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A representative nitrogen doped titanium dioxide sample (denoted as N-doped TiO_2) was prepared by treating nanotubular titanic acid (H_2Ti_2O_5·H_2O,NTA in short) in flowing NH_3 at an elevated temperature of 500℃ for 4 h.The sub-band structure within Eg of N-NTA-500 was determined by means of photoluminescence (PL) spectrometry and ultraviolet-visible light-near infrared (UV-Vis-NIR)diffusion reflectance spectrometry (DRS).Moreover,the photocatalytic activity of N-NTA-500 was evaluated by performing the redox reaction of propylene under the irradiation of alternate on-and-off visible light,using commercially available P-25 titania as a comparison.It was found that a large amount of single-electron-trapped oxygen vacancies (denoted as Vo~·) formed a sub-band within the Eg of N-NTA-500,contributing to extend the optical absorption of N-doped TiO_2 into visible light region.The sub-band width,the bottom energy above the top edge of valence band,and the top energy below the bottom edge of conduction band were determined to be 0.24 eV,1.79 eV,and 0.99 eV,respectively.Results and discussion:Each of the four PL spectra at an excitation wavelength of 360-390 nm can be deconvoluted into two bands.The peak positions of the two deconvoluted PL bands for each of the four PL spectra at λ_(ex)=360-390 nm are unchanged and located at 465 nm and 570 nm,respectively,implying that they are not originated from the back band-to-band transition but the e~--h~+recombination on two surface states.The band at 465 nm represents the emission on shallow surface state (2.66 eV above valence band edge),while that band at 570 nm represents the emission on deep surface state (2.17 eV above valence band edge).Moreover,the PL peak intensity decreases with increasing excitation energy,indicating that the photogenerated energy-enriched electrons are firstly relaxed to the bottom of the conduction band and then transferred to the surface state,and the larger the value of E_(ex),the more the energy loss.Therefore,it can be concluded that the excitation at λ_(ex)≤400 nm for N-NTA-500 is dominated by band-to-band transition,while E_g of the N-doped TiO_2 is identical to that of TiO_2 (anatase),I.e.,3.1 eV at 400 nm.The emission mechanism with respect to band-to-band transition of N-NTA-500 in an excitation wavelength range of 360-390 nm is depicted.