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

The synthesis of nanocrystalline, p-type delafossite metal oxides (CuMO2) viahydrothermal methods has been explored for a variety of energy conversion and storageapplications. However, isolation of the pure phase ternary product is challenging due to the facilegrowth of unwanted, binary byproducts (CuO, Cu2O, and M2O3) which could ultimatelyinfluence the optoelectronic properties of the resulting nanocrystals. Here, we report on theoptimized hydrothermal synthesis of CuCrO2nanocrystals to limit the production of suchbyproducts. This material possesses a wide band gap and high reported conductivity, making itattractive for applications as the hole transport layer in a variety of heterojunction solar cells. Animportant 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 ofhydrothermal synthesis in addition to the fact that CuCrO2purity was maximized at a ratio of4:3 Cr/Cu, consistent with the proposed stoichiometric reaction: 4Cr3++ 3Cu2++20OH--> 3CuCrO2+ CrO42-+10H2O. Using a 4:3 ratio of Cr/Cu starting materials and allowing thesynthesis to proceed for 60 h eliminates the presence of CuO beyond detection by powder X-raydiffraction (pXRD). Furthermore, washing the solid product in 0.5 M NH4OH removes Cu2O and Cr2O3impurities, leaving behindthe isolated CuCrO2product as confirmed using pXRD and inductively coupled plasma mass spectrometry.

Automated summary

The optimized hydrothermal synthesis of CuCrO2nanocrystals can limit the production of byproducts and result in a high-purity CuCrO2product. This material has a wide band gap and high conductivity, making it suitable for various heterojunction solar cells.