| Nowadays, the world faces two big problems that are the energy crisis and environmental pollution. The solar energy has attracted extensive attentions as the clean and renewable energy. Photocatalytic water-splitting to generate hydrogen utilizing solar energy and solar cells are the important ways to develop and take full advantage of the solar energy. For photocatalytic water-splitting, the semiconductor based photocatalyst determines the photocatalytic activity. The ideal semiconductor photocatalyst should satisfy many requirements such as good stability, suitable band gap and matched band edge with the water redox potentials et al. However, very few semiconductors can satisfy these requirements. Searching and engineering efficient and stable semiconductor photoeatalyst is a hot research topic. For solar cells, dye-sensitized solar cells (DSSC) have extensively investigated due to its simple process, low price and high efficiency. To enhance the power conversion efficiency of DSSC, we should develop high efficient photoanode semiconductive film and stable and broad spectral responsive sensitized dyestuff. Based on the density functional theory (DFT) calculations, we have studied stability, band structure and other properties of ZnO and MoS2. To achieve their application in photocatalytic hydrogen generation and DSSC, we engineer their band structure by doping and surface ligand functionalization. The main conclusions are summarized below:(1) Band structure engineering of ZnO by anion-cation codoping for enhanced photocatalytic activity. The optical absorption of ZnO is only limited in ultraviolet region due to its too wide band gap. To improve the photocatalytic activity, we proposed anion-cation codoping to narrow the band gap of ZnO and enhance its optical absorption in visible light region. The anion-cation codoping can not only effectively reduce the band gap of ZnO via introducing the dopant levels, but also enhance the stability stemming from the strong electrostatic attraction between the n-type and p-type dopants. Furthermore, the calculated results show that the compensated (Ti+C) and noncompensated (Sc+C) and (Cr+C) codoped ZnO are considered as the strong candidates for photocatalytic water-splitting because of their narrowed band gaps, potentially reduced electron-hole recombination centers, proper band edge positions, enhanced optical absorption and good stability.(2) Band structure engineering of ZnO by isovalent anion-cation codoping for enhanced photocatalytic activity. Based on previous work, we found that the dopant levels introduced by monodoping and noncompensated codoping might form new electron-hole recombination centers, which would reduce the photocatalytic activity. To solve this problem, we propose isovalent anion-cation codoping. This method can not only effectively reduce the band gap of ZnO, but also can not form deep localized levels and partially occupied impurity levels and therefore suppresses the electron-hole recombination. More importantly, the (Cd+Te) codoped ZnO may be considered as a compelling candidate for photocatalytic water-splitting in virtue of its suitable band gap and matched band edge positions for water redox. This isovalent codoping approach may be applied in other wide-band-gap semiconductors to engineering their band structure.(3) Achieved photocatalytic hydrogen production activity of Monolayer MoS2 by surface ligand functionalization. Monolayer MoS2 (ML-MoS2) has a direct band gap of 1.8 eV, which is ideal for solar energy absorption. However, the band minimum (CBM) of ML-MoS2 is below the hydrogen redox potential of water, which limits its application in photocatalytic water-splitting. Our calculations show that surface ligand functionalization can alter the positions of the band edges. The band shifts come from the intrinsic dipole of ligand itself and the induced dipole at the interface of ligand/MoS2. The ligand coverage, ligand functionalization and the substrate have great influence on the band edges of MoS2. More importantly, the hybrid C6H5CH2NH2/MoS2/graphene structure may be compelling candidate as they satisfy stringent requirements of photocatalytic water-splitting.(4) The application of organic dye molecule adsorbed MoS2 in dye-sensitized solar cells. To investigate the possibility of MoS2 applying in dye-sensitized solar cells, we choose the suitable organic dye molecule to adsorb on the surface of the monolayer MoS2. The calculated results show that three phenyl-conjugated oligoene dyes D1, D2 and D3 can effectively adsorb on MoS2, their LUMO levels are matched with the CBM of MoS2, which ensure the charge injection. The power conversion efficiency of these organic dye molecules sensitized MoS2 is as high as 17%. The presence of graphene can not only enhance the stability of monolayer MoS2, but also improve photoinduced charge-separation. More importantly, the hybrid D3/MoS2/graphene structure is considered as compelling candidate of dye-sensitized solar cells because it has good stability, matched band structure, reduced carrier recombination and high power conversion efficiency. Our calculations can provide theoretical support for MoS2-based photoanode of dye-sensitized solar cells. |
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