Ordered mesoporous silica-based inorganic nanocomposites
Graphical abstract
HAADF TEM image of gold nanoparticles in amine-functionalized MCM-41 (from Ref. [22]).
Introduction
Developing the ability to control material properties such as composition, structure, and morphology at the nanometer scale will lead to both intellectual advances in material design as well as societal benefits through enabling of new technologies and improvements in existing ones. In the last 10 years the field of nanotechnology has witnessed an explosive growth. Take for example the synthesis of nanoparticles, nanowires, and other multidimensional nanostructures. Chemists now have the ability to synthesize materials with highly tailored compositions, chemical structures, and particle sizes. Thus nanoparticles are beginning to make contributions and inroads into fields as diverse as magnetic materials, biological sensing, and chemical warfare agent sensing/destruction.
Ordered mesoporous silicas (OMS) are another class of materials possessing unique properties at the nanoscale. since their initial discovery by the Mobil labs in the early 1990s [1], [2] these materials have been heavily investigated [3], [4], [5], [6], [7], [8]. These porous solids have desirable properties including highly uniform pore sizes in the 2–10 nm range that possess long-range order, despite that the matrix material is amorphous silica. Originally it was hoped that these materials would be large-pore versions of zeolites, i.e. possess comparable thermal stability and acidity [9]. This would enable, among other things, the cracking of large hydrocarbons too large to enter the pores of zeolites. While this has not come to pass, OMS phases show considerable promise as model supports for a variety of applications, as well as intriguing possibilities as nanocontainers, wherein chemistry can be performed to generate novel nanocomposites. The functionalization of OMS materials, particularly chemical grafting of homogeneous catalysts, and their use as catalyst supports is an active field of study and has been extensively reviewed elsewhere [9], [10], [11], [12], [13]. OMS have also been studied extensively for separations, drug release, and a variety of other potential applications. The use of OMS phases as containers for nanoparticle/nanowire growth is the focus of this Review/Perspectives article. That these materials possess uniform pores possessing long-range ordering, and that the pore surface contains silanol groups facilitating well-developed silane chemistry for tuning surface properties makes these materials in many ways model supports for assembling nanocomposite materials. Fig. 1 summarizes some of the basic properties of the most frequently studied OMS phases and the methods one can use to analyze them [13].
This contribution summarizes the generation of OMS-nanoparticle and -nanowire composites. While much of this work originates from the catalysis community, works are beginning to emerge where the inorganic phase occluded in the OMS pores has applications beyond catalysis (e.g. magnetic, optical materials). In this Review/Perspectives article, the literature since 2004 will be reviewed, focusing on the synthesis of metal nanoparticles and nanowires. Earlier works have been reviewed and summarized elsewhere [14], [15], [16], [17]. The review will emphasize the various synthesis methodologies used to date, attempting to correlate and understand how the material synthesis method impacts the properties of the occluded phase. Also whether powders or films of OMS materials were used as the substrate will be noted. OMS thin films are a potentially attractive matrix phase as they are more amenable for device fabrication and also present a straightforward means to orient the nanostructured material formed. The article will conclude with the authors’ perspectives on the future of this area, some unresolved problems facing the field, and potential opportunities.
Section snippets
Metal nanoparticles in OMS phases
Most studies of OMS-nanoparticle composites have focused on forming metal nanoparticles in OMS, particularly of late transition metals such as silver, gold, platinum, and palladium. There are several driving forces for this including their relative ease of synthesis, the existing body of literature from the catalysis community of metal particles on other substrates and their catalytic relevance, particularly for platinum and palladium. Gold is a relatively late arrival in the catalysis
Bimetallics
By contrast to the reports of single metal nanoparticles, there is a relative dearth of literature in the area of bimetallic compounds. Beyond the early work from Thomas’ lab [17], [68], [69] reporting bimetallic particles formed via decomposition of mixed metal carbonyl precursors, the literature in this area is sparse. Based on some of the reports below the authors believe this area is rich for future studies.
King et al. [70] reported the synthesis of several bimetallic nanoparticle systems
Non-metals
In addition to work on metal nanoparticles, there are numerous reports of metal chalcogenides, most notably ZnS, CdS, and ZnSe. The investigation of these materials, while extremely interesting, has been sufficiently reviewed elsewhere [73], [74], [75]. There are also numerous reports of metal oxide clusters OMS phases which have similarly been reviewed previously. However, one area that has been only sparingly investigated is the synthesis of supported metal carbide/nitride/phosphide clusters
Future prospects, challenges, and opportunities
As one can see the catalysis community has been the driving force behind work in this area. The consequence is that the bulk of the effort has been on using powders as a support, a focus on late transition metal particles, and the reliance of catalytic testing to evaluate metal properties, at times at the expense of other analytical methods. Homogeneity of the supported metal phase will prove crucial as the field moves to areas such as optical/magnetic materials.
Advances in the ability to
Acknowledgment
The authors acknowledge the National Science Foundation (CTS-0329386, CTS-0624813) for supporting their research in OMS materials.
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Novel one-pot synthesis of 4H-chromene derivatives using amino functionalized silica gel catalyst
2014, Chinese Chemical LettersCitation Excerpt :Recently, mesoporous silica (OMS) and related materials have become popular as catalysts and adsorbents and have found extensive applications as containers for cluster/nanowire growth, etc. [7]. These materials have several advantages such as tunable pore diameter and high surface area, for which they have emerged as effective heterogeneous catalysts in organic synthesis [8]. Aminopropyl functionalized mesoporous silica materials are reported to be effective base catalysts for Knoevenagel condensation [9], Aldol condensation [10], Michael addition [9], epoxidation reactions [11], and Claisen–Schmidt condensations [12].
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2012, Microporous and Mesoporous MaterialsCitation Excerpt :Thus, there is a need to investigate and discover new synthetic methodologies for the production of homogeneous metal containing M41S materials. In general, metal containing mesoporous materials are prepared by the addition of a metal salt to the preformed material [9], or the co-condensation of metal containing siloxane or organosiloxane precursors [10]. However, these materials do not always remain intact during the harsh conditions required for calcination, with aggregation of the metal species or destruction of the fragile organic moieties often occurring.