Self-Organized Plasmonic Nanowire Arrays Coated with Ultrathin TiO2 Films for Photoelectrochemical Energy Storage

Abstract

The strategic field of renewable energy production and storage requires novel nanoscale platforms that can feature competitive solar energy conversion properties. Photochemical reactions that promote energy storage, such as water splitting and oxygen-hydrogen evolution reactions, play a crucial role in this context. Here, we demonstrate a novel photoelectrochemical device based on large-area (cm²) self-organized Au nanowire (NW) arrays, uniformly coated with ultrathin TiO₂ films. The NW arrays act both as transparent nanoelectrodes and as a plasmonic metasurface that resonantly enhances the very weak visible photocurrent generated by a prototype photoelectrochemical oxygen evolution reaction. We demonstrate a polarization-sensitive plasmon-enhanced photocurrent that reaches a gain of about 3.8 in the visible spectral range. This highlights the potential of our novel nanopatterned plasmonic platform in photochemistry and energy storage.

Figure 1

(a,b) AFM topography of the quasi-1D nanopatterned glass template and the corresponding cross-section profile (blue line in panel a). (c,d) Sketch of the Au glancing angle evaporation onto the faceted templates and scanning electron microscopy (SEM) image (backscattered electron signal) of the self-organized Au nanowire arrays. The white scale bar corresponds to 2 μm and the red circles highlight the lateral interconnections between the nanowires.

Figure 2

(a,b) Picture of the large-area plasmonic nanoarrays featuring enhanced light scattering and sketch of the configuration used for the in situ electrical transport characterization, respectively. Two-wire sheet resistance measurements have been performed in situ parallel to the long axis of the NW arrays with two electrodes facing each other at a 2 cm distance. (c) Longitudinal sheet resistance plotted as a function of the local Au thickness (h) deposited on top of the exposed facets. (d) Optical transmission of the Au NW arrays detected for longitudinal (TE—black line) and transversal (TM—red line) polarization of the incident beam with respect to the NW long axis, as sketched in panel b. (e) Schematic illustration of the system (structure/chemical composition) and interconnection to the electrochemical cell.

Figure 3

(a,b) Photoelectrochemical current detected by illuminating the samples NW10 (blue line, panel a), NW5 (red line, panel b), and their corresponding flat Au–TiO₂ films acting as references (black lines) with an unpolarized monochromatized light source. The light is turned on for a fixed time interval (50 s, corresponding to the yellow boxes) on both the NW arrays and the reference device, increasing the wavelength from the near-UV to the visible spectrum at the 20 nm step. The illumination wavelengths are shown on the upper blue axis. (c) Extract of photoelectrochemical current measurements for illumination between 520 and 640 nm relative to the NW5 device (red bar) and its corresponding reference (black bar). (d) Plot of photocurrent gain G = I_NW/I_flat as a function of the illuminating wavelength for samples NW10 (blue dots) and NW5 (red dots).

Figure 4

(a,b) Photocurrent measured under linearly polarized illumination at different wavelengths for samples NW10 and NW5, respectively. The black squares correspond to the Au NWs-TiO₂ device illuminated with TE polarization, while the red dots correspond to a TM polarization (both polarizations are defined in Figure 2b). The blue triangles refer to a flat sample illuminated with polarization corresponding to a TM polarization for the Au NWs-TiO₂ device. (c) Relative photocurrent gain for sample NW5 under polarized illumination, normalized to the Ref5 sample. The red dots correspond to a TM polarization, while the black squares correspond to a TE polarization. The blue line corresponds to the transmission spectrum of the sample with a TM polarization.