Hydrothermal Doping Regulation Of Biomass Derived Carbon Quantum Dots And Applications In Photovoltaic And Sensing Fields

As a new kind of carbon nanomaterials,carbon quantum dots（CQDs）have a variety of surface functional groups that are adjustable,which is the basis for many applications.Therefore,how to carry out precise regulation to achieve the optimization of the target performance has become an important scientific problem.In the preparation methods of CQDs,hydrothermal method can conduct one-pot doping regulation,and it is regarded as a green technology when biomass is further used as the precursor.In this study,natural plants were used as carbon sources to conduct self-doping regulation and heteroatomic doping regulation of CQDs by hydrothermal method.The formation and doping mechanism of CQDs were discussed by analyzing the changes of their morphologies and structures.The effects of regulation on ultraviolet-visible light absorption and photoluminescence characteristics of CQDs were studied to optimize the photoelectric performance of quantum dot-sensitized solar cells（QDSCs）when CQDs used as sensitizers.In addition,the application of CQDs in the field of pressure sensing is enriched.The dual functional coupling of fluorescence and piezoelectricity is realized,and the mechanism of the formation of piezoelectric phase induced by CQDs is analyzed.The main achievements are listed as follow.（1）Five natural plants used as precursors,the CQDs were self-doped with nitrogen atoms successfully.Among them,allium fistulosum derived CQDs showed the best optical performance and had up-conversion fluorescence.With the increase of CQDs sensitization time,the energy conversion efficiency（PCE）of QDSCs increased first and then decreased,and the best photoelectric performance was obtained at 24 h.The influence of the type and content of dopants on the microstructure（size）and the three characteristics（i.e.,spectrum,energy level structure and functional groups）of CQDs finally lead to the difference of the photoelectric properties of QDSCs.The appropriate size of CQDs is conducive to chemical bonding with Ti O₂ and appropriate band gap.The energy level structure affects the charge transfer among CQDs,Ti O₂ and electrolyte.The increase content of nitrogenous functional groups is beneficial to enhance the absorption spectrum of CQDs.Among the three dopants,1,3-diaminopropane obtained the best PCE,followed by urea and thiourea.With the increase content of dopant,photoelectric performance of QDSCs was continuously improved,which was attributed to the increase of nitrogenous functional groups in CQDs.But there is a limit,probably because nitrogen atoms are saturated in CQDs.（2）The formation process of biomass derived CQDs generally includes five successive stages:hydrolysis,dehydration,polymerization,carbonization and passivation.The main components of natural plants include carbohydrates（such as glucose,maltose,cellulose,etc.）and proteins,etc.Macromolecules will go through the hydrolysis process first.Carbohydrates（except monosaccharides）will be hydrolyzed to glucose and fructose,etc.,while proteins hydrolyzed to polypeptides and amino acids,etc.These hydrolysis products are good precursors for the preparation of CQDs,sometimes with adding specific dopants（such as amine）.A series of chemical reactions between small molecules produce dehydration,polymerization and carbonation and passivation process.For example,carboxyl group and hydroxyl group can be esterified;carboxyl group and amine group can be amidated.These reaction processes will also produce cyclization.Therefore,CQDs are formed doping with nitrogen and other atoms.（3）Since the absorption spectrum of CQDs based QDSCs is mainly concentrated in the ultraviolet region,two new application strategies are proposed.One is to construct the ultraviolet photovoltaic window（UPWs）,that is,the photoanode is thinned to further increase the light transmittance of the device（the average visible light transmittance is 35.5%）.In this way,UPWs can generate electricity（PCE=0.493%）without affecting indoor lighting,and effectively block the entry of ultraviolet rays.On the other hand,CQDs and Cd S quantum dots（QDs）were used to build a co-sensitized solar cell.The PCE was 40.9%higher than that of Cd S QDSCs.The reason is that the fluorescence emitted by CQDs can be absorbed by Cd S QDs,and then the light absorption capacity of the sensitized photoanode is enhanced.The photoluminescence efficiency of the sensitized photoanode is weakened,indicating that the recombination of electron and hole is reduced.Cd S QDs and CQDs probably have formed energy level arrangement of type I,which lowers the electron transfer barrier.（4）A fluorescent pressure sensor based on Ti O₂/CQDs nanocomposite was constructed.First,CQDs act as a chemical bridge to strengthen the bonds between Ti O₂nanoparticles,helping to build resilience against damage.In addition,smaller CQDs are beneficial to increase the contact surface between CQDs and Ti O₂ nanoparticles,and enhance the linear range and degree between the current response and the applied pressure.Secondly,unpaired electrons of nitrogenous functional groups in CQDs can promote the contribution of CQDs to nanocomposites,thus improving their fluorescence properties.Moreover,the dopant can improve the carrier mobility of the nanocomposite by adjusting the chemical bond composition of CQDs,thus improving the sensitivity of the pressure sensor.The Ti O₂/CQDs sensor with the best performance had a sensitivity of 5.13%k Pa^-1（R²=0.965）,and showed bright white fluorescence under ultraviolet light irradiation.In addition,the influence of the thickness of nanocomposite film on the pressure sensing was studied,and the sensor thickness of30μm had the best sensing performance.（5）With the introduction of CQDs,the content ofβphase in PVDF-HFP/CQDs film was increased by 14.4%,and the sensitivity of the constructed flexible piezoelectric sensor（FPS）was improved by 42.4%（R²>99%）.The FPS exhibited good electrical stability under repeated dynamic loads.The self-powered FPS has potential applications in anti-collision alarm systems,finger bending detection,voice recognition and vibration monitoring.The mechanism ofβphase formation induced by CQDs in PVDF-HFP may be that the larger specific surface area and abundant surface defects of CQDs provide more potential induced active sites forβphase formation in PVDF-HFP.As the crystal nucleus of PVDF-HFP,CQDs changed the crystallization kinetics of PVDF-HFP.In addition,the abundant surface functional groups in the shell of CQDs will facilitate the chemical bonds between CQDs with PVDF-HFP.For example,the carboxyl,hydroxyl,and amine groups of CQDs form hydrogen bonds with the fluorine atoms in PVDF-HFP,which drags the PVDF-HFP chain straighter,and the fluorine atoms are pulled to the same side,resulting in the formation ofβphase with zigzag conformation（TTT）instead ofαphase with curly conformation（TGTG’）.