Methods/Protocols pubs.acs.org/cm
Hands-on Guide to the Synthesis of Mesoporous Hollow Graphitic Spheres and Core−Shell Materials Johannes Knossalla, Daniel Jalalpoor, and Ferdi Schüth* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany ABSTRACT: In this work we present a detailed preparation method for mesoporous hollow graphitic spheres (HGS) that has been developed in our laboratory over recent years. The aim of this description is to enable the reader to reproduce the procedure by highlighting important steps, conditions, and challenges during the synthesis. HGS have initially been developed as a carbon support to enhance the stability of metal catalysts in the oxygen reduction reaction (ORR) of PEM fuel cells via pore confinement. The HGS are synthesized in a multistep procedure employing a core−shell silica template, DVB as carbon source, and iron as graphitization catalyst. The silica template is removed by leaching with hydrofluoric acid yielding the mesoporous carbon support, where metal catalysts can be introduced via incipient wetness method followed by a reduction in hydrogen. The whole procedure allows high control over product parameters such as core or shell diameter and graphitization degree. Thus, it can be adapted and tuned to match the desired properties of high performance materials for various potential applications.
1. INTRODUCTION In order to improve the control of porosity and the morphology of porous materials on the nanoscale, many different synthesis methods have been developed over recent decades.1−3 Also in our group many projects have dealt with the controlled synthesis of porous materials with various textural properties for different applications. Several pathways for the synthesis of stable catalysts rely on encapsulation strategies based on hollow shell materials with pores in their shell.4,5 To introduce porosity into materials in a controlled manner, often a templating method is applied as a key synthesis technique. The templating process can generally be subdivided into soft and hard templating. Soft templating involves the application of nonrigid templates, while stiff and rigid materials are used for hard templating. Nanocasting is a form of hard templating process, in which voids of a rigid mold with structures on the nanoscale are infiltrated with the material to be cast, or a precursor for the material, respectively. Subsequently, the mold is removed to produce a negative replica.3 Both templating techniques, hard and soft templating, are commonly employed for the synthesis of nanostructured materials. Soft templates are very sensitive to the selected synthesis parameters, which impede the specific control of particle morphology and, for core−shell materials, of core or shell thicknesses, respectively. In contrast, hard templates allow a precise control of pore sizes, morphologies, and so on. Among the most frequently used materials for hard templating are silica, polymeric, or carbon materials. The hard templating process usually consists of a three step procedure: (i) the template formation, (ii) the deposition of the material to be molded, and (iii) the template removal. Often a modification of the template material’s surface is needed to modify the © 2017 American Chemical Society
interaction with the precursor and thus to improve the replication. This multistep procedure is in most cases more complicated than the application of soft templates, as those can usually be removed by washing, an evaporation step, or calcination, which also reduces the probability of damage to the just formed structure. However, the synthesis of such materials can be rather tricky and depends on small, but crucial, factors, which are often not described in detail in publicationsnot because one does not want to reveal the details but more due to difficulties in describing them in detail in the limited space typically available. Moreover, sometimes the synthesis chemist is not even aware of these tiny details, because he or she always runs the synthesis in the same manner and considers certain points as trivial and/ or obvious. To give a detailed, exemplary synthesis guide on porous core−shell materials, a hollow, porous, partially graphitic carbon material was selected. These hollow graphitic spheres (HGS) are synthesized in a multistep process both involving hard templating and soft templating. In the synthesis, a silica core−shell hard template was used, with a solid core and a mesoporous shell, which served as a scaffold for the final product. The resulting hollow, porous, partially graphitized carbon spheres (HGS) have a typical void diameter of 170 nm and a shell thickness of 40 nm. They have a very narrow pore size distribution in the size range of 3−4 nm predefined by the mesoporous silica shell. HGS were developed as supports for PEM fuel cell catalysts, to enhance stability and durability of the catalyst, providing controlled pore sizes and morphologies to encapsulate catalytically active nanoparticles. The detailed Received: June 25, 2017 Revised: July 27, 2017 Published: July 27, 2017 7062
DOI: 10.1021/acs.chemmater.7b02645 Chem. Mater. 2017, 29, 7062−7072
Chemistry of Materials
Methods/Protocols
Table 1. Chemicals Used in the Synthesis
a
abbreviation
name
puritya (%)
CAS reg. no.
TEOS EtOH NH4OH OTMS N2/Ar DVB AIBN Fe(NO3)3·9H2O HF CaCl2 HCl
tetraethyl orthosilicate ethanol ammonium hydroxide (aq) octadecyltrimethoxysilane nitrogen/argon divinylbenzene 2,2′-azobis(methylbutyronitril) iron nitrate nonahydrate hydrofluoric acid (aq) calcium chloride hydrochloric acid (aq)
98 ≥99.8 (GC) 28−30 90 99.999 80 (tech.) >98 >98 40 93 (anhydrous) 37−38
78-10-4 64-17-5 1336-21-6 3069-42-9 1321-74-0 13472-08-7 7782-61-8 7664-39-3 10043-52-4 7646-01-0
Purchased from Sigma-Aldrich; gases supplied by Air Liquide and Praxair.
water used was filtered by a Millipore device to ensure a high quality of the products, even though the purity of distilled water might be sufficient. 2.2. Equipment. Basically, the synthesis can be reproduced without special laboratory equipment (Table 2), although
synthesis description is giving access to a broad range of potential applications by only slight variations of the synthesis protocol, enabling the possibility to tune the material properties with respect to the application demands. For instance, prior to the silica core formation, metal nanoparticles can be synthesized and coated with surfactants. The following formation of the solid silica core proceeds on the metal particles, resulting in encapsulated yolk−shell materials (described below).4,5 Furthermore, instead of a precursor polymer for pure carbon, polymers containing heteroatoms may be used, so that the resulting material after pyrolysis contains these heteroatoms.6 Thus, the precise procedure reported herein provides a blueprint and starting point for the synthesis of many other complex solids. The aim of this work is thus to give hands-on experience on the material’s synthesis and highlight important steps and techniques to improve reproducibility. A comprehensive review of the variety of core−shell hollow materials is not within the scope of this work and the reader is referred to the following comprehensive reviews.1,3,7 These reviews, incidentally, can also provide inspiration for further modification of the protocols.
Table 2. Employed Laboratory Equipment for the Synthesis Method General Equipment balance (at least three decimal places) centrifuge (up to 9000 rpm) heating/stirring plate (up to 600 rpm and >150 °c) and oil bath refrigerator vacuum pump (for pressures
