Singapore - Nanomaterials: Formation in a flash

The nanoscale titania pattern before and after heat-treatment

A new lithography technique enables the production of nanoscale patterns of titania for high-tech applications

Titanium dioxide, or titania, is an inorganic material commonly used as a whitening agent in food and toothpaste. It is also used as one of the main active ingredients in sunscreens. The properties that make titania useful in commercial applications — namely its whitening ability and high refractive index — are now being exploited in a wide range of technological applications.

One particular area of interest has been the application of titania in dye-sensitized solar cells — devices that can be used to convert sunlight into electricity. Such application often requires the formation of intricate surface patterns, with the key limiting factors for development being cost and speed of processing. Now, Ramakrishnan Ganesan, Mohammad Saifullah and co-workers1 at the A*STAR Institute of Materials Research and Engineering have described the use of a technique called step-and-flash imprint lithography (SFIL) to produce such patterns on the nanoscale.

“The precursor method to SFIL is thermal nanoimprint lithography, which is extremely time-consuming as it requires temperature-cycling processes to form a pattern,” explains Saifullah. “A mold could be pressed into a heated (and softened) resist material or a liquid precursor could be forced into a mold and then hardened upon heating.”

Newer processes eliminate the need for heating by using irradiation with ultraviolet (UV) light to harden the polymer. Although this process may be ideal for organic polymer materials, it is more problematic when using inorganic materials such as titania as the liquid precursor materials are highly viscous and do not spread easily. As a result, the dispensing nozzle may sometimes become blocked.

The chemicals used to make titania can also be unstable in solution, so the team had to identify a mixture of components that offered a combination of stability and low viscosity. “We found that an allyl functionalized titanium complex was stable in combination with other polymer precursors,” explains Saifullah. The final component of the mixture is a photoinitiator — which starts the polymerization process upon irradiation with UV light.

The mixture was dispensed onto the surface in the form of droplets, and the mold pressed into place to help the liquid spread. Irradiation with UV light results in hardening of the pattern, after which the mold can be removed. A final heating step burns away the organic material, leaving behind a shrunken version of the original pattern made from titania (see image). Significantly, the aspect ratio of the pattern is maintained after the heat-treatment process.

“Our current method is quite specific to titania, but after tackling this most important material, we hope to develop similar procedures for other inorganic materials,” says Saifullah.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering

References

1. Ganesan, R. et al. Direct patterning of TiO₂ using step-and-flash imprint lithography. ACS Nano 6, 1494–1502 (2012). | article

Singapore - Nanoparticle synthesis: Joined at the hip

The exposed nature of the gold surface in Janus nanoparticle gold-titania hybrids (left) leads to greater catalytic activity than eccentric (center) and concentric (right) structure. The protective titania coating confers durability on the catalyst.

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Hybrid 'Janus' nanoparticles made from gold and titania have high catalytic activity and extraordinary durability

As recently as twenty-five years ago, chemists considered gold to be one of the most inert metallic elements, until the discovery that nanoscale-sized dispersions of gold had high catalytic activity forced a re-think of old principles. Researchers soon found that gold nanoparticles could promote many industrially important reactions, such as the removal of harmful carbon monoxide gas from emission streams. Whilst the benefits of nanoscale gold are well-attested, preparing the material in a durable and reusable form remains a significant challenge that limits its uptake by manufacturers.

Work by the teams of Ming-Yong Han of the Institute of Materials Research and Engineering and Yong-Wei Zhang from the Institute of High Performance Computing both at A*STAR has revealed that the stability of gold nanoparticle catalysts can be enhanced by coating them with protective titania (TiO2) layers1. Conceived by co-author Zhi Wei Seh, an A*STAR National Science Scholar, this new technique produces so-called Janus nanostructures that retain nearly all the catalytic activity of bare gold nanoparticles without suffering from irreversible aggregation that diminishes the reactivity of the latter.

Named after the twin-faced Roman god of beginnings and transitions, Janus nanostructures join two or more equal-sized components together through very small junctions — an arrangement that maximizes the active surface area of each substance. The beneficial effects of pairing gold nanoparticles with titania is well known, but until the work by A*STAR researchers, a detailed understanding of the mechanism by which these two species fuse together had proved elusive.

Han and co-workers used an unconventional chelating compound called titanium diisopropoxide bis(acetylacetonate) to nucleate the growth of TiO2 onto gold at extremely slow rates. By carefully controlling the addition of this reagent to rod- and spherical-shaped gold nanoparticles, the researchers observed three distinct nanostructures (see image): a Janus geometry; a partially encapsulating ‘eccentric’ geometry; and a ‘concentric’ core-shell arrangement.

Catalytic experiments revealed that the reactivity and durability of gold-titania Janus structures have unique advantages over other nanoparticles. Due to the exposed nature of their gold surfaces, the former catalyze the reduction of the molecule 4-nitro phenol at much faster rates than eccentric and concentric nanoparticles whose gold surfaces are more confined. Furthermore, the protective TiO2 coating of the hybrid catalysts allowed them to be reused repeatedly with little loss of activity. In contrast, bare gold nanoparticles agglomerated into un-reactive clumps after just five usage cycles.

Futher theoretical investigations by the team revealed that the formation of Janus nanostructures as the energetically stable species is promoted by the addition of smaller volumes of the titania precursor — a finding that may help the researchers generate other metal–oxide hybrids for catalytic applications in the near future.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering and the Institute of High Performance Computing

References

1. Seh, Z. W. et al. Anisotropic growth of titania onto various gold nanostructures: Synthesis, theoretical understanding, and optimization for catalysis. Angewandte Chemie International Edition 50, 10140–10143 (2011). | article