Supporting Information

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1 Supporting Information CO 2 -Induced Phase Engineering: A Protocol for Enhanced Photoelectrocatalytic Performance of 2D MoS 2 Nanosheets Yuhang Qi, Qun Xu*, Yun Wang*, Bo Yan, Yumei Ren, Zhimin Chen Table of Contents 1. Experimental sections 2. Supplementary Figures Figure S1. The schematic process. Figure S2. SEM images of the MoS 2 nanosheets. Figure S3. Low-magnification TEM of the MoS 2 nanosheets. Figure S4. Lateral size of the 2D nano-flake distribution. Figure S5. AFM images of the exfoliated MoS 2 nanosheets. Figure S6. Thickness distribution of the exfoliated MoS 2 nanosheets. Figure S7. HRTEM of lateral heterostructures. Figure S8. HRTEM of phase boundary. Figure S9. Raman characterizations of MoS 2 nanosheets. 3. Computational details 4. References 1

2 1. Experimental sections Materials: Commercially available MoS 2 powder was purchased from Sigma Aldrich (Fluka, Product Number: 69860). According to the description of products, the starting MoS 2 powder has representative lateral particle sizes in the range of 6-40 µm. Ethanol used in all experiments was purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used without further purification since the reagent is of analytical grade. CO 2 with purity of 99.99% was provided by the Zhengzhou Shuangyang Gas Co. and used as received. Aqueous solution was prepared with double-distilled water. Preparation of lateral heterostructures of MoS 2 nanosheets: Exfoliation Process: MoS 2 powder (50 mg, 6-40 mm, Sigma-Aldrich Reagent Inc.) was added to 100 ml flask. 10 ml of ethanol/water mixtures with ethanol volume fractions of 50% were added as dispersion solvents. The dispersion in the sealed flask was sonicated in the bath for 6 h, and then the dispersion was centrifuged at 5000 rpm for 15 mins to remove aggregates. The supernatant (top three quarters of the centrifuged dispersion) was collected by pipette. Phase transformation process. The supernatant was then quickly transferred into the supercritical CO 2 apparatus composed mainly of a stainless steel autoclave (50 ml) with a heating jacket and a temperature controller. The autoclave was heated to K, and CO 2 was then charged into the autoclave to the desired pressure under stirring. After a reaction time of 6 h, the gas was slow released. Finally the dispersion was collected. Characterization. The morphologies of MoS 2 powder and sediment were characterized by field-emission SEM (JEOR JSM-6700F). Tapping-mode AFM (Nanoscope IIIA), HRTEM (JEOL JEM-2100F), and TEM (JEOL JEM-2100) were used to study the morphology of the nanomaterials. UV/Vis spectra (Shimadzu UV- 240/PC) were measured to evaluate MoS 2 dispersions concentration. The Raman 2

3 measurements were carried out on a Renishaw Microscope System RM2000 with a 50mWAr + laser at nm. Photoelectrochemical (PEC) Measurements: The PEC measurements were taken using an electrochemical workstation (CHI660D) with a typical three-electrode cell. The as-prepared sample was used as the working electrode, a Ag/AgCl electrode and Pt wire were used as reference and counter electrode, respectively. 0.5 mol L 1 Na 2 SO 4 was used as the electrolyte. The working electrodes were prepared by dropping the suspension onto the surface of a clean fluorine-doped tin oxide (FTO) glass substrate. The light ON-OFF switches were set as 200s when measuring the I-t curves of the absolute values under visible light. The bias for the measurement was set as -0.7 V. 3

4 2. Supplementary Figures Figure S1. The schematic process to produce 2D atomic heterostructures of MoS 2 nanosheets from layered bulk materials in the CO 2 /ethanol/water system environment. 4

5 Figure S2. SEM images (a, b) bulk MoS2 (c, d) exfoliated MoS2 nanosheets have been treated with supercritical CO2 for 6 h (353.2 K, 18MPa). 5

6 Figure S3. TEM images of MoS 2 nanosheets that have been treated with supercritical CO 2 for 6 h (353.2 K, 18MPa). 6

7 Figure S4. Lateral size of the exfoliated MoS 2 nanosheets distribution. The dynamic light scattering (DLS) is a convenient and efficient method to estimate the size of the MoS 2 nanosheets. 1,2 The obtained MoS 2 nanosheets show the polydispersity of their lateral dimensions based on the DLS pattern, which ranges mainly between 50 and 250 nm, and the lateral dimension is in accord with the above-mentioned SEM and TEM images. 7

8 Figure S5. Typical AFM images of exfoliated MoS 2 nanosheets. As shown in Fig. S4, the thickness of the exfoliated MoS 2 nanosheets is 1-2 nm, close to the theoretical thickness value of single layer MoS 2 in the range of nm. 3 Therefore the AFM image confirmed again that bulk MoS 2 crystals were exfoliated into mono-layer or few-layer nanosheets. 8

9 Figure S6. Thickness distribution of the MoS 2 nanosheets. The statistical analysis based on AFM measurements (Figure S4) indicates that the MoS 2 nanosheets have various thicknesses, while most of them are in the range of 1-3 layers. 9

10 10

11 Figure S7. HRTEM image of the lateral heterostructures of MoS 2 obtained at different ranges. 11

12 Figure S8. a) HRTEM image of the lateral heterostructures of MoS2. b) A filtered image of a). Inset: fast Fourier transform. c) zoom-in image of the zone indicated by the blue square in b). d) Schematic illustration of the formation mechanism for 1T@2H-MoS2 heterostructures. 12

13 Figure S9. Raman spectra recorded using a 356 nm laser for bulk MoS 2, 2H-MoS 2 nanosheets and 1T@2H-MoS 2 nanosheets. Through the Raman mapping of MoS 2 shown in Figure S7, we can confirm that the few-layered MoS 2 sheets were successfully fabricated. The bulk MoS 2 samples shows bands at 375 (E 1 2g) and 403 (A 1g ) cm 1. The intensities of the bands become significantly enhanced. And it can be clearly seen from the Raman map that the full-widths at half-maximum values are obviously increased in the obtained products than in the bulk samples, which are possibly attributed to phonon confinement by facet boundaries. 4,5 Moreover it can found that the frequency of E 1 2g peak increases while that of the A 1g peak decreases for 2H-MoS 2. And both the frequency of E 1 2g and A 1g peak increase for 1T@2H-MoS 2. 13

14 3. Computational details All density functional theory (DFT) computations were performed using the Vienna ab initio simulation package (VASP) based on the projector augmented wave (PAW) method. 6,7 Electron-ion interactions were described using standard PAW potentials, with valence configurations of 4s 2 4p 6 5s 2 4d 4 for Mo (Mo_sv_GW), 3s 2 3p 4 for S (S_GW), 2s 2 2p 2 for C (C_GW_new) and 2s 2 2p 4 for O (O_GW_new). A plane-wave basis set was employed to expand the smooth part of wave functions with a cut-off kinetic energy of 520 ev. For the electron-electron exchange and correlation interactions, the functional parameterized by Perdew-Burke-Ernzerhhof (PBE)), 8 a form of the general gradient approximation (GGA), was used throughout. Since traditional DFT calculations cannot correctly include the non-local van der Waals (vdw) interactions, the DFT calculations with dispersion corrections may affect the interaction between layers of 2D materials. 9,10,11 Herein, the DFT-D3 method has been used for dispersion corrections. 12 The single-layer (SL) MoS 2 has been modelled with the (2 2) surface cell, which is separated by a vacuum region of at least 15 Å. When the geometries were optimized, all the atoms were allowed to relax until the Hellmann-Feynman forces were smaller than 0.001eV/Å. The convergence criterion for the electronic self-consistent loop was set to 10-5 ev. We performed Brillouin-zone integrations using a gamma-centered (4 4 1) k-point grid. The adsorption energy E ad was calculated as following, E ad = [E mol/surf (n E mol + E surf )]/n where E mol is the energy of an isolated CO 2 molecule; E surf is the energy of clean SL slab; and E mol/surf is the total energy of the surface with adsorbed CO 2 molecules. Here, n is the number of CO 2 adsorbed on the SL MoS 2 in each unit cell. A Å 3 unit cell was used for the calculations on the isolated CO 2 molecule using a Γ-only k-point grid. According to the equation to calculate the adsorption energy, a negative value of E ad indicates an exothermal process. 14

15 Figure S10. Illustration of theoretical calculation about layered structure of the 2H phase (a) and 1T phase (b), respectively. Computational details: DFT-D3 E cut = 520 ev. (Yellow - sulfur, and grey - molybdenum) 15

16 Figure S11. Illustration of theoretical calculation about single layered structure of the 2H phase (a) and 1T phase (b), respectively. The total energies demonstrate that the 2H phase is more thermodynamically stable than 1T, both with bulk and SL structures. 16

17 4. References [1] Gopalakrishnan, D.; Damien, D.; Shaijumon, M. M., MoS 2 Quantum Dot-Interspersed Exfoliated MoS 2 Nanosheets. Acs Nano 2014, 8, [2] Wang, Y. C.; Ou, J. Z.; Balendhran, S.; Chrimes, A. F.; Mortazavi, M.; Yao, D. D.; Field, M. R.; Latham, K.; Bansal, V.; Friend, J. R.; Zhuiykov, S.; Medhekar, N. V.; Strano, M. S.; Kalantar-zadeh, K., Electrochemical Control of Photoluminescence in Two-Dimensional MoS 2 Nanoflakes. Acs Nano 2013, 7, [3] Frindt, R. F. Single Crystals of MoS 2 Several Molecular Layers Thick, J Appl Phys 1966, 37(4): [4] Lee, C.; Yan, H.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S., Anomalous Lattice Vibrations of Single- and Few-Layer MoS 2. Acs Nano 2010, 4, [5] Li, H.; Zhang, Q.; Yap, C. C. R.; Tay, B. K.; Edwin, T. H. T.; Olivier, A.; Baillargeat, D., From Bulk to Monolayer MoS 2 : Evolution of Raman Scattering. Adv Funct Mater 2012, 22, [6] Kresse, G.; Hafner, J., Ab Initio Molecular Dynamics for Liquid Metals, Physical Review B 1993, 47(1): 558; [7] Kresse, G.; Furthmüller, J.,Ab Initio Pseudopotentials for Electronic Structure Calculations of Poly-Atomic Systems Using Density-Functional Theory, Physical Review B 1996, 54(16): [8] Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple, Phys Rev Lett 1996, 77(18): [9] Bjorkman, T.; Gulans, A.; Krasheninnikov, A. V.; Nieminen, R. M., Van Der Waals Bonding in Layered Compounds from Advanced Density-Functional First-Principles Calculations. Phys Rev Lett 2012, 108. [10] Dobson, J. F.; White, A.; Rubio, A., Asymptotics of the Dispersion Interaction: Analytic Benchmarks for van der Waals Energy Functionals. Phys Rev Lett 2006, 96. [11] Klimes, J.; Bowler, D. R.; Michaelides, A., Van der Waals Density Functionals 17

18 Applied to Solids. Phys Rev B 2011, 83. [12] Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J Chem Phys 2010,