Microstructure Construction and Photoelectrochemical Water Splitting Investigation of Ta3N5 Photoanode

Author:Shi Zhan

Supervisor:zou zhi gang zuo shi cheng


Degree Year:2019





Solar energy can be converted into clean,storable hydrogen energy directly in photoelectrochemical cell(PEC cell).A PEC cell consists of a cathode,an anode,external circuit and electrolyte.One or both of the electrodes are made of semiconductor materials that are able to absorb sunlight to drive the water splitting reaction.PEC water splitting mainly contains three steps:(1)the semiconductor electrode absorbs photons,of which the energy is no less than the band gap,to generate electron-hole pairs;(2)the electron-hole pairs get separated by the built-in electric field within the semiconductor-electrolyte junction and transf’er to cathode-electrolyte interface(electrons)or anode-electrolyte interface(holes)respectively;(3)the photo-generated electrons or holes participate in water reduction or oxidation reactions Therefore,the overall solar-to-hydrogen efficiency depends on the light absorption efficiency,the charge separation efficiency and the charge carrier injection efficiency.Particularly,the water oxidation reaction is a four-electron transfer reaction with slow kinetics,so that the photoanode is currently the bottleneck of PEC systemTa3N;is a promising photoanode material.The band gap of Ta3N5 is about 2.1 eV,which corresponds to light absorption up to 590 nm and a maximum solar-to-hydrogen efficiency of 15.9%.Its conduction band and the valence band straddles the water reduction potential and oxidation potential.which in theory enables Ta3N5 photoanode to accomplish water splitting reaction without external bias.However,the efficiency of Ta3N5 photoanode is restricted by the large carrier effective mass(which is 2.70 m0/3.56 m0,0.21 m0/0.66 m0,0.85 m0/0.26 m0 for electron/hole along kx,ky and kz axis,or a,b,c axis),the short lifetime(<10 ps),and the low mobility(1.3-4.4 cm-2V-1 s-1).In order to solve the charge carrier transport problem of Ta3N5,it is beneficial to construct nanostructures to enlarge the semiconductor-electrolyte interface,as well as the volume ratio of space charge layer.On this basis,taking advantage of the anisotropy of carrier transport properties by controlling the exposing facets will further enhance the charge separation efficiency.In this dissertation,the details of single-crystalline Ta3N5 nanostructure growth via flux synthetic method are investigated.The relationship between the PEC performance and the facet exposure of Ta3N5 photoanode is revealed.The research contents are as follow:(1)Fabrication of single-crystalline Ta3N5 nanorod array photoanode and its PEC performance.Nanostructure construction can significantly improve the saturated photocurrent of photoanodes,especially for those of poor charge transport properties.Single-crystalline Ta3N5 nanorod array film is fabricated on Ta substrate via flux synthetic method with KI flux.The nanorod with a rectangle cross section grows along[100]orientation,the lateral facets of which are(010)and(001)facets.The nanorod single crystal grows with bottom-up mechanism.The absorption of K+ and I-on(010)and(001)facets slows the growth rate along[010]and[001]orientation,which finally leads to the nanorod shape.Compared with the poly-crystalline planar film,fabricationof single-crystalline nanorod array enlarges the volume ratio of space charge layer and shortens the diffusion path of photo-generated hole,as well as reduces the concentration of grain boundary and defect.These factors bring enhanced charge separation efficiency and the overall PEC performance.(2)Fabrication of(001)facets preferentially exposed Ta3N5 nanoflake array photoanode and its PEC performance.Preferentially exposing hole-accumulation facets or high-activity facets is an effective method to accelerate PEC water splitting.(001)facet preferentially exposed Ta3N5 nanoflake array photoanode is fabricated via flux synthetic method using RbCl as flux.Using semi-closed reactor obviously slows the draining rate of the flux,so that the growth rate of Ta3N5 crystal can be controlled.The adsorption of Rb+ on(001)facet slows the growth rate in[001]direction,leading to the exposure of(001)facet.Photo-assisted deposition and photo-assisted electro-deposition of tracing metal oxides demonstrate the(001)facet is favorable for hole accumulation.Though theoretical calculation,the reason for the spacial separation of photo-generated holes may be the high surface potential of(001)facet and the small hole effective mass along[001]orientation.The preferential exposure of(001)facet further enhances the charge separation efficiency compared to the nanorod array photoanode.Low onset potential of 0.6 VRHE and high photocurrent of 5.6 mA cm-2 at 1.23 VRHE are obtained.(3)Fabrication wedge-shaped ultrathin Ta3N5 flake and its PEC performance.The ultrathin materials have many merits for PEC water splitting such as ultra-short carrier migration path and abundant surface reaction sites.Thinning Ta3N5 is a potential approach to enhance the PEC performance.Single-crystalline wedge-shaped untrathin Ta3N5 flake is fabricated with alkali metal halide-carbonate mixed flux.The preferential exposure facets can be controlled by changing the alkal metal ions.When using Csl-Cs2CO3 flux,for the first time(010)facet becomes the preferential exposure facets.The adsorption of Cs+ on the(010)facet slows the growth rate along[010]orientation,which leads to the exposure of(010)facet.The solubility of Ta5+ in molten Csl flux is smaller than Ta(5-x)+,so that the use of carbonate in the flux to create oxidative environment is pivot due to its low acitivity than other oxygen sources.The valence state of tantalum ions are lifted from 0 to +5 slowly,maintaining a low supersaturation of Ta5+ in the molten flux,making the growth rate of step growth parallel to(010)facet much larger than the growth rate of 2D nucleus perpendicular to the(010)facet.