14948
J. Phys. Chem. C 2008, 112, 14948–14954
Iodine-Doped TiO2 Photocatalysts: Correlation between Band Structure and Mechanism Sachiko Tojo, Takashi Tachikawa, Mamoru Fujitsuka, and Tetsuro Majima* The Institute of Scientific and Industrial Research (SANKEN), Osaka UniVersity, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan ReceiVed: June 5, 2008; ReVised Manuscript ReceiVed: July 7, 2008
Iodine-doped TiO2 powders (I-TiO2) prepared via hydrothermal treatment have absorption in the region of ultraviolet (UV) and visible light, and were used as a photocatalyst with irradiation of UV or visible light. The I-TiO2 powders were characterized by XRD, TEM, EDS, XPS, FTIR, and steady-state UV-vis diffuse reflectance spectra (DRS), and their photocatalytic activities were investigated based on the photodegradation of 4-chlorophenol (4-CP) in water. A higher photodegradation efficiency of 4-CP was observed for I-TiO2 under UV- and visible-light irradiation, when compared to the undoped TiO2. The transient behavior of the photogenerated charge carriers, such as trapped electrons (e-) and holes (h+), and the one-electron oxidation dynamics of substrates during UV or visible laser flash photolysis of undoped TiO2 and I-TiO2 powders were investigated using time-resolved diffuse reflectance (TDR) spectroscopy. The time evolution of transient signals indicated that the long-lived photogenerated h+ were formed upon the laser excitation of I-TiO2 powders, while no trapped e- were observed. From the experimental results, it is suggested that the recombination of e--h+ pairs is inhibited because the doping I sites act as trapping site to capture the e- during the I-TiO2 photocaltalytic reaction. Furthermore, the trapped h+ generated in I-TiO2 have no significant oxidation reactivity toward substrates, such as aliphatic and aromatic compounds, adsorbed on the surface under both UV- and visible-light irradiation. Introduction The efficient utilization of solar energy is one of the major goals of modern science and engineering that will have a great impact on technological applications. Of the materials being developed for photocatalytic applications, titanium dioxide (TiO2) remains the most promising because of its high efficiency, low cost, chemical inertness, and photostability.1-4 However, the widespread technological use of TiO2 is impaired by its wide band gap (3.2 eV), which requires UV light for photocatalytic activation. A current area of research in this field is to modify TiO2 so that it is sensitive to visible light. One approach is to substitute for a Ti site. Anpo et al. substituted Mn4+, Cr3+, or V3+ (V4+) at lattice positions of Ti4+ in TiO2 by a metal ionimplantation method and enhanced the photocatalytic activity of TiO2.5,6 Another case is the doping of TiO2 with nonmetal atoms, such as nitrogen (N),7-13 sulfur (S),14-17 carbon (C),18-21 and fluorine (F).22,23 Hashimoto et al. substituted the N atom at lattice positions of O in TiO2 by annealing anatase TiO2 powders under NH3 gas flow.8 Also in this case the photocatalytic activity depends on the content of nonmetal atoms and on the method of preparation. The trapping of photogenerated holes (h+) at midgap states above the top of the valence band will cause a decrease in their oxidation power. In addition, the doping may induce instability of the TiO2, owing to the introduction of lattice distortion and bond weakening. Recently, by using the time-resolved diffuse reflectance (TDR) spectral measurements, our research group found that h+ generated during the visible-light irradiation of N-, S-, and C-doped TiO2 nanoparticles were quickly trapped in the doping sites. The trapped h+ have a poor reactivity because of their low oxidation potentials compared with those of the adsorbates, * Corresponding author. Telephone: +81-6-6879-8495. Fax: +81-6-68798499. E-mail:
[email protected].
such as methanol (CH3OH) and aromatic sulfides.24-26 On the other hand, the reduction properties or indirect oxidation properties via reactions with surface intermediates due to oxygen reduction or water oxidation would be maintained upon irradiation with visible light because the h+ trapped at the N, S, or C doping sites do not serve as effective charge recombination centers.24 Recently, Cheng et al. have synthesized I-doped mesoporous titania with a bicrystalline framework and shown the high visible photocatalytic activity for the photodegradation of methylene blue.27 Cai et al. have reported that I-TiO2 shifted its optical absorption to the longer wavelength and enhanced the photocatalytic activities, such as photodegradation of methylene blue, during the visible-light irradiation.28 Fu et al. also reported the preparation, characterization, and electronic structure of multivalency iodine (I7+/I-) doped TiO2.29 It is well-known that the I atom has several redox states for the cationic and anionic doping sites. However, the role of the I doping site and the reaction mechanism have not been elucidated. In addition, no clear experimental evidence has been given on the origin of the visible-light response for I-TiO2. Herein we studied the structural and optical characteristics of I-TiO2 nanoparticles and their photocatalytic reactions under UV- and visible-light irradiation. The influence of I atom doped in the TiO2 on the carrier dynamics of photogenerated electrons (e-) and h+ was investigated using nanosecond TDR spectroscopy. The relationship between the carrier trapping at the doping sites and the redox processes is examined. On the basis of the experimental results, we discuss the features of the band structures of I-TiO2, and correlation of the band structure and the photocatalytic activity under UV- and visible-light irradiation.
10.1021/jp804985f CCC: $40.75 2008 American Chemical Society Published on Web 08/30/2008
Iodine-Doped TiO2 Photocatalysts
Figure 1. Powder XRD patterns of TiO2 (a) and I-TiO2 (b-f): powders calcined at 400 (a), 300 (b), 350 (c), 400 (d), 450 (e), and 500 °C (f). (Key: A, anatase; R, rutile; B, brookite.)
Figure 2. TEM image (a) and EDS spectrum (b) of I-TiO2 calcined at 400 °C.
Results and Discussion XRD Measurements. Figure 1 shows the powder X-ray diffraction (XRD) patterns of the undoped TiO2 calcined at 400 °C (a) and as synthesized I-TiO2 powders calcined at 300-500 °C (b-f). It could be concluded from Figure 1 that I-TiO2 shows peaks corresponding to anatase phase and undoped TiO2 shows peaks corresponding to anatase and brookite phases. The hydrothermal synthesis of titania typically produces a mixture of brookite and anatase. However, the hydrothermal synthesis produced I-TiO2 with the anatase form, selectively. Apparently I-TiO2 was transformed from anatase to rutile above 400 °C since the XRD peaks assigned to rutile increased as shown in parts b-f. Because the transformation temperature is known to be ∼700 °C for undoped TiO2, it is suggested that the presence of I in I-TiO2 has the effect of unrestraining this phase transformation. The I doping may induce instability of the TiO2, owing to the introduction of lattice distortion and bond weakening. TEM and EDS Measurements. Figure 2a shows a typical transmission electron microscopy (TEM) image of the partly agglomerated I-TiO2 calcined at 400 °C. The crystallites of I-TiO2 were found to have diameters in the range of 7-10 nm. The diameter of undoped TiO2 was also found to be about 10
J. Phys. Chem. C, Vol. 112, No. 38, 2008 14949 nm. We further investigated to determine the composition of I-TiO2 using an energy dispersive X-ray spectrometer (EDS). An EDS spectrum is presented in Figure 2b. In addition to the strong Ti peaks, weak I atomic peaks at 3.923 and 28.60 keV were observed to be assigned to I-LR and I-KR, respectively. However, the EDS result cannot distinguish the oxidation number of I atom. XPS and ATR-FTIR Studies. Figure 3a shows X-ray photoelectron spectroscopy (XPS) survey spectra of I-TiO2 calcined at 400 °C, indicating the existence of Ti, O, and I elements. The high resolution XPS spectra with scanning over the following three areas were analyzed: the binding energies for the Ti 2p region around 460 eV (b), the I 3d region around 620 eV (c), and the O 1s region around 530 eV (d). As shown in Figure 3b, the XPS peaks in the Ti 2p region appear at 458.2 (Ti 2p3/2) and 463.8 (Ti 2p1/2) eV for I-TiO2. The binding energy of Ti 2p3/2 shifted to a positive value by 0.6 eV, when compared to that of undoped TiO2 (458.8 eV, data not shown). The XPS spectrum of the I 3d region in panel c shows two doublet peaks at 623.6 (I 3d5/2) and 635.0 (I 3d3/2) eV and at 620.1 (I 3d3/2) and 631.6 (I 3d5/2) eV. The intensities of doublet peak at 620.1 and 631.6 eV are much weaker than those at 623.6 and 635.0 eV. The weak doublet peak was equivalent to that in the I2.30 The main peak at 623.6 eV was 0.6 eV higher than that in the HIO3 (623.0 eV).30 The atomic content of I incorporated into the lattice of I-TiO2 was roughly estimated to be about 4% from the relative sensitivity factor (RSF) method. There was no detectable peak due to I7+ at the binding energy of 624.0 eV, which was reported by Fu et al.29 Thus it can be inferred that the oxidation state of the doped I is assigned to I5+.31 These results suggest that cationic I5+ ions substitute for Ti4+ and are present in the I-O-Ti bond, as reported by Cai and Liu.28 It should be noted that the I 3d5/2 doublet peak of I5+ decreased with a significant increase in the peaks of I- at 620.1 and 631.6 eV during measurement (inset of Figure 3c). The fact clearly indicates that doped I5+ acts as an electron acceptor to produce I-. The XPS spectrum of the O 1s region shows a major peak at 529.2 eV attributed to the bulk oxygen (O2-) and a weak peak at 531.4 eV due to the presence of surface hydroxyl (OH) groups.32,33 As shown in Figure 4, the binding energy of O 1s shifted to a negative value by 0.8 eV, when compared to that of undoped TiO2, suggesting that a partial electron transformation from the I to O in I-O-Ti bonds results in an increase of the electron density on O. The apparent Osurface OH/Obulk O2- ratio is determined to be 0.07 for I-TiO2, which is 3.6-times lower than that of TiO2. The results indicate that the amounts of surface OH groups were significantly decreased by I doping into the TiO2. The attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy was utilized to detect the surface OH groups and water adsorption on the surface, as shown in Figure 5. For undoped TiO2, an overlapped broad peak at 3375 and 3120 cm-1 and a weak peak at about 1626 cm-1 were observed. The broad peak at 3375 cm-1 is characterized as surface Ti-OH and hydrogen-bonded molecular H2O species, while the absorption at 3120 cm-1 is associated with water complexes that are strongly bound to the TiO2 surface.34 The weak absorption at 1626 cm-1 is assigned to the deformation vibration for H-O-H bonds of the physisorbed water.35-37 On the other hand, no clear peaks were observed for I-TiO2, indicating that the I-TiO2 surface is more hydrophobic than undoped TiO2.
14950 J. Phys. Chem. C, Vol. 112, No. 38, 2008
Tojo et al.
Figure 3. XPS spectra of I-TiO2 calcined at 400 °C: survey spectrum (a) and high resolution XPS spectra for Ti 2p (b), I 3d (c), and O 1s (d). The inset in panel c shows the spectral change during measurement.
Figure 4. XPS spectra of O 1s observed for I-TiO2 and TiO2.
Figure 5. ATR-FTIR spectra observed for I-TiO2 and TiO2.
UV-Vis Absorption Spectroscopy. The steady-state diffuse reflectance spectra of undoped TiO2 and I-TiO2 powders at different calcined temperatures are shown in Figure 6a. The spectrum of N-doped TiO2 (N-TiO2) powder synthesized in this work is also shown for comparison. The photoabsorption in the visible region observed for I-TiO2 was much stronger than those of TiO2 and N-TiO2 powders. The absorbance values at 355
Figure 6. Steady-state diffuse reflectance spectra of TiO2, N-TiO2, and I-TiO2 powders (a) and plots of the square root of the Kubelka-Munk function (F(R)) versus the photon energy (Eph) (b).
nm (3.50 eV) were 0.64, 0.62, and 0.65 for the undoped, N-doped, and I-doped TiO2, respectively; on the other hand, those at 450 nm (2.75 eV) were 0.04, 0.10, and 0.17, respectively. As shown in Figure 6b, the plots of the Kubelka-Munk functions (F(R)) against the photon energy (Eph) indicate that the band gaps of TiO2 and N-TiO2 are 2.95 and 2.90 eV, respectively.38 In contrast, the band gaps of I-TiO2
Iodine-Doped TiO2 Photocatalysts
Figure 7. Comparison of the photocatalytic decomposition of 4-CP in the presence of TiO2 (triangles), I-TiO2 (circles), and ST-01 (squares) as monitored by the changes in absorbance at 280 nm after UV- (a) and visible-light (>440 nm) (b) irradiation. C0 indicates the initial concentration of 4-CP. The inset in panel a shows the UV-light irradiation time dependence of absorption spectra of 4-CP obtained for I-TiO2. The inset in panel b shows the effect of calcined temperature on the photodegradation of 4-CP obtained for I-TiO2.
are roughly estimated to be 1.4-2.2 eV. It is obvious that the absorption edges shift toward longer wavelength with increasing calcined temperature. Photocatalytic Degradation of 4-CP. To compare the photocatalytic activities of undoped TiO2, commercial anatase TiO2 (ST-01, Ishihara Sangyo), and I-TiO2 calcined at 400 °C, we examined the photocatalytic degradation of 4-chlorophenol (4-CP) in water at neutral pH upon UV- and visible-light irradiation as shown in Figure 7. There was no obvious change in the concentration of 4-CP in the dark for 3 days. Under UVlight irradiation, the photocaltalytic activity of I-TiO2 was higher than those of undoped TiO2 and ST-01 (Figure 7a). Under visible-light irradiation at the wavelength longer than 440 nm, only I-TiO2 showed the photocatalytic activity (Figure 7b). The absorption spectra of 4-CP at various UV-irradiation times are shown in the inset of Figure 7a. The characteristic absorption of 4-CP at 280 nm almost disappeared after 90 min, but the broad absorption at
