The GO/PES membrane was prepared with vacuum filtration method. The GO aqueous solution was filtrated with a PES ultrafiltration membrane. As a results, the GO/PES membrane was obtained, that the surface is GO layer and the beneath is the PES layer.

At first, the surface of the GO layer was yellow.

When stored in air for one month at the room temperature, the surface of the GO/PES membrane became black.

After two months, the surface became darker.

Someone suggests GO became rGO during the storage.

Then, what happened during the storage?

Did some chemical in air react with GO?