Synthesis Of ZnO And TiO2 Based Semiconductor Photoanodes And Study Of Their Photoelectrochemical Performance

Photoelectrochemical?PEC?water splitting is very significant for sustainable development since it is a potential new energy technology.The PEC cell consists semiconductor working electrode?photoelectrode?,counter electrode and electrolyte.Taking photoanode as an example,the whole PEC process can be described as follow: Under the illumination,the semiconductor photoanode generates the photo-excited electrons and holes,the photo-induced electrons migrate to the cathode under the external bias,and the H+ ions around the cathode are reduced into hydrogen after getting the electrons.While the holes paticipate in the water oxidation reaction after diffusing to the surface of the semiconductor.Obviously,the key point is to investigate and fabricate the semiconductor photoelectrode which possess excellent photoelectrochemical activity and stability.The PEC performance is mainly influenced by two factors.For one side,the materials should be able to absorb more visible light to generate photo-excited carriers as more as possible.On the other hand,the charge separation is very important,that is,the photoelectrode should suppress the charge recombination and extend the life time of the photo-excited carriers,making more and more electrons and holes can take part in the water splitting reaction effectively.Among various semiconductor materials,TiO₂ and ZnO are low-cost,physical and chemical stable,non-toxic and environmental friendly,which make them very potential photoelectrode materials.However,their band gap is large,which limits their visible response.Also,the photogenerated electrons and holes recombine significantly during the PEC process,which restrict their photoelectric conversion efficiency.In this dissertation,we prepared the photoanodes with different morphology and structure via different methods,and achieved the improved PEC performance,the main working contents are summerized as follow:1.ZnO nano film were deposited on the FTO substrates by a dc magnetron sputtering method,then,the Al film were deposited onto ZnO via a thermal evaporatation process,and the thickness and the morphology of the Al film can be controlled by the evaporating time easily.The visible absorption of Al/ZnO film was improved,and the photocurrent density increased more under visible radiation.By anlyzing the impedence,we found the kinetics of Al/ZnO is faster than bare ZnO,which could be ascribed to the Al film decoration,faciliating the chare migration at the interface.2.Preparing the 3D flower-like nano structured TiO₂ photoanodes?TiO₂ NFs?with a simple hydrothermal method,and how the hydrothermal time affect the morphology was discussed,and we proposed the growth mechanism of the nanoflowers.Then,the PEC performance of the samples were tested.Acorrding to the results,the 3D flower-like nano structures enlarge the specific area and provide more effective transport paths for electrons.3.The as-prepared TiO₂ NFs were coated with Mo S₂ and GO nanosheets by a dipping method.Acordding to the experiment,TiO₂ NFs/Mo S₂ and TiO₂ NFs/GO photoanodes could absorb more visible light,which contributtes to the narrowed band gap after Mo S₂ and GO decorating.Moreover,the interfacial heterojunction of the composite tuned the migration of the electrons and holes,which promotes the holes diffusing to the surface and taking part in the water oxidation.4.The TiO₂ NFs were dipped in the Na BH4 solution for a period of time for reducing treatment.By analyzing the experiment results,the existence of Ti3+ ions and oxygen vacancies were confirmed.The self-doped R-TiO₂ NFs showed greatly enhanced PEC performance.The induced oxygen vacancies increase the donor density remarkbaly,causing the upward shift of the Fermi level,leading to a larger band bending level,which faciliates the charge separation at the interface.