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Fabrication of TiO2-Pt Coaxial Nanotube Array Schottky Structures for Enhanced Photocatalytic Degradation of Phenol in Aqueous Solution Huan Chen, Shuo Chen, Xie Quan,* Hongtao Yu, Huimin Zhao, and Yaobin Zhang Key Laboratory of Industrial Ecology and EnVironmental Engineering (Ministry of Education), School of EnVironmental and Biological Science and Technology, Dalian UniVersity of Technology, Dalian 116024, China ReceiVed: February 7, 2008; ReVised Manuscript ReceiVed: April 14, 2008
Well-aligned TiO2-Pt coaxial nanotube array schottky structures on Ti substrate (TiO2-Pt/Ti) were successfully fabricated by direct current (DC) electrodeposition using anodic aluminum oxide (AAO) templates and the subsequent atmospheric pressure chemical vapor deposition (APCVD) technique. Environmental scanning electron microscopy (ESEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy-dispersive X-ray spectra (EDX), and X-ray diffraction patterns (XRD) indicated that the as-prepared samples were a vertically well-aligned TiO2-Pt coaxial nanotube array, and the outer TiO2 nanotube was anatase with the preferential orientation of (101) plane. The asymmetry of the current-voltage (I–V) curve revealed that a schottky barrier had been formed between TiO2 and Pt. The enhanced separation of photogenerated holes and electrons was demonstrated by surface photovoltage (SPV) and photocurrent measurement. For the degradation of phenol under UV light irradiation, the TiO2-Pt coaxial nanotube array exhibited a much higher photocatalytic efficiency (up to 87%) than did the TiO2 nanotube array, and the kinetic constant of it was 2.3 times as great as that of the TiO2 nanotube array. Introduction Because of its unique physical and chemical properties, TiO2 has attracted considerable research in many areas such as photocatalysis,1,2 water splitting,3–5 dye-sensitized solar cells,6,7 and generation of hydrogen gas.8,9 To date, suffering from low quantum efficiency, the photocatalytic potential of TiO2 is not fully realized. An increase in either the recombination lifetime of charge carriers or the interfacial electron-transfer rate constant is expected to result in higher quantum efficiencies for photocatalysis.10 Applying a bias potential serves as a conventional method to suppress hole-electron recombination, but it requires an extra power supply, restricting its practical applications. Efforts to employ semiconductor–semiconductor or semiconductor–metal composites have been explored to facilitate charge rectification in the semiconductor nanostructures and improve the charge separation efficiency.11–15 When a small band gap semiconductor (SC-A) is coupled with a large band gap semiconductor (SC-B) with a more positive conduction band (CB) level, their valence bands (VB) and conduction bands are suitably disposed, which align as shown in Figure 1a. If both of them can be illuminated and activated simultaneously, the electron-transfer and hole-transfer can occur from one semiconductor to the other. Because the CB level of SC-A is more negative than that of SC-B and the VB level of SC-B is more positive than that of SC-A, the photogenerated electrons will transfer to the CB of SC-B, participating in reduction reaction, while photogenerated holes transfer to the VB of SC-A, participating in oxidation reaction. Thereby, charge carriers recombination can be hindered, whereas the oxidation ability of the nanocomposite system is provided by holes on the VB level of SC-A, which is not as positive as the VB level of SC-B. The more positive is the VB level suggests stronger oxidation ability; however, the more negative * To whom correspondence should be addressed. Phone: +86-411-84706140. Fax: +86-411-8470-6263. E-mail:
[email protected].
VB holes are utilized in the semiconductor–semiconductor system. In this regard, the oxidation potential of this system is not fully realized. As shown in Figure 1b, metal with a high work function coupled with a semiconductor (SC) can make the best use of the oxidation ability of VB holes in SC. When SC is activated, photogenerated electrons will immediately transfer to metal, participating in a reduction reaction, while the holes will move to the surface, participating in an oxidation reaction. In this way, a semiconductor–metal system can realize efficient separation of photogenerated charge carriers. With respect to efficient utilization of energy, the semiconductor–metal nanocomposite photocatalyst has more advantages than does the semiconductor–semiconductor nanocomposite photocatalyst. As an example, Chen et al.16 reported a novel TiO2-Pt nanotube powder, which could separate the photogenerated holes and electrons efficiently and exhibited photocatalytic ability for degradation of toluene using the holes of TiO2. Studies of semiconductor–metal nanocomposites focused on the photocatalytic properties; however, there was no work on the current-voltage(I–V)characteristicandthephotocurrent-potential behavior of them. The primary reason might be that most nanocomposites reported were used in a powder form, and the studies above-mentioned were hard to develop without a conductive substrate connecting to the nanocomposite. Moreover, the powder form makes it difficult to separate photocatalysts from the solution after the reaction. These disadvantages could be overcome by fabricating semiconductor–metal nanocomposites on a conductive substrate. On the other hand, photocatalysts consisting of a highly ordered 1D nanostructure such as nanotube or nanowire, which align vertical to the substrate, are of particular interest, because they possess a high surface to volume ratio, as well as provide a convenient way for photogenerated electrons to transfer to the conductive substrate, both of which will improve photocatalytic efficiency.
10.1021/jp8011393 CCC: $40.75 2008 American Chemical Society Published on Web 06/03/2008
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Figure 1. The working principles of a semiconductor–semiconductor system and a semiconductor–metal system.
SCHEME 1: Schematic Diagrams of the Process To Fabricate a TiO2-Pt Coaxial Nanotube Arraya
a (1) AAO template with evaporated Ag film, (2) deposition of Pt nanotubes by DC electrodepostion, (3) dissolving AAO template, and (4) deposition of TiO2 nanoparticles by APCVD.
Herein, we reported the fabrication of TiO2-Pt coaxial nanotube array photocatalysts with a schottky barrier. The I–V characteristic and photocurrent-potential behavior of the sample, which are both intrinsically important for further understanding of the photocatalysis process, were investigated. Photocatalytic activity was evaluated using phenol as the test substance. Moreover, the electron-transfer mechanisms of the photocatalyst were discussed. Experimental Procedures Preparation of the TiO2-Pt/Ti Coaxial Nanotube Array. The process of realizing the ordered TiO2-Pt coaxial nanotube array is outlined in Scheme 1. In brief, the ordered Pt nanotube array was prepared by using a direct current (DC) electrodepostion process with the assistance of an anodic aluminum oxide (AAO) template. Subsequently, atmospheric pressure chemical vapor deposition (APCVD) technology was applied to fabricate the TiO2-Pt array. The ordered AAO membranes (Anodisc 47, 0.1 µm) were purchased from Whatman. The thickness of membrane is nominally 60 µm, and the pore diameter is about 100-150 nm. Prior to the electrodeposition of Pt nanotube, a thin film of Ag (approximately 400 nm) was first evaporated onto one side of the AAO template to act as a conductive contact, and then the AAO template was placed in a 2 g L-1 H2PtCl4 solution17 containing 1.2 mM hydrochloric acid18 for 20 min to ensure that the inner wall of the pore channel was wetted. The potentiostatic DC electrodeposition was carried out with a potential of -0.35 V19 for 10 min at room temperature, using a three-electrode system with the Ag-coated AAO template as the working electrode, a Pt foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. After electrodeposition, the AAO template with Pt nanotubes was washed with double-distilled water and fixed on a surfacetreated Ti foil using high-purity silver paint. To expose the Pt nanotube array, the AAO template was dissolved in a 3.0 M NaOH solution. The deposition of TiO2 particles was carried out in an APCVD system using the methods described by Yu et al.20 The Pt nanotube array was placed in a tubular quartz
reactor and heated in argon flow at a constant flow rate of 500 sccm. When the temperature reached 320 °C, titanium(IV) isopropoxide as titanium source was fed continuously into the tubular quartz reactor through a capillary at a rate of 0.05 mL min-1. After deposition for 10 min, the argon flow was stopped and the sample was annealed in air at 430 °C for 60 min to convert the amorphous phase of TiO2 to a crystalline one. The TiO2 nanotube array on a Ti foil (TiO2/Ti) was prepared using the methods described by Quan et al.21 to make a comparison with the TiO2-Pt coaxial nanotube array. Characterization. The morphology of the samples was observed using an environmental scanning electron microscope (ESEM Quanta 200 FEG) and a transmission electron microscope (TEM FEI-Tecnai G2 F30 S-Twin) equipped with an energy-dispersive X-ray spectrometer (EDX). The phase of the samples was identified by an X-ray diffractometer (Shimadzu LabX XRD-6000) employing Cu KR radiation at 40 kV and 30 mA over the 2θ range of 20-80°. The current-voltage (I–V) characteristics were analyzed by a micromanipulator manual probe station (model 4200). The photogenerated charge carriers separation and transfer ability of the samples were investigated using a lock-in-based surface photovoltage (SPV) measurement system, which consists of a monochromator (model Omni-λ 3005) and a lock-in amplifier (model SR830-DSP) with an optical chopper (model SR540) running at a frequency of 20 Hz. All of the SPV measurements were performed at room temperature. Photocurrent Measurements. Photocurrent densities were measured using a CHI electrochemical analyzer (CH Instruments 650B, Shanghai Chenhua Instrument Co. Ltd.) in a standard three-electrode configuration with the TiO2-Pt/Ti electrode as a photoanode, a Pt foil as the counter electrode, and a SCE as the reference electrode. A 300 W high-pressure mercury lamp (Beijing Huiyixin Lighting Co.) was used as the UV light source (a principal wavelength of 365 nm, the power density of incident light I0 ) 0.75 mW cm-2). Photocatalytic Degradation of Phenol. The photocatalytic degradation was performed in a 100 mL cuboid quartz reactor with 40 mL of reaction solution, and the effective area of the
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Figure 2. ESEM images of the Pt nanotube array and the TiO2–Pt nanotube array. (a) Top-view of the Pt nanotube array, (b) top-view of the TiO2–Pt nanotube array, and (c) cross-section view of the TiO2–Pt nanotube array.
Figure 3. TEM images of (a) Pt nanotubes, (b) a TiO2–Pt coaxial nanotube, (c) the HAADF image of a TiO2–Pt coaxial nanotube, and (d,e) EDX elemental mapping of the marked region corresponding to Pt and Ti, respectively.
photoanode is 1 cm2. A 300 W high-pressure mercury lamp (Beijing Huiyixin Lighting Co.) was used as the UV light source (I0 ) 2.0 mW cm-2). Phenol was selected as the test pollutant, and the initial concentration was 5 mg L-1 using 0.01 M Na2SO4 as electrolyte. The concentration of phenol was determined by HPLC (Waters 2695, Photodiode Array Detector 2996) with a SunFire C18 (5 µm) reverse-phase column at 30 °C. The mobile phase was methanol and water (v:v ) 0.6:0.4) at a flow rate of 1.0 mL min-1, and the detection wavelength was set at 280 nm. Results and Discussion Morphology Examination. Figure 2 shows ESEM images of the as-prepared Pt nanotube array (Figure 2a) and TiO2–Pt nanotube array (Figure 2b,c). It can be clearly seen that the Pt nanotubes were open at the top end with an average outer diameter of 100-150 nm, depending on the pore diameter of the AAO template. The Pt nanotubes were apart from each other and vertical to the Ti foil substrate, and the density of them was estimated as 1.3 × 109 cm-2 from the image. After deposition of TiO2 particles onto the Pt nanotube array, the TiO2-Pt coaxial nanotube array was obtained. The average outer diameters of TiO2-Pt coaxial nanotubes were larger than that of Pt nanotubes, and the top sides of them were covered with the deposited TiO2 nanoparticles. Figure 3a shows a typical TEM image of Pt nanotubes. Two Pt nanotubes could be seen from the image, whose surface was rather smooth, and the wall thickness was about 30 nm. Figure 3b shows a TEM image of a TiO2-Pt coaxial nanotube. As shown in Figure 3b, TiO2 nanoparticles covered the surface of the Pt nanotube densely, forming another tube wall with thickness of 40-50 nm outside the Pt nanotube, which could be clearly distinguished from the inside Pt nanotube wall. The high-resolution TEM (HRTEM) image of the TiO2 particles (inset of Figure 3b) showed that the material was crystalline with the interplanar spacing of 0.35 nm, which corresponded to the (101) plane of anatase TiO2. The composition of the coaxial nanotube was further determined using EDX in STEM
Figure 4. XRD patterns of (a) Ti, (b) Pt/Ti, and (c) TiO2 Pt/Ti.
mode. Figure 3c shows a representative high-angle annular dark field (HAADF) image of the coaxial nanotube. Comparing the elemental maps to the original HAADF image confirmed that the inner tube wall corresponded to Pt (Figure 3d), whereas the outer tube wall corresponded to TiO2 (Figure 3e). XRD Analysis. Figure 4 shows XRD patterns of Ti, Pt/Ti, and TiO2-Pt/Ti. To distinguish Ti substrate, the XRD pattern of the Ti is displayed in Figure 4a. As shown in Figure 4b, dominant Pt peaks were indexed to the (111) and (200) plane, and all of the other dominant peaks were responsible for the Ti substrate. A peak was observed at 25.2° from the XRD patterns in Figure 4c, corresponding to anatase (101), whereas no rutile phase was observed. I-V Characterization. The I–V curve of TiO2-Pt/Ti is presented in Figure 5. These data were obtained at room temperature and were reproducible as the potential was scanned from -3 to 3 V. The I–V curve showed great asymmetry; that is, the current increased sharply with the positive bias potential and reached nearly 0.015 mA at 3 V, whereas only a low reverse current of 0.004 mA was seen at -3 V. Because the obtained I–V curve of great asymmetry is a typical feature of a schottky barrier, it can be confirmed that a schottky barrier had been
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Figure 5. I–V curve of the TiO2-Pt/Ti and Pt/Ti measured with a micromanipulator manual probe station.
Chen et al.
Figure 7. Short-circuit photocurrent density vs time plotted for TiO2-Pt/Ti and TiO2/Ti in 0.01 M Na2SO4 solution under UV light illumination (I0 ) 0.75 mW cm-2).
Figure 6. SPV spectra of TiO2-Pt/Ti, TiO2/Ti, and Pt/Ti.
formed between TiO2 and Pt. As a benefit from the schottky barrier, photogenerated electrons would transfer from TiO2 to Pt, and the recombination of photogenerated charge carriers would be suppressed. The inset gives the I–V curve of Pt/Ti. The linear curve was typical of ohmic contact,22 which indicated that Pt was a good conductor and there existed a good electrical connection between Pt and Ti foil substrate. Surface Photovoltage (SPV) Spectra. SPV spectroscopy is a good tool for studying photogenerated charge carriers in nanostructured materials. It can detect an optical absorption spectrum by the illumination-induced change in the surface potential due to the drift, accumulation, and recombination of photogenerated carriers.23 Figure 6 shows the SPV spectra of TiO2-Pt/Ti, TiO2/Ti, and Pt/Ti. Pt/Ti showed no response to light illumination. In contrast, TiO2-Pt/Ti exhibited distinguished SPV response, and its performance was evidently higher than that of TiO2/Ti. It is generally accepted that the higher SPV signal suggests the higher separation rate of photogenerated charge carriers.24 TiO2-Pt/Ti showing a much higher SPV signal than TiO2/Ti benefited from the schottky barrier within it. While TiO2-Pt/Ti was illuminated by UV light, electrons generated on TiO2 transferred from the outer TiO2 nanotubes to the inner Pt nanotubes immediately. Thus, the separation ability of photogenerated hole-electron pairs was promoted and a much higher SPV signal could be obtained. Photocurrent Measurements. As shown in Figure 7, TiO2-Pt/ Ti has a strong instant photoresponse to the UV light illumination. The short-circuit photocurrent density of the TiO2-Pt/Ti was as great as 6 times that of the TiO2/Ti. This demonstrated that the separation rate of photogenerated holes and electrons increased due to the formation of a schottky barrier, which was in accordance with the SPV measurements. More photogener-
Figure 8. Photocurrent density vs potential plotted for TiO2-Pt/Ti and TiO2/Ti in 0.01 M Na2SO4 solution under UV light illumination (I0 ) 0.75 mW cm-2).
ated electrons collected from the TiO2-Pt/Ti suggested more photogenerated holes survived from recombination or from the longer lifetime the holes had. Therefore, it could be speculated that TiO2-Pt/Ti, with a schottky barrier inside, possessed better photocatalytic activity than did TiO2/Ti. As illustrated in Figure 8, TiO2-Pt/Ti showed different effects of bias potential on photocurrent. When the applied potential increased from -0.3 to 0.1 V, the photocurrent density of TiO2-Pt/Ti increased rapidly until reaching its maximum, 0.19 mA cm-2, and after that it appreciably decreased. What accounts for it was that with the increase of applied positive potential, the driving force for separation of holes and electrons was weakened due to the thinning of the space charge layer.20 It is worth noting that TiO2-Pt/Ti reached its maximum at 0.1 V, and this was because the coated TiO2 layer was not completely “depleted”. The photocurrent density of TiO2/Ti increased with the applied potential in the range of -0.3 to 0.3 V and would finally achieve a saturated one if the applied potential increased further. Photocatalytic Degradation of Phenol. It is expected that both the high surface to volume ratio of the nanotube array structure and the increased ability of the separation of photogenerated charge carriers benefited from a schottky barrier can make TiO2-Pt/Ti possess an enhanced photocatalytic activity. The photocatalytic activity of the TiO2-Pt/Ti was evaluated by photocatalytic degradation of phenol under UV light illumination. For comparison, photocatalytic degradation of phenol using TiO2/Ti was performed, and the direct photolysis of phenol was a control experiment as shown in Figure 9. After
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Figure 9. Process of photocatalytic degradation of phenol, under UV light illumination (I0 ) 2.0 mW cm-2). The effective areas of both photoanodes are 1 cm2.
Figure 10. Schematic illustration of the schottky barrier between TiO2 and Pt.
TABLE 1: Kinetic Constants and Regression Coefficients of Phenol Photocatalysis under UV Light (I0 ) 2.0 mW cm-2) process
kinetic constants (k, h-1)
R2
photocatalytic degradation with TiO2-Pt/Ti photocatalytic degradation with TiO2/Ti direct photolysis
0.501 0.210 0.086
0.998 0.997 0.988
4 h of UV light illumination, 29.9% of phenol was degraded in a direct photolysis process without any photocatalyst. The degradation efficiency of phenol by a photocatalytic process was over 87% when the TiO2-Pt/Ti existed. Only 55.6% of phenol was photocatalytic degraded when using TiO2/Ti, under the same conditions. The linear correlation between ln(C0/Ct) and t (C0 is the initial concentration of phenol, Ct is the concentration of phenol at time t, and k is kinetic constant) suggested a pseudofirst-order reaction in all of the processes. The corresponding kinetic constants and regression coefficients were given in Table 1. Under the same experimental conditions, the kinetic constant of phenol photocatalysis with TiO2-Pt/Ti was 2.3 times as great as that of the TiO2/Ti. Discussion on Electrons-Transfer Mechanisms. In aqueous solution, the photocatalytic mechanism of organic compound oxidation could be an integration of direct holes oxidation or oxidation initiated by hydroxyl radicals produced via the photogenerated holes.25 Therefore, it is preferred that the surface that can provide holes is exposed to the organic substance solution. In our case, the TiO2-Pt coaxial nanotube was built, TiO2 as the outer nanotube and Pt as the inner nanotube. Because Pt has a higher work function than does TiO2, when they are connected, electron migrates from TiO2 to Pt until a thermodynamic equilibrium is reached.26 At this time, the two Fermi levels are equal. The bands of TiO2 bend upward toward the surface. The electrical contact has formed a space charge layer, and the layer is “carriers depleted”. Therefore, the interface of Pt and TiO2 can form a schottky barrier with a barrier height (Φb) that is the energy difference between the work function of the Pt (Φm) and the electron affinity of the TiO2 (Ex) (see Figure 10).27
Φb ) Φm - Ex
(1)
When TiO2-Pt is exposed to UV light, the Fermi level of TiO2 moves upward to a new quasi-Fermi level due to the photogenerated electrons.28 Under the effect of the schottky barrier, the photogenerated electrons would continuously transfer across the TiO2-Pt interface to Pt and finally access the external circuit, and thus recombination is effectively suppressed. The
Figure 11. Schematic illustration of TiO2-Pt/Ti and the transfer pathway for photogenerated electrons.
holes, on the other hand, are free to diffuse to the TiO2 surface where the oxidation of organic substance can occur (depicted as Figure 11). So, once electrons are photogenerated at outer TiO2 nanotubes, the schottky barrier between TiO2 and Pt will capture them to inner Pt nanotubes, and the conductive substrate connected with Pt nanotubes allows electrons to flow to the external circuit. This simultaneous electron-transfer can greatly increase the charge carrier separation, limit their recombination, and enhance the photocatalytic efficiency. Conclusions Well-aligned TiO2-Pt coaxial nanotube array schottky structures were successfully fabricated by combining DC electrodeposition with the APCVD method. The present results showed that the as-prepared TiO2-Pt schottky structure materials had a much higher photocatalytic activity than that of bare TiO2, indicating that the formation of a schottky barrier between TiO2 and Pt played a great role in effective separation of photogenerated holes and electrons. A similar methodology can be adopted to fabricate other photocatalyst array schottky structures such as TiO2-Ag, TiO2-Pd, etc. A better understanding of the schottky structures will help us design more novel materials in the future, which have potential applications in many fields, such as photocatalysis, water splitting, optoelectronic materials, and solar cells. Acknowledgment. We would like to thank the National Science Foundation of Distinguished Young Scholars of China (No. 20525723) and the National Nature Science Foundation China (Nos. 20507003 and 20337020) for financial support. References and Notes (1) Frank, S. N.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 303.
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