Hydrothermal Synthesis of Highly Crystalline Zwitterionic Vinylene-Linked Covalent Organic Frameworks with Exceptional Photocatalytic Properties

Abstract

Ionic covalent organic frameworks (COFs) featuring both crystallinity and ionic characteristics have attracted tremendous attention in recent years. Compared with single anionor cation-containing ionic COFs, zwitterionic COFs possess unique functionalities beyond single ionic COFs such as tunable charge density and superhydrophilic and highly ion-conductive characteristics, endowing them with huge potential in various applications. However, it remains a considerable challenge to directly synthesize robust, highly crystalline zwitterionic COFs from the original building blocks. Herein, we report a green hydrothermal synthesis strategy to prepare highly crystalline zwitterionic vinylene-linked COFs (ZVCOFs) from the predesigned zwitterionic building block by utilizing 4-dimethylaminopyridine (DMAP) as the high-efficiency catalyst for the first time. Detailed theoretical calculations and experiments revealed that both the high catalytic activity of DMAP and the unique role of water contributed to the formation of highly crystalline ZVCOFs. It was found that the participation of water could not only remarkably reduce the activation energy barrier and thus enhance the reaction reversibility but also enable the hydration of zwitterionic sites and facilitate ordered layered arrangement, which are favorable for the ZVCOF crystallization. Benefiting from the highly π-conjugated structure and hydrophilic characteristic, the obtained ZVCOFs achieved an ultrahigh sacrificial photocatalytic hydrogen evolution rate of 2052 μmol h −1 under visible light irradiation with an apparent quantum yield up to 47.1% at 420 nm, superior to nearly all COF-based photocatalysts ever reported. Moreover, the ZVCOFs could be deposited on a support as a photocatalytic film device, which demonstrated a remarkable photocatalytic performance of 402.1 mmol h −1 m −2 for hydrogen evolution.

Ionic Covalent Organic Frameworks in Adsorption and Catalysis

“…Ionic COFs with polar skeletons or chains are suitable for overcoming these limitations since features as below: (1) the elaborated modulation of band gaps which can regulating the holeelectron separation; (2) the high degree of polarization improve the electron and charge transfer; (3) ionic groups can enhance the adsorption and accessibility of reactants. [46] In 2018, Zang et al encapsulated anionic clusters (Mo 3 S 13 ) 2À in EB-COF through the electrostatic attraction and open channels for the photocatalytic HER. [47] The porous framework provided the accessibility of catalytic sites, while the interaction between cationic sites and anionic cluster ensured the full encapsulation in channels which can enhance the mass transport.…”

Polymer photocatalysts for photocatalytic hydrogen peroxide production

“…4,[49][50][51][52][53] Many polymer semiconductors can be obtained as nanosheets with atomic layer thickness through exfoliation. The ultrathin nanosheets facilitate the exposure of active sites and carrier separation, [54][55][56][57] thereby achieving high quantum efficiency.…”

Molecular Engineering for Modulating Photocatalytic Hydrogen Evolution of Fully Conjugated 3D Covalent Organic Frameworks

“…The fully conjugated COFs not only have the general merits of normal ones, but also have the extended π-conjugated structures that benefit for the charge delocalization and transport. These merits endow this category of COFs exceptional semiconducting properties, [5] magnetism, [6] sensing capabilities, [3] as well as electro-/photocatalysis, [7] etc. Therefore, fully conjugated COFs exhibit immense potential in various fields such as electronics, energy storage/conversion, and environmental fields.…”