J. Phys. Chem. C 2007, 111, 12987-12991
12987
TiO2-Multiwalled Carbon Nanotube Heterojunction Arrays and Their Charge Separation Capability Hongtao Yu, Xie Quan,* Shuo Chen, and Huimin Zhao School of EnVironmental and Biological Science and Technology, Key Laboratory of Industrial Ecology and EnVironmental Engineering (Ministry of Education, China), Dalian UniVersity of Technology, Dalian 116024, China ReceiVed: April 12, 2007; In Final Form: June 1, 2007
Aligned multiwalled carbon nanotubes (MWNTs) coated with TiO2 nanoparticles were fabricated on a titanium foil by atmospheric pressure chemical vapor deposition (CVD). Their morphology was characterized by environmental scanning electron microscopy (ESEM) and transmission electron microscopy (TEM). The results revealed MWNTs were aligned, and each MWNT with well-graphited walls was coated with TiO2 nanoparticles. Measured by a micromanipulator manual probe station, the current-voltage (I-V) curve of aligned MWNTs on the titanium substrate (MWNT/Ti) was almost linear due to ohmic MWNT-Ti contacts, whereas for aligned MWNTs coated with TiO2 nanoparticles on titanium substrate (TiO2-MWNT/Ti), the I-V curve revealed rectifying behavior, and this rectification demonstrated that a heterojunction had been formed between TiO2 and the MWNT. The photoelectrochemical investigations certified that the TiO2-MWNT heterojunction could minimize recombination of photoinduced electrons and holes and had a more effective photoconversion capability than the aligned TiO2 nanotubes on the titanium substrate (TiO2/Ti).
Introduction TiO2 possesses superior photoelectric properties and has been widely investigated in water splitting,1,2 photocatalysis,3,4 solar cells,5,6 and sensors.7,8 Most of these applications suffer from a dissatisfactory quantum efficiency. The rapid recombination of photoinduced electrons and holes greatly limits the enhancement of quantum efficiency9 and makes essential an inhibition of the recombination of charge carriers. Applying a bias potential serves as a conventional method to suppress the rate of holeelectron recombination, but it requires an extra power supply, restricting its practical applications, especially in cells. With recent development in the improvement of photocatalysis ability of TiO2, research on composite materials of TiO2 and carbon nanotubes (CNTs) has received extensive attention, due to the large specific surface area and unique electrical properties of CNTs. In initial research, CNTs were dispersed into a TiO2 matrix10-15 and enhanced photocatalysis ability was obtained, but the enhancement was limited because some vacant surface of CNTs shaded the light if dispersed CNTs were excessive. To solve this problem, TiO2 has been coated uniformly on dispersed CNTs,16-20 on aligned multiwalled carbon nanotubes (MWNTs) on quartz substrate,21 and on aligned single-walled carbon nanotube (SWNT) bundles on silicon substrate.22 Some researchers predicted that these kinds of TiO2-CNT composite materials could exhibit heterojunction function that might inhibit the recombination of photoinduced carriers. However, to our knowledge, no report finding actual charge separation has been published to date. A heterojunction, of course, can provide a potential driving force for the separation of photoinduced charge carriers, but not all connections of two different materials will create a heterojunction, and even a real heterojunction will not show its * Corresponding author. Phone: +86-411-84706140. Fax: +86-41184706263. E-mail:
[email protected].
charge separation ability without a proper route for charge transfer. Therefore, a particular design is indispensable to create a TiO2-CNT heterojunction with excellent charge separation ability. For the purpose, the following points were taken into consideration when we fabricated TiO2-CNT heterojunction material. Aligned MWNTs were selected as a support, both because MWNTs compared to SWNTs are stronger and could provide more landing sites for different-sized nanoparticles’ coating and because well-aligned MWNT arrays compared to disordered CNTs have a space between each tube that would beneficially disperse nanoparticles. Moreover, these MWNT walls should be well-graphited, and defects in the surface of walls should be kept to a minimum, so that these MWNT walls could play significant roles in forming the heterojunction and in transferring photoinduced electrons. Finally, an electrically conductive substrate that acted as an electrode was applied for emigrating photoinduced electrons to an external circuit. Based on these considerations, we have successfully created TiO2MWNT heterojunction arrays on titanium substrate, and their abilities to minimize recombination and transfer electrons to external circuit are presented in this paper. Experimental Section Our equipment is similar to that used by Andrews et al.23 in fabricating aligned MWNT arrays on plain quartz substrates. The first step was to grow MWNT arrays on titanium substrate. The process of preparing a substrate involved first mechanically polishing and rinsing a titanium foil in ultrasonic baths of acetone and deionized water for 3 min and 5 min in turn; next, chemically etching it by immersion in a mixture of HF and HNO3 acid (HF/HNO3/H2O ) 1:4:5 in volume) for 30 s; and last, rinsing it in deionized water and drying it in argon flow at room temperature. The pretreated titanium foil was put into a tubular quartz reactor in a furnace and heated in argon flow. When the substrate temperature reached 800 °C, the solution
10.1021/jp0728454 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/14/2007
12988 J. Phys. Chem. C, Vol. 111, No. 35, 2007 of the catalyst and carbon source (1.0 g of ferrocene dissolved in 25 mL of xylene) was fed continuously into the tubular quartz reactor through a capillary at a rate of 0.045 mL/min. At the outlet of the capillary (about 220 °C), the solution was immediately volatilized and swept into the reaction zone by a combined flow of argon (1000 mL/min) and hydrogen (100 mL/ min). After 5 min of reaction, both the ferrocene-xylene solution and hydrogen flow were turned off and the substrate was allowed to cool in argon flow. When the temperature dropped to 450 °C, argon was shut off and imported air removed amorphous carbon to purify MWNTs on Ti (MWNT/Ti). In a second step, when the substrate temperature reached 320 °C, argon swept titanium(IV) isopropoxide (TTIP), instead of the ferrocene-xylene solution, as a precursor into the reaction zone. After 15 min, the reaction was finished, and the substrate was annealed in the air at 430 °C for 1 h with a heating rate of 2 °C/min, to convert the amorphous phase of TiO2 to a crystalline one. Then the substrate was allowed to cool to room temperature, and TiO2-MWNT heterojunction arrays on Ti (TiO2MWNT/Ti) were obtained. To compare with TiO2-MWNT/ Ti, TiO2 nanotubes arrays on a titanium foil (TiO2/Ti) were prepared using the methods described by Quan et al.4 The morphology and crystallographic structure of the heterojunctions were characterized by environmental scanning electron microscopy (ESEM Quanta 200 FEG), transmission electron microscopy (TEM FEI-Tecnai G2 20), and X-ray diffraction (XRD) using a diffractometer with Cu KR radiation (Shimadzu Lab X XRD-6000). The current-voltage (I-V) characteristics were measured by a micromanipulator manual probe station (model 4200). To evaluate the charge separation capability of heterojunction arrays as prepared, photocurrent densities were measured using an electrochemical station (CH Instruments 650B, Shanghai Chenhua, China) in a conventional three-electrode configuration with the heterojunction array electrode as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The electrolyte was 0.01 M Na2SO4 aqueous solution, and the UV light source was a 300 W high-pressure mercury lamp (Beijing huiyixin Light, China), with a main wavelength of 365 nm. The UV intensity was 0.75 mW cm-2, which was measured by a radiometer (Photoelectric Instrument Factory Beijing Normal University, model UV-A) in µW cm-2. Results and Discussion Figure 1 displays the SEM images of MWNT arrays before (Figure 1a) and after (Figure 1b) coating with TiO2 nanoparticles. The thickness of the MWNT layer was about 5 µm, which was measured from the SEM image (figure not shown). When MWNTs are coated by TiO2, the nanotube diameters are significantly larger than those of pristine MWNTs. The TEM image of an MWNT (Figure 2a) displays a rather smooth and well-graphitized parallel wall, which can facilitate electron transfer. In contrast, Figure 2b depicts a nanotube after coating; the surface of the MWNT wall is covered by compact TiO2 nanoparticles, some particles with clear crystal lattice, and the interplanar spacing is about 0.35 nm, which corresponds to the distance between two (101) planes of anatase TiO2. Because TiO2 nanoparticles could only descend into the space between MWNTs from the array surface, the descent deepness was limited in some bended channel and nanoparticles would land once the surface of MWNT walls block their descent; consequentially, there is a boundary of coated and uncoated areas. Figure 2c illustrates that the MWNT wall near the substrate is
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Figure 1. SEM images of the MWNT array (a) and TiO2-MWNT array (b) (scale bar: 5 µm).
vacant, and the wall far from the substrate is coated compactly. Because TiO2 nanoparticles attached to the MWNT are sparse near this boundary, they can be clearly seen to be about 15 nm in diameter. From TEM images, the diameter and wall thickness of this MWNT are about 50 and 20 nm. Figure 3 shows the XRD patterns of Ti, MWNT/Ti, and TiO2-MWNT/Ti. To distinguish Ti peaks, the XRD pattern of the titanium substrate is displayed in Figure 3a. In Figure 3b, MWNT depositions weaken Ti peaks; the most intense peaks correspond to graphite (002) reflection and TiC (111) reflection. TiC peaks reveal the existence of a TiC layer between titanium substrate and MWNTs, whose presence can be interpreted that MWNTs have reacted with titanium to form conductive TiC.24,25 Due to this TiC layer, an ohmic contact between Ti and MWNT was formed, which could remove the barrier from the MWNTTi interface, thus permitting electrons to pass through the interface more easily.26 Figure 3c indicates the anatase diffraction peaks and decreased peak intensities of TiC, MWNT, and Ti attributable to the TiO2 coating. The electrical characteristics of TiO2-MWNT heterojunction were determined by a micromanipulator manual probe station, whose contact configuration is shown schematically in Figure 4. Figure 5 identifies the I-V characteristics of MWNTs/Ti, TiO2-MWNT/Ti, and TiO2/Ti. The current of TiO2/Ti is almost
TiO2-MWNT Heterojunction Arrays
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Figure 3. XRD patterns of (a) Ti, (b) MWNT/Ti, and (c) TiO2MWNT/Ti.
Figure 4. Schematic diagram of the contact configuration for currentvoltage (I-V) measurements (with a micromanipulator manual probe station).
Figure 5. Current-voltage (I-V) characteristic of the MWNT/Ti, TiO2-MWNT/Ti, and TiO2/Ti measured with a micromanipulator manual probe station.
Figure 2. TEM images of the graphited wall of MWNT (a), the wall after coating (b), and the boundary between the coated and uncoated part (c).
zero, whereas the I-V plot of MWNTs/Ti is approximately linear, since the interface barrier had disappeared and the formation of a conductive TiC interlayer had created a lowresistance ohmic contact between the MWNT and Ti. In the case of TiO2-MWNT/Ti, the current shows the difference in the forward and in the reverse direction; an obvious asymmetry is observed, and rectifying behavior demonstrates the TiO2MWNT heterojunction has a p-n junction property. In general, TiO2 is an n-type semiconductor, but in this situation, the electrons move more freely in MWNTs, and this leaves an excess of valence band holes in the TiO2 to migrate to the surface, so the TiO2 behaves as a p-type semiconductor.27 The space charge region of the heterojunction promoted separation of photoinduced carriers, so more holes could participate in the redox reactions instead of recombination. Furthermore, separated electrons were easy to transfer to an external circuit along the MWNT and through the interface of MWNT-Ti, given the
12990 J. Phys. Chem. C, Vol. 111, No. 35, 2007
Figure 6. Photocurrent density vs potential plotted for TiO2-MWNT/ Ti and TiO2/Ti in 0.01 M Na2SO4 solution with UV light of 0.75 mW cm-2.
graphited wall of the MWNTs and a low-resistance ohmic contact between the MWNT and the titanium substrate. Photoelectrochemical properties were investigated in a 0.01 M Na2SO4 solution. The dark current of TiO2/Ti is always near zero under our conditions, but because of rectification, the dark current of TiO2-MWNT/Ti increased in accordance with a positive potential, which is very different from TiO2/Ti. To avoid the effect of dark currents, we studied charge separation capability with a pure photocurrent, which can be calculated by subtracting the dark current from the total current. Pure photocurrent, the result of electrons transferred to the external circuit, can be affected by two factors. One is the effective quantity of TiO2; the more effective quantity of TiO2 is provided, the more electrons and holes could be induced. The other is the rate of hole-electron recombination, which can be decreased by applying a bias potential to TiO2/Ti. Figure 6 shows the different effects of bias potential on photocurrent for the TiO2 nanotube and the TiO2-MWNT heterojunction. The TiO2 nanotube is a simplex semiconductor; hence, photoinduced electrons and holes are separated by an external electric field, so the photocurrent increased with bias potential. TiO2-MWNT is a heterojunction; hence, photoelectrons and holes are separated by the interior electric field of space charge layer. However, this interior electric field would be weakened with increase of bias potential because the thickness of the space charge layer decreased with the bias; thus, less electron-hole pairs could be separated and the photocurrent decreased. In Figure 6, the photocurrent density of TiO2/Ti with a bias potential of more than 0.6 V was obviously higher than the maximal photocurrent density of TiO2-MWNT/Ti, so the effective quantity of TiO2 in TiO2-MWNT/Ti was less than that in TiO2/Ti. However, the photocurrent density of TiO2-MWNT/Ti can arrive at its maximum with a small positive bias potential and was larger than that of TiO2/Ti at the same potential until 0.45 V. For TiO2-MWNT/Ti, the photoinduced hole-electron recombination was restrained by the internal electrostatic field in the junction region, and separated electrons emigrated easily to the external circuit through the interface between the MWNT and the substrate. It is important to note that the effective quantity of TiO2 in the TiO2-MWNT heterojunction can be increased by prolonging the deposition time of TiO2; we selected a less effective quantity of TiO2 to ensure that the enhancement of the photocurrent be contributed by the heterojunction. According to the photocurrents of TiO2-MWNT/Ti and TiO2/Ti in Figure 7, the short-circuit photocurrent density of TiO2-MWNT/Ti was more than 0.17 mA cm-2, whereas that of TiO2/Ti was less than 0.03 mA cm-2; the short-circuit photocurrent of the
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Figure 7. Short-circuit photocurrent density vs time plotted for TiO2MWNT/Ti (a), TiO2/Ti (b), and photocurrent density vs time plotted for TiO2/Ti with 0.2 V bias potential (c). The electrolyte is 0.01 M Na2SO4 solution, and photointensity is 0.75 mW cm-2.
Figure 8. Schematic diagram of the electron-transfer path.
heterojunction arrays was more than 5 times that of TiO2 nanotube arrays. A 0.2 V bias potential increased the photocurrent density of TiO2/Ti to 0.1 mA cm-2, but it remained less than that of TiO2-MWNT/Ti. On the basis of the former characterization, charge transfer in TiO2-MWNT/Ti can be indicated in three steps (Figure 8): photoinduced hole-electron pairs separate at the interface between TiO2 and MWNT, electrons transfer along MWNT, and electrons pass through the interface between MWNT and Ti to the external circuit. To elucidate the function of the TiO2MWNT heterojunction, our experiment optimized the electron transport path using three corresponding steps: (1) a low chemical vapor deposition (CVD) temperature yielded smaller TiO2 nanoparticles with more effective surfaces, and more holeelectron pairs can be excited when these smaller nanoparticles are irradiated with UV light; (2) purification of MWNT by annealing in air removed amorphous carbon, producing a good contact between TiO2 nanoparticles and the outside wall of the MWNT, so that holes and electrons immediately separated under the internal electrostatic field in the junction region and then moved to opposite directions to minimize recombination; (3) since an ohmic MWNTs-Ti contact was formed, the separated electrons were transferred to the external circuit through the electric interface, and the separated holes could participate in the redox reaction, with a corresponding enhancement of the overall quantum efficiency. Conclusions We have successfully fabricated TiO2-MWNT arrays on titanium substrate by a successive CVD method. This TiO2MWNT/Ti has charge separation ability because of the TiO2-
TiO2-MWNT Heterojunction Arrays MWNT heterojunction, the excellent MWNT walls, and the ohmic contact of the MWNT and titanium substrate. The shortcircuit photocurrent of the heterojunction arrays as prepared was more than 5 times that of TiO2 nanotube arrays. With enhanced activities to minimize recombination and transfer electrons, heterojunction arrays provide many potential applications, such as photocatalysis and sensors, which will be explored in our next work. Acknowledgment. This work was supported jointly by the National Nature Science Foundation P. R. China (Project No. 20640160447) and the National Science Fund for Distinguished Young Scholars of China (Project No. 20525723). References and Notes (1) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5, 191. (2) Park, J. H.; Kim, S.; Bard, A. J. Nano Lett. 2006, 6, 24. (3) Wu, J.; Zhang, T. Langmuir 2005, 21, 6995. (4) Quan, X.; Yang, S.; Ruan, X.; Zhao, H. EnViron. Sci. Technol. 2005, 39, 3770. (5) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (6) Paulose, M.; Shankar, K.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Phys. D: Appl. Phys. 2006, 39, 2498. (7) Wang, G.; Wang, Q.; Lu, W.; Li, J. J. Phys. Chem. B 2006, 110, 22029. (8) Zhang, S.; Jiang, D.; Zhao, H. EnViron. Sci. Technol. 2006, 40, 2363.
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