Supporting Information. Controlled Electrodeposition of Photoelectrochemically

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1 Supporting Information Controlled Electrodeposition of Photoelectrochemically Active Amorphous MoSx Cocatalyst on Sb2Se3 Photocathode Jeiwan Tan, Wooseok Yang, Yunjung Oh, Hyungsoo Lee, Jaemin Park, and Jooho Moon * Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea * jmoon@yonsei.ac.kr S-1

2 Figure S1. SEM images of (a) bare Sb2Se3 and (b) TiO2/Sb2Se3 microstructures. S-2

3 (a) CI-MoS x /FTO 1st 5th 20th 80th (b) AI-MoS x /FTO 1st 5th 20th 80th Potential / V RHE Potential / V RHE (c) (d) CI-MoS x AI-MoS x FTO 100 nm FTO 100 nm (e) Potential / V RHE CI-MoS x /FTO AI-MoS x /FTO Figure S2. Cyclic voltammograms of 1, 5, 20, and 80 electrodeposition cycles of (a) CI-MoSx and (b) AI-MoSx deposited onto FTO substrate. Corresponding cross-sectional microstructure of (c) CI-MoSx/FTO and (d) AI-MoSx/FTO. (e) Electrochemical HER performance of CI-MoSx/FTO and AI-MoSx/FTO. S-3

4 Figure S3. Schematic illustration of the activation procedure of CI-MoSx and AI-MoSx. S-4

5 x10 Sb 2 Se 3 TiO 2 / Sb 2 Se 3 MoS x / TiO 2 / Sb 2 Se 3 Pt / TiO 2 / Sb 2 Se 3 5 ma cm Potential / V RHE Figure S4. Comparison of the PEC performance of Sb2Se3, TiO2/Sb2Se3, activated CI- MoSx/TiO2/Sb2Se3, and Pt/TiO2/Sb2Se3. S-5

6 R HF Potential V vs. RHE R HF Potential V vs. RHE E CB E CB -0.5 E F E VB R MF Surface state R LF HER 0.0 E F E VB R LF HER 0.0 Sb 2 Se 3 TiO 2 CI-MoS x Electrolyte 2.5 Sb 2 Se 3 TiO 2 AI-MoS x Electrolyte Figure S5. A comparison of CI-MoSx/TiO2/Sb2Se3 and AI-MoSx/TiO2/Sb2Se3 band diagrams, after activation. Only CI-MoSx showed the surface state positioned at the forbidden state within the band gap of MoSx, which introduced additional RMF, but dramatically reduced the charge transfer resistance (RLF). ECB, EF, and EVB represent the energy states of the conduction band, fermi level, and valence band, respectively. S-6

7 (a) 0 off (b) 0 off off CI-MoS x at 0 V RHE AI-MoS x at 0 V RHE CI-MoS x at -0.2 V RHE AI-MoS x at -0.2 V RHE bubble elimination Time / sec Time / sec Figure S6. Stability test of CI-MoSx/TiO2/Sb2Se3 and AI-MoSx/TiO2/Sb2Se3 at (a) 0 VRHE and (b) -0.2 VRHE. S-7

8 Table S1. Resistance and capacitance values obtained from deconvolution of the EIS data. R ohmic R HF CPE HF R MF CPE MF R LF CPE LF (Ω cm 2 ) (Ω cm 2 ) (F s n-1 cm 2 ) (Ω cm 2 ) (F s n-1 cm 2 ) (Ω cm 2 ) (F s n-1 cm 2 ) CI-MoS x/ TiO 2/Sb 2Se (n = 0.80) (n = 0.79) (n = 0.88) AI-MoS x/ TiO 2/Sb 2Se (n = 0.76) (n = 0.91) S-8

9 Table S2. Binding energies of representative XPS components, and FWHM values used in data fitting. Mo(IV) 3d 5/2 (ev) Mo(V) 3d 5/2 (ev) Mo(VI) 3d 5/2 (ev) S 0 2- S 2 2s (ev) 2s (ev) S 2-2s (ev) CI-MoS x/tio 2/Sb 2Se 3 before activation (FWHM) (2.02) (1.63) (1.38) (0.95) (1.96) (2.00) CI-MoS x/tio 2/Sb 2Se 3 after activation (FWHM) (1.75) (2.44) (1.59) - (1.08) (1.84) AI-MoS x/tio 2/Sb 2Se 3 before activation (FWHM) (2.00) (1.70) (1.42) (1.54) (0.83) (1.83) AI-MoS x/tio 2/Sb 2Se 3 after activation (FWHM) (1.75) (2.10) (1.67) (1.24) (1.25) (1.70) XPS fitting for a-mos x: Each X-ray photoelectron spectrum was calibrated using the C 1s peak with a binding energy of ev. The background was fitted using the Shirley method, and all fitting curves were determined by a Gaussian peak function. The Mo 3d spectrum was fitted using 3.14 ev peak separation between each pair of doublet peaks. This was reasonable, because the spin-orbit splitting of core levels is constant for a specific orbital of the element. (i.e., Mo 3d 5/2-3d 3/2 doublet separation is 3.14 ev). 1 Moreover, the full width half maximum (FWHM) for each pair of doublet peaks should be identical. Being an amorphous material, the fitted values of FWHM were slightly larger than those generally reported for crystalline MoS 2. The area ratio between each pair of doublets was assigned based on the degeneracy of the spin state (i.e., Mo 3d 5/2:Mo 3d 3/2 = 3:2). 2 Although the use of one equivalent singlet to fit the S 2s region is the common approach found in literature, each singlet peak was employed for three different chemical states for sulfur (S 0 2- for elemental sulfur, S 2, and S 2- for a-mos x). 3.4 Only spectra of the activated CI-MoS x sample could be reasonably fitted without the S 0 2s peak, whereas spectra of the other three samples required additional peaks for reasonable fitting (chisquare < 2). S-9

10 Supporting Information References (1) Lu, A.-Y.; Yang, X.; Tseng, C.-C.; Min, S.; Lin, S.-H.; Hsu, C.-L.; Li, H.; Idriss, H.; Kuo, J.-L.; Huang, K.- W.; Li, L.-J. High-Sulfur-Vacancy Amorphous Molybdenum Sulfide as a High Current Electrocatalyst in Hydrogen Evolution. Small 2016, 12, (2) Tran, P. D.; Tran, T. V.; Orio, M.; Torelli, S.; Truong, Q. D.; Nayuki, K.; Sasaki, Y.; Chiam, S. Y.; Yi, R.; Honma, I.; Barber, J.; Artero, V. Coordination Polymer Structure and Revisited Hydrogen Evolution Catalytic Mechanism for Amorphous Molybdenum Sulfide. Nat. Mater. 2016, 15, (3) Morales-Guio, C. G.; Tilley, S. D.; Vrubel, H.; Grätzel, M.; Hu, X. Hydrogen Evolution from a Copper(I) Oxide Photocathode Coated with an Amorphous Molybdenum Sulphide Catalyst. Nat. Commun. 2014, 5, (4) Vrubel, H.; Hu, X. Growth and Activation of an Amorphous Molybdenum Sulfide Hydrogen Evolving Catalyst. ACS Catal. 2013, 3, S-10