Facile regeneration and photocatalytic activity of CuO-modified silver bromide photocatalyst

Materials Science in Semiconductor Processing 40 (2015) 613–620

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Facile regeneration and photocatalytic activity of CuO-modified silver bromide photocatalyst Yunfang Wang n, Xue Zhang, Jianxin Liu, Yawen Wang, Donghong Duan, Caimei Fan Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 4 May 2015 Received in revised form 13 July 2015 Accepted 14 July 2015

CuO-modified silver bromide (AgBr/CuO) crystal was successful synthesized by a facile method at room temperature. The physical and chemical properties of AgBr/CuO crystals were carefully detected through X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy (DRS), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and Electron spin resonance (ESR) techniques. The photocatalytic activity and stability of AgBr/CuO hybrid were evaluated by photocatalytic degradation of methyl orange (MO) under visible light irradiation. The AgBr/CuO sample exhibited high photocatalytic activity, degrading 92% MO after irradiation for 40 min, which was 3.8 times higher than that of pure AgBr. Both the experimental scavenging results and characterization results revealed that O2
acts as the main active specie. Based on above, the high photocatalytic performance is mainly attributed to the abundant of oxygen vacancies, and which further generate lots of superoxide radicals. Moreover, the method by using bromide water to rejuvenate AgBr/CuO could well maintain the photocatalytic activity and stability without any environment pollution. & 2015 Elsevier Ltd. All rights reserved.

Keywords: CuO-modified Silver bromide Catalyst regeneration Photocatalytic mechanism Oxygen vacancy

1. Introduction To utilize solar energy, a number of researchers focus on the development of photocatalysts in a wide region of solar spectrum as photocatalysis is a great potential for the decomposition of various organic contaminates in water or air pollutants. Accordingly, a numerous of novel functional materials have been explored, such as metal surface modified catalyst [1], heterojunction type catalyst [2,3], graphene-based composite photocatalysts [4] and new monomer photocatalysts [5,6]. In recent years, the silver bromide semiconductor material as an obviously higher photocatalytic activity for the efficient removal of organic species has been studied in the field of photocatalysis research. Although silver bromide has many advantages, during the photocatalytic process, the silver ions on the surface of silver bromide will combine with photo-induced electrons and inevitably generate silver atoms (Ag0) during the photocatalytic process, which is the light corrosive of silver bromide [7]. Recently, many composite photocalalysts (such as AgBr/Ag3PO4 [8,9], Ag/AgBr/TiO2 [10,11], AgBr/SiO2 [12], and Fe(III) modified AgBr [13]) have been widely developed, which solve the repeatability problem of silver bromide to some extent, but the light corrosive problem of silver bromide remains unresolved. n

Corresponding author. Fax: þ 86 351 6018554. E-mail address: [email protected] (Y. Wang).

http://dx.doi.org/10.1016/j.mssp.2015.07.036 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

Nowadays, cupric oxide has been a hot topic because of its interesting property as a safety and environmental friendliness semiconductor. Especially the nonstoichiometric p-type CuO with a narrow band gap (about 1.7 eV) is easy to adsorb oxygen molecules forming oxygen vacancies, and then generate reactive oxygen species, thereby it is extensively used in various applications such as catalysis [14,15], gas sensors [16], biological applications [17] and solar energy transformation [18–20]. However, so far, the study of using copper oxide modified silver bromide has not been reported yet. In our work, we have fabricated AgBr/CuO composites to further enhance photocatalytic performance of silver bromide. In this way, the CuO is partially embedded into the surface of AgBr and easy to absorb oxygen molecules forming oxygen vacancies, which can present strong adsorption sites for O2 on AgBr. The O2 adsorbed on the oxygen vacancies can capture photogenerated electrons, simultaneously producing O2
radical groups, which are benificial to promote the photodegradation of organic substrates. The other purpose is to solve the light corrosive problem of silver bromide. To this end, bromine water was chosen as a regenerating agent and copper oxide as modification compound to prepare a reusable and highly efficient solar photocatalyst. Furthermore, new insights in the mechanism of MO photodegradation and regeneration for AgBr/CuO composites are also discussed in this paper.

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2. Materials and methods 2.1. Preparation of photocatalysts CuO, AgNO3, KBr, ethylene glycol and bromide water were analytical reagents (A.R.) and purchased from Sinopharm used without further purification. CuO-modified silver bromide particle was prepared through a precipitation process in a dark condition at room temperature. 0.1 g of CuO was added in 20 mL ethylene glycol under ultrasonication for 30 min, and 1.5 g of silver nitrate and 1.1 g of potassium bromide were added. After stirring for 30 min vigorously, the gray green precipitate was collected by centrifugation, washed with distilled water thoroughly and dried at 60 °C for 6 h. The CuO-modified silver bromide sample was denoted as AgBr/CuO (the molar ratio of AgBr and CuO is about 6.4:1). The procedure adopted for the preparation of AgBr/CuO hybrid photocatalyst is depicted in Scheme 1. For comparation, pure AgBr was prepared with the same precipitation process without add CuO. 2.2. Regeneration of photocatalysts After MO photodegradation in an aqueous solution under visible-light irradiation, the AgBr/CuO photocatalyst was centrifuged, washed thorough with distilled water, and dried at 60 °C for 6 h. The final sample was named as Used-AgBr/CuO. In typical regeneration reaction, the Used-AgBr/CuO photocatalyst was dispersed in 20 mL bromine water (3%) for 30 min. The regenerated product was subsequently collected by centrifugation, washed thoroughly with distilled water, and dried at 60 °C for 6 h. This sample was named as Re-AgBr/CuO. The procedure adopted for the regeneration process of AgBr/CuO hybrid photocatalyst is depicted in Scheme 2. And also we used the same method to collect Used-AgBr and Re-AgBr. 2.3. Characterization of photocatalysts The physical property of AgBr/CuO, Re-AgBr/CuO, AgBr and ReAgBr were systematically investigated. Powder X-ray diffraction (XRD) measurements were performed on the D/max-2500 instrument using Cu Kα radiation (λ ¼0.15406 nm) at a scanning rate of 0.02°/s. The morphologies (FESEM) of samples were observed with a field-emission scanning electron microscopy (JSM-7001F), and the energy-dispersive spectroscope (EDS) was investigated using a QX200 detector. Diffuse reflectance spectra (DRS) of the samples were evaluated by a Cary 3000 spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were detected

CuO

on an ESCAL-AB 250Xi spectrometer, and the spectra were calibrated to the C 1s peak at 284.8 eV. Fluorescence emission spectra (PL) were carried on an FLsp920 fluorescence spectrophotometer. Room temperature electron spin resonant (ESR) was recorded on a Bruker model JES FA200 spectroscopy at room temperature with the signals of radicals trapped by 5, 5-dimethyl-1-pyrroline N-oxide (DMPO). The setting is as followed: center field is 323.734 mT, microwave freguency is 9055.173 MHz and the power is 0.99800 mW. 2.4. Photocatalytic activity The photocatalytic activities of the samples were evaluated by photocatalytic degradation of MO in an aqueous solution under visible-light irradiation. The visible light was obtained from a 500 W Xe lamp with a 420 nm cut off filter produced by Shanghai Lansheng Electronics Company Limited. In each experiment, 0.1 g of as-prepared sample was dispersed in 100 mL of 15 mg L
1 MO solution. Prior to illumination, the suspension was stirred for 20 min to reach the absorbance equilibrium. During photocatalysis, 4 mL of the suspension was periodically sampled every 10 min and centrifuged to remove the photocatalyst particles. The upper clear liquid was analyzed using a Varian Cary 50 UV–vis spectrophotometer, and the characteristic absorption of MO at 464 nm was determined to monitor photocatalytic degradation rate.

3. Results and discussion 3.1. XRD analysis Fig. 1 displays the XRD patterns of pure AgBr, AgBr/CuO, ReAgBr/CuO and Used-AgBr/CuO hybrid photocatalysts. It is observed that CuO is monoclinic phase (JCPDS no.65-2309) [21], while AgBr is cubic phase (JCPDS no. 06-0438) [22]. As shown in the pattern Fig. 1(b) and (c), AgBr/CuO and Re-AgBr/CuO hybrids exhibit coexistence of both AgBr and CuO phases. Furthermore, as shown in Fig. 1(b) and (d), compared with AgBr/CuO, the diffraction peak at 38.15° (JCPDS no. 02-0931) [9] assigned to Ag0 was found in UsedAgBr/CuO after three recycling run, indicating that serious photocorrosion of AgBr occurred in the process of photocatalysis. However, the diffraction peak of Ag0 in the XRD patterns of the ReAgBr/CuO (Fig. 1(c)) is apparently disappeared, suggesting that Ag0 originated from light corrosion process transformed to Ag þ after regeneration. In addition, the diffraction peaks of Re-AgBr/CuO have no obvious difference from that of AgBr/CuO except that the

AgNO3

KBr

Keep stiring for 30 min

Washed with distilled water and dried at 60 °C

AgBr/CuO

CuO+EG Scheme 1. Preparing procedure of AgBr/CuO hybrid photocatalysts.

Y. Wang et al. / Materials Science in Semiconductor Processing 40 (2015) 613–620

615

Visible light Add 3% bromine water 20 mL Washed for 3 times

Re-AgBr/CuO

Used AgBr/CuO

AgBr/CuO

Scheme 2. Regeneration procedure of AgBr/CuO hybrid photocatalysts.

(410)

1.0

(d) CuO

(c)

CuO

Absorbance/a.u.

Intensity (a.u.)

(400)

0

Ag

1.4 1.2

(222)

(111)

(220)

(200)

AgBr

0.8 0.6 0.4

(b) 0.2

(a)

CuO AgBr Re-AgBr AgBr/CuO Re-AgBr/CuO

0.0 200

300

400

2theta( )

500

600

700

800

Wavelength/nm

Fig. 1. XRD patterns of (a) AgBr, (b) AgBr/CuO, (c) Re-AgBr/CuO and (d) Used-AgBr/ CuO.

2.8 2.6

3.2. DRS analysis Fig. 2(a) describes the UV–vis diffuse reflectance spectra of the pure AgBr, CuO, AgBr/CuO, Re-AgBr and Re-AgBr/CuO samples. It can be seen that the absorption band edges correspond to AgBr/ CuO and pure AgBr appeared at 477.7 nm and 483.3 nm respectively, in which an obvious blue shift in the direct band edge due to the decrease of the particle size [23]. Moreover, the absorption band edges of Re-AgBr/CuO and Re-AgBr match exactly with AgBr/ CuO and AgBr respectively, which manifested that silver bromide can be restored completely through regeneration process without any impurity. The enhanced adsorption of Re-AgBr/CuO around the 500 nm is attributed to the presence of oxygen vacancies. This feature indicated that the Re-AgBr/CuO could absorb more visible light. The band gap energy of a semiconductor can be calculated by the following formula:

(

(αhv)n /2 = A hv − Eg

)

Where α, h, v, Eg and A represent the abosorption coefficient, Planck constant, light frequency, band gap energy, and proportionality constant, respectively. The values of n are 1 for AgBr and CuO [24,25]. According to the plot of (αhv)1/2versus the band gap energy (hv) is shown in Fig. 2(b). The band gap energies (Eg) of AgBr and AgBr/CuO have been calculated to be 1.88 and 1.98 eV

2.4 2.2

1/2

2.0

(α hv)1/2/eV

intensities of diffraction peaks are slightly higher than that of AgBr/CuO. Thus, we conclude that the regeneration method using bromine water (3%) significantly restores the crystal structure and improves the crystallinity of Used-AgBr/CuO.

1.8 1.6 1.4 1.2 1.0 0.8

AgBr

0.6

Re-AgBr

0.4

AgBr/CuO

0.2 0.0 1.5

Re-AgBr/CuO

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

hv/eV Fig. 2. UV–vis spectra and band gaps of as-synthesized samples: (a) UV–vis spectra and (b) band gaps.

respectively. That means the band gap of catalyst modified copper oxide is widen, which is helpful to improve the oxidation reduction abilities of electrons and holes, leading to enhance the photocatalytic activity of AgBr/CuO hybrids. Furthermore, the conduction band (CB) and valence band (VB) of the semiconductor can be calculated according to the empirical formula [26]:

EVB = X − Ee + 0.5Eg

ECB = EVB − Eg Where ECB and EVB are the conduction and valence band edge potentials, respectively; x is the electronegativity of the

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Fig. 3. FESEM images of (a) AgBr, (b) AgBr/CuO, (c) Re-AgBr/CuO and (d) EDS of sample (b).

Fig. 3 shows the FESEM images of pure AgBr, AgBr/CuO and ReAgBr/CuO. Fig. 3(a) and (b) reveal that the probably size of AgBr/ CuO was 700 nm and that of AgBr was 1400 nm, indicating that the modification of CuO makes the smaller particles of catalyst. Compared with AgBr/CuO sample, the Re-AgBr/CuO had the same morphology and much smaller size particles (Fig. 3(b) and (c)). This strongly demonstrates that the regeneration process makes the photocatalytic particles smaller. The EDS result (Fig. 3(d)) suggested that the AgBr/CuO hybrids is composed of Ag, Br, O, and Cu elements, further confirming the existence of CuO, and the amount of CuO is about 2.49 at%.

surface. Compared with CuO/AgBr, the binding energies of the Cu 2p3/2 and Cu 2p1/2 in Re-AgBr/CuO sample were shifted to 934.1 and 953.9 eV respectively, as shown in Fig. 4(c). This result attributes to abundant of adsorbed oxygen atoms could attract the electrons on the orbital of Cu 2p result in reducing the binding energy Cu 2p3/2[27]. The comparison of O 1s spectra in Fig. 4(d) revealed that the O 1s peak located at 532.25 eV was fitted to vacancy oxygen and superoxide [28– 30]. Moreover, the relative of O 1s/Cu 2p is 1.14 for Re-AgBr/CuO and 0.97 for AgBr/CuO, which indicating that the mole ratio of O and Cu for Re-AgBr/CuO is greater than the stoichiometric ratio of CuO (O/Cu¼1). This mainly because there are a great quantity of gain boundary and rough surface on the catalyst, these sites are easy to adsorption oxygen molecules and form a nonstoichiometric compound [27,31]. Abundant adsorbed oxygen molecules are one of the ways to reach higher oxidizability of CuO and keep the Cu element in the 2 þ state. The amount of the CuO was calculated to be ca. 1.6% according to the XPS results. The XPS results confirm the presence of vacancy oxygen in the ReAgBr/CuO samples.

3.4. XPS analysis

3.5. PL analysis

The XPS survey spectra of AgBr/CuO and Re-AgBr/CuO are shown in Fig. 4. It is clearly observed from Fig. 4(a) that both samples show the binding energy peaks of Ag, Br, Cu, and O elements. As shown in Fig. 4(b), the binding energies of Ag 3d5/2 and Ag 3d3/2 peaks were located at 367.6 and 373.6 eV respectively, consistent with the presence of Ag þ on the AgBr surface. For CuO/AgBr, the binding energies of the Cu 2p3/2 and Cu 2p1/2 were located at 935.25 and 954.35 eV respectively are corresponding to the presence of Cu2þ on the CuO

The PL properties of the photocatalysts can provide useful information about the electronic structures and photochemical properties of semiconductor materials, by which information such as the separation and recombination characteristics of the photoexcited e
–h þ pairs and surface oxygen vacancies can be obtained. The PL emission spectra of AgBr/CuO and Re-AgBr/CuO were excited by 370 nm at room temperature. As shown in Fig. 5, it could be found that the broad visible photo-luminescence around

semiconductor. Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV vs. NHE). The x value for AgBr is 5.81 eV. Thus, the ECB and EVB of AgBr are calculated to be þ0.37 eV and þ2.25 eV, respectively. 3.3. FESEM analysis

Y. Wang et al. / Materials Science in Semiconductor Processing 40 (2015) 613–620

Ag3d

617

Ag3d5/2 Ag3d3/2 Cu2p O1s

Br2p

Counts(a.u.)

Counts(a.u.)

Br3d

Re-AgBr/CuO

Re-AgBr/CuO

AgBr/CuO AgBr/CuO

0

200

400

600

800

1000

1200

365

370

Binding energy/eV

375

Binding energy/eV

O1s Cu2p3/2

Counts(a.u.)

Cu2p1/2 Counts(a.u.)

Re-AgBr/CuO

AgBr/CuO

920

Re-AgBr/CuO

AgBr/CuO

930

940

950

960

970

525

530

Binding energy/eV

535

540

Binding energy/eV

Fig. 4. XPS spectra of as-synthesized AgBr/CuO hybrids and Re-AgBr/CuO hybrids: (a) survey, (b) Ag 3d, (c) Cu 2p and (d) O 1s.

AgBr/CuO

Re-AgBr/CuO

Intensity(a.u.)

Intensity/a.u.

Re-AgBr/CuO

AgBr/CuO

AgBr

430

480

530

580

630

Magnetic (mT)

680

Wavelength(nm)

Fig. 6. ESR spectra of AgBr, AgBr/CuO and Re-AgBr/CuO under visible light irradiation (λ 4400 nm).

Fig.5. PL spectra of as-synthesised AgBr/CuO and Re-AgBr/CuO samples.

3.6. ESR analysis 500 nm for AgBr/CuO and Re-AgBr/CuO is result from oxygen vacancy and the latter have more oxygen vacancies, in good agreement with the result of O 1s in XPS pattern. The above results strongly demonstrate that a quantity of oxygen vacancies exist on the surface of CuO-modified AgBr hybrids which lead to the effective separation of e
–h þ pairs. Therefore, it is clear that the formation of more oxygen vacancies in Re-AgBr/CuO catalyst contributes to its improved photocatalytic performance.

Room temperature electron spin resonant (ESR) analysis is utilized to investigate the active species of the photocatalysts. Fig. 6 shows the ESR spectra of the pure AgBr, AgBr/CuO and ReAgBr/CuO. For the pure AgBr, no apparent signal is detected. In contrast, for the AgBr/CuO and Re-AgBr/CuO, a symmetrical sharp signal appearing at g ¼2.004 gives clear evidence that the AgBr/ CuO hybrids contains a large number of oxygen vacancies [32]. In addition, the signal with g values of g1 ¼2.011, g2 ¼2.001 and

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photocatalytic performance. To further investigate the stability and reusability of the AgBr/CuO photocatalyst, three cycles were carried out, and experimental results are displayed in Fig. 7(b). Under the same conditions, the ultimate degradation rate of MO over Re-AgBr/CuO was maintained at 92% and nearly had no difference from the fresh AgBr/CuO hybrids. It can be drawn a conclusion that the using bromine water treatment is a very effective method of rejuvenated AgBr/CuO photocatalysts. More importantly, the method can be used continuously without restrictions.

0.07 0.06

k ( min-1)

0.05 0.04 0.03 0.02

3.8. Detection of reactive oxygen species

0.01 0.00

AgBr/CuO

AgBr

CuO

photolysis

RUN3

RUN4

1.2

RUN1

RUN2

1.0

C/C

0.8

0.6

0.4

0.2

0.0 -20

0

20

40

60

80 100 120 140 160 180 200 220 240 260

Time/min Fig. 7. The rate constant (k) of the MO decomposition by various processions (a) and cycling runs of Re-AgBr/CuO and Used-AgBr/CuO (b).

g3 ¼ 1.997 is known to be O2
, which is formed by photogenerated electrons [33]. From the Re-AgBr/CuO, the g value of signal a shift to a bit lower gradually which suggested that the crystal lattice distortion increases. Therefore, this result is consistant with the XPS and PL analysis suggested that the oxygen vacancies were formed on the surface of AgBr/CuO, which were benefit for the creation of superoxide ions to promote the photocatalytic activity. 3.7. Photocatalytic performance of CuO-modified silver bromide

To further investigate the improved photocatalytic performance of CuO-modified AgBr and propose a reaction route, we determined to carry out the active species trapping experiments. During the photocatalytic oxidation process, a series of reactive species, such as ∙OH h þ and ∙O2
are supposed to be involved. The experiments in valuing reactive oxygen species were similar to the photocatalytic degradation experiment, 1 mmol scavenger was introduced into the MO solution before catalyst added. In this study, ammonium oxalate (AO), tert-butanol (TB), and H2O2 were introduced to acted as the scavengers for h þ , ∙OH and ∙O2
into the reaction system respectively, and the obtained results are illustrated in Fig. 8. It can be clearly seen from Fig. 8 that the addition of TB in the solution has little effect on the photocatalytic degradation of MO, suggesting that ∙OH does not play a major role during the photocatalytic oxidation process. On the contrary, the photocatalytic degradation of MO is obviously inhibited after the addition of H2O2 and AO. According to the above results, it can be drawn a conclusion that ∙O2
is vital specie and h þ is also quite important in the degradation process of MO under visible light irradiation, which is correspond with the ESR spectra result. 3.9. Possible mechanism of CuO-modified silver bromide photocatalyst 3.9.1. Regeneration mechanism Because the conduction band potential of AgBr (about 0.205 eV) is more positive than the reduction potential of H þ , the fresh AgBr decomposes to Ag0 through combination with the photogenerated electrons of AgBr under visible light irradiation during the successive photocatalytic degradation cycles, and thus loses its photocatalytic activity. In view of the redox potential of AgBr/Ag is 0.0713 V, whereas the Br2/Br
pairs has a redox potential of 1.066 V in the acidic conditions which is sharply higher than that of AgBr/Ag0, and thus bromine water was chosen as

The photocatalytic performances of the CuO-modified silver bromide samples were evaluated by photocatalytic degradation of MO under visible light irradiation. In general, the photocatalytic degradation of MO obeys pseudo-first order kinetics. At low initial concentration, a pseudo-first order reaction model was used to describe the experimental data as follows:

1.0

0.8

ln (C0/C ) = kt C/C0

where k is apparent reaction rate constant, C0 is the adsorption equilibrium concentration of MO and C is the concentration at reaction time t. Fig. 7(a) displays the corresponding photocatalytic rate constant k of various processions. As shown in Fig. 7(a), in the presence of CuO and in absence of photocatalyst (photolysis), both of k values are about zero. These imply that the photocatalytic degredation of MO by CuO and the photolysis of MO are negligible. It can be seen that the k value being 0.0687 and 0.018 min
1 for AgBr/CuO and pure AgBr respectively, that is, the k value for AgBr/ CuO is 3.8 times higher than that for AgBr. These results suggest the AgBr/CuO photocatalyst show an obviously improved

0.6

0.4

0.2

0.0 -20

H2O2 AO TB No quencher

0

20

40

60

Time/min Fig. 8. Effects of different scavengers on the degradation of MO in the presence of AgBr/CuO hybrids under visible light irradiation.

Y. Wang et al. / Materials Science in Semiconductor Processing 40 (2015) 613–620

619

.

O2-

VO∙∙ +0.37eV

e-

e-

e-

e-

e-

e-

e-

e-

ee- e- e- e e- e- e-

E/eV

+0.46eV

O2

AgBr

hv

CuO

hv

.

+1.99eV +2.16eV +2.25eV

O2

h+ h+ h+ h+ h+ h+ h+ h+ h+

h+ h+

+ h+ h h+ h+

OH-/H2O ·OH

h+

Br 0 OH Br Scheme 3. Possible photocatalytic mechanism of MO over CuO-modified AgBr hybrids under visiblelight irradiation.

regenerating agent to rejuvenate the active Ag0 to highly photocatalytically active AgBr at room temperature. In addition, cheap copper oxide can be used as a catalyst during the regeneration process. The regeneration reaction is as followed: CuO

Ag + Br2⟹2AgBr

(1)

Where, copper oxide is a catalyst. As shown the above experimental results, for the rejuvenated CuO-modified AgBr, it not only restores into AgBr particles from active Ag0, but also improves photocatalytic activity through the regeneration process. This further proved that using bromine water is an effective regeneration method for AgBr/CuO photocatalysts. 3.9.2. Photocatalysis mechanism The experimental results of reactive oxygen species demonstrated that  O2
and h þ play an important part in the MO degradation activity of CuO-modified AgBr composites. XPS analysis and ESR spectra show that the oxygen vacancies appear on the surface of catalyst. Based on the detection results of active species and a series of characterizations of photocatalysts, a possible photocatalytic reaction mechanism for AgBr/CuO hybrid photocatalyst is illustrated in Scheme 3. The possible defects equation follows the reaction below [34–36]: •• 2CuO+1/2O2 ⇄ 2Cu•Cu + V Cu + 3Oo

4. Conclusions

(2)

2Oo ⇄ O2 + V o•• + 4e−

(3)

AgBr + hν → AgBr(e− + h+)

(4)

O2 + V o•• + e− → •O−2

(5)

O2
þmethyl orange-intermediates-CO2 þH2O

(6)

h þ þOH
-OH

(7)

OH-methyl orange-intermediates-CO2 þH2O

(8)

h þ þBr
-Br0

(9)

Br0 þmethyl orange-intermediates-CO2 þH2O

•• where VCu , Oo and Vo•• are copper vacancy, lattice oxygen and oxygen vacancy, respectively. On the one hand, in the air, the nonstoichiometry and rough surface of CuO are expected to absorb abundant oxygen molecules so that generating lattice oxygen in reaction (2) and formation of oxygen vacancies through reaction (3). Subsequently, under visible light irradiation, electrons of AgBr semiconductor are excited from the VB to CB, leaving h þ in the VB. And then, the oxygen vacancies of nonstoichiometric copper oxide compound will combine with electrons lying above the conduction band edges of silver bromide and cupric oxide to form active species  O2– as shown in Eq. (5). Finally, the  O2– reacts with MO molecules to mineral end-products in the reaction (6). On the other hand, the VB edge potentials of AgBr (EVB ¼ þ 2.25 eV) and CuO (EVB ¼ þ 2.16 eV) are a little positive than E0(  OH/OH
)(þ1.99 eV), the photogenerated holes on the VB edge of AgBr are transmitted to the VB of CuO particles which demonstrated that the photogenerated holes can provide sufficient potential to oxidize OH
to OH as shown in Eq. (7). In addition, the photogenerated holes on the VB edge of AgBr particles correspond to the oxidation of Br
to Br0, and then the OH and Br0 atoms are the reactive radical species, MO molecules can be oxidized thoroughly.

(10)

An effectively respond to visible-light photocatalyst of CuOmodified AgBr which filled with oxygen vacancies on the surface was synthesized via an anion-exchange precipitation method and regenerated by bromine water. The high photocatalytic performance of AgBr/CuO in decomposing methyl orange can be mainly attributed to the oxygen vacancies on the surface of AgBr/CuO composite which will be beneficial to effective separation of e
– h þ pairs and formation of active species ∙O2–. The regenerated method makes smaller catalyst particles, produce more oxygen vacancies, higher photocatalytic activity and stability, which indicated that this green regenerated method may facilitate AgBr/ CuO practical application in environmental issues.

Acknowledgements This work was financially supported by the Youth National Natural Science Foundation of China (No. 21206105) and the National Natural Science Foundation of China (No. 21176168).

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