Preparation And Photoelectric Performance Of Graphene Quantum Dots And Their Composites

Graphene Quantum Dots?GQDs?are graphene nanoparticles with fluorescence emission and the particle size less than 100 nm.As a new type of carbon material,GQDs have excellent optical,electrical,magnetic,environmentally friendly and biocompatible properties.GQDs have attractive application potential in the fields of energy environment,biomedicine and electronic equipment.However,the preparation methods and application research of GQDs are still in the ascendant,some researchers have found that the factors affecting the photoelectric properties of GQDs include preparation of raw materials,synthesis routes,molecular size and surface structure.In addition,GQDs are used as sensitizers for semiconductor catalytic materials and have a significant enhancement effect on the catalytic performance of the catalyst.This paper mainly focuses on the controllable preparation of GQDs and the functional modification of GQDs,the synthesis control,surface morphology,photoelectric properties and photocatalytic properties of GQDs/hollow porous structure TiO₂?H-TiO₂?and GQDs/cerium oxide?CeO₂?composites were systematically studied.New research results have been achieved in the preparation of GQDs,GQDs/H-TiO₂and GQDs/CeO₂ composites with enhanced photocatalytic performance.?1?GQDs were prepared by"top-down"methods using three-dimensional network graphene as raw material.The synthesis conditions of GQDs were optimized,and a preparation method of GQDs with high yield,easy scale control and strong photoluminescence performance was proposed.The current density of the GQDs under visible light is 0.025?A/cm² and its stable and reversible.The fluorescence at520 nm is stable and unaffected by the excitation wavelength.The preparation of functional GQDs is an important way to develop its application.We prepared chiral GQDs,fluorescence-enhanced GQDs and rare earth Eu³⁺coordinated GQDs by simple GQDs surface modification.These functionally modified GQDs show good optics characteristic.?2?We prepared a large specific surface area and porous H-TiO₂ by"template method",and combined with GQDs by hydrothermal method to obtain GQDs/H-TiO₂composites.H-TiO₂ with a specific surface area of 223.48 m²/g provides a large number of attachment sites for GQDs.The small size distribution and high solubility of GQDs play an important role in the uniform distribution of GQDs on the surface of H-TiO₂.The composites GQDs/H-TiO₂ enhanced the absorption of visible light,and the recombination rate of photogenerated electrons and holes decreased.The band gap of GQDs-3.0/H-TiO₂ was as low as 2.8 eV,and the photocurrent density was 1020?A/cm²,showing a good photoelectric activity.At the same time,the photocatalytic degradation rate of the prepared GQDs-3.0/H-TiO₂ composite were 9.1 times higher than that of H-TiO₂,and the photocatalytic activity of rhodamine B?RhB?was significantly improved under visible light.?3?The photogenerated electrons and holes of the rare earth compound CeO₂ were easily recombined.We have prepared GQDs/CeO₂ composites by combining them with GQDs to improve the catalytic performance of CeO₂.GQDs in the GQDs/CeO₂composites are uniformly coated around each CeO₂ crystal,and the CeO₂ assembly is induced to have an ordered conical morphology structure,which effectively increases the reactive sites and effective contact interfaces.Among them,the composite GQDs-4.5/CeO₂ absorption edge broadens to 475 nm with the best visible light response,with extremely low electron-hole recombination rate and optimal photoelectric performance.The composite materials form a heterogeneous structure with staggered energy levels.The photogenerated electrons are transferred from the GQDs conduction band to the CeO₂ conduction band,which effectively prolongs the carrier lifetime,exhibits enhanced photocatalytic activity,and decomposes RhB photocatalytic reaction under visible light.The rate is up to 13.8 times than of CeO₂.It can be seen that GQDs/CeO₂ composites have a significant enhancement effect in enhancing the photocatalytic activity of CeO₂.