Research On The Preparation Of Highly Luminescent Carbon Dots And Its Functional Composite Materials

Carbon dots (CDs), as a new type of luminescent carbonaceous nanomaterials that was first reported in 2004, have attracted widespread attention in recent years owing to their low prices, non-toxicity, good biocompatibility and excellent optical properties. The fluorescent quantum yield (FQY) of most CDs synthesized so far is low and most of the work focused on their liquid states photoluminescence (PL) properties. Thus, to explore simple and efficient methods for the preparation of high luminescent CDs and the research on their PL properties in solid states is imperative. In addition, due to their excellent light-induced electron transfer property, CDs are expected to be used as sensitizers to improve the photoelectric and photocatalytic properties of semiconductors. However, the related research is still at an initial stage so far. It is of great significance to prepare highly luminescent CDs and study related composite materials.In this paper, focusing on the preparation and related composite materials of CDs, we proposed a one-step hydrothermal method to synthesize highly luminescent water-soluble CDs, investigated their PL properties in solid states and preliminary studied their photoelectric sensitizing effect on TiO2 nanorods. In addition, we designed and synthesized CDs/BiOBr semiconductor composite photocatalysts with a wide spectrum utilization of sunlight and discussed the photocatalytic mechanism. The main contents are as follows:1. One-step hydrothermal synthesis of highly luminescent water-soluble CDs. With citric acid as carbon source and organosilane coupling agent as surface coating agent, organosilane-functionalized fluorescent CDs were prepared. The preparation and modification were carried out simultaneously through hydrolysis, polymerization, carbonization and coating. The resulted carbon dots showed rich surface functional groups, good water solubility, and the highest FQY was 82% with the size about 5 nm. The preparation conditions such as reaction solvent, carbon source/silane ratio, reaction time and temperature were optimized. The effects of excitation wavelength, concentration, pH, dispersing solvent and metal ions on their luminescence property were discussed. The carbon dots showed good solubility in water, methanol and ethanol, maintaining efficient luminescence and stability in the pH range of 5-10, which was conducive to biological applications. The PL intensity and wavelength changed with the increasing concentration. The carbon dots were insensitive to most of metal ions except for Hg2+ and Fe3+, which could lead to fluorescence quenching. In addition, the resulting CDs showed good stability and there was no obvious decrease of PL intensity after being stored for 3 months.2. Thanks to surface methoxysilyl groups on the silane-functionalized CDs, CDs/gel glass composites were developed, which maintained the structure and luminescent property of CDs in liquid states through covalent bonds and showed good UV/IR shielding effect, achieving uniform dispersion, high doping concentration and the inhibition of fluorescence quenching. The CDs monoliths exhibited good UV, partial visible and IR absorption properties. Based on their film-forming, good light absorption and photoinduced charge transfer properties, CDs were used as non-metallic green sensitizers to sensitize the TiO2 nanorods anode. After spin coating-heating process, the CDs and TiO2 formed tight combination with the hydrolysis and condensation of CDs. The morphology, optical and optoelectronic properties of the system were characterized and the composites structure exhibited higher photocurrent. The sensitizing role of CDs was preliminary discussed. The CDs extended the light response range and enhanced the utilization of sunlight, suppressed the photogenerated electron-hole recombination and prolonged the lifetime of carriers owing to their good photoinduced charge transfer and electron reserve capacity properties.3. The design and building of CDs/BiOBr nanocomposites photocatalysts. The CDs and BiOBr achieved close combination by simple electrostatic adsorption and subsequent heat treatment due to their opposite electronegativity. The characterization of structure and morphology confirmed that CDs distributed evenly on the BiOBr sheets with interaction between them. Photocatalytic experiments were carried out using Rhodamine B as the target degradation pollutant. The results indicated that the loading of CDs improved both the UV and visible light catalytic performance of BiOBr. The effect of CDs loading on the photocatalytic was studied and the optimal loading was 2 ml and showed good stability after 5-cycle experiments. In addition, the enhancing mechanism of CDs for BiOBr composites was discussed. The significant enhancement in both UV and visible photoactivity could be ascribed to the extended the light response range and improved absorption and utilization of the sunlight. The electrons could be exported rapidly, inhibiting the electron-hole pair recombination. As an electron "reservoir", the electron shuttled between the CDs and BiOBr easily and prolonged the carriers’ lifetime. Thus, the CDs enhanced the photocatalytic activity of BiOBr under both UV and visible light.