Among different crystal structures and morphologies of TiO2, vertically oriented anatase nanotube arrays show the highest activity for solar energy conversion. However, the long-standing bottleneck is the fact that TiO2 is a wide band gap semiconductor, limiting its activation to the deep-blue and ultraviolet spectral region that contains but a small fraction (5%) of the incident solar energy. Herein, density functional theory (DFT) has been used to compare between density of states of bulk and nanotube forms of TiO2 by using different concentrations of N atoms. The wave functions were described using two different techniques; linear combination of atomic orbital (LCAO) and plane wave. Our results showed a shift in the calculated bandgap for bulk TiO2 only for small concentrations of N atoms as dopant. For TiO2 nanotube, the bandgap decreases as the concentration of N atoms increases. The effect of the diameter of TiO2 nanotubes on their optical and structural properties has also been investigated and discussed in details. Our study presents a protocol to fine tune the optical, electronic and structural properties of TiO2 for energy conversion applications.


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