Article pubs.acs.org/JPCC
Doping Behavior of Zr4+ Ions in Zr4+-Doped TiO2 Nanoparticles Jingsheng Wang,† Yanlong Yu,† Sha Li,† Limei Guo,† Enjun Wang,‡ and Yaan Cao*,† †
The MOE Key Laboratory of Weak Light Nonlinear Photonics, TEDA Applied Physics Institute and School of Physics, Nankai University, Tianjin 300457, China ‡ Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China
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S Supporting Information *
ABSTRACT: TiO2 nanoparticles doped with different concentrations of Zr4+ ions were prepared by the sol−gel method and annealed at different temperatures. X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and high resolution transmission electron microscopy (HRTEM) techniques were used to investigate the existing states and doping mechanism of dopants as well as the phase transition of the Zr4+-doped TiO2 samples. It was revealed that the doping behavior of introduced Zr4+ ions was closely related to the doping concentration. The Zr4+ ions would replace the lattice Ti4+ ions directly in substitutional mode at a certain annealing temperature. Moreover, if the concentration of doped Zr4+ ions is high enough, excess Zr4+ ions would form ZrTiO4 on the surface of TiO2. In addition, the phase transition temperature from anatase to rutile increases significantly after doping Zr4+ ions, due to their larger electropositivity and radius than those of Ti4+ ions. Our results may afford a better understanding on the doping mechanism and aid in the preparation of Zr-doped TiO2 with high photoelectric performance. the influence of Zr4+ doping on the structure and the phase transition of TiO2, and the existing states of Zr4+ ions under different conditions. In this work, a series of TiO2 doped with different concentrations of Zr4+ ions were prepared via the sol−gel method and annealed at different temperatures. We studied the doping mode and existing states of Zr4+ ions in TiO2 and the conversion from substitutional doping Zr4+ ions to ZrTiO4 as well as the influence on the phase transition temperature of TiO2.
1. INTRODUCTION As one of the most important functional materials, TiO2 has been extensively investigated for its wide applications from catalysis to photoelectrochemistry.1−10 However, owing to its large band gap, pure TiO2 cannot use solar energy efficiently, especially visible light. Doping TiO2 with metal or nonmetal elements is considered to be one of the most efficient methods to make full use of solar light. Asahi et al. found that nitrogendoped TiO2 exhibits enhanced visible-light response and improved photocatalytic activity under visible-light irradiation.11 Zhao et al. reported that doping B in TiO2 could extend the spectral response to the visible region.12 Choi and coworkers reported that doping TiO2 with Fe, Mo, Ru, Os, Re, V, Rh, Co, and Al is an effective method to enhance photocatalytic activity.13,14 A series of other metal or rare earth ions such as W6+, V5+, Ce4+, Zr4+, Fe3+, Cu2+, La3+, Pd2+, Cr3+, Ag+, Sm3+, Nd3+, and Pr3+ have been also investigated.15−20 As a titanium subgroup element, Zr has the same valence shell structure (n − 1)d2ns2 and valence state as Ti, so Zr4+-doped TiO2 has attracted much interest from researchers. Chang and Doong21 prepared a series of Zr4+-doped TiO2 via a sol−gel method. Venkatachalam et al.22 reported that Zr4+ doping of TiO2 presented enhanced catalytic activity. Lukáč et al.23 researched the influence of different temperature treatments on the photocatalytic performance of Zr4+-doped TiO2. Liu et al.24 reported the synthesis and photocatalytic activity of Zr-doped TiO2 nanotube arrays. However, there are few systematic reports on the doping mechanism of Zr4+ ions in TiO2, such as © 2013 American Chemical Society
2. EXPERIMENTAL METHODS 2.1. Sample Preparation. At room temperature, a certain amount of ZrOCl2 was added into 50 mL of anhydrous ethanol solution. After mixing equably under vigorous stirring, 15 mL of tetrabutyl titanate and 3 mL of deionized water were added dropwise into the ethanol solution. The pH of the mixture was 0.8, controlled by adding concentrated HCl (12 mol/L). The obtained sol was stirred continuously until the formation of gel, which was aged for 24 h at room temperature. The as-prepared gel was dried at 100 °C for 10 h, and then triturated to powder in an agate mortar. The powder was calcined at different temperatures for 2.5 h. A series of zirconium-doped TiO2 Received: August 1, 2013 Revised: November 26, 2013 Published: November 27, 2013 27120
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catalysts with different concentrations of zirconium were prepared by changing the amount of ZrOCl2 added in the ethanol solution. Pure TiO2 samples were prepared with the same procedures, but without adding ZrOCl2. The pure and Zr4+-doped TiO2 were designated as P-T and D-T-x, respectively, where T stands for the annealing temperature and x represents the nominal molar percentage concentration of Zr4+ ions in all metal ions (Zr4+ and Ti4+) in TiO2. In all experiments, deionized water (ρ = 18.2 MΩ cm−1) was used. All of the chemicals were of analytical grade. 2.2. Characterization. X-ray diffraction (XRD) patterns were acquired on a Rigaku D/max 2500 X-ray diffraction spectrometer (Cu Kα, λ = 1.540 56 Å). The average crystallite size was calculated according to the Scherrer formula (D = kλ/ B cos θ). X-ray photoelectron spectroscopy (XPS) measurements were carried out with an ESCA Lab 220i-XL spectrometer by using an unmonochromated Al Kα (1486.6 eV) X-ray source. All of the spectra were calibrated to the binding energy of the adventitious C 1s peak at 284.8 eV. Raman spectra were taken on a Renishaw inVia Raman microscope by using the 785 nm line of a Renishaw HPNIR 785 semiconductor laser. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) analyses were conducted using a JEM-2010FEF, for which the samples were prepared by applying a drop of ethanol suspension onto an amorphous carbon-coated copper grid and dried naturally.
3. RESULTS AND DISCUSSION 3.1. Phase Transition and Structure of Zr4+-Doped TiO2. Figure 1 shows the XRD spectra of pure TiO2 (Figure 1A) and Zr 4+ -doped TiO 2 samples with a Zr 4+ ion concentration of 20% (Figure 1B) annealed at different temperatures, respectively. In Figure 1A, it can be found that P-100 exhibits an amorphous structure, and P-200, P-300, P400, and P-500 exhibit an anatase structure.25 Besides the peaks of anatase, a weak peak at around 27.3° is found in P-500, indicating the formation of slight rutile. Compared with P-500, the diffraction peaks of rutile increase and those of anatase decrease in P-600, which means the phase transition from anatase to rutile occurs at 600 °C. Anatase is transformed into rutile completely when the annealing temperature is above 700 °C. As shown in Figure 1B, D-100-20 and D-200-20 show amorphous structure and D-T-20 (300 ≤ T ≤ 900) samples exhibit an anatase structure. For the D-1000-20 sample, the diffraction peaks at 27.3 and 35.8° are observed respectively, corresponding to the (110) and (101) planes of rutile. Hence, the phase transition temperature from anatase to rutile is at around 1000 °C for the D-T-20 sample, which is about 400 °C higher than that for pure TiO2 (600 °C). This result demonstrates that the phase transition temperature of Zr4+doped TiO2 from anatase to rutile is enhanced compared with pure TiO2. Furthermore, besides the peaks of anatase, a weak peak of ZrTiO4 around 30.3° is observed in the D-800-20 sample, whose peak intensities increase gradually when the annealing temperature rises from 800 to 1000 °C.26 Figure 2 shows the XRD patterns of a series of Zr4+-doped TiO2 samples with different concentrations of Zr4+ ions annealed at 500 °C (Figure 2A) and 1000 °C (Figure 2B), respectively. In Figure 2A, the pure TiO2 and D-500-x (x = 5, 10, 20, and 30) samples exhibit anatase structure. Besides the diffraction peaks of anatase, the peaks of ZrTiO4 are observed in D-500-40.26 In Figure 2B, pure TiO2, D-1000-5, and D-1000-
Figure 1. XRD patterns of (A) pure TiO2 and (B) D-T-20 samples. Inset of (B) displays the enlargement of (101) plane.
10 show mainly rutile structure. For D-1000-20, D-1000-30, and D-1000-40, besides the diffraction peaks of rutile and ZrTiO4, the diffraction peaks of anatase are observed, due to the increased phase transition temperature caused by the doped Zr4+ ions. 3.2. Doping Mode and Existing States of Zr4+ Ions in TiO2. 3.2.1. XRD. It is documented that there are usually two kinds of doping mode, interstitial and substitutional, for doped metal ions in oxides. As the ionic radius of Zr4+ (72 pm) is larger than that of lattice Ti4+ (53 pm), the doped Zr4+ ions doping in interstitial mode could be excluded. In the case of substitutional mode, the doping Zr4+ ions will replace the lattice Ti4+ ions and thus occupy the positions of the lattice Ti4+ ions, leading to an increase of the lattice parameters and cell volume, accompanied by a shift to lower diffraction angles of XRD peaks, compared with pure TiO2. It is clearly seen that the diffraction peaks of anatase for D-500-x and rutile for D-1000-x samples gradually shift to lower angles with the increase of Zr4+ ion concentration compared with those of pure TiO2 (insets of Figure 2A,B), and the cell parameters and cell volume of Zr4+doped TiO2 samples are larger than those of pure TiO2 (Tables S1−S3 in the Supporting Information), which indicates that the doping Zr4+ ions enter the lattice of TiO2 in substitutional mode. It is also found from the inset of Figure 1B that the diffraction peak of the (101) plane in Zr4+-doped TiO2 is located at the same angle (25.0°) when annealed from 300 to 800 °C. This implies that the Zr4+ ions exist in TiO2 in substitutional mode when the annealing temperature is below 27121
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Figure 3. Raman spectra of (A) D-500-x and (B) D-1000-x samples. Figure 2. XRD patterns of (A) D-500-x and (B) D-1000-x samples. Insets of (A) and (B) display enlargement of (101) and (110) planes.
a Zr−O bond is formed in the lattice. Moreover, the ionic radius of Zr4+ (72 pm) is larger than that of lattice Ti4+ (53 pm), so the length of the Zr−O bond is longer than that of the Ti−O bond in the lattice. As a result, a tensile stress in the lattice is induced, leading to the decrease of vibrational energy of the 144 cm−1 peak (Eg) corresponding to the O−Ti−O bond bending vibration and the 396 cm−1 peak (B1g) corresponding to the O−Ti−O bond symmetric bending vibration.27 Therefore, the peaks at 144 cm−1 (Eg) and 396 cm−1 (B1g) for Zr4+-doped TiO2 samples shift to lower frequencies compared with the pure TiO2. However, the tensile stress due to the Zr−O bond in the lattice cannot change the vibrational energy of the 516 cm−1 peak (A1g and B1g) corresponding to the Ti−O stretching vibration and the 638 cm−1 peak (Eg) corresponding to the O−Ti−O bond symmetric stretching vibration;27 thus no shift of the peaks around 516 and 638 cm−1 is observed. In Figure 3B, the peaks of rutile at about 144 cm−1 (B1g), 234 cm−1 (two phonon process), 447 cm−1 (Eg), and 612 cm−1 (A1g) were observed for P-1000, D-1000-5, and D-1000-10, respectively.29 The peaks of ZrTiO4 for D-1000-20, D-1000-30, and D-1000-40 were observed.28 Furthermore, the peaks of anatase are observed for the D-1000-20, D-1000-30, and D1000-40 samples, due to the high phase transition temperature of Zr4+-doped TiO2 samples. Moreover, it can be seen that the Eg peak shifts to lower frequency with increase of concentration of Zr4+ ions from 5 to 30% compared with pure TiO2 (Figure 3B), indicating that the doping Zr4+ ions enter the lattice of rutile in substitutional mode. The Eg mode (447 cm−1) corresponds to the librational motion of oxygen atoms along the c-axis, while the A1g mode (612 cm−1) corresponds to the
800 °C. Furthermore, the diffraction peaks of anatase in Zr4+doped TiO2 gradually shift to higher angles, while a new diffraction peak of ZrTiO4 (30.3°) appears and becomes stronger with the increase of the annealing temperature from 800 to 1000 °C, implying the substitutional doped Zr4+ ions in TiO2 become saturated at 800 °C and change into ZrTiO4 gradually when the annealing temperature is above 800 °C. In addition, the diffraction peaks of ZrTiO4 are detected for D500-40 (Figure 2A) and 10% for D-1000-10 (Figure 2B). These results imply that the doped Zr4+ ions in substitutional mode are saturated when the Zr4+ ions increase to a certain concentration, and the residual Zr4+ ions form ZrTiO4 whether the structure of TiO2 is anatase or rutile. The chemical states and the doping mode of Zr4+ ions will be confirmed by Raman spectroscopy, XPS, and HRTEM in sections 3.2.2 and 3.2.3. 3.2.2. Raman Spectroscopy. Figure 3 shows Raman spectra of D-500-x (Figure 3A) and D-1000-x (Figure 3B) samples. In Figure 3A, the typical anatase peaks at about 144 cm−1 (Eg), 194 cm−1 (Eg), 396 cm−1 (B1g), 516 cm−1 (A1g and B1g), and 638 cm−1 (Eg) were observed, respectively.27 Besides the peaks of anatase, two weak peaks at about 281 and 332 cm−1 were observed for D-500-40, which is ascribed to the characteristic Raman peaks of ZrTiO4.28 Compared with those of pure TiO2, the peaks at about 144 cm−1 (Eg) and 396 cm−1 (B1g) shift to lower vibrational frequencies with increase of the concentration of Zr4+ ions (Figure 3A), and no shift is observed for 516 cm−1 (A1g and B1g) and 638 cm−1 (Eg). This result suggests that the introduced Zr4+ ions enter the lattice of anatase in substitutional mode. Since the lattice Ti4+ ion is replaced by a Zr4+ ion, 27122
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Ti−O stretching vibration.30 When lattice Ti4+ ions were substituted by the Zr4+ ions, the O2− ions could be bound more tightly due to the greater electropositivity of Zr4+ ions than that of Ti4+ ions. As a result, the librational motion of oxygen atoms along the c-axis is inhibited, leading to a decrease of the vibrational energy of oxygen atoms’ librational motion corresponding to the Eg mode (447 cm−1), resulting in a shift to lower frequency. However, the substituted Zr4+ ions in rutile structure have little effect on the two phonon process (234 cm−1) and the A1g mode (612 cm−1) corresponding to the Ti−O stretching vibration energy. Thus, the peaks at 234 and 612 cm−1 almost do not shift with the increase of Zr4+ ion concentration. 3.2.3. XPS and HRTEM. XPS and HRTEM measurements were carried out to further investigate the existing states of Zr4+ ions in TiO2. Figure 4 shows the XPS spectra of Zr 3d for D-
500-5, D-500-40, D-1000-5, and D-1000-40, respectively. The double peaks (181.8 and 184.1 eV) in the Zr 3d spectra of D500-5 and D-1000-5 samples could be ascribed to Zr 3d5/2 and Zr 3d3/2 of the doping Zr4+ ions in substitutional mode, since the peak of Zr 3d5/2 (181.8 eV) locates between that of ZrO2 (183.5 eV)31 and metallic Zr (179.0 eV).32 Besides the peaks of Zr4+ ions in substitutional mode, the double peaks (182.2 and 184.6 eV) in the Zr 3d spectra of D-500-40 and D-1000-40 are ascribed to Zr 3d5/2 and Zr 3d3/2 of ZrTiO4.33 The Zr 3d spectra indicate that only the doped Zr4+ ions in substitutional mode exist for D-500-5 and D-1000-5, and the doped Zr4+ ions form ZrTiO4 with little of them in substitutional mode for D500-40 and D-1000-40. In Ti 2p spectra (Supporting Information, Figure S2A), compared with P-500 (458.6 eV) and P-1000 (458.5 eV), the binding energies of Ti 2p3/2 for D-500-5 (458.7 eV), D-500-40 (458.7 eV), D-1000-5 (458.6 eV), and D-1000-40 (458.6 eV) increase, because the electronegativity of Zr (1.6) is larger than that of Ti (1.5). This result implies that Zr4+ ions are doped into the TiO2 lattice in substitutional mode. For D-500-40 and D-1000-40, new doublet peaks at 458.8 eV were ascribed to the Ti 2p of ZrTiO4. Similar results are observed for the O 1s spectra (Supporting Information, Figure S2B). In addition, the atom percentages of Zr, Ti, and O elements calculated from the XPS spectra are shown in Table S4 in the Supporting Information. For P-500, the sample consists of TiO2 and few surface O species. For D-500-5, besides TiO2 and surface O species, 2.47% doped Zr4+ ions in substitutional mode is observed. For D-500-40, besides TiO2 and surface O species, 5.86% doped Zr4+ ions in substitutional mode and 9.47% ZrTiO4 is found. Furthermore, for P-1000 and D-1000-x (x = 5 or 40), the results are similar to the aforementioned results. Figure 5 shows the HRTEM images of P-500, P-1000, D500-5, D-500-40, D-1000-5, and D-1000-40, respectively. The fringe spacing (d) of the (101) anatase crystallographic plane for D-500-5 (3.55 Å, Figure 5c) increased compared with P-500 (3.52 Å, Figure 5a), implying that Zr4+ ions are doped into anatase TiO2 lattice in substitutional mode for D-500-5 since the ionic radius of Zr4+ (72 pm) is larger than that of the lattice Ti4+ (53 pm).34 The fringe spacing (d) of the (110) rutile
Figure 4. XPS spectra of Zr 3d for D-500-5, D-500-40, D-1000-5, and D-1000-40 samples.
Figure 5. HRTEM images of (a) P-500, (b) P-1000, (c) D-500-5, (d) D-1000-5, (e) D-500-40, and (f) D-1000-40 samples. 27123
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ature is closely related to the concentration of doped Zr4+ ions. The phase transition temperature for D-900-5 and D-900-10 increases to 900 °C and further increases to 1000 °C for D1000-20, D-1000-30, and D-1000-40, compared with pure TiO2 (P-600). As the concentration of Zr4+ ions is more than 20%, the phase transition temperatures of Zr4+-doped TiO2 samples are almost the same (1000 °C) due to the saturation of the substitutional Zr4+ ions, implying the substitutional Zr4+ ions have saturated in Zr4+-doped TiO2, inhibiting the increase of the phase transition temperature, and ZrTiO4 has little effect on the phase transition. Therefore, it could be deduced that the phase transition temperatures of Zr4+-doped TiO2 samples increase with the increase of Zr4+ ions when they are below saturation concentration and remain unchanged above saturation. Also, the formation of ZrTiO4 in Zr4+-doped TiO2 sample is related to the annealing temperature and the concentration of doping Zr4+ ions. Figure 7 shows XRD patterns of the Zr4+-
crystallographic plane for the D-1000-5 sample (3.28 Å, Figure 5d) is larger than that for the P-1000 sample (3.19 Å, Figure 5b), which also implies that Zr4+ ions exist in substitutional mode in rutile TiO2. Furthermore, for D-500-40 (Figure 5e) and D-1000-40 (Figure 5f), besides the fringe spacing of the TiO2, a fringe spacing of 2.91 Å corresponding to the (111) plane of ZrTiO4 is found in both D-500-40 and D-1000-40 samples, respectively (Figure 5e,f), which indicates the formation of ZrTiO4 on the surface of TiO2. 3.2.4. UV−Vis Diffuse Reflectance Spectroscopy (DRS). UV−vis DRS is used to investigate the photoelectric properties of Zr-doped TiO2 with different concentrations annealed at a certain temperature, as shown in Figure S1 in the Supporting Information. It is revealed that the absorption spectra are different for Zr-doped TiO2 with different concentrations at a certain temperature, such as 500 (Supporting Information, Figure S1A) and 1000 °C (Supporting Information, Figure S1B). When the concentration is below 20%, the visible absorbance is enhanced due to the doped Zr4+ in substitutional mode; when the concentration is above 20%, a blue shift is found for the absorption edge caused by the formation of ZrTiO4. 3.3. Mechanism of the Phase Transition and the Formation of ZrTiO4 in Zr4+-Doped TiO2. It can be deduced that the temperature of the phase transition from anatase to rutile for Zr4+-doped TiO2 samples increases from 600 °C (pure TiO2) to 900 or 1000 °C, shown in Figure 6 and Table
Figure 7. XRD patterns of Zr4+-doped TiO2 samples with lowermost precipitating temperature of ZrTiO4 with different concentrations of doping Zr4+ ions.
doped TiO2 samples with the lowermost precipitating temperature of ZrTiO4 with different concentrations of Zr4+ ions. It can be seen that the lowermost precipitation temperature of ZrTiO4 is about 500 °C for D-500-40, 600 °C for D-600-30, 800 °C for D-800-20, and 900 °C for D-900-10, respectively. However, no diffraction peak of ZrTiO4 for D-1000-5 is observed. These results indicate the lowermost precipitation temperature of ZrTiO4 is gradually decreased with the increase of Zr4+ ion concentration. It can be known from Figures 1 and 2 that the precipitation of ZrTiO4 means the substitutional Zr4+ ions reached saturation in sample, and the ZrTiO4 is formed by the residual Zr4+ ions under a definite annealing temperature. Thus, it can be deduced that the saturation concentration of Zr4+ ions in substitutional mode in Zr4+-doped TiO2 sample gradually decreases with the increasing of annealing temperature. Therefore, the saturation concentration of doped Zr4+ ions in substitutional mode under different annealing temperatures is shown in Figure 7 and Table S5 in the Supporting Information. For the same concentration of the doping Zr4+ ions, e.g., 20%, Zr4+ ions are doped into the TiO2 lattice in substitutional mode when the annealing temperature is below 800 °C due to nonsaturation of the substitutional doping Zr4+ ions in the sample. When the annealing temperature is higher than 800 °C, the lattice vibration of Zr4+-doped TiO2 sample is enhanced and part of the substitutional doping Zr4+ ions can gain enough energy to escape from the bondage of the lattice to the surface of TiO2 and react with surface O2− and TiO2 to form ZrTiO4,
Figure 6. XRD patterns of Zr4+-doped TiO2 samples with phase transition temperature from anatase to rutile with different concentrations of Zr4+ in TiO2.
S5 in the Supporting Information. This is caused by the doping Zr4+ ions in substitutional mode in the TiO2 lattice which can prevent the conversion from anatase to rutile during calcination.35 Since the Zr4+ ion is more electropositive than the Ti4+ ion, the electron cloud in each TiO2 particle might be loosely held, thus favoring formation of the less dense anatase phase. In other words, the tight packing arrangements required for rutile phase formation are suppressed by the substitutional doping Zr4+ ion in the TiO2 lattice.35 Besides, as the phase transition is always accompanied by the motion of ions,36 and the radius of the Zr4+ ion (72 pm) is larger than that of the Ti4+ ion (53 pm), it costs more energy for the Zr4+ ion to move than for the Ti4+ ion during the phase transition. Thus, the doped Zr4+ ions in substitutional mode result in the increase of the phase transition temperature from anatase to rutile. In addition, it is clearly seen from Figure 6 and Table S5 in the Supporting Information that the phase transition temper27124
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accompanied by the release of the lattice energy and a decrease of the saturation concentration. The higher the annealing temperature is, the easier it is for the Zr4+ ions to escape from the lattice. Thus, if the annealing temperature is high enough, e.g., 1000 °C, the majority of the substitutional doping Zr4+ ions will transform into ZrTiO4 on the surface of TiO2. At the same annealing temperature, e.g., 500 °C, when the concentration of the doping Zr4+ ions is lower than the saturation concentration, all of the Zr4+ ions can enter the TiO2 lattice in substitutional mode, causing lattice distortion with the large ionic radius of Zr4+ (Zr4+, 72 pm; Ti4+, 53 pm).34 The lattice distortion would increase with the increase of doped Zr4+ ions. When the concentration of doped Zr4+ ions is higher than the saturation concentration, e.g., 40%, the redundant Zr4+ ions can transform into ZrTiO4 on the surface to maintain the minimum lattice energy of the sample.
4. CONCLUSION In summary, for doping of Zr4+ ions in TiO2, besides the Zr4+ ion that can be doped into the lattice in substitutional mode, the formation of ZrTiO 4 depends on the saturation concentration of the doped Zr4+ ions at a certain annealing temperature. The substitutional Zr4+ ions in the lattice can inhibit the conversion from anatase to rutile, leading to an increase the phase-transition temperature. These results can afford help in understanding the doping mechanism much better and in preparing materials with high photoelectric performance.
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ASSOCIATED CONTENT
S Supporting Information *
Crystal size, cell parameters, and cell volume calculation of the samples, diffuse reflectance UV−vis spectra of D-500-x and D1000-x samples, XPS spectra of Ti 2p and O 1s for D-500-5, D500-40, D-1000-5, and D-1000-40 samples, atom percentage calculations from XPS spectra, and different phases of pure TiO2 and Zr-doped TiO2 samples. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +86-22-66229310. Tel.: +86-22-66229598. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Nos. 51072082, 51372120, 51302269, and 21173121).
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REFERENCES
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