N-Doped TiO2 Prepared from Microwave-Assisted Titanate Nanotubes

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N-Doped TiO2 Prepared from Microwave-Assisted Titanate Nanotubes (NaxH2-xTi3O7): The Effect of Microwave Irradiation during TNT Synthesis on the Visible Light Photoactivity of N-Doped TiO2 Hsin-Hung Ou, Shang-Lien Lo,* and Ching-Hui Liao Research Center for Environmental Pollution Prevention and Control Technology, Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan Rd., Taipei 106, Taiwan

bS Supporting Information ABSTRACT: This study aimed to characterize N-doped TiO2 prepared by thermally treating microwave-assisted titanate nanotubes (TNTs, NaxH2-xTi3O7) in an Ar/NH3 atmosphere. The effect of intercalated Na(I) within TNTs on the visible light photoactivity and the N-doping mode was investigated as well. By evaluating the performance of photocatalytic oxidation of phenol under the visible region, the photoactivity of N-doped TiO2 prepared from TNTs is 3 times higher than that of N-doped TiO2 prepared from P25 TiO2. Characterizations, including HR-TEM, XRD, XPS, NH3-TPD, UV-vis DRS, and SBET, indicate that the substitutional N-doping mode, the O-Ti-N linkage, is mainly responsible for narrowing the band gap and eventually enhancing the visible light photoactivity. Furthermore, the doping mechanism is significantly dependent on the presence of intercalated Na(I) within TNTs. The O-Ti-N linkage, owing to the substitutional doping, is apparent for TNTs with a low content of intercalated Na(I), whereas the presence of the higher amount of intercalated Na(I) leads to the formation of the Ti-N-O linkage that is considered as an interstitial doping mode. Also, the presence of intercalated Na(I) during the doping process results in the formation of Na2Ti6O13 instead of an inert TiN crystallinity, which is advantageous to enhancing the photoactivity of N-doped TiO2 due to the effect of interphase electron transfer between Na2Ti6O13 and TiO2.

1. INTRODUCTION Since the discovery of N-doped TiO2 by Asahi et al.,1,2 considerable efforts have been undertaken to study N-doped TiO2 thin films and powders.3-24 In comparison to the unamended TiO2, N-doped TiO2 can be effectively illuminated with visible light. Several possibilities were proposed to account for the enhancement of visible light photoactivity.1-7 Asahi et al.1 revealed that the narrowing of the band gap was attributed to the mixing of N 2p and O 2p states, whereas Irie et al.3 indicated that the formation of an isolated narrow band above the valence band was responsible for the visible light photoactivity. Two doping modes, including substitutional and interstitial N dopants, in TiO2 were also reported. For the substitutional N-doping mode, the N atom is bound to three Ti atoms by replacing a lattice O in TiO2, whereas the N atom is bound to one or more O atoms in the interstitial N-doping mode and results in the lattice defects, such as NO2- or NO3-.25-29 However, the visible light photoactivity of the N-doped is still limited owing to the low content of doped N whose atomic concentration within N-doped TiO2 is below 2% in most cases.8 On the other hand, TiN, an inert crystalline phase that forms during the sintering process, has an inhibitive effect on the photoactivity of N-doped TiO2.7,9,10 To the best of our knowledge, few studies, so far, attempted to prepare N-doped TiO2 that has a high doping amount of N without the presence of TiN. r 2011 American Chemical Society

The successful N doping into the TiO2 lattice has been reported to include two key factors: the geometric and electronic accommodation of N.7 Titanate nanotubes (TNTs, NaxH2-xTi3O7) appear to meet the above requirements because they has been confirmed to provide more accommodation for aqueous NH3 intercalation owing to the ion exchangability.30-32 This specific feature is likely advantageous to the doping capacity of N as TNTs are used to prepare N-doped TiO2. In fact, several efforts have succeeded at preparing N-doped TNTs and N-doped TiO2 by using TNTs as a precursor.9,10,33-35 Tokudome and Miyauchi33 first examined the visible light activity of N-doped TNTs. A following effort was also carried out by Geng et al.,35 who indicated that the doping N is likely located at the interstitial sites rather than substitute O2- ions in the TNT lattice. In a related study about the N-doped TiO2 nanoparticles prepared from TNTs, Feng et al.10 carried out a comprehensive study in which the single-electron-trapped oxygen vacancy is suggested to be responsible for the visible light photoactivity. All the above TNTs, however, were synthesized via a conventional hydrothermal method in which the intercalated Na(I) Received: August 11, 2010 Revised: January 19, 2011 Published: February 21, 2011 4000

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The Journal of Physical Chemistry C within TNTs may be removed after an acid-washing treatment. In our previous study, we have demonstrated a facile method to synthesize TNTs by applying microwave irradiation during hydrothermal treatment.31,36,37 The amount and intensity of intercalated Na(I) within TNTs are enhanced with the applied level of microwave irradiation. Furthermore, the Ti-N linkage, which has been evidenced to be responsible for the visible light photoactivity of N-doped TiO2, forms after the photocatalytic oxidation of NH3/NH4þ over microwave-assisted TNTs.31 Despite that several studies have succeeded in the preparation of N-doped TiO2 from TNTs, no report, so far, examined the effect of intercalated Na(I) on the doping performance of N within the TiO2 lattice. In the present study, N-doped TiO2 was prepared by thermally treating microwave-assisted TNTs in an Ar/NH3 atmosphere. The effect of Na(I) within TNTs, which is induced by the applied microwave irradiation during TNT synthesis, on the visible light photoactivity of N-doped TiO2 was investigated.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. Microwave-assisted TNTs (NaxH2-xTi3O7) were synthesized by a microwave hydrothermal method, as reported in our previous studies.31,36,37 In a typical synthesis process, 0.6 g of P25 TiO2 along with 70 mL of NaOH (10 N) was first stirred for 0.5 h in a Teflon container that was then transferred into a double-walled vessel consisting of an inner Teflon liner and an outer shell of high-strength Ultem polyetherimide. This vessel was subsequently moved into a microwave digestion system (Ethos Touch Control, MILESTONE Corporation). The reaction time, temperature, and irradiation power were computer-controlled. During the microwave hydrothermal treatment, the slurry was treated at 130 C for 3 h under two levels of irradiation power, 70 and 700 W. After the synthesis reaction, the resulting powders were then neutralized with 0.5 N HCl and deionized water in order to remove NaCl formed in excess. TNTs was obtained after vacuum freezedrying (-58.8 C and 100-200 mTorr). TNTs synthesized at an irradiation power of 70 and 700 W are designated as TNT70W (Na0.32H1.68Ti3O7) and TNT-700W (Na1.04H0.96Ti3O7), respectively.31 N-doped TiO2 (TNTiO2) was prepared by treating TNTs at the temperature of interest (500-700 C) for 2 h in a gaseous mixture (80 vol % Ar/20 vol % NH3). The flow rate of the mixture was controlled at 70 mL min-1. For comparison, P25 TiO2 was also treated under the same conditions as that of TNTiO2, and the samples obtained were denoted as PNTiO2. 2.2. Characterization. BET surface areas (SBET) of TNTiO2 were determined by nitrogen adsorption at -196 C with an automated gas adsorption analyzer (Micromeritics, ASAP 2010). A high-resolution transmission electron microscope (HR-TEM, Philips Tecnia 20 G2 S-Twin) was used to observe the morphology and the diffraction pattern of samples. The determinations of crystalline phases of samples were carried out with an X-ray powder diffractometer (XRPD, Philips X’ Pert Pro MPD) that is equipped with a Cu KR X-ray source (λ = 1.5405 Å) operating at a voltage of 40 kV and a current of 30 mA. The patterns were recorded from 10 to 80 with a scan rate of 2 min-1. The phase molar ratio of anatase and rutile was also calculated according to the Spurr and Myers methods.38 The analysis of the X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD) on TNTiO2 was carried out by a spectrometer equipped with a Mg KR X-ray source. All binding energies (BEs) were referenced to

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the C 1s peak at 284.6 eV. The spectra were fitted using a nonlinear least-squares fitting program (XPSPEAK) with a linear background and to the 80% Gausssian/20% Lorentzian peak shape. NH3 temperature-programmed desorption (NH3-TPD) was used to determine the surface acidity of TNTs. The standard procedure involved purging the reactor with He gas for 0.5 h and heating the powders at 100 C for 1 h to remove the impurities and water adsorbed on the TNTs. The resulting powders were subsequently treated by a mixture of 95% Ar/5% NH3 with the flow rate at 50 mL min-1. The temperature ramping was 5 C min-1 (25-700 C), and the temperature remained constant for 20 min after reaching 700 C. Diffuse reflectance spectra were obtained on dry pressed disk samples by using a UV-vis spectrometer (PerkinElmer Lambda 35) equipped with an integrating sphere assembly. 2.3. Photocatalytic Reaction. Photocatalytic oxidation of phenol was carried out in a 250 mL quartz reactor equipped with a water circulation facility at the outer wall of the reactor. The concentrations of phenol and N-doped catalyst samples were prepared at 60 and 300 mg L-1 in a 200 mL solution. The slurry was stirred with a magnetic stirrer at a constant speed to make well-mixed suspensions. After the adsorption of phenol over the catalysts reached equilibrium, the solution was illuminated for 24 h by using a blue-light-emitting diode (BLED, NSPB500S Nichia Kagaku), which emits a principal wavelength at 470 nm. Air was continuously purged throughout the reaction. The pH value of the initial reaction slurry was around 6 and increased to 6.5 after the photocatalytic reaction was finished. Sample aliquots were collected at appropriate time intervals and filtered using cellulose acetate syringe membrane filters (Millipore, 0.22 μm). The phenol concentration was determined using a high-performance liquid chromatograph (Waters 600E) equipped with a UV detector set at 270 nm (Waters 486 detector).

3. RESULTS AND DISCUSSION 3.1. Appearance and Morphology of TNTiO2. The colors of the samples sintered at elevated temperatures (500-700 C) were pale yellow, yellow, yellowish green, and dark green. In fact, yellow TiO2 is indicative of the successful doping of N into the TiO2 lattice and exhibits a good absorbance of visible light.3,17,39 The dark green was only observed for TNTiO2-700W sintered at 700 C, which may be attributed to a mixture of yellow (N-doped TiO2) and blue (Ti3þ due to exceeding NH3 treatment). The change of the crystalline phase was accompanied by the conversion of the tube morphology to a mixture of rods and particles for TNTiO2-70W (Figure 1a) and to bundles for TNTiO2-700W (Figure 1b). A rough examination of the crystalline phase within TNTiO2 was determined by indexing the d-spacings shown in the HR-TEM images (Figure 1c,d). Fourier transformation of a selected area in the HR-TEM images revealed the distances that we identified as anatase, rutile, and Na2Ti6O13 for TNTiO2-70W (Figure 1e) and Na2Ti6O13 for TNTiO2-700W (Figure 1f). In addition, the vivid spot pattern is also indicative of the highly oriented crystallinity. 3.2. Crystal Structure of TNTiO2. For the thermally treated samples, no N-related crystalline phase and peak shift were observed in XRD patterns (Figure 2). This result indicates the N doping into the crystal lattice and no change of the average cell dimension during the doping process.4,19,20 For both TNTiO270W and TNTiO2-700W, the anatase phase appears along with 4001

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Figure 2. XRD patterns for (a) TNTiO2-70W and (b) TNTiO2-700W sintered at elevated temperatures.

Figure 1. HR-TEM and Fourier transform images of TNTiO2-70W (a, c, e) and TNTiO2-700W (b, d, f) sintered at 700 C.

the rutile phase at 500-650 C until Na2Ti6O13 emerges at 700 C at the expense of anatase and rutile. The anatase phase still dominates for TNTiO2-70W at 700 C, whereas Na2Ti6O13 becomes apparent for TNTiO2-700W. This result reveals that the presence of intercalated Na(I) within TNTs, which is induced by microwave irradiation during TNT synthesis, appears to change the route of phase transformation because TNT-700W contains a larger amount of intercalated Na(I) than TNT70W.31,36 The formation of Na2Ti6O13 is attributed to the topotactical connection of adjacent (Ti3O7)2layers within TNTs36 and has also been evidenced to exhibit excellent photoactivity because the corresponding tunnel structure provides a number of active sites.40,41 On the other hand, there is no TiN crystalline phase in XRD patterns at the elevated temperatures. The previous reports had indicated that TiN appears at 700 C as conventional hydrothermal TNTs or TiO2 was sintered in a NH3 atmosphere.9,10,42 In fact, the existence of TiN in N-doped TiO2 has been confirmed to negatively affect the visible light photoactivity owing to its inertness. For TNTiO2, the formation of TiN is suppressed and substituted by Na2Ti6O13 because Na within TNTs is involved in the doping process. The molar ratios of the anatase and rutile phases were also determined with Spurr and Myers methods.37 The ratio for each sample was obtained by averaging the results from five XRD measurements. It was found that the standard error due to the

lack of instrument reproducibility is about 3%. The ratio of Na2Ti6O13 in the case of TNTiO2-70W is not considered because the peak intensity of Na2Ti6O13 is too weak. For TNTiO2-700W sintered at 700 C, the approximate ratio of Na2Ti6O13 was calculated by considering the intensities of the highest diffraction peaks of anatase and Na2Ti6O13. As demonstrated in Table 1, the anatase ratio for TNTiO2-70W decreases with sintering temperature, whereas the ratios of anatase and rutile remain at a consistent level for TNTiO2-700W until Na2Ti6O13 emerges at 700 C. The increase in the rutile phase ratio with TNTiO2-70W is a common phase transformation from anatase to rutile. In contrast, the anatase content for TNTiO2700W remains almost constant until 700 C, which is owing to the fact that the intercalated Na(I) within TNTs is advantageous to the thermal stability of TNTs.36,43,44 3.3. Chemical Structure Characterization. XPS determinations were also conducted for the investigation of the chemical state of doped N within TNTiO2 (Figure 3). Regarding the N 1s spectra of TNTiO2-70W sintered at 500 C (Figure 3a), peaks at 399.4 and 403.1 eV can be assigned to NH3 and N-Na, respectively.6,16,31 NH3 was observed only for this case because TNT-70W adsorbs more NH3 versus TNT-700W, as shown in NH3-TPD spectra (Figure 5). On the other hand, the peak appearing at 396.2 eV is attributed to the O-Ti-N linkage within TNTiO21-3 rather than the TiN crystalline phase because no typical binding energy of TiN (397.2 eV) was found in the XPS spectra of TNTiO2.45 The intensity of the O-Ti-N linkage increases at the expense of NH3 as the sintering temperature goes up, whereas the Ti-O-N linkage shows up at 700 C. The O-Ti-N linkage is a typical substitutional N doping, which has been reported to be owing to the fact that N atoms within NH3 are bonded to three Ti atoms and replace 4002



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Table 1. Phase Molar Ratio, Band Gap, SBET, and Doping Amount of N of TNTiO2-70W and TNTiO2-700W Sintered at Elevated Temperatures phase molar ratio (mol %) band gap (eV)

doping amount of N (atom %)

SBET (m2 g-1)

10

2.90

0.65

171

13

2.75

0.18

159

84 84

16 16

2.45 2.00

0.74 2.04

129 110

TNTiO2-70W (700 C)

76

24

1.35

3.37

78

TNTiO2-700W (500 C)

74

26

2.55

0.09

210

TNTiO2-700W (550 C)

77

23

2.45

0

201

TNTiO2-700W (600 C)

73

27

2.29

0.74

181

TNTiO2-700W (650 C)

73

27

2.25

1.59

154

TNTiO2-700W (700 C)

27

73 (Na2Ti6O13)

2.25

1.67

132

catalysts

anatase

TNTiO2-70W (500 C)

90

TNTiO2-70W (550 C)

87

TNTiO2-70W (600 C) TNTiO2-70W (650 C)

rutile (Na2Ti6O13)

Figure 3. N 1s XPS spectra for (a) TNTiO2-70W and (b) TNTiO2700W sintered at elevated temperatures.

lattice oxygen in the TiO2 matrix. On the other hand, the TiO-N linkage, an interstitial N-doping mode, forms from N atoms bonded to one or more lattice oxygen.22,26-28 In contrast, a N-related species was not observed with TNTiO2-700W sintered at 500 C (Figure 3b), which reassures us that TNT-700W is inferior to TNT-70W in adsorbing gaseous NH3. As the sintering temperature increased, only the O-Ti-N linkage was observed for TNTiO2-700W at 600 C, whereas three peaks appearing at 396.3, 400.1, and 403.4 eV for TNTiO2700W at 700 C were assigned to the O-Ti-N, Ti-O-N, and Na-N, respectively.4,6,15,20-22,46 The formation mechanisms of O-Ti-N and Ti-O-N for TNTiO2-700W are supposed to be the same as that of TNTiO2-70W. One phenomenon worthy to mention is that the substitutional N-doping mode dominates for TNTiO2-70W, whereas the interstitial one is more apparent for

TNTiO2-700W (Figure 3). In other words, the presence of intercalated Na(I) within TNTs appears to suppress the formation of the O-Ti-N linkage. In fact, Di Valentine et al.28 indicated that the formation of the O-Ti-N linkage is accompanied by the oxygen vacancies. Oxygen vacancies, meanwhile, have been reported as not being allowed to form in the presence of Na(I) within TNTs.47 Therefore, a suggested explanation on the Na effect is that Na likely compensates for the oxygen vacancies and inhibits the formation of substitutional N doping. It could be seen that Ti 2p spectra of TNTiO2 consists of two peaks at around 459.5 eV (Ti 2p3/2) and 465.3 eV (Ti 2p1/2) (Figure 4a,b). A slight shift of Ti 2p to lower binding energy was noticed as the sintering temperature increased, which is likely attributed to the interaction between titanium atoms and nitride anions, fewer electron acceptors than oxide ions.4,14,20,48 In other words, the partial electrons of N within Ti-N transferred to the Ti owing to the substitution of O by N (3.44), which has a lower electronegativity than O (3.04).49 This result reassures that N incorporates into the TNT lattice and substitutes for O. The shift to lower binding energy is more apparent for TNTiO2-70W than for TNTiO2-700W, which corresponds to the analysis of N 1s spectra that the O-Ti-N linkage is prominent for TNTiO270W. Regarding the O 1s spectra of TNTiO2-70W and TNTiO2700W, curve fitting indicated the presence of a shoulder peak in addition to the main peak for the TNTiO2 (Figure 4c,d). The main peak can be assigned to the crystal lattice oxygen (529.8 eV), whereas the shoulder one is principally contributed by the surface adsorbed OH (531.8 eV).31,50 Another peak showed at around 532.7 eV in the case of TNTiO2 sintered at 700 C as the shoulder peak was deconvoluted. This unapparent peak could be attributed to the O-Ti-N or Ti-O-N.11,51 Despite that there is a debate over this assignment, the peak is certainly of the oxidation of Ti-N bonds. Furthermore, the BEs at 530 eV for TNTiO2-70W and TNTiO2-700W shifted negatively by 0.3-0.6 and 0.15-0.3 eV in comparison with the TNT-70W and TNT700W (data not shown), respectively. This result is also indicative of successful N doping into TNTs or TiO2 after thermal treatment in a NH3 atmosphere.4,5,9,10 On comparison of the N-doped TiO2 prepared from conventional TNTs reported by Feng et al.,10 only NO (NO-Ti linkage) was observed by XPS analysis as TNTs were sintered at 400-600 C. They attributed the formation of NO to the reaction between NH3 and O of TiO2-x(Vo•)x, a photoactive 4003



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Figure 4. Ti 2p XPS spectra for (a) TNTiO2-70W and (b) TNTiO2-700W and O 1s XPS spectra for (c) TNTiO2-70W and (d) TNTiO2-700W sintered at elevated temperatures.

Figure 5. NH3-TPD spectra for TNT-70W, TNT-400W, and TNT700W.

center in N-doped TiO2. Their result is distinct from what is presented in our study, which may be owing to the different synthesis condition of the TNTs. In fact, the characterizations of TNTs have been revealed to be significantly dependent on the presence of Na(I) within TNTs.43,44 As stated in our previous studies, the intercalation intensity and amount of Na(I) within TNTs are enhanced with the power level of the microwave irradiation during TNT synthesis.31,36 The intercalated Na(I) of TNTs synthesized via a conventional hydrothermal method is removed once TNTs were treated by acid-washing.43,44 Therefore, the chemical state of N is totally distinct because the feature of N-doped TiO2 prepared from microwave-assisted TNTs is different from that prepared from conventional hydrothermal TNTs. 3.4. Surface Acidity of TNTs Prepared under Different Power Levels of Microwave Irradiation. For further exploring the effect of microwave irradiation during TNT synthesis on the doping performance, NH3-TPD was also carried out to examine the surface acidity of TNTs synthesized under different power levels of microwave irradiation. Figure 5 apparently shows that the amount of released NH3 decreases with increasing power level of the microwave irradiation during TNT synthesis. For the case of TNT-70W, the broad temperature range can be attributed

to different modes for NH3 intake, which includes the physisorbed or chemisorbed NH3 on the outer surface of TNT-70W, and the intercalated NH3 into the zigzag space of TNT-70W. In addition, our previous study has indicated that TNTs with high amounts of intercalated H(I) are able to provide more substitution sites for aqueous NH3.31 Both the aforementioned results can be explained by the fact that TNTs with high amounts of intercalated H(I) behave as a Brønsted acid.52 The surface acidities of TNTs calculated by the capability toward NH3 uptake are 18.1, 4.7, and 1.1 μmole g-1 for TNT-70W, TNT-400W, and TNT-700W, respectively. The result of TPD analysis directly evidence the XPS conclusion in that the N amount is doped more in TNT-70W than in TNT-700W because TNT-70W exhibits a better affinity toward NH3 adsorption. 3.5. Optical Feature of TNTiO2. As demonstrated in Figure 6, the absorbance shoulders for all TNTiO2 samples are above 400 nm, which indicates that TNTiO2 can be excited in the visible region. For TNTiO2-70W (Figure 6a), the absorbance shoulder apparently shifts with sintering temperature. The remarkable absorbance for visible light is attributed to the formation of the O-Ti-N linkage, leading to the narrowing of the energy band gap.1,2 This phenomenon can also be explained in terms of the formation of an intra-band gap located above the valence band, due to substitution of oxide centers by nitride centers and/or to the interstitial introduction of nitride into the oxide lattice.27,53 In contrast, the shift of the absorbance shoulder for TNTiO2-700W appears to stop at 600 C. No further enhancement on the absorbance of visible light was observed (Figure 6b). This result is owing to the fact that there is no significant increase in the doping amount of N for TNTiO2700W sintered at temperatures above 600 C (Table 1). The transformed Kubelka-Munk function was also exhibited by plotting the [F(R¥)]0.5 value against excitation energy to determine the apparent band-gap energy (the insets in Figure 6).24,54 The band gaps for TNTiO2 are demonstrated in Table 1. As expected, TNTiO2 exhibits a narrower band gap than that of TNTs whose band gap is about 3.0 eV. The band gap of TNTiO2-70W is a function of the doping amount of N, whereas 4004



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Figure 7. Time-dependent degradation of phenol over PNTiO2 prepared at 600 C, TNTiO2-70W prepared at 700 C, and TNTiO2-700W prepared at 650 C.

Figure 6. Diffuse reflection UV-vis absorbance for TNTiO2-70W and TNTiO2-700W at elevated temperatures. (The inset shows the corresponding plots of the Kubelk-Munk (KM) function versus photo energy (E).)

above 600 C, there is almost no change on the band gap for TNTiO2-700W despite that there is an increase in the doping amount of N. In other words, the presence of Na(I) definitely results in the different doping mechanism and eventually leads to the change of optical features. By taking into account the XPS analysis, it can be reassured that the narrowing of the band gap for TNTiO2 is dependent on the formation of the O-Ti-N linkage rather than Ti-O-N ones. 3.6. Photocatalytic Oxidation of Phenol over TNTiO2. The time-dependent degradations of phenol over PNTiO2, TNTiO270W, and TNTiO2-700W are presented in Figure 7. With no illumination, 60-70% of phenol with an initial concentration of 60 mg L-1 degraded in 5 min after mixing with TNTiO2. The adsorption equilibrium was reached by 6 h, at which the concentration of phenol for TNTiO2-70W and TNTiO2-700W was about 17 and 23 mg/L, respectively. This result indicates that a strong photoinduced adsorption of phenol over TNTiO2 took place as a result of doped N and the relatively high SBET of TNTiO2, as shown in Figure 8 and Table 1. In addition, almost no phenol degradation was observed when no catalyst existed in the photocatalytic reaction. This result is attributed to the fact that the light emitted from the BLED is behind the optical absorbance spectrum of phenol. Therefore, the phenol oxidation can be exclusively owing to the contribution by the visible light photoactivity of TNTiO2. Figure 8 demonstrates the dependence of the visible light photoactivity of the TNTiO2 toward phenol oxidation on the doping amount of N and the sintering temperature. The photocatalytic oxidation of phenol over N-doped TiO2 prepared from

Figure 8. Dependence of phenol degradation on the sintering temperature and doping amount of N for (a) PNTiO2, (b) TNTiO2-70W, and (c) TNTiO2-700W (error bars represent a 95% confidence interval).

P25 TiO2 (PNTiO2) was also carried out to compare the visible light photoactivity of TNTiO2. As shown in Figure 8a, the photoactivity of PNTiO2 reaches a maximum at 600 C. A further increase in the sintering temperature leads to a decrease in the photoactivity despite that the atomic concentration of N remains around 0.5% at temperatures above 600 C. This phenomenon appears to be owing to several possibilities, 4005


The Journal of Physical Chemistry C including the phase transformation of anatase to rutile, the formation of the TiN phase, and the decrease in SBET.19,42,55 Figure 8 also revealed that the oxidation efficiencies of phenol over TNTiO2-70W and TNTiO2-700W are higher than that of PNTiO2 by about 3 and 2 times, respectively. Regarding the N-doped TiO2 prepared from the conventional hydrothermal TNTs, Wang et al.9 presented a maximal C3H6 removal efficiency as the N-doped TiO2 was prepared at 600 C. A further increase in the sintering temperature to 700 C resulted in a dramatic decrease in the removal efficiency owing to the formation of the TiN crystalline phase. In our case, no TiN formed, as stated in the previous result, because TNTiO2 was prepared using microwaveassisted TNTs as a precursor. Table S1 (Supporting Information) was organized as well to compare the visible light photoactivity of our N-doped TiO2 with other ones. The SBET of TNTiO2 shown in Table 1 indicates that the SBET is not a predominant factor in the phenol oxidation. This is because the decreasing SBET, which may be attributed to the loss of void sites, the agglomeration of nanoparticles, and the collapse of the porous structure, appears not to have a negative effect on the efficiency of phenol oxidation. On the other hand, the phenol oxidation over TNTiO2 is definitely a function of the doping amount of N. By considering the XPS analysis and phenol degradation over TNTiO2, it can be concluded that the OTi-N linkage has the decisive effect on the visible light photoactivity of TNTiO2. Despite that Ti-O-N forms at 700 C, the O-Ti-N linkage remains predominant for TNTiO2-70W and there is no further improvement on the efficiency of phenol oxidation for TNTiO2-700W. In other words, the substitutional N-doping mode rather than the interstitial one is responsible for the visible light photoactivity of TNTiO2 toward phenol oxidation. This result corresponds to the several previous reports despite that the assignment of the N dopant is still under debate.1,2,11,47 Regarding the effect of crystallinity, Table 1 presents that the optimal ratio of the anatase to the rutile phase is 3:1 for either TNTiO2-70W (700 C) or TNTiO2-700W (600 C). The mixed phases of anatase and rutile have been reported to exhibit higher activity than their pristine composition.56-58 Such enhancement can be attributed to the effect of interphase electron transfer (IPET), which has been proved to prolong the lifetime of photoelectrons and photoholes and eventually enhance the efficiency of photocatalytic oxidation.59,60 Interestingly, there is no dramatic decrease in the efficiency of phenol degradation with TNTiO2-700W sintered at 700 C as the content of anatase phase considerably decreases. This result can be also attributed to the IPET effect between anatase and Na2Ti6O13.

4. CONCLUSION Because the crystallinity of P25 TiO2 powder is relatively high, the incorporation of N into the TiO2 lattice is difficult. Here, we prepared N-doped TiO2 from microwave-treated TNTs whose geometric structure is advantageous for N doping. The characterizations of TNTiO2 are significantly dependent on the intercalated Na(I) within TNTs, which is induced by the microwave irradiation during TNT synthesis. Several critical findings about TNTiO2 are listed as follows: 1 NH3 can be effectively adsorbed over TNTs because TNTs behave as a Brønsted acid owing to the intercalated H(I) within the TNT structure. Therefore, the atomic concentration of doped N within TNTiO2 is about 3.3 atom %,

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which is higher than most cases reported in the previous studies. 2 The presence of intercalated Na(I) within TNTs changes the route of phase transformation for which the Na2Ti6O13, rather than TiN, prefers to form during the thermal doping process. In addition, TNTiO2 has a bicrystalline framework dominated by the anatase phase, which is beneficial to the photocatalytic performance owing to the IPET effect. 3 The presence of Na(I) appears to dominate the doping mechanism of N into TNTiO2 as well. The formation of the O-Ti-N linkage owing to the substitution doping is more apparent for TNTs with a low content of Na(I), whereas a further increase in the Na(I) content within TNTs initiates the interstitial doping process, the formation of the TiN-O linkage. In addition, the atomic concentration of N remained at an unchanged level as TNTiO2 with a high amount of intercalated Na(I) was prepared at temperatures above 600 C. 4 The visible absorbance increase with increasing N doping amount and the O-Ti-N linkage within TNTiO2. In other words, the O-Ti-N linkage within TNTiO2 is responsible for the visible light photoactivity rather than the Ti-O-N linkage. The oxidation efficiency of phenol over TNTiO2 is 3 times higher than that over N-doped TiO2 prepared from P25 TiO2. This is because TNTiO2 does not suffer from the relatively low SBET, low doping amount of N, and the formation of the inert TiN crystalline phase.

’ ASSOCIATED CONTENT

bS

Supporting Information. A comparison of the photocatalytic performance of the prepared materials with some reference samples. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: þ886-2-2392-8821. Telephone: þ886-2-2362-5373. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors gratefully acknowledge the funding from the National Science Council of Taiwan (NSC 95-2221-E-002-143MY3). ’ REFERENCES (1) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. (2) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2002, 295, 627–628. (3) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483–5486. (4) Chen, X.; Burda, C. J. Phys. Chem. B 2004, 108, 15446–15449. (5) Sathish, M.; Viswanathan, B.; Viswanath, R. P.; Gopinath, C. S. Chem. Mater. 2005, 17, 6349–6353. (6) Li, H.; Li, J.; Huo, Y. J. Phys. Chem. B 2006, 110, 1559–1565. (7) Chen, H.; Nambu, A.; Wen, W.; Graciani, J.; Zhong, Z.; Hanson, J. C.; Fujita, E.; Rodriguez, J. A. J. Phys. Chem. C 2007, 111, 1366–1372. (8) Burda, C.; Lou, Y.; Chen, X.; Samia, A. C. S.; Stout, J.; Gole, J. L. Nano Lett. 2003, 3, 1049–1051. 4006


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N-Doped TiO2 Prepared from Microwave-Assisted Titanate Nanotubes

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