Gas adsorption for (BET) surface area of graphene oxide based thin films

[mic*]

I am using a Micromeretics ASAP2020 machine. The sample is ~25mg of reduced graphene oxide based film.

The problem is I am getting adsorption isotherms showing negative volumes of adsorbate (N2) entering the tube as P/P0 increases.

The sample was degassed prior to testing at 250°C for 3 hours then backfilled with N2. As a side note, TGA says the sample retains >99% mass up to 400°C.

Any ideas kind people?

 

[mjc123]

Check your apparatus for air leaks. Replace the O rings if necessary. Air slowly leaking into the instrument would reduce the apparent drop in pressure due to adsorption, giving you low adsorption values - in extreme cases, negative values. I have encountered this problem occasionally.

 

[mic*]

Thanks for the advice mjc123. I am pretty sure I have no leaks. I ran an activated carbon sample for comparison and it produced a lovely microporous isotherm.

There is something going on with the sample. I thought perhaps it was thermally related. The sample itself consists of little chopped up pieces of film. My thought was that maybe it is not getting cold enough because there is limited mechanism for heat transfer from the sample to the N2 bath. Otherwise I was thinking it was diffusion related? But I am a bit stuck.

I have run this material numerous times with virtually zero or else actual negative values of gas adsorbed. How can one explain the negative volumes if not through sample outgassing?

 

[mjc123]

I wouldn't use an activated carbon to check for leaks. The specific surface area tends to be very high, and the amount of nitrogen adsorbed by the sample dwarfs the amount admitted via a leak. Leaks show up most with low surface area samples. What is the expected SSA of your graphene oxide, do you know? However, if you're sure there isn't a leak, then I can't think of anything other than sample outgassing. Have you tried degassing for longer before the experiment? I usually degas carbons overnight at 300°C - will your sample stand this? If you're worried about heat transfer, why not mix your sample with some inert (non-reacting, even at the degassing temperature of 300°C) powder of much lower specific surface area? Then if you get a sensible answer, divide it by the mass fraction of graphene oxide in your mixture.

 

[mic*]

TGA suggests it will be fine so I am actually doing that already :)

I expect it to be a high SSA material - the rGO. Unless all the graphene sheets have restacked, which is possible but not probable. But I see ypur point. I might run the same size sample of graphite as a comparison. Cheers