In recent years, two-dimensional (2D) materials [1–4] have become of interest due to their unique properties and various applications that emerge in the single-layer limit. The most typical and earliest 2D material confirmed experimentally is graphene [5, 6], which possesses unique electronic, optical, and pyroelectric properties. In line with the continuous exploration and research of 2D materials, an increasing number of various 2D materials have been successfully synthesized through experiments [7, 8]. MXenes (include 2D transition metal nitrides, carbides, and carbonitrides) are a rapidly developing family of 2D materials with nearly 30 members experimentally synthesized and dozens of them investigated theoretically [9, 10]. They are widely applied in various applications because of their unique physiochemical properties, including outstanding electronic [11], photocatalytic [12], and mechanical [13] properties. Recently, Hong et al [14], during the chemical vapor deposition growth of nanolayered MoN₂, introduced elemental Si to passivate its surface. As a consequence, centimeter-scale monolayer films of MoSi₂N₄ can be grown. According to the report, MoSi₂N₄ monolayer comprises the N–Si–N–Mo–N–Si–N atomic layer, and displays a non-magnetic nature, high strength, and outstanding stability. Meanwhile, its optical properties and bandgap were analyzed via the Tauc plot. It is found that MoSi₂N₄ monolayer presents a high optical transmittance (average of 97.5% ± 0.2%) in the visible range, as well as exhibits an indirect bandgap semiconductor structure (bandgap ∼1.94 eV).

The advent of the MoSi₂N₄ monolayer [15, 16] has inspired widespread attention in optoelectronics, valleytronics, spintronics, and so on. Yu et al [17] investigated the pristine lattice thermal conductivity of MoSi₂N₄ monolayer based on the first principles phonon Boltzmann transport equation. The study showed that MoSi₂N₄ monolayer displays high lattice thermal conductivity in 300 ∼ 800 K. Meanwhile, Bafekry et al [18] used a hybrid density functional theory to explore the thermal, mechanical, electronic, optical, and photocatalytic properties. The investigations revealed that pristine MoSi₂N₄ monolayer has good thermoelectric properties, and is a promising photocatalyst for water splitting and CO₂ reduction. In addition, Yang et al [19] predicted that peak-to-valley conversion characteristics could be achieved in the MoSi₂N₄ monolayer.

However, current research on the properties and performance of the modified MoSi₂N₄ monolayer is still incomplete. It is well known that the modification of alkali metal adsorption on semiconductor materials will modify their electronic structure and optoelectronic properties because alkali metal atoms can easily lose electrons. In recent years, many research groups have reported the optoelectronic properties of alkali-metal-adsorbed 2D materials [20, 21]. For example, Jin et al [22] used density functional theory to study alkali-metal-adsorbed graphene and discovered that the alkali metal adsorption can the change optoelectronic properties of graphene. Cui et al [23] studied alkali-metal-adsorbed g-GaN using first principles calculations, and has shown that the optical absorption of g-GaN is extended due to the decoration of alkali metals. However, the fully electronic, optical, and photocatalytic properties of alkali-metal-adsorbed MoSi₂N₄ monolayer are still unclear.

In this paper, the structural, electronic, optical, and photocatalytic properties of alkali-metal-adsorbed MoSi₂N₄ monolayer were calculated via first principles calculations. Lithium(Li), sodium(Na), and potassium(K) atoms are adsorbed onto the 3 × 3 × 1 supercell of the MoSi₂N₄ monolayer, respectively, to study the influence of the decoration of alkali metals. The most stable adsorption sites, work function, band structure (BS), density of state (DOS), optical constants of pristine and alkali-metal-doped MoSi₂N₄ monolayers were studied in detail. Furthermore, the band alignments of alkali-metal-adsorbed MoSi₂N₄ monolayer were analyzed to show the potential application in photocatalysis. Our findings suggest that the alkali-metal-adsorbed MoSi₂N₄ monolayer is a potential 2D material for photocatalytic water splitting and CO₂ reduction.

The VASP [24, 25] (Vienna ab initio Simulation Package) code was used to conduct first principles calculations in this work. The Perdew–Burke–Ernzerhof (PBE) [26–28] in a generalized gradient approximation (GGA) was used to optimize the geometric structure. The cutoff energy is 500 eV. Since the local density approximation (LDA) typically underestimates the equilibrium atomic distances while overestimating the model system's adsorption energy, PBE/GGA can provide more precise estimations for geometric optimization. For the electronic and optical analysis, the convergence criterion for the self-consistent electronic loop was set to 10–6 eV. Meanwhile, a 3 × 3 × 1 k-point grid in the first Brillouin zone was applied. The maximum allowed force was 0.03 eV Å⁻¹, and the maximum stress was 0.05 GPa.

The adsorption energy [29] for one Li, Na, or K atom adsorbed onto the MoSi₂N₄ monolayer can be determined aswhere E_total, E_pris, and E_Li/Na/K denote the total energy of alkali-metal-adsorbed MoSi₂N₄, pristine MoSi₂N₄, and a free alkali metal atom, respectively. Negative (positive) values reveal that the adsorption is an exothermic (endothermic) reaction.

From the perspective of quantum mechanics, the interaction between photons and electrons in the system is described in terms of time-dependent perturbations of the ground electronic state, and the photon absorption or emission result in transitions between unoccupied and occupied states. The spectrum produced by excitation can be regarded as a joint DOS between the conduction bands (CB) and the valence bands (VB). The imaginary part ϵ₂(ω) of the dielectric function [29, 30], which is a function of the photon frequency, can be described aswhere k is the reciprocal lattice vector, ω is the incident photon frequency, u is the vector defining the polarization of the incident electric field, and the superscripts c and v denote the CB and VB, respectively. The real part ϵ₁(ω) of the dielectric function, since the dielectric function shows a causal response, can be gained by the imaginary part of the Kramers−Kroning relations. Then, the optical absorption coefficient α(ω) can be obtained from ϵ₁(ω) and ϵ₂(ω) [31]:

The top and side views of the alkali-metal-adsorbed MoSi₂N₄ monolayer structure and the three different adsorption sites are illustrated in figure 1. The top view illustrates that the pristine MoSi₂N₄ monolayer, packing N, Si, and Mo atoms in a honeycomb lattice, is a 2D crystal with the optimized lattice constant a = 2.897 Å. Li, Na, and K atoms were decorated on the surface of the MoSi₂N₄ monolayer, which constitutes the alkali-metal-adsorbed MoSi₂N₄ monolayer. From the side view, the MoSi₂N₄ monolayer is composed of a N–Si–N–Mo–N–Si–N atomic layer, which can be regarded as a MoN₂ layer sandwiched between two slightly buckled honeycomb Si–N layers. We compared three different adsorption sites to determine the most stable sites of Li, Na, and K atoms adsorbed onto the MoSi₂N₄ monolayer. The T_N, T_Si, and T_Mo sites are directly above the N, Si, and Mo atoms. The adsorption energies of alkali-metal-decorated MoSi₂N₄, used to compare their stability, are calculated and shown in table 1. The data shows that all the adsorption energies of the three different sites are negative, indicating that the adsorption process of alkali metals onto the MoSi₂N₄ monolayer is exothermic, which means all adsorption systems are stable. Furthermore, the T_Mo is the most stable site due to the lowest adsorption energy. Therefore, the following discussion only refers to alkali metals decorated on T_Mo adsorption positions.

Table 2 displays the lattice parameters and bond lengths of alkali-metals-decorated MoSi₂N₄ monolayer systems with four configurations. The lattice constant of the pristine MoSi₂N₄ monolayer is 2.897 Å, which is consistent with previous reports [18]. Furthermore, the lattice parameters of Li-, Na- and K-doped MoSi₂N₄ are slightly bigger than that in pristine MoSi₂N₄ monolayer. Still, the lattice parameters of alkali-metals-doped MoSi₂N₄ do not increase with the tuning of the atomic number of alkali metal atoms. The bond length of Si-X (the bond length between Si and alkali metal atoms) or N-X (the bond length between N and alkali metal atoms) were shown in table 2. The results indicate that the bond lengths of Si-X or N-X increase as the atomic radius of alkali metal atoms increases. In addition, to study the bonding and interaction between the MoSi₂N₄ monolayer, we calculated the average atomic charge populations of pristine and alkali-metal-decorated MoSi₂N₄, as shown in table 3. Due to the introduction of alkali metal atoms, the charge population of the MoSi₂N₄ monolayer has changed. The results indicate that the charges of Mo and N vary slightly, but Si atoms gain electrons after the adsorption of alkali metals. There are ∼0.99 ∣e∣, 0.71 ∣e∣ and 0.84 ∣e∣ transfer from Li, Na and K atoms to Si atoms of MoSi₂N₄ monolayer.

To explore the effect of alkali metal adsorption on the electronic structure of the MoSi₂N₄ monolayer, we investigated the BS and DOS of pristine and alkali-metal-adsorbed MoSi₂N₄ monolayers, as shown in figures 2 and 3, respectively. Figure 2(a) shows that the pristine MoSi₂N₄ monolayer exhibits an indirect bandgap semiconductor structure since the maximum valence band (VBM) and minimum conduction band (CBM) are at two different k points. The calculated bandgap was 1.89 eV, which agrees with the values of experimental measurement (1.94 eV) in previous reports [14].

The band structures of the alkali-metal-adsorbed MoSi₂N₄ monolayer are shown in figures 2(b)–(d). The Li-, Na-, and K-adsorbed MoSi₂N₄ monolayers exhibit an indirect bandgap, which is similar to the pristine MoSi₂N₄ monolayer. Compared to the pristine system, the number of energy bands in the CB of the adsorbed systems increases. Besides, the CB of the doped systems shifts toward the low-energy region. Hence, the bandgaps of the doped systems are reduced. The corresponding bandgaps are 1.73 eV, 1.61 eV, and 1.75 eV for the Li-, Na-, and K-adsorbed MoSi₂N₄ monolayers. The bandgap reduction demonstrates that the BS of the MoSi₂N₄ monolayer can be regulated and modified by the adsorption of alkali metals. From the viewpoint of optical properties, less energy is required to complete the transition of electrons from the VB to the CB in an alkali-metal-adsorbed MoSi₂N₄ monolayer, which means that the photocatalytic properties of the MoSi₂N₄ monolayer will be improved after the introduction of alkali metals. Meanwhile, the optical absorption edge of the alkali-metal-adsorbed MoSi₂N₄ monolayer will shift to the low-energy region compared with the pristine MoSi₂N₄ monolayer. Comparing four different configurations, the smallest bandgap of Na-adsorbed MoSi₂N₄ monolayer (1.61 eV) may imply the highest photocatalyst.

Furthermore, the influence of alkali metal dopants in higher concentrations on the BS of MoSi₂N₄ monolayer, especially the effect on the bandgap, has been studied using the same method. The variation for the bandgaps as a function of the adsorbed alkali metal atoms is shown in figure 3(a). The results indicate that the bandgap significantly decreases as the number of doped alkali metal atoms increases. When an alkali metal atom is adsorbed onto the MoSi₂N₄ monolayer, the Na atom adsorbed case shows a more obvious modulation effect. However, when three alkali metal atoms are doped, the corresponding bandgaps are 1.42(3Li), 1.46(3Na), and 1.47 eV(3K), respectively. The bandgap values of the 3Li-, 3Na-, and 3K-adsorbed MoSi₂N₄ monolayers are close, implying that the bandgap does not depend on dopant types when three alkali metal atoms are adsorbed onto the MoSi₂N₄ monolayer. Interestingly, the bandgaps of the MoSi₂N₄ monolayer decorated with Li dopants decreased almost linearly as the number of Li atoms increased.

In addition, the BS of the 3Li-adsorbed MoSi₂N₄ monolayer is calculated and shown in figure 3(b). The result demonstrates that the CBM moves down below the Fermi level, converting these 2D materials into an n-type semiconductor. Meanwhile, the upper VB of the 3Li-adsorbed MoSi₂N₄ monolayer shifts toward the high energy range as a whole. The CBM and the VBM approach each other, resulting in a reduction in the bandgap. The reduction in the bandgap means that the photocatalytic activity of the MoSi₂N₄ monolayer will improve as the alkali metal concentration increases. Because the band structures of 3Na(K)-adsorbed MoSi₂N₄ monolayer are similar to that of 3Li-adsorbed MoSi₂N₄ monolayer, we just show the 3Li-adsorbed case as representative.

The total density of states (TDOS) and partial density of states (PDOS) of the pristine MoSi₂N₄ monolayer are shown in figure 4(a). The lower VB (from −3.5 to −6 eV) was mainly contributed by the N 2p, while the upper VB (from −3.5 to 0 eV) was contributed primarily by the Mo 4d state. Besides, the CB originates from the hybridization of the Mo 4d and the Si 3p states. Figures 4(b)–(d) present the DOS of the adsorbed systems. The impurity bands of alkali metal s states locate above the Fermi energy, which is partially occupied with a bandwidth of ∼1.7 eV. Meanwhile, the s states of the alkali metals are hybridized with the N 2p, Si 3p, and Mo 4d states at 0.2 ∼ 1.9 eV, indicating the impurity state generated by the alkali metal mainly contributes to the CB. The impurity state is the reason for the increased number of energy bands in the CB. In addition, the alkali metal atoms interact with other atoms and resonate at ∼1 eV, which also proves that the alkali-metal-adsorbed MoSi₂N₄ systems are stable.

Furthermore, some occupied levels are introduced into the Si 3p states below the Fermi energy. In this case, the electronic in-band transition from the occupied bands to the unoccupied bands under irradiation may cause intense absorption in the long-wavelength visible region. These occupied levels introduced into the Si 3p states reduce the bandgap of alkali-metal-adsorbed MoSi₂N₄ monolayer. Specifically, the alkali metal s state is above the Fermi level, so the introduction of the alkali metal s state does not directly cause the CB to move down. But the alkali metals lose electrons after adsorption, and the Si 3p state shifts to the low-energy region due to the electrons obtained by Si. In figures 3(a)–(d), it can be seen that the CBM of Si 3p states is from 0.15 eV (pristine) to −0.28(Li), −0.34(Na), and −0.32 eV(K), which causes the CB to move down and reduces the bandgap.

The work function [32, 33] is a critical factor to counterpoise the optoelectronic properties of materials. We investigate the work function variations of the surface in the following section. The work function (Φ) of materials can be defined as the minimum energy required to remove an electron from the surface. The variation in Φ is closely related to the change of surface conductivity, especially the single layer. Accurate calculation of Φ is also helpful in determining the direction of charge flow on the surface. Traditionally, the work function is defined as Φ [33]:where E_F and D were the Fermi level and the electronic potential at a vacuum region away from the surface, respectively. The decrease (increase) of work function can be induced by either a reduced (enhanced) surface dipole or a rising (lowering) of its intrinsic volume of Fermi energy. To investigate the influence of alkali metal adsorption on work function, we calculated the work function of pristine and adsorbed MoSi₂N₄ monolayer systems. Figure 5(a) presents the work function of pristine MoSi₂N₄ monolayer with the calculated work function was 4.80 eV. Figure 5(b) shows the variation of work function in the alkali-metal-adsorbed MoSi₂N₄ monolayer. The calculated work functions are 2.03 eV, 1.50 eV, and 1.43 eV for Na-, Li-, and K-adsorbed MoSi₂N₄ monolayers. As can be seen, the work function values were significantly reduced following the decoration of the Li, Na, and K atoms. As a result, the work function of the MoSi₂N₄ monolayer can be adjusted with different alkali metal absorption. Furthermore, the decreased work function reveals that the alkali-metal-adsorbed MoSi₂N₄ monolayer can be applied to field-emission devices.

To verify our prediction for the influence of the modified electronic structure on the enhanced photocatalytic behavior, we computed the optical constants of alkali-metal-decorated MoSi₂N₄ monolayer. Figure 6(a) displays the real part ϵ₁(ω) of pristine and Li-, Na-, and K-decorated MoSi₂N₄ monolayer systems. The pristine MoSi₂N₄ monolayer exhibits a static dielectric constant ϵ₁(0) of 3.8. For alkali-metal-adsorbed MoSi₂N₄ monolayer, the ϵ₁(0) are 13.9 (Li), 14.9 (Na) and 13.7 (K) respectively. The result indicates that the ϵ₁(0) of the doped systems are markedly larger than that of the pristine system. Hence, the optical properties of MoSi₂N₄ monolayer are tunable and can be modified by decorated alkali metals. It is well known that the optical properties of the material are highly dependent on its electronic BS. It has also been discovered in the previous article [34, 35] that the dielectric constant increases as the energy gap of the material decreases. Therefore, the decoration of alkali metals reduced the bandgap, resulting in the increase of the dielectric constant. The high dielectric constant value will reduce the charge carrier recombination rate, thereby improving the photovoltaic performance of the solar cell. The imaginary part ϵ₂(ω) for pristine and Li-, Na-, and K-decorated MoSi₂N₄ monolayers was demonstrated in figure 6(b). For pristine MoSi₂N₄ monolayer, three main peaks are located at 5.2 eV, 7.1 eV, and 13.2 eV. The peak at 5.2 eV originates from the electronic transition between the N 2p states in the lower VB and the Si 3p states in the CBM (figures 2 and 3). The electronic transition between Si 3p and Mo 4d states in the VB results in a peak at 6.50 eV. The weak peak at 13.2 eV is due to the electronic transition between N 2p and Si 3p states (not drawn in the electronic structure). For the alkali-metal-adsorbed MoSi₂N₄ monolayer, the peak at 5.2 eV found in the pristine MoSi₂N₄ monolayer disappeared. However, a new peak, which originates from the electronic transition between the Si 3p and s states of the alkali metal cations, arises at 0.72 eV after the introduction of Li, Na, and K atoms. In addition, in a higher energy range than around 7 eV, the spectral shapes of all the MoSi₂N₄ monolayer systems were almost same. Thus, the different alkali metal dopants mainly influence the optical properties in the low-energy range.

The absorption coefficients α(ω) for the pristine and Li-, Na-, and K-decorated MoSi₂N₄ monolayers were calculated from the obtained ϵ₁(ω) and ϵ₂(ω). Figure 7(a) shows the absorption spectra of the MoSi₂N₄ monolayer systems with four different configurations. For the pristine MoSi₂N₄ monolayer, the absorption coefficient gradually decreases in the visible range (380 ∼ 780 nm). When the wavelength is greater than 780 nm, the α(ω) of the pristine MoSi₂N₄ monolayer is almost 0, which means that the pristine MoSi₂N₄ monolayer can hardly absorb infrared light. In contrast, the absorption coefficient numbers increase remarkably with the wavelength increase after alkali metals are adsorbed. The α(ω) at 380 nm are 0.34(pristine), 0.57(Li), 1.09(Na), and 0.89(K) (×10⁵ cm⁻¹), while the α(ω) at 780 nm are 0.02(pristine), 0.92(Li), 1.34(Na), and 1.14(K) (×10⁵ cm⁻¹), respectively. Compared to the three configurations of MoSi₂N₄ monolayer systems, the Na-decorated MoSi₂N₄ monolayer shows a stronger visible light absorption ability due to the largest absorption coefficient. The above results reveal that alkali metal adsorption is an effective method to influence the optical absorption capacity and photocatalytic activity of MoSi₂N₄ monolayer because of the enhanced visible light absorption.

The phenomenon of enhanced absorption is consistent with the electronic structure (shown in figure 2). The bandgap of the alkali-metal-doped MoSi₂N₄ monolayer is smaller than that of the pristine MoSi₂N₄ monolayer. Meanwhile, the Na-adsorbed MoSi₂N₄ monolayer with the smallest bandgap corresponds to the most significant optical absorption coefficient. Furthermore, the absorption edges of alkali-metal-doped MoSi₂N₄ monolayer systems shift to lower energy (higher wavelength), and then the redshift occurs. Due to the sharp increase in infrared absorption, alkali-metal-adsorbed MoSi₂N₄ monolayer systems can also be used for long-wavelength optoelectronic devices, such as infrared light-emitting diodes and infrared detectors.

The photocatalytic properties [35, 36] of pristine and alkali-metal-decorated MoSi₂N₄ monolayers for water splitting and CO₂ reduction were investigated. The relative positions of the band edges are one of the most vital criteria for the photocatalytic process, and the CBE (conduction band edge) must lie above the H⁺/H₂ reduction level, while the VBE (valence band edge) must lie below the O₂/H₂O oxidation level. The CBE and VBE can be calculated according to the following relations found in [37, 38]:andWhere x [39] is the geometric average value of the electronegativity of the constituent atoms, 4.5 eV is the free energy of the electrons relative to the vacuum level.

Based on the above calculations, the redox potential of pristine and alkali-metal-adsorbed MoSi₂N₄ monolayers for water splitting and CO₂ reductions relative to the NHE (normal hydrogen electrode) were calculated and are shown in figure 7(b). The result reveals that the pristine MoSi₂N₄ monolayer can be used for photocatalytic water splitting. In addition, the E_VBM and E_CBM of alkali-metal(Li-, Na-, and K)-adsorbed MoSi₂N₄ monolayer systems are more positive and negative than the oxidation level and the reduction level of water, respectively; hence, they are active for both the photo-oxidation and photo-reduction of water. By comparison, the E_VBM of the K-decorated MoSi₂N₄ monolayer is up-shifted ∼0.15 eV over the pristine MoSi₂N₄ monolayer, while the E_CBM is almost similar to that of the pristine MoSi₂N₄ monolayer. This indicates that the reduced ability of H⁺ is maintained for the K-decorated MoSi₂N₄ monolayer with an increased tendency to generate oxygen. On the other hand, the CBE and VBE positions of Na(Li)-adsorbed MoSi₂N₄ monolayer systems approach the H⁺/H₂ reduction level and O₂/H₂O oxidation level, inducing Li- and Na-adsorbed MoSi₂N₄ monolayers exhibit a higher potential tendency for water splitting. Based on the overall consideration of the redox potential and photon absorption coefficient, we believe that the Na-adsorbed MoSi₂N₄ monolayer will show the best performance for photocatalytic water splitting.

Then, for the photocatalytic CO₂ reduction, the E_CBM of the Na-adsorbed MoSi₂N₄ monolayer lies below the CO₂/HCOOH reduction level, which indicates that it cannot complete the process of reducing CO₂ to HCOOH. However, the Na-adsorbed MoSi₂N₄ monolayer can reduce CO₂ to other substances like HCHO, CO, CH₄, and CH₃OH. Besides, the E_CBM of both Li- and K-adsorbed MoSi₂N₄ monolayer systems are more negative than the CO₂/HCOOH reduction level. Thus, the electrons in E_CBM of the Li(K)-adsorbed MoSi₂N₄ monolayer have enough potential to transfer CO₂ to HCOOH, HCHO, CO, CH₄, and CH₃OH. Compared with Li and K dopants, the Li-adsorbed MoSi₂N₄ monolayer shows a much better ability to reduce CO₂.

Based on the calculated bandgap variation with the number of alkali metal atoms adsorbed onto the MoSi₂N₄ monolayer (figure 3(a)), we computed the E_CBM and E_VBM for 1∼3 homogeneous alkali metal atoms adsorbed onto the MoSi₂N₄ monolayer, as shown in table 4. According to relations (5) and (6), found in [37, 38], for a fixed bandgap value, the positions of the band edges mainly depend on the constituent atoms' average electronegativity of the material. The electronegativity of alkali metals decreases as the atomic number increases. According to previous articles [39], the electronegativity of one Li, Na, and K atom is 2.58 eV, 2.32 eV, and 1.92 eV, respectively. With the increase of alkali metal doping concentration, the electronegative gap among Li-, Na-, and K-adsorbed MoSi₂N₄ monolayers increases. As a consequence, the band edges of K-adsorbed MoSi₂N₄ monolayer become more negative than that of Li- and Na-adsorbed MoSi₂N₄ monolayers. It can be seen that the E_VBM of the 3K-adsorbed MoSi₂N₄ monolayer is more negative than the O₂/H₂O level, resulting in the oxidation process of water impossible.

However, the data also reveals that 1 ∼ 3 Li(Na)-adsorbed MoSi₂N₄ monolayer can be used for photocatalytic water splitting. Besides, the CBE and VBE positions of the MoSi₂N₄ monolayer adsorbed by different concentrations of Li(Na) atoms are closer to the H⁺/H₂ reduction level and O₂/H₂O oxidation level compared to the pristine MoSi₂N₄ monolayer, respectively. Thus, Li and Na with higher doping concentrations can also improve the efficiency of photocatalytic water splitting. Interestingly, with the increase of the concentration of Li dopant, the photocatalytic efficiency increases. Conversely, for Na dopant, the reduction ability for water splitting increases with the increase of doping concentration, while the oxidation ability shows the opposite trend. Therefore, Li and Na's adsorption plays a significant role in improving the photocatalytic efficiency of MoSi₂N₄ monolayer, which will substantially expand the potential application of MoSi₂N₄ monolayer in the field of photo-electrochemistry.

In this study, the structural, electronic, optical, and photocatalytic properties of alkali-metal(Li, Na, and K)-adsorbed MoSi₂N₄ were investigated using first principles calculations. The negative adsorption energy reveals the stability of the alkali-metal-adsorbed MoSi₂N₄ monolayer structure. The electronic structure analysis shows that the pristine MoSi₂N₄ monolayer is an indirect bandgap semiconductor (bandgap is ∼1.89 eV). By contrast, the bandgaps of the Li-, Na-, and K-adsorbed MoSi₂N₄ monolayers are 1.73 eV, 1.61 eV, and 1.75 eV, respectively. Besides, the bandgap of the MoSi₂N₄ monolayer significantly decreases as the number of doped alkali metal atoms increases. The bandgaps of 3Li-, 3Na-, and 3K-adsorbed MoSi₂N₄ turned to 1.42 eV, 1.46 eV, and 1.47 eV.

The work function of the MoSi₂N₄ monolayer is 4.80 eV. It is noteworthy that the work function was significantly reduced due to the adsorption of alkali metals. One Li-, Na-, and K-adsorbed MoSi₂N₄ monolayer systems display ultra-low work functions of 1.50 eV, 1.43 eV, and 2.03 eV. Furthermore, the optical investigations indicate that alkali metal adsorption substantially increases the dielectric constant, and the visible light absorption range and coefficient of MoSi₂N₄ are also increased . Through the photocatalytic study, Li- and Na-adsorbed MoSi₂N₄ monolayers can be applied as a promising photocatalyst for water splitting due to enhanced photo-absorption ability and proper redox potential. While the Li-adsorbed MoSi₂N₄ monolayer shows the best potential applications for photocatalytic CO₂ reduction. In conclusion, due to their unique physical and chemical properties, alkali-metal-adsorbed MoSi₂N₄ monolayers are outstanding potential candidates for novel photocatalytic materials and optoelectronic devices.