Our research is focused on designing and making new hybrid and inorganic nanomaterials for enabling technologies, with emphasis on functional thin films, micropatterns and low-dimensional nanostructures via chemical synthesis.
We use colloidal solutions of nanoparticles, inorganic oligomers, complex clusters, or quasi-low dimensional nanostructures such as dispersed nanowires or nanosheets, as ‘building blocks’ for the synthesis of nanomaterials. These objects, with dimensions of ~0.5 to ~100 nm, are assembled for the construction of nanomaterials.
Most inorganic materials find their application in the form of thin or ultrathin films. A few examples of functional nanostructured inorganic materials that have been developed in this laboratory are shown in Figure 1 below.
Figure 1. (A) Ultrathin ZrO2 film; (B) Amorphous titania membrane; (C) Microsieve-supported MCM-48 layer as ionic gate; D. Mesoporous silica membrane.
Figure 1a is a scanning electron microscope (SEM) image of the cross-section of an ultrathin high-k dielectric ZrO2 film on Si made from sintered nanoparticles (on native oxide). Due to the small size of the precursor particles, the temperature at which a dense film was formed by sintering was several hundreds of degrees lower than in normal ceramics. Other electronic materials that the group is working on are Pb(Zr,Ti)O3 (PZT) and BaTiO3 nanoparticles and thin films.
Figure 1b and 1d are examples of a nanoporous amorphous titania film of ~100 nm thickness on a porous support. The titania layer has pores with a size of ~0.9 nm and can act as a filtration membrane to filter out molecules with molecular weights as low as 400 g/mol from water. Figure 1D shows a similar kind of membrane made of mesoporous MCM-48 silica. It has slightly larger ordered pores of 2-3 nm diameter.
Microsieve-supported ion-selective gate made of the same silica MCM-48 phase is shown in Figure 1c. These gates, with a size of less than 1 micrometer thickness, have been used successfully to actively transport selected ions from one side of the gate to another.
In recent years, our group has been working on relatively cheap and upscalable methods to pattern functional inorganic materials on micrometer and nanometer scale. Such patterns could find application in electronics, sensors, and energy materials technologies (solar cells, fuel cells, etc.). We use soft-lithographic techniques for this purpose and can make single and multiple layer patterns with resolutions down to 100 nm on inch scale. Shown below in Figure 2 are a few examples from the work that we carried out with a VIDI research grant (Vernieuwingsimpuls) from the Dutch Science Foundation (NWO).
Figure 2 (a): Micropatterned surface with oriented ZnO nanorods; (b): PZT pattern with 380 nm wide lines on silicon; (c): Dual layer line pattern consisting of BaTiO3 (bottom layer) and Y-stabilized ZrO2 (2nd layer); (d) Micropatterned surface with clusters of single-crystalline nanowires.
In 2006 we invented a novel hybrid organosilane material that has molecular sieving properties and can be used for the separation of water from organic solvents, or separation of gases based on molecular size differences. The material is a sol-gel organic-inorganic hybrid that combines the advantageous properties of ceramics, like thermal and chemical stability, with the hydrolytic stability and flexibility of polymers. It is stable at high temperatures in the presence of steam, outperforming any other molecular separation membrane made till date.
Figure 3: (left) Artist’s impression of hybrid molecular separation membrane, showing how the nature of the organic moiety in the membrane affects the separation selectivity; (right) Scanning electron microscope picture of the cross section of a 100 nm thick hybrid silica layer with a pore size < 0.3 nm on top of a mesoporous gamma alumina layer.
The material was patented together with ECN and has been commercialized under the name HybSi®. Hybrid molecular separation membranes have a high potential to make it into large-scale industrial applications for selective removal of water from biofuels and solvents, or separation of hydrogen. Two stories (in Dutch) on the discovery of this material, the background of sol-gel membranes and their development and testing can be found on Kennislink.nl in two articles (#1 from 2008; #2 from 2011). We are currently still working on the development of novel materials for these applications.
We make metal and metal oxide nanowires by templated electrodeposition in the pores of polycarbonate track-etched membranes and anodic alumina membranes. The wires have diameters of 50-500 nm, and wire lengths of 3-30 micrometer. The example shown in Figure 4a is a nanowire with a metallic Ni core, and a iron oxide shell.
Figure 4: (a) Ni-Fe2O3 core-shell nanowires; (b) titania nanosheets; (c) principle of dielectrophoresis for nanowire positioning and alignment; (d) a 100 nm diameter nanowire between two gold micro-electrodes
We also made segmented metal-metal oxide nanowires such as gold-zinc oxide Au|ZnO and silver-zinc oxide Ag|ZnO that show current-rectifying diode behavior. They can be considered as nanoscale electronic circuit elements. An example of a nanowire between two gold microelectrodes is shown in Figure 1d. We use dielectrophoresis (Figure 1c) to position and align the wire from solution in as non-uniform electrical field.
Figure 1b shows a non-continuous film of crystalline titania nanosheets. The sheets have a thickness of less than 1.0 nanometer, but their lateral dimensions are in some cases > 50 micrometer. Nanosheets are the inorganic equivalent of graphene and are under investigation for a number of possible applications.
For more information, please contact Prof. dr. ir. André ten Elshof.