Manganese Doping of MoSe2 Promotes Active Defect Sites for

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Manganese Doping of MoSe2 Promotes Active Defect Sites for Hydrogen Evolution Vasu Kuraganti,†,‡,# Akash Jain,§,# Ronen Bar-Ziv,†,∥ Ashwin Ramasubramaniam,*,⊥ and Maya Bar-Sadan*,†,‡

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†

Department of Chemistry and ‡Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel § Department of Chemical Engineering and ⊥Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States ∥ Department of Chemistry, Nuclear Research Center Negev, Beer-Sheva 84190, Israel S Supporting Information *

ABSTRACT: Transition-metal dichalcogenides (TMDs) are being widely pursued as inexpensive, earth-abundant substitutes for precious-metal catalysts in technologically important reactions such as electrochemical hydrogen evolution reaction (HER). However, the relatively high onset potentials of TMDs relative to Pt remain a persistent challenge in widespread adoption of these materials. Here, we demonstrate a one-pot synthesis approach for substitutional Mn-doping of MoSe2 nanoflowers to achieve appreciable reduction in the overpotential for HER along with a substantial improvement in the charge-transfer kinetics. Electron microscopy and elemental characterization of our samples show that the MoSe2 nanoflowers retain their structural integrity without any evidence for dopant clustering, thus confirming true substitutional doping of the catalyst. Complementary density functional theory calculations reveal that the substitutional Mn-dopants act as promoters, rather than enhanced active sites, for the formation of Se-vacancies in MoSe2 that are known to be catalytically active for HER. Our work advances possible strategies for activating MoSe2 and similar TMDs by the use of substitutional dopants, not for their inherent activity, but as promoters of active chalcogen vacancies. KEYWORDS: catalysis, electrocatalysis, 2D materials, transition-metal dichalcogenides, density functional theory

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active catalytic sites.14 Other studies have suggested that doping of layered TMDs produces new catalytic sites at the dopant atoms or at the adjacent Mo- or Se-atoms.16−28 Here, we propose further activation of 2H-MoSe2 for HER via Mn-doping in a one-pot low-temperature synthesis with high yield. Mn is a nonprecious metal (in contrast to platinumgroup metals usually used as doping agents for catalysis) with affordable cost. The pristine nanoflowers of MoSe2 are slightly Se-deficient and have a large surface area with multiple exposed edge sites and defects, but a significant enhancement in their catalytic activity toward hydrogen evolution is achieved by Mndoping. Using density functional theory (DFT), we calculate the free-energy hydrogen adsorption (ΔGH) on pristine and defective MoSe2 monolayers, with and without Mn-doping, to understand the combined effect of defects and dopants on HER activity. We calculate and compare the free-energy reaction of H adsorption and Se-vacancy formation and elucidate the role of Mn-dopants in altering the limiting

INTRODUCTION Transition-metal dichalcogenides (TMDs) are emerging as potential candidates for use as noble metal-free electrocatalysts for the hydrogen evolution reaction (HER, 2H+(aq) + 2e ⇄ H2(g), that is, the cathodic half-reaction of water-splitting).1−7 Specifically, the edges of the thermodynamically stable phases 2H-MoS28 and 2H-MoSe24,7 are highly active toward HER, but their basal plane is practically inert due to nonoptimal values of the Gibbs free-energy for H adsorption.7,9−11 Thus, many studies have focused on maximizing the number of edges sites in TMDs.4,5,12 Alternatively, it was demonstrated that defect engineering can activate the inert basal plane of 2HMoS2.10,11,13−15 In particular, defects such as S-vacancies form new catalytic sites in the basal plane because the gap states around the Fermi level allow hydrogen to bind directly to exposed Mo-atoms.13 Chalcogen vacancies have been shown to induce a significant boost in the catalytic activity toward HER,13 and subsequent theoretical studies have shown a similar but even more pronounced effect for MoSe2.15 In another study, formation of various defects in MoS2 was examined, concluding that in addition to S-vacancies, defects such as Mo−Mo bonds, S bridges, and so forth produce similar © XXXX American Chemical Society

Received: March 31, 2019 Accepted: June 25, 2019 Published: June 25, 2019 A

DOI: 10.1021/acsami.9b05670 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Scanning electron microscopy images of (a) pristine MoSe2 nanoflowers, (b) 1.7% Mn-doped MoSe2 nanoflowers, and (c) 2.4% Mndoped MoSe2 nanoflowers. TEM images of (d) low-and (e) high-resolution pristine MoSe2 nanoflowers. The HAADF STEM image of 2.4% Mndoped MoSe2 nanoflowers (f) and the elemental EDS mapping of the same frame showing (g) Mo, (h) Se, and (i) Mn.

Table 1. Summary of Composition, Electrochemical Overpotential, Tafel Slope, Charge-Transfer Resistance, Double-Layer Capacitance, and Electrochemical Surface Area of Pristine and Mn-Doped MoSe2 Nanoflowers % Mn (Mn/metals × 100)

Se/metal ratio (Se/metals)

η, over potential (mV)

Tafel slope (mV dec−1)

Rct (Ω)

double-layer capacitance (mF/cm2)

ECSA (cm2)

0 1.7 2.4

1.84 1.90 1.82

225 192 167

83 73 60

210 126 52

2.3 2.5 7

57 60 175

apart, in accordance with the unit cell of MoSe2 (Figure 1a−f). According to elemental mapping by scanning transmission electron microscopy (STEM)−energy-dispersive spectroscopy (EDS), the Mn-dopants are uniformly distributed throughout the nanoflower with the parent elements Mo and Se (Figure 1g−i). Powder X-ray diffraction (XRD) confirmed the hexagonal 2H-MoSe2 crystal structure without secondary phases and without notable shifts in the peak position due to Mn-doping (Figure S1), ruling out intercalation of Mn into the interlayer van der Waals gap. The catalytic activity for HER of the Mn-doped MoSe2 nanoflowers was measured in acidic conditions. Hydrogen production in acidic conditions gains much interest due to its potential application in proton exchange membrane electrolyzers. In addition, the reaction mechanism changes when working in alkaline conditions because of the waterdissociation process, making the use of calculations to understand catalytic activity more complex. The Mn-doped MoSe2 nanoflowers exhibited high performance in 0.5 M H2SO4 (Table 1 and Figure 2). The polarization curves (without iR correction) of Mn-doped MoSe2 nanoflowers show a decreasing trend in the overpotential for the HER at a current density of 10 mA cm−2 from 225 mV for pristine

potentials for these reactions. Our DFT calculations show that Mn-dopants play a key role in improving the HER activity of MoSe2 by promoting the formation of active Se-vacancy sites, while exerting lesser influence on H adsorption on the basal plane near substitutional sites.

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RESULTS For the purpose of this study, which is to gain insights into the intrinsic nature of the catalytic activity, we developed a synthetic process where the morphology of the nanostructures is, by and large, preserved upon doping. We synthesized 1.7% and 2.4% Mn-doped nanoflowers of MoSe2 using colloidal chemistry by a previously reported procedure (see Supporting Information for details).29−32 The colloidal route offers the synthesis of a substantial amount of material with high yield and reproducibility, using low temperatures, and without the need to use specific substrates or a second annealing step, showing an advantage over other complex synthetic methods.33−35 Scanning and transmission electron microscopy images show a similar morphology of flower-like structures for all samples in the range of 300−500 nm with multiple exposed edges comprising 2−5 atomic layers spaced 0.63−0.65 nm B


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the improved HER electrocatalytic efficiency of the Mn-doped MoSe2, electrochemical impedance spectroscopy was conducted to study the electrode kinetics under HER conditions. The charge-transfer resistance (Rct) of the samples was estimated from the Nyquist plots (Figure 2c) by fitting the semicircle in the high-frequency range. The Rct values showed lower charge-transfer resistance for Mn-doped samples compared to pristine MoSe2, indicating a faster charge-transfer process. Furthermore, electrochemical double-layer capacitances (Cdl) were measured to evaluate the electrochemical surface area (ECSA) of the various catalysts (Figure S3). As shown in Table 1, the 2.4% Mn-doped MoSe2 exhibits ECSA that is threefold higher than that of MoSe2, indicating the high fraction of exposed effective active sites, which is responsible for the excellent HER activity. To evaluate the durability of the electrocatalyst, accelerated potential sweeps were conducted repeatedly on the electrodes at a scan rate of 50 mV s−1. After long-term cycling, the Mn-doped catalyst showed very good electrocatalytic stability in 0.5 H2SO4 (Figure S2). The results shown here present lower overpotential for HER compared to recently reported values for other TMD nanostructures such as hybrid structures of Mn-doped MoS2/reduced graphene oxide,37 NiSe−MoSe2 hybrids,38 and even Ru-doped MoSe2 for similar degree of substitution.32 To achieve insights into the possible mechanistic role of Mndopants in enhancing the HER activity of MoSe2, we turn to first-principles DFT studies. First, we note that substitution of an Mo(IV) d2 atom by an Mn(IV) d3 atom introduces one unpaired d-electron (net magnetic moment of +1 μB) and leads to the emergence of gap states within the band gap of the pristine MoSe2 layer; this is exactly analogous to previous reports on Mn-doped MoS2.39,40 The introduction of these gap states closer to the Fermi level is expected to enhance hydrogen adsorption as shown below. Second, we also find (Figure S4) that the Mn-doped monolayers are thermodynamically stable with heats of formation that are intermediate between those for the ground-state phases of pristine MoSe2 and MnSe2 monolayers. Thus, the structures that we study for electrocatalytic activity in more detail below can indeed be expected to be experimentally relevant. The free-energy of hydrogen adsorption (ΔGH) is a useful descriptor for HER activity in acid41−43 and, as per the Sabatier principle,44 an ideal HER catalyst has ΔGH ≈ 0 eV.43 Figure 3 displays the calculated values of ΔGH under standard conditions [300 K and a potential of 0 V vs reversible hydrogen electrode (RHE)] at Se top sites and Se-vacancy sites on the basal plane of pristine and Mn-doped MoSe2. The free-energy for hydrogen adsorption was calculated as

Figure 2. (a) Polarization curves, (b) Tafel slopes, and (c) Nyquist plots of Mn-doped MoSe2 nanoflowers. Dashed lines show the performance of a benchmark, commercial Pt/C catalyst.

ΔG H = ESL + H − ESL −

MoSe2 to 167 mV for 2.4% Mn-doped nanoflowers (Figure 2a). The HER kinetics of the catalysts were analyzed by corresponding Tafel plots (η vs log j) (Figure 2b) by fitting the linear portions of the Tafel plots to the Tafel equation (η = b log j + a, where j is the current density and b is the Tafel slope). A Tafel slope of 60 mV dec−1 is observed for the 2.4% Mn-doped MoSe2 catalyst, much smaller than the slopes for pure MoSe2 nanoflowers (83 mV dec−1). This result suggests that HER proceeds via the Volmer−Heyrovsky mechanism with faster kinetics for the Mn-doped nanoflowers, implying that the free-energy barrier of the discharge step becomes comparable with that of the following desorption step, resulting in the slope of 60 mV dec−1.5,36 To further confirm

E H2 2

+ ΔEZPE − T ΔS

(1)

where ESL+H, ESL, and EH2 are the DFT total energies of a single layer (SL) of MoSe2 (pristine or Mn-doped) with an adsorbed hydrogen atom, the SL alone, and a H2 molecule in the gas phase, respectively; ΔEZPE and ΔS are the differences in zeropoint energies and entropies, between an adsorbed hydrogen atom and its reference state in the H2 molecule. In agreement with the previous reports,7−11 we find that the basal plane of pristine MoSe2 is inert toward HER with a highly unfavorable ΔGH of nearly 2 eV at a Se top site (Figure 3a; Table 2). On inserting a single Mn-atom into MoSe2, ΔGH at the Se top site adjacent to the Mn-atom (Figure 3a) decreases to ∼1 eV. This C


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Figure 3. Free-energy of H adsorption (ΔGH) at (a) Se sites and (b) Se-vacancies (VSe) on the basal plane and edges of single-layer MoSe2 with and without Mn-doping, calculated at standard conditions (T = 300 K and 0 V vs RHE). (c) Atomic models illustrating the various H adsorption sites studied; Se-atoms removed to create vacancies are highlighted in different colors.

while the thermodynamics of HER is much more favorable at edges than on the basal plane,7,45 the Mn-dopant systematically increases ΔGH by 60−90 meV. In the case of the Mo edge with the 50% Se coverage, the shift in ΔGH is beneficial, producing a near optimal value of ΔGH ≈ 0; however, for the other two edges, the increase in ΔGH is detrimental to the HER thermodynamics. We therefore conclude that the experimentally observed enhancement of HER activity is unlikely to arise merely from the electron-donating effect of Mn. It is well known that chalcogen point defects in TMDs are active for several catalytic reactions including HER.10,11,13,46 In particular, the introduction of a Se-vacancy in the MoSe2 monolayer results in an ideal active site for HER with ΔGH ≈ 0 eV (Figure 3). For Mn-doped MoSe2, we calculated the Sevacancy formation energy as a function of distance from the substitutional Mn-atom and found it to be energetically favorable (by ∼0.8 eV) to form a Se-vacancy at the nearestneighbor (NN) site of the Mn-dopant relative to the formation energy in pristine MoSe2 (Figure S7). In light of this result, we also examined the formation energy of a Se-vacancy at, and in the vicinity of, edges of MoSe2 nanoribbons. Considering both basal plane and edge sites, we find that the Mn-dopant reduces the formation energy of an adjacent Se-vacancy (Figure 4) over a range of 0.12−0.80 eV. We attribute this effect to the excess d-electron of Mn relative to Mo, which lowers the energetic cost of dangling bonds associated with Se-vacancy formation. These results indicate that Mn-dopants are more likely to be associated with Se-vacancies in the basal plane and at edges. We have not explored the influence of Mn-dopants on the formation energies of vacancy clusters here, although it is known that pre-existing chalcogen vacancies can reduce the energetic cost for formation of neighboring vacancies, leading to catalytically active chalcogen vacancy clusters.10,44 In Figure 3b and Table 2, we report ΔGH for hydrogen adsorption at Sevacancies adjacent to Mn-dopants in the basal plane and at various edges. For an Se-vacancy site adjacent to a single Mn-

Table 2. Free-energy of H-Adsorption (ΔGH) and Limiting Potentials (vs RHE) for HER (UL,HER) and DSR (UL,DSR) at the Basal Plane and Edges of Undoped and Mn-Doped MoSe2 undoped MoSe2

Mn-doped MoSe2

adsorption site

ΔGH (eV)

UL,HER (V)

UL,DSR (V)

ΔGH (eV)

UL,HER (V)

UL,DSR (V)

basal plane basal plane, VSe Se edge Se edge, VSe Mo edge (θSe = 0.5) Mo edge (θSe = 0.5), VSe Mo edge (θSe = 1.0) Mo edge (θSe = 1.0), VSe

+1.95 +0.01

−1.95 −0.01

−1.28

+1.06 −0.09

−1.06

−0.88

0.00 −0.18 −0.08

0.00

−0.47

+0.07 −0.12 −0.02

−0.07

−0.07

+0.08

−0.08

+0.13

−0.13

+0.50

−0.50

+0.56

−0.56

+0.38

−0.38

−0.51

−0.26

−0.16

−0.20

−0.06

effect of the Mn dopant on the hydrogen adsorption energy is localized to the immediate vicinity (nearest neighbors) of the defect, and adsorption sites that are further away are equivalent to those on undoped/pristine MoSe2 (Figure S5). While, in principle, the decrease in ΔGH is substantial, the thermodynamics (ΔGH ≈ 1 eV) are still not ideal for HER. As Mndopants in the MoSe2 monolayer have attractive interactions (Figure S6), we also considered the role of two Mn-atoms at various neighbor separations (Mo substitutional sites) and found that the influence of such clusters on ΔGH is negligible and cannot improve the HER thermodynamics (Figure S5). As an alternative to basal sites, we studied catalytic sites at Se edges and at Mo edges with 50% (θSe = 0.5) and 100% (θSe = 1.0) Se-coverage (Figure 3a); Table 2 displays ΔGH calculated at the most stable binding sites along these edges. In general, D


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reaction free-energy, ΔG, at an applied electrode potential, URHE, is calculated as50 ΔG = ΔE + ΔEZPE − T ΔS − neURHE

where ΔE is the DFT (0 K) reaction energy, ΔEZPE is the difference of zero-point energies of products and reactants, T is the temperature (300 K at standard conditions), ΔS is the change in entropy for the reaction, and n is the number of (H+ + e−) pairs transferred. At URHE = 0 V (low overpotential range), both HER and the DSR are thermodynamically uphill on single-layer, Mn-doped or pristine MoSe2 (Figure S9). Upon increasing the cathodic potential, the formation of a Sevacancy by transfer of two consecutive H+ + e− pairs to MoSe2 becomes exergonic at URHE = −1.28 and −0.88 V on undoped and Mn-doped MoSe2, respectively. However, the Volmer step (eq 3), which is essential for HER to proceed, is still energetically uphill at this point, and larger cathodic potentials (URHE = −1.95 and −1.06 V on undoped and Mn-doped MoSe2, respectively) are required to render this step exergonically. Once a Se-vacancy is formed, though, HER proceeds with very low limiting potentials, with or without Mn-dopants, via the Volmer−Heyrovsky mechanism (as deduced from experiments)

Figure 4. Formation energy of a Se-vacancy in the basal plane (BSL); at Se edges and Mo edges at 50% (θSe = 0.5) and 100% (θSe = 1.0) Secoverage, respectively; and at near-edge sites on Mo edges. Structural models of these sites are displayed in Figure 3c. The vacancy formation energy is calculated as Evacancy = EH2Se + EMoSe2−v(+Mn) − EMoSe2(+Mn) − EH2, where EH2Se, EMoSe2−v(+Mn), EMoSe2(+Mn), and EH2 are the energies of the H2Se molecule, pristine or Mn-doped SL-MoSe2 containing a Se-vacancy, pristine or Mn-doped SL-MoSe2 without Sevacancies, and the H2 molecule, respectively.

atom, ΔGH decreases only slightly (by ∼0.1 eV) from its value in undoped MoSe2. At edges, the influence of the Mn-dopant on H adsorption is much more variable: at the Se edge and Mo-θSe = 0.5 edge, the effect is weakly destabilizing (by 60 and 70 meV, respectively); at the Mo-θSe = 1.0 edge, the effect is strongly stabilizing (by 0.44 eV). For completeness, we also considered the case of a Se-vacancy site between a two-atom Mn cluster (1NN or 2NN positions); as seen from Figure S8, such sites bind the hydrogen atom with a significantly higher ΔGH (−0.2 to −0.5 eV). Hence, it is likely that Se-vacancy sites within regions of Mn-clustering are poisoned by H, especially at higher cathodic potentials. In short, our thermodynamic calculations indicate that Mn-dopantsin particular, single atomspromote the formation of Sevacancies in the basal plane and at edges, and that this effect is more energetically significant than changes in hydrogen adsorption energies brought about by doping. Chalcogen vacancy defects are naturally generated during the synthesis of TMDs but can also be generated postsynthesis by methods such as plasma treatments, electron/ion irradiation, or annealing.47,48 Recently, a new route was suggested for generation of S-vacancy in MoS2 via electrochemical reduction of S to H2S.10 Similarly, Se-vacancies can also be generated in the basal plane of MoSe2 via an electrochemical deselenization reaction (DSR),9 in which the proton-coupled electron transfer converts Se-atoms to H2Se gas, via *Se + (H+ + e−) → *SeH

(2)

*SeH + (H+ + e−) → H 2Se(g) + *

(3)

(4)

* + (H+ + e−) → *H

(5)

*H + (H+ + e−) → H 2(g) + *

(6)

The behavior of limiting potentials at edge sites is more nuanced: a few broad trends may nevertheless be gleaned from the data in Table 2. First, at undoped Se and Mo-θSe = 0.5 edges, HER is already favorable at ∼0 V, and Mn-doping increases these limiting potentials consistent with the destabilizing effect on H adsorption noted before; this holds true both with and without Se-vacancies. The exception to this trend is the Mo-θSe = 1.0 edge where the Mn−Se vacancy complex appreciably reduces the limiting HER potential (by ∼0.5 V) due to strong stabilization of H adsorption. Second, Mn-doping consistently reduces the limiting potential for DSR and brings the limiting potentials to 0.1−0.2 eV for all Mndoped edges. Lastly, for the Mo-θSe = 1.0 edge, the limiting potential for DSR is lower than the corresponding value for HER both with and without Mn-doping. This result suggests that the Mo-θSe = 1.0 edge could evolve toward lower Secoverage, which is beneficial considering that the Mo-θSe = 0.5 edge is optimally active for HER. We emphasize that these limiting potentials indicate general trends and should not be directly compared to the experimental onset potentials as the calculations here are limited to understanding the first DSR or HER event on a pristine basal plane/edge, which is far from being a representative of the complex catalyst surface. Based on the above thermodynamic analyses, we may draw a few mechanistic insights into the role of Mn-dopants in enhancing the HER activity of MoSe2. While Mn reduces appreciably the adsorption free-energy of hydrogen on the MoSe2 basal plane, the limiting potentials are still excessively large. Mn-dopants could promote the formation of basal-plane Se-vacancies during synthesis of the Mn-doped samples due to the significant reduction in formation energies (∼0.8 eV), but further electrochemical generation of such basal-plane vacancies still requires large negative overpotentials. The reaction thermodynamics at MoSe2 edges is conducive to HER, and Mn-dopants consistently reduce the limiting potential for electrochemical Se-vacancy generation at edges.

where *Se is the reacting Se-atom in the basal plane and * is the corresponding Se-vacancy formed. HER is also initiated via eq 2 (Volmer step) and the subsequent Tafel or Heyrovsky step to produce H2(g)49 and is in direct competition with DSR. To understand the propensity for HER versus deselenization on our (doped) MoSe2 samples, we employed the computational hydrogen electrode50 model and calculated reaction free energies (Table 2; Figure S9 for these two reactions). The E


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calculations, one H atom was adsorbed on the surface of the ML and Mo/Se edges, and dipole corrections60 were applied normal to MoSe2 sheets and along the width of the nanoribbons.

The DSR limiting potentials at edges are much lower than for basal-plane vacancies and, moreover, competitive with HER limiting potentials. Overall, Mn-dopants appear to act as promoters for the formation of Se-vacancies, which, once formed, are excellent active sites for HER. While we have only focused on one class of point defects as an example here, grain boundaries within nanoflowers could also play an important role in HER, and it would be interesting to understand the interplay of Mn-dopants with these defects in the future.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05670.

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CONCLUSIONS In conclusion, we developed and synthesized Mn-doped MoSe2 nanoflowers that enabled us to compare the catalytic activity of the pristine and doped samples. The doping of Mn via the solvothermal approach is a one-pot synthesis pathway to produce substantially doped materials with high yield. The Mn was evenly distributed throughout the structure, and XRD showed no minor phases or expansion of the structure due to intercalation and hence, we concluded that the Mn is a substitutional dopant. DFT calculations revealed that Mndoping is an efficient approach for promoting the DSR at lower cathodic potentials thereby generating Se-vacancies, which are the active catalytic sites for HER. Our work thus advances possible strategies for activating MoSe2 and similar TMDs by the use of dopants, not as inherently active sites, but as promoters for the formation of active chalcogen vacancies.

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ASSOCIATED CONTENT

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Detailed synthesis protocols, XRD diffraction patterns of the Mn-doped samples, stability measurements, electrochemical surface area (ECSA) measurements, and additional DFT calculations (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.R.). *E-mail: [email protected] (M.B.-S.). ORCID

Ronen Bar-Ziv: 0000-0003-3082-7845 Ashwin Ramasubramaniam: 0000-0001-6595-7442 Maya Bar-Sadan: 0000-0002-1956-8195 Author Contributions #

Equal contribution.

Notes

The authors declare no competing financial interest.

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EXPERIMENTAL SECTION

ACKNOWLEDGMENTS A.J. and A.R. gratefully acknowledge research support from the National Science Foundation NSF-CBET-1803614. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1548562. This research was supported by the United StatesIsrael Binational Science Foundation (BSF) and the United States National Science Foundation (NSF), grant no. 2017642.

Undoped and Mn-doped MoSe2 nanoflowers were synthesized by the colloidal method described elsewhere.30 In essence, Se−ODE stock solution was mixed with the required quantities of cation precursors (Na2MoO4·2H2O and MnCl2·4H2O) and added to 8 mL oleic acid and 2 mL of 1-octalamine. The solution was heated to 240 °C, and then, 2 mL of ODE−Se stock solution (0.1 M) was injected into the reaction vessel, and the temperature was raised to 300 °C and maintained for 30 min. The electrochemical HER activity was examined by a linear scan voltammeter in 0.5 M H2SO4 (pH ≈ 0.25) solution using a standard three-electrode electrochemical workstation. The catalyst ink was prepared by adding 4 mg catalyst and 2 mg Vulcan carbon black in a mixture of 200 μL of ethanol, 800 μL of water, and 80 μL Nafion solution (5%). A detailed description of the synthesis, the electrochemical measurements, and the experimental instrumentation is available in the Supporting Information. Computational Methods. DFT calculations were performed using Vienna Ab initio Simulation Package (version 5.4.1).51,52 Core and valence electrons were described using the projector-augmented wave method,53,54 and electron exchange and correlations were described using the Perdew−Burke−Ernzerhof55-generalized gradient approximation. All calculations are spin-polarized to account for the magnetic moment of the Mn-dopants.39 A kinetic energy cutoff of 400 eV was established from convergence tests, and Brillouin zone integrations were performed with a Gaussian smearing of 0.05 eV. Structural optimization was performed using the conjugate-gradient algorithm until the Hellmann−Feynman force on each atom was below 0.01 eV/Å. The relaxed lattice parameters for the 2H-MoSe2 monolayer are 3.32 Å (experimental value, 3.29 Å56). All adsorption and dopant calculations for the MoSe2 ML basal plane were performed in a 6 × 6 MoSe2 supercell with periodic boundary conditions and a minimum of 10 Å of vacuum between periodic images of the 2D sheets. The Brillouin zone was sampled using a 2 × 2 × 1 Γ-centered k-point mesh. For studies of Mo and Se edges, we used a 4 × 4 MoSe2 supercell, as in previous studies7,57−59 with periodic boundaries applied along one direction (producing a nanoribbon) and at least 10 Å of vacuum in the nonperiodic directions (Figure S12). The Brillouin zone for the nanoribbons was sampled using a 1 × 3 × 1 Γ-centered k-point mesh. For ΔGH

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NOTE ADDED AFTER ASAP PUBLICATION Due to a production error, the term “free-energy” was mistakenly deleted from the paper multiple times and posted on the Web on July 3, 2019. The paper was corrected, and the corrected version was reposted on July 5, 2019.

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Manganese Doping of MoSe2 Promotes Active Defect Sites for

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