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Electronic Supplementary Information

Synthesis of MoS2 from [Mo3S7(S2CNEt2)3]I for Enhancing

Photoelectrochemical Performance and Stability of Cu2O Photocathode

Toward Efficient Solar Water Splitting

Pravin S. Shinde ab Patricia R. Fontenot, c James P. Donahue, c Joseph L. Waters,bd Patrick

Kung,bd Louis E. McNamara,e Nathan I. Hammer,e Arunava Gupta,abf and Shanlin Pan *, ab

a Department of Chemistry, bCenter for Materials for Information Technology, dDepartment of

Chemical and Biological Engineering, fDepartment of Electrical and Computer Engineering, The

University of Alabama, Tuscaloosa, Alabama 35487, United States

c Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States

e Department of Chemistry and Biochemistry, University of Mississippi, Oxford, Mississippi

38655, United States

* Corresponding Author:

E-mail: span1@ua.edu

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2018

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Table of Contents

Cyclic voltammetry (CV) study of MoS2 precursors----------------------------------------------- III

Fig. S1. CV of ferrocene in DMF and description. ---------------------------------------------------- III

Fig. S2. CVs of [Mo3S7(S2CNEt2)3]I, [Mo3S7(S2CNMe2)3]I, and [NH4]2[Mo3S13] precursors in

anhydrous DMF. ------------------------------------------------------------------------------------------- IV

Fig. S3. Polarization curves of MoS2 on GCE at different rpms; Koutecky–Levich plots. ------- V

Fig. S4. Polarization curves of MoS2/FTO electrodes fabricated at 450°C. ----------------------- VI

Fig. S5. CVs of bare FTO, as-grown MoS2 and N2-annealed MoS2 electrodes from freshly

prepared and N2-saturated Na2SO4. ---------------------------------------------------------------------- VI

Fig. S6. CVs of bare FTO, as-grown MoS2, and N2-annealed MoS2 electrodes from a freshly-

prepared and N2-saturated Na2SO4 electrolytes recorded at different scan rates. ---------------- VII

Fig. S7. CVs of Pt disc electrode for ORR from N2-saturated, freshly-prepared and O2-saturated

Na2SO4 electrolytes recorded at different scan rates. ------------------------------------------------ VIII

Fig. S8. EDS analysis of Cu2O and MoS2/Cu2O samples. ------------------------------------------- IX

Fig. S9. Cross-sectional SEM of MoS2-modified Cu2O and its EDS mapping. ------------------- IX

Fig. S10. SEM images of N2-annealed MoS2 and N2-annealed Cu2O on FTO. -------------------- X

Fig. S11. Photocurrent responses of N2-annealed MoS2/Cu2O with different spin-coated layers

and N2-annealed Cu2O photocathodes. ------------------------------------------------------------------ XI

Fig. S12. XRD and SEM of N2-annealed Cu2O before and after PEC measurement. ------------ XI

Fig. S13. Photocurrent responses of as-grown MoSx-protected Cu2O photocathode. ----------- XII

Fig. S14. IPCE spectra of the Cu2O and MoS2/Cu2O under backside illumination. ------------- XII

Fig. S15. Long-term photostability tests of as-grown Cu2O and N2-annealed MoS2-modified

Cu2O photocathodes. ------------------------------------------------------------------------------------- XII

Fig. S16. XRD of as-grown Cu2O and MoS2/Cu2O photoelectrodes before and after long-term

stability tests. --------------------------------------------------------------------------------------------- XIII

Fig. S17. SEM images revealing surface morphology of as-grown Cu2O and MoS2/Cu2O

photoelectrodes before and after long-term stability tests. ----------------------------------------- XIV

References ------------------------------------------------------------------------------------------------ XIV

III

Cyclic voltammetry study of MoS2 precursors. Prior to studying redox behaviour of MoS2 precursors, the ferrocenium/ferrocene (Fc+/Fc) couple was used as an internal standard for calibration of the electrochemistry set-up. The cyclic voltammogram (Figure S1) of ferrocene in DMF reveals the occurrence of Fc+/Fc couple at +0.477 V vs. Ag/AgCl. The cyclic voltammograms of both [NH4]2[Mo3S13] and [Mo3S7(S2CNR2)]+I− (R = Me or Et) in anhydrous N,N-dimethylformamide display strong similarity to one another (Figure S2). All of them show an irreversible feature at ~−0.75 V vs. Ag/AgCl followed by a reversible reduction at approximately −1.32 V. The dithiocarbamate clusters show an additional irreversible wave upon scanning to ~−1.92 V, which is not apparent in the voltammogram of [NH4]2[Mo3S13]. None of the complexes supports any reversible process in the anodic direction. An electrochemical study of [NH4]2[Mo3S13] in CH2Cl2 by Garriga, Llusar and coworkers revealed reductions at −1.03 V and −1.27 V ([Cp2Fe]+/[Cp2Fe] at + 0.44 V) but no satisfactory explanation about the reversibility of these processes was provided.1 The irreversibility observed for the first cathodic process may be attributed to transformation of bridging S2

2− ligand to bridging monosulfide, S2−, by extrusion of elemental sulphur from the equatorial positions. This interpretation is suggested by calculations showing the LUMO of [Mo3S13]2− to be predominantly a σ* character between sulphur p orbitals of the bridging S2

2− ligands.2 The initial reductions observed for [Mo3S7(S2CNR2)]+I− (R = Me or Et) are notably similar to that in [Mo3S13]2− both in approximate potentials and qualitative appearance. Thus, the essential basis for irreversibility is likely the same as in [Mo3S13]2−. The reversible following reductions found both in [Mo3S13]2− and [Mo3S7(S2CNR2)]+I− (R = Me or Et) are tentatively assigned to reduction of the [Mo3S4]n core.

1.21.00.80.60.40.20.0-0.2

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Figure S1. Cyclic voltammogram of ferrocene in DMF showing a ferrocenium/ferrocene couple at +0.477 V vs. Ag/AgCl.

IV

Figure S2. Cyclic voltammograms of (A) [Mo3S7(S2CNEt2)3]I, (B) [Mo3S7(S2CNMe2)3]I, and (C) [NH4]2[Mo3S13] in anhydrous DMF. Insets show their corresponding structures.

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Figure S3. Polarization curves of MoS2 recorded at (A) disc and (B) ring electrodes at different rotation speeds (viz. 500 to 5000 rpm). Insets show the Koutecky–Levich plots constructed at −0.36 V vs. RHE over the entire rotation frequency range at both disc and ring electrodes. Scan rate: 50 mV s−1; Electrolyte: 0.5 M H2SO4.

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E / V vs. SCE

FTO and MoS2/FTO 1 mM, 5L spin-coated, N2+450oC

FTO Mo3S4(S2CNEt2)4+ DMF (NH4)2[Mo3S13] + DMF [Mo3S7(S2CNEt2)3]I + DMF [Mo3S7(S2CNMe2)3]I + DMF (Bu4N2)[Mo3S13] + CH2Cl2 [Mo3S7(S2CNMe2)3]I + THF

-0.4 -0.2 0.0 0.2 0.4 0.6E / V vs. RHE

Figure S4. Polarization curves of MoS2/FTO electrodes fabricated at 450°C in nitrogen environment. Scan rate: 50 mV s–1; Electrolyte: 0.5 M H2SO4.

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FTO As-grown MoSx/FTO (1min) As-grown MoSx/FTO (15 h) N2-annealed MoS2/FTO

-0.4 -0.2 0.0 0.2 0.4 0.6Potential / V vs. RHE

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FTO As-grown MoSx/FTO (1min) As-grown MoSx/FTO (15 h) N2-annealed MoS2/FTO

-0.4 -0.2 0.0 0.2 0.4 0.6Potential / V vs. RHE

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FTO As-grown MoSx/FTO (1min) As-grown MoSx/FTO (15 h) N2-annealed MoS2/FTO

-0.4 -0.2 0.0 0.2 0.4 0.6Potential / V vs. RHE

A

B

Figure S5. Cyclic voltammograms of bare FTO, as-grown MoS2 (air-dried for 1 min and 15 h), and N2-annealed MoS2 electrodes recorded at the scan rate of 50 mV s−1 from (A) freshly prepared and (B) N2-saturated 0.5 M Na2SO4 electrolytes.

VII

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N2-purged Na2SO4 (pH=7.0)

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Figure S6. Cyclic voltammograms showing catalytic activity for proton reduction by bare FTO (A, B), as-grown 1-min air-dried MoS2 (C, D), as-grown 15-h air-dried MoS2 (E, F), and N2-annealed MoS2 (G, H) electrodes from a freshly-prepared and N2-saturated 0.5 M Na2SO4 electrolytes recorded at different scan rates (viz. 20, 50, 100, 200, 500, and 1000 mV s−1).

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ORR@Pt in N2-purged0.5 M Na2SO4 (pH=7.0)Different scan rates

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Ambient air (pH=6.7) N2-Steady (pH=7.0) N2-Live (pH=7.0) O2-Steady (pH=6.4) O2-Live (pH=6.4)

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A B

C D

Figure S7. Cyclic voltammograms showing catalytic ORR activity during proton reduction by Pt electrode from (A) 2 h N2-saturated, (B) freshly-prepared (ambient), and (C) 2 h O2-saturated 0.5 M Na2SO4 electrolytes recorded at different scan rates (viz. 20, 50, 100, 200, 500, and 1000 mV s−1). (D) Overlay of cyclic voltammograms for possible ORR activity at the scan rate of 50 mV s−1 for Pt electrode in 0.5 M Na2SO4 electrolyte with or without oxygen. CVs were also recorded with in-situ (live) purging of O2 and N2 gases at the flow rate of 30 mL cm−1.

IX

Cu2O MoSx/Cu2O

Cu2OCu: 89.6 wt%O: 10.4 wt%

MoS2/Cu2OCu: 89.7 wt%O: 9.6 wt%Mo: 0.4 wt%S: 0.3 wt%

Figure S8. EDS spectra of Cu2O and MoS2/Cu2O samples for elemental confirmation.

Figure S9. Cross-sectional SEM image (40,000X) of MoS2-modified Cu2O revealing coverage of MoS2 on Cu2O. A drift in the mapped image is observed due to longer exposure time.

X

The surface coverage of MoS2 ( ) on Cu2O is estimated using a relation below:3𝑛𝑀𝑜𝑆2

𝑛𝑀𝑜𝑆2= 𝜌𝑀𝑜𝑆2

∗ 𝑁𝐴 ∗ 𝑡𝑀𝑜𝑆2/𝑀𝑊𝑀𝑜𝑆2

where, is the density of MoS2, NA is the Avogadro’s number, is the film 𝜌𝑀𝑜𝑆2

𝑡𝑀𝑜𝑆2

thickness, is the molar mass of MoS2.𝑀𝑊𝑀𝑜𝑆2

= (5.06 g cm−3 * 6.023 1023 molecules mol−1 * 0.0000040 cm) /

𝑛𝑀𝑜𝑆2

(160.07 g mol−1)= 7.6158 1023 10-7 molecules cm−2

= 7.6158 ± 0.6 1016 molecules cm−2

Figure S10. SEM images of (A) N2-annealed MoS2 and (B) N2-annealed Cu2O on FTO substrate at lower and higher magnifications.

XI

Figure S11. Photocurrent responses of (A) N2-annealed MoS2-protected Cu2O photocathodes (prepared with different spin-coated layers; 1L, 2L, and 4L) and (B) N2-annealed Cu2O photocathode (3-segment measurement). Electrolyte: 0.5 M Na2SO4 solution (pH ~6.7); Light source: a 300 W Xe lamp; Illumination: simulated 1 sun (100 mW cm−2).

XII

Figure S12. (A) Normalized X-ray diffraction patterns and (B) SEM images (showing porous surface morphology) of N2-annealed Cu2O photoelectrode before and after PEC measurement.

XIII

Figure S13. Photocurrent responses of as-grown MoSx-protected Cu2O photocathode (with 3L spin-coated MoSx). Electrolyte: 0.5 M Na2SO4 with and without 0.1 M K2HPO4 solution; Light source: a 300 W Xe lamp; Illumination: simulated 1 sun (100 mW cm−2).

Figure S14. IPCE spectra of as-grown Cu2O and MoS2/Cu2O photocathodes at ~0.2 V vs. RHE (−0.4 V vs. SCE) under backside illumination. Electrolyte: 0.5 M Na2SO4 solution (pH ~6.7).

XIV

Figure S15. Long-term photostability tests of as-grown Cu2O and MoS2-modified Cu2O photocathodes at an applied potential of 0 V vs. RHE in 0.1 M phosphate buffered (pH ~7) 0.5 M Na2SO4 electrolyte.

XV

Figure S16. Normalized X-ray diffraction patterns of as-grown Cu2O and MoS2-modified Cu2O photocathodes before and after the 1 h photostability tests.

XVI

Figure S17. SEM images of (A) as-grown Cu2O and (B) MoS2-modified Cu2O photocathodes before and after the 1 h photostability tests.

References1. Garriga, J. M.; Llusar, R.; Uriel, S.; Vicent, C.; Usher, A. J.; Lucas, N. T.; Humphrey, M. G.; Samoc, M., Dalton Trans. 2003, 0, 4546-4551.2. Müller, A.; Wittneben, V.; Krickemeyer, E.; Bögge, H.; Lemke, M., Z. Anorg. Allg. Chem. 1991, 605, 175-188.3. Ray, S.; Steven, R. T.; Green, F. M.; Höök, F.; Taskinen, B.; Hytönen, V. P.; Shard, A. G., Langmuir 2015, 31, 1921-1930.