J . Phys. Chem. 1993,97, 2646-2648
2646
Electrode Modification by the Sol-Gel Method Ondrej Dvorak’ and M. Keith De Armond’ Chemistry Department, New Mexico State University, Las Cruces, New Mexico 88003 Received: April 6, 1992
Silica gel films prepared by the sol-gel route were used to modify platinum and transparent indium oxide electrodes. The silicon dioxide layer reduces electron transfer to the electrode but, due to the porous structure of the silica gel, does not completely prevent it. The tris(2,2’-bipyridyl)ruthenium(II) cation is incorporated into the gel layer, and the cation retains electrochemical and photoelectrochemical activity. The comparison of electrochemical and photochemical data was used to evaluate attachment of the ruthenium complex to the silica gel matrix.
Electrochemistry was performed in a standard three-electrode arrangement with platinum auxiliary electrode and a silver Modified electrodes have been used in a wide range of chloride reference electrode with water as the solvent. A solution applications: chemical analysis’+2 and electrochemical catalysi~.~.~ of 0.01 M NaCl was used as the supporting electrolyte and To accomplish these purposes, an electrode can be modified by reference electrode solution. Both solutions were separated by four different techniques: (1) incorporationof modifying particles an asbestos fiber diaphragm. The redox agent (iodide anion, into the electrode body (e.g., into graphite paste), (2) direct methylviologen cation, [Ru(NH3)612+, or tris(2,2’-bipyridyl)attachment of modifying particles,5 (3) application of films from ruthenium) was added to the supporting electrolyte solution in modifying particles (e.g., conductive polymers), and (4) incor0.001 M concentration for the study of electron transfer across poration of modifying particles into the supporting matrix. the gel film. The diffusion coefficients were determined by a Particles in this context mean molecules, ions, enzymes, clusters, potentiostatic pulse method. The electrochemical analyzer BAS etc. lOOA (Bioanalytical Systems, Inc.) was used as a voltage source The sol-gel route6.’ offers a possibility of preparing ceramicand current acquisition device. like films under rather mild conditions that enable incorporation The photoelectrochemical experiments were performed in the of temperature-sensitive particles into such layers. Thus, interTeflon three-electrode cell described elsewhereZOwith one change esting particles embedded in very inert and stable matrices are where the I T 0 auxiliary electrode20 was replaced by a NiCr wire attainable. To our knowledge, this technique has been used in coil. The reference electrode was the same as for the electrophotochemistry,s-15in analytical chemistry,I6and in biotechnology chemistry experiments. A lock-in amplification technique was researchI7.l8 but not in electrochemistry. used to detect the small photocurrent. A 450-W Hg lamp was This paper describes some examples of electrode modification used as light source with the 435-nm region isolated by filters as by a sol-gel silicondioxidegel film doped with tris(2,2’-bipyridyl)previously described.21 The light intensity was monitored by a ruthenium(I1) chloride, which retains its electrochemical activity YSI-Kettering Model 65 radiometer. The experimental arand exhibits an anodic photoeffect. The comparison of photorangement for cell polarization and data acquisition was described chemical and electrochemical data is used to estimate the active el~ewhere.~~,~~ fraction of the encapsulated complex. The electron transfer The gel film absorption spectra were obtained on an on-line between a redox couple in solution and the silica gel film modified computer-modified Cary- 14 spectrophotometer; the emission electrode is also investigated. spectra were done with an LS-100 luminescence spectrometer from Photon Technology International. The slide was oriented Experimental Section parallel to the excitation beam with the covered side closer to the sample mirror. The silica gel films were prepared by hydrolysis and condenTetramethyl orthosilicate, Si(OCH3)499%,tris( 2,2’-bipyridyl)sation of tetramethyl orthosilicate (TMOS). First, 1 drop of ruthenium( 11) chloride hexahydrate, hexaammineruthenium(II1) surface agent Triton X10011q19 was dissolved in 1 mL of ethanol. chloride. Triton X 100 reduced, and methylviologen dichloride One drop of this solution was mixed with 4 drops of ethanol, 8 hydrate (1,l’-dimethyL4,4’-bipyridinium dichloride) were purdrops of water, and 1 drop of 0.01 M HCl solution in water.I9 chased from Aldrich, dehydrated ethyl alcohol (200 proof) was After careful mixing, 3 drops of TMOS were added and carefully purchased from Quantum Chemical Corp., US1 Division, hymixed. The resulting clear solution was aged for 30 min, and drochloric acid was purchased from Fisher Scientific Corp., then indium tin oxide (ITO) transparent electrodes (CG-80INsodium chloride was purchased from J.T. Baker, and sodium CUV from Delta Technologies) were covered by spin coating. The platinum disk electrode (MF-2013 from Bioanalytical iodide was purchased from EM Science. Deionized water was used for all experiments. Systems, Inc.) was covered by horizontal spreading. The gel film was dried in an air stream for several minutes and then dried overnight at 55 OC. For the doped film preparation, about 0.02 Results g of tris(2,2’-bipyridyl)ruthenium(II) chloride hexahydrate was Tris(2,2’-bipyridyl)ruthenium(II) cation is not affected by the dissolved in a water-ethanol mixture before adding the TMOS. gelation processes and retains photochemical14~19and electroThe I T 0 electrodes were washed with water and ethanol before chemical activity. No changes in the absorption spectra were covering by the gel film. The platinum electrode was cleaned by observed. However, the silica gel film is very thin; thus, the polishing with 1-pm diamond polish (Bioanalytical Systems, Inc.). maximum absorbance at 452 nm was -0.1. If we use the value of the extinction coefficient (14 600 L/(mol cm)) from water ’ On leave from the J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Dolejskova 3 182 23, Prague, Czechoslovakia. solution,24the concentration of the complex in the film is 7 X
Introduction
0022-3654/93/2091-2646%04.00/0 0 1993 American Chemical Society
Electrode Modification by the Sol-Gel Method
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2647
20
I
lo
1 . . I
I
5
15
I
I
500
600
h (nm)
700
Number of cycles
Figure I. Emission spectrum of tris(2,2’-bipyridyl)ruthenium(lI) chloride insilicagel film. Excitation wavelength: 453 nm. Thecurveisan average of 200 scans.
n
10
a a .-
Figure 3. Anodic voltammetric peak current dependence on the number of performed cycles. Experimental conditions and convention are the same as in Figure 2.
54
.-
0-
I
/
I
I
-5 -
I
0 I 1.5
1
1
I
I
E0
0.5
Figure 2. Cyclic voltammogram of I T 0 electrode modified by a silica gel layer with incorporated tris(2,2’-bipyridyl)ruthenium(lI) chloride. Supporting electrolyte: 0.01 M NaCl in water. Reference electrode: silver chloride in 0.01 M NaCI. Polarization rate: 50 mV/s. Electrode area: 1.75 cm2. The negative current is anodic.
TABLE I: Comparison of Cyclic Voltammetry Data of Tns( 2,2’-bipy~dyl)ruthenium(II) Cation Oxidation in Silica Gel Film on ITO, in Solution on Bare ITO,and in Solution on Bare Platinum’ E‘, mV Ec, mV AE, mV
Si02 gel film
IT0
Pt
1200
1 I65
1045
800 400
900 265
980 65
0 Ea and ECdenote the anodic and cathodic peak potentials, respectively. A E is the difference between peak potentials. All potentials are against the silver chloride electrode immersed in 0.01 M NaCI.
mollcmz. The gel film estimated thickness is less than 0.5 pm (from the interference pattern) and, thus, is much less precise than the complex content determination from absorbance and coulometry data. Consequently, the Ru complex concentration per film square unit is preferred. The maximum luminescence intensity is blue-shifted by 25 nm relative to the 607-nm water solution value.24 The emission spectra are given in Figure 1. The cyclic voltammogram of tris(2,2’-bipyridyl)ruthenium(11) cation modified silica gel film on I T 0 is shown in Figure 2. The characteristic potential parameters are summarized in Table I, together with values for tris(2,2’-bipyridyl)ruthenium(II) cation oxidation on a bare I T 0 electrode and bare platinum electrodes. Potential cycling leads to a decrease in the complex concentration in gel (Figure 3), but the steady state is established after a
I
-0.2
-0.4
I
Figure 4. Cyclic voltammograms of 0.001 M hexaammineruthenium(11) cation on bare Pt electrode (A) and on Pt electrode covered with silica gel film (B). Polarization rate: 100 mV/s. Electrode area: 2 mm2. Supporting electrolyte, reference electrode, and current convention are the same as in Figure 2.
reasonable time. Soaking of the gel in water without polarization has no influence on complex concentration in gel. The electrochemical behavior fits neither linear diffusion nor typical thin-layer behavior; thus, the voltammetry data are not sufficient for concentration determination due to the lack of adequate theory. The concentration of the ruthenium complex in silica gel film was determined by coulometry to be 2 X le9 mol/cm2 for fresh gel and about a quarter of that at the steady state. The doped silica gel film exhibits an anodic photoeffect under illumination with 435-nm light in the presence of the iodide anion in solution. The photocurrent monotonously increases with potential until the potential of iodide anion oxidation is reached. Further measurement is unreliable. The quantum yield, determined from monochromatic light intensity, gel absorbancy, and photocurrent density, is quite small (
