Heterometallic Benzenehexathiolato Coordination Nanosheets: Periodic Structure Improves Crystallinity and Electrical Conductivity

Abstract

Coordination nanosheets are an emerging class of 2D, bottom‐up materials having fully π‐conjugated, planar, graphite‐like structures with high electrical conductivities. Since their discovery, great effort has been devoted to expand the variety of coordination nanosheets; however, in most cases, their low crystallinity in thick films hampers practical device applications. In this study, mixtures of nickel and copper ions are employed to fabricate benzenehexathiolato (BHT)‐based coordination nanosheet films, and serendipitously, it is found that this heterometallicity preferentially forms a structural phase with improved film crystallinity. Spectroscopic and scattering measurements provide evidence for a bilayer structure with in‐plane periodic arrangement of copper and nickel ions with the NiCu2BHT formula. Compared with homometallic films, heterometallic films exhibit more crystalline microstructures with larger and more oriented grains, achieving higher electrical conductivities reaching metallic behaviors. Low dependency of Seebeck coefficient on the mixing ratio of nickel and copper ions supports that the large variation in the conductivity data is not caused by change in the intrinsic properties of the films. The findings open new pathways to improve crystallinity and to tune functional properties of 2D coordination nanosheets.

Two-Dimensional Metal–Organic Framework Acts as a Hydrogen Evolution Cocatalyst for Overall Photocatalytic Water Splitting

“…Moreover, we found that the coexistence of CoO x , an O 2 evolution cocatalyst, ,, could further enhance the photocatalytic activity for overall water splitting by NiBHT/SrTiO 3 :Al. The dual cocatalyst strategy with H 2 and O 2 evolution cocatalysts, such as RuO 2 /NiO, Pt/Co 3 O 4 , and CoOOH/RhCrO x , has been used widely to improve the photocatalytic activity for water splitting. ,,− CoO x was first deposited onto SrTiO 3 :Al (denoted as CoO x /SrTiO 3 :Al) by the photodeposition method, which was combined with NiBHT by the impregnation method (denoted as NiBHT/CoO x /SrTiO 3 :Al). The gas production rates of CoO x /SrTiO 3 :Al were inferior to NiBHT/SrTiO 3 :Al (Figure f), while NiBHT/CoO x /SrTiO 3 :Al (1 wt % NiBHT and 0.5 wt % CoO x as Co ions) boasted a 4-fold increase in water splitting activity over NiBHT/SrTiO 3 :Al.…”

“…From these results, we concluded that NiBHT functioned stably as a H 2 evolution cocatalyst in the photocatalytic water splitting reaction. Moreover, we found that the coexistence of CoO x , an O 2 evolution cocatalyst, 5,27,36 could further enhance the photocatalytic activity for overall water splitting by NiBHT/ SrTiO 3 :Al. The dual cocatalyst strategy with H 2 and O 2 evolution cocatalysts, such as RuO 2 /NiO, Pt/Co 3 O 4 , and CoOOH/RhCrO x , has been used widely to improve the photocatalytic activity for water splitting.…”

“…NiBHT was synthesized through a liquid−liquid interfacial coordination reaction between BHT and nickel(II) acetate [Ni(OAc) 2 ] (Scheme 1, experimental setup in Figure 1a) according to the previous method with modifications. 27 First, the saturated solution of benzenehexathiol (BHT) was made by dispersing BHT (6 mg) in degassed CH 2 Cl 2 (150 mL, 99.5%, Kanto Chemical Co., Inc.) in a glass beaker under a strict inert atmosphere. The CH 2 Cl 2 solution was then covered with degassed pure water (100 mL) to form a double layer.…”

“…22−26 NiBHT also features high electrical conductivity (∼100 S cm −1 ). 27 By combining NiBHT with aluminumdoped strontium titanate (SrTiO 3 :Al), a representative watersplitting photocatalyst, 28,29 one-step overall photocatalytic water splitting was achieved with H 2 and O 2 in a 2:1 stoichiometric ratio. A photocatalytic system (NiBHT/CoO x / SrTiO 3 :Al), where CoO x denotes cobalt oxide as an oxygen evolution reaction (OER) cocatalyst, featured a stable photocatalytic operation.…”

“…The Ni II (μ 2 -S 4 ) core is expected to undertake catalytic activities. In fact, electrocatalytic properties have been reported for the metal chalcogenolene-based 2D MOF series, including hydrogen evolution reaction (HER). − NiBHT also features high electrical conductivity (∼100 S cm –1 ) . By combining NiBHT with aluminum-doped strontium titanate (SrTiO 3 :Al), a representative water-splitting photocatalyst, , one-step overall photocatalytic water splitting was achieved with H 2 and O 2 in a 2:1 stoichiometric ratio.…”