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The metal-assisted chemical etching technique was applied on the p-types of Si wafer in the solution of AgNO3/HF for study etching mechanism. The conductor transfer electrons from Si wafer to the network of Ag dendrites was assigned for some Ag dendrites that connect between two networks of Ag dendrites and of Ag particles. The etching method was also applied for the modification of Si thick films as anode materials in lithium-ion batteries. The specific discharge capacity of etched anode remained 74% after 30 cycles of charge and discharge, comparing with 33% in the case of unmodified Si anode.
The metal-assisted chemical etching (MACE) technique, with the advantage of simplicity and low cost, has been widely used to manufacture SNWs for many different applications[6-9]. In this method, metal particles play the role as the catalyst for etching process in the present of HF solution. The simplest way to perform this technique is the soaking of Si wafers in an etching solution containing HF and metal salts. Among the metals used as the catalyst for the etching process, silver is the best candidate not only because it is inexpensive but also because it can be used to produce SNWs with high uniformity and highly efficient etching. Although it was affirmed that the growth of silver dendrites plays an important role in the etching process[1-3], the mechanism of the etching process as well as the growth of silver dendrites still remains controversial[2,3,9] and further researches are still needed. In this paper, the MACE method was used to etch p-types of silicon wafer (100) in order to fabricate SNW arrays. Based on the experimental results, The mechanism of chemical etching of silicon assisted by silver particles has been studied. The same method has also been applied for modification of Si thick films as anode materials in lithium-ion batteries. Electrochemical characteristics of the etched anode were investigated and compared with that of the unmodified anode.
2. EXPERIMENTAL DETAILS
2.1. Etching process
Si (100) wafers (p-type, Boron doped; 10-30 Ωcm; 500 µm thick) were used in this work. The detail about cleaning process can be found in somewhere[1-3]. After cleaning, the silicon wafers were separately etched in a Teï¬‚on beaker at room temperature. Two etching solutions were used: one solution contained 0.02 mol/l AgNO3 and 4.6 mol/l HF (solution 1) and the another one contained 0.01 mol/l AgNO3 and 4.6 mol/l HF (solution 2). The solution of HNO3 was used to remove the silver dendrites after etching process.
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Si thick ï¬lms were deposited on copper foil from a SiH4/Ar gas mixture using a plasma-enhanced CVD system. The silicon thick film was etched in the solution 1 and then rinsed under spout by deionized water for the elimination of Ag dendrites.
For further investigations, all sample after etching were dried in a vacuum oven at 80 oC for 2 hours. The morphology of the samples was observed by NOVA NanoSEM200 ï¬eld emission scanning electron microscope.
2.2. Electrochemical testing
Half cells were fabricated in a dry room, and assembled in a polyethylene bag using the Si thick films as the working electrodes, polyethylene as the separator and lithium metal foils as the counter electrodes. The liquid electrolyte was 1M LiPF6 in ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate (1:1:1 volume ratio). Galvanostatic discharge-charge cycling was carried out in the potential range of 2-0V (versus Li/Li+) at a current density of 250µAcm-2.
3. RESULTS AND DISCUSSION
3.1. Etching mechanism
Fig. 1a, 1b and 1c shows the cross-sectional SEM images of the Si wafers etched in solution 1 respectively for 30, 60 and 90 min after the removal of Ag dendrites. It can be seen that the well-aligned vertical Si nanowire arrays were readily obtained. The diameters of the SiNWs can be observed to be in the range of 40-300 nm.
Fig. 1. SEM image showing the cross section of the Si wafers etched in solution 1 for 30 (a), 60 (b) and 90 (c) min. The top view of Si wafer presented in figure b before (d) and after treatment by ultrasonic wave (e).
The top view of the Si wafer in Fig. 1b was shown in Fig. 1d. In this figure, the SNWs are clustered together at the tip and the distribution of SNWs was not clear. For further observation, the Si wafer after etching was subjected to ultrasonic treatment for 10 minutes. The top view of Si wafer after treatment by ultrasonic wave was shown in Fig. 1e. Almost SNWs was cracked. The diameters of the SiNWs observed in this figure agreed with those obtained from Fig. 1b. However, some silicon columns with diameters of approximately 1 µm were also observed. It is likely that these columns are the group of SNWs with a smaller size. Fig. 1e also showed the separation of the SiNWs from each other.
Fig. 2. Etch rate as function of etching time. Straight line is guide to the eye.
Fig. 2 shows the etch rate as a function of the etching time in Si wafer. Data in the plot was calculated from SEM images using image analyzer. It can be seen that the etching length increased quite linearly with etching time in the period from 15 to 120 min.
The deposition of Ag particles onto Si surface can be explained by Gerischer model in the case of semiconductor/electrolyte solution interfaces. Follow that, it have two simultaneous processes occur at the Si surface: the cathodic reaction (reduction of Ag+ ions produces metallic Ag deposits) and the anodic reaction (oxidation of Si result in the dissolution of Si into solution).
Immediately after the deposition process, silver particle will play the role as the catalyst for etching process. Although the etching mechanism remains controversial, we agree with the mechanism based on the catalytic activity of the silver particles on the silicon surface that was proposed in . Follow this mechanism, the silver particles will catalyze the oxidation of silicon by capturing electrons from it, resulting in the dissolution of the silicon underneath the silver particles in HF acid. However, the dynamic mechanism for the growth of Ag dendrites are unclear in this paper, especially the transfer of electrons as well as Ag+ ions in the etching solution.
Fig. 3. SEM image showing the top view of Si wafer etched in solution 2 for 1 second (a) and 5 seconds (b), the cross-sectional view of Si wafer etched in solution 2 for 60 min (c), and the cross-sectional view of Si thick film etched in solution 1 for 6 min (d).
To investigate the additional effects of the silver particles on the etching process, as well as on the growth of the silver dendrites, the concentration of AgNO3 was decreased to slow down the reaction rate. Fig. 3a and 3b shows the SEM top view images of the Si wafer etched in solution 2 for respectively 1 and 5 seconds. It can be seen that silver particles with diameters of 30-90 nm were immediately deposited on the Si surface after 1 second. As time goes by, the silver particles become bigger and connected to each other (Fig. 3b). From the comparision between Fig. 3b and Fig. 1d, it can be concluded that the Ag network, as observed in Fig. 3b, moved into Si wafer during etching process and resulted in the formation of SNWs.
The cross-sectional view of the Si wafer etched in solution 2 for 60 min was shown in Fig. 3d. Some of the silver dendrites were grown from the bottom holes that originate from the etching process. These silver dendrites are connected to each other and cover the top of the silicon nanowire array. In this figure, it is clear that the Ag dendrites grow towards the solution. From this observation, it is likely that the growth of the Ag dendrites results from the deposition of Ag+ ions that come from solution. This conclusion contradict with the mechanism proposed by T. Qiu et al., but agree with the recent report of J. Fang et al.[10,11].
From the arguments presented above, we believe that during nucleation process, some of the Ag particles in the Ag network will grow into the silver dendrites following the diffusion-limited aggregation model. Since a concentration gradient exists between the silicon surface and solution, these silver dendrites will grow faster and consume almost all of the electrons that the silver-network obtains from the etching process. The consumption of electrons by the silver dendrites and the existence of a concentration gradient will restrict the nucleation at any other positions at silver-network. Finally, the silver-network continuously sinks further into the silicon wafer and the silver dendrites grow larger and larger. The growth of Ag dendrites resulted in the formation of a network of silver dendrites. From this time, the network of silver dendrites receives the electrons supplied by the silver network that embedded in the Si nanowire array. Because in a conducting network, every position is equal from an electrical point of view, the etching rate is uniform over the whole surface of the silicon wafer.
The cross-section of the Si thick film etched in solution 1 also showed in Fig. 3d. As shown in this figures, the etching process resulted in the formation of the Si columns on the Cu substrate with diameters in the range of 0-00 nm. Although most of the Ag dendrites was eliminated by rinsing, some Ag dendrites still remain on the top of Si columns.
3.2. Electrochemical characteristics
Fig. 4a compares the fading degree and coulombic efï¬ciency of the pristine and etched Si thick films used as electrodes. In the case of pristine film, the discharge capacity was unstable during initial cycles. The fading process began after the 10th cycle and resulted in a decrease of the discharge capacity of c.a. 77% after 30 cycles. Together with the fading process, the coulombic efï¬ciency of the pristine film become unstable after 10 cycles as shown in figure. This fact should be attributed for the cracking of Si anode because of the volume extending during lithium insertion. Unlike pristine film, the discharge capacity as well as coulombic efï¬ciency of the etched film quite stable during cycling process. After 30 cycles, the discharge capacity still remain 74%.
Fig. 4. (a) comparison of fading degree and coulombic efï¬ciency of pristine and etched Si thick films used as electrodes; (b) profile of voltage vs. capacity for pristine and etched Si thick films used as electrodes at 1st, 5th, 15th and 30th cycles.
The profile of the voltage versus capacity for the pristine and etched Si thick films was shown in Fig. 4b. Despite of the contribution of Ag dendrites to the weight of the anode, the specific discharge capacity of the etched Si thick film remain 998 mAhg-1, about 3 times higher than that of novel carbon material, comparing with 767 mAhg-1 in the case of pristine film.
SiNWs were prepared via a metal-assisted chemical etching method. We believe that Ag+ ions were directly deposited on the Ag dendrites by getting electrons from them. The network of Ag dendrites do not receive electrons directly from Si wafer but form the network of Ag particles soaking in Si wafer. To transport electrons, Some Ag dendrites play the role as conducting channels connecting between two networks. The good electrochemical performance of etched Si films showed the advantage of simple etching method for the preparation of silicon nanowire anode in lithium-ion batteries.
Acknowledgment: Nguyen Si Hieu appreciates for Mr. Sang-Ok Kim's experimental support in field of scanning electron microscopy..