Low-Temperature Nitrogen Doping in Ammonia Solution for Production of N-Doped TiO₂-Hybridized Graphene as a Highly Efficient Photocatalyst for Water Treatment

To facilitate the potential application of TiO2 as an efficient photocatalyst, the modulation of its band gap and electrical structure is of great significance. Herein, we report a very simple nitrogen (N)-doping method to obtain N-doped TiO2, which is hybridized with graphene sheets at a temperature as low as 180 °C and using an ammonia solution as the N source and reaction medium. X-ray photoelectron spectroscopy analysis revealed that the atomic N level could reach 2.4 atomic percent when the reaction time was 14 h. Photoluminescence (PL) emission spectra indicated that N-doped TiO2/graphene composites have drastically suppressed TiO2 PL intensity when compared to undoped TiO2, confirming the lower recombination rate of electron–hole pairs in the N-doped TiO2. Additionally, photodegradation data showed that the decomposition rate of methylene blue with our N-doped TiO2/graphene photocatalyst is twice as fast as a commercial Degussa P25 catalyst. Furthermore, density functional theory (DFT) calculations demonstrate that N doping creates empty states in the band gap of the TiO2 that are below the Fermi energy of graphene. Thus, when N-doped TiO2 is brought into contact with graphene, these states become filled by electrons from the graphene, shifting the TiO2 bands upward relative to the graphene. Such a shift in band alignment across the TiO2/graphene heterojunction makes transfer of the photoexcited electron more energetically favorable. This work provides a very convenient chemical route to the scalable production of N-doped TiO2/graphene photocatalyst for potential applications in wastewater treatment.