Formation of silver metal films by photolysis of silver salts of high molecular weight carboxylic acids

Thin Solid Films, 218 (1992) 109-121

109

Formation of silver metal films by photolysis of silver salts of high molecular weight carboxylic acids Yoshiro

Yonezawa,

Yoshiaki

Konishi*

and Hiroshi Hada

Department of Industrial Chemistry, Faculty of Engineering, Kyoto University, Kyoto 606 (Japan)

Katsuhiko

Yamamoto

and Hideyuki

Ishida

Toray Research Centre Inc., 3-3 Sonoyama, Otsu, Shiga 520 (Japan)

Abstract

The photochemical formation of silver metal films from silver salts of high molecular weight carboxylic acids has been studied. On irradiation with a low pressure mercury lamp in wet air at room temperature, the films first became yellow-brown in colour owing to the formation of colloidal silver. After prolonged irradiation, the irradiated surface of the silver salt films of natural high molecular weight carboxylic acids (silver alginate, silver pectate) readily changed into silver mirrors. When silver alginate films were photolysed at 77 K, a broad absorption band below 600 nm and a small peak near 335 nm, probably due to silver clusters, appeared. In this method, silver atoms in the various aggregation states (atoms, clusters, colloids, bulk metal) can be formed by changing photolysis conditions. The preparation of matrix-supported silver films reported by other research groups has been reviewed briefly.

deposits [2]:

1. Introduction

Ag + + 2H20 Molecular photochemistry in gaseous and liquid phases and the photochemistry of solids have been extensively studied [1]. In contrast, the photochemistry of molecules and solids at the gas-solid and liquidsolid interfaces has not been adequately studied despite its relevance to many practical applications. We have been interested in the photodeposition of metals from homogeneous phases since this is a typical example of photochemically induced phase formation. Detailed studies have been carried out on the photoreduction of silver ions (Ag ÷) in aqueous solutions [2] and in aqueous suspensions of semiconductors (ZnO [3], TiO2 [4]). In the initial stage, Ag ÷ ions are reduced by a suitable reducing agent (e-(Red)) to silver atoms (Ag°). Then, they gradually change into clusters (Agn), colloids (Ag c) and finally bulk metal (Ag M) through nucleation, growth and aging stages by aggregation: Ag + + e - ( R e d ) nAg °

, Ag~

, Ag o ~ Ag c

(1) ~ Ag M

(2)

Photolysis of aqueous silver perchlorate (AgC104) solution at 2 = 2 5 3 . 7 n m light gave rise to silver *Present address: Department of Chemical Engineering, Faculty of Engineering, Tokushima University, Minami-Josanjima, Tokushima 770, Japan.

0040-6090/92/$5.00

h,., AgO + 4H + + 02

(3)

Kubal reported photolysis of a precipitate formed by addition of polyalkylacrylate to a silver nitrate (AgNO3) solution [5]. We prepared colloidal silver and gold by photolysis of AgC104 and chlorauric acid solutions respectively in the presence of protective agents such as sodium dodecylsulphate, sodium alginate and colloidal silica [6]. Irradiation of aqueous AgC104 solution containing sodium dodecylsulphate and isopropanol (i-C3H7OH) with 253.7 nm light caused the reaction 2Ag+ + i_C3HTOH h,., 2Ag0 + 2H ÷ + (CH3)2CO

(4)

We have used these reactions to fabricate the A g - A u composite colloids [7]. Extinction spectra of colloidal silver, gold and A g - A u were interpreted on the basis of Mie theory [8, 9]. Vogler et al. photolysed azide complexes such as Ag(PPh3)2N3 and [Au(N3)2](Ph 3-=triphenylphosphine) to form metal organosols [10]: 2Ag(PPha)2N 3 by, 2Ag0 + 4PPh 3 + 3N 2

(5)

It has long been known that irradiation of silver salts of low molecular weight carboxylic acids, e.g. acetic acid, propionic acid and oxalic acid, with UV light forms metallic silver. Tompkins and coworkers observed quantitative formation of silver and carbon

© 1992 -- Elsevier Sequoia. All rights reserved

110

Y. Yonezawa et al. / Formation of Ag .films by photolysis

dioxide (CO2) from silver oxalate (Ag2(COO)2) [11]: Ag2 (COO)2 ~

2Ag ° + 2CO2

(6)

We photolysed thin films of silver salts of alginic acid [12, 13]. On irradiation with a 15 W sterilization lamp in wet air, a large amount of silver was precipitated and the irradiated surface finally changed into a clear silver mirror with a low sheet resistance. Thin films of inorganic and organic materials are technologically important. In particular, thin metal films have received much attention for a long time [14, 15]. Many fabrication techniques, either physical or chemical deposition and dry or wet processes, have been developed. In this article, we review photodeposition of silver films from silver salts of high molecular weight carboxylic acids. Photolysis at room temperature and film characterization are first described. Then, preliminary results of the photolysis at liquid nitrogen temperature is presented. The preparation of silver films by other research groups is discussed briefly.

AgNO3 contained in the films, and dried again in the dark. The films on the plates (1-5 gm in thickness) were colourless and transparent. Hereafter, the following abbreviations will be used: SA, silver alginate; SP, silver pectate; SCMC, silver carboxymethycellulose; SPA, silver polyacrylate; SPM, silver polymethacrylate. Replacement of Na + by Ag + ions and the absence of AgNO3 in the films were verified using UV and IR spectrometry. Transmittance spectra in the UV region and IR absorption spectra of the SA films are compared with those of sodium alginate and alginic acid films in Figs. 2 and 3 respectively [12, 17]. Although UV absorption of sodium alginate and alginic acid was discernible only at wavelengths shorter than 250 nm, a broad absorption band extending to 300 nm was seen in the SA films (Fig. 2). This absorption is assigned to charge transfer excitation from a carboxylate ion to an Ag + ion. A characteristic frequency of the antisymmetrical stretching vibration of the carboxylate ion (RCOO-) in the metal salts of carboxylic acids shifts considerably according to the kind of metal ion [18].

2. Experimental methods COOH

H

H

COOH

2.1 Silver salts o f high molecular weight carboxylic acids

We have chosen polysaccharides and their derivatives alginic acid, pectic acid and carboxymethylcellulose in this study [16]. Synthetic high polymers having carboxyl groups, polyacrylic acid and polymethacrylic acid were chosen as reference materials. The chemical structures of those compounds are shown in Fig. 1. Alginic acid, (C5HvO4COOH)x , is a block heteropolymer of D-mannuronic acid and L-guluronic acid produced by brown algae. Pectic acid is a free polygalacturonic acid, (CsHvO4COOH)x , contained in the cell wall of land plants in the form of pectic substance. Carb o x y m e t h y l c e l l u l o s e , ( C s H v O 4 C H z O C H z C O O H ) x , is a carboxymethyl derivative of cellulose. Polyacrylic acid, (CH2CHCOOH)x, and polymethacrylic acid, (CHzC(CH3)COOH)~, are addition polymers of vinyl compounds. Commercially available carboxylic acid and sodium salts were used. They were sodium alginate, sodium pectate, sodium carboxymethylcellulose, sodium polyacrylate and polymethacrylic acid. Sodium salts were purified by reprecipitation [13]. Polymethacrylic acid was used as supplied. Thin films of silver salts of high molecular weight carboxylic acids were prepared by an ion exchange method. A 0.5 wt.% aqueous solution of sodium salts or polymethacrylic acid was dropped on glass or quartz plates (0.05 ml cm-2). The plates were dried in air at room temperature and then immersed in AgNO3 solution to substitute Na+(sodium salts) or H + (free acid) ions with Ag + ions. The plates were then washed with distilled water to remove sodium nitrate and excess

A -

o

OH HO H

OH HO H

H

H

COOH

H

H

H

COOH

COOH

B H

OH

H

H

OH

CH~

OH

H

CIZX)H

H

OH

OH

C CI-~OCI-~(::OOH

H

(:)H

CH~COC)H

- - CH?-CH--C H2-CH--CH2-CH--C H2- CH - -

-I

COOH

CH3 -

-

I

COOH

CH3

I

I

COOH

CH3

COOH

CH3

CH2-C--CH2-C--CH - C - - C H 2 - ( ~ - I

COOH

I

CO(:O

2 I

COOH

D

I

E

CO()H

Fig. 1. Structural formulae of natural and synthetichigh molecular weight carboxylic acids: A, alginic acid; B, pectic acid; C, carboxymethylcellulose;D, polyacrylicacid; E, polymethacrylicacid.

Y. Yonezawa et al. / Formation of Ag films by photolysis 100 I

i

,

i

i

.

i

,

--

v

C

"

50

C s. I--

220

z~o

260

2Bo

s00

320 3~0

Wavelength (nm)

Fig. 2. Transmittance spectra of thin films of silver alginate (spectrum A), sodium alginate (spectrum B), and alginic acid (spectrum C) on quartz plates.

~100i

.

.

.

.

.

40 34 30 26 22

.

.

19 17

.

.

.

.

ia)

.

15 13 115 10.5 9.5 8.5 75 6.5 xlO0

ib)

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- lOO

~-

(c)

~

SO

OI

40

i

~

i

1

i

i

i

1

i

,

,

1

34 30 26 22 19 17 15 13 11.5 105 9.5 8.5 7.5 6,5 XlO0

Wavenumber ( cm -1)

Fig. 3. IR absorption spectra of thin films of (a) sodium alginate, (b) alginic acid, and (c) silver alginate ( - - , before irradiation; - - -, after 120 min of irradiation with UV light, the arrows indicating the development of two bands at 1725 cm-1 and 1260cm ~). In Fig. 3, the 1725 cm -~ band of alginic acid ( R C O O H ) disappeared and the 1610cm -~ ( N a +) or 1585cm -~ (Ag +) band developed instead. A band at 13001400 c m - ' is due to the symmetrical stretching vibration of the R C O O - ion. The distinct 1585 cm -1 band in the SA films confirmed that most of N a + ions were substituted by Ag + ions. We dissolved the silver salt films with concentrated nitric acid and analysed the amount of total Ag + by means of atomic absorption spectrometry. The ratios of Ag + substitution were about 89% (SA), 82% (SP), 87% (SCMC), 75% (SPA) and 21% (SPM). 2.2. Photolysis o f silver salt films A 15 W sterilization lamp was used as the light source to photolyse silver salt films in a sample holder

111

in air at r o o m temperature. For photolysis under evacuated conditions, a 200 W low pressure mercury lamp was used. In this case, the sample was held inside a quartz cryostat equipped with four windows for photolysis and spectroscopic measurements. The incident radiation of 253.7 nm light on the sample from a 15 W lamp, as measured by a ferrioxalate actinometer [19], was 3.5 × 1014-7.0 × 10 ~5 photons cm 2s ~. The number of photons from the 2 0 0 W lamp was about 1.6 times larger than from the 15 W lamp. The amount of photolytic silver in the SA films was analysed by two methods. The irradiated film was dissolved in a m m o n i a water [20]. The resultant brown solution containing colloidal silver, ammine complex of silver, and a m m o n i u m alginate was neutralized with acetic acid. In the first method, colloidal silver was purified by dialysis to remove Ag + ions with a semipermeable membrane [6, 13]. After that, deposited silver was dissolved in concentrated nitric acid and then submitted to quantitative analysis by atomic absorption spectrometry. In the second method, the amount of Ag + was determined by differential potentiometric titration [20]. The amount of photolytic silver was estimated by subtraction of the amount of Ag + after photolysis from the total number of Ag + ions at the beginning. These two methods yielded practically the same results. However, these methods were not applicable to SCMC, SPA and SPM films because of low solubility of the irradiated films in a m m o n i a water. 2.3. Film characterization Transmittance spectra and I R absorption spectra were recorded on a U V - v i s i b l e spectrophotometer [13, 17] and an I R spectrophotometer [12] respectively. The electrical sheet resistance of the film surface ( 1 cm 2 area) was measured by an electrometer [ 13, 21]. The morphology of the films was examined by a high resolution scanning electron microscope of the field emission (FE) type [13]. The X-ray diffraction ( X R D ) data of the films were collected in a diffractometer system with Cu K s radiation [13]. The X-ray fluorescence ( X R F ) analysis was carried out with an energy-dispersive X-ray fluorescence spectrometer [13]. Almost all measurements were carried out with the films attached to the glass or quartz plates as obtained. The films were stripped from the plates for measurements of I R spectra.

3. Photolysis of silver salts of high molecular weight carboxylic acids

3.1. Photochemistry o f silver alginate On irradiation with the 15 W sterilization lamp in wet air (relative humidity, more than 70%) at r o o m temperature, the SA film became first y e l l o w - b r o w n in

l 12

Y. Yonezawa et al. / Formation of Ag films by photolysis

100

I

I

]

I

i

U

c

C

_~. 50

D

"

c I,,-

0

300

400 500 600 Wavelength (nm)

700

Fig. 4. Variation in the transmittance spectra of silver alginate films on the quartz plate with irradiation time: spectrum A, 0 min; spectrum B, 5 rain; spectrum G, 15 rain; spectrum D, 30 rain; spectrum E, 60 rain; spectrum F, 120 rain.

colour. When irradiation was continued, the irradiated surface of the film gradually acquired a metallic lustre and finally changed into a bright silver mirror [12, 13]. The variation in the transmittance spectra of the film with irradiation is given in Fig. 4. Absorption bands at 2 < 300 nm and 2 = 410-420 nm appeared and developed with irradiation time. These bands are assigned to photolytic silver. The latter band, which is characteristic of colloidal silver, gradually became broader with irradiation and finally the distinct band was smeared out, resulting in a continuous absorption band of the silver metal films. The dependence of the amount ANAg of photolytic silver on irradiation time is given in Fig. 5. The conversion efficiency of the Ag + ions which existed at the beginning (ca. 1.0 lamolcm -2) was about 55% after

.

.

.

.

12

180 min of irradiation. The initial quantum yield of silver deposition was in the range 0.02-0.05. When the SA film was photolysed in dry air at room temperature, the amount of photolytic silver obtained was smaller than in wet air and the initial quantum yield was about 0.01 [20]. As shown in Fig. 3, the IR absorption spectra of the SA films exhibited two characteristic absorption bands arising from the R C O O - structure at 1585 cm -1 (antisymmetrical stretching vibration)and 1410cm -~ (symmetrical stretching vibration). When the SA films were irradiated, the intensities of those bands decreased and two new bands developed at 1725 cm -1 (strong) and at 1260cm -~ (weak) [12]. The frequencies of the new bands agree with those of the carboxyl group (RCOOH) in the alginic acid. It appears that R C O O H was produced during the reaction. However, it was found by gas chromatography that the SA films evolved CO2 during the irradiation [20]. The ratio of the amount of photolytic silver to CO2 was 4.0-5.0. A primary photochemical process in the SA films is assumed to be one-electron transfer from the carboxylate ion to the bound Ag + ion, giving rise to a silver atom (Fig. 6). The carboxyl radical (RCOO') either decomposes into a secondary radical (R') and CO2 or is converted to carboxylic acid (RCOOH) as a result of hydrogen extraction from the C - H bond of the polymer chain. Electron spin resonance (ESR) spectra of the SA films photolysed under evacuated conditions at 77 K were measured [22]. An ESR spectrum at 77 K exhibited four lines. When the sample was warmed, the four-line signal gradually vanished and, finally, a singlet signal remained at room temperature. It is probable that the initial spectrum consists of the overlap of two spectra. After subtraction of the contribution of the singlet signal, a quartet assigned to R" having a sixmembered ring structure resulted.

3.2. Properties of silver metal films from silver alginate The SA films produced an electrically conductive surface at the irradiated side (front side). The variation e

RCOO-A¢~ n Ag"

RCOO-<7"+ Ag -~) Ag=+RCOO , ( A~ )n

RCOO ~ 1 ~ +H"*

+ CO2 RCOOH

60 120 180 ] r radi ation time (mi n) Fig. 5. Growth of the amount ANAg of deposited silver and accompanying change in electrical sheet resistance, R of the irradiated surface of silver alginate films on quartz plates with irradiation time. ANAg and R are given per square centimetre.

H

H

Fig. 6. Reaction scheme showing photolysis of silver alginate, hv denotes the photon energy of 253.7 n m light.

Y. Yonezawa et al. / Formation of Ag films by photolysis

....~:< .......

(a)

113

,~,~

(b)

(c)

'1 ~ITI

Fig. 7. FE scanning electron micrographs of silveralginate films on the glass plate: (a) before irradiation; (b) after 30 min of irradiation; (c) after 180 min of irradiation. A thin layer of Pt-Pd was deposited on the surface of the film in (a) by sputtering. The Pt Pd sputtering was not done for the films in (b) and (c). in electrical sheet resistance with irradiation is given in Fig. 5. The resistance of the unirradiated film was about l010 ~. However, it decreased sharply with irradiation and attained some 102~ after 100min of irradiation [13]. In the final stage, the film surface had a high electrical conductivity, which was more than 108 times higher than that of the original film. On the contrary, the sheet resistance of the back side of the film facing the plate and the bulk resistance between the front and back sides of the film did not undergo a considerable change. At the same time, the back side did not have a mirror-like appearance but rather was granular. FE scanning electron microscopy (SEM) micrographs of the SA films before and after irradiation are shown in Fig. 7. Before irradiation, the surface looked smooth. Photolysis for 30 min generated colloidal silver particles ( 1 0 - 5 0 n m in diameter) sparsely distributed on the surface. After prolonged irradiation, the film surface was densely covered with colloidal silver [13]. This change in morphology is responsible for the decrease in the electrical resistance in Fig. 5. A preliminary observation of the cross-section of the SA films by transmission electron microscopy showed that silver films consisted of aggregates of colloidal silver particles (Fig. 8) [23]. An interesting aspect is that photolytic silver has a tendency to precipitate at the front side of the film. This is consistent with the decrease in the electrical sheet resistance at the irradiated surface. The

presence of smaller silver particles in the inside of the film is also observed. The X R D profile of the irradiated films consisted of sharp diffraction lines (Fig. 9). Colloidal silver particles seem to be in a highly crystalline state corresponding to the f.c.c, silver lattice [ 13]. On the contrary, non-irradiated films exhibit a broad diffraction peak assignable to the SA polymer. The X R F spectra of SA films are given in Fig. 10. The L~ emission intensities of silver proved to be nearly equal before and after photolysis. The almost constant surface concentration of the total amount of silver (silver atoms and ions) means that Ag ÷ ions are converted to silver atoms within the same film area by photolysis [13]. We have observed another structural change of the thin silver layers which occurs over a period of several tens of days at room temperature [13]. The film surface which had been irradiated for 40 min changed considerably during dark storage for a period of about 90 days. An FE SEM micrograph of the aged film reveals large crystals ( 1 0 0 - 5 0 0 n m in size) (Fig. ll(a)). The FE SEM observation of a cross-section of the same film on the plate (Fig. 1 l(b)) reconfirms the existence of such crystals on the front side. X R D and X R F analyses provide evidence that those crystals do not consist of silver oxides (AGO, Ag20) but of silver metal (Fig. 12). The X R F analysis of a 0.1-1 ~tm2 area of the film surface revealed that the emission intensity of silver

l 14

Y. Yonezawa et al. / Formation o f Ag/ilms by photolysis

t

(a)

I

I

3 IJm

(b)

I

0.5 IAm

Fig. 8. Transmission electron micrographs of a cross-section of a silver alginate film exposed to UV light for 120 min. (a) Lower magnification micrograph. The arrow is the direction of incident light for photolysis. (b) Enlarged micrograph of (a).

r

i

r

I

3

.O

"G C

c

C

20

XO

6'0

~0

100 (a)

(a)

I

I

I

2.0

4.0

I

6.0

i

A

Ag

g

~g

C

X

c

v

~

E N

2?

40

60

80

100

2D

20 (degrees)

4O

~0

Energy (keV)

(b)

(b)

Fig. 9. X R D profiles for silver alginate films on glass plates (a) before and (b) after 180 rnin of irradiation.

Fig. 10. X R F spectra for silver alginate films on glass plates (a) before and (b) after 180 min of irradiation.

from the region lacking large silver crystals was considerably smaller than that from the region of large silver crystals (Fig. 13). It is likely that large crystals have grown at the expense of colloidal particles around them. However, it is significant that such changes did

not occur when the film surface was densely covered with colloidal silver particles. In fact, the FE SEM observation did not reveal any substantial change when the SA film was irradiated for 180 min and stored in the dark for several months.

Y. Yonezawa et al. / Formation of Ag films by photolysis

I (a)

(b)

115

I 1 ~ [TI

Fig. 11. FE scanning electron micrographs of a silver alginate film irradiated for 40 min and then stored in the dark at room temperature for 90 days: (a) irradiated surface; (b)cross-section. A Pt Pd layer was deposited by sputtering on the divided surface in (b).

3.3. Photochemistry o f silver salts other than silver alginate We have found a number of high molecular weight carboxylic acids whose silver salts could produce photolytic silver on irradiation with the 15 W sterilization lamp [12, 24-26]. In the transmittance spectra of the SP, SCMC, SPA and SPM films, broad absorption bands were observed for 2 < 300 nm. They are assigned to charge transfer excitation from the carboxylate ion to the Ag + ion. The variation in the transmittance spectra of the films with irradiation is given in Fig. 14. An absorption band characteristic of colloidal silver appeared at 2 = 4 2 0 - 4 3 0 n m (SP), 410-420nm (SCMC), 450-460 nm (SPA) or 420-450 nm (SPM). Prolonged irradiation caused a metallic lustre. However, the irradiation time necessary for development of continuous absorption of the metal films depended on the kind of carboxylic acid. Although the formation of silver mirrors was fairly easy for the SA and SP films and not very difficult for the SCMC films, a much longer irradiation time was needed for the SPA and SPM films. In the IR absorption spectra, the intensity of the antisymmetrical stretching vibration of the RCOOstructure of the SCMC and SPA films decreased and the band corresponding to RCOOH appeared with increasing irradiation time [12]. ESR study of these films indicated formation of some alkyl radicals from

RCOO" [22]. These observations suggest that photolysis of silver salts of high molecular weight carboxylic acids generally proceeds as shown in Fig. 6. The variation in the electrical sheet resistance of the SP and SCMC films with irradiation is given in Fig. 15. The resistance of the films decreased with irradiation time and reached values of 102-103~ after 100 min of irradiation. FE SEM micrographs of the SP and SCMC films before and after irradiation are shown in Fig. 16. After prolonged irradiation, the film surface was densely covered with colloidal silver. 3.4. Photolysis o f silver alginate at low temperature According to a preliminary experiment, the initial quantum yield of photolysis of the SA films immersed in water decreased by 1/3-1/2 when the temperature was lowered from 30 °C to 0 °C. In view of the remarkable effect of wet air in forming silver mirrors, the formation of colloidal silver and silver mirrors would become difficult if photolysis were carried out under evacuated conditions at low temperature. We have thus tried to illuminate SA films in a cryostat under evacuated conditions with the 200 W low-pressure mercury lamp. The variation in the absorption spectra with irradiation at room temperature is given in Fig. 17. The colloidal absorption band at 2 = 4 1 0 n m and a small shoulder around 335 nm appeared. Although the

116

Y. Yonezawa et al. / Formation of Ag films by photolysis ..~

i

i

I

i

ij °_

_c3

--

,

20

40

60 20 ( d e g r e e s )

80

100

Ag

_=

(a)

I AgO Ag20_

,

Ag20 (a)

IlJl

Ih I

=.u,

I;

I

It

,,I

I

~ I

0.0

,

02

OA OE

OB 1.0 ×10

i

Si I i

,,

,

I

..fi

=¢

"G .,,,., C

0~ 16

L_

02

0.4 0.6 Energy(keY)

OB 1.0 x 10

(b)

C ¢-

2.0

4.0

6.0

Fig. 13. X R F spectra of 0.1-1 gm 2 areas of a silver alginate film irradiated for 40 rain and then stored in the dark at room temperature for 90 days: (a) emission from the region lacking in large silver crystals; (b) emission from the region of a large silver crystal. The emission intensity is normalized to the X R F peak of silicon.

E n e r g y (keY) (b) Fig. 12. (a) X R D profile and (b) X R F spectrum of a silver alginate film irradiated for 40 min and then stored in the dark at room temperature for 90 days. The diffraction patterns for bulk silver and silver oxides (AGO, Ag20 ) are shown for comparison.

colloidal absorption band developed with irradiation time, prolonged irradiation did not cause silver mirror formation. Moreover, the optical density of the colloidal absorption was smaller than that obtained in wet air. The results of photolysis at 77 K are shown in Fig. 18. In this case, even colloidal absorption was not evident after 180 min of irradiation. On the contrary, a broad absorption band extending from 600 nm to less than 200 nm and a small peak around 335 nm, probably due to silver clusters, developed. The irradiated film was gradually heated in the dark under evacuated conditions until room temperature was reached and the change in absorption spectra was monitored (Fig. 18). An increase in colloidal absorption (2 = 415 nm) and the appearance of a weak 295 nm peak, as well as decrease in the 335 nm peak, were observed. As the final optical density of colloidal absorption was always smaller than that observed after photolysis at room temperature for the same irradiation time, the amount of photolytic silver at 77 K was smaller than that obtained at room temperature.

The lattice distance of bulk silver, 4.086 A, is smaller than the average distance of Ag + ions, 10A, in the SA films [21]. According to Welker and Martin [27], the absorption spectrum of Ag o isolated in solid argon (5 K) consists of several lines, e.g. 299, 304, 315 nm. Ozin reported the optical spectrum of Ag o in high molecular weight paraffin wax ( 1 0 - 1 2 K; 2 = 312-315, 334-338, 3 5 7 - 3 6 0 n m ) [28]. The absence of those bands and the presence of a broad absorption below 600nm, together with a peak at 2 = 335 nm (77 K), imply diffusion and aggregation of silver atoms and small clusters, resulting in the formation of somewhat larger clusters. A bright silver mirror was not formed unless irradiation was performed in wet air at room temperature. Water molecules contained in the film exposed to an atmosphere of high relative humidity could act as a plasticizer, which facilitates the segmental motions of the polymer and promotes migration of colloidal metal particles. The tendency for photolytic silver to precipitate mainly at the front side of the film, despite relatively homogeneous absorption of UV light, implies that the air-polymer film interface plays the role of an absorbing barrier to the Brownian motion of colloidal particles. The diffusion of colloidal particles has been further confirmed by the growth of large silver crystals at the film surface in the time scale of several tens of days at room temperature.

Y. Yonezawa et al. / Formation of Ag .films by photolysis

I 17

4. Matrix-supported silver films and their applications

,°° t ~

\A'

A

=

/

E

0 300

400

500

Wavelength

600

700

(nm)

(a)

100 A.~

7 , ~- -"-5-' ' ~- ~D--:' -: -KI-:-:----:-~-. . ~. .,./., i~"---~ , ' /--"

~50" E

/

E

/-\.

J

300

400

500

600

700

Wavelengt h (n m) (b) Fig. 14. Variation in transmittance spectra of silver salts of high molecular weight carboxylic acids with irradiation (the films were on quartz plates): (a) spectra A, B, C, silver pectate films; spectra A', B', C', silver carboxymethylcellulose films; (b) spectra D, E, F, silver polyacrylate films; spectra D'. E', F', silver polymethacrylate films. The irradiation times were as follows: spectra A, A', D, D', 0 min; spectra B, B' E, E', 30 rain; spectra C, C', F, F', more than 180min.

141

,

,

,

,

,

,

12 1_-~~/A lO

r¢-

A number of techniques have been developed to fabricate matrix-supported metal films: matrix-supported metal clusters, metal-matrix composites, surface-metallized films and metal mirrors. In general, the wet process of thin film fabrication is carried out under milder conditions than the dry process. Chemical reduction, electrochemical reduction, photochemical reduction and various particle collection methods (spinning, dipping, draining methods) have been employed for the fabrication of silver films by wet processes [29]. Electroplating and electroless plating are industrially important technologies [30]. Chemical silvering, an application of the silver mirror reaction, has long been used to produce mirrors and Dewar bottles [31]. Recently, Yogev and coworkers reported a new type of thin silver film, a silver metal liquid-like film (MELLF), which forms at the interface between water and a halocarbon solvent [32]. The MELLF was produced by the chemical reduction of ammoniacal AgNO~ with hydrazine sulphate in the presence of anisic acid and a suitable surfactant. Although the UV-visible reflectance spectrum of the MELLF resembled that of vacuum-deposited silver films, the electrical conductance was rather small. Gordon et al. obtained mobile silver interlayers by simply mixing an aqueous silver sol with a solution of metal complex in a halocarbon solvent [33]. Zhao and Fendler succeeded in fabricating two-dimensional silver particulate films by electrochemical reduction at the surface of the monolayer at the air water interface [34]. Anionic amphiphilic compounds such as arachidic acid and dialkyl polymerizable phosphate surfactants were used for monolayer formation on the subphase of AgNO3 solution. The silver film looked like a mirror and the resistivity was fairly small. The procedures mentioned in this paragraph seem to bear a resemblance to the spinning, dipping and draining methods of metal films deposition [29]. Mazur et al. described a process for electrochemical reduction of metal ions in polyimide films [35]. Zero-valent metals (silver, gold, copper) were deposited as an interlayer, a thin electrically conductive and optically reflective layer embedded within polymer films (P = polymer): e (electrode) + P ~

P'-

(7)

0

'

6;

'

Irradiation

1:20

'

AgBF4 + P"

~Ago + BF 4 + P

I;0

Ii rne ( r n i n )

Fig. 15. The change in electrical sheet resistance R of the irradiated surface of silver pectate (curve A) and silver carboxymethylcellulose (curve B) films on quartz plates with irradiation time. R is given per square centimetre.

Although the silver film facing the electrolyte solution looked mirror smooth, that in contact with the electrode was granular. The reflectivity and electrical conductivity of the silver mirror were fairly large. Annealing at 220-420 °C caused sintering of the silver

118

Y. Yonezawa et al. / Formation of Ag films by photolysis

I

~a)

I

3 pm

I

(c)

I

,I

4.3 ~ m

|

313m

(b)

I

I

~d~

I la m

Fig. 16. FE scanning electron micrographs of (a), (b) silver pectate and (c), (d) silver carboxymethylcellulosefilms on the glass plate: (a), (c) before irradiation; (b), (d) after 24 h of irradiation. A thin layer of platinum was deposited on the surface of the films by sputtering.

metal interlayers. I f metal ions and reducing agents are made to diffuse into the polyimide films along opposite directions, chemical reduction occurs, giving rise to metal interlayers. Manring and coworkers reported a "countercurrent diffusion" of AgBF 4 and NaBF4 in the electroless deposition of silver interlayers [36]. The reflectivity of both sides and the electrical conductivity were very large. A similar method was extended by

K u r o k a w a to the preparatio n of several polymer films (cellulose, cellulose acetate, polyvinlyalcohol) incorporating silver interlayer or colloidal particles [37]. Barraud and coworkers deposited monolayers of behenic acid on CaF 2 plates by the L a n g m u i r - B l o d g e t t (LB) technique [38]. Immersion of the films into A g N O 3 solution formed LB films of silver behenate by ion exchange. Silver clusters and colloids were

Y. Yonezawa et al. / Formation of Ag films by photolysis

41'

'

i

i

i

i

i

i

E

C

2 ¢11

._o O.

O

0

i

I

300

i

I

i

I

400 500 Wavelengt h (n m )

i

I

600

I

7IX)

Fig. 17. Absorption spectra of silver alginate films on quartz plates before and after irradiation. The films were irradiated with a 200 W low pressure mercury lamp under evacuated conditions at room temperature. The irradiation times were as follows: spectrum A, 0 min; spectrum B, 6 min; spectrum C, 15 min; spectrum D, 30 min; spectrum E, 60 rain.

1

f

I

i

I

[

I

I

>.

"~

a b

'

0.

,

300

5 Wavelength (rim)

'

6

700

Fig. 18. Absorption spectra ( - - ) of silver alginate films on quartz plates before and after irradiation. The films were irradiated with a 200 W low pressure mercury lamp under evacuated conditions at liquid nitrogen temperature (77 K). The irradiation times were as follows: spectrum A, 0min; spectrum B, 60min; spectrum C, 180 min. - , absorption spectra measured intermittently as the film was gradually warmed from 77 K to room temperature (a ~ e) under evacuated conditions in the dark.

119

circuits on solid substrates [42]. Thermography based on the chemical reduction of silver salts of organic acids, e.g. the dry silver process (3M Company), has been studied [43]. Mazur et al. proposed a write-once, high density optical information storage system based on sintering silver interlayers in polyimide films [35]. Enhanced spectroscopy and photochemistry of small metal particles have attracted considerable attention [44,45]. Surface-enhanced Raman scattering (SERS) and IR absorption with a silver overlayer have been applied for new high sensitivity surface probes of industrial materials [46, 47]. We have started working on Raman scattering from the SA films and obtained preliminary results which show photolytic colloidal silver at the film surface induces SERS, giving rise to strong Raman signals of the symmetrical stretching vibration of the RCOO- group adsorbed on the colloidal silver (1390 cm-J) [48]. Raman scattering studies of the liquid-like silver interlayers have been carried out by Yogev and Efrima and coworkers [32] and by Gordon et al. [33]. In the field of non-linear optics, a great deal of interest has arisen in metal colloid suspensions in liquid and solid media. These suspensions hold promise for applications involving high optical non-linearities [49]. The chemical reactions and catalytic activity of small metal particles have been the subjects of current research by chemists and chemical engineers. Recently, Henglein and coworkers have carried out extensive studies on radiation chemistry of colloidal metal particles. In particular, catalytic reactions of organic free radicals at the colloidal silver-aqueous solution interface have been studied [50]. We have started such a line of research into the photochemistry of colloidal silver particles and suggested a novel "photo-acetone method" available for preparation of small colloidal silver and gold particles [51].

5. Conclusions

generated by exposure of the LB films to hydrazine vapour. Sakai et al. obtained Ag-Nafion composite films by the ion exchange-chemical reduction method [391. The vacuum codeposition of silver vapour and rare gas onto a cooled substrate has become a standard technique for spectroscopic studies of silver clusters [27, 28, 40]. Andrews and Ozin deposited silver particles in high vacuum conditions with very low vapour pressure liquids at 250-260 K [41]. Matrix-supported silver films seem to have many future applications. Surface metallization of thin films has been attempted for circuit pattern formation and imaging materials. Tabei et al. proposed a "photochemical circuit process" for the formation of printed

We have studied the fabrication of silver films by photolysis of high molecular weight silver compounds. Details of photochemical reactions and film characterization have been given. A special feature of our method is the photoreduction of metal ions under fairly mild conditions (room temperature, wet air, low intensity lamp and low toxicity compounds). In this method, UV light from a low pressure mercury lamp is used for the production of silver films. Silver atoms in various aggregation states (atoms, clusters, colloids, deposits, bulk metal) can be produced and stabilized in the form of polymer-matrix-supported metal films under controlled reaction conditions: matrix-supported clusters, silver-polymer composites, surface-metallized films and

120

Y. Yonezawa et al. / Formation of Ag ,films by photolysis

silver mirrors. The photolysis conditions can easily be controlled by changing light intensity, wavelength, irradiation time, temperature, humidity, Ag + content, polymer and film thickness. The polymer matrix plays the role not only of the support for obtaining photolytic silver, but also as the reagent for photolysis. The irradiation time necessary for the appearance of metallic lustre and silver mirror formation depends on the kinds of high molecular weight carboxylic acids used, as well as on other reaction conditions. The predominance of SA, SP and SCMC films over SPA and SPM films is at least in part due to the presence of the six-membered ring having one carboxyl group per monomer unit. We have found many common features between our work and that reported by other groups regarding the preparation and characterization of silver films despite the diverse fabrication techniques. Hence, silver salts of high molecular weight carboxylic acids can be regarded as interesting starting materials for matrix-supported metal films; these have attracted considerable attention from a technological viewpoint because of many unique properties different from those of bulk metals.

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