Synthesis of nano-TiO₂ assisted by glycols and submitted to hydrothermal or conventional heat treatment with promising photocatalytic activity

Preparation of different TiO₂ photocatalysts

All chemicals were of analytical or HPLC grade and were used as received. Ultrapure water obtained from an Elix 5 Milli-Q^® water purification system was employed in all experiments. TiO₂ samples were synthesized by the sol–gel method, using different glycols (ethylene glycol, diethylene glycol or polyethylene glycol 300) (Sigma Aldrich), and heat treatment in a conventional oven or hydrothermal system.

The TiO₂Gx photocatalyst was obtained from the mixture, under magnetic stirring, of 10 mL of Ti (IV) isopropoxide (Aldrich, 97%) and 50 mL of glycol (where x = 1 when 886 mmol of ethylene glycol (Vetec, 99.5%) were used, x = 2 for 527 mmol diethylene glycol (Vetec, 99.5%), and x = 3 when polyethylene glycol 300 (Fluka) was used). After 2 h of stirring, a mixture containing 10 mL of ultrapure water and 90 ml of acetone (Synth, 99.5%) was added to the suspension and kept under stirring for 2 h. The white precipitate was separated with the aid of a centrifuge (9,000 rpm for 20 min), followed by washing several times with ethanol to remove residues of glycol, followed by washing three times with distilled water.

For the preparation of heat-treated photocatalysts in a conventional oven (TiO₂GxM), after washing the powder was dried at 70 °C under reduced pressure and sintered at 400 °C for 2 h. After centrifugation and washing the decanted oxide prepared using hydrothermal treatment, TiO₂GxHT, was submitted to the hydrothermal reactor under a pressure of approximately 13.8 bar at 200 °C for 4 h. Subsequently, it was dried at 70 °C for 24 h.

Characterization of the photocatalysts

High resolution electronic transmission images were obtained using a Jeol, JEM-2100, Thermo scientific Transmission Electron Microscope. The particle size and spacing between crystalline planes were calculate with the free software “ImageJ”.

X-ray diffraction analyses (XRD) using a Shimadzu XRD600 powder diffractometer operating at 40 kV and 120 mA, employing Cu K α (λ= 1,54148 Å) radiation. The diffractograms were collected between 10° ≤2θ ≤ 90° under a rate of 0.5° min⁻¹. Crystalline silicon was used as the diffraction standard. X-ray diffratogram of the oxides were refined by the method of Rietveld using the FullProf software, with fitting criteria (Factor S - Goodness of Fit) was employed as the ratio between the weight factor (R_wp) and the expected factor (R_exp), which should be closer to 1. The fit parameters can be found in the (Table S1).

N₂ adsorption–desorption isotherms were obtained using an ASAP 2010 analyzer (Micrometrics). The specific area were analyzed using the Brunauer, Emmett and Teller (BET) model and the Barrett-Joyner-Halenda (BJH) model for the porous volume (Barrett, Joyner & Halenda, 1951).

Raman spectra were acquired at room temperature using a Bruker spectrometer model RFS 100/S, samples were excited at 1064 nm with laser operating at 100 mW. Diffuse reflectance spectra of the synthesized oxides were acquired using an UV-1650PC Spectrometer (Shimadzu), at room temperature and potassium bromide was used as reference. The band gap energy being estimated by the Kubelka–Munk function (Patterson, Shelden & Stockton, 1977).

Photocatalytic activity

In all photocatalytic assays, 100 mg L⁻¹ of the catalyst was added to 31 mg L⁻¹ dye Ponceau 4R (trisodium (8Z)-7-oxo-8-[(4-sulfonatonaphthalen-1-yl)hydrazinylidene]naphthalene-1,3-disulfonate, CI 16255, Sigma-Aldrich, 75%) aqueous solution (pH = 6.9) under magnetic stirring. The experimental setup was previously described in detail (Oliveira et al., 2012). Information about the radiation source and experimental data were available in (Machado et al., 2008; Santos et al., 2015).

The photocatalytic system was kept at 40 ± 2 °C and under stirring for 30 min in the dark to reach the adsorption equilibrium. Control measurements in the dark were performed and in the absence of a catalyst to evidence the role of TiO₂ in the photochemical reaction. Aliquots were taken at 20 min intervals, filtered and analyzed by spectrophotometry, following the discoloration at 507 nm using a Shimadzu spectrophotometer model 1650PC and by Total Organic Carbon (TOC) measurements, using a Shimadzu TOC-VCPH/CPN analyzer.

Photocatalytic activity

The photocatalytic activity of the different synthesized oxides was evaluated in terms of the degradation of the azo-dye Ponceau 4R. The control experiment, in the absence of any photocatalyst, reveals extremely low levels of dye discoloration (4.0%) and mineralization (13%) after 140 min of irradiation (Fig. S2). The degradation efficiency presented by the different photocatalysts is summarized in Table 2.

The oxides thermally treated by hydrothermal via were more efficient than conventional heat treatment in promoting the degradation and mineralization of the dye under study. Calcination in a conventional oven led to an increase in the crystallinity of the materials, as seen by the XRD data, and a decrease in the surface area, which ended up compromising the photocatalytic activity of these oxides.

The photocatalytic performance exhibited by the samples synthesized in the presence of different glycols and thermally treated by hydrothermal via can be attributed to the coexistence of anatase and brookite the high surface area, mesoporosity, and more appropriate particle sizes. Crystalline materials with smaller particle sizes are more likely to exhibit expressive photocatalytic properties (Ohno et al., 2001).

Although the TiO₂G3M photocatalyst also presents itself as a mixture of polymorphs anatase and brookite, it did not show significant photocatalytic activity, probably related to its smaller surface area.

The increase in photocatalytic activity of samples that present anatase and brookite can be explained by the synergism between these polymorphs. Although anatase and brookite present a very close E_g (Machado et al., 2012; Patrocinio et al., 2015), theoretical calculations have shown that the energies of the conduction and valence bands of anatase phase are slightly lower than the corresponding energy levels of brookite (Li et al., 2008), suggesting a certain ease of migration of electrons from brookite to anatase. Thus, the holes are more available for oxidation reactions. In addition, the energy barrier between these polymorphs will tend to hinder the recombination among charge carriers. Therefore, with an extended life span, holes in the brookite valence band have a greater chance to oxidize organic matter, while electrons “trapped” in anatase may favor reduction reactions, leading to an increase in the photocatalytic activity (Li, Ishigaki & Sun, 2007; Patrocinio et al., 2015).

A complex degradation mechanism is expected in heterogeneous photocatalysis (Hoffmann et al., 1995; Ahmed et al., 2010). The reactions occur initially at the solid-solution interface and involve reactive species generated on the surface of the excited photocatalyst or by direct interaction between the excited photocatalyst and the substrate (Oliveira et al., 2012; Santos et al., 2015). In the degradation under study, the discoloration of the dye is probably related to the homolytic scission of the azo group. Hydroxyl radicals (HO^⋅), formed in the solid-solution interface, may be responsible for this process (Kumar & Devi, 2011). Table 2 presents data on the percentage of discoloration in the reactions mediated by the oxides synthesized in this study. The best performances occurred using oxides submitted to hydrothermal treatment.

The mineralization process follows a Langmuir–Hinshelwood kinetics (Hoffmann et al., 1995), being of pseudo-first order in relation to the dye, as show in Fig. 6. The rate constants are listed in Table 2.

In these assays, it was found that only 4.0% of the dye was adsorbed in the photocatalyst (Fig. S2), suggesting that the observed mechanism occurs mainly through the photodegradation of organic matter, certainly by the action of reactive oxygen species (ROS), such as HO^⋅ or O₂^⋅−, with the predominant action of the HO^⋅ radicals, a very strong oxidizing agent (standard reduction potential of HO^⋅/H₂O 2.38 V_vs. NHE) (Hoare, 1985). Accordingly, and based on the characterization of the photocatalysts, we can propose a mechanism, shown in Eqs. (1)-(9) which must occur at the solid-solution interface, where TiO₂ (A) is the anatase polymorph and TiO₂ (B) is the brookite polymorph. As result of the photoexcitation of the catalyst (1), the e⁻/h⁺ pairs are generated; recombination processes (2) compete with the electron trapping in the polymorph anatase (3) and holes in the brookite polymorph (4 and 5), generating the reactive species responsible for the degradation of the dye (5, 6 and 7). In the valence and conduction bands, the oxidation (8) and reduction (9) reactions occur, respectively, resulting in degradation products.

(1)${\displaystyle Ti{O}_2\left(A+B\right)+h\nu \left(UV\right)\to Ti{O}_2\left({e_{cb}}^{-}+{h_{vb}}^{+}\right)}$ (2)${\displaystyle e^{-}\left(cb\right)+h^{+}\left(vb\right)\to Ti{O}_2\left(A+B\right)+h\nu \left(orheat\right)}$ (3)${\displaystyle Ti{O}_2\left(A\right)\left({e_{cb}}^{-}\right)+O_2\to Ti{O}_2+O}$ (4)${\displaystyle Ti{O}_2\left(B\right)\left({h_{vb}}^{+}\right)+H_{2}O\to Ti{O}_2+H^{+}+H{O}^{•}}$ (5)${\displaystyle Ti{O}_2\left(B\right)\left({h_{vb}}^{+}\right)+O{H}^{-}\to Ti{O}_2+H{O}^{•}}$ (6)${\displaystyle O_2^{-•}+H^{+}H{O_2}^{•}}$ (7)${\displaystyle P4R+O{H}^{•}\to Degradationproducts}$ (8)${\displaystyle P4R+{h_{vb}}^{+}\to productoxidation}$ (9)${\displaystyle P4R+{e_{cb}}^{-}\to productreduction}$

The TiO₂G1HT oxide, present the best photocatalytic performance (k_app = 5.9 ×10³ min⁻¹; R = 0.9824), because the availability of reactive species becomes proportionally higher as the concentration of the dye decreases, since the concentration of these species is practically constant during the photocatalytic process (França et al., 2016). Therefore, P4R undergo fragmentation at the beginning of the reaction (Fig. 6 - Inset), which should favor a faster mineralization.

Analyzing the spectrum presented in the Inset of Fig. 6, it can be seen that at the end of the photocatalytic process, the band centered at 500 nm, referred to an electronic transition with a major component π → π* (Oliveira et al., 2012), involving the naphthalenic structures and the azo group, associated with the coloring of the dye, decreases significantly. The formed products should not present new or significant absorption bands in the analyzed region, suggesting that the degradation not only induces a quick discoloration of the dye (Table 2), as they should also cause a significant fragmentation of the dye structure, whose fragments should not absorb significantly in the monitored region of the electromagnetic spectrum.

The coexistence of anatase and brookite in the TiO₂ synthesized with different glycols and treated by a hydrothermal via at low temperature, minimized the recombination rate of the e ⁻/h⁺ pairs, thus allowing the holes to be available for oxidation reactions. In addition, the correlation of physical and chemical factors, such as high surface areas and porosity, high photon absorption capacity in the UV-visible region and crystallinity considerably improved the photocatalytic activity of these oxides.