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

[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.

 

[Borek]

My bet would be on a mechanism similar to the precipitate maturation (which is based on thermodynamics similar to the Ostwald maturation).

 

[zbikraw]

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.