As I understood, Hamideh was trying to grow MnO2 nanocrystals by hydrothermal method using KMnO4 and MnSO4 using water. She could not detect the formation of MnO2 in her sample by XRD.

Now, while 140 C for hydrothermal treatment I think adequate, the pH control is a key parameter in hydrothermal synthesis (as mentioned by Shridhar and Adam (the mixture solution should be basic in nature with pH in between 8 and 10). If the pH of the reaction mixture is low (which I think is the main problem in this case), instead of MnO2, manganese hydroxide can be formed.

As a test, why do not you air-anneal the obtained sample at about 500 C for a couple of hrs and measure its XRD spectrum again. If after this treatment you get XRD peaks correspond to MnO2, then for sure the pH of the reaction mixture was the problem.

Probably through FTIR you can also detect the signals correspond to manganese hydroxide (can try that too).

Can't understand your question.. you synthesized MnO2 on a paper substrate and annealed at 140C for 12 hours??

Or probably you meant "synthesized MnO2 based on a published procedure"... then your MnO2 is amorphous.. 140C is pretty low temperature for annealing.. but generally MnO2 be it amorphous or crystalline should show a peak at 37-38.

Yes, based on a published procedure...

thank you for explanation.

Your description is not complete. There is no mention of pH control. And as pointed out by Sridhar, you cannot use 140 C for annealing. The most is you can get the water out. You probably need to go to high temperature or 500 C or more to get the metal oxide. Getting it to nano size is a different thing altogether!

Hamideh is probably doing an hydrothermal treatment at a autoclave and not a annealing process.

If you followed a paper, you may want to contact the authors for guidance on that specific synthesis. In your case, hydrothermal is very dependent upon the pressure within the vessel during the heating protocol. Perhaps, you did not achieve a high enough pressure to crystallize the material.

As I understood, Hamideh was trying to grow MnO2 nanocrystals by hydrothermal method using KMnO4 and MnSO4 using water. She could not detect the formation of MnO2 in her sample by XRD.

Now, while 140 C for hydrothermal treatment I think adequate, the pH control is a key parameter in hydrothermal synthesis (as mentioned by Shridhar and Adam (the mixture solution should be basic in nature with pH in between 8 and 10). If the pH of the reaction mixture is low (which I think is the main problem in this case), instead of MnO2, manganese hydroxide can be formed.

As a test, why do not you air-anneal the obtained sample at about 500 C for a couple of hrs and measure its XRD spectrum again. If after this treatment you get XRD peaks correspond to MnO2, then for sure the pH of the reaction mixture was the problem.

Probably through FTIR you can also detect the signals correspond to manganese hydroxide (can try that too).

Let's assume that you've followed a sound published paper, your best shot is to get in touch with the authors for some clarifications on their synthesis.

Yes, the pH is probably a key point, it seems it was a process of dissolution-reprecipitation un-controlled, leading to amorpohus phases. You can do rapidly ATD-TG to see if you have exothermic peak at lw temperatures, is more easiliy that FT-IR.

First dry your sample in oven and anneal the powdered sample upto 600 C for more than 6 hours. than take XRD you will get some peaks and if you want more crystalline peak than anneal it at higher temperatures.

i think that instead of MnO2, manganese hydroxide can be obtained .Probably through FTIR you can detect the signals correspond to manganese hydroxide and if you get it Anneal the obtained sample at about 500 C in air for 3 hr.

The pH of my solution is about 5.5-6! (pH of dionized water in our lab is 5.5)

So there wasn't problem in pH parameter.

the autoclave container has no leakage, so the pressure is OK.

I think it's better I try annealing!

Dear Hamideh,

I see the pH of your DI water is a bit low (than ususal). While I do not doubt your pH meter, While performing annealing (in air) the earlier prepared samples, you can prepare another sample at higher pH value (e.g. pH 9) and you might obtain MnO2 NPs you wanted to get from your hydrothermal synthesis. It would be even nicer if you increase the temperature of hydrothermal treatment to 160 C instead of present 140 C.

Best of luck.

Thank you Mr Pal...

but how I can increase the pH ?

What material is allowed to add into my solution to increase pH?

As you are using KMnO4, I think KOH would be most suitable for incresing the pH of your reaction mixture. However, you can use NaOH too. In any case wash you product at least 5 times with water and finally by ethanol.

There may be probably two main issues that you have to pay attention concerning the XRD analysis:

1) If you have really tiny nanoparticles, even they are crystalline, the XRD peak will be broad and with low intensity. So maybe you will not see the XRD pattern you are expecting.

2) You got the oxide you want but with low cristallinity. In this case, as before, the XRD peaks will be broad and with low intensity.

I belive that you have low cristallinity nanoparticles. Some ajustment during the hydrothermal treatment should be done, something like increase the temperature to 160/180 oC. You can try some ajust in the synthesis methos like pH suggested by the colleagues.

I can do annealing but the problem is I must: 1- synthesis nanoparticles of MnO2

2- synthesis the carbon nanotubes decorated with MnO2 nanoparticles

so need to XRD analysis of both them and I have to synthesis MnO2 and CNT/MnO2 in a same way!!

Can I have the reference of the paper to help you after reading it?

Thank you

I think the mixing of these two compounds in autoclave at such conditions makes new amorphous structure or low crystalline material. You can do more investigations to identify the materials after mixing, such as IR, elemental analysis, and TGA ... etc.

Frederic Chaput:

Sure Mr Chaput

Thank you, I'll read the paper.

.. concerning the comment on the pH of water... dissolved (atmospheric) CO2 gets it down to ca. 5.7 so I am not surprised to see values close to this value. What is quite sure is that hydrothermal synthesis works in basic environment (as most synthesis reactions) so I would also check the pH of the mother liquor before autoclaving to be sure it's not acid (the suggested use of KOH is probably the best solution.. but be also sure to prepare your deionised water and use it immediately. The one you are using for sure remained in open air for long time).

As for the XRD analysis, my experience says that Manganese oxides can be a bit of a nightmare. You could definitely see broad peaks if the domains are nano. However I have seldom seen spherical domains so expect to see anisotropic broadening (broad and narrow peaks). In any case "broad" does not mean "invisible". You can easily demonstrate that already a 2 nm powder gives clear peaks. What most people forget is that the area of the peak is proportional also to the quantity of powder so the peaks can be broad but they should be there if you have sufficient powder.

So what puzzles me is that you say that "no peaks were observed". Physics says that if you have a powder specimen and if you have measured it correctly, you MUST ALWAYS see some signal. it is impossible to have zero scattering of any type. Even an amorphous gives some scattering. So before starting to comment on the preparation I would go back and check why you did not get anything out of the diffractometer. And perhaps be sure that you did not get anything. Is the pattern completely flat or you see some humps? Is it counted enough (the one in the paper you shared is of very pooooooooor quality. Just useful to see that there is a phase and it is layered). Try plotting it in log scale or measuring the specimen again e.g. with a wider step and counting longer. How much powder did you measure?

Optical microscopy and SEM could also help you a lot to understand what's going on.

Thanks for your guidance Mr Leoni...

the analysis show very very small peaks. I study some papers and I found they decreased the pH of the mother liquor before autoclaving. (almost pH=1)