Investigations on Structural, Electronic and Optical Properties of MoS₂/CDs Heterostructure via First-Principles Study

Abstract

Much effort has been made for MoS₂/CDs heterostructure application in the field of photocatalysts. However, the impacts of functional groups of CDs on the properties of the heterostructure are ambiguous. Here, the impacts of hydroxyl, carbonyl, and carboxyl groups of CDs on the structural, electronic, and optical properties of MoS₂/CDs’ heterostructure were investigated by conducting a first-principles study. The calculated energy band structure and band gap of monolayer MoS₂ were consistent with the experimental values. The band gap of MoS₂ was obviously decreased after the construction of MoS₂/CDs and MoS₂/CDs–hydroxyl/carboxyl, thus helping to improve the light adsorption range. However, the band gap of MoS₂/CDs–carbonyl was slightly increased compared with that of monolayer MoS₂. The CDs with functional groups can spontaneously bind on 2D-MoS₂ and form a stable MoS₂/CDs heterostructure. It was confirmed that the MoS₂/CDs’ heterostructure belongs to the typical type-II band alignment, which contributes to the separation of photogenerated charge and hole. Notably, the carbonyl and carboxyl groups on the CDs obviously reduced the optical absorption intensity of the MoS₂/CDs in the ultraviolet region. The hydroxyl groups have little effect on optical absorption intensity. Thus, the CDs with more hydroxyl groups are beneficial to produce a higher photocatalytic performance. This paper reveals the impacts of surface functional groups and provides a promising approach for designing the MoS₂/CDs’ heterostructure to enhance the photocatalytic properties.

1. Introduction

Molybdenum disulfide (MoS₂) is one of the most studied novel material among transition metal sulfides, due to its potential applications in electronic and optoelectronic devices [1,2,3], hydrogen evolution reaction [4,5,6], and energy storage [7,8,9]. However, MoS₂ also has some disadvantages in practical applications, including the inert basal plane, untunable band gap, and low conductivity [10,11]. Therefore, in order to activate the basal plane, enhance conductivity, and tune band-gap, MoS₂-based heterostructures and hybrid systems have been built. The MoS₂-based heterostructures include carbon materials (graphene, graphene oxide, graphene quantum dot, carbon dot, and carbon nanotube), noble metals (Ag, Au, and Pt), TiO₂, Cu₂O, MoO₃, and other two-dimensional materials (WS₂, WSe, BN, and Mxenes) [12,13,14,15,16,17,18,19,20,21], exhibiting the better performance in terms of electronics, optoelectronics, catalytics, and energy storage.

Carbon materials play a key role in these MoS₂-based heterostructures, due to their unique electronic properties. However, some unexpected functional groups always are produced from the preparation of carbon materials such as graphene, graphene oxide, and carbon dot. The impacts of functional groups on MoS₂-based heterostructures are vague. In this paper, we chose carbon dots (CDs) as a representative carbon material for researching the impacts of functional groups on the electronic properties. Carbon dots are a common carbon material, because they have extensive applications in photocatalysis [22], bio-imaging [23], sensor [24], and light-emitting diodes [25], due to high quantum yields and tunable light-emission bands. Chen et al. [26] prepared 1L-MoS₂/CDs ultra-thin light emission films with full-color tunable photoluminescence, which greatly extended the light emission bands of MoS₂. Badhulika et al. [16] first fabricated a 2D-MoS₂-CDs-based flexible broadband photodetector covering the entire range of the electromagnetic spectrum, due to the combination of UV illumination of CDs and visible and NIR illumination of MoS₂. To date, the facile and green preparation of CDs is the carbonization of carbon sources, including glucose [27], citric acid [26], sugar [28], and chia seed [16]. Many experiments have demonstrated that the hydroxyl, carbonyl, and carboxyl groups exist in the edge of CDs due to the incomplete carbonization [27,29]. It is well-known that the surface functional groups have a strong influence on electronic structures, optical characteristics, and conductivity [26,30,31,32,33]. However, due to the difficulty of synthesizing CDs with a specific functional group, the influence of a specific functional group is hard to study through experimentation. Therefore, it is of great significance to study it through simulation calculations.

Density functional theory (DFT) calculation is a powerful and scientific technology that is used to research the intrinsic properties of nanomaterials and provide theoretical guidance for experiments and applications. Some papers have reported the effects of vacancies or heterostructure on the structural, electronic, and optical properties of monolayer MoS₂ by DFT [30,31]. However, the influences of the surface properties of CDs on the properties of MoS₂/CDs were less investigated through simulation. In this study, the effects of hydroxyl, carbonyl, and carboxyl groups of CDs on the structural, electronic, and optical properties of MoS₂/CDs’ heterostructure were revealed by applying density functional theory. This study contributes to improving the optical property of MoS₂/CDs’ heterostructure.

2. Computational Details

All the first-principles calculations were performed within the framework of the plane-wave pseudopotential density functional theory (DFT) implemented in CASTEP code with ultrasoft pseudopotential method on the basis of DFT [34,35,36]. The generalized gradient approximation (GGA) with Perdew–Bruke–Ernzerhof (PBE) functional and van der Waals correction proposed by the Grimme potential were employed [34,37]. This method has widely been employed to study the structural, electronic, and photoelectronic properties of MoS₂-based heterostructure [34,38,39,40]. All structures were relaxed until the force on each atom was less than 0.01 eV Å⁻¹. The energy cutoff was 400 eV. The two-dimensional (2D) periodic boundary conditions were considered along the growth directions of the MoS₂/CDs layer. The Monkhorst–Pack k-point mesh was sampled with a separation of 0.05 and 0.015 Å⁻¹ in the Brillouin zone during the relaxation and calculation periods, respectively.

3. Results and Discussions

First, a 4 × 4 supercell of MoS₂ monolayer composed of 16 molybdenum atoms and 32 sulfur atoms was constructed as a substrate (Figure 1a). A zigzag-edged C₂₄H₁₂ structure was used, and all edge carbon atoms of CDs were assumed to be fully passivated by H atoms to eliminate the dangling bonds (Figure 1b) [34,41]. It has been widely accepted that the carboxyl, hydroxyl, and carbonyl are three dominant functional groups on the CDs. Therefore, the CDs-T (T is carboxyl, hydroxyl, and carbonyl, respectively; CDs-T represents that only one functional group T is on the CDs) were constructed for studying the impacts of functional groups on the structure, electronic, and optical absorption of MoS₂/CDs. In order to match the experimental data, the functional groups were place on the edge of CDs. After geometry optimization, the lattice constants of the MoS₂ monolayer were a = b = 3.191 Å and α = β = 90°, γ = 120°, which is comparable to the experimentally measure for the bulk MoS₂ [42]. To evaluate the structural stability of MoS₂/CDs-T, the binding energies were calculated by using the Equation (1) [39].

E_b = E_(MoS2/CDs-T) − E_(MoS2) − E_(CDs-T)

(1)

where E_(MoS2/CDs-T), E_(MoS2), and E_(CDs-T) are the total energies of MoS₂/CDs-T, MoS₂, and CDs-T, respectively.

Figure 2a shows the E_b and h (the distance between MoS₂ and CDs and CDs-T) in the heterostructures. The optimized h value ranges from 3.267 to 3.358 Å, and the closest distance is 3.267 Å of MoS₂/CDs-OH. The bigger carboxyl groups on the CDs increase the distance due to the steric effect. The negative binding energy suggests that CDs spontaneously form the MoS₂/CDs’ heterostructure and keep it in a stable state. A negative binding energy indicates that the two substances can spontaneously bind and form a stable heterojunction. Conversely, a positive binding energy indicates that the two substances cannot bind spontaneously. Therefore, the carboxyl, hydroxyl, and carbonyl groups on the CDs can still form the stability of the MoS₂/CDs-T heterostructures. Figure 2b,c shows the band structure and partial density of the MoS₂ states. The direct band gap (the conduction band minimum and the valence band maximum are located at the K symmetry point indicating) of 1.75 eV (Figure 2b) for monolayer MoS₂ is in good agreement with other theoretical values (1.78 eV [43], 1.74 eV [34], and 1.70 eV [43]) and experimental results (1.75 eV [44] and 1.80 eV [45]). As shown in Figure 2c, the valence bands and the conduction bands are essentially hybridized from the Mo 4d and S 3p orbitals, and this is in accordance with the previous calculation results [34,46].

Figure 3 displays the PDOS of CDs and CDs-T; the insets in these figures are the corresponding band structures. Electron transfer and sharing between functional groups and carbon atoms lead to changes in the electronic structure. The valence bands of CDs from −5 to −2 eV are mainly hybridized from C 2p and H 1s orbitals, while the valence bands from −2 to 0 eV and the conduction bands from 0 to 4 eV are composed only of C 2p orbital (Figure 3a). Moreover, the energy levels of CDs are discrete, owing to the quantum confinement [34]. The energy gap of CDs between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is 2.859 eV, which is really agreement with previously reported results (2.890 eV [34] and 2.90 eV [41]). It is worth noting that the energy gaps calculated by DFT are lower than the experimental result (5.56 eV [47] and 6.82 eV [48]), which results from the most used local and semi-local approximations for exchange–correlation functional in standard DFT calculation liable to underestimate energy gaps [46]. After hydroxyl, carbonyl, and carboxyl were added to CDs, the energy gaps of CDs-T between LUMO and HOMO were 2.637, 2.236, and 2.110 eV, respectively. As seen in Figure 3b–d, the valence bands of CDs-T from −5 to −2 eV and the conduction bands from 3.5 to 5 eV are hybridized from C 2p, O 2p, and H 1s orbitals, while the valence bands from −2 to 0 eV and the conduction bands from 0 to 3.5 eV are hybridized from major C 2p and a small part portion of O 2p orbitals. The bottom of the conduction bands and top of the valence bands of MoS₂/CDs-T consist of the hybridization from the C 2p and O 2p orbitals.

The PDOS and band structure of MoS₂/CDs and MoS₂/CDs-T heterostructures are shown in Figure 4. In the LUMO, the conduction bands of these heterostructures are mainly hybridized from Mo 4d and S 3p orbitals. There are energy levels under the Femi level. For MoS₂/CDs, MoS₂/CDs-OH, and COOH, these energy levels are mainly composed of the C 2p orbital. However, the energy level at the Femi level of MoS₂/CDs-O is hybridized from the C 2p and O 2p orbitals. The electronic structure indicates that the Mo 4d, S 3p, C 2p, and O 2p orbitals are the main way of photo-induced electron. Under the illumination, the electrons are transferred from CDs to MoS₂ and leave holes on the CDs, which contribute to the separation of electron and hole. The energy bands of MoS₂/CDs, MoS₂/CDs–hydroxyl, and carboxyl are reduced to 1.496, 1.320, and 1.248 eV, respectively, while the energy band in MoS₂/CDs–carbonyl is 1.772 eV.

The optical absorption of the catalyst plays a key role in the photocatalytic process. Figure 5a shows the optical absorption spectra of these samples, namely CDs-T, MoS₂, and MoS₂/CDs-T. The frequency-dependent dielectric matrix, ε (ω) = ε₁ (ω) + iε₂ (ω), is calculated, where ε₁ (ω) and ε₂ (ω) are the real and imaginary parts of the dielectric function, respectively. The ε₁ (ω) is determined by summation over electronic states, using the Equation (2). [49]:

$${\mathit{\epsilon }}_1(\mathit{\omega })=1+\frac{2}{\pi }P{{\displaystyle \int }}_0^{\infty }\frac{{\mathit{\epsilon }}_2\left({\mathit{\omega }}^{\prime }\right){\mathit{\omega }}^{\prime }}{{\mathit{\omega }}^{\prime 2}-{\mathit{\omega }}^2+i\eta }\mathrm{d}{\mathit{\omega }}^{\prime }$$

(2)

The ε₂ (ω) is obtained by using the Kramers–Kronig relationship [45]:

$${\mathit{\epsilon }}_2(\mathit{\omega })=\frac{4{\pi }^2{e}^2}{\Omega }\underset{q\to 0}{\lim }\frac{1}{q^2}{\sum }_{c,v, k}2{\mathit{\omega }}_k\delta \left({\mathit{\epsilon }}_{ck}-{\mathit{\epsilon }}_{vk}-\mathit{\omega }\right)\times \langle {\mu }_{ck+e_{\propto }q}|{\mu }_{vk}\rangle \langle {\mu }_{vk+e_{\beta }q}|{\mu }_{vk}\rangle$$

(3)

The light adsorption intensity is evaluated according to the Equation (4). [45]:

$$\mathsf{\alpha }(\mathrm{E})=\frac{4\pi e}{hc}{\left\{\frac{{\left[{\mathit{\epsilon }}_1^2+{\mathit{\epsilon }}_2^2\right]}^{1/2}-{\mathit{\epsilon }}_1}{2}\right\}}^{1/2}$$

(4)

The calculated ε₁ (ω) and ε₂ (ω) of dielectric function versus energy of MoS₂, CDs-T, and MoS₂/CDs-T are shown in Figure 5. The peak A (3.01 eV), B(4.96 eV), and C(10.94 eV) can be observed in the imaginary part of MoS₂. These peaks are located from 0 to 30 eV, due to the absorptive transition from the valence bands to the conduction bands. According to the structure analysis, the peaks of A, B, and C result from the transition of S 3p into Mo 4d conduction bands [50], the hybridization orbitals between S 3p and Mo 4d into Mo 4d conduction bands [51], and σ bonding between S 3p and Mo 5s into Mo 4d conduction bands [52], respectively. For MoS₂/CDs-T, they have similar profiles with that of MoS₂. Notably, the imaginary parts of MoS₂/CDs-T move slightly toward lower energies because of the effects of heterostructure [53]. The calculated static constants of MoS₂ is about 5.0 eV, which is consistent with other reported values of 5.3 eV calculated by the generalized gradient approximation (GGA) with Perdew–Bruke–Ernzerhof functional [54]. It is obvious that the static dielectric constants of MoS₂/CDs-T are bigger than those of of MoS₂. Figure 6a shows the optical absorption spectra of all samples. There is strong absorption in the range of 200 to 700 nm for the MoS₂ monolayer, which is potential for applications in sunlight-driven photocatalysis. The main peak at about 450 nm is attributed to the transition from the occupied Mo-4d orbital to the unoccupied S-3p orbital [55]. However, the MoS₂ monolayer has a low adsorption capacity at the long wavelength end, and this results from the electronic structure of the monolayer. The absorption peaks of CDs are mainly located at the ultraviolet region from 200 to 400 nm, and the absorption intensity is relatively low. The obvious red shifts of the absorption peaks and the reduction of the absorption intensities can be observed after the MoS₂/CDs’ heterostructures were constructed. Compared to the MoS₂ monolayer, the absorption intensities ranging from 200 nm to the long wavelength were obviously increased by CDs. However, the carbonyl and carboxyl groups of CDs decrease the absorption intensities of MoS₂/CDs in the ultraviolet region. In contrast, the hydroxyl groups have a slight impact on the absorption. It can be concluded that CDs are suitable for promoting the photocatalysis of MoS₂ due to the enhanced adsorption intensity. However, the carbonyl and carboxyl groups have a negative effect on the photocatalysis of MoS₂/CDs, causing a decrease in adsorption intensity.

To investigate the redox ability of MoS₂/CDs-T, the valence band maximum (VBM) and conduction band minimum (CBM) relative to the vacuum energy level were calculated according to the Equations (5) and (6). [55]:

E_VB = φ = V (∞) − E_F

(5)

E_CB = E_VB − E_g

(6)

where V (∞), E_F, and E_g are the electrostatic potential in a vacuum region, the Fermi level of the neutral surface system, and the band gap, respectively. As shown in Figure 6b, the LUMO and HOMO of CDs-T are placed at higher energy states than the CBM and VBM of MoS₂ monolayer, respectively. It indicated that MoS₂/CDs heterostructures can construct the typical type-II band alignments [34], which promote photo-induced charge separation and transfer. Combining with the results from PDOS and band structures (Figure 4), the photo-generated electron of C 2p orbitals of CDs and CDs-T will transfer to the S 3p and Mo 4d hybrid orbitals (Figure 6c).

4. Conclusions

In summary, the impacts of hydroxyl, carbonyl, and carboxyl of the CDs on the structural, electronic, and optical properties of MoS₂/CDs were studied by employing density function theory. The more negative binding energies (−0.36 eV to −1.17 eV) between MoS₂ and CDs-T suggest that the hydroxyl, carbonyl, and carboxyl groups contribute to spontaneous combination and the stability structure. The three groups show opposite effects on the band gap of MoS₂/CDs-T. The hydroxyl and carboxyl greatly reduce the band gap (1.32 and 1.25 eV, respectively) compared with the band gap of MoS₂ and MoS₂/CDs. However, the carbonyl group slightly increases the band gap (1.77 eV). The heterostructure of MoS₂/CDs-T belongs to typical type-II band alignment, which is beneficial to the separation of photo-induced charge and hole. Notably, the carbonyl and carboxyl groups of CDs reduce the absorption intensity of MoS₂/CDs. In the preparation of MoS₂/CDs, the generation of carbonyl and carboxyl groups should be avoided as much as possible. Increasing the content of hydroxyl groups in carbon quantum dots is more conducive to improving the photocatalytic performance.