Preparation Of Functionalized Carbon Quantum Dots And Their Application In Photovoltaic And Photocatalytic Hydrogen Evolution

Recently,Carbon quantum dots?CQDs?have received much attention for being a new generation luminescent nanomaterials due to their fascinating optical properties,such as excellent chemical and fluorescence stability,high quantum efficiency,superior aqueous dispersibility,low-cost and largescale production,low toxicity,and visible light absorption.In the pasted decades,CQDs have demonstrated wide application in solar cells,light emitting diode,bioimaging,fluorescent probe,and photocatalysis.However,there are still some challenges need to be solved for further application.For example,CQDs show an obviously decreased fluorescent intensity and quantum yield in solid resulting from the aggregation of CQDs.This fluorescence-quenching character in solid would have an adverse effect on its down-conversion efficiency when CQDs are employed in solar cells.Likewise,the rapid charge recombination in CQDs is also disadvantage to the catalytic efficiency of CQDs when they are used for hydrogen evolution in visible light.Thus,in this dissertation we show two kinds CDQs systems with elaborated design and use it in crystalline silicon solar cells and photocatalytic hydrogen evolution in visible light.The main contents and results of our work are as follows:In the first part where CQDs are applied in crystalline silicon solar cells,a high-fluorescent silanized carbon quantum dots?CQD-SI?was prepared via hydrothermal approach using citric acid as carbon source and KH792 as passivating agent.The high-resolution transmission electron microscopy?HR-TEM?shows that the average particle size of CQD-SI was 3.5 nm.It has an excellent water solubility and dispersibility.XPS and FTIR spectra indicats that the CQD-SI consisted of mainly carboxyl group,hydroxyl group,Si-C,Si-O et.al.When combined with EVA,the fluorescence quantum efficiency of hybrid material reaches 20.68%.In addition,the obtained CQD-SI/EVA composite film can be used as luminescent down-conversion layer in the polysilicon solar cell.After adding CQD-SI/EVA composite film with 2%mass fraction,the J_scc and PCE of polycrystalline silicon solar module increase from 35.47 mA/cm² to 35.98 mA/cm²and from 17.44%to 17.86%,respectively.Furthermore,the CQD-SI/EVA layer significantly improve the external quantum efficiency of the module,especially in the UV region.Our designed CQD-SI system may accelerate the application of CQDs in crystalline silicon solar cells,since it simply solves the dispersion problem and keeps the transparency of sealing polymer materials.In the second part for CQDs being used as photocatalyst for hydrogen evolution in visible light,a core-shell type carbon dot B₁N₂CQD with boron-doped carbon dot as core and nitrogen-doped carbon dot as shell was obtained via two-step hydrothermal synthesis.Nitrogen co-doping carbon dot BNCQD were also prepared in a similar method.The morphologies,optical properties and surface chemical structures of CQDs are investigated through various characterizations,including TEM,XRD,UV-Vis,FTIR and XPS et.al.Generally,B₁N₂CQD shows the most broadened visible light response region and obviously strongest fluorescence intensity.In addition,B₁N₂CQD also demonstrates the highest photocurrent intensity and lowest resistance value,indicating an enhanced charge-separation efficiency due to the formation of specific core-shell structure.Finally,the obtained CQDs are employed as photocatalysts for hydrogen evolution in visible light.As expected,B₁N₂CQD displays the highest hydrogen producing efficiency with a value of 0.3 umol/h.Our designed B₁N₂CQD may offer a solution to effectively suppress the photo-generated charge recombination and thus improve the catalytic efficiency of CQDs.