Synthesis of Ag₂CO₃/Bi₂WO₆ heterojunctions with enhanced photocatalytic activity and cycling stability

Abstract

Hierarchical Bi₂WO₆ nanoarchitectures with a size of 2–3 μm were prepared via a facile microwave-assisted solution-phase reaction process. Monodisperse spherical Ag₂CO₃ nanoparticles with an average size of about 10 nm were deposited onto the surface of the Bi₂WO₆ nanoarchitectures to form a novel Ag₂CO₃/Bi₂WO₆ heterojunction structure through a facile in situ precipitation–deposition method. The obtained samples were characterized using XRD, XPS, SEM, TEM (HRTEM), UV-vis DRS and nitrogen adsorption–desorption techniques. The photocatalytic evaluation demonstrates that the decoration with Ag₂CO₃ nanoparticles significantly enhances the photocatalytic activity of Bi₂WO₆ and the photocatalytic performance is greatly influenced by the content of deposited Ag₂CO₃. The 30 wt% Ag₂CO₃-loaded Bi₂WO₆ sample exhibited the highest photocatalytic activity for the degradation of rhodamine B (RhB) under visible light irradiation. Meanwhile, it also possesses excellent cycling stability and superior photocatalytic performance toward other pollutants. The dramatically enhanced photocatalytic activity and stability can be mainly ascribed to well-matched energy bands and heterojunctions between Ag₂CO₃ and Bi₂WO₆, which can effectively improve the separation of photo-induced electron–hole pairs at the heterojunction interfaces.

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1. Introduction

Heterogeneous photocatalysts have attracted tremendous attention due to their great potential for decomposing organic pollutants and for converting photon energy into chemical energy.^1–4 Among the novel photocatalysts, Bi₂WO₆ has been proven to be a promising catalyst for photo-chemical reactions owing to its suitable bandgap (∼2.8 eV) and strong oxidizing power.^5–7 It is widely accepted that the photocatalytic performance of a catalyst is very closely related to its surface structure and crystallinity, including its morphology, size, specific surface area, crystal planes and defects.^8–10 Bi₂WO₆ as a layer-structured semiconductor, composed of corner sharing WO₆ octahedra (WO₄)²⁻ sheets and bismuth oxide (Bi₂O₂)²⁺ sheets,¹¹ could be more easily obtained with three-dimensional (3D) hierarchical architectures by assembling one-dimensional (1D) nanosheets. Many studies found that the 3D hierarchical architectures could effectively increase the specific surface to volume ratio, organic pollutant adsorption and surface active sites.^12,13

As is well known, in addition to the adsorption behavior and photoresponse range, for an ideal photocatalyst the separation efficiency and oxidation power of the photogenerated carriers are vital considerations. Nonetheless, the bare Bi₂WO₆ photocatalysts have a high recombination of photoinduced electron–hole (e⁻–h⁺) pairs which leads to low quantum efficiency.^14,15 Recently, numerous attempts have been made to prolong the lifetime of photogenerated e⁻–h⁺ and promote carrier transport processes of Bi₂WO₆, including impurity doping,^16,17 noble metal sensitization,^18–20 and heterojunction structure formation.^21–28

The coupling of two semiconducting structures with matching energy levels to form a heterojunction can effectively improve the absorption and utilization of visible light. More importantly, a nanojunction system can remarkably change the electrical properties of the composite semiconductors, resulting in the enhanced separation, transport and oxidation (or reduction) power of the photogenerated carriers.^29,30 Thus, the coupling of Bi₂WO₆ with other semiconductors has been widely employed to improve the photocatalytic performance of Bi₂WO₆. For example, AgBr/Bi₂WO₆, Ag₂S/Bi₂WO₆, BiOBr/Bi₂WO₆, TiO₂/Bi₂WO₆, Bi₂S₃/Bi₂WO₆ and g-C₃N₄/Bi₂WO₆ systems have been shown to be effective in suppressing the recombination of photogenerated e⁻–h⁺ pairs.^21–28

Recently, Ag₂CO₃ has been regarded as a new high efficiency visible light photocatalysis material. Compared with the well-known N-doped TiO₂, Ag₂CO₃ usually shows an excellent visible light utilization rate and excellent photocatalytic activity.^31,32 Although Ag₂CO₃ is unstable in its pure crystal form owing to its self-photocorrosion, previous studies suggested that Ag₂CO₃ could maintain its stability through the growth of heterostructured composites. Yu et al.³³ reported the formation of Ag₂O/Ag₂CO₃ heterostructured photocatalysts via phase transformation routes. This result indicates that the heterostructure interface effectively facilitates charge transfer and suppresses the recombination of photogenerated e⁻–h⁺, resulting in extremely high activity and stability. Similarly, the synthesis of Ag₂CO₃ composite photocatalysts have been reported, such as Ag₂CO₃/g-C₃N₄, Ag₂CO₃/AgX, Ag₂CO₃/Ag/AgBr, and their visible-light-driven photocatalytic activity and stability have also been explored.^34–36 Herein, we intend to design a novel Ag₂CO₃/Bi₂WO₆ heterojunction photocatalyst with matching energy levels. The aim is to achieve an improvement in the visible-light photocatalytic activity of Bi₂WO₆ and overcome the Ag₂CO₃ self-photocorrosion disadvantage.

Moreover, the microwave-assisted approach has been accepted as a promising method for the preparation of Bi₂WO₆.³⁷ Compared with traditional hydrothermal synthesis methods, microwave-assisted synthesis is simple, rapid, uniform, efficient, economical, and environmentally friendly.³⁸ Herein, in this work, we firstly prepared the Bi₂WO₆ microspheres using a facile microwave-assisted solution-phase reaction process. Then, monodisperse Ag₂CO₃ nanoparticles were deposited onto the surface of the Bi₂WO₆ nanoarchitectures to form a novel Ag₂CO₃/Bi₂WO₆ heterojunction. As expected, the as-prepared photocatalysts exhibited excellent photocatalytic activity and outstanding cycling performance for the degradation of rhodamine B (RhB) dye under visible light irradiation. Meanwhile, they also possess extremely high photocatalytic activities for other pollutants such as methyl orange (MO), methyl blue (MB), Congo red (CR), and hexavalent chromium (Cr(VI)). Furthermore, the possible mechanism of the enhanced photocatalytic activity and stability for the Ag₂CO₃/Bi₂WO₆ heterojunctions was also discussed in detail.

2. Experiment section

2.1 Catalyst synthesis

All the chemicals were analytical grade purity and used without further treatment. The hierarchical Bi₂WO₆ microspheres were prepared using a MARS 5 Microwave System (Matthews, NC, USA). In the system, the parameters of time and temperature of reaction were regulated by a user-defined computer program. The automatic temperature-control system allowed continuous monitoring and control of the internal temperature of the reaction system by continuously adjusting the applied microwave power. In a typical preparation procedure, Bi(NO₃)₃·5H₂O (3.5 mmol) was dissolved in 5 mL of acetic acid (36 wt%) to form a clear solution with magnetic stirring, then 30 mL of an aqueous solution of Na₂WO₄·2H₂O (2 mmol) was added dropwise to the obtained solution. The mixture was allowed to stir for another 1 h and was then transferred to a 50 mL microwave (MW) Teflon container and kept at 200 °C for 6 h.

For the synthesis of the Ag₂CO₃/Bi₂WO₆ composites, the as-synthesized Bi₂WO₆ (0.5 mmol) powders were ultrasonically dispersed into 20 mL of deionized water. Subsequently, AgNO₃ solution (0.05 M) and NH₃·H₂O solution (0.1 M) were added dropwise to the above suspension in sequence under magnetic stirring. Finally, NaHCO₃ solution (0.05 M) was added into the mixture at a rate of less than 0.3 mL min⁻¹. The precipitate was collected by centrifugation, washed with deionized water and ethanol, and dried at 60 °C for 12 h. The Ag₂CO₃/Bi₂WO₆ composites with different mass ratios were fabricated by changing the added amount of the AgNO₃, NH₃·H₂O and NaHCO₃ solutions under the AgNO₃/NH₃·H₂O/NaHCO₃ volume ratios of 1 : 1 : 2. The samples are denoted as A-BWO-1, A-BWO-2, A-BWO-3 and A-BWO-4 when the weight percentages of Ag₂CO₃ in the Ag₂CO₃/Bi₂WO₆ composites are 10%, 20%, 30% and 40%, respectively. Pure Ag₂CO₃ was prepared by the same method without the addition of Bi₂WO₆. Moreover, N-doped TiO₂ was obtained according to the literature procedure.³⁹

2.2 Characterization

The X-ray diffraction (XRD) data were recorded on a Bruker D8 X-ray diffractometer with a Cu-Kα radiation source (λ = 1.5406 Å). Scanning electron microscopy (SEM) images were taken using a JSM-6701F scanning electron microscope with an accelerating voltage of 15 kV. Energy Dispersive X-ray (EDX) analysis was also performed on a JSM-6701F. Transmission electron microscopy (TEM) images were obtained on a JEM-2100 electron microscope at an accelerating voltage of 200 kV. The X-ray photoelectron spectroscopy (XPS) analysis was carried out using an AXIS-ULTRA DLD-600W photoelectron spectrometer with Al K1 radiation. The UV-vis diffuse reflectance spectroscopy (DRS) was carried out using a Hitachi U-4100 spectrophotometer. Nitrogen adsorption–desorption isotherms were collected on an Autosorb-iQ sorption analyzer and analyzed using the Brunauer–Emmett–Teller (BET) equation and Barret–Joyner–Halenda (BJH) model.

2.3 Photocatalytic activity test

The photocatalytic performance of the as-prepared samples was evaluated by measuring the degradation of RhB. In a typical photocatalytic measurement, the samples (0.1 g) were put into a solution of RhB dye (100 mL, 10 mg L⁻¹), which was then irradiated with a 300 W Xe arc lamp to provide visible light with λ ≥ 420 nm by a cutoff filter. Before the suspensions were irradiated they were magnetically stirred for 60 min in the dark to complete the adsorption–desorption equilibrium between the dye and the photocatalyst. The degradation results were analyzed using a UV-vis spectrophotometer (UV-2550, Shimadzu, Japan). Before the spectroscopy measurements, these photocatalysts were removed from the photocatalytic reaction system by centrifugation. The same test for RhB was also carried out on MB, MO and CR. The Cr(VI) was tested using the diphenylcarbazide (DPC) method introduced by Clesceri et al.⁴⁰

3. Results and discussion

3.1 Structure and property analysis

The structural characteristics and phase composition of the samples were investigated using powder XRD. As shown in Fig. 1, the diffraction peaks of the pure Bi₂WO₆ and Ag₂CO₃ samples match well with the standard crystalline structure of the orthorhombic phase of Bi₂WO₆ (JCPDS no. 73-1126) and the monoclinic phase of Ag₂CO₃ (JCPDS no. 70-2184), respectively. For the Ag₂CO₃/Bi₂WO₆ heterojunction samples, except for all the diffraction peaks of Bi₂WO₆, the peaks at 2θ of 18.6°, 20.5°, 32.6°, 33.6°, 37.1°, 39.6°, 44.3°, and 51.3° are attributed to the pure monoclinic phase of Ag₂CO₃. Meanwhile, the intensities of the corresponding diffraction peaks of Ag₂CO₃ strengthened gradually along with the increasing Ag₂CO₃ content in the Ag₂CO₃/Bi₂WO₆ composites. Metallic Ag or impurity peaks were not detected in any of the samples, indicating that the Ag₂CO₃/Bi₂WO₆ composites are only composed of Ag₂CO₃ and Bi₂WO₆ phases.

Fig. 1 XRD patterns of the pure Bi₂WO₆, Ag₂CO₃ and Ag₂CO₃/Bi₂WO₆ composite photocatalysts with different Ag₂CO₃ content.

The elemental composition and oxidation states of the as-prepared samples were further analyzed using XPS. Fig. 2a shows the XPS survey spectra of pure Bi₂WO₆, Ag₂CO₃ and the A-BWO-3 composite. The overview spectrum of the composite demonstrates that Bi, W, O, C and Ag exist, further confirming that the sample was composed of Bi₂WO₆ and Ag₂CO₃. Fig. 2b–f show the high-resolution spectra of the Bi 4f, W 4f, O 1s, C 1s and Ag 3d regions for the A-BWO-3 sample, respectively. The peaks at binding energies (E_b) of 163.3 and 158.0 eV (Fig. 2b) belong to Bi 4f_5/2 and Bi 4f_7/2 of the Bi³⁺ ions,⁴¹ which are shifted toward lower binding energies compared with that of pure Bi₂WO₆ (164.7 and 159.4 eV, Fig. S2†). This appearance was also found in the XPS spectra of W 4f and O 1s. Such results could be ascribed to the interaction between Bi₂WO₆ and Ag₂CO₃ resulting in an inner shift of Bi 4f, W 4f and O 1s orbitals.⁴² The W 4f_5/2 and W 4f_7/2 peaks (Fig. 2c) are located at 36.3 and 34.2 eV with a spin–orbital separation of 2.1 eV, which corresponds to W⁶⁺ according to previous reports.⁴³ The wide and asymmetric XPS of O 1s peaks (Fig. 2d) can be deconvoluted into four peaks, the peaks at E_b of 528.9, 529.7 and 530.5 eV correspond to Bi–O, W–O and the crystal lattice oxygen of Ag₂CO₃, respectively.^44,45 The weak peak at 531.7 eV corresponds to surface absorbed oxygen species.⁴⁶ In Fig. 2e, the C 1s spectrum was deconvoluted into three peaks: the peak at E_b of 283.6 eV could be attributed to the carbon element of Ag₂CO₃, which is similar to what was reported by Dong et al.,⁴⁷ and the other two peaks were mainly ascribed to the adventitious hydrocarbon from the XPS itself. Fig. 2f gives the high-resolution XPS spectrum of Ag 3d. The peaks at E_b of 366.9 and 372.9 eV correspond to the Ag 3d_5/2 and Ag 3d_3/2 of Ag⁺, respectively.²² This clearly suggests that no Ag metal is formed, which is in accordance with the results of the XRD analysis. Therefore, by combining the XPS and XRD investigations, the results confirmed that there were both Bi₂WO₆ and Ag₂CO₃ species in the Ag₂CO₃/Bi₂WO₆ composite samples.

Fig. 2 XPS survey spectra (a) and high-resolution XPS spectra of Bi 4f (b), W 4f (c) O 1s (d), C 1s (e) and Ag 3d (f) regions for the A-BWO-3 composite.

The morphology of the as-synthesized samples was examined using SEM, as shown in Fig. 3. It can be clearly seen that bare Bi₂WO₆ is uniformly dispersed and presents 3D hierarchical architectures with a diameter of 2–3 μm (Fig. 3a). When Ag₂CO₃ is deposited onto the 3D hierarchical Bi₂WO₆ microspheres via a facile precipitation–deposition process (Fig. 3b–e), the resulting Ag₂CO₃/Bi₂WO₆ composite exhibits a similar morphology and size compared to that of bare Bi₂WO₆. With increasing Ag₂CO₃ content, it is easy to observe that a large amount Ag₂CO₃ nanoparticles were dispersed uniformly onto the surface of the hierarchical Bi₂WO₆ microspheres. Furthermore, the EDX spectrum of the A-BWO-3 composite is shown in Fig. 3f. The composite contains only Bi, W, O, C and Ag elements except for elements of Au from the supports, which consists well with the XRD and XPS results.

Fig. 3 SEM images of bare Bi₂WO₆ (a), A-BWO-1 (b), A-BWO-2 (c), A-BWO-3 (c) and A-BWO-4 (d) composite photocatalysts; EDX spectrum of the A-BWO-3 composite photocatalyst (f).

Further information regarding the microstructure of the prepared samples was obtained from the TEM and HRTEM images. Fig. 4a and b show TEM images of bare Bi₂WO₆ with different magnifications. It is easy to observe that the hierarchical Bi₂WO₆ microspheres have a zigzag brim, which is in fact assembled from several laminar Bi₂WO₆ nanoplates with a quadrate shape. Specifically, each quadrate shaped nanoplate has a relatively smooth surface with an average length of about 60 nm and a width of about 40 nm. Fig. 4c and d show the TEM images of the A-BWO-3 composite. The locations of the Ag₂CO₃ nanoparticles on the surface of the Bi₂WO₆ nanoplates are indicated by arrows in the TEM image (Fig. 4d). It shows that some monodisperse spherical nanoparticles with a size of about 10 nm cover the surface of the Bi₂WO₆ nanoplates. Fig. 4e shows the corresponding selected area electron diffraction (SAED) pattern of the composite, which shows several diffraction rings, indicating that the nanoplates are polycrystalline in structure. The diffraction fringes can be indexed to the (113), (006), (206) and (313) planes for the orthorhombic crystal structure of Bi₂WO₆, which is in accordance with the XRD results. The HRTEM image (Fig. 4f) shows that the Ag₂CO₃ nanoparticles tightly adhere to the surface of the Bi₂WO₆ nanoplates. The lattice fringe spacings of 0.315 and 0.266 nm correspond to the (113) plane of orthorhombic Bi₂WO₆ and the (130) plane of the monoclinic Ag₂CO₃, respectively. Thus, the above results indicate the formation of heterojunctions between Ag₂CO₃ and Bi₂WO₆.

Fig. 4 TEM images of bare Bi₂WO₆ (a and b) and the A-BWO-3 heterojunction photocatalyst (c and d); SAED and HRTEM images of the A-BWO-3 heterojunction photocatalyst (e and f).

Fig. 5 shows the fabrication method for the Ag₂CO₃/Bi₂WO₆ heterojunction photocatalysts via a three-step process. Firstly, the Bi₂WO₆ hierarchical microspheres are obtained by a facile microwave-assisted solution-phase reaction process. Afterwards, the AgNO₃ and NH₃·H₂O solutions were added to the Bi₂WO₆ suspension. The formed Ag(NH₂)²⁺ can be bound tightly to the surface of the hierarchical Bi₂WO₆ microspheres due to chemical adsorption. After that, with the addition NaHCO₃, the Ag(NH₂)²⁺ ions attached to the surface of Bi₂WO₆ react with HCO₃⁻ to generate Ag₂CO₃ nanoparticles. Furthermore, added NH₃·H₂O is an important influencing factor for the regulation of the Ag₂CO₃ grain size and formation of nano-heterojunctions. In the absence of NH₃·H₂O, a large number of short nanorods of Ag₂CO₃ exist on the brim of the Bi₂WO₆ nanoplates (Fig. S3†).

Fig. 5 Schematic diagram of the fabrication of the Ag₂CO₃/Bi₂WO₆ heterojunctions.

Fig. 6 displays the nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curves of the as-prepared Bi₂WO₆ and A-BWO-3 heterojunction photocatalysts. The isotherm of the samples can be categorized as a type IV isotherm and an H3-type hysteresis loop (0.75 < P/P₀ < 0.99), suggesting the presence of mesoporous structures with slit-like pores in these two samples.⁴⁸ According to the fitting analysis with the BET equation, the surface area of the hierarchical Bi₂WO₆ microspheres is about 19.2 m² g⁻¹. After decoration with the Ag₂CO₃ nanoparticles, the heterojunction photocatalysts still possess a relatively large surface area of 16.8 m² g⁻¹. Furthermore, the main pore size distribution (inset of Fig. 6) in these two samples is similarly in the range of 10–15 nm, thus implying the Ag₂CO₃ nanoparticles only covered the surface of Bi₂WO₆ and the pore size was retained well.

Fig. 6 Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curves (inset) of the pure Bi₂WO₆ and the A-BWO-3 heterojunction photocatalysts.

The light absorption properties of the as-prepared samples were investigated using UV-visible diffuse reflectance spectroscopy. As shown in Fig. 7, the absorption spectrum of pure Bi₂WO₆ extends from the UV region to visible light at about 450 nm. Compared with pure Bi₂WO₆, the Ag₂CO₃/Bi₂WO₆ heterojunctions show a significant redshift of the absorption edge and display surprisingly strong absorption around 450–800 nm. This observation clearly indicates that the Ag₂CO₃/Bi₂WO₆ heterojunctions show more intense absorption within the visible light range. This could be extremely advantageous for the degradation of pollutants owing to the light harvesting ability having a great impact on the photocatalytic reaction process. Furthermore, it could be seen that the improvement in the visible light absorption intensity is inconspicuous with increasing Ag₂CO₃ content. It might be because the excess Ag₂CO₃ will impede the light coming into contact with the Bi₂WO₆ crystal and will inhibit the interfacial transport of electron–hole pairs between Ag₂CO₃ and Bi₂WO₆.⁴⁹ In addition, the optical band gap was calculated following the equation:⁵⁰

αhν = A(hν − E_g)^n/2

where α, h, ν, A and E_g are the absorption coefficient, Plancks constant, frequency, proportionality constant and bandgap, respectively, and the n value is determined by the type of transition (the n value is 4 for Ag₂CO₃ and Bi₂WO₆ with indirect transition).^19,33,51 A plot of (αhν)^1/2 versus hνgives the bandgap as shown in the inset of Fig. 7. The bandgaps of Ag₂CO₃ and Bi₂WO₆ estimated from the intercept of the tangent to the plot are 2.17 and 2.74 eV, respectively. The bandgap of as-prepared pure Ag₂CO₃ is lower than that of the Ag₂CO₃ crystals (∼2.30 eV) obtained by the reaction of NaHCO₃ with AgNO₃.^33,34 This could be attributed to the different preparation method and phase composition of Ag₂CO₃.

Fig. 7 UV-vis diffuse reflectance spectra of the as-prepared samples and the bandgap (inset) of pure Bi₂WO₆ and pure Ag₂CO₃.

3.2 Photocatalytic activity

The photocatalytic performance of the as-prepared samples was evaluated by measuring the degradation of RhB and MO under visible light irradiation. For comparison, the respective performances of P25 and prepared N-doped TiO₂ (N-TiO₂) photocatalysts were also presented and the results are given in Fig. 8. As shown in Fig. 8a, RhB can only be slightly degraded under visible light irradiation without a catalyst, indicating that the self-degradation effect of RhB could be ignored. Compared to pure Ag₂CO₃ and Bi₂WO₆, the Ag₂CO₃/Bi₂WO₆ heterojunctions can effectively improve the photocatalytic activities. The highest activity is observed for the A-BWO-3 sample, and the degradation rate of RhB can exceed 95% within 60 min. Additionally, it is noted that the A-BWO-3 heterojunction photocatalyst exhibits remarkably superior photocatalytic activity under the same conditions compared to P25 and N-TiO₂. Fig. 8b shows the changes in the absorption spectrum of the RhB solution with the temporal evolution. The major absorption peak of RhB around 553 nm diminished gradually with the increasing visible light irradiation time in the presence of A-BWO-3. Meanwhile, the suspension loses color gradually, which further indicates that the structure of RhB has been destroyed. The pseudo-first-order reaction kinetics of the photocatalytic degradation of the RhB solution was investigated and the results are given in Fig. S4† and Table 1. The rate constant k is 0.0489 min⁻¹ for the A-BWO-3 heterojunction photocatalyst, which is much higher than those of P25 (0.0014 min⁻¹), N-TiO₂ (0.0124 min⁻¹), pure Ag₂CO₃ (0.0195 min⁻¹) and pure Bi₂WO₆ (0.0272 min⁻¹). These results indicated that RhB could be degraded more efficiently by the A-BWO-3 heterojunction photocatalyst.

Fig. 8 Photocatalytic activity of the as-prepared samples and the adsorption spectra in the presence of A-BWO-3 for the degradation of RhB (a and b) and MO (c and d) under visible light irradiation.

Table 1 Pseudo-first-order rate constants in the photocatalytic degradation of RhB (k₁) and MO (k₂)

Sample	k1 (min^−1)	k2 (min^−1)
Pure-Ag2CO3	0.0195	0.0132
Pure-Bi2WO6	0.0272	0.0005
A-BWO-1	0.0297	0.0110
A-BWO-2	0.0343	0.0342
A-BWO-3	0.0489	0.0598
A-BWO-4	0.0423	0.0532
Degussa P25	0.0014	0.0017
Prepared N-TiO2	0.0124	0.0108

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Furthermore, MO was also selected as the other typical target compound to further evaluate the photocatalytic activity of the as-prepared samples. As shown in Fig. 8c and d, A-BWO-3 decomposed about 95.2% of MO after 50 min under visible light irradiation. For comparison, only 4.7%, 13.2%, 43.1%, and 54.8% of MO is removed under the same conditions using pure Bi₂WO₆, P25, N-TiO₂ and pure Ag₂CO₃, respectively. The photodegradation process of MO was found to follow pseudo-first-order kinetics and the rate constant for the A-BWO-3 heterojunction photocatalyst is higher than the sum of pure Bi₂WO₆ and pure Ag₂CO₃ (Fig. S4† and Table 1). The photocatalytic decomposition rate constant of A-BWO-3 for MO is 35 and 5 times greater than those of P25 and N-TiO₂, respectively. Moreover, the data, shown in Table 1, revealed that the reactivity of the heterojunction photocatalysts for the decomposition of RhB is more related to Ag₂CO₃ content. It is considered that the synergistic effect between Bi₂WO₆ and Ag₂CO₃ plays a dominant role in the reaction process. Whereas, for the degradation of MO, the significantly enhanced photocatalytic activity of Bi₂WO₆ is mainly due to the modification effect of Ag₂CO₃ on Bi₂WO₆. It can thus be seen that the actual active component of the heterojunction photocatalysts for the degradation of RhB and MO is different. At the same time, the experimental data showed that the photocatalytic activity of Ag₂CO₃/Bi₂WO₆ enhanced remarkably with increasing Ag₂CO₃ content. Nevertheless, when the Ag₂CO₃ content exceeds 30 wt%, the photocatalytic activity decreases, suggesting that the optimal Ag₂CO₃ content in Ag₂CO₃/Bi₂WO₆ is 30 wt%. The optimum content of Ag₂CO₃ in the heterojunction should be related to the recombination rate of photogenerated e⁻–h⁺ pairs and light absorption efficiency. A similar phenomenon was also observed for other systems such as Ag₂CO₃/g-C₃N₄³⁴ and AgBr/Ag₂CO₃.⁵²

To further investigate the validity of the photocatalytic activity of the A-BWO-3 heterojunction photocatalyst, the degradation experiments of A-BWO-3 for other pollutants such as MB, CR, and Cr(VI) were performed. As shown in Fig. 9, the A-BWO-3 heterojunction photocatalyst could remove more than 95% of these pollutants after 60 min. All these results convincingly demonstrate that the formation of a heterogeneous interface in Ag₂CO₃/Bi₂WO₆ can effectively promote the photoactivity of pure Bi₂WO₆.

Fig. 9 Photocatalytic activity of A-BWO-3 for degradation of MB, CR, Cr(VI), MO and RhB after 60 min visible light irradiation.

3.3 Catalyst stability

In addition to the photocatalytic activity, the stability of a catalyst is a vital consideration for practical applications. Thus, the stability of pure Ag₂CO₃ and A-BWO-3 was investigated. As shown in Fig. 10, it was found that the Ag₂CO₃ nanoparticles almost lose their activity after two recycles. This result can be attributed to the reduction of large amounts of Ag⁺ ions in the Ag₂CO₃ lattice by the photogenerated electrons (Ag₂CO₃ + 2e⁻ → 2Ag⁰ + CO₃²⁻) during the recycle reaction.^31,32 The excess Ag could act as the recombination center of electrons and prevent the visible light absorption of Ag₂CO₃, leading to the photocatalytic activity being extremely lower. Conversely, A-BWO-3 still maintains an 85% degradation rate after five recycles for RhB. Fig. S5 and S6† show the XRD and XPS patterns of the freshly obtained A-BWO-3 and its recovered samples after two and five cycles of photocatalysis. It is found that there is a slight change in the position and intensity of the peaks after five cycles of the photodegradation reaction, demonstrating that Ag₂CO₃ remains stable and only a relatively small number of Ag⁰ nanoparticles were formed on the surface of Ag₂CO₃.

Fig. 10 The recycling tests of the as-prepared Ag₂CO₃ and A-BWO-3 heterojunction photocatalyst for the photodegradation of RhB.

3.4 Possible photocatalytic mechanism

To confirm the main active species in the photocatalytic process, we performed in situ trapping experiments of free radicals and holes for the degradation of RhB by the A-BWO-3 heterojunction system. As shown in Fig. 11, disodium ethylenediaminetetraacetate (EDTA, a quencher of h⁺), 1,4-benzoquinone (BQ, a quencher of ˙O²⁻) and isopropanol (IPA, a ˙OH scavenger) were added into the reaction liquid. We observed that the addition of 2 mM EDTA had a significant effect on the k_app compared with no scavenger under the same conditions, suggesting that h⁺ may play a dominant role in the photocatalytic reaction process of RhB degradation. Whereas, the addition of 2 mM IPA or BQ had a relatively lesser influence on the k_app, meaning that ˙OH and ˙O²⁻ played a secondary role and, to a lesser extent, participated in the photocatalytic reaction process.

Fig. 11 Comparison of the rate constant k_app for the photocatalytic degradation of RhB by A-BWO-3 in the presence of various scavengers, the concentration of all added scavengers is 2 mM.

For the hierarchical Ag₂CO₃/Bi₂WO₆ heterojunctions, the enhancement mechanism of photocatalytic activity could be further revealed by the energy band matching principle for two semiconductors. The top of the valence band (VB) and the bottom of the conduction band (CB) of Ag₂CO₃ were calculated to be 2.60 and 0.43 eV (vs. NHE), respectively, and the VB and CB of Bi₂WO₆ were estimated to be 3.23 and 0.49 eV (vs. NHE), respectively (some relevant details of the energy band estimation are provided in the ESI†). It was widely accepted that the excellent photocatalytic activity results from efficient separation and smooth transport of the photogenerated electrons and holes, a schematic illustration of the possible heterojunction enhancement mechanism for photocatalytic activity in Ag₂CO₃/Bi₂WO₆ is shown in Fig. 12. It can be seen that an electron is excited from the valence band to the conduction band in both Ag₂CO₃ and Bi₂WO₆, thereby generating an electron–hole pair under visible light irradiation. Once heterogeneous interfaces are formed between Bi₂WO₆ and Ag₂CO₃, the photogenerated electrons on the Ag₂CO₃ nanoparticles will easily transfer to the CB of Bi₂WO₆ through the closely contacted interfaces, as the conduction band of Ag₂CO₃ has a more negative potential than that of Bi₂WO₆. Meanwhile the photoinduced holes could migrate from the VB of Bi₂WO₆ to that of Ag₂CO₃. Therefore, the photoexcited electrons and holes are separated effectively due to the ingenious heterojunction design under visible light irradiation. Meanwhile, the photocorrosion of the Ag₂CO₃ nanoparticles can be further restrained because photoinduced electrons will fast transfer to the CB of Bi₂WO₆ rather than react with the Ag⁺ ions of Ag₂CO₃ (Ag⁺ + e⁻ → Ag⁰). The captured electrons on the CB of Bi₂WO₆ can react with dissolved oxygen to give ˙O²⁻,⁵³ and the holes with strong oxidizing power in the VB of Ag₂CO₃ can react with H₂O to generate ˙OH or directly oxidize the pollutants adsorbed.^36,54 Based on the above results, we believe that the well-matched bands of the Ag₂CO₃/Bi₂WO₆ heterojunctions can promote interfacial carrier transfer processes and prolong the lifetime of the photogenerated holes and electrons, resulting in high photocatalytic activity and stability.

Fig. 12 Schematic illustration of the photocatalytic reaction process and the possible charge carrier transfer of the Ag₂CO₃/Bi₂WO₆ heterojunctions under visible light irradiation.

In addition, after two cycles of the photodegradation reaction, a small number of Ag⁰ nanoparticles were formed on the surface of Ag₂CO₃ (Fig. S5 and S6†), which can act as an electron-rich collective.³² These enriched electrons on the Ag⁰ nanoparticles will participate in the multi-electron transfer routes (O₂ + 2e⁻ + 2H⁺ = H₂O₂(aq.); O₂ + 4e⁻ + 4H⁺ = 2H₂O(aq.)).⁵⁵ While Ag⁰ may also become a recombination center of e⁻–h⁺ pairs,⁵⁶ too many Ag⁰ nanoparticles on the Ag₂CO₃ surface could prevent the visible light absorption of Ag₂CO₃,³⁴ which would lead to the decrease of activity in the recycling reactions. With increasing the number of recycle tests, a relatively stable Ag–Ag₂CO₃ system can be formed. The self-stability mechanism has also been discussed in some other silver-containing compounds.^57,58 Therefore, the Ag–Ag₂CO₃/Bi₂WO₆ system may be a vital factor for achieving high stability in the photocatalytic reaction process.

4. Conclusion

In summary, a novel hierarchical Ag₂CO₃/Bi₂WO₆ heterojunction catalyst has been successfully fabricated by in situ depositing monodisperse spherical Ag₂CO₃ nanoparticles onto the surface of Bi₂WO₆. The decoration with Ag₂CO₃ nanoparticles significantly enhanced the photocatalytic activity of Bi₂WO₆ and the 30 wt% Ag₂CO₃ content sample exhibited the highest photocatalytic activity for the degradation of RhB and MO. Furthermore, Ag₂CO₃/Bi₂WO₆ heterojunctions have excellent cycling stability. The enhanced photocatalytic activity and stability is not only attributed to the narrow bandgap of Ag₂CO₃ and the extended optical response, but also benefits from the appropriate band structure matching that enhances the photo-generated carrier separation in the heterogeneous interface between Ag₂CO₃ and Bi₂WO₆. Consequently, we believe that such a heterojunction system is promising for extension to other silver-containing heterostructured photocatalysts with highly efficient photocatalytic performances.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51264015), Yunnan Provincial Science and Technology Innovation Talents scheme – Technological Leading Talent of China (No. 2013HA002), The Applied Basic Research Fund Project of Yunnan Province & Kunming University of Science and Technology of China (No. 14118930), as well as the Kunming Scientific and Technological Project of China (No. 2014-04-A-H-02-3085).

References

1. A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253–278.
2. S. Y. Dong, J. L. Feng, M. H. Fan, Y. Q. Pi, L. M. Hu, X. Han, M. L. Liu, J. Y. Sun and J. H. Sun, RSC Adv., 2015, 5, 14610–14630.
3. M. Anpo and M. Takeuchi, J. Catal., 2003, 216, 505–516.
4. J. Tian, Y. H. Sang, G. W. Yu, H. D. Jiang, X. N. Mu and H. Liu, Adv. Mater., 2013, 25, 5075–5080.
5. A. Kudo and S. Hijii, Chem. Lett., 1999, 1103–1104.
6. L. W. Zhang, Y. J. Wang, H. Y. Cheng, W. Q. Yao and Y. F. Zhu, Adv. Mater., 2009, 21, 1286–1290.
7. H. F. Cheng, B. B. Huang, Y. Y. Liu, Z. Y. Wang, X. Y. Qin, X. Y. Zhang and Y. Dai, Chem. Commun., 2012, 48, 9729–9731.
8. H. L. Xu, W. Z. Wang and W. Zhu, J. Phys. Chem. B, 2006, 110, 13829–13834.
9. H. Wang, J. Gao, T. Q. Guo, L. Guo and J. H. Li, Chem. Commun., 2011, 48, 275–277.
10. D. S. Kim and S. Y. Kwak, Appl. Catal., A, 2007, 323, 110–118.
11. F. Amano, K. Nogami and B. Ohtani, J. Phys. Chem. C, 2009, 113, 1536–1542.
12. Z. Chen, L. W. Qian, J. Zhu, Y. P. Yuan and X. F. Qian, CrystEngComm, 2010, 12, 2100–2106.
13. F. Cao, W. D. Shi, L. J. Zhao, S. Y. Song, J. H. Yang, Y. Q. Lei and H. J. Zhang, J. Phys. Chem. C, 2008, 112, 17095–17101.
14. J. Ren, W. Z. Wang, S. M. Sun, L. Zhang and J. Chang, Appl. Catal., B, 2009, 92, 50–55.
15. C. M. Liu, J. W. Liu, G. Y. Zhang, J. B. Zhang, Q. S. Wu, Y. Y. Xu and Y. Q. Sun, RSC Adv., 2015, 5, 32333–32342.
16. M. Shang, W. Z. Wang, L. Zhang and H. L. Xu, Mater. Chem. Phys., 2010, 120, 155–159.
17. S. Guo, X. F. Li, H. Q. Wang, F. Dong and Z. B. Wu, J. Colloid Interface Sci., 2012, 369(5), 373–380.
18. T. L. Thompson and J. T. Yates, Chem. Rev., 2006, 106, 4428–4453.
19. D. J. Wang, G. G. Xue, Y. Z. Zhen, F. Fu and D. S. Li, J. Mater. Chem., 2012, 22, 4751–4758.
20. J. Yang, X. H. Wang, Y. M. Chen, J. Dai and S. H. Sun, RSC Adv., 2015, 5, 9771–9782.
21. D. J. Wang, L. Guo, Y. Z. Zhen, L. L. Yue, G. G. Xue and F. Fu, J. Mater. Chem. A, 2014, 2, 11716–11727.
22. O. Mehraj, B. M. Pirzada, N. A. Mir, S. Suitana and S. Sabir, RSC Adv., 2015, 5, 42910–42912.
23. Y. L. Li, Y. M. Liu, J. S. Wang, E. Uchaker, Q. F. Zhang, S. B. Sun, Y. N. Huang, J. Y. Li and G. Z. Cao, J. Mater. Chem. A, 2013, 27, 7949–7956.
24. Y. F. Hou, S. J. Liu, J. H. Zhang, X. Cheng and Y. Wang, Dalton Trans., 2014, 43, 1025–1031.
25. M. Shang, W. Z. Wang, L. Zhang, S. M. Sun, L. Wang and L. Zhou, J. Phys. Chem. C, 2009, 113, 14727–14731.
26. Z. J. Zhang, W. Z. Wang, L. Wang and S. M. Sun, ACS Appl. Mater. Interfaces, 2012, 4, 593–597.
27. Y. J. Wang, X. J. Bai, C. G. Pan, J. He and Y. F. Zhu, J. Mater. Chem., 2012, 22, 11568–11573.
28. Y. L. Tian, B. B. Chang, J. L. Lu, J. Fu, F. N. Xi and X. P. Dong, ACS Appl. Mater. Interfaces, 2013, 5, 7079–7085.
29. X. F. Gao, W. T. Sun, G. Ai and L. M. Peng, Appl. Phys. Lett., 2010, 96, 153104.
30. H. F. Cheng, B. B. Huang, P. Wang, Z. Y. Wang, Z. Z. Lou, J. P. Wang, X. Y. Qin, X. Y. Zhang and Y. Dai, Chem. Commun., 2011, 47, 7054–7056.
31. S. H. Guo, J. X. Bao, T. Hu, L. B. Zhang, L. Yang, J. H. Peng and C. Y. Jiang, Nanoscale Res. Lett., 2015 DOI:10.1186/s11671-015-0892-5.
32. G. P. Dai, J. G. Yu and G. Liu, J. Phys. Chem. C, 2012, 116, 15519–15524.
33. C. L. Yu, G. Li, S. Kumar, K. Yang and R. C. Jin, Adv. Mater., 2014, 26, 892–898.
34. Y. F. Li, L. Fang, R. X. Jin, Y. Yang, X. Fang, Y. Xing and S. Y. Song, Nanoscale, 2015, 7, 758–764.
35. H. J. Dong, G. Chen, J. X. Sun, Y. J. Feng, C. M. Li, G. H. Xiong and C. D. Lv, Dalton Trans., 2014, 43, 7282–7289.
36. J. J. Li, Y. L. Xie, Y. J. Zhong and Y. Hu, J. Mater. Chem. A, 2015, 3, 5474–5481.
37. X. F. Cao, L. Zhang, X. T. Chen and Z. L. Xue, CrystEngComm, 2010, 13, 306–311.
38. L. K. Pan, X. J. Liu, Z. Sun and C. Q. Sun, J. Mater. Chem. A, 2013, 1, 8299–8326.
39. B. Chi, L. Zhao and T. Jin, J. Phys. Chem. C, 2007, 111, 6189–6193.
40. L. S. Clesceri, A. E. Greenberg and A. D. Eaton, Standard methods for the examination of water and wastewater, American Public Health Association, Washington, 1998, pp. 366–368.
41. J. Wu, F. Duan, Y. Zheng and Y. Xie, J. Phys. Chem. C, 2007, 111, 12866–12871.
42. T. Yan, Q. Yan, X. D. Wang, H. Y. Liu, M. M. Li, S. X. Lu, W. G. Xu and M. Sun, Dalton Trans., 2015, 44, 1601–1611.
43. H. P. Li, W. G. Hou, X. T. Tao and N. Du, Appl. Catal., B, 2015, 172–173, 27–36.
44. W. J. Yang, B. Ma, W. C. Wang, Y. W. Wen, D. W. Zeng and B. Shan, Phys. Chem. Chem. Phys., 2013, 15, 19387–19394.
45. C. Dong, K. L. Wu, X. W. Wei, X. Z. Li, L. Liu, T. H. Ding, J. Wang and Y. Ye, CrystEngComm, 2014, 16, 730–736.
46. D. J. Wang, Y. Z. Zhen, G. L. Xue, F. Fu, X. M. Liu and D. S. Li, J. Mater. Chem. C, 2013, 1, 4153–4162.
47. H. J. Dong, G. Chen, J. X. Sun, C. M. Li, Y. G. Yu and D. H. Chen, Appl. Catal., B, 2013, 134–135, 46–54.
48. T. S. Natarajan, H. C. Bajaj and R. J. Tayade, CrystEngComm, 2014, 17, 1037–1049.
49. J. X. Xia, J. Di, S. Yin, H. Xu, J. Zhang, Y. G. Xu, L. Xu, H. M. Li and M. X. Ji, RSC Adv., 2014, 4, 82–90.
50. M. A. Butler, J. Appl. Phys., 1977, 48, 1914–1920.
51. C. M. Li, G. Chen, J. X. Sun, Y. J. Feng, J. J. Liu and H. J. Dong, Appl. Catal., B, 2015, 163, 415–423.
52. H. Xu, J. X. Zhu, Y. X. Song, W. K. Zhao, Y. G. Xu, Y. H. Song, H. Y. Ji and H. M. Li, RSC Adv., 2014, 4, 9139–9147.
53. W. J. Yang, B. Ma, W. C. Wang, Y. W. Wen, D. W. Zeng and B. Shan, Phys. Chem. Chem. Phys., 2013, 15, 19387–19394.
54. L. S. Zhang, K. H. Wong, H. Y. Yip, H. Chun, J. C. Yu, C. Y. Chan and P. K. Wong, Environ. Sci. Technol., 2010, 44, 1392–1398.
55. H. G. Yu, R. Liu, X. F. Wang, P. Wang and J. G. Yu, Appl. Catal., B, 2012, 111–112, 326–333.
56. R. Georgekutty, M. K. Seery and S. C. Pillai, J. Phys. Chem. C, 2008, 112, 13563–13570.
57. H. G. Yu, L. Liu, X. F. Wang, P. Wang, J. G. Yu and Y. H. Wang, Dalton Trans., 2012, 41, 10405–10411.
58. X. F. Wang, S. F. Li, H. G. Yu, J. G. Yu and S. W. Liu, Chem.–Eur. J., 2011, 17, 7777–7780.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18938a

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