| Photoelectrochemical(PEC)water splitting producing H2 contributes to constructing the clean renewable energy system taking H2 as energy carrier,which is considered as a promising approach to cope with the challenges of environmental pollution and energy crisis.The efficient photoanode semiconductors are required to improve the oxygen evolution reaction and meet the practical demands of PEC water splitting,since the water oxidation suffers from large over-potentials and sluggish kinetics.Three main steps are carried out for PEC water oxidation on photoanodes,including(1)the generation of hole-electron pairs in semiconductors absorbing the incident photons whose energy are not less than the bang gaps of semiconductors,(2)the separation of hole-electron pairs in the built-in electric field of space charge layer and the transport of holes to the semiconductor-electrolyte interface(SC|E),and(3)the transfer of holes at SC|E to oxidize water for oxygen evolution.Besides the improvements of the optical and electric properties of photoanodes,the modifications of electrocatalysts,inert protection layers and/or effective light absorbers are usually applied at SCIE to achieve the efficient and stable PEC water splitting.Therefore,the photo-generated charge carriers in semiconductors should undergo the charge separation and transport,the charge transfer at the interface between semiconductor and modification layer,and the water splitting at the interface between modification layer and electrolyte.It is important to understand the functions of the modification layers on photoelectrodes,and to study the transfer and energy loss of charge carriers as well as the kinetics of water oxidation at multiphase interfaces for efficient PEC performances.In this thesis,our research is focused on interfacial water oxidation on Ti4+-doped?-Fe2O3(Ti-Fe2O3)photoanodes.The electrocatalysts,inert overlayers and metal particles with surface plasmonic resonance(SPR)are employed to modify Ti-Fe2O3 photoanodes to study their functions on the separation and transfer of photo-generated charge carriers and the kinetics of water oxidation at photoanode-electrolyte interfaces.We describe the properties required for effective electrocatalysts,investigate the charge recombination at the interface between semiconductor and SPR excimer,and study the effects of ion-permeability of inert overlayers on interfacial water oxidation.The main research contents are as follows:(1)Electrocatalysts improve the performance of water oxidation on photoanodes.The semiconductor-electrocatalyst-electrolyte interfaces were constructed by modifying the high-crystalline and low-crystalline CoGa2O4 on Ti-Fe2O3 photoanodes.The low-crystalline electrocatalyst is found to be ion-permeable and to present the bulk catalytic properties,of which the moderate ion constraint contributes to catalytic activation and stability.The low-crystalline electrocatalyst is more suitable to promote the performance of water oxidation and decrease on-set potential of Ti-Fe2O3 photoanodes.In addition,the hole extraction and charge transfer are improved,so the photocurrent density at 1.23 VRHE is increased to 1.45 mA cm-2.(2)Electrocatalysts adjust charge separation and injection of photoanodes.The semiconductor-electrocatalyst-electrolyte interfaces were built by depositing CoOOH on Ti-Fe2O3 at different bath temperatures,bringing about gradual ion-permeabilities of oxygen evolution catalysts(OECs).We find that the efficient OECs require high catalytic activation as well as the ability to form impactful heterojunction,which could promote charge separation and charge transfer.Adjusting ion-permeabilities of OECs would affect the interface energetics and kinetics of water oxidation for enchanced efficiencies of charge separation and injection,so the photocurrent density at 1.23 VRHE could be increased to about 1.90 mA cm-2.(3)Surface passivation and SPR enhance the utilization of absorbed photons for photoanodes.The semiconductor-passivation layer-SPR metal-electrolyte interfaces were formed by modifying Al2O3 and Au nanoparticles on Ti-Fe2O3.The SPR effects of Au nanoparticles contribute to improve the light absorption,and Al2O3 overlayer is used to passivate the surface states on photoanodes,suppressing interface recombination between Au and Ti-Fe2O3.Surface passivation promotes the plasmon-induced energetic carriers to participate into PEC water oxidation,improving the utilization of absorbed photons.As a consequence,the incident photo-to-current efficiency at 589 nm is increased twice(4)Inert overlayers adjust the interfacial charge transfer and injection efficiency of photoanodes.The semiconductor-inert overlayer-electrolyte interfaces were constructed for Ti-Fe2O3 coated with Al2O3 overlayers having different ion-permeabilities.The compact and ion-impermeable Al2O3 overlayer could passivate the surface states on photoanodes and suppress the back reaction.In contrast,the ion-permeable Al2O3 fails to passivate surface states,and ineffectively prevents the back reaction.Nevertheless,this overlayer could promote the charge transfer and remarkably improve the forward reaction of water oxidation.As a result,the injection efficiency of photoanodes is improved,and the photocurrent density at 1.23 VRHE is increased to about 2.38 mA cm-2. |
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