**5.1 Titanium dioxide (TiO2) nanomaterials as a photoelectrode**

Various typed metal oxide semiconductors such as TiO2, ZrO2, SnO2, ZnO, Nb2O5, Fe2O3, Al2O3, (binary compounds) and and ternary compounds such as SrTiO3 and Zn2SnO4 can all act as photoelectrodes in DSSC due to their electronic structures which are characterized by a filled valence band and an empty conduction band [7, 92]. Among these heterogenous semiconductors, TiO2 is the most widely used photoelectrode material because of the large amount of grain boundary and the large interface between the TiO2 surface, dye and the electrolyte in the solar cell, further defects in the crystal structure are expected. In TiO2, the Ti ions are in a distorted octahedral environment and formally have a Ti4+(3d0 ) electronic configuration. The different obital hybridized structure of valence band (VB) and conduction bands (CB) at Ti 3d states occur the decreased the transition probability of electrons to the VB when the electron–hole recombination probability is reduced [92]. Therefore, in view of the electronic configuration and recombination probability TiO2 are the best choice as photoelectrodes among 3d transition metal oxides. TiO2 exsist naturally four commonly known crystalline polymorphs, i.e., anatase (tetragonal, Eg = 3.23 eV), rutile (tetragonal, Eg = 3.05 eV), brookite (orthorhombic, Eg = 3.26 eV) and metastable forms (monoclinic, orthrhombic, cotunnute) [93]. These structures can be described in terms of chains of TiO6 octahedra. The crystal structures differ in the distortion of each octahedron and by the assembly pattern of the octahedra chains [94]. The crystalline strucuture of TiO2 can be seen in **Figure 17** [95]. Both rutile and anatase have tetragonal structure with a = 0.46 nm and c = 0.29 nm (rutile); a = 0.3782 nm and c = 0.9502 nm (anatase). Brookite has orthorhombic structure with a = 0.5456 nm, b = 0.9182 nm, and c = 0.5143 nm and it is very hard to synthesize in the laboratory, while the rutile and anatase can be easily prepared. For solar cell application, anatase structure is more prferred because of 0.1 eV higher the Fermi level, lower recombination rate of electron–hole pairs and lower formation temperature [96].

photoelectrode because of its structural advantages such as high surface area, submicro- or meso-porous structure for light scattering function and better infiltration of electrolyte. There are several different efforts to produce spheres [101, 102]. Nevertheless, there are limitations for the quality control, large-scale production and flexibility. In this book, a simple electro-spraying technique is introduced [103, 104]. As a first step, well-dispersed TiO2 suspensions is very important to make a continuous fluid jet. **Figure 18** show the three phase statues during E-sparying process: (i) TiO2 suspension zone, which strong electric field extracts droplets from Taylor cone (the non-sphere formation), (ii) mixed zone of TiO2 suspension and solid, which non-spheres and spheres are formed by solvent evaporates, leading to the droplet shrink, and (iii) solidified TiO2 zone, depositing

In this systmem, the formation of tight cluster sphere filled sphere is caused by

desireable sphere typed TiO2 film, it is necessary to controll several parameters such as electric field, feed rate, a size of tip and a distance between the nozzle and FTO substrate. It is noted that the the sphere size is controlled by changing the TiO2 concentration and the mixture of solvent in the dispersion solution. In order to find the optimazed condition, TiO2 SPs with the different concentration are tested at 6 μm thick film. **Figure 19(a)** show the SEM results for E-sprayed film prepared from the different concentration at 1 wt%, 3 wt%, 5 wt% and 10 wt%. With increasing the concentration, the diameters of sphere are almost linearly increased, while the number of molecules calculated from UV–vis absorption spectra of desorbed sensitizers reveal that a TiO2 NSs with 5 wt% have about 11.6% and 13.9% higher value than that of 1 wt% and 10 wt% TiO2 NSs film, respectively (see

Experiemntally, E-spraying at the low weight percent of TiO2 make it hard to achieve the competely formed sphere over 8 μm thick film. Therefore, in this reaserch, about 5 wt% TiO2 suspension in EtOH is used as the best condition. The detailed phase diagram accroding to the electrospraying parameters can be seen in the literature [105]. **Figure 19(c)** displays the thickness profiler on the various

*The schematic diagram of the formation of hierarchically structured 0D TiO2 Nanosphere (NS).*

the ultrafast evaporation of alcholic solvent under an electric field. For the

the TiO2 NSs onto the conductive glass.

*A New Generation of Energy Harvesting Devices DOI: http://dx.doi.org/10.5772/intechopen.94291*

**Figure 19(b)**).

**Figure 18.**

**213**

**Figure 17.**

*The schematic TiO2 structure and the DOS calculated band structure for (a) anatase, (b) rutile and (c) brookite TiO2. The big green spheres represent Ti atoms and the small red spheres represent O atoms. Reprinted from [95].*

Thermodynamic calculations based on calorimetric data predict that rutile is the stablest phase at all temperatures, exhibiting lower total lower total free energy than metastable phases of anatase and brookite. The small differences in the Gibbs free energy (4 20 kJ/mole) between the three phases suggest that the metastable polymorphs are almost as stable as rutile at normal pressures and temperatures [97]. The physical and chemical properties of TiO2 nanocrystals are affected not only by the intrinsic electronic structure, but also by their size, shape, organization, and surface properties. For example, if the particle sizes of the three crystalline phases are equal, anatase is most thermodynamically stable phase below 10 15 nm, brookite is most stable between 11 35 nm, and rutile is most stable at sizes greater than 35 nm [98]. Herein, interesting morphologies and properties have recently attracted considerable attention and many nanostructured TiO2 materials, such as nanotubes, nanorods, nanofibers, nanosheets, and interconnected architectures, mesoporus material such as inverse opal and photonic crystal have been fabricated and applied in PV devices [35, 83, 99, 100]. In order to be effective photoelectode, several parameters such as morphologies, pore volume, and the crysltalinity of TiO2 influence the charge transport and recombination processes.
