Materials Research Progress on Carbon Quantum Dots-Titanium Dioxide Composite Photocatalysts

With the photocatalytic technology, people can make further progress in tackling the potential threat of environmental pollution and energy shortage, by effectively utilize solar energy to degrade pollutants in water or atmosphere and to generate hydrogen via photocatalyzed water splitting reactions. Titanium dioxide (TiO 2 ), currently the most widely used photocatalyst with excellent photocatalytic activity, high chemical stability, low price and non-toxicity, suffers great limitation in application due to its wide band gap and rapid recombination of electrons and holes. Over the past decade or more, researchers have developed a series of methods to improve the photocatalytic activity of TiO 2 , including adjusting TiO 2 crystal and morphology, sensitizing TiO 2 using quantum dot or organic dye, depositing precious metal on catalyst surface, doping transition metal ion or non-metal ion. The sensitization of TiO 2 with quantum dot refers to the combination of TiO 2 and quantum dots, which can adjust the band width of TiO 2 , broaden the photoresponse range of the photocatalyst. However, the traditional quantum dots mostly contain toxic heavy metal ions, and will no doubt be a hazard to environment and human health. This urges intensive research efforts to seek non-toxic fluorescent nanomaterials, among which carbon quantum dots (CDs or CQDs) have displayed impressive potential and representativeness. CDs, consisting of sp 2 /sp 3 hybridized carbon atoms and holding various surface functional groups, enjoy the advantages of abundant ingredient, stable physical and chemical performance, non-toxicity, good biocompatibility, ease of functionalization, and excellent resistance to photobleaching, compared with the traditional quantum dots. In addition, CDs possess photoinduced electron transfer character and photosensitivity, and part of them display excellent up-conversion photoluminescence. By integrating CDs and TiO 2 , we can obtain CDs-TiO 2 composite photocatalyst which surpasses the traditional hazardous quantum dots by virtue of its low toxicity. And on the other hand, it has an enhanced ultraviolet light absorption, and also extended visible light absorption and near-infrared light absorption, and moreover, can inhibit the recombination of photogenerated electrons and holes, thus effectively promoting the photocatalytic performance and having broad application prospects in the field of photocatalysis. Previous works intend to improve the photocatalytic activity of CDs-TiO 2 photocatalysts mainly from the prospectives of regulating TiO 2 and regulating CDs. The former mainly includes the adjustment of TiO 2 crystal and crystal surface, the control of TiO 2 morphology and the hybridization of TiO 2 . The latter mainly includes the hybridization of CDs, the change of the particle size and loading capacity of CDs. This review analyzes the hypothesized photocatalytic mechanism of CDs-TiO 2 composite photocatalysts, and describes the research results with respect to the above mentioned regulation methods. Finally, it briefly discusses the preparation and application of CDs-TiO 2 as well as the future development trend. © 2018, Materials Review Magazine. All right reserved.