First-principles Calculations On The Effect Of Doping And Interface On The Electronic Structure And Band Edge Potentials Of Ta₃N₅

Photoelectrochemical water splitting is an effective and promising solution towards the energy crisis and environment pollution.The poor durability and efficiency of photoanode restrict the further development of photoelectrochemical water splitting.Ta3N5 is a promising candidate for the commercial photoelectrode.Though the superior property of Ta3N5,there are still two problems which restrains the further development of Ta3N5.Firstly,the durability of Ta3N5 is poor and the photocurrent decays seriously in a short time.Secondly,the onset potential performance is far from the theoretical prediction.The high onset potential and the poor photocurrent in low bias limit the energy transfer efficiency.To enhance the performance of Ta3N5 photoanode,many strategies have been developed,for instance,doping,constructing nanostructure,surface engineering,et al.It is convinced that doping proper transition metal atoms into Ta3N5 is an effective strategy to boost the performance of Ta3N5.In our laboratory,Ti-doped Ta3N5 shows an improved photocurrent and onset potential performance,therefore,in this thesis,we explains the role of doping Ti in Ta3N5 in the first work.To understand the universality in transition metal doped Ta3N5,Sc?Ti?V?Zr and Hf doped Ta3N5 systems have been calculated.Inspired by previous work related to interface science,and combining the fact that Ta3N5/water interface has not been studied sufficiently,we have studied the effect of the interface of Ta3N5/water theoretically.The details of the calculations on the doping and interface of Ta3N5 is shown as followsFirst-principles calculations on the transition metal doped Ta3N5 systems.The experiments prove that with Ti doping,the onset potential of Ta3N5 shift negatively.To understand the effect of the concentration of Ti,we performed the calculations of different doping concentration in a Ta3N5 supercell by using HSE06 method.According to the previous work,oxygen impurity is difficult to remove because of the low formation energy.From the viewpoint of experiments,the Ta3N5 is synthesized by nitriding its metal oxide precursor,resulting in the difficulty in removal of oxygen impurity in as prepared Ta3N5,Therefore,oxygen impurity should be taken into consideration when calculating Ti-doped Ta3N5 system.The calculating results demonstrate that Ti-O codoped system is the most stable configuration with the lowest formation energy among all the doped configurations,indicating that oxygen impurity will stabilize the Ti doped Ta3N5.Both Ti and O have little influence on the bandgap of Ta3N5.However,with the oxygen content increasing,the band edge potential of Ta3N5 versus vacuum potential shift negatively while the band edge potential of Ti doped Ta3N5 shift positively.In Ti-O codoped Ta3N5,the band edge potential shift negatively with the increasing O concentration while the Ti has the opposite effect.The flat band potential of Ta3N5 determines the theoretical onset potential of Ta3N5 photoanode,hence,the negative shift of band edge potential is detrimental to the onset potential performance.That is to say,the oxygen impurity is not profitable to the onset potential of Ta3N5 while doping Ti can compensate the unfavorable effect of oxygen impurity.To further understand the doping effect of transition metal,Sc?Ti?V?Zr and Hf doped Ta3N5 systems were calculated The results indicate that the farther to the left in the periodic table and the more doping concentrations,the more positive shifts of band edge potentials versus vacuum level.This indicates that Sc doping is the optimized option to compensate the detrimental effect of oxygen impurity on the onset potential.However,the formation energy implies the instability of Sc-doped system.Ti?Zr and Hf doped Ta3N5 show the almost same effect on the shift tendency of the band edge potentialsFirst-principles molecular dynamics(FPMD)simulations were performed to study the behaviors of electronic structure and band edge potentials of Ta3N5/water interface.In the Ta3N5/water interface model,the charge density difference and bader charge analysis suggest that the electrons transfer from the surface layer of Ta3N5 to the adsorbed water and surface dipole are formed,which results in the band edge potential of Ta3N5 shifts negatively relative to vacuum level.This demonstrates that the adsorbed water is harmful to the onset potential performance of Ta3N5 photoanode.With the ratio of dissociated water at Ta3N5/water interface increasing,more electrons transfer from the surface layer of Ta3N5 to the adsorbed water and a stronger surface dipole are formed,resulting in a further negative shift of band edge potential of Ta3N5.Combining recent experiments and our DFT calculations,we suggest that surface midification is a promising way to enhance the ABPE(Applied Bias Photon-to-Current Efficiency)efficiency of Ta3N5.This work focus on the effect of transition metal doping and Ta3N5/water interface on the electronic structure and band edge potential of Ta3N5.The results explain the high onset potential observed in experiments.Combining our calculations and experiments,we propose promising strategies towards the high performance of Ta3N5.