Photoelectrochemical characterization of the synthetic crednerite CuMnO2

Abstract

High quality crednerite CuMnO 2 was prepared by solid state reaction at 950°C under argon flow. The oxide crystallizes in a monoclinically distorted delafossite structure associated to the static Jahn-Teller (J-T) effect of Mn 3? ion. Thermal analysis showed that it converts reversibly to spinel Cu x Mn 3-x O 4 at *420°C in air and further heating reform the crednerite above 940°C. CuMnO 2 is p-type, narrow semiconductor band gap with a direct optical gap of 1.31 eV. It exhibits a long-term chemical stability in basic medium (KOH 0.5 M), the semi logarithmic plot gave an exchange current density of 0.2 lA cm -2 and a corrosion potential of *-0.1 V SCE . The electrochemical oxygen insertion/desinsertion is evidenced from the intensity-potential characteristics. The flat band potential (V fb = -0.26 V SCE ) and the holes density (N A = 5.12 9 10 18 cm -3 ) were determined, respectively, by extrapolating the curve C -2 versus the potential to the intersection with C -2 = 0 and from the slope of the MottSchottky plot. From photoelectrochemical measurements, the valence band formed from Cu-3d wave function is positioned at 5.24 ± 0.02 eV below vacuum. The Nyquist representation shows straight line in the high frequency range with an angle of 65°ascribed to Warburg impedance originating from oxygen intercalation and compatible with a system under mass transfer control. The electrochemical junction is modeled by an equivalent electrical circuit thanks to the Randles model.

Oxygen storage capacity and thermal stability of the CuMnO2–CeO2 composite system

“…† Such behaviour can be attributed to the transformation of spinels and CuO back into a crednerite Cu 1+y 0 Mn 1Ày 0 O 2 phase, accompanied by the release of O 2 . 18,32 During the cooling process under owing air from 1000 C, there is a sharp continuous weight increase up to around 880 C, which can be attributed to the re-oxidation of the crednerite phase to CuO and spinel oxides. The slight weight increase from 880 to 600 C can be attributed to the composition adjustment of spinels with the cooling temperature, as determined by XRD shown in Fig.…”

“…18,28 In addition, the Cu x Mn 3Àx O 4 spinel, one of the oxidation products of CuMnO 2 , has been reported to show the coexistence of Mn 3+ /Mn 4+ , implying a possible increase in the total oxygen uptake ability under an oxidative atmosphere. 27,29,30 However, pure crednerite CuMnO 2 oxide is usually synthesized using the solid-state reaction at very high temperatures 15,31,32 or the ion-exchange reaction with a long reaction time. 32 Such a high reaction temperature or slow chemical conversion rate for reversible redox processes is impractical for many technological applications.…”

“…27,29,30 However, pure crednerite CuMnO 2 oxide is usually synthesized using the solid-state reaction at very high temperatures 15,31,32 or the ion-exchange reaction with a long reaction time. 32 Such a high reaction temperature or slow chemical conversion rate for reversible redox processes is impractical for many technological applications. 18 Therefore, it is important to improve the oxygen diffusion ability and the overall oxygen storage capacity of crednerite CuMnO 2 at low temperatures.…”

Disclosing the Structural, Electronic, Magnetic, and Morphological Properties of CuMnO2: A Unified Experimental and Theoretical Approach

“…CuMnO 2 material, called crednerite, is a very intriguing delafossite derivative which can be potentially applied in several fields such as catalyst, photoelectrochemical cells, and multiferroic devices, among others. − Regarding the crystalline structure, this material shows a distorted arrangement in comparison with the hexagonal delafossite structure because of the Jahn–Teller effect of Mn 3+ ions. This cation has crystal-field splitting in e g orbitals due to the 3d 4 (t 2g 3 e g 1 ) electronic configuration.…”

Relationship between morphological and physical properties in nanostructured CuMnO2

“…Available information from the literature on synthesises indicates that the CuMnO 2 compound can be synthesized via several methods including hydrothermal methods [5,6], ion exchange reactions [10], solid state reactions [13,17,18] and sol -gel techniques [19,20]. The crystal structure of the C2/m space group and lattice parameters of samples from different preparations are essentially consistent with each other experiment, but their physical properties differ dramatically.…”