Synthesis of Nanomaterials

Assembling nanoscale building blocks into well-defined superstructures is of tremendous technological interest: It allows tailoring the nanomaterials, optimizing their performance, and facilitates their implementing into devices. It is crucial that low-dimensional metal nanostructures like nanowires, nanotubes or nanoplates possess favorable properties such as a high surface area, a high density of active sites, continuous electronic conduction, and high reactivity.

Ion track membranes and nano materials

SEM image of an ion track-etched membrane
SEM image of an ion track-etched membrane

In order to manufacture these membranes polymer foils (PC, PET, PI etc.) are irradiated with heavy ions which penetrate the polymer and leave cylindrical damage zones along their tracks. These areas can be removed selectively by chemical etching. By varying the etching conditions, both size and shape of the resulting pores or channels can be controlled.

Metallic nanowires and -tubes

SEM image of electrochemically produced gold nanowires
SEM image of electrochemically produced gold nanowires

In a further process step the nanochannels are filled electrochemically with metals (Cu, Pt, Au etc.) yielding nanowires, which can be freed by dissolution of the polymer template. Besides morphological parameters such as surface topography, microstructure and crystallinity, physical properties like electrical conductivity and thermal stability are investigated. The wires disintegrate upon heating considerably below the melting point of the bulk material, forming a row of spheres (Rayleigh instability), an effect that is explored in detail.

TEM image of gold nanowires
TEM image of gold nanowires

Metal structures can also be obtained by an electroless approach. To generate surface selectivity the polymeric membrane is covered with metallic nanoparticles as a first step. In the subsequent autocatalytic redox reaction of a metal complex and a reducing agent, the initial particles act as nuclei for the metal deposition. Depending on the reaction conditions, polycrystalline wires or tubes are obtained.

Fabrication of nanostructures by means of electroless deposition.
Fabrication of nanostructures by means of electroless deposition.
SEM image of isolated nanocrystalline gold tubes
SEM image of isolated nanocrystalline gold tubes

Nanotubes exhibit exceptional physical and chemical properties, e.g. plasmon resonance or an considerably enhanced catalytic activity compared to bulk material. Main research areas are the control of morphological parameters, the correlation of structure and properties and the utilization of this knowledge in further experiments. Possible applications include catalysis, separation or sensor technology.

SEM image of gold nanotubes
SEM image of gold nanotubes

Another topic of interest is the development of novel synthesis methods for nanotubes, circumventing known problems like insufficient mass transport or homogeneous nucleation.

Oxidic nanowires and –tubes, thin films

SEM image of polycrystalline ZnO wires
SEM image of polycrystalline ZnO wires

Alternatively the ion track etched templates are filled with metal oxides by different means with a focus on functional materials (e.g. semiconducting or magnetic oxides), whose properties qualify them for applications in electro analysis, heterogeneous catalysis or photonics. Like the other nanostructures, they are mainly characterised by SEM, TEM and XRD as well as specific experiments.

SEM image of a nanocrystalline ZnO film
SEM image of a nanocrystalline ZnO film

The template syntheses mentioned so far are not limited to wire- or tube-like structures. They can be realized with arbitrarily formed polymer surfaces and thus be used for the preparation of thin films.

Sulf-supported and hybrid nano-architectures

When low-dimensional metal nanostructures are used as individual nano-objects, they are difficult to handle and organize because rather mobile (which has implications regarding their release), and, due to their tiny size, limited in the strength they interact with their environment.

By combining hundreds of millions up to billions of such building blocks into free-standing 3D architectures, we augment the advantages of the individual nanostructures with those of an ordered superstructure, which provides mechanical robustness, a high nanostructure density, percolating conduction paths, high porosity, efficient diffusive access, and greatly simplifies macroscale handling. For instance, interconnected and free-standing networks composed of metal nanowires or nanotubes can be employed as highly active and durable catalysts, or as fast-responding, sensitive and selective sensors.

In this context, we use ion-track technology to design template membranes for the deposition of complex 3D nano-architectures. Moving beyond structural design, we also expand the compositional range of our synthetic processes. While the areal density, alignment, size and general shape of our nanoscale building blocks can be precisely defined by the template fabrication conditions, the choice of the applied deposition or modification reaction controls the composition and nanostructure of the products. Combined, both aspects open up a huge parameter space for optimizing the nanomaterial properties.

Fabrication of complex nano-architectures.
Fabrication of complex nano-architectures.

We also explore synthetic concepts based on symmetry breaking and directed crystal growth for directly growing anisotropic nanostructures on various surfaces, without the need for templating steps. While such nanostructures tend to be more random in nature, their production is straightforward and easy to scale. While such reactions do not rely on templates, we also combine them with our templating strategies to further increase the structural complexity of our products.

In Situ Transmission Electron Microscopy Analysis of Thermally Decaying Polycrystalline Platinum Nanowires
T. Walbert, F. Muench, Y. Yang, U. Kunz, B.-X. Xu, W. Ensinger, L. Molina-Luna
ACS Nano 2020, 14, 9, 11309–11318
DOI: 10.1021/acs.analchem.8b00902
Electroless Nanoplating of Iridium: Template‐Assisted Nanotube Deposition for the Continuous Flow Reduction of 4‐Nitrophenol
M. C. Scheuerlein, F. Muench, U. Kunz, T. Hellmann, J. P. Hofmann, W. Ensinger
ChemElectroChem 7, 3496-3507, 2020
DOI: 10.1002/celc.202000811
Dual metastability in electroless plating: Complex inertness enabling the deposition of composition‐tunable platinum copper alloy nanostructures
T. Stohr, J. Brötz, M. Oezaslan, F. Münch
Chemistry – A European Journal 26, 3030-3033, 2020
DOI: 10.1002/chem.202000158
Electrocatalytic applications of platinum-decorated TiO2 nanotubes prepared by a fully wet-chemical synthesis
M. Antoni, F. Muench, U. Kunz, J. Brötz, W. Donner, W. Ensinger
Journal of Materials Science 52, 7754–7767, 2017
DOI: 10.1007/s10853-017-1035-4
Conformal Solution Deposition of Pt-Pd Titania Nanocomposite Coatings for Light-Assisted Formic Acid Electrooxidation
F. Muench, G. A El-Nagar, T. Tichter, A. Zintler, U. Kunz, L. Molina-Luna, V. Sikolenko, C. Pasquini, I. Lauermann, C. Roth
ACS Applied Materials & Interfaces 11, 43081-43092, 2019
DOI: 10.1021/acsami.9b12783
Facile wet-chemical synthesis of differently shaped cuprous oxide particles and a thin film: Effect of catalyst morphology on the glucose sensing performance
C. Neetzel, F. Muench, T. Matsutani, J.C. Jaud, J. Broetz, T. Ohgai, W. Ensinger
Sensors and Actuators B: Chemical 214, 189-196, 2015
DOI: 10.1016/j.snb.2015.03.011
Metal Nanotube/Nanowire-Based Unsupported Network Electrocatalysts
F. Muench
Catalysts, 8, 597, 2018
DOI: 10.3390/catal8120597
Highly-Ordered Supportless Three-Dimensional Nanowire Networks with Tunable Complexity and Interwire Connectivity for Device Integration
M. Rauber, I. Alber, S. Müller, R. Neumann, O. Picht, C. Roth, A. Schökel, M.E. Toimil-Molares, W. Ensinger
Nano Letters 11, 2304–2310, 2011
DOI: 10.1021/nl2005516
Electrodeposition and electroless plating of hierarchical metal superstructures composed of 1D nano- and microscale building blocks
F. Muench, E.-M. Felix, M. Rauber, S. Schaefer, M. Antoni, U. Kunz, H.-J. Kleebe, C. Trautmann, W. Ensinger
Electrochemistry Communications 65, 39-43, 2016
DOI: 10.1016/j.electacta.2016.03.188
Nucleation‐Controlled Solution Deposition of Silver Nanoplate Architectures for Facile Derivatization and Catalytic Applications
F. Muench, R. Popovitz-Biro, T. Bendikov, Y. Feldman, B. Hecker, M. Oezaslan, I. Rubinstein, A. Vaskevich
Advanced Materials 30, 1805179, 2018
DOI: 10.1002/adma.201805179
Shape-selective electroless plating within expanding template pores: Etching-assisted deposition of spiky nickel nanotube networks
T. Böttcher, S. Schaefer, M. Antoni, T. Stohr, U. Kunz, M. Dürrschnabel, L. Molina-Luna, W. Ensinger, F. Muench
Langmuir 35, 4246-4253, 2019
DOI: 10.1021/acs.langmuir.9b00030