ArticleHeptanuclear brucite disk with cyanide bridges in a cocrystal and tracking its pyrolysis to an efficient oxygen evolution electrode
Graphical abstract
The [CoII7(μ3-CN)6(mmimp)6][CoIICl3N(CN)2]·3CH3OH co-crystal containing the unprecedented µ3-CN bridged Brucite disk forms core-shell Co@NC nanostructures, where TG-MS technique was used to determine the best pyrolysis condition in inert atmosphere. The optimized samples treated at 600 °C realize enhancement of the OER with ultralow an overpotential of 257 mV.
Introduction
The construction of material systems through precise chemical synthesis and design in molecular level is a unified dream of chemists and materialists [1], [2], [3], [4], [5]. As a hub for energy conversion, electrocatalytic materials are impetus in renewable energy conversion with intriguing applications [6], [7], [8], [9], [10]. Therefore, how to precisely regulate the electrocatalyst by chemical control has attracted much attention [11], [12], [13], [14]. Among electrocatalytic materials, nanocomposites have demonstrated fascinating advantages and made important progress, especially the as-resulted core-shell nanostructures with synergic effect on high surface areas and electroactive sites [15], [16], [17], [18]. Of note, metal–organic frameworks (MOFs) and simple organic-inorganic composites as precursors under proper thermal decomposition are an effective way to prepare electrocatalytic materials with core-shell nanostructures [18], [19], [20], [21], [22], [23]. Coordinative cluster has displayed signs of success as potential precursor, with outstanding advantage for achieving subtly regulation on precursor structure through control on the number and kinds of metal ions, metal ion coordination geometry, peripheral ligand, inner bridge ligand, and inter/intra-cluster interaction. Compared with MOFs, the 0D coordination clusters are isolated molecular where the metal core is well surrounded by multiply peripheral ligands. The simultaneous carbonizing of the peripheral ligands during pyrolysis of the cluster promotes the effective carbon cladding and segregation of the metal core, avoiding interference from framework connection leading complicated pyrolysis. In addition, the existence of inner bridge in cluster, like CN−, CSN−, N3− and others, provide additional chance to induce in-situ N-doping, which is beneficial to improvement of electrocatalytic activity [24], [25], [26]. However, studies on the precursors of coordination molecular clusters have yet not been reported. Meanwhile, although pyrolysis has been used as a general method for the preparation of electrocatalytic materials, the mechanism of its pyrolysis process has been few studied systematically [27], [28]. In this regard, it is urgent to explore and track the mechanism of pyrolysis process, as well as to design a precursor at the molecular level to achieve control and optimization of derivative electrocatalysts.
In our previous work, we have been interested in designing different types of MOFs accurately, and finally realized their functional regulation through post-synthesis modification, such as catalysis, conduction and magnetism [29], [30], [31]. Furthermore, we have been engaged in the assembly process and mechanism of coordination molecular clusters and their post-synthesis modification or thermal transformation regulation [32], [33], [34]. For some special systems, reasonable design and controlled synthesis can be implemented to control the formation of different internal bridges, different metal ions, complex counter ions and co-crystal clusters [35], [36], [37]. In particular, a step-by-step assembly process at room temperature was found when Schiff bases were used as the key chelating ligands, which led to co-crystal heptanuclear disk clusters starting from the Ni1 via Ni2, Ni4 and Ni7+1 intermediate species [38]. Driven by above mentioned experience, we recently design a 3-MeOsalophen-ligated cobalt complex as a precursor to obtain a Co@CoOx-NC core-shell nanostructures with decent oxygen evolution reaction (OER) performance [39]. Therefore, these work bases inspire us to investigate the pyrolysis process of well-designed coordination molecular clusters to obtain a core-shell nanostructure with high electrochemical activity.
In this work, we have chosen the N, O-double chelating Schiff base ligand to design and synthesis a brucite cationic disk Co7 cluster with discrete [CoCl3N(CN)2]2− anion. And for the first time, the introduction of the μ3-CN− inner bridge was achieved by in-situ decomposition of dicyanamide ions under solvothermal reaction. Interestingly, the rarely observed μ3-CN bonded through the nitrogen was the bridging ligand at the core of the disk. However, thermal analysis proved to be more interesting with the observation of nanoparticles consisting of cubic cobalt core and N-doped carbon (Co@NC). Moreover, TG-MS technique was used to study the pyrolysis process and revealed the substituents on the ligand and be cleaved to generate a large number of active sites promoting the polymerization and retention of the organic fragments for forming the core–shell nanostructure. Here, we detail the exceptional properties and provide evidence of the above conclusion.
Section snippets
Synthesis of Co7+1
CoCl2·6H2O (1 mmol, 0.238 g) and NaN(CN2) (1 mmol, 0.089 g) was added to a solution of the Hmmimp (1 mmol, 0.165 g) in CH3OH (15 mL). Then 0.1 mL triethylamine was added and the mixture was stirred for 10 min before being placed in a 23 mL Teflon-lined autoclave and heated at 80 °C for 2 d. The autoclave was then cooled over a period of 3 h at a rate of 20 °C/h, and the dark green crystals of Co7+1 were collected by filtration, washed with CH3OH and dried in air. Yield (based on Co): ca. 38.7%.
Results and discussion
The novelty of [Co7(mmimp)6(CN)6] [CoCl3N(CN)2]·3CH3OH is the first presence of μ3-CN− bridges at the centre of the Brucite cationic disk (Fig. 1), in place of the usual μ3-OH, -OCH3 or -N3 with retention of the planarity [35], [37]. The cyanide exhibits the very rare bonding via the nitrogen atom. The second chemical variation is the co-crystallization with an anion of a cobalt complex. The third is a structural point where the flat cations are not stacked as found in the trigonal symmetry
Conclusions
In summary, a novel co-crystalline cluster [CoII7(μ3-CN)6(mmimp)6] [CoIICl3N(CN)2]·3CH3OH having a unique Brucite disk with in-situ generated μ3-CN bridge was obtained and packed orthogonally. The decomposition of dicyanamide avoids the direct use of highly toxic cyanide. Its low oxygen content proceeds with a gradual thermal decomposition under inert atmosphere to Co@NC nanostructures, which is tracked and systematically analysed by TG-MS. The products, identified by microscopy and
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China for Distinguished Young Scholars (21525101), the BAGUI Talent Program and Scholar Program (2014A001), the National Natural Science Foundation of China (21805074 and 21661008), the Natural Science Foundation of Hubei Province (2017CFA006 and 2018CFB151), the Natural Science Foundation of Guangxi Zhuang Autonomous Region (2017GXNSFDA198040). MK is supported by the Centre National de la Recherche Cientifique (CNRS, France).
Author contributions
Ming-Hua Zeng conceived the idea, co-wrote the paper and supervised the whole experimental procedure and data analysis. Mohamedally Kurmoo and Xu Peng edited the manuscript. Jian-Qiang Zhao, Dandan Cai, Jun Dai and Xu Peng performed the experiments, analyzed the data and wrote the manuscript. All the authors discussed the results, commented on and revised the manuscript.
Jian-Qiang Zhao joined Prof. Ming-Hua Zeng’s group as a Master’s student at Guangxi Normal University and has received his M.S. degree in 2019. His research is focused on 3d-coordination clusters, especially their synthesis and pyrolysis leading to nanomaterials with electrocatalysis properties.
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Jian-Qiang Zhao joined Prof. Ming-Hua Zeng’s group as a Master’s student at Guangxi Normal University and has received his M.S. degree in 2019. His research is focused on 3d-coordination clusters, especially their synthesis and pyrolysis leading to nanomaterials with electrocatalysis properties.
Xu Peng obtained his Ph.D. degree in Inorganic Chemistry at University of Science and Technology of China in 2017. He currently works at College of Chemistry and Chemical Engineering as associate professor, Hubei University. His research is focused on controlled pyrolysis of the low-dimensional 3d metal solid for their applications in energy storage and conversion areas.
Ming-Hua Zeng obtained his Ph.D. degree from Sun Yat-sen University in 2004. Then he started his individual research in Guangxi Normal University and obtained the position of full professor in 2006. His earlier research focused on the synthesis, properties, structural transformation and post-synthetic modification of coordination clusters and metal–organic frameworks. Currently his interest has turned to the coordination molecular cluster chemistry in solution as well as their assembly processing, mechanism and application.