Facile preparation of an Ag/AgVO₃/BiOCl composite and its enhanced photocatalytic behavior for methylene blue degradation

Abstract

BiOCl and AgVO₃ have aroused great interest as photocatalysts in environmental remediation. They could be combined to improve their photocatalytic activity. A novel Ag/AgVO₃/BiOCl composite photocatalyst was produced via a facile ultrasound assisted hydrothermal method. The as-prepared samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (UV-DRS), photoluminescence (PL) emission spectroscopy and Brunauer Emmett Teller (BET) specific surface area analysis. It is revealed that the Ag/AgVO₃/BiOCl composite was successfully synthesized with a large specific surface area, mesoporous structure, enhanced light absorption performance and good recyclability. The photocatalytic activity for methylene blue (MB) degradation was investigated under visible light irradiation. The Ag/AgVO₃/BiOCl composite photocatalyst exhibited superior photocatalytic activity, and about 93.16% of MB was removed within 60 minutes of irradiation, which was better than that of pure BiOCl (29.24%) and Ag/AgVO₃ (37.52%). The enhanced photocatalytic activity could be attributed to the effective visible light absorption and separation of electrons and holes. Therefore, it is reasonable to believe that the Ag/AgVO₃/BiOCl composite photocatalyst has great potential in environmental remediation.

---

1. Introduction

With the development of urbanization and industrialization, environmental problems have become increasingly serious.¹ Photocatalysis based on semiconductors has been investigated due to the demand for organic dye degradation.^2–4 Organic dyes, like methylene blue (MB) and methyl orange (MO), are toxic and do not decompose quickly.^5–7 Photocatalytic degradation of MB is high performance and cost-effective, and it seldom discharges any dangerous chemicals.⁸ It provides a promising way to address environmental challenges, such as energy consumption and sustainability.⁹ Many semiconductors like TiO₂ and ZnO have been extensively utilized as effective photocatalysts for the degradation of organic dyes.^10,11 However, TiO₂ has a few major drawbacks in its photoexcitation domain.¹² Recently, bismuth oxychloride (BiOCl) has been studied due to its outstanding photocatalytic performance and environmental properties.¹³ It has been reported that BiOCl exhibited better performance than TiO₂ for Rhodamine B (RhB) degradation.¹⁴ BiOCl is a p-type semiconductor with a wide bandgap of about 3.12 eV.^15–17 The highly anisotropic layered structure features [Bi₂O₂]²⁺ layers sandwiched between two slabs of chloride ions which could efficiently induce the separation of photo-generated electron–hole pairs and enhance the photocatalytic activity.^18,19 So far, it has been reported that BiOCl doped with other materials could surprisingly enhance the photocatalytic performance compared to pure BiOCl. Duo et al. reported photocatalytic activity for MO degradation by BiPO₄/BiOCl heterojunction composites.¹⁶ Cao et al. reported BiOCl/m-BiVO₄ composites for RhB degradation.²⁰ Zuo et al. studied BiOCl/Bi₂MoO₆ composites with photocatalytic activity in degrading MO.²¹ In our previous study, Liu et al. synthesized a three-dimensional BiOCl_0.75Br_0.25/graphene microsphere for RhB degradation.²² However, due to its wide bandgap, BiOCl is not suitable for visible light irradiation, which limits its practical applications.^5,23 It is essential to modify BiOCl by coupling it with another narrow-bandgap material to improve the visible light absorption and promote the separation of photogenerated electron–hole pairs.³

Silver vanadium oxides (SVOs) such as AgVO₃ (α-AgVO₃ and β-AgVO₃), Ag₂V₄O₁₁, Ag₃VO₄ and Ag₄V₂O₇ have attracted more and more attention owing to their perfect electrical conductivity and potential applications in photocatalysts. For SVOs, the unique hybridization of the valence bands leads to a narrow bandgap, and this makes them potentially applicable as visible-light-sensitive photocatalysts.^24,25 Typically, β-AgVO₃ exhibited a much higher charge capacity than α-AgVO₃.²⁶ However, its low capability for separating electron–hole pairs became one of the limitations for its practical application.^26,27 Modified Ag nanoparticles could improve the separation of electrons and holes and enhance the absorption of the materials in visible light, resulting in better photocatalytic performance.^28,29 Reports about the synthesis and application of Ag/AgVO₃/BiOCl composites are still scarce.

In this study, we demonstrated a facile approach to prepare a novel Ag/AgVO₃/BiOCl composite photocatalyst using an ultrasound assisted hydrothermal method. The photocatalytic activity of the Ag/AgVO₃/BiOCl composite was measured by the degradation of MB under visible light irradiation. A feasible mechanism for the MB degradation was proposed. Cycling experiments consisting of five cycles were carried out to verify the stability of the Ag/AgVO₃/BiOCl composite.

2. Experimental

2.1 Materials

Bismuth nitrate pentahydrate (Bi (NO₃)₃·5H₂O) was purchased from Xi long Chemical Co., Ltd. Silver nitrate (AgNO₃) was purchased from Sinopharm Chemical Reagent Co., Ltd. Ammonium metavanadate (NH₄VO₃) was purchased from Shanghai Shan Pu Chemical Co., Ltd. Sodium hydroxide (NaOH) and absolute ethanol were purchased from Tianjin Hengxing Chemical Reagent Co., Ltd. Hydrochloric acid (HCl) was purchased from Zhuzhou City Star Glass Co., Ltd. All reagents were of analytical grade and used directly without further purification.

2.2 Synthesis of the Ag/AgVO₃/BiOCl composite

The BiOCl was prepared by a modified method.³⁰ Typically, 1.940 g of Bi(NO₃)₃·5H₂O was dissolved in 20 mL of 1.5 mol L⁻¹ HCl to get a transparent solution. Then, 0.72 mol L⁻¹ NaOH was added dropwise into the above solution with constant stirring until precipitations appeared in the mixed solution. Then the mixed solution was stirred for 10 minutes. Finally, the precipitates were filtrated and washed several times with deionized water, followed by drying at 80 °C overnight.

Then, different stoichiometric amounts of as-prepared BiOCl were dispersed in 20 mL of deionized water with the aid of ultrasonication. The Ag/AgVO₃/BiOCl composite was synthesized via a hydrothermal method:²⁷ 20 mL of 0.11 mol L⁻¹ AgNO₃ solution and 20 mL of 0.10 mol L⁻¹ NH₄VO₃ solution were successively added to the BiOCl suspension and the mixture was stirred for 180 min. The obtained mixture was transferred into a Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. The system was then allowed to cool down to room temperature naturally. The collected product was centrifuged and washed with deionized water and absolute ethanol four times, and then dried under vacuum at 80 °C. For comparison, the un-doped Ag/AgVO₃ was also prepared by the same procedure without the addition of BiOCl. The as-synthesized Ag/AgVO₃/BiOCl composite was labeled as AB2.5, AB5.0 and AB10.0 with 2.5, 5.0 and 10.0 wt% of BiOCl to Ag/AgVO₃, respectively.

2.3 Characterization

X-ray powder diffraction (XRD) patterns of the samples were obtained using a Rigaku D/Max 2500 diffractometer with Cu Kα radiation (λ = 1.5406 Å) under 40 kV, 250 mA. Surface elemental composition analyses were conducted using X-ray photoelectron spectroscopy (XPS) spectra obtained on a Thermo ESCALAB 250Xi instrument. The surface morphologies of the samples were observed using scanning electron microscopy (SEM) with a HITACHI S4800 model. The transmission electron microscopy (TEM) images were obtained using a FEI TECNAI G2-F20 machine. UV-visible diffused reflectance spectra (UV-DRS) of the samples were obtained using a Hitachi UV-3310 diffuse reflection instrument. The photoluminescence (PL) spectra of the photocatalysts were obtained using a F4600 (Hitachi, Japan) photoluminescence detector with an excitation wavelength of 384 nm. The Brunauer Emmett Teller (BET) specific surface area and total pore volume of the samples were measured by nitrogen adsorption–desorption analysis with an automatic Micromeritics 3 Flex instrument.

2.4 Photocatalytic experiment

The photocatalytic degradation of MB was carried out in a 250 mL beaker containing 100 mL of MB solution (7 mg L⁻¹) and photocatalyst (50 mg). The visible light irradiation was generated by a 300 W xenon lamp (14 V, 16 A) with a 420 nm cutoff filter. The mixed solution was magnetically stirred in the dark for 1 h to reach adsorption–desorption equilibrium. Then the solution was exposed to visible light with continuous stirring. At certain time intervals, a certain amount of liquid was collected and centrifuged at 8000 rpm for 10 min to remove the photocatalyst particles. The MB concentration of the residual liquid was analyzed using a UV-vis spectrophotometer (UV-2550, SHIMADZU Corporation, Japan) at its maximum absorption wavelength of 664 nm.

3. Results and discussion

3.1 XRD analysis

The purity and crystallinity of the as-prepared samples were confirmed by X-ray diffraction (XRD) and are depicted in Fig. 1. For Ag/AgVO₃, all peaks are attributed to the monoclinic phase of β-AgVO₃ (JCPDS no. 29-1154) and the face-centered cubic Ag (1 1 1) phase (JCPDS no. 04-0783).²⁷ For BiOCl, all peaks could be well indexed as the tetragonal phase of BiOCl (JCPDS no. 06-0249).³¹ And the intensity of the BiOCl characteristic peaks is weak due to the low proportion of BiOCl to Ag/AgVO₃.²⁰ The result indicates that the Ag/AgVO₃/BiOCl composite is well crystallized and was prepared successfully.

Fig. 1 XRD patterns of the Ag/AgVO₃, AB2.5, AB5.0, AB10.0 and BiOCl samples.

3.2 XPS analysis

X-ray photoelectron spectroscopy (XPS) measurements were carried out to determine the surface chemical composition and oxidation states of elements for the as-prepared samples. The XPS survey spectrum of the AB5.0 sample is shown in Fig. 2a. Chemical binding energies observed at approximately 530.18 eV, 159.43 eV, 198.51 eV, 368.01 eV and 517.03 eV are assigned to O 1s, Bi 4f, Cl 2p, Ag 3d and V 2p, respectively, which suggests that the AB5.0 composite is composed of O, Bi, Cl, Ag and V elements.²⁷ The C 1s peak can be ascribed to the adventitious hydrocarbon from the XPS instrument itself.^17,32 In Fig. 2b, the O 1s region is matched with two peaks at 530.2 eV and 532.0 eV. The main peak at 532.0 eV is related to the binding energy of Bi–O bonds in the [Bi₂O₂]²⁺ layered structure of BiOCl.^22,33 The peak at 530.2 eV is assigned to the binding energy of V–O in the AgVO₃ system.^34,35 In Fig. 2c, the peaks at 164.73 eV and 159.44 eV are assigned to the binding energies of the Bi 4f_5/2 and Bi 4f_7/2 spin–orbital splitting photoelectrons of Bi³⁺ in the BiOCl chemical state.^16,36 In Fig. 2d and S1,† the peaks at 198.14 eV and 199.75 eV are assigned to the binding energies of the Cl 2p_3/2 and Cl 2p_1/2 spin–orbital splitting photoelectrons of Cl⁻ in the BiOCl chemical state.^36,37 However, a shift of 0.36 eV to the higher binding energies of 198.50 eV and 200.14 eV is observed in the AB5.0 composite, implying that the chemical surroundings of Cl⁻ have changed.³³ This tiny shift indicates that Ag⁺ (or AgVO₃) has been doped into the lattices of BiOCl.^16,34,36 From the XPS spectra of Ag 3d in Fig. 2e, two peaks at 368.1 eV and 374.1 eV are assigned to the binding energies of Ag⁺ 3d_5/2 and Ag⁺ 3d_3/2, and two peaks at 368.5 eV and 374.6 eV are assigned to the binding energies of Ag⁰ 3d_5/2 and Ag⁰ 3d_3/2.^28,34 In Fig. 2f, two peaks at 517.08 eV and 524.41 eV are assigned to the binding energies of V⁵⁺ 2p_5/2 and V⁵⁺ 2p_3/2 in Ag/AgVO₃.^26,28 The result indicates the successful preparation of the AB5.0 composite.²⁶

Fig. 2 XPS spectra of the as-synthesized AB5.0 sample: (a) survey scan, (b) O 1s, (c) Bi 4f, (d) Cl 2p, (e) Ag 3d, and (f) V 2p.

3.3 SEM and TEM analysis

The morphology of the as-prepared samples was determined by SEM and TEM analysis. As shown in Fig. 3a, the typical SEM image of pristine BiOCl exhibits agminated flower-like microspheres with a diameter of 400–500 nm, and the microspheres are composed of many thin nanosheets. This morphology is different from conventional BiOCl bulk plate architectures.¹⁶ In Fig. 3b, the SEM image displays an overall view of Ag/AgVO₃ microrods with a length of 4–10 μm and a width of 270–300 nm. Fig. 3c shows the morphology of the AB5.0 composites. Some irregular shaped particles are successfully deposited on the surface of the Ag/AgVO₃ microrods. Then samples were further characterized by TEM analysis. As shown in Fig. 3d and e, the typical TEM image further reveals that the flower-like structure is constructed of nanosheets with an average thickness of 6–10 nm.^36,38 In addition, the chemical composition of the samples is confirmed by X-ray energy dispersive spectroscopy (EDS) analysis. Fig. 3f proves the presence of O, Bi, Cl, Ag and V elements. The atomic ratio (At%) of every element is shown in Table S1.† The result is in agreement with the XRD results.

Fig. 3 SEM images of (a) BiOCl, (b) Ag/AgVO₃, and (c) AB5.0 samples; TEM images of (d) BiOCl and (e) AB5.0 samples; EDS spectrum of (f) the AB5.0 sample.

3.4 UV-vis diffuse reflectance spectroscopy analysis

The optical absorption property is one of the most important properties to characterize the optical properties of semiconductors. UV-visible diffuse reflectance spectra in the wavelength range of 200–700 nm for Ag/AgVO₃, BiOCl and the Ag/AgVO₃/BiOCl composite are shown in Fig. 4a. The absorption edge of BiOCl is located at around 400 nm, while Ag/AgVO₃ has a broad absorption scale over the whole visible light region. For the Ag/AgVO₃/BiOCl composite, the absorption edge exhibits a red-shift to the visible light region until 600 nm, and the intensity increases more than that of BiOCl. Moreover, the absorbance is higher than that of Ag/AgVO₃ in the high wavelength region (600–800 nm), and the slopes of these two curves are different. This could be attributed to the integrative effect of the Ag/AgVO₃ microrods formed on the surfaces of the BiOCl microspheres.²⁷ Compared with the other samples, the Ag/AgVO₃/BiOCl composite produced a remarkable enhancement of the absorption intensity and the extension of the edge in the visible light region, implying that the Ag/AgVO₃/BiOCl composite can be used as an excellent photocatalyst under visible light irradiation.^9,10 The bandgap energy could be calculated by the following formula:³⁵

αhν = A(hν − E_g)^n/2 (1)

where α, ν, E_g and A are the absorption coefficient, light frequency, the bandgap energy, and a constant, respectively. n is determined by the type of optical transition of a semiconductor (n = 1 for a direct transition and n = 4 for an indirect transition). For BiOCl and AgVO₃, the value of n is 4 and 1, respectively. As illustrated in Fig. 4b and c, the bandgap energy of as-prepared BiOCl and Ag/AgVO₃ is estimated to be 3.12 eV and 2.10 eV, respectively, from the intercept of the tangent to the line. The result is in agreement with former reports.^16,27

Fig. 4 (a) UV-vis diffuse reflectance spectra of different samples; the band gap energy of (b) BiOCl and (c) Ag/AgVO₃ samples.

3.5 Nitrogen adsorption–desorption analysis

The specific surface area and total pore volume of the samples were measured by nitrogen adsorption–desorption analysis, and the corresponding curves are shown in Fig. 5. The inset shows the corresponding pore-size distribution, which implies the presence of mesopores in the size range of 2–50 nm.²² The mesoporous structure is more efficient for the photocatalytic degradation of organic pollutants in water.^35,39 As shown in Table 1, the BET surface area values of the samples are 13.47 m² g⁻¹ (Ag/AgVO₃), 3.81 m² g⁻¹ (BiOCl) and 13.84 m² g⁻¹ (AB5.0 composite). The total pore volume of the Ag/AgVO₃, BiOCl and AB5.0 composite is 0.029 cm³ g⁻¹, 0.014 cm³ g⁻¹ and 0.035 cm³ g⁻¹, respectively. In summary, the AB5.0 composite possesses a larger specific surface area and pore volume, which could improve the adsorption of dye molecules and enhance the photocatalytic activity.^36,37

Fig. 5 N₂ adsorption–desorption isotherms of the Ag/AgVO₃, BiOCl and AB5.0 samples. Inset: the corresponding pore-size distribution.

Table 1 BET surface area and total pore volume of the Ag/AgVO₃, BiOCl and AB5.0 samples; the calculated values of energy bandgap (E_g), valence band edge potential (E_VB) and conduction band edge potential (E_CB) of the Ag/AgVO₃ and BiOCl samples

Samples	BET characterization		Optical property (eV)
	Surface area (m^2 g^−1)	Total pore volume (cm^3 g^−1)	Eg	EVB	ECB
Ag/AgVO3	13.47	0.029	2.10	2.41	0.31
BiOCl	3.81	0.014	3.12	3.52	0.40
AB5.0	13.84	0.035

---

3.6 Photoluminescence spectra analysis

Photoluminescence (PL) spectra have been widely used to disclose the migration, transfer, and separation efficiency of photogenerated charge carriers in semiconductor materials.³⁵ A lower PL intensity indicates a lower recombination of the charge carriers, implying that more electrons and holes can take part in the oxidation and reduction reactions, resulting in the higher photocatalytic activity.^40,41 In Fig. 6, all samples were studied using PL spectra analysis with an excitation wavelength of 384 nm. The main emission peak center at the visible light area about 771 nm can be ascribed to the band gap recombination of electron–hole pairs. The relative intensity of the Ag/AgVO₃/BiOCl composite is lower than that of the pure BiOCl, implying that Ag/AgVO₃ formed on the surface of BiOCl could efficiently inhibit the recombination of electrons and holes, which is attributed to the efficient charge transfer between BiOCl and Ag/AgVO₃.²⁷ The emission intensity of the PL spectra for the Ag/AgVO₃/BiOCl composites is in the order of AB10.0 > AB2.5 > AB5.0. The result proves that Ag/AgVO₃/BiOCl composites with suitable proportions are very effective for the suppression of electron–hole pair recombination, which is a necessary factor for improving the degradation of MB.³³

Fig. 6 PL spectra of as-prepared samples with the excitation wavelength of 384 nm.

3.7 Photocatalytic degradation performance

To determine the photocatalytic activity of the as-prepared samples, the degradation of MB was investigated under visible light irradiation. Before light irradiation, we performed a dark experiment with the composite to investigate the adsorption–desorption equilibrium (Fig. S2†). After 1 h, the adsorption–desorption equilibrium had been achieved. Then, the following degradation test was carried out. Fig. 7a presents the variation of the MB concentration (C_t/C₀) according to the irradiation time over different samples. A blank experiment without any photocatalyst was also conducted for comparison. The MB degradation efficiency of the AB2.5, AB5.0 and AB10.0 composite is 73.51%, 93.16% and 87.33%, respectively, which are all better than those of the pure BiOCl (29.24%) and Ag/AgVO₃ (37.52%).

Fig. 7 (a) Photocatalytic degradation of MB over different samples under visible light irradiation; (b) pseudo first-order kinetics fitting plots; (c) changes in the UV-vis absorption spectra of MB over the AB5.0 composite every 10 min under visible light irradiation; (d) photocatalytic degradation of MB over different dosages of AB5.0.

The enhanced photocatalytic activity of the composites may be ascribed to these four reasons. (1) The presence of Ag/AgVO₃ greatly extended the scale of light absorption and remarkably enhanced the intensity of light absorption compared to pure BiOCl. (2) MB dye molecules can easily adsorb on the surface of BiOCl and further be degraded under visible light irradiation due to the dye sensitization.¹⁴ The flowerlike hierarchical microstructure can not only enhance the surface area, but also facilitate the transfer of the dye molecules during the photocatalytic process.^15,42 With the combination of BiOCl and Ag/AgVO₃, the separation of photogenerated electron–hole pairs and the transportation of electrons were efficiently facilitated compared to pure Ag/AgVO₃.⁴³ (3) For Ag nanoparticles, as they have excellent conductivity and a strong electron trapping ability (as investigated by Zhao et al. with fluorescence emission spectra analysis²⁸), the separation of electrons and holes could be improved, as well as the interfacial charge transfer.²⁹ And the modified Ag nanoparticles can induce localized surface plasmon resonance (SPR) from the collective oscillation of the surface electrons and remarkably enhance the absorption of the samples in visible light, resulting in a better photocatalytic performance (as investigated by Zhao et al. with UV-vis adsorption spectra analysis).²⁸ (4) Moreover, a large specific surface area could prevent the agglomeration of BiOCl microspheres, facilitate the transfer of the dye molecules during the photocatalytic process and improve the surface reaction rates.^15,42

To quantitatively understand the reaction kinetics of the MB degradation, the pseudo-first-order kinetics model was used to fit the curves:

−ln(C_t/C₀) = kt (2)

where k (min⁻¹) and t (min) are the apparent first-order rate constant and irradiation time, and C₀ (mg L⁻¹) and C_t (mg L⁻¹) are the initial concentration of MB and the remaining concentration of MB at each time, respectively. The model fitting plots and corresponding k values are shown in Fig. 7b. It is indicated that the values of k were in the order of AB5.0 > AB10.0 > AB2.5 > Ag/AgVO₃ > BiOCl with values of 0.04437, 0.03523, 0.02153, 0.00675 and 0.00493 min⁻¹, respectively. The sequence demonstrated that the photocatalytic activity of the Ag/AgVO₃/BiOCl composite was closely related to the amount of BiOCl. An increase of the amount of BiOCl (10%) would lead to a decrease in the photocatalytic efficiency, for the reason that the excessive BiOCl would widen the bandgap of the photocatalytic composite and weaken the availability under visible light irradiation.¹⁵ And the k value of the AB5.0 composite was 9 times that of pure BiOCl, highlighting the outstanding photocatalytic performance of the AB5.0 composite. The temporal evolution of the spectral changes of the MB over the optimum AB5.0 photocatalyst was also detected and is depicted in the Fig. 7c. The MB dye manifested a maximum absorption band at 664 nm. The concentration of the MB at 664 nm was continuously decreased over time and approached zero after 60 min, which implied the degradation of the auxochrome group of the MB dye.⁴⁴ Fig. 7d presents the effect of different concentrations of photocatalyst for the degradation of MB. The degradation rate of MB was associated with the amount of active sites in the Ag/AgVO₃/BiOCl photocatalytic oxidation systems. The degradation rate was limited at a low concentration, for there were not enough active sites. When the concentration of photocatalyst was increased, the photocatalytic activity improved. However, this beneficial effect would drop off when the concentration was in excess, for the penetration of visible light irradiation was weakened. The result demonstrated that the photocatalytic activity was determined by the amount of active sites and the penetration of visible light irradiation.⁴⁵ The optimum concentration is 0.5 g L⁻¹.

3.8 Possible photocatalytic mechanism

The photocatalytic activity depended on the generation, transfer and separation of photogenerated electron–hole pairs. Band positions of Ag/AgVO₃ and BiOCl were calculated by the following empirical formulas:³³

E_VB = X − E^e + 0.5E_g (3)

E_CB = E_VB − E_g (4)

where E_VB is the valence band edge potential, E_CB is the conduction band edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, E^e is the energy of free electrons on the hydrogen scale (about 4.5 eV), and E_g is the bandgap energy of the semiconductor. The X values of AgVO₃ and BiOCl are 5.86 eV and 6.46 eV, respectively. As shown in Table 1, according to the calculation, the VB and CB edge potentials of Ag/AgVO₃ are 2.41 eV and 0.31 eV, respectively. And the VB and CB edge potentials of BiOCl are 3.52 eV and 0.40 eV, respectively. The result is consistent with former reports.^16,27 To further investigate the photocatalytic mechanism, EDTA, isopropanol (IPA) and methyl alcohol (MeOH) were respectively introduced as the scavenger to quench active holes (h⁺), hydroxyl radicals (˙OH) and superoxide radicals (˙O₂⁻), respectively. As shown in Fig. 8a and b, in the presence of EDTA, the degradation of MB was significantly inhibited compared with no scavenger, indicating that the crucial active species was active holes (h⁺). Furthermore, the photocatalytic degradation rate also decreased with the addition of IPA and MeOH, suggesting that both ˙OH and ˙O₂⁻ species also participated in the photocatalysis reactions.

Fig. 8 (a) Effects of different reactive species scavengers on the photocatalytic degradation of MB by AB5.0 under visible light irradiation; (b) histogram of k value.

On the basis of the above-described experimental results, the mechanism of the photocatalytic degradation of MB dye over the Ag/AgVO₃/BiOCl composite under visible light irradiation is proposed, as illustrated in Fig. 9. When the Ag/AgVO₃/BiOCl composite is subjected to visible light irradiation, the electrons (e⁻) can be excited from the valence band (VB) to the conduction band (CB) of Ag/AgVO₃, simultaneously leaving behind a hole in the valence band. However, BiOCl can hardly be excited by visible light due to the wide band gap energy.¹⁴ Because the CB of Ag/AgVO₃ (0.31 eV) is more negative than that of BiOCl (0.40 eV), the excited electrons on the CB of Ag/AgVO₃ would flow into the CB of BiOCl.²⁶ The MB molecules adsorbed on the surface of the BiOCl are excited by visible light irradiation to generate electrons and then the electrons are injected from the excited MB molecules to the CB of BiOCl.¹⁴ Thus, the electron–hole pairs are separated effectively. Then, electrons in the Ag/AgVO₃/BiOCl composite can be trapped by O₂ to form ˙O₂⁻ reactive species, followed by the generation of ˙OH.²⁴ Lastly, these reactive species, including ˙O₂⁻, ˙OH and h⁺, could directly oxidize the MB dye molecules.

Ag/AgVO₃ + visible light → Ag/AgVO₃ (e⁻ + h⁺) (5)

AgVO₃ (e⁻) + Ag → AgVO₃ + Ag (e⁻) (6)

Ag/AgVO₃ (e⁻) + BiOCl → Ag/AgVO₃ + BiOCl (e⁻) (7)

MB + visible light → MB⁺ + e⁻ (8)

e⁻ + absorbed O₂ → ˙O₂⁻ (9)

(h⁺, ˙O₂⁻, ˙OH) + MB dye → degradation products (10)

Fig. 9 Schematic illustration of the band structures and possible photocatalytic reactions of the Ag/AgVO₃/BiOCl photocatalyst.

3.9 Recyclability and stability

The stability of catalysts is an important issue for their practical applications. Therefore, the degradation of MB by a AB5.0 composite photocatalyst which was reused for five runs under the same conditions was investigated. As shown in Fig. 10a, the photocatalytic activity of the AB5.0 composite photocatalyst remained almost unchanged after five cycles, confirming good recyclability. The XRD pattern of the used AB5.0 sample was compared with that of a fresh AB5.0 sample (Fig. 10b). As we have seen, the intensity of Ag in the used AB5.0 sample is higher than that in the fresh AB5.0 sample, indicating an increase of Ag nanoparticles. The result is consistent with the report of Zhao et al. that Ag nanoparticles may be generated under light irradiation for the photosensitivity of silver ions.^38,46

Fig. 10 (a) Five cycles in the presence of AB5.0 under visible light irradiation; (b) XRD patterns of the used AB5.0 sample and a fresh AB5.0 sample.

4. Conclusions

In summary, a novel Ag/AgVO₃/BiOCl composite photocatalyst was produced via a facile ultrasound assisted hydrothermal method. BiOCl microspheres were uniformly dispersed and integrated with Ag/AgVO₃ microrods. The photocatalytic activity for MB degradation under visible light irradiation was investigated. The Ag/AgVO₃/BiOCl composite photocatalyst exhibited a much higher photocatalytic activity than pure BiOCl and Ag/AgVO₃. In particular, the degradation efficiency of the AB5.0 composite is 93.16% within 60 min. The enhanced photocatalytic activity could be attributed to the effective visible light absorption, charge transportation and efficient separation. The Ag/AgVO₃ microrods greatly extended the scale of light absorption and remarkably enhanced the intensity of light absorption. The BiOCl microspheres acted as a transporter to separate photogenerated electron–hole pairs. And the modified Ag nanoparticles can induce localized surface plasmon resonance (SPR) from the collective oscillation of the surface electrons and remarkably enhance the absorption of the samples in the visible light. Active holes (h⁺) as the dominant active species generated during the photocatalytic process were assumed to interact with the pollutants. In addition, the Ag/AgVO₃/BiOCl composite photocatalyst exhibited excellent stability. The synergetic effect between noble metal oxide and semiconductor provided insight into new possibilities for potential application in the field of wastewater treatment.

Acknowledgements

The authors gratefully acknowledge the financial support provided by Collaborative Innovation Center of Resource-Conserving & Environment-friendly Society and Ecological Civilization, the National Natural Science Foundation of China (No. 71431006, 21276069).

References

1. H. Wang, X. Yuan, Y. Wu, H. Huang, X. Peng, G. Zeng, H. Zhong, J. Liang and M. Ren, Adv. Colloid Interface Sci., 2013, 195–196, 19–40.
2. H. Wang, X. Yuan, G. Zeng, Y. Wu, Y. Liu, Q. Jiang and S. Gu, Adv. Colloid Interface Sci., 2015, 221, 41–59.
3. N. Wetchakun, S. Chainet, S. Phanichphant and K. Wetchakun, Ceram. Int., 2015, 41, 5999–6004.
4. C. Dong, X. Xiao, G. Chen, H. Guan and Y. Wang, Mater. Chem. Phys., 2015, 155, 1–8.
5. J. R. Reddy, S. Kurra, R. Guje, S. Palla, N. K. Veldurthi, G. Ravi and M. Vithal, Ceram. Int., 2015, 41, 2869–2875.
6. X. Wen and L. Tang, J. Mol. Catal. A: Chem., 2015, 399, 86–96.
7. A. Singh, D. P. Dutta, A. Ballal, A. K. Tyagi and M. H. Fulekar, Mater. Res. Bull., 2014, 51, 447–454.
8. H. Wang, X. Yuan, Y. Wu, G. Zeng, X. Chen, L. Leng, Z. Wu, L. Jiang and H. Li, J. Hazard. Mater., 2015, 286, 187–194.
9. X. Yuan, H. Wang, Y. Wu, X. Chen, G. Zeng, L. Leng and C. Zhang, Catal. Commun., 2015, 61, 62–66.
10. H. Wang, X. Yuan, Y. Wu, G. Zeng, X. Chen, L. Leng and H. Li, Appl. Catal., B, 2015, 174–175, 445–454.
11. H. Wang, X. Yuan, Y. Wu, X. Chen, L. Leng and G. Zeng, RSC Adv., 2015, 5, 32531–32535.
12. Z. Zarghami, M. Ramezani and M. Maddahfar, Mater. Lett., 2015, 152, 21–24.
13. M. Zhang, Y. Liu, L. Li, H. Gao and X. Zhang, Catal. Commun., 2015, 58, 122–126.
14. K. Li, Y. Tang, Y. Xu, Y. Wang, Y. Huo, H. Li and J. Jia, Appl. Catal., B, 2013, 140–141, 179–188.
15. K. Wang, C. Shao, X. Li, X. Zhang, N. Lu, F. Miao and Y. Liu, Catal. Commun., 2015, 67, 6–10.
16. F. Duo, Y. Wang, X. Mao, X. Zhang, Y. Wang and C. Fan, Appl. Surf. Sci., 2015, 340, 35–42.
17. L. Li, M. Zhang, Y. Liu and X. Zhang, J. Colloid Interface Sci., 2014, 435, 26–33.
18. Y. Mi, L. Wen, Z. Wang, D. Cao, Y. Fang and Y. Lei, Appl. Catal., B, 2015, 176–177, 331–337.
19. X. Gao, X. Zhang, Y. Wang, S. Peng, B. Yue and C. Fan, Chem. Eng. J., 2015, 263, 419–426.
20. J. Cao, C. Zhou, H. Lin, B. Xu and S. Chen, Appl. Surf. Sci., 2013, 284, 263–269.
21. Y. Zuo, C. Wang, Y. Sun and J. Cheng, Mater. Lett., 2015, 139, 149–152.
22. Y. Liu, X. Yuan, H. Wang, X. Chen, S. Gu, Q. Jiang, Z. Wu, L. Jiang and G. Zeng, RSC Adv., 2015, 5, 33696–33704.
23. V. Sivakumar, R. Suresh, K. Giribabu and V. Narayanan, Solid State Sci., 2015, 39, 34–39.
24. P. Ju, H. Fan, B. Zhang, K. Shang, T. Liu, S. Ai and D. Zhang, Sep. Purif. Technol., 2013, 109, 107–110.
25. J. Ren, W. Wang, M. Shang, S. Sun, L. Zhang and J. Chang, J. Hazard. Mater., 2010, 183, 950–953.
26. W. Zhao, Y. Guo, Y. Faiz, W.-T. Yuan, C. Sun, S.-M. Wang, Y.-H. Deng, Y. Zhuang, Y. Li, X.-M. Wang, H. He and S.-G. Yang, Appl. Catal., B, 2015, 163, 288–297.
27. W. Zhao, Y. Guo, S. Wang, H. He, C. Sun and S. Yang, Appl. Catal., B, 2015, 165, 335–343.
28. W. Zhao, J. Li, Z. B. Wei, S. Wang, H. He, C. Sun and S. Yang, Appl. Catal., B, 2015, 179, 9–20.
29. X. Hu, C. Hu and J. Qu, Mater. Res. Bull., 2008, 43, 2986–2997.
30. X. Zhang, T. Guo, X. Wang, Y. Wang, C. Fan and H. Zhang, Appl. Catal., B, 2014, 150–151, 486–495.
31. H.-Y. Hao, Y.-Y. Xu, P. Liu and G.-Y. Zhang, Chin. Chem. Lett., 2015, 26, 133–136.
32. J. Cao, B. Xu, H. Lin, B. Luo and S. Chen, Catal. Commun., 2012, 26, 204–208.
33. F. Duo, Y. Wang, C. Fan, X. Mao, X. Zhang, Y. Wang and J. Liu, Mater. Charact., 2015, 99, 8–16.
34. H. Xu, H. Li, G. Sun, J. Xia, C. Wu, Z. Ye and Q. Zhang, Chem. Eng. J., 2010, 160, 33–41.
35. X. Lin, X. Guo, W. Shi, F. Guo, H. Zhai, Y. Yan and Q. Wang, Catal. Commun., 2015, 66, 67–72.
36. C. Huang, J. Hu, S. Cong, Z. Zhao and X. Qiu, Appl. Catal., B, 2015, 174–175, 105–112.
37. F. Dong, Y. Sun, M. Fu, Z. Wu and S. C. Lee, J. Hazard. Mater., 2012, 219–220, 26–34.
38. C. Belver, C. Adán, S. García-Rodríguez and M. Fernández-García, Chem. Eng. J., 2013, 224, 24–31.
39. L.-P. Zhu, G.-H. Liao, N.-C. Bing, L.-L. Wang, Y. Yang and H.-Y. Xie, CrystEngComm, 2010, 12, 3791.
40. F. Xie, X. Mao, C. Fan and Y. Wang, Mater. Sci. Semicond. Process., 2014, 27, 380–389.
41. G.-T. Pan, C.-M. Huang, P.-Y. Peng and T. C. K. Yang, Catal. Today, 2011, 164, 377–383.
42. Y. Mi, L. Wen, Z. Wang, D. Cao, Y. Fang and Y. Lei, Appl. Catal., B, 2015, 176–177, 331–337.
43. K. Zhang, C. Liu, F. Huang, C. Zheng and W. Wang, Appl. Catal., B, 2006, 68, 125–129.
44. X. Yu, L. Huang, Y. Wei, J. Zhang, Z. Zhao, W. Dai and B. Yao, Mater. Res. Bull., 2015, 64, 410–417.
45. X. Gao, X. Zhang, Y. Wang, S. Peng, B. Yue and C. Fan, Chem. Eng. J., 2015, 273, 156–165.
46. L. Xu, Y. Wei, W. Guo, Y. Guo and Y. Guo, Appl. Surf. Sci., 2015, 332, 682–693.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra21453j

---