Facet-Dependent Heterostuctures with Interfacial Modification for Photocatalytic Water Splitting

Author:Wei Ting Cha

Supervisor:liu li min


Degree Year:2019





Solar energy has received extensive attention due to its abundant exploitable resource and environmentally friendliness.Hydrogen is an ideal energy fuel due to its high energy density,storage,transportation,and pollution-free when being used.Solar-to-hydrogen conversion thought photocatalytitc water splitting is one of the most effective solutions to current energy and environmental problems.However,photocatalytic technology still faces enormous challenges in industrial applications.The most important problem is the low photocatalytic efficiency caused by photo generated carrier recombination.With this problem,how to improve photocatalytic efficiency is a hot topic in current material research.To construction of semiconductor heterostructure is regarded as an effective method to achieve photoelectron and hole separation.Nowadays,heterostructures formed by different material have been developed.However,the transfer process of charge at the interface of heterostructure is not only affected by the semiconductor Fermi level,but also closely related to the exposed crystal facets,defect structure and so on.How to regulate the interface charge behavior and obtain effective improvement of photocatalytic performance has become the focus of future research.In this paper,by constructing a crystal heterostructure,the effects of defect modulation,multi-field synergy,and cocatalyst loading on photo-generated charge separation were studied.And the application of the photocatalysts for photocatalytic hydrogen production was explored.The thesis mainly comprises the following three parts:(1)Multi-electric field synergistic photocatalyst based on defect modulated ZnO/BiVO4 crystal face heterostructure for photocatalytic oxygen evolution.The difference of energy levels between crystal facets of semiconductors provides favorable conditions for constructing the electric field.How to adjust the separation of charge by the enhanced electric field through the electronic structure of the heterostructure is still an important problem to be solved.ZnO/BiVO4 heterostructure was constructed by loading ZnO nanorod array on the surface of facets BiVO4.Multi-electric field modulation strategy contribute to effective separation of charge.Studies have shown that BiVO4 exhibit spontaneous charge separation under photoexcitation due to the crystal facets effect.Photogenerated electrons and holes are sperated on the {010} and {110} crystal facets of BiVO4,respectively.The polarity of the ZnO nanorods makes the charge transfer anisotropic.Electrons transported rapidly along the c-axis allows the photogenerated electrons and holes to be effectively separated.The defects have an important influence on the interface transfer process of charge.After oxygen vacancies defect modulated in the heterostructures,the Z-scheme mechanism is exhibited between the BiVO4 {101} facet and the supported ZnO defect level.Under the synergistic effect of Z-scheme charge transfer process and the photocatalytic built-in electric field,photocatalytic performance to oxidize water under visible light is increased by an order of magnitude,and the oxygen production rate of 68 μmol h-1 is achieved.The quantum yield measured is 5.0%at 450 nm,which is an order of magnitude higher than that of BiVO4.The photocatalytic organic decomposition has also been greatly improved.(2)Defect modulated Cu2O/TiO2 heterostructure for photocatalytic water splitting.Cu2O is regarded as a promising photocatalyst with great application prosepects due to its high visible light activity.However,the stability problem caused by photocorrosion is still the main problem that restricts its application.How to construct a highly efficient and stable copper-based composite photocatalyst through heterostructure design and electronic structure regulation is still a new field to be developed.Therefore,TiO2 photocatalysts with {001} and{101} facets were selected as substrate.Cu2O is loaded onto the two crystal facets of TiO2 to construct heterostructure.The heterostructure with oxygen vacacies is obtain to improve charge separation efficiency for photochemical stability.The research shows that the defect engineering and crystal facets synergistic strategy can make the Z-scheme between photogenerated electrons of {101} facet TiO2 and holes of Cu2O.Z-scheme system can effectively avoid the photooxidation of Cu2O to form CuO.The hydrogen production rate reached 32.6 mmol h-1 g-1 in the full-spectrum with the stability up to 32 hours.This indicates that the synergistic effect of facet engineering and defect molulation is an effective method for developing Z-scheme photocatalysts with high performance.(3)Single atom cocatalysts on faceted TiO2 photocatalyst for photocatalytic hydrogen production.The construction of Schottky junctions formed between noble metal and semiconductor is an effective method to enhance the separation of photogenerated charges.The single atom cocatalyst loading not only provides more active sites for the photocatalytic reaction,but also provides a new way to reduce the photocatalyst cost.Therefore,single atom Au and Pt cocatalysts loading on the {001} and {101} facets TiO2 surface was investigated.The effect of the synergy between the single atom cocatalyst and the TiO2 crystal facet in the photocatalytic water reduction was analyzed.Results show that the noble metal can effectively capture photogenerated electrons,thereby suppressing the recombination of charge carriers and effectively improving the photocatalytic efficiency.Combining the advantages of crystal face and single atom cocatalyst,Pt loading on {001} facet TiO2 shows high photocatalytic activity with hydrogen production rate of 21.9 mmol h-1 g-1.This study provides a new idea for the application of single atom assisted TiO2-based photocatalysts in photocatalysis.