# Sustainable Organic Dyes from Winemaking Lees for Photoelectrochemical Dye-Sensitized Solar Cells
## 3 Results and Discussion  
### Nanostructured Semiconductor  
FEG-SEM microscopy evaluated the morphology of the synthesized TiO\({}_{2}\). Figure 2 illustrates the presence of aggregated nanoparticles with a size between 5-50 nm. After the screen-printing process of titania paste, a thermal treatment is required to obtain a layer with the most suitable characteristics. As expected for this type of oxide, the aggregation of TiO\({}_{2}\) nanoparticles, as well as their average size, increased by increasing the calcination temperature, passing from about 10 nm for the sample that was treated at 400 \({}^{\circ}\)C to about 28 nm for the sample treated at 600 \({}^{\circ}\)C.  
N\({}_{2}\) physisorption at \(-\)196 \({}^{\circ}\)C to evaluate its textural properties as well as the porosity formed during the sol-gel synthesis characterizes the nanostructured titania semiconductor (Table 1). The isotherms of the samples, both before and after calcination (at 400 \({}^{\circ}\)C or 600 \({}^{\circ}\)C) are of type IV according to the IUPAC classification, with a hysteresis loop indicating an ordered pore size distribution in the mesoporous region (Figure 3). Nanostructured titania sample before calcination showed very high specific surface area, SA\({}_{\text{BET}}\), of 206 m\({}^{2}\) g\({}^{-1}\), a pore diameter distribution lower than 20 nm and a pore volume of 0.281 cm\({}^{3}\) g\({}^{-1}\). A correlation between the calcination temperature process and the textural features of these materials  
\begin{table} \begin{tabular}{c c c c c} \hline \hline
**Thermal** & & & & **Average** \\
**Treatment (\({}^{\circ}\)C)** & **SA\({}_{\text{BET}}\)****a (m\({}^{2}\) g\({}^{-1}\))** & **V\({}_{\text{P}}\)****b (cm\({}^{3}\) g\({}^{-1}\))** & **A\({}^{\circ}\)****c (wt\%)** & **R\({}^{\text{d}}\)****d (wt\%)** & **Crystallite** \\  & & & & & **Size\({}^{\text{e}}\)******(nm)** \\ \hline
**As-prepared** & & & & & \\
**TiO\({}_{\text{2}}\)** & 206 & 0.281 & 100 & - & 6.50 (A) \\
**TiO\({}_{\text{2}}\).400** & 136 & 0.236 & 100 & - & 8.1 (A) \\
**TiO\({}_{\text{2}}\).600** & 12 & 0.036 & 98 & 2 & 28.1 (A) \\ \hline \hline \end{tabular}   
* BET specific surface area determined by N\({}_{2}\) physisorption at \(-\)196 \({}^{\circ}\)C; b Cumulative pore volume calculated at P/P0=0.98 by N2 physisorption at \(-\)196 \({}^{\circ}\)C; c Anatase polymorph (wt%) obtained by X-ray diffraction, Equation (2); d  
can be noticed. By increasing the calcination temperature from 400 \({}^{\circ}\)C to 600 \({}^{\circ}\)C, the surface area drops from 136 to 12 m\({}^{2}\) g\({}^{-1}\), and the cumulative pore volume significantly decreases, passing from 0.236 to 0.036 cm\({}^{3}\) g\({}^{-1}\), respectively. This phenomenon can be attributed to the reorganization of the crystalline structure towards a thermodynamically more stable structure, with an increase of the nanoparticles average size, as observed by the FEG-SEM micrographs. Figure 4 shows the diffractograms of titania samples before and after calcination. The diffraction peaks of the as-prepared and calcined at 400 \({}^{\circ}\)C samples can be indexed, which is in agreement with the values of standard cards, as typical anatase phase (JCPDS, PDF card no. 21-1272) of TiO\({}_{2}\). When the calcination temperature increases to 600 \({}^{\circ}\)C, the typical rutile (110) reflection becomes noticeable at 20 = 27.4\({}^{\circ}\), showing a partial transformation of anatase phase into rutile polymorph (JCPDS, PDF card no. 21-1276). The peak broadening that was observed for the as-synthesized titania sample suggests the formation of small crystallites during the particle growth in the sol-gel synthesis process. As the calcination temperature increases, anatase and rutile peaks become sharper and more intense, due to particles aggregation, which leads to the growth of larger crystal domains. The average size of anatase results to be 6.50 \(\pm\) 0.07 nm for the as-prepared sample and 28.1 \(\pm\) 0.4 nm for the titania treated at the highest temperature (600 \({}^{\circ}\)C).