Computational exploration and screening of novel Janus MA₂Z₄ (M = Sc−Zn, Y−Ag, Hf−Au; A=Si, Ge; Z=N, P) monolayers and potential application as a photocatalyst

Abstract

By high-throughput calculations, 13 thermally and environmentally stable Janus MA₂Z₄ monolayers were screened from 104 types of candidates. The 13 stable monolayers have very high charge carrier concentrations (×10¹⁵ cm⁻²), which are better than those of the well-known graphene and TaS₂. Because of their excellent conductivity, the 6 monolayers with band gaps less than 0.5 eV are identified as potential electrode materials for hydrogen evolution reaction applications. For potential applications as photoelectric or photocatalytic materials, bandgaps (E_g-HSE) higher than 0.5 eV remained, which resulted in 7 potential candidates. Based on optical absorption analysis in the visible-light range, H-HfSiGeP₄ and H-MoSiGeP₄ have higher absorption ability and optical conductivity, which is quite impressive for optoelectronic, solar cell device, and photocatalysis applications. Additionally, the transmittance coefficient of Janus MA₂Z₄ monolayers is approximately 70%−80% in the visible-light range, which implies that these monolayers show good light transmittance. For potential applications as photocatalysts, the redox potential and charge effective mass analysis indicate that H-HfSiGeP₄, H-MoSiGeP₄, T-ScSiGeN₄, and T-ZrSiGeN₄ are suitable photocatalysts for CO₂ reduction reactions. Using high-throughput identification, 13 types of new and stable Janus MA₂Z₄ monolayers were explored, and the basic properties and potential applications were investigated, which can reduce the time for experiments and provide basic data for the material genome initiative.

Graphical abstract

Keywords

Janus MA ₂Z ₄ / high-throughput identification / charge carrier concentration / electronic structure / optical properties

1 Introduction

Two-dimensional (2D) materials have attracted extensive attention because of their unique properties, including high specific surface, excellent stability, and low cost. Therefore, experimental and theoretical studies have been widely performed because of their potential applications in electronics and optoelectronics, field-effect transistors, catalysis, and energy storage [1]. Diverse types of 2D materials with good properties and diverse potential applications have been widely investigated, including phosphides [2], and g-C₃N₄ [3] transition-metal chalcogenides [4]. However, few 2D candidates have practical and industrialized applications, such as the hydrogen evolution reaction (HER), NO removal or CO₂ reduction. Therefore, it is urgent to explore new 2D materials or mechanisms by calculations and experimental efforts to satisfy the extending requirements of the applications.

In 2020, Ren et al. [5] first synthesized MoSi₂N₄ using a chemical vapor deposition method. This material exhibited semiconducting behavior (band gap ~1.94 eV), high strength (~66 GPa), and excellent environmental stability. Because of its excellent physical and chemical properties, the MoSi₂N₄ monolayer has attracted considerable attention. Li et al. [6] investigated the possibility of MoSi₂N₄ as both the cathode and anode of Zn–air batteries and revealed the mechanism of Zn storage. Ma et al. [7] investigated the adsorption behavior of environmental gas molecules on pristine and defective MoSi₂N₄ and indicated that this material could be applied as a highly sensitive and reusable gas sensor. Ye et al. [8] predicted the performances of double-gate metal oxide semiconductor field effect transistors (MOSFETs) based on monolayer MoSi₂N₄, and the results revealed that monolayer MoSi₂N₄ was a promising alternative for transistor channel materials in the post-silicon era. Optical analysis shows that the MoSi₂N₄ monolayer is a promising candidate as a photocatalyst for hydrogen evolution [9] and CO₂ reduction [10, 11].

Due to the aforementioned outstanding performance and various applications, many researchers have explored the family of 2D (2D) materials, including atom doping, adsorption, and atomic layer replacement, to develop new types of 2D materials [2, 12]. Two-dimensional Janus structures have attracted increasing attention because of the unique properties caused by their out-of-plane asymmetry. Janus structures have been successfully achieved in many 2D materials, such as Janus graphene [13], silicone [14], PtSSe [15], TiXY (X/Y = S, Se and Te) [16], VSSe [17, 18], SnSSe [19], TMDs [20, 21], and M₂XY (M = Ga, In; X/Y = S, Se, Te) [22]. Therefore, the Janus structure of MoSi₂N₄ can also be achieved, and some researchers have reported the stability and potential applications of the Janus structure of the MoSi₂N₄ family, such as MoSiGeN₄ and WSiGeN₄ monolayers. Guo et al. [23] predicted that Janus MSiGeN₄ (M = Mo and W) monolayers had very good dynamic, mechanical and thermodynamic stability and were indirect band-gap semiconductors. Yu et al. [24] reported Janus MoSiGeN₄ and WSiGeN₄ structures with excellent stabilities that can be applied as photocatalysts. Binh et al. [25] constructed ultrathin graphene and Janus MoGeSiN₄ van der Waals heterostructures and investigated the Schottky barriers and interfacial electronic properties. However, there is no systematically exploration and screening of Janus MA₂Z₄ monolayers.

Therefore, the systematic exploration and screening of stable Janus MA₂Z₄ (M = Sc-Zn, Y−Ag, Hf−Au; A = Si, Ge; Z = N, P) monolayer are urgent. This work applied high-throughput calculations to screen 104 types of Janus MA₂Z₄ monolayers and explored 13 stale candidates. The charge carrier concentrations, band gap, density of states, and optical properties were investigated. The potential applications as photocatalysts were also investigated. Using high-throughput calculations, 13 types of new and stable Janus MA₂Z₄ monolayers were explored, which can reduce the time for the experiment and provide basic data for the material genome initiative.

2 Computational details

The Vienna ab initio simulation (VASP) package [26] based on the first-principle method is applied in this calculation. The GGA (generalized gradient approximation)-PBE (Perdew−Burke−Ernzerhof) [27] exchange-correlation function was applied for geometry optimization, while the electronic and optical properties were calculated by Heyd−Scuseria−Ernzerhof (HSE06) method [28]. The convergence thresholds in energy and force on each atom were 2.0×10⁻⁶ eV and 0.002 eV/Å, respectively. The energy cutoff was set to 450 eV for the plane-wave basis. Brouin zone integration was performed on a 3 × 3 × 1 k-point grid, and the rationality was verified by calculations with a higher cutoff energy and a k-point grid.

The interactions between photons and electrons can be described in terms of time-dependent perturbations of the ground electronic state based on quantum mechanics. The electron transitions between different states are caused by photon absorption or emission. The optical spectra can be accepted by the joint density of states in the conduction and valence bands. The imaginary part \left[{\epsilon }_2\right(\omega \left)\right] can be written as follows [29, 30]:

(1)\begin{array}{rl}{\epsilon }_2& \left(q\to O_{\hat{u}},h\omega \right)=\\ & {\displaystyle \frac{2\pi e^2}{\mathrm{\Omega }{\epsilon }_0}}{\sum }_{k,v,c}{\left|⟨{\psi }_{\mathit{k}}^c|\mathit{u}\cdot r|{\psi }_{\mathit{k}}^{\mathrm{v}}⟩\right|}^2\delta (E_{\mathit{k}}^c-E_{\mathit{k}}^v-E),\end{array}

where u is the vector defining the incident electric field’s polarization, and k is the reciprocal lattice vector. The superscripts c and v denote the conduction band (CB) and valence band (VB), respectively, and \omega is the incident photon frequency. Using Kramers−Kroning relations, the real part (ε₁) of the dielectric function can be obtained from ε₂. Then, {\epsilon }_1\left(\omega \right) and {\epsilon }_2\left(\omega \right) can be applied to obtain the optical spectra as the absorption coefficient \left(\alpha \right(\omega \left)\right), reflectivity R\left(\omega \right), and refractivity index \left[n\right(\omega \left)\right]:

(2){\alpha \left(\omega \right)=\left[\sqrt{{{\epsilon }_1}^2\left(\omega \right)+{{\epsilon }_2}^2\left(\omega \right)}-{\epsilon }_1\left(\omega \right)\right]}^{1/2},

(3)n\left(\omega \right)={\left[\sqrt{{{\epsilon }_1}^2\left(\omega \right)+{{\epsilon }_2}^2\left(\omega \right)}+{\epsilon }_1\left(\omega \right)\right]}^{1/2}/\sqrt{2},

(4)R\left(\omega \right)={\left|{\displaystyle \frac{\sqrt{{\epsilon }_1\left(\omega \right)+\mathrm{i}{\epsilon }_2\left(\omega \right)}-1}{\sqrt{{\epsilon }_1\left(\omega \right)+\mathrm{i}{\epsilon }_2\left(\omega \right)}+1}}\right|}^2.

3 Results and discussion

3.1 Structures and dynamic stability

Fig.1 demonstrates the element compositions and chemical structures for the Janus MA₂Z₄ monolayers, where M represents 3d, 4d and 5d transition-metal (TM) elements, A denotes Si and Ge elements, and Z is for N and P elements (M = Sc-Zn, Y−Ag, Hf−Au; A = Si, Ge; Z = N, P). The Janus MA₂Z₄ materials have a sandwich structure: the inner part is the transition-metal dichalcogenide-like MZ₂ monolayer (including H and T phases), which is covered by two hexagonal AZ monolayers. Therefore, there are two proposed structures of H- and T-phases for every Janus MA₂Z₄ monolayer, denoted as Janus H(T)-MA₂Z₄ in Fig.1. Considering the radioactive, toxic and unstable nature, the transition metals Tc, Cd, Hg and La-Lu were not considered in this study. Considering the stochastic combination of element compositions (26 × 1 × 2) and chemical structures (2), 104 types of Janus MA₂Z₄ monolayers are prepared for the following calculations.

The thermal and environmental stabilities are important to the practical application of Janus MA₂Z₄. The phonon dispersions were calculated to evaluate the stability of the 104 candidates. There are 13 candidates with no imaginary frequency in the first Brillouin zone, which verifies their dynamical stability with no soft modes. The phonon dispersions of H-MoSiGeN₄ and T-ScSiGeN₄ are shown in Fig.2(a, c) as the representative, and the phonon dispersions of the residual 11 candidates are shown in Figs. S1 and S2.

Then, the thermal stability of the 13 selected Janus MA₂Z₄ was evaluated by AIMD (ab initio molecular dynamics) simulations. During the AIMD simulations, the supercells were set to be 2 × 2, and the temperature was 300 K with a Nose thermostat. The simulation time was 5 ps, and the step time was 1 fs. As shown in Fig.2(b, d), the temperature and energy fluctuate around the equilibrium values without any sudden changes for H-MoSiGeN₄ and T-ScSiGeN₄. The residual 11 candidates show similar phenomena. Therefore, the 13 selected Janus MA₂Z₄ monolayers can maintain their structural integrity at room temperature, which indicates their good thermal stability.

3.2 Electronic properties of Janus MA₂Z₄ monolayers

The electronic properties were determined to screen the Janus MA₂Z₄ monolayers for different applications. The PBE and HSE06 band structure calculations were performed for the 13 monolayers. Janus MA₂Z₄ monolayers with HSE06 band gaps (E_g-HSE) lying outside the 0.5−3.26 eV were left out from this subset, which resulted in 7 potential photocatalysts (H-ZnSiGeN₄, H-MoSiGeN₄, H-MoSiGeP₄, H-HfSiGeP₄, H-WSiGeN₄, T-ScSiGeN₄, T-ZrSiGeN₄). The band structures of the 7 candidates are shown in Fig.3, and the band gap values are tabulated in Tab.1. This criterion was chosen to ensure that the selected Janus MA₂Z₄ was optically active for photoelectric, photovoltaic or photocatalysis applications.

The partial density of states (PDOSs) and total density of states (TDOS) of H-MoSiGeN₄ and T-ScSiGeN₄ were investigated. Fig.4(a) shows the PDOSs and TDOS of H-MoSiGeN₄. E_VB (valence band edge) is mainly contributed by Mo 4d and N 2p orbital hybridization. E_CB (conduction band edge) is mainly contributed by the Mo 4d orbital. For the T-ScSiGeN₄ case in Fig.4(b), E_VB is mainly contributed by the Sc 3d and N 2p orbital hybridization. E_CB is mainly contributed by the Sc 3d orbital. The DOS for the residual 7 candidate are shown in Fig. S3. These materials have similar phenomena: E_VB is mainly contributed by TMd and N/P p orbital hybridization, while E_CB is mainly contributed by the d orbitals of TMs.

The charge carrier concentration character is one of the most important factors to evaluate potential applications of semiconductors. Herein, based on the DOS data, the charge carrier concentrations were calculated and analyzed. Fermi−Dirac distribution can be approximated as a step function at very low temperatures. Therefore, the carrier densities (n) can be obtained by integrating the electronic TDOS around the Fermi level (E_F). To verify the reliability of the calculations, we also calculated the carrier density of graphene and TaS₂. Based on this method, the calculated carrier concentration of graphene is 3.15 × 10¹³ cm⁻², which is quite comparable with other reports [(2−5) × 10¹³ cm⁻²] [31, 32]. For the TaS₂ case, the calculated carrier concentration is 0.85 × 10¹⁵ cm⁻², which is very close to a previous report (1.01 × 10¹⁵ cm⁻²) [33]. Therefore, our calculation method is reasonable and can be applied in the following investigations. Based on this method, the carrier densities of stable Janus MA₂Z₄ monolayers are listed in Tab.1 and Tab.2. The carrier concentrations of the 13 stable monolayers are on the order of 10¹⁵ cm⁻², which are better than those of the widely applied materials graphene and TaS₂. The highest case is T-ZrSiGeN₄, which is 2.33 × 10¹⁵ cm⁻². Therefore, these stable Janus MA₂Z₄ monolayers can be applied as potential electrodes or photoelectric devices because of their high charge carrier density. Additionally, they are good potential photocatalysts because the high carrier density can facilitate carrier transfer and is favorable for contact between the photo-induced charge and the molecules on the surface.

The band gaps (E_g-PBE and E_g-HSE) below 0.5 eV (H-CrSiGeP₄, H-ZnSiGeP₄, T-ScSiGeP₄, T-ZnSiGeP₄, T-YSiGeP₄, T-PdSiGeP₄) are tabulated in Tab.2. Because of the band gaps below 0.5 eV and high conductivities (carrier concentrations in magnitude of 10¹⁵ cm⁻²), all 6 candidates are excellent conductive materials, which are helpful for electron transfer to accelerate the proton−electron recombination during the hydrogen evolution reaction (HER). Therefore, the 6 candidates are identified as suitable electrode materials for HER applications because the excited electrons in the materials can cross this low band gap and induce metallic properties at room temperature.

Because of the high charge carrier concentrations, the Janus MA₂Z₄ monolayers are proposed to be promising photo-electric materials applications such as solar cells, light-emitting diodes, and field emitters. For these applications, the work function is an important factor to evaluate the conversion efficiency and device performance. The work function ( \Phi) is formally defined as [34]

(5)\Phi =I-E_F,

where I is the potential energy at an infinite distance from the system, and E_F is the Fermi level. Tab.1 and Tab.2 show the calculated work functions of the stable Janus MA₂Z₄ monolayers. It is the surface dipole formed by the interaction between atoms that should be responsible for the observed variation in the work function.

3.3 Optical properties of the Janus MA₂Z₄ monolayers

Optical property investigations can provide basic data to evaluate potential applications as photoelectric or photocatalytic materials. The dielectric constant is the synopsis of these optical properties. The static (when \omega =0) dielectric constant {\epsilon }_1\left(0\right) was calculated and is tabulated in Tab.1. Previous reports suggest that materials with a lower band gap show higher static dielectric constants [35]. In Tab.1, the band gap decreased with increasing static dielectric constant. Materials with a higher static dielectric constant show a lower charge carrier recombination rate, which facilitates solar cell performance and photocatalysis. Therefore, materials with higher static dielectric constants, such as H-ZnSiGeN₄ H-MoSiGeP₄ and H-HfSiGeP₄, will be more suitable for solar cells and photocatalysis applications.

Fig.5 (a) and (b) depict the energy-dependent parameters of the imaginary part of the dielectric constants {\epsilon }_2\left(\omega \right) for Janus MA₂Z₄ monolayers (E_g-HSE higher than 0.5 eV). The imaginary parts of the dielectric functions {\epsilon }_2\left(\omega \right) of H-MoSiGeN₄ and T-ScSiGeN₄ are analyzed as representations. For H-MoSiGeN₄ [the black line in Fig.5(a)], the main peak is located at 8.94 eV, which corresponds to one of the intrinsic plasma frequencies. The peak originates from an electronic transition from the edge of the N 2p level in the valence band at approximately −6.80 eV to the edge of the Mo 4 d level in the conduction band at approximately 2.20 eV [Fig.4(a)]. The peak at 4.19 eV originates from electronic transitions from the edge of the N 2p level in the valence band at approximately −2.02 eV to the edge of the conduction band of Mo 4 d states at approximately 2.21 eV. For T-ScSiGeN₄ [the black line in Fig.5(b)], the main peak is located at 8.72 eV, which corresponds to one of the intrinsic plasma frequencies. The peak originates from an electronic transition from the edge of the N 2p level in the valence band at approximately −6.20 eV to the edge of the Sc 3 d level in the conduction band at approximately 2.80 eV [Fig.4(b)]. The peak at 2.10 eV is attributed to the intraband transitions from the edge of the Sc 3d level in the valence band at −1.96 eV to the edge of the N 2p states at 0 eV. The other Janus MA ₂Z₄ monolayers are not analyzed in detail. However, the peaks are mainly contributed from the electron transition between N/P p orbitals and TM d orbitals.

Fig.5 (c, d) depict the energy-dependent parameters of the imaginary part of the dielectric constants {\epsilon }_2\left(\omega \right) for Janus MA₂Z₄ monolayers with E_g-HSE less than 0.5 eV. The imaginary parts of the dielectric functions {\epsilon }_2\left(\omega \right) of H-CrSiGeP₄ and T-PdSiGeP₄ are analyzed as representations. For H-CrSiGeP₄ (the black line in Fig.5), the main peak is located at 5.97 eV, which corresponds to one of the intrinsic plasma frequencies. For T-PdSiGeP₄ [the green line in Fig.5(d)], the main peak is located at 2.72 eV, which corresponds to one of the intrinsic plasma frequencies. The peaks are mainly contributed from the electron transition between N/P p orbitals and TM d orbitals.

The optical properties of the material, such as its refractive index (n) or absorption coefficient (α), are critical for several display and optical applications. For example, in camera lenses for smartphones or optical components for augmented reality devices, glasses with varying refractive index of 1.2−2.5, are often preferred. For solar energy conversion applications, a higher absorption coefficient in the visible light range is preferred.

The photo-absorption range and coefficient are important factors to assess the potential applications in photovoltaic, photocatalysis or other optoelectronic applications. The main optical absorption peaks of the Janus MA₂Z₄ monolayers are in the ultraviolet region (Fig.6). For E_g-HSE higher than 0.5 eV cases [Fig.6(a, b)], in the visible-light range, H-HfSiGeP₄ and H-MoSiGeP₄ have higher absorption abilities, which are potentially suitable for photocatalysis applications. For E_g-HSE less than 0.5 eV cases [Fig.6(c, d)], the main optical absorption peaks of the Janus MA₂Z₄ monolayers are in the ultraviolet region also. However, in the visible-light range, H-CrSiGeP₄ and T-PdSiGeP₄ have one additional absorption peak, indicating the two materials have higher photo absorption ability than the other cases.

Then, Fig.7 shows the functions n(ω) (refractivity index), R(ω) (reflectivity), and L(ω) (energy loss) of the seven Janus MA₂Z₄ monolayers. The H-MoSiGeN₄ monolayer is analyzed as a representation [the black line in Fig.7(a1)]. In the lower energy range (<5.00 eV), the reflectivity and refractivity are relatively low. The ~5.00−10.00 eV photon-energy range is characterized by stronger reflectivity, and the refractivity and refractivity spectra have similar tendencies. When the electrons traverse the material, there is some energy loss, which can be described with the energy-loss function. The highest peak is located at approximately 18.89 eV inFig.7(c1), which is related to the existence of plasma oscillations. Additionally, the peaks of the energy-loss spectra are near the trailing edges of the reflection spectra. For the H-MoSiGeN₄ case, there are wave troughs at approximately 4.95 and 11.11 eV for R(ω), which correspond to the wave crest of L(ω). The other systems of Janus MA₂Z₄ monolayers have similar tendencies. H-MoSiGeP₄ and H-HfSiGeP₄ are in the ideal refractive index range and preferred for application as camera lenses.

The degree of electron transport in the substance can be described with the optical conductivity spectrum. In Fig.8, the Janus MA₂Z₄ monolayers exhibit a significant optical conductivity under ultraviolet light. H-HfSiGeP₄, H-MoSiGeP₄, H-CrSiGeP₄, T-PdSiGeP₄ show better optical conductivity in visible light range, which indicates that the photo-induced charge carriers can quickly be transferred to the surface, which is quite impressive for optoelectronic, solar cell device, and photocatalysis applications.

Then, the transmittance spectra indicate the transmittance of the materials, which is also an important factor for the design of optical devices and can be calculated by the following formula [36]:

(6)T={\displaystyle \frac{{(1-R)}^2\exp (-\alpha d)}{1-R^2\exp (-2\alpha d)}},

where α is the absorption coefficient, R is the reflectivity, and d is the thickness of the material. The multiple reflections between front and back surfaces of the film are ignored. d is set to 50 nm in this work. As shown in Fig.9, in the visible-light range, the transmittances of H-HfSiGeP₄ and H-MoSiGeP₄ are less than 70%, which implies that more photons are absorbed or reflected. However, the transmittance spectra are approximately 70%−80% in the visible-light range, which implies that the Janus MA₂Z₄ monolayers show good light transmittance.

3.4 Potential applications as photocatalysts

The relative effective masses of photo-induced charge carriers are important for photovoltaics or photocatalysis, which can determine the charge separation efficiency. Based on the electronic band structure results, the effective mass of electrons (e⁻) and holes (h⁺) of Janus MA₂Z₄ monolayers were calculated via parabolic fitting to the conduction band minimum (CBM) and valence band maximum (VBM), respectively. The effective masses of e⁻and h⁺ were calculated as follows:

(7)m^{\ast }=\pm {\displaystyle \frac{{\hslash }^2}{{\mathrm{d}}^{2}E/\mathrm{d}{k}^2}},

where m^{\ast } is the effective mass of the charge carrier, and {\mathrm{d}}^{2}E/\mathrm{d}{k}^2 is the coefficient of the second-order term in a quadratic fit of E(k) curves for the band edge. The relative ratio (D) of the effective masses plays a significant role in photocatalysis and is evaluated as follows:

(8)D={\displaystyle \frac{m_h^{\ast }}{m_e^{\ast }}}.

By definition, the photogenerated e⁻−h⁺ pairs are more inclined to be separated when D is higher, and e⁻ and h⁺ can migrate and enrich on the surface of materials.

The effective mass results of photoinduced e⁻−h⁺ of 7 selected Janus MA₂Z₄ monolayers and some reported materials are tabulated in Tab.1 and S1. The electron effective mass ( m_{\mathrm{e}}^{\ast }) of the selected Janus MA₂Z₄ monolayers is lighter than Bi₄O₅Br₂ [29], BiVO₄ [37], g-C₃N₄ [38], or TiO₂ [39], which are well-known photocatalysis or photovoltaic materials. Therefore, based on the charge effective mass analysis, the selected 7 Janus MA₂Z₄ monolayers have higher photocatalytic activity than the above three photocatalysts.

For photocatalysis applications, redox potential lines are a prerequisite for the catalytic reaction. For example, in photocatalytic NO removal or H₂O splitting, the CBM must be more negative than the redox potential of H⁺/H₂ (0 V vs. NHE), while theVBM must be more positive than the redox potential of O₂/H₂O (1.23 V) [40, 41]. Therefore, it is important to calculate the redox potential and evaluate the potential applications of the materials.

Herein, the energy positions of the VBM and CBM of Janus MA₂Z₄ monolayers are calculated as follows [42]:

(9)E_{VB}=X-E_e+0.5E_{\mathrm{g}},

(10)E_{CB}=E_{VB}-E_{\mathrm{g}}.

X is the absolute electronegativity of the semiconductor, which is accepted by the geometric mean of the absolute electronegativity of the constituent atoms. With this method, the calculated X values of WO₃ and CdS are 6.59 and 5.18, respectively, which are almost the same as those in previous reports [43, 44] and verify that this method can be applied in future research. E_e is the energy of free electrons on the hydrogen scale (4.50 eV). E_g is accepted to have an HSE06 function. Tab.1 shows the VBM andCBM of various Janus MA₂Z₄ monolayers calculated from Eqs. (9) and (10). Based on these judgment rules, H-HfSiGeP₄, H-MoSiGeP₄, T-ScSiGeN₄, T-ScSiGeN₄ and T-ZrSiGeN₄ are suitable for the CO₂ reduction reaction (CO₂RR) [45], as shown in Fig.10.

Based on the redox potential analysis, another issue in the CO₂RR catalyst design is to obtain the energy-containing product (CH₄, CO, etc.), which is closely related to the activation energies (E_a) of radical reaction intermediates. Herein, we calculated the energy changes for all possible elementary hydrogenation steps of the CO₂RR on Janus MA₂Z₄ monolayers. Based on the redox potential analysis, T-ScSiGeN₄ and H-HfSiGeP₄ may have better photocatalytic performance, and the CO₂RR pathways were investigated, as shown in Fig.11.

The 1st step is CO₂ activation. The activation energies (E_a1) of H-HfSiGeP₄ and T-ScSiGeN₄ are 0.06 and −0.09 eV, respectively, which indicates that CO ₂ can be easily activated. T-ScSiGeN₄ can automatically adsorb CO₂ because of the negative activation energy. Therefore, the first step can easily occur.

The 2nd hydrogenation (H⁺/e⁻) reactions are energetically prone to form OCOH* (Step 2-2) and energetically preferred than OCHO* (Step 2-1). Since OCOH* will always form CO* by releasing one H₂O molecule, the CO production can be the main product in CO₂ reduction by Janus MA₂Z₄.

Then, after the hydrogenation reaction, OCOH* will be further reduced to CO* with the release of one H₂O molecule. The activation energies (E_a3) of H-HfSiGeP₄ and T-ScSiGeN₄ are 1.43 and 1.09 eV, respectively. The final step is CO release. The activation energies (E_a4) of H-HfSiGeP₄ and T-ScSiGeN₄ are −0.06 and 0.09 eV, respectively, which implies that CO can be easily released on the surface. The reaction processes to reduce CO ₂ to CO are listed below according to the abovementioned analysis.

Step 1: CO₂(g) → CO₂*,

Step 2-1: CO₂* + H⁺ + e⁻ → OCHO*,

Step 2-2: CO₂* + H⁺ + e⁻ → OCOH*,

Step 3: OCOH* + H⁺ + e⁻ → CO* + H₂O ↑,

Step 4: CO* → CO ↑.

Given the analysis above, four monolayers can accept the redox potential requirement for the CO₂RR: H-MoSiGeP₄, H-HfSiGeP₄, T-ScSiGeN₄, and T-ZrSiGeN₄. However, the photo absorption of H-MoSiGeP₄ and H-HfSiGeP₄ is the strongest in the visible-light range, and the relative ratio (D) of the electron/hole effective mass is larger in the four monolayers. Therefore, H-MoSiGeP₄ and H-HfSiGeP₄ are proposed as the best CO₂RR catalysts.

4 Conclusion

In summary, using high-throughput calculations, 104 types of Janus MA₂Z₄ materials were prepared in the initial space for screening and evaluation. The thermal and environmental stability investigations indicate that 13 candidates are stable. Because of their excellent conductivity, the 6 monolayers with band gaps below 0.5 eV were identified as potential electrode materials for hydrogen evolution reaction applications. For potential applications in photoelectric applications, Janus MA₂Z₄ monolayers with HSE06 band gaps (E_g-HSE) above 0.5 eV remained, which resulted in 7 potential candidates. Based on the DOS analysis, E_VB is mainly contributed by TM d and N/P p orbital hybridization, while E_CB is mainly contributed by the d orbitals of TMs. The carrier concentrations of the 13 stable monolayers are on the order of 10¹⁵ cm⁻², which are better than those of well-known graphene and TaS₂. The highest case is T-ZrSiGeN₄, which is 2.33 × 10¹⁵ cm⁻². Based on the optical absorption analysis, in the visible-light range, H-HfSiGeP₄ and H-MoSiGeP₄ have higher absorption ability and optical conductivity, which is quite impressive for optoelectronic, solar cell device, and photocatalysis applications. The transmittance spectra of the Janus MA₂Z₄ monolayers are approximately 70%−80% in the visible light range, which implies that these monolayers show good light transmittance. For potential applications as a CO₂RR photocatalyst, the redox potential and charge effective mass analysis indicate that H-HfSiGeP₄, H-MoSiGeP₄, T-ScSiGeN₄, and T-ZrSiGeN₄ are suitable for the CO₂RR. By high-throughput identification, 13 types of new and stable Janus MA₂Z₄ monolayers were explored, and the basic properties and potential applications were investigated, which can reduce the time for experiments and provide basic data for the material genome initiative.

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