MXene-based Nanostructures for Water Splitting Process Using the Density Functional Theory

Alireza Rastkar Ebrahimzadeh ¹ (Department of Physics, Faculty of Basic Sciences, Azarbaijan Shahid Madani University, Tabriz, Iran)
Sima Rastegar ² (Department of Physics, Faculty of Basic Sciences, Azarbaijan Shahid Madani University, Tabriz, Iran)
Jaber Jahanbin Sardroodi ³ (Department of Chemistry, Faculty of Basic Sciences, Azarbaijan Shahid Madani University, Tabriz, Iran)

Submited date : 2020-10-31 accepted date : 2021-01-18

Keywords: DFT, Photocatalyst, Water splitting, MXene, Hybrid Functional,

Abstract :

Solar energy reserving and conversion into usable chemical energy with semiconductor photocatalysts help a promising method to solve both energy and environmental issues. Green and efficient energy technologies are crucial where nanoscience could change the paradigm shift from fossil fuels to renewable sources. One of the most attractive cases is solar energy utilization to earn electricity or chemical fuel based on semiconductor nanomaterials ability to function as photocatalysts promoting various oxidation and reduction reactions under sunlight. Recently, two-dimensional (2D) materials have attracted particular focus because of their charming properties. We report on a novel class of two-dimensional photocatalysts for hydrogen generation via water splitting. In this paper, by Density Functional Theory (DFT) calculations, we investigated Hf2CO2 as two-dimensional transition metal carbides, referred to as MXene, to understand its photocatalytic properties. Using this method, we theoretically investigated the structural, electronic, and optical properties of MXene-based nanostructures such as Hf2CO2 that calculated using GGA-PBE and HSE06 functionals. The lattice constant for GGA-PBE functional for Hf2CO2 is 3.3592A°. The calculated band gaps for GGA-PBE and HSE06 functionals for two-dimensional Hf2CO2 MXene were 0.92 and 1.75 eV, respectively. This MXene-based nanostructure also exhibits excellent optical absorption performance. Hence, Hf2CO2 is a promising photocatalytic material.

Full-Text:



MXene-based Nanostructures for Water Splitting Process Using the Density Functional Theory

 

ABSTRACT

Solar energy reserving and conversion into usable chemical energy with semiconductor photocatalysts help a promising method to solve both energy and environmental issues. Green and efficient energy technologies are crucial where nanoscience could change the paradigm shift from fossil fuels to renewable sources. One of the most attractive cases is solar energy utilization to earn electricity or chemical fuel based on semiconductor nanomaterials' ability to function as photocatalysts promoting various oxidation and reduction reactions under sunlight. Recently, two-dimensional (2D) materials have attracted particular focus because of their charming properties. We report on a novel class of two-dimensional photocatalysts for hydrogen generation via water splitting. In this paper, by Density Functional Theory (DFT) calculations, we investigated Hf2CO2 as two-dimensional transition metal carbides, referred to as MXene, to understand its photocatalytic properties. Using this method, we theoretically investigated the structural, electronic, and optical properties of MXene-based nanostructures such as Hf2CO2 that calculated using GGA-PBE and HSE06 functionals. The lattice constant for GGA-PBE functional for Hf2CO2 is 3.3592A°. The calculated band gaps for GGA-PBE and HSE06 functionals for two-dimensional Hf2CO2 MXene were 0.92 and 1.75 eV, respectively. This MXene-based nanostructure also exhibits excellent optical absorption performance. Hence, Hf2CO2 is a promising photocatalytic material.

Keywords: MXene; Photocatalyst; Water Splitting; DFT; Hybrid Functional

 

 

INTRODUCTION

The escalating consumption of fossil fuels has driven the development of energy generation methods to increase sustainability and reduce negative effects on the environment [1,2]. Nowadays, the much consumption of fossil fuels (natural gas, coal, oil, etc.) by population growth and continuous improvement of living standards have dramatically enhanced the world's demand for green energy. The large intake of fossil fuels has led to huge carbon dioxide (CO2) emissions and global environmental challenges. Hence, various research groups are efforting to find stable ways by using various strategies, such as solar energy, hydropower, wind energy, geothermal energy, and biomass energy [3–8]. Solar energy is considered one of the most promising options due to its ultimately unlimited supply. While natural photosynthesis is a biochemical process to transform sunlight, water, and carbon dioxide into carbohydrates and oxygen with very high efficiency, artificial photosynthesis is a human-made chemical process that biomimics the natural process of photosynthesis for catching and storing the energy from sunlight in the chemical bonds of solar fuels [9–14]. Photocatalytic water splitting and light-driven carbon dioxide reduction are two important applications. For the past decades, various materials (organic, inorganic) have been designed for these purposes, such as graphene [15], TiO2 [16], WO3 [17], ABO3 [18], Fe2O3 [19], WS2 [20], MoS2 [21], Bi2O3 [22] and BiVO4 [23] etc. However, all these materials have some deficiencies, such as improper band gap or band edge positions, limited visible-light response, larger electronic defect density, and small surface area. Therefore, it is crucial to identify materials or a design strategy to overcome these deficiencies towards overall high efficiency for practical civil applications [24,25].

Photocatalytic water splitting can play an essential role in future renewable energy systems by presentation a way to directly convert solar energy into chemical energy in the form of hydrogen [26,27]. There are three steps in the process of water splitting using a semiconductor photocatalyst:

(1) Light absorption, resulting in electrons in the valence band is excited into the conduction band when a photon with energy major than the semiconductor's band gap energy is absorbed.

(2) Separation and diffusion of the photo-excited electrons and holes to the surface.

(3) The holes and electrons reduce surface reactions in which water oxidized and protons, respectively, to produce O2 and H2 [28,29].

The photocatalytic water splitting process involves the generation of electrons and holes by photocatalysts through absorbing solar energy, the migration of the generated electrons and holes to the semiconductor surface, and the redox reaction of water to form H2 and O2 on the surface. In order to achieve these three steps, an ideal photocatalyst should meet the requirements of:

(1) A primary requirement for a suitable photocatalyst is that the conduction band minimum (CBM) should be higher (more negative) than the hydrogen reduction potential (H+/H2) and the valence band maximum (VBM) should be lower (more positive) than the water oxidation potential (H2O/O2) [30]. (2) Second, the smallest band gap for a semiconductor to be used as a photocatalyst is 1.23 eV.

Due to the wide range of applications of 2D nanomaterials in materials science, including energy storage [34], sensing [35], catalysis [36], and electronic devices [37] like field-effect transistors [38], caused to the 2D materials identify as a considerable research field.  Recently, a new family of 2D materials has emerged known as MXenes. MXenes are a new family of 2D transition metal carbides/nitrides produced by selective chemical etching of "A" from MAX phases, where M is a transition metal, A is an IIIA or IVA element, and X is C or N [39]. Hydrogen produced by the direct splitting of water via a semiconductor photocatalyst under sunlight is considered an alternative energy resource to fossil fuels and a promising solution to severe environmental problems [40]. 

        MXenes has highly regarded by researchers all over the world for its specific features. Currently, the potential of MXenes is due to their use as electrodes for supercapacitors [41], Li–S batteries [42], and Li-, Na- and K-ion batteries [43, 44]. All the MXenes are metals with a high electron density near the Fermi level [45]. Hence, all properties of MXenes could also be modified by surface functionalization [46-48]. Therefore, such materials convert from a metallic state to a semiconductor state due to surface functionalization by O or F groups. Also, the band gap can be tailored by surface functionalization [46, 47].

        Band gap engineering is an essential technology for designing new materials and devices for semiconducting, optoelectronic, and optical applications [49, 50].

        2D materials have two desirable advantages for water splitting: they have large surface active sites for photocatalytic reactions. The other is the thin layer allowing photogenerated electrons and holes to migrate rapidly, decreasing the possibility of electron-hole recombination and increasing the quantum yield [51-55].

In this paper, we study two-dimensional Hf2CO2 nanostructure from the family of MXenes. An important aim in our research is to study methods to discover photocatalyst for the water-splitting process. For this purpose, we calculate the structural, electronic, and optical properties of Hf2CO2 MXene, and using these properties, we find that it is a suitable and promising photocatalyst for the water-splitting process. For this work, we use the Vienna ab initio simulation package (VASP). Since the GGA-PBE always underestimates the band gap while a computationally more expensive Heyd–Scuseria– Ernzerhof (HSE06) hybrid functional [56] has been proven to provide accurate values agreeing well with experiments in a vast variety of systems.

 

COMPUTATIONAL METHOD

The computational studies based on Density Functional Theory (DFT) in conjunction with projector augmented wave (PAW) potentials, as implemented in the Vienna ab initio simulation package (VASP) [57], which includes hybrid methods. Hybrid procedures generally increase the precision of computed band gaps compared with pure DFT computations [58]. The k-points of 11*11*1 and 13*13*1 were automatically generated by the Monkhorst–Pack scheme [59] used for structural optimization and static self-consistent calculations, respectively. The cut-off energy is set to be 520 eV. The optical absorption properties of the 2D semiconductor photocatalyst were studied using the calculation of dielectric constants () at a given frequency. For this purpose, we used the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional with 11*11*1 k-point mesh. The atomic structures were analyzed by using the VESTA code.

RESULTS AND DISCUSSION

Among the two-dimensional MXene-based nanostructures with a hexagonal lattice, we can mention Hf2CO2. In this two-dimensional nanostructure, the C atom sandwiched between two layers of transition metal atoms functionalized with oxygen, shown in Fig. 1. The model shown in Fig. 1 is the most stable configuration after optimized geometry for the top (Fig. 1(a)) and side (Fig. 1(b)) views for two-dimensional MXene nanostructure, which is functionalized with oxygen.

 

 

 

 

 

 

 

 

 

 

 

Fig. 1. (a) Top and (b) side views of the most stable configurations for two-dimensional Hf2CO2 MXene.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 2. Energy diagram based on unit cell volume for two-dimensional Hf2CO2 MXene and fitted curve.

To calculate the lattice parameter of Hf2CO2, corresponding simulated unit cells were optimized. Calculating the total energy of optimized primitive unit cells at different lattice parameters around the equilibrium lattice parameter and fitting the data with the Murnaghan equation of state [16], the obtained value of the equilibrium lattice parameter together with other theoretical result has been shown in Tabel 1. Fig. 2 shows the energy diagram based on unit cell volume for two-dimensional Hf2CO2 MXene and its fitted curve using the Birch-Murnaghan equation. This process was performed to obtain the best lattice constant for the structure two-dimensional Hf2CO2 MXene. The optimized lattice constant for Hf2CO2 using the GGA-PBE is 3.3592, which is in excellent agreement with experimental and theoretical works [60]. The bond length for two-dimensional Hf2CO2 MXene is indicated in Table 2.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 1. Lattice parameter for two-dimensional Hf2CO2 MXene.

 

 

Table 2. Bond length for two-dimensional Hf2CO2 MXene.

Lattice parameter ()
a (Present)	a (ref. 60)	a (ref. 61)	a (ref. 62 )	a (ref. 63 )
3.3592	3.2660	3.273	3.266	3.271

 

 

Table 3. Energy gap for two-dimensional Hf2CO2 MXene.

Bond Length ()
Hf1-Hf2	Hf1-C	Hf2-C	Hf1-O	Hf2-O
3.41	2.34	2.49	2.13	2.14

Energy gap (eV)
Eg (GGA-PBE) (Present)	Eg (HSE06) (Present)	Eg (HSE06) (ref. 60)	Eg (HSE06) (ref. 62)
0.92	1.75	1.79	1.657

 

 

 

 

 

To explore the electronic properties of Hf2CO2, the electronic band structure that is pivotal in determining the electronic features has been determined for its structural geometry. The band structures of Hf2CO2 for GGA-PBE and HSE06 functionals were computed and are presented in Fig. 3. The computed band structures indicate that two-dimensional Hf2CO2 MXene, whether for GGA-PBE and HSE06, are semiconductors. In the present study and investigated reliable validations references for 2D Hf2CO2 MXene calculated bond length and energy gap indicated in Tables 2 and 3, respectively. The corresponding energy gap calculated with GGA-PBE and HSE06 functionals is 0.92 and 1.75 eV, respectively.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 3. Band structure for Hf2CO2 MXene for (a) GGA-PBE and (b) HSE06 functionals.

 

 

The density of states describes the distribution of electrons in the energy spectrum. One of the significant results obtained from their curvature is the compound’s band gap. We have calculated the partial and total density of states using GGA-PBE and HSE06 approximations. The partial and total density of states for Hf2CO2 MXene with GGA-PBE and HSE06 functionals are presented in Fig. 4(a) and Fig. 4(b), respectively. The calculation of the partial density of states (PDOS) for M2CT2 materials is due to discovering the relationship between the band characteristics and the types and geometries of surface groups. As shown in Fig. 4, the p states from the C atom and the d states from the transition metal atom Hf are strongly hybridized, consistent with the fact that MXenes are assembled mainly through the bonds between C and transition metal atoms. The p states from the surface group O hybridize with the d states from the transition metal atom Hf, denoting strong covalent bonds between surface groups and transition metal atoms, which have theoretically verified to be responsible for the improvement of the mechanical flexibility of functionalized MXenes [48]. The most critical issue is that the two-dimensional Hf2CO2 MXene displays a more massive band gap than 1.23 eV, requiring the minimum value for photocatalytic water splitting. In chemistry, HOMO and LUMO are types of molecular orbitals. The HOMO and LUMO are the highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. Hence, we show HOMO and LUMO in Fig. 4(a) and 4(b).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 4. The total density of states (TDOS) and projected density of states (PDOSs) of  Hf2CO2 MXene for (a) GGA-PBE and (b) HSE06 functionals.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 5. Band edge positions and band gaps obtained by HSE06 functional for two-dimensional Hf2CO2 MXene.

 

In addition to the band gap size, we also computed the band edge positions for Hf2CO2 MXene. Band-edge positions are another predominant factor in facilitating the water-splitting reaction because the CBM and VBM must be higher (more negative) and lower (more positive) than the hydrogen reduction potential of H+/H2 and the water oxidation potential of H2O/O2, respectively. As shown in the schematic diagram (Fig. 5), the water oxidation and reduction processes for Hf2CO2 are thermodynamically feasible. Furthermore, this material's conduction band positions are high enough with respect to the water reduction potential, which means that the water reduction reaction's driving force is strong. Hence, two-dimensional Hf2CO2 material is a potential photocatalyst to drive the water-splitting reaction characterized by its appropriate band gap and band edge positions.

 

Optical properties play an essential role in determining and classifying materials that take advantage of solar cell systems. The study of optical properties is based on the calculation of absorption coefficients, which can be directly reached from the dielectric function. The optical spectrum provides a comprehensive source of information to study band structure, electronic properties, excitations, and network oscillations. Dielectric function is determined by the response of crystals to applied electromagnetic fields. One of the most critical optical quantities is the complex dielectric function, which describes the optical properties of a compound and is expressed by the following equation:

 

Where  and  are the real and imaginary parts for dielectric function, respectively. The imaginary part of the dielectric function obtained from the following relation:

 

Where e is the electron charge,  is the unit vector of polarization the electric field,  and  are wave functions conduction and valence bands in K, respectively, and    are related energies to these states.

The real contribution of the dielectric function is obtained by using the imaginary part as well as the Kramers-Kronig relation in the form of the following equation (2) [64].

 

The dielectric function has two intra-band and inter-band contributions. We do not consider indirect interband transitions that have a small contribution to the material's optical properties [65, 66]. We can obtain real contribution changes based on Kramers–Kronig transformations by having imaginary contribution changes [67].

The band structures of the Hf2CO2 indicate their visible-light absorption and possible photocatalytic applications. Therefore, we calculated the dielectric constants of the two-dimensional Hf2CO2 MXene using the HSE06 functional to confirm their optical absorption features. Dielectric function reality and imaginary parts are indicated in Fig. 6(a) and 6(b) for a two-dimensional Hf2CO2 structure.

From the dielectric function's real contribution, the static dielectric value is obtained, which is the dielectric function's value at zero energy. Fig. 6(a) illustrates the real contribution of the dielectric function for the 2D Hf2CO2 Mxene. According to Fig. 6(a), it can be seen that these structures show anisotropic behaviors in different directions.

As shown in Fig. 6(b), the imaginary part's positive value  indicates the light absorption at a given frequency. The imaginary part  results illustrate that 2D Hf2CO2 MXene functionalized by the –O group has visible light absorption due to the energies' positive values below 3.0 eV. The sizeable optical absorption of Hf2CO2 would guarantee high efficiency in the utilization of solar energy. Thus, this material has visible-light absorption and can probably be used as a visible-light-driven photocatalyst.

As shown in Fig. 6(b), the dielectric function's imaginary value up to 2eV is quiet that illustrated only intra-band transitions occur in this area. However, after this energy, this contribution suddenly increases, which indicates the absorption. Hence, after this sudden increase, inter-band transitions occur. Therefore, using the imaginary contribution, the dielectric function can be recognized optical gap of the material.

 

 

 

 

 

 

 

Fig. 6. Dielectric function for Hf2CO2, for (a) the real and (b) imaginary part.

 

In the previous sections, in order to understand the photocatalytic behavior of Hf2CO2 for the process of water splitting, we calculated some of the inherent features like the structural, electronic, and optical properties. Consequently, to validate the present study's obtained results, we will use the previous theoretical and experimental analysis [60,62,68].

 In a study aimed at investigating the photocatalytic properties of Hf2CO2 by ab initio calculations, they examined 48 two-dimensional (2D) transition metal carbides, also referred to as MXenes, to understand their photocatalytic stuff. Among the studied two-dimensional structures was Hf2CO2. They used the density functional theory method for all calculations. The optimized lattice constant for Hf2CO2 using the GGA-PBE was calculated at 3.2660 Å. The corresponding energy gap calculated HSE06 hybrid functional obtained 1.79 eV. Also, they showed that Hf2CO2 MXene exhibits excellent optical absorption performance in the wavelength ranging approximately 300 to 500 nm. Consequently, their calculations indicated that 2D Hf2CO2 is the just opportunity for photocatalysts for possible high-efficiency photocatalytic water splitting [60]. 

In other work, the thermal and electrical properties of oxygen-functionalized M2CO2 (M = Ti, Zr, Hf) MXenes are investigated using first-principles calculations. Their results show that Hf2CO2 is determined to be a semiconductor with a band gap of 1.657 eV and to have high and anisotropic carrier mobility [62].

Generally, it can be evaluated the efficacy of research studies by using theoretical and experimental investigation. The validity of its results is proved based on compliance with the relevant reference. Based on this point of view, we have reviewed and evaluated the present study results by conforming to reliable experimental studies. Using these reliable scientific references to validate research results is a prevalent and formal way to achieve this goal. For the first time, in experimental research work, the application of two-dimensional (2D) layered transition metal carbides, MXenes, as electrocatalysts for the HER. Their computational screening study of 2D layered M2XTx (M = metal; X = (C, N); and Tx = surface functional groups) predicts Mo2CTx to be an active catalyst candidate for the HER. They also understand trends in HER activity among MXenes. Their study also contains the density functional theory (DFT) calculations to determine ΔGH, a descriptor of HER activity, to a first approximation [68].

We have reviewed many such studies in detail on the structure of Hf2CO2 with different goals [61,63,69], but to validate our results, the mentioned cases seem enough.

As we mentioned in the sections on computed quantities, the values obtained for the lattice constant after optimization are 3.3592 Å, which we reached by applying the density functional theory method. According to Fig. 3(a) and 3(b), the measured amount of band gap for the 2D Hf2CO2 MXene using the GGA-PBE functional and HSE06 hybrid functional is 0.92 and 1.97 eV, respectively. In addition to the band gap size, we computed the band edge positions for 2D Hf2CO2 MXene. Also, as shown in Fig. 6(a) and 6(b), this structure has excellent optical absorption. Therefore, according to these interpretations, it can be said that this structure is a suitable photocatalyst for the water-splitting process. According to the research studies, to validate our work results, the agreement of the calculated results and the interpretations attributed to the studied structure with the validation reference to evaluate the accuracy and precision of the obtained quantities can be seen.

 

CONCLUSION

MXene-based materials have many attractive properties such as high surface area, tunable electronic structure, and high electronic conductivity. The mentioned features are proper for electrocatalysis applications. In order to further explore their application, it is needful to understand the MXene materials better. The present study systematically investigated the structural, electronic band structure, and optical properties of novel two-dimensional Hf2CO2 MXene using DFT calculations.

In this paper, to thoroughly explore the properties of MXenes as a photocatalyst for water splitting, we investigated the structural and electronic properties of Hf2CO2 MXene. The results indicate surface-functionalized M2C-type MXenes, such as Hf2CO2, turn out to be semiconductors from GGA-PBE functional. Due to underestimate the band gaps in the calculations with the GGA-PBE functional, we computed the electronic structures using an HSE06 hybrid density functional. As shown in Fig. 3(b) and 4(b), a necessary condition for the water-splitting process was that the band gap for a suitable photocatalyst is 1.23 eV. Because this requirement could not be met by a GGA-PBE functional, we did this using an HSE06 hybrid functional, which caused the band gap to increase from 0.92 to 1.75 eV.  Also, we illustrate the alignment of water reduction and oxidation potential for the band edges of MXenes to investigate the thermodynamic possibility of the water oxidation and reduction. Hence it seems that for Hf2CO2 MXene, the water oxidation and reduction processes are thermodynamically feasible. Furthermore, the optical absorption properties of two-dimensional Hf2CO2 are analyzed based on the dielectric function's calculated imaginary part. It can seem that this two-dimensional MXene-based nanostructure has a very high optical absorption and is suitable as a photocatalyst for the water-splitting process.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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