纳米材料和纳米结构第五讲.ppt

纳米材料和纳米结构纳米材料和纳米结构溶胶溶胶-凝胶法凝胶法Sol-gel Processing第五讲第五讲uBased on inorganic polymerization reactions（无机聚合反应）（无机聚合反应）uUsed to prepare inorganic oxides,such as glasses or ceramics,especially multicomponent systems（多多组组分分体体系）系）uBe able to control the structure on nanoscale from the earliest stages of processinguEasy to obtain nanostructured grains with compositional homogeneity（组组分分均均匀匀性性）,higher purity,lower processing temperatures over both conventional ceramic processing and traditional glass melting by controlling the chemical additives（添加剂）（添加剂）and processing,but without adopting vacuum conditions1 General IntroductionThe sol-gel technique to material synthesis is based on some organic precursors（先先驱驱物物）,and the gels may form by network growth from an array of discrete particles or by formation of an interconnected 3-D net work by the simultaneous hydrolysis and polycondensation（缩缩聚聚作作用用）of organometallic（有有机机金金属属）precursors.The size of the sol particles depend upon some variable factors such as pH,solution composition,and temperature etc.By controlling the experimental conditions,one can obtain the nanostructured target materials in the form of powder or thin film.Principles Based on the difference of the precursors used,the sol-gel processing is roughly divided into three groups:u Silica（二氧化硅）（二氧化硅）sol-gel processingu Metal alkoxide（金属醇盐）（金属醇盐）processingu Pechini-type processing2 Experimental ApproachThe whole silica sol-gel processing could be divided into three steps:hydrolysis and polycondensation,gelation and drying.2.1 Silica Sol-Gel ProcessingLiquid alkoxide precursor and Si(OR)4 is mixed with water and acid or ammonia,where R represents CH3,C2H5,or C3H7 and acid or ammonia acts as catalyst.The following hydrolysis reaction will take place:Step 1 Hydrolysis and PolycondensationThe hydrated silica tetrahedra（水水合合硅硅氧氧四四面面体体）interact in a condensation reaction（缩缩合合反反应应）,forming bonds:Linkage of additional tetrehedra occurs as a polycondensation reaction and eventually results in a SiO2 network,while the water and alcohol expelled from the reaction remain in the pores of the net work.The hydrolysis and polycondensation reactions initiate at numerous sites within the TMOS+H2O solution as mixing occurs.When sufficient interconnected bonds are formed in a region,they respond cooperatively as colloidal particles or a sol.Initial concentration of water Type and concentration of catalyst Types of silicon alkoxide(methyl 甲基甲基,ethyl 乙基乙基,butyl丁基丁基,pentyl戊基戊基,esters脂脂,etc.)Alcohol 醇醇(methyl,ethyl,butyl,pentyl)mixture used Reaction temperature Final size determination of spherical silica particlesThe shape and size of polymeric（聚聚合合的的）structural units are determined by the relative values of the rate constants for hydrolysis(kH)and polycondensation reactions(kC)To reduce the particle size,kH should be larger than KCFast hydrolysis and slow condensation favor formation of linear polymersSlow hydrolysis and fast condensation lead to larger,bulkier,and more ramified（网网眼眼状状的的）polymers Determination of the structure of the gel kH increases linearly with the concentration of H+or H3O+in acidic media and with the concentration of OH-in basic medium;For basic conditions,distribution of polysilicate species is much broader,polymers often with a high cross-linking degree;For acidic conditions,there is a low cross-linking degree due to steric crowding（立体皱纹形成）（立体皱纹形成）;Base catalysis produces colloidal particles giving rise to meso-or macro-porous xerogels（干凝胶）（干凝胶）composed of dense,non-porous particles;Acid catalysis gives weakly branched polymeric sols and gels leading after drying to highly microporous xerogels with a very fine texture.Electrolyte Concentration:Dominant factor in controlling kH kH varying in the different solvents as follows:acetonitrile 乙脲乙脲 methonal 丙烷丙烷 dimethylformamide 二甲基甲酰胺二甲基甲酰胺 dioxane 二恶烷二恶烷 formamide 甲酰胺甲酰胺;kH for acetonitrile is 20 times larger than kH for formamide;Additionally 10-fold increase in kH occurs when the temperature varies from 20C to 45.5 C;The nature of the alkoxy（烷氧基）（烷氧基）groups on the silicon atoms also influences the rate constant,as a general rule,the longer and the bulkier the alkoxide group,the slower the rate constant.Other factors affecting kHIn fact,the condensation and the hydrolysis reaction are not separated in time but take place simultaneously.It has been well established that the presence of H3O+in the solution increases the rate of the hydrolysis reaction,whereas OH-ions increase the condensation reaction.As for the solvent effect,formamide decreases the hydrolysis rate but slightly increase the condensation rate.As the sol particles grow and collide,condensation occurs and macroparticles begin to form;The sol becomes a gel when it can support a stress elastically.The change is gradual as more and more particles become interconnected and this is typically defined as gelation（凝凝胶胶化化）point or gelation time tg;All subsequent stages of processing depend on the initial structure of the wet gel formed in the reaction bath during gelation;Polymerization reactions are usually thermally activated.In view of the dependence of tg on solution pH,the gelation can be either acid or base catalyzed.Step 2 GelationThe anion（阴阴离离子子）and solvent（溶溶剂剂）also play a role in the kinetics of gelation.The trend is the longer and the larger the solvent molecule,the longer the gelation time.Effect of water content on gelationThe amount of water for hydrolysis has a dramatic influence on gelation time.Generally,an increase of the amount of water will decrease the gelation time for low water content,whereas it increases the gelation time for the high water content owing to the dilution effect.Effect of solvent on gelationSmall-angle X-ray scattering(SAXS)studies show that the radius of primary particles is between 1 and 2 nm and can be modeled by rings and chains of three to four silica tetrahedra.The secondary particle has a radius of 5-20 nm depending on the experimental conditions.Gelation occurs when the secondary particles are linked to each other and form a three-dimensional network across the sample.Characterization of the primary and secondary particlesTMOSTMOSTMOSTMOS基金属醇盐中一次、二次颗粒形成示意图基金属醇盐中一次、二次颗粒形成示意图基金属醇盐中一次、二次颗粒形成示意图基金属醇盐中一次、二次颗粒形成示意图 The activation barrier to aggregation increases linearly with the size of two equal particles.Thus the rate of aggregation would decrease exponentially with their size.Smaller particles will aggregate with larger ones at a much higher rate.Thus two distributions of particles are predicted,small newly formed particles and large aggregating particles.In gelation process,it should be noticed that the polycondensation reactions continue to occur within the gel network as long as neighbouring silanol（硅硅烷烷醇醇）groups(SiOH)in a newly formed gel are close enough to react.This increases the connectivity of the network and its factual dimension（分形维度）（分形维度）.To control the condensation reaction in the gelation process,the large concentration of silanol could be pre-protected.The basic idea is to cap the SiOx(OH)y(OMe)z formed during sol-gel processing of Si(OMe)4 by organic groups and thus to create an inert surface.二氧化硅气凝胶硅烷醇表面的化学修饰二氧化硅气凝胶硅烷醇表面的化学修饰Drying is a necessary procedure to remove the solvent which is condensed into the gel during gelation processDuring the drying cycle,solvents are evaporated at the atmospheric surface and the solid-liquid interface is replaced by a solid-vapor interface Stresses developed during drying may cause gels to crack catastrophically（灾难性地）（灾难性地）A small pore size(20 nm)can cause lager capillary stress,while a large pore size and a stronger network may reduce the stresses and thereby reduce the amount of cracking Step 3 Dryingu Addition of surfactants;u hypercritical evaporation(超临界蒸发超临界蒸发),which avoids the solid-liquid interface;u Obtaining monodispersed pore sizes by controlling the rates of hydrolysis and condensation.Ways of decreasing liquid surface energy to avoid cracking in drying process The decrease in the volume of the gel(the volume of liquid lost by evaporation),87%;Reaching the critical point,characterized by the increment of the strength of the network,10%;The pores being substantially emptied and no further dimensional changes occurred,3%.Three stages of drying processuReactions of metal with alcoholuDirect method:Li,Na,K,Cs,Sr,Ba;uWith catalyst(I2,HgCl2,HgI2):Be,Mg,Al,Tl,Sc,Y;uReactions of halides（卤化物）（卤化物）with alcoholu Direct method:B,Si,P;u Ammonia（氨）（氨）method:Si,Ge,Ti,Zr,Hf,Nd,Ta,Fe,Sb,V,Ce,U,Th,Pu;u Sodium alkoxide（醇钠）（醇钠）method:Ga,In,Si,Ge,Sn,Fe,As,Sb,Bi,Ti,Th,U,Se,Te,W,La,Pr,Nd,Sm,Y,Yb,Er,Gd,Ho,Ni,Cr;2.2 Metal Alkoxide ProcessingStep 1 Synthesis of Metal AlkoxideuReactions of metal hydroxides and oxides with alcohol;uAlcohol interchange reactions;uTransesterification（酯交换作用）（酯交换作用）reactions;uReactions of dialkylamides（二烷基酰胺）（二烷基酰胺）with alcohol.In addition to monometal alkoxides,some heterobimetal alkoxide with same molar ratio of two elements as target composed oxides can also be synthesized as precursors.Solid non-volatile compounds:metals with strong electropositive;Low-volatile polymeric compounds:heavier multivalent metals;Volatile monomeric covalent liquid:light transition metals Possible phases of metal alkoxideThe degree of oligomerization（齐齐聚聚）or polymerization can directly induces differences in the processing and in the resulting final structures of multicomponent systems.Most sol-gel processes are preferably accomplished in a common solvent,alcohol（醇醇）is the substance usually chosenThe choice of alcohol should be made by availability,toxicity（毒毒性性）,solubility of the possible salts and the properties such as low boiling point.Some possible selections are methanol（甲甲醇醇）,isopropyl alcohol（异丙醇）异丙醇）etc.After different optimized precursors are mixed together,the final solution should be stirred long enough to ensure homogeneity.An acid or a base can also be used to hydrolyze the solution.Acid or basic hydrolyzing agents will affect the pore structure of the gelStep 2 MixingJust as in silica sol-gel processing,basic conditions tend to produce hydrolyzed monomers which will create more densely,highly branched,cross-linked polymers and result in a larger pore size upon gelation;In contrast,acidic conditions tend to produce hydrolyzed monomers which condense into a more linear,lightly cross-linked net work;The particulate granular morphology can be accomplished via hydrolytic polycondensation of alkoxides under acidic conditions;The honeycomb morphology of the material are usually formed under basic conditions with ammonia.Effect of solution pH value Water may be added along with acid or base to further tailor the pores controlling reactions;The effect of water/alkoxide ratio during the hydrolytic condensation on the behavior of the resultant oxides is considerable;The higher the content of the hydrolysis water,the finer the texture of matrix.Effect of water addition Other factors in addition to water/alkoxide ratio also affect the kinetics of network forming reactionsu Types of alkyl（烷基）（烷基）groups in the alkoxideu Types of host mediums（基底媒质）（基底媒质）u Molecular separation of speciesu Catalystsu Temperature The use of the larger alkyl groups of alkoxides will be helpful for achieving nanostructured product The cold-water-hydrolyzed materials appear coarse granular（粗颗（粗颗 粒）粒）,but the hot-water-hydrolyzed ones have a fine and fluffy（起毛（起毛 的）的）structureEffect of other factors The drying procedure is routinely performed as having been described elsewhere.In some cases,the hypercritical drying is chosen to eliminate cracking due to stresses.Step 3 DryingStep 4 Thermal DecompositionAfter drying,a porous and homogeneous aerogel（气凝胶）（气凝胶）can be obtained.In this step,the complex decompositions of organic precursor take place,and the organic substances added for preparation of gels are almost completely removed,leading to amorphous powders.Sintering is a procedure to collapse the pore structure and solidify the gel,in which pore reduction occurs due to a process of particle bonding by thermal energy;Increase in particle size is always accompanied with the increase in sintering temperature,although sintering at higher temperature for longer time can produce better crystallized samples;Oxygen-rich atmospheres are usually used to ensure a clean burnout of organics and to enhance the formation of target crystal phase at lower temperature;Calcinating（煅烧）（煅烧）is often adopted to obtain further crystallizationStep 5 Sintering and Calcination Pechini processing involves the dissolution of metal salts in a mixture of a weak hydroxycarboxylic acid（羟羟基基羧羧酸酸）such as citric acid（柠柠檬檬酸酸）,and a polyhydroxy alcohol（多多羟羟基基醇醇）such as ethylene glycol（乙乙二二醇醇）.When the solution is heated at a certain temperature usually lower than 100C,a viscous（粘粘稠稠）organic precursor develops,which can then be converted to the oxide by post heating.2.3 Pechini ProcessingMixtures of reagent-grade（试试剂剂等等级级）CA(citric acid 柠柠檬檬酸酸)and EG(ethylene glycol 乙乙二二醇醇)are stirred in beakers（烧烧杯杯）on hot plates.These well mixed samples are then aged in a drying oven at 100 C for three days.Two basic chemical reactions to make ceramic precursors:(1)Chelation(螯合作用螯合作用)between complex cations and citric acid,leading toStep 1 Formation of A Gel(2)Polyesterification（聚聚酯酯化化）of the above chelate with glycol（乙乙二二醇醇）in acidified solution,leading to A soft and porous resin（树树脂脂）intermediate is always the initial requirement for making a single-phase,fine-grain,and non-agglomerated powder.The processing parameters including mass ratio of the organic substances to metal salts,water content in the mixture,pulverization（粉碎，雾化）粉碎，雾化）before calcination,and calcination conditions should be properly controlled.The thermal decomposition and sintering/calcination steps for Pechini-type process are the same as those for alkoxide method.Step 2&3 Thermal Decomposition,Sintering/Calcination A growing number of magnetic,electronic and optical applications require the nanoscaled materials as thin layers on plannar substrates.A solution containing the desired oxide precursor can be applied to a substrate by spin coating（旋旋涂涂法法）or dip coating（浸浸渍渍提提拉法）拉法）method.2.4 Sol-gel Thin Film Processing Being used in a batch process（成批处理）（成批处理）or a continuous process when the substrate is long,flexible sheet Objects in different shapes and all external surfaces could be coated Film thickness could be changed by controlling both viscosity of the sol-gel and the removal rate of the substrateDip Coating Be more of a batch process in which one must use a rigid flat disk,plate,or a slightly curved or bowl or lens Involving a deposition stage followed by spin-up and spin-down stages Been widely used to apply sol-gel thin film to semiconductor substratesSpin CoatingViscosity:must be kept within a reasonable range,i.e.,the solution must be hydrolyzed past the percolation threshold Time-to-gel:film formation,drying and cre