Ag4P2O7 composite photocatalyst and enhanced photocatalytic performance

Materials Science and Engineering B 189 (2014) 70–75

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Synthesis of AgBr/Ag4 P2 O7 composite photocatalyst and enhanced photocatalytic performance Limin Song a,∗ , Yamiao Li a , Haifeng Tian a , Xiaoqing Wu b,∗ , Sheng Fang a , Shujuan Zhang c,∗ a College of Environment and Chemical Engineering & State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin Polytechnic University, No. 399 Binshuixidao Street, Xiqing District, Tianjin 300387, People’s Republic of China b Institute of Composite Materials, Tianjin Polytechnic University, No. 399 Binshuixidao Street, Xiqing District, Tianjin 300387, People’s Republic of China c College of Science, Tianjin University of Science & Technology, No. 29 Shisan Street, Kaifa District, Tianjin 300457, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 13 March 2014 Received in revised form 1 July 2014 Accepted 9 August 2014 Available online 21 August 2014 Keywords: AgBr/Ag4 P2 O7 , Photocatalysis Methylene blue

a b s t r a c t Ag4 P2 O7 and AgBr/Ag4 P2 O7 photocatalysts were synthesized by a simple precipitation reaction, and their structures were characterized by X-ray powder diffraction, scanning electronic microscopy, Brunauer–Emmett–Teller surface area analysis, X-ray photoemission spectroscopy, UV–vis absorption spectroscopy, photoluminescence, and surface photovoltage techniques. Photocatalytic degradations of methylene blue over Ag4 P2 O7 and AgBr/Ag4 P2 O7 were also studied under visible light irradiation. AgBr/Ag4 P2 O7 exhibited excellent photocatalytic efficiency, with the activity higher than that of Ag4 P2 O7 . The band gap and valence band of Ag4 P2 O7 were 2.67 eV and 1.93 eV, respectively. Ag4 P2 O7 failed to produce • OH radicals by directly oxidizing OH− because the oxidation potential of OH− was 2.7 eV. Therefore, electrons excited by light reacted with O2 to produce • O2 − radicals that then formed • OH radicals. Finally, • OH radicals oxidized MB molecules. The matching energy bands of Ag4 P2 O7 and AgBr in the AgBr/Ag4 P2 O7 heterocatalysts effectively separated the photo-induced charges, which may enhance the photocatalytic activity of Ag4 P2 O7 . © 2014 Elsevier B.V. All rights reserved.

1. Introduction Photocatalytic oxidation, a well-known promising technology for wastewater treatment, can oxidize pollutants to H2 O and CO2 cheaply and efficiently without giving rise to secondary pollution [1–5]. Numerous photocatalysts, such as TiO2 [5], WO3 [6], ZnO [7], ZnS [8], AgX [9], BiOX [10], and their composite materials [11–15], have been systemically studied. However, new types of photocatalysts are still in need in order to improve the activity and stability. Ye et al. [16,17] reported a Ag3 PO4 photocatalyst with high activity for organic compound degradation and water decomposition [18]. In addition to Ag3 PO4 , other PO4 3− -containing photocatalysts, such as Ti3 (PO4 )2 [19], BiPO4 [20], Cu(OH)PO4 [21] and Li9 Fe3 (P2 O7 )3 (PO4 )2 [22], also have high activities. Two PO4 3− anions generate P2 O7 4− after losing one H2 O molecule. Therefore, the photocatalytic properties of pyrophosphates may be similar to those of phosphates. At present, the photocatalytic performances of Ag4 P2 O7 have not yet been reported. In order to further enhance the

∗ Corresponding author. Tel.: +86 22 83955458; fax: +86 22 83955458. E-mail address: [email protected] (S. Zhang). http://dx.doi.org/10.1016/j.mseb.2014.08.001 0921-5107/© 2014 Elsevier B.V. All rights reserved.

photocatalytic ability, a composite photocatalyst AgBr/Ag4 P2 O7 was synthesized and studied in detail. Their structures and the effects of conditions on the photocatalytic performance were also investigated. Moreover, the kinetics of the activity and photocatalytic mechanism were analyzed. 2. Experimental 2.1. Synthesis of samples All reagents were bought from Tianjin Chemical Reagent Company (China). The reagents (99 wt.%) were used without further purification. Deionized water was used for all the processes. Ag4 P2 O7 was prepared by a precipitation reaction. In a typical process, 0.92 g AgNO3 and 0.52 g K4 P2 O7 were dissolved in 20 mL of deionized water, respectively, to which the solution of K4 P2 O7 was then dropwise added under stirring. The above mixture was stirred for 10 min to get white precipitates which were centrifuged, washed with distilled water and absolute alcohol several times, and dried in a vacuum oven at 80 ◦ C for 3 h. Br-Ag4 P2 O7 was prepared by the procedure similar to that described above, with the only difference being a 0.1 g Br ionic liquid (C8 H15 BrN2 ) was added to K4 P2 O7 solution.

L. Song et al. / Materials Science and Engineering B 189 (2014) 70–75

× AgBr

2.3. Activity measurement Photocatalytic activity under visible-light irradiation was evaluated by using methylene blue (MB) as the model substrate. In a typical process, 100 mL of MB (10 mg/L) aqueous solution and 0.3 g photocatalyst powders were mixed in a quartz photoreactor. Prior to a photocatalytic reaction, the photocatalyst suspension was sonicated to reach adsorption equilibrium in dark. The above solution was photoirradiated by using a 300 W Xe lamp ( > 400 nm) as the light source under continuous stirring. At a defined time interval, the concentration of MB in the photocatalytic reaction was analyzed by using a UV–vis spectrophotometer at 665 nm. 2.4. Measurement of hydroxyl radicals The formation of • OH radicals under visible-light irradiation was detected by a photoluminescence (PL) method [23]. Typically, 5 mg samples were added to an 80 mL aqueous solution containing 0.01 mol/L NaOH and 3 mmol/L terephthalic acid, and the solution was stirred for 10 min in dark. The reaction was carried out under visible-light irradiation. At a defined time interval, the solution in the system was collected, centrifuged, and analyzed by PL (excited at 325 nm). 3. Results and discussion 3.1. Characterization of photocatalysts Fig. 1 shows the XRD patterns of Ag4 P2 O7 (Fig. 1a) and AgBr/Ag4 P2 O7 (0.1 g Br ionic liquid; Fig. 1b). Fig. 1a shows three strong peaks at 27.14◦ , 28.79◦ , and 32.41◦ , which can be attributed to the hexagonal phase of Ag4 P2 O7 (JCPDS card no. 37-0187). A part of peaks shifted to right, which can be attributed to the difference

20

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50

(3 3 0)

×

(1 4 0 ) (0 3 18 ) (0 0 24 ) (1 1 24 )

(2 2 9 ) (2 2 11 ) (2 2 12 ) (1 1 20 )

×

(2 2 2 ) (0 0 18 )

a

(1 1 9 ) (1 1 10 ) (1 1 11 ) (1 1 12 )

2.2. Characterization of samples ˚ X-ray diffraction (XRD, Rigaku D/max 2500,  = 1.5406 A, 40 kV, 40 mA) was performed to investigate the crystalline phase and crystallite size of samples. The morphologies of samples were observed by field emission scanning electron microscopy (FESEM, Hitachi, S-4800) at the accelerating voltage of 10 kV. Brunauer–Emmett–Teller (BET) specific surface area was analyzed by nitrogen adsorption with a Micromeritics ASAP 2020 nitrogen adsorption apparatus. UV–vis absorption spectra were measured on a Shimadzu UV-2550PC spectrophotometer equipped with an integration sphere, with BaSO4 as the reference. Photoluminescence spectra (PL) were recorded by using a Cary Eclipse photoluminescence analyzer. Surface photovoltage (SPV) was measured on a home-made solid-junction photovoltaic cell. Monochromatic light was obtained by passing light from a 500 W xenon lamp through a double prism monochromator. Binding energy (BE) was investigated by an X-ray photoelectron spectrometer (XPS, Pekin-Elmer PHI5300) and calibrated by the standard peak of adventitious carbon (C1s ).

×

b

(1 1 13 )

Counts (a.u.)

Nitrogen-doped TiO2 (N-TiO2 ) was synthesized using a sol–gel route. In a typical process, 15 mL of tetrabutyl titanate (C16 H36 O4 Ti) was dissolved in 40 mL of anhydrous ethanol at room temperature. Acetic acid (CH3 COOH) (10 mL) and deionized water (5 mL) were added to 40 mL of anhydrous ethanol at room temperature. The second solution was dropwise added to the first one. Then 5 mL of NH3 ·H2 O (35 wt.%) was added to the above mixture after stirring for 30 min. Subsequently, the resultant sol was dried at 65 ◦ C for 12 h in air to remove residual water and alcohol. Finally, the asobtained white xerogel was calcined at 500 ◦ C for 3 h in static air. The as-prepared product was ground into fine powders.

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60

2Theta/degree Fig. 1. X-ray diffraction patterns of the resulting products. (a) Ag4 P2 O7 , (b) BrAg4 P2 O7 (0.1 g Br-ionic liquid).

between preparation methods. No impurity was observed in the XRD pattern, indicating that the as-prepared Ag4 P2 O7 was a pure phase. The crystal structure of the as-obtained sample could not be examined because the crystal cell parameters or plane indices of Ag4 P2 O7 have not been included in the JCPDS card. Fig. 1b shows four diffraction peaks at 31.09◦ , 44.41◦ , and 55.59◦ , corresponding to the AgBr (2 0 0), (2 2 0), and (2 2 2) lattice planes (JCPDS card no. ˚ respectively. Hence, the AgBr/Ag4 P2 O7 79-0149, a = b = c = 5.775 A), composite photocatalysts had been prepared successfully. In addition to the AgBr peaks, all other peaks in Fig. 1b can be assigned to the Ag4 P2 O7 phase (JCPDS card no. 37-0187). The average crystallite size (ds ) was calculated by the Scherrer equation based on the stronger peaks (planes) of Ag4 P2 O7 (Fig. 1a and b): D=

0.94 ˇ cos 

where D is the average crystallite size in angstroms,  is the ˚ ˇ is the full width at wavelength of X-ray radiation ( = 1.5406 A), half-maximum, and  is the diffraction angle. The sizes of Ag4 P2 O7 and AgBr/Ag4 P2 O7 particles were calculated as 32 nm. Since adding AgBr to Ag4 P2 O7 did not change the particle size, excess AgBr did not affect the size of Ag4 P2 O7 crystals. The FESEM images of microscale Ag4 P2 O7 and AgBr/Ag4 P2 O7 (0.1 g Br ionic liquid) particles are shown in Fig. 2. The shape of Ag4 P2 O7 is slice-like, and microslices with the mean diameter of 0.36 ␮m (SFig. 1a) uniformly disperse on the holder. Besides, the sphere-like AgBr/Ag4 P2 O7 (0.1 ␮m in average diameter) (Fig. 2b) is smaller than Ag4 P2 O7 . According to the XRD patterns of Ag4 P2 O7 (Fig. 1a), hexagonal phase is responsible for the sheet shape. There were some like-hexagonal sheets (Fig. 2a), and thin AgBr particles severely inhibited the growth of Ag4 P2 O7 particles (Fig. 2b), making them gathering into a mass. Therefore, excess AgBr changed the morphology and size of Ag4 P2 O7 particles. As exhibited in SFig. 1a and b (SFig. suggests the figure in support information), the sizes of Ag4 P2 O7 and AgBr/Ag4 P2 O7 particles are distributed homogeneously. Fig. 3 shows obvious absorptions of Ag4 P2 O7 and AgBr/Ag4 P2 O7 (0.1 g Br ionic liquid) from 400 nm to 800 nm, suggesting the sensitivity of the two samples to visible light. However, the absorption intensity of AgBr/Ag4 P2 O7 was higher than that of Ag4 P2 O7 in the visible light region, indicating that AgBr/Ag4 P2 O7 can be easily excited by visible light. SFig. 2 shows that the absorption threshold of AgBr/Ag4 P2 O7 slightly red-shifts, indicating a smaller band gap of AgBr/Ag4 P2 O7 than that of Ag4 P2 O7 . The band gap energies

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0.5

Br-Ag4P2O7

Photovoltage (mV)

0.4

0.3

0.2

0.1

Ag4P2O7

0.0 200

300

400

500

600

700

800

Wavelength (nm) Fig. 4. SPV spectra of the as-synthesized samples.

Fig. 2. SEM images of (a) Ag4 P2 O7 and (b) Br-Ag4 P2 O7 (0.1 g Br-ionic liquid).

Absorbtion (a.u.)

of Ag4 P2 O7 and AgBr/Ag4 P2 O7 were estimated as 3.0 and 2.85 eV, respectively, suggesting that excessive AgBr slightly reduced the excitation energy of Ag4 P2 O7 . Consequently, photo-induced electrons and holes were easily generated, which was conducive to boosting the photocatalytic activity of Ag4 P2 O7 . Fig. 4 shows the SPV spectra of Ag4 P2 O7 and AgBr/Ag4 P2 O7 . Of the SPV peaks at 325–610 nm, those at around 400–600 nm can be assigned to the band-to-band transition of Ag4 P2 O7 . The peaks at around 325–400 nm may be introduced by some organic contamination. The AgBr/Ag4 P2 O7 peak slightly red shifted, and the SPV intensity was significantly higher than that of pure Ag4 P2 O7 due to

250

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650

700

3.2. Activity of photocatalysts Fig. 5 shows the UV–vis spectra (200–800 nm) of MB solutions under 0–30 min of visible light irradiation in the presence of AgBr/Ag4 P2 O7 (0.1 g Br ionic liquid). The strongest absorption peaks at 665 nm stemmed from outer electron transitions of MB molecules, and the peaks at 616 nm were the maximal absorption from MB dimmers. The absorption peaks at 246 and 292 nm can be attributed to the ␲ → ␲* and n → ␲* electronic transitions of the naphthyl rings, and those at 330 nm correspond to the conjugated naphthyl rings in MB molecules. Fig. 5 shows that the absorption intensity of MB molecules decreases with increasing time. As suggested by the disappearance of the characteristic adsorption peaks at 246, 295, 330, and 665 nm after 30 min, the MB structure was completely destructed. Generally, MB molecules are initially oxidized by being attacked at S position or being methyl-removed at amine position. There are two possible mechanisms for degradation of MB, one being the direct oxidation of MB and the other being two-electron reduction to colorless structure of Leuco-MB (LMB). The second process can be verified by the absorption at Table 1 Physical parameters of Ag4 P2 O7 and Br-Ag4 P2 O7 .

Br-Ag4P2O7

Ag4P2O7

Br− dopant in Ag4 P2 O7 . Since SPV is generated from surface photoelectron migration, the high-intensity photo-voltage generated on the AgBr/Ag4 P2 O7 surface contributed to the separation of photoinduced electrons and holes, which may significantly enhance the photocatalytic activity of Ag4 P2 O7 [24]. N2 adsorption analysis was conducted to evaluate the specific surface area and porosity of the as-synthesized Ag4 P2 O7 and AgBr/Ag4 P2 O7 samples (Table 1). The BET surface area and total pore volume of AgBr/Ag4 P2 O7 , but not the crystal size and average pore size, decreased compared with those of Ag4 P2 O7 due to Br− dopant. SFig. 3 shows the pore-size distribution curves of the two samples. The two peaks of Ag4 P2 O7 and AgBr/Ag4 P2 O7 at 4.3 and 4.7 nm, as well as at 7.5 and 9.7 nm, corresponded to the small intra-aggregated mesopores and large inter-aggregated mesopores, respectively.

750

800

Wavelength (nm) Fig. 3. UV–vis absorption spectra of the as-synthesized samples.

850

Sample

ds (nm)

SBET (m2 /g)

Vp (cm3 /g)

dp (nm)

Ag4 P2 O7 Br-Ag4 P2 O7

32 31

4.4 0.7

0.008 0.002

3.456 3.442

Definitions: ds , crystal size; SBET , BET surface area; Vp , total pore volume; dp , BJH average pore size.

L. Song et al. / Materials Science and Engineering B 189 (2014) 70–75 3.5

1.8 1.6

10 mg/L 20 mg/L 30 mg/L 40 mg/L

3.0

0 min 10 min 20 min 30 min

1.2 1.0

2.5

ln(C0/Ct)

1.4

Intensity (a.u.)

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Wavelength (nm)

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Irradiation time (min)

Fig. 5. The UV–vis absorption spectra of solution in the process of the photocatalytic degradation of MB on Br-Ag4 P2 O7 (0.1 g Br-ionic liquid).

256 nm. The absence of such peak in Fig. 5 indicates direct oxidation of MB molecules. The photocatalytic activity of AgBr/Ag4 P2 O7 for MB exceeded that of undoped Ag4 P2 O7 (Fig. 6). The degradation rate of MB on AgBr/Ag4 P2 O7 was 96% within 30 min under visible light irradiation, whereas those of Ag4 P2 O7 and N-TiO2 were only around 73% and 29% respectively. The activity of AgBr/Ag4 P2 O7 was 1.3 and 3.3 times those of Ag4 P2 O7 and N-TiO2 , respectively. In other words, excessive AgBr helped improve the photocatalytic activity of Ag4 P2 O7 . Moreover, the effects of initial MB concentration on photocatalytic degradation were investigated in the range of 10–40 mg/L (SFig. 4). Given that the amount of AgBr/Ag4 P2 O7 (0.1 g Br ionic liquid) was fixed, total active sites remained constant for absorption, and the degradation rate of MB decreased with increasing MB concentration. Furthermore, the degradation rates of MB are still over 94% after five cycles (SFig. 5), verifying that the as-prepared AgBr/Ag4 P2 O7 (0.1 g Br ionic liquid) has a good lifetime. Generally, Ag-containing photocatalysts were unstable under light irradiation. Therefore, XRD after degrading MB (after the first run) was performed (SFig. 6). The peaks at 27.8◦ , 33.6◦ , and 35.2◦ can be assigned to Ag3 O4 (40–1054). However, the XRD peaks of Ag were absent. Ag nanoparticles were yielded because Ag+ ions reacted with e-first after photocatalytic reaction, and Ag3 O4 was produced due to the reaction of Ag particles with O2 in the dye

100

Decomposion ratio of MB (%)

10

80

AgBr/Ag4P2O7

Fig. 7. The relationship between ln(C0 /Ct ) and irradiation time (Br-Ag4 P2 O7 , 0.1 g Br-ionic liquid). Table 2 Relating kinetic parameters on the photodegradation of MB. C0 (mg/L)

k (min−1 )

R2

10 20 30 40

0.1067 0.0707 0.0533 0.0260

0.9977 0.9981 0.9928 0.9998

solution. Ag3 O4 , as a stable material, was responsible for the stable photocatalytic performance of AgBr/Ag4 P2 O7 . 3.3. Kinetics analysis The photodegradation of MB over AgBr/Ag4 P2 O7 can be well fitted by first-order kinetics based on a previous reference [7]: ln

C  0

Ct

= kt

where C0 and Ct are the concentrations of MB at time 0 and t, respectively, and k is the observed pseudo first-order rate constant. As shown in Fig. 7, ln(C0 /Ct ) is linearly related with reaction time. Meanwhile, the correlation coefficients (R2 ) in Table 2, which are higher than 0.99, further demonstrated a linear relationship between ln(C0 /Ct ) and reaction time. The photodegradation rate constants of AgBr/Ag4 P2 O7 were calculated as 0.1067, 0.0707, 0.0533 and 0.026 min−1 respectively (Table 2), following a decreasing photocatalytic activity order with rising MB concentration. As evidenced by the decreasing k with rising MB concentration, there was a competition for active sites on the catalyst surface (Fig. 8), being consistent with the outcomes in SFig. 4.

Ag4P2O7

60

3.4. Hydroxyl radicals (• OH) and photocatalytic mechanism

40

20

N-TiO2

0 0

5

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30

35

Irradiation time (min) Fig. 6. Comparative experiments decomposing MB on N-TiO2 , Ag4 P2 O7 , and AgBr/Ag4 P2 O7 (0.1 g Br-ionic liquid).

To identify the active species during the photocatalysis of Ag4 P2 O7 and AgBr/Ag4 P2 O7 , the formation of • OH radicals on Ag4 P2 O7 and AgBr/Ag4 P2 O7 under visible light irradiation was examined by the PL method. SFig. 7A and B show the changes in the PL spectra of terephthalic acid solution with increasing irradiation time in the presence of Ag4 P2 O7 and AgBr/Ag4 P2 O7 , respectively. The PL intensity at 425 nm gradually increased with elapsed irradiation time, confirming that the fluorescence originated from the reaction of terephthalic acid with • OH formed by AgBr/Ag4 P2 O7 photocatalysis. For comparison, Ag4 P2 O7 was also examined under the same conditions. The PL intensity of Ag4 P2 O7 increased with

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Potential (eV) vs NHE (pH=0)

30000

e-

25000

-1.00

e

e-

20000

Counts (s)

-

0 2.63eV

15000

1.00 2.6eV

10000

2.00

h+ h+

3.00

5000

0 -10

-5

0

5

10

15

20

25

30

35

40

45

Binding energy (eV) Fig. 8. Valence-band XPS spectra, and position of conduction band (CB) and valence band (VB) (B) of the as-synthesized samples. Table 3 Concentration of H2 O2 in the photodegradation process of MB. Time (min)

H2 O2 (mg/L)

10

15

20

25

30

1.585

1.658

1.806

1.904

1.953

extended time, but the PL intensity of AgBr/Ag4 P2 O7 was higher (about 1.2 times) than that of Ag4 P2 O7 at the same irradiation time. Hence, the higher photocatalytic activity of AgBr/Ag4 P2 O7 was reasonable. In the oxidation of liquid phase, reactive oxygen species may include holes (h+ ), H2 O2 , • O2 − and • OH radicals. • OH radicals were generated mainly from two ways, one being oxidation of OH− to • OH radicals by holes and the other being formation of • OH radicals by multistep reductions of • O2 − radicals [25]. The H2 O2 amount of the photodegradation reaction, which was measured by a potassium titanium oxalate colorimetric method [26], increased with prolonged reaction (Table 3). Hence, • OH radicals formed by the multistep reductions of • O2 − radicals but not the hole oxidation of OH− because H2 O2 was an intermediate of the multistep reaction [27].• OH radicals were the predominant species for oxidizing MB molecules, following the processes below [27,28]: O2 + 2H+ + 2e− → H2 O2

(1)

H2 O2 + e− → • OH + OH−

(2)

• OH

(3)

+ MB → CO2 + H2 O

Fig. 9. The enhanced mechanism diagram of the photocatalytic activity of BrAg4 P2 O7 .

4. Conclusion In summary, Ag4 P2 O7 and AgBr/Ag4 P2 O7 photocatalysts were successfully synthesized by a simple one-step method. AgBr also increased the interfacial charge transfer and inhibited the recombination of electron–hole pairs. Therefore, the photocatalytic activities of AgBr/Ag4 P2 O7 were significantly enhanced in the presence of small amounts of AgBr distributed over Ag4 P2 O7 . Acknowledgement This work was supported by Natural Science Foundation of Tianjin of China (Grant 14JCYBJC20500). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mseb. 2014.08.001. References [1] [2] [3] [4] [5] [6]

3.5. Mechanism for the enhanced photocatalytic activity of AgBr/Ag4 P2 O7 The band-gap structures of Ag4 P2 O7 and AgBr in the AgBr/Ag4 P2 O7 heterocatalysts are shown in Fig. 9, which are responsible for the activity changes of this photocatalytic reaction. Ag4 P2 O7 absorbed visible light and produced photo-generated electrons and holes. Photo-generated electrons on the HOMO orbits of Ag4 P2 O7 can be injected into the conduction band of AgBr because HOMO of Ag4 P2 O7 is higher than the valence band of AgBr. At the same time, the photo-generated electrons bound adsorbed O2 to produce • O2 − radicals that were then reduced to • OH radicals. Finally, OH radicals triggered the oxidation reaction. In general, the matching energy level and hybrid role of AgBr and Ag4 P2 O7 enable photogenerated electrons and holes to quickly migrate to a different surface of the catalyst, which promote effective separation of electrons and holes, thus enhancing the photocatalytic activity.

h+

[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

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