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Sulfur-Doped Highly Ordered TiO2 Nanotubular Arrays with Visible Light Response Xinhu Tang and Dongyang Li* Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada ReceiVed: October 30, 2007; In Final Form: January 10, 2008
The future of TiO2-based photocatalysts strongly depends on their structural optimization so as to obtain high activity as well as visible light response. Recently we successfully fabricated sulfur-doped highly ordered TiO2 nanotubular arrays by potentiostatic anodization of titanium foils, followed by annealing in a flow of H2S at 380 °C. The as-prepared arrays were characterized using field emission scanning electron microscopy, differential scanning calorimetry, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), and photoelectrochemical measurements. The results clearly show that the vertically oriented nanotubular arrays are highly ordered with a wall thickness of 10 nm. After being annealed at 380 °C, the original TiO2 nanotubular arrays were transformed from an amorphous structure to an anatase phase with a crystallization enthalpy of 324.6 J/g. With treatment in a H2S atmosphere, sulfur ions were incorporated into oxygen atom sites in the TiO2 nanotubes to form O-Ti-S bonds, confirmed by high-resolution XPS of S2p and Ti2p and XRD analysis. The sulfur doping caused the absorption edge of TiO2 to shift into the lower energy region, making the nanotubular arrays active under visible light irradiation up to 650 nm, demonstrated by UV-vis DRS and photocurrent measurements.
1. Introduction Since water splitting on a single-crystalline TiO2 (rutile) anode under ultraviolet (UV) irradiation was reported first by Fujishima and Honda in 1972,1 TiO2-based photocatalytic materials have been extensively studied over the past three decades due to their chemical stability, nontoxicity, and high photocatalytic activities. Studies demonstrate that the bulk and surface properties of photocatalytic materials largely determine their photocatalytic activity.2-4 Very recently, TiO2 nanotubes have attracted increasing attention due to their outstanding charge transport properties that enable a variety of applications.5,6 The one-dimensional nanotubes create a better opportunity to harvest sunlight more efficiently than the randomly oriented nanoparticles or nanotubes prepared by the sol-gel process.7 Besides, the nanotubular structure can improve the photoexcited charge carrier lifetime by more than an order of magnitude.8-10 The TiO2 nanotubular arrays were found to produce a photoanodic response 10 times higher than that of a TiO2 nanoparticle (P-25) film under the same illumination condition.11 It is expected that the TiO2 nanotubes could be significantly improved if their structureproperty relationship is established and mechanisms involved are fully clarified. TiO2 is a semiconductor with a wide band gap (3.2 eV) and can only utilize UV light (e388 nm), the energy of which makes up only 4-5% of the whole solar spectrum. From the viewpoint of solar energy utilization, the development of a photocatalyst that can utilize visible light (λ > 400 nm) efficiently is indispensable. For producing a visible-light-driven photocatalyst, doping transition-metal ions,12,13 depositing some noble metals,14-16 and coupling metallic oxides17,18 or nonmetal * To whom correspondence should be addressed. E-mail: dongyang.li@ ualberta.ca. Phone: (780) 492-6750. Fax: (780) 492-2881.
elements (e.g., N, S, and C)19-21 into a TiO2 lattice to narrow its band gap have been reported. However, the photocatalytic activity of the cation-doped TiO2 often decreases even in the UV region due to the thermal instability or an increase in carrierrecombination centers. Recently, nonmetal element doping attracted increasing attention. Nonmetal elements having atomic orbitals (e.g., N2p, S3p, and C2p) with a potential energy higher than that of the O2p atomic orbital are introduced into TiO2. In this case, new valence bands can be formed instead of a pure O2p atomic orbital, which results in a decrease in the band gap energy without affecting the conduction band level. Nitrogendoped and carbon-doped TiO2 nanotubes were reported by several research groups,7,11,22,23 which showed beneficial effects of the doped elements on photocatalytic properties. However, although the element-doping approach is beneficial, the photocatalytic activity of conventional TiO2 doped with nonmetal elements is still very low, including S-doped TiO2. It is expected that a combination of element doping and TiO2 nanotubular structure could result in a marked increase in the photocatalytic activity of TiO2 under both UV and visible light. To the authors’ knowledge, however, to date no studies on S-doped TiO2 nanotubular arrays have been reported. The objective of this study is to investigate possible enhancement of the photocatalytic activity of TiO2 by incorporating S-doping treatment with synthesis of TiO2 nanotubular arrays. In this paper we report our recent studies of doping sulfur into TiO2 nanotubular arrays. Since sulfur has a larger ionic radius compared to nitrogen and carbon, it was expected that sulfur could considerably modify the electronic structure of TiO2 and thus result in larger influences on its photocatalytic properties. We fabricated highly ordered TiO2 nanotubular arrays by potentiostatic anodization and then incorporated sulfur ions into the TiO2 lattice by annealing in a H2S atmosphere. This study demonstrates that the doped sulfur ions effectively
10.1021/jp710468a CCC: $40.75 © 2008 American Chemical Society Published on Web 03/19/2008
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narrowed the TiO2 band gap and rendered photocatalytic activity under visible light illumination. 2. Experimental Details 2.1. Fabrication. Similar to the processes reported by Vitiello et al.,7,11,22 highly ordered TiO2 nanotubular arrays were fabricated by potentiostatic anodization, followed by annealing in a H2S atmosphere. Briefly, titanium foils with a thickness of 0.25 mm (99.5%, Sigma-Aldrich) were ground using silicon carbide papers up to 1200 grit and finally polished using 0.05 µm alumina powder. The foils were then cleaned with acetone, then ethanol, and finally DI water in an ultrasonic cleaner. The foils were subjected to potentiostatic anodization at 60 V in a two-electrode electrochemical cell with a platinum foil (12 mm × 12 mm) as the counter electrode for 12 h in ethylene glycol (99+%, enzyme grade, Fisher Scientific) electrolyte containing 0.25 wt % ammonium fluoride (99.3%, ACS reagent, Fisher Scientific) and 2 vol % water. All experiments were done at room temperature. The as-prepared TiO2 nanotubular arrays were finally annealed at 380 °C for 14 h in a tube furnace (Thermocraft, Inc.) with a heating/cooling rate of 0.75 °C min-1 and a H2S flux of 10 mL min-1. For comparison, an undoped sample, referred to as annealed TiO2 nanotubular arrays, was treated under identical thermal conditions but in the air atmosphere. 2.2. Characterization. The crystalline structure were determined by low-angle X-ray diffraction (XRD), using a Rigaku rotating Co anode system (40 kV, 160 mA) operated in the continuous scanning mode. The Co KR X-ray incidence angle and scan rate were set to 3° and 2 deg min-1, respectively. Electron diffraction analysis and imaging were carried out on a JEOL 2010 transmission electron microscope equipped with a Noran UTW X-ray detector and a JEOL JSM6301FXV scanning electron microscope with a field emission electron source running at 5 kV. The crystalline structure was also analyzed with selected area diffraction (SAD) patterns. Chemical state analysis was carried out by X-ray photoelectron spectroscopy (XPS) using a Kratos AXIS Ultra X-ray photoelectron spectrometer. A monochromatic Al source, operating at 210 W with a pass energy of 20 eV and a step of 0.1 eV, was utilized. All XPS spectra were corrected using the C1s line at 284.6 eV. Curve fitting and background subtraction were accomplished using the Casa XPS software package (version 2.3.13). The thermal behaviors of the TiO2 nanotubular arrays were recorded using a differential scanning calorimeter (model Q20, TA Instruments) equipped with a finned air cooling system from room temperature to 600 °C at a scan rate of 5 °C min-1 in a nitrogen flux of 50 mL min-1. The obtained differential scanning calorimetry (DSC) curve was analyzed using the Universal Analysis 2000 software package (TA Instruments). The obtained results are repeatable. The DSC curves and XRD patterns of TiO2 nanotubular arrays were measured at least three times. 2.3. Response to Visible Light. A scanning UV-vis spectrophotometer (model Cary 400, Varian) equipped with a Labsphere diffuse reflectance accessory was used to collect the reflectance spectra of TiO2 nanotubular arrays over a range of 240-800 nm at a scan speed of 300 nm min-1, using a Labsphere DRA-CA-50M as the reflectance standard. The photoelectrochemical properties of the sulfur-doped TiO2 nanotubular arrays were characterized using a three-electrode photoelectrochemical cell with nanotubular arrays as the working electrode, a saturated calomel electrode as the reference, and a platinum foil with dimensions of 12 mm × 12 mm as the counter electrode in 0.1 mol L-1 Na2SO4 solution. The sample was
Figure 1. FE-SEM images of TiO2 nanotubular arrays fabricated at 60 V for 12 h in ammonium fluoride/ethylene glycol solution: (A) top view, (B) bottom view, (C) cross-sectional view, and (D) magnified view of (C).
pressed against an O-ring in the photoelectrochemical cell, leaving an area of 0.785 cm2 exposed to the light source through a quartz window. A 50 W fiber optic illuminator (model 190, Dolan-Jenner Industries, Inc.) was used as the light source and a cutoff filter to remove any radiation below 400 nm to ensure illumination by visible light only. A scanning potentiostat (model PC4-750, Gamry) was used to perform a potentiodynamic scan from 0.05 to 1.5 V vs SCE at a rate of 5 mV s-1 and measure the generated current. The photocurrent transient was also measured at fixed bias potential, 0.1 V vs SCE, with a light pulse of 100 s under visible light illumination. 3. Results and Discussion 3.1. Field Emission Scanning Electron Microscopy (FESEM) Observation. Figure 1 presents the illustrative top view, bottom view, and cross-sectional view of a representative TiO2 nanotubular array observed by FE-SEM. The vertically oriented nanotubular arrays are around 36 µm in length, with a roughness factor (the real surface area per unit geometric surface area) of 960 (tubular packing, 140 nm inner diameter, 10 nm wall thickness) and a length-to-width aspect ratio of ∼250 (length 36 µm, average tube inner diameter 140 nm). Certainly, we were able to achieve longer nanotubular arrays by altering the composition of the electrolyte or optimizing the anodization conditions (e.g., potential, time). However, as reported,4,24 there is an optimal length for nanotubes: a shorter nanotube cannot fully absorb the light, while for a sufficiently long nanotube, the photoconversion efficiency suffers from recombination of the photogenerated electron-hole pairs. In addition, as we observed, longer TiO2 nanotubular arrays much more easily cracked and fell from the substrate during annealing treatment for crystallization or sulfur doping. 3.2. Structure and Composition Analysis. The originally synthesized TiO2 nanotubular arrays were amorphous and needed to be transformed into an anatase crystalline structure by annealing treatment to achieve higher photocatalytic activity.25 To determine the optimum annealing temperature, the DSC curve of TiO2 nanotubular arrays with a weight of 2.550 mg was obtained at 5 °C min-1 in a nitrogen atmosphere. As shown in Figure 2, the transformation from an amorphous structure to the crystalline phase begins at 218.1 °C and is completed at
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Figure 2. DSC curve of the original TiO2 nanotubular arrays, determined at a ramp rate of 5 °C min-1.
Figure 4. (A) High-resolution XPS spectra of S2p in the sulfur-doped TiO2 nanotubular arrays and (B) a comparison of high-resolution Ti2p XPS spectra between the sulfur-doped and the annealed nanotubular arrays. Figure 3. XRD patterns of the original, the annealed, and the sulfurdoped TiO2 nanotubular arrays.
357.6 °C, and the maximum exothermic peak corresponding to crystallization of TiO2 nanotubular arrays is at 341.8 °C. It is much lower than the reported crystallization temperatures to obtain crystalline anatase TiO2 from the amorphous structure and should be ascribed to the nanoscale dimension or the very highly ordered nanotubular architecture. Through integrating the exothermic peak, we obtain the crystallization enthalpy, 324.6 J g-1, of the fabricated TiO2 nanotubular arrays. Consequently, sulfur-doped TiO2 nanotubular arrays were produced by annealing the original nanotubular samples at 380 °C in a H2S atmosphere. For comparison, a nanotubular sample was annealed at 380 °C in the air atmosphere. The XRD patterns of the sulfurdoped, the annealed, and the original nanotubular arrays are presented in Figure 3. As shown, both the annealing and doping treatment made TiO2 nanotubular arrays transform from an amorphous structure to the anatase phase (ICCD PDF 21-1272), and there are no observable structural differences between the annealed and the sulfur-doped TiO2 nanotubular arrays. However, diffraction peaks of the sulfur-doped sample slightly shift to a lower angle, in comparison with those of its annealed counterpart. This happened because some oxygen ions in the TiO2 crystalline lattice could be replaced by sulfur ions, whose ionic radii are bigger than that of oxygen. Full profile structure refinement of XRD data using the Rietveld program from the Jade 7.0 software package (Materials Data, Inc.) showed that the lattice parameters of the annealed TiO2 structure were a ) b ) 3.78898(0.001464) and c ) 9.4952(0.003792), while those
of the sulfur-doped TiO2 nantubes were a ) b ) 3.79299(0.000968) and c ) 9.50264(0.002984). It is clear that the lattice parameters increased along all three directions (a, b, and c axes) as sulfur was doped, especially in the c axis direction, implying that oxygen ions in the TiO2 lattice could be replaced by ions with larger radii. It may need to be mentioned that we did not measure the sulfur atom content distribution along the longitudinal direction of the nanotubular arrays. The sulfur atom content may gradually decrease along the nanotubes from the top to the bottom, depending on the doping temperature and duration. However, since the doping duration was 14 h, steep changes in the doped S concentration along the nanotubes are not expected. The sulfur distribution controlled by atomic diffusion is adjustable by varying the doping temperature and duration. To obtain more detailed information on the chemical state of sulfur-doped TiO2 nanotubular arrays, high-resolution XPS of S2p core levels in the sulfur-doped TiO2 nanotubular arrays were measured and are shown in Figure 4A. The S2p state has a broadened peak because of the overlap of the split sublevels, 2p3/2 and 2p1/2 states, with separation of 1.2 eV by spin-orbit coupling.26 Clearly, two broadened peaks of S2p were observed at around 167.70 and 163.88 eV in the sulfur-doped nanotubular arrays (Figure 4A). According to Umebayashi et al.27 and Sayago et al.,28,29 for adsorbed sulfur dioxide (SO2) molecules on a TiO2 semiconductor surface, the typical peak of the S2p state is located between 166 and 170 eV. Thus, the higher energy peak can be ascribed to surface-adsorbed SO2 molecules, which may come from an impurity of H2S gas or generated through
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Figure 5. Representative high-resolution TEM images and corresponding SAD patterns of the annealed nanotubes (A, bright field; B, dark field; C, SAD) and the sulfur-doped nanotubes (D, bright field; E, dark field; F, SAD).
the decomposition and oxidation of H2S in the annealing process, compared to titanium sulfides in which sulfur atoms are bonded to Ti and have their S2p peak between 160.7 and 163.7 eV.30 Consequently, the resultant sulfur compound in the TiO2 nanotubular arrays is neither S-Ti-S nor S-O. Some mixed state such as S-Ti-O (with a binding energy between those of S-O and S-Ti) could exist as Zhao et al. suggested,31 which is occupied by 89% of S atoms according to the corresponding peak areas. Besides, in comparison with the Ti2p binding energy of the annealed TiO2 nanotubular arrays and previously reported data,32 we found that the binding energy of Ti2p in the sulfurdoped nanotubular arrays was lower with 0.12 eV for 2p3/2 and 0.13 eV for 2p1/2, compared to those of Ti2p in the annealed sample, as shown in Figure 4B. This suggests a decrease in the ionic bond nature of Ti4+ due to the substitution of oxygen by sulfur, which has a lower electronegativity value. 3.3. High-Resolution TEM Observation. Representative bright- and dark- field TEM images and diffraction patterns of the annealed and the sulfur-doped nanotubes are presented in Figure 5. Parts A and B of Figure 5 show an annealed nanotube, and parts D and E of Figure 5 show some sulfur-doped nanotubes. The corresponding diffraction patterns of these nanotubes are also given in Figure 5C,F. As shown, the annealed and sulfur-doped TiO2 nanotubes have similar morphologies in terms of the tubular diameter and the wall thickness (around 10 nm). The sulfur-doping treatment at 380 °C with a heating/ cooling rate of 0.75 °C min-1 did not lead to any structural damage to the nanotubular arrays. The SAD patterns in Figure 5 reveal seven clear characteristic diffraction rings, which correspond to the (101), (004), (200), (105), (211), (204), and (215) planes and are consistent with the XRD patterns. 3.4. Activity under Visible Light Illumination. UV-vis DRS spectra of the annealed and sulfur-doped TiO2 nanotubular arrays are presented in Figure 6. As illustrated, the absorption edge of the annealed nanotubular arrays is less than 400 nm, while the sulfur-doped sample shows a noticeable shift of its absorption edge to the visible light range up to 650 nm. This shift is attributed to a decrease in the band gap energy of TiO2, resulting from the substitution of oxygen by sulfur ions, as discussed earlier.
Figure 6. UV-vis DRS spectra of the annealed and the sulfur-doped TiO2 nanotubular arrays, respectively.
To investigate the influence of the applied potential on the photocurrent magnitude, potentiodynamic scans on the annealed and sulfur-doped TiO2 nanotubular arrays were performed from 0.05 to 1.5 V vs SCE with a scanning rate of 5 mV s-1 under visible light illumination. Results of the experiment are presented in Figure 7. As shown, in comparison with the annealed sample, a significantly larger photocurrent was determined at the sulfurdoped TiO2 nanotubular arrays. Meanwhile, the generated photocurrent becomes strong with increasing applied bias potential. The higher the applied potential, the higher the photocurrent generated at the sulfur-doped TiO2 nanotubular array electrode. The profile of the increase in photocurrent is in good agreement with the photocurrent behavior on a semiconductor electrode predicted by the Gartner model.33 For the annealed sample without doping, no photocurrent should be detected over the entire voltage range under study, due to the lower energy of light relative to the band gap energy of anatase TiO2. Therefore, the observed photocurrent of the annealed nanotubular arrays could be ascribed to possible
TiO2 Nanotubular Arrays with Visible Light Response
J. Phys. Chem. C, Vol. 112, No. 14, 2008 5409 Research Council of Canada (NSERC). We thank Prof. SangYeup Park at Kangnung National University in Korea for stimulating discussions on the subject and thank Dr. Weifeng Wei for his assistance in transmission electron microscopy imaging, as well as Dr. Heng Chen for providing facilities to carry out sulfur doping. We would also like to thank Dynetek Industries Ltd. and Deloro Stellite Inc. for their support to this project. References and Notes
Figure 7. Dependence of the generated photocurrent on the potential applied to the annealed and the sulfur-doped TiO2 nanotubular arrays under visible light illumination in a 0.1 M Na2SO4 solution. The inset shows the transient photocurrent generated under pulse visible light illumination at a fixed bias potential of 0.1 V (SCE) for the sulfurdoped TiO2 nanotubular arrays.
pollutants on electrode surfaces or impurities or species in the Na2SO4 solution. Furthermore, the transient photocurrent measured at a fixed bias potential of 0.1 V vs SCE with a visible light pulse of 100 s is also shown in the inset of Figure 7. The transient photocurrent density decays to 3.5 µA cm-2 in the first stage, obeying the Cottrell equation.34 When visible light reaches the sulfur-doped TiO2 nanotubular arrays, the transient current quickly increases to 7.5 µA cm-2 and remains constant until the illumination is turned off, when the current decays to the background current density (∼3.5 µA cm-2) again. The present experiments indicate that the sulfur-doped TiO2 nanotubular arrays are sensitive to visible light and can generate a sustainable steady photocurrent under visible light illumination. 4. Conclusions Highly ordered and sulfur-doped TiO2 nanotubular arrays were synthesized by potentiostatic anodization in an ethylene glycol based electrolyte, followed by annealing in a flow of H2S at 380 °C with a very low heating/cooling rate. The results show that the vertically oriented nanotubular arrays are highly ordered with a wall thickness of around 10 nm. During annealing at 380 °C, the TiO2 nanotubular arrays transformed from an amorphous structure to the anatase phase with a crystallization enthalpy of 324.6 J/g-1. The substitution of oxygen by sulfur ions caused a significant shift of the absorption edge from 400 to 650 nm, attributed to the band gap narrowing that originated from the sulfur doping and mixing the S3p states with VB, which was confirmed by high-resolution XPS analysis of S2p and Ti2p, as well as the XRD full profile structure refinement. The sulfur-doped TiO2 nanotubular arrays exhibit photocatalytic activity under visible light, demonstrated by DRS and photocurrent measurements. Acknowledgment. This work was sponsored by Alberta Science and Innovation and the Natural Science and Engineering
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