Preparation and Characterization of Alumina Nanoparticles in Deionized Water Using Laser Ablation Technique

Al2O3 nanoparticles were synthesized using laser ablation of an aluminum (Al) target in deionized water. Nd:YAG laser, emitted the light at a wavelength of 1064 nm, was used as a light source. The laser ablation was carried out at different energies of 1, 3, and 5 J. The structure of ablated Al particles suspended in deionized water was investigated using X-ray diffraction (XRD). The XRD patterns revealed that the ablated Al particles transformed into γ-Al2O3. The morphology of nanoparticles was investigated by field emission scanning electron microscopy (FE-SEM). The FE-SEM images showed that most of the nanoparticles obtained from all the ablated laser energies have spherical shape with a particle size of less than 100 nm. Furthermore, it was observed that the particle size increased with increasing the laser energy. The absorption spectra of Al2O3 nanoparticles suspended in deionized water were recorded at room temperature using UV-visible spectroscopy. The absorption spectra show a strong peak at 210 nm arising from the presence of Al2O3 nanoparticles. The results on absorption spectra are in good agreement with those investigated by XRD which confirmed the formation of Al2O3 nanoparticles during the laser ablation of Al target in deionized water.


Introduction
Metal oxide nanoparticles have been extensively developed in the past decades. They have been widely used in many applications such as catalysts, sensors, semiconductors, medical science, capacitors, and batteries [1][2][3][4][5][6]. Among them, aluminum oxide (Al 2 O 3 ) and its compounds have long been known for more than a century, for example, aluminum oxide hydroxide (AlOOH) and aluminum trihydroxide (Al(OH) 3 ). Al 2 O 3 or alumina generally refers to corundum. It is a white oxide. Alumina has several phases such as gamma, delta, theta, and alpha. However, the alpha alumina phase is the most thermodynamically stable phase. In general, alumina has many interesting properties, for example high hardness, high stability, high insulation, and transparency [7]. Alumina is also widely used in the fire retard, catalyst, insulator, surface protective coating, and composite materials [8][9][10][11][12]. Al 2 O 3 nanoparticles can be synthesized by many techniques including ball milling, sol-gel, pyrolysis, sputtering, hydrothermal, and laser ablation [13][14][15][16][17][18]. Among them, the laser ablation is a widely used technique for the synthesis of nanoparticles since it can be synthesized in gas, vacuum or liquid. This technique offers several advantages such as rapid and high purity process compared with other methods [19]. Furthermore, nanoparticles prepared by the laser ablation of materials in liquid are easier to be collected than those of in gas atmosphere.
In the recent years, Al 2 O 3 nanoparticles were synthesized in liquid using a short pulse laser with the pulse width in the range of nanosecond [20][21][22]. Furthermore, Al target was used as a starting material for laser ablation in those works. Therefore, in this study we reported the synthesis  of Al 2 O 3 from Al powders using laser ablation in deionized water with a long pulsed Nd:YAG laser. The laser pulse widths adopted in this work were 2.5, 6, and 9.5 ms to obtain the output laser energies of 1, 3, and 5 J, respectively. The particle size and morphology of synthesized nanoparticles obtained at different laser energies were investigated by field emission scanning electron microscopy (FE-SEM). The optical property of synthesized nanoparticles was carried out using UV-visible spectroscopy. The structure of the synthesized nanoparticles was investigated using X-ray diffraction (XRD) technique.

Material Preparation for Laser Ablation.
Aluminum powder with purity of 99.99% (American Elements) and an average particle size of 35 μm were pressed in a stainless steel mold by a hydraulic with a pressure of 100 bars to obtain the aluminum pellet with a diameter of 15 mm and 5 mm thick. The aluminum pellet was used as a target for laser ablation. The experimental setup of the laser ablation is shown in Figure 1. The aluminum pellet was placed at the bottom of a glass vessel with a diameter of 30 mm and 50 mm in height. Deionized water with a volume approximately of 10 mL was poured into the vessel until its level was approximately 5 mm above the target. The Nd:YAG (Miyachi, ML-2331B) that emitted the laser light at a wavelength of 1064 nm was used for the laser ablation. The laser ablation was carried out at different laser energies of 1, 3, and 5 J with a repetition rate of 2 Hz. The laser beam was focused by a 50 mm focallength lens onto the aluminum pellet. For each energy of laser ablation, the aluminum target was ablated with 5,000 pulses for the total time of about 40 min. After the laser ablation, the Al 2 O 3 particles suspended in deionized water was obtained.

Characterization.
After the laser ablation of Al target in deionized water was carried out, the Al particles suspended in deionized water were characterized to confirm that they transformed into Al 2 O 3 . The ablated Al particles suspended in deionized water were used for the optical measurement. The optical absorption spectra were recorded at room temperature using a UV-Vis spectrophotometer (JASCO, V570). Then, the liquid sample was dropped on silicon wafers and dried in an oven at 40 • C for 4 h. The dried-particles sample on silicon substrates was used for investigating the structure and morphology. The structure of particles was investigated using X-ray diffraction (PANalytical, X'Pert Pro) with CuK α radiation and a pattern recorded at grazing incidence angle (3 • ) in the 2θ range of 20 • -80 • with a scan step of 0.02 in a scan time of 5 s. The morphology of particles was investigated using field emission scanning electron microscopy (Hitachi, S4700).

Results and Discussion
3.1. The Structure of Ablated Particles. The structure of ablated particles was investigated by XRD measurement. To identify the structural difference between ablated particles and Al, the XRD measurement for Al target was also carried out. Figure 2 shows the XRD patterns of Al target and particles synthesized from laser energies of 1, 3, and 5 J. The observed peaks in Figure  laser energy. This might lead to the conclusion that the particle concentration, suspended in deionized water, obtained from a laser energy of 5 J is higher than those of 1 and 3 J. It should be pointed out that the phase transitions in aluminum oxide occur in the following order [23]: Since γ-Al 2 O 3 occurs at the lowest temperature compared with those of another phases, the most likely phase of Al 2 O 3 to be formed after laser ablation of Al target in deionized water is γ-Al 2 O 3 .

Morphology of Al 2 O 3
Nanoparticles. The morphology of dried Al 2 O 3 nanoparticle samples obtained from the laser ablation of aluminum pellet in deionized water at different energies was investigated using field emission scanning electron microscopy. For comparison, the morphology of aluminum powder was also investigated. Figure 3(a) shows the FE-SEM image of the aluminum powder used as the target. It revealed that the morphology of the aluminum was a nonspherical shape with the size in the range of 1-35 μm. Figures  3(b)-3(d) show the FE-SEM images of the Al 2 O 3 nanoparticles synthesized in deionized water with different laser energies of 1, 3, and 5 J, respectively. It is clearly observed that the morphology of the synthesized Al 2 O 3 nanoparticles is mostly spherical shape. These results are in good agreement with those previously reported in reference [20][21][22].
The particle size distribution of the Al 2 O 3 nanoparticles could be obtained by measuring the size of total particles of about 200 particles in the FE-SEM image using the Image J program. The plots of the particle size distribution of the Al 2 O 3 nanoparticles are shown in Figure 4. The average particle size of the Al 2 O 3 nanoparticles is summarized in Table 1. As seen in Figure 4, the particle size of the Al 2 O 3 nanoparticles synthesized in deionized water with laser energies of 1, 3, and 5 J are ranged from 10 to 60, 10 to 70, and 10 to 110 nm, respectively. The average particle size of the Al 2 O 3 nanoparticles as given in Table 1 showed that the smallest particle size of the Al 2 O 3 nanoparticles was obtained from a laser energy of 1 J and found to be 26 nm with the narrow particle size distribution in the range of 10-60 nm. It is seen that the higher laser energy seemed to promote more collisions between the vapour atoms/ions, to coalesce within the ablated plume and eventually to form larger particles. It should be noted that the spot size of the laser at the target surface is 0.03 mm 2 . Hence, the laser fluence (J/cm 2 ) can be determined and the results are also given in Table 1.

Optical Property of Al 2 O 3 Nanoparticle in Deionized
Water. Figure 5 shows the opacity of deionized water after the laser ablation of Al with different laser energies. It is clearly observed that the opacity of deionized water increases with increasing laser energy. It can be concluded that the opacity of deionized water is proportional to the number of Al 2 O 3 nanoparticles suspended in deionized water. Hence, the Al 2 O 3 nanoparticles obtained from a laser energy of 5 J are higher than those obtained from 3 and 1 J. Figure 6 shows the absorption spectra of deionized water with suspended Al 2 O 3 nanoparticles obtained from laser energies of 1, 3, and 5 J. A strong absorption peak at 210 nm is clearly observed which confirms the presence of Al 2 O 3 nanoparticles in deionized water [24,25] and not those of aluminum [26][27][28]. As seen in Figure 6, the difference of absorbance is due to the different Al 2 O 3 nanoparticle concentrations suspended in deionized water. To confirm the dependence of the absorbance on the Al 2 O 3 nanoparticle concentration, all the samples of deionized water with suspended Al 2 O 3 nanoparticles were diluted with deionized water to obtain different water concentrations of C/2, C/4, C/8, and C/16, where C is the initial concentration. All the diluted deionized water samples were used  for absorption measurements. Figure 7(a) shows the typical absorption spectra of the deionized water with different Al 2 O 3 nanoparticle concentrations obtained from a laser energy of 5 J. It is seen that the absorbance decreases with decreasing concentration. The plot between Al 2 O 3 nanoparticle concentration and absorbance is shown in Figure 7(b). Figure 7(b) shows that the absorbance depends linearly on the Al 2 O 3 nanoparticle concentration. Similar results were also obtained for laser energies of 1 and 3 J.

Conclusion
We have successfully demonstrated that γ-Al 2 O 3 nanoparticles can be synthesized from Al powder by pulsed laser ablation in deionized water. Al powder was pressed to obtain the Al pellet and used as a target for laser ablation. The advantage of using compressed Al powder is that we can sieve the Al powder to obtain small particle size for starting materials of laser ablation. Therefore, it is easier to obtain Al 2 O 3 nanoparticles compared with those obtained from laser ablation of Al plate. The effect of laser energy on the particle size was investigated. It was found that Al 2 O 3 particle size ablated at a low laser energy was smaller than that obtained from high laser energy. However, high laser energy yielded high Al 2 O 3 concentration than that of low laser energy.