Catalyst Characterization Science - American Chemical Society

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Surface Spectroscopy of Platinum-Cadmium SulfidePerfluorosulfonate Polymer Systems N. Kakuta, J. M. White, A. Campion, A. J. Bard, M. A. Fox, S. E. Webber, and M. Finlayson Department of Chemistry, The University of Texas—Austin, Austin, TX 78712 Films on Pt/CdS/Nafion have been constructed for use i n investigations of photoassisted H2 production from H2O. These films were characterized using scanning electron microscopy, x-ray photoelectron spectroscopy and x-ray diffraction. Induction periods, observed in the photoassisted H2 production, correlate with the extent of reduction of partially oxidized Pt to Pt° as measured by the Pt(4f) binding energy changes. Two crystal forms of CdS (cubic and hexagonal) were detected by x-ray d i f f raction. The scanning electron micrographs show that samples dominated by the cubic form have a very rough surface compared to the hexagonal form. The S(2p) binding energies indicate that the surface of the hexagonal form is rich in sulfate ion, whereas the surface of the cubic form is dominated by sulfide ion. Over the past several years, interest in photoassisted reactions involving semiconductors has expanded (1). Among the interesting relatively small bandgap materials is CdS (bandgap=2.4 eV). In operating systems, CdS has been stabilized with respect to photodecomposition by the addition of sacrificial reagents (2). Recently, the cleavage of water in a colloidal CdS/Pt/RuO^ system has been reported (3) and has led to increased interest in this system (4). While these colloidal systems are very interesting, another approach with considerable promise involves localizing the photoactive species and other catalytic components in a polymer membrane. This has recently been accomplished for CdS in polyurethane (5) and Nafion (6). This kind of system lends itself to characterization using surface analytical techniques while at the same time preserving many of the small particle features of colloids. In this paper we examine how the CdS crystal structure and the state of the Pt in a CdS/Pt/Nafion system influences the rate of hydrogen generation in a system containing a sacrificial electron donor, S^-. A detailed account of the photoassisted H2 production rates is given elsewhere (6b). NOTE: Nafion is Du Pont's brand name for perfluorosulfonate polymers. 0097-6156/ 85/0288-O566$06.00/0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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KAKUTA ET AL.

Surface Spectroscopy of Pt-CdS-Perfluorosulfonate Polymer

Experimental P l a t i n u m was added t o N a f i o n b e f o r e i n c o r p o r a t i n g CdS i n o r d e r t o a v o i d the r e d u c t i o n of CdS d u r i n g the p l a t i n i z a t i o n p r o c e s s . Nafion (DuPont 117, 0.018 cm t h i c k ) f i l m s were soaked i n P t ( N H ) I (0.1 mM) s o l u t i o n f o r 4 h r . The amount o f the P t complex i n c o r p o r a t e d was determined by measuring the o p t i c a l a b s o r p t i o n change i n the l i q u i d phase. The f i l m s were s u b s e q u e n t l y reduced w i t h NaBH^ (0.1 M) s o l u t i o n f o r one day t o produce P t m e t a l d i s p e r s e d throughout the polymer f i l m . The amount o f P t was found t o be about 0.02 mg cm 2. CdS was i n c o r p o r a t e d i n t o these p l a t i n i z e d f i l m s u s i n g two methods. I n the f i r s t , the f i l m was soaked i n a 0ά(Ν0β)2 (0.5 M) s o l u t i o n f o r 1 h r , washed w i t h w a t e r , soaked i n b o i l i n g HNO3 (0.03 M, pH=2.1) s o l u t i o n , t r e a t e d w i t h b u b b l i n g H^S f o r 10 min and f i n a l l y washed w i t h b o i l i n g w a t e r . The r e s u l t i n g f i l m s were b r i g h t orange. I n the second method, the p r e p a r a t i o n d i f f e r e d o n l y i n the H S treatment w h i c h was c a r r i e d out a t room temperature i n d i s t i l l e d w a t e r . I n b o t h cases the amount of CdS i n c o r p o r a t e d was determined by p o l a r o g r a p h i c r e d u c t i o n of Cd^+ t o be 1.3 mg cm~2. The p h o t o d r i v e n p r o d u c t i o n of hydrogen was c a r r i e d out u s i n g 1 cm χ 1 cm f i l m s i n a 10 ml P y r e x t e s t tube c o n t a i n i n g 3 ml of 0.1 M N a S s o l u t i o n (pH=13). P r i o r t o i r r a d i a t i o n , these s o l u t i o n s were degassed by b u b b l i n g N t h r o u g h them. I r r a d i a t i o n was"done w i t h a 1 kW Xenon lamp. The l i g h t was f i l t e r e d through w a t e r t o remove most of the IR r a d i a t i o n . The amount of hydrogen formed was measured w i t h a gas chromatograph. The f i l m s were c h a r a c t e r i z e d u s i n g x - r a y powder d i f f r a c t i o n (XRD), x - r a y p h o t o e l e c t r o n s p e c t r o s c o p y (XPS) and s c a n n i n g e l e c t r o n m i c r o s c o p y (SEM). The p h o t o e l e c t r o n s p e c t r o s c o p y u t i l i z e d a Vacuum G e n e r a t o r s ESCA Lab I I system w i t h Mg(Kot) r a d i a t i o n . B i n d i n g e n e r g i e s (BE) were measured w i t h r e s p e c t t o the s u r f a c e C ( l s ) peak (284.5 eV) w h i c h was always p r e s e n t i n these f i l m s . Scanning e l e c t r o n m i c r o s c o p y was done w i t h a JEOL JSM-35C system.

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3

2

2

-

2

2

2

R e s u l t s and

Discussion

Pt/CdS/Nafion. F i g u r e 1 shows the time dependence of the e v o l u t i o n o f hydrogen f o r the i r r a d i a t e d f i l m s . The f i r s t p r e p a r a t i o n method g i v e s the h e x a g o n a l form ( 7 ) . I n b o t h c a s e s x - r a y l i n e w i d t h a n a l y s i s i n d i c a t e s p a r t i c l e s i z e s o f 20 nm. The H g e n e r a t i o n r a t e i s a t l e a s t t h r e e times f a s t e r f o r the c u b i c form than f o r the hexagon a l form. G e n e r a l l y s p e a k i n g , the XRD and SEM a n a l y s i s i n d i c a t e s s m a l l p a r t i c l e s o f CdS and P t i n i n t i m a t e c o n t a c t . F i g u r e 1 a l s o shows an i n d u c t i o n p e r i o d d u r i n g w h i c h the r a t e of H p r o d u c t i o n a c c e l e r a t e s . A l t h o u g h t h i s e f f e c t i s more p r o nounced f o r the hexagonal CdS sample, i t i s p r e s e n t i n b o t h . Further r e s u l t s , summarized i n F i g . 2, emphasize t h i s i n d u c t i o n p e r i o d . The curves l a b e l l e d (1) and (2) i n v o l v e d r e p e a t e d use o f the same f i l m . Between runs (1) and (2) the f i l m was washed i n b o i l i n g w a t e r . C l e a r l y the r a t e of H e v o l u t i o n was s i g n i f i c a n t l y f a s t e r i n the second r u n . Repeated e x p e r i m e n t s gave d a t a s u p e r i m p o s a b l e on c u r v e (2). 2

2

2

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

567


568

CATALYST CHARACTERIZATION SCIENCE

Time

F i g u r e 1. Comparison o f H g e n e r a t i o n from p l a t i n i z e d h e x a g o n a l CdS ( c u r v e A) and c u b i c CdS ( c u r v e B) i n N a f i o n 117. The i n s e r t shows x - r a y powder d i f f r a c t i o n s p e c t r a o f CdS i n t h e s e f i l m s . 2



46.

KAKUTA ET AL.


0

20

40

60

80

1 0 0 (min)

Time 2

F i g u r e 2. Time dependence o f hydrogen p r o d u c t i o n i n a 1 cm N a f i o n f i l m c o n t a i n i n g CdS (1.3 mg c n r ) and P t (0.02 mg c m " ) . Curves (1) and (2) a r e r e p e a t e d runs w i t h t h e same f i l m . The dashed c u r v e i s f o r a d i f f e r e n t f i l m p r e t r e a t m e n t . See t e x t f o r details. 2


2

569

570


I n a n o t h e r e x p e r i m e n t , a new f i l m was p r e t r e a t e d by b u b b l i n g H2 a t 1 atm and 300 Κ t h r o u g h the s o l u t i o n f o r 0.5 h r p r i o r t o the N treatment d e s c r i b e d above. The a c t i v i t y of t h i s f i l m i s shown i n F i g . 2, dashed c u r v e . I t i s c l o s e r t o (2) t h a n ( 1 ) , and the i n d u c t i o n time i s r e l a t i v e l y s h o r t . T h i s i n d i c a t e s t h a t , d u r i n g the i n i t i a l s t a g e s of i r r a d i a t i o n w i t h a f r e s h f i l m , the a c t i v i t y i s enhanced by some p r o c e s s i n v o l v i n g hydrogen. S i m i l a r r e s u l t s have r e c e n t l y been r e p o r t e d by Aspnes and H e l l e r ( 8 ) . They proposed an a u t o c a t a l y t i c model f o r p h o t o a c t i v e systems i n v o l v i n g metal/compound semiconductor i n t e r f a c e s . To e x p l a i n i n d u c t i o n times i n CdS systems ( 9 ) , they suggest t h a t h y d r o gen i n c o r p o r a t e d i n the s o l i d lowers the b a r r i e r t o charge t r a n s f e r a c r o s s the i n t e r f a c e and t h e r e b y a c c e l e r a t e s H p r o d u c t i o n r a t e s . W h i l e our r e s u l t s f o l l o w the same t r e n d and may i n v o l v e s i m i l a r e f f e c t s , s u r f a c e a n a l y s i s u s i n g XPS i n d i c a t e s t h a t the o x i d a t i o n s t a t e of P t i s a l s o s i g n i f i c a n t . The P t ( 4 f ) b i n d i n g energy was 72.1 eV on f r e s h f i l m s . A f t e r i r r a d i a t i o n i n the manner d e s c r i b e d above, the BE dropped t o 71.3 eV. For a P t f o i l , the measured BE was 71.2 eV. We c o n c l u d e t h a t whole P t was p a r t i a l l y o x i d i z e d i n the f r e s h f i l m s and t h a t d u r i n g the e a r l y s t a g e s of hydrogen p r o d u c t i o n , i t was reduced t o Pt°. T h i s change i n o x i d a t i o n s t a t e i s , we b e l i e v e , r e l a t e d t o the change i n H p r o d u c t i o n a c t i v i t y . To i n v e s t i g a t e t h i s f u r t h e r , we probed the s t a t e of P t b e f o r e CdS i n c o r p o r a t i o n i n t o the f i l m . A f t e r P t i n c o r p o r a t i o n and r e d u c t i o n w i t h NaBH4, the P t ( 4 f ) BE was 71.1 eV, i n d i c a t i n g t h a t P t complexes were c o m p l e t e l y reduced t o Pt°. P t i n f i l m s w i t h o u t CdS can be o x i d i z e d by b o i l i n g i n n i t r i c a c i d as i n d i c a t e d by the f i l m s becoming c l e a r . Thus, the o x i d i z e d P t s p e c i e s p r e s e n t i n f r e s h C d S / P t / N a f i o n f i l m s i s formed a t some p o i n t d u r i n g the CdS i n c o r p o r a t i o n process. Thus, w h i l e not e x c l u d i n g the p r o c e s s proposed by Aspnes and H e l l e r ( 8 ) , we c o n s i d e r the r e d u c t i o n o f p a r t i a l l y o x i d i z e d P t as r e s p o n s i b l e , i n p a r t , f o r the observed i n d u c t i o n p e r i o d (10).


2

2

2

2 +

CdS/Nafion. Scanning e l e c t r o n m i c r o s c o p y (SEM) of the CdS f i l m s prepared by b o t h methods r e v e a l e d a s t r i k i n g d i f f e r e n c e i n the s u r f a c e roughness of c u b i c and hexagonal CdS i n N a f i o n ( F i g . 3 ) . E x a m i n a t i o n of a c r o s s - s e c t i o n e d f i l m by SEM d i d not show any mor p h o l o g i c a l d i f f e r e n c e s i n the two c r y s t a l forms i n the i n t e r i o r o f the N a f i o n . X-ray f l u o r e s c e n c e demonstrated t h a t Cd was d i s t r i b u t e d throughout the N a f i o n , w i t h a s l i g h t l y h i g h e r c o n c e n t r a t i o n n e a r the outer surface ( F i g . 3). The XPS o f the Cd(3d) r e g i o n shows ( F i g . 4) t h a t the c u b i c CdS f i l m has a C d ( 3 d / 2 ) BE of 405.6 eV, w h i l e the h e x a g o n a l CdS f i l m has a peak a t 406.4 eV. A f t e r A r bombardment f o r a s h o r t time of the h e x a g o n a l CdS f i l m , the peak p o s i t i o n does not s h i f t but the integrated intensity increases. F i g u r e 5 shows XPS f o r the S(2p) r e g i o n . The c u b i c CdS f i l m has peaks a t 161.7 and 168.9 eV. Hexagonal CdS has o n l y one peak a t 169.6 eV w h i c h a f t e r A r bombardment d e c r e a s e s i n i n t e n s i t y and broadens. Based on a s y s t e m a t i c comparison w i t h b i n d i n g e n e r g i e s r e p o r t e d on a v a r i e t y o f cadmium and s u l f u r compounds ( 1 1 ) , we a s s i g n the C d ( 3 d / 2 ) 405.6 eV peak t o Cd + i o n of CdS and the S(2p) 161.7 eV peak t o S ~ i o n of CdS. 5

+

+

2

5

2



KAKUTA ET AL.


F i g u r e 3. SEM photographs of h e x a g o n a l CdS/Nafion and c u b i c CdS/Nafion.


572


405.6

Cd(3d)


/l

406.5

U

J1VJ 1

1

402

1

405

1

408

411

414

417

(eV)

Binding energy F i g u r e 4. Cd(3d) XPS s p e c t r a o f hexagonal CdS/Nafion (A) and c u b i c CdS/Nafion (B) f i l m s .

S(2p)

169.6 I A

/

'

168.9

\

/l61.7\ 1

160

1

163

Β

I .-1

J

ι

166

169

172

175

(eV)

Binding energy F i g u r e 5. S(2p) XPS s p e c t r a o f hexagonal CdS/Nafion (A) and c u b i c CdS/Nafion (B) f i l m s .


46.

KAKUTA ET AL.


To e l u c i d a t e the h i g h e r BE peaks [S(2p) 168.9, 169.6 e V ] , we examined the N a f i o n f i l m i t s e l f because i t has -SO3" c a t i o n exchange s i t e s . Only C ( l s ) and F ( I s ) peaks were o b s e r v e d . No S(2pJ peak was observed even a f t e r A r bombardment. T h i s r e s u l t s u g g e s t s t h a t the s u r f a c e c o n c e n t r a t i o n o f c a t i o n exchange s i t e s i n v o l v i n g -SO3 i s low. S u l f u r l o c a t e d w e l l below the s u r f a c e w o u l d , of c o u r s e , hot be de t e c t e d by XPS. We a s s i g n the h i g h e r BE S(2p) peak t o a s u r f a c e s u l f a t e species (11). The s u r f a c e a t o m i c r a t i o s (Cd/F, S/F and C ( n a f i o n ) / F ) c a l c u l a t e d from the XPS peak a r e a s a r e l i s t e d i n T a b l e I . The r e s u l t s i n d i c a t e t h a t the cubic-CdS f i l m has a h i g h e r s u r f a c e c o n c e n t r a t i o n o f CdS t h a n t h e h e x a g o n a l f i l m . I n b o t h , the C ( l s ) / F ( l s ) r a t i o i s s i m i l a r to Nafion i t s e l f . The h i g h e r BE f o r the Cd(3d) peak f o r the f i l m c o n t a i n i n g hex a g o n a l CdS can be u n d e r s t o o d i n terms of s u r f a c e c h a r g i n g o f the sample under x - r a y i r r a d i a t i o n . S i n c e the s u r f a c e o f the h e x a g o n a l sample i s not as r i c h i n CdS as the c u b i c sample, we e x p e c t the N a f i o n t o p l a y a g r e a t e r r o l e i n the f o r m e r , and t h i s would l e a d t o a g r e a t s u r f a c e c h a r g e . S h i f t s t o h i g h e r BE and e x t e n s i v e broaden i n g would then be e x p e c t e d , as observed. T h i s c o n c l u s i o n based on XPS i s c o n s i s t e n t w i t h the SEM d a t a . We c o n c l u d e t h a t the s u r f a c e d e n s i t y o f CdS i s h i g h e r f o r the c u b i c CdS and t h a t the h e x a g o n a l CdS f i l m s u r f a c e c o n t a i n s a l a r g e amount o f s u l f a t e i o n . One can s p e c u l a t e t h a t the p r e s e n c e o f s u l f a t e may c o n t r i b u t e t o t h e lower a c t i v i t y o f the h e x a g o n a l form f o r the p h o t o a s s i s t e d p r o d u c t i o n o f hydrogen.


+

Table I .

XPS Peak R a t i o s

(a) Cd(3d

5 / 2

)/F(ls)

a-CdS

β-CdS

0.04

8.4

S(2p)/F(ls)

0.02

C(ls)/F(ls)

0.1

(a)

( b )

1.6 0.1

standard. N a f i o n F ( I s ) peak was

^ T h i s peak was Acknowledgments

used as

assigned to

' species. SO " 2

We acknowledge f i n a n c i a l s u p p o r t of t h i s work by the Gas R e s e a r c h I n s t i t u t e ( C o n t r a c t No. 5982-260-0756). NSF support o f t h e x - r a y p h o t o e l e c t r o n s p e c t r o m e t e r by an equipment g r a n t , CHE-8201179, i s a l s o acknowledged.

Literature Cited 1.a. Sato, S.; White, J. M.; Chem. Phys. Lett. 1980, 72, 83. b. Kawai, T.; Sakata, T.; Chem. Phys. Lett. 1980, 72, 87. c. Dunn, W. W.; Bard, A. J.; Nov. J. Chim. 1981, 5, 651.


573



574

2.a. E l l i s , A.B.; Kaiser, S.W.; Wrighton, M.W. J. Am. Chem. Soc. 1976, 98, 6885. b. Inoue, I.; Watanabe, T.; Fugishima, Α.; Honda, K. Bull. Chem. Soc. Jpn. 1979, 52, 1243. 3. Kalyanasundaram, K.; Borgarello, E.; Gratzel, M. Helv. Chim. Acta 1981, 64, 632. 4.a. Matsumara, M.; Sato, Y.; Tsubomura, H. J. Phys. Chem. 1983, 87, 3897. b. Kuczynski, J. P.; Milosavijevic, Β. H.; Thomas, J. K. J. Phys. Chem. 1983, 87, 3368. c. Aruga, T.; Domen, K.; Naito, S.; Onishi, T.; Tamaru, K. Chem. Lett. 1983, 1037. 5. Meissner, D.; Memming, R.; Kastening, B. Chem. Phys. Lett. 1983, 96, 34. 6.a. Krishnan, M.; White, J. R.; Fox, Μ. Α.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 7002. b. Mau, A. W.-H.; Huang, C.-B.; Kakuta, N.; Bard, A. J.; Campion, Α.; Fox, Μ. Α.; White, J. M.; Webber, S. E. J. Am. Chem. Soc. (in press). c. Kuczynski, J. P.; Milosavijevic, Β. H.; Thomas, J. K. J. Phys. Chem. 1984, 88, 980. 7. Powder Diffraction File, ASTM, 1977. 8. Aspnes, D. E.; Heller, A. J. Phys. Chem.1983, 87, 4919. 9. Borgarello, E.; Kalyanasundaram, K.; Gratzel, M.; Pelizzetti, E. Helv. Chim. Acta 1982, 65, 243. 10. Meyer, M.; Wallberg, C.; Kurihara, K.; Fendler, J. H. J. Chem. Soc., Chem. Commun. 1984, 90. 11.a. Outka, D. Α.; Madix, R. J. Surface Sci. 1984, 137, 242. b. Furuyama, M.; Kishi, K.; Ikeda, S. J. Electron. Spectrosc. Relat. Phenom. 1978, 13, 59. RECEIVED August 6, 1985


Catalyst Characterization Science - American Chemical Society

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