Preparation Of Some Novel Graphene Quantum Dots And Their Application In Metal Ions Detection And Photocatalysis

As novel fluorescent carbon nanomaterials,graphene quantum dots?GQDs?,usually represents single or few layers to tens of layers of graphene with size less than 100 nm.Owing to the pronounced quantum confinement and edge effects,GQDs show much more advantages including low toxicity,excellent solubility,stable luminescence,high biocompatibility and tunable band gap compared with traditional fluorescent materials.Heteroatom doping or surface functionalization can tune the optical and electrical properties of GQDs including electronic structure,band gap,chemical reactivity,and optical properties,that increase fluorescence quantum yield,and tune chemical selectivities.In recent years,GQDs have attracted much attention in photocatalysis and ion detection.In this dissertation,facile approaches were developed to prepare three new GQDs?S-GQDs1,S-GQDs2 and O-GQDs?by using water-phase molecular fusion with 1,3,6-trinitropyrene,different S sources or O source in one-step hydrothermal process.The application of the obtained GQDs in ions detection and photocatalysis was studied.Detailed work was summarized as follows:?1?One-pot synthesis of sulfur-doped graphene quantum dots?S-GQDs1?was proposed based on water-phase molecular fusion with 1,3,6-trinitropyrene,Na2 S,and Na OH in a hydrothermal process.The prepared S-GQDs1 gave a stable yellow-green emission and exhibited excitation-independent photoluminescence behaviors with maximum excitation/emission wavelength at 490/535 nm,respectively.As investigated by atomic force microscopy?AFM?,the synthesized S-GQDs1 possessed monolayer-graphene thickness.As illustrated by transmission electron microscope?TEM?,the synthesized S-GQDs1 exhibited high crystallinity and uniform size 3 nm.Successful doping of S atoms in GQDs lattices was proven by X-ray photoelectron spectroscopy?XPS?characterization.S atoms with-C-SOx-C-?x=2,3,and 4?sulphone bridges were indeed covalently bonded to the framework of GQDs.The S-GQDs1 had drastically changed surface chemistry and showed a selective and sensitive response to Pb²⁺.Based on the fluorescence quenching of S-GQDs1 by Pb²⁺,direct fluorescence sensor for Pb²⁺ detection was developed.Under the optimized conditions at p H 7.0 and 5 min reaction time,the linear response ranged from 0.1 to 140.0 ?M with a detection limit of 0.03 ?M was obtained.?2?Sulfur-doped graphene quantum dots?S-GQDs2?with bright blue emission have been prepared by a facile one-pot hydrothermal treatment.1,3,6-trinitropyrene,which has a mother nucleus structure similar with graphene,was chosen as the carbon source and 3-mercaptopropionic acid was employed for S-doping and carboxyl groups modification.The synthesized S-GQDs2 was characterized by AFM,TEM and XPS.Results indicated that S-GQDs2 possessed single layer graphene structure with mean size of about 2.5 nm and presented an excitation-independent photoluminescence behavior with maximum excitation/emission wavelength at 360/450 nm,respectively.Successful S-doping into the framework of GQDs with-C-S-C-covalent bond and-C-S-?O2?-C-sulphone bridges.The sulfur-doping of GQDs drastically improved their electronic and chemical properties,which afforded the S-GQDs2 a sensitive response to Ag+ ions.Furthermore,the S-GQDs2 was successfully explored as a sensing probe for Ag+ detection with high sensitivity and selectivity.A wide linear range of 0.1-130.0 ?M with a low detection limit of 30 n M was obtained.?3?The synthesized S-GQDs1 was applied as nanophotocatalyst.Due to remarkable visible light absorption and high separation ratio of photogenerated charges,an enhanced photocatalytic performance for removing basic fuchsin under visible light irradiation was achieved.The experimental parameters for photo-degradation of basic fuchsin including p H value,dye concentration and dosage of S-GQDs1 were optimized.Results demonstrated that degradation efficiency reached maximum value at p H 7.0,5.0 mg L-1 of dye concentration and 20% S-GQDs1?volume fraction?,respectively.The photocatalytic reaction rate of the S-GQDs1 was 9 times and 45 times higher than that of S-free GQDs and self-photocatalysis,respectively.?4?An effective O-GQDs/TNSs composite photocatalyst was prepared via a facile electrostatic flocculation route with O-GQDs as green photocatalytic promoter.The obtained O-GQDs/TNSs heterojunction was characterized by X-ray diffraction?XRD?,TEM,X-ray energy-dispersive spectrophotometer?EDS?and electrochemical impedance spectroscopy?EIS?.The results demonstrated that the composite exhibited an intimate coupling of O-GQDs and TNSs.A significant visible light absorption and therefore the enhanced separation efficiency of photogenerated charges was explored.The O-GQDs/TNSs composites showed enhanced photocatalytic performance for the photodegradation of Rhodamine B?Rh B?under visible light irradiation in comparison with rs-TNSs and O-GQDs.Furthermore,this composite exhibited superior recyclability and stability.The photocatalytic reaction rate of the O-GQDs/TNSs composites was ²2 times and ⁵ times higher than those of pure O-GQDs and rs-TNSs,respectively.We further explored the photocatalytic ability of different dyes and on hydrogen production,there we elected three types of the dyes,crystal violet?CV?,reactive blue?RB?and alizarin red?AR?,respectively.These results implied that the O-GQDs/TNSs photocatalysts had excellent applicability and generality under visible light,the rate of H2 production which was 68 times higher than that of the pure rs-TNSs.Possible mechanism was proposed,superoxide radicals??O₂^-?and photogenerated holes?h+?were the main active species in the photocatalytic process.