OFFICE OF NAVAL RESEARCH

FINAL REPORT

Nfor

Contract N0014-4-85-K-821

R&T Code 413a001 ---01

Replaces Old

Task #056-123

"Thin Films from Solvated Metal Atoms and Metal-Metal Bonded Compounds"

by.

Kenneth J. KlabundeDepartment of ChemistryKansas State UniversityManhattan, KS 66506

Reproduction in whole, or in part, is permitted for any purpose of theUnited States Government

*This document has been approved for public release and sale; its

distribution is unlimited.

i o n For

U:anuic: od Elju4stification__

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Distribution/ D TIPECCE0

* Availability Codes D T ICiDist Special

JUL2 71988

4.

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" , •

%ECU.,ITY CLASSIFICATION OF THIS PAGE (*%on Data Ent ..reII

REPORT DOCUMENTATION4 PAGE REDISRCINI. REPORT NUMBER 2. GOVT ACCESSION NO. 3. REC NS C ATALOG NUMBER

Fin~al Report4. TITLE (and Subtite) 5. TYPE OF REPORT a PERIOD COVERED

"Thin Films from Solvated Metal Atoms and Final Report August 1985-Met-al-Metal Bonded Compounds" 1988

6. PERFORMING ORG. REPORT NUMBER

7. AUTHOR(s) 8. CONTRACT OR GRANT NUMBER(-)

Kenneth J. Klabunde N0014-4-85-K-821

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMEN-' -P.=, TASK<

Depatmen of hemitryAREA & WORK UNIT NUMBERS

Kansas State University 1596Manhattan, Kansas 66506

1 1. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

July 198813. NUMBER OF PAGES

14. MONITORING AGENCY NAME & AODRESS(il different from Controlling Office) I5. SECURITY CLASS. (of this report)

.0Chemistry Program UnclassifiedOffice~~~~S. DfNvlRsac ECLASSI FICATION/ DOWN GRADING800 N. Quincy Avenue SCHEDULE

Arlington, VA 22217 ______________

16. DISTRIBUTION STATEMENT (of thi. Report)

17. DISTRIBUTION STATEMENT (of the casract entered In Block 20. If dilloent from Reoiat)

NV. I$. SUPPLEMENTARY NOTES

% I9. KCEY WORDS (Continuo, on reverse ad. if necessary and Identify by block number)

metal atoms, solvated, nonaqueous colloids, metal particles, clustering,living colloids, palladium, gold, indium, thin films, chemical liquiddeposition

20. ABSTRACT (Continue on reverse aide if nocoo.ctV and Identify by block number)

-(attached)

DD I JAN 7 1473 EDITION OF INOV 65 IS OBSOLETE- ___ --

S/N 0102.LF.Q14-660?SECURITY CLASSIFICATION OF THIS PAGE (Whe.n Data Entered,

ABSTRACTJ

Metals such as Pd, Pt, Cu, Ag, Ga, In, Ge, Sn, and Pb are evaporated

under vacuum and the vapors (atoms) cocondensed at 77,K with excess organicsolvents. In this way solvated metal atoms are produced.. Upon warmup toroom temperature metal atom agglomeration occurs in certain solvents toyield stable colloidal particles in solution. In many cases these are thefirst examples of non-aqueous colloids of these metals, and they are verynovel in that they are free of contaminating reducing agents, halide ions,etc., and they are living colloids--by removal of solvent metallic films canbe grown on various substrates under very mild conditions. Characterizingthese colloidal particles and the films therefrom is an important part ofthis project.

A second area is the proposed synthesis of new metal-metal bondedcompounds, eg. R Al-AlR2', as possible new Chemical Vapor Depositionmaterials for thin film production. Unusual synthetic approaches, someinvolving metal vapors, are underway. Compounds containing Al, Ga, In, andAs are contemplated. The advantage of such material, on the microscopiclevel, is that two metal atoms could be deposited at a time on a hotsubstrate target, theieby generating films of novel

0 structure/stoichiometry (when mixed metals are being codeposited). -..'

A. Brief Description of Results

Our colloid and surface work continued and we have reported on Au, Pd,and In colloidal dispersions in organic solvents. Films of indium metal andindium oxide were prepared and studied. We also found that stable Au and Pdcolloids can be prepared in liquid styrene, and this purple solution can becontrollably polymerized to metal doped polystyrene. This is the firstpreparation of stable metal colloids such as these in a useful monomermedium. Unusual stabilization mechanisms for these colloidal particles areof interest:

(1) the particles appear to scavenge electrons to become negativelycharged, and (2) strong salvation processes must be important.Electrophoresis studies, electron microscopy, electrolytic additions,plasmon absorption spectroscopy, and conductivity studies have been valuablein characterization of particles and films.

A second major emphasis this year has been the attempted synthesis ofAl-Al, Al-As, Ga-Ga, and Ga-As bonded organometallic compounds. We have metwith limited success. However, we have further elucidated the chemistry ofAl and Ga atoms, and As dimer. We have discovered a facile synthesis of

2R Ga 2X which may be an important discovery since it would allow a betterroute o R 3Ga compounds for CVD processes.

B. Recent Findings (July 1987 to July 1988)

The dispersion of metal vapor into fluorocarbon solvents has been ofinterest in recent months. Metal atom clustering in cold fluorocarbons donot stabilize colloidal suspensions at room temperature, and metal powder

•precipitates. How-ver, a very interesting phenomenon is that thesefluorocarbon derived metal powder are "soluble" in organic solvents. A goodexample is gold powder derived from perfluoro-tri-n-butylamine. Uponextraction (or treatment of the filter cake) with acetone, red colloidalsuspensions are obtained. The gold particles "dissolved" in this way aresmaller than those obtained directly from metal atoms dispersed in acetone.

For example, fluorocarbon derived particles are 10-40A, while acetonederived are 50-90A. Furthermore, the particles retain fluorocarbon; thatis, according to X-ray fluorescence they are true gold but their surfacesappear to be fluorocarbon coated, thus their strange molecular behavior(ability to be dissolved and small size).

We have also discovered that monomers such as styrene andmethylmethacrylate can stabilize colloidal dispersions of Cu, Ag, Au, andPd. Polymerization of these colored solutions yields homogeneous metaldoped polymers. This procedure works extremely well and allows thesynthesis of various loadings of metal doped polymers.

C. A Listing of Technical Reports Submitted

1. S. T. Lin, M. T. Franklin, and K. J. Klabunde, "Non-Aqueous ColloidalGold. Clustering of Metal Atoms in Organic Media, 12," Langmuir, 2,259 (1986).

2. K. J. Klabunde, editor, "Thin Films From Free Atoms and Particles,"'Academic Press, Orlando (1985).

* 3. K. J. Klabunde, "Introduction to Free Atoms and Particles," Chapter in 2above.

4I 4. G. Nieman and K. J. Klabunde, "Clustering of Free Atoms and Particles:Polymerization and Film Growth," Chapter in 2 above.

5. M. Franklin and K. J. Klabunde, "Living Colloidal Metal Particles fromSolvated Metal Atoms: Clustering of Metal Atoms in Organic Media",Sym. High Energy Processes in Organomet. Chem., K. Suslick, editor, ACSSym. No. 333, pg. 246 (1987).

6. G. Cardenas-Trivino, K. J. Klabunde, and B. Dale, "Living ColloidalPalladium in Non-Aqueous Solvents. Formation, Stability and FilmForming Properties. Clustering of Metal Atoms in Organic Media. 14",Langmuir, 3, 986 (1987).

7. G. Cardenas-Trivino, K. J. Klabunde, and B. Dale, "Thin Metallic Filmsfrom Solvated Metal Atoms", Proceedings of SPIE, Vol. 821, 206 (1987).

8. K. Starowieyski and K. J. Klabunde, "Reactions of Vapors of Some Metalsand Metal Oxides with Organometallics of Main Group Elements",

Submitted to App, Organomet, Chem,

9. G. Cardenas-Trivino and K. J. Klabunde, "Characterization of MetallicThin Films Prepared by Chemical Liquid Deposition (CLD): LivingColloidal Metal Particles in Non-Aqueous Solvents", Journal of theChemical Society of Chile, in press.

10. G. Cardenas-Trivino and K. J. Klabunde, "Thermal Degradation Studiesand Scanning Electron Microscopy Studies of Pd Thin Films", XVIIILatin-American Chemical Congress, Santiago, Chile, January 1988.

11. G. Cardenas-Trivino and K. J. Klabunde, "Synthesis, Stability, andStructure of Metal Colloidal Dispersions in Non-Aqueous Solvents",XVIII Latin-American Chemical Congress, Santiago, Chile, January 1988.

r

12. J. Habdas and K. J. Klabunde, "Preparation of Colloidal Au, Ag, Pd, andCu in Styrene, Polystyrene, Methylacrylate and Polymethylacrylate", tobe submitted to Macromolecules.

13. E. Zuckerman and K. J. Klabunde, "Metal Clusters or Metal Colloids?"Clustering of Metal Atoms in Fluorocarbon Media." Submitted toChemistry of Materials.

D. Importance of Past ONR Supported Work

The major contributions have been the discovery that colloidal metalparticles can be prepared by agglomeration of solvated metal atoms,exploring part of the scope of this process, learning about particlestabilization mechanisms, and demonstrating that metal films can be preparedfrom these colloidal solutions. This work has opened up a new dimension incolloid chemistry and potentially in thin film production. 1 2

The attached reprints (two of thirteen technical reports)1 ' discussthe history of colloidal metal particles and emphasize the novelty of ourdiscovery. As a partial reiteration of the reasons such colloidal solutionsare unique: (1) they are non-aqueous, (2) no metal salts reducing processis used so no biproducts are present as impurities, (3) the metal particlesare "living"' in the sense that simply by solvent removal particle growthoccurs until films result, (4) films can be deposited on irregularly shapedobjects, (5) the method allows particle deposition inn polymer matrices, and(6) particle size can be controlled by metal: solvent ratios, solventchoice, and warm-up procedures.

Industrial interest in these colloidal solutions as film precursors hasbeen extremely gratifying. Shortly after announcing our findings (afterfiling for patent protection), a series of events took place: a majorcompany has expressed their interest to license the patent and is paying thepatent fees (the P. I. is a consultant for this company)4 the findings weresummarized in three research and development magazines; the P. I. wasinvited to speak at an international meeting of optical engineers (SPIEmeeting in San Diego); reprint requests have been numerous; phone calls andletters from numerous industrial firms requesting more information andsamples were received (the names of these forms can be provided on aconfidential basis).

The firms interested in this technology invariably wanted to know if wecould produce films on irregularly shaped objects such as metal parts,screens, and fabrics. Often such substrates cannot be coated byconventional vacuum evaporation/deposition methods. Another general themewas the need to deposit metal particles on or in composite materials ororganic polymers. These and other interactions convince us that the work weare doing is certain to be useful to many high technology companies.

(These same scientists have expressed considerable general interes ina book the P. I. edited on "Thin Films from Free Atoms" and Particles.")

I'

I

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E. Equipment Acquired

During the ONR supported period some important pieces of equipment havebeen built or purchased and brought into operation.

(1) Computerized Mossbauer Spectrometer with low temperature (as low as4-K)

(2) Electrophoresis equipment(3) Thin film spray apparatus(4) Conductivity measuring devices(5) Additional metal vapor deposition equipment

RI F. People Who Have Participated in the Prolect

G. Cardenas-Trivino (Postdoc)

Ellis Zuckerman (Postdoc)J. Habdas (Postdoc)

M. Franklin (Grad. Student)

S. T. Lin (Postdoc)S. Antrim (Undergrad.)K. Starowieyski (Visiting Prof.)

0 G. Nieman (Visiting Prof.)B. Dale (Collaborating Physicist)B. J. Tan (Grad. Student)

References

(1) G. Cardenas-Trivino, K. J. Klabunde, and B. Dale, Langmuir, 3, 986(1987).

(2) M. Franklin and K. J. Klabunde, Symposium on High Energy Processes inOrganomet. Chem., K. Suslick, editor, ACS Sym. No. 333, pg. 246 (1987).

(3) Patent Application, K. J. Klabunde, "Stable Colloidal Metal Dispersionsand Conductive Films Therefrom," Serial No. 853,0277 filed April 17,1986. A continuation in part "Substrates Coated with Solvated Clustersof Metal Particles," CIP of Serial No.. 853,027 filed March 2, 1987.

(4) B. J. Spalding, "High Hopes for Metal Vapor Chemistry," Chemical Week,July 16, pg. 34-35 (1986); Chemical and Eng. News, Sept. 22, pg. 41(1986) Advanced Coatings and Surface Technology (a monthly intelligenceservice from Technical Insight, Vol. 1, No. 1. pg. 2 January (1988).

(5) K. J. Klabunde, editor, "Thin Films from Free Atoms and Particles,"Academic Press, Orlando (1985).

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Reprinted from LANGMUIR, 1987, 3, 986.Copyright © 1987 by the American Chemical Society and reprinted by permission of the copyright owner.

Living Colloidal Palladium in Nonaqueous Solvents.Formation, Stability, and Film-Forming Properties.

Clustering of Metal Atoms in Organic Media. 14

% Galo Cardenas-Trivino,' Kenneth J. Klabunde,* and E. Brock Dale2

5/ Department of Chemistry and Department of Physics, Kansas State University, Manhattan,31" Kansas 66506

Received August 25, 1986. In Final Form: April 21, 1987

Palladium atoms in cold acetone, ethanol, and other organic solvents cluster to form living colloidalparticles. The Pd colloids are stable for months at room temperature in acetone, and their particle sizes(about 8 nm) vary slightly according to the solvent:Pd ratio and warming procedures. The particles arestabilized by both steric stabilization (solvation) and by incorporation of negative charge. They are "living"in the sense that production of larger particles and films can be accomplished simply by solvent removal.The Pd films formed at room temperature by solvent evaporation have semiconductor properties (ratherthan metal-like conductivity) due to the incorporation of substantial portions of organic solvent (and smallamounts of solvent fragments). Upon heat treatment the films become smoother, give up organic solvent,and decrease in resistivity. These are the first examples of nonaqueous Pd colloids and the first examplesof film formation from such colloidal solutions.

Introduction and Background new approach to the preparation of stable metal colloids

Although colloidal metal particles in aqueous media are in nonaqueous media based on the process of atom clus-well-known, 3 preparations in nonaqueous media have been tering at low temperature. 4 This process appears to behampered by low stability of the colloids presumably due very wide in scope, and the resultant colloidal particles areto ineffective stabilization mechanisms (low solvent di.. free of interfering ions and impurities usually present inelectric constant, low viscosity, etc.) and preparative aqueous preparations. In addition, they are "livingmethods. However, we have recently reported a successful, colloids" since particle growth to films occurs under very

imild conditions and can be induced simply by solventevaporation. In this paper we report details on nonaqueous

(1) On leave from Departamento de Quimica, Universidad de Con- palladium colloids.cepcion, Casilla 3-C, Concepcion, Chile.

(2) Department of Physics.(3) Turkevich, J.; Stevenson. P. C.; Hillier, J. Discuss. Faraday Soc. (4) Lin, S. T.; Franklin, M. T.; Klabunde, K. J. Langmuir 1986, 2,

1951, I1, 55-75. 259-260.

* 0743-7463/87/2403-0986$01.50/0 0 1987 American Chemical Society

- - - - - - ls~rw r rw -, ----- rwyr~rrwr'-s-n-

Living Colloidal Palladium in Nonaqueous Solvents Langmuir, Vol. 3, No. 6,1987 987

Palladium and Platinum Colloids. Some years ago Table I. Electrophoresis Studies of Pd-Acetone ColloidsRampino and Nord5 prepared Pd and Pt catalysts in which velocity of particle sizesynthetic high molecular weight polymers were used as solvent conch, M migration, mm/h (average), nmprotective coatings (steric stabilization of colloidal parti- acetone 0.0308 3.3 8cles).6 Palladium-methyl methacrylate was prepared by acetone 0.0360 3.3 8the addition of glacial acetic acid to methyl methacrylate acetone 0.00384 8.0 8(MMA) followed by the addition of palladium chloride acetone 0.01700 8.0 6solutions. Similarly, palladium-poly(vinyl alcohol) was acetone 0 .0 138b 8.3 8prepared and shown to be a very efficient hydrogenation acetone 0.02280 6.0 6catalyst. The palladium-poly(vinyl alcohol) and plati- acetone 0.0200 c 8acetone 0.0096 10.0 8num-poly(vinyl alcohol) catalysts were equally active in ethanol 0.0416 3.0 8acid, neutral, and alkaline media.

Stable aqueous Pt sols (colloidal particles) have been Au wire connected to left electrode on bottom reactor, also Cuprepared by the citrate reduction of choroplatinic acid.7 wire connected to ground. 'A Pd-Au bimetallic colloid. 'It wastimpossible to carry out electrophoresis since the colloid mixed withIt was found that the Pt particle size and the extent of the solvent. In this experiment during the warmup the left elec-reduction of the chloride salt increased with temperature. trode was connected to the negative pole of a battery. d During theThese Pt sols with particle diameters in the range 1.5-4 warmup the left electrode was connected to the positive pole of anm exhibited great resistance to coagulation by electrolytes battery.such as LiCl, NaCI, KCI, CsCl, MgCI2, SrCI2, CaCI2, BaCI2,and AI1d3 . It was also found that H2 treatment caused The colloidal particles obtained at stage H were air-stableparticle growth to about 5 nm. and remained in solution indefinitely. A series of black

Colloid Stabilization. There are two principle mech- colloidal solutions using different concentrations andanisms for stabilization of metallic colloids: (1) electro- slightly different procedures was prepared (Table I). Instatic, colloidal particle charging due to adsorption of in- order to learn more about these particles several mea-nocent ions in the solutions (such as chloride ion), and (2) surements and studies were carried out.steric, solvent ligation or polymer adsorption or ligation. Colloidal Particle Studies. (1) Electrophoresis.Addition of polymers to aqueous or nonaqueous colloid Generally, aqueous metallic colloidal particles carry some

* solutions can have significant stabilization effects due to negative charge,8 and the rate of migration of these par-steric features or due to depletion stabilization (rejection ticles to a positive pole can be determined as the electro-of polymer molecules from the interparticle region is not phoretic mobility (us,). For our 0.0308 and 0.0360 M Pd-favored thermodynamically due in large part to proper acetone solutions (8-nm particles) the velocity of migrationsolvent choice).6 ,9 0 Polymer stabilization is very im- was reproducibly 3.3 mm/h (see Table I).portant industrially for such products as paints, inks, and field strength = X = E/l = -12.67 V/23.5 cmfood emulsions and for oil recovery, waste treatment, -0.539 V/cmetc.1 ,I112 Biological systems such as milk and blood are

also affected in similar ways. velocity = v - 3.3 mm/3600 s = 9.17 XIn our system, where only metal particles and solvent 10-4 mm/s (for 8-nm particles) = 9.17 X 10-5 cm/s

are present (no ions and no polymers), the question of astabilization mechanism is quite intriguing and will be uE = v/X = (9.17 x 10- 1 cm/s)/(-0.539 V/cm)

, discussed.discussd -17.0 X 10- 5 cm2/(V S) = -1.7 X 10- 8 m 2/(V s)

Results The above value of ME is similar to those reported for aThe fvariety of aqueous colloidal particles, e.g., colloidal gold,

-olThe following scheme was used to prepare stable Pd- 30-40 x 10- (<100-nm particle diameter); colloidal plat-solvent colloidal solutions' with acetone as an example: inum, 20 x 10"6 (<100 nm); colloidal lead, 12 X 10-6 (<100

77K nm); and oil droplets, 32 X 10-5 cm2/(V s) (2000 nm)."48

Pd atoms + CH13COCH3 concondam It is interesting to note the similarities of these valuess slow warmup (.5 hto room temperature regardless of particle size, which suggests that larger

Pd(CH3 COCH) 1 particles possess much higher overall charge.stage I The Debye-Hyckel approximation may be used to ex-

(Pd) (CH3 COCH3 )y press the charge density as a function of potential if it isstage II low.'6 The potential at the surface of the particle is de-

fined as the r potential The potential can be calculatedaccording to the convention of Hunter" and the H Mckel

• (5) Rampino, L. D.; Nord, F. F. J. Am. Chem. Soc. 1941, 63, equation:2745-2749.

(6) Hirtzel, C. S.; Rajagopalum, R.; Colloidal Phenomena: Advanced ME f 4IhODP/6IIb i= 2eoD /3iTopics; Noyes: New Jersey, 1985; pp 88-97.

(7) Furlong, D. N.; Launikonis, A.; Sassee, W. H. F. J. Chem. Soc., where for acetone D is the dielectric constant (20.7), to theFaraday Trans. 1 1984, 80, 571-588.

(8) (a) Booth, F. Progr. Biophys. Chem. 1953, 3, 131. (b) Bull, H. B.Physical Biochemistry, 2nd ed.; Wiley: New York, Chapman and Hall:London, 1951. (13) Metal atom chemistry and techniques have been described in:

(9) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Klabunde, K. J. Chemistry of Free Atoms and Particles; Academic: NewSoc. 1983, 105, 2574-2579. York, 1980.

(10) Water-polymer-solvent systems have also been reported: (a) (14) Jirgensons, B.; Straumanis, M. E. Colloid Chemistry; Macmillan:Ledwith. A. Chem. Ind. (London) 1956,1310. (b) Blumencron, W. Med. New York, 1962; pp 132-133.Moatssachr. 1957, 11, 89. (15) Shaw, D. J. Introduction to Colloid and Surface Chemistry, 2nd

( 1) Sato, T.; Ruch, R. Stabilization of Colloidal Systems by Polymer ed.; Butterworths: London, 1970; pp 157-158.Adsorption; Marcel Dekker: New York, 1980. (16) Hiemenz, P. C. Principles of Colloid and Surface Chemistry;

(12) (a) Napper, D. H. J. Colloid Interface Sci. 1977,58,390-407. (b) Lagowski, J. J., Ed.; Marcel Dekker. New York, 1977; pp 453-466.Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Aca- (17) Hunter, R. J. Zeta Potential in Colloid Sciences; Ottewil, R. H.,

* demic: London, 1983. Rowell, R. L, Eds.; Academic: New York, 1981; p 63, 69.

988 Langmuir, Vol. 3, No. 6, 1987 Cardenas-Trivino et al.

Table II. Pd Films Prepared from Colloid Solutions with Organic Solventscolloid particle

solvent concn, M % Pd % C % He size, nm

acetone 0 .0236-0.0 588b 79.70 4.90 0.58 10-14acetone 0.0588 (500 *C)Y 88.47 2.30 C.07ethanol 0.0178-0.0416 73.43 4.40 0.85 unresolvedacetone 0 .0 138d 25.35 (55 .29)d 5.45 0.77 8thf 0.0148 79.70 3.30 0.56 62-propanol 0.0108 79.90 6.00 1.10 6

*"Microanalyses were obtained from Galbraith Laboratories. bAverage from several colloids within thie range of concentrations. cFilmafter heating at 500 *C. The % C varied from 0.4 to 2.5 over several samples. The % H varied from 0.03 to 0.07, while the % Pd variedfrom 88.3 to 88.5. d (Au-Pd)-acetone colloid; % Au in parentheses.permittivity (8.854 X 0-U 1 F/m), and 1 the solvent viscosity Table III. Products Evolved upon Heating Films Derived

(3.16 X 10-4 N-s/m 2). from Pd-Acetone Colloidal Solutions

AE = [(Q)(8.854 X 10-12)(20.7)f]/[3.16 X 10-4] temp, @C products (relative %)=(.8)1 ) 100 H20 (25.6), CO (41.2), CO (11.8), (CH)2C=0 (13.2)

(2.58 x 106)S 200 H20 (8.4), CO (27.7), C02 (2.3), CH3CH-CHCH3

(-1.7 X 10-8)(2.58 X 106) (24.7), (CH 3) 2C-O (36.5)fl350 H 20 (5.9), CO (21.2), C02 (3.1), C3H (2.7), C4H2 (0.3),

-4.39 X 10-2 V = -44 mV C4H4 (0.2), C2HOH (0.7), CH1CH-'CHCH3 (28.8),- ; (CH2)2C==0 (37.b

The above value for P compares well with those reported 'The film was obtained by stripping the acetone solvent under.-,* for a variety of aqueous sols, i.e., 18-58 mV." Such corn- vacuum at room temperature for 3 h. Initial concentration of the

parisons are tenuous, however, since the equations derived colloid was 0.0521 M.and the data accumulated in the literature are for aqueoussystems. Much more work with nonaqueous media is -

needed.It is interesting to note that when a gold ground wire 3k.•

or a wire connected to either pole of a 12-V battery wasimmersed in the solution as colloid formation took place, - .more highly charged particles resulted, and r potentials" . , .of greater than 100 mV were calculated (Table I compares -. ' ..rates of migration).

(2) Flocculation. Various electrolytes were added to , ..the Pd/acetone colloidal solutions in order to induce JP , .flocculation.8 Three solutions, 0.01 M Nal, Cal2, and AIBr , :. ', -were prepared in acetone, respectively. Addition of Nal %.r . %

solution to a 0.0175 M Pd/acetone colloid in a NaPd ratio A'.,Adtoof 1:1 caused flocculation to begin in 5 tin at room tenperature. Addition of CaI solution to the same olloid in R."

* the same ratio caused flocculation to begin in 3 min. ZFinally, addition of AIBr3 solution to the colloid in thesame M:Pd ratio induced flocculation in 2 min. These . a-.:. " ~results are in agreement with data reported by Furlong7 "- = "

in which high-valent cations induced flocculation faster - ..... -

than monovalent cations. Addition of water to the colloid Figure 1. Transmission electron micrograph of particles derived* induced flocculation only after 120 h at room temperature. from a Pd-acetone colloid (very dilute solution; 2.5 X 10 mag-

(3) pH Measurements. No evidence for increased nification). (About 8-nm particles.)acidity or basicity was obtained by measuring the pH ofthe colloidal solutions. The same values for pure acetone films by spraying the solution as an aerosol. Stage IV waswere observed (pH 7.7). obtained by heating stage III under vacuum or in a stream

(4) Other Solvents. Ethanol, isopropyl alcohol, and of N2 . Films from stages I and IV were analyzed andacetone-isopropyl alcohol mixtures all worked very well characterized in a variety of ways as follows.

* as solvents for Pd colloidal formation and stabilization. Thin-Film Analysis. (I) Elemental Analyses andEthanol allowed slow flocculation after about 24 h, al- Pyrolyses. Stage III, after drying at 10-3 Tort for 3 h, stillthough with the other solvents indefinite colloid stability contained substantial portions of carbon and hydrogenwas observed (room temperature). (Table U). Vacuum pyrolysis at 500 *C to reach stage IV

Thin Films. Stage II can be converted to stages III and caused the evolution of mainly acetone along with someIV: carbon dioxide.

solvent evaporation A more detailed pyrolysis study using GC-MS was* (Pd),(CH3COCH 3), carried out at temperatures of 25, 100, 200, and 350 °C.

stage II Table III indicates the products that were successivelymetal-like Pd film heat metal film evolved. At the lower temperatures acetone was the mainmetaie Pd ftal Il product while at higher temperatures butenes, propene,

stage HII stage IV and other similar products were evolved.

Stage III was prepared by slowly dripping the colloidal (2) IR Studies. Infrared analysis of the stage III filmsolution onto a substrate. Solvent evaporation was speeded indicated the presence of only adsorbed acetone. Strongest

* by applying a vacuum or N2 gas flow or by using a warm absorption was at 2980 and 1740 cm-, similar to puresubstrate. Stage III was also prepared in uniform thin acetone.

-..,

SLiving Colloidal Palladium in Nonaqueous Solvents Langmuir, Vol 3, No. 6, 1987 989

i?:7

SAl,

Figure 2. Scanning electron micrograph of film (stage 111) derivedfrom a Pd-acetone colloid (1.1 X 105 magnification).

Table IV. Resistivities of Films (Stages III and IV)Derived from Pd-Acetone Colloids

resistance, thickness, resistivity psolution concn, M fl/cm 2 (X102), 0 Cm0.0165 71-934 40-60 3.5-4.60.0165b 43 40 2.20.0298 82-107 65 53-690.0228 49-109 4.0 1.9-4.40.01381 380 1.6 6.1Pd film' 357 0.28 0.10Pd bulk' 11 x 10"cis-[CH(AsF) 0.14]

/ h 0.18. r'F.T-TCNQ' h 0.14

a Values varied within this range by measuring resistivities ofdifferent sections of the film. bThe film was heated for I h at 200*C, and then resistance was measured. 'A Pd-Au bimetallic col-loid solution. d A pure Pd flm for comparison (prepared by vacu-um deposition of Pd vapor). 'Handbook of Chemistry, 57th ed.;CRC: Boca Raton, FL, Vol. 20, p E84. tPolyacetylene, doped with4.Fs. IMeasured on a single crystal in the direction of greatestconductivity. h Not reported.

Figure 3. Scanning electron micrographa of films (stage IV)(3) SEM and TEM Studies. Dilute solutions were derived from a Pd-acetone colloid: (top) after heating at 18000C

dripped onto carbon-coated copper grids so that acetone (3 X 10' magnification); (bottom) after heating at 500 *C (1 Xevaporation left isolated particles. According to trans- 104 magnification).mission electron microscopy (TEM) the particles werespherical and had a tendency to link together (Figures 1 solvents and they are living colloids in that the particlesand 2). grow to films under very mild, controlled conditions. We

Scanning electron microscopy (SEM) showed that the will now consider what is known about formation andfilm formed by acetone evaporation was made up of a stabilization mechanisms for these materials.series of strands or chains of colloidal Pd particles that The growth of colloidal particles from solvated atomswere intertwined. When the film was heated, these strands is in competition with the reaction of the atoms andcollapsed to a more uniform film (Figure 3). growing particles with solvent medium.1s As would be

(4) Resistivity. Films of different thickness (2.8-65/zm) expected, one process being favored over the other dependswere prepared by dripping the colloidal solutions onto a on what metal and what solvent are interacting. In the

* glass plate. The values of thickness and resistance are case of nonpolar solvents and reactive metals, such as Fe,summarized in Table IV. After the films were heated, Co, or Ni, extensive bond-breaking reactions have beenresistivity decreased. observed (e.g., Ni with pentane).19 However, more polar

(5) Reaction with (C6H5 )2P(C2H5). After solvent solvents with better ligating properties tend to solvate theevaporation at room temperature the resultant Pd film (25 metal species with less bond breaking of the solventmg) was treated with excess (CeHs) 2P(C2Hs) under nitro- molecules. In other words, solvent molecules complex withgen. After the mixture stirred for 48 h under nitrogen the the metal species rather than fragments of the solvent.volatiles were removed, separated, and identified. Besides In the case of Pd-acetone colloidal particles, we haveexcess phosphine, only acetone (0.39 mg or 6.8 x 10-3 found no evidence for adsorbed species other than acetonemmol) was collected. This corresponds to about 1.6% of itself. Thus, vacuum treatment of stage III at room tem-the total weight of the sample. perature and 100 *C yielded only acetone as a volatile

Discussion(18) Klabunde, K. J.; Tanks, Y. J. Malec. Catal. 1983, 21, 57-79.The novel features regarding these materials are that (19) Davis, S. C.; Severon, S.; Klabunde. K. J. J. Am. Chem. Soc.

* the particles are stable toward flocculation in nonaqueous 1981, 103, 3024-3029.

990 Langmuir, Vol. 3, No. 6, 1987 Cardenas-Trivino et al.

product. At higher temperatures products were evolved are based on aqueous systems), it is clear that these neg-that were probably formed from catalytic/pyrolytic de- atively charged particles will repel each other and thereforecomposition of acetone. In addition, ligand displacement aid their stabilization. l" potentials are indicative of sub-by excess (C6Hd)2P(CH5) yielded only acetone, and IR stantial electronic stabilization.studies suggest that the only displaceable organic material How is this negative charge acquired? One possibilityis acetone itself. But note that it is quite strongly coor- is that free radicals are involved, perhaps formed by py-dinated, requiring a vacuum and warming for just partial rolytic decomposition of small amounts of acetone on theremoval. A strong solvation mode is apparently important. hot metal vaporization source or by reactions of acetoneOn the metal cluster surface a variety of binding schemes with metal atoms. A number of radiolvsis studies of metal

' may be operational, as suggested by Weinberg and Tem- colloids in water-acetone solutions indicate that organicpleton2" for acetone on a Ru(001) surface: radicals do transfer electrons to the particles which act as

. - electron reservoirs (and can behave as catalysts for water

O-C O-C reduction).22.23

- M CH 3 95K \ /cICH 3 I [-CH3 (CH 3)2CtOH + (Ag),m - (CH3)2C'-OH + (Ag),-M M-M

d"CH. 3 CH 3COCH3 + H+ + (Ag);-J, M_-CZH

M C-H 3 If free radicals were involved in our system, the generationoI of H + in solution would be expected. However, we have

0- found no evidence of H+ in our solutions nor have we found

As our Pd particles grow to hundreds of atoms, solvent any radical recombination products that might be ex-molecules would be incorporated within the particles and pected. Therefore, we do not believe free radicals areon the outside. As growth continues, some solvent mole- important in the generation of negatively charged metalcules must be displaced by incoming atoms and smaller particles in our system.metal particles. Eventually the particle growth stops. At A second possibility is that the electron affinity of thewhat point it stops (ultimate particle size) depends on the particles may allow them to acquire electrons from theinitial metal concentration in the matrix and the matrix reaction vessel walls, electrodes, and solvent medium. Such

0 warmup procedure. a process would help explain the need for a slow warmupMetal Concentration. Initial metal concentration can procedure in order to yield stable colloidal solutions since

affect colloid particle size in a kinetic way, since it is un- scavenging of electrons may be a slow process.likely particle growth is reversible under such conditions.21 Actually, this type of electrostatic charging of colloidalOnce a Pd-Pd bond is formed, it does not break. There- particles is not uncommon. Oil droplets, for example,

, fore, in a dilute solution of atoms, the frequency of en- scavange electrons from aqueous solution.1 4

counters will be lower. As the metal atom-solvent matrix If scavenging occurs during the warmup period, wewarms and the atoms and the forming particle become reasoned that by inserting a gold ground wire into themobile, it is the number of encounters that occur during solution during colloid formation some change would bethe period before particle stabilization that is important. realized. Indeed, with this procedure the resulting PdAnd if metal concentration becomes too high, particle size particles became more highly negatively charged accordingbecomes too large, causing precipitation. Similar behavior to electrophoresis studies (Table I). The next step was tohas been encountered for gold colloids in acetone. 21 In- place a wire attached to the negative pole of a 12-V batteryterestingly, however, gold particle size could be more easily into the solution during colloid formation. In this casecontrolled by concentration effects. 21 With palladium we electrophoretic behavior changed markedly, and mea-

, invariably obtained particle sizes of 6-12 nm. Low con- surements were impossible due to uncontrolled mixing. Acentrations of Pd still yielded 6-8-nm particles, and high last case was to attach the wire to the positive pole of theconcentrations of Pd yielded 8-12-nm particles plus much battery, and this again caused a significant change in thelarger particles that precipitated. Thus, there is a distinct behavior of the colloid. The migration rate was the highestpreference for an 8-nm average particle size for Pd in measured (Table I), and the colloid was very stable. Sinceacetone as well as for Pd in ethanol. We do not fully a circuit was not complete, either pole of the battery simply

* understand this selectivity yet, although particle stabili- served as a reservoir of electrons, and more electrons werezation must be the key, as discussed below, available, yielding more negatively charged particles.

Particle Stabilization. We believe particle growth On the basis of the above observations, we believestops because of two factors. The first comes under the particle stabilization occurs slowly during the warmupheading of steric stabilization.' Solvent molecules must period by steric effects (solvation) and by electronic effects,be displaced and reordered on the surface of a Pd cluster where the growing particles develop and possess a suffi-if another cluster is to chemically bind to it. As the par- ciently high electron affinity that electron scavenging from

- ticles (clusters) become more massive the kinetic energy the reactor environment is possible. This scavenging cangoes down, and perhaps the energy requirement for solvent be affected by the presence of electron sources, and elec-displacement/reordering becomes large compared to the trophoretic mobilities increased. Such experimental ma-kinetic energy of the sluggish larger particles. nipulations hold promise for controlling electrophoretic

A second mode of stabilization is electronic in nature. mobilities and perhaps particle size.Electrophoresis experiments clearly show that the Pd Further support for this electronic stabilization mech-particles bear negative charge. Although it is difficult to anism is found in our studies of electrolyte additions. Itdetermine accurately the number of negative charges each is known that electrolytes added to aqueous metal colloidsparticle possesses (formulas derived for such calculations aid the breakdown of the charged double layer, which in

turn allows particle flocculation.'"-" Our studies with

(20) Templeton, M. K.; Weinberg, W. H. J. Am. Chem. Soc. 1985, 07, electrolytes yielded similar results. The electrolytes with774-779.

(21) Franklin, M. F.; Klabunde, K. J. High Energy Processes in Or.ganometallic Chemistry; Suslick, K. S., Ed.; ACS Symposium Series 333; (22) Henglein, A. J. Am. Chem. Soc. 1979.83, 2209-2216.

* American Chemical Society: Washington, DC, 1987; pp 246-259. (23) Henglein, A.; Lillie, J. J. Am. Chem. Soc. 1981, 103, 1059-1066.

Living Colloidal Palladium in Nonaqueous Solvents Langmuir, Vol. 3, No. 6, 1987 991

the more highly charged cations caused flocculation more under vacuum by removal of the liquid N2 filled Dewar for 1.5qtickly (AIl3 > Ca2+ > Na+). This is a classic case showing h.not only the existence of charged colloidal particles but Upon meltdown a black solution was obtained. After additionalso that a charged double layer must exist." of nitrogen the solution was allowed to warm for another 0.5 h

Living Colloids - Films. From Table II it is evident to room temperature. The solution was siphoned out under N2.- t*, into Schlenk ware. Based on Pd evaporated and acetone inlet

that substantial portions of organic residue remain in the the solution molarity could be calculated.films after solvent stripping at room temperature. We Effects of a Ground Wire and a Battery-Attached Wire.found that the films were susceptible to oxidation, as might Several experimenth were carried out where a gold wire wasbe expected, and oxygen (by difference) ranged as high as connected to an electrode inside the reactor so that it reached25%. If care was taken to prevent oxidation, an empirical the bottom of the reactor. A copper wire was attached to the upper

* .-"Z formula of about Pd5 C-H 20 2 was determined. An average part of the electrode external to the vacuum chamber. This wireof all determinations indicated PdgCH 7O11. After treat- was either grounded or attached to the negative or positive polement at 500 *C, causing the evolution of some organic of the 12-V storage battery. Colloidal solutions obtained by usingmaterial, an average empirical formula of PdC103 was these modifications did not show any marked changes in stability,determined, but electrophoretic mobilities increased.

Electrophoresis Experiments. The electrophoresis exper-Earlier discussion suggested that while in solution the iments were carried out by using a glass U-tube of 11.0 cm with

colloidal particles are solvated by acetone, and other or- a stopcock on the base to connect a perpendicular glass tube (13

ganic fragments were not detected. However, upon solvent cm long X 35 cm high)."'-m Platinum electrodes were attached% ,stripping it is obvious from the empirical formulas that to the top of the U-tube and through a ground glass joint to the

acetone must be breaking up, accompanied by some oxi- pole of a 12-V battery. The acetone was placed in the U-tube,dation. Some acetone is still present since it is the main and then the colloid solution was added slowly thragi. ,he sidevolatile product evolved at 300 *C (Table III) and the only tube. The migration rate was determined on the basis of thevolatile product displaced by (C6HS) 2P(C2 H5). Since the aNerage of the displacement in each side of the U-tube. A typicalremaining fragments must be very rich in carbon and ox- experiment was carried out for a period of 3 h at 25 9C.

Electrolyte Additions. A study of flocculation times wasygen, the formation of palladium carbides and palladium carried out by using a 0.010 M Na! solution. In a test tube 2 mLoxides is likely. of colloidal solution (0.0175 M) and 2 mL of Nal were added at

Electron microscopy studies show that the individual room temperature (25 *C). After 5 min flocculation of the colloidcolloidal particles are spherical and have a tendency to link was observed.together in chains. The initial film appears to be made A solution of 0.010 M Cal2 in acetone was also prepared. Byup of a network of Pd particle chains (Figure 2). Heating use of the same ratio as before, flocculation of the colloid began

- the film causes these chains to collapse to a more uniform after 3 min. Finally, a 0.010 M AIBr3 acetone was prepared.film (Figure 3). Addition to the colloid in the same amount as before induced

-. Resistivities of these films are of interest. Table IV lists flocculation after 1 min at room temperature. Complete floccu-values determined for 1 cm2 films of varying thickness lation was observed after 10, 8, and 7 min, respectively.

In other experiments water was added to the colloid solution,(0.2-60 jim). The initial films are conductive and increase and after 120 h flocculation was observed.in conductivity after heating. They behave more like GC-MS Experiments. GC-MS pyrolysis was carried out bysemiconductors than pure metals, and actually their re- using a Porapak Q 6-ft column (flow rate 35 mL/min) attachedsistivities are similar to those of doped organic polymers.' to a Finnigin 4000 quadrupole GC-MS. The sample was placed

in a stainless steel tube 10 cm long connected to a four-way valve.Conclusions One of the outlets was attached to a Porapak Q column interfaced

with the M.S. The stainless steel tube containing a portion ofPalladium atoms dispersed in excess acetone (or other Pd colloid film (stage Ill was placed in a furnace connected to

solvents) begin to cluster upon warming. The properties a Variac provided with a digital quartz pyrometer to measure theof the resultant colloidal particles depend slightly on initial temperature. Three pyrolyses were performed at 100, 200, andmetal concentrations, warmup procedures, and the 350 *C with the Pd-acetone film (from colloid, 0.0521 M).availability of electrons. During colloid formation the Addition of (C6H5)2P(C2H5). A Pd film was prepared by

-., d particles are stabilized by solvation effects and by elec- evaporating the solvent from a 0.035 M colloid solution. A 25-mgtronic effects due to electron scavenging (the Pd particles sample of the film was treated with 1.5 mL of (C6HS)2 P(C2H5)

* behave as electron sinks). Upon solvent removal, films of (5.6 mmol) under a nitrogen atmosphere. After 48 h at roomintertwined chains of spherical Pd colloidal particles (still temperature with stirring, the dark solution became lighter. The

volatiles were pumped out through 263 and 77 K traps. The 77containing organic residues) are formed. Heating the film K trap contained only acetone (0.39 mg or 6.8 X 10- 3 MMol),

causes these chains to collapse to a uniform film with a identified by gas-phase IR.decrease in resistivity. Some organic residue remains in SEM and TEM Studies. Electron micrographs were obtainedthe films. on a Jeol, Temscan-100 CXIl combined electron microscope and

a Hitachi HV-IIB (TEM) operated at 2 X I0 magnification. The* Experimental Section specimens for TEM were obtained by placing a drop of the colloid

SPreparation o T lPd-Acetone o . e metal solution on a copper grid coated by a carbon film. The samplesa reato n dof a Typical Colloid. for SEM were placed between two copper grids one of which wasatom reactor has been described previously.iihg As a typical coated by a carbon film.example, a W-A20 3 crucible was charged with 0.80 g of Pd metal Resistity Studies. Films of different thickness (02.8-65 um)(one piece). Acetone (300 mL, dried over K2CO3 ) was placed in were prepared by dripping the colloidal solutions onto a glass platea ligand inlet tube and freeze-pump-thaw-degassed with several edged with silicon rubber adhesive resin. The acetone was allowedcycles. The reactor was pumped down to 1 X 10' Torr while the eged wt siirbe aese resnTe acetone slocrucible was warmed to red heat. A liquid N2 filled Dewar was o evaporate. Resistivities were measured by scraping the siliconplaced around the vessel, and Pd (0.5 g) and acetone (189 g) were rubber away from the edges of the film, which was then trimmedcodeposited over a 1.0-h period. The matrix was dark-brown at to rectangular shape. iL was men connectea to electrodes on eachthe end of the deposition. The matrix was allowed to warm slowly end by vapor deposition of an opaque film of aluminum or copper.to.s.x To get a reliable contact on aluminum, it was necessary to apply

a spot of silver paint over the aluminum. This was not necessary

(24) Wegner, G. Angew. Chem., Int. Ed. Engl. 1981, 20, 361-381.(25) Klabunde. K. J.; Timms. P. L. Ittel, S.; Skell, P. S. Jnorg. Synth.

1979, 19, 59-86. (26) Shaw, D. J. Electrophoresis; Academic: New York, 1969.

* 992

with copper electrodes. The resistance of each sample was metal films using either KBr pellets or Fluorolube yielded onlymeasured with a Keitley 178 Model digital multimeter. the vapor evidence for Pc-_ (2980 cm-1) and PO (1740 cm - ) showing the samedepositions were carried out with a Veeco Model VS-90 metal shape as the acetone standard.evaporator. the values of thickness and resistance are summarizedin Table IV. Acknowledgment. The support of the Office of Naval

Solubility Studies. The solubility of the Pd-acetone film Research is acknowledged with gratitude. We also thank(0.0236 M) was tested with the following solvents: acetone, Rarh is akno h graitude. We also taethanol, THF, DMSO, benzene, toluene, and pentane. The films Matthew T. Franklin for helpful discussions and Larry L.were completely insoluble after 24 h of contact with stirring at Seib for assistance with the SEM-TEM experiments. Also25 °C. we want to thank Dr. Ileana Nieves for her assistance in

Infrared Studies. Infrared spectra were recorded on a obtaining spectra and Thomas J. Groshens for assistancePerkin-Elmer PE-1330 infrared spectrometer. IR studies of the with the mass spectrometer.

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