Titania, the naturally occurring oxide of titanium, is inexpensive, non-toxic, chemically stable with various structural forms, and attracts great attention in photocatalysis for photocatalytic degradation of a variety of heavily degradable organic pollutants, photocatalytic water splitting, and solar energy conversion. Among the three most known crystalline forms of TiO2 (brookite, anatase, and rutile), anatase and rutile are usually used for photocatalytic purposes. Despite the higher band gap energy value (3.2 eV for anatase vs. 3.0 eV for rutile), anatase has been shown as a better photocatalyst than rutile. TiO2 is considered a semiconductor with a wide band gap, therefore it can be activated under UV light. There are several approaches to decrease the band gap energy and to promote the photocatalytic activity in the region of visible light, such as (i) addition (doping) of different anions or cations, causing lattice defects during the doping, mainly oxygen vacancies, which promote photocatalytic activity in the visible range of light; (ii) using non-stoichiometric titanium oxides (Magneli phases) filled with lattice defects of oxygen vacancies type; (iii) irradiation of TiO2 by ionizing radiation (electron beam, X-rays or γ-rays) or iv) reinforcing with carbon nanostructures (e.g. carbon nanotubes, CNB), creating TiO2/CNTs nanocomposites. This study aimed to observe the changes in structure and consequently on band gap energy of TiO2 nanoparticles produced by sol-gel procedure, after treatment by ionizing irradiation (X-rays and e-beam) and after reinforcing with multi-walled and single-walled carbon nanotubes (TiO2/MWCNTs and TiO2/SWCNTs nanocomposites).