Effect of synergy on the visible light activity of B, N and Fe co-doped TiO₂ for the degradation of MO

Abstract

Single doped, co-doped and tri-doped TiO₂ with B, N and Fe are successfully synthesized by using the hydrothermal method. The samples are characterized by X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS). The photocatalytic activities of the samples are evaluated for degradation of methyl-orange (MO, 20 mg L⁻¹) in aqueous solutions under visible light (λ > 420 nm). The results of XRD suggest that all the catalysts present anatase crystal. All the doping catalysts show higher photoactivities than pure TiO₂ under visible light irradiation. In the single nonmetal doped TiO₂, the localized dopant levels near the valence band (VB) are responsible for the enhancement of photoactivies. Fe³⁺ impurity level formed under the conduction band (CB) induces the high photocatalytic activities of iron doped TiO₂. In the co-doped and tri-doped catalysts, the B 2p and N 2p acceptor states contribute to the band gap narrowing by mixing with O 2p states combined with the overlapping of the conduction band by the iron “d” orbital, resulting in improvement of the photo-performance under visible light irradiation. Iron co-doped with boron catalyst shows low photoactivity under visible light due to the absence of Fe³⁺ impurity levels at the bottom of the conduction band. In addition, the XPS results indicate the presence of synergistic effects in co-doped and tri-doped catalysts, which contribute to the enhancement of photocatalytic activities.

---

1. Introduction

Titanium oxides, as one of the most promising photocatalysts, have been used for degradation of organic pollution, photocatalytic dissociation of water and solar energy conversion.^1–4 Because the wide band gap of TiO₂ hampers practical applications, many approaches have been undertaken to extend the activity of TiO₂ into the visible range.^5–11 There are a number of publications reporting the beneficial influence of the substitution of oxygen in TiO₂ by nonmetals such as nitrogen, carbon, sulfur and boron on bringing activity of the photocatalysts into the visible range.^12–19 Among all nonmetal-doped TiO₂, nitrogen doped TiO₂ nanomaterials have been found to exhibit superior photocatalytic activity under visible light due to its effective narrowing of the band gap of TiO₂.¹⁶ Compared with single nitrogen doping, simultaneous doping of nitrogen and other atoms has attracted many researchers, due to its higher photocatalytic performance and peculiar characteristics. For example, Li et al.^20,21 have found nitrogen co-doped with fluorine in TiO₂ had a higher photo activity under visible light than TiO₂ single doping with nitrogen or fluorine. Yang²² and co-workers found that TiO₂ nanomaterials co-doped with carbon and nitrogen showed high photocatalytic activity due to the synergistic effect of carbon and nitrogen. Lu et al.²³ have prepared the TiO₂ co-doped with boron and nitrogen and ascribed its high photocatalytic activity under UV and visible light to the existence of synergistic effects between boron and nitrogen.

Doping with transition metals is also used to improve the absorption properties of TiO₂.^24–29 Choi et al.³⁰ conducted a systematic study on the photocatalytic activity of TiO₂ nanoparticles doped with 21 transition metal elements and found that doping with Fe³⁺, Mo⁵⁺, Ru³⁺, Os³⁺, Re⁵⁺, V⁴⁺ and Rh³⁺ significantly increased photoactivity in the liquid-phase photodegradation of CHCl₃, while Co³⁺ and Al³⁺ doping decreased the photoactivity. Among these transition metal elements, Fe³⁺ was considered as an interesting dopant of TiO₂ and most of the investigations have been carried out on preparation of Fe-doped TiO₂ using chemical and physical methods.^31–40 Yamashita et al.³¹ have found significant improvement in the degradation of aqueous 2-propanol by Fe³⁺-ion-implanted TiO₂ under visible light irradiation. However, iron doping can easily act as charge carriers which are recombination centers, harmful to the photo activity of TiO₂. In order to minimize its role as recombination centers, TiO₂ co-doped with iron and other nonmetal elements are prepared by many researchers. Kim⁴¹ and co-workers synthesized boron–iron co-doped anatase TiO₂ using a modified sol–gel method and the presence of boron and iron caused a red shift in the absorption band of TiO₂. Cong et al.⁴² have successfully synthesized the nano-TiO₂ co-doped with nitrogen and iron(III) which showed high photocatalytic activity under visible light.

To date there are many published articles related to single doped and co-doped TiO₂. However, the research on tri-doping, especially relating to iron, boron and nitrogen tri-doped TiO₂, is rather rare. Herein we would like to prepare single doped, co-doped and tri-doped TiO₂ with B, N and Fe using the hydrothermal method and investigate its photo-performance under visible light. The mechanisms for the photocatalytic activities of different doping catalysts are also investigated in this article.

2 Experimental

2.1 Preparation of photocatalysts

Tetrabutyl titanate (6.7 mL TBOT) was added to ethanol (30 mL), while stirring the solution for 0.5 h, and then Fe(NO₃)₃·9H₂O (0.02, 0.04, 0.08 g corresponding to 0.2 at%, 0.5 at%, 1.0 at% of Fe/Ti mol%) was added to the above solution to obtain solution A. H₃BO₃ (0.12 g) and urea (0.6 ml) were dissolved in the ethanol (36 mL), and then deionized water (2.0 mL) and HNO₃ (0.50 mL) was successively added to the solution under vigorous stirring to obtain solution B. Solution A was added drop wise to solution B, while stirring the mixture for 4 h. The resultant mixture was then transferred into a 100 mL Teflon-inner-liner stainless steel autoclave which was kept under 453 K for 12 h. After the hydrothermal treatment, the precipitate was washed, dried and ground to obtain the nano-particles of TiO₂ tri-doped with nitrogen, boron and iron denoted as (mFe, N, B)–TiO₂, where m describes the mol% of iron to titanium.

(B,N)-TiO₂ was prepared by a similar method without the addition of Fe(NO₃)₃·9H₂O. By addition of 0.04 g Fe(NO₃)₃·9H₂O in solution A, (Fe,N)-TiO₂ and (Fe,B)-TiO₂ were prepared by the similar procedure without the addition of H₃BO₃ and urea, respectively. 0.04 g Fe(NO₃)₃·9H₂O was added in solution A to prepare Fe-TiO₂ without the addition of H₃BO₃ and urea. N-TiO₂ was synthesized without the addition of H₃BO₃ and Fe(NO₃)₃·9H₂O and the catalyst B-TiO₂ was prepared without the addition of urea and Fe(NO₃)₃·9H₂O. Without the addition of Fe(NO₃)₃·9H₂O, urea and H₃BO₃, pure TiO₂ was prepared using the same method.

2.2 Characterization

X-Ray diffraction (XRD) patterns of all samples were collected in the range 20–80° (2θ) using a Rigaku D/MAX 2550 diffractometer (Cu-K radiation, λ = 1.5406 Å), operated at 40 kV and 100 mA. The crystallite size was estimated by applying the Scherrer equation to the full width at half-maximum (fwhm) of the (101) peak of anatase and the (110) peak of rutile, with α-silicon (99.9999%) as a standard for the instrumental line broadening. The instrument employed for XPS studies was a Perkin-Elmer PHI 5000C ESCA system with Al Kα radiation operated at 250 W. The shift of the binding energy due to relative surface charging was corrected using the C1s level at 285.0 eV as an internal standard. The UV-vis absorbance spectra were obtained for the dry-pressed disk samples using a Scan UV-vis spectrophotometer (Varian, Cary 500) equipped with an integrating sphere assembly, using BaSO₄ as the reflectance sample. The spectra were recorded at room temperature in air within the range 200–800 nm.

2.3 Measurements of photocatalytic activities

The photocatalytic activity of each sample was evaluated in terms of the degradation of methyl-orange (MO, 20 mg L⁻¹). MO was chosen as a model pollutant to evaluate the photocatalytic activities. The photocatalyst (0.07 g) was added to a 100 mL quartz photoreactor containing 70 mL of a 20 mg L⁻¹ MO solution. The mixture was stirred for 30 min in the dark in order to reach the adsorption–desorption equilibrium. A 1000-W tungsten halogen lamp equipped with a UV cut-off filters (λ > 420 nm) was used as a visible light source (the average light intensity was 60 mW cm⁻².) and a 300-W high-pressure Hg lamp for which the strongest emission wavelength is 365 nm was used as a UV light source (the average light intensity was about 1230 μW cm⁻²). The lamp was cooled with flowing water in a quartz cylindrical jacket around the lamp and ambient temperature was maintained during the photocatalytic reaction. At the given time intervals, the analytical samples were taken from the mixture and immediately centrifuged, then filtered through a 0.22 μm Millipore filter to remove the photocatalysts. The filtrates were analyzed by recording variations in the absorption in UV-vis spectra of MO using a Cary 100 ultraviolet visible spectrometer.

3 Results and discussion

3.1 X-Ray diffraction

XRD patterns of the samples are shown in Fig. 1. It can be seen that the diffraction peaks of all samples are ascribed to the peaks of TiO₂ anatase phase. These observations indicate that there has been virtually no phase change in TiO₂ during the doping, co-doping and tri-doping process. Furthermore, no significant characteristic peak of boron oxide and iron oxide can be found in boron and iron doped TiO₂, due to lower amount of boron and iron. The crystal sizes of all the samples are estimated using the Scherrer equation:⁴³
Where B is the half-height width of the diffraction peak of anatase, K = 0.89 is a coefficient, θ is the diffraction angle, and λ is the X-ray wavelength corresponding to the Cu-Kα irradiation, and the detailed data are shown in Table 1. It is suggested that the particle size decreased obviously after the nitrogen doping which is consisted with our previous work.⁴⁴ The crystallite sizes of catalysts doped with nitrogen are much smaller than that of the catalysts doped with iron or boron. Excluding the impact of nitrogen, the process of co-doping and tri-doping exhibit little effect on the particle size as shown in Table 1. In addition to this, there is no change in the “d” space values, which implies that the iron, boron and nitrogen modification in doping, co-doping and tri-doping samples do not change the average unit cell dimension.⁴⁴

Table 1 Some structural characteristics of different samples

	Crystallite size/nm	d-Spacing/Å
Fe-TiO2	8.01	3.5
B-TiO2	8.86	3.5
(Fe,B)-TiO2	8.22	3.5
N-TiO2	6.58	3.5
(Fe,N)-TiO2	6.14	3.5
(Fe,B,N)-TiO2	6.32	3.5

---

Fig. 1 XRD patterns of Fe-TiO₂, B-TiO₂, N-TiO₂, (Fe,B)-TiO₂, (Fe,N)-TiO₂, and (0.5Fe,B,N)-TiO₂.

3.2 UV-vis diffuse reflectance spectra

The diffuse reflectance spectra of doped, co-doped and tri-doped samples are shown in Fig. 2. The absorptions of Fe-TiO₂, B-TiO₂ and (Fe,B)-TiO₂ are similar and stronger in the range of wavelengths from 400 to 600 nm. After nitrogen doping the absorptions of catalysts are drastic and stronger in the range of wavelengths from 400 to 800 nm. The absorptions of N-TiO₂, (Fe,N)-TiO₂ and (0.5Fe,B,N)-TiO₂ are similar and present a stronger red shift compared to those of Fe-TiO₂, B-TiO₂ and (Fe,B)-TiO₂ samples. It is expected that nitrogen doping contributed to the red shift because of the narrowing of the band gap by the N species.^42,44

Fig. 2 UV–vis diffuse reflectance spectra of Fe-TiO₂, B-TiO₂, N-TiO₂, (Fe,B)-TiO₂, (Fe,N)-TiO₂, and (0.5Fe,B,N)-TiO₂.

3.3 XPS spectra

Fig. 3 shows the XPS spectra for the Fe 2p region and their fitting curves of doped, co-doped and tri-doped samples. It can be seen that there is a peak at approximately 709.5–710.5 eV, which is attributed to the presence of the Fe³⁺ state.^45–47 The peak observed at a higher BE (713.3–714.3 eV) indicates the presence of Fe²⁺ ions.^47,48 It can be speculated that Fe²⁺ is detected due to photocatalytic reduction from Fe³⁺ to Fe²⁺ by conduction-band electron of the catalysts during XPS measurement in vacuum.⁴⁹ For the catalyst Fe-TiO₂, the peak at 709.5 eV corresponds to the Fe³⁺ entered by substitution of part of the O sites (Ti–O–Fe).⁵⁰ Another peak at 713.3 eV is attributed to the presence of Fe²⁺ (see Fig. 3(a)). When TiO₂ is co-doped with iron and nitrogen, there is a new peak at 706.8 eV which is the result of the formation of Fe–N on the surface of the catalyst (see Fig. 3(c)). The peak at 706.4 eV ascribed to Fe–B also can be observed when iron and boron were doped simultaneously.^51,52 To the catalyst of (Fe,B)-TiO₂, the Fe²⁺ characteristic peak disappears compared with Fe-TiO₂ (see Fig. 3(c)). The tri-doped catalysts show three peaks at 707.6, 710.5 and 714.3 eV which are attributed to the presence of Fe–B, Fe³⁺ and Fe²⁺ states, respectively (see Fig. 3(d)).

Fig. 3 XPS spectra of Fe 2p for Fe-TiO₂ (a), (Fe,N)-TiO₂ (b), (Fe,B)-TiO₂ (c), and (0.5Fe,B,N)-TiO₂ (d) samples.

The XPS spectra for the B1s region of B-TiO₂ shows two peaks at the BE of 188.8 and 191.9 eV as shown in Fig. 4(a). The former is the characteristic peak of Ti–B and the later corresponds to boron weaved into the interstitial TiO₂ structure, existing in the form of a Ti–O–B structure,⁵³ which is consistent with the result reported by Jiang and co-workers.⁵⁴ Some researchers found that BE peaks for B1s in FeB and Fe₂B were 187.9 eV and 188.3 eV respectively.^51,52 The peak at 190.5 eV for B1s of (Fe,B)-TiO₂ in Fig. 4(b) reveals the existence of Ti–O–FeB. The oxygen bonded to FeB induces the reduction of B1s electron density, resulting in the increasing BE of B1s (a change from 187.9 eV to 190.5 eV). Fig. 4(c) reveals the XPS spectra of (B,N)-TiO₂ and the BE peaks for B1s are 186.5 eV and BE at 189.7 eV which should be attributed to B1s in Ti–B and Ti–B–N, respectively.^23,51 When TiO₂ tri-doped with iron, nitrogen and boron, there appear four characteristic peaks at 185.9, 188.5, 190.8 and 192.6 eV seen from Fig. 4(d). The peak located at BE of 188.5 eV should be attributed to Ti–B–N. The peak at 190.8 eV indicates the existence of Fe–B in tri-doped catalysts. The peaks of BE above 192.6 eV are attributed to boron oxides such as B₂O₃.

Fig. 4 XPS spectra of B1s for B-TiO₂ (a), (Fe,B)-TiO₂ (b), (B,N)-TiO₂ (c), and (0.5Fe,B,N)-TiO₂ (d) samples.

3.4 Evaluation of photocatalytic activity

The decomposition of MO (20 mg L⁻¹) in visible light irradiation of TiO₂ doped with different elements is carried out and compared with that of pure TiO₂ (shown in Fig. 5). The results indicate that the photocatalytic activities are greatly improved by the doping of nitrogen, boron and iron. To the single doped samples, N-TiO₂ shows the higher decomposition rate than other catalysts such as Fe-TiO₂ and B-TiO₂. For the co-doped samples, except for (Fe,B)-TiO₂, the catalysts of (Fe,N)-TiO₂ and (N,B)-TiO₂ all show excellent photocatalytic activities which are higher than that of single doped samples. The photo degradation rate of catalyst is not improved by co-doped with iron and boron, whose degradation rate is even lower than iron single doped TiO₂. The tri-doped TiO₂ shows high photocatalytic activity under visible light and the photodegradation rate increases with increasing Fe/Ti ratios. The tri-doped catalyst with an Fe/Ti mole ratio of 0.5 at% presents the highest photocatalytic activity.

Fig. 5 Photodegradation rate of MO under visible light illuminated for 5 h on pure TiO₂, doping, co-doping and tri-doping samples.

3.5 Mechanism for the photocatalytic activity under visible light irradiation

For the single doped catalysts, the photocatalytic activities of these are improved by the doping of metal or nonmetal elements compared with the pure TiO₂ as shown in Fig. 6. The XPS spectra of Fe2p for Fe-TiO₂ sample indicate the existence of Fe³⁺ and Fe²⁺ states as shown in Fig. 3(a). Fe³⁺ ions in the Ti⁴⁺ matrix form an impurity level under the CB and can act as a shallow trap to trap the photogenerated electron, which can restrain the recombination of photogenerated electrons and holes (the schemes as shown in Fig. 6(a)).⁵⁴ Fe³⁺ ions trap photogenerated electrons to form Fe²⁺ which induces the detection of Fe²⁺ during XPS measurement in vacuum (without any hole scavenger as shown in Fig. 6(a)), that is, the detection of Fe²⁺ indirectly confirms the formation of Fe³⁺ impurity level under CB which can narrow the band gap of TiO₂ and induce the improvement of photocatalytic activity of Fe-TiO₂ under visible light. For the samples of B-TiO₂ and N-TiO₂, photocatalytic performance under visible light is also improved by the nonmetal doping due to the band gap narrowing of TiO₂ as shown in Fig. 6(a). Oxygen atom in TiO₂ is substituted by nitrogen or boron atom and a narrow substitute band (dopant level) is formed above VB. As a result, the binding energy of TiO₂ is lowered by a significant shift.¹³

Fig. 6 Various proposed schemes illustrating the possible changes that might occur to the band-gap electronic structure of anatase TiO₂ on doping with various non-metals, metals: (a) B-TiO₂ and N-TiO₂ with localized dopant levels near the VB and Fe-TiO₂ with an impurity level formed under the CB; (b) band-gap narrowing resulting from broadening of the VB by B co-doping with N-TiO₂ and overlapping of the CB by Fe co-doping with N-TiO₂; (c) (Fe,N,B)-TiO₂ with localized dopant levels to broaden the VB by nonmetals doping and an impurity level formed to overlap the CB by iron doping.

When TiO₂ is co-doped by nonmetals such as nitrogen and boron, there exists a synergistic effect. In our experiment, the nitrogen and boron are simultaneously doped in TiO₂ by a hydrothermal method. To the band-gap electronic structure of (B,N)-TiO₂, the mixing of B 2p with O 2p states will effectively lead to the band gap narrowing of titania, and the mixing of the N 2p states of nitrogen atoms with the VB which results into the further decrease of band gap energy as shown in Fig. 6(b). Our XPS spectra of B1s for (B,N)-TiO₂ results indicate the presence of synergetic effects between boron and nitrogen. The peak at the BE of 189.7 eV is attributed to Ti–B–N, which is the result of a synergistic effect. Lu et al.²³ have found the role of Ti–B–N bonding in the formation of additional impurity states in the upper VB by the density functional theory (DFT) calculations method. Their calculation results suggested that B and N can be doped on the pristine surface of TiO₂ to form Ti–B–N. The effect of B and N co-doping can change the surface electronic states of TiO₂ dramatically. They found that the B and N co-doping introduces more surface electronic states and changes the gaps between these states, which may have an important influence on the lifetime of excitons. The formation of Ti–B–N leads to the construction of a favorable surface structure that facilitates the separation and transfer of charge carriers. Thereby in our case, it seems that the presence of Ti–B–N structure on the (B,N)-TiO₂ surface acts as an efficient co-catalyst which facilitates the separation and transfer of charge carriers and that the N species contributes to the visible light absorption as shown in Fig. 2.^42,44 The synergistic effects of creating visible light absorption and then promoting visible light induced carriers are responsible for the high visible-light photocatalytic activity of (B,N)-TiO₂. The above results are contrary to the work of Lambert and co-workers, who did not find the synergistic effect of (B,N)-TiO₂.⁶

The probable schematic diagram of (Fe,N)-TiO₂ is proposed in Fig. 6(b) by considering the effects of nitrogen and iron ions co-doping. The co-doping of N and Fe induces the narrowing band gap of TiO₂. The overlap of the CB due to Ti (d orbital) and Fe (d orbital) combined with the N impurity level formed near the VB results in the decrease of band gap as shown in Fig. 6(b).⁵⁵ The Fe³⁺ ion can trap the photogenerated electrons while the nitrogen can trap the photogenerated holes, which enhances the utilization efficiency of photogenerated electrons and holes. For the catalyst of (Fe,N)-TiO₂, the electrons can be excited from the N impurity level to the Fe³⁺ impurity level or from the VB to the Fe³⁺ impurity level or from the N impurity level to the CB under visible light irradiation. Therefore, the quantity of the photoincuced electrons and holes is improved compared with the pure TiO₂ and single doped TiO₂. The band gap narrowing and a large amount of photogenerated charges induce its enhancement of photo-performance.

It is worth noting that when the iron is co-doped with boron, the photocatalytic activity of the catalyst is not improved and its photodegradation rate of MO even lower than Fe-TiO₂, as shown in Fig. 5. From the XPS spectra of Fe 2p for (Fe,B)-TiO₂, the characteristic peak of Fe²⁺ has disappeared which suggests the absence of Fe³⁺/Fe²⁺ impurity levels under CB. That means the Fe doping can not contribute to the narrowing band gap of (Fe,B)-TiO₂. For the catalyst of (Fe,B)-TiO₂, the band gap can not be availably narrowed by the Fe and B co-doping resulting into its lower photocatalytic activity compared with (Fe,N)-TiO₂. This is further supported by the results of Ti 2p XPS spectra as follows. As we all know, photogenerated electrons and holes could either recombine or move to the surface to react with species adsorbed on the surface. The formation of Fe³⁺ impurity levels can narrow the band gap and trap electrons to form Fe²⁺ defective sites. Some of the photogenerated electrons can also be reacted with Ti⁴⁺ to form a reduced state of the titanium ions due to the overlap of Ti CB (the CB consisting of Ti “3d” orbital and Fe “3d” orbital).⁵⁶Fig. 7 shows the Ti 2p XPS of different samples. The spectrum of pure TiO₂ exhibits a symmetrical profile, while the (Fe,N,B)-TiO₂ exhibits a shoulder at the lower binding energy side of the main peak, as circled in Fig. 7. This shoulder peak has been previously observed for oxygen-treated TiO₂ and it originates from the reduced state of the titanium ions.^57,58 However, there is no shoulder peak observed for (Fe,B)-TiO₂, as shown in Fig. 7, which indicates the absence of reduced titanium due to the disappearance of the Fe³⁺/Fe²⁺ impurity level which can trap the photogenerated electrons and transfer them to Ti⁴⁺ to form reduced titanium ions. Therefore, the photocatalytic activity of (Fe,B)-TiO₂ is not enhanced by the co-doping of Fe and B due to the absence of Fe³⁺ impurity level which plays an important role in charge transfer.

Fig. 7 Ti 2p XPS spectra of pure TiO₂, (Fe,B)-TiO₂ and (Fe,N,B)-TiO₂.

Although the catalyst of (Fe,B)-TiO₂ shows a low photoactivity and the N tri-doped with (Fe,B)-TiO₂ presents excellent photo-performance in visible light irradiation, (Fe,N,B)-TiO₂ presents high photoactivity due to the synergistic effects of metals and nonmetals. The results of the XPS measurements indicate the presence of synergistic effects between N and B due to the appearance of a Ti–B–N characteristic peak at 188.5 eV, as shown in Fig. 4(d). The B and N synergistic effect can induce a change of surface electronic states resulting in the promotion of visible light induced carriers and introduce new states around the upper VB close to the Fermi level. Meanwhile, the introduction of iron produces the impurity level at the bottom of the CB as shown in Fig. 6(c). The new states introduced by B and N combined with the Fe³⁺ impurity level induce the narrowing band gap of (Fe,N,B)-TiO₂ which enhances the photoactivity of tri-doped TiO₂. (Fe,N,B)-TiO₂ shows the highest photodegradation rate when the Fe/Ti mole ratio is 0.5 at% due to the large excess amount of Fe³⁺ ions thought to occupy active sites on the surface of N-TiO₂ and to function as a recombination center between electrons and holes.⁵⁹

4 Conclusions

Single doped, co-doped and tri-doped TiO₂ with iron, boron and nitrogen are successfully synthesized by hydrothermal method. All the doping catalysts have higher photocatalytic activities than pure TiO₂ under visible light irradiation. Co-doped TiO₂ presents higher photoactivities than single doped TiO₂ while the tri-doping is better than co-doping. The (0.5Fe,N,B)-TiO₂ catalyst shows the highest degradation rate in all the doped catalysts. The band gap of the doped catalysts is narrowed by the formation of nonmetal dopant levels near the VB and Fe³⁺ impurity level under the CB responsible for the enhancement of photocatalytic activities under visible light. In addition, the synergistic effect of Ti–B–N formed on the surface of the catalysts can act as an efficient co-catalyst to inhibit the recombination of photogenerated electrons and holes, which can improve the photo-performance of (B,N)-TiO₂ and (Fe,N,B)-TiO₂.

Acknowledgements

This work has been supported by National Nature Science Foundation of China (20773039, 20977030); National Basic Research Program of China (973 Program, 2007CB613301, 2010CB732306), and the Ministry of Science and Technology of China (2007AA05Z303).

References

1. M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69.
2. A. Fujishima, T. N. Rao and D. A. Tryk, J. Photochem. Photobiol., C, 2000, 1, 1.
3. C. Hu, J. C. Yu, Z. Hao and P. K. Wong, Appl. Catal., B, 2003, 42, 47.
4. J. C. Yu, W. Ho, J. Yu, H. Yip, P. K. Wong and J. Zhao, Environ. Sci. Technol., 2005, 39, 1175.
5. H. Kisch, S. Sakthivel, M. Janczarek and D. Mitoraj, J. Phys. Chem. C, 2007, 111, 11445.
6. S. In, A. Orlov, R. Berg, F. Garcia, S. Pedrosa-Jimenez, M. S. Tikhov, D. S. Wright and R. M. Lambert, J. Am. Chem. Soc., 2007, 129, 13790.
7. J. F. Zhu, Z. G. Deng, F. Chen, J. L. Zhang, H. J. Chen, M. Anpo and J. Z. Huang, Appl. Catal., B, 2006, 62, 329.
8. Y. M. Wu, J. L. Zhang, L. Xiao and F. Chen, Appl. Catal., B, 2009, 88, 525.
9. Y. M. Wu, J. L. Zhang, L. Xiao and F. Chen, J. Phys. Chem. C, 2009, 113, 14689.
10. M. Y. Xing, J. L. Zhang and F. Chen, Appl. Catal., B, 2009, 89, 563.
11. M. Y. Xing, J. L. Zhang and F. Chen, J. Phys. Chem. C, 2009, 113, 12848.
12. C. Burda, Y. B. Lou, X. B. Chen, A. C. S. Samia, J. Stout and J. L. Gole, Nano Lett., 2003, 3, 1049.
13. H. Irie, Y. Watanabe and K. Hashimoto, J. Phys. Chem. B, 2003, 107, 5483.
14. S. Sakthivel and H. Kisch, Angew. Chem., Int. Ed., 2003, 42, 4908.
15. B. Tryba, T. Tsumura, M. Janus, A. W. Morawski and M. Inagaki, Appl. Catal., B, 2004, 50, 177.
16. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269.
17. T. Ohno, Z. Miyamoto, K. Nishijima, H. Kanemitsu and X. Y. Feng, Appl. Catal., A, 2006, 302, 62.
18. W. Zhao, W. Ma, C. Chen, J. Zhao and Z. Shuai, J. Am. Chem. Soc., 2004, 126, 4782.
19. K. Y. Jung, S. B. Park and S. K. Ihm, Appl. Catal., B, 2004, 51, 239.
20. D. Li, H. Haneda, S. Hishita and N. Ohashi, Chem. Mater., 2005, 17, 2588.
21. D. Li, H. Haneda, S. Hishita and N. Ohashi, Chem. Mater., 2005, 17, 2596.
22. D. M. Chen, Z. Y. Jiang, J. Q. Geng, Q. Wang and D. Yang, Ind. Eng. Chem. Res., 2007, 46, 2741.
23. G. Liu, Y. N. Zhao, C. H. Sun, F. Li, G. Q. Lu and H. M. Cheng, Angew. Chem., Int. Ed., 2008, 47, 4516.
24. J. Kiwi and M. Graetzel, J. Phys. Chem., 1986, 90, 637.
25. K. E. Karakitsou and X. E. Verykios, J. Phys. Chem., 1993, 97, 1184.
26. J. F. Zhu, F. Chen, J. L. Zhang, H. J. Chen and M. Anpo, J. Mol. Catal. A., 2004, 216, 35.
27. K. Nagaveni, M. S. Hegde and G. Madras, J. Phys. Chem. B, 2004, 108, 20204.
28. S. Klosek and D. Raftery, J. Phys. Chem. B, 2001, 105, 2815.
29. H. Kisch, L. Zang, C. Lange, W. F. Maier, C. Antonius and D. Meissner, Angew. Chem., Int. Ed., 1998, 37, 3034.
30. W. Choi, A. Termin and M. R. Hoffmann, J. Phys. Chem., 1994, 98, 13669.
31. H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, B. Neppolian and M. Anpo, Catal. Today, 2003, 84, 191.
32. J. A. Navio, G. Colon, M. I. Litter and G. N. Bianco, J. Mol. Catal. A: Chem., 1996, 106, 267.
33. Y. H. Zhang, S. G. Ebbinghaus, A. Weidenkaff, T. Kurz, P. J. Nidda, K. V. Klar, M. Guengerich and A. Reller, Chem. Mater., 2003, 15, 40283.
34. J. Soria, J. C. Conesa, V. Augugliaro, L. Palmisano, M. Schiavello and A. Sclafani, J. Phys. Chem., 1991, 95, 274.
35. M. I. Litter and J. A. Navio, J. Photochem. Photobiol., A, 1996, 98, 171.
36. K. T. Ranjit and B. Viswanathan, J. Photochem. Photobiol., A, 1997, 108, 79.
37. E. Piera, M. I. Tejedor, M. E. Zorn and M. A. Anderson, Appl. Catal., B, 2003, 46, 671.
38. J. Navio, G. Colon, M. Macias, C. Real and M. Litter, Appl. Catal., A, 1999, 177, 111.
39. J. F. Zhu, F. Chen, J. L. Zhang, H. J. H. J. Chen and M. Anpo, J. Photochem. Photobiol., A, 2006, 180, 196.
40. C. Adan, A. Bahamonde, M. Fernandez-Garcia and A. Martinez-Arias, Appl. Catal., B, 2007, 72, 11.
41. R. Khana, S. W. Kima, T. J. Kima and C. M. Namb, Mater. Chem. Phys., 2008, 112, 167.
42. Y. Cong, J. L. Zhang, F. Chen, M. Anpo and D. He, J. Phys. Chem. C, 2007, 111, 10618.
43. J. Lin, Y. Lin, P. Liu, M. J. Meziani, L. F. Allard and Y. P. Sun, J. Am. Chem. Soc., 2002, 124, 11514.
44. Y. Cong, J. L. Zhang, F. Chen and M. Anpo, J. Phys. Chem. C, 2007, 111, 6976.
45. L. Zhang, Y. F. Zhu, Y. He, W. Li and H. B. Sun, Appl. Catal., B, 2003, 40, 287.
46. J. G. Yu, H. Yu, C. H. Ao, S. C. Lee, J. C. Yu and W. Ho, Thin Solid Films, 2006, 496, 273.
47. F. Iacomi, D. Mardare, M. N. Grecu, D. Macovei and I. Vida-Simiti, Surf. Sci., 2007, 601, 2692.
48. M. Descostes, F. Mercier, N. Thromat, C. Beaucaire and M. Gautier-Soyer, Appl. Surf. Sci., 2000, 165, 288.
49. T. Morikawa, T. Ohwaki, K. Suzuki, S. Moribe and S. Tero-Kubota, Appl. Catal., B, 2008, 83, 56.
50. Q. C. Ling, J. Z. Sun, Q. Y. Zhou, Q. Zhao and H. Ren, J. Photochem. Photobiol., A, 2008, 200, 141.
51. G. Mavel, J. Escard, P. Costa and J. Castaing, Surf. Sci., 1973, 35, 109.
52. D. J. Joyner, O. Johnson and D. M. Hercules, J. Am. Chem. Soc., 1980, 102, 1910.
53. Y. Huang, W. K. Ho, Z. H. Ai, X. Song, L. Z. Zhang and S. C. Lee, Appl. Catal., B, 2009, 89, 398.
54. D. Chen, D. Yang, Q. Wang and Z. Jiang, Ind. Eng. Chem. Res., 2006, 45, 4110.
55. J. Zhang, Y. Hu, M. Matsuoka, H. Yamashita, M. Minagawa, H. Hidaka and M. Anpo, J. Phys. Chem. B, 2001, 105, 8395.
56. H. B. Jiang and L. Gao, Mater. Chem. Phys., 2003, 77, 878.
57. L. Q. Wang, D. R. Baer and M. H. Engelhard, Surf. Sci., 1994, 320, 295.
58. Y. Adachi, S. Kohiki, K. Wagatsuma and M. Oku, J. Appl. Phys., 1998, 84, 2123.
59. T. Ohno, N. Murakami, T. Chiyoya and T. Tsubota, Appl. Catal., A, 2008, 348, 148.

---