TiO2 Nanosheet Heterojunctions for

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Role of SnS2 in 2D-2D SnS2/TiO2 Nanosheet Heterojunctions for Photocatalytic Hydrogen Evolution Linqiang Sun, Zhicheng Zhao, Shun Li, Yiping Su, Liang Huang, Ningning Shao, Fei Liu, Yibin Bu, Haijun Zhang, and Zuotai Zhang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00122 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019

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ACS Applied Nano Materials

Role of SnS2 in 2D-2D SnS2/TiO2 Nanosheet Heterojunctions for Photocatalytic Hydrogen Evolution

Linqiang Sun,†,§,‡ Zhicheng Zhao,†,‡ Shun Li,*,†,∥ Yiping Su,† Liang Huang,⊥ Ningning Shao,† Fei Liu,† Yibin Bu,† Haijun Zhang⊥ and Zuotai Zhang*,† † School

of Environmental Science and Engineering, Guangdong Provincial Key Laboratory of

Soil and Groundwater Pollution Control, Southern University of Science and Technology, Shenzhen 518055, Guangdong, P. R. China ∥

Academy for Advanced Interdisciplinary Studies, Southern University of Science and

Technology, Shenzhen 518055, Guangdong, P. R. China § Shenzhen ⊥

Technology University, Shenzhen 518118, Guangdong, P. R. China

The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and

Technology, Wuhan 430081, P. R. China

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Abstract: Constructing heterojunctions with face-to-face interface disclose a new avenue for accelerating the charge separation in semiconductor photocatalysts toward highly efficient solar water splitting systems. Here, a novel SnS2/TiO2 2D–2D ultrathin nanosheet heterostructure was fabricated via a hydrothermal route in two steps. The obtained 2D–2D SnS2/TiO2 nanojunction has not only provided large contact areas, but also shortened the charge transport distance, resulting in significantly enhanced photocatalytic H2 evolution property. The H2 generation rate obtained for the optimized sample reaches 652.4 μmol h–1 g–1, far exceeding (~8 times) that of pristine TiO2 and SnS2 ultrathin nanosheets under simulated solar irradiation. Moreover, further analysis reveals a photoinduced carrier transfer from TiO2 to SnS2 at the interface junction, which causes a partial reduction of the SnS2 cocatalyst to SnS in the nanocomposite during the photocatalytic reactions. These results not only demonstrate that the construction of 2D–2D heterojunction is a promising approach to improve the photocatalytic H2 production activity of nanostructured semiconductor photocatalysts, but also provide new understanding into the role and evolution of SnS2 nanosheets during the photocatalytic reaction process in heterogeneous photocatalyst systems.

Key words: Solar water splitting; Two dimensional materials; Nanojunction; Cocatalyst; Interfacial charge transfer

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1. Introduction The direct conversion of sunlight into chemical fuels such as H2 has been regarded as one of the most promising routes of producing renewable energy. Photocatalytic water splitting using nanostructured semiconductors offers a very simple, environmental friendly, and lowcost approach to sustainable hydrogen production.1-2 After the first report of Honda–Fujishima effect,3 tremendous works have been devoted in the field of photocatalysis, and various semiconductor based photocatalytic materials have been developed.4-5 However, the conversion efficiency of photon energy into H2 of the studied photocatalytic materials is limited by three factors: (1) poor absorption ability of the visible light; (2) fast recombination of photoexcited carriers; and (3) low surface catalytic reaction efficiency. In recent years, many strategies have been proposed to overcome these bottlenecks, including cocatalysts loading,6 crystal facet engineering,7 band gap engineering,8-9 and construction of heterojunctions,10-12 etc. To suppress the recombination of charge carriers, noble metal nanoparticles, including Pt, Ag, Au, and Rh, have been widely used as efficient cocatalysts loading on semiconductor photocatalysts.13-15 However, the small contact area between semiconductor and zero-dimensional (0D) noble metal nanoparticles severely hinders the charge transfer at the interface. Moreover, the price of noble metals are generally very high, which greatly impede their industrial applications. Therefore, the accurate design of highly conductive and cheap cocatalysts without using noble metal with larger interface region is very important to further promote the solar-driven hydrogen evolution of heterostructure semiconductor photocatalyst systems. In the past decade, two-dimensional (2D) layered materials have been emerging as potential candidates for solar water splitting resulting from their unique structural property, 3



including high specific surface area and abundant surface active sites.16-19 In particular, the fabrication of 2D-2D layered nanoheterojunctions with large interfacial area and close contact can significantly promote the transportation of charges at the interface of the heterojunction owing to the strong electronic coupling effect, which is a crucial factor for promoting the photocatalytic activity.20-34 As an earth abundant and low cost material, layered transition metal dichalcogenides (TMDs) are considered to be attractive candidates in the field of photocatalytic water splitting, which can act as narrow band gap light absorber, electron conducting media, as well as active sites for catalytic reactions.35-36 Several metal sulfides, such as MoS2, WS2, NiS2, CdS, and CuS, have been used in heterostructure photocatalysts.37-46 As an important layered transition metal sulfide, tin disulfide (SnS2), with narrow band gap (2.18–2.44 eV) and high electrical conductivity, has attracted increasing attention toward solar-driven hydrogen evolution reaction and organic pollutant degradation.47-50 Up to now, a few works have been reported the combination of SnS2 with different semiconductor photocatalysts, such as SnS2/TiO2,51-57 SnS2/g-C3N4,20,

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SnS2/SnO2,59-60 and SnS2/RGO,61 which showed enhanced

photocatalytic performance. Despite of these progresses, the construction of 2D–2D layered photocatalysts using SnS2 has been relatively less studied. Moreover, understanding of the role and evolution of SnS2 during the photocatalytic reaction has not clearly demonstrated yet. Herein, we have synthesized a 2D–2D photocatalyst consisting of TiO2 nanosheets and ultrathin SnS2 nanosheets via an in situ solution based synthetic route. The as-prepared 2D-2D SnS2/TiO2 heterojunction exhibits significant enhancement on solar-driven photocatalytic hydrogen generation activity, compared to pure 2D SnS2 and TiO2 nanosheets. The 2D-2D junction ensures close interfacial contact between TiO2 and SnS2 ultrathin nanosheets, which is 4


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highly beneficial for separating the photogenerated carriers. Furthermore, a partial transformation of SnS2 to SnS was observed after the photocatalytic reaction.

2. Experimental 2.1 Synthesis of TiO2 nanosheets The TiO2 nanosheets was synthesized following a typical experimental procedure as reported.62 Ti(OBu)4 (8 mL) and HF solution (3 mL) were injected into a Teflon-lined reactor (50 mL) in sequence. After that, ethanol absolute (10 mL) was added into the above solution under magnetic stirring. Subsequently, the Teflon-lined autoclave was kept at 180 oC for 16 h in an oven. The obtained powder was separated by centrifugation, and washed with ethanol, NaOH solution (0.1 M), and deionized water for five times. Finally, the as-obtained powder sample was dried at 60 oC overnight. 2.2 Synthesis of 2D-2D SnS2/TiO2 nanosheets The as-prepared TiO2 nanosheets powder (50 mg) was dispersed ultrasonically in deionized water (50 mL). Then, different amount of SnCl4•5H2O (0.05, 0.1, 0.5 and 1 mmol) was dissolved under magnetic stirring for 4 h. The samples were denoted as 0.05-SnS2/TiO2, 0.1-SnS2/TiO2, 0.5-SnS2/TiO2, and 1-SnS2/TiO2. Subsequently, thioacetamide (0.3 g) was added and continued stirring for 0.5 h. The solution was transferred into a 100 mL Teflon-lined autoclave and was kept at 95° for 6 h in an oven. The obtained powder was separated by centrifugation and washed with ethanol followed by deionized water for several times. Finally, the samples were dried at 60 oC overnight in an oven.

3. Results and discussion

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Scheme 1. Synthesis process of the 2D-2D SnS2/TiO2 heterostructures. The fabrication process of the SnS2/TiO2 heterostructures is illustrated in Scheme 1, which involves a two-step hydrothermal synthesis procedure. The characterizations of the samples are described in the Supporting Information in detail. The crystal structure of the as-prepared TiO2, SnS2, and SnS2/TiO2 composites was investigated by X-ray diffraction (XRD). As shown in Fig. 1a, all diffraction peaks of the pure TiO2 and SnS2 sample match well with the tetragonal anatase TiO2 (JCPDS No. 04-0477) and the hexagonal SnS2 (JCPDS No. 23-0677), respectively. The comparatively broad diffraction peaks of SnS2 suggest the small crystal size of the sample. As for the composites, both TiO2 and SnS2 phases were confirmed from the XRD patterns, maintaining the same crystal structures as the pure ones. In addition, the peak intensity of SnS2 increases gradually as increasing the SnCl4•5H2O content from 0.1 to 1 mmol. Meanwhile, Raman spectra (Fig. 1b) of the samples were also obtained. Raman bands centered at 313 and 204 cm–1 correspond to the first-order A1g and Eg mode of hexagonal SnS2, respectively.50, 54 The peaks located at 149, 395, 515, and 640 cm–1 correspond to the Eg (1), B1g (1), B1g (2), and Eg (3) modes of anatase phase TiO2, respectively.54,

63

Therefore, both Raman and XRD

characterizations indicate the exclusive presence of anatase phase TiO2 and hexagonal phase SnS2 in the hybrids, and no evidence demonstrates the presence of impurities. 6


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Figure 1. (a) XRD patterns and (b) Raman spectra of TiO2, SnS2, and SnS2/TiO2 composites. The TEM image of the pure SnS2 exhibit very small sheet-like morphology with only few nanometers (Fig. S1a), and the lattice fringes of 0.592 nm match well with the (001) crystal plane of SnS2 (Fig. S1b). Typical SEM image (Fig. S2a) of the SnS2/TiO2 heterojunction sample (taking 0.1-SnS2/TiO2 as an example) shows well-defined plate-like morphology with a mean lateral size of about 100 mm, maintaining the same structural features with the pure TiO2 nanosheets.62, 64 The EDS spectrum (Fig. S2b) demonstrates the existence of Ti, O, Sn, and S elements in the composite, indicating the coexistence of TiO2 and SnS2. The TEM image shown in Fig. 2a clearly depicts sheet-like shape of the SnS2/TiO2 heterojunction with a thickness of about 10 nm. In addition, the atomic force microscopy (AFM) image clearly shows a sheet morphology of the SnS2/TiO2 hybrid, and the height profiles (Fig. 2b) reveal that the 7



heterojunction nanosheet has a thickness of ~10 nm. High-resolution transmission electron microscopy (HRTEM) characterizations were further carried out, which discloses more detailed information of the SnS2/TiO2 2D-2D nanojunction. As shown in Fig. 2c, many irregular shaped sheet-like structures (highlighted by the red dot line) with distinct dark contrast are distributed on the TiO2 nanosheets, suggesting the face-to-face growth of ultrathin SnS2 nanosheets on the TiO2 parent substrate. The selected area electron diffraction (SEAD) pattern (inset of Fig. 2c) indicate that the TiO2 nanosheets are single-crystalline with exposed (001) facets.62, 65 However, no diffraction spot of SnS2 was observed, which should be due to the ultrasmall size of the SnS2 nanosheets, in accordant with the XRD analysis. In addition, the magnified HRTEM image (Fig. 2d) of the interface of these two phases clearly displays lattice fringes of 0.236 and 0.592 nm, corresponding to the (004) plane of TiO2 and the (001) plane of SnS2, respectively, which clearly demonstrate the formation of 2D-2D heterojunctions. To further confirm the loading of SnS2 on the surface of TiO2, high-angle dark-field scanning TEM (HAADF-STEM) image and the corresponding EDS elemental mapping of the 2D-2D nanocomposite were also performed. The HAADF-STEM image (Fig. 2e) exhibits distinct contrast, which indicates the existence of more than one phases. Moreover, as expected, the EDS elemental mapping shows uniform distribution of Ti and O elements, while non-homogeneous dispersion of Sn and S elements on the nanosheets. These results clearly demonstrate that a 2D-2D heterojunction structure is formed between the TiO2 and SnS2 ultrathin nanosheets. The large contact area and intimate interfacial contact is highly beneficial for charge transfer, which may result in an improved photocatalytic water splitting performance.

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Figure 2. (a) TEM, (b) AFM (inset shows height cutaway view), (c and d) HRTEM images (inset of Fig. 2c shows the SAED patterns), (e) HAADF-STEM image, and the corresponding EDS element mapping images of (f) Ti, (g) O, (h) Sn, and (i) S for 0.1-SnS2/TiO2. X-ray photoelectron spectroscopy (XPS) survey scan (0.1-SnS2/TiO2) illustrated in Fig. 3a shows that the sample contains Ti, O, Sn, and S elements, which is in agreement with the EDS results. The peaks located at 459.0 eV and 464.7 eV (Fig. 3b) are assigned to the binding energies of Ti 2p3/2 and 2p1/2, respectively. The O 1s peaks shown in Fig. 3c can be deconvoluted into two peaks with binding energies of 530.2 eV and 531.7 eV, which belong to Ti–O bonds and hydroxyl groups, respectively.66, 67 For S 2p core level (Fig. 3d), peaks at 161.6 and 162.8 eV are ascribed to S 2p3/2 and S 2p1/2 of SnS2, respectively. The XPS spectrum of Sn in the 3d region (Fig. 3e) shows the binding energy of Sn 3d5/2 and Sn 3d3/2 peaks at 486.6 and 495.0 eV respectively, corresponding to Sn4+ of SnS2. To reveal the role and evolution of SnS2, XPS characterizations of the SnS2/TiO2 composite and pristine SnS2 after photocatalytic reaction was carried out, as discussed in the following parts.

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Figure 3. (a) XPS survey scan spectra of the 0.1-SnS2/TiO2 composite and high resolution XPS spectra of (b) Ti 2p, (c) O 1s, (d) S 2p, (e) Sn 3d, and (f) Sn 3d XPS spectra of the SnS2 nanosheets before and after photocatalytic reaction under simulated sunlight irradiation. The optical absorption spectra of the TiO2, SnS2, and SnS2/TiO2 heterojunctions was measured by UV-vis diffuse reflectance spectroscopy (Fig. 4). The pure TiO2 nanosheets show absorption edge at approximately 400 nm in the UV region. According to previously reports, SnS2 has a relatively narrow band gap of ~2.2 eV.47, 58, 68 However, the absorption of the pure SnS2 nanosheets covers the entire solar spectrum form UV to visible-light region without a sharp optical transition, which should be related with their ultrathin structural feature and high conductivity. Further works on understanding of this phenomenon is under way. After 10


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combining the SnS2 ultrathin nanosheets with TiO2, the absorption spectra onset of the hybrids lies between the pure SnS2 and TiO2, and gradually shifts to longer wavelength as increasing the proportion of SnS2 in the nanocomposite.

Figure 4. UV−Vis DRS of TiO2, SnS2, and different SnS2/TiO2 composites. The photocatalytic water splitting H2 production activities over the as-prepared photocatalysts were examined under simulated solar light irradiation (Xe lamp, 100 mW/cm2) with methanol as the hole scavenger (experimental details are described in the Supporting Information). Fig. 5a and b displays the hydrogen production rates with pure TiO2, SnS2 and different SnS2/TiO2 nanohybrids as photocatalysts. The generated hydrogen increases steadily with irradiation time (Fig. 5a) for all samples. As shown in Fig. 5b, the pure TiO2 and SnS2 sample exhibit a very weak photocatalytic H2 production rate of 74.3 and 86.8 μmol h-1 g-1, respectively, which should be due to their high recombination rate of photoexcited carriers. The mechanically mixed SnS2/TiO2 sample demonstrate similar H2 production rate as the pure TiO2 and SnS2 sample. In contrast, all 2D-2D SnS2/TiO2 composites present significantly enhanced photocatalytic activities. The 0.1-SnS2/TiO2 nanohybrid reaches the optimal hydrogen production rate of 652.4 μmol h–1 g–1, which is about 8 times of the pure TiO2 and SnS2. 11



However, further increasing the amount of SnS2 in the hybrid leads to a slightly decreased H2 production activity (621.5 and 526.7 μmol h-1 g-1 for 0.5-SnS2/TiO2 and 1-SnS2/TiO2, respectively). This is because that the excessive SnS2 can hinders the absorption of incident light and generation of the excited carriers by TiO2. In addition, excessive SnS2 may also act as charge recombination centers, resulting in relatively low photocatalytic performance. Similar results have been reported in different semiconductor heterojunction systems such as CuS/ZnS 44

and Cu-ZnIn2S4/MoS2.24 In addition, we have also performed the photocatalytic hydrogen

test of 0.1-SnS2/TiO2 composite under visible light irradiation (λ > 420 nm), which shows that no hydrogen was produced (Fig. 5a). This result indicate that SnS2 was mainly acting as cocatalyst in the composite and serving as electrons sink to separate electrons from TiO2, rather than visible light sensitizer. Moreover, the BET surface areas of the sample were estimated based on N2 adsorption measurements (Fig. S3). The surface area of the SnS2/TiO2 hybrid increases as increasing the loading amount of SnS2, which does not follow the photocatalytic activity, demonstrating that the surface area is not dominant in determining the catalytic performance.

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Figure 5. (a) Photocatalytic H2 evolution curves with different catalysts and (b) corresponding H2 evolution rates. The photoelectrochemical (PEC) measurements were performed to get a deeper understanding on the separation and transportation of photoexcited carriers (experimental details are described in the Supporting Information). As shown in Fig. 6, all samples display rapid transient photocurrent response for periodic light on-off cycles collected at 0 V vs. Ag/AgCl. It is clearly seen that the photocurrent densities of all SnS2/TiO2 hybrids are higher than that of pure TiO2, indicating that the loading of SnS2 greatly facilitates the carrier separation and transportation in TiO2. The photoresponse of the hybrids follows the order of 0.1-SnS2/TiO2 > 0.5-SnS2/TiO2 > 1-SnS2/TiO2, and the optimal photocurrent density achieved for 0.1-SnS2/TiO2 (10 μA cm–2) is about 10 times of bare TiO2. These results are consistent with the photocatalytic water splitting H2 production measurements. Noteworthy, in the present 13



work the pure SnS2 exhibits the highest photoresponse. In most cases, the photocurrent are in accordance with photocatalytic H2 evolution activity. However, the photocatalytic water splitting for H2 production are dependent on several factors including surface catalytic reaction and so on. Therefore, high photocurrent does not necessarily mean high H2 production activity. In our case, the high photocurrent of the SnS2 nanosheets should due to its high conductivity. Moreover, we also measured transient photocurrent response under visible light. The photocurrent density (10 μA cm–2) under UV-Vis light irradiation is more than 10 times higher than that of the same sample exposed by visible light (λ > 420 nm) (Fig. S4). These results also indicate that the SnS2 ultrathin nanosheets may act as cocatalyst in the hybrid SnS2/TiO2 materials, rather than narrow bandgap photosensitizer.

Figure 6. Transient photocurrent of different samples. To get a deep understanding of the recombination behavior of the photoexcited electronhole pairs in the SnS2/TiO2 hybrid, time-resolved photoluminescence spectra (TRPL) were further obtained, as shown in Fig. 7. All the decay curves could be well-fitted by a biexponential function,69 and the results are listed in Table S1. The 0.1-SnS2/TiO2 exhibits the longest decay time (4.4 ns) compared to pristine TiO2 (2.8 ns) and SnS2 (1.8 ns), suggesting 14


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reduced recombination rate of the photoexcited electron-hole pairs in the 2D-2D hybrids. These results are accordant with the photocatalytic H2 production performance and PEC properties as discussed above.

Figure 7. Time-resolved photoluminescence decay spectra of TiO2, SnS2, and 0.1-SnS2/TiO2. To reveal the role and evolution process of SnS2, we performed XRD and XPS characterizations on the SnS2/TiO2 composite after photocatalytic reaction under simulated sunlight irradiation for 2 h. As displayed in Fig. S5, XRD pattern of the 0.1-SnS2/TiO2 sample clearly proves the formation of SnS besides SnS2 after the photocatalytic reaction process. The ratio of SnS2 and SnS was estimated to be 1:8.5 from the XRD patterns. XPS analysis were also conducted, as shown in Fig. 3. Negligible change was observed for Ti 2p spectra (Fig. 3b). However, different from the as-prepared 0.1-SnS2/TiO2 sample, two new peaks corresponding to Sn2+ appeared at 485.3 and 493.7 eV after photocatalytic reaction, indicating the formation of SnS. In addition, the S 2p spectrum also demonstrates the coexistence of S 2p3/2 and S 2p1/2 of SnS and SnS2 (Fig. 3d). For O 1s spectrum, a new peak centered at 532.1 eV appeared, which is associated with Ti-O-Sn bonding energy. These results clearly indicate the electron transfer from TiO2 to SnS2 during the photocatalytic reaction process, which causes the partial reduction 15



of SnS2 to SnS. To further prove this hypothesis, we also conducted XPS analysis on pure SnS2 nanosheets under the same experiment condition (Fig. 3f), and no change was observed, demonstrating that pure SnS2 is highly stable under light illumination, in good agreement with previous reports.70 Therefore, we can conclude that the reduction of SnS2 in the hybrid should be due to the transfer of high energy electrons from the conduction band (CB) of TiO2 under solar light illumination.

Figure 8. (a) Schematic illustration of the photoinduced electron transfer process at the interface of the 2D-2D SnS2/TiO2 photocatalyst for hydrogen production, and (b) the corresponding band energy diagrams and charge transfer path. A tentative mechanism for the enhanced photocatalytic water splitting for hydrogen production using the 2D-2D SnS2/TiO2 nanojunction is proposed in Fig. 8. First, DFT calculation (Fig. S6) was performed for SnS2 and SnS. Consistent with previous reports, the calculated bandgap of SnS (1.37 eV) is narrower than that of SnS2 (2.50 eV), and the electron 16


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affinity of SnS is smaller than that of SnS2.70 Noteworthy, the bottom of the CB of both SnS2 and SnS are higher than the potential for H2 production. Upon simulated sunlight irradiation, the electrons in the valence band (VB) of TiO2 are excited to the CB, creating electron–hole pairs. Due to the energy difference of the CB level of TiO2 (–4.20 eV) and SnS2 (–4.28 eV), the photogenerated electrons in the CB of TiO2 will easily transfer to the 2D nanojunction interface and then migrate to SnS2. The calculated CB band position favors the electron transfer from TiO2 to SnS2, in accordance with our experimental results that SnS2 is serving as cocatalyst. Subsequently, the electrons will move quickly to the surface SnS2 due to the excellent conducting ability and the ultrathin morphology of the nanosheets. Moreover, the transferred electrons from TiO2 to SnS2 cause the partial reduction of SnS2 to SnS. As a result, these SnS2/SnS nanosheet composites can serve as cocatalyst to conduct photogenerated electrons efficiently from TiO2 to SnS2/SnS, where H2 is generated by the electrons. Meanwhile, the holes generated at the VB of TiO2 continuously react with methanol as sacrificial agents. The photocatalytic H2 production mechanism are expressed by the following equations: TiO2/SnS2 + hν → TiO2 (h+)/SnS2 (e–)

(1)

SnS2 + 2e– → SnS + S2–

(2)

SnS + 2H+ + S2– → SnS2 + H2

(3)

Similar phenomenon has been reported in other sulfide based heterojunction photocatalysts such as CuS/ZnS.44 In addition, we performed cycling photocatalytic hydrogen production experiments (Fig. S7), which demonstrate that the 2D-2D SnS2/TiO2 maintain good stability after three cycles. This result further supports the proposed mechanism above. It should be noted that the formation of 2D-2D structure play an important role in determining the 17



excellent H2 production peroperty. The large interface area and intimate contact of the 2D-2D junction can facilitate photoinduced interfacial charge transfer, inhibit the recombination of electron-hole pairs, and provide more active reaction sites for H2 evolution, resulting in enhanced photocatalytic activity.

4. Conclusions In summary, a novel 2D-2D SnS2/TiO2 heterojunction was synthesized based on a solution chemical route for photocatalytic water splitting for H2 production under simulated sunlight irradiation. The tight attachment and large contact interfaces allow the effective charge transfer, leading to the excellent solar-driven hydrogen production activity of the 2D-2D heterojunctions. Both photoelectrochemical and photoluminescence measurements indicate the reduced recombination rate of the photoinduced carriers in the hybrid system. In addition, a reduction of partial SnS2 to SnS in the SnS2/TiO2 hybrid after the photocatalytic reactions was confirmed by XPS and XRD analysis, and thus the SnS2/SnS hybrid ultrathin nanosheets are mainly serving as an electron sink and cocatalyst. Our work gives new insight into interfacial charge transfer in the SnS2/TiO2 junctions, which provides important implications for other sulfide based hybrid systems and related phenomena.

ASSOCIATED CONTENT Supporting Information. Materials characterization, photocatalytic and photoelectrochemical measurements, DFT computational methods, TEM images of pure SnS2 nanosheets, SEM and EDS images of 0.1SnS2/TiO2, nitrogen adsorption-desorption isotherms, photocurrent of 0.1-SnS2/TiO2 under visible light (λ > 420 nm), XRD pattern of 0.1-SnS2/TiO2 after simulated solar light irradiation, 18


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calculated band diagram of SnS2 and SnS, cycling experiments for photocatalytic hydrogen production, and calculated average lifetime τ for TiO2, 0.1-SnS2/TiO2 and SnS2. AUTHOR INFORMATION Corresponding Author [email protected]; [email protected] Author Contributions ‡ L. S. and Z. Z. contributed equally to this work.

Conflicts of interest There are no conflicts to declare.

Acknowledgments This work was supported by the Shenzhen Science and Technology Innovation Committee (Grant No. JCYJ20180504165648211 and JCYJ20170817111443306), and Shenzhen Freedom Exploration Project (Grant No. K16295046). Additional supports were provided by the NSFC (Grant No. 51802143 and 21802066) and Guangdong Provincial Key Laboratory of Soil and Groundwater Pollution Control (Grant No. 2017B030301012).

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Graphical Abstract

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TiO2 Nanosheet Heterojunctions for

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