J. Phys. Chem. 1994,98, 9247-9251
9247
Synthesis and Minimum Energy Structure of Novel Metal/Silica Clusters A. N. Patil and R. P. Andrest School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47906
N. Otsuka School of Materials Science and Engineering, Purdue University, West Lafayette, Indiana 47906 Received: March 7, 1994’
The synthesis of nanometer diameter Au/SiO2, Ag/SiO2, and Cu2O/SiO2 clusters is described. These heteronuclear clusters are grown as metal/silicon clusters of controlled composition and controlled size using a gas aggregation source and, by means of gas phase annealing, are transformed into “structured” particles. The final metal/silica or metal oxide/silica clusters are formed by air exposure. The atomic structures of these nanometer-scale particles are determined by depositing them on thin carbon substrates and studying them using electron diffraction and transmission electron microscopy. The minimum energy structures are single crystals of AuO, Ago, and Cu20 encapsulated in amorphous SiOl.
Introduction C. N. R. Rao and his students have carried out a number of fundamental studies on the structural, electronic, and chemical properties of supported metallic and bimetallic particles.’“ A major theme of this research is the importance of interactions between particle and substrate as particle size decreases and the implications of these interactions on catalysis. In an effort to control such interactions and thereby produce novel chemical activity, we have developed a gas aggregation source capable of producing heteronuclear clusters of controlled composition and size.7-9 If these nanometer diameter particles are well annealed as free particles in the gas phase, it is expected that they will assume a minimum energy structure characteristic of their atomic composition and size and that this structure will be retained when they are soft landed onto a solid support. In this way perhaps supported particles having novel properties can be synthesized. The minimum energy structure of clusters, containing two atomic species that are immiscible in the bulk solid, is expected to consist of two domains each enriched in one species. Perhaps the most common structure of this type is the “bicrystal” particle with two domains separated by a planar interface. When the surface free energy of one of the components is much smaller than that of the other, however, the minimum energy cluster can also have a “fish-eye” morphology with a core consisting of the atomic species with the higher surface free energy and a skin consisting of the atomic species with the lower surface free energy. We report the results of experiments designed to produce nanometer diameter “fish-eye” clusters having a metal core encapsulated by a thin layer of silica. In order to synthesize these particles, we first produce an aerosol mixture of metal/ silicon clusters entrained in an inert gas. The clusters are then heated and cooled in the gas phase to promote phase segregation. Exposure of these clusters to oxygen results in the production of metal/silica or metal oxide/silica particles. The results of experiments with Au/Si, Ag/Si, and Cu/Si are reported. Experimental Section Figure 1 shows a schematic diagram of a gas aggregation source known as a multiple expansion cluster source (MECS), developed at Purdue University. A detailed description of the MECS can be found elsewhere.’-g Typical conditions used in the present study are listed in Table 1. In order to produce metal/silicon e Abstract
published in Advance ACS Abstracts, August 15, 1994.
0022-3654/94/2098-9247%04.50/0
OVEN GAS
METAL o SILICON QUENCH GAS
YHEATE
0,
PUMP
ZONE
PUMP
OVEN GAS
IOVEN
ZONE^
Figure 1. Schematic of the MECS for synthesis of metal/silicon clusters.
TABLE 1 ~~~
operating conditions oven gas oven pressure oven temperature quench gas ratio of quench gas oven gas reactor pressure heating zone temperature sample chamber pressure
He 100-150 Torr 1200-1600 O C He or Ar 2 4 40-60
Torr
950-1350 O C
lt5-10-Torr 6
clusters, two carbon crucibles, one containing metal and one containing silicon, were placed in the oven zone. Metal and silicon were coevaporated in the presence of H e or a He/H2 mixture. H e (99.995%, AIRCO, Murray Hill, NJ) that had been contacted with Cu fillings at 400 “C was used. In some experiments with Au/Si and Ag/Si a 90% He/lO% H1 mixture was used. As there is a temperature gradient along the length of the oven, the evaporation rate from a crucible depends on its position in the oven. By placing the crucibles at appropriate positions, the relative evaporation of metal and silicon can be controlled, thereby controlling the composition of the metal/silicon clusters synthesized. The evaporation rate from a crucible can also be controlled by varying the size of the crucible aperture. The superheated vapor mixture from the oven undergoes sonic expansion into the reactor region. It is cooled rapidly by mixing with room temperature quench gas. H e or Ar(99.997% pure, Q 1994 American Chemical Society
9248 The Journal of Physical Chemislry. Vol. 98, No. 37, 1994
Patil et al.
E
.. .
. ..
.: . .. , . ~. ...,. . . , .. ,. .
m~
Figure 3. Bright field micrograph of AujSi02 cIus1crs (Au/Si ratio:
formed from AujSi clustcrs that are annealed in the gas phase by heating lo 1200 OC and then cooling to 25 'C. 75/25)
Figure 1. Bright field micrograph of Au/Si02 clu~ters(Au/Si ratio: 90/10) formcd from AujSi dusters that are annealed in the gas phase by heating to I100 'C and then cooling to 25 "C.
AIRCO, Murray Hill, NJ) that had been contacted with Cu fillings at 400 'C was used as quench gas. Use of AI as the quench gas enhances cluster-cluster agglomeration yielding a widersizedistributionanda larger meandiameter. The residence timeinthereactorzonewas I-IOms. Bycontrollingthisresidence time, the mean size of the clusters can be changed. The clusters grow initially by accretion of single atoms of metal or silicon and eventually by cluster-clusteragglomeration. The rateof clustercluster agglomeration depends on the quench ratio (molar ratio of quench gas to oven gas) and the type of quench gas used. The diameters of the clusters synthesized in the present study could be easily varied in the range 1-30 nm. Theclusters areannealed in thegas phase by flowing thecluster aerosol through an alumina tube (3/4 in. id., 30 in. length, Vesuvius McDaniel Refractory, Beaver Falls, PA) that passes through a Lindberg furnace (General Signal, Watertown, WI). The clusters are first heated to the temperature of the furnace and then cooled to room temperature. The length of the heating zone is about 12 in. The length of the cooling zone is about 8 in. The total residence time in the alumina tube is about 1 s.The furnace temperature can be set as high as I500 "C. However, evaporation from the clusters becomes appreciable at temperatures above 1300 OC. The cluster aerosol exiting from the annealing region flows through a capillary into a vacuum chamber maintained at I o d Torr. The resulting cluster beam is deposited on 5 nm thick amorphous carbon films supported on 400 mesh nickel or copper grids (Ernest Fullam Inc., Latham, NY). After deposition the samples were removed from the vacuum system for analysis. Transmission electron micrographs (TEM) were taken using a JEOL 2000 FX analyticalelectron microscope. The metal/silicon ratio for a cluster sample was estimated by depositing a thin film ofclusterson a TEM gridand using EDAX toestimate themetal/ silicon ratio of the film! High-resolution transmission electron micrographs (HRTEM) of individual clusters were also taken using a JEOL 2000 EX microscope with point to point resolution of0.2nm. BeforeTEManalysisthesampleswerealwaysexposed to air and the clusters were oxidized, as discussed below. In order to study the effects of prolonged annealing, samples of Au/Si02 clusters supported by amorphous carbon films on Ni TEM grids were subsequently heat treated. These samples were placed in an alumina boat positioned in the center section of an alumina tube inserted into a Hoskins furnace (National Element Inc., Troy, MI). A steady flow of inert gas (He or AI) mixed with IO% Hz at 1 atm pressure was maintained in the alumina tube. The samples were heated for 5-15 h. Results and Discussion
Figure 2 shows Au/SiOz clusters with an estimated 90/10 atom % Au/Si ratio that were obtained from Au/Si clusters
.
. .
~-... .,. -..~.~.-,-.-;al
93'
Figure+ Bright field micrograph of A u j S i 0 2 cIus1crs similar tothose in Figure 3 except that they arc deposited on a carbon substrate and annealed at 500 'C for 14 h.
heated in the gas phase to 1100 O C . The melting temperature of a bulk Au/Si sample with this composition is about 900 "C. Thelargestcluster in themicrographisabout 20nmindiameter. This cluster has a 'fish-eye" structure. It has a single highcontrast region corresponding to a gold rich core. A thin, lowcontrast layer surrounding this gold-rich domain can also be detected. This corresponds to a silicon rich domain. X-ray photoelectron spectroscopy (XPS) analysis of a film of these clusters deposited on graphite indicates that the gold atoms are zero valent (Au') and that the silicon atoms are oxidized (SiOz). Electron diffraction analysis of a film of these clusters deposited on an amorphouscarbon substrate indicates that thecoreof these clusters is fcc gold. Figure3 showsAu/SiOzclustes with anestimated 75/25atom 96 Au/Si ratio, obtained from Au/Si clusters heated in the gas phase to 1200 OC. The melting temperature of a bulk Au/Si smaple with thiscompositionisabout600DC.'oThesilicaregion in these clusters does not surround the gold region. The cluster morphologyisthatofa'bicrystal" particle withside by sidesilica and gold regions. ClustersamplescorrespondingtoFigures2and 3 weredeposited on carbon TEM grids and annealed at 500 O C for about 14 h. This treatment did not change the structureof the clusters shown in Figure 2. However, after this prolonged annealing theclusters shown in Figure 3 are transformed into 'fish-eye" particles, as shownin Figure4. Thesilicon rich phasenow surroundsthegold phase. Although the quality of encapsulation in Figure 4 is not as good as that in Figure 2, the indication is that a structure consisting of a spherical core of Au covered by an outer layer of SiOl is the most stable or minimum energy structure. If this is the case, why are the clusters in Figure 3 not 'fish-eye" particles? The equilibrium phase diagram for Au/Si is quite simple, exhibiting a single eutectic at 18.6 i 5 atom % S i and 363 3 "C and nearly complete immiscibility of Au and Si below this temperature.I0 This binary system has the interesting property, however, of yielding metallic glasses and metastable crystalline compounds. In fact a Au/Si sample with a composition near the eutectic was the first metallic glass ever made." Although the
*
Novel Metal/Silica Clusters
The Journal of Physical Chemistry. Vol. 98, No. 37. 1994 9249
MORE Si THAN THE EUTECTIC *(SOLIDI
/
MORE M THAN THE EUTECTIC rmrsil . . Lloulo
\
LlWlD
Figure 6. Bright field micrograph of AulSiOi cluster^ formed SOLID
from
Au/Si clusters that are not annealed (Au/Si ratio: 65/35).
AIR
M: METAL
FigureS. Schematicrepresentation ofthesegregationkinetiaofnanoscalc Au/Si clusters.
cooling rate imposed on the clusters in the annealing region is fairly slow (-10' "C/s), Si clusters melted and cooled in the same manner as the Au/Si clusters in Figures 2 and 3 vitrify to form amorphous silicon particles, which when exposed to air are transformed into amorphous SiOzparticles.I2 Thus, it is possible that the nanoscale Au/Si clusters also vitrify. Unfortunately, the clusters are examined only after they are exposed to oxygen at room temperature. No direct evidence of metallic glass or crystalline compound clusters was observed. On the basis of TEM analysis of the final Au/SiOz particles and the Au/Si phase diagram, a rough picture of the segregation kinetics of clusters havingdiametersof20 nmor lesscan bepostulated. This process is represented schematically in Figure 5 . When a liquid cluster having a composition richer in gold than theeutectic composition is cooled, a Au rich phase solidifies first. The remaining liquid phase wets the Au phase and surrounds it. Ator belowtheeutectictemperature, thisthinliquidlayersolidifis to encapsulate the Au rich core with a Si rich skin. On exposure toO2theouter layerbecomesamorphousSiOzandtheinnercore becomes fcc Au. When a liquid cluster having a composition richer in silicon than the eutectic composition is cooled. a silicon rich phase solidifies first. The remaining gold rich liquid does not wet this silicon rich phase. Thus, at or below the eutectic temperature the cluster solidifies into side by side silicon rich and gold rich domains. On exposure to O2 the silicon rich domain becomes amorphousSi02and thegold rich domain becomes fcc Au. With further annealing the particle transforms into the more stable "fish-eye" structure by diffusion of SiOz to encapsulate fcc Au. This slow diffusive or viscous transformation has also been observed when Si/SiOz clusters are annealed." Here SiOz encapsulates crystalline Si. Thesegregation kinetics of silicon rich clusters with diameters greater than approximately 20 nm is more complicated. Figure 6 shows a bright field micrograph of relatively large Au/SiOz clusters formed from Au/Si aggregates that were not annealed in the gas phase. It appears that each cluster is an aggregate of a number of small amorphous domains. The diameter of these small domains is about 2-5 nm. The composition of the clusters in Figure 6 is estimated to be 65/35 atom 9o Au/Si. There is no observable segregation into silicon rich and metal rich regions.
Figure 7. Bright field micrograph of a Au/SiOl cluster (Au/Si ratio: 65/35) farmed from a A u j S cluster that is annealed in the gas phase at 1000 OC.
The rather loose structure of these clusters is not characteristic ofunannealed Auclustersofsimilarsire,whichtend tobecompact spheres. Figure 7 shows a TEM micrograph of a cluster grown under the same conditions as for Figure 6 but annealed in the gas phase a t 1000 OC. The corresponding melting temperature of a bulk Au/Si sample with this composition is about 800 oC.lo The micrograph indicates that the cluster contains numerous highcontrast gold rich domains with diameters of approximately 5 nm embedded in a silicon rich domain. This "emulsion-like" structure may bean indication that vitrification and/or metastable compound formation is occurring. Subsequent annealing of "emulsion-like" particles supported on a carbon TEM grid transforms them into 'fish-eye" particles. Figure 8 shows a TEM micrograph of a cluster grown under the same conditions as above but annealed in the gas phase a t 1350 O C . The TEM micrograph reveals a "fish-eye" particle. The uniformity of the silica coating was confirmed by rotating the TEM substrate with respect lo the electron beam. All the clusters in this sample exhibited a uniform amorphous silica layer. It is felt that there was substantial evaporation and subsequent condensation occurring in the annealing region during this experiment. The evaporation and recondensation ofthese atoms may enable the "fish-eye" structure seen in Figure 8 to form. Figure 9 shows the microdiffraction pattern of a single Au/ SiOz cluster from this sample. This pattern was taken a t the smallest beam size (about I O nm) of the JEOL 2000 FX and corresponds to the spot pattern of a single crystal of fcc gold.14 Thus, the relatively large gold core in this cluster is a single fcc crystal. No spots corresponding to S i 0 2 were observed. Figure I O shows a HRTEM micrograph of a Au/Si02 cluster fromthissample. Inorder toobtain a latticeimage,thesubstrate was tilted with respect to the electron beam. The (1 1 I ) lattice
9250
Patil et al.
The Journal oJPhysica1 Chemistry, Vol. 98, No. 37, 1994
e
Flgure 8. Bright field micrograph of a Au/Si02 cluster (Au/Si ratio: from a Au/Si cluster that is annealed in the gas phase
65/35) formed at 1350 ' C .
Figure 11. Bright field micrograph o l A g / S V 2 cIustcrs (Ag/Si ratio: 9515) formed from a Ag/Si cluster that is anncalcd in the gas phase at
I Fipro 9. Microdiffraction pattcm of a Au/SiO> cluster (Au/Si ratio: 65/35) formed from a AujSi cluslcr that i s annealed in the gas phase at 1350 'C.
Figure 12. Bright field micrograph of A g / S 0 2 clutcrs (Ag/Si ratio: 35/65) formed from Ag/Si clusters that arc annealed in the gas phase
f i p r c IO. High-resolution 1 r a n m " w l n ele;tr.m m u y r i p h ofa Au/ SO2cluster {Au/St r m o 65 3!) formed i r m a Au SI iluslcr that is anncalcd in the gas phase at 1350 ' C
planes of Au are clearly visible in this image. The Au core of the clustcr appears to be very spherical and unfaceted. Thcre is no indication of any long-range order in the silica outer layer of this cluster. We havealsosynthesizedsilver/silicaandcopper/silica clusters. Ag/Si and Cu/Si have higher solid-solid diffusivities than is the case for Au/SI.",'~ Hence, the transformation of Ag/Si and C u p clusters into their minimum energy structures should be easier than is the case for Au/Si clusters. No "bicrystal" or "emulsion-like" particles were seen for the
960 "C.
at 960 'C. Ag/Si/O or the Cu/Si/O systems. Upon gas phase annealing 'fish-eye" particles are always formed. Figure I I shows a bright field micrograph ofAg/Si02clusters with an estimated 9515 atom % Ag/Si ratio, obtained from Ag/ Si clusters annealed at 960 OC. These clusters all have a silver core surrounded by a silica layer. The composition of these clusters lieson thesilver sideoftheeutecticcomposition (90atom%Ag). Figure 12 showsa bright field micrograph ofAg/Si02clusters with an estimated 35/65 atom % Ag/Si ratio that were obtained from Ag/Si clusters annealed a t 960 OC. Although the composition of these clusters lies on the silicon side of the eutectic, they also are "fish-eye" particles. Electron diffraction reveals that the core of these Ag/Si02 clusters is fcc Ag. According to the bulk phase diagrams of Au/Si and Ag/Si. no stable silicide exists a t rmm temperature.I0 For the Cu/Si system, however, there are several stable silicides.I0 Thus, dependingupon itscomposition. a stableCu/Si cluster containing two silicide phases, silicon and a silicide, or copper and a silicide can form in the gas phase. For the ternary system of Cu/Si/O, however,stabletielinesexist betweenC~20andSiO~andbetween CuOandSi02. Thus, upon exposure to air, Cu/Si clustenshould oxidize to copper oxide and silica. Figure 13 shows a bright field micrograph of an oxidized Cu/ Si cluster that was annealed in the gas phase at 1200 OC. The cluster has a "fish-eye" structure made up of two regions, a lowcontrast silicon rich outer layer and a high-contrast copper rich core. In order to identify the oxidation state of the copper. microdiffraction patterns were taken from individual clusters.
Novel Metal/Silica Clusters
The Journal of Physical Chemistry. Vol. 98. No. 37. 1994 9251
Figure 13. Bright field mtcrographafaCu?O S~O~rlustcr(Cu/Stratio: 75/25)formed from a Cu/S clustcr that IF anncaled in the gar phase a1 1200 OC
composition is on the Au side of the bulk eutectic composition. When the cluster composition is on the silicon side of the bulk eutecticcomposition,either 'bicrystal" or 'emulsion-like" clusters are formed. Further annealing transforms these particles into the "fish-eye" structure, which seems to be the minimum energy structure for the Au/SiO system. The Ag/SiO and Cu/SiO systems yield only 'fish-eye" clusters with a metal rich core surrounded by a silica layer. The core regions of these silicaencapsulated clusters appear to be single crystals of Au", Ago, and Cu20, respectively. The silica layers always appear amorphous. These novel nanoparticles are expected to exhibit interesting electronic and chemical properties. Metal clusters encapsulated in an ultrathin oxide skin provide a whole new class of potentially interesting heterogeneous catalysts. Acknowledgment. This work was partially funded by the National Science Foundation under Grant ECS-9117691. The authors would like to acknowledge the helpful comments of one of the referees with regard tovitrification phenomena in the Au/ Si system. References and Notes ( I ) Subbanna. 0.N.; Rao, C. N. R. Mom.Res. Bull. 1986.21. 1465. (2) Sanker. 0.;VasudNan, S.; Rao. C. N. R. J . Phys. Chem. 1988.92.
Figure 14. Microdiffraction pattern of a Cu20ISiOi cluster (Cu/Si ratio: 75/25) formed from a CujSi cluster that is annealed in the gas phase at 1200 'C.
1878. (3) Rao, C. N. R.; Kulkami. G. U.; Kannan. K. R.; Chaturvcdi. S.J. Phys. Chcm. 1992,96,7379. (4) Vijayakrishnan. V.; Rao. C. N. R. Sur/. Sci. Lett. 1991.255,LS16. ( 5 ) Vijayakrirhnan. V.: Santra. A. K.: Seshadri, R.: Nagarajan. R.: hadcep, T.: Rao. C. N. R. Sur5 Sci. Lctl. 1992. 262. L87. ( 6 ) Rao. C. N. R.: Vijayakrishnan. V.; Aiyer, H. N.; KulLarni, 0.U.: Subbanna, G.N. J. Phys. Chem. 1993.97, I I 157. (7) Park, S. B. Ph.D. Thesis, Purdue University. West Lafayette. IN, .no0
,700,
Figure 14showsa microdiffractionpattern from a singleoxidized Cu/Si cluster from the same sample as Figure 13. The spot pattern corresponds to that of a single Cu20 crysta1.l' No spots corresponding to Si or Si02were detected.
(8) Patil. A. N. Ph.D. Thesis, Purduc University. West Lafayette. IN, 1994. (9) Choi, E.;Andres.R. P. lnPhysicsondChcmisr,y yolS" Clwlrrs; h a , P.. Raa. B. K.. Khanna. S. N., Eds.: Plenum Press: New Yark. 1987;
Summary and Conclusions
Metal...
It is possible to synthesize nanoscale metal/silicon clusters of controlled sizeandcontrolled composition usingagasaggregation source. Metal/silicon clusters annealed in the gas phase and thenexposed toair exhibit different morphologiesdependingupon the annealing temperature, cluster composition, and metal. For the Au/SiO system, 'fish-eye" clusters with a thin layer of silica coating a core of gold are always observed when the cluster
(12) Patil. A. N.; Andrcs, k. P. Unpublished. (13) Ajayan, P. M.; lijima. S.J. Am. Cerom. Soe. 1992.75.999. (14) Hirch,P.;Howie.A.;Nicholson.R.B.;Pashlcy.D.W.;Whelan,M. J. Eleclron Microscopy of Thin Cryslols;Rokn E.Kricger Publishing Co.:
"._ lil
I
(10) Binary Alloy Phllse Diagrams, Massalski. T. B., Ed.; S d c t y of Metalr Park, .. . . OH, 1986. Phus. Lctl. 1967..~ IO.. 284. (11) Chcn. H. IS.: . Turnball. D. Anal. ~ ~ r ,~ r ~
~
Malabar. FL, 1977. (IS) Rollcrt,F.;Stolwijk,N.A.; Mchrer,H. J.Phys.DI987.20. 1148. (16) Frank, W.:Gasele. U.; Mehrcr, H.: Seeger. A. In Diffusion in Crysloysrolline Solidr: Murch, G. F.. Nowick. A. S., Edr.: Academic Press: New Vork. 1984;p 63.
