The UV–visible diffuse reflectance spectra show the light absorption properties of mesoporous TiO2spheres with different amount of N doping, shown in Fig. 6. For comparison, the spectrum of undoped mesoporous TiO2sphere is also illustrated. The band gap absorption edge of 0-N-TiO2 sample is approximately 400 nm and does not exhibit noticeable absorption in the visible region. However, interestingly, all of the N doped TiO2catalysts display distinct and meaningful absorption covering the whole visible region of 400–800 nm. Therefore, we can conclude that the degree of N doping could influence the visible light absorption of mesoporous TiO2spheres. Nevertheless, instead of reducing the recombination of photo electrons and holes, too much N can result in the formation of photoelectron recombination centers.