**5. Conclusions**

There are also other studies that focus on designing water splitting systems by mixing TiO<sup>2</sup>

ability to dissociate water exposing an exothermic dissociation energy of −0.7 eV with a small activation barrier of 0.04 eV. A relatively larger system is investigated by Pastore and

Ru polypyridyl dye, acting as a linker between the oxides, is attached to both the anatase

shows the orientation of the Ru‐dye with respect to the oxides. Electronic structure analysis

cally active and efficient devices, one has to use adequate theoretical methods. Although DFT

systems, (c). Reprinted with the permission from Ref.~\cite{pastore2015} Copyright (2015) American

**Figure 4.** Optimized molecular structure of Ru‐dye in its partially deprotonated form, grafted to the (TiO<sup>2</sup>

.2H<sup>2</sup> \rm\_2<sup>2</sup>

tions, it also has some limitations. Accurate results can be obtained by increasing system size,

reproduce the correct behavior at the surface and increase the calculated accuracy, increasing system size also increases computational effort. In the case of AIMD runs required computa‐ tional source becomes even more expensive than DFT calculations. Therefore, one needs to find a compromise between the accuracy and the computational cost. In this regard, Harris

(110) surface. Results show that the proposed mixed‐metal oxide has a promising

surfaces via phosphonic acid and malonate groups, respectively. **Figure 4**

and TiO2

.

O3

VB, its unoccu‐

\rm\_2<sup>2</sup> )2 \

\rm\_2<sup>2</sup>

complex using DFT. In the designed complex

showing a metallic‐like character.

complexes and develop photocatalyti‐

‐based complexes for photocatalytic applica‐

O nanoparticle (b), and tethered across the TiO<sup>2</sup>

slab. While the thickness of the slab has to be sufficient to

doped

with another metal oxide. Graciani et al. [95] modeled water adsorption on the Ce<sup>2</sup>

/Ru‐dye/IrO<sup>2</sup>

While the occupied molecular orbitals of the dye are located within the IrO<sup>2</sup>

shows that both the HOMO and LUMO are located on IrO<sup>2</sup>

pied orbitals are distributed over the CB of both IrO<sup>2</sup>

**4. Challenges of DFT‐based simulations**

is a powerful tool to analyze and screen TiO2

i.e., increasing layers of the TiO<sup>2</sup>

cluster (a), to the (IrO22

In order to understand the physical properties of TiO<sup>2</sup>

\rm\_2<sup>2</sup> )2 \rm\_{56}<sup>2</sup>

rutile TiO2

200 Titanium Dioxide

TiO2

rm\_{82}<sup>2</sup>

and IrO<sup>2</sup>

\rm\_2<sup>2</sup>

Chemical Society

De Angelis [96] who modeled TiO<sup>2</sup>

(101) and IrO<sup>2</sup>

Search for renewable energy sources leads scientists to benefit from sunlight and convert pho‐ ton energy to chemical/electric energy using TiO<sup>2</sup> ‐based materials. Although photocatalytic water splitting and DSSC applications are accomplished using TiO<sup>2</sup> surface, large band gap of the oxide limits absorbing photons in the visible spectrum thus hindering device efficiency. Therefore, functionalizing TiO<sup>2</sup> surface by adsorbing photosensitizers and/or water reduc‐ tion/oxidation catalysts, by metal/nonmetal deposition, or by mixing with other oxides, the optical response of the complex can be shifted from UV to the visible region. This is the crucial requirement in designing promising, robust, and scalable photocatalysts toward water split‐ ting and DSSC applications.

Together with the improvement in the computational power, today, DFT is an important tool to obtain optimized geometries of the complexes, analyze electronic structures, model many spectroscopic techniques, determine intermediate states of the reactions, and so on. In particular, it is powerful for modeling several TiO<sup>2</sup> ‐based materials and testing their physi‐ cal/chemical/optical characteristics for photocatalytic applications. Although there are some limitations in DFT, several new exchange‐correlation density functionals and van der Waals correction schemes have been proposed to increase the flexibility and accuracy of the model. Using DFT, all phases of TiO<sup>2</sup> surfaces, i.e., rutile, anatase, and TiO2 nanoparticles can be modeled. The outcome of the simulations serves as an initial knowledge on the systems for scientists without experimental effort.
