Mesolamellar composite of TiN and CTAB using fluoride ion bridge: synthesis, mechanism & characterization

doi:10.4236/ns.2010.25057

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Vol.2, No.5, 464-468 (2010) Natural Science

http://dx.doi.org/10.4236/ns.2010.25057

Mesolamellar composite of TiN and CTAB using fluoride

ion bridge: synthesis, mechanism & characterization

Tumbavanam Venkateswaran Anuradha

Department of Materials Engineering, Indian Institute of Science, Bangalore, India; [email protected]

Received 29 December 2009; revised 10 February 2010; accepted 5 March 2010.

ABSTRACT

In an attempt to synthesize hexagonal mesopo-

rous titanium nitride, a mesolamellar composite

based on titanium nitride and cetyltrimethylam-

monium bromide (CTAB) is obtained by the

sol-gel route involving templating at 80oC. The

mechanism underlying the above synthesis is

discussed for the first time in the literature and

till date there are no reports on the synthesis of

mesoporous nitrides. The above mesolamellar

composite is found to form an oxide of titania

(anatase) upon heat treatment at 335oC for 1h.

Keywords: Mesolamellar; Templating; Sol-Gel

Method; XRD; FT-IR; SEM

1. INTRODUCTION

Mesoporous materials are generally produced by the

templating approach using various surfactant species as

template. The template acts like an imprint molecule

which could then be extracted by either calcination or

solvent extraction leaving behind pores in the regions

where they are present. At the first critical micelle con-

centration (cmc1), the surfactant species forms self-as-

sembled aggregates with a spherical shape which chan-

ges to a rod shape on further increase in the concentra-

tion of the surfactant species (cmc2). Different liquid

crystalline phases with cubic, hexagonal and lamellar

structures are formed by further increase in the concen-

tration of the surfactant species (above cmc2) which

indeed acts as the template for the production of meso-

porous materials.

There are a few reports based on the synthesis of me-

solamellar phases based on zirconia [1] and silica [2].

Certain species like F- ions and acetylacetone are found

to favour the formation of lamellar phases when present

in the reaction medium [3,4]. There are no reports on the

synthesis of mesoporous materials based on TiN due to

the non-availability of suitable metal-organic precursor

that could be used for the sol-gel synthesis. The precur-

sors of TiN are also found to be highly oxophilic and

hence there is always a possibility that they will result in

the formation of oxynitrides rather than pure nitrides.

The present experiments involved the formation of a me-

solamellar composite based on TiN and CTAB in the

medium enriched with F- ions by sol-gel route at 80oC.

The mechanism underlying the above synthesis is dis-

cussed in this communication in more detail for the first

time in the literature.

2. EXPERIMENTAL PROCEDURES

2 g of TiN, 2 g of CTAB, 25 cc of HF-HNO3 mixture

and 25 cc of distilled water were taken. TiN is dissolved

in 1:1 solution of HF-HNO3 mixture (9:1) in water

forming species like [Ti1-xFxNy]3n+ to which an aqueous

solution of CTAB is added to result in the yellow col-

ored solution which is kept in the oven for 3 h at 80oC

and then it is kept aside at RT for 24 h. The yellow solid

thus obtained has the composition [CTA] (H2O)n [Ti1-x

FxNy]. Thus a mesolamellar composite based on TiN and

CTAB is formed with the F- ion acting as the bridge be-

tween them. The lamellar composite obtained above is

characterized by x-ray diffraction (XRD) and FT-IR spe-

ctroscopy.

JEOL diffractometer (reflection type-model 8030) is

used for the characterization of the materials by x-ray

diffraction (XRD). Cu target is used. A voltage of 30 kV,

a current of 20 mA, and a step angle of 0.020o with de-

tection times of 0.5 sec at each step are used. A JEOL

2000FX-IItransmission electron microscope operating at

the accelerating voltage of 200 kV is used for TEM

studies. The powder samples are crushed thoroughly and

dispersed in a suitable organic solvent (acetone or high

purity ethanol/methanol) and subjected to ultrasonication

before supporting on the carbon-coated grids for loading

in the TEM equipment. FT-IR studies are carried out

using Perkin Elmer FT-IR equipment. Differential Scan-

ning Calorimetry (DSC) is performed with a Perkin

Elmer DSC-2C calorimeter to study the thermal stability

of the material. The measurements are carried out under

dynamic argon flow condition. Before starting the ex-

T. V. Anuradha / Natural Science 2 (2010) 464-468

465

periment the base line was corrected and the calibration

of the instrument is done using pure indium. A heating

rate of 20 K/min is used.

3. RESULTS & DISCUSSION

Porous materials i.e. materials with non-lamellar mesos-

tructures that are stable enough to be preserved after the

removal of the structure-directing amphiphile could only

be synthesized under aqueous conditions with the utili-

zation of cationic surfactants [5,6] and the other attempts

yielded lamellar phases only [7,8]. It is believed that a

larger variety of surfactants in the alcoholic medium

with a small quantity of water would result in the forma-

tion of non-lamellar materials [9]. But the utilisation of

alcoholic medium is avoided in this investigation since a

highly acidic medium is required for the dissolution of

titanium nitride. The only constraint in not making use

of the alcoholic medium for the synthesis is the

non-availability of a suitable organo-metallic precursor

for titanium nitride. There are continuous efforts to syn-

thesize mesoporous materials based on nitrides like sili-

con nitride for the catalytic applications [10,11] which

involves the ammonolysis of SiCl4 in organic solvent.

But the handling of halides of titanium like TiCl4 or the

organo-metallic precursor like (TiCl4L2) (where L =

ether, amine and pyridine) is very difficult at room tem-

perature since they are both air-sensitive and moisture -

sensitive.

Figure 1 shows the XRD of the mesolamellar com-

posite after templating TiN onto CTAB using F- ion as

the bridge at 80oC. The lamellar nature of the composite

is confirmed by XRD studies which is again reconfirmed

using SEM studies [12,13]. The lamellar nature is found

to be accompanied by atomic scale ordering and registry

of the layers [14] where the first peak has appeared at d

 19.4 Ao with the following peaks at d  {19.4/2 Ao,

19.4/3 Ao, 19.4/4 Ao). The clearly resolved six orders of

the diffraction peak as well as the plain baseline in the

20-30o 2 range suggest that the TiN layers sandwiched

between the surfactants are well crystallized as in the

case of lamellar TiO2 mesophase which was produced by

Lin et al. [15] using CTAB as the templating surfactant

for tetrabutylorthotitanate (precursor of TiO2) under ba-

sic conditions. This high degree of condensation of the

inorganic building units may be different from those of

the silicate [16,17] and aluminophosphate phase meso-

phases [18,19] whose frameworks are amorphous in

nature. The reflections present in the higher 2 region

are found to represent the internal structure involving the

organization of the lamellae and thus could not be in-

dexed to any impurities, but they could be indexed to an

orthorhombic cell with a reasonable justification and this

result compares very well with the mesocomposite of

zinc phosphate produced by Huo et al. [20].

Figure 2 gives the FT-IR spectra of CTAB and meso-

composite respectively. FT-IR studies of the composite

material showed many of the vibration modes like C-H&

C-N stretching and C-H bending similar to that of the

template i.e. CTAB which is used as standard for com-

parison, but the peaks had significantly different intensi-

ties and line widths in this case and there are some addi-

tional peaks which could be assumed to be due to the

transition metal co-ordination with the template. FT-IR

studies clearly indicated the formation of composite

where CTAB is intact. The peaks appeared at 3651 cm-1

and 3587 cm-1 corresponding to the O-H stretching fre-

quencies in the physisorbed water molecules and also the

peak at 1627 cm-1 corresponds to the bending mode as-

sociated with the physisorbed water molecules.

Differential Scanning Calorimetric (DSC) analysis in-

dicated the decomposition behaviour of the lamellar

Figure 1. XRD of the mesolamellar composite of TiN and

CTAB after templating at 80oC.

Figure 2. FT-IR spectrum of (a) CTAB; (b) mesolamellar

composite of TiN and CTAB.

T. V. Anuradha / Natural Science 2 (2010) 464-468

466

composite with three distinct stages of reaction. Figure

3 gives the ‘DSC’ trace of the mesolamellar composite

in argon atmosphere [12] The first split peak (endother-

mic) around 100oC is due to the desorption of water in-

dicating the presence of two different kinds of water

molecules associated with the mesocomposite structure

namely, water of hydration and associated (hydrogen

bonded) water. The second peak at 335oC, which is exo-

thermic in nature, indicates the decomposition of the

organic surfactant. But the appearance of the last two

peaks at 391oC and 500oC is not completely understood.

The XRD patterns of the mesocomposite after heating it

at 335oC, 391oC and 500oC for 1h, are shown in Figure

4. They indicated the formation of anatase form of nano-

crystalline titania (TiO2) at all the above temperatures

and average crystallite sizes less than 10 nm. Figure 5

gives the bright field TEM image of anatase after heat

treatment at 335oC for 1h, with its selected area electron

diffraction pattern and Figure 6 shows its corresponding

dark field TEM image. The dark field TEM image indi-

cated the presence of particles less than 10 nm though

agglomeration is also present.

Intercalated structure is indicated by TEM studies

with the average particle size less than 10 nm [11] which

in turn is composed of [Ti1-xFxNy]3n+and [CTA]+ with

the bridging F- ions. Hence it could be assumed that

the F- ion acts as bridge between positively charged

[Ti1-xFxNy]3n+ and [CTA]+ species. The bilayers of CTAB

between the layers of TiN are assumed to be interdigited

since it is believed that hydrated [Ti1-xFxNy]3n+ species is

large and thus interdigitation of the surfactant tails al-

lows the surfactant headgroups to be spaced well apart

which compares well with the earlier report on iron ox-

ide/surfactant composites [14] where the change in la-

mellar ‘d’ spacing with surfactant carbon chain length

for hydrated and dehydrated Fe(II) ions are studied.

Figure 7 demonstrates the mechanism underlying the

synthesis of lamellar mesophase based on TiN which is

found to follow the counter ion (here F- ion) mediated

pathway in Liquid Crystal Templating (LCT) approach.

Titania is found to be stable in rutile form when the par-

ticle size exceeds 14 nm and below this critical size,

anatase phase is stable [21]. Titania obtained by the heat

treatment of the mesolamellar composite of titanium

nitride fluoride and CTAB at 335oC for 1h is found to be

anatase with the average particle size less than ~10 nm.

4. CONCLUSIONS

There are no reports on the synthesis of mesostructured

nitrides till date in the literature and thus it is most re-

markable to produce the nanostructured mesolamellar

composite of titanium nitride fluoride and CTAB with an

average particle size less than ~ 10 nm by templating

TiN onto CTAB in the highly acidic medium enriched

with F- ions (pH ~ 2) at 80oC. The interlamellar spacing

is found to be ~ 19 Ao. The mechanism underlying the

above synthesis is also discussed for the first time where

F- ions act as the bridge between positively charged

[Ti1-x Fx Ny]3n+ and [CTA]+ species. The bilayers of

CTAB between the layers of TiN are interdigited since

that hydrated [Ti1-x Fx Ny]3n+ species are large and thus

Figure 3. DSC trace of the mesolamellar composite of TiN and

CTAB.

Figure 4. XRD pattern of the mesocomposite of TiN and

CTAB, after heat treatment at (a) 335oC; (b) 391oC; (c) 500oC,

for 1 h.

Figure 5. (a) The bright field TEM image of TiO2 (anatase)

obtained from the mesolamellar composite after heat treatment

at 335 oC for 1 h, and (b) its corresponding selected area elec-

tron diffraction pattern.

T. V. Anuradha / Natural Science 2 (2010) 464-468

467

Figure 6. The dark field TEM image of TiO2

(anatase) obtained from the mesolamellar com-

posite after heat treatment at 335 oC for 1 h.

Figure 7. The mechanism underlying the syn-

thesis of mesolamellar composite of titanium

nitride and CTAB in the medium enriched

with F- ions.

interdigitation of the surfactant tails allows the surfactant

headgroups to be spaced well apart. The above lamellar

composite has resulted in the formation of anatase form

of titania upon heat treatment at 335oC for 1h with the

average particle size less than 10 nm.

5. ACKNOWLEDGEMENTS

The support of the Council for Scientific & Industrial Research (India)

to T. V. Anuradha through a Senior Research Fellowship is gratefully

acknowledged.

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