Effect of oxygen activity on the n-p transition for pure and Cr-doped TiO2

Titanium dioxide, TiO2, is commonly considered as an n-type semiconductor. The present work shows that TiO2 exhibits both n- and p-type semiconducting properties. It is shown that p-type properties may be imposed either by increasing of oxygen activity or incorporation of acceptor-type ions, such as chromium. This work reports the n-p transition for both pure and Cr-doped TiO2 (0.05 at % Cr) single crystals at elevated temperatures (1023 K – 1323 K) in the gas phase of controlled oxygen activity. The n-p transition was determined by the measurements of thermoelectric power as a function of oxygen activity in the range 10 Pa - 105 Pa. It is shown that the n-p transition line for Cr-doped TiO2 exhibits a sharp change at 1173 K that is related to the conversion in chromium oxidation state from trivalent below 1173 K to six-valent above. This effect, which is reflective of a dual mechanism of chromium incorporation into the rutile structure of TiO2, can be used for imposition of desired semiconducting properties that are needed for specific applications, such as solar-to-chemical energy conversion.