Fabrication And Photoelectrochemiacl Properties Of Silicon Composition Structure

To relieve the pressure from energy and environment crisis due to the development of human society,using solar energy to split water to produce oxygen and hydrogen gas is a very ideal scheme.Hydrogen gas has high heat valuve,it is a clean energy.In the process of water splitting,the photoelectrochemical(PEC)materials are very crucial,they not only decide the efficiency of PEC cell,but also affect the lifee time of PEC cell.As one of the most commonly used semiconductor materials,silicon has high carriers mobilities and its mature technologies for fabrication and modification,which is regarded as a very promising material for water splitting.However,its shortages should not be neglected.The practical application of Si is restricted by the unsatisfied potocurrent densitis,bad stability in aqueous solution and sluggish kinetic in oxygen evolution reaction(OER).Aiming to these problems,this thesis carries out these modification as following:(1)N-type WO3 and p-type Si can be assembled into a composite structure called"Z-scheme," which is a high efficiency model for overall water splitting.However,due to the existence of Schottky barrier,its relatively low photocurrent density is still a great challenge for application.Here,a modified "Z-scheme,structure by inserting a W interlayer is fabricated.A great enhancement of photocurrent density over 10 times is achieved,which can be attributed to the introduction of the ohmic contacts between W interlayer with both WO3 and Si layers and the elimination of Si-O bands at the interface.(2)Some special micro-nano structures on the surface of Si are successfully fabricated by metal-asisted chemical etching(MaCE).The contrable and ordely nanoporous arrays,nanorods arrays and nanowires arrays are prepared by adjusting the geography of nobel metal catalysts and changing the recipe of etching solution.These special miro-nano structures can effectively expand the surface,decreasing the onset potential,increasing the photocurrent density of Si photoelectrodes,facilitating the desorption of gas product.The satuated photocurrent density of nanoporous Si is 6 mA/cm2 greater than that of pristine-Si,the onset potential of nanoporous Si is 0.14 V smller than that of pristine-Si.(3)Nanostructured Si as the high efficiency photoelectrode material is hard to keep stable in aqueous for water splitting.Covering a passivation layer on the surface of Si is an effective way to pro vent from oxidation.However,it is still not clear in the difference in mechanisms and effects between insulating oxide materials and oxide semiconductor materials as passivation layers.Here,we compare the passivation effects,the PEC properties,and the corresponding mechanisms between the HffD2/nanoporous-Si and the TiO2/nanoporous-Si by I-V curves,Motte-schottky(MS)curves,and electrochemical impedance spectroscopy(EIS).Although the saturated photocurrent densities of the TiO2/nanoporous Si are lower than that of the HfD2/nanoporous Si,the former is more stable than the later.(4)Developing highly efficient photoanode of PEC cell to meet industrial requirements remains a challenge,we effectively engineer the onset potential of the photoanode though doping of V into Co oxide film by a low-cost way of magnetron co-sputtering deposition on p+n junction Si cell(p+nSi/CoVO).The highest photocurrent density of p+nSi/CoVO is 29.15 mA/cm2(at 1.23 V vs.reversible hydrogen electrode(RHE))and highest photovotage is 608 mV.Moreover,a successful decrease of the onset potential by 40 mV to 1 V(at 1 mA/cm2)and 3.6-fold increase in incident photons to current conversion efficiency(IPCE)are achieved comparing to those of p+nSi/CoO.We find that the doped V atoms can decrease the charge transfer resistance of Co3O4 film,serve as Lewis acid to improve local pH value and facilitate the generation of Co(?)oxo-bridged species to reduce kinetics of oxygen evolution reaction(OER),which can obviously decrease the onset potential of p+nSi/CoVO in OER.This work highlights a general approach to further improve the performance of OER catalysts by engineering active sites and local chemical environment through element doping.