Study On Localized Surface Plasmon Resonance Effect In Cu/TiO₂ Composite Photocatalytic Material System

Energy shortage and environmental pollution have always been two practical problems facing social development.As one of high-efficiency solar energy harvesting/conversion techniques,semiconductor-based photocatalytic technology can efficiently utilize so lar energy to produce clean energy and purify the environment.Since the advent of photocatalytic technology,researchers have invested tremendous efforts to develop efficient photocatalytic materials.Among the plethora of semiconductor-based photocatalysts,metal-semiconductor composite materials,especially plasmmoic metal-semiconductor composite materials have received the most tremendous attention on account of unique localized surface plasmon resonance(LSPR)effect of metals which can broaden the spectral response range by resonance absorption and simultaneously,increase the quantum conversion efficiency by local electric field enhancement.This ultimately results in remarkably improved solar energy conversion efficiency,thereby plasmonic metal-semiconductor system has been attracting burgeoning interest in the field of photocatalytic technology.Of particular note,Cu has been overwhelmingly explored by virtue of their peculiar intrinsic advantages compared with other plasmonic metals,including ric h abundant storage,low-cost,and plasmon resonance absorption peaks located in the visible light region.Therefore,combination of Cu and TiO ₂ to build efficient photocatalytic materials is expected to achieve large-scale production applications of photocatalytic technology.It is generally accepted that the LSPR effect of ordinary spherical Cu nanoparticles at approximately 580 nm;meanwhile,the interband transition of their electrons from the valence band to the Fermi level occurs at the wavelength belo w?590 nm.As such,overlap results in a significant attenuation of the LSPR effect of Cu nanoparticles;on the other hand,the oxidative property of Cu nanoparticles also definitely deteriorate the LSPR effect of Cu.Based on these effects,this paper studied the generation,regulation of the LSPR effect of Cu/TiO₂ photocatalyst,and the influence of Cu oxides(i.e.Cu₂O)on LSPR effect and photocatalytic performance of Cu/TiO ₂ photocatalyst,and exploer the influence of some practical factors on LSPR eefe ct of Cu under actual conditions by the finite-difference time-domain method(FDTD)and first principles calculation.The main research contents and results are as follows:(1)Firstly,Cu/TiO₂ photocatalyst were successfully prepared by sol-gel method and wet chemical method.The LSPR effect of Cu was evaluated by photocatalytic degradation of methyl orange and played a positive role in photocatalysis.Secondly,The LSPR effect of Cu/TiO ₂ nanospheres with different configurations was simulated by the FDTD method.The change rule and trend of LSPR effect of Cu/TiO₂ nanospheres was discussed in detail by changing the actual parameters such as the direction of incident light,the size of the nanospheres,and the environmental medium contant.The intrinsic mechanism of the LSPR effect produced by Cu/TiO₂was analyzed by the DFT.The results show that when the Cu nanosphere gradually sink into the TiO ₂nanosphere to form a core-shell configuration,the LSPR absorption of the Cu nanosphere can gradually red-shift and shift the interband transition region,so LSPR effect can be effectively excited.Through density of states analysis,the interface state of the Cu/TiO ₂heterostructure is the key fac tor for these red-shifts.Among the three kinds of Cu/TiO₂ configurations of"Loading","Embedding"and"Core-shelling",the"Embedding"configuration exhibits better photocatalytic performance,owing to less affected the incident direction of light,effectively excited LSPR wavelength and the local electric field,and more reaction sites for the photo-redox reaction.(2)The LSPR characteristics of Cu@Cu ₂O core-shell nanostructure formed b y layer-by-layer oxidation of Cu to Cu ₂O are simulated by FDTD.Base d on the experimental SEM images,all possible configurations of the Cu/Cu ₂O/TiO₂ ternary composite structure were reasonably constructed.The interface properties of the binary hetero-structures of Cu/Cu₂O and Cu₂O/TiO₂ were calculated by first principles calculation.The calculation results show that the presence of Cu ₂O in the Cu/TiO₂ hetero-structure can directly regulate the LSPR peak beyond the interband transition range.The Cu@Cu₂O/TiO₂ configuration significantly weakens the interface electric field intensity generated by the LSPR effect of Cu.However,the Cu/TiO ₂/Cu₂O(where the Cu core and TiO ₂ nanoparticles can be in direct contact)successfully improved the interface electric field intensity,which produced by the LSPR effect of Cu.In addition,the Fano resonance is introduced in Cu/TiO₂/Cu₂O configuration to further enhance the interfacial electric field enhancement.Meanwhile,the hot electrons generated by the LSPR effects of Cu core readily cross the lower Schottky barrier in C u/TiO₂ interface.Based on the analysis of electronic structures,the three types of band alignment may exist in the Cu/Cu₂O/TiO₂ ternary composite photocatalysts,and the possible electron transfer processes are discussed in details.(3)The LSPR charact eristics of Cu nanosphere array/TiO ₂ composite photoelectrodes with different configurations,different arrangements,different periods,and oxidation were simulated by FDTD method.The results show that when the closely arranged hexagonal Cu nanosphere ar ray is completely sunk into the TiO₂film,the absorption enhancement is the strongest and does not affect the absorption of TiO₂ in the ultraviolet region.Moreover,because of the scattering effect between Cu nanospheres and the strong interaction betwee n Cu and TiO₂,a strong absorption appears in the wavelength of 450-550 nm,which enables the Cu nanosphere array/TiO ₂ composite photoelectrode to can achieve the wide light respnose from 300-1000 nm.In addition,the quadrupole mode oscillation in the hexagonal nanosphere array is favorable for coupling light into the underlying TiO₂ to promote the light absorption of TiO ₂.However,the presence of Cu ₂O greatly weakens the LSPR effect of Cu nanospheres,but the presence of the Cu ₂O covering can increase the light absorption around 500 nm wavelength,and provides a new interface for the separation of photo-generated carriers and more active site s are used for the reduction reaction.In photocatalysis,due to the serious recombination of photocarrier at the interface,we take the local electric field strength as the main index to measure the photocatalytic efficiency.Taken together,a hexagonal nanosphere array with a gap of 2 nm may greatly enhance the water-splitting performance of the photoelectrode in the photoelectrochemical system.In this paper,the generation mechanism of the LSPR effect in the Cu/TiO ₂hetero-structure is systematically studied by the FDTD method a nd first principles calculation.And an effective approach is propo sed to adjust the LSPR deviation from the interband transitions of Cu.The influence mechanism of Cu ₂O oxide layer on the LSPR effect of Cu nanospheres was analyzed in detail,and the clarification of the positive role of Cu ₂O in separating photogenerated carriers.By constructing a periodic Cu nanosphere array substantially increase the LSPR effect of Cu for photocatalytic water splitting reaction.The above results provide a theoretical basis for realizing large-scale practical application of Cu/TiO₂ composite photocatalysis.