Enhanced visible light activity and mechanism of TiO2 codoped with molybdenum and nitrogen

Materials Science and Engineering B 178 (2013) 425–430

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Enhanced visible light activity and mechanism of TiO2 codoped with molybdenum and nitrogen Xiuwen Cheng a,b,d , Xiujuan Yu a,b,∗ , Biying Li c , Lei Yan c , Zipeng Xing a,b , Junjing Li d a

Department of Environmental Science and Engineering, Heilongjiang University, Xuefu Road 74, Nangang District, Harbin 150080, PR China Key Laboratory of Chemical Engineering Process & Technology for High-Efficiency Conversion, College of Heilongjiang Province, Harbin 150080, PR China State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), Department of Environmental Science and Engineering, Harbin Institute of Technology, Huanghe Road 73, Nangang District, Harbin 150090, PR China d College of Resource and Environment, Northeast Agricultural University, Wood Sreet 59, Xiangfang District, Harbin 150030, PR China b c

a r t i c l e

i n f o

Article history: Received 12 July 2012 Received in revised form 26 November 2012 Accepted 6 January 2013 Available online 20 January 2013 Keywords: TiO2 Mo-N-co-doped Visible light Photocatalysis Phenol

a b s t r a c t Mo-N-codoped TiO2 was synthesized by using ammonium molybdate tetrahydrate and ammonia water as the sources of Mo and N, respectively. The resulting materials were characterized by X-ray diffraction (XRD), X-photoelectron spectroscopy (XPS) and UV–vis light diffuse reflection spectroscopy (DRS). Furthermore, the activity enhanced-mechanism was proposed. XRD results indicated that codoping favored the formation of anatase and improved the anatase crystallinity. XPS analysis revealed that N was incorporated into the lattice of TiO2 through substituting lattice O and coexisted in the substitutional forms. Meanwhile, Mo was incorporated into the lattice of TiO2 through substituting Ti and coexisted in the forms of Mo6+ and Mo5+ . DRS showed that the light absorption in visible region was improved by codoping, leading to a narrower band gap and higher visible activity for the degradation of phenol than that of others. The enhanced activity was attributed to the high anatase crystallinity, large amount of surface oxygen vacancies, intense light absorption and narrow band gap. © 2013 Elsevier B.V. All rights reserved.

1. Introduction During the past decades, titanium dioxide (TiO2 ) has been extensively studied as a photocatalyst for the degradation of pollutants in waste water treatment due to its high oxidizing capacity, inexpensiveness, non-toxicity and photostability [1–3]. However, TiO2 (anatase TiO2 ) with a wide band gap (∼3.2 eV) can only be excited by UV light, which accounts for only a small fraction (∼5%) of the solar spectrum at the earth’s surface [4]. To effectively make full use of visible light, many methods, such as doping, sensitization, and coupling, were developed [5–11]. Among which, the simplest and most feasible approach seemed to be N-doping, that was, doping with N element into the lattice of TiO2 was usually regarded as the most effective and feasible approach to narrow the band gap of TiO2 , thereby resulting in the improvement of visible light photocatalytic activity. In addition, other studies demonstrated that TiO2 exhibited a very low quantum yield due to its high recombination efficiency of photogenerated electron and hole pairs (e− /h+ ) [12,13]. Anpo et al.

∗ Corresponding author at: Department of Environmental Science and Engineering, Heilongjiang University, Harbin, Heilongjiang Province, PR China. Tel.: +86 451 86608549; fax: +86 451 86413259. E-mail address: [email protected] (X. Yu). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.01.009

[14] reported that the photogenerated electrons and holes can collide in the bulk of TiO2 catalyst, resulting in such a result that only a few electrons can migrate to the surface of TiO2 and be captured by the surface adsorbed O2 . As demonstrated, the surface adsorbed oxygen species played an important role in improving the photocatalytic activity of TiO2 photocatalyst. Recently, it was demonstrated [15] that doping with a transition metal could introduce a new energy level below the conduction band of TiO2 , thereby improving the separation efficiency of the photogenerated electron and hole (e− /h+ ) pairs. Furthermore, the doping energy level of foreign ions could also improve the photoresponse of TiO2 , which was beneficial to the enhancement of visible light photocatalytic activity. Very recently, it was reported that the visible light response and photocatalytic activity for the degradation of pollutants can be further improved by co-doping [16–18]. In this study, we presented a TiO2 -based photocatalyst codoped with Mo and N, which can be synthesized through a simple sol–gel method. N was incorporated into the lattice of TiO2 through substituting oxygen atoms, and coexisted in the substitutional forms of N-Ti and O-Ti-N in codoped TiO2 ; while Mo was incorporated into the lattice of TiO2 and coexisted in the form of Mo6+ and Mo5+ . As expected, the Mo-N-TiO2 sample exhibited higher visible light response and photocatalytic activity than that of mono element doped TiO2 (such as Mo-TiO2 and N-TiO2 ) and undoped TiO2 . Furthermore, the activity enhanced mechanism was also discussed in detail.

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2. Experimental

A(101)

All chemicals used in this study were analytical grade and used without further purification. Deionized (DI) water was used in all our experiments.

The preparation of TiO2 samples in this work was similar to that described in previous paper [19], employing a sol–gel method with Ti(OBu)4 as raw material. Firstly, 10 mL of Ti(OBu)4 was mixed with 40 mL of absolute ethanol in a dry atmosphere. Then, the mixed Ti(OBu)4 -C2 H5 OH solution was added dropwise into another mixture which consisting of 10 mL of absolute ethanol and 12 mL dilute nitric acid (1:5, volume ratio between nitric acid and deionized water) at room temperature under roughly stirring to carry out hydrolysis. Subsequently, the yellowish transparent sol was yielded after continuously stirring for 120 min. After the obtained sol was aged for 6 h at room temperature and dried for 48 h at 80◦ C, the TiO2 xerogel was obtained. Finally, pure TiO2 nano-particle was gained by calcining the TiO2 xerogel at 400◦ C for 4 h. Mo and N co-doped TiO2 nano-particles (Mo-N-TiO2 ) could be synthesized using the same process. The desired amount of ammonium molybdate tetrahydrate dissolved into appropriate amount of ammonia water was added dropwise to the mixed water/ethanol solution prior to the hydrolysis of Ti(OBu)4 . The remaining procedures were the same as described above. For comparison, mono element doped TiO2 (such as Mo-doped TiO2 and N doped TiO2 ) nano-particles were also synthesized from the same procedure by the addition of ammonium molybdate tetrahydrate solution and ammonia water instead of deionized water, respectively.

Relative intensity/a.u.

2.1. Sample preparation

d c R(110)

b

B(121)

a e f g

20

30

40

50

60

70

2 theta/degree Fig. 1. XRD patterns of TiO2 (a), Mo-TiO2 (b), N-TiO2 (c), Mo-N-TiO2 (d) JCPDS-211272 (e), JCPDS-21-1276 (f) and JCPDS-29-1360 (g).

equilibrium. At given time intervals (120 min), the analytical samples were taken and centrifuged to remove the remaining particles. The filtrates were analyzed using the colorimetric method of 4aminoantipyrineby with an UV–vis spectrophotometer (UV-2550) at the wavelength of 510 nm after centrifugation and filtration. 3. Results and discussion 3.1. Measurements of XRD and Raman

2.2. Characterization The crystalline structures of the as-synthesized samples were analyzed using an X-ray diffraction (XRD) with a D8 advance diffractometer (Bruker, Germany) equipped with Cu K␣ radiation. The accelerating voltage and applied current were held at 40 kV and 40 mA, respectively. Raman spectra were recorded with a Jobin Yvon HR 800 Raman spectrometer, and the used excitation wavelength was 457.9 nm with an Ar ion laser beam. To investigate the surface composition and chemical states of the samples, X-ray photoelectron spectroscopy (XPS) was conducted with a PHI-5700 ESCA system with Al K␣ X-ray source. All the binding energies were calibrated with respect to the C 1s peak at 284.6 eV of the surface adventitious carbon, and relative quantitative analysis was carried out using the sensitivity factors supplied by the instrument itself. The UV–vis diffuse reflection spectrum (UV–vis DRS) of the samples were recorded with a Model Shimadzu UV-2550 spectrophotometer equipped with an integrating sphere and using BaSO4 as reference. 2.3. Evaluation of photocatalytic activity Phenol is a kind of widely used chemicals, which is harmful to our health and environment. Thus, it is often chosen as a model pollutant to evaluate the photocatalytic activity of the as-synthesized samples. In this study, the visible light photocatalytic experiments were carried out in a self-made photochemical reactor and the light was provided from a side of the reactor by a 350 W arc Xenon lamp with a 420 nm cut-off filter to ensure the desired irradiation, which was placed at about 15 cm from the reactor. Aqueous suspensions of phenol (20 mL, 50 mg L−1 ) were placed in a vessel, and 20 mg of as-synthesized TiO2 sample was added. Prior to irradiation, the suspensions were magnetically stirred in the dark for about 30 min to ensure the establishment of adsorption/desorption

Fig. 1 showed the XRD patterns of the as-synthesized TiO2 samples. Clearly, undoped and Mo doped TiO2 contained anatase (JCPDS file NO. 21-1272), rutile (JCPDS file NO. 21-1276) and brookite (JCPDS file NO. 29-1360) with anatase phase in the majority according to their peak intensities, while N doped and Mo-N-codoped TiO2 samples did not exhibit any additional phase except for the anatase. Generally speaking, brookite is a transitional phase from anatase to rutile in the heating process. Thus, it can be induced that the addition of ammonia water favored the formation of anatase and inhibited the formation of rutile. Moreover, N-doping exhibited superior inhibiting effect in the phase transformation than that of Mo-doping. Further, it was worth noting that the crystallinity was obviously improved for the N dope and Mo-Nco-doped TiO2 samples, which may arose from the doping effect of the nitrogen species. In addition, compared to undoped TiO2 , no peaks related to Mo and N species were detected, which may be caused by the following two aspects. One was that the concentration of dopants was so low that it cannot be detected by XRD. The other was that the radius of titanium ion (0.605 nm) was similar with that of Mo ions (0.59 nm) and Mo ions may be incorporated into the lattice of TiO2 and occupied some of the tita˚ nium lattice sites. Additionally, the Ti O bond length (1.998 A) ˚ [20]. Furthermore, full was very close to Ti N ones (2.079 A) profile structure refinement of XRD data using the Rietveld program MULTI-PATTERN showed that the lattice parameters of pure TiO2 structure were a = b = 3.7830 and c = 9.4309, while Mo-TiO2 , N-TiO2 and Mo-N-codoped TiO2 structures were a = b = 3.7963 and c = 9.4493, a = b = 3.8073 and c = 9.4597, and a = b = 3.8344 and c = 9.4730, respectively. Thus, it can be deduced that Mo and N elements were weaved into the crystal structure, and substituted titanium and oxygen atoms, respectively. Moreover, by applying the Scherrer formula [21] on the anatase (1 0 1) diffraction peaks, the average crystallite size of the undoped TiO2 , Mo-TiO2 ,

A

A

Raman shift/cm

A

A

c b

30000

a

200

300

400

500

600

700

b

TiLMM

OKLL

Ti2s

N1s

Mo3d

a d

0 100

C1s

a d

Ti3p Ti3s

60000

c b

Itensity/a.u.

Intensity/a.u.

90000

Ti2p

A Intensity/a.u.

120000

427

O1s

X. Cheng et al. / Materials Science and Engineering B 178 (2013) 425–430

800

-1

200

Raman shift/cm

3.2. Measurement of XPS XPS analysis was performed to investigate the chemical composition and state of the as-synthesized Mo-N-TiO2 . Fig. 3 showed a comparison of the XPS survey spectra of undoped TiO2 and MoN-TiO2 samples, respectively. For the undoped TiO2 sample, it only contained C, O and Ti elements with sharp photoelectron peaks appearing at binding energies of 458 (Ti 2p), 529.25 (O 1s) and 284.62 eV (C 1s). The atomic composition of C, O, and Ti elements were 14.71, 58.39, and 26.90 at %, respectively. The C 1s peak (Binding energy = 284.62 eV) was attributed to the adventitious carbon from the XPS instrument itself. On the contrary, the Mo-N-TiO2 sample not only contained C, O and Ti elements, but also a small amount of Mo and N atom (binding energies at 230.05 and 399.7 eV,

800

1000

1200

Fig. 3. XPS survey spectra of pure TiO2 (a) and Mo-N-TiO2 (b) photocatalysts. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)

respectively), which probably originated from the dopants during the synthetic process. The atomic compositions of C, O, Ti, Mo and N elements were 14.56, 57.81, 26.04, 0.15 and 1.44 at%, respectively.

a

2500

Mo3d Mo3d5/2 (VI) Mo3d3/2 (V)

2400

Intensity/cps.

N-TiO2 and Mo-N-TiO2 was found to be 8.3, 8.5, 10 and 11.4 nm, respectively, suggesting that the addition of ammonia water had a promotion effect on the growth of crystallite sizes. The phenomenon can be explained by the fact that the addition of ammonia water increased the hydrolysis of titanium alkoxides, together with the change of the surface property of TiO2 , and making the surface of TiO2 OH− caped, so the adjacent OH− caped TiO2 grains will combine to form bigger particles by dehydration reaction. Thus, the crystallization was enhanced, resulted in the growth of the crystallite size. Raman spectra of the as-synthesized TiO2 samples were shown in Fig. 2. As seen from Fig. 2, the peaks at 147, 197, 396, 515 and 638 cm−1 was assigned to the characteristics of anatase phase [22], demonstrating that anatase was the predominant phase structure in each sample. Interestingly, no trace of rutile and brookite in TiO2 and Mo-TiO2 samples was detected, implying that the amount of rutile and brookite was too low to be checked out, which was in accordance with the XRD results. Furthermore, as seen from the insert pattern of Fig. 2, the Eg mode at 147 cm−1 shifted to the blue (toward the low wave-number region) lightly with the introduction of Mo and N species, which may be caused by the increasing of surface oxygen vacancy [23]. As demonstrated, oxygen vacancy is beneficial to the enhancement of photocatalytic efficiency. In addition, it should be noted that the blue-shift of the as-synthesized TiO2 samples can be ranged in the following order: Mo-N-TiO2 > N-TiO2 ≈ Mo-TiO2 > TiO2 . Therefore, it can be proposed that Mo-N-codoped TiO2 sample should most probably possess higher photocatalytic activity for the degradation of phenol than that of others.

600

Binding energy/eV

Mo3d5/2 (V)

2300

Mo3d3/2 (VI)

2200

2100 226

228

230

232

234

236

238

240

242

Binding energy/eV 2700

b

399.6 eV

N1s

2600 2500

Intensity/cps

Fig. 2. Raman spectra of TiO2 (a), Mo-TiO2 (b), N-TiO2 (c), and Mo-N-TiO2 (d) photocatalysts. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)

400

2400 2300

396.8 eV

2200 2100 2000 392

394

396

398

400

402

Binding energy/eV Fig. 4. High-resolution XPS spectra of Mo 3d (a) and N 1s (b) for Mo-N-TiO2 photocatalyst. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)

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X. Cheng et al. / Materials Science and Engineering B 178 (2013) 425–430 0.8

Photocatalytic degradation Adsorption degradation

0.6

Intensity/a.u.

(α hν)

d 0.4

0.4

a b

0.2

c 0.0

a

2.1

2.8

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Photon energy/eV

b

N-TiO2 60

Mo-TiO2 40

20

0.0 300

400

500

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Mo-N-TiO2

80

d

c

0.6

Phenol degradation rate/%

0.8

700

800

TiO2

0

Different TiO2 samples

Wavelength/nm Fig. 5. UV–vis diffuse reflectance spectra of TiO2 (a), Mo-TiO2 (b), N-TiO2 (c), and Mo-N-TiO2 (d) photocatalysts. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)

Fig. 4a showed the high-resolution XPS spectrum of the Mo 3d region, taken on the surface of the Mo-N-TiO2 sample. As seen from this figure, two peaks located at about 232.4 and 235.6 eV were attributed by Mo 3d5/2 and Mo 3d3/2 electronic state of Mo6+ [24], respectively. However, at the Mo 3d region, two additional peaks at 231.2 and 234.5 eV were detected, respectively, which can be attributed to the presence of Mo5+ in the lattice of TiO2 [25]. Thus, we can conclude that Mo ions were incorporated into the lattice of TiO2 through substituting titanium ions and coexisted in the forms of Mo6+ and Mo5+ in codoped TiO2 . Furthermore, we can propose that a part of Mo6+ ions was reduced to lower state, such as Mo5+ in this case. Fig. 4b showed the corresponding high-resolution XPS spectrum of the N 1s region taken for the Mo-N-TiO2 sample. Clearly, the N 1s peaks were broad and asymmetric, indicating that there were at least two kinds of chemical states according to the binding energy range from about 392 to 404 eV. After fitting, two peaks were obtained at 396.8 and 399.6 eV, respectively. The minor peak at about 396.8 eV was attributed to the substitutional N in the Ti-N (␤-N) structure [8], while the major peak at 399.6 eV was attributed to the presence of substitutional N such as N-Ti-O in the TiO2 lattice [26], which was the natural result of partial substitution of O-Ti-O by nitrogen element. Therefore, it can be concluded that two substitutional forms of N as Ti-N and O-Ti-N coexisted in Mo-N-TiO2 .

P25 TiO2

Fig. 6. Photodegradation rates of phenol solution on different TiO2 photocatalysts.

from the electron transition from N 2p level to the conduction band of TiO2 [27]. However, compared to the undoped TiO2 , the Mo-NTiO2 sample showed two predominant features: (a) the appearance of a new absorption in the region of 390–500 nm, (b) the greatly improved absorbance in the range of 500–700 nm. Such a strong light absorbance in the visible light region was consistent with the yellowish characteristic of the Mo-N-TiO2 sample. The absorption in the visible light region of the as-synthesized samples arranged the order as follows: d > c > b > a, which was attributed to the contribution of Mo and N species. The enhancement of absorption in the visible region could increase the number of photogenerated electrons and holes to participate in the photocatalytic reaction, which could enhance the photocatalytic efficiency of TiO2 . Based on the above analysis, we can conclude that by the addition of ammonia water, the light absorption in visible region was sharply improved. Furthermore, the Mo-N-TiO2 samples were more sensitive to the visible light than that of mono-doped and undoped TiO2 . Besides, the Kubelka-Munk functions [28] could be applied to calculate the band gap energies of the as-synthesized TiO2 samples by plotting [F(R)·E]1/2 versus energy of light, and the results were shown in the inset in Fig. 5. After calculating, the band gaps were approximately 3.09, 2.84, 2.44 and 2.31 eV for the sample TiO2 , Mo-TiO2 , N-TiO2 and Mo-N-TiO2 , respectively, revealing that the band gap of TiO2 was narrowed by Mo and/or N doping. Furthermore, we can conclude that the Mo-N-TiO2 should excite more light to participate in the photoreactions and possess higher visible photocatalytic activity than that of others.

3.3. Measurement of DRS 3.4. Photocatalytic activity The optical properties of the as-synthesized TiO2 samples have been measured by UV–vis diffuse reflectance spectra. As displayed in Fig. 5, all of the samples showed a typical absorption in the UV region, which was assigned to the intrinsic band gap absorption of TiO2 corresponding to the electron transitions from the valence band to conduction band. As shown by curves b and c in Fig. 5, in comparison to the undoped TiO2 , the absorption edge was significantly red-shifted to the visible region for both Mo-TiO2 and N-TiO2 samples. Compared with the undoped TiO2 , Mo-TiO2 presented an optical absorption edge at about 440 nm in the visible light region, which may be caused by the doping energy level of metal ions below the conduction band, suggesting that the visible light absorption of Mo-TiO2 arose from the electronic transition from valance band to the doping energy level. As shown in Fig. 5, N-TiO2 showed a distinct absorption in the region of 390–500 nm, which was a typical absorption feature of N doped TiO2 and arose

In order to explore the photocatalytic activity of the as-synthesized TiO2 samples under visible light irradiation, degradation of phenol was investigated and shown in Fig. 6. Comparing with the photocatalytic degradation, so small amount of direct photolysis of phenol (1.9%) could be neglected. As shown in Fig. 6, the photocatalytic degradation rate of phenol solution on TiO2 sample can be greatly improved by Mo and/or N doping. Notably, undoped TiO2 showed very low photocatalytic efficiency under visible irradiation, while mono Mo and N doped TiO2 exhibited a relatively high visible light activity, Mo-N-TiO2 displayed even higher visible capability for the photodegradation of phenol than that of TiO2 , Mo-TiO2 and N-TiO2 , for which 91.8% of phenol could be degraded after 2 h of visible light irradiation. The results were consistent with the conclusion achieved from Fig. 2, and also verified the speculations of Fig. 5. The higher visible light photocatalytic

X. Cheng et al. / Materials Science and Engineering B 178 (2013) 425–430

hν

e- CB

produce active • OH radicals (Eq. (5)). Meanwhile, the photogenerated holes (hVB + ) can be captured by the surface adsorbed H2 O to form active species • OH radicals (Eq. (6)), which were known to be the most oxidizing species. Eventually, phenol were mineralized into small molecules, such as CO2 and H2 O (Eq. (7)).

O2

e-

e-

e-

Mixed level

O2(surf)-

Mo6+ + eCB − → Mo5+

3.2 eV

B

A

Mo

C ·OH

h+ N2p h+

h+

VB

h

OH(surf)-

+

H2O Fig. 7. Scheme of photocatalytic mechanism of Mo-N-TiO2 photocatalyst. A: electrons migrated from valence band to conduction band, B: electrons migrated from valence band to Mo level, C: electrons migrated from N 2p level to conduction band.

activity of Mo-N-TiO2 catalyst might be attributed to the high anatase crystallinity, large amount of surface oxygen vacancy, intense light absorption in the visible region and narrow band gap energy. As referenced [29,30], the energy level of conduction band of TiO2 (−0.29 eV vs. NHE) was higher than the O2 /• O2 − redox potential (about 0 eV vs. NHE), thus the electron in conduction band of TiO2 can be captured by O2 . In this study, the band gap of Mo doped TiO2 was 2.84 eV, while pure TiO2 was 3.09 eV, indicating that the doping energy level of Mo was 0.25 eV lower than the conduction band of TiO2 , and 0.04 eV higher than the O2 /• O2 − redox potential. Taking into account the former results, the possible reason that Mo-N-TiO2 exhibited a high photocatalytic activity for the degradation of phenol under visible light irradiation can be explained using the scheme shown in Fig. 7. In the schematic diagram, undoped TiO2 exhibited a relatively low visible light ( > 420 nm) photoactivity for the degradation of pollutants due to its wide band gap energy (3.09 eV, process A). After codoping with Mo and N, electrons can be excited simultaneously from the valance band of TiO2 to Mo-doping energy level (process B) and from the N 2p energy level to the conduction band (process C). Meanwhile, it was possible for the photogenerated electrons at the conduction band to fall onto the doping energy level of Mo species. Furthermore, the photogenerated electrons can easily transfer from the conduction band and the Mo doping energy level to the surface of the materials and then captured by the adsorbed O2 , thereby enhancing the separation efficiency of photogenerated charge carriers. For mono Mo and N doped TiO2 , only process B and C occurred under visible light irradiation. However, codoping with Mo and N elements could increase the quantity of photogenerated charge carriers, compared to undoped and mono-doped TiO2 catalysts. As a result, more departed photogenerated electrons and holes (h+ /e− ) could participate in the photodegradation process, resulting in higher photocatalytic efficiency than that of undoped and mono-doped TiO2 catalysts. Further, the process of visible light photocatalytic oxidation of phenol was speculated as followed. Firstly, electrons and holes were generated under visible light irradiation (Eq. (1)). Mo-N-TiO2 + h → hVB + + eCB −

(1) −)

429

Then, the photogenerated elctrons (eCB can reduce the Mo6+ species quickly (Eq. (2)). Subsequently, the Mo5+ can be directly captured by the adsorbed O2 molecules on the surface of TiO2 (Eq. (3)) and subsequently to form hydrogen peroxide (H2 O2 ) (Eq. (4)), which can further react with the photogenerated electrons to

5+

•O 2

−

+ O2(ads) → Mo −

6+

(2) + • O2 −

+

(3)

+ eCB + 2H → H2 O2

(4)

H2 O2 + eCB − → • OH + OH−

(5)

hVB + + OH− → • OH

(6)

• OH

(7)

+ C6 H5 OH → ··· → CO2 + H2 O

As we known, photogenerated electrons and holes were crucial to the formation of active species hydroxyl groups. Generally speaking, large amount of active hydroxyl groups corresponds to high photocatalytic activity. Therefore, it can be concluded that the Mo-N-TiO2 should produce a greater number of hydroxyl group yield and a higher photocatalytic activity. 4. Conclusions Based on the above analysis, the following conclusions can be drawn: (1) Mo-N-TiO2 nano-particle with high visible light photocatalytic activity was successfully synthesized through simple sol–gel reactions in the presence of ammonium molybdate tetrahydrate and ammonia water. (2) Codoping with nitrogen and Mo elements favored the formation of anatase and inhibited the formation of rutile so as to enhance the crystallinity of anatase TiO2 . (3) The surface oxygen vacancy and light absorption in visible region were greatly improved by codoping with Mo and N. (4) The enhancement of visible light photocatalytic activity was ascribed to the high anatase crystallinity, large amount of surface oxygen vacancy, intense light absorption in visible region and narrow band gap energy. Acknowledgments This work was supported by National Natural Science Foundation of China for Youth (21106035) and Youth Scholar Backbone Supporting Plan Project for general colleges and universities of Heilongjiang province (1151G034). References [1] M.S. Hoffman, S. Martin, W. Chio, Chemical Reviews 95 (1995) 69–96. [2] I.K. Konstantinou, T.A. Albanis, Applied Catalysis B: Environmental 49 (2004) 1–14. [3] M. Fujihira, Y. Satoh, T. Osa, Nature 293 (1981) 206–208. [4] M.I. Litter, Applied Catalysis B: Environmental 23 (1999) 89–114. [5] E.Y. Bae, W.Y. Choi, Environmental Science and Technology 37 (2003) 147–152. [6] W. Choi, A. Termin, M.R. Hoffman, Journal of Physical Chemistry 98 (1994) 13669–13679. [7] G.R. Bamwenda, S. Tsubota, T. Nakamura, M. Haruta, Journal of Photochemistry and Photobiology A: Chemistry 89 (1995) 177–189. [8] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [9] S. Kohtani, A. Kudo, T. Sakata, Chemical Physics Letters 206 (1993) 166–170. [10] T. Ohno, T. Mitsui, M. Matsumura, Chemistry Letters 32 (2003) 364–365. [11] O. Diwald, T.L. Thompson, T. Zubkov, E.G. Goralski, S.D. Walck, J.T. Yates, Journal of Physical Chemistry B 108 (2004) 6004–6008. [12] H.B. Yu, S. Chen, X. Quan, H.M. Zhao, Y.B. Zhang, Environmental Science and Technology 42 (2008) 3791–3796. [13] K. Naeem, F. Ouyang, E-Journal of Chemistry 6 (2009) S422–S428. [14] M. Anpo, K. Chiba, M. Tomonari, S. Coluccia, M. Che, M.A. Fox, Bulletin of the Chemical Society of Japan 64 (1991) 543–551. [15] E. Arpac, F. Sayılkan, M. Asiltürk, P. Tatar, N. Kiraz, H. Sayılkan, Journal of Hazardous Materials 140 (2007) 69–74. [16] Y. Cong, J.L. Zhang, F. Chen, M. Anpo, D.N. He, Journal of Physical Chemistry C 111 (2007) 10618–10623.

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