Influence of the catalyst volume on the morphological transition of SiOx nanostructures

Accepted Manuscript Influence of the catalyst volume on the morphological transition of SiOx nanostructures Sun-Woo Choi, Han Gil Na, Suyoung Park, Seon Jae Hwang, Myeong Soo Cho, Changhyun Jin PII:

S0925-8388(16)32883-3

DOI:

10.1016/j.jallcom.2016.09.131

Reference:

JALCOM 38965

To appear in:

Journal of Alloys and Compounds

Received Date: 18 February 2016 Revised Date:

9 September 2016

Accepted Date: 12 September 2016

Please cite this article as: S.-W. Choi, H.G. Na, S. Park, S.J. Hwang, M.S. Cho, C. Jin, Influence of the catalyst volume on the morphological transition of SiOx nanostructures, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.09.131. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Influence of the catalyst volume on the morphological transition

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of SiOx nanostructures

Changhyun Jinb,* a

Sensor System Research Center, Korea Institute of Science and Technology, 14-gil 5 Hwarang-

Department of Materials Science and Engineering, Inha University, Incheon 402-751, Republic

of Korea c

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ro, Seongbuk-gu, Seoul 136-791, Republic of Korea b

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Sun-Woo Choia,‡, Han Gil Nab,c,‡, Suyoung Parkd, Seon Jae Hwange, Myeong Soo Choe,

Advanced Materials and Manufacturing Technology, Inha University Business Incubator,

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Incheon 402-751, Republic of Korea d

Helmut-Fischer Korea, 462, Dogok-ro, Songpa-gu, Seoul 05574, Republic of Korea

e

Inha Analytical Instrumentation Center, Inha University, 253 Yonghyeon-dong, Incheon 402-

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751, Republic of Korea

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S.-W.C. and H.G.N. contributed equally as first authors. * Correspondence to: [email protected]

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ABSTRACT

Two different-sized SiOx nanowires and microtubes were grown from the surface of Si substrate,

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according to the distribution of Sn nanoparticles on the Si substrate. The numerous Sn nanoparticles obtained from pre-deposited SnO2 films on the tube play two roles simultaneously: the first is to act as a type of catalyst for promoting a rapid nucleation and growth, and the

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second is to decrease the melting point of the Si substrate. In order to maintain the phase equilibrium, changes at a morphological (nanowire, microtube, and split loops) and elemental

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compositional (aggregation and dispersion) level are involved in the Sn volume.

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1. Introuduction Using a conventional gas-phase transport-based approach [1,2], all types of onedimensional (1D) semiconducting oxide materials have been fabricated in the shape of

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nanorods [3-5], nanopillars [6], nanobelts [7-9], nanoribbons [10-12], nanowires (NWs) [13-15], and nanotubes [16]. Compared to their bulk equivalents, 1D semiconducting oxide nanostructures have attracted increasing attention in the fields of transistors [17,18],

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sensors [19,20], electrodes [21,22], solar cells [23,24], light emitting diodes [25,26], and so on. This is mainly due to their extraordinary properties, including high surface-to-

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volume ratio, defect-free crystals, and quantum confined structures of state. Even though many different approaches have been studied in order to modify the micro/nanosized morphological characteristics, the vapor-liquid-solid (VLS) method, especially without pre- or post-treatment steps (such as catalyst and annealing), may be the most

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fundamental and useful technique to obtain the desired geometry of semiconducting oxides. However, conventional VLS mechanism is still difficult to control the precise variability of light powder-shaped precursors due to their easy sublimation and evacuation

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corresponding to given process temperature and inlet gas pressure, respectively. Therefore, other more sophisticated synthetic strategies, i.e., superlattice [27], core-shell

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[16], hierarchical [28], doped [29], and tubular [30] structures have been developed in order to achieve the satisfactory results. However, these approaches also suffer from several drawbacks, namely additional functionalization treatments, expensive costs for extra processes, and unexpected mechanical damage from post-manipulation during the entire procedure.

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In this study, we present the synthesis of micro and nanosized SiOx (1
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as catalyst to produce the final SiOx structures. Furthermore, no additional Si source or catalysts were needed in this system. The role of Sn NPs, the effect of supporting powders, and their

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reactions are discussed with regard to the morphological transitions.

2. Experimental procedure

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The simple experimental procedures were shown in SI (Fig. S2). Firstly, SnO2 thin films were synthesized onto a horizontal alumina tube furnace using Sn powders (3 g) and oxygen (10 cm3 min–1). No substrates or catalysts were used because the objective was to form a SnO2 thin film on the inside surface of the alumina tube. The furnace was

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maintained at 900°C for 1 h. In order to make a SnO2 film thick enough to supply Sn NPs, the same cycle was repeated over 10 times. The system was cooled before each subsequent cycle and the thickness of SnO2 films for one cycle was approximately 100

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nm. Then, this preformed thick film may cause the carbothermal reductive dissociation of SnO2 with graphite at high temperature (over 1050°C) as follows [31]. (Eq. 1)

SnO2 + CO → SnO + CO2

(Eq. 2)

SnO + CO → Sn + CO2

(Eq. 3)

SnO + 2C → Sn + CO2

(Eq. 4)

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SnO2 + C → SnO + CO

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The Sn NPs may also lead to SiOx structures resulting from lowering the melting point of the Si substrate in order to maintain a Si-Sn phase equilibrium at different temperatures and compositions [32].

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Secondly, a p-type Si(100) substrate was placed upside-down on the alumina boat at the center of the tube furnace after deposition of SnO2 films onto horizontal alumina tube as described above. The as-synthesized micro and nanosized SiOx tubes and wires,

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respectively, were fabricated using supporting materials such as In2O3 (1 g) and graphite (0.1 g) powders in the alumina boat under the same thermal evaporation method.

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Specifically, In2O3 powders may provide the possibility of Sn4+ substitution instead of In3+ sites, accelerating the SiOx nucleation and growth [33]. As the carrier gas, pure Ar (200 cm3 min–1) was used to maintain a 0.1 mTorr total internal pressure in the tube at 1100°C for 1 h. After completing the entire process, the furnace was cooled to room

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temperature and the original Si substrate has been changed to white-colored SiOx structures due to binary equilibrium between Sn and Si [32]. In order to determine the morphology of the samples, field emission scanning electron

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microscopy (FE-SEM, Hitachi S-4200, 20 kV) was used. The microstructures of the SiOx microtubes and NWs were characterized by transmission electron microscopy (TEM,

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Phillips CM-200, 200 kV) and high-resolution TEM (HR-TEM) techniques. X-ray diffraction (XRD, Philips X’pert MRD) was also carried out using a Cu-Kα radiation to identify the structural properties of the samples. Energy-dispersive X-ray (EDX) analysis was performed for elemental composition. For the TEM preparation, detailed procedure is as follows: (1) Samples were collected after scratching the surface of Si substrate. (2) Collected samples placed into a capsule for hardening the samples with epoxy. (3)

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Capsule was cut up to 100 nm thick by ultramicrotomy (RMC MTX) without chemical contamination. (4) Sections were left floating on deionized (DI) water. (5) Sections retrieved from the water were mounted on a Cu grid. (6) Final samples were dried in the

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oven.

3. Results and discussion

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SEM images of the contact area morphology between Sn particles and Si substrate are shown in Fig. 1. At first, traces of both tetragonal and spherical structures (about 2 µm in

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size) are randomly formed on the surface of the Si substrate (Fig. 1(a)). After the surface of the Si substrate collapsed, several Sn aggregations with neighboring particles were exposed. As mentioned above [31], numerous Sn NPs originate from pre-deposited SnO2 films. From the high-magnification SEM image shown in Fig. 1(b), a spherical Sn particle

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embedded in the Si substrate is observed, forming a crater-like pattern. The EDX result of spherical Sn particle is shown in SI (Fig. S3). Then, at the interface between the Si substrate and the Sn particle, SiOx bundles formed due to the lowering of the Si melting

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point according to the Si-Sn phase diagram [32], lifting the Sn particle. The SEM image shown in Fig. 1(c) indicates that the Sn particle shown in Fig. 1(b)

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grew slowly along the axial direction changing from a spherical into an ellipsoidal shape. Both SEM images (Fig. 1(b-c)) suggest that not only the nucleation and growth processes consisted of ternary elements (Si-Sn-O), but most compositions contain only Si and O due to the small solid state solubility of Sn atoms under certain process conditions [32]. Interestingly, Sn NPs may occasionally detach from the Si substrate (Fig. 1(d)) possibly due to high inlet gas pressures, or local fluid turbulence. Fig. 1(d) also illustrates a crater-

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like configuration at the upper part, as well as a detachment of the Si surface at the lower part. Fig. 2 shows SEM images of the as-synthesized SiOx NWs grown from dispersed small

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Sn NPs. The observed randomly oriented, wire-like nanostructures have diameters of a few tens of nanometers and lengths of a few tens of micrometers. Fig. 2(a) presents typical SiOx NWs in a damaged area, while Figs. 2(b-c) display the uniform SiOx NWs.

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These partially nucleated regions indirectly suggest that the Sn NPs supply is influenced by process conditions such as temperature, gas pressure, and concentration of Sn. In

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addition, the growth mechanism of most NWs can be considered as VLS, because only the Si surface contacted with Sn (acting as a type of catalyst where nanostructures start to grow), even if there is no evidence of a specific image of a tip in this study. A series of enlarged SEM images (Figs. 2(d-f)) show 1D structures without any aggregated or three-

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dimensional arrangements. No significant difference in morphology was observed among the samples, with the exception of a few thicker and longer NWs (Fig. 2(e)). The EDX results (SI (Fig. S4)) confirm that wire-like nanostructures are consisted of SiOx structures

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with numerous Sn nanoparticles.

However, in the case of bigger Sn NPs (due to Sn aggregation) ranging from 2 µm to

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11 µm (SI (Fig. S5), in other words, when the diffusion rate of the Sn NPs was faster, leading to the aggregation in one point (i.e., boundary between Sn and Si substrate indicated by yellow circles in Fig. 3(a) or tip regions (tip areas) of SiOx structures indicated by red circles) the morphology of SiOx structures possessed a tubular shape (Figs. 3(b-d)). In order to decrease the grain boundary area (surface energy) of these Sn NPs, grain boundary migration and grain growth of Sn NPs occurred at both regions

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(boundary and tip areas) in the form of an aggregation phenomenon as seen in SI (Fig. S6). When Sn aggregation at the bottom leads to tubular SiOx formation (Figs. 3(b-d)), the tip regions, having higher Sn concentrations compared to that of other regions, are not

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allowed to grow along the axial direction due to a much larger and heavier Sn aggregated tip. Therefore, the additional energy is entirely consumed by splitting up the SiOx microtubes into various loops at the head region. These morphological characteristics

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differ completely from those of SiOx NWs in Fig. 2. The main reason capable of explaining the difference between wires and tubes is primarily based on the Sn catalyst

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volume size. Apart from morphological transition, the SEM images shown in Figs. 3(b-d) imply three critical possibilities: firstly, the morphology of the Sn tip is divided corresponding to the direction and number of split loops, and consequently, needle holelike SiOx structures are formed; secondly, we could observe that the grown SiOx samples

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have tubular structures assembled with several loops; and thirdly, the axial bonding force in each loop is relatively stronger than the radial bonding force. Under an excess of energy, and after the formation of bigger Sn tips, extremely split

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SiOx morphologies are observed (Fig. 4). The structures (Figs. 4(a-b)) appear to have a spade-like profile, split into two parts. The hollow inner part is in good agreement with

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the result shown in Fig. 3. Based on the enlarged SEM images shown in Figs. 4(c-d), the final tubular SiOx structures, having about <2 µm diameter and < 500 nm, respectively, also revealed that the combination of two tubular SiOx structures could form a new and bigger SiOx microtube. Hence, both split morphologies for the SiOx structures are possible, and can originate from the top and bottom simultaneously, depending on the mechanism that is more predominant.

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To understand the mechanism of Sn-embedding into the SiOx matrix, the TEM results shown in Fig. 5 include the low-magnification, high-magnification, and HR-TEM images, respectively. From the low-magnification TEM image (Fig. 5(a)), the tubular morphology

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of the SiOx structure reveals a gradually diminishing diameter from <5 µm at the bottom (or top) to the top (or bottom), maintaining a narrowing tendency. There is no significant difference compared to previous SEM images. The microstructural properties of SiOx

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tubes are shown in high-magnification (Fig. 5(b)) and HR-TEM (Fig. 5(c)). The randomly distributed black peapod-like Sn NPs are observed in the SiOx matrix. The average size of

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Sn NPs ranges between 1 and 3 nm. In addition, the clear fringes of the Sn NP in Fig. 5(c) indicate that the Sn NP is monocrystalline, while the SiOx matrix has an amorphous structure, because of the absence of regular fringe patterns. The interplanar spacing in monocrystalline Sn was 0.292 nm and it was indexed to the (200) lattice plane of

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tetragonal Sn (JCPDS 04-0673). In order to support this microstructural analysis related to both amorphous SiOx and crystalline Sn, the XRD patterns are added to SI (Fig. S7), even though a small amount of In remains in the final samples.

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On the other hand, the randomly dispersed Sn peapods in the SiOx matrix play a catalyst role for the new-produced SiOx nanostructures. These Sn peapods act as sites for nucleation and

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growth of secondary SiOx nanostructures, irrespective of their locations, as seen in SI (Fig. S8). Therefore, we can conclude that precise information on catalyst volume for Si-based structures, using this new synthetic approach, can allow many other micro/nanomaterials to be appropriately synthesized for suitable applications in micro/nanoscience and technology.

4. Conclusions

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Based on the volume differences of Sn NPs obtained from pre-deposited SnO2 films, we were able to adjust precisely the three different types of SiOx structures. The Sn NPs play two roles simultaneously in the SiOx formation: the first is to act as a type of catalyst, and the second is to

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decrease the melting point of Si on the substrate. When small Sn NPs are used, a SiOx structure with a wire-like morphology is obtained. However, bigger Sn particles produce SiOx structures with a tube-like morphology. Specifically, tubular SiOx structures have a needle hole-like and a

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spade-like split shape, when an excess of energy is provided to the samples. With this new synthetic method, it is possible embed Sn NPs into the SiOx matrix, where Sn peapods can cause

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the nucleation and growth of secondary SiOx nanostructures. This technique enables various semiconducting oxide materials to have unprecedented new morphologies and functionalizations.

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Acknowledgement

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This study was supported by the KU Research Professor Program of Konkuk University.

Appendix A. Supplementary data

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Supplementary data associated with this article can be found, in the online version, at [details of any supplementary data available should be include here].

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Figure captions Fig. 1. SEM images of the surface of Si substrate: (a) randomly distributed Sn particles on the Si

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substrate, (b) spherical Sn particle on the Si substrate, (c) evolution of the Sn particle, (d) craterlike pattern after Sn detachment.

Fig. 2. SEM images of SiOx NWs grown from Si substrate: (a-c) low-magnification, (d-f) high-

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magnification.

Fig. 3. SEM images of needle hole-like SiOx microtubes grown from the Si substrate: (a)

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aggregated Sn particles and SiOx microtubes, (b-d) high-magnification SEM images of SiOx microtubes with split loops at the head region.

Fig. 4. SEM images of spade-like split SiOx microtubes: (a-b) typical spade-like split SiOx

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microtubes, (c-d) high-magnification SEM images of split SiOx microtubes at the bottom region. Fig. 5. TEM images of SiOx microtubes: (a) typical SiOx microtube image, (b) enlarged Sn NPs

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in the SiOx matrix, (c) HR-TEM of (b).

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Fig. 1. SEM images of the surface of Si substrate: (a) randomly distributed Sn particles on the Si substrate, (b) spherical Sn particle on the Si substrate, (c) evolution of the Sn particle, (d) craterlike pattern after Sn detachment.

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Fig. 2. SEM images of SiOx NWs grown from Si substrate: (a-c) low-magnification, (d-f) highmagnification.

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Fig. 3. SEM images of needle hole-like SiOx microtubes grown from the Si substrate: (a) aggregated Sn particles and SiOx microtubes, (b-d) high-magnification SEM images of SiOx microtubes with split loops at the head region.

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Fig. 4. SEM images of spade-like split SiOx microtubes: (a-b) typical spade-like split SiOx microtubes, (c-d) high-magnification SEM images of split SiOx microtubes at the bottom region.

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Fig. 5. TEM images of SiOx microtubes: (a) typical SiOx microtube image, (b) enlarged Sn NPs in the SiOx matrix, (c) HR-TEM of (b).

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Supplementary Information (SI)

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Figure S1. Elemental compositions of SiOx structures at three different regions.

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Figure S2. Schematic illustration showing two different synthetic processes of the SnO2 thin

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films and SiOx nanostructures.

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Figure S3. Elemental composition at the spherical particle.

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Figure S4. Elemental composition of wire-like SiOx structures with Sn nanoparticles.

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Figure S5. Histogram of different micro-sized Sn particles.

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Figure S6. Schematic diagram for grain boundary migration and growth.

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Figure S7. XRD pattern for the Sn-pea-podded SiOx microtubes.

Figure S8. SEM images of secondary SiOx nanostructure formation: (a) new-produced SiOx nanostructures over the entire SiOx microtubes, (b) enlarged view of new-produced SiOx structures.

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Highlights Two different-sized SiOx nanowires and microtubes were investigated.




We have studied the catalyst volume effects in amorphous SiOx growth mechanism.




The Sn peapods in SiOx structures have resulted from pre-deposited SnO2 films.

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