Defining the Role of Cr 3+ as a Reductant in the Hydrothermal Synthesis of CuCrO 2 Delafossite.

The synthesis of nanocrystalline, p-type delafossite metal oxides (CuMO 2 ) 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, Cu 2 O, and M 2 O 3 ) which could ultimately influence the optoelectronic properties of the resulting nanocrystals. Here, we report on the optimized hydrothermal synthesis of CuCrO 2 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 Cr 3+ as the reductant used to reduce Cu 2+ to Cu + . This was confirmed by detection and quantification of CrO 4 2- as a product of hydrothermal synthesis in addition to the fact that CuCrO 2 purity was maximized at a ratio of 4:3 Cr/Cu, consistent with the proposed stoichiometric reaction: 4Cr 3+ + 3Cu 2+ + 20 OH - → 3CuCrO 2 + CrO 4 2- + 10 H 2 O. 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 NH 4 OH removes Cu 2 O and Cr 2 O 3 impurities, leaving behind the isolated CuCrO 2 product as confirmed using pXRD and inductively coupled plasma mass spectrometry.