The optical absorption edge of brookite TiO2

The optical absorption edge of brookite TiO2 was measured at room temperature, using natural crystals. The measurements extend up to 3.54 eV in photon energy and 2000 cm−1 in absorption coefficient. The observed absorption edge is broad and extends throughout the visible, quite different from the steep edges of rutile and anatase. No evidence of a direct gap is seen in the range measured. The spectral dependence of the absorption strongly suggests that the brookite form of TiO2 is an indirect-gap semiconductor with a bandgap of about 1.9 eV.