Falk Muench

Ligand-optimized electroless synthesis of silver nanotubes and their activity in the reduction of 4-nitrophenol

A facile electroless plating procedure for the controlled synthesis of nanoscale silver thin films and derived structures such as silver nanotubes was developed and the products were characterized by SEM, TEM and EDS. The highly stable plating baths consist of AgNO(3) as the metal source, a suitable ligand and tartrate as an environmentally benign reducing agent. Next to the variation of the coordinative environment of the oxidizing component, the influence of the pH value was evaluated. These two governing factors strongly affect the plating rate and the morphology of the developing silver nanoparticle films and can be used to adapt the reaction to synthetic demands. The refined electroless deposition allows the fabrication of homogeneous high aspect-ratio nanotubes in ion track etched polycarbonate. Template-embedded metal nanotubes can be interpreted as parallelled microreactors. Following this concept, both the silver nanotubes and spongy gold nanotubes obtained by the use of the silver structures as sacrificial templates were applied in the reduction of 4-nitrophenol by sodium borohydride, proving to be extraordinarily effective catalysts.

Fabrication of Single Cylindrical Au-Coated Nanopores with Non-Homogeneous Fixed Charge Distribution Exhibiting High Current Rectifications

We designed and characterized a cylindrical nanopore that exhibits high electrochemical current rectification ratios at low and intermediate electrolyte concentrations. For this purpose, the track-etched single cylindrical nanopore in polymer membrane was coated with a gold (Au) layer via electroless plating technique. Then, a non-homogeneous fixed charge distribution inside the Au-coated nanopore was obtained by incorporating thiol-terminated uncharged poly(N-isopropylacrylamide) chains in series to poly(4-vinylpyridine) chains, which were positively charged at acidic pH values. The functionalization reaction was checked by measuring the current-voltage curves prior to and after the chemisorption of polymer chains. The experimental nanopore characterization included the effects of temperature, adsorption of chloride ions, electrolyte concentration, and pH of the external solutions. The results obtained are further explained in terms of a theoretical continuous model. The combination of well-established chemical procedures (thiol and self-assembled monolayer formation chemistry, electroless plating, ion track etching) and physical models (two-region pore and Nernst-Planck equations) permits the obtainment of a new nanopore with high current rectification ratios. The single pore could be scaled up to multipore membranes of potential interest for pH sensing and chemical actuators.

4-(Dimethylamino)pyridine as a Powerful Auxiliary Reagent in the Electroless Synthesis of Gold Nanotubes

Gold nanotubes of small particle sizes down to 5 nm and high aspect ratios were synthesized in ion track etched polycarbonate following a rational reaction design. 4-(Dimethylamino)pyridine (DMAP) was employed to adjust the electroless deposition by interfering with the autocatalytically active gold surface. Modification of the pH value and DMAP concentration led to a wide range of products which were characterized by SEM, TEM, and EDS. Filigree nanotubes of 10-15 nm wall thickness and 5.0 ± 2.1 nm grain size were obtained as well as robust and free-standing structures proving homogeneous deposition along the whole template length of 30 μm. Template-supported gold nanotubes were applied in the UV-vis monitored reduction of 4-nitrophenol by sodium borohydride under pseudo-first-order conditions. They proved to be a reliable microfluidic system of excellent catalytic activity coming up with an apparent rate constant of 1.3 × 10(-2) s(-1). Despite a high flow rate, the reaction showed 99% conversion after a distance of just 60 μm.

Electroless synthesis of platinum and platinum–ruthenium nanotubes and their application in methanol oxidation

A dependable and mild deposition procedure for the first electroless synthesis of platinum nanotubes in ion track etched polycarbonate was developed. It utilizes ethylenediamine to adjust the reactivity of Pt(IV) towards reduction and allows the controlled deposition of nanoscale platinum films, wires and tubes at room temperature. Single crystal structure determination proves the formation of Pt(II) as a side product next to elemental platinum. Highly polycrystalline nanostructures of 100 to 900 nm diameter and up to 30 μm length were obtained and characterized by TEM, SEM, EDS and XRD. The platinum nanotubes showed high activity in the electrooxidation of methanol in acidic environment. To illustrate the possibilities for synthesizing bimetallic nanotubes in the presented system, ruthenium was introduced by electroless and spontaneous deposition methods. The corresponding surface-normalized current densities are strongly dependent on the preparation method and can surpass commercial fuel cell catalysts, confirming the efficiency and flexibility of electroless metal plating in the preparation of nanomaterials.

Fabrication of porous rhodium nanotube catalysts by electroless plating

A versatile electroless plating procedure for the fabrication of rhodium nanomaterials was developed, leading to deposits consisting of loosely agglomerated metal nanoparticles. By using carbon black as the substrate, supported rhodium nanoparticle clusters were obtained. In combination with ion track etched polymer templates, the deposition protocol allowed the first direct synthesis of rhodium nanotubes. Polymer dissolution provided access to well defined, supportless and free-standing rhodium nanotubes of nearly cylindrical shape, 300 nm opening diameter, 28 μm length and 50 nm wall thickness. The characterization by SEM, TEM, EDS and XRD confirmed the purity of the deposit, displayed a small particle size of approximately 3 nm and revealed gaps in the range of a few nanometers between the rhodium particles. BET analysis verified the presence of pores of <5 nm. To evaluate the electrocatalytic potential of the rhodium nanotubes, they were applied in the amperometric detection of hydrogen peroxide. Compared to classical nanoparticle-based sensing concepts, improved performance parameters (sensitivity, detection limit, and linear range) could be achieved.

Template-based synthesis of metallic Pd nanotubes by electroless deposition and their use as catalysts in the 4-nitrophenol model reaction

The method of electroless plating offers a facile synthesis route to high-aspect ratio metal nanotubes. Despite the simplicity of the method, the traditional approach involves hazardous and toxic chemicals; to change this Green Chemistry has come into play. Since both, nanotechnology and Green Chemistry, have become important research topics, the combination of these offers new opportunities. According to the concept of Green Chemistry, Pd nanotubes were produced by using nonhazardous chemicals. The challenge was to rebuild the sensitisation and activation process of electroless plating and to find green reaction parameters for controlling the auto-catalysed heterogeneous nucleation of Pd. The produced Pd nanotubes were characterised by SEM and EDX. Furthermore TEM characterisation was done for determining the structural properties of Pd NTs. The efficiency of the reaction was quantified by ICP-OES and XPS measurements. To illustrate that green synthesized nanomaterials can be compared with conventionally prepared catalysts, the Pd nanotubes were tested in a model reaction to determine their catalytic activity. For this purpose the reduction of 4-nitrophenol to 4-aminophenol with NaBH4 was chosen, which also finds application in the synthesis of acetaminophen.

Thermal stability of electrodeposited platinum nanowires and morphological transformations at elevated temperatures.

Pt nanowires were prepared by template electrodeposition using ion track etched polymer membranes and analysed with respect to their thermal stability. Driven by Rayleigh instability, the polycrystalline Pt nanostructures experienced structural transformations and finally fragmented into linear chains of nanospheres at temperatures much below the melting point of bulk Pt. Morphological changes were systematically studied by electron microscopy and compared with previously reported results on other metal nanowires and theoretical predictions. In addition, nanowires could readily be interconnected to two-dimensional assemblies by taking advantage of the rapid diffusion processes. This study will help to predict the durability of integrated nanowires and contributes to the understanding of thermal-induced transformations for polycrystalline nanowires.

Free-Standing Networks of Core–Shell Metal and Metal Oxide Nanotubes for Glucose Sensing

Nanotube assemblies represent an emerging class of advanced functional materials, whose utility is however hampered by intricate production processes. In this work, three classes of nanotube networks (monometallic, bimetallic, and metal oxide) are synthesized solely using facile redox reactions and commercially available ion track membranes. First, the disordered pores of an ion track membrane are widened by chemical etching, resulting in the formation of a strongly interconnected pore network. Replicating this template structure with electroless copper plating yields a monolithic film composed of crossing metal nanotubes. We show that the parent material can be easily transformed into bimetallic or oxidic derivatives by applying a second electroless plating or thermal oxidation step. These treatments retain the monolithic network structure but result in the formation of core-shell nanotubes of altered composition (thermal oxidation: Cu2O-CuO; electroless nickel coating: Cu-Ni). The obtained nanomaterials are applied in the enzyme-free electrochemical detection of glucose, showing very high sensitivities between 2.27 and 2.83 A M-1 cm-2. Depending on the material composition, varying reactivities were observed: While copper oxidation reduces the response to glucose, it is increased in the case of nickel modification, albeit at the cost of decreased selectivity. The performance of the materials is explained by the network architecture, which combines the advantages of one-dimensional nano-objects (continuous conduction pathways, high surface area) with those of a self-supporting, open-porous superstructure (binder-free catalyst layer, efficient diffusion). In summary, this novel synthetic approach provides a fast, scalable, and flexible route toward free-standing nanotube arrays of high compositional complexity.

Nano- and microstructured silver films synthesised by halide-assisted electroless plating

Electroless silver plating baths were modified with different amounts of chloride and bromide, and the effect on the deposition kinetics and the morphology of the resulting silver films was evaluated. The baths were based on silver nitrate and tartrate as the metal source and the reducing agent. Ethylenediamine was used as the complexing agent to suppress silver halide precipitation. With increasing halide concentration, a reduction in the deposition rate and a decreased tendency towards three-dimensional nucleation was found. Bromide affected the plating reaction more strongly than chloride. The deposit morphologies range from coarse-grained, compact particle aggregates over bimodal structures composed of island-like microparticles and smaller particles of varying geometry to shape-controlled films dominated by plates with a triangular or hexagonal shape. The fabrication of silver films of adjustable micro- and nanostructure is relevant for various applications such as heterogeneous catalysis, sensing and plasmonics. As an example for structural tailoring enabled by the outlined reaction system, we created a biomimetic, self-cleaning coating possessing a static contact angle of 165 ± 3° and a tilt angle of <3°. To this end, a hydrophobic metal surface was designed which exhibits a superimposed roughness on the micrometre and submicron scale. The former was defined by the silver deposition, the latter by consecutive galvanic replacement. To achieve superhydrophobic properties, the metal surface was coated with an alkanethiol self-assembled monolayer.

Metal Nanotubes and Nanowires with Rhombohedral Cross-Section Electrolessly Deposited in Mica Templates

Electroless plating is a facile wet-chemical process for the creation of metal thin films on arbitrary substrates, which can be used to produce intricate nanomaterials. In this study, we demonstrate how nanotubes and nanowires can be electrolessly deposited in the rhombohedral pores of ion-track etched muscovite mica templates. Mutual optimization of the activation and plating reactions proved to be essential for the fabrication of well-defined nanostructures of an aspect ratio (length-to-diameter) of up to approximately 70. By repeating the activation procedure utilizing the redox couple Sn(II) and Ag(I), a high density of Ag nanoparticle seeds could be deposited on the template surface, which was required to initiate metal film nucleation with nanoscale homogeneity. Furthermore, it was necessary to adapt the plating reaction to ensure sufficient diffusion of the reagents into the depth of the template pores. To prove the flexibility of the process and to evaluate the effect of the intrinsic film morphology on the shape of the resulting nanostructures, three different plating reactions were applied (Ag, Au, Pt). If the size of the deposited metal particles approached the dimension of the template pores, only wire-like structures of moderate shape conformity were obtained. Electroless plating protocols which yield homogeneous deposits consisting of small nanoparticles allowed exact replication of the pore shape. Under consideration of the above-mentioned requirements, electroless plating displays an effective and versatile route toward the fabrication of parallel arrays of angular metal nanotubes and nanowires in the chemically and thermally robust mica templates. By simply immersing the templates in aqueous plating solutions for an appropriate time, well-defined metal nanomaterials for application in, for example, plasmonics, catalysis, or molecular separation are obtained.