P. Peng

Dual role of monolayer MoS2 in enhanced photocatalytic performance of hybrid MoS2/SnO2 nanocomposite

The enhanced photocatalytic performance of various MoS2-based nanomaterials has recently been observed, but the role of monolayer MoS2 is still not well elucidated at the electronic level. Herein, focusing on a model system, hybrid MoS2/SnO2 nanocomposite, we first present a theoretical elucidation of the dual role of monolayer MoS2 as a sensitizer and a co-catalyst by performing density functional theory calculations. It is demonstrated that a type-II, staggered, band alignment of ∼0.49 eV exists between monolayer MoS2 and SnO2 with the latter possessing the higher electron affinity, or work function, leading to the robust separation of photoexcited charge carriers between the two constituents. Under irradiation, the electrons are excited from Mo 4d orbitals to SnO2, thus enhancing the reduction activity of latter, indicating that the monolayer MoS2 is an effective sensitizer. Moreover, the Mo atoms, which are catalytically inert in isolated monolayer MoS2, turn into catalytic active sites, making the mo...

Band structure engineering of monolayer MoS2: a charge compensated codoping strategy

The monolayer MoS2, possessing an advantage over graphene in that it exhibits a band gap whose magnitude is appropriate for solar applications, has attracted increasing attention because of its possible use as a photocatalyst. Herein, we propose a codoping strategy to tune the band structure of monolayer MoS2 aimed at enhancing its photocatalytic activity using first-principles calculation. The monodoping (halogen element, Nd) introduces impurity states in the gap, thus decreasing the photocatalytic activity of MoS2. Interestingly, the NbMoFS codoping reduces the energy cost of doping as a consequence of the charge compensation between the niobium (p-dopant) and the fluorine (n-dopant) impurities, which eliminates the isolated levels (induced by monodopant) in the band gap. Most importantly, the NbMoFS codoped MoS2 has more active sites for photocatalysis. These results show the proposed NbMoFS codoped monolayer MoS2 is a promising photocatalyst or photosensitizer for visible light in the heterogeneous semiconductor systems.

Tuning near-gap electronic structure, interface charge transfer and visible light response of hybrid doped graphene and Ag3PO4 composite: Dopant effects.

The enhanced photocatalytic performance of doped graphene (GR)/semiconductor nanocomposites have recently been widely observed, but an understanding of the underlying mechanisms behind it is still out of reach. As a model system to study the dopant effects, we investigate the electronic structures and optical properties of doped GR/Ag3PO4 nanocomposites using the first-principles calculations, demonstrating that the band gap, near-gap electronic structure and interface charge transfer of the doped GR/Ag3PO4(100) composite can be tuned by the dopants. Interestingly, the doping atom and C atoms bonded to dopant become active sites for photocatalysis because they are positively or negatively charged due to the charge redistribution caused by interaction. The dopants can enhance the visible light absorption and photoinduced electron transfer. We propose that the N atom may be one of the most appropriate dopants for the GR/Ag3PO4 photocatalyst. This work can rationalize the available experimental results about N-doped GR-semiconductor composites, and enriches our understanding on the dopant effects in the doped GR-based composites for developing high-performance photocatalysts.

Electronic properties and photoactivity of monolayer MoS2/fullerene van der Waals heterostructures

van der Waals (vdW) heterostructures have attracted immense interest recently due to their unusual properties and new phenomena. Atomically thin two-dimensional MoS2 heterostructures are particularly exciting for novel photovoltaic applications, because monolayer MoS2 has a band gap in the visible spectral range and exhibit extremely strong light–matter interactions. Herein, first-principles calculations based on density functional theory is used to investigate the effects of vdW interactions on changes in the electronic structure, charge transfer and photoactivity in three typical monolayer MoS2/fullerene (C60, C26, and C20) heterostructures. Compared to monolayer MoS2, the band gap of the heterostructures is smaller, which can enhance the visible light absorption and photoinduced electrons transfer. The amount of charge transfer at interface induced by vdW interaction depends on the size of fullerenes. Most importantly, a type-II, staggered band alignment can be obtained in the MoS2/C20 heterostructure, leading to significantly reduced charge recombination and thus enhanced photocatalytic activity. These results reveal that fullerene modification would be an effective strategy to improve the photocatalytic performance of semiconductor photocatalysts.

Hybrid TiO2/graphene derivatives nanocomposites: is functionalized graphene better than pristine graphene for enhanced photocatalytic activity?

Graphene (GR) and its derivatives are generally assumed to be electron shuttles in order to explain the improved photocatalytic activity of their nanocomposites (such as TiO2/GR). However, it fails to account for the experimental results, which demonstrate that the photocatalytic activity of TiO2/reduced graphene oxide (RGO) is higher than that of TiO2/GR. Herein, we explore the underlying mechanism for the enhanced photocatalytic activity of TiO2/RGO (GR) by comparing several influential factors: band gap, band alignment near the gap, optical absorption, and active sites, via first-principles calculations. The results show that the small band gap, the type-II staggered band alignment, and the negatively charged O atoms as active sites in photocatalytic reactions are likely to be key factors for the photocatalytic activity of TiO2/RGO being better than that of TiO2/GR, partly offering a physical interpretation for related experimental results. Interestingly, the enhanced photocatalytic activity of TiO2/graphane (GRH) is also predicted. These results suggest that functionalized GR is most likely better than pristine graphene at improving the photocatalytic activity of TiO2/GR-based semiconductor photocatalysts.

Native vacancy defects in bismuth sulfide

Bismuth sulfide (Bi2S3) exhibits excellent photocatalytic activity under visible light. We perform first-principles, density-function theory (DFT) calculations of the electronic structure for the Bi2S3 with native vacancy to facilitate its applications by gaining insight into the role of native defects. We find that the Bi vacancies are effective p-type defects for Bi2S3, while the S vacancies induce an intermediate level appearing in the band gap. Besides one Bi vacancy, the native vacancy defect at other four inequivalent positions in Bi2S3 leads to a reduction of band gap. Moreover, the change of band gap depends on the position of native vacancy defect. The results indicate that the native defects are the most likely physical cause for the scattered band gaps obtained by experiments. The influence of native vacancy defects on the photocatalytic properties of Bi2S3 is also discussed.

THE ELECTRONIC AND OPTICAL PROPERTIES OF X-DOPED SrTiO3 (X = Rh, Pd, Ag): A FIRST-PRINCIPLES CALCULATIONS

The electronic and optical properties of X-doped (X = Rh, Pd, Ag) cubic SrTiO3 in perovskite structure are investigated using first-principles calculations. The strength of the Ti–O bonds near the substitutional X impurity is found to be weakened by the shorter X–O bonds. Three types of electronic characteristics due to X-doping are demonstrated. X-doping decreases the band gap of SrTiO3, extending the optical absorption edge to visible light. Although Pd-doped SrTiO3 has the greatest absorption in the visible light region, its photocatalytic activity is lower than that of Rh-doped SrTiO3, because the intermediate bands from the 4d orbitals of the Pd dopant act as recombination centers. The theoretical results coincide with the available experimental results.

Interfacial Interactions in Monolayer and Few-Layer SnS/CH3NH3PbI3 Perovskite van der Waals Heterostructures and Their Effects on Electronic and Optical Properties

A high light-absorption coefficient and long-range hot-carrier transport of hybrid organic-inorganic perovskites give huge potential to their composites in solar energy conversion and environmental protection. Understanding interfacial interactions and their effects are paramount for designing perovskite-based heterostructures with desirable properties. Herein, we systematically investigated the interfacial interactions in monolayer and few-layer SnS/CH3 NH3 PbI3 heterostructures and their effects on the electronic and optical properties of these structures by density functional theory. It was found that the interfacial interactions in SnS/CH3 NH3 PbI3 heterostructures were van der Waals (vdW) interactions, and they were found to be insensitive to the layer number of 2D SnS sheets. Interestingly, although their band gap decreased upon increasing the layer number of SnS, the near-gap electronic states and optical absorption spectra of these heterostructures were found to be strikingly similar. This feature was determined to be critical for the design of 2D layered SnS-based heterostructures. Strong absorption in the ultraviolet and visible-light regions, type II staggered band alignment at the interface, and few-layer SnS as an active co-catalyst make 2D SnS/CH3 NH3 PbI3 heterostructures promising candidates for photocatalysis, photodetectors, and solar energy harvesting and conversion. These results provide first insight into the nature of interfacial interactions and are useful for designing hybrid organic-inorganic perovskite-based devices with novel properties.

Dramatically Enhanced Visible Light Response of Monolayer ZrS2 via Non-covalent Modification by Double-Ring Tubular B20 Cluster

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.

Dual functions of 2D WS2 and MoS2?WS2 monolayers coupled with a Ag3PO4 photocatalyst

The photocatalytic performance of semiconductors can be improved by coupling two-dimensional (2D) layered materials. Understanding the underlying mechanism of this phenomenon at the electronic level is important for the development of photocatalysts with a high efficiency. Here, we first present a theoretical elucidation of the dual functions of 2D layered material as a sensitizer and a co-catalyst by performing density functional theory calculations, taking WS2 and a lateral heterogeneous WS2–MoS2 monolayer as examples to couple with a promising photocatalyst Ag3PO4. The band alignment of a staggered type-II is formed between Ag3PO4 and the 2D monolayer with the latter possessing the higher electron affinity, resulting in the robust separation of photoexcited charge carriers between them, and indicating that the 2D monolayer is an effective sensitizer. Interestingly, the W (Mo) atoms, which are catalytically inert in the isolated 2D monolayer, turn into catalytic active sites, making the 2D monolayer a highly active co-catalyst in hybrids. A better photocatalytic performance in the coupled lateral heterogeneous WS2–MoS2 monolayer and Ag3PO4 can be expected. The calculated results can be rationalized by available experiments. These findings provide theoretical evidence supporting the experimental reports and may be used as a foundation for developing highly efficient 2D layered materials-based photocatalysts.