- Email: [email protected]
Time dependence of nucleation and growth of silver nanoparticles
Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Time dependence of nucleation and growth of silver nanoparticles Javed Ijaz Hussain a , Abou Talib a , Sunil Kumar b , Shaeel Ahmed AL-Thabaiti c , Athar Adil Hashmi a , Zaheer Khan a,c,∗ a
Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110025, India Department of Chemistry, University of Delhi, New Delhi 110007, India c Department of Chemistry, Faculty of Science, King Abdul Aziz University, P.O. Box 80203, Jeddah 21413, Saudi Arabia b
a r t i c l e
i n f o
Article history: Received 18 December 2010 Received in revised form 20 February 2011 Accepted 25 February 2011 Available online 5 March 2011 Keywords: Silver-mirror Nanoparticles Glucose Ammonia Kinetics
a b s t r a c t This paper describes a simple procedure based on the modified silver-mirror process for the preparation of colloidal silver quantum dots (diameter ≤7 nm). All reactions were monitored with respect to reaction time by using UV–visible studies to understand the growth kinetics and the influence of different [ammonia], [glucose] and temperature on the formation of silver nanoparticles. Shape of the reaction–time curves (sigmoid facing up and sigmoid facing down) strongly depend on the [NH3 ]. Glucose concentrations have no significant effect on the progress of the reaction. For the nucleation and growth processes, surface plasmon absorbance is directly proportional and increase–decrease with increasing [NH3 ], respectively. Transmission electron microscopy (TEM) results show that the quantum dots are aggregated in an irregular manner, resulting in the formation of branches-like structures of silver. The activation energy, enthalpy and entropy of activation have been evaluated. Mechanism consistent with the observed kinetics results has been suggested. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Advanced nanoparticles and/or nanoclusters of silver are one of the most commonly utilized nanomaterials due to their antimicrobial properties, high electrical conductivity, and unique optical properties because they support surface plasmons [1,2]. At specific wavelengths of light the surface plasmons are driven into resonance and the silver nanomaterials have a distinct color that is a function of their size, shape, and environment. The plasmon resonance is responsible for their yellow color in solution. Any visible change to the color of the nanoparticles in solution typically indicates that the aggregation state of the nanoparticles has changed. Great effort has been devoted to the synthesis of oneand two-dimensional (1D and 2D) monodispersed nanocrystals of silver [3–6]. Qi and Dai prepared silver nanoclusters with various morphologies ranging from the leaflike to flowerlike hierarchical structures from the silver mirror reaction (Tollen’s test) by removing the copper oxide layer from commercially available copper foils under different conditions [7]. Huang and Mau have demonstrated by using silver Tollen’s reaction that the carbon nanotubes prepared by the pyrolysis of iron(II) phthalocyanine can selectively
∗ Corresponding author at: Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110025, India. Tel.: +91 11 26981717; fax: +91 11 2698 0229. E-mail address: [email protected] (Z. Khan). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.02.048
grow on a SiO2 surface when the SiO2 wafer is patterned by silver [8]. Role of different surfactants as growth modifiers in the synthesis of silver nanoparticles and their effect on the on fundamental characteristics of silver nanoparticles prepared by modified Tollens process was studied by Soukupova and his coworkers [9]. Novel silver nanomaterials having different morphologies such as nanocubes, nanotubes, etc. structures also are produced from the silver mirror reaction [10–15]. Yu and Yam [16] developed a method to the preparation of silver nanparticles of various morphologies such as triangular particles, rods, wires and cubes, in water by nhexadecyltrimethylammonium bromide modified silver mirror reaction at 120 ◦ C. The Tollens’ test is important in carbohydrate chemistry, for proof of structure. The test is specific for reducing sugars. The purpose of adding the ammonia water is to weaken the reduction of silver ion [17], which makes more beautiful silver. Ag+ + e− → Ag0
E0 = 0.799 V
Ag(NH3 )+ + e− → Ag0 + 2NH3
(1) E0 = 0.373 V
(2)
The Eq. (2) indicates that ammonia forms a complex with silver ion, which is more difficult to reduce than silver ion itself. Addition of the first ammonia results in the exclusion of not one but two water molecules from the inner solvation shell, selectively reducing the equilibrium constant for the first association. This is not surprising because silver ion forms a more stable complex with ammonia
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
than with water. If silver ion is employed without ammonia, the silver ion is reduced so quickly that colloidal silver metal would appear. The solution would become a black and cloudy liquid. The main reaction involved in this process has been long employed in the electroless deposition of silver to generate reflective mirrors on solid supports. In this paper, we report a very simple, low-cost and green chemical method to the preparation of silver nanoparticles on the basis of the modified “silver mirror” reaction. To the best of our knowledge, study on time dependent UV–visible absorption behavior of silver nanoparticles prepared using different concentrations of ammonia and reducing sugar in aqueous media is limited in literature. To our surprise, we observed that the [ammonia] plays an important role in regulating the course of silver nanoclusters thus formed. In this work, we have used the different NH3 and glucose concentrations to determine how the size distribution and optical properties of the Ag-nanoparticles would be affected in the presence of ammonia and/or glucose. A possible formation mechanism for the observed silver nanoclusters is also discussed.
2.0
1.5
Absorbance
24
1.0
0.5
0.0 350
400
450
500
550
600
650
700
750
Wavelength (nm) 2. Materials and methods 2.1. Materials Silver nitrate (AgNO3 , ≥98%) was obtained from Aldrich. Glucose was obtained from Merck with purity greater than 98%. Aqueous NH3 solution (25%) was purchased from Merck, India. n-Hexadecyltrimethylammonium bromide (98%) was Fluka product. The chemicals were used without any further purification. All the solutions were prepared in triply distilled and CO2 -free water, first time from alkaline KMnO4 . Glassware was cleaned prior to use with freshly prepared aqua regia (HCl:HNO3 = 3:1), rinsed with deionized water sequentially. 2.2. Methods 2.2.1. Preparation of silver nanoparticles Upon addition of the glucose aqueous solution, reaction mixture containing AgNO3 and ammonia turned milky yellow. The turbidity disappeared after mixing an aqueous solution of HTAB and transparent yellow color appeared as the reaction proceeds indicating the silver nanoparticles formation as well as role of HTAB [18,19]. It is believed that most of the Ag-particles formed after incorporating the Ag–ammonia complex (vide infra) and glucose into the Stern layer of HTAB micelles by virtue of the presence of water in the Stern layer of the micelles. The initial concentrations of the reagents and temperatures in the reaction system were as follows: AgNO3 (2.0 × 10−4 to 14.0 × 10−4 mol dm−3 ), glucose (reducing agent, 5.0 × 10−4 to 30.0 × 10−4 mol dm−3 ), HTAB (2.0 × 10−4 to 15.0 × 10−4 mol dm−3 ), NH3 (0.5 × 10−4 to 10.0 × 10−4 mol dm−3 ) and temperature (30–40 ◦ C). 2.2.2. Transmission electron microscopy (TEM) Transmission electron microscopic (TEM) micrographs and selected area electron diffraction (SADE) data were acquired with a Hitachi H 7500 electron microscope operating at 80 kV. Samples were prepared by drying a drop of a dilute dispersion of synthesized silver nanoparticles onto a carbon-coated grid and evaporating the solvent in air at room temperature. 2.2.3. Kinetic measurements The two necked reaction vessel (fitted with double walled condenser to arrest the evaporation) containing silver nitrate, ammonia and HTAB was thermally equilibrated at room temperature (40 ± 0.1 ◦ C) and to this was added a solution of glucose, pre-equilibrated at the same temperature. The progress of silver sol formation was followed spectrophotometrically (UV–Visible
Fig. 1. UV–visible spectra of reaction solution containing AgNO3 , ammonia, glucose and HTAB as a function of time at 40 ◦ C. Reaction conditions: [Ag+ ] (=20.0 × 10−4 mol dm−3 ), [HTAB] (=10.0 × 10−4 mol dm−3 ), [glucose] (=10.0 × 10−4 mol dm−3 (), (), (䊉) and 20.0 × 10−4 mol dm−3 (), [ammonia] (=4.0 × 10−4 mol dm−3 () and 10.0 × 10−4 mol dm−3 (), (䊉), ), time (=30 min ()), (), () and 60 min (䊉)).
Recording Spectrophotometer, UV-260 Shimadzu) by monitoring the absorbance at different time intervals at 425 nm (max of silver sol, vide infra) using a sampling technique. The apparent first-order rate constants (kobs , s−1 ) were calculated from the slopes of the plots of ln a/(1 − a) versus time with a fixed-time method [20,21]. 2.2.4. Product identification Paper chromatographic technique was used to identify the oxidation product of glucose using n-butanol–acetic acid–water in a ratio of 4:1:5 as the eluent. Paper chromatograms were developed by AgNO3 –NaOH–Na2 S2 O3 (specific for the detection of aldonic acids) [22]. Seemingly, lactones, which was formed in the rate determining step, was hydrolyzed to form the gluconic acid in neutral medium. 2.2.5. Free radical detection The presence of free radicals in the reaction mixture was evaluated by using acrylonitrile monomer. The monomer solution (5.0 cm3 ) was added to a reaction mixture (10 cm3 , 0.01 mol dm−3 AgNO3 , 5 cm3 , 0.01 mol dm−3 ammonia, 10 cm3 , 0.01 mol dm−3 glucose and 5 cm3 , 0.01 mol dm−3 HTAB). After 20 min, a white precipitate appeared as the reaction proceeded. The positive response indicated in situ generation of free radicals in the reaction mixture. Control experiments (with glucose or AgNO3 only) did not show a precipitate formation. 3. Results and discussion 3.1. Preliminary studies The UV–visible spectra of the resulting yellow color were recorded as a function of time with a spectrophotometer in quartz cuvette of 1 cm path length. Fig. 1 shows some typical UV–visible spectra of the solution (AgNO3 , HTAB, ammonia and glucose) for different periods of time with a strong surface plasmon absorption band around 425 nm (Fig. 1; ) characteristic of spherical silver nanoparticles of wide size distribution [23]. As the reaction proceeds the color of the solution started to change from colorless to light yellow and then brown. Inspections of these results suggesting
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
25
1.2
1.8
Growth 1.0
Absorbance at 425 nm
Absorbance at 425 nm
1.5
1.2
Growth
0.9
0.6
0.8
0.6
0.4 Nucleation 0.2
0.3 Nucleation
0.0
0.0 0
50
100
150
0
200
40
80
Time (min)
120
160
200
Time (min)
Fig. 2. Plots showing the effect of [ammonia] on the growth of Ag-nanoparticles. Reaction conditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [glucose] = 10.0 × 10−4 mol dm−3 ; [HTAB] = 5.0 × 10−4 mol dm−3 ; temp. = 40 ◦ C; [ammonia] = 1.0 (), 2.0 (䊉), 3.0 () and 4.0 × 10−4 mol dm−3 ().
Fig. 3. Plots showing the effect of [ammonia] on the growth of Ag-nanoparticles. Reaction conditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [glucose] = 10.0 × 10−4 mol dm−3 ; [HTAB] = 5.0 × 10−4 mol dm−3 ; temp. = 40 ◦ C; [ammonia] = 5.0 (), 6.0 (䊉), 7.0 (), 8.0 (), 9.0 () and 10.0 × 10−4 mol dm−3 ().
that shape of the each spectrum strongly depend on the [reactants]. At lower [NH3 ] (=4.0 × 10−4 mol dm−3 ; (Fig. 1; ), spectra of silver sol consist a broad shoulder at 525 nm in the whole visible region of the spectrum. We did not observe any change in the shape and absorbance of this spectrum up to 60 min. On the other hand, at higher [NH3 ] (=10.0 × 10−4 mol dm−3 ), shape of the spectrum, position of the shoulder and appearance of sharp peak at 425 nm with time were observed. The surface plasmon resonance absorption of silver nanoparticles have the shorter wavelength band in the visible region at 425 nm is due to the transverse electronic oscillation. These observations consistent with previously reported methods to the preparation of silver nanoparticles by modified silver-mirror reaction [16,24].
(figure not given). The values of apparent rate constants are depicted graphically in Fig. 5 as the kobs –[ammonia] profile. We see that the initially, kobs values increases until it reaches a maximum then decreases with [NH3 ] (it can be noted here that no significant changes was observed in the rate constants at higher [NH3 ] (≥5.0 × 10−4 mol dm−3 ). To explain the [NH3 ] dependence of rate of silver nanoparticles formation, it is necessary to consider the following equilibrium.
In aqueous solution, [NH3 ] plays an important role in determining stability, size, shape and the size distribution of the Ag-nanoparticles [12,16]. In order to see the effect of [NH3 ] on the growth of silver nanoparticles, a series of experiments were preformed at constant [Ag+ ] (=10.0 × 10−4 mol dm−3 ), [glucose] (=10.0 × 10−4 mol dm−3 ), [CTAB] = 5.0 × 10−4 mol dm−3 , temp. (=40 ◦ C) and varying [NH3 ] (=1.0 × 10−4 mol dm−3 to 10.0 × 10−4 mol dm−3 ), and the reaction is monitored spectrophotometrically, unusual kinetic behavior was observed (Figs. 2 and 3). As can be seen in these figures, plots of absorbance versus time clearly indicate that the formation of silver sol has an induction period (nucleation) followed by autocatalysis (growth). Interestingly, sigmoid shape (facing up; [NH3 ] from 1.0 × 10−4 mol dm−3 to 4.0 × 10−4 mol dm−3 ; Fig. 2) of the absorbance-time curves bend abruptly towards the time axis (facing down; [NH3 ] from 5.0 × 10−4 mol dm−3 to 10.0 × 10−4 mol dm−3 ; Fig. 3). The absorbance increased sharply and slightly, respectively, at lower and higher [NH3 ], but it increased smoothly at longer times. The apparent first-order rate constants were calculated from the slopes of the plots of ln a/(1 − a) versus time, where a = At /A˛ and At and A˛ are the absorbance’s at times t and ˛, respectively [20,21]. At lower [NH3 ] (from 1.0 × 10−4 mol dm−3 to 5.0 × 10−4 mol dm−3 ), the plots are linear (Fig. 4). On the other hand, as [NH3 ] increased from ≥6.0 × 10−4 mol dm−3 , the deviation in the linearity commenced
(3)
(pKb = 7.3; Ref. [17])
For which we can write expression (4) pKb = pH + log
[Ag+ ][NH3 ]2
(4)
[Ag(NH3 )2 ]+
In presence of ammonia, Ag+ ions exists mainly [Ag(NH3 )2 ]+ because Ag+ has strong affinity toward ammonia (Ag+ /Ag standard
0.5 0.0 -0.5
ln a/(1-a)
3.2. Effect of [ammonia] on the growth of Ag-nanoparticles
Kb
Ag+ + 2NH3 [Ag(NH3 )2 ]+
-1.0 -1.5 -2.0 -2.5 -3.0 0
50
100
150
200
Time (min) Fig. 4. Plots between ln(a/(1 − a) versus time: Reaction conditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [glucose] = 10.0 × 10−4 mol dm−3 ; [HTAB] = 5.0 × 10−4 mol dm−3 ; temp. = 40 ◦ C; [ammonia] = 1.0 (), 2.0 (䊉), 3.0 (), 4.0 () and 5.0 × 10−4 mol dm−3 ().
26
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
2.0
104 kobs (s-1)
1.5
1.0
0.5
0.0 0
2
4
6
8
10
104 [ammonia] (mol dm-3) Fig. 5. Effect of [ammonia] on kobs . Reaction conditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [glucose] = 10.0 × 10−4 mol dm−3 ; [HTAB] = 5.0 × 10−4 mol dm−3 ; temp. = 40 ◦ C.
redox potential decreases from +0.80 for uncomplexed Ag+ /Ag to +0.38 V for [Ag(NH3 )2 ]+ [17]). This causes decrease in the reduction rate of silver ions upon complexation. As a results, the percentage of [Ag(NH3 )2 ]+ species increases with [NH3 ] which in turn, decreases the reaction rate. This suggests that [Ag(NH3 )2 ]+ is the reactive species in the solution. Fig. 6 shows the variation of absorbance with increasing [ammonia] at two different time intervals. For a short reaction time (20 min (䊉); nucleation path) the surface plasmon absorbance is directly proportional to the [ammonia]. A drastic change in the absorbance behavior is observed at the end of the reaction (200 min (); growth path). As the reaction proceeded, the intensity of the silver sols increased to its maximum value, then decreases with [ammonia], while the absorbance was unchanged at higher [NH3 ] and this value remained constant for at least 2 months. In fact, when the reaction time was increased to 200 min, there are
several processes (reduction, nucleation, coagulation, aggregation and coalescence) likely to coexist. An increase in [NH3 ] in the presence of fixed Ag+ ions results in a decrease in the reduction rate (Fig. 5), and thus is reflected in the particle size. Initially, nucleation results in the formation of small spherical nanoparticles. In the latter stage of particle growth, the limited presence of nuclei would lead to larger particles due to the involvement of aggregation and/or deposition. This shows that slow nucleation leads to thermodynamically stable spherical nanoparticles and properties (e.g., optical absorption) of silver nanoparticles are strongly dependent on their size and the size distribution. To confirm the morphologies of the particles, TEM analysis was carried out. The typical TEM images for the solutions containing 5.0 × 10−4 mol dm−3 and 7.0 × 10−4 mol dm−3 NH3 are shown in Fig. 7(A), (B) and (C), (D), respectively. These images show agglomerates of small grains (quantum dots or tinier particles; size less that 7 nm) and some dispersed spherical nanoparticles (average size is ca. 20 nm). As can be seen in these figures, no significant change in the particle size, shape and the size distribution can be observed, when the reaction mixture containing different [NH3 ]. This indicates that the silver nanoparticles obtained are stable at this stage. The TEM analyses corroborate well with the results drawn from the corresponding reaction–time curves (Fig. 3). The TEM illustrated in Fig. 8 shows the presence of face centered cubic (fcc) particles. Diffraction rings can be seen when corresponding selected area electron diffraction of quantum dots was conducted, further corroborate the small size and crystalline nature of the silver nanoparticles [25]. At the higher magnifications the crystalline nature of the metal cores are visible by the appearance of lattice fringes which identical to a lattice spacing {1 1 1} of bulk silver metal. The TEM images in Fig. 7(A) and (B) indicate that the quantum dots are aggregated and/or deposited on to the surface of Ag0 , reduction species of Ag+ , in a irregular manner, resulted to the formation of beautiful silver (branches-like structures; Fig. 7(C) and (D). These results are in good agreement with the results of Xia et al. to the synthesis and characterization of stable aqueous dispersions of silver nanoparticles through the Tollens process [10].
3.3. Effect of [glucose] on the growth of Ag-nanoparticles 2.0
Absorbance at 425 nm
1.6
1.2
0.8
0.4
0.0
0
2
4
6
8
10
104 [ammonia] (mol dm-3) Fig. 6. Effect of [ammonia] on the absorbance of Ag-nanoparticles formation after 20 (䊉) and 200 min () of mixing of silver nitrate (10.0 × 10−4 mol dm−3 ), glucose (10.0 × 10−4 mol dm−3 ) in presence of HTAB (5.0 × 10−4 mol dm−3 ) at 40 ◦ C.
The particle formation is also affected by the strength of the reducing agent and its molecular structure, which influences its ability to adsorb on the growing particle. Therefore, the effect of [glucose] [range: (2.0–30.0) ×10−4 mol dm−3 ] was studied at constant [Ag+ ] (=10.0 × 10−4 mol dm−3 ), [NH3 ] (=5.0 × 10−4 mol dm−3 ) and [HTAB] (=5.0 × 10−4 mol dm−3 ). Reaction–time curves are depicted graphically in Fig. 9. Interestingly, we did not observed any significant change in path (nucleation and growth) of silver sols formation. However, rate constants first increased until it reached a maximum, then decreased with [glucose], i.e., kobs (×10−5 ) were 0.0, 3.8, 8.6, 11.6, 13.1, 11.2, 9.4 and 8.6 s−1 at [glucose] 0.0, 3.0, 5.0, 10.0, 12.0, 15.0 and 20.0 × 10−4 mol dm−3 , respectively. The increase–decrease behavior of rate constants may be explained in terms of adsorption of glucose onto the surface of metallic silver particles, which in turn, increases the Fermi level of particles [26]. The neutral nucleophiles and neutral stabilizing polymers have strong effect on the surface plasmon absorption band of silver and/or metal nanometer particles and donate the electron density to the particles via lone pairs of electrons. The presence of 6 –OH groups, are responsible to the adsorption of glucose onto the surface of silver particles which easily donates the lone pairs electrons leading to the formation of stable water-soluble nanosize silver particles.
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
27
Fig. 7. TEM images of Ag-nanoparticles. Reaction conditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [glucose] = 10.0 × 10−4 mol dm−3 ; [HTAB] = 5.0 × 10−4 mol dm−3 ; [ammonia] = 5.0 × 10−4 mol dm−3 (A), (B) and 7.0 × 10−4 mol dm−3 (C), (D).
1.0 0.9
Absorbance at 425 nm
0.8 0.7 Growth
0.6 0.5 0.4 0.3
Nucleation
0.2 0.1 0.0 0
50
100
150
200
Time (min)
Fig. 8. SADE images of randomly selected Ag-nanoparticles of Fig. 7(B).
Fig. 9. Plots showing the effect of [glucose] on the growth of Ag-nanoparticles. Reaction conditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [ammonia] = 5.0 × 10−4 mol dm−3 ; [CTAB] = 5.0 × 10−4 mol dm−3 ; temp. = 40 ◦ C; [glucose] = 5.0 (), 10.0 (), 15.0() and 20.0 × 10−4 mol dm−3 (䊉).
28
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
3.4. Mechanism
at lower concentrations of these variables. The following rate-law can be derived from Scheme 1.
Glucose is known to exist in two forms in solution, for example, pyranoid and furanoid forms. Out of these, only the pyranoid form is claimed to be involved in oxidation reactions, which is also exists in equilibrium between ␣- and -pyranose forms with free aldehyde form as an intermediates.
OH
H HO HO
H
H
OH H
H
HO HO
OH
H
H
3.5. Effect of HTAB, pH and activation parameters
O
The effect of [HTAB] (=2.0 × 10−4 to 15.0 × 10−4 mol dm−3 ) was studied at constant [Ag+ ] (=10.0 × 10−4 mol dm−3 ), [NH3 ] (=5.0 × 10−4 mol dm−3 ), [glucose] (=10. × 10−4 mol dm−3 ) and temperature (=40 ◦ C). Absorbance increases with [HTAB] (from 2.0 × 10−4 to 6.0 × 10−4 mol dm−3 ) but at higher [HTAB] (≥7.0 × 10−4 mol dm−3 ), yellowish turbidity appeared instead of transparent yellow color, suggesting that silver colloids were formed and temporarily stabilized by HTAB, but they were unstable due to their high energy resulting from the small particle size (Fig. 7) [16]). Some experiments were also performed to see the formation of AgBr nanoparticles via reaction of AgNO3 with HTAB counter ions. We did not observed the appearance of any color, turbidity and/or precipitate upon the addition of HTAB (from 2.0 × 10−4 to 6.0 × 10−4 mol dm−3 ) in an ammonical silver nitrate solution ([Ag+ ] = 10.0 × 10−4 mol dm−3 ) + [NH3 ] = 5.0 × 10−4 mol dm−3 ) for 200 min at room temperature. These results may be explained in terms of the stability of [Ag(NH3 )2 ]+ complex. Counter ions, Br− , could not replaced the coordinated ammonia molecules from the inner salvation shell of Ag+ , which ruled out the formation of AgBr as the reaction product under normal experimental conditions. The pH of the reaction medium is an important parameter for the preparation of silver nanoparticles with different morphologies but the control of pH and of ionic strength is not as straightforward in micellar solutions as in ordinary solvents. Silver sol solution undergoes acid hydrolysis or is unstable in aqueous solutions of [H+ ] > 1.0 × 10−3 mol dm−3 . The formation of long-lived oligomeric silver clusters has been observed only in the neutral–acidic solution [26]. Tondre et al. [29] have advised to avoid the use of even buffer solutions to maintain pH of micellar solution. However, a series of experiments were performed in order to see any change in the macroscopic pH of the working solution in presence of HTAB (=5.0 × 10−4 mol dm−3 ). The pH values were found to be nearly
H
OH OH
H
It has been confirmed that the -anomer should be more reactive than ␣-anomer [27]. On the other hand, different types of silver species which are designated as Ag0 , Ag2 + , Ag3 2+ , Ag4 2+ , Ag8 Ag+ or Ag9 + and Ag6 4+ have been prepared in solution by the chemicaland photo-reduction of Ag+ ions in aqueous solutions, zeolite cages and frozen organic glasses. Only Ag4 2+ species can be stabilized for along time in presence of a polyanion even under air and growth stops at the stage [26]. Henglein et al. [28] investigated the formation mechanisms of colloidal Ag particles due to the generation of hydrated electrons in pulse radiolysis. They demonstrated that the reduction of Ag+ ions to Ag0 atoms and the transformation of Ag0 atoms into oligomeric Ag clusters. All these considerations, along with the above kinetic results, lead to the proposal of the following mechanism for the reduction of Ag+ to metallic Ag0 particles (Scheme 1). In Scheme 1, Eq. (5) represents complex formation between the reactive species of Ag+ , [Ag(NH3 )2 ]+ , and glucose. Eq. (6) is a onestep, one-electron oxidation–reduction mechanism which gives Ag0 and a free radical, which in turn, reduces another molecule of ammonia–silver complex to Ag0 , metallic silver. The complexation of the formed Ag0 atoms with Ag+ ions yields Ag2 + ions and then the Ag2 + ions dimerize to Ag2+ 4 (Eqs. (8) and (9) [26]. fast
Ag+ + Ag0 −→Ag2 + fast
Ag2 + + Ag2 + −→
(8)
Ag4 2+ (Ag − sol)
(9)
As the appearance of silver sol has complicated kinetic behavior with respect to [NH3 ] and [glucose], the rate-law has been derived
Ag+ + 2NH3
Kb
[Ag(NH3)2]+
OH
H HO HO
H
H
OH
H
O
OH H
H
OH + [Ag(NH3)2]
+
Kc
HO HO
H
O OH H
H
O H - [Ag(NH3)2]
+
(5)
H
(Complex) OH
H HO HO
H
H
H
OH
H
O OH
OH - [Ag(NH3)2]+
k1
HO HO
H
H
H
O
OH
H
.
OH + Ag 0 + 2NH3 + H+
(6)
+ Ag 0 + 2NH3 + H+
(7)
(radical) OH
H HO [Ag(NH3)2] + HO +
H H
H
OH
OH
H
O
.
(10)
Eq. (10) clearly explains the first-order dependence of the reaction each on [NH3 ] and [glucose], respectively.
OH
H
O
d[yellow color] = Kb Kc k1 [(Ag(NH3 )2 )+ ][glucose] dt
OH Fast
HO HO
H H
H
O
OH
O
(lactone) Scheme 1. Mechanism to the formation of metallic silver.
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
reaction–time curves. The nucleation rate was found to be directly proportional to the [ammonia]. Coagulation and coalescence processes were dominant in the growth of nanoparticles at higher [ammonia]. We have also deduced the kinetics that path of silver nanoparticles is independent on the concentrations of glucose. Single-crystal habit was also observed in the tiny particles. The out come of this work may hold prospects for the development a design route for experimental studies on the interconnecting quantum devices in future.
-4.16
log(kobs)
29
-4.32
References
-4.48 3.20
3.24
3.28
3
10 /T Fig. 10. Arrhenius plot to the formation Ag-nanoparticles. Reaction con[ammonia] = 5.0 × 10−4 mol dm−3 ; ditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [CTAB] = 5.0 × 10−4 mol dm−3 ; [glucose] = 10.0 × 10−4 mol dm−3 .
constant with increasing [ammonia] (pH = 9.3 ± 0.2 at [ammonia] values of 2.0, 4.0, 6.0, 8.0 and 10.0 × 10−4 mol dm−3 ). It is not surprising from the fact that ammonia is a weak base; ionization constant = 1.9 × 10−5 . In order to calculate the various activation parameters, the reaction was studied at three different temperatures (30, 35 and 40 ◦ C) at constant [Ag+ ], [NH3 ], [glucose] and [HTAB]. The values of activation parameters (Ea , H# and S# ) were calculated by using Arrhenius (k = Ae−Ea /RT ; log kobs versus 1/T (Fig. 10)) and Eyring (k = (kB T/h)e−H(#) − TS(#) /RT ; log kobs /T versus 1/T) equations and found to be Ea = 70 kJ mol−1 , H# = 67 kJ mol−1 and S# = −109 J K−1 mol−1 . A higher value of activation energy clearly suggests that the transfer of electron from Ag+ to glucose, oxidation–reduction, is slow due to the complexation between Ag+ and ammonia, led to the formation of [Ag(NH3 )]+ , thus resulting in the reduction potential of free Ag+ [17]. The fairly high value of H# indicates that the transition state is highly solvated. The large decrease in S# shows that the transition state is well structured in the micellar phase. Activation energy is much higher in comparison to the Ea reported in the literature to the formation of silver nanoparticles by the silver-mirror reaction in presence of anionic SDS micelles [30]. This is not very surprising judging from the fact that nature of the surfactants play an important role in the association, penetration, solubilization and/or stabilization of reactants as well as products into the micellar-pseudo phase. It is certainly possible that the positive charge on [Ag(NH3 )2 ]+ forms an ion pair (–OSO− 3 –[Ag(NH3 )2 ]+ ) with the negative head group of SDS micelles which concentrate the [Ag(NH3 )2 ]+ within the small volume through the electrostatic interactions in to the reaction site. As a results, the reaction proceeds more fast in presence of SDS than HTAB. 4. Conclusions In summary, we have demonstrated for the first time that the silver-mirror reaction is a suitable chemical and an easy route to the preparation of quantum dots, which underwent an adhesion and coalescence process to assemble into branches-like beautiful silver nanostructures. It has been proven that an increase in [ammonia] in the reaction mixture from 4.0 × 10−4 mol dm−3 to 5.0 × 10−4 mol dm−3 leads to an abruptly change in the shape of the
[1] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chemistry and properties of nanocrystals of different shapes, Chem. Rev. 105 (2005) 1025–1102. [2] J.A. Dahl, B.L.S. Maddux, J.E. Hutchison, Toward greener nanosynthesis, Chem. Rev. 107 (2007) 2228–2269. [3] Y. Sun, Y. Xia, Shape-controlled synthesis of gold and silver nanoparticles, Science 298 (2002) 2176–2179. [4] R. Jin, Y.C. Cao, E. Hao, G.S. Metraux, G.Z. Schatz, C.A. Mirkin, Controlling anisotropic nanoparticle growth through plasmon excitation, Nature 425 (2003) 487–490. [5] Y. Sun, B. Mayers, Y.N. Xia, Transformation of silver sanospheres into nanobelts and triangular nanoplates through a thermal process, Nano Lett. 3 (2003) 675–679. [6] E.C. Hao, K.L. Kelly, J.T. Hupp, G.C. Schatz, Synthesis of silver nanodisks using polystyrene mesospheres as templates, J. Am. Chem. Soc. 124 (2002) 15182–15183. [7] L. Qu, L. Dai, Novel silver nanostructures from silver mirror reaction on reactive substrates, J. Phys. Chem. B 109 (2005) 13985. [8] S. Huang, A.W.H. Mau, Selective growth of aligned carbon nanotubes on a silverpatterned substrate by the silver mirror reaction, J. Phys. Chem. B 107 (2003) 3455–3458. [9] J. Soukupova, I. Kvitek, A. Panacek, T. Nevecna, R. Zboril, Comprehensive study on surfactant role on silver nanoparticles (NPs) prepared via modified Tollens process, Mater. Chem. Phys. 111 (2008) 77–81. [10] Y. Yin, Z.-Y. Li, Z. Zhong, B. Gates, Y. Xia, S. Venkateswaran, Synthesis and characterization of stable aqueous dispersions of silver nanoparticles through the Tollens process, J. Mater. Chem. 12 (2002) 522–527. [11] S.-H. Zhang, Z.-X. Xie, Z.-Y. Jiang, X. Xu, J. Xiang, R.-B. Huang, L.-S. Zheng, Synthesis of silver nanotubes by electroless deposition in porous anodic aluminium oxide templates, Chem. Commun. (2004) 1106–1107. [12] D. Yu, V.W.-W. Yam, Controlled synthesis of monodisperse silver nanoparticles in water, J. Am. Chem. Soc. 126 (2004), 1320013201. [13] S. Sangsuk, Preparation of high surface area silver powder via Tollens process under sonication, Mater. Lett. 64 (2010) 775–777. [14] T. Textor, M.M.G. Fouda, B. Mahltig, Deposition of durable thin silver layers onto polyamides employing a heterogeneous Tollens’ reaction, Appl. Surf. Sci. 256 (2010) 2337–2342. [15] A.-T. Le, P.T. Huy, P.D. Tam, T.Q. Huy, P.D. Cam, A.A. Kudrinskiy, Y.A. Krutyakov, Green synthesis of finely-dispersed highly bactericidal silver nanoparticles via modified Tollens technique, Curr. Appl. Phys. 10 (2010) 910–916. [16] D. Yu, V.W.-W. Yam, Hydrothermal-induced assembly of colloidal silver spheres into various nanoparticles on the basis of HTAB-modified silver-mirror reaction, J. Phys. Chem. B 109 (2005) 5497–5503. [17] D.V. Goia, E. Matijevic, Preparation of monodispersed metal particles, New J. Chem. (1998) 1203–1215. [18] (a) T. Pal, T.K. Sau, N.R. Jana, Reversible formation and dissolution of silver nanoparticles in aqueous surfactant media, Langmuir 13 (1997) 1481–1485; (b) V.K. Sharma, R.A. Yngard, Y. Lin, Silver nanoparticles: green synthesis and their antimicrobial activities, Adv. Colloid Interface Sci. 145 (2009) 83–96. [19] C.A. Bunton, The dependence of micellar rate effects upon reaction mechanism, Adv. Colloid Interface Sci. 123 (2006) 333–343. [20] K. Esumi, T. Hosoyo, A. Yamahira, K. Torigoe, Formation of gold and silver nanoparticles in aqueous solution of sugar-persubsituted poly(amidoamine) dendrimers, J. Colloid Interface Sci. 226 (2000) 346–352. [21] N. Ahmad, M.A. Malik, F.M. Al-Nowaiser, Z. Khan, A kinetic study of silver nanoparticles formation from paracetamol and silver(I) in aqueous and micellar media, Colloids Surf. B: Biointerfaces 78 (2010) 109–114. [22] M.A. Malik, S.A. Al-Thabaiti, Z. Khan, Kinetics of oxidation of d-glucose by permanganate in aqueous solution of cetyltrimethylammonium bromide, Colloids Surf. A: Physicochem. Eng. Aspects 337 (2009) 9–14. [23] J. Xie, J.Y. Lee, D.I.C. Wang, Y.P. Ting, Silver nanoparticles: from biological to biomimetic synthesis, ACS Nano 1 (2007) 429–439. [24] L. Kvitek, R. Prucek, A. Panacek, R. Novotny, J. Hrbac, R. Zboril, The influence of complexing agent concentration on particle size in the process of SERS active silver colloid synthesis, J. Mater. Chem. 15 (2005) 1099–1105. [25] H. Huang, Y. Yang, Preparation of silver nanoparticles in inorganic clay suspensions, Compos. Sci. Technol. 68 (2008) 2948–2953.
30
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
[26] A. Henglein, Physicochemical properties of small metal particles in solution: “microelectrode” reactions, chemisorption, composite metal particles, and the atom-to-metal transition, Phys. Chem. 97 (1993) 5457–5471. [27] M. Rudrum, F.D. Shaw, The structure and conformation of some monosaccharides in solution, J. Chem. Soc. (1965) 52–57. [28] (a) P. Mulvaney, A. Henglein, Long-lived nonmetallic silver clusters in aqueous solution: a pulse radiolysis study of their formation, J. Phys. Chem. 94 (1990) 4182–4188; (b) T. Linnert, P. Mulvaney, A. Henglein, H. Weller, Long-lived nonmetallic silver clusters in aqueous solution: preparation and photolysis, J. Am. Chem. Soc. 112 (1990) 4657–4664.
[29] W. Richmond, C. Tondre, E. Krzxanowiska, J. Szymanowski, Metal extraction in micellar media: a comparison of copper complexation rates and ultrafiltration yields of two isomeric (E)-1-(2-hydroxy-5-alkylphenyl) 1-alkanone oximes, J. Chem. Soc. Faraday Trans. 91 (1995) 657–664. [30] S.K. Mehta, S. Chaudhary, M. Gradzielski, Time dependence of nucleation and growth of silver nanoparticles generated by sugar reduction in micellar media, J. Colloid Interface Sci. 343 (2010) 447–453.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Time dependence of nucleation and growth of silver nanoparticles Javed Ijaz Hussain a , Abou Talib a , Sunil Kumar b , Shaeel Ahmed AL-Thabaiti c , Athar Adil Hashmi a , Zaheer Khan a,c,∗ a
Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110025, India Department of Chemistry, University of Delhi, New Delhi 110007, India c Department of Chemistry, Faculty of Science, King Abdul Aziz University, P.O. Box 80203, Jeddah 21413, Saudi Arabia b
a r t i c l e
i n f o
Article history: Received 18 December 2010 Received in revised form 20 February 2011 Accepted 25 February 2011 Available online 5 March 2011 Keywords: Silver-mirror Nanoparticles Glucose Ammonia Kinetics
a b s t r a c t This paper describes a simple procedure based on the modified silver-mirror process for the preparation of colloidal silver quantum dots (diameter ≤7 nm). All reactions were monitored with respect to reaction time by using UV–visible studies to understand the growth kinetics and the influence of different [ammonia], [glucose] and temperature on the formation of silver nanoparticles. Shape of the reaction–time curves (sigmoid facing up and sigmoid facing down) strongly depend on the [NH3 ]. Glucose concentrations have no significant effect on the progress of the reaction. For the nucleation and growth processes, surface plasmon absorbance is directly proportional and increase–decrease with increasing [NH3 ], respectively. Transmission electron microscopy (TEM) results show that the quantum dots are aggregated in an irregular manner, resulting in the formation of branches-like structures of silver. The activation energy, enthalpy and entropy of activation have been evaluated. Mechanism consistent with the observed kinetics results has been suggested. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Advanced nanoparticles and/or nanoclusters of silver are one of the most commonly utilized nanomaterials due to their antimicrobial properties, high electrical conductivity, and unique optical properties because they support surface plasmons [1,2]. At specific wavelengths of light the surface plasmons are driven into resonance and the silver nanomaterials have a distinct color that is a function of their size, shape, and environment. The plasmon resonance is responsible for their yellow color in solution. Any visible change to the color of the nanoparticles in solution typically indicates that the aggregation state of the nanoparticles has changed. Great effort has been devoted to the synthesis of oneand two-dimensional (1D and 2D) monodispersed nanocrystals of silver [3–6]. Qi and Dai prepared silver nanoclusters with various morphologies ranging from the leaflike to flowerlike hierarchical structures from the silver mirror reaction (Tollen’s test) by removing the copper oxide layer from commercially available copper foils under different conditions [7]. Huang and Mau have demonstrated by using silver Tollen’s reaction that the carbon nanotubes prepared by the pyrolysis of iron(II) phthalocyanine can selectively
∗ Corresponding author at: Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110025, India. Tel.: +91 11 26981717; fax: +91 11 2698 0229. E-mail address: [email protected] (Z. Khan). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.02.048
grow on a SiO2 surface when the SiO2 wafer is patterned by silver [8]. Role of different surfactants as growth modifiers in the synthesis of silver nanoparticles and their effect on the on fundamental characteristics of silver nanoparticles prepared by modified Tollens process was studied by Soukupova and his coworkers [9]. Novel silver nanomaterials having different morphologies such as nanocubes, nanotubes, etc. structures also are produced from the silver mirror reaction [10–15]. Yu and Yam [16] developed a method to the preparation of silver nanparticles of various morphologies such as triangular particles, rods, wires and cubes, in water by nhexadecyltrimethylammonium bromide modified silver mirror reaction at 120 ◦ C. The Tollens’ test is important in carbohydrate chemistry, for proof of structure. The test is specific for reducing sugars. The purpose of adding the ammonia water is to weaken the reduction of silver ion [17], which makes more beautiful silver. Ag+ + e− → Ag0
E0 = 0.799 V
Ag(NH3 )+ + e− → Ag0 + 2NH3
(1) E0 = 0.373 V
(2)
The Eq. (2) indicates that ammonia forms a complex with silver ion, which is more difficult to reduce than silver ion itself. Addition of the first ammonia results in the exclusion of not one but two water molecules from the inner solvation shell, selectively reducing the equilibrium constant for the first association. This is not surprising because silver ion forms a more stable complex with ammonia
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
than with water. If silver ion is employed without ammonia, the silver ion is reduced so quickly that colloidal silver metal would appear. The solution would become a black and cloudy liquid. The main reaction involved in this process has been long employed in the electroless deposition of silver to generate reflective mirrors on solid supports. In this paper, we report a very simple, low-cost and green chemical method to the preparation of silver nanoparticles on the basis of the modified “silver mirror” reaction. To the best of our knowledge, study on time dependent UV–visible absorption behavior of silver nanoparticles prepared using different concentrations of ammonia and reducing sugar in aqueous media is limited in literature. To our surprise, we observed that the [ammonia] plays an important role in regulating the course of silver nanoclusters thus formed. In this work, we have used the different NH3 and glucose concentrations to determine how the size distribution and optical properties of the Ag-nanoparticles would be affected in the presence of ammonia and/or glucose. A possible formation mechanism for the observed silver nanoclusters is also discussed.
2.0
1.5
Absorbance
24
1.0
0.5
0.0 350
400
450
500
550
600
650
700
750
Wavelength (nm) 2. Materials and methods 2.1. Materials Silver nitrate (AgNO3 , ≥98%) was obtained from Aldrich. Glucose was obtained from Merck with purity greater than 98%. Aqueous NH3 solution (25%) was purchased from Merck, India. n-Hexadecyltrimethylammonium bromide (98%) was Fluka product. The chemicals were used without any further purification. All the solutions were prepared in triply distilled and CO2 -free water, first time from alkaline KMnO4 . Glassware was cleaned prior to use with freshly prepared aqua regia (HCl:HNO3 = 3:1), rinsed with deionized water sequentially. 2.2. Methods 2.2.1. Preparation of silver nanoparticles Upon addition of the glucose aqueous solution, reaction mixture containing AgNO3 and ammonia turned milky yellow. The turbidity disappeared after mixing an aqueous solution of HTAB and transparent yellow color appeared as the reaction proceeds indicating the silver nanoparticles formation as well as role of HTAB [18,19]. It is believed that most of the Ag-particles formed after incorporating the Ag–ammonia complex (vide infra) and glucose into the Stern layer of HTAB micelles by virtue of the presence of water in the Stern layer of the micelles. The initial concentrations of the reagents and temperatures in the reaction system were as follows: AgNO3 (2.0 × 10−4 to 14.0 × 10−4 mol dm−3 ), glucose (reducing agent, 5.0 × 10−4 to 30.0 × 10−4 mol dm−3 ), HTAB (2.0 × 10−4 to 15.0 × 10−4 mol dm−3 ), NH3 (0.5 × 10−4 to 10.0 × 10−4 mol dm−3 ) and temperature (30–40 ◦ C). 2.2.2. Transmission electron microscopy (TEM) Transmission electron microscopic (TEM) micrographs and selected area electron diffraction (SADE) data were acquired with a Hitachi H 7500 electron microscope operating at 80 kV. Samples were prepared by drying a drop of a dilute dispersion of synthesized silver nanoparticles onto a carbon-coated grid and evaporating the solvent in air at room temperature. 2.2.3. Kinetic measurements The two necked reaction vessel (fitted with double walled condenser to arrest the evaporation) containing silver nitrate, ammonia and HTAB was thermally equilibrated at room temperature (40 ± 0.1 ◦ C) and to this was added a solution of glucose, pre-equilibrated at the same temperature. The progress of silver sol formation was followed spectrophotometrically (UV–Visible
Fig. 1. UV–visible spectra of reaction solution containing AgNO3 , ammonia, glucose and HTAB as a function of time at 40 ◦ C. Reaction conditions: [Ag+ ] (=20.0 × 10−4 mol dm−3 ), [HTAB] (=10.0 × 10−4 mol dm−3 ), [glucose] (=10.0 × 10−4 mol dm−3 (), (), (䊉) and 20.0 × 10−4 mol dm−3 (), [ammonia] (=4.0 × 10−4 mol dm−3 () and 10.0 × 10−4 mol dm−3 (), (䊉), ), time (=30 min ()), (), () and 60 min (䊉)).
Recording Spectrophotometer, UV-260 Shimadzu) by monitoring the absorbance at different time intervals at 425 nm (max of silver sol, vide infra) using a sampling technique. The apparent first-order rate constants (kobs , s−1 ) were calculated from the slopes of the plots of ln a/(1 − a) versus time with a fixed-time method [20,21]. 2.2.4. Product identification Paper chromatographic technique was used to identify the oxidation product of glucose using n-butanol–acetic acid–water in a ratio of 4:1:5 as the eluent. Paper chromatograms were developed by AgNO3 –NaOH–Na2 S2 O3 (specific for the detection of aldonic acids) [22]. Seemingly, lactones, which was formed in the rate determining step, was hydrolyzed to form the gluconic acid in neutral medium. 2.2.5. Free radical detection The presence of free radicals in the reaction mixture was evaluated by using acrylonitrile monomer. The monomer solution (5.0 cm3 ) was added to a reaction mixture (10 cm3 , 0.01 mol dm−3 AgNO3 , 5 cm3 , 0.01 mol dm−3 ammonia, 10 cm3 , 0.01 mol dm−3 glucose and 5 cm3 , 0.01 mol dm−3 HTAB). After 20 min, a white precipitate appeared as the reaction proceeded. The positive response indicated in situ generation of free radicals in the reaction mixture. Control experiments (with glucose or AgNO3 only) did not show a precipitate formation. 3. Results and discussion 3.1. Preliminary studies The UV–visible spectra of the resulting yellow color were recorded as a function of time with a spectrophotometer in quartz cuvette of 1 cm path length. Fig. 1 shows some typical UV–visible spectra of the solution (AgNO3 , HTAB, ammonia and glucose) for different periods of time with a strong surface plasmon absorption band around 425 nm (Fig. 1; ) characteristic of spherical silver nanoparticles of wide size distribution [23]. As the reaction proceeds the color of the solution started to change from colorless to light yellow and then brown. Inspections of these results suggesting
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
25
1.2
1.8
Growth 1.0
Absorbance at 425 nm
Absorbance at 425 nm
1.5
1.2
Growth
0.9
0.6
0.8
0.6
0.4 Nucleation 0.2
0.3 Nucleation
0.0
0.0 0
50
100
150
0
200
40
80
Time (min)
120
160
200
Time (min)
Fig. 2. Plots showing the effect of [ammonia] on the growth of Ag-nanoparticles. Reaction conditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [glucose] = 10.0 × 10−4 mol dm−3 ; [HTAB] = 5.0 × 10−4 mol dm−3 ; temp. = 40 ◦ C; [ammonia] = 1.0 (), 2.0 (䊉), 3.0 () and 4.0 × 10−4 mol dm−3 ().
Fig. 3. Plots showing the effect of [ammonia] on the growth of Ag-nanoparticles. Reaction conditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [glucose] = 10.0 × 10−4 mol dm−3 ; [HTAB] = 5.0 × 10−4 mol dm−3 ; temp. = 40 ◦ C; [ammonia] = 5.0 (), 6.0 (䊉), 7.0 (), 8.0 (), 9.0 () and 10.0 × 10−4 mol dm−3 ().
that shape of the each spectrum strongly depend on the [reactants]. At lower [NH3 ] (=4.0 × 10−4 mol dm−3 ; (Fig. 1; ), spectra of silver sol consist a broad shoulder at 525 nm in the whole visible region of the spectrum. We did not observe any change in the shape and absorbance of this spectrum up to 60 min. On the other hand, at higher [NH3 ] (=10.0 × 10−4 mol dm−3 ), shape of the spectrum, position of the shoulder and appearance of sharp peak at 425 nm with time were observed. The surface plasmon resonance absorption of silver nanoparticles have the shorter wavelength band in the visible region at 425 nm is due to the transverse electronic oscillation. These observations consistent with previously reported methods to the preparation of silver nanoparticles by modified silver-mirror reaction [16,24].
(figure not given). The values of apparent rate constants are depicted graphically in Fig. 5 as the kobs –[ammonia] profile. We see that the initially, kobs values increases until it reaches a maximum then decreases with [NH3 ] (it can be noted here that no significant changes was observed in the rate constants at higher [NH3 ] (≥5.0 × 10−4 mol dm−3 ). To explain the [NH3 ] dependence of rate of silver nanoparticles formation, it is necessary to consider the following equilibrium.
In aqueous solution, [NH3 ] plays an important role in determining stability, size, shape and the size distribution of the Ag-nanoparticles [12,16]. In order to see the effect of [NH3 ] on the growth of silver nanoparticles, a series of experiments were preformed at constant [Ag+ ] (=10.0 × 10−4 mol dm−3 ), [glucose] (=10.0 × 10−4 mol dm−3 ), [CTAB] = 5.0 × 10−4 mol dm−3 , temp. (=40 ◦ C) and varying [NH3 ] (=1.0 × 10−4 mol dm−3 to 10.0 × 10−4 mol dm−3 ), and the reaction is monitored spectrophotometrically, unusual kinetic behavior was observed (Figs. 2 and 3). As can be seen in these figures, plots of absorbance versus time clearly indicate that the formation of silver sol has an induction period (nucleation) followed by autocatalysis (growth). Interestingly, sigmoid shape (facing up; [NH3 ] from 1.0 × 10−4 mol dm−3 to 4.0 × 10−4 mol dm−3 ; Fig. 2) of the absorbance-time curves bend abruptly towards the time axis (facing down; [NH3 ] from 5.0 × 10−4 mol dm−3 to 10.0 × 10−4 mol dm−3 ; Fig. 3). The absorbance increased sharply and slightly, respectively, at lower and higher [NH3 ], but it increased smoothly at longer times. The apparent first-order rate constants were calculated from the slopes of the plots of ln a/(1 − a) versus time, where a = At /A˛ and At and A˛ are the absorbance’s at times t and ˛, respectively [20,21]. At lower [NH3 ] (from 1.0 × 10−4 mol dm−3 to 5.0 × 10−4 mol dm−3 ), the plots are linear (Fig. 4). On the other hand, as [NH3 ] increased from ≥6.0 × 10−4 mol dm−3 , the deviation in the linearity commenced
(3)
(pKb = 7.3; Ref. [17])
For which we can write expression (4) pKb = pH + log
[Ag+ ][NH3 ]2
(4)
[Ag(NH3 )2 ]+
In presence of ammonia, Ag+ ions exists mainly [Ag(NH3 )2 ]+ because Ag+ has strong affinity toward ammonia (Ag+ /Ag standard
0.5 0.0 -0.5
ln a/(1-a)
3.2. Effect of [ammonia] on the growth of Ag-nanoparticles
Kb
Ag+ + 2NH3 [Ag(NH3 )2 ]+
-1.0 -1.5 -2.0 -2.5 -3.0 0
50
100
150
200
Time (min) Fig. 4. Plots between ln(a/(1 − a) versus time: Reaction conditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [glucose] = 10.0 × 10−4 mol dm−3 ; [HTAB] = 5.0 × 10−4 mol dm−3 ; temp. = 40 ◦ C; [ammonia] = 1.0 (), 2.0 (䊉), 3.0 (), 4.0 () and 5.0 × 10−4 mol dm−3 ().
26
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
2.0
104 kobs (s-1)
1.5
1.0
0.5
0.0 0
2
4
6
8
10
104 [ammonia] (mol dm-3) Fig. 5. Effect of [ammonia] on kobs . Reaction conditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [glucose] = 10.0 × 10−4 mol dm−3 ; [HTAB] = 5.0 × 10−4 mol dm−3 ; temp. = 40 ◦ C.
redox potential decreases from +0.80 for uncomplexed Ag+ /Ag to +0.38 V for [Ag(NH3 )2 ]+ [17]). This causes decrease in the reduction rate of silver ions upon complexation. As a results, the percentage of [Ag(NH3 )2 ]+ species increases with [NH3 ] which in turn, decreases the reaction rate. This suggests that [Ag(NH3 )2 ]+ is the reactive species in the solution. Fig. 6 shows the variation of absorbance with increasing [ammonia] at two different time intervals. For a short reaction time (20 min (䊉); nucleation path) the surface plasmon absorbance is directly proportional to the [ammonia]. A drastic change in the absorbance behavior is observed at the end of the reaction (200 min (); growth path). As the reaction proceeded, the intensity of the silver sols increased to its maximum value, then decreases with [ammonia], while the absorbance was unchanged at higher [NH3 ] and this value remained constant for at least 2 months. In fact, when the reaction time was increased to 200 min, there are
several processes (reduction, nucleation, coagulation, aggregation and coalescence) likely to coexist. An increase in [NH3 ] in the presence of fixed Ag+ ions results in a decrease in the reduction rate (Fig. 5), and thus is reflected in the particle size. Initially, nucleation results in the formation of small spherical nanoparticles. In the latter stage of particle growth, the limited presence of nuclei would lead to larger particles due to the involvement of aggregation and/or deposition. This shows that slow nucleation leads to thermodynamically stable spherical nanoparticles and properties (e.g., optical absorption) of silver nanoparticles are strongly dependent on their size and the size distribution. To confirm the morphologies of the particles, TEM analysis was carried out. The typical TEM images for the solutions containing 5.0 × 10−4 mol dm−3 and 7.0 × 10−4 mol dm−3 NH3 are shown in Fig. 7(A), (B) and (C), (D), respectively. These images show agglomerates of small grains (quantum dots or tinier particles; size less that 7 nm) and some dispersed spherical nanoparticles (average size is ca. 20 nm). As can be seen in these figures, no significant change in the particle size, shape and the size distribution can be observed, when the reaction mixture containing different [NH3 ]. This indicates that the silver nanoparticles obtained are stable at this stage. The TEM analyses corroborate well with the results drawn from the corresponding reaction–time curves (Fig. 3). The TEM illustrated in Fig. 8 shows the presence of face centered cubic (fcc) particles. Diffraction rings can be seen when corresponding selected area electron diffraction of quantum dots was conducted, further corroborate the small size and crystalline nature of the silver nanoparticles [25]. At the higher magnifications the crystalline nature of the metal cores are visible by the appearance of lattice fringes which identical to a lattice spacing {1 1 1} of bulk silver metal. The TEM images in Fig. 7(A) and (B) indicate that the quantum dots are aggregated and/or deposited on to the surface of Ag0 , reduction species of Ag+ , in a irregular manner, resulted to the formation of beautiful silver (branches-like structures; Fig. 7(C) and (D). These results are in good agreement with the results of Xia et al. to the synthesis and characterization of stable aqueous dispersions of silver nanoparticles through the Tollens process [10].
3.3. Effect of [glucose] on the growth of Ag-nanoparticles 2.0
Absorbance at 425 nm
1.6
1.2
0.8
0.4
0.0
0
2
4
6
8
10
104 [ammonia] (mol dm-3) Fig. 6. Effect of [ammonia] on the absorbance of Ag-nanoparticles formation after 20 (䊉) and 200 min () of mixing of silver nitrate (10.0 × 10−4 mol dm−3 ), glucose (10.0 × 10−4 mol dm−3 ) in presence of HTAB (5.0 × 10−4 mol dm−3 ) at 40 ◦ C.
The particle formation is also affected by the strength of the reducing agent and its molecular structure, which influences its ability to adsorb on the growing particle. Therefore, the effect of [glucose] [range: (2.0–30.0) ×10−4 mol dm−3 ] was studied at constant [Ag+ ] (=10.0 × 10−4 mol dm−3 ), [NH3 ] (=5.0 × 10−4 mol dm−3 ) and [HTAB] (=5.0 × 10−4 mol dm−3 ). Reaction–time curves are depicted graphically in Fig. 9. Interestingly, we did not observed any significant change in path (nucleation and growth) of silver sols formation. However, rate constants first increased until it reached a maximum, then decreased with [glucose], i.e., kobs (×10−5 ) were 0.0, 3.8, 8.6, 11.6, 13.1, 11.2, 9.4 and 8.6 s−1 at [glucose] 0.0, 3.0, 5.0, 10.0, 12.0, 15.0 and 20.0 × 10−4 mol dm−3 , respectively. The increase–decrease behavior of rate constants may be explained in terms of adsorption of glucose onto the surface of metallic silver particles, which in turn, increases the Fermi level of particles [26]. The neutral nucleophiles and neutral stabilizing polymers have strong effect on the surface plasmon absorption band of silver and/or metal nanometer particles and donate the electron density to the particles via lone pairs of electrons. The presence of 6 –OH groups, are responsible to the adsorption of glucose onto the surface of silver particles which easily donates the lone pairs electrons leading to the formation of stable water-soluble nanosize silver particles.
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
27
Fig. 7. TEM images of Ag-nanoparticles. Reaction conditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [glucose] = 10.0 × 10−4 mol dm−3 ; [HTAB] = 5.0 × 10−4 mol dm−3 ; [ammonia] = 5.0 × 10−4 mol dm−3 (A), (B) and 7.0 × 10−4 mol dm−3 (C), (D).
1.0 0.9
Absorbance at 425 nm
0.8 0.7 Growth
0.6 0.5 0.4 0.3
Nucleation
0.2 0.1 0.0 0
50
100
150
200
Time (min)
Fig. 8. SADE images of randomly selected Ag-nanoparticles of Fig. 7(B).
Fig. 9. Plots showing the effect of [glucose] on the growth of Ag-nanoparticles. Reaction conditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [ammonia] = 5.0 × 10−4 mol dm−3 ; [CTAB] = 5.0 × 10−4 mol dm−3 ; temp. = 40 ◦ C; [glucose] = 5.0 (), 10.0 (), 15.0() and 20.0 × 10−4 mol dm−3 (䊉).
28
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
3.4. Mechanism
at lower concentrations of these variables. The following rate-law can be derived from Scheme 1.
Glucose is known to exist in two forms in solution, for example, pyranoid and furanoid forms. Out of these, only the pyranoid form is claimed to be involved in oxidation reactions, which is also exists in equilibrium between ␣- and -pyranose forms with free aldehyde form as an intermediates.
OH
H HO HO
H
H
OH H
H
HO HO
OH
H
H
3.5. Effect of HTAB, pH and activation parameters
O
The effect of [HTAB] (=2.0 × 10−4 to 15.0 × 10−4 mol dm−3 ) was studied at constant [Ag+ ] (=10.0 × 10−4 mol dm−3 ), [NH3 ] (=5.0 × 10−4 mol dm−3 ), [glucose] (=10. × 10−4 mol dm−3 ) and temperature (=40 ◦ C). Absorbance increases with [HTAB] (from 2.0 × 10−4 to 6.0 × 10−4 mol dm−3 ) but at higher [HTAB] (≥7.0 × 10−4 mol dm−3 ), yellowish turbidity appeared instead of transparent yellow color, suggesting that silver colloids were formed and temporarily stabilized by HTAB, but they were unstable due to their high energy resulting from the small particle size (Fig. 7) [16]). Some experiments were also performed to see the formation of AgBr nanoparticles via reaction of AgNO3 with HTAB counter ions. We did not observed the appearance of any color, turbidity and/or precipitate upon the addition of HTAB (from 2.0 × 10−4 to 6.0 × 10−4 mol dm−3 ) in an ammonical silver nitrate solution ([Ag+ ] = 10.0 × 10−4 mol dm−3 ) + [NH3 ] = 5.0 × 10−4 mol dm−3 ) for 200 min at room temperature. These results may be explained in terms of the stability of [Ag(NH3 )2 ]+ complex. Counter ions, Br− , could not replaced the coordinated ammonia molecules from the inner salvation shell of Ag+ , which ruled out the formation of AgBr as the reaction product under normal experimental conditions. The pH of the reaction medium is an important parameter for the preparation of silver nanoparticles with different morphologies but the control of pH and of ionic strength is not as straightforward in micellar solutions as in ordinary solvents. Silver sol solution undergoes acid hydrolysis or is unstable in aqueous solutions of [H+ ] > 1.0 × 10−3 mol dm−3 . The formation of long-lived oligomeric silver clusters has been observed only in the neutral–acidic solution [26]. Tondre et al. [29] have advised to avoid the use of even buffer solutions to maintain pH of micellar solution. However, a series of experiments were performed in order to see any change in the macroscopic pH of the working solution in presence of HTAB (=5.0 × 10−4 mol dm−3 ). The pH values were found to be nearly
H
OH OH
H
It has been confirmed that the -anomer should be more reactive than ␣-anomer [27]. On the other hand, different types of silver species which are designated as Ag0 , Ag2 + , Ag3 2+ , Ag4 2+ , Ag8 Ag+ or Ag9 + and Ag6 4+ have been prepared in solution by the chemicaland photo-reduction of Ag+ ions in aqueous solutions, zeolite cages and frozen organic glasses. Only Ag4 2+ species can be stabilized for along time in presence of a polyanion even under air and growth stops at the stage [26]. Henglein et al. [28] investigated the formation mechanisms of colloidal Ag particles due to the generation of hydrated electrons in pulse radiolysis. They demonstrated that the reduction of Ag+ ions to Ag0 atoms and the transformation of Ag0 atoms into oligomeric Ag clusters. All these considerations, along with the above kinetic results, lead to the proposal of the following mechanism for the reduction of Ag+ to metallic Ag0 particles (Scheme 1). In Scheme 1, Eq. (5) represents complex formation between the reactive species of Ag+ , [Ag(NH3 )2 ]+ , and glucose. Eq. (6) is a onestep, one-electron oxidation–reduction mechanism which gives Ag0 and a free radical, which in turn, reduces another molecule of ammonia–silver complex to Ag0 , metallic silver. The complexation of the formed Ag0 atoms with Ag+ ions yields Ag2 + ions and then the Ag2 + ions dimerize to Ag2+ 4 (Eqs. (8) and (9) [26]. fast
Ag+ + Ag0 −→Ag2 + fast
Ag2 + + Ag2 + −→
(8)
Ag4 2+ (Ag − sol)
(9)
As the appearance of silver sol has complicated kinetic behavior with respect to [NH3 ] and [glucose], the rate-law has been derived
Ag+ + 2NH3
Kb
[Ag(NH3)2]+
OH
H HO HO
H
H
OH
H
O
OH H
H
OH + [Ag(NH3)2]
+
Kc
HO HO
H
O OH H
H
O H - [Ag(NH3)2]
+
(5)
H
(Complex) OH
H HO HO
H
H
H
OH
H
O OH
OH - [Ag(NH3)2]+
k1
HO HO
H
H
H
O
OH
H
.
OH + Ag 0 + 2NH3 + H+
(6)
+ Ag 0 + 2NH3 + H+
(7)
(radical) OH
H HO [Ag(NH3)2] + HO +
H H
H
OH
OH
H
O
.
(10)
Eq. (10) clearly explains the first-order dependence of the reaction each on [NH3 ] and [glucose], respectively.
OH
H
O
d[yellow color] = Kb Kc k1 [(Ag(NH3 )2 )+ ][glucose] dt
OH Fast
HO HO
H H
H
O
OH
O
(lactone) Scheme 1. Mechanism to the formation of metallic silver.
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
reaction–time curves. The nucleation rate was found to be directly proportional to the [ammonia]. Coagulation and coalescence processes were dominant in the growth of nanoparticles at higher [ammonia]. We have also deduced the kinetics that path of silver nanoparticles is independent on the concentrations of glucose. Single-crystal habit was also observed in the tiny particles. The out come of this work may hold prospects for the development a design route for experimental studies on the interconnecting quantum devices in future.
-4.16
log(kobs)
29
-4.32
References
-4.48 3.20
3.24
3.28
3
10 /T Fig. 10. Arrhenius plot to the formation Ag-nanoparticles. Reaction con[ammonia] = 5.0 × 10−4 mol dm−3 ; ditions: [Ag+ ] = 10.0 × 10−4 mol dm−3 ; [CTAB] = 5.0 × 10−4 mol dm−3 ; [glucose] = 10.0 × 10−4 mol dm−3 .
constant with increasing [ammonia] (pH = 9.3 ± 0.2 at [ammonia] values of 2.0, 4.0, 6.0, 8.0 and 10.0 × 10−4 mol dm−3 ). It is not surprising from the fact that ammonia is a weak base; ionization constant = 1.9 × 10−5 . In order to calculate the various activation parameters, the reaction was studied at three different temperatures (30, 35 and 40 ◦ C) at constant [Ag+ ], [NH3 ], [glucose] and [HTAB]. The values of activation parameters (Ea , H# and S# ) were calculated by using Arrhenius (k = Ae−Ea /RT ; log kobs versus 1/T (Fig. 10)) and Eyring (k = (kB T/h)e−H(#) − TS(#) /RT ; log kobs /T versus 1/T) equations and found to be Ea = 70 kJ mol−1 , H# = 67 kJ mol−1 and S# = −109 J K−1 mol−1 . A higher value of activation energy clearly suggests that the transfer of electron from Ag+ to glucose, oxidation–reduction, is slow due to the complexation between Ag+ and ammonia, led to the formation of [Ag(NH3 )]+ , thus resulting in the reduction potential of free Ag+ [17]. The fairly high value of H# indicates that the transition state is highly solvated. The large decrease in S# shows that the transition state is well structured in the micellar phase. Activation energy is much higher in comparison to the Ea reported in the literature to the formation of silver nanoparticles by the silver-mirror reaction in presence of anionic SDS micelles [30]. This is not very surprising judging from the fact that nature of the surfactants play an important role in the association, penetration, solubilization and/or stabilization of reactants as well as products into the micellar-pseudo phase. It is certainly possible that the positive charge on [Ag(NH3 )2 ]+ forms an ion pair (–OSO− 3 –[Ag(NH3 )2 ]+ ) with the negative head group of SDS micelles which concentrate the [Ag(NH3 )2 ]+ within the small volume through the electrostatic interactions in to the reaction site. As a results, the reaction proceeds more fast in presence of SDS than HTAB. 4. Conclusions In summary, we have demonstrated for the first time that the silver-mirror reaction is a suitable chemical and an easy route to the preparation of quantum dots, which underwent an adhesion and coalescence process to assemble into branches-like beautiful silver nanostructures. It has been proven that an increase in [ammonia] in the reaction mixture from 4.0 × 10−4 mol dm−3 to 5.0 × 10−4 mol dm−3 leads to an abruptly change in the shape of the
[1] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chemistry and properties of nanocrystals of different shapes, Chem. Rev. 105 (2005) 1025–1102. [2] J.A. Dahl, B.L.S. Maddux, J.E. Hutchison, Toward greener nanosynthesis, Chem. Rev. 107 (2007) 2228–2269. [3] Y. Sun, Y. Xia, Shape-controlled synthesis of gold and silver nanoparticles, Science 298 (2002) 2176–2179. [4] R. Jin, Y.C. Cao, E. Hao, G.S. Metraux, G.Z. Schatz, C.A. Mirkin, Controlling anisotropic nanoparticle growth through plasmon excitation, Nature 425 (2003) 487–490. [5] Y. Sun, B. Mayers, Y.N. Xia, Transformation of silver sanospheres into nanobelts and triangular nanoplates through a thermal process, Nano Lett. 3 (2003) 675–679. [6] E.C. Hao, K.L. Kelly, J.T. Hupp, G.C. Schatz, Synthesis of silver nanodisks using polystyrene mesospheres as templates, J. Am. Chem. Soc. 124 (2002) 15182–15183. [7] L. Qu, L. Dai, Novel silver nanostructures from silver mirror reaction on reactive substrates, J. Phys. Chem. B 109 (2005) 13985. [8] S. Huang, A.W.H. Mau, Selective growth of aligned carbon nanotubes on a silverpatterned substrate by the silver mirror reaction, J. Phys. Chem. B 107 (2003) 3455–3458. [9] J. Soukupova, I. Kvitek, A. Panacek, T. Nevecna, R. Zboril, Comprehensive study on surfactant role on silver nanoparticles (NPs) prepared via modified Tollens process, Mater. Chem. Phys. 111 (2008) 77–81. [10] Y. Yin, Z.-Y. Li, Z. Zhong, B. Gates, Y. Xia, S. Venkateswaran, Synthesis and characterization of stable aqueous dispersions of silver nanoparticles through the Tollens process, J. Mater. Chem. 12 (2002) 522–527. [11] S.-H. Zhang, Z.-X. Xie, Z.-Y. Jiang, X. Xu, J. Xiang, R.-B. Huang, L.-S. Zheng, Synthesis of silver nanotubes by electroless deposition in porous anodic aluminium oxide templates, Chem. Commun. (2004) 1106–1107. [12] D. Yu, V.W.-W. Yam, Controlled synthesis of monodisperse silver nanoparticles in water, J. Am. Chem. Soc. 126 (2004), 1320013201. [13] S. Sangsuk, Preparation of high surface area silver powder via Tollens process under sonication, Mater. Lett. 64 (2010) 775–777. [14] T. Textor, M.M.G. Fouda, B. Mahltig, Deposition of durable thin silver layers onto polyamides employing a heterogeneous Tollens’ reaction, Appl. Surf. Sci. 256 (2010) 2337–2342. [15] A.-T. Le, P.T. Huy, P.D. Tam, T.Q. Huy, P.D. Cam, A.A. Kudrinskiy, Y.A. Krutyakov, Green synthesis of finely-dispersed highly bactericidal silver nanoparticles via modified Tollens technique, Curr. Appl. Phys. 10 (2010) 910–916. [16] D. Yu, V.W.-W. Yam, Hydrothermal-induced assembly of colloidal silver spheres into various nanoparticles on the basis of HTAB-modified silver-mirror reaction, J. Phys. Chem. B 109 (2005) 5497–5503. [17] D.V. Goia, E. Matijevic, Preparation of monodispersed metal particles, New J. Chem. (1998) 1203–1215. [18] (a) T. Pal, T.K. Sau, N.R. Jana, Reversible formation and dissolution of silver nanoparticles in aqueous surfactant media, Langmuir 13 (1997) 1481–1485; (b) V.K. Sharma, R.A. Yngard, Y. Lin, Silver nanoparticles: green synthesis and their antimicrobial activities, Adv. Colloid Interface Sci. 145 (2009) 83–96. [19] C.A. Bunton, The dependence of micellar rate effects upon reaction mechanism, Adv. Colloid Interface Sci. 123 (2006) 333–343. [20] K. Esumi, T. Hosoyo, A. Yamahira, K. Torigoe, Formation of gold and silver nanoparticles in aqueous solution of sugar-persubsituted poly(amidoamine) dendrimers, J. Colloid Interface Sci. 226 (2000) 346–352. [21] N. Ahmad, M.A. Malik, F.M. Al-Nowaiser, Z. Khan, A kinetic study of silver nanoparticles formation from paracetamol and silver(I) in aqueous and micellar media, Colloids Surf. B: Biointerfaces 78 (2010) 109–114. [22] M.A. Malik, S.A. Al-Thabaiti, Z. Khan, Kinetics of oxidation of d-glucose by permanganate in aqueous solution of cetyltrimethylammonium bromide, Colloids Surf. A: Physicochem. Eng. Aspects 337 (2009) 9–14. [23] J. Xie, J.Y. Lee, D.I.C. Wang, Y.P. Ting, Silver nanoparticles: from biological to biomimetic synthesis, ACS Nano 1 (2007) 429–439. [24] L. Kvitek, R. Prucek, A. Panacek, R. Novotny, J. Hrbac, R. Zboril, The influence of complexing agent concentration on particle size in the process of SERS active silver colloid synthesis, J. Mater. Chem. 15 (2005) 1099–1105. [25] H. Huang, Y. Yang, Preparation of silver nanoparticles in inorganic clay suspensions, Compos. Sci. Technol. 68 (2008) 2948–2953.
30
J.I. Hussain et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 381 (2011) 23–30
[26] A. Henglein, Physicochemical properties of small metal particles in solution: “microelectrode” reactions, chemisorption, composite metal particles, and the atom-to-metal transition, Phys. Chem. 97 (1993) 5457–5471. [27] M. Rudrum, F.D. Shaw, The structure and conformation of some monosaccharides in solution, J. Chem. Soc. (1965) 52–57. [28] (a) P. Mulvaney, A. Henglein, Long-lived nonmetallic silver clusters in aqueous solution: a pulse radiolysis study of their formation, J. Phys. Chem. 94 (1990) 4182–4188; (b) T. Linnert, P. Mulvaney, A. Henglein, H. Weller, Long-lived nonmetallic silver clusters in aqueous solution: preparation and photolysis, J. Am. Chem. Soc. 112 (1990) 4657–4664.
[29] W. Richmond, C. Tondre, E. Krzxanowiska, J. Szymanowski, Metal extraction in micellar media: a comparison of copper complexation rates and ultrafiltration yields of two isomeric (E)-1-(2-hydroxy-5-alkylphenyl) 1-alkanone oximes, J. Chem. Soc. Faraday Trans. 91 (1995) 657–664. [30] S.K. Mehta, S. Chaudhary, M. Gradzielski, Time dependence of nucleation and growth of silver nanoparticles generated by sugar reduction in micellar media, J. Colloid Interface Sci. 343 (2010) 447–453.