Supporting information: Engineering the Edges of MoS 2 (WS 2 ) Crystals for Direct Exfoliation into Monolayers in Polar Micromolecular Solvents

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1 Supporting information: Engineering the Edges of MoS 2 (WS 2 ) Crystals for Direct Exfoliation into Monolayers in Polar Micromolecular Solvents Xiao Hai ζ, Kun Chang* ζ, Hong Pang, Mu Li, Peng Li, Huimin Liu, Li Shi and Jinhua Ye* # Graduate School of Chemical Science and Engineering, Hokkaido University, Sapporo , Japan. Photocatalytic Materials Group, International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki , Japan. # TU-NIMS Joint Research Center, School of Materials Science and Engineering, Tianjin University, 92 Weijin road, Tianjin, P. R. China. Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin , P. R. China. ζ These authors contributed equal to this work. * Chang.Kun@nims.go.jp; Jinhua.Ye@nims.go.jp 1

2 Figure S1: SEM image of the commercial MoS 2 with an average lateral size larger than 2 um. Figure S2: Measurement of the interlayer distance of as-prepared bulk MoS 2 crystal. 2

3 Figure S3: (a) XRD patterns of the as-prepared and commercial WS 2. The standard diffraction peak position of 2H-WS 2 (PDF ) is displayed at the bottom of diagram. An estimation of the lateral size from the fullwidth at half-maximum of the (110) reflection is about 180 nm. Furthermore, the (002) peak is also characterized by a shift to a lower angle as compared to commercial WS 2. (b) SEM image. It can found that the as-prepared WS 2 precursor is composed of nanoflakes with a lateral size less than 200 nm. 3

4 Figure S4: TEM images of the exfoliated nanosheets at the beginning (a) and with further sonication (b). The bulk crystals are first decomposed into individual thick nanoflakes. Then, these thick nanoflakes will be further exfoliated into thin nanosheets under continuous sonication. 4

5 Figure S5: Optimization of MoS 2 dispersions in ethanol. (a) Concentration of nanosheets after sonication for different times. The concentration of exfoliated MoS 2 nanosheets was quickly increased in the first 48 h and slowed down, reaching the maximum in exfoliation. So, we sonicated the bulk precursors in all polar micromolecular solvents for 48 h. (b) Concentration of nanosheets after centrifugation at different rates. The obtained dispersions centrifuged at different rates were studied by AFM to check the exfoliated nanosheets. We found that the best centrifugation process was first centrifuged at 2000 rpm (RCF 760g) for 1 h, the top 2/3 supernatants were carefully collected by pipette. The obtained supernatants were added into a new glass vial and sonicated again for another 4 h. Then the monolayer dispersions were collected by pipette followed by another centrifugation of the supernatants at 4000 rpm (RCF 3020g) for 1 h. (c) Photograph of the obtained monolayer MoS 2 nanosheets dispersed in ethanol. 5

6 Figure S6: Example spectra of the second derivative of the absorption spectra of MoS 2 dispersions in different polar micromolecular solvents used to preciously assess the peak position of the A-exciton and B-exciton. To exclude the contribution from size-dependent scattering background, the peak position was collected form the second derivative of the absorption spectra 1. The accurate position of A- and B-exciton was determined to be located at 670 and 615 nm, respectively. 6

7 Figure S7: (a-e) Optical absorption spectra versus concentration of MoS 2 dispersions in different polar micromolecular solvents, insets are the absorption at A-exciton with the change of concentration. The absorption intensity at A-exciton scaled linearly with concentration for all solvents. (f) Photographs of dispersions in quartz cuvettes with a pathlength of 2 mm. 7

8 Figure S8: (a-e) Absorption at A-exciton versus concentration of WS 2 dispersions in different micromolecular solvents. The absorption at A-exciton also scaled linearly with concentration for all solvents. (f) Photograph of dispersions in quartz cuvette under the illumination of incandescent. 8

9 Figure S9: (a) AFM and (b) 3D model images of the MoS 2 dispersions deposited on mica substrates by pipet and let them dry naturally. The high fluidity and volatility features of these solvents allow the dispersed nanosheets to quickly and uniformly deposit on mica substrates. 9

10 Figure S10: AFM images of obtained monolayer MoS 2 nanosheets. 10

11 Figure S11: AFM images of obtained monolayer WS 2 nanosheets. 11

12 Figure S12: Concentration dependent photoluminescence spectra of (a) monolayer MoS 2 dispersion and (c) monolayer WS 2 dispersion. PL intensity of the A-exciton as a function of concentration for (b) MoS 2 monolayer and (d) WS 2 monolayer. We found the strongest PL intensity obtained from mg/ml dispersion for MoS 2 and mg/ml dispersion for WS 2, respectively. 12

13 Figure S13: PL spectra of bulk, partial monolayer and entire monolayer dispersions with the same concentration for (a) MoS 2 with a concentration of mg/ml and (b) WS 2 with a concentration of mg/ml. Bulk dispersion was obtained by adding a certain amount of bulk crystal into ethanol, then sonicated for 10 min. Partial monolayer dispersion was obtained by adding a certain amount of bulk crystal into ethanol, then sonicated for 48 h and collected by pipet followed by centrifugation at 1500 rpm for 20 min. Figure S14: Average lateral size measured by particle size distribution analysis for (a) MoS 2 and (b) WS 2 monolayers. 13

14 Figure S15: Absorption spectra of dispersions versus standing days. (a) MoS 2 monolayers dispersed in water. (b) MoS 2 monolayers dispersed in ethanol. (c) WS 2 monolayers dispersed in water. (d) WS 2 monolayers dispersed in ethanol. 14

15 Figure S16: Zeta potential measurement of (a) MoS 2 and (b) WS 2 dispersions in ethanol with a concentration of 0.1mg/mL. 15

16 Figure S17: (a) Catalytic hydrogen activities of MoS 2 monolayers versus ph value. We evaluated the catalytic hydrogen activities of MoS 2 monolayers by loaded them on CdS for photocatalytic hydrogen production. Light source: 300-W Xe lamp with a L42 cut-off, λ > 420 nm; Reaction solution: 300 ml lactic acid solution (30 vol.%, ph=1.45), ph value was adjusted by NaOH; Catalyst: 200 mg, cocatalyst: 1.5wt %. (b) Photocatalytic hydrogen evolution system. 16

17 Figure S18: Photocatalytic H 2 evolution activities of MoS 2 /CdS and WS 2 /CdS under visible light irradiation. Light source: 300-W Xe lamp with a L42 cut-off, λ > 420 nm; Reaction solution: 300 ml sodium lactate solution (30 vol. % lactic acid adjusted by NaOH, ph=5.0); Catalyst: 200 mg; MoS 2 /CdS with 2 wt% cocatalyst and WS 2 /CdS with 3 wt% cocatalyst. Bottom: the photographs of MoS 2 /CdS and WS 2 /CdS composites. 17

18 Figure S19: Photocatalytic H 2 evolution activities of Pt/CdS under visible light irradiation. Light source: 300-W Xe lamp with a L42 cut-off, λ > 420 nm; Reaction solution: 300 ml lactic acid solution (30 vol. %, ph=1.45), 300 ml sodium lactate solution (30 vol.% lactic acid adjusted by NaOH, ph=5.0), 300 ml Na 2 S-Na 2 SO 3 solution (ph=14); Catalyst: 200 mg; cocatalyst: 0.65 wt% 2. 18

19 Figure S20: SEM images of pure CdS (a,b), MoS 2 /CdS (c) and WS 2 /CdS (d). Pure CdS surface is very smooth, the MoS 2 and WS 2 monolayers can be easily and uniformly adsorbed on the surface of CdS due to their soft and small size features. 19

20 Figure S21: Comparison of absorption spectra of MoS 2 dispersions and MoS 2 /CdS (a), WS 2 dispersions and WS 2 /CdS (b). Inset of (a) is the second derivative of the absorption spectra. Peak positon of the A-exciton is closely related to the nanosheets thickness, it will be red-shifted with the increase of nanosheets thickess 1. After loaded on CdS, peak positions of the A-exciton of MoS 2 and WS 2 do not have any movement compared with MoS 2 and WS 2 dispersions. This result indicates that MoS 2 and WS 2 monolayers are individually absorbed on the surface of CdS, and avoid restacking. 20

21 Figure S22: Photocatalytic H 2 evolution activities of MoS 2 /CdS and WS 2 /CdS composites with different annealing temperature. Light source: 300-W Xe lamp with a L42 cut-off, λ > 420 nm; Reaction solution: 300 ml sodium lactate solution (30 vol.% lactic acid adjusted by NaOH, ph=5.0); Catalyst: 200 mg; MoS 2 /CdS with 2 wt% cocatalyst and WS 2 /CdS with 3 wt% cocatalyst. Supplementary reference (1) Backes, C.; Smith, R. J.; McEvoy, N.; Berner, N. C.; McCloskey, D.; Nerl, H. C.; O Neill, A.; King, P. J.; Higgins, T.; Hanlon, D.; Scheuschner, N.; Maultzsch, J.; Houben, L.; Duesberg, G. S.; Donegan, J. F.; Nicolosi, V.; Coleman, J. N. Nat. Commun. 2014, 5, (2) Chang, K.; Li, M.; Wang, T.; Ouyang, S. X.; Li, P.; Liu, L. Q.; Ye, J. H. Adv. Energy. Mater. 2015, 5,