Combinatorial screening of photoanode materials - Uniform platform for compositional arrays and macroscopic electrodes

Abstract

A new work flow for high-throughput evaluation of complex metal oxides for water splitting relies on the use of commercial material printer for library production and scanning photoelectrochemical microscopy (SPECM) for hit identification. The printing process is optimized into several directions using as an example binary mixtures of Zn2+ and Fe3+ salts as precursors. Firstly, temporary barrier layers from eicosane are printed to confine the spots to a particular region and to allow larger drop sizes without merging of neighboring droplets. Secondly, a glycerol droplet is printed before the application of metal salts to serve as a carrier liquid that is non-volatile at room temperature. The metal salt solutions are printed from ethylene glycol solutions into the carrier droplets. With these precautions, the drop remained as liquid phase on the substrate with their lateral position locked by the eicosane grid. This avoids problems with the early crystallization of metal salts between successive printing and ensured complete mixing of the precursor solution. The liquid drops are then thermally processed by temperature-programmed heating up to 600 °C allowing the sequential drying of the spots, evaporation of the grid structures and formation of ferrites. The same procedure is used to cover macroscopic electrodes with a large number of spots of identical composition to verify hits in laboratory photoelectrochemical cells and to use photocurrent transient and chopped light voltammetry for their further characterization. Structural characterization of the composition (Zn0.17Fe0.83Ox) with highest activity is performed by Raman and photoelectron spectroscopy. The opportunity to use the same printing protocols to form compositional libraries as well as extended single component electrodes allows screening of medium-sized arrays and the fast transition to more in depth single component experimentation using exclusively standard laboratory instrumentation.