Date Published: January 30, 2019
Publisher: John Wiley and Sons Inc.
Author(s): Juan Su, Guo‐Dong Li, Xin‐Hao Li, Jie‐Sheng Chen.
2D layered materials with atomic thickness have attracted extensive research interest due to their unique physicochemical and electronic properties, which are usually very different from those of their bulk counterparts. Heterojunctions or heterostructures based on ultrathin 2D materials have attracted increasing attention due to the integrated merits of 2D ultrathin components and the heterojunction effect on the separation and transfer of charges, resulting in important potential values for catalytic applications. Furthermore, 2D/2D heterostructures with face‐to‐face contact are believed to be a preferable dimensionality design due to their large interface area, which would contribute to enhanced heterojunction effect. Here, the cutting‐edge research progress in 2D/2D heterojunctions and heterostructures is highlighted with a specific emphasis on synthetic strategies, reaction mechanism, and applications in catalysis (photocatalysis, electrocatalysis, and organic synthesis). Finally, the key issues and development perspectives in the applications of 2D/2D layered heterojunctions and heterostructures in catalysis are also discussed.
The definition of heterojunction, originally developed from the semiconductor–semiconductor (S–S) junction, has now been extended to the scope of metal–semiconductor (M–S) junction and even untypical heterostructures of semiconductors and ionic conductors.1, 2 Generally speaking, the coupled interface of two components in a heterojunction could generate a band alignment or a rectifying contact after the equilibration of the Fermi levels (or work functions) at the interface according to the Anderson’s rule or Schottky–Mott rule for S–S or M–S junctions, respectively.1, 2, 3 There is a consensus in the literature that the relocalization of charge carriers at the interface of heterojunctions may facilitate the catalytic performance of the as‐fabricated materials or devices.1, 2, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18
Since 2D materials successfully synthesized by mechanical exfoliation, an increasing number of synthetic methods, like chemical exfoliation and chemical vapor deposition (CVD) methods, have been developed to obtain 2D materials with various nanostructures. For example, there are more opportunities to obtain 2D materials with high crystallinity but small‐scale production through mechanical exfoliation method; chemical exfoliation methods such as electrochemical40 and solvents assistance28, 41 exfoliation are apt to realize large‐scale production of 2D materials but without precise control of lateral size and thickness; and chemical vapor deposition method can form monolayer 2D materials42 and is also suitable for their wafer‐scale synthesis.43 Thanks to the synthetic variety of 2D materials, a wide variety of building blocks and techniques are applied to fabricate 2D/2D heterostructures. Here, the synthesis schemes of 2D/2D heterostructures are categorized as “ex situ assembly methods” and “in situ growth methods,” which are introduced in detail as following.
Besides the influence of compositions and dimensionality of constituent 2D materials, the highly coupled interface, especially based on ultrathin 2D materials, always plays key role in facilitating the catalytic performance of 2D/2D heterostructures via electron relocalization induced i) ultrafine nanostructure or interfacial defects and thus novel active sites; ii) band bending and thus enhanced redox power of active sites; iii) high‐speed electron transfer path at interface.
Although the materials in their 3D forms have long been used as catalysts, the materials with 2D layered structure exhibit considerable changes in their electronic structure and offer new opportunities for chemical/structural modification and catalytic reactions.4, 23, 24, 25, 26, 27 Design of heterostructures based on 2D layered materials is one of the promising routes for flexibly controlling materials’ chemical reactivity, which would enhance catalytic performance and provide new possibilities in catalysis through the heterojunction effect on the transfer and separation of charge at interface.4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 29, 60, 61, 62 Over the past decade, 2D/2D layered heterostructures have been investigated and applied in a variety of catalytic reactions, such as photocatalysis, electrocatalysis, and organic synthesis.
In summary, this review highlights the general strategies and recent progress in the 2D/2D heterojunctions and heterostructures, which are excellent candidates for fundamental research and potential catalytic applications due to their unique structural physicochemical and electronic properties: i) integrating the merits of their components, such as 2D ultrathin structure, large surface area, and physicochemical/electronic properties; ii) the separation or transfer of charges can be promoted for desirable properties or functions; and iii) versatile options (including component, thickness, defect, fabrication technology, contact distance, etc.) can be designed to tune properties and thus impact application performance. Driven by the above‐mentioned advantages and potential values of 2D/2D heterostructures, increasing amounts of remarkable achievements have been made in the past few years. Nevertheless, in light of the challenges remaining in practical applications of catalysis, e.g., catalytic efficiency, selectivity associated with yields and pollution, environmentally friendly, and cost, there is still much work to pursue with the aims of developing desired catalyst and reaction system for practical applications.
The authors declare no conflict of interest.