Investigation of Nanostructure and Photocatalytic Stability of Mesoporous CuCrO2 Delafossite using Analytical Electron Microscopy

Abstract

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 bimetallic-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 co...

The effects of Bi alloying in Cu delafossites: A density functional theory study

“…17 PEC response for some Cu delafossites has already been demonstrated. [27][28][29][30] Unfortunately, the efficiencies of these photo-conversion processes are not satisfactory, largely due to the inefficient absorption of visible light. Recent theoretical study has revealed that the poor optical absorption is due to the inversion symmetry of their special crystal structure.…”