The Preparation Of Two Dimensional Micro-Nano Structural Semiconductor Photoanodes And Their Photoelectrochemical Performance

Energy and environmental issues are two major challenging subjects among the biggest challenges in the current world.With the growth of worldwide industry,this will accelerate the need and consumption for energy.Nowadays the mainly energy source is still chemical fuels including oil,coal,natural gas and so on.Lots of consumption with chemical fuels has induced serious environmental issues.Another reason is that those chemical fuels are non-renewable,and it will run out in the future.Therefore,it is the thing that very important to develop the new renewable energy.Hydrogen is the energy source that has the properties of high combustion value,high efficiency and clean,which is very promising to solve energy storage and environmental crisis.Since the discovery of semiconductor photocatalytic water splitting in 1972,it has been attracted extensive investigation and concern.Especially the development and progress in nanotechnology and nano-materials in the 21^stt century make more application of using semiconductor nano-materials into the field of photocatalytic and photoelectrochemical（PEC）water splitting.This dissertation is on the basis of semiconductor nano-materials that could absorb visible light and their PEC performance.There are mainly three parts.In the first part,large-scale,triangular pore arrays on GaP were successfully prepared by a simple and high-efficient approach of electrochemical etching under high field.The morphology and structural characterizations of GaP were conducted by scanning electron microscope,X-ray diffraction,UV-vis diffuse reflectance spectra,and so on.The measurements including photovoltage,photocurrent density,and the incident photon conversion efficiency（IPCE）were used to characterize the photoelectrochemcial performance of porous and bulk GaP.The experiment results indicated that the porous GaP exhibited much higher photo-activity in compared with the bulk GaP.The photocurrent of the porous GaP exceeded on order of magnitude higher than that of bulk material under 0.1 V compared to the reversible hydrogen electrode（RHE）,which indicated the porous structure could facilitate the separation of photo-induced carrier charges and their collection to make the porous GaP display higher photoresponse.The structure of triangular pore arrays cooperated with its depth determined the PEC performance.During the formation of porous GaP,the etching time will affect the pore size and depth.Through the PEC performance measurement for different pore size and depth GaP photoelectrode,the optimal etching time could be found.At the same time,the optimal structure of GaP could be determined.Finally,upon the porous structure,siginificantly enhanced hydrogen production has also observed,which indicated that the porous GaP should have important potential in photocatalytic hydrogen generation.On average,hematite has smaller bandgap than GaP.So hematite can absorb more visible range light in compared with GaP,which makes hematite possess higher theoretical solar-to-hydrogen（STH）effiency.More importantly,hematite is much more stable than GaP in the electrolyte without containing sacrificial agent.Part 2 was on basis of iron foil substrate,and began with preparation of hematite under different annealed temperature.The PEC performance of hematite photoelectrodes prepared by different sinter temperature was investigated,and the hematite photoanode annealed under 600oC exhibited the optimal PEC performance.Next step was to further improve the PEC performance hematite photoanode.Two methods were used.The first one was to construct the heterojunction with CuO.The heterojunction CuO/α-Fe₂O₃ could benefit to realize the photo-induced charger separation and transport,which would attribute the photocurrent density increase of hematite photoanode.The other approach was using plasma to treat the surface of hematite.After plasma treatment,the photocurrent density could be improved,which may result from the increased carrier density.Therefore,plasma treatment could enhance the PEC performance of hematite photoanode.At last,hematite photoanode prepared at different oxygen partial pressure under 600oC for 1 h was investigated.Oxygen partial pressure would affect the PEC performance of hematite photoanode under the same annealed temperature.Electrochemical impedance spectroscopy（EIS）was used to analyze the effect of hematite photoanode from oxygen partial pressure.Part 3 mainly explored the preparation of large-area and uniform hematite nanoflakes and sought one way to activate photo-activity for hematite nanoflakes photoanode.The nano-structure hematite photoanode could benefit the charger collection compared to planar structure,which was demonstrated in theoretical investigation.However,the hematite photoanode without any decoration showed low photo-activity.Therefore,it is necessary to active the photoactivity for hematite photoanode in order to improve PEC performance.On one hand,we chose high purity iron foil and made pre-treated using mechanical polish to pave the way to anneal in air.Mechanical polish played an important role in the formation of large area and uniform hematite nanoflakes.On the other hand,in order to activate the photoactivity of hematite photoanode,we first used air plasma to treat hematite nanoflakes to activate their photoactivity.The experiment investigation indicated air plasma treatment could introduce oxygen vacancies into the hematite nanoflakes.The plasma intensity and treated time would affect the amount of oxygen vacancies.Increasing plasma treated time and intensity benefited to make more oxygen vacancies.Through PEC measurement,the hematite nanoflake photoanode with rich oxygen oxygen vacancies exhibited much higher photocurrent density than the photoelectrode with poor oxygen.The nanoflake photoanode with the optimized plasma treatment yields an IPCE of35.4%at 350 nm under 1.6 V vs RHE without resorting to any other cocatalysts,an effiency far exceeding that of the pristineα-Fe₂O₃ nanoflakes（2.2%）.In order to find the reason behind the high PEC performance resulted from plasma treatment for hematite photoanode,we performed Mott-Schottky measurements.Through analysis and calculation,the nanoflake photoanode exhibited higher donor density after plasma treatment.The increase of donor density would facilitate to improve the conductivity of hematite photoanode,and promote the charge transport.At the same time,we measured the EIS for hematite nanoflake photoanode with and without plasma treatment.The results from EIS indicated that theα-Fe₂O₃ nanoflake with rich oxygen vacancies exhibited that charge transfer resistances decreased and capacitances increased,implying oxygen vacancies promoted charge separation and transport during the interface between hematite and electrolyte.Therefore,theα-Fe₂O₃ ultrathin nanoflake photoanode showed high PEC performance that attributed to good light absorption resulted from its bandgap,high efficiency photo-induced charge collection resulted from its nanostructure,and effective charge separation and transport resulted from rich oxygen vacancies.