Hollow Al₂O₃ spheres prepared by a simple and tunable hydrothermal method

Abstract

Hollow nanostructures are of great interest in many current and emerging areas of technology. In this paper, uniform hollow spheres of alumina were successfully synthesized by a simple and tunable hydrothermal treatment and calcination process. The effects of preparation parameters on the properties of the spheres, for example, surface area, particle diameter and thickness of the shell, were explored. The microstructure, morphology, and chemical composition of the resulting materials were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetry, ¹³C solid state NMR, and N₂ adsorption–desorption techniques. Based on the characterizations of as-prepared products, a new formation mechanism of the alumina hollow spheres was proposed.

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Introduction

Hollow spheres which integrate hollow interiors or voids with mesoporous shells have recently attracted scientific and technological interests due to their outstanding features of low density, presence of mesoporous channels on the shell, high permeability and tunable properties.¹ By adjusting the texture and structure, hollow spheres are proposed to have a broad range of applications in the fields of catalysis, controlled release of various substances, photonic band-gap crystals, microcapsule reactors, chemical sensors etc.^2–7

Over the past decades, many efforts have been made in the development of strategies for the design and fabrication of the hollow capsules, to take advantage of the enhanced properties endowed by the hollow structures. These methods include interfacial synthesis, ultrasound mediated, surface polymerization, Ostwald ripening, gas-blowing and spray-drying.^8–15 Among them, template method has been widely employed to fabricate hollow spheres with porous shells as the shape and cavity size of the formed hollow structures can be directly determined by the templates.^16–19 Recently, the colloidal carbonaceous sphere with functional groups and reactive surfaces was used as a green and novel template to synthesize a variety of materials with hollow structures,^20–22 e.g. Li and co-workers prepared a series of metal oxide hollow spheres, such as SnO₂, Ga₂O₃, TiO₂, ZrO₂ and noble-metal nanoparticles, combining surface-layer-adsorption with calcinations process.^23–25 However, these synthesis strategies are often complicated. Besides, the yield of the metal oxides and the thickness of the shell wall are limited due to the adsorption capability of the hydrophilic surfaces of carbonaceous spheres for metallic cations. Thus, there is still a challenge to develop simple, controllable, and environmentally friendly methods for the synthesis of hollow spheres.

Of the various methods, hydrothermal synthesis is advantageous in terms of its single-step, time-saving, and low-energy-consumption.²⁶ Titirici et al.²⁷ had synthesized metal oxides hollow spheres of Fe₂O₃, Ni₂O₃, Co₃O₄, CeO₂, MgO, and CuO via hydrothermal approach, which demonstrated it's universal for obtaining hollow spheres of metal oxides. Formation of clusters of the products did not occur because the metallic cations were uniformly dispersed in the spheres rather than over their surfaces. However, the yield of this hydrothermal process is usually lower than predicted when considering the concentration of the dispersed metal ions.

Porous Al₂O₃ particles have been used extensively due to their good thermal and chemical stability, high specific surface area, and availability.^28,29 Cai et al. synthesized hierarchical boehmite hollow core–shell and hollow microspheres by a facile hydrothermal method with sulfate and sodium tartrate as additives, respectively.^30,31 Zhang et al. synthesized Co₃O₄/Al₂O₃ hollow core–shell microspheres via a solvothermal approach in the presence of trisodium citrate.³² Glucose has been widely used as a template agent and organic fuel for the generalized synthesis of metal oxide hollow spheres by the hydrothermal technique.^33,34 Rattle-type carbon–alumina core–shell spheres had been synthesized by Zhou et al. using colloidal carbon spheres as hard templates.³⁵ However, the detailed hydrothermal reaction and calcination conditions and their effects on morphology and properties of hollow spheres had not been studied thoroughly yet.

In the present paper, uniformed alumina hollow spheres with large cavities, relatively high surface area, and porous structures were prepared through hydrothermal approach combined with calcinations treatment. The preparation conditions of alumina hollow spheres were explored systematically, and a new formation mechanism was proposed based on the characterization results.

Experimental procedures

Materials

All the chemicals were analytical grade from Sinopharm Chemical Reagent limited corporation and were used without further purification.

Synthesis of hollow Al₂O₃ microspheres

Al₂O₃ hollow spheres were fabricated by a hydrothermal approach of mix aqueous solutions of glucose and aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O) with molar ratios of 16, 8, 4, 2 and 1. Typically, 6.48 g glucose and 6.76 g Al(NO₃)₃·9H₂O was dissolved in 90 mL of distilled water under vigorous stirring. After ultrasonic dispersion for 10 min, the mixed solution was placed in a 150 mL Teflon-lined stainless steel autoclave, followed by heated to 160 °C for 6 h. A puce or black powdery mass comprising of hydrated alumina and carbon particles was collected by filtration and washed with distilled water and ethanol for several times followed by drying at 80 °C for 8 h. Then, the metal oxide–carbon composites were calcined in air for 3 h to remove the carbon core. The Al₂O₃ samples calcined at 550, 750, 850 and 950 °C were denoted as Al-550, Al-750, Al-850, and Al-950, respectively. When 8 g Al(NO₃)₃·9H₂O was used, the alumina hollow spheres prepared with different glucose : metal salt molar ratios (2 : 1, 4 : 1, 8 : 1 and 16 : 1) were denoted as CAl-2, CAl-4, CAl-8, and CAl-16, respectively. 10 mL ethanol or urea of 0.2 M were also used as additives to prepared alumina hollow spheres.

Characterization

Powder X-ray diffraction (XRD) patterns of the samples were examined on a D8 Advance X-ray diffractometer (Bruker Corporation, Germany) using CuKα radiation at a scan rate of 0.05° 2θ s⁻¹. The particle size and morphology of the oxide hollow spheres were observed on a JSM-7001F Field Emission Scanning Electron Microscope (JEOL, Japan) at an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) analyses were carried out on a JEM-2010 electron microscope (JEOL, Japan) using a 200 kV accelerating voltage. The Brunauer–Emmett–Teller (BET) surface area of the powders was analyzed by nitrogen adsorption in a Tristar 3020 nitrogen adsorption apparatus (Micromeritics, U.S.A.). All the samples were degassed at 150 °C prior to nitrogen adsorption measurements. The BET surface area was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P₀) range of 0.06–0.2. Adsorption isotherms were used to determine the pore-size distributions for the samples by the Barrett–Joyner–Halenda (BJH) method, assuming a cylindrical pore model. The volume of nitrogen adsorbed at the relative pressure (P/P₀) of 0.994 was used to determine the pore volume. IR (FT-IR) spectra were measured on a Perkin-Elmer 580B IR spectrophotometer using KBr pellet technique. ¹³C solid state NMR spectra were acquired on a Bruker IIITM 600 MHz NMR spectrometer (14.09 T) at room temperature. The ¹³C NMR spectra were obtained at frequencies of 151.01 MHz. The chemical shifts in ¹³C NMR spectra were referred to the adamantane. Thermogravimetry analysis (TG) and differential thermal analysis (DTA) were conducted on a Rigaku TG thermal analyzer with a heating rate of 10 °C min⁻¹.

Results and discussion

Phase, morphology, and structure

The phase structure and relative crystallinity of the prepared samples were investigated by XRD (Fig. 1). It was observed that no characteristic peaks of crystalline alumina were observed before calcination (Fig. 1(a)), which confirms that after hydrothermal treatment, the Al³⁺ were evenly dispersed in the carbon particles as amorphous clusters.^35,36 As can be seen in Fig. 1(b), the Al₂O₃ hollow sphere prepared at 550 °C was amorphous,³⁷ suggesting that the existence of glucose probably inhibited the crystallization of Al₂O₃. The crystallization of γ-Al₂O₃ was initiated at 750 °C, and the intensity of γ-alumina peaks and crystallite size increased with the increase of calcinations temperature (Fig. 1(c and d)). The average crystallite size of the γ-Al₂O₃ calculated using the Scherrer formula was around 10.6 nm for the sample treated at 850 °C. The structures of both γ-Al₂O₃ and α-Al₂O₃ were observed after calcination at 950 °C (Fig. 1(e)).³⁸

Fig. 1 XRD patterns of (a) precursor, (b) Al-550, (c) Al-750, (d) Al-850, and (e) Al-950.

SEM and TEM were used to further investigate the morphology and microstructure of the samples. The carbon spheres with diameters around 330 nm produced via the hydrothermal treatment of glucose aqueous solution were observed (Fig. 2(a)). With the addition of Al(NO₃)₃, the Al³⁺ ions may act as both catalyst and adhesion agent, accelerating the hydrothermal reactions and facilitating the carbonization of the carbohydrate. As a result, the average diameter of the carbon spheres increased to about 6.20 μm (Fig. 2(b)). Considerable shrinkage (from approximately 6.20 μm to 1.90 μm in diameter) of the structures was found after heat treatment (Fig. 2(c)), revealing a transition from loosely adsorbed Al³⁺ to a dense network in the hollow spheres.²⁷ Meanwhile, the energy-dispersive X-ray spectrum (EDS) analysis of the precursor shown in Fig. 2(d) indicated that the major elemental composition was carbon which revealed that the aluminum was well dispersed in the uncalcined precursor.

Fig. 2 SEM images of (a) carbon spheres, (b) precursor, (c) Al₂O₃ hollow spheres, and (d) selected area EDX pattern of the precursor.

The SEM micrographs of precursors and Al₂O₃ prepared with different hydrothermal conditions were shown in Fig. 3. No product was obtained after 2 h while the diameters of the precursors prepared at 3 h and 8 h increased from 5.4 μm to 6.9 μm (Fig. 3(a–c)). As the reaction proceeded, more nuclei appeared and more reactants were consumed, so the mean diameter of the products stopped increasing or even decreased. SEM images of the samples derived from different hydrothermal treatment temperature were shown in Fig. S1.† As the hydrothermal temperature increased from 140 °C to 180 °C, the average diameter of the Al₂O₃ increased from 1.30 μm to 1.90 μm. Moreover, with the temperature increased, more irregular products were produced. It can also be seen that the concentration of glucose had notable effect on the diameters of the samples, whose average sizes increased from 1.80 μm to 2.40 μm (Fig. 3(d and e)). When the glucose : Al³⁺ ratios were 4 : 1 and 2 : 1, uniform hollow spheres of Al₂O₃ were prepared. However, with the ratio reduced to 1 : 1, no spherical sample was obtained which indicated that when the hydrothermal reaction proceeded, hydrated alumina nanoparticles and carbon particles formed simultaneously.^39,40 Relatively high concentration of glucose improved the dispersion of Al³⁺, preventing the hydrolytic precipitation of Al³⁺ that enhanced the formation of spherical products under the effect of surface tension.

Fig. 3 SEM images of the precursors prepared at 160 °C for (a) 3 h, (b) 4 h, and (c) 8 h; and SEM images of the Al₂O₃ prepared with (c) 0.8 M glucose, 0.2 M Al(NO₃)₃; (d) 0.4 M glucose, 0.2 M Al(NO₃)₃; (e) 0.2 M glucose, 0.2 M Al(NO₃)₃.

The calcination process was the key step in the formation of the hollow spheres. SEM images of the samples prepared from different heating rates and different calcination temperatures were shown in Fig. 4. It was found that with the heating rate of 5 °C min⁻¹, the carbonaceous materials may react vigorously with oxygen, releasing large amount of carbon dioxide and steam in short time that resulted in holes on the shell. Hollow and broken spheres could be seen in Fig. 4(a), from which the thickness of the shell was estimated ca. 1 μm. As the calcination temperature increased from 550 °C to 750 °C, the average diameter of Al₂O₃ decreased slightly (from 1.80 μm to 1.70 μm) which may result from the structural stability of the samples prepared by hydrothermal method.

Fig. 4 SEM images of the Al₂O₃ calcined at 550 °C with heating rate of (a) 1 °C min⁻¹, and (b) 5 °C min⁻¹; and calcined at (c) 650 °C, and (d) 750 °C. (Areas I and II marked with red dots in (a) highlight the broken spheres.)

Additives were found to have significant influence on the morphology and structure of the products. It was difficult to prepare intact Al₂O₃ hollow spheres when small amount of ethanol was added as can be proved by SEM images of the Al₂O₃ (Fig. 5(a)). In addition, the surfaces of the hollow spheres prepared were rougher than those obtained without ethanol as can be seen in Fig. S2.† It might because that the polarity of the solution decreased with the addition of ethanol, and the reactivity of the Al³⁺ ions in the solution increased,⁴¹ which resulted in the increasing of the hydrolysis–precipitation that aggregated into nanoparticles with irregular morphologies instead of being dispersed by char formed from glucose. When urea was added, the mean diameter of the precursor reduced to 1.90 μm, meanwhile a featureless surface morphology was observed in Fig. 5(b). As a result, urea can be used to adjust the size and yield of the products. Significantly, hollow Al₂O₃ microspheres with rougher surfaces were produced using aluminium chloride (Fig. 5(c)), while two different morphologies (Fig. 5(d)) were obtained in the presence of aluminium sulfate. This variation may be related to the different hydrated ionic radius and coordinating ability of Cl⁻, NO₃⁻, and SO₄²⁻,^42,43 that need to be further confirmed.

Fig. 5 SEM images of the precursors prepared with 10 mL ethanol (a) or 0.2 M urea (b) as additives; and SEM images of the Al₂O₃ prepared using aluminium chloride (c) or aluminium sulfate (d) as aluminium sources.

The obtained hollow spheres were further investigated by TEM. Fig. 6 depicted the alumina hollow spheres prepared with different glucose : metal salt molar ratios (4 : 1, 8 : 1 and 16 : 1). The thickness of the shell of the sample prepared with glucose : metal salt ratio of 16 : 1 was around 100 nm. However, as the metal concentration increased, a much denser packing and thicker shell was observed. Both HRTEM and SAED (Fig. 6(d)) indicated the poor crystallinity of the Al₂O₃ calcined at 550 °C, in accordance with the XRD result (Fig. 1(b)). HRTEM in Fig. S3† proved the not perfectly smooth surfaces of the hollow spheres, and the presence of nanoparticles. The as-prepared precursor was embedded in epoxy resin and ultramicrotomed to study the distribution of the Al³⁺ inside the carbon matrix. As can be seen from the elemental mapping analysis of the cross section, the Al³⁺ were evenly distributed throughout the precursor (Fig. S4†). This may derive from the good affinity of reactive oxygen functional groups such as –OH and –COO– groups on the carbonaceous tiny spheres with the Al³⁺ ions. The tiny carbonaceous spheres polymerized to form larger carbonaceous particles, and at the same time the Al species were embedded into the carbon medium.^44,45

Fig. 6 TEM images of the Al₂O₃ prepared with glucose : Al(NO₃)₃ of (a) 4 : 1, (b) 8 : 1, and (c) 16 : 1; (d) HRTEM, and SAED (inset) corresponding of (b).

The FTIR measurement was used to identify the functional groups of as-prepared carbon spheres, uncalcined precursor, and hollow Al₂O₃ spheres (Fig. 7). For the carbon spheres, the bands at 1700 and 1620 cm⁻¹ could be attributed to C=O and C=C vibrations, respectively, suggesting the aromatization of glucose during hydrothermal treatment.⁴⁶ The bands in the range of 1000–1300 cm⁻¹ could be assigned to the C–OH stretching and OH bending vibrations, implying the existence of large amount of residual hydroxyl groups which were beneficial to the adsorption of Al³⁺. For the precursor, the bands in the range of 1000–1300 cm⁻¹ were weakened as the Al³⁺ might interact with the active functional groups. The bands at 620, 780, and 870 cm⁻¹ could be ascribed to Al–O vibrations and Al–OH wagging or rocking mode of molecular water in amorphous alumina.⁴⁷ Furthermore, the band at around 1120 cm⁻¹ might be ascribed to the (O) H⋯O–H vibration of amorphous alumina.⁴⁸ The broad band at about 550–850 cm⁻¹ for the Al₂O₃ hollow spheres could be assigned to the Al–O vibrations of Al₂O₃ in which aluminum ions were in both tetrahedral (AlO₄) and octahedral (AlO₆) sites.⁴⁹

Fig. 7 FTIR spectra of (a) carbon sphere, (b) precursor, (c) the Al₂O₃ hollow spheres calcined at 550 °C.

Final chemical structure of the carbon spheres and precursor were analyzed by ¹³C MAS NMR (Fig. S5†). A large number of signals from 0 ppm to 210 ppm for carbon spheres (Fig. S5a†), suggested the presence of a complex network of aliphatic or O–CH_x (δ < 100 ppm), aromatic or C=C (100–160 ppm cross-peak connection), carboxylic acids (160–190 ppm), and aldehydes or ketones carbonyl (190–210 ppm) groups.^{50,51 13}C NMR spectrum of the precursor (Fig. S5b†) was very similar to that of the carbon spheres, which proved that the glucose played an important role in hydrothermal reaction. The tiny difference between two spectra might be caused by the addition of Al³⁺.

TG–DTA curves for the pure carbon spheres and as-prepared precursor were shown in Fig. 8. Two stages of weight loss can be observed for the carbon spheres: the first weight loss might be attributed to the dehydration and densification of surface layer of the carbon spheres; the second weight-loss could be associated with the decomposition of the aromatic rings of the inner layer.⁵² The TG–DTA curve of the precursor was similar to that of the carbon spheres before 500 °C (Fig. 8(b)) except that the first exothermic peak of the precursor shifted to a higher temperature (325 °C). While the second exothermic peak shifted to a lower temperature (405 °C), which may because that the existence of Al³⁺ inhibited the dehydration and densification of the surface layer and promoted the oxidation of aromatic rings of the inner layer.

Fig. 8 TG–DTA curves for (a) the carbon spheres, and (b) as-prepared precursor.

Physical properties

Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curves for the samples of CAl-2, CAl-4 and CAl-8 were shown in Fig. 9. Isotherm of type IV with a typical hysteresis loop at the P/P₀ range of 0.4–0.8 can be observed for all samples, indicating the presence of mesopores. The pore structure parameters of the samples were listed in Table 1. The pore size distributions calculated, by the BJH method from the desorption data, showed that most of the pores in the Al₂O₃ spheres were centered at about 9–11 nm. The mesopores in the samples might originate from the removal of carbonized glucose materials in the inorganic–organic precursors and the stacking of the nanoparticles constituting the Al₂O₃ hollow spheres. The surface area and pore volume of hollow spheres can be adjusted by varying the glucose : Al³⁺ ratios. It was noteworthy that, from molar ratios 2 : 1 to 8 : 1, the surface area of the samples increased from 12 to 112 m² g⁻¹. The specific surface area and pore volume of the samples decreased with the increase of calcination temperature.

Fig. 9 Nitrogen adsorption–desorption isotherms (a), and the corresponding pore size distribution curves (b) of (A) CAl-8, (B) CAl-4, and (C) CAl-2.

Table 1 Physical properties of the samples studied

Sample	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore size (nm)
Al-550	104	0.20	7.6
Al-650	93	0.18	7.5
Al-750	84	0.17	7.6
CAl-2	12	0.02	10.9
CAl-4	33	0.05	9.1
CAl-8	112	0.14	9.0

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Formation mechanism

The formation mechanism of the Al₂O₃ hollow sphere can be proposed as following: firstly, the carbohydrate was subjected to dehydration, polymerization and aromatization to form carbon spheres. The hydrophilic shell of the carbon particles promoted the distribution of Al³⁺ ions through coordination or electrostatic interactions, due to the effect of OH and C=O groups on the surface.⁴⁹ Secondly, upon subsequent dehydration, nonpolar aluminum-containing carbon spheres assembled in a manner analogous to the mechanism by which a detergent emulsified a mixture of oil and water. Subsequent loss of water by self assembly led to further coalescence of the microscopic spheres to form larger spheres.⁵³ Finally, during the process of calcination, the outer layer carbon core was removed, and the particles size decreased. At the same time, the uniform distribution of Al-ions inside the carbon spheres during heating would diffuse outwards and in the process be engaged with oxygen to form (the very strongly bound) alumina spheres, while carbon could be leaving as (among other species) gaseous CO or CO₂, through the forming alumina shells, which could be a process responsible for creating the pores.⁵⁴ The synthetic route of the hollow spheres was illustrated in Scheme 1.

Scheme 1 Schematic of the synthesis of alumina hollow spheres.

Conclusions

In conclusion, a very simple and scalable synthetic technique towards Al₂O₃ hollow spheres had been explored using hydrothermal method. After the hydrothermal treatment of mixtures of carbohydrates with Al(NO₃)₃, carbon spheres with metal precursors tightly embedded in were obtained. The removal of carbon resulted in hollow spheres, which were composed of closely packed nanoparticles with a high surface areas. The thickness of the shell wall was around few hundreds of nanometers. The structure, size, and composition of hollow or composite particles were tunable. In addition, this method might offer an alternative route to synthesize mixed metal oxide hollow nanostructures with improved physical and chemical properties for applications in electronics, magnetism, optics and catalysis.

Acknowledgements

This work was financially supported by Innovation Key Program of the Chinese Academy of Sciences (KGCX2-YW-215), “Strategic Priority Research Program-Climate Change: Carbon Budget and Related Issues” of the Chinese Academy of Sciences (XDA05010109, 05010110), the Instrument Developing Project of the Chinese Academy of Sciences (YZ201139), the Chinese Academy of Sciences the strategic pilot science and technology projects – key technology and engineering demonstration of carbon dioxide capture, use and storage (XDA070401), National Natural Science Foundation of China (21306217) and the Key Science and Technology Program of Shanxi Province, China (MD2014-09).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Five figures showing SEM images, TEM images, TEM-EDS images, and ¹³C MAS NMR spectra of the selected samples. See DOI: 10.1039/c4ra12917b

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