Ultrasonics Sonochemistry xxx (2013) xxx–xxx

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Ultrasonics Sonochemistry

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Photocatalytic properties of TiO2 and TiO2/Pt: A sol-precipitation,sonochemical and hydrothermal approach

1350-4177/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ultsonch.2013.05.018

⇑ Corresponding author. Tel.: +386 1 477 39 92; fax: +386 1 477 38 75.E-mail address: vojka.zunic@ijs.si (V. Zunic).

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

Vojka Zunic a,⇑, Marija Vukomanovic a, Sreco D. Škapin a, Danilo Suvorov a, Janez Kovac b

a Jozef Stefan Institute, Advanced Materials Department, Jamova cesta 39, 1000 Ljubljana, Sloveniab Jozef 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 a b s t r a c t

Article history:Received 31 August 2012Received in revised form 21 May 2013Accepted 26 May 2013Available online xxxx

Keywords:TiO2

TiO2/PtSol-precipitationHydrothermal synthesisSonochemical synthesisPhotocatalytic activity

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 andmedia. To evaluate and compare the physical properties of the prepared materials, X-ray diffraction anal-ysis, BET measurements, FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS) and electron micros-copy (TEM, HRTEM, SAED) were applied. The results showed changes to the TiO2 phase composition andcrystallinity, the specific surface area as well as the platinum’s particle shape and size, depending on themethod of synthesis. To determine the photocatalytic efficiency of the prepared materials, the photocat-alytic discoloration of the methylene blue solution was evaluated using UV–Vis spectroscopy. The impor-tant properties required for a high photocatalytic activity, related to the surface characteristics and thephase composition, were determined in terms of the synthesis method. It was concluded that the opti-mum characteristics were obtained when using the hydrothermal approach, where the TiO2 had twophases, i.e., – anatase and rutile, a Pt-phase in the form of nanoparticles and adsorbed Pt-molecular spe-cies, as well as the presence of available free surface hydroxyl groups. Such characteristics had a criticalinfluence on the photocatalytic activity of the final material.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Since the discovery of the photoelectrochemical decompositionof water during irradiation with light [1], titanium dioxide (TiO2)has attracted a great deal of attention as a photocatalytic materialfor the selective synthesis of organic compounds [2], for the puri-fication 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, thefollowing physical properties must be considered: the crystalstructure, the phase composition, the crystallinity, the particle sizeand/or the specific surface area [9–13]. Among the three naturalmodifications of TiO2 (anatase, rutile and brookite), anatase is be-lieved to be the most photocatalytically effective phase [14]. Thehigher Fermi level, greater capacity to absorb oxygen, increasedsurface hydroxylation, faster interfacial charge transfer and lowerrecombination rate for the charge carriers are the proposed reasonsfor anatase’s higher photocatalytic activity when compared torutile [9]. However, it has also been reported that a biphasic TiO2

consisting of anatase and rutile exhibits an even higher

photocatalytic activity than single-phase TiO2 anatase, due thesynergy effect between the anatase and rutile particles, which en-hances the life time of the charge carriers [15–17]. The crystallinityof the TiO2 is of high importance since it influences the charge car-riers’ life time. Highly crystalline materials contain fewer struc-tural defects, which act as scattering centers for thephotogenerated electrons and holes and as a consequence promotetheir recombination [18,19]. The primary particle size has, in gen-eral, a major effect on the photocatalytic activity. The optimumparticle size for a high photocatalytic activity is a result of the com-peting effects of the volume recombination, the surface recombina-tion, the migration of the electrons and holes, the absorption oflight, the defects and the specific surface area [12,20,21].

Among the most common synthesis methods for the prepara-tion of nano-crystalline TiO2 are the precipitation method, thesol–gel method, the hydrothermal and solvothermal methods,the micro-emulsion method, the combustion method, the gas-phase method, spray pyrolysis, etc. [9]. The physical and chemicalproperties of TiO2 depend on the preparation route employed. Theprecipitation method enables control of the particle size and thesize distribution [22]. Usually, the TiO2 precipitates are semi-crys-talline and require an additional thermal treatment at a tempera-ture higher than 400 �C to induce the crystallization [23]. A veryhomogeneous TiO2 can be prepared using the hydrothermal(aqueous media) and solvothermal (organic media) methods at a

al ap-

Table 1Sample notation.

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relatively low reaction temperature [24]. Compared to the precip-itation method, the hydrothermally prepared TiO2 nano-particlesare mono-dispersed and less agglomerated [24]. A relatively novelsynthesis method for the preparation of nano-crystalline materialsis the sonochemical method [25]. The advantages of a sonochemi-cal synthesis over conventional methods of preparing TiO2 are theuniform distribution/dispersion of the nanoparticles, a higher spe-cific surface area, better thermal stability, and increased phase pur-ity [26–28].

In recent years many efforts have been made to improve thephotocatalytic activity of TiO2 in the near UV and Vis portionsand to shift the absorption edge of the TiO2 anatase to the Vis partof the light spectra [9]. One of the approaches that enable animprovement of the photocatalytic activity is the attachment ofnoble metals with a large work function onto the surface of theTiO2 [29]. The surface platinization, which was introduced byKraeutler and Bard [30], has been a frequently used surface-modi-fication technique. The enhancement of the photocatalytic activityof the TiO2 attached with Pt particles is explained by the electrontransfer in the TiO2 to the attached Pt particles. This leads to a de-crease in the electron–hole recombination, as well as to an efficientcharge-carrier separation [31]. However, the reported effects of Pton the photocatalytic activity of TiO2 are contradictory, since thefinal properties of TiO2/Pt nano-composites depend on the prepa-ration method and on the added amount of noble metal [32–40].

The aim of this study was to prepare TiO2 nano-powders andTiO2/Pt nano-composites via three synthesis methods – the sol-precipitation method, the hydrothermal method and the sono-chemical method – derived from the same precursors and media,in order to determine the influence of the synthesis method onthe final properties of the TiO2 and TiO2/Pt. It was planned to com-pare the different physical properties of the prepared materials andevaluate their influences on the photocatalytic activity. Specialemphasis will be on an evaluation of the contribution of the sur-face properties and the phase composition of the resulting materi-als on the activity.

Samplenotation

Precipitation method

TiO2sp Sol-precipitation followed by thermal treatmentTiO2ht Hydrothermal synthesis followed by thermal treatmentTiO2sc Sonochemical precipitation followed by thermal treatmentTiO2/Pt_sp Sol-precipitation followed by thermal treatmentTiO2/Pt_ht Hydrothermal synthesis followed by thermal treatmentTiO2/Pt_sc Sonochemical precipitation followed by thermal treatment

2. Experimental

2.1. Starting materials

For the syntheses of the TiO2 nano-powders titanium (IV) n-butoxide (TNB; TiO4H36C16, 98%, Alfa Aesar), 1-Butanol (C4H9OH,99%, Alfa Aesar), nitric acid (HNO3, 65%, Merck) and ultrapurewater were used. The Pt nanoparticles and their composites withTiO2 were prepared using chloroplatinic acid hexahydrate (H2-

PtCl6�6H2O, Sigma Aldrich). To prepare a starting solution thatwas stable at room temperature (T = 22 ± 5 �C) the following molarratio of the precursors was used:n(TNB):n(BuOH):n(H2O):n(HNO3) = 1:15:1000:10.

Fig. 1. XRD pattern of the (a) sol-precipitated TiO2, (b) sonochemically precipitatedTiO2 and (c) hydrothermally synthesized TiO2.

2.2. Preparation of the TiO2 nano-powders

The TiO2 nano-powders were prepared by the three differentsynthesis methods followed by a thermal treatment. The titaniumprecursor titanium(IV) n-butoxide (V = 1.91 ml) was dissolved in 1-Butanol (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. Atransparent Solution 3 (pH = 1) was formed, from which the TiO2

particles were precipitated via three different methods, whichare described below. In order to obtain a highly crystalline TiO2

and to remove the organic residuals originating from the precur-sors, the formed precipitates were thermally treated at 400 �C for3 h in a reducing atmosphere (Ar/H2 = 96/4).

Please cite this article in press as: V. Zunic et al., Photocatalytic properties of Tproach, Ultrason. Sonochem. (2013), http://dx.doi.org/10.1016/j.ultsonch.2013

(a). Sol-precipitation. The Solution 3 precipitated when heated at80 �C for 10 h. Such precipitates were thermally treated at400 �C for 3 h in a reducing atmosphere.

(b). Hydrothermal synthesis. The Solution 3 was transferred to aTeflon-lined stainless-steel autoclave and heated at 240 �Cfor 10 h. The precipitates that formed were separated fromthe liquid by centrifugation with 4500 rpm for 10 min, driedat 80 �C and thermally treated in a reducing atmosphere at400 �C for 3 h.

(c). Sonochemical precipitation. The Solution 3 was transferredinto a Suslick reactor and heated to 80 �C. Afterwards, thesonication was initiated. The following parameters wereused: time of sonication t = 3 h, pulse on:off = 02:01s, ampli-tude 80%, power P = 600 W and frequency f = 20 kHz (Ultra-sonic Processor for High Volume Applications VCX 750,Newtown, Connecticut, USA). The formed precipitates wereseparated with centrifugation, dried and thermally treatedin a reducing atmosphere at 400 �C for 3 h.

Table 1 summarizes the precipitation methods used to achievea 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 pre-cursor, chloroplatinic acid hexahydrate (m = 0.019 g), was dis-solved in 50 ml of ultrapure water. The amount of used Ptprecursor 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 sameways as described in Section 2.2 (a–c). For the formation of thePt metal particles the obtained precipitates were thermally treated

iO2 and TiO2/Pt: A sol-precipitation, sonochemical and hydrothermal ap-.05.018

V. Zunic et al. / Ultrasonics Sonochemistry xxx (2013) xxx–xxx 3

at 400 �C for 3 h in a reducing atmosphere (Ar/H2 = 96/4). The pre-pared TiO2/Pt nano-composites were denoted as described inTable 1.

2.4. Characterization techniques

X-ray powder-diffraction analysis was preformed utilizing aBruker AXS D4 Endeavor diffractometer with a Cu Ka X-ray source.The data were collected over the 2h range 20–60� with a step sizeof 0.04 and a count time of 5 s. The average particle size was calcu-lated from the X-ray line broadening, according to the Debye–Scherrer equation (Eq. (1)):

d ¼ kkB cos h

ð1Þ

where k = 0.9 is the Scherer coefficient, k = 0.15406 nm is the X-raywave length, B is the full width at half maximum (FWHM) of the dif-fraction peak – (101) for anatase and (110) for rutile – and h is thediffraction angle.

Fig. 2. Bright field TEM images with the corresponding SAED pattern of the (a) sol-precipiTiO2.

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The specific surface area (sBET) was measured with the Bru-nauer–Emmett–Teller method using the Micromeritics Gemini II2370 nitrogen-adsorption apparatus (Norcross, GA).

The morphological characteristics were analyzed with trans-mission electron microscopy utilizing a TEM, JEOL-JEM-2100 oper-ating at 200 kV.

The FTIR spectroscopy was performed using a Spectrum One(Perkin-Elmer) FTIR spectrometer. In this case the attenuated totalreflectance (ATR) technique was applied and the recording wasperformed in the spectral range between 4000 and 1000 cm�1 witha spectral resolution of 4 cm�1.

The XPS analyses were carried out on a PHI-TFA XPS spectrom-eter produced by Physical Electronics Inc. The sample surfaceswere excited by X-ray radiation from a monochromatic Al source.The high-energy-resolution spectra were acquired with an energyanalyzer operating at a resolution of about 0.6 eV and a pass energyof 29 eV. During the data processing the spectra from the surfacewere aligned by setting the C1s peak at 285.0 eV, which is charac-teristic for C–C bonds.

tated TiO2, (b) sonochemically precipitated TiO2 and (c) hydrothermally synthesized

iO2 and TiO2/Pt: A sol-precipitation, sonochemical and hydrothermal ap-.05.018

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

2.5. Measurements of the photocatalytic activity

The photocatalytic activity of the prepared samples in theaqueous media was evaluated by monitoring the discolorationof the organic azo dye methylene blue (C16H18N3SCl). Some7.5 ml of the dye solution (2.67 � 10�5 M; 10 mg/l) and 15 mgof the TiO2 powder (2 g/l) were placed in a beaker of 5 cm indiameter. The distance between the sample and the light was8 cm. Prior to the illumination the suspension was left in the darkto reach the adsorption/desorption equilibrium. The UV irradia-tion was carried out using fluorescent Blacklight Blue LampsL18–73 (18 W; k = 300–400 nm) and the Vis irradiation was car-ried out using the LED Parathom PAR 16 (5 W, k = 400–700 nm),both supplied by Osram. At specific time intervals 700 ll of thesuspension was taken from the beaker and centrifuged to removethe TiO2 particles. The change in the absorbance of the dye solu-tion was measured utilizing the UV–Vis-NIR spectrometer (Shi-madzu UV–Vis-NIR 3600). After the measurement the dyesolution and the TiO2 particles were returned to the beaker. Thephotocatalytic efficiency of the prepared materials was comparedwith the commercially available photocatalyst Aeroxide TiO2 P25(Envonik Industries AG).

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

Table 2The 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 16 23 / 62TiO2sc 7 / / 87TiO2ht 17 19 / 54TiO2/Pt_sp 14 15 / 62TiO2/Pt_sc 7 / ? 72TiO2/Pt_ht 16 16 / 66

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

3. Results and discussion

3.1. TiO2 nano-powders

The physical properties of the TiO2 were analyzed to establishthe influence of the synthesis method on the final properties ofthe TiO2.

The phase identification based on X-ray diffraction analysis re-vealed that the crystallization and phase composition of the TiO2

were influenced by the method of synthesis (Fig. 1). The sol-precip-itated TiO2 and hydrothermally synthesized TiO2 were a biphasicTiO2 consisting of anatase and rutile. However, the intensity ofthe diffraction peaks characteristic for the rutile was lower forthe hydrothermally synthesized TiO2. The sonochemically precipi-tated TiO2 crystallized only in the form of anatase.

The TiO2 synthesized via the sol-precipitation method consistedof particles with an average crystallites size of 16 nm for anataseand 23 nm for rutile. The determined average particle size for thehydrothermally synthesized TiO2 was 17 nm for the anatase crys-tallites and 19 nm for the rutile crystallites. The smallest particleswere obtained with the sonochemically prepared TiO2. The averageparticle size for the anatase was 7 nm. The reason for the smallerparticle size of the sonochemically synthesized TiO2 is the ultra-sound irradiation. It generates many localized hot spots withinthe solution, which promote the polycondensation of the BTi–OHspecies. This results in the homogeneous formation of a large num-ber of seed nuclei, which leads to a smaller particle size [41]. Themeasured specific surface area was 62 m2/g for the sol-precipitatedTiO2, 87 m2/g for the sonochemically precipitated TiO2 and 54 m2/gfor the hydrothermally synthesized TiO2.

Transmission electron microscopy (TEM, HRTEM and SAED)analyses (Fig. 2) revealed that the different preparation methodsresulted in different morphological characteristics of the finalTiO2 nano-powders. The particle size observed with the TEM wasin agreement with the calculated size. TiO2 prepared by the sol-precipitation method (Fig. 2a) consisted of uniformly sized nano-particles with a slight tendency to agglomerate. The particlesexhibited 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 differentshape. Besides cube-like and truncated tetragonal bipyramidalparticles, particles with a spherical shape were present. The

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

3.2. TiO2/Pt nano-composites

The attachment of Pt affected the TiO2 phase composition andcrystallinity in the case of the sol-precipitated and sonochemicallyprecipitated TiO2/Pt. The formation of the sol-precipitated TiO2/Ptnano-composites (Fig. 3a) was accompanied by an intensity de-crease in the diffraction maxima characteristic for rutile. Theattachment of the Pt particles on the TiO2 surface caused a surfacemodification that enhanced the anatase–rutile phase transforma-tion [40]. Such a Pt-attachment effect for the sol-precipitation pre-pared samples was also evident with the determined averageparticles size of the rutile, which was smaller for the TiO2/Pt_spthan for the TiO2sp, i.e., 15 and 23 nm, respectively (Table 2). Sono-chemically precipitated TiO2/Pt (Fig. 3b) crystallized in the form ofbiphasic TiO2 with anatase as the major phase and brookiteappearing only in traces. The attachment of Pt particles did notinfluence the average particles size of the anatase for the sono-chemically precipitated materials. Since the brookite phase existedonly in traces, an accurate determination of its crystallite size fromthe diffraction peaks characteristic for brookite was not possible.The presence of Pt did not significantly alter the TiO2 phase compo-sition, the crystallinity and the average particle size for the hydro-thermally prepared TiO2/Pt (Fig. 3c).

The measured specific surface area for the prepared nano-com-posites was the highest for the sonochemically prepared TiO2/Pt,

iO2 and TiO2/Pt: A sol-precipitation, sonochemical and hydrothermal ap-.05.018

V. Zunic et al. / Ultrasonics Sonochemistry xxx (2013) xxx–xxx 5

which was 77 m2/g. The lower values of the specific surface areafor the sol-precipitated and hydrothermally synthesized TiO2/Ptwere due to the larger TiO2 particle size of such materials whencompared to the sonochemically precipitated TiO2/Pt.

The TEM analysis of the TiO2/Pt (Fig. 4) revealed nano-compos-ites with Pt nanoparticles attached to the surface of TiO2. The Ptparticles’ morphology (size distribution, distribution over theTiO2-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 TiO2-surface (Fig. 4a). Such a distribution at a low pH was also observedelsewhere [42,43]. A similar distribution of Pt nanoparticles overthe surface of the TiO2 was also observed with the sonochemicallyprecipitated TiO2/Pt_sc (Fig. 4b). However, in this case the size ofthe Pt particles was smaller (2–10 nm). A different situation wasobserved with the hydrothermally synthesized TiO2/Pt_ht nano-composites (Fig. 4c) that contained sphere-like and polyhedralPt-nanoparticles. The size of the Pt spheres was between 5 and13 nm, while the size of the Pt polyhedral particles was between20 and 45 nm. It should be mentioned that in this case some ofthe Pt particles formed a composite with the TiO2 particles whilesome of them were present as a discrete phase.

Fig. 4. Bright field TEM images with the corresponding SAED patterns and the HRTEMPt_sc and (c) hydrothermally synthesized TiO2/Pt_ht.

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3.3. Surface chemistry

The hydroxyl groups adsorbed on the surface of the photocata-lyst are very important since they form strong hydroxyl radicalswith the photogenerated holes. The hydroxyl radicals are able tooxidize the organic compounds adsorbed on the TiO2 surface andtherefore play an important role in the photocatalytic reactionmechanism [9]. Using FTIR spectrometry, information about theappearance of surface hydroxyl modes was obtained. Accordingto the literature data [44] the various surface hydroxyl modes arisefrom the fact that the oxygen atom of the OH group can coordinateto several different neighboring metal atoms. The bands corre-sponding to the vibrations of free (isolated Ti–OH) surface hydrox-yls occur at the highest frequency (wave number). The hydroxylgroups, hydrogen bonded one to another (vicinal) or bonded towater molecules chemically adsorbed on the TiO2 surface, vibrateat a lower frequency [44,45]. The FTIR spectrums of the preparedTiO2 nano-powders and TiO2/Pt nano-composites are presentedin Fig. 5. The broad absorption bands between 3600 and2800 cm�1, with a maximum at around 3400 cm�1, were distinc-tive for the TiO2sp, TiO2sc, TiO2ht and TiO2/Pt_ht, but less pro-nounced for the TiO2/Pt_sp. Vibrations at such frequencies are

images of the (a) sol-precipitated TiO2/Pt_sp, (b) sonochemically precipitated TiO2/

iO2 and TiO2/Pt: A sol-precipitation, sonochemical and hydrothermal ap-.05.018

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.

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

characteristic for the superposition of the mOH mode of the interact-ing hydroxyl groups (i.e. involved in hydrogen bonds) and the sym-metric and anti-symmetric mOH modes of the molecular watercoordinated to the Ti4+ cations [44,45]. Such absorption bandswere not observed in the FTIR spectrum of the TiO2/Pt_sc, whichindicated the absence of surface hydroxyl groups in this materialor indicated that these groups were occupied so their vibrationswere enabled. Accordingly, we would expect these materials tohave a different response during the photocatalytic testing. Theband around 1630 cm�1 was ascribed to the bending mode of themolecular water.

To understand the influence of the surface chemistry on thephotocatalytic activity we performed XPS on some of the preparedcomposites. Using the XPS technique it is possible to determine thechemical status of the elements on the surface. Fig. 6 shows theoxygen O1s spectra from samples TiO2/Pt_ht and TiO2/Pt_sc. In

Fig. 6. High resolution XPS spectra of O 1s for the TiO2 samples (a) T

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both spectra we recognized two components using a fitting proce-dure. The component at 530.2 eV of the binding energy corre-sponds to oxygen atoms bounded in an oxide structure (TiO2)and the component at 532.1 eV corresponds to OH groups presenton the surface of TiO2 [46]. From Fig. 6 we calculated that the OHcomponent is 23% of the total O1s peak for the sample TiO2/Pt_htand 6% of the total O1s for the sample TiO2/Pt_sc. This shows thatmany more OH groups are present on the sample TiO2/Pt_ht.

Another interesting result obtained with the XPS spectroscopyis the spectrum of Pt 4f (showed for the sample TiO2/Pt_sc). In gen-eral, the Pt signal was low due to the low concentration of Pt par-ticles. The Pt 4f spectrum is composed from the doublet Pt 4f7/2and Pt 4f5/2, separated by 3.33 eV. We decomposed the two dou-blets of the measured spectrum Pt 4f. The Pt 4f7/2 peak of the firstdoublet is at a binding energy of 71.3 eV, which we assign to thePt0 metallic valence state. The 4f7/2 of the second doublet is at a

iO2/Pt_ht, (b) TiO2/Pt_sc and (c) Pt 4f for the sample TiO2/Pt_sc.

iO2 and TiO2/Pt: A sol-precipitation, sonochemical and hydrothermal ap-.05.018

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 ofthe 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.

Table 3UV and Vis first-order reaction constants kapp (min�1) for the TiO2 and the TiO2/Pt.

Sample kapp (min�1)UV

Correlationcoefficient (r2)

kapp (min�1)Vis

Correlationcoefficient (r2)

TiO2sp 0.00600 0.99367 0.00130 0.99045TiO2sc 0.01200 0.97253 0.00040 0.91179TiO2ht 0.03100 0.97810 0.00340 0.94832TiO2/Pt_sp 0.05000 0.99553 0.00150 0.99794TiO2/Pt_sc 0.00800 0.99468 0.00120 0.99067TiO2/Pt_ht 0.13200 0.86629 0.00570 0.92448P25 0.08800 0.98230 0.00140 0.98365

V. Zunic et al. / Ultrasonics Sonochemistry xxx (2013) xxx–xxx 7

binding energy of 72.5 eV, which we assigned to the Pt2+ valencestate [47]. This confirmed that the TiO2 particles contain two Pt-species attached to their surface: (i) elemental Pt within the Ptnanoparticles and (ii) the Pt molecular species with a +2 oxidationstate.

3.4. Photocatalytic activity of the TiO2 nano-powders and TiO2/Ptnano-composites

The mechanism that leads to the better photocatalytic acitivityof the TiO2 attached to noble metals is well known and desribed indetail elsewhere [29,48]. On the basis of such a mechanism we ex-pected that the TiO2/Pt nano-composites would exhibit a higherphotocatalytic activity than the bare TiO2.

The photocatalytic activity was determined by monitoring thephotocatalytic discoloration of the organic azo dye named methy-lene blue. According to the literature data [49] the photocatalyticdegradation of methylene blue follows an apparent first-orderreaction mechanism (Eq. 4), which is in agreement with the gener-ally observed Langmuir–Hinshelwood model:

ln C ¼ lnðC0Þ � kt ð2Þ

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

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exclude the self-degradation of the model organic substance underirradiation with UV or Vis light, blank tests with no catalyst wereperformed. Additionally, blank tests with a catalyst and no irradia-tion were also performed. The results revealed that the UV- or Vis-light-induced degradation of methylene blue was practically negli-gible in the absence of a photocatalyst or when no irradiation waspresent.

Irradiation with UV light of the prepared TiO2 nano-powders inmethylene blue solution (Fig. 7a) induced the photocatalytic activ-ity that is represented as a decrease of the relative concentration ofthe dye with time. Based on the apparent first-order reaction, theUV degradation rates (Fig. 7b) were determined to be 0.006,0.012 and 0.031 min�1 for the TiO2sp, TiO2sc and TiO2ht, respec-tively (Table 3). It was previously reported that the bulk properties

iO2 and TiO2/Pt: A sol-precipitation, sonochemical and hydrothermal ap-.05.018

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 ofthe 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.

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

of the material determine the charge carriers’ photogeneration anddiffusion, while the surface properties influence the organic mole-cules’ adsorption and formation of the species important for thephotocatalytic reaction [10–21,44]. In the present study significantcorrelations were revealed by comparing the properties of the pre-pared TiO2 nano-powders. The phase composition, average particlesize and specific surface area of the TiO2ht and TiO2sp were similar.However, the UV degradation rate of the TiO2ht was 5-times higherthan that of the TiO2sp. The exact reason for this difference is notclear yet, but it can be related to the surface properties of the mate-rials (the surface hydroxyl groups and the surface defects) obtaineddifferent 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 thermaltreatment as a consequence of the high concentration of ligandmolecules (water and butanol). Such defects could be responsiblefor the more active sites at which the water could dissociate, thuscontributing to an enhanced formation of hydroxyl radicals, whichare 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. Dueto the phase composition of the prepared materials, it is supposedthat the Vis-light photocatalytic activity was induced by the sur-face-adsorbed dye. A mechanism for the Vis-light-induced photo-catalytic activity of an azo dye in the presence of TiO2 hasalready been described [50–52]. Visible-light absorption of thedye results in the formation of photo-excited electrons that can

Please cite this article in press as: V. Zunic et al., Photocatalytic properties of Tproach, Ultrason. Sonochem. (2013), http://dx.doi.org/10.1016/j.ultsonch.2013

be transferred to the TiO2 conduction band, leading to the reduc-tion of molecular oxygen and the oxidative decomposition of theTiO2 surface-adsorbed methylene blue [52]. Such a mechanismcan explain the Vis-light photocatalytic activity of the preparedTiO2 nano-powders.

Presence of the Pt particles resulted in an enhancement of thephotocatalytic degradation of the methylene blue under UV irradi-ation for the hydrothermally synthesized TiO2/Pt and the sol-pre-cipitated TiO2/Pt (Fig. 7c and d). Such an enhancement isascribed to the presence of Pt particles that are able to store thephotogenerated electrons [29], thus contributing to an improvedcharge-carrier separation. However, the sonochemically precipi-tated TiO2/Pt_sc did not show any improvement of the photocata-lytic performance. On the contrary, the attachment of Pt particlesto the surface of the TiO2 resulted in a lower photocatalytic activityof the TiO2/Pt_sc when compared to the bare TiO2sc. As a confirma-tion of the previously described FTIR results and the hypothesismade according to the presence of the free OH groups, it can beconcluded that the reason for the decreased photocatalytic activityof the TiO2/Pt_sc was the absence or occupations of the free surfacehydroxyl groups. It is possible that there is an interaction betweenthe free hydroxyl groups on the surface of TiO2 with Pt nano-particles.

Compared to the TiO2 nano-powders, the TiO2/Pt nano-compos-ites exhibited a higher photocatalytic activity under Vis irradiation.In the literature [53] it was reported that the Vis-light inducedphotocatalytic activity of the TiO2/Pt nano-composites could be

iO2 and TiO2/Pt: A sol-precipitation, sonochemical and hydrothermal ap-.05.018

V. Zunic et al. / Ultrasonics Sonochemistry xxx (2013) xxx–xxx 9

assigned to the presence of Pt molecular species present on thesurface of the TiO2. In our case Pt2+ was detected that formsPt(OH)2 complexes.

According to the obtained results, the major influence on the UVand also on the Vis light photocatalytic activity belongs to the sur-face properties. In the case of the UV irradiation, the higher photo-catalytic activity is correlated with the presence of free hydroxylgroups. Moreover, the influence of the surface chemistry could bea possible explanation for the Vis light induced photocatalyticactivity, where the presence of the Pt-molecular species (i.e.,Pt(OH)2 complexes) on the surface of the TiO2/Pt nano-compositescontributes to the enhanced Vis photocatalytic activity.

4. Conclusion

Due to the different growth of the TiO2 nano-materials and theirnano-composites with Pt obtained by sonochemical, hydrothermaland sol conditions, their physico-chemical properties were signifi-cantly influenced. It was determined that the surface propertiesand the phase composition were very dependent on the growthrout of the material and they had the most important influenceon their photocatalytic activity. The high content of free OH groupstogether with the presence of the Pt-species, had the most impor-tant contribution to the efficacy of the photocatalytic activity in thesynthesized material. While the presence of Pt nanoparticles en-hanced the UV activity of the nano-composites, the adsorbed Pt-molecular species enhanced their Vis activity. It was also con-cluded that the methods that enable anatase/rutile transformationand formation of a biphase system have a significant benefit on thephotocatalytic activity of the final products.

Acknowledgements

The authors wish to thank Dr. Polona Umek of the Jozef StefanInstitute for her help with the FTIR measurements.

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