Pt: A sol-precipitation, sonochemical and hydrothermal approach

Ultrasonics Sonochemistry xxx (2013) xxx–xxx

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Photocatalytic properties of TiO2 and TiO2/Pt: A sol-precipitation, sonochemical and hydrothermal approach Vojka Zˇunicˇ a,⇑, Marija Vukomanovic´ a, Srecˇo D. Škapin a, Danilo Suvorov a, Janez Kovacˇ b a b

Jozˇef Stefan Institute, Advanced Materials Department, Jamova cesta 39, 1000 Ljubljana, Slovenia Jozˇef Stefan Institute, Department of Surface Engineering and Optoelectronics, Jamova cesta 39, 1000 Ljubljana, Slovenia

a r t i c l e

i n f o

Article history: Received 31 August 2012 Received in revised form 21 May 2013 Accepted 26 May 2013 Available online xxxx Keywords: TiO2 TiO2/Pt Sol-precipitation Hydrothermal synthesis Sonochemical synthesis Photocatalytic activity

a b s t r a c t In this work we prepared TiO2 nano-powders and TiO2/Pt nano-composites via three synthesis methods (sol-precipitation, sonochemical method, hydrothermal method) starting with the same precursors and media. To evaluate and compare the physical properties of the prepared materials, X-ray diffraction analysis, BET measurements, FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS) and electron microscopy (TEM, HRTEM, SAED) were applied. The results showed changes to the TiO2 phase composition and crystallinity, the specific surface area as well as the platinum’s particle shape and size, depending on the method of synthesis. To determine the photocatalytic efficiency of the prepared materials, the photocatalytic discoloration of the methylene blue solution was evaluated using UV–Vis spectroscopy. The important properties required for a high photocatalytic activity, related to the surface characteristics and the phase composition, were determined in terms of the synthesis method. It was concluded that the optimum characteristics were obtained when using the hydrothermal approach, where the TiO2 had two phases, i.e., – anatase and rutile, a Pt-phase in the form of nanoparticles and adsorbed Pt-molecular species, as well as the presence of available free surface hydroxyl groups. Such characteristics had a critical influence on the photocatalytic activity of the final material. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Since the discovery of the photoelectrochemical decomposition of water during irradiation with light [1], titanium dioxide (TiO2) has attracted a great deal of attention as a photocatalytic material for the selective synthesis of organic compounds [2], for the purification of water [3,4] and air [5], for the disinfection of water [6] and surfaces [7] and for self-cleaning and anti-fogging [8] materials. In general, for a high photocatalytic efficiency of the TiO2, the following physical properties must be considered: the crystal structure, the phase composition, the crystallinity, the particle size and/or the specific surface area [9–13]. Among the three natural modifications of TiO2 (anatase, rutile and brookite), anatase is believed to be the most photocatalytically effective phase [14]. The higher Fermi level, greater capacity to absorb oxygen, increased surface hydroxylation, faster interfacial charge transfer and lower recombination rate for the charge carriers are the proposed reasons for anatase’s higher photocatalytic activity when compared to rutile [9]. However, it has also been reported that a biphasic TiO2 consisting of anatase and rutile exhibits an even higher

⇑ Corresponding author. Tel.: +386 1 477 39 92; fax: +386 1 477 38 75. E-mail address: [email protected] (V. Zˇunicˇ).

photocatalytic activity than single-phase TiO2 anatase, due the synergy effect between the anatase and rutile particles, which enhances the life time of the charge carriers [15–17]. The crystallinity of the TiO2 is of high importance since it influences the charge carriers’ life time. Highly crystalline materials contain fewer structural defects, which act as scattering centers for the photogenerated electrons and holes and as a consequence promote their recombination [18,19]. The primary particle size has, in general, a major effect on the photocatalytic activity. The optimum particle size for a high photocatalytic activity is a result of the competing effects of the volume recombination, the surface recombination, the migration of the electrons and holes, the absorption of light, the defects and the specific surface area [12,20,21]. Among the most common synthesis methods for the preparation of nano-crystalline TiO2 are the precipitation method, the sol–gel method, the hydrothermal and solvothermal methods, the micro-emulsion method, the combustion method, the gasphase method, spray pyrolysis, etc. [9]. The physical and chemical properties of TiO2 depend on the preparation route employed. The precipitation method enables control of the particle size and the size distribution [22]. Usually, the TiO2 precipitates are semi-crystalline and require an additional thermal treatment at a temperature higher than 400 °C to induce the crystallization [23]. A very homogeneous TiO2 can be prepared using the hydrothermal (aqueous media) and solvothermal (organic media) methods at a

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relatively low reaction temperature [24]. Compared to the precipitation method, the hydrothermally prepared TiO2 nano-particles are mono-dispersed and less agglomerated [24]. A relatively novel synthesis method for the preparation of nano-crystalline materials is the sonochemical method [25]. The advantages of a sonochemical synthesis over conventional methods of preparing TiO2 are the uniform distribution/dispersion of the nanoparticles, a higher specific surface area, better thermal stability, and increased phase purity [26–28]. In recent years many efforts have been made to improve the photocatalytic activity of TiO2 in the near UV and Vis portions and to shift the absorption edge of the TiO2 anatase to the Vis part of the light spectra [9]. One of the approaches that enable an improvement of the photocatalytic activity is the attachment of noble metals with a large work function onto the surface of the TiO2 [29]. The surface platinization, which was introduced by Kraeutler and Bard [30], has been a frequently used surface-modification technique. The enhancement of the photocatalytic activity of the TiO2 attached with Pt particles is explained by the electron transfer in the TiO2 to the attached Pt particles. This leads to a decrease in the electron–hole recombination, as well as to an efficient charge-carrier separation [31]. However, the reported effects of Pt on the photocatalytic activity of TiO2 are contradictory, since the final properties of TiO2/Pt nano-composites depend on the preparation method and on the added amount of noble metal [32–40]. The aim of this study was to prepare TiO2 nano-powders and TiO2/Pt nano-composites via three synthesis methods – the solprecipitation method, the hydrothermal method and the sonochemical method – derived from the same precursors and media, in order to determine the influence of the synthesis method on the final properties of the TiO2 and TiO2/Pt. It was planned to compare the different physical properties of the prepared materials and evaluate their influences on the photocatalytic activity. Special emphasis will be on an evaluation of the contribution of the surface properties and the phase composition of the resulting materials on the activity. 2. Experimental 2.1. Starting materials For the syntheses of the TiO2 nano-powders titanium (IV) nbutoxide (TNB; TiO4H36C16, 98%, Alfa Aesar), 1-Butanol (C4H9OH, 99%, Alfa Aesar), nitric acid (HNO3, 65%, Merck) and ultrapure water were used. The Pt nanoparticles and their composites with TiO2 were prepared using chloroplatinic acid hexahydrate (H2PtCl66H2O, Sigma Aldrich). To prepare a starting solution that was stable at room temperature (T = 22 ± 5 °C) the following molar ratio of the precursors was used: n(TNB):n(BuOH):n(H2O):n(HNO3) = 1:15:1000:10.

(a). Sol-precipitation. The Solution 3 precipitated when heated at 80 °C for 10 h. Such precipitates were thermally treated at 400 °C for 3 h in a reducing atmosphere. (b). Hydrothermal synthesis. The Solution 3 was transferred to a Teflon-lined stainless-steel autoclave and heated at 240 °C for 10 h. The precipitates that formed were separated from the liquid by centrifugation with 4500 rpm for 10 min, dried at 80 °C and thermally treated in a reducing atmosphere at 400 °C for 3 h. (c). Sonochemical precipitation. The Solution 3 was transferred into a Suslick reactor and heated to 80 °C. Afterwards, the sonication was initiated. The following parameters were used: time of sonication t = 3 h, pulse on:off = 02:01s, amplitude 80%, power P = 600 W and frequency f = 20 kHz (Ultrasonic Processor for High Volume Applications VCX 750, Newtown, Connecticut, USA). The formed precipitates were separated with centrifugation, dried and thermally treated in a reducing atmosphere at 400 °C for 3 h. Table 1 summarizes the precipitation methods used to achieve a variety of TiO2 nano-powders and the sample notation. 2.3. Preparation of the TiO2/Pt nano-composites For the preparation of the TiO2/Pt nano-composites the Pt precursor, chloroplatinic acid hexahydrate (m = 0.019 g), was dissolved in 50 ml of ultrapure water. The amount of used Pt precursor was 1 weight (wt.)% of the amount of titanium precursor. Such a Pt-aqueous solution (V = 50 ml) was added to Solution 3 (V = 50 ml), as described in Section 2.2., and treated in the same ways as described in Section 2.2 (a–c). For the formation of the Pt metal particles the obtained precipitates were thermally treated

Table 1 Sample notation. Sample notation

Precipitation method

TiO2sp TiO2ht TiO2sc TiO2/Pt_sp TiO2/Pt_ht TiO2/Pt_sc

Sol-precipitation followed by thermal treatment Hydrothermal synthesis followed by thermal treatment Sonochemical precipitation followed by thermal treatment Sol-precipitation followed by thermal treatment Hydrothermal synthesis followed by thermal treatment Sonochemical precipitation followed by thermal treatment

2.2. Preparation of the TiO2 nano-powders The TiO2 nano-powders were prepared by the three different synthesis methods followed by a thermal treatment. The titanium precursor titanium(IV) n-butoxide (V = 1.91 ml) was dissolved in 1Butanol (V = 7.7 ml) to form Solution 1. The nitric acid (V = 3.9 ml) was diluted in the ultrapure water (V = 100 ml) to form Solution 2. Afterwards, Solution 2 was added dropwisely to Solution 1. A transparent Solution 3 (pH = 1) was formed, from which the TiO2 particles were precipitated via three different methods, which are described below. In order to obtain a highly crystalline TiO2 and to remove the organic residuals originating from the precursors, the formed precipitates were thermally treated at 400 °C for 3 h in a reducing atmosphere (Ar/H2 = 96/4).

Fig. 1. XRD pattern of the (a) sol-precipitated TiO2, (b) sonochemically precipitated TiO2 and (c) hydrothermally synthesized TiO2.

Please cite this article in press as: V. Zˇunicˇ et al., Photocatalytic properties of TiO2 and TiO2/Pt: A sol-precipitation, sonochemical and hydrothermal approach, Ultrason. Sonochem. (2013), http://dx.doi.org/10.1016/j.ultsonch.2013.05.018

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at 400 °C for 3 h in a reducing atmosphere (Ar/H2 = 96/4). The prepared TiO2/Pt nano-composites were denoted as described in Table 1. 2.4. Characterization techniques X-ray powder-diffraction analysis was preformed utilizing a Bruker AXS D4 Endeavor diffractometer with a Cu Ka X-ray source. The data were collected over the 2h range 20–60° with a step size of 0.04 and a count time of 5 s. The average particle size was calculated from the X-ray line broadening, according to the Debye– Scherrer equation (Eq. (1)):

d¼

kk B cos h

ð1Þ

where k = 0.9 is the Scherer coefficient, k = 0.15406 nm is the X-ray wave length, B is the full width at half maximum (FWHM) of the diffraction peak – (1 0 1) for anatase and (1 1 0) for rutile – and h is the diffraction angle.

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The specific surface area (sBET) was measured with the Brunauer–Emmett–Teller method using the Micromeritics Gemini II 2370 nitrogen-adsorption apparatus (Norcross, GA). The morphological characteristics were analyzed with transmission electron microscopy utilizing a TEM, JEOL-JEM-2100 operating at 200 kV. The FTIR spectroscopy was performed using a Spectrum One (Perkin-Elmer) FTIR spectrometer. In this case the attenuated total reflectance (ATR) technique was applied and the recording was performed in the spectral range between 4000 and 1000 cm1 with a spectral resolution of 4 cm1. The XPS analyses were carried out on a PHI-TFA XPS spectrometer produced by Physical Electronics Inc. The sample surfaces were excited by X-ray radiation from a monochromatic Al source. The high-energy-resolution spectra were acquired with an energy analyzer operating at a resolution of about 0.6 eV and a pass energy of 29 eV. During the data processing the spectra from the surface were aligned by setting the C1s peak at 285.0 eV, which is characteristic for C–C bonds.

Fig. 2. Bright field TEM images with the corresponding SAED pattern of the (a) sol-precipitated TiO2, (b) sonochemically precipitated TiO2 and (c) hydrothermally synthesized TiO2.

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2.5. Measurements of the photocatalytic activity The photocatalytic activity of the prepared samples in the aqueous media was evaluated by monitoring the discoloration of the organic azo dye methylene blue (C16H18N3SCl). Some 7.5 ml of the dye solution (2.67  105 M; 10 mg/l) and 15 mg of the TiO2 powder (2 g/l) were placed in a beaker of 5 cm in diameter. The distance between the sample and the light was 8 cm. Prior to the illumination the suspension was left in the dark to reach the adsorption/desorption equilibrium. The UV irradiation was carried out using fluorescent Blacklight Blue Lamps L18–73 (18 W; k = 300–400 nm) and the Vis irradiation was carried out using the LED Parathom PAR 16 (5 W, k = 400–700 nm), both supplied by Osram. At specific time intervals 700 ll of the suspension was taken from the beaker and centrifuged to remove the TiO2 particles. The change in the absorbance of the dye solution was measured utilizing the UV–Vis-NIR spectrometer (Shimadzu UV–Vis-NIR 3600). After the measurement the dye solution and the TiO2 particles were returned to the beaker. The photocatalytic efficiency of the prepared materials was compared with the commercially available photocatalyst Aeroxide TiO2 P25 (Envonik Industries AG).

3. Results and discussion 3.1. TiO2 nano-powders The physical properties of the TiO2 were analyzed to establish the influence of the synthesis method on the final properties of the TiO2. The phase identification based on X-ray diffraction analysis revealed that the crystallization and phase composition of the TiO2 were influenced by the method of synthesis (Fig. 1). The sol-precipitated TiO2 and hydrothermally synthesized TiO2 were a biphasic TiO2 consisting of anatase and rutile. However, the intensity of the diffraction peaks characteristic for the rutile was lower for the hydrothermally synthesized TiO2. The sonochemically precipitated TiO2 crystallized only in the form of anatase. The TiO2 synthesized via the sol-precipitation method consisted of particles with an average crystallites size of 16 nm for anatase and 23 nm for rutile. The determined average particle size for the hydrothermally synthesized TiO2 was 17 nm for the anatase crystallites and 19 nm for the rutile crystallites. The smallest particles were obtained with the sonochemically prepared TiO2. The average particle size for the anatase was 7 nm. The reason for the smaller particle size of the sonochemically synthesized TiO2 is the ultrasound irradiation. It generates many localized hot spots within the solution, which promote the polycondensation of the BTi–OH species. This results in the homogeneous formation of a large number of seed nuclei, which leads to a smaller particle size [41]. The measured specific surface area was 62 m2/g for the sol-precipitated TiO2, 87 m2/g for the sonochemically precipitated TiO2 and 54 m2/g for the hydrothermally synthesized TiO2. Transmission electron microscopy (TEM, HRTEM and SAED) analyses (Fig. 2) revealed that the different preparation methods resulted in different morphological characteristics of the final TiO2 nano-powders. The particle size observed with the TEM was in agreement with the calculated size. TiO2 prepared by the solprecipitation method (Fig. 2a) consisted of uniformly sized nanoparticles with a slight tendency to agglomerate. The particles exhibited a cube-like and truncated tetragonal bipyramidal shape. The TiO2 nano-powders obtained with the sonochemical synthesis (Fig. 2b) contain nanoparticles with a uniform size and a different shape. Besides cube-like and truncated tetragonal bipyramidal particles, particles with a spherical shape were present. The

hydrothermally synthesized TiO2 (Fig. 2c) exhibited a similar morphology to the sol-precipitated particles.

3.2. TiO2/Pt nano-composites The attachment of Pt affected the TiO2 phase composition and crystallinity in the case of the sol-precipitated and sonochemically precipitated TiO2/Pt. The formation of the sol-precipitated TiO2/Pt nano-composites (Fig. 3a) was accompanied by an intensity decrease in the diffraction maxima characteristic for rutile. The attachment of the Pt particles on the TiO2 surface caused a surface modification that enhanced the anatase–rutile phase transformation [40]. Such a Pt-attachment effect for the sol-precipitation prepared samples was also evident with the determined average particles size of the rutile, which was smaller for the TiO2/Pt_sp than for the TiO2sp, i.e., 15 and 23 nm, respectively (Table 2). Sonochemically precipitated TiO2/Pt (Fig. 3b) crystallized in the form of biphasic TiO2 with anatase as the major phase and brookite appearing only in traces. The attachment of Pt particles did not influence the average particles size of the anatase for the sonochemically precipitated materials. Since the brookite phase existed only in traces, an accurate determination of its crystallite size from the diffraction peaks characteristic for brookite was not possible. The presence of Pt did not significantly alter the TiO2 phase composition, the crystallinity and the average particle size for the hydrothermally prepared TiO2/Pt (Fig. 3c). The measured specific surface area for the prepared nano-composites was the highest for the sonochemically prepared TiO2/Pt,

Fig. 3. XRD pattern of the (a) sol-precipitated TiO2/Pt_sp, (b) sonochemically precipitated TiO2/Pt_sc and (c) hydrothermally synthesized TiO2/Pt_ht.

Table 2 The average particle size and measured specific surface area for the prepared TiO2 nano-powders and TiO2/Pt nano-composites. Sample

danatase (nm)

drutile (nm)

dbrookite (nm)

sBET (m2/g)

TiO2sp TiO2sc TiO2ht TiO2/Pt_sp TiO2/Pt_sc TiO2/Pt_ht

16 7 17 14 7 16

23 / 19 15 / 16

/ / / / ? /

62 87 54 62 72 66

? Due to the low intensity of the diffraction peaks characteristic for the brookite an average brookite particle size determination was not possible.

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which was 77 m2/g. The lower values of the specific surface area for the sol-precipitated and hydrothermally synthesized TiO2/Pt were due to the larger TiO2 particle size of such materials when compared to the sonochemically precipitated TiO2/Pt. The TEM analysis of the TiO2/Pt (Fig. 4) revealed nano-composites with Pt nanoparticles attached to the surface of TiO2. The Pt particles’ morphology (size distribution, distribution over the TiO2-surface and shape) was influenced by the synthesis method. The sol-precipitated TiO2/Pt_sp nano-composites contain 5– 15 nm-sized spheres, homogeneously distributed over the TiO2surface (Fig. 4a). Such a distribution at a low pH was also observed elsewhere [42,43]. A similar distribution of Pt nanoparticles over the surface of the TiO2 was also observed with the sonochemically precipitated TiO2/Pt_sc (Fig. 4b). However, in this case the size of the Pt particles was smaller (2–10 nm). A different situation was observed with the hydrothermally synthesized TiO2/Pt_ht nanocomposites (Fig. 4c) that contained sphere-like and polyhedral Pt-nanoparticles. The size of the Pt spheres was between 5 and 13 nm, while the size of the Pt polyhedral particles was between 20 and 45 nm. It should be mentioned that in this case some of the Pt particles formed a composite with the TiO2 particles while some of them were present as a discrete phase.

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3.3. Surface chemistry The hydroxyl groups adsorbed on the surface of the photocatalyst are very important since they form strong hydroxyl radicals with the photogenerated holes. The hydroxyl radicals are able to oxidize the organic compounds adsorbed on the TiO2 surface and therefore play an important role in the photocatalytic reaction mechanism [9]. Using FTIR spectrometry, information about the appearance of surface hydroxyl modes was obtained. According to the literature data [44] the various surface hydroxyl modes arise from the fact that the oxygen atom of the OH group can coordinate to several different neighboring metal atoms. The bands corresponding to the vibrations of free (isolated Ti–OH) surface hydroxyls occur at the highest frequency (wave number). The hydroxyl groups, hydrogen bonded one to another (vicinal) or bonded to water molecules chemically adsorbed on the TiO2 surface, vibrate at a lower frequency [44,45]. The FTIR spectrums of the prepared TiO2 nano-powders and TiO2/Pt nano-composites are presented in Fig. 5. The broad absorption bands between 3600 and 2800 cm1, with a maximum at around 3400 cm1, were distinctive for the TiO2sp, TiO2sc, TiO2ht and TiO2/Pt_ht, but less pronounced for the TiO2/Pt_sp. Vibrations at such frequencies are

Fig. 4. Bright field TEM images with the corresponding SAED patterns and the HRTEM images of the (a) sol-precipitated TiO2/Pt_sp, (b) sonochemically precipitated TiO2/ Pt_sc and (c) hydrothermally synthesized TiO2/Pt_ht.

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Fig. 5. IR spectra of the prepared materials: (a) TiO2sp, (b) TiO2sc, (c) TiO2ht, (d) TiO2/Pt_sp, (e) TiO2/Pt_sc and (f) TiO2/Pt_ht.

characteristic for the superposition of the mOH mode of the interacting hydroxyl groups (i.e. involved in hydrogen bonds) and the symmetric and anti-symmetric mOH modes of the molecular water coordinated to the Ti4+ cations [44,45]. Such absorption bands were not observed in the FTIR spectrum of the TiO2/Pt_sc, which indicated the absence of surface hydroxyl groups in this material or indicated that these groups were occupied so their vibrations were enabled. Accordingly, we would expect these materials to have a different response during the photocatalytic testing. The band around 1630 cm1 was ascribed to the bending mode of the molecular water. To understand the influence of the surface chemistry on the photocatalytic activity we performed XPS on some of the prepared composites. Using the XPS technique it is possible to determine the chemical status of the elements on the surface. Fig. 6 shows the oxygen O1s spectra from samples TiO2/Pt_ht and TiO2/Pt_sc. In

both spectra we recognized two components using a fitting procedure. The component at 530.2 eV of the binding energy corresponds to oxygen atoms bounded in an oxide structure (TiO2) and the component at 532.1 eV corresponds to OH groups present on the surface of TiO2 [46]. From Fig. 6 we calculated that the OH component is 23% of the total O1s peak for the sample TiO2/Pt_ht and 6% of the total O1s for the sample TiO2/Pt_sc. This shows that many more OH groups are present on the sample TiO2/Pt_ht. Another interesting result obtained with the XPS spectroscopy is the spectrum of Pt 4f (showed for the sample TiO2/Pt_sc). In general, the Pt signal was low due to the low concentration of Pt particles. The Pt 4f spectrum is composed from the doublet Pt 4f7/2 and Pt 4f5/2, separated by 3.33 eV. We decomposed the two doublets of the measured spectrum Pt 4f. The Pt 4f7/2 peak of the first doublet is at a binding energy of 71.3 eV, which we assign to the Pt0 metallic valence state. The 4f7/2 of the second doublet is at a

Fig. 6. High resolution XPS spectra of O 1s for the TiO2 samples (a) TiO2/Pt_ht, (b) TiO2/Pt_sc and (c) Pt 4f for the sample TiO2/Pt_sc.

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Fig. 7. The degradation rates profiles and the corresponding apparent first-order kinetics rate plots for the UV photocatalytic degradation of methylene blue in the presence of the prepared TiO2 nano-powders (a and b) and the TiO2/Pt nano-composites (c and d). The photocatalytic activity was compared with the commercial P25.

binding energy of 72.5 eV, which we assigned to the Pt2+ valence state [47]. This confirmed that the TiO2 particles contain two Ptspecies attached to their surface: (i) elemental Pt within the Pt nanoparticles and (ii) the Pt molecular species with a +2 oxidation state. 3.4. Photocatalytic activity of the TiO2 nano-powders and TiO2/Pt nano-composites The mechanism that leads to the better photocatalytic acitivity of the TiO2 attached to noble metals is well known and desribed in detail elsewhere [29,48]. On the basis of such a mechanism we expected that the TiO2/Pt nano-composites would exhibit a higher photocatalytic activity than the bare TiO2. The photocatalytic activity was determined by monitoring the photocatalytic discoloration of the organic azo dye named methylene blue. According to the literature data [49] the photocatalytic degradation of methylene blue follows an apparent first-order reaction mechanism (Eq. 4), which is in agreement with the generally observed Langmuir–Hinshelwood model:

ln C ¼ lnðC 0 Þ  kt

ð2Þ

where C0 and C are the initial concentrations of the dye at time zero and at time t, respectively, and kapp is the apparent first-order reaction constant. Based on this apparent first-order kinetic mechanism the degradation reaction constants were determined and compared with the commercial photocatalyst TiO2 P25 from Degussa. To

exclude the self-degradation of the model organic substance under irradiation with UV or Vis light, blank tests with no catalyst were performed. Additionally, blank tests with a catalyst and no irradiation were also performed. The results revealed that the UV- or Vislight-induced degradation of methylene blue was practically negligible in the absence of a photocatalyst or when no irradiation was present. Irradiation with UV light of the prepared TiO2 nano-powders in methylene blue solution (Fig. 7a) induced the photocatalytic activity that is represented as a decrease of the relative concentration of the dye with time. Based on the apparent first-order reaction, the UV degradation rates (Fig. 7b) were determined to be 0.006, 0.012 and 0.031 min1 for the TiO2sp, TiO2sc and TiO2ht, respectively (Table 3). It was previously reported that the bulk properties

Table 3 UV and Vis first-order reaction constants kapp (min1) for the TiO2 and the TiO2/Pt. Sample

kapp (min1) UV

Correlation coefficient (r2)

kapp (min1) Vis

Correlation coefficient (r2)

TiO2sp TiO2sc TiO2ht TiO2/Pt_sp TiO2/Pt_sc TiO2/Pt_ht P25

0.00600 0.01200 0.03100 0.05000 0.00800 0.13200 0.08800

0.99367 0.97253 0.97810 0.99553 0.99468 0.86629 0.98230

0.00130 0.00040 0.00340 0.00150 0.00120 0.00570 0.00140

0.99045 0.91179 0.94832 0.99794 0.99067 0.92448 0.98365

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Fig. 8. The degradation rates profiles and the corresponding apparent first-order kinetics rate plots for the Vis photocatalytic degradation of methylene blue in the presence of the prepared TiO2 nano-powders (a and b) and the TiO2/Pt nano-composites (c and d). The photocatalytic activity was compared with the commercial P25.

of the material determine the charge carriers’ photogeneration and diffusion, while the surface properties influence the organic molecules’ adsorption and formation of the species important for the photocatalytic reaction [10–21,44]. In the present study significant correlations were revealed by comparing the properties of the prepared TiO2 nano-powders. The phase composition, average particle size and specific surface area of the TiO2ht and TiO2sp were similar. However, the UV degradation rate of the TiO2ht was 5-times higher than that of the TiO2sp. The exact reason for this difference is not clear yet, but it can be related to the surface properties of the materials (the surface hydroxyl groups and the surface defects) obtained different preparation procedures. It was previously reported [44] that the hydrothermal synthesis enables the preparation of TiO2 with more surface defects, that were stabilized during the thermal treatment as a consequence of the high concentration of ligand molecules (water and butanol). Such defects could be responsible for the more active sites at which the water could dissociate, thus contributing to an enhanced formation of hydroxyl radicals, which are very important for a high photocatalytic activity. Compared to the UV photocatalytic activity, in the Vis regime (Fig. 8) the prepared TiO2 exhibited lower degradation rates. Due to the phase composition of the prepared materials, it is supposed that the Vis-light photocatalytic activity was induced by the surface-adsorbed dye. A mechanism for the Vis-light-induced photocatalytic activity of an azo dye in the presence of TiO2 has already been described [50–52]. Visible-light absorption of the dye results in the formation of photo-excited electrons that can

be transferred to the TiO2 conduction band, leading to the reduction of molecular oxygen and the oxidative decomposition of the TiO2 surface-adsorbed methylene blue [52]. Such a mechanism can explain the Vis-light photocatalytic activity of the prepared TiO2 nano-powders. Presence of the Pt particles resulted in an enhancement of the photocatalytic degradation of the methylene blue under UV irradiation for the hydrothermally synthesized TiO2/Pt and the sol-precipitated TiO2/Pt (Fig. 7c and d). Such an enhancement is ascribed to the presence of Pt particles that are able to store the photogenerated electrons [29], thus contributing to an improved charge-carrier separation. However, the sonochemically precipitated TiO2/Pt_sc did not show any improvement of the photocatalytic performance. On the contrary, the attachment of Pt particles to the surface of the TiO2 resulted in a lower photocatalytic activity of the TiO2/Pt_sc when compared to the bare TiO2sc. As a confirmation of the previously described FTIR results and the hypothesis made according to the presence of the free OH groups, it can be concluded that the reason for the decreased photocatalytic activity of the TiO2/Pt_sc was the absence or occupations of the free surface hydroxyl groups. It is possible that there is an interaction between the free hydroxyl groups on the surface of TiO2 with Pt nanoparticles. Compared to the TiO2 nano-powders, the TiO2/Pt nano-composites exhibited a higher photocatalytic activity under Vis irradiation. In the literature [53] it was reported that the Vis-light induced photocatalytic activity of the TiO2/Pt nano-composites could be

Please cite this article in press as: V. Zˇunicˇ et al., Photocatalytic properties of TiO2 and TiO2/Pt: A sol-precipitation, sonochemical and hydrothermal approach, Ultrason. Sonochem. (2013), http://dx.doi.org/10.1016/j.ultsonch.2013.05.018

V. Zˇunicˇ et al. / Ultrasonics Sonochemistry xxx (2013) xxx–xxx

assigned to the presence of Pt molecular species present on the surface of the TiO2. In our case Pt2+ was detected that forms Pt(OH)2 complexes. According to the obtained results, the major influence on the UV and also on the Vis light photocatalytic activity belongs to the surface properties. In the case of the UV irradiation, the higher photocatalytic activity is correlated with the presence of free hydroxyl groups. Moreover, the influence of the surface chemistry could be a possible explanation for the Vis light induced photocatalytic activity, where the presence of the Pt-molecular species (i.e., Pt(OH)2 complexes) on the surface of the TiO2/Pt nano-composites contributes to the enhanced Vis photocatalytic activity. 4. Conclusion Due to the different growth of the TiO2 nano-materials and their nano-composites with Pt obtained by sonochemical, hydrothermal and sol conditions, their physico-chemical properties were significantly influenced. It was determined that the surface properties and the phase composition were very dependent on the growth rout of the material and they had the most important influence on their photocatalytic activity. The high content of free OH groups together with the presence of the Pt-species, had the most important contribution to the efficacy of the photocatalytic activity in the synthesized material. While the presence of Pt nanoparticles enhanced the UV activity of the nano-composites, the adsorbed Ptmolecular species enhanced their Vis activity. It was also concluded that the methods that enable anatase/rutile transformation and formation of a biphase system have a significant benefit on the photocatalytic activity of the final products. Acknowledgements The authors wish to thank Dr. Polona Umek of the Jozˇef Stefan Institute for her help with the FTIR measurements. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37–38. [2] R. Terzian, N. Serpone, J. Photochem. Photobiol., Chem. 89 (1995) 163–175. [3] A. Fernández, G. Lassaletta, V.M. Jiménez, A. Justo, A.R. González-Elipe, J.-M. Herrmann, H. Tahiri, Y. Ait-Ichou, Appl. Catal., B 7 (1995) 49–63. [4] S.M. Rodríguez, C. Richter, J.B. Gálvez, M. Vincent, Sol. Energy 56 (1996) 401– 410. [5] C.H. Ao, S.C. Lee, Chem. Eng. Sci. 60 (2005) 103–109. [6] S. Gelover, L.A. Gómez, K. Reyes, Ma.T. Leal, Water Res. 40 (2006) 3274–3280. [7] K.P. Kühn, I.F. Chaberny, K. Massholder, M. Stickler, V.W. Benz, H.-G. Sonntag, L. Erdinger, Chemospere 53 (2003) 71–77. [8] Q.-C. Xu, D.V. Wellia, M. Alam Sk, K.H. Lim, J.S.C. Loo, D.W. Liao, R. Amal, T.T.Y. Tan, J. Photochem. Photobiol., C 210 (2010) 181–187. [9] O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004) 33–177. [10] G. Liu, X. wang, Z. Chen, H.-M. Cheng, G.Q. Lu, J. Colloid Interface Sci. 39 (2009) 331–338. [11] V. Puddu, H. Choi, D.D. Dionysiou, G. Li Puma, Appl. Catal., B 94 (2010) 211– 218.

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Please cite this article in press as: V. Zˇunicˇ et al., Photocatalytic properties of TiO2 and TiO2/Pt: A sol-precipitation, sonochemical and hydrothermal approach, Ultrason. Sonochem. (2013), http://dx.doi.org/10.1016/j.ultsonch.2013.05.018