I am synthesizing {001} facets TiO2 particles by addition polyvinyl alcohol (PVA) to hydrothermal solution along with HF, and isopropanol : water. I observed that when hydrothermal time is increased from 1hr to 10hrs, the morphology of TiO2 particles also changed. For example at 1 hr of hydrothermal treatment the shape of TiO2 particles were of decahedral geometry, at 2hrs of hydrothermal time formation of microscopes occurs, at 3 hrs of hydrothermal treatment the size of micro spheres increases, at 5 hrs of hydrothermal time the formation of sheet occurs within the microsphere, at 10 hrs of hydrothermal time the sheets are made independent of micro spheres.
Any suggestions for the plausible mechanism of this shape evolution with increase in hydrothermal time by addition of PVA?.
FESEM image is attached.
FESEM of {001} facet TiO2 films prepared at hydrothermal time of 1 hr (a), 2 hr (b), 3 hr (c), 5 hr (d), and 10 hr. Other conditions are 110 mL water solution, 60 mL iso – propanol, hydrothermal temperature of 180˚C, HF = 0.03M, pH = 2.8 (natural pH of hydrothermal solution) and PVA = 8mg.
Your comparing experiments are not supported, I can't have any answer for you!!!
I suggest: Firstly, the surface of such micro sphere is not smooth and the micro spheres are possible to be with hollow structure, you should observe the microstructure of them clearly by HRTEM at least. Secondly, too much HF could lead to the erosion of TiO2, try to decrease the concentration of HF for comparing experiments.
Best wishes for you!
It would help to know the device configuration in which the hydrothermal experiments are carried out. The hydrothermal technique applied over the lengthy period of the experiments can be very deliterious. It would be surprising if the material you synthesize has significant crystallinity. If a crystalline phase is formed, the extended hydrothermal exposure should distort the material significantly over the long periods of exposure. Is the pH of the solution 2.8 after the experiments are completed. I ask this question because you have quite a cocktail of materials and over the extended periods of the experiment these materials can and will be converted and affect the agglomeration in very different ways. As Yunyu Cai notes, HF may be a problem. However, potential intermediates are also a consideration. Can you check the pH of the hydrothermal solution after 1,2 3,5 and 10 hours. I would try these assessments before HRTEM simply because I think the results you will obtain will be too divergent to cast light on the conflicting processes. Good luck!
First, you do not tell how the evolution without addition of PVA looks like. PVA can act in different ways, and will probably alter the relative growth kinetics of the different TiO2 facets. This means, an originally compact crystal shape could change into a flat one as one can see in your image (a). The intrinsic crystal shape is very important in shape evolution. Second, organic molecules could act as surface directing agents, which help to form aggregates from the smaller TiO2 particles, similar to zeolite growth from hydrotermal solution.
From the pictures you supply I would propose the following sequence: First, TiO2 nucleates and grows with a shape close to its ideal minimum surface energy form (Wulff shape). Possibly nucleation is suppressed (addition of PVA?) and new grains nucleate in the vicinity of the existing, hence globular microcrystalline spheres evolve, pretty much like spheroidal crystals. In the course of time the solution gets increasingly depleted in the mineralizing agent, and new nucleation cedes completely. Then competitive growth between different orientations within one microsphere takes place, leading to the observed blade-like crystals which are the winners of the growth competition. This crosscutting morphology is very common for many natural minerals like the gypsum or barite 'desert rose'.
very good explain, Frank. That is same as what I think. Firstly, agglomeration occurs. Then with the elapse of time, the nanobuilding blocks are separately crystallized, leading to the distortion of spherical structure and the formation of nanoplates.
In addition to Frank Wendler answer, I would recomend that you run some structure/composition characterization on the powders produced with different time of hydrothermal reaction, as you may as well have produced some non-TiO2 materials. On figures B, C and D the grains with different shape could also present different composition and/or crystalline structure. XRD would be a start which could be complemented by EDS over one grain at a time.
Dear Frank Wendler,
Thank you for your very detail comment. When no PVA was added the shape of TiO2 particles was sheet like morphology with exposed {001} facets as shown in attached file.
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