Photochemical/photoeletrochemical Behaviors Of Several Typical N-type Semiconductors And The Electrochemical Performance Of Their Composite Films

With the growth of world population and development of economy,it is extremely urgent to find clean and sustainable energy to satisfy the growing global demand for energy.As the main energy source delivered from the extraterrestrial space,solar energy attracts great interests from researchers.They proposed different kinds of utilization strategies,including solar energy-heat energy conversion,solar energy-electricity conversion and solar energy-chemical conversion paths.Among them,solar energy-chemical conversion path promises solar light conversion and storage simultaneously,which includes natural photosynthesis and artificial photocatalytic/photoelectrocatalytic reactions.Herein,we select two typical photocatalytic/photoelectrocatalytic reactions.One is photodeposition method based on n-type semiconductor since it is a green and conventional preparation method for material driven by solar energy and it is proposed for the application in electrochemical system.The other one is photocatalytic/photoelectrocatalvtic water oxidation,since it is an important half reaction,which can be coupled with reductive reactions such as water reduction.CO2 reduction.and N2 reduction.But photocatalytic/photoelectrocatalytic water oxidation suffers from poor kinetics.The deep investigation of photocatalytic/photoelectrocatalytic water oxidation is of significant importance.The details are as follows.Hydrogenated TiO2 nanotube arrays with enhanced electronic conductivity and visible light absorbance are used as photoactive substrates.TiO2/Mn3O4 composite films are prepared by photodeposition process.The photodeposition mechanisms of Mn3O4 on TiO2 are investigated.Physical characterizations including SEM,TEM and XPS indicated Mn3O4 nanorods photodeposited on TiO2 nanotubes uniformly from bottom to top.The as-formed 3D structures exhibit more electrochemical active sites and larger contact area between electrode and electrolyte.Electrochemical tests demonstrate excellent supercapacitor performance of as-prepared composite materials with specific capacity of 508 F g-1 at current density of 0.7 A g-1 and 228 F g-1 at current density of 35.7 A g-1.exhibiting outstanding rate performance.No obvious capacity decay is observed after 10000 cycles of charge-discharge test at current density of 3.6 A g-1,demonstrating good long-term charge-discharge stability.Oxygen-defective Fe2O3/CoOx nanorod array composite films are prepared by a facile and environmentally friendly photodeposition method using Fe2O3 as photoactive subtrate followed by annealing in Ar/H2 atmosphere.Compared with the original Fe2O3/CoOx sample,the optimized oxygen-defective Fe2O3/CoOx composite film electrode annealed at 250 ? has a 120 mV negative shifting of overpotential at current density of 10 mA cm-2 and lower Tafel slope.Moreover,the optimized oxygen-defective Fe2O3/CoOx composite film electrode exhibits significant OER stability for 16 h.These may be attributed to numerous active sites on oxygen-defective CoOx and its weak crystallinity that promise excellent activity as well as stability.Furthermore,the amount of oxygen vacancies in CoOx can be finely tuned by changing annealing temperature.Based on XPS spectra and electrochemical analysis results,we show the relationship between the electronic states of Co and its electrocatalytic OER activity.The results may find broad applications in designing highly efficient catalysts for electrochemical process.TiO2 nantube arrays are used as photoactive substrates for photodeposition of CoOx.SEM images demonstrate CoOx nanoparticles grew on TiO2 nanotube surface uniformly from bottom to top.The as-prepared TiO2/CoOx composite films are treated with in-situ electrochemical reduction during electrocatalytic hydrogen evolution reaction(HER)tests.The electrocatalytic HER test results show that the HER activity of TiO2/CoOx composite films are enhanced through in-situ electrochemical reduction.XRD patterns and Raman spectra indicate no obvious phase change of composite films after in-situ electrochemical reduction.XPS spectra suggested Ti4+ species are reduced to Ti3+ species(oxygen vacancies),which enhance the electronic conductivity of TiO2 resulting in better HER performance.In the meanwhile,CoOx doesn't show obvious electronic structure change,exhibiting excellent stability.On the other hand,water oxidation reaction mechanisms are investigated using BiVO4 photoanode as a research platform.A series of experiments are designed to probe the dual-path involving the protonation and deprotonation reaction of M-OOH intermediate.The photoelectrochemical behaviors of BiVO4 are investigated in electrolytes with different pH values.It is revealed that water oxidation tends to form H2O2 through protonation of M-OOH intermediate when the surface proton is abundant.The kinetic isotope effect results show that faradaic efficiency of O2 in water oxidation decreases,demonstrating harder deprotonation of M-OOD after H was replaced by D.And this result further indicates the rate-limiting step might involve proton transfer process,suggesting the importance of deprotonation of M-OOH in water oxidation process.Similar results are observed on platform of WO3 photoanode,demonstrating that this mechanism is not specific to BiVO4 system.