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?

  • $\begingroup$ As usual is difficult to propose an answer without looking at the whole spectra. When you say Abs, is the value at a given wavelength, is the entire spectrum shifting up (down), does it change in shape? In the specific case water is volatile enough to alter concentration. Especially in summer if it happens that you are in the northern emisphere. Else it could be some aggregation, but the spectrum would likely change in its appearance $\endgroup$ – Alchimista Jul 21 '17 at 17:42
  • $\begingroup$ Alchimista, the whole spectrum is shifting down but the shape does not change. I am in the northern hemisphere. $\endgroup$ – Meguna Jul 21 '17 at 18:33
  • $\begingroup$ Then the absorbance is decreasing for graphene as well? If the spectrum is shifting down cannot be that your solution gets concentrated.... $\endgroup$ – Alchimista Jul 21 '17 at 18:41
  • $\begingroup$ Oh, sorry, I miswrote that. The black phosphorus spectrum shifts down. The graphene spectrum shifts up. $\endgroup$ – Meguna Jul 21 '17 at 18:42
  • $\begingroup$ Than look in the literature (no idea at what level you are working) because everything as dots nanoparticles could of course change upon time. As you depict the story, a simple evaporation of water can do that. With water I would be surprised if it take place while measuring. But it can happen at storage level. If you have a small amount of solution/suspension in a relatively large container and you do not shake it, it can even happen because some water evaporate and condense at the top. You take the sample with a pipette and Voilà, you have already a bigger concentration than supposed..... $\endgroup$ – Alchimista Jul 21 '17 at 19:02

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