Preparation Of ZnO, ZnO/CdS And ZnO/PbS Modified TiO₂ Nanowire Arrays And Their Photoelectric Properties

Recently,semiconductor heterojunction micro/nanostructures with superior photoelectronic performance have been widely used as photoelectrodes to produce clean energy hydrogen through photoelectrochemical(PEC)and solar light photocatalytic water splitting.However,single-phase semiconductor heterojunction micro/nanostructure always suffers from high recombination rate of photogenerated charge carriers,limiting conversion efficiency of the solar energy.It has been demonstrated that the photoelectronic performances such as PEC and solar light photocatalytic water splitting of the semiconductor micro/nanostructures can be significantly improved via constructing heterojunction structures.In this thesis,TiO2/ZnO,TiO2/ZnO/CdS and TiO2/ZnO/PbS nanowire arrays(NWAs)were fabricated by modifying TiO2 nanowire arrays with ZnO,ZnO/CdS and ZnO/PbS nanostructures,respectively.The photoelectronic performances of the as-fabricated heterojunction nanowire arrays were studied systematically.This thesis mainly focuses on the following works.Using titanium(?)butoxide and concentrated hydrochloric acid as raw materials,the TiO2 NWAs were prepared by a simple hydrothermal method.Then,using zinc nitrate as raw material,the TiO2/ZnO heterojunction NWAs were constructed by a seed-assisted hydrothermal method.Under irradiation of the simulated solar light,the photocurrent-time characteristic curves were measured by using a standard three-electrode system in 0.1 M Na2SO4 electrolyte.The photocurrent density of the as-prepared TiO2/ZnO heterojunction NWAs photoelectrode was 6 times higher than that of pure TiO2 NWAs photoelectrode at 0 V vs.Ag/AgCl.The photocurrent density of the as-prepared TiO2/ZnO heterojunction NWAs photoelectrode was 3 times higher than that of pure TiO2 NWAs at 0.8 V vs.Ag/AgCl.The average hydrogen production rate of the TiO2/ZnO heterojunction NWAs photoelectrode in the PEC water splitting to produce hydrogen was 2.7 times higher than that of the pure TiO2 NWAs photoelectrode.The mechanism for the enhanced photoelectronic activities of TiO2/ZnO heterojunction NWAs was studied in detail.TiO2/ZnO/CdS heterojunction NWAs were constructed by modifying TiO2 NWAs with ZnO/CdS nanostructures using a successive ionic layer adsorption and reaction(SILAR)method.The light absorption measurements showed that the prepared TiO2/ZnO/CdS heterojunction NWAs exhibited enhanced visible light harvesting ability.Using a mixed aqueous solution of 0.35 M sodium sulfide(Na2S)and 0.25 M sodium sulfite(Na2SO3)as the electrolyte,the photocurrent density of the as-prepared TiO2/ZnO/CdS heterojunction NWAs photoelectrode was 130 times higher than that of pure TiO2 NWAs photoelectrode in a at 0 V vs.Ag/AgCl under illumination of visible light.The mechanism for improved photoelectronic performances of the prepared TiO2/ZnO/CdS heterojunction NWAs was also investigated.The results show that a type ? band allignment is formed between CdS and ZnO semiconductors in the TiO2/ZnO/CdS heterojunction structure.That means that conduction band of CdS was more negative than that of ZnO and the valence band of ZnO was more positive than the valence band of CdS.This band allignment promotes the efficient separation of photogenerated charge carriers in TiO2/ZnO/CdS heterojunction structure,leading to enhanced photoelectronic activities.TiO2/ZnO/PbS heterojunction NWAs were fabricated by modifying TiO2 NWAs with ZnO/PbS nanostructures using a SILAR method.The as-fabricated TiO2/ZnO/PbS heterojunction NWAs show enhanced visible light absorption as confirmed by light absorption measurements.Using a mixed solution of 0.35 M Na2S and 0.25 M Na2SO3 as electrolyte,the photocurrent density of TiO2/ZnO/PbS heterojunction NWAs photoelectrode was 60 times higher than that of pure TiO2 NWAs photoelectrode at 0 V vs Ag/AgCl under visible light irradiation.The electrochemical impedance spectroscopy(EIS)was measured and used to study the transfer and separation of photoinduced charge carriers of the prepared TiO2/ZnO/PbS heterojunction NWAs.The results demonstrated that the charge carries were separated efficiently,resulting in significantly enhanced photoelectronic activities.