| Metal-Organic Frameworks(MOFs)are hybrid maternals that combine both organic and inorganic functional components.Over the last decades,MOFs are emerging as a new type of promising photocatalysts with semiconductor like attributes and another kind of important new porous materials besides zeolite and carbon nanotubes.It has been widely used in catalysis,sensing,energy storage and gas adsorption and separation.The large specific surface area,porous and unsaturated metal sites and the diversity of structure and function make MOFs materials be of great research potential.The shortage of energy and environmental pollution are two major crises in our society.In order to solve the two problems,photocatalytic technology emerges at the historic moment.Since hydrogen is a clean fuel with unlimited supply and easy transportation,it is of great significance to study how to obtain hydrogen from aqueous solutions.Because of being able to use light energy to decompose water into hydrogen and oxygen,photocatalytic water splitting technology happens to be developing rapidly.Besides,hydrogen energy is clean and able to solve the problem of energy shortage.Therefore,photocatalytic water splitting has been a research focus of photocatalytic technology.However,researchers generally focused on the photocatalytic hydrogen production or oxygen production half reaction with the presence of sacrifice agents.Thus,how to realize photocatalytic overall water splitting is still a great challenge.Metal-organic frameworks(MOFs),in which metal ions are coordinated to organic ligands forming one-,two-,or three-dimensional structures,possess semiconductor-like attributes and can be promising photocatalysts.Compared with traditional semiconductors,unique advantages make MOFs as photocatalysts:large surface area,high porosity and well-ordered pore channels,tunability of structure and function.Large surface area provides more active sites for photocatalysis.High porosity and well-ordered pore channels are beneficial for introducing guest molecules into frameworks,contacting with active sites directly.Besides,taking advantage of structure and function tunability,MOFs can be adjusted by changing or modifying metal sites and organic ligands.Moreover,unsaturated metal sites in MOFs can also promote the photocatalytic reactions.Many MOFs have been used for water reduction but little for water oxidation.And so far,no MOF has been reported to be able to realize overall water splitting.As heterogeneous photocatalysts,MOFs have attracted more and more attentions.In terms of photocatalysts,successful application systems mainly focus on MOFs constructed by Zr-carboxylic acid and Ti-carboxylic acid.There is only some study shows that Al-based MOFs have photoelectric responses but no researches on photocatalytic applications.We choose to design and synthesize the above three kinds of traditional MOFs materials and explore their photocatalytic water splitting applications.In this thesis,a series of traditional Zr,Al and Ti-based MOFs were synthesized,modified and innovatively designed to obtain a series of new MOF structures for photocatalytic hydrogen production,oxygen production and even for overall water splitting.The specific contents are as follows:In chapter one,the basic properties of MOFs materials are introduced,including preparation methods and practical applications.Secondly,the basic principles of semiconductor photocatalytic technology and the material systems for photocatalytic water splitting are briefly summarized.Then,the research progress of MOFs in the field of photocatalysts is described,and the advantages of MOFs in photocatalysts,photocatalytic mechanisms and charge transfer process are introduced.Finally,the significance of the selected topic and research content of this thesis are clarified.In chapter two,we introduce Ce4+ into UiO-67 to form UiO-67-Ce by replacing some bpdc of UiO-67 with bpydc-Ce(namely,bpydc coordinated to a Ce4+ complex)and show that the photocatalytic H2 evolution rate is greatly enhanced over UiO-67-Ce than UiO-67.Our analysis suggests that the introduction of Ce weakens the LMCT process,and the energy transfer occurs between bpdc and bpydc-Ce.The introduction of Ce not only promotes the energy transfer,but also acts as active sites for H2 evolution.In chapter three,two kinds of MOFs MIL-53(Al)and NH2-MIL-53(Al)were synthesized by using Al3+ as the metal nodes and terephthalic acid(H2TA)or amino terephthalic acid(H2ATA)as the organic ligands,respectively.(1)the most common Al based MOFs composed of terephthalic acid(denoted as Al-TA)and 2-amineterephthalic acid(denoted as Al-ATA)were prepared.Their photocatalytic activities for O2 evolution from water were investigated.With the help of CoPi,the oxygen production performance of both MOFs has been improved.Furthermore,their photoelectrical and photophysical properties were investigated,and based on which,the photocatalytic mechanism was proposed.(2)In addition,by virtue of the strong affinity between Ag+ and-NH2 to control the synthesis rate of Ag3PO4 in the in-situ precipitation method,the Al-ATA:Ag3PO4 compound was prepared.Its property towards photocatalytic oxygen evolution was studied.The experimental results show that Al-ATA improves the stability and oxygen evolution efficiency of Ag3PO4,and we can improve the photocatalytic activities of MOFs by means of constructing composite materials with traditional inorganic semiconductor photocatalysts.In chapter four,taking advantage of the tunability of structure and function in MOFs,we design and prepare two kinds of MOFs photocatalysts,Al-ATA-Ni and MIL-125(Ti)-CoPi-Pt,which can realize photocatalytic overall water splitting with H2 and O2 producted in close proximity(2:1).(1)Al-ATA MOF is a photocatalyst for oxygen evolution from water,with the benzene ring of ATA2-as the site for O2 evolution.It is known that compounds with Ni2+ ions are efficient cocatalysts for hydrogen production,and the amino groups present in the pores of Al-ATA MOF can act as anchors for the Ni2+ cations.Thus,we incorporated Ni2+ ions into Al-ATA by coordination to the amino groups,the resulting modified MOF(Al-ATA-Ni)would have both the H2 and O2 evolution units in close proximity and hence might be an efficient photocatalyst for overall water splitting.In this section,we confirm this hypothesis.(2)Pt and CoPi are co-precipitated onto MIL-125(Ti)in two steps,which would act as H2 and O2 reaction sites,respectively.As expected,the as-prepared MIL-125(Ti)-CoPi-Pt produces stoichiometric H2 and O2 evolution from pure water with no external bias.Photoelectrical results suggest that CoPi and Pt play essential roles in decreasing the overpotential of O2 and I-I2 evolution from water,respectively.This may shed new light on developing new efficient overall water splitting MOFs based photocatalysts.In chapter five,we find a very simple and efficient method to enhance the stability of a representative Ti-based MOF,Ti-ATA(ATA = 2-aminoterephthallate ion)in water for days,without sacrificing its porosity and photocatalytic activity.The stability of Ti-ATA in water is greatly improved when it is treated with aqueous solutions containing metal ions Mn+ with empty d orbitals(Y3+,La3+,Ce3+,Nd3+ and Tm3+)or partially-filled d orbitals(Cr3+).The sixth chapter is the summary and prospect part.The work in this thesis and the conclusions we obtained are summarized,the innovation points of our work is elaborated,the existing problems and shortcomings in the work are proposed,and the future research work is also prospected.In summary,through the structural design and preparation of a series of metal-organic framework materials and the study on the property of photocatalytic water splitting,we realize the photocatalytic overall water splitting by using MOFs,which has extremely important research values. |
PDF Full Text Request