Investigation On Ag@SnO₂ Core-shell Nanoparticles Incorporated ZnO Nanorods For UVPD

UV photodetectors?UVPD?can be used for various applications as light-wave communications,flame alarm,and ozone and pollution monitoring,which became more important with the ecological problems nowadays.Photoanode is one of the most important parts of UVPD.ZnO nanorod is a prominent material for photoanode.Here,we tried to enhance the photoinduced current of the photoanode,by incorporated core-shell Ag@SnO2nanoparticles.The enhancement should be due to localized surface plasmon resonance energy transfer?inherent of Ag nanoparticles?and by increasing the oxygen hole traps.By using ZnO nanorods for the photoanode material different UVPD are built and tested.The I^-/I3^-couple has been used for electrolyte and the counter electrode was made of platinum.First a ZnO seeds layer has been coated over an FTO substrate by magnetron sputtering process,then the seeds layers have been annealed,finally hydrothermal methods have been used to grow the nanorods from the seed base.Different annealing temperatures have been tested,150°C,250°C,350°C,and 450°C.The 350°C annealing temperature improved the crystallinity the uniformity and the adhesion of the seeds,thus of the nanorods.Two different ZnO²⁺sources have been tried for hydrothermal process.The best results were for zinc acetate,it allowed a better vertically oriented growth of the nanorods.The potentials were not affected by the quality of the nanorods,but the photoinduced currents were.For the best UVPD a short circuit current of 0,529mA under a light power of 3,92mW was measured,the efficiency was 6,6%of power conversion.The shape of the current curves,while the light is repeatedly turn on/off,showed a fast-sharp drop just after illumination.The Ag@SnO2 nanoparticles were synthesized by a two steps process:first a chemicalreduction to produce Ag nanoparticles,then a precipitation method using sodium stannate to coat with a SnO2 shells.Different amounts of sodium stannate were used to create different shell thicknesses.The effect of the surrounding medium caused red shifts on absorbance spectra after coated with SnO2 shells.From Mie theory it was possible to calculate the mean thicknesses of the shells,for a sodium stannate added volume of 2ml,3ml and 4ml,respectively,they were2nm,4nm and 6nm.No Ag/Sn or Ag/SnO2 compound peaks were found from XRD,so the Ag core and the SnO2 shell conserved their structural organizations.Ag nanoparticles crystallized with a face-centered cubic structure,while the SnO2 shells showed a trigonal cassiterite form.The dynamic light scattering showed that the shells were uniformed all around the core,it seemed that no holes appeared in the shell,so ensured the stability of the Ag nanoparticles.The incorporations of core-shell Ag@SnO2 nanoparticles were done by immerging for two hours the ZnO nanorods photoanodes into different water solutions of Ag@SnO2.Three different solutions have been used,two of them were composed of Ag@SnO2 nanoparticles with a shell thickness of 2nm,but with different concentration.The third solution was composed of 10nm thick shell nanoparticles.The Raman spectra confirmed the presence of Ag@SnO2 nanoparticles after incorporation.Raman peaks were enhancement by a factor 10after the incorporation of Ag@SnO2.From the absorbance tests a bump was found between400nm and 500nm wavelength,it is characteristic to the nanoparticle surface resonance plasmon wavelength.For every photodetector with incorporated Ag@SnO2 nanoparticles,an enhancement of the photocurrent was measured.For a similar number of incorporated nanoparticles,the ones with a shell thickness of 10nm showed a photocurrent of 0,597mA,for a thickness of 2nm it was0,735mA.This last result was the highest improvement,it produced a power of 0,388mW.Comparing with an UVPD without any incorporation,there is an enhancement of 45%.Two effects are responsible,the energy transfer from localized surface plasmon resonance near-field,and the recombination restraining of photogenerated electrons.The potentials and dark currents were not affected by the incorporation.According to Nyquist plots incorporation of Ag@SnO2nanoparticles increases the interfacial resistance between the electrolyte and the photoanode,it reduces the recombination rate at the interface.When the solution concentration used to incorporate the Ag@SnO2 core-shell nanoparticles decreased,the enhancement of the photocurrent also decreased.For no-incorporation,during long illumination,the photocurrent decreased.It was due to a slow hole trap recovering rate.However,when Ag nanoparticles were incorporated,they tend to bend the ZnO conduction band,accelerating the hole trap recovering rate,reducing the recombination inside the nanorod.When the shell around the Ag nanoparticles is too thick,this bending reduces,becoming not significant to affect the hole trap recovering.The photoluminescence of the different photoanodes were also measured.The peak intensity at 350nm,corresponding to excitonic recombination of ZnO conduction band electrons,was enhanced by a factor 7 for the incorporation of Ag@SnO2 with 2nm thick shell,while no enhancement was found for 10nm thick shell.