FULL PAPER DOI: 10.1002/asia.201301646
Enhanced Photocatalytic Hydrogen Evolution over Hierarchical Composites of ZnIn2S4 Nanosheets Grown on MoS2 Slices Guohui Tian,[a, b] Yajie Chen,[a] Zhiyu Ren,[a] Chungui Tian,[a] Kai Pan,[a] Wei Zhou,[a] Jiaqi Wang,[a] and Honggang Fu*[a] Abstract: A highly active hierarchical MoS2/ZnIn2S4 composite catalyst was synthesized in situ by using a facile controlled-growth approach through a solvothermal process. During the solvothermal reaction, 2D ultrathin curled ZnIn2S4 nanosheets grew on the surface of MoS2 slices, which could help to form a more-homogeneous mixture, effective interfacial contact, and strong interactions between the ZnIn2S4 nanosheets and the MoS2 slices. The inti-
mate contact between ZnIn2S4 and MoS2 favored the formation of junctions between the two components, thereby improving the charge separation and prolonging the mean lifetime of the electron–hole pairs. Moreover, growing ZnIn2S4 nanosheets by visibleKeywords: charge transfer · composite materials · molybdenum · nanostructures · photochemistry
Introduction
of semiconductors and co-catalysts are critically dependent on their spatial integration; therefore, the construction of the optimal structure morphology and the formation of surface heterojunctions are of vital importance.[7] For example, Yin et al. fabricated well-defined 3D graphene/metal-oxide nanoparticle hybrids by using a new and general approach with low-valence metal ions as the precursors. The prepared hybrids exhibited excellent capacitive deionization performance and have potential applications in other areas, such as catalysis and supercapacitors.[8] In the photocatalytic process, the co-catalyst cooperates with the light harvester to facilitate the charge separation and increase the lifetime of the photogenerated electron– hole pair, whilst lowering the activation barriers for H2 or O2 evolution. Thus, the use of a co-catalyst leads to an increase in overall photocatalytic performance, including in terms of activity, selectivity, and stability. In general, the efficiency of a given photocatalytic system is dependent on the ability of the co-catalyst to support reductive and/or oxidative catalysis.[9] In particular, the structural characteristics and intrinsic catalytic properties of a co-catalyst are important. Photocatalytic/catalytic systems based on abundantly available materials are certainly desirable for large-scale hydrogen production for future energy production based on water and sunlight; indeed, the widespread use of Pt may be limited by its scarcity and high cost and, thus, the development of non-noble-metal alternatives is desirable.[10] Recently, 2D graphene was shown to be a good co-catalyst. Metal oxides (sulfur) supported on 2D graphene nanosheets and their composites have been extensively investigated for their photocatalytic properties.[11] In particular, the construction of hybrids with graphene-like thin-layered heterojunctions,
As is well-known, hydrogen energy is a potentially significant alternative form of storable and clean energy in the future.[1, 2] Over the past few years, photocatalytic H2 production has attracted a lot of attention and it appears to be a promising strategy because it is clean, low-cost, and environmentally friendly by utilizing solar energy.[3, 4] In practice, a single catalyst generally shows relatively low photocatalytic activity, so a co-catalyst is often introduced to improve the performance.[5] In recent years, nanocomposites of a semiconductor and a co-catalyst, which combine the unique properties of the individual nanostructures and may show synergistic effects, have attracted great attention in photocatalysis and other applications.[6] Moreover, the performance
[a] Dr. G. Tian, Dr. Y. Chen, Z. Ren, Dr. C. Tian, K. Pan, W. Zhou, J. Wang, Prof. H. Fu Key Laboratory of Functional Inorganic Material Chemistry Ministry of Education of the People’s Republic of China Heilongjiang University Harbin 150080 (P. R. China) Fax: (+ 86) 4518666-1259 E-mail: [email protected] [b] Dr. G. Tian Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion College of Heilongjiang Province School of Chemistry and Materials Science Heilongjiang University Harbin 150080 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201301646.
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light catalysis on MoS2 slices afforded a higher number of available catalytically active sites. So, the photocatalytic hydrogen-evolution performance of the hierarchical MoS2/ZnIn2S4 composite was significantly enhanced, owing to a synergistic effect of these factors. This work could provide new insights into the fabrication of a highly efficient and low-cost non-noble-metal co-catalyst for visible-light H2 generation.
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Results and Discussion
such as graphene-gC3N4, can allow the prompt migration of light-induced charge, thereby resulting in an efficient photocatalytic reaction.[12] Similar to graphene, as a typical layered transition-metal sulfide, MoS2 has also been extensively investigated.[13, 14] MoS2 is a typical example of the layered transition-metal dichalcogenide family. It is composed of covalently bonded S Mo S sheets that are bound by weak van der Waals forces and is particularly important for lithium batteries and sensing applications.[15] The potential application of MoS2 as a co-catalyst for photocatalytic H2 production has also received great attention (e.g., MoS2/CdS, TiO2/MoS2/graphene composites) and the introduction of MoS2 can significantly enhance the H2-production rate.[16] Recently, C. N. R. Rao’s group has done a lot of work on the synthesis and application of MoS2.[17] They prepared graphene (EG)-MoS2 composites that showed very high H2-evolution activity. Moreover, they also studied the influence of the polytypes of MoS2 (few-layer 2H-MoS2 and single-layer 1T MoS2) on the H2-evolution activity and stability.[17b] Similar to graphene, the potency of MoS2 as a co-catalyst in improving the photocatalytic activity has been demonstrated for some photocatalysts. However, in most of those studies, MoS2 was loaded onto the surface of the catalysts (e.g. graphene, gC3N4).[18] In these examples, it increased the light-blocking effect of the co-catalyst, thereby decreasing the light utilization by the catalysts. Moreover, there are only very weak interactions between the two components in composites that are obtained by physical mixing and by general chemical synthesis methods, which can prohibit the direct chemical contact between the two components, thereby greatly decreasing the synergetic catalytic effect of guest MoS2 and the host catalyst.[19] Therefore, to effectively inhibit the recombination of photogenerated electron and holes and to improve the synergetic catalytic effect of the composite, it is important to construct composite catalysts with strong interactions between MoS2 and the main catalyst. Herein, for the first time, we synthesized hierarchical MoS2/ZnIn2S4 composites in situ by the controlled growth of ZnIn2S4 nanosheets on MoS2 slices through a solvothermal process. The exfoliated 2D platform structure of MoS2 slices was an excellent supporting matrix for the in situ growth of the ZnIn2S4 nanosheets. This method can help to form junction structures between ZnIn2S4 nanosheets and MoS2 slices, with stronger interactions compared with their physical mixtures, which helps the interfacial charge transfer and decreases self-agglomeration. The synergistic effect of the strong interactions from these junctions between ZnIn2S4 nanosheets and MoS2 slices and the higher number of available catalytically active sites can contribute significantly to the enhanced photocatalytic hydrogen-evolution activity under visible-light irradiation in the absence of noble metal Pt.
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XRD analysis was performed to investigate the structures of the obtained samples. As shown in Figure 1 a, pristine ZnIn2S4 shows diffraction peaks that correspond to the (006), (102), and (110) crystal planes (2q = 21.6, 27.8, and
Figure 1. XRD patterns of a) ZnIn2S4, b) the MoS2/ZnIn2S4 composite (15 wt % MoS2), and c) MoS2.
47.28, respectively), which can be indexed to the hexagonal phase of ZnIn2S4 (JCPDS No. 65-2023).[20] For the pure MoS2 sample (Figure 1 c), the XRD peaks at 2q = 14.5, 32.7, 39.7, 49.8, and 58.48 can be assigned to the (002), (100), (103), (105), and (110) planes in the orthorhombic phase of MoS2 (JCPDS No. 37-1492).[21] In the MoS2/ZnIn2S4 composite, the XRD pattern (Figure 1 b) displays two types of diffraction peaks: besides the diffraction peaks that are assigned to the orthorhombic phase of MoS2, all of the additional peaks are well-matched to the hexagonal phase of ZnIn2S4, thus indicating the presence of ZnIn2S4 nanosheets coated on the MoS2 slices. The morphology and microstructure of the obtained hierarchical MoS2/ZnIn2S4 composites were investigated by FESEM and TEM (Figure 2). For comparison, the SEM image of exfoliated MoS2 slices was also recorded (Figure 2 A), which showed a 2D sheet-like morphology with sizes varying from 500 nm to several micrometers in length and width. The TEM image (Figure 2 A, inset) confirmed that the exfoliated MoS2 was several nanometers thick and belonged to few-layer 2H-MoS2 (p-type semiconductor).[17b] Figure 2 B shows that the ultrathin ZnIn2S4 nanosheets uniformly grew on both sides of the MoS2 slices. These ZnIn2S4 nanosheets were interconnected with each other, thereby forming 3D nanosheet networks. Figure 2 C shows a TEM image of a MoS2/ZnIn2S4 composite (15 wt % MoS2), in which the thin ZnIn2S4 nanosheets cover the surface of the MoS2 slices. HRTEM analysis (Figure 2 D, magnification of the dashed square in Figure 2 C) clearly shows the lattice fringes, thus suggesting a well-defined crystal structure. The fringes with a lattice spacing of 0.27 nm corresponded to the (100) plane of MoS2.[22] The lattice spacing was about 0.324 nm, which could be assigned to the (102) plane of hex-
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smooth surface of the original sample (Figure 2 A), some nanoparticles were formed on the surface of the MoS2 slices after the initial nucleation reaction (Figure 3 A, B), mainly because the surface of the MoS2 slices could provide highenergy nucleation sites for the nucleation and growth of ZnIn2S4 nanocrystals. Moreover, there were oxygen functional groups (e.g., hydroxy, glycerol) on the surface. These functional groups could act as anchor sites and bind with Zn2+ and In3+ metal cations. During the solvothermal reaction, the anchored Zn2+ and In3+ metal cations reacted in situ with glycerol to form metal glycerolate complexes, which then further reacted with S2 ions to form ZnIn2S4 on the surface of MoS2 slices. Moreover, the stacking forces between the ZnIn2S4 nuclei and the basal planes of MoS2 were also beneficial for in situ growth on the surface of the MoS2 slices. Therefore, the ZnIn2S4 would gradually grow along the initial nuclei of ZnIn2S4 and the ZnIn2S4 nanosheets would grow larger and more densely along the surface of the MoS2 slices (Figure 3 C, D) with the extension of the reaction time. According to this study, the growth process of MoS2/ZnIn2S4 composites during the hydrothermal process is shown in the Supporting Information, Figure S2. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical states of Mo and S in the MoS2/ ZnIn2S4 composite (15 wt % of MoS2). The high-resolution XPS spectrum (Figure 4 A) showed that the binding energies of the Mo 3d3/2 and Mo 3d5/2 peaks in pure MoS2 were locat-
Figure 2. A) SEM and TEM (inset) images of MoS2 slices; B) SEM image of a hierarchical MoS2/ZnIn2S4 composite (15 wt % MoS2); C) lowmagnification TEM image of a hierarchical MoS2/ZnIn2S4 composite (15 wt % MoS2); D) HRTEM image of the dashed square in (C).
agonal-phase ZnIn2S4.[23] Moreover, a distinguished and coherent interface between the continuity of the lattice fringes of MoS2 and ZnIn2S4 was observed, which indicated the formation of a p–n junction and could afford better charge separation and efficient electron transfer within the hybrid structure compared with the physical mixtures and pure ZnIn2S4. For comparison, the corresponding SEM images of the MoS2/ZnIn2S4 composites with different amounts of MoS2 are shown in the Supporting Information, Figure S1. If the MoS2 content was high (20 wt %), only a low density of ZnIn2S4 nanosheets were grown on the surface of the MoS2 slices. However, the excess ZnIn2S4 nanosheets self-assembled to form ZnIn2S4 microspheres at low MoS2 content (5 wt %). To further illustrate the formation process of hierarchical MoS2/ZnIn2S4 composites, time-dependent experiments were performed. As shown in Figure 3 A, compared to the
Figure 4. XPS spectra of A) Mo 3d, B) S 2p, C) In 3d, and D) Zn 2p peaks in MoS2/ZnIn2S4 (15 wt % MoS2), ZnIn2S4, and MoS2.
ed at 232.5 and 229.3 eV, respectively, thus suggesting that Mo4+ existed in the pure MoS2.[22] After ZnIn2S4 nanosheets were grown on the surface of MoS2 slices, the Mo 3d3/2 and Mo 3d5/2 peaks shifted to 232.1 and 228.8 eV, respectively.[24] In the MoS2/ZnIn2S4 composite, the S 2p1/2 and S 2p3/2 peaks (Figure 4 B), which corresponded to MoS2 and ZnIn2S4, were also slightly lower than those of MoS2 (163.25 and
Figure 3. SEM images of MoS2/ZnIn2S4 samples that were prepared from different solvothermal reaction times: a) 0.5, b) 1, c) 4, and d) 10 h.
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162.25 eV) and ZnIn2S4 (162.6 and 161.6 eV). The binding energies of In 3d (444.9 and 452.4 eV) and Zn 2p (1021.8 and 1044.6 eV) in the MoS2/ZnIn2S4 composite were slightly higher than those of ZnIn2S4 (Figure 4 C, D).[25] These results indicate that there are strong interactions between the ZnIn2S4 nanosheets and the MoS2 slices in the composite, driven by the probable electron transfer and delocalization of firmly contacted MoS2 layers and ZnIn2S4 nanosheets.[11, 25] Figure 5 shows UV/Vis absorption spectra of ZnIn2S4 and the MoS2/ZnIn2S4 composites with various MoS2 content. For the pure ZnIn2S4 nanosheets, a significant absorption at l > 400 nm was found. Moreover, for the MoS2/ZnIn2S4 com-
Figure 6. A) Surface photovoltage spectra of a) ZnIn2S4 and b) the MoS2/ ZnIn2S4 composite (15 wt % MoS2). B) Transient photovoltage spectra of a) ZnIn2S4 and b) the MoS2/ZnIn2S4 composite (15 wt % MoS2). Figure 5. UV/Vis diffuse reflectance spectra of a) ZnIn2S4 and MoS2/ ZnIn2S4 composites with different MoS2 content: b) 5, c) 10, d) 15, and e) 20 wt %.
pared to pure ZnIn2S4, the intensity of the SPV response for the MoS2/ZnIn2S4 composite is significantly enhanced. This result demonstrates that there must be charge transfer between the MoS2 slices and ZnIn2S4 nanosheets and that the photogenerated charge-separation efficiency in ZnIn2S4 is enhanced, owing to the introduction of MoS2 slices as a substrate. The transient photovoltage (TPV) technique, which is an effective method for investigating the dynamic properties of photoinduced charge carriers in semiconductor materials,[27] was also used to further investigate the transfer properties of the photogenerated electrons from n-type semiconductor ZnIn2S4 to p-type semiconductor MoS2.[17b] As shown in Figure 6 B, the MoS2/ZnIn2S4 composite exhibited a prolonged mean lifetime of electron–hole pairs with respect to ZnIn2S4, which further confirmed that the p–n junction between MoS2 and ZnIn2S4 in the hierarchical MoS2/ZnIn2S4 composite greatly retarded the recombination of electron– hole pairs in the excited ZnIn2S4. Electrochemical impedance spectroscopy (EIS) is a powerful tool for studying charge-transfer processes in three-electrode systems[28] and EIS Nyquist plots of the two samples are shown in Figure 7. It has been reported that the intermediate-frequency response is associated with electron transport and -transfer at the ZnIn2S4/electrode interface.[29] The hierarchical MoS2/ZnIn2S4 composite shows a smaller semicircle in the mid-frequency region in comparison to ZnIn2S4, which indicates faster interfacial charge trans-
posites (15 and 20 wt % MoS2), there were an additional two absorption peaks, located at about 480 and 670 nm, thus leading to a broad absorption in the visible-light region.[22] The corresponding color of the composites also turned dark gray. These result shows that the addition of MoS2 slices increased the absorbance of visible light.[14] Because of the increased visible-light absorbance, a more-efficient utilization of the solar energy could be realized. Therefore, it can be inferred that the introduction of MoS2 slices as the growing substrate of ZnIn2S4 nanosheets is effective for the visiblelight response of the MoS2/ZnIn2S4 composites. Surface photovoltage (SPV) is an effective technique for studying the photophysics of excited states that are generated by photon absorption and can effectively reflect the separation and recombination of photoinduced charge carriers.[26] To probe the effect of strong interactions at intimate junctions between ZnIn2S4 nanosheets and MoS2 slices on the photogenerated charge-transfer properties, the SPV technique was employed. The SPV spectrum of a MoS2/ZnIn2S4 composite is shown in Figure 6 A. A SPV response appears if photogenerated charge carriers are separated in space; a stronger photoelectric signal corresponds to higher chargeseparation efficiency. For pure ZnIn2S4 and the MoS2/ ZnIn2S4 composite, there is a clear surface photovoltage response within the range 350–700 nm. In particular, com-
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Figure 7. Nyquist plots of a) ZnIn2S4 and b) the MoS2/ZnIn2S4 composite (15 wt % MoS2) electrodes in an aqueous solution that contained 0.25 m Na2SO3/0.35 m Na2S under visible-light irradiation.
fer.[30, 31] So, the introduction of MoS2 slices can benefit charge transfer and -separation in the MoS2/ZnIn2S4 system and, thus, decrease charge recombination because of the formation of junctions between MoS2 and ZnIn2S4. Overall, MoS2 slices can act as both a substrate for ZnIn2S4 nanosheets and as an electron collector and -transporter in the composite, thereby effectively inhibiting charge recombination and, thus, contributing to the improved photocatalytic H2-production activity. Photocatalytic H2-production reactions under visible-light irradiation ( 420 nm) were performed on the hierarchical MoS2/ZnIn2S4 composites and their photocatalytic activities are shown in Figure 8 A. ZnIn2S4 alone exhibited relative low activity, because of the rapid recombination of conduction-band (CB) electrons and valence-band (VB) holes, whilst MoS2 alone was almost inactive for photocatalytic H2 production. However, when ZnIn2S4 nanosheets were grown on the surface of MoS2 slices, the rate of H2 evolution on the hierarchical MoS2/ZnIn2S4 photocatalysts by using MoS2 as a co-catalyst was enormously increased compared with ZnIn2S4 and MoS2 alone. These results suggest a synergistic effect between MoS2 slices and ZnIn2S4 nanosheets. First, the hierarchical MoS2/ZnIn2S4 composites can provide a larger number of available catalytically active sites, because ZnIn2S4 nanosheets grow on the MoS2 slices and avoid the excess aggregation of pristine ZnIn2S4. Second, the enhanced catalytic activity of H2 evolution can be attributed to improved photoinduced charge transport and -separation, owing to the formation of p–n junctions between MoS2 and ZnIn2S4, as confirmed by the SPV, TPV (Figure 6), and EIS analyses (Figure 7). A possible mechanism for photocatalytic H2 evolution over MoS2/ZnIn2S4 is proposed in Figure 8 B. The band-gaps in MoS2 and ZnIn2S4 are about 1.9 and 2.3 eV, respectively. As a p-type semiconductor, the conduction band (CB) in MoS2 is lower than that in n-type semiconductor ZnIn2S4.[32] Therefore, the photoexcited electrons in the CB of ZnIn2S4 can be transferred quickly into that of MoS2, which are accessible to adsorbed H+ ions (from water ionization) to produce H2. The sacrificial reagent will complement the electrons to the remaining holes in the VB of
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Figure 8. A) Photocatalytic H2 evolution of ZnIn2S4, MoS2, and MoS2/ ZnIn2S4 composites with different MoS2 content. Conditions: catalyst (80 mg), an aqueous solution (100 mL) that contained 0.25 m Na2SO3/ 0.35 m Na2S as sacrificial reagents, a 300 W Xe lamp that was equipped with a cut-off filter (l > 420 nm). B) Proposed mechanism for photocatalytic H2 evolution from the hierarchical MoS2/ZnIn2S4 composite induced by visible-light irradiation.
ZnIn2S4. This process greatly enhances the separation ability of the photoexcited electrons and holes. After charge separation, photocatalytic oxidation and reduction reactions take place on different parts of the catalyst and both regions must be accessible to the reactants. Thus, in heterojunction photocatalysts, in which one component is grown or deposited on the other, one must have control over the coverage, because both overly complete and sparse coverage of one component on the other would decrease the catalytic efficiency. Therefore, the amount of MoS2 is an important factor that affects photocatalytic activity; on increasing MoS2 content in the nanocomposites, the rate of H2 evolution increased first and then decreased. The optimum amount of MoS2 was about 15 wt %, at which the hierarchical MoS2/ZnIn2S4 sample showed the highest activity in H2 production (78 mmol h 1), four-times higher than that of pure ZnIn2S4 (19 mmol h 1). A further increase in the amount of MoS2 (20 wt %) resulted in a decrease in the photocatalytic hydrogen evolution, presumably owing to the shading effect.[18b] Superfluous MoS2 in the composites should increase the opacity and light scattering, thereby leading to a decrease in light absorption by the reaction suspension solution, as
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found in MoS2/ZnIn2S4 photocatalysts.[2, 30] Moreover, the presence of lower than 15 wt % MoS2 in the sample indicated that a small amount of MoS2/ZnIn2S4 junctions were formed. Recycling experiments under the same conditions (see the Supporting Information, Figure S3) demonstrated that the composite catalyst showed slight deactivation on recycling. On the one hand, a high concentration of sacrificial S2 / SO32 in the solution suppresses the oxidation of S2 on the composite by complementing the electrons to the remaining holes in the VB of ZnIn2S4, whereas, on the other hand, oxidation/corrosion of MoS2 and ZnIn2S4 by photogenerated holes occurs over the MoS2/ZnIn2S4 catalysts. These two competing processes for the photogenerated holes lead to slight deactivation of the catalyst. Further studies on photocatalysts based on hierarchical MoS2/ZnIn2S4 composites are still needed to improve their activity and stability for practical applications. To investigate the effect of interactions at junctions between MoS2 and ZnIn2S4 on the photocatalytic activity of hierarchical MoS2/ZnIn2S4 photocatalysts, a physical mixture of MoS2 and ZnIn2S4 was used as a reference sample. The hierarchical MoS2/ZnIn2S4 composite showed a much-higher rate of H2 evolution than the physical mixture of MoS2 and ZnIn2S4 with the same amount of catalyst (Figure 8 A). This result indicates that the intimate interfacial contact and strong interactions between ZnIn2S4 nanosheets and MoS2 slices are crucial for interelectron transfer between the two components. Compared with the interface provided by random collisions, a steadier and tighter interface that is formed by the direct growth of ZnIn2S4 nanosheets on the 2D planar structure of MoS2 slices can lead to easier charge transfer and more-efficient separation of electron–hole pairs before recombination.
Experimental Section Exfoliation of Layered Commercial MoS2 Slices The exfoliation of layered commercial MoS2 was performed according to a literature procedure.[33] Commercial MoS2 powder (60 mg) was dispersed in an EtOH/water mixture (20 mL, 45 % v/v EtOH). The sealed flask was sonicated for 8 h and then the dispersion was centrifuged at 3000 rpm for 20 min to remove any aggregates. Finally, the supernatant (the exfoliated MoS2 slices) was collected and used to synthesize MoS2/ ZnIn2S4 composites. Synthesis of MoS2/ZnIn2S4 Composites In a typical synthesis of a MoS2/ZnIn2S4 composite, the exfoliated MoS2 slices (MoS2/ZnIn2S4 = 5, 10, 15, and 20 wt %) were dispersed in EtOH (15 mL) by ultrasonication for 30 min. Subsequently, glycerol (5 mL), In(NO)3·4.5 H2O (0.3820 g), Zn(AC)2·6 H2O (0.1110 g), and l-cysteine hydrochloride monohydrate (C3H7NO2S·HCl·H2O, 0.5272 g) were added into the solution under stirring, and the mixture was stirred for 20 min. Then, the mixed solution was transferred into a Teflon-lined stainless steel autoclave (50 mL), which was heated to 180 8C and maintained at that temperature for 10 h. After cooling to room temperature, the as-synthesized solid products were rinsed several times with EtOH and dried at 70 8C for 12 h. For comparison, blank ZnIn2S4 was prepared in the absence of MoS2 under the same experimental conditions. Characterization X-ray diffraction (XRD) of the powder samples was performed on a Bruker D8 Advance diffractometer with monochromatized CuKa radiation (l = 0.15418 nm). TEM and high-resolution TEM (HRTEM) images of the samples were recorded by using a JEOL 2100 microscope at an accelerating voltage of 200 kV; SEM was performed on a Hitachi, S-4800. UV/Vis diffuse reflectance spectroscopy of the samples was performed on a UV/Vis spectrophotometer (Shimadzu UV-2550). Electrochemical impedance spectroscopy (EIS) of thin films of these materials was performed on a computer-controlled IM6e impedance measurement unit (Zahner Elektrik, Germany) in an aqueous solution that contained 0.25 m Na2SO3 and 0.35 m Na2S under UV light by applying sinusoidal perturbations of 10 mV under a bias of 0.8 V within the frequency range 0.05– 100 kHz. Surface photovoltage (SPV) measurements were performed a home-made apparatus. The powder samples were sandwiched between two indium tin oxide (ITO) glass electrodes and the change in surfacepotential barrier in the presence of light and in the dark corresponded to the SPV signal. The raw SPV data were normalized with a Model Zolix UOM-1S (China) illuminometer. For the transient photovoltage (TPV) measurements, the sample chamber consisted of an FTO electrode, a mica spacer (thickness: 10 mm) as an electron isolator (to prevent the photoinduced electrons in the semiconductor from being directly injected into the electrode), and a platinum wire gauze electrode (transparency: about 50 %). The construction had a sandwich-like structure, which was comprised of the FTO-electrode/sample/mica/platinum-gauze-electrode, and the components were cascaded directly in sequence without further treatment. During the measurements, the platinum gauze electrode was connected to the core of a BNC cable, which provided the input signal to the oscilloscope. The samples were excited from the platinum wire gauze electrode with the laser radiation pulse (355 nm, 50 mJ cm 2, pulse width: 5 ns) of a third harmonic Nd:YAG laser (Polaris II, New Wave Research). The intensity of the pulse was determined on an EM500 single channel Joulemeter (Molectron). The TPV signals were recorded on a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix).
Conclusions In this study, we have demonstrated the in situ synthesis of hierarchical MoS2/ZnIn2S4 composites by the confined growth of ZnIn2S4 nanosheets on the surface of exfoliated MoS2 slices. The exfoliated MoS2 can not only serve as a confined platform for growing ZnIn2S4 nanosheets with abundant exposed active sites, but also an electron acceptor and transporter for favorable charge separation. Compared with MoS2 alone and ZnIn2S4 alone, the unique hierarchical MoS2/ZnIn2S4 hybrid structure enabled strong interactions, owing to the formation of p–n junctions between two components and facilitated efficient electron transfer for a higher hydrogen-production rate under visible-light irradiation without expensive Pt loaded as co-catalyst. This study reveals the possibility of using a MoS2 co-catalyst as an ideal support material for enhancing the photocatalytic hydrogenevolution activity. We anticipate that our proposed synthetic route to MoS2/ZnIn2S4 composites will open up a new way to prepare MoS2-based composite materials for photocatalysis and solar cells.
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Photocatalytic Hydrogen Production Photocatalytic H2 evolution from water was performed by using an online photocatalytic hydrogen-production system (AuLight, Beijing, CEL-SPH2N). A powder sample of the catalyst (80 mg) was suspended in an aqueous solution (100 mL) that contained 0.25 m Na2SO3 and 0.35 m Na2S. The reaction was performed by irradiating the suspension with the light of a 300 W Xe lamp (AuLight, CEL-HXF-300, Beijing) that was equipped with an optical filter (l > 420 nm) to cut-off the light in the ul-
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traviolet region. Prior to the reaction, the mixture was de-aerated by evacuation to remove any dissolved O2 and CO2. Gas evolution, which was only observed under photoirradiation, was analyzed by an on-line gas chromatograph (SP7800, TCD, 5 molecular sieves, N2 carrier, Beijing Keruida Limited). To evaluate the photostability, the photocatalyst after the first 3 h run of the photochemical reaction was separated from the suspension, washed with water, and dried at 60 8C; then, the recovered photocatalyst was used in the next run of the photoreaction under the same conditions.
[16]
[17]
[18]
Acknowledgements We gratefully acknowledge the support of this research by the Key Program Projects of the National Natural Science Foundation of China (21031001), the National Natural Science Foundation of China (51272070, 91122018, 20971040, 21371053, 21001042, 21376065, and 21201058), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (No 708029), the Program for Innovative Research Team in University (IRT-1237), the Special Research Fund for the Doctoral Program of Higher Education of China (20112301110002), the NSFC Projects of International Cooperation and Exchanges (51310105017), the Special Fund of Technological Innovation Talents in Harbin City (No. 2012RFQXG111), and the Scientific Research Fund of Heilongjiang Provincial Education Department (12531502).
[19] [20]
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FULL PAPER Photocatalysis Guohui Tian, Yajie Chen, Zhiyu Ren, Chungui Tian, Kai Pan, Wei Zhou, Jiaqi Wang, Honggang Fu* &&&&—&&&& Enhanced Photocatalytic Hydrogen Evolution over Hierarchical Composites of ZnIn2S4 Nanosheets Grown on MoS2 Slices
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Rolling stones gather no MoS: MoS2/ ZnIn2S4 composites were prepared in situ by the controlled growth of ZnIn2S4 nanosheets on MoS2 slices. The synergistic effect of strong interactions between ZnIn2S4 nanosheets and
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MoS2 slices at the p–n junctions and the higher number of available catalytically active sites contribute to enhanced photocatalytic hydrogenevolution activity under visible-light irradiation in the absence of Pt.
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Enhanced Photocatalytic Hydrogen Evolution over Hierarchical Composites of ZnIn2S4 Nanosheets Grown on MoS2 Slices Guohui Tian,[a, b] Yajie Chen,[a] Zhiyu Ren,[a] Chungui Tian,[a] Kai Pan,[a] Wei Zhou,[a] Jiaqi Wang,[a] and Honggang Fu*[a] Abstract: A highly active hierarchical MoS2/ZnIn2S4 composite catalyst was synthesized in situ by using a facile controlled-growth approach through a solvothermal process. During the solvothermal reaction, 2D ultrathin curled ZnIn2S4 nanosheets grew on the surface of MoS2 slices, which could help to form a more-homogeneous mixture, effective interfacial contact, and strong interactions between the ZnIn2S4 nanosheets and the MoS2 slices. The inti-
mate contact between ZnIn2S4 and MoS2 favored the formation of junctions between the two components, thereby improving the charge separation and prolonging the mean lifetime of the electron–hole pairs. Moreover, growing ZnIn2S4 nanosheets by visibleKeywords: charge transfer · composite materials · molybdenum · nanostructures · photochemistry
Introduction
of semiconductors and co-catalysts are critically dependent on their spatial integration; therefore, the construction of the optimal structure morphology and the formation of surface heterojunctions are of vital importance.[7] For example, Yin et al. fabricated well-defined 3D graphene/metal-oxide nanoparticle hybrids by using a new and general approach with low-valence metal ions as the precursors. The prepared hybrids exhibited excellent capacitive deionization performance and have potential applications in other areas, such as catalysis and supercapacitors.[8] In the photocatalytic process, the co-catalyst cooperates with the light harvester to facilitate the charge separation and increase the lifetime of the photogenerated electron– hole pair, whilst lowering the activation barriers for H2 or O2 evolution. Thus, the use of a co-catalyst leads to an increase in overall photocatalytic performance, including in terms of activity, selectivity, and stability. In general, the efficiency of a given photocatalytic system is dependent on the ability of the co-catalyst to support reductive and/or oxidative catalysis.[9] In particular, the structural characteristics and intrinsic catalytic properties of a co-catalyst are important. Photocatalytic/catalytic systems based on abundantly available materials are certainly desirable for large-scale hydrogen production for future energy production based on water and sunlight; indeed, the widespread use of Pt may be limited by its scarcity and high cost and, thus, the development of non-noble-metal alternatives is desirable.[10] Recently, 2D graphene was shown to be a good co-catalyst. Metal oxides (sulfur) supported on 2D graphene nanosheets and their composites have been extensively investigated for their photocatalytic properties.[11] In particular, the construction of hybrids with graphene-like thin-layered heterojunctions,
As is well-known, hydrogen energy is a potentially significant alternative form of storable and clean energy in the future.[1, 2] Over the past few years, photocatalytic H2 production has attracted a lot of attention and it appears to be a promising strategy because it is clean, low-cost, and environmentally friendly by utilizing solar energy.[3, 4] In practice, a single catalyst generally shows relatively low photocatalytic activity, so a co-catalyst is often introduced to improve the performance.[5] In recent years, nanocomposites of a semiconductor and a co-catalyst, which combine the unique properties of the individual nanostructures and may show synergistic effects, have attracted great attention in photocatalysis and other applications.[6] Moreover, the performance
[a] Dr. G. Tian, Dr. Y. Chen, Z. Ren, Dr. C. Tian, K. Pan, W. Zhou, J. Wang, Prof. H. Fu Key Laboratory of Functional Inorganic Material Chemistry Ministry of Education of the People’s Republic of China Heilongjiang University Harbin 150080 (P. R. China) Fax: (+ 86) 4518666-1259 E-mail: [email protected] [b] Dr. G. Tian Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion College of Heilongjiang Province School of Chemistry and Materials Science Heilongjiang University Harbin 150080 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201301646.
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light catalysis on MoS2 slices afforded a higher number of available catalytically active sites. So, the photocatalytic hydrogen-evolution performance of the hierarchical MoS2/ZnIn2S4 composite was significantly enhanced, owing to a synergistic effect of these factors. This work could provide new insights into the fabrication of a highly efficient and low-cost non-noble-metal co-catalyst for visible-light H2 generation.
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Results and Discussion
such as graphene-gC3N4, can allow the prompt migration of light-induced charge, thereby resulting in an efficient photocatalytic reaction.[12] Similar to graphene, as a typical layered transition-metal sulfide, MoS2 has also been extensively investigated.[13, 14] MoS2 is a typical example of the layered transition-metal dichalcogenide family. It is composed of covalently bonded S Mo S sheets that are bound by weak van der Waals forces and is particularly important for lithium batteries and sensing applications.[15] The potential application of MoS2 as a co-catalyst for photocatalytic H2 production has also received great attention (e.g., MoS2/CdS, TiO2/MoS2/graphene composites) and the introduction of MoS2 can significantly enhance the H2-production rate.[16] Recently, C. N. R. Rao’s group has done a lot of work on the synthesis and application of MoS2.[17] They prepared graphene (EG)-MoS2 composites that showed very high H2-evolution activity. Moreover, they also studied the influence of the polytypes of MoS2 (few-layer 2H-MoS2 and single-layer 1T MoS2) on the H2-evolution activity and stability.[17b] Similar to graphene, the potency of MoS2 as a co-catalyst in improving the photocatalytic activity has been demonstrated for some photocatalysts. However, in most of those studies, MoS2 was loaded onto the surface of the catalysts (e.g. graphene, gC3N4).[18] In these examples, it increased the light-blocking effect of the co-catalyst, thereby decreasing the light utilization by the catalysts. Moreover, there are only very weak interactions between the two components in composites that are obtained by physical mixing and by general chemical synthesis methods, which can prohibit the direct chemical contact between the two components, thereby greatly decreasing the synergetic catalytic effect of guest MoS2 and the host catalyst.[19] Therefore, to effectively inhibit the recombination of photogenerated electron and holes and to improve the synergetic catalytic effect of the composite, it is important to construct composite catalysts with strong interactions between MoS2 and the main catalyst. Herein, for the first time, we synthesized hierarchical MoS2/ZnIn2S4 composites in situ by the controlled growth of ZnIn2S4 nanosheets on MoS2 slices through a solvothermal process. The exfoliated 2D platform structure of MoS2 slices was an excellent supporting matrix for the in situ growth of the ZnIn2S4 nanosheets. This method can help to form junction structures between ZnIn2S4 nanosheets and MoS2 slices, with stronger interactions compared with their physical mixtures, which helps the interfacial charge transfer and decreases self-agglomeration. The synergistic effect of the strong interactions from these junctions between ZnIn2S4 nanosheets and MoS2 slices and the higher number of available catalytically active sites can contribute significantly to the enhanced photocatalytic hydrogen-evolution activity under visible-light irradiation in the absence of noble metal Pt.
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XRD analysis was performed to investigate the structures of the obtained samples. As shown in Figure 1 a, pristine ZnIn2S4 shows diffraction peaks that correspond to the (006), (102), and (110) crystal planes (2q = 21.6, 27.8, and
Figure 1. XRD patterns of a) ZnIn2S4, b) the MoS2/ZnIn2S4 composite (15 wt % MoS2), and c) MoS2.
47.28, respectively), which can be indexed to the hexagonal phase of ZnIn2S4 (JCPDS No. 65-2023).[20] For the pure MoS2 sample (Figure 1 c), the XRD peaks at 2q = 14.5, 32.7, 39.7, 49.8, and 58.48 can be assigned to the (002), (100), (103), (105), and (110) planes in the orthorhombic phase of MoS2 (JCPDS No. 37-1492).[21] In the MoS2/ZnIn2S4 composite, the XRD pattern (Figure 1 b) displays two types of diffraction peaks: besides the diffraction peaks that are assigned to the orthorhombic phase of MoS2, all of the additional peaks are well-matched to the hexagonal phase of ZnIn2S4, thus indicating the presence of ZnIn2S4 nanosheets coated on the MoS2 slices. The morphology and microstructure of the obtained hierarchical MoS2/ZnIn2S4 composites were investigated by FESEM and TEM (Figure 2). For comparison, the SEM image of exfoliated MoS2 slices was also recorded (Figure 2 A), which showed a 2D sheet-like morphology with sizes varying from 500 nm to several micrometers in length and width. The TEM image (Figure 2 A, inset) confirmed that the exfoliated MoS2 was several nanometers thick and belonged to few-layer 2H-MoS2 (p-type semiconductor).[17b] Figure 2 B shows that the ultrathin ZnIn2S4 nanosheets uniformly grew on both sides of the MoS2 slices. These ZnIn2S4 nanosheets were interconnected with each other, thereby forming 3D nanosheet networks. Figure 2 C shows a TEM image of a MoS2/ZnIn2S4 composite (15 wt % MoS2), in which the thin ZnIn2S4 nanosheets cover the surface of the MoS2 slices. HRTEM analysis (Figure 2 D, magnification of the dashed square in Figure 2 C) clearly shows the lattice fringes, thus suggesting a well-defined crystal structure. The fringes with a lattice spacing of 0.27 nm corresponded to the (100) plane of MoS2.[22] The lattice spacing was about 0.324 nm, which could be assigned to the (102) plane of hex-
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smooth surface of the original sample (Figure 2 A), some nanoparticles were formed on the surface of the MoS2 slices after the initial nucleation reaction (Figure 3 A, B), mainly because the surface of the MoS2 slices could provide highenergy nucleation sites for the nucleation and growth of ZnIn2S4 nanocrystals. Moreover, there were oxygen functional groups (e.g., hydroxy, glycerol) on the surface. These functional groups could act as anchor sites and bind with Zn2+ and In3+ metal cations. During the solvothermal reaction, the anchored Zn2+ and In3+ metal cations reacted in situ with glycerol to form metal glycerolate complexes, which then further reacted with S2 ions to form ZnIn2S4 on the surface of MoS2 slices. Moreover, the stacking forces between the ZnIn2S4 nuclei and the basal planes of MoS2 were also beneficial for in situ growth on the surface of the MoS2 slices. Therefore, the ZnIn2S4 would gradually grow along the initial nuclei of ZnIn2S4 and the ZnIn2S4 nanosheets would grow larger and more densely along the surface of the MoS2 slices (Figure 3 C, D) with the extension of the reaction time. According to this study, the growth process of MoS2/ZnIn2S4 composites during the hydrothermal process is shown in the Supporting Information, Figure S2. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical states of Mo and S in the MoS2/ ZnIn2S4 composite (15 wt % of MoS2). The high-resolution XPS spectrum (Figure 4 A) showed that the binding energies of the Mo 3d3/2 and Mo 3d5/2 peaks in pure MoS2 were locat-
Figure 2. A) SEM and TEM (inset) images of MoS2 slices; B) SEM image of a hierarchical MoS2/ZnIn2S4 composite (15 wt % MoS2); C) lowmagnification TEM image of a hierarchical MoS2/ZnIn2S4 composite (15 wt % MoS2); D) HRTEM image of the dashed square in (C).
agonal-phase ZnIn2S4.[23] Moreover, a distinguished and coherent interface between the continuity of the lattice fringes of MoS2 and ZnIn2S4 was observed, which indicated the formation of a p–n junction and could afford better charge separation and efficient electron transfer within the hybrid structure compared with the physical mixtures and pure ZnIn2S4. For comparison, the corresponding SEM images of the MoS2/ZnIn2S4 composites with different amounts of MoS2 are shown in the Supporting Information, Figure S1. If the MoS2 content was high (20 wt %), only a low density of ZnIn2S4 nanosheets were grown on the surface of the MoS2 slices. However, the excess ZnIn2S4 nanosheets self-assembled to form ZnIn2S4 microspheres at low MoS2 content (5 wt %). To further illustrate the formation process of hierarchical MoS2/ZnIn2S4 composites, time-dependent experiments were performed. As shown in Figure 3 A, compared to the
Figure 4. XPS spectra of A) Mo 3d, B) S 2p, C) In 3d, and D) Zn 2p peaks in MoS2/ZnIn2S4 (15 wt % MoS2), ZnIn2S4, and MoS2.
ed at 232.5 and 229.3 eV, respectively, thus suggesting that Mo4+ existed in the pure MoS2.[22] After ZnIn2S4 nanosheets were grown on the surface of MoS2 slices, the Mo 3d3/2 and Mo 3d5/2 peaks shifted to 232.1 and 228.8 eV, respectively.[24] In the MoS2/ZnIn2S4 composite, the S 2p1/2 and S 2p3/2 peaks (Figure 4 B), which corresponded to MoS2 and ZnIn2S4, were also slightly lower than those of MoS2 (163.25 and
Figure 3. SEM images of MoS2/ZnIn2S4 samples that were prepared from different solvothermal reaction times: a) 0.5, b) 1, c) 4, and d) 10 h.
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162.25 eV) and ZnIn2S4 (162.6 and 161.6 eV). The binding energies of In 3d (444.9 and 452.4 eV) and Zn 2p (1021.8 and 1044.6 eV) in the MoS2/ZnIn2S4 composite were slightly higher than those of ZnIn2S4 (Figure 4 C, D).[25] These results indicate that there are strong interactions between the ZnIn2S4 nanosheets and the MoS2 slices in the composite, driven by the probable electron transfer and delocalization of firmly contacted MoS2 layers and ZnIn2S4 nanosheets.[11, 25] Figure 5 shows UV/Vis absorption spectra of ZnIn2S4 and the MoS2/ZnIn2S4 composites with various MoS2 content. For the pure ZnIn2S4 nanosheets, a significant absorption at l > 400 nm was found. Moreover, for the MoS2/ZnIn2S4 com-
Figure 6. A) Surface photovoltage spectra of a) ZnIn2S4 and b) the MoS2/ ZnIn2S4 composite (15 wt % MoS2). B) Transient photovoltage spectra of a) ZnIn2S4 and b) the MoS2/ZnIn2S4 composite (15 wt % MoS2). Figure 5. UV/Vis diffuse reflectance spectra of a) ZnIn2S4 and MoS2/ ZnIn2S4 composites with different MoS2 content: b) 5, c) 10, d) 15, and e) 20 wt %.
pared to pure ZnIn2S4, the intensity of the SPV response for the MoS2/ZnIn2S4 composite is significantly enhanced. This result demonstrates that there must be charge transfer between the MoS2 slices and ZnIn2S4 nanosheets and that the photogenerated charge-separation efficiency in ZnIn2S4 is enhanced, owing to the introduction of MoS2 slices as a substrate. The transient photovoltage (TPV) technique, which is an effective method for investigating the dynamic properties of photoinduced charge carriers in semiconductor materials,[27] was also used to further investigate the transfer properties of the photogenerated electrons from n-type semiconductor ZnIn2S4 to p-type semiconductor MoS2.[17b] As shown in Figure 6 B, the MoS2/ZnIn2S4 composite exhibited a prolonged mean lifetime of electron–hole pairs with respect to ZnIn2S4, which further confirmed that the p–n junction between MoS2 and ZnIn2S4 in the hierarchical MoS2/ZnIn2S4 composite greatly retarded the recombination of electron– hole pairs in the excited ZnIn2S4. Electrochemical impedance spectroscopy (EIS) is a powerful tool for studying charge-transfer processes in three-electrode systems[28] and EIS Nyquist plots of the two samples are shown in Figure 7. It has been reported that the intermediate-frequency response is associated with electron transport and -transfer at the ZnIn2S4/electrode interface.[29] The hierarchical MoS2/ZnIn2S4 composite shows a smaller semicircle in the mid-frequency region in comparison to ZnIn2S4, which indicates faster interfacial charge trans-
posites (15 and 20 wt % MoS2), there were an additional two absorption peaks, located at about 480 and 670 nm, thus leading to a broad absorption in the visible-light region.[22] The corresponding color of the composites also turned dark gray. These result shows that the addition of MoS2 slices increased the absorbance of visible light.[14] Because of the increased visible-light absorbance, a more-efficient utilization of the solar energy could be realized. Therefore, it can be inferred that the introduction of MoS2 slices as the growing substrate of ZnIn2S4 nanosheets is effective for the visiblelight response of the MoS2/ZnIn2S4 composites. Surface photovoltage (SPV) is an effective technique for studying the photophysics of excited states that are generated by photon absorption and can effectively reflect the separation and recombination of photoinduced charge carriers.[26] To probe the effect of strong interactions at intimate junctions between ZnIn2S4 nanosheets and MoS2 slices on the photogenerated charge-transfer properties, the SPV technique was employed. The SPV spectrum of a MoS2/ZnIn2S4 composite is shown in Figure 6 A. A SPV response appears if photogenerated charge carriers are separated in space; a stronger photoelectric signal corresponds to higher chargeseparation efficiency. For pure ZnIn2S4 and the MoS2/ ZnIn2S4 composite, there is a clear surface photovoltage response within the range 350–700 nm. In particular, com-
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Figure 7. Nyquist plots of a) ZnIn2S4 and b) the MoS2/ZnIn2S4 composite (15 wt % MoS2) electrodes in an aqueous solution that contained 0.25 m Na2SO3/0.35 m Na2S under visible-light irradiation.
fer.[30, 31] So, the introduction of MoS2 slices can benefit charge transfer and -separation in the MoS2/ZnIn2S4 system and, thus, decrease charge recombination because of the formation of junctions between MoS2 and ZnIn2S4. Overall, MoS2 slices can act as both a substrate for ZnIn2S4 nanosheets and as an electron collector and -transporter in the composite, thereby effectively inhibiting charge recombination and, thus, contributing to the improved photocatalytic H2-production activity. Photocatalytic H2-production reactions under visible-light irradiation ( 420 nm) were performed on the hierarchical MoS2/ZnIn2S4 composites and their photocatalytic activities are shown in Figure 8 A. ZnIn2S4 alone exhibited relative low activity, because of the rapid recombination of conduction-band (CB) electrons and valence-band (VB) holes, whilst MoS2 alone was almost inactive for photocatalytic H2 production. However, when ZnIn2S4 nanosheets were grown on the surface of MoS2 slices, the rate of H2 evolution on the hierarchical MoS2/ZnIn2S4 photocatalysts by using MoS2 as a co-catalyst was enormously increased compared with ZnIn2S4 and MoS2 alone. These results suggest a synergistic effect between MoS2 slices and ZnIn2S4 nanosheets. First, the hierarchical MoS2/ZnIn2S4 composites can provide a larger number of available catalytically active sites, because ZnIn2S4 nanosheets grow on the MoS2 slices and avoid the excess aggregation of pristine ZnIn2S4. Second, the enhanced catalytic activity of H2 evolution can be attributed to improved photoinduced charge transport and -separation, owing to the formation of p–n junctions between MoS2 and ZnIn2S4, as confirmed by the SPV, TPV (Figure 6), and EIS analyses (Figure 7). A possible mechanism for photocatalytic H2 evolution over MoS2/ZnIn2S4 is proposed in Figure 8 B. The band-gaps in MoS2 and ZnIn2S4 are about 1.9 and 2.3 eV, respectively. As a p-type semiconductor, the conduction band (CB) in MoS2 is lower than that in n-type semiconductor ZnIn2S4.[32] Therefore, the photoexcited electrons in the CB of ZnIn2S4 can be transferred quickly into that of MoS2, which are accessible to adsorbed H+ ions (from water ionization) to produce H2. The sacrificial reagent will complement the electrons to the remaining holes in the VB of
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Figure 8. A) Photocatalytic H2 evolution of ZnIn2S4, MoS2, and MoS2/ ZnIn2S4 composites with different MoS2 content. Conditions: catalyst (80 mg), an aqueous solution (100 mL) that contained 0.25 m Na2SO3/ 0.35 m Na2S as sacrificial reagents, a 300 W Xe lamp that was equipped with a cut-off filter (l > 420 nm). B) Proposed mechanism for photocatalytic H2 evolution from the hierarchical MoS2/ZnIn2S4 composite induced by visible-light irradiation.
ZnIn2S4. This process greatly enhances the separation ability of the photoexcited electrons and holes. After charge separation, photocatalytic oxidation and reduction reactions take place on different parts of the catalyst and both regions must be accessible to the reactants. Thus, in heterojunction photocatalysts, in which one component is grown or deposited on the other, one must have control over the coverage, because both overly complete and sparse coverage of one component on the other would decrease the catalytic efficiency. Therefore, the amount of MoS2 is an important factor that affects photocatalytic activity; on increasing MoS2 content in the nanocomposites, the rate of H2 evolution increased first and then decreased. The optimum amount of MoS2 was about 15 wt %, at which the hierarchical MoS2/ZnIn2S4 sample showed the highest activity in H2 production (78 mmol h 1), four-times higher than that of pure ZnIn2S4 (19 mmol h 1). A further increase in the amount of MoS2 (20 wt %) resulted in a decrease in the photocatalytic hydrogen evolution, presumably owing to the shading effect.[18b] Superfluous MoS2 in the composites should increase the opacity and light scattering, thereby leading to a decrease in light absorption by the reaction suspension solution, as
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found in MoS2/ZnIn2S4 photocatalysts.[2, 30] Moreover, the presence of lower than 15 wt % MoS2 in the sample indicated that a small amount of MoS2/ZnIn2S4 junctions were formed. Recycling experiments under the same conditions (see the Supporting Information, Figure S3) demonstrated that the composite catalyst showed slight deactivation on recycling. On the one hand, a high concentration of sacrificial S2 / SO32 in the solution suppresses the oxidation of S2 on the composite by complementing the electrons to the remaining holes in the VB of ZnIn2S4, whereas, on the other hand, oxidation/corrosion of MoS2 and ZnIn2S4 by photogenerated holes occurs over the MoS2/ZnIn2S4 catalysts. These two competing processes for the photogenerated holes lead to slight deactivation of the catalyst. Further studies on photocatalysts based on hierarchical MoS2/ZnIn2S4 composites are still needed to improve their activity and stability for practical applications. To investigate the effect of interactions at junctions between MoS2 and ZnIn2S4 on the photocatalytic activity of hierarchical MoS2/ZnIn2S4 photocatalysts, a physical mixture of MoS2 and ZnIn2S4 was used as a reference sample. The hierarchical MoS2/ZnIn2S4 composite showed a much-higher rate of H2 evolution than the physical mixture of MoS2 and ZnIn2S4 with the same amount of catalyst (Figure 8 A). This result indicates that the intimate interfacial contact and strong interactions between ZnIn2S4 nanosheets and MoS2 slices are crucial for interelectron transfer between the two components. Compared with the interface provided by random collisions, a steadier and tighter interface that is formed by the direct growth of ZnIn2S4 nanosheets on the 2D planar structure of MoS2 slices can lead to easier charge transfer and more-efficient separation of electron–hole pairs before recombination.
Experimental Section Exfoliation of Layered Commercial MoS2 Slices The exfoliation of layered commercial MoS2 was performed according to a literature procedure.[33] Commercial MoS2 powder (60 mg) was dispersed in an EtOH/water mixture (20 mL, 45 % v/v EtOH). The sealed flask was sonicated for 8 h and then the dispersion was centrifuged at 3000 rpm for 20 min to remove any aggregates. Finally, the supernatant (the exfoliated MoS2 slices) was collected and used to synthesize MoS2/ ZnIn2S4 composites. Synthesis of MoS2/ZnIn2S4 Composites In a typical synthesis of a MoS2/ZnIn2S4 composite, the exfoliated MoS2 slices (MoS2/ZnIn2S4 = 5, 10, 15, and 20 wt %) were dispersed in EtOH (15 mL) by ultrasonication for 30 min. Subsequently, glycerol (5 mL), In(NO)3·4.5 H2O (0.3820 g), Zn(AC)2·6 H2O (0.1110 g), and l-cysteine hydrochloride monohydrate (C3H7NO2S·HCl·H2O, 0.5272 g) were added into the solution under stirring, and the mixture was stirred for 20 min. Then, the mixed solution was transferred into a Teflon-lined stainless steel autoclave (50 mL), which was heated to 180 8C and maintained at that temperature for 10 h. After cooling to room temperature, the as-synthesized solid products were rinsed several times with EtOH and dried at 70 8C for 12 h. For comparison, blank ZnIn2S4 was prepared in the absence of MoS2 under the same experimental conditions. Characterization X-ray diffraction (XRD) of the powder samples was performed on a Bruker D8 Advance diffractometer with monochromatized CuKa radiation (l = 0.15418 nm). TEM and high-resolution TEM (HRTEM) images of the samples were recorded by using a JEOL 2100 microscope at an accelerating voltage of 200 kV; SEM was performed on a Hitachi, S-4800. UV/Vis diffuse reflectance spectroscopy of the samples was performed on a UV/Vis spectrophotometer (Shimadzu UV-2550). Electrochemical impedance spectroscopy (EIS) of thin films of these materials was performed on a computer-controlled IM6e impedance measurement unit (Zahner Elektrik, Germany) in an aqueous solution that contained 0.25 m Na2SO3 and 0.35 m Na2S under UV light by applying sinusoidal perturbations of 10 mV under a bias of 0.8 V within the frequency range 0.05– 100 kHz. Surface photovoltage (SPV) measurements were performed a home-made apparatus. The powder samples were sandwiched between two indium tin oxide (ITO) glass electrodes and the change in surfacepotential barrier in the presence of light and in the dark corresponded to the SPV signal. The raw SPV data were normalized with a Model Zolix UOM-1S (China) illuminometer. For the transient photovoltage (TPV) measurements, the sample chamber consisted of an FTO electrode, a mica spacer (thickness: 10 mm) as an electron isolator (to prevent the photoinduced electrons in the semiconductor from being directly injected into the electrode), and a platinum wire gauze electrode (transparency: about 50 %). The construction had a sandwich-like structure, which was comprised of the FTO-electrode/sample/mica/platinum-gauze-electrode, and the components were cascaded directly in sequence without further treatment. During the measurements, the platinum gauze electrode was connected to the core of a BNC cable, which provided the input signal to the oscilloscope. The samples were excited from the platinum wire gauze electrode with the laser radiation pulse (355 nm, 50 mJ cm 2, pulse width: 5 ns) of a third harmonic Nd:YAG laser (Polaris II, New Wave Research). The intensity of the pulse was determined on an EM500 single channel Joulemeter (Molectron). The TPV signals were recorded on a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix).
Conclusions In this study, we have demonstrated the in situ synthesis of hierarchical MoS2/ZnIn2S4 composites by the confined growth of ZnIn2S4 nanosheets on the surface of exfoliated MoS2 slices. The exfoliated MoS2 can not only serve as a confined platform for growing ZnIn2S4 nanosheets with abundant exposed active sites, but also an electron acceptor and transporter for favorable charge separation. Compared with MoS2 alone and ZnIn2S4 alone, the unique hierarchical MoS2/ZnIn2S4 hybrid structure enabled strong interactions, owing to the formation of p–n junctions between two components and facilitated efficient electron transfer for a higher hydrogen-production rate under visible-light irradiation without expensive Pt loaded as co-catalyst. This study reveals the possibility of using a MoS2 co-catalyst as an ideal support material for enhancing the photocatalytic hydrogenevolution activity. We anticipate that our proposed synthetic route to MoS2/ZnIn2S4 composites will open up a new way to prepare MoS2-based composite materials for photocatalysis and solar cells.
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Photocatalytic Hydrogen Production Photocatalytic H2 evolution from water was performed by using an online photocatalytic hydrogen-production system (AuLight, Beijing, CEL-SPH2N). A powder sample of the catalyst (80 mg) was suspended in an aqueous solution (100 mL) that contained 0.25 m Na2SO3 and 0.35 m Na2S. The reaction was performed by irradiating the suspension with the light of a 300 W Xe lamp (AuLight, CEL-HXF-300, Beijing) that was equipped with an optical filter (l > 420 nm) to cut-off the light in the ul-
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traviolet region. Prior to the reaction, the mixture was de-aerated by evacuation to remove any dissolved O2 and CO2. Gas evolution, which was only observed under photoirradiation, was analyzed by an on-line gas chromatograph (SP7800, TCD, 5 molecular sieves, N2 carrier, Beijing Keruida Limited). To evaluate the photostability, the photocatalyst after the first 3 h run of the photochemical reaction was separated from the suspension, washed with water, and dried at 60 8C; then, the recovered photocatalyst was used in the next run of the photoreaction under the same conditions.
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Acknowledgements We gratefully acknowledge the support of this research by the Key Program Projects of the National Natural Science Foundation of China (21031001), the National Natural Science Foundation of China (51272070, 91122018, 20971040, 21371053, 21001042, 21376065, and 21201058), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (No 708029), the Program for Innovative Research Team in University (IRT-1237), the Special Research Fund for the Doctoral Program of Higher Education of China (20112301110002), the NSFC Projects of International Cooperation and Exchanges (51310105017), the Special Fund of Technological Innovation Talents in Harbin City (No. 2012RFQXG111), and the Scientific Research Fund of Heilongjiang Provincial Education Department (12531502).
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FULL PAPER Photocatalysis Guohui Tian, Yajie Chen, Zhiyu Ren, Chungui Tian, Kai Pan, Wei Zhou, Jiaqi Wang, Honggang Fu* &&&&—&&&& Enhanced Photocatalytic Hydrogen Evolution over Hierarchical Composites of ZnIn2S4 Nanosheets Grown on MoS2 Slices
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Rolling stones gather no MoS: MoS2/ ZnIn2S4 composites were prepared in situ by the controlled growth of ZnIn2S4 nanosheets on MoS2 slices. The synergistic effect of strong interactions between ZnIn2S4 nanosheets and
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MoS2 slices at the p–n junctions and the higher number of available catalytically active sites contribute to enhanced photocatalytic hydrogenevolution activity under visible-light irradiation in the absence of Pt.
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