High-Energy Processes in Organometallic Chemistry - American

measurements, electron microscopy, GC-MS, resistivity, and related studies. ... to be from 1 to 5000 nm in diameter and are not easily precipitated, f...
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Chapter 15 Living Colloidal Metal Particles from Solvated Metal Atoms: Clustering of Metal Atoms in Organic Media Downloaded by NANYANG TECHNOLOGICAL UNIV on August 20, 2015 | http://pubs.acs.org Publication Date: February 26, 1987 | doi: 10.1021/bk-1987-0333.ch015

Matthew T. Franklin and Kenneth J. Klabunde Department of Chemistry, Kansas State University, Manhattan, KS 66506

A review of preparative methods for metal sols (colloidal metal particles) suspended in solution is given. The problems involved with the preparation and stabilization of non-aqueous metal colloidal particles are noted. A new method is described for preparing non-aqueous metal sols based on the clustering of solvated metal atoms (from metal vaporization) in cold organic solvents. Gold-acetone colloidal solutions are discussed in detail, especially their preparation, control of particle size (2-9 nm), electrophoresis measurements, electron microscopy, GC-MS, resistivity, and related studies. Particle stabilization involves both electrostatic and steric mechanisms and these are discussed in comparison with aqueous systems. Graham coined the term " c o l l o i d " to describe suspensions of small p a r t i c l e s in a l i q u i d . (J_) Such particles are g e n e r a l l y c o n s i d e r e d to be from 1 to 5000 nm in diameter and are not easily precipitated, f i l t e r e d , or observed by ordinary o p t i c a l microscopes. The topic of this paper i s metallic c o l l o i d a l p a r t i c l e s , often c a l l e d metal sols, with special emphasis on non-aqueous media. Some h i s t o r y of gold sols i s appropriate. Gold has held the attention of mankind for thousands of y e a r s . Attempts to make the " p e r f e c t metal" (gold) from imperfect metals was the realm of alchemists, and over the centuries great m e d i c i n a l powers were a s c r i b e d to g o l d . And as early as 300 A . D . there are references to the consumption of gold f l u i d to prolong l i f e . (2) It i s doubtful that t h i s was a c o l l o i d a l solution of gold since aqua regia(^) was unknown to early Chinese a l c h e m i s t s , and more l i k e l y amalgams of Hg-Au were a c t u a l l y consumed. Centuries l a t e r more recipes for aurum p o t a b i l e ( d r i n k a b l e gold) appeared which were aqueous gold c o l l o i d solutions prepared by dissolving gold in aqua regia followed by treatment (chemical reduction of AuCl~ or HAuCl^ to Au metal p a r t i c l e s ) with e t h e r e a l o i l s . These solutions were usually then treated with chalk to neutralize the acid before being NOTE: This chapter is part 15 in a series.

0097-6156/87/0333-0246$06.00/0 © 1987 American Chemical Society

In High-Energy Processes in Organometallic Chemistry; Suslick, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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consumed. F a b u l o u s c u r a t i v e powers were a t t r i b u t e d to these solutions, e s p e c i a l l y toward heart d i s e a s e . And i n 1618 A n t o n i p u b l i s h e d Panacea Aurea: Auro Potabile(iJ) which centered on the treatment of venereal diseases, dysentery, epilepsy, tumors and more with drinkable gold. A d d i t i o n a l s i m i l a r books appeared, (5) and Helcher pointed out that the addition of b o i l e d s t a r c h n o t i c e a b l y i n c r e a s e d the s t a b i l i t y of the p r e p a r a t i o n . In 1802 Richter(6) mentioned that the shades of color i n purple gold solutions and ruby g l a s s were due to the p r e s e n c e of f i n e l y d i v i d e d g o l d . He correlated the colors with p a r t i c l e s i z e i n a q u a l i t a t i v e way.(jp And Fulhame(7) noted in 1794 that she could dye s i l k cloth various shades of purple with c o l l o i d a l gold s o l u t i o n s . C a s s i u s ( 8 ) and Glauber(9) were also involved in using c o l l o i d a l gold as a coloring agent. Faraday p u b l i s h e d the f i r s t s c i e n t i f i c investigations of gold sols.(H)) He u s u a l l y reduced aqueous s o l u t i o n s of A u C l with phosphorous. However, he a l s o experimented with sparkirrg gold wires. He concluded that gold was present i n the s o l u t i o n s as elemental g o l d , and that color depended on p a r t i c l e s i z e . He also discovered f l o c c u l a t i o n by addition of an e l e c t r o l y t e N a C l . Other workers also used sparking of metals under water to produce sols of Pb, Sn, Au, Pt, B i , Sb, A s , T I , Ag, and Hg. (11-12) As the years passed aqueous c o l l o i d a l gold was studied extensively. The c l a s s i c work of Svedberg, (J_3 ) Zsigmondy , (1J* ) K o h l s c h u t t e r , ( 1_5 ) and T u r k e v i c h ( J_6 ) must be n o t e d . Zs igmondy , (V4 ) u s i n g a s l i t microscope, was able to study the "seeding" phenomenon and found that gold p a r t i c l e s already present p r e f e r e n t i a l l y grew as more gold s a l t was reduced. T h i s and r e l a t e d work Π 72,1,8 ) i m p r o v e d our understanding of p a r t i c l e nucleation and growth. Non-Aqueous C o l l o i d a l Metal S o l u t i o n s . I t has been d i f f i c u l t to prepare c o l l o i d a l gold i n non-aqueous media due to limitations in preparative methods (low s a l t s o l u b i l i t i e s , s o l v e n t r e a c t i v i t y , e t c . ) , and the f a c t that the low d i e l e c t r i c constant of organic solvents has hindered s t a b i l i z a t i o n of the p a r t i c l e s . In aqueous solution the gold p a r t i c l e s are s t a b i l i z e d by adsorption of innocent ions, such as chloride, and thus s t a b i l i z e d toward f l o c c u l a t i o n by the formation of a charged double l a y e r , which is dependent on a solvent of high d i e l e c t r i c c o n s t a n t . Thus, i t seemed that such electronic s t a b i l i z a t i o n would be poor i n organic media. In spite of these d i f f i c u l t i e s , some l i m i t e d successes have been r e p o r t e d . Svedberg(J_3) struck an e l e c t r i c arc in a glass tube under organic l i q u i d s . Gas flow through the tube c a r r i e d some of the metal p a r t i c l e s into the solution ( l i q u i d methane or isobutanol were used). Mayer used a s i m i l a r method using very high voltage w i t h organic-water mixtures. (1_9) More recently, Kimura and Bandow reported a s i m i l a r method where metals were evaporated and swept into a cold trap containing ethanol, with some success.(20) Additional successes have been reported: Svedberg(2J_) used an a l t e r n a t i n g c u r r e n t d i s c h a r g e to d i s p e r s e small pieces of metal; Natanson(22) obtained c o l l o i d a l copper i n an acetone/toluene/1pentanol mixture by r e d u c i n g C u C l with z i n c powder; Janek and Schmidt(23) added a g o l d / c i t r a t e aqueous sol to an a l c o h o l / t o l u e n e mixture followed by h e a t i n g and c o o l i n g and found that some gold 2

American Chemical Society Library 1155 16th St., N.W. In High-Energy Processes in Organometallic Chemistry; Suslick, K.; ACS Symposium Series; American Chemical Washington, D . C Society: 2DQ36Washington, DC, 1987.

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c o l l o i d a l p a r t i c l e s remained in the alcohol rich layer; M a r i n e s c u ( 2 Ά ) r e p o r t e d t h a t s o n i c a t i o n o f a l k a l i m e t a l s at t h e i r mp i n kerosene y i e l d e d p y r o p h o r i c c o l l o i d s ; Y a m a k i t a ( 2 5 ) u s e d fats, o r g a n i c a c i d s , a l c o h o l s , and o t h e r o r g a n i c s as r e d u c i n g agents f o r Au 0 and o b t a i n e d s u c c e s s e s p e c i a l l y w i t h f a t s and f a t t y a c i d s ; and L e a w l t h ( 2 6 ) was a b l e t o r e d u c e A u C l - i n w a t e r w i t h d i a z o e t h a n e f o l l o w e d by m i x i n g w i t h o r g a n i c s which gave some g o l d p a r t i c l e s i n the o r g a n i c phase.

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2

P r o t e c t i v e C o l l o i d s . Another approach i n p r e p a r i n g and s t a b i l i z i n g m e t a l c o l l o i d s i s by a d s o r p t i o n o f macromolecules on t h e i r s u r f a c e s . A w i d e v a r i e t y o f m a t e r i a l s h a v e b e e n u s e d i n c l u d i n g gummy g e l a t i n o u s l i q u i d s , (J_0) a l b u m i n , (27) I c e l a n d i c moss, (28) l a t e x , (22) p o l y v i n y l p y r r o l i d o n e , (29 ) a n t i b o d i e s , ( 30 ) c a r b o w a x 20M , ( 3 j ) p o l y v i n y l p y r i d i n e , (_3_p a n d v a r i o u s polymer-water/oil-water m i x t u r e s . (32) These s t u d i e s c l e a r l y i n d i c a t e that "steric s t a b i l i z a t i o n " of metal c o l l o i d s i s a l s o important (along with electronic stabilization).(33) Results

and D i s c u s s i o n

More d i r e c t and s u c c e s s f u l m e t h o d s f o r t h e p r e p a r a t i o n o f n o n ­ aqueous metal s o l s are d e s i r a b l e . E s p e c i a l l y v a l u a b l e would be a method t h a t a v o i d s the metal s a l t r e d u c t i o n s t e p ( a n d t h u s a v o i d s c o n t a m i n a t i o n by o t h e r r e a g e n t s ) , a v o i d s e l e c t r i c a l discharge methods which decompose o r g a n i c s o l v e n t s , and a v o i d s m a c r o m o l e c u l e stabilization. Such a method would p r o v i d e p u r e , n o n a q u e o u s m e t a l c o l l o i d s and s h o u l d make e f f i c i e n t use o f p r e c i o u s m e t a l s e m p l o y e d . S u c h c o l l o i d s would be v a l u a b l e t e c h n o l o g i c a l l y i n many ways. They would a l s o be v a l u a b l e to study so that more c o u l d be l e a r n e d a b o u t p a r t i c l e s t a b i l i z a t i o n m e c h a n i s m s i n non-aqueous media, o f which l i t t l e i s known at the p r e s e n t t i m e . We have r e p o r t e d n u m e r o u s s t u d i e s o f the c l u s t e r i n g o f metal atoms i n non-aqueous ( o r g a n i c ) m e d i a . (J3jO And v e r y r e c e n t l y our p r e l i m i n a r y r e p o r t o f s t a b l e n o n - a q u e o u s g o l d s o l s a p p e a r e d . (35) The approach has been t o d i s p e r s e metal atoms i n e x c e s s c o l d o r g a n i c s o l v e n t thus forming s o l v a t e d metal atoms. Upon w a r m i n g atoms c l u s t e r i n g takes p l a c e moderated by s o l v a t i o n . Particle (cluster) growth i s e v e n t u a l l y i n h i b i t e d and s t o p p e d by s t r o n g l y bound s o l v e n t molecules. As mentioned a b o v e , t h e r e i s some p r e c e d e n t f o r t h i s a p p r o a c h f o u n d i n e a r l i e r work on the p r e p a r a t i o n o f a c t i v e metal s l u r r i e s , ( 3 6 ) d i s p e r s e d c a t a l y s t s , ( 3 7 ) and metal atom c l u s t e r i n g i n polymer o i l s . (38) H o w e v e r , o u r i n i t i a l r e p o r t ( 3 5 ) was the f i r s t d e s c r i b i n g the p r e p a r a t i o n o f s t a b l e m e t a l s o l s by t h e solvated m e t a l atom method. The e x p e r i m e n t a l a p p a r a t u s used c o n s i s t e d o f a s t a t i o n a r y metal a t o m - v a p o r r e a c t o r w h i c h has b e e n d e t a i l e d i n t h e l i t e r a t u r e e a r l i e r . (_39) M e t a l was e v a p o r a t e d (-0.1 to 0 . 5 g) and c o d e p o s i t e d a t - 1 9 6 ° C w i t h e x c e s s o r g a n i c s o l v e n t v a p o r (~ 40-150 mL). The f r o z e n m a t r i x was a l l o w e d t o warm under c o n t r o l l e d c o n d i t i o n s , a n d u p o n m e l t i n g s t i r r i n g was c o m m e n c e d . A f t e r w a r m i n g t o room t e m p e r a t u r e s t a b l e c o l l o i d a l s o l u t i o n s were o b t a i n e d a n d s y p h o n e d o u t under N . 0

In High-Energy Processes in Organometallic Chemistry; Suslick, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

15. FRANKLIN AND KLABUNDE

Colloidal Metal Particles

249

A v a r i e t y o f m e t a l s h a v e now b e e n i n v e s t i g a t e d i n our l a b o r a t o r y i n c l u d i n g F e , C o , N i , P d , P t , C u , A g , A u , Z n , C d , I n , Ge, S n , and P b . S o l v e n t s employed have been a c e t o n e , e t h a n o l , T H F , diethylether, dimethylsulfoxide, dimethylformamide, p y r i d i n e , triethylamine, ispropanol, isopropanol-acetone, toluene, pentane, and w a t e r . A c e t o n e - A u w i l l be d i s c u s s e d i n d e t a i l h e r e i n , a l o n g with Ag.

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0

0

Au + C H C C H 3 3 0

->

0

(CH CCH_) Au 3 3 m 0

*

(CH CCH ) Au 3

stable

3

y

z

purple

.ufSjiO

warm