Synthesis of CaF 2 Nanostructures from Calcium Silicide Powders in Diluted Aqueous HF Solution

CaF 2 nanostructures were synthesized from Ca-silicide powders by a diluted aqueous HF treatment. Commercially-available CaSi 2 crystal powders and calcium silicide powders prepared by mechanical alloying were used as the source materials, and CaF 2 nanosheet bundles and nanobunches of the CaF 2 nanoparticles were obtained, respectively. The morphological property of the resulting CaF 2 nanostructures was characterized by electron microscopy. It was found that the morphology of the resulting products depended on the starting materials. In addition, the growth mechanism of the CaF 2 nanostructures was discussed from a topological synthesis point of view.


Introduction
Low-dimensional materials have attracted much interest because of their enhanced or modified optical, electronic and mechanical properties compared to those of the bulk materials. CaF2 is widely known as one of the excellent optical materials, and its thermoluminescent property has been investigated [1][2][3][4]. Recently, CaF2 nanoparticles (NPs) were applied in theranostics for diagnostic analysis, optical imaging, cell labeling, drug delivery and tumor therapy, as shown in Ref. 5. In addition, dental applications of the CaF2 NPs were reported [6]. Moreover, the anti-bacterial activity and cytotoxicity of CaF2 NPs were investigated [7].
To synthesize nanostructures, the topological synthesis technique has been widely used. As one of the examples, CaSi2 is used for the formation of a Si-based two-dimensional nanostructure [8]. The synthesis of CaF2 nanosheets has been reported, but the product consists of CaF2 in three morphologies: square or rectangular nanosheets, irregularly-shaped particles, and sparse squareshaped platelets. The platelets can be separated by centrifugation, but the nanosheets and nanoparticles remain mixed in the samples subjected to characterization and reactivity studies [9]. It has also been demonstrated that the morphology of the resultant products depends on the position and the bonding direction of the Si atoms in the initial crystals [10].
Multiple silicide phases, such as CaSi2, Ca5Si3, CaSi, and Ca2Si, exist in the Ca-Si system. CaSi2 has a zintl phase, trigonal structure (CaSi2-type) (R3m, No.166). In addition, it was reported that a calcium silicide prepared by mechanical alloying has a cubic structure, if it was prepared under the appropriate conditions described in the literature [11]. In this study, the synthesis of CaF2 nanostructures was examined, and morphological control of the nanostructures was investigated from the view point of topological synthesis.

Experiments
Commercially-available (CA) CaSi2 crystal powders (ca.1~10 µm, 2N, Kojundo Chemical Lab. Co., Ltd.) were dipped in an aqueous HF (~0.3, 3 mol/L) solution at room temperature for 5 to 60 min. After the products were filtered for about one hour, substances that remained on the filter paper (JIS P 3801 5C, α-cellulose, pore size 1µm) were dried at room temperature. Calcium silicide powders formed by a mechanical alloying (MA) procedure were also used as the starting materials. The powders were alloyed from fine Si powder (APS ≤ 50 nm, 98%) and Ca granules (2~4 mm, 2N). The Ca and Si powders were weighed in the molar ratio of Ca:Si=2:1. The powders and four 1/4 inch steel milling balls were placed in a steel vial under an argon atmosphere, then the vial was sealed by an O-ring. The milling was carried out using a SPEX 8000 vibratory mill for 50 h [11]. The milled powders were denoted as "MA-calcium silicide powder" in this paper, and were also treated in the HF solution by a 15 min dipping and subsequent filtration process.
The morphological and structural properties of the products were characterized by X-ray diffraction (XRD) measurements, field-emission scanning electron microscopy (FE-SEM) with energy dispersion spectroscopy (EDS), conventional transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) with Fast Fourier Transform (FFT). For the TEM sample preparation, the products were dispersed in a small amount of ethanol, then transferred onto a lacey-carbon-coated copper grid and dried.

Results and discussion
The structural and morphological properties of the CaF2 product synthesized by the dipping and subsequent filtration treatment using the HF (3 mol/L) solution are described in this section as follows, except the result of the product shown in Fig.2(d). First, the products synthesized using the CA-CaSi2 crystal powders and the MA-calcium silicide powders were examined by XRD measurements. For both cases, it was confirmed that the CaSi2 phase still remained after a 5 min. dipping and subsequent filtration treatment, but after a 15 min. dipping and subsequent filtration treatment, the CA-CaSi2 and MA-calcium silicides were decomposed as shown in Fig.1. The lattice parameter of CaF2 (0.5462 nm) is well-matched with that of silicon (0.5431 nm), thus it is difficult to distinguish the diffraction peak corresponding to only CaF2 [12,13]. It is necessary to identify the crystalline phase with the chemical distribution analysis as described later. The morphology of the products synthesized using the CA-CaSi2 powders is described. The products shown in Figs. 2(a)-(c) were synthesized from CA-CaSi2 powders by a 15 min dipping and subsequent filtration treatment, which show features of the intermediate process of transforming from the CaSi2 powders to nanosheets. It was found that the product showed a powder form, as shown in Fig.2(a), and the powder was formed by a collection of nanosheets, as shown in Fig.2(b). In the crosssectional view TEM, the nanosheet bundle structure was clearly seen with a collection of nanosheets having a thickness of about 50 nm overlapping each other along with a void space in Fig.2(c). To observe the initial stage of structural transformation from CaSi2 crystals to CaF2 nanostructures, the synthesis from the CA-CaSi2 powders by a 15 min dipping and subsequent filtration treatment using the HF (0.3 mol/L) solution was examined, and the result is shown in Fig. 2(d) as a cross-sectional TEM image of the product. It is found that the powder was torn and split into the layers, and the CaF2 were distributed between the exfoliated layers of the original powder.
Focusing on the nanosheet structures in the products synthesized from the CA-CaSi2 powders for a 60 min dipping and subsequent filtration treatment using the HF solution (3 mol/L), the products are shown in Figs. 2(e)-(g). It was found that three types of nanosheets were mainly observed. The first one is a moderately facetted nanosheet shown in Fig. 2(e), which was identified as a CaF2 nanosheet based on the EDS mappings and HRTEM observations described later. The second one is an unknown crumpled large nanosheet shown in Fig. 2(f). As shown in Fig. 2(g), the last one is the well-facetted Si nanosheets, which was identified using EDS mappings and HRTEM observations [14]. In this paper, further characterizations of the products focused on only one kind of nanosheet, as only shown in Fig. 2(e).
As already described, the characterization of the chemical distribution is important in order to identify the crystalline phase. The EDS mapping of the products synthesized from the CA-CaSi2 powders with the 15 min dipping and subsequent filtration treatment is shown in Fig. 3(a). It was confirmed that the Si atoms were extracted from the CA-CaSi2 powder, and calcium fluoride was formed by the HF treatments. In the mappings, Si is slightly inhomogeneously distributed, corresponding to the oxygen distribution. The chemical distribution of the nanosheets shown in Fig.  2(e) is also shown in Fig. 3(b). In the image, several small irregular-shaped species are also observed, which may be broken during the TEM sample preparation process. The formation of calcium fluoride is confirmed again in the range of the nanosheet size.
A detailed HRTEM observation was made for the nanosheets as shown by the arrows in Fig. 3(b), and the HRTEM images are shown in Fig.4. The stepped structure of the nanosheet is clearly observed, as shown in Fig. 4(a). In Fig. 4(b), several domains connected to each other to form the sheet form are observed, which means that the nanoheet is polycrystalline. HRTEM image (Fig. 4(c)) and corresponding FFT pattern (Fig.4(d)), taken in a part of the domain shown in Fig.(b), agrees with the lattice image and the ideal diffraction pattern of CaF2 with the [1 # 01] observation direction. In the image, faint additional FFT spots were also observed by the formation of polycrystalline phases. Along with the EDS mappings, the nanosheet is identified as CaF2, but the sheet is polycrystalline.
In Ref. 9, the formation mechanism of the polycrystalline nanosheets is considered as follows: the fast formation of CaF2 in the limited interlayer spaces [∼0.5 nm between (Si -)n sheets] accounts for the observed nanosheet morphology. Additionally, CaF2 formation is templated between the silicon layers of the CaSi2 as the fluoride ions first react with the interlayer calcium ions. After the CaF2 formation is completed, there are no dangling bonds attaching CaF2 to the silicide layers, and  the CaF2 nanosheets are easily released as the silicide is consumed. Thus, both the confinement and templating direct the two-dimensional growth of CaF2 in this system. On the other hand, the polycrystalline CaF2 nanosheets with thicker layer thickness (around 50 nm or less) and wider area (order of sub-micro to micrometers) were obtained in this study. By comparing the results in this study with the growth phenomena of the CaF2 nanosheets in the literature, it is considered that the two-dimensional growth of CaF2 imperfectly takes place, presumably because of the higher reaction ratio of CaSi2 with HF under the examined synthesis condition in this study. The CaF2 crystals having sheet-like morphology were deposited, then, developed to free standing nanosheets after the original powder was decomposed. The polycrystalline deposition of the condensed CaF2 layers leads the formation of the polycrystalline nanosheets. To improve the crystalline quality of the nanosheets, epitaxial or templating deposition of CaF2 on the surface of the exfoliated layers is important. In addition, the formation of a biproduct of H2SiF6 or CaSiF6 was also reported [9]. It should be pointed out to improve the crystalline quality of the nanosheets that the formation of biproducts of H2SiF6, CaSiF6 and CaSiF6•2H2O would play important roles to form nanosheets of the CaF2 crystal, because the precipitates obtained from the residual solution, in which these compounds would be included, were condensed in the sheet morphology [14]. Further investigation of the nanosheet synthesis technique to improve the quality of the nanosheets will be required. Next, the morphology of the CaF2 crystals prepared using the MA-calcium silicide was observed. It was found that the product had a powder form, and the powder was formed by a collection of smaller particles, which looks like sandstone, as shown in Figs.4(a) and (b). The TEM observation of part of the product scratched off the powder shows that a bunched structure was formed, and it consisted of nanoparticles with a diameter around 10 nm, as shown in Fig.4(c). The HRTEM image and the corresponding FFT pattern in Fig.4(d) revealed that the nanoparticles in the nanobunch showed that the CaF2(111) planes of the domains were preferentially distributed along the directions shown by the red dotted arrows. The angles of 71°correspond to that between the CaF2 (111) and (1 # 11) planes, and used as a visual guide. It also shows the superposition of the FFT patterns of CaF2 denoted as CaF2 111, and those of the MA-calcium silicide shown as CS1 and CS2. The actual phase of the MA-calcium silicide has not been identified [11], thus, CS1 and CS2 are used to describe the patterns. It is considered that the nanoparticles were then formed, and the CaF2(111) planes were distributed parallel to each other, presumably at the interface between two adjacent particles.
Fluorine atoms were diffused into the CaSi2 crystals, then CaF2 layers were formed in the crystals [15]. The formation of the CaF2 layers was also reported for the use of CaGe2 crystals [16]. The crystallographic symmetry of the CaSi2 and CaGe2 starting crystal was maintained even during the transformation from CaSi2 to CaF2 by the Si extraction and F incorporation processes. It was also confirmed that the Si atoms were extracted from the MA-calcium silicide powders and CaF2 was formed by the HF treatments, as shown in the SEM images and corresponding EDS mappings of the powder consisting of the CaF2 nanobunches or nanoparticles in Figs.5 and 6. The morphological control of the CaF2 nanostructure was demonstrated by the HF treatment of the appropriate Ca-silicide as the starting material. As the starting material, when the CA-CaSi2 crystals were used, CaF2 nanosheet bundles were formed. On the other hand, when MA-calcium silicide was used, CaF2 nanobunches or nanoparticles were formed. It was reported that that MAcalcium silicide has a cubic phase structure, and showed a semiconducting property [11]. The difference in the crystalline structure of the starting materials affects the morphology of the final products. The characteristic morphology of each product of the CaF2 nanostructures can be developed into applications that take advantage of each property as already mentioned. Further control of nanoscale morphological property would be possible for designing the nanoscale architectures.

Conclusion
The CaF2 nanostructures were synthesized from Ca-silicide powders by an HF treatment. It was found that the morphology of the resulting products depends on the starting materials. The morphological control of the CaF2 nanostructure was demonstrated by the HF treatment of the appropriate Ca-silicide as the starting material. As the starting material, when CA-CaSi2 crystals were used, polycrystalline CaF2 nanosheet bundles were formed. On the other hand, when MAcalcium silicide was used, CaF2 nanobunches or nanoparticles were formed. The results suggest that further control of the morphological and structural properties of the CaF2 nanostructures would be possible by a simple chemical reaction in an HF solution using calcium silicides as the source materials, which would allow us to apply the nanostructures to many applications in various fields.