Novel Two-Dimensional Janus MoSiGeN₄ and WSiGeN₄ as Highly Efficient Photocatalysts for Spontaneous Overall Water Splitting

Abstract

Searching for highly efficient and eco-friendly photocatalysts for water splitting is essential for renewable conversion and storage of inexhaustible solar energy but remains a great challenge. Herein, based on the new emerging two-dimensional (2D) material of MoSi2N4, we report novel Janus MoSiGeN4 and WSiGeN4 structures with excellent stabilities and great potentials in photocatalytic applications through first-principles calculations. Comprehensive studies show that MoSi2N4, MoSiGeN4, and WSiGeN4 exhibit semiconductor characteristics with an indirect gap, appropriate band gaps, and strong optical absorbance in the visible spectrum. Excitingly, by constructing Janus structures, an intrinsic electric field is realized that enhances the spatial separation and anisotropic migration of photoexcited electrons and holes. Further, this strategy can also alter the band alignment to provide an adequate photoexcited carrier driving force for water redox reactions. Moreover, the surface N vacancy can effectively lower the energy demand of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) so that the catalytic process can be self-sustained under the potential provided by the photocatalyst alone. Particularly, the overall water splitting can proceed simultaneously and spontaneously on the surface of MoSiGeN4 and WSiGeN4 when pH is 3 or ≥8, respectively. These explorations offer new prospects for the design of highly efficient photocatalysts.

Stacking Engineering: A Boosting Strategy for 2D Photocatalysts

“…Figure b gives their plane averaged electrostatic potentials normal to the basal plane. The value of discontinuity (ΔΦ) between the vacuum levels of the top and bottom layers is found to be 0.12 eV for AB-MoSi 2 N 4 , which is comparable or even larger than that of other polarized materials. ,, The mechanism for such an electrostatic potential difference is the separation between negative and positive charge centers due to the stacking-induced out-of-plane centrosymmetry and mirror symmetry breaking, which is different from previously reported heterostructures or intrinsic polarized systems − and can also be extended to other 2D photocatalysts. We expect such an electric polarization could create an interfacial built-in electric field and affect the charge transfer processes.…”

“…Importantly, the energy difference between the CBM (VBM) and the reduction (oxidation) potential level of water is 0.70 (0.16) eV, implying the high redox abilities of AB-MoSi 2 N 4 . Recent literature found that the perfect surface of MoSi 2 N 4 is inert to the hydrogen evolution reaction and oxygen evolution reaction, 35 which means that other modification methods such as vacancy and strain may be needed in experiments. 35,39 As the built-in electric field and type-II band alignment are realized, this will in turn call into play the prolonged lifetime of photogenerated carriers and favorable for photocatalysts, but to what extent in the context of stacking engineered polarized multilayers remains an open question.…”

“…Recent literature found that the perfect surface of MoSi 2 N 4 is inert to the hydrogen evolution reaction and oxygen evolution reaction, 35 which means that other modification methods such as vacancy and strain may be needed in experiments. 35,39 As the built-in electric field and type-II band alignment are realized, this will in turn call into play the prolonged lifetime of photogenerated carriers and favorable for photocatalysts, but to what extent in the context of stacking engineered polarized multilayers remains an open question. To quantitatively analyze the lifetime of photogenerated electron−hole recombination, herein, the evolutions of state populations of the excited electrons are calculated using time-domain density functional theory combined with NAMD.…”

“…First, the band gap has to be moderate for utilizing incident visible or infrared light. Second, its edge positions need to span redox potentials of specific reactant to provide a driving force for the reaction. , Here, we only focus on evaluating the photocatalytic potential of MoSi 2 N 4 for water splitting to validate our conjecture and concentrate on discussing the impact of stacking on the charge dynamics and external strain effect as shown below. According to the standard reaction mechanism of water splitting, the band edge positions are required to fit the standard reduction potential of H + /H 2 (−4.44 eV vs the vacuum level) and the oxidation potential of O 2 /H 2 O (−5.67 vs vacuum level).…”

“…Importantly, the energy difference between the CBM (VBM) and the reduction (oxidation) potential level of water is 0.70 (0.16) eV, implying the high redox abilities of AB-MoSi 2 N 4 . Recent literature found that the perfect surface of MoSi 2 N 4 is inert to the hydrogen evolution reaction and oxygen evolution reaction, which means that other modification methods such as vacancy and strain may be needed in experiments. , …”