3.3.1 Synthesis of Co-pyrazole complex
First, 150 mg of Co(OAc)₂·4H₂O (0.847 mmol, Sigma-Aldrich) was dissolved in 50 mL ethanol. To synthesize Co-pyrazole complex, 50 mL of an aqueous solution of pyrazole (C₃H₃N₂H, 2.542 mmol (173.1 mg), Sigma-Aldrich) was poured gradually in the above solution at room temperature, then the mixture was stirred for 2 h at 60 oC. The solution was kept overnight without stirring and the Co- pyrazole precipitates were collected through vacuum filtration and dried at 60 oC in an oven.
3.3.2 Synthesis of Co3O4-(am-MoSx)
The required amount of Co-pyrazole complex was dispersed and sonicated in an aqueous solution of (NH₄)₂MoS₄ (0.769 mmol (200.1 mg), Sigma-Aldrich) for 1 h. Then, the mixture was transferred into a Teflon-lined hydrothermal reactor, heated to 120 oC, and maintained for a required time of 8, 12, or 24 h. The black powder of Co3O4-(am-MoSx) was collected from the vacuum filter and dried at 60 oC in an oven.
58 3.3.3 Synthesis of Co9S8-MoS2 heterostructure
As-prepared Co3O4-(am-MoSx) was transferred into an alumina boat and annealed in a tubular furnace at a required temperature (300-500 oC) for 2 h in H2 environment. To fabricate Co9S8-MoS2/NF, Ni foam was sonicated first in ethanol followed by acetone for 30 minutes and then treated with 3.0 M HCl to remove the oxide layer and washed several times with distilled water. The pre-cleaned nickel foam was dip-coated in the ink of as-synthesized Co3O4-(am-MoSx) and dried at 60 oC in an oven. The Co9S8-MoS2/NF (1.2 mg/cm2) electrode was obtained after reduction of Co3O4-(am-MoSx)/NF at 400
oC for 2 h in H2 environment.
3.3.4 Synthesis of single phases of Co9S8 and MoS2
For Co9S8 synthesis, 100 mg of Co(OAc)2·4H2O (0.565 mmol, Sigma-Aldrich) and 860 mg of thiourea (SC(NH2)2, 11.298 mmol, Sigma-Aldrich) were dissolved in a 50 mL of distilled water with stirring for 2 h. The solution was heated hydrothermally at 180 oC for 12 h. Then, the powder was prepared by vacuum filtering and dried at 60 oC in an oven. The powder was reduced at 400 oC for 2 h in H2 to obtain reduced Co9S8. Similarly, the MoS2 was synthesized by (NH4)6Mo7O24·4H2O (0.405 mmol (500 mg), Sigma-Aldrich) with 1.232 g of thiourea (16.183 mmol, Sigma-Aldrich) in a hydrothermal reactor and the reductive annealing was followed as well.
3.3.5 Materials characterizations
The phase of heterostructure was determined by powder X-ray diffraction (XRD, PANalytical pw 3040/60 X’pert) using Cu Kα radiation. The Raman analysis was conducted using 532 nm laser (AFM‐
Raman, WITec, alpha300S). Chemical states of the surface were identified by X-ray photoelectron spectroscopy (XPS, Thermo-Fisher, K-alpha). X-ray absorption fine structure (XAFS) measurements were conducted to investigate local structures of Co9S8-MoS2 on 7D beamline of Pohang Accelerator Laboratory (PLS-II, 3.0 GeV, Korea). As for Co9S8-MoS2 phase on NF, the spectra for K-edges of Mo (E0=20000 eV) and Co (E0=7709 eV) were taken in a fluorescence mode at room temperature under helium atmosphere. Spectra for other materials including references were acquired through transmission detection. The obtained data were analyzed with Athena in the IFEFFIT 1.2.11 suite of software programs and FEFF9 code. A field-emission scanning electron microscope (FE-SEM, Hitachi, S-4800, 15 kV) was used to analyze the surface structure. A transmission electron microscope (TEM, JEOL, JEM-2100) and high-resolution transmission electron microscopy (HR-TEM, JEOL, JEM-2100F) were utilized to investigate structural information, elemental mapping, and chemical compositions of the heterostructures.
3.3.6 Electrochemical measurements
59
All electrochemical analyses were conducted in a three-electrode cell using a rotating disk electrode (RDE, PAR Model 636 RDE) connected with a potentiostat (Ivium Technologies) at room temperature.
An Ag/AgCl (3.0 M KCl) electrode and a graphite rod were used as reference and counter electrodes, respectively. All potentials in different pH of electrolytes were converted to a reversible hydrogen electrode (RHE) by the equation; ERHE = E(Ag/AgCl) + 0.059 pH + 0.209. The ink of catalyst was prepared as follows: 10 mg of as-synthesized powder were sonicated for 1 h in the mixture (1.0 ml) of distilled water and ethanol including 10 μL of 5 % Nafion solution. The 10 μL of ink was dropped onto a glassy carbon electrode and dried at 60 ℃ for 10 min to form the working electrode (catalyst loading:
526.3 μg/cm2).
The electrocatalytic performances of HER were measured in aqueous 1.0 M KOH (pH=14), 0.1 M phosphate buffer solution (PBS, pH=7), and 0.5 M H2SO4 (pH=0) at a scan rate of 5 mV s−1 with a rotation speed of 1600 rpm (iR-corrected). The chronopotentiometry (CP) test was carried out at -20 mA/cm2 and the chronoamperometry (CA) test was measured alternately at -0.14 and -0.2 V vs. RHE for 60 h. The electrochemical impedance spectroscopy (EIS) results were fitted with Z-view software evaluated in the frequency range of 100 kHz to 1 mHz with a modulation amplitude of 10 mV.
Cyclovoltammetry (CV) cycles were performed to estimate the electrochemical active surface area (ECSA) in the range of -0.8 to -0.6 V vs. Ag/AgCl with different sweep rates between 20 and 100 mV s−1.
3.3.7 Theoretical Section
All density functional theory (DFT) calculations were conducted using the Vienna Ab initio Simulation Package (VASP) source code.31 The atomic pseudopotentials were described using the Projector Augmented Wave (PAW) method as provided by the package.32 A plane-wave basis set with an energy cutoff of 400 eV and the Perdew-Burke-Ernzerhof (PBE) type gradient-correlated functional was employed to present the exchange-correlation potential.33 We used Dudarev et al.’s Hubbard U approach to treat the strong on-site Coulomb interaction in localized orbitals and exchange interaction between the 3d electrons in transition metal sulfides correctly.34 The Hubbard parameter 𝑈𝑒𝑓𝑓 = 3.50 𝑒𝑉 was set for 3d orbitals of Co.35 We calculated the surface energy for Co9S8 slab to define the Co9S8 nanoparticle and adsorption direction on the MoS2 monolayer. The surface energies were calculated as 𝐸𝑠𝑢𝑟𝑓𝑎𝑐𝑒 =12(𝐸𝑠𝑙𝑎𝑏− 𝐸𝑏𝑢𝑙𝑘). The 1 nm Co9S8 nanoparticle consists of 38 Co and 38 S atoms with (111) plane, which is the most stable surface index, and it is used to model the Co9S8
nanoparticle on the 6x6x1 MoS2 monolayer. There are two configurations for the adsorption site of the Co9S8 nanoparticle on the MoS2 monolayer, S-top and Mo-top. Each binding energy is calculated by 𝐸𝐵 = [𝐸𝑀𝑜𝑆2+ 𝐸𝐶𝑜9𝑆8 𝑁𝑃] − 𝐸𝐶𝑜9𝑆8 𝑁𝑃−𝑀𝑜𝑆2. We finally decided the model structure as described in
60
Figure 3.19a, and all of the models were free to relax until the self-consistent forces reached 0.02 eV/A.
The k-point grids were sampled 3x3x1 for geometry optimization and 9x9x1 for calculating projected density of state (PDOS) and charge density. The sulfur defect formation energies were calculated as
∆G = [𝐺𝑀−𝑛𝑆− 𝑛𝜇𝑆] − 𝐺𝑀, where n is number of the sulfur defect and 𝜇𝑆 is chemical potential of sulfur (S). We obtained the value of d-band center by using 𝐸𝑑−𝑏𝑎𝑛𝑑𝑀 =∑ 𝐷𝑁𝑖 𝑖𝑀𝐸𝑖
∑ 𝐷𝑁𝑖 𝑖𝑀 , where M is index of target atom, N is number of the fast Fourier transfer (FFT) grid for DOS calculation, 𝐷𝑖𝑀 is PDOS for M atom at 𝑖-th grid and 𝐸𝑖 is energy for 𝑖-th grid, and the summations ranged from –50 eV to 5 eV.36-38 The charge accumulation and depletion were obtained as 𝜌𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 = 𝜌𝐶𝑜9𝑆8−𝑀𝑜𝑆2− (𝜌𝑀𝑜𝑆2+ 𝜌𝐶𝑜9𝑆8) . The x-y averaged charge difference is plotted along the z-direction as 𝜌(𝑧) =
∑𝑁𝑥𝑥=1∑𝑁𝑦𝑦=1𝜌(𝑥,𝑦,𝑧)
𝑁𝑥𝑁𝑦 , where ρ(x, y, z), 𝑁𝑥 and 𝑁𝑦 indicate the FFT grid data of charge difference, number of x grid and number of y grid, respectively. The value of the transferred charge is obtained by using Bader charge analysis.39-42 The Gibbs free energies for hydrogen adsorption were defined by ∆GH =
∆EH+ ∆EZPE− T∆SH , where ∆EZPE and ∆SH are the zero-point energy and entropy difference between the phase for gas state and phase for adsorbed state on the active site. ∆EH , hydrogen adsorption energy, was calculated by ∆EH = EH∗− E∗− EH+(𝑎𝑞), where EH∗ is the total energy of hydrogen adsorbed state, E∗ is the total energy of state for catalyst surface without hydrogen atom and E𝐻+(𝑎𝑞) is the energy of proton in the ionic phase and it is described as EH+(𝑎𝑞)=1
2𝐸H2(𝑔)− 𝑅𝑇𝑙𝑛(10)pH , where 𝐸H2(𝑔) is the energy of a gas phase hydrogen molecule.43 In this study, we determined the T and pH value as 300 K and 14, respectively, to depict the experimental condition, standard temperature and pressure (STP) and alkaline medium.