Investigation of Nanostructure and Photocatalytic Stability of Mesoporous CuCrO 2 Delafossite using Analytical Electron Microscopy Miaofang Chi * , Peng Zhang ** , and Eric McFarland ** * Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 ** Department of Chemical Engineering, University of California, Santa Barbara, California, 93117 Investigating new sources of renewable energy has been a major focus in the scientific community during the last decade and continues to attract much attention [1]. Photovoltaics — used in the conversion of solar energy to electrical power — are considered a promising technology to meet the requirements of a clean, non-fossil fuel in the future. The photoelectrochemical cell (PEC), utilizing semiconducting surfaces for catalysis, has been intensively studied as a method to generate hydrogen (H 2 ). Novel catalyst compounds, which meet all the essential criteria for a PEC system, i.e. a small energy band gap (E g ) for absorption of visible light, chemical stability for reuse and storage, a negative conduction band potential and an efficient conversion rate, are being investigated. Cu + X 3+ O 2 oxides, where X denotes a transition metal, crystallize in the delafossite structure, have been reported as leading candidates to meet these PEC requirements, primarily as a result of their interesting band-gap modulation, long-term chemical stability, and low-cost [2,3]. Mesoporous CuCrO 2 delafossite structures were successfully synthesized by nanocasting methods using KIT-6 as a template. Copper nitrate and chromium nitrate precursors were melted together at 60°C or dissolved in methanol before impregnation into the silica KIT-6 template. After calcination at ~1000°C in Ar, delafossite nanostrutcures were obtained after removal of the silica template by treatment in a NaOH solution. The synthesized mesoporous CuCrO 2 delafossite is a p-type semiconductor with a small band gap of ~1.38 eV. However, the most efficient H 2 generation for this material takes place under irradiation with a wavelength of ~400nm, which is shorter than the expected value (~900 nm). Two potential explanations for this controversial result include (1) the existence of surface defects and (2) a low electronic conductivity in the bulk delafossite, both of which could result in the recombination of photo-generated electrons and holes. To investigate the surface structure and electronic structure of the mesoporous CuCrO 2 , a combination of STEM and EELS was used. A C s -corrected FEI Titan 80/300 TEM/STEM equipped with a Gatan Imaging Filter was used for this study. Bright-field (BF) and dark-field (DF) STEM images were recorded simultaneously to study the crystal structure of the CuCrO 2 delafossite. The fine structures of Cr-L, Cu-L, and O-K edges have been studied to reveal the electronic structure of the material. The oxidation states of Cu and Cr were investigated by EELS. Mesoporous CuCrO 2 samples exposed for different periods of light illumination were evaluated to understand the catalytic stability. In addition, we have also studied bi- metallic-doped mesoporous CuCrO 2 to understand how dopants affect the microstructure and photoactivity. The as-synthesized mesoporous CuCrO 2 microstructure is characterized by pore sizes of ~10 nm and an inter-pore spacing of ~17 nm (Figure 1). The mesoporous CuCrO 2 morphology maximizes the solar-to-chemical conversion efficiency as a result of a very high surface/volume ratio and an enhanced degree of crystallinity of the CuCrO 2 nanoparticles. The hydrogen-generating efficiency of Microsc Microanal 15(Suppl 2), 2009 Copyright 2009 Microscopy Society of America doi: 10.1017/S143192760909713X 1406 https://doi.org/10.1017/S143192760909713X Downloaded from https://www.cambridge.org/core. IP address: 54.163.42.124, on 09 Jun 2020 at 11:12:30, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.