Facile preparation of hydroxyl−functionalized mica nanosheets assisted by plasma treatment

The efficient exfoliation of mica, a naturally layered material, into two-dimensional (2D) nanosheets has received much attention due to its low price, good chemical stability, and better shielding function against UV light. However, fast and simple exfoliation of mica in a large-scale face a great challenge. In this work, we developed a simple and effective method for obtaining OH−functionalized mica nanosheets (MNs). The process involved calcination, plasma treatment, and ultrasonic exfoliation, resulting in a yield of 7.535%. Furthermore, the effects of sonication time, solvent type and particle sizes of mica were investigated. The conditions for the preparation of MNs were determined: mica calcination, plasma treatment, and sonication in ethanol for 5 h. XPS and FT−IR demonstrated that more hydroxyl groups were introduced to mica after the plasma treatment, which facilitated the exfoliation of mica.


Introduction
After the successful preparation of graphene, other two-dimensional (2D) nanosheets such as boron nitride, molybdenum disulfide, black phosphorus, and MXene have garnered significant interest in various fields due to their exceptional properties, including 2D superconductivity, direct band gap, and quantum spin Hall effect [1][2][3][4][5][6]. For instance, boron nitride nanosheets (BNNSs) are often referred to as 'white graphene' due to their remarkable thermal conductivity, stability, impressive mechanical properties, and wide band gap, which attributes them useful in a variety of applications, such as dielectric substrates, electronic packaging, and high −power devices [7][8][9][10][11]. White mica is a naturally occurring layered silicate material, consisting primarily of Al 2 O 3 and SiO 2 . Its structure unit comprises an octahedral silicate sheet sandwiched between two tetrahedral silicate sheets, with each layer separated by a cation plane [12,13]. Sericite is a type of white mica that has fine scale and measures less than 0.01 mm, which has become increasingly popular due to its cost-effectiveness, excellent chemical stability, and effectiveness shielding against ultraviolet light [14]. Moreover, it can be easily exfoliated into 2D nanosheets. Like other types of nanosheets, mica nanosheets (MNs) possess excellent resistance to permeation and high temperatures, making them ideal candidates for use in the development of large−scale anticorrosive coatings [15,16]. In contrast to graphene, MNs are considered insulating materials that can effectively prevent microcurrent reactions that accelerate metal corrosion when the coating is destroyed [17]. The surface of MNs features hydrophilic functional groups that can easily undergo modification and be incorporated into waterborne polymeric coatings. Furthermore, the exceptional UV light shielding properties of MNs make them more advantageous in protecting organic coatings that are heavily exposed to the external environment over extended periods of time, as compared to BNNSs [18,19]. However, unexfoliated mica is too thick to effectively construct a labyrinthine effect within an anticorrosive coatings that is thin. Furthermore, while extensive research has been conducted on 2D nanosheets like graphene, boron nitride, molybdenum disulfide, and MXene, the study of mica exfoliation for MNs remains underdeveloped [20][21][22][23]. Therefore, there is still a great challenge in achieving simple and efficient preparation of MNs on a large scale. Any further distribution of this work must maintain attribution to the author-(s) and the title of the work, journal citation and DOI.
Inspired by the exfoliation of graphene, various methods have been developed to prepare 2D nanosheets, including ball milling [24,25], chemical exfoliation [26], high shear mixing [27], electrochemical exfoliation [28], and liquid phase exfoliation [29][30][31]. Among them, liquid phase exfoliation is a popular and cost−effective method for creating nanosheets, making it the preferred method for producing MNs [32]. Fu et al employed vigorous stirring to disperse mica in N, N−dimethylformamide (DMF) resulting in the production of a small amount of MNs following ultrasound and centrifugation [17]. Ding et al utilized the hydrothermal technique to incorporate lithium chloride (LiCl) into mica layers resulting in the production of MNs [33]. This process involves the substitution of Li + for K + in between the layers at elevated high pressure and temperature. Ding et al prepared MNs by subjecting mica raw materials to a process that involved calcination, acidification, and intercalation with cetyltrimethylammonium bromide (CTAB) [34]. However, the previous study involved numerous steps and harsh conditions and lacked details concerning the impacts of various solvents, liquid phase exfoliation times, and other factors.
In our study, we aimed to improve the preparation steps for MNs following thermal activation and acidification. We compared these optimized steps to the conventional preparation method, and presented five modified processes. This work explored the impact of raw material particle size, sonication time, and solvent type. The research was comprehensive, with XRD, SEM, AFM, and TEM characterizations indicating that MNs can be effectively exfoliated to a thickness of just a few layers.

Results and discussion
2.1. Morphology and structure of mica nanosheets The MNs were prepared by calcination, plasma treatment, and sonication according to our previous work [11], whose morphology was characterized by SEM, TEM, and AFM.  [35], indicating that the capability of the preparation process for the few layer MNs. Figures 1(g)−(i) show the TEM images of MNs. The MNs presented a thin and transparent structure, whose average transverse size was larger than 1 μm, similar to that of the nanosheets in SEM. Figure 1(h) shows the typical edge region of the MNs, where the nanosheets were observed as 1∼3 overlapping layers on the edge, revealing the successful preparation of MNs. The region of the high-resolution TEM image displays an ordered lattice structure of MNs (figure 1(i)). The distance between the lattice planes was about 0.433 nm, which matched the d (111) spacing of 0.4326 nm for mica. Besides, the electron diffraction pattern further confirms the crystal nature of the nanosheets. Figure 2(a) shows the FT−IR spectra of the raw mica and MNs. The Si−O−Si stretching vibration at around 1000 cm −1 , the strong absorption region of Si−O bending vibration between 800∼600 cm −1 , and the stretching vibration of Si−O 4 tetrahedron at 600∼400 cm −1 showed little change before and after the exfoliation of mica [36]. This result proved that the structure of mica was not destroyed after calcination, plasma, and ultrasonic treatment. Moreover, a stronger O−H stretching vibration at 3624 cm −1 was observed after oxygen plasma treatment due to the introduction of more hydroxyl groups. Additionally, it is also obvious that there is a weak peak around 1648 cm −1 , which is attributed to the bending vibration of H 2 O [37].
X−ray diffraction (XRD) of the unexfoliated mica and MNs is displayed in figure 2 [38]. It indicates that the lattice of mica was activated and exfoliated after thermal activation, plasma treatment, and ultrasonic treatment, while the crystal structure was not damaged.
X−ray photoelectron spectroscopy (XPS) spectra of the unexfoliated mica and MNs are shown in figures 2(c)−(e), which had O 1s, Si 2p, Al 2p and K 2p components. Compared with the unexfoliated mica, the oxygen content of the exfoliated MNs increased, while the levels of other elements remained almost unchanged (figure 2(c)). Furthermore, the binding energy of oxygen and silicon was shifted. These suggest that the state between oxygen, silicon and other atoms was partially altered, potentially due to the surface etching resulting from the plasma treatment.

Effect of preparation processes on mica nanosheets
Generally, MNs were prepared through thermal modification, acid activation, sodium modification, and intercalation of CTAB through ion exchange [39]. During the exfoliation process, the layer spacing of mica was altered. In terms of practical applications, it is more crucial to focus on optimizing the preparation process to minimize production costs rather than achieving complete exfoliation of nanosheets. In this study, we analyzed and compared various methods for preparing MNs as illustrated in figure 3. The specific details of these methods can be found in Experimental section 4. Figure 3(a 3 ) shows that the thickness of MNs prepared by method A is about 8.428 nm and the nanosheet thickness distribution mainly ranges from 5 to 30 nm ( figure 4(a)). The thickness of MNs by method B is as low as 2.048 nm due to the introduction of oxygen plasma treatment ( figure 3(b 3 )), which is twice the theoretical thickness of MNs (∼1 nm). The nanosheets prepared by method B range from 1.5 to 4.5 nm, with a predominance of nanosheets of about 2 nm thickness ( figure 4(b)). As the acidification process was inserted in method C, mica turned from white to brownish yellow. The corresponding nanosheet thickness changes from 1 to 30 nm, indicating that the MNs contain single−layer and multi−layers (figure 3(c 2 ) and figure 4(c)). This result showed that the nanosheets can be thin assisted by the acidification process. As the acidification and oxygen plasma treatment was combined in method D, the resulting MNs have a thickness of ∼5 nm due to the increased hydroxyl content (figures 3(d 2 ) and 4(d)). The method E reported by Liang et al was chosen as a reference [40]. The obtained MNs have a thickness of 9−15 nm after thermal modification, acid activation, sodium modification and CTAB intercalation.
The yields of MNs made by all the methods were measured. It can be seen that the yield of MNs increased with the increase of process steps, with the highest yield of 9.47% for method E ( figure 5(a)). Method A has the lowest yield of 5.97%. At an initial mica concentration of 10 mg ml −1 , the yield of MNs made by method A was 0.597 mg ml −1 . Pan et al obtained a MN yield of about 0.25 mg ml −1 by a similar method E (with the same initial concentration). It is clear that all the methods reported here have high yields as compared with the previous work.  Figure 5(b) shows the XRD results of MNs prepared by different methods. The intensity of their diffraction peak (002) was reduced compared with mica, which proved that the mica was exfoliated. After thermal activation, the mica structure changed. As the activation degree increased, the mica absorbed more energy during the heating process, and the lattice vibration was enhanced, leading to exfoliation. After acidification, the intensity of the mica diffraction peak (002) was further reduced, which implied better exfoliation for MNs.

Effect of preparation conditions on mica nanosheets
Method B was chosen for subsequent nanosheets preparation based on the obtained MNs being the thinnest with larger layer spacing. Additionally, the difference in yield between the preparation methods was not significant. In order to improve the convenience and efficiency of preparing large−scale MNs, we further optimized the conditions for method B.

Effect of solvents on the yield of MNs
The polarity of the solvent is closely related to the yield of nanosheets, making it a crucial factor to consider in the preparation process of MNs. The mica raw material GB−4 was chosen to be exfoliated in different solvents for 5 h to explore the effects on the yield. Isopropyl alcohol (IPA), N−methylpyrrolidone (NMP), ethanol, water, and N, N−dimethylformamide (DMF) were selected, and the yields corresponding to different solvent types were shown in figure 5(c). Compared to DMF, the yields of IPA and NMP decreased to 3.09% and 5.71%, and the yield using water as solvent was 4.965%, but the yield in ethanol was higher than that in DMF, reaching 7.535%, an increase of 12.55%. According to the Hansen solubility parameter theory, the excellent exfoliation effect of  EtOH can be attributed to the good match between the surface tension of EtOH and the surface energy of mica [41].

Effect of sonication time on the yield of MNs
The mica raw material GB−4 was selected as exfoliated material and EtOH as exfoliating solvent, and the sonication time was varied. The yield increased with the increase of the sonication time at 1 ∼5 h, and the maximum yield was 6.695% ( figure 5(d)). This is mainly due to the continuous shear force acting on mica during the sonication process, which makes the layers tend to separate from each other. There was a corresponding slight decrease in yield at 8 h of ultrasound time, but as the ultrasound time reached 12 h, the yield increased again to 7.11%. Based on the principle of reducing the instrumentation time and saving energy, 5 h was chosen as the final sonication time for this process.

Effect of different particle sizes on the yield of MNs
As shown in figure 5(e), after determining the solvent type and sonication time, three different raw material particle sizes of mica were chosen to explore the effect of the particle sizes on the yield. The main parameters of the three different raw material particle sizes are displayed in Experimental section 4. The yield of the prepared MNs increased with the decrease of the raw material particle size, and the highest yield of 7.535% was achieved by GB−4, which showed that the size of the raw material had a great influence on the yield of MNs. This is mainly due to the fact that the larger particle size needs to overcome the greater force between the layers during the exfoliating process, so the yield is lower.

Investigation of structural changes of mica by oxygen plasma treatment
In 1997, Liu et al treated mica with oxygen plasma and found that its oxygen content increased, [42] and Achour et al studied the introduction of hydroxyl groups to boron nitride via oxygen plasma to improve wettability [43]. It is inferred that more hydroxyl groups were introduced to the mica surface due to the plasma treatment. When the glass surface is treated with plasma, the surface bonds are activated to form oxygen−containing groups such as hydroxyl groups, which can change the wettability of the surface [44]. The principle is that plasma etching will cause the exposed silicon atoms on the surface to graft hydroxyl groups. The mica after plasma treatment was characterized by FT−IR spectra. As shown in figure 6(a), the hydroxyl peaks of the mica around 3600 cm −1 were significantly enhanced after plasma treatment, indicating that the plasma treatment introduced more hydroxyl groups. Figures 6(b)−(d) display the XPS spectra od the mica before and after treatment of plasma, and the corresponding results are summarized in table 1. The ratio of oxygen to silicon content was selected as a means of determining whether the oxygen content of the samples had been elevated. As shown in table 1, the O/Si ratio

Conclusions
Plasma treatment was introduced to produce the mica nanosheet. Five different preparation methods and the effects of sonication time and solvent type were investigated. The conditions for the preparation of MNs were determined: mica calcination, plasma treatment, and sonication in ethanol for 5 h. XPS and FT−IR demonstrated that more hydroxyl groups were introduced to mica after the plasma treatment, which facilitated the exfoliation of mica.

Mica specifications
In our study, the specifications of mica are mainly GB-2, GB-3 and GB-4. These three detailed parameters (number of mesh, average partcle size, 325 mesh sieve residual rate) are shown in table 2 below.

Preparation of mica nanosheets
Method A: Mica powder GB−4 (the same as below) was calcined at 800°C for 2 h. 1.0 g mica was placed in 200 ml DMF and sonicated for 8 h. The resulting dispersion was centrifuged at 1500 rpm for 10 min, and the supernatant was filtered and dried at 80°C for 48 h under vacuum to obtain mica nanosheet powder. Method B: Mica powder was calcined at 800°C for 2 h. After calcination, the powder was treated with air plasma for 10 min to obtain hydroxylated mica powder. 1.0 g mica was placed in 200 ml of different solvents (IPA, NMP, EtOH, H 2 O, DMF) and sonicated for 8 h. The resulting dispersion was centrifuged at 1500 rpm for 10 min, and the supernatant was filtered and dried at 80°C for 48 h under vacuum to obtain mica nanosheet powder.
Method C: Mica powder was calcined at 800°C for 2 h. 3.0 g of the powder was dispersed into 100 ml of 5.0 mol l −1 HNO 3 at 95°C with mechanical stirring for 5 h, filtered and washed with hot water to achieve neutrality, then dried to remove excess water. 1.0 g of the above mica powder was dispersed into 200 ml DMF and sonicated for 8 h. The resulting dispersion was centrifuged at 1500 rpm for 10 min, and the supernatant was filtered and dried at 80°C for 48 h under vacuum to obtain mica nanosheet powder.
Method D: Mica powder was calcined at 800°C for 2 h. 3.0 g of the powder was dispersed into 100 ml of 5.0 mol l −1 HNO 3 at 95°C with mechanical stirring for 5 h, filtered and washed with hot water to achieve neutrality, then dried to remove excess water. The acidified mica powder was treated with air plasma for 10 min to obtain hydroxylated mica powder. 1.0 g of the above mica powder was dispersed into 200 ml DMF and sonicated for 8 h. The resulting dispersion was centrifuged at 1500 rpm for 10 min, and the supernatant was filtered and dried at 80°C for 48 h under vacuum to obtain mica nanosheet powder.
Method E: Mica powder was calcined at 800°C for 2 h. 3.0 g of the powder was dispersed into 100 ml of 5.0 mol l −1 HNO 3 at 95°C with mechanical stirring for 5 h, filtered and washed with hot water to achieve neutrality, then dried to remove excess water. Subsequently, 3.0 g of the above powder was dispersed into a 100 ml saturated NaCl solution at 95°C, mechanically stirred for 3 h, filtered and washed several times with hot water to remove NaCl, and dried to remove excess water. Then, 1.5 g of the resulting powder and 4.6 g of The surface morphology of MNs was characterized by Zeiss Sigma 300 scanning electron microscope and FEI talosf200s transmission electron microscope. The SEM sample preparation process of MNs was carried out by dispersing low concentration MNs in EtOH and ultrasonically dispersing them well, then dropping them on high-crystal silicon wafers and drying them in an oven at 60°C for 24 h. Due to the poor conductivity of MNs, they were sprayed with gold before the test to improve the conductivity of MNs and obtain high-quality SEM images. The well-dispersed MNs dispersion was dipped in a 230-mesh copper mesh ordinary micro-grid membrane and dried in an oven at 60°C for 24 h to obtain a TEM test sample. The height characterization of MNs is realized by Bruker Dimension ICON atomic force microscope. The sample preparation operation is to drop the diluted MNs dispersion on the high-crystal silicon wafer and dry it at 60°C for 24 h. The mode of AFM is intelligent tapping mode, and the brand of probe is ScanAsyst Air.

Crystal structure of mica nanosheets
Empyrean x−ray diffraction (XRD) instrument was used to study the crystalline structure unit cell strength and other information of MNs after various treatments, in order to determine whether MNs has been effectively exfoliated. The specific parameter settings are as follows : the diffraction angle range is 5 ∼ 10°and 5 ∼ 50°; the step length is 0.02°; the working voltage is 40 kV ; the working current is 40 mV.

Chemical composition of mica nanosheets
The principle of Fourier transform infrared spectroscopy (FT−IR) is that when infrared light is used to irradiate the surface of the sample molecule, it can selectively absorb radiation in certain wavenumber ranges, thereby generating characteristic vibration or rotational absorption peaks. In this paper, Spectrum100SystemB FTIR was used to scan MNs in the wavelength range of 4000 ∼ 400 cm −1 , and the scanning mode was transmission mode. Thermo Scientific K−α X−ray photoelectron spectrometer (XPS) were used to determine the surface chemical composition of MNs. The specific parameters are as follows: Excitation source: Al Kα ray (hv = 1486.6 eV); beam spot: 400 μm; operating voltage: 12 kV; filament current: 6 mA.