Effect of Size-Dependent Thermal Instability on Synthesis of Zn2 SiO4-SiO x Core–Shell Nanotube Arrays and Their Cathodoluminescence Properties
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Vertically aligned Zn2SiO4-SiO x (x < 2) core–shell nanotube arrays consisting of Zn2SiO4-nanoparticle chains encapsulated into SiO x nanotubes and SiO x -coated Zn2SiO4 coaxial nanotubes were synthesized via one-step thermal annealing process using ZnO nanowire (ZNW) arrays as templates. The appearance of different nanotube morphologies was due to size-dependent thermal instability and specific melting of ZNWs. With an increase in ZNW diameter, the formation mechanism changed from decomposition of “etching” to Rayleigh instability and then to Kirkendall effect, consequently resulting in polycrystalline Zn2SiO4-SiO x coaxial nanotubes, single-crystalline Zn2SiO4-nanoparticle-chain-embedded SiO x nanotubes, and single-crystalline Zn2SiO4-SiO x coaxial nanotubes. The difference in spatially resolved optical properties related to a particular morphology was efficiently documented by means of cathodoluminescence (CL) spectroscopy using a middle-ultraviolet emission at 310 nm from the Zn2SiO4 phase.
KeywordsNano-template Core–shell nanotube Cathodoluminescence Zinc Silicate Rayleigh instability Kirkendall effect
Nanotubes made of carbon and diverse inorganic compounds have continuously attracted significant attention due to their unique fundamental physical properties and many potential applications [1–3]. Inorganic nanotube generation strategy can generally be classified into two categories: first, one-step self-organization such as self-rolling and/or Ostwald ripening; second, two-step template-based fabrication through either scarification or recently developed solid-state reaction utilizing the Kirkendall effect [4–6]. From a viewpoint of final device integration, aligned nanotube arrays are highly desirable. In addition to the direct epitaxial growth of nanotube arrays on lattice-matched substrates, a solid-state reaction under thermal annealing and using readily available nanostructure arrays as templates could be an efficient way to generate novel chemically complex, multiphase nanotube arrays. However, due to a considerable increase in the surface-to-volume ratio with decreasing a nanomaterial size, size-dependent thermal stability and melting of the nanostructured templates during annealing must be carefully addressed.
ZnO nanowire (ZNW) arrays are one of the most common aligned nanostructures owing to their naturally preferable epitaxial growth at a relatively low temperature. They have widely been utilized as templates for the synthesis of lattice-matched GaN and SiC nanotubes, [7, 8] ZnO-related semiconducting heterojunctions [9, 10], and ZnO-based ternary compound nanostructures . In most cases the latter have been considered to be even more important than pure-phase ZnO ones [4–6]. Although solid-state reactions by thermal annealing based on a ZNW template have been used to synthesize ZnO-based ternary compound nanotubes, [4–6, 11] no research has been carried out on the influence of size-dependent template thermal stability on the final characteristics of such tubes.
In this study, this phenomenon was thoroughly demonstrated for the case of Zn2SiO4. While employing a one-step solid-state reaction and the ZNW array templates, vertically aligned Zn2SiO4-SiO x core–shell nanotube arrays (ZSO), i.e., Zn2SiO4-nanoparticle chains encapsulated in SiO x nanotubes, SiO x -coated polycrystalline and single-crystalline Zn2SiO4 coaxial nanotubes were simultaneously obtained. Furthermore, a cathodoluminescence (CL) study, as a noninvasive and high spatial-resolution characterization tool, was employed to detect and analyze local structural and optical properties of the nanostructures. The structural and optical differences were effectively identified by transmission electron microscopy (TEM) paired with CL spectroscopy. Finally, the size-dependent thermal instability induced formation mechanism was proposed.
The ZNW templates were grown on a ZnO-film-coated silicon substrate using a vapor phase transport, as we previously reported . The synthesis of ZSO nanotube arrays was carried out in a vacuum tube furnace with an outer diameter of 24 mm and a length of 1,200 mm. A 0.2 g powder mixture of SiO2 (Alfa Aldrich, 99.9%), activated carbon (as a reductant), and Si with equal molar ratios was placed at the center of the tube. A piece of the ZNW template was placed downstream of the tube at ~14 cm away from the source. The tube was sealed and evacuated to a base pressure of ~2 Pa. The furnace was then heated to 1,100°C at a rate of 24°C min−1 and kept at this temperature for 2 h. The local temperature of the substrate was about 1,000°C due to the temperature gradient along the tube furnace. A constant flow of high-purity Ar gas was fed into the tube at a flow rate of 80 sccm (standard cubic centimeters per minute) and a pressure of 100 Torr throughout entire heating/cooling. After the furnace was naturally cooled to room temperature, the surface color of ZNW templates changed from black to white–gray.
The ZNW templates and ZSO samples were characterized by a powder X-ray diffraction (XRD; Rigaku, Ultima III, 40 kV/40 mA with Cu K α radiation), a scanning electron microscope (SEM; JEOL, JSM-6700F), a high-resolution field-emission transmission electron microscope (TEM; JEOL, JEM-3000F), a high-angle annular dark-field scanning transmission electron microscope detector (HAADF-STEM) (JEOL JEM-3100FEF), and an energy-dispersive X-ray spectrometer (EDX). CL spectra were recorded at room temperature in an ultrahigh-vacuum SEM with a Gemini electron gun (Omicron, Germany) at an accelerating voltage of 10 kV and a current of 1 nA.
Results and Discussion
It will be of interest to investigate the formation mechanism during simultaneous generation of different tube structures, especially regarding the Zn2SiO4-nanoparticle chains, for the first time observed here in a ternary compound nanomaterial. We believe that nanoscale thermodynamics of ZNW is an important factor. It has been experimentally demonstrated that the melting point of Cu, Zn, and Sn nanowires will significantly decrease with a decrease in wire diameter [19–21]. Based on molecular dynamics simulations, the same diameter dependency has also been found for GaN nanowires . In addition, under heating in air, ZnO nanorods start to melt at 750°C,  much lower temperature than the melting point of a bulk form. Therefore, it is reasonable to assume that the melting point of ZNW would also decrease with diameter decreasing.
It is worth mentioning that by using plasma-enhanced chemical vapor deposition (PECVD) of a thin amorphous Si film (~10 nm) on the top half of a ZNW template followed by vacuum annealing, Zhou et al. fabricated vertically aligned Zn2SiO4 nanotube/ZnO nanowire heterojunction arrays. Our results undoubtedly demonstrate that prior to ZnO template utilization for reliable making ternary compound nanotubes, problems associated with their thermal instability must be seriously taken into account and solved. The good news is that due to many diameter-controllable synthesis methods of ZNWs reported to date, finding templates with uniform diameters for the subsequent unique one specific ternary nanotube syntheses looks highly plausible. Furthermore, the Rayleigh instability applied to ZNW provides a structuring technique that produces long chains of ZnO-based compound semiconducting nanospheres which may find potential applications in nanoscale photonic devices .
In conclusions, vertically aligned Zn2SiO4-nanoparticle chains encapsulated into SiO x nanotubes, SiO x -coated polycrystalline and single-crystalline Zn2SiO4 coaxial nanotubes were simultaneously fabricated by a one-step solid-state reaction using ZNW array templates. It is found that the nanotubes of different morphologies can be easily identified by CL spectroscopy mapping technique. The apparent size-dependent nonuniform nanotube generation was due to the strong size-dependent thermodynamic behavior of ZNWs. With diameter increasing of starting ZNW, the formation mechanism changes from decomposition of “etching”, to Rayleigh instability, and then to Kirkendall effect, resulting in polycrystalline Zn2SiO4-SiO x coaxial nanotubes, single-crystalline Zn2SiO4-nanoparticle-chain-embedded SiO x nanotubes, and single-crystalline Zn2SiO4-SiO x coaxial nanotubes, respectively.
Electronic Additional material
This work was supported by the World Premier International Center for Materials Nanoarchitectonics (MANA) Project tenable at the National Institute for Materials Science (NIMS), Tsukuba, Japan. The authors thank Drs. A. Nukui, M. Mitome, and Mr. K. Kurashima for the continuous technical support and kind help.
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