RIHA ET AL. VOL. 7 ’ NO. 3 ’ 2396–2405 ’ 2013 www.acsnano.org 2396 February 12, 2013 C 2013 American Chemical Society Atomic Layer Deposition of a Submonolayer Catalyst for the Enhanced Photoelectrochemical Performance of Water Oxidation with Hematite Shannon C. Riha, †,‡,2 Benjamin M. Klahr, §,2 Eric C. Tyo, ) So ¨ nke Seifert, ^ Stefan Vajda, †, ) ,z Michael J. Pellin, †,‡ Thomas W. Hamann, §, * and Alex B. F. Martinson †,‡, * † Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States, ‡ ArgonneÀNorthwestern Solar Energy Research (ANSER) Center, Argonne National Laboratory, Argonne, Illinois 60439, United States, § Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322, United States, ^ X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States, ) Department of Chemical and Environmental Engineering, School of Engineering & Applied Science, Yale University, New Haven, Connecticut 06520, United States, and z Nanoscience and Technology Division, Argonne National Laboratory, Argonne, Illinois 60439, United States. 2 These authors contributed equally. S olar radiation is a renewable energy source that has the capacity to meet global energy demands in a carbon- neutral fashion. As such, many devices have been developed and optimized to capture and convert solar energy into electricity. 1,2 However, because solar en- ergy is diffuse and intermittent, an effi- cient and economically viable method to concentrate and store energy harvested from the sun must be realized. One me- thod is to store solar energy in the form of chemical bonds ; much like in photo- synthesis ; through photoelectrochemical (PEC) water splitting. 3À6 Since Fujishima and Honda's seminal report, 7 extensive re- search has been devoted toward identi- fying a material capable of meeting the strict requirements necessary for large-scale implementation. 1,4,8À13 Hematite (R-Fe 2 O 3 ) is one promising ma- terial that meets many of the requirements for the water oxidation half reaction: (i) it has a suitable band gap of 2.0À2.1 eV; (ii) it is stable under water oxidation conditions, often in alkaline electrolytes; and (iii) it is composed of earth-abundant and nontoxic elements, making it environmentally be- nign and inexpensive. 8À10,14 Despite such promising attributes, the overall water split- ting efficiency of hematite photoanodes falls well short of the theoretical maximum efficiency. 4,15 A short hole collection dis- tance, coupled with comparatively long light penetration depths, is one factor limit- ing the efficiency of hematite photoanodes. * Address correspondence to martinson@anl.gov, hamann@chemistry.msu.edu. Received for review December 6, 2012 and accepted February 12, 2013. Published online 10.1021/nn305639z ABSTRACT Hematite photoanodes were coated with an ultrathin cobalt oxide layer by atomic layer deposition (ALD). The optimal coating ; 1 ALD cycle, which amounts to <1 monolayer of Co(OH) 2 /Co 3 O 4; resulted in significantly enhanced photoelectrochemical water oxidation performance. A stable, 100À200 mV cathodic shift in the photocurrent onset potential was observed that is correlated to an order of magnitude reduction in the resistance to charge transfer at the Fe 2 O 3 /H 2 O interface. Furthermore, the optical transparency of the ultrathin Co(OH) 2 /Co 3 O 4 coating establishes it as a particularly advantageous treatment for nanostructured water oxidation photoanodes. The photocurrent of catalyst-coated nanostructured inverse opal scaffold hematite photoanodes reached 0.81 and 2.1 mA/cm 2 at 1.23 and 1.53 V, respectively. KEYWORDS: hematite . Fe 2 O 3 . photoelectrocatalysis . water oxidation . electrochemical impedance spectroscopy . X-ray absorption spectroscopy . XANES ARTICLE