Au/TiO2 Nanoparticles with High Photocatalytic Activity

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Titanium dioxide can be used for treatement of polluted waters. Photo by Alan Liefting.

Titanium dioxide can be used for treatment of polluted waters. Photo by Alan Liefting.

Researchers from Karlsruhe Institute of Technology (Germany) found a new route to prepare composite nanoparticles made of titanium dioxide and gold; their method allowed a better control over the amount and the shape of gold nanoparticles used. These composite materials show improved photocatalytic activity with UV and visible light; which makes them suitable for several biological applications.

Titanium Dioxide

Titanium dioxide or titania (TiO2) is a material used for many different applications, due to its photocatalytic properties. Under suitable light irradiation, TiO2 can react with water vapor and/or oxygen present in the atmosphere, to generate reactive oxygen species (ROS). Such active species can subsequently interact with other molecules present on the surface of the material, leading to further chemical reactions.

Such photocatalytic behavior is due to the semiconductor nature of TiO2; in fact light irradiation generates charges (negative electrons and positive holes) across the bandgap.

TiO2 photocatalytic properties are employed in several fields, which include organic pollutants degradation (treatment of industrial waters), water sterilization (elimination of bacteria from drinking water), catalysis for oxidation reactions (i.e. water splitting) and enzyme activation.

Photocatalytic Activity Improvements

Although the mechanisms behind the photocatalysis are well known, several parameters can affect TiO2 photocatalytic activity.

The activity can be higher, for instance, if TiO2 is used in the form of nanoparticles; in this case the dimensions and the shape of the particles are crucial for the materials’ performances.

Moreover, the combination with other materials can also have a noticeable effect on the photocatalytic activity; noble metal particles such as gold (Au) or silver (Ag), for instance, can affect the photocatalytic activity. For this reason, there has been a lot of research in this field and several methods to prepare composite material Au/TiO2 or Ag/TiO2 materials were investigated.

Structure of the material. Photo by Marko Miljevic.

Structure of the material: titania and gold nanoparticles connected by the organic molecule Photo by Marko Miljevic.

Novel Preparation Method

Researchers from Karlsruhe Institute of Technology (Germany) developed a new method to prepare Au/TiO2 nanoparticles (NPs); the results were published in Advanced Functional Materials on the 9th of September 2013.

Dr. Ljiljana Fruk, one of the scientists involved in the study, explains the novelty of her work to Decoded Science as follows:

“What we did was to use TiO2 NPs and attach gold NPs on their surface. To prepare Au NPs, we considered a stabilizer which is different from the ones normally employed; we used N-(3-4-dihydroxyphenethyl)-5-(1,2-dithiolan-3-yl)pentamide (LA-DA). The particularity of LA-DA is its bifunctionality; this means that it can form chemical bonds with both TiO2 and gold, acting as a bridge.

In this way, we managed to have a much better control of the shape, the size and the amount of Au NPs attached on titania surface.”

Microscopy image of the material. Larger particles: titania; smaller sarker particles: gold. Photo by Pascal Bockstaller.

Microscopy image of the material. Larger particles are titania; smaller sarker particles are gold. Photo by Pascal Bockstaller.

Photocatalytic Performances

Dr. Fruk and her coworkers prepared composite materials with different quantities of gold, between 0.1 to 50 % weight; in this way they could see which  Au concentration had the best effect on the photocatalytic activity.

“To test the photocatalytic activity we monitored the reaction of an enzyme (horseradish peroxidase) with an organic molecule (ampliflu red) under either UV or visible light irradiation (λ= 365 or 470 nm). This reaction leads to the formation of a fluorescent molecule (resorufin); hence, measuring the intensity of the fluorescent signal, we could see how active the material was.”

The results showed that the addition of gold improves the photocatalytic performances remarkably. With UV irradiation, a gold concentration of 2 % causes a six-fold increase of the activity in comparison with the untreated NPs with no gold; higher Au content does not make any additional difference. With visible light, similar results are possible, although a higher gold content is necessary (50 %).

Promising Results, Possible Applications

According to Dr. Fruk, “We used a simple method to prepare a composite material which has a much better properties than the commercial one. The activity with visible light is also particularly interesting; this means these systems can be used in biological applications. Moreover, enzymes such as horseradish peroxidase can be activated by light on this surface; this is also interesting, as you normally need hydrogen peroxide for the activation.

Although nanomedicine and biosensing are the sectors most interesting for us, in principle these materials could be used also as antibacterial agents or as catalysts.”

Reacting With Light: Photocatalytic Activity

With new research, science improves the preparation of nanoparticles which have a better reaction to light. Could nanoparticles make the Earth a greener place?  These new compounds have many applications, including removing pollution from water, so it’s very possible.

Sources

Miljevic, M.et al., Enhanced Photocatalytic Activity of Au/TiO2 Nanocompsite Prepared Using Bifunctional Bridging Linker. (2013). Advanced Functional Materials. Accessed September 12, 2013.

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