A. Thermal behavior of the gel precursor

Figure 1 shows the thermal behavior of the [C16mim]Br-based dry gel in the temperature range of room temperature to 1200 °C. Two major exothermic peaks were observed at 425 and 800 °C in the DSC curve. The peak at 425 °C is related to the decomposition of the metal nitrates and the metal-ionic liquids, a corresponding 50% weight loss was obtained at the same temperature from the TG curve. The peak at 800 °C with no weight loss was attributed to exothermic crystallization of Na₂Al_2xO_3x+1 (JCPDS-291164). Based on the results, the calcination route was set as follows: the precursor was heated to 450 °C for 2 h for decomposition and then 800–1100 °C for 3 h for the phase formation.

FIG. 1.

TG-DSC curves of the dry gel prepared with [C16mim]Br.

We can see in Fig. 2 that the product obtained was primarily amorphous at the temperature below 800 °C. While at 900 °C, an intermediate phase, Na₂Al_2xO_3x+1 (JCPDS 29–1164),^{Reference Li, Wen, Wu, Zhang and Liu5} formed. At temperatures >1000 °C, a phase transition occurs. It is indicated by the XRD results that the final powder is a mixture of Na-β″-Al₂O₃ and some Na-β-Al₂O₃ (JCPDS No. of 48-1258 and 19-1173, respectively, simply described as Na-β″/β-Al₂O₃). As seen, Na-β″/β-Al₂O₃ powder with good crystallinity was obtained after the precursor was further heated at 1100 °C. This synthesis temperature for Na-β″/β-Al₂O₃ was 150 °C lower than that of the conventional solid-state reaction method or most wet chemical methods reported.^{Reference Virkar, Miller, Cutler and Gordon3,Reference Eddy and Rhodes17,Reference Virkar18} According to Scherrer equation, the crystallite size was estimated to be about 50 nm. The chemical formula of the final synthesized powder was determined to be Na_1.61Li_0.29Al_10.70O₁₇ according to chemical composition analysis results.

FIG. 2.

XRD patterns of the [C16mim]Br-based dry gel and the powders calcined at different temperatures.

B. FTIR analysis of the precursor

Fourier transform infrared spectroscopy (FTIR) was used to qualitatively illustrate the interaction between [C16mim]Br and metal ions. Figure 3 shows the FTIR profile of the dried gel heated at 120 °C for 2 days at ambient pressure. The pure [C16mim]Br was also tested for comparison. The characteristic bands at 3450 and 1180 cm⁻¹ were distinguished in the spectrum of [C16mim]Br with metal ions, which were not observed in the spectrum of the pure [C16mim]Br sample. According to the standard spectrum,^{Reference Zhao, Hu, Zhang, Guan and Yu19} the adsorption band at 3450 cm⁻¹ could be attributed to the hydrogen bond in C position of the imidazolium ring. The characteristic band at 1180 cm⁻¹ indicated the stretching vibration of the imidazolium ring. Both bands revealed the combination of [C16mim]Br with the metal ions.

FIG. 3.

FTIR patterns of the dry gel and [C16mim]Br.

C. Morphology of the powders

Morphologies of the powders prepared in different solvents were observed by TEM. Typically, as seen in Fig. 4(d), the particles prepared with [C16mim]Br solvent were nano-sized and highly dispersed. A regular nanorod shape of about 50 nm in diameter and 200–300 nm in length was observed. The diameter was uniform for almost all particles. However, some abnormally long nanorods were found as shown in Fig. 4(e).

FIG. 4.

TEM photographs of the powders prepared with different ionic liquid templates, (a) pure water, (b) [C4mim]Br, (c) [C12mim]Br, (d and e) [C16mim]Br.

Powders were prepared in different ILs. The results showed that the ILs play an important role in controlling the particle shape of the calcined product. As shown in Fig. 4(a), in the absence of ILs, the particles showed typical hexagonal plate type morphology of Na-β″/β-Al₂O₃ with the size in the range of tens to 300 nm. Figures 4(b) and (c) show the morphologies of the particles fabricated by changing the ILs to [C4mim]Br and [C12mim]Br, respectively. In particular, the former product was composed of nanoparticles with a shape greatly different from that of the powder prepared in [C16mim]Br. Anyway, the final products exhibited the tendency to form rod-type particles, although it was highly agglomerated when a short alkyl chain IL was used. As seen, the sample prepared with [C12mim]Br solvent displayed nanorod shape with the size much smaller than those fabricated using [C16mim]Br, but the particles were aggregated. These results indicate that the morphology of Na-β″/β-Al₂O₃ powders is strongly dependent on the type of IL or the length of the alkyl chain in this work.

The influence of the amount of ionic liquid on the synthesis was also investigated. Figures 5(a) and (b) show the morphology of the powders obtained with 10 and 100-fold amount of [C16mim]Br as designed in the preparation procedure (i.e., M = 0.94 and 9.4 mol/L, respectively). It was found that excessive ILs did not affect the average particle size of the final calcined powder, indicating no obvious effect of ionic liquid on the morphology of the products at higher IL concentration.

FIG. 5.

TEM photographs of the powders prepared with different concentrations of [C16mim]Br ionic liquid in the reaction systems, (a) 10-fold and (b) 100-fold.

D. Mechanism for the formation of Na-β″/β-Al₂O₃ nanorods

Till now, it is still difficult to fully understand the mechanism of the nanorod-formation process, though some work has been reported in the literature.^{Reference Schmidt, Ellison, Holliday, Kubin and Trujillo20–Reference Rautaray, Banpurkar, Sainkar, Limaye, Pavaskar, Ogale and Sastry22} Based on the results obtained in our work and previous work, it is believed that the ILs acted as the templates, which can form different structures by self-assembly under different conditions. The proposed principle for the formation of the Na-β″/β-Al₂O₃ nanorods is schematically illustrated in Fig. 6. Figure 6(a) shows the structure of [C16mim]Br,^{Reference Guerra-Abreu, Pinoa, Anderson and Afonso23} which consists of one [C16mim]⁺ cation and one Br⁻ anion. While dissolved in water, the ILs can be connected by weak interactions of C–H···O and O–H···Br, then a 1D chain was formed with the long alkyl groups on two sides^{Reference Virkar18} [Fig. 6(b)]. The added aluminum isopropoxide hydrolyzed, and Al(OH)₃ (Ref. Reference Zeng and Yang24) was produced, which was connected with the [C16mim]⁺ cations by weak interactions originating from the hydrogen bond formed between H atom at C in the imidazolium ring and O atom in the hydroxyl of Al(OH)₃ along the 1D chain, as shown in Fig. 6(c). Then a one-dimensional precursor containing the components of Na-β″/β-Al₂O₃ would be formed, as shown in Figs. 6(d) and (e).

FIG. 6.

Proposed growth mechanism of Na-β″/β-Al₂O₃ nanorods with ionic liquid as the template, (a) structural illustration of ILs, (b) proposed hydrogen bond in ILs, (c) aggregation of the hydrogen bond connected ILs, (d) precursor rods formed in the gel, (e) Na-β″/β-Al₂O₃ nanorods after calcination.

According to the Wulff facet theorem, the shape of a crystal is determined by the relative specific surface energies associated with the facets of the crystal.^{Reference Zhu, Wang, Qi and Hu25} This theorem implied that one could control the final shape of a crystal by changing the free energies of the various crystallographic surfaces and thus to alter their growth rates. For the obtained precursor, the facet perpendicular to the bar means that the Na-β″/β-Al₂O₃ precursor attaches more easily to this facet and this facet has the largest relative specific surface energies, indicating that grain growth along the bars is preferred to other directions. It was suggested that in the precursor, the cations were preferred to bind to the facets that were not perpendicular to the bars, resulting in the largest relative specific surface energies of the facets perpendicular to the bars. In a typical grain growth process, Na-β″/β-Al₂O₃ nanocrystals rapidly grew primarily along the bar axis and coalesced to form nano-bars.

Why did the morphologies of the products differ when different ILs were used? It would be related to the morphology of the IL aggregates shown in Fig. 6(b), which favorably increases their interactions and minimizes unfavorable interactions with water.^{Reference Bhargava and Klein26} Based on the results from Wang’s group,^{Reference Wang, Wang, Zhang, Zhang and Zhao27,Reference Zhang, Liang, Wang and Li28} the aggregation of ionic liquids depends on the alkyl chain length of their cations, that is, the aggregation of [C16mim]Br was stronger than that of [C4mim]Br and [C12mim]Br. So the length of the lines in Fig.6(c) is longer. While the amount of ILs increased, less Al(OH)₃ was connected with one [C16mim]⁺ cation, which is directly correlated to the generation of the nucleation centers for Na-β″/β-Al₂O₃. Therefore, the increase in the amount of ILs influences the morphology of Na-β″/β-Al₂O₃ to some extent.

E. Performance of Na-β″/β-Al₂O₃ nanorods

Figure 7 shows the N₂ adsorption–desorption isotherms of the nanorod Na-β″/β-Al₂O₃ powders. The BET surface area was 81.3 m²/g compared with 8.1 m²/g obtained by the SSR process. Figure 8 compares the shrinkage behavior among the green bars prepared by SSR, sol–gel process, and the IL-assisted method. As seen, there was no obvious difference for the on-set temperature for sintering as a result of the formation process. But the total shrinkage from room temperature to 1500 °C was 12.9% and 11.4%, respectively, for the IL-based and sol-gel based samples and only 7.1 % for the SSR sample. The slopes at the end of the curves for both IL-assisted and sol–gel samples were steep, indicating their shrinkages were far from ending at 1500 °C. The end part of the curve for the SSR sample had an inflection point, meaning that the shrinkage is finished. For determining the formula of the powder, we did the ICP-OES analysis, and the molar ratio of 1.61:0.29:10.70:17 for Na:Li:Al:O was obtained.

FIG. 7.

Nitrogen adsorption–desorption isotherms of powders by SSR and IL processes.

FIG. 8.

Temperature dependence of linear shrinkage of different powders.