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J. Phys. Chem. B 1999, 103, 104-107
Diffusional Mediation of Surface Electron Transfer on TiO2 Scott A. Trammell and Thomas J. Meyer* Department of Chemistry, UniVersity of North Carolina at Chapel Hill, CB# 3290, Chapel Hill, North Carolina 27599-3290 ReceiVed: June 8, 1998; In Final Form: October 29, 1998
The results of a kinetic study are reported for cross-surface electron transfer in the reversible oxidation of [OsII(bpy)2(4,4′-(CO2H)2bpy)](PF6)2 to OsIII surface adsorbed to nanocrystalline TiO2 films on optically transparent ITO (tin-doped indium oxide) electrodes. The kinetics are sensitive to the extent of surface loading. From monolayers adsorbed from CH3CN, a percolation threshold of ∼60% was measured. Adsorption isotherms for [OsII(bpy)2(4,4′-(CO2H)2bpy)](PF6)2 in EtOH and CH3CN show that surface coverage from EtOH is ∼60% of that from CH3CN at similar adsorbate concentrations. Apparent charge-transfer diffusion coefficients in 0.1 M [N(n-C4H9)4](PF6) in CH3CN measured by chronoabsorptometry are Dapp ) 1.4 × 10-9 cm2/s (Γ ) 1.1 × 10-10 mol/cm2, CH3CN) and 1.4 × 10-11 cm2/s (Γ ) 7 × 10-11 mol/cm2, EtOH). Electron-transfer mediation of the oxidative component of the OsIII/II surface couple occurs in the presence of added Ru(bpy)32+ by stepping the potential past the Ru(bpy)33+/2+ wave. Mediation of both oxidative and reductive components occurs with added Os(bpy)32+. The apparent diffusion coefficient for oxidation of the OsIII/II surface couple in the presence of Ru(bpy)32+ increases linearly with the concentration of added mediator.
Introduction In a recent publication, Bonhoˆte et al. reported on the kinetics of cross-surface electron transfer for an organic triarylamine couple adsorbed to nanocrystalline TiO2 or ZrO2 or to Al2O3 on optically transparent fluorine-doped SnO2 electrodes.1 We report here the results of a related study based on the surface adsorbed OsII complex, [Os(bpy)2(4,4′-(CO2H)2bpy)](PF6)2 (1), and mediation of the surface couple by redox “carriers” in the external solution.
Experimental Section Materials. The solvents CH3CN (Baxter, B&J, high purity) and C2H5OH (AAPER Alcohol and Chemical Co, absolute) were used as received. [N(n-C4H9)4](PF6) (TBAH) (Aldrich, 98%) was recrystallized twice from ethanol. Indium oxide (ITO: In2O3, Sn) coated glass slides were purchased from Delta Tech. Ltd, Stillwater, MN. [Os(bpy)3](PF6)2 and [Ru(bpy)3](PF6)2 were available from previous studies. The procedure used for the preparation of [Os(bpy)2(4,4′-(CO2H)2bpy)](PF6)2 was reported previously.2 Electrode Preparation. The TiO2 colloid was prepared by a literature procedure.3,4 A portion of ITO was masked with Scotch tape, and the TiO2 colloid was spread with a glass rod. The resulting films were allowed to dry for 30 min, and the electrodes were cut into ∼9 mm wide pieces. The electrodes were heated to 400 °C for 30 min over O2, cooled, and placed in solutions (0.1-2) × 10-4 M in metal complex as PF6- salts in CH3CN or EtOH for 16 h. Film thicknesses of 3-4 µm were measured by profilometry on dry films. Measurements. Cyclic voltammetric experiments were conducted with a PAR 273 potentiostat by using a standard three-
electrode configuration in a three-compartment cell in 0.1 M TBAH in CH3CN. The reference electrode was Ag/AgNO3 (0.1 M TBAH and 0.01 M AgNO3 in CH3CN), which was 300 mV more positive than SCE as measured by the ferrocene+/0 couple at 0.40 V vs SCE. The counter electrode was Pt, and the working electrode was the derivatized TiO2 electrode. UV-visible measurements were made with a HP-8452 diode array spectrometer and referenced against a solvent blank (CH3CN). The TiO2 electrodes were placed in a 1 cm cuvette containing CH3CN and positioned against the side of the cell for each measurement. The number of moles of metal complex per square centimeter of projected surface area of the nanocrystalline film, Γpro (mol/cm2), was calculated from the relationship, A(λ) ) Γpro σ(λ). A(λ) is the absorbance of the film, and σ(λ) is the absorption cross section in units of cm2/ mol obtained from the decadic molar extinction coefficient (M-1 cm-1) by multiplication by 1000 cm3/L. In the analyses, (650 nm) ) 3400 M-1 cm-1 and (498 nm) ) 14 300 M-1cm-1 (measured in CH3CN) were used for adsorbed [OsII(bpy)2(4,4′(CO2H)2bpy)](PF6)2. A monolayer surface coverage of ∼1.1 × 10-10 mol/cm2 for [OsII(bpy)2(4,4′-(CO2H)2bpy)](PF6)2 can be calculated if spheres of 7 Å radii (estimated by using van der Waals radii) are closely packed on a flat surface, neglecting the PF6- counterions.5 A surface roughness factor, η, defined as the ratio of the effective surface area of the nanocrystalline films to its projected area, can be calculated to be 760 by dividing Γpro for a surface fully loaded from CH3CN by 1.1 × 10-10 mol/cm2. The actual coverage, Γ, is given by Γ ) Γpro/η. Langmuir isotherms were calculated from the relationship, Γ ) Γo[M]/([M] + 1/K) with Γo as the coverage for a fully loaded surface in mol/cm2. Γ is the equilibrium coverage at a defined molar concentration, [M]. K is the equilibrium constant for surface binding. The Langmuir isotherms were measured after the electrodes were placed in solutions 0.1-2 × 10-4 M in metal complex as PF6- salts in CH3CN or EtOH for 5 days. Chronoabsorptometry measurements in which absorption changes are monitored after a potential step were conducted in
10.1021/jp9825258 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/17/1998
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J. Phys. Chem. B, Vol. 103, No. 1, 1999 105
Figure 1. (A) Cyclic voltammograms of [Os(bpy)2(4,4′-(CO2H)2bpy)](PF6)2 on TiO2 adsorbed from CH3CN measured in 0.1 M TBAH in CH3CN vs Ag/AgNO3 at scan rates (V) of 500, 200, 50, and 20 mV/s. (B) Variation of anodic peak current, ip,a, with V1/2; Γ ) 1.1 × 10-10 mol/cm2.
spectroelectro chemical cells having path lengths of 1 or 4 mm. The cell contained two compartments attached to the base, each of which contained a counter and reference electrode. The cell was attached to a base that could be placed in an HP-8452 diode array spectrometer. Spectra were recorded at various time intervals by using the kinetic software package used to control the spectrometer. Results In Figure 1A is shown a series of cyclic voltammograms of [OsII(bpy)2(4,4′-(CO2H)2bpy)](PF6)2 adsorbed to TiO2 from CH3CN (TiO2-OsII) measured as a function of scan rate (V) in CH3CN 0.1 M in TBAH. In Figure 1B is shown a plot of oxidative peak current, ip,a vs V1/2, showing the V1/2 dependence expected for a diffusional process. ∆Ep (∆Ep ) Epa - Epc) is significantly scan rate dependent, varying from 275 mV at V ) 10 mV/s to 900 mV at V ) 500 mV/s, characteristic of slow electron transfer. The dynamics of surface electron transfer were studied by chronoabsorptometry in which time-dependent absorbance changes are monitored following a potential step. The results of typical experiments for electrodes loaded from CH3CN are illustrated in Figure 2. The spectrum of [OsII(bpy)2(4,4′-(CO2H)2bpy)](PF6)2 on TiO2, which is dominated by metal-to-ligand (MLCT) charge transfer absorption bands, is shown in Figure 2A. The maximum at 498 nm was monitored following potential steps first to 1.3 V vs Ag/AgNO3 (1.6 V vs SCE), which is well past E1/2 for the OsIII/II couple at 0.60 V (0.90 V vs SCE),5 to study Os(II f III) electron transfer. Almost complete oxidation of the monolayer on the nanocrystalline TiO2 electrode from Os(II) to Os(III) (93 ( 5% from absorbance measurements) takes place within 2 min. The potential was then stepped to 0 V to observe Os(III f II) electron transfer. The early time absorption changes in the chronoabsorptometry experiments follow the t1/2 dependence predicted by the Cottrell equation,6 eq 1, modified for monitoring absorption changes (Figure 2C).
Figure 2. (A) [OsII(bpy)2(4,4′-(CO2H)2bpy)](PF6)2 and [OsIII(bpy)2(4,4′-(CO2H)2bpy)](PF6)2 on TiO2 before and after a potential step to 1.3 V vs Ag/AgNO3 for 2 min in 0.1 M TBAH in CH3CN; Γ ) 1.1 × 10-10 mol/cm2. (B) Time-dependent oxidation of [Os(bpy)2(4,4′(CO2H)2bpy)]2+ to OsIII following potential steps to 1.3 V vs Ag/AgNO3 and then to 0 V. Absorbance-time changes, At, monitored at 498 nm. (C) ∆A as a function of the square root of time with A0 the absorbance at t ) 0 and ∆A ) A0 - At.
∆A )
2AmaxDapp1/2t1/2 dπ1/2
(1)
In eq 1, ∆A is the absorbance change at time t and Amax the absorbance at t ) 0. The film thickness is d, and the apparent charge-transfer diffusion coefficient in cm2/s is Dapp. Chronoabsorptometry measurements on TiO2-OsII loaded from CH3CN were also conducted as a function of surface coverage. As shown in Figure 3A, Dapp dramatically increases past a coverage of 6.8 × 10-11 mol/cm2, which is ∼60% of the total surface coverage. This is equally apparent in Figure 3B in which the mole fraction of monolayer oxidized to OsIII, χ(OsIII), after 60 s is plotted as a function of the fractional coverage F. Similar results were obtained on surfaces of comparable coverages loaded from EtOH. Adsorption isotherms for 1 on
106 J. Phys. Chem. B, Vol. 103, No. 1, 1999
Figure 3. (A) Conditions as in Figure 2. Dapp for the oxidation of 1 on TiO2 plotted as a function of fractional surface coverage, F. (B) Mole fraction of 1 oxidized in 60 s plotted as a function of F.
Trammell and Meyer × 10-11 mol/cm2, K ) 7.4 × 104 M-1) is ∼60% of that in CH3CN (Γo ) 1.1 × 10-10 mol/cm2, K ) 4.4 × 105 M-1). There is a striking contrast between the two surfaces in the time scales for electron transfer. In CH3CN 0.1 M in TBAH, almost complete oxidation of TiO2-OsII loaded from EtOH occurs in ∼1 h (Dapp ) 1.4 × 10-11 cm2/s), but in less than 2 min for an electrode loaded from CH3CN (Dapp ) 1.4 × 10-9 cm2/s). This is a surface coverage effect. Electrodes with similar coverage loaded from EtOH or CH3CN have similar Dapp values within experimental error. Electrochemical Mediation by Redox Relays. Electrontransfer mediation of surface electron transfer for partly loaded surfaces (∼60%) was investigated with Ru(bpy)32+ or Os(bpy)32+ added to the external solution. In Figure 5B are shown absorption-time plots for TiO2-OsII loaded at 60% coverage in the presence of 3 × 10-5 M Ru(bpy)32+ added to the external solution. The same results were obtained with electrodes loaded from either EtOH or CH3CN as long as surface coverages were comparable. In these experiments, the potential was stepped past the RuIII/II couple (E1/2 ) 0.96 vs Ag/AgNO3) to 1.3 V and then to 0 V. The resulting absorption-time profiles provide evidence for selective mediation of oxidative electron transfer. In the presence of 3 × 10-5 M Os(bpy)32+ there is evidence for both oxidative and reductive mediation but reduction is more pronounced; see Figure 5C. Coactivation of both oxidation and reduction in the presence of both is illustrated in Figure 5D. The apparent diffusion coefficient for activation of surface Os(II f III) oxidation was evaluated as a function of added Ru(bpy)32+ by using eq 1. As shown in Figure 6, in CH3CN, Dapp increases linearly with [Ru(bpy)32+], both for monolayers loaded from CH3CN at ∼60% coverage and monolayers loaded from EtOH. Discussion
Figure 4. Adsorption isotherms for 1 on TiO2 loaded from CH3CN (]) and EtOH (0) at 25 C.
TiO2 were measured in both CH3CN and EtOH. Adsorption follows the Langmuir isotherm relation as shown in Figure 4. On the basis of these results, adsorption from EtOH (Γo ) 7.0
Consistent with earlier results,1,4 surface oxidation and reduction of [Os(bpy)2(4,4′-(CO2H)2bpy)](PF6)2 adsorbed on TiO2 occurs to the underlying ITO electrode. Peak currents in cyclic voltammograms vary with V1/2, consistent with net diffusional charge transfer. In this case, the redox sites are immobilized, with the adsorbed complexes forming a monolayer
Figure 5. Conditions as in Figure 2. Time-dependent oxidation and rereduction of [Os(bpy)2(4,4′-(CO2H)2bpy)]2+ on TiO2 (Γ ) 6.8 × 10-11 mol/cm2): (A) following a potential step to 1.3 V and a subsequent step to 0 V; (B) with 3.0 × 10-5 M added [Ru(bpy)3](PF6)2 in the external solution; (C) with 3.0 × 10-5 M [Os(bpy)3](PF6)2 added; (D) with both at 3.0 × 10-5 M. The absorption-time changes were monitored at 650 nm.
Diffusional Mediation of Surface Electron Transfer
J. Phys. Chem. B, Vol. 103, No. 1, 1999 107 for oxidation (Dapp ) 3.3 × 10-10 cm2/s). Rereduction is thermodynamically nonspontaneous (∆G° ) 0.36 eV) and slow. The value of Dapp (
