Low temperature hydrothermal synthesis mechanism and thermal stability of p-type CuMnO2 nanocrystals

Abstract

We first report an oxidation–reduction reaction mechanism for the hydrothermal synthesis of CuMnO₂ nanocrystals at the low temperature of 80 °C. In detail, CuMnO₂ nanocrystals with 50–100 nm size could be obtained based on a hydrothermal reaction from the starting materials of Cu(NO₃)₂, Mn(NO₃)₂/MnCl₂/MnSO₄ and NaOH at 80 °C for 24 h. During the hydrothermal reaction, Cu²⁺ was reduced to Cu⁺ and Mn²⁺ was oxidized to Mn³⁺, and finally CuMnO₂ nanocrystals formed. The TGA analysis indicates the studied CuMnO₂ was stable in a N₂ atmosphere and the CuMnO₂ remained unchanged at a high sintering temperature (500 °C). Moreover, the Mott–Schottky plot results demonstrate that CuMnO₂ nanocrystals show p-type semiconductor behaviour. The flat band potential (E_fb) position and carrier density of the CuMnO₂ nanocrystals are 0.28 V vs. reversible hydrogen electrode (RHE) and 2.46 × 10¹⁸ cm⁻³, respectively. After the CuMnO₂ nanocrystals were sintered at 300 °C in N₂, the E_fb position shows a negative shift to 0.23 V vs. RHE and the carrier density increased to 1.58 × 10¹⁹ cm⁻³. This quick and facile method opens a new route for the preparation of other delafossite oxides, which can be used in some optoelectronic devices such as photoelectrochemical cells and photovoltaic cells.