Abstract
Using optical in-situ measurements in an electrochemical environment, we study the electrochemical tuning of the transmission spectrum of films from the nanoporous gold (NPG) based optical metamaterial, including the effect of the ligament size. The long wavelength part of the transmission spectrum around 800 nm can be reversibly tuned via the applied electrode potential. The NPG behaves as diluted metal with its transition from dielectric to metallic response shifted to longer wavelengths. We find that the applied potential alters the charge carrier density to a comparable extent as in experiments on gold nanoparticles. However, compared to nanoparticles, a NPG optical metamaterial, due to its connected structure, shows a much stronger and more broadband change in optical transmission for the same change in charge carrier density. We were able to tune the transmission through an only 200 nm thin sample by 30%. In combination with an electrolyte the tunable NPG based optical metamaterial, which employs a very large surface-to-volume ratio is expected to play an important role in sensor applications, for photoelectrochemical water splitting into hydrogen and oxygen and for solar water purification.
Highlights
Charge carrier injection or extraction is viable route to significantly modify the optical response of very thin layers
We have recently shown that the optical properties of nanoporous gold (NPG) can be qualitatively described by modeling the NPG as a cubic grid of crossing gold wires[18]
The transmission experiments indicate that the transition from dip to plateau in the spectrum depends on the dealloying potential, on the ligament size
Summary
As a basis for the NPG samples served ca. 200 nm thick, 6 Carat, i.e Au25Ag75 (mass%) white gold leafs (Wasner Blattgold) that were glued to 1 mm thick optical glass slides, using epoxy adhesive (Araldite LY564 with hardener Aradur 2954 (Huntsman), mixing ratio 3:1 by mass). During the first few cycles of the potential applied on the NPG the optical transmission underwent a non-reversible change[20] This change is assigned to an enhanced mobility of gold atoms associated with a transient increase in surface diffusivity during the lifting of the oxygen adsorbate layer at the positive end of the potential scale[34,35]. The Faraday current results from molecular oxygen impurities that cannot be avoided since the in situ cell is open to air Superimposed to these extrinsic features is the intrinsic capacitive behavior of clean gold surfaces, which gives rise to the nearly constant current during positive going scans between 0 and 0.4 V
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