**Acknowledgements**

On the anatase (101) surface, dissociation of water close to the oxygen vacancy is energetically favored compared to molecular adsorption. Tilocca and Selloni [44] have done a detailed calculation of energy barrier between the molecular and dissociated states. When

pathway. Thus, the dissociation of water on O vacancy of anatase (101) is also attributed to

DFT calculations showed that the (111) facet has a much higher surface energy of 1.61 J/m2

which is attributed to the large percentage of under-coordinated atoms. They also found that this material has much higher photocatalytic activity than other surfaces. Here, there are Ti3

atoms with the ratio 1:3 on the (111) surface. According to our model, the Ti<sup>3</sup>

could contribute two electrons participating interaction. Thus, it is expected that two water

In conclusion, we use the anatase (211) surface as an idea model surface, containing one Ti4

*ab initio* calculations show that the (211) surface is indeed a high reactivity surface with a high

rated bond. Studies of water adsorption suggest two distinct states of adsorbed water on the

each unsaturated Ti bond contributes 2/3 charge in average. As a necessary requirement of chemical reaction, the dissociation of water only occurs when Ti atoms provide more elec-

at surface with high surface energy while the dissociation will not happen for surface with low surface energy. Besides traditional DFT total energy calculation, this model is considered in a fundamental way. We can also declare that the bond charge offered by surface Ti atoms

and applicable to both rutile and anatase surfaces including defects, e.g., step edges and

trons to oxygen in water than H atom. Then, we reach to a conclusion that only Ti<sup>4</sup>

can dissociate water. The controversy about whether Ti5

. In addition, the four-coordinated Ti4

bonds have a stronger chemical reactivity in comparison to the Ti5

atom will eventually become Ti4

under-coordinated atoms in unit cell, to investigate their distinct properties. Our

atoms bridging this O atom tune into Ti5

and then dissociates through a dissociation

) are found on the (111) anatase TiO2 surfaces. Xu

site. It could explain the (111) surface has much

atoms with two unsaturated

sites and the other to dissociated water on

by breaking bond to neighboring O atom

surface. Furthermore, the model is generic

atoms will play a critical role in water decomposi-

structure, we propose a simple bond-charge counting model where

atoms with one unsatu-

atom or

can dissociate water

single crystals exposed (111) facet. Their

and

,

atom

and Ti5

a surface O atom is removed, the Ti6

, respectively. Water oxygen bonds to Ti<sup>4</sup>

 *atom*

et al [34] reported that they prepared anatase TiO2

Recently, the three-coordinated Ti atom (Ti<sup>3</sup>

molecules could be dissociated on a single Ti3

higher photocatalytic activity than other surfaces.

(211) surface, one related to molecular water on Ti5

is the mechanism for water dissociation on TiO2

sites. These results indicate that the Ti4

Ti4

that Ti4

18 Titanium Dioxide

and Ti5

**4. Conclusions**

surface energy of 0.97 J/m2

tion. According to TiO2

equivalent Ti4

vacancies.

is resolved that Ti5

and one Ti5

Ti4

atom.

*3.2.5. Surfaces with Ti3*

We acknowledge the useful discussions with Y.G. Cao. This work was supported by the National Science Foundation of China (Grants No. 10774177, No. 10634070, and No.11174297), and the National Basic Research Project (Grants No. 2010CB923002 and No. 2011CB922204), and the Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China.
