Substituent and anchor group effects in bonding to TiO2 Single Crystal Surfaces: Enhancing Solar Conversion Efficiency and Corrosion Inhibition.

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With concerns over the environmental impact of fossil fuel use and the issue of sustainability of fossil fuels, generation of fuels such as hydrogen or methanol and direct generation of electricity from nanoengineered materials is an area of considerable interest. Titanium dioxide (TiO2), for example, has long been known to be capable of producing hydrogen from water under illumination and can also be used to generate a photo voltage. However TiO2 absorbs in the ultraviolet region of the electromagnetic spectrum, and the amount of ultraviolet at the earth's surface is relatively low, when compared to visible light. One approach to enhance absorption of visible light is to functionalise the oxide with a material which will absorb in the visible region of the solar spectrum followed by charge transfer to the TiO2 which then drives the photoelectrochemical reaction or produces the photovoltage. This has led to the advent of dye sensitised solar cells, or the so called Graetzel cell which have an efficiency of around 15 %. These dyes are often bound to the nano structured TiO2 by carboxylic acid (COOH) groups to form Ti-O-C-O-Ti + H at the surface. The ordering, and therefore the packing density, and strength of bonding thought to play a part in the efficiency of the cells, since ultimately they control the charge transfer from the dye to the surface. Defects in the TiO2 substrate are also thought to contribute to limitations in efficiency.Changing the chemistry of substituent groups on benzoic acids affects the acidity of the carboxyl group, and make it more or less likely to give up its proton when it attaches to the oxide material. We wish to study whether this change in the acidity has an effect on the boding strength of the molecules. We also wish to investigate whether having three oxygen atoms, as found in phosphonic acid (R-P=O(OH)2) where R is an organic side chain or ring (here we will study R=C6H5). Synchrotron radiation allows us to utilise tunable X-rays to determine the geometry of the molecules when they are adsorbed on the surface. It also allows us to look at where the electrons are in the material and recently we discovered that we can "see" electrons being injected into the TiO2 substrate when a dye-molecule is attached to the surface, via a spectroscopic fingerprint. We wish to use this fingerprint to determine how efficiently charge is injected into the surface. In addition the techniques available at the synchrotron allow us to determine the chemistry of the molecule, and importantly its stability. Clearly any dye molecule must remain stable in order for a device to work, and loss of the bonding to the substrate would have a negative impact. This fundamental work should allow chemists designing dyes to choose the most appropriate side groups to ensure strong bonding to the TiO2 and to obtain the maximum efficiency for a particular dye.