Defining the Role of Cr³⁺ as a Reductant in the Hydrothermal Synthesis of CuCrO₂ Delafossite

Abstract

The synthesis of nanocrystalline, p-type delafossite metal oxides (CuMO2) via hydrothermal methods has been explored for a variety of energy conversion and storage applications. However, isolation of the pure phase ternary product is challenging due to the facile growth of unwanted, binary byproducts (CuO, Cu2O, and M2O3) which could ultimately influence the optoelectronic properties of the resulting nanocrystals. Here, we report on the optimized hydrothermal synthesis of CuCrO2 nanocrystals to limit the production of such byproducts. This material possesses a wide band gap and high reported conductivity, making it attractive for applications as the hole transport layer in a variety of heterojunction solar cells. An important aspect of this work is the consideration of Cr3+ as the reductant used to reduce Cu2+ to Cu+. This was confirmed by detection and quantification of CrO42– as a product of hydrothermal synthesis in addition to the fact that CuCrO2 purity was maximized at a ratio of 4:3 Cr/Cu, consistent with the proposed stoichiometric reaction: 4Cr3+ + 3Cu2+ + 20 OH– → 3CuCrO2 + CrO42– + 10 H2O. Using a 4:3 ratio of Cr/Cu starting materials and allowing the synthesis to proceed for 60 h eliminates the presence of CuO beyond detection by powder X-ray diffraction (pXRD). Furthermore, washing the solid product in 0.5 M NH4OH removes Cu2O and Cr2O3 impurities, leaving behind the isolated CuCrO2 product as confirmed using pXRD and inductively coupled plasma mass spectrometry