Chemical passivation of metal oxides

Cuprous oxide is an energy conversion material with good bulk properties that suffers surface breakdown in aqueous photoelectrochemical cells.

cu2o 300dpi
Photoreduction reactions at cuprous oxide surfaces are more efficient at reducing interfacial cuprous cations to copper metal than reducing aqueous-phase species. Can chemical passivation shut down this reaction?

Cuprous oxide is natively a p-type material, so it performs reduction electrochemistry under illumination. Electrons freed from the bulk migrate to the surface to do some reduction reaction and they’ll reduce whatever is the easiest to reduce without prejudice. In non-aqueous solutions, that is likely some dissolved oxidized species, however, in water, it’s usually easier to reduce interfacial cuprous cations to copper metal. Copper metal makes a bad photovoltaic junction to cuprous oxide, so wouldn’t it be great if we could passivate that cuprous oxide surface?

How do we know it’s a surface
problem and not a bulk problem?

xiang_fig_a

Cuprous oxide achieves open circuit photovoltages in excess of 800 mV in the decamethylcobatoscene+/0 redox couple in acetonitrile with litium perchlorate as the supporting electrolyte under ELH-simulated AM 1.5 illumination. This is the blue trace in the JV plot to the right.

In contrast to the nonaqueous performance, cuprous oxide electrodes achieve significantly lower photovoltages and reduced currents. Contacting aqueous-phase methyl vionlogen+/0, illuminated p-Cu2O achieves a Voc< 200 mV (purple trace). That cuprous oxide electrodes achieve higher performance in non-aqueous redox couples compared to aqueous-phase couples indicates that water is promoting the degredation of the cuprous oxide surface.1

What are we going to do about it?

Let’s come back to our cartoon of the cuprous oxide surface:

cu2o 300dpi
Are you sick of this picture yet?

If cuprous oxide is stable in organic (non-aqueous) solvents and not stable in water, then we have to separate the cuprous oxide from water with some kind of interfacial layer. How are we going to do this? Stay tuned to find out!

Cu2O and MQP’s

Interested in learing about the surface structure and passivation of curprous oxide? Contact Prof. Grimm to learn how this could be your MQP!


1. “820 mV open-circuit voltages from Cu2O/CH3CN junctions”. C. X. Xiang, G. M. Kimball, R. L. Grimm, B. S. Brunschwig, H. A. Atwater, N. S. Lewis. Energy Environ. Sci., 20114, 1311-1318. 10.1039/C0EE00554A