Early attempts to make hematite electrodes for water oxidation focused on planar geometries, whose related surface and bulk properties were studied extensively. The conclusions of these studies resulted in a set of design criteria with the goal to maximize the electrode’s water oxidation performance. Nanostructuring is one of the most important design criterion, as it enhances light absorption and reduces the distance the charge carriers have to traverse before being collected and moved to a counter electrode. Furthermore, the interaction of light with ordered arrays of nanostructures results in resonance effects in both the material and voids, which enhance the charge carrier generation, increasing water oxidation efficiency. It was also shown that the surface properties of nanostructured hematite electrodes is strongly dependent on the patterning method, material growth and the annealing approaches.
In this work, we fabricate arrays of free-standing hematite nanorods and study their surface states with transmission electron microscopy. The fabrication process comprises electroplating of iron in anodized alumina membranes followed by etching of the latter to reveal an array of bare, free-standing iron nanorods. This enables an efficient thermal treatment, resulting in hematite nanorods. Annealing was performed in a box furnace in air at different temperatures and for different durations with slow heating and cooling rates. Raman spectroscopy was used to study the effects of the annealing conditions, revealing well defined hematite patterns for samples annealed at 350oC and higher temperatures and 2 or more hours. This is a considerably lower annealing temperature than what was reported before. Using Electron Energy loss spectroscopy on individual rods, a trend of Fe3+ to Fe2+ valence states from the center to the very edges was found by evaluating the L3/L2 intensity line ratios and chemical shifts. This reduction of iron has been related in rapid annealing works to the reduction of surface trap states, improving performance. The combination of ordered nanostructuring, efficient fabrication process and surface states make the hematite nanorods electrodes excellent candidates for photocatalytic water splitting.