| Antireflective coatings play an important role in a number of fields,including photovoltaic devices,optical components,displays and optical sensors,etc.,which can effectively improve the working efficiency of photovoltaic devices and light-emitting devices,enhance the luminous flux and sensitivity of optical systems.Antireflective coatings can be divided into single layer,multi-layer and gradient index coatings according to the different structure.Ideal antireflective coatings should have omnidirectional as well as broadband antireflective capabilities to adapt to different incident angles and wavelengths of light.Although there are a variety of methods for preparing antireflective coatings,it is still a challenge to prepare antireflective coatings in large areas at low cost.As a natural polymer material,chitin has good renewability and biodegradability.The chitin nanofibers isolated from chitin are rigid,rod-like and have high length to diameter ratio.On the one hand,the nanoscale diameter of chitin nanofibers can ensure that the coatings constructed from them are highly transparent.On the other hand,the rigid,rod-like shape with a high aspect ratio enables the formation of nanoscale porous structures when stacking,thereby reducing the refractive index of the coating.On this basis,the gradient index coatings were further constructed,In this paper,chitin nanofibers(ChNFs)with positive,negative charge and zwitterionical charge were prepared from chitin by a series of modification methods.The positively and negatively charged chitin nanofibres were used as building blocks to construct porous coatings by casting and layer-by-layer assembly.By controlling and optimizing geometrical parameters of the coatings,such as thickness and porosity,the refractive index of the coatings were adjusted,and the omnidirectional antireflection of the ChNFs coatings was achieved.By controlling and optimising the stacking sequence and number of antireflective coatings with different refractive indices(single refractive index homogeneous layer)in the gradient index coating,the omnidirectional broadband antireflective capability of the coating was improved.The specific research content and main conclusions of this paper are as follows:(1)Positively charged ChNFs(E-PD-ChNFs)were prepared using doubly modified(esterified-partially deacetylated)chitin,and negatively charged ChNFs(E-ChNFs)were prepared using esterified-modified chitin,and based on the above two kinds of ChNFs,singlelayer antireflective coatings were prepared by self-assembly of ChNFs.The effects of concentration of the E-ChNFs and E-PD-ChNFs suspensions on the antireflective properties of the coatings were investigated.The effects of the concentration of E-ChNFs and E-PDChNFs suspensions and the coating times(each alternate coating of E-PD-ChNFs and EChNFs suspensions was denoted as a double layer)on the antireflective performance of the coatings were investigated.The results showed that the ChNFs coating with number of bilayers of 5,prepared by ChNFs suspension with concentration of 0.1%,had a refractive index of 1.36 and a light transmission gain of 3.7% on the glass.Moreover,the coating has omnidirectional antireflective property.(2)In order to further simplify the preparation process of ChNFs,zwitterionically charged ChNFs(ZC-ChNFs)containing both carboxyl and amino groups were prepared by one-step modification of oxalic acid esterification combined with ultrasonic crushing.The length and width of ZC-ChNFs decreased with the increase of the concentration of oxalic acid,and their suspensions could be stably dispersed in both acidic and basic conditions.Therefore,by adjusting the p H value,carboxyl group could be positively charged and amino group can be negatively charged,and ZC-ChNFs with positive and negative charges could be obtained.The ZC-ChNFs coatings were prepared by casting and layer-by-layer self-assembly methods,respectively,and the effects of the p H value of ZC-ChNFs suspension and the size of ZC-ChNFs on the antireflective properties of the coating was studied.The results show that the refractive index of ZC-ChNFs coatings prepared by the layer-by-layer self-assembly method was 1.36,and the transmittance gain on the slide substrate couls reach 3.5%.(3)In order to further reduce the porosity of ChNFs coatings and improve the antireflective properties of the coatings,the strategy of using pore-forming agent sodium dihydrogen citrate(SDCA)in the preparation process of ZC-ChNFs coatings was adopted.The ZC-ChNFs/SDCA coating with a lower refractive index(1.29)was successfully constructed.The effects of the content of pore-forming agent on the porosity,refractive index,thickness and antireflective properties of the coating were studied,and the geometric parameters of the ChNFs coatings were regulated.The results showed that the porosity of the ZC-ChNFs/SDCA coating reached 59.74% at 40% SDCA addition,and the refractive index of the coating was the lowest,which was 1.29.The transmittance gain of this coating on the substrate of the slide reached 6.4%.In addition,homogeneous layers with different refractive indices were prepared,which provided strong support and guarantee for the subsequent construction of gradient refractive index coatings.(4)In order to improve the antireflective ability of ChNFs coating in a wide wavelength range,the gradient index antireflective coatings of ChNFs were constructed based on the homogeneous layer with different refractive indices prepared by the above work.By adjusting the stacking sequence and number of homogeneous layers with different refractive indices,the distribution of the gradient refractive index and its variation trend were regulated,and the effect of the gradient refractive index coating on the antireflective performance in a wide wavelength range was studied.The results showed that the quintuple layer gradient index coating with the quadratic function(a < 0)distribution had an excellent antireflective effect in a wide wavelength range,and increased the transmittance gain of the glass by 6.6% at 550 nm and 7.6% at 650. |
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