| The development and utilization of renewable energy is the premise and guarantee of sustainable development,and solar energy is one of the high-quality resources among them.Therefore,the research on converting the endless solar energy into other available energy has attracted much attention.Splitting water into hydrogen and oxygen via a photoelectrochemical procedure is a promising approach to directly convert solar energy into clean energy fuel.Such a typical photoelectrochemical water splitting procedure can be divided into three steps:light absorption to generate charges,separation and transport of photogenerated charge carriers,and finally,water reduction and oxidation reaction.After that,solar energy can be converted into storable chemical energy.As the carrier for realizing the entire photoelectrochemical process,metal oxide is one kind of excellent candidates due to its appropriate energy band position,adjustable band gap and good stability.Herein,we investigated the transport and separation of photogenerated charges between electrolyte/cocatalyst/semiconductor electrode phases by using the metal oxide-based photoelectrodes as objects.The photogenerated charge transport,separation and reaction kinetics on electrode surface and bulk were discussed.Furthermore,on that basis,the photoelectrodes were further optimized and exhibited enhanced PEC performance.The dissertion is summarized as follow.(1)In order to solve the surface reaction kinetics problem of n-Si nanowire arrays(Si NW)photoelectrode,TiO2 film and non-noble bimetallic NiMoO4 nanosheets cocatalyst were deposited on Si NW photoelectrode.Such a structure effectively promotes charge transportation in semiconductor electrode,resulting in reduced charge carrier recombination.Besides,thanks to the robust OER activity of NiMoO4 nanosheets,fast surface reaction rate was also obtained.As a result,the NiMoO4/TiO2/Si NW photoanode delivered a photocurrent density as high as 8.7 mA cm-2(2.5 V vs RHE),which is 111 times higher than that of Si NW photoanode.Furthermore,the TiO2 film improves the stability of Si-based photoelectrode efficiently.Finally,a Si-based photoanode with extremely enhanced performance was obtained.(2)By intergrating non-noble metal NiMoOxS4-x nanosheets cocatalyst with planar p-Si wafers,along with an ultra-thin TiO2 film as the protective layer,photogenerated charge transfer at the interface between the semiconductor electrode and cocatalyst,and reaction rate of photoelectrode surface were efficiently improved.Compared with the Si wafer photocathode,the resulting TiO2/NiMoO4-xSx/TiO2/Si wafer photocathode delivered 12 times higher photocurrent density,with the onset potential positively shifting 0.34 V,which indicates significant improvement for Si-based photocathode.(3)Combining p-type metal oxide CuO with n-type semiconductor ZnO to form a three-dimensional branched heterosrtuctured photocathode.The photocurrent density of CuO/b-ZnO reached 3 mA cm-2(0 V vs RHE),and the ratio of photo to dark current density is 6.4,while the ratio for the pristine CuO photocathode is only 2.7.The enhanced PEC performance is attributed to the synergistic effects of promoted charge separation and transfer in the bulk of photoelectrode,prolonged carrier lifetime and reduced charge recombination.(4)Aiming at the inferior charge separation ability in the two-dimensional structure SnS2 nanoplate arrays photoelectrode due to reduced space charge layer,we designed and prepared a two-dimensional SnS2/SnO2 heterostructured photoanode,which can enlarge the space charge layer of SnS2 photoanode,and provide powerful driving force for charge separation and transfer in the bulk of photoelectrode,efficiently improving higher charge separation efficiency.Consequently,the photocurrent density of the SnS2/SnO2 heterojunction photoanode is 0.7 mA cm-2 at 1.23 V vs RHE,which is the highest photocurrent density for wet-chemically prepared SnS2 based photoanodes. |
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