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Single Atomic Vacancy Catalysis Jieun Yang, Yan Wang, Maureen J Lagos, Viacheslav Manichev, Raymond Fullon, Xiuju Song, Damien Voiry, Sudip Chakraborty, Wenjing Zhang, Philip E Batson, Leonard Feldman, Torgny Gustafsson, and Manish Chhowalla ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b05226 • Publication Date (Web): 09 Aug 2019 Downloaded from pubs.acs.org on August 9, 2019

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Single Atomic Vacancy Catalysis Jieun Yang,† Yan Wang, † Maureen J. Lagos,‡ Viacheslav Manichev,§,¶ Raymond Fullon,! Xiuju Song,# Damien Voiry,' Sudip Chakraborty,- Wenjing Zhang,# Philip E. Batson,¶, 2 Leonard Feldman, †

!2

Torgny Gustafsson,

2

Manish Chhowalla†, #,*

Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge, CB3 0FS, UK.

‡ Department

of Materials Science and Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada

§ Department

of Chemistry and Chemical Biology, Rutgers University, Piscataway, 08854, NJ, USA

¶

Department of Physics and Astronomy, Rutgers University, Piscataway, 08854, NJ, USA Materials Science and Engineering, Rutgers University, Piscataway, 08854, NJ, USA

# International

Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology, Shenzhen University, Shenzhen 518060, China

Institut Europeen des Membranes, Universite de Montepellier , Montepellier, France. Discipline of Physics, Indian Institute of Technology (IIT) Indore-453552, M. P., India. Institute of Advanced Materials, Devices, and Nanotechnology, Rutgers University, Piscataway, NJ, USA *Corresponding Author: [email protected]

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ABSTRACT Single atom catalysts provide exceptional activity. However, measuring the intrinsic catalytic activity of a single atom in real electrochemical environments is challenging. Here, we report the activity of a single vacancy for electrocatalytically evolving hydrogen in two dimensional (2D) MoS2. Surprisingly, we find that the catalytic activity per vacancy is not constant but increases with their concentration – reaching a sudden peak in activity at 5.7 x 1014 cm-2 where the intrinsic turn over frequency (TOF) and Tafel slope of a single atomic vacancy was found to be ~ 5 s-1 and 44 mV/dec, respectively. At this vacancy concentration, we also find local strain of ~ 3% and a semiconductor to metal transition in 2D MoS2. Our results suggest that, along with increasing the number of active sites, engineering the local strain and electrical conductivity of catalysts is essential in increasing their activity.

KEYWORDS: hydrogen evolution reaction, single vacancy, molybdenum disulfide, helium ion microscope, scanning transmission electron microscope

Single atom catalysts possess intrinsically high catalytic activity and have the added economic benefit of low material utilization.1-3 Typically, single atom activity is measured by depositing thin layers of catalyst layers on carbon (or some other support) or by designing complexes with single atoms.4-6 However, the catalytic activity of the entire electrode is measured and contribution from individual atoms is inferred rather than directly measured. Atomic sites and their activity have been identified in highly controlled scanning tunneling microscope under ultra-high vacuum conditions.7 However, measuring activity from a well-defined number of active sites in electrochemical environments is challenging and therefore individual site activity is difficult to extract.


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Recently, there has been significant interest in using atomically flat two-dimensional (2D) materials such as MoS2 as catalysts for the hydrogen evolution, CO2 reduction and other reactions.8-10 The catalytic activity of MoS2 has been attributed to metallic edges,7,11 metallic 1T phase,8,12 and/or sulfur vacancies in the basal plane.13,14 The atomically flat nature of 2D MoS2 allows lithographic fabrication of micro-electrochemical cells with a well-defined surface exposed to the electrolyte so that catalytic parameters such as the turnover frequency (TOF) and number of active sites can be accurately determined.15 The micro-electrochemical cells also allow study of how extrinsic factors such as the electrical conductivity of 2D materials and the charge transfer resistance between the support and catalyst influence the performance.15 Finally, lithographic patterning also enables micro-electrochemical measurements of activity only from the edges or the basal plane of the flat 2D catalysts.15

RESULTS AND DISCUSSION We used single layer 2D MoS2 grown by chemical vapor deposition (CVD) shown in Figure 1a as the catalyst for the hydrogen evolution reaction (HER). The as-grown CVD MoS2 monolayers are semiconducting 2H phase and their basal plane is catalytically inactive in the absence of sulfur vacancies.14-16 To activate the basal plane, we utilized a helium ion microscope (HIM) to controllably introduce individual sulfur vacancies by varying the dose of helium ions (see Methods section for details). We found that the helium ion beam is able to gently remove sulfur atoms one by one with no detectable extraneous damage to the material. This is in contrast to ion beams of larger atoms such as Ar and Ga where substantial disorder in the material along with vacancies is generated.14,17 The vacancy concentration and type were identified by examining the atomic contrast in High Angle Annular Dark Field (HAADF) scanning transmission electron microscope


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(STEM) imaging as shown in Figure 1b. The concentration and type of vacancies as indicated by yellow (single S atom missing), orange (two S atoms missing) and green (single Mo atom missing) circles in Figure 1b were obtained by analyzing the contrast of the atoms in tens of HAADF images at each dosage. The HIM was performed on monolayer MoS2 so that vacancies in all three atomic layers (top and bottom sulfur layers and middle Mo layer) can be easily imaged and counted. In HAADF images, the highest contrast is from Mo atoms and single sulfur and double sulfur vacancies show ~30% and ~45% less intensity, respectively, than Mo atoms as indicated by the intensity profile in Figure 1c. It can be seen that the molybdenum vacancy contrast (line 2) is decreased to 0% in Figure 1c. The atomic resolution HAADF STEM of as grown and HIM treated samples are shown in Figures 2a – 2f. As-synthesized MoS2 has vacancy concentration of 6 × 1013 cm-2 (Figure 2a). The yellow, orange and green circles indicate single sulfur vacancies, double sulfur vacancies, and single molybdenum vacancies, respectively. The majority of vacancies were found to be single and double sulfur vacancies for concentrations of

2.6 × 1014 cm-2. At higher concentrations,

molybdenum vacancies were also observed (Figure 2d and 2e). The fraction of each type of vacancy as a function of ion dosage is shown in Figure 2g. The vacancy concentration and the Mo:S ratio with helium ion dosage are given in Figure 2h. The as-deposited MoS2 monolayer by CVD is known to contain sulfur vacancies.15 Exposure of the CVD sample to HIM leads to controlled increase in S vacancy concentration up to 2.6 × 1014 cm-2, above which a few Mo vacancies are also observed. At very high ion dosages (> 40 ions-nm-2), substantial S and Mo vacancies form and the atomic structure collapses, consisting of Mo chains with stoichiometry of Mo:S ~ 1:1.2 (see Supporting Figure S1).


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The catalytic activity [current density (mA – cm-2) versus. Potential (V)] of the monolayered MoS2 with different vacancy concentrations was measured using micro-electrochemical cells shown in Figure 3a in which only the basal plane of the MoS2 flakes was exposed to the electrolyte and the edges were covered by Poly(methyl methacrylate) (PMMA). Microelectrochemical cells allow lithographic fabrication of windows with well-defined microscale area so that the exact number of atoms exposed to the electrolyte is known. Thus, by counting the number of vacancies within the area exposed to the electrolyte, it is possible to extract the activity of vacancies. The polarization curves as a function of vacancy concentration are shown in Figure 3b along with those of Pt for comparison. It can be seen that as the vacancy concentration increases, the catalytic activity improves – achieving the lowest overpotential and highest current density at a concentration of 5.7 × 1014 cm-2. Above this value, the catalytic properties degrade – despite the much higher sulfur vacancy concentration. The Tafel slope values were extracted from the polarization curves and are shown in Figure 3c. It can be seen that at low (< 5.7 × 1014 cm-2) and highest vacancy concentrations (1.5 × 1015 cm-2), the Tafel slopes are > 90 mV-dec-1, which suggests that the reaction is limited by the adsorption of protons on the vacancy sites.18,19 The Tafel slope (44 mV-dec-1) at vacancy concentration of 5.7 × 1014 cm-2 suggests that the reaction is limited by desorption of protons.19 We also measured the turnover frequency (TOF, number of hydrogen molecules evolved per second) with vacancy concentration as shown in Figure 3d. We obtain TOFs of >1,000 s-1 and > 10,000 s-1 at overpotential of 200 mV and 300 mV, respectively. Figure 3e shows the intrinsic turnover frequency with vacancy concentration. The variation of TOF and the exchange current density (extracted at 0 V) as a function of the vacancy concentration is summarized in Figure S2a. It can be seen that both values remain largely unchanged with introduction of vacancies, unlike the overpotential and Tafel slope values that improve gradually


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(See Figure S2b). However, at a concentration of 5.7 × 1014 cm-2, there is a dramatic peak in both the TOF (~ 5 s-1) and the exchange current density (~ 0.1 mA-cm-2). As a reference, Pt (111) has an exchange current density of 0.5 – 1 mA-cm-2 corresponding to TOF of ~ 1 s-1.7 Our theoretical results suggest that the increase in catalytic activity at 5.7 × 1014 cm-2 is unlikely to be due to Mo vacancies. This is consistent with the literature in which Mo vacancies have been shown be inactive and it is the Mo atom that mitigates the HER.20 Using the data in Figure 3, it is possible to extract the activity of a single vacancy. That is, since we know precisely the area of the catalyst exposed to the electrolyte, the current density of the catalytic reaction in mA – cm-2 can be accurately obtained. We also know the precise number of vacancies per cm2, making it is possible to extract the current per vacancy. The polarization curves, Tafel slopes and TOF of activity from single vacancies are shown in Figures 4a – 4c. At a vacancy concentration of 5.7 × 1014 cm-2, the intrinsic turn over frequency (TOF) and Tafel slope of a single atomic vacancy is extracted to be ~ 5 s-1 and 44 mV/dec, respectively. Surprisingly, the results suggest that the catalytic activity of a vacancy is not constant. That is, the activity of a vacancy can be enhanced or diminished by controlling their concentration. We closely examined the macroscopic properties and local atomic structure of the material with number of vacancies and found that the structure of MoS2 single layer remains largely undisturbed for vacancy concentrations below 5.7 × 1014 cm-2. That is, Raman spectroscopy (Figure 5a) and HAADF STEM imaging do not reveal any substantial deviation of the atomic structure from the as-synthesized material at low vacancy concentrations (

2.6 × 1014 cm-2). It can be seen that

as-synthesized 2H-MoS2 (black curve) shows peaks at 401 cm-1 for the E2g and 385 cm-1 for the A1g mode. As the vacancy concentration increases, the peak position of E2g downshifts while that of A1g mode upshifts and the peak at ~227 cm-1 that is related to disorder-induced Raman scattering


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increases slightly (Figure 5a). However, photoluminescence (Figure 5b) and electrical measurements (Figure 5c) reveal that subtle changes in the electronic structure occur even at vacancy concentrations below 5.7 × 1014 cm-2. For example, the PL intensity decreases, indicative of increased doping, with introduction of vacancies.21,22 The field effect transistor (FET) measurements shown in Figure 5c reveal a semiconductor to metal transition with increasing vacancy concentration. The FET results show that at 5.7 × 1014 cm-2, the material is metallic and has the highest conductivity at 0 V gate bias. The metallic behavior at high vacancy concentrations can be attributed partially to doping.23 A close examination of the atomic structure near the vacancy reveals the presence of local strain. The local atomic structure of a single layer MoS2 with a vacancy concentration of 5.7 × 1014 cm-2 is shown in Figure 6a, which reveals that the atomic planes deviate substantially from their equilibrium positions. In Figure 6b, the change in interatomic Mo-Mo distance is observed to deviate

10 pm immediately around the vacancy site, which correlates to ~ 3% strain in the

material at a vacancy concentration of 5.7 × 1014 cm-2. In our previous work,8 we demonstrated that ~ 3% strain in monolayered transition metal dichalcogenides leads to an increase in the density of states at the Fermi level, which in turn enhances the conductivity and lowers the free energy to ~ 0 for the limiting reaction in HER. Other studies have also reported presence of ~ 3% strain and sulfur vacancies leads to

~ 0 for the HER.8,16 Thus, the combination of increased

conductivity due to increase in density of states from vacancies, which facilitates electron transport to active sites, along with desirable thermodynamic conditions due to local strain leads to an increase catalytic activity at vacancy concentration of 5.7 × 1014 cm-2.

CONCLUSIONS


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We have investigated the catalytic activity at varying vacancy concentrations and extracted the activity from a single atomic vacancy in MoS2. Our results indicate that along with increasing the number of active sites, the increase in electrical conductivity and presence of strain are essential for improving the catalytic performance of MoS2 for HER. The electrical conductivity improves the charge transfer kinetics,23 while the presence of strain improves the thermodynamics.16 Our results provide an understanding of the activity of single atomic vacancies. The rational control of their concentration provides a method for extracting the activity of individual vacancies. Our results show that the activity of a vacancy is strongly dependent on its local atomic and electronic structure. Understanding these fundamental relationships should provide pathways for designing effective catalysts.


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Figure 1. a) Optical microscope image of CVD-grown MoS2. b) HAADF-STEM image of a single layer MoS2 with vacancy concentration of 5.7 × 1014 cm-2 showing different types of vacancies: Single sulfur vacancy (yellow circles, Vs), double sulfur vacancy (orange circles) and single molybdenum (green circle). c) Intensity profiles along lines 1-4 in image (b).


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Figure 2. He ion irradiated single-layer 2H-MoS2. (a-f) Atomic resolution HAADF-STEM images of single layer of MoS2 with varying vacancy concentrations. The yellow, orange and green circles indicate single sulfur vacancies, double sulfur vacancies, and single molybdenum vacancies, respectively. (a) As-synthesized CVD MoS2 has a vacancy concentration of 6 × 1013 cm-2. (b) He


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ion dose of 0.3 He+nm-2. The vacancy concentration of MoS2 is 1 × 1014 cm-2. (c) MoS2 with vacancy concentration of 2.6 × 1014 cm-2. (Irradiation of He ions with 8 He+nm-2). (d) A dose of 30 He+nm-2 and vacancy concentration was counted as 5.7 × 1014 cm-2. (e) At 100 He+nm-2, pores greater than atomic vacancies are introduced in monolayer MoS2. The atomic arrangement at the edge of the pore is distorted. (f) MoS2 is heavily damaged by high dose of He ions (500 He+nm-2). There is substantial loss of sulfur and molybdenum and the crystallinity of MoS2 is largely destroyed (Scale bar is 1 nm, a-f) (g) The fraction of different vacancies as a function of ion dosage: Single sulfur vacancies (Vs), double sulfur vacancies (V2s), single molybdenum vacancies (VMo) and double molybdenum vacancies (V2Mo). (h) Mo:S ratio with He ion dosage. The atomic ratio was determined by counting S and Mo atoms from the intensity profiles.


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Figure 3. Electrochemical measurements at different vacancy concentrations in MoS2. (a) Optical microscope image of microcell (left image). This cell enables accurate estimation of number of active sites. Scheme of the electrochemical microcell (right image). Single layer MoS2 on SiO2 is contacted by lithographically patterned Au electrode and glassy carbon and Ag/AgCl are used as a counter electrode and reference electrode, respectively. Entire substrate is covered with PMMA except He ions irradiated basal plane of MoS2. Electrolyte solution interacts only with the exposed MoS2 basal plane. (b) Polarization curves for 2H-MoS2 with vacancy concentration of 6 × 1013 cm-2 (black line), 1 × 1014 cm-2 (blue line), 2.6 × 1014 cm-2 (yellow line), 5.7 × 1014 cm-2 (orange line), 1 × 1015 cm-2 (green line) and Pt wire (red line) measured in 0.5M H2SO4 with a scan rate of 5 mVs-1. (c) Tafel slopes of He ion irradiated MoS2 obtained from polarization curves in (b). (d) TOF values of the MoS2 with vacancy concentration of 6x1013 cm-2 (black line), 1x1014 cm-2 (blue line), 2.6 × 1014 cm-2 (yellow line), and 5.7 × 1014 cm-2 (orange line), and other MoS2-based catalysts reported in the literature.7,14,16,24-26 MoS2 basal planes with sulfur vacancies (green),16 strained MoS2 basal plane with sulfur vacancies (purple),14 MoS1.65 nanocrystals (grey square) and edge sites of MoS2 (red star).27 (e) TOF at 0 V versus RHE as a function of vacancy concentration and other MoS2 with sulfur vacancies from literature.14, 16, 26, 27


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Figure 4. The activity of a single vacancy. (a) Polarization curves for a single vacancy at concentrations of 6 × 1013 cm-2 (black line), 1 × 1014 cm-2 (blue line), 2.6x1014 cm-2 (yellow line), 5.7 × 1014 cm-2 (orange line), and 1 × 1015 cm-2 (green line) extracted from Figure 3b. (b) Tafel slopes of single vacancy obtained from 4a. (c) TOF of single vacancy. The value matches with Figure 3d.


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Figure 5. (a) Raman spectra and (b) PL spectra of monolayer MoS2 as a function of helium ion irradiation. Measurements were performed using a green laser (514 nm) excitation wavelength. The laser power was

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