双金属氢氧化物

Hexagonal nanosheets from the exfoliation of Ni 2+-Fe 3+LDHs:a route towards layered multifunctional materials †Gonzalo Abell a n,a Eugenio Coronado,*a Carlos Mart ı-Gastaldo,a Elena Pinilla-Cienfuegos a and Antonio Ribera abReceived 13th May 2010,Accepted 20th June 2010DOI:10.1039/c0jm01447hHere we report the synthesis of a crystalline micrometric-sized hexagonal-shaped Ni 2+-Fe 3+LDH by following a modified homogeneous precipitation method.The exfoliation of the material in formamide leads to stable suspensions of hexagonal nanometric sheets,which have been extensively characterized.Our data confirm that the intrinsic properties of the bulk material are retained by these segregated nanosheets,thus opening the door for their use in the development of layered multifunctional materials.Exfoliation of layered hydroxides,defined as the segregation of these lamellar inorganic solids into single entities through soft chemistry methods,1can be regarded as a versatile synthetic route to prepare stable suspensions of sheets,typically exhibiting homogeneous micrometric lateral size and nanometric thickness.Concerning materials science,these 2D nanosheets offer very interesting features:(a)owing to the nanometric thickness of these layers,they may exhibit new physical properties different from the bulk;(b)compared to other low dimensional nano-materials (0D or 1D),their two-dimensional structure facilitates their processability.In fact,these layers can be re-assembled into highly oriented films by employing Langmuir–Blodgett (LB)or Layer-by-Layer (LbL)techniques;2(c)they can lead to the design of new multifunctional materials combining sophisticated phys-ical properties through the rational choice of the constituting building blocks and the precise control of their arrangement in the solid state.3Among these layered materials,we were particularly interested in the layered double hydroxides (LDHs)compounds.The struc-ture of these anionic clays is closely related to that of the brucite,Mg(OH)2,with partial substitution of some of the divalent posi-tions with M 3+cations.This fact generates an excess of positive charge in the layers which must be balanced by the presence of anions and water molecules interleaved between the layers according to the formula:[M II 1Àx M III x (OH)2](A n À)x/n $yH 2O.In this way,their exfoliation in organic solvents produces steady colloidal suspensions of positively charged nanosheets,4which in the presence of anionic components,can be further re-assembled into sandwich-like layered materials as result of the presence of attractive Coulombic interactions.5In addition,LDHs offer a wide plethora of intrinsic magnetic,optical or catalytic proper-ties,6which ultimately will be retained by the segregated layers andtransferred to these layered super-structures.Concerning magne-tism,we recently reported the presence of spontaneous magneti-zation below 15K in Ni 2+-Fe 3+LDHs prepared by the traditional coprecipitation method,as result of the combination of strong antiferromagnetic superexchange interactions between the in-plane ions mediated by the hydroxyl bridge and much weaker dipolar interactions between the layers.7Unfortunately,the synthesis of crystalline Fe 3+-based LDHs with hexagonal morphology in an extendable controlled manner has been elusive so far.More generally,the synthesis of non-Al 3+-based LDHs is very disfavoured mainly due to the specific amphoteric behaviour of Al(OH)3.Except for remarkable efforts,8the use of traditional coprecipitation methods,based on the direct combination of the constituting ions in a basic medium,has just led to the isolation of amorphous phases.Although these materials can indirectly enhance its crystallinity after an additional hydrothermal treat-ment,they cannot be effectively exfoliated.9Here we report how,by following a modified homogeneous precipitation method,whose novelty resides in the use of the chelating agent triethanolamine (TEA;C 6H 15NO 3)as auxiliary reagent,permits the synthesis of a highly crystalline micrometric-sized hexagonal-shaped [Ni 1-x Fe x (OH)2](CO 3)x/2(x ¼0.25)LDH.Besides,this material can be readily ion-exchanged with nitrate anions through the so-called ‘‘acid-salt treatment’’,10permitting its quantitative exfoliation in formamide.Regarding the potential magnetic functionality of these Ni 2+Fe 3+-LDH nanosheets,7this work paves the way for their use in the design of advanced multifunctional materials.ExperimentalSynthesisAll chemical reagents,Ni(NO 3)2$6H 2O,Fe(NO 3)3$9H 2O,Al(NO 3)3$9H 2O,NaNO 3,HNO 3,C 6H 15NO 3(triethanolamine,TEA)and CO(NH 2)2(urea)were used as received from commercial suppliers (Sigma-Aldrich and Fluka)without further purification.Synthesis of NiFe-CO 3LDH (1):LDHs were synthesized following a modified homogeneous precipitation method by using urea and TEA as ammonium releasing reagent (ARR)and chelating agent,respectively.aUniversidad de Valencia (ICMol),Catedr atico Jos e Beltr a n n 2,46980Paterna,Spain.E-mail:eugenio.coronado@uv.es;Fax:+34963543273;Tel:+34963544415bFundaci on General de la Universidad de Valencia (FGUV),Amadeu de Savoia n 4,46010Valencia,Spain†Electronic Supplementary Information (ESI)available:Further details concerning the experimental procedure and physical characterization of the materials here reported including X-ray diffraction,SEM images,EDAX metallic analysis,FT-IR and TG/DTA studies.See DOI:10.1039/c0jm01447h/PAPER /materials |Journal of Materials ChemistryD o w n l o a d e d b y H e F e i U n i v e r s i t y o f T e c h n o l o g y o n 10 J a n u a r y 2012P u b l i s h e d o n 02 A u g u s t 2010 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 0J M 01447HView Online / Journal Homepage / Table of Contents for this issueIn a typical procedure,the nitrate salts of the metals were dissolved in 50mL of deionized purged water together with TEA,in order to reach a total metal cation concentration of 20mM in the final solution,with x ¼0.25and a concentration of TEA equimolar with iron concentration,being x ¼[Fe 3+]/([Fe 3+]+[Ni 2+]).Next,50mL of an aqueous solution of urea (35mM)were added.The amount of urea was fixed to be 1.75times the total metal concentration.The resulting dark brown mixture was placed in a 125cm 3stainless steel Teflon-lined autoclave and heated up to 150 C in a preheated oven.After 48h,the auto-clave was left to cool down at room temperature and the resulting yellowish fine powder was filtered and dried in a vacuum.The pH value of the remaining solution was found to be around 8.IR (KBr pellet)n /cm À1:3446(br,st);1637.0(m);1358(sh,m);735(st);492(sh,m).EDAX (calculated for Ni 0.75Fe 0.25):Found Ni ¼75.2%(75.0%);Fe ¼24.8%(25.0%).Water content (TG):12.9%.Anal.calc.for [Ni 0.75Fe 0.25(OH)2](CO 3)0.125$0.8H 2O (F w ¼113.35):C,1.32;H,3.19%.Found:C,1.72;H,2.89%.Preparation of NiFe-NO 3LDH (2):Nitrate-exchange was carried in an excess of nitrate anion.In a typical procedure,1g of 1was immersed in a round-bottom flask containing 1000mL of an aqueous solution of NaNO 3(1.5M)and HNO 3(0.005M).This mixture was mechanically stirred under inert atmosphere during 48h.Afterwards,the resulting yellowish product was filtered,washed thoroughly with deionized water and dried in vacuum at room temperature.IR (KBr pellet)n /cm À1:3448(br,st);1637.0(m);1385(sh,m);1352(w);654(st);486(sh,m).EDAX (calculated for Ni 0.75Fe 0.25):Found Ni ¼75.13%(75.0%);Fe ¼24.87(25.0%).Water content (TG):6.7%.Anal.calc.for [Ni 0.75Fe 0.25(OH)2](NO 3)0.250$0.5H 2O (F w ¼125.51):N,2.79;H,3.21%.Found:N,2.95;H,2.39;C,0.44%.Exfoliation of the NiFe-NO 3LDH:2was finely powdered and dispersed in degassed formamide at a concentration of 1g L À1.Then,the mixture was vigorously stirred at 600rpm for three days.To facilitate the exfoliation,the dispersion was successively sonicated in an ultrasonic water bath (Branson 5510)during three intervals of 20min periodically during the three days,until the turbidity of the dispersion remained constant.Finally,the yellowish translucent colloidal suspension was centrifuged at 2000rpm for 10min to remove the remaining non-exfoliated material.The resulting emulsion clearly exhibited the Tyndall effect when irradiated with a laser beam,thus confirming the presence of exfoliated LDH layers (see Fig.2in the main text).Restacked NiFe-CO 3LDH (R-NiFe-CO 3;3):This sample was isolated by adding an aqueous solution of Na 2CO 3(250mL,1M)to 250mL of a freshly prepared suspension of exfoliated layers of 2in formamide.The mixture was mechanically stirred;with immediate appearance of the yellowish precipitate.Mechanical stirring was maintained for 5additional minutes and finally the suspension was kept static,without stirring during 12h,until no sign of turbidity was detected.Afterwards,the resulting finely divided powder was filtered,washed thoroughly with deionized water and dried in vacuum at room temperature.IR (KBr pellet)n /cm À1:3485(br,st);1637.0(m);1358(w);1360(st);727(st);521(sh,m);494(sh,m).EDAX (calculated for Ni 0.75Fe 0.25):Found Ni ¼74.2%(75.0%);Fe ¼25.8%(25.0%).Water content (TG):14.4%.Anal.calc.for [Ni 0.74Fe 0.26(OH)2](CO 3)0.125$0.7H 2O (F w ¼112.08):C,1.34;H,3.06%.Found:C,1.95;H,2.65;N,0.28%.Physical characterizationMetallic atomic composition of bulk samples was determined by means of electron probe microanalysis (EPMA)performed in a Philips SEM-XL30equipped with an EDAX microprobe.Carbon,nitrogen and hydrogen contents were determined by microanalytical procedures by using a LECO CHNS-932in the Universidad Complutense de Madrid.Particle morphologies and dimensions were studied with a Hitachi S-4100scanning electron microscope at an accelerating voltage of 20keV,over metalized samples with a mixture of gold and palladium during 30s.Infrared spectra were recorded in a FT-IR Nicolet 5700spec-trometer in the 4000–400cm À1range with a nominal resolution of 0.4cm À1,using powdered samples diluted in KBr pellets.Ther-mogravimetric analysis of all compounds were carried out with a Mettler Toledo TGA/SDTA 851apparatus in the 25–800 C temperature range under a 10 C min À1scan rate and an air flow of 30mL min À1.X-ray powder diffraction patterns were collected with a Siemens d-500X-ray diffractometer (Cu-K a radiation;l a ¼1.5418 A)equipped with a rotating anode D-max Rigaku operating at 80mA and 45kV.Samples were mounted on a flat sample plate.Profiles were collected in the 2.5 <2q <70 range with a step size of 0.05 .Dynamic light scattering (DLS)measurements were carried out at 25 C with a Zetasizer Nano ZS instrument (Malvern Instruments Ltd.)on a freshly prepared emulsion of exfoliated LDH in formamide (1g L À1).Mean hydrodynamic diameter and correlation functions were deter-mined by accumulative analysis.HRTEM studies of exfoliated nanosheets were carried out on a JEM-2010microscope (JEOL,Japan)operating at 200kV.Samples were prepared by dropping a suspension of the fresh exfoliated sample in formamide on a carbon-coated copper grid.The digital analysis of the HRTEM micrographs was done using DigitalMicrographTM 1.80.70for GMS 1.8.0by Gatan.Atomic Force Microscopy (AFM)measurements were collected in a Multimode atomic force microscope from Veeco.Typically,a fresh diluted emulsion resulting from the exfoliation of 2in formamide was deposited onto a clean Si wafer by dip coating.The images were obtained with a Si tip (fr and K of ca.300kHz and 42N m À1,respectively)using tapping-mode in air at room temperature.Images were recorded with 512Â512pixels and 0.5–1Hz scan rate.Pro-cessing and analysis of the images were carried out using the Nanotec WSxM software.11Results and discussionNiFe-CO 3LDH (1)was synthesized from aqueous solutions of the nitrate salts of Fe 3+and Ni 2+,TEA and urea under hydro-thermal conditions,according to the homogeneous precipitation method (see the Experimental section).TEA has been widely employed in the synthesis of mesoporous materials according to the so called ‘‘atrane route’’,in view of its ability to chelate metallic ions forming complexes which remain inert towards hydrolysis.12This feature pushed us to employ it as an auxiliary reagent in the synthesis of LDHs,since it could prevent the formation of non-soluble metallic oxides,maintaining a constant ratio of the LDH constituting metals in solution.On the other hand,urea,acting as an ammonia releasing reagent (ARR),isD o w n l o a d e d b y H e F e i U n i v e r s i t y o f T e c h n o l o g y o n 10 J a n u a r y 2012P u b l i s h e d o n 02 A u g u s t 2010 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 0J M 01447Hresponsible for gradually increasing the pH of the synthetic gel in a controlled manner under the reaction conditions.13Fig.1shows the X-ray diffraction pattern of 1.It exhibits the typical profile for LDH materials with sharp intense peaks at low theta values,whereas they become weaker and less defined at higher angular values.No residual peaks were observed thus confirming the absence of contaminant phases.By assuming rhombohedral symmetry,the (00l)peaks,related to the stacking periodicity along the c axis,and the (110)in-plane peak were indexed and permitted calculating the unit cell parameters of 1(Table 1).From these parameters,we estimated a basal spacing(BS)of 7.8 Ain excellent agreement with that reported for other carbonate-intercalated LDHs.13These data,together with the metal analysis,FT-IR and TG/DTA studies (SI4,SI5(ESI†)and Table 2)permit estimating a molecular formula of [Ni 0.75Fe 0.25(OH)2](CO 3)0.125$0.8H 2O for 1.According to scan-ning electron microscopy (SEM)images,this material exhibits an homogeneous distribution of well-defined hexagonal-shaped particles with lateral sizes of ca.1m m (SI1).Exfoliation of carbonate-intercalated LDHs is far unfav-ourable due to the divalent nature of carbonate,which intro-duces stronger electrostatic interactions than other monovalent anions.4For this reason,the starting NiFe-CO 3(1)was trans-formed into NiFe-NO 3(2)through an ion exchange reaction byimmersing it into an aqueous salt-acid mixed solution of NaNO 3and HNO 3.According to Fig.1,the X-ray diffraction of 2showsa small increase in the BS of 0.1 A(Table 1),slightly lower than that expected for nitrate-intercalated LDHs.14This fact together with the presence of low intensity CO 32Àvibration modes in the FT-IR data (SI4)seems to indicate the presence of residual carbonate anions in the interlamellar space.The presence of sharp intense diffraction peaks,with no signature of contami-nant phases,indicates that the ion exchange step proceed with no remarkable loss of crystallinity.In view of the SEM studies (SI2),the micrometric size and hexagonal morphology of the starting material is also maintained after decarbonation.Besides,the constant values of the metal ratio and a parameter permits dis-carding any acid-induced chemical change affecting the LDH layers.According to all these data and the TG/DTA analysis (SI4,SI5and Table 2),2can be formulated as [Ni 0.75Fe 0.25(OH)2](NO 3)0.250$0.5H 2O.Steady yellowish colloidal emulsions of [Ni 0.75Fe 0.25(OH)2]0.25+layers were easily obtained by mechanically stirring and soni-cating a suspension of 2in formamide.The experiments here reported were carried out with concentrations of 1g L À1,but preliminary results indicate that this system can undergo quan-titative exfoliation even at higher concentrations (ca.10g L À1).The presence of exfoliated LDH layers was firstly confirmed by the appearance of Tyndall light scattering when irradiating these emulsions with a laser beam (Fig.2).Direct information con-cerning the size of the exfoliated layers in the formamide dispersion was obtained from Dynamic Light Scattering (DLS)studies (Fig.3a).Even though the average hydrodynamic diameter of LDH dispersions is commonly biased towards the larger particles,15in this case we observe a homogeneous Gaussian distribution of the size of the particles centeredaroundFig.1X-Ray powder diffraction patterns and peaks indexation of 1(black),2(red)and 3(blue).The graphical scheme in the right side shows the structural model for these layered compounds.Table 1X-Ray diffraction data and unit cell parameters for 1–32Q [ ](hkl )Calc.param.a / A(003)(006)(009)(110)a c BS 111.4022.9034.4559.95 3.0823.347.78Æ0.02211.2722.6034.3559.75 3.1023.557.85Æ0.01311.4022.9034.4059.903.0823.347.78Æ0.03aa ¼2d 110;c ¼d 003+2d 006+3d 009;BS ¼c /3.Table 2Metal ratio of the synthetic gels (calculated)and the isolated compounds (experimental)as estimated from Electronic Probe Micro-analysis (EPMA),water content deduced from the thermogravimetric analysis and molecular formulae calculated for 1–3x a Water contentMolecular formula Calc.Exp.Wt.[%]Mol.10.250.2512.90.8[Ni 0.75Fe 0.25(OH)2](CO 3)0.125$0.8H 2O 20.250.25 6.70.5[Ni 0.75Fe 0.25(OH)2](NO 3)0.250$0.5H 2O 30.250.2610.70.7[Ni 0.74Fe 0.26(OH)2](CO 3)0.125$0.7H 2Oax ¼[Fe 3+]/([Ni 2+]+[Fe 3+]).D o w n l o a d e d b y H e F e i U n i v e r s i t y o f T e c h n o l o g y o n 10 J a n u a r y 2012P u b l i s h e d o n 02 A u g u s t 2010 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 0J M 01447H960nm.Fig.3b,c shows high-resolution transmission electron microscopy (HRTEM)images of the NiFe-LDH nanosheets obtained from dropping a freshly prepared emulsion of 2on a carbon-coated copper grid.The exfoliated nanosheets show generally hexagonal morphologies with weak homogeneous contrast,revealing their uniform nanometric thickness,and lateral sizes coincident with those extracted from DLS studies.Fig.3d shows a selected area electron diffraction (SAED)pattern acquired from one of the exfoliated nanosheets.It exhibits a hexagonal arrangement of diffraction peaks as expected for the in-plane symmetry intrinsic to LDH layers.Indexation of the SAED pattern under a hexagonal closed packed structure (hcp)yielded a ¼2.9 A,in good agreement with the values estimated from the indexation of the powder diffraction patterns of thebulk samples (3.08and 3.10 A,for 1and 2,respectively).This point seems to confirm that these nanosheets do not experience any chemical change during the exfoliation process but retain theinternal atomic structure of the LDH layers from the pristine layered bulk material.Fig.4displays atomic force microscopy (AFM)images collected by dip coating a clean Si wafer in a fresh emulsion of 2.The image covering a 6Â6m m area (Fig.4a)shows the presence of particles with lateral sizes coincident once again with the DLS experiments.In some cases,the presence of regular shapes with internal angles close to 120 ,reminiscent of the hexagonal morphologies of the non-exfoliated material,can be observed.The thickness of these nanosheets was studied with more detail in several areas.According to Fig.4b,we observe the presence of an average height profile of 0.8nm corresponding to the presence of a single unilamellar nanosheet.The observed deviation with respect to the crystallographic thickness of a single brucite layer (0.48nm),has been already observed in several AFM studies of exfoliated layered hydroxides and generally attributed to the adsorption of solvent molecules and residual counterions present in the medium.2b,5,15,16Finally,to demonstrate the ability of these single layers to act as macromolecular cations,RNiFe-CO 3(3;where R accounts for restacked)was obtained by adding an excess of Na 2CO 3to the exfoliated NiFe-LDH nanosheets.The presence of intense elec-trostatic interactions between the positively charged layers and the anionic moieties mediates the re-assembling of the original layered structure.On basis of the X-ray diffraction data (Fig.1),the restacked material exhibits the typical pattern for LDHs,with the same unit cell parameters and BS exhibited by 1(Table 1).Besides,the barely broader profile and less intense reflections observed denote a crystallinity quite similar to the original material (1).The observed decrease of the crystallinity must be probably related to the presence of a bigger amount of stacking faults and overall disorder intrinsic to the exfoliation/restacking route.Additionally,a minor impurity related to the presence of a Ni,Fe spinel-like oxide was detected.This fact is not surprising and similar features have been previously observed in the synthesis of related NiFe LDHs through co-precipitation.17Nevertheless,the chemical nature of the LDH layers remains mainly unaffected as confirmed by the constant metallic compo-sition,FT-IR and TG/DTA characterization (SI4,5and Table 2).ConclusionsIn summary,we have successfully overcome the synthetic prob-lems inherent to the preparation of non-Al 3+-containingLDHsFig.2Picture of the colloidal emulsion obtained from the exfoliation of NiFe-NO 3LDH in formamide.The Tyndall effect,resulting from the scattering of the irradiated beam by the particles in solution,confirms the presence of exfoliated LDHnanosheets.Fig.3(a)Distribution of sizes as extracted from DLS studies.(b)HRTEM image of the exfoliated nanosheets.(c)Zoom-in showing their hexagonal morphology.(d)SAED exhibiting a hexagonal arrangement of the diffraction peaks,as expected from the intrinsic symmetry of the LDH layers.The indexed peaks have been employed for estimating the a parameter (see maintext).Fig.4(a)Tapping-mode AFM image of the exfoliated NiFe-LDH nanosheets deposited on a silicon wafer substrate.(b)Height profile concordant with that expected for a single layer.D o w n l o a d e d b y H e F e i U n i v e r s i t y o f T e c h n o l o g y o n 10 J a n u a r y 2012P u b l i s h e d o n 02 A u g u s t 2010 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 0J M 01447Hby following an slightly-modified homogeneous precipitation method which employs TEA as auxiliary reagent.This procedure permits obtaining [Ni 1Àx Fe x (OH)2](CO 3)x/2(x ¼0.25)LDHs with well-defined hexagonal shapes and micrometric size in excellent yields.Preliminary results indicate that this method-ology permits also controlling the in-plane composition of the layers in the 0.2<x <0.3interval,without remarkable effects on the crystallinity,particle size or morphology of the material.In a second step,the starting precursor can be easily nitrate-exchanged in order to produce stable colloidal emulsions of NiFe-LDH nanosheets in formamide.The resulting unilamellar 2D nanometric objects have been extensively characterized by means of DLS,HRTEM and AFM experiments,demonstrating that the internal atomic structure of the LDH layers remains mainly unaffected despite the exfoliation step.Finally,we have validated the suitability of these nanosheets as cationic macro-molecular building blocks by re-assembling the carbonate-intercalated LDH in the presence of an excess of anions.In view of the magnetic functionality exhibited by NiFe-LDHs,7this work must be considered as the first step in the development of advanced multifunctional materials,which will result from the combination of the physical properties intrinsic to these cationic 2D nanosheets with those introduced by electronically active anionic discrete complexes,18or even more complex extended layers.AcknowledgementsFinancial support from the EU (MolSpinQIP)and the ERC (SPINMOL Advanced Grant to EC),the Spanish Ministeriode Ciencia e Innovaci on,with FEDER co-financing (Project Consolider-Ingenio in Molecular Nanoscience and projects MAT2007-61584and CTQ-2008-06720)and the Generalitat Valenciana (Prometeo Program)are gratefully acknowledged.We also acknowledge P.Amor o s and D.Beltr a n for helpfulscientific discussions,Efr e n Navarro-Moratalla for the aesthetic picture illustrating Fig.2and agre for his help with the experimental work.Notes and references1A.Jacobson,J.Mater.Sci.Forum ,1994,1,152–153;T.Sasaki,M.Watanabe,H.Hashizume,H.Yamada and H.Nakazawa,J.Am.Chem.Soc.,1996,118,8329–8335;R.Abe,K.Shinohara,A.Tanaka,M.Hara,J.N.Kondo and 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