The formation of hollow nanoparticles of alumina was detected when nanostructured carbon–aluminum material, synthesized by composite electrode sputtering in an electric arc, was annealed in oxygen. The synthesized material was characterized by the methods of transmission electron microscopy, thermogravimetry, and roentgen-phase analysis. It is shown that the alumina is the γ-phase; the typical size of particles is 6 - 12 nm and they have a wall thickness of 2 - 3 nm. The mechanism of formation of the hollow nanoparticles of alumina is suggested.
Ceramics based on Al2O3 are widely used by modern industry as a construction material with several unique properties such as high mechanical strength and hardness, heat resistance, chemical inertness, and insulation characteristics. Another important application of materials based on Al2O3 is the creation of various catalytically active complexes for the oil industry and cleaning of industrial emissions.
The main physical and chemical methods used for the synthesis of nanocrystalline materials are described in review [
The application of an electric arc for synthesis of nanomaterials relates to the pioneering work [
Experiments were carried out in the DC electric arc under pressure of buffer gas (helium) of 25 torr at an arc current of 100 А. The sputtered electrode (anode) was made of a metal-carbon composite rod 70 cm in length and 7 mm in diameter with a С:Al weight ratio of 15:1. The sprayed material was deposited on a cooled screen. Then, the synthesized composite material was annealed in air at temperatures of 400˚C - 950˚C. The synthesized material was analyzed by transmission electron microscopy (ТЕМ), thermogravimetry (TGA), and roentgenphases analysis (XRD) in the angle range of 10˚ - 75˚ with a step of 2θ = 0.05˚ and storage time of 3 s at every point; monochromatic CuKα radiation was applied (λ = 1.5418 A).
The metal-carbon deposit produced is relatively uniform material. The image of this material obtained via TEM is shown in
The ТЕМ pictures of synthesized material do not differ qualitatively from the pictures of material obtained by sputtering of the pure graphite electrode without aluminum. In particular, in the pictures it is impossible to distinguish the outline of separate aluminum particles. The element analysis (EDAX) detected aluminum in the synthesized material, and there were no lines of aluminum crystalline structure in the XRD spectra. The XRD spectra of graphite (1), pure carbon material (2), and aluminum-carbon material (3) obtained by sputtering of graphite (2) and aluminum-carbon (3) electrodes are shown in
It can be seen in
The thermogravimetric analysis was carried out in air at a temperature of up to 1200˚C with a linear increase during two hours. According to
The temperature history of the material’s morphology was studied during annealing. The material was annealed in air at 400, 550, 700 and 950˚C for 1 hour at every temperature. The TEM images are shown in Figures 4(a)-(d) for different annealing temperatures.
During annealing, gradual structurization of the material occurs and at 900˚C a significant part of the material is represented by hollow shells. According to the image analysis, with a rise in temperature the shape of the structures in the material becomes more spherical and the typical size of observed carbon structures changes from 10 - 30 nm in the initial material to 6 - 14 nm in alumina after annealing at 950˚C. The size distribution of particles measured via TEM image processing is shown in
Elemental analysis of the material after annealing shows that there is no carbon in the sample. According to roentgen-phase analysis, the synthesized hollow shells are the γ-phase of alumina. The XRD table data for γ-Al2O3 (curve 1) and synthesized material (curve 2) are compared in
The peaks positions of the spectra coincide, that allows concluding that the synthesized material is γ-Al2O3. The lines spreading in the angle range of 20 - 50 result in its overlapping. This phenomenon is obvious due to a small dimension of the coherent region of crystal structure of hollow nanoparticles.
Therefore, plasma-arc synthesis of aluminum—graphite material followed by annealing in an oxygen-bearing atmosphere allowed the synthesis of hollow γ-Al2O3 nanoparticles with a typical size of 6 - 14 nm and wall thickness of 2 - 3 nm.
One of the most important scientific questions arising from the results of the investigation is the determination of the mechanism of formation of hollow nanoparticles of alumina. According to our studies we suggest the following mechanism. Following the first stage of plasma synthesis by atomic sputtering of aluminum, joint condensation of carbon and aluminum occurs in which aluminum is partially or totally carbidized, and the size of crystallites in the synthesized material is no larger than 2 nm. The structure of the synthesized material is represented by carbon agglomerates with a typical size of 15 - 30 nm (
material, the initial weight concentration of aluminum of 6.7% in the initial particles of metal-carbon material with a size of 30 nm is equivalent to the amount of aluminum oxide in a spherical hollow particle with a diameter of 10 nm and wall thickness of 2.5 nm; this result agrees with the experimental data.
Hollow nanoparticles of γ-Al2O3 were synthesized by the method of electric-arc sputtering of a composite electrode in inert gas followed by oxidation. The effect of the temperature of annealing in an oxygen-bearing atmosphere on the morphology of the synthesized material was analyzed. The mechanism of formation of the hollow nanoparticles was suggested. We can state that this new material is of particular interest for catalytic applications and materials science.
The work was financially supported by The Ministry of education and science of Russian Federation, State Contract No.11.519.11.5001.