Construction And Performance Of Nanocrystals Photocatalysis Systems For Water Splitting And Carbon Dioxide Reduction

Photocatalysis for solar fuels,i.e.,utilizing renewable solar energy to drive water splitting or carbon dioxide（CO₂）reduction to produce storable chemical fuels such as hydrogen（H₂）,carbon monoxide（CO）,etc.The processes of photocatalysis for solar fuel involves several key steps including light absorption,carrier migration,and surface reactions,and the reaction paths for different solar fuels are not identical,which requires the efficient synergy between light absorbing materials,co-catalyst materials,even interface materials.The unique properties of colloidal nanocrystals,such as abundant active-sites,composition,size-tunability and solution processability,have led to the broad application prospects towards solar energy conversion.However,the coupling modes and mechanisms of colloidal nanocrystals applied in photocatalysis system for solar fuel are still unclear.Oriented by function,this dissertation aims to realize rational construction of photocatalysis system for solar fuel based on colloidal nanocrystals via organic-liquid-phase controlled synthesis and surface-interface engineering strategies,and regulate the transfer of carriers and surface reactions.Meanwhile,the related performance-improvement-mechanism was investigated.The specific research contents of this dissertation are listed as follows:To overcome the bottleneck of sluggish kinetics for water oxidation,monodisperse and ultrafine colloidal nickel（Ni）nanoparticles were prepared via an organic-liquid-phase synthesis method.Subsequently,the Ni nanoparticles were anchored on a bismuth vanadate（Bi VO₄）substrate through facile drop coating and annealing treatment.Under standard simulated sunlight irradiation,Ni/Bi VO₄ photoanode achieves a photocurrent density of 4.41 m A/cm² at 1.23 V vs.RHE,which is around 3.2 higher than that of the pristine Bi VO₄ photoanode.The photocurrent density of Ni/Bi VO₄ photoanode only exhibits a 4.7%decay within 10 h test at 1.23 V vs.RHE.Furthermore,the characterization results of the synchrotron radiation-based X-ray absorption spectroscopy（XAS）,Raman spectrum,etc.,reveal that an ultrathin amorphous Ni OOH layer is in-situ generated on the Ni surface after the activation process,forming a Ni@Ni OOH co-catalysts.Finite element current density distribution simulation reveals that Ni core has the current collector effect.The Ni@Ni OOH co-catalyst can perfectly couple the high electrical conductivity of metallic Ni and high oxygen evolution activity of Ni OOH,contributing to the enhanced carrier transport efficiency,water oxidation dynamics and stability of Bi VO₄ photoanode.To improve the hydrogen evolution performance of Ni,a strategy of high-temperature-metal-organic phosphidation was used to realize the conversion of colloidal Ni nanoparticles to Ni₂P nanocrystals.Subsequently,Ni₂P nanocrystals were deposited on surface of TiO₂/Si photocathode.Under standard simulated sunlight irradiation,the resulting Ni₂P/TiO₂/Si photocathode demonstrates a remarkable photocurrent density as high as-11.00 m A/cm² at 0 V vs.RHE,which is much higher than that of TiO₂/Si and Si photocathode.This impressive photoelectrochemical（PEC）performance can be attributed to the key function of the Ni₂P nanocrystals,which provide abundant hydrogen evolution active-sites.To further verify the hydrogen evolution activity of Ni₂P nanocrystals,an organic liquid co-heating solution was employed to in-situ embed Ni₂P nanocrystals on g-C₃N₄ nanosheets.A Ni（δ⁺）-N（δ^-）chemical coupling in Ni₂P/g-C₃N₄hybrid composite is confirmed by synchrotron radiation-based XAS.Under light irradiation,Ni₂P nanocrystals are able to extract photo-generated electrons from g-C₃N₄through Ni（δ⁺）-N（δ^-）coupling and accelerate hydrogen evolution reaction.Ni₂P/g-C₃N₄photocatalyst achieves a hydrogen production rate as high as 2.85 mmol g^-1 h^-1,which is6.7 folds higher than that of pristine g-C₃N₄.To achieve a stable and highly selective PEC reduction of CO₂,the colloidal Zn Te semiconductor nanocrystals and Ag-Cu bimetallic colloidal nanocrystals were prepared by thermal injection and Galvanic displacement methods,respectively.The Ag-Cu/TiO₂/Zn Te photocathode with metal-insulator-semiconductor（MIS）structure was fabricated by the atomic layer deposition technique.Owing to the unique MIS structure,fast interface charge-carriers migration,high catalytic selectivity,and excellent cell stability are integrated together.For Ag-Cu/TiO₂/Zn Te MIS photocathode,an extraordinary and highly stable photocurrent density of-5.10 m A/cm² was achieved at-0.20 V vs.RHE for syngas-producing with a ratio of CO as high as 87%under standard simulated sunlight irradiation.The band energy analysis and transient absorption（TA）spectroscopy reveal the efficient transport mechanism of carriers in Ag-Cu/TiO₂/Zn Te photocathode.In addition,density functional theory（DFT）calculations reveal that the Ag-Cu bimetallic nanocrystals co-catalysts areable to significantly lower the energy barrier in the step of key COOH intermediate formation for CO₂ reduction to CO,thus improving the catalytic selectivity of CO₂ reduction.