Silicon Photoanode Modified By Ni-Fe-based Particles With Core-shell Structure For Water Oxidation

Facing the severe energy and environmental crisis increasingly,renewable energy becomes favored worldwide gradually.Converting abundant solar energy to hydrogen energy with high calorific value via photoelectrochemical（PEC）water-splitting cells,is an effective way to mitigate energy and environmental crisis.The main factors limiting the energy conversion efficiency of photoelectrochemical reactions are the low charge separation and injection efficiency resulting from the recombination of photogenerated charges at the interface between the photoelectrode and electrolyte.In order to improve the charge separation and injection efficiency of the photoelectrode,modifying the surface of the photoelectrode,such as constructing a metal-insulator-semiconductor（MIS）functional layer,depositing cocatalysts and so on,is used widely.However,surface modification with thick continuous film will affect the light absorption of the photoelectrode and reduce the photoelectric conversion efficiency.The deposition of core-shell structured nanoparticle cocatalysts is an effective method to balance the contradiction between light absorption loss and charge separation/injection efficiency improvement.On the one hand,island-distribution of nanoparticles can reduce the light absorption loss resulting from surface modification effectively;on the other hand,charge separation and injection efficiency can be improved via depositing core-shell structures formed by materials with different work functions to construct a gradient-enhanced junction electric field.The purpose of this thesis is to prepare an efficient and stable silicon-based photoanode.In order to improve the photoelectrochemical performance of n-type silicon（n-Si）photoanodes,following methods are used:1.an MIS structure between n-Si and core-shell structured nanoparticles with island-distribution.2.a gradient-enhanced junction electric field generated at the interface between electrode and electrolyte.3.the‘pinch-off’effect of the electric field between particles.These methods increase the photogenerated current density,reduce the onset potential and improve the stability of n-Si photoanodes efficiently.Meanwhile,the main factors affecting the PEC water-splitting performance are also discussed.The main research contents of this article are as followed:（1）Core-shell structured Ni@Fe nanoparticles modifying n-type silicon photoanodes（n-Si/Si O_x/Ni@Fe）by electrochemical deposition reduce light absorption loss,increase photogenerated voltage and improve charge separation efficiency.In this thesis,n-type silicon photoanodes modified by crystalline Ni@Fe nanoparticles with core-shell structure（n-Si/Si O_x/Ni@Fe）were prepared by electrochemical deposition.Compared with continuous Ni films,Ni@Fe nanoparticles with island-distribution reduce light absorption loss effectively.In the core-shell structure of Ni@Fe nanoparticles,the Ni core represents a good photogenerated hole collector and forms a MIS structure with n-Si,which increases the band bending in the depletion layer of semiconductors and improves the electrical conductivity of the whole device;a Fe OOH layer with a higher work function（6.3 e V）than Ni（5.0 e V）would be formed on the surface of Fe shells via oxidation during the PEC reaction,which forms a high-efficiency photoanode（n-Si/Si O_x/Ni@Fe@Fe OOH）with a work function gradient-enhanced junction electric field,further increasing the band bending in the depletion layer of semiconductors and improving the separation efficiency of photogenerated charges and photogenerated voltage.The core-shell structured nanoparticles with island-distribution reduce the light absorption loss,meanwhile,improve the separation efficiency of photogenerated charges and the efficiency of charge extraction and injection.（2）Core-shell structured Ni@NiFe layered double hydroxide（LDH）nanoparticles modifying n-type silicon photoanodes（n-Si/Si O_x/Ni@Ni_xFe_yOOH）by electrochemical deposition increase the photogenerated voltage and decrease the onset potential.Ni@Fe nanoparticles with island-distribution can reduce the light absorption loss caused by modified layers and improve the charge separation efficiency.However,the Fe OOH,which acts as a high work function layer on the surface of the Ni@Fe nanoparticles,is growth on the surface of Fe shell in situ spontaneously during OER.It limits the composition of the high work function layer and the field strength of the gradient-enhanced junction electric field.In order to further increase the photogenerated voltage and decrease the onset potential,n-type silicon photoanodes modified by Ni@NiFe LDH nanoparticles with core-shell structure（n-Si/Si O_x/Ni@Ni_xFe_yOOH）were prepared by electrochemical deposition.Compared with Ni@Fe nanoparticles,the work function of the interface layer could be adjusted by the Ni-Fe ratio of NiFe LDH for Ni@NiFe LDH nanoparticles.It was shown by photoelectrochemical measurement that the photoelectrochemical performance of photoelectrodes varies with the Ni-Fe ratio of NiFe LDH.When the molar ratio of Ni-Fe is 3:7,the photoelectrode showed a lower onset potential of 1.0 V_RHE,and the photocurrent density reached 37.5 m A cm^-2 at 1.48 V_RHE.This result indicates that composition regulation is an effective method to change the efficiency of photogenerated charge separation and injection at the interface between photoanode and electrolyte.