N-doped carbon quantum dots/TiO2 hybrid composites with enhanced visible light driven photocatalytic activity toward dye wastewater degradation and mechanism insight

• NCQDs were successfully doped onto TiO2 via a simple and efficient low-temperature process.
• NCQDs/TiO2 exhibited superior photocatalytic activity than TiO2 under visible light irradiation.
• NCQDs played important roles for MB degradation under visible light irradiation.

N-doped carbon quantum dots/TiO2 (NCQDs/TiO2) hybrid composites were prepared via a simple and efficient low-temperature process. The resulted composites were characterized by various techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, scanning electron microscope, transmission electron microscopy and high-resolution TEM, and UV–vis diffuse reflectance spectra. The results indicated that the NCQDs were successfully introduced onto the TiO2 surface. The photocatalytic activity of the NCQDs/TiO2 hybrid composites were investigated by using Methylene blue (MB) as the target pollutant. The photodegradation efficiency of MB over 1NCQDs/TiO2 was 86.9% within 420 min, which was much higher than those of pristine TiO2 (53.8%) and the hybrid composites with other NCQDs contents (lower than 80.1%). This suggested that the NCQDs played important roles for MB degradation under visible light irradiation. The functions of NCQDs were discussed and a new synergistic mechanism was proposed for the degradation of MB on the surface of NCQDs/TiO2 hybrid composites. The enhanced activities were attributed to the interfacial transfer of photogenerated charges from the conductive band (CB) of TiO2 to NCQDs, leading to effective charge separation of TiO2.

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