Materials Research Bulletin 48 (2013) 1137–1142

Effects of electrolytes and surfactants on the morphology and stability ofadvanced silver nano-materials

Abdullah Yousif Obaid a, Shaeel Ahmed AL-Thabaiti a, E.H. El-Mossalamy a, Javed Ijaz Hussain b,Zaheer Khan b,*a Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21413, Saudi Arabiab Nano-science Research Lab, Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110 025, India

A R T I C L E I N F O

Article history:

Received 9 April 2012

Received in revised form 29 November 2012

Accepted 4 December 2012

Available online 13 December 2012

Keywords:

A. Nanostructures

B. Chemical synthesis

B. Crystal growth

D. Fermi surface

D. Surface properties

A B S T R A C T

The impact of electrolytes, stabilizing and/or capping agents on morphology of colloidal silver nano-

materials (AgNPs) has been studied spectroscopically. Sodium thiosulfate acts as reducing-, stabilizing-

and damping-agents. Stoichiometric ratios of S2O32� and Ag+ ions were responsible to the formation

stable and prefect transparent dark yellow colored AgNPs. The S2O32�-stabilized AgNPs were

significantly more stable in inorganic electrolytes (NaNO3, Na2SO4, Na2CO3 and KBr). S2O32� is adsorbed

more strongly than the used other anions. The addition of cetyltrimethylammonium bromide (CTAB) and

sodium dodecylsulfate (SDS) has significant effects on the absorbance of S2O32�-stabilized AgNPs which

can be rationalized in terms of electrostatic attraction and repulsion between the adsorbed S2O32� ions

on to the surface of AgNPs and cationic and/or anionic head groups of used surfactants, respectively.

Transmission electron microscopy images suggest that AgNPs are polydispersed, spherical and

exhibiting an interesting irregular morphology.

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Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Different polymeric materials, surfactants and electrolytes(mono-, di- and poly-valent) are usually used as stabilizers toprevent agglomeration and precipitation of silver and goldnanoparticles in solution [1–5]. For instance, Henglein andcoworkers [6,7] reported the surface plasmon damping effects ofvarious nucleophilic reagent (KI, NaSH, C6H5SH), organic solvent,CCl4, molecular gases (H2S, O2) and some metal cations (Ag+, Ba2+,Cd2+, Ni2+, Hg2) on the surface chemistry of colloidal silverprepared by radiation method using 2-propanol, AgClO4 andpolyphosphate as reductant, oxidant and stabilizer, respectively.Chemisorbed effect was interpreted in terms of the donation ofelectron density from the silver particles to the adsorbed cations.Henglein also monitored the stepwise growth of first silver clustersAg+ ion reduction in aqueous solution using by spectroscopictechniques [8]. Chen et al. [4] prepared citrate and polyvinylpyr-rolidone coated AgNPs and time-resolved dynamic light scatteringwas used to measure the aggregation kinetics over a range ofmonovalent, NaCl and divalent, CaCl2 and MgCl2 electrolytesconcentrations. Divalent electrolytes were more efficient indestabilizing the citrate-coated AgNPs. Decher reported the

* Corresponding author. Tel.: +91 11 26981717; fax: +91 11 2698 0229.

E-mail address: drkhanchem@yahoo.co.in (Z. Khan).

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.12.009

adsorption of poly(diallyldimethylammonium chloride), poly(al-lylamine hydrochloride) and neutral polyethylene glycol onto thesurface of silver nanoparticles using layer-by-layer technique [9].Ryu synthesized the highly concentrated silver nanosol bypolyelectrolytes for inkjet method and suggested that molar ratios(R = [COO�]/[Ag+]) has significant impact on the stability. They alsopointed out that the adsorption of weak polyelectrolytes on thenanoparticle surface is a more complicated process owing to thefact that the linear charge density of the weak polyelectrolyte isstrongly influenced by the solution pH and ionic strength [10].Solomon et al. [11] synthesized clear yellow-, dark yellow-, violet-and grayish-silver sols in large amounts of sodium borohydride,which acts as reducing, stabilizing and damping agents. Differenttypes of nanomaterials like copper, zinc, titanium, magnesium,gold, alginate and silver have come up but silver nanoparticleshave proved to be most effective as it has good antimicrobialefficacy against bacteria, viruses and other eukaryotic micro-organisms [12,13].

Sodium thiosulfate has potential applications in the fields ofiodometry, photographic processing, gold extraction, analyticaland medicinal chemistry. It is also used as a common dechlor-inator, antidote and management of extravasations for the removalof chlorine from drinking water, cyanide poisoning and chemo-therapy, respectively. Hayden et al. demonstrated the dual role ofsodium thiosulfate as a potent antioxidant and chelator of calciumfor the clinical syndrome of calciphylaxis [14]. To the best of our

0 20 40 60 80 10 0 12 0 14 0 16 00.0

0.1

0.2

0.3

0.4

0.5

Abs

orba

nce

at 4

00 n

m

Time (min)

104[S2O

32-] (mol dm-3)

4 8 12 16

nuclea tion

growth

adsorption

Fig. 1. Reaction–time plots to the formation of AgNPs as a function of [S2O32�] at

30 8C. Reaction conditions: [Ag+] = 4.0 � 10�4 mol dm�3, [S2O32�] = 4.0 (&), 8.0

(*), 12.0 (~), and 16.0 � 10�4 mol dm�3 (!).

A.Y. Obaid et al. / Materials Research Bulletin 48 (2013) 1137–11421138

knowledge, the use of thiosulfate as reductant, stabilizer, adsor-bate and/or damping agents was not available and examined in theexisting literature despite the fact that, color, morphology andstability of silver nanoparticles strongly depends on the method ofpreparation, nature of reductant and presence of stabilizers [15–20]. The ultimate goal of our research is to establish the role ofthiosulfate in the synthesis and stability of AgNPs in presence ofelectrolytes (NaNO3, Na2SO4, Na2CO3 and KBr) and surfactants(CTAB and SDS) because the syntheses of nanostructures of silverin high yield and in a wide range of shapes are challenging tasks.This requires the understanding of the nuclei formation and theinfluence of reaction species on nuclei morphology. In addition, theinfluence of electrolytes and surfactants on the stability of alreadysynthesized AgNPs is reported for the first time.

2. Experimental

2.1. Materials and instruments

Double distilled (first time from alkaline permanganate),deionized and CO2-free water was used as solvent throughoutthe studies in the preparations of reagent solutions. AgNO3 (E.Merck, 99%), sodium thiosulfate (Aldrich, 99%), cetyltrimethylam-monium bromide (Fluka, 98%), sodium dodecylsulfate (BDH,99.9%) and all inorganic electrolytes (NaNO3, Na2SO4, Na2CO3

and KBr) were used as received. However, purity of the CTAB waschecked by the reported method. UV–vis spectrophotometer (UV-260 Shimadzu, with 1 cm quartz cuvettes), Transmission electronmicroscope (JEOL, JEM-1011, Japan) and Fisher Scientific digital pHmeter 910 fitted with a combination electrode were used for thecharacterization, morphology determination (samples were pre-pared by placing a drop of silver sol on carbon-coated TEM gridswith an accelerating voltage of 100 kV, microscope was equippedwith a Mega view II digital camera and size distribution of theparticles was determined by using UTHSCSA Image Tool 2.00software) and measurements of pH of the working solutions,respectively.

2.2. Preparation of perfect transparent colored silver sol

Noble metal nanoparticles exhibit a unique UV–vis absorptionband derived from collective oscillation of conduction electronsupon interaction with electromagnetic radiation, which is knownas localized surface plasmon resonance. Therefore, UV–visabsorption spectra have been proved to be quite sensitive to theformation of silver sol [21]. In a typical experiment, a yellowcolored silver sol was prepared on mixing an equimolar solutionsof Ag+ ions = 8.0 � 10�4 mol dm�3 and[S2O3

2�] = 8.0 � 10�4 mol dm�3 at room temperature (vide infra).

2.3. Kinetic measurements

Required volume of thiosulfate was added into the two-neckedreaction vessel, equipped with a double-surface condenser toprevent evaporation, containing silver nitrate and electrolytes (ifneeded) to initiate the reaction. The reaction volume was always50 cm3. The progress of reaction (formation of colored silver sol)was followed spectrophotometrically by pipetting out aliquots atdefinite time intervals and measuring the absorbance of at 400 nm.The apparent first-order rate constants (kobs, s�1) were calculatedfrom the slopes of the plots of ln a/(1 � a) versus time (vide infra)[22]. Other details of kinetic procedure were the same as describedpreviously [20,23]. The pH of the reaction mixture was alsomeasured at the end of each kinetic experiment and observed thatpH drift during the course of the reaction is very small (with in0.04 unit).

3. Results and discussion

3.1. Characterization and kinetics

Color of aqueous silver nitrate solution was changed fromcolorless to pale yellow, yellow and finally brownish yellowturbidity appeared upon addition of sodium thiosulfate within thetime of mixing at room temperature. Therefore, the choice of thebest conditions for the preparation of perfect transparent AgNPs isa critical problem that we address first. The stoichiometry of thereaction as determined by visual observation was found to be 1:1,that is, 1 mol S2O3

2� reacts with 1 mol of Ag+ to give quite stableyellow brown color which is unstable in excess of [S2O3

2�] (Fig. 1).When [S2O3

2�] was varied from 1.0 � 10�4 mol dm�3 while using1.0 � 10�4 [AgNO3], we did not observed the formation of AgNPs(Table 1). The damping in the absorbance as a function of theconcentration of added adsorbate might be due to the chemisorp-tion and formation of less than a monolayer of S2O3

2� onto thepositive surface of AgNPs [24]. In case of S2O3

2�, negative charge isdistributed at the oxygen atoms instead of localizing on a S atom.At high concentrations, the delocalized distribution of negativecharge causes the interaction of one S2O3

2� anion with severalnanoparticles. As results, the aggregation of free AgNPs takes placeand induces the damping of absorbance.

To see the nature of yellowish brown color, the spectrum ofreaction mixture was recorded at the end of the reaction. The mostcharacteristic part of colloidal and/or nano silver is the surfaceplasmon resonance absorbance observable in the 250–650 nmregions [21]. Fig. 2 shows a large band covering the whole visibleregion of the spectrum. The spectral shape, peaks position andstability of silver species present in solution strongly depends onthe nature of reducing agent, acidity of the reaction mixture andnature of stabilizing agent [25]. Further, Rayleigh’s law was alsoused to obtain insight into the colloidal nature of silver sol. The plotof log(absorbance) versus log(wavelength) is linear with slope ca. –4.60 indicating the colloidal nature of silver species. These findingsare in good agreement with the observations of other investigators[21]. The different inorganic salts (NaNO3, Na2SO4, Na2CO3 andKBr) were added in the aqueous silver sol solution. In presence ofthese salts, we did not observed the appearance of any precipitate,which suggests that stable silver species is present in the form ofcolloidal particles of silver under our experimental conditions, i.e.,stiochiometric ratio of Ag+ and S2O3

2� ions. Our observations,formation of stable S2O3

2�–AgNPs are in good agreement to thepolyvinylpyrrolidone-, branched polyethyleneimine-, NaBH4-, and

Table 1Effect on the stability of AgNPs when [S2O3

2�], [CTAB] and [SDS] is varied.

[S2O32�]/[Ag+] 104 [CTAB] (mol dm�3) 103 [SDS] (mol dm�3) Absorbance, color and stability

2.0 0.0 0.0 Pale yellow turbidity

1.0 0.4, dark yellow; stable

2.0 0.1, light yellow, unstable

3.0 0.08, yellow, unstable

4.0 0.03, yellowish turbidity

1.0 10.0 0.0 Yellow turbidity

0.20 10.0 0.50, light yellow, stable

0.12 10.0 0.70, yellow, stable

0.10 10.0 1.01, dark yellow, stable

0.08 10.0 1.04, dark yellow, stable

1.0 10.0 0.42, dark yellow; stable

2.0 10.0 0.41, light yellow, unstable

3.0 10.0 0.43, yellow, unstable

4.0 10.0 0.42, yellowish turbidity

20

A.Y. Obaid et al. / Materials Research Bulletin 48 (2013) 1137–1142 1139

citrate-stabilized AgNPs through steric repulsion, ionization ofprimary, secondary, and tertiary amine groups, electrostaticallycaused by adsorption of negatively charged BH4

� ions, andionization of the polar citrate carboxyl groups onto the surfaceof AgNPs [2,4,11,26].

To assess the role of [S2O32�] in the reduction of Ag+ ions (silver

sol formation), a series of kinetic runs were performed. Unusualkinetic behavior was observed when a fixed concentration of Ag+

ions was allowed to react with varying concentrations of S2O32�

(from 6.0 � 10�4 to 16.0 � 10�4 mol dm�3). Apparent first-orderrate constants were found to be decreases with [S2O3

2�] as thereaction proceeded (Figs. 1 and 3). These observations are anindicative to the adsorption of S2O3

2� onto the positive surface ofresulting AgNPs through electrostatic interactions, which in turn,increases the Fermi levels of the resulting AgNPs. These results arein good agreement with the observations of Henglein (the neutralnucleophiles and neutral stabilizing polymers have strong effect onthe surface resonance plasmon absorption band of metalnanometer particles and donates the electron density to theparticles via lone pairs of electrons [21]). The influence of [Ag+](from 2.0 � 10�4 to 25.0 � 10�4 mol dm�3) was also studied atfixed [S2O3

2�] = 2.0 � 10�4 mol dm�3. The reaction follows first-order kinetics with respect to [Ag+] (kobs = 19.0, 20.0, 19.8 and20.2 � 10�4 s�1 for 2.0, 8.0, 10.0, 12.0 and 16.0 � 10�4 mol dm�3

[Ag+], respectively) [27]. The rate law is, therefore, represented byEq. (1).

d½silver sol�dt

¼ kobs½Agþ�½S2O2�3 �T (1)

300 400 500 600 700 8000.0

0.1

0.2

0.3

0.4

0.5

0.6

Abs

orba

nce

Wavelength (nm)

Fig. 2. Absorption spectra of colloidal silver sol. Reaction conditions:

[Ag+] = 8.0 � 10�4 mol dm�3, [S2O32�] = 8.0 � 10�4 mol dm�3 and temperature = 30 8C.

3.2. Mechanism and rate-law

S2O32� ions are known to provide two different coordination

sites: at the O-, and S-centers that can coordinate with Ag+ ions andwith metallic nanoparticles. Sulfur has been established as themost susceptible to gain electrons from the oxidizing agents [28].The sulfur is not as electronegative as oxygen, the sulfur–Ag bondis less polar than the oxygen–Ag bond. Thus, a large partialnegative charge resides on the oxygen atom, activating it forprotonation. This accounts for the non-availability of the electronpair at the seat of the reaction. Therefore, high reactivity ofterminal S atom seems because of the presence of a lone pair ofelectron on S atom. Therefore, the following mechanism isproposed for the AgNPs (Scheme 1).

In Scheme 1, Eq. (2) represents a complex formation betweenAg+ and S2O3

2�. The rate-determining step is a one-step, oneelectron oxidation–reduction taking place in Eq. (3). This reactionresults in the formation of Ag0 and radical. Henglein et al. [8,29]reported the formation of various species like Ag2

+, Ag32+, Ag4

2+,Ag8Ag+ or Ag9

+ and Ag64+ in aqueous solution using the radiation

method for the reduction of Ag+ ions and suggested that Ag42+ can

be stabilized for along time in presence of a polyanion even underair and growth stops at the stage of this species [21]. Therefore, Ag+

adsorb on the surface of Ag0 leading to the formation of Ag2+ ions

(Eq. (5)) and then the Ag2+ ions dimerize to yield Ag4

2+ (yellow-color silver sol; Eq. (6)).

6 8 10 12 14 160

5

10

15

104 k

obs

(s-1

)

104[S2O32- ] (mo l dm-3 )

Fig. 3. Effect of [S2O32�] on kobs. Inset: plot showing the effect of [S2O3

2�] on the

absorbance of AgNPs. Reaction conditions: [Ag+] = 6.0 � 10�4 mol dm�3,

temperature = 30 8C.

O S O + 2Ag+

O-

OS-

OAg

SAgO S

(complex)

Kc(2)

(4)

(5)

(6)

fast

+ Ag0kO S O

OAg

SAg

OAg

S O

SO

(radical)

slow

2 O

S

OAgOS dimerization O S SS S

OAg OAg

OO

O

(Ag2S4O6)

Ag0 + Ag+ Ag2+

2 Ag2+ Ag4

2+

(yellow color-Ag sol)

(3)

Scheme 1. Mechanism to the reduction of Ag+ ions by S2O32�.

300 400 500 600 700 8000.00

0.05

0.10

0.15

0.20

0.25

0.30

Abs

orba

nce

Wavelength (nm)

Fig. 5. Effect of [NaNO3] on the absorbance of S2O32�-stabilized AgNPs. The

concentration of AgNPs was diluted from the original solution by 4 times.

300 400 500 600 700 8000.00

0.05

0.10

0.15

0.20

0.25

Abs

orba

nce

Wavelength(nm)

Fig. 6. Effect of [Na2CO3] on the absorbance of S2O32�-stabilized AgNPs. The

concentration of AgNPs was diluted from the original solution by 4 times.

A.Y. Obaid et al. / Materials Research Bulletin 48 (2013) 1137–11421140

3.3. Electrolytes and morphology

The shape of the plasmon absorption band of colloidal silver isvery sensitive to the adsorbed substances. The absorption band isdamped and its maximum is shifted to shorter or longerwavelengths, depending on the nature of the adsorbed speciesand its concentrations. Henglein and coworkers pointed out thatthe changes in the shape of the absorption band, one can determinewhether adsorption of an additive has occurred [24]. Figs. 4–7show the effect of the different anions, SO4

2�, NO3�, CO3

2� and Br�,on the absorbance of AgNPs when these ions are added in differentamounts. Surprisingly, shape of the spectra remains unaffected inpresence these electrolytes. On the other hand, absorbance wasenhanced (hyperchromic-shift) by increasing the [KBr] (from

300 40 0 50 0 60 0 70 0 80 00.00

0.05

0.10

0.15

0.20

0.25

Abs

orba

nce

Wavelength (nm)

Fig. 4. Effect of [Na2SO4] on the absorbance S2O32�-stabilized AgNPs. The

concentration of AgNPs was diluted from the original solution by 4 times.

300 40 0 50 0 60 0 70 0 80 00.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Abs

orba

nce

Waveleng th ( nm)

Fig. 7. Effect of [KBr] on the absorbance of S2O32�-stabilized AgNPs. The

concentration of AgNPs was diluted from the original solution by 4 times.

Fig. 8. TEM images and SADE patterns of AgNPs in absence (A) and presence of CTAB

(B) and (C). Reaction conditions: [Ag+] = 2.0 � 10�4 mol dm�3,

[S2O32�] = 2.0 � 10�4 mol dm�3 and [CTAB] = 8.0 � 10�4 mol dm�3.

A.Y. Obaid et al. / Materials Research Bulletin 48 (2013) 1137–1142 1141

2.5 � 10�3 to 7.5 � 10�3 mol � dm�3) without affecting thenanoparticle dispersivity and no change is observed in the shapeof the spectra (Fig. 7). Interestingly, no turbid of AgBr was detectedin presence of KBr (solutions were non-opalescent before as wellafter the addition of Br� as viewed by the naked eye). This resultindicated that essentially all Ag+ ions were transformed to the Ag0

during the redox process and/or adsorbed on the surface ofS2O3

2�–Ag42+ cluster. Thus, we may safely conclude that added

anions (SO42�, NO3

�, and CO32�) and Br� ions could not detach the

adsorbed S2O32� and Ag+ from the surface of Ag4

2+, respectively.Initially, redox kinetics of S2O3

2� and Ag+ reaction was studiedat constant initial [S2O3

2�] and [Ag+] in the presence of CTAB and/or SDS at 30 8C. The observations are summarized in Table 1. Atlower [CTAB] (<3.0 � 10�4 mol dm�3), the reaction mixturesbecomes turbid and a gray precipitate appears where as, the

formation of yellow color is observed in [CTAB](>5.0 � 10�4 mol dm�3). It was found that formation of stableperfect transparent silver sol depend upon the nature of the headgroup of the surfactants, i.e., cetyltrimethylammonium bromide(CTAB) and sodium dodecyl sulfate (SDS). Interestingly, theabsorbance and reaction path changed very rapidly in presenceof CTAB (Table 1), while no effective change in absorbance wasobserved in presence of SDS for a short reaction time, i.e., 2 h.Taking into account the role of CTAB and SDS, it is to be noted thatboth surfactants have different behavior in the stabilization ofsilver particles. This may be attributed to solubilization/incorpo-ration/complexation/adsorption/repulsion/interaction of the reac-tant species S2O3

2�, Ag+ and/or silver particles into the micellarphase. S2O3

2� gets incorporated electro statically into the Sternlayer of CTAB micelles and forms ion-pair with the positive groupof CTAB. Such types of interactions are not favorable in presence ofSDS due to the repulsive forces.

The morphology and crystalline nature of the AgNPs weredetermined by the transmission electron microscope image andselected-area diffraction pattern of the synthesized AgNPs asshown in Fig. 8. AgNps are basically quantum dots or tinierparticles with some bigger spherical and polyl-dispersed with tworanges of size (one is approximately �3 nm, the other �25 nmmeasured from photographs of the TEM images, Fig. 8A). On theother hand, Fig. 8B indicates that the quantum dots, spherical, andsome irregular shaped with size ca. 9 and 35 nm AgNPs whichaggregated into small rods also formed in presence of8.0 � 10�4 mol dm�3 CTAB solution (S2O3

2�e/Ag+ ratio = 1.0). Thuswe may safely conclude that CTAB acts as a capping and/or shape-directing agent [18–20]. Diffraction rings can also be seen whenSADE was conducted on the quantum dots AgNPs (Fig. 8C)indicating the presence pure small size nano-crystalline materials.

4. Conclusions

In summary, a simple, green-synthetic route to prepare advancesilver nanomaterials, reduced, stabilized and damped by sodiumthiosulfate has reported in this paper. Addition of an excess ofS2O3

2� caused the damps of AgNPs absorbance and yellowish-white turbidity appeared instead of perfect transparent yellowcolored. The S2O3

2� stabilized AgNPs are stable in presence ofadditives, NaNO3, Na2SO4, and Na2CO3 whereas KBr causes theiraggregation. These electrolytes are not in position to detach theadsorbed S2O3

2� from the surface of nanoparticles. CTAB acts as ashape-directing and capping agent where SDS has no significanteffect on the growth of AgNPs under our experimental conditions.

Acknowledgments

This project was funded by the Deanship of Scientific Research(DSR), King Abdulaziz Univeristy, Jeddah, under grant no. (320/130/1431). The authors, therefore, acknowledge DSR with thanksfor technical and financial support.

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