How water induces phase change of TiO2 after acid-etching?

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SUMMARY

The discussion focuses on the phase change of TiO2 after acid-etching with a hot HCl-H2SO4 solution, leading to the formation of nano-scale features on the surface. These features, measuring 20 to 30 nm, develop over two weeks in water but not in air. The mechanism is hypothesized to be similar to precipitate maturation, influenced by thermodynamic factors and surface defects that promote dissolution and crystal growth. This process is crucial for producing commercially viable small crystals, as larger crystals tend to form in imperfect shapes.

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  • Understanding of TiO2 surface chemistry
  • Familiarity with acid-etching techniques, specifically HCl-H2SO4
  • Knowledge of thermodynamics related to phase changes
  • Basic principles of crystallization and nanoparticle formation
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Material scientists, chemists, and engineers involved in nanomaterials research and development, particularly those focused on TiO2 applications and surface modifications.

hyunmkim
It is reported that TiO2 thin surface amorphous layer left after acid-etching with hot HCl-H2SO4 solution slowly changes from the original flat surface to a layer rich in nano-scale features (granules attached to the surface) of 20 ~ 30 nm in size. You can see these nano-features in the picture at bottom right from Fig 1 of "http://www.medscape.com/viewarticle/777297_3". These nano-features appear in 2 weeks after immersing the titanium which has been etched to make the surface rough with hot HCl-H2SO4. These nano-features develop only in water, not in air. I am wondering how these nano-features are generated from the smooth layer of Ti surface.
 
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My bet would be on a mechanism similar to the precipitate maturation (which is based on thermodynamics similar to the Ostwald maturation).
 
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Neglecting thermodynamics, titania slightly dissolves in both acidic and basic solution in water, and probably some other polar solvents. On the surface you should have "defects" or other highly energetic places, where dissolution is preferred. The same or other "energetic" places are favourable for new crystals forming on old ones. The effect is very minute and proceeds slowly, but is main reason for producing commecially small crystals, as big ones usually forms in imperfect shape. When big crystals are needed one prefers growth from melt, without solvent.
 
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