Three-dimensional nanoporous heterojunction of monolayer MoS2

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Three-dimensional nanoporous heterojunction of monolayer MoS2@rGO for photo-enhanced hydrogen evolution reaction Yongzheng Zhang, Jing Du, Ziqian Wang, Min Luo, Yuan Tian, Takeshi Fujita, Qikun Xue, and Mingwei Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00234 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Energy Materials

Three-dimensional Nanoporous Heterojunction of Monolayer MoS2@rGO for Photo-enhanced Hydrogen Evolution Reaction Yongzheng Zhang†, Jing Du†, Ziqian Wang‡, Min Luo†, Yuan Tian‡, Takeshi Fujita†, Qikun Xue†, §, and Mingwei Chen†, ‡, ǁ, * †

WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

‡

Department of Materials Science and Engineering, Johns Hopkins University, Baltimore,

MD 21218, USA §

Department of Physics, Tsinghua University, Beijing 100084, China

ǁ

CREST, Japan Science and Technology Agency, Saitama 332-0012, Japan

*

Corresponding Author: M. W. Chen (email: [email protected])

ACS Paragon Plus Environment

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ABSTRACT: Two-dimensional (2D) semiconductor materials with a large surface area, tunable bandgap and high catalytic activity are emerging as a new class of photoelectrocatalysts. However, the 2D photoelectrocatalysts usually suffers from lower photoelectrochemical efficiency because of the challenges in effectively utilizing the 2D photoelectrocatalysts in three-dimensional electrodes with a reserved surface area and 2D electronic properties. Here we report a novel nanoporous heterojunctions comprised of 3D nanoporous reduced graphene oxide (rGO) as visible light absorber and 2D monolayer MoS2 as electron transfer bridge and HER electrocatalyst for photoelectrochemical hydrogen production. With a designed band alignment between monolayer MoS2 and rGO and a large surface and contact interface, the photoelectrocatalyst shows an outstanding photo-enhanced HER activity with a high solar-to-hydrogen conversion efficiency. KEYWORDS: monolayer MoS2; 3D nanoporous reduced graphene oxide; heterojunction; hydrogen evolution reaction; photoelectrocatalyst. INTRODUCTION Two-dimensional (2D) materials, such as graphene, phosphorene (BP) and transition metal dichalcogenides (TMDs), have attracted great interest over the past decades due to their unique electronic and optical properties and large specific surface area.1-8 Recent advances in creating heterostructure based on 2D atomic crystals also provide new opportunities for the applications of 2D materials as electrocatalysts and photocatalysts toward oxygen reduction reaction9-11, water splitting12-18 and CO2 activation19-21. In particular, hydrogen production by water splitting is one of the most efficacious approaches to produce the clean and renewable hydrogen energy. However, the large overpotential (-1.23 V) for water splitting leads the sluggish reaction kinetics and low energy efficiency. Platinum and its alloys are the bestknown catalysts for hydrogen evolution reaction (HER) and can dramatically enhance the


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reaction kinetics at a very low potential for high efficiency energy conversion.22-25 However, the practical application is restricted by the high material costs. Recently, earth-abundant and low-cost 2D MoS2 crystals are emerging as a promising alternative of Pt-based catalysts for HER.26-33 In particular, the direct band gap of monolayer MoS2, different from metallic Pt catalysts, opens the possibility to utilize solar energy for HER by forming a heterostructure with a paired 2D semiconductor. The p-n heterojunctions uniting MoS2 flakes and nitrogen doped rGO sheets have recently been synthesized for water splitting by utilizing the tunable band gaps, high electric conductivity and high photoresponsivity of rGO.34-35 The heterostructured photoelectrocatalysts have demonstrated the photo-enhanced HER kinetics by light illumination. However, the calculated photoelectrochemical efficiency is very poor and only about 0.49%.35 This is mainly because the technical challenge in retaining the large accessible electrochemically active surface area and high electron conductivity in the 2D p-n heterojunctions constructed by discrete nanosheets. Inspired by our recent finding that a three-dimensional bicontinuous nanoporous architecture can significantly improve the photoresponsivity of rGO for visible light absorption,36 in this study we develop a novel p-n heterostructure by depositing 2D monolayer MoS2 on free-standing 3D nanoporous rGO (MoS2@rGO) via chemical vapor deposition (CVD) (Figure 1) for photo-enhanced HER. The open and bicontinuous nanoporous nanoarchitecture of 2D p-n heterojunctions effectively retains the high electrochemical active surface areas of the 2D materials, and provides a large contact interface between n-type MoS2 monolayers and p-type rGO substrate for fast photoexcited electron transfer and high-efficiency visible light absorption.


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RESULTS AND DISCUSSION

Figure 1. Design and synthesis of monolayer MoS2@3D nanoporous reduced graphene oxide. (a) Fabrication process of MoS2@rGO; (b) STEM image of 3D nanoporous MoS2@rGO; (c) Schematic image of MoS2@rGO; (d) Schematic illustration of band alignment and mechanism of photo-excited charge transfer in MoS2@rGO. The fabrication process of MoS2@rGO is schematically illustrated in Figure 1a. Freestanding nanoporous MoS2@rGO heterojunctions with a large geometric area of ~3 cm × 3 cm were synthesis for photoelectrocatalytic measurements. The bicontinuous 3D nanoporous structure of MoS2@rGO is shown in Figure 1b. For comparison, monolayer MoS2 is also deposited on pure nanoporous graphene (MoS2@gra). The pristine monolayer MoS2 with 2H (hexagonal) phase is an n-type semiconductor with a direct bandgap of ~1.9 eV. The positions of the valence band maximum (VBM) and conduction band minimum (CBM) of 2H MoS2 locate at -6.28 eV and -4.38 eV relative to vacuum level.37-39 In contrast, the band structure of rGO strongly depends on the concentration of oxygen.40-42 Density functional theory (DFT) calculations of rGO with 15 at.% O (Figure S1, Supporting Information) suggests that the rGO has a p-type direct bandgap of 1.1 eV at the K point in reciprocal space. The positions of


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the VBM and CBM for rGO are at -4.62 eV and -3.52 eV relative to vacuum level. The schematic illustration of the band alignment and proposed mechanism of photo-excited charge transfer in the MoS2@rGO heterojunction are shown in Figure 1d. The vacuum level is at 0 eV and the dashed line at -4.44 eV is the potential of the standard hydrogen electrode relative to vacuum level.43-44 Note that the position of CBM of rGO is beyond that of MoS2 and the potential of standard hydrogen evolution locates below the CBM of MoS2. Thus, 3D nanoporous rGO has a good band alignment with monolayer MoS2 and can form a type II (staggered) heterojunction structure with effective photo-excited charge transfer. When the heterojunction is irradiated with visible light, the electrons are excited from the VBM to the CBM of rGO. The designed band offset of CBM between MoS2 and rGO facilitates the photo-excited electron transfer and promotes hole-electron separation with the formation of electron-rich MoS2 for HER. Furthermore, the photo injected electrons produce an extra positive bias during HER process, which will further enhance the catalytic performance of the MoS2@rGO catalyst. Experimentally, the photoresponsivity of np-rGO is tuned by reduction time to have the band alignment with 2H MoS2 monolayers. The photoresponsivity of MoS2@rGO characterized by reduction time of rGO are shown in Figure S7 in Supporting Information. The best performed heterojunction (MoS2@1h-rGO) has an oxygen concentration of 13.8 at.% (Table S1, Supporting Information), which is close to the DFT prediction of 15 at.%.


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Figure 2. Optical characterization of MoS2@gra and MoS2@rGO. (a) Raman spectra of MoS2@gra and MoS2@rGO; (b) UV-Vis spectra of gra, MoS2@gra, 3D nanoporous rGO and MoS2@rGO. Raman spectra of MoS2@gra and MoS2@rGO are shown in Figure 2a. The Raman peaks at ~405 and ~385 cm-1 represent the out-of-plane mode (A1g) and the in-plan mode (E12g) of 2H MoS2.45-46 The successful deposition of MoS2 on the nanoporous graphene and rGO substrates are also confirmed by X-ray diffraction (Figure S4, Supporting Information). The frequency difference (∆f) between A1g and E12g is a reliable quantity to characterize the layer number of MoS2. The values of ∆f=20.2 cm-1 for MoS2@gra and 20.5 cm-1 for MoS2@rGO suggest that MoS2 in both samples is monolayer.46-47 The Raman peaks at 1354, 1583 and 2712 cm-1 are the D, G and 2D bands of graphene, respectively. Raman spectra of the 3D nanoporous rGO show a more intense D band and a weaker 2D band compared with pure nanoporous graphene. Since the relative intensity ratios ID/IG and I2D/IG are indication of the graphene quality,48-49 the ratios, ID/IG=1.1 and I2D/IG=0.4 for 3D nanoporous rGO, ID/IG=0.1 and I2D/IG=1.8 for pure nanoporous graphene, suggest that the delocalized π conjugation is impaired and there is more geometric lattice disorder introduced by oxidation and reduction process in the 3D nanoporous rGO. Optical absorption spectra of the nanoporous graphene, MoS2@gra, nanoporous rGO and MoS2@rGO are shown in Figure 2b.


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In contrast to the nanoporous rGO and MoS2@rGO which exhibit photo absorption mainly in the visible light region, no absorption can be observed from nanoporous graphene and MoS2@gra because of the zero bandgap of pure nanoporous graphene in Dirac cones at the Brillouin zone. Despite the fact that monolayer 2H MoS2 is a semiconductor with a direct bandgap, the absorption cannot be seen due to the effective light absorption of graphene and possible transfer of hot electrons from MoS2 to graphene,50 The band gap energies of 3D nanoporous rGO and MoS2@rGO can be estimated from the curves of (αhλ)2 versus photon energy as shown in the insets in Figure 2b. The measured band gaps of 3D nanoporous rGO at ~1.15 eV in both samples are in good agreement with the DFT calculations (Figure S2, Supporting Information). The band structure of the rGO has also been investigated by the cyclic voltammetry (CV) method by measuring the positions of the valence band maximum (VBM) and conduction band minimum (CBM) as shown in Figure S3. Moreover, a broad peak appearing in the vicinity of 675 nm of the UV-Vis spectrum of MoS2@rGO is assigned to the A- exciton of monolayer MoS2 with the energy of 1.83 eV.31, 51-52


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Figure 3． XPS spectra of (a) C 1s in MoS2@gra; (b) C 1s in MoS2@rGO; (c) Mo 3d in MoS2@gra; and (d) Mo 3d in MoS2@rGO. The binding state of MoS2@gra and MoS2@rGO were investigated by X-ray photoelectron spectroscopy (XPS) (Figure 3). Both samples show a prominent C-C bond at 284.5eV. However, the width of graphitic carbon becomes broader after oxidation and reduction processing, which is associated with the mixed states of sp2 and sp3 hybridized orbitals. Higher peak intensities of the C-O (286.5 eV), C=O (287.5 eV), and C(O)OH (289.0 eV) bonds are visible in the MoS2@rGO (Figure. 3b), which are less intense in the MoS2@gra (Figure 3a), as a consequence of remnant oxidization in the rGO.36 From the XPS spectra of Mo 3d for both samples shown in Figure 3c and 3d, two intense Mo 3d5/2 and Mo 3d3/2 doublets can be observed at 228.8 eV and 232.7 eV, which has been characterized as the Mo-S bond of 2H phase MoS2.53


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Figure 4. Structure and chemistry of MoS2@gra. (a) Bright-field TEM image showing the 3D nanoporous morphology of MoS2@gra. The inserted electron diffraction pattern demonstrates the existence of MoS2 and nanoporous graphene in the sample; (b) and (c) HAADF-STEM images showing the morphology and lattice structure of MoS2@gra; (d) EDS mappings of MoS2@gra.

Figure 5. Structure and chemistry of MoS2@rGO. (a) Bright-field TEM image showing the 3D nanoporous morphology of MoS2@rGO. The inserted electron diffraction pattern


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demonstrates the existence of MoS2 and rGO in the sample; (b) and (c) HAADF-STEM images of the morphology and lattice structure of MoS2@rGO; (d) EDS mappings of MoS2@rGO. The microstructure and chemical mappings of MoS2@gra and MoS2@rGO are characterized with scanning transmission electron microscope (STEM) as shown in Figure 4 and Figure 5. The low magnification STEM (Figure 1c) and SEM images (Figure S5, Supporting Information) suggest that rGO maintains the bicontinuous 3D nanoporous structure after oxidation, reduction and CVD. The MoS2 monolayers grown on rGO show weak contrast with a triangular shape (Figure 5b). The atomic structure of MoS2 from the high-resolution STEM image verifies the high crystallinity of the monolayer crystals (Figure 5c). The curved lattices, marked by the yellow dash lines, indicate that obvious lattice distortion is introduced into the MoS2 monolayers when the 2D crystals grow on the curved internal surface of nanoporous rGO. It has been demonstrated that such lattice distortion can enhance HER activities by changing local electronic structure of MoS2.54 The selected area electron diffraction pattern (the inset of Figure 5a) exhibits two sets of diffraction spots. The inner six diffraction spots represent monolayer MoS2 and the outer six diffuse diffraction spots are from rGO. The average size of the triangular MoS2 monolayers on 3D nanoporous rGO is about ~50 nm, which is smaller than that (~100 nm shown in Figure 4b) on pure nanoporous graphene, indicating that the more defective rGO may promote the nucleation of MoS2 monolayers during CVD growth and the smaller MoS2 size with increased edge sites is expected to benefit the HER activity.55 The STEM elemental mappings of C, O, Mo and S elements taken by energy dispersive X-ray spectroscopy (Figure 4d and 5d) reveal the uniform coverage of monolayer MoS2 on graphene and rGO at micrometer-scale. The Composition analyses are shown in Table S1. The MoS2@rGO sample has a higher oxygen content of ∼10.4% comparing to 1.3% in the MoS2@gra sample. The ratio of Mo/C in


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MoS2@rGO and MoS2@gra is 0.101 and 0.103. The estimated loading amounts of MoS2 on nanoporous graphene and rGO are estimated to be 51.9 wt.% and 49.6 wt.%, respectively.

Figure 6. Photo-enhanced electrocatalytic performances of MoS2@gra and MoS2@rGO. (a) Polarization curves with/without light irradiation; (b) Corresponding Tafel plots; (c) Column diagrams of onset potential, tafel slope and overpotential at 10 mA/cm2. The photo-assisted electrocatalysis of MoS2@rGO is tested in a 0.5 M H2SO4 solution by cyclic voltammetry using modified three-electrode electrochemical system with a solar simulator. Figure 6a and 6b show the cathodic polarization curves and Tafel slopes of MoS2@gra and MoS2@rGO with/without light irradiation. For comparison, a Pt electrode is used as the reference. Compared with the performances of nanoporous graphene and rGO (Figure S6, Supporting Information), the HER activities of samples with monolayer MoS2 are


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significantly higher. Importantly, light irradiation can obviously improve the HER performances of rGO and MoS2@rGO while the performances of nanoporous graphene and MoS2@gra are insensitive to light irradiation. The MoS2@rGO has an onset potential of -76 mV and a Tafel slope of 101 mV/dec in dark. When exposed to visible light, it exhibits a much lower onset potential of -48mV and a smaller Tafel slope of 61 mV/dec under 1 sun illumination. Correspondingly, the solar-to-hydrogen (STH) conversion efficiency of the MoS2@rGO catalyst is 1.2%, which is calculated by the equation (1) with short-circuit current density of 0.97 mA/cm2 (Figure S8a). The obvious light response of the MoS2@rGO, but not MoS2@gra, evidences that rGO plays an important role in the photo-enhanced HER catalysis. Moreover, the much higher HER activities of MoS2@rGO, in comparison with nanoporous rGO with/without light irradiation, demonstrate that the monolayer MoS2 is an intrinsically active HER catalyst and plays an key role as an electrocatalysts in the heterojunction. The features of HER activities of the samples, including onset potentials, Tafel slopes and overpotentials at 10 mA/cm2 are summarized in Figure 6c. In addition to the high electrocatalytic activity, the cycling stability of MoS2@rGO is tested by polarization curves measurements in the acidic electrolyte under 1 sun illumination (Figure S10a, Supporting Information). Negligible loss of the cathodic current density even after 2000 cycles indicates high electrochemical stability of the heterojunction. The excellent electrochemical stability of the heterostructured photoelectrocatalyst is also verified by Raman spectra (Figure S10b, Supporting Information) of the sample after 2000 cycle testing. Visible structure changes cannot be seen.


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Figure 7. Electrochemical properties of MoS2@gra and MoS2@rGO. (a), Electrochemical active surface areas reckoned by Cdl. (b), Nyquist plots with/without light irradiation.

The electrochemical active surface areas of MoS2@gra and MoS2@rGO are reckoned by measuring the double-layer capacitance (Cdl) from the CV results at different scan rates (Figure S11, Supporting Information). The twice of Cdl for each sample is extracted by plotting j=ja - jc at a given potential (0.2 V vs. RHE) against the CV scan rates as shown in Figure 7a, where ja and jc are anode current density and cathode current density, respectively. The Cdl of MoS2@rGO (35.1 mF/cm2) is about 1.9 times larger than that of the MoS2@gra (18.6 mF/ cm2), demonstrating the proliferation of the electrochemical active sites in the MoS2@rGO catalyst. Since the j of MoS2@rGO at a given potential (0.2 V vs. RHE) is unchanged with and without light irradiation (Figure S12, Supporting Information), the light irradiation does not make detectable changes to the electrochemical active surface area. Electrochemical impedance spectroscopy (EIS) is employed to investigate the HER kinetics of MoS2@rGO at electrode/electrolyte interfaces. The Nyquist plots of the MoS2@gra and MoS2@rGO are recorded at the potential of 250 mV as shown in Figure 7b. The semicircles represent the charge transfer resistance (Rct) of H+ reactions at the


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electrode/electrolyte interfaces. Both the MoS2@gra and MoS2@rGO electrodes exhibit obvious charge transfer resistance. However, the value of Rct of the MoS2@rGO is much lower than that of MoS2@gra, suggesting that faster electron transfer at the interfaces between electrolyte and MoS2@rGO. Importantly, the Rct of MoS2@rGO becomes obviously smaller under light irradiation while MoS2@gra is insensitive to the light illumination, further demonstrating that the photoexcitation leads to the enhanced reaction kinetics of HER by providing additional hot electrons for effective charge transfer. For HER in acid media, the kinetics of Volmer reaction for reducing protons to hydrogen (H+ + e− → Had) is closely related to the onset potential and Tafel slope of HER.56-57 Apparently, the formation of electron-rich MoS2 by photoexcitation benefits the proton reduction and thus lower onset potentials and Tafel slope. When the samples are irradiated with visible light, the electrons are excited from the VBM to the CBM of rGO. The relative band offset between the CBM of MoS2 and rGO facilitates hot electrons transfer in sequence to MoS2 and interfaces with electrolyte, leading to hole-electron separation. The hot electrons at HER active sites of MoS2@rGO promote the reduction reaction kinetics and supply an extra built-in electric field. Both of them will enhance the catalytic activity of MoS2@rGO with reduced onset potential and Tafel slope. Besides the high photoresponsivity of nanoporous rGO, the appropriate band alignment between monolayer MoS2 and rGO also promotes the efficient hot electron transfer from rGO to MoS2 for photo-enhanced HER catalysis. Although 2H MoS2 is a semiconductor and can generate hot electrons in the system, the amount of photo-generated electrons from MoS2 should be very small because of the lower light absorption of the monolayer crystals. Since the HER activity of semiconducting MoS2 mainly arises from the active edge sites, increasing the density of active edge sites and electric conductivity are important strategies in improving the HER activities of 2D MoS2. Depositing monolayer MoS2 on large surface and conductive nanoporous rGO significantly


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improves the HER catalysis of MoS2 with enlarged electrochemical active surface area and enhanced charge transport. Combining with the effect of photoexcitation, the heterostructured photoelectrocatalyst MoS2@rGO shows superior HER performances (Table S2, Supporting Information). In particular, the significant enhancement in HER kinetics from light illumination may open a new field of developing photo-enhanced 2D HER catalysts based on 3D nanoporous rGO. CONCLUSIONS In summary, a novel heterojunction, 2D monolayer MoS2@3D nanoporous rGO, has been successfully fabricated for high efficiency photo-enhanced HER. When exposed in the visible light, the built-in electric field created by space charge of the heterojunction suppresses the electron-hole recombination and promotes the formation of electron-rich MoS2 for catalyzing HER at a lower onset potential and a smaller Tafel slop with a higher solar-tohydrogen conversion efficiency of 1.2% under 1 sun illumination. In the heterostructured photoelectrocatalyst, 3D nanoporous rGO serves as a visible light absorber and a conductive substrate while 2D monolayer MoS2 attached on rGO acts as the acceptors of hot electrons for photo-enhanced electrocatalysis towards HER. By tuning the band structure of rGO, the proper band alignment between MoS2 and rGO has been achieved, which plays an important role in hot electron transfer from rGO to catalytically active MoS2 for high photoelectrochemical efficiency. This work presents a novel pattern of 3D material nanoporous rGO and 2D MoS2 hybrid catalysts and may paves a new way to enhance the HER performance by utilizing solar energy. METHODS Synthesis of Nanoporous Graphene by CVD method.


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Nanoporous graphene was prepared by a CVD method using nanoporous Ni as the catalyst and porous template for graphene growth. The nanoporous Ni templates were obtained by de-alloying 100 µm thick Ni30Mn70 foil in 1 M (NH4)2SO4 to etch away Mn at 50 °C.58 The as-prepared Ni template was then inserted into a quartz tube and annealed at 900 °C under 2500 sccm Ar and 10 sccm H2 for 3 min. After the pretreatment, benzene (0.5 mbar, 99.8%, anhydrous) as the carbon source was conveyed with the gas flow of Ar (2500 sccm) and H2 (100 sccm) at 900 °C for 2 min for the graphene growth. Then the furnace was rapidly cooled down to room temperature. Finally, the graphene coated nanoporous Ni were immersed into 2.0 M HCl solution to dissolve the nanoporous Ni template. After washed in deionized water for five times, free-standing nanoporous graphene sheets with the thickness about 80 µm were obtained. Preparation of Nanoporous Graphene Oxide and Reduced Graphene Oxide. The Nanoporous Graphene Oxide was prepared by a modified Hummer method. A freestanding graphene sheet was put into the mixture of KMnO4 (1.5 g), concentrated H2SO4 (10 mL), and NaNO2 (0.1 g) without stirring at room temperature. To keep the original 3D nanoporous structure, the oxidization time was kept for 1 h. The reaction was terminated by aqueous H2SO4 (5%) and H2O2 (30%) mixture solution. The obtained GO sheet was then washed with a mixture solution of aqueous H2SO4 (3%) and H2O2 (0.5%) for three times. The nanoporous GO sheets were then transferred into the 30ml Hydrazine hydrate for reduction treatment at room temperature. The free-standing 3D reduced nanoporous graphene oxide samples with various reduction states were acquired by controlling the reduction time at room temperature. Preparation of MoS2 on Nanoporous Graphene (MoS2@gra) and MoS2 on 3D Reduced Nanoporous Graphene Oxide (MoS2@rGO).


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The MoS2@gra and MoS2@rGO samples were synthesized using a low-pressure chemical vapor deposition method. MoCl5 (Sigma-Aldrich, purity >99.5%) and S (Wako Pure Chemical Industry, purity 99%) powders were used as the precursors and free-standing 3D nanoporous graphene and reduced 3D nanoporous graphene oxide as substrates. Growth chamber was pumped down to a base pressure of 5 × 10−2 mbar before growth. Then temperatures of the sources and the substrates were set at 80 °C for MoCl5, 120 °C for sulfur and 800 °C for the substrates, respectively. For monolayer MoS2, the growth process was performed for 1 h with 500 sccm Ar as carrier gas under pumping. Structural characterization. The microstructures of MoS2@gra and MoS2@rGO samples were studied by a ﬁeldemission scanning electron microscope (JEOL JIB-4600F) and a ﬁeld-emission transmission electron microscope (JEOL JEM-2100F, 200 kV) with double spherical aberration correctors for both the probe-forming and image-forming objective lenses. Chemical compositions of the samples were measured in STEM mode with an energy dispersive X-ray spectrometer. Raman spectra were recorded using a micro-Raman spectrometer (Renishaw InVia RM 1000) with an incident wavelength of 514.5 nm at a low laser power of 2.0 mW. The binding states of elements in the samples were characterized by an X-ray photoemission spectrometer (AXIS ultra DLD, Shimadzu) in a vacuum of 10−7 Pa with an Al Kα (mono) anode. The optical properties were investigated by ultraviolet–visible (UV–Vis) spectrophotometer (JASCO V-650 spectrophotometer). The range of 400–850 nm is collected with a standard component of a Ф60 mm integrating sphere. Optical band gap are calculated from UV-Vis absorption spectra based on Tauc Plots (αhλ)2 versus photon energy. Electrochemical measurements.


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A modified three-electrode system with a solar simulator was used in electrochemical measurements. Ag/AgCl was used as the reference electrode. A graphite sheet was used as counter electrode. The electrolytes used in the electrochemical tests were 0.5 M H2SO4 solution. All the electrochemical experiments were conducted using an electrochemical workstation (Ivium Technologies B.V.) and were performed at room temperature. The reference electrode was calibrated to reversible hydrogen potential (RHE) using a platinum electrode for both working and counter electrodes and converted to RHE according to the Nernst equation: ERHE= 0.0592pH + EoAg/AgCl (EoAg/AgCl=0.197 V) and all data presented were corrected for iR losses. The sweep rates used in the cyclic voltammetry studies were 10 mV s−1. EIS measurements were carried out at different potentials with the frequencies ranging from 10−2 to 105 Hz with AC voltage of 5 mV. The HER durability of the MoS2@rGO was tested for 48 h under 1 sun illumination. The STH conversion efficiency were calculated according to the following equation59:

STH =

(mA/cm ) × 1.23() × (1) (mW/cm )

Where jsc is short-circuit current density, ηF is Faradic efficiency, Ptotal is the incident illumination power density, 100 mW/cm2. The short-circuit current density (jsc) was measured in 0.5 M Na2SO3 by a two-electrode cell equipped with Pt wire as counter electrode. Supporting Information. Supplementary on experimental results and discussion. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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Three-dimensional nanoporous heterojunction of monolayer MoS2

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