Nanostructures transcend grain boundaries in metals

Scientists in Poland claim they can overcome grain boundaries to build uniform nanoscale structures on metal surfaces. 

Coatings made of nanostructures with precisely selected sizes and shapes make it possible to control material properties. But the ability to coat large metal surfaces with precisely formed nanostructures has long-eluded materials scientists. 

Polycrystalline metals are composed of multiple crystallites separated by interfaces called grain boundaries. The interface is a defect where oxide growth initiates during anodisation, disrupting the formation of ordered nanostructures. 

Researchers at the Institute of Nuclear Physics of the Polish Academy of Sciences believe they have the solution. 

'The most common and cheapest method is to use two-step anodisation, but this approach does not overcome the microstructure of a titanium substrate,' says Juliusz Chojenka, who led the research. By combining nanosphere lithography with anodisation, they pre-pattern the titanium surface so that nanotube formation is guided rather than dictated by the metal’s microstructure.

Titanium was chosen for its practicality, functionality and biocompatibility. A reagent-grade titanium substrate was used with 99.7% purity, to reduce the occurrence of nucleation sites. 

First, the sample was polished to minimise defects – using sandpaper, then a solution of perhydrol and colloidal silica suspension. After pre-patterning the samples with a monolayer of 287nm polystyrene nanospheres, a thin, 30nm, titanium layer was deposited via e-beam evaporation, creating a honeycomb-like mask for subsequent anodisation.

Anodisation occurs in an ethylene-glycol-based electrolyte containing 0.3% ammonium bifluoride and 1.5% deionised water at 60V for 90 minutes. The electrolyte is pre-aged using a sacrificial titanium electrode to avoid mask dissolution. 

The resulting titanium oxide nanotubes are 15µm long, with inner-tube diameters ranging from 60-79nm, depending on the length of plasma etching of the polystyrene nanospheres. 

For larger areas, says Chojenka, the results are the same. Annealing at 450°C and 650°C produces mechanically stable titanium dioxide nanotubes suitable for further functional testing. 

Standard titanium anodisation is usually subject to severe physical limitations due to the size of the crystal grains in the metal matrix. Preceding anodisation with nanoparticle lithography allows the electric field distribution to be modified on the titanium surface, forcing long-range ordering of the nanostructures during anodisation. 

'Our approach builds on earlier works, as we are able to merge two nanopatterning techniques that are both cost-effective and scalable,' Chojenka explains. The method also allows precise control over nanotube diameter and spacing, which can influence catalytic efficiency. 

'We expect to increase the efficiency of the catalytic activity for smaller nanotube diameters, but only if the density of the nanotubes of different diameters is kept at the same level,' he adds.

Potential applications are diverse. Chojenka believes large-scale, hexagonally ordered, titanium dioxide nanotubes could act as catalysts, sensors, memristive devices, or drug-delivery matrices.

Large-area anodisation is commonly used for protective coatings on jewellery, bicycles and car components. And while nanosphere lithography is not commercially available, it is rapidly developing for this purpose

Chojenka says, 'Anodisation is scaleable to metres, and for nanosphere lithography, we tested samples up to 4in [10cm] diameter. Reports by other authors show, however, that this technique can be rescaled to even bigger areas. Novel electronics or targeted drug delivery systems do not require more than a few square centimetres – for these applications, it would be one or two orders of magnitude higher than we currently achieve.'

The researchers plan to investigate application in other transition metals. 'Metals that form nanotubes after anodisation, such as tantalum or iron, should show similar results to our findings,' says Chojenka. 'For metals that do not, for example tin or zinc, ordering may only be on the surface.'

While there is more work to be done in scaling for industrial applications, the research seeks to demonstrate that when it comes to growing nanotubes on metals, boundaries need not be barriers.