In this article, direct coating of ZnO on PECVD-grown multi-walled carbon nanotubes (MWCNTs) is achieved using atomic layer deposition (ALD). Transmission electron microscopy investigation shows that the deposited ZnO shell is continuous and uniform, in contrast to the previously reported particle morphology. The ZnO layer has a good crystalline quality as indicated by Raman and photoluminescence (PL) measurements. We also show that such ZnO layer can be used as seed layer for subsequent hydrothermal growth of ZnO nanorods, resulting in branched CNT-inorganic hybrid nanostructures. Potentially, this method can also apply to the fabrication of ZnO-based hybrid nanostructures on other carbon nanomaterials.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are

Chu, DP

Fan, HJ

Li, C

Li, XL

Milne, WI

Zhang, Y

PubMed

In this article, direct coating of ZnO on PECVD-grown multi-walled carbon nanotubes (MWCNTs) is achieved using atomic layer deposition (ALD). Transmission electron microscopy investigation shows that the deposited ZnO shell is continuous and uniform, in contrast to the previously reported particle morphology. The ZnO layer has a good crystalline quality as indicated by Raman and photoluminescence (PL) measurements. We also show that such ZnO layer can be used as seed layer for subsequent hydrothermal growth of ZnO nanorods, resulting in branched CNT-inorganic hybrid nanostructures. Potentially, this method can also apply to the fabrication of ZnO-based hybrid nanostructures on other carbon nanomaterials

Apollo (Cambridge)

NANO EXPRESSAtomic Layer Deposition of ZnO on Multi-walled CarbonNanotubes and Its Use for Synthesis of CNT–ZnOHeterostructuresX. L. Li • C. Li • Y. Zhang • D. P. Chu •W. I. Milne • H. J. FanReceived: 7 June 2010 / Accepted: 26 July 2010 / Published online: 7 August 2010 The Author(s) 2010. This article is published with open access at Springerlink.comAbstract In this article, direct coating of ZnO on PEC-VD-grown multi-walled carbon nanotubes (MWCNTs) isachieved using atomic layer deposition (ALD). Transmis-sion electron microscopy investigation shows that thedeposited ZnO shell is continuous and uniform, in contrastto the previously reported particle morphology. The ZnOlayer has a good crystalline quality as indicated by Ramanand photoluminescence (PL) measurements. We also showthat such ZnO layer can be used as seed layer for sub-sequent hydrothermal growth of ZnO nanorods, resulting inbranched CNT–inorganic hybrid nanostructures. Poten-tially, this method can also apply to the fabrication of ZnO-based hybrid nanostructures on other carbon nanomaterials.Keywords ZnO  Atomic layer deposition  Carbonnanotubes  Hybrid nanomaterials  Hydrothermal  SolarcellsIntroductionCarbon nanotube (CNT) and inorganic composite materialshave attracted much attention recently due to their potentialapplication such as in photocatalyst, gas sensors, superca-pacitors, and field emission devices [1]. It has been reportedthat the optical and electrical properties of CNT–inorganiccomposites can be enhanced compared to the individualconstituents [2]. For example, CNT films when employed asconducting scaffolds in a TiO2 based photoelectrochemicalcell showed an enhancement of the photoconversion effi-ciency by a factor of two [3].CNT–ZnO represents one of most important members ofthe CNT–inorganic composites family due to the fact thatZnO is a n-type semiconductor with a direct wide band gap(3.37 eV) and large exciton binding energy (60 meV) [4].For instance, an ultrafast nonlinear optical switchingbehavior has been observed from ZnO-coated CNTs [5].The coaxial heterostructured nanotubes with a p-channelCNT combined with an n-channel ZnO shell may beintegrated into logical inverters [6].Various synthesis strategies based on physical andchemical process toward CNT–inorganic hybrids havebeen established so far, as summarized in recent reviewarticles [1, 7]. ALD is a cyclic self-limiting depositionmethod, which is capable of conformal and uniform coat-ing of thin films at the atomic level. It has been applied todeposit a variety of materials including oxides and metalson various nonplanar high-aspect-ratio substrates [8]. ALDon CNTs has been an interesting topic, but there are rela-tively few reports compared to ALD on inorganic orpolymers nanostructures. ALD coating CNTs with contin-uous amorphous Al2O3 layers has been reported by Kimet al. [9]. However, as for direct ALD ZnO on CNTs, the sofar available reports show only ZnO nanoparticles ormorphological poor-defined ZnO layer. While such poorly-defined tube–particle structure is shown useful for fieldemission applications [10, 11], more homogeneous coat-ings are desirable for CNT-based photonic devices. Kimet al. [12] achieved relatively smooth ZnO coating on CNTusing a thin ALD alumina buffer layer. However, theX. L. Li  H. J. Fan (&)Division of Physics and Applied Physics, School of Physicaland Mathematical Sciences, Nanyang Technological University,21 Nanyang Link, Singapore 637371, Singaporee-mail: fanhj@ntu.edu.sgC. Li  Y. Zhang  D. P. Chu  W. I. MilneElectrical Engineering Division, Engineering Department,University of Cambridge, 9 JJ Thomson Avenue,CB3 0FA Cambridge, UK123Nanoscale Res Lett (2010) 5:1836–1840DOI 10.1007/s11671-010-9721-zexistence of an Al2O3 buffer layer breaks the direct contactof ZnO to the CNTs and thus prevents the charge transfer, aprocess needed for the functions of photoelectrochemicalcells. Furthermore, the low optical quality of such ZnOlayers, as seen from the weak UV emission, will hinder thephotonic application of such hybrid nanostructures [12].In this work, we report the direct ALD of ZnO on ver-tical-aligned multi-walled carbon nanotube arrays. Theresulting ZnO layers have a well-defined morphology andhigher smoothness compared to the discontinuous nano-particles in previous reports. We also demonstrate that thedeposited ZnO layer can be used as a seed layer for thehydrothermal growth of ZnO nanorods. This provides anew method for the fabrication of CNT–ZnO three-dimensional (3-D) hybrid nanostructures, which might beuseful as photoelectrochemical anode materials. The PLproperties of the ZnO-coated CNTs and CNT–ZnO 3-Dnanotrees will be discussed.Experiment DetailsThe vertical-aligned MWCNT arrays were grown byplasma-enhanced chemical vapor deposition (PECVD)reported elsewhere [13]. ALD of ZnO was conducted usinga Beneq system (TFS 200) at 200C using diethylzinc(Zn(C2H5)2, DEZ) and water as the zinc and oxygensource, respectively. High purity N2 was the process gas inour experiment. During the deposition, the reactionchamber was maintained at 1.0 mbar with a steady N2steam at 200 SCCM (cubic centimeter per minute). EachALD cycle consisted of a 250-ms precursor pulse and 10-spurging time with N2. The relatively short precursorexposures and long purging times were adopted in order toachieve uniform coatings on the closely stacked CNTsarrays. The shell thickness was controlled by the numbersof the ALD cycles. A typical deposition consists of 80cycles. For ALD of alumina (AlxOy), trimethylaluminum[Al(CH3)3] and water were used as aluminum and oxygensource, respectively. A thickness of 7 nm was obtainedfrom 60 cycles.ZnO nanorods growing on vertical-aligned MWCNTswere synthesized using the standard hydrothermal method.The ALD ZnO-coated CNTs substrate was immersed into a35-mL aqueous solution of equimolar zinc nitrate[Zn(NO3)26H2O] and hexamethylenetetramine (C6H12N4)in an autoclave. The reaction was conducted at 95C for5 h. After reactions, the substrate was removed from thesolution, rinsed with deionized water, and dried with N2.The morphology of the as-fabricated samples wascharacterized using a JEOL JSM-6700F field emissionscanning electron microscope (FESEM) and a JEOL JEM-1400F transmission electron microscope (TEM). PL andRaman measurements were taken with a Renishaw systemusing 325- and 532-nm laser as the excitation source,respectively.Results and DiscussionFigure 1a shows the TEM image of the typical morphologyof the PECVD CNTs prior to deposition. Most of thenanotubes are multiwall tubes, and the average diameter ofthe CNTs is about 7 nm. Figure 1b shows the typical TEMimage of ALD ZnO-coated CNTs. Clearly, the depositedZnO shell is continuous and uniform along the tube. TheZnO shell thickness is about 18 nm, corresponding to agrowth rate of 0.22 nm per cycle. This value is in consis-tent with the regular ALD ZnO process [14–16]. Figure 1dand e show the SEM images of the ZnO-coated alignedCNTs. As can be seen, the ALD process did not affect theoverall alignment of the CNTs and that CNTs on the wholesubstrate area were coated with a ZnO shell. For compar-ison, the result of direct ALD of 7 nm AlxOy on the sameCNTs is also shown in Fig. 1e. As expected, the amor-phous AlxOy layer exhibits long range continuity andsmoothness. The growth rate of AlxOy is 0.13 nm per cycle,which is consistent with the previous report [16].There are several factors that affect the morphology ofthe ALD ZnO shell on CNTs. The first one is the surfaceconfiguration of the CNTs. As a micromolecular form ofcarbon, CNT can be regarded as graphitic layers (sp2-hybridized carbon atoms) rolled up into a cylindrical form.A perfect CNT is chemically inert. However, there gener-ally exist defects on the tube wall, such as bending in thenanotube, the finite size of crystalline domains, sp3-hybridized bonds, or functional groups created by oxida-tion [17, 18]. These defects or functional groups make theCNT surface reactive to the atomic species of an ALDprecursor. The Raman spectrum of our PECVD MWCNTs(Fig. 2 curve a) shows a strong D band, indicating theexistence of defects on the tube wall. Compared toMWCNTs, the surfaces of the single-walled carbon nano-tubes (SWCNTs) are known to have less structural defectsor impurity sites on the tube walls. This is the reason whyALD on SWCNTs is generally more challenging than onMWCNTs. The same argument holds true for deposition ongraphene. In the ALD work by Min et al. [10], ZnO par-ticles were deposited on the SWCNTs. It is most likely thatthe nanoparticles were formed selectively on the defectivesites or impurities on the nanotubes wall, which providechemisorptions sites for DEZ molecules.The second factor is the ALD processing parameters. Itis known that one ALD cycle consists of two half-chemicalreactions. After each-half cycle, the excess precursor needsto be thoroughly purged out; otherwise, it will contribute toNanoscale Res Lett (2010) 5:1836–1840 1837123a chemical vapor deposition (CVD) reaction. With theoccurrence of CVD reactions, the growth rate will behigher than a regular ALD process. In our experiment, toexclude the unwanted CVD reaction, we used short pre-cursor exposures and long purging times. The growth rateof ALD ZnO, 0.22 nm/cycle, in our experiment is com-parable to the previously reports based on DEZ and water[14, 15], indicating that there is negligible CVD reaction inour case. Furthermore, the alignment of CNTs also matters.Note that in Ref. [11] and [12], the authors used randomlyoriented MWCNTs for ZnO ALD. Compared to the verti-cal-aligned CNTs in our cases, there is much less freespace between the tubes, which makes the purge of theexcess ALD precursors after each semi-cycle difficult.Subsequently, additional CVD reactions may occur. Thiscould explain why only poor-defined ZnO agglomerateswere observed on CNTs in previous work [11, 12]. It is alsonoted that the growth rate (0.35 nm/cycle) was muchhigher than the regular ALD process (0.22 nm/cycle),further implying the occurrence of additional CVD reac-tions in their experiment [12]. In our experiment, we usedvertical-aligned MWCNTs and a longer purge time toexclude possible CVD reactions. This contributes to theimproved conformity of ALD ZnO.Lastly, ZnO tends to crystallize and texture alongc-orientation even at low temperatures. As predict from thefirst-principle simulations, an uncompensated polarityexists in ultrathin ZnO films [19]. The involvement of thepolarity of ZnO nanoclusters during the ALD explains whythe ZnO shells are not as smooth as the amorphous AlxOyon CNT, as seen from the TEM image (Fig. 1e) and pre-vious reports [9, 12].Raman spectrum of CNTs prior to (curve a in Fig. 2)and after (curve b in Fig. 2) ALD ZnO were measured. Asseen, a G band appears at about *1580 cm-1 corre-sponding to sp2-hybridized carbon and a D band at*1346 cm-1 originating from disordered carbon [17, 18].In addition to the D band, the D0 band as a shoulder of theFig. 2 Raman spectrum of CNTs before (a) and after (b) ALD ofZnO. Inset: Raman spectrum in the range of 150–650 cm-1 of ZnO-coated CNTsFig. 1 a TEM image of PECVD-grown CNTs. b TEM image of ALD ZnO-coated CNTs. c and d SEM images of ALD ZnO-coated CNTs.e TEM image of ALD AlxOy-coated CNTs1838 Nanoscale Res Lett (2010) 5:1836–1840123G band appears at about 1614 cm-1, which also originatesfrom the disorder features due to the finite size effect of thecrystalline domain or lattice distortion [18]. The intensityratio of the D0 over the G band (ID0/IG) increases with adecrease in the graphite crystalline domain. The D bandand D0 band peak are very strong, indicating that the CNTswe used have considerable number of defects on the sur-face. After ALD ZnO those D band, D0 band and G bandpeaks changed only slightly. Additional Raman peaks fromthe ZnO shells appeared; the relatively strong peaks at 427and 567 cm-1 correspond to E2high and A1 (LO) modes,respectively, and the peaks at 199, 321, and 1106 cm-1 areattributed to 2E2low, E2high-E2low, and 2LO, respectively.These Raman peaks are consistent with the previousreports [20].Figure 3 shows the room-temperature PL spectrum ofALD ZnO-coated CNTs, in which the UV emission peak atabout 390 nm (3.18 eV) corresponds to the near-band-edgeemission of ZnO crystal. An intensive broad visible emis-sion due to defects or impurities is also observed. It appearsto be a superposition of two main components at *560 and*630 nm, a feature similar to the PL of ZnO nanowirereported by Fan et al. [21]. Emission in the green spectrarange is commonly observed in bulk and nanostructure ZnOand the origin is still under debate [22, 23]. The orange–redemission is generally associated with oxygen interstitial[24]. Compared to the only available report so far on PL ofALD ZnO [12], the UV to visible emission ratio of oursample is significantly higher. That might be due to the highcrystalline quality of the ZnO shell in our case.Low-temperature hydrothermal growth is a popularmethod for synthesizing ZnO nanorods on any type ofsubstrates. Based on hydrothermal growth of ZnO onnanostructured substrates, hierarchical heterogeneousnanowires can be realized [25–27]. A prerequisite for ZnOhydrothermal growth of nanorods is the seed layer, and thequality of the seed layer (e.g., crystallinity, smoothness,orientation). Here, we demonstrate that the deposited ZnOshell can also be used as seed layer for hydrothermalgrowth of ZnO nanorods on CNTs, without any furtherannealing process. Figure 4a shows the schematics of thegrowth processes of the CNT–ZnO 3-D hybrid structure.Figure 4b shows the SEM images of the synthesized CNT–ZnO 3-D hybrid structure. The densely packed ZnOnanorods are aligned roughly perpendicularly to the axis ofthe tubes. The size of the branched ZnO nanorods is about30 nm in the diameter and several hundred nm in length.Figure 4c shows the room-temperature PL spectrum ofCNT–ZnO 3-D hybrid structure. It shows a strong UV peakFig. 3 Room-temperature PL spectra of ALD ZnO-coated CNTsFig. 4 a Schematics of the growth process, b SEM image, and c PLspectrum of the CNT–ZnO branched nanostructure. Inset: magnifiedSEM imageNanoscale Res Lett (2010) 5:1836–1840 1839123positioned at 385 nm with full width at half maximum(FWHM) of about 21 nm and a low and broadened peak inthe visible range. This is consistent with the generallyobtained PL spectra of ZnO nanorods growth by hydro-thermal methods [27]. Compared with the ALD ZnO seedlayer on CNTs, the UV emission intensity and the ratio ofUV/Visible peak of the CNT–ZnO 3-D hybrid structure aremuch higher. This is not surprising as the ZnO nanorodsare single crystalline, while the ALD ZnO is a polycrys-talline thin layer.ConclusionDirect ALD of ZnO thin layers on PECVD-grown CNTshas been successfully achieved without pretreatment of theCNTs. The deposited ZnO shell is continuous and uniformalong the tube long axis. Raman and PL studies reveal thatthe ZnO shells are of reasonably good crystalline quality,in contrast to the ALD ZnO shell on CNTs through anAl2O3 buffer layer in the previous report. Also we havedemonstrated that the ALD ZnO can be used as seed layerfor hydrothermal growth of ZnO nanorods on CNTs,forming a CNT–ZnO 3-D hybrid nanostructure, whichcould be useful materials for electronic or energy-relatedapplications.Open Access This article is distributed under the terms of theCreative Commons Attribution Noncommercial License which per-mits any noncommercial use, distribution, and reproduction in anymedium, provided the original author(s) and source are credited.References1. D. Eder, Chem. Rev. 110, 1348 (2010)2. J.M. Haremza, M.A. Hahn, T.D.K.S. Chen, J. Calcines, Nano.Lett. 2, 1253 (2002)3. A. Kongkanand, R.M. Domı´nguez, P.V. Kamat, Nano. Lett. 7,676 (2007)4. J. Zhong, A.H. Kitai, P. Mascher, W. Puff, J. Electrochem. Soc.140, 3644 (1993)5. Y.W. Zhu, H.I. Elim, Y.L. Foo, T. Yu, Y.J. Liu, W. Ji, J.Y. Lee,Z.X. Shen, A.T.S. Wee, J.T.L. Thong, C.H. Sow, Adv. Mater. 18,587 (2006)6. R. Martel, T. Schmidt, H.R. Shea, T. Hertel, P. Avouris, Appl.Phys. Lett. 73, 2447 (1998)7. D. Tasis, N. Tagmatarchis, A. Bianco, M. Prato, Chem. Rev. 106,1105 (2006)8. M. Knez, K. Nielsch, L. Niinisto¨, Adv. Mater. 19, 3425 (2007)9. J.S. Lee, B. Min, K. Cho, S. Kim, J. Park, Y.T. Lee, N.S. Kim,M.S. Lee, S.O. Park, J.T. Moon, J. Cryst. Growth 254, 443 (2003)10. Y.S. Min, E.J. Bae, J.B. Park, U.J. Kim, W. Park, J. Song, C.S.Hwang, N. Park, Appl. Phys. Lett. 90, 263104 (2007)11. J.M. Green, L. Dong, T. Gutu, J. Jiao, J.F. Conley Jr, Y. Ono,J. Appl. Phys. 99, 094308 (2006)12. D.S. Kim, M.S. Lee, R. Scholz, M. Knez, U. Go¨sele, J. Fallert,H. Kalt, M. Zacharias, Appl. Phys. Lett. 93, 103108 (2008)13. S. Hofmann, M. Cantoro, B. Kleinsorge, C. Casiraghi, A. Parvez,J. Robertson, C. Ducati, J. Appl. Phys. 98, 034308 (2005)14. A.W. Ott, R.P.H. Chang, Mater. Chem. Mater. Phys. 58, 132(1999)15. E. Guziewicz, I.A. Kowalik, M. Godlewski, K. Kopalko, V. Osinniy,A. Wojcik, S. Yatsunenko, E. Lusakowska, W. Paszkowicz,M. Guziewicz, J. Appl. Phys. 103, 033515 (2008)16. J.W. Elam, S.M. George, Chem. Mater. 15, 1020 (2003)17. W.Z. Li, H. Zhang, C.Y. Wang, Y. Zhang, L.W. Xu, K. Zhu,S.S. Xie, Appl. Phys. Lett. 70, 2684 (1997)18. A. Hirsch, Angew. Chem. Int. Ed. 41, 1853 (2002)19. J. Goniakowski, C. Noguera, L. Giordano, Phys. Rev. Lett. 98,205701 (2007)20. R. Cusco´, E. Alarco´n-Llado´, J. Iba´n˜ez, L. Artu´s, J. Jime´nez,B. Wang, M.J. Callahan, Phys. Rev. B 75, 165202 (2007)21. H.J. Fan, R. Scholz, F.M. Kolb, M. Zacharias, Appl. Phys. Lett.85, 4142 (2004)22. A.B. Djurisic, Y.H. Leung, Small 2, 944 (2006)23. M.D. McCluskey, S.J. Jokela, J. Appl. Phys. 106, 13 (2009)24. F.H. Leiter, H.R. Alves, A. Hofstaetter, D.M. Hofmann,B.K. Meyer, Phys. Status Solidi B 226, R4 (2001)25. S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84, 2287(1988)26. W.D. Zhang, Nanotechnology 17, 1036 (2006)27. C.W. Cheng, B. Liu, H.Y. Yang, W.W. Zhou, L. Sun, R. Chen,S.F. Yu, J.X. Zhang, H. Gong, H.D. Sun, H.J. Fan, ACS NANO 3,3069 (2009)1840 Nanoscale Res Lett (2010) 5:1836–1840123

Atomic Layer Deposition of ZnO on Multi-walled Carbon Nanotubes and Its Use for Synthesis of CNT-ZnO Heterostructures.

1840 Nanoscale Res Lett

Alarco ´n-Llado ´,

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2964460

Atomic Layer Deposition of ZnO on Multi-walled Carbon Nanotubes and Its Use for Synthesis of CNT-ZnO Heterostructures.

Abstract

Similar works

Full text

Available Versions

Apollo (Cambridge)