Enhancement of the optical absorption of carbon group elements doped ZnS in the visible light range
Graphical abstract
Introduction
In order to solve the serious environmental problems and the depletion of fossil fuels effectively, a new form of energy has been searching for representing a viable alternative to the fossil fuels [1], [2], [3], [4]. Hydrogen (H2), as a clean, cheap, renewable energy has attracted more attention [5], [6]. Recently, photocatalytic water splitting under sun light by some catalysts is becoming a promising way for the generation of H2 [7], [8], [9]. In fact, the conversion of solar light into hydrogen through TiO2 photocatalytic from water splitting has been firstly reported by Fujishima and Hongda in 1972 [10]. More semiconductors, such as NaTaO3, KTaO3, K3Ta3B2O12 [11], [12], [13], [14], [15], have been reported as photocatalytic materials for the splitting water to produce hydrogen. Nowadays, more works have been devoted to enhancing the efficiency of the photocatalytic activity of these materials. In 2009, Valle et al. [16] found that the Zn concentration influenced the activity of Cd1-xZnxS for water splitting obviously. In the same year, Kudo and Miseki found that NaTaO3 shows a higher water splitting rates with the addition of NiO [17].
As one of the most important II-VI semiconductors [18], ZnS has a wide band gap of about 3.7 eV at room temperature [19]. Due to its high refractive index, high effective dielectric constant and wide wavelength band gap, ZnS has wide technological applications such as filters, reflectors and planar wave guide [20]. ZnS has been used to splitting water to produce hydrogen [21]. However, the wide energy band gap makes it cannot be effectively driven by solar light. Fortunately, the gap can be tuned by using various dopants. For example, Manzoor et al. [22] reported that the energy band gap of ZnS can be significantly reduced by doped Cu. Tang et al. [23] found that the doped-Pd can also result in similar effect for ZnS.
The visible light is the main component of the solar light reaching the surface of the earth. Therefore, the photocatalytic water splitting driven by visible light is one of the most important ways to utilize the solar energy. The absorption of ZnS in visible light range can also be enhanced by doping with various dopants. In 1993, Bhargava et al. [24] found that the quantum efficiency of Mn-doped ZnS increased and the optical properties were improved. In 2013, ZnS quantum dots as green nanophotocatalysts were successfully synthesized without the use of high temperature or any organic solvent by Rajabi et al. [25]. They found that the absorption of ZnS quantum dots in visible light and UV light range increased by doping with Fe3+. Moreover, the doped structure shows a higher photocatalytic activity in comparison with the pristine one. Recently, Prasad et al. [26] found that the photocatalytic activity of ZnS is significantly enhanced by doped Cu. However, the possibility to use them to split water remains unexplored. But in 2011, Bi-doped ZnS hollow spheres were successfully synthesized using a simple cation exchange method by Zhang et al. [27]. They found that the positions of the valence band maximum (VBM) and the conduction band minimum (CBM) for Bi-doped ZnS satisfied the requirements for H2-production, meanwhile, the UV and visible light photocatalytic activity for H2-production of ZnS was enhanced. The requirements for photocatalytic H2-production from water splitting will be stated in the following text.
Recently, (C, Sn, Pb)-doped ZnS has been successfully synthesized in experiment [28], [29], [30]. However, to the best of our knowledge, neither theoretical nor experimental investigation had been performed for carbon group elements doped ZnS to perform photocatalytic water splitting for hydrogen production, except for Pb-doped ZnS [30]. In the present work, the electronic and optical properties for the pristine and doped ZnS have been investigated based on the first principles density functional theory (DFT). The mechanism of the enhancement for carbon group elements doping ZnS in the visible light range is explored and the feasibility of carbon group elements doped ZnS to perform photocatalytic hydrogen production from water splitting has also been investigated.
Section snippets
Computational details
The doped structures have been constructed based on the 2 × 2 × 2 supercell of ZnS (including 64 atoms) with one Zn atom substituted by the carbon group elements. The doped structures are represented with C@Zn, Si@Zn, Ge@Zn, Sn@Zn, and Pb@Zn, respectively. All the structures have been fully optimized with the exchange and correlation potentials of Perdew–Burke–Ernzerhof (PBE) parameterization under the generalized gradient approximation (GGA). Due to the inherent error of the self-interaction,
Geometrical structures
The opted doped structures have similar lattice. By using VESTA [37], we plot and present the lattice of Pb@Zn as the sample in Fig. 1, and the other structures are omitted for the brevity. Table 1 lists the optimized lattice parameters, bond length and the energy band gap of these structures. From Table 1, one can find that our lattice parameters for the pristine ZnS are a = b = c = 5.436 Å, which is in excellent agreement with the experimental value [38], [39]. The changes of the lattice
Conclusion
In summary, we have investigated the effects of photocatalytic water splitting for carbon elements doping ZnS (C@Zn, Si@Zn, Ge@Zn, Sn@Zn, and Pb@Zn) using the first-principles DFT with the meta-GGA + MBJ potential. The results of the electronics properties demonstrate that the energy band gaps of all the doped structures decrease. However, the energy band gaps and levels of the VBM and CBM still satisfy the requirement of water splitting. Therefore, as the pristine ZnS, all the considered
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) under Grant Nos. NSFC-11374132 and NSFC-11574125, as well as the Taishan Scholars project of Shandong Province (ts201511055).
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