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Langmuir 2008, 24, 3422-3428
Multivalency Iodine Doped TiO2: Preparation, Characterization, Theoretical Studies, and Visible-Light Photocatalysis Wenyue Su, Yongfan Zhang, Zhaohui Li, Ling Wu, Xuxu Wang, Junqian Li, and Xianzhi Fu* Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou UniVersity, Fuzhou 350002, P. R. China ReceiVed June 10, 2007. In Final Form: NoVember 29, 2007 Multivalency iodine (I7+/I-) doped TiO2 were prepared via a combination of deposition-precipitation process and hydrothermal treatment. The as-prepared samples were characterized by X-ray diffraction, transmission electron microscopy, Brunauer-Emmett-Teller surface area, UV-vis diffuse reflectance spectra, X-ray photoelectron spectroscopy, surface photovoltage spectroscopy, and electric-field-induced surface photovoltage spectroscopy. The electronic structure calculations based on the density functional theory revealed that upon doping, new states that originated from the I atom of the IO4 group are observed near the conduction-band bottom region of TiO2, and the excitation from the valence band of TiO2 to the surface IO4- is responsible for the visible-light response of the I-doped TiO2. The as-prepared I-doped TiO2 showed high efficiency for the photocatalytic decomposition of gaseous acetone under visible light irradiation (λ > 420 nm). A possible mechanism for the photocatalysis on this multivalency iodine (I7+/I-) doped TiO2 under visible light was also proposed.
Introduction Photocatalysis is environmentally friendly, capable of performing at room temperature, and it can treat organic pollutants at extremely low concentrations. It has been widely employed in the treatment of all kinds of organic contaminants.1-5 However, due to its large band gap (3.2 eV), the currently used photocatalyst TiO2 can only absorb a small fraction of solar energy, thus restricting its practical applications. In view of the efficient utilization of solar energy, it is indispensable to develop a photocatalyst with high activities under visible light. Over the past several years, considerable effort has been made to extend the absorption of TiO2 to the visible light region. Combinations of TiO2 with various narrow band gap semiconductors like CdS, Fe2O3, Cu2O, Bi2O3, and ZnMn2O46-10 have been built, and various transition metals11-15 or nonmetal atoms16-24 have been doped into TiO2 to enhance the photo* Correspondingauthor.Fax: +86-591-83738608.E-mail:
[email protected]. (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37-38. (2) Adachi, M.; Murata, Y.; Takao, J.; Jiu, J. T.; Sakamoto, M.; Wang, F. M. J. Am. Chem. Soc. 2004, 126, 14943-14949. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69-96. (4) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735-758. (5) Fox, M. A.; Duby, M. T. Chem. ReV. 1993, 93, 341-357. (6) Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett. 1990, 174, 241-246. (7) Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1992, 96, 6834-6839. (8) Wu, L.; Yu, J. C.; Fu, X. Z. J. Mol.Catal. A 2006, 244, 25-32. (9) Yu, J. C.; Wu, L.; Lin, J.; Li, P. S.; Li, Q. Chem. Commun. 2003, 15521553. (10) Ho, W. K.; Yu, J. C.; Lin, J.; Yu, J. G.; Li, P. S. Langmuir 2004, 20, 5865-5869. (11) Anpo, M.; Takeuchi, M. J. Catal. 2003, 216, 505-516. (12) Klosek, S.; Raftery, D. J. Phys. Chem. B 2001, 105, 2815-2819. (13) Zhao, W.; Chen, C. C.; Li, X. Z.; Zhao, J. C.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 2002, 106, 5022-5028. (14) Kato, H.; Kudo, A. J. Phys. Chem. B 2002, 106, 5029-5034. (15) Litter, M. I. Appl. Catal. B 1999, 23, 89-114. (16) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 54835486. (17) Sakthivel, S.; Janczarek, M.; Kisch, H. J. Phys. Chem. B 2004, 108, 19384-19387. (18) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269-271. (19) Nakamura, R.; Tanaka, T.; Nakato, Y. J. Phys. Chem. B 2004, 108, 1061710620.
catalytic activities of TiO2 under visible light. Among these modifications, the doping of nonmetal atoms into TiO2 has been proved to be an effective method. Asahi et al.18 first reported that N-doped TiO2 showed enhanced photocatalytic activities for the degradation of methylene blue and gaseous acetaldehyde. Khan et al.20 reported a C-doped TiO2 with a higher photocurrent density and photoconversion efficiency for water splitting. Asai et al.23 also reported enhanced photocatalytic activities under visible light on S-doped TiO2. Recently Cai and Liu reported that cationic iodine-doped TiO2 shows visible light photocatalytic activities in the decomposition of phenol25 and methylene blue.26 According to Cai and Liu, the iodine exists as I5+ in the iodinedoped TiO2 hydrothermal prepared from iodic acid and tetrabutyltitanate. It is expected that the chemical state of iodine in I-doped TiO2 would affect its photocatalytic activity to some degree. Studying I-doped TiO2 with iodine in different chemical states is therefore interesting and is absolutely necessary in view of developing I-doped TiO2 visible light photocatalysts. Thus, in this paper, we reported the preparation, characterization, and electronic structure of multivalency iodine (I7+/I-) doped TiO2 and its visible-light photocatalytic activity as evaluated by the decomposition of gaseous acetone. A possible mechanism for the photocatalysis on this multivalency iodine (I7+/I-) doped TiO2 under visible light was also proposed. Experimental Section Materials. Titanium sulfate [Ti(SO4)2, 99.9%] and potassium iodate (KIO3, 99.9%) were purchased from Shanghai Chemical Reagent Co. Terephthalic acid (TA, high-purity grade) was from (20) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 22432245. (21) Irie, H.; Watanabe, Y.; Hashimoto, K. Chem. Lett. 2003, 32, 772-773. (22) Zhao, W.; Ma, W. H.; Chen, C. C.; Zhao, J. C.; Shuai, Z. G. J. Am. Chem. Soc. 2004, 126, 4782-4783. (23) Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. Appl. Phys. Lett. 2002, 81, 454-456. (24) Yu, J. C.; Ho, W. K.; Yu, J. G.; Yip, H. Y.; Wong, P. K.; Zhao, J. C. EnViron. Sci. Technol. 2005, 39, 1175-1179. (25) Hong, X. T.; Wang, Z. P.; Cai, W. M.; Lu, F.; Zhang, J.; Yang, Y. Z.; Ma, N.; Liu, Y. J. Chem. Mater. 2005, 17, 1548-1552. (26) Liu, G.; Chen, Z.; Dong, C.; Zhao, Y.; Li, F.; Lu, G. Q.; Cheng, H. M. J. Phys. Chem. B 2006, 110, 20823-20828.
10.1021/la701645y CCC: $40.75 © 2008 American Chemical Society Published on Web 02/27/2008
MultiValency Iodine Doped TiO2 J&K China Chemical Ltd. All other chemicals were analytical grade and used without further treatment. Preparation. Ti(SO4)2 (2.5 g) was dissolved in 75 mL of distilled water containing 1.12 g of potassium iodate (KIO3). After the reaction mixture had been stirred for 8 h, the resultant white slurry was transferred to a Teflon-lined autoclave and reacted at 373 K for 24 h. The resultant yellow powder was centrifuged, washed with deionized water, and evaporated at 333 K before calcination at 575 K for 2 h. For comparison, pure TiO2 was prepared in a similar way without adding KIO3. Characterization. X-ray diffraction (XRD) patterns were collected on a Philips XPERT MPD X-ray diffractometer with Cu KR radiation. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. Data were recorded at a scan rate of 0.02° 2θ s-1 in the 2θ range from 10° to 80°. The crystallite size was calculated from X-ray line broadening by the Scherrer equation: D ) 0.89λ/β cos θ, where D is the average crystal size in nm, λ is the Cu KR wavelength (0.154 06 nm), β is the full-width at half-maximum, and θ is the diffraction angle. Lattice parameters were calculated by a least-squares fitting using Pearson VII functions and the Winfit program. Errors for XRD-derived parameters were estimated from 10% position (cell volume and parameters) and fwhm (particle size and strain) deviations. UV-vis diffuse reflectance spectra (DRS) were recorded on a Varian Cary 500 Scan UV-vis-NIR spectrometer with BaSO4 as the background between 200 and 800 nm. The Brunauer-Emmett-Teller (BET) surface area (SBET) was determined by nitrogen adsorption-desorption isotherm measurements at 77 K on a Micromeritics ASAP 2010 system. The samples were degassed in vacuum at 473 K until a pressure lower than 10-6 Torr before the actual measurements. All the samples were degassed at 473 K before the measurement. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained by JEOL model JEM 2010 EX instrument at the accelerating voltage of 200 kV. The powder particles were supported on a carbon film coated on a 3 mm diameter fine-mesh copper grid. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantum 2000 XPS system with a monochromatic Al KR source and a charge neutralizer. All the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. The surface photovoltage spectroscopy (SPS) was measured on an Edinburgh FL/FS900 spectrophotometer equipped with a lock-in amplifier (SR830) synchronized with a light chopper (SR540). The powder sample was sandwiched between two ITO glass electrodes, and monochromatic light was obtained by passing light from a 500 W xenon lamp (CHF-XQ500W) through a double prism monochromator (SBP300). The slit widths of entrance and exit were 2 and 1 mm, respectively. Photocatalytic Activity. The photocatalytic activity of the asprepared sample was determined through the degradation of gaseous acetone in a 2500 mL glass container. The container was connected to a quartz reactor with an inner size of 15 × 15 × 2 mm and interfaced to a gas chromatograph (GC, Hewlett-Packard 4890). A closed circulation reaction system, including the container, the reactor, and the GC, was established with the aid of a circulation pump. The quartz reactor was irradiated with a 500 W tungsten halogen lamp positioned inside a cylindrical Pyrex vessel and surrounded by a circulating water jacket (Pyrex) to cool the lamp. A cutoff filter was placed outside the Pyrex jacket to completely remove all wavelengths less than 420 nm to ensure irradiation with visible light only. A 0.8 g portion of the as-prepared sample was packed into the quartz reactor. Prior to the photodegradation experiment, the catalyst was allowed to reach a steady state with acetone in the dark overnight, and the equilibrium concentration of acetone was ca. 400 ppm. The flow rate of the system was 20 mL/min, and the reaction temperature was controlled at 32 ( 1 °C by an air-cooling system. The concentrations of residual acetone and the produced CO2 were measured at an interval of 20 min by gas chromatography equipped with a Porapak R column, a flame ionization detector, and a thermal conductivity detector. Terephthalic Acid Fluorescence Probe. The existence of •OH can be attested by the terephthalic acid (TA) fluorescent method,
Langmuir, Vol. 24, No. 7, 2008 3423
Figure 1. XRD patterns of I-TiO2 and TiO2. since •OH can react with TA and generate luminescent TAOH27,28 as shown in eq 1. •OH + TA f TAOH
(1)
A 40 mg portion of as-prepared photocatalysts was added to 80 mL of solution containing 3 mM TA in a quartz reactor. Prior to irradiation, the suspensions were magnetically stirred in the dark for 0.5 h to ensure the establishment of an adsorption/desorption equilibrium. At given irradiation time intervals, 3 mL of the suspensions was collected, centrifuged, and filtered through a Millipore filter (pore size, 0.22 µm) to remove the photocatalyst particles. The fluorescence spectra of the resultant solution were measured on an Edingburgh F-900 fluorescence spectrophotometer. Electronic Structure Calculations. To make insight into the effects introduced by iodine on the photocatalytic activity, further theoretical investigations were carried out to study the electronic structures of the system. In the present work, the (101) surface of anatase TiO2 was used since it is the thermodynamically most stable surface.29 A periodic slab model with 18 atomic layers was adopted to simulate the anatase TiO2(101) surface, and a supercell consisting of a (2 × 1) surface unit cell was employed. During the structural optimization for the slab, the atoms on the bottom seven layers were fixed to the bulk positions while other atoms including adsorbates were allowed to relax in all directions. The first-principles method with plane-wave basis set and ultrasoft pseudopotentials was used to optimized the configurations of the TiO2(101) surface, as implemented in the Vienna ab initio simulations package (VASP).30,31 The Perdew-Wang exchange-correlation was adopted, and the effects of spin polarization were considered. The energy cutoff and Monkhorst-Pack k-point grid were set to 396 eV and 3 × 3 × 1, respectively. To evaluate the atomic contribution to the electronic structure, further calculations based on the linear combination of atomic orbital (LCAO) were carried out to obtain the information about the band structures and density of states. In the calculations, the all-electron basis sets 8-6411G31d and 8-411G were adopted for Ti and O atoms, respectively, which have been specifically constructed to describe the electronic structure of TiO2.32,33 For I atom, Hay and Wadt’s relativistic effective core potentials (RECPs) were used, 46 electrons were incorporated into pseudopotentials, and a (4s2p/3s2p) basis set was adopted.34 To obtain a good prediction of the band gap,35,36 Becke’s three-parameter hybrid functional (B3LYP) was used in this stage. All the band-structure calculations were performed by using the CRYSTAL program on the structures optimized by plane(27) Mason, T. J.; Lorimer, J. P.; Bates, D. M.; Zhao, Y. Ultrason. Sonochem. 1994, 1, S91-95. (28) Hirakawa, T.; Nosaka, Y. Langmuir 2002, 18, 3247-3254. (29) Bredow, T.; Jug, K. Surf. Sci. 1995, 327, 398-408. (30) Kresse, G.; Furthmuller, J. Phys. ReV. B 1996, 54, 11169-11186. (31) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15-50. (32) Zicovich-Wilson, C. M.; Dovesi, R. J. Phys. Chem. B 1998, 102, 14111417. (33) Towler, M. D.; Allan, N. L.; Harrison, N. M.; Saunders, V. R.; Mackrodt, W. C.; Apra, E. Phys. ReV. B 1994, 50, 5041-5054.
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Table 1. Physicochemical Properties of I-TiO2 and Pure TiO2 Samples from N2 Sorption Analysis and XRD Results cell parameters (Å)
sample
SBETa (m2 g-1)
Vpb (cm3 g-1)
DBJHc (nm)
st straind (%)
crystallite size (nm)
a)b
c
cell volume (Å3)
I-TiO2 TiO2
176.0 98.7
0.34 0.30
6.1 10.6
2.289 0.872
7.6 23.7
3.811(1) 3.786(7)
9.540(4) 9.499(1)
138.550 136.209
a BET surface area, calculated from the linear part of the BET plot (P/P0 ) 0.05-0.3). b Total pore volume, taken from the volume of N2 adsorbed at P/P0 ) 0.995. c Average pore diameter, estimated using the desorption branch of the iostherm and the Barrett-Joyner-Halenda (BJH) formula. d The microstrain in the anatase lattice, calculated by Rietveld profile fitting using Pearson VII functions and Si standard sample.
wave calculations.37 Our previous investigations indicated that this two-step procedure could produce reasonable results.38,39
Results and Discussion X-ray Diffraction. X-ray diffraction (XRD) patterns of the as-prepared I-doped TiO2 and undoped TiO2 are shown in Figure 1. Both samples show peaks at 2θ values of 25.4°, 37.8°, 48.2°, 54.0°, and 55.2° which correspond to anatase (101), (004), (200), (105), and (211) crystal planes (JCPDS 21-1272). No other crystalline phase relating to iodine could be observed on I-doped sample. The diffraction peaks for I-doped TiO2 are apparently weaker and broader and indicate a smaller crystallite size compared to undoped TiO2. The average crystallite sizes calculated from the Scherrer equation for the I-doped and undoped TiO2 are 7.6 and 23.7 nm, respectively, and the microstrain of the TiO2 matrices increases accordingly from 0.872 to 2.289% with iodine doping (Table 1). It is obvious that iodine doping can inhibit the crystallite growth, probably due to the repulsion among the adsorbed iodine species. BET and TEM. The TEM images (Figure 2) reveal that both samples contain almost uniform particles in nearly spherical morphology and their sizes are comparable with those obtained from XRD pattern according to the Scherrer equation. Some aggregations among the particles can be observed. I-doped TiO2 shows higher surface area and pore volume (176 m2/g, 0.35 cm3/g) as compared to the undoped TiO2 (98.7 m2/g and 0.30 cm3/g) (Table 1). XPS. The X-ray photoelectron spectroscopy (XPS) measurement on I-doped TiO2 reveals the presence of Ti, O, I, C elements (Figure 3), and the banding energies for Ti 2p3, O 1s, I 3d5, and C 1s are located at 458.7, 529.9, 620.1 and 284.8 eV, respectively. The C element is ascribed to the adventitious hydrocarbon from the XPS instrument itself. The high-resolution XPS spectra of the I 3d5 region (inset in Figure 3) show doublet peaks at 624 and 619 eV, respectively, and suggest that I exists in multivalency in the I-doped sample. The binding energy at 624.0 eV should be ascribed to I7+, while the other peak at 619 eV originates from negatively charged iodine (I-) species.40 A similar binding energy for I 3d5 has been observed on mixed-valency I7+/I- in a high(34) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310. (35) Muscat, J.; Wander, A.; Harrison, N. M. Chem. Phys. Lett. 2001, 342, 397-401. (36) Zhang, Y. F.; Lin, W.; Li, Y.; Ding, K. N.; Li, J. Q. J. Phys. Chem. B 2005, 109, 19270-19277. (37) Saunders, V. R.; Dovesi, R.; Roetti, C.; Causa, M.; Harrison, N. M.; Orlando, R.; Zicovich-Wilson, C. M. CRYSTAL98 User Manual; University of Torino: Torino, 1998. (38) Zhang, Y. F.; Li, J. Q.; Liu, Z. F. J. Phys. Chem. B 2004, 108, 1714317152. (39) Lin, W.; Zhang, Y. F.; Li, Y.; Ding, K. N.; Li, J. Q.; Xu, Y. J. J. Chem. Phys. 2006, 124, 054704-1-8. (40) Handbook of X-ray Photoelectron Spectroscopy; Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. H., Muilenberg, G. E., Eds.; Perkin-Elmer Corp.: Eden Prairie, MN, 1979. (41) Hwang, S. J.; Park, D. H.; Choy, J. H. J. Phys. Chem. B 2004, 108, 12044-12048. (42) Xin, B.; Jing, L.; Ren, Z.; Wang, B.; Fu, H. J. Phys. Chem. B 2005, 109, 2805-2809. (43) Nauka, K.; Kamins, T. I. J. Electrochem. Soc. 1999, 146, 292-295. (44) Kronik, L.; Shapira, Y. Surf. Sci. Rep. 1999, 37, 1-5.
Tc superconducting IBi2Sr1.5-xLaxCa1.5Cu2Oy. The ascription of the binding energy at 627.5 and 624.5 eV to I5+ by Cai and Liu’s previous report are disputable, since they are much higher than that of HIO3 (623.1 eV). No peak around 623.1 eV can be observed and indicated that the iodate ion may undergo disproportionation to form heptavalent iodine (I7+) and negatively charged iodine (I-) species during the hydrothermal process, as shown in reaction 2.
4IO3- f 3IO4- + I-
(2)
The calculated Gibbs free energy (∆rGm) of reaction 2 is -10.84 kJ/mol at 373.15 K and therefore the reaction is thermodynamically feasible under the hydrothermal condition. Since I7+ exists in the form of periodate (IO4-) and the ionic radius of I- (0.216 nm) is much larger than that of O2- (0.124 nm) or Ti4+ (0.068 nm), the substitution of Ti4+ or O2- ions in the lattice with IO4-/I- species (at a high iodine doping level of 7 wt % as evaluated by AAS) would obviously affect the anatase matrix. This is not the case observed, since the position of the diffraction peaks of I-doped TiO2 are exactly the same as that of undoped TiO2. So it is possible that the I7+/I- species are dispersed on the surface of anatase particles. This is opposite to what Cai and Liu had reported, that cationic I5+ substitutes for Ti4+ in the lattice. Theoretical Calculations. The adsorptions of iodine species on two types of anatase TiO2(101) surface, namely, the perfect surface and the defect surface that is created by removing one bridging oxygen from the top layer of (2 × 1) supercell, have been investigated. For the perfect TiO2(101) surface, there are two kinds of Ti atom, one is six-coordinated and another is five-coordinated. The results of structural optimizations indicate that the IO4 group can be chemisorbed on the 5-fold Ti atom through one of its oxygen. In this configuration, the Ti-O length is 2.034 Å (Figure 4A) and the calculated adsorption energy is about 0.79 eV with respect to the pristine perfect surface and the free IO4 group. However, for the I atom, it is only bound to the perfect surface weakly, and the distance between I and 5-fold Ti atoms is about 3.1 Å. When one of the bridging oxygen is removed from the perfect surface, the four-coordinated Ti atoms are created. For the adsorptions of IO4 group on this defect surface, two possible configurations are considered, as shown in Figure 4B,C. If in the initial structure one oxygen atom of IO4 is located at the bridging site, the dissociation of the IO4 group is observed (Figure 4B), in which the bond between the O and I atoms is broken and the rest of the IO4 group is adsorbed on a 5-fold Ti atom by another oxygen atom. Therefore, in the final structure, all Ti atoms at the top layer are six-coordinated. If one O atom of IO4 is atop of a 4-fold Ti atom, another configuration is obtained, and in this structure (Figure 4C), all Ti atoms on the top layer are fivecoordinated. Examining the total energies of two structures shows that the first configuration is more stable than the second one by about 2.12 eV, which implies that the IO4 group tends to be
MultiValency Iodine Doped TiO2
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Figure 2. TEM images and SAED (inset) of (A) iodine-doped TiO2 and (B) undoped TiO2.
Figure 4. Side views of the optimized configurations for the adsorption of IO4 group on the perfect (A) and defect (B and C) anatase TiO2(101) (2 × 1) surfaces, the adsorption of I atom on defect surface (D), and the coadsorption of I and IO4 on defect surface (E). In the figures, the Ti, O, and I atoms are denoted by the small, medium, and large spheres, respectively. It is noted that in part B, one I-O bond of the IO4 group is broken. The lengths (in angstroms) of bond between adsorbate and TiO2 substrate are also shown.
Figure 3. XPS survey spectrum of iodine doped TiO2 samples. The inset shows the I 3d5 peaks in the 620 eV region.
dissociated on the defect surface. Compared with the perfect surface, I atom can be adsorbed on the defect surface and it almost occupies the bridging site between two Ti atoms (Figure 4D). In addition, we have also investigated the structure of the coexistence of I and IO4 species. As shown in Figure 4E, the I atom and IO4 group can be simultaneously attached to the defect anatase TiO2(101) surface, and the corresponding I-Ti and O-Ti
bond lengths are about 2.767 and 1.932 Å, respectively. Moreover, it is noted that the number of sites for the adsorption of IO4 group is larger than that of I atom, which is responsible for the disproportionate distribution of iodine species as discussed in the section 3.3 According to the eq 2, in the following, we only focus on the case of coadsorption of I and IO4 species. The calculated band structure for the perfect anatase TiO2(101) (2 × 1) surface is displayed in Figure 5A, and the predicted band gap (BG) is about 3.7 eV, comparable to the experimental range of values of 3.2-3.5 eV.45-48 Similar to rutile phase, the (45) Qian, X. M.; Qin, D. Q.; Song, Q.; Bai, Y. B.; Li, T. J.; Tang, X. Y.; Wang, E. K.; Dong, S. J. Thin Solid Films 2001, 385, 152-158. Tang, H.; Levy, F.; Berger, H.; Schmid, P. E. Phys. ReV. B 1995, 52, 7771-7774.
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Figure 6. UV-Vis DRS of I-doped TiO2 (solid) and undoped TiO2 (dashed).
Figure 5. The calculated band structure for the perfect anatase TiO2(101) (2 × 1) surface (A) and the band structure (B) and atomic density of states (C) of the defect surface after the coadsorption of the I and IO4 group. In parts A and B, the Fermi level (namely, the top of valence band) is taken as the energy zero. In part B, the dashed lines indicate the sub-bands originating from the I and IO4 group.
valence band mainly consists of oxygen states, while the Ti states give rise to the conduction band. Figure 5B presents the band structure for the defect surface after the coadsorption of I and IO4 group. It is interesting that several new energy bands appear in the BG of TiO2. The distributions of density of states of different atoms are also calculated and displayed in Figure 5C. It can be seen clearly that these additional bands are dominated by the states of I atom at the bridging site and those of IO4 group, and the new states that originated from the I atom of the IO4 group are found near the conduction-band bottom (CBB) region of TiO2. Although the energy gap between the highest O 2p band and the lowest Ti 3d band is slightly larger (about 0.1 eV) than that of undoped TiO2, the electrons in the VB can be excited to the new states by absorption of visible light. So these new states are beneficial for broadening the sensitive light wavelength and make possible photocatalytic degradation of organic contaminants under visible light irradiation. Examining the energy position of the top of VB (Figure 5A,B), the band potentials of I-doped TiO2 shift downward significantly (about 2.7 eV) with respect to that of undoped TiO2. This indicated that VB of I-doped TiO2 should have a stronger oxidative power than that of undoped TiO2. So it is reasonable that I-doped TiO2 shows a higher photocatalytic activity than undoped TiO2. UV-Vis DRS and SPS Spectra. UV-vis DRS of I-doped and undoped TiO2 are shown in Figure 6. The onset of the absorption edge for undoped TiO2 is 380 nm and is consistent with the intrinsic band gap absorption of pure anatase TiO2 (ca. 3.2 eV). I-TiO2 shows enhanced absorptions in the range from 400 to 550 nm, accompanied by the change in color from white to yellow. The absorptions in the visible region may be induced (46) Duzhko, V.; Timoshenko, V. Y.; Koch, F.; Dittrich, T. Phys. ReV. B 2001, 64, 075204-1-7. (47) Strelhow, W. H.; Cook, E. L. J. Phys. Chem. Ref. Data 1973, 2, 163-199. (48) Tang, H.; Berger, H.; Schmid, P. E.; Levy, F.; Burri, G. Solid State Commun. 1993, 87, 847-850.
by a sub-band-gap transition corresponding to the excitation from the valence band of TiO2 to the doped iodine species, as discussed in the theoretical study. The SPS technique can provide a rapid and straightforward investigation of this sub-band-gap transition. Since the sub-band-gap transition probability is low because the involved surface state is localized, the introduction of an appropriate external electric field could magnify the SPS signal. The SPS technique, especially when combined with the electricfield-modified technique,43 can provide important information about the carrier separation and transfer behavior. Without a bias electric field, the strong SPS response peak at 332 nm observed on undoped TiO2 can be attributed to the electron transition from valence band to conduction band of TiO2 (O 2pfTi 3d). However, the intensity of the SPS signal on I-doped TiO2 decreased, which indicates that the doping iodine can act as traps to capture the photoinduced electron and inhibit the recombination of the electron-hole pairs44 (Figure 7A). The introduction of an appropriate external electric field could magnify the SPS signal, and the SPS response changes of the I-doped TiO2 and undoped TiO2 in the presence of an external electric field are shown in parts B and C of Figure 7, respectively. The EFISPS spectra of undoped TiO2 only show one signal in the presence of the external field. However, a broad photoresponse in the visible light region observed in the EFISPS spectra of I-doped TiO2 can be ascribed to the sub-band-gap transition corresponding to the electron transition from the valence band of TiO2 to the doped iodine species. The SPS results are consistent with the UV-vis absorption behaviors. Photocatalytic Activity. The photocatalytic activity of I-TiO2 for the degradation of gaseous acetone under visible-light irradiation was investigated. Figure 8 shows the concentration changes of acetone and CO2 in the system as a function of visible light irradiation time. It is observed that I-doped TiO2 exhibited obvious photocatalytic activity under visible light illumination. Within 300 min illumination, more than 94% of the initial CH3COCH3 (400 ppm) was degraded, and about 83% of the degraded CH3COCH3 was mineralized to CO2 (935 ppm) over I-doped TiO2. On the contrary, neither undoped TiO2 nor P25 TiO2 shows photocatalytic activity for acetone decomposition under visible light irradiation. The as-prepared I-doped TiO2 also shows photocatalytic activity on the degradation of MO (see the Supporting Information). Mechanism Discussion. On the basis of the experimental and calculated results, a possible sub-band-gap transition mechanism was proposed for the visible light photoactivity on this multivalency I doped TiO2. The addition of I creates IO4-/I- species. The redox potential E0(I7+/I-) ) 1.24 V lies between the conduction band Ecb(TiO2) ) -0.5 V and the valence band Evb(TiO2) ) 2.7 V (versus NHE); therefore, the excitation of an electron from the valence band of TiO2 to the surface I species is feasible. Theoretical results reveal that new states originating
MultiValency Iodine Doped TiO2
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Figure 9. Proposed photocatalytic mechanisms over I-TiO2.
Figure 7. The surface photovoltage spectroscopy (SPS) spectra (A) of pure TiO2 and I-TiO2 without external electric field. SPS spectra of (B) pure TiO2 and (C) I-TiO2 at different external electric fields.
Figure 8. The photooxidation of acetone over I-TiO2 and TiO2 under visible light irradiation. The symbols O and b represent C3H6O and CO2, respectively.
from the I atom of the IO4 group are observed near the conductionband bottom (CBB) region of TiO2 and again suggest a possible electron excitation from the valence band of TiO2 to the surface IO4-. This could induce the visible light response of I-doped TiO2. The holes left on the valence band of TiO2 can oxidize hydroxyl to give hydroxyl radicals (•OH), which can degrade the organic pollutants. In the meantime, the electrons can reduce IO4- to I-. A detailed illustration of this mechanism is shown in Figure 9. This mechanism can be corroborated by the following experiments. One is the terephthalic acid (TA) fluorescence probe
Figure 10. Fluorescence spectra recorded during visible light illumination on I-TiO2 sample in terephthalic acid solution (3 mmol/ L, excitation at 320 nm).
method and the other is the XPS characterization of the I-doped TiO2 after the photocatalytic reaction. It is known that TA reacts with •OH to give a highly luminescent TAOH and could be used as a sensitive probe in detecting •OH. The change of the fluorescent signal corresponding to TAOH in the illumination of I-doped TiO2 in terephthalic acid solution is shown in Figure 10. It is observed that the intensity of the fluorescence at 430 nm increases gradually and indicates the formation of •OH in the visible light illuminated I-doped TiO2. On the contrary, the visible light illuminated TiO2 did not give any fluorescent signal under otherwise similar condition. The terephthalic acid probe method confirms that photogenerated holes left in the valence band of I-doped TiO2 do oxidize hydroxyl to give •OH under visible light illumination. The XPS measurement on the I-doped TiO2 after reacting for 15 h is shown in Figure 11. A decrease in the intensity at a binding energy of 624 eV accompanied by an increase in the intensity at a binding energy of 619 is observed. This indicates that during the photocatalytic reaction, I7+ acted as an electron acceptor and was reduced to I-. The efficient electron scavenging by IO4- results in an efficient separation of electron-hole. However, the evolved CO2 was calculated to be 3500 ppm (ca. 3.5 × 10-3 mol) for the total 15 h reaction. In the meantime, only about 1.4 × 10-4 mol of IO4- has been changed to I-, as determined from the XPS spectra. The nonstoichiometry between the evolved CO2 and the amount of IO4- transformed to I- gives an indication that it is not a simple suicide reaction. It is probably that part of the I- species can be oxidized back to IO4- by the photogenerated holes and the I-doped TiO2 can regain part of
3428 Langmuir, Vol. 24, No. 7, 2008
Su et al.
appear in the band gap of TiO2. A sub-band-gap transition corresponding to the excitation from the valence band of TiO2 to the doped iodine species contributes to the visible light response of I-doped TiO2. The as-prepared I-doped TiO2 exhibits obvious photocatalytic activity in the degradation of gaseous acetone under visible light irradiation. Our result indicates that to a certain degree a cycle between IO4- and I- can be maintained, although the as-prepared I-doped TiO2 is deactivated after a long-term photocatalytic reaction. An in-depth study on this system is still going on in our laboratory.
Figure 11. I3d5 XPS spectra for fresh I-TiO2 and I-TiO2 after photocatalytic degradation of acetone for 15 h (inset).
its photocatalytic activity (Figure 9). Therefore, this reaction could be best described as an IO4--sensitized photocatalytic reaction.
Conclusions Multivalency I-doped TiO2 is prepared by a combination of a deposition-precipitation process and hydrothermal treatment. XPS measurement reveals that the iodine exists in both I7+ and I- chemical states in the as-prepared I-doped TiO2. Theoretical studies show that upon iodine doping, some iodine energy bands
Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20573020, 20537010, 20677009, and 20571015), National Basic Research Program of China (973 Program, 2007CB613306), 973 Specialize Program (2007CB616907), the National High Technology Research and Development Program of China (863 project 2006AA03Z340), the Health Ministry Foundation of China (WKJ2005-2-003), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (20060386001). Supporting Information Available: A figure showing the photodegradation of MO over I-doped TiO2, undoped TiO2, and Degussa P25 under UV (λmax ) 365 nm) or visible light (λ g 420 nm) irradiation. This information is available free of charge via the Internet at http://pubs.acs.org LA701645Y
