Preparation of Hollow Nitrogen Doped Carbon via Stresses Induced Orientation Contraction

Abstract

be made into well-faceted morphologies of rhombic dodecahedrons or cubic hexahedrons. [13,14] A critical step for finally getting the hollow structures by using the template method is to remove the templates, which normally requires that the template materials have poorer thermal/chemical stability than the desired species attached on the templates. ZIF8 templates can be easily etched in a solution system, [13,[15][16][17] owing to their chemical instability. But some literatures also show that an annealing process can also remove ZIF8, [18][19][20] which especially in our detailed investigation, is not due to the poor thermal stability.In particular, when we coated a layer of polymerized dopamine (PDA) on rhombic dodecahedron nanoparticles of ZIF8, and annealed the ZIF8@PDA composites at a temperature of 900 °C, the obtained product is nitrogen doped carbon (NC) hollow structured materials, indicating that ZIF8, just like a hard template, can be removed at high-temperature for constructing the hollow interiors. While interestingly, when ZIF8 was solely calcined at 900 °C, it results in same shape, solid NC dodecahedron nanoparticles, indicating ZIF8 itself can well retain the original structures during heating treatment. This observation implies that the hollow structures obtained by annealing the composites are absolutely not derived from poor thermal stability of the ZIF8 cores and the existence of PDA layer may also contribute to the hollowing procedure.Considering the unusual observation, we then gave this hollowing process a detailed investigation and proposed a "stresses induced orientation contraction" mechanism to address it. Particularly, pure ZIF8 rhombic dodecahedrons would gradually decompose, shrink, and transfer into NC along temperature increase, in which the uniform shrinkage stresses applied on different directions could be well balanced, resulting in NC with well inherited shapes but smaller sizes. While for ZIF8@PDA composites, due to the intense PDA-ZIF8 interface interactions, the PDA layer strongly drags the ZIF8 outward to restrain their contraction, thus leading to accumulated stresses in the central part of ZIF8 dodecahedron nanoparticles and gradually damaged interior cores for creating the hollow structures. Besides, the achieved NC materials demonstrate a very high catalytic activity to ORR owing to the nice hollow structure and very high content of active pyridinic N. Indeed this work gives an absolutely novel Hollow structured materials are widely applicable in various fields. Although many routes have been explored for getting such materials, a strategy mainly based on physical effect is still deficient. Herein, a "stresses induced orientation contraction" mechanism for preparation of hollow structures is reported. The composites constructed by zeolite imidazolate framework-8 (ZIF8) cores and polymerized dopamine (PDA) shells, upon annealing, form intensive interfacial interactions, which drag the ZIF8 cores outward to restrain their shrinkage. The gradually accumulated stresses in...

Yolk–Shell Fe/Fe4N@Pd/C Magnetic Nanocomposite as an Efficient Recyclable ORR Electrocatalyst and SERS Substrate

“…Taking into account the weight change of the thermogravimetric analysis together with acid leaching results, the carbon contents of FFPC‐1, FFPC‐2, and FFPC‐3 were calculated as 13.69, 13.71, and 11.62 wt%, respectively (detailed analysis shown in the Supporting Information). More interestingly, with Pd content increasing, the oxidation of iron was pushed to a lower temperature (Figure S10, Supporting Information) due to a lower Pd content resulting in less defects carbon shell, which is more effective in inhibiting the rapid oxidation of iron . The content of carbon from the SEM‐energy dispersive spectrometer (EDS) data (35.9 wt%, Figure S11, Supporting Information) is much higher than that of calculated.…”

Hollow Carbon‐Based Nanoarchitectures Based on ZIF: Inward/Outward Contraction Mechanism and Beyond

“…In 2018, the hollowing mechanism of ZIF@PDA derived hollow carbon structures was demonstrated. [ 116,117 ] With solid evidences such as TGA and temperature‐dependent TEM results, a “stresses induced orientation contraction” mechanism was proposed (Figure 11d). During the carbonization process, the phase evolution of ZIF‐8@PDA composites can be divided into three stages.…”

Transition Metal and Nitrogen Co‐Doped Carbon‐based Electrocatalysts for the Oxygen Reduction Reaction: From Active Site Insights to the Rational Design of Precursors and Structures

“…Nontoxic and sustainable biomolecule dopamine is well known for its great potential in spontaneous self‐polymerization and coating on corresponding substrates [81] . Thus, it can derive N‐doped carbon coating layers during high‐temperature heat treatment and limit the size of compounds under the coating layers [82] . The PDA molecules can effectively chelate metal ions through abundant catechol and amine functional groups, which is beneficial to obtaining active M−N x sites [83] .…”

“…The PDA molecules can effectively chelate metal ions through abundant catechol and amine functional groups, which is beneficial to obtaining active M−N x sites [83] . The thickness of the PDA coating can be effectively controlled by regulating the monomer concentration and the polymerization time [82] . As shown in Figure 8a, a core‐shell hybrid Co−N/CNT catalyst was synthesized via pyrolysis of PDA coated carbon nanotubes (CNTs) with adsorbed cobalt [80] .…”

“…The PDA coating probably drove the construction of some special structure. For example, the controlled hollow structure was formed by a mechanism of “stresses induced orientation contraction”, which allowed the intensive interfacial interactions between the PDA coating shells and the zeolite imidazolate framework‐8 (ZIF‐8) cores to drag the internal ZIF‐8 cores outward to restrict their shrinkage [82] . A similar shell‐core structure was also discovered in the composite of PDA coating and ZIF‐67, [88] which was advantageous in improving the catalytic activity and mass transfer of ORR.…”