Synthesis of MoS 2 and MoSe 2 films with vertically aligned layers

Transcription

1 Supporting Information for Synthesis of MoS 2 and MoSe 2 films with vertically aligned layers Desheng Kong 1, Haotian Wang 2, Judy J. Cha 1, Mauro Pasta 1, Kristie J. Koski 1, Jie Yao 1, and Yi Cui 1,3,* 1 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA 2 Department of Applied Physics, Stanford University, Stanford, CA 94305, USA 3 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA * yicui@stanford.edu

2 Contents Figures S1: TEM images at select tilt angles for tomography. S2: Analysis of TEM tomogram. S3: Composition characterization. S4: Estimation of the edge site density of nanoparticulate MoS 2 electrodes. S5: Nyquist plots of impedance spectroscopy analysis of the electrochemical cells. S6: Optical transmittance of edge-terminated MoS 2 and MoSe 2 films. S7: MoS 2 films grown on rough glassy carbon substrates and the corresponding electrochemical measurements. S8: The rapid sulfurization process directly applied to molybdenum foil. Tables S9: Summary of AFM measurements. S10: Summary of electrochemical measurements. Movies S11: Reconstructed MoS 2 film viewing at an arbitrary viewing angle. S12: Reconstructed MoS 2 film viewing from the top.

3 Figures S1. TEM images at select tilt angles for tomography. Each image corresponds to a dimension of nm2. A series of TEM images are captured by tilting the sample from -64o to 64o with respect to the electron beam. The grains show up in the TEM image when the layers are aligned with the beam. Accordingly, a majority of the grains are clearly visible at low tilt angles, as expected from the edge surface texture. Au colloid nanoparticles (from Ted Pella, Inc.) were deposited on the film as fiducial markers to track sample drifts during tilting for post-alignment of the tilt series.

4 S2. Analysis of TEM tomogram. (a) Two orthogonal cross-sections of the reconstructed film. The tilted grains are clearly revealed in the vertically-sliced cross-section. Images of the sliced cross-sections are used to calculate the tilt angles of the layers. When the layers are roughly parallel to the slice plane, the grains appear either missing or exhibit much larger layer spacing in the cross-sections, as evident by the empty spaces between two readily identified grains (indicated by the yellow color regions). For these grains, another orthogonal slice plane is used for the analysis. (b) A schematic representation of the tilt angle θ, interlayer distance d, and spacing of the layer s on the film surface. The majority of grains are perpendicular to the substrate, with small tilt angles less than 20 o. We note that a large tilt angle may modify the density of active sites by changing the spacing, s, of the layers with respect to the interlayer distance, d, via the relation, s d / cos( θ) =. We calculate the average spacing between layers for MoS 2 film considering observed tilt angles and yield not affect the density of active sites significantly. s = d. It suggests the small tilt angles do mean 1.07 S3. Composition characterization. The characterizations are performed inside a TEM. (a) Electron energy spectrum (EELS) collected from a sulfurized sample reveal spectral features of Mo-M (shaded in green) and (shaded in yellow) edges. The dotted lines represent power-law fits for background subtraction. Using

5 Hartree-Slater cross-sections for Mo and S in Digital Micrograph, elemental analysis of the spectrum shows a Mo atomic percentage of (32.9±5.4) %, consistent with the expected stoichiometry for MoS 2. (b) Energy dispersive spectrum (EDS) from a selenized sample. Counts for Se core edge in EELS are too low, so EDS is used to analyze the selenized sample. Integration of the spectrum yields a Mo atomic percentage of (35.1±0.4) %, close to the expected stoichiometry for MoSe 2 phase. The observed Ni peaks are from the Ni TEM grid. S4. Estimation of the edge site density of nanoparticulate MoS 2 electrodes. The low inter-layer conductivity of MoS 2 suggests single-layer nanoplatelets are likely the optimal structure for nanoparticulate electrodes 1. Single-layer MoS 2 nanoplatelets with hexagonal or triangular morphology can be grown on Au (111) surface by vapor-phase synthesis under ultrahigh vacuum condition 2. Here, we use a simplified model to roughly estimate the edge site density of such nanoparticulate MoS 2 electrodes, as illustrated in the inset. MoS 2 nanoplatelets are monodispersed single-layer triangles with an edge dimension of x in a hexagonal array separated by a distance of 2x. The number of edge site density as a function of the nanoplatelet dimension is presented as the blue curve in this figure. The range of edge site densities obtained from reported experiments is indicated by the light blue region 2, whereas the edge site density value of edge-terminated MoS 2 film developed in this study is marked as the black dotted line. Rather small nanoplatelets of ~1nm in size should be used to obtain similar edge site density as the edge-terminated MoS 2 film, which are likely challenging to grow. The rapid sulfurization/selenization technique represents a facile approach to achieve extremely high edge-site density by exposing edge-terminated surface.

6 S5. Nyquist plots of impedance spectroscopy analysis of the electrochemical cells. 1 cm 2 glassy carbon (GC) electrode, edge-terminated MoS 2 film on glassy carbon (MoS 2 /GC) and edge-terminated MoSe 2 film on glassy carbon (MoSe 2 /GC) are used as working electrodes respectively. The measurement is performed at zero potential versus RHE. The series resistances are 2.3 Ω for the GC cell, 2.0 Ω for the MoS 2 /GC cell and 2.4Ω for the MoSe 2 /GC cell. The series resistance primarily comes from wiring (e.g. cables, alligator clips) and the electrolyte, where the resistance of MoS 2 or MoSe 2 films is negligible. S6. Optical transmittance of edge-terminated MoS 2 and MoSe 2 films. MoS 2 and MoSe 2 thin films are semi-transparent across optical wavelengths and highly transparent in near infrared wavelengths. Inset: digital

7 photo of MoS 2 (left) and MoSe 2 (right) films grown on 1 1 cm 2 quartz substrates. These films may potentially be integrated into solar water splitting cells as HER catalyst, where the optical properties can be turned with film thickness. S7. MoS 2 films grown on rough glassy carbon substrates and the corresponding electrochemical measurements. (a) The morphology of a rough, lapped glassy carbon substrate resolved by SEM. (b) Raman spectrum from a typical MoS 2 film grown on the rough glassy carbon substrate, confirming the film is edge-terminated with vertically aligned layers. We intentionally coat the substrate with a slightly thicker Mo film of 20 nm prior to sulfurization, to ensure complete coverage on the substrate. (c) Polarization curves of MoS 2 films grown on a mirror-polished glassy carbon substrate and on a rough, lapped glassy carbon substrate. The polarization curves have been corrected by a small ohmic drop. The series resistance of MoS 2 /Rough GC cell by impedance spectroscopy analysis is 3.3 ohm with very small contribution from the working electrode. (d) Corresponding Tafel plots. The MoS 2 film grown on the rough glassy carbon substrate exhibits a Tafel slope of 86 mv/decade and an exchange current density of A/ cm 2. It is recognized that Tafel slope is related, in a complex way, to the reaction pathway and absorption conditions of the active sites, which varies significantly with different sample preparation. The lack of sufficient understanding of the critical factors influencing the Tafel slope of MoS 2 often complicates the interpretation of the observed improvement 1. However, it is still a bit surprising to observe the dramatic improvement of Tafel slope simply by simply modifying the substrate morphology. It represents a new opportunity to improve the overall performance of MoS 2 HER catalyst in addition to chemical doping 5 and high specific surface area design 6.

8 S8. The rapid sulfurization/selenization process can be directly applied to molybdenum foil. Here, we summarize our preliminary studies on MoS 2 and MoSe 2 films grown on molybdenum foil. (a) Digital photos of pristine Mo foil (top), sulfurized foil (middle), and selenized foil (bottom). Each photo corresponds to an area of 3 3 cm 2. (b) X-ray diffraction patterns of sulfurized foil (top) and selenized foil (bottom). For sulfurized foil, a single (100) peak of MoS 2 phase is identified in addition to Mo peaks (marked by *), revealing edge-surface texture of MoS 2 films grown on the foil. For selenized foil, (100) and (110) peaks of MoSe 2 phase are observed besides Mo peaks (marked by *), confirming edge-surface texture of MoSe 2 film grown on the foil. A broad, unidentified peak is also observed (marked by o), likely coming from small amount of impurity or non-crystalline phase. (c) Polarization curves of MoS 2 and MoSe 2 films grown on Mo foil, showing increased cathodic current as compared with the films grown on glassy carbon substrate. (d) Corresponding Tafel plots. The MoS 2 film grown on Mo foil exhibit a Tafel slope of 75 mv/decade, and an exchange current density of A/ cm 2. MoSe 2 film grown on Mo foil has a Tafel slope of 68 mv/decade, and an exchange current density of A/ cm 2. We notice the exchange current densities of these films are slightly smaller than those grown on glassy carbon substrates, suggesting edge sites are partially active, which requires further study.

9 Tables Roughness (nm) Surface Area / Geometric Area Glassy Carbon (GC) MoS 2 film / GC MoSe 2 film / GC S9. Summary of AFM measurements. AFM measurements are performed on glassy carbon, edge-terminated MoS 2 and MoSe 2 electrodes. We measured multiple locations of the substrates and report the averaged value. These electrodes are very smooth, and the specific surface area of these electrodes is almost the same as their geometric area. Analysis of AFM data is based on XEI program (from Park Systems Corp). MoS 2 MoSe 2 Tafel Slope log (j 0 A cm -2 ) Tafel Slope (mv/decade) log (j 0 A cm -2 ) (mv/decade) Sample Sample Sample Sample S10. Summary of electrochemical measurements. The average exchange current density is A/cm 2 for MoS 2 and A/cm 2 for MoSe 2.

10 Movies S11. Reconstructed MoS 2 film viewing at an arbitrary viewing angle. This movie shows the cross-sections of the reconstructed MoS 2 film moving throughout the film thickness. Individual layers are clearly resolved in the tomogram. The movie confirms that the layers are extended across the entire depth of the film. S12. Reconstructed MoS 2 film viewing from the top. This movie shows the top viewing of cross-sections of the reconstructed MoS 2 film moving throughout the film thickness. Individual layers are clearly resolved in the tomogram. The evolution of the stripe patterns is the result of slight tilting of individual grains. REFERENCES 1. Laursen, A. B.; Kegnaes, S.; Dahl, S.; Chorkendorff, I. Energy & Environmental Science 2012, 5, (2), Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Science 2007, 317, (5834), Jäger-Waldau, A.; Lux-Steiner, M.; Jäger-Waldau, R.; Burkhardt, R.; Bucher, E. Thin Solid Films 1990, 189, (2), Jäger-Waldau, A.; Lux-Steiner, M. C.; Bucher, E.; Scandella, L.; Schumacher, A.; Prins, R. Applied Surface Science 1993, 65/66, (0), Bonde, J.; Moses, P. G.; Jaramillo, T. F.; Norskov, J. K.; Chorkendorff, I. Faraday Discussions 2009, 140, Chen, Z.; Cummins, D.; Reinecke, B. N.; Clark, E.; Sunkara, M. K.; Jaramillo, T. F. Nano Letters 2011, 11, (10),