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Synthesis of Nanoparticles of CuI, CuCrO4, andCuS in Water/AOT/Cyclohexanone and Water/TX‐100 + i‐Propanol/Cyclohexanone ReverseMicroemulsionsSoma Biswas a , Samik Kumar Hait a , Subhash Chandra Bhattacharya a & Satya PriyaMoulik aa Department of Chemistry, Centre for Surface Science , Jadavpur University , Kolkata,700 032, IndiaPublished online: 17 Mar 2008.

To cite this article: Soma Biswas , Samik Kumar Hait , Subhash Chandra Bhattacharya & Satya Priya Moulik (2005)Synthesis of Nanoparticles of CuI, CuCrO4, and CuS in Water/AOT/Cyclohexanone and Water/TX‐100 + i‐Propanol/Cyclohexanone Reverse Microemulsions, Journal of Dispersion Science and Technology, 25:6, 801-816, DOI: 10.1081/DIS-200035591

To link to this article: http://dx.doi.org/10.1081/DIS-200035591

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Synthesis of Nanoparticles of CuI, CuCrO4, and CuS in Water/AOT/Cyclohexanone and Water/TX-1001 i-Propanol/Cyclohexanone

Reverse Microemulsions

Soma Biswas, Samik Kumar Hait, Subhash Chandra Bhattacharya, and Satya Priya Moulik*

Department of Chemistry, Centre for Surface Science, Jadavpur University, Kolkata, India

ABSTRACT

The phase behaviors of two potential ternary and quaternary microemulsion forming

systems (water/AOT/cyclohexanone (Cy) and water/TX-100/i-propanol/Cy) have

been studied. The nanoparticles of copper iodide, CuI; copper chromate, CuCrO4;

and copper sulfide, CuS have been synthesized in these microemulsion media at

different [water]/[amphiphile] mole ratio. The formed nanoparticles of the studied

copper salts have been characterized by the UV-visible, fluorescence and Fourier

transform infrared (FTIR) spectroscopy as well as by the dynamic light scattering

(DLS), X-ray diffraction (XRD), and transmission electron microscopic (TEM)

methods. From the absorption spectra of the colloidal dispersions of the salts CuI,

CuCrO4, and CuS, their band gap values have been evaluated. The rate of growth of

the insoluble copper salts in the nanowater compartments of the quaternary

microemulsion has been followed by the DLS method.

Key Words: Microemulsion; Ternary; Quaternary; Nanoparticles.

INTRODUCTION

The synthesis of nanoparticles using water/oilmicroemulsion medium is a potential preparative

route.[1,2] The method is simple and soft; no extreme con-

dition such as high temperature or pressure[3] or a special

arrangement is required. In practice, either two w/omicroemulsions of similar composition with the reacting

components separately taken in the nanodomain of

them[4–6] are mixed together or, a component taken

in the microemulsion is treated with the solution of a

reacting component from outside[7] to yield the desired

product in the dispersed phase. These dispersed particles

can then be characterized using physical techniques,[8,9]

and isolated[5,6] following a simple procedure. They

can be used as specialized products like catalyst,[7]

801

DOI: 10.1081/DIS-200035591 0193-2691 (Print); 1532-2351 (Online)

Copyright # 2004 by Marcel Dekker, Inc. www.dekker.com

*Correspondence: Satya Priya Moulik, Department of Chemistry, Centre for Surface Science, Jadavpur University, Kolkata 700 032,

India; Fax: 91-33-414-6266; E-mail: spmcss@yahoo.com.

JOURNAL OF DISPERSION SCIENCE AND TECHNOLOGY

Vol. 25, No. 6, pp. 801–816, 2004

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semiconductor,[8,9] magnetic material,[10] optoelectronic

device material,[11] laser,[12] resonant tunneling devices

(RTD),[13] material for low porosity filter,[14] etc.

According to literature report,[8] the nanomaterials of

oxides, sulfides, silicates, chromates, etc. prepared by

the microemulsion route have narrow size distribution

and are thus advantageous; the formation of mono-

disperse particles may also result.

It is quite understandable that to achieve the above

goal, a stable w/o microemulsion with large monophasic

zone is advantageous,[2,15] and the stability requirement

of the system with respect to temperature and salt

environment should be high.[2] The search for a con-

venient system or systems is thus an essential aspect of

nanoparticle synthesis in microemulsion medium. The

ternary and quaternary systems involving Triton X-100

(p-tert-octyl phenoxy polyoxy ethylene (9.5) ether),[6]

cetyl trimethyl ammonium bromide (CIAB),[15] AOT

[Na-bis(2-ethyl hexyl) sulfosuccinate],[1,4,5] etc. as sur-

factants; butanol, pentanol, isopropanol,[6,16,17] etc. as

cosurfactants and hydrocarbon oils (heptane, octane,

isooctane, decane, etc.)[4 –6] are often used for the

preparation of microemulsions. Although a surfactant in

combination with a cosurfactant works much better in the

formation of microemulsion, the versatile surfactant

AOT can achieve this goal[1,18] without a cosurfactant

and can make the system simpler reducing a four com-

ponent (water/surfactant/cosurfactant/oil) system into

a three component (water/surfactant/oil) one.Although the microemulsion route of preparation

of nanoparticles has good potential, so far the method

has been moderately used.[1,2,7,15,19] We have recently

reported convenient way of synthesis and characteriza-

tion of copper ferrocyanide,[4] lead chromate[5] in

water/AOT/heptane, and tungstic acid[6] in water/TX-100/alkanol/heptane w/o microemulsion media.

In this paper, we have reported the preparation and

characterization of nanoparticles of CuI and CuS in

quaternary w/o microemulsion system of water/TX-100/i-propanol/cyclohexanone (Cy), and that of CuCrO4 in

the quaternary and ternary (water/AOT/Cy) w/o micro-

emulsions. We have used cyclohexanone as the oil,

which has not been used earlier. It is economical than

hydrocarbon and other oils, and can form a very large

monophasic microemulsion zone. The phase behaviors

of the above mentioned ternary and quaternary systems

have been studied together with the synthesis of CuI,

CuS, and CuCrO4 in them at different [water]/[amphiphile]

mole ratio. The formed products have been characterized

by the Fourier transform infrared (FTIR), x-ray diffrac-

tion (XRD), absorption and emission spectroscopy,

dynamic light scattering (DLS) and transmission electron

microscopic (TEM) techniques. The compound CuI is

used as catalyst,[20] pesticide and industrial disinfectant;

CuS is used as lubricant additives and pigments,[16,21,22]

and CuCrO4 has applications as paint, pigment, and

preservative.[23] For the above mentioned specific uses

and applications, the materials in fine dispersion is

advantageous, and hence their synthesis in nanosized

scale is worthy of investigation.

EXPERIMENTAL

Materials

The surfactants, TX-100 [p-tert-octyl phenoxy

polyoxy ethylene(9.5) ether], and 99% pure AOT

[sodium bis(2-ethyl hexyl)-sulfosuccinate] were purchased

from Sigma, USA. The oil Cy used was of E Merck,

Germany. Copper sulfate, potassium iodide, potassium

chromate, and sodium sulfide were AR grade products

of BDH, UK. The cosurfactant, isopropyl alcohol

(i-PrOH) was obtained fromMerck, India. It was distilled

following standard procedure for purification. Doubly

distilled conductivity water was used in all preparation.

Methods

Preparation of Quaternary Microemulsion

The quaternary microemulsion was prepared by

titrating either the mixture of Cy, TX-100, and i-PrOH

with water or the mixture of water, TX-100, and

i-PrOH with Cy. For both the procedures, TX-100 and

i-PrOH, were taken at a constant mass ratio. The

mixture of TX-100 and i-PrOH was dissolved in Cy or

in water, as the case may be, in varied amounts taken

in different stoppered test tubes to form solutions of

different concentrations. They were placed in a thermo-

stated water bath and stirred until at the equilibrium

temperature clear solutions were obtained. The samples

prepared in oil were titrated with doubly distilled

water, and those prepared in water were titrated with

Cy from a microburet at a constant temperature until

the solutions became just turbid indicating phase separa-

tion. The mixtures were stirred well for a sufficient length

of time to detect the stable end point monitored visually

against a dark back ground by illuminating the sample

with light. The effect of concentration of salts (KI,

K2CrO4, CuSO4, and Na2S) on the phase behaviors was

also studied. The obtained data points in the phase

formation experiments were converted into wt% of the

components and plotted as pseudoternary and tetrahedral

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phase diagrams. In the phase study, 1 : 2, 1 : 1, and 2 : 1

mass ratios of TX-100 and i-PrOH were used.

Preparation of Ternary Microemulsion

The ternary microemulsion was also prepared

following the same procedure as earlier. Here, either

the solution of AOT in Cy was titrated with water or

AOT in water was titrated with Cy. No cosurfactant

was needed for the preparation of microemulsion as

double tailed AOT can conveniently stabilize the w/osystem. The results in mass% were used to construct

the triangular phase diagram.

Preparation of Nanoparticles

Colloidal nanoparticles of CuI were prepared in

water/TX-100þ i-PrOH (1 : 1w/w)/Cy microemulsion

medium by the method of mixing. Two w/o microemul-

sions, one containing CuSO4 (0.5mmol dm23) and

another containing potassium iodide (1mmol dm23) at

different [water]/([TX-100]þ [i-PrOH]) mole ratio (v)

from 2 to 20 were prepared. The CuSO4 containing

microemulsion was maintained slightly acidic with

glacial acetic acid to avoid the formation of Cu(OH)2.

After thermal equilibration, KI containing micro-

emulsion was added dropwise to CuSO4 containing

microemulsion under constant stirring condition. The

concentrations of the reacting components were main-

tained at 1 : 2 molar ratio of CuSO4 : KI, so that the

formation of CuI was according to the reaction,

2CuSO4þ 4KI ¼ 2CuI# þ I2þ 2K2SO4. The final con-

centration of copper iodide formed was maintained at

0.25mmol dm23.

Following the above protocol, CuCrO4 nanoparticles

were also prepared by the reaction, CuSO4þK2CrO4 ¼

CuCrO4#þ K2SO4 using [salt] of 0.5mmol dm23

in the microemulsion both quaternary, H2O/TX-100þi-PrOH (1 : 1w/w)/Cy and ternary, H2O/AOT/Cy.The concentration of AOT in the ternary microemulsion

was maintained constant at 0.25mol dm23. The

[CuCrO4] was also maintained at 0.25mmol dm23,

whereas the v was varied from 2 to 30. This concen-

tration of the nanoparticle was found to be optimum

for the microemulsion stability at all the v values.

The CuS nanoparticles were prepared in quaternary

microemulsion medium using the aforesaid mixing pro-

cedure. The reacting components were taken in 1 : 1mol

ratio so that after mixing, the concentration of the final

product in the sample became 0.25mmol dm23. The

product, CuS formed by the reaction, CuSO4þNa2S ¼

CuS#þNa2SO4, was brown in color, and was obtained

at v values ranging between 2 and 14.

The 0.25mmol dm23 concentration of all the

synthesized nanoparticles was found to be adequate for

absorbance and fluorescence measurements. The nuclea-

tion of the molecularly formed products started imme-

diately after mixing. Owing to high exchange dynamics

of the dispersed water droplets, the product formation

was instantaneous and the particle growth was fairly

quick. The system was sensitive to temperature. With

increasing temperature, the solution became turbid indi-

cating phase separation (by way of clouding of TX-100)

which became clear on cooling; the process was rever-

sible. The phase behavior and other properties of this

microemulsion system were studied at 296K.

The solution properties of the synthesized nanoparti-

cles in the ternary and quaternary microemulsion systems

were studied after aging the preparations for 2–3 days.

The TEM measurements were done 2 weeks after their

preparation.

Isolation of Nanoparticles from Microemulsion

The colloidal nanoparticles containing w/o micro-

emulsion were added to excess acetone with stirring;

the system got destabilized. The mixture was then centri-

fuged for 5min at 5000 rpm, the solid particles settled

down at the bottom. The supernatant clear solution was

then decanted, and the particles were washed several

times with butanol and then with acetone to remove

other components. The thoroughly washed solid particles

were stored under acetone. For CuI containing nanopar-

ticles care was taken to avoid contamination of air, so

that the white salt of Cuþ could not be oxidized to the

bluish white Cuþ2 salt.

Optical Characterization

Spectrophotometry

The absorption spectra of the prepared samples were

recorded with a Shimadzu UV-visible 160A spectro-

photometer, Japan, in the wave length range of 200–

1100 nm for all the systems at 296K+ 0.1, using a

pair of matched quartz cuvettes of optical path length

of 1.0 cm.

Spectrofluorimetry

The fluorescence spectra were taken in a Fluorolog

F-IIIA spectrofluorimeter (Spex) with a slit width of

2.5 nm at 296K. The solutions were excited at 360 nm

for CuS in quaternary microemulsion systems.

Ternary and Quaternary Microemulsion Forming Systems 803

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Dynamic Light Scattering

The DLS measurements were taken in a DLS

spectrophotometer, DLS-700 of Otsuka Electronics,

Japan, accessed with a 5mW He–Ne laser operated at

632.8 nm. All measurements were taken at 908 angle.

Samples were filtered several times through 0.22mm

Millipore filters. The temperature of measurements was

controlled by a Neslab RTE-100 circulating water bath

of +0.18 accuracy. The collected DLS intensity data

were processed with necessary software to obtain hydro-

dynamic diameter (dh), diffusion coefficient (D), and

the polydispersity index (PDI; the ratio of the standard

deviation in dh to the average dh of the prepared colloidal

nanoparticles in the microemulsion medium). A PDI

value greater than 0.1 denotes polydisperse system.

Essentially, the instrument measured the diffusion coeffi-

cient (D) of the dispersed droplets taken to be spherical

and the average diameter (dh) was estimated in terms

of the Stokes–Einstein equation,[4]

dh ¼kT

3phDð1Þ

where dh, h, k, and T are the hydrodynamic diameter

of the droplet, the viscosity coefficient of the medium,

the Boltzmann constant, and the absolute temperature,

respectively. Assuming the nanodroplets are polydis-

perse solid spheres with high scattering power, the

average diameter kdl was obtained from the relation,[24]

dh ¼ f1þ 5ðPDIÞ2gkdl ð2Þ

The effect of variation of v and temperature on the size

parameters of the formed nanodispersions of both bare

and nanoparticle loaded microemulsions were examined.

It is evident from Eq. (2) that at low PDI (0.1 when the

system is taken as monodisperse), dh � kdl. Giving

weightage to the PDI, it is convenient to present data

and its analysis in terms of kdl rather than dh. In the

present report, the kdl values have been used for analysis

and comparison, because the PDI values were mostly

greater than 0.1.

Structural Characterization

FTIR Spectra

Fourier transform infrared (FTIR) spectroscopic

measurements were taken in Perkin–Elmer (RXI)

Spectrophotometer. Samples of isolated particles of pre-

pared nanoparticles were mixed with KBr, homogenized

and converted into pellets under a pressure of 8 ton and

the spectra (% transmittance with wave number) were

taken thereafter. The FTIR spectra of the prepared nano-

particles in water and in microemulsion medium were

then compared for equivalence.

XRD Measurements

The x-ray powder diffraction measurements were

taken in a Philips PW 1710 diffractometer using nickel

filtered Cu-Ka radiation at 1.54 A. The resultant intensity

data were processed using an in-built PC-APD diffraction

software to monitor the peak position and its corres-

ponding intensity data correctly. The samples were placed

on a glass slide and the measurements were taken con-

tinuously from 108 to 708 angles at 0.028 interval.

TEM Measurements

TEM measurements of the samples were taken in

a JEOL JEM 100 CX TEM with a 60 kV accelerating

voltage. The dispersions of CuI and CuS prepared in

quaternary w/o microemulsion and CuCrO4 in both

quaternary and ternary microemulsions were placed on

carbon-coated 400 mesh copper grids, allowed to dry

at room temperature before taking measurements. The

obtained micrographs were then examined for particle

shape and size.

RESULTS AND DISCUSSION

Phase Behaviors of Quaternary System

The pseudoternary phase diagrams of the system,

water/(TX-100þ i-PrOH)/Cy at TX-100 : i-PrOH ratios

of 1 : 2, 1 : 1, and 2 : 1 (w/w) at 296 K are presented in

Fig. 1. Of the three, the ratio 1 : 1 (A) has shown a mod-

erate difference from the other two towards the oil

corner. The area of the single-phase zone for this ratio

is to some extent lower than that for both 1 : 2 and 2 : 1.

All the isotropic monophasic microemulsion zones

extend over a wide range in the diagram. The tetrahedral

representation of the 1 : 1 TX-100 : i-PrOH derived

system is illustrated in Fig. 2(A). The interior of the

prolate-shaped caged area represents the immiscible or

the heterogeneous zone, whereas its exterior regions

are monophasic. The triangular face of the tetrahedron

demarcated by Cy– i-PrOH–TX-100 represents a total

miscibility region. The major portions of the other

faces are monophasic, the minor biphasic regions are

separated by the surface of the prolate-cage. Other

fascinating tetrahedral representations of quaternary

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microemulsion systems employing plant and vegetable

oils have been recently reported by us.[25–27] The

quaternary microemulsion system was fairly temperature

sensitive. The compositions with v ¼ 8, 10, 12, 14, and

20 were stable up to temperatures 498C, 408C, 37.58C,

358C, and 308C, respectively.

Salt Effect on Phase Behavior

The effect of the nanoparticle forming salts CuSO4,

K2CrO4, and KI on the phase behaviors of the quaternary

system at 1 : 1 (w/w) TX-100 : i-PrOH ratio was

examined. Three compositions in the single-phase zone

identified as 1, 2, and 3 in the phase diagram [Fig. 1(B)]

were selected and three concentrations 5, 50, and

100mmol dm23 of each salt were used. To the mixed

system of TX-100, i-PrOH, and Cy corresponding to the

compositions 1, 2, and 3, aqueous salt solutions of above

mentioned concentrations were added until phase separ-

ation occurred. These points are plotted in Fig. 1(B–D)

for 5, 50, and 100mmol dm23 salt solutions, respect-

ively. In the diagram, the arrowheads indicate the direc-

tion of increasing concentration of the salts for the

compositions (1, 2, and 3) indicated by the symbols

given in the legend. These points represent the boundary

compositions between the mono- and biphasic states. It

was observed that for CuSO4, the threshold phase separ-

ating compositions crowded towards the left of the

diagram reasonably above the boundary obtained

without salt. In the case of K2 CrO4, the phase demarcat-

ing points for the composition 1 grazed towards left with

increasing concentration along the boundary obtained in

the absence of salt. Those for the other two compositions,

2 and 3, were closely similar to that obtained with CuSO4.

Figure 1. Pseudoternary phase diagram of water/TX-100/i-PrOH/cyclohexanone at 2 : 1, 1 : 1, and 1 : 2 (w/w) TX-100 : i-PrOHratios with and without salts at 296K. 1F, monophasic; 2F, biphasic. The Apex values of 1 refer to 100% of the components. Studied

composition for salt effect. (A) without salt. TX 100þ i-PrOH as 2 : 1 (B), TX 100þ i-PrOH as 1 : 1 (W), and TX 100þ i-PrOH as

1 : 2 (O). (B) CuSO4, (C) K2CrO4, (D) KI. Each of them was taken at TX 100þ i-PrOH as 1 : 1. Compositions of 1: Wa/TX 100þ

i-PrOH/Cy as 47.44/24.23/28.33; 2: Wa/TX 100þ i-PrOH/Cy as 44.04/24.79/31.16 and 3: Wa/TX 100þ i-PrOH/Cy as

19.72/26.64/53.64. [Salt]: 0mmol dm23 (A); 5mmol dm23 (W); 50mmol dm23 (O); and 100mmol dm23 (r).

Ternary and Quaternary Microemulsion Forming Systems 805

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The salt KI generated boundary points similar to those

observed without KI. The points (irrespective of the com-

positions 1, 2, and 3) shifted to the left with increasing

concentration of KI as indicated by the arrowheads. The

effect of the salt Na2S on the phase behavior of the qua-

ternary microemulsion system (herein studied) was not

separately examined. It was observed that under the exper-

imental protocol, the presence of Na2S did not bring any

visible change.

The stability of the synthesized nanoparticles

depended on their overall concentration. For a stable for-

mulation of the studied water/amphiphile at different v,

a concentration of 0.25mmol dm23 of all the three

products was found to be suitable. Thus, the effects of

salts on the changing phase behavior of the quaternary

system were not significant at the studied conditions

for the synthesis of nanoparticles.

Phase Behavior of Ternary System

The ternary phase diagram of water/AOT/Cysystem at 303 K is represented in Fig. 2(B) wherein the

zone inside the boundary is biphasic and the region

outside the boundary is homogeneous or clear. The

87% area of the triangle is monophasic whereas only

13% is biphasic. Compared with the quaternary system

of water/TX-100/i-PrOH/Cy, the ternary system per-

formed better, and was expected to be convenient for

synthesizing nanoparticles. Such a wide monophasic

zone is very seldom achieved in practice with other

types of oil. It would be then interesting to study the

phase behaviors of different ternary and quaternary

microemulsion forming systems using Cy as the oil,

and examine their prospects for general and specific

applications. Such systems would be economically

viable. The maximum concentration of Cu2Fe(CN)6and PbCrO4 prepared in water/AOT/heptane micro-

emulsion was reported[4,5] to be 0.025mmol dm23. The

present ternary microemulsion system using Cy for

heptane as the oil has shown much greater product

holding capability (0.25mmol dm23); thus, Cy has a

good prospect for the synthesis of nanoparticles by the

microemulsion route. As the ternary system (water/AOT/Cy) was sparingly used in this study, the effects

of the salts on the phase behavior of the system were

not studied.

Optical Characterization

Absorption Spectra

The dispersed or colloidal solutions of CuI, CuCrO4,

and CuS have been subjected to UV-visible spectral

study. The results are illustrated in Fig. 3(A–C). All the

curves follow a regular sequence with respect to v

([water]/[amphiphile]). The colloidal CuI [Fig. 3(A)] in

the quaternary microemulsion medium has exhibited

two absorption peaks around 346 nm and 975 nm; the

absorbance of the first increased with decreasing v,

i.e., the water pool size with a small blue shift from

348 to 346 nm. A similar trend in the absorbance has

been reported earlier for colloidal solution of Cu2Fe(CN)6prepared in water/AOT/n-heptane microemulsion.[4]

In the aqueous medium, excess KI solubilized CuI

Figure 2. (A) Tetrahedral representation of the phase diagram

of Wa/TX-100/i-PrOH/Cy system at 1 : 1 (w/w) TX-100 :

i-PrOH ratio at 296K. The interior of the prolate cage with

major axis along the direction of Wa and Cy is biphasic; the

outside boundaries are monophasic. (B) Ternary phase diagram

of Wa/AOT/Cy at 303K. 1F, monophasic; 2F, biphasic.

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forming CuI432; two peaks at 285 and 348 nm then

appeared. The peak at 285 nm due to CuI432 intensified

whereas the latter at 348 nm for I2 got reduced with

increasing presence of KI. In the microemulsion

medium, the peak corresponding to CuI432 ion was

absent owing to stoichiometric reaction of CuSO4 with

KI. The 346 nm peak corresponds to I2 as in aqueous

and also in microemulsion medium KI gives peak at

the same position. The 975 nm peak corresponds to

Cu2þ (since CuSO4 containing microemulsion has also

shown this peak). With the decrease in v, enhancement

in the absorbance of colloidal CuI has been observed

with a blue shift due to size quantization effect.[8,28,29]

The lowering in size (also supported by TEM) has

caused a widening of the HOMO–LUMO gap.

The pure K2CrO4 in water as well as in w/o micro-

emulsion has produced two absorption bands at 275 and

371 nm. The CuSO4 does not have a characteristic

absorption band in this region. Therefore, the 362 nm

peak [Fig. 3(B)] in the ternary microemulsion system

corresponds to the formed colloidal CuCrO4 dispersion.

Here also the absorbance has increased with decreasing

v with a blue shift of lmax from 364 to 362 nm, and

363 to 346 nm in the quaternary and ternary systems,

respectively. In both the media, the absorbance of the

peak at 976 nm has also increased with decreasing v

with an associated blue shift from 976 to 965 nm. The

same trend has been also observed for the colloidal

CuS solution in the quaternary microemulsion rep-

resented in Fig. 3(C). The blue shifts of lmax from 360

to 346 nm as well as from 983 to 977 nm, respectively,

associated with increase in absorbance with decreasing

v or particle size were similar to the observations

found for the CuI and CuCrO4 systems. The size quanti-

zation effect has been thus found for all the three syn-

thesized nanoparticle systems. The effect arises for

colloidal particles having sizes smaller to the natural

length scale of the exciton or the Bohr exciton radius

(electron–hole pair), i.e., confinement of the electron

and the hole in two separate spheres than the confinement

of the electron–hole pair in a single sphere as in bulk

semiconductor with the quantization of the energy

levels. The confinement induces quantization of the bulk

electronic bands such that crystallites have discrete

electronic transitions that shift to the higher energies

with decreasing size. With decreasing water pool size,

Figure 3. (A) Absorption spectra of 0.25mmol dm23 CuI prepared in quaternary w/o microemulsion system at v values 2, 4, 6, 8,

10, 12, 14, and 20 at 296K. (B) Absorption spectra of 0.25mmol dm23 CuCrO4 prepared in ternary w/o microemulsion system at v

values 2, 4, 6, 8, 10, 12, 14,16, 18, 20, 25, and 30 at 303K. (C) Absorption spectra of 0.25mmol dm23 CuS prepared in quaternary w/omicroemulsion system at v values 2, 4, 6, 8, 10, 12, 14, and 20 at 296K. Inset of (A), (B), and (C) show absorption spectra of the

nanodispersions of higher wavelengths (850–1100 nm).

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as effective mass of electron decreases owing to the for-

mation of hole in a crystal lattice, the energy difference

increases and as a consequence l decreases resulting in

a blue shift. Therefore, the length of the exciton diameter

depends on the extent of the electron delocalization and

on the effective mass of the charge carrier. The spectra of

the CuI, CuCrO4, and CuS have evidenced the quantum

confinement effect related to the formation of nanometer

size particles. These visible spectral data around the

absorbance maximum have been processed in terms of

Eq. (3) to obtain the value of the band gap of the colloidal

nanoparticles encapsulated in the w/o microemulsion

medium.[8,9]

ð1hnÞ2 ¼ Cðhn� 1gÞ ð3Þ

where hn is the photon energy, 1 is the molar extinction

coefficient, 1g is the band gap of the prepared nanoparti-

cles, and C is a constant.

The graphical determination of the band gap for

the CuI nanoparticles at different v is shown in Fig. 4

as a representative illustration. From the plot between

(1h)n2 and hn, the 1g has been obtained from the intercept

on the X-axis at (1hn)2 ¼ 0. A tendency of curvature in

the plot has been observed for lower v. The intercepts

of the initial linear portion of the plots have been used

for the estimation of 1g (inset Fig. 4). The band gap is

the photon energy. It has been observed that the deter-

mined band gap values of CuI, CuCrO4, and CuS nanopar-

ticles have hardly depended on their particle size obtained

in the range of v ¼ 2–30. The 1g values for CuCrO4

are the averages of the results obtained in the ternary

and quaternary systems: the two w/o microemulsion

systems have not evidenced any sizeable difference

in the 1g values. The 1g values determined were 1.20,

1.20, and 1.22 eV for CuI, CuS, and CuCrO4, respectively.

Following the same procedure of 1g value 3.23 eV for

lead chromate has been recently reported by Moulik

et al.[5] The 1g values of 1.11, 0.6, and 0.5 eV for

CuI, CuS, and CuCrO4, respectively, have been reported

in literature.[30] The microemulsion derived materials

have higher magnitudes of 1g particularly for CuS and

CuCrO4.

Fluorescence Spectra

The colloidal dispersions in w/o microemulsion

medium have exhibited emissions with characteristic

excitonic bands at 418 nm for CuI (in quaternary system),

420 nm for CuCrO4 (in quaternary system), 425 nm for

CuCrO4 (in ternary systems), and 432 nm for CuS (in

quaternary system) when excited at 346, 362, 360, and

360 nm, respectively. The emission intensity of all the

colloidal materials in the quaternary microemulsion

system has decreased with increasing v [Fig. 5(A–C)].

The absorption of light by the molecular aggregates has

decreased (Fig. 3) by way of overlap of their energy

levels making the electronic transition unfavorable with

a consequence of decrease in the emission intensity

with increasing v, which is attributed to the size quanti-

zation effect.

For CuCrO4 in the ternary microemulsion medium,

the emission intensity has initially decreased up to

v ¼ 8, then it has increased. An explanation is not at

hand for this disorderly observation.

The fluorescence spectra of CuI and CuCrO4 were

structured. The peaks were broad and there were blue

shifts with decreasing v, i.e., with decreasing particle

size. The broad peaks were due to the overlap of

normal emission with the exciton emission from the

trap states arising from defects in the crystals. Such

effects for Cd3P2 and CdTe have been reported in litera-

ture.[31] Better spectral manifestation of the trap states

can be found taking emission measurements at low

temperature (liquid N2 temperature) reported by Kapito-

nov et al.[32,33] This was not done in the present study for

want of facility. The trapped states may origin from

defects[34] on the surface or in the volume. Lower sized

particles have large surface to volume ratio; the prob-

ability of the trapped states from surface defects is

expected to be higher. In sufficiently small semiconduc-

tor nanoparticles, exciton self-trapping can also arise.[42]

In the present study, the structure in the spectra (Fig. 5)

decreased with increasing particle size or v evidencing

the fair contribution of surface defects to the emission

process. The resultant emission behavior (including the

Figure 4. Plot of (1hn)2 vs. hn for determining the band gaps

of colloidal CuI particles prepared in quaternary w/o micro-

emulsion at v values: 2 (B), 4 (W), 6 (O), 8 (r), 10 (A), 12

(†), 14 (4), and 20 (P). Inset shows the two extreme cases,

v ¼ 2 and 20.

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trapped contribution) followed the order CuI ,

CuCrO4 , CuS. More intimate study on the emission

characteristics of the synthesized nanoparticles is

required for a better understanding of their interaction

with photon energy.

Dynamic Light Scattering

DLS is a potential method for measuring the par-

ticle size, its translational diffusion coefficient, and

polydispersity of the particles when examined at the

state of dilute dispersion. When a coherent beam of

light (as in a laser) interacts with colloidal particles,

the fluctuation of the intensity of light due to Brownian

motion of the particle takes place, and the intensity

correlation function provides information on the trans-

lational diffusion coefficient (D) of the scattering

particles and hence the average hydrodynamic diameter

(kd l) according to Eqs. (1) and (2).

The temperature induced average diameter, kdl and

the diffusion coefficient, D of bare quaternary w/omicroemulsion obtained from DLS measurements are

presented in Table 1. The kdl and D values have

increased and decreased, respectively, with v at a

constant temperature. The temperature has shown an

increasing effect on kdl and consequently a decreasing

effect on D.

At a constant [amphiphile] with increasing v,

the interface has become continuously sparsed with the

stabilizer amphiphile, thereby its surface density has

decreased making the droplets unstable and coalesce to

enlarge by way of fusion. The growth of the particles

in microemulsion has been considered to be due to inter-

droplet mass exchange and aggregation. Lower inter-

facial density of the amphiphile, collision, and effective

interparticle contact has led to the growth of the

droplet dimension with increasing v.

Figure 5. (A) Fluorescence spectra of 0.25mmol dm23 CuI

prepared in quaternary w/o microemulsion system at v values

2, 4, 6, 8, 10, 12, 14, and 20. (B) Fluorescence spectra of

0.25mmol dm23 CuCrO4 prepared in quaternary w/o micro-

emulsion system at v values 2, 4, 6, 8, 10, 12, 14, and 20.

(C) Fluorescence spectra of 0.25mmol dm23 CuS prepared in

quaternary w/o microemulsion system at v values 2, 4, 6, 8,

10, 12, 14, and 20.

Table 1. DLS derived temperature dependent average hydro-

dynamic diameter (kdl) of bare quaternary w/o microemulsion

at 296K.

v Temperature kdl (nm)

D � 107

(cm2 sec21)

8 38.0 3.6 19.1

48.4 5.5 13.0

50.7 10.2 7.0

10 27.1 2.5 26.3

30.1 2.9 23.4

32.9 4.2 16.2

34.9 4.5 15.2

37.2 6.3 10.9

38.6 9.7 7.10

40.3 10.1 6.90

41.2 11.3 6.20

12 29.2 2.8 24.3

33.7 4.1 16.6

36.2 5.9 11.6

36.4 8.5 8.10

38.2 11.2 6.20

14 27.8 2.5 26.7

29.2 5.7 11.7

31.1 7.5 9.00

35.5 12.3 5.60

36.9 19.5 3.50

20 27.0 13.2 5.10

29.0 13.5 5.00

30.8 30.3 2.20

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The dependence of the droplet size of bare micro-

emulsion on temperature has been observed to be a

reversible phenomenon. In the quaternary micro-

emulsion system, with increasing temperature TX-100

got dehydrated and partly transferred to the oil phase;

the amphiphile—poor droplets coalesced to adjust for

the increased interfacial tension. The kinetics of the

growth process got activated by the increased thermal

energy. The whole process was found to be reversed on

lowering the temperature.

The average diameter (kdl) of CuI and CuCrO4 nano-

particles obtained in the quaternary w/o microemulsion

medium, and also that of CuCrO4 in the ternary medium

with their diffusion coefficient (D) are presented in

Tables 2 and 3.

The DLS results on CuI nanoparticles in the quater-

nary microemulsion medium are presented in Fig. 6

with a comparison of the results on bare microemulsion.

The size of the microemulsion with and without CuI

appreciably differed at v . 12; that with CuI was

greater than that without CuI (Table 1). The diffusion coef-

ficients have accordingly decreased with increasing v.

The results presented in Tables 2 and 3 have also evi-

denced increasing kdl and decreasing D with increasing

temperature at different v as reasoned out above. In

the ternary system, CuCrO4 embedded droplet size

was greater than without CuCrO4. The trend in D was

obviously reverse. The size variation between bare and

nanoparticle embedded droplets was more for CuCrO4

than CuI.

The average size of CuS particles registered by

the DLS method was large (Table 4), it was much larger

than the size of the water droplets in the microemulsion

in which the material was synthesized. The D values

were consequently much lower. The growth of the CuS

did not remain limited in the water droplets. Instead the

Table 2. Temperature effect on the DLS derived average

hydrodynamic diameter (kdl) and the diffusion coefficient

(D) of CuI nanoparticles prepared in quaternary w/o micro-

emulsion at 296K.

v Temperature

kdl

(nm)

D � 107

(cm2 sec21)

8 30.4 2.7 25.3

36.8 4.5 15.3

50.5 6.2 11.5

10 31.3 3.9 17.3

35.1 4.5 15.1

45.5 6.6 10.7

12 27.8 3.1 22.0

34.0 5.0 13.6

36.3 6.6 10.5

36.4 7.6 9.10

37.9 7.8 8.80

14 27.5 5.5 12.1

31.4 6.1 11.1

35.2 7.4 9.30

38.0 7.8 8.90

39.5 12.2 5.70

20 29.5 16.0 4.20

30.5 19.4 3.50

31.7 21.9 3.10

33.4 25.2 2.70

Figure 6. Profiles of average diameter (kdl) and diffusion

coefficient (D) with v for the quaternary w/o microemulsion

systems without (B) and with (W) CuI (0.25mmol dm23) nano-

particles.

Table 3. DLS data of average hydrodynamic diameter (kdl)

and the diffusion coefficient (D) of colloidal CuCrO4 prepared

in w/o microemulsion at 296K.

v Temperature kdl (nm)

D � 107

(cm2 sec21)

In quaternary microemulsion

14 24.7 27.5 2.40

20 23 57.3 1.10

In ternary microemulsiona

6 29.9 7.9 (3.6) 8.5 (18.3)

8 29.8 17.2 (6.9) 3.9 (9.8)

10 29.7 24.3 (13.2) 2.8 (5.0)

12 29.8 32.9 (12.5) 2.0 (5.5)

14 28.5 43.2 (12.6) 1.5 (5.4)

16 28.7 41.9 (18.7) 1.6 (3.6)

18 28.8 43.7 (24.5) 1.5 (2.8)

aParenthetic values for bare microemulsion.

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particles grew large and were stabilized by way of surface

adsorption of the amphiphile TX-100 and butanol. Such

stabilization of CuS by the polar polyoxyethylene chain

of TX-100 was reported by Haram et al.[16] The large

growth of nano-CuS particles will be further discussed in

connection with TEM results.

Time Dependent Growth of Nanoparticles in the

Quaternary System

The time dependent growth of the nanoparticles

of CuCrO4 (at 1.5mmol dm23) in the quaternary

microemulsion at v ¼ 20 is depicted in Fig. 7 wherein

the ratio of the scattering intensity at a given time (It)

and that after mixing (I0) is profiled against time. The

curve has manifested the phenomena of nucleation

leading to particle growth. The growing particles contri-

buted to the increase in the scattering intensity It/I0 withtime. The rate of growth has been asymptotic and stabil-

ized after 200min. The pattern of growth for CuI was

different than CuCrO4; there was an induction period

of �20min, after which the nature was sigmoidal with

maximization at �125min. For CuS, the particle growth

was faster, the intensity maximized at 20min and then

the intensity ratio declined. This was due to the interfer-

ence among the intensities originated from the multiple

scattering centers in the large particles having dimensions

greater than 0.05l (or l/20)[35] where l was the wave-

length of the laser beam (632 nm) used in the study. The

DLS results in Tables 2–4 revealed that CuS particles

were larger than CuI and CuCrO4 at comparable v and

the condition for interference was applicable to CuS. Rela-

tively large It/I0 values for CuS in the initial stage also

corroborated the formation of large size of CuS particles.

Detailed kinetics of the nucleation and growth of

nanoparticles have been recently studied.[36–42] Such an

attempt on the systems herein reported would be

worthwhile. This is contemplated to be taken up in future.

Table 4. DLS data of average hydrodynamic diameter (kdl)

and the diffusion coefficient (D) of colloidal CuS prepared in

quaternary w/o microemulsion at 296K.

v Temperature

kdl

(nm)

D � 107

(cm2 sec21)

2 26.4 44.0 1.5

4 24.1 61.0 1.1

6 25.4 66.0 1.0

8 23.7 73.0 0.9

10 29.6 90.0 0.8

14 31.0 98.0 0.7

Figure 7. It/I0 vs. time profiles for the growth of 1.5mmol dm23 CuI, CuCrO4, and CuS nanoparticles at v ¼ 20 in the quaternary

microemulsion system.

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Structural Characterization

FTIR Spectra

The FTIR spectra of CuCrO4 isolated from water

and microemulsion are presented in Fig. 8. The bands

of CuCrO4 prepared in water and in microemulsion

media nearly appeared at comparable positions

suggesting the formation of the same compound in both

the media. The spectra of CuCrO4 prepared in micro-

emulsion have shown broadening of bands at lower

frequencies.[43] CuI and CuS were not done owing to

their linear symmetry, for which peaks should appear

at the far-IR region.

XRD Measurements

The XRD patterns of CuI and CuS are shown in

Figs. 9 and 10, respectively. The crystal structures of

CuI prepared both in water and in microemulsion

look similar. The corresponding Miller indices,

obtained form the x-ray data are (0 0 2), (0 1 1),

(1 1 0), and (1 1 2). The compound prepared in micro-

emulsion has shown sharp peaks than that prepared

in water suggesting more crystallinity in the sample.

CuI has a C6V symmetry and the corresponding

indices have been compared with the standard data

file.[44] The Miller indices for CuS prepared both in

water and in microemulsion have shown similarity;

they are (1 0 1), (1 0 2), (1 0 3), (1 0 5), (1 1 0), (1 0 8),

(1 0 1 0), and (1 1 8). The Miller indices obtained for

CuCrO4 are (0 1 2), (0 0 6), (1 1 0), (1 0 4), and (0 1 8).

The XRD diagram for CuCrO4 is not shown to save

space.

Transmission Electron Microscopy

The TEM measurements on CuI, CuCrO4, and CuS

nanoparticles obtained in ternary and quaternary microe-

mulsions at different v were taken. The particle size has

increased with v. This is in conformity with the DLS

results presented in Tables 2–4. The TEM evaluated

dimensions are presented in Table 5. The results are

mostly on the products obtained from the quaternary

microemulsion system. The particles were fairly mono-

disperse. In Fig. 11, the TEM observations for CuI,

CuS, and CuCrO4 particles in both ternary and quatern-

ary microemulsion media at v ¼ 2, and for CuI and

CuCrO4 in quaternary medium at v ¼ 20 are illustrated.

At v ¼ 2, the particles were spherical except for CuCrO4

which was parallelepiped in quaternary microemulsion.

The particles were also spherical at v ¼ 20. The TEM

derived results were higher for CuS and CuCrO4 than

expected corroborating the large particle size obtained

by the DLS method.

Figure 8. IR spectra of CuCrO4 nanoparticles isolated from (A) water and (B) quaternary microemulsion.

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CONCLUSIONS

The following conclusions can be drawn from the

results.

1. Both the ternary and quaternary microemulsion

systems with Cy as the oil produced very large

monophasic zones and thus have good appli-

cation prospect.

2. The synthesized nanoparticles (CuI, CuCrO4, and

CuS) in the above w/o microemulsions formed

colloidal dispersions having characteristic

UV-visible and fluorescence features with blue

shifts of their maxima and increasing intensity

with decreasing particle size.

3. The band gaps of CuI, CuCrO4, and CuS nano-

particles were close and higher than literature

reports.

4. The growth of the particles in the nanowater

compartments monitored by the DLS method

maximized at 120, 200, and 20min for CuI,

CuCrO4, and CuS, respectively. The CuS has

shown a decline in the scattering intensity

after maximization due to interference among

the intensities emitted from multiple scattering

centers of the particles.

5. The synthesized materials in the quaternary

microemulsion system were mostly spherical

and monodisperse at v ¼ 2; the CuCrO4 par-

ticles at v ¼ 2 were parallelepipeds.

Figure 9. X-ray diffraction spectra of CuI nanoparticles

isolated from (A) water and (B) quaternary microemulsion.

Corresponding Miller indices are shown on each peak.

Figure 10. X-ray diffraction spectra of CuS nanoparticles

isolated from (A) water and (B) quaternary microemulsion.

Corresponding Miller indices are shown on each peak.

Table 5. Transmission electron microscopy Results on the shape and size of the synthesized nanoparticles.

v 2 8 12 14 20 30

CuI in quaternary microemulsion

d (nm) 3 (spherical) 6 (spherical) — — 25 (spherical) —

CuS in quaternary microemulsion

d (nm) 38 (spherical) 60 (spherical) — 95 (nearly

spherical)

— —

CuCrO4 in quaternary microemulsion

d (nm) 8/6.5 (parallelepiped) 24 (spherical) — — 54 (spherical) —

CuCrO4 in ternary microemulsion

d (nm) 14 (spherical) — 30 (spherical) — — 65 (spherical)

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ACKNOWLEDGMENT

S. Biswas thanks the Council of Scientific and

industrial Research (CSIR), Govt. of India for a

Senior Research Fellowship. S. K. Hait thankfully

acknowledges the award of a Senior Research Fellow-

ship by the University Grants Commission (UGC),

Govt. of India. Thanks are also extended to the UGC

for an Emeritus Fellowship to S. P. Moulik. We thank

Mr. S. N. Dey of IICB, Kolkata and Mr. S. N. Dutta of

SINP, Kolkata, respectively, for taking TEM and XRD

measurements.

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Received August 7, 2003

Accepted June 1, 2004

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