Characterization and evaluation of the photocatalytic activity of oxides based on TiO2 synthesized by hydrolysis controlled by the use of water/acetone mixtures

New photocatalysts based on TiO2 were synthesized and characterized. The synthesis involved the controlled hydrolysis of titanium tetraisopropoxide using water containing different proportions of acetone. X-ray diffraction analyses combined with Raman spectroscopy revealed crystalline oxides characterized by the coexistence of the anatase and brookite phases. The Rietveld refinement of diffractograms showed that the presence of acetone in the synthesis process influenced the composition of these crystalline phases, with the proportion of brookite growing from 13% to 22% with the addition of this solvent in the synthesis process. The BET isotherms revealed that these materials are mesoporous with surface area approximately 12% higher than that of the oxide prepared from hydrolysis using pure water. The photocatalytic potential of these oxides was evaluated by means degradation tests using the dyes Ponceau 4R and Reactive Red 120 as oxidizable substrates. The values achieved using the most efficient photocatalyst among the synthesized oxides were, respectively, 83% and 79% for mineralization, and 100% for discoloration of these dyes. This same oxide loaded with 0.5% of platinum and suspended in a 5:1 v/v water/methanol mixture, produced 56 mmol of gaseous hydrogen in 5 h of reaction, a specific hydrogen production rate of 138.5 mmol h−1g−1, a value 60% higher than that achieved using TiO2 P25 under similar conditions.


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The diffuse reflectance spectra were obtained using a double beam UV-1650 (Shimadzu) 115 spectrophotometer, estimating the band energy by Kubelka-Munk treatment (Patterson, Shelden 116 & Stockton, 1997). In these measures, barium sulfate was used as reference. 117 The N 2 adsorption-dessorption isotherms were obtained using an ASAP 2020 118 (Micrometrics) analyser. The adsorption data were analysed by the method proposed by 119 Brunauer, Emmett and Teller (BET) for the surface area and the method of Barrett-Joyner-120 Halenda (BJH) for pore volume. 121 Transmission electron microscopy (TEM) images were obtained using a JEM-2100 (Jeol) 122 microscope. In the preparation of the samples, suspensions containing the powders dispersed in 123 acetone were used with the aid of a cutting-edge ultrasound. These suspensions were deposited 124 on copper grids and air dried. From the images, obtained with the aid of the image editing 125 software "ImageJ", it was possible to calculate the particle size randomly selecting 126 approximately 100 particles per image.
127 Photocatalytic assays 128 4 L of an aqueous solution containing 100 mg L -1 of the photocatalyst were used in the 129 photodegradation assay, in combination with a concentration equivalent to 12.0 ppm of dissolved 130 organic carbon of the dye -corresponding to 31.3 mg L -1 of P4R or 43.5 mg L -1 of RR120 -used 131 as oxidizable substrates. Detailed experimental assembly for the photodegradation assays was 132 described in a previous study (Oliveira et al., 2012). 133 A commercial high-pressure mercury lamp (HPLN) of 400 W (Philips, 2015) without the 134 protective bulb was employed as radiation source. Under this condition, its estimated photonic 135 flux in the UVA was of 3.3 x 10 -6 Einstein/s (Machado et al., 2008), with an irradiance inside the 136 reactor of 100 W / m 2 . During discoloration and dye mineralization monitoring, aliquots were 137 collected every 20 minutes, in a total reaction interval of 140 minutes. The dyes discoloration 138 was monitored by varying the absorbance of the solutions with the reaction time, without pH 139 correction. Monitoring was done in the maximum absorbance wavelength in the visible of each 140 dye -507 nm for P4R and 512 nm for RR120 -using a UV-1201 (Shimadzu) spectrophotometer. 141 Mineralization was monitored from dissolved organic carbon (DOC) measurements, using a 142 TOC-VCPH/CPN (Shimadzu) analyser, aiming to identify the most efficient photocatalyst. For 143 this, the experiments were restricted to the monitoring of P4R photodegradation. The most 144 efficient photocatalyst was also submitted to photodegradation tests using Remazol Red 145 (RR120), comparing its performance with that presented by the commercial catalyst Evonik 146 Degussa TiO 2 .

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These assays were conducted at least in triplicate and separately for each dye.

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The reuse of the most efficient photocatalyst was evaluated using P4R as oxidizable 149 substrate. For this, after each reaction the photocatalyst was separated from the supernatant by 150 decanting, washed with distilled water, centrifuged and dried at 70°C for 24 hours, and then 151 reused under the same described conditions using a new load of the same dye. Each test was 152 performed in quadruplicate in order to compensate for the losses that occurred during the 153 washing of the catalyst, in order to ensure a constant mass of this in each cycle. Subsequently, hydrogen production assays were done using the most effective 155 synthesized photocatalyst, as well as the commercial catalyst Evonik Degussa TiO 2 and the W1 156 oxide. In these experiments, the concentration of catalyst was similar to that used in the assays of 157 dye degradation, being this oxide loaded by photoreduction with 0.5% m/m of Pt, furnished by a 158 solution of hexachloroplatinic acid. So, the Pt-loaded photocatalyst was then suspended in 750 159 ml of a water/methanol mixture containing 20% v/v of methanol, this last being used as 160 sacrificial reagent. These assays occurred under continuous stirring. The pH of the reaction 161 medium was adjusted in 6.2 using solutions 0.1 mol L -1 of HCl or NaOH. Finally, the potential 162 of reuse of the photocatalyst used in such assays was evaluated in at least three photocatalytic 163 cycles. In the reuse assays, only the pH adjustment of the reaction medium was performed at the 164 beginning of each new cycle. The first cycle was equivalent to the first hydrogen production test, 165 carried out for five hours. Thus, the total reaction time was 15 hours.

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For all photocatalytic assays the results are the averages of at least three individual 167 experiments.

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For operator protection and better use of radiation produced by the lamp, the reactor was 169 positioned in a box internally covered with aluminum film,   The reactor, built in borosilicate glass, has a cooling jacket connected to a thermostat bath on 175 its outside which keeps the temperature of the reaction medium stabilized at 20ºC throughout the 176 reaction. Before each experiment, the reactor was purged with N 2 for 20 minutes to eliminate 177 dissolved gases, especially oxygen. The same HPLN lamp reported above was used as radiation 178 source. For analysis of the gases produced during the reaction, aliquots of 1 mL of these gases 179 were collected at intervals of 30 minutes of reaction, in a total period of 5 hours. These samples 180 were analyzed at 230°C in a Shimadzu GC-17A gas-phase chromatograph equipped with thermal 181 conductivity detector (TCD) and a Carboxen™ 1010 Plot capillary column. Argon, with flow of 182 40 ml min -1 , was employed as carrier gas. The mean size and mean deformation of crystallite were calculated from the data 197 obtained from the Rietveld refinement, as presented in Table 1 Table 1 -Percentage of crystalline phase, crystallite size and medium deformation, obtained by 203 Rietveld refinement for synthesized oxides.

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Rietveld refinement data demonstrates that the percentage of brookite phase increases 206 from 13% to 22% with the addition of acetone as co-solvent in the hydrolysis of titanium 207 tetraisopropoxide. Despite this, the increase in the proportion of acetone from 25% to 75% did 208 not result in an equivalent increase in the percentage of brookite phase, suggesting that the use of 209 acetone only interfered in hydrolysis, affecting the organization of critical nuclei in the 210 oligomeric network of titanium, in order to preorder the crystallization of the mentioned phase. 211 On the other hand, the average crystallite size of the anatase phase was about 30% lower for W1-212 75, compared to the other oxides, including the W1, where there was no addition of acetone 213 during its synthesis. This suggests that the excess of acetone should promote a significant 214 reduction in the average crystallite size of the anatase phase, favoring the increase in the average 215 crystallite size of brookite. Thus, the mean deformation of the crystallite follows the same trend, 216 i.e., if the secondary phase becomes larger it will present larger deformations, when compared 217 with the primary phase.

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As in the X-ray diffractograms, the Raman spectra also evidence the mixed composition 219 of two crystalline phases, Fig 3. In all oxides, five main bands attributed to the anatase phase are 220 observed respectively at 145 cm -1 (E g ), 198 cm -1 (E g ), 399 cm -1 (B 1g ), 519 cm -1 (B 1g ) and 640 cm -221 1 (E g ) (Sahoo et al., 2009). Between 200 and 500 cm -1 four bands of lower intensity are observed: 222 at 247 cm -1 (A 1g ), 323 cm -1 (B 1g ), 368 cm -1 (B 2g ) and 456 cm -1 (B 2g ), attributed to the phase 223 brookite. In addition to these bands, this phase features a band of greater intensity around 150 224 cm -1 which may be superimposed with the band identified at 145 cm -1 , attributed to anatase, thus 225 influencing the width of the E g Raman mode (Di Paola, Bellardita & Palmisano, 2013; Iliev, 226 Hadjiev & Litvinchuk, 2013).   As for N 2 adsorption and desorption of these oxides, Fig 6, the analysis of the adsorption-256 desorption isotherms suggests that they are type IV (IUPAC., 1985), characteristic of 257 mesoporous materials with an average pore diameter between 2 and 50 nm, Table 2. Hysteresis 258 profiles are very close to those of type H2, associated with more complex mesoporous structures, 259 in which the distribution of pore sizes and their shape are not well defined (Guan-Sajonz et al., 260 1997). It is also evident that the photocatalysts W1-50 and W1-75, synthesized by hydrolysis 261 using the highest percentages of acetone, present slightly more steeper isotherms compared to the 262 oxides W1 and W1-25, also exhibiting greater heterogeneity in pore distribution compared to 263 these same oxides.    Table 2 presents the morphological parameters related to the synthesized oxides. In 270 general, oxides obtained from hydrolysis using water/acetone mixtures did not undergo 271 significant morphological changes, since for W1 the oxide porosity is practically the same 272 presented by W1-50 and W1-75. On the other hand, the surface area of these two oxides is 273 between 10 and 12% larger than that of W1. This may favor the adsorption of organic matter on 274 their surfaces, which can consequently favor the photocatalytic efficiency. In addition, it was 275 observed an inverse correlation between the surface area and the average particle size, except for 276 the W1-25 that presented wide variation on its particle size.

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The TEM images, Fig 7, suggest a dense aspect to the particles, which have irregular 279 spherical shape and a strong tendency to aggregation, giving rise to clusters of TiO 2 . This should 280 be related to the high level of hydrolysis provided by the synthesis method (Jiang, Herricks & 281 Xia, 2003). However, agglomeration appears to have been minimized by the addition of acetone 282 as co-solvent in hydrolysis, evidencing that its use decreased the hydrolysis rate of the precursor. 283 This, consequently, should favor particle dispersion. On the other hand, the particle sizes 284 estimated from these images do not suggest a role of acetone on this property, as can be seen by 285 the values estimated for the particle size: (  The Table 3 presents the photocatalytic performance of the synthesized oxides and of the 294 commercial oxide TiO 2 -P25, in the degradation of the two azo dyes used as oxidizable substrates 295 in this study. For comparative purposes, the dyes were also submitted to direct photolysis, in order 296 to evidence the role of the photocatalysts in the photodegradation.  Table 3 -Photocatalytic performance of synthesized oxides and TiO2-P25 compared with direct 299 photolysis, in the degradation of the dyes Ponceau 4R (P4R) and Remazol Red 120 (RR120).  Suplementary Information (Supplementary Figures 3, 4 and 5). 307 The expected low efficiency both in degradation and discoloration via direct photolysis, 308 compared to the results achieved by the photocatalysts can be related to the energy of the 309 incident photons, provided by the radiation source (Machado et al., 2008), and to the very low 310 rate of formation of radical species, produced by homolytic scission of labile bonds present in 311 these dyes (Kumar et al., 1999). 312 In the experiments involving the participation of the photocatalysts, the degradation 313 occurred more efficiently due the participation of reactive oxygen species, among them the 314 hydroxyl radicals (HO . ) and superoxide radical-ions (O 2 .-), generated mainly by water 315 decomposition. Such species, due their low selectivity (Machado et al., 2012), together with 316 secondary radical species produced during the photocatalytic process, tend to promote the 317 oxidation of organic substrates present in the reactional medium (Oancea & Meltzer, 2014; 318 Santos at al 2015b). The dissolved oxygen, present in the aqueous medium, as example, when 319 reduced by the semiconductor, contributes with the formation of O 2 .and perhydroxyl radicals, 320 which, although less oxidizing than HO . (Machado et al., 2012), are very important in promoting 321 the degradation of organic substrates.

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The values of the apparent rates of discoloration and mineralization, observed in the 323 reactions mediated by the photocatalysts evaluated in the present study, Supplementary Figure  324 3 -Supplementary Information, suggest that these reactions occur in two stages, following 325 kinetics of apparent pseudo-first order. Initially, the reaction occurs at a rate lower than in the 326 second stage, when the apparent rate constant, in some cases, is three times higher. The higher 327 rate constant in the second stage should be a consequence of the more favored adsorption of the 328 fragments of organic matter formed in the first stage of the process, combined with the good 329 availability of oxygen and water, important for the formation of radicals responsible for the 330 oxidation of organic matter (França et al., 2016).

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The mineralization of P4R mediated by the oxides W1-25, W1-50 and W1-75 increased 332 respectively 5.7%, 18.6% and 24.3% more than the result obtained using W1, when 70% 333 mineralization was achieved. It is noteworthy that the hydrolysis process which gave rise to this 334 oxide, occurred exclusively in the presence of water. It should be noted that the mineralization 335 achieved using TiO 2 P25 as photocatalyst was only 8% higher than that obtained when W1-75 336 was employed.

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Although the mineralization and discoloration of P4R conducted using W1-75 presented 338 the best performance among the synthesized oxides, the result observed was only 4.8% higher 339 than that achieved using W1-50. Considering the proportion of acetone used in the synthesis of 340 W1-75 and its limited photocatalytic performance, W1-50 was then considered as the most 341 effective catalyst for mineralizing P4R, being therefore preferably applied in the following stages 342 of the present study. Since W1-25 presented intermediate performance to that observed for the 343 W1 and W1-50 catalysts, evaluating its efficiency, regarding the degradation of RR120, was 344 therefore considered unnecessary.

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The good photocatalytic activity presented by these oxides, in particular the W1-50, can 346 be attributed mainly to the mixed composition of the phases and high crystallinity obtained after 347 heat treatment, confirmed by the XRD and Raman spectra. The presence of an additional phase 348 tends to introduce defects that tend to favor the photocatalytic activity of a photocatalyst 349 (Kandiel et al., 2010). Brookite, for having conduction band approximately 0.14 eV more

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Reuse assays were performed using the recycled W1-50 in the photocatalytic degradation 355 of the dye P4R. The recycled W1-50 was separated by decantation after the first photocatalytic 356 test. It was then washed with distilled water, centrifuged and dried at 70°C for 24 hours. After 357 this procedure, the recycled oxide was used to promote the degradation of P4R present in a new 358 solution. The discoloration level remained at 100% while the mineralization performance 359 decreased about 30%. This loss of performance should be related to photocatalyst poisoning 360 caused by species adsorbed on the catalyst at the end of each photocatalytic cycle, compromising 361 the availability of active sites (Nakhjavani et al., 2015). It is important to consider that the 362 recycled catalyst was not submitted to any prior purification procedure aiming the removal of 363 contaminants incorporated by adsorption the previous cycles. The discoloration and 364 mineralization profiles, as well as the kinetics of discoloration and mineralization in this reuse 365 assay, are available in the Supplementary Information, Supplementary Figure 4. 366 Table 3 also presents the performance of the oxides W1, W1-50 and TiO 2 P25 in the 367 mineralization and discoloration of the dye RR120. In this case, although RR120 has a more 368 complex chemical structure than P4R, presenting two azo groups and two triazine groups, the 369 performance achieved by W1-50 was comparable to that presented when using TiO 2 P25 370 differing only by the kinetic constants of mineralization (k min ). The residual total organic carbon 371 (TOC) observed after degradation of both P4R and RR120 (Supplementary Figures 3 and 5 377 Photocatalytic hydrogen production 378 The profiles of hydrogen production as function of the reaction time, Fig 8, show a 379 superior performance of W1 and W1-50 compared to TiO 2 -P25.  The process mediated by W1-50 produced approximately 56 mmols of gaseous H 2 , while 384 in the same period TiO 2 P25 produced 43% less. On the other hand, W1 produced approximately 385 3% less hydrogen than W1-50. In addition, it is explicit that the production of H 2 using the 386 oxides presented in this study increased until the end of the assay, suggesting that the 387 photocatalytic process was still in its propagation stage. H 2 production using TiO 2 P25 presented 388 a different profile, suggesting typical accommodation of processes in stages near termination. It 429 photostability, reproducibility and significant yield in H 2 production during these experiments 430 may be related to the absence of contaminants in the catalyst in the different cycles. Certain 431 oxides based on TiO 2 , obtained from associations, anchoring and doping with other substances, 432 show losses in the capability of H 2 production as the photocatalytic cycles succeed. The reason 433 for this has been pointed out as being due the photodesorption of compounds associated or 434 anchored or by photoreduction of metals on TiO 2 surface, thus contaminating the reaction sites 435 (Zhang et al., 2013;Yuan et al., 2015).  During the photodegradation assays, the W1-50 was defined as the most effective 451 photocatalyst based on P4R degradation, when 83% mineralization and 100% discoloration were 452 achieved. In reuse assays using the same catalyst and new charges of the same dye, it was 453 possible to achieve the same level of discolouration. However, the mineralization was impaired 454 by the lack of previous treatment of the catalyst between the cycles of reuse, reaching only 58% 455 of mineralization. On the other hand, in the degradation of the dye RR120 the performance of 456 W1-50 was comparable to that obtained using TiO 2 P25, with 100% discoloration and 79% 457 mineralization.

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Regarding the photocatalytic production of hydrogen using W1-50 as a catalyst, 56 459 mmols of gaseous hydrogen were produced in 5 hours of reaction, which corresponds to a 460 specific hydrogen production rate (SHPR) of 138.5 mmol h -1 g -1 , a value 60% higher than that 461 achieved when TiO 2 P25 was employed. In addition, the reuse assays demonstrated the very 462 good photostability and effectiveness of W1-50, which also ensured an increase of 10% in SHPR 463 in the succession of cycles.