Formation of Si-based Nanosheet Bundles and Morphological Modication of CaSi2 Crystals by Thermal Treatment Using Chloride Compounds

Si-based nanosheet bundles were formed by Ca-atom extraction from CaSi2 crystals by thermal treatment using FeCl2, FeCl3, NH4Cl, and MgCl2. The morphological property of the Si-based nanosheet bundles was examined. Though the Si-based nanosheets were formed under the stated thermal treatment conditions using FeCl3, the formation of iron-oxide phases was accelerated at higher temperatures and longer duration times for the highest thermal treatment temperatures using FeCl2. Thinner and thicker nanosheets were obtained using NH4Cl and MgCl2, respectively. The morphological properties of the Si-based nanosheet bundles were compared to each other and their formation mechanism was discussed in terms of the thermal instability of the chlorides, their vapor pressure and thermal treatment temperature profiles. [DOI: 10.1380/ejssnt.2018.218]


I. 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 [1,2]. A nanosheet bundle is also one of the important structures for technological applications [3]. The formation of Si nanosheet bundles by Ca extraction from CaSi 2 crystals by thermal treatment with metal chlorides has been previously reported [4][5][6]. However, details of the structural property of the treated nanosheets have not been clarified. Recently, the structural property of the Si-based nanosheet bundle formed using FeCl 3 was reported [7]. It was revealed that the structures of the nanosheets were basically the same as that of the Si-based nanosheets synthesized by inositol hexakisphosphate, C 6 H 18 O 24 P 6 , (IP6) [8]. However, the detailed structures of the nanosheets are only slightly modified compared to that of the IP6-treated nanosheets. It is important to clarify the difference in the detailed structures of the nanosheets, which depends on the nanosheet synthesis procedure.
In this paper, the morphological property of the nanoproducts formed using several chloride compounds, such as FeCl 2 , FeCl 3 , NH 4 Cl, and MgCl 2 , was characterized. It should be noted that water absorbing (deliquescent) metal chlorides, especially MgCl 2 , were used as the * This paper was presented at the 8th International Symposium on Surface Science, Tsukuba International Congress Center, Tsukuba, Japan, October 22-26, 2017. † Present address: Chongqing University of Arts and Sciences, NO. 319 Honghe Avenue, Yongchuan, Chongqing City, 402160, P. R. China ‡ Corresponding author: tatsuoka.hirokazu@shizuoka.ac.jp source material. The morphological property of the Sibased nanosheet bundles was compared to each other and discussed in terms of their thermal instability and deliquescency of the chloride compounds and thermal treatment temperature profiles.

II. EXPERIMENTAL
The CaSi 2 micro-walls were initially grown on Si(111) substrates [9]. Commercially-available CaSi 2 crystal powders were also used as the source material. The Si-based nanosheet bundles were then formed by Ca-atom extraction from the CaSi 2 crystals by thermal treatment with FeCl 2 , FeCl 3 , NH 4 Cl, or MgCl 2 . The chloride sources were stored in about 30-50% humidity. For the thermal treatment of the CaSi 2 crystals in the vapors, the Si substrates with the CaSi 2 micro-walls or CaSi 2 powders and the chloride powders were placed on opposite sides of each other inside a sealed stainless-steel cell in a N 2 atmosphere (Ar atmosphere for only MgCl 2 source) with detected oxygen of less than 0.1%. When using the chlorides and CaSi 2 crystals, except for the NH 4 Cl-CaSi 2 powder, the cell was heated to the highest thermal treatment temperature of 550 • C to 800 • C and the temperature was maintained for 0 min to 300 min. The heaters were then turned off, and the cell was naturally cooled. The temperature profile of the thermal treatment is shown in Fig. 1(a). In the figure, only for the cases of, for example, the highest thermal treatment temperatures of 550 • C, 600 • C to 650 • C and their duration times of 0 min, 0 min and 10 min, respectively, are shown. It is noted here that the "0 min" of the duration time of the highest thermal treatment temperature means the heaters were immediately turned off, when the cell temperature reaches the highest treatment  temperature. The nanosheet formation reactions have already begun during a rise in the cell temperature. After the thermal treatment process, the treated nanosheets were washed in ethanol a few times for a few minutes to remove the residual chloride compounds. When using MgCl 2 , the resulting powders were further washed by an HCl solution (35-37 w/w%) for 1 h to remove MgO formed as a by-product. For the use of the NH 4 Cl-CaSi 2 powder, the NH 4 Cl and CaSi 2 powders were located in a loosely sealed quartz container, which was loaded into a vacuum chamber, then thermally treated. The thermal temperature profile for this case is also shown in Fig. 1(b). Further detailed information about the treatment conditions of the micro-walls and nanosheet bundles is described elsewhere [6,7,9]. Throughout this paper, for example, these nanosheets treated under the FeCl 3 vapor are denoted here as "FeCl 3 -treated" nanosheets, etc.
The morphological and structural properties of the nanosheets were characterized by field-emission scanning electron microscopy (FE-SEM) with energy dispersion spectroscopy (EDS), conventional transmission electron microscopy (TEM), high-resolution TEM (HRTEM) with Fast Fourier Transform (FFT), and scanning transmission electron microscopy (STEM) with EDS. For the TEM sample preparation, the products were scratched off the substrates using a sharp-edged blade and dispersed in a small amount of ethanol, then transferred onto a laceycarbon-coated copper grid and dried.  Figure 2(a-d) shows a series of SEM images of the FeCl 3 -treated nanosheets formed from the CaSi 2 microwalls and rooted on the Si substrates with the thermal treatment conditions at (a) the highest thermal treatment temperature of 550 • C and its duration time for 5 min, (b) 600 • C for 0 min, (c) 650 • C for 0 min, and (d) 650 • C for 10 min. The nanosheet formation reactions have already begun during a rise in the cell temperature, even though the duration time is "0 min" at the highest thermal treatment temperature. The nanosheets with a thickness of several to a few tens of nanometers were obtained, and the nanosheets were stacked with a small void space to form a bundle. The small void space is formed by the volume reduction caused by the Ca-atom extraction. It was confirmed that the Si-based nanosheet bundles were formed on the Si substrate under the all of the stated thermal conditions. The nanosheets are formed along the Si{111} planes inclined from the Si substrate, and the geometric arrangements of the nanosheets or the c-planes of the CaSi 2 crystals are described elsewhere [9]. Figure 2(e) shows an STEM image and corresponding EDS mappings of the FeCl 3 -treated nanosheets formed on the Si substrate with the thermal treatment condition of 550 • C for 5 min. It is shown from the EDS mappings, for nanosheet (A), Ca atoms still remain, and O atoms are distributed the same as that of the Ca atoms. Cl atoms also remain as particles and correspond to the Ca-atom distribution. The Ca-atom extraction was not completed, and it is considered that the distributions of the O and Cl atoms cause oxidation of the Ca atoms and the formation of CaCl 2 particles, respectively. Fe atoms are slightly distributed in some regions. On the other hand, for nanosheet (B), almost only Si atoms are observed, and the Ca-atom extraction is almost complete. Figure 3 shows (a) an SEM image and corresponding EDS mappings of the FeCl 3 -treated nanosheet bundle formed from the CaSi 2 powders with the thermal treatment condition of 540 • C for 60 min, and (b) crosssectional view of Si-based nanosheet bundles formed from the CaSi 2 powder with the treatment condition of 540 • C for 60 min. The EDS mappings show that the nanosheet bundle mainly consisted of Si, and Fe and O are partially distributed as particles around the bundles caused by the formation of an iron-oxide and/or an iron-hydroxide. It was found that the nanosheets with the thickness of a few tens of nanometers were obtained. Figure 4(a-d) shows a series of SEM images of the FeCl 2 -treated nanosheet bundles and other products formed and rooted on the Si substrates with the thermal treatment conditions of (a) the highest thermal treatment temperature of 555 • C and its duration time for 10 min, (b) 570 • C for 10 min, (c) 650 • C for 0 min, and (d) 650 • C for 10 min. The nanosheet formation reactions have already begun during a rise in the cell temperature, even though the duration time is "0 min" at the highest ther- ness of several to tens of nanometers were also observed at the low treatment temperature. The nanosheets are stacked with a small void space to form a bundle. In addition to the formation of the nanosheet bundles, nanorods were formed around the nanosheet bundles, with the increasing highest thermal treatment temperature and the duration time.  Fig. 4(c, f) is Fe : O ∼ 1 : 2, and that in Fig. 4(d) is Fe : Si : O ∼ 2 : 1 : 4. It is considered that iron-oxide was formed around 570-650 • C [10][11][12], and Fe 2 SiO 4 was formed at 650 • C for 10 min [13]. Figure 5(a) shows an SEM image and corresponding EDS mappings of the FeCl 2 -treated nanosheet bundle formed from the CaSi 2 powders with the thermal treatment condition of 550 • C for 80 min, and (b) crosssectional view of the nanosheet bundle formed from the CaSi 2 powder with the treatment condition of 555 • C for 3 min. The EDS mappings show that the nanosheet bundle mainly consisted of Si. In addition, Fe and O are partially distributed in the same way as the particles around the bundles, as well as Ca and Cl. It was found that the nanosheets with the thickness of a few tens of nanometers were observed.   tively. It was confirmed that MgO was hardly observed when using anhydrous magnesium chloride as the source material.

III. RESULTS AND DISCUSSION
It was reported that the nanosheets were synthesized by a solid state reaction using metal chlorides, such as FeCl 2 and NiCl 2 [14]. In this case, the metal silicides were formed accompanied by the formation of stable CaCl 2 and nanoflakes based on the metathesis reactions. However, it is noted that the Si-based nanosheets were formed for all of the reactants, and no metal-silicide alloy was formed in this study. By-products, CaCl 2 and iron-hydroxide and/or oxide remained around the nanosheet bundles, not between the sheets nor in the sheets. It is not likely that the reactants of the metal chloride compounds directly react with the CaSi 2 crystals. The Ca extraction was successfully demonstrated. From Fig. 2(e), it is considered that the Ca extraction takes place layer by layer. The anisotropic reaction was reported for the case of the HCl reaction with CaSi 2 for the siloxene synthesis in the HCl aqueous solution [15]. The simple extrapolations of the reaction rates along the Si(111) and Si(110) planes around ∼400 • C are 1 mm/s and 0.1 mm/s, respectively. It is much higher than the reaction rate roughly up to ∼50 nm/s. Thus, the actual reaction rate by the thermal treatment is diffusion controlled, namely, the supply of the tentatively-formed HCl or Cl 2 vapors [10][11][12]16]. Assuming that the existence of residual H 2 O and/or O 2 with the chlorides due to their deliquescency and in the N 2 (or Ar) atmosphere would not be negligible, HCl or Cl 2 directly reacts with CaSi 2 , and the chloride compounds play a role as a source to supply HCl or Cl 2 , the following discussions are provided.
Though Si-based nanosheets were formed under the thermal treatment conditions using FeCl 3 , the formation of the iron-oxide phases was accelerated at the higher temperatures and the longer highest thermal treatment duration time using FeCl 2 . The chemical reactions of FeCl 2 with residual H 2 O and O 2 supplied by its deliquescency and/or in the atmosphere are as follows [10], during the initial stage: during the middle stage, during the last stage, and these reactions would take place at the position of the source materials, during the initial stage of the thermal treatment, because of its lower vapor pressure of FeCl 2 , for example, ∼10 −4 Pa around 300 • C, and a partial pressure of FeSi 2 at the temperature of 550 • C increases to ∼10 2 Pa [17,18]. The formation of iron oxides at the higher thermal treatment temperature is caused by the transport of FeCl 2 to the CaSi 2 crystals. The chemical reactions of FeCl 2 with H 2 O and O 2 are also reported as [11], In addition, numerical calculations of the following reactions were as follows [12]: The HCl and/or Cl 2 molecules are transported to the CaSi 2 crystals because of their higher vapor pressures, for example, ∼10 5 Pa at −85.2 • C for HCl, ∼10 5 Pa at −34.2 • C for Cl 2 , [19] then extract the Ca atoms. The Ca atom extraction by HCl is described as follows: [20,21], On the other hand, for the case of using FeCl 3 , the partial pressure of ∼10 2 Pa is obtained at the lower temperature around 300 • C [18]. The possible dissociation reactions of FeCl 3 are 2FeCl 3 (s) ⇔ Fe 2 Cl 6 (g), Fe 2 Cl 6 (g) ⇔ 2FeCl 3 (g), Fe 2 Cl 6 (g) ⇔ 2FeCl 2 (s) + Cl 2 (g) at 160−420 • C or 333−697 • C, 2FeCl 3 (s) ⇔ 2FeCl 2 (s) + Cl 2 (g) at 160−220 • C [16].
The dissociation of FeCl 3 starts to take place around 160 • C [16], and at this temperature, the partial pressure of FeCl 2 is about 10 −4 Pa, which is too low to evaporate, thus the FeCl 2 remains as a solid. On the other hand, the partial pressure of Cl 2 is higher, and the partial pressure of FeCl 2 is further reduced based on the law of mass action for the vapor pressure [23]. The Cl 2 molecules are preferentially transported to the CaSi 2 crystals, which causes the Ca extraction by [5], The deliquescency of FeCl 3 is much higher than that of FeCl 2 . The FeCl 3 source includes much H 2 O due to its high deliquescency. The chemical reactions of FeCl 3 with residual H 2 O supplied by its deliquescency and/or in the atmosphere are given by [24].
The reactions of FeCl 3 with H 2 O and O 2 are also reported as [10], In addition, the following reactions are discussed [25]: HCl. Because these reactions take place at a lower temperature compared to the highest thermal treatment temperature, these hydroxides and oxides mainly remain with the source materials. The HCl molecules are transported to the CaSi 2 crystals to extract the Ca atoms.
Considering the thermal treatment temperature profile, these reactions take place from room temperature. Cl 2 and/or HCl with the higher vapor pressures preferentially react with CaSi 2 , then the Ca atoms are bound to the Cl atoms, and are extracted from the silicide as CaCl 2 . Based on the enthalpy of formation of CaSi 2 (−12.0 kcal/g-atom), HCl (−11.0 kcal/g-atom) and CaCl 2 (−63.4 kcal/g-atom) [26], the CaCl 2 is much more stable compared to CaSi 2 and HCl.
When using FeCl 2 as the source material, during the initial stage of the thermal treatment process, the HCl mainly plays the role to form the Si nanosheets. With the increasing temperature, FeCl 2 is also transported to the substrate, then iron oxide and its related materials are deposited. On the other hand, when using FeCl 3 as the source material, during the initial stage of the thermal treatment process, not only HCl, but also Cl 2 play the role to form the Si nanosheets. In addition, FeCl 3 is also actively evaporated due to its higher vapor pressure. However, the transport of Fe chlorides to the CaSi 2 crystals is lower, because of dissociations of the Fe chlorides.
When using NH 4 Cl as the source material, the dissociation of NH 4 Cl, such as, takes place at 338 • C to 400 • C [27,28]. HCl molecules are significantly transported to the CaSi 2 crystals, thus the nanosheets are effectively formed by the direct dis-sociation of the source materials. Only about a 10-min duration time is necessary to obtain the Si-nanosheets, even though the CaSi 2 powder is used. Thus, the feature of the nanosheet formation caused by NH 4 Cl shows a more similar to that of siloxene formation caused by HCl. Thinner nanosheets were formed compared to those of the other nanosheets formed using FeCl 2 , FeCl 3 , and MgCl 2 .
For the case of using MgCl 2 as the source material, the dissociation of MgCl 2 with H 2 O, such as, at 300 • C, MgOHCl → MgO + HCl at 550 • C, takes place [29]. HCl molecules are transported to the CaSi 2 crystals, thus the nanosheets are formed. Because of the higher treatment temperature of 800 • C, thicker nanosheets were formed by reconstruction of the Si network in the crystals compared to those of the other nanosheets formed using NH 4 Cl, FeCl 2 , and FeCl 3 .
It has been demonstrated that the Si-based nanosheet bundles were formed on the Si substrates or in the powder formed from CaSi 2 crystals using FeCl 2 , FeCl 3 , NH 4 Cl, and MgCl 2 . The morphology of the nanosheet bundles depends on the reactant and the thermal treatment conditions. This discussion is based on the assumption that HCl and Cl 2 , generated by the thermal instability and deliquescency of the chloride compounds, play an important role in forming the Si-based nanosheet bundles. However, the discussion well explains the experimental results of the Si-based nanosheet formation. Thus, this nanosheet formation model is proposed to explain the thermal treatment condition dependence, including the reactant, on the structural and morphological properties of the Si-based nanosheets.

IV. CONCLUSION
Si-based nanosheet bundles were formed by Ca atom extraction from the CaSi 2 crystals by thermal treatment using FeCl 2 , FeCl 3 , NH 4 Cl, and MgCl 2 . Though Si-based nanosheets were formed under the thermal treatment conditions using FeCl 3 , the formation of iron-oxide phases was accelerated at the higher temperatures and longer duration time of the highest thermal treatment temperatures using FeCl 2 . Thinner and thicker nanosheets were obtained using NH 4 Cl and MgCl 2 , respectively. The nanosheet formation model, based on the thermal instability and deliquescency of the chloride compounds, their vapor pressure, and the thermal treatment temperature profile is proposed to explain the reactant dependence of the structural property of the Si-based nanosheets. In the model, HCl and Cl 2 play an important role to form the Sibased nanosheets. The morphological modifications of the nanosheet bundles have been successfully demonstrated by the appropriate thermal treatments with the specific starting materials. Further characterizations of the bundle structures will be expected to evaluate the enhanced electric, thermal and thermoelectric properties of the Sibased materials.