Nanoscaled inorganic materials by molecular design

Ralf Riedel

Prof. Riedel studied chemistry and got his PhD in Inorganic Chemistry in 1986. Between 1986 and 1992 he joined the Max-Planck-Institute for Metals Research and the Institute of Inorganic Materials at the University of Stuttgart. In 1992 he finished his habilitation in Inorganic Chemistry. Since 1993 he has been Professor at the Institute of Materials Science at the Technische Universität Darmstadt. Prof. Riedel is Fellow of the American Ceramic Society and was awarded with the Dionyz Stur Gold Medal for merits in natural sciences. He is a member of the World Academy of Ceramics and Guest Professor at the Jiangsu University in Zhenjiang, China. In 2006 he was awarded with the honorary doctorate of the Slovak Academy of Sciences in Bratislava and in 2009 with the honorary Professorship at the Tianjin University in Tianjin, China. Presently, he is Dean of the Materials and Earth Sciences Department at the Technische Universität Darmstadt. His current research interest is focused on (i) synthesis and properties of advanced ceramics and (ii) ultra-high pressure synthesis of new materials.

In this themed issue you will find a series of articles focused on inorganic nanomaterials synthesized by molecular precursors. The research studies reported here have been financially supported by the German Science Foundation (DFG), Bonn, Germany, in the frame of a so-called “Priority-Programme” SPP 1181 entitled “Nanoscaled Inorganic Materials by Molecular Design: New Materials for Advanced Technologies”. This research programme was funded between 2005 and 2011 with a total budget of ca. €10 million. The interdisciplinary research team included natural scientists as well as mechanical engineers and was comprised of ca. 30 principal investigators from various German Universities working on more than ten different topics related to nanoscaled inorganic materials.

The aim of this priority programme was to develop concepts for the production of novel multifunctional inorganic materials with a tailor-made nanoscaled structure. Industrial demands on future technologies have created a need for new material properties which exceed by far those of materials known today and which can only be produced by designing the respective microstructure at the nanoscale. Furthermore, the increasing miniaturisation of components calls for new process technologies allowing reliable production of materials at and below the micrometre scale. In particular, inorganic–organic hybrid materials as well as amorphous and polycrystalline ceramics have to be considered as novel material classes and are produced by means of cross-linking routes in various states of condensation. In accordance with the so-called “bottom-up” approach, specific inorganic molecules are to be assigned to higher molecular networks and solid-state structures in the form of molecular nanotools by means of condensation and polymerization processes. This method aims at linking organic components to inorganic structures producing materials inaccessible by thermodynamically controlled chemical syntheses. Therefore, the experimental studies were focused on the development of solids derived from molecular units via kinetically controlled synthesis processes in the interface between molecular and solid-state chemistry enabling specific adjustments to the solid-state properties.

Thus, the ultimate objective of the priority program was to systematically study the “bottom-up” approach with regard to the synthesis and exploration of novel materials in order to establish the technological fundamentals for their development and potential use. Possible fields of application for materials produced at the nanoscale are key technologies of the 21st century such as transport systems, information technology, energy as well as environmental systems and micro- or nano-electromechanical systems. The correlation between the structure of the molecular precursor and the nanostructure of the derived materials and their properties provided the focal point for the detailed experimental studies reported here.

In particular nanostructured materials for applications in fuel cells, Li-ion batteries and photovoltaic systems, high-temperature resistant nanosized carbides and nitrides with a variety of integrated functional (sensing, electrical and thermal conductivity, piezoresistivity, etc.) and mechanical (ultra-high hardness, creep resistance, etc.) properties suitable for e.g. environmental and thermal barrier coatings (EBC and TBC) have been studied. Furthermore, nano-porous ceramics for catalytic, bio- and food technology applications as well as complex topological nanostructures for medical and biological applications have been in the focus of the priority programme. In the following the individual research projects as part of the coordinated programme are briefly highlighted:

In the work of P. Tölle et al. (DOI: 10.1039/C2CS15322J) enhanced proton transport by using functionalized pore channels of inorganic molecular sieves like Si-MCM-41 or benzene containing periodic mesoporous organosilica (PMO) with hexagonal ordering is described. Beneficial effects for the proton conduction at elevated temperatures of hybrid membranes made of different polymers (Nafion®, functionalized polysiloxanes and polyoxadiazoles) and from functionalized nanoparticles compared to the pure polymer membranes have been demonstrated. Different theoretical approaches to gain insight into the atomistic mechanisms of proton transport in confined spaces are presented. The limits of these approaches as well as relations to other materials are also considered.

Nanostructured materials promise fundamental advances in efficient energy storage and/or conversion, in which surface processes and transport kinetics play determining roles. The review of L. Dimesso et al. (DOI: 10.1039/C2CS15320C) describes some recent developments in the synthesis and characterization of composites which consist of lithium metal phosphates coated on nanostructured carbon architectures. This contribution highlights some first new progress in using different three dimensional nanostructured carbon architectures as support for the phosphate based cathode materials (e.g. LiFePO₄, LiCoPO₄) to develop lithium batteries with high energy density, high rate capability and excellent cycling stability resulting from their huge surface area and short distance for mass and charge transport.

Fabrication and characterization of nanostructured titania films with integrated function from inorganic–organic hybrid materials for solar cell applications is the topic of the article by M. Rawolle et al. (DOI: 10.1039/C2CS15321A). The morphology of the titania layer has to be controlled on several length scales in order to enhance the effectiveness and performance of titania-based photovoltaics. Possible preparation methods for tuning the morphology of titania range from chemical vapor deposition to sol–gel synthesis, where the latter constitutes a promising route to nanostructured titania in combination with amphiphilic block copolymer templates. Via the sol–gel method a wide diversity of structures, including lamellae, nanoparticles, nanorods, nanowires, nanotubes, nanovesicles and sponge-like nanoscale networks can be achieved. Newly synthesized tailored block copolymers allow for the integration of further functions during the titania fabrication step. For example, the blocking layer, which is important for solid-state titania based solar cells, can be included in the titania tailoring step.

The synthesis of nanosized titania is also the focus of the critical review of T. Fröschl et al. (DOI: 10.1039/C2CS35013K). This contribution highlights the formation of crystalline nanoscale titania particles via solution-based approaches without thermal treatment. In particular, sol–gel processes via glycolated precursor molecules as well as the miniemulsion technique are applied to selectively synthesize titania particles of distinct polymorphism and with distinct crystal morphology, surface area, and particle dimensions. Functional properties of these materials such as heterogeneous catalysis and electrochemical energy storage for battery materials are exemplary discussed.

Cellular and nanoporous carbides are the focus of the work of L. Borchardt et al. (DOI: 10.1039/C2CS15324F). The design of new materials with well-defined pore size and high accessible surface area plays not only a key role for advanced catalysis, filtration and electrochemical devices but also for the generation of macroporous SiC-supports for diesel soot filter applications, particle filters, air purification systems, burner applications and solar receivers. Non-oxide ceramics based on carbides show excellent thermal stability and are thus valuable supports for high temperature catalytic applications. In this contribution, the generation of highly porous carbide materials with surface areas up to 800 m² g⁻¹ obtained by the molecular precursor approach is described. Highly porous carbons and hierarchically structured systems are synthesised from porous SiC materials (CDC = carbide derived carbons) with extremely high surface areas up to 2800 m² g⁻¹. This type of porous ceramics shows outstanding performance in catalytic and electrochemical applications (fuel cell, electric double layer capacitor) and also in protein adsorption.

The work of M. Veith et al. (DOI: 10.1039/C2CS15345A) deals with bi-phasic nanostructures for functional applications. Here, 0-D and 1-D metal–metal oxide or semiconductor–metal oxide heterostructures, such as co-axial core–shell, super lattice and composite nanospheres or nanowires are studied. These heterostructures have been shown to exhibit superior or new functional properties compared to their individual constituent compounds. In particular, the novel synthesis of single source precursor-derived nano-spheres, nano-wires and nano-loops is demonstrated. The bi-phasic nature of the wires is discussed in terms of a disproportionation of aluminium into the transient “AlO” which is followed by self-assembling of aluminium and alumina. Various nanostructures may be obtained by simply altering the deposition parameters. In each case, the bi-phasic nature (Al core and Al₂O₃ shell) stays identical which opens up a wide spectrum of applications ranging from biomedical, optical, bio-mimetical up to adhesion.

The contribution of E. Ionescu et al. (DOI: 10.1039/C2CS15319J) highlights the synthesis and properties of silicon-based ceramic nano-composites derived from modified or functionalized silicon containing preceramic polymers such as poly(organo)siloxanes and poly(organo)silazanes. Polymer-derived ceramic nano-composites show excellent behavior under extreme conditions (e.g. ultra-high temperatures as well as in oxidative and corrosive environments) and have consequently an enormous potential as materials for structural applications. They also exhibit interesting functional features such as optical, electrical, and magnetic properties which make them candidate materials for high temperature sensors, micro glow plugs, electrochemical devices, and MEMS/NEMS operating under harsh conditions.

Polymer derived ceramics are also the topic of the article presented by M. Zaheer et al. (DOI: 10.1039/C2CS15326B). Applications of these materials ranging from high performance functional or protective coatings, sensors, nano-composites, and ceramic fibres are discussed. In particular, polymer-derived ceramics modified with late transition metals are synthesized in order to broaden their property profile as well as their field of applications. Late transition metals are ideally suited since they form metallic phases under reductive pyrolysis conditions. In this way, two completely different types of materials in terms of bonding nature, namely ceramics and metals, are combined and structured in one material on an atomic or nanoscale.

Nanostructured ceramic hard materials are reported by D. Rafaja et al. (DOI: 10.1039/C2CS15351C). Superhard nano-composites show superior mechanical properties which surpass the corresponding single crystalline materials and microstructured solids. When compared to diamond, carbides, borides, and nitrides are of interest due to their high temperature oxidation resistance and compatibility with iron containing alloys. This tutorial review compares methods of synthesis, properties and microstructural characteristics of ultra-hard bulk boron nitride nano-composites and Ti/Al/(Si)/N- and Ti/Si/N-coatings. Microstructural approaches are applied to gain an understanding of the mechanisms effective in nitride-based nano-composites in order to control and improve the physical and chemical properties relevant for their application.

Another critical review by R. K. Joshi and J. J. Schneider is concerned with the assembly of one dimensional inorganic nanostructures into 2D and 3D architectures (DOI: 10.1039/C2CS35089K). The arrangement of 1D nanomaterials (tubes, wires, rods) into ordered 2D or 3D micro or macro sized structures is of great technological importance for the further development of high performance electronic devices like field effect transistors, chemo- and biosensors, catalysts, or for energy material applications. Materials of interest are 1D arranged carbon based structures, metal oxides and metal based substances as well as hybrid compounds or composites as tools for the construction of 2D and 3D architectures.

With these focused research topics, the priority programme has contributed to the significant progress that has been achieved recently in terms of a systematic bottom-up synthesis approach to novel nanoscaled materials with tailored and complex property profiles. Nanoscaled inorganic materials are one of the most promising and exciting classes of materials for the key technologies of the 21st century. Recent developments in this area are highlighted in this themed issue and include a selection of synthesis, characterization and processing techniques applied for the production of novel structural and functional materials and composites with advanced nanostructured features.

In the name of all colleagues who contributed to the research in the frame of the priority programme, I acknowledge the German Science Foundation for funding of the SPP 1181. In particular, I wish to thank Dr Burkhard Jahnen from the DFG who continuously supported the needs and requirements of all principal investigators of the priority programme with respect to various administration and organization matters. I also thank all the authors of this themed issue for their great enthusiasm in compiling excellent manuscripts in their respective areas of expertise. Last but not least I wish to thank Prof. Ulrich Wiesner, Cornell University, USA, for encouraging me to organize this themed issue and the Editor, Dr Robert Eagling, and staff of Chem Soc Rev for their great support in getting the themed issue accomplished and finally published.

Footnote

† Part of a web theme on the topic of nanomaterials (Deutsche Forschungsgemeinschaft SPP1181/Nanomaterials program).

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