Synthesis under Normal Conditions, Morphology and Composition of AlF3 Nanowires

1. Introduction

Metal fluorides are promising for applications in catalysis [1,2,3,4,5,6], as ionic conductors [7] and luminescent materials [8,9,10,11]. One of the most important representatives of these binary compounds is aluminum fluoride (AlF₃), which is an inorganic crystalline substance under normal conditions. Due to its electrophysical properties, it finds application as a catalyst in organic synthesis, an additive for electrolyte to lower the melting point, fluxes for oxide separation and removal [12], an element of multilayer optical structures [13], a material for fabrication of far-infrared mirrors [14]. It is also a promising candidate for fiber lasers in the mid-infrared region [15]. AlF₃/polyimide films increase dielectric permeability as well as reduce leakage currents compared to conventional polyimide films [16]. Interestingly, AlF₃ can also be used to prevent tooth discoloration [17]. AlF₃ can be used as a film to cover lithium-sulfur batteries to prevent the "shuttle effect" that causes a rapid decrease in capacity [18]. Synthesis of AlF₃ in the form of one-dimensional (1D) structures, such as nanorods, nanowires (NWs) and nanotubes, enables a high surface to volume ratio. This makes 1D AlF₃-based structures promising for use in supercapacitors and nano-probes of different types [19,20,21,22,23]. There are several methods for producing 1D structures, including AlF₃-based NWs [24,25,26,27,28,29,30], each characterized by a complicated technological process. Particularly, it is difficult to obtain AlF₃ without the use of aggressive HF. One of the most popular methods for synthesizing AlF₃ nanoparticles is the fluorolitic sol-gel synthesis [31], which can be described by the system of equations

M-OR + H-F → M-F + ROH

(1)

nM-F → (-N-F-M-F)n/2

(2)

In the first reaction, the water as a reactant is completely replaced with hydrogen fluoride, which leads to the formation of M-F bonds. The second reaction, if performed correctly, leads to the formation of either metal oxides or nanoscale metal fluorides. Fluoride ions tend to form stable bonds. Consequently, there are not many examples of crystalline fluorides containing fluorine at the ends. The main problem in the synthesis of metal fluorides is their high lattice energy, which makes it difficult to form regular crystal structures [32]. On top of that, the existing technological methods require relatively high temperatures (450–500 °C), as well as additional raw materials for the intermediate reactions. Thus, the development of simple and robust methods to synthesize AlF₃ NWs is a challenging task.

This paper presents the synthesis method and the results of morphological and compositional characterization of AlF₃ NWs. The NWs are obtained in the direct reaction of hydrofluoric acid solution with aluminum under normal conditions. The morphology and composition of the NW samples are studied versus the concentrations of hydrofluoric acid reacting with pure aluminum.

2. Materials and methods

The growth experiments were carried out as follows. A sample of mass m = 2.435 g was placed in a 70 ml volume of hydrofluoric acid in a polypropylene vessel. At the beginning, the entire area of the sample was covered with a white film, and bubbles gradually began to appear in the volume. Twenty-five minutes later, the reaction transitioned to the active stage, with fast outgassing. Two hours later, the sample dissolved completely. The solution was left to await precipitation. Twenty-four hours later, a crystal formed, which was treated with 2-methanol propane to prevent contact with the solution residue.

The morphology and composition of the original aluminum and synthesized samples were studied using a scanning electron microscope (SEM) Zeiss Supra25 (Carl Zeiss), equipped with an energy dispersive x-ray analyzer (EDX) Ultim Max 100 (Oxford Instruments). Aluminum used for the production of 1.7-liter pots by KALITVA COOK was a raw material for the study (Al_f). In the analysis of the alloy by EDX method, the following composition was determined: Al=98.95%, Si=0.67%, Ca=0.05%, Fe=0.21%, Ni=0.13%, and Zn=0.02%. Granulated pure Al (Al_p) was also used, with an average pellet mass of m = 0.246 g. Hydrofluoric acid from "Sigma Tech" was used as an oxidizer. NT-MDT NTEGRA THERMA was used for atomic force microscopy (AFM) analysis.

The crystal is transparent, has a characteristic odor, and resembles table salt crystals in brittleness. A series of experiments with different concentrations of hydrofluoric acid were performed, with the volume of the solution always kept at 40 ml. For all samples a precipitate in the solution after the formation of crystals was observed. Table 1 shows the acid concentrations and the ratio of the mass of the initial metal over the mass of the precipitate. Comments referring to these samples are given later in the text. In addition, a similar series of experiments with Al_f was performed.

3. Results and discussion

In the first experiment it was assumed that AlF₃∙H₂O was obtained. A known method of obtaining aluminum fluoride by neutralizing hydrofluoric acid with aluminum hydroxide 3HF + AL(OH)₃ = AlF₃ + 3H₂O at 90–95 °C for one hour. During the reaction, aluminum fluoride crystallizes as AlF₃∙H₂O, after which the paste is dehydrated at 350 °C [33]. It is possible that the additives in the original food-grade aluminum alloy served as a catalyst for the reaction. However, this reasoning was not confirmed by a series of tests with Al_f pellets.

According to the AFM results obtained, exposure to HF disturbed the integrity of the original surface, forming a stepped-grid structure. As can be seen, the flaking of the structure occurred uniformly, regardless of variations in the height of the original relief. Based on the assumption that the destruction of the sample material occurred under the catalytic effect of a certain alloy constituent, we can conclude that its greatest concentration was in the places under the "steps" where dissolution was the most intense. A series of tests with hydrofluoric acid of different concentrations was also carried out.

As the concentration of hydrofluoric acid in the solution increased, the initial structure of the resulting precipitate changed. During the reaction of sample 6.1 with 8% solution, the precipitate obtained during the reaction copied the shape of the container in which the experiment took place in Figure 1b. The sample has a sandwich structure: a transparent layer in the middle and a white salt-like surface. In the experiment with sample 1.1, the precipitate did not form a structure but fell out as flakes. In addition to the precipitate, a thin layer of unreacted Al remained in the container. It is interesting that samples 1.1-1.2 turned out to be unstable and after two weeks acquired a macro-porous structure, as shown in Figure 1a.

The chemical composition of the obtained samples was investigated using EDX, while SEM visualization represented the morphology of the output structures. Figure 2 shows a comparative analysis of the spectra of the initial aluminum and the studied substance. According to the peaks, we can conclude that the resulting material consists mainly of Al and F. The calculated atomic percentages of different elements are the following: F=71.17%; Al=29.53%; Si=0.57%; Fe=0.08%; Ni=0.08%, and Zn=0.01%. Figure 3 shows the images obtained by scanning SEM in the analysis of sample 6.1.

Figure 3 shows that the surface consists of an ensemble of 1D microwires. The average height of 1D structures is 4 µm, the length of the hexagon side is 0.82 µm, and the surface density is in the order of 10⁴ 1/cm². EDX was also used to determine the composition. The analysis revealed that the percentage of Zn in the microwires was higher than in the matrix as a whole. It is interesting that, depending on the concentration of the solution with which the starting material interacted, the size of the wires and their geometry change. For example, Figure 4 shows an SEM image of sample 5.1, where the NWs are thinner than in sample 6.1.

The average height of these NWs is 1 µm, the length of the hexagon side is 0.26 µm, and the surface density is in the order of 3 × 10⁴ 1/cm². Under normal conditions, the thermodynamically stable α-AlF₃ phase is a rhombic structure similar to that of perovskite. The β-AlF₃ rhombic phase has a hexagonal structure [34]. β-AlF₃ is synthesized by two methods: by dehydration of α-AlF₃∙3H₂O and by growing crystals from chloride flux [14]. There are other metastable phases of AlF₃ (tetragonal and cubic), whose structures are little studied. When analyzing the structures using SEM, a tendency of charging of the samples was observed. From the first sample to the sixth, the charging capacity of the sample increased, due to which the obtained images had a noise background.

SEM of sample 4.1 revealed that the NWs have a nearly perfect cylindrical shape (Figure 5), and that overall pattern of the NWs resembles a herringbone branch. The average height of these NWs is 4 µm, the average diameter is around 0.4 µm; and the surface density is in the order of 4 × 10⁴ 1/cm². During the study of the formation of 1D structures, an interesting fact was discovered - NWs did not form in all reactions. In the synthesis of sample 1.1, no NWs were formed, and the microstructure of the material is a rough surface.

For a clearer result, we performed a series of experiments with particularly pure Al (Al_p). Table 2 presents the experimental data (in the synthesis of sample 5.2 the mass of the initial aluminum was lower than average, which may have affected the overall dependence of the given ratios). Figure 6 shows the SEM images of the obtained samples. The data of the Al_f and Al_p reactions appear very similar.

The sample in Figure 6a was obtained during the reaction of Al_p with 8% solution. The geometry of 1D structures is similar to that shown in Figure 4. According to the SEM data, we can also say that the obtained samples exhibit the ability to accumulate charge. We clearly observe a structural transformation of the surface, from a completely smooth surface with the presence of local dislocations to a complete coverage of the surface with NWs.

The difference is also observed in the macrostructure. In the 8% solution, the sample is completely white without the formation of an additional phase, while in the 40% solution the precipitate is flake-like crystallites. The flakes formed with Al_p were completely transparent, and when we tried to examine the structure using SEM (with a probe energy of 20 kEv) at magnification, the sample began to crack. The microstructural surface was smooth, the inhomogeneities present on it represented dislocation defects, and the 1D structures were inside the clefts. In the samples with a solution concentration of 32.6%, 1D structures were nanoscale and did not have hexagons at their base, but rather resembled needles in Figure 6c.

Figure 7 shows the EDS data, from which the compositions on the surface of sample 5.2 and in the gap are slightly different. The macrostructure of the resulting compound based on food-grade aluminum is in a polycrystalline state. One of the reasons for the presence of crystallites is the level of purity of the laboratory room and local temperature gradients. As noted earlier, the reaction was violent, which is why the phase transitions may have been exothermic in nature. Dislocation transitions are clearly visible. All the resulting samples obtained from both Al_f and Al_p are stoichiometric compounds of AlF₃. In the case of Al_f, the total impurity is less than 1%.

4. Conclusions

To sum up, we have demonstrated the possibility of obtaining 1D AlF₃ structures under normal conditions. Due to the ease of the production method, these AlF₃ NWs can be applied in sensorics, high-performance electronics, and scanning probe microscopy. The obtained stoichiometric Al_p-based AlF₃ NWs can also be used as a dielectric in supercapacitors due to their predisposition to active charge accumulation. It is known that the use of Alp in the AlF₃/SiO₂/Si system allows one to localize a high density of fixed negative charges in fluorine vacancies, serving as a source of a strong drift field [15]. The robustness of the technological method under normal conditions enables the fabrication of new composite materials based on AlF₃, obtaining multilayer structures with 1D nanocrystals with interesting electro-physical properties that can be used for the design of different microelectronic components.