21499
2005, 109, 21499-21501 Published on Web 10/29/2005
Silver Clusters on Silver Sulfide Nanocrystals: Synthesis and Behavior after Electron Beam Irradiation L. Motte*,† and J. Urban‡ Laboratoire LM2N, URA CNRS 1662, UniVersite´ P. et M. Curie, 4 Place Jussieu, Paris, France, and Fritz-Haber-Institut der MPG, Faradayweg 4-6, 14195 Berlin, Germany ReceiVed: July 30, 2005; In Final Form: October 11, 2005
Nanocomposite crystals, (Ag)x(Ag2S)y with x < y, are synthesized in micellar media. The generation of Ag clusters on Ag2S nanocrystals is attributed to the reduction of mobile Ag+ ions in the Ag2S nanocrystals by sulfur derivatives. The proportions in the composite material can be modulated by electron beam irradiation. Using dodecanethiol as surface passivating agent, 2D self-organizations of these nanocomposite crystals are produced.
The study of inorganic nanocomposites has been of considerable interest in recent years because of their novel properties, which are a combination of those of the original inorganic materials.1 Here, we report a new type of nanocomposite, made up of silver sulfide semiconductor and silver metal. Bulk silver sulfide has been considered for photoimaging and photodetection in the IR region,2,3 while in the nanometer scale range, Ag2S nanocrystals are known to play an important role in photographic sensitivity.4-7 Usually, Ag2S nanocrystals are synthesized either by solid-phase conversion of silver particles suspended in polymers8 and zeolites,9 or by the coprecipitation reaction between silver and sulfide ions in the presence of a stabilizing agent such as, thiols10-12 or polymers13,14 or in reverse micelles.15-17 In the latter case, it has been shown that reverse micelles act as nanoreactors and provide convenient media for obtaining size and shape controlled nanocrystals.18 Recently, it has been demonstrated that Ag2S nanocrystals act as a catalyst for the reduction of Ag+ ions by various reductants such as hydroquinone or sodium sulfite Na2SO3. This catalytic reduction19 is accelerated by irradiation. It was suggested, without direct evidence, that the formation of metallic silver proceeds on the surface of Ag2S nanocrystals. On the other hand, using an STM tip made of Ag2S crystal, nanoscale Ag particle is produced at the apex of this tip by applying a negative bias voltage to the Pt sample.20 The particle could be gradually shrunk and disappeared when the polarity of the sample bias voltage was switched to positive. In this case, the formation of silver particle is explained by the reduction of mobile Ag+ ions in the Ag2S tip by the tunneling electrons from the Pt sample, and the silver particle is formed by the precipitation of the Ag atoms at the apex of the tip. With a positive bias voltage, Ag atoms in the Ag particle are oxidized to Ag+ ions and the Ag+ ions redissolve in the Ag2S tip. In this communication, nanocomposite crystals, (Ag)x(Ag2S)y with x < y are synthesized in micellar media, and it is shown * Corresponding author. † Universite ´ P. et M. Curie. ‡ Fritz-Haber-Institut der MPG.
10.1021/jp0542322 CCC: $30.25
that the proportions in the composite material can be modulated by electron beam irradiation. Using dodecanethiol as surface passivating agent, these nanocomposite crystals are extracted from micellar media and self-organized in two dimensions. Ag2S nanocrystals are synthesized as described previously15-17 (see appendix). At the end of the synthesis, two samples are produced having average diameters of 2.3 and 7.6 nm with size distributions of 33% and 15%, respectively. The high-resolution transmission electron microscopy (HRTEM)21 images of the two samples show nanocrystals of a composite nature (Figures 1A and 1B). The Ag2S nanocrystals have the well-known lattice constants of the stable monoclinic β-type crystal modification.22 A small cluster (dark contrast) is attached to the nanocrystal (lighter contrast), and the shape of the nanocrystals appears more elongated than circular, especially for the particle shown in Figure 1B. The power spectra (square of the Fourier transform of the image) of these two images (Figures 1C and 1D) show dominant reflections characteristic of the monoclinic structure of the bulk Ag2S phase. These reflections correspond to the (010) reflection with 0.686 nm lattice spacing, the (020) reflection with 0.343 nm lattice spacing, and the (030) reflection with 0.231 nm lattice spacing. This indicates that the Ag2S nanocrystals in Figures 1A and B are nearly in the [001] orientation. Moreover, antiphase domains and antiphase boundaries can be observed on Ag2S nanocrystals (Figures 1A and 1B).23,24 The outer diffuse rings in the power spectra (Figures 1C and 1D), corresponding to the attached clusters, are characteristic of metallic silver, with a lattice constant of 0.236 nm that is typical for the (111) reflection of elemental silver. This lattice constant is clearly different from that of monoclinic Ag2S. From these data it is concluded that for any Ag2S nanocrystals size, small metallic silver clusters are bound at the nanoparticle interface. This result is quite surprising. The absorption spectra of the two samples show only a long absorption tail between 250 and 800 nm, characteristic of Ag2S nanocrystals.13-15 No absorption band in the 380-430 nm region due to the development of pure colloidal silver can be observed due to the small size. © 2005 American Chemical Society
21500 J. Phys. Chem. B, Vol. 109, No. 46, 2005
Figure 1. HRTEM images of Ag2S nanocrystals near the [001]orientation with a smaller attached polycrystalline Ag nanocrystal. The power spectra are shown next to the images. The nanocrystals are synthesized at w ) 5 (A) and w ) 40 (B), see Appendix.
To make sure that silver clusters are produced during the synthesis and not by exposure to the electron beam, as already observed by K. Terabe et al.,20 the electron beam is focused first onto an arbitrary spot and afterward shifted and focused again on various nanocrystals that were not previously exposed to the beam. Patterns similar to those shown in Figure 1 are produced, indicating that the Ag clusters attached to Ag2S nanocrystals are formed during particle growth in the reverse micelles. A systematic study of the extended exposure of the Ag2S nanocrystals to the electron beam shows an increase in
Letters the silver metallic cluster size with the duration of the exposure (Figure 2). Note that the silver clusters have a polycrystalline phase. From this it is concluded that silver ions are reduced in the Ag2S nanocrystal with a concomitant increase in the size of the attached Ag nanocrystal. The data presented above clearly show that polycrystalline silver metal clusters are produced during the silver sulfide nanocrystals growth. To explain the formation of an Ag cluster on the Ag2S nanocrystal, we have to take into account several parameters. We know from the literature that Ag+ ions play an important role in nanocrystal growth.25,26 Furthermore, a recent paper shows that the chemical reduction of Ag+ ions by SO32is catalyzed by Ag2S nanocrystals.19 According to the conventional redox potential of Ag+/Ag (+0.8 V) and S/S2- (-0.44 V), the chemical reduction of Ag+ is possible. Let us consider our experimental conditions. The reverse micelles (AOTNa/ isooctane/water) contain an excess of S2- ions, and AgAOT (in isooctane) is injected in the solution. Immediately after AgAOT (Figure 3) injection, the solution turns yellow, indicating the instantaneous formation of Ag2S nanocrystals. The system remains optically clear, indicating that reverse micelles are still formed. Because the S2- ions are in excess inside the droplet, a slow reduction takes place. Moreover, due to the presence of an excess of S2- ions, it can be expected that the Ag2S nanocrystal surface is S2- rich, i.e., having surface donor defects. A possible mechanism for the formation of Ag metal clusters on Ag2S nanocrystals is that sulfur ion vacancies at the nanocrystal surface favor the reduction by sulfur derivatives of Ag+ ions coming from the internal structure of Ag2S particles. Hence the nucleation (Ag atom formation) takes place on the surface of the Ag2S nanocrystal acting as a center for subsequent reduction of Ag+ ions. The confinement of the reactants (S2ions and R-SO3-) inside the micelles containing Ag2S nanocrystals favors the chemical reduction of mobile Ag+ ions in the Ag2S nanocrystals. This agrees with the mechanism proposed
Figure 2. Change in (Ag2S)x(Ag)y nanocomposite crystal upon exposure to the beam in the TEM. There is an increase in the size of the polycrystalline Ag nanoparticles after exposure to the electron beam at the expense of the Ag2S nanocrystals. The image at the left-hand side was recorded after 1 s, corresponding to an exposure to a dose of 1.5 × 105 electrons/nm2, the center image was recorded after 60 s exposure to a dose of 9 106 electrons/nm2, and the image at the right was recorded after 180 s exposure to a dose of 2.7 × 107 electrons/nm2.
Letters
J. Phys. Chem. B, Vol. 109, No. 46, 2005 21501 Appendix
Figure 3. Chemical structure of silver bis(2-ethyl-hexyl) sulfosuccinate.
The synthesis of Ag2S nanocrystals is made in reverse micellar solution, which contains droplets of water in oil stabilized by a monolayer of surfactant.25 With sodium bis(2ethyl-hexyl) sulfosuccinate, usually called NaAOT, as surfactant, a micellar system has two very important properties for coprecipitation reactions: (i) the droplet size increase linearly with the water content, w ) [H2O]/[NaAOT] and (ii) some of the collisions, due to Brownian motion, between droplets are efficient and an exchange process between water pools occurs. Synthesis of Ag2S nanocrystals requires a functionalized surfactant such as silver bis(2-ethyl-hexyl) sulfosuccinate, called AgAOT (Figure 3). The Ag2S nanocrystals are obtained by addition of a solution of AgAOT dispersed in isooctane to a micellar solution (NaAOT/isooctane/water, [NaAOT] ) 0.1 mol‚L-1) containing sulfide ions S2- (Na2S salt). The synthesis is carried out with an excess of sulfide ions ([S2-] ) [Ag+] ) 8. 10-4 mol‚L-1). The size of the particles is tuned from 2 to 10 nm by changing the size of the droplets, i.e., the water content.13 By using water contents (w) of 5 and 40, the average diameters of Ag2S nanocrystals are 2.3 and 7.6 nm with size distributions of 33% and 15%, respectively. References and Notes
Figure 4. 2D organization of (Ag)x(Ag2S) nanocomposite crystals, TEM image.
in the literature for the growth of metallic silver on Ag2S.19,20 This is also supported by the fact that, on exposure to an electron beam, the Ag cluster size increases (Figure 2) and a direct reduction of Ag+ ions occurs. By addition of dodecanethiol to the reverse micellar system containing (Ag)x(Ag2S) particles, a size-selective precipitation is induced. The nanocrystallites coated by dodecanethiol are then dispersed in heptane and form an optically clear solution. By evaporation of a drop of solution on a TEM grid, 2D organization of these nanocomposite crystals is observed, Figure 4. In summary, we have shown that composite nanocrystals, in the form of small (Ag)x clusters attached to (Ag2S)y nanocrystals, are obtained using a micellar system as a chemical nanoreactor. To our knowledge, this is the first example and direct evidence where a coprecipitation reaction and chemical reduction take place leading to the formation of this type of nanocrystals. This is mainly attributed to the confinement of the reactants inside the water pool of the reverse micelles. These data are a direct proof of hypotheses made for catalytic reduction reaction of Ag+ ions in which formation of silver clusters at the interface of Ag2S nanoparticles was suggested.19 Acknowledgment. The authors wish to thank Professor M. P. Pileni for fruitful discussions.
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