critical free energy, B is a constant, σ is the interfacial energy, V is the volume of the molecule, k is the Boltzmann constant, T is the absolute temperature, and S is the supersaturation.

The two parameters (σ and S) cause the critical nucleation free energy (ΔG*) to decrease. Accordingly, crystal nuclei are considerably easier to form in this micro-phase-separated nano hybrid system. With the crystal growth and the template guide, the 1.20 nm of period ordered lamellar superstructured crystalline zirconia were generated via the Brij-58 supramolecular template.

Therefore, nuclei can be formed in priority in this micro-phase separated hybrid and zirconia crystallizes in situ under the control of the Brij-58 template, and then it grows into layered particles between the bilayer Brij-58 templates.

4. Summary

In this article, we successfully used Brij-58 as an independent soft template to synthesize self-assembled thermostable lamellar crystalline zirconia without post-treatments. Brij-58 played the role of the template in the preparation of ordered crystalline layered nanostructure zirconia. On the one hand, it promoted zirconia nucleation and growth, and on other hand, it induced a self-assembled layered nanostructure assembly. Moreover, the R-HT route well promoted the Brij-58 supramolecular template role and may be useful for the syntheses of other self assembly nanostructured materials with good thermostabilities. The possible mechanism may be described by the novel Brij-58 templating synthesis.

Acknowledgements

This study is supported by the National Natural Science Foundation of China (50902043 and 21104057) and Natural Science Foundation of Hebei Province (E2009000081 and E2015202069). We greatly appreciate the reviewers' valuable comments.

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