P Dopants Triggered New Basal Plane Active Sites. and Enlarged Interlayer Spacing in MoS 2. Evolution

Transcription

1 Supporting Information P Dopants Triggered New Basal Plane Active Sites and Enlarged Interlayer Spacing in MoS 2 Nanosheets towards Electrocatalytic Hydrogen Evolution Peitao Liu a, Jingyi Zhu b, Jingyan Zhang a, Pinxian Xi c, Kun Tao a, Daqiang Gao a *, Desheng Xue a. a Key Laboratory for Magnetism and Magnetic Materials of MOE, Key Laboratory of Special Function Materials and Structure Design of MOE, Lanzhou University, Lanzhou , P. R. China. b Department of Physics and Astronomy, Clemson Nanomaterials Center and COMSET, Clemson University, Clemson, SC 29634, USA c Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and The Research Center of Biomedical Nanotechnology, Lanzhou University, Lanzhou, , P. R. China The two authors have equal contribution to this work * Corresponding author: gaodq@lzu.edu.cn (D. Q. Gao). 1

2 EXPERIMENTAL SECTION Material Preparation. The P doped MoS 2 nanosheets are synthesized by hydrothermally reacting with sodium molybdate dihydrate [Na 2 MoO 4 2H 2 O], thioacetamide [CH 3 CSNH 2 ] and ammonium dihydrogen phosphate [NH 4 H 2 PO 4 ] as the Mo, S and P sources. In a typical synthesis, 1.5 g Na 2 MoO 4 2H 2 O and 2.2 g CH 3 CSNH 2 are dissolved in 40 ml deionized water under magnetic stirring. Different weight of NH 4 H 2 PO 4 of 0.1, 0.2 and 0.4 g are added in the solution to fabricate series P doped MoS 2. After being magnetically stirring for 1 h, the above solution is transferred into a Teflonlined stainless steel autoclave with a capacity of 50 ml. Then the autoclave is sealed and heated at 200 C for 24 h. For convenience, we denoted these doped samples as P1, P2 and P3, respectively, as well as P0 for pure MoS 2 nanosheets. Material Characterization. X-ray diffraction (XRD, X' Pert PRO PHILIPS with Cu Kα radiation) and Raman (Jobin-Yvon LabRam HR80 spectrometer Horiba Jobin Yvon, Inc. with a 532 nm line of Torus 50 mw diodepumped solid-state laser under backscattering geometry) were employed to study the crystal structure of the products. The samples' morphology was obtained by the scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, TecnaiTM G2 F30, FEI, USA). The bonding characteristics of the products were captured by the X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra). The element concentration is measured by the inductively coupled plasma atomic emission spectroscopy (ICP-AES) with the uncertain of ±0.1. 2

3 Electrocatalytic Hydrogen Evolution. To fabricate the electrochemical testing electrode, 10 mg of catalysts and 10 mg of carbon black (Vulcan XC72) are added in to 50 ml Petroleum ether with the sonicate of 2 hrs and then dried at 40 Further, the mixture of 3 mg of this catalyst, 1470 µl N, N-Dimethyl Formamide and 30 µl Nafion-117 solution are added into a 5 ml container to disperse the catalysts in the ink. Finally, 12 µl of the fresh catalyst ink is dropped onto a glassy carbon electrode (0.071 cm 2 geometrical area, Pine Research Instrument) and dried at room temperature where the loading amount is estimated to be 0.32 mg cm -2 for the samples. Electrochemical measurements are carried out with the electrochemical workstation (CHI 660E, USA) in a standard three-electrode cell, where the glassy carbon (GC) coated with various samples is the working electrode (WE), Ag/AgCl and the carbon rod are the reference and counter electrode, respectively. Linear sweep voltammetry (LSV) is performed with the scan rate of 2 mv/s in 0.5 M H 2 SO 4 at the room temperature, where the electrolyte is first purged by high purity N 2 gas. The cyclic voltammetry (CV) measurements are carried out at the scan rate of 100 mv/s to reveal the stability performance of the catalysts. During the electrochemical testing, all the electrochemical measurements are ir-corrected by the equation of E(RHE)=E(Ag/AgCl) mv. The Nyquist plots are tested at overpotential of 280 mv with frequencies changing from 10 khz to 0.1 Hz, where the impedance dates are fitted to a simplified Randles circuit to obtain the series and charge-transfer resistances. 3

4 Calculation details. Density functional theory (DFT) calculations in our case are performed using the Vienna ab initio simulation package (VASP) and the Perdew Burke Ernzerhof exchange correlation functional correction 1. A 450-eV kinetic energy cutoff is chosen for plane-wave basis set, and Monhorst Pack k-point sampling is used. The atomic structures for all models are fully relaxed with selfconsistency accuracy of 10-4 ev reached for electronic loops and the residual forces were within 0.02 ev/å for geometry optimizations 2. We use the HSE06 hybrid functional to study the electronic properties (DOS, PDOS, partial charge density). For MoS 2 monolayer, the calculated lattice constants are a = b = 3.18 Å after fully relaxation and the vacuum slab of 12 Å is inserted in z direction for surface isolation to prevent interaction between two neighboring surfaces. A supercell is adopted to assess the electronic property and hydrogen evolution activity. To study the HER performance of MoS2 slabs with different interlayer gap, two layers slabs are employed with AB stacking and the lattice constant c were set 1.49 and 2.15 nm for MoS nm and MoS nm, respectively. The calculated lattice parameters are a = b = 3.18 Å, which agrees with the experimental value. The formation energy of N substituted S was calculated in neutral states by the formula E =E E +μ, μ, where E is the total energy of the supercell with P substitution (S site, Mo site or hole site), E is the total energy of pristine supercell, μ, and μ are chemical potentials of isolated S, Mo and P atom, respectively. The Gibbs free energy change ( G H* ) is expressed as follows 3 : = +,where, and are the adsorption energy of atomic hydrogen on the given surface, zero point energy correction and entropy change of H* adsorption, respectively. The zero point energy correction can be estimated by the equation = H 1/2 H, where H and H are calculated by 4

5 vibration frequency calculation. At 1 bar and 300 K, T S is approximately -0.2 ev. The value of is calculated as =E tot E sub 1/2E H2, where E tot and E sub are the energies of H absorbed systems and the clean given surface, respectively, and E H2 is the energy of molecular H 2 in the gas phase. (1) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS 2 Transistors. Nat. Nanotechnol. 2011, 6 (3), (2) de Chialvo, M. R. G.; Chialvo, A. C. Hydrogen evolution reaction: Analysis of the Volmer-Heyrovsky-Tafel Mechanism with a Generalized Adsorption Model. J. Electroanal. Chem. 1994, 372 (1 2), (3) Yang, Y.; Lun, Z.; Xia, G.; Zheng, F.; He, M.; Chen, Q. Non-precious Alloy Encapsulated in Nitrogen-doped Graphene Layers Derived from MOFs as an Active and Durable Hydrogen Evolution Reaction Catalyst. Energy Environ. Sci. 2015, 8 (12), Figure S1. SEM results of pure and P doped MoS 2 nanosheets. 5

6 Figure S2. The XPS results for P doped MoS 2 nanosheets. Figure S3. The Raman results for P doped MoS2 nanosheets. 6

7 Figure S4. Stability test for pure MoS 2 and P doped MoS 2 nanosheets. Figure S5. Mott-Schottky plots for pure MoS 2 and P doped MoS 2 nanosheets. 7

8 Figure S6. CV for pure and P doped MoS 2 nanosheets electrocatalysts at different scan rates of 20, 40, 60, 80, 100, 120, 140, 160, 180 mv/s from inner to out, respectively. (a) P0, (b) P1, (c) P2 and (d) P3. Figure S7. The CV curves in 1.0 M PBS for pure MoS 2 and P doped MoS 2 nanosheets. 8

9 Figure S8. Polarization curves recorded for P doped MoS 2 nanosheets (P3) before and after CV cycles in (a) 1.0 M PBS and (c) 1.0 M KOH. Potentiostatic measurements of P doped MoS 2 nanosheets (P3) at an overpotential of 200 mv in (b) 1.0 M PBS and (d) 1.0 M KOH. Figure S9. The SEM and XRD results for catalyst P3 after long-time stability test. 9

10 Figure S10. Three calculated models of P doped MoS 2 monolayer. The formation energies of these three cases are 0.67 ev, 10.9 ev and 1.12 ev, respectively, indicating that P dopants are liable to substitute the S sites. Figure S11. The calculated result of absorption site of H on the P site. 10

11 Figure S12. The side view and top view of the MoS 2 : (a) side view and (b) top view for MoS nm, and (c) side view and (d) top view for MoS nm. The purple, yellow balls represent Mo, S atoms, respectively. 11

12 Figure S13. The side view and top view of the P doped MoS 2 : (a) side view and (b) top view for P doped MoS nm, and (c) side view and (d) top view for P doped MoS nm. The purple, yellow and pompadour balls represent Mo, S and P atoms, respectively. 12

13 Figure S14. Calculated free energy diagram for HER on MoS 2 with different interlayer space of 0.65 nm and 0.91 nm, as well as the calculated models respectively. Table S1. HER properties of reported MoS 2 -based catalysts. Catalyst Onset overpotential (mv vs. RHE) η at J = 10 ma/cm 2 (mv) Tafel slope (mv/dec) Reference P-doped MoS 2 (P3) This work MoS 2 /N-RGO Adv. Energy Mater. 2016, 6, MoS 2 /CoSe Nat. Commun. 2015, 6, CoP/MoS 2- CNT Catal. Sci. Technol., 2016, 6, M-MoS Nat. Commun. 2016, 7, N- Carbon/MoS ACS Appl. Mater. Interfaces 2016, 8, 3558 C-MoS Nanotechnology, , MoS 2 -flake Nano Lett. 2016, 16, 4047 MoS 2 with S depletion hollow MoS 2 microspheres ACS Nano 2016, 10, ACS Appl. Mater. Interfaces 2016, 8, 5517 Li-MoS ACS Nano 2014, 8,

14 Amorphous carbon/ MoS 2 edge-oriented MoS Nanoscale, 2014, 6, Adv. Mater. 2014, 26, 8163 MoS 2 /MoO Nanoscale, 2015, 7, 5203 MoS Adv. Mater. 2014, 26,