Supporting Information

Transcription

1 Supporting Information Topotactic Growth of Edge-Terminated MoS2 from MoO2 Nanocrystals Christian Dahl-Petersen 1, 2, Manuel Šarić 3, Michael Brorson 1, Poul Georg Moses 1, Jan Rossmeisl 4, Jeppe Vang Lauritsen 2, Stig Helveg 1,* 1 Haldor Topsoe A/S, Haldor Topsøes Allé 1, DK-2800 Kgs. Lyngby, Denmark 2 Interdisciplinary Nanoscience Center (inano), Aarhus University, Gustav Wieds Vej 14, DK Aarhus C, Denmark 3 Technical University of Denmark, Department of Physics, Fysikvej, DK-2800 Kgs. Lyngby, Denmark 4 University of Copenhagen, Department of Chemistry, Universitetsparken 5, DK-2100 Copenhagen, Denmark *Correspondence to, sth@topsoe.com (S. Helveg)

2 Statistical evaluation of MoS 2 observations. The presence of e-mos2 and p-mos2 is evaluated from high-resolution TEM images of samples after sulfidation at 250 and 300 C in 1 mbar of 10% H2S in H2. The images were acquired of areas, which were monitored by TEM during the sulfidation (in situ), and of areas, which were only exposed to the electron beam after sulfidation (post mortem). From the images the number of nanoparticles, N, is evaluated as the number of MoO2 particles observed in situ or post mortem. Each particle is characterized according to the presence of e-mos2 and p-mos2 in the corresponding image. The accumulated counts relative to the total number of imaged nanoparticles are listed in the table below. All the observations are obtained for particle sizes of 5 to 20 nm in diameter positioned at a maximum of 20 nm from the edge of the holes in the perforated heating chip membrane using either the Protochips Aduro or the FEI NanoEx system. Particles with e- MoS2 and p-mos2 are counted once per mode regardless of the number of MoS2 sheets grown. MoS2 growth is counted based on visual inspection and is denoted as sheets longer than 1 nm in length, in accordance with ref. 1. Table 1 shows that e-mos2 and p-mos2 formed at both 300 o C and 250 o C and that few nanoparticles did not contain the characteristic MoS2 lattice fringes at 250 o C while all did at 300 o C. Moreover, Table 1 indicates that more particles with p-mos2 and fewer with e-mos2 was formed at 300 o C than at 250 o C. Although a more elaborate analysis may be needed for confirmation of this finding, the trend is in line with the expectation that p-mos2 should be prevalent at even higher temperatures. 2

3 Table 1 In situ Post mortem Sulfidation temperature 250 o C Number of nanoparticles, N Number of nanoparticles with p-mos2 relative to N 80% 75% Number of nanoparticles with e-mos2 relative to N 14% 13% Sulfidation temperature 300 o C Number of nanoparticles, N Number of nanoparticles with p-mos2 relative to N 100% 100% Number of nanoparticles with e-mos2 relative to N 11% N.A.

4 Atomic density considerations. According to the monoclinic crystal structure of MoO2 and the hexagonal crystal structure of MoS2, the density of Mo atoms in a single sheet of MoO2(20-2) and MoS2(002) is: MoO2(20-2): 7.4 Mo/nm 2 MoS2(002): 11.6 Mo/nm 2 Figures 2 and 3, and S4 and S5 in the Supporting Information show a threefold periodicity for which three MoO2(20-2) lattice planes match one MoS2(002) sheet at the MoO2 and e-mos2 interface, suggesting that a MoS2(002) sheet emerges from a projection of Mo atoms from three sheets of MoO2(20-2) into one MoS2(002) sheet. The projection of three MoO2(20-2) sheets into one of the same length yields a Mo atom density of: Mo/nm 2 = 22.2 Mo/nm 2 This density is close to twice the density of the MoS2(002) plane, which contains 11.6 Mo/nm 2. In the experiments, the excess amount of Mo-atoms as compared to the MoS2(002) plane are probably expelled and contribute to p-mos2, which forms on the exterior surface of the MoO2 nanoparticles and simultaneous with e-mos2 (Figure S9 in the Supporting Information). The mechanism in Figure 4 also consistently accounts for the expelled atoms in the form of an outward extension of the e-mos2 sheets having a similar length as the sheets formed inward from the original MoO2 surface. For the MoO2(110) surface, the Mo atom density in the [-11-1]-rows corresponds to a Mo-Mo inter-spacing for the half-filled row and fully filled row configurations of: Lhalf-filled [-11-1] row = 0.74 nm Lfull-filled [-11-1] row = 0.37 nm

5 Lfull-filled [-11-1] row = 0.37 nm matches closely the shortest Mo-Mo distance along the MoS2[110] rows in the MoS2(002) plane, which is LMoS2 [110] row = 0.32 nm. The close lattice matching corroborates that the stepwise displacement and merger of pairwise half-filled [-11-1]-rows in the MoO2(110) surface generates a one-row high MoS2 sheet. The Mo atom densities in the different MoO2(010) and (110) surfaces are: MoO2(010): 1 Mo / (0.28 nm 0.24 nm) = 14.7 Mo/nm 2 Full row MoO2(110): 1 Mo / (0.34 nm 0.28 nm) = 10.5 Mo/nm 2 Half row MoO2(110): 0.5 Mo / (0.34 nm 0.28 nm) = 5.25 Mo/nm 2 Hence, the MoO2(010) is more densely packed than the MoO2(110) in both the full row and half row configurations and thus expected to be less reactive.

6 Theoretical details. The following specifies the calculation of the data points given in Figure 4e. a) Reference points defined by the sulfided surfaces and the chemical potentials of H2S and H2O. b) Mo surface restructuring of the MoO2(010) and (110) surfaces. For the MoO2(010) surface calculated as: DEb 010 = Eb Ea 010 and for the MoO2(110) surface: DEb 110 = Eb Ea 110 where Ea is the DFT total energy of the initial sulfided surface and Eb is the DFT total energy of the reconstructed surface. The factor of 2 arise from the unit cell after point a) being repeated twice to accommodate the space required for surface reconstructions. c) Reconstruction and sulfidation of the MoO2(010) and (110) surfaces. For the MoO2(010) surface calculated as: and for the MoO2(110) surface: DEc 010 = Ec Ea HH2O 6 GH2S DEc 110 = Ec Ea HH2O 7 GH2S where Ec is the calculated total energy of the surface after both sulfidation and reconstruction of an entire MoO2[101] or [-11-1] row of Mo-atoms and GH2O and GH2S are calculated free energies for H2O and H2S, respectively. d) Adding two MoS2 sheets on the MoO2(110) surface. The energy is calculated as: DEd 110 = Ed GH2O 15 GH2S ½ EMo4O8

7 where Ed is the calculated total energy for the MoO2(110) surface with four [-11-1] row high e- MoS2 sheets, and ½ EMo4O8 is half a unit cell equivalent to 2 MoO2. The additional Mo-atoms relative to the initial sulfided surface are referenced to the bulk MoO2 (EMoO2).

8 Figure S1. MoO2 nanocrystals in the as-prepared and sulfided states. a, A TEM image of the ammonium heptamolybdate tetrahydrate precursor after heating at 450 C, in accordance with the sample preparation procedure described in the Methods section. The inserts show Fast Fourier Transform (FFT) images of the red and yellow framed particles. b, c Close-ups of the two nanocrystals framed by red and yellow squares in (a) with an outline of crystal lattice planes from the FFTs in (a). f TEM image of the region in (a) after sulfidation at 250 C in 1 mbar 10% H2S in

9 H2 for 300 min. g, h Close-ups of the two nanocrystals framed by red and yellow squares in (f). d, e Ball models of the MoO2 crystal structure along the [111] and [010] directions. Blue spheres correspond to Mo-atoms and red spheres to O-atoms. i, j Simulations of the TEM image of the structures in (d) and (e). The simulated contrast pattern in (i) compares well with the observed contrast pattern in (g) (white frame) and the simulated contrast pattern in (j) compares well with the observed contrast pattern in (h) (white frame). The agreement between the simulated and observed contrast patterns supports the assumed MoO2 crystal structure.

10 Figure S2. MoO2 nanocrystal shape analysis. a TEM image of as-prepared MoO2 nanoparticles with round (R), square (S), needle-like (N) and irregular (I) shapes. b High-resolution TEM image of a square shaped nanoparticle. The crystal lattice fringes reveal a MoO2 nanocrystal imaged along [100] and terminated by (010), (001) and (011) facets. c High-resolution TEM image of a needle-shaped nanoparticle. The crystal lattice fringes correspond to a MoO2 nanocrystal terminated by an extended (100) facet and shorter (001) and (10-1) facets. Based on 12 different particles the ratio of the nanocrystal height and width is d(100):d(102)~ 1:2.7±0.6. Thus, (b) and (c) suggest that the MoO2 nanocrystals can obtain a desk-like shape including (100), (110) and (010) facet terminations as indicated in the sketch (e). d High-resolution TEM image of an irregular shaped nanoparticle. The crystal lattice fringes reveal a MoO2 nanocrystal imaged along [-324] and terminated by (2-12) and (211) facets. The present synthesis procedure therefore results in nanocrystal shapes with low and high-index facet terminations.

11 Figure S3. TEM images of MoO2 nanocrystals exposed to H2 or H2S/H2. a, b TEM images of MoO2 nanocrystals (a) before and (b) after the exposure to 0.1 mbar of 100% H2 at 250 C for 300 min, showing no significant changes of the crystal structure of the MoO2 particles. The inserts show Fast Fourier transforms (FFTs) of the images. In the FFTs, the superimposed circles indicate lattice spacing of (I) 0.48 nm, corresponding to MoO2 (010) and (001) lattice planes, (II) 0.34 nm, corresponding to MoO2 (011) and (110) lattice planes, and (III) 0.24 nm, corresponding to MoO2 (200), (20-2) and (111) lattice planes, respectively. c, d TEM images of MoO2 nanocrystals (c) before and (d) after the exposure to 1 mbar of 10% H2S in H2 at 250 C for 300 min. The inserted close-ups show the emergence of p-mos2 by the distinct lattice structure in the MoO2 surface with lattice spacing of 0.62 nm, corresponding to the MoS2 (002) lattice planes.

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13 Figure S4. Crystal lattice analysis of e-mos2 formed at 250 o C. a-f High-resolution TEM images and corresponding FFT images, after sulfidation for 300 min at 250 C in 1 mbar 10% H2S in H2, as well as simple ball-models of MoO2 nanocrystals in the observed crystal orientation. The e- MoS2 regions are marked by open arrowheads. The MoO2 [20-2] lattice vectors are directly resolved in (a, d-f), and are inferred from projected lattice vectors resolved in the FFTs in (b-c).

14 The MoS2(002) lattice spacing is strained at the MoO2:MoS2 interface and relaxes toward the bulk value (0.62 nm) at the e-mos2 surface (as in Figure 3). In (a-f), the MoS2(002) retains, however, the strained value of nm to the e-mos2 surface, which is caused by the short extension of the MoS2 sheets that prevents lattice relaxation in these examples. In the ball models, blue balls refer to Mo atom positions and red balls denote O atom positions.

15 Figure S5. Time-resolved TEM images of MoO2 nanocrystals in situ during sulfidation. The displayed images are cropped from images extracted from image series at times of (a, b, c) <0 min, 70 min and 250 min, (d, e, f) <0 min, 70 min and 250 min and (g, h, i) <0 min, 95 min and 165 min. The time of 0 min corresponds to the time of gas introduction in the experiment. The open arrowheads indicate locations of e-mos2. Sulfidation conditions: Total pressure of 1 mbar of 10% H2S in H2 at 250 C.

16 Figure S6. Crystal lattice analysis of e-mos2 formed at 300 C. a-b TEM images of e-mos2 on MoO2 nanocrystals after sulfidation at 300 C in 1 mbar 10% H2S in H2 for 240 min. In (b), the MoS2(002) has the strained value of 0.73 nm at the MoO2 interface and relaxes towards the bulk value of 0.62 nm close to the e-mos2 surface (as in Figure 3).

17 Figure S7. Temporal evolution of the e-mos2 sheet length. Based on the TEM images in Supplementary Video 1, the average length and the corresponding standard deviation of e-mos2 sheets are evaluated as a function of time during sulfidation (Time = 0 min corresponds to the introduction of 1 mbar of 10% H2S in H2 at 250 C). After 300 min of sulfidation, the average length converged at 2.8 nm. This length is shorter than the size of the MoO2 nanocrystal and probably reflects that the transport of sulfur into the bulk of the MoO2 crystal lattice is impeded by additional energy barriers.

18 Figure S8. Examination of electron beam illumination on e-mos2 and p-mos2. The distributions of sheet lengths are evaluated based on samples after sulfidation at 250 C for 300 min in 1 mbar of 10% H2S in H2. a Length distribution of e-mos2 sheets for sample regions that were illuminated by the electron beam during sulfidation (in situ) and b length distribution of e-mos2 sheets in sample regions that were only illuminated by the electron beam after sulfidation was terminated (post mortem). c Length distribution of p-mos2 sheets for sample regions that were illuminated by the electron beam during sulfidation (in situ) and d length distribution of p-mos2 sheets in sample regions that were only illuminated by the electron beam after sulfidation was terminated (post mortem). Sheets shorter than a threshold length of 1 nm are not included to reduce uncertainty from defocus effects. 1 The observations are binned into 0.25 nm (e-mos2) and 0.2 nm (p-mos2) intervals. The total number of particles analyzed (N), the length mean (µ) and its standard deviation

19 (σ) are given in the displays. In comparison, particles observed in situ and post mortem show similar distribution and projected average lengths indicating that the applied imaging protocol was sufficient to suppress electron-beam-induced alterations.

20 Figure S9. Sulfidation-induced volume expansion of MoO2. TEM images of MoO2 nanocrystals before (left column) and after (right column) sulfidation in 1 mbar of 10% H2S in H2. a The

21 sulfidation lasted for 50 min at 300 C and b-e for 300 min at 250 C. p-mos2 is marked by an open arrowhead and shows interlayer distance of approximately 0.63 nm. An outline (black line) of the nanocrystal periphery before sulfidation is superimposed on the images. The consistent observation of the crystal zone-axis before and after sulfidation in (b, e) and the isotropic expansion of the nanoparticle in (a-e) show that only negligible particle rotation occurred and that p-mos2 grew beyond the initial periphery as an added layer on the exterior surface of the nanocrystals.

22 Figure S10. Ball models of the sulfided MoO2(110) and (010) surfaces. a Top and side view of the sulfided MoO2(110) surface. b Top and side view of the sulfided MoO2(010) surface. The structural models were obtained using DFT (Methods). In the ball models, Mo atoms in full [-11-1] rows on MoO2(110) and full [101] rows on MoO2(010), as in the corresponding bulk structures, are denoted by dark blue spheres, Mo in half-filled [-11-1] rows (half row) on MoO2(110) are denoted by light blue spheres, O atoms are denoted by red spheres and S atoms are denoted by yellow spheres. The numbers superimposed on the ball models correspond to row numbers in Figure 4 and Figure S11 in the Supporting Information.

23 Figure S11. Atomic mechanism for topotaxy of e-mos2 on MoO2(010). a-c The topotactic transformation is illustrated by ball models of the atomic structures at an inclined projection (left) and sketches of the [101] oriented atomic rows with Mo in the center and the anionic content at the rim (right). The color coding is dark blue for Mo in [-11-1] rows with the bulk density (full row), red for O and yellow for S. Crystal directions are quoted with reference to the MoO2 lattice. The surface structures are obtained by DFT calculations. a Initial state for the sulfided MoO2(010) surface (indicated by the dashed line). b The sulfided MoO2(010) surface in (a) after displacement of row 1. c The sulfided MoO2(010) surface in (b) after displacement of row 2 and surface O-S exchange of exposed oxide surface sites. For each state (a-c), the calculated Gibbs free energy per unit cell is given in Figure 4e. Crystal directions are quoted with reference to the MoO2 lattice.

24 Figure S12. Mass spectrometry of the sulfiding gas. Mass spectrometry during an in situ sulfidation experiment of the 10% H2S in H2 gas mixture, showing ion currents corresponding to gases: H2, H2S, S, HS, H2O and O. After 5 min at a constant pressure of 1 mbar, a constant gas composition is obtained. The mass spectra are acquired at the second differential pumping stage of the electron microscope and reflect the major constituents in the gas introduced into the sample.

25 Video S1. The video accompanies Figure 2. The movie shows a MoO2 nanocrystal during the exposure to 1 mbar of 10% H2S in H2 at 250 C. The images are recorded with 15 min intervals, with the first image equivalent to 45 min of gas exposure. The movie is displayed with 2 frames per second. Each frame corresponds to an image series of 5 exposures, which are post-aligned using cross-correlation and summed to improve the image signal-to-noise ratio. Supplementary references: 1 Hansen, L. P., Johnson, E., Brorson, M. & Helveg, S. Growth Mechanism for Single- and Multi-Layer MoS2 Nanocrystals. J. Phys. Chem. C 2014, 118, Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. High-Rate, Gas-Phase Growth of MoS2 Nested Inorganic Fullerenes and Nanotubes. Science 1995, 267,