Biao Wanga,
Xukai Luoa,
Junli Changa,
Xiaorui Chena,
Hongkuan Yuana and
Hong Chen*ab
aSchool of Physical Science and Technology, Southwest University, Chongqing 400715, People’s Republic of China. E-mail: chenh@swu.edu.cn
bKey Laboratory of Luminescent and Real-Time Analytical Chemistry, Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, People’s Republic of China
First published on 23rd May 2018
Two-dimensional (2D) hafnium disulfide (HfS2) has been synthesized and is expected to be a promising candidate for photovoltaic applications, and at the same time the hexagonal BN sheet (h-BN) and graphene-like C3N4 sheet (g-C3N4) have also been fabricated and are expected to be applied in photocatalysis. In this work, we employ hybrid density functional theory to investigate HfS2-based van der Waals (vdW) heterojunctions for highly efficient photovoltaic and photocatalytic applications. HfS2/h-BN and HfS2/g-C3N4 heterostructures with direct bandgaps and efficient charge separation are both typical type-II semiconductors and have potential as photovoltaic structures for solar power. Moreover, compared with h-BN and g-C3N4 single-layers, HfS2/h-BN heterostructures with 6% tensile strain and HfS2/g-C3N4 heterostructures with 9% tensile strain have moderate bandgaps, whose optical absorption is obviously enhanced in the ultraviolet-visible (UV-VIS) light range and whose bandedges are suitable for photocatalytic water splitting. HfS2/h-BN heterostructures with 6% applied strain, being different from HfS2/g-C3N4 heterostructures with 9% strain, possess a direct bandgap and show complete separation of the photoinduced electron–hole pairs. Thus the HfS2/h-BN heterojunction with 6% strain has bright prospects for use in visible light photocatalytic water splitting to produce hydrogen.
Because HfS2 is predominantly an ionic crystal with a moderate bandgap and a “two-dimensional” layered structure bonded by weak van der Waals forces,9 it has been inspiring researchers to study its properties from bulk to monolayer.10–12 Recently, few-layered hafnium disulfide nanosheets with a 1T-structure have been successfully made by chemical vapor deposition, which shows their bright prospects in the fields of photodetectors7 and field effect transistors13 due to their ultrafast photoresponse time, high photosensitivity and excellent field effect responses. Furthermore, theoretical calculations suggest that the monolayers of 1H- and 1T-HfS2 have potential applications as photocatalysts for water splitting.14 However, individual HfS2 nanosheets similar to graphitic carbon nitride (g-C3N4)15 are liable to rapidly recombine photogenerated electron–hole pairs and reduce solar conversion and photocatalytic efficiency.
For the sake of reducing bandgaps and enhancing the separation of the photoinduced electron–hole pairs,16 a number of vdW heterojunctions composed of different single-layer materials that are combined together by vdW forces have been manufactured and have been predicted to acquire more desirable properties.2,8,17–24 At present, 2D HfS2/phosphorene heterojunctions25 as field effect transistors and HfS2-based nanocomposites as Z-scheme type photocatalysts for hydrogen production26 have been studied, whose structures and photocatalytic mechanism are more complex than those of type-II heterojunctions. Because of the high cost and difficulty in manufacturing 2D heterojunctions in the laboratory, theoretical research is necessary to predict whether or not they can be used as catalysts for hydrogen evolution by water splitting.26 Moreover, the g-C3N4 and hexagonal boron nitride (h-BN) monolayers have become elementary slabs for vdW heterostructures.27 Here, we mainly investigate whether the bilayer vdW heterostructures with combined HfS2 and h-BN (g-C3N4) layers can form standard type-II heterojunctions, which are deemed to be able to enhance the separation of electron–hole pairs. In addition, their band gaps and band edge positions, the effect of the biaxial strain, the density of states and optical absorption spectra are calculated by density functional theory.
Structure | ε (%) | R (Å) | Eb (eV) | Φ (eV) |
---|---|---|---|---|
HfS2/h-BN | 3.89 | 3.40 | −0.11 | 6.09 |
HfS2/g-C3N4 | −1.16 | 3.35 | −0.58 | 6.74 |
As shown in Fig. 1, the equilibrium distances of the fully relaxed nanocomposites, marked as R in Table 1, are 3.40 Å and 3.35 Å for HfS2/h-BN and HfS2/g-C3N4 heterojunctions, respectively, and represent the space between the S atom in the HfS2 layer and the other layer. The equilibrium distances of the studied heterostructures are close to those of other vdW heterostructures,8,24 which are typical distances of vdW heterostructures. Therefore, the vdW correction of interactions has been applied when optimizing the structure and calculations. The binding energy (Eb) is defined by the equation: Eb = Ex/HfS2 – (Ex + EHfS2), where Ex/HfS2, Ex and EHfS2 represent the total energy of the nanocomposites, the independent single-layers (x = h-BN or g-C3N4) and the isolated HfS2 monolayer, respectively. The computed binding energies of the HfS2/h-BN and HfS2/g-C3N4 heterojunctions (Table 1) are −0.11 eV and −0.58 eV respectively, which means that these nanocomposites are stable.
HfS2/h-BN and HfS2/g-C3N4 heterostructures are both direct bandgap semiconductors, whose bandgaps have been observed to reduce to 1.78 eV and 1.29 eV, as presented in Table 2. Because of these suitable bandgaps which are less than 1.8 eV, these heterostructures can effectively take advantage of solar power. It’s well known that the redox potentials of water are connected to the pH value.38 The reduction potential for H+/H2 is EH+/H2 = −4.44 eV + pH × 0.059 eV, and the oxidation potential for O2/H2O is EO2/H2O = −5.67 eV + pH × 0.059 eV. As shown in Table 2, Eg, EVBM and ECBM denote the bandgap and the energy levels of the valence band maximum (VBM) and the conduction band minimum (CBM) compared to those of the vacuum level, respectively. Unfortunately, the CBMs of these heterojunctions shown in Fig. 3 aren’t higher than the reduction potential of water at pH = 0, which indicates that these heterostructures may not be used for water splitting to produce hydrogen. However, the light-generated electrons and holes are completely separated in HfS2/h-BN and HfS2/g-C3N4 heterostructures. Fig. 4 shows the total density of states (TDOS) and partial density of states (PDOS). The CB in the HfS2/h-BN heterojunction is composed of Hf 5d and S 3p states that originate from the HfS2 slab, while the VB of the heterostructure largely consists of the N 2p state which stems from the h-BN layer. Thus, the CB and VB in the heterojunction are seated in two different slabs. Similarly, the CB in HfS2/g-C3N4 is composed of Hf 5d and S 3p states located in the HfS2 layer, while the VB is composed of C 2p and N 2p states located in the other layer. Therefore, both of the heterostructures form typical type-II semiconductors, which can improve the efficiency of utilizing sunlight. With moderate bandgaps and full separation of photogenerated electron–hole pairs, HfS2/h-BN (g-C3N4) heterostructures have profound development potential as photovoltaic materials for solar power, except for in water splitting.
Structure | Eg (eV) | EVBM (eV) | ECBM (eV) | Bandgap | Straddling water redox potentials |
---|---|---|---|---|---|
HfS2/h-BN | 1.78 | −6.51 | −4.74 | Direct | No |
HfS2/h-BN with −6% strain | 0.62 | −5.77 | −5.14 | Direct | No |
HfS2/h-BN with −3% strain | 1.27 | −6.21 | −4.94 | Direct | No |
HfS2/h-BN with 3% strain | 2.03 | −6.63 | −4.60 | Direct | No |
HfS2/h-BN with 6% strain | 2.28 | −6.71 | −4.43 | Direct | Yes |
HfS2/h-BN with 9% strain | 2.52 | −6.80 | −4.28 | Direct | Yes |
HfS2/g-C3N4 | 1.29 | −7.19 | −5.90 | Direct | No |
HfS2/g-C3N4 with −6% strain | 0.01 | −5.34 | −5.33 | Direct | No |
HfS2/g-C3N4 with −3% strain | 0.38 | −5.60 | −5.22 | Direct | No |
HfS2/g-C3N4 with 3% strain | 1.92 | −6.65 | −4.73 | Indirect | No |
HfS2/g-C3N4 with 6% strain | 2.29 | −6.88 | −4.59 | Direct | No |
HfS2/g-C3N4 with 9% strain | 2.72 | −7.15 | −4.43 | Indirect | Yes |
In previous studies it was confirmed that the in-plane biaxial strains can effectively adjust the optical absorption and electronic properties of vdW heterostructures.8,24 In order to explore their potential application in the field of photocatalytic water splitting, HfS2/h-BN and HfS2/g-C3N4 heterostructures with biaxial strains have been comprehensively investigated. During relaxation of the geometric structure, the lattice constant lx is fixed to imitate the strain on the heterostructures. Strains are defined as follows: η = (lx − l0)/l0, where l0 and lx represent the lattice of the original heterostructures and strained heterojunctions. Employing the in-plane biaxial compressive and tensile strain on these nanocomposites, the value of η ranges from −6% to 9%, as shown in Fig. 5, where the bandedge positions are presented as a function of the applied strains. With increasing tensile strains, the lattices of the heterostructures become larger, which indicates that the distance of the nearest atoms is enlarged. On the basis of tight-binding theory,39 their energy-bands will narrow down and the bandgaps will get bigger. Consequently, the enlargement of the bandgaps is accompanied by an increase in the biaxial strain as shown in Fig. 5. Due to their porosity in the g-C3N4 layer,40 the bandedge variation with strain in the HfS2/g-C3N4 heterojunction is different from that in the HfS2/h-BN heterojunction. Applying 6% or 9% tensile strain, the bandedge of the HfS2/h-BN heterostructure can straddle water redox potentials. Similarly, that of the HfS2/g-C3N4 heterostructure with 9% tensile strain can straddle water redox potentials as well. However, HfS2/g-C3N4 with increasing pressure, whose bandgap approaches to zero, can transform from semiconductor to metal. With the aim of meeting the requirements for photocatalytic water splitting, a HfS2/h-BN heterostructure with 6% strain and HfS2/g-C3N4 heterostructure with 9% strain were investigated using the following calculations.
Fig. 5 The bandedge positions of the HfS2/h-BN heterojunction (a) and HfS2/g-C3N4 heterojunction (b) as a function of biaxial strain. |
The HfS2/h-BN heterojunction with 6% applied tensile strain is a direct bandgap semiconductor, whose bandgap is calculated to be 2.28 eV. The VBM and CBM of the heterojunction are both at the G point, as plotted in Fig. 6. Meanwhile the HfS2/g-C3N4 heterostructure with 9% applied tensile strain, whose VBM is at the G point and CBM is between the G and K points, is an indirect bandgap semiconductor with a bandgap of about 2.72 eV. Considering the moderate and direct bandgap, the photocatalytic performance of the HfS2/h-BN heterojunction with 6% strain is superior to that of the other heterostructures.
Fig. 6 The band structures of the HfS2/h-BN heterostructure with 6% strain (a) and HfS2/g-C3N4 heterostructure with 9% strain (b) compared to the vacuum level. |
The TDOS and PDOS of the HfS2/h-BN heterostructure with 6% strain and HfS2/g-C3N4 heterostructure with 9% strain are presented in Fig. 7. The CB of the HfS2/h-BN heterostructure with 6% strain is mainly composed of Hf 5d, Hf 6s and S 3p orbitals derived from the HfS2 layer, while the VB of the heterojunction is primarily composed of the N 2p state stemming from the h-BN layer. Obviously, this heterojunction is a classic type-II semiconductor, where the CB and VB are located on different layers in favor of complete separation of the electron–hole pairs and separate production of hydrogen and oxygen. For the HfS2/g-C3N4 heterostructure with 9% strain, the VB of the heterojunction is composed of N 2p and S 3p orbitals that originate from both layers, and the CB is composed of the Hf 5d, Hf 6s and S 3p orbitals. As a result, the HfS2/g-C3N4 heterostructure with 9% strain is incapable of completely separating the photoinduced carriers. Therefore, considering the efficient division of the electron–hole pairs, the HfS2/h-BN heterostructure with 6% strain is more suitable for photocatalysis than the HfS2/g-C3N4 heterostructure with 9% strain.
Fig. 7 The TDOS and PDOS for the HfS2/h-BN heterostructure with 6% strain (a) and HfS2/g-C3N4 heterostructure with 9% strain (b). |
Moreover, the optical absorption spectra of the HfS2/h-BN heterojunction (with 6% strain) depicted in Fig. 9(a) are obviously enhanced in the ultraviolet range in contrast to those of the HfS2 single-layer. Compared with the optical absorption spectrum of the h-BN monolayer, it can only be speculated that the significant red shift of the HfS2/h-BN heterojunction’s absorption spectrum (with 6% strain) is mainly induced by a transition from the N 2p to Hf 5d orbital, as shown in Fig. 5(a) (Fig. 7(a)), which is important for strengthening the heterojunction’s photocatalytic activity. Consequently, with an ideal band gap, complete charge separation and visible-light response, the HfS2/h-BN heterojunction can be used in the field of photovoltaics. On account of its moderate bandgap, well positioned band edge, effective charge separation and reinforced UV-VIS light absorption, the HfS2/h-BN heterojunction with 6% strain can be used for photocatalytic water splitting.
As shown in Fig. 9(b), the optical absorption edge of the HfS2/g-C3N4 heterojunction (with 9% strain) shifts to a longer wavelength range. In particular, the HfS2/g-C3N4 heterojunction takes full advantage of visible light, which is similarly derived from a transition from the N 2p to Hf 5d orbital, as shown in Fig. 5(b). Because of its relatively narrow bandgap, efficient charge separation and visible-light response, the HfS2/g-C3N4 heterojunction is a promising material for photovoltaic applications. With an indirect and moderate bandgap, the HfS2/g-C3N4 heterojunction with 9% applied strain has a well positioned band edge but isn’t able to completely separate charge carriers. Therefore, the photocatalytic performance of the HfS2/h-BN heterojunction with 6% strain is superior to that of the HfS2/g-C3N4 heterojunction with 9% strain.
In addition, applying biaxial strain can effectively adjust the electronic properties of HfS2/h-BN and HfS2/g-C3N4 heterojunctions. With pressure of a certain extent, the bandgap of the HfS2/g-C3N4 heterojunction can reduce to 0 eV, which may indicate that the heterojunction transforms from semiconductor to metal. The band edges of HfS2/h-BN composites with 6% tensile strain and HfS2/g-C3N4 heterojunctions with 9% tensile strain are both able to straddle water redox potentials. The photocatalytic performance of a HfS2/h-BN heterojunction with 6% strain, which is a typical type-II semiconductor, has advantages over that of a HfS2/g-C3N4 heterojunction with 9% strain. Because of the moderate band gap, effective charge separation, well positioned band edge and reinforced UV-VIS light absorption, HfS2/h-BN heterojunctions with applied strains are a promising photocatalyst for water splitting. These calculations pave the way for exploiting high-performance HfS2-based photocatalysts.
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