Ti3+ states induced band gap reduction and enhanced visible light absorption of TiO2 nanotube arrays: Effect of the surface solid fraction factor

• TiO2 layers were produced by anodizing a Ti substrate at different potentials.
• EIS is an appropriate method to follow the surface states properties of TiNT.
• UV pretreatment provides a high photoactivity in visible light.
• Higher is the surface solid fraction factor of TiNT higher is the visible light photoconversion.
• Ti3+ states induced band gap reduction.

In this work, a simple strategy was used to prepare Ti3+ surface defect states through the simple UV irradiation of TiO2 nanotubes (TiNT) formed in ethylene glycol/NH4F/H3PO4/H2O mixture at different anodizing potentials. The band gap electronic states created by Ti3+ are mostly responsible for the effective narrowing of band gap.The effect of anodizing potential on surface states of TiO2 nanotubes/electrolyte interface has been studied under UV irradiation. A quantitative and qualitative analysis of surface states of TiNT formed at different anodizing potential was performed using capacitance measurements. These measurements allow us to provide evidence for the presence of monoenergetic surface states in the band gap. The energy levels and the density of surface states have been determined. Both density and energetic location are very dependent on the geometric characteristics. Relatively wide distribution of these states in a range of electric energy of 0.17–0.35 eV below the edge of the conduction band was obtained according to the anodizing potential. The optical absorption at 630 nm may be attributed to deep level states originated from oxygen vacancies associated with Ti3+. The presence of energetic level states and elevated surface solid fraction factor of TiNT are the determining factors in enhancing the photoactivity in visible light as evidenced by high photocurrent densities up to 16 mA cm−2.