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MoS2 hybrids for boosted photocatalytic hydrogen evolution under visible light
Journal of Alloys and Compounds 798 (2019) 553e559
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Ultrathin 2D/2D ZnIn2S4/MoS2 hybrids for boosted photocatalytic hydrogen evolution under visible light Lixian Huang a, b, 1, Bin Han b, 1, Xihe Huang b, Shujie Liang b, Ziqi Deng b, Weiyi Chen b, Miao Peng b, Hong Deng a, b, * a
School of Environment and Energy, Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters (Ministry of Education), Guangdong Engineering and Technology Research Center for Environmental Nanomaterials, South China University of Technology, Guangzhou 510006, China b School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 December 2018 Received in revised form 3 May 2019 Accepted 13 May 2019 Available online 16 May 2019
A well-connected hetero-layered ZnIn2S4/MoS2 composite was fabricated via an electrostatic selfassembly process to boost photocatalytic hydrogen evolution under visible-light irradiation. Ultrathin nanosheets contribute to efficient charge transfer by reducing electron diffusion length. In the sheet-onsheet heterostructure, ultrathin ZnIn2S4 nanosheets (NSs) and MoS2 NSs were well connected with each other to form a large and intimate contact interface, which features an efficient separation and migration of photoinduced charge carriers. Moreover, MoS2 NSs endow the composite with abundant active sites and lowers its overpotential for water reduction. Consequently, the ultrathin hetero-layered ZnIn2S4/ MoS2 composite with optimized loading of MoS2 achieves a hydrogen production rate of 4.974 mmol g1 h1, which is ca. 50 times as much as that of pure ZnIn2S4, and 2.2 times as much as that of its ZnIn2S4/Pt counterpart. This work provides inspiration for the fabrication of well-contacted heterolayered structure for photocatalytic hydrogen evolution. © 2019 Elsevier B.V. All rights reserved.
Keywords: Photocatalytic hydrogen evolution Ultrathin nanosheets Interfacial contact ZnIn2S4 MoS2
1. Introduction Solar-driven hydrogen production, a promising approach to produce sustainable renewable fuels, has been attracting increasing attention due to its potential in relieving severe energy crisis and environmental pollution [1e6]. Since the original work on photocatalytic water splitting on TiO2 [7], a large number of photocatalysts have been explored for hydrogen evolution [8e16]. Among them, ZnIn2S4, a kind of visible-light-responsive semiconductor, has received much attention owing to its suitable bandgap, high chemical stability, and environmental friendliness [17e21]. Particularly, the unique structure of ultrathin twodimensional (2D) ZnIn2S4 nanosheets (NSs), which originates from the reduced electron diffusion length and exposed surface atoms, promotes separation of photoinduced charges and enriches its active sites [22e27]. For example, Yang et al. recently reported
* Corresponding author. School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. E-mail address: [email protected] (H. Deng). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.05.162 0925-8388/© 2019 Elsevier B.V. All rights reserved.
that single-unit-cell ZnIn2S4 layers exhibited higher photocatalytic H2 production rate than the bulk ones [28]. However, the photocatalytic activity of ZnIn2S4 NSs itself is still unsatisfactory for the shortage of active sites and rapid recombination of photoinduced charge carriers [29,30]. Loading cocatalysts has been widely acknowledged as an effective strategy to boost the photocatalytic efficiency of H2 evolution. Cocatalysts not only provide more active sites for H2 evolution but also promote the separation of photo-generated charges [31e36]. MoS2, a 2D transition metal dichalcogenide has been certified as an ideal cocatalyst in photocatalytic hydrogen evolution due to the active sulfur atoms on its exposed edges, which strongly bond with the Hþ in water and thus serve as the active sites for proton reduction [33,37e42]. Especially, ultrathin MoS2 NSs with large edge-to-volume ratio tend to expose more sulfur edges for H2 production. Moreover, compared with its 0D/2D and 1D/2D counterparts, the ultrathin 2D/2D structure, with face-to-face interfacial contact, is able to provide multipath and reduced distance for electrons. Therefore, electrons can migrate across the interface and reach the surface more efficiently, thereby facilitating the separation of photo-generated electrons and holes [43e47]. As a result,
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constructing ultrathin 2D/2D ZnIn2S4/MoS2 nanocomposite is of great interest yet rarely reported in literature. Herein, ultrathin ZnIn2S4 and MoS2 NSs are employed to construct the ultrathin 2D/2D ZnIn2S4/MoS2 (ZIS/M) composite for photocatalytic H2 evolution via an electrostatic self-assembly process. The obtained well-contacted ultrathin 2D/2D composite exhibits lower activation potentials for H2 evolution and more effective separation of photoinduced charge carriers in comparison with pure ZnIn2S4. Consequently, the ultrathin 2D/2D ZIS/M nanocomposite achieves an enhanced H2 production rate of 4.974 mmol g1 h1 under visible-light irradiation, which is ca. 50 and 2.2 times as much as that of pure ZnIn2S4 ultrathin NSs and its ZnIn2S4/Pt counterpart, respectively. 2. Results and discussion 2.1. Materials synthesis and characterization Scheme S1 illustrates the electrostatic self-assembly process of 2D/2D ZnIn2S4/MoS2 (ZIS/M). Briefly, both ultrathin ZnIn2S4 nanosheets (NSs) and MoS2 NSs are negatively charged with zeta potentials of 45.7 mV and 23.8 mV in aqueous dispersion, respectively (Fig. S1). After being modified with poly (diallyldimethylammonium chloride) (PDDA), the zeta potential of MoS2 aqueous dispersion was changed to þ36.5 mV. Electrostatic attraction between positively charged PDDA-MoS2 NSs and negatively charged ZnIn2S4 NSs provides the preconditions for the fabrication of a hetero-layered structure with good interfacial contact [45]. The composites of MoS2 and ZnIn2S4 (ZIS/M) were denoted as ZIS/0.25% M, ZIS/0.5% M, ZIS/0.75% M, and ZIS/1% M, with theoretical weight percentage controlled to be 0.25, 0.5, 0.75 and 1 wt%, respectively. The real content of MoS2 in ZIS/M is measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), as shown in Table S1, which reveals that the mass ratio of MoS2 in as-prepared samples approaches its theoretical value. Powder X-ray diffraction (XRD) was employed to determine the crystalline phase of as-prepared MoS2, ZnIn2S4 and ZIS/M composites. The XRD pattern of MoS2 exhibits three broad peaks at 14.4 , 33.3 and 56.2 , as shown in Fig. S2, which can be indexed to (002), (101) and (106) planes of the hexagonal phase of MoS2 (JCPDS No. 37-1492), respectively. For pure ZnIn2S4 (Fig. 1), three distinct diffraction peaks at 21.5 , 27.6 , 47.1 match well with (006), (102), and (110) crystallographic planes of the hexagonal ZnIn2S4 (JCPDS No. 65-2023), respectively. After loading various
amounts of MoS2 onto it, all XRD patterns of ZIS/M composites present the same diffraction peaks as that of pure ZnIn2S4, indicating that the introduction of MoS2 has no impact on the crystalline phase of ZnIn2S4. However, no distinct peaks of MoS2 can be found in the XRD patterns of ZIS/M composites, which is ascribed to the low diffraction intensity, low loading amount, and high dispersion of MoS2 in the nanocomposites. Morphology and microstructure of as-prepared samples were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). As shown in Figs. S3A and B, SEM images of both ZnIn2S4 and MoS2 present flake-like morphology with a lateral size of a few micrometers and several hundred nanometers, respectively. Notably, it is observed that TEM images of as-prepared ZnIn2S4 (Fig. 2A) and MoS2 (Fig. 2C) also exhibit layered structures with the nearly transparent feature, which is in agreement with the ultrathin nature presented in SEM. Additionally, high-resolution TEM (HRTEM) images of ZnIn2S4 (Fig. 2B) and MoS2 (Fig. 2D) show distinct lattice fringes of 0.32 nm and 0.62 nm, matching well with the (102) lattice plane of ZnIn2S4 and (002) plane of MoS2, respectively. Atomic force microscopy (AFM) was used to determine the thickness of as-prepared products. The average thickness of ZnIn2S4 and MoS2 NSs is ca. 2.53 nm and 1.05 nm, respectively, as revealed in Fig. 2E and D, further confirming the ultrathin nature of the as-prepared nanosheets. After the assembly of ZnIn2S4 and MoS2 ultrathin NSs, the ZIS/M composite exhibits a sheet-on-sheet structure (Fig. 2G) with large contact surface. Moreover, interlaced lattice fringes of ZnIn2S4 (0.32 nm) and MoS2 (0.62 nm) can be found in the HRTEM image of the 2D/2D composite, as shown in Fig. 2H, revealing an intimate interfacial contact between ZnIn2S4 and MoS2. The large and intimate contact interface provides multiplex channels for carriers migration, accelerating efficient charge spatial separation [48,49]. Energy-dispersive X-ray spectroscopy (EDX) and elemental mappings of ZIS/M composite were used to further identify the constituent elements of the composite and the distribution of MoS2. The EDX result shown in Fig. S3C demonstrates the existence of In, Zn, Mo, and S elements in ZIS/M composite. Corresponding elemental mappings (Fig. 2I-M) show that the elements are homogeneously distributed throughout the ZIS/M composite, confirming the fabrication of hetero-layered structure with sufficient interfacial contact. X-ray photoelectron spectroscopy (XPS) was carried out to confirm the composition and chemical states of elements in the asobtained products. Fig. S4A illustrates the full-range XPS spectra of ZIS/M, ZnIn2S4 and MoS2. Compared with pure ZnIn2S4, a peak of Mo 3d appears in the spectrum of ZIS/M, which is further displayed clearly in the magnifying figure in Fig. S4B. Fig. 3AeB shows the high-resolution XPS spectrum of ZIS/M composite in the Mo 3d and S 2p region. Peaks at 228.7 eV and 231.9 eV can be assigned to Mo 3d5/2 and 3d3/2, respectively [50], suggesting the existence of Mo4þ. For S element of ZIS/M composite, peaks at 225.8eV, 161.9 eV and 162.9 eV can be assigned to S 2s, S 2p3/2 and 2p1/2, respectively, characteristic of S2 [28]. As revealed in Fig. 3C and D, In 3d peaks of ZIS/M at 445.03 eV and 452.6 eV can be indexed to In3þ, while Zn 2p peaks at 1021.83 eV and 1044.82 eV can be indexed to Zn2þ valence state [28]. XPS results further confirm the existence of MoS2 in the ZIS/M hybrids. Additionally, when compared with pure ZnIn2S4, the binding energy of In 3d and Zn 2p in the composite remains unchanged, implying that loading MoS2 would not affect the chemical structure of ZnIn2S4. 2.2. Photocatalytic hydrogen generation
Fig. 1. XRD patterns of pure ZnIn2S4 and ZnIn2S4/MoS2 (ZIS/M) composites loaded with various amounts of MoS2.
The photocatalytic hydrogen evolution using the as-prepared samples was tested under visible-light irradiation (l > 400 nm)
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Fig. 2. TEM images of ZnIn2S4 (A), MoS2 (C), and ZIS/M composite (G). HRTEM images of ZnIn2S4 (B), MoS2 (D), and ZIS/M composite (H). AFM images of ZnIn2S4 (E) and MoS2 (F). Corresponding elemental mappings of ZIS/M composite (I-M).
Fig. 3. High-resolution XPS spectra of Mo 3d (A) and S 2p (B) in ZIS/M composites. High-resolution XPS spectra of ZIS/M and ZnIn2S4 in the In 3d (C) and Zn 2p (D) region.
with lactic acid served as a sacrificial agent. No H2 could be detected without light irradiation or photocatalysts, suggesting that the photocatalytic hydrogen evolution is a light-driven process. As shown in Fig. 4A, a small amount of hydrogen could be detected by using pure ZnIn2S4 as photocatalyst. However, the photocatalytic activity of ZnIn2S4 alone was really unsatisfactory with a hydrogen evolution rate of 0.099 mmol g1 h1, which is attributed to the severe recombination of photoinduced electrons and holes [51,52]. After loading MoS2 onto ultrathin ZnIn2S4, there was a remarkable enhancement in photocatalytic activity.
Additionally, the amount of loaded MoS2 dramatically affected the photocatalytic activity of ZIS/M. Hydrogen evolution rates of ZIS/M composites increased along with MoS2 mass ratio. The highest H2 evolution rate for ZIS/0.75%M reached 4.947 mmol g1 h1, which is approximately 50 times as much as that of using pure ZnIn2S4 NSs. However, further loading of MoS2 led to the decrease of hydrogen evolution rate. This result can be ascribed to a shading effect caused by an excessive amount of black MoS2 covering the surface of ZnIn2S4, which shields the incident light and thus prevents the generation of photoinduced charge carriers [53e55]. No hydrogen
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Fig. 4. Photocatalytic H2 evolution rates over bulk ZnIn2S4, pure ZnIn2S4 NSs, MoS2 and ZIS/M loaded with various amounts of MoS2, ZnIn2S4/0.75% Pt and bulk ZIS/0.75% M (A). Recycling photoactivity test of 2D/2D ZIS/0.75% M (B).
could be detected when using MoS2 alone as a photocatalyst, suggesting that MoS2 is inactive for photocatalytic hydrogen evolution. Noble metal Pt is routinely used as a typical cocatalyst in promoting photocatalytic hydrogen production [56e58]. In this study, ZnIn2S4/0.75% Pt was also synthesized with its photocatalytic activity tested as a comparison. As revealed in Fig. 4A, the hydrogen generation rate of hetero-layered ZIS/0.75% M is ca. 2.2 times as much as that of ZnIn2S4/0.75% Pt. This result clearly demonstrates that 2D/2D ZIS/M composites have excellent photocatalytic activities and highlights the effectiveness of ultrathin MoS2 NSs as a cocatalyst in photocatalytic hydrogen evolution. To further verify
the advantages of ultrathin nanosheets as well as 2D/2D heterolayered structure, control experiments were performed with bulk ZnIn2S4 and bulk ZIS/M composites, respectively. As revealed in Fig. 4A, the hydrogen evolution rate of ZnIn2S4 NSs surpasses that of its bulk counterpart. The result may owe to ultrathin structure of ZnIn2S4 NSs, which not only leads to efficient charge transfer from the bulk to the surface but also maximizes the exposure of the surface to the reactant solution and light [59,60].Considering the interfacial contact between a light-harvesting semiconductor and a cocatalyst plays an important role in the separation of photoinduced electrons and holes [61,62], control experiments using 2D/
Fig. 5. Steady-state photoluminescence (PL) spectra (A), time-resolved transient photoluminescence (PL) decay transient photocurrent responses (B), transient photocurrent responses (C), and linear sweep voltammetry (LSV) curves of ZnIn2S4 and ZIS/0.75% M (D).
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2D-contacted hybrids and bulk composites were carried out to investigate the impact of interfacial contact areas. Bulk ZnIn2S4 and MoS2 were synthesized and coupled via electrostatic self-assembly. XRD patterns and SEM images of as-prepared bulk samples are shown in Fig. S5, exhibiting their crystalline phase and morphology. It is observed in Fig. 4A that the hetero-layered ZIS/M composite exhibits a higher hydrogen evolution rate than ZIS/M system consisting of bulk ZnIn2S4 and MoS2. Compared with 2D/2D heterolayered structure, the bulk composite lacks sufficient contact between components. That suggests the large and intimate contact interface in hetero-layered structure facilitates the photocatalytic hydrogen generation, which is ascribed to promoted separation of photoinduced charge carriers. Cycling tests were performed to examine the durability of the 2D/2D ZIS/M system. The photocatalytic hydrogen evolution rate of ZIS/0.75% M shows a negligible loss after 4 consecutive cycles under visible-light irradiation (Fig. 4B), suggesting the hetero-layered ZIS/M composites are very stable.
2.3. Origin of the enhanced performance To understand the origin of boosted photocatalytic hydrogen evolution over the hetero-layered ZIS/M composite, a series of photo- and electrochemical comparative characterizations of pure ZnIn2S4 and ZIS/0.75% M have been carried out. UVevis diffuse reflectance spectra (DRS) were collected to investigate the optical properties of the as-prepared samples. As shown in Fig. S6A, both pure ZnIn2S4 and ZIS/0.75% M display a wide range of light absorption from ultraviolet light to visible light, indicating their response to visible light. Especially, DRS of ZIS/0.75% M composite
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shows an increased absorption, which is ascribed to the introduced black MoS2, matching with the color change from light-yellow for pure ZnIn2S4 to gray-yellow for ZIS/0.75% M (Figs. S6B and C). The band gap of pure ZnIn2S4 and ZIS/0.75% M were estimated to be 2.33 eV and 2.35 eV, respectively, using the KubelkaMunk function [63e65]. The similar absorption curves and band gaps manifest that MoS2 was located on the surface of ZnIn2S4 rather than inside of the crystal grating, which is in accordance with the XRD results. Steady-state photoluminescence (PL) and time-resolved photoluminescence measurements were carried out to study the efficiency of charge separation and transfer. Fig. 5A shows PL emission spectra of pure ZnIn2S4 and ZIS/0.75% M. Compared with pure ZnIn2S4, the spectrum of ZIS/0.75% M exhibits a remarkable decrease in PL intensity. In time-resolved PL spectra (Fig. 5B), a reduced emission lifetime of ZIS/0.75% M as compared with that of pure ZnIn2S4 can be observed. It is widely acknowledged that PL emission originates from the recombination of charge carriers [66,67]. Therefore, the diminished PL intensity and decreased emission lifetime collectively indicate better carrier separation ability of hetero-layered ZIS/M. The elevated efficiency of electrons migration in hetero-layered ZIS/M composite can be further verified by transient photocurrent response. As shown in Fig. 5C, the hetero-layered ZIS/0.75% M has an increased photocurrent compared with that of pure ZnIn2S4, indicating that loaded MoS2 favors charge separation and migration. Fig. 5D displays linear sweep voltammetry (LSV) curves of pure ZnIn2S4 and ZIS/0.75% M, in which the ZIS/M composite displays a higher cathodic current than pure ZnIn2S4. The LSV result suggests that ZIS/M composite possesses lower overpotentials for water reduction compared with that of pure ZnIn2S4, leading to an enhanced photocatalytic
Scheme 1. Proposed mechanism for the photocatalytic H2 generation with pure ZnIn2S4 (A, C) and 2D/2D ZIS/M (B, D) under visible-light irradiation with lactic acid as the sacrificial agent.
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hydrogen evolution performance. In addition, nitrogen adsorptiondesorption isotherms of pure ZnIn2S4 and ZIS/0.75% M (Fig. S7) display similar Brunauer-Emmett-Teller surface areas (SBET) of 31.82 and 36.63 m2/g, respectively, as well as the similar pore volume, implying that surface area makes no significant contribution to the enhancement of photocatalytic activity. Collectively, the aforementioned photo- and electrochemical comparative characterizations confirm that the enhanced photocatalytic hydrogen evolution over hetero-layered ZIS/M composite is derived from the efficient separation of charge carriers and lower activation potentials for hydrogen evolution. On the basis of the above characterizations and analyses, a tentative mechanism of photocatalytic hydrogen evolution using 2D/2D ZIS/M composite is proposed and illustrated in Scheme 1. Upon visible-light irradiation, photoinduced electrons in the valence band (VB) of ZnIn2S4 are excited to the conduction band (CB), generating photogenerated electron-hole pairs. However, the photoinduced charge carriers would recombine quickly within ZnIn2S4 NSs without cocatalysts, as revealed in Scheme 1A and C, leading to the unsatisfactory photocatalytic activity. After loading MoS2 NSs, a sheet-on-sheet structure with sufficient interfacial contact is constructed (Scheme 1B and D). According to MottSchottky (M-S) plots in Fig. S8, the flat of the CB positions for the as-prepared ZnIn2S4 and MoS2 were determined to be 1.26 V and 0.31 V versus Ag/AgCl (1.06 V and 0.11 V vs. NHE), respectively. The band alignment of ZnIn2S4 and MoS2 are shown in Fig. S9. In the ZIS/M heterostructure, photoinduced electrons in the light harvester ZnIn2S4 tend to move to the CB of MoS2 cocatalyst through the intimate-contacting interface, achieving spatial separation of photogenerated charge carriers. What's more, large and intimate contact interface between two components provides multiplex channels and reduced diffusion length for electrons transfer from ZnIn2S4 to MoS2, which offers exposed sulfur atoms as active site and low overpotentials for water reduction. Finally, Hþ bonding with active sulfur atoms on exposed edges of MoS2 is reduced to hydrogen gas by photogenerated electrons.
3. Conclusions In summary, ultrathin 2D/2D ZnIn2S4/MoS2 hybrids have been successfully constructed via electrostatic self-assembly of ultrathin ZnIn2S4 and MoS2 NSs for photocatalytic hydrogen evolution. With the optimized 0.75 wt% of MoS2, the obtained hetero-layered composite achieves a boosted hydrogen production rate of 4.974 mmol g1 h1, which is ca. 50 times and 2.2 times as much as that of pure ZnIn2S4 and ZnIn2S4/0.75% Pt, respectively. The enhanced photocatalytic activity of 2D/2D ZIS/M composite might be ascribed to the following factors: (i) quantum confinement effect of ultrathin ZnIn2S4 and MoS2 NSs leads to efficient charge transfer; (ii) large and intimate contact interface between ZnIn2S4 and MoS2 NSs accelerates the separation of photoinduced electrons and holes; (iii) ultrathin MoS2 NSs provide plentiful active sites and lower activation potentials for hydrogen production. This work will provide inspiration for the fabrication of well-contacted ultrathin 2D/2D photocatalysts for photocatalytic hydrogen evolution.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 21777046), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06N569), and the Science and Technology Project of Guangzhou (No. 201803030002).
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Ultrathin 2D/2D ZnIn2S4/MoS2 hybrids for boosted photocatalytic hydrogen evolution under visible light Lixian Huang a, b, 1, Bin Han b, 1, Xihe Huang b, Shujie Liang b, Ziqi Deng b, Weiyi Chen b, Miao Peng b, Hong Deng a, b, * a
School of Environment and Energy, Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters (Ministry of Education), Guangdong Engineering and Technology Research Center for Environmental Nanomaterials, South China University of Technology, Guangzhou 510006, China b School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 December 2018 Received in revised form 3 May 2019 Accepted 13 May 2019 Available online 16 May 2019
A well-connected hetero-layered ZnIn2S4/MoS2 composite was fabricated via an electrostatic selfassembly process to boost photocatalytic hydrogen evolution under visible-light irradiation. Ultrathin nanosheets contribute to efficient charge transfer by reducing electron diffusion length. In the sheet-onsheet heterostructure, ultrathin ZnIn2S4 nanosheets (NSs) and MoS2 NSs were well connected with each other to form a large and intimate contact interface, which features an efficient separation and migration of photoinduced charge carriers. Moreover, MoS2 NSs endow the composite with abundant active sites and lowers its overpotential for water reduction. Consequently, the ultrathin hetero-layered ZnIn2S4/ MoS2 composite with optimized loading of MoS2 achieves a hydrogen production rate of 4.974 mmol g1 h1, which is ca. 50 times as much as that of pure ZnIn2S4, and 2.2 times as much as that of its ZnIn2S4/Pt counterpart. This work provides inspiration for the fabrication of well-contacted heterolayered structure for photocatalytic hydrogen evolution. © 2019 Elsevier B.V. All rights reserved.
Keywords: Photocatalytic hydrogen evolution Ultrathin nanosheets Interfacial contact ZnIn2S4 MoS2
1. Introduction Solar-driven hydrogen production, a promising approach to produce sustainable renewable fuels, has been attracting increasing attention due to its potential in relieving severe energy crisis and environmental pollution [1e6]. Since the original work on photocatalytic water splitting on TiO2 [7], a large number of photocatalysts have been explored for hydrogen evolution [8e16]. Among them, ZnIn2S4, a kind of visible-light-responsive semiconductor, has received much attention owing to its suitable bandgap, high chemical stability, and environmental friendliness [17e21]. Particularly, the unique structure of ultrathin twodimensional (2D) ZnIn2S4 nanosheets (NSs), which originates from the reduced electron diffusion length and exposed surface atoms, promotes separation of photoinduced charges and enriches its active sites [22e27]. For example, Yang et al. recently reported
* Corresponding author. School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. E-mail address: [email protected] (H. Deng). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.05.162 0925-8388/© 2019 Elsevier B.V. All rights reserved.
that single-unit-cell ZnIn2S4 layers exhibited higher photocatalytic H2 production rate than the bulk ones [28]. However, the photocatalytic activity of ZnIn2S4 NSs itself is still unsatisfactory for the shortage of active sites and rapid recombination of photoinduced charge carriers [29,30]. Loading cocatalysts has been widely acknowledged as an effective strategy to boost the photocatalytic efficiency of H2 evolution. Cocatalysts not only provide more active sites for H2 evolution but also promote the separation of photo-generated charges [31e36]. MoS2, a 2D transition metal dichalcogenide has been certified as an ideal cocatalyst in photocatalytic hydrogen evolution due to the active sulfur atoms on its exposed edges, which strongly bond with the Hþ in water and thus serve as the active sites for proton reduction [33,37e42]. Especially, ultrathin MoS2 NSs with large edge-to-volume ratio tend to expose more sulfur edges for H2 production. Moreover, compared with its 0D/2D and 1D/2D counterparts, the ultrathin 2D/2D structure, with face-to-face interfacial contact, is able to provide multipath and reduced distance for electrons. Therefore, electrons can migrate across the interface and reach the surface more efficiently, thereby facilitating the separation of photo-generated electrons and holes [43e47]. As a result,
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constructing ultrathin 2D/2D ZnIn2S4/MoS2 nanocomposite is of great interest yet rarely reported in literature. Herein, ultrathin ZnIn2S4 and MoS2 NSs are employed to construct the ultrathin 2D/2D ZnIn2S4/MoS2 (ZIS/M) composite for photocatalytic H2 evolution via an electrostatic self-assembly process. The obtained well-contacted ultrathin 2D/2D composite exhibits lower activation potentials for H2 evolution and more effective separation of photoinduced charge carriers in comparison with pure ZnIn2S4. Consequently, the ultrathin 2D/2D ZIS/M nanocomposite achieves an enhanced H2 production rate of 4.974 mmol g1 h1 under visible-light irradiation, which is ca. 50 and 2.2 times as much as that of pure ZnIn2S4 ultrathin NSs and its ZnIn2S4/Pt counterpart, respectively. 2. Results and discussion 2.1. Materials synthesis and characterization Scheme S1 illustrates the electrostatic self-assembly process of 2D/2D ZnIn2S4/MoS2 (ZIS/M). Briefly, both ultrathin ZnIn2S4 nanosheets (NSs) and MoS2 NSs are negatively charged with zeta potentials of 45.7 mV and 23.8 mV in aqueous dispersion, respectively (Fig. S1). After being modified with poly (diallyldimethylammonium chloride) (PDDA), the zeta potential of MoS2 aqueous dispersion was changed to þ36.5 mV. Electrostatic attraction between positively charged PDDA-MoS2 NSs and negatively charged ZnIn2S4 NSs provides the preconditions for the fabrication of a hetero-layered structure with good interfacial contact [45]. The composites of MoS2 and ZnIn2S4 (ZIS/M) were denoted as ZIS/0.25% M, ZIS/0.5% M, ZIS/0.75% M, and ZIS/1% M, with theoretical weight percentage controlled to be 0.25, 0.5, 0.75 and 1 wt%, respectively. The real content of MoS2 in ZIS/M is measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), as shown in Table S1, which reveals that the mass ratio of MoS2 in as-prepared samples approaches its theoretical value. Powder X-ray diffraction (XRD) was employed to determine the crystalline phase of as-prepared MoS2, ZnIn2S4 and ZIS/M composites. The XRD pattern of MoS2 exhibits three broad peaks at 14.4 , 33.3 and 56.2 , as shown in Fig. S2, which can be indexed to (002), (101) and (106) planes of the hexagonal phase of MoS2 (JCPDS No. 37-1492), respectively. For pure ZnIn2S4 (Fig. 1), three distinct diffraction peaks at 21.5 , 27.6 , 47.1 match well with (006), (102), and (110) crystallographic planes of the hexagonal ZnIn2S4 (JCPDS No. 65-2023), respectively. After loading various
amounts of MoS2 onto it, all XRD patterns of ZIS/M composites present the same diffraction peaks as that of pure ZnIn2S4, indicating that the introduction of MoS2 has no impact on the crystalline phase of ZnIn2S4. However, no distinct peaks of MoS2 can be found in the XRD patterns of ZIS/M composites, which is ascribed to the low diffraction intensity, low loading amount, and high dispersion of MoS2 in the nanocomposites. Morphology and microstructure of as-prepared samples were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). As shown in Figs. S3A and B, SEM images of both ZnIn2S4 and MoS2 present flake-like morphology with a lateral size of a few micrometers and several hundred nanometers, respectively. Notably, it is observed that TEM images of as-prepared ZnIn2S4 (Fig. 2A) and MoS2 (Fig. 2C) also exhibit layered structures with the nearly transparent feature, which is in agreement with the ultrathin nature presented in SEM. Additionally, high-resolution TEM (HRTEM) images of ZnIn2S4 (Fig. 2B) and MoS2 (Fig. 2D) show distinct lattice fringes of 0.32 nm and 0.62 nm, matching well with the (102) lattice plane of ZnIn2S4 and (002) plane of MoS2, respectively. Atomic force microscopy (AFM) was used to determine the thickness of as-prepared products. The average thickness of ZnIn2S4 and MoS2 NSs is ca. 2.53 nm and 1.05 nm, respectively, as revealed in Fig. 2E and D, further confirming the ultrathin nature of the as-prepared nanosheets. After the assembly of ZnIn2S4 and MoS2 ultrathin NSs, the ZIS/M composite exhibits a sheet-on-sheet structure (Fig. 2G) with large contact surface. Moreover, interlaced lattice fringes of ZnIn2S4 (0.32 nm) and MoS2 (0.62 nm) can be found in the HRTEM image of the 2D/2D composite, as shown in Fig. 2H, revealing an intimate interfacial contact between ZnIn2S4 and MoS2. The large and intimate contact interface provides multiplex channels for carriers migration, accelerating efficient charge spatial separation [48,49]. Energy-dispersive X-ray spectroscopy (EDX) and elemental mappings of ZIS/M composite were used to further identify the constituent elements of the composite and the distribution of MoS2. The EDX result shown in Fig. S3C demonstrates the existence of In, Zn, Mo, and S elements in ZIS/M composite. Corresponding elemental mappings (Fig. 2I-M) show that the elements are homogeneously distributed throughout the ZIS/M composite, confirming the fabrication of hetero-layered structure with sufficient interfacial contact. X-ray photoelectron spectroscopy (XPS) was carried out to confirm the composition and chemical states of elements in the asobtained products. Fig. S4A illustrates the full-range XPS spectra of ZIS/M, ZnIn2S4 and MoS2. Compared with pure ZnIn2S4, a peak of Mo 3d appears in the spectrum of ZIS/M, which is further displayed clearly in the magnifying figure in Fig. S4B. Fig. 3AeB shows the high-resolution XPS spectrum of ZIS/M composite in the Mo 3d and S 2p region. Peaks at 228.7 eV and 231.9 eV can be assigned to Mo 3d5/2 and 3d3/2, respectively [50], suggesting the existence of Mo4þ. For S element of ZIS/M composite, peaks at 225.8eV, 161.9 eV and 162.9 eV can be assigned to S 2s, S 2p3/2 and 2p1/2, respectively, characteristic of S2 [28]. As revealed in Fig. 3C and D, In 3d peaks of ZIS/M at 445.03 eV and 452.6 eV can be indexed to In3þ, while Zn 2p peaks at 1021.83 eV and 1044.82 eV can be indexed to Zn2þ valence state [28]. XPS results further confirm the existence of MoS2 in the ZIS/M hybrids. Additionally, when compared with pure ZnIn2S4, the binding energy of In 3d and Zn 2p in the composite remains unchanged, implying that loading MoS2 would not affect the chemical structure of ZnIn2S4. 2.2. Photocatalytic hydrogen generation
Fig. 1. XRD patterns of pure ZnIn2S4 and ZnIn2S4/MoS2 (ZIS/M) composites loaded with various amounts of MoS2.
The photocatalytic hydrogen evolution using the as-prepared samples was tested under visible-light irradiation (l > 400 nm)
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Fig. 2. TEM images of ZnIn2S4 (A), MoS2 (C), and ZIS/M composite (G). HRTEM images of ZnIn2S4 (B), MoS2 (D), and ZIS/M composite (H). AFM images of ZnIn2S4 (E) and MoS2 (F). Corresponding elemental mappings of ZIS/M composite (I-M).
Fig. 3. High-resolution XPS spectra of Mo 3d (A) and S 2p (B) in ZIS/M composites. High-resolution XPS spectra of ZIS/M and ZnIn2S4 in the In 3d (C) and Zn 2p (D) region.
with lactic acid served as a sacrificial agent. No H2 could be detected without light irradiation or photocatalysts, suggesting that the photocatalytic hydrogen evolution is a light-driven process. As shown in Fig. 4A, a small amount of hydrogen could be detected by using pure ZnIn2S4 as photocatalyst. However, the photocatalytic activity of ZnIn2S4 alone was really unsatisfactory with a hydrogen evolution rate of 0.099 mmol g1 h1, which is attributed to the severe recombination of photoinduced electrons and holes [51,52]. After loading MoS2 onto ultrathin ZnIn2S4, there was a remarkable enhancement in photocatalytic activity.
Additionally, the amount of loaded MoS2 dramatically affected the photocatalytic activity of ZIS/M. Hydrogen evolution rates of ZIS/M composites increased along with MoS2 mass ratio. The highest H2 evolution rate for ZIS/0.75%M reached 4.947 mmol g1 h1, which is approximately 50 times as much as that of using pure ZnIn2S4 NSs. However, further loading of MoS2 led to the decrease of hydrogen evolution rate. This result can be ascribed to a shading effect caused by an excessive amount of black MoS2 covering the surface of ZnIn2S4, which shields the incident light and thus prevents the generation of photoinduced charge carriers [53e55]. No hydrogen
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Fig. 4. Photocatalytic H2 evolution rates over bulk ZnIn2S4, pure ZnIn2S4 NSs, MoS2 and ZIS/M loaded with various amounts of MoS2, ZnIn2S4/0.75% Pt and bulk ZIS/0.75% M (A). Recycling photoactivity test of 2D/2D ZIS/0.75% M (B).
could be detected when using MoS2 alone as a photocatalyst, suggesting that MoS2 is inactive for photocatalytic hydrogen evolution. Noble metal Pt is routinely used as a typical cocatalyst in promoting photocatalytic hydrogen production [56e58]. In this study, ZnIn2S4/0.75% Pt was also synthesized with its photocatalytic activity tested as a comparison. As revealed in Fig. 4A, the hydrogen generation rate of hetero-layered ZIS/0.75% M is ca. 2.2 times as much as that of ZnIn2S4/0.75% Pt. This result clearly demonstrates that 2D/2D ZIS/M composites have excellent photocatalytic activities and highlights the effectiveness of ultrathin MoS2 NSs as a cocatalyst in photocatalytic hydrogen evolution. To further verify
the advantages of ultrathin nanosheets as well as 2D/2D heterolayered structure, control experiments were performed with bulk ZnIn2S4 and bulk ZIS/M composites, respectively. As revealed in Fig. 4A, the hydrogen evolution rate of ZnIn2S4 NSs surpasses that of its bulk counterpart. The result may owe to ultrathin structure of ZnIn2S4 NSs, which not only leads to efficient charge transfer from the bulk to the surface but also maximizes the exposure of the surface to the reactant solution and light [59,60].Considering the interfacial contact between a light-harvesting semiconductor and a cocatalyst plays an important role in the separation of photoinduced electrons and holes [61,62], control experiments using 2D/
Fig. 5. Steady-state photoluminescence (PL) spectra (A), time-resolved transient photoluminescence (PL) decay transient photocurrent responses (B), transient photocurrent responses (C), and linear sweep voltammetry (LSV) curves of ZnIn2S4 and ZIS/0.75% M (D).
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2D-contacted hybrids and bulk composites were carried out to investigate the impact of interfacial contact areas. Bulk ZnIn2S4 and MoS2 were synthesized and coupled via electrostatic self-assembly. XRD patterns and SEM images of as-prepared bulk samples are shown in Fig. S5, exhibiting their crystalline phase and morphology. It is observed in Fig. 4A that the hetero-layered ZIS/M composite exhibits a higher hydrogen evolution rate than ZIS/M system consisting of bulk ZnIn2S4 and MoS2. Compared with 2D/2D heterolayered structure, the bulk composite lacks sufficient contact between components. That suggests the large and intimate contact interface in hetero-layered structure facilitates the photocatalytic hydrogen generation, which is ascribed to promoted separation of photoinduced charge carriers. Cycling tests were performed to examine the durability of the 2D/2D ZIS/M system. The photocatalytic hydrogen evolution rate of ZIS/0.75% M shows a negligible loss after 4 consecutive cycles under visible-light irradiation (Fig. 4B), suggesting the hetero-layered ZIS/M composites are very stable.
2.3. Origin of the enhanced performance To understand the origin of boosted photocatalytic hydrogen evolution over the hetero-layered ZIS/M composite, a series of photo- and electrochemical comparative characterizations of pure ZnIn2S4 and ZIS/0.75% M have been carried out. UVevis diffuse reflectance spectra (DRS) were collected to investigate the optical properties of the as-prepared samples. As shown in Fig. S6A, both pure ZnIn2S4 and ZIS/0.75% M display a wide range of light absorption from ultraviolet light to visible light, indicating their response to visible light. Especially, DRS of ZIS/0.75% M composite
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shows an increased absorption, which is ascribed to the introduced black MoS2, matching with the color change from light-yellow for pure ZnIn2S4 to gray-yellow for ZIS/0.75% M (Figs. S6B and C). The band gap of pure ZnIn2S4 and ZIS/0.75% M were estimated to be 2.33 eV and 2.35 eV, respectively, using the KubelkaMunk function [63e65]. The similar absorption curves and band gaps manifest that MoS2 was located on the surface of ZnIn2S4 rather than inside of the crystal grating, which is in accordance with the XRD results. Steady-state photoluminescence (PL) and time-resolved photoluminescence measurements were carried out to study the efficiency of charge separation and transfer. Fig. 5A shows PL emission spectra of pure ZnIn2S4 and ZIS/0.75% M. Compared with pure ZnIn2S4, the spectrum of ZIS/0.75% M exhibits a remarkable decrease in PL intensity. In time-resolved PL spectra (Fig. 5B), a reduced emission lifetime of ZIS/0.75% M as compared with that of pure ZnIn2S4 can be observed. It is widely acknowledged that PL emission originates from the recombination of charge carriers [66,67]. Therefore, the diminished PL intensity and decreased emission lifetime collectively indicate better carrier separation ability of hetero-layered ZIS/M. The elevated efficiency of electrons migration in hetero-layered ZIS/M composite can be further verified by transient photocurrent response. As shown in Fig. 5C, the hetero-layered ZIS/0.75% M has an increased photocurrent compared with that of pure ZnIn2S4, indicating that loaded MoS2 favors charge separation and migration. Fig. 5D displays linear sweep voltammetry (LSV) curves of pure ZnIn2S4 and ZIS/0.75% M, in which the ZIS/M composite displays a higher cathodic current than pure ZnIn2S4. The LSV result suggests that ZIS/M composite possesses lower overpotentials for water reduction compared with that of pure ZnIn2S4, leading to an enhanced photocatalytic
Scheme 1. Proposed mechanism for the photocatalytic H2 generation with pure ZnIn2S4 (A, C) and 2D/2D ZIS/M (B, D) under visible-light irradiation with lactic acid as the sacrificial agent.
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hydrogen evolution performance. In addition, nitrogen adsorptiondesorption isotherms of pure ZnIn2S4 and ZIS/0.75% M (Fig. S7) display similar Brunauer-Emmett-Teller surface areas (SBET) of 31.82 and 36.63 m2/g, respectively, as well as the similar pore volume, implying that surface area makes no significant contribution to the enhancement of photocatalytic activity. Collectively, the aforementioned photo- and electrochemical comparative characterizations confirm that the enhanced photocatalytic hydrogen evolution over hetero-layered ZIS/M composite is derived from the efficient separation of charge carriers and lower activation potentials for hydrogen evolution. On the basis of the above characterizations and analyses, a tentative mechanism of photocatalytic hydrogen evolution using 2D/2D ZIS/M composite is proposed and illustrated in Scheme 1. Upon visible-light irradiation, photoinduced electrons in the valence band (VB) of ZnIn2S4 are excited to the conduction band (CB), generating photogenerated electron-hole pairs. However, the photoinduced charge carriers would recombine quickly within ZnIn2S4 NSs without cocatalysts, as revealed in Scheme 1A and C, leading to the unsatisfactory photocatalytic activity. After loading MoS2 NSs, a sheet-on-sheet structure with sufficient interfacial contact is constructed (Scheme 1B and D). According to MottSchottky (M-S) plots in Fig. S8, the flat of the CB positions for the as-prepared ZnIn2S4 and MoS2 were determined to be 1.26 V and 0.31 V versus Ag/AgCl (1.06 V and 0.11 V vs. NHE), respectively. The band alignment of ZnIn2S4 and MoS2 are shown in Fig. S9. In the ZIS/M heterostructure, photoinduced electrons in the light harvester ZnIn2S4 tend to move to the CB of MoS2 cocatalyst through the intimate-contacting interface, achieving spatial separation of photogenerated charge carriers. What's more, large and intimate contact interface between two components provides multiplex channels and reduced diffusion length for electrons transfer from ZnIn2S4 to MoS2, which offers exposed sulfur atoms as active site and low overpotentials for water reduction. Finally, Hþ bonding with active sulfur atoms on exposed edges of MoS2 is reduced to hydrogen gas by photogenerated electrons.
3. Conclusions In summary, ultrathin 2D/2D ZnIn2S4/MoS2 hybrids have been successfully constructed via electrostatic self-assembly of ultrathin ZnIn2S4 and MoS2 NSs for photocatalytic hydrogen evolution. With the optimized 0.75 wt% of MoS2, the obtained hetero-layered composite achieves a boosted hydrogen production rate of 4.974 mmol g1 h1, which is ca. 50 times and 2.2 times as much as that of pure ZnIn2S4 and ZnIn2S4/0.75% Pt, respectively. The enhanced photocatalytic activity of 2D/2D ZIS/M composite might be ascribed to the following factors: (i) quantum confinement effect of ultrathin ZnIn2S4 and MoS2 NSs leads to efficient charge transfer; (ii) large and intimate contact interface between ZnIn2S4 and MoS2 NSs accelerates the separation of photoinduced electrons and holes; (iii) ultrathin MoS2 NSs provide plentiful active sites and lower activation potentials for hydrogen production. This work will provide inspiration for the fabrication of well-contacted ultrathin 2D/2D photocatalysts for photocatalytic hydrogen evolution.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 21777046), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06N569), and the Science and Technology Project of Guangzhou (No. 201803030002).
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