I have a solution of black phosphorus quantum dots suspended in water that I prepared by sonicating black phosphorus powder in n-methyl-2-pyrrolidone and centrifuging to isolate the quantum dots. Repeat measurements of the absorbance of this solution over a period of days resulted in a decrease in the absorbance; I know this to be due to the instability of the black phosphorus when exposed to water, air, and light.
However, I also have a solution prepared in the same way using graphene nanoplatelets instead of black phosphorus. Testing the graphene solution over a period of days resulted in an increase in absorbance. Why might the absorbance be increasing in the case of the graphene in water?