AgNbO3 composite with enhanced visible-light photocatalytic activity

Applied Surface Science 273 (2013) 159–166

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Synthesis and characterization of AgBr/AgNbO3 composite with enhanced visible-light photocatalytic activity Cheng Wang a , Jia Yan b , Xiangyang Wu a,∗ , Yanhua Song b , Guobin Cai b , Hui Xu a,∗ , Jiaxiang Zhu a , Huaming Li b a b

School of the Environment, Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, PR China School of Chemistry and Chemical Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, PR China

a r t i c l e

i n f o

Article history: Received 20 November 2012 Received in revised form 30 January 2013 Accepted 4 February 2013 Available online 11 February 2013 Keywords: AgBr/AgNbO3 Photocatalytic Visible-light Methylene blue

a b s t r a c t A novel AgBr/AgNbO3 composite was synthesized by a two-step method. The physical and chemical properties of catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), diffuse-reflection spectra (DRS), and photocurrent techniques. The photocatalytic performance of the samples was evaluated by photocatalytic oxidation of methylene blue (MB) dye under visible-light irradiation. The XRD, SEM-EDS, and XPS analyses indicated that the heterojunction structure had formed in the composite. The DRS analysis showed that AgBr formed on the surface of AgNbO3 had promoted the optical absorption in the visible region and made it possible to enhance the photocatalytic activity. The results indicated that the AgBr/AgNbO3 heterojunction had exhibited a much higher photocatalytic activity than the pure AgNbO3 . The mechanism of the AgBr/AgNbO3 composite with enhanced heterojunction photocatalytic activity was researched. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Photocatalytic technology has become an important method to control environmental pollution [1,2]. Many researchers concentrated on TiO2 because of its stability, non-toxicity and low-cost [3–5]. However, the low utilization of visible-light limited its practical applications. Nowadays, two major kinds of research have attracted the public’s attention: on the one hand, TiO2 -based photocatalysts are modified in order to improve their photocatalytic activity; on the other hand, new photocatalytic materials are being developed to achieve higher photocatalytic activity [6]. In many literatures, it was well known that semiconductor photocatalysis had been highly expected to be a perfect green technology. After several decades of persistent investigation, all kinds of novel photocatalysts which could work under visible-light irradiation had been discovered to replace the conventional TiO2 to obtain higher efficient photocatalytic activity. In recent studies, Ag-based photocatalysts have been attracted much attention and some researchers have proved that some Ag-based materials had photocatalytic activity under visible-light irradiation, such as Silver Carbonate [7], Silver Vanadates [8], Silver Orthophosphate [9], etc. AgNbO3 as an Ag-based material was considered to have good optical and electrical properties [10]. In the

∗ Corresponding authors. Tel.: +86 511 88791798; fax: +86 511 88791798. E-mail addresses: [email protected] (X. Wu), [email protected] (H. Xu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.02.004

ultraviolet or visible-light irradiation, AgNbO3 could decompose formaldehyde to produce H2 in aqueous solution [11]. However, its photocatalytic activity for decomposition of organic pollutants was not high. Therefore, it is necessary to enhance the photocatalytic activity of AgNbO3 . In other research, some investigators have showed that AgNbO3 –NaNbO3 and AgNbO3 –SrTiO3 which were based on AgNbO3 have been proved to perform a higher photocatalytic activity than the original AgNbO3 [12,13]. Thus, with necessary modification and compound on the original AgNbO3 , we will gain a novel photocatalyst, which has a higher photocatalytic activity and stability under light irradiation. In recent years, the silver halide (AgX)-based catalysts have been reported to show excellent catalytic activity in the degradation of organic dyes, disinfect and the reduction of carbon dioxide [14–21]. Silver bromide (AgBr) is an important narrow band gap semiconductor and it can be used as the photosensitive material with a direct band gap of 4.29 eV (289 nm) and an indirect band gap of 2.64 eV (470 nm) [22]. AgBr itself is unstable in the pure crystal form, but if it is composited with the semiconductor photocatalysts, the stability of the generated composites can be improved, such as AgBr/WO3 [23], AgX/Ag3 PO4 (X = Cl, Br, I) [24], AgBr/H2 WO4 [25]. In the above materials, it can be noticed that the heterojunction containing AgBr can significantly enhance the photocatalytic activity and also maintain the optical stability to a certain extent. The probable reason is that electron–hole pairs have been efficiently separated because of the formation of the heterojunction structure between AgBr and substrates.

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However, few works about the synthesis and the photocatalytic properties of the AgBr/AgNbO3 composite have been reported until now. In the present work, a novel AgBr/AgNbO3 composite was synthesized by a two-step method. The X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM) analyses had been used to characterize its structure. The photocatalytic activity of AgBr/AgNbO3 was evaluated with methylene blue (MB) as a model contaminant. The experimental results confirmed that AgBr/AgNbO3 exhibited much higher photocatalytic activity than that of the pure AgNbO3 . The stability, the kinetics and possible photocatalytic mechanism of the AgBr/AgNbO3 composite were also studied.

Intensity(a.u)

AgBr AgNbO3

AgBr/AgNbO (27.4wt%) 3 AgBr/AgNbO (23.2wt%) 3 AgBr/AgNbO (18.5wt%) 3 AgBr/AgNbO3(13.1wt%) AgBr/AgNbO (7.0wt%) 3 AgNbO 3

10

20

30

40

50

60

70

80

2 ()

2. Experimental

Fig. 1. XRD pattern of AgBr/AgNbO3 and AgNbO3 samples.

2.1. Preparation of the photocatalysts Synthesis of AgNbO3 : Nb2 O5 and AgNO3 were mixed by 1:2 as the molar ratio. Then, the mixture was fully grinded. It was put into the crucible, and was took into a muffle furnace. Then, it was heated at 880 ◦ C for 5 h. Finally, the resulted material was grinded completely. Synthesis of AgBr/AgNbO3 : Firstly, 0.2488 g of the obtained AgNbO3 and 0.0387 g 1-hexadecyl-3-methylimidazolium bromide ([C16 mim]Br) were dispersed in the solution with 8 mL of ethylene glycol, and the suspension was sonicated for 10 min. Then, 0.0170 g AgNO3 was added into 2 mL ammonia (25 at %). By adding dropwise to the above solution, the other solution was obtained. Then, the obtained mixture was stirred magnetically in a 90 ◦ C oil bath for 6 h. After the mixture was cooled to room temperature, the products were separated centrifugally and washed with deionized water and absolute ethanol for three times. Then the products were dried at 50 ◦ C for 6 h. Other substances containing different weight proportions of AgBr could also be obtained with the same method. 2.2. Characterization of photocatalysts The crystalline phases of AgBr/AgNbO3 composites were analyzed by XRD using a Bruker D8 diffractometer with Cu-K␣ ˚ in the range of 2 = 10◦ –80◦ . The morpholradiation ( = 1.5418 A) ogy and structure of the as-prepared samples were examined with SEM by a JEOL JSM-7001F field-emission microscope. The chemical composition of the samples was determined by EDS. TEM micrographs were taken with a JEOL-JEM-2010 (JEOL, Japan) operated at 200 kV. Ultraviolet visible (UV–vis) diffuse reflection spectra were measured using a UV–vis spectrophotometer (Shimadzu UV2450, Japan) in the range of 200–800 nm. BaSO4 was used as the reflectance standard material. XPS analysis was performed on an ESCALa b MKII X-ray photo-electron spectrometer using Mg-K␣ radiation. 2.3. Photocatalytic activity Photocatalytic activity of the sample was evaluated by the degradation of MB under 300 W Xe lamp with a 400 nm cutoff filter. 0.075 g photocatalysts were added into 75 ml MB (10 mg/L) in a Pyrex potocatalytic reactor. Prior to irradiation, the suspensions were magnetically stirred for 30 min in the dark to ensure that the MB could reach the absorption–desorption equilibrium on the photocatalyst surface. Furthermore, all the experiments were performed at 30 ◦ C under constant stirring. At certain time intervals,

3 mL solution was sampled and centrifuged to remove the photocatalyst particles. Then, the filtrates were analyzed by recording variations of the absorption band maximum (664 nm) in the UV–vis spectra of MB by using a Shimadzu UV-2450 spectrophotometer. The photocatalytic degradation efficiency (E) of MB was obtained by the following formula: E=



1−

C C0



× 100% =



1−

A A0



× 100%

where C is the concentration of the MB solution at reaction time t, C0 is the adsorption/desorption equilibrium concentration of MB (at reaction time 0); and A and A0 are the corresponding values. 3. Results and discussion 3.1. XRD analysis In order to confirm the crystalline structure of the AgBr/AgNbO3 samples, the XRD study was carried out. Fig. 1 shows the XRD patterns of the AgBr/AgNbO3 heterojunction with different AgBr contents. All the diffraction peaks of AgNbO3 could be exactly indexed as the perovskite structure (JCPDF 52-0405). The main diffraction peaks appeared at 32.21◦ , 37.96◦ , 39.79◦ , 45.98◦ , 57.43◦ , 67.37◦ , and 76.52◦ . Compared with the pure AgNbO3 crystals, small diffraction peaks of the AgBr crystals (JCPDS 06-0438) have been detected. As is shown in Fig. 1, it can be seen that AgBr formed in the composites. The above result revealed that the samples have contained AgNbO3 and AgBr. 3.2. SEM and TEM analyses Fig. 2 shows typical SEM and TEM images of the asprepared AgNbO3 and AgBr/AgNbO3 (18.5 wt %) particles. From Fig. 2(A), it can be found that the size of the pure AgNbO3 catalyst is larger than the AgBr/AgNbO3 composite in Fig. 2(B) and (C). These AgNbO3 particles with different sizes cluster together. In the AgBr/AgNbO3 samples, AgBr with the regular small spherical shape loads on the surface of the composites. EDS pattern shows that the AgBr/AgNbO3 contains the elements of Ag, Br, Nb, and O. Moreover, Fig. 2(E) shows the TEM images of a typical AgBr/AgNbO3 composite heterostructure. Fig. 2(E) clearly exhibits that small particles disperse over the edge of large AgNbO3 particles, and they constitute a kind of composite with a heterojunction structure. Therefore, the TEM analysis further confirmed the coexistence of AgBr and AgNbO3 in the samples. The

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Fig. 2. SEM images of (A) AgNbO3 , (B and C) AgBr/AgNbO3 , (D) EDS image of AgBr/AgNbO3 (18.5 wt %), (E) TEM image of AgBr/AgNbO3 composite (18.5 wt %).

combination of TEM, SEM and EDS analytical methods indicated that the AgBr particles were doped on the surface of the AgNbO3 and the heterostructure was formed in the AgBr/AgNbO3 composite.

confirmed that there were both AgNbO3 and AgBr species in the heterojunction structure.

3.3. XPS analysis

To investigate the optical properties, the as-prepared samples were analyzed by DRS, and the absorption spectra of AgNbO3 and AgBr/AgNbO3 is shown in Fig. 4(A). From Fig. 4(A), it can be found that the absorption range of the AgBr/AgNbO3 composites apparently increased in the visible-light and a red shift appeared after the addition of AgBr particles. These results were attributed to the interaction between AgBr and AgNbO3 in the samples. The red shift of the absorption wavelength indicated that the photocatalyst could absorb more photons. Therefore, the red shift in the absorption band could be favorable for photocatalytic reaction. The band gap energy of a semiconductor could be calculated by the following formula:

Fig. 3(A) shows the XPS spectra of the Ag, Nb, Br and O peak regions in the AgBr/AgNbO3 powder. Any contamination, besides carbon, could be found in XPS analysis. The Ag 3d3/2 and Ag 3d5/2 peaks were identified at 373.59 and 367.38 eV, respectively, suggesting the presence of Ag+ (Fig. 3(B)) [26]. In Fig. 3(C), the peak of Br 3d at 68.19 eV was due to the crystal lattice of Br− in AgBr [16]. According to the reports, in the niobate, binding energy of Nb5+ was between 206.5 and 207.2 eV and Nb4+ was at 205.5 eV [27–29]. In Fig. 3(D), binding energy of Nb 3d was at 207.12 eV, so Nb existed as Nb5+ in the samples. Moreover, the XPS peak of the O 1s was at 530.59 eV, as is shown in Fig. 3(E). Therefore, with the combination of the XRD, SEM-EDS, TEM and XPS investigation, the results

3.4. DRS analysis

2

(Ahv) = hv − Eg

162

C. Wang et al. / Applied Surface Science 273 (2013) 159–166

Fig. 3. XPS spectra of (A) AgBr/AgNbO3 , (B) Ag 3d, (C) Br 3d, (D) Nb 3d, and (E) O 1s.

where h, v, Eg and A were Planck constant, light frequency, band gap energy, and absorbance, respectively. Therefore, in Fig. 4(B), Eg of AgNbO3 was elicited to be 2.86 eV. Eg of AgBr/AgNbO3 (18.5 wt %) was found to be 2.50 eV according to the above formula. This result indicated that doped AgBr could narrow the band gap of catalysts, which might be beneficial to improving the photocatalytic activity of the composite.

3.5. Photocatalytic activity of the samples With MB dye as a contaminant, the photocatalytic activity of the pure AgNbO3 and AgBr/AgNbO3 materials were tested, and the result is shown in Fig. 5. With the absence of photocatalyst, MB self-degradation was almost negligible. It was found that

all AgBr/AgNbO3 heterojunction photocatalysts exhibited higher photocatalytic activity than the pure AgNbO3 catalyst. It could be clearly seen that the photocatalytic degradation efficiency of MB was 74.62% and 46.28% for AgBr/AgNbO3 (18.5 wt %) and pure AgNbO3 after irradiation for 3 h, respectively, which indicated that MB could be degraded more efficiently by AgBr/AgNbO3 than by pure AgNbO3 . It was interesting that the AgBr/AgNbO3 (18.5 wt %) heterojunction exhibited the highest photocatalytic degradation efficiency. The photocatalytic activity of the AgBr/AgNbO3 composite increased remarkably with the increasing AgBr content, but at the higher AgBr concentration (>18.5 wt %), the photocatalytic activity decreased. So it suggested that the optimal AgBr content in AgBr/AgNbO3 existed when the mass ratio was 18.5 wt %. In order to investigate the synergy effect between AgNbO3 and AgBr in the composite, a series of experiments were studied. As

C. Wang et al. / Applied Surface Science 273 (2013) 159–166 1.1

163

100

(A)

1.0

90

0.9

80

Absorbance(a.u)

0.8

Red shift

C/C 0 100%

0.7 0.6 0.5

AgBr/AgNbO (7.0wt%) 3

0.4

AgBr/AgNbO (13.1wt%) 3

0.3

AgBr/AgNbO (18.5wt%) 3

0.0139g AgBr 0.0611g AgNbO3 Mixed AgBr and AgNbO3

AgNbO 3

200

50

30

AgBr/AgNbO (27.4wt%) 3

0.1

60

40

AgBr/AgNbO (23.2wt%) 3

0.2

70

20

300

400

500

600

700

800

Wavelength(nm)

AgBr/AgNbO3(18.5wt%) 0.0

0.5

1.0

1.5

2.0

2.5

3.0

irradiation time(h)

10

(B) 8

AgNbO3 (Ah )

2

6

4

AgBr/AgNbO3(18.5wt%)

2

0 2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

h (eV) Fig. 4. (A) UV–vis diffuse reflectance spectra of the samples and (B) estimated band gap of AgNbO3 and AgBr/AgNbO3 catalysts.

is shown in Fig. 6, it can be seen that the photocatalytic activity of AgBr/AgNbO3 (18.5 wt %) composite (75 mg) was much higher than that of the mathematical sum of AgBr (13.9 mg) and AgNbO3 (61.1 mg), in which they contain the same weight of visible-lightactive components as in AgBr/AgNbO3 (18.5 wt %). The above was 100 90 80

C/C0 100%

70 60 50 AgBr/AgNbO (7.0wt%) 3

Fig. 6. Comparison of photocatalytic activity of different photocatalysts with the same weight of each visible-light-active component: 13.9 mg AgBr (black line), 61.1 mg AgNbO3 (red line), mathematical sum (blue line), and 75.00 mg AgBr/AgNbO3 (18.5 wt %) containing 13.9 mg AgBr and 61.1 mg AgNbO3 (green line) on the degradation of MB under visible-light irradiation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

due to the decrease in recombination rate of electron–hole pairs in the photocatalytic reaction. Fig. 7 is the absorption spectrum chart showing that the MB has been degraded in the photocatalyst. It can be seen in Fig. 7(B), with the increase of the illumination time and the presence of AgNbO3 , the degradation rate of MB was relatively low and the maximum absorption peak of MB wavelength had a slightly offset. However, after AgBr doping, with reaction time increased, the absorbance value of MB significantly decreased, and the wavelength of maximum absorption of MB had a significant offset. The absorption peak gradually shifted from 664 nm to approximative 600 nm, as is shown in Fig. 7(A). According to the reports, the blue shift of absorption wavelength might be demethylation into Azure B (AB), azure A (AA), azure C (AC), and there are similar reports in other literatures [30–32]. 3.6. Kinetics The photocatalytic degradation kinetics of MB was investigated, and the results are shown in Fig. 8. It was found that the photodegradation process followed a pseudo-first-order reaction. From the data of Fig. 8, the pseudo-first-order rate constant (k) for MB degradation with the pure AgNbO3 under visible-light irradiation was 0.207 h−1 with the R2 of 0.998. For AgBr/AgNbO3 photocatalyst with AgBr contents 7.0 wt %, 13.1 wt %, 18.5 wt %, 23.2 wt % and 27.4 wt %, the corresponding degradation rate constant k were estimated to be 0.326 h−1 , 0.328 h−1 , 0.500 h−1 , 0.420 h−1 and 0.424 h−1 , respectively. The rate constant k of the AgBr/AgNbO3 (18.5 wt %) photocatalyst was up to 2.5-fold faster than that of the bare AgNbO3 . The pseudo-first-order constants and relative coefficients are summarized in Table 1.

AgBr/AgNbO (13.1wt%) 3

40

3.7. Cycle experiment

AgBr/AgNbO (18.5wt%) 3

30

AgBr/AgNbO (23.2wt%) 3 AgBr/AgNbO (27.4wt%) 3

20

AgNbO 3

0.0

0.5

1.0

1.5

2.0

2.5

irradiation time(h) Fig. 5. The photocatalytic activity of the samples.

3.0

In addition to photocatalytic efficiency, the stability of photocatalysts was also very important for practical application. To evaluate the stability of the AgBr/AgNbO3 hybrid, we conducted the repeatability experiments of MB degradation over AgBr/AgNbO3 (18.5 wt %). From the result, although the activity of the catalyst had decreased after three cycles, the photocatalytic degradation

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C. Wang et al. / Applied Surface Science 273 (2013) 159–166 1.6

(A) 1.4

AgBr/AgNbO3(7.0wt%)

1.4

irradiation time/h

AgBr/AgNbO3(13.1wt%) 1.2

0h 0.5h 1h 1.5h 2h 2.5h 3h

1.0 0.8 0.6

AgBr/AgNbO3(18.5wt%) AgBr/AgNbO3(23.2wt%)

1.0

-Ln(C/C0)

Absorbance(a.u)

1.2

AgBr/AgNbO3(27.4wt%) AgNbO3

0.8

0.6

0.4

0.4 0.2

0.2 0.0

0.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

irradiation time(h) 200

400

600

800

Fig. 8. Kinetic fit for the degradation of MB with the pure AgNbO3 and AgBr/AgNbO3 samples.

wavelength/nm 1.6 1.4

(B)

irradiation time/h

-7

8.0x10

1.0 0.8 0.6

0h 0.5h 1h 1.5h 2h 2.5h 3h

-7

7.0x10

-7

6.0x10

Photocurrent(A)

Absorbance(a.u)

1.2

0.4 0.2 0.0

-7

5.0x10

-7

4.0x10

-7

3.0x10

AgBr/AgNbO3(18.5wt%)

-7

2.0x10

-7

-0.2 200

1.0x10 400

600

800

0.0

wavelength/nm

AgNbO3

-7

Fig. 7. Absorption spectral changes of MB under visible light irradiation: (A) AgBr/AgNbO3 (18.5 wt %) as photocatalyst and (B) AgNbO3 as photocatalyst.

efficiency maintained at about 60%. This may be due to the loss of the catalyst during the cycle experiments. It could be found that AgBr/AgNbO3 had certain stability from the results of the cycle experiments. It was interesting because Ag-based photocatalysts were usually unstable, such as Ag3 PO4 [33], Ag2 CO3 [34]. However, when Ag-based photocatalyst was modified by Ag, graphene, or other materials, the photogenerated electrons formed in the conduction band (CB) of the photocatalysts could be captured quickly, then the photocorrosion of the semicondutors would be suppressed. It had already been proved by some literatures, such as Ag/Ag3 PO4 [35], GO/Ag3 PO4 [36]. In the AgBr/AgNbO3 system, photons from the visible-light irradiation could be absorbed by AgNbO3 and AgBr, and the photogenerated electrons and holes were produced. Due to the suitable band structure of the AgNbO3 and AgBr, the electrons formed in the CB of AgNbO3 could be transferred to the CB of AgBr, and the transferred electrons could

-1.0x10

0

100

200

300

400

500

600

Time(s) Fig. 9. The photocurrent of AgNbO3 and AgBr/AgNbO3 samples.

participate in the photocatalytic reaction quickly so that the electrons and holes could separate efficiently. Therefore, AgNbO3 and AgBr could keep the original structure and the AgBr/AgNbO3 composites had good stability. 3.8. Photocurrent It was well known that semiconductors exhibited unique photoelectric properties owing to their intrinsic band-gap structure, which provided a simple and economical light-to-electric conversion approach for various energy-related applications. Therefore, the photoelectric conversion properties over the pure AgNbO3 and AgBr/AgNbO3 (18.5 wt %) had been investigated in detail. As is shown in Fig. 9, AgBr/AgNbO3 (18.5 wt %) exhibited a

Table 1 Pseudo-first-order rate constant for MB photocatalytic oxidation under different photocatalysts. Series

Photocatalyst

1 2 3 4 5 6

AgBr/AgNbO3 AgBr/AgNbO3 AgBr/AgNbO3 AgBr/AgNbO3 AgBr/AgNbO3 AgNbO3

(7.0 wt %) (13.1 wt %) (18.5 wt %) (23.2 wt %) (27.4 wt %)

The first order kinetic equation

k (h−1 )

R2

−ln(C/C0 ) = 0.326 t −ln(C/C0 ) = 0.328 t −ln(C/C0 ) = 0.500 t −ln(C/C0 ) = 0.420 t −ln(C/C0 ) = 0.424 t −ln(C/C0 ) = 0.207 t

0.326 0.328 0.500 0.420 0.424 0.207

0.997 0.991 0.986 0.979 0.989 0.998

C. Wang et al. / Applied Surface Science 273 (2013) 159–166

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Table 2 Absolute electronegativity, estimated band gap, conduction band edge, and valence band for AgBr and AgNbO3 . Semiconductor

Absolute electronegativity (X)

Estimated energy band gap, Eg (eV)

Conduction band edge (eV)

Valence band edge (eV)

AgBr AgNbO3

5.8 5.12

2.60 2.86

0 −0.81

2.60 2.05

Fig. 10. Photocatalytic mechanism diagram of the AgBr/AgNbO3 samples.

higher photocurrent than AgNbO3 , while the pure AgNbO3 crystals almost did not perform the photo-electric behavior, which was consistent with the result of their photocatalytic activity for MB dye degradation. Therefore, when AgBr and AgNbO3 were compounded and formed the heterojunction structure, the composite had a good response of photocurrent. 3.9. The possible photocatalytic mechanism To explain the photocatalytic process of AgBr/AgNbO3 photocatalyst, a possible mechanism based on the band structure of the metal-semiconductor heterojunction was proposed. The band positions of AgBr and AgNbO3 could be calculated by the following empirical formula: ECB = X − EC − EVB = ECB + Eg

1 Eg 2

(1)

conducive to the migration of the electron and hole pairs, so the composite had a higher photocatalytic performance. 4. Conclusions In short, the AgBr/AgNbO3 heterojunction material had been successfully synthesized by the two-step synthesis method. The AgBr/AgNbO3 heterojunction exhibited higher photocatalytic activity than the pure AgNbO3 for the degradation of MB. The formation of the AgBr/AgNbO3 heterojunction was beneficial to the efficient separation of electrons and holes, which could be favorable for the enhancement of photocatalytic activity. It was also found that the photocatalytic reaction followed the pseudo-first-order kinetic model in the presence of the heterojunction photocatalyst. Acknowledgements

(2)

where X is the absolute electro-negativity of the atom semiconductor, expressed as the geometric mean of the absolute electro-negativity of the constituent atoms, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy; EC is the energy of free electrons of the hydrogen scale (4.5 eV); Eg is the band gap of the semiconductor; ECB is the conduction band potential and EVB is the valence band potential. By the above formula, the valence band and the conduction band position of AgBr and AgNbO3 are shown in Table 2. The photocatalytic mechanism is shown in Fig. 10. From Fig. 10, it can be seen that when AgBr/AgNbO3 composite photocatalyst was irradiated, AgBr and AgNbO3 were both excited. Electron was excited to the AgNbO3 conduction band, then the electron could be migrated to the conduction band of the AgBr under the effect of electric field in the composite body, and subsequently retransferred to the surface of the solid catalyst to participate in the photocatalytic reaction. The hole appearing in the AgBr of valence band could be migrated to the AgNbO3 of valence band. Thus, photo-generated electrons and holes could be effectively separated. The presence of AgBr/AgNbO3 composites with the heterojunction structure was

The authors genuinely appreciate the financial support of this work from the National Nature Science Foundation of China (21007021, 21076099 and 21177050), Society Development Fund of Zhenjiang (SH2011011), Postdoctoral Foundation of China (2012M521014), and Doctoral Innovation Fund of Jiangsu (CXLX120666). References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemannt, Environmental applications of semiconductor photocatalysis, Chemical Reviews 95 (1995) 69–96. [2] J.M. Herrmann, Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants, Catalysis Today 53 (1999) 115–129. [3] J. Li, D. Xu, Tetragonal faceted-nanorods of anatase TiO2 single crystals with a large percentage of active {1 0 0} facets, Chemical Communications 46 (2010) 2301–2303. [4] J.B. Joo, Q. Zhang, M. Dahl, I. Lee, J. Goebl, F. Zaera, Y.D. Yin, Control of the nanoscale crystallinity in mesoporous TiO2 shells for enhanced photocatalytic activity, Energy & Environment Science 5 (2012) 6321–6327. [5] B.S. Liu, K. Nakata, M. Sakai, H. Saito, T. Ochiai, T. Murakami, K. Takagi, A. Fujishima, Mesoporous TiO2 core–shell spheres composed of nanocrystals with exposed high-energy facets: facile synthesis and formation mechanism, Langmuir 27 (2011) 8500–8508.

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