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Chemical Engineering Journal 170 (2011) 326–332

Contents lists available at ScienceDirect

Chemical Engineering Journal

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imply heating to remove the sacrificial core TeO2 nanowires and to generateubular nanostructures of metal oxides

youn Woo Kima,∗, Han Gil Nab, Ju Chan Yangb

Division of Materials Science and Engineering, Hanyang University, 17 Haengdang Dong, Seongdong Gu, Seoul 133-791, Republic of KoreaDivision of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea

r t i c l e i n f o

rticle history:

a b s t r a c t

This paper reports a novel method for fabricating nanotubes, in which the nanowire-templates were

eceived 21 October 2010eceived in revised form 2 March 2011ccepted 16 March 2011

eywords:ellurium oxide

removed by simple heating in air. The morphology and structure of the composite nanowires werecontrolled by varying the heating temperature. High-temperature heating was found to be effectivein removing the core TeO2 nanowires. The photoluminescence study indicated that the emission of thecore-shell nanowires originated mainly from the ZnO shell. The increased visible emission with increas-ing heating temperature was attributed to the increase in the number of structural defects in the ZnO

n ban

anostructureshotoluminescence

shell. The visible emissio

. Introduction

Since the discovery of carbon nanotubes in 1991 [1], one-imensional (1D) nanostructured materials, including nanorods,anowires and nanotubes, have received considerable attention onccount of their remarkable physical properties and potential appli-ations. Among the 1D structures, tubular or hollow nanostructuresave attracted particular interest for their special technologicalpplications due to not only the superior optical, electrical, ther-al and mechanical properties, but also to their high efficiently

nd activity caused by their high porosity and large surface area2].

Many researchers have prepared inorganic nanotubes through aariety of approaches, including plasma discharge [3], the rolling upf thin films [4], thermal evaporation [5,6], hydrothermal process7], sono-electrochemical methods [8], anodization [9,10], and cat-lyzed transport reaction [11]. Despite their scientific significancend potential applications, the in situ fabrication of nanotubesas some shortcomings, in which precise control of the nan-tube dimensions is limited. Accordingly, a casting method haseen illuminated, in which nanostructures with hollow interi-rs are commonly prepared by coating the surfaces of templatesith thin layers of the desired materials, followed by selective

emoval of the templates through wet chemical etching [12,13]r calcination [12,14,15]. However, wet chemical methods haveeveral disadvantages, including incomplete removal of the tem-late, difficulty in achieving a small enough diameter, and possible

∗ Corresponding author. Tel.: +82 10 8428 0883.E-mail address: hyounwoo@hanyang.ac.kr (H.W. Kim).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.03.066

d was blue-shifted with increasing annealing temperature.© 2011 Elsevier B.V. All rights reserved.

environmental pollution. A range of calcination methods have beenimplemented. Caruso et al. employed polystyrene latex particles astemplates, which were removed by exposure to tetrahydrofuran[12]. Ji et al. used iced lipid nanotubes as templates, in which sub-sequent sol–gel process/calcination resulted in the transition metaloxide nanotubes [15]. Goldberger et al. removed the ZnO nanowiretemplates in NH3 or H2 ambient [16]. These methods require a com-plicated process with a precisely controlled scheme or an expensivegas.

This communication reports the fabrication of metal oxidenanotubes using a simple heating method. Compared to theconventional calcination method, this technique provides a larger-process-window, low-cost, clean and simpler process. In thismethod, TeO2 nanowires were used as a sacrificial template, whichcan be removed easily by a simple heating technique (Fig. 1). To ourknowledge, this is the first report of the use of TeO2 nanowires asa sacrificial template for the formation of nanotubular structures.Ultimately, hollow ZnO nanotubes were obtained. Zinc oxide (ZnO)is a promising material for applications as transparent electrodes[17], gas sensors [18], acousto-optical devices [19], nanolasers[20], piezoelectric devices [21] and short-wavelength optoelec-tronic devices owing its wide direct band-gap of 3.37 eV and alarge exciton binding energy of 60 meV. ZnO is also a biosafe andbiocompatible material that can be used directly for biomedicalapplications without a coating [22].

In particular, ZnO nanotubes with a hollow structure will have a

variety of potential applications. The large thickness ratio of thewall to the diameter suggests good field emission performancesimilar to the large aspect ratio of carbon nanotubes [23]. A largesurface area–volume ratio in ZnO tubular structures would pro-vide an effective way of optimizing the performance of a range of

H.W. Kim et al. / Chemical Engineerin

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S-2 (Supplementary materials) presents the diameter distribution

ig. 1. Schematic outline of the process sequence for the generation of metal oxideanotubes.

evices including dye-synthesized photovoltaic cells [24], catalysts,hotocatalysts [25], gas sensors [26], hydrogen storage [27], lightmitting diodes [28,29], and biosensors [30].

. Experimental details

.1. Synthesis of core-shell nanowires and subsequent thermaleating

Core TeO2 nanowires were prepared by heating the source Teowders in a high-temperature vertical tube furnace in N2 ambientt a flow rate of 2 standard liters per min (slm) (Fig. 1). A schematiciagram of the experimental apparatus is reported elsewhere [31].e powder (99.9%-purity) was used as the source material andlaced at the lower holder in the center of the quartz tube. Thei plate coated with a 3-nm Au layer was placed in the upperolder, which acted as a substrate for collecting the growth prod-cts. The Au layer was sputtered by ion sputterer (Emitech, K757X).he vertical distance between the powders and the substrate was

pproximately 7 mm. The substrate was set to 400 ◦C in a nitrogenN2) gas flow, in which the gas flow rate was 2 standard liter per

in (slm). After a typical 1 h deposition process, the substrate wasooled and removed from the furnace.

g Journal 170 (2011) 326–332 327

Atomic layer deposition (ALD) of the ZnO shell layers on the coreTeO2 nanowires was carried out using diethylzinc (DEZn) and H2Oas the Zn and O precursors, respectively (Fig. 1). The vaporizationtemperature of DEZn and H2O was set to 35 and 18 ◦C, respectively[32]. Ar carrier gas with flow rates of 100 sccm and 0 sccm was usedto deliver the DEZn and H2O to the chamber, respectively. Ar gaswith a flow rate of 400 sccm was used for the line and chamberpurge. The DEZn injection time was 5 s, which was divided into 1 sfor sourcing without the carrier gas and 4 s for delivery with an Argas flow rate of 100 sccm. The H2O injection time was 5 s under avariety of power conditions of remote VHF ICP (27.12 MHz, 200 W).A sufficient time of 15 s was used to purge the sources so as toprevent intermixing of the precursor and reactant. The number ofALD cycles was set to 100. The substrate temperature was 200 ◦C.Subsequently, the core-shell nanowires were thermally annealedin air ambient at temperatures ranging from 400 to 800 ◦C (Fig. 1).The pressure was maintained at less than 2 Torr by using the rotarypump.

2.2. Characterization

Scanning electron microscopy (SEM) images were obtainedusing a Hitachi S-4200. For SEM observation, the samples werecoated with Au using a turbo sputter coater (Emitech K575X,Emitech Ltd., Ashford, Kent, UK). Glancing angle (0.5◦) X-ray diffrac-tion (XRD) was performed using a Philips X’pert MPD system withCuK�1 radiation. Transmission electron microscopy (TEM) andenergy-dispersive X-ray (EDX) spectroscopy were carried out usinga Philips CM 200 operating at 200 kV. The PL spectra were mea-sured at room temperature on a SPEX-1403 photoluminescencespectrometer using the 325 nm line from a He–Cd laser (Kimon,Japan).

3. Results and discussion

Fig. 2a shows an SEM image of the TeO2 nanowires used as tem-plates, indicating the large-scale production of the nanowires. TheXRD pattern of the TeO2 nanowires in Fig. 2b shows that almostall peaks could be indexed to tetragonal tellurium oxide with lat-tice constants of a = 4.81 A and c = 7.61 A, which are consistent withthe standard literature values (JCPDS no. 42-1365). The enlargedSEM image and low-magnification TEM image showed that thenanowires had maintained their circular-rod-like and continuousmorphology despite the shell coating (Fig. 2c and d). On the otherhand, the TeO2/ZnO core-shell nanowires exhibited XRD peakswith respect to tetragonal TeO2, orthorhombic TeO2 (JCPDS no.09-0433) and hexagonal ZnO (JCPDS no. 36-1451) (Fig. 2e). Accord-ingly, the ZnO shell layer was comprised of a crystalline phase. XRD(Fig. 2b and e) confirmed the conversion of a tetragonal TeO2 to anorthorhombic TeO2 structure at approximately 200 ◦C, which wasthe substrate temperature during the ALD process. This is in con-trast to previous reports in which an orthorhombic to tetragonalphase transition in TeO2 thin films occurred during thermal anneal-ing [33]. S-1 (Supplementary materials) shows the lattice-resolvedTEM image and selected area electron diffraction (SAED) pattern ofthe TeO2/ZnO core-shell nanowires, confirming that the shell layerwas comprised of a hexagonal ZnO phase.

Subsequently, thermal annealing was carried out at 400, 600and 800 ◦C. Fig. 3 shows the changes in the structure and morphol-ogy of the TeO2/ZnO core-shell nanowires after thermal annealing.

of the samples annealed at 400, 600 and 800 ◦C based on sta-tistical analysis of the SEM images. The maxima of the diameterdistributions of the samples annealed at 400, 600 and 800 ◦C were130–140 nm, 80–90 nm, and 70–80 nm, respectively. The mean

328 H.W. Kim et al. / Chemical Engineering Journal 170 (2011) 326–332

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ig. 2. (a) SEM image and (b) XRD pattern of the as-synthesized TeO2 nanowires. (he TeO2/ZnO core-shell nanowires.

iameter of the nanowires annealed at 400, 600 and 800 ◦C, whichas calculated from the graph of the diameter distributions, was

31 nm, 96 nm and 85 nm samples, respectively. The nanowireiameter decreased with increasing annealing temperature.

The XRD pattern of 400 ◦C-annealed sample on the right-and-side of Fig. 3a revealed the presence of tetragonal TeO2,rthorhombic TeO2, rhombohedral TeO3 (JCPDS no. 85-0747), andubic Au (JCPDS no. 04-0784). Some TeO2 phase may have trans-ormed to TeO3 by thermal annealing at 400 ◦C. In addition, the XRDatterns of the samples annealed at 600–800 ◦C showed no signs ofellurium oxide-related peaks (Fig. 3b and c).

Fig. 4a shows a low magnification TEM image of the core-shellanowire annealed at 400 ◦C. Fig. 4b shows the corresponding SAEDattern, indicating that it is comprised of diffusion rings, whichre associated with hexagonal ZnO, orthorhombic TeO2 and rhom-ohedral TeO3. The lattice-resolved TEM image shown in Fig. 4cevealed the lattice planes of hexagonal ZnO and rhombohedraleO3.

On the other hand, the 600 ◦C-annealed nanowires (Fig. 4d)xhibited a contrasted image with the core and shells as bright andark regions, respectively. The line concentration profiles shown

n the upper-right inset showed that the relative amount of Te wasonsiderably smaller than Zn, indicating that the proportion of Teas reduced significantly by thermal annealing at 600 ◦C. The SAEDattern (Fig. 4e) and lattice-resolved TEM image (Fig. 4f) coinciden-ally indicated that the 600 ◦C-annealed sample consisted mainlyf the hexagonal ZnO phase. In addition, the ring-like SAED patternevealed the samples to be polycrystalline.

Fig. 5a is a low-magnification TEM image of the core-shell

anowires annealed at 800 ◦C. The nanostrucure had a relativelyough surface. In conjunction with S-2, it is believed that the nanos-ructure was silghtly contracted and deformed. Furthermore, someoids were observed in the nanostructure. Fig. 5b is a correspond-ng line concentration profile that exhibits a valley-like profile

image and (d) TEM image of a TeO2/ZnO core-shell nanowire. (e) XRD pattern of

for Zn and O with a negligible amount of Te. Both SAED patterns(Fig. 5c) and lattice-resolved TEM images (Fig. 5d) showed that thenanostructure consisted mainly of a polycrystalline hexagonal ZnOstructure.

It is unclear why the core TeO2 nanowires could be removedefficiently by thermal heating. The vaporization of solid TeO2 canbe activated at higher temperatures. Since the contribution of allother tellurium-bearing gas species, such as (TeO2)2, TeO, (TeO)2,Te2, to the total pressure was estimated to be <5% [34], it wassurmised that the vapor consisted mainly of monometric TeO2(g)species. Expression (1) shows the temperature dependence of theTeO2 vapor pressure [35].

TeO2(s) = TeO2(g)

log p (atm) = 8.067 − 12, 000T

(846 < T < 1006 K) (1)

The vapor pressures of TeO2 at 400 and 600 ◦C are 1.7 × 10−10

and 2.1 × 10−6 atm, respectively, suggesting a drastic increase inthe TeO2 vapor pressure at higher temperatures. This indicates thata variety of materials with an acceptably low vapor pressure can bea candidate for sacrificial templates.

Since the boiling temperature of TeO2 is 1245 ◦C, we reveal thatthe TeO2 nanowires have been evaporated at temperatures whichare considerably lower than their boiling temperature. We surmisethat this observation is due to the size effect, in which the evapo-ration temperature of nanocrystals is found to be size-dependentand decreases with decreasing size, i.e. diameter of nanoparticlesand nanorods, or thickness of thin films [36]. A TGA curve from

the bundles of TeO2 nanowires tested at 760 Torr indicates thatthe evaporation occurred at about 816 ◦C, which is considerablylower than their boiling temperature (1245 ◦C) (Supplementarymaterials, S-3). By the way, in the present paper, the evaporationof TeO2 nanowires seemed to start below 800 ◦C. The heating of

H.W. Kim et al. / Chemical Engineering Journal 170 (2011) 326–332 329

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Fig. 3. SEM images and associated XRD patterns of the TeO2/Z

he TeO2/ZnO core-shell nanowires was carried out at a pressuref <2 Torr, with the rotary pump. On the contrary, the TGA experi-ents (S-3) were performed at a pressure of 760 Torr. Accordingly,e expect that the actual TGA curve of the TeO2/ZnO core-shellanowires in the present paper will shift to the left, in compari-on to the curve of TeO2 nanowires tested at 760 Torr. Under theower ambient pressure, TeO2 is supposed to evaporate at a loweremperature.

Fig. 6 shows the PL spectra of various samples measured at roomemperature. The PL spectrum of the uncoated TeO2 nanowiresorresponds to the band centered at approximately 2.7 eV in thelue region (Supplementary materials, S-4). A similar blue bandas been observed with TeO2 crystals, which is probably excitonic

n nature [37]. In terms of Gaussian fitting analysis, the PL spec-

rum of the ZnO-coated TeO2 nanowires was deconvoluted into twomission bands peaked at approximately 2.3 eV (green) and 3.2 eVultraviolet (UV)), respectively (S-4). This was attributed to the typ-cal emission from ZnO, with no blue emission from TeO2 beingbserved, presumably due to the covering effect. The UV emission

e-shell nanowires annealed at (a) 400, (b) 600, and (c) 800 ◦C.

band peaking at 3.2–3.3 eV corresponds to near band edge (NBE)emission, which is associated with the excitons in ZnO [38,39]. Thebroad green (or visible) emission band is related to emission fromthe deep trapping sites corresponding to defects, such as oxygenvacancies in ZnO [38–41].

Annealing at 400 ◦C did not noticeably change the PL spectrumof the as-synthesized sample. In terms of Gaussian fitting analy-sis, the PL spectra of the ZnO-coated TeO2 nanowires annealed at600–800 ◦C were deconvoluted into two emission bands at approx-imately 2.5 eV (green) and 3.2 eV (ultraviolet (UV)), respectively(S-4). The peak position of visible emission for ZnO-coated TeO2nanowires was blue-shifted to 2.5 eV after thermal annealing athigher temperatures (600–800 ◦C). The visible emission was quitebroad, covering a wide range of energy/wavelengths. The emission

of the sample annealed at 400 ◦C extended from approximately 1.7(red) to 2.6 eV (blue). In addition, the visible emission of the sam-ples annealed at 600–800 ◦C extended from approximately 2.0 (red)to 2.9 eV (blue). The yellow and green luminescence bands wereassociated mainly with VZn and OZn defects, respectively [42,43]. At

330 H.W. Kim et al. / Chemical Engineering Journal 170 (2011) 326–332

Fig. 4. (a) Low-magnification TEM image, (b) SAED pattern, and (c) lattice-resolved TEM image of TeO2/ZnO core-shell nanowires annealed at 400 ◦C. (d) Low-magnificationTEM image (inset: EDX concentration profile along the line drawing across the diameter of the coated nanowire), (e) SAED pattern, and (f) lattice-resolved TEM image ofTeO2/ZnO core-shell nanowires annealed at 600 ◦C.

H.W. Kim et al. / Chemical Engineering Journal 170 (2011) 326–332 331

F ine drawing across the diameter of a coated nanowire, (c) SAED pattern, and (d) lattice-r

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ig. 5. (a) Low-magnification TEM image, (b) EDX concentration profile along the lesolved TEM image of TeO2/ZnO core-shell nanowires annealed at 800 ◦C.

igher temperature, many oxygen atoms in air ambient enter the Znacancy to generate OZn. With increasing annealing temperature,ore oxygen atoms form OZn resulting in a gradual decrease in the

oncentration of VZn in the ZnO shell. Accordingly, at higher anneal-ng temperatures, the yellow emission may shift to green emission,ontributing to the overall blue-shift in the visible emission band.

Fig. 7 presents the relative intensities of the visible and UV emis-ion for various samples. Figs. 6 and 7 show that the intensity ofisible emission from the ZnO-coated TeO2 nanowires increasesith increasing annealing temperature, whereas that of UV emis-

ion is relatively unaffected by temperature. Accordingly, we reveal

ig. 6. PL spectra of (a) TeO2 nanowires and (b–e) TeO2/ZnO core-shell nanowires,hich were (b) as-fabricated, (c) 400 ◦C-annealed, (d) 600 ◦C-annealed, and (e)

00 ◦C-annealed.

Fig. 7. Relative intensities of visible and UV emissions for TeO2/ZnO core-shellnanowires.

that a larger number of structural defects in the ZnO shell has beengenerated during the annealing process at higher temperatures.

4. Conclusions

Tubular structures of ZnO were fabricated by heating ZnO-coated TeO2 nanowires. The ZnO-coated TeO2 nanowires wereproduced by coating ZnO shell layers on core TeO2 nanowires

by ALD. XRD showed that the core-shell nanowires annealed at600–800 ◦C were comprised only of a hexagonal ZnO phase. On theother hand, those heated at 400 ◦C revealed additional peaks fortellurium oxides. TEM and EDX showed that the 800 ◦C-annealedsample had contracted and deformed into a hollow tubular struc-

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32 H.W. Kim et al. / Chemical Engi

ure. The room-temperature PL spectra of the ZnO-coated TeO2anowires revealed visible and UV emission due to the ZnO shell.he intensity of visible emission increased with increasing anneal-ng temperature, whereas that of UV emission was relativelynaffected. This was attributed to an increase in the number oftructural defects in the ZnO shell by annealing at higher tem-eratures. The overall blue-shift of the visible emission band by

ncreasing the annealing temperature was attributed to the changen VZn to OZn defects.

cknowledgements

This research was supported by National Nuclear R&D Pro-ram through the National Research Foundation of Korea (NRF)unded by the Ministry of Education, Science and Technology2010-0018699).

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.cej.2011.03.066.

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