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Article

Promising M2CO2/MoX2 (M = Hf, Zr; X = S, Se, Te) Heterostructures for Multifunctional Solar Energy Applications

1
Multiscale Computational Materials Facility, and Key Laboratory of Eco-Materials Advanced Technology, College of Materials Science and Engineering, Fuzhou University, Fuzhou 350100, China
2
College of Materials, Xiamen University, Xiamen 361005, China
3
Department of Ocean and Mechanical Engineering, Florida Atlantic University, Boca Raton, FL 33431, USA
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(8), 3525; https://doi.org/10.3390/molecules28083525
Submission received: 23 March 2023 / Revised: 10 April 2023 / Accepted: 15 April 2023 / Published: 17 April 2023
(This article belongs to the Section Materials Chemistry)

Abstract

:
Two-dimensional van der Waals (vdW) heterostructures are potential candidates for clean energy conversion materials to address the global energy crisis and environmental issues. In this work, we have comprehensively studied the geometrical, electronic, and optical properties of M2CO2/MoX2 (M = Hf, Zr; X = S, Se, Te) vdW heterostructures, as well as their applications in the fields of photocatalytic and photovoltaic using density functional theory calculations. The lattice dynamic and thermal stabilities of designed M2CO2/MoX2 heterostructures are confirmed. Interestingly, all the M2CO2/MoX2 heterostructures exhibit intrinsic type-II band structure features, which effectively inhibit the electron-hole pair recombination and enhance the photocatalytic performance. Furthermore, the internal built-in electric field and high anisotropic carrier mobility can separate the photo-generated carriers efficiently. It is noted that M2CO2/MoX2 heterostructures exhibit suitable band gaps in comparison to the M2CO2 and MoX2 monolayers, which enhance the optical-harvesting abilities in the visible and ultraviolet light zones. Zr2CO2/MoSe2 and Hf2CO2/MoSe2 heterostructures possess suitable band edge positions to provide the competent driving force for water splitting as photocatalysts. In addition, Hf2CO2/MoS2 and Zr2CO2/MoS2 heterostructures deliver a power conversion efficiency of 19.75% and 17.13% for solar cell applications, respectively. These results pave the way for exploring efficient MXenes/TMDCs vdW heterostructures as photocatalytic and photovoltaic materials.

1. Introduction

The production of sustainable and renewable energy can effectively tackle both the increasing global energy demand and environmental pollution issues [1]. Harnessing solar energy via photocatalytic hydrogen productions and photovoltaic solar cells have been proposed as promising solutions, which optimize the utilization of solar energy in a cost-effective and efficient way [2,3,4]. The search for efficient photocatalytic and photovoltaic materials is one of the most daunting tasks in the use of solar energy nowadays. Since graphene has been successfully produced and applied [5,6,7,8], great attention has been paid to 2D materials, for instance, transition metal carbides/nitrides (MXenes) [9,10,11], carbonitrides [12] transition metal dichalcogenides (TMDCs) [13,14,15], black phosphorene [16] and silicene [17]. Herein, MXenes, first successfully divested from the MAX phase in 2011 [18,19], can be expressed by Mn+1XnTx (n = 1~3), in which M is early transition metals, X refers to carbon or nitrogen, and T indicates surface terminations, such as hydroxyl groups, oxygen, or fluorine [10,20]. Usually, the functionalization of the MXenes surface makes the modulation of electronic properties more efficient [21,22], which enhances the possibility of solar energy-related applications [23,24,25,26].
On the other hand, the structural formula of TMDCs is generally expressed as MX2, where M refers to the transition metal elements (W, Mo, Re, etc.), and X represents the elements (S, Se, and Te). TMDCs materials exhibit a similar three-layer structure, where a transition metal atom single layer is sandwiched between two hexagonal chalcogenide atom planes. TMDCs have garnered significant interest owing to their emergent properties in the realm of light-emitting and photonic devices [27,28,29]. Among the TMDCs family, MoS2, MoSe2, and MoTe2 stand out with their distinguished physical and chemical properties, such as good stability, flexibility, electronic conductivity, optical, and catalytic properties [30,31,32,33]. However, the applications of transition metal dichalcogenides (TMDCs) in photocatalytic and photovoltaic applications are hindered by limitations such as inadequate spatial separation of electron-hole pairs, substantial photo-corrosion, and high light transmittance [34]. Consequently, a promising approach to enhance electron-hole separation in TMDCs is the construction of heterostructures with 2D semiconducting materials.
To further enhance the properties of 2D materials, vdW heterostructures with weak vdW interactions between vertical layers have been proposed [35,36]. Based on the interlayer coupling effect, 2D vertical vdW heterostructures exhibit distinct advantages derived from each constituent material, thereby yielding unique and promising features [37,38]. MXenes and TMDCs are potential candidates to form vdW heterostructures. For instance, MoSe2/Ti3C2 [39] and MoSe2/Ti3C2O2 [40] integrate the properties of their individual components, which show potential applications in photocatalysis, photovoltaics, and optoelectronics. The abundance and possibilities of combining MXenes and TMDCs together promote the investigation of MXene/TMDC vdW heterostructures of general interest and great importance. It is worth noting that the oxygen-functionalized MXenes materials Hf2CO2 and Zr2CO2 possess suitable band gaps for solar energy harvesting applications. The construction of heterostructures using M2CO2 (M = Hf, Zr) and TMDCs MoX2 (X = S, Se, Te) with the same hexagonal 2D lattice and restricted lattice mismatch not only serves as a remedy for the aforementioned drawbacks of MoX2 materials but also exhibits significant potential for optoelectronic and photocatalytic applications.
In this work, we constructed the M2CO2/MoX2 (M = Hf, Zr; X = S, Se, Te) heterostructures to investigate their photocatalytic hydrolysis and photoelectric conversion mechanism to evaluate the potential in photocatalytic and photovoltaic applications. Using first-principles calculations, we performed a comprehensive study on the structural stability, electronic structure, photocatalytic mechanism, and optoelectronic properties of M2CO2/MoX2 heterostructures. Interestingly, the Zr2CO2/MoSe2 and Hf2CO2/MoSe2 heterostructures have the potential for promising candidates for overall water splitting, owing to their band gaps and band edge positions that are suitable for photocatalytic water splitting. Moreover, the estimated maximum power conversion efficiencies of Hf2CO2/MoS2 and Zr2CO2/MoS2 heterostructures are pretty excellent and are considerably competitive with other existing heterostructures. These findings will diversify catalyst options of MXenes/TMDCs vdW heterostructures for photocatalytic hydrogen production and solar cell energy storage.

2. Experimental Section

2.1. Computational Details

Our density functional theory (DFT) calculations were performed by using the VASP package [41] together with the ALKEMIE platform [42,43]. The exchange-correlation interactions between electrons were carried out using the projector-augmented wave (PAW) [44] generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) functional [45]. The cutoff energy for the plane-wave-basis was set to 500 eV. For both geometry optimization and electronic structure calculation, the 2D Brillouin zone was installed with a Γ-centered k point of 12 × 12 × 1 mesh. To eliminate the interactions of periodic two-layer adjacent heterostructures, a vacuum layer larger than 20 Å was added along the z-direction of the 2D models. To have the assurance that the heterogeneous structures were fully optimized, the precision energy and precision force were required to be within 10−5 eV and 0.01 eV/Å, respectively. The Grimme’s DFT-D3 method was used to include the long-range vdW interactions [46,47]. The HSE06 hybrid density was functional and was utilized for the precise determination of bandgap values [48]. To verify the lattice dynamic stability of heterostructures, the phono dispersion curves were calculated using Phonopy code [49] with a 3 × 3 × 1 supercell. In order to investigate the thermodynamic stability of M2CO2/MoX2 heterostructures at room temperature (300 K), ab initio molecular dynamics (AIMD) simulations were performed with a supercell of size 3 × 3 × 1 [50,51].

2.2. Data Analysis

To quantify the lattice constant differences between different 2D structures, the degree of lattice mismatch K is defined in the theoretical calculation of heterostructures as [52]:
K = 2 | a 1 a 2 | a 1 + a 2
where a 1 and a 2 are the lattice constants of the two monolayers forming the heterostructures. To examine the thermodynamic stabilities of 2D heterostructures, the formation energy E f is obtained according to [53]:
E f = E M 2 CO 2 / MoX 2 E M 2 CO 2 E MoX 2
where E M 2 CO 2 / MoX 2 is the total energy of the M2CO2/MoX2 heterostructures, E M 2 CO 2 and E MoX 2 are the total energies of pristine M2CO2 and MoX2 monolayers, respectively. On the other hand, we have also calculated the 2D heterostructure binding energy E b to assess the strength of vdW interactions according to:
E b = ( E M 2 CO 2 / MoX 2 E M 2 CO 2 / MoX 2 h ) / S h
where E M 2 CO 2 / MoX 2 h is the sum of the total energies of M2CO2 and MoX2 monolayers fixed in the corresponding heterostructure lattice, and S h represents the 2D unit cell area. The absorption coefficients of 2D materials α ( ω ) are derived from [54]:
α ( ω ) = 2 ω [ ε 1 2 ( ω ) + ε 2 2 ( ω ) ε 1 ( ω ) ] 1 / 2 ε 2
where ε 1 and ε 2 are the real and imaginary parts of the optical dielectric functions, respectively. The carrier mobility of 2D materials μ was calculated by [55]:
μ = 2 e 3 C 2 D 3 K B T | m | 2 E 1 2 E 1
where e , , C 2 D , K B , T , m and E 1 are the electron charge, reduced Planck constant, 2D elastic modulus, Boltzmann constant, temperature, carrier effective mass, and deformation potential constant, respectively.

3. Results and Discussion

First, we analyzed the geometries and electronic structures of MoX2 (X = S, Se, Te) and M2CO2 (M = Zr, Hf) monolayers. The optimized lattice constants using DFT-D3 functionals of the MoS2, MoSe2, MoTe2, Zr2CO2, and Hf2CO2 monolayers are 3.16, 3.29, 3.52, 3.30, and 3.25 Å, respectively. The corresponding projected band structures are illustrated in Figure S1. MoX2 monolayers are direct band gap semiconductors, where the conduction band minimum (CBM) and the valence band maximum (VBM) are located at the K point. The HSE06 band gaps of MoS2, MoSe2, and MoTe2 monolayers are 2.23, 2.00, and 1.63 eV. While Zr2CO2 and Hf2CO2 exhibit indirect bandgap semiconductor features, whose band gap values are 1.68 and 1.69 eV, respectively. The calculated results (listed in Table S1) are consistent with the previous literature [56,57,58,59,60,61,62,63].
Furthermore, by perpendicularly combining the monolayers, M2CO2/MoX2 (M = Hf, Zr; X = S, Se, Te) heterostructures were built. The lattice mismatches observed between the M2CO2 and MoX2 monolayers fall within the reasonable range from 0.1% to 7.9%, which indicates the feasibility of establishing M2CO2/MoX2 heterostructures. Herein, to investigate the stability of M2CO2/MoX2 heterostructures, six distinct stacking configurations were examined. Taking Zr2CO2/MoS2 as an example, Figure 1 displays the top and side views of the diverse stacking structures examined, and Table S2 presents the respective calculated total energies. Details of the most stable structures after structural optimization under the van der Waals correction algorithm are presented in Table S3. It is noted that stacking II is the most stable model for most systems. The only exception is that stacking IV is the most stable configuration for Hf2CO2/MoTe2 heterostructure. In the following, the most stable stacking configurations were used for further electronic structure calculations. The formation energies presented in Table S3 demonstrate the energetic favorability of these heterostructures, as evidenced by their negative or minimally positive values [64]. Additionally, all heterostructures are classical van der Waals heterostructures, whose optimized interlayer distances and binding energy are around 3 Å and −20 meV/Å2 [65], respectively. Moreover, the negative binding energy corresponds to the exothermic reaction, which further confirms the feasibility from the thermodynamics point of view.
In order to further confirm the dynamic and thermal stabilities of the M2CO2/MoX2 heterostructures, we performed phonon dispersion calculations and AIMD simulations. For one thing, Figure 2 illustrates the phonon dispersion curves for the M2CO2/MoX2 heterostructures. Owing to the existence of eight atoms within each unit cell of the heterostructure, a total of 24 spectral lines are generated in Figure 2, encompassing 21 optical modes and 3 acoustic modes. Furthermore, minimal imaginary frequencies are observed from the phonon dispersion curves, which could be eliminated by depositing the M2CO2/MoX2 heterostructures onto suitable substrates or applying slight strain [66,67]. For another, it can be seen from the energy and structure evolutions in Figure 3 that total energies change in small ranges with temperature and atoms vibrate only slightly around the equilibrium position after a simulation time of 9 ps. Overall, M2CO2/MoX2 vdW heterostructures show good lattice dynamic and thermal dynamic stabilities.
To gain a more comprehensive comprehension of the electronic structures, the projected HSE06 band structures of the M2CO2/MoX2 heterostructures are illustrated in Figure 4. It can be seen that Hf2CO2/MoTe2 and Zr2CO2/MoTe2 show direct band gap structures, where both VBM and CBM locate at the M point. On the other hand, the other heterostructures present indirect band gap features. Table S4 lists the calculated band gaps of the most stable configurations for M2CO2/MoX2 heterostructures. As listed in Tables S1 and S4, the band gaps of Hf2CO2/MoS2, Hf2CO2/MoSe2, Hf2CO2/MoTe2, Zr2CO2/MoS2, Zr2CO2/MoSe2 and Zr2CO2/MoTe2 heterostructures are comparatively lower than those of their corresponding monolayers, with values of 1.35, 1.64, 0.66, 1.11, 1.69, and 1.13 eV, respectively. This observation indicates that the electron can be excited and more accessible with less light energy. In Hf2CO2/MoS2 and Zr2CO2/MoS2 heterostructures, the VBM are primarily influenced by the contribution of the M2CO2 layers, while the CBM are predominantly determined by the MoX2 layers. In contrast, the VBM and CBM are dominated by MoX2 and M2CO2 monolayers in the other heterostructures, respectively. Interestingly, all the M2CO2/MoX2 heterostructures are categorized as type-II heterostructures, which enables the effective separation of photo-generated charge carriers [36].
To further explore the charge transfer between M2CO2 and MoX2 monolayers, the charge density difference Δ ρ can be calculated from the following [68]:
Δ ρ = ρ M 2 CO 2 / MoX 2 ρ M 2 CO 2 ρ MoX 2
where the ρ M 2 CO 2 / MoX 2 , ρ M 2 CO 2 , and ρ MoX 2 are the charge densities of the M2CO2/MoX2 heterostructures, M2CO2 and MoX2 monolayers, respectively. The 3D iso-surface and planar average of the charge difference densities along the z-direction are illustrated in Figure 5. Except for Zr2CO2/MoS2 heterostructure, the charges consume in the MoX2 sides and accumulate in the M2CO2 regions. From the Bader charge analysis listed in Table S5, we found that 0.0066, 0.0104, 0.0157, 0.0094, and 0.0197 electrons transfer from MoX2 to M2CO2 slabs in the Hf2CO2/MoS2, Hf2CO2/MoSe2, Hf2CO2/MoTe2, Zr2CO2/MoSe2, and Zr2CO2/MoTe2 heterostructures, respectively. On the contrary, for Zr2CO2/MoS2, there are 0.0158 electrons from Zr2CO2 to the MoS2 side. The charge transfer contributes to the formation of built-in electric fields between the monolayers, which drives the photo-generated electrons and holes in opposite directions and further promotes the separation of electrons from holes [69]. It has been observed that the degree of charge transfer between layers exhibits a strong correlation with photocatalytic and photovoltaic activities. The electron localization functions (ELF) further visualize the detailed chemical bonding features in M2CO2/MoX2 heterostructures. Figure 6 presents the 2D contour plots of ELF in the (110) plane. It shows that the ELF bond points of the vdW bonding are about 0.040 for M2CO2/MoX2 heterostructures, which confirms the vdW interaction between M2CO2 and MoX2 layers.
To understand the chemical driving force for the water-splitting reaction, the band edge alignments of the M2CO2, MoX2 monolayers, and M2CO2/MoX2 heterostructures in conjunction with the work functions were investigated, as shown in Figure 7. The energy levels of the VBM and CBM of the MoS2, MoSe2, Hf2CO2, and Zr2CO2 monolayers satisfy the requirements of the redox potential of water splitting. However, the VBM of the MoTe2 monolayer is higher than the oxidation-reduction potential. The result agrees well with the previous report [60]. Herein, the work functions for MoS2, MoSe2, MoTe2, Zr2CO2, and Hf2CO2 monolayers are 5.68, 5.06, 4.60, 5.23, and 5.17 eV, respectively. The disparity in the work function across monolayers contributes to the electron transfer in the vdW heterostructures until the Fermi energy level reaches equilibrium, which is beneficial to form built-in electric fields. Herein, Hf2CO2/MoSe2 and Zr2CO2/MoSe2 heterostructures with suitable band edge alignments for water oxidations and reductions, render them effective photocatalysts for overall water splitting.
To obtain the charge carrier transport characteristics, we calculated the carrier effective masses and mobilities in M2CO2/MoX2 heterostructures along the x and y directions with the orthorhombic lattices. We have rebuilt the hexagonal cell to an orthorhombic cell for the carrier mobility calculations, as shown in Figure S2 by taking Zr2CO2/MoS2 as an example. The effective masses ( m ), deformation potentials ( E 1 ), 2D elastic modulus ( C 2 D ), and carrier mobilities ( μ ) are summarized in Table 1. On the one hand, the predicted hole mobilities of M2CO2/MoX2 heterostructures are much larger than the electron mobilities. On the other hand, the carrier mobilities of M2CO2/MoX2 exhibit the characteristic of high anisotropy. Generally, electron mobilities along the x direction are greater than the y direction, and vice versa, hole mobilities along the y direction are greater than the x direction. Therefore, the electrons tend to go through the x direction on the MoX2 side, while the holes demonstrate a predilection for migration along the y-axis within the M2CO2 domain of the heterostructures. Interestingly, the hole mobilities of Hf2CO2/MoS2 and Hf2CO2/MoTe2 heterostructures in the y direction have reached 9140.34 cm2 V−1s−1 and 7592.80 cm2 V−1s−1, respectively, which are greater than silicon (~1400 cm2 V−1s−1) [70]. The strong anisotropic carrier mobility in M2CO2/MoX2 heterostructures can reduce the rate of electron-hole recombination, which is favorable to redox reactions and the photoelectric conversion process.
As the optical property is one of the most important indicators for considering materials in solar energy conversion applications, subsequently, we investigated the optical absorption coefficient of the M2CO2/MoX2 vdW heterostructures. Figure 8 depicts the absorption coefficient curves of the M2CO2/MoX2 heterostructures and the corresponding monolayers using HSE06. In general, M2CO2/MoX2 heterostructures exhibit significant enhancement in the optical absorption coefficient (blue curves in Figure 8), featuring multiple high absorption peaks in both the visible and ultraviolet regions. The absorption coefficient of M2CO2/MoX2 is over two-fold higher than that of M2CO2 and notably exceeds that of the corresponding MoX2 monolayers. It is worth noting that Hf2CO2/MoS2 and Zr2CO2/MoS2 heterostructures exhibit stronger responses to infrared light than monolayers as well. Moreover, compared to M2CO2 and MoX2 monolayers, the absorption peaks of the vdW heterostructures undergo a slight redshift due to the narrowed bandgap. Therefore, the formation of heterostructures enhances optical response, showing promising potential for M2CO2/MoX2 heterostructures in photocatalysis and photovoltaic applications.
According to the previous analysis, Zr2CO2/MoSe2 and Hf2CO2/MoSe2 heterostructures are ideal materials for photocatalytic water splitting. The EVBM of Zr2CO2/MoSe2 and Hf2CO2/MoSe2 are more positive than the redox potential of O2/H2O (1.23 eV corresponds to the potential of −5.67 eV at pH = 0), while the ECBM is more negative than the redox potential of H+/H2O (0 eV corresponds to the potential of −4.44 eV at pH = 0). Meanwhile, they are also type-II semiconductors, which avoid the recombination of photo-generated carriers. Therefore, to further reveal the photocatalytic mechanism, we take the Zr2CO2/MoSe2 and Hf2CO2/MoSe2 heterostructures as examples to further analyze the adsorption and dissociation processes of water molecules on the surface of M2CO2, as presented in Figure 9a. Under light irradiation, electrons in the Zr2CO2/MoSe2 heterostructure are excited from the valence bands to the conduction bands, while an equal amount of holes remain in the valence bands. Immediately afterward, electrons are transferred from the CBM of MoSe2 to Zr2CO2, and holes are from the VBM of Zr2CO2 to MoSe2, thus forming an internal electric field that effectively accelerates the electron-hole recombination. Combined with the projected band diagram, the hydrogen evolution reaction (HER) occurs on the Zr2CO2 surface and the oxygen evolution reaction (OER) takes place on the MoSe2 surface. For Hf2CO2/MoSe2 heterostructure, the photocatalysis mechanism is the same. Here, we analyzed the HER processes on the Zr2CO2 and Hf2CO2 surfaces of the Zr2CO2/MoSe2 and Hf2CO2/MoSe2 heterostructures. Based on the crystal structure features, effective adsorption sites A~C and D~F were considered on the surfaces of Zr2CO2 and Hf2CO2, as shown in Figure S3. The adsorption models were subjected to complete relaxation and the adsorption energies of water molecules E ads H 2 O / surface were calculated from [71]:
E ads H 2 O / surface = E tot H 2 O / surface E tot H 2 O E tot surface
where E tot H 2 O / surface , E tot H 2 O and E tot surface are the total energy of the absorption system, individual water molecules, and pristine surface, respectively. Herein, the location with the lowest adsorption energy is where the HER takes place. The optimized models and the corresponding adsorption energies after water adsorption at different sites are exhibited in Figure S4 and S5. Different absorbed models obtain adsorption energies from −0.061 to −0.338 eV. All the cases with negative adsorption energies indicate that the water molecule adsorption processes are thermodynamically favorable. It is observed that the adsorption energy is lower in the case of oxygen atoms as adsorption atoms, which indicates that water molecules are more stable on the surface of the heterojunction with oxygen atoms. For the Zr2CO2 and Hf2CO2 surfaces, the optimal adsorption sites are C and F, where the HER will take place most possibly.
Furthermore, the generation processes of H2 on the Zr2CO2 and Hf2CO2 surfaces of Zr2CO2/MoSe2 and Hf2CO2/MoSe2 heterostructures have been illustrated in Figure 9b. For Zr2CO2/MoSe2 heterostructure, the isolated H adatoms exhibit an inclination to migrate in closer proximity to one another under a chemical driving force of −0.21 eV. This driving force is termed as the energy differential between two proximal hydrogen atoms (marked as Step-I and set to 0) and two remote hydrogen atoms (marked as Step-II) on the Zr2CO2 surface. Afterward, H atoms will form H2 molecules (marked as Step-III) accompanied by a chemical driving force of −4.48 eV (from −0.21 eV of Step-II to −4.69 eV of Step-III) on the Zr2CO2 surface, while the system is less energetic and more thermodynamically stable. The generated hydrogen will be departed easily from the surface (marked as Step-IV), as only 0.12 eV energy is required (from −4.69 eV of Step-III to −4.57 eV of Step-IV). On the other hand, the process of producing H2 molecules on the Hf2CO2 surface of the Hf2CO2/MoSe2 heterostructure is similar. The energies taken for the gradual approach of two distant H atoms to form an H2 molecule and then to detach from the surface are −0.30, −6.13, and 0.08 eV, respectively.
To evaluate the application of M2CO2/MoX2 heterostructures in solar cells, we conducted an estimation of the power conversion efficiency (PCE, η ), which describes the ability of M2CO2/MoX2 heterostructure materials to transform solar energy into electrical energy, proposed by Scharber et al. [72]:
η = 0 . 65 ( E g d Δ E c 0.3 ) E g P ( ϖ ) h ϖ d ( ϖ ) 0 P ( ϖ ) d ( ϖ )
where 0.65 is the band-fill factor, P ( ϖ ) is the AM1.5 solar energy flux at the photon energy ϖ , and E g d is the donor band gap. Δ E c is the donor and acceptor conduction band offset. To better understand the conduction band offset, Figure S6 presents the band structures of individual monolayers that have been arranged in the 2D lattice of the corresponding van der Waals heterostructure, based on the band edge data. The donor band gap E g d , conduction band offset Δ E c , and calculated PCE ( η ) of M2CO2/MoX2 heterostructures are listed in Table S6. Figure 10 shows simulated solar cell PCE as well as the charge carrier transfer route in M2CO2/MoX2 heterostructures. Interestingly, the maximum PCEs of the Hf2CO2/MoS2 and Zr2CO2/MoS2 heterostructures are calculated to be 19.75% and 17.13% (red star highlighted in Figure 10a), respectively. Remarkably, the donor band gaps of Hf2CO2/MoS2 and Zr2CO2/MoS2 heterostructures are in the range of the ideal band gap for the best light absorption characteristics of solar cells [73,74]. As shown in Figure 10b, photon absorptions in M2CO2/MoX2 heterostructures with high PCEs generate excited-free carriers to develop photocurrent more efficiently.

4. Conclusions

In conclusion, we have systematically investigated the geometrical structures, electronic structures, optical properties, photocatalytic and photovoltaic applications of M2CO2 (M = Hf, Zr), MoX2 (X = S, Se, Te) monolayers and corresponding M2CO2/MoX2 vdW heterostructures based on density functional theory calculations. Firstly, the most stable configurations of these heterostructures have been determined by the formation energy. The thermal and lattice dynamic stabilities of the M2CO2/MoX2 heterostructures are demonstrated by the negligible fluctuations of total energy and atomic equilibrium positions with temperature during ab initio molecular dynamics simulations, coupled with the scarcity of imaginary frequencies present in the phonon dispersion curves. Secondly, the calculated bandgap values of these heterostructures exhibit a reduced extent in comparison to those of the associated monolayers. It is noted Hf2CO2/MoTe2, and Zr2CO2/MoTe2 heterostructures display direct band gap characteristics, which are favorable for solar light absorption. Moreover, it is found that built-in polarization electric fields generated near the interfaces, as well as the high and anisotropic carrier mobility, can facilitate the photo-generated carrier separation to improve the photoelectric conversion process. In contrast to the M2CO2 and MoX2 monolayers, the M2CO2/MoX2 heterostructures display a heightened optical absorption effect, predominantly within the ultraviolet and visible light spectra. Interestingly, the M2CO2/MoX2 heterostructures all exhibit the intrinsic type-II semiconductors, the VBM and CBM primarily influenced by the contribution of different layers, which effectively hinder the recombination of electron-hole pairs. Lastly, the Zr2CO2/MoSe2 and Hf2CO2/MoSe2 heterostructures are considered highly prospective contenders for water splitting, given their appropriate band gaps and band edge positions that furnish ample driving force for the redox reaction of water. Furthermore, the designed Hf2CO2/MoS2 and Zr2CO2/MoS2 heterostructures can achieve PCE values of 19.75% and 17.13%, respectively. The present study reveals that M2CO2/MoX2 vdW heterostructures are potential candidates for photocatalytic and photovoltaic device applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28083525/s1, Figure S1. The projected band structure of (a) MoS2, (b) MoSe2, (c) MoTe2, (d) Zr2CO2, (e) Hf2CO2 monolayers. (f) The first Brillouin zone and high symmetry points of the hexagonal 2D lattice; Figure S2. An orthorhombic lattice instead of the traditional hexagonal lattice was adopted to calculate the intrinsic responses to uniaxial strain in the Zr2CO2/MoS2 heterostructure; Figure S3. Different adsorption sites of the water molecule on the (a) Zr2CO2 and (b) Hf2CO2 surfaces of the Zr2CO2/MoSe2 and Hf2CO2/MoSe2 heterostructures; Figure S4. Optimized structures and adsorption energies of the water molecule with H as the adsorption atom at (a) A, (b) B, (c) C adsorption site and O as the adsorption atom at (d) A, (e) B, (f) C adsorption site on the Zr2CO2 surface of the Zr2CO2/MoSe2 heterostructure; Figure S5. Optimized structures and adsorption energies of the water molecule with H as the adsorption atom at (a) D, (b) E, (c) F adsorption site and O as the adsorption atom at (d) D, (e) E, (f) F adsorption site on the Hf2CO2 surface of the Hf2CO2/MoSe2 heterostructure; Figure S6. The HSE06 band structures of mutually independent monolayers fixed in the heterostructure lattices for (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2 and (f) Zr2CO2/MoTe2; Table S1. The lattice constants a (Å), band gaps Eg (eV) and band gap types of M2CO2 (M = Zr, Hf) and MoX2 (X = S, Se, Te) monolayers; Table S2. The total energy (eV) of different stacking configurations for M2CO2/MoX2 heterostructures; Table S3. The lattice constants a (Å), interlayer distance d (Å), degree of lattice mismatch K, formation energy Ef (meV) and binding energy Eb (meV/Å2) for the most stable configurations of M2CO2/MoX2 heterostructures; Table S4. The calculated band gap Eg (eV) of the most stable configurations for M2CO2/MoX2 heterostructures; Table S5. The total charge transfer amounts between MoX2 and M2CO2 in the M2CO2/MoX2 heterostructures; Table S6. The conduction band offset ΔEc (eV), donor band gap E g d (eV), and calculated power conversion efficiency (PCE) η (%) of M2CO2/MoX2 heterostructures for solar cell applications. References [56,57,58,59,60,61,62,63] are cited in the supplementary materials.

Author Contributions

Conceptualization, J.W. and B.S.; methodology, Y.Z.; software, Y.Z., B.W. and B.S.; validation, Q.C., R.X., Z.C., Z.H. and M.L.; formal analysis, J.W. and B.S.; investigation, J.W. and B.W.; resources, Y.Z. and B.S.; data curation, J.W., Q.C., R.X., J.L. and M.L.; writing—original draft preparation, J.W. and B.W.; writing—review and editing, C.W. and B.S.; visualization, J.W.; supervision, B.W. and B.S.; project administration, B.W. and B.S.; funding acquisition, C.W., B.W. and B.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key Research and Development Program of China (No. 2022YFB3807200), the National Natural Science Foundation of China (No. 21973012), the Natural Science Foundation of Fujian Province (Nos. 2020J01351, 2020J01474, 2021H6011, 2021J01590, 2018J01754), and the “Qishan Scholar” Scientific Research Startup Project of Fuzhou University.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic views of the Zr2CO2/MoS2 heterostructures stacked by varying the position of MoS2 layers. The upper layer and the lower layer represent MoS2 and Zr2CO2, respectively. I to VI correspond to the six configurations.
Figure 1. Schematic views of the Zr2CO2/MoS2 heterostructures stacked by varying the position of MoS2 layers. The upper layer and the lower layer represent MoS2 and Zr2CO2, respectively. I to VI correspond to the six configurations.
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Figure 2. The phonon dispersion curves of (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2, and (f) Zr2CO2/MoTe2 heterostructures.
Figure 2. The phonon dispersion curves of (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2, and (f) Zr2CO2/MoTe2 heterostructures.
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Figure 3. The evolutions of total energy and structure snapshots from AIMD simulations of (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2, and (f) Zr2CO2/MoTe2 heterostructures.
Figure 3. The evolutions of total energy and structure snapshots from AIMD simulations of (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2, and (f) Zr2CO2/MoTe2 heterostructures.
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Figure 4. The projected band structures of (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2 and (f) Zr2CO2/MoTe2 heterostructures using HSE06 functional. The Fermi energy level is set to zero.
Figure 4. The projected band structures of (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2 and (f) Zr2CO2/MoTe2 heterostructures using HSE06 functional. The Fermi energy level is set to zero.
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Figure 5. The 3D iso-surfaces and plane-averaged charge density differences along the z-direction for: (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2, and (f) Zr2CO2/MoTe2 heterostructures. The depletion and accumulation of electrons are represented by magenta and blue contours, respectively.
Figure 5. The 3D iso-surfaces and plane-averaged charge density differences along the z-direction for: (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2, and (f) Zr2CO2/MoTe2 heterostructures. The depletion and accumulation of electrons are represented by magenta and blue contours, respectively.
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Figure 6. ELF contour plots projected on the (110) plane of (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2 and (f) Zr2CO2/MoTe2 heterostructures.
Figure 6. ELF contour plots projected on the (110) plane of (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2 and (f) Zr2CO2/MoTe2 heterostructures.
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Figure 7. Band edge alignments of the M2CO2, MoX2 monolayers, and M2CO2/MoX2 vdW heterostructures.
Figure 7. Band edge alignments of the M2CO2, MoX2 monolayers, and M2CO2/MoX2 vdW heterostructures.
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Figure 8. The absorption coefficient of (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2, and (f) Zr2CO2/MoTe2 heterostructures together with the pristine monolayers using HSE06 functional.
Figure 8. The absorption coefficient of (a) Hf2CO2/MoS2, (b) Hf2CO2/MoSe2, (c) Hf2CO2/MoTe2, (d) Zr2CO2/MoS2, (e) Zr2CO2/MoSe2, and (f) Zr2CO2/MoTe2 heterostructures together with the pristine monolayers using HSE06 functional.
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Figure 9. (a) The schematic illustrations of the charge transfer paths and overall water splitting of Zr2CO2/MoSe2 (Hf2CO2/MoSe2) heterostructure. (b) The generation processes of hydrogen on the Zr2CO2 (Hf2CO2) surface of Zr2CO2/MoSe2 (Hf2CO2/MoSe2) heterostructure.
Figure 9. (a) The schematic illustrations of the charge transfer paths and overall water splitting of Zr2CO2/MoSe2 (Hf2CO2/MoSe2) heterostructure. (b) The generation processes of hydrogen on the Zr2CO2 (Hf2CO2) surface of Zr2CO2/MoSe2 (Hf2CO2/MoSe2) heterostructure.
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Figure 10. (a) A simulated PCE map refers to the donor band gap and conduction band offset of M2CO2/MoX2 heterostructures. (b) Schematic illustration of free charge carrier transfer path in M2CO2/MoX2 heterostructure solar cells.
Figure 10. (a) A simulated PCE map refers to the donor band gap and conduction band offset of M2CO2/MoX2 heterostructures. (b) Schematic illustration of free charge carrier transfer path in M2CO2/MoX2 heterostructure solar cells.
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Table 1. The effective mass m* (m0), deformation potentials E1 (eV), elastic moduli C2D (N/m), and carrier mobility μ (cm2 V−1s−1) of MoX2/MoX2 heterostructures.
Table 1. The effective mass m* (m0), deformation potentials E1 (eV), elastic moduli C2D (N/m), and carrier mobility μ (cm2 V−1s−1) of MoX2/MoX2 heterostructures.
SystemDirectionCarrier TypeE1C2Dm*μ
Hf2CO2/MoS2xe−10.48428.341.1641.00
h2.06428.34−0.554707.47
ye−9.40421.712.0016.71
h1.33421.71−0.619140.34
Hf2CO2/MoSe2xe8.44419.800.51323.12
h2.80419.80−0.661743.19
ye7.63417.612.0923.01
h2.72417.61−0.582393.98
Hf2CO2/MoTe2xe7.94409.120.56295.46
h−3.75409.12−1.30243.67
ye5.87404.392.2233.41
h−1.78404.39−0.497592.80
Zr2CO2/MoS2xe−11.47394.340.8065.10
h5.21394.34−0.72396.73
ye−11.49335.301.3719.08
h2.88335.30−0.611555.99
Zr2CO2/MoSe2xe6.82368.680.76192.50
h1.32368.68−0.873928.96
ye4.43392.742.6839.24
h2.19392.74−0.603227.15
Zr2CO2/MoTe2xe10.55323.538.850.52
h−3.29323.53−1.64155.33
ye6.10369.172.9516.00
h−3.81369.17−0.551192.02
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MDPI and ACS Style

Wen, J.; Cai, Q.; Xiong, R.; Cui, Z.; Zhang, Y.; He, Z.; Liu, J.; Lin, M.; Wen, C.; Wu, B.; et al. Promising M2CO2/MoX2 (M = Hf, Zr; X = S, Se, Te) Heterostructures for Multifunctional Solar Energy Applications. Molecules 2023, 28, 3525. https://doi.org/10.3390/molecules28083525

AMA Style

Wen J, Cai Q, Xiong R, Cui Z, Zhang Y, He Z, Liu J, Lin M, Wen C, Wu B, et al. Promising M2CO2/MoX2 (M = Hf, Zr; X = S, Se, Te) Heterostructures for Multifunctional Solar Energy Applications. Molecules. 2023; 28(8):3525. https://doi.org/10.3390/molecules28083525

Chicago/Turabian Style

Wen, Jiansen, Qi Cai, Rui Xiong, Zhou Cui, Yinggan Zhang, Zhihan He, Junchao Liu, Maohua Lin, Cuilian Wen, Bo Wu, and et al. 2023. "Promising M2CO2/MoX2 (M = Hf, Zr; X = S, Se, Te) Heterostructures for Multifunctional Solar Energy Applications" Molecules 28, no. 8: 3525. https://doi.org/10.3390/molecules28083525

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