Introduction

Nanoparticles and microparticles of metallic materials are widely known to exhibit properties that differ from those of the corresponding bulk materials. Hence, metallic particles are used in fields related to catalysis, optics, and biotechnology [1,2,3]. Metallic Al particles have also been investigated for potential applications as fuels, energy storage materials, and pigments [4,5,6]. Many reported methods for synthesizing metallic Al particles involve the electrical evaporation or explosion of metallic Al wire in a gaseous medium, which is a well-known gas-phase process for synthesizing particles [7,8,9,10,11,12]. Although this gas-phase process works well, it requires equipment that is expensive, consumes a large amount of electric power, and is complicated and dangerous to perform.

An alternative method to produce metallic particles is liquid-phase reactions. Most such reactions involve the reduction of metal ions in an aqueous phase. This process can easily produce a large amount of metallic particles and is therefore suitable for industrial production. Particles of noble metals such as Au, Pt, and Ag can be synthesized via the reduction of metal ions in aqueous solution [13,14,15]. However, the synthesis of less-noble metals in aqueous solution is difficult. For example, the corrosion of metallic Al in aqueous electrolytes leads to oxidation of metallic Al by the reaction 2Al + 6H2O → 2Al(OH)3 + 3H2↑ [16], which makes both the synthesis and long-term storage of metallic Al particles difficult. Metallic Al particles can be synthesized via a process based on the reduction of Al3+ ions in organic solvents [17, 18]. The organic solvents are used to inhibit the oxidation derived from aqueous electrolytes. However, the use of organic solvents increases the environmental impact.

Typical methods for producing metals and metal alloys are electrolytic processing (electrolysis) techniques such as electrodeposition and electroplating [19,20,21,22,23]. The electrolytic deposition of metallic Al has been reported [24,25,26]. The electrolysis process does not usually involve reducing reagents and therefore can increase the purity of the metal. Electrolysis usually forms a metallic film on the electrode via the deposition of metal nanoparticles. If the metallic particles deposited onto the electrode can be dispersed in the electrolyte, the dispersion can yield a colloidal solution of metallic particles. From this viewpoint, several researchers have synthesized metallic particles using electrolysis [27,28,29]. Methods for the electrolytic synthesis of metallic Al particles, in particular, have also been reported [30,31,32]. In these methods, an organic solvent, ionic liquid, and strong reducing reagent are used, which creates a large environmental load, introduces high costs, poses a danger to human health and the environment, and leads to adulteration of the product by impurities that originate from the reducing reagent. Unsurprisingly, the synthesis of metallic Zn particles in water is also challenging. Nevertheless, our research group has synthesized metallic Zn particles via electrolysis in aqueous solution.

In the present study, we extend our method used to prepare metallic-Zn-particle colloidal solutions in water by electrolysis to prepare high-purity metallic Al particles via a simple method with low environmental impact, which is also challenging.

Experimental

Materials

Aluminum nitrate nonahydrate (Al(NO3)3∙9H2O) (98.0%, Kanto Chemical Co.) was used as received as the starting reagent for preparing metallic Al particle colloidal solutions. Metallic Al plates with dimensions of 20 × 75 × 0.5 mm3 were used as the electrodes. A commercial metallic Al plate (purity: 99.7%, dimensions: 45 × 120 × 0.5 mm3, code #: 1–126-0119, Kenis) was divided into metallic Al plates with the desired size. All the aqueous solutions were prepared using water purified by ion exchange and distillation with an Advantec RFD372NC water distillation apparatus.

Preparation

Figure 1 shows an illustration of set-up of fabrication of metallic Al nanoparticles by electrolysis in aqueous solution. The electrolytic solution was a 0.1 M Al(NO3)3 aqueous solution prepared using purified water, in which the oxygen was removed by N2 bubbling. One hundred milliliters of the electrolyte solution was placed in a beaker with a capacity of 100 mL. Electrolysis was carried out at room temperature using a two-electrode system with a constant current in the electrolytic solution. The Al plates were used as both the anode and the cathode. At the anode, Al3+ should be supplied to the electrolyte solution, as represented by the reaction Al → Al3+  + 3e−. At the cathode, the reaction Al3+  + 3e−  → Al should occur, where Al3+ is supplied from the reagent used to prepare the electrolyte and by the reaction Al → Al3+  + 3e− at the anode. Metallic Al should be produced through deposition of Al nuclei followed by growth of the nuclei into Al nanoparticles on the cathode. The two electrodes were submerged in 2 cm of the electrolytic solution such that the distance between them was 2 cm. The voltage was applied using an A&D AD-8724D DC stabilized power supply to maintain a current constant of 1.0 A. The electrolysis time was 60 min. To disperse the Al nanoparticles deposited onto the cathode into the electrolytic solution prior to the formation of metallic bulk Al, the bottom of the beaker was irradiated with ultrasonic waves generated by a Honda Electronics W-113 ultrasonic cleaner (oscillation frequency: 28 kHz) during electrolysis.

Fig. 1
figure 1

Illustration of set-up of fabrication of metallic Al nanoparticles by electrolysis in aqueous solution

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Characterization

The morphology and crystal structure of the particles were investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD), respectively. TEM imaging was performed using a JEOL JEM-2100 microscope operating at 200 kV. The TEM samples were prepared by dropping the colloidal solution onto a collodion-coated Cu grid and evaporating its dispersant. The volume-averaged particle size and the standard deviation of the particle size distribution were determined using dozens of particle diameters measured in the TEM images. The XRD measurements were carried out using a Rigaku Ultima IV X-ray diffractometer with a CuKα radiation source operated at 40 kV and 30 mA. The XRD samples were powders of the particles. The particles in the colloidal solution were washed by repeated centrifugation and decantation to remove the supernatant, followed by the addition of water and redispersion by shaking using a vortex mixer. The washing process was performed three times, and the particle powder was obtained by centrifuging the colloidal solution, removing its supernatant by decantation in the third washing process, and drying the slightly wet particle powder in a Yamato DP-31 vacuum drying oven equipped with an oil-sealed rotary vacuum pump (ULVAC GCD-136X).

Results and discussion

Electrolysis at room temperature (25 °C)

Figure 2a shows a photograph of the electrolytic solution before electrolysis. The solution was transparent at this point. Figure 2b shows a photograph of the electrolytic solution prepared by electrolysis at 25 °C. The transparent electrolytic solution became a slightly opaque and grayish colloid solution after electrolysis. No significant precipitation occurred after a few hours of preparation, as far as the naked eye could determine. The solution could therefore be regarded as colloidally stable. The opaqueness implied the production of nanoparticles that caused light scattering.

Fig. 2
figure 2

Photographs of a Al(NO3)3 aqueous solution and colloidal solutions prepared by electrolysis at b 25 °C, c 35 °C, and d 45 °C

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Figure 3a shows a TEM image of particles contained in the colloidal solution. Several particles were observed, and their size was 76.3 ± 30.2 nm. Pattern (a) in Fig. 4 shows the XRD pattern of the particles obtained at room temperature. No remarkable peaks were detected, which indicates that the particles were either amorphous or crystallites that were too fine to be detected.

Fig. 3
figure 3

TEM images of nanoparticles in colloidal solutions prepared by electrolysis at a 25 °C, b 35 °C, and c 45 °C

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Fig. 4
figure 4

XRD patterns for nanoparticles in colloidal solutions prepared by electrolysis at a 25 °C, b 35 °C, and c 45 °C. ●: cubic Al

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Electrolysis under heating

Figure 2c, d show photographs of the electrolytic solutions prepared after the electrolysis at 35 and 45 °C, respectively. Like the product obtained at 25 °C, those obtained at 35 and 45 °C were slightly opaque, grayish, and colloidally stable solutions. Figure 3b, c show TEM images of particles in the colloid solutions. The particle sizes were 77.0 ± 31.0 nm for product obtained at 35 °C and 84.7 ± 38.2 nm for those obtained at 45 °C. Thus, the particle size increased with increasing temperature of the electrolytic solution. The particles collided with greater energy at higher temperatures, which resulted in aggregation of the nanoparticles and an increase of the average particle size.

Figure 4 shows the XRD results. Profile (b) is for particles produced at 35 °C. Peaks were detected at 38.5°, 44.7°, 65.1°, and 78.2°. These peaks were assigned to the (111), (200), (220), and (311) planes of metallic Al (cubic), respectively, according to previously published results [26, 33] and an ICDD reference pattern (#00-004-0787). These peak assignments confirmed the production of metallic Al particles. An increase in temperature during electrolysis was considered to promote reduction of Al3+ ions and crystallization of particles. By applying the Scherrer equation to the XRD linewidth for the (111) plane peak, we determined that the average metallic Al crystal size was 55.7 nm. The observed particle size is larger than the crystal size determined by XRD analysis, although they are similar. This result suggests that the metallic Al particles were a mixture of single crystals and polycrystals of metallic Al. However, the formation of a single crystals cannot be confirmed from XRD powder diffraction analysis alone, and analyses using other analytical techniques, such as electron-backscatter diffractometry and selected-area electron diffractometry, are required to demonstrate the formation of a single crystal.

Profile (c) in Fig. 4 is for nanoparticles produced at 45 °C. Peaks assigned to the (111), (200), (220), and (311) planes of metallic Al (cubic) appeared at 38.5°, 44.7°, 65.1°, and 78.2°, respectively. The average Al crystal size was 59.3 nm; in this case also, the particle size was larger than the crystal size but the sizes were similar. The metallic Al nanoparticles were similarly speculated to be a mixture of single crystals and polycrystals of metallic Al. The crystal size increased from 55.7 to 59.3 nm when the temperature was increased from 35 to 45 °C. This temperature dependence of the particle size reconfirms that higher temperatures promote the crystallization of particles. Notably, no peaks attributable to phases other than metallic Al (e.g., Al oxide) were observed in the XRD profiles for the products obtained at either temperature. According to Rai et al. [34], oxidation of metallic Al occurs via the diffusion of oxygen. Such oxidation was not observed in the present work, which indicates that the obtained metallic Al particles were chemically stable, although the mechanism for this high chemical stability remains unclear.

Gas evolution

Bubbles were generated on the anode in all the electrolysis experiments. The reaction 2H2O → O2 + 4H+  + 4e−, which results in gas evolution, must have occurred because the electrolyte solutions contained no substances prone to oxidation other than water. The standard potential of the reaction O2 + 4H+  + 4e−  ⇌ 2H2O is + 1.229 V [35]; thus, the reaction 2H2O → O2 + 4H+  + 4e− does not progress easily. Because electrons in the anode moved toward the power supply via current flow, electrons were depleted at the anode. Therefore, the reaction 2H2O → O2 + 4H+  + 4e− progressed to the right, which led to O2 evolution at the anode. Thus, the bubbles were regarded as O2 gas produced by the oxidation of water. Because the current flow was provided by electrons generated by both the reactions 2H2O → O2 + 4H+  + 4e− and Al → Al3+  + 3e− at the anode, the O2 evolution that resulted from the reaction 2H2O → O2 + 4H+  + 4e− would have disturbed the reaction Al → Al3+  + 3e− at the anode. Thus, O2 evolution may have reduced the efficiency of the production of metallic Al particles because this disturbance did not supply Al3+ ions to the electrolyte solution. The development of a highly efficient method for synthesizing metallic Al particles is a challenge and will be addressed in future work.

Conclusions

A method for fabricating metallic Al particles in aqueous solution was proposed. An aqueous colloidal solution of metallic Al nanoparticles with a cubic crystal structure was prepared by reducing Al3+ ions via electrolysis onto metallic Al-plate electrodes under ultrasonic irradiation and heating. The particle sizes were 76.3 nm at 25 °C, 77.0 nm at 35 °C, and 84.7 nm at 45 °C, and the crystal sizes were 55.7 nm at 35 °C and 59.3 nm at 45 °C. Higher temperatures led to greater collisions among the nanoparticles, which promoted particle growth followed by crystal growth. The metallic Al particles were chemically stable not only in aqueous solution but also in air. In conclusion, we demonstrated that chemically stable and colloidally stable metallic Al particles can be synthesized via electrolysis in water. Our future challenge is to develop a highly efficient synthesis method.