Titanium dioxide, TiO2, is one of the most interesting transition metal oxides because of the following properties: 1) high refractive indices of anatase (2.54) and rutile (2.79), which result in high surface reflectivity, 2) high surface area which leads to a higher density of localized states, composed of electrons with energies between the conduction and valence bands, 3) relatively slow rate of charge carrier recombination and energy gaps beyond 3.0 eV, 4) facile interconversion of the +3 and +4 oxidation states, and 5) high absorption in the ultraviolet region of the spectrum. Titania occurs in the form of two important polymorphs, the metastable anatase and stable rutile. These polymorphs exhibit different properties and consequently different photocatalytic performances. Anatase is irreversibly transformed to rutile at elevated calcination temperatures  .
The main industrial applications of titania are as white pigment in paints, food colouring, cosmetics, toothpastes, polymers etc., as filler in heterogeneous catalysis (promoter, catalytical substrate), in photocatalysis. Regarding the energy sector, titania is employed in water electrolysis for hydrogen production and dye-sensitised solar cells (DSSCs). In addition, TiO2 finds application in decontamination technologies and waste-water treatment as adsorbent     . In this context, the study of possible synthetic procedures for preparation of mesoporous titania solids with high surface area and adjustable mesoporous structure is of particular interest in both catalytic applications and environmental decontamination technologies    .
Among the matrix-based synthesis techniques used, the reverse microemulsions belong to the category of combined colloidal systems, which allow particle size and consequently surface area control of TiO2 solids     . Generally, the use of surfactants (e.g. Triton X), offers in addition to the steric stabilization, adjustable micro-environment of the reaction and subsequently particle growth control and selectivity towards certain crystalline phases     .
Regarding the matrix-free methods in aqueous and organic solutions, there are many bibliographic data that describe synthetic pathways of TiO2 solids resulting in high surface area related to the porosity and particle size of the solids  . The matrix-free methods are the solvothermal and hydrothermal processes, pyrolysis, thermal decomposition, aqueous alkalic/acidic precipitation, liquid, and solid combustion techniques, mechanochemical and electrochemical methods, sol-gel procedure, gas phase condensation, microwaves heating synthesis and ultrasound methods. Among the different matrix-free methods the present study is focused on two synthetic processes 1) the colloid formation in aqueous acidic solutions (pH 2) and 2) the solvothermal method using benzyl alcohol and dried acetic acid.
The formation of titania nanocolloids is achieved successfully by hydrolysis and condensation of titanium alkoxides in aqueous media. The hydrolysis of the precursor compound in the presence of excess water is fast and is completed in seconds . The formation of titania colloids at high r ratios (r = [H2O]/[M]) is of interest, because under such conditions small size particles are formed. Furthermore, the solution pH strongly affects the size distribution of the nanoparticles. At pH 2 a homogenous supernatant of well-formed particles is created, whereas at solution pH > 2 the reaction results in the formation of ill-formed solid phases     .
The solvothermal synthesis includes organic solvents both as catalysts and occasionally as components of the solid phases at increased temperature (>100˚C) and pressure (several bars) . The solvothermal process can be defined as chemical reaction or transformation in an organic solvent like methanol , 1,4-butanol , toluene  under supercritical conditions . The particle morphology, crystalline phase and surface chemistry of the solids could be controlled by adjusting the precursor composition, reaction temperature, pressure, solvent properties and aging time  . The solvothermal method provides excellent chemical homogeneity and the possibility to form unique metastable structures at low reaction temperatures . The use of solution additives governs crystalline growth and prevents particle aggregation due to the effective dispersion of nano-crystallites    .
The present study aims to compare of texture and surface characteristics of mesoporous TiO2 solids prepared using Triton X reverse micelles of varying tail size matrix and TiO2 solids synthesized in matrix-free systems 1) through aqueous precipitation at pH 2 and 2) by a solvothermal method in a non-aqueous phase.
2.1. Reverse Micelle-Mediated Synthesis of Titania
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 .
2.2. Synthesis of Titania Colloids in Aqueous Media (pH = 2)
For comparison purposes titania nano-particles prepared by modified method of hydrolysis and digestion Ti(OiPr)4 solution . 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.
2.3. Solvothermal Synthesis of TiO2 Using Benzyl Alcohol and Glacial Acetic Acid (Non-Aqueous Phase)
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 .
3. Results and Discussion
3.1. Surface Characteristics of Titania Solids Calcined at 400˚C
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  . 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 . The specific pore volume of the material has a value 0.393
Figure 1. N2 adsorption isotherms and corresponding pores diameter distributions of TiO2 solids calcined at 400˚C, prepared by three different reverse micelles, hydrolysis in aqueous system and solvothermal synthesis.
Table 1. Porosity data of TiO2 solids prepared by three different reverse micelles, hydrolysis in aqueous system and solvothermal synthesis after calcination at 400˚C.
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 .
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 . 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  .
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
Figure 2. X-ray diffractograms of TiO2 samples calcined at 400˚C for 2 h via three different Triton X reverse micelles, colloidal aqueous systems and solvothermal synthesis.
Figure 3. Infrared spectra of TiO2 samples calcined at 400˚C for 2 h via three different reverse micelles, colloidal aqueous systems at pH 2 and solvothermal method.
Table 2. XRD data of TiO2 solids calcined at 400˚C for 2 h via three different reverse micelles, colloidal aqueous systems at pH 2 and solvothermal method.
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  .
Table 3. Bibliographic and experimental data of FTIR peaks of TiO2 solids via three different reverse micelles, colloidal aqueous systems and solvothermal method    .
3.2. Effect of the Synthetic Path and Calcination at 500˚C on the Texture Characteristics of TiO2
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 . On the other hand, the XRD peaks corresponding to solids obtained from the
Figure 4. N2 ad/desorption isotherms and corresponding pores size distributions of TiO2 solids calcined at 500˚C for 2 h, prepared via matrix-assisted and matrix-free synthetic paths.
Figure 5. XRD diagrams of TiO2 particles calcined at 500˚C for 2 h prepared via matrix-assisted and matrix-free synthetic paths.
Table 4. Surface characteristics of TiO2 solids calcined at 500˚C for 2 h synthesized via matrix-assisted and matrix-free synthetic paths.
colloidal aqueous system at pH 2 become more intense with increasing calcination temperature and correspond to the crystalline planes of rutile  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.
3.3. Textural Changes of Titania Solids after Calcination at 700˚C
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
Figure 6. X-ray diffractograms of TiO2 solids calcined at 700˚C for 2 h prepared via matrix-assisted and matrix-free synthetic paths.
Table 5. XRD data of TiO2 solids calcined at 500˚C for 2 h from three different reverse micelles, aqueous colloidal method and solvothermal synthesis.
use of Triton X-100 correspond exclusively to crystalline planes of anatase  . 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, LXRD = 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 . 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 . The peaks that correspond to anatase structure are relatively very small.
The crystallite size  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
Figure 7. Infrared spectra of TiO2 materials calcined at 700˚C for 2 h prepared via matrix-assisted and matrix-free synthetic paths.
Table 6. XRD measurements of TiO2 solids calcined at 700˚C for 2 h prepared via matrix-assisted and matrix-free synthetic paths.
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  , supporting the previous results.
3.4. DR-UV-Vis Solid State and SEM Studies of TiO2 Samples without Matrix Use Increasing Calcination Temperature
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) . 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
Figure 8. Correlation comparative graph of energy gap with calcination temperature of all the studied TiO2 samples increasing calcination temperature.
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 . 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 . 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 .
Figure 11 summarizes data of surface characteristics of titania solids prepared by five different methods and calcined at different temperatures.
From the diagrams above it can be concluded that:
1) Gels extraction by cyclohexane leads to significant increase of surface area and specific pore volume and improvement of surface characteristics (smaller crystalline size and smaller endo-atomic distances in crystal lattice) of produced titania solids from reverse micelles to an extent that they outweigh the solids prepared in matrix-free media.
Figure 9. SEM images of titania solids obtained from aqueous colloidal systems and calcined at (a) 400˚C; and (b) at 600˚C for 2 h. Red circles highlight some clearly resolved titania crystallites.
(a) (b) (c) (d)
Figure 10. SEM images of TiO2 solids prepared by the solvothermal method and calcined at 400˚C in (a) 500× and (b) 1500× magnifications; (c) at 600˚C and (d) at 700˚C for 2 h (500× magnification). Red circles highlight some clearly resolved titania crystallites.
2) At all calcination temperatures solids obtained using reverse micelles present smaller values of specific volume and mean pore diameter and higher values of endo-atomic distances compared to solids prepared in matrix-free media.
3) With increasing calcination temperature the crystal particle size increases at a slower rate for solids obtained using reverse micelles compared to those formed in matrix-free media.
4) The solids corresponding to the surfactant Triton X-100 and the solvothermal method are characterized by the greatest thermal stability and resistance to sintering.
The results discussed herein augur well for the use of organized media such as micelles, including reverse micelles, as microreactors for the synthesis of well characterized porous samples with desirable characteristics.
Figure 11. Calcination temperature effect on porosity (SpA, Vsp and dDFT) and XRD characteristics (LXRD and dBragg) of TiO2 samples prepared via matrix-assisted and matrix-free synthetic paths.
We thank Professor Ioannis Giapintzakis and co-workers, for providing access to SEM facilities (Department of Mechanical & Manufacturing Engineering at the University of Cyprus). Also, we thank Professor Ioannis Pashalidis for providing access to the DRUV-Vis solid state facilities.
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