Improving The Charge Separation Efficiency Of Ta₃N₅ Photoanodes By Ation Doping And Heterostructures

Photoelectrochemical(PEC)water splitting is a promising technology for converting solar energy into hydrogen energy.However,the solar-to-hydrogen(STH)conversion efficiency of PEC cell is greatly limited by the low efficiency of photoanode because of the slow surface water oxidation kinetics.Therefore,the development of cost-effective photoanode candidate towards the large-scale application of PEC water splitting is of particular important.Among various candidates investigated,tantalum nitride(Ta3Ns),stands out for its favorable optical absorption at 600 nm,theoretical maximum STH of 15.9%,and suitable band edge alignments,has been identified as attractive semiconductor photoanode.However,one of the major limitations of Ta3N5 photoanode is its relatively positive photocurrent onset potentials at around 0.6-1.1 VRHE,which is far from the flat band potential of Ta3N5(ca.0 VRHE).Such positive onset potential necessitates the application of external bias to reach the desired photocurrent density,thus lowering the STH conversion efficiency.The origin of such positive onset potential of Ta3N5-based photoanodes generally attributes to the fast carrier recombination(<10 ps),resulting from its poor electrical mobility,typically<4 cm2 V-1 s-1.Therefore,an effective strategy for improving the charge separation of Ta3Ns and developing Ta3N5 photoanodes that can split water with low(or even without)externally-applied bias,is extremely desirable.Based on the viewpoint of photo-induced charges strengthening separation,strategies related to morphology tailoring,orientation controlling,element doping,cocatalyst loading and heterojunction construction were used to modifying Ta3N5 photoanode in this dissertation,with an aim at effective separation of photo-induced charges and reducing photocurrent onset potential of Ta3N5 photoanode for water oxidation,thus improving the STH conversion efficiency.The main contents and conclusions of the study are as follows:Oriented growth of Sc3+-doped Ta3N5 nanorod achieving low onset potential for photoelectrochemical water splitting.Morphological and electronic structure dual modulation of Sc3+ doped Ta3N5 nanorod is developed by flux-assisted oriented crystal growth route to significantly minimize the drawbacks of Ta3N5 photoanode,including:(1)Crystallographic-oriented growth minimizes the negative effect of electronic structure anisotropy of Ta3N5;(2)Single-crystalline Ta3N5 nanorods can simultaneously reduce crystal defects and expose more active surface area;(3)Replacing Ta5+ by Sc3+ causes the concurrent substitution of O2-for N3-,thus tuning the energy level alignment to effectively boost the photovoltage.As a result,the optimized Ta3N5 photoanode modified with Co(OH)x co-catalyst achieves a record photocurrent onset potential of 0.4 VRHE,which is about 400 mV lower than the conventional Ta3N5 particles photoanode,and thereby affording a maximum HC-STH conversion efficiency of 0.82%.Mg2+-doped Ta3N5 nanorod coated with a conformal CoOOH layer for efficient photoelectrochemical water splitting.Mg2+ doped Ta3N5 nanorods photoanodes are designed and fabricated to overcome charge transport limitations in bulk Ta3N5 for PEC water splitting.Benefiting from the significantly enhanced carrier density and electrical conductivity by the addition of Mg2+,the optimized Mg2+ doped Ta3N5 photoanodes exhibit a considerable high PEC activity with an AM 1.5G photocurrent of 1.5 mA cm-2 at 1.23 VRHE.Moreover,based on the Mg2+ doped Ta3N5 nanorods,we concentrate on tuning the kinetics of holes transfer with a active CoOOH cocatalysts by a feasible electrodeposited method.Eventually,the optimal photoanode presents an AM 1.5G photocurrent up to 9.5 mA cm-2 at 1.6 VRHE,accompanied by a value of 6.5 mA cm-2 at 1.23 VRHE and a low onset potential of 0.6 VRHE,as well as the photoanode maintains about 70%of the initial photocurrent value after 70 min irradiation at 1.0 VRHE.Nanostructured TaON/Ta3N5 as type-? heterojunction photoanode with improved charge separation efficiency.Heterostructured TaON/Ta3N5 photoanodes were designed and fabricated for the first time.We found that the TaON could be served as the partner of the heterojunction electrode for enhancing the charge separation of Ta3N5.By optimizing the TaON thickness,the optimized TaON/Ta3N5 heterojunction photoanode,without any co-catalysts,shows 350 mV negative shift of photocurrent onset potential to 0.65 VRHE compared to that of the Ta3N5 photoanode.The design and fabrication scheme can be readily extended to other(oxy)nitrides semiconductors for heterojunction construction.