Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2014. Supporting Information for Small, DOI: 10.1002/smll.201401598 Perpendicularly Oriented MoSe 2 /Graphene Nanosheets as Advanced Electrocatalysts for Hydrogen Evolution Shun Mao, Zhenhai Wen,* Suqin Ci, Xiaoru Guo, Kostya (Ken) Ostrikov, and Junhong Chen*
Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2013. Supporting Information Perpendicularly-oriented MoSe 2 /Graphene Nanosheets as Advanced Electrocatalysts for Hydrogen Evolution Shun Mao 1, Zhenhai Wen 1 *, Suqin Ci 1, Xiaoru Guo 1, Kostya (Ken) Ostrikov 2, and Junhong Chen 1 * [1] Prof. J. Chen, Dr. Z. Wen, Dr. S. Mao, Dr. S. Ci, X. Guo, Department of Mechanical Engineering University of Wisconsin-Milwaukee 3200 North Cramer Street Milwaukee, Wisconsin 53211, United States E-mail: jhchen@uwm.edu, wenzhenhai@yahoo.com [2] Prof. K. Ostrikov Plasma Nanoscience CSIRO Materials Science and Engineering P.O. Box 218, Lindfield, NSW, 2070, Australia Experimental Section MoSe 2 /GN preparation: A schematic illustration of the experimental setup for PECVD/CVD growth of MoSe 2 /GN hybrids is shown in Fig. S1. A graphite disc (6 mm diameter, 0.283 cm 2 surface area) was used as the substrate to grow the catalysts. GNs were first grown on the graphite disc using a PECVD method with parameters similar to our previous reports. The MoSe 2 nanosheets were then grown on the GN surface by a CVD method using MoCl 5 and Se as the precursors. After CVD growth, the MoSe 2 /GN hybrids were characterized and used as the electrocatalyst for HER without any further treatment. MoSe 2 /GN hybrids characterization: A Hitachi (H 9000 NAR) transmission electron microscope (TEM) and a Hitachi (S-4800) scanning electron microscope (SEM) were used for the structure characterization of the prepared materials. The EDS elemental mapping data were obtained using the Hitachi S-4800 scanning electron microscope. XRD was performed on a Scintag XDS 2000 X-ray powder diffractometer with monochromated CuK α radiation (λ=1.5418 Å). Raman spectroscopy was conducted with a Renishaw Raman spectrometer (Inc 1000B) with a HeNe laser. 1
Electrochemical measurements: A saturated silver chloride electrode was used as the reference electrode in all measurements and was converted to the reversible hydrogen electrode (RHE) scale via the Nernst equation. The electrochemical measurements were carried out in 0.5 M H 2 SO 4 at room temperature using a CHI 600 electrochemical workstation (CHI Inc., USA). The three-electrode cell consisted of an Ag/AgCl electrode as the reference electrode, Pt as the counter-electrode, and a glassy carbon electrode loaded with various catalysts as the working electrode, respectively. For comparison, a commercial Pt/C catalyst (HP 10 wt.% platinum on Vulcan XC-72, 0.05 mg, Fuel Cell Store) was used and prepared on the glassy carbon electrode (3 mm diameter, 0.071 cm 2 surface area) using a conventional method with Nafion as the binder. The performance of the HER catalysts was measured by using a linear sweep voltammetry from +0.315 to -0.785 V versus RHE with a scan rate of 5 mv/s. Electrochemical impedance spectroscopy was measured in the frequency range from 0.1 to 100,000 Hz at a constant potential -0.385 V vs. RHE. The stability of the catalysts was evaluated by testing the catalysts at a constant potential (-0.15 V versus RHE) for continuous operation of 6,000 seconds. Figure S1. Schematic illustration of the experimental setup for the MoSe 2 /GN and MoS 2 /GN production through a PECVD/CVD method with the graphite disc as the substrate. Graphite rods were cut into thin discs and polished using abrasive papers. The discs were then washed with DI water several times and dried in air. The home-built CVD system was constructed with a tube furnace and a quartz tubing (1 inch diameter). The growth of the hybrids was conducted through two steps. In the first step, GNs were grown on the graphite disc by a 2
previously reported PECVD method. In a typical procedure, Ar and CH 4 flows were introduced into the reactor (700 ºC) with two separate flows: a dry Ar flow (0.9 lpm) and a wet Ar/CH 4 flow through a water bubbler flask (Ar: 0.1 lpm, CH 4 : 0.1 lpm). The plasma reactor was designed with a tungsten pin cathode (3.5 kv) pointing toward the graphite disc (grounded) with a distance of 0.8-1.0 cm. GN growth duration was 10 minutes without any catalyst. In the second step, the CVD growth of the MoSe 2 and MoS 2 nanosheets were carried out using MoCl 5 and Se/S as the precursors. The MoCl 5 (0.05 g, 95%, Sigma-Aldrich) and Se/S powder (0.2 g, 99.5%, Sigma-Aldrich/Alfa Aesar) were placed in two alumina boats; and the graphite disc was placed in the Se/S boat at downstream. Before growth, both boats were placed outside the furnace, and after the furnace was heated to a reaction temperature (950 ºC for MoSe 2 and 550ºC for MoS 2 ), the boats were moved slowly into the furnace. The reaction lasted for 20 minutes under Ar atmosphere (0.125 lpm) and cooled naturally with an Ar flow. The digital images show the graphite discs with different materials. To study the catalytic activity of the catalysts in HER, the graphite disc with MoSe 2 /GN and MoS 2 /GN was attached to the glassy carbon electrode with colloidal silver paste (Ted Pella) to ensure good electrical conductivity. Typical masses of pure MoSe 2 nanosheets and MoSe 2 /GN hybrids are around 0.25 mg under current preparation conditions. 3
Figure S2. (a-d) SEM, TEM, and HRTEM images of the GN. The SEM images show that a layer of GN, consisting of graphene sheets, was produced through PECVD and uniformly covered the graphite disc surface. From the TEM image, the graphene sheets show a crumpled structure and form a porous 3D network. On the top area of the GN, individual graphene sheets are found and the typical size of the graphene sheet is dozens of nanometers. The high-resolution (HRTEM) image indicates that the graphene sheet has 4 8 layers, which is in accordance with a previous report. The Raman spectrum of GN shows three distinct bands, i.e., the G band (~1598 cm -1 ) assigned to the first-order scattering of the E 2g phonon from sp 2 carbon, the D band (~1323 cm -1 ) resulting from the structural imperfections and defects on the carbon basal plane; and the 2D band (2639 cm -1 ) due to the double resonant phonons. The intensity ratio of D band to G band I(D)/I(G) (2.44) is high for the PECVDgrown GN, which is consistent with previous results that perpendicularly-oriented graphene of small size has high I(D)/I(G) value. 4
Figure S3. SEM images of MoSe 2 nanosheets directly grown on a graphite disc. The perpendicularly-oriented MoSe 2 nanosheets show similar stucture and mophology to the MoSe 2 nanosheets grown on the GN layer. Figure S4. XRD spectra of (a) MoSe 2 and (b) MoS 2 nanosheets on the graphite disc. The XRD spectrum of MoSe 2 shows diffraction peaks corresponding to (002), (100), (103), (105), (110), and (112) crystal facets from hexagonal 2H-MoSe 2 ; while the MoS 2 spectrum shows diffraction peaks corresponding to (002), (100), (103), (105), and (110) crystal facets of hexagonal 2H-MoS 2. Strong diffraction peaks of graphite are also found in both samples. 5
Figure S5. (a, b, c, d) SEM images and EDS elemental mappings of the MoSe 2 /GN hybrids. (e) EDS profile of the MoSe 2 /GN hybrids. (f) Raman spectrum of the MoSe 2 nanosheets. To further investigate the hybrid structure, SEM image and corresponding EDS elemental mappings were performed on the as-prepared MoSe 2 /GN hybrids. Based on the elemental mapping data, the Mo, Se, and C elements are clearly evidenced in the MoSe 2 /GN hybrids; and the distributions and intensities of the signals match well with the MoSe 2 nanosheets and GN structures shown in the SEM image. The Raman spectrum provides information on the orientation of MoSe 2 nanosheets on the GN layer. The MoSe 2 nanosheets showed characteristic A 1g and E 1 2g Raman modes located at 242.4 and 285.4 cm -1, respectively. The Raman peak corresponding to the out-of plane Mo Se phonon mode (A 1g ) is preferentially excited for the edge-terminated perpendicularly-oriented nanosheets, which results in a much higher A 1g peak intensity compared to the E 1 2g peak. The calculated intensity ratio of A 1g to E 1 2g is around 10:1, which is close to that of the perpendicularly-oriented MoSe 2 on Si nanowire (13:1). This indicates that the MoSe 2 nanosheets are perpendicularly-oriented on the GN layer with a predominant edge-terminated structure. 6
Figure S6. Specific mass catalytic activities of MoSe 2 /GN hybrids, MoSe 2 nanosheets, and the commercial Pt/C catalyst. (a) Polarization curves of the catalysts measured in an Arsaturated 0.5 M H 2 SO 4 solution with a scan rate of 5 mv/s. (b) Corresponding Tafel plots of the polarization curves. Figure S7. (a, b) SEM images of MoS 2 nanosheets grown on GN with a graphite disc as the substrate. (c) Cross-section SEM image of the MoS 2 /GN hybrids. (d, e, f, g) TEM and HRTEM images of MoS 2 /GN hybrids. The hybrid structures of MoS 2 nanosheets and GN are clearly seen from the SEM and TEM images. (h) SAED patterns of the MoS 2 /GN hybrids. 7
Figure S8. (a, b, c, d) SEM images and EDS elemental mappings of the MoS 2 /GN hybrids. (e) EDS profile of the MoS 2 /GN hybrids. (f) Raman spectrum of the MoS 2 nanosheets. From the elemental mapping results, the Mo, S, and C elements are clearly evidenced in the MoS 2 /GN hybrids; and the distributions and intensities of the signals match well with the MoS 2 nanosheets and GN structures shown in the SEM image. The characteristic Raman peaks at 380, 408, and 455 cm 1 correspond to the E 1 2g, A 1g, and longitudinal acoustic phonon modes of 2H-MoS 2, respectively. The peak at 187 cm 1 was found in previously reported second-order Raman spectrum of 2H-MoS 2. The peak at 226 cm -1 is not normally seen in the Raman spectrum of 2H-MoS 2, but was found in Raman spectra of freshly prepared MoS 2 single layer aqueous suspension and fresh restacked MoS 2 film; this peak is possibly coming from the superlattice and distorted octahedral structure of MoS 2. 8
Figure S9. Electrocatalytic performance of MoS 2 /GN hybrids. (a) Polarization curves of the catalysts measured in an Ar-saturated 0.5 M H 2 SO 4 solution with a scan rate of 5 mv/s. (b) Nyquist plots of the MoS 2 /GN hybrids and MoS 2 nanosheets grown on the graphite disc, showing the imaginary part versus the real part of impedance. (c) Polarization curves of the MoS 2 /GN hybrids and MoS 2 nanosheets after ir correction compared with a commercial Pt/C catalyst. (d) Corresponding Tafel plots of the polarization curves. 9
Figure S10. SEM images of MoSe 2 /GN catalysts after durability test. From SEM images, there is no obvious deformation of perpendicularly-oriented MoSe 2 nanosheets after the durability test, indicating good stability of the MoSe 2 /GN catalyst in HER. RHE conversion A saturated silver chloride electrode was used as the reference electrode in all measurements. The measured potentials versus the Ag/AgCl reference electrode were converted to the reversible hydrogen electrode (RHE) scale via the Nernst equation: (1) where E RHE is the converted potential versus RHE, E Ag/AgCl is the experimental potential measured against the Ag/AgCl reference electrode, and E o Ag/AgCl is the standard potential of Ag/AgCl at 25 C (0.1976 V). The electrochemical measurements were carried out in 0.5 M H 2 SO 4 (ph = 0.3) at room temperature; therefore, E RHE = E Ag/AgCl + 0.215 V. Comparison of the catalytic activities of layered MoSe 2 /MoS 2 catalysts Table. S1 summarizes the key electrochemical parameters of some previously reported MoSe 2 /MoS 2 catalysts for HER. From this table, it is clearly shown that the catalytic performance of MoSe 2 /GN is superior in MoSe 2 -based catalysts; and the catalytic performance of MoS 2 /GN is comparable to the best MoS 2 -based catalysts ever reported. React ion HER Table S1 Catalytic activities of MoSe 2 and MoS 2 -based catalysts for HER. Catalyst Onset Overpotential Tafel Production potential Electrolyte (V vs. RHE) slope method (V vs. at 10 ma/cm 2 (mv/dev) RHE) Ref. MoSe 2 /GN CVD 0.5 M H 2 SO 4-0.05 0.159 61 This work MoSe 2 /rgo Hydrothermal 0.5 M H 2 SO 4-0.05 0.15 69 S1 MoSe 2 CVD 0.5 M H 2 SO 4-0.11 0.25 59.8 S2 MoSe 2 CVD 0.5 M H 2 SO 4-0.2 > 0.4 105-120 S3 MoS 2 /GN CVD 0.5 M H 2 SO 4-0.025 0.145 62 This work MoS 2 /rgo Solvothermal 0.5 M H 2 SO 4-0.1 0.15 41 S4 Amorphous Electrochemic MoS 2 film al deposition 1 M H 2 SO 4-0.1 0.21 40 S5 MoS 2 CVD 0.5 M H 2 SO 4-0.15 0.187 43 S6 10
References: S1. H. Tang, K. Dou, C.-C. Kaun, Q. Kuang and S. Yang, Journal of Materials Chemistry A, 2014, 2, 360-364. S2. H. Wang, D. Kong, P. Johanes, J. J. Cha, G. Zheng, K. Yan, N. Liu and Y. Cui, Nano Letters, 2013, 13, 3426-3433. S3. D. Kong, H. Wang, J. J. Cha, M. Pasta, K. J. Koski, J. Yao and Y. Cui, Nano Letters, 2013, 13, 1341-1347. S4. Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong and H. Dai, Journal of the American Chemical Society, 2011, 133, 7296-7299. S5. D. Merki, S. Fierro, H. Vrubel and X. Hu, Chemical Science, 2011, 2, 1262-1267. S6. M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. Li and S. Jin, Journal of the American Chemical Society, 2013, 135, 10274-10277. 11