One-Dimensional ?-Fe₂O₃ And ZnO Photocatalysts For Photoelectrochemical Application

Excellent photocatalytic performance of zinc oxide?Zn O?and hematite??-Fe₂O₃?are a currently hopeful material for photoelectrochemical?PEC?water splitting.Due to its favorable energy band gap?2.0-2.2 e V?,economic viability,decent chemical stability in oxidative solutions and abundance,a theoretically STH efficiency of Fe2O3 is 16.8%.However,the STH efficiency of hematite is far low than 16.8%,due to weakly electrical conductivity,the short photogenerated charge carriers lifetime,short hole diffusion length?2-4 nm?,depressed oxygen evolution kinetics,and disadvantageous conduction band potential in regard to the water reduction leve.Zinc oxide?Zn O?is another promising material for the application in photoelectrochemial water oxidation due to higher electron mobility.However,the drawbacks of being large band gap?3.37 e V?and low photocorrosion resistant restrict its application in practical sense.Great efforts were concentrated on the modification of electronic structure to enhance their PEC activity.In the aspect of energy shortage and environmental pollution have important research significance.In this thesis,we used wet chemical methods to synthesize ?-Fe₂O₃ nanomaterials,and Zn O nanowires were prepared by hydrothermal method,we conducted the surface modification of Zn O nanowires and hematite nanomaterials which have been prepared by a lot of methods to improve their phtotoelectrochemical activity.In the second section of this thesis,we demonstrate a fast and low-cost wet chemical routes to chemically stabilize Zn O nanowires?NWs?by conformal growth of narrow band gap Fe2O3 shell,allowing their operation in strongly alkaline solution and efficient utilization of solar spectrum.This shell forms an intact interface with the wurtzite Zn O NWs,and its thickness can be controlled and optimized by varying reaction time.The Fe2O3 shell is subsequently activated by introducing a amorphous iron phosphate overlayer via annealing in phosphine atmosphere.The existence of this over layer endows Zn O/Fe2O3 core-shell NWs with substantially suppressed surface recombination,possibly through two mechanisms: passivating surface state and accelerating process of surface catalysis.Beneficial from this,the post PH3-treated samples also exhibit longer lifetime and faster transport for photogenerated electrons,relative to pristine Zn O/Fe2O3 NWs.In comparison to bare Zn O nanowire,the enhanced photocurrent for Zn O/Fe2O3 core-shell NWs is observed because of the heterojunction effect.This photocurrent is further improved after post-treatment with PH3.The optimized sample show a photocurrent as high as 2.4 m A·cm-2 at 1.23 V vs RHE?under illumination of AM 1.5 G,100 m W·cm-2?,which is 3 times greater with respect to untreated samples?0.8 m A·cm-2?.This achievement suggest the importance of rational design of electrode configuration as well as substantial potential of overlayer-based passivation of trap state for assembling stable and high efficient photoelectrochemial water oxidation devices.In the third section of this thesis,Firstly,Cu O is obtained by electrodeposition,etching and high temperature annealing,then using the wet chemical method gets Sn4+ doped ?-Fe2O3 nanotubes,because of Sn4+ion radius is close to the ionic radius of Fe3+,this makes Sn4+dopant is superior to other dopant ions.Sn dopant serves as an electron donor and increases the carrier density of hematite nanotubes.In low temperature?300??sodium phosphate was annealed to produce PH3,using PH3 activated Fe2O3 to form amorphous Fe PO4,as a result,the oxygen vacancies are decreased and the surface states are effectively suppressed.Compared to pristine Fe2O3 nanotubes,PH3 treatment samples presented a good photocurrent response,its photocurrent is 0.8 m A·cm-2?1.23 V vs RHE?,while the photocurrent of Sn4+doped Fe2O3 nanotubes is 0.6 m A·cm-2?1.23 V vs RHE?.In addition,we are looking forward to applying the thought is that codoped metal and nonmetal to other semiconductors.In the forth section of this thesis,Zn O nanowire is acquired by hydrothermal method,then we use a wet chemical method to gain the etched Zn O nanowire.Not only the etched Zn O nanowire increased the specific surface area,but also its could produce more photogeneate electrons and holes,which increased carrier concentration,thus enhance their phtotoelectrochemical activity,the photocurrent of the etched Zn O nanowire is 0.64 m A·cm-2?1.23 V vs RHE?,while pristine Zn O is 0.4 m A·cm-2.Zn O/Zn S nanowire is successfully obtained by using solution sulfurization process.Surface modification with Zn S shell layer plays a robust functional role on electrode which reveals against photo chemical corrosion than Zn O electrode.Efficient electron transfer from Zn S conduction band to Zn O conduction band and reduced hole recombination at sample's surface improves the photocurrent.The photocurrent of Zn O/Zn S is 1.42 m A·cm-2?1.23 V vs RHE?.Zn O/Zn S structures promote fast electron hole separation,due to Zn S functionalized.