11 E l e c t r o c h e m i s t r y of S i l a n e - D e r i v a t i z e d I r i d i u m
C. A. LUNDGREN and C. E. RICE
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Bell Laboratories, Holmdel, NJ 07733
The properties of silane-derivatized iridium and anodic iridium oxide (AIROF) electrodes have been studied by cyclic voltammetry in tetraethylammonium perchlorate/acetonitrile solutions. Both electrodes react with silanes such as dichlorosilylferrocene (DCSF) to give persistently bonded silylferrocene monolayers based on geometric area. This contrasts with the behavior of anodized platinum (Pt/PtO), which gives considerable polymerization with DCSF, resulting in layers of variable and unpredictable thickness. The electrochemical behavior of these three derivatized electrodes, and the information this provides about the nature of their oxide films are discussed. In the course of our studies of electrochromic iridium oxide i n nonaqueous electrolytes (1,2^,3^ ), we recognized that i t has unique properties which might make i t an interesting and useful electrode for the study of the electrochemistry of surface-bound molecules. This e l e c t r o n i c a l l y conducting oxide can be grown anodically i n films of accurately known thickness (4). I t i s a hydrous oxide, with ample acidic protons available for reaction with chloro- and alkoxysilanes. Yet i t i s quite electrochemically inert i n nonaqueous electrolytes lacking small ions. In addition, we anticipated that the reaction of anodic iridium oxide with silanes might provide useful information about the surface chemistry of this unusual material, which i s not only a good electrochromic but also an excellent electrocatalyst for oxygen evolution (5). Subsequently we found that iridium reacts easily with silanes even without preanodization, giving persistent films of monolayer coverage with reproducible electrochemical properties (6). By cont r a s t , the more widely studied Pt/PtO electrode requires a lengthy anodization pretreatment before derivatization, and gives films of variable and unpredictable coverage Ç7,J3,9).
0097-6156/82/0192-0197 $6.00/0 © 1982 American Chemical Society Miller; Chemically Modified Surfaces in Catalysis and Electrocatalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
CHEMICALLY MODIFIED SURFACES
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In a previous communication, we gave a detailed description of the electrochemical properties of s i l y l f errocene on iridium (J>). In this paper we b r i e f l y review this work, and compare dérivatized iridium, Pt/PtO, and anodic iridium oxide electrodes. We show that the results of derivatization can give useful and sometimes unexpected information about the nature of the oxides on these electrodes. Experimental
Downloaded by FUDAN UNIV on April 11, 2017 | http://pubs.acs.org Publication Date: July 2, 1982 | doi: 10.1021/bk-1982-0192.ch011
H
sicl
D i c h l o r o s i l y l f e r r o c e n e ( F e C i Q 8 2 ) (DCSF) was synthesized by a published procedure (10). Other silanes (as described below) were used as received from Petrarch Chemicals. The synthesis and derivatizations were performed i n a dry box under a nitrogen atmosphere. Electrodes were made of 0.25 mm. Pt and I r sheet, and each had a t o t a l geometric area of 0.8 cm^. Electrode cleaning procedures are detailed elsewhere (6). Pt was anodized on 0.5 M H S0, by potential cycling between the hydrogen and oxygen evolution potentials u n t i l the c y c l i c voltammogram was constant («2-3 hours) and then held at +1.1V versus SCE u n t i l the current decayed to a small value (11). Anodic iridium oxide f i l m (AIROF) electrodes with thicknesses from 10 to 135 nnw were grown by potential c y c l ing of I r i n 0.5 M H S0 from -0.25 to +1.25 V versus SCE (4) ; growth times f o r the thickest films were 10-15 minutes. The prepared I r , Pt/PtO, and AIROF electrodes were kept under vacuum for several hours at room temperature to eliminate surface water. They were then reacted with 0.01 M silane solutions i n toluene or ether f o r times varying from 1-27 hours at room temperature. Cyclic voltammetric studies were performed using a three electrode c e l l consisting of the derivatized working electrode, a platinum counter electrode, and a Ag/Ag nonaqueous reference electrode (+0.182+0.002 V versus SCE) (6). Tetraethylammonium Perchlorate (TEAP) i n a c e t o n i t r i l e was used as the electrolyte throughout. Care was taken f o r the s t r i c t exclusion of water and oxygen during solution preparation and electrochemical measurements • 2
2
4
+
Results and Discussion The c y c l i c voltammograms of underivatized I r and Pt/PtO electrodes i n 0.2 M TEAP/acetonitrile are shown i n Figure 1. A comparison of the two electrodes shows that I r has a 200 mv. wider potential window between the points where electrolyte breakdown occurs, and 30 percent less residual (capacitive) current than Pt/PtO. The c y c l i c voltammogram of an underivatized AIROF electrode i s shown i n Figure 2. I t d i f f e r s from "clean" iridium i n having a s l i g h t l y smaller potential window and considerably greater (and thickness dependent) capacitive current (for example, a 125 nm. f i l m had approximately s i x times more residual current than bare I r ) .
Miller; Chemically Modified Surfaces in Catalysis and Electrocatalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
11.
LUNDGREN
AND
SUane-Derivatized Iridium
RICE
199
t
Downloaded by FUDAN UNIV on April 11, 2017 | http://pubs.acs.org Publication Date: July 2, 1982 | doi: 10.1021/bk-1982-0192.ch011
10 ft a
I
-10
1
-.5
I
I
0 +.5 POTENTIAMvs Ag)
I
1.0
Figure 1. Comparison of residual current for underivatized Pt/PtO and Ir electrodes having the same area. Scan speed 0.05 V/s.
-0.4 V
+0.5V POTENTIAL (Vvs Ag)
Figure 2. Cyclic voltammograms of underivatized AIROF electrode, scan speed 0.05 V/s (solid line); silylferrocene-derivatized AIROF, scan speeds 0.05 V/s ( ) and 0.2 V/s ( AIROF thickness 120 nm.
Miller; Chemically Modified Surfaces in Catalysis and Electrocatalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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CHEMICALLY MODIFIED SURFACES
Halo- and alkoxysilanes react with acidic protons on oxide surfaces according to reaction (1), R SiX + a-OH -> HX + a-0-SiR
Downloaded by FUDAN UNIV on April 11, 2017 | http://pubs.acs.org Publication Date: July 2, 1982 | doi: 10.1021/bk-1982-0192.ch011
3
3
(1)
where X i s a halide or alkoxide and a represents the oxide surface Silanes with a single X group can only form monolayer films of surface-bound molecules, while silanes with two or three Xs can polymerize to form thicker films i f water i s present i n the reaction solution or within the oxide i t s e l f . Both I r and anodized Pt (Pt/PtO) are reactive toward silanes. However, the extent of reaction i s different i n the two cases i n ways that give information about the nature of the oxide films on these electrodes. For example, at reaction times greater than about four hours, iridium always reacts with the same amount of dichlorosilylferrocene. Integration of the ferrocene-fericinium redox wave i n the c y c l i c voltammogram (Figure 3) gives a coverage approximately what one would expect for a closely packed monolayer (S-yxlO-^^mol/cm geometric area). This must mean that the native oxide on I r i s rather "dry", with a uniformly hydroxylated surface. By contrast, Pt/PtO always gives greater than monolayer coverage when reacted with DCSF (see Figure 3), and this coverage varies rather unpredictably from electrode to electrode. Reported coverage ranged from 4-280xl0~ mol/cnr (10). Thus, the anodic oxide on platinum must contain adsorbed water; i t i s also p l a i n that the amount of oxide formed by the anodization process i s not very reproducible. Many electrochemical parameters (such as EQ of the ferrocene-fericinium couple which ranged from +0.47 to +0.55 V versus SCE for E C8)), the width at half height of the redox peak, A E (the difference between anodic and cathodic peak potentials, which ranged from 30 to 80 mv. (8)) of silylferrocene-derivatized Pt/PtO also show a f a i r amount of v a r i a b i l i t y , again, we think, due to variations i n oxide thickness and water content. While the electrochemical properties of silylferrocene on iridium deviate from those expected for an ideal surface-attached electroactive species ( E Q ^ ^ ranged from+0.329 to +0.353 V versus Ag/Ag+; AE ranged from 48 to 87 mv. (
