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Super-long aligned TiO2/carbon nanotube arrays

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2010 Nanotechnology 21 505702 (http://iopscience.iop.org/0957-4484/21/50/505702) View the table of contents for this issue, or go to the journal homepage for more

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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 nanotube arrays Yang 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, Beijing Institute of Technology, Beijing 100081, People’s Republic of China 2 Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China 3 Department of Chemical Engineering, Case School of Engineering, Case Western Reserve University, Cleveland, OH 44106, USA E-mail: [email protected] and [email protected]

Received 9 August 2010, in final form 27 October 2010 Published 22 November 2010 Online at stacks.iop.org/Nano/21/505702 Abstract 5 mm long aligned titanium oxide/carbon nanotube (TiO2 /CNT) coaxial nanowire arrays have been prepared by electrochemically coating the constituent CNTs with a uniform layer of highly crystalline anatase TiO2 nanoparticles. While the presence of the TiO2 coating was confirmed by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and x-ray diffraction, the resultant TiO2 /CNT coaxial arrays were demonstrated to exhibit minimized recombination of photoinduced electron–hole pairs and fast electron transfer from the long TiO2 /CNT arrays to external circuits. This, in conjunction with the aligned macrostructure, facilitates the fabrication of TiO2 /CNT arrays for various device applications, ranging from photodetectors to photocatalytic systems. Thus, the millimeter long TiO2 /CNT arrays represent a significant advance in the development of new macroscopic photoelectronic nanomaterials attractive for a variety of device applications beyond those demonstrated in this study. (Some figures in this article are in colour only in the electronic version)

coat vertically aligned CNTs (VA-CNTs) with TiO2 to form a coaxial structure, which allows the nanotube framework to provide good mechanical stability, high thermal/electrical conductivity and the large surface/interface area necessary for efficient optoelectronic and sensing devices [27, 28]. The structure of aligned arrays will facilitate surface modification for adding novel surface/interfacial characteristics to the TiO2 and VA-CNT hybrids [29, 30], and allow the constituent nanotube devices to be collectively addressed through a common substrate/electrode [31, 32]. On the other hand, super-long nanostructures such as nanotubes [33–36], nanoribbons [37] and nanowires [38, 39] have attracted increasing attention in recent years, which offer significant advantages over their shorter counterparts for various potential applications, including multifunctional composites [40, 41], sensors [42–44] and nanoelectronics [45, 46]. In addition, their millimeter-order lengths provide novel opportunities for translating electronic, thermal and other physical properties through individual nanostructures over

1. Introduction TiO2 possesses superior optoelectronic properties which are promising for applications in water splitting [1–3], photocatalysis [4–8], environmental protection [9], solar cells [10–14] and sensors [15, 16]. However, many of these applications suffer from electron–hole recombination, which limits, for example, the efficiency of TiO2 photocatalysis. To reduce charge recombination and enhance reactivity in photocatalytic and optoelectronic systems, TiO2 has been combined with CNTs to form TiO2 /CNTs nanocomposites with the unique one-dimensional electronic structure, large surface area, good chemical and thermal stability, and excellent mechanical properties of CNTs [17]. So far, various approaches including chemical vapor deposition (CVD) [18, 19], hydration/dehydration method [20, 21], sol–gel method [22–24] and electrodeposition [25] have been reported for the synthesis of TiO2 /CNT composites. In this regard, we have developed the electrophoresis method [26] to 0957-4484/10/505702+07$30.00

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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 length makes it very easy to perform measurement of transport properties and fabricate devices on large scales. Therefore, the synthesis and development of super-long nanostructures is of both fundamental and applied significance. To the best of our knowledge, however, all the previously synthesized TiO2 /CNT hybrids have a length in only the micrometer scale, and there is still no report on millimeter-order long TiO2 /CNT arrays. The recent advance in the synthesis of super-long VACNTs provides new opportunities for developing super-long aligned TiO2 /CNT arrays. Along with others [47–49], we have successfully achieved the preparation of super-long VA-CNTs with millimeter-order lengths by means of chemical vapor deposition (CVD) techniques [33, 50], which establishes the base for the synthesis of super-long TiO2 /CNT arrays. In this work, we will demonstrate the use of superlong CNTs not only as the support for deposition with TiO2 , but also as the template for producing aligned TiO2 /CNT arrays. The resultant TiO2 /CNT coaxial nanowire arrays with a macroscopic length can be readily manipulated for device construction. Fast photocurrent responses and photocatalytic decomposition of organic substances will also be demonstrated in this paper.

The resultant samples were rinsed with distilled water and annealed at 430 ◦ C in air for 30 min to convert the TiO2 nanoparticles into the anatase phase before characterization. 2.2. Property and morphology characterization All of the electrochemical measurements were carried out by using a CHI660D electrochemical station. Electrochemical impedance spectroscopy (EIS) was recorded by applying an AC voltage of 0.01 mV amplitude in the frequency range of 0.01–100 000 Hz with the initial potential (0 mV) in 0.01 M K2 SO4 solution using a three-electrode cell, where the TiO2 /CNT arrays acted as the working electrode, a platinum wire as the counter electrode and the Ag/AgCl as the reference electrode. The photocurrent was generated by exposure 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 reactor containing 5 ml MB solution. Before degradation, the equilibria of adsorption/desorption of MB on TiO2 -coated CNTs, bare CNTs and TiO2 particles were obtained by respectively immersing the samples into the MB solution in the dark for several days. The morphology of the samples was examined by scanning (SEM, 10 kV, JSM-7500F) and transmission (TEM, 120 kV, 7650B, Hitachi) electron microscopy. X-ray diffraction (XRD) patterns were obtained by using a Netherlands 1710 diffractometer with graphite monochromatized Cu Kα ˚ with a voltage of 40 kV and irradiation (λ = 1.54 A) current of 30 mA. Raman spectra were recorded using a RM 2000 Microscopic Confocal Raman Spectrometer (Renishaw PLC, UK) with a 633 nm laser. The UV–vis absorption was measured with a 5300PC UV–vis spectrophotometer.

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 Si substrate were synthesized by the water-assisted CVD method similar to our previous report [33]. The electrodeposition of TiO2 was performed in a three-electrode cell with a CHI660D electrochemical station (CH instruments, Shanghai, China). Super-long CNT arrays were directly connected to a Cu wire by conducting resin as a working electrode (figure 1(a)). Pt wire and Ag/AgCl (3 M KCl) were used as counter and reference electrodes, respectively. Prior to electrodeposition, the as-prepared CNTs were activated in the electrolyte by a cyclic voltammetric (CV) scan within the potential range of ±3.0 V for two cycles at a scan rate of 50 mV s−1 in order to improve their hydrophilicity. A constant potential of −0.1 V was then applied for a certain time to deposit TiO2 nanoparticles along individual CNTs. The electrolyte used in the present study consists of 3 M KCl solution, 10 mM H2 O2 and 10 mM Ti(SO4 )2 , similar to that reported by Zhang [25].

3. Results and discussion Figure 1 shows the electrode of a 5 mm long CNT array before (a) and after (b) electrodeposition of TiO2 . Clearly, the original CNT array (figure 1(a)) still maintained its structure without obvious changes after electrodeposition (figure 1(b)). The structural integrity of long TiO2 /CNT arrays provides additional advantages for construction of various devices, as demonstrated below. Figure 2(a) shows the super-long CNT array contains wellaligned CNTs with a diameter of approx. 15 nm. Upon 2

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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.

with the electrolyte and prevents the deposited TiO2 from congregation between CNTs. Figure 2(d) also shows the individual CNTs extrude from the broken section, indicating the uniform coating of TiO2 on CNTs within the whole arrays once again. The uniform coating of TiO2 along CNTs was also confirmed by TEM images. As shown in figure 3, the original CNTs have a diameter of approx. 15 nm (figure 3(a)). However, the diameter of the TiO2 -coated CNTs increased to approx. 100 nm (figure 3(b)), in good consistency with the SEM observation in figures 2(c) and (d). Some amorphous carbon on the surface of the CNTs (figure 3(a)) was probably introduced during the sample preparation for TEM characterization. The TiO2 coating layer was composed of nanoclusters of less than 10 nm, which was further revealed in

electrodeposition for 5 min, the CNT diameter increased to approx. 30 nm due to the deposition of TiO2 nanoclusters along the sidewall of the CNTs (figure 2(b)). Increasing the electrodeposition time up to 15 min caused more deposition of TiO2 nanoparticles along the CNT length (figure 2(c)) and the diameter of TiO2 -coated CNTs increased to approx. 100 nm. We observed the uniform deposition of TiO2 nanoparticles on individual CNTs with the spaces between the VA-CNTs being unfilled. This was further confirmed by the cross-section view of the deliberately broken sample shown in figure 2(d), which exhibited the inner individual CNTs of the arrays were coated with TiO2 and the free space among CNTs still remained unoccupied. These results indicate the electrochemical CV pre-treatment process prior to electrodeposition of TiO2 is quite successful in increasing the compatibility of CNTs 3

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Figure 4. High resolution TEM image of TiO2 -coated CNTs. The inset shows the lattice fringes of TiO2 nanocrystal. Figure 6. XRD pattern of the TiO2 -coated CNTs.

The TiO2 in the form of the anatase phase with a bandgap of approx. 3.2 eV could be considered as a wide-bandgap photoelectronically active semiconductor [52] and had been widely used as a class of photocatalysts for environmental protection by decomposing organisms with UV irradiation [53]. Therefore, the newly prepared superlong TiO2 /CNT coaxial nanowires are expected to exhibit photoelectronic properties. In particular, the well-defined surface/interface structures between the TiO2 and CNT phases and unique photoelectronic properties of the TiO2 /CNT coaxial nanowires should provide important advantages for various optoelectronic applications. The millimeter length of TiO2 /CNT arrays (figure 1) allows us to readily fabricate the device for photocurrent investigation. The inset in figure 7(a) shows a schematic set-up for the I –V measurements. Under light illumination, the current is overall higher than that in the 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 V was also revealed in figure 7(b). While a pulsed UV light beam (254 nm) has been used to excite a photocurrent in our previous study [26], the super-long TiO2 /CNTs prepared in this work allow us to directly use a common daylight lamp to generate a strong photocurrent response with good repeatability due to the CNT-enhanced charge transport and associated minimization of the electron–hole combination [18, 54]. These results indicate a direct electron/hole injection between the TiO2 layer and the underlying metallic CNTs through photoexcitation of TiO2 [53] as well as more effective electron transport along super-long CNT arrays. The increase of dark current after each light illumination (figure 7(b)) is probably due to the charge accumulation within the high-surface-area TiO2 /CNTs. The dark current can achieve a constant level and the strength of the photocurrent increases with the increase of exposed time (figure 7(c)), indicating the reliable stability and repeatability of the TiO2 /CNTs towards light stimulation. To further investigate the electron-transfer properties of TiO2 /CNT–electrolyte interface under light, EIS was also carried out in 0.01 M K2 SO4 electrolyte. The less impedance arc radius in Nyquist plots indicates the faster electron transfer [55]. As we can see in figure 8, the Nyquist plots for

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

the high resolution TEM as shown in figure 4. Apart from the well-graphitized parallel walls of CNTs, we found almost all of the TiO2 nanoclusters were crystalline. A typical lattice fringe of one TiO2 nanoparticle was illustrated in figure 4, which clearly revealed the interplanar distance of approx. 0.35 nm, corresponding to the spacing between two (101) planes of anatase TiO2 . The Raman spectrum of TiO2 -coated CNTs with the 633 nm laser excitation is similar to what we reported previously [26]. There are several peaks at 144, 397, 518 and 639 cm−1 , characteristic of the photoelectronically active TiO2 anatase (figure 5). Two other predominant bands derived from CNTs appear at 1380 and 1580 cm−1 , attributed to the D- and G-bands of CNTs, respectively [26]. The intensity of the D-band for TiO2 -coated CNTs is higher than that of CNTs, probably due to the influence of interaction between electrodeposited TiO2 and CNTs. X-ray diffraction (XRD) pattern of TiO2 -coated CNT arrays 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 overlapped with the (101) of TiO2 , and no diffraction peaks associated with the rutile structure of TiO2 was observed in figure 6. The (101) peak at 25.35◦ is in good agreement with the interplanar distance of anatase TiO2 obtained from high resolution TEM (figure 4). 4

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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 of TiO2 -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 for photocurrent measurements. (c) The photocurrent response of TiO2 -coated CNT arrays after 3000 s (20 s on–off interval).

Figure 9(a) shows a simple proof-of-concept set-up for a simulated capillary reactor to degrade MB solution. The tip of a glass burette was filled with the as-prepared TiO2 /CNT arrays (figure 9(a), inset), then the 1 × 10−5 M MB solution was injected into the tube. Under the light illumination by using a daylight lamp, the MB solution goes through the TiO2 /CNT arrays with a flow rate of 0.25 ml min−1 . The original MB solution with a blue color becomes colorless after flowing out from the tube, indicating the MB has been effectively degraded. For comparison, we also investigated the photocatalytic properties of bare CNTs and TiO2 particles with the same amount. UV–vis absorption spectra of the MB solution before and after going through samples are clearly shown in figure 9(b), revealing that approx. 84% MB in the collected solution has been decomposed by TiO2 /CNTs. However, the change of absorption intensity for CNTs is negligible and that for TiO2 is mild, with a degradation efficiency of only 28%. These results, once again, demonstrate the high-efficiency photocatalytic behavior of as-prepared TiO2 /CNT arrays. The simulated capillary reactor developed here provides a continuous mode for degradation of MB solution, which is a scalable way for effective decomposition of organic compounds and pollutants beyond MB solution demonstrated here. A controlled experiment carried out under the same conditions but without light illumination did not show any color change.

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

CNT arrays in dark and under light have similar linear features without showing semicircular regions, indicating low charge transfer resistance and negligible photoresponse of CNTs [19]. The arc radius of TiO2 /CNTs in the dark is much larger than that under light, revealing that the light illumination can induce faster electron transfer in the TiO2 /CNT–electrolyte interface. The 5 mm long TiO2 /CNT arrays provide additional advantages for easy fabrication of a photocatalytic system.

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 filled tubes, respectively.

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4. Conclusion

[12] Chen Y, Kim H C, McVittie J, Ting C and Nishi Y 2010 Synthesis of TiO2 nanoframe and the prototype of a nanoframe solar cell Nanotechnology 21 185303 [13] Jennings J R, Ghicov A, Peter L M, Schmuki P and Walker A B 2008 Dye-sensitized solar cells based on oriented TiO2 nanotube arrays: transport, trapping, and transfer of electrons J. Am. Chem. Soc. 130 13364–70 [14] O’Regan B and Gr¨atzel M 1991 A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films Nature 353 737–40 [15] Wang G, Wang Q, Lu W and Li J 2006 Photoelectrochemical study on charge transfer properties of TiO2 -B nanowires with an application as humidity sensors J. Phys. Chem. B 110 22029–34 [16] Zhang S, Jiang D and Zhao H 2006 Development of chemical oxygen demand on-line monitoring system based on a photoelectrochemical degradation principle Environ. Sci. Technol. 40 2363–8 [17] Dai L M 2006 Carbon Nanotechnology: Recent Developments in Chemistry, Physics, Materials Science and Device Applications (Amsterdam: Elsevier) [18] Yu H T, Quan X, Chen S and Zhao H M 2007 TiO2 –multiwalled carbon nanotube heterojunction arrays and their charge separation capability J. Phys. Chem. C 111 12987–91 [19] Yu H T, Quan X, Chen S, Zhao H M and Zhang Y B 2008 TiO2 –carbon nanotube heterojunction arrays with a controllable thickness of TiO2 layer and their first application in photocatalysis J. Photochem. Photobiol. A 200 301–6 [20] Yao Y, Li G H, Ciston S, Lueptow R M and Cray K A 2008 Photoreactive TiO2 /carbon nanotube composites: synthesis and reactivity Environ. Sci. Technol. 42 4952–7 [21] Lee S W and Sigmund W M 2003 Formation of anatase TiO2 nanoparticles on carbon nanotubes Chem. Commun. 780–1 [22] Wang S, Ji L J, Wu B, Gong Q M, Zhu Y F and Liang J 2008 Influence of surface treatment on preparing nanosized TiO2 supported on carbon nanotubes Appl. Surf. Sci. 255 3263–6 [23] Akhavan O, Abdolahad M, Abdi Y and Mohajerzadeh S 2009 Synthesis of titania/carbon nanotube heterojunction arrays for photoinactivation of E. coli in visible light irradiation Carbon 47 3280–7 [24] Jitianu A, Cacciaguerra T, Benoit R, Delpeux S, B´eguin F and Bonnamy S 2004 Synthesis and characterization of carbon nanotubes–TiO2 nanocomposites Carbon 42 1147–51 [25] Jiang L C and Zhang W D 2009 Electrodeposition of TiO2 nanoparticles on multiwalled carbon nanotube arrays for hydrogen peroxide sensing Electroanalysis 21 988–93 [26] Yang Y D, Qu L T, Dai L M, Kang T S and Durstock M 2007 Electrophoresis coating of titanium dioxide on aligned carbon nanotubes for controlled syntheses of photoelectronic nanomaterials Adv. Mater. 19 1239–43 [27] Dai L and Mau A W H 2000 Surface and interface control of polymeric biomaterials, conjugated polymers, and carbon nanotubes J. Phys. Chem. B 104 1891–915 [28] Dai L M 2004 Intelligent Macromolecules for Smart Devices: From Materials Synthesis to Device Applications (New York: Springer) [29] Chen Q, Dai L, Gao M, Huang S and Mau A 2001 Plasma activation of carbon nanotubes for chemical modification J. Phys. Chem. B 105 618–22 [30] Qu L T and Dai L M 2007 Polymer-masking for controlled functionalization of carbon nanotubes Chem. Commun. 3859–61 [31] Dai L, Patil A, Gong X, Guo Z, Liu L, Liu Y and Zhu D 2003 Aligned nanotubes ChemPhysChem 4 1150–69 [32] He P and Dai L 2004 Aligned carbon nanotube-DNA electrochemical sensors Chem. Commun. 348–9

In summary, we have successfully fabricated 5 mm long aligned TiO2 /CNT coaxial nanowire arrays by electrodeposition. The TiO2 was deposited uniformly along individual CNTs in the form of anatase nanocrystals. The as-prepared TiO2 /CNT super-long coaxial nanowires were demonstrated to have the capability to minimize recombination of photoinduced electron–hole pairs, and transfer electrons rapidly from long TiO2 /CNT arrays to the external circuit. The TiO2 /CNT arrays can be macroscopically handled and fabricated into scalable photocatalytic devices for degradation of organic pollutants directly related to environmental issues. Combined with the fast photocurrent response, the millimeter long TiO2 /CNT arrays represent a significant advance in the development of new photoelectronic nanomaterials for diverse device applications, ranging from photodetectors to photocatalysts and to many other optoelectronic systems.

Acknowledgments This work was supported by BIT, the 111 Project B07012 in China, NSFC (21004006) and the Program for the New Century Excellent Talents in University (NCET). L Dai thanks the financial support from AFOSR (FA9550-10-1-0546).

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