Taba
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Practical Guide to g-C3N4
As I have done a lot of research on g-C3N4 I have decided to make this practical guide on how to synthesize and use g-C3N4 for other people who also
want to contribute in understanding and fully utilizing this material.
g-C3N4 is used as a photocatalyst and catalyst support. Advantages over other catalysts are high surface area, easy synthesis and that it does not
contain any metals. It is commonly used as a catalyst for CO2 reduction or for Water splitting. In its pure form g-C3N4 is pretty bad at both of these
reactions, but it is improved significantly by doping it with other materials.
There are lot of ways to synthesize g-C3N4, but most of these are not practical. In general you can heat most compounds which contain both nitrogen
and carbon to 600°C and will receive some g-C3N4. But there is little reason to use complex molecules like tetrazoles to synthesize g-C3N4 when you
can just heat simple molecules like Urea, Thiourea, Melamine or similar ones. The only reason to use other precursors is to introduce more defects by
slightly altering nitrogen or carbon content.
I have found that for some reason a 2:1 mixture of Urea and Thiourea polymerizes especially well, even at temperatures below 600°C.
An example procedure to synthesize g-C3N4 by pyrolysis is following:
10g of Urea are added to a crucible. The crucible is covered and heated to 600°C in an oven for 2 hours. The g-C3N4 is should be washed with water
and/or ethanol and be seperated by filtration or centrifugation.
There is not really a need for an inert atmosphere in this synthesis as the pyrolysis releases a lot of ammonia which will protect the g-C3N4 from
oxidation.
Lowering the temperature of the synthesis will result in increased reaction time, smaller polymer size, increased hydrogen content, worse
semiconductor properties and more defects. Higher temperature will also result in more defects, but also more degradation and yield loss.
If you want the g-C3N4 to have a different morphology you can try carrying the mixture out in molten salts, for example eutectic mixtures of alkali
chlorides, as they wont interfere with the polymerization.
There are a lot of more nieche ways to synthesize g-C3N4. For example you can synthesize g-C3N4 electrochemically from melamine and cyanuric chloride.
I have tried this and have not succeeded, needless to say I do not recommend this.
A synthesis that as far as I know was discovered by me, is introducing Melamine to a relatively cold argon plasma in a microwave. This method is not
really practical unless you are trying to synthsize a lot of g-C3N4. I will not further discuss this method in this post but maybe in a future one.
If you are for some reason researching yet another new method for synthesizing g-C3N4, here is a list of things I have tried which have not worked:
Ball milling melamine in different conditions (oxidizing, acidic, basic, and with small amounts of graphite) (it will become grey sludge). Reducing
TCCA with magnesium (results in some red crystals). Heating a mixture of copper(I)oxide and Melamine in a microwave (resuls in black sludge).
A method not yet tried, but I think is worth trying, is converting melamine into its triisonitrile by reacting it with chloroform and then
polymerizing that by heat and maybe transition metal catalysis. This reaction could maybe be carried out at a much lower temperature than normal
pyrolysis methods and increase the yield.
g-C3N4 can be exfoliated into this layers by ultrasonicing it for a few hours in water. This seems to be working quite well in my experience. Drying
the g-C3N4 can be done by mixing in acetone, which will knock the g-C3N4 out of suspension. The water acetone mixture can then be decanted. After this
has been repeated 4 times the acetone will quickly evaporate and will leave behind mostly dry g-C3N4.
If you are unsure if you really have made g-C3N4 there are a few quick tests to ensure that you have been successful.
The color (unless you have doped it with something) should be offwhite to yellow, normally a more yellow color means a high degree of polymerization
or a deviation from the normal C N ratio.
g-C3N4 should not release anymore ammonia when being heated.
If you have an IR or any other actually good analytical method you can easily tell it apart from your educts or not fully polymerized intermediates.
Another way is to test if it will degrade dyes when combined with them in water and put into sunlight. You should only put a very small amount of dye
into the solution, as the degradation is very slow. Suspending and ultrasonicating the g-C3N4 makes this process much faster (it will still take a few
days). If you have doped your g-C3N4 this sometimes will give false positives, as metal complexes may also be able to degrade the dye.
Pure g-C3N4 is not very catalytically active. I can be made more reactive by doping it with transition metals or by adding Photosensitizers. Adding
non-metals or semi-metals like sulfur, chlorine, flourine, phosphorous, silicon or boron or tweaking the carbon nitrogen ratio can also increase
efficiency, although it is less commonly used.
In general non-metals or semi-metals should be introduced before polymerization, while transition metals or photosensitizers should most commonly be
added by ultrasonication after the g-C3N4 has already formed, although there are many exceptions.
Cu(I)oxide particles can for example be added onto the exfoliated g-C3N4 sheets by reducing Cu(II) salts with aldehydes or ascorbic acid in a basic
solution while ultrasonicing the mixture.
g-C3N4 is also often combined with r-GO (reduced graphene oxide) or semiconductors to increase its surface area and electron transfer properties.
Using the g-C3N4 in a photochemical reaction is pretty straightforward. For example to use it to reduce CO2 you just need to mix it with water, put a
balloon filled with CO2 over it, although atmospheric CO2 also works, it just takes longer, and put it into the sunlight. To analyze the reduction
products you sadly will probably need very good analysis methods as GC-MS. This is because reasearch into g-C3N4 is still very new and the efficiency
we currently have is very bad. Using the g-C3N4 for water splitting photocatalysis is probably easier, as you can just test the volume of the gas
being created. A more general test to roughly estimate the efficiency of your g-C3N4 sample is to test its dye degradation capabilities. I have always
used methylene blue, but I am sure other dyes work just as well.
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bnull
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Could you gives us details of one of your runs? Also, some references you consulted.
Someone tried urea and aluminum oxide about four years ago (Making Graphitic carbon nitride (g-C3N4) from urea).
Quod scripsi, scripsi.
B. N. Ull
P.S.: Did you know that we have a Library?
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Taba
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I just looked at the post you linked in your reply. The yield looks pretty normal, maybe slightly below average. The consistency of the product should
be more flaky, but maybe the aluminium oxide makes it form these strange balls. I have actually never tried catalysing the pyrolysis with aluminium
oxide as it works pretty well on its own without any catalysis.
Here are a few of the papers I used:
https://doi.org/10.1016/j.apsusc.2020.148293 -> co2 reduction mechanism
http://dx.doi.org/10.1021/acssuschemeng.7b00575 -> carbon doped g-C3N4
http://dx.doi.org/10.1016/j.jcis.2017.01.111 -> a more simple carbon doped g-C3N4
http://dx.doi.org/10.1016/j.mtsust.2023.100430 -> c3n4 doped with ferrate for co2 reduction
http://dx.doi.org/10.1016/j.apsusc.2020.148293 -> co2 reduction using cu(I) doped g-C3N4 theoretical analysis
http://dx.doi.org/10.1016/j.apsusc.2021.150448 -> co2 reduction using cu(I) doped g-C3N4, this one is pretty good i think, it is very simple to
replicate
http://dx.doi.org/10.1016/j.carbon.2018.12.104 -> c3n4 synthesis by pyrolysis using ammonium thiocyanate and urea mixture
https://doi.org/10.1002/smtd.202201013 -> c3n4 for co2 reduction review, this paper helped me when I was just getting started
I never read any papers about c3n4 for water splitting, as my research was mostly focused on co2 reduction and synthesis of c3n4.
I don't know what details of my runs you are looking for. The pyrolysis synthsis methods are just as simple as heating the stuff in a covered crucible
and then washing it after. On some of the other methods, which are not simple at all, I will make a dedicated post. If you have any specific questions
on any procedures mentioned here I whould be glad to answer them tho.
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bnull
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Quote: Originally posted by Taba  | | I don't know what details of my runs you are looking for. The pyrolysis synthsis methods are just as simple as heating the stuff in a covered crucible
and then washing it after. On some of the other methods, which are not simple at all, I will make a dedicated post. If you have any specific questions
on any procedures mentioned here I whould be glad to answer them tho. |
Just the usual stuff but, since you'll be writing about the other methods, my questions would be sort of pointless.
Thanks for the references, I'll give them a look.
Quod scripsi, scripsi.
B. N. Ull
P.S.: Did you know that we have a Library?
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