Ion-exchange preparation for visible-light-driven photocatalyst AgBr/Ag₂CO₃ and its photocatalytic activity

Abstract

The AgBr/Ag₂CO₃ composite was synthesized by an ion-exchange reaction. The physical and chemical properties of the catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), transmission electron microscopy (TEM), 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, TEM, and XPS analyses indicated that the heterojunction structure had been obtained. The results indicated that the AgBr/Ag₂CO₃ heterojunction had exhibited a much higher photocatalytic activity than the pure Ag₂CO₃. The enhancement of photocatalytic activity was related to the efficient separation of electron–hole pairs because of the stagger band potentials between AgBr and Ag₂CO₃.

---

1. Introduction

Semiconductor photocatalysis as a green technology has provided an alternative way for environment purification and solar energy conversion.^1–4 For the past few decades, TiO₂, ZnO^5,6 and other such kinds of materials have been studied. Due to their wide band gap, only the ultraviolet light (4% of the solar light spectrum) can be absorbed, which greatly limits their visible-light photocatalytic activity and practical application. More and more studies have been carried out for the development of visibile-light sensitive photocatalysts, such as BiOX (X= Cl, Br, I),^7–9 BiVO₄¹⁰ and Ag₃PO₄,¹¹ which could absorb and highly utilize the solar energy.

In recent studies, Ag-based semiconductors have been used as high-efficient photocatalyst, such as AgInW₂O₈,^12,13 Ag₃VO₄,¹⁴ and Ag₂Mo₃O₁₁.¹⁵ According to the report,¹⁶ when the p-block elements incorporated into the Ag₂O, it might broaden the bandgap, enhance oxidative ability and improve the photocatalytic stability. C element as a nonmetallic p-block element is different from the traditional metallic p-block elements,¹⁷ which can be incorporated into Ag₂O to form the Ag₂CO₃ photocatalyst. As a visible-light-driven photocatalyst, Ag₂CO₃ has shown a high photocatalytic activity for the photodegradation of organic pollutant molecules under visible light.¹⁶ However, the recycle experiment of organic pollutant decomposition indicated that Ag₂CO₃ was unstable and the Ag⁺ ions in Ag₂CO₃ were reduced to metal Ag.¹⁸ Therefore, it is necessary to modify the original Ag₂CO₃ to further enhance the photocatalytic activity and the stability of the semiconductor.

Silver halide (AgX)-based catalysts have been reported to exhibit excellent photocatalytic activity in the degradation of organic dyes.¹⁹ Under visible light irradiation, silver bromide (AgBr) can absorb photons to generate electron–hole pairs and then the organic pollutants can be efficiently degraded. However, AgBr is unstable in the pure crystal form because of photocorrosion. Under light irradiation, the stability of the AgBr-based photocatalysts can be improved when AgBr was coupled with other semiconductors, such as ZnO,²⁰ WO₃,²¹ Ag₃PO₄,²² and C₃N₄.²³ The AgBr composite materials can significantly enhance the photocatalytic activity and also maintain the optical stability to a certain extent, which is probably for the reason that the heterojunction structure formed between AgBr and substrates can contribute to the efficient separation of electrons and holes. Therefore, if AgBr/Ag₂CO₃ hybrid materials are prepared, the heterojunction may be formed between AgBr and Ag₂CO₃, which is likely to enhance the photocatalytic activity and the stability of Ag₂CO₃. To the best of our knowledge, there has been no report about the synthesis and photocatalytic properties of the AgBr/Ag₂CO₃ until now.

In the present work, the AgBr/Ag₂CO₃ composite was synthesized by an ion-exchange reaction. To characterize its structure, 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. The photocatalytic activity of AgBr/Ag₂CO₃ was evaluated with methylene blue (MB) as a model contaminant. The experimental results confirmed that AgBr/Ag₂CO₃ exhibited much higher photocatalytic activity and stability than the pure Ag₂CO₃. The relationship between the photocatalytic property and the structure of hybrid materials was discussed. The mechanism of photocatalytic reaction was also proposed.

2. Experimental

2.1 Preparation of the photocatalysts

The synthesis of Ag₂CO₃ was achieved by a simple precipitation reaction. Firstly, 0.265 g of Na₂CO₃ and 0.849 g of AgNO₃ were dissolved in deionized water, respectively. Then the Na₂CO₃ solution was added dropwise into AgNO₃ solution while being under stirring and the resulting mixture was stirred for another 1 h. Finally, the yellow green precipitations were achieved, rinsed with deionized water and absolute ethanol for three times and dried at 50 °C for 6 h.

AgBr/Ag₂CO₃ was prepared by an ion-exchange reaction. Initially, 0.276 g Ag₂CO₃ was dispersed in 30 mL deionized water and sonicated for 30 min. Subsequently, 0.155 g 1-hexadecyl-3-methylimidazolium bromide ([C₁₆mim] Br) was dissolved in deionized water, and added dropwise into the above solution. After being stirred 3 h, the mixture was washed with deionized water and absolute ethanol for three times and dried at 50 °C for 6 h. The AgBr/Ag₂CO₃ (AgBr content: 21.41 wt%) was obtained. Other composites with different weight proportion of AgBr were named as AgBr/Ag₂CO₃ (6.38 wt%), AgBr/Ag₂CO₃ (11.93 wt%), AgBr/Ag₂CO₃ (16.97 wt%), AgBr/Ag₂CO₃ (21.41 wt%), AgBr/Ag₂CO₃ (25.41 wt%) and they can also be obtained with the same above method.

2.2 Characterization of photocatalysts

The crystalline phases of AgBr/Ag₂CO₃ composites were analyzed by XRD using a Bruker D8 diffractometer with Cu-Kα radiation (λ = 1.542 Å) in the range of 2θ = 10–80°. The morphology and the 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 micro-graphs were taken with a JEOL-JEM-2010 (JEOL, Japan) operated at 200 kV. Ultraviolet visible (UV-vis) diffuse reflection spectra (DRS) were measured using a UV-vis spectrophotometer (Shimadzu UV-2450, Japan) in the range of 200–800 nm. BaSO₄ was used as the reflectance standard material. XPS analysis was performed on an ESCALab MKII X-ray photo-electron spectrometer using Mg-Kα radiation.

2.3 Photocatalytic activity

Photocatalytic activity of the sample was evaluated by the degradation of MB. A 300 W Xe lamp as the light source was set in the middle of the photocatalytic reactor. To provide the visible light for the photocatalytic reaction, a 400 nm cutoff filter was set beside the lamp. Moreover, the 100 mL quartz glass beaker which contained the photocatalyst and MB solution was set next to the cutoff filter and the distance between the center of the beaker and the lamp was about 10 cm. They were all at the same height and the light could irradiate the beaker evenly. In all the photocatalytic experiments, the lamp and the quartz glass beaker were set at the same place. So the light intensity was fixed.

In the experiment, 0.075 g of each sample was added into 75 mL of MB solution (10 mg L⁻¹). Prior to illumination, the suspension was magnetically stirred in the dark for 30 min to reach adsorption–desorption equilibrium of MB on catalyst surface. At the irradiation time intervals of 3 min, the sample was extracted and centrifuged to remove the catalyst particles. Then, the filtrates were analyzed by recording variations of the maximum absorption band (664 nm) in the UV-vis spectra of MB by using a Shimadzu UV-2450 spectrophotometer.

2.4 Photoelectrochemical measurements

The photocurrents were measured with an electrochemical analyzer (CHI660B, Chen Hua Instruments, Shanghai, China) in a standard three-electrode system, which employed a platinum wire as the counter electrode, a Ag/AgCl as the reference electrode and a ITO as working electrode, respectively. 2 mg sample powder was dispersed ultrasonically in 400 μL of ethylene glycol, and 20 μL of the resulting colloidal dispersion (5 mg mL⁻¹) was drop-cast onto a piece of ITO slice with a fixed area of 0.5 cm² and dried in air at room temperature. All the photocurrent measurements were performed at a constant potential of 0 V. A 0.1 M Na₂SO₄ aqueous solution was used as the supporting electrolyte for photocurrent measurements. A 500 W Xe arc lamp was utilized as the light source. The Nyquist plots were recorded within the frequency range 100 MHz to 100 kHz. The samples were irradiated for several minutes until the photocurrent was stable. Then we irradiated and shielded the samples at intervals of 20 seconds to get the photocurrent analysis.

3. Results and discussion

3.1 XRD analysis

Fig. 1 exhibits the typical XRD patterns of the AgBr/Ag₂CO₃ and Ag₂CO₃ samples. In Fig. 1, all the diffraction peaks of the as-prepared samples were in good agreement with Ag₂CO₃ (JCPDS file no. 26-0339) and no other impurities were found, such as metallic silver, which indicated the purity of the as-prepared Ag₂CO₃. Except the peaks of pure Ag₂CO₃ crystals, several additional diffraction peaks associated with AgBr crystals (JCPDS file no. 06-0438) had also been detected in the XRD patterns of AgBr/Ag₂CO₃ samples. Besides, with the increasing content of AgBr, the intensity of diffraction peaks at 2θ = 30.96°, 44.35°, 56.04° of the AgBr/Ag₂CO₃ samples increased. The above results revealed that Ag₂CO₃ and AgBr/Ag₂CO₃ heterojunction had been obtained.

Fig. 1 XRD patterns of Ag₂CO₃ and AgBr/Ag₂CO₃ composites.

3.2 XPS analysis

The surface composition of the AgBr/Ag₂CO₃ samples was analyzed by XPS. The survey scan XPS spectra of Ag₂CO₃ and AgBr/Ag₂CO₃ are shown in Fig. 2. Carbon, oxygen, silver and bromine were detected in the samples and no other impurities were found. The peaks of Ag at 367.89 eV and 373.89 eV which are shown in Fig. 2B can be attributed to the Ag 3d_5/2 and Ag 3d_3/2 binding energies, respectively.²⁴ In particular, the Ag peaks of Ag₂CO₃ were at 368.13 eV and 374.13 eV (Fig. 2B). Such a shift may be due to the interaction between AgBr and Ag₂CO₃. Fig. 2C shows that all the samples have C peaks at 284.69 eV and 288.69 eV, which can be attributed to carbon contamination and C 1s. Moreover, O 1s peak at 531.22 eV was also shown in Fig. 2D. In Fig. 2E, the peak of Br 3d at 68.49 eV was due to the crystal lattice of Br⁻ in AgBr. All these results suggested that AgBr had been obtained on the surface of Ag₂CO_3. To further confirm the morphology of the AgBr and AgBr/Ag₂CO₃, SEM and TEM analyses were used.

Fig. 2 (A) XPS survey spectra of Ag₂CO₃ and AgBr/Ag₂CO₃ (21.41 wt%) composite, (B) Ag 3d, (C) C1s, (D) O1s and (E) Br 3d.

3.3 SEM and TEM analyses

The SEM images of Ag₂CO₃ and AgBr/Ag₂CO₃ (21.41 wt%) are presented in Fig. 3(A and B), and the corresponding EDS result of AgBr/Ag₂CO₃ (21.41 wt%) is also shown in the Fig. 3C. Fig. 3A displays that Ag₂CO₃ particles had the size of 0.5–0.7 μm with relatively smooth surface. Compared with the pure Ag₂CO₃, the particle size and the morphology of the AgBr/Ag₂CO₃ (21.41 wt%) hybrids were approximately similar to those of Ag₂CO₃ except for the relatively crude surface (Fig. 3B). This was for the reason that the grain-like AgBr particles were in direct contact with the Ag₂CO₃ surface. To further confirm the AgBr/Ag₂CO₃ structure, the EDS analysis of the composite was shown in Fig. 3C. The result showed that sample consisted of Ag, Br, C, O elements, which further proved that AgBr exited in the AgBr/Ag₂CO₃ hybrids.

Fig. 3 SEM images of (A) Ag₂CO₃, (B) AgBr/Ag₂CO₃ (21.41 wt%) composites, (C) EDS image of AgBr/Ag₂CO₃ (21.41 wt%) composites.

Furthermore, the TEM images of Ag₂CO₃ and AgBr/Ag₂CO₃ (21.41 wt%) are also shown in Fig. 4. Compared with Ag₂CO₃, it is obvious that there were small AgBr nanoparticles dispersed on the surface of large Ag₂CO₃ particles. The size of AgBr on the surface of Ag₂CO₃ was in the range of 20–50 nm. The two types of semiconductor materials combined together closely. Therefore, all these analyses have proved that the heterojunction was formed between AgBr and Ag₂CO₃ in the composites.

Fig. 4 TEM images of (A) Ag₂CO₃, (B and C) AgBr/Ag₂CO₃ (21.41 wt%) composites.

3.4 Diffuses reflectance UV-vis spectra of the samples

The optical absorbance of Ag₂CO₃ and AgBr/Ag₂CO₃ samples was measured by UV-vis diffuses reflectance spectra. From Fig. 5, it can be found that Ag₂CO₃ had an absorption edge around 470 nm and exhibited absorption in the visible-light ranges. With the increasing AgBr content, the AgBr/Ag₂CO₃ hybrids had stronger absorption in the visible region, which was consistent with the color of the samples changing from yellow to gray and then to dark. This may be for the reason that the heterojunction formed between AgBr and Ag₂CO₃ had changed the optical properties of the photocatalyst. Combining the AgX (X = Cl, Br, I) photocatalyst contributed to the improvement of the optical property of the substrate. This phenomenon can still be seen in many other works, such as AgBr/TiO₂,²⁵ AgBr/ZnO,²⁶ AgX/graphite-like C₃N₄ (X = Br, I).²³ However, in some regions of visible-light spectrum, the absorption intensity is not linearly when varying the AgBr loadings. DRS reveals surface absorption information and only a small fraction of AgBr existed on the surface of composite which can be recorded in the UV-Vis DRS.²⁷ From the result, it might be concluded that heterojunction had obviously changed the optical properties of Ag₂CO₃. In the photocatalytic reaction, light-absorbing property of the samples was crucial for the photocatalysis.

Fig. 5 UV-vis diffuse reflectance spectra of Ag₂CO₃ and AgBr/Ag₂CO₃ samples.

3.5 Photocurrent analysis

As we all know, the separation efficiency of electrons and holes makes a great influence on the photocatalytic reaction. The photocurrent represents the level of the separation efficiency: the high the photocurrent was, the better the electron and hole separation efficiency would be. As is shown in Fig. 6, AgBr/Ag₂CO₃ (21.41 wt%) exhibited a 60% higher photocurrent than Ag₂CO₃, which means the hybrids had the higher electron and hole separation efficiency. It was in correspondence with the result of their photocatalytic activity for MB dye degradation.

Fig. 6 Photocurrent profiles of Ag₂CO₃ and AgBr/Ag₂CO₃ (21.41 wt%) samples.

3.6 Photocatalytic activity of the samples

The photocatalytic activity of as-prepared samples were evaluated by the degradation of MB under visible light. MB self-degradation and the dark adsorption of MB over samples could be neglected. As is shown in Fig. 7A, all AgBr/Ag₂CO₃ heterojunction photocatalysts exhibit higher photocatalytic activity than the pure Ag₂CO₃. With the increasing AgBr content, the photocatalytic activity of AgBr/Ag₂CO₃ had been promoted gradually. AgBr/Ag₂CO₃ exhibited the highest photocatalytic degradation efficiency when AgBr content reached to 21.41 wt% and 97.87% of MB was removed after 18 min irradiation. However, the photocatalytic activity of AgBr/Ag₂CO₃ samples decreased when the AgBr content was more than 21.41 wt%. It suggested that the optimal content of AgBr in heterojunction photocatalyst was 21.41 wt%. This phenomenon could also be seen in other systems, such as AgBr/WO₃²¹ and AgBr/BiPO₄.²⁸ The reason why the high content of AgBr remarkably could affect the photocatalytic degradation efficiency of AgBr/Ag₂CO₃ could be concluded as follows: when there was high content of AgBr on the surface of Ag₂CO₃, AgBr particles would be agglomerating (the TEM image was not displayed here), which significantly impacted the size and the dispersion of AgBr. Many literatures reported that the size and the dispersion of nanoparticles on the surface of the semiconductors could affect the catalytic activity.²⁹ After agglomerating, the large size of AgBr could weaken the anchored force between the substrate and AgBr, destructing the heterojunction structure, which would limit the photocatalytic activity.

Fig. 7 (A) Photodegradation of MB by Ag₂CO₃ and AgBr/Ag₂CO₃ samples under visible light, (B) photodegradation of 4-CP by Ag₂CO₃ and AgBr/Ag₂CO₃ samples under visible light, (C) comparison of photocatalytic activity of different photocatalysts with the same weight of each visible-light-active component: 15.97 mg (blue line), 59.03 Ag₂CO₃ mg (red line), mathematical sum (pink line), and 75.00 mg AgBr/Ag₂CO₃ (21.41 wt%) containing 15.97 mg AgBr and 59.03 mg Ag₂CO₃ (black line) on the degradation of MB under visible light.

To further investigate the relationship between the microstructure and the property, we evaluated the photocatalytic activity on another representative colorless model organic pollutant 4-chlorophenol (4-CP) under visible light irradiation (λ ≥ 400 nm), eliminating the influence, such as dye sensitization. As can be seen in Fig. 7B, after first 1 h visible light irradiation, both Ag₂CO₃ and AgBr/Ag₂CO₃ (21.41 wt%) degraded the 4-CP rapidly and about 20% and 30% of 4-CP were degraded, respectively. After 6 h irradiation, the degrade efficiency by Ag₂CO₃ and AgBr/Ag₂CO₃ (21.41 wt%) for 4-CP decomposition reach 35% and 55% at last. Thus, AgBr/Ag₂CO₃ had better photocatalytic activity than Ag₂CO₃.

Besides, Fig. 7C shows that AgBr/Ag₂CO₃ (21.41 wt%) had higher photocatalytic activity than the mathematical sum of AgBr (15.97 mg) and Ag₂CO₃ (59.03 mg). The content of each component in the photocatalyst was the same as in AgBr/Ag₂CO₃ (21.41 wt%), but the photocatalytic degradation efficiency of MB was 43.71% and 97.87%, respectively. It was significantly proved that there were interaction between AgBr and Ag₂CO₃ in the composites, and the heterojunction structure could be formed in the composites.

Fig. 8 shows the change of absorption spectra of MB aqueous solution with the presence of Ag₂CO₃ and AgBr/Ag₂CO₃ (21.41 wt%) under visible light, respectively. Compared with Fig. 8A, it could be found that the absorption peak at λ = 664 nm decreased rapidly with the increasing time in the presence of AgBr/Ag₂CO₃ (21.41 wt%). Besides, in Fig. 8B, it could be found that the maximum absorption wavelength of MB was a slight blue shift in the change of absorption spectra of MB aqueous solution in the presence of AgBr/Ag₂CO₃ (21.41 wt%) under visible light irradiation, which implied N-demethylation process was occurring in the photocatalytic oxidation of MB.^30,31 The TOC removal of the degraded MB solution under visible light for 30 min was tested (Fig. 8C). The result shows that the decay of TOC was 42%, 58%, 60%, 66%, 68% and 50% for Ag₂CO₃, AgBr/Ag₂CO₃ (6.38 wt%), AgBr/Ag₂CO₃ (11.93 wt%), AgBr/Ag₂CO₃ (16.97 wt%), AgBr/Ag₂CO₃ (21.41 wt%) and AgBr/Ag₂CO₃ (25.41 wt%) composite, respectively. And the decay of TOC get highest efficience of 68% in the presence of AgBr/Ag₂CO₃ (21.41 wt%) composite. Therefore, in the presence of the AgBr/Ag₂CO₃ composites, examination of the spectral variations suggested that MB was N-demethylated in a stepwise manner, with cleavage of the MB chromophore ring structure occurring concomitantly as evidenced by the decrease in TOC (Fig. 8C).

Fig. 8 Absorption spectral changes of MB under visible light irradiation: (A) Ag₂CO₃, (B) AgBr/Ag₂CO₃ (21.41 wt%) composites, (C) the TOC removal of the MB solution in the presence of Ag₂CO₃ and AgBr/Ag₂CO₃ samples for 30 min.

3.7 Stability of AgBr/Ag₂CO₃ photocatalyst

The stability of the photocatalyst is another vital consideration for the photocatalyst except for the photocatalytic activity. To evaluate the stability of the AgBr/Ag₂CO₃ (21.41 wt%) hybrid, the repeatability experiments of MB degradation over AgBr/Ag₂CO₃ (21.41 wt%) were conducted and the result is shown in Fig. 9. After 4-time repeatability, the photocatalytic degradation efficiency over AgBr/Ag₂CO₃ decreased slightly due to the loss of the catalyst. As reported by other researchers, Ag₂CO₃ was unstable and could be easily decomposed by light irradiation. After 5-time repeatability, the degradation efficiency of Ag₂CO₃ would decrease from 50% to 2%.³² This may be because that after irradiation the electrons could reduce Ag⁺ ions in Ag₂CO₃ to form metallic Ag, which would decrease the photocatalytic activity of Ag₂CO₃.^18,32,33

Fig. 9 Cycling runs of AgBr/Ag₂CO₃ (21.41 wt%) composite under visible light irradiation.

However, when the AgBr photocatalyst combined with Ag₂CO₃, the heterojunction was formed between AgBr and Ag₂CO₃ and the degradation efficiency of AgBr/Ag₂CO₃ (21.41 wt%) was still beyond 92% after 4-time cycles, indicating the introduction of AgBr could also greatly improve the stability of the composite. Upon exposure to the visible light, both AgBr and Ag₂CO₃ were excited to generate the electron–hole pairs. The staggered band potentials of the AgBr and Ag₂CO₃ semiconductors were beneficial to reducing the recombination of photoexcited carriers in the AgBr/Ag₂CO₃ hybrids. Moreover, the staggered band potentials also contributed to the stability of AgBr/Ag₂CO₃ hybrids. The photogenerated electrons (e⁻) could transfer from AgBr to Ag₂CO₃ and then quickly transfer to the surface of the semiconductor, and then were trapped by the dissolved oxygen to form other oxidative species. This would prevent electrons coupling with the interstitial Ag⁺ to form metallic Ag⁰ and suppress the photocorrosion of the photocatalyst.

Besides, in some other works, such as AgI/BiOI,³⁴ Ag₂O/Ag₂CO₃,³³ AgBr/AgNbO₃,³⁵ the staggered band potentials were also beneficial for the electron–hole transformation to suppress the photocorrosion occurrence.

3.8 The possible photocatalytic mechanism

The highly efficient photocatalytic performance of the AgBr/Ag₂CO₃ hybrids were associated with their heterojunction structure, which resulted in an effective photo-excited electron and hole separation in the two materials. To further investigate the band structure, the band edges of Ag₂CO₃ and AgBr were evaluated by Mulliken electronegativity theory

E_CB = X − E_C − 1/2 E_g

E_VB = E_CB + E_g

where X is the absolute electro-negativity of the atom semi-conductor, 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 and the X value for Ag₂CO₃ and AgBr are 6.02¹⁸ and 5.80³⁵ from the reported literature, respectively; E_C is the energy of free electrons of the hydrogen scale (4.50 eV); E_g is the band gap of the semiconductor; E_CB is the conduction band potential and E_VB is the valence band potential. According to band potential calculation, the conduction band edge of AgBr (0 eV) was more negative than that of Ag₂CO₃ (0.37 eV), while the valence band edge of Ag₂CO₃ (2.67 eV) was more positive than that of AgBr (2.60 eV). Upon exposure to the visible light, both AgBr and Ag₂CO₃ were excited to generate the electron–hole pairs. Electron which was excited to AgBr conduction band could migrate to the conduction band of Ag₂CO₃. Besides, the hole which was present in the Ag₂CO₃ valance band would transform to the valance band of AgBr. The similar results had been reported in the previous literature.³⁵ The stagger band potentials of the hybrids were beneficial to increasing the separation efficiency of the electrons and holes.³⁶ This was also coincided with the results of the photocurrent analysis, as is shown in Fig. 6.

It is generally accepted that the dye and organic pollution can be photodegraded by reactive oxygen species, such as h⁺ and ˙OH.³⁷ Therefore, in order to get incisive insights into essential of the photoreaction mechanism of the dye over the photocatalyst, a series of radical trapping experiments were performed by using tert-butanol^38,39 and ethylene diamine tetraacetic acid,⁴⁰ which were introduced as effective scavengers for hydroxyl radicals and holes, respectively. Fig. 10 shows that the photodegradation of MB was slightly inhibited by the addition of tert-butanol (hydroxyl radicals scavenger) under visible light irradiation, indicating that the free hydroxyl radicals were not the major active oxidizing species in the photocatalytic process. However, photocatalytic activity was intensively suppressed when ethylene diamine tetraacetic acid (holes scavenger) was introduced, which indicated that holes were the main active oxidizing species involved in the photoreaction process.

Fig. 10 Plots of photogenerated active species trapped in the system of photodegradation of MB by AgBr/Ag₂CO₃ (21.41 wt%) under visible light irradiation.

Owing to the heterojunction structure of AgBr/Ag₂CO₃, it had higher separation efficiency for the electrons and holes to provide more holes to oxidize dye molecules, in this way the composite had a higher photocatalytic performance. The possible photocatalytic process is shown in Fig. 11.

Fig. 11 Proposed mechanism for the photodegradation of MB over AgBr/Ag₂CO₃ composite.

4. Conclusions

In summary, the AgBr/Ag₂CO₃ heterojunction material used for photodegradation of MB under visible-light irradiation had been successfully synthesized by the ion-exchange reaction. The AgBr/Ag₂CO₃ heterojunction exhibited higher photocatalytic activity than the pure Ag₂CO₃ and AgBr/Ag₂CO₃ (21.41 wt%) had the highest photocatalytic activity. The heterojunction structure of AgBr/Ag₂CO₃ could also maintain the stability of photocatalyst and the photocatalytic activity of AgBr/Ag₂CO₃ did not even change after 4-time cycles.

Acknowledgements

The authors genuinely appreciate the financial support of this work from the National Nature Science Foundation of China (no. 21177050 and 21206060), Natural Science Foundation of Jiangsu Province (BK20130513), Postdoctoral Foundation of Jiangsu Province (1302167C), University Natural Science Research of Jiangsu (13KJB430007) and Doctoral Innovation Fund of Jiangsu Province (CXLX13-651).

References

1. M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69–96.
2. Z. G. Zou, J. H. Ye, K. Sayama and H. Arakawa, Nature, 2001, 414, 625–627.
3. F. E. Osterloh, Chem. Mater., 2008, 20, 35–54.
4. X. B. Chen, S. H. Shen, L. J. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503–6570.
5. X. G. Han, Q. Kuang, M. S. Jin, Z. X. Xie and L. X. Zheng, J. Am. Chem. Soc., 2009, 131, 3152–3153.
6. A. Mclaren, T. Valdes-Solis, G. Q. Li and S. C. Tsang, J. Am. Chem. Soc., 2009, 131, 12540–12541.
7. X. Zhang, Z. H. Ai, F. L. Jia and L. Z. Zhang, J. Phys. Chem. C, 2008, 112, 747–753.
8. J. X. Xia, S. Yin, H. M. Li, H. Xu, Y. S. Yan and Q. Zhang, Langmuir, 2011, 27, 1200–1206.
9. J. X. Xia, S. Yin, H. M. Li, H. Xu, L. Xu and Y. G. Xu, Dalton Trans., 2011, 40, 5249–5258.
10. S. S. Dunkle, R. J. Helmich and K. S. Suslick, J. Phys. Chem. C, 2009, 113, 11980–11983.
11. J. F. Ma, J. Zou, L. Y. Li, C. Yao, Y. Kong, B. Y. Cui, R. L. Zhu and D. L. Li, Appl. Catal. B: Environ., 2014, 144, 36–40.
12. B. Hu, L. H. Wu, S. J. Liu, H. B. Yao, H. Y. Shi, G. P. Li and S. H. Yu, Chem. Commun., 2010, 46, 2277–2279.
13. S. Y. Song, Y. Zhang, Y. Xing, C. Wang, J. Feng, W. D. Shi, G. L. Zheng and H. J. Zhang, Adv. Funct. Mater., 2008, 18, 2328–2334.
14. H. Xu, H. M. Li, L. Xu, C. D. Wu, G. S. Sun, Y. G. Xu and J. Y. Chu, Ind. Eng. Chem. Res., 2009, 48, 10771–10778.
15. M. Feng, M. Zhang, J. M. Song, X. G. Li and S. H. Yu, ACS Nano, 2011, 5, 6726–6735.
16. H. J. Dong, G. Chen, J. X. Sun, C. M. Li, Y. G. Yu and D. H. Chen, Appl. Catal. B: Environ., 2013, 134–135, 46–54.
17. N. Umezawa, S. X. Ouyang and J. H. Ye, Phys. Rev. B, 2011, 83, 035202(1–8).
18. G. P. Dai, J. G. Yu and G. Liu, J. Phys. Chem. C, 2012, 116, 15519–15524.
19. H. Xu, H. M. Li, J. X. Xia, S. Yin, Z. J. Luo, L. Liu and L. Xu, ACS Appl. Mater. Interfaces, 2011, 3, 22–29.
20. C. L. Wu, L. Shen, Y. C. Zhang and Q. L. Huang, Mater. Lett., 2012, 66, 83–85.
21. J. Cao, B. D. Luo, H. L. Lin and S. F. Chen, J. Hazard. Mater., 2011, 190, 700–706.
22. Y. P. Bi, S. X. Ouyang, J. Y. Cao and J. H. Ye, Phys. Chem. Chem. Phys., 2011, 13, 10071–10075.
23. H. Xu, J. Yan, Y. G. Xu, Y. H. Song, H. M. Li, J. X. Xia, C. J. Huang and H. L. Wan, Appl. Catal. B: Environ., 2013, 129, 182–193.
24. H. Zhang, G. Wang, D. Chen, X. J. Lv and J. H. Li, Chem. Mater., 2008, 20, 6543–6549.
25. W. X. Wang, L. Q. Jing, Y. C. Qu, Y. B. Luan, H. G. Fu and Y. C. Xiao, J. Hazard. Mater., 2012, 243, 169–178.
26. B. Krishnakumar, B. Subash and M. Swaminathan, Sep. Purif. Technol., 2012, 85, 35–44.
27. L. Kong, Z. Jiang, H. H. Lai, R. J. Nicholls, T. C. Xiao, M. O. Jones and P. P. Edwards, J. Catal., 2012, 293, 116–125.
28. H. Xu, Y. G. Xu, H. M. Li, J. X. Xia, J. Xiong, S. Yin, C. J. Huang and H. L. Wan, Dalton Trans., 2012, 41, 3387–3394.
29. V. Subramanian, E. E. Wolf and P. V. Kamat, J. Am. Chem. Soc., 2004, 126, 4943–4950.
30. T. Zhang, T. Oyama, A. Aoshima, H. Hidaka, J. Zhao and N. Serpone, J. Photochem. Photobiol. A, 2001, 140, 163–172.
31. T. Zhang, T. Oyama, S. Horikoshi, H. Hidaka, J. Zhao and N. Serpone, Sol. Energy Mater. Sol. Cells, 2002, 73, 287–303.
32. C. W. Xu, Y. Y. Liu, B. B. Huang, H. Li, X. Y. Qin, X. Y. Zhang and Y. Dai, Appl. Surf. Sci., 2011, 257, 8732–8736.
33. C. L. Yu, G. Li, S. Kumar, K. Yang and R. C. Jin, Adv. Mater. DOI:10.1002/adma.201304173.
34. H. F. Cheng, W. J. Wang, B. B. Huang, Z. Y. Wang, J. Zhan, X. Y. Qin, X. Y. Zhang and Y. Dai, J. Mater. Chem A, 2013, 1, 7131–7136.
35. C. Wang, J. Yan, X. Y. Wu, Y. H. Song, G. B. Cai, H. Xu, J. X. Zhu and H. M. Li, Appl. Surf. Sci., 2013, 273, 159–166.
36. X. Zong, H. J. Yan, G. P. Wu, G. J. Ma, F. Y. Wen, L. Wang and C. Li, J. Am. Chem. Soc., 2008, 130, 7176–7177.
37. H. Tada, Q. L. Jin and H. Kobayashi, ChemPhysChem, 2012, 13, 3457–3461.
38. Y. Y. Li, J. S. Wang, H. C. Yao, L. Y. Dang and Z. J. Li, J. Mol. Catal. A: Chem., 2011, 334, 116–122.
39. X. Zhang, L. Z. Zhang, T. F. Xie and D. J. Wang, J. Phys. Chem. C, 2009, 113, 7371–7378.
40. S. Feng, H. Xu, L. Liu, Y. H. Song, H. M. Li, Y. G. Xu, J. X. Xia, S. Yin and J. Yan, Colliods Surf. A, 2012, 410, 23–30.

---