Abstract
19 min readOpen AccessCCS ChemistryRESEARCH ARTICLE7 Dec 2022Strain-Assisted Single Pt Sites on High-Curvature MoS2 Surface for Ultrasensitive H2S Sensing Zhenggang Xue, Chun Wang, Yujing Tong, Muyu Yan, Jiangwei Zhang, Xiao Han, Xun Hong, Yafei Li and Yuen Wu Zhenggang Xue NEST Laboratory, Department of Physics, Department of Chemistry, College of Science, Shanghai University, Shanghai 200444 Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), School of Chemistry and Materials Science, National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026 , Chun Wang Jiangsu Collaborative Innovation Centre of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023 , Yujing Tong Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), School of Chemistry and Materials Science, National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026 , Muyu Yan Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), School of Chemistry and Materials Science, National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026 , Jiangwei Zhang Dalian National Laboratory for Clean Energy, State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Xiao Han Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), School of Chemistry and Materials Science, National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026 , Xun Hong Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), School of Chemistry and Materials Science, National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026 , Yafei Li Jiangsu Collaborative Innovation Centre of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023 and Yuen Wu *Corresponding author: E-mail Address: [email protected] Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), School of Chemistry and Materials Science, National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026 Dalian National Laboratory for Clean Energy, Dalian 116023 https://doi.org/10.31635/ccschem.022.202101628 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Engineering the local three-dimensional structure of metal sites has an important effect of maximizing the activity and selectivity of single-atom site catalysts. Here, we engineered a strain-assisted single Pt sites structure on a highly curved MoS2 surface to enhance its H2S sensor property. By introducing N-methyl-2-pyrrolidone (NMP) as guiding molecules, a multilayer MoS2 structure with bending base planes was achieved. This bending behavior could inject not only uniform in-plane strain into the original inert MoS2 basal plane but also introduce sufficient accessible sites to anchor Pt monomers. Further experimental and theoretical results showed that the high-curvature MoS2 surface endowed 0.8% stretch strain onto the low-coordinated single Pt sites with a unique "tip" effect, which led to more accumulation of electrons around the Pt species, thereby accelerating the electric transfer process between H2S and supports. The final catalyst delivered pronouncedly enhanced H2S sensing response and response speed at room temperature. Our proposed strain-assisted strategy might create a new path to design highly active single-atom site catalysts for gas sensors. Download figure Download PowerPoint Introduction One frontier in the synthesis of single-atom site catalysts includes locating highly distributed metal atoms onto the desired positions to form a site-special geometric framework so as to improve the intrinsic activity of metal sites.1–6 For example, in the case of transition metal dichalcogenides (TMDs) materials comprised active edges and inert basal plane, selectively modifying the single metal atoms at the edge or in-plane sites is crucial for their catalytic performance due to differences in coordinated environment and steric configuration of metal sites. Many attempts have proven that metal atoms around the coordinatively unsaturated edge regions possess unique geometric and electronic configurations, and thus, exhibit superior activity during the catalytic process.7,8 However, these edge regions usually only occupy a little part over the whole TMDs supports, which severely restrict the concentrations and density of metal sites. One effective strategy used to solve this problem is to sufficiently activate the inert basal plane of TMDs materials as active regions to anchor the metal species.9,10 Some physical or chemical methods have been reported to fully utilize inert basal atoms, such as constructing vacancies,11,12 manipulating strain,13,14 creating distortion,15 inducting doped atoms,16–19 and so on. Among these, injecting lattice strain into the TMDs planes could create the active surface and induce the strain-assisted single metal sites, which might be beneficial to improve the catalytic activity. However, the strain-related single-atom site catalysts supported on active TMDs surfaces have rarely been studied. Here, we present a strain-assisted single Pt site on a highly curved MoS2 surface to enhance the H2S sensor property. In our design, the N-methyl-2-pyrrolidone (NMP) was employed as guiding solvent molecules to construct multilayer fullerene-like MoS2 configurations. This closed structure introduced uniform in-plane strain and effectively prevented strain recovery of MoS2 layers. The X-ray absorption fine structure (XAFS) characteristics and density functional theory (DFT) calculation further revealed the high-curvature MoS2 surface endowed 0.8% stretch strain onto the low-coordinated single Pt sites. The final Pt decorated bending MoS2 (denoted as Pt-B-MoS2) enhanced the sensing response of H2S gas markedly and reduced the response/recovery time at room temperature. Further calculations and simulations indicated that the special Pt sites could enhance the H2S adsorption and showed a unique tip effect to induce more accumulation of electrons, which further accelerated an electric transfer between H2S and supports. Experimental Methods Materials Ammonium tetrathiomolybdate [(NH4)2MoS4], NMP, ethanol, hydrazine hydrate (N2H4·H2O) were purchased from Shanghai Chemical Reagents (Shanghai, China). Chloroplatinic acid (H2PtCl6·6H2O) was obtained from Sigma-Aldrich (Shanghai, China). Deionized water was used throughout this study. All chemicals were used as received without further purification. Preparation of S-MoS2 and B-MoS2 The B-MoS2 was prepared by a facile solvothermal method: Typically, 0.5 mmol of ammonium tetrathiomolybdate was mixed with 80 mL of NMP solution and stirred for 0.5 h. Then 2 mL N2H4·H2O was added to the solution and heated to 80 °C in an oil bath under gentle reflux. After stirring for 12 h, the black-brown products were harvested and washed three times with water, then dried at 60 °C overnight. The as-B-MoS2 precursors obtained were heated to 800 °C at a rate of 5 °C/min and kept for 2 h under a flowing Ar2 atmosphere. The S-MoS2 was prepared by the same process except that H2O replaced the NMP solution as a corresponding solvent. Preparation of Pt-B-MoS2 and Pt-S-MoS2 In a typical procedure, 150 mg B-MoS2 (or S-MoS2) powder was dispersed in 20 mL water and ultrasonic mixed for 10 min. After stirring for 1 h, 0.375 mL of H2PtCl6 aqueous solution (Pt: 4 mg/mL) was added to the solution mixture, followed by stirring for 4 h. Then the as-formed precipitates were washed three times with water and dried at 60 °C under a vacuum. Characterization Powder X-ray diffraction (XRD) measurements were recorded on a Rigaku Miniflex-600 (Tokyo, Japan) operated at 40 kV voltage and 15 mA current using a Cu Kα radiation (λ = 0.15406 nm) at a step width of 8°/min. Scanning electron microscopy (SEM) was performed on JSM-6700F (JEOL, Tokyo, Japan). Transmission electron microscope (TEM) images were recorded on a Hitachi-7700 (Tokyo, Japan) set at 100 kV. The high-resolution TEM and high-angle annular dark-field scanning TEM (HAADF-STEM) images were recorded on an FEI Tecnai G2 F20 S-Twin high-resolution transmission electron microscope (HRTEM; Hillsboro, OR, United States) set at 200 kV, and a JEOL JEM-ARM200F TEM/STEM (Tokyo, Japan), respectively, with a spherical aberration corrector worked at 300 kV. Through-focal HAADF series were acquired at nanometer intervals, with the first image under-focused (beyond the beam exit surface) and the final image over-focused (before the beam entrance surface). Then the images were aligned manually to remove the sample drift effects. X-ray photoelectron spectroscopy (XPS) experiments were performed at the Catalysis and Surface Science End station at the BL11U beamline of National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. Soft X-ray absorption spectroscopy (Soft-XAS, S L-edge) was carried out at BL12B X-ray Magnetic Circular Dichroism (XMCD) station and BL10B photoemission end-station of National Synchrotron Radiation Laboratory (NSRL, Hefei in China) in total electron yield (TEY) mode. The samples were coated on double-sided carbon tape for characterization. UV–vis absorption spectra were collected by an Agilent Cary 60 spectrophotometer (Santa Clara, CA, United States). Room-temperature electron paramagnetic resonance (EPR) spectra were obtained using a JEOL JES-FA200 spectrometer (300 K, 9.062 GHz; Tokyo, Japan). XAFS measurement and data analysis: The X-ray absorption fine structure data were collected at 1W1B station in Beijing Synchrotron Radiation Facility (BSRF). The storage rings of BSRF was operated at 2.5 GeV with a maximum current of 250 mA. The X-ray absorption near edge structure (XANES) data were recorded in fluorescence mode. All samples were pelletized as disks of 13 mm diameter using poly(1,1-difluoroethylene) powder as a binder. The acquired extended X-ray absorption fine structure (EXAFS) data were processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages.20 The EXAFS spectra were obtained by subtracting the post-edge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, χ(k) data in the k-space were Fourier transformed to real (R) space using a Hanning window (dk = 1.0 Å−1) to separate the EXAFS contributions from different coordination shells. Fabrication and response test of the gas sensor Gas sensing tests were performed on a commercial CGS-8 Gas Sensing Measurement System (Beijing Elite Tech Company Limited), and the gas sensing performances of the sensors were measured using a data acquisition system and subsequent transformation to a response. First, the products were mixed with ethanol to form a paste and coated on the sensor substrates. After drying in the air, the substrates were welded into the circuit board. All tests were implemented at room temperature (25 °C). The H2S sensitivity of the sensor was defined as S = (Rg − Ra)/Ra = ΔR/Ra, where Ra was the baseline resistance of the sensors when exposed to air, and Rg was the resistance of the sensors when exposed to gas analytes. The sensitivity of the oxidizing gas NO2 sensor was defined as S = (Ra − Rg)/Ra = ΔR/Ra. Results and Discussion The strategies used to activate the basal plane and introduce active Pt sites in MoS2 are illustrated in Figure 1a. The straight MoS2 (denoted as S-MoS2) nanosheet with normal crystal arrangement was synthesized by facilely reducing the (NH4)2MoS4 precursors through N2H4 reagent in the water solution at 80 °C (see Experimental Procedures). The TEM and HRTEM images ( Supporting Information Figure S1) showed that the S-MoS2 nanosheets possessed layer-assembled morphology, and no obvious bending structure was observed. The lattice spacing of S-MoS2 was 0.66 nm, consistent with the (002) surfaces of 2H-MoS2.9 Interestingly, when the water solution was replaced by NMP reagent, the MoS2 layer was inclined to curve and close, establishing a hollow fullerene-like nanosphere with an average shell thickness of ∼7 nm ( Supporting Information Figures S2 and S3). The HAADF-STEM image in Figure 1c indicated that the bending MoS2 (denoted as B-MoS2) was highly curved at an atomic scale, and the interconnected fullerene spheres supported the high-quality MoS2 surface. XRD patterns in Supporting Information Figure S4 suggested that both S-MoS2 and B-MoS2 were typical structures of the trigonal prismatic (2H) phase (PDF no. 37-1492).21 Based on the HAADF-STEM image in Figure 1c, the detailed lattice strain components of εxx and εyy were displayed in Supporting Information Figure S5. The average strain distribution of εxx and εyy ranged from ∼−10%to 10% and 0% to 30%, respectively, indicating that the visual curvature induced the generation of noticeable strain between the atoms. More representative zones were investigated, as shown in Supporting Information Figures S6 and S7. The hollow B-MoS2 sphere exhibited uniform layer numbers of ∼11, consistent with the TEM images in Supporting Information Figure S3. The HAADF-STEM images and the corresponding simulation configurations of straight and bending MoS2 configurations in Figure 1b clearly showed the differences in atomic array and direction. Moreover, compared with S-MoS2, slight redshifts of the Raman E 2 g 1 and A1g peaks were observed in B-MoS2 (Figure 1d), suggesting that a tensile strain was introduced into the B-MoS2 sample, in accordance with previous reports.10 Furthermore, the UV–vis spectrum ( Supporting Information Figure S8a) of B-MoS2 showed an apparent redshift of A and B excitons with respect to the S-MoS2, ascribed mainly to the structural stretch.22 The Mo 3d XPS analysis in Supporting Information Figure S8b showed that the doublet peaks of Mo in B-MoS2 shifted to lower binding energy compared with Mo in S-MoS2, suggesting a reduced oxidation state of the Mo species in B-MoS2. The Mo XANES characterization is displayed in Supporting Information Figure S9a: The pre-edge centroid of B-MoS2 shifted into lower energy compared with S-MoS2, suggesting a decreased oxidation of Mo in B-MoS2. This result agreed well with the XPS analysis. Moreover, the EXAFS spectrum of B-MoS2 ( Supporting Information Figure S9b) and S-MoS2 ( Supporting Information Figure S9c) and quantitative analysis of EXAFS fitting ( Supporting Information Table S1) showed that the average Mo-S bond length in B-MoS2 was 2.43 Å, which was longer than that of S-MoS2 (2.33 Å). Figure 1 | (a) The schematic diagram for forming process of Pt-B-MoS2. (b) The HAADF-STEM images and corresponding simulating configurations of S-MoS2 and B-MoS2. (c) The HAADF-STEM image of the B-MoS2 sample. (d) Raman spectra for the S-MoS2 and B-MoS2. Inset: the HAADF-STEM image of B-MoS2. (e) The HAADF-STEM image of Pt-B-MoS2. Download figure Download PowerPoint Further, when Pt species were introduced to the curved B-MoS2 surface, the HAADF-STEM image (Figure 1e) clearly showed that the isolated Pt atoms (circled in Figure 1e) distributed uniformly over the B-MoS2 surface (denoted as Pt-B-MoS2), with no apparent agglomeration. More representative images are shown in Supporting Information Figure S10. Energy-dispersive X-ray analysis ( Supporting Information Figure S11) exhibited homogeneous distribution of Mo, S, and Pt compositions on the bending frameworks. Moreover, we explored the chemical environment of Pt species on the supports employing XPS: For pristine B-MoS2, the S 2p XPS displayed a characteristic doublet peak at ∼163.4 and ∼162.2 eV (Figure 2a). After functionalization by the Pt atoms, the doublet peaks of S 2p in Pt-B-MoS2 shifted to higher binding energy (0.3 eV left shift) compared with S in B-MoS2. Similarly, the S L-edge XANES spectra in Supporting Information Figure S12 exhibited a lower peak intensity of Pt-B-MoS2 compared with origin B-MoS2. These changes occurred mainly because of the spontaneous reduction of Pt4+ species on MoS2 surface and the formation of Pt-S coordinate bond, resulting in electrons' transfer from S to Pt; thus, modifying the electrical environment of S atoms.8,23 In addition, the same peak shift of Mo 3d was also observed in Supporting Information Figure S13, in which Mo in Pt-B-MoS2 shifted to higher energy compared with B-MoS2.17 The Pt 4f XPS analysis in Figure 2b showed that the Pt binding energy in Pt-B-MoS2 was higher than that of the Pt nanoparticles (Pt0, at ∼71.2 eV) but lower than PtO2 (Pt4+, at ∼74.0 eV), indicating its ionic Ptδ+ (0 < δ < 4) nature. However, for the Pt loading of S-MoS2 samples (donated as Pt-S-MoS2), the Pt exhibited an additional peak at ∼74.0 eV, suggesting the existing higher oxidation state of the Pt4+ species. This was attributed mainly to the differences in local geometric configuration and electronic environment of the Pt single atoms on special supports. Moreover, the XPS results showed that the contents of surface Pt in Pt-B-MoS2 was ∼0.83 wt %, which is higher than that in Pt-S-MoS2 (0.69 wt %) and closed to the inductively coupled plasma atomic emission spectroscopy results of ∼0.91 wt % ( Supporting Information Table S2). The EPR analysis ( Supporting Information Figure S14) revealed no detection of notable sulfur vacancy peak (at g = 2.003) in the B-MoS2 samples,11,24 demonstrating negligible defect sites over the support surface for plausible capture of dissociative metal monomers. Therefore, the Pt atoms on the bending MoS2 surface were inclined to disperse and stabilize only through the strong covalent metal-support interaction,23,25 distinct from defect-stabilized Pt atoms in Pt-S-MoS2. Figure 2 | (a) The S 2P XPS of Pt-B-MoS2 and B-MoS2. (b) The Pt 4f XPS of Pt-B-MoS2 and Pt-S-MoS2. (c) Pt L-edge XANES spectra. (d) The EXAFS spectra (e) corresponding fitting curve of Pt L3 edge. (f) The diffusion path of Pt atoms on B-MoS2 surface and the analysis of diffusion barrier. Download figure Download PowerPoint Next, the Pt L-edge XANES characterization (Figure 2c) revealed the oxidation states of Pt in Pt-B-MoS2 and Pt-S-MoS2, in which the white line intensity was situated between the PtS2 and Pt foil. The EXAFS spectrum (Figure 2d) displayed a notable peak at ∼2.2 Å, contributed by the Pt-S coordination path. No Pt–Pt characteristic peak (at ∼2.8 Å) was detected for both Pt-B-MoS2 and Pt-S-MoS2 samples, confirming the atomic dispersion of Pt on the supports. Interestingly, the intensity of Pt in Pt-B-MoS2 was weaker than that in Pt-S-MoS2, suggesting a lower coordinated number (CN) of Pt atoms in Pt-B-MoS2. Furthermore, the first-shell EXAFS fitting curve (Figure 2e) and k space fitting curve obtained ( Supporting Information Figure S15) validated that the central Pt in Pt-B-MoS2 coordinated directly with three S atoms, with an average bond length of 2.34 Å ( Supporting Information Table S3), which were 0.8% longer than that of that of Pt-S-MoS2. In contrast, the Pt in Pt-S-MoS2 showed a higher CN of ∼6.0 ( Supporting Information Figure S16 and Table S3), which might have led to an increase in valence states of Pt, in accordance with the XPS results. The best-fitting results of PtS2 nanoparticles and the corresponding Pt L-edge wavelet transform contour plots are displayed in Supporting Information Figures S17 and S18 and Table S3, respectively. Only one maximum peak at ∼5.9 Å−1 was discovered in all Pt-B-MoS2, Pt-S-MoS2 and PtS2 samples, regarded as the Pt-S coordination path. No obvious Pt–Pt bond was observed, demonstrating highly distributed single Pt atoms in Pt-B-MoS2 and Pt-S-MoS2 catalysts. To further corroborate the Pt-sites configuration on the special support, theoretical investigations were implemented employing DFT methods. As seen in Figure 2f, the curved MoS2 framework was and the Pt species were introduced into the special surface. that the Pt could stabilize on the of the Mo to three S atoms, consistent with the EXAFS results. the diffusion of Pt with the bending surface additional energy of eV to the hollow site and eV to the of S structural for this Moreover, when we introduced or into this curved MoS2 was different from the or single atoms in straight MoS2 supports. The three-dimensional of the proposed Pt-B-MoS2 and Pt-S-MoS2 theoretical are shown in Supporting Information Figure the Pt in Pt-B-MoS2 from the MoS2 plane, and the of in-plane S atoms were due to the bending In to the catalytic activity of S-MoS2, B-MoS2, Pt-S-MoS2, and Pt-B-MoS2 samples, performance tests were For an sensing when the reducing at the MoS2 surface, the supports electrons from the gas molecules, to a in and an increase in the sensors were exposed in the the behavior of the molecules in resistance The of a metal such as Pt as the chemical and electrical to the of species and the electron transmission First, the Pt-B-MoS2 sensors were exposed to H2S ranged from 0.5 to 10 at room temperature. As seen in Figure on the of H2S the Pt-B-MoS2 sensor exhibited and response and recovery the baseline of the Pt-B-MoS2 sensors in the in with of B-MoS2 ( Supporting Information Figure Moreover, the as S = (Rg − Ra)/Ra = for all sensors with H2S concentrations (Figure In the B-MoS2 sensor exhibited an enhanced H2S response compared with S-MoS2, suggesting that introducing the bending the active MoS2 surface to the adsorption of H2S gas In the Pt-B-MoS2 sensor to 10 exhibited and higher than of Pt-S-MoS2 B-MoS2 and S-MoS2 respectively. This was ascribed mainly to the highly active Pt atoms on the MoS2 both the gas adsorption and the of H2S species and the subsequent electron the for H2S in environment is 10 such a sensing response of reported H2S sensors ( Supporting Information Figure and Table and the detection More performances on the different MoS2 are displayed in Supporting Information Table Furthermore, the Pt-B-MoS2 sensor exhibited a response of at an H2S of 100 However, the of detection for Pt-S-MoS2 sensor H2S is only 1 the sensing could not be for S-MoS2 and B-MoS2 sensors the of H2S was 5 The response and recovery times of the samples at H2S concentrations were further and The Pt-B-MoS2 sensors showed a response and recovery times (25 and 20 at 10 H2S than of Pt-S-MoS2 and B-MoS2 and and S-MoS2 and This increase in response and recovery speed that the Pt configuration the adsorption of the gas and electron thus, the intrinsic activity of catalysts. In addition, the of S-MoS2, B-MoS2, Pt-S-MoS2, and Pt-B-MoS2 sensors different at 10 were further (Figure The response of Pt-B-MoS2 sensor for H2S was at times higher than carbon and indicating a This sufficiently that the Pt sites with configuration binding sites to H2S selectively and the The tests are displayed in Figure in which the response time and recovery time of Pt-B-MoS2 sensors during The measurement was performed in Supporting Information Figure suggesting a superior of Pt-B-MoS2 sensors. Figure | (a) of Pt-B-MoS2. (b) The of S-MoS2, B-MoS2, Pt-S-MoS2, and Pt-B-MoS2 in different H2S The (c) response and (d) recovery time of samples at H2S (e) The selectivity of S-MoS2, B-MoS2, Pt-S-MoS2, and Pt-B-MoS2 in different (f) The tests of Pt-B-MoS2 sensors. Download figure Download PowerPoint DFT calculations were to the between Pt-B-MoS2 catalysts and different of S-MoS2, B-MoS2, Pt-S-MoS2, and Pt-B-MoS2 surfaces were on the experimental results (Figure In the adsorption energy of B-MoS2 eV) was than that of S-MoS2 eV) with H2S molecules, which that the introduced by bending structure could effectively activate the MoS2 supports and gas well with previous Furthermore, the gas adsorption was further the Pt was introduced into the B-MoS2 or S-MoS2 supports. the Pt-B-MoS2 structure exhibited the adsorption = eV) than that the Pt-B-MoS2 surface was beneficial for adsorption and with the H2S The electric density distribution of the Pt-B-MoS2 framework was further As displayed in Figure the Pt more electrons over the bending surface, and thus, revealed a around the Pt species. The highly electric configuration was proven to as active sites for and the process such as and Therefore, the tip effect induced by Pt sites the adsorption and with H2S To into the between supports and H2S molecules during the density of states analysis of and were The atomic of and are displayed in Figure with we that the and in possessed a noticeable the indicating a strong between these Therefore, the Pt sites might have as a crucial of and thus, enhanced the electric transfer between H2S and Pt sites. Figure further of electronic transfer in in which the and accumulation regions are by the and respectively. of electrons transfer from the H2S to the Pt-B-MoS2 which might have led to a in and an increase in well with the experimental
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