Effect of hydrogen on the microstructure and electrochemical properties of Si nanoparticles synthesized by microwave plasma

Materials Chemistry and Physics 180 (2016) 332e340

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Effect of hydrogen on the microstructure and electrochemical properties of Si nanoparticles synthesized by microwave plasma Jeongboon Koo, Jeongeun Lee, Joonsoo Kim, Boyun Jang* Advanced Materials and Devices Laboratory, Korea Institute of Energy Research, 152, Gajeong-ro, Yuseong-gu, Dajeon, 305-343, Republic of Korea

h i g h l i g h t s
We synthesized Si nanoparticles by an atmospheric microwave plasma process.
We investigated the effects of injected H2 on the microstructures of Si nanoparticles.
Swirling H2 was critical, due to the formation of vortex flow in plasma.
The synthesized Si nanoparticles had core (crystalline Si)-shell (SiOx) structures.
The electrochemical properties depend on its core-shell structures as LIB anode.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 March 2015 Received in revised form 1 June 2016 Accepted 4 June 2016 Available online 17 June 2016

We synthesized silicon (Si) nanoparticles using an atmospheric microwave plasma process, and investigated the effects of hydrogen (H2) injection on their microstructure during the synthesis. Two nozzles were applied to inject H2 (swirling and rectilinear H2). Our microstructural analysis indicated that the amount and method of H2 injection were critical for completion of the reaction from silicon tetrachloride (SiCl4) to Si, as well as to obtain highly crystalline Si nanoparticles. The swirling H2 was especially critical due to its formation of vortex flow, which allowed relatively long residence time of the H-ions in plasma. The Si nanoparticles synthesized by the atmospheric plasma process had core-shell structures that consisted of crystalline Si cores with amorphous SiOx shells of 5e15 nm thickness. We also investigated the feasibility of the synthesized Si nanoparticles as anode materials in a lithium-ion battery (LIB). For the core-shell structured Si nanoparticles, we obtained the first reversible capacity of 1204 mAhg1, and a capacity retention of 82.2% at the 50th cycle. © 2016 Elsevier B.V. All rights reserved.

Keywords: Nanostructures Semiconductors Arc discharges Microstructure Phase transitions Electrochemical properties

1. Introduction Si nanoparticles are known to be widely applicable to fields such as photovoltaics [1e3], data storage [4e6], and optoelectronics [7,8], due to their eco-friendly properties and abundance on Earth. The large and mature infrastructure for Si-based electronics is another benefit for the development of Si nanoparticles. Recently, intensive research has been conducted on the application of Si nanoparticles to lithium-ion batteries (LIBs) [9e11]. Si is known to have the highest theoretical capacity of Li-ions, 4200 mAhg1 (Li4.4Si), and a relatively low discharge voltage (delithiation at 0.4 V). However, Si exhibits more than 400% volumetric change during the charge-discharge process of the battery, which leads to

* Corresponding author. E-mail address: [email protected] (B. Jang). http://dx.doi.org/10.1016/j.matchemphys.2016.06.015 0254-0584/© 2016 Elsevier B.V. All rights reserved.

fast capacity fading. The use of Si nanoparticles can provide a solution for such huge volumetric change, because the nanoparticles can efficiently relieve mechanical stresses inside the anode. Various processes can be employed to synthesize Si nanoparticles, such as solid, liquid, and gas phase reactions [12e15]. Among them, gas phase reaction is the most efficient way to synthesize Si nanoparticles, and offers high reaction speed and low contamination. In particular, the plasma processes are known to be effective approaches as gas phase reactions for the synthesis of nanoparticles, because electrons, ions and radicals can significantly enhance the reaction rate. Among various plasma processes used in nanoparticle production, the microwave plasma method enables nanoparticle synthesis with high production rates at ambient pressure. In addition, a microwave plasma torch can provide good stability and uniform temperature field without electrodes [16]. Kumar et al. studied the microwave plasma synthesis of Si powders

J. Koo et al. / Materials Chemistry and Physics 180 (2016) 332e340

through reduction of SiCl4 [17]. Wu et al. also reported a system for destroying SiCl4 and producing nano-sized Si particles based on a microwave plasma jet at atmospheric pressure [18]. In the present study, we synthesize Si nanoparticles using SiCl4 as a starting material by an atmospheric microwave plasma technique. The following reaction shows that H2 is the main parameter for the synthesis of Si from SiCl4: SiCl4 þ 2H2 4 Si þ 4HCl

333

gases were injected via two nozzles: a rectilinear gas nozzle, and a swirling gas nozzle. The SiCl4 vapour and H2 were injected through the rectilinear gas nozzle positioned at the top of the injector. N2 and additional H2 were injected via the swirling nozzle placed on the inner wall of the injector. Mass flow controllers precisely controlled the flow rates of all gases. A quartz tube with the diameter of 38 mm was inserted on top of the reactor with a cooling jacket, and the plasma formed inside the tube.

(1)

However, to obtain Si nanoparticles, we must consider the above reaction in terms of the thermodynamics and kinetics in plasma where the reaction was carried out. Obviously, the method to add H2, as well as its amount, determines the microstructures of synthesized Si nanoparticles. We injected H2 through two different nozzles into plasma with varying its amount, and investigated the effects of H2 on the nanoparticle microstructures. Furthermore, we applied Si nanoparticles synthesized with varying amounts of H2 as active materials in LIB anodes, and studied their feasibility.

2. Experimental procedures 2.1. System for the synthesis of Si nanoparticles Fig. 1 shows a schematic diagram of atmospheric microwave plasma system used in the synthesis of Si nanoparticles. The system consisted of a power generator, feeder, reactor, trap, and scrubber. The power generator consisted of a microwave power supply (6 kW, Richardson Inc., SM1208), magnetron (2.45 GHz, Richardson Inc., TM060.1), coupler, 3-stub tuner, and tapered waveguide. The microwave radiation generated from the magnetron was delivered through the waveguide, which was perpendicularly connected with a reactor [19]. We could maximize the electric field induced by the microwave radiation by adjusting the 3-stub tuner, and the reflected power was typically less than 5% of the forward power. The feeder part consisted of a reactant bubbler, a pre-heater, and an injector with a swirling nozzle. Inside the bubbler, SiCl4 (99%, Wako Inc.) was bubbled by the injection of argon (Ar) carrier gas, and evaporated by the pre-heater. Finally, SiCl4 vapour was injected with other gases through the injector. Inside the injector, three

2.2. Process and parameters for the synthesis of Si nanoparticles After we injected the N2 and H2 (called swirling H2) via the swirling gas nozzle into the quartz tube, we applied microwave radiation of 2.5 kW power to the swirling gases. Spark discharge ignition formed plasma inside of the quartz tube. The plasma was also swirling, which served to prevent the quartz tube from melting by the high temperature plasma instead of the sheath gas. Subsequently, we injected additional H2 (called rectilinear H2) via the rectilinear gas nozzle. SiCl4 precursor at 25  C in the bubbler was bubbled by 50 sccm of Ar gas, and vaporized by passing through the pre-heater at 200  C. When the plasma stabilized, we injected 350 sccm of the vaporized SiCl4 with 50 sccm of Ar via the rectilinear nozzle into the quartz tube. The injected SiCl4 dissociated into Si and Cl in the plasma, and Si nanoparticles nucleated and grew in the quartz tube. The Si nanoparticles then attached to the inner wall of reactor that was surrounded by a cooling water jacket. The Cl-ions associated with H-ions to form HCl, which vented into the scrubber, and was neutralized. The working pressure was atmospheric. After synthesis for 1hr, we turned off the plasma power, and removed the residual gases from inside the reactor by N2 flushing. Table 1 shows the process conditions for the synthesis of Si nanoparticles. To study the effects of H2 on the microstructure of Si nanoparticles, we prepared samples under the same process conditions, except for variation of the amount of swirling and rectilinear H2 during synthesis. The remaining conditions were constant at the applied power of 2.5 kW, and the amount of swirling N2, carrier Ar, and SiCl4 were 25 slpm, 50 sccm, and 350 sccm, respectively. In the case of H2, we introduced 0e8 slpm of swirling H2 and 0e15 slpm of rectilinear H2. Table 1 indicates that we named

Rectilineal H2 MW-power supply

pre-heater

SiCl4 bubbler

injector 2.45 GHz magnetron

coupler

3-stub tuner

wave guide

Swirling gas (N2 & H2) quartz tube vent

reactor scrubber

trap Fig. 1. Schematic diagram of an atmospheric microwave plasma system.

Carrier Ar

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Table 1 Process conditions of microwave plasma for the synthesis of Si nanoparticles. Sample no.

Swirling H2 (slpm)

Partial pressure of swirling H2

Rectilinear H2 (slpm)

Partial pressure of rectilinear H2

A1 A2 A3 A4 A5 A6

0 3 8 3 3 3

0.00 0.08 0.18 0.10 0.09 0.07

10 10 10 0 5 15

0.28 0.26 0.23 0.00 0.15 0.35

the samples A1 ~ A6 for convenience. 2.3. Microstructural analysis To monitor the optical properties of nanoparticles synthesized with and without swirling H2, we measured the absorption spectra of the synthesized Si nanoparticles in the range of 240e1000 nm by UVevis spectrophotometry (Shimadzu Inc., SolidSpec-3700). The crystallinity of samples was investigated by X-ray diffraction (XRD; Rigaku Inc., HPC-2500) analysis using Cu Ka1 radiation (k ¼ 0.15406 nm) with a scan speed of 3 min1 (40 kV, 100 mA). The microstructures of samples were measured using scanning electron microscopy (SEM; Hitachi Inc., S-4800) and high resolution transmitted electron microscopy (HR-TEM; Jeol Inc., JEOL2100F) analysis. We obtained the average particle size of the synthesized Si nanoparticles by measuring and averaging the size of 100 nanoparticles in the SEM images. The atomic concentration ratio of each sample was measured by energy dispersive spectrometry (EDS; HORIBA Inc., EMAX 7200-H). We performed X-ray photoelectron spectra (XPS; KRATOS Inc., AXIS Ultra DLD) to investigate the chemical bonding environment of Si in each sample at a pass energy of 40 eV using Al Ka (1486.6 eV) source. 2.4. Electrochemical characterization We prepared the electrodes by coating with a homogeneous slurry containing the synthesized Si nanoparticles as an active material (60 wt%), Denka black (20 wt%, Denka Inc.) as a conducting agent, and polyamide imide (PAI; 20 wt%, PNS Technology Inc.) as a binder on Cu foil. The slurry was dried for 30 min at room temperature, and heated for 3 h at 300  C with Ar purge. The loading level of electrode on the Cu foil was typically 0.3 mgcm2, and the electrode density of all samples was 0.2e0.3 gcc1. To determine the electrochemical performance, we used the above electrodes to construct coin-type cells (CR2032), which we assembled using a separator (Celgard 2400), a counter electrode (Li metal foil), and an electrolyte of 1 M LiPF6 in a mixture of ethylene carbonate/diethyl carbonate with fluorinated ethylene carbonate (EC:DEC ¼ 1:1 (v/v) with 3 vol% FEC, Soulbrain Inc.). The assembled half-cells were aged at 40  C for 24 h, after which we measured their electrochemical properties by loading the cells on a multi-channel battery system (WBCS 3000, WonATech Inc.) at 25  C. The discharge (Li-insertion, lithiation) process was performed galvanostatically at a constant current (CC)/constant voltage (CV). The charge (Li-extraction, delithiation) process was carried out at a CC. The cut-off voltage was from 0.01 to 2.0 V, and the current density of the anode for each sample was 252 mAcm2. 3. Results and discussion Fig. 2 shows (a) typical photo images of plasma, and (b) absorbance spectra of synthesized nanoparticles without and with swirling H2. The insets of Fig. 2 (b) are their photo images. The volume and color of the plasma were critically dependent on the

types and flow rates of injected gases. Therefore, observation of the plasma volume and color could provide an indication in analysis of the relations between the process conditions and the properties of synthesized nanoparticles. The left image of Fig. 2 (a) shows that pink-colored plasma formed when only 25 slpm of swirling N2 was injected. Injection of SiCl4 and H2 increased the plasma volume, and the image on the right shows the flame we observed at the end of the plasma. Generally, plasma volume determines reaction time, which refers to the residence time of SiCl4 in plasma. Thus, the smaller plasma volume results in a shorter reaction time (residence time of SiCl4), which could affect the various properties of synthesized nanoparticles. We specially designed the plasma torch to use swirling gas to concentrate the plasma; the type and flow rate of the swirling gas could be much more critical to the plasma volume than those of the rectilinear gas. In addition, the plasma color depended on the total amount of H2. Originally, the color of SiCl4 plasma without H2 flow was white, and it gradually changed to orange with increase in the amount of injected H2. The darker orange indicates a larger amount of ionized H in the plasma. Fig. 2 (b) shows that the color of the synthesized nanoparticles was also critically sensitive to the injection of swirling H2. Without injection of swirling H2, the absorbance was almost zero in the UVevisible range higher than 300 nm, and the inset image shows the synthesized powder appeared white. These spectra and the powder color were similar to those of SiO2 [20]. Whereas, when swirling H2 was injected, we detected much higher absorbance in the UVevisible range with a broad band from 280 to 500 nm. The inset image shows that the powder’s color was ocher, which is the typical color of Si nanoparticles [21]. All of the above results confirm that swirling H2 was a key process parameter to determine plasma volume and color, and to gain Si nanoparticles. We investigated in detail the dependence of the amount of swirling H2 on the microstructures of synthesized nanoparticles. Fig. 3 shows XRD patterns of Si nanoparticles synthesized with various amounts of swirling H2. The other process conditions, including the power and flow rates of swirling N2, rectilinear H2 and SiCl4 vapour, were identical. Fig. 3 (a) shows the single broad band ranging from 15 to 35 we observed for the A1 powder without swirling H2. This broad band indicated homogeneous amorphous phase, such as Si or SiOx. Meanwhile, for the powder synthesized with swirling H2, Fig. 3 (b) and (c) show the peaks at 28.3 , 47.2 , 56.1, 69.1 and 76.3 that we clearly detected, which we assigned to the (111), (220), (311), (400) and (331) diffraction planes of cubic-Si (JCPDS No. 75-0589). For the A2 powder synthesized with 3 slpm of swirling H2, the peak intensities according to Si were relatively high, but we still detected the amorphous phase. In the case of A3 powder with 8 slpm of swirling H2, the relative intensity of crystalline Si peaks was slightly lower than that of A2, but the difference was negligible. We did not expect to only detect amorphous phase without swirling H2, because a large amount of rectilinear H2 (10 slpm) was still injected to the plasma with the same applied power. The above results confirm that there was a minimum amount of swirling H2 to gain crystalline Si phase, regardless of the injection of a large amount of rectilinear H2.

J. Koo et al. / Materials Chemistry and Physics 180 (2016) 332e340

(a)

(b)

335

w/ swirling H2

0.5

Absorbance Units

w/o swirling H2

N2

N2 + H2 + SiCl4

0.4 0.3 0.2 0.1 0.0 300

400

500

600

700

800

900

1000

Wavelength (nm) Fig. 2. (a) Photo images of microwave plasma, and (b) absorbance spectra of synthesized nanoparticles without and with swirling H2. Inset shows photo images of each sample.

Fig. 3. XRD patterns of Si nanoparticles synthesized with (a) A1, (b) A2, and (c) A3, and standard pattern of cubic-Si (JCPDS No. 75-0589).

Fig. 4 shows SEM images of the Si nanoparticles synthesized with (a) 0 (A1), (b) 3 (A2), and (c) 8 (A3) slpm of swirling H2. In the case of A1 powder without injection of swirling H2, the sample exhibited random shape, which was typically observed in

amorphous Si or SiOx phase [22,23]. The average size of particles in the A1 powder was 15e50 nm smaller than for the other samples. For A2 powder synthesized with 3 slpm of swirling H2, Fig. 4 (b) shows the spherically shaped particles with necking among the particles we detected. Spherical shapes typically indicated a crystalline Si phase [22,23]. The particle size ranged from 30 to 130 nm, and we also detected the agglomeration of the Si nanoparticles. For A3 powder synthesized with 8 slpm of swirling H2, the nanoparticles exhibited a similar agglomeration to that of A2. The particles of A3 ranged in size from 35 to 150 nm. The inset table indicates the atomic concentration ratio of each sample. Note that we observed Cl at every point of the sample tested for the A1 powder. In contrast, we detected much less or no elemental Cl in the A2 and A3 powder, regardless of the measured position. Finally, we should mention that we observed O in all of the samples. The O observed might be from surface oxidation after synthesis of the nanoparticles, or from the formation of amorphous SiOx during synthesis. Although we used no oxidation gas used during the synthesis, there must be O, because we carried out the synthesis at atmospheric pressure. From the results up to now, we could interpret the formations of crystalline Si nanoparticles as follows. As Fig. 2 shows, without swirling H2, the plasma volume and color were relatively small and white, respectively. Thus, sufficient reaction of Eq. (1) we mention in the introduction was not carried out in plasma to gain Si nanoparticles. The insufficient reaction without swirling H2 resulted in amorphous phase detected in XRD pattern. The presence of elemental Cl in this amorphous phase gives good evidence of the

Fig. 4. SEM images of Si nanoparticles synthesized with (a) A1, (b) A2, and (c) A3. Inset tables indicate the weight and the atomic concentration ratios of each sample.

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insufficient reactions. Meanwhile, as the XRD patterns and SEM images show, with swirling H2, the plasma volume increased with darker color, which enhanced the reaction to form crystalline Si phase. Consequently, swirling H2 is a key process parameter to form crystalline Si nanoparticles, due to the enhanced reaction in plasma. We will discuss the detailed reaction mechanism later in this research. In addition, the amorphous phases in all the samples should be SiOx phase, which played the role of agglomeration of Si nanoparticles. Rectilinear H2 injection was another main process parameter that determined the microstructure of synthesized Si nanoparticles. Fig. 5 shows XRD patterns of Si nanoparticles synthesized with various amounts of rectilinear H2. We varied only the rectilinear H2 from 0 to 15 slpm, and kept the other process conditions constant during the synthesis. For all samples, we simultaneously injected 3 slpm of swirling H2. The XRD patterns show that crystalline Si nanoparticles formed for all samples, and we also detected amorphous phase, regardless of the amount of rectilinear H2. For A4 powder without injection of rectilinear H2, the peak intensity according to crystalline Si was low, despite the injection of 3 slpm of swirling H2. The relative intensity continuously increased with increasing injection of rectilinear H2. We could not inject more rectilinear H2, because higher flow rates of rectilinear H2 over 15 slpm turned off the plasma. The above results show that both the swirling and rectilinear H2 determined the microstructures of synthesized Si nanoparticles, but the swirling H2 was a more sensitive parameter than the rectilinear H2. From the above results, including XRD and SEM measurements of the nanoparticles produced with various H2 gas injection conditions, we could interpret the reaction mechanism as follows. To obtain Si from SiCl4, intermediate reactions must be carried out in the following sequence: H2 / 2H

(2)

H þ SiCl4 / SiCl3 þ HCl,

(3)

H þ SiCl3 / SiCl2 þ HCl,

(4)

H þ SiCl2 / SiCl þ HCl,

(5)

H þ SiCl / Si þ HCl

(6)

Fig. 5. XRD patterns of Si nanoparticles synthesized with (a) A4, (b) A5, (c) A2, and (d) A6.

When there was insufficient time or H-ion to carry out the above reactions in plasma, intermediate phases such as SiCl3, SiCl2 and SiCl could form [18]. Such intermediate phases might change into chlorosilane, such as SiH3Cl, SiH2Cl2, and SiHCl3. Those phases are known to easily transform into SiOx when exposed to air [24,25]. In addition, in terms of thermodynamics, only 2 mol of H2 is required to complete the reaction shown in Eq. (1). Thermodynamically considering the amount of SiCl4 injected per minute, only 0.4 slpm of H2 is enough to complete the reaction. However, even when much more H2 was injected than thermodynamically required, we failed to gain homogeneous Si. Instead, we obtained intermediate phases or only small amounts of amorphous Si. We could explain this result in terms of kinetics. Specifically, the gas velocities in torch were too fast to complete the thermodynamic reaction shown in Eq. (1); in other words, flown gas could not have sufficient time to complete the reaction in plasma. It is more reasonable to assume that even if we supplied a thermodynamically sufficient amount of H2, the reactions between Eq. (2) and Eq. (6) partly occurred. To complete this reaction, there are two approaches; the first approach is to increase the reaction probability, and the second one is to increase the reaction time. We could gain a high reaction probability by increasing the relative amount of injected gas, which is a reason to inject a much higher amount of H2 than the thermodynamically calculated one in this study. Also, we could obtain a long reaction time (meaning a long residence time) of gas by injection of swirling gases, as we discuss below. All of the above analysis implies that swirling H2 was more critical to determine the final microstructure and chemical composition of nanoparticles. We could explain all of the results by the swirling phenomenon of H2 in plasma. Fig. 6 shows a schematic diagram of the gas flows in plasma torch. Solid and dashed lines indicate swirling and rectilinear H2 gas flow in the torch, respectively. Unlike the rectilinear gas, swirling gas formed a vortex flow in the plasma, which resulted in a long residence time of the H-ions in the plasma. Thus, the reaction of SiCl4 and H2 to Si could be enhanced to form a discrete shape, with less remaining Cl element. Increased injection of swirling H2 resulted in the formation of more Si nanoparticles. When the concentration of Si nanoparticles in the plasma exceeded a critical point, Fig. 4 (b) shows that necking, or so-called sintering among the particles, began. As rectilinear H2 alone could not have enough residence time to form a Si phase, there was also not enough time to form crystalline Si phase in plasma. Meanwhile, the long residence time of swirling H2 in the plasma gave more time to form crystalline Si phase, which is the reason we observed crystalline phase with a small amount of swirling H2 in the XRD patterns. Consequently, swirling injection ensured a sufficient residence time of the H2 to complete the reaction from SiCl4 to Si, the formation of crystalline phase. Unfortunately, 8 slpm of swirling H2 was the maximum, because higher flow rates turned off the plasma in this system. On the other hand, we injected rectilinear H2 gas via a nozzle located at top of the injector without any angle, thus the rectilinear H2 gas did not form an H2 vortex flow in the plasma. Therefore, the residence time of rectilinear H2 might be shorter than that of swirling H2 gas. As mentioned above in our discussion of the XRD patterns, even though the crystallinity of Si nanoparticles was high, we still detected the amorphous phase in all samples. To identify the distribution of the crystalline and amorphous phases, we performed HR-TEM analysis. Fig. 7 shows the (a) TEM, (b) HR-TEM, and (c) Si and (d) O STEM mapped images of A2 powder synthesized with 3 slpm of swirling H2 and 10 slpm of rectilinear H2. The inset shows the fast Fourier transform pattern of the image. Fig. 7 (a) shows that almost all particle shapes were spherical, and all the particles connected to each other with different phase. It seems that the crystalline Si was surrounded by an amorphous phase. Fig. 7 (b)

J. Koo et al. / Materials Chemistry and Physics 180 (2016) 332e340

337

Rectilineal gas 5~15o

Swirling gas

Microwave

Fig. 6. Schematic diagram of gas flows in plasma torch: swirling gas (solid line) and rectilinear gas (dash line).

Fig. 7. (a) TEM, (b) HR-TEM, and (c) Si and (d) O STEM mapped images of Si nanoparticles (A2). Inset shows a fast Fourier transform pattern of the image.

shows the thickness of the amorphous phase surrounding crystalline nanoparticle was as thick as 10 nm, which indicated that amorphous phase detected in XRD was not from amorphous

nanoparticles, but from the shell and necking phase. Those shells were much thicker than the native oxide film, indicating that the amorphous phase formed during the synthesis. In our previous

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research, the native oxide layer formed after the synthesis of nanoparticles had the thickness of only 1e3 nm [22,23]. In addition, we found discrete lattice fringes in the core of a Si nanoparticle. Spots in the electron diffraction pattern also indicated the formation of a single crystalline phase, as the inset of Fig. 7 (b) shows. In addition, the amorphous phase of the thick shell surrounding the crystalline core must be SiOx formed in an atmospheric process. The STEM mapped images of Fig. 7 (c) show homogeneous Si, while Fig. 7 (d) shows we mainly detected O in the shell. From these results, we can infer that we synthesized core-shell structured nanoparticles in the process, wherein amorphous SiOx shells surrounded the crystalline Si cores. Finally, for further investigations of the microstructures of the Si nanoparticles synthesized without (A1) and with (A2) swirling H2, Fig. 8 shows that we obtained (a) XPS, and (b) Raman spectra. The inset of Fig. 8 (a) shows the overall spectra of the sample. For the A1 powder synthesized without swirling H2, we observed only one broad band ranging from 101.5 to 106 eV. This indicates that the particle surface was composed almost completely of SiOx (x  2). Meanwhile, in the case of the A2 synthesized with 3 slpm of swirling H2 and 10 slpm of rectilinear H2, we detected a band of SieSi binding (99.3 eV), as well as that of SieO binding. This result is typically observed in core-shell nanoparticles as described in HRTEM analysis. In addition, the amount of SiOx phase seemed to be much higher than that of the Si phase, because of the relatively high intensity of the SieO band. However, it is not proper to compare the amount of SiOx with that of Si, because the spectra are only from the surface of nanoparticle. To estimate the amount of SiOx in nanoparticles, Fig. 8 (b) shows that the Raman spectrum is more proper. In the case of A1 powder, we detected broad band peaking at 498 cm1 in addition to relatively sharp band peaking at 516 cm1, which we attribute to crystalline Si. We note that we also detected the sharp band according to crystalline Si phase in A1 powder, which we considered as pure amorphous SiOx phase in XRD and XPS measurement. We can explain this result as a small crystalline Si core with a thick SiOx shell. Specifically, the crystallite size was too small to observe in XRD pattern, and SiOx shell was so thick that we could not observe Si core in the XPS spectrum. In the Raman spectrum of A2, as expected, we mainly detected a peak according to crystalline Si. We intend to conduct further study of the relation between Si core and SiOx shell. All of the above results suggest that the amount of H2 injection determines the microstructures of core (Si) and shell (SiOx) structured nanoparticles. The

thick SiOx phase could play an important role as a buffer layer when the nanoparticles are applied as an anode material in LIBs. We applied the Si nanoparticles synthesized with various amounts of injected H2 as active materials for anodes of LIBs, to study their feasibilities. Fig. 9 shows the cyclic performances of Si nanoparticles synthesized with various amounts of (a) swirling,

Fig. 9. Cyclic performances of the anodes prepared with Si nanoparticles synthesized with various amount of (a) swirling, and (b) rectilinear H2.

Fig. 8. (a) Chemical binding spectra, and (b) Raman spectra of Si nanoparticles synthesized without (A1), and with (A2) swirling H2.

J. Koo et al. / Materials Chemistry and Physics 180 (2016) 332e340

and (b) rectilinear H2. In the case of A1 powder synthesized without swirling H2, the first reversible capacity was only 422.1 mAhg1, with initial columbic efficiency (ICE) of 19.0%. We might attribute this result to the amorphous SiOx phase from incomplete reaction, as mentioned in the microstructural analysis. Meanwhile, for the A2 and A3 powders synthesized with various swirling H2, the first reversible capacities and ICE were 1203.8 (29.3%) and 1274.2 mAhg1 (34.3%), respectively, which showed no significant difference. We can attribute the noticeably low initial capacities and ICE to the PAI binder, which is known to react with Li when not properly terminated. The capacity retentions at the 50th cycle, however, were as high as 82.2 (for A2) and 94.8% (for A3), respectively. We should mention that the reversible capacity increased slightly during the first 13 cycles in the case of A3 powder, which has often been observed in nanoparticles that did not react with Li within the first cycle, due to their huge surface area. We calculated the capacity retention of A3 powder based on the maximum capacity to be 86.6%, similar to that of the A2 powder. According to our previous research, those capacity retentions of A2 and A3 were higher than that of commercial Si nanoparticles [9,26]. We might attribute those retention enhancements of A2 and A3 to the buffer layer surrounding Si nanoparticles, as the TEM analysis shows (Fig. 7 (b, d)). Fig. 9 (b) shows that the A5 powder synthesized with 5 slpm of rectilinear H2 also exhibited similar electrochemical properties to those of A2; the first reversible capacity was 1208.8 mAhg1 with ICE of 27.2%, and the capacity retention at the 50th cycle was 78.6%. On the other hand, the A6 powder synthesized with 15 slpm of rectilinear H2 displayed relative improvement of the first reversible capacity (1755.5 mAhg1) and ICE (37.2%). However, the capacity retention of A6 powder was 71.6% at the 50th cycle, which was relatively low compared with the others. Fig. 5 (d) shows that we can explain this result by the increased amount of crystalline Si phase in the samples. The higher amount of crystalline Si increased the first reversible capacity, as well as ICE. Instead, the retention decreased due to large volumetric change of Si during the cycles. From the above results, although the electrochemical properties of the synthesized Si nanoparticles were similar to each other excluding A1 processed without swirling H2, large amounts of H2 gas must be injected for the formation of crystalline Si phase to gain a high specific capacity of more than 1200 mAhg1. In addition, the good cyclic performances might be due to the core (Si) and shell (SiOx) nanostructures we describe in the microstructural analysis. Fig. 10 shows the differential capacity (dQ/dV) profiles of the Si nanoparticles synthesized (a) without (A1), and (b) with (A2)

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swirling H2. In the case of A1 powder synthesized without swirling H2, we observed three reduction peaks of around 0.47, 0.62 and 0.96 V (vs. Li/Liþ) during the first Li-insertion. These peaks correspond to the decomposition of electrolyte and the formation of solid-electrolyte interphase (SEI) layers on the electrode surface. From the second Li-insertion, we observed only a small reduction peak at 0.05 V, which corresponds to an amorphous LixSi phase. During the Li-extraction from Si nanoparticles, we observed a broad oxidation band around 0.4 V, which corresponds to de-alloying to form amorphous Si phase. As a result, only the amorphous phase participated in alloy/de-alloying. As mentioned in relation to the XPS and Raman spectra, however, the A1 powder consisted of a small amount of crystalline Si core with thick SiOx shell. This crystalline Si core might not participate in the alloying/de-alloying with Li-ions because of too thick a SiOx shell. A2 powder synthesized with swirling H2 showed a reduction peak at 0.01 V corresponding to a destruction of crystalline Si phase (amorphization of lattice) with a peak at 0.70 V due to SEI formation during the first Li-insertion. After the second Li-insertion, we observed reduction peaks corresponding to alloy phases including crystalline L15Si4 at 0.18 V, as well as amorphous LixSi at 0.05 V, at the same time. The profile of Li-extraction showed an oxidation peak corresponding to de-alloying of crystalline L15Si4 at 0.45 V, with broad band to that of amorphous LixSi phases around 0.33 V. This profile also indicates typical alloying and de-alloying of crystalline Si and amorphous SiOx phase. From the results above, we can clearly interpret the difference of phase changes during charge-discharge process as the difference of crystallinity of Si phase between A1 (without swirling H2) and A2 (with swirling H2) powders.

4. Conclusions We synthesized Si nanoparticles using atmospheric microwave plasma, and investigated their microstructures and electrochemical properties according to varying amounts and nozzles of injected swirling and rectilinear H2. A larger amount of H2 was required than that calculated thermodynamically for completion of the reaction from SiCl4 to Si and obtaining a highly crystalline Si phase. Specifically, we produced only amorphous SiOx phase into which intermediate phases were oxidized at atmospheric pressure without swirling H2 injection. Whereas, a small amount of swirling H2 injection enabled the highly crystalline Si phase to appear. The critical dependence of the microstructures on the swirling H2 might be due to the formation of vortex flow, which provides a relatively

Fig. 10. dQ/dV profiles of the anodes prepared with Si nanoparticles synthesized without (A1), and with (A2) swirling H2.

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long residence time of the H-ions in the plasma. Rectilinear H2 also enhanced the probability of reacting with SiCl4 in plasma, which resulted in increase of the synthesized particle’s crystallinity in proportion to its injected amount. HR-TEM, XPS and Raman analysis showed that the crystalline Si nanoparticles were surrounded by a thick amorphous SiOx phase; when those nanoparticles were applied as anode materials of LIBs, this phase could provide a good buffer layer. Electrochemical characterization revealed the coreshell nanoparticles exhibited good cyclic performance with a first reversible capacity of about 1200 mAg1, and capacity retention of approximately 80% at the 50th cycle. As a future work, we intend to carry out further electrochemical analysis of those nanoparticles to investigate their relations with the microstructures in detail, such as core-shell or necking, and enhance the ICE of synthesized nanoparticles. Acknowledgements This work was conducted under the framework of the Research and Development program of the Korea Institute of Energy Research (KIER) (B6-2454). References [1] M. Stupca, M. Alsalhi, T.A. Saud, A. Almuhanna, M.H. Nayfeh, Enhancement of polycrystalline silicon solar cells using ultrathin films of silicon nanoparticle, Appl. Phys. Lett. 91 (2007) 063107. [2] C.Y. Liu, Z.C. Holman, U.R. Kortshagen, Hybrid solar cells from P3HT and silicon nanocrystals, Nano Lett. 9 (2009) 449e452. [3] G. Conibeer, M. Green, R. Corkish, Y. Cho, E.C. Cho, C.W. Jiang, T. Fangsuwannarak, E. Pink, Y. Huang, T. Puzzer, T. Trupke, B. Richards, A. Shalav, K.L. Lin, Silicon nanostructures for third generation photovoltaic solar cells, Thin Solid Films 511e512 (2006) 654e662. , K. Chan, A silicon [4] S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E.F. Crabbe nanocrystals based memory, Appl. Phys. Lett. 68 (1996) 1377e1379. [5] J.K. Lee, K.B. Smith, C.M. Haynerb, H.H. Kung, Silicon nanoparticlesegraphene paper composites for Li ion battery anodes, Chem. Commun. 46 (2010) 2025e2027. [6] P. Dimitrakis, E. Kapetanakis, D. Tsoukalas, D. Skarlatos, C. Bonafos, G.B. Asssayag, A. Claverie, M. Perego, M. Fanciulli, V. Soncini, R. Sotgiu, A. Agarwal, M. Ameen, C. Sohl, P. Normand, Silicon nanocrystal memory devices obtained by ultra-low-energy in-beam synthesis, Solid-State Electron. 48 (2004) 1511e1517. [7] L. Pavesi, L.D. Negro, C. Mazzoleni, G. Franzo, F. Priolo, Optical gain in silicon

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