English

• Two-dimensional metal chalcogenides with atomically layered structure, due to their special optoelectronic and photochemical properties, are ideal low-dimensional system for photocatalysis, photodetection and optoelectronic devices [1-5]. Among them, SnS₂ has become one of the most promising two-dimensional semiconductors and attracted considerable attention in the field of photocatalysis due to its effective visible-light absorption, chemical stability and low toxicity [6, 7]. However, the photocatalytic activity of SnS₂ is still far below the actual requirement. SnS₂ suffers from sluggish separation of photoexcited carriers due to the serious recombination of excited electron-hole pairs. Therefore, in order to eliminate the bottleneck of the photoelectron conversion process in SnS₂, we must suppress the recombination and accelerate the separation of the photoexcited carrier [8-11].

  In recent years, it has been reported that defect engineering can effectively promote the separation of photoexcited carriers in photocatalysts [12-16]. For instance, the introduction of anion vacancies can serve as traps to capture photoexcited electrons, thus preventing their recombination with photoexcited holes [17-20]. Chun Du et al. reported that the sulfur vacancies can effectively regulate the electronic structure and introduce defect electronic states in ZnIn₂S₄ semiconductor. This defect electronic states can trap photoexcited electrons and promotes the separation efficiency of charge carriers, achieving 7.8 times higher hydrogen evolution rate than that of pristine ZnIn₂S₄ [21]. Similar occurrence was also reported in BiOBr with oxygen vacancies and In₂S₃ with sulfur vacancies [22, 23]. On the other hand, a great deal of works has been devoted to accelerate the transfer of charge carriers in metal sulfides through partial oxidization of the metal and in turn promote photocatalytic activities [24, 25]. For example, Xu Huichang et al. demonstrated that photoexcited electrons can be transferred directionally from the conduction band of the In₂S₃ lattice to that of the In₂O₃ lattice, while the photoexcited holes in the valence band of the In₂O₃ lattice can be transferred to that of the In₂S₃ lattice [26]. This strategy significantly facilitates the transfer of photoexcited electron - holes into active sites, thereby improving the photocatalytic activity.

  For these reasons, we proposed a simple method to introduce simultaneously sulfur vacancies and in-plane SnS₂/SnO₂ heterojunction into SnS₂ nanosheets, aiming to improve the photocatalytic activity of SnS₂ photocatalyst. In this work, we fabricated SnS₂ nanosheets with sulfur vacancies through high energy ballmilling in argon atmosphere. Compared with pure SnS₂ nanosheets (Pure SnS₂), the SnS₂ nanosheets with sulfur vacancies (Vs-SnS₂) exhibit better photocatalytic activity due to the sulfur vacancies that efficiently promote the separation of photoexcited electron-hole pairs. Furthermore, the SnS₂ powder was ball-milled in the air. Not only sulfur vacancies but also oxygen atoms are introduced and embedded into the SnS₂ sheets. The in-plane SnS₂/SnO₂ heterojunctions further reduce the recombination of photoexcited electrons-holes. As a result, the SnS₂ nanosheets with sulfur vacancies and in-plane SnS₂/SnO₂ heterojunction (Vs-SnS₂-O) exhibit optimal photocatalytic activity, with roughly six times higher photodegrading rate for methyl orange and four times higher photocatalytic reduction rate of Cr⁶⁺ than pure SnS₂ nanosheets.

  The defective SnS₂ nanosheets were prepared through high energy ball-milling (see the details in Supporting information). The structure and phase purity of the products were characterized by powder X-ray diffraction (PXRD). As shown in Fig. 1a, the PXRD patterns for all three samples can be indexed to layered structure of hexagonal SnS₂ (P-3m1, ICSD card No. 42566, a = b = 3.645(1) Å, c = 5.891(1) Å, as shown in Fig. S1 in Supporting information) [27], indicating that the as-prepared defective SnS₂ nanosheets maintain its original SnS₂ structure. The morphology of Vs-SnS₂-O nanosheets was characterized by scanning electron microscope (SEM) and transmission electron microscopy (TEM). The morphology is similar to that of pure SnS₂ nanosheets and Vs-SnS₂ nanosheets. As shown in Fig. S₂ (Supporting information), the sheet-like morphology of Vs-SnS₂-O shows sizes ranging from 100 nm to 200 nm. Additionally, the magnified TEM image in Fig. 1c inset clearly reveals layered structure. The fine structure of Vs-SnS₂-O nanosheets was further studied by high resolution transmission electron microscope (HRTEM). As shown in Fig. 1d, HRTEM image of Vs-SnS₂-O nanosheets displays discontinuous lattice fringes, corresponding to the interlayer distance of about 3.15 Å with a dihedral angle of 60° between the (100) and (010) planes of SnS₂. At the same time, another lattice fringe shows interplanar spacings of 3.34 Å and 1.58 Å with dihedral angle of 90°, corresponding to the (002) and (110) planes of the tetragonal SnO₂. These evidences indicate the presence of SnO₂ lattice confined in Vs-SnS₂-O nanosheets. In comparison, the crystal fringes of pure SnS₂ nanosheets and Vs-SnS₂ nanosheets are related to the single SnS₂ lattice as shown in Figs. S3 and S4 (Supporting information). Furthermore, the main Raman peak at 310 cm^-1 of all three samples can be assigned to SnS₂, as shown in Fig. 1b [28]. The weak Raman peak at 472 cm^-1 observed only in Vs-SnS₂-O nanosheets, however, are attributed to the Sn-O bonding in SnO₂ [29]. Which further confirms the presence of SnO₂ lattice in Vs-SnS₂-O nanosheets and is consistent with the results of HRTEM.

  Figure 1

  

  Figure 1.  Characterization of the obtained products. (a) PXRD patterns and (b) Raman spectra of as-prepared three samples. (c) The TEM image and (d) the corresponding HRTEM image of Vs-SnS₂-O nanosheets, clearly displaying the two lattice configuration of SnS₂ and SnO₂.

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  The electron paramagnetic resonance (EPR) spectra were used to detect the presence of sulfur vacancies in samples of Vs-SnS₂ and Vs-SnS₂-O. As shown in Fig. 2a, both Vs-SnS₂ and Vs-SnS₂-O samples exhibit a sharp and nearly equal intensity EPR signals located at g-value of 2.003, demonstrating the existence of sulfur vacancies with similar concentration [21]. However, there is almost no EPR signal in pure SnS₂ nanosheets, confirming that there is nearly no sulfur vacancy. Furthermore, the chemical states of Sn, S, and O in three samples were analyzed by X-ray photoelectron spectroscopy (XPS). Only Vs-SnS₂-O nanosheets has a significant XPS peak of O 1s compared with pure SnS₂ and Vs-SnS₂ as shown in Fig. 2b. The detailed XPS spectrum of O 1s shown in Fig. 2c displays a peak at 530.5 eV, which is consistent with the binding energy of O^2- in SnO₂ [30]. The content of lattice oxygen in Vs-SnS₂-O is approximately 2.6% as calculated from the peak area of S 2p XPS spectra (Fig. 2b). The O 1s peak at 531.9 eV can be attributed to the oxygen atoms chemisorbed at the surface [30]. Moreover, the XPS spectrum of Sn 3d shown in Fig. 2d can be fitted to peaks of Sn⁴⁺. Compared with pure SnS₂ nanosheets (486.88 eV, 495.37 eV), the binding energy of Sn 3d in Vs-SnS₂ and Vs-SnS₂-O (486.69 eV, 495.18 eV) experiences a red shift of 0.19 eV, indicating the existence of sulfur vacancies [31]. Additionally, the binding energy and the intensity of S 2p of Vs-SnS₂ and Vs-SnS₂-O (161.49 eV, 162.65 eV) become lower and weaker relative to the pure SnS₂ (161.69 eV, 162.85 eV) as shown in Fig. S5 (Supporting information). All of these evidences further confirm the existence of sulfur vacancies in Vs-SnS₂ and Vs-SnS₂-O due to lowcoordination of sulfur [21]. The XPS spectra of three samples are consistent with the EPR results. All these results demonstrate that we had successfully prepared the SnS₂ nanosheets with sulfur vacancies and in-plane SnS₂/SnO₂ heterojunction.

  Figure 2

  

  Figure 2.  EPR and XPS analysis of as-prepared three samples (a) EPR spectra. (b) XPS survey spectra. (c) O 1s XPS spectra and (d) Sn 3d XPS spectra of the three samples.

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  In order to reveal the effect of sulfur vacancies and in-plane heterojunction on the photocatalytic activity of SnS₂ nanosheets, the photodegradation of methyl orange (MO) and photocatalytic reduction of Cr⁶⁺ were performed under a 300 W Xe lamp without any auxiliary catalysts. As shown in Fig. 3a, 100 mg pure SnS₂ nanosheets can photocatalyticly degrade all the MO in 100 mL of 10 mg/L MO aqueous solution within 60 min. Accelerated photodegradation rates are detected for Vs-SnS₂ and Vs-SnS₂-O. Under the same condition, Vs-SnS₂ nanosheets photodegrade all MO within 20 min, the rate is about three times faster than that of pure SnS₂ nanosheets, indicating that the sulfur vacancies in the Vs-SnS₂ nanosheets can enhance photocatalytic activity. With sulfur vacancies and in-plane heterojunction, Vs-SnS₂-O nanosheets photodegraded all MO within 10 min under the same condition, roughly 6 times faster than that of pure SnS₂ nanosheets. This experiment indicates that the in-plane SnS₂/SnO₂ heterojunction can further improve the photocatalytic activity in Vs-SnS₂-O nanosheets. Similar trends were observed in the tests of photocatalytic reduction of Cr⁶⁺. As shown in Fig. S6a (Supporting information), pure SnS₂ nanosheets can photocatalyticaly reduce all Cr⁶⁺ in 150 mg/L K₂CrO₄ aqueous solution within 120 min. Meanwhile, Vs-SnS₂ and Vs-SnS₂-O finished the photocatalytic reduction within 50 min and 30 min, respectively. Furthermore, Vs-SnS₂-O nanosheets show a small decrease in photocatalytic activity even after five consecutive cycles, as shown in Fig. 3b and Fig. S6b (Supporting information). The small decrease of cycle performance is mainly caused by the descending adsorption of MO on Vs-SnS₂-O photocatalyst during cyclic photocatalysis (Fig. 3b). In this respect, the PXRD patterns, HRTEM images and XPS spectra shown in Figs. S7-9 (Supporting information) strongly demonstrated that the sulfur vacancies, the phase as well as in-plane heterojunction characteristic of Vs-SnS₂-O did not occur obvious variation after photocatalysis, verifying the favorable photostability.

  Figure 3

  

  Figure 3.  (a) Photocatalytic degradation of 10 mg/L MO solution in the presence of pure SnS₂, Vs-SnS₂, Vs-SnS₂-O and without any catalyst for comparison tests. (b) Photocatalytic MO degradation performance of Vs-SnS₂-O (red line) and pure SnS₂ (black line) in five cycles.

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  Obviously, the photocatalytic activity of Vs-SnS₂-O is distinctly superior to those of pure SnS₂ and Vs-SnS₂. Considering that all samples have similar surface area (Fig. S10 in Supporting information), three key factors that determine the catalytic activity of a semiconductor are photoabsorption, separation of photoexcited electron-hole pairs and transfer of these charge carriers. The photoabsorption properties of as-prepared samples can be evaluated by the UV–vis-NIR absorption spectra. As shown in Figs. 4a and b, Vs-SnS₂ has superior photoabsorption capacity (Eg_Vs-SnS2 = 1.91 eV) compared to that of pure SnS₂ (Eg_pureSnS2 = 2.12 eV) owing to the introduction of sulfur vacancy. The photoabsorption capacity of Vs-SnS₂-O (Eg_Vs-SnS2-O = 2.26 eV) is slightly weaker than that of pure SnS₂. The weak photoabsorption of Vs-SnS₂-O results from the size effect by the introduction of inplane SnO₂ lattice in SnS₂ nanosheets. The introduction of one inplane SnO₂ lattice in SnS₂ splits the original nanosheet into two nanosheets of smaller sizes, and two SnO₂ lattices further result in three splitting SnS₂ nanosheets and so forth. Therefore, smaller SnS₂ nanosheets lead to the blue shift of absorption edge because of the size effect (Fig. 4b). However, the sample of Vs-SnS₂-O shows the highest photocatalytic activity, suggesting that other factors aside from photoabsorption determine photocatalytic activity.

  Figure 4

  

  Figure 4.  (a) UV–vis-NIR absorption spectra and (b) (ahv)² versus (hv) plots of pure SnS₂, Vs-SnS₂ and Vs-SnS₂-O. (c) PL spectra. (d) Time-resolved PL decay spectra. (e) Electrochemical impedance spectroscopy (EIS) measurements and (f) periodic on/off photocurrent response at 0.4 V.

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  It is well known that the separation of photoexcited electronholes pairs is the decisive process in photocatalytic reaction. Photoluminescence (PL) emission spectra is a powerful tool to investigate the efficiency of charge separation in photocatalysts. The PL emission spectra of all three samples at the constant 375 nm excitation wavelength are shown in Fig. 4c. The pure SnS₂ nanosheets display the strongest emission spectrum peak at 437 nm due to the recombination of the photoexcited electronhole pairs [32, 33]. However, the samples of Vs-SnS₂ and Vs-SnS₂-O have much weaker PL peaks owing to the existence of sulfur vacancies, demonstrating the lower recombination and more efficient separation of charge carriers. Furthermore, time-resolved photoluminescence (TRPL) decay spectra shown in Fig. 4d were measured to investigate the migration process of photoexcited carriers. As depicted in the Table 1, the average PL lifetime (τ_A2) of Vs-SnS₂ nanosheets is prolonged to 4.42 ns compared with that of pure SnS₂ nanosheets (τ_A1 = 1.27 ns), principally attributing to the sulfur vacancies that can accommodate photoexcited electron and restrained the electron-hole recombination. Apparently, Vs-SnS₂-O has the longest average lifetime (τ_A3 = 5.00 ns compared to those of Vs-SnS₂ (τ_A2 = 4.42 ns) and pure SnS₂ (τ_A1 = 1.27 ns). Here, the PL emission spectra and TRPL decay spectra demonstrate that the introduction of sulfur vacancies can effectively trap electrons to inhibit the recombination of photoexcited carriers. In-plane heterojunction can further boost the separation of the photoexcited electron-hole pairs, resulting in the most efficient photocatalytic activity in Vs-SnS₂-O nanosheets.

  Table 1

  

  Table 1.  Photoluminescence lifetime parameters of the as-prepared three products.

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  Furthermore, the photo-electrochemical analysis is an effective method to characterize the generation and transport properties of photoexcited carriers. The electrochemical impedance spectroscopy of all three samples was measured in a solution of 0.5 mol/L Na₂SO₄ aqueous solution (pH 7.5) under AM 1.5 G at 100 mW/cm² illumination. As shown in Fig. 4e, compared to pure SnS₂ and Vs-SnS₂, Vs-SnS₂-O has the lowest electrochemical impedance, suggesting the most efficient photoexcited carriers transport on the surface of Vs-SnS₂-O. Furthermore, the periodic on/off photocurrent response was carried out, as demonstrated in Fig. 4f. Vs-SnS₂-O exhibits the highest photocurrent response intensity, illustrating most efficient generation and transport properties of photoexcited carrier due to the existence of sulfur vacancies and in-plane heterojunction in Vs-SnS₂-O. These results further confirm that SnS₂ nanosheets with sulfur vacancies and inplane heterojunction have the most efficient photoexcited charge transfer performance.

  To further understand the effect of sulfur vacancies on the promotion of charge carrier separation of SnS₂ nanosheets, electronic structure calculations based on the density functional theory (DFT) were performed. As shown in Fig. 5a, both the conduction band minimum and valence band maximum of pristine SnS₂ are dominantly contributed by the states of S-3p orbitals. Upon introducing sulfur vacancies in SnS₂ structure, a new peak just below the conduction band appears in the density of states as shown in Fig. 5b. The sulfur vacancies induce more electrons around neighboring Sn atoms which act like intermediate electronic states of Sn²⁺-5 s and S-3p orbitals in the forbidden band of SnS₂ (purple area in Fig. 5b). This new defect electronic states not only causes the red shift of absorption edge (Fig. 4a), but also act as shallow electronic traps. The shallow electron trap energy level induced by sulfur vacancy can effectively suppress the recombination of excited electron and holes, and promotes the charge carriers separation. Hence, the carrier lifetime is extended and more charge carriers participate in the photocatalytic reaction directly, which helps to significantly improve the photocatalytic performance of Vs-SnS₂-O and Vs-SnS₂ compared with pure SnS₂ nanosheets. Additionally, a sulfur vacancy induce two electrons around neighboring Sn⁴⁺ atom which acting as Sn(II)S (a wellknown photovoltaic material [34]) also benefitting for photocatalytic activity.

  Figure 5

  

  Figure 5.  The density of states of pristine SnS₂ (a) and Vs-SnS₂ with sulfur vacancies (b) calculated by Vienna ab-initio simulation package (VASP) (the Fermi energy is shifted by setting valence band maximum (VBM) as zero). Purple area: defect electronic states.

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  Furthermore, the in-plane SnS₂/SnO₂ heterojunction in Vs-SnS₂-O could favor the vectorial displacement of electrons and holes travel from one to another owing to the close work functions for SnS₂ (4.2 ~ 4.6 eV) and SnO₂ (4.7 eV) [35], which is similar to the mechanism of built-in electric-field, an shown in Fig. S11 (Supporting information). This in-plane SnS₂/SnO₂ heterojunction can strongly promotes the separation and transfer of photoexcited electron-hole pairs. Additionally, SnO₂ lattice in Vs-SnS₂-O acting as nano-oxide also is benefit for the organics degradation [36, 37], which is similar to the previous reported Co₃O₄ [38, 39]. The shallow electron trap energy level induced by sulfur vacancy can effectively suppress the recombination of excited electron and holes, and the in-plane SnS₂/SnO₂ heterojunction can strongly promotes the separation and transfer of photoexcited electronhole pairs. Thus, the sulfur vacancies and in-plane SnS₂/SnO₂ heterojunction of Vs-SnS₂-O synergistically improve the charge carrier separation and transfer efficiency, as shown in Fig. S12 (Supporting information), which promotes the efficient utilization of charge carriers and improves the photocatalytic activity of Vs-SnS₂-O. This interpretation is consistent with the results of photoluminescence spectrum and time-resolved transient photoluminescence decay spectra. As a result, the SnS₂ nanosheets with sulfur vacancies and in-plane SnS₂/SnO₂ heterojunction (Vs-SnS₂-O) has the most efficient photocatalytic performance.

  In summary, we have presented a simple way of high energy ball-milling to promote the photocatalytic activity of SnS₂ nanosheets by simultaneously introducing sulfur vacancies and inplane SnS₂/SnO₂ hererojunction. The sulfur vacancies in SnS₂ structure introduce shallow electronic traps near the valence band maximum. This shallow electronic traps can accommodate the photoexcited electrons to prevent the recombination with photoexcited holes, facilitating the separation of photoexcited electrons and holes in photocatalytic process. Moreover, the inplane SnS₂/SnO₂ heterojunction in the Vs-SnS₂-O further accelerates the photoexcited charge-carrier separation, as confirmed by PL and TRAL delay spectra. As a result, the SnS₂ nanosheets with sulfur vacancies and in-plane heterojunction (Vs-SnS₂-O) exhibited a stable and remarkably enhancement of photocatalytic activity compared to pure SnS₂ nanosheets. The current work provides a feasible method for the design of high-efficiency photocatalysts with stable performance through simple defective engineering.

  Declaration of competing interest

  This work has not been published before, nor is it under consideration for publication elsewhere. We have no conflicting commercial interest, other than being inventors of patents and patent disclosures that have been filed based on the technology described here. We have all read and discussed the manuscript, for which I am the corresponding author.

  Acknowledgments

  This work was financially supported by National Key Research And Development Program (No. 2016YFB0901600), CAS Center for Excellence in Superconducting Electronics, the Key Research Program of Chinese Academy of Sciences (Nos. QYZDJ-SSW-JSC013 and KGZD-EW-T06), National Natural Science Foundation of China (Nos. 21871008 and 21801247), and Jingdezhen Science and Technology Bureau (No. 20192GYZD008-21). Science Foundation for Youth Scholar of State Key Laboratory of High Performance Ceramics and Superfine Microstructures (No. SKL 201804).

  Appendix A. Supplementary data

  Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.07.052.

  
  
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• 

  Figure 1  Characterization of the obtained products. (a) PXRD patterns and (b) Raman spectra of as-prepared three samples. (c) The TEM image and (d) the corresponding HRTEM image of Vs-SnS₂-O nanosheets, clearly displaying the two lattice configuration of SnS₂ and SnO₂.

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  Figure 2  EPR and XPS analysis of as-prepared three samples (a) EPR spectra. (b) XPS survey spectra. (c) O 1s XPS spectra and (d) Sn 3d XPS spectra of the three samples.

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  Figure 3  (a) Photocatalytic degradation of 10 mg/L MO solution in the presence of pure SnS₂, Vs-SnS₂, Vs-SnS₂-O and without any catalyst for comparison tests. (b) Photocatalytic MO degradation performance of Vs-SnS₂-O (red line) and pure SnS₂ (black line) in five cycles.

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  Figure 4  (a) UV–vis-NIR absorption spectra and (b) (ahv)² versus (hv) plots of pure SnS₂, Vs-SnS₂ and Vs-SnS₂-O. (c) PL spectra. (d) Time-resolved PL decay spectra. (e) Electrochemical impedance spectroscopy (EIS) measurements and (f) periodic on/off photocurrent response at 0.4 V.

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  Figure 5  The density of states of pristine SnS₂ (a) and Vs-SnS₂ with sulfur vacancies (b) calculated by Vienna ab-initio simulation package (VASP) (the Fermi energy is shifted by setting valence band maximum (VBM) as zero). Purple area: defect electronic states.

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  Table 1.  Photoluminescence lifetime parameters of the as-prepared three products.

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