A novel self-separating silicon nanowire thin film and application in lithium-ion batteries

Nano silicon structures are important materials for modern electronic devices and have been widely researched with regard to photoelectricity, thermoelectricity, and lithium-ion batteries. However, since the nano silicon structures fabricated by conventional methods cannot be separated from silicon substrates, reuse of the substrate is restricted. Here, we propose a simple fabrication method to separate the nano silicon structures from the silicon substrates, which allows the reuse of the substrates. The fabrication was processed at room temperature, which allows large-area fabrication and is not restricted by the substrate thickness. Honeycomb structures of different length scales observed on both the nano silicon structure and the substrate suggest that the separation occurred due to the amplification of the silicon crystal defects. The nano silicon structures comprised porous silicon with an excellent specific surface area of 480 m2 g−1 and a mean pore diameter of 5.7 nm. Moreover, the nano silicon structures show good potential as anode materials for lithium-ion batteries wherein the measured reversible capacity was 1,966 mAh g−1 after 100 cycles. Based on the proposed method and morphological characteristics, the fabricated nano silicon structures can be considered a low-cost material with suitable applications in the energy field.


Introduction
Owing to their unique and stable properties [1][2][3][4], nano silicon structures are promising materials in various applications, such as photoelectric conversion [5][6][7][8][9][10][11], thermoelectric conversion [12], and lithium-ion batteries [13]. They can be prepared using physical vapour deposition [14], chemical vapour deposition [15], laser ablation [16], thermal evaporation [17], molecular beam epitaxy [18], or chemical etching [11]. Among these methods, the metal-assisted chemical etching (MACE) has been widely used to fabricate nano silicon structures owing to its simplicity and lower cost [19][20][21][22][23]. However, this method requires the use of 200-600 μm thick silicon wafers [17,24,25], and the silicon substrates cannot be fully utilised since the nano silicon structures cannot usually be separated from the underlying substrates. In this work, we propose a simple method to fabricate nano silicon structures on silicon substrates using MACE. Thin films of the fabricated nano silicon structures, composed of silicon nanowires (SiNWs) and porous nano silicon layers, can be peeled off from silicon substrates without any external force. Therefore, in this paper, they are termed as self-separating SiNW thin films. The primary advantage of the method is that the SiNW thin films can be formed and separated from silicon wafer easily which would enable large-area preparation; moreover, the entire fabrication and separation processes can be performed in only 15 min at room temperature. Furthermore, this also allows the silicon substrates to be reused, and the application of the fabricated SiNW would not be restricted by the thickness of the substrate. The separation mechanism of the self-separating SiNW thin films is believed to be due to the amplification of the defects in the silicon crystal because honeycomb structures of varying length scales were observed at the surface of the silicon substrate and the bottom of the self-separating SiNW film.
The morphology of the self-separating SiNW thin film was investigated by scanning electron microscopy (SEM), and the results showed that the multilayer nano silicon structure was composed of porous SiNW clusters and porous nano silicon layers (figure 1). The porosity of the self-separating SiNW thin film was excellent with a surface area of 480 m 2 g −1 and mean pore diameter of 5.7 nm. In addition to this, the reversible capacity of a lithium-ion battery containing the self-separating SiNW thin film as a anode was 1,966 mAh g −1 after 100 cycles, which indicates its potential use in lithium-ion batteries [26,27].

Fabrication and characterization method
2.1. Fabrication process of self-separating SiNW thin film Figure 2(a) shows the fabrication and separation mechanisms of the self-separating SiNW thin film. Both the fabrication and separation of the thin film were done using the MACE method with different concentrations of the etching and separation solutions. The fabrication process of the self-separating SiNW thin film is shown in the figure 2(b). This was performed at room temperature (300 K) and using a (100) oriented n-type Si wafer (0.0007 Ω cm). Initially, silver particles were deposited on the substrate surface using electroless silver plating (ESP). For this, the substrate was cleaned with 1.7 M HF for 1 min to remove any native oxide . The SiNW thin film developed a branching structure, indicating that it had separated from the silicon substrate (figure 2(b)-6). Finally, the silicon substrate and the thin film containing silver particles were rinsed three times with water to remove the etching solution, then the thin ilm containing silver particles can be obtained. In order to obtain thin film without silver particles, the thin film was immersed in 1.85 M HNO 3 for 20 min to remove the silver particles as shown in figure 2(b)-7.

Characterization of morphology, porosity, and lithium-ion battery performance
Since the self-separating SiNW film is easily destroyed by static electricity, before taking it out of water, the film was stored on a copper foil with aluminium paste. After taking it out of water and drying, the film was characterised using a SEM (JSM-7001F) and an optical microscope (VHX-7000). On the other hand, the porosity of the self-separating SiNW thin film was characterized by its surface area and mean pore diameter. The surface area was determined by the Brunauer-Emmett-Teller (BET) method (BELSORP-MINIX). For this, ultra-high purity N 2 and He was used at 77.35 K, and the pore size distribution was determined by the Barrett-Joyner-Halenda (BJH) method. Li-storage performance of the self-separating SiNW thin film were characterised by galvanostatic chargedischarge measurements using a conventional two-electrode-type cell (HSCell, Hohsen). The measurements were conducted using a galvanostat (Toyo System, TOSCAT-3200). To prepare the working electrode, the selfseparating SiNW thin film (30 wt%) and single-walled carbon nanotube (60 wt%), purchased from Meijo Nanocarbon Co. Ltd (EC2.0), were mixed with polyvinylidene difluoride binder (10 wt%) in an appropriate quantity of ethanol. The obtained slurry was applied on a Cu foil and dried at 80°C for at least 2 h in vacuum prior to use. For the experiments, 1 mol l −1 LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) solution (volume of EC:DEC = 1:1) was used as the electrolyte. A lithium foil (Aldrich) was used as the counter and quasi reference electrode. A porous polyethylene/polypropylene film was used as the separator. Both the cell setup of the cell and charge-discharge measurements were performed in an Ar-filled glove box to avoid air exposure and contamination. Specific capacity values were calculated based on the mass of the self-separating SiNW thin film on the working electrode.

Morphology characterisation and separation mechanism
The morphologies of the self-separating SiNW thin film before (figures 3(a)-(b)) and after separation (figures 3(c)-(f) and 3(i)) were observed by SEM and optical microscopy (OM). From figures 3(a)-(b), it can be observed that the SiNW thin film consists of two parts, namely the 10-μm-long SiNWs at the top and 1-μmthick silicon layer at the bottom. The porous silicon layer was formed during the etching process. After dripping the separation solution, even though the length of the SiNWs remained the same, they clustered owing to lateral etching [28,29] of highly doped silicon, and the separation occurred along the bottom of porous layer ( figure 3(c)). In addition, as shown in figure 3(d), the thickness of the porous silicon layer increased from 1 μm prior to separation to 1.8 μm after separation. Two layers can be observed in the porous layer compared to the single-layer before separation, which can be due to the increase in the concentration of H 2 O 2 ( figure 3(d)). After the separation, distorted honeycomb structures with their sizes varying from a few nanometres to a few micrometres were observed with OM and SEM at the bottom of the SiNW thin film (figures 3(e)-(f)).
The formation mechanism of the porous silicon layer is considered because of the considerably high dislocation density in the highly doped silicon. The silver ions entering the silicon substrate can rapidly diffuse through the defects and, hence, etching is considerably easy. In addition to this, hydrogen gas generated during etching could have affected the etching direction and aided in the formation of a porous silicon layer. After dripping the separation solution with a higher concentration of H 2 O 2 (1.3 M), the etching reaction with the silver ions inside the silicon was intensified. The reason for the separation is assumed that the connections between the porous silicon layer and the silicon substrate were easily broken due to the generation of a large amount of hydrogen. The self-peeling occurred along the bottom of the porous silicon layer and the thin film separated from the silicon substrate.

Hypothesis of defect amplification in silicon crystal
After the SiNW thin film separated from the silicon substrate, a swirl defect structure [30,31] was observed on the surface of the silicon substrate ( figure 3(g)). The swirl defects are a macroscopic manifestation of the dislocations in the crystal [32]. The top view of the thin film shown in figure 3(i) suggests that the differing density of the distribution of SiNW clusters could also have been affected by the swirl defects. In addition, after the separation, honeycomb structures were observed on the surface of silicon substrate at a micrometre-scale ( figure 3(h)), and the bottom of the separated thin film at a micrometre-scale ( figure 3(e)) and nanometre-scale ( figure 3(f)). As the honeycomb structures (figures 3(e)-(f) and 3(h)) were similar to those on the defect of silicene [33,34], it is possible that the observed honeycomb structure is an amplification of the defects in the silicon crystal at various scales; that is, a particular solution concentration during MACE might be capable of magnifying the crystal defects from nanometre scale to micrometre scale, resulting in a honeycomb morphology.

Porosity characterisation and lithium-ion battery performance
Nitrogen adsorption/desorption isotherm for the thin film is shown in figure 4(a) and the pore size distribution of the thin film is shown in figure 4(b). The surface area and the mean pore diameter of the thin film was 480 m 2 g −1 and 5.7 nm, respectively. Additionally, three peaks were observed in the pore size distribution curve shown in figure 4(b), which could be related to the three structures, namely the SiNW clusters and the two porous silicon layers, observed in the SEM images.
The performance of the self-separating SiNW thin film when used as an anode in a lithium-ion battery is shown in figure (5). When the thin film devoid of silver particles was used as the anode, the reversible capacity of the lithium-ion battery was 499 mAh g −1 after 100 cycles, whereas the reversible capacity was 1,966 mAh g −1 (after 100 cycles) when the thin film containing silver particles was used as the anode. Comparing to reviewed results, a reversible capacity of 1,966 mAh g −1 is higher than majority of the lithium-ion batteries containing nano silicon [35,36]. A recent result has reported the first discharge capacities of the silicon nanowire film electrode by highly doped silicon as 3,615 mAh g −1 , however, the charge capacity was 1,569 mAh g − 1 and decreased to around 1,200 mAh g −1 after 15 cycles, and the results after 15 cycles were not given [37]. In our study, the reversible capacity has shown much higher stability during the first 100 cycling period, and was still 1,966 mAh g −1 after 100 cycles. The stability could be related to the higher doping concentration of silicon as a larger number of crystal defects could provide stability for the lithiation/delithiation process [38]. In addition, the large surface area of negative electrode materials is a crucial factor to improve the capacity of lithium batteries [39]. The surface area of the SiNWs film fabricated in our study was 480 m 2 g −1 , which is larger than most of the current SiNWs made of highly doped silicon [40]. Therefore, the thin film containing silver particles is believed  . Performance of a lithium-ion battery containing the self-separating SiNW thin film with silver particles (red dots) and without silver particles (blue dots). The reversible capacity of the lithium-ion battery was 499 mAh g −1 (without Ag) and 1,966 mAh g −1 (with Ag) after 100 cycles. as a potential anode material for lithium-ion batteries. Moreover, the lower cost of fabrication of the thin film could help reduce the price of lithium-ion batteries. The difference in the performance between the thin films with and without silver particles could be due to the fact that, upon removal of silver particles with HNO 3 , the silicon dioxide were formed, which could have adversely affected the storage of lithium ions during the charging or discharging of the battery.

Conclusion
In this study, we report a simple and economical method to fabricate nano silicon structures over a large area at room temperature. Moreover, this method is not limited by the thickness of the silicon substrate and allows the substrate to be reused. In terms of morphology, the self-separating SiNW thin film consisted of SiNW clusters on top and porous silicon layers at bottom. Honeycomb structures were observed on the bottom of separated thin film and the surface of silicon substrate, indicating the amplification of the defects in the silicon crystal. Moreover, the mean pore diameter of the self-separating SiNW thin film was 5.7 nm, and the surface area was 480 m 2 g −1 . In addition to this, the reversible capacity of a lithium-ion battery containing the self-separating SiNW thin film with sliver particles was observed to reach 1,966 mAh g −1 after 100 cycles. Therefore, it can be concluded that the method developed in this study can be used to economically fabricate nano silicon structures, which is possible to reduce the cost of anode materials in lithium-ion batteries and provide a potential manufacturing direction for nano silicon structures.