Background:
Borene and molybdenum disulfide are promising two-dimensional functional materials due to their superior high specific surface area. However, due to its instability and inert substrate, it does not perform well in hydrogen evolution reactions. Heterostructure is an effective structure that combines the advantages of each component to achieve synergistic performance. Pan Yong, Southwest Petroleum UniversityThe two-dimensional heterostructure of borene MOS2 was constructed by first-principles method to improve its application characteristics in hydrogen evolution catalytic reactions.
Models and calculation methods
AdoptionDMOL3 module of the Materials Studio packageThe electrocatalytic hydrogen evolution performance and electronic properties are studied theoretically, the electron exchange correlation is calculated based on the PBE functional under the Generalized Gradient Approximation (GGA), the core electrons are processed by DNP basis set, and the reaction in the real aqueous solution is simulated by Cosmo. The heterogeneous structure under the four stacking methods is shown in Figure 1.
Figure 1Heterostructure of borene MOS2 with different stacking methods
Results & Discussion
Through the calculation of formation energy, it was found that borene@mo-s MOS2 and borene@mo MOS2 were more stable. Figure 2 shows the model, band diagram and density of states of the two heterostructures. It can be found that the electronic structure of the two stacking methods is almost identical, and the subsequent calculations will be carried out with borene@mo-s mos2. Figure 3 shows the electronic properties of the remaining three models, with the original borene having metallic properties. The band gap of the original 2H phase molybdenum disulfide is about 1696 ev。This pseudogap restricts electron transport between the conduction band and valence band, which is detrimental to catalytic performance.
There is no energy gap in borene @mos2 heterostructure. It is shown that the heterojunction can adjust the pseudo-gap of the semiconductor. This is attributed to the redistribution of charges around the heterogeneous interface, which improves the electron transport of the original molybdenum disulfide. Molybdenum exhibits strong delocalized behavior between the conduction and valence bands near the Fermi level. The electronic state of the B atom is hybridized at the same time as the occupied and unoccupied states of the Mo atom and the S atom, showing delocalized behavior. In addition, the synergistic effect of borene @s2 MOS2 is stronger, which is conducive to the progress of HER.
Figure 2Electronic structure of borene@mo-s mos2 and borene@mo mos2
Figure 3Bands and densities of states of borene@mo-s mos2, borene@s1 mos2, and borene@s2 mos2
The proton h adsorption position was designed according to Figure 4. It was found that borene@mo-s mos2 had the best HER activity at HE (free energy was -0.).086ev）。The optimal adsorption site for borene @s1 MOS2 is HA, and its hydrogen-adsorption Gibbs free energy is about -0037ev。The optimal adsorption site of borene @s2 mos2 is also Ha, and its hydrogen adsorption Gibbs free energy is about -0024ev。At this time, the HER activity was much higher than that of the MOS2 electrocatalyst alone, indicating that the composition of heterogeneous heterostructures was an effective way to optimize the catalytic hydrogen evolution performance of the system.
Figure 4Borene @mos2 hydrogen evolution location
The volcanic curve of the catalyst and the theoretical overpotential were then calculated, as shown in Figure 5. In the volcanic curve, the catalytic ability of the peak position is better, so the two heterostructures of borene@s1 MOS2 and borene@s2 MOS2 have better HER activity. This conclusion is also confirmed by the theoretical overpotential, and the smaller the overpotential, the easier the H reduction process is to proceed. These results indicate that the borene@s2 MOS2 catalyst can provide higher exchange current and lower energy loss during hydrogen adsorption. This contributes to the adsorption and release of hydrogen by borene @s2 mos2 in the hydrogen evolution reaction, which has the best catalytic activity in the hydrogen evolution reaction.
Figure 5Heterojunction hydrogen, free energy and volcanic curves
The changes in energy bands and density of states after hydrogen evolution of the three heterostructure catalysts are shown in Figure 6. Although the electron number of the H atom is only 1, the complex interaction after adsorption on the surface of the catalyst changes the electronic properties of the whole system. Comparing the electronic properties of the original structure (Fig. 3), it is found that the adsorption structure has significantly more energy bands than the original structure in the valence band region. This phenomenon is particularly evident in the band plots of borene @s2 mos2, which is the main reason why the catalytic hydrogen evolution reaction performance of borene @s2 mos2 is better than that of the other two heterojunctions. Although the adsorption of hydrogen weakens the interaction between borene and MOS2 to a certain extent, the hybridization of hydrogen and two-dimensional materials optimizes the carrier migration capacity of the overall structure.
At the same time, the position of H in the borene@mo-s MOS2 structure causes electron depletion in the two layers of materials at the same time, resulting in the enhancement of H hybridization, which is also the reason for the large free energy of hydrogen evolution. At the HA site of borene@s1 mos2, the adsorption of hydrogen appears to interact only with borene. Molybdenum disulfide is not involved in this process. The hydrogen adsorption site in the borene@s2 MOS2 heterojunction is encapsulated in an electron depletion region, followed by the accumulation of electrons, and S is involved in the whole dynamic reaction process, confirming that this structure has the potential to be an excellent HER catalyst.
Figure 6The bands and densities of states of borene@mo-s mos2, borene@s1 mos2, and borene@s2 mos2 after hydrogen evolution
The light absorption curve of the catalyst can intuitively reflect the light excitation ability of the catalyst in the infrared visible ultraviolet spectrum. Figure 7 shows the wavelength of the absorption spectra of borene@mo-s MOS2, borene@s1 MOS2, and borene@s2 MOS2. All borene MOS2 heterojunctions have strong absorption in the UV region. Two peaks appear in the wavelength range of 100 nm and 200 nm, indicating that they have excellent absorption of ultraviolet light. In the visible wavelength range, the three borene MOS2 heterojunctions also have a certain degree of light response.
However, the optical activity of these three heterojunctions is weaker in the infrared region, and the optical absorption coefficient in the ultraviolet region (30 100 nm) is greater than that of the other two heterojunctions for the borene @s2 mos2 structure, indicating that borene@s2 mos2 can absorb more photons. Stronger light absorption means that more photogenerated carriers can be activated, inducing the progression of HER and OER.
Figure 7Heterojunction light absorption spectroscopy
Conclusions and prospects
In this paper, the catalytic reaction activity is further improved by constructing two materials with poor intrinsic HER activity through heterojunction. The catalyst with excellent HER ability was calculated by stacking method and selection of H adsorption site. This can provide a research basis for the heterostructure of other new 2D materials to enhance the electronic activity.
Bibliographic information
yu, e., pan, y. (2022). catalytic properties of borophene/mos2 heterojunctions for hydrogen evolution reaction under different stacking conditions. journal of materials chemistry a.