Oxygen defect dependent variation of band gap, Urbach energy and luminescence property of anatase, anatase–rutile mixed phase and of rutile phases of TiO2 nanoparticles

• Vacuum annealed TiO2 has small band gap and high Urbach energy.
• The Urbach energy is higher in mixed phase TiO2 than that in pure anatase or rutile phase.
• The visible emission peaks are due to excitons and charged oxygen defects.
• Oxygen vacancies are responsible for the long lifetime of the carriers.

TiO2 nanoparticles are prepared by a sol–gel method and annealed both in air and vacuum at different temperatures to obtain anatase, anatase–rutile mixed phase and rutile TiO2 nanoparticles. The phase conversion from anatase to anatase–rutile mixed phase and to rutile phase takes place via interface nucleation between adjoint anatase nanocrystallites and annealing temperature and defects take the initiate in this phase transformation. The samples are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–vis and photoluminescence spectroscopy (PL). Anatase TiO2 exhibits a defect related absorption hump in the visible region, which is otherwise absent in the air annealed samples. The Urbach energy is very high in the vacuum annealed and in the anatase–rutile mixed phase TiO2. Vacuum annealed anatase TiO2 has the lowest emission intensity, whereas an intense emission is seen in its air annealed counterpart. The oxygen vacancies in the vacuum annealed samples act as non-radiative recombination centers and quench the emission intensity. Oxygen deficient anatase TiO2 has the longest carrier lifetime. Time resolved spectroscopy measurement shows that the oxygen vacancies act as efficient trap centers of electrons and reduce the recombination time of the charge carriers.

Ti3+ forms shallow trap centers and oxygen vacancies form both shallow and deep trap centers. These defects reduce the band gap and increases carrier concentration.Figure optionsDownload as PowerPoint slide