hydrogen evolution reaction

Transcription

1 Conducting MoS 2 nanosheets as catalysts for hydrogen evolution reaction Damien Voiry, Maryam Salehi, Rafael Silva +, Takeshi Fujita #, Mingwei Chen #, Tewodros Asefa +, Vivek Shenoy, Goki Eda $,$$, and Manish Chhowalla, * Rutgers University, Materials Science and Engineering, 607 Taylor Road, Piscataway, New Jersey 08854, USA. + Rutgers University, Department of Chemistry and Chemical Biology, 610 Taylor Road, Piscataway, New Jersey 08854, USA # WPI Advanced Institute for Materials Research, Tohoku University, Sendai , Japan. JST, PRESTO, Honcho Kawaguchi, Saitama , Japan Rutgers University, Department of Chemical and Biochemical Engineering, 98 Brett Road, Piscataway, New Jersey 08854, USA. Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA. $ National University of Singapore, Physics Department and Graphene Research Centre, 2 Science Drive 3, Singapore $$ National University of Singapore, Chemistry Department, 3 Science Drive 3, Singapore

2 Supporting information Electrode preparation All the electrodes were prepared via solution process. Thin films of MoS 2 were prepared alternatively by drop-casting or by vacuum filtration (25-nm pore size membrane from Millipore), delaminated and transferred on the glassy carbon electrode. The electrodes were then used as prepared in the case of 1T MoS 2. To convert the 1T to the 2H phase, the electrodes were further annealed without any additional transfer at 300 o C under vacuum with a 80 sccm Ar/H 2 flow for 15 minutes. The amount of deposited catalyst on the electrode is ~ 50 µg/cm 2. Finally the electrode was caped with 3µL of 0.5% Nafion solution (Sigma Aldrich) to protect the active material. Materials characterization MoS 2 structure can be well understood from Raman and XPS spectroscopy. Indeed 2H-MoS 2 has strong Raman signature at ~ 380 cm -1 and 410 cm -1 corresponding to in-plane E 1 2g and out-ofplane A 1g respectively. By XPS, 2H-MoS 2 signals can be identified from the signals at 229 ev, 232 ev for Mo3d 5/2 and Mo3d 3/2. Similarly from the high-resolution spectra, the binding energy for sulfur atoms in the 2H structure is expected at and 163 ev. Upon lithium intercalation, the trigonal prismatic phase (2H phase) of MoS 2 is known to be less stable than the phase corresponding to a octahedral coordination (1T phase) of Mo atoms. This phase transformation involves deep modification in the Raman and XPS spectra. In Raman spectroscopy, new

3 vibration modes are allowed and peaks called J 1, J 2 and J 3 have been indentified for 1T-MoS 2 prepared via n-butyllithium. Similar signals have been identified here for the exfoliated MoS 2 prepared here using LiBH 4 at ~ 160 cm -1, 230 cm -1, 330 cm -1 corresponding to J 1, J 2 and J 3 peaks (Fig. S1). E 1 2G peak is also found to decrease in intensity. This is attributed to the decrease of the 2H content since E 1 2G mode is originated from the 2H structure (Jimenez, Phys. Rev. B., 1991). The modification of the crystal phase changes the electronic structure from semiconducting (2H- MoS 2 ) to metallic (1T-MoS 2 ). Thus the change of the Fermi level can be then detected by XPS and the deconvolution of the spectra reveals the 1T components at ~ ev and ~ ev for Mo3d (Fig. S2a). These positions are slightly lower than in the case of 2H MoS 2 (~ ev and 232 ev). (Ref 22). The same downshift is also identifiable in the S2p regions (Fig. S2b). From the deconvolution of the XPS spectra, the 1T content is estimated to be ~ 80%. During annealing, the 2H phase can be restored as reported previously (Ref 19). These modifications can be followed by Raman and XPS (Fig. S1,2) and we found that the pristine 2H structure is virtually restored (>95%) at 300 C annealing. 3

4 Figure S1. Raman spectra of chemically 1T (as-exfoliated) and 2H MoS 2 (300 C) thin films. Figure S2. XPS spectra from the Mo3d (a) and S2p (b) of chemically exfoliated MoS 2 thin films as-exfoliated and after 300 C annealing.

5 Optoelectronic characterization Photoluminescence measurements were performed using 514 nm (2.41 ev) Raman laser as excitation. We have used the photoluminescence (PL) measurements to follow the restoration of the semiconducting 2H phase of MoS 2. Thin films of MoS 2 were prepared by vacuum filtration (Ref 19), transferred on silicon wafer with a 300 nm-oxide top layer and annealed. As-exfoliated films don t show any PL due to the predominance of the metallic 1T phase. As the annealing temperature increases, the 2H is restored as observed by Raman and XPS spectroscopy. Films annealed at a temperature of 150 C and higher show PL signal at 655 nm (1.9 ev) indicating direct band gap electronic structure coherent with single layer MoS 2 (Ref 21). The PL signal increases with the temperature and we found highest PL intensity for thin films annealed at 300 C in perfect agreement with chemically exfoliated MoS 2 prepared via n-butyllithium (Ref 22) (Fig. S3a). A and B band from the 2H phase of MoS 2 have also been detected by UV-vis absorption whereas the absorbance spectrum of 1T MoS 2 consists in a continuous decrease typical of a metallic behavior (Fig. S3b). 5

6 Figure S3. (a) Photoluminescence spectra on single layer flakes of MoS 2 thin film deposited on SiO 2 substrate using 514 nm laser. (b) UV-visible spectra of MoS 2 thin film on quartz for as deposited and 300 C -annealed films. MoS 2 nanosheet analysis By adjusting the contrast, MoS 2 nanosheets can be observed by Scanning Electron Microscope (SEM) as previously demonstrated for graphene oxide (Cote, J. Am. Chem. Soc., 2009) (Fig. 1). Extensive study on flakes size before and after intercalation reveals that there is no significant change in the morphology of the MoS 2 aggregates whereas no exfoliated nanosheets was observed at this step of the process (Fig. S4a,b). The effect of the sonication on the size of the nanosheets was also investigated by doing statistical analysis of the SEM/AFM images (Fig. S4c,d). The Gaussian-shape histograms demonstrates that without sonication, the flakes size are 20% larger with an average size of ~800 nm, which considerably bigger than MoS 2 nanoclusters (~10 nm) previously studied for HER (Ref 11,12).

7 Figure S4. SEM images of MoS 2. SEM images of bulk MoS 2 crystal (a) and intercalated with Li cations (b). Flakes size distribution in chemically exfoliated MoS 2 with sonication (c) and without sonication (d). Statistics are based on hundreds of flakes. Iodine treatment MoS 2 was first exfoliated in water following the above procedure and water was then replaced by acetonitrile. The MoS 2 nanosheets were treated with a 0.15 M iodine solution in acetonitrile 7

8 for a period of time between 5h and 10 days. Iodine-treated MoS 2 was then washed thoroughly with acetonitrile, isopropanol, ethanol and water. The absence of iodine in the final material was confirmed by XPS spectroscopy (Fig. S5). Figure S5. (a) XPS survey spectra of as-exfoliated MoS 2 and iodine-treated MoS 2. Inset: Magnification of the I 3d region. No iodine signal (I 3d 5/2 expected at ~ 620 ev) can be detected. (b) High resolution XPS spectra from the Mo 3d region obtained for as-exfoliated MoS 2 and iodine-treated MoS 2 showing that the 1T phase is preserved. Zeta Potential measurements Zeta potentials of 1T MoS 2, iodine-treated 1T MoS 2 and bulk 2H MoS 2 were measured using Malvern Instrument Zetasizer Nano-ZS90. Each sample was first dispersed at 0.5 mg/ml in 1/1 DI water/pbs buffer (ph ~ 7.4) solution and sonicated for 30 mins. 1 ml of the dispersion thus

9 obtained was poured into a disposable capillary cell (Malvern, DTS1070) and used for measurement of zeta-potential. The values are reported as an average of three measurements. Entry Zeta-potential (, mv) 1T MoS ± 3.9 Iodine-treated 1T MoS ± 1.9 Bulk 2H MoS ± 2.7 Table S1: Zeta potential values of 1T MoS 2, iodine-treated 1T MoS 2 and bulk 2H MoS 2 at ph ~ 7.4. Comparison of the HER activity from 1T MoS 2 and iodine-treated 1T MoS 2 The effect of the iodine treatment on the HER activity of exfoliated 1T MoS 2 is presented in the Fig. S6. Large reduction of the overpotential and improved Tafel slope is obtained in the case of iodine-treated MoS 2 nanosheets. These results indicate the beneficial effect of charge removal from the surface of the nanosheets. 9

10 Figure S6. (a) Polarization curves of 1T MoS 2 and iodine-treated 1T MoS 2. (b) Tafel slopes of ~ 65 mv/dec and ~ 40 mv obtained from 1T MoS 2 and iodine-treated 1T MoS 2 respectively. MoS 2 edge oxidation MoS 2 nanosheets have been partially oxidized by saturating the MoS 2 solution with oxygen for few days and partial oxidation was confirmed by XPS spectroscopy in both as-exfoliated and annealed MoS 2 (Fig. S7). Peaks at ~ and ~ ev representing Mo 3d 3/2 and Mo 3d 5/2 respectively indicate the presence of Mo 6+. From the deconvolution of the XPS spectra, we found that ~ % of the MoS 2 has been oxidized and virtually no further oxidation has been introduced during the annealing. The Mo 6+ peaks progressively decreases upon cycling and after the 150 th cycles, 6-10% of the sample is oxidized.

11 Figure S7. XPS spectra from edge-oxidized 1T MoS 2 (a) and edge-oxidized 2H MoS 2 (b) after oxidation, the first and 150 th cycle. Impedance measurements AC impedance measurements were performed in the same configuration at η = V from 10 6 to 0.1 Hz with an alternating current voltage of 10 mv. We have found that the charge-transfer resistance (Z f ) is significantly reduced in the case of the 1T MoS 2 : 30 Ω compare to 2H MoS 2 : 360 Ω (Fig. S8). Such high difference is attributed to the high conductivity of the metallic nature of 1T MoS 2. The impedance of edge-oxidized 1T MoS 2 is not significantly modified (Z f = 100 Ω) proving that the charge transfer does not suffer from the oxidation of the edge. At the opposite, edge-oxidized 2H MoS 2 demonstrates a dramatic increase of Z f up to approximately 1800 Ω. Similarly we can note that the Tafel slope of the edge-oxidized 1T MoS 2 remains virtually constant whereas the Tafel slope increases from 85 mv/dec up to 186 mv/dec of edge- 11

12 oxidized 2H MoS 2 (Fig. 2b). Thus together with the Tafel plots, the AC impedance spectroscopy indicates that the oxidation perturbs profoundly the reaction mechanism. The series resistances are slightly reduced in the case of the metallic 1T phase whereas the relatively high resistance in the case of 2H MoS 2 is attributed to a higher wiring resistance (Table S2). Figure S8. Nyquist plots of 1T and 2H MoS 2 compared with edge-oxidized 1T and 2H MoS 2 performed at η = V from 10 6 to 0.1 Hz with an alternating current voltage of 10 mv. Series resistances (Ω) 1T MoS 2 7 Edge-oxidized 1T MoS 2 8

13 2H MoS 2 35 Edge-oxidized 2H MoS 2 12 Table S2. Series resistances for 1T MoS 2, edge-oxidized 1T MoS 2, 2H MoS 2 and edge-oxidized 2H MoS 2. When adding SWCNT to 1T MoS 2, we observed a decrease of the Tafel slope from 40 mv to 60 mv attributed to the increase of the interfacial resistance between the carbon nanotubes and MoS 2. The reduction of the rate of the reaction is also detected by impedance spectroscopy with a dramatic increase of the charge-transfer resistance from 30 Ω up to 180 Ω for 1T MoS 2 and 1T MoS 2 /SWCNT respectively (Fig. S9). 13

14 Figure S9. Nyquist plots of 1T MoS 2 compared with 1T MoS 2 /SWCNT performed at η = V from 10 6 to 0.1 Hz with an alternating current voltage of 10 mv. MoS 2 /SWNT composite electrode Single-wall carbon nanotubes (SWNT, Thomas Swan, batch #: K1713) have been used as received without any enrichment in one type of tube. The amount of metallic tubes is estimated to be ~ 33.3% versus ~ 66.6% for semiconducting. SWCNT have been dispersed using sodium dodecylsulfate (SDS, Sigma Aldrich) as surfactants following the protocol developed by Vigolo et al. (Vigolo, Science, 2000). Namely 25 mg of SWNTs (0.08 wt%) with 100 mg of SDS (0.2 wt%) in 30 ml of water and then tip-sonicated for 10 min (25W, 40% amplitude). The dispersion

15 was further centrifuged at 1500 rpm (~ 700 g) for 5 min to remove to SWNT aggregates and the final concentration of dispersed SWNT reaches 0.68 mg/ml. The MoS 2 /SWNT composite electrodes were prepared by mixing together the two solutions followed by vacuum filtration over 25nm pore size membrane (Millipore). Large amount of water was used to remove the excess of SDS in the film. Films were then delaminated on the surface of water and transferred on the glassy carbon electrode. The presence of carbon nanotubes in the MoS 2 /SWNT films was confirmed by Raman spectroscopy (Fig. S10). D, G and 2D band from the nanotubes can be observed at 1340 cm -1, 1595 cm -1 and 1672 cm -1. The absence of any modification of the signals from either SWNTs or MoS 2 nanosheets indicates that both materials don t react when in contact. Details of the radial breathing mode (RBM) of the nanotubes as well as E 1 2G and A 2G modes of MoS 2 are shown in Fig. S5b. 15

16 Figure S10. Raman spectra from the different MoS 2 /SWNT thin films deposited on SiO 2 without annealing and after 300 C-annealing. The radial breathing mode (RBM) of the nanotubes as well as E 1 2G and A 2G modes of MoS 2 are presented in figure b. The peak at 520 cm -1 is coming from the SiO 2 wafer. Conductivity measurements In order to investigate the electrical properties of the different materials, MoS 2 thin films were deposited after vacuum filtration on silicon wafer with a 300 nm oxide top layer and further annealed if needed. The MoS 2 /SWNT composite thin films were prepared following the same procedure. The film thicknesses have been measured by AFM and were typically around 5 nm. Gold electrodes with 20 µm separation were thermally evaporated and XPS measurements have confirmed that the evaporation doesn t change the metallic 1T phase concentration in the films. Additional electrochemical measurements The reactivity of chemically exfoliated MoS 2 toward hydrogen evolution has been systematically measured in hydrogen-saturated and nitrogen-saturated solution. Indeed it is reasonable to think that nitrogen-saturated electrolyte could artificially enhance the activity of MoS 2 by generating a driving force towards the hydrogen evolution. Polarization curves from 1T MoS 2 measured in N 2

17 and H 2 -sparged solutions are presented in Fig. S11. No significant change can be observed indicating that the driving force is negligible. Figure S11. Polarization curves of 1T MoS 2 in N 2 and H 2 -sparged 0.5 M H 2 SO 4 electrolyte solution. 17