Employing Electrochemical Potential Cycling to Controlling Surface to Volume Ratio of Nanoporous Gold and Photoelectron Injection into Electrolyte
Authors: Ebrahimi Fatemeh , Wu Xinyan , Mameka Nadiia , Petrov Alexander , Eich Manfred 
Published: February 2024

Nanoporous metals made by alloy corrosion are bicountinuous porous networks that possess nanoscale ligaments and pores and thus a large surface-to-volume ratio. Their ligament and pore size can be tuned between a few nanometers and several microns via thermal annealing [1], which makes them attractive material systems for a variety of applications, including catalysis, sensing, and energy storage. In particular, dealloying-derived nanoporous metals, present novel opportunities for exploring the impact of surfaces on electro-photocatalytic activity of metallic nanostructures [2]. Yet, achieving the ligament and pore sizes in the dealloyed metallic networks below 10 nm, which is especially crucial for understanding an efficient electrochemical charge transfer, is challenging so far.

In this study, we demonstrate a successful fabrication of nanoporous gold (npAu) thin films via electrochemical dealloying with the ligament size down below 10 nm on quartz substrate. We study the material by means of in situ photoelectrochemical setup while simultaneously varying the electrode potential in 0.5 M H2SO4 electrolyte. To probe the size effect on the photocurrent response, we exploit cyclic voltammetry (CV) in order to promote coarsening of the ligaments of npAu networks. By varying a number of CV cycles, we were able to induce subtle changes in the ligament size, therefore modifying the surface-to-volume ratio of the nanoporous metals at the lower limit of the nanoscale.

Our results show that this approach provides an excellent foundation for future optical investigations of surface effects on the internal quantum efficiency of high-surface area metallic photoelectrodes.

Our study demonstrates that modifying the surface-to-volume ratio of the sample by electrochemistry allows us to control the hot electron generation and injection processes in nanoscale plasmonic systems. Moreover, the internal quantum efficiency of the npAu network photoelectrode is significantly influenced by the size of its ligaments. In contrast to prior findings that suggested efficiency would level off with decreasing ligament size [3], we have observed a continuous nonlinear increase in quantum efficiency as the ligament size decreased from 16 to 8 nm. This result is attributed to the increase in absorption by the electrons colliding with the surface (Landau damping) and increase of emission of this electrons into the electrolyte due to possibility of multiple collisions with the surface.

References:

[1]. Wang, Jianli, et al. "Effect of thermal coarsening on the thermal conductivity of nanoporous gold." Journal of materials science 47 (2012): 5013-5018.

[2]. Scandura, Gabriele, et al. "Nanoporous Dealloyed Metal Materials Processing and Applications─ A Review." Industrial & Engineering Chemistry Research (2023).

[3]. Graf, Matthias, et al. "Surface-to-volume ratio drives photoelelectron injection from nanoscale gold into electrolyte." ACS catalysis 9.4 (2019): 3366-3374.