1 Introduction

Nanoparticles of multi-phases materials refer to systems that their sizes are usually in the range of 1–100 nm. These materials often exhibit novel chemical, electrical, optical, mechanical or magnetic properties, which are different from their bulk and the individual atomic constituents [1, 2]. Nanoscale materials also provide large surface areas compared with their micro and macro counterparts, which brings opportunities to function as adsorbents, catalysts, etc. [3].

Within all kinds of nanostructure materials, much attention is paid to the synthesis of nanocrystalline ceramics recently. It has shown that mechanical properties of ceramics strongly depend on their nanostructure, which closely relates with the shape and size of the ceramic particles [4]. Among various ceramics, alumina, especially nanosized alumina is utilized in many areas of modern industry such as electronics, metallurgy, optoelectronics, ceramic composites, wear protection, refractories, catalysis (as catalytic supports and catalysts) in petroleum refining, automotive emission control and hydrogenation [5–10]. Alumina has many advantages, such as that it is hard, highly resistant towards bases and acids, allowing very high temperature applications and having excellent wear resistance [11, 12]. According to the arrangement of oxygen anions, this interesting ceramic material exists in two broad categories, including a face-centered cubic (fcc), and a hexagonal close-packed (hcp) arrangement. The Al2O3 structures based on fcc lattice include γ, η (cubic), θ (monoclinic), and δ (either tetragonal or orthorhombic), whereas the Al2O3 structures based on hcp packing are α (trigonal), κ (orthorhombic) and χ (hexagonal). Except γ and α-alumina that are thermally stable, the other phases are unstable at room temperature and called transition phases [13].

Several methods to synthesize nano-alumina are categorized into physical and chemical methods. Physical methods include mechanical milling [14], laser ablation [15], flame spray [16], thermal decomposition in plasma [17], chemical methods include sol-gel processing [18], hydrothermal [19], precipitation [20], combustion methods [21], vapor deposition [22] and microemulsion [23]. The co-precipitation and sol-gel methods are most commonly used for synthesis of nanoparticles. In each method, different parameters like pH, reaction temperature and reaction time, concentration of the initial solution and material, have important role on getting ceramic powders with desired shape and size.

Parida et al. [24] synthesized spherical nano sized gamma alumina with surface area of 140–190 m2/g and crystallite size 4.7–5.7 nm by control precipitation method using aluminum nitrate as precursor and four different precipitating agents including ammonium bicarbonate, ammonium carbonate, sodium bicarbonate and sodium carbonate annealing at 550 °C. They also concluded that the sample prepared by ammonium bicarbonate exhibited excellent fluoride adsorption capacity that was ascribed to the highest surface area of nanoparticles (190 m2/g). Potdar et al. [25] successfully utilized γ-Al2O3 powder by precipitation/digestion method at calcinations temperature 550 °C using aluminum nitrate and sodium carbonate. The particles had spherical shape with size of 4.5 nm and showed BET surface area of 220 m2/g. It was found that the performance of γ-Al2O3 catalyst was comparable with that of commercially catapal-B calcined sample, due to it’s high surface area, surface acidity and better crystallinity. The sol-gel method is based on the phase transformation of the sol obtained from metallic alkoxides or organo-metallic precursors. This sol, which is a solution containing particles in suspension, is polymerized at low temperature in order to form a wet gel. The solvent is removed by drying the gel and the next step is a proper heat treatment [2]. Rogoja et al. [26] prepared α-aluminum by sol-gel method using aluminum chloride and aluminum triisopropylate as two different precursors. The powders obtained after drying the gel were heat treated at 1,000 and 1,200 °C for 2 h. They concluded that alumina powder was obtained at the nanometric scale, both for the utilized inorganic and organic precursors, with different structures. Mirjalili et al. [27] reported that sol-gel method was used to synthesize ultrafine nano α-alumina particles using an aqueous solution of aluminum isopropoxide and 0.5 mol/L aluminum nitrate hydrate. Sodium dodecylbenzene suffocate and sodium bis-2-ethylhexyl sulfosuccinate were also used as surfactant stabilizing agents. Their results indicated that the addition of sodium dodecylbenzene suffocate and sodium bis-2-ethylhexyl sulfosuccinate affected the particle size, the shape of the produced nanoparticles and the degree of aggregation. The sodium dodecylbenzene sufonate produced better dispersion and finer particles in range of 20–30 nm compared to sodium bis-2-ethylhexyl sulfosuccinate.

The aim of this study is to characterize various types of alumina nanopowders synthesized by two different methods including co-precipitation and a simple sol-gel procedure, and compare the effect of these methods on the final structure, size and phase transformation temperature of Al2O3 nano particles. They are facile and practicable processes to obtain dispersed nanoalumina with low-cost and without pollution. Another aspect of this work is to study the effect of precipitant concentration on the final structure of nanoparticle in the co-precipitation method.

2 Experimental

2.1 Synthesis

2.1.1 Synthesis of alumina by sol-gel method

Aluminum nitrate (Al(NO3) 3·9H2O), citric acid (C6H8O7), 1,4-butandiol (C4H10O2), ethanol (C2H5OH) and deionized water were used as starting materials. Initially, 0.1 mol aluminum nitrate and 30 mL 1, 4-butandiol were gradually added to the aqueous-alcoholic solution including 100 mL deionized water and 100 mL ethanol, then it was placed on a hot plate. 0.3 mol citric acid dissolved in 40 mL deionized water was added to the solution and continuously stirred at 40 °C for 15 min until a colloidal sol was prepared. On the next step, the sol was heated in the oil bath at 80 °C to evaporate the solvent and prepare gel. After 18 h the beaker containing the aerogel was placed on a hot plate at 120 °C and concurrently heated from the top by IR radiation for 4 h. The obtained sol-gel precursors were dried at 200 °C in an oven and grinded into powders after 2 h. The pale brown powders,which obtained after drying the gel, were divided into three parts and were heat treated at 750, 1,000 and 1,250 °C for 2 h, respectively.

2.1.2 Synthesis of alumina by co-precipitation method

The starting materials were aluminum trichloride (AlCl3), ethanol and 25 % ammonia solution as precipitant agent. At first, AlCl3 was dissolved in 150 mL ethanol and a little amount of distilled water was added to get a transparent solution. In the next step, 60 mL NH3 was added to the stirred AlCl3·6H2O solution drop by drop with the rate of 2.5 mL/min until precipitation became white as gelation of Al3+ cations in the form of hydroxides Al(OH)3. After filtering in vacuum system, drying at 200 °C for 2 h in an oven, and annealing at 750, 1,000 and 1,250 °C for 1 h, a white fine alumina nano-powder was obtained to study the effect of precipitant concentration on the final structure of nanoparticles. Other samples were prepared with 10 and 30 mL NH3 under the same conditions.

2.2 Characterization

Phase identification and the structural evolution of the powders during temperature processing were performed by D8 Advance Bruker Diffractometer using CuK α (λ = 0.15406 nm) radiation in the 2θ range from 20° to 70°. The full widths at half-maximum of the XRD lines (FWHM) at 2θ was used to calculate the average crystallite size (D) using Scherrer’s formula. The morphology, the microstructure, homogenity and the particle size of alumina powders were examined with the transmission electron microscope (TEM, LEO 912 AB) and the scanning electron microscope (SEM, Leo 430i). Specific surface areas were measured using the BET-N2 technique in the P/P 0 range of 0.09–0.3 and outgas temperature of 200 °C. The specific surface area (SSA) was converted into particle size assuming that the particles are closed sphere with smooth surface and uniform size using Eq. (1)

$$ D_{\text{BET}} = 6\times 10^{ 3} /\left( {d_{\text{th}} S_{\text{BET}} } \right), $$
(1)

where d th is the theoretical density of the material under consideration, D BET the average particle size (nm), and S BET the SSA (m2/g) [28]. To get information on the presence of organic materials and study the phase conversion, FTIR spectroscopy was carried out by KBr disc technique using the FTIR spectrometer (Perkin Elmer-RXI) in the range 4,000–400 cm−1.

3 Result and discussion

3.1 XRD analysis

Figure 1a–c show the comparable XRD patterns of powders prepared by different methods and calcined temperature. As shown in Fig. 1a, there are three very broad peaks in the pattern at 750 °C for the sol-gel sample, which are difficult to index according to the JCPDS data indicating that the powder is possibly amorphous. The black color of powders at this temperature shows the existence of organic materials, and confirms XRD pattern, where these compounds have prevented particles from forming crystal structure. However, the diffraction peaks are a little sharper and attributed to γ-alumina for the sample prepared by 10 mL NH3. The peaks become sharper and more with increasing NH3 concentration, clearly indicating that the more the precipitant concentration, the more the sediment particles. All diffraction peaks exhibit high degree of broadness due to the formation of nanocrystals [28]. The characteristic peaks of γ-Al2O3 is improved for the sol-gel sample with increasing calcination temperature up to 1,000 °C, but there is a shift on the position of other three sample’s peaks using precipitation method that can be attributed to δ phase (see Fig. 1b). Although the diffraction peaks of δ and γ are very close to each other, especially overlapping in some position, we can indicate that γ-alumina coexists with δ-alumina [29]. At this temperature for the sample prepared by 10 mL NH3, sharp peaks of α appear indicating γ- to α-Al2O3 phase transition starts more quickly than the other samples. It seems that due to the lack of additional ingredients at this sample lower temperature is required to break chemical compounds, and phase transformation occurs faster. At 1,250 °C for both methods single phase α-alumina is completely formed (see Fig. 1c). The diffraction pattern is extremely sharp indicating the existence of the highly crystalline material.

Fig. 1
figure 1

XRD patterns of four different Al2O3 nano-powders prepared with sol-gel and co-precipitation method annealing at different temperatures (Pre describes co-precipitation method)

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3.2 BET analysis and particle size

Tables 1 and 2 show XRD parameters and calculated crystallite sizes using Scherrer’s formula for nanoparticles prepared by co-precipitation and sol-gel respectively. The particles size were found to be in the range of 4–8 nm for γ-alumina with only slight increase for delta phase to 6–11 nm, however the amount of α-Al2O3 increases drastically to 27–32 nm. It shows a trend that the average crystallite size is larger at higher calcination temperature, which is related to the grain growth. The effect of synthesis methods on the surface area of alumina is remarkable, as shown in Table 3. After calcination at 750 °C, the BET surface area of sample prepared by precipitation method is 206.2 m2/g and for those prepared by the sol-gel method is 30.87 m2/g. It should be noted that the sample prepared by precipitation process obtained without using structure-directing reagents such as surfactants. This can avoid the surfactant removal step, which causes the collapse of the final structure and leads to the very high surface area [30]. We also calculated average particle size with Eq. (1) using alumina density of 3.98 g/cm3 in Table 3.

Table 1 Summary of XRD measurements for Al2O3 prepared by co-precipitation method
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Table 2 Summary of XRD measurements for Al2O3 prepared by sol-gel method
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Table 3 Surface area of Al2O3 nano particles and their calculated crystallite size at 750 °C
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3.3 Microscopic observations

Figure 2a–f show the SEM images of the particles produced from sol-gel and co-precipitation methods. The sol-gel particles have elongated shape with uniform distribution unlike the strong agglomeration and varied size of precipitation samples. The reason is that in the sol-gel process the open structure of gelatinous state formed from precursor allowed alumina crystallites agglomeration freely [31, 32]. By eye inspection, the primary particle size is increased after calcinations due to the agglomeration of particle. To get more information about morphological features of the powders, we applied TEM studies. Figure 3a–e represent TEM image of nanoparticles prepared by two methods annealing at 1,250 °C. These reveal that the alumina particles synthesized by co-precipitation method have different shapes with the size of 10–50 nm such as nearly spherical or irregular hexagonal, whereas sol-gel alumina nanoparticles are more spherical with uniform structure having the size distribution of 10–20 nm.

Fig. 2
figure 2

SEM image of Al2O3 nano particles prepared by different method calcining at 750 °C a sol-gel, b co-precipitation (10 mL NH3), c co-precipitation (60 mL NH3), and 1,250 °C d sol-gel, e co-precipitation (10 mL NH3), f co-precipitation (60 mL NH3)

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Fig. 3
figure 3

TEM images of Al2O3 nanoparticles prepared with a, b co-precipitation calcined at 750 °C, c and d co-precipitation calcined at 1,250 °C, and e sol-gel calcined at 1,250 °C

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3.4 FTIR analysis

To support our conclusion drawn from XRD results that calcined sample at 700 °C were γ and higher temperature yielded α, we carried out systematic IR studies (see Fig. 4 a–b). Both methods show an intense band centered around 3,500 cm−1 and the other around 1,600 cm−1 are assigned to O–H stretching and bending modes of adsorbed water or alcohol, respectively [21]. The wide band appearing between 500 and 900 cm−1 corresponds to the vibrational frequencies of co-ordinate O–Al–O bond, showing nano amorphous Al2O3 [33]. This wide band is divided into two peaks. The peaks in the region 500–750 cm−1 are assigned to ν-AlO6, and the other at 800 cm−1 is assigned to ν-AlO4, indicating tetrahedral and octahedral coordination existence in γ-Al2O3 [24–34]. For precipitation method this band shows that gamma phase is divided at 750 °C, whereas for sol-gel method this happened at 1,000 °C. It is in agreement with the presence of γ-alumina diffraction observed in the XRD pattern. For the samples calcined at 1,250 °C two significant spectroscopic bands near 600–650 and 450 cm−1 appear, which are identified to the characteristic absorption band of α-Al2O3 with a corundum structure [33].

Fig. 4
figure 4

FTIR spectra of Al2O3 nanopowders prepared with a co-precipitation and b sol-gel

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4 Conclusions

In summary, alumina nanostructures with different morphologies have been synthesized through two different approaches including sol-gel and co-precipitation. These can be concluded as following:

  1. (i)

    Synthesis method and reaction temperature play key roles to control the phase formation of the alumina nanocrystals.

  2. (ii)

    The powder synthesized by sol-gel process need higher temperature for γ-crystallization in comparison with precipitation method. Although, in the precipitation method, we have γ phase at 750 °C, in the sol-gel process this phase formed at 1,000 °C.

  3. (iii)

    At 1,000 °C for sol-gel method, the most intense diffraction peaks were identified as γ-alumina, but for precipitation process δ-alumina together with the presence of γ-alumina have been observed.

  4. (iv)

    The temperature at which α-alumina is formed is lower in precipitation method using the lowest precipitant concentration that we have seen relatively sharp peaks of α phase at 1,000 °C for 10 mL NH3.

  5. (v)

    With increasing temperature up to 1,250 °C, high intensity and sharp peaks of α-alumina have been observed for both samples that show very high crystallization at this temperature.

  6. (vi)

    According to electron microscope images, the size of α-alumina nanoparticles obtained using sol-gel (10–20 nm) is smaller and more uniform than those obtained using precipitation method (10–50 nm) at 1,250 °C.

  7. (vii)

    The results show that BET surface area of sample prepared by precipitation method (206.2 m2/g) is higher than that prepared by sol-gel technique (30.72 m2/g), which will make them attractive in the field of catalysis and adsorption applications.