Electronic Supplementary Information

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1 Electronic Supplementary Information Hydrogen evolution catalyzed by MoS 3 and MoS 2 particles Heron Vrubel, Daniel Merki and Xile Hu* Laboratory of Inorganic Synthesis and Catalysis, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), SB-ISIC-LSCI, BCH 3305, Lausanne, CH 1015, Switzerland * To whom correspondence should be addressed. xile.hu@epfl.ch S1

2 Ohmic drop correction: The ohmic drop correction of polarization curves has been performed according to the method given in the literature [1-3]. The overpotential η (V) observed during an experiment is given by equation (1): η = a + bln j + jr (1) where a (V) is the Tafel constant, b (V dec -1 ) is the Tafel slope, j (A cm -2 ) is the current density and R (Ω cm 2 ) is the total area-specific uncompensated resistance of the system, which is assumed to be constant. The derivative of Eq. (1) with respect to current density gives Eq. (2) from which b and R can be easily obtained by plotting dη/dj as a function of 1/j. dη dj = b j + R (2) The estimation of R allows correcting the experimental overpotential by subtracting the ohmic drop jr according to equation (3): η corr = η jr (3) During the calculations, the derivative dη/dj was replaced by their finite elements Δη/Δj estimated from each pair of consecutive experimental points. Reference [1] D.M. Schub, M.F. Reznik, Elektrokhimiya, 21 (1985) 937 [2] N. Krstajic, S. Trasatti, Journal of Applied Electrochemistry. 28 (1998) 1291 [3] L.A. De Faria, J.F.C. Boodts, S. Trasatti, Journal of Applied Electrochemistry, 26 (1996) 1195 S2

3 O 1s C 1s O KLL Mo 3p Mo 3d S 2p O 1s Sn 3d C 1s O KLL Sn 3p Mo 3p Mo 3d S 2p Fig. S1. XPS survey spectra of the MoS 3 particles before (top) and after (bottom) polarization. S3

4 0-2 Current density (ma/cm 2 ) Potential (V vs. RHE) 53 μg/cm 2, scan 1 53 μg/cm 2, scan 2 53 μg/cm 2, scan 3 53 μg/cm 2, scan 4 53 μg/cm 2, scan 5 53 μg/cm 2, scan 6 Fig. S2. Consecutive polarization curves of a freshly prepared MoS 3 -modified FTO electrode (drop casting) recorded at ph = 0 (1.0 M H 2 SO 4 ); scan rate: 5 mv/s. Table S1. HER activity of drop-casted MoS 3 -modified FTO electrodes according to polarization measurements. Loading (μg/cm 2 log j ) 0 (log[ma/cm 2 Tafel Slope ]) (mv/decade) j η=150 (ma/cm 2 ) j η=200 (ma/cm 2 ) a b c d e a Determined at η = mv; b Determined at η = mv; c Determined at η = mv; d Determined at η = mv; e Determined at η = mv. S4

5 Table S2. HER activity of spray-casted MoS 3 -modified FTO electrodes according to polarization measurements. Loading (μg/cm 2 ) log j 0 (log[ma/cm 2 Tafel Slope ]) (mv/decade) j η=150 (ma/cm 2 ) a b c d e j η=200 (ma/cm 2 ) a Determined at η = mv; b Determined at η = mv; c Determined at η = mv; d Determined at η = mv; e Determined at η = mv. S5

6 2 0-2 Current density (ma/cm 2 ) b a e d c a: 8 μg/cm 2 b: 16 μg/cm 2 c: 32 μg/cm 2 d: 64 μg/cm 2 e: 128 μg/cm Potential (V vs. RHE) 0.0 a -0.5 current density (ma/cm 2 ) b loading (μg/cm 2 ) a: 150 mv overpotential b: 200 mv overpotential Fig. S3 (Top) Polarization curves of spray-casted MoS 3 -modified glassy carbon electrodes recorded at ph = 0 (1.0 M H 2 SO 4 ); scan rate: 5 mv/s. (Bottom) Loading-dependence of current densities. S6

7 Table S3. HER activity of spray-casted MoS 3 -modified glassy carbon electrodes according to polarization measurements. Loading (μg/cm 2 ) log j 0 (log[ma/cm 2 Tafel Slope ]) (mv/decade) j η=150 (ma/cm 2 ) a b c d e j η=200 (ma/cm 2 ) a Determined at η = mv; b Determined at η = mv; c Determined at η = mv; d Determined at η = mv; e Determined at η = mv Current density (ma/cm 2 ) MoS 3, no MWCNT MoS 3 : MWCNT = 1 : 1 MoS 3 : MWCNT = 1 : Potential (V vs. RHE) Fig. S4 Polarization curves of spray-casted MoS 3 -modified glassy carbon electrodes, with MWCNT as additive, recorded at ph = 0 (1.0 M H 2 SO 4 ); scan rate: 5 mv/s. The loading of MoS 3 is always 130 μg/cm 2. S7

8 Fig. S5 The home-made working electrode filled with conductive graphite powder. S8

9 MoS3 particles drop-casted on FTO (ca. 16 μg). Consecutive polarization curves in 1.0 M H2SO4 (Ti counter electrode not separated). 1 0 Current density (ma/cm 2 ) scan 1 scan 2 scan 3 scan 4 scan 5 scan Potential (V vs. RHE) DM4-10-Tcell Ti Fig. S6 Consecutive polarization curves of a freshly prepared MoS 3 -modified FTO electrode (drop casting) recorded at ph = 0 (1.0 M H 2 SO 4 ); scan rate: 5 mv/s. Ti electrode was used as a counter electrode. S9

10 Mo 3d 3/ Mo 3d 5/ Experimental data I 2p 3/2 I 2p 1/2 II 2p 3/2 II 2p 1/2 Curve fitting Fig. S7 XPS spectra of the MoS 3 -modified electrode after five minutes of electrolysis. (Top) Mo spectrum; (bottom) S 2p spectrum. S10

11 Mo 3d 3/ Mo 3d 5/ Experimental data I 2p 3/2 I 2p 1/2 II 2p 3/2 II 2p 1/2 Curve fitting Fig. S8 XPS spectra of the MoS 3 -modified electrode after ten minutes of electrolysis. (Top) Mo spectrum; (bottom) S 2p spectrum. S11

12 Mo 3d 3/ Mo 3d 5/ Experimental data I 2p 3/2 I 2p 1/2 II 2p 3/2 II 2p 1/2 Curve fitting Fig. S9 XPS spectra of the MoS x species prepared by reduction of MoS 3 with NaBH 4. (Top) Mo spectrum; (bottom) S 2p spectrum. S12

13 1 0-1 Current density (ma/cm 2 ) MoS x - from reduction by NaBH 4 MoS Potential (V vs RHE) Fig. S10 Polarization curves of FTO electrodes modified by MoS x species prepared by reduction with NaBH 4, and by MoS 3 particles recorded at ph = 0 (1.0 M H 2 SO 4 ); scan rate: 5 mv/s. The electrodes were made using the same loading of catalysts and drop casting. S13

14 Mo 3d 5/ Mo 3d 3/ Experimental data I 2p 3/2 I 2p 1/2 II 2p 3/2 II 2p 1/2 Curve fitting S14

15 Fig. S11 XPS spectra and electron diffraction pattern of the MoS particles. The Mo/S ratio is 1:3.1. For S 2p region; binding energies (ev): doublet I: 2p 3/2, 162.0; 2p 1/2, 163.2; doublet II: 2p 3/2, 163.3; 2p 1/2, S15

16 Mo 3d 5/ Mo 3d 3/ S 2s Experimental data S 2-2p 3/2 S 2-2p 1/2 Curve fitting S16

17 Fig. S12 XPS spectra and electron diffraction pattern of the MoS particles. The Mo/S ratio is 1:2. S17

18 Mo 3d 5/ Mo 3d 3/ Binding Energy (ev) S18

19 162.3 S 2p1/ S 2p3/ Fig. S13 (Top) TEM image (left) and electron diffraction pattern of commercial MoS 2 particles. (Middle) XPS Mo spectrum of commercial MoS 2 particles. The spectrum shows a peak at lower binding energies, which is the S 2s peak. (Bottom) XPS S spectrum of commercial MoS 2 particles. The S/Mo ratio is 2.1 for MoS 2 particle. S19