Electric Field Assisted Capillary Force Self-assembled SiO₂ Rod And Its Application In Metamaterial-based Photocatalysis

The self-assembly technology has been attracting people’s attention for a long time,this technique is widely used in the preparation of periodic structures such as photonic crystals.Generally,the self-assembly method of spherical colloidal particles is relatively mature,and a large area can be obtained;for rod colloidal particles with slightly complicated shapes,it is still a great challenge to prepare the array structure of rods with uniform orientation by self-assembly.Arrays composed of rods with anisotropic shapes are expected to be used in optical fields such as metamaterials,for example,by adjusting the light,the light absorption efficiency is increased,thereby improving the photocatalytic performance of the rod array structure.At present,the alternating electric field has been successfully applied to the construction of colloidal rod ordered arrays.Although the self-assembly of rods under the effect of the original electric field has obtained a rod array with uniform orientation,its self-assembly area is still limited.This article focuses on the problem of large-area self-assembled colloidal rods and their impact on metamaterial-based photocatalysis.Several parts of work were carried out:the preparation of silica colloidal balls and colloidal rods,the combination of electric field and capillary force to prepare large-area rod arrays with uniform orientation,Preparation of titanium dioxide and platinum coated rod arrays and their photocatalytic performance test,and electromagnetic simulation of materials:（1）Monodisperse silica rods and silica spheres were prepared by hydrolysis of tetraethyl orthosilicate.In an environment containing polyvinylpyrrolidone and 1-Pentanol,a silica rod with a diameter of about 200 nm and a length of about 1.6μm was prepared by the hydrolysis reaction of tetraethyl orthosilicate.In the absence of polyvinylpyrrolidone and 1-Pentanol,silica spheres with a diameter of about 300 nm were obtained.The effects of changes in the amount of reagents and reaction conditions on the colloidal silica morphology were further studied.The main five factors were:polyvinylpyrrolidone,ammonia solution,tetraethyl orthosilicate,the filtration step,and the centrifugation step.For polyvinylpyrrolidone with a molecular weight between24000-58000,the results show that the lower the molecular weight,the easier it is to obtain fibrous silica colloid.The rod prepared at a molecular weight of 40,000 is good,with a length of about 1.6μm and regular morphology;Lower concentration of ammonia solution will make the rod longer,and the longest reaches more than 2.6μm;The pre-hydrolyzed ethyl orthosilicate will form a spherical"head"at the front end of the rod.（2）The electric field assisted capillary force method was used to explore the influencing factors of the surface area of the self-assembled structure and the orientation of the rods.The rod array structure with 20%coverage area and the dense ball structure with 90%coverage area were obtained.The electric field assisted capillary force method was used to explore the surface area of the self-assembled structure and the orientation of the rod.After using silica balls to verify the feasibility of self-assembly by capillary force,the electric field capillary force self-assembly scheme was applied to the silica rod,and six more important factors were identified during the experiment:solution concentration,temperature,glass gap width,surface tension,electric field strength,electric field frequency strength.The results show that when the solution evaporation rate is greater than settling velocity of colloidal particles,large-area colloidal particles can be obtained.Increased temperature can increase solution evaporation rate.When the temperature is40°C and the concentration is 10wt%,the ITO glass gap is at 80μm,the surface area of the densely packed silica spheres reaches about 90%of the ITO glass substrate.For colloidal rods,it is more important to consider the balance between the thermal convection brought by high temperature and the vertical orientation driven by an electric field.The strong thermal convection will limit the orientation of the rod under the electric field,so the area of the rod array with the same orientation cannot reach the area of the self-assembly of colloidal balls.The final arrangement is a densely packed silica rod array at 25℃and an initial voltage of 4.5V.The surface area accounts for about 20%.（3）The structure of TiO₂ and Pt-coated rod arrays was prepared through experiments.The effect of the addition of precious metal Pt on the metamaterial-based photocatalytic properties was systematically investigated.Atomic layer deposition method was used to dielectrically coat the three-dimensional rod array,and finally,from the inside to the outside,a layered structure of SiO₂-TiO₂-Pt-TiO₂ was obtained.By changing the number of deposition circles to control the content of Pt,SEM and EDS determined the content of Pt.The absorption at 350nm-1000nm was tested by spectrometer,and the results showed that the absorption rate increases as the number of deposition cycles increases,up to 94%.When the number of deposition circles is less than 80,the absorption peak at425 nm is red-shifted to 449 nm,and when it is greater than 80 turns,a blue shift occurs to 427 nm.At the same time,the Pt-coated materials prepared by the chemical deposition method are compared.The results show that the longer the reaction time of the chemical deposition method,the lower the material absorption rate.Finally,the photocatalytic properties of the material were tested,and the results showed that The photocurrent response of the sample deposited with 100 turns of Pt is up to 210n A/mm²,which was about 4 times that of the sample without Pt.It is determined that Pt has a certain effect on the photoelectric conversion rate of metamaterials.As the small range of Pt content increases,the photoelectric conversion rate also increases,so the photocatalytic efficiency also increases.Using the FDTD method,the absorption curves of the TiO₂ and Pt-coated silica rod arrays and the absorption curves of the TiO₂ and Pd-coated silica rod arrays are simulated.The results show that with the increase of Pt thickness,the absorption peak at420nm is red-shifted to 450nm and then blue-shifted to 400nm.The highest absorption rate is more than 93%.The increase in the thickness of Pd also causes the absorption peak of the material at 450 nm to red-shift to 500 nm and then blue-shift to 400 nm.It is determined that when the Pt thickness is 2nm-6nm,the light absorption performance of the metamaterial is better,and at the same time predicts the effect of Pd as a metal layer on the light absorption performance of the material.