Fabricaiton And Photoelectrochemical Property Of Titanium Dioxide Composite Structures

In this thesis,photoelectrochemical(PEC)properties of theTiO2 semiconductor nanowire arrays photoanode are studied for hydrogen production.Different methods are used to prepare TiO2 nanowire arrays(TiO2 NWAs)photoanode decorated with the other martials to form heterojunctions,and we try to reveal the cocatalytic reaction mechanism of composite materials.First,we synthesized TiO2 nanowire arrays photoanode and its composite materials through a series of nanomaterial synthesis methods,including hydrothermal method,chemical precipitation method,electrochemical deposition method,and solid-phase reaction method.Then,through modem characterization methods to study the phase composition,size,morphology,and light absorption characteristics of PEC materials.The activity of semiconductor photoanode was evaluated by the performance of the photoelectrochemical water-splitting.The construction and interfacial properties of heterojunctions were studied,and the relationship between the microstructure and properties of PEC materials was revealed to understand the photoelectrochemistry mechanism.The specific research content is divided into the following parts:(1)Nanostructured TiO2@Ag2O p-n heterojunction arrays were fabricated by combining the hydrothermal method with the chemical-bath method,and they were used as a photoelectrode in photoelectrochemical cell to produce hydrogen.Nanostructured TiO2 nanowire arrays are combined with Ag2O nanoparticles to produce a large number of nano p-n heterojunctions.Under simulated AM 1.5 irradiation(100 mW/cm2),the photocurrent density of the TiO2@Ag2O p-n heterojunctions arrays anode at a bias of 1.23 V vs.RHE is 1.61 mA/cm2,which is twice times of that of the pristine TiO2 nanowire arrays.In the absorption range of 350-450 nm,IPCE of the TiO2@Ag2O nanowire array is higher than that of the original TiO2 nanowire arrays.As the incident wavelength is about 385 nm,the highest value of IPCE achieved by the TiO2@Ag2O nanowire array photoanode is 53%.However,IPCE of the pristine TiO2 nano wire arrays photoanode is only 41%at the same wavelength.Because of the p-n heterojunction interface between p-Ag2O and n-TiO2,this enhancement is due to effective electron-hole separation.We verified the above conclusions by the electrochemical impedance spectroscopy(EIS),including Mott-Schottky plots and Nyquist plots.TiO2@Ag2O p-n heterojunction arrays were expected to have the considerably potential applications insolar water splitting.(2)ZnCo-layered double hydroxides(ZnCo-LDH)and Co(OH)2 samples were obtained by solid-phase synthesis at room temperature.The above two materials have good electrochemical activity for oxygen precipitation reaction.The synthesis method does not require complicated equipment and complicated synthesis procedures.The morphology of ZnCo-LDH isthe layered structure,while the morphology of Co(OH)2 is thehexagonal nanosheet.ZnCo-LDH shows great electrocatalytic property for OER with low initial potential,low Tafel slope and good stability.The initial potentials of ZnCo-LDH and Co(OH)2 are 1.42 V vs.RHE and 1.53 V vs.RHE,respectively.At a current density of 10 mA/cm2,the overpotentials of the ZnCo-LDH layered structure and the Co(OH)2 hexagonal nanosheets were 19 mV and 30 mV,respectively.As a kind material with Zn doping Co(OH)2,the conductivity of ZnCo-LDH not been improved.However,due to the good layered structure,ZnCo-LDH shows a higher activity of electrochemical oxygen evolution reaction.A novel,simple one-step solid-phase reaction method was developed to synthesize ZnCo-LDH with layered structure and Co(OH)2 with hexagonal nanosheet structure.This method without any solvent is convenient,green,and high yield.ZnCo-LDH synthesized by this method are expected to play a role in practical industrial applications of electrocatalytic water splitting.(3)By using a simple electrochemical method,ZnCo-LDH was decorated on TiO2 nanowires fabricating a TiO2/ZnCo-LDH core/shell arrays photoanode.The electrocatalytic water oxidation property of ZnCo-LDH improves the photoelectrochemical performance of TiO2 photoelectrode.The high-efficiency photoelectrochemical performance is attributed to the excellent electrocatalytic water oxidation activity of ZnCo-LDH.This material can quickly transfer the photogenerated charge to the solid/liquid interface on the surface of the photoanode,which is beneficial to the water oxidation reaction.Under the entire bias of the PEC reaction,the charge transfer efficiency of TiO2@ZnCo-LDH nanowire array photoanode is significantly higher than that of the original TiO2 nanowire array photoanode;meantime,the difference in photogenerated charge separation efficiency between the two is small,and the two curves of photogenerated charge transfer efficiency have a cross as the applied bias increases.When the applied bias potential is 1.23 V vs.RHE,the photogenerated charge transfer efficiency of the TiO2@ZnCo-LDH nanowire array photoanode increases to 65%,and under the same conditions,the charge transfer efficiency of the pristine TiO2 nanowire arrays photoanode is only 55%.(4)Efficient charge transfer and separation play a significant role in determining the photoelectrochemical water-splitting performance of photocatalysts.Here,hierarchical nanowire arrays(NWAs)containing the TiO2 nanowire core and nickel-cobalt layered double hydroxide(NiCo-LDH)shell were synthesized by combining a hydrothermal method with a facile electrochemical-deposition process.The hierarchical structure not only forms heterojunctions between the TiO2 core and NiCo-LDH shell but also provides active sites for water-splitting reaction,achieving the charge transfer efficiency up to 87%and the charge separation efficiency up to 42%at 1.23 V vs.reversible hydrogen electrode(RHE)under one sun illumination.The excellent PEC water-splitting performance of TiO2@NiCo-LDH core-shell NWAs is attributed to the enhanced charge transfer and separation,resulting from the decoration of the NiCo-LDH cocatalyst.This facile and cost-effective strategy for integrating the light-harvesting TiO2 semiconductor and NiCo-LDH cocatalyst into a hierarchical core-shell nanostructure can be potentially applied in energy conversion and environmental applications.