Super-long aligned TiO2/carbon nanotube arrays

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IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 21 (2010) 505702 (7pp) doi:10.1088/0957-4484/21/50/505702

Super-long aligned TiO2/carbon nanotubearraysYang Zhao1, Yue Hu1, Yan Li1, Han Zhang1, Shaowen Zhang1,Liangti Qu1, Gaoquan Shi2 and Liming Dai3

1 Key Laboratory of Cluster Science, Ministry of Education, Department of Chemistry, BeijingInstitute of Technology, Beijing 100081, People’s Republic of China2 Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China3 Department of Chemical Engineering, Case School of Engineering, Case Western ReserveUniversity, Cleveland, OH 44106, USA

E-mail: lqu@bit.edu.cn and liming.dai@case.edu

Received 9 August 2010, in final form 27 October 2010Published 22 November 2010Online at stacks.iop.org/Nano/21/505702

Abstract5 mm long aligned titanium oxide/carbon nanotube (TiO2/CNT) coaxial nanowire arrays havebeen prepared by electrochemically coating the constituent CNTs with a uniform layer ofhighly crystalline anatase TiO2 nanoparticles. While the presence of the TiO2 coating wasconfirmed by scanning electron microscopy, transmission electron microscopy, Ramanspectroscopy and x-ray diffraction, the resultant TiO2/CNT coaxial arrays were demonstrated toexhibit minimized recombination of photoinduced electron–hole pairs and fast electron transferfrom the long TiO2/CNT arrays to external circuits. This, in conjunction with the alignedmacrostructure, facilitates the fabrication of TiO2/CNT arrays for various device applications,ranging from photodetectors to photocatalytic systems. Thus, the millimeter long TiO2/CNTarrays represent a significant advance in the development of new macroscopic photoelectronicnanomaterials attractive for a variety of device applications beyond those demonstrated in thisstudy.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

TiO2 possesses superior optoelectronic properties whichare promising for applications in water splitting [1–3],photocatalysis [4–8], environmental protection [9], solarcells [10–14] and sensors [15, 16]. However, many of theseapplications suffer from electron–hole recombination, whichlimits, for example, the efficiency of TiO2 photocatalysis.To reduce charge recombination and enhance reactivity inphotocatalytic and optoelectronic systems, TiO2 has beencombined with CNTs to form TiO2/CNTs nanocompositeswith the unique one-dimensional electronic structure, largesurface area, good chemical and thermal stability, and excellentmechanical properties of CNTs [17].

So far, various approaches including chemical vapor depo-sition (CVD) [18, 19], hydration/dehydration method [20, 21],sol–gel method [22–24] and electrodeposition [25] have beenreported for the synthesis of TiO2/CNT composites. In thisregard, we have developed the electrophoresis method [26] to

coat vertically aligned CNTs (VA-CNTs) with TiO2 to forma coaxial structure, which allows the nanotube frameworkto provide good mechanical stability, high thermal/electricalconductivity and the large surface/interface area necessary forefficient optoelectronic and sensing devices [27, 28]. Thestructure of aligned arrays will facilitate surface modificationfor adding novel surface/interfacial characteristics to the TiO2and VA-CNT hybrids [29, 30], and allow the constituentnanotube devices to be collectively addressed through acommon substrate/electrode [31, 32].

On the other hand, super-long nanostructures such asnanotubes [33–36], nanoribbons [37] and nanowires [38, 39]have attracted increasing attention in recent years, whichoffer significant advantages over their shorter counterparts forvarious potential applications, including multifunctional com-posites [40, 41], sensors [42–44] and nanoelectronics [45, 46].In addition, their millimeter-order lengths provide novelopportunities for translating electronic, thermal and otherphysical properties through individual nanostructures over

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http://dx.doi.org/10.1088/0957-4484/21/50/505702mailto:lqu@bit.edu.cnmailto:liming.dai@case.eduhttp://stacks.iop.org/Nano/21/505702

Nanotechnology 21 (2010) 505702 Y Zhao et al

Figure 1. Digital camera images of 5 mm long CNT arrays connected with a Cu wire before (a) and after (b) electrodeposition of TiO2.

large length scales. More importantly, the macroscopic lengthmakes it very easy to perform measurement of transportproperties and fabricate devices on large scales. Therefore, thesynthesis and development of super-long nanostructures is ofboth fundamental and applied significance. To the best of ourknowledge, however, all the previously synthesized TiO2/CNThybrids have a length in only the micrometer scale, and thereis still no report on millimeter-order long TiO2/CNT arrays.

The recent advance in the synthesis of super-long VA-CNTs provides new opportunities for developing super-longaligned TiO2/CNT arrays. Along with others [47–49], we havesuccessfully achieved the preparation of super-long VA-CNTswith millimeter-order lengths by means of chemical vapordeposition (CVD) techniques [33, 50], which establishes thebase for the synthesis of super-long TiO2/CNT arrays.

In this work, we will demonstrate the use of super-long CNTs not only as the support for deposition with TiO2,but also as the template for producing aligned TiO2/CNTarrays. The resultant TiO2/CNT coaxial nanowire arrays witha macroscopic length can be readily manipulated for deviceconstruction. Fast photocurrent responses and photocatalyticdecomposition of organic substances will also be demonstratedin this paper.

2. Experimental section

2.1. Synthesis of the TiO2-coated CNT arrays

Super-long VA-CNTs with a length of approx. 5 mm on an Sisubstrate were synthesized by the water-assisted CVD methodsimilar to our previous report [33]. The electrodeposition ofTiO2 was performed in a three-electrode cell with a CHI660Delectrochemical station (CH instruments, Shanghai, China).Super-long CNT arrays were directly connected to a Cu wireby conducting resin as a working electrode (figure 1(a)). Ptwire and Ag/AgCl (3 M KCl) were used as counter andreference electrodes, respectively. Prior to electrodeposition,the as-prepared CNTs were activated in the electrolyte bya cyclic voltammetric (CV) scan within the potential rangeof ±3.0 V for two cycles at a scan rate of 50 mV s−1 inorder to improve their hydrophilicity. A constant potential of−0.1 V was then applied for a certain time to deposit TiO2nanoparticles along individual CNTs. The electrolyte used inthe present study consists of 3 M KCl solution, 10 mM H2O2and 10 mM Ti(SO4)2, similar to that reported by Zhang [25].

The resultant samples were rinsed with distilled water andannealed at 430 ◦C in air for 30 min to convert the TiO2nanoparticles into the anatase phase before characterization.

2.2. Property and morphology characterization

All of the electrochemical measurements were carried out byusing a CHI660D electrochemical station. Electrochemicalimpedance spectroscopy (EIS) was recorded by applying anAC voltage of 0.01 mV amplitude in the frequency rangeof 0.01–100 000 Hz with the initial potential (0 mV) in0.01 M K2SO4 solution using a three-electrode cell, wherethe TiO2/CNT arrays acted as the working electrode, aplatinum wire as the counter electrode and the Ag/AgCl asthe reference electrode. The photocurrent was generated byexposure of TiO2/CNT arrays to a daylight lamp (100 W).The photocatalytic degradation of methyl blue (MB) solution(1 × 10−5 M) was performed in a simulated capillary reactorcontaining 5 ml MB solution. Before degradation, theequilibria of adsorption/desorption of MB on TiO2-coatedCNTs, bare CNTs and TiO2 particles were obtained byrespectively immersing the samples into the MB solution inthe dark for several days.

The morphology of the samples was examined byscanning (SEM, 10 kV, JSM-7500F) and transmission (TEM,120 kV, 7650B, Hitachi) electron microscopy. X-ray diffrac-tion (XRD) patterns were obtained by using a Netherlands1710 diffractometer with graphite monochromatized Cu Kαirradiation (λ = 1.54 Å) with a voltage of 40 kV andcurrent of 30 mA. Raman spectra were recorded using a RM2000 Microscopic Confocal Raman Spectrometer (RenishawPLC, UK) with a 633 nm laser. The UV–vis absorption wasmeasured with a 5300PC UV–vis spectrophotometer.

3. Results and discussion

Figure 1 shows the electrode of a 5 mm long CNT arraybefore (a) and after (b) electrodeposition of TiO2. Clearly, theoriginal CNT array (figure 1(a)) still maintained its structurewithout obvious changes after electrodeposition (figure 1(b)).The structural integrity of long TiO2/CNT arrays providesadditional advantages for construction of various devices, asdemonstrated below.

Figure 2(a) shows the super-long CNT array contains well-aligned CNTs with a diameter of approx. 15 nm. Upon

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Figure 2. SEM images of super-long CNT arrays after electrodeposition of TiO2 for different times: (a) 0 min, (b) 5 min and (c) 15 min, and(d) a cross-section view of (c) broken deliberately.

Figure 3. Typical TEM images of a CNT before (a) and after (b) electrodeposition of TiO2 for 15 min.

electrodeposition for 5 min, the CNT diameter increased toapprox. 30 nm due to the deposition of TiO2 nanoclustersalong the sidewall of the CNTs (figure 2(b)). Increasing theelectrodeposition time up to 15 min caused more deposition ofTiO2 nanoparticles along the CNT length (figure 2(c)) and thediameter of TiO2-coated CNTs increased to approx. 100 nm.We observed the uniform deposition of TiO2 nanoparticles onindividual CNTs with the spaces between the VA-CNTs beingunfilled. This was further confirmed by the cross-section viewof the deliberately broken sample shown in figure 2(d), whichexhibited the inner individual CNTs of the arrays were coatedwith TiO2 and the free space among CNTs still remainedunoccupied. These results indicate the electrochemical CVpre-treatment process prior to electrodeposition of TiO2 isquite successful in increasing the compatibility of CNTs

with the electrolyte and prevents the deposited TiO2 fromcongregation between CNTs. Figure 2(d) also shows theindividual CNTs extrude from the broken section, indicatingthe uniform coating of TiO2 on CNTs within the whole arraysonce again.

The uniform coating of TiO2 along CNTs was alsoconfirmed by TEM images. As shown in figure 3, the originalCNTs have a diameter of approx. 15 nm (figure 3(a)).However, the diameter of the TiO2-coated CNTs increasedto approx. 100 nm (figure 3(b)), in good consistencywith the SEM observation in figures 2(c) and (d). Someamorphous carbon on the surface of the CNTs (figure 3(a)) wasprobably introduced during the sample preparation for TEMcharacterization. The TiO2 coating layer was composed ofnanoclusters of less than 10 nm, which was further revealed in

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Figure 4. High resolution TEM image of TiO2-coated CNTs. Theinset shows the lattice fringes of TiO2 nanocrystal.

Figure 5. Raman spectra of original CNTs and TiO2-coated CNTs.

the high resolution TEM as shown in figure 4. Apart from thewell-graphitized parallel walls of CNTs, we found almost all ofthe TiO2 nanoclusters were crystalline. A typical lattice fringeof one TiO2 nanoparticle was illustrated in figure 4, whichclearly revealed the interplanar distance of approx. 0.35 nm,corresponding to the spacing between two (101) planes ofanatase TiO2.

The Raman spectrum of TiO2-coated CNTs with the633 nm laser excitation is similar to what we reportedpreviously [26]. There are several peaks at 144, 397, 518and 639 cm−1, characteristic of the photoelectronically activeTiO2 anatase (figure 5). Two other predominant bands derivedfrom CNTs appear at 1380 and 1580 cm−1, attributed to theD- and G-bands of CNTs, respectively [26]. The intensityof the D-band for TiO2-coated CNTs is higher than that ofCNTs, probably due to the influence of interaction betweenelectrodeposited TiO2 and CNTs.

X-ray diffraction (XRD) pattern of TiO2-coated CNTarrays in figure 6 shows the typical peaks of anatase TiO2,attributable to the (101), (004), (200), (211) and (204) planes,respectively [26, 51]. The peak of (002) of CNTs is overlappedwith the (101) of TiO2, and no diffraction peaks associatedwith the rutile structure of TiO2 was observed in figure 6. The(101) peak at 25.35◦ is in good agreement with the interplanardistance of anatase TiO2 obtained from high resolution TEM(figure 4).

Figure 6. XRD pattern of the TiO2-coated CNTs.

The TiO2 in the form of the anatase phase with abandgap of approx. 3.2 eV could be considered as awide-bandgap photoelectronically active semiconductor [52]and had been widely used as a class of photocatalysts forenvironmental protection by decomposing organisms withUV irradiation [53]. Therefore, the newly prepared super-long TiO2/CNT coaxial nanowires are expected to exhibitphotoelectronic properties. In particular, the well-definedsurface/interface structures between the TiO2 and CNT phasesand unique photoelectronic properties of the TiO2/CNTcoaxial nanowires should provide important advantages forvarious optoelectronic applications. The millimeter length ofTiO2/CNT arrays (figure 1) allows us to readily fabricate thedevice for photocurrent investigation. The inset in figure 7(a)shows a schematic set-up for the I –V measurements. Underlight illumination, the current is overall higher than that inthe dark through the bias potential range from −1.0 to 1.0 V(figure 7(a)), indicating the effective photoexcitation of TiO2.The photocurrent response of TiO2/CNTs with a bias of 0.2 Vwas also revealed in figure 7(b). While a pulsed UV light beam(254 nm) has been used to excite a photocurrent in our previousstudy [26], the super-long TiO2/CNTs prepared in this workallow us to directly use a common daylight lamp to generate astrong photocurrent response with good repeatability due to theCNT-enhanced charge transport and associated minimizationof the electron–hole combination [18, 54]. These resultsindicate a direct electron/hole injection between the TiO2 layerand the underlying metallic CNTs through photoexcitation ofTiO2 [53] as well as more effective electron transport alongsuper-long CNT arrays. The increase of dark current after eachlight illumination (figure 7(b)) is probably due to the chargeaccumulation within the high-surface-area TiO2/CNTs. Thedark current can achieve a constant level and the strength ofthe photocurrent increases with the increase of exposed time(figure 7(c)), indicating the reliable stability and repeatabilityof the TiO2/CNTs towards light stimulation.

To further investigate the electron-transfer properties ofTiO2/CNT–electrolyte interface under light, EIS was alsocarried out in 0.01 M K2SO4 electrolyte. The less impedancearc radius in Nyquist plots indicates the faster electrontransfer [55]. As we can see in figure 8, the Nyquist plots for

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Figure 7. (a) Current–potential curves of TiO2-coated CNT arrays in dark and under light illumination. (b) A typical photocurrent response ofTiO2-coated CNT arrays with an applied bias of 0.2 V (10 s on–off interval). Inset in (a) is the schematic diagram of the set-up forphotocurrent measurements. (c) The photocurrent response of TiO2-coated CNT arrays after 3000 s (20 s on–off interval).

Figure 8. The Nyquist plots of the super-long CNTs and TiO2-coatedCNTs in dark and under light illumination.

CNT arrays in dark and under light have similar linear featureswithout showing semicircular regions, indicating low chargetransfer resistance and negligible photoresponse of CNTs [19].The arc radius of TiO2/CNTs in the dark is much larger thanthat under light, revealing that the light illumination can inducefaster electron transfer in the TiO2/CNT–electrolyte interface.

The 5 mm long TiO2/CNT arrays provide additionaladvantages for easy fabrication of a photocatalytic system.

Figure 9(a) shows a simple proof-of-concept set-up for asimulated capillary reactor to degrade MB solution. The tipof a glass burette was filled with the as-prepared TiO2/CNTarrays (figure 9(a), inset), then the 1 × 10−5 M MB solutionwas injected into the tube. Under the light illumination byusing a daylight lamp, the MB solution goes through theTiO2/CNT arrays with a flow rate of 0.25 ml min−1. Theoriginal MB solution with a blue color becomes colorlessafter flowing out from the tube, indicating the MB has beeneffectively degraded. For comparison, we also investigatedthe photocatalytic properties of bare CNTs and TiO2 particleswith the same amount. UV–vis absorption spectra of the MBsolution before and after going through samples are clearlyshown in figure 9(b), revealing that approx. 84% MB inthe collected solution has been decomposed by TiO2/CNTs.However, the change of absorption intensity for CNTs isnegligible and that for TiO2 is mild, with a degradationefficiency of only 28%. These results, once again, demonstratethe high-efficiency photocatalytic behavior of as-preparedTiO2/CNT arrays. The simulated capillary reactor developedhere provides a continuous mode for degradation of MBsolution, which is a scalable way for effective decompositionof organic compounds and pollutants beyond MB solutiondemonstrated here. A controlled experiment carried out underthe same conditions but without light illumination did not showany color change.

Figure 9. (a) A simulated capillary reactor for degradation of MB solution. The inset shows the tube tip filled with super-long TiO2/CNTs.(b) UV–vis absorption spectra of the MB solution before and after going through bare CNT arrays, pure TiO2 particles and TiO2/CNTs filledtubes, respectively.

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4. Conclusion

In summary, we have successfully fabricated 5 mm longaligned TiO2/CNT coaxial nanowire arrays by electrodepo-sition. The TiO2 was deposited uniformly along individualCNTs in the form of anatase nanocrystals. The as-preparedTiO2/CNT super-long coaxial nanowires were demonstrated tohave the capability to minimize recombination of photoinducedelectron–hole pairs, and transfer electrons rapidly from longTiO2/CNT arrays to the external circuit. The TiO2/CNT arrayscan be macroscopically handled and fabricated into scalablephotocatalytic devices for degradation of organic pollutantsdirectly related to environmental issues. Combined with thefast photocurrent response, the millimeter long TiO2/CNTarrays represent a significant advance in the developmentof new photoelectronic nanomaterials for diverse deviceapplications, ranging from photodetectors to photocatalystsand to many other optoelectronic systems.

Acknowledgments

This work was supported by BIT, the 111 Project B07012in China, NSFC (21004006) and the Program for the NewCentury Excellent Talents in University (NCET). L Dai thanksthe financial support from AFOSR (FA9550-10-1-0546).

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1. Introduction2. Experimental section2.1. Synthesis of the TiO2 -coated CNT arrays2.2. Property and morphology characterization

3. Results and discussion4. ConclusionAcknowledgmentsReferences