Stimulated Electrocatalytic Hydrogen Evolution Activity of MOF‐Derived MoS2 Basal Domains via Charge Injection through Surface Functionalization and Heteroatom Doping

Abstract The design of MoS2‐based electrocatalysts with exceptional reactivity and robustness remains a challenge due to thermodynamic instability of active phases and catalytic passiveness of basal planes. This study details a viable in situ reconstruction of zinc–nitrogen coordinated cobalt–molybdenum disulfide from structure directing metal–organic framework (MOF) to constitute specific heteroatomic coordination and surface ligand functionalization. Comprehensive experimental spectroscopic studies and first‐principle calculations reveal that the rationally designed electron‐rich centers warrant efficient charge injection to the inert MoS2 basal planes and augment the electronic structure of the inactive sites. The zinc–nitrogen coordinated cobalt–molybdenum disulfide shows exceptional catalytic activity and stability toward the hydrogen evolution reaction with a low overpotential of 72.6 mV at −10 mA cm−2 and a small Tafel slope of 37.6 mV dec−1. The present study opens up a new opportunity to stimulate catalytic activity of the in‐plane MoS2 basal domains for enhanced electrochemistry and redox reactivity through a “molecular reassembly‐to‐heteroatomic coordination and surface ligand functionalization” based on highly adaptable MOF template.

room temperature, MoS-CoS or MoS-CoS-Zn were collected by centrifugation, washed by DI water for three times, and the dried in an oven at 60℃ for 12 h.
Synthesis of MoS-CoS-Zn-1pot. 0.05 g of Na 2 MoO 4 .2H 2 O, 0.04 g of Co(NO 3 ) 2 .6H 2 O and 0.01 g of Zn(NO 3 ) 2 .6H 2 O and 0.2 g of TU were dissolved in 25 mL of DI water. The obtained solution was then transferred into a 50 ml Teflon-lined autoclave, which was sealed and maintained at 220℃ for 24 h.
After cooling down room temperature, the product was collected by centrifugation, washed by DI water for three times, and the dried in an oven at 60℃ for 12 h. The elemental mapping of the materials was performed by energy-dispersive X-ray spectroscope (EDX) attached to the JEOL JEM-2010 F TEM instrument. The crystal structure was obtained by X-ray powder diffraction (XRD) by collecting the patterns using a diffractometer (GADDS XRD system, Bruker AXS) equipped with a CuKα radiation source (λ= 1.54 Å). X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantera x-ray photoelectron spectrometer with a monochromated Al Kα radiation. Raman spectrum was obtained using a 532 nm excitation laser with a WITec Raman instrument. Nitrogen adsorption-desorption isotherms were obtained at 77.3 K using the (Quantachrome) NOVA-1200 System and pore size distribution was calculated by BJH method based on desorption isotherm.
The UPS measurements were carried out using the SINS beamline of Singapore Synchrotron Light Source equipped with a VG Scienta R4000 analyser. [1] The work-functions (WFs) were measured with an excited photon energy of 60 eV and the sample is biased at -10 V. The following formula was used to calculate work function: where, is the instruments work function ( ), is the kinetic energy at the secondary electron edge, which is at the peak position of the differentiate curve.
Zn and Co K-edge X-ray absorption fine structure (XAFS) data was recorded using XAFCA beamline at the Singapore Synchrotron Light Source (SSLS). [2] Zn and Co foil were applied for the energy correction of the Theoretical methodology. First-principles calculations were performed based on spin-polarized density functional theory (DFT) using the generalized gradient approximation of Perdew, Beurke and Ernzerhof (GGA-PBE) for the exchange-correlation functional, [3] implemented in the Vienna ab-initio Simulation Package (VASP). [4][5] The ion-electron interaction was described by the projector augmented-wave (PAW) method. [6] The wave functions were expanded in a plane-wave basis with a cutoff energy of 550 eV. The first Brillouin zone was sampled with a gamma-centered 15×15×1 k-mesh for the MoS 2 unit cell. The electronic and ionic convergence criteria were set to 10 -4 eV and 0.01 eV/Å for energy and force, respectively. A vacuum of around 20 Å was used on the surface of MoS 2 to eliminate the interaction between periodic images. A large 6×6×1 supercell was adopted to investigate the effects of doping of Zn atoms and the adsorption of CoS 2 ZnN clusters on MoS 2 . Both the long-range van der Waals interaction [7][8] and the dipole correction [9][10] were included in our calculations. The optimized lattice constant of the MoS 2 unit cell is 3.185 Å, in agreement with the reported result [11] .
Gibbs free energy for adsorbed hydrogen, , is a good descriptor to evaluate the catalytic activity towards HER, [12][13]  with an adsorbed CoS 2 ZnN cluster. and represent the change in the zero-point energy (ZPE) and the change in the entropy of hydrogen upon the hydrogen adsorption. Since the entropy of the adsorbed H is very small, eV at K. [14] The optimal HER activity can be achieved as goes to zero, where both the hydrogen adsorption and the subsequent desorption can be facilitated. [12][13]    Zoomed-in view of the XRD patterns. XRD peaks of the Co-MOF match well with the experimental and simulated XRD patterns of ZIF-67. [15] As shown in Figure S3a, the Co 8 Zn 1 -MOF exhibits sharp diffraction peaks, indicating that it exhibits a highly crystalline structure. Moreover, diffraction pattern of the This was further supported by investigating the zoomed-in XRD patterns. Figure S3b shows that peaks of Co-MOF are slightly shifted to lower angles, confirming the substitutional constitution of slightly larger zinc atoms for cobalt atoms.  Figure S4a-e, molar ratio of Zn/Co in the bimetallic templates can be tuned strategically without compromising the morphological merits, endowing the possibility to control selective metal substitution. Figure S4f supports the successful tuning of the cobalt and zinc contents.  The precursors form non-uniform large-sized microspheres (>2 µm) as shown in Figure S6a,b.         Table S2), thereby allowing a rational Zn loading comparison. As shown in Figure S14a and        Table S2), indicating compositional stability of the electrocatalyst. XRD pattern in Figure S20d also reveals existence of MoS 2 and CoS 2 (similar to the XRD pattern before stability test), suggesting that the crystal structure and phase of the electrocatalyst do not change during HER testing.          Figure   S30), and corresponding adsorption energies.   Figure S30). These hydrogen adsorption sites are also presented in Figure 5b.