The Optical Anisotropy and Photogenerated Carriers Recombination Mechanism in Ta3N5 Films

Author:Fu Gao

Supervisor:yu tao zuo shi cheng


Degree Year:2016





Nowadays,the large-scale application and exploitation of fossil energy have caused severe environmental problems and energy crisis.To develop renewable and environmental friendly energy sources is the only way to realize the sustainable development of human society.Solar energy is always widely accepted as the best renewable energy source,it distributes across the globe.Although the energy density is low,it is a widow’s cruse.Photoelectrochemical water splitting cells(PEC)can utilize the dispersed solar energy to produce restorable hydrogen,it has drawn much attention due to the advantage of low cost,environment-friendly preparation and easy operation.To realize its large scale application in the future,it had been reported that the solar to hydrogen(STH)efficiency must be larger than 10%.Semiconductors with proper band position to reduce H+ or oxidize H2O,ideal band gap to absorb enough sunlight to generate electrons and holes efficiently,play the key role in the PEC system.Among the various semiconductors,Ta3Ns is considered to be the most promising candidate to meet the requirements of the large-scale application of PEC.It owns a band gap of 2.1 eV,corresponding to a theoretical STH efficiency as high as 15.9%.The conduction and valence band of Ta3N5 cross the oxidation-reduction potential of water,in other words,it can split water into hydrogen and oxygen without applied potential theoretically.Over ten years has passed since Ta3N5 was firstly introduced into the PEC system,Ta3N5 photoanode still suffers from high onset potential and low STH efficiency.It is believed that the recombination of photogenerated carriers at the surface and bulk defects accounts for the low STH efficiency.It is an urgent step to understand the recombination mechanism of the photogenerated carriers in Ta3N5 and its optical properties.In this work,by using the laser induced photoluminescence(PL)spectrum,we have successfully researched recombination mechanism of the photogenerated carriers in Ta3N5,discovered that there were three kinds of radiative recombination mechanism of the photogenerated carriers in Ta3N5 We have firstly found the relationship of the optical anisotropy and proposed effective experimental methods to reduce the typical defects in Ta3N5 nanocrystals.The main research contents are as follows:Exploring the mechanism of photogenerated carriers recombination in Ta3Ns thin films.By comparing the temperature dependent laser induced PL spectra of Ta3N5 films prepared at different nitration temperatures,we discovered three PL peaks at 561,580 and 750 nm,corresponding to the recombination of oxygen-related impurity phase,band to band recombination along b and c-axis and the surface oxygen-related defects.By improving the nitration temperature or annealing in the hydrogen atmosphere at low temperature,the oxygen-related surface defects can be removed,but the oxygen impurity phase stood still due to its highly thermodynamics stability.Zr2ON2 crystal template inducing orientated growth of(023)face of Ta3N5 films.By controlling the Zr/Ta atom ratio and the hydrolysis rate of the organic precursor solution,we successfully prepared Ta3Ns films at different levels of dominated growth of(023)face.By analyzing the UV-visible,Raman,PL and X-ray diffraction spectra,we discovered that as the dominated growth along a-axis strengthened,the absorption band edge at 500 nm disappeared and the band to band recombination along b and c-axis became weak.As the dominated growth along a-axis suppressed the bulk recombiantion of photogenerated carriers along the b and c-axis,the bulk charge separation efficiency in the Ta3Ns photoanode improved,resulting with the photocurrent onset potential negatively shifted 0.4 V.Addition of Mg2+ enhancing the crystallization of Ta3N5film,Mg-Zr co-doped improved the photoelectrochemical property of Ta3Ns photoanode at low potential.The organic precursor solution contained a Mg/Ta atom ratio of 0.01 were spin-coated on fused silicon.After oxidation and nitridation,we got Ta3N5 films with or without Mg2+.Comparing the optical properties of the Mg-Ta3N5 films nitrated in wet and dry NH3 atmosphere,we discovered that,the surface MgO could not only increase crystallinity of Ta3N5,but also favor the formation of Ta3N5 phase in wet NH3 atmosphere.In addition,Mg-Zr co-doped Ta3N5 was much more easily realized than Zr or Mg doped due to its lower formation energy.Mg-Zr co-doping could improve the carriers density of the(023)preferential growth Ta3N5 photoanode,resulting with an improvement in the photocurrent at low potential.Fabricating TasNs photoanode with smooth surface by electron beam evaporation.By adjusting the evaporation rate and annealing temperature in the air,Ta3N5 on Pt/Ti/SiOx/SiO2/Si substrate with different particle sizes and porosity were prepared.At 1.23 V vs RHE,a photocurrent density of 5.7 mA/cm2 was prepared.In addition,by a simple dip-molding method,the smooth Ta3N5 photoanode with a 100 nm-thick Co(OH)x on the surface showed a photoelectrochemical stability of 9.5 h at 1.24 V vs RHE.