Received Date:
01 September 2017 Available Online:
15 November 2017
Fund Project:
Project supported by the National Natural Science Foundation of China (Nos. 21327807, 21775043), the Program of Introducing Talents of Discipline to Universities (No. B16017) and Natural Sciences and Engineering Council of Canada (No. RGPIN-2013-201697)
Abstract:
Steady-state cyclic voltammetry has several advantages that make it extremely useful for characterization of ultramicroelectrodes, such as rapid electrode performance assessment and size determination. However, due to the nanoscale size effect, the slight variations in geometry of the nanoelectrodes can lead to significant perturbations in its performance. In this study, electrochemical experiments and finite-element simulations using COMSOL Multiphysics software were conducted to characterize the steady-state current dependence on electrode radii, geometries and recesses. To study the above characterics, the Pt nanodisk electrodes with different sizes were fabricated using a laser-assisted wire pulling method on a P-2000 laser puller. Sluggish current responses were obtained for electrodes with a radius smaller than 80 nm in a 5 mmol/L ferrocene (Fc) CH3CN electrolyte solution containing 0.2 mol/L Tetra-n-butylammonium hexafluorophosphate (TBAPF6). The experimental results agree very well with the simulations. It was discovered that the sluggish responses are due to the kinetically limited electron transfer resulting from the slow reaction rate relative to diffusion. Moreover, a minimum, constant steady state current value was measured once the RG value (i.e., the ratio of overall electrode radius to active electrode radius) was greater than 3. However, the current increased obviously with the RG value decreased below this value due to the enhanced mass transport. In addition, the steady-state voltammetric responses of recessed nanoelectrodes were investigated. It was found that the steady-state current decreased rapidly as the recess depth increased and a classical sigmoidal shape current response was obtained recovering from a sluggish current response. The unique current responses resulted from the restriction of the diffusion of redox molecules in a deep channel to the electrode. To verify the correlation of recess depth to current response, a model was built based on two-dimensional axial symmetry and the obtained simulation results were consistent well with the experimental data. Our findings offer an understanding on the relationship between the nanoelectrode geometry and steady-state cyclic voltammetry, which can provide insight into their electrochemical behaviors.
Figure 1.
2D axisymmetric models for a nanodisk electrode and recessed nanoelectrode. (a) (d+r)/r reflects RG value, where r is the diameter, d is the length of insulator. (b) R and Lh represent the diameter and the recess depth of the recessed nanoelectrode, di is the length of insulator. The blue part is the electrolyte solution
Figure 2.
Simulated and experimental results of nanodisk electrode. Simulated (a) and experimental (c) voltammograms of nanodisk electrodes in an CH3CN/0.2 mol/L TBAPF6 solution containing 5 mmol/L Fc. Scan rate is 20 mV/s. Smoothed lines drawn through the voltammetric datas are replotted in panels (b) and (d) after normalization. Insert figure (a) is the amplifying image of the nanoelectrode with radius of 30 nm
Figure 4.
Steady-state voltammograms of nanodisk electrodes (r=300 nm) with different RG values. The gray rectangle is a further enlarged view of portions
Figure 5.
Simulated steady-state voltammograms of recessed nanoelectrode. Simulated steady-state voltammograms (a) and normalized current (b) of a recessed nanoelectrode (diameter 80 nm) with different recess depths from 0~200 nm. (c) SEM images and structure diagram of a recessed nanoelectrode. (d) Simulated (blue line) and experimental (red line) results of recessed nanoelectrode. Insert figure (d) is steady-state current response of nanodisk electrode (black line) and recessed nanoelectrode.
Ma, W.; Ma, H.; Chen, J. F.; Peng, Y. Y.; Yang, Z. Y.; Wang, H. F.; Ying, Y. L.; Tian, H.; Long, Y. T. Chem. Sci. 2017, 8, 1854. doi: 10.1039/C6SC04582K
Figure 1
2D axisymmetric models for a nanodisk electrode and recessed nanoelectrode. (a) (d+r)/r reflects RG value, where r is the diameter, d is the length of insulator. (b) R and Lh represent the diameter and the recess depth of the recessed nanoelectrode, di is the length of insulator. The blue part is the electrolyte solution
Figure 2
Simulated and experimental results of nanodisk electrode. Simulated (a) and experimental (c) voltammograms of nanodisk electrodes in an CH3CN/0.2 mol/L TBAPF6 solution containing 5 mmol/L Fc. Scan rate is 20 mV/s. Smoothed lines drawn through the voltammetric datas are replotted in panels (b) and (d) after normalization. Insert figure (a) is the amplifying image of the nanoelectrode with radius of 30 nm
Figure 4
Steady-state voltammograms of nanodisk electrodes (r=300 nm) with different RG values. The gray rectangle is a further enlarged view of portions
Figure 5
Simulated steady-state voltammograms of recessed nanoelectrode. Simulated steady-state voltammograms (a) and normalized current (b) of a recessed nanoelectrode (diameter 80 nm) with different recess depths from 0~200 nm. (c) SEM images and structure diagram of a recessed nanoelectrode. (d) Simulated (blue line) and experimental (red line) results of recessed nanoelectrode. Insert figure (d) is steady-state current response of nanodisk electrode (black line) and recessed nanoelectrode.
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