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Metal (Ag, Pt)-MoS2 Hybrids Greenly Prepared Through Photochemical Reduction of Femtosecond Laser Pulses for SERS and HER Pei Zuo, Lan Jiang, Xin Li, Bo Li, Peng Ran, Xiaojie Li, Liangti Qu, and Yong Feng Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00579 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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ACS Sustainable Chemistry & Engineering

Metal (Ag, Pt)−MoS2 Hybrids Greenly Prepared Through Photochemical Reduction of Femtosecond Laser Pulses for SERS and HER ‖

Pei Zuo,† Lan Jiang,†,‡ Xin Li,*, † Bo Li, † Peng Ran, † Xiaojie Li, † Liangti Qu, Yongfeng Lu§ †

Laser Micro/Nano-Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute

of Technology, 5 South Zhongguancun Street, Haidian District, Beijing 100081, China, ‡Laser Micro/Nano-Fabrication Laboratory, Department of Mechanical Engineering, Tsinghua University, 30 Shuangqing Road, Haidian District, Beijing, 100084, China, ‖Key Laboratory of Cluster Science, Ministry of Education, School of Chemistry, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing 100081, China, §Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0511, USA *E-mail address: [email protected] (Xin Li). KEYWORDS: multilayer MoS2, femtosecond laser irradiation, photogenerated electrons, in situ decoration, metal−MoS2 nanohybrids, chemical sensing, hydrogen production

ABSTRACT: MoS2-based nanohybrids have garnered extensive research interest for enhancing chemical catalytic performance, application of biochemical sensing, and inducing phase

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transition of MoS2. This work presents a novel green method to prepared Ag−MoS2 and Pt−MoS2 nanohybrids through the photogenerated electrons of MoS2 nanosheets induced by using femtosecond laser pulses. Metal (Ag, Pt) nanoparticles are reduced by capturing the photogenerated electrons of MoS2, and in situ decorated on MoS2 nanosheets, thus forming Ag−MoS2 and Pt−MoS2 nanohybrids, respectively. The proposed method doesn’t need other chemical reagents except for the metal salts necessary for supplying metal cations, which commendably avoids the introduction of reagent byproducts to the reaction mixture, toxicity, and chemical or environmental contamination. This method also emphasizes the extensive application fields of MoS2. For example, the prepared Ag−MoS2 hybrids reveal excellent surface enhanced Raman scattering performance with the enhancement factor reaching 1.32×107 and the detection limit low to 10-11 M; the prepared 7.6% Pt−MoS2 hybrids with C exhibit enhanced hydrogen evolution reaction activity with low Tafel slope of 25 mV/decade and high turnover frequency per exposed Mo of 11.15 H2 s-1 at 220 mV; demonstrating the remarkable prospects of MoS2-based hybrids in chemical/biological molecule sensing as well as hydrogen production applications.

INTRODUCTION Two-dimensional transition metal dichalcogenides (TMDs) have attracted substantial research attention for their uniquely electronic properties and huge application potential in various areas such

as

sensors,

spin-

and

valley-tronics,

thermoelectrics,

field-effect

transistors,

superconductors, and batteries.1-4 Molybdenum disulfide (MoS2) is a typical semiconductor of the family of TMDs, with crystal structure of a Mo-layer intercalated two S-layers, and atoms (Mo and S) in a layer are bonded through strong covalent force, whereas the adjacent layers are


2

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interacted by means of weak Van der Waals force.5 Due to the advantages of natural abundance, inexpensive price, good stability, and significant chemical activity,5-6 MoS2 crystal has obtained extensive research attention, also displayed appealing application potentials in chemical catalysis,7 functional composite synthesis,8 biochemical sensing,9 and energy conversion.10 To expand the potential applications and improve the application performance of MoS2, MoS2-based nanohybrids have been valued, specifically the MoS2 composited with metal nanoparticles (NPs). These metal NPs can induce local surface-plasmon resonance to activate photoelectro catalysis for hydrogen evolution reaction (HER) and enhance the ability of MoS2 for light emission or absorption; can improve surface-enhanced Raman scattering (SERS) sensitivity of MoS2 hybrids for biochemical sensing application; can promote electron transport in electrochemical process to improve the electrochemical-catalytic property of MoS2; and can result in phase transition of MoS2 from semiconducting (2H) to metallic (1T) state.5, 11-14 Recent approaches to prepare metal−MoS2 hybrids primarily include four categorizations: physicalrelated method including physically depositing metal on MoS2 and mixing/putting ready-made metal NPs with/on MoS2;13,

15-17

chemical-related method including chemical synthesis,

solvothermal method, self-assembly method, and wave-assisted chemical reduction;11-12,

18-22

spontaneous reduction of Au NPs on MoS2;22-23 photochemical synthesis method including halogen lamp and laser irradiation.5, 22, 24-26 Among them, photochemical synthesis method is a green and simple method, where MoS2 acts as photocatalyst and metal ions can be directly reduced and decorated on photoexcited MoS2 surfaces, without using reductants.25 The irradiation source in photochemical synthesis method primarily includes halogen lamp and laser. In preparing metal−MoS2 hybrids by halogen lamp irradiation, although no reductants is needed, other one or two kinds of chemical agents are still used, and only one kind of metal−MoS2


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hybrids is prepared with the same irradiation conditions.22, 24 In addition, recent methods of laser induced metal on MoS2 are mainly two-step method or used for bulk or supported MoS2 flakes/films.5, 25-26 Hence, it is also essential to develop a simple one-step, environment friendly, in situ, and highly adaptable approach to preparing various liquid-phase metal−MoS2 hybrids for multiple applications. In the work, a novel method was developed by using femtosecond laser pulses to nonthermally induce photogenerated electrons (negative charge) of MoS2, which could interact with metal (Ag, Pt) cations, leading to the reduction and in situ deposition of metal NPs on MoS2 nanosheets that formed metal−MoS2 nanohybrids. Compared with previous reported methods, this method has several advantages such as simplicity, the absence of chemical contamination, green, strong adaptability, and in situ growth. Characterization analyses indicated the formation of metal (Ag, Pt) NPs on MoS2 nanosheets and high crystalline of the metal NPs, as well as and their doping effect on MoS2. This method has great potential for improving the application effect in chemical/biological sensing, enhancing electrochemical catalytic performances or activating photoelectro-catalysis for HER, and enhancing light absorption/emission ability of MoS2-based hybrids; for example, the obtained Ag−MoS2 hybrids and Pt−MoS2 hybrids respectively exhibited highly sensitive SERS activity and obviously excellent HER activity.

EXPERIMENTAL SECTION Materials, reagents, and experiment preparation: MoS2 nanosheet dispersion (1 mg/mL) were purchased from XFNANO, INC (Nanjing, China). AgNO3 aqueous solution was purchased from Sinopharm Chemical Reagent Beijing Co., Ltd (Beijing, China). K2PtCl4 powder was purchased from Thermo Fisher Scientific (China) Inc (Shanghai, China). Before the laser irradiation experiment, the MoS2 dispersion was ultrasonically re-dispersed for 3 min, then MoS2


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dispersion was evenly mixed with prepared AgNO3 solution (2.0 mM/L) at the volume ratio of 1:1, and the MoS2 dispersion also was evenly mixed with prepared K2PtCl4 solution (2.1 mM/L) at the volume ratio of 1:1 or 2:1. Femtosecond pulse irritation synthetizing Ag−MoS2 and Pt−MoS2 hybrids: The femtosecond laser pulses were emitted from a Ti: Sapphire laser (35-fs) system with wavelength of 800 nm and repetition rate of 1 kHz. Then laser pulse beam was focused by using a 5X microscope object lens (Olympus NA = 0.15) and vertically income to the internal of the metal salt (AgNO3 or K2PtCl4) and MoS2 mixed solution (2 mL) in transparent glass containers that were placed on a displacement platform. The processing parameters of the laser were as follows: pulse energy = 4 µJ; scan speed = 2000 µm/s; scan spacing = 20 µm; and scan time = 2 h. Characterization of Ag−MoS2 and Pt−MoS2 hybrids: Transmission electron microscopy (TEM) was conducted using a JEM-2100F transmission electron microscope. X ray photoelectron spectroscopy (XPS) was conducted using a PHI Quantera X ray photoelectron spectrometer. Scanning electron microscopy (SEM) was conducted using a field emission scanning electron microscope (Quanta 200FEG). The atomic force microscope (AFM) was conducted using a SPM960 AFM. UV-vis spectra were obtained using an Agilent Cary-5000 UV-vis near-infrared spectrophotometer. Energy dispersive spectrometer (EDS) was performed using a JEM2010 transmission electron microscope. The liquid samples were moderately dropped onto polished Si substrates for XPS, Raman, PL, SEM, and AFM characterization. SERS detection of Ag−MoS2 hybrids: SERS was performed using a Renishaw-InViaReflex spectrometer with light sources of 532 and 633 nm. In the SERS detection using R6G as the probe, 0.25 mL of Ag−MoS2 hybrid dispersion and 1 mL of R6G solution were mixed together and subsequently ultrasonically dispersed for 3 min. Finally, 10 µL mixture was


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dropped onto a clean glass slide for testing. Other analytes (CV and 4-MBA molecules) were also detected similarly. Electrocatalytic test of Pt−MoS2 hybrids: The HER catalytic activity of the Pt−MoS2 hybrids was tested using a electrochemical workstation (CHI 760D) in a standard three-electrode system, with rotating disc glassy carbon electrode (GCE), Ag/AgCl (in 3.5 M KCl solution) electrode, and graphite rod used as the working, reference, and counter electrode, respectively. In this process, 40 µL of Pt−MoS2 hybrids with C solution was loaded on the GCE (diameter=5 mm) for drying, and the electrode was subsequently coated with 5 µL of Nafion solution (5%). Pt/C powder (1.6 mg) (20 wt% platinum on Vulcan carbon black, Alfa Aesar) was ultrasonically dispersed in a mixed solvent containing 40 µL Nafion (5%) and 1 mL ethanol for 1.5 h, next, about 35 μL of Pt/C dispersion was deposited onto the GCE, serving as the commercial Pt−C reference electrode after drying in air. All measurements were performed in 0.5 M H2SO4 solution (purged with pure N2). IR compensations were conducted for polarization curve test of all catalyst samples, and the obtained potentials were against the RHE through RHE calibration.

RESULTS AND DISCUSSION The size, thickness, atomic lattice, and atomic vibration of the pristine liquid-phase multilayer MoS2 nanosheets are illustrated in Figure S1-S4. Figure 1 shows the schematic of proposed method to in situ decorate MoS2 with noble metal (Ag, Pt) NPs (more detailed operation is described in Experimental section). Briefly, a 35-fs-800-nm femtosecond laser pulse beam was focused by an objective lens into the internal of the mixed solution of MoS2 nanosheets and metal salts. The photon energy of femtosecond laser pulses with wavelength of 800 nm was 1.55 eV, and the optical bandgap of multilayer MoS2 was approximately 1.3 eV; hence, MoS2 can be excited through single-photon absorption process, which can be easily realized by femtosecond


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laser pulses for its nonlinear susceptibility and ultrafast absorption.27 Upon the irradiation of femtosecond laser pulses, electrons of MoS2 were excited from valence band to conduction band through photon absorption, yielding electron–hole pairs. The photogenerated electrons (negative charge) of MoS2 induced by femtosecond laser pulses served to interact with the metal cations (Ag+, Pt2+) that absorbed on MoS2 nanosheets, and the positive charge (hole) was estimated to engage in water oxidation in the presence of moisture.24,

28-30

The metal cations (Ag+, Pt2+)

capturing and interacting with the photogenerated electrons of MoS2 were reduced to metal atoms (Ag, Pt) to form metal NPs, which were in situ deposited on MoS2 nanosheets, forming metal (Ag, Pt)–MoS2 nanohybrids. The more detailed mechanism of charge transfer from MoS2 to metal cations in the photochemical reduction process are shown in Supporting Information and Figure S5,S6. In addition, the reduction of metal cations by capturing the electrons produced through laser-induced water ionization could be almost ignored, which is analyzed in supporting information (Figure S7,S8).

Figure 1 Schematic of in situ decorating MoS2 with noble metal (Ag or Pt) NPs. M represents metal.

Laser-induced Ag−MoS2 hybrids. After femtosecond laser scanning the mixed solution of MoS2 nanosheets and metal salts, different metal NPs (Ag or Pt NPs) were in situ synthesized on MoS2 nanosheets, by using different metal precursors, namely AgNO3 or K2PtCl4. As shown in the TEM images in Figure 2a,c, Ag NPs were uniformly decorated on MoS2 nanosheets (more


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TEM images are presented in Figure S9), and the average size of the Ag NPs on MoS2 was approximately 2.76 nm (the diameter distribution of which is shown in Figure S10). Figure 2b shows the selected-area electron diffraction (SAED) pattern of Ag–MoS2 hybrids, which was conducted by keeping basalplane of MoS2 sheets perpendicular to electron beam and identified three diffraction rings, assigned to the {100}MoS2, {111}Ag, and {220}Ag with lattice spacing of 0.27, 0.24, and 0.14 nm, respectively. The {111}Ag and {220}Ag suggested an face centred cubic (FCC) lattice of the Ag NPs, in which the stronger diffraction ring of {111}Ag suggested the primary orientation. Figure 2d shows the HRTEM image of Ag NPs on MoS2, revealing the cognizable lattice fringes, lattice spacing of 0.24 nm in accordance with the value of Ag (111), and lattice spacing of 0.27 nm assigned to lattice orientation of {100}MoS2. The TEM analysis clearly demonstrated the simultaneous presence of Ag and MoS2 (which was also proved by element composition mapping of Mo, S, and Ag elements shown in Figure S11), and in-situ growth of highly crystalline Ag NPs on MoS2 surface, also indicated that MoS2 {100} was beneficial to the growth of Ag NPs along its {111} crystal plane, which can be due to the lowest surface energy of {111} plane of FCC structures and hence a relatively small lattice mismatch with MoS2 {100}.26, 31-33 The XPS spectra of Ag 3d indicated its chemical composition (Figure 2e). Two peaks at 373.4 and 367.4 eV were attributed to Ag 3d3/2 and Ag 3d5/2 orbitals, respectively. The peak at 373.4 eV was assigned to Ag0 NPs, and the peak at 367.4 eV was assigned to Ag+, which might have resulted from the residual silver salt and the easy oxidation of Ag NPs in the air.34-35 To reveal the interaction between Ag and MoS2 and the modification of MoS2, Raman and XPS characterization of MoS2 decorated with Ag NPs were performed. Two active modes, namely E2g1 and A1g, were shown in Raman spectra of MoS2 (Figure 2f), where the in-plane E2g1


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mode arises from the opposite vibration of two S-atoms with reference to the Mo-atom between them, whereas the out-plane A1g mode arises from the opposite vibration of only two S-atoms.5,36 Compared with Raman modes of original MoS2, the E2g1 mode of Ag–MoS2 hybrids obviously downshifted as a result of the softening of E2g1 vibrations, suggesting that the interaction between the Ag NPs and MoS2 obviously softened the lateral vibration between S and Mo atoms, which demonstrated the n-doping effect of Ag NPs on MoS2.12, 19, 23, 37 And the charge transfer mechanism of n-doping effect of Ag NPs on MoS2 is shown in Figure S12. Figure 2g,h show the XPS Mo and S spectra of the obtained Ag–MoS2 nanohybrids, and the XPS Mo and S spectra of pristine MoS2 nanosheets are shown in Figure S13. Figure S13a and 2g show the detailed peak assignments of Mo 3d for pristine MoS2 and Ag–MoS2 samples, which show six peaks, attributed to S 2s orbital of divalent sulfur, Mo4+3d5/2 and Mo4+3d3/2 orbital of tetravalent molybdenum, and Mo6+3d3/2 orbital of hexavalent molybdenum, respectively. Among them, peaks around 228.2eV and 231.5eV are attributed to Mo4+3d5/2 and Mo4+3d3/2 components in 1T phase of MoS2, and peaks around 229.2eV and 232.6eV are attributed to Mo4+3d5/2 and Mo4+3d3/2 components in 2T phase of MoS2.38-40 Figure S13a indicates the simultaneous presence of 2H and 1T phases in the pristine MoS2 nanosheets, and the phase of 1T was slightly more than that of 2H. In addition, there was a Mo6+ peak in Figure S13a, indicating the oxidation of purchased pristine MoS2 nanosheets. Figure 2g indicates that the 2H phase of MoS2 was inhibited and 1T phase was enhanced in the obtained Ag–MoS2 hybrids, indicating obvious phase change of MoS2 from 2H to 1T. Figure 2g also indicates the further oxidation of MoS2 in the process of preparing Ag−MoS2 hybrids. Figure S13b and 2h show the detailed assignments of S 2p for pristine MoS2 and Ag−MoS2 samples, which show two peaks, attributed to S 2p3/2 and S 2p1/2 orbital of divalent sulfur, respectively. Figure S13b indicates that the amount of S


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atoms in S 2p1/2 orbital was larger than that in S 2p3/2 orbital in the pristine MoS2 nanosheets. However, Figure 2h indicates that the amount of S atoms in S 2p1/2 orbital was smaller than that in S 2p3/2 orbital, indicating the decrease of binding energy (valence state) of S atoms in Ag−MoS2 hybrids when comparing with the pristine MoS2, which might be attributed to the phase change of MoS2 from 2H to 1T as shown in the Mo 3d spectra (Figure 2h). The above XPS changes of MoS2 in Ag−MoS2 hybrids, phase change from 2H to 1T, might attributed to the comprehensive effect from these aspects: laser irradiating MoS2 can result in phase change of MoS2 from 1T to 2H;38,

41

doping effect of Ag on MoS2 can improve the free carrier

concentration of MoS2, resulting in phase change of MoS2 from 2H to 1T;38-39, 42-44 AgO derived from the oxidation of Ag NPs in the air and residual metal salts might also doping or induce phase change of MoS2. And the comprehensive effect of phase change from 2H to 1T was stronger.


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Figure 2 TEM, Raman, and XPS analysis of Ag−MoS2 hybrids. a) Low-resolution TEM image of Ag NPs synthesized on MoS2 nanosheets (scale bar, 100 nm). b) SAED pattern of the Ag–MoS2 hybrids (scale bar, 10 1/nm). c) TEM image (scale bar, 10 nm) and d) HRTEM image (scale bar, 5 nm) of Ag NPs on MoS2. e) XPS Ag 3d spectra of synthesized Ag−MoS2 hybrids indicating the chemical composition of Ag. f) Raman spectra of pristine MoS2 nanosheets and Ag−MoS2 hybrids. XPS (g) Mo 3d and (h) S 2p spectra of synthesized Ag−MoS2 hybrids.


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Laser-induced Pt−MoS2 hybrids. As shown in TEM images in Figure 3a,c, Pt NPs were uniformly decorated on the surface of MoS2 nanosheets (more TEM images are presented in Figure S14), and the average size of the Pt NPs was approximately 2.33 nm (the diameter distribution of which is shown in Figure S15). The SAED pattern of Pt–MoS2 hybrids is shown in Figure 3b, which was conducted by keeping basalplane of MoS2 sheets perpendicular to electron beam and identified four diffraction rings, assigned to the {100}MoS2, {111}Pt, {220}Pt, and {116}MoS2 with lattice spacing of 0.27, 0.23, 0.14, and 0.13 nm, respectively. The strong and sharp diffraction ring of {100}MoS2 suggested the primary orientation of MoS2. The {111}Pt, and {220}Pt suggested an FCC lattice of the Pt NPs, in which the strongest diffraction ring of {111}Pt suggested the primary orientation. Figure 3d shows the HRTEM picture of Pt NPs on MoS2, revealing the cognizable lattice fringes, lattice spacing of 0.23 nm in accordance with the value of Pt (111), and lattice spacing of 0.27 nm assigned to lattice orientation of {100}MoS2. The TEM analysis clearly demonstrated the simultaneous presence of Pt and MoS2 (which was also proved by element composition mapping of Mo, S, and Pt elements shown in Figure S16), and in-situ growth of highly crystalline Pt NPs on MoS2 surface, also indicated that MoS2 {100} was beneficial to the growth Pt NPs along its {111} crystal plane, which can be due to the lowest surface energy of the {111} plane of FCC structures and hence a relatively small lattice mismatch with MoS2 {100}.26, 31-33 The XPS spectra of Pt 4f indicated its chemical composition (Figure 3e). The double peaks at lower binding energies of 72.0 and 75.4 eV were assigned to Pt0 NPs that were bonding to MoS2 molecules.45 The double peaks at higher binding energies of 73.4 and 77.1 eV were assigned to Ptδ+, which might result from the residual platinum salt.46-48 To reveal the interaction between Pt and MoS2 and the modification of MoS2, Raman and XPS characterization of MoS2 decorated with Pt NPs were performed. Compared with Raman


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modes of original MoS2, the E2g1 mode of Ag–MoS2 hybrids obviously upshifted as a result of the stiffening of E2g1 vibrations, as shown in Figure 3f, suggesting that the interaction between the Pt NPs and MoS2 obviously stiffened the lateral vibration between S and Mo atoms, which indicated the p-doping effect of Ag NPs on MoS2.12, 19, 23, 37 And the charge transfer mechanism of p-doping effect of Pt NPs on MoS2 is shown in Figure S17. Figure 3g,h show the XPS Mo and S spectra of the obtained Pt–MoS2 nanohybrids, and the XPS Mo and S spectra of pristine MoS2 nanosheets are shown in Figure S13. Figure S13a and 3g show the detailed peak assignments of Mo 3d for pristine MoS2 and Pt−MoS2 samples, which show six peaks, attributed to S 2s orbital of divalent sulfur, Mo4+3d5/2 and Mo4+3d3/2 orbital of tetravalent molybdenum, and Mo6+3d3/2 orbital of hexavalent molybdenum, respectively. Among them, peaks around 228.2eV and 231.5eV are attributed to Mo4+3d5/2 and Mo4+3d3/2 components in 1T phase of MoS2, and peaks around 229.2eV and 232.6eV are attributed to Mo4+3d5/2 and Mo4+3d3/2 components in 2T phase of MoS2.38-40 Figure S13a indicates the simultaneous presence of 2H and 1T phases in the pristine MoS2 nanosheets, and the phase of 1T is more than that of 2H. In addition, there is a Mo6+ peak in Figure S13a, indicating the oxidation of purchased pristine MoS2 nanosheets. Figure 3g indicates that the 2H phase of MoS2 was enhanced and 1T phase was slightly inhibited in the obtained Pt−MoS2 hybrids, indicating slight phasechange of MoS2 from 1T to 2H. Figure 3g also indicates the future oxidation of MoS2 in the process of preparing Pt−MoS2 hybrids. Figure S13b shows the detailed assignments of S 2p for pristine MoS2, which shows two peaks, respectively assigned to S 2p3/2 and S 2p1/2 orbital of divalent sulfur. Whereas, the detailed assignments of S 2p for obtained Pt−MoS2 hybrids in Figure 3h shows four peaks: not only the two peaks assigned to S 2p3/2 and S 2p1/2 orbital of divalent sulfur, but also the peak at 164.6 eV and 169.6 eV, which are attributed to unbound S and oxidized S4+. The appearance of unbound/


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unsaturated S indicated the breaking of Mo−S bonds of MoS2 in Pt metal salt solution by femtosecond laser. And the appearance of S4+ indicated the future oxidation of unbound S. The above XPS changes of MoS2 in Pt−MoS2 hybrids, phase change from 1T to 2H, might attributed to the comprehensive effect from these aspects: laser irradiating MoS2 can result in phasechange of MoS2 from 1T to 2H;38,

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doping effect of Pt on MoS2 can improve the free carrier

concentration of MoS2, resulting in phase change of MoS2 from 2H to 1T;38-39, 42-44 residual metal salts might also doping or induce phase change of MoS2; femtosecond laser inducing the obvious breaking of Mo-S bonds, indicating strong processing effect of femtosecond laser on MoS2 itself, and this should be due to the different physicochemical properties of different metal salt solution and the deeper color of Pt salt solution (brown) than that of Ag salt solution (transparent) contributing to more laser kept in solution and less laser throughout. And the comprehensive effect of phase change from 1T to 2H was stronger. According to above analysis, it can also show that femtosecond laser can not only induce photogenerated electrons of MoS2 for the reduction of metal cations, but also directly induce the ablation (such as bonds breaking) of MoS2, which is one of the common features of femtosecond laser and can be affected by the different surrounding environment of processed materials.27, 49-51


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Figure 3 TEM, Raman, and XPS analysis of Pt–MoS2 hybrids. a) Low-resolution TEM image of Pt NPs synthesized on MoS2 nanosheets (scale bar, 100 nm). b) SAED pattern of the Pt–MoS2 hybrids (scale bar, 10 1/nm). c) TEM image (scale bar, 10 nm) and d) HRTEM image (scale bar, 5 nm) of Ag NPs on MoS2. e) XPS Pt 4f spectra of synthesized Pt−MoS2 hybrids indicating the chemical composition of Pt. f) Raman spectra of pristine MoS2 nanosheets and Pt−MoS2 hybrids. XPS (g) Mo 3d and (h) S 2p spectra of synthesized Pt−MoS2 hybrids.


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Enhanced Raman effect of Ag–MoS2 hybrids for SERS. SERS is a very useful microanalytical tool for the detection of organic/biological molecules.51-54 The dominant materials as SERS substrate are mainly noble metals for their high enhancement factors (EFs) attributed to the electromagnetic mechanism (EM) effect, which is the local electromagnetic field enhancement around metallic structures.52 However, recent researches suggest some novel flat substrates for SERS, namely 2D materials, and their EFs are mainly attributed to chemical mechanism (CM) effect (for instance, charge transfer between detected molecules and substrate materials) rather than EM effect.5 These 2D material substrates are suitable to combine with metallic micro/nano structures to utilize the EM and CM effect, simultaneously.17, 52-53, 55 Hence, the study of noble metal–MoS2 hybrids as SERS substrates is recently valued for few years,24, 26, 53

indicating their tremendous potential as outstanding SERS substrates. Here, the synthesized Ag–MoS2 hybrids were applied as a SERS substrate, and the SERS

activity were explored with R6G serving as probed molecules (Figure 4a). The detailed SERS sample preparation is presented in Experimental section. Through S-π coordinating interaction, R6G molecules can be adsorbed onto the surface of MoS2 hybrids.5 Three Raman spectra are tested, shown in Figure 4b, and the Raman peak at 612 ± 2 cm−1 (Table S1 shows the assignments of main R6G bands) was selected to calculate the EF for quantitatively analyzing the effect of Raman enhancement. The EF calculation was referenced from previous reports26, 53 and is as expressed in Equation 1: EF =

I sers N ref × I ref N sers

(1)

where Raman signal of R6G solution dropped onto clean glass slide was as the reference. The calculated EF of the Ag–MoS2 hybrids was 1.32 × 107 (related data for calculating EF is shown in Table S2), showing a sensitive SERS activity. Moreover, the Raman spectrum of R6G (as


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reference) and R6G on MoS2 nanosheets were compared to understand the CM effect of MoS2, and the EF of R6G molecules on the MoS2 nanosheets was calculated approximating to 18.8 (related data for calculating EF is shown in Table S2), demonstrating the effect of Raman enhancement on MoS2 surface. Due to S-π coordinating interaction between R6G and MoS2, the charge transfer process can proceed between them, which can induce CM enhancement.5 The charge transfer in the chemical mechanism of SERS is the interaction between MoS2 and detected organic molecules under the excitation of Raman laser; and the conduction band of MoS2 is lower compared with the excited state of R6G, which can contribute to accepting the photoinduced electrons of R6G under light excitation.53,

56

The charge transfer process was

depicted in Figure 4f. In addition, the sulfur atoms of MoS2 are on the surface, and there are polar-covalent Mo−S bonds whose polarity is vertical to MoS2 surface, therefore interface dipole–dipole coupling might simultaneously occur with charge transfer, and it also could induce an effective Raman enhancement.5, 52 In order to future investigate the SERS performance of the Ag−MoS2 hybrids and make our work more systematic, we conducted the SERS detection for different R6G concentrations and the Raman signal mapping of R6G on SERS sample to investigate the detection limit and uniformity of the SERS substrates. Figure 4c shows the variation of R6G Raman intensity at about 612±2 cm-1 with the variation of its concentrations. It shows that with the decrease of the concentrations of detected R6G molecules, its Raman intensity decreased slowly, then decreased fast, at last decreased slowly; this change trend was consistent with other published work17. And the detection limit of our SERS substrate can be low to 10-11 M, indicating the ability of ultra-low concentration detection. To examine the uniformity of our SERS substrate, Raman mapping image of 60 × 60 µm2 area by using peak intensity at about 612±2 cm−1 was obtained, as shown


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in Figure 4d, and the calculated relative standard deviation (RSD) of Raman intensity at 612±2 cm−1 was about 15% over 121 grid cells. These results indicated the homogeneous signals on our Ag−MoS2 SERS substrate. Moreover, other analytes were investigated, including crystal violet (CV) and 4-mercaptobenzoic acid (4-MBA). As illustrated in Figure 4e, the Raman signals of the two types of analyte solution dropped onto clean glass slide (as reference) were weak in intensity and almost invisible, instead, their Raman signal on Ag–MoS2 hybrid substrates were obviously enhanced, indicating that the Ag–MoS2 hybrid substrates exhibited good SERS adaptability.

Figure 4 SERS performance of Ag–MoS2 hybrids as SERS substrates. a) Schematic illustration of the Ag– MoS2 hybrids and their use for SERS detection. b) Raman spectra of R6G solution, R6G on MoS2 substrate, and R6G on Ag–MoS2 hybrid substrate. c) Variation of Raman intensity with the variation of R6G concentrations. d) Raman mapping image of 60 × 60 µm2 area by using peak intensity at about 612±2 cm−1 on the Ag−MoS2 SERS substrate. e) Raman spectra of e1) CV solution and CV on Ag–MoS2 hybrid substrate, and e2) 4-MBA solution and 4-MBA on Ag–MoS2 hybrid substrate. f) Electron transfer between R6G and MoS2 (relative position of energy levels were taken from literature56).


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Electrocatalytic activity of Pt–MoS2 hybrids for HER. Hydrogen represents a kind of clean and renewable energy sources, and electrocatalytic HER by splitting water is considered to be a critical pathway to sustainable and efficient hydrogen production.7, 12, 57-58 Recently, the most effective HER electrocatalysts are noble metals, especially Pt-group metals, but the high cost and low abundance largely prevent their practical application.57-62 Hence, it is significant to develop alternative or half-alternative electrocatalysts with low cost and high abundance for effective HER. Recent researches have highlighted that 2D MoS2-based materials can be considered as promising HER electrocatalysts due to their special atomic structures, low cost, and high earth abundance.7,

57-58, 60

However, pristine MoS2 as a semiconductor has poor

electron-transport property and needs large overpotential to achieve high HER turnover frequencies, although layered MoS2 nanosheets have highly exposed active edges.12,

63-64

To

promote the electron transport rate and further improve the HER activity, MoS2 can therefore be hybridized with electrically-conductive metal NPs (for instance, Pt), which can improve charge transport along inter-planar orientations and serve as spacers to suppress restacking.22, 64 Here, the obtained Pt–MoS2 hybrids were applied as electrocatalysts to explore the electrocatalytic activity for HER (Figure 5a). To define the electrocatalytic activity of the obtained Pt–MoS2 hybrids, the electrocatalytic activity of commercial hydrogenation Pt catalyst (20 wt% Pt on Vulcan carbon black, referred to as Pt–C) were used for comparison. In order to decrease the Pt loading and improve the HER performance of the prepared Pt−MoS2 hybrids, we decreased the use of Pt and added moderate carbon black in the obtained Pt−MoS2 hybrid solution (7.6% Pt−MoS2 hybrids with C). The mass of all components in the prepared Pt−MoS2 hybrids with C are shown in Table S3, and the calculation for loading percentage (wt%) of Pt in Pt–MoS2 with C are shown in Supporting Information.


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The polarization curves of current density (J) against potential of pristine MoS2 nanosheets, the Pt–MoS2 hybrids with C, and commercial Pt–C catalyst are shown in Figure 5b, indicating different eletrocatalytic activities toward HER. The Pt–MoS2 hybrids with C shows a near-zero onset potential vs reversible hydrogen electrode (RHE), approximate with that of commercial Pt– C catalyst, obviously lower than that of pristine MoS2 nanosheets (-0.103 V). The cathodic current density of pristine MoS2 nanosheets was weak, hardly increasing with the negative potential, and no significant H2 evolution (J = 10 mA/cm2) was observed. However, a significant H2 evolution (J = 10 mA/cm2) was clearly observed at -0.041 V for the Pt–MoS2 hybrids with C, suggesting an obvious increase in the catalytic activity of Pt−MoS2 hybrids with C, although this potential value was slightly higher than that of commercial Pt–C catalyst (-0.030 V). Overall, the cathodic current density of 7.6% Pt−MoS2 hybrids with C in our work was lower than that of commercial 20% Pt–C catalyst, which can be expected. However, to obtain further insight into the HER performance of prepared Pt–MoS2 hybrids with C, Tafel plots of various catalysts were calculated (Figure 5c), the slope of which is another key parameter to evaluate HER catalytic activity. The resulting Tafel slope of the Pt–MoS2 hybrids with C was approximately 25 mV/decade, which was lower than that of commercial Pt–C catalyst (32 mV/decade) and obviously lower than that of pristine MoS2 nanosheets (63 mV/decade), indicating a considerable enhancement in the catalytic activity of MoS2 hybridized with Pt NPs and C, even better than that of commercial Pt–C catalyst. The earlier onset of significant H2 evolution and smaller Tafel slope suggest that the hydrogen adsorption was closer to equilibrium and confirm the excellent metallic conductivity of Pt–MoS2.65-66 In addition, we calculated the exchange current density, which is the intersection of the extrapolated linear part of Tafel plots and the X-axis.67 And the


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calculated exchange current density of Pt–MoS2 hybrids with C was 0.72 mA/cm2, lower than that of the commercial Pt–C catalyst (3.89 mA/cm2). In order to future investigate the HER performance of the Pt–MoS2 hybrids with C and make our work more systematic, the per-site turnover frequency (TOF) was calculated, and its impedance and current stability were tested. Figure 5d shows the TOF per Pt and Mo atom curves for Pt–MoS2 hybrids with C and commercial Pt–C catalyst (calculation process and other relative calculation are shown in Supporting Information). It shows that the TOF per Pt of Pt– MoS2 hybrids with C was higher than that of commercial Pt–C catalyst at low overpotential, but lower than that of the commercial Pt–C catalyst at high overpotential. And the calculated TOF per exposed/active Mo of the obtained Pt–MoS2 hybrids with C reached up to 11.15 H2 s-1 at 0.22 V. Figure 5e shows the Nyquist plots and relevant equivalent circuit (Randles circuit) of three samples. According to Randles circuit and based on diameter of semicircular Nyquist plots, charge transfer resistance (Rct) of pristine MoS2, 7.6% Pt–MoS2 hybrids with C, and 20% Pt–C catalyst were respectively 5651, 6.25, and 1.35 Ω, indicating the obviously lower impedance of Pt–MoS2 hybrids with C than pristine MoS2, although slightly higher than that of the commercial Pt–C catalyst. The durability of Pt–MoS2 hybrids with C by applying scanning at a constant overpotential is shown in Figure 5f. After 5×103 seconds, the degradation of current density was about 9.6%, indicating good stability. In addition, cyclic voltammetry and electrochemical surface area (ECSA) of two Pt based catalysts were tested (Figure S20 and S21) and calculated (calculation process are shown in Supporting Information). And the calculated ECSA of 7.6% Pt–MoS2 hybrids with C and 20% Pt–C catalyst were respectively 126.526 and 59.032 m2/g, indicating higher activity and use-efficiency of Pt metal in 7.6% Pt–MoS2 hybrids with C sample than in commercial 20% Pt–C catalyst in HER application. According to the aforementioned


21


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analysis of Raman and XPS results, the excellent HER activity of low-loading (7.6%) Pt–MoS2 with C can be attributed to not only the high use-efficiency of Pt NPs itself but also the electron doping effect on MoS2 and the effect of added moderate C.

Figure 5 Electrocatalytic performance of obtained Pt–MoS2 hybrids. a) Schematic illustration of the Pt–MoS2 hybrids and the catalytic mechanism of H2 generation. b) Polarization curves and c) the corresponding Tafel plots of pristine MoS2 nanosheets, 7.6% Pt–MoS2 hybrids with C, and 20% Pt–C catalyst. d) TOF per Pt and Mo for Pt–MoS2 hybrids with C and Pt–C catalyst. e) Nyquist plots and relevant equivalent circuit of pristine MoS2, Pt–MoS2 hybrids with C, and Pt–C catalyst. f) Plot of current density (j) versus time for Pt–MoS2 hybrids with C at a constant overpotential.

CONCLUSIONS A novel method was developed to induce photogenerated electrons (negative charge) of MoS2 via femtosecond laser pulses, through which metal (Ag, Pt) NPs were reduced without any chemical reductant, and in situ decorated on MoS2 nanosheets, thus forming metal–MoS2 nanohybrids. TEM, Raman, and XPS characterization indicated the formation of highly


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crystalline metal (Ag, Pt) NPs on MoS2 nanosheets, the doping effect of metal NPs on MoS2, and the modification of MoS2. In addition, the Ag–MoS2 hybrids exhibited sensitive SERS activity with EF reaching 1.32 × 107 and detection limit low to 10-11 M, demonstrating huge potential for chemical/biological molecule sensing. Furthermore, the Pt–MoS2 hybrids exhibited prominent HER activity with low Tafel slope to 25 mV/decade and high TOF per exposed Mo of 11.15 H2 s-1 at 220 mV, demonstrating great potential for future hydrogen production applications.

ASSOCIATED CONTENT Supporting Information. The characterizations of pristine MoS2 nanosheets; extended analysis; extended TEM figures; diameter distributions of Ag and Pt NPs; extended XPS spectrum; Raman frequencies and assignments of R6G; table about related data for calculating EF; table and calculation for loading amounts (wt%) of Pt in Pt–MoS2 hybrids with C; calculation for TOF. These material are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] (Xin Li). Phone: 86-10-68914524 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The research was supported by the National Key R&D Program of China (Grant No. 2017YFB1104300), and National Natural Science Foundation of China (NSFC) (Grant No. 51775047).

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Table of Contents Graphic The preparation of metal−MoS2 nanohybrids don't need any chemical reductants except for metal precursor, avoiding the introduction of reagent byproducts, toxicity, and chemical or environmental contamination.


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Metal (Ag, Pt)âMoS2 Hybrids Greenly Prepared ... - ACS Publications

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