Direct observation of melting behaviors at the nanoscale under electron beam
and heat to form hollow nanostructures†
Chun-Wei Huang,
aCheng-Lun Hsin,
aChun-Wen Wang,
aFu-Hsuan Chu,
aChen-Yen Kao,
aJui-Yuan Chen,
aYu-Ting Huang,
aKuo-Chang Lu,
cWen-Wei Wu*
aand Lih-Juann Chen*
bReceived 26th March 2012, Accepted 25th May 2012 DOI: 10.1039/c2nr30724c
We report the melting behaviours of ZnO nanowire by heating ZnO–Al2O3core–shell heterostructures
to form Al2O3nanotubes in an in situ ultrahigh vacuum transmission electron microscope
(UHV-TEM). When the ZnO–Al2O3core–shell nanowire heterostructures were annealed at 600C under
electron irradiation, the amorphous Al2O3shell became single crystalline and then the ZnO core
melted. The average vanishing rate of the ZnO core was measured to be 4.2 nm s1. The thickness of the Al2O3nanotubes can be precisely controlled by the deposition process. Additionally, the inner
geometry of nanotubes can be defined by the initial ZnO core. The result shows a promising method to obtain the biocompatible Al2O3nanotubes, which may be applied in drug delivery, biochemistry and
resistive switching random access memory (ReRAM).
Introduction
Nanomaterials with remarkable mechanical, field emission, electrical, magnetic, optical and chemical properties have been extensively investigated.1–4 At the nanoscale, the fundamental
properties of bulk materials can change. One of the most prominent examples is the decrease in melting point of nano-particles.5,6 The effects of size on the melting temperature of metallic nanoparticles embedded in an insulating, high melting temperature matrix, such as Pb nanoparticles in amorphous SiO2
or Al2O3 matrix, were investigated.7 In addition, the electron
beam irradiation was found to influence the structure modifica-tion of small Au particles,8due to the electron beam charging and heating of the nanoparticles. Several reports have addressed the importance of the irradiation of the high energy electrons,9which
can cause the physical effects on nanostructures and lead to interesting results in technological applications of these nanosystems.
Metal oxides have attracted great attention in the past decades. Among them, Al2O3 is one of the most important
materials for its potential applications in drug delivery, biochemistry,10,11 optics,12 resistive switching random access memory (ReRAM) and electronics. Several methods have been proposed to fabricate Al2O3 nanomaterials with a variety of
geometries, such as nanoparticles, nanorods, nanowires and nanotubes.13–15 Among them, the template-assisted method is
representative for fabricating nanotubes. Generally, the fabri-cation process involves three steps: first, formation of the template; second, formation of the core–shell nanostructures; finally, removal of the templates for the designed geometry.16
Porous anodic alumina,17 alumina nanotubes,18,19 cholesterol
nanotubes20 and carbon nanotubes21–23 have been used as templates. Various methods to remove the template were also investigated. Therefore, the understanding of the removal process is essential for the design of more exquisite nano-structures. For example, ZnO reacted with ammonia24 and
hydrogen at high temperatures.25Yang et al.26,27had studied the
irradiation-induced local etching mechanism. Meanwhile, for the purpose of direct examination, in situ transmission electron microscope (TEM) has been proven to be a powerful tool for atomic-level observation, including phase/shape transformation, molten nanofluidic migration and electromigration.28–40Here, we
report the observation of the irradiation of ZnO–Al2O3core–
shell nanostructures to form the Al2O3nanotubes in an
ultra-high-vacuum transmission electron microscope (UHV-TEM). The ZnO nanostructures were used as templates and removed via electron irradiation to form the Al2O3nanotube from the ZnO–
Al2O3core–shell nanostructure at 600C. The shell structure was
formed by atomic layer deposition (ALD) process. A new formation mechanism has been proposed and the effect of heat and electron irradiation were identified by the in situ TEM observation.
aDepartment of Materials Science and Engineering, National Chiao Tung
University, No.1001, University Rd., East Dist., Hsinchu City 300, Taiwan. E-mail: wwwu@mail.nctu.edu.tw; Fax: +886-3-5724727; Tel: +886-3-5712121-55395
bDepartment of Materials Science and Engineering, National Tsing Hua
University, Hsinchu 300, Taiwan. E-mail: ljchen@mx.nthu.edu.tw
cDepartment of Materials Science and Engineering National Cheng Kung
University, Tainan 701, Taiwan
† Electronic supplementary information (ESI) available: Proof of the electron irradiation effect and three in situ TEM videos as dynamic observation of the ZnO–Al2O3 core–shell nanowire heterostructures
forming the Al2O3 nanotube at 600C under electron irradiation and
the disappearance of the ZnO core. See DOI: 10.1039/c2nr30724c
Cite this: Nanoscale, 2012, 4, 4702
www.rsc.org/nanoscale
PAPER
Experimental
Here, single-crystalline ZnO nanowires were synthesized on a Si (100) substrate in a three-zone furnace using the carbothermal reduction method through the vapor–liquid–solid mecha-nism.41–45Before loading into the furnace, Au was deposited as catalysts with a high vacuum e-beam evaporator. Mixed powders of zinc oxide and carbon with a mass ratio of 2 : 1 was placed in an alumina boat at the high-temperature (950C) zone while the substrate was placed at the low temperature (750C) zone. The temperatures were elevated at the rate of 10C per min and held for 90 min. The furnace was then cooled down to room temperature. Argon and oxygen were introduced at flow rates of 100 and 10 sccm, respectively. The ZnO–Al2O3 core–shell
nanostructure was prepared with an ALD system. 25 nm thick alumina was deposited at 250 C with trimethylaluminum (TMA) as the source. The processing pressure of introducing the chemical precursors and Ar purging was 210 and 193 mTorr, respectively. In our processing, a total of 268 cycles was used, resulting in an alumina thickness of 25 1 nm, which corre-sponds to an average growth rate of 0.093 nm per cycle. A field emission scanning electron microscope (FESEM) JEOL JSM-6500F was used to examine the morphology of core–shell nanowires. High-resolution lattice imaging and line scan were performed with a JEOL 2100F TEM equipped with an energy dispersive spectrometer (EDS). In situ TEM observations were carried out in a JEOL 2000V UHV-TEM, which has a base pressure of 3 1010Torr. During the in situ TEM observation, the temperature was raised to 600 C to investigate the morphological change of the ZnO–Al2O3 core–shell nanowire
heterostructure with electron beam irradiation. The in situ TEM was equipped with a video recorder with 1/30 s time resolution.
Results and discussion
Fig. 1 is the schematic illustration of the nanotube formation process. The as-synthesized ZnO nanowire in Fig. 1(a) is grown by the vapor–liquid–solid growth process. The first step is to form the core–shell nanostructure with an ALD system (from (a) to (b)). Al2O3nanotubes could be achieved when the reaction
temperature was raised to 600C by electron irradiation on the
core–shell nanowire in the UHV-TEM. From Fig. 1(b) and (c), the core ZnO nanowire is shown to have vanished and the Al2O3
nanotube shell has crystallized.
A SEM image of the as-synthesized ZnO nanowires is shown in Fig. 2(a) and Fig. 2(b) depicts the high magnification SEM image of the as-synthesized ZnO nanowires in Fig. 2(a). The lengths of the nanowires are about 10–15mm and the diameters of the nanowires are in the range of 45–55 nm. Fig. 2(c) shows the SEM image of ZnO–Al2O3core–shell nanowires and Fig. 2(d) is
the corresponding magnified SEM image. A distinct contrast of the core–shell can be seen. After 268 cycles of Al2O3coating, the
uniform shells around the ZnO nanowires are evident.
Fig. 3 shows TEM images and EDS spectra which reveal the composition and structure of the core–shell nanowire. In Fig. 3(a), the image shows a smooth interface between the core and shell. The [0001] growth direction of the NW was identified by the diffraction pattern and the zone axis is [1210]. Fig. 3(b) is the HRTEM image corresponding to Fig. 3(a). The HRTEM image indicates the crystallinity of the single-crystalline ZnO core and the amorphous Al2O3shell. Both the fast Fourier transform
diffraction pattern (FFTDP) in the inset of Fig. 3(a) and the lattice spacing in Fig. 3(b) show that the ZnO core is single crystalline. The energy-dispersive X-ray spectra (EDS) line scan across the wire confirmed the presence elements of Zn, Al and O with a core–shell distribution.
The images in Fig. 4 were taken from the in situ TEM video, revealing the melting and vanishing of the ZnO core. During the in situ observation, the samples were found to be sensitive to the electron irradiation and the resulting energy deposition would cause significant perturbations in the system, including collective charge oscillations and irreversible atomic displacement.46The
electron irradiation on the core–shell nanowire is expected to give rise to energy transfer and heating of the nanowire. On the other hand, the Al2O3shell with high melting temperature will
withstand the heating, which melt the ZnO nanowire, like a heat barrier.47We can expect that a high interfacial energy between
Fig. 1 Schematic illustration of the formation process of Al2O3
nano-tubes. (a) Single-crystal ZnO nanowires grown by the vapor–liquid–solid mechanism, (b) ZnO–Al2O3core–shell nanowires coated with a uniform
Al2O3layer by ALD, and (c) Al2O3nanotubes formed by electron
irra-diation and heating.
Fig. 2 (a) Low magnification and (b) high magnification SEM images of ZnO nanowires on the silicon substrate. (c) Low magnification and (d) high magnification SEM images of the ZnO–Al2O3core–shell nanowires.
Inset shows the ZnO core and Al2O3shell with a sharp interface. The
image contrast was reversed to highlight the core–shell structure.
Al2O3 and ZnO could be the driving force of the observed
reaction. Furthermore, the ZnO nanowire alone was intact under the same irradiation condition (ESI, Fig. S-1†). For comparison, we have designed an additional experiment to anneal ZnO–Al2O3
core–shell nanowires under the same heating conditions but without electron irradiation, for which no change was observed. The results from the series of experiments indicate that electron irradiation and the concurrent presence of the Al2O3shell are
crucial for the melting of the ZnO core.
For phase transition from solid to liquid,48 based on the
thermodynamic model for the prediction of melting point reduction of the solid phase, surface melting will occur at the surface tension transition point (ESI, Video-1†). This result is well coordinated with the report of size dependent melting of nanocrystals.49In Fig. 4(a), it is clear that the melting occurred at the interface between the ZnO core and the Al2O3shell. When the
ZnO surface layer became liquid, the droplet would coalesce to change the morphology to minimize the interfacial energy (Fig. 4(b) and (c)).
Since the ZnO–Al2O3core–shell nanowire was detached from
the substrate by ultrasonication, the ZnO–Al2O3 core–shell
nanowire was broken at the bottom of the as-grown samples. Then the ZnO nanowire is enclosed by the Al2O3shell with a
opening at one end. The molten ZnO was drained by the chamber vacuum. Then the molten ZnO droplets receded very quickly along the length of the wire, leaving the coarse ZnO core empty and forming the nanotube. Therefore, the coarse ZnO core was formed (Fig. 4(c)). When the temperature was sufficient to melt the remaining ZnO core, the ZnO liquid is drained away quickly to the open side, as shown in Fig. 4(d). Eventually, the ZnO nanowire completely vanished, leaving the empty Al2O3
nano-tube (Fig. 4(e)). When the liquid ZnO drained away at the open end, it was expected to find it accumulated at one end of the nanotube (ESI, Fig. S-2†). We believe that it is to balance the pressure between the region inside the tube and the outside vacuum environment. A schematic illustration depicting the reaction process is presented in Fig. 4(f) (the green part is ZnO and the other is Al2O3).
Fig. 5 shows the plot of the length of hollow portion as a function of reaction time. It is apparent that there are two different stages. The first stage corresponds to the heating and melting of the ZnO core under the electron irradiation. In the mean time, the Al2O3 shell functions as a protective cover to
prevent the outflow of the ZnO as its temperature approaches the melting point. After 150 s of preheating, the vanishing rate of ZnO increased under electron irradiation. This is attributed to the fact that the ZnO core was liquidized completely, increasing the fluidity (ESI, Video-2†). Based on our results, the average vanishing rate of ZnO core has been measured to be 4.2 nm s1.
Additionally, the ZnO core was liquidized and always dis-appeared along the [0001] growth direction, leaving clear and straight single crystalline Al2O3nanotubes at high temperature
(ESI, Video-3†).
The microstructure and composition of the nanotube were characterized by TEM. Fig. 6(a) shows the low magnification image of the Al2O3nanotube, revealing the hollow structure. The
Fig. 3 (a) TEM image of a ZnO–Al2O3core–shell nanowire. Inset shows
the FFT diffraction pattern of the ZnO core. (b) A HRTEM image of the single crystalline ZnO core and the amorphous Al2O3shell. (c) EDS line
scan of the core–shell nanowire. The energy-dispersive X-ray spectra line scan across the nanowire also confirmed the presence of elements of Zn, Al and O with a core–shell distribution. Al shows a gorge-like intensity profile, being consistent with the core–shell nanostructure.
Fig. 4 A series of images showing the formation process. The melting occurred at the interface between the ZnO core and the Al2O3shell. (a)–(c)
Phase transition of ZnO from solid to liquid. (d) and (e) ZnO liquid drains away and the Al2O3nanotube was formed. The remaining ZnO in the core
was in the solid phase. (f) Schematic illustration of the formation process. First, the melting of ZnO occurs at the ZnO–Al2O3interface; then the
liquid ZnO drains away at the open end. Finally, the ZnO nanowire completely disappears and the empty Al2O3nanotube is in place.
Fig. 5 Plot of the length of vanishing ZnO as a function of time. There are two different stages. The first stage corresponds to the heating and melting of the ZnO core under the electron irradiation. After 150 s of preheating, the vanishing rate of ZnO increased under electron irradiation.
EDS line scan in Fig. 6(a) explicitly shows the chemical distri-bution after the reaction. The intensity profile demonstrates the tubular Al2O3 structure and confirms the absence of Zn,
indi-cating that the ZnO core has been completely removed and only Al2O3 nanotube remains. The HRTEM image in Fig. 6(b)
depicts the crystalline structure of the Al2O3 nanotube, the
d(111)¼ 0.47 nm and d(002)¼ 0.45 nm. With electron irradiation at
600C for 10 min, the amorphous Al2O3shell was crystallized. It
was found that the Al2O3nanotube could be formed under high
intensity of irradiation at an appropriate heating temperature. Based on the computational modeling of the FFT diffraction pattern and crystal plane spacing from the HRTEM image, the alumina shell was confirmed to be of cubic phase.
The melting was found to be different from the previous report.50–55No Kirkendall effect between ZnO core and Al
2O3
shell was detected in our work since the outer Al2O3shell was
intact. Otherwise, the outer shell could have the composition of ZnxAlyOzinstead of Al2O3. Moreover, the electron irradiation
effect was different from the report by Yang et al.26 In their
report, the shell of the nanotube was composed of original amorphous alumina and a number of embedded ZnO. In our result, single crystalline Al2O3was formed and no Zn element
found in the Al2O3shell layer via local etching of the ZnO core by
electron-beam irradiation. It is worthwhile to mention that the Al2O3 nanotube can be considered as a representative shell
utilizing the ZnO nanowire as a template. A variety of ZnO nanostructures has been synthesized by a number of growth methods. It is feasible to fabricate clean, flat and hollow Al2O3
nanostructures by ALD on inner ZnO core with various geom-etries (ESI, Fig. S-3†). The controllable formation process will be essential in future applications of the nanoscale cylindrical capacitors, drug delivery and optical transmission.
Conclusions
In summary, we have fabricated single crystalline Al2O3
nano-tube and investigated the interface melting of ZnO–Al2O3core–
shell structure under electron irradiation in UHV-TEM at 600C. The average vanishing rate of ZnO core was measured to be 4.2 nm s1. The geometry of Al2O3nanotube can be controlled
by the inner ZnO template. The amorphous Al2O3nanostructure
can become crystalline by the electron irradiation and heating. The study demonstrates a promising method to obtain the biocompatible Al2O3hollow nanostructures, which may play a
crucial role in nanomedicine, biochemistry, ReRAM and MEMS units in nanoelectronics.
Acknowledgements
W.W.W., L.J.C. and K.C.L. acknowledge the support by National Science Council through grants 2628-E-009-023-MY3, 99-2120-M-007-011, 98-2221-E-007-104-2628-E-009-023-MY3, and 100-2628-E-006-025-MY2.
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