Dramatically Enhanced Visible Light Response of Monolayer ZrS2 via Non-covalent Modification by Double-Ring Tubular B20 Cluster
© The Author(s). 2016
Received: 30 July 2016
Accepted: 4 November 2016
Published: 10 November 2016
The ability to strongly absorb light is central to solar energy conversion. We demonstrate here that the hybrid of monolayer ZrS2 and double-ring tubular B20 cluster exhibits dramatically enhanced light absorption in the entire visible spectrum. The unique near-gap electronic structure and large built-in potential at the interface will lead to the robust separation of photoexcited charge carriers in the hybrid. Interestingly, some Zr and S atoms, which are catalytically inert in isolated monolayer ZrS2, turn into catalytic active sites. The dramatically enhanced absorption in the entire visible light makes the ZrS2/B20 hybrid having great applications in photocatalysis or photodetection.
Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) have intriguing properties that make them highly suitable for many fields including lithium-ion battery, solar cell, and catalysis . Thanks to the dramatic progress in recent experimental advances, many kinds of few-layer or monolayer TMDs have been successfully prepared [2, 3]. However, any one of pure-layered TMDs is not always a perfect material for different applications. To achieve superior performance for some specific cases, various strategies have been developed to engineer the chemical, physical, and electronic properties of 2D TMDs [1, 4, 5]. In particular, coupling 2D TMDs with other materials to create novel functional van der Waals (vdW) heterostructures receives growing significant attention .
As one of representative group IVB-TMDs, zirconium disulfide (ZrS2) has attracted considerable attention and shows great potential in photodetectors , solar cells , and photocatalysis , due to its good thermodynamic stability, environmental friendliness, high sensitivity, and low-cost production. In recent years, monolayer ZrS2 keeping these advantageous qualities have been successfully fabricated by various methods [9–11]. The band gap of bulk ZrS2 is around 1.70 eV [12, 13], while it is very interesting that mono-, bi-, and trilayer ZrS2 have an indirect band gap with 2.01, 1.97, and 1.94 eV [8, 14], respectively, indicating that it undergoes a transition of band gap when the dimensionality decreases from 3D to 2D. Due to its appropriate band gap, monolayer ZrS2 can utilize the maximum portion of the solar visible light. However, the measured efficiency in solar hydrogen production of monolayer ZrS2 is quite low compared with the theoretical value owing to its conduction band maximum (CBM) slightly lower than the reduction level of hydrogen [15, 16]. To overcome the drawbacks, many methods have been explored to improve the photocatalytic performance of ZrS2. Among them, combining with other semiconductors, such as graphene, g-C3N4, h-BN, and ZnO, has been demonstrated to be an effective strategy to enhance the stability and photocatalytic activity of ZrS2 [16, 17].
Boron (1s2 2s2 2p2) can form a wide variety of clusters with fascinating properties, as its neighbor carbon (1s2 2s2 2p1) which is well known showing distinct solid-state allotropes like chains, rings, and fullerenes . The related study of boron clusters can date back to nearly 30 years ago . A lot of boron fullerenes, such as B80 and B100 [20, 21], have been studied theoretically. Recently, Zhai et al. have firstly observed the all-boron fullerene B40 in experiment , triggering renewed interest in these boron clusters [23–25]. Herein, we for the first time study the structural and electronic properties of hybrid monolayer ZrS2/B20 vdW heterostructure to explore its potential applications in solar energy conversion by using large-scale density functional theory (DFT) computations. Here, double-ring tubular B20 cluster is taken as the typical boron cluster, motivated by its special structure and properties. As a stable non-planar structure formed by 20 boron atoms with high symmetry, double-ring tubular B20 is considered to be an important structure due to the 2D-to-3D transition of boron cluster: the boron clusters prefer 2D structures up to 19 atoms and favor 3D structures beginning at 20 atoms in terms of experimental and computational studies [26–29]. More importantly, the band gap of B20 ring is about 1.2~1.4 eV , suggesting that its spectral response covers the entire visible region, even extending to near-infrared light. It is speculated that the role of B20 ring in the hybrid is multiple. The calculated results show that compared to pure monolayer ZrS2, the ZrS2/B20 hybrid displays dramatically enhanced visible light response, making it to be great potential in solar energy conversion.
Results and Discussion
The distribution of electric potential in the ZrS2/B20 hybrid will be altered due to the interfacial charge transfer. To display the quantitative analysis, the profile of the planar averaged self-consistent electrostatic potential for the ZrS2/B20 hybrid as a function of position in the z-direction is displayed in Fig. 4a. One can see that the electrostatic potential at two B atomic planes is lower than that at their middle region in ring B20 cluster, and obvious potential difference between the Zr atomic plane and two S atomic planes can be observed, rendering a typical S-Zr-S sandwich distribution. Note that the potential at the upper S atomic plane is slightly higher than that at the lower S atomic plane (upper −25.05 eV, lower −25.26 eV), verifying that the S atoms at the upper layer lose some electrons due to the ring B20 cluster modification (as displayed in Fig. 4b). The potential at the monolayer ZrS2 plane is much lower than that at ring B20 cluster, resulting into a large potential difference between the two constituents. The built-in potential at the interface promotes the separation of electron-hole pairs. Moreover, under light irradiation, the separation and migration of photogenerated carriers at the interface will be more effective due to the appearance of this built-in potential, i.e., the existence of a potential well can effectively hinder the recombination of photogenerated charge carriers in the ZrS2/B20 hybrid. The results suggest that the ZrS2/B20 hybrid would be a potential photocatalyst with high quantum efficiency.
In summary, we have studied the electronic structure, charge transfer, and optical properties of the ZrS2/B20 hybrid by using DFT calculation. It is found that the band gap and near-gap electronic structure of the monolayer ZrS2 can be tuned by the non-covalent modification of double-ring tubular B20 cluster. The interfacial charge transfer results into some positively charged Zr atoms and negatively charged S atoms in the hybrid, thus to be active sites, which are initially catalytically inert in the isolated monolayer ZrS2. The ZrS2/B20 hybrid exhibits dramatically enhanced absorption in the entire visible light due to its small band gap and unique near-gap electronic structure caused by interfacial interaction. These results suggest that not only the ZrS2/B20 hybrid would be an active photocatalyst or photodetector in the main part of the solar spectrum, and ever poor illumination of interior lighting, but also ring B20 cluster modification would be an effective strategy to tune the performance of monolayer TMDs.
This work was supported by the National Natural Science Foundation of China (Grant No. 51428101) and the Undergraduate Students Research Program of School of Physics and Electronic, Hunan University (No. USRP201609).
WQH and GFH proposed the work and revised the paper. YS calculated the first principles results and wrote the manuscript. HYW, HMY, KY, and PP have devoted valuable discussion. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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