Novel Ag₂S quantum dot modified 3D flower-like SnS₂ composites for photocatalytic and photoelectrochemical applications

Abstract

Novel 3D flower-like Ag₂S/SnS₂ composites were fabricated by a hydrothermal and ion exchange method. Uniform Ag₂S quantum dots were homogeneously interspersed on 3D flower-like SnS₂. The samples were characterized through X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) analysis. As expected, the as-prepared Ag₂S quantum dot modified 3D flower-like SnS₂ composites exhibited enhanced photoelectrochemical (PEC) performance and photocatalytic activities. The photocurrent density of 3% 3D flower-like Ag₂S/SnS₂ at 2.0 V (vs. Ag/AgCl) (0.65 mA cm⁻²) was about 3.25 times higher than that (0.2 mA cm⁻²) of 3D flower-like SnS₂. The photocatalytic activity of 3D flower-like Ag₂S/SnS₂ composites was assessed through the degradation of methyl orange and the photocatalytic H₂ evolution performance under visible light irradiation. The coupling of SnS₂ and Ag₂S quantum dots could notably promote the photocatalytic activity. The experimental results indicated that 3% 3D flower-like Ag₂S/SnS₂ composites showed the best photocatalytic performance for the degradation of methyl orange. These composites also exhibited a high H₂ evolution rate of 574.7 μmol h⁻¹ g⁻¹ under visible light irradiation, approximately 5.57 times higher than that of pure 3D flower-like SnS₂. Based on the calculation, radical trapping tests and ESR, a plausible mechanism for increased photoactivity was proposed. This work provides experimental insight into the design of low-cost photocatalysts for highly efficient photodegradation and photocatalytic H₂-production.

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1. Introduction

The growing serious environment and energy problems have been attracting much attention. Semiconductor photocatalysis have been applied for contaminant treatment and H₂ evolution from water splitting using ultraviolet (UV) or visible light irradiation.^1,2 However, semiconductor photocatalysis involves the recombination of photo-induced carriers.^3,4 Therefore, the coupling of two semiconductor photocatalysts to form a heterogeneous semiconductor photocatalyst is regarded as an effective strategy to overcome the above limitation and further extend the photoabsorption range.^5,6

As one of the most important semiconductor chalcogenide nanostructures, SnS₂ with a band gap of ca. 2.2 eV displays a wide photoelectrochemical response in the region of visible light.^1,7 To our knowledge, SnS₂ nanomaterials have a lot of advantages, including low cost, non-toxicity, good stability and oxidative and thermal stability in air, and so on.^8–11 So as to upgrade the photocatalytic activity of pure SnS₂, several materials including TiO₂,¹² SnO₂,^13,14 WO₃,¹⁵ La₂Ti₂O₇,¹⁶ g-C₃N₄,^17,18 Bi₂S₃ ¹⁹ and BiOBr ²⁰ were coupled with SnS₂. Compared to a single semiconductor, the coupling between two different semiconductors has proved to be successful in developing better performance photocatalysts, which is due to assembling heterostructures to enhance their photochemical conversion property. As an important chalcogenide semiconductor, Ag₂S has a narrow band gap (about 1.0 eV), which can harvest solar light and apply to the photocatalytic field.^21,22 There are a large number of reports on the synthesis of Ag₂S-based photocatalysts. For example, Ag/Ag₂S/CuS,²³ ZnS/CdS/Ag₂S,²⁴ Ag₂S/ZnS,²⁵ Ag₂S/Bi₂WO₆,²⁶ Ag₂S/TiO₂ ²⁷ and so on have been used for the photocatalytic degradation of organic pollutants. In addition, Ag₂S as a cocatalyst for assisting hole transfer is beneficial for enhancing the photocatalytic H₂ evolution.^28–30 In view of the above, coupling of SnS₂ with other narrow band gap photocatalysts, such as Ag₂S quantum dots, to form a heterostructure is expected to prevent the recombination of photo-generated electron–hole pairs and broaden the absorption wavelength.

Here, we synthesized 3D flower-like Ag₂S/SnS₂ composite photocatalysts by a simple facile ion-exchange method for the first time. Under visible light irradiation, photodegradation was determined by the removal of methyl orange (MO), and the photocatalytic hydrogen evolution over the 3D flower-like Ag₂S/SnS₂ composites was investigated in Na₂S and Na₂SO₃ aqueous solution. The results show that the 3D flower-like Ag₂S/SnS₂ composites exhibit significantly improved photocatalytic performance compared with that of pristine SnS₂, and a plausible mechanism for the increased photoactivity was proposed.

2. Experimental section

2.1 Preparation

2.1.1 Preparation of 3D flower-like SnS₂ microspheres. 3D flower-like SnS₂ microspheres were synthesized using a solvent thermal process dissolving SnCl₄·5H₂O and thioacetamide (TAA) in 50 mL polyethylene glycol (PEG-600) with stirring, and the above mixture was transferred into a Teflon-lined stainless steel autoclave and then maintained at 180 °C for 12 h. After the solvent thermal reaction, the autoclave was air-cooled. A brown sample was collected, and washed with ethanol and deionized water several times. Finally, the sample was dried at 60 °C for 12 h.

2.1.2 Preparation of 3D flower-like Ag₂S/SnS₂ composites. 0.1 g as-prepared SnS₂ samples were scattered in 40 mL deionized water and stirred for 30 min, and a certain quantity of AgNO₃ solution (0.1 mol L⁻¹) was added to the above suspension with further stirring for 1 h. Afterwards, the product was washed with deionized water and absolute ethanol, followed by drying at 60 °C overnight. Based on the different mass percentages of AgNO₃, the quality of the compounds of Ag₂S, denoted as 0.5% Ag₂S/SnS₂, 1% Ag₂S/SnS₂, 3% Ag₂S/SnS₂, and 5% Ag₂S/SnS₂ composite, was controlled.

2.2 Characterization

The crystal constitution of the resultant samples was characterized using a Shimadzu X-ray diffraction (XRD, Cu-Kα radiation, λ = 1.5406 Å) instrument in the range of 10 to 80° at 7° min⁻¹. Raman spectra was recorded at room temperature using a Renishaw InVia micro-Raman spectrometer with excitation at 785 nm. The morphologies were observed by scanning electron microscopy (SEM) equipped with an energy dispersive X-ray detector (EDX, Thermo NORAN system 7) and transmission electron microscopy (TEM) performed on a JEOL-JEM-2010 (JEOL, Japan) operating at 200 kV. An ES-CALab PHI500 X-ray photo-electron spectrometer was used to record the X-ray photoelectron spectra (XPS) of the samples with Mg-kα radiation. UV-vis diffuse reflectance spectra (DRS) was recorded on a UV-2450 Shimadzu UV-vis spectrophotometer (Japan), with BaSO₄ used as the substrate. The photoluminescence (PL) spectra of the photocatalyst were obtained using a Varian Cary Eclipse spectrometer with an excitation wavelength of 325 nm. The electron spin resonance (ESR) signals of spin-trapped radicals by the spin-trap reagent DMPO (Sigma Chemical Co.) were recorded on a Bruker model ESR JEOL FA-200 spectrometer in water for hydroxyl radicals and in methanol for superoxide radicals, respectively.

2.3 Photocatalytic degradation and hydrogen production

The photocatalytic degradation activity of the as-prepared samples was assessed through the degradation of MO. The sample (70 mg) was dispersed into MO solution (70 mL, 10 mg L⁻¹) in a cylindrical quartz vessel and magnetically stirred in the dark for 1 h to attain the adsorption–desorption equilibrium. A 300 W Xe lamp was then used as the light source that was equipped with a cutoff filter (λ > 420 nm) during the photocatalytic process. At an interval of 30 min, the absorbance of MO was measured by using a UV-2450 spectrometer at 463 nm after centrifugation. The photocatalytic hydrogen output experiments were carried out in a 100 mL Pyrex flask at atmospheric pressure and ambient temperature, and the flask's openings were sealed with silicone rubber septums. A 300 W Xe arc lamp with a UV cutoff filter (>400 nm) positioned 20 cm away from the reactor was used as the light source to trigger the photocatalytic reaction. In a characteristic photocatalytic experiment, 50 mg of samples were added into a 100 mL mixed aqueous solution consisting of 0.35 M Na₂S and 0.25 M Na₂SO₃. Before irradiation, the catalyst suspension was scattered through an ultrasonic bath for 10 min, followed by bubbling with nitrogen by the reactor for 40 min to get rid of the dissolved oxygen wholly and guarantee that the reactor was in an anaerobic circumstance. A magnetic stirrer was used to keep the photocatalyst particles in the suspension state. The amount of produced H₂ gas was determined by gas chromatography (GC-14C, Shimadzu, Japan, TCD) with nitrogen as a carrier gas and a column with 5 Å molecular sieves.

3. Results and discussion

3.1 Structure and property characterization

The XRD patterns of 3D flower-like Ag₂S/SnS₂ composites and the pure 3D flower-like SnS₂ sample are presented in Fig. 1A. As displayed in Fig. 1(A), the diffraction peaks at 2θ = 15.00°, 28.28°, 32.16°, 41.90°, 50.04°, 52.56°, 54.98°, 58.54°, 62.72° and 67.17° matched well with the (0 0 1), (1 0 0), (1 0 1), (1 0 2), (1 1 0), (1 1 1), (1 0 3), (2 0 0), (004) and (2 0 2) crystal faces, respectively. The results were consistent with SnS₂ (JCPDS card no. 23-0677).^31,32 The XRD patterns of pure Ag₂S are presented in Fig. S1;† the diffraction peaks are consistent with Ag₂S (JCPDS card no. 65-2356).³³ As shown in Fig. 1A(b–e), compared with the diffraction patterns of bare SnS₂, the diffraction peaks of SnS₂ in the composites were only gradually weakened and no XRD peaks of Ag₂S were detected for 3D flower-like Ag₂S/SnS₂ composites because of the low content of Ag₂S (0.5–5%) and high dispersion of Ag₂S quantum dots on the surface of 3D flower-like SnS₂.³⁴ The presence of Ag₂S will be demonstrated by other means of representation. Fig. 1B presents the Raman spectrum of 3D flower-like SnS₂ and 3% 3D flower-like Ag₂S/SnS₂ composites. For SnS₂, the strong peak at 312 cm⁻¹ can be indexed to the A_1g mode of SnS₂.^35–38 The Raman spectrum of 3D flower-like Ag₂S/SnS₂ composites displays a slightly weaker peak of SnS₂ at 312 cm⁻¹ (A_1g), which is in agreement with the results of XRD.

Fig. 1 (A) XRD patterns for 3D flower-like SnS₂ and 3D flower-like Ag₂S/SnS₂ composites; (B) Raman spectra of 3D flower-like SnS₂ and 3% 3D flower-like Ag₂S/SnS₂ composites.

Fig. 2A and B demonstrate the SEM images of the as-prepared 3% 3D flower-like Ag₂S/SnS₂ composites. It can be seen that the composites are consistent flower-like spheres with an average diameter of 2.0–2.5 μm and Ag₂S quantum dots cannot be seen on the surface of 3D flower-like SnS₂. In order to further confirm the presence of Ag elements, the element distribution was examined by EDS and EDX-mapping; Sn, Ag and S elements can be seen in the 3% 3D flower-like Ag₂S/SnS₂ composite (Fig. 2C and D). Fig. 2C shows clearly that the 3D flower-like Ag₂S/SnS₂ composite is comprised of Sn, Ag and S elements, indicating the formation of the Ag₂S/SnS₂ composite. The EDX elemental mapping of the 3% 3D flower-like Ag₂S/SnS₂ composite (Fig. 2D) showed that Ag was highly dispersed in the 3% 3D flower-like Ag₂S/SnS₂ composite. As shown in Fig. S2,† it can be seen that Ag₂S appears in the agglomerated state and is reunited into larger particles. The microstructure of the 3% 3D flower-like Ag₂S/SnS₂ composite was further characterized by means of TEM (Fig. 2E and F). Fig. 2E and F show that the 3D flower-like SnS₂ is comprised of several self-assembled irregular plates and Ag₂S quantum dots were grown on the surface of 3D flower-like SnS₂. As shown in Fig. S3,† we have carried out the high resolution TEM (HR-TEM) investigation of the 3% Ag₂S/SnS₂ composite, which shows Ag₂S QDs on the 3D flower-like SnS₂ and reveals the resolved lattice fringes of the SnS₂ (100) planes with a spacing of 0.315 nm and the Ag₂S (101) planes with a spacing of 0.341 nm.^18,33 It can be seen that the size of Ag₂S quantum dots is less than 10 nm. Therefore, it is noteworthy that we synthesized Ag₂S quantum dot modified 3D flower-like SnS₂ composites in this work.

Fig. 2 (A) and (B) SEM images; (C) EDS spectrum and (D) mapping distribution of Sn, S, and Ag; (E) and (F) TEM image of 3% 3D flower-like Ag₂S/SnS₂ composites.

Fig. 3 shows the XPS survey spectra and the corresponding Sn 3d, S 2p, Ag 3d high-resolution spectra for 3D flower-like SnS₂ and the 3% 3D flower-like Ag₂S/SnS₂ composite. The survey spectrum in Fig. 3A shows clearly that pure 3D flower-like SnS₂ is composed of Sn and S elements while the 3% 3D flower-like Ag₂S/SnS₂ composite is composed of Sn, S and Ag elements. Two obvious peaks at 487.1 and 495.6 eV could be attributed to the Sn 3d_5/2 and Sn 3d_3/2 orbitals of Sn⁴⁺ in SnS₂, respectively (Fig. 3B).^39,40 In relation to SnS₂, the Sn 3d XPS peak in the 3% 3D flower-like Ag₂S/SnS₂ composite shows a slight offset and the intensity of the peak has a certain degree of weakening. As shown in Fig. 3C, it is clear that the binding energies of S 3d_3/2 (162.1 eV) and S 3d_1/2 (163.3 eV) in 3D flower-like SnS₂ are higher than those of 3D flower-like Ag₂S/SnS₂ composites (161.9 eV and 163.2 eV), which are characteristic of S²⁻.^20,41,42 At the same time, the intensity of the S 2p peak has a certain degree of weakening. In general, the Sn 3d and S 2p peaks are offset, mainly due to the introduction of Ag₂S quantum dots affecting the chemical environment of Sn⁴⁺ and S²⁻. Two peaks (Fig. 3D) at 368.5 and 374.5 eV are attributed to Ag 3d_5/2 and Ag 3d_3/2, respectively, which are characteristic of Ag⁺, indicating that Ag mainly exists in the form of Ag₂S.^43–45 In conclusion, according to the XRD, Raman and XPS results, it can be confirmed that Ag₂S/SnS₂ composites have been successfully synthesized.

Fig. 3 XPS of 3D flower-like SnS₂ and 3% 3D flower-like Ag₂S/SnS₂ composites. (A) Full survey spectra and the high resolution XPS spectra of the samples: (B) Sn 3d and (C) S 2p; and (D) Ag 3d of 3% 3D flower-like Ag₂S/SnS₂ composites.

Fig. 4 displays the UV-vis diffuse reflectance spectra (DRS) of 3D flower-like SnS₂ and 3D flower-like Ag₂S/SnS₂ composites. For SnS₂, the absorption edge is about 550 nm, which broadens the visible-light absorption region (Fig. 4A). Fig. S4† displays the UV-vis diffuse reflectance spectra (DRS) of pure Ag₂S which exhibit a strong light absorption from ultraviolet to visible light. In contrast, Ag₂S/SnS₂ composites exhibit an obvious enhanced visible light absorption at the wavelength ranging from 550 to 800 nm, which indicates that the incorporation of Ag₂S quantum dots can effectively extend the visible light absorption edge of SnS₂. According to the theory of optical absorption for indirect semiconductors, the band gap energy (E_g) of 3D flower-like SnS₂ is calculated according to the following equation:^20,46

ahv = A(hv − E_g)²

where a, h, v, A and E_g are the absorption coefficient, the Planck constant, the light frequency, a constant and the band gap energy, respectively. By extrapolating the linear portion of (ahv)^1/2vs.(hv) curves (Fig. 4B), the E_g of SnS₂ is estimated to be about 2.23 eV, which is in agreement with the previous paper results.⁴⁷ The data of the band gap information and VB-XPS results are shown in Fig. S5.† As shown in Fig. S5A,† the E_g of all 3D flower-like Ag₂S/SnS₂ composites with a Ag₂S weight percentage of 0.5%, 1%, 3% and 5% was estimated to be about 2.22, 2.18, 2.13 and 2.08 eV, respectively. It can be seen that the band gap of the composites becomes narrower than 3D flower-like SnS₂ as the Ag₂S content increases. The valence band (VB) of 3D flower-like Ag₂S/SnS₂ composites was measured from the VB-XPS spectra and is shown in Fig. S5B.† The VB of all 3D flower-like Ag₂S/SnS₂ composites with a Ag₂S weight percentage of 0.5%, 1%, 3% and 5% was estimated to be about 1.58, 1.47, 1.35 and 0.89 eV, respectively. It can be observed that as the silver sulfide content increases, the valence band of the Ag₂S/SnS₂ composites becomes more and more negative.

Fig. 4 (A) UV-vis spectra of 3D flower-like SnS₂ and 3D flower-like Ag₂S/SnS₂ composites; (B) band gap energies of the 3D flower-like SnS₂.

3.2 Photocatalytic degradation and H₂-production activity

The photocatalytic activity of the 3D flower-like Ag₂S/SnS₂ composites was assessed for the degradation of MO (10 mg L⁻¹) under visible light irradiation. As shown in Fig. 5, the 3D flower-like SnS₂ exhibited a photocatalytic efficiency of 21.8% in 240 min. When Ag₂S and SnS₂ are coupled to construct the composites, even if the Ag₂S content is extremely low, the photocatalytic activity is dramatically enhanced. The photocatalytic activity of all 3D flower-like Ag₂S/SnS₂ composites with a Ag₂S weight percentage of 0.5%, 1%, 3% and 5% is higher than that of 3D flower-like SnS₂. The optimal photocatalytic performance was obtained when the Ag₂S content was 3%; that is, almost 59.1% of MO was degraded in 240 min. It is suggested that the Ag₂S content has a significant influence on the photocatalytic activity of 3D flower-like SnS₂. At the same time, Fig. 6 shows the catalytic stability test of 3% 3D flower-like Ag₂S/SnS₂ composites. It was shown that 3% 3D flower-like Ag₂S/SnS₂ composites maintained their catalytic photoactivity in 240 min without appreciable deactivation after four recycling processes.

Fig. 5 Photocatalytic degradation of MO over the samples under visible light.

Fig. 6 The cycling runs of 3% 3D flower-like Ag₂S/SnS₂ composites for the photodegradation of MO under visible light.

The photocatalytic hydrogen evolution of 3D flower-like Ag₂S/SnS₂ composites was assessed to further illustrate the enhanced photocatalytic activity (Fig. 7). The 3D flower-like SnS₂ showed a poor hydrogen evolution rate (103.2 μmol h⁻¹ g⁻¹). However, the 3% 3D flower-like Ag₂S/SnS₂ composite displayed a higher hydrogen production activity of about 574.7 μmol h⁻¹ g⁻¹, which is approximately 5.6 times higher than that of pure SnS₂. The obvious photocatalytic and hydrogen production activities are attributed to the efficient visible light harvesting of Ag₂S quantum dots and the efficient charge separation.

Fig. 7 Hydrogen evolution rate over 3D flower-like SnS₂ and 3% 3D flower-like Ag₂S/SnS₂ photocatalysts from aqueous solution containing Na₂S + Na₂SO₃ sacrificial reagents under visible light irradiation (0.05 g, 100 mL (0.35 M Na₂S/0.15 M Na₂SO₃), 300 W Xe lamp, t = 3 h).

3.3. Possible mechanism for the increased photocatalytic activity of 3D flower-like Ag₂S/SnS₂ composites

The transient photocurrent response of pure SnS₂ and 3% 3D flower-like Ag₂S/SnS₂ under visible light irradiation was studied by photoelectrochemical experiments (Fig. 8). It can be seen that the 3D flower-like Ag₂S/SnS₂ composite shows higher photocurrent intensity than pure SnS₂. The results revealed that Ag₂S can speed up the separation of the photo-generated carriers. This observation suggests that the formed 3D flower-like Ag₂S/SnS₂ composite allows for the more efficient separation of photo-generated carriers compared to SnS₂ alone. Fig. 9 shows the linear sweep voltammetry measurements of the 3D flower-like SnS₂ and 3% 3D flower-like Ag₂S/SnS₂ composites at a scan rate of 10 mV s⁻¹. The complete sweep procedure was applied under visible light or dark conditions. Obviously, the photocurrent of 3% 3D flower-like Ag₂S/SnS₂ under visible light is distinctly higher than that of 3D flower-like SnS₂, because the heterostructure considerably enables the separation of photo-induced carriers and restrains the recombination of electron–hole pairs.^48,49 It can be seen that the photocurrent density of 3% 3D flower-like Ag₂S/SnS₂ at 2.0 V (vs. Ag/AgCl) (0.65 mA cm⁻²) is about 3.25 times higher than that of 3D flower-like SnS₂ (0.2 mA cm⁻²). To further verify the charge transport behavior in this composite, electrochemical impedance spectroscopy (EIS) has been carried out. Fig. 10 shows the EIS spectra of the 3% 3D flower-like Ag₂S/SnS₂ composite which shows that it has a smaller radius, suggesting that the lower transfer barrier of electrons and faster interfacial charge transfer occurred at the composite interface. In consequence, it can be concluded that Ag₂S speeds up the efficient charge transfer between Ag₂S and SnS₂ facilitating the carrier separation, which is responsible for the increased photocatalytic activity.

Fig. 8 The transient photocurrent responses of 3D flower-like SnS₂ and 3% 3D flower-like Ag₂S/SnS₂ composites under visible-light irradiation.

Fig. 9 Linear sweep voltammogram curves for 3D flower-like SnS₂ and 3% 3D flower-like Ag₂S/SnS₂ composites.

Fig. 10 Electrochemical impedance spectroscopy (EIS) of 3D flower-like SnS₂ and 3% 3D flower-like Ag₂S/SnS₂ composites under visible-light irradiation.

The photoinduced carrier separation efficiency of a photocatalyst can be investigated using photoluminescence (PL) analysis. Fig. 11 shows the PL spectra of 3D flower-like SnS₂ and 3% 3D flower-like Ag₂S/SnS₂ composites monitored at an excitation wavelength of 325 nm. The PL spectra for all of the samples show some principal emission peaks centered at approximately 422, 470, and 555 nm. Comparatively, the 3% 3D flower-like Ag₂S/SnS₂ composite shows lower intensity than pure SnS₂. This phenomenon indicates that the 3% 3D flower-like Ag₂S/SnS₂ composite shows slower recombination of the charge carriers, which may lead to enhanced photocatalytic activity.

Fig. 11 PL spectra of 3D flower-like SnS₂ and 3% 3D flower-like Ag₂S/SnS₂ composites.

The ESR spin-trap with DMPO technique was also performed on illuminated 3D flower-like SnS₂ and the 3% 3D flower-like Ag₂S/SnS₂ composite. As illustrated in Fig. 12A, there was no signal in the dark; while the light was on, it could be observed that the signal of DMPO-˙OH for the 3% 3D flower-like Ag₂S/SnS₂ composite is much stronger than that for 3D flower-like SnS₂, which suggests that ˙OH is the main reaction during the photocatalytic process. Additionally, the signal of DMPO-˙O₂⁻ could be detected when exposed to light (Fig. 12B). And the peak intensity of the 3D flower-like Ag₂S/SnS₂ composite was stronger than that of pure SnS₂ after the irradiation, suggesting that ˙O₂⁻ also played a vital role in the enhanced photocatalytic performance. ESR demonstrates that ˙OH and ˙O₂⁻ are the principle active species of the 3D flower-like Ag₂S/SnS₂ composite in the photooxidation procedure under visible light illumination. To further elucidate the reaction mechanism in depth in the photocatalytic system, free radical (˙OH, ˙O₂⁻ and h⁺) trapping experiments over the 3% 3D flower-like Ag₂S/SnS₂ composite were conducted, as shown in Fig. 13. In these radical species trapping experiments, isopropyl alcohol (IPA), argon (Ar) and triethanolamine (TEOA) were used as the scavengers of ˙OH, ˙O₂⁻ and h⁺ in the photocatalytic reaction, respectively. It can be seen that IPA, Ar and TEOA had significant effects on the photocatalytic activity of the composites, which suggests that ˙OH, ˙O₂⁻ and h⁺ are the main active species in the photocatalytic reaction. The results of ESR and trapping experiments are also consistent with the above results, which fully proves that ˙OH, ˙O₂⁻ and h⁺ are the main active species.

Fig. 12 DMPO spin-trapping ESR spectra recorded at ambient temperature in aqueous solution. 3D flower-like SnS₂ and 3% 3D flower-like Ag₂S/SnS₂ composite dispersion for DMPO-˙OH (A), as well as CH₃OH dispersion for DMPO-˙O₂⁻ (B) under visible irradiation.

Fig. 13 Effects of various scavengers on the photocatalytic efficiency of 3D flower-like Ag₂S/SnS₂ composites.

According to the above results and discussion, a probable mechanism for MO photodegradation and the photocatalytic H₂ production of the present 3D flower-like Ag₂S/SnS₂ composite is proposed, as shown in Fig. 15. The valence band (VB) of SnS₂ was measured by using the VB-XPS spectra, as shown in Fig. 13. Based on the DRS analysis in Fig. 4B and the equation E_CB = E_VB − E_g, the valence band (VB) and conduction band (CB) potentials of bare SnS₂ are about 1.91 eV and −0.34 eV, respectively. As shown in Fig. 15(a), the valence band potential (1.91 eV) is more negative compared with the redox potential for ˙OH/OH⁻ (+1.99 eV),⁵⁰ and thus OH⁻ cannot be oxidized to ˙OH by the photogenerated holes of the 3D flower-like Ag₂S/SnS₂ composite. Meanwhile, O₂/˙O₂⁻ (−0.33 eV) or O₂/H₂O₂ (+0.695 eV)²⁹ is more positive than the CB potential (−0.34 eV) of SnS₂, and thus O₂ can obtain these electrons and be reduced to yield ˙O₂⁻ or H₂O₂, and then ˙OH can be formed by the reaction between ˙O₂⁻ or electrons and H₂O₂.^51,52 As a consequence, the highly active photogenerated h⁺ of the 3D flower-like Ag₂S/SnS₂ composite can also directly oxidize MO and the above reactive species can effectively decompose the adsorbed MO molecule. For the photocatalytic H₂ production (Fig. 14b), when the photocatalyst is irradiated, electrons and holes would be generated. Because the CB and valence band (VB) energy levels of Ag₂S are bracketed by those of SnS₂, both electrons and holes will be transferred to Ag₂S. On the one hand, electrons transferred to Ag₂S cannot generate H₂ because the CB of Ag₂S is lower than that of the reduction potential of H⁺/H₂. So, Ag₂S can neither generate H₂ nor pass electrons from its VB to H⁺.⁵³ On the other hand, the electrons from SnS₂ will react with the protons from water and generate H₂. On the other side of the catalyst, holes will be transferred to Ag₂S, which will promote the carrier separation, and Ag₂S acts as an oxidation site. Thus, it is reasonable to consider that the better performance of the 3D flower-like Ag₂S/SnS₂ composite is due to the enhanced hole transfer via its shallower VB edge of Ag₂S. So, the loading of the cocatalyst Ag₂S in the composite can further accelerate the hole transfer to promote the separation of electron–hole pairs; the electrons can be scavenged by H⁺ to generate H₂. In all, this composite enhanced the photogenerated charge separation, which then caused enhanced photoactivity for either pollutant degradation or H₂ production.

Fig. 14 VB-XPS spectra of SnS₂.

Fig. 15 Proposed mechanism for (a) the photodegradation of MO and (b) photocatalytic H₂ production over 3D flower-like Ag₂S/SnS₂ composites.

4. Conclusions

In conclusion, a novel 3D flower-like Ag₂S/SnS₂ composite photocatalyst was fabricated through a solvent thermal method and in situ method. The 3% 3D flower-like Ag₂S/SnS₂ composite exhibits the optimal photocatalytic activity and high stability for the photodegradation of MO and photocatalytic H₂ production. The vital role of Ag₂S in the enhanced photocatalytic performance can improve the charge separation efficiency. According to ESR and the trapping experiments, ˙OH, ˙O₂⁻ and h⁺ are the main active species in the photocatalytic reaction. In addition, a photocatalysis mechanism was proposed for photodegradation and H₂ production. Moreover, Ag₂S-semiconductors are expected to be applied in the development of photocatalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No. 21777063, 21407065), the Natural Science Foundation of Jiangsu Province for Youths (BK20140533), the China Postdoctoral Science Foundation (2015 T80514), the Jiangsu University Scientific Research Funding (No. 14JDG052), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qi00513j

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