Fabrication of a novel ternary heterojunction composite Ag₂MoO₄/Ag₂S/MoS₂ with significantly enhanced photocatalytic performance

Abstract

In this work, a novel ternary heterojunction composite Ag₂MoO₄/Ag₂S/MoS₂ was successfully fabricated via a facile two-step method for the first time. The results of the photocatalytic experiments toward degradation of RhB and TC showed that the as-prepared ternary composite exhibited outstanding catalytic efficiency. Compared to the photocatalytic degradation rate for Ag₂MoO₄ and Ag₂MoO₄/Ag₂S, the rate for RhB was 5 and 2 times higher and for TC was 12 and 2 times higher, respectively. The significantly enhanced catalytic efficiency can be ascribed to the formation of a ternary heterojunction with well-matched band positions, which facilitates the charge mobility and transfer, restrains the recombination of charge carriers, and increases light absorption and specific surface area. This study reveals that the photocatalytic performance of the fabricated ternary heterojunction is superior to that of a binary heterojunction, and artful integration of semiconductors to form a ternary heterojunction is an effective strategy to remarkably improve the photocatalytic performance of semiconductors. The as-prepared products present a potential application for environmental remediation.

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1. Introduction

Recently, molybdate matrix materials have caught wide attention due to their vast applications in wide fields such as antibacterial materials,^1–3 photocatalysis,^4–8 photoluminescence,^9–12 lasers,^13–15 and up-conversion.^16,17 As a typical molybdate matrix material, silver molybdate (Ag₂MoO₄) has been widely studied in many different areas owing to its excellent properties such as high electrical conductivity,^18,19 excellent antimicrobial activity,^20,21 and environmentally friendly nature.²² However, due to its wide energy band (E_g = 3.0 eV) with narrow sunlight absorption and fast recombination of charge carriers, its application in photocatalysis is limited.²³ There have been numerous attempts to improve its photocatalytic activity, such as morphology and structural mediation,^24–27 element doping,²⁸ and heterojunction construction.^29–31 Among these modification strategies, heterojunction construction has proven to be an effective method to restrain recombination of charge carriers and increase light absorption, resulting in a significant improvement of photocatalytic activity.^32–38

Ag₂S is a narrow bandgap semiconductor (E_g ≈ 1.0 eV) with a broad and strong light absorption in the entire solar spectrum.³⁹ It has received great attention as a promising photocatalyst or photosensitizer for photocatalysis.⁴⁰ However, the photocatalytic performance of pristine Ag₂S is far from meeting the demands of real applications due to its photocorrosion and the fast recombination ratio of photoinduced charge carriers.³⁹ In order to restrain the photocorrosion and recombination of the charge carriers, construction of heterojunctions with wide bandgap semiconductors is an efficient strategy. It can not only restrain the photocorrosion and recombination of the charge carriers for Ag₂S, but also modify wide bandgap materials by broadening their light absorption ranges.^41,42 For example, Ag₃PO₄/Ag₂S, BiVO₄/Ag₂S and TiO₂/Ag₂S have been constructed and they show much improved photocatalytic performances for both Ag₂S and wide bandgap semiconductors.^43–46

MoS₂ is a typical two-dimensional (2D) structure semiconductor with a narrow band gap (1.2–1.9 eV), excellent electrical carrier mobility, high chemical reactivity and good optical properties.^47–49 It has been widely investigated as an effective catalyst for H₂ evolution and degradation of pollutants. Notably, as an effective co-catalyst for H₂ evolution, it can be considered as a promising alternative to the noble metal Pt.^50–52 Nevertheless, the high recombination ratio of electron–hole pairs limited its wide application in photocatalysis.⁵³ Various methods have been applied to increase the separation efficiency of charge carriers for MoS₂.^54,55 Among them, combining with other semiconductors to form a heterojunction has proven to be an effective strategy. For example, MoS₂/Ag₂S, MoS₂/g-C₃N₄ and MoS₂/COF have been reported. The results show that these constructed heterojunctions have highly improved separation efficiency of charge carriers and photocatalytic activity.^51,56–58

Although the binary heterojunctions Ag₂MoO₄/Ag₂S and MoS₂/Ag₂S have been studied and have shown improved photocatalytic performance in contrast to their pristine monomers, their wide application with higher photocatalytic activity is still limited.^51,59 Moreover, they still suffer from structural instability issues in aqueous solution.⁶⁰ Thus, determining how to further improve their photocatalytic performances remains a challenge.

If a molybdate-based binary heterojunction is further integrated into a ternary heterojunction, whether its photocatalytic performance can be improved is worth studying. Interestingly, previous studies revealed that the photocatalytic performances of some ternary heterojunctions were superior to that of their binary heterojunctions.^53,56 Inspired by this, in the present work, we rationally integrated Ag₂MoO₄, Ag₂S and MoS₂ together to form a novel ternary heterojunction photocatalyst by a facile two-step method. The assembled composite, consisting of rod-like Ag₂MoO₄, nanoparticle-like Ag₂S and flower-like MoS₂, exhibited an outstanding photocatalytic activity for degradation of rhodamine B (RhB) and tetracycline (TC) under simulated sunlight irradiation. The greatly enhanced photocatalytic activity compared with the pristine monomers, binary composites, and previous reported molybdate-based and sulfide-based catalysts,^30,42 can be ascribed to the rationally assembled ternary heterojunction with intimate interface contact and well-matched energy bandgaps, which not only promote the transfer and separation of the charge carriers, but also increase light absorption and specific surface area. To the best of our knowledge, this is the first time the ternary heterojunction composite Ag₂MoO₄/Ag₂S/MoS₂ has been fabricated and its photocatalytic performances have been studied in detail. The constructed ternary heterojunction exhibits advantages in photocatalytic performance, implying its potential application in environmental protection and energy conversion fields. This work provides a new strategy to highly improve photocatalytic performance of metal molybdate and sulfide for environmental remediation and energy conversion.

2. Experimental

2.1 Materials

Silver nitrate (AgNO₃), ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O), thioacetamide (TAA), thiourea (CN₂H₄S), polyvinyl pyrrolidone (PVP, k-30), rhodamine B (RhB), methylene blue trihydrate (MB), polyvinylidene fluoride (PVDF), sodium sulfate (Na₂SO₄), p-benzoquinone (BQ), triethanolamine (TEOA), and isopropanol (IPA) were obtained from Sinopharm Chemical Reagent Corp. (P.R. China). Milli-Q water was used in all of the experiments.

2.2 Synthesis of flower-like MoS₂

Flower-like MoS₂ was synthesized by a facile hydrothermal method.⁵¹ In a typical experiment, (NH₄)₆Mo₇O₂₄·4H₂O (0.210 g) and CN₂H₄S (0.456 g) were dissolved in water (30 mL) with magnetic stirring for 10 min. Then, PVP (0.1 g) was slowly added into the above solution with stirring for 20 min. A clear solution was obtained, which was transferred into a 50 mL Teflon lined stainless steel autoclave, heated to 200 °C in an oven and maintained at this temperature for 10 h. After the autoclave was cooled down to room temperature naturally, the resulting product was collected by centrifugation and washed with water and ethanol respectively several times. Finally, the obtained sample was dried at 60 °C overnight.

2.3 Synthesis of the Ag₂MoO₄/Ag₂S/MoS₂ ternary composite

As shown in Scheme 1, the Ag₂MoO₄/Ag₂S/MoS₂ ternary composite was prepared by a one-pot approach with two steps. In a typical procedure, a specific amount of MoS₂, AgNO₃ (0.5096 g) and PVP (0.5 g) were dispersed in 30 mL water with sonication for 20 min. Then, (NH₄)₆Mo₇O₂₄·4H₂O (0.2648 g) in 20 ml water and TAA (5.6 mg) in 5 ml water were added to the above solution, respectively. The solution was magnetically stirred for 4 h. The resulting precipitate was washed several times with water and ethanol, respectively. Finally, the obtained product was dried at 60 °C overnight (Scheme 1).

Scheme 1 A schematic illustration of the synthesis procedure of Ag₂MoO₄/Ag₂S/MoS₂.

The binary composite Ag₂MoO₄/Ag₂S was prepared using the same procedure except for the addition of MoS₂. The as-prepared Ag₂MoO₄/Ag₂S with different mole ratios of Ag₂S (1%, 5% and 8%, respectively) are denoted as Ag₂MoO₄/Ag₂S-X (X = 1, 5, and 8, respectively). Ag₂MoO₄/Ag₂S-5 was used as a representative to be characterized and as a reagent to prepare Ag₂MoO₄/Ag₂S/MoS₂. Ag₂MoO₄/Ag₂S/MoS₂ with different mole ratios of MoS₂ (0.1%, 0.5%, 1%, and 10%, respectively) were denoted as Ag₂MoO₄/Ag₂S-Y (Y = 0.1, 0.5, 1, and 10, respectively). Ag₂MoO₄/Ag₂S/MoS₂-0.5 is used as a representative to be characterized.

2.4 Characterizations

The powder X-ray diffraction (XRD) patterns were measured by a Rigaku D/max-2500 X-ray diffractometer using Cu-Kα radiation. The transmission electron microscopy (TEM) measurements were performed on an HT7700 electron microscope (HITACHI, Japan). The scanning electron microscopy (SEM) and the energy dispersive spectroscopy (EDS) were carried out on a Phenom ProX (PHENOM, Netherland). X-ray photoelectron spectra (XPS) were recorded on a Thermo ESCALAB 250XI (ThermoFisher Scientific, America). UV-vis-NIR diffuse reflectance spectra (DRS) were measured on a Cary 5000 spectrophotometer (Agilent, China). N₂ adsorption–desorption curves and pore size analysis (BET) were conducted by ASAP2460 (ASAP2020, America). The photoluminescence (PL) spectra were obtained using an FS5 fluorescence spectrophotometer (Edinburgh Instruments, UK) with an excitation wavelength of 280 nm.

2.5 Photocatalytic experiments

The photocatalytic experiments were performed on an SGY-IB multifunction photochemical reactor (Nanjing Sidongke Electric Co. Ltd) with a 300 W Xe lamp (PL-X500D) as the light source. In a typical experiment, 10 mg of the photocatalyst was suspended in a quartz cuvette containing 20 mL of RhB aqueous solution (20 mg L⁻¹). The suspension was stirred in the dark for 30 min to establish an adsorption–desorption equilibrium between the photocatalyst and the target contaminants before irradiation. The photoreaction vessels were then exposed to Xe lamp irradiation. At regular time intervals, the photoreacted solutions were analyzed by recording the variation of the absorption band maximum (554 nm) of RhB in the UV-vis spectrum.

2.6 Photoelectrochemical and Electrochemical Measurements

The photoelectrochemical properties of Ag₂MoO₄/Ag₂S/MoS₂ were studied on a CHI 660E electrochemical workstation using a standard three-electrode system. The working electrode was prepared according to the following procedure. Ag₂MoO₄/Ag₂S/MoS₂ (8 mg) was ground in a mixture of N-methyl pyrrolidone (NMP) (300 μL) and polyvinylidene fluoride (PVDF) solution (2.5%, 200 μL) for 30 min to produce a slurry. The as-obtained slurry was spread onto the conductive surface of the ITO glass (10 × 20 mm²) to form a film and dried in a 60 °C oven to obtain the Ag₂MoO₄/Ag₂S/MoS₂/FTO working electrode. Na₂SO₄ solution (0.1 M) was used as the electrolyte, and platinum tablets and a saturated calomel electrode (SCE) served as the counter electrode and the reference electrode, respectively. The photocurrent responses were measured in 0.1 M Na₂SO₄ solution (20 mL) in the presence of O₂ (1 atm) at room temperature under the irradiation of a 300 W Xe lamp. Amperometric I–t curves were obtained at a basic 0.0 V (vs. SCE electrode) with light on–off switches of 30 s.⁶¹ The Nyquist plots were determined over the frequency range of 100–10⁶ Hz with an ac amplitude of 10 mV at the open circuit.

3. Results and discussion

3.1 Characterizations

The crystal phases of the as-products were obtained by X-ray diffraction (XRD) analysis. As shown in Fig. 1, bare cubic-phase Ag₂MoO₄ showed characteristic XRD peaks at 27.06°, 31.84°, 33.29°, 38.63°, 47.83°, 50.91°, 55.80°, 65.70° and 66.56°, corresponding to the (220), (311), (222), (400), (422), (511), (440), (533), and (622) lattice planes (JPCDS No. 08-0473), respectively.^5,30,38 The diffraction peaks (2θ) of Ag₂S appeared at 2θ values of 28.9°, 31.5°, 34.3°, 36.8°, 37.7°, 40.7°, 43.4°, 46.2° and 53.2°, corresponding to the (111), (−112), (−121), (121), (−103), (031), (200), (−123) and (−213) planes of monoclinic phase Ag₂S (JCPDS No. 14-0072), respectively.^39,62 The pure MoS₂ displayed characteristic XRD peaks at 14.47°, 32.93°, 33.94° and 58.28°, corresponding to the (002), (102) and (008) planes of hexagonal MoS₂ (JCPDS No. 37-1492), respectively.⁵¹ The composite of Ag₂MoO₄/Ag₂S/MoS₂ did not show the peaks of Ag₂S and MoS₂ due to their low contents and high dispersity. However, as shown in Fig. S1 (ESI), the peaks of Ag₂S and MoS₂ can be observed with the increase of their contents. This validates that the ternary heterojunction composite is successfully prepared.

Fig. 1 XRD patterns of the prepared samples.

The morphologies of the samples were analyzed by TEM images (Fig. 2). As shown in Fig. 2, pure Ag₂MoO₄ and MoS₂ exhibit irregular rhombic polyhedron-like and flower-like morphologies, respectively. Ag₂S is small sphere nanoparticles anchored on the surface of Ag₂MoO₄ and MoS₂. Fig. 2d shows that the assembled heterojunction composite with an intimate-contact interface does not change the morphologies of its constituent monomers. The high-resolution TEM (HR-TEM) image (Fig. 2e) of Ag₂MoO₄/Ag₂S/MoS₂ reveals the highly crystalline nature of the products. The lattice spacings of 0.20 and 0.14 nm correspond to the (102) and (232) monoclinic phase Ag₂S nanoparticles. The lattice spacings of 0.239 and 0.27 nm correspond to the (320) crystallographic planes of cubic Ag₂MoO₄ and the (100) crystallographic planes of hexagonal MoS₂, respectively.^7,63–65 The results of the SEM images are shown in Fig. 3, which are consistent with the results of the TEM images. The HRTEM and SEM images further confirm the successful synthesis of the ternary heterojunction.

Fig. 2 (a–d) The TEM images of Ag₂MoO₄, MoS₂, Ag₂MoO₄/Ag₂S, and Ag₂MoO₄/Ag₂S/MoS₂, respectively. (e) The HRTEM images of Ag₂MoO₄/Ag₂S/MoS₂.

Fig. 3 (a–d) SEM images of Ag₂S, MoS₂, Ag₂MoO₄ and Ag₂MoO₄/Ag₂S/MoS₂, respectively.

The EDX elemental mapping images and EDX spectrum (Fig. 4) show that all of the elements including Ag, Mo, O and S are detected in the composite, confirming that the exact ingredients are desirably combined to form the composite. The atomic percentages of the elements determined by EDX are consistent with the theory values, demonstrating that the heterojunction composite is successfully prepared.

Fig. 4 (a–e) Elemental mapping images and (f) the EDX spectrum of Ag₂MoO₄/Ag₂S/MoS₂.

X-ray photoelectron spectroscopy (XPS) analysis was performed to identify the chemical states and compositions of the prepared products. As shown in Fig. 5a, the XPS survey spectrum shows that the composition of Ag₂MoO₄/Ag₂S/MoS₂ presents the peaks of Ag, Mo, O, S and adventitious C. The obtained binding energies of these elements are calibrated by referencing the binding energy of 284.6 eV for C 1s.⁶ As shown in Fig. 5e, two strong peaks located at binding energies 367.7 eV and 373.8 eV can be ascribed to Ag 3d_5/2 and Ag 3d_3/2 of Ag⁺ in Ag₂MoO₄ and Ag₂S, respectively.⁴⁵ In the Mo 3d spectrum (Fig. 5c), two predominant peaks at 232.5 eV and 235.6 eV are assigned to the Mo 3d_5/2 and Mo 3d_3/2 of Mo⁶⁺ in Ag₂MoO₄, respectively.³⁴ In addition, two weak peaks at 226.2 eV and 231.8 eV are attributed to the 3d_5/2 and 3d_3/2 of Mo⁴⁺ in MoS₂, respectively.⁶⁶ In the O 1s spectrum (Fig. 5b), the peak at 530.4 eV is indexed to the Mo–O bonds in Ag₂MoO₄.³⁰ The S 2p spectrum (Fig. 5d) shows two peaks at 161.7 eV and 162.9 eV, which can be ascribed to S 2p_3/2 and S 2p_1/2, respectively.^67,68

Fig. 5 XPS spectra of (a) the whole spectrum, (b) O 1s, (c) Mo 3d, (d) S 2p, and (e) Ag 3d spectra for the Ag₂MoO₄/Ag₂S/MoS₂.

The light absorption properties of the samples were measured by the UV-vis diffuse reflectance spectra. As shown in Fig. 6a, both Ag₂S and MoS₂ exhibit a strong and full light absorption from 200 to 1100 nm. The pure Ag₂MoO₄ displays an absorption edge of 446 nm with low absorption at the visible region. When the Ag₂MoO₄ is coupled with Ag₂S to form a binary heterojunction or further coupled with MoS₂ to form a ternary heterojunction, its absorption edge has a remarkable red-shift with an obvious enhanced light absorption toward visible light. Moreover, in contrast to the binary heterojunction, the ternary heterojunction displays a bigger red-shift and a stronger light absorption. The red-shift of the absorption edge and increase in light absorption intensity can contribute to the enhancement of the photocatalytic activity of the catalysts.

Fig. 6 (a) UV-vis diffuse reflection spectra and (b and c) Tauc plots of the prepared samples.

The optical band gap energy (E_g) for the semiconductor catalyst can be calculated from the Tauc plots using the formula:

αhv = A(hv − E_g)^n/2 (1)

In which α, h, v, A and E_g represent the absorption coefficient, Planck's constant, light frequency, proportionality, and band gap energy, respectively. n equals 1 or 4, depending on whether the optical transition is direct or indirect, and n is 4 herein.^7,49,69 Thus, the E_g values for Ag₂MoO₄, Ag₂S, MoS₂, Ag₂MoO₄/Ag₂S-5.0 and Ag₂MoO₄/Ag₂S/MoS₂-0.5 are calculated to be 2.95, 0.97, 1.57, 2.63 and 2.61 eV, respectively (Fig. 6b). In contrast to bare Ag₂MoO₄, the band gap energies for the binary and ternary composites show an obvious decrease. Moreover, the band gap energy for the ternary composite is lower than that of the binary composite. The decrease of band gap energy caused by the formation heterojunction is consistent with the enhanced light absorption.

The surface areas and porous structures of the samples were measured by N₂ adsorption–desorption experiments (Fig. 7). As shown in Fig. 7, all of the samples exhibit type IV isotherms with hysteresis loops, suggesting the mesoporous structures of the materials. The Brunauer–Emmett–Teller surface areas (S_BET) were calculated to be 13.35, 18.35, and 30.19 m² g⁻¹ for Ag₂MoO₄, Ag₂MoO₄/Ag₂S and Ag₂MoO₄/Ag₂S/MoS₂, respectively. It is clear that the formation of the heterojunction can increase S_BET and the ternary heterojunction has the biggest S_BET and pore volume (Table 1). The increased S_BET and pore volume can offer more actives sites, which is favorable for the reactants’ adsorption and contributes to the enhancement of photocatalytic activity for the catalysts.

Fig. 7 N₂ adsorption–desorption isotherms for the samples.

Table 1 Surface area, pore size, and pore volume of the samples

Samples	Surface area (m^2 g^−1)	Average pore size (nm)	Pore volume (cm^3 g^−1)
Ag2MoO4	13.35	16.71	0.0558
Ag2MoO4/Ag2S	18.35	25.27	0.1159
Ag2MoO4/Ag2S/MoS2	30.19	15.41	0.1163

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The photoluminescence (PL) spectra were measured to determine the recombination rate of the photo-induced charge carriers. As shown in Fig. 8, the PL intensities of Ag₂MoO₄/Ag₂S and Ag₂MoO₄/Ag₂S/MoS₂ are much weaker than that of bare Ag₂MoO₄, indicating a great decrease of the recombination rate of the charge carriers in the heterojunctions. The Ag₂MoO₄/Ag₂S/MoS₂ exhibits the lowest PL intensity, demonstrating that the formed ternary heterojunction has a lower recombination rate of charge carriers in contrast to the binary heterojunction. In addition, the PL peaks shift to a longer wavelength after the formation heterojunction, owing to their interaction.^70,71

Fig. 8 Photoluminescence emission spectra of the samples.

To further study the separation efficiency of the charge carriers, the transient photocurrent response and electrochemical impedance spectroscopy (EIS) were conducted. As shown in Fig. 9a, the photocurrent density of Ag₂MoO₄/Ag₂S is higher than that of pure Ag₂MoO₄, and Ag₂MoO₄/Ag₂S/MoS₂ is higher than that of Ag₂MoO₄/Ag₂S, respectively. This implies that the formation of the heterojunction can promote the separation efficiency of charge carriers, and the formed ternary heterojunction has a higher separation efficiency compared with the binary heterojunction. As shown in Fig. 9b, the Nyquist semicircle radius of the Ag₂MoO₄/Ag₂S is smaller than that of the bare Ag₂MoO₄, indicating its lower interface charge transport resistance. Ag₂MoO₄/Ag₂S/MoS₂ displays the smallest EIS radius, suggesting its lowest resistance for charge transport. All of the results of the PL, photocurrent response and EIS are consistent, which demonstrates that the formation of the Ag₂MoO₄/Ag₂S/MoS₂ ternary heterojunction is the most effective strategy to improve transfer and separation and inhibit the recombination of the interface charge carriers.

Fig. 9 (a) Transient photocurrent response and (b) EIS Nyquist plots of the as-prepared samples.

3.2 Photocatalytic Properties

The photocatalytic performances of the prepared products were evaluated through the degradation of RhB and TC. As shown in Fig. 10a, pure Ag₂MoO₄, Ag₂S and MoS₂ show poor photocatalytic activities. The removal ratios of RhB within 15 min irradiation are only 42%, 11% and 19% for Ag₂MoO₄, Ag₂S and MoS₂, respectively. The composites show higher photocatalytic activities compared with the pristine samples, and Ag₂MoO₄/Ag₂S/MoS₂ exhibits the most superior photocatalytic performance in all of the samples. The degradation ratios of RhB with 15 min irradiation are 74.8% and 93.9% for Ag₂MoO₄/Ag₂S and Ag₂MoO₄/Ag₂S/MoS₂, respectively. The apparent rate constants of the catalysts toward the photocatalytic degradation of RhB are calculated to be 0.036 min⁻¹, 0.092 min⁻¹ and 0.189 min⁻¹ for Ag₂MoO₄, Ag₂MoO₄/Ag₂S and Ag₂MoO₄/Ag₂S/MoS₂, respectively (Fig. 10b). Ag₂MoO₄/Ag₂S/MoS₂ exhibits the highest rate constant, which is about 2 and 5 times higher than those of Ag₂MoO₄/Ag₂S and Ag₂MoO₄, respectively.

Fig. 10 (a and c) Photocatalytic degradation curves for RhB and TC. (b and d) Kinetic fits for the degradation of RhB and TC over various as-prepared samples under simulated sunlight irradiation.

For the photocatalytic degradation of TC, its degradation ratio by Ag₂MoO₄/Ag₂S/MoS₂ is 42.8% after 100 min irradiation, while it is only 3.7% and 24.9% for pure Ag₂MoO₄ and Ag₂MoO₄/Ag₂S, respectively (Fig. 10c). The apparent rate constant of the TC degradation for Ag₂MoO₄/Ag₂S/MoS₂ is 0.0054 min⁻¹, which is about 13 and 2 times higher than that of pristine Ag₂MoO₄ and Ag₂MoO₄/Ag₂S, respectively (Fig. 10d).

The better photocatalytic performance of the composites can be ascribed to the formation of heterojunctions, which promote the transfer and separation of the charge carriers and increase light absorption as well as specific surface area. In contrast to Ag₂MoO₄/Ag₂S, Ag₂MoO₄/Ag₂S/MoS₂ displays a higher photocatalytic activity. This can be ascribed to the introduction of MoS₂, which further improves the transfer of charge carriers and further increases light absorption as well as specific surface area (Fig. 6–9). The influence of the coupled amounts of Ag₂S and MoS₂ was investigated. As shown in Fig. 10 and Fig. S2 (ESI), the introduction of Ag₂S and MoS₂ can improve the photocatalytic performance of Ag₂MoO₄, nevertheless, excessive Ag₂S and MoS₂ may cause agglomeration, which inhibits the incoming light absorption and the transfer of the charge carriers, resulting in a decrease of photocatalytic efficiency. It was found that the optimized amount of Ag₂S and MoS₂ in the composite Ag₂MoO₄/Ag₂S/MoS₂ is 5% and 0.5% of molar ratio, respectively.

As the specific surface area of Ag₂MoO₄/Ag₂S/MoS₂ is larger than that of Ag₂MoO₄/Ag₂S and Ag₂MoO₄, we conducted a control experiment to distinguish the effect of the surface area and the charge separation and light absorption here. We blocked the surface of the sample by covering it with an inert layer of SiO₂. It was found that after covering with a layer of SiO₂, the apparent rate constant of the RhB degradation for Ag₂MoO₄/Ag₂S/MoS₂ is about 2 times higher than that of Ag₂MoO₄/Ag₂S (Fig. S3, ESI†), which is similar to the result without a covering of SiO₂ (Fig. 10b). This indicates that the surface area may not play a crucial role, but the charge separation and light absorption play an essential role in the photocatalytic reactions in this work.

Besides the high photocatalytic efficiency, the stability of the catalyst is another important factor for practical application. Thus, photocatalytic recycling experiments were performed to evaluate the stability of the catalyst Ag₂MoO₄/Ag₂S/MoS₂. All the used samples were collected together after each cycle test by centrifugation, washed, and vacuum-dried at 80 °C for 12 h. As shown in Fig. 11, the degradation efficiency of the catalyst does not decrease obviously after 3 cycles, demonstrating that the as-prepared catalyst Ag₂MoO₄/Ag₂S/MoS₂ has some stability. However, Fig. 11 shows a small decrease of degradation efficiency after 3 cycles. The reduced degradation efficiency in the cycle experiments may be ascribed to the photocorrosion of Ag₂S.⁴¹

Fig. 11 Repeated photocatalytic experiments for the degradation of RhB using Ag₂MoO₄/Ag₂S/MoS₂ as the catalyst under simulated sunlight irradiation.

4. Photocatalysis mechanism

For an in-depth investigation of the photodegradation mechanism, capture experiments toward the radicals were carried out. Herein, different scavengers were employed in the RhB photodegradation. Benzoquinone, TEOA, IPA and AgNO₃ are applied as the sacrificial agents for superoxide ions (˙O₂⁻), holes (h⁺), hydroxyl radicals (˙OH) and electrons (e⁻), respectively. Fig. 12 shows the degradation ratios of RhB in the presence of various trapping agents under simulated sunlight irradiation for 15 min. It reveals that the degradation ratio of RhB does not change in the presence of AgNO₃, indicating that the electrons are not the active species. The degradation ratios of RhB reach 80.4% and 91.0% in the presence of BQ and IPA, respectively, which are slightly less than those in the absence of trapping agents (93.9%). These results imply that ˙O₂⁻ and ˙OH do not play a crucial role toward the RhB degradation. However, the degradation ratio significantly reduces to 11.0% with the addition of TEOA. This indicates that h⁺ is the main active species, which plays a crucial part in the degradation of RhB. Therefore, the influence sequence of the active species for RhB degradation is h⁺ > ˙O₂⁻ > ˙OH.

Fig. 12 Trapping experiments by using different scavengers in RhB degradation over Ag₂MoO₄/Ag₂S/MoS₂.

The CB and VB of the as-prepared photocatalysts can be calculated according to the Mulliken electronegativity theory:⁶⁹

E_VB = X − E_e + 0.5E_g (2)

E_CB = E_VB − E_g (3)

where E_CB and E_VB are the conduction band and valence band relative to the normal hydrogen electrode (NHE), E_e is the energy of free electrons (4.5 eV), E_g is the band gap of the photocatalysts, and X is the electronegativity value of the semiconductors. The X values for Ag₂MoO₄, Ag₂S and MoS₂ are 5.92 eV, 4.96 eV and 5.32 eV, respectively.^67,72,73 On the basis of the formula and the obtained band gaps, the E_VB and E_CB values are calculated to be 2.90 eV and −0.05 eV for Ag₂MoO₄, 0.95 eV and −0.02 eV for Ag₂S, and 1.61 eV and 0.04 eV for MoS₂, respectively.

Based on the results of the trapping experiments and the calculated values of the CB and VB for the catalysts, the tentative mechanism for the ternary composite is illustrated in Fig. 13. Ag₂MoO₄ and Ag₂S are excited and engender photoinduced electron–hole pairs under the simulated solar light irradiation. MoS₂, in the ternary heterojunction, acts as a good electron acceptor due to its two-dimensional structure and excellent electrical carrier mobility.^56,74 Both of the generated electrons on the CB of Ag₂MoO₄ and Ag₂S can rapidly transfer to that of MoS₂. The transfer of the electrons between the interfaces of the semiconductors can effectively inhibit the recombination of the electrons and holes. The holes on the VB of Ag₂MoO₄ and Ag₂S can directly oxidize the organic molecules into small molecules, e.g., CO₂ and H₂O. Meanwhile, the accumulated electrons on the CB of MoS₂ can react with the adsorbed O₂ to form ˙O₂⁻ radicals. These formed ˙O₂⁻ radicals and subsequently generated ˙OH radicals can also participate in the photocatalytic degradation reaction. The photocatalytic reaction processes are illustrated below.

Ag₂MoO₄ + hν = h⁺ + e⁻

Ag₂S + hν = h⁺ + e⁻

Ag₂MoO₄ (e⁻_CB) + Ag₂S (e⁻_CB) → MoS₂ (e⁻_CB)

MoS₂ (e⁻) + O₂ = ˙O₂⁻

˙O₂⁻ + 2H⁺ + e⁻ = H₂O₂

H₂O₂ + e⁻ = OH⁻ + ˙OH

h⁺ + ˙O₂⁻ + ˙OH + Pollutants = CO₂ + H₂O + degradation intermediate

Fig. 13 The possible photocatalytic degradation mechanism for Ag₂MoO₄/Ag₂S/MoS₂.

5. Conclusions

In summary, a novel ternary heterojunction hybrid Ag₂MoO₄/Ag₂S/MoS₂ was successfully fabricated via a facile two-step method. In contrast to the pristine catalysts, the as-prepared composite exhibits superlative photocatalytic performances. The tremendously enhanced photocatalytic performances can be ascribed to the formation of a ternary heterojunction, which effectively inhibits the recombination of the photoinduced charge carriers and increases light absorption and the specific surface area. The empirical findings in this study offer a new strategy to rationally design Ag₂MoO₄-based photocatalytic systems with significantly promoted photocatalytic performances for application in environmental protection issues.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the Innovative research team of high-level local universities in Shanghai, National Natural Science Foundation of China (No. 21271126), Program for Innovative Research Team in University (No. IRT 13078), and National 973 Program (No. 2010CB933901).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nj04290k

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