Effect of Nature of Surfactant on the Formation of SeAgβ 2−

Nanoparticles and Optical Properties of SeAgβ 2− and ZnS/ SeAgβ 2−

Nanocomposite

V.Andal1 * and G.Buvaneswari2b

1KCG college of Technology, Chennai, India

2VIT University, Vellore, India

aandal.che@kcgcollege.com,

bgbuvaneswari@vit.ac.in

Keywords: Chemical preparation; semiconductor; Optical properties; Electron microscopy; X-ray diffraction

Abstract. Surfactant assisted synthetic route was followed to prepare silver selenide (β-Ag2Se)

nanoparticles. The effect of three different surfactants viz., Triton X100, SDS and CTAB in the

formation of silver selenide nanoparticles had been examined. Pure and crystalline β-Ag2Se

nanophase was obtained in the presence of Triton X100 and SDS. However, the presence of CTAB

leads to metallic silver formation. Nano Composite of β-Ag2Se and ZnS was fabricated in the

presence of glycine as a molecular linker. The products were characterized by different techniques

such as XRD, FT-IR, SEM and TEM. Room temperature photoluminescence spectrum of the ZnS/

β-Ag2Se nanocomposite exhibited two emission peaks at around 286 nm and 392 nm with enhanced

intensity (λex = 250 nm).

Introduction

Silver selenide, Ag2Se exists in two polymorphic forms: low temperature orthorhombic and

high temperature body centered cubic structures [1]. The polymorphs exhibit distinct properties: the

low temperature form shows semiconducting behaviour with low band gap, low thermal

conductivity, etc., and high temperature polymorph is a super ionic conductor with mobile Ag+ ions

[1]. Such properties make these materials promising for various potential applications such as

thermoelectric application [2], non-linear optical devices [3], batteries [4], ion selective electrodes

[5], photolithography [6] etc. Interestingly, Beck et al. observed magneto resistance in modified

composition of silver selenide [7].

The reported routes of bulk synthesis of metal chalcogenides include direct elemental

reaction at high temperatures under vacuum for longer duration [8], metal oxides with hydrogen

chalcogenides [9], reaction of metal with potassium selenide under inert atmosphere [10], elemental

reaction in liquid ammonia or n-butylamine [11], sonochemical reaction between silver nitrate and

selenium in ethylenediamine [12], hydrothermal synthesis [13] etc. Similarly various routes have

been reported for the synthesis of nano Ag2Se. Wang et al have reported room temperature

preparation of nanocrystalline Ag2Se using AgNO3, Se and KBH4 in pyridine [14]. Khanna and Das

[15] have made silver selenide nanopowder using cycloalkeno – 1, 2, 3 – selenadiazole and silver

nitrate. Batabyal et al synthesized silver selenide nanocrystals by chemical route using ammoniacal

silver nitrate and sodium selenosulfate in aqueous medium [16]. However, many of the reported

procedures are associated with drawbacks such as longer duration of synthesis [17], application of

ultrasonic or γ irradiation [18], high temperature or high pressure [13], expensive and toxic organic

precursors [15] etc. Despite the availability of various methods of synthesis, reports on the

synthesis of silver selenide by micellar technique or surfactant assisted synthesis are limited.

Compounds of similar composition with different morphologies exhibit difference in their

properties [19]. Such morphological changes can be achieved by different synthetic routes. In the

Journal of Nano Research Vol. 30 (2015) pp 96-105 Submitted: 20.08.2014Online available since 2015/Mar/02 at www.scientific.net Revised: 29.12.2014© (2015) Trans Tech Publications, Switzerland Accepted: 29.12.2014doi:10.4028/www.scientific.net/JNanoR.30.96

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 58.68.70.125-05/03/15,08:11:25)

present study, nano β-Ag2Se is prepared by surfactant assisted solution route using three different

surfactants such as Cetyl trimethyl ammonium bromide, Sodium dodecyl sulphate and Triton X100.

The present study investigates the effect of surfactants on the phase formation and surface

morphology of nano β-Ag2Se.

Core shell nanocomposites are of current interest because of their widespread applications.

Enhanced or modified properties are observed with respect to thermal, chemical, mechanical,

optical properties etc [20]. The core shell nanocomposites are classified as inorganic core shell

nanoparticles, organic-inorganic hybrid core shell nanoparticles, inorganic core organic shell

nanoparticles and polymeric core shell nanoparticles. Binary semiconductor nanoparticles coming

under the inorganic core shell nanoparticles are used for fluorescent bio imaging [20].

It is observed that the optical property of the sulphide and selenide nanomaterials strongly depends

on the size of the particles and the surface quality [21]. Surface quality can be further improved by

suitable coating or shell material [21]. For example, increase in peak intensity of PL spectra of

CdSe/ZnSe core/shell nanocrystals have been observed by Lee et al [21]. ZnS an important

semiconductor finds application in luminescent materials [22], infrared windows [23], flat panel

displays [24], photo catalysis [25] and photovoltaic devices [26]. The composite of ZnS with other

materials such as CdS, CdSe etc. has resulted in modified optical properties. Dabbousi et al reported

on the improved luminescence in the case of (CdSe)ZnS core-shell composite [27]. Loukanou et al

observed a change in photoluminescence from yellow to orange when CdS is converted into

CdS/ZnS [28]. ZnS shell over ZnS:Ag nanocrystals showed significant enhancement in the

photoluminescence quantum yield [29]. The current work reports on the fabrication of ZnS/β-Ag2Se

nano composite and to enhance the photoluminescence behaviour of nano β-Ag2Se.

Experimental

Materials

The reagents used were: Silver nitrate (99.9%), Sodium selenate (99%), Hydrazine

hydrochloride (97%) obtained from CDH Ltd. Sodium borohydride (98%), Triton X100 (99%),

Sodium dodecyl sulfate (98%), Cetyl trimethyl ammonium bromide (98%), ZnCl2 (98%) and Na2S

(97%) were purchased from sd. fine-Chemical limited, India and Pyridine (99%) from Thomas

Baker Ltd. Selenium powder was freshly prepared by reducing sodium selenate using hydrazine

hydrochloride.

Synthesis of nano β-Ag2Se

Three types of surfactants used were Cationic (Cetyl trimethyl ammonium bromide), anionic

(Sodium dodecyl sulphate) and non-ionic (Triton X100).

Solution (10 ml) of each surfactant (TX100 2.0 mmol, CTAB 0.4 mmol and SDS 0.2mmol)

was made in distilled water. To 20 ml of pyridine, 5 ml of surfactant solution was added. To the

mixture, 0.0792 g ‘Se’ powder was added and stirred for 10 mins (I). To another 5 ml of the

surfactant solution, 10 ml of silver nitrate solution (0.3397g AgNO3) was added (II). Part I and part

II were mixed well for 10 mins. To the mixture, freshly prepared NaBH4 (0.3397g in 10 ml) was

introduced dropwise. While adding NaBH4 to the mixture black particles were formed. The contents

were stirred at room temperature (~ 30°C) for 4 h for the completion of reaction. The black

precipitate was collected, washed with distilled water and air dried.

Synthesis of ZnS/β-Ag2Se nanocomposite

The prepared nano SeAgβ 2− using TX100 (0.02 g) was well dispersed in 20 ml ethanol

by sonicating for 10 mins. To the dispersion, 10 ml of aqueous glycine (2mM) was added and

stirred for 10 mins. To the mixture, ZnCl2 (0.008 mM, 10 ml) and Na2S (0.008 mM, 10 ml)

solutions were added successively. The suspension was centrifuged and the collected powder was

washed with distilled water and air dried. The same process was repeated in the absence of glycine.

Journal of Nano Research Vol. 30 97

Characterization

The obtained products were characterized by powder X-ray diffraction method using CuKα

radiation Bruker (D8 Advance) X-ray diffractometer. Morphological studies were carried out by

Scanning Electron Microscopy (FEI Quanta FEG 200). TEM studies were carried out using Philips

(C8 model). UV-Vis and PL spectra were recorded using Jobin Yvon Fluorometer. FT-IR

measurements were executed using KBr disc technique (FT-IR spectrometer- Thermo Nicolate

Company Avatar 330).

Results and Discussion

Synthesis of nano β-Ag2Se

The surfactants used in the current work are given in Fig. 1. It is noted that the nature of the

surfactant influences the formation of nano β-Ag2Se. The powder X-Ray diffraction patterns of the

products obtained by using Triton X100, SDS and CTAB are given in Figures 2a-c. The results

demonstrate the effect of surfactants on the formation of nano β-Ag2Se. In the presence of the

surfactants Triton X100 and SDS, pure and nanocrystalline silver selenide is formed. All the peaks

appear in the diffraction pattern are indexed based on the orthorhombic silver selenide, SeAgβ 2−

[JCPDS file # 24-1041].

On the other hand, the powder X-ray diffraction pattern of the product obtained in the

presence of CTAB as a surfactant indicates the formation of metallic silver as a major phase.

The mechanism of formation of silver selenide involves two steps [30]: (i) selenium forms a

complex with pyridine (ii) the reactive complex interacts with the freshly generated metallic silver

and forms the binary selenide. In order to understand the role of surfactants in the formation of β-

Ag2Se, the products obtained from surfactant mixed (Py)2Se are separated and analysed by FT-IR

spectroscopy. The infrared spectra are compiled in Figure 3. The spectra of the products obtained

using Triton X100, SDS and CTAB are compared with the reported IR spectra of the respective

pure surfactants [31 - 33] (Table 1 & 2).

Fig. 1. Surfactants: Triton X100 (a), Sodium dodecyl sulphate (b) and Cetyl trimethyl ammonium

bromide (c)

From the Table 1 and 2 it is noted that the characteristic vibrational modes of TX100 such as

νsym (C–H), νasym (C–H), δs (C–H) aromatic rings, C-O, ρr (CH2) n and S=O(str), νsym (C–H) and

νasym (C–H) of SDS show negligible shift in the IR spectra of surfactants mixed (Py)2Se. This

confirms that there is no interaction between the surfactants and selenium of (Py)2Se and thus it

combines with freshly formed silver and yields silver selenide.

O

O

H

n

OS

OO

O- Na+

N Br

a

b

c

98 Journal of Nano Research Vol. 30

10 20 30 40 50 60 70

a

Inte

nsity (a.u

)

2θ

(204)

(213)

(023)

(214)

(114)

(212)

(220)

(123)

(004)(0

32)

(201)

(113)

(200)

(031)

(013)

(121)

(112)

(120)

(102)

(111)

(002)

b

c

(220)Ag(200)Ag

(111)Ag

Fig. 2. Powder X-Ray diffraction patterns of the products obtained using Triton X100 (a),

SDS (b) and CTAB (c)

Fig. 3. Infrared spectra of Triton X100 + (Py)2Se (a), SDS + (Py)2Se (b) and CTAB +(Py)2Se (c)

3000 2500 2000 1500 1000 500

Tra

nsm

itta

nce (

%)

wavenumber(cm-1)

b

a

c

Journal of Nano Research Vol. 30 99

Table 1 Comparison of FT-IR spectral details of Triton X100 and Triton X100 +(Py)2Se.

Table 2 Comparison of FT-IR spectral details of SDS and SDS + (Py)2Se.

Curve c in the Fig. 3 reveals that the product obtained in the presence of CTAB shows the

absence of peaks characteristic to symmetric and asymmetric –CH2 stretching vibrations in the

region 2,935and 2,871 cm-1

respectively. Similarly, the peaks attributed to –C–H vibrations of –N–

CH3 group and –C–N stretching vibrations do not appear in the respective regions such as 1,559,

1,473 cm-1

and 1,253, 1,218 cm-1

[33]. Such observations prove the modification in the head group

region of CTAB due to possible strong interaction between CTA+ and Se(Py)2. This results in the

prevention of direct reaction between ‘Se’ and ‘Ag’. Thus, it is presumed that due to the

unavailability of ‘Se’ to combine with ‘Ag’ for the formation of Ag2Se, the added silver nitrate is

reduced by sodium borohydride to metallic silver (Fig. 4) and it is confirmed by powder X-Ray

diffraction analysis (Fig. 2c).

Scanning electron microscopic analysis of nano silver selenide obtained in the presence of

Triton X100 and SDS shows slight change in the particle morphology. The SEM micrographs are

shown in Fig. 5(a) and 5(b). Examination of the surface morphology reveals agglomeration of

smaller size particles (100-200 nm).

Peak assignment SDS SDS + (Py)2Se

-S=O (stretching vibrational mode

of sulphonic acid)

1219 1254

1083 1110

νsym (C–H) 2850.4 2848

νasym (C–H) 2918.9 2920

Peak assignment TX100 TX100 + (Py)2Se

νsym (C–H) 2,850 2850

νasym (C–H) 2,950 2920

δs (C–H) aromatic 1,500

1600

1630

C-O 1300

1100

1380

ρr (CH2) n 800 880

100 Journal of Nano Research Vol. 30

a

Fig. 4. Reaction pathway depicting the formation of silver in the presence of CTAB

Fig. 5. SEM images of SeAgβ 2− obtained using Triton X100 (a) and SDS (b)

Synthesis of ZnS/ SeAgβ 2− nanocomposite

The presence of SeAgβ 2− and ZnS in the nanocomposite ZnS/ β-Ag2Se is confirmed by

powder X-Ray analysis (Fig. 6(a)). The XRD patterns are indexed based on JCPDS file No.24-1041

for β-Ag2Se and JCPDS file No. 89-2427 for zinc blende ZnS. No separate peak is observed in the

pattern to indicate the presence of glycine. The powder XRD pattern (Fig. 6(b)) of the product

obtained in the absence of glycine shows lesser number of diffraction peaks corresponding to ZnS.

This confirms the role of glycine in anchoring the two phases efficiently.

FT-IR analysis provides an evidence to prove that glycine acts as an interlink between two

semiconductor (β-Ag2Se and ZnS) particles. The IR spectra of glycine, β-Ag2Se @glycine and

ZnS@glycine@ β-Ag2Se are compiled in Fig. 7. The spectral analysis is detailed in Table 3.

Comparison of the IR spectrum of Ag2Se@glycine with that of glycine indicates an interaction

between silver selenide and glycine through COO- group [34, 35]. The peaks appear at 1111, 698

and 611 cm-1

due to ‘CO’ group are broadened in β-Ag2Se @glycine. Presence of the peaks at

b a

Journal of Nano Research Vol. 30 101

4000 3500 3000 2500 2000 1500 1000 500

14121612

698

1111

Tra

nsm

itta

nce %

w avenum ber(cm-1

)

1633

1410

a

b

c

1633, 1500, 1410 cm-1

confirms that the amino group of glycine is not affected during the formation

of β-Ag2Se @glycine. However, these peaks are broadened in Fig. 7(c) indicating that the amino

group end of β-Ag2Se @glycine gets affected on the addition of zinc chloride and sodium sulphide

which leads to the formation of ZnS.

Fig. 6. Powder XRD patterns of ZnS/ β-Ag2Se composite in the presence of glycine (a) and in the

absence of glycine (b) (unmarked peaks are due to β-Ag2Se)

Fig. 7. FTIR spectra of glycine (a), β-Ag2Se @glycine (b) and ZnS@glycine@β-Ag2Se (c)

TEM image (Fig.8) of the composite reveals the formation of nanosized particles (20 nm) of

one of the materials on the surface of another material. The base material provides its surface for the

nucleation of nanocrystals at random sites. The overall morphology of the composite depicts surface

embedded crystallites.

10 20 30 40 50 60 70

Inte

nsit

y (

a.u

)

*

*

****

**

**

*

b

a

*ZnS

*

*

2θ

102 Journal of Nano Research Vol. 30

Fig. 8. TEM image of ZnS/ β-Ag2Se composite

Optical Properties

Very limited reports are available in the literature on the pholotoluminescence of β-Ag2Se.

For instance, Cao et al reported on the photoluminescence of β-Ag2Se complex nanostructures with

different excitation wavelengths [36]. There are numerous reports on other chalcogenide systems

either as a pure material or in the form of composite [27, 28, 37, 38, 39]. The absorption spectra of

β-Ag2Se, ZnS and ZnS/ β-Ag2Se are shown in Figure 9. The UV absorption peaks are indicative of

the band gap of the semiconductor ZnS and β-Ag2Se. The calculated band-gap value of the ZnS and

β-Ag2Se nanoparticle was 3.92 eVand1.8eV, which is blue shifted from that of bulk ZnS (340 nm,

Eg = 3.65 eV) and β-Ag2Se (Eg=0.08eV) . Increasing of band gap energies of ZnS and β-Ag2Se

nanostructures could be an indication of the quantum confinement effect due to decreasing size of

structures. The composite exhibits negligible shift in the absorption wavelength with respect to β-

Fig. 9. UV-Vis spectra of β-Ag2Se (a), Fig. 10. Photoluminescence spectra of (250 nm

ZnS/ β-Ag2Se (b) and ZnS (c) excitation) β-Ag2Se (a) and ZnS/ β-Ag2Se (b)

(b1-1:1, b2-2:1 and b3-1:2 molar ratios of

ZnS:β-Ag2Se)

20nm

200 300 400 500 600 700 800

c

b

a

Ab

so

rban

ce (

a.u

)

wavelength(nm)

250 300 350 400 450 500

1000000

1500000

2000000

b3

b1

b2

a

wavelength (nm)

Inte

nsit

y (

a.u

)

Journal of Nano Research Vol. 30 103

Ag2Se but a small increase in absorption intensity is noticed. Compared to ZnS, a blue shift in

absorption wavelength is observed. Similar observation is reported by Azhniuk et al [41]. They

noticed a slight absorption increase in the case of CdSe/HgSe and CdSe/Ag2Se and a slight red shift.

In the case of (CdSe)ZnS core-shell composite, a slight red shift in the absorption with respect to the

core CdSe is reported [27]. However, in the current study on β-Ag2Se /ZnS, no such shift is noticed

which could be due to the fact that in the current method the composite formed contains particles of

one of the phases on the surface of another without forming a homogeneous shell.

Room temperature PL spectra of bare β-Ag2Se and ZnS/ β-Ag2Se composite are compared in

Fig. 10. The excitation wavelength is 250 nm. Examination of the PL spectra of ZnS/ β-Ag2Se (Fig.

10 (b1) and(b3)) indicate a two fold increase in intensity of two emission peaks appearing at ~

280 nm and ~ 390 nm compared to the emission intensities of β-Ag2Se. Thus, the composite shows

an enhanced photoluminescence compared to β-Ag2Se. The effect of change in the concentration of

β-Ag2Se and ZnS on the photoluminescence is studied. The PL spectra obtained for ZnS/β-Ag2Se

molar ratios 2:1 and 1:2 are compiled in Fig. 10(b2) and (b3). Comparing the spectrum Fig. 10 (b1)

with that of 10 (b2) reveals an increase in the concentration of ZnS leads to slight decrease in the PL

intensity. Fig. 10 (b3) indicates that an increase in the concentration of β-Ag2Se slightly increases

the PL intensity compared to ZnS: β-Ag2Se in 1:1 ratio.

Conclusions

The current study reports the influence of surfactants on the synthesis of phase pure silver

selenide. Presence of surfactants such as nonionic Triton X100 and anionic SDS leads to the

formation of pure nano β-Ag2Se. The presence of cationic surfactant CTAB prohibits the reaction

between pyridine selenium complex and silver and leads to the reduction of added silver nitrate to

silver. A nanocomposite of ZnS and β-Ag2Se has been prepared. The room temperature PL studies

(excitation wavelength = 250 nm) on the composite (ZnS/ β-Ag2Se) display an enhanced intensities

with respect to two emissions at ~ 280 nm and ~ 390 nm.

Acknowledgements

The authors thank VIT University for providing all required facilities to carry out the

experiments.

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Journal of Nano Research Vol. 30 10.4028/www.scientific.net/JNanoR.30 Effect of Nature of Surfactant on the Formation of β-Ag2Se Nanoparticles and Optical Properties of β-

Ag2Se and ZnS/β-Ag2Se Nanocomposite 10.4028/www.scientific.net/JNanoR.30.96

DOI References

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