The Synthesis And UV Absorption Property Research And Its Applications Of Carbon And Silicon Quantum Dots And Their Composite Materials

In view of the narrow absorption range,or toxicity,or skin allergies,or environmental pollution caused by circulation and accumulation of chemical UV absorbers,,and low efficiency,poor gas permeability and low light transmittance of physical shielding agents,and no breakthrough in the development of a broad-spectrum UV absorbers and the extreme lack of aqueous UV absorbers.Whereas,the UV absorption spectrum and absorption efficiency of silicon and carbon quantum dots are controllable,and they also have good compatibility,low toxicity or non-toxicity,low cost,and stability under strong acid and alkali,optical radiation and high temperature.By now,the study of silicon and carbon quantum dots as UV absorbers have not been reported before we carried out this research,thus,we have carried out a challenging and valuble research topic on silicon and carbon quantum dots as UV absorbers.Here,we have carried out theoretical research and experimental verification on the UV absorption properties of silicon and carbon quantum dots.Theoretically,the relationship between the UV-absorbing properties of quantum dot surface groups was studied,the effects of environmental factors(pH,light,heat)on the absorption efficiency were discussed,the energy transfer process after UV absorption was analyzed by infrared imaging,the UV absorption mechanism of silicon quantum dots and carbon quantum dots was firstly proposed.Experimentally,the anti-UV radiation effect of quantum dots was tested by human body sunscreen test,the anti-yellowing and photodegradation effects of quantum dots were studied by comparing commercial UV absorbers,by assembling quantum dots into inorganic materials,a series of multi-functional composites were realized,a UV-fully absorbed warm white LED was obtained by one-step synthesis of quantum dot polymer composites.Further,we prepared oil-soluble silicon quantum dots and studied the compatibility between them and high molecular materials and anti-aging properties.The details and progress of the studies as follows:(1)Silicon quantum dots(SiQDs)and carbon quantum dots(CQDs)with broad-spectrum UV absorption were synthesized by one-step hydrothermal method through surface group regulation,and the two types of quantum dots showed excellent UV absorption properties under the temperature range of 25°C?150°C and varying pH from 1 to14,which meets the processing conditions and application environment of most related industrial products.Through application tests,it was found that silicon quantum dots have better anti-yellowing property against UV radiation than commercial waterborne UV absorbers(BP-4),and can effectively prevent UV photocatalysis of Rhodamine B.In addition,through theoretical calculations,the synthesized silicon quantum dots have a higher sun protection index,which is basically the same as the sun protection performance of carbon quantum dots,and the human body sunscreen test further carried out based on carbon quantum dots showed that the emulsion prepared using quantum dots and PVA achieves the sunscreen effect of commercial similar products simulating the test conditions of industrial test standards.Importantly,we have proposed the UV absorption mechanism of SiQDs and CQDs,respectivey.The UV absorption of SiQDs is derived from the indirect band gap transition of???,while the UV absorption of CQDs is derived from the n??*and???*electronic transitions caused by the aromatic conjugate structure,and confirmed the absorbed UV radiation energy convert into thermal energy by infrared imaging.(2)Based on the similar refractive index between CaF₂and PE that indicates high transmittance of visible light,and the high stability of BaSO₄ that is a commonly additive used in polymer material,we have assembled them into quantum dot composites with CQDs and SiQDs,respectively,which endowed CaF₂ and BaSO₄ with multifunctional properties,such as UV absorption performance,light conversion performance,and even nano-toughening of nano CaF₂ and BaSO₄.Then,PE films with CQDs@CaF₂or SiQDs@BaSO₄ added were prepared by mixing with PE particles,through the UV accelerated aging test,which presented more significant UV aging resistance than pure PE film,and the fluorescence spectrum showed that the two types of PE films have an ultraviolet-to-blue light function,which not only absorbs ultraviolet radiation that inhibits plant growth,but also emits blue light(main peak:?450 nm)that promotes plant growth.In addition,the CaF₂ and BaSO₄ nanocrystallizations realized by adding CQDs or SiQDs respectively,can be used to improve the mechanical properties of PE films.(3)Based on the technical problems of poor compatibility between inorganic materials and polymers that blocks the transmittance of visible light,to overcome the technical difficulties of the formation of micelles when mixing silane,octadecene and CA-Na,we proposed a two-step synthesis strategy and successfully synthesized oi-soluble silicon quantum dots with full-band UV absorption and strong fluorescence emission(F-OSiQDs).By mixing with PE through heat treatment,F-OSiQDs have very good compatibility with PE,and the prepared F-OSiQDs/PE film not only has excellent full UV absorption,but also has high light transmittance of visible light and extraoridinary transparency.Besides,F-OSiQDs have significant plasticizing effect on PE films,and because of its low toxicity or non-toxicity,F-OSiQDs are expected to replace existing commercial UV absorbers,and to improve UV aging resistance of polymers.(4)A inside-out method for synthesizing quantum dot composites is proposed.By the mothond,a composite material of polysiloxane-embedded silicon quantum dots(P-E-SiQDs)is prepared that present the fluorescence quantum efficiency up to 82.8%,and its physical structure was confirmed by HRTEM.Solid NMR,FT-IR and XPS test analysis confirmed that the luminescence properties of P-E-SiQDs were determined by the two closed-loop structures formed by deamination reactions,and the two closed-loop structures were confirmed to determine the characteristic emissions at the main peaks of 450 nm and 521nm,respectively.In addition,the composite has excellent heat resistance and light stability,and encapsulates a warm white LED light source using a 395nm LED chip that present no UV and no blue emission,and show the CRT up to 73 and a luminous efficacy of up to 76.2 lm/W.This work proposes a new strategy for the preparation of quantum dot polymer composites,and opens up a new technical route for preparing healthy warm white LED light source.