Study on Preparation and Photoelectrochemical Properties of W03-based Nanostructured Photoanodes

Author:Xiao Yong Zuo

Supervisor:zhang wei de


Degree Year:2018





Increasing global environmental deterioration is becoming a serious concern,leading to an exponential increase in scientific interest in renewable energy as a technology to replace fossil fuels.Photoelectrochemical(PEC)water splitting,which directly converts sunlight to hydrogen fuels,is a promising renewable energy technology.Semiconductors,used as photoelectrodes,provide the best known method for converting solar energy to electrical energy or chemical fuels.As a typical wide bandgap semiconductor,tungsten trioxide(WO3)has been widely used as a promising photoanode due to its suitable bandgap of approximately2.7 e V and great efforts have been made to promote the separation of photogenerated carriers.However,the bandgap of WO3 is still too large to realize sufficient absorption of the solar spectrum and its fast photo-generated charge recombination.Moreove,although the top of valence band of WO3 is suitable for O2 evolution under visible light,the bottom of the conduction band of this material is located at a more positive potential than the potential of water reduction.In this paper,several efforts have been taken to improve the PEC performance of WO3,such as metal doping,quantum dot sensitization,nanostructure-controlled growth and combining with other semiconductors to build heterostructures.The details are as follows:(1)A two-step hydrothermal process for preparing Ni-doped WO3 nanoplate arrays(NPAs)is developed,and the obtained samples were used as a photoanode to produce a highly active and stable electrocatalyst for photoelectrochemical(PEC)water splitting under AM 1.5G solar illumination at 100 mW cm-2.The NPAs are formed as a single-phase solid solution with high purity.XPS measurement verifies the binding energy of W element is negatively shifted due to the substitution of W6+by Ni2+in the monoclinic lattice and thereby the formation of Ni-O-W bonds.Notably,the two-step 3 at%Ni-doped WO3 NPAs exhibits the highest PEC performance,compared with pure and one-step Ni-doped WO3 NPAs.At 1.0V(vs Ag/AgCl),the current density of two-step 3 at%Ni-doped WO3 is 0.80 mA/cm2,which is about 1.74-and 2.35-fold of that of WO3(0.46 mA/cm2)and one-step 3 at%Ni-doped WO3(0.34 mA/cm2),respectively.Ni doping is an effective strategy for changing the bandgap of WO3 and leads to a red shift in the edge of the action spectra.The changes of band gap and ECSA are playing important roles on improving the activity of the WO3photoanode.(2)Ultrasmall Mo S2 quantum dots(QDs)are exploited as surface sensitizers to boost the photoelectrochemical(PEC)properties of Mo S2/WO3.The Mo S2/WO3 photoanode is prepared by decorating 2D WO3 nanoplate arrays with ultra-small Mo S2 QDs via a facile and effective assembly method.The interspersing of Mo S2 QDs,not only broadens the photoabsorption region,but also provides more active sites for the surface reactions and enhances charge separation of the photogenerated electron-hole pairs,leading to a 2.13-fold enhancement in photocurrent density.Mo S2/WO3 photoanodes hold potentialities for the straightforward building of molecular level devices for PEC production.The heterojunction structure endows the photoelectrode with enhanced photoconversion efficiency and increased the density of charge carriers.Through the EIS and Mott-Schottky analyses,a conclusion can be drawn that this MoS2 QDs/WO3 film displays advantages of effective separation,restraining the recombination of photo-generated electron-hole pairs and efficient electron transport properties,so the PEC performance was enhanced.(3)The novel visible-light-driven WO3 nanorod arrays with oriented(200)facets were successfully synthesized via a facile solvothermal process using mixed ethanol and water as solvents.Subsequently,WS2 QDs derived from bulk WS2 powder via ultrasonication method were interspersed on WO3 nanorod arrays.The presence of suitable volume ratio of ethanol is a key factor to achieve high exposure of the WO3(200)facets.The morphologies,microstructures and optical properties of the prepared samples were systematically investigated.The results illustrated that the preferential exposure on the(200)facets of WO3nanorods as well as the efficient relative electrochemically active surface areas provided by WS2 QDs could facilitate the charge transfer between WS2 QDs and the(200)facets of WO3nanorods.(4)ZIF-8 and ZIF-67 are confirmed to work as cocatalysts for suitable band alignment,benefiting the charge transfer/separation of ZIFs/WO3 photoanodes.The absorbance edges of ZIF-8/WO3 and ZIF-67/WO3 hybrids are similar to that of WO3 without any obvious change.However,the ZIF-67 afforded absorption in the visible-light range.Hence,ZIF-67/WO3 has a significantly extended range in visible light.The band alignment is beneficial for the separation of photogenerated electron-hole pairs.The photogenerated holes were then transferred from WO3 to ZIFs,facilitating the oxidation reaction in the water.