a b c Supplementary Figure S1

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1 a b c Supplementary Figure S1 AFM measurements of MoS 2 nanosheets prepared from the electrochemical Liintercalation and exfoliation. (a) AFM measurement of a typical MoS 2 nanosheet, deposited on Si/SiO 2 substrate, gives an average thickness of ~ 1.0 nm, confirming that single-layer MoS 2 was successfully fabricated. (b-c) AFM images of many MoS 2 nanosheets deposited on Si/SiO 2 substrates. S1

2 Supplementary Figure S2 High angle annular dark field scanning transmission electron microscopy (HAADF STEM) images of synthesized hybrid materials. HAADF STEM images of (a) Pd-MoS 2 and (b) Pt- MoS 2 hybrid nanomaterials. HAADF STEM image gives a contrast that is proportional to the square of the atomic number of the scattering material (Z 2 ). Since Pd and Pt have larger atomic numbers than MoS 2, the metal nanoparticles appeared bright in the dark field image, distinguishable from the MoS 2 nanosheets. S2

3 Supplementary Figure S3 TEM analysis of Pd-MoS 2 hybrid nanomaterials. (a) SAED pattern of a Pd-MoS 2 hybrid nanomaterial, which is also shown in Figure 1b. The six spots of Pd that are aligned with the six spots of {100} MoS2 show a corresponding lattice spacing of Å. Hence, they can be assigned to both the {111} Pd and 1/3{422} Pd planes with theoretical lattice spacings of 2.21 and 2.35 Å, respectively. (b-c) HRTEM images of Pd NPs on MoS 2, which are also shown in Figure 1C-D in the main text. (d-h) FFT-generated SAED patterns of Pd NPs indicated in (b-c). S3

4 Supplymentary Figure S4 HRTEM image of a typical MoS 2 nanosheet. Defect regions can be observed possibly due to the Li intercalaion and exfoliation induced structure damage, as well as the surface oxidation. Examples of surface defects are indicated by circles. S4

5 Supplementary Figure S5 TEM analysis of Pt-MoS 2 hybrid nanomaterials. (a-c) HRTEM images of Pt NPs on MoS 2, which are also shown in Figure 2d-f in the main text. (d-f) FFT-generated SAED patterns of Pt NPs indicated in (a-c). S5

6 Supplementary Figure S6 TEM analysis of Ag-MoS 2 hybrid nanomaterials. (a) TEM image of Ag NPs synthesized on an MoS 2 nanosheet. (b) SAED pattern of Ag-MoS 2 hybrid nanomaterial with the electron beam perpendicular to the basal plane of MoS 2 nanosheet. (c-d) HRTEM images of Ag NPs on MoS 2, showing the distinguishable lattice fringes for Ag and MoS 2. Insets in (c-d): FFT generated SAED of the NPs in (c-d) marked by arrows. In (b), the six spots of {202} Ag planes, with a corresponding lattice spacing of ~1.4 Å, are aligned with the six spots of {110} MoS2 with a lattice spacing of ~1.6 Å. Besides, another six spots of Ag with corresponding lattice spacing of Å show the alignment with six spots of {100} MoS2. These six spots of Ag can be assigned to both the {111} Ag and 1/3{242} Ag planes with theoretical lattice spacings of 2.32 and 2.46 Å, respectively. The HRTEM images in (c-d) show the lattice spacing of 2.7 Å with the six-fold symmetry, which can be assigned to the {100} planes of MoS 2 nanosheet. In (c), an Ag NP show the hexagonal lattice spacing of 2.5 Å corresponding to the 1/3{422} reflections. The FFT generated diffraction pattern with the six-fold symmetry confirms the (111) orientation of the Ag NP. In (d)an Ag NP shows lattice fringes with inter-plane spacings of 2.3 and 2.0 Å, which can be attributed to the {111} Ag and {220} Ag planes, respectively. The FFT generated diffraction pattern indicates that this Ag NP is oriented along the (101) zone axis. Based on the SAED and HRTEM data, two types of epitaxial relationship between the Ag NPs and MoS 2 nanosheet were confirmed and defined by (1) [1 2 1] Ag [100] MoS2 and (101) Ag (001) MoS2 and (2) [110] Ag [100] MoS2 and (111) Ag (001) MoS2, respectively. S6

7 Supplementary Figure S7 TEM analysis of Au-MoS2 hybrid nanomaterials. (a) TEM image of Au NPs synthesized on an MoS 2 nanosheet. (b) SAED pattern of the Au-MoS 2 hybrid nanomaterial with the electron beam perpendicular to the basal plane of MoS 2 nanosheet shows the diffraction rings of Au{111}, {200} and {220}, without the preferred alignment with the spots of MoS 2 {100} or {110}. (c) HRTEM image of a portion of an Au NP and MoS 2 nanosheet shows the hexagonal lattice fringes of 2.7 Å corresponding to the MoS 2 {100} planes. Crystal defects are observed in the Au NP as indicated by the arrows. Therefore, the epitaxial growth of Au on MoS 2 was not observed. S7

8 Intensity (a.u.) Mo 4+ (1T) Mo 4+ (2H) Mo 6+ S 2- Mo3d Binding Energy (ev) Supplementary Figure S8 XPS Mo3d spectrum of as-prepared MoS 2 nanosheets. The spectrum indicates the coexistence of the 2H and 1T phases. Energy values were calibrated by using the C1s level of ev. S8

9 Supplementary Figure S9 Schematic illustration of the crystal structure of MoS 2. (a) Out of plane and (b) in plane view of the 2H and 1T MoS 2 structures. S9

10 Supplementary Figure S10 SAED pattern of several Ag nanoplates lying on an MoS 2 nanosheet. The 1/3{422} Ag and {201} Ag spots are aligned with the {100} MoS2 and {110} MoS2 spots, respectively. S10

11 Supplementary Figure S11 Energy-dispersive X-ray spectroscopy (EDS) analysis of Pt-MoS 2 hybrid nanomaterials deposited on a Si/SiO 2 substrate. The Pt content in the Pt-MoS 2 hybrid nanomaterials is 36 wt%. S11

12 Supplementary Figure S12 TEM image of commercial 10% Pt on activated charcoal. The Pt nanoparticles are in the size of 1-3 nm. S12

13 Supplementary Figure S13 XPS Pt4f spectrum of Pt-MoS 2 hybrid nanomaterials deposited on Si/SiO 2 substrate. Energy values were calibrated by using the C1s level of ev. The sub-bands at 72.3 ev, 73.2 ev, and 74.9 ev can be assigned to Pt δ+, Pt 2+ and Pt 4+, respectively The absence of Pt 0 and the presence of the major Pt δ+ bands likely result from the substrate-catalyst interaction, i.e., the electrons transfer from the Pt NPs to the MoS 2 nanosheet 52,54. S13

14 Supplementary Figure S14 HRTEM analysis of Pt-MoS 2 hybrid nanomaterials obtained after the photochemical reaction proceeded for 30 min. (a) HRTEM image of Pt clusters nucleated on MoS 2 nanosheet. (b) Enlarged TEM image of a typical Pt cluster. Its inter-atomic distance R Pt-Pt, as indicated by a red dashed line in (b), is given in (c). S14

15 Supplementary Figure S15 TEM image of Pt NPs synthesized by photochemical reduction of K 2 PtCl 4 in the absence of MoS 2 nanosheets. The particles are ~30 nm in size and aggregated. S15

16 Supplementary Figure S16 TEM analysis of metal-mos 2 hybrid nanomaterials prepared in the absence of surfactants. TEM images of (a-b) Pd-MoS 2 and (c) Ag-MoS 2 hybrid nanomaterials prepared in the absence of surface capping molecules such as PVP or CTAB. S16

17 Supplementary References: 51 Dablemont, C. et al. FTIR and XPS study of Pt nanoparticle functionalization and interaction with alumina. Langmuir 24, (2008). 52 Sen, F. & Gokagac, G. Different sized platinum nanoparticles supported on carbon: an XPS study on these methanol oxidation catalysts. J. Phys. Chem. C 111, (2007). 53 Borodko, Y., Ercius, P., Pushkarev, V., Thompson, C. & Somorjai, G. From single Pt atoms to Pt nanocrystals: photoreduction of Pt 2+ inside of a PAMAM dendrimer. J. Phys. Chem. Lett. 3, (2012). 54 Murgai, V., Raaen, S., Strongin, M. & Garrett, R. F. Core-level and valence-band photoemission study of granular platinum films. Phys. Rev. B 33, (1986). S17