Facile synthesis of a novel WO₃/Ag₂MoO₄ particles-on-plate staggered type II heterojunction with improved visible-light photocatalytic activity in removing environmental pollutants

Abstract

A novel WO₃/Ag₂MoO₄ heterojunction has been synthesized through a facile precipitation method with Ag₂MoO₄ particles firmly deposited on the surface of WO₃ nanoplates, forming “particles-on-plate” type II heterojunction structures. This heterojunction exhibited improved photocatalytic activities for the degradation of rhodamine B (RhB), 4 chlorophenol (4-CP) and tetracycline hydrochloride (TC) under visible-light irradiation compared to pure Ag₂MoO₄ and WO₃. In addition, the heterojunction with 10 wt% Ag₂MoO₄ displays the best photocatalytic performance, which was about 2 times better than that of pure WO₃ or Ag₂MoO₄. The TC photodegradation rate reaches up to 91% within 90 min visible light irradiation. Furthermore, the photocatalytic efficiency of the Ag₂MoO₄/WO₃ heterojunction is 1.3 times higher than that of the mixture of the two individual photocatalysts. This remarkable enhanced photocatalytic performance results from the staggered bandgap between Ag₂MoO₄ and WO₃, which can suppress the recombination of electron–hole pairs efficiently. Moreover, based on the radical trapping experiment, the superoxide radical anions (·OH) and photogenerated holes (h⁺) are the crucial active oxidizing species.

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1. Introduction

Photocatalysis has aroused significant attention recently due to its efficiency, environmental friendliness and convenience for solving the energy and environment crisis. The key to photocatalysis is to construct an efficient and stable photocatalyst.^1,2 To date, various kinds of photocatalysts have been reported such as TiO₂,^3,4 Ag₃PO₄,^5–7 Bi₂WO₆,^8–11 MoS₂,^12,13 C₃N₄,^14,15 BiVO₄ (ref. 16) and WO₃.^17–19 Among them, WO₃ is regarded as a promising photocatalyst due to its strong adsorption in visible light, photocorrosion resistance, and stable physicochemical properties. Nevertheless, its practical application is largely restricted due to its comparatively narrow band gap (2.4 to 2.8 eV) and lower light energy conversion efficiency.²⁰ The commonly used strategies to improve the photocatalytic performance include ion doping,²¹ heterojunctions,^22–25 and morphological modulation.^10,26 Among those strategies, the heterojunction is proven to be most widely used because of its effectiveness and convenience for improving the photocatalytic activity, as highlighted in our recent review.²² Furthermore, we have also constructed some nanojunction systems such as Bi₂O₃–Bi₂WO₆ (ref. 11) and Ag₃PO₄–GQDs composites;⁷ these heterojunction photocatalysts exhibited higher photocatalytic activity than their bare components. Therefore, constructing WO₃ based heterojunction photocatalyst should be an effective way to improve the photocatalytic performance of WO₃. Up to date, various WO₃-based heterojunctions have been reported, such as WO₃/BiVO₄,¹⁸ WO₃–C₃N₄,^27,28 WO₃/CdS,²⁹ WO₃/Ag,³⁰ WO₃/RGO³¹ and CNTs–WO₃.³² Compared with bare WO₃, the construction of these heterojunctions enhanced the photocatalytic performance in different degrees. However, it is still highly desirable to design novel WO₃-based heterojunctions to further improve the photocatalytic performance of pure WO₃. Over the last decade, silver molybdate (Ag₂MoO₄) has attracted extensive attentions in photoluminescence,³³ biological sterilization³⁴ and electrocatalytic oxygen reduction reaction,³⁵ and it has been shown as an excellent semiconductor to modify other semiconductors to improve their photocatalytic performance,^36–38 such as Ag/AgCl/Ag₂MoO₄,³⁹ Ag@Ag₂MoO₄–AgBr composite,^40,41 Ag₂MoO₄–Ag₃PO₄.⁴² As the band edges of Ag₂MoO₄ match well with those of WO₃, it can be expected that the WO₃/Ag₂MoO₄ heterojunction would be a photocatalyst with a staggered bandgap and therefore exhibit a high performance. Also, it is desirable to construct such heterojunction, as it has never been done so far.

In this study, Ag₂MoO₄–WO₃ heterojunctions with various Ag₂MoO₄ mass percentages have been developed with a facile precipitation method. Their photocatalytic activities have been investigated with the photodegradation of rhodamine B (RhB), 4 chlorophenol (4-CP) and tetracycline hydrochloride (TC) under visible-light irradiation. And the mechanism of the improved photocatalytic performance of the heterojunction photocatalysts has also been proposed based on the characterization and analysis results.

2. Experimental

2.1. Chemicals

Degussa TiO₂ with particle size 25 nm (P25), TC and 4-CP were purchased from Shanghai Macklin Biochemical Co., Ltd. Na₂MoO₄·2H₂O, Na₂WO₄·2H₂O, AgNO₃, HCl, ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd.

2.2. Preparation of tungsten oxide (WO₃) nanoplates

WO₃ nanoplates were synthesized according to ref. 31. In a typical synthesis, Na₂WO₄·2H₂O (200 mg) were dissolved in deionized water (10 mL). Subsequently, 5 mL HCl (35%) was slowly added to the above solution with continuous stirring. The suspension was then transferred to a 45 mL Teflon-lined stainless-steel autoclave and heated at 140 °C for 8 h. The sample was collected by centrifugation with ethanol and deionized water for several times and dried by vacuum freeze drier. Finally, the samples obtained were annealed at 300 °C for 30 min.

2.3. Preparation of silver molybdate (Ag₂MoO₄) and Ag₂MoO₄–WO₃

The surface decoration of WO₃ with Ag₂MoO₄ nanoparticles (NPs) was accomplished by a simple in situ deposition method. Briefly, a certain amount of WO₃ (0.375, 0.75, 1.125 and 1.5 g) was dispersed into 40 ml deionized water and sonicated for 30 min to give a WO₃ aqueous suspension. Then, AgNO₃ (5 mM, 40 mL) was dissolved in the above suspension under constant stirring. After that, Na₂MoO₄·2H₂O (2.5 mM, 40 mL) was added drop by drop into the above solution. After continually stirring for 4 h in the dark, the obtained yellow precipitate of Ag₂MoO₄–WO₃ was separated by centrifugation with deionized water and ethanol for several times and dried at 30 °C for 6 h in vacuum drier. For comparison, pure Ag₂MoO₄ was also prepared with this method without WO₃.

2.4. Characterization

Scanning Electron Microscopy (SEM) and Transmittance Electron Microscopy (TEM) were performed on field emission scanning electron microscope (Hitachi S-4800) and field emission electron microscope (FEI TECNAI G2 F20 200 kV) to reveal the morphology of the catalysts. Powder X-ray diffractometer (XRD) patterns were recorded on Bruker D8 ADVANCE to analysis the phases of the catalysts. UV-vis diffuse reflectance spectra (DRS) were recorded on a UV-vis spectrophotometer (Shimadzu Corporation, UV-3600), and the specific surface area (BET) measurements were performed on a N₂ adsorption specific surface & pore size analysis instrument (3H-2000PS1, Beishide Instrument S&T). Photoluminescence (PL) spectra were measured on the F-7000, Hitachi spectrophotometer at room temperature, and the wavelength of excitation light was 325 nm.

2.5. Photocatalytic test

The photocatalytic performances were evaluated with the photodegradation of RhB, 4-CP and TC under visible-light irradiation (300 W Xe lamp with a cut-off filter, λ > 420 nm). The photo of the reaction setting up was shown in Fig. S1.† In each typical experiment, 100 mg catalyst was added into 100 mL RhB, 4-CP or TC solution (10 mg L⁻¹). To establish the adsorption–desorption equilibrium between the catalysts and the pollutants, the suspension was magnetically stirred in dark for 60 min before the light irradiation. During irradiation, 2.0 mL suspension was collected at the regular intervals and centrifuged to remove the catalysts in RhB solution or filtered through a Navigator syringe filter of 0.22 μm in TC and 4-CP solution. The concentrations of RhB were determined by UV-6000PC spectrophotometer at the characteristic absorption peak of RhB (553 nm), and the concentrations of 4-CP and TC were recorded with the high-performance liquid chromatography (HPLC, Agilent 1100 series) equipped with a UV detector at 280 nm. The flow rate of mobile phase (TC: 8% methanol, 72% oxalic acid and 20% acetonitrile; 4-CP: 80% methanol and 20% water) was set at 0.5 mL min⁻¹.

To determine the free radical in the progress of photodegradation, 6 mM AgNO₃, 1 mM disodium ethylene diamine tetraacetate (Na₂-EDTA), 1 mM p-benzoquinone (BQ) or 1 mM isopropanol (IPA) were employed as sacrifice agents in the RhB (100 mL, 10 mg L⁻¹) photocatalytic process.

2.6. Photoelectrochemical measurements

The photoelectrochemical measurements were conducted on a CHI660E electrochemical workstation using a standard three-electrode system in which a saturated calomel electrode (SCE), a platinum sheet (2 cm²) and a FTO glass (2 cm²) coated with Ag₂MoO₄, WO₃ and 10% Ag–W film were used as reference, counter and working electrodes, respectively. While 0.5 M Na₂SO₄ solution was used as the electrolyte. In addition, the potential of the EIS measurements was the open-circuit potential. The photocurrent responses of the samples were determined under inconsecutive irradiation by 300 W Xe lamp. During the experiment, the test potential was 0.5 V vs. SCE.

3. Results and discussion

3.1. Preparation of Ag₂MoO₄/WO₃ heterojunctions

A series of Ag₂MoO₄/WO₃ heterojunctions were prepared by a facile precipitation–deposition method, as shown in Fig. 1. Firstly, WO₃ plates were synthesized by the reaction of Na₂WO₄·2H₂O and HCl during the hydrothermal treatment. Subsequently, Ag₂MoO₄ NPs were in situ anchored onto the plate-like WO₃ substrate by the reaction of Na₂MoO₄ and AgNO₃ at room temperature. The resulting catalysts with different Ag₂MoO₄ weight percentage of 5, 10, 15 and 20% are abbreviated as 5, 10, 15, and 20% Ag–W respectively.

Fig. 1 Schematic illustration for the preparation of Ag₂MoO₄/WO₃ heterojunction.

3.2. Characterization of photocatalysts

The morphology of WO₃ and Ag–W heterojunctions have also been characterized by SEM. As shown in Fig. 2A and B, the WO₃ displays a uniform nanoplate morphology, with an average length of 150–200 nm, which can also be further demonstrated by the TEM image (Fig. 2C) and the HRTEM (Fig. 2D) which showed the facet (200) is the dominating facet. After the surface decoration with Ag₂MoO₄, the obtained Ag–W heterojunction retain the nanoplate morphology (Fig. 3A). Notably, TEM images of Ag–W heterojunctions shows that there are plenty of small Ag₂MoO₄ nanoparticles distributed on WO₃ nanoplates (Fig. 3B and C). In addition, the high-resolution TEM (HRTEM) image (Fig. 3D) clearly displays two sets of lattice fringes with inter-plane spacings of 0.364 nm and 0.233 nm, which can be well assigned to the values for the (200) crystal plane of monoclinic WO₃ (JCPDS card no. 43-1035) and the (400) crystal plane of cubic Ag₂MoO₄ (JCPDS card no. 08-0473), respectively. Furthermore, the EDS pattern (Fig. 4A) reveals that Ag–W heterojunctions were composed of W, Ag, O and Mo elements. The molar ratio of W to Mo is 73 : 2.1, suggesting that the weight ratio of Ag₂MoO₄ to WO₃ species is equal to 0.09 : 1. The above results confirm the formation of Ag–W heterojunctions.

Fig. 2 (A) SEM, (B) enlarged SEM of WO₃ nanoplates, (C) TEM image and (D) HRTEM images of WO₃ nanoplates with dominating 200 facets.

Fig. 3 (A) SEM, (B and C) TEM, and (D) HRTEM images of 10% Ag–W heterojunction: Ag₂MoO₄ nanoparticles were selectively deposited on 200 facets.

Fig. 4 (A) EDS of 10% Ag–W heterojunction; (B) XRD patterns of as-prepared 10% Ag–W heterojunction compared with JCPDS of Ag₂MoO₄ and WO₃.

The crystal phases of all as-prepared photocatalysts were tested by the XRD to demonstrate the phase and crystallinity (Fig. 4B). The standard XRD pattern of Ag₂MoO₄ and WO₃ have also been supplied. All the diffraction peaks of pure WO₃ can be well indexed to monoclinic WO₃ (JCPDS card no. 43-1035), while all diffraction peaks of pure Ag₂MoO₄ can be well assigned to cubic Ag₂MoO₄ (JCPDS card no. 08-0473). For Ag–W heterojunctions, they showed the co-existence of Ag₂MoO₄ and WO₃. Major diffraction peaks with 2θ values of 23.1°, 23.6°, 24.4°, 26.6°, 28.9°, 34.2° and 49.9° could be perfectly indexed to the (002), (020), (200), (120), (112), (202) and (140) crystal planes of monoclinic WO₃ (JCPDS card no. 43-1035), respectively. Besides, additional diffraction peaks with 2θ values of 27.1°, 31.8°, 33.3°, 38.6°, 50.9° and 55.8° could be attributed to the (220), (311), (222), (400), (511) and (440) crystal planes of cubic Ag₂MoO₄ (JCPDS card no. 08-0473), respectively.

The UV-vis diffuse reflectance spectra of pure Ag₂MoO₄, WO₃ and 10% Ag–W heterojunction have been recorded in order to investigate the capacity of visible light absorption, as shown in Fig. 5A. Clearly, pure Ag₂MoO₄ shows a typical absorption at wavelength shorter than 400 nm, which accords previous reports.³⁵ Pure WO₃ exhibits an obviously absorption edge at ca. 500 nm, consistent with the previous reports.³¹ Interestingly, 10% Ag–W heterojunction displays a broad-spectrum photo absorption with an edge at ca. 530 nm, reflecting a red shift of photo absorption. This red shift of photoabsorption maybe resulting from the partial interfacial charge transfer between Ag₂MoO₄ and WO₃ which is caused by the staggered bandgap.¹¹ Therefore, VLD photocatalytic activity of 10% Ag–W heterojunction can be expected to be superior to that of Ag₂MoO₄ and WO₃.

Fig. 5 UV-vis diffuse reflectance spectra (A) and linear transformation (hν)–(Ahν)² curves (B) of Ag₂MoO₄, WO₃ and 10% Ag–W heterojunction.

The E_g (energy band gaps) of Ag₂MoO₄ and WO₃ can be estimated from a linear transformation (hν)–(Ahν)² curves, as Fig. 5B presents. Accordingly, E_g of Ag₂MoO₄ and WO₃ are calculated to be 3.38 and 2.54 eV, severally. In addition, the positions of the valence band (VB) and conduction band (CB) of photocatalysts have also been figured out by Mulliken electronegativity:³⁵

E_CB = X − E_C − 1/2E_g (1)

E_VB = E_CB + E_g (2)

Here, X is the absolute electronegativity of the corresponding semiconductors (Ag₂MoO₄ and WO₃), E_C is the energy of free electrons on the hydrogen scale (4.5 eV). According to the above equations, the E_CB and E_VB of Ag₂MoO₄ were calculated to be −0.28 and 3.1 eV, while those of WO₃ were calculated to be 0.82 and 3.36 eV, respectively.

The N₂ isothermal adsorption desorption curves of WO₃, Ag₂MoO₄ and 10% Ag–W heterostructures have also been recorded. As shown in Fig. 6, WO₃ and 10% Ag–W heterostructure exhibit a similar BET surface area of about 12 m² g⁻¹. In addition, according to the desorption branches in the inset of Fig. 6, the pore size distribution presents nanopores (∼26.5 nm) in 10% Ag–W heterostructure, which is usually benefit for the improvement of the photocatalytic activity.

Fig. 6 N₂ adsorption–desorption isotherms and the corresponding pore size distribution (inset) of WO₃, Ag₂MoO₄ and 10% Ag–W heterojunction.

3.3. Photocatalytic activity

The photocatalytic activities of Ag–W heterostructures with different weight ration of Ag₂MoO₄ and WO₃ were evaluated by the photodegradation of dye RhB, colorless 4-CP and antibiotic TC under visible light irradiation at room temperature. For comparison, pure WO₃, Ag₂MoO₄ and P25 have also been used as photocatalysts.

Fig. 7A presents the photodegradation efficiency of RhB over different photocatalysts. Clearly, all the Ag–W heterostructures (5% Ag–W, 10% Ag–W, 15% Ag–W and 20% Ag–W) exhibited remarkable enhanced photocatalytic performance in comparison with bare WO₃ and Ag₂MoO₄, and the RhB photodegradation efficiencies reached up to 95%, 97%, 89% and 73%, respectively within 160 min of reaction. Among these photocatalysts, 10% Ag–W heterostructures showed the most excellent photocatalytic performance, it's photocatalytic activity was more than 2 and 1.9 times than that of Ag₂MoO₄ and WO₃ respectively. In addition, the photodegradation efficiency of TC and 4-CP over Ag–W heterostructures with different Ag₂MoO₄ content have also been investigated respectively, 10% Ag–W heterostructures also showed the most excellent photocatalytic performance (Fig. S2 and S3†). Moreover, the load content of Ag₂MoO₄ had a prominent effect on the photocatalytic performance, too low or high content of Ag₂MoO₄ loading could not improve the separation of charges effectively, leading to an unsatisfactory photocatalytic performance. Similar results have also been observed by previous reports.^31,43,44 Furthermore, the photodegradation efficiency of 10% Ag–W (97%) was almost 3.88 times higher than that of P25.

Fig. 7 Degradation curves (A) and (B) rate constants (k) under the visible light irradiation (λ > 420 nm) over different photocatalysts.

Moreover, the degradation efficiency were further analyzed by using the pseudofirst-order model,⁴⁵ −ln(C/C₀) = kt. In this formula, C₀ represents the initial concentration of RhB, C represents the concentration of RhB at time t, and k represents the reaction rate constant. As shown in Fig. 7B, clearly, 10% Ag–W exhibited the highest k value (0.01991 min⁻¹), which was extremely higher than those of Ag₂MoO₄ (0.00482 min⁻¹) and WO₃ (0.00489 min⁻¹), and is more than 10 times that of TiO₂ (0.00158 min⁻¹). In addition, to further confirm the effect of the heterojunctions in Ag–W heterojunction, the degradation of RhB by using the mixture of two single components of Ag–W heterostructures with the same weights (90 mg WO₃ + 10 mg Ag₂MoO₄) as photocatalysts have also been conducted under visible light irradiation. The photodegradation efficiency over 10% Ag–W was almost 1.43 times than that of the mechanical mixture of WO₃ and Ag₂MoO₄ (68%), and the k over 10% Ag–W was more than 3 times than that of the mixture (0.00628 min⁻¹). All of these results demonstrated that there was a synergic effect between the two components (WO₃ and Ag₂MoO₄) in Ag–W heterostructures.

To preclude the dye sensitization effect during the photocatalytic degradation, colorless antibiotic TC and 4-CP have also been chosen as model pollutants. As shown in the illustrated HPLC chromatograms of TC and 4-CP solution at the regular intervals times (Fig. 8A). When TC was chosen as model pollutants, the characteristic peak of TC solution at retention time (5.8 min) decreased quickly (Fig. 8A). After 210 minutes irradiation, the degradation efficiency of TC reached up to 99%. Similar with the result of the HPLC chromatograms of 4-CP solution, the degradation efficiency of 4-CP reached up to 70% within 320 minutes (Fig. 8B), demonstrating the remarkable photodegradation of 10% Ag–W heterojunction.

Fig. 8 HPLC chromatograms of (A) TC (100 mL 5 mg L⁻¹) and (B) 4-CP (100 mL 5 mg L⁻¹) solutions over the 10% Ag–W heterojunction for different photocatalytic reaction times, inset: the photodegradation efficient column of (A) TC and (B) 4-CP under visible light irradiation (λ > 420 nm).

The stability of photocatalysts is one of most significant factors in practical applications. Hence, the photocatalytic stability of the 10% Ag–W heterojunction was researched by repeatability experiments for photodegradation of RhB under visible-light irradiation (Fig. 9A). After four consecutive runs, the photodegradation efficiency of 10% Ag–W heterojunction maintains 93%, and the k decrease slightly from 0.01991 min⁻¹ to 0.01679 min⁻¹ (Fig. S4†). The slight decrease of its activity should result from the loss of samples in the recycle experiments. All these results demonstrate the excellent recyclability of Ag–W heterojunction.

Fig. 9 (A) Cycling runs in photocatalytic photodegradation of RhB (10 mg L⁻¹, 100 mL) over 10% Ag–W heterojunction (100 mg) under visible light (λ > 420 nm); (B) effect of different scavengers on the degradation of RhB (10 mg L⁻¹, 100 mL) efficiencies over 10% Ag–W heterojunction.

3.4. Photocatalytic photodegradation mechanism

During the photocatalysis process, the photogenerated electrons (e⁻) and holes (h⁺) can transfer to the surface of catalyst and will generate sorts of reactive species, such as hydroxyl radicals (·OH), superoxide radicals (·O₂⁻) and so on, which play an important role in photodegrading organic contaminant.^46–48 Herein, we evaluated the dominating active species during the process of photodegradation of RhB. In the photodegradation experiments, AgNO₃, disodium ethylene diamine tetraacetate (Na₂-EDTA), benzoquinone (BQ) and isopropanol (IPA) which acted as the scavengers of e−, h⁺, ·O₂⁻, ·OH^38,49 were added into RhB suspensions before visible light irradiation respectively. Just as shown in Fig. 9B, when Na₂-EDTA and IPA were added into the RhB solution, the photocatalytic efficiency decreases sharply, the k value decrease to less than 1/10 of the original one (Fig. S5†), and the RhB degradation efficiency decreases from 94.9% to 50% or 19.3% respectively after 160 min of reaction. While the photocatalytic activities are influenced slightly with the addition of AgNO₃ and BQ. These results illustrate that h⁺ and ·OH are the primary active species which conduce to the photodegradation of RhB.

To investigate the electronic interaction between Ag₂MoO₄ and WO₃, the photocurrents of 10% Ag–W heterojunction, Ag₂MoO₄ and WO₃ electrodes have also been measured. To the best of our knowledge, the intensity of photocurrent reflects the transference speed of photogenerated electrons and holes. Clearly, all the electrodes showed fast and uniform photocurrent responses in Fig. 10A. Under visible light irradiation, the intensity photocurrent of 10% Ag–W heterojunction was almost 6 and 2 times than that of Ag₂MoO₄ and WO₃, respectively (Fig. 10A). The photocurrent enhancement of 10% Ag–W heterojunction manifested that Ag–W heterojunction can efficiently accelerate the separation of photogenerated electrons and holes due to the synergetic effect of Ag₂MoO₄ and WO₃ semiconductor. In addition, EIS analysis has also been measured to further study the electron transport property of the 10% Ag–W heterojunction, Ag₂MoO₄ and WO₃ electrodes. Usually, a smaller circular radius in EIS Nyquist plot represents a lower charge transfer resistance in the electrode.⁵⁰ Obviously, the EIS Nyquist plot of 10% Ag–W heterojunction showed the smallest circular radius among the three electrodes (Fig. 10B), suggesting the lowest charge transfer resistance. To further confirm the charge transfer property, the PL emission spectrum which can be used to evaluate the separation efficiency of photogenerated electron–hole pairs was employed (Fig. 10C). Generally, a weaker intensity of the PL emission spectra represents a lower charge recombination rate.⁵¹ As shown in Fig. 10C, 10% Ag–W heterojunction display the lowest intensity under the excitation of light at λ = 325 nm, further indicating the vital role in the suppression of the recombination of photogenerated charge carriers.

Fig. 10 Photocurrent density (A), EIS plots (B) and PL plots (C) of WO₃, Ag₂MoO₄ and 10% Ag–W heterojunction.

Based on the above results, the possible photodegradation mechanism for the enhanced photocatalytic activity of Ag–W heterojunction are analyzed. Two main reasons contributing to the improved photocatalytic performance of Ag–W heterojunction. On one hand, the improvement of the photoabsorption of visible-light enhances the utilization of solar energy and hence product more photogenerated holes and electrons, as shown in Fig. 11. On the other hand, the staggered bandgap and intimate contact between Ag₂MoO₄ and WO₃ greatly promote the charge separation and transfer in the Ag–W heterojunction.^52–59 Under visible light irradiation, both Ag₂MoO₄ and WO₃ can be excited to photogenerated electron–hole pairs. Since the potentials for CB and VB of Ag₂MoO₄ are more negative than that of Ag₂MoO₄, photogenerated electrons in the CB of Ag₂MoO₄ can be transferred to that of WO₃ while holes in the VB of WO₃ can be transferred to that of Ag₂MoO₄. Therefore, such charge transfer process can suppress the recombination of electron–hole pairs effectively, which has also been confirmed by the photocurrent measurements, EIS and PL emission spectrum. Consequently, numerous h⁺ accumulated on the VB of Ag₂MoO₄ can directly oxidize organic contaminant adsorbed on the surface of the photocatalyst or mineralized them indirectly through hydroxyl radicals (·OH) generated by the reaction of holes and H₂O or OH⁻.

Fig. 11 Schematic diagrams of electron–hole pair separation and the possible photodegradation mechanism over Ag–W heterojunction under visible-light irradiation.

4. Conclusions

In summary, we designed and prepared a novel the Ag–W heterojunction with different mass percentage though a facile precipitation method. These heterogeneous photocatalysts showed extremely superior photocatalytic performance than bare Ag₂MoO₄ and WO₃ for RhB/4-CP/TC photodegradation, and the 10% Ag–W exhibits the most excellent photocatalytic activity. Moreover, the mechanism of the enhanced photocatalytic activity of Ag–W heterojunctions has been analyzed based on the energy band positions and the free radical capture experiments. The enhancement of the photocatalytic performance is mainly attributed to the stagger band gap structure, improved the separation of photogenerated electron–hole pairs. Furthermore, the Ag–W heterojunction exhibits good stability with inappreciable loss after four cycling runs. Therefore, this work may provide new insight for developing VLD photocatalyst on application of environmental renovation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work has been financially supported by the Qingdao Science and Technology Program (16-5-1-32-jch, 17-3-3-77-nsh), the China Postdoctoral Science Foundation (2016M602110), the National Natural Science Foundation of China (31570541, 31170509, 21506109), the Key R&D project of Shandong province (GG201809130031) and the Primary Research and Development Plan of Shandong Province (2017GSF220001).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra07175j

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