Engineering. Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai

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1 Supporting Information Auto-Optimizing Hydrogen Evolution Catalytic Activity of ReS2 through Intrinsic Charge Engineering Yao Zhou 1, #, Erhong Song 1, #, Jiadong Zhou 2, #, Junhao Lin 3, Ruguang Ma 1, Youwei Wang 1, Wujie Qiu 1, Ruxiang Shen 1, Kazutomo Suenaga 3, Qian Liu 1, 4, *, Jiacheng Wang 1, 4, *, Zheng Liu 2, *, 1, 4, * Jianjun Liu 1 The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai , P. R. China 2 Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, Singapore , Singapore 3 National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba , Japan 4 Shanghai Institute of Materials Genome, 99 Shangda Road, Shanghai , P. R. China # These authors contributed equally. * jliu@mail.sic.ac.cn; jiacheng.wang@mail.sic.ac.cn; z.liu@ntu.edu.sg

2 Figure S1. Optimized structure of pristine ReS2 basal plane. Top view of the atomic structure of 1T phase ReS2 basal plane.

3 a PDOS (States/eV) Total Re S Energy (ev) b PDOS (States/eV) Total Re S Energy (ev) Figure S2. Electronic structure of pristine ReS2 and VRe-ReS2. Projected density of states (pdos) of optimized structure of (a) ReS2 and (b) VRe-ReS2. The Fermi energy is positioned as zero.

4 Figure S3. Optimized structure of H absorbed on pristine ReS2 and VRe-ReS2 system. (a-d) Optimized structure of H absorbed on S1, S2, S4, S5 atom of pristine ReS2; (e-j) Optimized structure of H absorbed on S1, S2, S3, S4, S5 and S6 atom of VRe-ReS2 system.

5 Figure S4. The GH* for this work and previously-studied TMDs 1-9 with activation/optimization.

6 Table S1. The correlation of GH* with electron amount transfer from S to H atom, which characterizes electrochemical potentials of different S-Re-Re structures to catalyze (H + +e - ) pair. Atom H G H* (ev) S S S S S S

7 a PDOS (States/eV) R4-d R3-d S2-p b R1-d R2-d S2-p c PDOS (States/eV) R4-d R3-d S2-p d R1-d R2-d S2-p e PDOS (States/eV) R4-d R3-d S3-p f R6-d R5-d S5-p g PDOS (States/eV) R4-d R3-d S3-p h R6-d R5-d S5-p Energy (ev) i PDOS (States/eV) R6-d R5-d S6-p j R1-d R2-d S6-p -1.5 k PDOS (States/eV) R6-d R5-d S6-p l R1-d R2-d S6-p Energy (ev) Energy (ev) Figure S5. Electronic structure analysis of VRe-ReS2. (a, b), (e, f),and (i, j) Projected density of state (pdos) of S2, S3, S5 and S6 atoms of VRe-ReS2. (c, d), (g-h) and (k, l) pdos of H absorbed on S2, S3, S5 and S6 atoms of VRe-ReS2. The Fermi energy is positioned as zero.

8 Table S2. The correlations of GH* with charge distributions of H-S and Re-Re, which are described by H-S bond distance. Atom l bond H-S (Å) G (ev) S S S S S S

9 Figure S6. (a) Correlation between H-S bond length and H adsorption free energy; (b) Hydrogen evolution GH* as a function of the gap state energy of VRe-ReS2. (c) Illustration of calculation of ΔGap state energy. The upper panel and lower panel show the pdoss of non-h adsorption and H- adsorption of VRe-ReS2, respectively.

10 Note 1. Methods to calculate ΔGap state energy and explanation of results. In order to describe electron-acceptance ability of Re-Re bond, gap state energy change of Re-Re bonds between valence band maximum (VBM) and conduct band minimum (CBM) before (Figure S6c upper panel) and after hydrogen adsorption (Figure S6c lower panel) were calculated. Three steps are needed to get ΔGap state energy: 1. non-h2 adsorption: ΔE1 = E(CBM) - E(VBM). E(CBM) and E(VBM) are determined by Re-d band. 2. after H2 adsorption: ΔE2 = E(CBM) - E (VBM). 3. ΔGap state energy = ΔE2 -ΔE1. When one sulfur atoms (SA2, SA4, SI2) connected two Re-Re bonds, step 1-3 should do twice separately for each Re-Re bonds. Then, adding two ΔGap state energy for each Re-Re bonds and finally get the total ΔGap state energy. Other sulfur atoms (SA1, SA3, SI1) only need to go through from step 1 to 3 once. If ΔGap state energy < 0, dangling bonds of Re moves downshift toward Fermi level, illustrating the electron acceptance of Re-Re during H2 adsorption. The more negative the value of ΔGap state energy, the stronger electron accepted ability of Re-Re. If ΔGap state energy > 0, means the electron loss of Re-Re. When it comes to SIn (n = 1, 2), nearly no change of gap state energy was observed, showing an extremely weak Re-Re effect on regulating electron transfer which is also consistent with pdos results.

11 Figure S7. SEM image of as-prepared VRe-ReS2 films with continuous size from hundredmicrometer scale. Note 2. Role of potassium iodide (KI) on Re precursor vapor pressure and growth rate To understand the role of KI, we discuss the reaction and the growth of ReS2 via thermodynamics. First, to make the reaction happen, both metal and sulfur precursors are required. Although the later can be continually supplied during the reaction by heating the sulfur powder, metal precursors are not readily to control. The low-melting-point metal sulfides are highly active and may evaporate in a short time. According to previous reports, metal oxides, such as ReO3, are readily for evaporation due to their low melting point (usually below 800 C). As a result, previous work usually took use of ReO3 as Re precursor to synthesize ReS2 via chemical vapor deposition process (CVD). 10 Due to production of oxidizing metal oxides intermediates during CVD process, sulfur vacancy usually tends to be created. In order to avoid the interruption of sulfur vacancy, non-oxide

12 metal precursors (e. g. Re) are needed. However, metal Re is difficult to evaporate because of its very high melting point (3180 C). Mixing this high-melting-point metal precursor with salt will produce a molten solution. The vapor pressure of molten salt is much higher than many metal precursors. The vapor pressure of KI can be written as: B log 10( P) A (1) T C Where P is the vapor pressure (bar), A = 6.6, B = , C= For T = 650 C, the pressure of KI can be estimated to be around 19 Pa. As a comparison, at the same temperature, the vapor pressure for Re is only Pa, as shown in Figure S8. Given that, the solubility of metal is at the order of ppm, the metal/molten salt will dramatically increase the vapor pressure of Re by at a few orders. Figure S8. Comparison of vapor pressure of Re and KI, illustrating temperature dependence of vapor pressure.

13 Increased vapor pressure of reactants leads to the reduced reaction barrier. According to the relationship between vapor pressure and Gibbs free energy: ΔG = ΔG0 - RTIn P P0 (2) Where vapor pressure P increases, along with Gibbs free energy ΔG decreases. T dedicates to the reaction temperature. When ΔG is not above 0, reactions can happen spontaneously. As a result, lowered T is required when P gradually increased to achieve ΔG = 0, suggesting that KI efficiently induces lowered synthetic temperature in this work. As for the growth rate, vapor pressure also can impact the growth rate from the aspect of vapor concentration. At a fixed vapor pressure, mass flow rate directly determines the domain size of TMDs. Due to the depositions growth rate G(x) is in proportion to mass flux J(x) in equation 3, 11 the enhanced Re vapor pressure leads to the lifted mass flux, which in turn increases the deposition growth rate.

14 Figure S9. Optical image of sample of ReS2 film without adding of KI. The continuity of film only reaches up to ~10 μm, which is much inferior to that of VRe-ReS2 (~hundred micrometers), showing facilitated effect of KI to obtain large-scaled film. a b 1 L E g Intensity (a.u.) Ag E g E g E g A g S contributed E g Raman Shift (cm -1 ) Figure S10. (a) The XPS survey of sample VRe-ReS2. (b) Raman spectrum for as-prepared ReS2 films of monolayer. Raman spectrum of monolayer was collected with a 532 nm excitation laser. Sample VRe-ReS2 films display similar phonon vibration modes, just like the data shown in Figure S10, in accord

15 with reported work. 12 The presence of large number of Raman peaks is attributed to the low crystal symmetry of ReS2 and associated with fundamental Raman modes (Eg- and Ag-like) coupled to each other and to acoustic phonons. 13 Four peaks (145.9, 153.6, and cm -1 ) are assigned to in-plane Eg-like mode and the peak at cm -1 corresponds to out-of-plane Ag-like mode. The observed vibrational modes in the cm -1 range are associated with the A1g and E1g 14, 15 modes of ReS2. Figure S11. EDX data of sample VRe-ReS2. Obtained films with signals of Re and S are readily detected by energy dispersive spectrum (EDS), while Si and O come from the supporting SiO2/Si substrates

16 Table S3. The EDX elemental analysis of VRe-ReS2, ReS2, VMo-MoS2 and MoS2. Sample ID Re Mo S Si O Metal (at%):s (at%) (at%) (at%) (at%) (at%) (at%) VRe-ReS :2 VMo-MoS :2 c Figure S12. (a) STEM. No visible Re vacancy was observed; (b) XPS spectrum of Re 4f of sample ReS2. There is no extra peak in high binding energy region ascribed to VRe in addition to finger

17 prints of original ReS2; (c) The STEM image of VMo-MoS2. VMo were highlighted in the red circles. As for Re signal of XPS of sample ReS2, only two typical peaks located at ~42 and 44.5 ev which are ascribed to ReS2, can be observed (Figure S12). With comparison, the peak at 47.7 ev in sample VRe-ReS2 is not belonged to any Re-related compounds or Re metal, which suggests a new state of Re (Figure 4g in the main text). It is reported that peak of Re 4f will appear at relative high binding energy than other Re-related compounds 16 when the formation of Re vacancy, supporting that the peak locating at 47.7 ev belonged to Re vacancy. As for the STEM image of VMo-MoS2 shown in Figure S12c, the absence of Mo atoms can be clearly observed (marked with red circles) without sulfur vacancy, confirming the Mo vacancy formation. Table S4. The XPS elemental analysis of VRe-ReS2 with different VRe concentrations. V Re concentration (at%) C (at%) O Re S

18 Figure S13. (a-d) The STEM images of VRe-ReS2 with different VRe concentrations. (e) Concentration of VRe calculated from Re (at%) : S (at%) from XPS results; (f) LSV curves after

19 ir correction of samples with different concentration of VRe. The concentration of VRe varied from 0.3 % to 3.0 %, while sample with 1.7 % in concentration of VRe shows the best HER activity. The 1.7 % sample corresponds to sample in main text named as VRe-ReS2. According to the XPS elemental analysis (Table S4), the VRe concentrations were calculated based on the Re (at%) and S (at%). The existence of C and O are ascribed to the conductive tape substrate for measurement. In pristine ReS2, the ratio of Re (at%) : S (at%) should be 1:2. After introduction of VRe, the ratio of Re (at%): S (at%) denoted as a : b will be reduced. The Re: S of four samples are 0.99:2, 0.97:2, 0.95:2 and 0.91:2, respectively. The VRe concentration was calculated as follows: C(VRe) = (1-a)/(a+b). Besides the XPS results, STEM images were also performed to confirm the VRe concentration in each sample, which is nearly consistent with XPS results.

20 0.1 G H* (ev) S Vs-ReS2 H + + e - H * 1/2H 2 Reaction coordinate Figure S14. ΔGH* of VRe-ReS2 and S4 atom in VS-ReS2. a b S1 S2 S3 S4 Figure S15. (a) Top view of atomic structure of H absorbed on S1, S2, S3, S4 vacancy of VS-ReS2 system. (b) The correlation of formation energy (Ef ) and hydrogen adsorption energy ( GH* ) of S1, S2, S3, S4 vacancy of VS-ReS2 system, respectively.

21 Table S5. Comparison of HER activity of VRe-ReS2 in this work with mono-/few-layered TMDs and some bulk TMDs electrocatalysts measured in 0.5 M H2SO4. Sample ID Overpotential at 10 ma cm -2 (η10, mv vs. RHE) Monolayered or few-layered TMDS ΔGH* (ev) TOFs (s -1 ) Ref. Monolayer VRe-ReS This work 20 nm 2H-TaS2 ~ [ 17 ] Strained VS-MoS [ 1 ] 1T-MoS2 <-200 ~ [ 18 ] VS-MoS2 <-300 ~ [ 19 ] Monolyaer MoS2 on Au < [ 2 ] Vs-MoS2 nanocrystals ~ [ 20 ] 2H-monolyaer MoS2-201 ~ [ 21 ] Mo edge 2H-MoS2 basal plane [ 2 1] 1T -MoS2 Mo edge [ 22 ] Monolayer MoS2 ~ [ 23 ] Bulk TMDs and other catalysts H-TaS2-60 ~ [ 24 ] H-NbS2-50 ~ [ 24 ] TaS2 200 nm in thickness [ 17 ] rgo/wxmo1-xs [ 25 ] 1T-MoSe2 nanosheets [ 26 ] Co-doped MoS [ 3 ] VS2 crystal [ 27 ] MoS2 on N-carbon nanoboxes [ 28 ] ReS2 on Au foil ~ [ 29 ] Bulk ReS2 nanosheets [ 30 ] NiCo2Px Nanowires [ 31 ] Mo2C@N-carbon [ 32 ]

22 Note 3. Calculation of the turnover frequency. The turnover frequency is calculated using the current density j and the active si4te density N according to Equation 6, = j/(2 q) N TOF = Total number of H 2 atoms per second Total number of active sites per unit area (6) Where q = C is the elementary charge, and 2 accounts for 2 H atoms per H2 molecule. To calculate the turnover frequency per surface S atom (TOFs), the S atom density (NS) is estimated to be from the ReS2 lattice constant ~6.4 Å. Note 4. Illustration of HER mechanisms. The HER is generally accepted to proceed via the steps (Equation 3-5) noted below, where in the initial proton discharge to form adsorbed H (Equation 3) is followed by either the recombination of the adsorbed H to form H2 (Equation 4) or electrochemical desorption of the adsorbed intermediate to form H2 (Equation 5). M + H + + e - MH(Tafel mechanism) (3) MH + MH H2 + 2M (Heyrovsy mechanism) (4) or

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