FIRST-PRINCIPLES CALCULATIONS ON MOLYBDENUM SULFIDE TWO-DIMENSIONAL STRUCTURE WITH CHEMICAL REACTIVE BASAL PLANE FOR HYDROGEN EVOLUTION REACTION
Hydrogen evolution reactions (HER) have been considered as one of the most promising pathway to convert and utilize solar energy, the efficiency of which relies mostly on the catalyst used. Molybdenum disulfide has shown its potential as a non-noble-metal catalyst for the hydrogen evolution reactions. However, its performance is limited by fractional active edge sites, poor electrical transport, and hydrogen coverage. Intensive efforts have been made to engineer molybdenum disulfide and exploring new phases of Mo-S compounds to improve the HER performance. Among them, a recent synthesized two-dimensional (2D) sub-stoichiometric molybdenum sulfide (s-MoSx) might be a promising candidate as an electrochemical catalyst for HER, owing to its under-coordinated Mo atoms and chemically reactive basal plane.
In this study, first-principles calculations were carried out to study the HER performance catalyzed by the sub-stoichiometric molybdenum sulfide, in which the density-functional theory (DFT) based Vienna ab initio simulation package (VASP 5.4.4) was used with the projector augmented wave (PAW) pseudopotentials, the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, and plane-wave basis.
We consider three potential structures by inter-clustering the Mo6S8 Chevrel units into 2D square structures based on the experimental observation. The most stable structure is shown in Figure 1, where both lattice spacing and lattice configuration of the simulated STM results are consistent with the experimental measurements. This suggests that the proposed structure is a promising candidate to resolve the experimental grown 2D s-MoSx structure.
The stability of this Mo6S4 2D structure is evidenced by the calculated phonon dispersion and molecular dynamics (MD) simulation at room temperature. As Figure 2 shows, there is no negative frequency in the phonon dispersion spectra, and the bond length variation is within 0.3 Å. The calculated projected density of states further reveal that the Mo6S4 2D structure is metallic with dominant contribution from Mo d orbital near Fermi level due to much under-coordinated Mo ions in the structure. This infers the chemical reactivity in the basal plane of this 2D Mo6S4 structure.
It has been found that the Gibbs free energy for adsorbed hydrogen, ΔGH, is a good descriptor to evaluate the catalytic activity towards HER, where a ΔGH~0 eV is related to an optimum HER efficiency of the catalyst. The calculated ΔGH of the Mo6S4 2D structure is shown in Figure 3, as well as other reference catalysts. We note that the Mo6S4 2D structure is chemically reactive towards HER, in which its catalytic performance is slighter better than that of 2H-MoS2 edges. More importantly, the HER performance can be improved by applying compressive strain, and is not affected by the increased hydrogen coverage. All these outcomes are desired for practical HER applications.
In conclusion, using first-principle calculations, we predict a sub-stoichiometric stable two-dimensional metallic molybdenum sulfide, Mo6S4, and show that the energetically favourite basal-plane Mo top site is chemically reactive for HER as supported by the calculated hydrogen adsorption Gibbs free energy. We also find that the HER catalytic activity of the Mo top site is insensitive to hydrogen coverage and can be further improved by the application of biaxial compressive strains. The predicted HER activity of the Mo6S4 2D structure is further confirmed by the experimental measurement. Our results present a promising structure to understand the novel 2D phase of the recent synthesized s-MoSx film, and further show its potential application as an effective catalyst for HER.
Reference:
Yang T., et al. Hydrogen Evolution Catalyzed by a Molybdenum Sulfide Two-Dimensional Structure with Active Basal Planes, ACS Appl. Mater. Interfaces, 2008, 10, 22042–22049.
#!/bin/bash #PBS -o job_log.stdout #PBS -e job_log.stderr #PBS -N docking #PBS -l select=1:ncpus=8 #PBS -q parallel8 ./autodock4 -p compound_1.dpf -l compound_1.dlg & pid=$! ; PID_LIST+=" $pid"; ./autodock4 -p compound_2.dpf -l compound_2.dlg & pid=$! ; PID_LIST+=" $pid"; ./autodock4 -p compound_3.dpf -l compound_3.dlg & pid=$! ; PID_LIST+=" $pid"; ./autodock4 -p compound_4.dpf -l compound_4.dlg & pid=$! ; PID_LIST+=" $pid"; ./autodock4 -p compound_5.dpf -l compound_5.dlg & pid=$! ; PID_LIST+=" $pid"; ./autodock4 -p compound_6.dpf -l compound_6.dlg & pid=$! ; PID_LIST+=" $pid"; ./autodock4 -p compound_7.dpf -l compound_7.dlg & pid=$! ; PID_LIST+=" $pid"; ./autodock4 -p compound_8.dpf -l compound_8.dlg & pid=$! ; PID_LIST+=" $pid"; wait $PID_LIST
If the user is familiar with interpreted languages such as PERL and Python, the similar implementation can be easily achieved. The aforementioned commands of autodock4 applications in the job submission file can be replaced by a PERL script.
As shown in Example 2, the idea is to use a “system” call in combination with a “for” loop to execute command of my application in background. More PERL functions can be integrated in the script to check if the processes are completed successfully or even directly process output files.
Example 2. PERL script (partial) to distribute jobs in a node
for($i=1; $i<=8; $i++)
{
system("./autodock4 -p compound_$i.dpf -l compound_$i.dlg &");
}
By applying the script, users can query queue types of “serial”, “parallel8”, “parallel12” and “parallel24” for the mass serial applications as our projects. According to my experience, the job turnover rate can be increased by more than 5 fold compared to exclusive “serial” jobs. Of course, the increase will also depend on the total job loading on the HPC system. In this article, we provide examples of bash shell script and PERL script. Other shell scripts (e.g. C shell, Bourne shell, Korn shell…) or interpreted languages (e.g. Python) should have equivalent functions to achieve the implementations we hope the experience shared here can help the large NUS HPC users with facilitate their computational projects.
