The use of photocatalytic technology for water splitting is an important means to obtain hydrogen energy. At present, most photocatalysts have high photoexcitation carrier recombination rate, low visible light utilization efficiency, and few active sites for redox reactions, which are not conducive to photogenerated carriers participating in catalytic reactions. MOS2 is a potential photocatalyst material, but its hydrogen evolution performance is limited by the number of edge unsaturation sites (Mo and S) and the catalytically inert substrate.
Yao Man, Dalian University of Technologyet al. used N + F co-doped MOS2 to improve the photochemical activity by charge compensation. In addition, strain engineering was introduced to regulate the band-edge position of molybdenum disulfide, and the influence of doping and strain on the photocatalytic performance of MOS2 was introduced.
Models and calculation methods
This articleFor a calculation work of Professor Yao Man's research group at Dalian University of Technology, the CASTEP module of Materials Studio software package was used to study the hydrogen evolution performance theoreticallyThe reaction mechanism of moisture-resolved hydrogen with vacancies on the phosphide surface was revealed.
The electron exchange correlation is calculated based on the PEB functional under the Generalized Gradient Approximation (GGA), and the electron-ion interaction is described by the plane-wave ultrasoft pseudopotential method. Since the GGA-PBE function tends to underestimate the band gap, the HSE06 hybrid functional is used for band gap calculation. Fig. 1 shows the original molybdenum disulfide structure and N+F co-doped MOS2 structures.
Figure 1Side view of the original and doped molybdenum disulfide monolayer.
Structure & Discussion
First, the authors calculated the formation energy of the catalyst, the system is more stable when N+F is co-doped, and Mo-rich MoS2 is more stable when doped. Next, the adsorption of water molecules and proton H at different sites at the catalyst interface was examined, and it was obvious that the doping of heteroatoms could improve the HER and OER activities of molybdenum disulfide. The adsorption sites of the molecules are all near the n atom, indicating that the n atom plays a major role in improving the catalytic activity, and the doping of f atom can stimulate the activity of the n position, which synergistically activates the catalytic inert substrate of molybdenum disulfide. Among them, Type I has the most significant effect, and the ΔGH is -0081EV, the reaction rate determination step (O* OOH*) of oxygen evolution is 1751 EV (at the equilibrium potential of water oxidation, U=1.)23ev）。
Figure 2The formation energy of different structures of molybdenum disulfide and the reaction free energy of HER and OER.
Subsequently, the optoelectronic properties of MOS2 and Type I were described(Figure 3). MOS2 is a direct bandgap material, only the energy changes during the electron transition, the momentum does not change, and no additional phonon action is required, and it is easier to form photogenerated carriers, and the quantum efficiency is higher.
Type I is also a direct bandgap and the bandgap shrinks, effectively promoting electron migration. The charge transfer concentration is similarly increased under co-doping, and one S atom of the original molybdenum disulfide can be obtained from the Mo atom024 e。For Type I, the number of Mullikrn charges of Mo atoms flowing to N atoms and F atoms can be increased to 0., respectively58 e and 034 e, the catalyst bonding capacity is improved. Moreover, N+F doping can enhance the light absorption capacity of molybdenum disulfide in the visible region, which is conducive to the generation of photogenerated carriers.
(Figure 3). Mos2 and Type I were found in photoelectric properties diagram S7, and the doping position could not effectively improve the ability of H reduction. Next,The authors calculated the band gap of Type I under tensile and compressive strains, Figure S8 uses phonon spectra to confirm that the strain regulation does not affect the stability of the catalyst itself. Figure 4 shows the band arrangement of Type I at different strain rates, and it can be seen that the strain can fully adjust the band position of the catalyst, and the Type I catalyst has the ability to reduce H under compressive strain (at a strain rate of -2%).
Figure 4Band arrangement of Type I at different strain rates.
Finally, the HER and OER free energies of Type I catalysts under strain are studied. The strain effect affects the synergistic effect of N and F atoms on the catalyst and the coupling between Mo and S atoms, which in turn changes the electronic structure. In HER, the biaxial compressive strain retains excellent hydrogen evolution free energy. Although the compressive strain is 3%, it also has H reduction activity, but the band gap is transformed into an indirect band gap, which reduces the electron jumping ability. The Type I catalyst with a compressive strain of 2% was then calculated, and the overpotential is 1797 EV, although slightly higher than the overpotential of the original Type I catalyst (1751EV), but still maintains excellent OER activity. Combined with the principle of band alignment, it is easy to find that the Type I is suitable for water** reactions under compressive strain conditions.
Figure 5Type I: HER free energy and 2% strain rate at different strain rates are OER free energy.
Conclusions and prospects
In this paper, based on the idea of charge compensation, co-doped molybdenum disulfide is designed to improve the light absorption capacity, improve the mobility of catalytic active sites and photogenerated carriers, and then improve the reaction efficiency of HER and OER. Furthermore, the strain engineering adjustment band alignment is introduced, and it is found that both the oxidation and reduction of water can occur in the band gap region, which provides an important research basis for the catalytic ability of heteroatom co-doping and strain engineering regulation.
Bibliographic information
zhao, t., chen, j., wang, x., yao, m. (2022). enhanced photocatalytic activity of mos2 via n+ f codoping and strain engineering: a first-principles investigation. applied surface science, 154881.