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

In summary, the article discusses the formation of nano-scale features on the surface of titanium after acid-etching with hot HCl-H2SO4 solution. These features, measuring 20-30 nm in size, develop after 2 weeks of immersion in water and are not seen in air. It is speculated that the mechanism behind this process is similar to precipitate maturation, where dissolution and formation of crystals occur on highly energetic areas of the surface. This slow process results in the production of small, imperfect crystals, while larger ones are typically grown from a solvent-free melt.
  • #1
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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  • #2
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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  • #3
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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Related to How water induces phase change of TiO2 after acid-etching?

1. What is the phase change of TiO2 after acid-etching?

The phase change of TiO2 after acid-etching refers to the transformation of its crystal structure from anatase to rutile. This process is commonly known as the "phase transition".

2. How does water induce this phase change?

Water plays a crucial role in inducing the phase change of TiO2 after acid-etching. It acts as a catalyst, facilitating the movement of ions within the crystal lattice of TiO2, ultimately leading to the transformation from anatase to rutile.

3. What are the factors that affect the phase change of TiO2 after acid-etching?

There are several factors that can influence the phase change of TiO2 after acid-etching. These include the concentration and temperature of the acid solution, the duration of etching, and the purity and structure of the TiO2 sample.

4. Why is the phase change of TiO2 after acid-etching important?

The phase change of TiO2 after acid-etching has significant implications in various fields, such as photocatalysis, solar energy conversion, and biomedical applications. The anatase to rutile transformation can alter the properties and performance of TiO2, making it more suitable for specific applications.

5. Can the phase change of TiO2 after acid-etching be reversed?

No, the phase change of TiO2 after acid-etching is irreversible. Once the transformation from anatase to rutile occurs, it cannot be reversed back to its original state. However, the rate of transformation can be controlled by adjusting the etching conditions and parameters.

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