J . Phys. Chem. 1988,92, 5754-5760
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the model in which the colliding cluster heats only its projection on the surface. From the slope of the curves we find the ratio of the detection efficiency of the exoelectrons and the detection efficiency of the clusters, D / t = 1.2 X lo5 for the first series of measurements and D / t = 0.4 X lo5 for the second series. (The change in the value of D/c is caused by a change in the intensity of the argon lamp.)
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Figure 7. (a) Plot of the measured number of exoelectrons per second ((Iept) versus ZEQld 3 SJ’;CI(m) Z,(m)/m dm/J’;CI(m) d m ) for two series of measurements (dots and crosses) for case I. (b) Same as (a), for case 11. The slopes of the two lines give the value of D / c , 1.2 X 10’ for the top plot and 0.4 X lo5 for the bottom plot (see eq 7 ) .
For case I (Figure 7a) we approximated the curve of Figure 6a with a constant I e x ( m ) / m= 1.3 X 10” (dotted line). This approximation does not affect the result too much because the relative weight of the small clusters is low in the total exoelectron integral. For case I1 (Figure 7b) we approximated the curve of m2. Figure 6b with I e x ( m ) / m= 1.7 X Comparing the results for both cases, it is clear that we find the best linear relations in the second case. Therefore, we favor
Conclusions Using measured cluster distributions together with their exoelectron yield and a very simple theoretical model, we have tried to establish how the kinetic energy of a cluster is transferred to a metallic surface. Given the simplicity of the theory, the results are not disappointing, and we certainly can give a judgement about the two different models we have used. If a cluster would excite the total surface eventually covered, the yield per monomer would go to a constant value for large clusters. This must hold more generally and not only for our theoretical model. But it is in contradiction with our measurements. To fit the measurements, the energy input per surface unit has to rise with increasing cluster size, and then the number of exoelectrons per monomer increases with cluster size. The relation calculated for heating only the projection of the cluster gives satisfactory agreement, but of course other theories in which the energy input per unit surface increases with cluster size would also work. We have to use a surprisingly low value of the heat conductivity to fit the experimental result of reaching a surface temperature of 5000 K. What should be the reason for this? First of all, heat conductivity is normally measured in cases of microscopic reversibility. In calculations of the heat conductivity one also makes this assumption. But we may well be far away from this. In our case, it is conceivable that high k vector electrons are created in the collision. It would not be surprising if the energy exchange between electrons with very different k vectors is a relatively slow process. A second possibility is a collective excitation of the electrons of the metal by the collision, Le., the formation of a surface plasmon. This would delay the cooling of the hot spot and would formally give a lower heat conductivity.
Acknowledgment. We express our thanks to Dr. Jonkman for the use of his computer program and to Professor Even, who was largely responsible for the design and construction of the apparatus. This work was supported by the Netherlands Foundation for Chemical Research with financial aid from the Dutch Organization for the Advancement of Pure Research (N.W.O.).
Novel Silver Metal Liquidlike Fllms D. Yogev and S. Efrima* Department of Chemistry, Ben Gurion University of the Negev, POB 653, Beer Sheva. Israel 841 20 (Received: November 30, 1987; In Final Form: March 1 , 1988) We report the synthesis and some of the physical and chemical properties of novel silver films that are confined within two immiscible liquids. Visually there is a striking liquidlike behavior of the film (it flows and heals immediately upon rupture), combined with a high specular reflectivity. We discuss the structure of the film on the basis of physical measurements and the chemical behavior. Introduction The interest in thin films stems from their great importance in modem technology (microelectronics,l optics,2 heterogeneous ~atalysis,~ and coatings4) and from their high relevance to surface
science due to the large surface-to-volume ratio. It is not surprising, therefore, that thin films have attracted a large bulk of research. These efforts have dealt with two main types of films that are thick on the mokcular scale: (1) solid
(1) Leaver, R. D.; Chapman, B. N. Thin Films; Wykeham: London, 1971. (2) (a) Chopra, K. L. Thin Film Phenomena; McGraw-Hill: New York, 1969. (b) Macled, H. A. Thin Film Optical Fibers; Macmillan: New York, 1986. (3) See, for instance, Kalyanasundaran, K.; Gratzel, M. Springer Ser. Chem. Phys. 1984, 35, 11 1.
(4) (a) Lowenheim, F. A. Modern Electroplating, 2nd ed.; Wiley: New York, 1963. (b) Blum, W.; Hogaboom, G. B. Principles of Electroplating and Electroforming, 3rd ed.; McGraw-Hill: New York, 1949. (c) Graham, A. K. Electroplating Engineering Handbook, 2nd ed.; Reinhold: New York, 1968.
0022-365418812092-5754$01.50/0
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 5755
Novel Silver Metal Liquidlike Films
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Figure 1. Schematic view of the M E L L F system.
films-where the film itself is composed of a solid material or where the film is adjacent to a solid phase that imparts to the film its immobile character; (2) liquid films-where the material composing the film is a liquid or a gas and the substrate is either a liquid itself or its binding to the film does not freeze the lateral motion. We do not discuss monomolecular layers in this report. We present here a new type of thin films. These are films composed of a metal (silver) inserted between two immiscible liquids (Figure 1). Under certain feasible conditions, described below, one obtains metal liquidlike films (MELLF). By metal we mean not only that the (silver) metal is a major constituent but also that the film retains the metallic lusture, is highly reflective, and visually appears to be a metal. Liquidlike means that the film exhibits rheological properties of liquids: It can easily flow and sustain waves, and, more striking, it generally does not tear upon insertion of objects into it, or, when it does, the tear holds on short time scales. In the latter case the transient tear edges are not ragged but rather more rounded in appearance. The visual appearance of these films can be summarized by imagining a mercury sample, except that it is a thin film of silver a t room temperature. There are several reports in the literature of metal particles that were deposited on liquids. Some work under high-vacuum conditions with very low vapor pressure liquids (such as silicon Halpern6 coated liquid water with a gold sol using a helium stream that carried a gold vapor toward the liquid boundary. In contrast, our films are produced within the interfacial region of two liquids by using simple chemical deposition techniques. Their properties are also very unique. How does one introduce a new and unknown system? We chose to give, in this very first report, a general overview of the production, main properties, and structure of the films. We provide sufficient detail to make the system available to all and, on the other hand, give a broad presentation to show the variety, richness, and complexity of these very unique interfacial systems. In forthcoming reports each aspect and many more will be discussed separately, covering a large assortment of films, produced by different methods and exhibiting varying properties. The research on thin films has developed a multitude of techniques for the investigation of the structure of the films and their properties. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron diffraction are very powerful techniques but, unfortunately, require ultrahigh vacuum conditions. They could be applied to our systems only after treating them in such a way that must have affected their structure. These techniques, therefore, yielded only indirect, circumstantial information. Yet, this information was of importance especially when correlated with results obtained by other techniques. Other modern techniques such as LEED, EELS, etc., are even less suited to our systems. Even the conventional technique of measurement of UV-vis reflectivity of the MELLF posed some unique complexities, though (5) Blackboron, J. R.; Young, D. Metal Vapor Synthesis in Organometallic Chemistry No. 9; Springer-Verlag: Berlin, 1979. ( 6 ) Halpern, €3. L. J. Colloid Interface Sci. 1982, 86, 337.
such measurements can be performed in the natural state of the films and yield interesting information. These difficulties compelled us to resort to less conventional techniques, such as X-ray diffraction of the fluid film, Fresnel X-ray scattering,’ measurement of surface capillary waves? Raman spectroscopy of thin films, and others. Some of these measurements are still in their preparatory stages and will be described only in later reports. Finally it is of interest to point out the importance of these novel films to the study of surface phenomena and the understanding of surface forces and energies. Roughly speaking we have a “metal” film located between two liquids. Such a system is inherently unstable and is produced and stabilized only due to the interplay of surface and bulk energies. Indeed, as we show below, surfactants, and specific ones at that, have a very important effect on the MELLF. Flotation, as described for instance by Huh and Mason,9 is probably very relevant to the MELLF, especially as we will show that they are essentially colloid systems.
Experimental Section In this section we describe the production of one of the silver metal liquidlike films. Others can be produced varying the liquid phases, the reductant, the surfactant, and to some degree the relative concentrations of the various reactants. One adds 0.1 g of anisic acid (p-methoxybenzoic acid, Merck) to 100 mL of ammoniacal 0.05 M AgN03. Then one adds 0.03 g of FC143 (an anionic fluoro surfactant, ammonium perfluoroalkylcarboxylate, 100% active, produced by 3M ID no. 98-021 1-0367-0) and lets the mixture stand overnight. This solution (10 mL) is poured over 20-50 mL of dichloromethane (C12Me), and about 0.33 mL of 0.5% w/w hydrazine sulfate (BDH) in water are added dropwise. In a few minutes the silver begins to reduce to metallic silver, which produces a dark coloration in the aqueous phase. After a few hours the water clears and the silver deposits in the water-organic junction, forming a thin and very bright interfacial layer. The materials we use are AR quality, and the water is of 18-MQ conductivity, obtained from a Millipore Corp. ionic exchange and filter system. Depending on the surfactant and the organic phase, one may or may not have a (yellowish) coloration of the organic phase at the termination of the film production. One must emphasize that it is very easy to obtain black granular interfacial layers of silver (and other metals); however, the liquidlike behavior is obtained only under some strict procedures as described above. Results I . Production of the Films. Some history is in place to introduce some of the features of the synthesis stage. Initially we tried to produce the silver MELLF without adding a surfactant using a variety of reductants (mostly aldehydes). We tried to localize the chemical reduction to the interfacial region by dissolving the reductant in the organic phase while the silver ions were put into the top water phase. We found that anisaldehyde reduced the silver to a bright, liquidlike film, which would not rupture under insertion of glass rods or a moderate shaking of the system. However, we soon found out that the presence of anisic acid (or ion) was required as well; a pure aldehyde would not give the MELLF, producing granular films instead. Indeed, the first (7),(a) Pershan, P. J.; Als-Nielsen, J. Phys. Reu. Lett. 1984, 52, 759. (b) Als-Nielsen, J.; Pershan, P. J. Nucl. Insfrum. Methods 1983, 208, 545. (c) Als-Nielsen, J.; Christensen,F.; Pershan, P. J. Phys. Reo. Left. 1982,48, 1107. (d) Braslau, A,; Deutsch, M.; Pershan, P. C.; Weiss, H.; As-Nielsen, J.; Bohr, J. Phys. Reu. Lett. 1985, 54, 114. (8) (a) Hard, J.; Hamnerius, Y.; Nilsson, 0. J. Appl. Phys. 1976,47,2433. (b) Yoneda, K.; Tawata, M.; Hattori, S. Opt. Laser Technol. 1976,8,39. (c) Hard, S.; Neuman, R. D. J . Colloid Interface Sci. 1981,83,315. (d) Lofgren, H.; Neuman, R. D.; Saiven, L. E.; Davis, H.T. J. Colloid Interface Sci. 1984, 98, 175. (e) Hard, S.; Neuman, R. D. 1. Colloid Interface Sci. 1987, IIS, 73. (9) (a) Huh, C.; Scriven, L. E. J . Colloid Interface Sci. 1969,30, 323. (b) Huh, C.; Mason, S . G. J. Colloid Interface Sci. 1974, 47, 271.
5756 The Journal of Physical Chemistry, Vol. 92, No. 20. 1988
stage involves the deposition of a silver anisate salt in the interfacial region (as determined by elemental analysis). In fact. one can first add the acid, producing a white interfacial film, and then adding the reductant (anisaldehyde or others) gives the MELLF. Under such circumstances one can add the reductant even directly into the aqueous phase where the silver nitrate was added earlier and still obtain bright silver MELLF. If the silver anisate layer is not preformed, all our attempts to create MELLF by putting the silver ions and the reductant in the same phase produced only brown-black spongelike films. Even the use of standard techniques of producing silver mirrors1° would not work. Acids similar to anisic acid, such as pethoxybenzoic acid and 3-methoxybenzoic acid, worked just as well. Similarly, the corresponding aldehydes (Le., p-ethoxybenzaldehyde) also yielded silver MELLF. though anisaldehyde gave the brightest films. The main problem with the films produced as described above was that they were rather unstable. The (lower) organic phase would acquire a brown coloration, which grew in a matter of days into a strong turbidity. In parallel the MELLF gradually lost its luster and its "liquid" properties. Thus silver particles were "extracted" from the interfacial layer into the organic phase, producing a silver colloid, which eventually deposited on the bottom of the vessel To anticipate the discussion of the results, this was the first direct indication that the silver MELLF was composed of silver colloids, ample evidence for which has come forth as the work progressed. This is consistent with the yellowish coloration (and presence of colloidal particles) in the organic phase of some of the preparations. From the synthetic point of view, of course. the instability is undesirable. We overcame this problem by adding surfactants. We found that only anionic surfactants (carboxylates and sulfates) or surfactants capable of pmducing anionic "heads" were effective in stabilizing the silver MELLF over durations longer than a month. Furthermore, perfluoroalkyl backbones proved to be the most effective, though other types of "tails. worked reasonably well. The recipe we gave in the Experimental Section is mwt efficient in producing stable films. and the time required for the synthesis itself is minimal. The properties and stability of the films are strongly dependent on the precise composition of the reaction mixture. Changing the relative concentrations and certainly excluding one of the ingredients can make all the difference between a bright MELLF and a black, granular, sponge layer. I I . Properlies of the Films. In this subsection we describe various properties of the silver MELLF as produced according to the recipe given in the Experimental Section. A more detailed report presenting the results for a variety of films and conditions will be given in forthcoming publications. I1.a. Figure 2 is a photograph of the film taken from the water (top) side. The highly reflective, mirrorlike appearance is a p parent. The lower side of the film. that directed toward the organic phase, is black and resembles the granular films one usually obtains unless our procedures are used. We have tried to produce MELLF in systems where the aqueous phase is denser than the organic phase. This was done to determine whether the diffrent a p pearance of the two sides of the film was due to the physics of the deposition. or due to specific chemical interactions with the liquid phases. To date our efforts to produce MELLF in such 'inverted" systems have failed. Figure 3, top. shows a glass rod that was inserted into the film and dragged through it. N o tears are evident. Figure 3, bottom, shows a photograph of the film immediately after shaking the vessel. The waves sustained in the film are noticeable and are of an amplitude much larger than the apparent thickness of the film. 11.6. The reflectance of the film a t 90° incidence and a wavelength of 632.8 nm (He/Ne laser, Spectra Physics 155A) was 40%-50%. while the measured transmittance was only several (IO) V-n, 1978.
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.I FIw 7.. Photograph of film. lop vie- (water over dichloromethanc. rurlactant is FC143. an anionic perflwroalkyl carboxylate. 3M).
percent. The transmittance is similar to that of a continuous evaporated silver film several dozen nanometers thick. Some broadening of thc bcam upon reflection was noticed. indicating the presence of diffused scattering from the film. Visual inspection of the film over a lamp reveals an inhomogeneous structure ofthc film. Densitometer mappings of the film will be reponed elsewhere. Upon stirring or shaking the film. one notim motion within the film. so that the inhomogenmus structure rearranges itself. It rcscmbles currents in a lake or sca. The color of the film under white illumination is light purple. indicating a thickness in the range of lW60 nm in terms of continuous deposited silvcr films." I1.c. The electrical conductivity was measured with two silver electrodes connected to a conductometer or multimeter. The conductivity was the same as that of the water phase and the CI,Me phase in the absence of the silver MELLF. When the liquid phases were slouly evaporated. thc interfacial film deposited on the bottom of the vessel. but once again no detectable conductivity was observed. Under similar conditions a SO-nm-thickevaporated silver film eahibited considerable conductivity. The lack of conductivity of the MELLF. in principle. can stem either from a disconnectedness of the film itself or from wetting problems. 1l.d. Addition of IO'& (by weight) of NaNO, to the reaction mixture before the addition o f t h c reductant prevented the pro. dunion of thc MELLF. A gray interfacial deposit was observed. instead. When the salt was added only after the MELLF was produced, a significant deterioration of the brightness ofthe film was apparent. Addition o f a KCN solution caused an immediate destruction and almost total disappearance of the MELLF followed by the appearance o f a turbidity in the aqueous and organic phases. KI and Na,S,O, had a similar effect. This behavior was observed also under conditions of the exclusion of oxygen in a glovebox in an argon atmosphere. For comparison. thin evaporated silver films ( I I ) Compared to standards of evaporated silver in the thieknsi range IC-IW nm.
Novel Silver Metal Liquidlike Films
The Journal oJPhysica1 Chemistry, Vol. 92, No.20. 1988 5151
of cyanide was not that of oxidation by oxygen, as is the case for the continuous films (and as utilized by silversmiths to clean silver).
Figure 3. (top) Photograph o l silver MELLF wrth glas rod inserted (film as in Figure 2). (boltom) Photograph of the silver MELLF immediately alter shaking the vessel (film as in Figure 2).
(50-, 100.. and 200-nm thick) dissolved when treated with a cyanide solution (as expected) but only when exposed to air (oxygen). In a glovcbox under argon these evaporated silver films are stable indefinitely. This set of experiments showed that the mechanism of disruption of the silver MELLF under the influence
Instead, we propose that the cyanide ions adsorbed on silver particles in the MELLF and charged them accordingly, and the interparticle repulsion caused them to fly apart. This, of course, results in the disruption of the films. Such an explanation is consistent with the evidence that points to the colloid structure of the silver MELLF. To our knowledge this is a unique example of a salt effect causing further dispersion of a colloid, rather than coagulation. In contrast, a few d r o p of a concentrated solution of lanthanide or barium ions are enough to bring about a shrinking of the MELLF with a loss of the reflectivity and the ‘liquid” nature. A solid, granular film was obtained. This again is in line with the colloid picture, as these ions are well-known coagulants. ILe. We utilized the effect of cyanide to break up the MELLF and study its “fragments” by the dynamic light-scattering technique, using photon autocorrelation (Malvern 4700 spectrometer with k7032 correlator). After production of the silver MELLF we added cyanide and followed the scattering from the two liquid phases and compared it to the scattering seen from these phases before the disruption of the MELLF. Initially the water phase exhibited scattering from particles in the diameter range 13&1500 nm, with the (weighted) average at 450 nm. In the yellowish C12Me phase particles of 528-nm diameter were observed (ranging from 300 to 900 nm). In the noncolored organic phase no significant amount of scattering was seen. When the cyanide was added, the organic phase cleared and the average particle size reduced to 96 nm, with a tail of the size distribution reaching to about 450 nm. The aqueous phase, on the other hand, was enriched with larger particles. The average diameter increased to 1290 nm, with sizes ranging from 900 to 3000 nm. These experiments were conducted in the absence of oxygen. In the presence of oxygen the average particle size is smaller, and after the introduction of cyanide there is a definite decrease of the sizes as a function of time. For instance, the water phase may exhibit an average diameter of 355 nm before the addition of cyanide, 488 nm immediately after the disruption of the film, 250 nm I5 min later, and 197 nm half an hour after the introduction of cyanide. IIJ. Electron microscopy was also applied to our silver MELLF. As discussed above, such techniques required solid samples. Thus we would either evaporate the liquid from our MELLF or, more often, deposit the film on a TEM copper grid by pulling it gently from under the film. In both cases necessarily the films are damaged to a degree that we cannot determine. Therefore, the SEM and TEM results must be used with caution and only as a circumstantial piece of evidence. Figure 4 shows a TEM picture of a silver MELLF. The highly dispersed structure is very a p parent. Similar topology was seen also at smaller magnifications by using SEM. In Figure 4 one sees clusters with an average diameter of about 30 nm. However, these clusters have an internal structure as seen by the dark and light parts. The h s i c internal units are smaller than the resolution of the picture, Le.. about 5 nm. The electron diffraction from a dried film obtained in a TEM is shown in Figure 5 . As a comparison we show in Figure 6 the diffractionpattern from an evaporated silver film. The diffraction rings of silver ( f a a a n t e r e d cubic structure with unit cell of size 0.408 nm) appear in both pictures. However, the diffraction from the MELLF has rings that are not that of silver. By comparison we found them to belong to one or several of the following: anisic acid, its silver salt, the surfactant, or its salt with silver. The exact assignment of the additional diffraction rings was specific to the various MELLF preparations. 1I.g. A Perkin-Elmer A5 UV-vis spectrophotometer was used to obtain reflectance spectra from the silver MELLF. To study the films in their natural state, we slightly modified the spectrometer to allow for vertical incidence and detection, instead of the conventional horizontal layout. As a reference we used a 200nm-thick smooth silver film evaporated on glass and immersed in water.
5758 The Journal o/Physical Chemisrry. Val. 92, No. 20. 1988
Yogev and Elrima
Figure 4. TliM exposure 01 film (w;iler u\cr dichlon~iiictli.inc.w ~ I . ~ c t a n l is FC99. a n anionic perfluaroalkylsulfonatc. 3M). .Ihc Ihns dimension of the picture corresponds to 700 nm.
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