Fluorescence Mechanism and Properties of Carbon Quantum Dots Prepared from Cellulose Via Hydrothermal Method

Author:Wu Peng

Supervisor:liu shou xin


Degree Year:2019





Carbon quantum dots(CQDs)are novel zero-dimensional carbon materials with the characteristics of a particle size of less than 10 nm and photoluminescence properties.They have attracted widespread attention due to excellent optical properties,favourable water solubility,low toxicity,environmental friendliness,and good biocompatibility.CQDs also show favourable application prospects in the fields of medical imaging technology,environmental monitoring,chemical analysis,catalyst preparation,and energy development.Cellulose is the most widely distributed renewable resource in nature.It is significance to prepare CQDs with good optical properties using cellulose as a carbon source by suitable preparation methods,reasonable process conditions,and effective control measures.In this paper,CQDs(M-CQDs)and nitrogen-doped CQDs(N-CQDs)were prepared by hydrothermal carbonization method with cellulose as carbon source and ethylenediamine as nitrogen source,respectively.Ultra-small CQDs(s-CQDs)with excitation light independence were obtained by acid precipitation.Then,the fluorescence mechanism of M-CQDs was studied in detail by ultraviolet light oxidation,sodium borohydride reduction,and ethylenediamine modification.The regulation of the fluorescence properties of M-CQDs by surface functional grous was analyzed in-depth.Finally,the applications of M-CQDs and N-CQDs as heavy metal ions probes and N-CQDs and s-CQDs as photosensitizer werer explored.Specifically,the work of the thesis can be summarized into the following six aspects.The hydrothermal reaction conditions of preparing M-CQDs were optimized by orthogonal test.The results show that the quantum yield is up to 15.31%when the hydrothermal reaction temperature was 240℃,the solvent pH was 14,the hydrothermal reaction time was 12 h,and the microcrystalline cellulose concentration was 25 g/L.The morphology,structure,and optical properties of M-CQDs were systematically analyzed by means of TEM,XRD,XPS,FTIR,UV-Vis and fluorescence spectroscopy.The results show that M-CQDs are spherical nano-particles with graphitized carbon core structure,and there are groups,such as-COOH,-OH,and-C-O-C-,on they surface.Fluorescence emission of M-CQDs has characteristics of excitation light dependence and pH dependence.On the other hand,the fluorescence emission intensity and maximum emission wavelength of M-CODs are affected by the type and structure of surface groups,and the formation of groups of M-CQDs was influenced greatly by the hydrothermal temperature and hydrothermal time,and the formation of M-CQDs was affected by pH of the solvent.N-CQDs was prepared by hydrothermal method using ethylenediamine as nitrogen source.The optimized hydrothermal reaction conditions were 240℃,10 h,and N:C=0.8.Morphology analysis show that N-CQDs is a spherical-like nanoparticle with average particle size of 3.2 nm,and it has graphite-like carbon core structure with a lattice spacing of 0.21 nm.Structural analysis shows that there are nitrogen-containing and oxygen-containing groups such as C-N,C-O-C,C-C=O,and O-C=O/C=N on their surface.Compared with M-CQDs,the quantum yield of N-CQDs is significantly improved,which demonstrated N-doping can effectively improve the photoluminescence properties of M-CQDs.And the luminescence mechanism analysis found that the fluorescence enhancement of N-CQDs is derived from the electron donating effect of nitrogen-containing groups.The study also found that N-CQDs maintains stable fluorescence emission and high quantum yield in solution with pH≤11.The M-CQDs obtained by hydrothermal carbonization were separated into ultra-small CQDs(s-CQDs)and large CQDs(1-CQDs)with average particle diameters of 1.51 nm and 3.76 nm by acid precipitation method,and the differences in morphology,structure and optical properties of the two CQDs were analyzed contrastively.The results show that s-CQDs are amorphous CQDs,while 1-CQDs are nano-CQDs with a carbon core structure.The s-CQDs are richer in hydroxyl groups and carbonyl groups,and has higher quantum yield.Optical properties analysis found that s-CQDs have two independent fluorescence emission centers,while the fluorescence emission of 1-CQDs is excitation light dependence.By analyzing the structure and optical properties of s-CQDs,it is determined that the fluorescence emission of s-CQDs derives from its surface functional group,and the fluorescence mechanism is the surface state mechanism.In order to explore the luminescence mechanism of M-CQDs,the oxidation,reduction and surface modification of M-CQDs were carried out by UV oxidation,sodium borohydride reduction and ethylenediamine modification.Then oxidation-type CQDs(o-CQDs),reduced CQDs(r-CQDs),and amino-modified CQDs(m-CQDs)were obtained,and the surface group composition and content of M-CQDs were controlled the oxidation reaction time and reducing agent concentration.The differences in morphology,structure and optical properties of M-CQDs before and after chemical modification were analyze by TEM,XRD,XPS,FTIR,UV-Vis,NMR,Raman,and fluorescence spectroscopy.After that,the fluorescence source and fluorescence regulation mechanism of M-CQDs were inferred according to the variation of optical properties with the composition of groups of M-CQDs.The results show that the fluorescence emission capacity decreases as the-COOH groups of o-CQDs increase,and the green fluorescence emission of M-CQDs is related to the-COOH content.The r-CQDs exhibit the optical properties of fluorescence enhancement and blue shift,and the blue fluorescence emission is related to the content of-OH.The N modification changes the type and content of the surface groups of M-CQDs,and the quantum yield and fluorescence intensity of M-CQDs are improved,which suggested that the optical properties of M-CQDs are direct affected by surface defect states,and the blue and green fluorescence of M-CQDs can be regulated by changing the type and content of groups.The feasibility and fluorescence quenching mechanism of M-CQDs and N-CQDs as heavy metal ion(Mn+)probes were systematically investigated.The results show that the sensitivity of M-CQDs to most Mn+is weak and the selectivity is poor.However,N-CQDs has better selectivity for high valence and transition Mn+ and better sensitivity for the eighth and first subgroups Mn+.According to the results of structural characteristics of N-CQDs,UV-Vis absorption,and fluorescence lifetime of Mn+/N-CQDs,the mechanism of static fluorescence quenching of coordination reaction was used to explain the fluorescence quenching phenomenon.Based on the quenching mechanism,it is found that the electronic layer structure,ionization potential,pH,and reaction temperature all affect the fluorescence quenching efficiency of Mn+to N-CQDs.Furthermore,Fe3+was detected by N-CQDs when pH=1.The results show that the linear detection range was 10μM-500μM,the LOD was as low as 0.21 nM,and the fluorescence quenching of N-CQDs by Fe3+has good reversibility.N-CQDs/TiO2 and s-CQDs/TiO2 composite catalysts were prepared and used to degrade MB.The photolysis efficiencies of these composite catalyst were determined and the catalytic effect of two CQDs on TiO2 was analyzed.The results show that both N-CQDs and s-CQDs could promote the catalysis of TiO2,and the degradation rates of MB reached to 92%and 96.6%after 2 h photocatalytic reaction,respectively.The catalytic mechanism was the photosensitization of CQDs to TiO2.The excellent photosensitization of s-CQDs is attributed to the better electron donating ability,the small steric hindrance and the large specific surface area.N-CQDs/TiO2 and s-CQDs/TiO2 composite catalysts were prepared and used to degrade methylene blue.The photocatalytic properties of N-CQDs and s-CQDs as photosensitizers were investigated.The results show that the light absorption range of the two composite catalysts can be extended to the visible light region(400-700 nm),and the methylene blue can be effectively degraded under the visible light(λ≥400 nm),the degradation rate can be increased from 6.81%to 92%and 96.6%,respectively.The catalytic mechanism was the photosensitization of CQDs to TiO2.Except the photosensitization of s-CQDs,the excellent photocatalytic effect of s-CQDs/TiO2 is attributed to the better electron donating ability of-OH,the small steric hindrance,and the large specific surface area.