Ag nanoclusters on anatase single crystal TiO2 surfaces: the role of electronic structure in the enhanced photoactivity of Ag dosed TiO2 nanoparticles

Titanium dioxide (Titania/TiO2) is usually used in the form of a white powder. It exists in three different structures, rutile, anatase and brookite. It has uses which range from a white pigment (used in mayonnaise and toothpaste as well as paints) to generating electricity from sunlight where it is "activated" with a dye. Its main advantages are that it is not toxic and cheap to produce on a large scale. In the 1970's it was found that titania in the presence of ultra violet light could also be used to split water into hydrogen and oxygen, which could then potentially be used as fuels. However despite much active research the full potential in this area has not been realised. Related to this photocatalytic property however, titania has been used to breakdown organic chemicals and also kill bacteria in waste water. It is also this property which is used in self-cleaning windows which have a thin film of titania on them. Many of these applications use the titania consisting of particles a few tens or hundreds of nanometres in size, which forces the titania to adopt the anatase structure.
Recently it was found that adding nanometre sized clusters of silver to the nanoparticles of titania dramatically improved their photocatalytic activity. However it is not known why this is the case. The electronic structure (i.e. the states where electrons are located) in the bonds between the titanium and oxygen (the valence band) is thought to be key to the photocatalytic activity of titania. In this work we want to look in detail at these valence electrons using a synchrotron radiation source in Sweden. Synchrotron radiation allows us to tune the energy with which we probe our samples, which in turn allows us to map the contributions of the various elements to the valence band. The work requires relatively low energy light which is not currently available in the UK.
The aim is to understand how silver modifies the electronic structure of the titania and correlate this to the increased photocatalytic activity. By understanding how this system works we can use the minimum amount of silver to enhance the activity of the catalyst and therefore keep the costs of developing these catalysts down. We may also help other researchers identify other metals or materials which can increase the efficiency of the catalysts further.

Improving the photocatalytic activity of titania can potentially lead to cheap water splitting to form hydrogen and oxygen using sunlight. Clearly this would be a means to reducing fossil fuel consumption. In addition, titania is being widely investigated and indeed used for the cleaning of wastewater, acting both as a bactericide and also in the degradation of organic pollutants. Since titania is cheap this potentially impacts on the costs of waste water treatment and may benefit developing countries where access to clean drinking water is limited.

UK chemical companies may benefit from the research in that cheap photocatalysts capable of "cleaning" effluent before discharge back to the water supply will cut their costs both in UK and overseas facilities. These two points alone will lead to reduced costs in providing a cleaner and greener global economy.

More directly the research itself will provide direct training in the use of low energy synchrotron radiation techniques to two UK postgraduate students. These facilities are currently not available in the UK so this type of training is invaluable to the next generation of surface science and catalysis researchers to ensure these skills are not lost, particularly when the two new beamlines ARPES and VERSOX become available at the Diamond Light Source in 2015 and 2014 respectively.