A study of the photo-chemical properties of titanium dioxide.

The electron transfer properties of titanium dioxide pigments have been investigated by the adsorption of electron donors and acceptors and the detection of radical species by electron spin resonance spectroscopy. Electron transfer was found to be a thermal process, no photo-assistance being observed. A powerful electron acceptor, dichloro- dicyano p benzoquinone was used for a quantitative determination of the electron donor abilities of a number of pigments. The adsorption of electron acceptors from solution has been followed by ultra-violet absorption spectroscopy and ultra-violet and visible reflectance spectroscopy. Room temperature fluorescence of pigments and of compounds on pigments has been examined and compared with solution fluorescence. The photo-reduction of pigments in the presence of alcohols was studied and the formation of small quantities of photo-produced superoxide anion adsorbed on pigments has been confirmed. The photo-electric properties of pigments have been studied by use of a simple wet electrochemical cell containing a layer of pigment deposited on an inert substrate. Photo-voltages of a number of pigments have been measured and found to correlate well with their known durabilities in paint media; providing a rapid test for the estimation of pigment durabilities. The photo-voltages and equilibrium dark potentials are discussed in terms of the band model for titanium dioxide. Small photo-currents have been observed for pigments; anodic photo-currents flowing at positive biassing and small cathodic photo-currents at negative biassing. The initial photo-currents show a transient part which decays to leave a level photo-current. The spectral distribution of the photo-current has been determined and the temperature dependence of photo-voltage and photo-current examined. Anodic photo-currents have been sensitized by addition of a variety of alcohols to the electrolyte and are discussed in terms of the reaction of alcohols with photo-holes. Addition of small quantities of potassium iodide and bromide to the electrolytes have been found to decrease the photo-voltage, and for potassium iodide even resulting in a change in the direction of the photo-voltage. Anodic photo-currents are likewise decreased and can become cathodic.