Synthesis Of Lignin-derived Carbon Quantum Dots With Tunable Fluorescence And Their Photocatalytic Application

Lignin is the most productive natural aromatic oxygenated polymer in nature and is also a major component of paper black liquor and hydrolysis residues.The high value utilization of lignin will bring considerable economic and environmental value.Carbon quantum dots（CQDs）are monodisperse quasi-spherical carbon nanomaterials less than10 nm in size,which are not only chemically resistant,photoluminescent（PL）and easily chemically modified,but also possess excellent water solubility,biocompatibility and non-toxicity.CQDs are widely used in bio-imaging,chemical sensing,electrical energy storage and photocatalysis,etc.The preparation of CQDs nanomaterials from organic solid waste such as lignin has gradually become one of the research foci in this field.In this work,lignin-derived CQDs were prepared by a two-step process of"low-temperature depolymerization"coupled with"hydrothermal reaction",the directional regulation of the optical properties of lignin-derived CQDs was investigated,and the photocatalytic performance of lignin-derived CQDs-modified composites for the photocatalytic removal of pollutants was explored.It hopes to provide a new avenue for the high value utilization of lignin,the green preparation of CQDs and the synthesis of high-performance photocatalysts.The main research contents and conclusions are as follows:（1）Lignin-derived CQDs were prepared by a two-step process of"low-temperature depolymerization"coupled with"hydrothermal reaction",the"particle size cutting"method was used for separation.HR-MS,Raman,FT-IR,XPS and 2D-HSQC NMR were used to resolve the distribution patterns of the liquid-phase products in each component of lignin depolymerization process,to target the key precursors for the formation of CQDs,and to explore the pathways of lignin-based CQDs.The results showed that the molecular weights of the lignin acid-hydrolysis components C1～C5gradually increased,and the fluorescence of the resulting CQDs-1～5 changed from bright green fluorescence to faint blue fluorescence.C1～C3 surfaces contained rich-NH₂ and-OH reactive groups,while CQDs-1～3 had a sharply reduced-NH₂ content and contained abundant pyrrole and pyridine aromatic structures.C4,C5 and the resulting CQDs-4 and CQDs-5 retained the main aromatic structural units of lignin and there were no N-containing groups in the structure.It showed that polymeric lignin undergoes a"top-down"depolymerisation process when the C-O-C bond is broken after acid hydrolysis,followed by a"bottom-up"carbonisation of reactive groups such as-NH₂ and-OH,which are abundant on the supernatant liquid surface of the organic molecule,to form CQDs.（2）The optimal conditions for the two-step process of lignin low-temperature depolymerization coupled with hydrothermal reaction were obtained by adjusting the proportion,temperature and time of acid hydrolysis.Four lignin-derived CQDs with different PL centers were prepared were prepared by adjusting the type of organic acid containing various numbers of amino groups.HR-TEM,MALDI-TOF MS,Raman,FT-IR,XPS,UV-vis,PL and TRPL were used to resolve the conformational relationships between the morphological and structural properties of CQDs and to elucidate the fluorescence mechanism of lignin-derived CQDs in conjunction with DFT energy band theory calculations.The results showed that the optimal fluorescence quantum yield of23.6%was obtained for lignin-derived CQDs at an acid hydrolysis ratio of 1:1,an acid hydrolysis temperature of 90°C and a residence time of 60 min.As the graphitization degree（I_G/I_D）of the four CQDs increased sequentially from 1.06 to 1.55,the C=O content increased from 15.74%to 22.11%and their PL centers gradually red-shifted from 400 nm to 610 nm（blue→red）,and C-N content was critical to the values of the QYs of CQDs.The increase in graphitization within the carbon nucleus of CQDs led to a widening of theπ-electron delocalization.The presence of surface C=O groups corresponds to the introduction of additional energy levels betweenπandπ*.These two factors led to a progressive narrowing of the optical energy band gap and a red-shift in the emission wavelength of CQDs,thus obtaining a method for modulating the optical properties of lignin-derived CQDs.（3）Four composite photocatalysts（CQD/BOI）were prepared by using the tunable lignin-derived CQDs to modify Bi₇O₉I（BOI）.SEM,TEM-mapping,XRD,FT-IR and XPS were used to investigate the morphological structures of the four CQD/BOIs,and UV-vis DRS,PL,EIS,i-t and VBXPS were used to analyze their photochemical properties.The photocatalytic performance of different composite photocatalysts was evaluated by the photodegradation of tetracycline hydrochloride（TC）to obtain the optimal bandgap matching between lignin-based CQDs and Bi₇O₉I₃.The results showed that with the red-shift of the emission wavelength of the CQDs used for modification,the C-N group content in the four CQD/BOI increased in turn,and their light absorption range gradually broadened from the visible light to the infrared region（IR）.Among them,the red CQDs modified BOI composite photocatalyst showed the strongest photocurrent response and the smallest electrochemical impedance,resulting in the highest photodegradation efficiency of 94.7%for TC under simulated sunlight radiation for 60 min,and the degradation reaction rate was twice that of pure BOI.（4）Red lignin-derived CQDs were selected to modify the Bi₇O₉I₃/g-C₃N₄（BOI/CN）heterojunction,and Z-scheme ternary composite photocatalysts（CQD/BOI/CN）were designed and synthesized.The influence of CQDs on the morphological structure and photoelectrochemical properties of the composites was investigated by modulating the modification ratio of CQDs（0.2%～1.0%）.The photocatalytic performance of CQDs/BOI/CN was evaluated by their synergistic removal ability for levofloxacin（LEV）and hexavalent chromium(Cr⁶⁺).The mechanism of the effect of CQDs on the synergistic photocatalytic oxidation and reduction reaction of the composites was also resolved using EPR and radical trapping experiments.The results showed that the 0.5-CQD/BOI/CN composite photocatalyst exhibited significantly enhanced IR photoresponsiveness,with a photoreduction efficiency of up to 100%for Cr⁶⁺and a synergistic degradation efficiency of up to 94.8%for LEV under 60 min of light radiation.Its photoreduction and degradation reaction rate were 0.08725 and 0.03863 min^-1,which were 4.8 and 2 times higher than those of BOI/CN,respectively.In the Z-scheme composite photocatalytic system,CQDs acted as an electronic bridge connecting the conduction band of CN and the valence band of BOI,accelerating charge transfer and effectively separating electrons and holes,thus promoting the generation of·O₂^-and·OH radicals,leading to the enhanced photocatalytic performance of CQD/BOI/CN.