All chemicals were used without further purification. γ-Al₂O₃ with various morphologies was prepared as follows. First, 9.07 g NH₄Al(SO₄)₂·12H₂O, 0.2 g (PEG)-2000 and 15 g urea were dissolved into 100 mL deionized water under vigorously stirring till a transparent solution was formed. And then, the resulting solution was sealed in the 250 mL Teflon-lined stainless steel autoclave, and was placed in oven at different hydrothermal temperatures for 12 h. Finally, the precipitation was collected by centrifugation, washed with deionized water and ethanol for 4 times, and dried at 80℃. γ-Al₂O₃ was obtained by calcination of the collected precursors at 800℃ for 2 h with a heating rate of 10 ℃/min. The hydrothermal products synthesized at 90℃, 120℃ and 150 ℃ were named as Pre90, Pre120 and Pre150, respectively. Correspondingly, their calcined products were denoted as Alu90, Alu120 and Alu150.

X-ray diffraction (XRD, PANalytical X'Pert PRO, Almelo, Holland) and scanning electron microscope (SEM, ZEISS EVO 18, Oberkochen, Germany) were used to study the crystallinity and morphology of prepared powders, respecti--vely. The mesoporous microscopic features of the as-obtained γ-Al₂O₃ were characterized using a transmission electron microscope (TEM, JEM-2100, Tokyo, Japan). Average mesopore size distribution was measured by Barrett-Joyner-Halenda (BJH, Micromeritics ASAP2020, Atlanta, America) method from the desorption branch of isotherm. Photoluminescence analyses were performed with Jobin-Yvon Triax 320 spectrometer (PL, Jobin-Yvon Triax 320, Paris, France).

Figure 1(A) shows the typical XRD patterns of AlOOH prepared at 90℃, 120℃ and 150℃ for 12 h. XRD pattern of Pre90 in Fig. 1(A) presents that the as-obtained powder is amorphous, while the patterns of Pre120 and Pre150 can be readily indexed to the crystalline phase of AlOOH (JCPDS 01-072-0359). XRD patterns of Fig. 1(A) indicate that the hydrothermal temperature favors the particle’s crystallization. Generally, the crystal growth is determined by the rate of crystal formation and migration, which are influenced by the temperature. The lower temperature leads to the lower growth rate of crystals^{[ 22]}. Figure 1(B) presents the XRD patterns of the samples obtained by calcinations of AlOOH at 800℃ for 2 h. All the diffraction peaks can be identified as cubic γ-Al₂O₃ (JCPDS 00-001-1303). Figure 1(C) shows the small-angle XRD patterns of γ-Al₂O₃. The products show a strong diffraction peak at around 0.3º, indicating that the mesoporous structure exists in these samples.

Fig. 1

	Figure Option ViewDownloadNew Window
	Fig. 1 XRD patterns of AlOOH (A), together with their respective (B) wide- and (C) small-angle XRD patterns of γ-Al2O3 powder calcined at 800℃ for 2 h(a) Pre90; (b) Pre120; (c) Pre150

Figure 2 exhibits the different magnification SEM images of the as-obtained AlOOH. It shows that the sphere-like particle with non-uniform size is synthesized at 90℃ for 12 h in Fig. 2(A-B). The diameter of the sphere ranges from 1-5 μm and its surface is apparently smooth. Figure 2(C) and (D) show the SEM images ofPre120 which is composed of uniform and straight fibers with the length of 10-20 μm and the diameter of 200-500 nm. As the hydrothermal temperature increases to 150℃, a large number of flower-like particles with hierarchical structure are formed(Fig. 2(E-F)). The diameter of the flowers is 3-5 μm. Every flowers comprises numerous microfibers with length of 1-3 μm and a diameter of 20-30 nm, as shown in Fig. 2(E) and (F). The SEM results reveal that the hydrothermal temperature has an obvious effect on the morphology of the as-prepared precursor.

Fig. 2

	Figure Option ViewDownloadNew Window
	Fig. 2 SEM images of AlOOH(A-B)Pre90;(C-D)Pre120;(E-F)Pre150

Figure 3 shows the SEM images of γ-Al₂O₃ obtained by calcinations of the precursors. These samples show similar morphology to that of precursors. These results indicate that the transformation from AlOOH to cubic γ-Al₂O₃ without obvious morphology change and volume shrinkage, is known as a topotactical process^{[ 23, 24]}. The crystal tructure of AlOOH is orthorhombic, space group Amam (NO. 63) with a=0.369 nm, b=1.224 nm, c=0.286 nm, α= β = γ=90°, Z=4. The crystal structure of γ-Al₂O₃ is cubic, space group Fd-3m (NO. 227) with a= b= c= 0.79 nm, α= β= γ= 90°, Z=10. Comparing both above crystal structure, the transformation from AlOOH to γ-Al₂O₃ is involves only the vibration of corresponding axes other than change of the angle shift, which provides a basis for topotactical transformation^{[ 25]}.

Fig. 3

	Figure Option ViewDownloadNew Window
	Fig. 3 SEM images of γ-Al2O3 obtained from different hydrothermal products calcined at 800℃ for 2 h(A-B) Alu90; (C-D) Alu120; (E-F) Alu150

In order to observe the details of morphological structure of γ-Al₂O₃, the high-resolution transmission electron microscopy (TEM) is employed. Figure 4(A-B) present the TEM images and SAED patterns (inset) of Alu90. γ-Al₂O₃ solid sphere with non-uniform diameter can be observed from Fig. 4(A), and Fig. 4(B) presents the microsphere is assembled by small particles. The TEM images and SAED patterns (inset) of Alu120 (Fig. 4(C-D)) clearly display that γ-Al₂O₃ microfibers are prepared and the mesopores are formed homogenously in the fibers. Figure 4(E-F) show the TEM images and SAED patterns (inset) of Alu150. It is observed that the fiber-like blocks are composed of small crystals with mesopores of pore size less than 20 nm. The SAED patterns in Fig. 4 show that all the crystal phases of the products are polycrystalline. The TEM results present that γ-Al₂O₃ is consisted of small crystals and the mesopores structure is formed by decomposition of AlOOH and PEG 2000, which is consistent with SEM results.

Fig. 4

	Figure Option ViewDownloadNew Window
	Fig. 4 TEM image and a typical SAED pattern (inset) of γ-Al2O3(A-B) Alu90; (C-D) Alu120; (E-F) Alu150

Figure 5 presents the BJH pore-size distribution and N₂adsorption-desorption isotherm of γ-Al₂O₃ obtained by calcination of the precursors at 800℃. The pore size distribution curves of γ-Al₂O₃ in Fig. 5(A-B) exhibit that they have the same sized mesopores, while the pore size distribution of Alu150 (Fig. 5(C)) centers at 13 nm and 250 nm. The corresponding N₂ absorption-desorption isotherm curves in Fig. 5(D-F) are IV type with the pressure range of P/ P₀ at 0.4-0.9, 0.4-1.0 and 0.7-1.0, respectively. The curves of Fig. 5(D-E) with a hysteresis of H1 present that there exists mesopores, while the curve of Fig. 5(F) with an H4 hysteresis loop indicates that there exists silted pores due to the center cores in 3D structure. Meanwhile, the BET surface areas of the as-obtained powders are 125, 105 and 101 m²/g, and Langmuir surface areas are 173, 148 and 140 m²/g, respectively. The mesoporous structure of these products is beneficial to the potential applications in catalyst material.

Fig. 5

	Figure Option ViewDownloadNew Window
	Fig. 5 Barrett-Joyner-Halenda (BJH) pore-size distribution (A-C) and N2 adsorption and desorption isotherm (D-F) of γ-Al2O3 from different hydrothermal products(A, D) Alu90; (B, E) Alu120; (C, F) Alu150

Based on the above results, we propose a formation mechanism of different shaped γ-Al₂O₃, as presented in Fig. 6. The growth of AlOOH is composed of the process of precipitation from supersaturated solution and ostwald ripening. Urea hydrolyzes under hydrothermal conditions and boehmite precursor can be formed by the following reactions^{[ 26]}:

CO(NH₂)₂ + H₂O → CO₂↑ + 2NH₃ (1)

NH₃ + H₂O → NH₄⁺ + OH^- (2)

Al³⁺+ 3OH^- → AlOOH + H₂O (3)

Fig. 6

	Figure Option ViewDownloadNew Window
	Fig. 6 Schematic diagram of the formation of AlOOH precursor with various morphologies

At the initial of the hydrothermal process, AlOOH is formed due to the hydrolysis of urea, which is based on the mechanism of precipitation from supersaturated solution^{[ 27]}. When the hydrothermal temperature is kept at 90℃, the energy is not enough for crystal orientation, leading to the low growth rate of every crystal plane, and microsphere^{[ 28]}. As the temperature increases to 120℃, the crystal plane with grows preferentially, driven in part by relatively high surface energy and in part by a kinetic effect involving a cyclic generation of highly reactive adsorption sites^{[ 29]}. Eventually, AlOOH microfibers are fabricated. At 150℃, initially, the PEG micelle is formed due to the higher temperature resulting in the lower critical micelle concentration^{[ 30, 31]}. Then the AlOOH microfibers attach onto the PEG micelle, and the flower-like hierarchical structured AlOOH is formed^{[ 32]}. After calcination at 800℃, γ-Al₂O₃ is obtained with the morphology and volume of AlOOH remained according to Eq. (4), known as topotactical process.

2AlOOH → Al₂O₃ + H₂O (4)

Figure 7 shows the PL spectrum of micrometer-sized γ-Al₂O₃ with mesoporous structure. The PL spectra present that absorption and emission bands show almostno change with the crystal size and morphology, illustrating that the emission bands are attributed only to color centers. The spectrum is composed of a broad band, a strong peaklocated at 467 nm from emission of Xenon lamp and several weak peaks with excitation 325 nm. The broad band is consisted of three peaks at 398, 409 and 418 nm, ascribed to F centers, and the strong peak is at 467 nm with several shoulders at around 450, 481 and 490 nm, which are ascribed to the P centers^{[ 12, 33]}.

Fig. 7

	Figure Option ViewDownloadNew Window
	Fig. 7 Room temperature PL spectra of the as-prepared γ-Al2O3(a) Alu90; (b) Alu120; (c) Alu150

AlOOH microspheres, microfibers and 3D hierarchical structure can be synthesized at hydrothermal temperature of 90℃, 120℃ and 150℃, respectively. After calcination, γ-Al₂O₃ with similar morphologies with AlOOH precursors is formed through a topotactical thermal decomposition process. Owing to this formation process, the prepared γ-Al₂O₃ shows mesoporous properties and has large surface area, which might be useful for their potential application in catalyst. Besides, the PL results present that the as-synthesized γ-Al₂O₃ exhibits strong emission at room temperature.