Environ. Sci. Technol. 2007, 41, 4735-4740
Some Critical Structure Factors of Titanium Oxide Nanotube Array in Its Photocatalytic Activity HUI-FANG ZHUANG, CHANG-JIAN LIN,* YUE-KUN LAI, LAN SUN, AND JING LI State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
A highly ordered TiO2 nanotube array on Ti substrate was fabricated by using an electrochemical anodic oxidation method. The morphology, crystalline phase, and photoelectrochemical property of the nanotube array were characterized. The photocatalytic activity of the nanotube array was evaluated by the decolorization of methyl orange in aqueous solution using the different light sources. The effects of structure and morphology of the nanotube array on its photocatalytic activity were investigated. It was found that the photoabsorption behavior of the TiO2 nanotube film depended on the structures of the nanotube array. The nanotube array films exhibited a drastically enhanced photocurrent, and a higher photocatalytic activity compared with the TiO2 nanoparticle film prepared by the regular sol-gel method. The experimental results indicated that the film thickness markedly influenced the photocatalytic activity of nanotube array film, and the 2.5 µm-thick TiO2 nanotube array film appeared a maximum photodegradation efficiency to methyl orange. However, for a given nanotube length, the tube diameter was only very slightly affected the photocatalytic efficiency in this work. The explanation for some critical structure factors of TiO2 nanotube array in the photocatalytic activity was discussed as well.
Introduction The heterogeneous photocatalysis of titanium oxide (TiO2) has become a hot subject in recent years as it is an attractive technique for completely eliminating perpetual chemical pollutants in the environment by using solar or artificial light illumination. The unique advantages for TiO2 photocatalysis including low operation temperature, inexpensive, nontoxic, and relatively high chemical stability have led the relevant applications to the stage of commercialization (13). In the past few years, the colloidal and particulate TiO2 were widely used to photodegrade the pollutants in both the liquid and gaseous phase. But the suspended system encountered three vital technical problems: (a) the need for separation or filtration steps after the photodegradation reaction, (b) the particles aggregation especially at high concentrations, and (c) the problematic use in continuous flow systems (4, 5). To avoid these problems, various methods have been developed to prepare TiO2 films on the solid * Corresponding author phone:/fax: +86-592-2189354; fax: +86592-2189354; e-mail address:
[email protected]. 10.1021/es0702723 CCC: $37.00 Published on Web 05/26/2007
2007 American Chemical Society
support substrates including sol-gel (6, 7), sputtering (8, 9), chemical vapor deposition (10, 11), and liquid-phase deposition (12, 13). However, the efficiency of the immobilized system is much lower than that of the corresponding slurries, because of the inevitable reduction of the overall surface active area associated to catalyst immobilization (14). Compared with the TiO2 films mentioned above, TiO2 nanotube array film is expected to be a promising photocatalyst to overcome such drawbacks due to its great specific surface area. Moreover, the TiO2 nanotube array film possesses a very strong mechanical strength, because it grows directly on the titanium substrate by electrochemical anodic oxidation method, which is verified to be a relatively simple and efficient process for fabricating the nanostructured TiO2 films (15-21). And additionally the conductive support substrate is able to exhibit some interesting properties of photoelectrocatalysis and photoelectrochemistry (22-24). In our previous study, the TiO2 nanotube array prepared in 0.5 wt % HF electrolyte at 20 V and calcined at 450 °C had already showed a good photocatalytic activity (25). In this work, we concentrated our study on the influences of the thickness of the nanotube array film and the diameters of nanotubes on its photocatalytic activity, and the explanations for some critical structure factors of TiO2 nanotube array in the photocatalytic activity were studied as well. In this paper, the highly ordered TiO2 nanotube array films with various morphologies were fabricated by the anodic oxidation method. The morphology, structure and crystalline phase of TiO2 nanotube array were characterized by the scanning electron microscopy (SEM) and X-ray diffractometer (XRD). The photochemical property of TiO2 nanotube array was also examined by diffuse reflectance UV-vis spectroscopy (DRS) and photocurrent spectroscopy. The photocatalytic activity of the TiO2 nanotube array in methyl orange (MO) aqueous solution, as a typical pollutant in the textile industries, was evaluated and compared with that of the TiO2 nanoparticle film prepared by the sol-gel method.
Experimental Section Preparation of TiO2 Nanotube Array Films. The TiO2 nanotube array film was fabricated by the electrochemical anodic oxidation. Prior to electrochemical anodization, titanium samples (1.5 × 1 cm2) were mechanically ground with no. 400-1500 lb emery papers, then degreased in an ultrasonic bath in acetone, anhydrous ethanol, and deionized (DI) water, successively, followed by rinsing with DI water and drying in air. The electrochemical anodization was carried out under various conditions. The TiO2 nanotubes with the length of 0.4 µm were grown at 20 V for 20 min in the 0.5 wt % HF electrolyte, referred to as the short nanotubes. Relatively longer nanotubes were grown in the electrolyte of glycerol (1, 2, 3-propanetriol) and DI water mixed in a 2:1 volumetric ratio containing 0.5 wt % NaF, 0.2 M Na2SO4 at 20 V anodization for different times. After fabrication, the samples were rinsed with DI water. The as-anodized TiO2 nanotubes were amorphous. They were subsequently calcinated at 450 °C in air ambient for 2 h with heating and cooling rates of 5 °C/min to induce crystallization of anatase. Preparation of TiO2 Nanoparticle film. For comparison with the short nanotube array film, the 400 nm-thick TiO2 nanoparticle film on titanium substrate was prepared by using sol-gel method. Tetra-n-butyl titanate (TBT), ethanol, and ethyl acetoacetate (EAcAc) were used to prepare TiO2 sol. The TiO2 coating was formed on the titanium surface by a dip-coating method, and then the sample was calcinated at 450 °C for 30 min (26). The resultant film was controlled in VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. SEM cross-sectional images of TiO2 nanotube array films with different thicknesses: (a) 0.4 µm thick nanotube array film; (b) 3.5 µm thick nanotube array film. 400 nm-thick, anatase TiO2 nanoparticle film with particle size of about 25 nm and pore size of 10∼20 nm. Characterization of the Photocatalyst. The structures and morphologies of the prepared TiO2 nanotube arrays were characterized using a scanning electron microscope (SEM, LEO-1530). To examine on the thicknesses of the nanotube layers, the direct SEM cross-sectional thickness measurements were carried out by using the mechanically cut and cracked samples. The crystalline phase of the TiO2 nanotube arrays was analyzed with an X-ray diffractometer (Philips, Panalytical X’pert, Cu KR radiation). The photoabsorption properties of the nanotube array samples were investigated using a diffuse reflectance UV-visible (UV-vis) spectrometer (Varian, Cary 5000) with wavelength in the 300-600 nm range. The photoelectrochemical measurements were carried out in 0.1 M Na2SO4 using a LHX 150 Xe lamp, a SBP 300 grating spectrometer, and an electrochemical cell with a quartz window. The wavelength-dependent spectral response was measured in a two-electrode configuration with a platinum wire counter electrode at zero bias in the range of 300-600 nm. Photocatalytic Degradation Experiments. MO, as a wellknown sulfonated azo dye indicator, was chosen as a target compound. The initial concentration of the dye was 20 mg/L and the desired pH value of the MO solution (pH ) 3) was adjusted with H2SO4 solution. The analytic wavelengths selected for optical absorbance measurement was 508 nm. The photoactivity test was conducted in a quartz glass reactor of about 60 mm in diameter and 100 mm in height. The reactor was equipped with a water jacket to control the reaction temperature. The photoirradiation employed a 200 W high-pressure mercury lamp emitting at a wavelength of 365 nm as the UV light source or a 500 W tungsten-halogen lamp used to produce the simulated sunlight. A magnetic stirrer was used to provide a good mixing. At the beginning of a run, 30 mL MO solution was fed to the reactor, and air was bubbled through the gas disperser into the reactor. After 30 min premixing, to establish the adsorption/desorption equilibrium of MO on the photocatalyst surface, the run was started by illuminating the light source. The solution periodically taken from the reactor was analyzed by an UVvis spectrophotometer (Japan, UV-2100). The blank test was also carried out by irradiating MO homogeneous solution without TiO2 photocatalyst for checking the self-photolysis of MO. Attenuated Total Reflection Infrared Fourier Transform Spectroscopy (ATR IR) Analysis. This technique was applied to further identify the degradation degree of MO in molecular level. The FTIR spectra of the MO photocatalytic degradation process were recorded on a Nicolet 380 FTIR Spectrometer (Thermo Electron Corporation, U.S.) equipped with a DTGS (deuterated triglycine sulfate) detector. The thin TiO2 nanotube array film fabricated in 0.5 wt % HF electrolyte was used as the photocatalyst. During the photoactivity test, the residual dye and its degradation products of 0.1 mL volume 4736
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FIGURE 2. XRD patterns of TiO2 nanotube array film: (a) the asanodized sample; (b) short nanotubes (0.4 µm) with the diameter of 100 nm, calcined at 450 °C; (c) long nanotubes (2.5 µm) with the diameter of 100 nm, calcined at 450 °C. A and T represent anatase and titanium substrate, respectively. The inset shows SEM topview image of long TiO2 nanotube array after calcination. were sampled from the reactor by using the one-off injector at intervals of 30 min. The sample solution was dropped on the ZnSe reflection element at an incident angle of 45° for the ATR-FTIR measurement. Each spectrum comprises 32 co-added scans measured at a spectral resolution of 4 cm-1 in the 4000-650 cm-1 range. The overall volume of the sample solution is hardly changed during the FTIR measurement because the sample solution is kept in the special ATR-FTIR element, and only a few minutes are required for the ATRFTIR measurement. Spectral data were analyzed with the EZ Omnic version of Omnic.
Results and Discussion Morphology of TiO2 Nanotube Array. Figure 1 shows the SEM cross-sectional images of the well-aligned self-organized TiO2 nanotubes formed under the different preparation conditions. The TiO2 nanotubes grown at 20 V for 20 min in the 0.5 wt % HF electrolyte have the average length of 0.4 µm and the diameter of approximately 100 nm (Figure 1a). In the electrolyte containing 0.5 wt % NaF, 0.2 M Na2SO4 and glycerol, a series of TiO2 nanotube array films were fabricated. At the same anodization voltage (20 V), the nanotubes grown for anodization time of 1, 2, 5, 9 h have average lengths of 1.5, 2.5, 3.1, 3.5 µm, respectively, with almost the same tube diameter as the 0.4 µm length nanotubes. Figure 1b shows the cross-sectional image of the long nanotubes array with 3.5 µm thickness. When the anodization voltage changed from 10 to 25 V, the diameters of the nanotubes increased from 55 to 125 nm correspondingly.
FIGURE 3. Diffuse reflection spectra of TiO2 nanotube array films (a) different thicknesses of nanotube array films (tube diameter of 100 nm); (b) different diameters of nanotubes (tube length of 2.5 µm). XRD Analysis. The crystalline phase of the samples was identified with an X-ray diffractometer. Figure 2 depicts the XRD patterns of the TiO2 nanotube array films before and after calcined in air at 450 °C for 2 h. It is apparent that the as-anodized TiO2 only exhibits an amorphous structure, while the calcined samples show clearly the crystalline phase of anatase. It can be observed that the intensity of anatase peak increases with the increasing of the nanotube film thickness, whereas the intensity of titanium substrate peak decreases. It can be confirmed that the long nanotube array contains more of the anatase phase in comparison to the short nanotube array, based on the facts that the two nanotubes are comprised of similar tube diameter and calcinated in the same experimental conditions. From the inset picture, it is evident that even after calcination and conversion to anatase, the TiO2 nanotube array retains its structural integrity. DRS Analysis. Figure 3 illustrates the UV-vis diffuse reflection spectra of the prepared TiO2 nanotube array films by the different anodization conditions. The major absorbance at wavelengths less than 400 nm can be assigned to the intrinsic band gap absorption of TiO2. The UV-vis absorption behaviors of the nanotube array films with different thicknesses are distinctly different (Figure 3a). The long TiO2 nanotubes array increases the UV-vis absorption comparing with the short one, which can be attributed to increase of TiO2 content. For the short nanotube sample, two small and broad absorption peaks are apparent in the visible region as reported by Bahnemann and co-workers (27). The trapped hole exhibits the absorption at wavelength about 430 nm or even shorter, while the trapped electron at the Ti4+ center shows another absorption at around 500700 nm, so these phenomena can be identified to be the sub-band gap states of the TiO2 nanotube array. However, such a feature is not observed for the long nanotube sample. We presume that the number of trapped charge carriers per unit volume increases due to the larger internal surface area of the long nanotube array. Superposition of the spectra associated with trapped charge carrier can give rise to extremely broad the absorption peaks, even resulting in disappearance of peak shape. Three types of 2.5-µm-thick nanotube films, with tube diameter of 55, 100, and 125 nm, show different absorption behaviors in the visible region (Figure 3b). The absorbance decreases with the decreasing of the nanotubes diameters. The reason may be that the nanotube arrays with small pore diameters reflect significantly larger amounts of incident energy from their surfaces than their large pore counterparts due to a higher average dielectric constant (28). Comparison of Photochemical Property. Figure 4 shows the photocurrent spectra for three types TiO2 films. In comparison with the TiO2 nanoparticle film prepared by the regular sol-gel method, the TiO2 nanotube array films show
FIGURE 4. Plots of photocurrent versus wavelength for (a) 0.4 µm thick nanoparticle film; (b) 0.4 µm thick nanotube array film; (c) 2.5 µm thick nanotube array film measured in 0.1 M Na2SO4 solution. a significant increase in the photocurrent response. For example, at a wavelength of 350 nm, a 3.7 times higher photoresponse is observed for the thin nanotube array film compared to the nanoparticle film with a similar thickness. The result suggests that TiO2 nanotube array film is able to harvest light more effectively than the nanoparticle film under the same illumination. When the film thickness increases, more photons are absorbed and the photocurrent increases. This strong enhancement of photoresponse could be ascribed to an increased light penetration depth and better scattering within a regular pore structure. In addition, the photogenerated charge carriers in the TiO2 nanotube structure might be separated more efficiently than the TiO2 nanoparticle film because of the short diffusion distance in the tube wall and the high contact area between the photocatalyst and electrolyte (29-31). The responsive photocurrent intensity could reflect the overall photoelectron-conversion process. A higher photocurrent response means a lower electronhole recombination and higher photoelectron transfer efficiency for the nanotube array film, which could eventually benefit the corresponding photocatalytic reaction. Photocatalytic Degradation of MO Using TiO2 Nanotube Array Film. Figure 5 shows the absorption changes of MO during the photocatalytic decolorization using the 2.5-µmthick TiO2 nanotube array film as a photocatalyst. It is interesting to note that the absorption of the visible band at 508 nm drastically decreases after 30 min of continuous stirring in the dark. It is proved that the TiO2 nanotube array film possesses the excellent ability of adsorption, because of its great special surface area. Under the illumination of a high-pressure mercury lamp, all the absorption peaks in the VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. UV-visible spectra of MO at different time intervals (a) under the high-pressure mercury lamp illumination, (b) under the tungsten-halogen lamp illumination.
FIGURE 6. ATR-IR spectra of MO before and after photocatalytic degradation using short nanotube array as the photocatalyst: (a) initial MO solution (20 mg/L) before illumination; (b) after 1 h illumination; (c) after 2 h illumination. region of 200-600 nm monotonously decrease with the run time (Figure 5a). After a 15-min illumination, the absorption peaks disappear. However, a remarkable difference in the UV-vis spectra is observed when the tungsten-halogen lamp is used as the light source. The band at 508 nm decreases with time but a new band starts to form at 250 nm (Figure 5b). The hydrazine is suspected to be one of the intermediates responsible for the increment of 250 nm absorption peak (32). The absorption peak near 250 nm increases initially, and then decreases after 60 min. This trend suggests that the extended aromatic MO absorbs in the visible range and the aromatic ring absorbs in the range 200-270 nm. The polyaromatic rings in MO start to degrade creating the mono substituted aromatics in the first period, thus the band at 508 nm decreases in intensity and a new band at 250 nm appears. After 60 min both bands at 250 and 508 nm decrease. This indicates that the intermediate products are also degraded to form CO2 and H2O (33, 34). FTIR-ATR Spectra Analysis. The FTIR-ATR spectra of MO before and after photocatalytic degradation using the short TiO2 nanotube array as the photocatalyst are presented in Figure. 6. As is seen in the curve (a), the intense IR bands at 2969 and 2900 cm-1 are assigned to the stretching modes of the C-H of the methyl, and the rocking modes of the C-H of the phenyl ring are observed at ca. 1405, 1394, 1240, 1230 cm-1. The characteristic vibrational band at 1450 cm-1 is attributed to the aromatic stretching mode (CdC). The bands in the region 1100-1030 cm-1 are assigned to the C-N rocking modes and at 1027 cm-1 for the stretching mode of the -SO3Na (35, 36). It can be observed that the intensity of the 4738
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FIGURE 7. Comparison of photocatalytic degradation rates of MO for nanoparticle film and two different thicknesses of nanotubes films (a) under the high-pressure mercury lamp illumination, (b) under the tungsten-halogen lamp illumination. C0 and Ct are the initial and reaction concentration of MO aqueous solution, respectively. absorption bands decreases with the increasing of the illumination time. After 2 h, the all bands of MO almost disappear. This finding indicates that photocatalytic degradation of MO not only destroys the conjugate system but also destroys totally the whole molecule of MO to CO2 and H2O. Comparison of Photocatalytic Activity. Figure 7 demonstrates the kinetic behaviors of MO photodegradation by the TiO2 photocatalyst prepared with the different methods. It is obvious that the plot of ln(C0/Ct) versus illumination time represents a straight of line and the slope of linear
TABLE 1. Effect of the Thickness of TiO2 Nanotube Array Film on the First-Order Rate Constant k of Photocatalytic Degradation thickness of TiO2 nanotubes film (µm) under high-pressure mercury lamp illumination apparent rate constant k (min-1) correlation coefficient R2 under tungsten-halogen lamp illumination apparent rate constant k (min-1) correlation coefficient R2
0.4
1.5
2.5
3.1
3.5
0.0931 0.999
0.143 0.986
0.171 0.999
0.118 0.991
0.0858 0.976
0.0178 0.996
0.0188 0.996
0.0222 0.998
0.0210 0.998
0.0223 0.998
TABLE 2. The Effect of the Diameter of TiO2 Nanotubes on the First-Order Rate Constant k of Photocatalytic Degradation diameter of TiO2 nanotubes (nm) under high-pressure mercury lamp illumination apparent rate constant k (min-1) correlation coefficient R2 under tungsten-halogen lamp illumination apparent rate constant k (min-1) correlation coefficient R2
regression can be equal to the apparent first-order rate constant k. The blank experiments (in absence of TiO2 photocatalyst) reveal that the self-degradation of MO is almost negligible under either high-pressure mercury lamp or tungsten-halogen lamp illumination. Evidently, the apparent first-order rate constant of the photocatalytic degradation of MO with the assistance of TiO2 photocatalyst is significantly higher than that of MO self-photolysis. It is noted that the photocatalytic activity for the 2.5-µm-thick TiO2 nanotubes film is much higher than the 400 nm one, which is agreement with the result in Figure 4. When comparing the photodegradation activity of the thin nanotubes film with the nanoparticles film (the same thickness), it is obvious that the well-ordered nanotube array film is more efficient than the random nanoparticles film. This is ascribed to the more effective separation for the photogenerated electron-hole pairs and the higher internal surface area of the special nanotube array structure (37). Regardless of the light sources, the photocatalytic activities increase in the same order: nanoparticles film < thin nanotubes film < thick nanotubes film (25, 38). From the plot of absorption vs wavelength (Figure 5), a decrease in the absorbance at 508 nm reflects the degradation of MO on the TiO2 photocatalyst, thereby it can be used as a measure of the photocatalytic activity. Table 1 shows the effect of the film thickness on the first-order rate constant k of photocatalytic degradation. Under the high-pressure mercury lamp illumination, the photocatalytic degradation rate of MO initially increases with increasing of the thickness of TiO2 nanotube array film, and then has a down trend with an optimized efficiency of the MO photodegradation at the thickness of 2.5 µm in the experiments. Using the tungstenhalogen lamp as the light source, the apparent rate constant k increases from 0.0178 to 0.0222 min-1 as the thickness of TiO2 nanotube film increases from 0.4 to 2.5 µm, and then reaches an almost steady value at approximately 0.02 min-1. This finding means that the photocatalytic activity of TiO2 film is not only depended on its thickness and surface structure, but also the interaction between the incident photons and the structures of TiO2 nanotubes. It may be explained that the amounts of both absorption of the incident photons and adsorbed of MO increase with the increasing of film thickness, which is beneficial to achieve a greater photocatalytic degradation rate. However, the light intensity usually attenuates as it penetrates into the solid photocatalyst film (39). If the TiO2 film thickness is thicker than the light penetration depth, the bottom film absorbs few incident photons and serves as an inter support. So the photocatalytic
55
100
125
0.147 0.989
0.171 0.999
0.152 0.999
0.0257 0.983
0.0222 0.998
0.0233 0.999
degradation for a thick film inside becomes low. On the other hand, the photodegradation rate is also affected by the transport of species to and from the bulk solution. The species have a longer diffusion path to the interior TiO2 photocatalyst in the longer nanotube array film, and this may cause a decrease in photocatalytic degradation rate. The dependences of the apparent first-order rate constants of MO photocatalytic degradation on the tubes diameters are listed Table 2. It is worth noting that the k values are not remarkably different from each other while using the same light source. This result suggests that for a given nanotube length, the influence of the tube diameter on the photocatalytic efficiency of nanotube array film is slight. The possible reason may be on the one hand that the specific surface area of TiO2 nanotubes array is decreased with increasing of the diameter of nanotubes, resulting in a negative effect on photocatalytic activity. On the other hand, the increase of light transmittance with the increasing of pore size may have a positive effect on photocatalytic activity (40). Thus, the apparent rate constants are slightly affected by the diameter of nanotubes, if the thickness of the TiO2 film is kept the same.
Acknowledgments We gratefully acknowledge the financial support from the National Natural Science Foundation of China (50571085), and Technical Program of Fujian Province (2005HZ01-3).
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Received for review February 3, 2007. Revised manuscript received April 3, 2007. Accepted April 19, 2007. ES0702723
