Protonation‐Assisted Exfoliation of N‐Containing 2D Conjugated Polymers

Abstract

natural incorporation into the conjugated polymer frameworks mainly comprised of carbon atoms. [8][9][10][11] Consequently, the optoelectronic properties can be conveniently tailored in N-containing 2D conjugated polymers through different synthetic routes using different organic building blocks.On the other hand, compared to bulk 2D polymers, ultrathin nanosheets (<10 nm in thickness) are highly favorable for photocatalytic applications as the ultrathin feature offers several unique advantages in photocatalysis including more exposed active sites, enhanced photoresponsiveness and efficient charge separation. [12][13][14][15] Moreover, the ultrathin structure also enables facile integration with other materials to form heterostructures or hybrids to further regulate the electronic structure and charge separation, leading to enhanced photocatalytic performances. [16][17][18][19] Exfoliation of layered synthetic polymers is a formidable route for the scalable and efficient production of ultrathin 2D polymer nanosheets. [20][21][22][23] Unfortunately, direct exfoliation of 2D polymers still leads to low yields (normally the yield is <15%), which impedes further applications of ultrathin 2D polymer nanosheets. [24] To circumvent this problem, a more reliable exfoliation route is highly desired.The key to improving the exfoliation yield is to either weaken the interlayer interactions in layered polymers or enhance the affinity between the polymer surfaces and the dispersion media. [25][26][27] To tailor the interlayer interactions, guest substances such as concentrated H 2 SO 4 are intercalated into the interlayers of 2D polymers. [28] Although this method can achieve high yields, sometimes the chemical structures of the Ultrathin 2D conjugated polymer nanosheets are an emerging class of photocatalysts for solar-to-chemical energy conversion. Until now, the majority of ultrathin 2D polymer photocatalysts are produced through exfoliation of layered polymers. Unfortunately, it still remains a great challenge to exfoliate layered polymers into ultrathin nanosheets with high yields. In this work, a liquid-phase protonation-assisted exfoliation is demonstrated to enable remarkably improved exfoliation yields of various 2D N-containing conjugated polymers such as g-C 3 N 4 , C 2 N, and aza-CMP. The exfoliation yields are only 2-15% in pure water whereas they can be substantially improved to 41-56% in 12 m HCl. The exfoliated ultrathin nanosheets possess average thicknesses less than 5 nm and can be easily dispersed in aqueous solutions. More importantly, the exfoliated nanosheets exhibit significantly enhanced photocatalytic activity toward photocatalytic water splitting compared to their bulk counterparts. Further characterizations and computational calculations reveal that protonation of the heterocyclic nitrogen sites in the conjugated polymer frameworks can lead to strong hydrogen bonding between the polymer surfaces and water molecules, resulting in facilitated exfoliation of polymers into the liquid phase. This study ...

Recent Advances in Manifold Exfoliated Synthesis of Two‐Dimensional Non‐precious Metal‐Based Nanosheet Electrocatalysts for Water Splitting

“…The unique advantages of 2D NPMB nanosheet catalysts are concluded as follows: 1) the ultrathin 2D nanosheet structure with atomic thickness and large specific surface has the maximum possible numbers of exposed active sites; meanwhile, the exposed numbers of active sites is comparable to that of the whole atoms of monolayer [ 3,53,77 ] ; 2) the ultrathin 2D nanosheet structure, with numerous defects and unsaturated metal sites that arise from the delamination process, can facilitate the adsorption and desorption of reaction intermediates of water splitting; [ 48,78–80 ] 3) the ultrathin 2D nanosheet structure, with unique electronic structures of atom level, shortens the electron‐transfer path and reduces the free energy of elemental steps in HER and OER processes; [ 15,55,81,82 ] 4) the ultrathin 2D nanosheet structure, with marvelous accessible surface area, can serve them as the ideal platforms to combine other catalytic materials, to further adjust the states of electronic structures, so as to boost electrochemical behavior, i.e., heterostructure built with different unilamellar catalysts that own the diverse intrinsic advantages. [ 55,83–85 ] Besides, the ultrathin 2D nanosheet structure with atomic thickness is also an effective model for studying the real active species of NPMB nanosheet catalysts during the electrocatalytic reaction process, especially for the OER process in alkaline media. As can be seen from recent related examples, [ 86–92 ] the dynamics of electrocatalytic process of catalysts strongly depend on the external structure (morphology, crystallization, etc.)…”

2D Polymer Nanosheets for Membrane Separation

“…In order to address this issue, Zhang's group developed a protonation‐assisted LPE method to improve the exfoliation yield of various N‐containing 2D polymer nanosheets from only 2–15% in pure water to 41–56% in 12 m HCl. [ 29 ] The achieved ultrathin nanosheets had an average thickness of less than 5 nm, showing good dispersion in aqueous solutions. The high yield could be attributed to the fact that the presence of concentrated HCl would lead to the protonation of the N‐containing sites within the frameworks, which can produce strong electrostatic repulsion effect between adjacent layers and form hydrogen bonds with water molecules so that the neighboring layers within the bulk polymer could be easily exfoliated into ultrathin nanosheets.…”

CxNy: New Carbon Nitride Organic Photocatalysts

“…In 2019, X. Zhang et al reported 41%-56% exfoliation yields for C 3 N 4 , C 2 N, and aza-CMP in acid media (Zhang et al, 2019). In their work, C 2 N was also synthesized using HAB and HKH in NMP.…”