Capping effect of CTAB on positively charged Ag nanoparticles

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Physica E 33 (2006) 308–314 www.elsevier.com/locate/physe

Capping effect of CTAB on positively charged Ag nanoparticles Z.M. Suia, X. Chena,, L.Y. Wangb, L.M. Xua, W.C. Zhuanga, Y.C. Chaia, C.J. Yanga a

Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Jinan, Shandong 250100, PR China b School of Chemistry and Chemical Engineering, Jinan University, Jinan, Shandong 250022, PR China Received 13 February 2006; received in revised form 16 March 2006; accepted 17 March 2006 Available online 23 May 2006

Abstract A facial synthesis process of silver nanoparticles (NPs) capped by cetyltrimethylammonium bromide (CTAB) is reported with exploration for the capping effect of CTAB on particles’ stability and surface properties in aqueous medium. Multidisciplinary approaches including electrophoresis, UV-visible absorption spectroscopy, Fourier-transformed infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), thermal gravimetric analysis (TGA) and small angle X-ray scattering (SAXS) are conducted to systematically investigate surface charge and the adsorbed CTAB layer structure on Ag clusters. Obtained results indicate that CTAB molecules bind strongly to silver surface via their headgroups and form a bilayer shell. Detailed analysis of SAXS and NMR data and discussion on the interaction between CTAB molecules and NPs’ surface, provide a clearer model of capped molecules on Ag clusters. r 2006 Elsevier B.V. All rights reserved. PACS: 61.46.Df; 82.70.Dd Keywords: Silver nanoparticles; CTAB; Bilayer; SAXS

1. Introduction In the past several decades, nanoparticles (NPs) have been the focus of intense researches not only due to their novel properties which differ greatly from the bulk materials but also for their wide applications in the practical world [1]. Among them, silver NPs have been investigated extensively because of their application potential in catalysis, electrode and photographic processes, and as the substrates for surface enhanced Raman spectroscopy [2]. From 1979 [1f], a variety of methods to prepare Ag NPs have been reported in the literatures, including coprecipitation [3], sol–gel process [4], microemulsions [5], templated [6] and biomimetic syntheses [7]. In particular, the most commonly used processes are based on reactions performed in solution, which can easily dominate maintenance of the desired composition of the reaction mixtures and permit accurate control of the stoichiometry [1,3–8]. Corresponding author. Tel.: +86 531 88365420; fax: +86 531 88564750. E-mail address: [email protected] (X. Chen).

1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.03.151

To get monodisperse NPs, different kinds of stabilizers have been utilized, such as polymers, dendrimers, surfactants and other ligands, to overcome the van der Waals interaction between the nanoclusters which otherwise leads to agglomeration [8,9]. For long-term stability, researchers often employ surfactants as either stabilizers or templates in syntheses to decrease the surface energy, control the growth and shape of the particles, and act against aggregation [9,10]. Large numbers of anionic surfactants have been used to prepare negatively charged silver NPs, while only few researchers use cationic surfactants to get positively charged particles, probably due to undesired precipitate of silver halides as halogens are often employed as counterions [3d,8a,8b,8f–j]. In our previous research, we use a common cationic surfactant cetyltrimethylammonium bromide (CTAB) as stabilizer to produce Ag NPs with positive charge, and adopt an effective method to change silver nitrate to diamminesilver ion to avoid AgBr formation [11]. Such produced Ag NPs can keep steady and monodisperse for more than one month. Another challenge in this scope is to characterize and understand the adsorption and structural configuration of

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the capping agents on the surface of NPs, which is necessary for a predictive description of nanoparticle stability. Several studies concerning the adsorption of CTAB on a variety of surfaces have been carried out, and a bilayer structure is expected to be favorable in aqueous environment [10c,12]. Nikoobakht and El-Sayed gave the evidence of bilayer assembly of cationic surfactant on gold NPs dispersed in aqueous phase by FTIR and TGA experiments [10c]. Swami et al. extended this model to an interdigitated structure of a mixture containing two surfactants [12b]. Most researchers commonly believe that surfactant molecules may simply envelop metal clusters through touching of their headgroups on the particles’ surface. However, this is not a clear and real situation if we compare the results from small angle X-ray scattering (SAXS) and nuclear magnetic resonance (NMR) measurements to those of TEM, FTIR and TGA experiments. To get a more explicit configuration image of capped CTAB and systematically explore Ag NPs’ surface properties and stability under varied solution pH conditions, which has wide application in the areas of nanoscale materials and environmental processes, different characterization methods, especially SAXS and NMR experiments, have been adopted in this paper. From a novel point of view, we discuss the relationship between the thickness of CTAB capping layers and their molecular arrangement. Focuses are concentrated on the interaction between CTAB molecules and Ag clusters’ surface, and therefore the structure of surfactant-capped silver NPs, based on which a refined capping structure is established. 2. Experimental section 2.1. Materials AgNO3 (99+%), NaBH4 (99%) are purchased from Sigma-Aldrich Co. CTAB (AR), ammonia solution (AR), NaOH (AR) and HCl (AR) are supplied by Shanghai Chemical Agent Co. Ltd. All chemicals are used as received. High purity water (resistivity 18.0 MO cm) is used in this work. 2.2. Synthesis of silver NPs In a typical experiment, an aqueous solution containing 2.0
103 M AgNO3 and 0.4 M NH3 is prepared firstly, then CTAB is added to a concentration of 5.0
104 M. Another solution comprises 8.0
103 M NaBH4 and 5.0
104 M CTAB. Yellow silver colloidal dispersion is obtained by mixing these two solutions of equal volume drop by drop with vigorously stirring at ice-cold temperature for 4 h. Heat the mixture to drive out remanent NH3 and decompose the excess NaBH4. 2.3. Characterization of products UV-visible absorption spectra of samples are recorded by a HP 8453 diode array spectrophotometer in the range

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of 190–1100 nm, with the corresponding solvents as blank. Images of the particles are taken on a JEM-100 CX II transmission electron microscope (TEM) operating at a voltage of 100 kV. A drop of silver NPs dispersion is placed on a copper grid (240 mesh) with Formva support film. Size distribution is obtained from more than 500 particles on the TEM photographs. FTIR spectra of the samples in the form of KBr pallets are recorded on a Bruker Vector 22 spectrometer at 4 cm1 resolution. 1HNMR spectra are recorded on a JEOL FX-90Q spectrometer, and tetramethylsilane is used as an internal standard (chemical shifts are in d values). Thermal gravimetric analysis (TGA) of the sample is conducted on a TGA/SDTA 851e from 50 to 800 1C at a scanning rate of 10 1C/min under nitrogen atmosphere. SAXS experiments are operated at 298 K by means of a Kratky compact small-angle system equipped with a position sensitive detector (OED 50M from Mbraun, Graz, Austria) containing 1024 channels of width 54 mm. The range of scattering angle is chosen from h ¼ 0:05 to 6 nm1, where the magnitude of scattering vector h ¼ 2p sin y/l, 2y and l being, respectively, the scattering angle and incident X-ray wavelength of 0.1542 nm. The distance from sample to detector is 27.7 cm and the exposure time is 1000 s for each sample. 3. Results and discussion As described in our previous preliminary report [11], the obtained silver NPs are monodisperse but not aggregated, which are probably in polycrystalline states with facecentered cubic (FCC) lattice as confirmed by electron diffraction pattern. The measured average particle size is 13.573.3 nm. To characterize the surface charge of Ag NPs, a usual DC electrophoresis test is conducted. The deepened sol color near the cathode confirms the positive charge nature of NPs. Meanwhile, the UV-vis spectra for samples taken from different electrode regions are also compared with that of the original Ag NPs (see Fig. 1). A well-defined surface plasmon band with a maximum absorbance at 413 nm implicates a monodisperse Ag NPs sol. The enhanced absorption intensity of the sol near cathode further validates that the NPs are positively charged. 3.1. SAXS characterization To acquire more detailed information about the surface properties of Ag NPs, a SAXS experiment is employed for the sol. It has been accepted that only scattering from ideal two-phase system with sharp boundaries obeys Porod’s law [13]. If there are thermal electron movement or compositional heterogeneity within phases, density fluctuation will make additional contribution to the total scattering intensity and lead to a deviation from Porod’s law. In this case, a linear relation with a positive slope of ln [q3I(q)] versus q2 at high values of the scattering vector q can be fitted using formula ln ½q3 IðqÞ ¼ ln K þ s2 q2 , where I(q) is

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Fig. 1. UV-visible spectra of original Ag NPs (Ag–CTAB solution), anodic solution (the solution near anode) and cathodal solution (the solution near cathode).

interference function, the scattering vector q is defined as q ¼ 4p sin y/l with the scattering angle as 2y [14]. For widely separated systems (non-interacting systems), S(q)E1, and I(q) is therefore proportional to P(q). Under the condition of no interference effect on the scattering, I(q) and the distance distribution function p(r) (the probability to find pair of small volume elements at a distance r within the entire volume of the scattering particle) are correlated throughR an isotropic Fourier 1 transformation as pðrÞ ¼ ð1=2p2 Þ 0 IðqÞqr sinðqrÞ dq. The behavior of p(r) can provide information about the scattering particle shape and p(r) will reach zero for r ¼ 0 or values larger than the maximum dimension of particles, Dmax. From p(r),
the particle radius of gyration Rg is also RD RD 2 obtained as Rg ¼ 0 max pðrÞr2 drÞ=ð2 0 max pðrÞ dr , and the particle’s average diameter D ¼ 2.6Rg. Here, we use GNOM program to calculate p(r) from SAXS data of Ag NPs [14c]. The results are depicted in Fig. 3, from which we get Rg and Dmax values, respectively, as 6.0170.05 and 17.0 nm, corresponding to the diameter D of Ag NP as 15.670.13 nm. Although SAXS measurement reflects a root-mean-square average size, slightly larger than the arithmetic value from TEM [14], the obvious difference between SAXS and TEM results, 2–3 nm, should be related to the thickness of adsorbed CTAB layers on silver NPs. This deduction is based on the observation of a positive deviation of Porod’s law and the different measured objects: the whole NPs dimension with capping layer in SAXS measurement, while only the metal clusters in TEM observation. 3.2. pH effect

Fig. 2. SAXS data plotted as ln [q3I(q)] versus q2 for CTAB capped Ag NPs, indicating a positive deviation from Porod’s law.

the measured background-corrected scattering intensity and s is a parameter related to transitional boundary layer. In Fig. 2, a plot of ln [q3I(q)] versus q2 is displayed for Ag NPs. It is obvious that the curve in the high-angle region shows a positive slope, i.e., a positive deviation from Porod’s law. This suggests that the electron density does not change abruptly, but varies gradually over a certain distance range between the adsorbed CTAB molecules on Ag cluster and the bulk water. Because it is usually recognized that except for CTAB no other organic molecule can cap silver clusters in product solution, the deviation from Porod’s fitting is therefore the evidence indicating a transitional boundary layer of CTAB molecules formed between Ag clusters and the bulk water. It is also well known that the SAXS intensity I(q) of an isotropic solution containing monodisperse particles of small anisotropy is given by equation IðqÞ ¼ kPðqÞSðqÞ, where k is a factor related to the instrumental effects and particle number density; P(q) and S(q) correspond, respectively, to the particle form factor and interparticle

To further investigate the stability of Ag NPs in aqueous environment, pH of sol is changed from initial value (8.26) by adding NaOH or HCl and the corresponding solution UV-vis spectra are compared with that of the original sol. Fig. 4 shows such results at various pH values, which are recorded after 1-day keeping except only for the spectrum

Fig. 3. Distance distribution function p(r) of Ag NPs.

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Table 1 FTIR band positions of pure CTAB and silver NPs samples with their assignments Assignment

Symmetric and asymmetric stretching CH2 vibrations of alkyl chain Asymmetric and symmetric C–H Scissoring vibrations of CH3–N+ moiety C–N+ stretching bands Rocking mode of the methylene chain Fig. 4. UV-visible spectra of aqueous solutions of CTAB-capped silver NPs at various pH values. The pH values are adjusted by addition of HCl or NaOH.

at pH ¼ 1000 which is measured after 1 week. It can be clearly seen that stable Ag NPs could not be kept and they aggregate immediately at pH higher than 10, when the plasmon absorption around 413 nm is considerably depressed. The pH of 10 seems a stability limit, where the NPs keep stable for initial several days with an absorption peak near 413 nm (curve pH ¼ 100 ), but some gray precipitates can be observed after 1 week corresponding to no plasmon absorption (curve pH ¼ 1000 ). Under more acidic condition, Ag NPs are stable for more than 1 month. As we know, counterion Br in CTAB could be replaced by nucleophilic OH at basic condition, and CTAB is thus transformed to cetyltrimethylammonium hydroxide (CTAOH) [15]. It has been found that the charge on a CTAOH micelle will be reduced with addition of OH and it should also be true in the same way for the charge on the surface of CTAOH capped Ag NPs [16]. Therefore the electrostatic repulsion between NPs may be decreased and the reason why Ag NPs begin to aggregate at pH of 10 or higher can be explained. 3.3. FTIR investigation How about CTAB capped layer structure? FTIR data of pure and surface-bound CTAB molecules could give us some indications. It can be seen from the data summarized in Table 1 that the CH2 symmetric and asymmetric stretching vibrations of CTAB bound on silver NPs, respectively, lie at 2850 and 2917 cm1, the same as those of pure ones. But the CH2 stretching vibrational bandwidths are narrower for bound CTAB than those in free status [11]. The unchanged frequency positions suggest that there is no intermolecular interaction enhanced due to capping effect, and the conformation of methylene chains is maintained [17]. Although vibration frequencies of these bands are related to methylene configuration, the packing density in CTAB-capped Ag NPs is not so sufficient to produce any change in CH2 stretching modes [18]. The

Band position (cm1) Pure CTAB

Silver NPs

2850, 2917

2850, 2917

1482, 1430

1475, 1428

962

827, 946, 1004, 1130 717

730, 719

bandwidth change, however, reflects some changes from the structural order of the methylene chain groups in aggregate structure. Narrower bandwidths have been found to be resulted from a more ordered structure of the alkyl chains for bound CTAB molecules [18], which implies that CTAB molecules are arranged in some order on NPs surface. The obvious difference for asymmetric and symmetric C–H scissoring vibrations of CH3–N+ moiety between pure CTAB molecules and CTAB-capped Ag NPs reveals that CTAB adsorbs on metal surface via its headgroup. The broad peak at 1475 cm1 suggests existence of some unbound CTAB molecules in NPs sample, while shift to lower frequencies of bound CTAB exhibits an influence on CH3–N+ vibrations from capped Ag clusters. The singlets at 962 cm1 for pure CTAB and 946 cm1 for bound CTAB belong to C–N+ stretching bands, and the frequency shift is believed to be caused also by interaction between N-containing group and metal surface. The characteristic [C–Hymetal] vibration, however, has not been observed, indicating the absence of C–H binding to metal surface [19]. The broad new bands of CTAB capped on silver clusters at 827, 1004, and 1130 cm1 are also considered as stretching modes of C–N+ effected by metal surface, as described by El-Sayed for CTAB capped Au particles [10c]. The change of CH2 rocking mode from a doublet at 730 and 719 cm1 for pure CTAB to a singlet at 717 cm1 in Ag NPs sample also denotes a confinement effect of CTAB in capping structure [18]. All these experimental observations illuminate that CTAB molecules cap Ag clusters via their headgroups, while their methylene chains have a more ordered structure. Though FTIR can help to identify the bounding of CTAB on Ag surface, other measures are still needed to resolve the detailed capping structure. Table 2 lists data summarized from TGA measurements reported early [11]. The decomposition of bound CTAB molecules is found through two weight-loss steps with different rates, which denotes two different statuses of CTAB molecules capped on Ag clusters and indicates the possible formation of

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Table 2 The weight loss processes of pure CTAB and silver NPs samples with their assignments in TGA experiment Assignment

Desorption of moisture Thermal decomposition of CTAB molecules

Pure CTAB

Silver NPs

Weight loss range (1C)

Weight loss (%)

Weight loss range (1C)

Weight loss (%)

207–316

100

100–208 208–318 318–600

5 35 7

CTAB bilayer on Ag surface [12]. The first sharp weightloss step at 208–318 1C (centered at 282 1C) is considered from decomposition of CTAB outer layer with low desorption energy. While the next dull weight-loss step from 318 to 600 1C (centered at 445 1C) is attributed to decomposition of CTAB inner layer, in which the higher energy barriers come from the interactions between the surfactants’ headgroups and metal clusters. Therefore, it is confirmed that the capping CTAB molecules on Ag NPs are packed in a bilayer structure. 3.4. NMR measurement More detailed information about the interaction between CTAB and silver clusters can be obtained from NMR measurements [20]. Fig. 5 shows the results for pure CTAB and CTAB-capped Ag NPs (Ag–CTAB). The signals of aCH2 appear downfield at chemical shift d ¼ 3:39 ppm in Fig. 5A for pure CTAB and at 3.32 ppm in Fig. 5B for Ag–CTAB. Its multiplet appearance has been attributed to H–H coupling to b-CH2. There are narrow singlets, respectively, at 3.16 ppm in Fig. 5A and at 3.12 ppm in Fig. 5B due to the methyl protons of CTAB polar groups. Moving upfield, the broad humps at 1.80 ppm for pure CTAB and at 1.88 ppm for Ag NPs are b-CH2 signals. The large broad peaks at 1.28 and 0.86 ppm in both spectra are the combined resonances of the other methylenes in the alkyl chains and will be referred as the ‘‘main chain’’ peaks [20]. In addition, the protons of methyl on the top of CTAB alkyl chains have small singlets with the same chemical shifts, 0.86 ppm, before and after capping. Comparing the spectra of pure CTAB and Ag NPs, the chemical shift change of protons on the carbon near the nitrogen atom indicates that the Ag–CTAB ‘‘bonds’’ locate around CTAB headgroups. The larger changes of the protons on a-CH2 and b-CH2 than that on N–CH3, however, indicate that the capping of CTAB molecules on Ag clusters is through their headgroups with Ag cluster located between b-CH2 and nitrogen atom, i.e. CTAB molecules are tilted (but not vertical) on Ag NPs surface, as shown in Fig. 6A. 3.5. Discussion Spherical silver particles have a high electron density, which is quite different from H2O molecules. However, the

Fig. 5. NMR spectra for pure CTAB (A) and Ag–NPs (B) with different carbon atom indications in CTAB molecule.

positive deviation from Porod’s law indicates there is no distinct interface between the Ag NPs and bulk water, suggesting the presence of an adsorbed layer. The difference of NPs’ diameter from SAXS and TEM observation is 2–3 nm, corresponding to the thickness of the adsorbed CTAB layer. But as we know, CTABs, with molecular length about 2 nm and long flexible hydrophobic tails and positively charged hydrophilic headgroups, can

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The two weight-loss steps of CTAB in thermal investigation show evidence that there exist two different statuses of CTABs capped on silver clusters, indicating a bilayer packing on the metal surface. Based on it and the observed narrower FTIR bandwidths of CH2 symmetric and asymmetric stretching modes, which manifest a more ordered structure of the alkyl chains for bound CTAB molecules, it can be reasonably deduced that outside the inner CTAB layer located directly on Ag cores, there is another layer with more CTAB molecules which are anchored through hydrophobic interaction between alkyl chains, while leaving their headgroups extruding to solution bulk [10c]. Above analyses are supported by related molecular dynamics simulation [21], where CTA+ chains show a considerable degree of curvature when CTABs form bilayer on metal clusters’ surface. The molecules either in inner and outer layers do not pack in a full-extended state, but bend or twist to each other. Such arrangement may result in a much smaller thickness of adsorbed layer than twice length of CTAB molecules. Therefore, a clear and reasonable capping structure for CTAB on Ag can be drawn as shown in Fig. 6B. 4. Conclusion

Fig. 6. (A) A model for a single CTAB capped Ag–NP resulted from NMR test and (B) a three-dimensional model of CTAB capped Ag–NP in aqueous solution.

easily cover metal clusters’ surface to make them soluble in aqueous environment. However, we need to clarify whether CTAB molecules are in extended state to create a quite loose monolayer capped on Ag NPs, or they are twisted to form a dense bilayer. The positive charge of CTAB-capped NPs may reflect a possible image that CTABs wrap Ag core by their hydrophobic tail and arrange the hydrophilic headgroups outwards. However, the variations of FTIR bands for C–H symmetric and asymmetric scissoring modes in the N+(CH3)3 and the absence of the characteristic [C–Hymetal] vibration has shown us that the binding between CTABs and Ag surface is via their headgroups, as confirmed also by NMR chemical shift changes (Dd) of H atoms located around headgroup. In addition, the observation on the difference of proton Dd between methyls and C atoms beside N further suggests that CTAB molecules are bounded to Ag NP core by their headgroups and the cetyl chain is tilted to the surface. All of these exclude the possibility of forming only a CTAB monolayer on the silver cluster.

Based on results and discussions from different experimental measurements, a more distinct model for CTAB-capped and positively charged Ag NPs has been established. Except for the usual evidences from FTIR and TGA, SAXS and NMR methods could provide us more powerful supports about the formation of a capping CTAB bilayer on Ag clusters, which makes Ag NPs positively charged and much stable in aqueous solution especially with pHo10, when CTA+ ions adsorb on Ag surface and the resulted electrostatic repulsions prevent them from aggregation. Such capping bilayer structure is reasonably built based both on experimental data from SAXS and TEM tests, and also on preliminary molecular dynamics simulation in our laboratory [21]. In this model as shown in Fig. 6B, CTA+ chains show a considerable bending when they form the bilayer on Ag clusters’ surface, which is different from usual model reported previously [10c,12]. These curved or tilted chains produce a dual and dense mantle on Ag surface and thus protect it from aggregation. The structural characteristics obtained here should add some new insight for better understanding the capping mechanism of CTAB and other related surfactants. Acknowledgments We thank the financial supports from the National Natural Science Foundation of China (20073025, 20373035, 20573066) and Shandong Provincial Science Fund (Y2005B18).

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