Silicon-Catalyzed Growth of Amorphous SiOx Nanowires by Laser

Oct 29, 2013 - Division of Chemistry for Materials, Graduate School of Engineering, Mie University, 1577 Kurimamachiya, Tsu, Mie 514-8507. Japan...
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Silicon-Catalyzed Growth of Amorphous SiOx Nanowires by Laser Vaporization of Si and Si/SiO2 Keita Kobayashi,*,† Fumio Kokai,*,‡ Naoto Sakurai,‡ and Hidehiro Yasuda† †

Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, 7-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Division of Chemistry for Materials, Graduate School of Engineering, Mie University, 1577 Kurimamachiya, Tsu, Mie 514-8507 Japan

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ABSTRACT: Products from the laser vaporization of targets of four typesSi, (1:1) Si/SiO2, (1:9) Si/SiO2, and SiO2without the addition of any metal catalysts in the presence of high-pressure (0.9 MPa) Ar gas were characterized by transmission electron microscopy (TEM), high-resolution TEM, energy-filtered TEM, transmission electron diffractometry, energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, and powder X-ray diffractometry. Amorphous SiOx (0.8 ≤ x ≤ 2.0) nanowires (NWs) (10−40 nm in diameter and up to 1 μm long), having crystalline Si nanoparticles (NPs) covered with thin amorphous SiOx layers at the NW tips, were observed for the products from the Si and Si/SiO2 composite targets, while only amorphous SiOx NPs were produced from the SiO2 target. The O content relative to that of the Si in SiOx NWs was also increased with increasing the content of SiO2 in the targets. We propose a novel mechanism for the growth of amorphous SiOx NWs, in which nuclei of the NWs are formed on the Si-rich molten SiOx NPs due to precipitation of SiOx via its supersaturation. Si acts as both a catalyst to precipitate SiOx and a source material of the NWs. A successive supply of Si, SiO, O, and others to the molten NPs, their diffusion and precipitation, and the oxidation of the precipitated NWs result in the further growth of the NWs.

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INTRODUCTION Silicon-based nanostructures, such as nanowires (NWs), nanorods, and nanotubes, have attracted considerable research enthusiasm for their unique properties and promising applications in nanoscale electronics and optoelectronics.1,2 More recently, Si-based nanostructures such as Si NWs,3 Si/C NWs,4 Si/SiOx nanocoils,5 and SiO2/C nanocomposites6 have attracted a great deal of attention for their application as highperformance anode materials for rechargeable lithium ion batteries. Silicon and SiOx NWs have been grown by various synthesis methods including laser vaporization, chemical vapor deposition, physical evaporation, and template-assisted growth.1,2 At the same time, several efforts have been devoted to explaining their growth behaviors. The most well-known mechanism for NW growth is the metal-catalyzed vapor−liquid−solid (VLS) one, first proposed by Wagner and Ellis in 1964 for the growth of Si whiskers.7 In this VLS mechanism, first, a liquid-like alloy nanoparticle (NP) composed of a metal catalyst component (such as Fe, Au, etc.) and a NW material (such as Si) is formed. For example, in laser vaporization, laser irradiation was performed onto a Si target containing 10 wt % Fe in Ar gas, resulting in the formation of Si−Fe NPs.1,8 The liquid-like NP serves as the site for the dissolution of gas-phase reactant, its diffusion, and the nucleation site of a NW. Crystalline Si NW growth begins after the NP becomes supersaturated in reactant material and continues as long as the catalyst remains in a liquid-like state and the reactant is supplied. Other mechanisms have been proposed in addition to the VLS mechanism. For a crystalline Si NW surrounded by a layer of amorphous SiOx © 2013 American Chemical Society

synthesized by laser ablation and thermal evaporation, an oxideassisted mechanism has been proposed in which a SiO NP is thought to play a key role in the formation of the Si NW and SiOx layer.9 Crystalline Si NWs were synthesized without adding any catalysts by thermal evaporation10 and chemical vapor deposition.11,12 The growth of a Si NW on a NP was explained by a vapor−solid (VS) mechanism,11 in which Si vapor directly deposits on the seed NP to nucleate and grow the NW. The sites for the nucleation and growth of Si NWs are Si−metal alloy NPs,1,8 Si NPs covered with SiOx layers,9 and Si NPs.10,11 Not all of the sites of grown Si NWs have been identified yet. Similar to Si NW growth, SiOx (x ≤ 2) NWs can be grown by the VLS mechanism using metallic catalysts, such as Fe,2 Ga,13 Ga−In,14 In−Ni,15 Sn,16 Ni,17,18 and Ti.19 Compared to the easy growth of crystalline Si NWs, most of the grown SiOx NWs have been amorphous in nature. In addition, SiOx NWs and related nanostructures can be obtained without catalysts.2,19−24 The arrays of nanotubes and nanofibers19 and NWs2,20 of silica (SiO2) have been synthesized by thermal evaporation of Si and SiO2 mixed powders. The growth of wellaligned Si/SiOx composite NWs was demonstrated using SiCl4/H2 in a hot filament chemical vapor deposition reactor.21 Silica nanofibers were produced on Si substrates by heating in the flow of Ar/O2 mixed gas.22 A VS mechanism2,20,22 and important roles of gas-phase SiO, O2, and H2O species21,23 have Received: September 10, 2013 Revised: October 20, 2013 Published: October 29, 2013 25169

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18 kW/cm2, respectively, and the laser-irradiation time was set to 2 s. After laser vaporization using a single laser shot at different targets, the deposits in the chamber were collected for characterization. To attempt the removal of Si parts from NW structures, a portion of the deposit (30 mg) obtained by laser vaporization of a (1:1) Si/SiO2 target was treated by immersing in a 1% KOH aqueous solution at 50 °C for 1 min. The remaining deposit was then separated by vacuum filtration. Characterization. The structures of the products were characterized with an X-ray diffractometer (Rigaku, RU-200), a conventional TEM (Hitachi, H-800) operating at 100 or 200 kV, and an atomic resolution analytical TEM (JEOL, JEMARM200F/UHR) operating at 200 kV. The chemical compositions and electron states of the products were measured, respectively, with an EDS spectrometer (JEOL, JED-2300T) and postcolumn TEM energy filter (Gatan, GIF Quantum) mounted on the atomic-resolution analytical TEM system. The EFTEM images were also acquired by the postcolumn TEM energy filter.

been suggested for the growth of SiOx NWs and related structures. It is interesting that some SiOx NWs have been accidentally obtained with13,14 and without20,24 catalysts. In the growth of SiOx NWs, the sites for nucleation and growth were reported to be Si−metal alloy NPs2,13−19 and SiOx NPs.19,23 Thus, despite intensive efforts devoted to explaining the growth processes of Si and SiOx NWs by many groups, their growth mechanisms without using catalysts is not yet fully understood. Although the formation of Si NP seeds on an oxide surface can be controlled by the degree of reactant supersaturation, which is lower than that in homogeneous deposition,11 some factors, such as the driving force to nucleate and grow NWs anisotropically, are not very clear. Catalyst-free growth has the advantage of avoiding metal contamination.11 The growth mechanism should be clarified for the exact control of the morphology and composition of NWs. In this work, the characterization of deposits produced by continuous-wave laser vaporization of Si, (1:1) Si/SiO2, (1:9) Si/SiO2, and SiO2 targets without the addition of any catalysts in high-pressure (0.9 MPa) Ar gas was intensively carried out using transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy-filtered TEM (EFTEM), transmission electron diffractometry (TED), energy-dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), and powder X-ray diffractometry (XRD). Surprisingly, amorphous SiOx (0.8 ≤ x ≤ 2.0) NWs were only observed in the products from the Si and Si/SiO2 composite targets. Also, spherelike NPs composed of crystalline Si covered with thin SiOx layers existed at the tips of the NWs. We propose a novel Si-catalyzed VLS mechanism for the growth of SiOx NWs, in which the precipitation of SiOx from Si-rich molten SiOx NPs acts as a driving force to grow SiOx NWs. Si acts as both a catalyst to precipitate SiOx in the VLS mechanism and a source material of the NWs. Unlike previous metal-catalyzed VLS and VS mechanisms, in which metal−Si alloy or SiOx NPs serve as nucleation sites for SiOx NWs, in our growth mechanism, Sirich SiOx NPs play a crucial role as seeds for the nucleation of the NWs.

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RESULTS AND DISCUSSION Figure 1a−c shows TEM and HRTEM images of products from a Si target. NWs with diameters of 10−40 nm can be observed

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EXPERIMENTAL METHOD Materials and Reagents. Si (99% purity, 47 μm particle size) and SiO2 (99.5% purity, 45 μm particle size) powders were purchased from Sigma-Aldrich and used as received. Potassium hydroxide (KOH, 86% purity) was purchased from Kanto Chemical. Ar gas (99.995% purity) was obtained from Iwatani Industrial Gases. Synthesis. Si and SiO2 powders were used to prepare the laser vaporization targets. Two mixed powders of (1:1) and (1:9) Si/SiO2 in molar ratios and Si and SiO2 powders were pressed into disk plates with a 14 mm diameter and 8 mm thickness. Laser irradiation onto the four types of targets without the addition of any catalysts were carried out in the presence of high-pressure Ar gas as reported in a previous study using a Si target.26 A continuous-wave Nd:YAG laser (Lee Laser, Series 800, 1.06 μm wavelength, and 500 W peak power) was used for the vaporization of the targets at room temperature. The laser beam was focused on the targets through a quartz window installed in a stainless-steel chamber (110 mm in diameter and 150 mm long) filled with Ar gas at a pressure of 0.9 MPa at room temperature. The pressure of 0.9 MPa was chosen in accordance with the highest fraction (∼90%) of NWs in deposits.26 The laser spot size and the power density on the target were adjusted to 2 mm and about

Figure 1. (a) TEM image of product obtained from Si target, (b and c) HRTEM images of product magnified at two different tip regions, and (d) TED pattern obtained from a mixture aggregate of NWs and NPs.

(Figure 1a), and at their tips, spherelike NPs with diameters of 25−80 nm exist. As seen in the HRTEM images of tip portions (Figure 1, parts b and c), the NWs are in an amorphous state. In contrast, in the spherelike tip NPs, lattice fringes corresponding to the lattice distance (0.314 nm) and the interplanar angle (109.5°) of the {111} plane of crystalline Si are observed. The NPs are also found to be covered with thin (∼2 nm) amorphous layers. The TED pattern of the product (Figure 1d) shows distinct Debye−Scherrer rings which are assigned to a diamond-type structure Si according to JCPDS 27-1042. Although the TED pattern was obtained from a 25170

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mixture aggregate of NPs and NWs, it supports the crystalline phase of NPs as observed in the HRTEM images. Compared to the observation of tip portions consisting of NPs, it was fairly difficult to observe the other ends of NWs by TEM due to their overlapping each other probably because of cohesive features. However, as shown in Figure 2, we succeeded Figure 4. Plots of O/Si atomic ratios of NWs (red squares) and tip NPs (blue circles) estimated by intensity ratios of EDS spectra as a function of the component of the targets: Si, (1:1) Si/SiO2, and (1:9) Si/SiO2. Error bars show standard deviation.

(red squares) and tip NPs (blue circles) as a function of a component of the targets: Si, (1:1) Si/SiO2, and (1:9) Si/SiO2. Error bars show standard deviation. All the NPs obtained from the three targets consist of almost pure Si, and the O/Si ratios (0.05−0.23) of the NPs hardly vary even if the SiO2 content in the target is varied. In contrast, the O/Si ratios (0.83−2.00) of NWs significantly depend on the SiO2 contents in the targets: the O content in the NWs increases as the SiO2 content increases. The EELS of products in the region of Si L-shell excitation (Figure 5) also shows target-component dependency of

Figure 2. TEM image of NWs from one side to another side. Image is composed of four TEM images observed at different sample locations. A typical NW (denoted by an arrowhead) has a length of ∼1 μm and diameter of 10−30 nm.

in observing NWs from one side to another side, revealing that a large spherelike NP exists only at one end of a NW. A typical NW has a length of ∼1 μm. The diameter of the NW is not uniform along the direction of growth, ranging from 10 to 30 nm. The diameter of the spherelike NP existing at the one end of the NW is about 70 nm. We further investigated the compositions of NW and tip NP portions. Figure 3 shows typical EDS spectra observed for a

Figure 5. EELS spectra of (a) a NW obtained from a (1:9) Si/SiO2 target and of (b) a NW and (c) a tip NP obtained from a Si target in the region of Si L-shell excitation. The spectra of the NW and tip NP obtained from the Si target show additional peaks at ∼100 eV (denoted by an asterisk) which is the energy of the first peak of crystalline Si.

product stoichiometries and a difference in composition between the NW and NP. The energy-loss near-edge structure (ELNES) of the NW obtained from a (1:9) Si/SiO2 target (Figure 5a) resembles the ELNES of amorphous SiO2,27 while the ELNES of the tip NP obtained from a Si target (Figure 5c) shows a substantially different spectral pattern from SiO2 and is very close to the ELNES of crystalline Si,27 indicating that the NP is composed of crystalline Si. The ELNES of a NW produced from the Si target (Figure 5b) shows almost the same pattern as that of the NW obtained from the (1:9) Si/SiO2 target. However, the spectrum shows an additional peak at ∼100 eV (denoted by an asterisk) which is the same energy as the first peak of crystalline Si. Since amorphous SiO shows a similar ELNES pattern, the spectrum suggests that stoichiometry is SiO. These EELS results are basically consistent with those of the EDS data, while the O/Si ratios of the NWs from the (1:9) Si/SiO2 and Si targets in Figure 4 do not completely match with the ratios of ideal SiO2 and SiO. Additionally, we examined the spatial distribution of compositions in NWs and NPs using elemental mapping by EFTEM. Figure 6 shows the two sets of EFTEM images of products from a Si target observed at two different sampling positions. A comparison of bright field images (Figure 6, parts a and d) with Si L2,3-edge (Figure 6, parts b and e) and O K-edge

Figure 3. EDS spectra observed for a NW and a tip NP. The spectra consist of O Kα, Si Kα, and C Kα lines.

NW and a tip NP. The spectrum of the NW shows strong peaks at 0.53 and 1.74 keV corresponding to the respective O Kα and Si Kα lines. The EDS spectrum also shows a weak peak at 0.28 keV, which corresponds to the C Kα line and can be attributed to the amorphous carbon membrane of a TEM microgrid. In contrast to the NW spectrum, drastic decrease in the intensity of the O Kα peak can be found in the NP spectrum. This result suggests that tip NPs consist of almost pure Si, while NWs are formed by amorphous SiOx, being consistent with the HRTEM and TED of the NWs and NPs (Figure 1). The atomic ratio of O relative to Si (O/Si) was also determined from the peak intensities.26 The O/Si ratios of the NW and NP were 0.91 and 0.05, respectively. The O atoms detected in the tip NP probably originate from the outer thin amorphous layer. NWs similar to those from the Si target were observed for the deposits from the targets of (1:1) and (1:9) Si/SiO2, while NW fractions in the deposits were decreased. EDS spectra were taken for NWs and tip NPs, and O/Si ratios were estimated. Figure 4 shows the plots of the average O/Si ratios of NWs 25171

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concentration in the two types of NWs. The presence of tip NPs connected with thin and short Si NWs is much clearer in the Si L2,3-edge image of Figure 7b. Furthermore, we were able to remove parts of the tip Si NPs by the treating the NW samples in a 1% KOH solution. Figure 8, parts a and b, shows TEM images observed at two different

Figure 6. Two sets of EFTEM images observed at different sampling positions of the product from a Si target: (a and d) bright field images provided by filtered elastic electrons, (b and e) Si elemental maps provided by Si L2,3-edge, and (c and f) O elemental maps provided by O K-edge.

(Figure 6, parts c and f) images reveals largely different distributions of Si and O in the NWs and NPs. As seen in Figure 6, parts b and e, Si localizes in the central parts of tip NPs, while O is not detected in these regions. As seen in Figure 6, parts c and f, weak O signals are detected uniformly in the areas surrounding the NPs. This seems to be an indication of the surface of the NPs covered by amorphous SiOx, which is consistent with the weak O Kα line in the EDS spectrum (Figure 3) and the small O/Si ratios in the NPs (Figure 4). Similarly, weak Si and O signals are detected along the NWs, indicating that the NWs consist of SiOx as shown in EDS and EELS. We also note that, as seen in the lower part of Figure 6b, there are some tip NPs connected with thin and short Si NWs (∼20 nm in thickness and 100−200 nm long). These Si NWs are surrounded by SiOx outer layers. Similar EFTEM images were observed for the products from a (1:9) Si/SiO2 target (Figure 7), indicating the formation of SiOx NWs with tip NPs similar to those from a Si target. The analogous tendency of elemental distribution between the two products is consistent even if there is a difference in the O

Figure 8. TEM images observed at two different places of a deposit from a (1:1) Si/SiO2 target after treatment with KOH solution.

places of a deposit from a (1:1) Si/SiO2 target after treatment with KOH solution. The structures obtained indicate the parts of Si can be removed. The removal of Si seems to be achieved by an etching process with hydroxide ions and water reacting with Si by the following equation:28 Si + 2OH− + 2H 2O → SiO2 (OH)2 2 − + 2H 2

In addition to examining the deposits from the Si and Si/ SiO2 targets, we analyzed the deposit from a SiO2 target. No NW was observed in this deposit. Figure 9 shows a TEM image, TED pattern, HRTEM image, and EDS spectrum of the product from an SiO2 target. As seen in the TEM image (Figure 9a), NPs with diameters of 20−440 nm were observed. Since the corresponding TED pattern (inset of Figure 9a) shows only a halo pattern, these NPs are considered to be in an amorphous state. The amorphous structure of the NPs can also be confirmed by the HRTEM image (Figure 9b). In the EDS spectrum (Figure 9c), strong O Kα and Si Kα lines are observed. The compositions of O and Si atoms were estimated to be 35.8 and 64.2 atom %. The O/Si ratio is 1.79, which is close to that of SiO2. The additional weak peaks at 0.28 and 0.93 keV in the EDS spectrum correspond to C Kα and Cu Lα lines. These lines originate from the amorphous carbon membrane and Cu mesh of a TEM microgrid. Figure 10 shows powder XRD patterns of products obtained from Si, (1:1) Si/SiO2, (1:9) Si/SiO2, and SiO2 targets. XRD patterns of two products from Si and (1:1) Si/SiO2 exhibits strong three lines of (111), (220), and (311) reflections of Si that are overlapped by a broad signal. The three lines become weak in the XRD pattern of the product from (1:9) Si/SiO2. A broad signal is only observed in the XRD pattern of the product from SiO2. The broad signal is assigned to that from amorphous SiOx NPs. The changes observed in the four XRD patterns are caused by the amount of SiOx NWs having Si NPs at the tips relative to that of amorphous SiOx NPs and are consistent with the TEM images and TED patterns described above.

Figure 7. Two sets of EFTEM images observed at different sampling positions of the product from a (1:9) Si/SiO2 target: (a and d) bright field images provided by filtered elastic electrons, (b and e) Si elemental maps provided by Si L2,3-edge, and (c and f) O elemental maps provided by O K-edge. 25172

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molten SiOx NPs in a VLS growth process. Si acts as both a catalyst to precipitate SiOx and a source material of the NWs. After the formation of the nuclei, the anisotropic growth of the NWs, together with some processes such as the supply of Si, SiO, O, and others to the molten NPs and their diffusion and precipitation, occurs at high temperature. From the evaluation of more than 50 NWs grown from a Si target, the NW diameters were 45−80%, 42−63%, and 40− 58% of the tip NPs with diameters of about 20, 60, and 100 nm, respectively. These results indicate that the diameter difference between the NWs and NPs is larger in the NWs grown from larger molten NPs and the amount of precipitated SiOx relative to the volumes of the NPs is smaller in the growth of thicker NWs, while the larger molten NPs generally produce thicker NWs. Although the accurate reason remains unclear, the anisotropic precipitation of SiOx from the NPs may be attributed to limited growth area on the surface of molten NPs due to heterogeneous chemical abundance and temperature gradient in the NPs. Moreover, inhomogeneous diameter of NWs along the growth axis shown in Figure 2 may suggest that the growth area is enlarged during growth of the NWs. The growth of the NWs from larger Si-rich molten NPs may be based on supersaturation of SiOx at somewhat higher temperatures. As discussed by several groups,13,21,23 the source of oxygen, contained in the amorphous SiOx NWs produced even from Si targets, could have several origins. The source may come from residual O2 in the stainless steel chamber, H2O and O2 in the gas of Ar, or natural oxidation layers present on the surfaces of Si powders. The increase of O content in the SiOx NWs grown from Si/SiO2 targets (Figure 4) indicates that SiO2 in the targets is also an O source. The oxidation of the NWs probably occurs together with their growth. According to the binary phase diagram of Si−O,29 there is also a coexistence of liquid, composed of Si and O, and solid Si at temperatures of 1370− 1414 °C for the presence of O at low percentages of 0−25 atom %. The presence of tip NPs connected with thin and short Si NWs in the EFTEM images (Figures 6 and 7) can be explained by the precipitation of Si. The structures of NPs with short Si NWs may be formed at a late stage of the SiOx NW growth due to Si precipitation temperature lower than that of SiOx. The formation of Si-rich molten SiOx NPs even from the use of a (1:9) Si/SiO2 target may result from the higher boiling point of Si NPs than of SiOx NPs. The boiling points of bulk Si, SiO2, and SiO are 2355, 2230, and 1880 °C, respectively.30 The phase diagram of Si−O indicates that the precipitation of SiOx is not expected from molten SiOx (x ≈ 2) NPs. This is consistent with the formation of SiOx NPs from a SiO2 target.

Figure 9. (a) TEM image and corresponding TED pattern of a NP product from a SiO2 target, (b) HRTEM image of the NPs, and (c) EDS spectrum of the NPs.

Figure 10. XRD patterns of products obtained from (a) Si, (b) (1:1) Si/SiO2, (c) (1:9) Si/SiO2, and (d) SiO2 targets.

The findings of this study are apparently different from previous metal-catalyzed VLS and VS growth processes of SiOx NWs, in which the precipitation of SiOx from a liquid-like metal NP and the direct deposition of Si, SiO, and others on solid SiOx NPs were thought to play a key role in the growth of the SiOx NWs. From the observation of spherelike NPs composed of crystalline Si covered with thin SiOx layers at the tips of the NWs, a significant difference in the O/Si atomic ratios of the NWs and NPs, O/Si ratios of the NWs strongly depending on the SiO2 contents in the targets, and the elemental mapping in the EFTEM images as well as the strong correlation between the tip NP diameter and NW diameter,25 we believe that our amorphous SiOx NW growth is dominated by liquid-like molten SiOx NPs formed from laser-vaporized gaseous products such as Si, SiO, O, and others. Let us discuss how the SiOx (0.8 ≤ x ≤ 2.0) NWs are grown from the SiOx NPs. According to the binary phase diagram of Si−O,29 there is a coexistence of liquid, composed of Si and O, and solid SiO and/or SiO2 at high temperatures of 1370−1723 °C for the presence of O at specific percentages of 25−67 atom %. Consequently, we propose the precipitation of SiOx to form nuclei of SiOx NWs on Si-rich molten SiOx NPs. The precipitation of SiOx occurs via its supersaturation in Si-rich

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CONCLUSIONS The crystalline and amorphous features, atomic compositions, and morphology and structures of four types of products obtained by the laser vaporization of Si, (1:1) Si/SiO2, (1:9) Si/ SiO2, and SiO2 targets without the addition of any metal catalysts, were characterized by HRTEM and related advanced techniques. Amorphous SiOx (0.8 ≤ x ≤ 2.0) NWs (10−40 nm in diameter and up to 1 μm long) with crystalline Si NPs covered with thin SiOx layers at the NW tips were observed for the products from the Si and Si/SiO2 composite targets, while only amorphous SiOx NPs were produced from the SiO2 target. From several findings in this study such as a significant difference in the O/Si atomic ratios of the NWs and NPs, O/Si ratios of the NWs strongly depending on the SiO2 contents in 25173

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(15) Ma, R.; Bando, Y. In−Ni Microballs Catalyzed Growth of Dense and Highly Aligned Silica Nanowires. Chem. Phys. Lett. 2003, 377, 177−183. (16) Sun, S. H.; Meng, G. W.; Zhang, M. G.; Tian, Y. T.; Xie, T.; Zhang, L. D. Preparation and Characterization of Oriented Silica Nanowires. Solid State Commun. 2003, 128, 287−290. (17) Shin, D. H.; Kim, S.; Hong, S. H.; Choi, S. H.; Kim, K. J. Control of Amorphous Silica Nanowire Growth by Oxygen Content of Si-Rich Oxide. Nanotechnology 2010, 21, 045604. (18) Gallsen, G.; Reparaz, J. S.; Wagner, M. R.; Vierck, A.; Philips, M. R.; Thomsen, C.; Hoffmann, A. Titanium-Assisted Growth of Silica Nanowires: from Surface-Matched to Free-Standing Morphologies. Nanotechnology 2011, 22, 405604. (19) Wang, Z. L.; Gao, R. P.; Gole, J. L; Stout, J. D. Silica Nanotubes and Nanofibers Arrays. Adv. Mater. 2000, 12, 1938−1940. (20) Zhang, Y.; Wang, N.; He, R.; Liu, J.; Zhang, X.; Zhu, J. A Simple Method to Synthesize Si3N4 and SiO2 Nanowires from Si or Si/SiO2 Mixture. J. Cryst. Growth 2001, 233, 803−808. (21) Wu, J. J.; Wong, T. C.; Yu, C. C. Growth and Characterization of Well-Aligned nc-Si/SiOx Composite Nanowires. Adv. Mater. 2002, 14, 1643−1646. (22) Dai, L.; Chen, X. L.; Zhou, T.; Hu, B. Q. Aligned Silica Nanofibres. J. Phys.: Condens. Matter 2002, 14, L473. (23) Hu, J. Q.; Jang, Y.; Meng, X. M.; Lee, C. S.; Lee, S. T. A Simple Large-Scale Synthesis of Very Long Aligned Silica Nanowires. Chem. Phys. Lett. 2003, 367, 339−343. (24) Wei, Q.; Meng, G.; An, X.; Hao, Y.; Zhang, L. Synthesis and Photoluminescence of Aligned Straight Silica Nanowires on Si Substrate. Solid State Commun. 2006, 138, 325−330. (25) Kokai, F.; Inoue, S.; Hidaka, H.; Uchiyama, K.; Takahashi, Y.; Koshio, A. Catalyst-Free Growth of Amorphous Silicon Nanowires by Laser Ablation. Appl. Phys. A: Mater. Sci. Process. 2013, 112, 1−7. (26) Cliff, G.; Lorimer, G. W. The Quantitative Analysis of Thin Specimens. J. Microsc. 1975, 103, 203−207. (27) Schulmeister, K.; Mader, W. TEM Investigation on the Structure of Amorphous Silicon Monoxide. J. Non-Cryst. Solids 2003, 320, 143−150. (28) Sedel, H.; Crespregi, L.; Heuberger, A.; Baumgärten, H. Anisotropic Etching of Crystalline Silicon in Alkaline Solution. J. Electrochem. Soc. 1990, 137, 3612−3626. (29) Binary Alloy Phase Diagrams Vol. 2; Massalski, T. B., Murray, J. L., Bennett, L. H., Barker, H., Eds.; American Society for Metals: Metals Park, OH, 1986; p 1788. (30) CRC Handbook of Chemistry and Physics, 57th ed.; Weast, R. C., Ed.; CRC Press: Cleveland, OH, 1976; p B-155.

the targets, and elemental maps of the NWs and NPs, we conclude that Si-rich molten SiOx NPs are seeds and Si acts as a catalyst for the SiOx NWs’ growth, while the NWs attached to the NPs observed by TEM are final products after the growth process stops. The growth of SiOx NWs is explained by the precipitation of SiOx together with some processes, such as the supply of Si, SiO, O, and others to the molten NPs and their diffusion and precipitation. Oxidation of the growing SiOx NWs is also thought to be important.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +81-6-6879-7941. E-mail: [email protected] (K.K.). *Phone: +81-59-231-9422. E-mail: [email protected] (F.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.K. acknowledges the “Kakenhi (23510128)” Grant-in-Aid for Scientific Research provided by the Japan Society for the Promotion of Science in support for this work.

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dx.doi.org/10.1021/jp409039x | J. Phys. Chem. C 2013, 117, 25169−25174