J. Phys. Chem. C 2008, 112, 8939–8943
8939
Graphitized Carbon Nanotubes Formed in TiO2 Nanotube Arrays: A Novel Functional Material with Tube-in-Tube Nanostructure Lixia Yang, Shenglian Luo, Shaohuan Liu, and Qingyun Cai* State Key Laboratory of Chemo/Biosensing and Chemometrics, Department of Chemistry, Hunan UniVersity, Changsha 410082, People’s Republic of China ReceiVed: March 9, 2008; ReVised Manuscript ReceiVed: April 3, 2008
Carbon nanotubes with high orientation and low graphitization formed in TiO2 nanotube arrays were fabricated by annealing the latter in carbon atmosphere, constructing a novel functional material with tube-in-tube nanostructure. Compared with unmodified TiO2, coupled C-TiO2 photocatalyst shows an enhanced efficiency of photodecomposing methyl orange process due to the increasing carrier rate and stronger adsorbability as well as the unique mechanical nanostructure. Furthermore, the transition from anatase to rutile was suppressed by carbon, resulting in a high content of the photoactive anatase, which also benefits the high catalytic activity of C-TiO2 photocatalyst. 1. Introduction
2. Experimental Section
Composites consisting of TiO2 and carbon have received increasing attention because of their unusual photoelectrochemical and electronic properties. Three kinds of TiO2-carbon composite powders have been proposed: TiO2-mounted activated carbon, carbon-doped TiO2, and carbon-coated TiO2;1–5 each of them exhibits good photocatalytic activity. It is known that the electronic property of carbon is closely related to its graphitic nature.4 In combination with graphitic carbon, TiO2 photocatalysts5,6 would possess high carrier mobility, little recombination in the space charge region, bulk, and surface, and therefore a high photoactivity. Compared with TiO2 powders, TiO2 nanotube (NT) semiconductors contain much more free spaces in their interior that can be filled with active materials such as chemical compounds and noble metals, giving them a fundamental advantage over powders. Recently, particular interest has been given to the self-organized titania NT arrays fabricated by anodization because of the high orientation, uniformly stable structure, large internal surface area, and excellent electron percolation pathways for vectorial charge transfer between interfaces.7–12 Various modifications on titania NT arrays, such as doping TiO2 with C, B, or N to enhance the photoactivities,13–15 or chemically etching the titania NT to get through the NT for extending their application16, have been carried out. However, no attention has been given to the combination of carbon NTs (CNTs) and TiO2 NT arrays. The coupled NTs were expected to be a wonderful function material possessing unique characteristics based on the versatility of CNTs and special properties of TiO2 NT arrays.
Preparation of C-TiO2 Composite NT Arrays. Anodization of Ti was performed in a two-electrode configuration with titanium foil anode and platinum foil cathode in a dimethyl sulfoxide (DMSO) solution containing 2 wt% hydrofluoric acid (HF) at 40 V for 12 h. The as-prepared titania NTs of 8 µm length and 120 nm pore size were annealed at 550 °C in a conventional tube furnace in atmosphere for 5 h with heating and cooling rates of 2 °C/min to convert the amorphous phase to crystallinity. The sintered TiO2 specimen was placed in a graphite trough in which poly(ethylene glycol) 6000 was added. The content of poly(ethylene glycol) must be controlled in order to keep the top of the TiO2 NT open for further modification and application, which was fixed at 120 mg on the basis of experiments. Carbon was deposited on the TiO2 NTs by carbonizing poly(ethylene glycol) 6000 at 600 °C for 6 h with a heating and cooling rate of 1 °C/min in N2 atmosphere. Figure 1 shows the schematic drawing of the experimental configuration. Structural Characterization. The topology of the catalyst was characterized using a field-emission scanning electron microscope (FE-SEM) operating at 5 kV (JSM 6700F; JEOL, Tokyo, Japan). Energy-dispersive X-ray (EDX) spectrometers fitted to the electron microscopes were used for elemental analysis. Transmission electron microscopy (TEM) images were obtained using a JEM 3010 (JEOL, Tokyo, Japan) operating at 300 kV. Structural changes of the TiO2 samples characterized using a X-ray diffractometer (XRD, M21X, MAC Science Ltd., Japan) with Cu Ka radiation (λ ) 1.54178 Å). First-order Raman spectra was record using a Raman spectrometer (Renishaw System 2000) to study the fine structure of the specimen. Determination of Adsorbability and Photocatalytic Activity of the C-TiO2 Composite NT Arrays. In order to evaluate the adsorbability of the C-TiO2 NTs, a C-TiO2 NTs/Ti sheet (2 × 1 cm2) was placed in a 8 mL 1 × 10-5 M MO solution in which color change was observed. The photocatalytic activity was determined by detecting the degradation rate of 20 mL 1 × 10-5 M MO in the presence of a C-TiO2 NTs/Ti sheet with a size of 4 × 1 cm2 upon exposure to a UV irradiation (125W Hg lamp) at a power density of 50 mW/cm2. The C-TiO2 NTs/ Ti sheet was presaturated in 2 × 10-4 M MO solution for 12 h
In this work, the carbon layer uniformly coats the TiO2 NT inner wall, resulting in a tube-in-tube C-TiO2 composite NT array through a simple carbonizing process. The photocatalytic activity of the composite was investigated using methyl orange (MO) as the model compound. Because of the orderly tube-intube nanostructure, high photocatalytic efficiency is expected. Such a uniform interfacial structure between C and TiO2 is a lack of the carbon-modified TiO2 powders mentioned above.1–5 * Corresponding author.
10.1021/jp8020613 CCC: $40.75 2008 American Chemical Society Published on Web 05/27/2008
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Figure 1. Sketch of the experimental configuration.
in the dark since the C-TiO2 NTs possessed a high MO adsorbability. After variable irradiation times, MO solution was sampled to determine the absorbance. The concentration of MO in the solution was determined from the absorbance change at a wavelength of 642 nm using a spectrophotometer whose calibration curve had been determined in advance. For comparison, the same procedures were performed on a TiO2 NTs/ Ti sheet. 3. Results and Discussion Scanning electron microscope (SEM) micrographs in Figure 2 show the changes in top surface morphology of the TiO2 NTs after being coated by carbon. The gaps between the NTs (Figure 2a) have been filled by carbon in the C-TiO2 composite (Figure
2b). A discharge phenomenon is observed in the TiO2 SEM image in Figure 2a, which was due to the low electricconductivity of crystalline TiO2. But the SEM image of C-TiO2 in Figure 2b is clear, indicating that the electricconductivity of the photocatalyst has been enhanced by carbon. Figure 2c shows the cross section morphology of the C-TiO2 NTs, in which one CNT is clearly seen that is embedded in a TiO2 NT, depicting a typical tube-in-tube construction (marked by black arrows). The electric behavior of C-TiO2 NT arrays and the AC impedance analysis determined taking Fe2+/Fe3+ as the test probe depict that the C-TiO2 NTs are of good electric conductivity. The relative plots are provided in the Supporting Information, and the corresponding energy-dispersive spectroscopy (EDS) spectra in Figure 2d exhibit an obvious C peak in
Figure 2. FE-SEM images showing the top morphology of TiO2 NTs (a) and C-TiO2 NTs (b). (c) Cross section of the C-TiO2 NTs, depicting a typical tube-in-tube structure (marked by black arrows), and (d) the corresponding EDS spectrum with element analysis of C-TiO2 NTs. All the specimens were not Au sprayed.
Graphitized CNTs Formed in TiO2 NT Arrays
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Figure 3. TEM images showing (a) a bundle of C-TiO2 NTs, (b) the top morphology of the CNTs after dissolving TiO2 NTs, (c,d) exposed CNTs with residual TiO2 layer attached, showing a tube-in-tube construction, and (e) poorly aligned graphitic layers in a selected area taken from a CNT wall. Note that, for panels c and d, because of the tightness of the CNT clinging to the TiO2 NT and the difficulty to dissolve crystalline TiO2, clear, single CNTs are not observed by TEM.
the C-TiO2 composite, proving the presence of carbon. The element analysis embedded in Figure 2d shows that the weight percentages of C, Ti, and O in C-TiO2 NTs are 16%, 29.29%, and 54.71%, respectively. The TEM micrographs in Figure 3 also depict the morphologies of the C-TiO2 composite in detail. The bright-field lowresolution TEM image (Figure 3a) shows that the C-TiO2 NTs are uniformly translucent, indicating a good deposition of carbon layer in TiO2 NTs. After dissolving TiO2 NTs in 12 M HF, the top morphology of the remaining CNTs looks like interconnected rings, with a CNT wall thickness of 4-7 nm, as shown in Figure 3b. Figures 3c and 3d show the lateral view of the exposed CNTs on which a residual TiO2 layer can be clearly seen, revealing a typical tube-in-tube structure. The highresolution TEM (HRTEM) of Figure 3e taken from a carbon
wall of Figure 2d shows poorly aligned graphitic layers with an interlayer distance of 0.348 nm corresponding to the (002) plane of graphite, indicating that the CNTs are partly graphitized (not amorphous), which is essential to a high activity of C-TiO2 photocatalyst.3,5 The crystalline phases of TiO2 and carbon in C-TiO2 composite annealed in carbon atmosphere with different durations at 600 °C were identified by XRD, as shown in Figure 4a; a TiO2 specimen was also sintered at 600 °C without carbon for 2 h for comparison, the XRD spectrum of which is shown as the last line in Figure 4a. No other phases except the anatase and rutile phases were identified in C-TiO2 specimens. However, the relative intensity of anatase (101) to rutile (110) increases with prolonging the 600 °C sintering duration, suggesting the inhibiting effect of carbon in the phase transition
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Figure 5. (a) Color change of 1 × 10-5 M MO solution after placing a C-TiO2 NTs/Ti sheet in it for 30 min. (b) The irradiation time-dependent degradation rate of MO expressed as the change of MO concentration relative to the initial solution on C-TiO2 NTs and TiO2 NTs.
Figure 4. XRD (a) and Raman (b) analyses of TiO2 NTs annealed in vacuum at 600 °C for 2 h vs those C-TiO2 NTs annealed in carbon atmosphere with variable duration time at 600 °C. A: anantase; R: rutile; D: defect carbon; G: ordered graphitic.
from the photoactive anatase to the less active rutile.3 Relative to the intense TiO2 peaks, the graphitic carbon peak at 2θ ) 25.6° is too weak to see. In order to study the fine structure of C, Raman spectroscopy, a powerful and more surface-sensitive technique than XRD15 for the investigation of crystalline phases, was applied. Figure 4b shows the Raman spectra of unmodified TiO2 NTs and C-TiO2 composites. Two predominate Raman bands corresponding to carbon are seen: one at 1320 cm-1 corresponding to the carbon defect-induced Raman band (the D band), and another at 1589 cm-1 corresponding to the ordered graphitic structure (the G band).17,18 The appearance of a G band shows that carbon in the C-TiO2 composite is partly graphitized. In addition to the carbon bands, characteristic Raman bands at 146, 199.8, 399, 519, and 645 cm-1 corresponding to anatase modes are seen, while the rutile’s Eg mode at 448 cm-1,19,20 is only observed in the unmodified TiO2 NTs, suggesting that the combination of carbon suppresses the phase transition from anatase to rutile. The increase in the intensity of the 146 cm-1 band with prolonging duration time at 600 °C further confirms the above speculation and is consistent with the XRD results. The graphitization degree of CNTs can be enhanced with a higher carbonization temperature,5 which, however, is limited by the 680 °C collapse temperature at which the TiO2 NTs collapse.21,22
The adsorbability of the C-TiO2 composite was investigated by placing a C-TiO2 NTs/Ti sheet in 1 × 10-5 M MO solution and observing the color change; 30 min later, the MO solution is essentially colorless, as shown in Figure 5a, indicating the strong adsorbability of C-TiO2 NTs. But the color of MO solution with TiO2 NTs has almost no change (pictures are not presented), indicating that the CNTs are responsible for the high adsorbability of C-TiO2 photocatalyst. Figure 5b shows the change in MO concentration as a function of UV light irradiation time; the degradation rate of MO on C-TiO2 NTs is 2.18 times that on TiO2 NTs, which is 0.409 g cm-2 h-1 on C-TiO2 NT arrays and 0.195 g cm-2 h-1 on unmodified TiO2 NT arrays. The higher activity of the C-TiO2 composite is ascribed to the higher carrier mobility, less recombination, and strong adsorbability resulting from the CNTs. It is also considered that the higher active anatase content is helpful to the higher activity of C-TiO2 NTs. The higher photoactivity of the C-TiO2 NT composite is ascribed to the following aspects: (i) the graphitized CNTs formed in TiO2 NTs not only significantly enhance the adsorption capacity for MO, but also provide a large network to collect and rapidly transfer the photoexcited electrons from the conduction band of TiO2 during the photocatalytic process, which reduces the recombination between photoexcited electrons and holes and leaves more holes for the oxidization reaction of the adsorbed MO. The coupling between adsorption and photocatalytic reaction is achieved in a single process, resulting in a higher efficiency in photodegrading MO compared with the unmodified TiO2 photocatalyst. (ii) CNTs clinging to the TiO2 NT inner walls suppress anatase phase transformation to rutile during the carbonization process, resulting in a high photoactive anatase content, which is helpful to the high photocatalytic efficiency of the C-TiO2 NTs.
Graphitized CNTs Formed in TiO2 NT Arrays 4. Conclusions In summary, a novel tube-in-tube carbon-coated anodic TiO2 NT arrays has been facilely fabricated by annealing crystalline TiO2 NT arrays in carbon atmosphere. The highly oriented CNTs are partly graphitized at 600 °C as a result of the presence of TiO2 NTs, which serve as both the template and graphitization catalyst. The formed CNTs in turn suppress the phase transition from anatase to rutile, resulting in a high content of photoactive anatase. The strong adsorbability, high photocatalytic activity, and unique physical construction make C-TiO2 NT arrays a promising photocatalyst in purifying the contaminative water. In addition, the high electricconductivity of the C-TiO2 NT arrays can facilitate the electrodepositon of metal nanoparticles such as Pt, Au, or other active materials, which can further enhance the catalytic activity of the C-TiO2 photocatalyst. Except for the electricconductivity, the large internal specific area, high orientation, stable chemic/physical properties, and biocompatibility of C-TiO2 are all key factors in making C-TiO2 NT arrays ideal supports for application in photoelectrochemical and biochemical research fields. A relative investigation will be presented in another report. Acknowledgment. Funding for this work by the National Science Fund for Distinguished Young Scholars under Grant No. 50725825 is gratefully acknowledged. Supporting Information Available: The relative plots about the electric behavior of C-TiO2 NT arrays and the AC impedance analysis determined taking Fe2+/Fe3+ as the test probe depict that the C-TiO2 NTs are of good electric conductivity. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Takeda, N.; Torimoto, T.; Sampath, S.; Kuwabara, S.; Yoneyama, H. J. Phys. Chem. 1995, 99, 9986–9991.
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