Visualization of an Accelerated Electrochemical Reaction under an Enhanced Electric Field

Abstract

Locally enhanced electric fields produced by high-curvature structures have been reported to boost the charge transport process and improve the relevant catalytic activity. However, no visual evidence has been achieved to support this new electrochemical mechanism. Here, accelerated electrochemiluminescence (ECL) reactions emitting light are visualized for the first time at the heterogeneous interfaces between microbowls and the supporting electrode surface. The simulation result shows that the electric intensity at the interface with a high curvature is 40-fold higher than that at the planar surface. Consequently, local high electric fields concentrate reactive species to the heterogeneous interfaces and efficiently promote the charge transport reactions, which directly leads to the enhancement of ECL emission surrounding the microbowls. Additionally, the potential to induce visual ECL from a ruthenium complex drops to 0.9 V, which further illustrates the promotion of an electrochemical reaction with the aid of an enhanced electric field. This important visualization of electric field boosted electrochemical reactions helps to establish the proposed electron transfer mechanism and provide an alternative strategy to improve electrocatalytic efficiency.

Scheme 1

Schematic representation of the electrochemiluminescence imaging. The luminophore Ru(bpy)₃²⁺ and coreactant TPrA are oxidized at the heterogeneous interface between the microbowls and the ITO supporting electrode with the aid of enhanced electric field, generating the excited state Ru(bpy)₃^2+∗. The accelerated ECL emission is produced during the relaxation of Ru(bpy)₃^2+∗ back to the ground state.

Figure 1

Computed electric field and ECL images at the heterogeneous interface. (a) SEM images of Au microbowls in a face-down configuration on the supporting surface. (b) The simulated electric field at the ITO surface with Au microbowls (face-up view). The rainbow bar is the logarithm value of the electric field intensity. (c) The electric strength at the heterogeneous interface with the microbowl and planar ITO surface. (d) Bright-field, (e) ECL, and (f) overlapping image of Au microbowls on the ITO slide. The ECL images are recorded in 10 mM PBS (pH = 7.2) that contains 5 mM Ru(bpy)₃²⁺ and 50 mM TPrA.

Figure 2

Simulated electrochemical reaction near the heterogeneous interface. (a) The simulated concentration distribution of Ru(bpy)₃²⁺ before oxidation. The simulated concentration distribution of (b) Ru(bpy)₃²⁺ and (c) Ru(bpy)₃³⁺ after oxidation. (d) The simulated ECL intensity at the heterogeneous interface after oxidation.

Figure 3

ECL images of different microbowls: (a) ECL images of Al and Pt microbowls loaded at the ITO surface; (b) the enhancement ratio of ECL intensity between the heterogeneous interface (with different microbowl materials) and the ITO surface; (c) ECL images of Au microbowls with the size of 12 and 8 μm; (d) the enhancement ratio of ECL intensity between the heterogeneous interface (with different microbowl size) and the ITO surface; (e) the simulated electric field at the ITO surface with two adjacent microbowls; (f) the bright-field and ECL images from the adjacent microbowls. The ECL reagents are 5 mM Ru(bpy)₃²⁺ and 50 mM TPrA in 10 mM PBS.

Figure 4

ECL images at different voltages. The false-color ECL images of microbowls at the supporting ITO surface applied with different voltages. The color presents the ECL intensity.