Construction Of Three-dimensional Porous Ag/Cu₂O Structure With Dendrite For Efficient Photocatalytic Reduction Of CO₂

With the immoderate burning of irreplaceable fossil fuels causing a continuous rise of CO₂concentration in the atmosphere,and global warming is a major challenge in the world.CO₂ conversion to fuel by photo-technology provides an alternative pathway for the sustainability of the modern society.In recent years,cuprous oxide（Cu₂O）can respond to the visible light,can theoretically reduce carbon dioxide into different products owing to its suitable energy band structure,high theoretical photocurrent density(14.7 m A cm^-2),and solar cell conversion efficiency of about 18%.Therefore,it is a promising photocatalyst.However,the Cu₂O photo-generated electrons and holes recombination rate is too fast and the poor selectivity during the catalytic process limits its photocatalytic performance.Therefore,it is of great significance to modify Cu₂O photocatalyst to overcome its existing defects.In this thesis,by modifying the morphology of Cu₂O,the three-dimensional porous structure composed of one-dimensional dendrites was prepared to improve the light absorption and electron transport capabilities.The Schottky junction formed by Ag nanoparticles further enhanced the electron-hole separation ability,which reduced the oxidation phenomenon of the Cu₂O,and improved the CO selectivity during the photocatalytic process.（1）Three-dimensional（3D）porous Cu₂O with dendrite structure was prepared by electrodeposition combined with subsequent thermal oxidation.According to adjusting the deposition current density（0.025,0.1,0.7 and 1.3 A/cm²）,the dendritic three-dimensional porous Cu with the best morphology was obtained at 0.7 A/cm² current density.The resultant3D porous Cu samples were annealed at four different temperatures 170,220,270,and 320℃.For comparison,non-porous Cu₂O samples were prepared with low current density（0.025 and0.1 A/cm²）,and porous Cu₂O samples were prepared with high current density（0.7 and 1.3A/cm²）.The 3D porous Cu structure was completely transformed to Cu₂O with 220℃annealing,which exhibited high electrochemical specific surface area and good photocatalytic CO₂ reduction performance.The 3D porous Cu₂O photogenerated carriers(4.3×10²⁰ cm^-3)is2.5 times that of non-porous Cu₂O and the photocatalytic CO₂ reduction to CO(13.4 nmol cm^-2 h^-1)is 24 times that of non-porous Cu₂O.Especially,some C2 reduction product was observed for the 3D porous structure.This is because the 3D porous structure improves the mass transfer efficiency of CO₂ gas,and the nano-sized dendrite structure enhances the efficiency of light capture and photoinduced electron transportation.Furthermore,the rapid separation and high reaction activity of photoinduced holes and electrons result in the anti-photocorrosion properties of 3D porous Cu₂O.（2）On the basis of the above materials,Ag/Cu₂O composite structures were prepared by depositing Ag nanoparticles with different contents of Ag NO₃ solutions.The light absorption capacity and carrier separation efficiency of Cu₂O are significantly improved after Ag loading.The photocurrent density of the sample in the 0.04mol/L Ag NO₃ concentration reaches 75.6μA cm^-2,which is 1.7 times that of the original sample.The carrier concentration of Ag/Cu₂O-0.04 is 2.1×10²¹ cm^-3,which is nearly 5 times that of pure Cu₂O(4.3×10²⁰cm^-3),and the charge transfer resistance is also significantly reduced.In the photocatalytic CO₂ reduction performance,the CO yield is 22.52 nmol cm^-2 h^-1(43.04μmol g^-1 h^-1),which is about 1.5 times that of the original sample.The CO selectivity is 82.9%,which increase of23.3 percentage points hat of the Cu₂O.The improvement of CO selectivity indicates that Ag loading has an effect on the adsorption configuration of CO₂ on the surface of Cu₂O,the loading of Ag makes the adsorption configuration of from bidentate carbonic acid to monodentate carbonate.The Schottky structure composed of Ag/Cu₂O accelerates the surface reaction kinetics while accelerating the separation of photogenerated carriers.