Significant Enhancement in Visible Light Absorption of TiO2 Nanotube Arrays by Surface Band Gap Tuning

Band gap tuning of the wide band gap semiconductor, TiO2, has great importance due to its versatile properties in solar cell applications. Visible light activity of TiO2 can enhance the efficiency of the third-generation solar cells by acting itself as light absorbing material. In this study, we demonstrate a surface structure modification and band gap tuning of TiO2 nanotube arrays (TNTA) by anodization accompanied by a short-term, quick annealing process. This TNTA shows absorption both in the UV and entire visible range (400-700 nm, an increase by 350%). The effective band gap is found to be 1.7 eV. Through a detailed analysis we show that the significantly enhanced visible light absorption in the entire visible range is due to the substitutional and interstitial carbon atoms on the surface which introduces a structural disorder and localized states at the surface compared to the bulk. Based on the results from photoemission spectra, the probable band gap modeling shows a band bending at the surface. This behavior points to the excessive electronic conduction at the surface which has both merits and demerits in the applications of TNTAs in photocatalytic and photovoltaics in terms of surface recombination. This is confirmed by a solar cell device fabrication and testing.