The titania gels have been prepared by mixing the Ti(OⁱPr)₄ precursor compound with the three different reversed micelle mixtures of Triton X-100 (extra pure, Fisher Chemical), Triton X-114 (extra pure, Sigma-Aldrich) and Triton X-45 (extra pure, Sigma-Aldrich) as described elsewhere [17].

For comparison purposes titania nano-particles prepared by modified method of hydrolysis and digestion Ti(OⁱPr)₄ solution [23]. Initially, the precursor solution resulted from the addition of 5 ml Ti(OⁱPr)₄ in 15 ml isopropanol (99.95%, Fisher Chemical). Separately, an aqueous solution, which constitutes the hydrolysis catalyst, is formed by the dropwise addition of 8 M HNO₃ (70%, Sigma-Aldrich Chemical) in 250 ml de-ionized water until the pH is 2. The gel formation procedure started by the mixing of two mentioned above solutions under vigorous stirring. The hydrolysis of Ti(OⁱPr)₄ produced a turbid, milky, colloidal solution, which was heated at 65˚C overnight. The resulting suspension had a relatively high viscosity and a white-blue tint. Afterwards, the collected precipitate was washed several times with high purity ethanol (99.5%, Sigma-Aldrich) and dried at 100˚C under vacuum for several hours. In the end, parts of dried solid were calcined at temperatures 400˚C, 500˚C, 600˚C and 700˚C with temperature increase rate 6˚C/min for 2 h.

For this synthesis a mixture of 10 ml titanium isopropoxide (>97%, Sigma-Aldrich), benzyl alcohol (99.0%, Sigma-Aldrich) and glacial acetic acid (100%, Merck) at a molar ratio of 1:1:4, was employed as described in a previous work [16].

Figure 1 shows the N₂ ad/desorption isotherms and the corresponding pore size distributions of titania solids prepared by different synthetic routes and calcined at 400˚C: 1) TiO₂ precipitation in aqueous solutions (pH 2), 2) solvothermal method and 3) reverse micelle-mediated preparation using Triton-X surfactants of three different tail lengths. According to Figure 1 and the associated data, which are summarized in Table 1, all isotherms are type IV (a) with H2 (b) hysteresis loop [37] [38]. Moreover, the TiO₂ solids prepared by reverse micelles present significantly higher surface area and specific pore volume values and have smaller average pore diameters compared to TiO₂ solids obtained from the matrix-free synthetic procedures. On the other hand, the isotherm of the solvothermal synthesized TiO₂ presents larger H₂ type hysteresis loop, shifted at higher partial pressures. As the corresponding pores size distributions (Figure 1, Table 1) indicate the solids prepared by the solvothermal method after calcination at 400˚C exhibit a wider pore diameter distribution, with the mean diameter being around 16 nm [39]. The specific pore volume of the material has a value 0.393

cm³/g and the specific surface area is equal to 121.1 m²/g. The solids obtained by the precipitation in acidic aqueous solution and by matrix-assisted formation show distributions which are significantly narrower (e.g. smaller area and specific pore volume), are shifted to lower pore diameter values with a mean pore diameter equal to 5.06 nm.

This change in behaviour occurs because during the solvothermal method the formation of TiO₂ particles takes place under high pressure (3 MPa) and high temperature (150˚C) conditions resulting in greater crystallinity, smaller specific surface area and higher specific volume and mean pore diameter, due to the way the aggregation of primary particles occurs. Furthermore, the corresponding isotherm does not present any plateau at saturation pressure suggesting that the cohesion forces between the agglomerates are weak. Accordingly, the corresponding hysteresis loop (Figure 1) shifts at higher partial pressures. Nevertheless, the value of the specific surface area is relatively high (121.1 m²/g) as the benzyl alcohol and glacial acetic acid complexes act partly as structure directing agent affecting the structure and porosity of the formed titanium oxides.

The sequence is Triton X-114 > Triton X-100 > Triton X-45 > pH 2 > solvothermal method. The sequence for the homogeneous methods is due to cyclohexane extraction from the gels, since large amounts of organic residues are transferred from the gels and certainly from the inert of titania particles pores in the organic phase [40].

The diffraction peaks of XRD graphs (Figure 2) reflect the degree of crystallinity of the TiO₂ solids obtained by the five different synthetic procedures and calcined at 400˚C for 2 hours. According to the corresponding data, which are summarized in Table 2, the diffraction peaks of all solids indicate the formation of the thermally metastable anatase structure of TiO₂ [41]. However, in the XRD diffractogram of the titania solids obtained from precipitation in acidic aqueous systems one additional peak appears at 30.58˚, which is probably due to presence of small percent of rutile. Moreover, the diffraction peaks of these solids are very broad and of lower intensity ascribed to their colloidal/less crystalline structure. In contrast, the diffraction peaks of solvothermally prepared samples are more sharp and with higher intensity due to the higher crystallinity of the associated solids. The diffraction peaks of the matrix-assisted formation of TiO₂ solids are significantly broader because of their lower crystallinity. According to the XRD data the crystallinity of the solids increases as follows: TrX-114 < TrX-45 < TrX-100 < pH 2 < solvothermal.

The infrared spectra of the solids (Figure 3, Table 3) show five characteristic peaks at 3430 cm⁻¹, 2346 cm⁻¹, 1626 cm⁻¹, 1391 cm⁻¹ and 478 cm⁻¹ which are attributed to the stretching vibrations of surface hydroxyl groups, vibrations of adsorbed CO₂, bending vibrations of physisorbed water, surface residual organic groups or Ti-OH vibrations and stretching vibrations of Ti-O bonds of TiO₂ crystal lattice [42] [43].

It has to be noted that the peak at 1391 cm⁻¹ corresponding to the solvothermally prepared TiO₂ solid presents higher intensity than the corresponding peak

observed for the other solids. Similarly, the band at 478 cm⁻¹ is very extensive and somehow deformed because the corresponding solids are more crystalline, which is associated with increased number of Ti-O-Ti bonds due to extensive Ti-OH condensation. Generally, solids prepared through matrix-free synthetic procedures display more flattened band at 478 cm⁻¹ than solids prepared with the use of matrices [44] [45].

Figure 4 includes nitrogen ad/desorption isotherms and pore diameter distributions of TiO₂ samples prepared by five different synthetic paths and calcined at 500˚C. According to Figure 4 and the corresponding data summarized in Table 4 the solids prepared in matrix-free media present the highest BET surface area values and the order is as follows: pH 2 > solvothermal > TrX-100 > TrX-114 > TrX-45. This sequence is expected because even after calcination, organic residues originating from the reverse micelles are entrapped in the pores of TiO₂. In addition, the mean values of pores diameters corresponding to solids prepared using micelles are smaller than the corresponding values of solid formed in matrix-free media.

This points out that the use of surfactants with different polar chain length can affect in a specific way the porous and surface characteristics of the formed solids. Besides the changes in particle size, the specific surface area, pore volume and mean diameter of pores decline with decreasing the length of the polar tail of reverse micelles. In contrast to solids synthesized in matrix-free media, which exhibit random porous characteristics with greater mean pores diameter, the corresponding solids prepared using surfactants present systematic shift to lower values regarding the pore diameter, peaks high of the pores size distributions and surface area (Figure 4) as follows: pH 2 > solvothermal > TrX-100 > TrX-114 > TrX-45.

The XRD data corresponding to the five titania solids obtained from the five different synthetic paths and after calcination at 500˚C for 2 h (Figure 5) are in full agreement with the previous results. In addition, the XRD peaks of the solids obtained from the solvothermal method and the use of reverse micelles present sharp peaks, have higher intensity and correspond to the anatase structure [46]. On the other hand, the XRD peaks corresponding to solids obtained from the

colloidal aqueous system at pH 2 become more intense with increasing calcination temperature and correspond to the crystalline planes of rutile [47] and are indicated by squares above the peaks at 2θ values 30.58˚, 36.09˚, 42.46˚, 45.94˚ and 57.1˚ corresponding to (101), (200), (111), (210) and (220) planes, respectively. At a calcination temperature of 700˚C, rutile peaks are slightly shifted and located at 2θ values of 27.39˚, 35.94˚, 39.13˚, 41.15˚, 43.91˚, 54.2˚, 56.52˚, 63.91˚, 69.13˚ and 69.85˚, corresponding to (110), (101), (200), (111), (210), (211), (220), (002), (301) and (112) planes, respectively. Generally, independent of the synthetic path at increased calcination temperatures all TiO₂ solid are gradually transformed from anatase to rutile, which is the thermodynamically more stable form of TiO₂.

From the data in Table 5 it is obvious that the crystallinity of the solids is reduced by following order: solvothermal > TrX-114 > TrX-45 > TrX-100 > pH 2. Regarding the crystallite size, the order is as follows: pH 2 < TrX-100 < TrX-114 < TrX-45 < solvothermal. These results are in line with the foregoing discussion and show that solids of colloidal systems present greater surface area compared to solids produced by the solvothermal method. The TiO₂ samples from the Triton X-100 surfactant and solvothermal method are present higher thermal stability and resistibility to aggregation.

X-ray diffractograms of solids prepared from five different synthetic methods and after calcination at 700˚C are summarized in Figure 6.

According to Figure 6, the diffraction peaks become more narrow and of higher intensity after calcination at 700˚C due to proceeding crystallization and only the peaks of titania solids obtained from the solvothermal method and the

use of Triton X-100 correspond exclusively to crystalline planes of anatase [48] [49]. This is expected as these two materials are characterized by the highest thermal stability. In particular, the sample corresponding to Triton X-100 is the most stable of all and exhibits the smallest crystallite size, L_XRD = 28.72 nm (Table 6). It is obvious that for these materials the transformation of anatase to rutile takes place at temperatures greater than 700˚C. In the XRD diffractograms of the solids obtained from colloidal systems under acidic conditions the rutile peaks dominate as rutile is the titania structure, which is mainly formed under the given conditions [23]. The rutile appears at 2θ values: 27.39˚, 36.09˚, 39.13˚, 41.3˚, 43.91˚, 54.2˚, 56.52˚, 64.06˚, 68.99˚ and 69.71˚ and are associated with crystalline diffraction levels (110), (101), (200), (111), (210), (211), (220), (002), (301) and (112), respectively [50]. The peaks that correspond to anatase structure are relatively very small.

The crystallite size [51] of titanium oxides increases as follows: TrX-100 < TrX-114 < solvothermal < TrX-45 < pH 2. Generally, data show that TiO₂ solids obtained from the biggest amphiphilic TrX-100 present the biggest thermal stability, followed by the intermediate surfactant TrX-114 and the solids obtained from solvothermal method. Figure 7 shows FTIR spectra of titania samples prepared by the five different synthetic methods and calcined at 700˚C. From spectra (Figure 7) it is observed that band around 500 cm⁻¹ is widened even more

for solids obtained from reverse micelles and the solvothermal method compared to solids obtained from the aqueous colloidal system. In addition, the peaks at 521 cm⁻¹ and 671 cm⁻¹ in the spectra of samples of the colloidal system and the solid corresponding to smallest surfactant (Triton X-45), respectively, which are ascribed to vibrations of Ti-O-Ti and Ti-O bonds (Table 3) are characteristic for rutile phase [52] [53], supporting the previous results.

From the graph in Figure 8, which shows the correlation between the energy gap and the calcination temperature for the studied samples, it is clear that these results are in agreement with corresponding results obtained from the isothermal adsorption, XRD and FTIR measurements.

Generally, titania solids with anatase structure present energy gap values above 3.2 eV (3.23 - 3.6 eV), while rutile structure solids give an energy gap with lower values (3.02 - 3.24 eV) [1]. According to the data in Table 2, Table 5 and Table 6 the smaller particle sizes are related with greater energy gaps as it is the case of the solids from intermediate surfactant TrX-114 and colloidal aqueous systems at pH 2. On the other hand, the solids corresponding to TrX-114 show a big energy gap at 400˚C, which gradual declines with increasing temperature due to the gradual formation of the rutile phase. Materials from aqueous colloidal systems at pH 2 at 400˚C and 500˚C present a big energy gap, that is greater than this of pure rutile, because of coexistence of two titania phases, anatase and rutile. This is observed also in the case of solids obtained from smallest amphiphilic TrX-45 after calcination at 500˚C. At 600˚C and 700˚C materials obtained from colloidal systems at pH 2 show the smallest value of energy gap, because the solids are quantitatively transformed to rutile.

Finally, the smallest changes of energy gap with increasing calcination temperature are observed for the two solids corresponding to the biggest surfactant

TrX-100 and the solvothermal method. Moreover, the corresponding gaps have values above 3.2 eV, indicating that anatase is the predominant TiO₂ phase. It is noteworthy, that solids from solvothermal method above 500˚C present the greatest energy gap due to their increased thermal stability.

According to SEM measurements titania solids obtained from solvothermal methods are very well-formed and fine-grained powders with a mean size of aggregates around 48 μm (Figure 9(a) and Figure 10(a)). The mean size value of TiO₂ aggregates obtained using reverse micelles is 130 μm [17]. According to SEM images (Figure 9 and Figure 10) increasing calcination temperature affects the mean size of particles obtained from aqueous colloidal systems and the values are 48, 61 and 87 μm for 400˚C, 600˚C and 700˚C, respectively. This indicates particle sintering process with increasing calcination temperature and hence materials with reduced thermal stability [23]. On the other hand, the mean size of aggregates obtained from the solvothermal method present smaller changes with increasing temperature with values of 48, 47 and 56 μm for 400˚C, 600˚C and 700˚C, respectively, indicating high thermal stability of respective solids [41].