Introduction

Alumina (Al2O3) is one of the most commonly used inorganic materials in industrial chemistry due to its mechanical, electrical, thermal, and optical properties [1]. As one of the most important materials, it has been used in many applications as a catalyst or a catalyst support, absorbent, or surface coating [2]. It is also used in machine components [3], microelectronics [4], refractories [5], and optics and lasers [6]. Al2O3 is a structural complex oxide which has several different metastable phases and eventually converts to thermodynamically stable α-Al2O3. Alumina can generally be divided into two large groups according to crystal structures. These are the surface-centered cubic system (FCC) and the hexagonal cubic system (HCP) according to their position in the crystal structure of oxygen anions. The distribution of the cation in the crystal structure causes these subgroups to have many alumina polymorphs. The aluminum forms in the HCP crystal arrangement are shown in the form of chi (χ-), kappa (κ-), and alpha (α-) while gamma (γ-), eta (η-), delta(δ-) forms are shown in the FCC crystal arrangement. The stable single phase of alumina is an alpha form with high thermal stability, chemical resistance, and hardness and is obtained by a calcination processes at 1230 °C [7, 8]. Bohmite structure (AlOOH) is the most preferred starting material in alpha alumina synthesis. In the literature, various temperatures regarding the phase transformation are given but the most accepted temperatures can be shown as: bohmite→ γ (300 °C); γ → δ (850 °C); δ → θ (1100 °C); θ → α (≥ 1200 °C) [9,10,11,12].

Traditional processes for synthesizing α-Al2O3 involve mechanical milling [13, 14], vapor phase reaction [15, 16], precipitation [17], sol-gel [18], hydrothermal [19], and combustion methods [20]. All these methods have specific limitations. For instance, the precipitation method suffers from its complexity and long washing and aging time. The sol-gel method based on molecular precursors generally makes it necessary to use high-price alkoxides and it needs long gelation periods [21]. Hydrothermal synthesis is one of the most preferred synthetic ways to obtain inorganic oxides because of its ability to synthesize large crystals of high quality.

Chemical methods are simple ways to synthesize various crystals with different properties under moderate conditions which makes them indispensable. In chemical synthesis, particle formation takes place in three steps: nucleation, particle growth, and finishing. The particle growth step which is called Ostwald’s ripening is the most significant step in the process which occurs with the combination of the ions in the solution to form larger particles. Reducing the solubility of the growing particles in solution ends the particle growth [22]. During chemical synthesis, the existence of another substance which will limit the growth step in the reaction medium and which will not make any contribution to the reaction causes the particle size to be limited. For this purpose, the use of various surfactants and coating the particles with polymer, silica, carbon, or metal are widely used methods [23].

Surfactants are added to the precursor solution to delay the growth of crystals and to reduce the agglomeration [24]. Mirjalili et al. prepared ultrafine nano α-Al2O3 particles using an aqueous solution of aluminum isopropoxide and aluminum nitrate hydrate. They showed that addition of surfactants such as sodium dodecylbenzene sulfonates and sodium bis-2-ethylhexylsulfosuccinate affected the particle size, the shape of the produced nanoparticles and their degree of aggregation [18]. Zhu et al. synthesized mesopore alumina microfibers using PEG with different molecular weights (Mn = 400–20.000) as template by hydrothermal route. The obtained alumina microfibers have the different high surface area but the nearly same pore diameter about 3.5 nm. The results indicated that complete transformation to α-alumina has done at 1573 K, and the PEG template indeed played a role in the formation of the fiber-like alumina, but the mesopores within the alumina microfibers may be formed with the amorphous alumina framework during gases burning out in the thermal calcination process [25]. More recently, Ghanizadeh et al. reported that alumina seed had no effect on formation of transient alumina phase during hydrothermal treatment (≤ 220 °C) when processing was performed with aluminum chloride, ammonia solution, and TEAH (tetraethyl ammonium hydroxide) in the presence of Tween-80 and ß-alanine [26]. Surfactant supported synthetic routes were mostly used at the sol-gel synthesis of alumina powders in the literature which needs expensive starting materials and long gelation times [27,28,29]. At the present study, we introduced synthesis of anionic surfactant supported α-Al2O3 powders via hydrothermal route.

The aim of this study is the investigation of the α-Al2O3 formation in the presence of the anionic surfactant, sodium dodecyl sulfonate (SDS), as a structure directing agent. The effect of the calcination time on the structure is also investigated. The structural characterization was carried out using Raman, XRD, and FTIR spectroscopies. Structural transformation was interpreted supported by TGA analysis. Morphological identification was screened using SEM images. The total surface area of the samples was analyzed via BET absorption analysis.

Experimental

Chemicals

Aluminum nitrate nonahydrate (Al(NO3)3.9H2O) was used as the Al source. Sodium dodecyl sulfonate (SDS) was used as an anionic surfactant. Ammonia solution (35%) was the hydrolysing agent. All the other chemicals were used as received from commercial sources without further purification. The starting solutions were prepared using deionized water.

Synthesis

Surfactant assistant alpha alumina (α-Al2O3) structures were successfully prepared by using a hydrothermal technique. A precursor solution (0.3 M) was prepared initially by dissolving aluminum nitrate nonahydrate (Al(NO3)3.9H2O) in deionized water and stirred at room temperature for an hour. Surfactant solutions were added to the precursor solutions resulting in 1%, 3%, 5%, and 10% SDS/Al+3 mixtures, and the mixtures were stirred for another hour in order to get a homogenous solution. The pH value was adjusted to 10 ± 0.1 using 35% ammonia solution and the solutions were transferred to a Teflon-lined autoclave. The reactions were continued at 180 °C for 9 h. After cooling to room temperature, the products were precipitated by centrifuged at a rate of 12,000 rpm and purified by washing with deionized water several times. The obtained products were dried in a vacuum oven overnight at 40 °C. The crude products were calcinated at 1200 °C for 3 and 6 h, and the effects of the calcination time were investigated, as well (Scheme 1). The structural analyses of the obtained samples were studied by Raman and infrared spectroscopy (FTIR). Morphological characterizations were investigated via scanning electron microscopy (SEM). Thermogravimetric analyses were carried out using crude products to demonstrate the existence of surfactants in the structure. The surface area of the powders was measured by using an automated BET gas adsorption analyzer.

Scheme 1
scheme 1

Schematic illustration of synthesis of the Al2O3 powders

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Characterization

Structural analysis were studied via a Renishaw Invia Reflex Confocal (× 100 objective) Raman spectrometer with a 1800 lines/mm grating and 532-nm excitation energy at 200-mW laser power. X-ray diffraction analyses of the samples were performed by using the Rigaku Rad B-Dmax II powder diffractometer. The 2θ values were taken between 20° and 110° using Cu Kα (λ = 1.5405 Å) irradiation in 0.04° steps at a scanning speed of 4°/min. The dried samples were prepared for analysis by spreading 30 (± 2) mg sample into 5 cm2 area as a thin layer in order to minimize the deviations in the peak values and peak dilatation resulting from the sample thickness. Fourier transform infrared (FTIR) spectra were recorded on an FTIR spectrophotometer equipped with an attenuated total reflectance (ATR) accessory (Nicolet iS5 FTIR) in the range of 550–4000 cm−1 in ambient air at room temperature. Thermogravimetric Analysis (TGA) was performed with a Shimadzu TGA-50 thermal analyzer at a heating rate of 10°Cmin−1 using a 10-mg sample from 30 °C temperature to 900 °C under air atmosphere. Determination of the morphological properties of the prepared powders was carried out using a Ronteck xflash detector analyzer associated with a scanning electron microscope (Leo-Evo 40XVP). Incident electron beam energies from 3 to 13 keV were used. All samples were sputter coated with Au/Pd before the analyses. The surface area of the alumina samples was measured in a sorption analyzer, Quantachrome Nova, with N2 as the adsorbed gas, using the Brunauer-Emmett-Teller (BET) method. The BET measurements, nitrogen adsorption-desorption isotherms conducted at 77 K.

Result and discussion

The Raman spectroscopy technique has been extensively used to determine the structure, composition, and phase characterization of the materials. Figure 1 gives the Raman spectra of synthesized α-Al2O3 structures. The representation of optical bands in α-Al2O3 is given as follows:

$$ {2\mathrm{A}1}_{\mathrm{g}}+{2\mathrm{A}1}_{\mathrm{u}}+{3\mathrm{A}2}_{\mathrm{g}}+{2\mathrm{A}2}_{\mathrm{u}}+{5\mathrm{E}}_{\mathrm{g}}+{4\mathrm{E}}_{\mathrm{u}} $$

where 2A1g + 5Eg are Raman active modes, 2A2u + 4Eu are IR active modes, whereas 2A1u + 3A2g are neither Raman nor IR active modes. The α-Al2O3 gives rise to seven Raman active phonon modes. The Raman bands with peaks at ~ 378, 431, 453, 581, and 752 cm−1 have been assigned to Eg (external), and 416, 645 cm−1 peaks have been assigned to A1g modes [30,31,32].

Fig. 1
figure 1

Raman spectra of prepared Al2O3 powders calcined for 6 h

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XRD results of the synthesized alumina powders calcined at 1200 °C are shown in Fig. 2. The sharp peaks of α-Al2O3 at Bragg’s angle of 25.58, 35.16, 37.78, 43.36, 52.56, 57.76, 66.52, and 68.20° indicate the relatively large grain sizes and well-defined long-range order in structures [2, 33].

Fig. 2
figure 2

XRD spectra of prepared α-Al2O3 powders calcined for 6 h

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Figure 3 presents the ATR spectra of synthesized Al2O3 powders. The bands in the region of 400–1000 cm−1 were associated with stretching vibration of Al-O bonds in general. [34] According to the spectra, there are three major peaks shown at 549, 552, and 631 cm−1 which are identified to be the characteristic absorption bands of α-Al2O3 in octahedral structure [35]. Absorption at 2312 and 2285 cm−1 which did not appear at the spectra labeled as α-Al2O3, are stretching vibrations of the HCO3− group, indicating H2O and CO2 absorption in the prepared powders [36]. These peaks also indicate the presence of surfactant in the structure before the calcination.

Fig. 3
figure 3

FTIR spectra of prepared Al2O3 powders calcined for 6 h. *549 cm−1, **552 cm−1, and ***631 cm−1

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The TGA analyses were realized using the crude samples Figure 4 shows the alumina precursors which were synthesized using 1% (AA-1), 3% (AA-3), 5% (AA-5), and 10% (AA-10) surfactants and also the precursor in the absence of surfactant (AA-0). According to the thermogram of AA-0 sample, the weight loss up to 250 °C is associated with the solvents which remained inside the sample after drying and water loosely bound. The continuing weight loses after 250 °C can be attributed to the structural rearrangements which occur in regard to –OH group loses resulting from the phase transformation. It is clear that the AA-0 sample is thermally stable after around 550 °C. The thermogram of the AA-1 is very similar to AA-0 at the second stage because there is quite a small amount of surfactant inside the structure. The weight loss differences become more noticeable as the amount of surfactant increases. The surfactant inside the structure decomposes with the increasing temperature giving decay products of CO2 and H2O. This is the reason for the HCO3− absorption peak at 2312 and 2285 cm−1 in the FTIR spectra. It is reported that the alkyl chain of the surfactant, SDS, was completely removed below 200 °C, where the sulfate head group was lost in the region between 400 and 600 °C [37]. The weight loss at the second stage may be caused not only by the removal of the head group of the surfactant but also by the loss of water in the alumina framework. While the temperature increases the alumina dehydrate to a transitional phase and further to Al2O3 [38]. The weight loss continues at the range of analyzing temperature for all surfactant containing samples, and decomposition of the surfactant affects the morphological structure of the samples, as well.

Fig. 4
figure 4

TGA thermograms of prepared Al2O3 powders

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Figure 5 shows the SEM images of calcinated samples which are labeled at the up right corner of each image. It is clear from Fig. 5a that the alumina synthesized in the absence of surfactant appears as a bulk structure. After the addition of 1% SDS to the reaction medium, the resultant structure slightly changes (Fig. 5b). The morphology of the structures dramatically changes with the increasing amount of SDS in the reaction medium (Fig. 5c–d) and it becomes a porous structure when the SDS addition increases to 10% (Fig. 5e–f). According to the investigation of Sicard et al. [39], a strong interaction exists between the sulfate head groups and the alumina framework. Sintering likely occurs because the polar surfactant head group strongly interacts with the positive charges of the alumina surface and most likely embedded into the alumina walls. The further removals caused a ready sintering and collapse of the alumina framework upon surfactant removal by calcination which leads to a porous structure [38, 39].

Fig. 5
figure 5

SEM images of Al2O3 powders calcinated at 1200 °C for 6 h. a Without surfactant. b 1% SDS. c 3% SDS. d 5% SDS. e–f 10% SDS additions

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The calcination time also apparently changes the morphology of the synthesized alumina structures. Figure 6 provides the morphology of the structures calcinated at 1200 °C for 3 h. The alumina framework does not have enough time to collapse, and the resultant structures show worm-like morphology after 3-h calcination.

Fig. 6
figure 6

SEM images of Al2O3 powders calcined at 1200 °C for 3 h. a Without surfactant. b 1% SDS. c 3% SDS, d 5% SDS. e–f 10% SDS additions

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Total surface area of the samples is given at Table 1. It can be seen that with increasing surfactant content, the surface area of the sample increases. However, when calcination time increases to 6 h at the same calcination temperature, the total surface area nearly does not change because of the collapse of the alumina framework. These results are compatible with the SEM images.

Table 1 Total surface area of alumina powders at different calcination time
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Conclusion

α-Alumina structures were synthesized in the presence of the anionic SDS surfactant. The crude samples were calcinated at 1200 °C in air. The SDS molecules strongly interact with the positive charges of the alumina surface and because of these strong interactions, the surfactant head groups remain in the structure at high temperatures. The further removals cause a collapse of the alumina framework upon surfactant removal by calcination which leads to a porous bulk structure. The calcination time affects the morphology of the structures, as well. The structures show worm-like morphology when the calcination time decreases to 3 h. Phase transformation needs high temperature calcinations and a relatively long calcination time. However, increasing calcination time at 1200 °C provides the structure enough time to collapse with the effect of the high temperature, and the resulting structure becomes collapsed bulky material with nano-sized cavities.