Anatase nanoparticles (5–10 nm) generated on H-titanate nanotube surface (H-titanate/anatase) were prepared by an ingenious and simple method. H-titanate tubes were prepared by a hydrothermal reaction of Ti powder in concentrated NaOH solution and an ion exchange process with HNO₃ solution. After that, at a relatively low drying temperature (100 °C), a small quantity of anatase nanoparticles were in-situ formed on the H-titanate tubes surface by a surface dehydration reaction. In-situ transformation can form a strong interface coupling between H-titanate and anatase, which is conducive to accelerating charge transfer and improving its photocatalytic activity. In addition, the smaller average crystal size, the large specific surface areas (BET), the nanotubed and layered structure and the synergistic effect of dual phases would be beneficial to improving the photocatalytic efficiency.

Converting carbon dioxide (CO₂) into solar fuels through photocatalysis represents an appealing approach to tackling the escalating energy crisis and mitigating the greenhouse effect. In this study, using melamine–formaldehyde (MF) nanospheres as a nitrogen source, a N element was simultaneously doped into the TiO₂ nanoparticle structure supported by carbon hollow spheres using a one-step carbonization method to form a heterojunction N-CHS@N-TiO₂ (marked as (N-(CHS@TiO₂)). The composite showed superior photocatalytic activity in reducing CO₂ compared with TiO₂ and N-CHS: after 6 h of visible light irradiation, the CO yield was 4.3 times that of N-CHS and TiO₂; 6 h of UV irradiation later, the CO yield reached 2.6 times that of TiO₂ and 7 times that of N-CHS. The substantial enhancement in photocatalytic activity was attributed to the nitrogen simultaneously doped carbon hollow spheres and TiO₂, mesoporous structure, small average TiO₂ crystal size, large surface areas, and the heterostructure formed by N-CHS and N-TiO₂. The UV-vis diffuse reflectance spectra (DRS) exhibit a significant improvement in light absorption, attributed to the visible-light-active carbon hollow sphere and the N element doping, thereby enhancing solar energy utilization.