Adsorption of catechols at TiO2 single crystal surfaces.Charge transfer processes in photovoltaics and structure of novel biomedical materials.

This project will study the interaction of two molecules from a group of ring molecules called catechols, namely pyrocatechol and dopamine, with titanium dioxide (TiO2) surfaces. This interaction is of some interest for two reasons. Firstly it has been known for about 10 years or so that pyrocatechol (and other catechols including dopamine) adsorbed on TiO2 nanoparticles shifts the absorption of light from the ultraviolet region to the visible region of the electromagnetic spectrum. Since TiO2 is so cheap this may offer the potential for new cheap solar generation of electricity. It has been suggested in these types of cells, where a molecule is attached to a semiconductor surface that the strength of attachment is key in the efficient transfer of charge from the molecule to the surface. The other problem is that TiO2 is a photocatalyst capable of decomposing certain organic molecules so clearly the long term stability of the molecule must be understood and verified before this type of technology is developed. In addition, the catechol-TiO2 system is of interest as a possible targeted biomedical material. Unlike catechol, dopamine has a small chain on the side of the ring which can be grafted onto other molecules. In this way the dopamine can be attached to the surface of TiO2 nanoparticles and these functional molecules attached to the chain. Careful selection of the functional molecules can allow the nanoparticle TiO2 to respond to a specific stimulus. A polymer chain - Polytheyleneglycol (PEG) - effectively renders the nanoparticles invisible to the body's immune system. The inclusion of grafted temperature sensitive molecules along with the PEG means that at the site of an infection or disease the nanoparticles will clump together and form an opaque region in an x-ray. Since the particles are small they can be injected - thus quickly giving a surgeon information on where a problem may be located. Again one of the potential problems in these particles is the stability. We have been working with colleagues in the School of Pharmacy at Manchester, looking at the real nanoparticle systems but the surface structure is complicated by the presence of solvents and molecules used in the synthesis of the particles and chains. In this work we will use atomically clean surfaces and deposit carefully controlled amounts of pure catechols. Using the radiation facility at Elettra on Trieste we can determine a number of things about the nature of the chemistry at the surface including the orientation of the molecules and their stability over short timescales and different conditions. X-ray photoemission and absorption spectroscopies will be used to determine changes to the chemistry over time. In addition using a combination of x-ray absorption and photoemission we are able to infer the charge transfer time between the adsorbed molecules and the surface of the TiO2. We will study adsorption two different surfaces of TiO2 i.e. the rutile (110) and anatase (101), which arise from different crystal structures of TiO2. Anatase is the structure adopted by nanoparticulate TiO2 so our studies on this crystal will potentially give more realistic information. Rutile is a more widely studied material as it is easier to grow and obtain commercially. In fact we are one of the few groups who have carried out substantial research on anatase single crystal surfaces. Although some of what we have determined in previous work suggests organic acid molecules interact in similar ways on these two surfaces it is of some fundamental interest to determine whether this is also the case for the catechols. In addition the two different molecules will allow us to determine whether the presence of the side chain on the dopamine results in differences in the adsorption geometry or the chemical stability since this chain could potential react with the oxygen molecule through which the dopamine bonds to the surface.

The main beneficiaries of the scientific part of this research will be academics working in the area of charge transfer processes in dye sensitised solar cells. Since the mechanism of charge transfer in the system of interest is different to that proposed in the Graetzel type cells a detailed understanding could potentially lead to a new type of cell. In addition since functionalised TiO2 is of interest in biomedical materials academics in this area will benefit from this detailed study of the stability and bonding mechanisms of these catechol molecules. There may be benefits to industry developing dye sensitised solar cells and again to pharmaceutical/biomedical device industry in having information on the bonding mechanisms and charge transfer processes occurring in these systems. From the travel grant perspective the proposal offers good value for money. The science case has been peer reviewed by the Elettra peer review mechanism and the beamtime awarded as the research is deemed of fundamental interest in the fields outlined above, thus the cost of travel and subsistence for 2 UK scientists without the associated maintenance and staffing costs of the instrument is low. In addition the collaboration with the beamline staff will help to build further collaborations and will be an ideal opportunity for a young UK scientist (the PhD student) to obtain training using synchrotron radiation at an energy which is not currently available in the UK. It will also give them the opportunity to network with the scientists at Elettra. All of the results obtained will be published in high quality peer reviewed journals and at major UK and international conferences. The applicant is a member of the Joule centre and has actively been going out to the energy events organised by the N8 consortium and would welcome the chance to present data at these types of events specifically designed to promote synergy between academia and industrial partners