Applied Surface Science 200 (2002) 62±67
A study of phase transfer processes of Ag nanoparticles De-Gang Lia, Shen-Hao Chena,b,*, Shi-Yong Zhaoa, Xian-Ming Houa, Hou-Yi Maa, Xue-Geng Yanga a Department of Chemistry, Shandong University, Jinan 250100, PR China State Key Laboratory for Corrosion and Protection of Metals, Shenyang 110015, PR China
b
Received 3 April 2002; accepted 6 June 2002
Abstract With the protection of sodium oleate, Ag nanoparticles are produced through the reduction of AgNO3 with NaBH4 in an aqueous solution. The possible mechanism of phase transfer of the Ag nanoparticles was discussed. At a suitable concentration of sodium oleate, after adding NaH2PO4, the oleic acid molecule can change its position on the surface of Ag nanoparticles under the effects of water and toluene and become amphipathic. So most of the nanoparticles form a ®lm between water/toluene. For the case of a higher concentration of sodium oleate, excess sodium oleate will form a closed monolayer ®lm on the surface of the Ag nanoparticles. After adding NaH2PO4, the oleic acid molecule cannot move on the Ag nanoparticles surface, thus the colloid particles are hydrophobic but not amphipathic. So most of the particles transfer to the organic phase. UV±Vis spectra, TEM and conventional metallographic microscopy are used to characterize the Ag nanoparticles and nanoparticles ®lms. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Ag nanoparticles; Phase transfer; Mechanism; Amphipathic
1. Introduction Synthesis of nanoparticles was a developing ®eld in solid-state chemistry [1]. Due to the special size, nanoparticles have extremely exciting physicochemical and optoelectronic properties [2,3]. A variety of chemical methods have been used for the preparation of nanoparticles, such as chemical reduction with [4] or without [5] stabilizer, chemical reduction in reverse micelles [6], chemical vapor deposition or molecular beams [7], and the colloidal chemistry method [8]. Because of having many special properties in optics, * Corresponding author. Present address: Department of Chemistry, Shandong University, Jinan 250100, PR China. Fax: 86-531-8565167. E-mail address:
[email protected] (S.-H. Chen).
electricity, catalysis and sensitivity, the thin ®lm of nanoparticles has great application prospect and has been investigated extensively over recent years [9,10]. Many methods can be used for the formation of a thin ®lm of nanoparticles, such as sol±gel, electrodeposition, vapor deposition and self-assembly [11]. In this paper, oleate stabilized nanosized silver colloids were obtained. After adding NaH2PO4 to the solution, Ag nanoparticles transferred to the interface of water/toluene or toluene phase. An interesting result was that the concentration of sodium oleate affects the direction of phase transfer of Ag nanoparticles greatly. Only the proper concentration can lead to the transfer of Ag nanoparticles to the interface of water/toluene and formation of an interface ®lm. The interface ®lm can spontaneously climb up on the wet hydrophilous planar surfaces of glass and form a thin
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 0 6 - 2
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nanoparticles ®lm. UV±Vis spectra, TEM and conventional metallographic microscopy were used to characterize the Ag nanoparticles and the completeness of the nanoparticles ®lms. This paper focuses on the conditions of phase transfer and discusses its possible mechanism. 2. Experimental details 2.1. Preparation of Ag nanoparticles Ag hydrosols stabilized by sodium oleate were prepared according to Wang et al. [4]. About 2.5 ml of 1 10 3 M AgNO3 was added into an equal volume of 4 10 3 M NaBH4 with vigorous stirring at an ice-cold temperature. This process was repeated for various concentrations of sodium oleate. A yellowbrown colloidal solution stabilized by sodium oleate was obtained. The solution was continuously stirred until it was warmed to room temperature. The solution was then heated to about 80 8C and cooled back to room temperature. 2.2. Phase transfer of Ag nanoparticles Different amounts of NaH2PO4 were added into 20 ml mixtures of the Ag nanoparticles hydrosol and toluene. After 20 min of vigorous stirring, the aqueous phase gradually lost its color with the increasing amounts of NaH2PO4. An amaranth ®lm could be found at the interface of water/toluene after standing for a while and the toluene phase also became amaranth. 2.3. UV±Vis, TEM and conventional metallographic microscopy measurements UV±Vis spectra were taken on a Shimadzu UV-240 spectrophotometer. A copper grid coated with about 10 nm amorphous carbon ®lm was used to dredge up the ®lm from the water/toluene interface, which was examined with a Hitachi H-800 transmission electron microscope operated at 100 kV. The hydrophilic microscope slides of dimensions 7:5 2:5 cm2 were submerged into triply distilled water. After taken out, the slides were immersed into the mixture solutions at once. The Ag nanoparticles at the interface of water/
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toluene spontaneously climbed up the microscope slides. Ag nanoparticles ®lms clung to the slides while the slides were taken out. After the ®lms dried, they are detected with the XUL-03 metallographic microscopy. The images of these ®lms were taken with a KODAK DC240 numeral camera. In these experiments, triply distilled water and analytical reagents were used. If not declared otherwise, all the experiments were done at 20 2 8C. 3. Results and discussion Fig. 1a was the TEM image of interface ®lm of Ag nanoparticles at the interface of water/toluene when 2:5 10 4 M sodium oleate was the stabilizer and 0.05 g NaH2PO4 was added to a mixture of 10 ml Ag nanoparticles hydrosol and 10 ml toluene. The particle size distribution obtained from the enlarged TEM image of Fig. 1a was shown in Fig. 1b. The average particle size of the Ag nanoparticles was about 19.2 nm, with a standard deviation of 5.6 nm. NaH2PO4 was added into the solution composed of 10 ml of Ag nanoparticles hydrosol and 10 ml toluene with vigorous stirring. After 20 min, the mixture was separated out into three layers: the aqueous solution lost its color; the toluene phase became shallow amaranth; a dark amaranth thin ®lm emerged between aqueous and toluene phases. The transfer ef®ciency of Ag nanoparticles increased with the amount of NaH2PO4 used. This can be seen from the UV±Vis absorption spectra of Ag nanoparticles hydrosol containing 2:5 10 4 M sodium oleate after adding different NaH2PO4 (Fig. 2). There are characteristic absorption peaks at 410 2 nm similar to Wang et al. [12]. With the increasing of NaH2PO4, the heights of the characteristic absorption peaks of the hydrosol decrease. Although the toluene phase looked like light amaranth, it was colorless when taken out. There are no absorption peaks for the toluene phase. The apparently light amaranth was from the thin ®lms of Ag nanoparticles adsorbing to the sides of the beaker from the hydrosol/toluene interface. This was quite similar to Mayya and Sastry [13]. If a moistened microscope slide was immersed into the biphasic solution, the colloid Ag nanoparticles at the interface of aqueous/toluene will spontaneously climb up on it rapidly. A possible mechanism was discussed below.
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Fig. 3. UV±Vis spectra of Ag nanoparticles hydrosol containing 2:5 10 3 M sodium oleate (A) and organic phase (B) after adding different NaH2PO4: (a) 0.01 g; (b) 0.05 g; (c) 0.1 g.
Fig. 1. The TEM of Ag nanoparticles ®lm at the interface of water/ toluene (a) and the particle size distribution (b).
Fig. 2. UV±Vis spectra of Ag nanoparticles hydrosol containing 2:5 10 4 M sodium oleate after adding different NaH2PO4: (a) 0.01 g; (b) 0.05 g; (c) 0.1 g.
When higher concentrations of sodium oleate (2:5 10 3 M) were used as the stabilizer, the UV± Vis spectra of the aqueous phase and toluene phase can be seen in Fig. 3. Not only the aqueous phase but also the toluene phases exhibit intense characteristic absorption peaks of Ag nanoparticles. This indicates that some Ag nanoparticles have transferred into the toluene phase. Also, there exists an amaranth thin ®lm at the interface of water/toluene. Fig. 3B illustrates that more and more Ag colloid nanoparticles transfer to the organic phase from the aqueous phase with increasing of NaH2PO4 amount. The results of IR measurements show that, before adding NaH2PO4, in the hydrosol there was a strong interaction between the C=C bond of the adsorbed sodium oleate molecules and the surface of Ag nanoparticles [4]. Therefore, the carboxylate group (COO ) was exposed to the external environment. Because of the terminal hydrophilic group, Ag nanoparticles can disperse in aqueous solution steadily for at least 1 month. In organosols, the adsorption of carboxylic acid molecules on the Ag nanoparticles was stronger than that of the dissociated carboxylates [12]. After adding NaH2PO4, the carboxylate group of the sodium oleate adsorbing on the nanoparticle changes into carboxylic acid. When stirring the biphasic mixture vigorously, the colloid Ag nanoparticles can contact with toluene. So the oleate contacting with the toluene will change its direction (see Fig. 4a). The carboxylic acid head group adsorbs on the surface of Ag particles and the hydrophobic ``tail''
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Fig. 4. Schematic diagram of the colloid Ag nanoparticles' phase transfer when different concentrations of sodium oleate used as stabilizer: (a) 2:5 10 4 M; (b) 2:5 10 3 M.
of the hydrocarbon chain was directed towards the solvent. With the continuous stirring, more part of Ag nanoparticles can contact with the toluene. Thus more oleic acids change their adsorption direction. An interesting result was that the concentration of sodium oleate solution had a strong effect on the transfer of Ag nanoparticles. When the concentration of stabilizer was 2:5 10 5 or 1 10 4 M, stabilized colloid Ag nanoparticles could not be obtained. If the stabilizer was 2:5 10 4 M, the stabilized colloid Ag particles transferred to the interface of biphase after adding NaH2PO4. Most of the colloid particles migrated into the organic phase when 2:5 10 3 M stabilizer was used. This may be explained as below: as for the absent stabilizer, Ag nanoparticles cannot be capped well. So nanoparticles are unstable and easier to coagulation. For the moderate concentrations, such as 2:5 10 4 M sodium oleate, the molecules of sodium oleate on the surface of Ag nanoparticles could not form compact ®lms and could travel along the Ag nanoparticles surface. After adding NaH2PO4, oleic acid could change their direction of adsorption by stirring the biphasic solution vigorously. Under the effect of water and toluene, the colloid particle was separated into two segments, one was the hydrophobic ``tail'', the other was the hydrophilic part including oleates unchanging their adsorption direction and the BO2 anions etc. The hydrophobic terminal will direct towards the toluene phase as possible as they can
and the hydrophilic terminal will stay in aqueous phase. The amphipathy of the colloid particles was the reason that they can exist at the interface of the biphase. After the mixture has stood for a while, the colloid particles immobilize at the interface of aqueous/toluene (see Fig. 4a). This also explains why the colloid Ag nanoparticles at the aqueous/toluene interface will spontaneously climbed up the moistened microscopy slide rapidly. There was a thin water ®lm on the surface of the moistened hydrophilic slide. Once introduced into the mixture solution, a new interface was formed between the thin water ®lm and the toluene (see Fig. 5). The hydrophilic group of the colloid Ag particle can remove into the thin water ®lm and the hydrophobic tail still stayed in the toluene. Driven by the surface tension gradients [13] and the pressure of the toluene, the Ag nanoparticles climb up the slide along the new interface of water/toluene. The ®lm of Ag nanoparticles climbing up the slide could also be taken out together with the slide. This ®lm of Ag particles ¯oated on the surface of the thin water ®lm of the slide rather than adsorb on the slide. One experiment can con®rm this opinion. If a drop of water is dripped onto the slide, the colloid Ag nanoparticles ®lm will ¯oat on the surface of the water droplet. The microscopy slide was allowed to dry and the thin solid ®lm ®rmly adsorb on the slide. For the adequate stabilizer, for example 2:5 10 3 M of sodium oleate, the sodium oleate molecules
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Fig. 5. The schematic diagram of the thin ®lm of Ag nanoparticles climbing up on the surface of a hydrophilic slide.
formed a closed packed monolayer on the Ag particles (see Fig. 4b). After adding NaH2PO4, the part of the colloid Ag nanoparticles contacting with the toluene during stirring becomes hydrophobic. If fully stirred, the sodium oleates change their adsorbing directions and make the nanoparticles hydrophobic. Because of high concentration of sodium oleate molecules on the surface of the Ag nanoparticles, they cannot move on the particle and it was dif®cult for the particle to be amphipathic. Therefore, this kind of nanoparticles
transfers to the organic phase. There are also some Ag nanoparticles, which partly contact the toluene, that are amphipathic and immobilized at the interface of the water/toluene. At the same time, some particles did not contact the toluene all the time during the stirring and thus stayed in aqueous solution. In the experiment, most of the particles transferred into the toluene phase (see Fig. 2b) after abundantly stirring. Another result of experiment demonstrated the mechanism of the Ag nanoparticles' climbing up the slide. When a hydrophobic slide immersed into the biphase, the ®lm of Ag nanoparticles at the interface did not climb up but move down the slide instead. This can be explained: a thin organic ®lm adsorbs on the slide when it is cutting through the toluene. Therefore, a new interface also comes into being, one is the new thin toluene ®lm, and the other is the water. The hydrophobic part of the Ag nanoparticle will move into the organic phase and the hydrophilic part will stay in the water. Also, being driven by the surface tension gradients and the pressure of toluene, the Ag nanoparticles move down along the new water/toluene interface. The ef®ciency of phase transfer of Ag nanoparticles was so high that there was hardly any sign of the characteristic absorption peak near 410 nm on the UV±Vis absorption spectra of the aqueous phase or the organic phase after adding 0.1 g NaH2PO4 with the
Fig. 6. The images of ®lms of Ag nanoparticles formed on slides at different phase transfers: (a) 0.01 g; (b) 0.05 g; (c) 0.1 g; (d) 0.2 g.
D.-G. Li et al. / Applied Surface Science 200 (2002) 62±67
2:5 10 4 M sodium oleate used as stabilizer (see Fig. 2). Most of the Ag nanoparticles migrated to the interface of water/toluene. The concentration of NaH2PO4 also affected the coverage of the thin Ag nanoparticles ®lm adsorbing on the microscopy slide. Fig. 6 shows the images of the thin Ag nanoparticles ®lm formed on the slides after adding different NaH2PO4 obtained by the metallographic microscopy. In Fig. 6a, there were only some pieces of thin ®lms on the slide. Some ¯aws existed on the Ag nanoparticles ®lm for 0.05 g NaH2PO4 (see Fig. 6b). It can be clearly seen from Fig. 6b that the ®lm was monolayer. When 0.1 g NaH2PO4 was added into the Ag nanoparticles solution, a compact monolayer ®lm of Ag nanoparticles was obtained (see Fig. 6c). In Fig. 6d, the thin ®lm was not a monolayer because there was something else like patches on the ®lm. 4. Conclusion Excess sodium oleates can form a closed ®lm on the surface of Ag nanoparticles. After adding NaH2PO4, the molecule of oleic acid cannot move on the Ag nanoparticles surface. The colloid particles are dif®cult to become amphipathic. So most of the particles transfer to the organic phase. For a suitable concentration of sodium oleate, the molecule of oleic acid may change its position on the surface of the Ag particles under the effect of water and toluene and become amphipathic. Therefore most of the Ag nanoparticles form a ®lm on the interface of water/toluene. A new interface of water/toluene will form when immersing a wet hydrophilic slide into the mixture of hydrosol and toluene, so the amphipathic Ag nano-
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particles at the interface can climb up the slide along the new interface and form a thin ®lm. Acknowledgements This work was supported by the Special Funds for the Major State Basic Research Projects G19990650, the Chinese National Science Fund (no. 20173033) and Shandong Science Found for Distinguished Young Scientist (01BS20). References [1] C. Petit, P. Lixon, M.P. Pileni, J. Phys. Chem. 97 (1993) 12974±12983. [2] D.L. Klein, R. Roth, A.K.L. Lim, A.P. Alivwasatos, P.L. McEuen, Nature 389 (1997) 699±701. [3] C.P. Collier, R.J. Saykally, J.J. Shiang, S.E. Henrichs, J.R. Heath, Science 277 (1997) 1978±1981. [4] W. Wang, S. Efrima, O. Regev, Langmuir 14 (1998) 602±610. [5] L.M. Liz-Marzan, A.P. Philipse, J. Phys. Chem. 99 (1995) 15120±15128. [6] J. Lin, W.L. Zhou, A. Kumbhar, J. Wiemann, J. Fang, E.E. Carpenter, C.J. O'Connor, J. Solid State Chem. 159 (2001) 26±31. [7] J. Shi, S. Gider, K. Babcock, D.D. Awschalom, Science 271 (1996) 937±941. [8] M.P. Pileni, Langmuir 13 (1997) 3266±3276. [9] M. Sastry, A. Gole, S.R. Sainkar, Langmuir 16 (2000) 3553± 3556. [10] K. Akamatsu, N. Tsuboi, Y. Hatakenaka, S. Deki, J. Phys. Chem. B 104 (2000) 10168±10173. [11] V. Patil, K.S. Mayya, S.D. Pradhan, M. Sastry, J. Am. Chem. Soc. 119 (1997) 9281±9282. [12] W. Wang, X. Chen, S. Efrima, J. Phys. Chem. B 103 (1999) 7238±7246. [13] K.S. Mayya, M. Sastry, Langmuir 15 (1999) 1902±1904.
