Vapor–Solid Nanotube Growth via Sidewall ... - ACS Publications

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Vapor−Solid Nanotube Growth via Sidewall Epitaxy in an Environmental Transmission Electron Microscope Zhengfei Zhang,‡ Jialin Chen,‡ Hengbo Li, Ze Zhang, and Yong Wang* Center of Electron Microscopy and State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China S Supporting Information *

ABSTRACT: The growth of metal oxide nanotubes has been widely investigated; however, the mechanism regarding how nanotubes form remains elusive due to the lack of real time growth information. Here we report the growth of W18O49 nanotubes in an environmental transmission electron microscope. The real time observation of the growth dynamics indicates that the W18O49 nanotube is formed via the sidewall epitaxial growth on the leader W18O49 nanowire, which is different from the mechanism of nanowires coalescence proposed previously. Furthermore, our in situ results demonstrate that higher oxygen pressure leads to the growth of nanotubes, but low oxygen pressure results in the growth of nanowires. Such nanotube growth is presumably ascribed to the maximization of heat dissipation during fast growth. These findings may enrich our present understanding of the growth dynamics of metal oxide nanotubes and provide insight for fabricating metal oxide nanotubes.

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ince the discovery of carbon nanotubes,1,2 considerable efforts have been put into studying the growth mechanism of nanotubes (including metal oxides and semiconductors) and exploring possible applications for the nanotubes with unique chemical and physical properties. Various growth methods have been then developed to synthesize tube-like materials, including a template method,3 vapor deposition,4−6 anodic oxidation,7,8 and thermal oxidation.9,10 Among these methods, the mechanism of nanotubes growing from a vapor source is rather complicated. Several mechanisms have been proposed, such as catalyst involved tip-growth11 or root-growth,12 screw dislocation driven growth,13 and nanowires coalescence.9,14 In particular, for catalyst-free growth of metal oxide nanotubes such as W18O49, In2O3, MoO3, ZnO, and IrO2, which can be realized through thermal oxidization or physical vapor deposition, convincing evidence for the growth mechanism is still absent. On one hand, Wang et al.9 grew W18O49 microtubes by thermally oxidizing tungsten, and they proposed a whiskers aggregation mechanism for the formation of tube structure. Jeong et al.14 reported the growth of ZnO microtubes, and a similar nanowires coalescence mechanism was proposed. On the other hand, Chen et al.15 generated IrO2 nanotubes on the LiTaO3 substrate, and they thought the spiral growth due to slow deposition rate led to the formation of the tube structure. Nevertheless, without direct evidence, a clear picture can be hardly acquired due to the lack of real time information during the growth. In situ environmental transmission electron microscopy (ETEM) equipped with a gas-heating holder is a powerful tool, which has been successfully employed to uncover some critical images of the dynamics and kinetics of materials’ shape and © XXXX American Chemical Society

structure changes especially in surface science16−21 and crystal growth.22−26 To obtain the unique growth dynamics and reveal the growth mechanism of nanotubes, the in situ ransmission electron microscopy (TEM) observation of nanotube growth in real time is therefore necessary.27,28 However, because of the experimental difficulties, previous reports seldom concern in situ study of the growth mechanism of metal oxide tubule materials. Herein, we report the growth of W18O49 nanotubes by thermal oxidation of tungsten (W) filament under 0.095 Pa pure O2 at ∼700 °C in an E-TEM. The in situ TEM video (Supporting Information) shows the dynamics of sidewall step flow of W18O49 nanowires, which indicates that the grown W18O49 nanotubes grow via a sidewall epitaxy way: sidewall steps nucleate one by one at the root of leader nanowire, followed by the quick epitaxial growth of these steps on the sidewalls of the leader nanowire, which finally results in the formation of a tubular structure. The effect of oxygen pressure on the formation of nanotubes is investigated, and it is proposed that heat dissipation during fast nanowire lateral growth may affect the morphology of the final nanostructure. The growth experiment was carried out in an environmental TEM (H9500, HITACHI, Japan) which was equipped with a gas-heating holder. First, the TEM holder mounted with a pure W filament (diameter ∼50 μm, length ∼4 mm, purity 99.99%, HITACHI, Japan) was introduced into TEM column. The W Received: October 8, 2016 Revised: December 5, 2016 Published: December 8, 2016 A

DOI: 10.1021/acs.cgd.6b01483 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 1. SEM images of nanotubes and nanowires grown on tungsten (W) filament. (a) Low magnification SEM image of W filament after thermal oxidation in TEM for 2 h. Temperature was ∼700 °C and oxygen pressure was ∼0.095 Pa. (b) SEM image of nanotube grown on W filament near the oxygen gas inlet after thermal oxidation. (c) SEM image of nanowires grown on the W filament. The area where nanowires grow is ∼1 mm away from oxygen gas inlet. (d) SEM image of the area of W filament surface which is ∼2 mm away from oxygen gas inlet. (e) Plot of oxygen pressure and the distance from nozzle of the gas inlet. Copyright 2013 Hitachi High-Technologies Corporation.

Figure 2. . (a) TEM image of a typical tube-like structure grown on W filament via thermal oxidation in TEM. Temperature was ∼700 °C and oxygen pressure was ∼0.095 Pa. Bottom inset is electron diffraction pattern of the nanotube. Top inset is high resolution TEM image of the nanotube indicating the nanotube grows along the [010] direction. (b−d) TEM images of the atomic sidewall steps on three main low-index facets of W18O49 nanowires: (001), (1̅01) and (100).

Figure 3. (a−f) Series of TEM images captured during the growth of a W18O49 nanotube. Temperature was ∼700 °C and oxygen pressure was ∼0.095 Pa.

filament was then heated to the set temperature by the Joule effect in which the real time temperature of the tungsten filament was calibrated by a calibration curve between temperature and electron current through W filament,29 and then pure oxygen was introduced into the W filament area through a gas inlet near the W filament. The heating temperature and oxygen pressure were maintained at 700 °C and 0.095 Pa during the growth, respectively. The accelerate voltage of electron beam was 300 kV, and the electron beam current density was kept low enough (∼2 × 104 A/m2) to make sure that the nanotube growth could be captured in real time without being disturbed by electron beam irradiation.

Figure 1 shows the SEM images of different sites on the W filament surface after thermal oxidation (700 °C, 0.095 Pa O2) for 2 h in TEM. Figure 1a is a low magnification SEM image of the W filament. The oxygen (gas) inlet is located near the left end of the W filament. In the region near the gas inlet, tube-like structures with a diameter range of 100−200 nm and length of several hundred nanometers are found (Figure 1b); these tubes have a thickness range of 20−80 nm. While in the area ∼1 mm away from the gas inlet, only nanowires with a diameter range of 20−50 nm can be found (Figure 1c). Neither nanotubes nor nanowires were found in the region that was more than ∼2 mm away from the gas inlet. In situ TEM observation confirms that B

DOI: 10.1021/acs.cgd.6b01483 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 4. Diagram for the formation process of nanotube. Inset at left is the predicted morphology shape of W18O49.

nanowire growth occurs first in the region of the W filament near the oxygen inlet. We note that the oxygen pressure is not uniform along the axis of W filament: with the increase of distance from the nozzle of oxygen gas, the oxygen pressure decreases dramatically, as shown in Figure 1e. It should be pointed out that heat gradient along the axis of the W filament is not expected because the pure W filament with uniform diameter is heated by the Joule effect. Therefore, the morphology difference should be caused by oxygen pressure. Figure 2a shows the TEM image and the selected area electron diffraction (SAED) of an incomplete nanotube grown on the W filament. The SAED pattern indicates the nanotube is a single crystal. In the top inset, the interval distance of 0.378 nm corresponds to the (010) of W18O49 (JCPDS #712450), indicating the nanotube grows along the [010] direction. Note that the top of the nanotube is not fully flat but consisted of a step, as marked by the white rectangle. Several atomic sidewall steps can be found on the low-index facets of W18O49 nanowires including (001), (1̅01), and (100), as shown in Figure 2, panels b−d, respectively. To reveal the dynamic growth process of W18O49 nanotubes, in situ TEM experiments were carried out. Figure 3 shows a series of TEM images recorded from a typical nanotube during the initial growth. It can be seen that several steps nucleate on the sidewall of the leader nanowire (Figure 3a). The height of these sidewall steps are various with each other within a range from single molecular layer to over 10 nm. These steps flow quickly to the tip of the leader nanowire (Figure 3b−f) and disappear. It should be pointed out that no extra parallel nanowires can be found around the target nanowire during the whole growth. The sidewall step nucleation takes place on more than one kind of sidewall facet (marked by yellow arrow in Figure 3a); this is necessary for the formation of hollow structure; otherwise only a lamella structure will form. The growth rate of leader nanowire is much smaller than that of the steps on the sidewall during these sidewall steps flowing, which is reasonable because the step edge is more attractive for vapor molecular adsorption compared to the tip terrace.30 The flow rate of steps can reach to ∼10 nm/s and the smaller step has a larger flow rate. Note that the entire tube formation process was not captured because the time for the tube formation is too long (more than 2 h) and the oxygen pressure in TEM is unable to fully meet the requirement of the growth condition in a conventional CVD furnace, where a large scale of tungsten oxide tube structure grows by a similar thermal oxidation method.9,31−34 However, it is reasonable to believe that the

grown tube structures (Figures 1b and 2a) are formed via the same way of sidewall epitaxial step flow based on the observed initial growth process. The growth mechanism of W18O49 nanowires through thermal oxidation of tungsten has been systematically investigated in our previous work,25 in which a vapor−solid mechanism is proved to dominate the nanowires growth. The present experiment shares the same conditions, such as the W filament and heating temperature, and thus the growth mechanism of W18O49 nanotubes is believed to be a vapor− solid mechanism too. The vapor source for nanotube growth is supposed to be WO3 molecular provided by the sublimation of WO3, which is produced by the oxidation of W above 600 °C.25,35 According to the series of in situ TEM images (Figure 3), the growth process of W18O49 nanotubes can be depicted as follows: (1) WO3 vapor deposits on the surface of W filament to nucleate leader W18O49 nanowire. (2) Additional nucleation events take place attached to the root of the leader nanowire which results in the formation of epitaxial sidewall steps; subsequently these steps flow quickly to the tip of leader nanowire. (3) More steps nucleate and grow on the sidewall, which finally leads to the formation of a tube-like structure, as shown in Figure 4. There comes the primary question of why nanotubes form. It has been reported that heat dissipation might play an important role in the morphology of fast growing nanostructures at high temperature.8 Nanotubes with extra internal surface provide a larger surface for heat dissipation than a bundle structure during growth. In our experiment, the source for nanowires and nanotubes growth is vaporous WO3 coming from the sublimation of the reaction product between W and oxygen. When oxygen pressure is low, the oxidation of W filament to WO3 is slow and so is the production of WO3 vapor, which limits the nucleation and growth rates of W18O49 nanocrystals. Therefore, only nanowires with a smaller diameter are formed. In the higher oxygen pressure region, the nucleation and growth of W18O49 crystals are faster due to the higher oxidation rate of W which will yield more sublimated WO3. The probability of sidewall step nucleation is also higher in a higher oxygen pressure region; i.e., the lateral growth of leader nanowire is prevalent. Because the nanostructure is a single structure throughout the growth, the lateral growth may be influenced by the requirement of heat dissipation to minimize the energy of the crystal; therefore, a hollow structure instead of a bundle structure will form. C

DOI: 10.1021/acs.cgd.6b01483 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of National Natural Science Foundation of China (Nos. 51390474, 11234011, 11327901). The authors are greatly indebted to Dr. Chenghua Sun at Monash University and Prof. Shengbai Zhang at Rensselaer Polytechnic Institute for their intensive discussion and valuable suggestion.

Another question is why nanotubes instead of lamella form as lamella may also provide sufficient surface for heat dissipation. Jeong et al. synthesized ZnO microtubes via a vapor−solid method. They thought that the ZnO microtubes would like to form a regular hexagonal prism shape with {01− 10} sidewall facets due to the stability requirement under high growth temperature. Our case might be attributed to a similar reason, though the growth mode observed in our experiment is discrepant with the nanowires coalescence mechanism. According to the morphological prediction,21 the stable shape of W18O49 is predicted to be hexagonal prism morphology with [010] being the dominated growth direction and three facets including (100), (001), and (10̅ 1) being sidewall facets. We have confirmed the atomic steps on (100), (001), and (1̅01) sidewalls (Figure 2b−d), which is in accordance with the morphological prediction that low indexed facets which have a small surface energy determine the final shape of crystal. During lateral growth, steps will tend to nucleate on these three sidewall facets. The nucleation of each sidewall, which is driven by the chemical potential difference between vaporous WO3 and W18O49 crystal, might be stochastic, but the whole nanostructure with a single crystal has the requirement of energy minimization. Therefore, in consideration of the heat dissipation (say 700 °C high temperature in our experiment) and energy minimization of the whole structure during fast growth, the sidewall steps growth will result in the formation of a hollow nanostructure rather than a lamella or bundle structure. In summary, the growth of W18O49 nanotubes by thermal oxidation of tungsten filament was investigated by an E-TEM. In situ TEM observations manifest that the nanotubes form via a sidewall epitaxial growth way, in which crystal steps nucleate one by one by vapor deposition at root of leader nanowire and then grow epitaxially from the bottom to the top of leader nanowire. In particular, higher oxygen pressure is favorable for forming a tube-like structure, which is attributed to the requirement of maximal heat dissipation during fast crystal growth. The proposed growth mode is supposed to be ubiquitous in other oxide nanotubes and microtubules formation via a similar method.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01483. Real-time TEM video showing the side wall step flow during the formation of a single W18O49 nanotube at ∼700 °C in 0.095 Pa oxygen (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-0571-87952228. ORCID

Yong Wang: 0000-0002-9893-8296 Author Contributions ‡

Z.Z. and J.C. contributed equally.

Funding

National Natural Science Foundation of China (Nos. 51390474, 11234011, 11327901) Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.cgd.6b01483 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.cgd.6b01483 Cryst. Growth Des. XXXX, XXX, XXX−XXX