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

In summary: Micromeretics ASAP2020 machine. 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. In summary, the sample was degassed and the results show adsorption isotherms with negative volumes of adsorbate (N2) entering the tube as P/P0 increases. It is possible that there is something going on with the sample, but I am not sure what it is.
  • #1
mic*
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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?
 
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  • #2
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.
 
  • #3
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?
 
  • #4
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.
 
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Likes mic*
  • #5
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
 

1. What is gas adsorption and how is it used to measure the surface area of graphene oxide based thin films?

Gas adsorption is the process of measuring the amount of gas molecules that are able to adhere to the surface of a material. This method is commonly used to measure the surface area of graphene oxide based thin films because the highly porous structure of these films allows for a large amount of gas to be adsorbed, providing a more accurate measurement of the surface area.

2. What is the BET equation and how does it relate to gas adsorption for surface area measurements?

The BET (Brunauer-Emmett-Teller) equation is a mathematical model that describes the relationship between the amount of gas adsorbed and the relative pressure of the gas. This equation is commonly used in gas adsorption experiments to calculate the surface area of a material, including graphene oxide based thin films.

3. How does the choice of gas affect the accuracy of the surface area measurement for graphene oxide based thin films?

The choice of gas used in gas adsorption experiments can greatly impact the accuracy of surface area measurements for graphene oxide based thin films. It is important to choose a gas molecule that is small enough to penetrate the pores of the film, but not too small to be affected by surface interactions. Nitrogen is the most commonly used gas for surface area measurements, but other gases such as argon and krypton may also be used.

4. What are some common sources of error in gas adsorption experiments for graphene oxide based thin films?

One common source of error in gas adsorption experiments for graphene oxide based thin films is sample preparation. The film must be uniform and free of any impurities or defects to accurately measure the surface area. Another source of error is the choice of gas, as mentioned before, and the accuracy of the instruments used to measure the amount of gas adsorbed.

5. How can the surface area measurement of graphene oxide based thin films be used in practical applications?

The surface area measurement of graphene oxide based thin films can provide valuable information for various practical applications. For example, in the field of energy storage and conversion, a higher surface area allows for more active sites for reactions to occur, increasing the efficiency of devices such as batteries and fuel cells. Surface area measurements can also be used to optimize the performance of catalysts, as a higher surface area can increase the number of active sites for chemical reactions.

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