Delamination of Surfactant-Intercalated Brucite-Like Hydroxy Salts of Cobalt

Delamination of Surfactant-Intercalated Brucite-Like Hydroxy Salts of Cobalt and Copper and Solvothermal Decomposition of the Resultant

Colloidal Dispersions

Jacqueline.T.Rajamathi,?Anthony Arulraj,?N.Ravishankar,§James Arulraj,?and

Michael Rajamathi*,?

Materials Research Group,Department of Chemistry,St.Joseph’s College,36Lalbagh Road,

Bangalore560027,India,Department of Magnetism,Hahn-Meitner-Institut,Berlin,Germany,and Material Research Centre,Indian Institute of Science,Bangalore560012,India

Recei V ed June4,2008.Re V ised Manuscript Recei V ed July10,2008

Surfactant anion intercalated hydroxy salts of copper and cobalt of the formula M(OH)2-x(surf)x·m H2O[M)Cu, Co;surf)dodecyl sulfate,dodecyl benzene sulfonate,and x)0.5for Cu and0.67for Co]delaminate readily in 1-butanol to give translucent colloidal dispersions that are stable for months.The extent of delamination and the colloidal dispersion observed in these solids is higher than what had been observed for layered double hydroxides. The dispersions yield the corresponding nanoparticulate oxides on solvothermal decomposition.While the copper hydroxy salt forms～300nm dendrimer-like CuO nanostructures comprising nanorods of～10nm diameter,the cobalt analogue forms～20nm superparamagnetic particles of Co3O4.

Introduction

Delamination of layered solids is of current interest,as it is the?rst step in the preparation of layered composites with interesting properties.1-3Delamination has been reported in layered solids such as smectites,4graphite oxide,5metal dichalcogenides,6metal phosphates,7and layered oxides.8 Delamination of cationic clay minerals in aqueous medium is well-known and has been studied extensively due to the low layer charge densities and swelling properties of these clays.9 Delamination of anionic clays such as layered double hydroxides (LDHs)and hydroxy salts in aqueous medium is dif?cult,as they have high charge densities on the layers.However, delamination of these solids in organic solvents has been achieved through the intercalation of large surfactant ions in the interlayer region.

The?rst report of delamination of LDHs was due to Adachi-Pagano et al.10,11They intercalated dodecyl sulfate ions in Zn-Al LDHs and delaminated the surfactant-intercalated LDHs in https://www.360docs.net/doc/1410426611.html,ter,O’Leary et al.showed delamination of surfactant-intercalated LDHs in acrylate monomers.12Delami-nation behavior of the I-III LDHs,Li-Al-LDH that has a higher charge density than II-III LDHs,was reported by Singh et al.13 We have reported the delamination of surfactant-intercalated hydroxy double salts14and R-hydroxides of nickel and cobalt15 which are structurally and functionally similar to LDHs.16,17 Another class of anionic claylike hydroxide materials are hydroxy salts of the formula M(OH)2-x(A)x·m H2O,where M) Ni,Co,Cu;A-is a monovalent anion such as nitrate and acetate, and x)0.5or0.67.These are neutral layered solids whose structure is derived from that of brucite-like M(OH)2.Brucite consists of hexagonal packing of OH-ions,and the M2+ions occupy alternate layers of octahedral sites.18In the hydroxy salts, a part,x,of the hydroxyl ions of M(OH)2are replaced by the anions,A-.The anions are directly ligated to the metal ions in contrast to the cases of other anionic clays in which the anions are loosely held in the interlayer region.Though the anions are strongly bonded,it has been shown that these hydroxy salts exhibit anion exchange reactions.19,20It would be interesting to

*To whom correspondence should be addressed.E-mail:mikerajamathi@ https://www.360docs.net/doc/1410426611.html,.Fax:91-80-2224-5831.

?St.Joseph’s College.

?Hahn-Meitner-Institut.

§Indian Institute of Science.

(1)Jacobson,A.J.In Comprehensi V e Supramolecular Chemistry;Alberti,G., Bein,T.,Eds.;Elsevier:Oxford,1996;Vol.7,p315.

(2)(a)Venugopal,B.R.;Ravishankar,N.;Perrey,C.R.;Shivakumara,C.; Rajamathi,M.J.Phys.Chem.B2006,110,772.(b)Nethravathi,C.;Ravishankar, N.;Shivakumara,C.;Rajamathi,M.J.Power Sources2007,172,970.(c) Venugopal,B.R.;Ravishankar,N.;Rajamathi,M.J.Colloid Interface Sci.2008, 324,80.

(3)(a)Szekeres,M.;Szechenyi,A.;Stepan,K.;Haraszti,T.;Dekany,I.Colloid Polym.Sci.2005,283,937.(b)Hornok,V.;Erdohelyi,A.;Dekany,I.Colloid

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G.Appl.Clay Sci.1999,15,1.(c)Venugopal,B.R.;Sen,S.;Shivakumara,C.; Rajamathi,M.Appl.Clay Sci.2006,32,141.

(5)(a)Thiele,H.Kolloid Z.1948,111,15-9.(b)Matsuo,Y.;Tahara,K.; Sugie,Y.Carbon1997,35,113.(c)Nethravathi,C.;Rajamathi,M.Carbon2006, 44,2635.

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(7)(a)Kaschak,D.M.;Johnson,S.A.;Hooks,D.E.;Kim,H.N.;Ward, M.D.;Mallouk,T.E.J.Am.Chem.Soc.1998,120,10887.(b)Alberti,G.; Cavalaglio,S.;Dionigi,C.;Marmottini,https://www.360docs.net/doc/1410426611.html,ngmuir2000,16,7663.

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G.Chem.Mater.2001,13,4661.(b)Schaak,R.E.;Mallouk,T.E.Chem.Mater. 2000,12,3427.

(9)Jordan,J.W.J.Phys.Colloid Chem.1949,53,294.

(10)Adachi-Pagano,M.;Forano,C.;Besse,https://www.360docs.net/doc/1410426611.html,mun.2000,91.

(11)Leroux,F.;Adachi-Pagano,M.;Intissar,M.;Chauviere,S.;Forano,C.; Besse,J.-P.J.Mater.Chem.2001,11,105.

(12)O’Leary,S.;O’Hare,D.;Seeley,https://www.360docs.net/doc/1410426611.html,mun.2002,1506.

(13)Singh,M.;Ogden,M.I.;Parkinson,G.M.;Buckley,C.E.;Connolly, J.J.Mater.Chem.2004,14,871.

(14)Rajamathi,J.T.;Ravishankar,N.;Rajamathi,M.Solid State Sci.2005, 7,195.

(15)Nethravathi,C.;Harichandran,G.;Shivakumara,C.;Ravishankar,N.; Rajamathi,M.J.Colloid Interface Sci.2005,288,629.

(16)Morioka,H.;Tagaya,H.;Karasu,M.;Kadokawa,J.;Chiba,K J.Mater. Res.1998,13,848.

(17)Rajamathi,M.;Thomas,G.S.;Kamath,P.V.Proc.Indian Acad.Sci., Chem.Sci.2001,113,671.

(18)Ostwald,H.R.;Asper,R.In Preparation and Crystal Growth of Materials with Layered Structures;Leith,R.M.A.,Ed.;Reidel:Dordrecht,1977;p71.

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10.1021/la801730u CCC:$40.75 2008American Chemical Society

Published on Web08/26/2008

see if these solids could be delaminated by the methods employed for other anionic clays such as the LDHs and if the delamination behavior is in?uenced by the nature of bonding of the anions to the metal hydroxide layer.

We have earlier shown that the colloidal dispersion of delaminated R-hydroxides could be solvothermally decomposed to form nanoparticulate oxides of unusual shapes.21It would be interesting to see if the colloidal dispersions of the hydroxy salts also would yield similar results.

In this paper,we report the synthesis of surfactant-intercalated hydroxy salts of cobalt and copper,their delamination in alcohols, and the solvothermal decomposition of the resultant colloidal dispersion.

Experimental Section

Synthesis of Hydroxy Salts.Cobalt hydroxy nitrate was prepared according to the method reported by Rajamathi and Kamath.22A total of18g of Co(NO3)2was mixed with2g of urea and2mL of water in a beaker,and the mixture was placed in an air oven at 155-160°C.The mixture was stirred occasionally until it turned viscous.It was then removed from the oven and cooled to room temperature.The pink solid obtained was ground well and washed with copious quantities of water.Finally,it was washed with acetone

and dried in air at65°C to constant weight.Copper hydroxy acetate was prepared according to the method reported by Yamanaka et al.23A total of50mL of0.0094M cupric acetate was titrated against 50mL of0.1N sodium hydroxide solution with continuous vigorous magnetic stirring.The blue solid obtained was washed with copious quantities of water followed by acetone and dried in air at65°C to constant weight.

Synthesis of Surfactant Anion Intercalated Hydroxy Salts. Surfactant anion intercalated hydroxy salts were prepared by the anion exchange reactions of the hydroxy salts.A total of0.3g (0.814mmol)of cobalt hydroxy nitrate was added to a solution containing2.814g(9.762mmol)of sodium dodecyl sulfate or3.4g (9.762mmol)of sodium dodecyl benzene sulfonate in30mL of decarbonated water,and the mixture was stirred at room temperature for3days.The resultant solids were separated by centrifugation, washed with decarbonated water followed by acetone,and dried in air at65°C to constant weight.Surfactant-intercalated copper hydroxy salts were also prepared by the same procedure.

Delamination and Restacking.The delamination of surfactant-intercalated hydroxy salts was achieved by sonicating30mg of the sample in100mL of1-butanol for2h at60-70°C.The colloidal dispersions were allowed to stand overnight,and the undispersed solid was separated by centrifugation.The centrifugates were translucent colloidal dispersions,which were stable for more than 1month.

In each case,the hydroxy salt layers from the colloidal dispersions were restacked by(i)the evaporation of the colloidal dispersions at 110°C and(ii)the addition of an equal volume of acetone to the dispersions.The resultant products were washed with acetone and dried in air at65°C to constant weight.

Solvothermal Decomposition.The colloidal dispersions of dodecyl sulfate intercalated cobalt and copper hydroxy salts in 1-butanol were subjected to solvothermal decomposition.In each case,100mg of the sample,30mg of urea,and50mL of1-butanol were subjected to sonication.The resulting colloidal dispersion of the hydroxy salt was placed in a Te?on-lined stainless steel autoclave along with1mL of water and heated to200°C for18h.After the reaction,the autoclaves were cooled to room temperature under ambient conditions.The products were separated by centrifugation, washed with ethanol followed by acetone,and dried in air at65°C overnight.

Characterization.The materials were characterized by powder X-ray diffraction(Siemens D5005powder diffractometer,θ-2θBragg-Brentano geometry,Cu K R radiation,2°per min)and infrared spectroscopy(Nicolet model Impact400D FTIR spectrometer,KBr pellets,4cm-1resolution).In situ X-ray diffraction of the colloidal dispersions was carried out using a Philips X’pert Pro Diffractometer ?tted with a secondary graphite monochromator using Cu K R radiation.The samples obtained on solvothermal decomposition of the colloidal dispersions were characterized by transmission electron microscopy(JEOL200CX operated at160kV).Magnetization(0-5 T;5,10,350K)and dc-magnetic susceptibility(between5and350 K)of Co3O4were measured using a superconducting quantum interference device(SQUID)magnetometer(MPMS,Quantum Design).

Results and Discussion

Powder X-ray diffraction(PXRD)data con?rm the intercalation of the surfactant anions in both cobalt and copper hydroxy salts as seen in Figures1and2.Figure1a shows the PXRD pattern of the as prepared cobalt hydroxy nitrate.It matches well with the pattern reported in the literature22and can be indexed on a hexagonal cell with the unit cell parameters a)3.18?and

c)20.70?.While the basal spacing of the hydroxy nitrate is

6.9?,the basal spacing of the dodecyl sulfate(DS)exchanged cobalt hydroxy salt(Figure1b)has increased to29.2?(calculated from the00l re?ections),indicating the incorporation of the large surfactant anion.Unlike in the hydroxy nitrate,the10l and11l re?ections merge leading to two sawtooth shaped features starting at2θ)33°and58°,suggesting turbostratic disorder in the sample.24The observed basal spacing suggests that the alkyl chains of the anions are approximately perpendicular to the slabs and these chains of adjacent layers interpenetrate each other.11 The basal spacing in dodecyl benzene sulfonate(DBS)exchanged cobalt hydroxy salt(Figure1c)calculated from its00l re?ections is29.8?,con?rming the incorporation of the surfactant ion. Here,again we observe two sawtooth shaped features starting at2θ)33°and58°due to turbostratic disorder.

Figure2a shows the PXRD pattern of the as prepared copper hydroxyacetate,which matches well with what is reported in the literature.23,25It can be indexed on a monoclinic cell with a)

(21)Nethravathi,C.;Sen,S.;Ravishankar,N.;Rajamathi,M.;Pietzonka,C.; Harbrecht,B.J.Phys.Chem.B2005,109,11468.

(22)Rajamathi,M.;Kamath,P.V.Int.J.Inorg.Mater.2001,3,901.

(23)Yamanaka,S.;Sako,T.;Seki,K.;Hattori,M.Solid State Ionics1992, 53-56,527.

(24)Warren,B.E.;Bodenstein,P.Acta Crystallogr.1965,18,282.

(25)Yamanaka,S.;Sako,T.;Hattori,M.Chem.Lett.1989,10,1869

. Figure1.PXRD patterns of(a)cobalt hydroxy nitrate,(b)cobalt hydroxy dodecyl sulfate,and(c)cobalt hydroxy dodecyl benzene sulfonate.

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5.546?,b )

6.030?,c )9.361?,R )γ)90°,and )94.5°.The basal spacing of this sample calculated from the 00l re?ections is 9.4?.The DS-and DBS-intercalated copper hydroxy salts (Figure 2b and c)exhibit a series of 00l re?ections,indicating good ordering along the stacking direction.The observed basal spacings of 26.0and 31.3?con?rm the incorporation of the surfactant anions.In both samples,the in-plane re?ections are not observed clearly due to the strong oriented growth of the samples along the c -direction.The basal spacing values suggest that here too the alkyl chains of the anions are approximately perpendicular to the slabs and these chains of adjacent layers interpenetrate each other.11

The IR spectra of the DS-intercalated cobalt and copper hydroxy salts are shown in Figure 3a and b,respectively.The spectra of these solids are very similar in the range 4000-1000cm -1.In both cases,the strong and broad absorption centered at 3450cm -1is due to the O -H stretching of the H-bonded hydroxyl groups and interlayer water molecules.Absorptions in the region 2980-2820cm -1are due to the C -H stretching vibrations of the alkyl chain of the surfactant.The S d O stretching vibrations of the sulfate group are seen at 1220cm -1.In the IR spectrum of the DBS-intercalated cobalt hydroxy salt (Figure 3c),we observe absorptions due to O -H and C -H stretching at the same wavenumbers as the DS-intercalated hydroxy salts.The absorption due to the S d O stretching vibration is slightly shifted to 1190cm -1.Benzene ring related vibrations are observed at 1382,1135,1030,and 1005cm -1.In the case of DBS-intercalated copper hydroxy salt (Figure 3d),the O -H stretching vibration is sharper and it is observed at 3520cm -1,indicating poor H-bonding due to reduced interlayer water content.The absorption due to S d O stretching vibration is observed at 1220cm -1,and the benzene ring related vibrations are observed at 1407,1382,1135,1030,and 1005cm -1.

The DS-and DBS-intercalated hydroxy salts delaminate readily in 1-butanol on sonication at room temperature.The translucent colloidal dispersions obtained were stable for more than 1month.The extent of delamination and the stability of the colloidal dispersion are similar to what had been observed for hydrozincite type hydroxy double salts.14In all the cases,about 0.3g of the hydroxy salt can be dispersed in 1L of 1-butanol.The amount dispersed is slightly higher than what was observed for the

structurally related surfactant-intercalated R -hydroxides of cobalt and nickel.15This may be attributed to the fact that in the hydroxy salts the surfactant species is directly anchored to the layers while they are weakly bound to the layers in the R -hydroxides.The extent of delamination and the stability of the colloidal dispersions do not depend on the type of surfactants involved unlike in the case of Li-Al-LDHs.13The in situ XRD of the colloidal dispersions of the DS-and DBS-intercalated hydroxy salts are shown in Figure 4.The absence of 00l re?ections in all these patterns suggests complete delamination in all the cases.The layers from the delaminated colloidal dispersions could be restacked either by evaporation or by the addition of a polar solvent such acetone.The PXRD patterns of the samples

obtained

Figure 2.PXRD patterns of (a)copper hydroxy acetate,(b)copper hydroxy dodecyl sulfate,and (c)copper hydroxy dodecyl benzene

sulfonate.

Figure 3.IR spectra of (a)cobalt hydroxy dodecyl sulfate,(b)copper hydroxy dodecyl sulfate,(c)cobalt hydroxy dodecyl benzene sulfonate,and (d)copper hydroxy dodecyl benzene

sulfonate.

Figure 4.PXRD patterns of the colloidal dispersions of (a)cobalt hydroxy dodecyl sulfate,(b)copper hydroxy dodecyl sulfate,and (c)copper hydroxy dodecyl benzene sulfonate in 1-butanol.

11166Langmuir,Vol.24,No.19,2008Rajamathi et al.

through restacking are shown in Figure 5.Figure 5a and b shows the PXRD patterns of the restacked samples of cobalt and copper hydroxy dodecyl sulfate,respectively,obtained by the evaporation of the colloidal dispersions.In both the cases,the 00l re?ections are slightly more broadened and the basal spacing has increased slightly compared to the parent solid.This may be attributed to the incorporation of the solvent molecules in the interlayer region during restacking.10,11Figure 5c shows the PXRD pattern of the sample obtained when a mixture of colloidal dispersions of cobalt and copper hydroxy dodecyl sulfate was subjected to evaporation.There is only one set of 00l re?ections with a basal spacing of ～32?,suggesting random costacking of layers from both cobalt and copper hydroxy salts in this sample.

The PXRD patterns of the products obtained by solvothermal decomposition of DS-intercalated hydroxy salts are shown in Figure 6.The characteristic re?ections of Co 3O 4are observed in the decomposition product of the cobalt hydroxy salt (Figure 6a).The average particle size calculated using the Scherrer equation is ～20nm.In the case of the product obtained on the decomposition of copper hydroxy salt (Figure 6b),we observe re?ections due to tenorite,CuO.The average particle size is calculated to be ～15nm.

CuO particles obtained from copper hydroxy salt have an interesting shape.The low magni?cation transmission electron microscopy (TEM)image (Figure 7A)shows a number of dendrimer-like particles of ～250-300nm diameter.The high magni?cation image (Figure 7B)clearly indicates that each dendrimer-like particle is made of nanorods of diameter ～10nm.The selected area electron diffraction (SAED)pattern shown in Figure 7C matches well with the diffraction pattern of tenorite (JCPDS PDF:5-0661).Possibly,the decomposition of copper hydroxide layers creates a number of nuclei in close proximity and CuO rods grow from these nuclei to form the dendrimer-like structure.

The TEM image of Co 3O 4obtained on solvothermal decom-position of the hydroxy salt presented in Figure 8A shows particles of diameter 10-40nm with the average diameter being ～20nm.The shape of the Co 3O 4particles obtained here is very different

from what we obtained through solvothermal decomposition of surfactant-intercalated R -cobalt hydroxide in our earlier work.21While the R -cobalt hydroxide yielded cauli?ower-like particles,the structurally related cobalt hydroxy salt yields particles

of

Figure 5.PXRD patterns of the restacked samples of (a)cobalt hydroxy dodecyl sulfate and (b)copper hydroxy dodecyl sulfate obtained by evaporating the corresponding colloidal dispersions.A PXRD pattern of the sample obtained when a mixture of colloidal dispersions of cobalt hydroxy dodecyl sulfate and copper hydroxy dodecyl sulfate was evaporated is shown in

(c).

Figure 6.PXRD patterns of the nanoparticles obtained on solvothermal decomposition of the colloidal dispersions of (a)cobalt hydroxy dodecyl sulfate and (b)copper hydroxy dodecyl

sulfate.

Figure 7.Low magni?cation (A)and high magni?cation (B)TEM bright ?eld images of CuO particles obtained on solvothermal decomposition of the colloidal dispersion of copper hydroxy dodecyl sulfate.A SAED pattern of the particles is shown in

(C).

Figure 8.TEM bright ?eld image (A)and SAED pattern (B)of Co 3O 4particles obtained on solvothermal decomposition of the colloidal dispersion of cobalt hydroxy dodecyl sulfate.

Delamination of Brucite-Like Hydroxy Salts Langmuir,Vol.24,No.19,200811167

ill-de?ned shapes.The SAED pattern shown in Figure 8B matches well with the diffraction pattern of cubic Co 3O 4(JCPDS PDF:9-418).

We have measured the magnetic susceptibility and magnetiza-tion data of the Co 3O 4nanoparticles obtained from the decomposition of hydroxy salt.In Figure 9a (bottom),the ZFC-FC magnetic susceptibility of the Co 3O 4sample (obtained here from hydroxy salt)measured at a ?eld of 50G is shown.The temperature of the peak in the ZFC data represents the blocking temperature above which the magnetic susceptibility is inversely proportional to the temperature,and below this temperature magnetization does not change with in the time scale of the measurement.This is the reason why the ZFC and the FC data coincide above the blocking temperature and are different below it for a small applied ?eld of measurement.For samples with a distribution of small particle size,there is a corresponding distribution in the blocking temperature that needs to be taken into account in explaining the difference in the ZFC-FC data.26The difference in the ZFC-FC magnetic susceptibility of the Co 3O 4sample disappears when measured in the ?eld of 5000G (Figure 9a,top).

Isothermal magnetization data of the Co 3O 4nanoparticle sample are shown for 5,10,and 350K in Figure 9b.The low temperature data show large spontaneous magnetization values,and no hysteresis is observed.The spontaneous magnetization in the Co 3O 4nanoparticle can result from various combinations of contributions that include the uncompensated magnetic moment

of the two sublattice spin structures of an antiferromagnet due to the reduction in the particle size,27the magnetic moment due to canting of the sublattice magnetization,and the magnetic moment due to disordered spins in the surface of the particle.Further studies are needed to ascertain the relative importance of these contributions to the spontaneous magnetization of the Co 3O 4nanoparticles.The observed absence of hysteresis is expected of a magnetic material in the superparamagnetic state.28

It is interesting to note that the Co 3O 4obtained from R -cobalt hydroxide by an identical procedure in our earlier work 21showed a different magnetic behavior:it exhibited weak ferromagnetism.This difference may be attributed to the peculiar cauli?ower morphology of the Co 3O 4obtained in our previous work.21

Conclusion

Surfactant anion intercalated hydroxy salts of cobalt and copper delaminate better than LDHs in 1-butanol.The solvothermal decomposition of the colloidal dispersion of the copper hydroxy salts leads to dendrimer-like CuO particles that are a collection of ～10nm nanorods.The cobalt hydroxy salt yields ～20nm Co 3O 4nanoparticles that show superparamagnetic behavior.The results suggest that the solvothermal decomposition of monolayer colloidal dispersions of metal hydroxide materials could be an important route to interesting metal oxide nanomaterials.Acknowledgment.This work was funded by DST,New Delhi.We thank SSCU,IISc,Bangalore for PXRD facilities.

LA801730U

(26)Hansen,M.F.;Morup,S.J.Magn.Magn.Mater.1999,203,214.(27)Neel,L.Low Temperature Physics ;Gordon and Breach:New York,1962;p 413.

(28)Bean,C.P.;Livingston,J.D.J.Appl.Phys.1959,30,120S

.

Figure 9.(a)FC-ZFC curves of Co 3O 4particles at an applied ?eld of 50G (bottom)and 5000G (top).In the insets,FC inverse magnetic susceptibility data are shown.(b)Magnetization curves as a function of applied magnetic ?eld of Co 3O 4particles at 5,10,and 350K.

11168Langmuir,Vol.24,No.19,2008Rajamathi et al.