Interface photo-charge kinetics regulation by carbon dots for efficient hydrogen peroxide production

Abstract

Hydrogen peroxide (H₂O₂) is a multi-functional chemical for a range of industries, but the present H₂O₂ production requires complex processes, and leads to environmental pollution, etc. Solar water-splitting is one of the potential avenues to combine H₂O and O₂ into H₂O₂ through a cheap and clean way. Most of the photocatalysts involve multiple components and interfaces to improve the catalytic activity and energy conversion efficiency. However, it is difficult to regulate the photo-charge kinetics between the multi-interface catalyst, which hinders the practical application of photocatalysts. Here, we report a SnS₂/In₂S₃ type II heterostructure modified by carbon dots (SnS₂/In₂S₃/CDs) to highly improve the stability of sulfides and realize generation of H₂O₂ by the oxygen reduction reaction (ORR). Notably, in situ transient photovoltage measurements (TPV) were carried out to analyze the charge transfer process among SnS₂, In₂S₃ and CDs. The optimal SnS₂/In₂S₃/CD composite (n(Sn):n(In) = 50%) displays a prominent H₂O₂ production rate of 1111.89 μmol h⁻¹ g⁻¹ without any sacrificial agent under the conditions of normal pressure and neutral solution (pH = 7). The quantum efficiency (QE) of H₂O₂ production was calculated to be 3.9% under light (λ = 535 nm), and the solar energy conversion efficiency (SCC) was up to 1.02%, which is the highest known production of H₂O₂ from sulfides as photocatalysts. Our work provides a new way to regulate the photo-charge kinetics of the multi-interface catalyst using CDs to achieve the extremely efficient production of H₂O₂ by photocatalytic water-splitting.

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1. Introduction

Hydrogen peroxide (H₂O₂), as a multi-functional chemical, has been widely applied in lots of fields, such as chemical production, biomedicine, and water purification.^1–3 However, H₂O₂ is still synthesized by anthraquinone oxidation or the direct synthesis from H₂ and O₂ in industry. These methods need vast energy investment, multiple steps and may carry the risk of explosion.⁴ One potential avenue is to combine solar water-splitting and oxygen to form H₂O₂.⁵ During the photocatalytic process, H₂O can be oxidized to produce H₂O₂/O₂ by photo-holes. H₂O₂ can be produced simultaneously via the two-electron reduction of O₂ which is from air and produced by the catalyst.^6–8

Over the past decade, a range of oxide and organic photocatalysts were developed for H₂O₂ production, such as Pt/TiO₂,⁹ Au/BiVO₄,¹⁰ rGO/TiO₂/CoPi,¹¹ and ZIF-8/C₃N₄.¹² Most of the photocatalysts involve multiple components and interfaces to improve the catalytic activity and energy conversion efficiency.^13–18 Many multi-component and multi-interface catalysts have been proved to possess excellent photocatalytic performance compared to that of a single substance, such as heterojunctions, Z-schemes, co-catalysts, etc.^19–25 However, it is difficult to regulate the photo-charge kinetics of the multi-interface catalyst, which seriously hinders the practical application of photocatalysts.^26–29

Carbon dots (CDs), with a size of less than 10 nm, have become an attractive new material in the field of catalysis due to their abundant surface functional groups, unique electron storage ability, photoinduced electron transfer properties and efficient and stable reactive active centers.^30–36 Herein, we report that the SnS₂/In₂S₃/CD composite can achieve the production of H₂O₂ with high efficiency (1111.89 μmol h⁻¹ g⁻¹) and stability under visible light under the conditions of normal pressure and neutral solution (pH = 7) without any sacrificial agents. The quantum efficiency (QE) of this photocatalyst is up to 3.9% under a wavelength (λ) of 535 nm, which is the highest known production of H₂O₂ from sulfides as photocatalysts.^15,27,37,38 In this system, CDs can store the electrons generated by SnS₂ and In₂S₃, regulate the photo-charge kinetics in the multi-interface catalyst under visible light, and then further increase the electron–hole separation efficiency of SnS₂/In₂S₃/CDs. On the other hand, CDs are the photo-active sites for the oxygen reduction reaction (ORR) to produce H₂O₂.^14,39

2. Experimental

2.1 Synthesis of two-dimensional ultrathin SnS₂

Two-dimensional SnS₂ nanosheets were prepared through a one-step hydrothermal process. Briefly, 0.451 g (2 mmol) SnCl₂·2H₂O and 1.127 g (15 mmol) thioacetamide were mixed with 40 mL deionized water, and then stirred for 30 min. The mixture was heated up to 160 °C for 12 h and then the SnS₂ was collected and dried at 80 °C for 12 h after being washed with ethanol and deionized water.

2.2 Synthesis of three-dimensional In₂S₃

For comparison, three-dimensional In₂S₃ was also synthesized through a one-step hydrothermal route. Briefly, 0.382 g (1.0 mmol) In(NO₃)₃·4.5H₂O and 0.363 g (3 mmol) L-cysteine were mixed with deionized water (60 mL) and stirred for 30 min. The mixture was heated at 180 °C for 12 h. The In₂S₃ was collected and dried at 80 °C for 12 h after being washed with ethanol and deionized water.

2.3 Preparation of CDs

CDs were synthesized by the typical electrochemical method. Simply put, two graphite rods were placed in ultrapure water, and 30 V DC power was employed in the reaction. After 20 days of continuous stirring, the CD solution was acquired with a dark brown colour. After filtering the large size CDs through a three-layer filter paper device, the CD powder was obtained by freeze-drying.

2.4 Synthesis of SnS₂/In₂S₃/CDs

In our experiments, 0.382 g (1 mmol) In(NO₃)₃·4.5H₂O, 0.363 g (3 mmol) L-cysteine, 0.09 g (0.5 mmol) SnS₂, and 24 mg CDs were added to deionized water (60 mL) and stirred for 30 min. Then, the mixture was transferred into a Teflon-lined autoclave, which was heated for 12 h at 180 °C. Next, the SnS₂/In₂S₃/CDs were collected and dried at 80 °C for 12 h after being washed with ethanol and deionized water. For the preparation of SnS₂/In₂S₃/CDs, SnS₂ and In₂S₃ with different amounts (n(Sn):n(In) = 10%, 30%, 40%, 50%, 60%, 70% and 100%) were added to the compound. The detailed characterization test and experiment of photocatalysis for SnS₂/In₂S₃/CDs are included in the ESI.†

3. Results and discussion

The morphology of SnS₂/In₂S₃/CDs was characterized by using a scanning electron microscope (SEM) and transmission electron microscope (TEM). From Fig. 1a, the TEM image, HRTEM image and particle size distribution of monodisperse CDs were obtained, illustrating that CDs are particles with a diameter of 2–7 nm and lattice spacing of 0.21 nm, corresponding to the (100) crystal planes of graphitic carbon. From SEM images of SnS₂ and In₂S₃ (Fig. 1b and c), it was found that SnS₂ has a typical two-dimensional sheet structure with a size of 200 nm, and In₂S₃ has a three-dimensional spherical structure like a dandelion with a diameter of about 2 μm. The morphology and microstructure of the SnS₂/In₂S₃/CD composite in Fig. 1d show that the two-dimensional lamellar SnS₂ and the spherical structured In₂S₃ are tightly combined together. Fig. 1e–i show the elemental mapping images of SnS₂/In₂S₃/CDs, where the Sn, In, S, C elements are evenly distributed.

Fig. 1 (a) TEM image with size distribution and HRTEM image (inset) of CDs. SEM images of (b) SnS₂, (c) In₂S₃, and (d) SnS₂/In₂S₃/CDs. (e) HAADF-STEM image and the elemental mapping images of SnS₂/In₂S₃/CDs for (f) In, (g) Sn, (h) S and (i) C elements.

The crystal structures of SnS₂, In₂S₃, and SnS₂/In₂S₃/CDs were detected by X-ray diffraction (XRD). As shown in Fig. S1,† the relative intensity and peak position of SnS₂ and In₂S₃ are consistent with the standard cards JCPDS No. 22-951 and JCPDS No. 65-459, respectively, indicating the successful synthesis of SnS₂ and In₂S₃. In Fig. 2a, there are 6 distinct diffraction peaks of SnS₂ at 2θ = 15, 32.2, 28.4, 41.2, 50, and 52.6°, which are attributed to (001), (101), (100), (102), (110) and (111) reflections, and the sharp diffraction peaks of In₂S₃ at 2θ = 27.5, 28.8, 33.4 and 47.98° correspond to (311), (222), (400), and (440) crystal planes. It is noteworthy that the composite material SnS₂/In₂S₃/CDs has the characteristic peaks of both SnS₂ and In₂S₃, indicating successful combination of SnS₂ and In₂S₃. The important thing to note here is that XRD peaks of CDs cannot be measured in the XRD pattern of SnS₂/In₂S₃/CDs because of the small number of CDs and the weak diffraction intensity of CDs in the composite.

Fig. 2 (a) XRD patterns of SnS₂, In₂S₃ and SnS₂/In₂S₃/CDs (black, blue and red traces, respectively). (b) XPS survey spectrum of SnS₂/In₂S₃/CDs. (c) FTIR spectra of SnS₂/In₂S₃/CDs and CDs. (d) Raman spectra of SnS₂/In₂S₃/CDs, In₂S₃, SnS₂, and CDs.

The surface chemical and elemental states of SnS₂, In₂S₃, and SnS₂/In₂S₃/CDs were further studied using XPS technology. From the XPS full spectrum in Fig. 2b, it can be found that SnS₂/In₂S₃/CDs contain three elements of In, Sn, and S. At the same time, the full spectrum of SnS₂/In₂S₃/CDs can simultaneously identify the peaks of SnS₂ and In₂S₃, indicating that SnS₂ and In₂S₃ are combined successfully. The high-resolution XPS spectra of Sn 3d, In 3d, S 2p and C 1s for the SnS₂/In₂S₃/CD composite are shown in Fig. S2a–d.† As depicted in Fig. S2a,† two peaks of SnS₂/In₂S₃/CDs at 486.4 eV and 494.8 eV correspond to Sn 3d_5/2 and Sn 3d_3/2. Notably, the binding energies of Sn 3d_5/2 and Sn 3d_3/2 in SnS₂/In₂S₃/CDs both show a 0.5 eV negative shift compared to that of SnS₂, implying that SnS₂ has got the electrons in the composite. Meanwhile, according to Fig. S2b,† the peaks of In 3d_5/2 and In 3d_3/2 in SnS₂/In₂S₃/CDs (452.3 eV and 444.7 eV) show a 0.3 eV positive shift compared to that of In₂S₃, indicating that In₂S₃ has lost the electrons in the composite. As depicted in Fig. S2c,† the S 2p_3/2 and 2p_1/2 in SnS₂/In₂S₃/CDs correspond to 161.4 eV and 162.6 eV, respectively. Besides, the C 1s spectrum of the SnS₂/In₂S₃/CDs composite (Fig. S2d†) centered at 288.7 eV, 285.8 eV, and 284.5 eV can be attributed to the C=O, C–O, and C–C of CDs. As displayed in Fig. 2c, the FTIR spectrum proves the combination of SnS₂/In₂S₃ and CDs. The peaks at 3433 cm⁻¹, 1631 cm⁻¹, 1384 cm⁻¹ and 1331 cm⁻¹ of SnS₂/In₂S₃/CDs belong to the stretching vibrational peaks of the O–H, C=O, C=C, and C–O–C bonds in the CDs, respectively.³³ Besides, in Fig. 2d, the peaks at 1329 cm⁻¹ and 1567 cm⁻¹ of SnS₂/In₂S₃/CDs belong to the CDs and the peaks at 288 cm⁻¹ and 1433 cm⁻¹ of SnS₂/In₂S₃/CDs belong to the In₂S₃, indicating that Raman spectra further confirm the combination of SnS₂, In₂S₃, and CDs.

Fig. 3a shows a typical absorption range in the 420–800 nm region of SnS₂, In₂S₃, SnS₂/In₂S₃/CDs, indicating that the CDs can obviously enhance the visible light absorption capacity of SnS₂/In₂S₃/CDs. As depicted in Fig. 3b, the band gaps of SnS₂ and In₂S₃ are 1.93 eV and 2.01 eV, respectively. Furthermore, the valence bands of SnS₂ and In₂S₃ can be determined by ultraviolet photoelectron spectroscopy (UPS). As shown in Fig. 3c, the valence bands (VB) of SnS₂ and In₂S₃ were calculated to be 1.84 V and 1.36 V, respectively, while the conduction band (CB) values of SnS₂ and In₂S₃ were also calculated to be −0.09 V and −0.65 V (the specific calculation method is in S1†). According to Fig. 3d, the band gap structures of SnS₂ and In₂S₃ can be derived from the positions of the CB and VB, suggesting that SnS₂ and In₂S₃ both meet the requirements of photocatalytic water splitting.

Fig. 3 (a) UV-Vis absorption spectra of SnS₂/In₂S₃/CDs, In₂S₃, and SnS₂. (b) Plots of (αhν)²versus energy (hν) for SnS₂ and In₂S₃. (c) UPS spectrum of SnS₂ and In₂S₃. (d) Band structure diagram of SnS₂ and In₂S₃.

In Fig. 4a, the TPV overall diagrams of SnS₂, In₂S₃ and CDs are displayed. As shown in Fig. 4b, the charge extraction process of CDs (t_maxc) is the fastest compared to that of SnS₂ and In₂S₃, indicating that CDs have the fastest electron transmission capacity. Meanwhile, because it is easier for the lamellar structure of SnS₂ to transmit electrons than the three-dimensional spherical structure of In₂S₃, the charge extraction process of SnS₂ (t_max1) is faster than that of In₂S₃ (t_max2). According to Fig. 4c, the time attenuation constants of SnS₂ can be divided into two sections, namely (0.099) and (0.66), because the electron transport between the layers of SnS₂ can decrease the recombination rate of carriers. In contrast, the time attenuation constant of In₂S₃ was (0.123), suggesting that In₂S₃ has only a period of attenuation and the overall electron–hole recombination rate is faster compared to SnS₂. As displayed in Fig. 4d, the maximum charge extraction efficiency of In₂S₃ is greater than that of SnS₂ and CDs, suggesting that the main source of electrons and holes in the photocatalysis of SnS₂/In₂S₃/CDs is In₂S₃.

Fig. 4 (a) Comparison of the TPV with SnS₂, In₂S₃, and CDs. (b) Maximum extraction rate of the charge of SnS₂, In₂S₃ and CDs. (c) Charge extraction process of SnS₂, In₂S₃ and CDs. (d) Charge recombination process of SnS₂, In₂S₃ and CDs.

As shown in Fig. 5a, the charge extraction process of SnS₂/CDs (t_max3) is faster than that of SnS₂ (t_max1), indicating that CDs can increase the rate of charge transport in layer-to-layer of SnS₂. Meanwhile, the maximum charge extraction efficiency of SnS₂/CDs (0.017) is greater compared to SnS₂ (0.014), suggesting that CDs can promote the separation of carriers of SnS₂. In Fig. 5b, the time attenuation constants of SnS₂/CDs are (0.118) and (1.22), respectively. And (0.118) > (0.099), (1.22) > (0.66) mean that CDs can slow down the recombination rate of carriers from the angle of time attenuation constants. As depicted in Fig. 5c, the charge extraction process of In₂S₃ (t_max2) is a little faster than that of In₂S₃/CDs (t_max4), indicating that CDs slightly delay the transport rate of charges due to the charge storage function of CDs. Similarly, the maximum charge extraction efficiency of In₂S₃/CDs (0.03) is bigger than that of In₂S₃ (0.025), suggesting that CDs can promote the separation of carriers of In₂S₃. In Fig. 5d, the time attenuation constants of In₂S₃/CDs are (0.119) and (0.683), respectively. The second stage of time attenuation is caused by the secondary discharge process of CDs as good charge acceptors and donors.

Fig. 5 (a) Maximum extraction rate of the charge and charge extraction process of SnS₂ and SnS₂/CDs. (b) Charge recombination process of SnS₂ and SnS₂/CDs. (c) Maximum extraction rate of the charge and charge extraction process of In₂S₃ and In₂S₃/CDs. (d) Charge recombination process of In₂S₃ and In₂S₃/CDs.

In Fig. 6a, the i–t curve was used to evaluate the photocurrent response capability under six light-on and light-off cycles for SnS₂/In₂S₃/CDs, SnS₂/In₂S₃, In₂S₃, and SnS₂. Generally, the response intensity of photocurrent is closely related to photo-charge. Obviously, the photocurrent intensity of In₂S₃ is significantly stronger than that of SnS₂, indicating that In₂S₃ is mainly providing photogenerated charge in the photocatalytic process of SnS₂/In₂S₃/CDs, which is consistent with the result of the maximum charge extraction efficiency of SnS₂ and In₂S₃ in TPV. At the same time, the photocurrent intensity of SnS₂/In₂S₃ is significantly stronger than that of SnS₂ and In₂S₃, which proves that the heterojunction structure can increase the rate of photo-charge separation and the stability of sulfides. After adding CDs, the photocurrent intensity of SnS₂/In₂S₃/CDs is further enhanced, suggesting that the CDs further enhance the charge separation efficiency and stability of sulfides based on heterojunctions.

Fig. 6 (a) The photocurrent curves of the SnS₂/In₂S₃/CDs composite, SnS₂/In₂S₃ heterojunction, In₂S₃, and SnS₂. (b) The EIS curves of the SnS₂/In₂S₃/CDs composite, SnS₂/In₂S₃, In₂S₃, and SnS₂. (c) Maximum extraction rate of the charge and charge extraction process of SnS₂/In₂S₃ and SnS₂/In₂S₃/CDs. (d) Charge recombination process of SnS₂/In₂S₃ and SnS₂/In₂S₃/CDs.

Normally, a lower electron-transfer resistance inside the catalyst leads to a smaller semicircle arc in the EIS spectrum. As shown in Fig. 6b, SnS₂/In₂S₃/CDs has the smallest arc radius because of the presence of heterojunctions and CDs, indicating that the photo-electrons of SnS₂/In₂S₃/CDs have faster electron transmission capacity than that of SnS₂, In₂S₃ and SnS₂/In₂S₃. Meanwhile, the semicircle diameter of SnS₂ is smaller than that of In₂S₃, suggesting that it is easier for the two-dimensional lamellar structure of SnS₂ to transmit electrons than that of In₂S₃. As depicted in Fig. 6c, the charge extraction process of SnS₂/In₂S₃ (t_max5) is faster than that of SnS₂/In₂S₃/CDs (t_max6), indicating that the CDs are good electron donors and acceptors in SnS₂/In₂S₃/CDs. The continuous storage and release of electrons by the CDs during the electron transport process slow down the charge extraction rate. Meanwhile, the maximum charge extraction efficiency of SnS₂/In₂S₃/CDs (0.076) is 2.92 times that of SnS₂/In₂S₃ (0.026), suggesting that the addition of CDs can greatly promote the separation efficiency of carriers in SnS₂/In₂S₃/CDs. As shown in Fig. S4b and d,† the time attenuation constants of SnS₂/In₂S₃ are (0.076) and (0.258), respectively. And (0.076) < (0.099), (0.258) > (0.123) mean that after the formation of heterojunction between SnS₂ and In₂S₃, the electron–hole recombination rate of SnS₂ is accelerated and the electron–hole recombination rate of In₂S₃ is slowed down due to constant transfer of electrons from In₂S₃ to SnS₂. In Fig. 6d, the time attenuation constants of SnS₂/In₂S₃/CDs can be divided into three sections, namely (0.147), (0.177) and (0.752). And (0.076) is less than (0.147) and (0.258) is greater than (0.177) which illustrates that the addition of CDs slows down the electron–hole recombination rate of SnS₂ because of the ability of CDs to store electrons in the layers of SnS₂, and the CDs also decrease the electron–hole recombination rate of In₂S₃ due to the secondary discharge process of CDs.

The in situ TPV test was performed in different atmospheres shown in Fig. 7a and b. By integrating the curve of the in situ TPV, the amount of charge consumed during the oxidation and reduction reactions can be calculated. In the ACN solution with a N₂-saturated atmosphere, the charge generated by SnS₂/In₂S₃/CDs during light excitation and eventually consumed by the entire circuit was defined as Q₀. In the 5 vol‰ water/ACN (v/v) solution saturated with N₂, H₂O consumed charge as a water oxidation reaction (WOR), and the remaining charge was consumed by the entire circuit, which was defined as Q₁. Likewise, in the ACN solution saturated with O₂, O₂ as the ORR reactant consumes part of the charge, and the remaining charge was consumed by the entire circuit, which was defined as Q₂. As depicted in Fig. 7a, the SnS₂/In₂S₃/CDs displayed a higher photovoltage in the 5 vol‰ water/ACN (v/v) solution saturated with N₂, illustrating that H₂O consumed the charge of Q₁ − Q₀ (0.1133) in the WOR. According to Fig. 7b, SnS₂/In₂S₃/CDs consumed a lot of charge under the conditions of an O₂-saturated atmosphere, and the amount of charge consumed by the ORR is Q₀ − Q₂ (0.4817). Therefore, the ratio of (Q₁ − Q₀) and (Q₀ − Q₂) is the number of charges required for the SnS₂/In₂S₃/CDs to undergo the WOR and ORR. The half reaction rate ratio λ is 4.25 (0.4817/0.1133). At the same time, using the electrochemical test method in S6,† the oxygen evolution reaction (OER) rate constant k₁ (9.31 × 10⁻⁹) is also slower than that of the ORR rate constant k₂ (1.2 × 10⁻⁸), which is consistent with the results obtained by the in situ TPV (the specific calculation method is in S7†).⁴⁰ Besides, as displayed in Fig. 7c, the EPR spectra of SnS₂/In₂S₃/CDs support the existence of ˙O₂⁻ without the presence of ˙OH, suggesting that the ORR is the only way for H₂O₂ production by photocatalytic water splitting. In Fig. 7d, the electron transfer number of SnS₂/In₂S₃/CDs in the photocatalytic process can be calculated to be about 4, indicating that water is oxidized by photo-holes into O₂ instead of H₂O₂ during the photocatalytic reaction of SnS₂/In₂S₃/CDs.

Fig. 7 The in situ TPV pattern integral diagram of SnS₂/In₂S₃/CDs under (a) N₂-saturated ACN solution versus N₂-saturated 5 vol‰ water/ACN (v/v). (b) N₂-saturated ACN solution versus O₂-saturated ACN solution. (c) EPR spectra of SnS₂/In₂S₃/CDs, measuring the presence of ˙O₂⁻ and ˙OH under light-on and light-off. (d) i–t curves of SnS₂/In₂S₃/CDs using a rotating ring-disk electrode (RRDE) under light-on and light-off with N₂ saturation (0.1 M Na₂SO₄) and a rotating speed of 1600 rpm.

Fig. 8a shows the H₂O₂ generation rate with various catalysts. The production rate of SnS₂/In₂S₃/CDs was calculated to be 1111.89 μmol h⁻¹ g⁻¹, while that of SnS₂/In₂S₃ was 471.14 μmol h⁻¹ g⁻¹, that of In₂S₃ was 340.48 μmol h⁻¹ g⁻¹, and that of SnS₂ was 170 μmol h⁻¹ g⁻¹ in a 3 h reaction. SnS₂/In₂S₃/CDs exhibits the optimal photocatalytic ability, and the H₂O₂ production rate is 6.5 and 3.3 fold higher than that of pure SnS₂ and In₂S₃. The heterojunction formed by SnS₂ and In₂S₃ facilitates the separation of carriers, while the addition of CDs not only improves the light capture ability of the photocatalyst, as good electron acceptors and donors but also further facilitates the separation of carriers. As displayed in Fig. 8b, the catalyst exhibits excellent stability for five cycles with each cycle being performed for 3 h, indicating that the heterojunction structure and CDs enhance the stability of SnS₂/In₂S₃/CDs. As depicted in Fig. 8c, the effects of different molar ratios of Sn and In of the composite on the photocatalytic reaction were also explored. Clearly, the optimal content of molar ratios of Sn and In is 50%, and the optimal SnS₂/In₂S₃/CDs displays the highest photocatalytic activity with the H₂O₂ evolution rate of 1111.89 μmol h⁻¹ g⁻¹. The QE as a function of the absorption spectrum of SnS₂/In₂S₃/CDs is displayed in Fig. 8d, and is calculated to be 3.9% at the irradiation wavelength of 535 nm for 3 h (20 mg catalyst in 15 mL H₂O, the detailed calculations are displayed in S2†).

Fig. 8 (a) Evolution of H₂O₂ catalysed by SnS₂/In₂S₃/CDs, SnS₂/In₂S₃, In₂S₃ and SnS₂, (b) the cycling stability of SnS₂/In₂S₃/CDs for the photocatalysis of H₂O₂ production and (c) comparison of H₂O₂ evolution catalysed by SnS₂/In₂S₃/CDs with different molar ratios of Sn and In (1 : 10, 3 : 10, 4 : 10, 5 : 10, 6 : 10, 7 : 10 and 10 : 10) during the synthesis process. (d) Quantum efficiency (QE) performance (red points) versus UV-vis absorption spectrum of SnS₂/In₂S₃/CDs. Comparison of H₂O₂ evolution produced by SnS₂/In₂S₃/CDs in (e) different atmospheres (N₂/air/O₂) and using (f) different scavengers (AgNO₃/no scavenger/CH₃OH). All of the photocatalysis reactions were measured under visible light irradiation (λ ≥ 420 nm) for 3 h, with the system containing 20 mg powder dispersed in 15 mL H₂O.

As shown in Fig. 8e, the photocatalytic performance experiments of SnS₂/In₂S₃/CDs were performed under N₂, O₂, or air atmosphere conditions. It can be seen that the H₂O₂ produced by N₂ saturation (370.38 μmol g⁻¹ h⁻¹) was not completely inhibited and H₂O₂ produced by O₂ saturation (1751.41 μmol g⁻¹ h⁻¹) showed an obvious increase compared with air conditions (1111.89 μmol g⁻¹ h⁻¹), suggesting that oxygen is involved in the production of hydrogen peroxide from SnS₂/In₂S₃/CDs and the O₂ source of H₂O₂ produced by SnS₂/In₂S₃/CDs is composed of two parts from the air and the SnS₂/In₂S₃/CD WOR. Meanwhile, we have observed the dependence of kinetics on the concentration of O₂ in Fig. S5.† It shows that the H₂O₂ production of SnS₂/In₂S₃/CDs is related to Po₂, and with the increase of the reactant concentration, the rate of H₂O₂ generation gradually increases until it approaches the limit value which is caused by the limited number of active sites for the SnS₂/In₂S₃/CDs to adsorb and reduce O₂ when Po₂ is large enough. As depicted in Fig. S6,† before the detection of H₂O₂, we have detected the generation of H₂ using a GC-7900. There is no H₂ produced when SnS₂/In₂S₃/CDs was used as a photocatalyst under a N₂ or air atmosphere, indicating that SnS₂/In₂S₃/CDs only produces H₂O₂ even under a N₂ atmosphere. As shown in Fig. 8f, the photocatalytic performance of SnS₂/In₂S₃/CDs was displayed with different scavengers (AgNO₃/no scavenger/CH₃OH). AgNO₃ solution as an electron sacrificial agent (242.15 μmol g⁻¹ h⁻¹) and CH₃OH solution as a hole sacrificial agent (1426.56 μmol g⁻¹ h⁻¹) were added into the system also to verify the mechanism of hydrogen peroxide production by SnS₂/In₂S₃/CDs compared with the system without a scavenger (1111.89 μmol g⁻¹ h⁻¹), suggesting that electrons are involved in the reaction process of SnS₂/In₂S₃/CDs producing hydrogen peroxide. These are consistent with the mechanism process by which SnS₂/In₂S₃/CDs produces H₂O₂.

The mechanism of the photocatalytic reaction based on SnS₂/In₂S₃/CDs is depicted in Fig. 9. The charge transfer process between SnS₂ and CDs under the irradiation of visible light indicates that the electrons generated by SnS₂ will transfer to the CDs and enhance the electron transport process between the layers of SnS₂ (Fig. 9a). Similarly, the charge transfer process between In₂S₃ and CDs is displayed in Fig. 9b, suggesting that the electrons produced by In₂S₃ will also be transferred to the CDs and stored on the CDs for secondary discharge during the catalysis process. As shown in Fig. 9c, CDs display a more positive onset potential and a higher diffusion-limited current density compared with other sulfides, suggesting that CDs are the active sites in SnS₂/In₂S₃/CDs for the ORR due to its strong ORR process.^41,42

Fig. 9 (a) Schematic of photo-charge transfer between SnS₂ and CDs under visible light. (b) Schematic of photo-charge transfer between In₂S₃ and CDs under visible light. (c) ORR performance of CDs, SnS₂, In₂S₃, SnS₂/In₂S₃ and SnS₂/In₂S₃/CDs in O₂ saturation (0.1 M Na₂SO₄) with the rate of 1600 rpm. (d) The mechanism diagrams of solar water-splitting for the production of H₂O₂ over SnS₂/In₂S₃/CDs.

As depicted in Fig. 9d, when the SnS₂/In₂S₃/CDs was irradiated with visible light, SnS₂ and In₂S₃ will generate electron–hole pairs, respectively. And the electrons generated by SnS₂ and In₂S₃ will be transferred to the CDs to undergo reduction reaction with O₂ to produce H₂O₂. On the other hand, water will oxidize with holes on In₂S₃ to produce O₂. The WOR can further provide O₂ for ORR. In brief, the CDs in the multi-interface SnS₂/In₂S₃/CDs act as both the electron carriers of SnS₂ and In₂S₃ and the active centres of the ORR. The charge regulation of the CDs on the multi-interface further increases the efficiency of carrier separation between SnS₂ and In₂S₃. Meanwhile, it also prevents the damage of H₂O₂ to sulfides and greatly enhances the stability of sulfides.

Note that, CDs have adjustable characteristics in composition, size and surface functional groups, and their electronic structure and surface functional groups can be adjusted by further doping as well as further surface modification. There is plenty of room to achieve CDs to regulate photo-charge kinetics in multi-interfaces for efficient photocatalyst design.

4. Conclusions

The SnS₂/In₂S₃/CDs composite was prepared by a facile two-step hydrothermal treatment. The as-prepared photocatalyst displays superior activity with a yield of 1111.89 μmol h⁻¹ g⁻¹ and stability for H₂O₂ production without any sacrificial agent under the conditions of normal pressure and neutral solution (pH = 7). The QE value of the sole production of H₂O₂ was 3.9% at 535 nm and the SCC efficiency was determined to be 1.02%, which is the highest known production of H₂O₂ from sulfides as photocatalysts. In particular, the dual-functions of CDs contribute to the improved performance. CDs can not only store the electrons generated by SnS₂ and In₂S₃ under visible light to increase the electron–hole separation efficiency of SnS₂/In₂S₃/CDs, but also provide the active sites for SnS₂/In₂S₃/CDs to undergo the ORR reaction for producing H₂O₂. This work provides a creative way to regulate the kinetics of photo-induced charges in the multi-interface catalyst by combining CDs and then to achieve efficient production of H₂O₂ at normal pressure.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National MCF Energy R&D Program (2018YFE0306105), National Key Research and Development Project of China (2020YFA0406104), the Innovative Research Group Project of the National Natural Science Foundation of China (51821002), the National Natural Science Foundation of China (51725204, 21771132, 51972216 and 52041202), the Natural Science Foundation of Jiangsu Province (BK20190041), the Key-Area Research and Development Program of GuangDong Province (2019B010933001), the Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the 111 Project.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ta10231h

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