Abstract
Rutile-anatase phase mixed codoped TiO2 nanowires were designed and prepared by a two-step anodic oxidation method. The results of X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy confirm that the prepared codoped TiO2 nanowires exhibit intimately contacted anatase-rutile heterostructure with the rutile content of 21.89%. The X-ray photoelectron spectroscopy measurements show that nitrogen and iron atoms are incorporated into the titania oxide lattice, and the UV-visible absorption spectra show that the codoping of iron and nitrogen atoms could extend the absorption to visible light region. The photocatalytic activities of all the samples were evaluated by photocatalytic degradation of methylene blue under visible light irradiation. The codoped sample achieves the best response to visible light and the highest photocatalytic activities. The enhancement of photocatalytic activity for codoped sample should be ascribed to the synergistic effects of codoped nitrogen and iron ions and the anatase-rutile heterostructure.
1. Introduction
Titanium oxide is a multifunctional material with a wide variety of potential applications including solar cell [1], photocatalyst [2], water splitting [3], and gas sensors [4]. However, the wide band gap of TiO2 (3.2 eV for the anatase phase and 3.0 eV for the rutile phase) requires ultraviolet (UV) light for electron-hole separation, which is only 5% of the natural solar light [5]. It is of great significance to develop photocatalysts that can be used in visible light region to improve the photocatalytic efficiency. Numerous attempts have been made to optimize the band gap of TiO2 by different doping schemes, for example, nonmetal doping (such as C, S, N, I [6–11]), metal doping (such as Fe, V, Cr, Nb [12–15]), and nonmetal-metal codoping (such as codoping [16], codoping [17], codoping [18], and codoping [19]), to improve the photocatalytic activity. As a result, it was found that the monodoping can generate the recombination center inside the TiO2, which goes against the light-induced charge carriers’ migration to the surface [16, 20]. In compensated n-p codoping systems, the defect bands can be passivated and will not be effective as carrier recombination centers [16, 21] and the Columbic attraction between the n- and p-type dopants with opposite charge substantially enhances doping concentration [22]. Recently, noncompensated codoping was proposed by Zhu et al., for its distinctive merit of ensuring the creation of intermediate electronic bands in the gap region and enhancing photoactivity manifested by efficient electron-hole separation in the visible-light region [23].
It is well-known that powders consisting of anatase and rutile (a classical reference materials is Degussa P25, a mixture of 20% rutile, and 80% anatase) possess higher photoactivity than those of pure anatase. It was thought that the anatase-rutile heterojunction inside the TiO2 was the main reason for enhanced photoactivity [24]. Deák et al. calculated the band structure of rutile and anatase and validated that both bands of rutile lie higher than those of anatase, so in mixed systems, the holes are accumulated in the former while the electrons in the latter [25]. As pointed out earlier, the charge separation will enhance the photochemical activity of mixed phase TiO2 powders.
New interesting issue: does the noncompensated codoping with rutile-anatase mixing phase improve the visible light photocatalytic activity? It was reported that Fe-doped TiO2 and N-doped TiO2 exhibit enhanced visible light photocatalytic activity as compared to undoped TiO2, while codoping is the noncompensated codoping, but the light photocatalytic activity is seldom reported. Thus, in this study, codoped TiO2 nanowires with rutile-anatase mixed phase were designed and prepared. Compared with the undoped, Fe-doped and N-doped TiO2, codoped TiO2 gains the best photocatalytic activity under visible light irradiation and the mechanism of enhanced visible-light photocatalytic activity is discussed in detail.
2. Experimental Section
Titanium foils (0.6 mm thick, 99.5% purity, and cut in 1 cm × 2 cm) were used as the substrates for the growth of the TiO2 nanowire arrays. The titanium sheets were cleaned by sonicating in 1 : 1 acetone and ethanol solution, followed by being rinsed with deionized water and dried in airstream. The anodization was carried out in a two-electrode electrochemical cell with a graphite sheet as the cathode at a constant potential 60 V. A DC power supply (WYK-6010, 0–60 V, and 0–10 A) was used to control the experimental current and voltage for 1 h. The electrolyte contained 0.5 wt% NH4F, 5 mL H2O, and 195 mL ethylene glycol. After anodization, the specimens were cleaned in 10% HCl by ultrasonic immediately for 10 minutes and dried in airstream. The anodization process was repeated for several times.
The samples were then rinsed by ethanol for 5 minutes, after which some of them were submerged in mol/L Fe(NO3)3 solutions for 40 minutes for Fe doping. Postannealing in air at 700°C was employed to remove most of the organic and inorganic species encapsulated in the arrays and transform the amorphous titania to nanocrystalline TiO2. Nitrogen doping was carried out by annealing the samples in ammonia atmosphere at 500°C. The reagents—acetone (CH3COCH3), ethanol (C2H5OH), ammonium fluoride (NH4F), Fe(NO3)3, and ethylene glycol (C2H6O2) were of analytical-grade without further purification.
The crystal structures of samples were characterized by X-ray diffraction (XRD, Bruker AXS D8 Advance diffractions) using Cu radiation. The surface morphologies of the nanowire arrays were observed by scanning electron microscopy (SEM, S-4800) and high-resolution transmission electron microscopy (TEM 2010FEF HRTEM, JEOL, Japan). The X-ray photoelectron spectroscopy (XPS) experiments were performed on a VG Multilab2000 spectrometer to obtain the information on chemical binding energy of the TiO2 nanowires which was calibrated with the reference to the C 1s peak at 284.6 eV. The UV-visible absorption spectra were measured using a Cary 5000 UV-Vis-NIR spectrophotometer; BaSO4 was used as a reflectance standard in a UV-visible diffuse reflectance experiment. The photocatalytic activities under visible light irradiation were evaluated by the degradation of methyl blue irradiated by a 450 W xenon lamp. In the process, a TiO2 nanowire array with dimensions of 0.5 cm × 0.5 cm was immersed into 3 mL 10 mg/L methyl blue (MB) solution and placed below xenon lamp with 10 cm distance. The solution in the photoreactor was placed in dark for 30 minutes to reach the absorption-desorption equilibrium of the dye molecules on the sample surface.
3. Results and Discussions
Figure 1 shows the XRD patterns of the undoped, Fe-doped, N-doped, and codoped polycrystalline TiO2 samples. Compared with the standard patterns of the anatase phase TiO2 (JCPDS 21-1272) and the rutile phase (JCPDS 21-1276), no peaks of impurities are detected except the peaks of metal titanium coming from the substrates. The contents of rutile phase and anatase phase are calculated by the XRD results, using the method described by Zhu et al. [26] and listed in Table 1. The results show that the content of rutile phase in Fe-doped TiO2 (19.66 wt%) is lower than that of the undoped TiO2 (21.88 wt%), that is to say, the content of rutile decreases for the iron doping after the heating treatment, which indicates that the doping of iron may retard the transition from anatase to rutile. The content of rutile in the N-doped and codoped TiO2 are 22.48 wt% and 21.89 wt%. The average crystallite sizes were calculated by Scherrer’s formula (,, nm) and listed in Table 1, choosing the peaks at scattering angles of 25° and 27.5° for anatase and rutile, respectively. The average crystallite sizes for anatase phase in the undoped TiO2 and samples doped with Fe, N, are 32.92 nm, 25.27 nm, 27.60 nm, and 27.78 nm, while the average crystallite sizes for rutile phase are 30.57 nm, 25.30 nm, 26.93 nm, and 28.78 nm, respectively. These results reveal that doping with Fe and N element will retard the growth of the crystallite sizes, which is similar to those described in [27, 28].
Figures 2(a), 2(b), 2(c), and 2(d) present the illustrative top SEM images of typical undoped, Fe-doped, N-doped, and codoped TiO2 samples. The right part of Figure 2(d) is the SEM image of cross section of the codoped sample. As it can be seen, the length and the diameter of all the samples are about 5 μm and 90 nm. The nanowires are arranged one by one closely, ensuring that light can be effectively scattered by the rutile phase in the mixing-phase films [29].
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The crystalline structure of TiO2 is further examined by HRTEM. The HRTEM images of a single codoped TiO2 nanowire and the magnified local image in the white rectangle zone is shown in Figure 3. HRTEM observations reveal that the diameter of the nanoparticles is in a size range of 70–100 nm and TiO2 nanoparticles can be discriminated and exhibit a perfect lattice. As shown in Figure 3, the clear anatase-rutile interface is confirmed by HRTEM, where the lattice spaces are measured to be 0.189 nm along the (200) plane of anatase phase and 0.248 nm along the (101) plane of rutile phase, respectively. The formation of intimate contact between anatase and rutile phases is realized so that the charge transfer between the two phases can occur smoothly [30]. The anatase-rutile interface is the active site for photocatalysis and is one of the dominate factors for the enhancement of the photocatalytic activity reported elsewhere [31].
To investigate the composition and the chemical state of ions, XPS measurements were performed. Figure 4(a) is the XPS survey spectrum of the sample. The sharp peaks at 284.7 eV, 458.60 eV, 530.15 eV are corresponding to the binding energy of C 1s, Ti 2p3/2, O1s, and the two weak photoelectron peaks at 710.41 eV and 396.00 eV correspond to Fe 2p3/2, and N 1s, respectively. The peak at 284.7 eV, in general, is a signal of adventitious elemental carbon as reported in other works [32, 33]. No other impurity peaks were observed, which reveals that the sample contains Ti, O, Fe, and N four elements.
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Figure 4(b) is the N 1s core level spectra of codoped TiO2. It is found that there are two peaks with similar intensity at the bonding energies of 396.04 eV and 400.20 eV, respectively. Generally, the peak at 396 eV reflects the formation of N–Ti–O bonds, indicating the substitution of N ion for O ion [34–36]. The peak at 400 eV can be assigned to molecular nitrogen bonded to the surface defects or the N atoms bonding to O sites in TiO2, forming Ti–O–N bonds [37]. Figure 4(c) shows the high-resolution Fe 2p core level spectrum. As it can be seen, the binding energies of the Fe 2p XPS peaks are at 710.41 eV, indicating that the Fe element in the sample exists mainly in the +3 oxidation state [38]. The peak position of codoped TiO2 exhibits a shift about 0.3 eV to lower energy compared with that in Fe2O3 (710.7 eV for Fe 2p3/2) [38]. The variation in the elemental binding energy due to the difference in the chemical potential and polarizability of Fe ions in codoped TiO2 and Fe2O3 indicates that Fe3+ ion has incorporated into the TiO2 lattice and Fe–O–Ti bonds form in the codoped sample [39]. The calculations of the N 1s core level spectrum and Fe 2p core level spectrum show that the N and Fe content is 3.15% and 1.09%, respectively.
Figure 5(a) illustrates the UV-vis absorption spectroscopy of the undoped, Fe-doped, N-doped, and codoped TiO2. The undoped TiO2 exhibits the characteristic spectrum of TiO2 with its fundamental absorption sharp edge around 380 nm. Compared with the undoped TiO2, the absorption edge is shifted toward visible light range for Fe-doped, N-doped, and codoped samples and a significant enhancement of absorption visible light range is observed in N-doped and codoped samples. As it can be seen from Figure 5(a), the spectrum of codoped sample has the largest red-shift. The local spectra (Figure 5(b)) indicate that the onset of the absorption spectrum appears at 384 nm, 394 nm, 396.3 nm, and 405 nm, for undoped, Fe-doped, N-doped, and codoped TiO2, respectively. According to the equation , the band gaps of the undoped, Fe-doped, N-doped, and codoped TiO2 are 3.23 eV, 3.15 eV, 3.13 eV, and 3.06 eV. Figure 5(c) shows the plots of versus deduced from Figure 5(a). The band gap of each samples is determined by fitting the absorption spectra data according to the equation ( is the absorption coefficient; is the photo energy; is a constant number; and is the absorption band gap energy). As it can be seen that the band gaps of the undoped, Fe-doped, N-doped, and codoped TiO2 are 3.23 eV, 3.15 eV, 3.13 eV, and 3.06 eV, which are consistent with the results of Figure 5(b) and similar to those reported in [40, 41], but much higher than the values calculated by Romero-Gomez et al. [23]. The reason for this deviation may be that the doping content in experiments is much lower than that in the theoretical model.
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As to the origin of visible-light sensitivity in Fe, N and codoped TiO2, the possible mechanism can be proposed. It is known that the band gap of TiO2 can be tailored by doping with some nonmetal or metal elements. Herein, for the Fe-doped sample, the absorption edge red-shift could be understood by the band gap narrowing, which is the result of the induced sub-band-gap transition corresponding to the excitation of 3d electrons of Fe3+ to TiO2 conduction band [41, 42]. In nitrogen doped sample, the doped N ions could induce an add-on shoulder on the edge of the valence band maximum and the localized N 2p states above the valence band [40]. On the other hand, it is known that N has a lower valence state than O so that the incorporation of N must promote the synchronous formation of oxygen vacancies for the charge equilibrium in TiO2 [43]. Therefore, the band narrowing due to newly formed oxygen vacancies by N-doping in TiO2 cannot be neglected. As a result, the visible light response for N-doped TiO2 is attributed to both oxygen vacancies and the N 2p states. For codoped sample, compared with the N-doped TiO2, the Fe element is introduced into the lattice of TiO2 as the oxide state of Fe3+ by above discussion, so that Fe-doping is a p-type doping and can increase the oxygen vacancies induced by the hole doping of N, similar as [43]. Consequently, the codoped TiO2 results in the best response to visible-light and the largest red-shift because of the synergistic effect of Fe and N codopant.
To evaluate the photocatalytic activities of the undoped, Fe-doped, N-doped, and codoped samples, experiments were carried out for MB degradation in an aqueous solution under visible light irradiation by insert a filter ( nm) between the Xe-lamp and the samples. Figure 6 shows the photocatalytic degradation curves of MB catalyzed by the samples. The photocatalytic activity is in the order of the codoped TiO2 > N-doped TiO2 > Fe-doped TiO2 > undoped TiO2. The codoped TiO2 shows the best photocatalytic activity and leads to a removal of MB up to 57.42% in 4 hours.
It has been reported that oxygen vacancy induced by N doping or self-doping plays an important role in the photocatalytic activity of TiO2 catalyst by trapping the photoinduced electron and acting as a reactive center for the photocatalytic process [44, 45]. codoping on the photocatalytic activity in anatase-rutile mixed TiO2 nanowires, p-type Fe doping can increase the oxygen vacancies in N-doped TiO2 so that the photocatalytic activity of the sample is enhanced after introducing Fe3+ to form the codoped sample. It is known that the minimum of the conduction band of the rutile phase lies at higher in energy than that of the anatase phase by about 0.3-0.4 eV [25]. The Fe3+ energy level is just below the conduction band of anatase [41, 42]. The nitrogen doping could induce an add-on shoulder on the edge of the valence band maximum and the localized N 2p states above the valence band [40]. Therefore, after codoping, the anatase-rutile mixed TiO2 nanowires have complex band lineup, similar with the band lineup of rutile and anatase [25]. This complex staggered alignment of the bands effectively enhances the charge separation with migrating holes accumulating in rutile while electrons in anatase, thus enhancing photocatalytic activity, accordingly.
4. Conclusions
In conclusion, TiO2 nanowires with mixed rutile-anatase phase were synthesized by a two-step anodic oxidation method. Doping with nitrogen or iron could enhance the visible light absorption and the photocatalytic activity of TiO2 nanowires. The codoped TiO2 nanowire possesses the highest visible light absorption and the best photocatalytic activity for catalyzing the degradation of methylene blue. The increase of the quantity of O vacancy and the enhancement of the charge separation in the nanowires due to the complex staggered alignment of the bands leading by Fe and N codoping may be two of the reasons for the increase of photocatalytic activity in the codoped sample.
Acknowledgments
The authors would like to acknowledge the financial support from the National Basic Research Program (973 Program) (no. 2009CB939704 and no. 2009CB939705), the National Natural Science Funds (no. 51172166, 10974148, 61106005), National Science Fund for Talent Training in Basic Science (no. J0830310), and Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials (Hubei University).