Dear Mikhail,

You have asked a very key and important question, which scientists are still trying to fully understand.

We have studied hydrothermal synthesis of TiO2 nanoparticles and monitored the system in situ using synchrotron radiation EDXRD (in other words, we watched the crystal as they formed and grew in real time!!).

In hydrothermal process, the crystals do grow in the solution as a result of solvent evaporation and other chemical changes, depending on type of system under observation.

In simple terms, the formation of crystals in solutions occur in two main steps, nucleation followed by crystal growth. It takes few minutes (induction time, again depending on your system) before the nucleation starts (formation of nuclei) and then crystals start to grow by a number of possible mechanisms such as Ostwald ripening, polymorphic transformation etc.

Generally the crystal growth step takes longer time, and size of particles increase as you increase the reaction time. Furthermore, the crystallinity of crystals is also enhanced with longer reaction times. Please refer to our paper to see the hydrothermal process step by step in real time.

http://pubs.rsc.org/en/content/articlelanding/2015/ce/c4ce02270j#!divAbstract

All the best!

Dear Dr. Mohammad Rehan,

Thanks for the link, I have read the paper. In-situ observation of the hydrothermal reaction by EDXRD is a perfect way to understand hydrothermal synthesis.Would you be so kind to answer the following questions regarding the paper?

1. What do you mean under "HNO3 peptized sol-gels"(of hydrolysed TiO2)?

I found this in WIKI regarding peptization of TiO2 nanoparticles:

In the synthesis of titania (titanium dioxide) nanoparticles, peptization involves adsorption of quaternary ammonium cation on the titania surface. This causes the surface to become positively charged. Electrostatic repulsion of the primary particles in the agglomerated titania breaks up the agglomerate into primary particles"

However, did you investigate the precursor for hydrothermal synthesis? What was the particles size?

2. Aren't you surprised that the obtained crystals had rutile structure? I have not seen rutile TiO2 grown by hydrothermal synthesis before(but I have not read a ton of papers as well). According to the TEM images, there were particles with sizes of 5-15 nm. However, in [DOI: 10.1021/jp000499j] it was found by isothermal and isochronal annealings, that thermodynamic stability of the anatase and brookite phases depend on the particle size. So, the particle size of less than 11 nm corresponds to stability of the brookite phase, for particle sizes between 11 nm and 35 nm, the stable phase is anatase and rutile is the most stable phase at the sizes greater than 35 nm. Furthermore, the diffraction peak of the most stable plane of anatase TiO2, (101) peak, is very close to rutile (101) peak. So, why did not you observe it? I guess, the peak at ~ 25 degrees in the fig. 3 is for anatase (101)TiO2, right?

3. What else except TiO2 particles were in the products obtained from the hydrothermal synthesis? How did you separate particles(Did you?)?

4. What could be the nucleation centers for initiation of growth of TiO2 nanoparticles in the PTFE vessel?

It is very sad that the cooling process was not monitored. Did you compare particle sizes(evaluated using the Sherrer equation) of the last measured in-situ pattern with ex-situ ones?

Thanks!

Dear Mikhail,

In situ XRD indeed provide some useful information. We also did in situ XRD on the solvothermal synthesis of TiO2. On top of in situ XRD, we also did in situ small angle X-ray scattering. Here is the link of our paper if you are interested.

https://www.researchgate.net/publication/271273634_Understanding_Solvothermal_Crystallization_of_Mesoporous_Anatase_Beads_by_In_Situ_Synchrotron_PXRD_and_SAXS

Cheers

Fang

Article Understanding Solvothermal Crystallization of Mesoporous Ana...

Dear Dr. Fang Xia,

Thank you very much for the paper. It seems very interesting and I will definitely read it.

However, I have found answers to my questions in [doi:10.1016/S0151-9107(02)90012-7].

So, I would say that first of all, solvothermal growth involves the crystallization of a dissolved solid from a solution by changing thermodynamic stability of desired compound by controlling pH, temperature, pressure and composition of the solution that composes of solvent, precursors, mineralizers and other additives like surfactants or capping agents for control of crystal morphology. The set of reactions occurring in the solution is unique for each solvothermal system. In order to evaluate synthesis conditions suitable for quantitative precipitation of the phase of interest, the yield diagram of the system is obtained by Edisonian trial and error method or computational approach.

The crystallization is occurred in the solution, the final product is suspension. Nevertheless, what plays a role of nucleation centers in solution is still not clear to me. I think that it happens spontaneously through spontaneous growth critical-size nucleus. Probably, that is why the process takes so long. However, it is justified for some purposes that require use of fine metallic powders. Nonetheless, solvothermal synthesis has a myriad of variants and there are ones allowing obtaining thin films on substrates.

http://www.sciencedirect.com/science/article/pii/S0151910702900127

Dear Mikhail,

You are right. Thermodynamics is the pre-requisite for the formation of target material, but for materials synthesis, kinetics is also very important. We want to synthesis materials as fast as possible to minimize time and energy cost. Temperature and specific surface area usually play the most important role in controlling kinetics.

Depending on the size and density of the synthesized materials, they can form a suspension, or if density is high and particle is large, the synthesized material can also sink at the bottom of the autoclave as precipitate.

Nucleation event can be divided into homogeneous nucleation and heterogeneous nucleation. For homogeneous nucleation, crystal nucleate from homogeneous solution, which requires to overcome a high energy barrier, and hence is usually slow. On the other hand, for heterogeneous nucleation, crystal nucleate on pre-existing particles (acting as nucleate centre) or onto the autoclave wall, which requires to overcome a lower energy barrier, and is usually faster than heterogeneous nucleation. If we place a seed for the synthesis, then there is no nucleation event, crystal just grow onto the seed without creation a nucleus. 

Cheers

Fang

Dear Dr. Fang Xia,

Thank you very much for the explanations, they are very useful!

What do you think- are there other ways of in-situ investigation of hydrothermal synthesis process except ones using synchrotron radiation?

Best regards,

Mikhail

Well, we can do in situ XRD in our lab-based diffractometer, but synchrotron-based in situ XRD provides better quality dataset, especially for hydrothermal which involves a broad sovent peak.

Dear Dr. Fang Xia,

Sorry for the inaccurate formulation.

I meant other in-situ techniques.

I thought that synchrotron-based in situ techniques were the only ones for hydrothermal synthesis since it is hard to investigate something in autoclave, but synchrotron provides enough energy to penetrate it. Was I wrong?

What X-ray source do you use in your diffractometer?