Extremely Thin Photoelectrode Architectures for Photocatalysis
Authors: Thomann Isabell 
Published: February 2020

Introduction
Concepts from metamaterials, plasmonics and nanophotonics are highly promising to improve the design of future solar energy conversion devices. Here, I will describe how we use these concepts to design advanced photoelectrode architectures for driving two challenging photocatalytic reactions: water splitting and CO2 reduction. We focus on the light management aspect in extremely thin absorber structures and in plasmonic metal nanostructures, to achieve broadband omnidirectional solar light absorption while carefully choosing materials systems that allow for efficient charge separation and catalytic activity. Such thin-film absorber photoelectrodes hold promise for achieving enhanced charge carrier extraction, increased photovoltages, and the possibility to exploit hot carriers for purposes of driving chemical reactions. In addition, architectures made from two-dimensional materials building blocks allow us to tailor new electronic, catalytic and optical properties by engineering materials one layer at a time. However, light absorption in single-layer materials is generally small, and efficient photoconversion structures require nanophotonic concepts to enhance their optical properties.
Experimental
We are exploiting nanophotonics concepts to realize extreme light absorption in two-dimensional (2D) transition metal dichalcogenides. I will discuss the analytical models and three-dimensional electromagnetic simulations that we employ to engineer light absorption in these 2D materials and in plasmonic metal nanostructures, and describe our progress towards the experimental realization and characterization of such structures. Complementing these materials and device design efforts, we are developing an experimental characterization toolbox, including photoelectrochemical and spectroscopic techniques.
Materials
We have designed both plasmonic- and dielectric-based ultrathin photoelectrode structures 1. To create centimeter-scale nanostructured photoelectrodes, we have developed the synthesis of Anodic Aluminum Oxide (AAO) templates as generic deposition masks for plasmonic and non-plasmonic materials. The AAO masks can be transferred to a wide range of substrates (including rough transparent conducting oxides and metal oxides) and e-beam evaporation of metals of various sizes, shapes and periodicities can be accomplished. The AAO templates can also be used for lateral nanostructuring of two-dimensional materials. Sample characterization were performed by SEM, XRD, AFM. Photoelectrochemical measurements of water splitting were performed on plasmonic nanostructures, combined with suitable metal oxides for efficient charge separation.
Results and Discussion
Full-field electromagnetic simulations of our absorber structures suggest that they are suitable to substantially enhance the optical absorption over a broad wavelength range, covering regions of the solar spectrum that contain a large portion of the solar energy flux. The ultrathin nature of our structures also renders them relatively immune to efficiency reductions under non-normal incidence illumination. We will discuss first results on photoelectrode fabrication, and their materials-, optical and photoelectrochemical characterization.
Conclusions
This work is aimed at developing and characterizing advanced photoelectrode architectures for two challenging photocatalytic reactions, water splitting and CO2 reduction. Such structures will ultimately allow the deployment of solar-based approaches for driving large-scale applications in photoelectrochemistry and photocatalysis, and may hold promise for new generations of opto-electronic devices.
References
1. (a) Thomann, I.; Pinaud, B. A.; Chen, Z.; Clemens, B. M.; Jaramillo, T. F.; Brongersma, M. L., Plasmon Enhanced Solar-to-Fuel Energy Conversion. Nano letters 2011, 11 (8), 3440-3446; (b) Hägglund, C.; Zeltzer, G.; Ruiz, R.; Thomann, I.; Lee, H.-B.-R.; Brongersma, M. L.; Bent, S. F., Self-Assembly Based Plasmonic Arrays Tuned by Atomic Layer Deposition for Extreme Visible Light Absorption. Nano letters 2013, 13 (7), 3352-3357.