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Auto-Optimizing Hydrogen Evolution Catalytic Activity of ReS2 through Intrinsic Charge Engineering Yao Zhou, Erhong Song, Jiadong Zhou, Junhao Lin, Ruguang Ma, Youwei Wang, Wujie Qiu, Ruxiang Shen, Kazutomo Suenaga, Qian Liu, Jiacheng Wang, Zheng Liu, and Jianjun Liu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00693 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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Auto-Optimizing Hydrogen Evolution Catalytic Activity of ReS2 through Intrinsic Charge Engineering Yao Zhou1, 2, #, Erhong Song1, #, Jiadong Zhou3, #, Junhao Lin4, Ruguang Ma1, Youwei Wang1, Wujie Qiu1, Ruxiang Shen1, Kazutomo Suenaga4, Qian Liu1, 5, Jiacheng Wang1, 5, *, Zheng Liu3, *, Jianjun Liu1, 5, *

1

The State Key Laboratory of High Performance Ceramics and Superfine Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China.

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Center for Programmable Materials, School of Materials Science and Engineering, Nanyang

Technological University, Singapore 639798, Singapore.

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National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565,

Japan

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Shanghai Institute of Materials Genome, 99 Shangda Road, Shanghai 200444, P. R. China

# These authors contributed equally. *E-mail: [email protected]; [email protected]; [email protected]

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ABSTRACT Optimizing active electronic states responding to catalysis is of paramount importance for developing high-activity catalysts because thermodynamics itself may not favor forming an optimal electronic state. Setting monolayer transition metal dichalcogenide (TMD) ReS2 as a model for hydrogen evolution reaction (HER), we uncover that intrinsic charge engineering has an auto-optimizing effect on enhancing catalytic activity through regulating active electronic states. The experimental and theoretical results show that intrinsic charge compensation from S to Re-Re bonds could manipulate the active electronic states, allowing hydrogen to absorb the active sites neither strongly nor weakly. Two types of S sites exhibit the optimal hydrogen adsorption free energies (∆GH*) of 0.016 and 0.061 eV, which are the closest to zero corresponding to the highest HER activity. This auto-optimization via charge engineering is further demonstrated by higher turnover frequency (TOF) per sulfur atom of 1-10 s-1 and lower overpotential of -147 mV at 10 mA cm-2 than those of other TMDs through multi-scale activation and optimization. This work opens an avenue in designing extensive active catalysts through intrinsic charge engineering strategy.

Keywords: monolayer, transition metal dichalcogenides, metal-metal bonds, theoretical calculation, hydrogen evolution reaction

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Generating active electronic states responding to catalysis through controlling specific surface structure and making defects is fundamental base to design high-activity catalysts.1-8 However, it may still deviate from an optimal electronic state associated with a high catalytic activity due to thermodynamic driving force.9, 10 Therefore, further optimizations such as strain,11, 12 doping13 and substrate14 usually are required to regulate frontier electronic states. Starting from an inert structure, activation and optimization are usually simultaneously applied in catalyst design.15-17 Developing an efficient strategy to realize high catalytic activity is of much importance for pushing forward practical application of catalysts. A typical case of catalytic activation/optimization is transition-metal-based catalyst design for hydrogen evolution reaction (HER) which is considered as an important technology to solve energy crisis and environment pollution.18-20 Among all available catalysts, transition-metal dichalcogenides (TMDs) such as MoS2 and WS2, have attracted great interests as promising HER electrocatalysts.21-24 No matter which of edge sites and basal planes are used as active sites, a simple activation has its restriction to achieve an optimal catalytic activity. Therefore, a great deal of effort has been made to further optimize these active sites by engineering techniques.14, 25, 26

Sulfur vacancy of MoS2 has a catalytic activity with an overpotential of -250 mV at 10 mA

cm-2 (η10) and ∆GH* = 0.08 eV. Further strain optimizing sulfur vacancy in MoS2 can generate η10 of -170 mV and ∆GH* nearly close to 0 eV,11 showing the enhancement of further optimization. Substrate effect and chemical doping for activated MoS2 and WS2 are also demonstrated to have optimizing effect on HER catalytic activity. In nature, activation and optimization have different

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effects on making and regulating active electronic states of catalytic sites, respectively. Therefore, it is of much importance to develop high-efficiency strategy to optimize catalytic activity through regulating active electronic states.11

Figure 1. Schematic illustration of charge compensation effect on regulating active electronic states in TMDs (C = chalcogen, TM = metal). (a) Without TM-TM bonds, only gap states around Fermi level are generated by dangling atom (C); (b) with TM-TM bonds, charge in anionic C is transferred to TM-TM bonds due to uplifted states of TM-TM bonds.

Inspired by the conception of anion-cation charge exchange,27 we propose some TMDs (ReS2, ReSe2, WTe2 et al.) containing TM-TM σ bonds which would provide electron reservoir effect for regulating electronic state of active S. Charge compensation from anion to TM-TM σ bonds is very likely to take a further optimization role for active electronic states, enhancing catalytic activity. Figure 1a-b exhibit difference in active electronic states between TMDs without and with TM-TM bonds, respectively. Activated p-bands of chalcogen (pC) in TMDs without TM-TM do not have too much interaction with inactive TM bands and are isolated across Fermi-level. In 4


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contrast, active electronic states of TM-TM bonds and anionic atoms coexist near Fermi level. Due to p-d orbital hybridization between anions (C) and transition metals (TM) inducing redox reactions,11,

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this charge compensation from pC to σTM-TM may occur and have effect on

mediating charge distribution and electronic states of anions. The comparison suggests that TM-TM bonds may have a spontaneous optimization effect on active electronic states, possibly achieving a high catalytic activity. Therefore, it is of much importance to seek some TMDs with TM-TM bonds, which can form dual activations of C and TM-TM bonds. More importantly, TM-TM bonds have intrinsic charge engineering effect on optimizing active electronic state of C, possibly achieving a high catalytic activity. Here, among several TMDs with TM-TM bonds, monolayer ReS2 was selected as a catalyst model to describe charge regulation effect on optimizing catalytic activity. Compared to other TMDs with TM-TM bonds ReS2 has more types of TM-TM bonds27 to perform varied mediating effects of electronic states, which is more suitable as the model to deep understanding TM-TM bonds. The theoretical calculations predicted that the activated Re-Re bonds, serving as electron reservoirs for intrinsic charge regulation, enable ReS2 to achieve an optimal ∆GH* = 0 toward electrocatalytic HER. To verify auto-optimization effect, monolayer ReS2 with Re vacancy (VRe-ReS2) was synthesized via a potassium iodide (KI)-aided chemical vapor deposition (CVD) process. The activity of as-prepared monolayer ReS2 surpasses the reported monolayered TMDs without TM-TM bonds, confirming the theoretical prediction on charge regulating effects of TM-TM bonds on HER activity. This work highlights the inherent TM-TM bonds can be used to

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screen known materials as HER catalysts, providing a strategy for highly active catalyst design.

RESULTS AND DISCUSSION

Figure 2. Calculated hydrogen adsorption free energies of multiple active sites in VRe-ReS2. (a) The optimized structure of VRe-ReS2. (b) Enlarged defect structure based on the optimized structure. (c) ∆GH* in six exposed S atoms around VRe in VRe-ReS2, ReS2 without Re vacancy, VMo-MoS2, MoS2 without Mo vacancy. Here, these exposed S atoms are considered as the catalytic sites of HER. The Sn (n=1-6) correspond to the labeled number of S atoms in panel b.

To avoid some interruptions such as layer-dependence, interfaces or morphological effect,25, 28 monolayer 1T'-ReS2 was selected as the model to explore charge regulating effect of TM-TM bonds on catalytic activity. 1T'-ReS2 has a set of Re-Re metal bonds29-31 and distorted octahedral 6


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coordination between S and Re (Figure S1). Re-defect was introduced to activate S and Re-Re bonds through forming dangling bonds. Figure 2a-b exhibits the relaxed structure of monolayer ReS2 with Re-defect (VRe-ReS2). The activity of Re and S atoms (inactive R1-R3; active R4-R6 and S1-S6) is simply determined by whether they are originally bonded with defected Re in pristine structure. Projects density of states (pDOS) difference between pristine ReS2 and VRe-ReS2 (Figure S2) reveals VRe generates gap states contributed by activated S and Re-Re bonds (R3-R4 and R5-R6). It indicates that Re-Re bonds are very likely to participate internal redox reactions to optimize catalytic activity in S atoms, which is consistent with our prediction presented in Figure 1. ∆GH* is an effective descriptor for correlating theoretical predictions with experimental measurements of catalytic activity for various systems.32 The optimal value of ∆GH* is equal to 0.0 eV, where adsorbed atomic hydrogen is in a thermoneutral state and can perform efficient proton/electron-transferred and hydrogen release.33 The calculated ∆GH* (optimized structure see Figure S3) and detailed local structures on different six sulfur atoms of VRe-ReS2 are exhibited in Figure 2c. As a reference, S atoms in pristine ReS2 has the largest positive ∆GH* (1.425 eV) corresponding to the weakest S-H bonding strength. In comparison, active S atoms (S1-S6, six S toms) have much stronger bonding strength from -0.558 to 0.191 eV. More importantly, their catalytic activities strongly depend on activities of direct linked Re-Re bonds. S1 and S2 connected with inactive Re-Re bonds exhibit strong bonding strengths (-0.396 and -0.558 eV), whereas S3-S6 connected with active Re-Re bonds have moderate or weak bonding strengths

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(0.016, 0.061, 0.157 and 0.191 eV), respectively. Besides Re-Re activation effect, the relative coordinate structures between S and active Re-Re bonds also play an important role in determining catalytic activity. The W-shaped S atoms are found to have more catalytic activity than T-shaped S atoms. It is emphasized that S4 has near -zero ∆GH* (0.016 eV) which is better than the most of previously reported 2D TMDs (Figure S4). For comparison, the ∆GH* of VMo-MoS2 was calculated as -0.691 eV associated with a strong S-H bonding, requiring further optimization for reducing electron density of S sites. Without further optimization, a simple vacancy activation can generate an optimal ∆GH*≈ 0 in VRe-ReS2, implying a possible auto-optimization effect of active electronic states.

. Figure 3. Electronic structure analysis of VRe-ReS2. (a-c) Projected density of state (pDOS) before H absorbed on S5, S4 and S1 atoms of VRe-ReS2. (d-f) Partial orbitals of VRe-ReS2 in the energy range of -2.0~0 eV. The parts highlighted by red dashed circles correspond to active electrons which are responsible for electron transfer to bind H. (g) The correlation between ∆GH* and the transferred electron numbers from S to H on VRe-ReS2. The R2 value of linear fitting is 0.825. (h) The relationship of electron amount in S with ∆GH* for different T and W shapes of VRe-ReS2. 8


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The Re-Re-bond dependent catalytic activities of different S atoms can be analyzed by pDOS, hybridized orbital, quantitative charge distribution on S atoms (Figure 3 and Figure S5). PDOSs of S5, S4, S1, corresponding to weak, moderate, and strong adsorption strengths, exhibit active electronic state change by combining with Re-Re bond activity. In a strong adsorption, some high-energy occupied states (-0.5~0.5 eV) are accumulated across Fermi-level, which is attributed to negligible electronic compensation from S1 to inactive Re-Re bond (R1-R2). In contrast, charge compensation from S (S4 and S5) to activated Re-Re bonds (R3-R4 and R5-R6) occurs through p-σ orbital hybridization, reducing these high-energy occupied states and forming lower hydrogen adsorption energies. As shown in Figure 3d-f, these charge evolutions can be vividly illustrated by orbital contribution in active electronic states (0 ~ -2 eV), which was reflected by the amount of electron distribution surrounded by red circles. The charge compensation from S to Re-Re bonds are quantitatively correlated with ∆GH* for exhibiting charge regulation effect on catalytic activity. We plotted ∆GH* values of all six active sites as a function with transferred electron amount from S to H (Table S1) which are essentially equivalent with active electrons (gap state electrons) operating HER catalysis. As shown in Figure 3g, there exists a linear correlation between ∆GH* and active electron amount, indicating charge compensation effect on catalytic activity. The regulation effect is further verified by relationship between S-H bond length and ∆GH* (Figure S6a and Table S2). In nature, activity of Re-Re bonds corresponds to their electron-acceptance ability which can further be explained by

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band gaps of Re-Re bonds. A lower band gap usually has a larger electron-acceptance ability, which is consistent with our correlation of ∆GH* with band gap (Figure S6b and c, calculation details see Supplementary Note 1). Besides electron-acceptance ability of active Re-Re bonds, two types of coordinated structures between S and Re-Re bonds, W- and T-shape also show different electron acceptance ability. As shown in Figure 3h, W-shaped structures display clearly more electrons transferred from S to Re-Re bonds. It is attributed to orbital hybridized strength corresponding to electron transfer ability from S to Re-Re bonds. Above analysis indicates that intrinsic charge compensation plays an important role in optimizing electronic state and catalytic activity.34 The Re-Re bonds in different activity and coordinate structures form various regulation effect on charge distribution of S atoms, which presents a charge engineering effect on achieving optimal catalytic activity of ∆GH* = 0.0 eV. In nature, such optimization effect replaces additional engineering optimization such as substrate, strain and doping. To verify the charge engineering effect of TM-TM bonds on auto-optimizing HER activity, monolayer ReS2 with VRe (VRe-ReS2) was synthesized for HER testing. Figure 4a shows the reaction system for the growth of large-area continuous ReS2 film with tens to one hundred microns in size (Figure 4b and Figure S7). A detailed growth mechanism is discussed in Note 2 and Figure S8 of Supporting Information, showing that KI is helpful to increase the film size of monolayer ReS2 (Figure S9). After synthesis, several times DI water washing was applied to

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remove the left K and I. XPS survey (Figure S10a) confirms their removal, which is beneficial to discuss the effect of VRe and Re-Re bonds without other dopant’s interruption. The structural characterizations by atomic force microscopy (AFM) and Raman spectrum clearly display the formation of ReS2 monolayer (Figure 4c and S10b). Such large continuous monolayer through the KI-aided technique ensures maximum exposure of active sites in basal planes of as-grown ReS2 and reduces the proportion of edge-site active sites.11 This monolayered structure provides a well-defined system for observing VRe and Re-Re bonds, favoring to link the experimental results with theoretical calculations toward the HER.

Figure 4. (a) Schematic illustrationof VRe-ReS2 film by KI-assisted CVD approach. (b) Optical microscopyimage of VRe-ReS2 with selected areas. (c) AFM image of as-prepared ReS2 film. (d) STEM Z-contrast image with an inset of FFT patterns of monolayer VRe-ReS2. VRe were highlighted in the yellow circles. (e) XPS valence bands of ReS2 and VRe-ReS2. Inset is the magnified range between -2 to 3 eV. (f) Re 4f and (g) S 2p spectrum of sample VRe-ReS2 and ReS2. Re 4f peaks of VRe-ReS2 shifted to the position with

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higher binding energy compared to that of ReS2, whereas S 2p peaks shifted to the position with low binding energy, demonstrating the existence of charge transferring.

High-resolution Z-contrast scanning transition electron microscopy (STEM) image shows that VRe is omnipresent and can be seen apparently by the discontinuity of Re chains in the diamond-like unit cell, as highlighted by the dashed circles (Figure 4d). VRe are originated from the ReS2 decomposition induced Re evaporation.35 It is noted that S atoms are nearly invisible and the Fourier transform (FFT) pattern in the inset shows the distorted 1T′ structure of ReS2. The Re/S atomic ratio of 0.96:2 determined by energy-dispersive X-ray spectroscopy also indicates a slight deficiency of Re (Figure S11 and Table S3), agreeing with STEM observation. The electronic structure change induced by VRe was probed using XPS valence bands (Figure 4e). The shift of the position (0.94 eV) of valence band maximum (VBM) to a lower level of binding energy (0.23 eV) after introducing VRe demonstrates that more active electronic states were created by VRe, supporting the theoretical prediction of VRe activating effect. The charge transfer from S atoms to Re-Re bonds is further examined by experimental X-ray photoelectron spectroscopy (XPS) measurements. The binding energies of S 2p negatively shifted by ca. 0.19 eV by comparing with the pristine ReS2 (Figure 4f and g). In contrast, the binding energies of Re 4f are positively shifted by ca. 0.24 eV. These changes demonstrate electronic density decreases in S but increases in Re, reflecting a charge compensation from S to Re. Notably, the charge compensation phenomenon was not observed in MoS2,36 which illustrates the determinative effect of TM-TM bonds on charge compensation. The TEM and

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XPS observations combining with electronic structure analysis elucidate that VRe not only activates S and Re-Re bonds, but also induces charge transfer from S to Re-Re bonds. The previous studies indicate that TM vacancy in TMDs usually leads to too strong hydrogen binding between S and H since too much electrons are accumulated in active S sites. The present studies show that “extra” electrons in active S atom could be discharged into electron-deficient Re-Re bonds, which can reduce S-H bonding strength for optimizing ∆GH*. To evaluate the optimization effects of charge engineering, the HER catalytic activity of monolayer VRe-ReS2 was examined by using a three-electrode electrochemical cell in 0.5 M H2SO4, in comparison to glass carbon electrode (GCE), monolayer VMo-MoS2, monolayer MoS2 without VMo, monolayer ReS2 without VRe (See experimental section and Figure S12 in Supporting Information). Figure 5a shows linear-sweep voltammograms in the cathodic direction corrected for ohmic potential drop (iR) losses, normalized to the projected geometric area of the electrode. As expected, the blank GCE shows a poor HER activity. Both monolayer MoS2 and ReS2 have a very low HER activity, implying the inactive basal planes of these monolayers. This observation agrees well with their theoretical ∆GH* values (1.425 eV for ReS2 and 1.925 eV for MoS2, Figure 2c). The introduction of metal vacancies into the basal planes of MoS2 and ReS2 could moderately improve the HER activity of VMo-MoS2 and VRe-ReS2 monolayer. The calculated turnover frequency per surface S atom (TOFS) of VRe-ReS2 (1-10 s-1) is much higher than that of ReS2 (Figure 5b and Supplementary Note 3), indicating the exposed S sites around VRe are highly active for HER. The η10 shifts positively when concentration of VRe

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increases to no more than 1.7 % (concentration of VRe obtained from XPS results in Table S4) and achieves the top with 1.7 % VRe, while the continuing increase of VRe to 3 % leads to a decreased HER activity (Figure S13). The formation of vacancy facilitates the activation of basal plans. In contrast, the excessive vacancies declines the surface stability.11 However, the combination of VRe and TM-TM bonds results in a higher HER activity of VRe-ReS2 monolayer, consistent with the previously predicted ∆GH*. These control results demonstrate the activation effect of metal-vacancy on S atoms. These control results demonstrate the auto-optimization effect of charge engineering induced by metal vacancy-activated TM-TM bonds, leading to an optimal ∆GH*≈ 0 associated with best HER activity. In this work, the sulfur precursor (S powder) was excessive used to utmost reduce the effect of sulfur vacancy (Vs). The ∆GH* of sulfur Vs-ReS2 exhibit much more negative value (-0.248 eV, Figure S14, the detailed structures for calculation see Figure S15) than that of VRe-ReS2 (0.016 eV), demonstrating more active of VRe than that of Vs. The corresponding Tafel plots show that the presence of metal vacancies dramatically drops the Tafel slopes (Figure 5c). Only the introduction of VMo reduces the Tafel slope from 160 to 91 mV dec-1 in monolayer MoS2. In stark comparison, the combination of VRe and Re-Re bonds significantly decreases the Tafel slope from 151 to 69 mV dec-1. This Tafel slope shows the fast discharge of a proton is followed by rate-limiting electrochemical recombination with an additional proton37 (Supplementary Note 4). The presence of Re-Re bonds in VRe-ReS2 leads to a lower Tafel slope than that without TM-TM bonds, indicating that active Re-Re bonds around

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VRe could optimize the bonding of S and adsorbed H. Combining the XPS results with theoretical calculations, charge compensation from S to Re-Re bonds is identified to have optimizing effect for S-H bonding strength for VRe-ReS2 toward the HER compared to that of VMo-MoS2.

Figure 5. (a) Polarization curves of sample VRe-ReS2, VMo-MoS2, ReS2, MoS2, GCE and Pt/C. (b) TOFs comparison of monolayer VRe-ReS2 and ReS2. (c) Tafel plots of the cathodic sweeps of the polarization curves in panel a (same colour labelling). (d) Electrochemical impedance spectroscopy (EIS) Nyquist plots for VRe-ReS2 and VMo-MoS2. The data were fitted using the circuits shown in the inset. (e) Long stability test for VRe-ReS2 electrode at a current density of 10 mA cm-2. Inset is the Tafel plot of sample VRe-ReS2 after 1000 cycling. (f) Comparison of η10-∆GH* for TMDs with sole activation and further optimization by varied techniques. Samples in upper ellipse area shows the TMDs with activation and further optimization, whereas samples in lower ellipse area are only activated. The data except this work is excluded from references.11, 14, 25, 26, 38-42

In addition, electrochemical impedance spectroscopy shows a decreased charge-transfer resistance (Rct) for VRe-ReS2 (76 Ω) relative to VMo-MoS2 (102 Ω), showing the facilitative effect of Re-Re bonds on charge transfer between active S sites and proton in electrolyte (Figure 5e).17

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The active Re-Re bonds at VRe could accept extra electrons from active S sites to weaken the S-H adsorption strengthen, thus leading to more facile electrode kinetics for VRe-ReS2 for HER. Moreover, VRe-ReS2 exhibits outstanding long-term operation stability with minor changes in η10 and Tafel slope (Figure 5f and inset), suggesting the effectiveness of VRe-ReS2 toward HER process after cycling. The previous studies indicate that most 2D TMDs as HER catalysts require generating active sites (vacancy and edge sites) and further optimization of catalytic performance.11, 14, 25, 26, 38-42 Indeed, some additional regulations including Au substrate, Co-doping, mechanical strains make significant improvement in optimizing catalytic activity, achieving much lower η10 value and closer ∆GH* to 0 eV than those of TMDs with sole activations (Figure 5f). In this work, we found that the auto-optimizing ∆GH* is closer to zero than (or equal to) above-mentioned optimizing ones, together with the lowest η10 value (Figure 5f), demonstrating charge engineering effect of TM-TM bonds. In addition, comprehensive comparison of HER performance with other TMDs and non-noble-based catalysts were listed in Table S5. The performance of VRe-ReS2 is on the top among these catalysts, demonstrating the efficiency of TM-TM engineering effect. The present work provides a deep insight on optimizing catalytic activity by charge transfer and designing more HER catalysts of TMDs with TM-TM bonds.

CONCLUSIONS We employed first-principles calculations and experimental verification to demonstrate the

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charge engineering effect of TM-TM bonds on optimize HER catalytic activity of TMDs. A simple Re-vacancy in ReS2 monolayer can achieve a high catalytic activity of ∆GH*≈ 0 eV without additional optimization. Electronic structure analysis indicates Re-Re bonds have charge engineering optimization effect on active electronic states and catalytic activity in active S atoms. Experimental electrochemical characterizations show that a high activity for electrocatalytic HER with a low overpotential (-147 mV) to reach 10 mA cm-2, which is better than that of MoS2 through activation/optimization techniques, further verifying charge engineering optimizing effect of Re-Re bonds. The present work paves a way in designing high active catalysts by using charge compensation effect to optimize catalytic activity.

METHODS Synthesis of monolayer ReS2 with Re vacancy and without Re vacancy. The monolayer ReS2 with Re vacancy samples were synthesized in a quartz tube with 1 inch diameter. Flow gas of H2/Ar with a flow rate of 150 sccm was used as the carrier gas. Specifically, the alumina boat containing 2 mg Re powder (Alfa Aesar, purity 99.99%) as precursor and KI powder (Alfa Aesar, purity 99%) was put in the center of the tube. The Si substrate with 285 nm SiO2 top layer was placed on the alumina boat with surface down. Another alumina boat containing excessive S powder (Alfa Aesar, purity 99.5%, 100 mg) was put on the upstream of tube furnace with temperature of about 200 °C. The temperature ramped up to the 650 °C in 15 min, and kept at the reaction temperature for about 5 to 15 min. Then, the furnace was cooled down to room

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temperature gradually. Different volume ratios of H2/Ar (1%/99%, 3%/97%, 5%/95%, 10%/90%) were selected to obtain varied Re (at%): S (at%) in ReS2. The sample with Re (at%): S (at%) of 0.96:2 denoted as VRe-ReS2 in the main text was obtained using the flow rate of H2/Ar about 5%/95%.The samples ReS2 without Re vacancy was synthesized followed the majority process of monolayer ReS2 with Re vacancy, with 10 mg Re powder and 20 mg S powder as precursors. The carrier gas uses a flow rate of 60 sccm. Synthesis of monolayer MoS2. To compare VRe-ReS2 with the most common 2D catalysts without TM-TM bonds, monolayer MoS2 and VMo-MoS2 was synthesized by chemical vapor deposition process. The majority of synthetic process was similar with that of sample VRe-ReS2. MoO3 (Sigma-Aldrich, purity 99.5%) without addition of KI for MoS2 and with KI addition for VMo-MoS2 was involved in the reaction with sulfur powder at 200 °C, respectively. The EDS data shows the deficiency of Mo in VMo-MoS2 with Mo (at): S (S) of 0.96:2. Structural Characterization. Raman measurement with the excitation laser of 532 nm was performed using a WI Raman characterization TEC alpha 200R Confocal Raman system. Before Raman characterization, the system was calibrated with the Raman peak of Si at 520 cm-1. The laser powers are less than 1mW. TEM and STEM characterization. TEM samples were prepared with a poly (methyl methacrylate) (PMMA) assisted method. STEM was performed on a JEOL 2100F with a cold field-emission gun and an aberration corrector (the DELTA-corrector) operating at 60 kV. A Gatan GIF Quantum was used for recording the EELS spectra. The inner and outer collection angles for the STEM image (β1 and β2) were 62 and 129-140 mrad,

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respectively, with a convergence semi-angle of 35 mrad. The beam current was about 15 pA for the annular dark-field (ADF) imaging and EELS chemical analyses. XPS spectra were taken on an ESCALAB 250 X-ray photoelectron spectrometer with Al Kα. Samples transferred from substrate to GCE, were then immerged in acetone to remove PMMA. Conductive copper tape was used as the support for the samples stuck down from GCE. Surface morphologies of films were observed by field emission scanning electron microscopy (FESEM, FEI Magellan 400, 5kV). Electrochemical measurements. Electrochemical measurements were carried out in 0.5 M H2SO4 as electrolyte using a typical three-electrode cell consisting of a working electrode, a graphite carbon counter electrode and a saturated calomel reference electrode (SCE). The electrochemical cell was connected to an electrochemical workstation (CHI760) coupled with a rotating disk electrode (RDE) system (AFMSRCE3529, Pine Research Instrumentation, USA).A glassy carbon electrode (GCE) covered with catalyst samples was used as the working electrode. The potential, measured against a SCE electrode, was converted to the potential versus the reversible hydrogen electrode (RHE) according to ERHE = ESCE + EoSCE (0.2412) + 0.059×pH. Linear Sweep Voltammetry (LSV) was conducted in 0.5 M H2SO4 with a scan rate of 2 mV s-1 under 1500 rpm. The cyclic voltammograms (CVs) were obtained with rates of 100 mV s-1 in the potential range of 0.2-0.4 V vs. RHE. The current density vs. potential data plots were corrected for 90% ohmic compensation throughout the system. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the same configuration at a potential of

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150 mV from 100 KHz to 0.1 Hz. The polarization data for samples in 0.5 M H2SO4 initially and after 1000 CV sweeps between -0.2 and +0.2 V vs. RHE with scan rate of 50 mV s-1 were used to evaluate the stability performance. Computational Methods. All density functional theory (DFT) calculations were performed in the Vienna Ab initio simulation package (VASP).43 The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof exchange-correlation functional and a 500 eV cutoff for the plane-wave basis set were employed.44 The projector-augmented plane wave (PAW) was used to describe the electron-ion interactions.45 A set of (1×4×4) k-points were carried out for geometric optimization, and the convergence threshold was set as 10-6 eV in energy and 0.01 eV/Å in force, respectively. The empirical dispersions of Grimme (DFT-D2) was applied to account for the long-range van der Waals interactions.46 A 2 × 2 supercell and 15Å× 13 Å× 13 Å with a vacuum slab of 10 Å in X direction of single-layer ReS2 sheet with one Re removed was used to simulate VRe-ReS2 structure, which ensure negligible interaction between Re-vacancies. For the systems the free energy of the adsorbed state is calculated as ∆GH* = ∆EH* + ∆EZPE−T∆S

(1)

Where∆EH* is the hydrogen chemisorption energy, and ∆EZPE is the difference corresponding to the zero point energy between the adsorbed state and the gas phase. As the vibration entropy of H* in the adsorbed state is small, the entropy of adsorption of 1/2 H2 is∆SH≈−1/2ܵு଴మ , where ܵு଴మ is the entropy of H2 in the gas phase at the standard conditions.

AUTHOR INFORMATION 20


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Corresponding Author *E-mail: [email protected]; [email protected]; [email protected]

ACKNOWLEDGMENTS The authors thank the financial support from the National Key Research and Development Program of China (2016YFB0700204), the National Natural Science Foundation of China (51502327, 51602332, 51432010, 21573272, 51702345), and One Hundred Talent Plan of Chinese Academy of Sciences. J. Z. and Z. L. thank the financial support from supported by the Singapore National Research Foundation under NRF RF Award No. NRF-RF2013-08 and MOE Tier 2 grant MOE2016-T2-1-131. J. L. and K. S. acknowledge JST-ACCEL and JSPS KAKENHI (JP16H06333, JP25107003 and P16382) for financial support.

ASSOCIATED CONTENTS Supporting Information Available: Detailed structures for calculations and corresponding hydrogen adsorption free energies, pDOS, STEM images of VMo-MoS2, ReS2 and ReS2 with varied VRe concentration. This material is available free of charge via the Internet at http://pubs.acs.org.

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For Table of Contents Only

Large-area monolayer ReS2, syntehsized by a KI-assisted CVD process, shows an excellent HER activity with a low overpotential of 147 mV at 10 mA cm-2 and high turnover frequency per sulfur atom of 1-10 s-1. The theoretical calculation studies show that charge compensation happens from S to Re-Re bonds, achieving a hydrogen adsorption free energy ∆GH*≅ 0 eV.

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Auto-optimizing Hydrogen Evolution Catalytic ... - ACS Publications

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