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Langmuir 2008, 24, 2792-2798
Use of Reverse Micelles to Make Either Spherical or Worm-like Palladium Nanocrystals: Influence of Stabilizing Agent on Nanocrystal Shape K. Naoe,† C. Petit, and M. P. Pileni* Laboratoire LM2N, CNRS, UMR 7070, UniVersite´ Pierre et Marie Curie (Paris VI), BP 52 4 Place Jussieu, 75252 Paris Cedex 05, France ReceiVed September 19, 2007. In Final Form: NoVember 21, 2007
Depending on the water content, reverse micelles induce the formation of fcc metallic palladium worm-like nanocrystals made of spheres. After extraction from the nanoreactor, either spheres or worm-like nanocrystals are obtained, and it was found that the binding energy between the coating agent and the Pd surface is a key parameter in shape control (i.e., in the surface reconstruction).
1. Introduction During the last two decades, nanomaterials have been one of the most studied topics in chemistry and physics, and a large number of groups have developed new strategies to produce them. Reverse micelles were one of the first approaches,1,2 and their production has been followed by several other organometallic syntheses3,4 and phase transfer processes.5,6 In the reverse micelle method, the inner core of the reverse micelles is considered to be a nanoreactor, and the size of the nanocrystals is controlled by the state of the water molecules inside the water pools. This method1 has been used for the synthesis of semiconductor nanomaterials,7 metallic nanoparticles,8-11 and nanoalloys.12,13 Palladium nanocrystals act as excellent catalysts for the Heck reaction in synthetic organic chemistry14,15 and in many other applications such as environmental catalysis.16,17 They are also anticipated to be useful as hydrogen storage media for hydrogen fuel cells.18,19 For this, the size of the nanocrystals has to be rather small, and the coating agent has to be sufficient enough † Permanent address: Department of Chemical Engineering, Nara National College of Technology, Yamato-Koriyama, Japan.
(1) Pileni, M.-P. J. Phys. Chem. 1993, 97, 6961. (2) Petit, C.; Pileni, M. P. J. Phys. Chem. 1989, 92, 2282. (3) Murray, C. B.; Norris, D. J.; Bawendi. J. Am. Chem. Soc. 1993, 15, 8706. (4) Peng, X. G.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature (London, U.K.) 2000, 404, 59. (5) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 4, 1234. (6) Brust, M.; Fink, J.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1994, 4, 1234. (7) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598. (8) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (9) Lisiecki, I.; Pileni, M. P. J. Am. Chem. Soc. 1993, 115, 3887. (10) Petit, C.; Taleb, A.; Pileni, M. P. J. AdV. Mater. 1998, 10, 259. (11) Wikander, K.; Petit, C.; Holmberg, K.; Pileni, M.-P. Langmuir 2006, 22, 4863. (12) Yashima, M.; Falk, L. K. L.; Palmqvist, A. E. C.; Holmberg, K. J. Colloid Interface Sci. 2003, 268, 348. (13) Petit, C.; Rusponi, S.; Brune, H. J. Appl. Phys. 2004, 95, 4251. (14) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009. (15) Biffis, A.; Zecca, M.; Basato, M. J. Mol. Catal. A: Chem. 2001, 173, 249. (16) Nutt, M. O.; Hughes, J. B.; Wong, M. S. EnViron. Sci. Technol. 2005, 39, 1346. (17) Tanaka, H.; Uenishi, M.; Taniguchi, M.; Tan, I.; Narita, K.; Kimura, M.; Kaneko, K.; Nishihata, Y.; Mizuki, J. Catal. Today 2006, 117, 321. (18) Kuji, T.; Matsumura, Y.; Uchida, H.; Aizawa, T. J. Alloys Compd. 2002, 330-332, 718. (19) Kishore, S.; Nelson, J. A.; Adair, J. H.; Eklund, P. C. J. Alloys Compd. 2005, 389, 234.
to prevent coalescence and subtle enough to not interact too much with the nanoparticle surface. Furthermore, the crystallinity of the nanoparticles has to be very high to be efficient. Hence, several ligands have been used in recent years. A very low size distribution of Pd nanoparticles extracted from dendrimer templates into toluene in the form of an alkanethiol stabilizer20 has been reported, which seems to be tightly attached to the nanocrystal interface. A recent study has shown the importance of the thioalkyl chain length in the sulfidation of nanoparticles. For C18SH, the capping thiol appears not only on the surface but also in the bulk. This makes the nanoparticles totally inefficient, whereas with C12SH, the sulfidation remains on the surface, and the core stays crystallized.21 Poly-N-vinyl-2-pyrolidone (PVP) as a coating agent produces amorphous palladium nanocrystals.22 This was the focus of a study on behavior with platinum nanocrystals.23 In this case, dodecylamine as a stabilizing agent was loosely bound to the surface of the metal, and as a result, agglomerated Pt nanoparticles were formed.11 Furthermore, the change in size and shape with the coating agent used is still an open question.24 Digestive ripening consists of the addition of several ligands that are efficient in converting highly polydispersed particles to those that are nearly monodispersed and refluxing at the solvent boiling temperature.25,26 Via digestive ripening with gold nanomaterials, several ligands such as alkylthiols, amines, and phosphines are very efficient in converting highly polydispersed particles to nearly monodispersed ones. It has been shown that it is important to reflux the colloid only in the presence of the (20) Garcia-Martinez, J. C.; Scott, R. W. J.; Crooks, R. M. J. Am. Chem. Soc. 2003, 125, 11190. (21) Amallo-Lopez, J. M.; Giovanetti, L.; Craievich, A. F.; Vicentin, F. C.; Marin-Almazo, M.; Jose´-Yacaman, M.; Requejo, F. C. Physica B (Amsterdam, Neth.) 2007, 389, 150. (22) Lu, W.; Wang, W. L. B.; Wang, K.; Wand, Y.; Hou, I. G. Langmuir 2003, 19, 5887. (23) Dassenoy, F.; Phillippot, K.; Ould-Ely, T.; Amiens, C.; Lecante, P.; Snoeck, E.; Mosset, A.; Casanove, M. J.; Chaudret, B. New J. Chem. 1998, 22, 703. (24) Ramirez, E.; Jansat, S.; Philippot, K.; Lecante, P.; Gomez, M.; MasdeuBulto, A. M.; Chaudret, B. J. Organomet. Chem. 2004, 689, 4601. (25) Stoeva, S.; Klabunde, K. J.; Sorensen, C. M.; Dragieva, I. J. Am. Chem. Soc. 2002, 124, 2305. (26) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515.
10.1021/la702908y CCC: $40.75 © 2008 American Chemical Society Published on Web 02/01/2008
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Figure 1. TEM images, at various scales, of palladium nanoparticles made in reverse micelles at various water contents and observed 45 min after the two solutions were mixed. (A and B) w ) 10 and (C and D) w ) 20.
digestive ripening agent without a surfactant or other impurities.27 Recently, we demonstrated a novel, soft, digestive ripening technique.28 This technique is quite simple for producing spherical nanocrystals from worm-like nanostructures at room temperature, without refluxing, by adding excess alkylthiol. In the present study, we demonstrated the influence of the capping agent on the final nanocrystal shapes with the formation either of spherical nanocrystals or worm-like particles made up of spheres produced in reverse micelles. After extraction from micellar media, the change in the binding energy between the extractant molecule and the Pd surface may or may not induce surface reconstruction and produce either spherical or wormlike fcc Pd nanocrystals. 2. Experimental Procedures 2.1. Materials. Palladium (II) chloride (PdCl2, 99.9%) was from Sigma-Aldrich and was used as received. Sodium bis(2-ethylhexyl)sulfosuccinate (AOT), 99% purity, was from Fluka. The dodecylamine (C12NH2) and dodecylthiol (C12SH) used in the study (99%) were from Aldrich, as was the reducing agent, sodium borohydride, NaBH4 (99%). The organic solvents (toluene, isooctane, cyclohexane, and ethanol) were all analytical grade. Water was purified by a Millipore water system. All glassware was carefully cleaned in aqua regia and rinsed with large quantities of water before use. 2.2. Transmission Electron Microscopy (TEM) Measurements. TEM images were obtained using a JEOL 1011 operated at 100 kV with magnifications between 150 000× and 300 000×. TEM sample grids were prepared by placing a drop of a freshly prepared nanoparticle dispersion onto a grid placed on an adsorbing paper, which removed the excess solvent. To determine the mean nanoparticle size, D, and the corresponding size distribution, σ, around 500 nanoparticles were measured for each sample and presented in (27) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 2003, 15, 935. (28) Naoe, K.; Petit, C.; Pileni, M.-P. J. Phys. Chem. C 2007, 111, 16249.
a histogram. The standard deviation, σ, was calculated according to σ ) {[
∑(D - D) ]/[n - 1]} 2
1/2
i
where D is the average diameter. A polydispersity index was defined as the ratio σ/D. For high-resolution transmission electron microscopy (HRTEM) measurements of the crystallinity of the prepared palladium nanoparticles, a JEOL 2010 UHR instrument operated at 200 kV (LaB6) was used. 2.3. Syntheses of Palladium Nanocrystals. The PdCl2 solution (0.05 M) was prepared by dissolving 0.1773 g of PdCl2 in 20 mL of water and adding 115 µL of HCl (drop by drop) to obtain a transparent brown solution. Preparation of palladium nanoparticles in reverse micelles was achieved by mixing two solutions of reverse micelles, one containing the palladium salt and the other the reducing agent. In a typical experiment, 675 µL of 0.05 M PdCl2 aqueous solution (containing 33.75 µmol of Pd2+) was added to 15 mL of 0.25 M AOT in isooctane. The water-to-surfactant molar ratios (w ) [H2O]/[AOT]) were 5, 10, and 20, which correspond to droplet diameters of 1.5, 3, and 6 nm, respectively.1 An equal volume of reverse micelles but with freshly prepared NaBH4 solution (600 µmol) in the water pools was made ([NaBH4] in micellar solution was 0.02 M). Again, the reducing agent was present in large excess with a stoichiometric ratio Pd/NaBH4 of 1:17.8. Immediately after mixing the two reverse micelle solutions, the solution color turned from brown to black, indicating the formation of palladium nanocrystals. Extraction of the nanocrystals was performed as follows. After a variable reaction waiting time (5, 15, or 45 min), the stabilizing agent, dodecylamine (1.5 mL) or dodecylthiol (900 µL), was added to the reaction mixture, which was left for 1 h. The isooctane was then removed by evaporation, and the waxy residue was redispersed in 45 mL of ethanol followed by centrifugation at 5000 rpm for 10 mn. The black precipitate was redispersed in 30 mL of fresh ethanol and again centrifuged. This washing step was repeated again. After the washing with ethanol, the black precipitate was redispersed in 6 mL of toluene when C12NH2 was used as the stabilizing agent or 6 mL of cyclohexane when C12SH was used. The addition of another
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Figure 2. Formation kinetics of worm-like palladium nanocrystals made in reverse micelles at w ) 20. TEM images (A, C, and D), histogram of the spheres (B), and histogram of the short axis of the worm-like particles (E). (A and B) 5 min after mixing the two micellar solutions, (C) 15 min after mixing the two micellar solutions, and (D and E) 45 min after mixing the two micellar solutions. 10-20 µL of the stabilizing agent was required to obtain dispersion in the solvent. Each synthesis was carried out in duplicate.
3. Results and Discussion Reverse micelles were used to make palladium nanocrystals. At a very low water content (w ) 5), for any waiting time, no Pd nanoparticles were produced. At w ) 10, 45 min after mixing the two micellar solutions, a drop of solution was deposited on a TEM grid. Figure 1A,B shows the formation of discrete polydispersed spherical nanoparticles and some aggregated Pd nanoparticles. On increasing the water content to w ) 20, after the same waiting time (45 mn) and same experimental conditions (PdCl2, AOT, volume, etc.), worm-like particles were obtained as shown in Figure 1C,D. Hence, conversely to what has been
observed for very diversified nanomaterials such as metals, semiconductors, etc.,1,32,33 the content of the water droplets does not control the size of the spherical particles. With metallic palladium, the increase in the water pool induces a transition from polydispersed spheres to highly aggregated worm-like nanoparticles. The kinetic formation of the worm-like particles (at w ) 20) is described in Figure 2. Figure 2A shows, 5 min after mixing the solution, a rather large amount of spherical (29) Pan, C.; Pelzer, K.; Philippot, K.; Chaudret, B.; Dassenoy, F.; Lecante, P.; Casanove, M.-J. J. Am. Chem. Soc. 2001, 123, 7584. (30) Sun, Y.; Frenkel, L.; Isseroff, R.; Shondrun, C.; Forman, M.; Shin, K.; Koga, T.; White, H.; Zhang, L.; Zhu, Y.; Rafailovish, M. H.; Sokolov, J. C. Langmuir 2006, 22, 807. (31) Zinsli, P. E. J. Phys. Chem. 1979, 83, 3223. (32) Lisiecki, I. J. Phys. Chem. B 2005, 109, 12231. (33) Pileni, M. P. Langmuir 1997, 13, 3266.
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Figure 3. TEM images of palladium nanoparticles made in reverse micelles at various water contents and observed 45 min after the two solutions were mixed and then extracted. (A and B) Made in reverse micelles before extraction at w ) 10 (A) and w ) 20 (B). (C and D) Made in reverse micelles after extraction at w ) 10. (E and F) Made in reverse micelles after extraction at w ) 20.
particles with a 4 nm and 16.5% average size and size distribution, respectively (Figure 2B). Note that some worm-like and/or agglomerated spherical particles are already observed. In the histogram shown in Figure 2B, all the coalesced particles are not taken into account. On increasing the residence time in the reverse micelles to 15 min instead of 5 mn, there are rather large aggregates of worm-like particles as shown in Figure 2C. It has been impossible to obtain a histogram of the short axis, worm-like particles because not enough of these are isolated on the grid. A further increase in time (45 min) shows the formation of welldefined worm-like particles (Figure 2D) with an average short axis diameter and size distribution of 4.7 nm and 12.3%, respectively (Figure 2E). Hence, at a fixed water content (w ) 20), the first step in worm-like formation is the production of spherical nanoparticles indicating that in AOT reverse micelles,
Pd nanoparticles are not stable and tend to aggregate into wormlike particles. Furthermore, the fact that no metallic Pd particles are produced at w ) 5, whereas spheres are observed at higher water contents, is explained by the change in the water structure inside the droplets: at w ) 5, PdCl2 is surrounded by what looks like frozen water,37 whereas bulk water begins to exist at w ) 10 and is in its major state at w ) 20.31,34 Because of the similarity of the average diameters (4 and 4.7 nm) and the size distributions (16.5 and 12.3%) of the spherical particles produced 5 min after the reaction begins, the short axis observed after 45 min, and the fact that mainly spheres are produced at a low water content (w ) 10), it is reasonable to conclude that the worm-like particles (34) Pileni, M. P.; Hickel, B.; Ferradini, C.; Puchauld, J. J. Chem. Phys. Lett. 1982, 92, 308.
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Figure 4. Influence of the reaction time of Pd nanoparticles in reverse micelles (w ) 20) on the shape of nanocrystals after extraction by dodecylthiol. TEM images (A, C, and E) and histograms of the spheres (B, D, and F). (A and B) 5 min after the two micellar solutions were mixed. (C and D) 15 min after the two micellar solutions were mixed. (E and F) 45 min after the two micellar solutions were mixed.
are made of an association of spherical particles. This process yielding a worm-like structure was also observed in the case of semiconductor nanocrytsals.38,39 To support such a claim, let us first extract the particles produced at w ) 10 (Figure 3A) and w ) 20 (Figure 3B) with dodecylthiol (see the synthesis described previously) as a stabilizing agent ([C12SH] is 0.125 M). At w ) 10, a mixture of polydispersed spherical and worm-like particles is observed (Figure 3C,D), whereas at w ) 20, spherical particles are obtained (Figure 3E,F) with very few remaining worm-like particles. At w ) 10, C12SH, usually considered to be a strong stabilizing agent,29 is not highly efficient. This also is related to the water structure inside the water pool. Conversely to this, at a higher water content (w ) 20), C12SH is very efficient. The fact that the extraction produces spherical Pd nanoparticles confirms our claim that the worm-like particles observed in Figure
3B are made of associated spheres. To confirm the influence of C12SH, the following kinetic process was carried out: after the two micellar solutions were mixed during a given time, t, dodecylthiol was added to the micellar solution. The nanoparticles were then extracted from reverse micelles and dispersed in cyclohexane. For any waiting time, spherical nanoparticles are produced (Figure 4): 5 min (Figure 4A,B), 15 min (Figure 4C,D), and 45 min (Figure 4E,F); the average sizes and their distributions are 4.2, 4.2, and 4.9 nm and 10.6, 10.0, and 14.9%, respectively. The sizes and their distributions of spherical nanoparticles kept for 5 min in reverse micelles (Figure 2A) remain the same as those obtained after extraction (Figure 4A). In fact, the average diameters (Figures 2B and 4B) and size distributions are 4 and 4.2 nm and 16.5 and 10.6%, respectively. However, we note that some spherical particles still remain agglomerated and are not
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Figure 5. HRTEM micrographs of palladium nanocrystals made in reverse micelles (w ) 20). (A) Extracted by dodecylthiols and (B) extracted by dodecylamine. Figure 6. Worm-like particles (A and B), made in reverse micelles (45 min, w ) 20) and extracted with dodecylamine, at various scales.
taken into account in the histogram shown in Figure 4B. This clearly shows that the small numbers of aggregates observed in Figure 2A are made of spheres. Comparison (Figures 2E and 4F) of the short axes of worm-like particles observed after 45 min (4.7 nm and 12.3%) with the diameter of spherical particles extracted from these reverse micelles (4.9 nm and 14.9%) again shows very good agreement. These data confirm that the wormlike particles are made of spherical nanocrystals. HRTEM shows that the spherical nanoparticles are highly crystalline (Figure 5A) with a fcc structure that fully agrees with other research.22,30 This confirms that the synthesis of nanomaterials in reverse micelles produces highly crystalline materials and not amorphous ones. From these data, it is demonstrated that reverse micelles produce Pd spherical nanocrystals, which tend to agglomerate to form worm-like Pd particles. At this stage, we can ask if it is possible to maintain the wormlike structure and to observe a recrystallization of the spherical particles. To demonstrate this, let us use a stabilizing agent wellknown to have a weak binding affinity to palladium,24 such as dodecylamine (C12NH2). In the following discussion, the same experimental conditions (w ) 20, the particles remained for 45 min before extraction) were kept, and the stabilizing agent C12SH ([C12SH] was 0.125 M) was replaced by C12NH2 ([C12NH2] was 0.217 M). The worm-like particles made in reverse micelles (Figure 1C,D) were extracted with C12NH2 and dispersed in toluene. Conversely to what was observed with C12SH, the palladium particles retained the worm-like structures, as shown in Figure 6. They are elongated (Figure 6A,B) and not uniform as observed before extraction (Figure 1D). The fact that with C12NH2 as the coating agent the worm-like particles can be
extracted indicates that surface reconstruction took place in the presence of the additive. HRTEM confirms that the surface reconstruction process yielded worm-like particles characterized by a very high crystallinity (Figure 5B) with a fcc structure. The difference in the contrast confirms that the worm-like crystals were formed by recrystallization of coalesced spherical nanocrystals with thin regions between them where the atomic planes of both particles tend to align (see arrows in Figure 5B). This demonstrates that a given stabilizing agent allows the movement of Pd atoms between closed spheres and favors their reorganization in a fcc structure as bulk palladium. This confirms some claims that the nanoparticle shapes could change when they are coated with a stabilizing agent.35 However, this is not true for all the materials.36 Hence, fcc palladium metal nanocrystals produced in reverse micelles form worm-like particles composed of close-packed spheres that can either be broken into welldefined spherical nanocrystals or remain as highly crystalline elongated crystals. The strength of the bonding between the Pd surface and the coating agent is a key parameter in shape control. At this point, we can ask if C12SH could induce digestive ripening and favor the formation of spherical Pd nanocrystals, as observed when the worm-like nanocrystals were produced by a phase (35) Baletto, F.; Ferrando, R. ReV. Mod. Phys. 2005, 77, 371. (36) Pileni, M. P. J. Phys. Chem. C 2007, 111, 9019. (37) Pileni, M. P.; Chevalier, S. J. Colloid Interface Sci. 1983, 92, 326. (38) Tang, Z.; Kotov, N. A.; Giersig, M. Science (Washington, DC, U.S.) 2002, 297, 237. (39) Lu, W.; Gao, P.; Jian, W. B.; Wang, Z. L.; Fang, J. J. Am. Chem. Soc. 2004, 126, 14816.
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Figure 7. Effect of additional dodecylthiol on palladium nanocrystals synthesized with dodecylamine as a stabilizing agent. The concentrations of additional dodecylthiol are (A) 0.417 M, (B) 0.556 M, (C) 0.834 M, and (D) 3.34 M.
produced upon increasing the C12SH concentration. This is confirmed by HRTEM images where spherical (Figure 8A) and worm-like (Figure 8B) nanocrystals were observed. As stated previously, they are both crystallized in fcc arrangements. Note that the lattice fringes of two close spherical particles are not aligned with each other, demonstrating no connection between them (Figure 8A), whereas along the long axes of the worm-like nanocrystals, they are parallel even at their connections (Figure 8B). The slight influence of C12SH on the worm-like particles produced via reverse micelles markedly differs from what is observed when they are made via a phase transfer reaction,28 whereas they are crystallized in the same structure. This clearly indicates that, even if the data seem to be similar, the way the nanocrystals are made plays an important role. In the present case, the confinement of the reactants inside the water droplets could favor stronger interactions between the coating agent and the Pd surface and enhance the surface reconstruction of the nanocrystals.
4. Conclusion
Figure 8. HRTEM micrographs (A and B) of palladium nanocrystals synthesized with dodecylamine as a stabilizing agent after additional dodecylthiol was added. The concentration of the additional dodecylthiol was 0.834 M.
transfer reaction28 and coated with C12NH2. To answer this question, a large amount of C12SH ([C12SH] was 0.417, 0.556, 0.834, and 3.34 M) was added to the solution containing the worm-like particles coated by C12NH2 and dispersed in toluene. The solution was kept overnight at room temperature, and then a drop was deposited on a TEM grid. The progressive increase in the C12SH concentration from 0.417 to 3.34 M (Figure 7) shows a decrease in the contrast along the nanocrystals and the formation of less agglomerated and more elongated worm-like particles than those produced before C12SH addition (Figure 6). Furthermore, very small amounts of spherical particles were
Conversely to what has been observed for very diversified nanomaterials such as metals, semiconductors, etc.,1,32,33 water droplets do not control the size of spherical particles. With metallic palladium, the increase in the water pool induces the formation of worm-like particles made of an association of spherical nanocrystals. The change in the coating agent used to stabilize the worm-like particles (i.e., the binding energy between a molecule and the Pd surface) may or may not induce surface reconstruction and produce either spherical or worm-like fcc Pd nanocrystals The confinement of the reactants inside the water droplets could favor stronger interactions between the coating agent and the Pd surface and enhance surface reconstruction of the nanocrystals. Acknowledgment. K.N. thanks the Institute of National Colleges of Technology, Japan for a grant to visit the Universite´ Pierre et Marie Curie, Paris, where this work was performed. LA702908Y
