Water Splitting On Ta₃N₅ Based Nanoarray Heterojunction Photoanode

Photoelectrochemical（PEC）water splitting has become one of the most promising solar energy conversion technologies because it can convert solar energy into hydrogen energy with single product and easy storage.However,the PEC water splitting technology still faces many challenges,such as slow reaction kinetics and high overpotential,which seriously restrict the energy conversion efficiency.Therefore,the preparation of an efficient,stable and low-cost photoanode material is the basic premise towards to afford the large-scale application of PEC water splitting.Ta3N5 is a promising candidate photoanode material with a band gap of approximately 2.1 e V,whose theoretical maximum photocurrent and theoretical solar conversion efficiency density under AM 1.5G sunlight can reach 12.8 m A·cm-2 and15.9%,respectively.However,the releasing of Ta3N5 PEC performance has been seriously hindered on account of its low carrier mobility（1.3⁴.4 cm2·V-1·s-1）,fast carrier recombination rate（<10 ps）,and significant carrier effective mass anisotropy.In recent years,aiming to improve the PEC performance and achieve low-bias water splitting,thereby the ion doping,structuring heterojunctions,depositing functional layers or cocatalysts as effective approachs are considered to improve charge separation efficiency and injection efficiency.Among them,structuring heterojunctions and depositing functional layers are facing challenges due to the band structure mismatching and the poor interface contact quality for the two materials,as well as the easy oxidation for Ta3N5,which greatly limits the choice of constituent materials and preparation methods.In this paper,aiming to improve the charge separation and transport of electrodeelectrolyte interface for the Ta3N5 nanorod array photoanode,the Ta3N5/Sr Ta O2 N nanorod array photoanode with continuous solid-consolidation and solid-liquid junction structure is designed,and the atomic layer deposition（ALD）technology is used to deposit Al-doped Zn O（AZO）-Ti O2 functional layer on Ta3N5-Cu2 O nanorod array photoanode.The separation and transportation of photogenerated electron-hole pairs in Ta3N5 are promoted via the construction of heterojunction or the deposition of oxide layer and cocatalyst,so as to achieve reducing the onset potential and increasing the photocurrent density.The specific research content of this paper are as follows:（1）Oriented-growth Ta3N5/Sr TaO2N array heterojunction with extended depletion region for improved charge separation efficiency.In this paper,the Ta3N5 and Ta3N5/Sr Ta O2 N photoanodes were prepared by the molten salt high-temperature nitridation method and re-sintering.The thickness of the Sr Ta O2 N shell layer can be adjusted by changing the amount of the precursor Sr CO3,so as to explore the effect of different thicknesses of Sr Ta O2 N layers on the PEC performance of Ta3N5 photoanodes.TEM,XPS and M-S tests confirmed that Sr Ta O2 N was successfully grown on the surface of Ta3N5 nanorods and there was a strong interaction between them,accompanied by the formation of type-II heterostructure.It was found that the Ta3N5/Sr Ta O2 N photoanode with continuous Ta3N5-Sr Ta O2N（solid-solid）junction and Sr Ta O2N-electrolyte（solid-liquid）junction can make the photogenerated carriers located in the extended depletion layer as much as possible to maximize light absorption and promote the charge separation.Simultaneously,the diffusion distance of the holes to the electrolyte are shortened,thereby reducing the charge recombination in the bulk phase and making a greater potential gradient between the photoanode and the electrolyte.Consequently,more photogenerated holes can arrive the surface of the photoanode to participate water oxidation reaction.From these reasons,the Ta3N5/Sr Ta O2 N photoanode shows a more excellent PEC performance.（2）ALD-grown oxide protective layers on Ta3N5-Cu2 O n-p nanoarray heterojunction for improved photoelectrochemical water splitting.In order to further improve the stability and photocurrent density of the heterojunction photoanode,ALD technology is used to deposit Al-doped Zn O（AZO）-Ti O2 oxide functional layers on the Ta3N5-Cu2 O photoanode.Among them,the AZO layer can isolate the direct contact between the photoanode and the electrolyte,prevent the penetration of the electrolyte in the photoanode and passivate the surface state of the photoanode to increase the concentration of photogenerated carriers.The Ti O2 layer can effectively extract the charge in the AZO layer and improve the energy at the electrode-electrolyte interface,resulting in greater band bending.Finally,Co OOH is selected as a cocatalyst to modify the Ta3N5-Cu2O/AZO-Ti O2 photoanode.The stability of the photoanode is greatly improved under the synergistic effect of the cocatalyst and the oxide functional layers,achieving a low onset potential of 0.40 VRHE,and the photocurrent density of 4.61 mA·cm-2 at 1.23 V_RHE.