Interest in the sol-gel process of the transition metal alkoxides has been predominant among scientists for quite a long time. Until recently, its classical description concerning the reactivity and structure of molecular precursors in the kinetically curbed hydrolysis-polycondensation process had not been questioned. This has led to serious inquiries on how the hydrolysis of the homometallic oxides proceeds through the hydroxide intermediates. Minimal information has been collected concerning the hierarchy of contrasting smallest metal-oxo-alkoxy units, nanoparticles and clusters that appear in the commencement of a chain of the sol-gel alterations.
Dr. Andrei Kanaev, Professor Khay Chhor and PhD student Khley Cheng at the Laboratory of Sciences of Processes and Materials of C.N.R.S. in France proposed to look into the solvent effect on nucleation-growth of titanium-oxo-alkoxy nanoparticles. They hoped to dispute the unconsciously accepted classical description. Their work is now published in Chemical Physics Letters.
First, two thermostatic stock solutions containing titanium tetra-isopropoxide and water in alcohol solvents were added into the exocentric T-mixer through different inlet tubes at high Reynolds numbers ~6·103. Transfer of the stock solutions to the thermostatic reactor after the glove box operation was under dry nitrogen gas flow so as to curb from atmospheric contamination. The solution was then let to exit the mixer through an outlet tube that observes the Reynolds number and the flows were maintained equal by a nitrogen gas pressure. Dynamic light scattering and static light scattering methods were used to monitor size and light intensity of particles in the reactor solutions.
Automatic sampling mode was applied with the data accumulation periods of 60 seconds being observed. This method was seen to allow easy refutation of non-desirable events owing to occasional dust particles that emit strong light scattering spikes and the accumulation of good signal-to-noise ratio even from tiny particles.
The authors of this paper were able to observe that for both samples used, the appearance of a precipitate comes after definite time called the “induction period”. During this period, small nanoparticles appeared and grew by a mutual aggression. The researchers observed that the nanoparticle size in n-propanol was significantly smaller as compared with that in the isopropanol, while the size of the smallest nucleus was similar in both solvents. Also, they were able to realize that the reactivity of titanium oxo-alkoxy species in the isopropanol is higher as compared to the n-propanol. This was noted to have occurred since similar induction times were obtained with much lesser water content.
Based on the general description of the polycondensation and hydrolysis reactions, it was assumed that the two co-exist only when; the first one was fully reversible and the second has a critical condensation number upon which the reversibility is canceled. The amount of water consumed by the process was controlled by the critical hydrolysis ratio.
The research team noted that a better selection between possible hydrolysis ratio values could provide better and direct measurements of the induction period as opposed to the limiting value used for the experiments.
Based on the comparative study conducted it is notable that the nucleus size is stable in the beginning of the sol-gel process and maintains in the large parametric domain. Large hydrolysis ratios lead to the colloid unit becoming a Nano particle and at this point both the particle size and reaction kinetics are sensitive to the solvents. The results here suggest that the stability of the growing titanium oxo-alkoxy species declines with an increase of their size therefore supporting the hierarchical model of the sol-gel growth.
Khley Cheng, Khay Chhor, Andrei Kanaev. Solvent effect on nucleation-growth of titanium-oxo-alkoxy Nanoparticles. Chemical Physics Letters volume 672 (2017) pages 119–123
Laboratoire des Sciences des Procédés et des Matériaux, CNRS, Université Paris 13, Sorbonne Paris Cité, 93430 Villetaneuse, France.Go To Chemical Physics Letters