Novel functional materials
Nanocarbons such as graphene, carbon nanotubes (CNTs) and fullerenes possess extraordinary mechanical, electronic and thermal properties. Essentially, graphene is a 2D material that consists of a monolayer of hexagonally arranged carbon atoms. Initiated by a ground-breaking publication in 2004, graphene has since attracted tremendous attention as the world's thinnest material with outstanding electronic properties, which renders it a promising candidate for many applications in energy, microelectronic and information technology. As the 1D member of the nanocarbon family, CNTs can be seen as "rolled-up" graphene and has been first described in 1991. In addition to their extraordinary electronic and thermal properties, CNTs also exhibit exceptionally high mechanical strength and modulus along with a high resilience, making it the world's strongest and lightest material.
A concise overview about the structure and chirality as well as their synthesis and properties can be found here.
Carbon Nanotubes (CNTs)
We use a home-built floating catalyst chemical vapour deposition method (FCCVD) to grow powders and vertically-aligned "carpets" of multi-walled and single-walled CNTs. In short, a solution of ferrocene in toluene is injected into the pre-heated inlet before being carried by Ar gas flow into the quartz reaction chamber, pre-heated to 760 °C. The CNT characteristics (e.g. chirality, structure, length and diameter) can be varied by controlling several process parameters such as reaction time and temperature as well as catalyst and precursor composition. We also develop protocols for purification and functionalisation of CNTs. In collaboration with Dr. J. Vilatela (IMDEA Materials, Madrid, Spain) we produce and investigate unique CNT fibres, which are composited of individual CNTs held together solely by van-der-Waals interactions and are currently the world's strongest fibres.
Graphene is produced via atmospheric pressure chemical vapour deposition (APCVD) using Cu or Ni foils and foams as substrate, CH4 as carbon source and 95:5 Ar:H2₂ carrier gas. Depending on gas flow rates, substrate, growth temperature and cooling rate few-layered graphene films are prepared as determined by Raman spectroscopy.
Graphene oxide (GO)
To achieve gram scale production of single-layer graphene, graphite is first oxidized to graphite oxide which can then be exfoliated to graphene oxide (GO) via ultrasonication. GO is easily dispersed and the various functional groups of GO are suitable for a range of chemical reactions. The properties of the material can then be partly restored via reduction of GO to reduced graphene oxide (rGO).
Nanocarbon-inorganic hybrids constitute a novel class of multifunctional materials where an inorganic component is deposited onto a nanocarbon in the form of thin layers, films or nanoparticles. In contrast to composites, where these two components are simply mixed together, hybrids are prepared using bottom up techniques and thus allow for precise control over the morphology. This also creates a close and large interface that facilitates the processes of charge and heat transfer between the two components, thus resulting in additional synergistic effects.
D. Eder, Chem. Rev. 2010, 110, 1348–1385; DOI:10.1021/cr800433k.
C. J. Shearer, A. Cherevan, D. Eder, Adv. Mater. 2014, 26, 2295–2318; DOI:10.1002/adma.201305254.
We also reported on a simple non-covalent functionalization route that can be used to render pristine hydrophobic CNTs suitable for atomic layer deposition (ALD) growth of a variety of inorganic structures. In this work we demonstrate the potential of small aromatic molecules, including benzyl alcohol (BA), naphthalene carboxylic acid (NA) and pyrene carboxylic acid (PCA) to act as active nucleation sites and linking agents. Among all, PCA enables the deposition over a wider temperature range and without the typical surface corrosion induced by covalent functionalization.
N. Kemnade, C. J. Shearer, D. J. Dieterle, A. S. Cherevan, P. Gebhardt, G. Wilde, D. Eder, Nanoscale 2015, 7, 3028–3034; DOI:10.1039/C4NR04615C.
As a proof of concept, we demonstrated that the photocatalytic properties of such nanocarbon hybrids can be further improved by engineering the interfaces and morphology of the active layer. In contrast to the typical attachment of loose aggregates of metal oxide particles on CNTs, we have grown ultra-thin single-crystalline Ta₂O₅ films by utilizing the graphitic CNT surfaces as seed crystals for heterogeneous nucleation. This allowed not only to improve the charge transport within the oxide layer due to the increased crystallinity, but also to facilitate the electron extraction by the CNTs due to the formation of the tight interface.
A. S. Cherevan, P. Gebhardt, C. J. Shearer, M. Matsukawa, K. Domen, D. Eder, Energy Environ. Sci. 2014, 7, 791–796; DOI:10.1039/C3EE42558D.
In our attempts to understand the nature and extend of the charge transfer in such hybrid materials, we recently developed a new technique, the DETPM, which enables to deconvolute intrinsic effects of nanocarbons (e.g. based on the bolometric response) from charge transfer processes in nanocarbon-inorganic hybrids and composites by taking advantage of the different wavelength dependency of these two processes. We have successfully applied this method to two model hybrids CNTs-Ta2O5 and CNTs-TiO2, and have been able to confirm directly the presence of a charge transfer in both the systems.
A. S. Cherevan, D. Eder, Adv. Mater. Interfaces 2016, 3, 1600244; DOI:10.1002/admi.201600244.
Ordered mesoporous metal oxides/nitrides
To achieve the desired porosity of our materials, we use a combined sol-gel evaporation-induced self-assembly (EISA) process, that is also generally used to produce other mesoporous materials like mesoporous silica (i.e. MCM-41 or SBA-15). We use the commercial Pluronics® P123 and F127 and, more importantly, poly(isoprene-b-stryrene-b-ethylene oxide) (ISO) polymer as templates. The latter is provided by the group of Prof. Uli Wiesner (Cornell University, Ithaca, New York) and Tobias Dörr (Leibniz-Institut für Neue Materialien, Saarbrücken, Germany) and allows larger pore diameters (above 10 nm) and advanced pore structures than conventional templates. By tuning molar ratio between the hydrophilic organic-oxide precursors and the polymer we achieve oxide polymer hybrids with different pore architectures like 2D-hexagonal or 3D-alternating gyroid (GA). After removal of the block copolymer by calcination in air, we reach the ordered mesoporous inorganic materials that are of great interest for many applications due to their high active surface area.
Mesoporous semiconductors (like TiO₂, Ta₂O₅ or Nb₂O₅) with high porosity and large pore diameters are of great interest for catalytic applications, not only because of their high active surface area and accessibility of 3D pores for the reactant and product diffusion but also because of reduced transport limitations for photo excited species charges within the thin mesoporous walls. Mesoporous metal nitrides are produced by nitriding the mesoporous oxide samples.
These materials are characterized by SEM, TEM, XRD, SAXS, UV-VIS and BET/BJH. Photocatalytic tests (sacrificial water splitting) are performed in a closed home-build reactor.
A. S. Cherevan, S. Robbins, D. Dieterle, P. Gebhardt, U. Wiesner, D. Eder, Nanoscale 2016; DOI:10.1039/C6NR04430A.
Bioactive glasses of the SiO₂–CaO–P₂O₅ system can form a hydroxycarbonate apatite (HCA) layer on its surface when immersed in a physiological fluid (like human blood plasma). This HCA layer can build a bonding interface to the surrounding tissue and therefore enables the formation of new bone tissue. Furthermore, bioactive glasses with a controlled pore diameter in the mesopore range (as synthesized from block-copolymers, see above) can also act as drug carriers to ensure a local drug supply at the injured site of the tissue. Doping with different elements, like boron and aluminum, can influence both the bioactivity and mechanical properties of the bioactive glass. The latter approach is investigated in cooperation with Prof. Dr. C. Hellmich (Institut für Mechanik der Werkstoffe und Strukturen, TU Wien).
29Si and 31P MAS NMR provide valuable insights into the material's sturcure. Furthermore, we assess the bioactivity of our materials in vitro by measuring the formation of the HCA-layer in simulated body fluid (SBF) with common analysis methods such as XRD, FTIR and SEM.
S. Melchers, T. Uesbeck, O. Winter, H. Eckert, D. Eder, Chem. Mater. 2016, 28, 3254–3264; DOI:10.1021/acs.chemmater.5b04117.
Besides bioactive glasses, efforts are devoted to produce ordered mesoporous zeolites, such as titanium silicalite-1 (TS-1), for use in photocatalytic applications. The aim is to reduce the diffusion limitation for organic molecules imposed by micropores, thus enhancing the photocatalytic activity for the degradation of organic compounds from water. The complexity of creating a zeolitic system with both, well-ordered micro- and mesopores, constitutes an intriguing challenge on the synthesis.
P. Gebhardt, S. W. Pattinson, Z. Ren, D. J. Cooke, J. A. Elliott, D. Eder, Nanoscale 2014, 6, 7319–7324; DOI:10.1039/C4NR00320A.
Crystal-growth engineering of organic-inorganic halide perovskites
With high conversion efficiency and comparable low cost, organic-lead halide perovskite has emerged as promising solar cell material. As the efficiency race continues, questions on the control of the performance and on the working mechanism require being addressed. One part of our work is to optimize the microstructure of the perovskite layer.
With a two-step solution preparation, a PbI₂ film is first deposited on substrates, and then the crystallization during the immersion process is controlled via solvent tuning. Ethanol and other alcohols were used as the solvent for methylammonium iodide (MAI) instead of iso-propanol (IPA). SEM results indicate that compared with IPA, films prepared with EtOH and Phenol are composed of larger grains, ranging from 500nm to over 1µm. Detailed work is being done now, focused on increasing the coverage of the perovskite layer and studying the relationship between grain size and charge-transport behavior. Hopefully, our results can give more insights into the growth mechanism of perovskite crystals as well as the film.
Furthermore, due to the toxicity of Lead, we are also working at finding substitutes for Lead.