Chapter II - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/8415/6/06_chapter 2.pdf · Chapter...

Chapter II Chapter II Chapter II Chapter II

Adsorbent

Solvent

Solvent

NiAl-LDH

Synthesis of delaminated LDH: A facile two

step approach

Chem. Commun., 46 (2010) 1902

� One step scalable synthesis of phase pure nitrate containing

LDH

� Delamination of LDH achieved in both formamide and water

with similar particle dimensions

� pH dependent hexamine hydrolysis mechanism proposed

� Restacking of delaminaed LDH critically depends on the

solvent

CoAl-LDH

Synthesis of delaminated LDH: A facile

two step approach

2.1 Introduction

2.2 Experimental

2.2.1 Synthesis & delamination

2.2.2 Total delamination study

2.2.3 Sample characterization

2.3 Results and discussion

2.4 Conclusions

2.5 References

Synthesis of delaminated LDH Chapter-II

35

2.1 Introduction

The act of splitting or separating a laminate into separate layers is called

delamination and it is also called as exfoliation. The history of nanosheets, or exfoliated

sheets, dates as far back as from 1950s, when smectite-type clay minerals were reported

to disperse well in water and yield a colloidal suspension as a consequence of

spontaneous exfoliation or delamination [1-3]. Since 1970s, exfoliation of a wide range

of inorganic layered compounds, including metal chalcogenides, [4-6] metal phosphates,

and phosphonates, [7, 8] as well as layered metal oxides, [8-14] has been achieved

through an appropriate combination of interlayer cations and solvents. Unlike smectite

clays with a very weak interlayer interaction due to a low layer charge density, these

LDH layered compounds have a higher layer charge density and require chemical

modifications or interlayer composition changes to artificially promote the exfoliation

process assisted by a weak shear force. The thickness of the resulting sheet-like

crystallites is in the range of molecular dimensions, mostly around 1 nm, while the lateral

dimension is usually in the micrometer range, roughly corresponding to the size of the

parent lamellar crystals.

Nanosheets, as a new class of nanoscale materials, are important for physics in

low-dimensional systems because 2D quantum confinement may result in intriguing

properties that differ from those of bulk lamellar systems. Novel physical and chemical

properties, such as quantum confinement or surface effects, have indeed been gradually

revealed in nanosheets. A most important and attractive aspect of nanosheets is that they

are charge-bearing. Various nanoarchitectures, including aggregated flocculates,

multilayer thin films and hollow nanocapsules, can thus be readily fabricated by using the

nanosheets as building blocks. It is even possible to tailor superlattice like composites or

hybrid assemblies, incorporating inorganic ions, organic molecules or polymers and

metal nanoparticles as well as nanosheet counterparts. Sophisticated functionalities or

nano-devices may be rationally designed through the selection of nanosheets and the

combination with heterogeneous species, with precise control over the artificial

arrangement at a molecular scale [15].


36

Pioneering work on delamination of layered double hydroxide (LDH) was done

by Adachi-Pagano et al. using surfactant intercalated LDHs; surfactant intercalated LDH

[Zn2Al(OH)6][C12H25SO4·nH2O] was synthesized using ion exchange of ZnAlCl-LDH

and which on reflux in butanol at 120 °C for 16 h leads to delamination [16]. Translucent

colloidal solution obtained by this method remains stable for at least 8 months, up to 1.5

g of [Zn2Al-dodecylsulphate (DS)] per liter of butanol can be dispersed and similar

results have been obtained with higher alcohols such as pentanol and hexanol. They also

proposed that, this open the possibility of developing the chemistry of LDHs in non-

aqueous media, e.g. for electrochemistry applications and preparation of ultrathin films

by a soft chemistry route, interstratified LDH/LDH, LDH/clay, LDH/polymer or

nanoporous LDH/SiO2 materials with improved chemical or porosity properties.

Followed by them Gardner et al. synthesized alkoxide derivatives of LDH using co-

precipitation in methanol; which on dispersion in water leads to alkoxide hydrolysis and

formation of a nearly transparent colloidal LDH suspension [17]. Transparent and smooth

film was formed on the glass plate using these materials and other metal ion containing

alkoxide LDHs like NiAl, CoAl and ZnAl were also synthesized using this method.

Hibino and Jones have synthesized glycine containing LDHs which got delaminated

completely in formamide and the delamination process was extremely fast, completed

within few minutes [18]. They proposed that intercalated glycine attracts a large volume

of formamide into the interlayers due to hydrogen bonding, which causes the LDH sheets

to come apart in the solvent and this delaminated mixture is stable in a mixture of

formamide/water. In continuation of this work several other amino acids were introduced

in the view of increasing the interaction between the amino acid and formamide for

delamination. This result indicated that amino acids with greater affinity for formamide

can be used to prepare LDHs that can be delaminated and this result was promising for

the preparation of polymer-LDH nanocomposites [19]. The nature of the material

recovered from delaminated LDH is highly dependent on the drying process; when gently

dried, a well-ordered phase is obtained, either by freeze-drying or reconstruction, the

material becomes amorphous after evaporation of the solvent [20]. In situ XRD of the

delaminated LDH restacking showed that water content of LDH (DS) remain on the basal

surfaces, doesn’t play any significant role during exfoliation or in the macroscopic


37

separation of phases [21]. Leary et al. showed novel method for synthesizing polymer

LDH composites using delaminated LDH; surfactant intercalated LDH on delamination

gave high levels of inclusion in acrylate monomers which on subsequent polymerization

gave polyacrylates with the inorganic component still in the delaminated form [22].

These delaminated LDHs showed liquid crystal phase and phase separation process of

colloidal Mg/Al LDH dispersions involves the nucleation and growth of nematic droplets

[23]. Singh et al. delaminated lithium containing LDHs for the first time using surfactant-

exchanged intercalates and delamination was found to be dependent on the guest

surfactant structure in terms of both chain length and head group moiety. This method

was proposed to be the simple way to nanoparticulate plate-like gibbsite [24]. The

delaminating ability of surfactant intercalated LDHs, independent of nature of the metal

cation, increases with an increase in the size of the surfactant anion and alcohols such as

1-butanol, 1-hexanol, 1-octanol and 1-decanol showed best delamination [25].

Even though delamination was successfully achieved in organic solvents like

formamide and butanol, delaminaiton of LDH in green water solvent remains a challenge.

Hibino and Kobayashi synthesized lactate containing LDH which got delaminated in

water at room temperature to form stable translucent colloidal solutions and AFM results

showed that LDHs delaminate into single sheets with thicknesses that correspond to basal

spacings observed by XRD [26]. Followed by them Jaubertie et al. synthesized ZnAl-

lactate containing LDH in different Zn/Al molar ratio and delaminated in water using

sonication in all wet fractions [27].

Successful delamination of inorganic anion containing LDH was done for the first

time using ultrasonic treatment of NO3- containing LDH in formamide which

delaminated into single and double brucite layers (0.7-1.4 nm in thickness). Dispersions

of LDH-NO3 in formamide are transparent up to concentrations of at least 40 g/L

however concentrations higher than 5 g/L formed transparent gels (Fig. 2.1) [28].


38

Fig. 2.1 Gel formed by the exfoliated LDH in formamide - 10g/L (reproduced from ref.

28)

Anionic iron porphyrins immobilized on formamide exfoliated layered double hydroxides

showed good catalytic activity for oxidation of cyclooctene and cyclohexane using

iodosylbenzene as oxidant; however these big anions failed to intercalate in to the LDH

[29].

Several delaminated LDHs were synthesized using both organic and inorganic

anion intercalation, but researchers failed to characterize them completely; to overcome

this, Li et al. synthezied highly crystalline carbonate containing LDH of 10 µm size using

hexamine hydrolysis method. These highly crystalline materials were converted to nitrate

interlayer using salt-acid treatment, which got delaminated in formamide and

delamination was confirmed through PXRD and AFM. Layer by layer (LBL) assembly of

LDH and poly (sodium styrene 4-sulfonate) (PSS) anionic polymer onto the solid surface

to produce ultrathin nanocomposite films and this formation was monitored by UV-Vis

and PXRD measurements (Fig. 2.2) [30].

Fig. 2.2a UV-Vis absorbtion spectra of

multilayer films of (PSS/LDH)n on quartz

slide (reproduced from ref. 30)

Fig. 2.2b PXRD pattern of (PSS/LDH)n on

quartz slide (reproduced from ref. 30)


39

These fully characterized LDHs have been fabricated over polystyrene beads

through a synthetic route involving layer-by-layer assembly of LDH nanosheets obtained

through above method. Thermal decomposition of organic moieties and reconstruction of

LDH leads to hollow nanoshell of LDHs and confirmed by various characterization

techniques [31]. CoAl-LDH synthesized through above method was delaminated in

formamide and characterized through PXRD and Tyndall effect (Fig. 2.3).

Fig. 2.3a Photograph of exfoliated Co-Al

LDH nanosheets shows the Tyndall effect.

(produced from ref. 32)

Fig. 2.3b XRD pattern for the colloidal

aggregate centrifuged from the suspension.

(produced from ref. 32)

Thin film formed by the LBL assembly of these CoAl-LDH/PSS, showed

excellent magneto-optical response in the ultraviolet visible region [32]. Organic

intercalated LDHs synthesized using these highly crystalline LDH gave transparent

highly-oriented films with anion-exchangeability [33]. Sasaki et al. reviewed the

exfoliation of layered double hydroxides in formamide and proposed that future

challenges involve the complete delamination of LDH in water and synthesis of

delaminated LDHs with Co2+

-Fe3+

, Ni2+

-Fe3+

, Co2+

-Cr3+

etc., for probing novel electronic,

magnetic and optical properties [34].

To determine the factors controlling the orientation of the nanoparticle in films,

alkoxide containing LDHs films hydrolyzed LDH and the water control were studied by

TEM. An increase in surface-to-surface interactions was proposed as the driving force for

the formation of continuous-oriented films with enhanced adhesion to polar substrates.

This will enable the development of new applications for LDH films including microchip


40

sensors and reactors, environmental sensors, composite materials with improved barrier

properties, and inter-cellular drug delivery [35].

One step synthesis of LDH mono-layers were obtained using reverse

microemulsion method; the dodecylsulphate surfactant combines with the layers to act as

charge balancing anions and make the layers hydrophobic and this method can deliver

bulk quantities of uniform sized LDH monolayers [36].

Stable homogeneous suspensions containing monodispersed MgAl-LDH

nanoparticles were also reported using co-precipitation followed by hydrothermal at

different temperatures, the particle size can be precisely controlled between 40 and 300

nm and this method can be extended to variety of metal ions such as Ni2+

, Fe2+

, Fe3+

,

Co2+

, Cd2+

and Gd3+

[37, 38]. Decarbonation followed by delamination of carbonate

containing LDH was also observed in 1:1 (v/v) DMF-ethanol solvent mixture at ambient

temperature [39]. However these methods failed to give complete delamiantion of LDH

and formation of LBL assembly.

Followed by their successful delamination of highly crystalline ion exchanged

LDHs in formamide, Sasaki group also synthesized ternary-component M(II)-M′(II)-Al-

CO3LDHs (M(II) and M′(II) ) Fe, Co, Ni or Zn) and delaminated them in formamide

through ion exchange [40]. CoFe-LDH and CoNi-LDH with tunable composition were

synthesized using novel innovative topochemical oxidative intercalation process and

delaminated completely in formamide [41, 42]. Apart from common metal ions Unal has

synthesized the MgGa containing highly crystalline LDHs and delaminated them in

formamide [43]. Random co-stacking of CoAl and MgAl LDH nanosheets delaminated in

butanol showed different properties than LDH in which each layer contains Mg, Co and

Al and a physical mixture of MgAl and CoAl LDHs [44].

Several films preparations were reported in the literature using these delaminated

LDHs and their combination with poly anion using LBL assembly [45-47]. Iyi et al.

synthesized acetate containing LDHs using ion exchange method which got delaminated

in water immediately and self standing films were formed that can be exchanged with

inorganic and organic anions in the film state [48].


41

From mid 2008 onwards researchers started working on the various possibilities

of exploring these delaminated materials for different applications. Bulky anion like

thiacalix[4]arene was intercalated into the LDH using an osmotic swelling/restoration

reaction in formamide [49]. LBL assembly was made using charged LDH nanosheets and

negatively charged CdS nanoparticles of different sizes on F-doped SnO2 (FTO)

electrodes. This behaved as a n-type semiconductor photoelectrode in an acetonitrile

solution, regardless of the size of immobilized CdS particles but their efficiency for

photocurrent generation was greatly dependent on the stacked structure of the films [50].

Co-assembly of LDH nanosheets with carboxymethyl cellulose (CMC) was also reported

and thermal degradation temperature of CMC in the composite was raised by about 160

°C [51]. LDH films synthesized through vacuum suction method showed CO2

permselectivity; CO2/N2 and CO2/He gas mixtures showed ideal separation factors of

34.4 and 12.4 respectively [52]. These LDH derived films also can act as a precursor for

the synthesis of nanostructured ZnS and CdS films [53]. Highly ordered transparent

composite films of CdS and LDH have been fabricated using a template synthesis

method; the size of the CdS particles in the LDH host can be controlled by adjusting the

length of time of the reaction with H2S [54]. Sulforhodamine B/LDH and ruthenium (II)

complex anion/LDH showed ultrathin films with polarized luminescence [55,56] and

Poly(p-phenylene)/LDH, LBL films showed blue luminescence [57]. Recently Duan et

al. reviewed the synthesis, properties and applications of functional LDH films and they

also offered some perspectives for the design of future multifunctional LDH films [58]. A

cationic functional molecule bis(N-methyl-acridinium) (BNMA) was assembled with a

positively-charged LDH monolayer through a polyanion as the intermediary. These

materials showed periodic long-range ordered structure and well-defined yellow-green

photo-luminescence which demonstrate that they are potential candidates for light-

emitting materials [59].

Hsieh et al. followed a novel method for directly growing highly oriented Li-Al

LDH films on substrates such as glass, Si wafer and carbon cloth. The LDH film that was

fabricated on glass at 5 oC (~1.4 mm thick) exhibits good UV shielding (with only 9.7%

UV transparency) and a maximum of 56% transparency to visible light. Extra-high-

density Li-Al LDH platelets were formed on the carbon cloth surface, increased the


42

surface area of fibre without affecting the hydrophobic or hydrophilic nature [60]. A

facile protocol was reported for preparing colloidal layered double hydroxide/polyvinyl

alcohol (LDH/PVA) on electrospun nanofibrous mats via direct incorporation of low-

content LDH nanoplatelets. This gave uniform, smooth surface of electrospun

nanofibrous mats and enhanced temperatures of the onset decomposition and inflection

compared with other methods of preparation with super hydrophobicity [61]. Zhao et al.

reported a novel method of forming LDH hierarchical thin films over paper cloth and

sponge film using sol-gel deposition and subsequent in situ growth procedure. The

obtained films showed higher adsorption capacity and excellent ability to remove

sulforhodamine B, Congo red and Cr6+

ion in water treatment, compared with the

corresponding powder samples [62].

Erasable nanoporous antireflection coatings were made using delaminated LDH

LBL assembly. The mixed metal oxide film obtained after the calcination showed the

antireflection property and which can be erased using reconstruction method. This is a

novel approach to fabricate inorganic intelligent anti-reflection films with high

mechanical stability, strong adhesion to substrates and environmental friendliness for

viable long-term applications [63]. A hybrid graphene/Ni2+

-Fe3+

layered double

hydroxide material has been fabricated by the hydrothermal treatment of a mixed

suspension of the exfoliated graphite oxide and LDH precursors. The restacking of

graphene nanosheets were effectively prevented by the formation of Ni2+

-Fe3+

layered

double hydroxide platelets, and the graphene nanosheets exist in a complete exfoliation

state [64]. Formate containing NiAl-LDHs were synthesized using formamide hydrolysis

and which got delaminated to give nanosheets of 100-200 nm with a thickness of 9-12

nm [65].

Very recently PVA hybrid films with highly oriented graphene oxide and

delaminated CoAl-LDH were fabricated, which on in-situ reduction gave multilayer

nanocomposite films containing monolayer dispersed graphene and LDHs. These

heterogeneous composite films with monolayer dispersed and aligned graphene and

LDHs exhibited significantly improved electrical conductivity compared with the


43

unreduced one and are expected to find potential applications in electrodes and

multifunctional nanocomposites [66].

Besides MgAl-LDH, transition-metal bearing LDHs bestow broader technological

applications due to their special catalytic, electronic, optical and magnetic properties. In

comparison with other inorganic anions, nitrate-containing LDHs have a greater degree

of exfoliation and ion exchange capacity; however, the one-step synthesis of such nitrates

in interlayer always leads to carbonate impurities [28, 32]. Carbonate free nitrate

containing LDHs are synthesized by salt acid treatment of carbonate containing LDHs,

which requires energy intensive multi-steps. Hence we have made attempt to synthesize

nitrate containing LDHs in one step followed by their delamination in water.

2.2 Experimental

2.2.1 Synthesis & delamination

In a typical synthetic procedure 80 ml of 1M solution of divalent metal ion and

trivalent metal ion (nitrate salts) were mixed in a ratio 2:1; hexamethylenetetramine

(HMT) in a molar ratio of [M(II)+M(III)]:[HMT] of 1:1.5 (16.8 g) and 120 ml of

deionised water was added to this solution. The solution was mixed thoroughly, nitrogen

gas was purged for 30 min and was kept in an oil bath sharply at 80 oC (very subtle

temperature variations affected the phase purity) for four days without stirring. The

resulting material was filtered, washed with deionised water several times and dried in

vacuum at room temperature (yield-7 g). In whole process we have used CO2 free Milli-

Q water (18 ohm resistivity). Soon after the synthesis (pH of this slurry was around 5.0),

we have washed the material to neutral pH (6.5 to 7.0). Delamination in water was done

by taking 2.5 g of wet-LDH cake immediately after filtration in to a Teflon lined stainless

steel reactor to which 50 ml of deionised water was added and kept at 120 oC for 12 h in

nitrogen atmosphere. (pH after the hydrothermal treatment was 6.4, probably due to

release of nitrate ions from the interlayers). The undelaminated material was removed

after centrifuging at 2000 rpm for 30 min. The actual dispersion degree was obtained by

drying the samples under vacuum at room temperature, both before and after


44

hydrothermal treatment and computed from their difference. The value reported here is

an average value of 2.5 g/L with a variation of around ± 1 g/L.

2.2.2 Total delamination study

Total delamination of synthesized LDH was achieved by taking 0.050 g of dried

LDH powder and 50 cm3 of formamide (Aldrich) in a 100 ml conical flask, that was

sealed after purging nitrogen gas and vigorously agitated by a mechanical shaker at a

speed of 150 rpm for two days; the colloidal suspension was then centrifuged at 2000

rpm for 30 min; no residue was seen.

2.2.3 Sample characterization

Atomic Force Microscope (AFM) was done in Innova SPM with scan rate of 1Hz

to know the shape and size of the delaminated LDH platelets. 0.1 g/L of LDH dispersion

was deposited on a freshly cleaved mica substrate and images were obtained using

tapping mode. A RTESPA tip with 10 nm radius was used to achieve high resolution.

2.3 Results and discussion

Nitrate-containing LDHs of NiAl and CoAl without any carbonate impurity was

synthesized in one step using a well known HMT hydrolysis reaction.

10 20 30 40 50 60 70

10 20

00

600

3

c

b

a

Inte

ns

ity

, C

/s

2 Theta, deg

200

Fig. 2.4 XRD patterns of a) NiAl-NO3 LDH, b) CoAl-NO3 LDH c) CoAl-CO3 LDH

(Inset is the expanded region for clarity)


45

Earlier work on HMT hydrolysis reported that it was difficult to synthesize

crystalline LDH below 100 oC [67]; however, we have made crystalline LDHs at 80

oC

under optimized conditions. Fig. 2.4 shows the PXRD patterns of the NiAl-NO3 and

CoAl-NO3 LDH materials; PXRD of carbonate-containing LDH is also included,

synthesized as reported [32] for comparative purposes. Temperature and hexamine

concentration were varied and conditions were optimized to obtain nitrate containing

LDHs. We have also attempted to synthesize nitrate-containing MgAl, ZnAl, CaAl, CuAl

and CoFe LDHs; we ended up with different/multiple phases rather than the desired

nitrate pure phase. The powder X-ray diffraction patterns showed that only CoAl and

NiAl formed LDHs under these conditions.

10 20 30 40 50 60 70

e

d

c

b

a

Inte

ns

ity

, C

/s

2 Theta, deg

300

Fig. 2.5 PXRD pattern of a) ZnAl-LDH, b) MgAl-LDH, c) CaAl-LDH, d) CoFe-LDH, e)

CuAl-LDH

Fig. 2.5 clearly shows different phases obtained after hexamine hydrolysis. MgAl,

CaAl and CoFe failed to form LDH phase. CuAl formed mixed LDH like phase and ZnAl

formed NO3-LDH. Although ZnAl resulted in a single-phase nitrate containing LDH, the

yield was very low as compared to other LDHs and it was difficult to reproduce under

optimized conditions. Fig. 2.6 shows the PXRD patterns of ZnAl-LDHs synthesized at

different temperatures. Results showed that the final phase was highly sensitive to

temperature. Some results are not reproducible even after multiple attempts.


46

10 20 30 40 50 60 70

c

b

aIn

ten

sit

y,

C/s

2 Theta, deg

1000

Fig. 2.6 PXRD of a) ZnAl-80 oC, b) ZnAl-100

oC, c) ZnAl-80

oC repeated

Temperature and hexamine concentration variation studies were done to obtain MgAl-

NO3 LDH.

10 20 30 40 50 60 70

c

ba

Inte

ns

ity

, C

/s

2 Theta, deg

500

Fig. 2.7 PXRD of MgAl-LDHs a) MgAl-115 oC, b) MgAl-80

oC, c) MgAl-80

oC

prepared by taking double the concentration of hexamine

At higher temperature, we have obtained carbonate containing LDH phase along

with an unknown impurity phase. At the temperatures and hexamine concentrations

studied, this method was not a suitable one to prepare MgAl-NO3 LDH (Fig. 2.7).

Attempts were also made to synthesize samples with M(II)/Al ratios of 2, 3 and 4;

however, in all cases, the product had a ratio of about 2.0 (confirmed by ICP). The pH of

the reaction mixture was between 5 and 6.5 throughout the reaction and if the

temperature was increased slightly above 80 oC, the pH of the solution increased to more

than eight which resulted in the formation of carbonate-containing LDH.


47

10 20 30 40 50 60 70

b

aIn

ten

sit

y,

C/s

2 Theta, deg

300

Fig. 2.8 PXRD of a) CoAl-LDH-80 oC b) CoAl-LDH-90

oC

Fig. 2.8 clearly shows the temperature dependence. When the temperature was increased

to 90 oC, a highly crystalline carbonate containing LDH phase was obtained through

Leuckart reaction.

* Source of nitrate in LDH is from precursor only

Scheme 1 Hexamine hydrolysis mechanism

This prompts us to propose the following mechanism for hexamine hydrolysis:

once the temperature exceeds 80 oC, CO3

2- ions were formed through Leuckart reaction


48

leading to carbonate containing LDH; in contrast, if the temperature is less than 80 oC

nitrate-containing LDH (Scheme 1) is formed. Hexamine on hydrolysis yields ammonia

and formaldehyde/formic acid. Thus formed, ammonia increases the pH, resulting in the

precipitation of metal ions. The excess ammonia present in the system undergoes the

Leuckart reaction when the temperature is higher. This method may also be extended to

synthesize other LDHs that can be made between pH of 5 and 6.5. These synthesized

nitrates containing LDHs were characterized by FT-IR (Fig. 2.9).

4000 3500 3000 2500 2000 1500 1000 500

Tra

ns

mit

tan

ce

, %

c

b

a

Wavenumber, cm-1

Fig. 2.9 FT-IR spectra of a) CoAl-CO3 LDH, b) CoAl-NO3 LDH, c) NiAl-NO3 LDH

The broad band centered at 3450 cm-1

is due to O-H stretching mode of interlayer water

molecules and of hydroxyl groups. The absence of bands at 1360 and 790 cm-1

confirms

the absence of carbonate in the interlayer. The intense absorption band around 1384 cm-1

is attributed to the N-O stretching vibration of NO3- ions. It should be noted that it was

very difficult to synthesize completely nitrate containing LDH because both carbonate

and nitrate are present in the interlayers in the earlier work as evidenced by their FT-IR

spectra, while our samples showed only absorption due to nitrate [28]. Our methodology

has an advantage that it could also be scaled up for bulk synthesis of nitrate-containing

LDHs as our scale of synthesis was 35-70 times higher than in previously reported work

[67]. Total delamination study was done as reported earlier [30] to confirm the absence of

carbonate impurities; the resulting transparent colloidal suspension was centrifuged at

2000 rpm for 30 min, no residue was obtained which confirms the total delamination;

however when the colloidal suspension was centrifuged at 15000 rpm for 30 min, a glue

like material was seen (Fig. 2.10).


49

Fig. 2.10 Before and after centrifuge at 15000 rpm

ICP analysis of the glue-like residue (dried in a microwave oven) obtained after

centrifugation at 15000 rpm was done. No significant variation in the M(II)/Al ratio was

found between the parent sample and the centrifuged residue, which supports against the

possibility of dissolution of the LDHs and PXRD was also done for the obtained glue-like

residue.

5 10 15 20 25 30 35

5 10

III

cb

aInte

ns

ity

, C

/s

2 Theta, deg

50

Fig. 2.11 Total delamination of NiAl-NO3 LDH in formamide a) Delaminated LDH, b)

‘a’ after one day c) ‘a’ after 4 days (Inset is the expanded region for clarity)

The XRD pattern of this material showed no intense reflection at lower angles,

which corroborates the total delamination (Fig. 2.11). Broad reflection indicated by arrow

(I) at 2θ = 1.5-3.5o is due to aggregates of exfoliated nanosheets, 2θ = 7-15

o indicated by

arrow (II) is the crystallized LDH phase and broad reflection in the 2θ range 20-30o is

due to scattering of liquid formamide. Even though LDHs synthesized here are less


50

crystalline, their total delamination and excellent anion exchange capacity make these

materials promising.

Delamination is in general performed in high boiling and hazardous formamide.

However, the use of delaminated LDH sheets in multilayer film formation using LBL

self-assembly is most conveniently done under aqueous conditions. Hence, we have

studied the delamination of LDHs prepared by our method in water. Nitrate-containing

LDHs were delaminated successfully in water under optimized conditions. In previous

reports, delamination in water required a multistep organic intercalation procedure [26,

48]. We have achieved delamination by simple hydrothermal treatment of nitrate

containing LDHs synthesized through this novel route.

CoAl and NiAl LDHs

delaminated in formamide

CoAl and NiAl LDHs

delaminated in water

Fig. 2.12 Tyndall effect of delaminated LDH

Fig. 2.13 TEM images of a) CoAl-NO3 LDH dried and b) CoAl-LDH delaminated in

water


51

Dispersion degree of around 2.5 g/L was obtained which is sufficient for most

LBL assembly applications. It must however be mentioned here that a dried sample

prepared under vacuum failed to delaminate in water unlike the wet cake under similar

hydrothermal conditions. Delaminated transition metal containing LDHs in water showed

a Tyndall effect and are stable for up to eight months (Fig. 2.12). Fig. 2.13 shows TEM

images of the CoAl-NO3 LDH and CoAl-delaminated LDH in water. Similar sheet

morphology was retained even after delamination and hexagonal to circular platelets

ranging from 50-200 nm are seen. In general, carbonate and chloride containing LDHs

exhibit hexagonal platelets [68], the variation in the morphology might be due to

variation in the charge balancing anion or to the synthetic method adopted here. Such

water delaminated suspensions on microwave drying showed well-oriented nitrate

containing LDHs (2.45 GHz, 800 W, 8 min) (Fig. 2.14).

10 20 30 40 50 60 70

d

c

ba

Inte

ns

ity

, C

/s

2 Theta, deg

500

Fig. 2.14 PXRD of a) CoAl-NO3 LDH, b) NiAl-NO3 LDH, c) ‘a’ delaminated and

microwave dried, d) ‘b’ delaminated and microwave dried

Fig. 2.15 AFM images of CoAl-NO3 LDH delaminated in a) formamide and b) water


52

AFM analysis is the best method of characterizing these nano sheets. Fig. 2.15

shows the tapping mode AFM images of the CoAl-LDH delaminated in formamide and

water. Similar kinds of spherical and oblong objects with lateral dimensions of about

100-200 nm were seen irrespective of the medium and the dimensions are also

comparable with TEM results.

Fig. 2.16 a Cross sectional analysis of formamide delaminated LDH

Fig. 2.16 b Cross sectional analysis of water delaminated LDH

Cross sectional analysis of the images (Fig. 2.16 a, b) shows similar sheet

thickness dimensions ranging from 2-10 nm both in formamide and water, suggesting

that the medium does not affect the degree of exfoliation. To further understand the


53

restacking behavior, we centrifuged both water and formamide delaminated LDHs at

15000 rpm for 30 min. X-ray diffraction studies were done for these materials by placing

them on a glass plate in air at 25 oC and monitored at different times.

5 10 15 20 25 30 35

100

LDH

Water

cb

aInte

ns

ity

, C

/s

2 Theta, deg

Fig. 2.17 PXRD of water delaminated NiAl-LDH a) Immediately after centrifuging, b)

After 1 hour, c) After 3 days

10 20 30

50

LDH

Formamide

dc

ba

Inte

ns

ity

, C

/s

2 Theta, deg

Fig. 2.18 PXRD of formamide delaminated NiAl-LDH a) Immediately after centrifuging,

b) After 1 day, c) After 5 days, d) after 13 days

Water-delaminated LDH (Fig. 2.17) oriented within one hour and no further

variation in the crystallinity was observed on standing whereas formamide delaminated

LDH (Fig. 2.18) even after one day exposure in air showed no orientation. A weakly

crystalline LDH phase emerged after 5 days for formamide delaminated LDH whose

crystallinity marginally increased after 13 days of standing. Since water is less viscous

and low boiling than formamide, the mobility and re-organization of the delaminated

LDH nanoplatelets is better, which on drying leads to highly oriented LDHs. However,


54

formamide being viscous and high boiling might not facilitate such ordering of sheets,

resulting in poorly crystalline samples.

2.4 Conclusions

In summary, a two-step synthesis of delaminated transition metal containing LDH

was achieved using HMT hydrolysis wherein the temperature of hydrolysis was critical.

Total delamination was achieved in formamide confirming the absence of carbonate

impurities. AFM results show a similar extent of exfoliation irrespective of the medium;

on the contrary restacking behavior showed a strong dependence on the medium. Our

methodology may be applicable to other systems, such as ZnCr-Cl-LDH and CuCr-Cl-

LDH, which require low pH of precipitation (4.5-5.5).

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