The Micro-Optical Ring Electrode. 3:  Transient Photocurrent Studies of Photophysical−Electrochemical and Photophysical−Chemical−Electrochemical Systems

Abstract

The micro-optical ring electrode (MORE) is a photoelectrochemical device based on a ring microelectrode that uses the insulating material interior to the ring electrode as a light guide. In this paper, we describe the preparation and characterization of very thin ring MOREs with (ring inner radius)/(ring outer radius) > 0.99. Theoretically, we derive asymptotic analytical expressions for the time dependence of the diffusion-limited transient light-on photocurrent generated by two general types of photoelectrochemical systems: (a) the PE (photophysical-electrochemical) system, wherein the photoexcited species itself is directly detected on the ring; (b) the PCE (photophysical-chemical-electrochemical) system, wherein the photoexcited species undergoes a homogeneous electron transfer reaction prior to electrochemical detection. Experimentally, we establish that it is possible to use such MOREs to study the wavelength dependence of photocurrents derived from photoelectrochemically active systems, such as the Ru(bipy)3 2+/Fe3+ PCE system, demonstrating the potential utility of the MORE as a selective electroanalytical probe. We also use our expressions for the time dependence of photocurrents at the MORE to derive values for the photoelectrochemical kinetic parameters of this system, including the rate coefficient for the back reaction of photogenerated Ru(bipy)3 3+ (0.115 s(-1)) and the quantum efficiency for the primary redox products, Ru(bipy)3 3+ and Fe2+, escaping cage recombination, phi(CE) = 0.099.

The Micro-Optical Ring Electrode: Surface Finish and Photocurrent Magnitude

“…The behaviour of the MORE has previously been modelled and the photocurrent may be expressed by equation 1 where n-number of electrons exchanged, F-Faradays constant, Ds -Diffusion coefficient species S (m 2 s -1 ),[S]concentration of sensitizer (mol m -3 ), [A]concentration of acceptor (mol m -3 ), Ψquantum yield, Iph -Flux of light at electrode surface ((mol photons) m -2 s -1 ), k0, k2pseudo-first-order rate constants (s -1 ), k1secondorder rate constant (m 3 mol -1 s -1 ), xdistance into the solution from and normal to the plane of the electrode surface r (m), t time(S), ε-extinction coefficient at λ. This model makes several assumptions 2,3 including that the gold ring, inner insulator (fibre optic) and outer insulator (epoxy resin) all form part of the same plane.…”

“…Experimental setup Nova 2.0 software is then used to setup a chrono-amperometry experiment were a potential is held and the resulting current is measured. Previous work on this system has shown that a working potential of 0.5V was adequate to generate a photocurrent 3 . The experiment was left for 200 seconds at which point the light was switched on, an increase in the current is immediately seen and remains constant for the duration that the light is switched on (100 seconds).…”

Generator-collector double electrode systems: A review

“…[53][54][55][56] Micro ring electrodes have also been used to detect photochemically created species, where a ring electrode surrounds a fibre-optic light guide. [57][58][59] The light emitted from the fibre-optic wire can induce a photochemical reaction in the interior of the ring, the products of which can diffuse to the electrode and be detected. While not strictly a dual electrode technique, the underlying principles are very similar.…”

Microelectrode systems for the study of photochemical processes in solution

“…Notably, an optoelectrochemical probe consisting of an optical fibre coated in metal to form a ring ultramicroelectrode (UME) was introduced by Kuhn and Weber for the electrochemical detection of photogenerated species in solution [8]. This type of optical ring electrode was successfully developed further for imaging purposes [9,10] and kinetic studies of photochemical processes [11][12][13]. For example, the micro-optical ring electrode (MORE) was used to detect electro-active species with lifetimes <9 Â 10 À5 s [11] and to study photoelectrochemical kinetics [12].…”