Delicate topotactic conversion of coordination polymers to Pd porous nanosheets for high-efficiency electrocatalysis
Graphical abstract
Introduction
As an important member of two-dimensional (2D) nanomaterial family, noble metal-based nanosheets have recently aroused tremendous research interests due to their unusual electronic structures and physiochemical properties [[1], [2], [3], [4], [5]]. As a consequence of 2D confinement and surface effect, noble metal-based nanosheets show great promising applications in various fields, including electrocatalysis [[6], [7], [8]], photothermal therapy [9,10], heterocatalysis [11,12], gas sensor [13,14], and memory devices [15], etc. Stimulated by these extraordinary properties, tremendous efforts have been devoted to preparing 2D noble metal-based nanosheets by different synthetic approaches, including self-assembly process [[15], [16], [17]], hard template-engaged method [18,19], capping agent-confined synthesis [7,20,21], photochemical synthesis [22,23], 2D oriented attachment [24,25], and so forth. However, the majority of the as-fabricated noble metal nanosheets appear as rigid nanostructure with smooth and intact surface, which tend to irreversibly restack during device fabrication and materials processing, resulting in a great loss of accessible surface atoms (Fig. 1a). [13,26] Such phenomenon would inevitably lead to a detrimental impact on the utilization efficiency of noble metal atoms, especially in catalysis applications. In this regard, the porous nanosheets with rough surface and rich corner/edge atoms offer a very promising strategy for achieving outstanding catalytic performance in view of their remarkable features related to the high-efficiency utilization of both interior and exterior surface atoms as well as permeable channels for mass transport (Fig. 1b). To this end, rational synthesis of porous noble metal-based nanosheets is therefore highly demanded, yet still remains challenging.
Nanostructured coordination polymers (CPs) have been extensively served as versatile precursors to achieve various porous inorganic functional nanomaterials with controllable morphologies and tunable compositions which exhibit promising potentials in energy conversion and storage devices. [[27], [28], [29], [30], [31]] Nevertheless, the previously reported CP-derived nanomaterials mainly comprise porous carbon, transition metal oxides [29,32], chalcogenides [33] and phosphides [34], etc, rarely involving noble metal-based nanosheets. For the CP-derived fabrication of noble metal-based nanosheets, the synthetic challenges essentially lie in the following two aspects: (1) Due to the complicated coordination manners between different metal ions and organic bridged ligands, it is still difficult to precisely synthesize noble metal-based CP nanosheets through a universal approach. (2) It is also difficult to perfectly maintain the 2D sheet-like nanostructure during the topotactic conversion from CP precursor to target noble metal because of the thin thickness of the CP precursor. Among various CPs, Hofmann-type CP, M(L)nM'(CN)4⋅G (M = transition metal ions, L = monodentate or bidentate ligand, M' = Ni, Pd or Pt, and G = guest molecules), consists of 2D extended metal cyanide sheets made up of square-planar [M'(CN)4]2− centres equatorially connected with octahedral M(II) sites. [35,36] Due to the intrinsic anisotropy, Hofmann-type CP tends to form a 2D sheet-like nanostructure [37,38]. Furthermore, the removal of ligand from the parent Hofmann-type CP could effectively create numerous nanopores within its derived product. Therefore, it is envisaged that noble metal-based CP nanosheets would be ideal precursors for the fabrication of noble metal-based porous nanosheets.
As a proof of concept, herein we demonstrate a novel CP nanosheet-engaged approach to synthesize porous Pd nanosheets through sequential topotactic conversions for boosting the electrocatalysis of small molecules (Fig. 2). A Hofmann-type CP of Pd(H2O)2[Ni(CN)4]⋅xH2O with square plate-like nanostructure was firstly synthesized through a feasible hydrothermal approach, and used as template to regulate the morphology and size of target product. Afterwards, the pre-fabricated CP nanoplates were readily transformed into porous Pd-Ni oxide nanoplates upon a mild calcination. Finally, the as-synthesized porous Pd-Ni oxide nanoplates were subject to the repeated cyclic voltammetry (CV) treatments in 0.5 M H2SO4 solution to selectively dissolve NiO species and simultaneously electrochemically reduce the Pd oxide species into metallic Pd, resulting in the formation of porous Pd nanosheets. Remarkably, thanks to the highly open porous structure, which provides high density surface Pd atoms and easy molecular accessibility throughout the unique nanostructure, the porous Pd square nanosheets deliver extraordinary electrocatalytic properties for both the electrooxidation of liquid fuels (CH3OH and HCOOH) and the oxygen reduction reaction (ORR), as compared with commercial Pd black catalyst. The outstanding bifunctional electrocatalytic properties make the porous Pd square nanosheets be an efficient electrocatalyst for fuel cell devices. It is reasonable to anticipate that the present unique strategy will enlighten the rational design of noble metal-based porous nanosheets for diverse applications.
Section snippets
Reagents and chemicals
Potassium tetrachloropalladite(II) (K2PdCl4), potassium hexacyanonickelate(III) (K2Ni(CN)4) and sulfuric acid (H2SO4) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All the other reagents were of analytical grade and used without further purification. All of the aqueous solutions were prepared using Millipore water with a resistivity of 18.2 MΩ.
Preparation of Pd(H2O)2[Ni(CN)4]⋅xH2O square nanoplates
In a typical synthesis, 5 mL of 50 mM K2PdCl4 and 5 mL of 50 mM K2Ni(CN)4 aqueous solutions were mixed at room temperature.
Formation process and characterizations of the porous Pd square nanosheets
For the typical synthesis of Hofmann-type CP nanosheets, 5 mL of 0.05 M K2PdCl4 and 5 mL of 0.05 M K2Ni(CN)4 aqueous solutions were initially mixed at room temperature, forming light yellow K2PdCl4/K2Ni(CN)4 cyanogel in 20 min (Fig. S1). The resultant cyanogel was then hydrothermally treated at 160 °C for 1 h, generating a light blue product. The crystal structure of obtained product was identified by XRD pattern (Fig. 3a). All diffraction peaks correspond well to the Hofmann-type CP of Pd(H2O)2
Conclusion
In summary, for the first time, we have elaborately designed a novel CP-templated strategy to synthesize porous Pd square nanosheets. The CP templates of Pd(H2O)2[Ni(CN)4]⋅xH2O with square plate-like structure undergo thermal decomposition and electrochemical reduction as well as simultaneous selective dissolution of Ni species, finally transforming into porous Pd square nanosheets. Notably, this CP-engaged protocol can be potentially extendable to synthesize other noble metal porous nanosheets
Acknowledgements
The authors are grateful for the financial supports from National Natural Science Foundation of China (21503111 and 21576139), Natural Science Foundation of Jiangsu Higher Education Institutions of China (16KJB150020), Natural Science Foundation of Jiangsu Province (BK20171473), Natural Sciences and Engineering Research Council of Canada (NSERC) and China Scholarship Council (CSC, No. 201706860019). The authors also thank National and Local Joint Engineering Research Center of Biomedical
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