Silver-Copper Mixed Oxides and Soft Chemistry

If you catch your nearest colleague and ask him/her the following question: do any silver-copper mixed oxide exist? Chances are that he/she will answer “yes”. Many of them would indeed say so. This is not surprising if you think about the two metals separately, as they both form many different oxides (Ag₂O, AgO, Ag₂O₃, Ag3O4, Cu2O, CuO or Cu₄O₃). Both metals can even be found together in several compounds such as AgCuS, AgCuSe or AgCuPO₄. But does a mixed oxide really exist? Well, as it turns out, it did not until 1999; before the twenty first century, not a single example of a mixed silver-copper oxide was known, neither natural nor synthetic. Mother Nature can be that whimsical!

In 1999 then, Dr. Eva Tejada-Rosales and Prof. Pedro Gómez-Romero, in the Solid State Chemistry group in the Materials Science Institute of Barcelona (ICMAB-CSIC), discovered the first Ag-Cu mixed oxide, Ag₂Cu₂O₃, which is isoestructural with paramelaconite, Cu₄O₃, a Cu oxide containing Cu+1 and Cu+2. Incidentally, paramelaconite itself has not been synthesized in the lab until quite recently!

The interest in this new family came from trying to synthesize less toxic high temperature superconducting (HTSC) cuprates, by replacing Hg by Ag (a goal pursued by many labs around the world). Although no HTSC Ag-Cu oxide has been found to date, the new members of this recent family present quite interesting properties. Ag oxides are typically synthesized by means of high-temperature and high-pressure processes (in order to avoid Ag reduction during synthesis at high temperatures) which involve expensive equipment and are energetically very costly. In the case of the synthesis of Ag-Cu mixed oxides soft chemistry methods have proven to be very efficient.

Indeed, during my PhD thesis supervised by Prof. NIeves Casañ-Pastor, a novel synthetic approach, namely, electrochemical oxidation of suspensions, yielded the new phase AgCuO2 (initially formulated as Ag₂Cu₂O₄ since it was obtained by electrochemical oxidation of Ag₂Cu₂O_3, Electrochem. Comm. 2002). This oxide was also synthesized by chemical oxidation/precipitation and by by room temperature chemical oxidation with ozone. AgCuO₂ is isostructural with crednerite (CuMnO₂) which contains monovalent Cu and trivalent Mn cations. But despite sharing the same crystallographic structure, the electronic structure of AgCuO₂ is very different to that of CuMnO₂. AgCuO₂ presents indeed a very interesting electronic structure where both Ag and Cu have an intermediate oxidation state (+1,x and +2,y, respectively) and the charge is de-localized among all the atoms, as deduced from XPS and XAS studies (Journal of Physical Chemistry B, 2005). The use of ozone gave valuable information about its electronic structure and the formation mechanism, i.e. the formation of AgCuO₂ proceeds through oxidation of Ag atoms which then oxidise the Cu atoms in the oxide yielding the final mixed valence structure (J. Solid State Chem, 2005).Additionally, the values of x and y, i.e. the oxidation state of Ag and Cu, depend on the synthetic method used and are also sensitive to strong radiations. The Ag cations present a distorted 6-octahedral coordination, as shown in the figure.

PUBLICATIONS:

Reaction pathway of the hydrothermal synthesis of AgCuO₂ from in situ time-resolved X-ray diffraction

Liu, Hongjun; Grendal, Ola; Skjærvø, Susanne; Dalod, Antoine; van Beek, Wouter; Sekkat, Abderrahime; Einarsrud, Mari-Ann; Muñoz-Rojas, David*

Crystal Growth and Design, 2020, 20, 7, 4264–4272

Ag₂Cu₃Cr₂O₈(OH)₄:A new bidimensional silver-copper mixed -oxyhydroxide with in-plane ferromagnetic coupling

Nieves Casan-Pastor*, Jordi Rius, Oriol Vallcorba, Inma Peral, Judith Oró-Solé, Daniel Cook, Richard I Walton, Alberto Garcia and David Muñoz-Rojas*

Dalton Transactions, 2017,46, 1093-1104.

Silver-copper mixed oxides. The unprecedented family.

D. Muñoz-Rojas*

Materials Today, 14(3), 119, 2011.

High Conductivity in hydrothermally-grown AgCuO₂ single crystals verified using FIB-deposited nanocontacts

D. Muñoz-Rojas*, R. Córdoba, A. Fernández-Pacheco, J. M. De Teresa, G. Sauthier, J. Fraxedas, R. I. Walton, N. Casañ-Pastor

Inorganic Chemistry, 49, 10977–10983, 2010.

Transport properties and lithium insertion in the p-type semi-conductors AgCuO₂ and AgCu_0.5Mn_0.5O₂.

F. Sauvage*, D. Muñoz-Rojas*, K.R. Poeppelmeier, N. Casañ-Pastor

Journal of Solid State Chemistry, 182, 374–380, 2009.

Ag₂CuMnO₄: A new Silver Copper Oxide with Delafossite Structure.

D. Muñoz-Rojas, G. Subías, J. Oró-Solé, J. Fraxedas, B. Martínez, M. Casas-Cabanas, J. Canales-Vázquez, J. González-Calbet, E. García-González , R.I. Walton, and N. Casañ-Pastor*

Journal of Solid State Chemistry, 179, 3883-3892, 2006.

Electronic Structure of Ag₂Cu₂O₄. Evidence of Oxidized Silver and Copper and Internal Charge Delocalization.

D. Muñoz-Rojas, J. Fraxedas, G. Subias, P. Gómez-Romero, N. Casañ-Pastor*

Journal of Physical Chemistry B, 109, 6193-6203, 2005.

Room Temperature Solid State Transformation from Ag₂Cu₂O₃ to Ag₂Cu₂O₄ by Ozone Oxidation.

D. Muñoz-Rojas, J. Fraxedas, J. Oró, P. Gómez-Romero, N. Casañ-Pastor*

Journal of Solid State Chemistry, 178(1), 295-305, 2005.

Structural Study of Electrochemically-Synthesized Ag₂Cu₂O₄. A Novel Oxide Sensitive to Irradiation.

D. Muñoz-Rojas, J. Fraxedas, J. Oró, P. Gómez-Romero, N. Casañ-Pastor*

Crytal Engineering, 5, 459-467, 2002.

Electrochemically Induced Reversible Solid State Transformations: Electrosynthesis of Ag₂Cu₂O₄ by Room Temperature Oxidation of Ag₂Cu₂O_3.

D.Muñoz-Rojas, J. Oró, P. Gómez-Romero, J. Fraxedas, N. Casañ-Pastor*

Electrochemistry Communications, 4, 684-689, 2002.

Electrochemistry of Ag₂Cu₂O₃ and synthesis of Ag₂Cu₂O₄.

D. Muñoz-Rojas, E.Mª Tejada-Rosales, P. Gómez-romero, N. Casañ-Pastor*

Bol. Soc. Esp. Cerám. Vidrio, 41 [1], 55-58, 2002.