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 CrO4 2– 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 + CrO4 2– + 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.

Lithium Dependent Electrochemistry of p‐Type Nanocrystalline CuCrO2 Films

“…Figure S1 shows powder diffractograms collected pre‐ and post‐annealing to confirm the integrity of the CuCrO 2 solid. XPS studies on post‐annealed CuCrO 2 films also supported the conclusion that Cu 2+ sites are present to charge compensate for Cu + vacancies seen from ICP results, as previously reported [30] . Figure S2 shows Cu XPS spectra where the Cu region indicates a peak at 936.7 eV assigned to Cu 2+ .…”

“…CuCrO 2 nanocrystals were synthesized following a previously published procedure from our group. [30] Solid material produced from that study were further used for experiments reported here, thus some characterization methods have been previously reported. Powder diffractograms are shown in Figure 2a for the synthesized CuCrO 2 nanocrystals after washing in 0.5 M NH 4 OH for 24 hours.…”

“…XPS studies on post-annealed CuCrO 2 films also supported the conclusion that Cu 2 + sites are present to charge compensate for Cu + vacancies seen from ICP results, as previously reported. [30] Figure S2 shows Cu XPS spectra where the Cu region indicates a peak at 936.7 eV assigned to Cu 2 + . Cr and O XPS data are also shown in addition to the survey scan.…”

“…Synthesis of CuCrO 2 and CuGaO 2 nanocrystals An optimized hydrothermal synthetic procedure was followed as previously reported for delafossite CuCrO 2 nanocrystals [30] . 20 mmol of chromium nitrate nonahydrate [Cr(NO 3 ) 3 ⋅ ⋅9 H 2 O] (Alfa Aesar, 98.5 %) and 15 mmol copper nitrate hemipentahydrate [Cu(NO 3 ) 2 ⋅ 2.5 H 2 O] (Alfa Aesar, 98 %+) were added to 70 mL deionized H 2 O (18 MΩ cm, Milli‐Q) and stirred in an ice bath.…”

“…Synthesis of CuCrO 2 and CuGaO 2 nanocrystals An optimized hydrothermal synthetic procedure was followed as previously reported for delafossite CuCrO 2 nanocrystals. [30] After transferring the mixture to the Teflon cup and sealing in an acid digestion bomb (Parr), the vessel was placed into a box furnace (Lindberg Blue M) and heated to 240 °C for 60 hours. After cooling to room temperature, the bomb was removed and the resulting mixture was centrifuged to separate the solid product from solution.…”