Lattice -Mismatch-Induced Ultrastable 1T-Phase MoS2

Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX

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Lattice -Mismatch-Induced Ultrastable 1T-Phase MoS2−Pd/Au for Plasmon-Enhanced Hydrogen Evolution Bo Shang,† Xiaoqiang Cui,*,† Lin Jiao,† Kun Qi,‡ Yingwei Wang,‡ Jinchang Fan,† Yuanyuan Yue,§ Haiyu Wang,§ Qiaoliang Bao,∥ Xiaofeng Fan,† Shuting Wei,† Wei Song,∇ Zhiliang Cheng,⊥ Shaojun Guo,# and Weitao Zheng*,†

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†

Key Laboratory of Automobile Materials of MOE, School of Materials Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China ‡ Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, People’s Republic of China § State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China ∥ Department of Materials Science and Engineering, and ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), Monash University, Clayton, Victoria 3800, Australia ⊥ Department of Bioengineering, University of Pennsylvania, 210 South 33rd Street, 240 Skirkanich Hall, Philadelphia, Pennsylvania 19104, United States # Department of Materials Science and Engineering, and BIC-ESAT, College of Engineering, Peking University, Beijing 100871, China ∇ State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, 130012, P. R. China S Supporting Information *

ABSTRACT: Metallic 1T-phase transition metal dichalcogenides (TMDs) are of considerable interest in enhancing catalytic applications due to their abundant active sites and good conductivity. However, the unstable nature of 1T-phase TMDs greatly impedes their practical applications. Herein, we developed a new approach for the synthesis of highly stable 1T-phase Au/Pd-MoS2 nanosheets (NSs) through a metal assembly induced ultrastable phase transition for achieving a very high electrocatalytic activity in the hydrogen evolution reaction. The phase transition was evoked by a novel mechanism of lattice-mismatch-induced strain based on density functional theory (DFT) calculations. Raman spectroscopy and transmission electron microscopy (TEM) were used to confirm the phase transition on experimental grounds. A novel heterostructured 1T MoS2−Au/Pd catalyst was designed and synthesized using this mechanism, and the catalyst exhibited a 0 mV onset potential in the hydrogen evolution reaction under light illumination. Therefore, this method can potentially be used to fabricate 1T-phase TMDs with remarkably enhanced activities for different applications. KEYWORDS: Phase transition, ultrafast spectroscopy, electrocatalytic, molybdenum disulfide, hydrogen evolution

M

2H phase.22−24 The unstable nature of the 1T phase is attributed to its substantially higher energy relative to that of the 2H phase, thus resulting in transformation of the metastable 1T structure to a thermally preferred 2H phase, even under aging in air.22,25 As a result, the preparation of ultrastable 1T-MoS2 for practical applications remains a substantial challenge. Previous works showed that both mechanical strain and an electrostatic field could induce the

olybdenum disulfide (MoS2) has attracted considerable attention because of its excellent properties and potential applications in electrocatalysis,1 energy storage,2,3 and electronic devices.4,5 Metallic 1T-phase MoS2 shows superior performance in several applications, including hydrogen evolution reaction (HER) catalysts,6−10 actuators,11 supercapacitors,3 and edge contact transistors,12,13 due to its abundant active sites and excellent electrical conductivity.14−17 Unfortunately, the 1T-phase MoS2 synthesized by traditional approaches including chemical exfoliation,18 hot electron injection,19 electron beam irradiation,14 and metallic atom doping8,20,21 is metastable and tends to devolve back into the © XXXX American Chemical Society

Received: October 12, 2018 Revised: December 12, 2018

A

DOI: 10.1021/acs.nanolett.8b04104 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters 1T phase transition.19,26,27 The mechanical strain method developed by Zheng’s group required a periodic gold cone substrate to induce the strain.28 However, the phase transitions induced by these two physical methods are reversible and inappropriate for large-scale catalytic applications. The complicated synthetic procedures and the low yield limited the practical application of 1T MoS2. In this work, we developed a novel strategy for fabricating ultrastable 1T-MoS2 for widespread application based on a lattice mismatch mechanism. A bimetallic structure consisting of Pd NSs with a single-side-nucleated (SSN) Au nanoisland was designed and coassembled onto the surface of pristine 2HMoS2, resulting in a 1T phase transition at the contacted area, as shown in Scheme 1 and Figure S1. The lattice mismatch and

HER29 and also lowers the resistance, which facilitates the electron transport. The plasmonic electrons generated from the Au nanoislands tremendously enhanced the HER activity, which exhibited an onset potential of approximately 0 mV and a small Tafel slope of 49 mV dec−1 under visible light illumination. The 1T-phase MoS2 anchored by these Pd NSs showed a higher stability than the metastable counterpart obtained from the traditional chemical exfoliation method.22,24 Pd NSs with (111) facets were first synthesized according to a modified literature procedure.30 The synthesized Pd NSs were ultrathin with a thickness of 2.00 nm and an average diameter of 29.53 nm (Figure S2). Heterostructured SSN-Au/ Pd was further synthesized through epitaxial growth of gold nanoislands on the Pd NSs by including a small amount of Br− ions to prevent the nucleation of Au on the (100) edge sites.31,32 SSN-Au/Pd was characterized by TEM and HRTEM (Figure S3), showing that the Au nanoislands were dominantly localized on the (111) facets of the Pd NSs. SSN-Au/Pd was then assembled on MoS2 NSs (Figure S4) via van der Waals (vdW) preinteractions and sonication-induced covalent bonds.33 The phase transition was investigated by Raman spectroscopy (Figure 1a). The Raman spectra show differences between 2H-MoS2 and MoS2 assembled with SSN-Au/Pd (SSN-Au/Pd-MoS2). The suppression of the peaks at 377 and 407 cm−1 and the appearance of several new peaks at 199, 145, and 225 cm−1 corresponding to the phonon modes of 1TMoS2 indicate the formation of 1T-phase MoS2.34−37 This phase transition is ultrastable, even under irradiation by a powerful 1 mW 532 nm laser from the Raman laser source for 120 s.24 In contrast, the 1T MoS2 prepared by lithiation completely devolves into the 2H phase after only 30 s of

Scheme 1. Structural Model of SSN-Au/Pd-MoS2 and the Strain-Induced Phase Transition

covalent bonds between the Pd and S atoms generated a compressive strain that resulted in a 2H to 1T phase transition of MoS2. This phase transition was confirmed by the combined results of DFT calculations, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and femtosecond broadband transient absorption (TA) spectroscopy. The formation of the 1T MoS2 provides more active sites for the

Figure 1. (a) Raman spectra of 1T-phase MoS2 prepared by lithiation and SSN-Au/Pd-MoS2 and the same samples irradiated by a 532 nm laser source for different durations. (b) XPS spectra of Pd and Mo in pristine MoS2, pristine Pd NSs, and heterostructured MoS2-SSN-Au/Pd. The solid lines are the fitted curves, and the black dots are the experimental data. (c), (d) 2H and 1T structures of MoS2 assembled with Pd calculated by first principles. (e) Energies of 2H-MoS2 and 1T-MoS2 assembled with Pd calculated by first principles. B

DOI: 10.1021/acs.nanolett.8b04104 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 2. (a) TEM image of MoS2 NSs assembled with heterostructured SSN-Au/Pd. (b) High-magnification TEM image of SSN-Au/Pd-MoS2. (c) HRTEM image of MoS2 NSs assembled with heterostructured SSN-Au/Pd. (d) Selected area of HRTEM image of (c). The upper section represents the area with Pd and the bottom represents the uninteracted MoS2. (e) Magnified sections in (d) and the corresponding intensity profiles along the lines.

irradiation.22,24 The control experiment in which pristine ultrathin Pd NSs were assembled on MoS2 also exhibited a phase transition (Figure S5).19 The XPS results also confirm the occurrence of a phase transition by comparison of the changes in the binding energies of Mo and Pd before and after the assembly process (Figure 1b). The intensity of the PdII peak increased significantly after assembly with the MoS2 NSs, indicating that covalent bonds formed between the Pd and S atoms.33,38 The PdII doublet shifted by approximately 0.8 eV toward lower binding energies, indicating that the absorbed PdII ions were transformed into PdII−S.33 After assembly with heterostructured SSN-Au/Pd, the two peaks of Mo at 228.7 and 231.9 eV shifted to lower binding energies of 228.3 and 231.5 eV, respectively, indicating the formation of 1T-phase MoS2.6,39−41 The Au doublet in the XPS results remained unchanged after assembly on the MoS2 NSs, indicating that the Au did not interact with the MoS2 NSs (Figure S6). This result provides insight into the elaborate structure of the synthesized SSN-Au/Pd in which no Au atoms existed between the Pd and MoS2 lamellar structures, indicating that the Au nanoislands nucleated on only one side of the Pd NSs, as illustrated in Figure S7. In this case, the Pd NSs serve as double-sided tape between MoS2 and the Au nanoislands, binding these two structures together and inducing the observed phase transition. First-principles calculations were performed to elucidate the phase transition of MoS2 induced by the assembly of Pd. As shown in Figure 1c, Pd atoms adsorbed at the top sites of 2H MoS2 with an adsorption energy of 2.55 eV, which is defined by the eq 1: ΔE Pd = EMS + E Pd − EMS ‐ Pd

respectively. Interestingly, for 1T MoS2 (Figure 1d), the adsorption energy of a Pd atom trapped at a site adjacent to the top site is 2.88 eV, which is larger than that for adsorption on 2H MoS2. This result, in which Pd atoms prefer to adsorb to the 1T phase over the 2H phase, implies that the interaction between Pd sheets and the 2H phase can induce a phase transition from the 2H phase to the 1T phase. On the basis of our previous theoretical work,42 mechanical strain effectively triggers a phase transition from the semiconductive 2H phase to the metallic 1T phase. The Pd−Pd distance on the (111) surface of face-centered cubic (fcc) Pd is 2.8 Å, and the lattice constant a of MoS2 is 3.16 Å. This lattice mismatch between the (111) surface of Pd and MoS2 is proposed to induce strain and thus trigger the phase transition. When the Pd (111) surface is densely coated with 2H MoS2, the lattice of MoS2 is under compressive stress induced by Pd adsorption through the formation of covalent bonds between the Pd and S atoms.33 As shown in Figure 1e, the adsorption energy of Pd on 2H MoS2 increases quickly following compression of the lattice to match the Pd (111) surface. However, for the 1T phase, the adsorbed Pd more easily matches the bulk Pd lattice due to the free relaxation at the nearby top sites. On the basis of the formula in eq 2 with the 2H phase as the reference state, ΔE = E2H ‐ MS + E Pd − E1T ‐ MS ‐ Pd

(2)

the energy of Pd assembly with the 1T phase is lower than that with the strained 2H phase (Figure 1e). Therefore, under the compressive strain induced by the adsorbed Pd, the 1T phase becomes more stable than the 2H phase, and the phase transition occurs when MoS2 comes in contact with the Pd (111) surface. The structure and phase transition were further characterized by TEM and HRTEM (Figure 2). The TEM images

(1)

where EMS‑Pd, EMS, and EPd are the energies of MoS2 with adsorbed Pd atoms, a MoS2 slab, and isolated Pd atoms, C

DOI: 10.1021/acs.nanolett.8b04104 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters (Figure 2a,b) indicate that SSN-Au/Pd is uniformly loaded onto the surface of MoS2 without aggregation. Elemental mapping by energy-dispersive X-ray spectroscopy (EDX) confirmed the uniform assembly of the heterostructured SSN-Au/Pd (Figure S8). The HRTEM image in Figure 2c shows a clear interface between the boundary of the 1T and 2H phase. The darker area in the top-right corner corresponds to the region with Pd NSs. Figure 2d is the magnified area in the red square in Figure 2c. Two regions were selected from the top (with Pd) and bottom (without Pd) area of Figure 2d. By further evaluating the intensity profiles along the two lines in Figure 2e, the 1T structure can be clearly observed.14,17 The missing of the S peak between the two Mo atoms corresponds to the tetragonal coordination of the 1T MoS2. The distance between the two Mo atoms has also been compressed by 7% as shown in the intensity profiles. On the contrast, the area in the bottom of Figure 2d (without Pd NSs) exhibits a regular array of Mo atoms of the typical 2H phase. More electron microscopy data was also provided in Figure S9−11. The average Mo−Mo distances in 1T-phase and 2H-phase MoS2 were further calculated to be 2.85 and 3.02 Å, respectively (Figure S12), which further confirms the phase transition and the presence of compressive strain in MoS2, as predicted by DFT calculations. The distinct phase transition and efficient charge transfer were investigated by femtosecond broadband TA spectroscopy (Figure 3). Pristine MoS2 showed three exciton peaks at 685, 636, and 459 nm, corresponding to the A, B, and C excitons, respectively (Figure 3a).43 On the basis of the phase transitions in TMDs reported previously, the differences between the TA spectra of MoS2 with and without Pd NSs in this work indicate a novel 2H to 1T phase transition. The occurrence of the 2H to 1T phase transition can be explained as follows: (1) Compared to pristine MoS2, the exciton peak positions of Pd NSs-MoS2 and SSN-Au/Pd-MoS2 show slight blue shifts, as illustrated by the vertical red lines in Figure 3a−c, similar to previous observations reported by other groups.18,44 (2) A relatively flat transmission spectrum is a typical characteristic of 1T-phase MoS2 that is often caused by the free carriers present in the metallic 1T phase, which screen the electron− hole attraction and weaken the excitonic effects.22 Therefore, the observed significant decrease in ΔOD can be attributed to the phase transition of the MoS2 NSs, as shown in Figure 3a− c. To further evaluate charge transfer from SSN-Au/Pd to the MoS2 NSs, the carrier decays of pristine MoS2 and SSN-Au/ Pd-MoS2 were compared. The spectra of the SSN-Au/PdMoS2 change much faster than those of the pristine MoS2, as clearly seen from the decay dynamics measured at the A exciton (Figure 3d). By use of a triple exponential decay fitting analysis, expressed as eq 3:

Figure 3. (a)−(c) TA spectra of pristine MoS2, Pd NSs-MoS2, and SSN-Au/Pd-MoS2 measured at different delay times under 400 nm excitation. The vertical dashed red lines correspond to the exciton peak positions in pristine MoS2. (d) Normalized decay dynamics of the exciton states in (a)−(c), probed at the A exciton. The solid lines are the fitted curves. (e) Illustration of hot electron injection in heterostructured SSN-Au/Pd-MoS2. (f) Principle of plasmonic hot electron injection, where EF is the Fermi energy of Pd, φHE is the hot electron distribution energy, φSB is the Schottky barrier, EC is the conduction band, and EV is the valence band.

components. The shortest lifetime is due to carrier−carrier scattering45 or exciton−exciton annihilation.43,46 In the present heterostructure, the hot electron injection process from donor to MoS2 acceptor also occurs in such a time scale in femtoseconds, as shown in Figure S13.47 The intermediate decay time constant (τ2) is dominated by interband carrierphonon scattering.45 The slowest decay component (τ3), which is on the order of hundreds of picoseconds, is assigned to direct interband electron−hole recombination. On the basis of comparison of the lifetime components of the pristine MoS2 NSs to those of the heterostructured SSN-Au/Pd-MoS2, SSNAu/Pd-MoS2 had a shorter lifetime than the pristine MoS2 NSs over all three processes. The mechanism for these processes can be explained on the basis of the well-established hot carrier injection physical model,19 as illustrated in Figure 3e,f. When the assembled SSN-Au/Pd nanostructure is excited by a 400 nm laser, strong surface plasmon resonance is stimulated. Because the work function of Pd is approximately 4.8 eV, with an excitation laser of 400 nm (3.10 eV), the energies (φHE = 3.25 eV) of the hot electrons in Pd are high enough to overcome the Schottky barrier (φSB < 0.8 eV).48,49 Thus, an enormous amount of hot carriers are generated during nonradiative decay of the surface plasmon via Landau damping,47 and those hot carriers are then transferred from the SSN-Au/Pd heterostructure to MoS2 (Figure 3e). The decay time of the heterostructured SSN-Au/Pd then becomes shorter, as electron transfer leads to the formation of excitons

y = y0 + A1e−(x − x0)/ τ1 + A 2 e−(x − x0)/ τ2 + A 2 e−(x − x0)/ τ3 (3)

three lifetime components were extracted for the pristine MoS2 NSs and SSN-Au/Pd-MoS2. The values of τ1, τ2, and τ3 for the pristine MoS2 NSs are 0.59, 28.9, and 1101.6 ps, respectively, and for SSN-Au/Pd-MoS2, the values are 0.42, 16.6, and 272.9 ps. Thus, the carrier lifetimes are obviously shortened in the assembled heterostructure, as was also the case for Pd NSsMoS2 (Table S1). On the basis of the combination of these findings with those of previous reports,43,45,46 three different processes were concluded to contribute to the three lifetime D

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Figure 4. Linear sweep voltammetry (LSV) curves of pristine MoS2, Pd NSs-MoS2, Au NPs-MoS2, SSN-Au/Pd-MoS2, and commercial Pt/C in an N2-saturated 0.5 M H2SO4 aqueous solution in the dark (a) and under 1 W cm−2 xenon light illumination (b). (c) LSV curves of heterostructured SSN-Au/Pd-MoS2 before and after 1000 electrochemical scans. (d) Tafel plots and overpotentials. (e) Time dependence of the current density for MoS2-SSN-Au/Pd at a 60 mV overpotential vs RHE. The yellow areas represent periods of light illumination. (f) Nyquist plots of different samples, in which the solid lines represent the fitted curves. The inset shows a magnification of the area in the bottom left corner.

overpotential of nearly 0 mV was obtained for this heterostructured SSN-Au/Pd-MoS2 catalyst under light illumination. This catalyst also exhibited an extremely high stability (Figure 4c), as negligible change in activity was observed after 1000 electrochemical scans from 0 to −0.5 V versus the reversible hydrogen electrode (RHE). The small Tafel slope of 49 mV dec−1 indicates that the catalyst facilitates the HER through the Heyrovsky or Tafel mechanism instead of the common Volmer reaction (Figure 4d).51−53 Furthermore, a small Tafel slope corresponds to a faster increase in the current density, thus making the catalyst useful for practical applications.54,55 The time-dependent current densities of the materials at a constant overpotential of 60 mV are shown in Figure 4e. SSN-Au/Pd-MoS2 showed the highest enhancement with a 4-fold increase under 1 W cm−2 light illumination. This enhancement is 3 times higher than that in a previous report on MoS2−Au nanorods under more powerful 1.5 W (21.4 W cm−2) laser irradiation.56,57 Electrochemical impedance plots show that the charge transfer resistance of the MoS2 NSs decreased significantly from 960 Ω to only 20 Ω after assembly with the heterostructured SSN-Au/Pd (Figure 4f). Thus, the enhancement can be attributed to the excellent conductivity of the metallic 1T-phase MoS2 and the fast charge transfer rate between these two lamellar counterparts, which is consistent with the results of femtosecond broadband TA spectroscopy.58,59 The content of Pd in this material was 4.1% (Table S2), which is comparable to that in single-atom catalysts (∼2%) and keeps the price low for practical applications.60 In conclusion, a new strategy for inducing a stable phase transition in MoS2 NSs by the assembly of lamellar metallic counterparts was developed. Theoretical calculations elucidated that the mechanism of this novel compressive-straininduced phase transition arose from the lattice mismatch. The 1T-phase MoS2 anchored by Pd NSs was ultrastable because of the formation of covalent bonds between Pd and S. The largescale lamellar-interaction-induced phase transition and the excellent intrinsic conductivity of the as-formed 1T-phase

in MoS2, which accelerates the attenuation of hot electron induced by plasmon excitation of the assembled SSN-Au/Pd nanostructure (Figure 3f). Charge transfer and attenuation are much faster for metallic 1T MoS2 than for the semiconductive 2H phase, resulting in a shorter carrier lifetime. All these phenomena except for hot electron injection also occur in Pd NSs-MoS2 (Figure S5), indicating that the phase transition originates from the Pd−S covalent bonds and the lattice mismatch and not from the generated hot electrons.19 These results reveal the enhanced charge transfer efficiency of 1T MoS2, which favors electrochemical catalysis. The HER activity of SSN-Au/Pd-MoS 2 was further investigated by electrochemical measurements in a 0.5 M sulfuric acid (H2SO4) aqueous solution. Figure 4a shows the HER performances of the various catalysts without light illumination. The assembly of Pd NSs resulted in a significant improvement in the HER activity, similar to the results of our previous work, in which the onset potential shifted to the right by 150 mV.33 SSN-Au/Pd-MoS2 exhibited a low onset potential of 50 mV and a Tafel slope of 63 mV dec−1. Figure 4b shows that illumination improved the HER activities of all the materials (MoS2, Au NPs-MoS2, and SSN-Au/Pd-MoS2) because of the adsorption of light by the materials (Figure S14). SSN-Au/Pd-MoS2 showed the highest increase in the HER activity under illumination, in which the cathode current density reached 82 mA·cm−2 with an overpotential of 200 mV, 8 times higher than that of defect-rich MoS2.33,50 These results indicate extensive injection of photogenerated hot electrons, thus enhancing the HER activity. Surprisingly, MoS 2 assembled with spherical Au NPs showed a negligible enhancement in HER activity under illumination (Figure 4b and Figure S15), which was attributed to the poor conductivity and low charge transfer efficiency without the phase transition of MoS2 induced by the heterostructure assembly. Therefore, the Pd NSs are essential for inducing the phase transition of MoS2 and facilitating rapid hot electron transmission from the Au nanoislands to MoS2. As shown in Figure 4b, an ultralow E

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Nano Letters MoS2 significantly enhanced the HER activities.6 The heterostructured SSN-Au/Pd-MoS2 exhibited an onset potential of nearly 0 mV and a small Tafel slope of 49 mV dec−1 under light illumination. This work highlights a new way to enhance the intrinsic conductivity of the MoS2 basal plane and reveals a novel mechanism for inducing an ultrastable phase transition from 2H to 1T, which provides a facile and extraordinary strategy for fabricating multiphase 2D TMD materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b04104.

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Experimental section, figures of synthesis procedure of the MoS2-Pd/Au heterostructure, vertically aligned ultrathin Pd NSs and bar graphs of average diameters, TEM, HRTEM, SEM, EDX, and FFT images, Raman, XPS, UV−vis spectra, SSN-Au/Pd heterostructure illlustration, statistical data of the Mo−Mo distances, A exciton decay, and tables of carrier lifetimes and of Pd and Au content (PDF)

AUTHOR INFORMATION

Corresponding Authors

*X. Cui. E-mail: [email protected]. *W. Zheng. E-mail: [email protected]. ORCID

Xiaoqiang Cui: 0000-0002-5858-6257 Xiaofeng Fan: 0000-0001-6288-4866 Wei Song: 0000-0001-9814-419X Zhiliang Cheng: 0000-0003-3351-6874 Shaojun Guo: 0000-0003-4427-6837 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Programme of China (2016YFFA0200400), the Natural Science Foundation of China (51571100, 51872116, and 51602305), the Programme for JLU Science and Technology Innovative Research Team (JLUSTIRT, 2017TD-09), the start-up support from Peking University, the Young Thousand Talented Programme and the Fundamental Research Funds for the Central Universities. Q. Bao acknowledges support from the Australian Research Council (ARC) Future Fellowship (FT150100450) and the Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET, CE170100039). K. Qi acknowledges support from the China Postdoctoral Science Foundation (2018M633127) and the Natural Science Foundation of Guangdong Province (2018A030310602).

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DOI: 10.1021/acs.nanolett.8b04104 Nano Lett. XXXX, XXX, XXX−XXX