The Preparation And Photocathodic Research Of BiFeO₃ Ferroelectric Films For Water Splitting

Problems about energy and environment not only restrict the economy but also influence the human production,life and health.Using photocatalytic water splitting for hydrogen production is one of the effective means to solve such problems.Photoelectrochemical?PEC?properties of various semiconductors have been studied for dozens of years.To date,the most commonly used photoelectrodes in PEC systems are binary transition metal oxides,such as TiO₂ and ZnO.However,these materials have large band gap and can only absorb ultraviolet light,which greatly limits their application in water splitting.Due to the ferroelectric photovoltaic effect,the application of ferroelectric films in the field of PEC water splitting has attracted attention recently.Poling under external electric field can produce and reverse a depolarization field in the ferroelectric film,which can enhance the separation of photo-generated carriers.BiFeO₃?BFO?,a hot multiferroic material,is one of the representative of the ferroelectric materials.BFO has relatively small band gap?2.1-2.8 eV?,which can absorb the visible part of the solar spectrum.Therefore,the application of BFO in PEC electrode for hydrogen production has recently been focused on.However,BFO film as photoelectrode has low photoelectric conversion efficiency and small photocurrent.In order to improve its PEC performance,we mainly carried out the following two parts of research work in this thesis:?1?We use magnetron sputtering method to deposit the BFO polycrystalline films on ITO-coated glass,and then study their PEC properties after depositing Au nanoparticles with different sputtering time.The PEC performance of ITO/BFO/Au photoelectrode is obviously improved when comparing with the pure ITO/BFO.At 0.6 V vs.RHE,ITO/BFO/Au with 20 s deposition time has the greatest photocurrent density of-25?A/cm²,which is double of that from pure ITO/BFO photocathode.The localied surface plasmon resonance?LSPR?effect of Au Nps can enhance the visible-light absorption of ITO/BFO/Au photoelectrode,and this leads to the enhancement of its PEC performance.In order to improve the ferroelectric properties of BFO films,we introduced a 10 nm ZnO as the buffer layer between BFO and ITO.The ferroelectric properties was improved a lot due to the improvement of the surface flatness and the leakage current of the BFO film.The photocurrent at 0.6 V vs.RHE and onset potential increases to-62?A/cm² and 0.79 V vs.RHE,respectively,when ITO/ZnO/BFO/Au was polarized.?2?We introduced a carbon buffer layer between BFO and the Pt catalyst.Pt is the best performing catalyst discovered to date for the photocathode where H₂ evolved reaction?HER?happens.However,the work function E_f of the semiconductor BFO is?4.8 eV,much smaller than that of Pt??5.4 eV?.As a result,an upward barrier will form between Pt and BFO which impedes the electrons moving out from electrode surface to electrolyte.In this study,we tried to insert a conductive carbon layer between BFO and Pt to inhibit the formation of BFO/Pt Schottky barrier.Carbon has a work function of?4.2 eV.It forms a downward barrier with BFO which is favorable for the electron transfer on the BFO/carbon interface.Since it is highly conductive,thus electrons can go through the carbon/Pt interface.A facile,cost-effective fire treatment of BFO surface results in a porous carbon layer due to the cabonization of n-butane.Comparing with the ITO/BFO photocathode,significant enhanced photocathodic performance is observed in the ITO/BFO/carbon when the same Pt amount is loaded.The photocurrent at 0.6 V vs.RHE is-125?A/cm² and onset potentail is 1.2 V vs.RHE when the ITO/BFO/carbon/Pt photocathode was polarized.