Surface Energy Of Layered Double Hydroxides And Preparation And Properties Of Their Monolayer Nanosheets

Layered double hydroxides(LDHs)are a family of inorganic layered materials,comprising of layers with structural positive charge and exchangeable anions between the layers.They have a wide range of potential applications such as in biomedicine,energy transformation and storage,sensor,and environmental protection.Monolayer LDHs(ML-LDHs)commonly show better performances than bulk LDHs(B-LDHs).However,the synthesis of ML-LDHs is still a great challenge owing to the strong electrostatic attraction and hydrogen bonding between the positively charged layers and the intercalated anions.The surface free energy(?)and solubility parameters(?)are two important characteristic parameters to characterize the interactions between molecules of substance.However,the ?and ? parameters of LDHs are not well understood at present.There are intrinsic relationships between the two parameters,and several empirical ?-? equations have been proposed so far,but their application to solids is limited.This is because the molar volume(V)as a parameter exists in these equations while the V of solids is commonly hard to be obtained.Therefore,the development of ?—? equations without the parameter V is essential.In addition,the characteristic parameters matching(CPM)principle has been proposed and used to screen effective liquid media for liquid phase exfoliation(LPE)of van der Waals attraction-bonded materials(vdW-materials).Whether the CPM principle obtained from the vdW-materials can be applied to LDHs needs to be addressed.The colloidal stability is an important research topic in colloidal chemistry,but no reports have focused on the colloidal stability of dispersions of ML-LDHs nanosheets so far.It is well known that electrolytes can significantly affect the stability of colloids,and specific ion effects have attracted extensive attention.However,there is still a lack of reasonable(or universal)theoretical explanation for the specific ion effects.In the current work,the characteristic parameters(?and ? parameters)of LDHs as well as the preparation and properties of ML-LDHs were studied,involving the development of V-free ?-? equations and specific ion effect model and the electrocatalytic activity of CO32--type ML-LDHs for the oxygen evolution reaction(OER).The aim of this work is to deepen the understanding of the nature of LDHs,explain the origin of specific ion effects,and explore effective ways for large-scale production of ML-LDHs,providing information for their practical applications.The main contents and conclusions of this thesis:1.Correlations of surface free energy and solubility parametersSeveral empirical(or semi-empirical)?-? equations have been proposed.However,they are mostly applicable to non-polar liquids.Especially,these equations contain the parameter V,which limits their application to solid substances.In this work,the possible relationships between y and ? parameters were explored based on the reported data of solids,and six ?-? equations without the parameter V were developed by a trial and error fitting method.The ? parameters contain the total surface free energy(?t)and its dispersive(?d)and polar(?p)components.The ? parameters contain the Hildebrand parameter(?t)and the Hansen dispersive(?d),polar(?p),and hydrogen-bonding(?h)components.The as-established V-free ?-? equations are valid for most nonpolar and polar solids and liquids.These equations can not only deepen the understanding of the correlations of ?and? parameters,but also be used conveniently for mutual calculation between ? and ? parameters,which is beneficial to the application of the characteristic parameters for solid material engineering such as the LPE or synthesis of layered materials.2.The characteristic parameters and liquid phase exfoliation of LDHsThe y and ? parameters of LDHs with different metal cations(Mg-Al,Co2Al,Co2Fe and Ni2Fe)and anions(NO3-,Cl-,and CO32-)were estimated using the contact angle method and the V-free ?-? equations,to investigate the effect of compositions of LDHs on their characteristic parameters.In addition,the ?and ? parameters of Mg-Al LDHs were also estimated via the LPE method to verify the applicability of CPM principle to LDHs.The results show that the compositions have no obvious impact on all the characteristic parameters of LDHs,The ?t,yd,and ?p of the LDHs are estimated to be 47.6-56.9,33.7-39.4,and 10.1-18.8 mJ/m2,respectively,and their ?t,?d,?p and?h to be 29.6-33.5,17.2-17.9,9.5-12.5,and 23.3-26.1 MPa1/2,respectively.In comparison with vdW-materials,LDHs exhibit higher ? and ? values,especially,much larger ?h due to the strong electrostatic attraction and hydrogen bonding between the host layers and the guest anions.The characteristic parameter values obtained from LPE are close to those obtained from the contact angle method and V-free ?-? equations,indicating that the V-free ?-? equations are feasible to LDHs.When the solvents used have the characteristic parameters matching to those of the LDHs,the LPE of LDHs could occur,indicating that the CPM principle are suitable for the LPE of LDHs.Unfortunately,no obvious exfoliation was observed for the CO32--type LDHs in all the test liquids,indicating that the electrostatic attraction between the guest CO32-anions and the host LDHs layers is stronger than the solvation effect.Moreover,a new parameter called“surface free energy distance"(R?)was introduced,which can be used for screening effective solvents for the LPE of LDHs.Based on the mixed-solvent strategy,non-toxic glycol/PEG200 mixed liquids were found to be a good medium for the direct LPE of NO3-and Cl--type Mg-Al LDHs,producing monolayer nanosheets.3.Co-precipitation synthesis of ML-LDHs nanosheetsNi2Fe-NO3,Ni2Fe-CO3,and Co2Al-CO3 ML-LDHs nanosheets were synthesized via a co-precipitation method.36 solvents with different characteristic parameters(including the ? and ? parameters)were chosen to explore the correlation between the formation of ML-LDHs and the characteristic parameters of solvents.The results reveal that when the characteristic parameters of the solvents used are close to those of LDHs,ultrathin LDHs nanosheets can be obtained,demonstrating that the CPM principle is suitable for the selection of good solvents for the co-precipitation preparation of ML-LDHs.The so-obtained Ni2Fe-NO3 LDHs nanosheets are monolayer structure,while the Ni2Fe-CO3 and Co2Al-CO3 LDHs nanosheets are bi-or few-layer structures.The dispersions of ML-LDHs can be stable for at least six months without obvious precipitation,which is of great significance on their practical applications.The co-precipitation method has the advantages of mild reaction conditions,convenient operations,no need of special equipment,and easy industrial production.4.Solvothermal synthesis of CO32--type ML-LDHs nanosheetsThe solvothermal method was explored to synthesize CO32--type ML-LDHs.36 solvents with different ? and ? parameters were chosen to synthesize CO2Al-CO3,Co2Fe-CO3,Ni2Fe-CO3,and Mg2Al-CO3 LDHs,to explore the correlation between the formation of ML-LDHs and the characteristic parameters of solvents.The results reveal that,using the solvents that meet the CPM principle,CO32--type ML-LDHs with a thickness of ca.1 nm can be obtained,and their aqueous dispersions can be stable for at least six months without obvious precipitation.This work provides an effective way for the large-scale production of CO32--type ML-LDHs.5.Aggregation stability of ML-LDHs dispersionThe surface charge properties and aggregation behavior of positively charged Mg-Al-NO3 ML-LDHs dispersed in water in the presence of K+salts with different anions(involving mono-,di-,and trivalent anions)were investigated,to understand the stability of ML-LDHs dispersions and to explore the origin of the specific ion effects.The results reveal that an increase in the salt concentration can significantly decrease the effective surface charge density(?eff)of LDHs,leading to the aggregation of nanosheets.The critical coagulation concentration(CCC)or ionic strength(CCIS)of salts for nanosheets significantly decreases with an increase in the valence of anions.Specific ion effects,with a partially reverse Hofmeister series,were observed.On the basis of the Stern model and the DLVO theory,the relationship of CCC with ?eff and the ionic valences of salts(zi)was theoretically analyzed,which can accurately describe the dependence of CCC on the ?eff and zi,but cannot explain the origin of specific ion effects.To explore the origin of specific ion effects,a correlation of CCIS with the surface charge densities of the solid surface and the specific adsorption energy(Esc)of anions within the Stern layer was developed.Especially,the Esc was associated with the characteristic physical parameters of anions,and a "specific ion effects model" was proposed,which can accurately predict the CCISs of monovalent anions and divalent anions(CO32-and SO42-),proving the rationality of the model(its applicability for trivalent anions cannot be verified due to the lack of their characteristic physical parameter values).The model demonstrates that the specific ion effects observed can be attributed to the differences in ionic size,polarizability,and hydration free energy of different anions.This work not only deepens the understanding of specific ion effects on the colloidal stability but also provides useful information for the potential applications of ML-LDHs nanosheets.6.Electrocatalytic OER activity of CO32--type ML-LDHs.To examine the application performances of ML-LDHs,the electrocatalytic OER activity of Co2Al,Co2Fe,Ni2Fe,Ni2Cr,Co2Cr,NiMnFe,NiCoFe,and Ni2Fe0.5Cr0.5-CO3 ML-LDHs obtained via the solvothermal method were evaluated,and the effect of LDHs compositions on the electrocatalytic activity was investigated.The results show that Ni2Fe-CO3 ML-LDHs displays the highest electrocatalytic activity.Compared to the Ni2Fe-CO3 B-LDHs,Ni2Fe-CO3 ML-LDHs exhibits a sufficiently improved OER catalytic activity,which can be attributed to its larger specific surface area,higher electrochemical active surface area,more exposed active sites,and lower charge transfer resistance.In addition,Ni2Fe-CO3 ML-LDHs@NF composites in which LDHs were grown on the surface of NF were prepared via co-precipitation and solvothermal methods in optimized GC/DG(Rv=4/1)mixed liquids.The OER performance of the LDHs@NF composites were measured.The results show that the OER activity of LDHs could be further improved through this strategy,and that the LDHs@NF obtained from the solvothermal route shows better OER activity than that from the co-precipitation method.This work deepens the understanding of the application performance of ML-LDHs prepared from solvothermal and co-precipitation method,providing information for their application in electrocatalysis.